Synthesis and electrochemical characterization of
new high-voltage cathode materials for Lithium-ion
batteries
Silvia Calcaterra
Dissertação para obtenção do Grau de
Mestre em
Química
Silvia Calcaterra
Júri
Presidente:
Profª. Maria Matilde Soares Duarte Marques
Orientadores: Profª. Maria de Fátima Grilo da Costa Montemor
Prof. Roberto Tossici
Vogais:
Dra. Marilena Mancini
Profª. Maria Teresa Oliveira de Moura e Silva
Junho de 2013
Acknowledgements
I would like to thank Doctor Marilena Mancini, my tutor FAAM, Doctor Roberto Tossici, my tutor
UNICAM, Professor Roberto Marassi, Doctor Francesco Nobili and all doctoral candidates of
Electrochemistry Research Group of University of Camerino for the support that they offered me.
In addition, I would like to thank, for the precious collaboration in X-ray Diffraction analysis,
Doctor Gabriele Giuli of the Science Geologist Research Group of University of Camerino.
At the end, I would like to thank also Professor Maria de Fátima Grilo da Costa Montemor and
Professor Maria Matilde Soares Duarte Marques of Instituto Superior Técnico of Lisbon, where I
attend my foreign studies, for they professionalism and assistance.
2
Resumo
As baterias de Lítio recarregáveis têm grande aplicação em dispositivos electrónicos dado
serem caracterizadas por elevadas densidades de energia e ciclo de vida prolongado. As baterias de
Lítio representam a tecnologia mais promissora para futuros veículos eléctricos híbridos (HEVs) e
veículos eléctricos híbridos de tipo plug-in (PHEVs). O objectivo do presente trabalho foi o de
desenvolver materiais catódicos de alta voltagem para baterias de Lítio destinadas a aplicação em
automóveis. A investigação focou-se na síntese e caracterização estrutural, morfológica e
electroquímica da espinela LiNi0.5Mn1.5O4 e da correspondente estrutura dopada com Fe, em que os
iões Ni foram parcialmente substituídos por iões Fe. Foram aplicados diversos métodos sintéticos
para obter a estrutura de espinela, incluindo por reacção mecanoquímica em moinho de bolas e
usando o método sol-gel assistido por ácido cítrico, com diferentes razões ácido cítrico:metal total. O
pó sintetizado pelo método sol-gel demonstrou características estruturais, morfológicas e
electroquímicas superiores às dos pós preparados pelo processo mecanoquímico. A amostra
substituída com Fe demonstrou melhor desempenho que o material não dopado. Foi também
estudado o desempenho dos cátodos a baixa temperatura, tendo sido demonstrada uma boa
capacidade da estrutura dopada com Fe em comparação com o material não dopado na gama de
temperaturas entre 20 e -20 ºC.
Palavras-chave
Baterias de Lítio, alta voltagem, espinela, dopagem com ferro, método sol-gel, baixa
temperatura.
3
Abstract
Rechargeable Lithium-ion batteries have found wide application in the area of electronic devices
because they are characterized by high energy densities and prolonged cycle life. Lithium-ion
batteries are the most promising technology for future hybrid electric vehicles (HEVs) and plug-in
hybrid electric vehicles (PHEVs). The aim of the present work is to develop high-voltage cathode
materials for Lithium-ion batteries for automotive application. The research focused on the synthesis,
structural, morphological and electrochemical characterization of spinel LiNi 0.5Mn1.5O4 and its Fedoped structure, where Ni ions are partially substituted by Fe ions. Different synthetic methods have
been applied to obtain the spinel structure, including the mechanochemical reaction by ball-mill and
the citric acid-assisted sol-gel method, using different ratios citric acid:total metal amount. The sol-gel
synthesized powder shows superior structural, morphological and electrochemical characteristics in
comparison with the powders prepared by mechanochemical process. The Fe-substituted sample
shows higher cycling performances and rate capability than the pristine material. The low-temperature
performances of the cathodes have been also studied, by showing good capacity of the Fe-doped
structure with respect to the pristine one in the temperature range between 20 and -20 °C.
Key words
Lithium-ion batteries, high-voltage, spinel, iron doping, sol-gel method, low temperature.
4
SUMMARY
Acknowledgements...............................................................................2
Resumo...................................................................................................3
Palavras-chave.......................................................................................3
Abstract...................................................................................................4
Key words...............................................................................................4
Index of Figures......................................................................................7
Index of Tables......................................................................................10
Abbreviation List...................................................................................11
1. Introduction ..................................................................................... 13
1.1 Battery System ............................................................................. 15
1.2 Batteries Typologies ..................................................................... 17
1.2.1 Primary Batteries ................................................................. 17
1.2.2 Secondary Batteries ............................................................ 19
1.3 Lithium-ion batteries ..................................................................... 20
1.4 Materials for Lithium-ion batteries ................................................. 25
1.4.1 Anode Materials .................................................................. 25
1.4.2 Electrolyte ........................................................................... 27
1.4.3 Cathode Materials ............................................................... 30
1.5 High-voltage cathode materials for Lithium-ion batteries .............. 34
1.5.1 Synthetic Techniques .......................................................... 38
2. Aim of the Research........................................................................ 42
3. Experimental Techniques ............................................................... 43
3.1 Synthetic Techniques ................................................................... 43
3.2 Structural and Morphological Characterization Techniques .......... 44
3.3 Electrochemical Principles and Definitions ................................... 46
3.4 Electrochemical Characterization Techniques .............................. 47
4. Results and Discussion: evaluation of commercial LiMn2O4
cathode material ................................................................................. 51
5
4.1 Electrode preparation ................................................................... 51
4.1.1 Preparation technique of layer LMS1 .................................. 51
4.1.2 Preparation technique of layer LMS2 .................................. 52
4.2 Morphological characterization of layers LMS1 and LMS2 ........... 52
4.3 Electrochemical characterization of commercial LiMn2O4 ............. 53
5. Results and Discussion: LiNi0.5Mn1.5O4 based high-voltage
cathode materials................................................................................ 62
5.1 Synthesis by mechanochemical process ...................................... 63
5.1.1 Structural and Morphological characterization of the powders
synthesized by mechanochemical process .......................................... 64
5.1.2 Electrode preparation ......................................................... 69
5.1.3 Electrochemical characterization ........................................ 72
5.1.4 Conclusions ........................................................................ 77
5.2 Synthesis by sol-gel method ......................................................... 78
5.2.1 Synthetic procedure ........................................................... 79
5.2.2 Structural and Morphological characterization of the powders
synthesized by sol-gel method ............................................................. 81
5.2.3 Electrode preparation ......................................................... 86
5.2.4 Electrochemical characterization ........................................ 87
5.2.4.1 Electrochemical characterization of layer 3MNO-1 . 88
5.2.4.2 Electrochemical characterization of layer 2FNM-1 .. 95
5.2.4.3 Electrochemical characterization of layer 3FNM-1 .. 98
5.2.4.4 Conclusions and Future developments ................. 105
6. References..................................................................................... 107
6
Index of Figures
Figure 1.1. Trend of CO2 emission by fuel . ......................................................................................... 13
Figure 1.2. Ragone plot of the energy storage domains for the various electrochemical energy
conversion systems . ............................................................................................................................. 15
5
Figure 1.3. Scheme of the fundamental parts of a cell . ..................................................................... 17
5
Figure 1.4. Energy storage capability of primary battery systems . .................................................... 18
5
Figure 1.5. Energy storage capability of secondary battery systems ................................................. 20
Figure 1.6. Comparison between the working mechanisms of Li batteries. ......................................... 21
Figure 1.7. Schematic representation of the working mechanism of a rechargeable Lithium-ion
battery. ................................................................................................................................................... 21
Figure 1.8. Schematic view of fundamental components of Lithium-ion battery. ................................. 22
Figure 1.9. Two possible consequences of overcharge phenomenon both on laptop and mobile
phone. .................................................................................................................................................... 24
Figure 1.10. Three types of carbon
26
. .................................................................................................. 25
Figure 1.11. Structures of main organic solvents used in Lithium-ion batteries. .................................. 28
Figure 1.12. Schematic illustration of three crystal structures for cathode materials: a) layered
structure of LiCoO2, b) spinel structure of LiMn2O4, c) olivine structure of LiFePO4. ............................ 31
Figure 1.13. Result of Jahn-Teller distortion for spinel LiMn2O4: phase transition from cubic Fd3m to
tetragonal phase. ................................................................................................................................... 32
Figure 1.14. Voltage charge-discharge curves of various cathode materials . .................................... 33
Figure 1.15. Future applications for Lithium-ion batteries: hybrid electric (HEVs) and plug-in hybrid
electric (PHEVs) vehicles. ..................................................................................................................... 34
Figure 1.16. Energy densities of cathode materials LiNi0.5Mn1.5O4, LiMn2O4, LiCoO2 and LiFePO4
60
. 35
Figure 1.17. Comparison between the cyclic performance at room temperature (left) and the cyclic
performance at 55 °C (right) for LiNi0.5Mn1.5O4 (black trend)
60
. ............................................................ 36
Figure 1.18. Discharge profiles of the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples at various
C-rates
63
. .............................................................................................................................................. 37
Figure 1.19. Schematic illustration of the basic chemical reactions involved in the gel formation. ...... 40
Figure 3.1. Definitions of some important parameters for batteries. .................................................... 47
Figure 3.2. Schematic representation of a T-cell. ................................................................................. 48
Figure 3.3. Variation of applied potential with time in cyclic voltammetry. The sweep rate can be
defined as v = |dE/dt|. ............................................................................................................................ 49
Figure 4.1. SEM images of layer LMS1 (4 % SP) at different magnifications: 250X (left) and 1000X
(right). .................................................................................................................................................... 53
Figure 4.2. SEM images of layer LMS2 (2 % KB) at different magnifications: 250X (left) and 1000X
(right). .................................................................................................................................................... 53
Figure 4.3. Galvanostatic profiles at C/10-rate for electrodes LMS1.................................................... 54
7
Figure 4.4. Charge/discharge curves (left) and derivates of cycles (right) of electrodes LMS1. .......... 55
Figure 4.5. Galvanostatic profiles at C/10-rate for electrodes LMS2.................................................... 55
Figure 4.6. Charge/discharge curves (left) and derivates of cycles (right) of electrodes LMS2. .......... 56
Figure 4.7. Cycling performance at C/10-rate for: a) electrode LMS1 and b) electrode LMS2. ........... 56
Figure 4.8. Capacity retention for the electrodes LMS1 and LMS2. .................................................... 57
Figure 4.9. Galvanostatic profiles at different C-rates for electrode LMS1. ......................................... 57
Figure 4.10. Galvanostatic profiles at different C-rates for electrode LMS2. ....................................... 58
Figure 4.11. Cycling performance at different C-rates vs. cycle number for: a) electrode LMS1 and b)
electrode LMS2. .................................................................................................................................... 58
Figure 4.12. Discharge profiles for the 3
rd
cycle, of each C-rate, for: a) electrode LMS1 and b)
electrode LMS2. .................................................................................................................................... 59
Figure 4.13. Cyclic voltammetries of electrode LMS1. ......................................................................... 59
-1
Figure 4.14. Cyclic voltammograms of electrode LMS2 with a scan rate 0.1 mV s . .......................... 60
Figure 4.15. Comparison between the cyclic voltammograms (3
rd
cycle) of electrodes LMS1 and
LMS2. .................................................................................................................................................... 61
Figure 5.1. XRD pattern of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using agate
balls. ...................................................................................................................................................... 65
Figure 5.2. SEM images of LiNi0.5Mn1.5O4 powder (agate balls) at different magnifications: a) 1000X,
b) 5000X and c) 8000X.......................................................................................................................... 65
Figure 5.3. EDX spectra of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using agate
balls. ...................................................................................................................................................... 66
Figure 5.4. XRD pattern of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using steel
balls. ...................................................................................................................................................... 66
Figure 5.5. SEM images of LiNi0.5Mn1.5O4 powder (steel balls) after ball-mill treatment, at different
magnifications: a) 1000X and b) 5000X. ............................................................................................... 67
Figure 5.6. EDX spectra of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using steel
balls and after a ball-mill treatment. ...................................................................................................... 67
Figure 5.7. XRD pattern of LiFe0.1Ni0.4Mn1.5O4 powder synthesized by ball mill mechanism using agate
balls. ...................................................................................................................................................... 68
Figure 5.8. SEM images of LiFe0.1Ni0.4Mn1.5O4 powder (agate balls) at different magnifications: a)
1000X and b) 5000X.............................................................................................................................. 69
Figure 5.9. EDX spectra of LiFe0.1Ni0.4Mn1.5O4 powder synthesized by ball-mill mechanism, using
agate balls. ............................................................................................................................................ 69
Figure 5.10. Galvanostatic profiles at different C-rates for electrode 1MNO-1. ................................... 72
rd
Figure 5.11. Discharge profiles for the 3 cycle, of each C-rate, for electrode 1MNO-1. .................... 73
Figure 5.12. Cycling performance at different C-rates vs. cycle number for electrode 1MNO-1. ........ 73
Figure 5.13. Charge/discharge profiles at C/10-rate for electrodes 1FNM-1. ...................................... 75
-1
Figure 5.14. Cyclic voltammograms at scan rate of 0.05 mV s for electrode 1FNM-1. The peaks of
the 2
nd
cycle was highlighted. ................................................................................................................ 77
8
Figure 5.15. Formation of the high-viscous gel (citric acid : Li-Ni-Mn 0.3 : 1). ..................................... 79
Figure 5.16. The product obtained after heating process at 200 °C. ................................................... 79
Figure 5.17. Formation of the high-viscous gel (citric acid : Li-Ni-Mn-Fe 0.3 : 1)................................. 80
Figure 5.18. Formation of the high-viscous gel (citric acid : Li-Ni-Mn-Fe 1 : 1). ................................... 81
Figure 5.19. XRD pattern of powder 3MNO. ........................................................................................ 82
Figure 5.20. SEM images of powder 3MNO at different magnifications: a) 1000X and b) 5000X. ...... 82
Figure 5.21. XRD pattern of powder 2FNM. ......................................................................................... 83
Figure 5.22. SEM images of powder 2FNM at different magnifications: a) 1000X and b) 5000X........ 83
Figure 5.23. XRD pattern of powder 3FNM. ......................................................................................... 84
Figure 5.24. Comparison between XRD patterns of powders 3FNM and 2FNM. ................................ 85
Figure 5.25. SEM images of powder 3FNM at different magnifications: a) 1000X and b) 5000X........ 85
Figure 5.26. Charge/discharge profiles at C/10-rate for the electrodes 3MNO-1................................. 88
Figure 5.27. Comparison of cycling performance at C/10-rate between electrodes 3MNO-1. ............ 89
Figure 5.28. Comparison between a cell with an OCV of few seconds and a cell with an OCV of about
1 h, both performed with electrolyte LP71. ............................................................................................ 90
Figure 5.29. Comparison of discharge capacity profiles of the 3
rd
cycle at each C-rate for a) the
electrode cycled with LP71 and b) the electrode cycled with electrolyte FAAM. .................................. 91
Figure 5.30. Comparison of cycling performance at various C-rates for a) the electrode cycled with
LP71 and b) the electrode cycled with electrolyte FAAM. ..................................................................... 92
rd
Figure 5.31. Comparison of discharge capacities of the 3 cycle at C/5-rate at 20 °C, 0°C and -20 °C
for electrode 3MNO-1. ........................................................................................................................... 93
rd
Figure 5.32. dQ/dE vs. E curves at C/5-rate (3 cycle) at 20 °C, 0 °C and -20 °C for electrode 3MNO1. ............................................................................................................................................................ 93
Figure 5.33. Cycling performance for electrode 3MNO-1. .................................................................... 94
Figure 5.34. Charge/discharge profiles at C/10-rate for a) the electrode cycled with LP30, b) the
electrode cycled with electrolyte FAAM and c) the electrode cycled with LP71. .................................. 96
Figure 5.35. Comparison of cycling performance at C/10-rate for a) the electrode cycled with LP30, b)
the electrode cycled with electrolyte FAAM and c) the electrode cycled with LP71. ............................ 97
Figure 5.36. Comparison of discharge capacity profiles of the 3
rd
cycle at each C-rate for a) the
electrode 3FNM-1 cycled with electrolyte FAAM and b) the electrode 3FNM-1 cycled with LP71. ...... 98
Figure 5.37. Comparison of cycling performance at various C-rates for a) the electrode 3FNM-1
cycled with electrolyte FAAM and b) the electrode 3FNM-1 cycled with LP71. .................................... 99
Figure 5.38. dQ/dE vs. E voltage curve of LiFe0.08Ni0.42Mn1.5O4. ........................................................ 100
Figure 5.39. Cycling performance for electrode 3FNM-1. .................................................................. 101
rd
Figure 5.40. dQ/dE vs. E curves at C/5-rate (3 cycle) at 20 °C, 0 °C and -20 °C for electrode 3FNM1. .......................................................................................................................................................... 101
Figure 5.41. Cycling performance of electrode 3FNM-1 at 20 °C, 0 °C and -20 °C. .......................... 102
Figure 5.42. Discharge profiles of the pristine and Fe-substituted samples at various C-rates. ........ 103
Figure 5.43. dQ/dE vs. E voltage curve of Li1.02Ni0.5Mn1.5O4 and LiFe0.08Ni0.42Mn1.5O4. ..................... 103
9
Figure 5.44. Comparison of dQ/dE vs. E curves between pristine and Fe-doped samples cycled with
the electrolyte a) LP71 and b) electrolyte FAAM. ................................................................................ 104
Figure 5.45. Comparison of discharge cycling performances between pristine and Fe-substituted
samples at different temperatures. ...................................................................................................... 105
Index of Tables
4
Table 1.1. Electrochemical ESDs characteristics . .............................................................................. 14
Table 1.2. Main commercial primary batteries. ..................................................................................... 18
Table 1.3. Main commercial secondary batteries. ................................................................................ 19
Table 1.4. Theoretical irreversible and reversible capacities of several convertible oxides. ................ 26
a
b
c
Table 1.5. Physical properties of organic solvents at 25 °C ( 30 °C, 40 °C, 20 °C). .......................... 28
5
Table 1.6. Comparison of parameters of a Lithium-ion battery depending on cathode component ...33
Table 4.1. Percentage composition of layer LMS1. .............................................................................. 51
Table 4.2. Percentage composition of layer LMS2. .............................................................................. 52
Table 5.1. Comparison of bond dissociation energies of representative transition metals with oxygen
62
. ........................................................................................................................................................... 62
Table 5.2. Percentage composition of layer 1MNO-1. .......................................................................... 70
Table 5.3. Percentage composition of layer 2MNO-1. .......................................................................... 71
Table 5.4. Percentage composition of layer 1FNM-1. .......................................................................... 71
Table 5.5. Summary of cycling data for tested electrodes in different electrolyte systems. ................. 76
Table 5.6. Percentage composition of layer 3MNO-1. .......................................................................... 86
Table 5.7. Percentage composition of layer 2FNM-1. .......................................................................... 86
Table 5.8. Percentage composition of layer 3FNM-1. .......................................................................... 86
rd
Table 5.9. Summary of discharge capacities for the 3 cycle at each C-rate for electrodes 3MNO-1
cycled with different electrolytes. ........................................................................................................... 91
rd
Table 5.10. Summary of discharge capacities for the 3 cycle at each C-rate, at room temperature, for
electrode cycled with electrolyte FAAM and with LP71. ........................................................................ 99
10
Abbreviation List
ESD – Energy Storage Device
BEV – Battery Electric Vehicle
HEV – Hybrid Electric Vehicle
EV – Electric Vehicle
PHEV – Plug-in Hybrid Electric Vehicle
PDAs – Personal Digital Assistants
SHE – Standard Hydrogen Electrode
Li-GlCs – Lithium-Graphite Intercalation Compounds
SEI – Solid Electrolyte Interface
EC – Ethylene Carbonate
PC – Propylene Carbonate
DMC – Dimethyl Carbonate
DEC – Diethyl Carbonate
EMC – Ethyl Methyl Carbonate
LIBOB – Lithium Bis(Oxalate) Borate
BETI – Bisperfluoroethanesulfonimide
SPEs – Solid Polymer Electrolytes
GPEs – Gel Polymeric Electrolytes
PEO – Poly(Ethylene Oxide)
RTILs – Room Temperature Ionic Liquids
RPG – Radiated Polymer Gel
ED – Emulsion Drying
MS –Molten Salt
CCS – Carbon Combustion Method
XRD – X-ray Diffraction
SEM – Scanning Electron Microscopy
EDX – Energy Dispersive X-ray
LP30 – 1 M solution of LiPF6 in EC:DMC 1:1
LP71 - 1 M solution of LiPF6 in EC:DMC:DEC 1:1:1
Electrolyte FAAM – electrolyte provided by FAAM Spa company
KB – Ketjien Black
SP – Super P
LMS1, LMS2 – Lithium Manganese Oxide (LiMn2O4) layers
PVdF – Polyvinylidene Fluoride
NM2P - N-methyl-2-pyrrolidinone
1MNO-1 – Lithium Nickel Manganese Oxide (LiNi0.5Mn1.5O4, ball-mill, agate balls) layer
2MNO-1 - Lithium Nickel Manganese Oxide (LiNi0.5Mn1.5O4, ball-mill, steel balls) layer
11
1FNM-1 - Lithium Iron Nickel Manganese Oxide (LiFe0.1Ni0.4Mn1.5O4, ball-mill, agate balls)
layer
ICL – Irreversible Capacity Loss
3MNO – Lithium Nickel Manganese Oxide (Li1.02Ni0.5Mn1.5O4, sol-gel, citric acid:metals 0.3:1)
2FNM – Lithium Iron Nickel Manganese Oxide (LiFe0.08Ni0.42Mn1.5O4, sol-gel, citric
acid:metals 0.3:1)
3FNM – Lithium Iron Nickel Manganese Oxide (LiFe0.08Ni0.42Mn1.5O4, sol-gel, citric
acid:metals 1:1)
Fd3m – Face-centered cubic space group, cation disordering
P4332 – Cubic space group, cation ordering
3MNO-1 – layer (active material 82.0 %, SP 10.0 %, PVdF 8.0 %, NM2P 0.5 ml) prepared
with powder 3MNO
2FNM-1 – layer (active material 80.0 %, SP 12.0 %, PVdF 8.0 %, NM2P 0.5 ml) prepared
with powder 2FNM
3FNM-1 – layer (active material 80.0 %, SP 12.0 %, PVdF 8.0 %, NM2P 0.5 ml) prepared
with powder 3FNM
OCV – Open Circuit Voltage
EIS – Electrochemical Impedance Spectroscopy
XPS – X-ray Photoelectron Spectroscopy
12
1. Introduction
Nowadays there is a broad, global focus on energy as an economic, geopolitical and strategic
resource and the impact of energy consumption on the environment. Following this trend, investment
in the development and implementation of clean or renewable energy technologies, as well as energy
conservation, will be a major priority of worldwide governments and industry, subject to fluctuations in
the economy and the price of crude oil and natural gas. The emphasis will vary widely from nation to
nation, but the current energy economy must be changed into a cleaner and sustainable energy future
in order to reduce the carbon dioxide emissions and its levels in the atmosphere (Figure 1.1) that
have led to the climate changes and the increase of global warming.
1
Figure 1.1. Trend of CO2 emission by fuel .
Many efforts have been made in order to improve the renewable technologies based on solar,
wind and wave power and develop the electric automotive transportation.
Decarbonising transport is proving to be one of the largest R&D projects of the early 21st
century. There are around 1 billion auto-mobiles in use worldwide, satisfying many needs for mobility
2
in daily life . The automotive industry is therefore one of the largest economic forces globally,
3
employing nearly 10 million people and generating a value chain in excess of $3 trillion per year . The
ever increasing demand for personal mobility and near total dependence on liquid hydrocarbons
means that emission reductions from this sector will be particularly difficult. The development of
alternative fuels to petrol and diesel has been ongoing since the 1970s, initially in response to the oil
shocks and concerns over urban air pollution. More recently, low-carbon technologies are therefore
rapidly advancing with petrol and diesel hybrids, battery electric, hydrogen fuel cell and hybrids of the
two being developed by nearly every major manufacturer. Concerns about up-scaling production and
the ‘true’ environmental and social costs of biofuels means that hydrogen and electricity are widely
4
regarded as the sustainable transport fuels of the future .
Energy storage devices (ESDs) are systems which store energy in various forms such as
electrochemical, kinetic, pressure, potential, electromagnetic, chemical and thermal, using fuel cells,
13
batteries, capacitors, flywheels, compressed air, pumped hydro, super magnets, hydrogen etc. The
principal criteria of an ESD required for the specific automotive application are (i) the amount of
−1
energy in terms of specific energy (in Wh kg ) and energy density (in Wh kg
−1
-1
or in Wh L ), (ii) the
−1
electrical power (in W kg ) i.e. the electrical load required, (iii) the volume and mass, (iv) reliability, (v)
durability, (vi) safety, (vii) cost, (viii) recyclability and (ix) environmental impact. When choosing an
ESD, the following characteristics should be considered: specific power, storage capacity, specific
energy, response time, efficiency, self-discharge rate/charging cycles, sensitivity to heat, charge–
discharge rate lifetime, environmental effects, capital/operating cost and maintenance. For battery
electric vehicles (BEV), batteries with stored energies of 5–30 kWh for electric cars and up to 100 kW
h for electric buses are required; whereas hybrid electric vehicles (HEVs) hold 1–5 kW h of stored
energy, and focus more exclusively on high power discharge. Table 1.1 shows several types of
4
electrochemical ESDs and their characteristics .
4
Table 1.1. Electrochemical ESDs characteristics .
The energy contents of a system are expressed in terms of specific energy and energy density,
-1
-1
whereas the rate capability is expressed as specific power (W kg ) and power density (W L ). In order
to compare the power and energy capabilities, a representation known as the Ragone plot has been
developed. Figure 1.2 shows that fuel cells can be considered to be high-energy systems, whereas
supercapacitors are considered to be high-power systems. Batteries have intermediate power and
energy characteristics. As it is possible to see in the plot, there is some overlap in energy and power
of supercapacitors, or fuel cells, with batteries. Indeed, batteries with thin film electrodes exhibit power
characteristics similar to those of supercapacitors. The Ragone plot demonstrates clearly that no
single electrochemical power source can match the characteristics of the internal combustion engine.
High power and high energy (and thus a competitive behavior in comparison to combustion engines
and turbines) can best be achieved when the available electrochemical power systems are combined.
In such hybrid electrochemical power schemes, batteries and/or supercapacitors would provide high
power and the fuel cells would deliver high energy.
Batteries have found by far their applications in the market, establishing a specific market
position in comparison to supercapacitors and fuel cells. Supercapacitors are used as memory
protection in several electronic devices, while fuel cells are still in the development stage and many
efforts are focused on the research of specific applications in order to allow their diffusion in the
market. However, fuel cells established their usefulness in space applications. The most promising
future markets for fuel cells and supercapacitors are in the same application sector as batteries. This
means that supercapacitor and fuel cell development aim to compete with, or even to replace,
14
batteries in several application areas. In this way, if originally fuel cells were intended to replace
combustion engines and combustion power sources due to possible higher energy conversion
efficiencies and lower environmental impacts, are now under development to replace batteries to
power cellular telephones and notebook computers and for stationary energy storage. The motivation
of this change is simple: fuel cells cannot compete today with combustion engines and gas/steam
turbines because of much higher costs, inferior power and energy performance, and insufficient
5
durability and lifetime .
Figure 1.2. Ragone plot of the energy storage domains for the various electrochemical energy conversion
5
systems .
1.1 Battery System
A battery is an electrochemical device that converts the chemical energy contained in its active
materials directly into electric energy by a spontaneous redox reaction. With the term “battery” it is
6
meant a set of galvanic cells connected in series or in parallel in order to generate higher voltages ,
hence the basic electrochemical unit is the “cell”.
The cell consist of three major components (Figure 1.3):
 Anode: the negative electrode of a cell associated with oxidative chemical reactions that
release electrons into the external circuit.
 Cathode: the positive electrode of a cell associated with reductive chemical reactions that
gain electrons from the external circuit.
 Electrolyte: the ionic conductor which provides the medium for transfer the charge, as ions,
inside the cell between the anode and cathode. The electrolyte is typically a liquid, such as
water or other solvents, with dissolved salts, acids, or alkalis to impart ionic conductivity.
15
Some batteries use solid electrolytes or gel-type polymer electrolytes which are ionic
conductors at the operating temperature of the cell.
In a battery system the anode must be selected depending on these properties: efficiency as
-1
reducing agent, high coulombic output (A h g ), good conductivity, stability, ease of fabrication and
low cost. Metals are mainly used as the anode materials, in particular since the zinc has these
favorable properties is the predominant anode. Lithium, the lightest metal, with a high value of
electrochemical equivalence, has become a very attractive anode as suitable and compatible
electrolytes and cell designs have been developed to control its activity. With the development of
intercalation electrodes, lithiated carbons as well as lithium alloys are finding wide use as anode in
Lithium-ion technology.
The cathode, besides being an efficient oxidizing agent, must also be stable when it is in
contact with the electrolyte and have a useful working voltage. The most common cathode materials
are metallic oxides, while other cathode materials, such as the halogens and the oxyhalides, sulfur
and its oxides, are also used for special battery systems.
The electrolyte must be an excellent ionic conductor but not be an electronic conductor to not
cause internal short-circuiting. Other its important characteristics are non-reactivity with the electrode
materials, little properties variations with the change of temperature, safety in handling and low cost.
Most electrolytes are aqueous solutions, but there are important exceptions as, for example, in
thermal and lithium anode batteries, where molten salt and nonaqueous electrolytes are used to avoid
7
the reaction of the anode with the electrolyte . Each electrolyte is stable only within certain voltage
ranges because when this electrochemical stability window is exceeded, the decomposition of
electrolyte carries out. The voltage stability range depends on the electrolyte composition and its purity
level. The aqueous solvent-based electrolytes, thank their dielectric constants which favors stable
-1
ionic species, show high conductivity values, of the order of 1 S cm . Nonaqueous organic solvent-2
-3
-1
based systems (mainly used for Lithium batteries) have lower values of conductivity (10 - 10 S cm )
respect to the aqueous systems. Compared to water, most organic solvents have a lower solvating
power and a lower dielectric constant: this favors ion pair formation phenomenon. Ion pair formation
lowers the conductivity as the ions are no longer free and bound to each other. Organic electrolytes
show lower conductivities and much higher viscosities than aqueous electrolytes. Organic solventbased electrolytes are limited around to 4.6 V: if this voltage limit is exceeded a polymerization or
5
decomposition of the solvent system occurs .
Inside the cell, the anode and cathode are electronically isolated by a separator material to
prevent the internal short-circuiting, but this separator is permeable to the electrolyte in order to
7
maintain the desired ionic conductivity .
16
5
Figure 1.3. Scheme of the fundamental parts of a cell .
1.2 Batteries Typologies
Batteries are divided into three general classes: primary batteries that are discharged once and
discarded; secondary batteries, rechargeable batteries that can be discharged and then restored to
their original condition by reversing the current flow through the cell; and specialty batteries that are
designed to fulfill a specific purpose. The latter are mainly military and medical batteries that do not
find wide commercial use for various reasons of cost, environmental issues and limited market
5
application respect to the other two categories .
1.2.1 Primary Batteries
Primary batteries are used once, then discarded. They have the advantage of a low initial cost,
with the drawback of costing more over the long term due to the continuous requested replacements.
Generally, primary batteries have a higher capacity and initial voltage than secondary batteries, a
sloping discharge curve and a high energy density according to the ratio weight/volume. These
characteristics allow to use primary batteries in several common applications, such as toys, flashlights,
watches, clocks, hearing aids, radios and also in special uses as implanted medical devices, missiles,
weapons systems. Most primary batteries do not require special disposal although they produce a
8
greater amount of waste than rechargeable batteries .
A list of common commercial primary battery systems is shown in Table 1.2.
17
Nominal
Battery
Voltage
Anode
Cathode
(V)
Leclanché
(carbon-zinc)
Zn chloride
(carbon-zinc)
1.5
Zn foil
1.5
Zn foil
aq ZnCl2-
(natural)
NH4Cl
aq KOH
400
aq KOH
1000
Ag2O
aq KOH
525
treated
LiCF3SO3
MnO2
or LiClO4
Zn
electrolytic
powder
MnO2
Zn
Carbon
powder
(air)
1.5
Zn-Air
1.2
Ag-Zn
1.6
Li-MnO2
3.0
Li-foil
3.0
Li-foil
(CF)n
1.6
Li-foil
FeS2
Li-Carbon
Monofluoride
Li-Iron Sulfide
165
200
Alkaline
powder
-1
(Wh L )
aq ZnCl2
MnO2
Zn
Density
e
MnO2
electrolytic
Energy
Electrolyt
535
LiCF3SO3
635
or LiClO4
LiCF3SO3
500
or LiClO4
Table 1.2. Main commercial primary batteries.
The vast majority of primary batteries are the typical cylindrical alkaline - manganese cells, used
in devices that requiring a greater energy needs and the carbon - zinc, for low energy consumption
uses. Together cover 80 % of total volume of sold batteries.
A graphical representation of the energy storage capability of common types of primary
batteries is shown in Figure 1.4.
5
Figure 1.4. Energy storage capability of primary battery systems .
18
1.2.2 Secondary Batteries
Secondary batteries, also known as “storage batteries” or “accumulators” since they can be
considered as storage devices for electric energy, can be recharged electrically, after discharge, to
their original condition by passing current through them in the opposite direction to that of the
discharge current. They have the advantage of being more cost-efficient over the long term and
environmentally friendly. These electrochemical systems show lower capacity and initial voltage
respect to the primary batteries but they are characterized, besides their ability to be recharged, by
high power density, high self discharge rates, flat discharge curves and good low-temperature
performance.
Secondary batteries can be used in a wide range of applications, from power tools, laptop
computers, mobile phones, PDAs to traction devices and electric vehicles.
In the market there are many types of secondary batteries, but the most common are listed in
Table 1.3.
Nominal
Battery
Voltage
Energy
Anode
Cathode
Electrolyte
Density
-1
(V)
(Wh L )
Lead Acid
2.0
Pb
PbO2
aq H2SO4
70
Ni-Cd
1.2
Cd
NiOOH
aq KOH
100
Ni-Zn
1.7
Zn
NiOOH
aq KOH
100
1.2
MH
NiOOH
aq KOH
240
4.0
LixC6
Li(1-x)CoO2
Ni-Metal
Hydride
Li ion
LiPF6 in nonaqueous
solvents
400
Table 1.3. Main commercial secondary batteries.
A graphical representation of the energy storage capability of common types of secondary
batteries is shown in Figure 1.5.
19
5
Figure 1.5. Energy storage capability of secondary battery systems .
1.3 Lithium-ion batteries
The Lithium-ion batteries technology is undergoing rapid expansion, now representing the
largest segment of the portable battery industry and dominating the computer, cell phone and camera
power source industry. However, the diffusion of these secondary batteries are limited by use of
expensive components, which are not in sufficient supply to allow the industry to grow at the same
9
rate in the next decade . In addition, this type of batteries attracts much interest in the field of material
technology and others, in order to obtain high power devices for applications like electric vehicles
(EVs), hybrid electric vehicles (HEVs) and stationary energy storage
10
. While the energy storage is
the key factor for most consumer devices, for some larger applications, such as batteries in HEVs,
power is the fundamental criteria as the materials must be able to charge sufficiently fast to take
9
advantage of regenerative braking; otherwise, much of the gas savings are lost .
Lithium-ion battery has emerged from lithium metal battery to eliminate its safety problem,
although the former presents a lower energy density
11
. A comparison between these two technologies
are shown in Figure 1.6. The picture highlights that for the Li-metal battery (a), after 100 cycles, the
lithium dendrites growth affecting the efficiency and safety of the system. This phenomenon can be
avoided when the lithium electrode is replaced with an intercalation compound (b)
12
The name of “Lithium-ion battery” was given by T. Nagaura and K. Tozawa
.
13
, and the concept
of “Lithium-ion battery” was firstly introduced by Asahi Kasei Co. Ltd and they obtained patents over
the world
14
.
The working mechanism of a Lithium-ion cell, schematized in Figure 1.7, consists in the highly
reversible electrochemical reaction, usually called “lithium insertion” or “lithium intercalation” process
which may be described as the insertion/extraction of mobile lithium ions into a rigid host structure.
20
Figure 1.6. Comparison between the working mechanisms of Li batteries.
a) Rechargeable Li-metal battery, b) Rechargeable Li-ion battery
12
.
More specifically, during the discharge process lithium ions are extracted from the anode and
inserted into the cathode across the electrolyte. The reverse process occurs when the battery
charges. Thus, this battery is sometimes called as the “Rocking Chair Battery” or the “Swing Battery”
from the view point of lithium ions mobility into the system. The insertion/extraction of lithium ions into
solid host lattices on both electrodes is electronically compensated by the simultaneous electrons
migration from one electrode to the other one, storing and delivering electrical energy, during which
materials are oxidized or reduced. The energy density of a Lithium-ion battery depends on the lithium
storage capacity of the cathode and anode materials.
Figure 1.7. Schematic representation of the working mechanism of a rechargeable Lithium-ion battery.
A typical Lithium-ion battery (Figure 1.8) consists of a positive and a negative electrode
separated by a microporous polymer film and soaked in a liquid electrolyte, typically a lithium salt
dissolved in an organic carbonate or in a mixture of organic solvents. The film electrodes with a
thickness ranging from 30 to 120 μm are coated on metal foil current collectors. The negative
electrode material is typically a graphitic carbon or a layered structure which is combined with a
conductive agent, usually carbon black and binders and coated on a copper current collector. The
positive electrode material is a mixture of a lithium metal oxide with carbon black and/or graphite as
21
conductive agent and a binder and coated on an aluminium current collector. Binders are insulating
fluorine-containing polymers such as polyvinylidene difluoride (PVdF), necessary to achieve good and
stable mechanical contact between particles of active materials during cycling.
Figure 1.8. Schematic view of fundamental components of Lithium-ion battery.
Since the early 1990s, when Sony manufactured the first commercial Lithium-ion battery,
extensive efforts have been made to improve battery performances. Research and development have
focused on two general areas: electrochemistry and materials processing. Numerous methods have
been developed for the fabrication and assembly of Lithium-ion batteries, with particular attention on
the knocking down of the fabrication costs. In order to reduce the cost of Lithium-ion batteries
technology to the desired target, it will be necessary to improve materials processing and to introduce
quality control measures in the manufacturing process. There is little doubt that materials processing
and material development are critical to improving Lithium-ion battery performance. Much attempts
have been made in fabricating each component of this technology but more work is required to
optimize the processing conditions and to understand the effect of the processing on battery
performance. Further works are needed to reduce the safety issues, improve the cyclability, lower the
costs and reduce the environmental hazards associated with the manufacture of large-scale batteries
for hybrid and electric vehicles. For this purpose it would be important a shift from a nonaqueous to an
aqueous system for fabricating composite electrodes might significantly reduce the battery cost and
environmental effects. The key factors of this possible change, that have yet to be fully demonstrated,
are focused on the stabilization of aqueous solutions and reduction of the amount of remaining water
which could interfere with the system
15
. Therefore, breakthroughs in Lithium-ion batteries technology
are urgently required, with innovative, performing and durable material chemistries for both the
electrodes and the electrolyte sub-components. The principal objective is to identify materials
exhibiting higher performance and durability than those currently offered. At the moment Lithium-ion
battery technology consists of LiCoO2 and graphite, which is also the first generation of Lithium-ion
batteries and become successful thank its high energy density, but the worldwide R&D efforts focus
upon its replacement with alternative high capacity and low-cost materials, such as LiMn2O4 and
lithium nickel cobalt oxide. Another important aim of the research activity is to substitute the ethylene
22
carbonate - dimethyl carbonate with other electrolytes which do not suffer from decomposition under
4
oxidative regimes .
The present configuration of Lithium-ion batteries shows poor performances under critical
operating conditions, such as high or low temperatures, high charge/discharge rates and high
operating voltages.
Due to the potential application of Lithium-ion battery technology in the automotive industry, the
improvement of the electrochemical behaviour over a wide range of temperatures is the goal of
several research studies
16
. Generally, good charging performances are reached between 0 – 45 °C.
At low temperatures (below 0 °C), there are many problems such as the increased charge-transfer
resistance on the electrolyte – electrode interfaces, high polarization, limited diffusivity of lithium ions,
reduced ionic conductivity of the electrolyte and solid electrolyte interface (SEI) formed on the
electrodes
17
. At high temperatures, the Lithium-ion batteries may also suffer from capacity fading due
to the non-uniformity of SEI layer, electrolyte decomposition, current collector corrosion,
nanocrystalline deposits and phase segregation in the cathode
18
.
Batteries have the potential to be dangerous if they are not carefully designed or if they are
handled in a wrong manner. Cell manufacturers are conscious of these dangers and design safety
measures into the cells. Likewise, pack manufacturers incorporate safety devices into the pack
designs to protect the battery from out of tolerance operating conditions and where possible from
abuse. Once the battery has left the factory its fate is in the hands of the user. It is usual to provide
"Instructions For Use" with battery products which alert the end user to potential dangers from abuse
of the battery
19
.
Safety issues represent one of the most important drawbacks of Lithium-ion batteries because
they contain flammable organic electrolytes and, in extreme conditions, flame and smoke can be
produced. The problem of safety of these batteries depends on two main factors that are poor thermal
stability of electrode materials and the insufficient tolerance to overcharging.
The overcharge phenomenon occurs when the battery is inadvertently charged to a higher
voltage value than the specified one and generally it has a harmful influence on the battery
performance. Prolonged charging above 4.3 V the number of lithium ions transferred from cathode to
anode is higher than that the battery can accommodate and this leads to the formation of a metallic
lithium plate on the anode, while the cathode material becomes an oxidizing agent, loses stability and
produces carbon dioxide due to its reaction with the electrolyte. Not only CO2 can be produced during
the overcharge reaction: several varieties of gases like CO, H 2, CH4, C2H6 and C2H4 are evolved. As
consequence, the cell pressure rises up to the cell vents with flame or explodes (Figure 1.9). The
thermal runaway moves lower when the battery is fully charged; for Li-cobalt this threshold is between
130-150 °C, nickel-manganese-cobalt is 170-180 °C and manganese is 250 °C
23
19,20,21
.
Figure 1.9. Two possible consequences of overcharge phenomenon both on laptop and mobile phone.
Lithium-ion batteries are not the only technologies that are a safety hazard if overcharged.
Properly designed charging equipment is paramount for all battery systems. Battery manufactures
attempt to prevent overcharge by using several safety measures, among which overcharge protection
circuits, positive temperature coefficient resistors, pressure sensitive rupture disks, temperature
sensitive separators and more recently new researches have been focused on the use of various
types of electrolyte additives
22,23
.
24
1.4 Materials for Lithium-ion batteries
The adaptation of Lithium-ion technology to electronic devices and to hybrid-electric vehicles
and the improvement of its performances depending on the development of materials for the various
battery components. Many research efforts are focused on the study of new anode and cathode
materials, new types of electrolytes and separators in order to adapt Lithium-ion batteries to several
applications.
1.4.1 Anode Materials
Lithium metal is the most attractive material for anodic use in rechargeable batteries due to its
-1 24,25
potential (- 3.045 vs. SHE) and its high specific capacity (3860 mA h g )
. It has been widely used
as anode in primary lithium cells for more than two decades, but its employment in the secondary
batteries presents serious problems in terms of safety and cyclability because of prolonged
deposition/dissolution cycling causes dendrite formations of lithium metal on the electrode surface.
The substitution of lithium metal electrodes with insertion materials with relative high lithium
activities marked the beginning of the Lithium-ion batteries’ epoch. Several insertion materials,
including transition-metal oxides and chalcogenides, carbons, lithium alloys and polymers have been
proposed for negative electrodes.
Since the first appearance in the market of Lithium-ion batteries, graphite is the most used
material for the anode fabrication thank its characteristics in terms of safety, capacity, cyclability and
low voltage of the lithium insertion/de-insertion (the main insertion/de-insertion processes take place
+
between 0.25 and 0.05 V vs. Li /Li). The major improvement of the Lithium-ion battery technology in
terms of energy density has been reached by increasing the crystallinity of the carbon negative
electrode, i.e. by replacing amorphous carbon by graphite (Figure 1.10)
Figure 1.10. Three types of carbon
25
26
.
26,27
.
The electrochemical insertion of lithium into graphite is called “intercalation” and it occurs
through a stage mechanism with the reversible formation of well-indentified lithium-graphite
intercalation compounds (Li-GlCs)
28
. Lithium ions are progressively inserted between single graphite
layers up to LiC6, which corresponds to the maximum theoretical specific charge of 372 mA h g
-1
-1
(practical values of about 350 mA h g are generally reached), according with the following reaction:
-
xLi + 6C + xe ↔ LixC6
0<x<1
However, many problems of the current configuration are related to the use of graphite-based
anode, that present poor performances under some particular conditions, such as low temperatures,
high charge/discharge rates and high irreversible capacity in the first cycle due to the SEI formation.
There is a growing interest in finding alternative anode materials which present both larger
+
capacities and slightly more positive intercalation potentials vs. Li /Li, but not too high otherwise the
cell voltage value decreases, in order to reduce any risks of high-surface-area lithium plating at the
end of fast recharge.
Among the possible anode materials proposed to replace graphite-based electrodes, lithiummetal alloys seem be suitable candidates thank their high specific capacity
29
(Table 1.4). The
development of these kind of negative electrode materials is based on conversion of oxides into
lithium-metal alloys under near-equilibrium conditions.
Starting oxide
Reversible capacity
Irreversible capacity
Total capacity
mA g
SnO
875.36
398
1273
0.69
SnO2
782.43
711
1494
0.52
ZnO
493.92
659
1152
0.43
CdO
605.25
417
1023
0.59
PdO
540.32
240
780
0.69
mA g
-1
mA g
-1
Ratio rev/total
-1
Table 1.4. Theoretical irreversible and reversible capacities of several convertible oxides.
Tin based compounds (SnO2, SnSiO, Sn nanocrystalline, etc.)
27,29,30
have received particular
attention. Tin is easily available, relatively cheap and non-toxic material for electrodes, with a higher
-3
theoretical specific capacity than lithium, a large density (7.29 g cm ) and an elevated lithium diffusion
-8
2
-1
coefficient (~10 cm s at room temperature).
In the last years, anode materials based on titanium oxides are the promising candidates to
substitute carbonaceous anodes due to their low cost, low toxicity and elevated stability. Among
several titanium oxides for applications in Lithium-ion batteries, the spinel structure Li4Ti5O12 and TiO2
in crystalline form of anatase are the most studied and employed. The anatase form of TiO2 shows the
highest lithium storage ability respect to the other ones
-1
31
. The maximum theoretical capacity is 335
mA h g , but from a practical point of view, the reversible insertion corresponds to 0.5 lithium ions at
26
+
1.78 V vs. Li /Li
32
, equivalent to a capacity of 167 mA h g
-1
(Li0.5TiO2). Anatase TiO2 is safer than
graphite against overcharge phenomenon, but presents problems in terms of cycling stability and low
performances at high charge/discharge rates.
Recently, lithium titanate spinel, Li4Ti5O12, has been introduced for use as an anode material
providing high-power thermally stable cells with improved cycle life
33
. This insertion material is
-1
suitable for long-life Lithium-ion batteries, presents a rechargeable capacity value of 175 mA h g at
+
an extremely flat operating voltage of 1.55 V vs. Li /Li, which is above the reduction potential of most
organic electrolytes and not lead to the formation of SEI on the particle surface. In addition, lithium
titanate spinel is a “zero-strain” insertion material: lithium ions can be inserted and extracted without
cause no change of the lattice dimensions, leading to a high reversibility and minimum capacity fade
34
. Furthermore, lithium titanate spinel exhibits good electrochemical performances in terms of high
rate cyclability and low polarization due to the excellent lithium ion mobility
35
.
The commercial use of these electrode material is impeded because of it presents a low
conductivity that leads to initial capacity loss and poor rate capability. Different solutions have been
followed in order to improve the electronic conductivity of Li4Ti5O12, e.g. reducing the particle size,
doping with metal (Mg, Cr, Fe, Ni, V, Mn, Ag) and modifying its particles with carbon
36
.
1.4.2 Electrolyte
Generally, an electrolyte for Lithium-ion batteries includes an inorganic salt dissolved in organic
solvents with a large stability window. An appropriate electrolyte should have good ionic conductivity,
high chemical stability, low cost and ensure safety
26,37
. In particular, solvents with low melting point,
high boiling point and low vapour pressure are highly wanted.
The physical properties of some of the most common organic solvents are shown in Table 1.5,
while in Figure 1.11 there are their structural formulae.
27
Solvent
Acetonitrile (AN)
Melting Point
(°C, 1 atm)
- 45.72
Boiling
Dipole
Relative
Viscosity
Permittivity
(cP)
81.77
38
0.345
3.94
Point
(°C, 1 atm)
Moment
(D)
γ-Butirrolattone (BL)
- 42
206
39.1
1.751
4.12
Diethylether (DEE)
- 116.2
34.60
4.27
0.224
1.18
1,2-dimethoxyethane
- 58
84.7
7.20
0.455
1.07
Dimethylsulfoxide
(DME) (DMSO)
1,3-Dioxolane (DOL)
18.42
189
46.45
1.991
3.96
- 95
78
6.79
a
0.58
/
b
a
4.80
Ethylene carbonate (EC)
39 - 40
248
89.6
Methylformate (MF)
- 99
31.50
8.5
c
0.330
1.77
2-Methyltetrahydrofuran
3-Methyloxazolidinin-2-one
(MeTHF)
Propilen carbonate (PC)
Sulfolane (S)
Tetrahydrofuran (THE)
1.86
/
80
6.24
0.457
/
15.9
/
77.5
2.450
/
- 49.2
241.7
64.6
2.530
5.21
28.86
287.3
42.5
a
7.25
a
- 108.5
65.0
a
9.87
a
4.7
0.46
a
1.71
b
c
Table 1.5. Physical properties of organic solvents at 25 °C ( 30 °C, 40 °C, 20 °C).
Figure 1.11. Structures of main organic solvents used in Lithium-ion batteries.
Cyclic carbonate solvents, such as EC and PC, present elevate dielectric constants but also
show higher viscosity values than the linear carbonates DMC, DEC and EMC. To avoid any chemical
side-reactions with lithium, the water amount for the organic solvents must be less than 20 ppm.
Lithium hexafluorophosphate LiPF6 is at present the most common salt for commercial Lithium-ion
batteries, due to its high conductivity and compatibility with the usual cathode materials.
The most popular liquid electrolyte used in Lithium-ion battery by manufacturers and
researchers is LiPF6 salt dissolved in a binary or ternary solvent mixture of a chain and linear
carbonates, such as ethylene carbonate (EC) and dimethyl carbonate (DMC), diethyl carbonate (DEC)
and ethyl methyl carbonate (EMC). This trend is based on the following factors: (i) LiPF 6 passivates
and protects aluminium current collector, used for the cathode, (ii) EC has high dielectric constant to
supply high ionic conductivity, (iii) linear carbonate decreases the viscosity of the electrolyte system
and shows a good penetrating ability into polyolefin-based separator, (iv) the presence of linear
carbonates favour the formation of a stable SEI film on the graphite surface
linear carbonates, EMC has the best thermal compatibility with EC
38
. Among the mentioned
39
. Therefore, the binary
combination EC-EMC could produce an electrolyte composition that put together low liquids
28
temperature and high ionic conductivity, which otherwise should be obtained only from ternary or
40
quaternary solvents
. In addition, in comparison with the other two linear carbonates, EMC is more
stable towards the cathode and lithium inserted graphite compounds. A research study
38
revealed
that the solvent ratio (EC:EMC) does not affect on the electrochemical performance of both electrodes
but does affect the thermal compatibility and ionic conductivity, while the salt concentration (LiPF6)
influences the performance of the cell.
However, LiPF6 salt presents many disadvantages, since it is expensive, hygroscopic and yields
to the formation of toxic hydrofluoric acid (HF) for its reaction with water. For this reason, it must be
handled in a dry environment. In the latest decade, lithium bis(oxalate)borate (LIBOB) has spread as a
new fluorine-free salt with good stability to hydrolysis, good conductivity and lower costs
41
. Organic
salts have also been developed because of they are more stable to water and hence easier to handle.
In particular, lithium bisperfluoroethanesulfonimide (BETI) has attracted much attention since it offers
high conductivity, stability to water and it can be easily dried and does not cause aluminium corrosion
42
.
Solid polymer electrolytes (SPEs) show numerous advantages with respect to liquid
electrolytes. They present an excellent design flexibility that ensures high energy and power density in
each their applications and higher safety due to the lack of flammable, volatile organic solvents and
elevated viscosity. In addition, the polymer electrolyte allows ion exchange leading to the replacement
of the traditional porous separator, that means a further improvement in both energy density and
manufacturing cost thank to the simplified cell configuration. Unfortunately, these materials have a
poor ionic conductivity that seems be an insurmountable barrier for their practical applications. Several
studies demonstrated how the ionic conduction mechanism for solid polymer electrolytes, such as
PEO and other similar polyether-based media, occurred in the amorphous phases. This implies that
the problem of ionic conductivity can be resolved only operating ad high temperatures, since at room
temperature these materials have a high degree of crystallinity
43
. Many approaches have been
investigated in order to overcame intrinsic drawbacks of PEO-based solid polymer electrolytes
44
.
Another type of polymeric electrolyte is the gel polymeric electrolytes (GPEs), containing a small
amount of suitable organic liquid acting as a plasticizer. Compared with SPEs, GPEs encountered
more success in Lithium-ion cells technology thank their characteristics very close to that of liquid
electrolytic, such as ionic conductivity, electrochemical stability on both electrodes, safety and
resistance against mechanical and electric abuses
43
.
With the introduction of Lithium-ion batteries technology in EVs and the corresponding request
to increase the energy density, cathodes with higher operating voltages have been developed and
with them also electrolytes have been widely reconsidered, since the electrochemical performances of
a cell depend on the stability of electrolyte system. However, the high voltage stability of the
electrolytes has always been one of the main barriers to the application of these high operating
voltage materials
45
. Several research groups have focused their work on the development of new
types of electrolytes, such as room temperature ionic liquids (RTILs)
carbonate-based nonaqueous electrolytes
45
46
, on the reevaluation of
and on the use of additives, which can release different
tasks: (1) solid electrolyte interface forming improver, (2) cathode protection agent, (3), safety
29
protection agent, (4) LiPF6 salt stabilizer, (5) Li deposition improver and (6) other functions such as
ionic salvation enhancer, Al corrosion inhibitor and wetting agent
47
. RTILs have shown potential as
safe electrolytes in electrochemical applications. RTILs are molten salts at temperatures close to room
conditions and offer several attractive properties for Lithium-ion batteries systems, such as
electrochemical stability (4.0 - 5.7 V), thermal stability, high ionic conductivity, a wide liquid-phase
range, very low vapour pressure and non-flammability
46,48
. The most common RTILs present an
organic cation (e.g. alkylpyrazolium, alkylpyridinium, alkylpyrrolidinium, quaternary ammonium, etc)
6-
4-
combined with different large anions, with delocalized charge (PF , BF )
49
. It has been reported that
RTILs suppress the strong interface electrolyte reactions, which play a fundamental role in terms of
the lifetime and safety of Lithium-ion batteries
50
.
1.4.3 Cathode Materials
The positive electrode in rechargeable Lithium batteries is an insertion material
51
which has to
present high reversible lithium storage capacity and rapid solid-state lithium and electron transport.
Producers of Lithium-ion batteries, in order to find a feasible compromise among high energy density,
good current delivery and operational safety use different cathode materials.
Cathode materials are typically oxides of transition metals which undergo oxidation when lithium
is removed and reduction when it is inserted. During the lithium extraction, phase changes occur and
so stable crystal structures must be used. Obviously, cathode materials must be able to accept and
release lithium ion repeatedly (for recharging) and quickly to ensure high current.
The positive electrode materials used in Lithium-ion batteries are based on three fundamental
structures (Figure 1.12): layered oxide (such as lithium cobalt oxide), spinel (such as lithium
manganese oxide) or polyanion (such as lithium iron phosphate).
The first Lithium-ion battery launched in the market by Sony used LiCoO2 as cathode and it
remained the most commonly used positive active material for this technology. Lithium cobalt oxide
+
shows a flat operating voltage of 3.9 V vs. Li /Li and high gravimetric and volumetric specific capacity.
This material shows an high energy density, long runtime and low self-discharge and is the most
attractive for portable devices. Although lithium cobalt oxide is a successful cathode material, several
alternatives are being developed in order to lower cost and improve cycle stability. Cobalt is less
available, and thus more costly respect to the other transition metals (Ni, Mn, Fe) and in addition, its
employment implies environmental hazards due to its high toxicity. LiCoO2 can undergo easy
degradation when overcharged due to several reasons. One of them is that cobalt is dissolved into the
electrolyte when the de-insertion of lithium occurs leading to a less capacity to insert lithium ions
during discharge. As regards the synthesis of this cathode material it is known that stoichiometric
LiCoO2 can be difficulty obtained and hence a careful control during its heat treatment is needed to
improve performance during cycling.
30
Figure 1.12. Schematic illustration of three crystal structures for cathode materials: a) layered structure of
LiCoO2, b) spinel structure of LiMn2O4, c) olivine structure of LiFePO4.
LiNiO2 is a cathode material that has a higher energy density and is cheaper, but has the
disadvantages of be less stable and less ordered if compared with LiCoO 2. The lower degree of
ordering depends on the presence of nickel ions in the sites of lithium plane that impedes lithiation/delithiation mechanism. In order to increase the degree of ordering, cobalt can be added to LiNiO2 so
that nickel ions occupy the sites in nickel/cobalt plane rather than in the lithium plane. This addition
leads a new lithium cobalt/nickel oxide, with the approximate composition LiCo 0.2Ni0.8O2. The main
-1 52
advantage for this material is that a higher capacity ( 180 mA h g
respect to 137 mA h g
-1
for
LiCoO2) is achieved thank to the presence of cobalt into structure.
A cathode material in which nickel, manganese and cobalt are presented in equal amounts, i.e.
Li(Ni1/3Mn1/3Co1/3)O2 shows several advantages in terms of high capacity, good rate capability and the
possibility to operate at high voltages.
Another cathode material is the spinel of LiMn2O4, with space group Fd3m, in which manganese
occupies the octahedral sites and lithium predominantly occupies tetrahedral sites. Differently from
previous structures, in this case the paths for the lithium mobility are three-dimensional network of
channels rather than planes. LiMn2O4 offers different favourable aspects than LiCoO 2, such as the
lower price, greater safety, lower environmental impact and good thermal stability. Unfortunately, this
material has a pronounced capacity fading due to the dissolution of manganese in the electrolyte and
the Jahn-Teller distortion that leads to a transition of phase from cubic to tetragonal phase, in which
there is less symmetry and more disorder
53
(Figure 1.13). LiMn2O4 can be doped with iron, cobalt and
nickel. The addition of iron leads to an additional discharge plateau at high voltage while the presence
of cobalt improves the capacity retention during cycling. Among these possible choices, the most
common metal that is added to LiMn2O4 is nickel, which decreases the lattice parameter and the
electrical conductivity.
31
Figure 1.13. Result of Jahn-Teller distortion for spinel LiMn2O4: phase transition from cubic Fd3m to tetragonal
phase.
Generally, the most used stoichiometry is LiNi0.5Mn1.5O4 which shows high capacity, in particular
if it has a disordered spinel structure. Partial substitution of cobalt for nickel has been used to reduce
the formation of LixNi1-xO impurities which affect the electrochemical performances of cell during
cycling. As alternative to doping solution, LiMn2O4 can be coated with several metal oxides to allow an
improvement of capacity retention.
Another important class of cathode materials are phosphates of general formula LiMPO4 (M =
Fe, Mn, Co and Ni) with olivine structure. Among the various phosphates, LiFePO4 is that most
commonly used as cathode material for Lithium-ion batteries. In this case the insertion/extraction
reaction occurs through a reversible passage between lithiated and de-lithiated form, therefore a
stoichiometry LixFePO4 corresponding to a mixture of both phases: xLiFePO4 and (1-x)FePO4. The
formation of a two-phase mixture results in a relative flat discharge profile at 3.45 V that differs this
material from the other cathode materials which exhibit a trend potential vs. time with slope. LiFePO4
is less expensive and more environmental benign than the cobalt and manganese oxides and shows a
long cycle life and superior resistance to overcharge. Thank to its safety characteristics, LiFePO4
technology can be used in several fields, from vehicles up to electric devices. However, the material
exhibits low electronic and lithium ion conductivity that can be improved introducing conductive agents
in order to enhance the performance of olivine-structures chemistry as cathode
54,55
. In Table 1.6 are
summarized some typical parameters of Lithium-ion batteries in according to the chemistry of positive
electrode.
32
Theoretical capacity
-1
(mA h g )
LiCoO2
LiMn2O4
LiFePO4
145
148
170
Discharge plateau
(V)
Cycle life
3.6 ~ 3.7
4.1
3.45
> 500 cycle
> 300 cycle
> 1000 cycle
Thermal stability
Poor
Good
Very Good
Safety
Energy density
-1
(Wh L )
Normal
Better
Superior
560
420
333
Table 1.6. Comparison of parameters of a Lithium-ion battery depending on cathode component
54
.
Figure 1.14 shows the voltage charge-discharge profiles for various cathode materials and
compares their different electrochemical behaviour.
Figure 1.14. Voltage charge-discharge curves of various cathode materials
33
56
.
1.5 High-voltage cathode materials for Lithium-ion batteries
Rechargeable Lithium-ion batteries are now considered to be the technology for future hybrid
electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) to reduce CO2 emissions (Figure
1.15)
57
.
Figure 1.15. Future applications for Lithium-ion batteries: hybrid electric (HEVs) and plug-in hybrid electric
58
(PHEVs) vehicles .
For this regard, Lithium-ion batteries have to satisfy the requirements of higher energy and
power densities. Both the 4 V spinel LiMn2O4 and 3.4 V olivine LiFePO4 have drawn much attention as
Mn and Fe are abundant, inexpensive and environmentally benign and they offer higher rate capability
and better safety compared to the layered oxide cathodes. However, both LiMn 2O4 and LiFePO4 have
limited energy density because of their low capacity or operating voltage
59
. In particular, LiMn2O4
exhibits severe capacity fading on cycling mainly due to dissolution of Mn into the electrolyte via the
disproportionation reaction: 2Mn
3+
→ Mn
2+
+ Mn
4+
and Jahn-Teller distortion of trivalent Mn ions.
Therefore, various attempts have been made to enhance the cycling stability of the spinel LiMn2O4.
Considering the excellent intrinsic rate capability arising from the three-dimensional diffusion of
lithium ions in the spinel lattice, one way to improve the energy and power densities is to increase the
operating voltage. It has been reported that a partial substitution of manganese in spinel LiMn 2O4 by
other dopant ions to make LiMxMn2-xO4 (M = Al, Ni, Co, Fe, Cr, etc) leads to a effective improvement
of cycling performances and to deliver the capacity around a nearly flat operating voltage ~ 5 V, while
the capacity in the 4 V region (corresponding to the electrochemical activity of Mn) decreases. The
higher operating voltage is due to a binding energy of the M:3d eg electrons by at least 0.5 eV higher
than that of the Mn:3d eg electrons. The discharge capacity values and the voltage plateaus strongly
depend on the nature of the dopants and their content
57,59
.
Among the various compositions of 5 V spinel LiM xMn2-xO4 cathodes, LiNi0.5Mn1.5O4 has drawn
much attention because it displays a flat voltage profile at 4.7 V corresponding to the redox reactions
2+/3+
of Ni
and Ni
3+/4+
redox couples, while other compounds (LiM xMn2−xO4, with M = Cr, Fe, Co, Cu)
34
−1
exhibit two plateaus at 4.0 and 5.0 V, with a high discharge capacity (> 130 mA h g ) and with the
theoretical capacity of 146.7 mA h g
−1 60
. Depending on the synthesis temperature, two different crystal
structures can be obtained: the cubic P4332 space group (cation ordering) with one plateau and the
face-centered cubic Fd3m space group (cation disordering) with two plateaus at 4.7 V. From reported
studies, the LiNi0.5Mn1.5O4 with the Fd3m structure exhibits better electrochemical performances than
that with the P4332 structure
61
. Thank to its 4.7 V operating voltage, which is higher than 3.4 V for
LiFePO4, 3.9 V for LiCoO2 and 4.1 V for LiMn2O4, the spinel-structured LiNi0.5Mn1.5O4 delivers an
energy density of 650 Wh kg
−1
(Figure 1.16), about 20-30 % higher than the others. In addition, the
three-dimensional lithium diffusion pathways in the spinel lattices are beneficial to provide a high
power density
60
. In addition, this material has been demonstrated to show good cycling stability on
lithium extraction and insertion and good rate capability. LiNi 0.5Mn1.5O4 spinel material is fundamentally
different from pure Mn spinels as all the redox activities occur entirely at the Ni
remains as structure stabilizing ion during charging and discharging
2+
ion while the Mn
61
.
Figure 1.16. Energy densities of cathode materials LiNi0.5Mn1.5O4, LiMn2O4, LiCoO2 and LiFePO4
However, it has been reported
62
4+
60
.
that spinel LiNi0.5Mn1.5O4 material shows a non-negligible
capacity fading (Figure 1.17) during the electrochemical cycling, especially at elevated temperatures,
due to the structural and chemical instabilities resulting from the high spin Mn
3+
ions, which is closely
related with the oxygen deficiency of the spinel compound and the formation of impurity phases
(nickel-oxide-like peaks LixNi1-xO) during the heat treatment process. In an “ideal” case, no Mn
should be present in the stoichiometric compound (LiNi0.5Mn1.5O4) and all Mn
substituted by Ni
2+
ions. Unfortunately, a small amount of Mn
3+
3+
3+
ions
ions should be
is often observed in the compound
after the high temperature heat-treatment, leading to the existence of 4 V capacity. It has been
demonstrated that the manganese valence state in LiNi0.5Mn1.5O4 is highly susceptible to the synthetic
temperature range or the cooling rate of annealing process. Manganese ion is easy to be trivalent
35
above a certain critical temperature level and through a proper heating process the small amount of
3+
Mn
4+
ions can be completely transformed into Mn .
Figure 1.17. Comparison between the cyclic performance at room temperature (left) and the cyclic performance
at 55 °C (right) for LiNi0.5Mn1.5O4 (black trend)
60
.
Introducing an appropriate doping element with larger bond dissociation energy with oxygen
respect to that existing between Mn-O bond, the formation of impurity phases can be efficiently
suppressed preventing oxygen deficiency at the high synthesis temperatures as well as the dissolution
2+
of Mn
ions into electrolyte during electrochemical cycling. From the studies it is possible to assume
that the doping of the LiNi0.5Mn1.5O4 structure, with Fe, Ti, Cu, Co, Cr and Zn, leads to improve the
capacity retention and the rate capability in the 5 V region. Among these doping materials, iron, cobalt
and chromium favour the formation of the Fd3m structure, that means better electrochemical
performances.
It was tested that when LiNi0.5Mn1.5O4 material is doped with iron there is a decrease of the
polarization that leads to a reduced of electrolyte decomposition at high potential and an increase of
lithium ion transport
61
. In particular, it has been studied
63
that the substitution of Fe for Ni alone or for
both Ni and Mn improve the rate capability drastically, but the substitution of Fe for Ni alone leads to
both an effective enhance of the rate capability compared to the substitution of Fe for both Ni and Mn
and a reduction of the corresponding amount of Mn
4+
to Mn
3+
with an increase of electrochemical
activity in the 4 V region (Figure 1.18).
Generally, the rate capability of cathode materials for Lithium-ion batteries is influenced by the
lithium ion insertion/extraction kinetics. Insertion/extraction of lithium ions into/from the cathode
materials involves (i) lithium ion diffusion through the surface solid-electrolyte (SEI) layer, (ii) charge
transfer reaction and (iii) lithium ion diffusion in the bulk of the material, which imposes respectively
ohmic polarization, activation polarization, and diffusion polarization on the electrode. The process that
shows the slowest kinetics leads to the largest polarization and becomes the rate determining step.
36
Figure 1.18. Discharge profiles of the pristine LiMn1.5Ni0.5O4 and the Fe-substituted samples at various C-rates
63
.
The improvement of the cycling stability of LiMn2O4 due to the doping of the structure is usually
effective only at room temperature. At elevated temperatures its cycling behaviour is still poor due to
severe Mn dissolution by HF produced from F-containing inorganic electrolyte salt. A good method to
solve partially or completely this problem is the coating of the surface of LiMn2O4 with several
materials, such as MgO, Al2O3, SiO2, TiO2, ZnO, SnO2, ZrO2, conductive coatings and polymers.
These materials allow to obtain a better cycleability because HF does not react directly with cathode
particles (MOx + HF  MFy + H2O) and so the Mn
2+
dissolution, that is the reason of the active
material loss, is avoided. Furthermore, the coatings suppressing the phase transition improve the
structural stability and decrease the cation disorder in crystalline sites. This leads to decrease side
reactions and heat generation during cycling, to increase the conductivity of the cathode material and
to the removal of HF from electrolyte solutions
53
.
The coating becomes a fundamental practice when the cathode material consists in the 5 V
spinel LiNi0.5Mn1.5O4 because at this high operating voltages the LiFP6 salt in the electrolyte is
-
electrochemically unstable and the oxidation of PF6 proceeds according to the following reactions:
where traces of H2O, present in the electrolyte, act as source of protons. When the HF has been
formed, it can attack the surface of the electrode by the following autocatalytic corrosion reaction:
37
where M refers to transition metal. The high operating voltage of the 5 V spinel cathodes can increase
both the formation and concentration of HF, accelerating the corrosion rate of the active material and
the deposition rate of SEI components, such as LiF and LiOH, on cathode surface. These phenomena
cause the typical capacity fade for this material and the formation of a thicker SEI layer
59
.
The coating for the 5 V spinel cathode LiMn1.5Ni0.5O4 leads to a better cyclability, better rate
capability and better rate capacity retention but in particular, it decreases the irreversible capacity at
the first cycle.
It has been well-known that the development of 5 V cathode materials for Lithium-ion batteries
to employ in automotive industry depends on the stability of electrolyte at high operating voltages.
Some of the carbonate electrolytes exhibit a very good stability up to 4.9 V, ensuring considerable
electrochemical performances. This stability is strongly related to the stability and properties of the
cathode materials in the de-lithiated state, when the system reaches the upper limit potential
45
. During
charging process, water and acidic impurities (HF) are generated. A possible mechanism proposed by
Wang et al.
64
shows that the solvents are chemically oxidized by the oxygen released from the
cathode to generated H2O and CO2. The water presence hydrolyzes the LiPF6 salt to form acids such
as HF, that is the main source for the dissolution of the cathode materials
47
. At high-operating
voltages the HF formation rate increases and with it, increases also the corrosion rate of the active
material and the deposition rate of SEI components such as LiF and LiOH on cathode surface. The
“ideal” SEI layer should be thin in order to allow lithium-ion conduction. If a thick SEI layer is formed,
the surface resistance increases and a remarkable capacity fade occurs
59
.
1.5.1 Synthetic Techniques
Several literature articles deal with the synthetic routes to obtain cathode materials for Lithiumion batteries technology, since from the synthesis depend their electrochemical behaviour and
performances.
As regards the spinel LiNi0.5Mn1.5O4, the most commonly used synthesis are: solid-state
method, sol-gel method and co-precipitation method
38
65
.
I.
Solid state method
This is the most common method in which stoichiometric mixture of starting materials is ground
or ball-milled together and the resultant mixture is heat-treated in a furnace.
In the case of spinel LiNi0.5Mn1.5O4, appropriate amounts of starting materials, NiCl2·xH2O and
MnCl2·xH2O, are thoroughly mixed in the ratio of 1 : 3. Subsequently, 20% excess (NH4)2C2O4·H2O is
added to the mixture and then the mixture is ground to ensure complete reaction. After drying, the
mixture is calcined at 400 °C to form the precursor containing Ni–Mn. Stoichiometric amount of Li2CO3
is added and mixed thoroughly. Then the mixture containing Ni–Mn–Li is calcined at different
temperatures ranging from 700 to 900 °C
66
.
The purity of the material depends on the choice of the starting materials, calcination
temperature and time. The effect of various Ni precursors on the electrochemical performance has
been investigated and the results show that the best electrochemical performance is obtained from
Ni(NO3)2·6H2O precursor
electrolytic MnO2
67
. An improved solid state reaction is reported using Li2CO3, NiO and
68
.
However, this method has several disadvantages such as inhomogeneity, irregular morphology,
poor control of stoichiometry and uncontrollable particle growth
II.
69,70,71
.
Sol-gel method
The sol-gel method can overcome some disadvantages of conventional solid state method
thank to its low processing temperature, high homogeneity, possibility of controlling size and
morphology of the particles
72,73
.
Sol-gel method is used to prepare spinel LiNi0.5Mn1.5O4 by various research groups
57,74,75,76,77
.
Usually, stoichiometric amounts of lithium acetate [Li(CH 3COO)·2H2O], manganese acetate
[Mn(CH3COO)2·4H2O], and nickel acetate [Ni(CH3COO)2·4H2O] are dissolved in an appropriate
quantity of distilled water at room temperature. The solution is stirred at 50 °C and the citric acid is
added to the solution which acts as chelating agent in the polymeric matrix. The pH of the solution is
adjusted to 7.0 by slowly dropping ammonium hydroxide drop wise and continued stirring for 4 h. The
temperature of the solution is raised to 80–90 °C and continued stirring till the solution turned into
high-viscous gel. The resulting gel is dried at 80 °C for 24 h in a temperature controlled oven of an
accuracy of ± 1 °C. The LiNi0.5Mn1.5O4 precursor powder is ground to fine powder and calcined at 450
°C under oxygen flowing conditions with a constant heating followed by cooling rate at 4 °C min
−1
to
decompose organic constituents. The calcined powder is ground to a fine powder and re-sintered at
different temperatures and time under oxygen flowing conditions.
Figure 1.19 shows a schematic illustration of the reactions which lead to the gel formation. For
this synthesis, the citric acid : total metal content ratio can range from 0.3 : 1 to 1 : 1.
39
Figure 1.19. Schematic illustration of the basic chemical reactions involved in the gel formation.
III.
Co-precipitation method
Co-precipitation procedure to synthesize LiNi0.5Mn1.5O4 can easily be handled and the
precipitates are generated simultaneously and uniformly dispersed throughout the solution.
The precursor (Ni0.5Mn1.5)(OH)2 is firstly prepared by dissolving stoichiometric amounts of
Ni(CH3COO)2·4H2O, and Mn(CH3COO)2·4H2O in distilled water (cationic ratio of Ni:Mn = 1:3). The
aqueous solution is then precipitated by adding NaOH/NH 4OH solution along with continued stirring to
obtain mixed hydroxide precipitate. After filtering, washing and drying in a vacuum oven at 50–60 °C
overnight, the obtained precursor is mixed with required amount of LiOH and calcined at various
temperatures to get LiNi0.5Mn1.5O4 powders. This powder is prepared by co-precipitation using different
precursors such as metal sulfate, metal carbonate, and metal chlorides
IV.
78,79,80
.
Other methods
Nanomaterials have attracted much attention in the latest years thank to their chemical and
electronic properties due to their large surface area that ensures a major reactivity. These
characteristics are useful for energy purposes because electrodes based on nanomaterials have
emerged as a valuable choice for improving Lithium-ion battery performance, particularly in terms of
40
high rate capabilities. The nano-rod like LiNi0.5Mn1.5O4 spinel powder was prepared by polymer
assisted (PA) synthesis using polyethylene glycol
81
.
Homogeneous mixing of starting materials at the atomic scale was achieved by radiated
polymer gel (RPG) method in which the solution containing starting materials and acrylic acid to
synthesize LiNi0.5Mn1.5O4 spinel is polymerized under Co60 γ-ray irradiation (intensity 55–75 Gy
−1
min ).
50 nm sized LiNi0.5Mn1.5O4 having the Fd3m cubic spinel structure is readily prepared by the
emulsion drying (ED) method which can intermix cations (Li, Mn and Ni) on the atomic scale.
Some researchers employed mechanochemical process, to synthesize LiNi0.5Mn1.5O4 material,
in which planetary-type ball mill was adopted for the mechanical activation of the starting materials.
This method takes advantage of the high ball milling energy to initiate some reactions between the raw
materials in order to prepare highly homogeneous powders.
Other works reported a combinational annealing method to prepare the spinel LiNi 0.5Mn1.5O4
material. Since high temperature calcination results in oxygen loss, Ni deficiency and formation of
impurity phases, the cycling performance of the material would be deteriorated. Oxygen loss can be
recovered by low rate cooling or low temperature annealing. Therefore, in the combinational annealing
method, the mixed precursors were heated to high temperature and then quickly cooled down to low
temperature for isothermal annealing treatment. In this way, with a minimum oxygen loss and reduced
impurities amounts LiNi0.5Mn1.5O4 material achieves good electrochemical performances.
The main disadvantages of wet methods are high cost and complicated synthetic procedure.
Molten salt (MS) method is a simple technique to prepare complex oxide materials. This method is
based on the use of a salt with low melting point such as alkali metal sulfates, hydroxides, carbonates
and chlorides. Highly pure materials, with well defined crystal structure, can be prepared at relatively
low temperatures in MS method due to relatively higher diffusion rates between reaction components.
Various lithium salts can be employed to prepare lithium manganese oxides using this method
82,83
Combustion reaction method is also used for the preparation of various oxide materials
.
84,85
.
Some researchers used carbon combustion method (CCS) to prepare cubic LiNi 0.5Mn1.5O4 material
with a space group of Fd3m using carbon as fuel
86
. In this CCS method, the structure and particle
size could be adjusted by the amount of carbon used for combustion.
41
2. Aim of the Research
The present thesis work is the result of collaboration between the research electrochemistry
group of UNICAM and the R&D department of FAAM Spa company, leader in the production of energy
storage devices and electric vehicles.
The aim of this research work is to develop high-voltage cathode materials for Lithium-ion
batteries in automotive field focusing on their synthesis, structural, morphological and electrochemical
characterization.
The structural and morphological characterization of obtained compounds has been carried out
by using different techniques, such as X-ray Powder Diffraction (XRD), Scanning Electron Microscopy
(SEM) and Energy Dispersive X-ray analysis (EDX). The electrochemical characterization has been
carried out by using cyclic voltammetry technique and galvanostatic charge/discharge cycles both at
room and low temperatures.
A commercial sample of LiMn2O4 has been used as active material for electrode preparation in
order to have a commercial reference material. The electrochemical characterization of commercial
LiMn2O4 has been done by using two different conductive agents in the electrode composition in order
to test their effects on the electrochemical performances.
The research work includes the synthesis of spinel LiNi0.5Mn1.5O4 and Fe-doped LiNi0.5Mn1.5O4:
many efforts have been made for the selection of the best synthetic route enhancing both structural
and morphological properties as well as the electrochemical performance of these materials through
the optimization of synthetic conditions (precursors, annealing temperature, etc.) and the sample
treatment (i.e. ball-mill treatment to reduce the particle size). Two main synthetic ways were followed:
mechanochemical reaction by ball-mill and sol-gel method.
The electrochemical tests were carried out between 20 and -20 °C in order to verify the efficacy
of sol-gel synthesized high-voltage cathode materials for lower temperatures applications, as for
example in automotive field where a wider operating temperature range is required.
42
3. Experimental Techniques
3.1 Synthetic Techniques
In order to synthesize the spinel LiNi0.5Mn1.5O4 and its Fe-doped material, two different methods
were followed: the solid state method using ball-mill treatment and the sol-gel method.
I.
Solid state method
The solid-state method adopts planetary-ball milling, which employs balls of different materials
(zirconia, agate and steel), for the mechanical activation of the starting materials.
Proper amounts of Li2CO3, MnO2, Ni(OH)2 were initially ground in a mortar and then the mixture
was thoroughly ball-milled at well-defined speed and time. The ball-milled powder was collected and
subjected to heating processes in the flowing oxygen atmosphere
68,87
. The doping material (Fe2O3) is
added to the initial mixture in order to prepare the Fe-substituted sample.
II.
Sol-gel method
In order to synthesize spinel LiNi0.5Mn1.5O4 cathode material, stoichiometric amounts of lithium
acetate, nickel acetate and manganese acetate were dissolved in an appropriate quantity of distilled
water at room temperature. The solution was stirred at 50 °C and the citric acid (chelating agent) was
added at different ratios with the total metal amount in the solution. The pH of the solution was
adjusted to 7.0 by slowly dropping ammonium hydroxide drop wise and continued stirring for 4 h. The
temperature of the solution raised to 80–90 °C and continued stirring till the solution turned into highviscous gel. The resulted gel was dried at 80 °C for 24 h in a temperature controlled oven. The
precursor powder was ground to fine powder and calcined at 450 °C under oxygen flowing conditions
-1
with a constant heating followed by cooling rate at 4 °C min to decompose organic constituents. The
calcined powder was ground to a fine powder and re-sintered successively at 800 °C for 16 h under
-1
oxygen flowing conditions and heating and cooling rate was maintained at 2 °C min . To synthesize
the Fe-doped sample, ferric ammonium citrate – red (III) was chosen as doping raw material because
-1
it has good water solubility (S = 1200 g L at 20 °C)
88
43
.
3.2
Structural
and
Morphological
Characterization
Techniques
The structural and morphological characterization of synthesized powders and different
electrodes has been carried out by using several techniques, such as X-ray Powder Diffraction (XRD),
Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX).
I.
X-ray Powder Diffraction (XRD)
X-ray powder diffraction (XRD) is a rapid and nondestructive analytical technique primarily used
for phase identification of a crystalline material and can provide information on unit cell dimensions.
One of its main advantages is that it requires only a small amounts of finely grounded and
homogenized material.
X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline
sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic
radiation, collimated to concentrate and directed toward the sample. The interaction of the incident
rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy
Bragg's Law (nλ = 2d sinθ). This law relates the wavelength of electromagnetic radiation to the
diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then
detected, processed and counted. By scanning the sample through a range of 2θ angles, all possible
diffraction directions of the lattice should be achieved due to the random orientation of the powder.
Conversion of the diffraction peaks to d-spacing allows identification of the mineral because each
structure has a set of unique d-spacing. Typically, a comparison with standard reference patterns is
carried out.
A representative sample pattern can be produced by plotting the angular positions and
intensities of the resultant diffracted peaks of radiation. Narrow and well-defined peaks lead to
conclude that the sample has an elevated degree of crystallinity
II.
89
.
Scanning Electron Microscopy (SEM)
The scanning electron microscopy (SEM) uses a focused beam of high-energy electrons to
generate a variety of signals at the surface of solid specimens. The signals that derive from electronsample interactions reveal information about the sample including external morphology (texture) and
chemical composition.
44
In most applications, data are collected over a selected area of the sample’s surface generating
a two-dimensional image that displays spatial variations in these properties. Areas ranging from
approximately 1 cm to 5 µm in width can be imaged in a scanning mode using conventional SEM
techniques (magnification ranging from 20X to approximately 30000X, spatial resolution from 50 to
100 nm). SEM technique is also capable of performing analyses of selected point locations on the
sample; this approach is especially useful in qualitative or semi-quantitative chemical analysis (using
EDS).
Accelerated electrons beam in an SEM carries significant amounts of kinetic energy and this
energy is dissipated as a variety of signals produced by electron-sample interactions when the
incident electrons are decelerated in the solid sample. These signals include secondary electrons,
backscattered electrons, diffracted backscattered electrons, photons (characteristic X-rays), visible
light and heat. Secondary electrons and backscattered electrons are commonly used for imaging
samples: secondary electrons are most valuable for showing morphology and topography on samples
and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase
samples. X-ray generation is produced by inelastic collisions of the incident electrons with electrons in
discrete orbitals of atoms in the sample. As the excited electrons return to lower energy states, they
yield X-rays that are of a fixed wavelength (that is related to the difference in energy levels of electrons
in different shells for a given element). Thus, characteristic X-rays are produced for each element in
the sample. Although X-rays are generated by electron-sample interaction, no volume loss of the
sample occurs and therefore the SEM analysis can be considered a nondestructive technique. This
means that it is possible to analyze the same materials repeatedly
III.
90
.
Energy Dispersive X-ray analysis (EDX)
Interaction of an electron beam with a sample target produces a variety of emissions, including
X-rays. An energy dispersive detector is used to separate the characteristic X-rays of different
elements into an energy spectrum and a system software is used to analyze the energy spectrum in
order to determine the abundance of specific elements. EDX can be used to find the chemical
composition of materials down to a spot of a few microns and to create element composition maps
over a much broader area. Together, these capabilities provide fundamental compositional information
for a wide variety of materials.
A typical EDX spectrum is portrayed as a plot of X-ray counts vs. energy (in keV). Energy peaks
correspond to the various elements in the sample. Generally they are narrow and readily resolved, but
many elements yield multiple peaks and sometimes it can happen that a peak of specific element is
overlapped with another one. However, elements in low abundance will generate X-ray peaks that
may not be resolvable from the background radiation
45
91
.
3.3 Electrochemical Principles and Definitions
I.
Faraday’s Law
According to the general redox process reported below,
the amount of electrical charge transferred in an electrochemical reaction is given by Faraday’s
law:
where z is the number of equivalents per mole of reactant (or electrons per mole), F is the
-1
-1
Faraday constant (96 487 C equiv. or 26.8 A h equiv. ), m is the mass of the substance undergoing
electrochemical reaction and M is the molar mass of the substance. It is clear from Faraday’s law that
the mass of a given substance required for the transfer of a given amount of charge is proportional to
the equivalent mass. Of course, it is desirable that the mass of reactants required be minimized by
selecting electrode reactants of low equivalent mass. The direction of charge transfer depends upon
whether the electrode material is being oxidized or reduced. If a reduction reaction takes place, the
electrode is referred to as a cathode; if an oxidation reaction takes place, the electrode is an anode.
Thus, the negative electrode of a rechargeable cell is an anode during discharge, and acts as a
cathode during recharge. Correspondingly, the positive electrode is a cathode during discharge, and
acts as an anode during recharge
II.
92
.
Energy Density and Specific Energy
The electric power is the rate of doing work, measured in Watt (W). It is defined as:
where V is the electric potential and I represents the current. For a battery is useful to define the
power density, which indicates how much power a battery can deliver on demand. Power density can
-1
be expressed both as the ratio of the power available from a battery for its volume (W L ) and as the
-1
ratio of the power available from a battery for its mass, giving W kg .
46
-1
The energy density (W h L ) defines the amount of energy that an energy transformer, like
batteries, can store. The higher the energy density, the longer the runtime will be.
Since in many applications, in particular when a battery pack is considered inside electric or
hybrid vehicles, the weight of the battery becomes an important consideration. Therefore the goal is to
minimize the weight and volume of the battery for a given amount of energy stored (We).
th
-1
The theoretical maximum specific energy We (W h kg ) for any given cell reaction is given by
the Gibbs free energy change divided by the mass of reactants (or products):
where ni is the number of moles of reactant i involved in the reaction and Mi is the molar mass
of reactant i. If data are not available for ΔG, it can be estimated using measured cell potential data
(ΔE) and the known number of equivalents involved in the cell reaction:
-1
-1
Clearly, to achieve high specific energies (W h kg ) or high energy densities (W h L ), batteries
need electrode materials with high specific charge and high potential differences between the anode
and cathode.
In succession, several fundamental parameters for batteries are reported (Figure 3.1).
Figure 3.1. Definitions of some important parameters for batteries.
3.4 Electrochemical Characterization Techniques
The electrochemical measurements are performed employing T-shaped polypropylene type
cells (Figure 3.2) equipped with stainless steel current collectors. Disks of high-purity lithium foil are
used as counter and reference. A polypropylene separator (Celgard 2075) is used. Three types of
electrolytes have been used:
47

LP30 (Merck), 1 M solution of LiPF6 in EC:DMC 1:1,

LP71 (Merck), 1 M solution of LiPF6 in EC:DMC:DEC 1:1:1,

Electrolyte FAAM, whose composition cannot be displayed in this thesis because it is
under NDA (non-disclosure agreement). This electrolyte was supplied by FAAM.
Figure 3.2. Schematic representation of a T-cell.
The cells are assembled into dry-box. All the electrochemical characterizations are performed
using a galvanostat/potentiostat VMP2/Z by Bio-Logic and a CHI 660 electrochemical workstation (CH
Instruments).
I.
Cyclic Voltammetry
Cyclic voltammetry (CV) is the most widely used technique for acquiring qualitative information
about electrochemical reactions. CV provides information on redox processes, heterogeneous
electron-transfer reactions and adsorption processes. It offers a rapid location of redox potential of the
electroactive species. CV consists of scanning linearly the potential of a stationary working electrode
using a triangular potential waveform, forward from E1 to E2 and vice-versa for the backward scan, as
illustrated in Figure 3.3.
During the potential sweep,
the potentiostat
measures
the current resulting
from
electrochemical reactions due to the applied potential and therefore the cyclic voltammogram is a
current response as a function of the applied potential. When the electrochemical process is limited by
the diffusion (semi-infinite linear diffusion in the case of solutions, restricted diffusion in the case of thin
layer electrodes) the concentration gradients generated from the redox equation affect the current
values.
48
E
Emax
Ef
0
t
Emin
Figure 3.3. Variation of applied potential with time in cyclic voltammetry. The sweep rate can be defined as v =
|dE/dt|.
In general, as the applied potential approaches the characteristic E° for the redox process, a
cathodic current begins to increase until a peak is reached and then decreases upon the semi-infinite
linear diffusion conditions and to 0 when the process occurs on a thin layer. The peak currents are
respectively described by the following equations:
Where n is the number of moles of electrons transferred in the reaction, A is the area of the
electrode, C is the analyte concentration, D is the diffusion coefficient and ν is the scan rate of the
applied potential. These equations refer to ideal situations
93
. For thin layer electrodes, at relatively
high scan rates a current tail can observed.
For a reversible process, the cathodic and anodic peak currents are equal in magnitude (|ipc| =
|ipa|) and the cathodic peak potential (Epc) is 58/n mV more negative than the anodic peak potential
(Epa). These are important criteria for reversibility. In addition, the half-wave potential, which is used to
obtain the formal redox potential, is obtained by E1/2 = (Epc+Epa)/2. By decreasing the reversibility, the
difference between the two peak potentials increases.
The equilibrium potential for intercalation processes is given by the Nernst equation applied to
intercalation reactions, in which during the ion reversible insertion in the guest structure the reduction
of the guest species occurs:
where x represents the intercalation degree. The correspondent Nernst equation is:
49
Which corresponds to
If also the interactions between the charged species in the guest structure are considered, the
previous equation must be modified as follow:
Where z is the nearest neighbor site number, Φ is the interaction energy (Φ<0 for repulsive
interactions, Φ>0 for attractive interactions), a is a parameter which corresponds to 1 or 2, depending
on the fact that the intercalated ions and the electrons can form ion-electron couples or can
independently move
II.
94,95,96
.
Galvanostatic charge/discharge cycles
The basic characteristic of an electroactive intercalation compound is the thermodynamic
voltage-composition relation which corresponds to the equilibrium phase diagram of the system.
Basically a continuous dependence of the potential vs. composition corresponds to a solid-solution
single phase domain whereas a potential plateau corresponds to a two-phase domain. Other
properties of interest, in particular in view of possible applications as active electrode in a battery are
the potential window of electrochemical stability, kinetics and reversibility of the intercalation process.
During a galvanostatic charge/discharge test a fixed current is allowed to flow and the voltage
response is registered. The performance of a battery is determined as a function of its charge and
discharge conditions: a given rate is usually expressed as C/h, where h is the number of hours at
which the nominal charge of the battery (that involves both positive and negative electrode) will be
passed through. Studying a given electrode material, C is the charge corresponding to the total
expected oxidation/reduction of that electrode in one hour. It is sometimes convenient to consider the
-1
specific capacity of an intercalation electrode material per weight (mA h g ) and hence express the
-1
galvanostatic rate in current per active mass (m A g ).
From the theoretical capacity the charge/discharge rate can be determined and the anodic and
cathodic performances evaluated.
50
4. Results and Discussion: evaluation of commercial
LiMn2O4 cathode material
In this chapter, the morphological and electrochemical characterizations of electrodes
constituted by commercial LiMn2O4 (Merck) as active material, a cathode material for Lithium-ion
batteries, are shown.
The electrodes based on the commercial LiMn2O4 were prepared by using different conductive
agents: classical Super P carbon and Ketjien Black (KB), supplied by FAAM. The amount of KB in the
electrode preparation is lower than the amount of SP generally required, because of the higher
conductivity of KB according with the suggestions of the supplier. Therefore, 4 % of SP and 2 % of KB
were used in the electrode composition for LMS1 and LMS2, respectively. Small amounts of
commercial KS15 graphite, Timcal, have been introduced in the electrode composition. Polyvinylidene
fluoride (PVDF, Aldrich) was used as binder. Dried PVDF was used to prepare a 5% w/o solution in
anhydrous NM2P (N-methyl-2-pyrrolidinone, Aldrich) in argon atmosphere.
The aim of this study is to compare the influence of both the type of conductive agents and their
amounts on the electrochemical performances of LiMn2O4 electrodes.
4.1 Electrode preparation
4.1.1 Preparation technique of layer LMS1
The layer LMS1 was manufactured by preparing a slurry of LiMn2O4, graphite (KS15), Super P
and PVdF in NM2P, whose composition is shown in Table 4.1, with the use of an IKA homogenizer.
The mixture was stratified on aluminium foil through Doctor Blade technique and the thickness was set
at 200 µm. The obtained layer was dried on a hot plate at 50 °C, under hood, in order to remove
completely the solvent. The dried layer was pressed with roll press. Circular electrodes with a
diameter of 9 mm were cut and dried overnight at 120 °C under vacuum. Active material loading of
2
-2
electrodes (0.636 cm surface area) is in the range of 9.47-11.32 mg cm . The capacity of the
-1
electrodes was calculated considering a specific theoretical capacity of 148 mA h g .
LiMn2O4 (Merck)
84.0 %
Super P
4.0 %
KS15
4.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0 %
NM2P
65.8 %
Table 4.1. Percentage composition of layer LMS1.
51
4.1.2 Preparation technique of layer LMS2
The layer LMS2 was manufactured by preparing a slurry of LiMn 2O4, graphite (KS15), KB and
PVdF in NM2P, whose composition is shown in Table 4.2, that was stirred for 4 h using a magnetic
stirrer. Then the suspension was put for 12 minutes in an ultrasound bath. The mixture was stratified
on aluminium foil through Doctor Blade technique and the thickness was set at 200 µm. The obtained
layer was dried on a hot plate at 50 °C, under hood, in order to remove completely the solvent. The
dried layer was pressed with roll press. Circular electrodes with a diameter of 9 mm and a surface
2
area of 0.636 cm were cut and dried overnight at 120 °C under vacuum. The loading of active
-2
material is in the range of 4.23-6.07 mg cm . The capacity of the electrodes was calculated
-1
considering a specific theoretical capacity of 148 mA h g .
LiMn2O4 (Merck)
86.0 %
KB
2.0 %
KS15
4.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0%
NM2P
71.3 %
Table 4.2. Percentage composition of layer LMS2.
4.2 Morphological characterization of the layers LMS1 and
LMS2
The morphology of the layers was probed by Scanning Electron Microscopy (SEM) using a
Cambridge Stereoscan 360 with an electron acceleration potential of 20 kV.
Figure 4.1 shows the SEM images of layer LMS1 (4 % of SP) at different magnifications.
Figure 4.2 shows the SEM images of layer LMS2 (2 % of KB) at different magnifications.
From these images it is possible to observe that the distribution of the particle sizes, for both
layers, is not uniform. Probably, this phenomenon depends on the fact that the powders were ground
only by a mortar and hence with an inadequate energy and that the applied pressure was insufficient
to compact the layers.
52
Figure 4.1. SEM images of layer LMS1 (4 % SP) at different magnifications: 250X (left) and 1000X (right).
Figure 4.2. SEM images of layer LMS2 (2 % KB) at different magnifications: 250X (left) and 1000X (right).
4.3
Electrochemical
characterization
of
commercial
LiMn2O4
The electrochemical de-insertion/insertion reactions of lithium in the spinel Li xMn2O4 structure
occur at these voltages:
+
~ 3.9 V de-insertion/insertion of Li : 0.5<x<1
+
~ 4.1 V de-insertion/insertion of Li : 0.2<x<0.5
The electrochemical measurements were carried out at room temperature with T-shaped cells
using a galvanostat/potentiostat VMP2/Z by Bio-Logic, for galvanostatic cycles and a CHI 660
electrochemical workstation (CH Instruments), for cyclic voltammetries. The LP30 (Merck) electrolyte,
a 1 M solution of LiPF6 in EC:DMC 1:1, was used for both charge/discharge cycles and cyclic
+
voltammetries. For each electrochemical test, the potential window was set as 3-4.35 V vs. Li /Li.
53
In order to compare the electrochemical behaviour of electrodes obtained from layers LMS1 and
LMS2, respectively containing 4 % of SuperP and 2 % of KB as conductive carbons, several
experiments were carried out: galvanostatic cycles at C/10-rate, galvanostatic cycles at different C-1
rates, cyclic voltammetry at scan rate of 0.1 mV s and cyclic voltammetries at different scan rates.
I.
Galvanostatic cycles at C/10-rate
Figure 4.3 shows the galvanostatic profiles for electrodes LMS1. The irreversible capacity value
-1
is 4.75 mA h g .
LMS1-E_cyclesC10_15.m pr
Ew e vs. time
4,3
4,2
4,1
4
Ew e/V vs. L i
3,9
3,8
3,7
3,6
3,5
3,4
3,3
3,2
3,1
3
0
500.000
1.000.000
t im e /s
Figure 4.3. Galvanostatic profiles at C/10-rate for electrodes LMS1.
In order to interpret the electrochemical behaviour of these electrodes, the data were elaborated
taking in consideration (i) the charge/discharge profiles to observe in what way the discharge capacity
differs with the ageing of the system, (ii) the derivates of galvanostatic cycles and (iii) the cycling
performance.
Figure 4.4 shows the charge/discharge profiles of several cycles at C/10-rate and their
th
derivates. The system shows a good capacity retention (95.70 %) up to the 10 cycle. The discharge
st
rd
th
th
capacity values of 1 , 3 , 5 and 10 cycle are respectively 128.84, 127.70, 126.77 and 123.30 mA h
-1
g .From the dE/dt curves it is possible to observe that the operating voltages for the two insertion/de+
insertion processes (3.9 and 4.1 V vs. Li /Li) are close to the values reported in the literature.
th
Moreover, the electrochemical process presents a good reversibility up to the 10 cycle.
54
300
4.4
Derivative of cycle n°1
Derivative of cycle n°3
Derivative of cycle n°5
Derivative of cycle n°10
250
4.2
dE/dt (V/s * 100000)
4.0
+
E / V vs. Li /Li
200
3.8
1st cycle
3rd cycle
5th cycle
10th cycle
3.6
3.4
LMS1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
3.2
150
100
50
0
-50
LMS1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
-100
-150
3.0
-200
0
20
40
60
80
100
Specific Capacity / mA h g
120
140
3.0
3.2
3.4
3.6
-1
3.8
4.0
4.2
4.4
+
E / V vs. Li /Li
Figure 4.4. Charge/discharge curves (left) and derivates of cycles (right) of electrodes LMS1.
The same experiment was proposed also for an electrode LMS2. Figure 4.5 shows the
-1
galvanostatic profiles for this electrode. The irreversible capacity value is 4.89 mA h g , slightly higher
than the value of electrode LMS1.
LMS3-H_30cyclesC10_12.m pr
Ew e vs. time
4,3
4,2
4,1
4
Ew e/V vs. L i+/L i
3,9
3,8
3,7
3,6
3,5
3,4
3,3
3,2
3,1
3
0
500.000
1.000.000
1.500.000
t im e /s
Figure 4.5. Galvanostatic profiles at C/10-rate for electrodes LMS2.
Figure 4.6 shows the charge/discharge galvanostatic cycles at C/10-rate and their derivates for
th
this electrode. The system shows a good capacity retention up to 10
st
rd
th
cycle, as in the case of
th
electrode LMS1. The discharge capacity values of 1 , 3 , 5 and 10 cycle are respectively 103.73,
-1
116.84, 115.96 and 114.92 mA h g . These values suggest that the maximum reversible capacity for
this kind of electrodes, made with KB as carbon conductive agent, is reached after several cycles,
necessary to stabilize the electrode.
55
800000
4.4
400000
+
dE/dt (V/s * 100000)
4.0
E / V vs. Li /Li
Derivative of cycle n°1
Derivative of cycle n°3
Derivative of cycle n°5
Derivative of cycle n°10
600000
4.2
3.8
1st cycle
3rd cycle
5th cycle
10th cycle
3.6
3.4
LMS2
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
3.2
200000
0
-200000
LMS2
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
-400000
3.0
-600000
0
20
40
60
80
100
Specific Capacity / mA h g
120
3.0
3.2
3.4
-1
3.6
3.8
4.0
4.2
4.4
+
E / V vs. Li /Li
Figure 4.6. Charge/discharge curves (left) and derivates of cycles (right) of electrodes LMS2.
In order to compare the rate capability for the electrodes of the layers LMS1 and LMS2 tested
through galvanostatic cycles at C/10-rate, the cycling performances for the two electrodes were
reported in Figure 4.7.
-1
The electrode LMS1 (4 % of SP) delivers a discharge capacity of 99.30 mA h g after 32 cycles,
with a capacity retention of 77.1 %. Since, as definition, the end of cycle life of a cell correspond to 80
% of its nominal capacity, it can be observed that the cycle life of this cell corresponds to about 30
cycle at room temperature and at C/10-rate.
th
As regards the electrode LMS2 (2 % of KB), it is possible to see that at 30 cycle a discharge
capacity of 101.67 mA h g
-1
was delivered, with a capacity retention of 86.6 % (this value was
calculated excluding the first discharge value). This means that the end of cycle life of the cell,
differently from the electrode LMS1, has not been reached after 30 cycles at room temperature and at
C/10-rate.
a)
b)
120
-1
100
Specific Capacity / mA h g
Specific capacity / mA h g
-1
120
80
-1
Q charge / mAh g
-1
Q discharge / mA h g
60
40
LMS1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
20
100
80
-1
Q charge / mA h g
-1
Q discharge / mA h g
60
40
LMS2
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
20
0
0
0
5
10
15
20
25
30
0
35
5
10
15
20
25
30
Cycle number
Cycle number
Figure 4.7. Cycling performance at C/10-rate for: a) electrode LMS1 and b) electrode LMS2.
To highlight the different capacity retentions of tested electrodes, in Figure 4.8 the discharge
capacity trends are directly compared. As it is possible to see, the electrode LMS1 exhibits a more
noticeable capacity fading compared to the electrode LMS2.
56
140
Specific Capacity / mA h g
-1
120
100
LMS2, 2 % KB
LMS1, 4 % SP
80
60
40
Comparison of capacity retention in discharge
at C/10-rate
20
0
0
5
10
15
20
25
30
Cycle number
Figure 4.8. Capacity retention for the electrodes LMS1 and LMS2.
II.
Galvanostatic cycles at different C-rates
The rate performances of the electrodes LMS1 and LMS2 have been evaluated by cycling at
increasing rates: C/10, C/5, C/2, C, 2C up to 5C or 10C (Figure 4.9 and Figure 4.10). The irreversible
-1
-1
capacity value is 5.86 mA h g for electrode LMS1 and 4.49 mA h g for electrode LMS2.
Ew e vs. tim e
LMS1-G_cicli_varie_rates_01_GCPL_14.mpr
LMS1-G_cicli_varie_rates_02_GCPL_14.mpr #
4,3
4,2
4,1
4
Ew e/V vs. L i+/L i
3,9
3,8
3,7
3,6
3,5
3,4
3,3
3,2
3,1
3
0
100.000
200.000
300.000
400.000
t im e /s
Figure 4.9. Galvanostatic profiles at different C-rates for electrode LMS1.
57
500.000
Ew e vs. tim e
LMS3-L_cyclesatdiffC_01_GCPL_05.mpr
LMS3-L_cyclesatdiffC_02_GCPL_05.mpr #
LMS3-L_cyclesatdiffC_03_GCPL_05.mpr
4,6
4,4
Ew e/V vs. L i+/L i
4,2
4
3,8
3,6
3,4
3,2
3
0
100.000
200.000
300.000
400.000
500.000
t im e /s
Figure 4.10. Galvanostatic profiles at different C-rates for electrode LMS2.
Figure 4.11 shows the cycling performance of two electrodes at different C-rates vs. cycle
number. In both cases the reversibility of involved electrochemical processes for this cathode material
is preserved up to C/2-rate. Only at higher C-rates, a drastic capacity decrease occurs.
b)
-1
140
C/10
C/5
Q charge / mA h g
-1
Q discharge / mA h g
C/2
-1
C/10
100
80
60
2C
LMS1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
40
C/5
C/2
-1
C
Q charge / mA h g
-1
Q discharge / mA h g
140
120
120
Specific Capacity / mA h g
Specific Capacity / mA h g
-1
a)
20
100
C
80
60
LMS2
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
40
2C
20
10C
5C
0
0
0
5
10
15
20
25
30
0
Cycle number
5
10
15
20
25
30
Cycle number
Figure 4.11. Cycling performance at different C-rates vs. cycle number for: a) electrode LMS1 and b) electrode
LMS2.
Figure 4.12 shows the discharge profiles for the 3
rd
cycle, of each C-rate, for the two tested
-1
electrodes. Figure 4.12 a) shows that the electrode LMS1 delivers a capacity value of 0 mA h g when
it achieved 5C-rate. The best rate capability was recorded among C/10, C/5, C/2 and C-rates.
-1
Figure 4.12 b) shows that the electrode LMS2 delivers a capacity value of 0 mA h g when it
achieved 10C-rate. This cell presents a good rate capability up to C/2-rate.
These results suggest that at higher C-rates the electrodes lose their reversibility to insert/deinsert lithium ions in/from the crystal lattice of the spinel LiMn 2O4. The electrode LMS1 shows higher
discharge capacity values than the electrode LMS2.
58
a)
4.2
4.0
+
E / V vs. Li /Li
4.0
3rd cycle C/10
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
3rd cycle 10C
LMS2
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
4.4
+
E / V vs. Li /Li
4.2
b)
3rd cycle C/10
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
3rd cycle 5C
LMS1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
4.4
3.8
3.6
3.8
3.6
3.4
3.4
3.2
3.2
3.0
3.0
0
20
40
60
80
100
Specific Capacity / mA h g
120
140
0
20
-1
40
60
80
Specific Capacity / mA h g
100
120
140
-1
.
rd
Figure 4.12. Discharge profiles for the 3 cycle, of each C-rate, for: a) electrode LMS1 and b) electrode LMS2.
III.
Cyclic voltammetry
Cyclic voltammetry experiment was carried out by employing CHI 660 electrochemical
workstation.
-1
In order to evaluate the polarization of electrode LMS1, a cyclic voltammetry at 0.1 mV s and
one at higher scan-rates were carried out.
Figure 4.13 shows the cyclic voltammograms of the electrode LMS1 for these experiments. The
polarization effect is clearly visible both with the increasing of the cycle number (Figure a) and with
higher scan rates (Figure b).
0.2
LMS1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
scan rate 0.1 mV s
0.1
b)
-1
-1
0.0
1st Cycle
2nd Cycle
3rd Cycle
5th Cycle
10th Cycle
15th Cycle
-0.1
-0.2
LMS1
electrolyte LP30
(1M LiPF6 EC:DMC 1:1)
0.6
4.09 V
Specific Current / A g
-1
Specific Current / A g
0.8
4.22 V
a)
0.4
0.2
0.0
-0.2
-1
cycle at 0.1 mV s
-1
cycle at 0.2 mVs
-1
cycle at 0.5 mVs
-1
cycle at 1 mVs
-1
cycle at 2 mVs
-1
cycle at 5 mVs
-1
cycle at 10 mVs
-0.4
3.92 V
-0.6
4.05 V
-0.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
3.0
+
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
+
E / V vs. Li /Li
E / V vs. Li /Li
Figure 4.13. Cyclic voltammetries of electrode LMS1.
Figure 4.14 shows the cyclic voltammograms of the electrode LMS2 under the same conditions
of the previous one. The first cycle presents lower current peaks with respect to the next cycles. This
confirms the behaviour already observed by galvanostatic experiments of this kind of electrodes
(Figure 4.6) and suggests that an “activation” of the composite electrode material is necessary to
59
reach the optimal electrochemical performances. After 9 cycles, the current peaks are still well defined
and present the same intensity and potential values. This suggests a good kinetics and reversibility of
the system.
0.2
-1
Specific current / A g
4.20 V
LMS2
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
4.07 V
-1
scan rate 0.1 mV s
0.1
0.0
1st Cycle
2nd Cycle
3rd Cycle
4th Cycle
5th Cycle
6th Cycle
7th Cycle
8th Cycle
9th Cycle
-0.1
-0.2
3.0
3.2
3.94 V
3.4
3.6
3.8
4.07 V
4.0
4.2
4.4
+
E / V vs. Li /Li
-1
Figure 4.14. Cyclic voltammograms of electrode LMS2 with a scan rate 0.1 mV s .
rd
Figure 4.15 shows the comparison between the cyclic voltammograms (3 cycle) at 0.1 mV s
-1
of these two electrodes. The much smaller difference between anodic and cathodic potential peaks for
the electrode LMS2 (2 % of KB) compared to that for electrode LMS1 (4 % of SP) suggests that the
former has a faster lithium insertion/de-insertion rate. This suggest that the system in which 2 % of KB
is used as conductive agent shows better reversibility and kinetics than the system containing 4 % of
SP.
We can conclude that the use of KB as conductive agent instead of SP allows to increase the
reversibility of the system also with a smaller percentage of product in the electrode composition, that
allows higher percentage of the active material.
60
4.22 V
rd
Comparison between 3 cycles
-1
of CV (scan rate 0.1 mV s )
of two LMS (Merck) layers
Specific Current / A g
-1
0.2
4.09 V
4.20 V
4.07 V
0.1
0.0
-0.1
LMS2, 2 % KB
LMS1, 4 % SP
3.93 V
4.06 V
3.92 V
-0.2
4.05 V
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
+
E / V vs. Li /Li
rd
Figure 4.15. Comparison between the cyclic voltammograms (3 cycle) of electrodes LMS1 and LMS2.
In conclusion, the commercial LiMn2O4 used as high-voltage cathode material, shows typical
electrochemical behaviour affected by known drawbacks of this material: dissolution of manganese
into electrolyte, Jahn-Teller distortion due to Mn
on
3+
ions, change in crystal uniformity with cycling and so
97
. These phenomena leads to a rapid capacity fading of the electrodes upon cycling.
Interesting results have been observed by changing the composition of the manufactured layers
with the use of different conductive agent types and amounts. The electrode containing 4 % of
SuperP, LMS1, shows larger charge and discharge plateaus at both insertion/de-insertion lithium ions
voltages (~ 3.9 V and ~ 4.1 V) than the electrode containing 2 % of KB, LMS2. The electrodes LMS1
-1
and LMS2 deliver discharge capacity values of 128.35 and 103.20 mA h g , respectively.
While for the electrode LMS1 the capacity decreases with increasing the cycle number, for the
electrode LMS2 the capacity value increases with increasing the cycle number. This suggests that the
system containing KB, as conductive agent, needs several cycles to reach the optimal electrochemical
performance, probably due to a longer time required to obtain a good contact between active material
and the electrolyte.
Capacity fading is one of the major problems for the employment of spinel LiMn 2O4 as cathode
material in Lithium-ion batteries.
th
The electrodes LMS1 demonstrate a good capacity retention up to the 20 cycle. At the 30
th
cycle, it is possible to see a remarkable capacity loss together with the disappearance of the plateau
+
due to the insertion of lithium ions at the potential of 4.1 V vs. Li /Li. The same trend is observed for
the electrodes LMS2. These results are in agreement with the literature data and suggest that with the
ageing of the cell, the polarization effect increases and the insertion capacity of lithium ions into spinel
LiMn2O4 structure decreases. This capacity loss is directly correlate with the Jahn-Teller distortion of
the crystal lattice and with the Mn dissolution from the spinel structure into electrolyte and changes in
the electrode composition do not improve the stability of the electrodes.
61
5. Results and Discussion: LiNi0.5Mn1.5O4 based highvoltage cathode materials
In this chapter, the selection of the most suitable synthesis, in terms of safety, reaction
conditions and high purity of products, to obtain the pristine and Fe-doped LiNi0.5Mn1.5O4 spinel
structures is detailed. These materials play an important role for the development of Lithium-ion
batteries in electric and hybrid vehicles technology, because they are characterized by high operating
voltage and good electrochemical performances.
3+
The choice to employ trivalent iron ion, Fe , as doping transition metal ion, is mainly due to two
factors: i) its safety and ii) superior cycling performances and higher rate capabilities when it
substitutes only nickel in the LiNi0.5Mn1.5O4 cathode material
63
. Ions of transition metals such as Cr, Ti,
Zn, etc., are used as doping materials. Among them, trivalent chromium ion-doping shows very good
improvement of cyclic performances, but it is not environmental friendly. When the spinel
LiNi0.5Mn1.5O4 is doped through transition metals, the Mn
3+
2+
and/or Ni
ions substitution occurs. In this
way, the electrochemical performance of high-voltage LiNi0.5Mn1.5O4 improves because the doping
materials have larger bonding strength with oxygen and no Jahn-Teller distortion. Table 5.1 shows the
bond dissociation energies for the various transition metals involved with the high-voltage spinel. From
the table, bond strengths of Cr and Ti with oxygen exceed that of Mn, whereas Fe and Zn have
smaller bond strengths. However, the Fe-O bond shows a larger strength than that for Ni-O bond
62
.
This implies that Cr and Ti doping materials are suitable for substitution of Mn alone, Ni alone or both,
while the Fe doping improves the electrochemical performances of spinel LiNi0.5Mn1.5O4 when it
substitutes Ni alone.
Bond dissociation energies
-1
ΔHf (298 K) (kJ mol )
Bond
ΔHf
Mn-O
402
Ni-O
382
Cr-O
461
Fe-O
390
Ti-O
672
Zn-O
159
Table 5.1. Comparison of bond dissociation energies of representative transition metals with oxygen
62
.
Intense studies have been carried out in order to optimize the synthetic conditions, sample
treatments and electrode preparation because the electrochemical behaviour of high-voltage cathode
materials is correlated with these parameters.
62
Among the possible synthetic routes that normally are undertook to synthesize cathode
materials, mechanochemical process and sol-gel method are chosen. In order to avoid confusion
regarding among these techniques and the powder obtained from them, Chapter 5 is divided into two
parts as regards the synthetic procedure employed. The first one takes in consideration the
mechanochemical process, its improvements, the structural and morphological characterization of the
powders that are synthesized following this procedure, the electrode preparation and the
electrochemical tests. The second part deals with the same points considering the sol-gel method as
alternative synthetic route to overcome several problems of the mechanochemical process.
Electrochemical performances of sol-gel synthesized powders were tested both at room and lower
temperatures (0 °C and -20 °C).
5.1 Synthesis by mechanochemical process
The synthesis of high-voltage cathode materials by mechanochemical process is reported by
Oh et al.
98
. The mechanochemical process adopts planetary-ball milling, which employs balls of
different materials, for the mechanical activation of the starting materials. Following this reference,
both
pristine
LiNi0.5Mn1.5O4
and
its
Fe-doped
spinel
structure,
with
the
stoichiometry
LiFe0.1Ni0.4Mn1.5O4, have been synthesized. The obtained powders were characterized at the
structural, morphological and electrochemical levels.
Since grinding degrees depend on the materials balls, two different types of balls have been
used: agate balls and steel balls. Agate is a very interesting material because of its extreme hardness
(Mohs harness of ~ 7). The only grinding balls that have a higher hardness value are those made from
zirconia (Mohs hardness of ~ 10)
99
, but unfortunately they were not available in the lab during my
thesis. It has been found that agate balls are quite acceptable for most of applications where freedom
from grinding media contamination is crucial, like in synthesis of high performance cathode materials.
The steel grinding media balls present a high hardness but can contaminate the samples, mainly with
the iron.
The spinel structure LiNi0.5Mn1.5O4 was synthesized using both agate and steel balls, whereas
the Fe-doped sample was synthesized only with the employment of agate balls.
I.
Synthetic Procedure
In order to synthesize the pristine LiNi0.5Mn1.5O4 powder, stoichiometric amounts of Li2CO3,
Ni(OH)2, MnO2 were weighed and were poured into a mortar to grind them. Afterwards, the powder
was put into a jar of 50 ml together with 20 agate or steel balls (Ø 10 mm) and it was ball milled at a
speed of 400 rpm for 30 minutes. After that, the ball-milled powder was collected and subjected to a
63
first heat treatment at 600 °C for 8 h and a second one at 900 °C for 12 h in air, both at heating-rate of
-1
-1
5 °C min . The cooling process was set at rate of 5 °C min until 50 °C. After the heating and cooling
treatments, the powder was collected into a vial and put under vacuum in order to protect it from the
moisture attach. The same synthetic procedure was followed to obtain the Fe-substituted
LiNi0.5Mn1.5O4, adding the doping material Fe2O3 to the initial mixture.
Calculating the yield of each reaction, larger values than the fixed theoretical weight are
obtained. This leads to assume that some impurities could be presented inside the synthesized
powders.
5.1.1 Structural and Morphological characterization of the powders
synthesized by mechanochemical process
The structural characterization of synthesized powders was carried out by the X-ray diffraction
technique (XRD) using a Philips X-ray diffractometer with Cu Kα radiation, while their morphology was
investigated by SEM employing a Cambridge Stereoscan 360 with an electron acceleration potential
of 20 kV.
In addition, EDX analysis was conducted for each sample in order to verify the presence of
specific elements, such as Mn, Ni and Fe.
I.
LiNi0.5Mn1.5O4 powder synthesized using agate balls
The synthesized powder was characterized by X-ray diffraction (XRD) between 15° and 60°, a
step size of 0.02 and time/step of 1.00.
Figure 5.1 shows the X-ray diffraction pattern of the specific powder in which it is possible to
see a lot of undesirable peaks compared to spinel characteristic peaks of the (311), (400), (331)
reflections, due to LixNi1-xO impurity, the unreacted MnO2 (used as reagent) and the brass of sample
carrier plate.
64
(111)
1500
° MnO2 impurity (unreacted reagent)
* LixNi1-xO impurity
+ Brass
(220)
*
°°
°
(511)
500
(331)
(311)
(400)
+
(222)
Intensity (a.u.)
1000
+
*
*
0
15
20
25
30
35
40
45
50
55
60
65
2  (degree)
Figure 5.1. XRD pattern of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using agate balls.
The morphology, particle shape and the homogeneity of the LiNi0.5Mn1.5O4 powder were probed
by Scanning Electron Microscopy (SEM). Figure 5.2 shows the SEM images for the synthesized
powder with different magnifications. As it is possible to see, the powder presents a patchy particle
distribution with visible aggregates that suggest an incomplete formation of the desired product and
the existence of impurities (MnO2 crystals). These images confirmed the considerations about both the
amount of obtained sample higher than the theoretical and the presence of impurities observed by
XRD analysis. Particle size has been estimated in the range 5-20 µm.
Figure 5.2. SEM images of LiNi0.5Mn1.5O4 powder (agate balls) at different magnifications: a) 1000X, b) 5000X
and c) 8000X.
65
The qualitative presence of the metals was revealed by EDX analysis. The EDX spectrum
(Figure 5.3) for the LiNi0.5Mn1.5O4 powder shows the Mn Kα1 and Kβ1 peaks at 5.92 and 6.45 keV,
respectively and the Ni Kα1 peak at 7.49 keV and Kβ1 peak in the 8.20-8.27 keV range.
400
Mn (K1)
350
300
Counts
250
200
150
Mn (K 1)
100
Ni (K1)
50
Ni (K 1)
0
0
5
10
15
20
25
Energy (keV)
Figure 5.3. EDX spectra of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using agate balls.
II.
LiNi0.5Mn1.5O4 powder synthesized using steel balls
The synthesized powder was characterized by X-ray diffraction (XRD) between 10° and 70°, a
step size of 0.02 and time/step of 1.00.
Figure 5.4 shows the X-ray diffraction pattern where it is possible to see several peaks related
to the spinel characteristic (311), (400), (331) reflections, to LixNi1-xO impurity and to the unreacted
2500
(111)
MnO2 (used as reagent).
° MnO2 impurity (unreacted reagent)
* LixNi1-xO impurity
(440)
(511)
°
(531)
°°
(400)
*
(331)
500
(222)
1000
(311)
1500
(220)
Intensity (a.u.)
2000
°
0
20
30
40
50
60
70
2  (degree)
Figure 5.4. XRD pattern of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using steel balls.
66
Before to carry out the morphological characterization, the powder was ball-milled with 20 agate
balls at a speed of 350 rpm for 1 h and 30 minutes in order to reduce the particle size. Figure 5.5
shows the SEM images of the LiNi0.5Mn1.5O4 powder after ball-mill treatment, at different
magnifications. Making a comparison with the SEM images of the untreated ball-mill powder (Figure
5.2), it is possible to observe a decrease of the particle size, which varies in the range 0.2-5 µm,
although some aggregates are still present. Also in this case, SEM images confirmed the presence of
impurity phases.
Figure 5.5. SEM images of LiNi0.5Mn1.5O4 powder (steel balls) after ball-mill treatment, at different magnifications:
a) 1000X and b) 5000X.
The qualitative evaluation of the metals presence was carried out by EDX analysis. Figure 5.6
shows the EDX spectrum for the LiNi0.5Mn1.5O4 powder. The Mn Kα1 and Kβ1 peaks fall on 5.85 and
6.47 keV, respectively, while the Ni Kα1 peak falls on 7.47 keV and Kβ1 peak is in the 8.20-8.29 keV
range.
180
160
Mn (K1)
140
Counts
120
100
80
60
Mn (K 1)
40
Ni (K1)
20
Ni (K 1)
0
0
5
10
15
20
Energy (keV)
Figure 5.6. EDX spectra of LiNi0.5Mn1.5O4 powder synthesized by ball-mill mechanism, using steel balls and after
a ball-mill treatment.
67
III.
LiFe0.1Ni0.4Mn1.5O4 powder synthesized using agate balls
The synthesized powder was characterized by X-ray diffraction (XRD) between 10° and 70°,
with a step size of 0.02 and time/step 1.50.
Figure 5.7 shows the X-ray diffraction pattern of the synthesized powder. Also in this case there
are peaks of unreacted MnO2, while the undesirable LixNi1-xO peaks, close to the spinel characteristic
peaks of the (311), (400), (331) reflections, have weaker intensities compared to that present in XRD
of pristine material (Figure 5.1). In the literature it has been shown that the cationic substitutions help
to eliminate the formation of the LixNi1-xO impurity phases
63
. Nevertheless, the peaks due to impurities
are still visible in the diffraction pattern of LiFe0.1Ni0.4Mn1.5O4 powder. This suggest that the synthesis
needs further optimizations. In addition, the absence of (220) reflection indicates that the Fe-doped
(111)
material did not occupy the tetrahedral (8a) sites of lithium ion.
3500
° MnO2 impurity (unreacted reagent)
3000
* LixNi1-xO impurity
2000
*
°
10
15
20
25
30
35
40
45
50
(440)
(400)
* ° °
°
0
(531)
°
(511)
500
(222)
1000
(331)
1500
(311)
Intensity (a.u.)
2500
55
60
65
70
2  (degree)
Figure 5.7. XRD pattern of LiFe0.1Ni0.4Mn1.5O4 powder synthesized by ball mill mechanism using agate balls.
The particle morphology of the synthesized powder was analyzed by SEM. Figure 5.8 shows
the images of the synthesized powder at different magnifications. Despite the presence of some
agglomerates the particle size can be estimated in the order of 5-20 µm.
68
Figure 5.8. SEM images of LiFe0.1Ni0.4Mn1.5O4 powder (agate balls) at different magnifications: a) 1000X and b)
5000X.
Figure 5.9 shows the EDX spectrum for LiFe0.1Ni0.4Mn1.5O4 powder. Unfortunately, through this
analysis it is not possible to detect the presence of Fe doping metal since it is presented in small
amounts and its Kα1 peak (6.40 keV, from tabular data) is overlapped with the Mn Kβ1 peak (6.49 keV,
tabulated). The Fe Kβ1 peak falls on 7.07 keV with a low intensity, very close to the tabulated value
(7.06 keV). The Mn Kα1 and Kβ1 peaks are at 5.88 and 6.40 keV, respectively and the Ni Kα1 peak
falls on 7.47 keV and Kβ1 peak is in the 8.15-8.28 keV range.
Mn (K1)
400
Counts
300
Mn (K 1) Fe (K1)
200
Fe (K 1)
100
Ni (K1)
Ni (K 1)
0
0
2
4
6
8
10
12
14
16
18
20
Energy (keV)
Figure 5.9. EDX spectra of LiFe0.1Ni0.4Mn1.5O4 powder synthesized by ball-mill mechanism, using agate balls.
5.1.2 Electrode preparation
As previously seen, no powder with high purity degree was obtained by the mechanochemical
process. The presence of unreacted precursors crystals, such as MnO2, especially if they are in large
amount, compromises the electrochemical performances of the electrodes in which the synthesized
69
samples are the active materials. Three different electrode layers have been obtained from the
different compounds synthesized by ball-mill process:

the layer 1MNO-1 was obtained from LiNi0.5Mn1.5O4 powder, synthesized using agate
balls as grinding media balls;

the layer 2MNO-1 was obtained from LiNi0.5Mn1.5O4 powder, synthesized using steel
balls as grinding media balls;

the layer 1FNM-1 was obtained from LiFe0.1Ni0.4Mn1.5O4 powder, synthesized using
agate balls as grinding media balls.
I.
Layer 1MNO-1
The layer 1MNO-1 was manufactured by preparing a slurry of LiNi0.5Mn1.5O4, SuperP and PVdF
in NM2P, whose composition is shown in Table 5.2, that was stirred for 4 h using a magnetic stirrer.
LiNi0.5Mn1.5O4
82.0 %
SP
10.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0 %
NM2P
1.0 ml
Table 5.2. Percentage composition of layer 1MNO-1.
The mixture was stratified on aluminium foil through Doctor Blade technique and the thickness
was set at 200 µm. The obtained layer was dried on a hot plate at 50 °C, under hood, in order to
remove completely the solvent. The dried layer was pressed with roll press. Circular electrodes with a
2
diameter of 9 mm and a surface area of 0.636 cm were cut and dried overnight at 120 °C under
vacuum. The capacity of the electrodes was calculated considering a specific theoretical capacity of
-1
146.7 mA h g .
II.
Layer 2MNO-1
Before preparing the layer, the synthesized powder was ball-milled again for 2 h at a speed of
350 rpm in order to decrease particle size.
The layer 2MNO-1 was manufactured by preparing a slurry of LiNi0.5Mn1.5O4, SuperP and PVdF
in NM2P that was stirred for 5 h using a magnetic stirrer.
70
LiNi0.5Mn1.5O4
82.0 %
SP
10.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0 %
NM2P
0.2 ml
Table 5.3. Percentage composition of layer 2MNO-1.
The mixture was stratified on aluminium foil through Doctor Blade technique and the thickness
was set at 200 µm. The obtained layer was dried on a hot plate at 50 °C, under hood, in order to
remove completely the solvent. Several circular electrodes with a diameter of 9 mm were cut from the
dried layer. These electrodes were pressed in order to compact the electrodes and to guarantee a
good contact among current collector, active material and conductive agent. All obtained electrodes
were dried overnight at 120 °C under vacuum. The capacity of the electrodes was calculated
-1
considering a specific theoretical capacity of 146.7 mA h g .
III.
Layer 1FNM-1
The layer 1FNM-1 was manufactured by preparing a slurry of LiFe0.1Ni0.4Mn1.5O4, graphite
(KS15), SuperP and PVdF in NM2P that was stirred for 4 h using a magnetic stirrer. In this case, the
conductive agent was a mixture of KS15 graphite and SP carbon and the percentage of active
material was increased to 84%.
LiFe0.1Ni0.4Mn1.5O4
82.0 %
SP
4.0 %
KS15
4.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0 %
NM2P
0.2 ml
Table 5.4. Percentage composition of layer 1FNM-1.
The mixture was stratified on aluminium foil through Doctor Blade technique and the thickness
was set at 200 µm. The obtained layer was dried on a hot plate at 50 °C, under hood, in order to
remove completely the solvent. The dried layer was pressed with roll press. Circular electrodes with a
2
diameter of 9 mm and a surface area of 0.636 cm were cut and dried overnight at 120 °C under
vacuum. The capacity of the electrodes was calculated considering a specific theoretical capacity of
-1
148 mA h g .
71
5.1.3 Electrochemical characterization
The electrochemical measurements were carried out at room temperature with T-shaped cells
using a galvanostat/potentiostat VMP2/Z by Bio-Logic. The employed electrolytes were: a 1 M solution
of LiPF6 in EC:DMC:DEC 1:1:1 (LP71, Merck), a 1 M solution of LiPF6 in EC:DMC 1:1 (LP30, Merck)
and the electrolyte FAAM.
The electrochemical tests for the electrodes of layers 1MNO-1 and 1FNM-1 were carried out
over the potential range of 3.5-5 V. Unfortunately, several experiments carried out on the electrodes
2MNO-1 did not produced useful results, due to a very high mechanical instability of the electrodes
that leaded to their destruction after a few cycles. Therefore, a comparison between the
electrochemical performances of layers 1MNO-1 and 1FNM-1 is reported. All potentials are referred to
+
Li /Li.
I.
Layer 1MNO-1
Galvanostatic cycles at different C-rates (5 cycles at C/10, C/5 and C/2) have been carried out
by using a 1 M solution of LiPF6 in EC:DMC:DEC 1:1:1 as electrolyte.
Figure 5.10 shows the galvanostatic charge/discharge profiles at different C-rates for the
-1
electrode 1MNO-1. A relatively high irreversible capacity, of about 61.37 mA h g was obtained. From
the figure below, the plateau in the 4 V region of Mn
3+/4+
redox couple is depicted as well as the
plateaus around 4.7 V corresponding to the electrochemical activity of Ni
2+/3+
3+/4+
and Ni
redox couples.
The presence of two plateaus at this voltage value suggests that the synthesized active cathode
material could have a spinel structure with Fd3m symmetry.
Ew e vs. time
1MNO-1-D_CcyclesatdiffC_01_GCPL_09.mpr #
1MNO-1-D_CcyclesatdiffC_02_GCPL_09.mpr
5
4,8
4,6
Ew e/V vs. L i+/L i
4,4
4,2
4
3,8
3,6
3,4
3,2
0
50.000
100.000
150.000
200.000
t im e /s
Figure 5.10. Galvanostatic profiles at different C-rates for electrode 1MNO-1.
72
rd
Figure 5.11 shows the discharge profiles for the 3 cycle, at the different C-rates, for the tested
electrodes. The obtained discharge capacites (52.54 mA h g
-1
at C/10, 49.01 mA h g
-1
-1
at C/5 and
-1
41.54 mA h g at C/2) are lower than the theoretical values of 130 vs. 146.7 mA h g .
5.2
3rd cycle C/10
3rd cycle C/5
3rd cycle C/2
5.0
4.6
+
E/ V vs. Li /Li
4.8
4.4
4.2
4.0
3.8
1MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.6
0
10
20
30
40
Specific Capacity / mA h g
50
60
-1
rd
Figure 5.11. Discharge profiles for the 3 cycle, of each C-rate, for electrode 1MNO-1.
Figure 5.12 shows the obtained charge/discharge capacities of the tested electrode at different
C-rates vs. cycle number.
120
-1
Specific Capacity / mA h g
-1
Q charge / mA h g
-1
Q discharge / mA h g
100
80
C/10
C/5
60
C/2
40
1MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
20
0
0
2
4
6
8
10
12
14
16
Cycle number
Figure 5.12. Cycling performance at different C-rates vs. cycle number for electrode 1MNO-1.
Summarizing, the electrodes prepared by the LiNi0.5Mn1.5O4 powder, synthesized using agate
balls, show poor electrochemical performances in terms of high irreversible capacity at the first cycle,
low reversible capacity and low rate performances.
73
II.
Layer 1FNM-1
Galvanostatic cycles at C/10-rate have been carried out by using three types of electrolytes, i.e.
LP30 (1 M solution of LiPF6 in EC:DMC 1:1), LP71 (1 M solution of LiPF6 in EC:DMC:DEC 1:1:1) and
electrolyte FAAM.
th
Figure 5.13 shows the charge/discharge profiles, up to the 10 cycle, for electrodes 1FNM-1.
From plots it is possible to observe a well-defined plateau in the 4 V region due to the Mn
2+/3+
redox couple, two plateus at 4.7 V corresponding to the electrochemical activity of Ni
3+/4+
3+/4+
and Ni
and an additional small plateau above 4.9 V which can be attributed to the redox reaction of Fe
3+/4+
.
For all the electrodes, we can observe high irreversible capacities during first charge and low
reversible capacites during first discharge. In particular, the obtained values at the first cycle are 48.57
-1
-1
mA h g for the electrode cycled with electrolyte LP30, 49.27 mA h g for the electrode cycled with
electrolyte LP71 and 48.79 mA h g
-1
for the electrode cycled with electrolyte FAAM. The high
irreversible capacities and the low reversible capacities are probably due to the presence of impurities
in the synthesized powder that generate side reactions with the used electrolytes at high-operating
voltages leading to poor electrochemical performances.
The obtained capacities for the different cells are reported in Table 5.5.
74
5.2
a)
5.0
4.6
1st cycle
3rd cycle
5th cycle
10th cycle
+
E / V vs. Li /Li
4.8
4.4
4.2
4.0
1FNM-1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
3.8
3.6
0
20
40
60
80
Specific Capacity / mA h g
100
-1
5.2
b)
5.0
4.6
1st cycle
3rd cycle
5th cycle
10th cycle
+
E / V vs. Li /Li
4.8
4.4
4.2
4.0
1FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.8
3.6
0
20
40
60
80
Specific Capacity / mA h g
100
-1
5.2
c)
5.0
4.6
1st cycle
3rd cycle
5th cycle
10th cycle
+
E / V vs. Li /Li
4.8
4.4
4.2
4.0
1FNM-1
electrolyte FAAM
3.8
3.6
0
20
40
60
Specific Capacity / mA h g
80
100
-1
Figure 5.13. Charge/discharge profiles at C/10-rate for electrodes 1FNM-1.
75
ICL*
-1
(mA h g )
Q
disch
-1
EC:DMC 1:1
EC:DMC:DEC 1:1:1
52.86
51.36
37.83
48.57
49.27
48.79
49.77
52.39
52.67
50.14
53.93
53.59
50.52
53.83
53.35
Electrolyte FAAM
3rd
disch
-1
(mA h g )
Q
5th
disch
-1
(mA h g )
Q
LP71
1st
(mA h g )
Q
LP30
10th
disch
-1
(mA h g )
* ICL = Irreversible Capacity Loss.
Table 5.5. Summary of cycling data for tested electrodes in different electrolyte systems.
The irreversible capacity at the first cycle is lower when the electrolyte FAAM is used. This
electrolyte contains a different solvent composition than the other two systems, but the same salt
LiPF6. The combination of ionic conductivity and viscosity of the electrolyte FAAM composition is
probably more appropriate for ensuring suitable reactions with the active material at the first cycle,
where chemical reactions between liquid electrolyte and active material take place beside
electrochemical reactions involving the electrolyte at high potentials. The lower ICL value obtained
from electrolyte FAAM system could depend on the formation of a more stable SEI layer on the
cathode surface, but further studies should be carried out in order to assess this statement. We can
observe that, whatever the electrolyte system used, the reversible capacity increases with the cycling,
suggesting also in this case that the contact between the active material and the electrolyte requires
some time to be completed.
Despite some difference in the irreversible capacity at the first cycle when a suitable solvent
combination is used in the electrolyte system, no significant improvements are observed in the
reversible capacity, that is much lower than the theoretical one. Moreover, the presence of impurities,
such as unreacted reagents and LixNi1-xO, in LiFe0.1Ni0.4Mn1.5O4 powder is a big limitation because it
affects the electrochemical performances of Fe-doped spinel LiNi0.5Mn1.5O4.
It is possible to characterize each redox reaction for this material by using a slow cyclic
-1
voltammetry, with a scan rate of 0.05 mV s . The electrolyte LP30 (EC:DMC 1:1) was used.
-1
Figure 5.14 shows the cyclic voltammograms at 0.05 mV s for electrode 1FNM-1.
Anodic and cathodic peaks in the 4 V region are due to the Mn
the 4.8 V region are due to the electrochemical activity of Ni
2+/3+
3+/4+
redox couple, large peaks in
3+/4+
and Ni
small peaks in the 5 V region are attributed to the redox reaction of Fe
3+/4+
redox couples and the
. The presence of these
peaks confirming the role of the iron in the increase of operating voltage for these materials. The
4+
peaks clearly visible in the 4 V region suggests that not all Mn is present as Mn . This result depends
76
on fact that the substitution of Fe
4+
amount of Mn
3+
to Mn
3+
for only Ni
2+
(LiFe0.1Ni0.4Mn1.5O4) reduces the corresponding
with an increase in the electrochemical activity in the 4 V region.
st
Well-defined peaks in the high-voltage region are visible only in the 1 and 2
nd
cycle. Probably,
the presence of impurities in the synthesized powder affects cycling performances.
0.04
Specific Current / A g
-1
0.03
4.84 V
1FNM-1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
4.73 V
0.02
5.00 V
4.07 V
0.01
0.00
4.87 V
4.01 V
-0.01
1st Cycle
2nd Cycle
3rd Cycle
-0.02
-0.03
3.6
3.8
4.0
4.68 V
4.61 V
4.2
4.4
4.6
4.8
5.0
5.2
+
E / V vs. Li /Li
-1
Figure 5.14. Cyclic voltammograms at scan rate of 0.05 mV s for electrode 1FNM-1. The peaks of the 2
nd
cycle
was highlighted.
5.1.4 Conclusions
As seen previously, no powder with high purity degree was obtained by the mechanochemical
process. The presence of unreacted precursors crystals, such as MnO 2, especially if they are in large
amount, compromises the electrochemical performances of the electrodes in which the synthesized
samples are the active materials and in addition does not allow to highlight the improvements due to
the doping of spinel structure.
These impurities together with a inhomogeneous particle distribution of the powders can be
mainly due to the unsuitable ball-mill treatment in terms of grinding media balls, time and energy.
Since the mechanochemical process presents several disadvantages, alternative synthetic
routes are preferred to it in order to obtain cathode powders with high degree of crystallinity and purity
that ensure good electrochemical performances.
77
5.2 Synthesis by sol-gel method
In order to overcome the disadvantages of conventional solid state method, pristine and Fedoped LiNi0.5Mn1.5O4 spinel cathode materials were synthesized using the sol-gel method.
The citric acid-assisted sol-gel method used to synthesize the samples is described in
57
.
The spinel pristine material was synthesized with the following stoichiometry Li1.02Ni0.5Mn1.5O4
57
because the extra lithium content is used to remove the Mn from trivalent to tetravalent, thus
3+
minimizing the impact of any Jahn-Teller distortion due to the presence of Mn . The stoichiometry of
Fe-doped material was taken from
63
and corresponds to LiFe0.08Ni0.42Mn1.5O4. It has been reported
that this composition offers a combination of high capacity and excellent cyclability due to the
substitution of Fe for Ni alone. The enhance of electrochemical performances by Fe doping is
attributed to the i) stabilization of the structure with cation disorder (Fd3m) in the 16d octahedral sites
of the spinel lattice that leads to the suppression of the phase transition during the cycling, ii)
suppression of the formation of a thick SEI layer due to the Fe-enrichment and Ni-deficiency on the
surface, iii) production of Mn
3+
and consequent electronic conductivity increase and iv) reduction of
polarization phenomenon arising from fast charge transfer kinetics and lithium ion diffusion kinetics in
the bulk.
It is well-known that the electrochemical performances of high-voltage cathode materials
strongly depend on the synthetic conditions. A citric acid-assisted sol-gel synthesis for high-voltage
cathode materials is affected from different parameters, such as the type of precursors, the citric acid :
total metal amount ratio and annealing temperature. Among them, the citric acid : total metal amount
ratio has a key role in the formation of composite cathode materials that show good electrochemical
performances.
In this work, three samples were synthesized (one bare and two Fe-substituted samples) by
citric acid-assisted sol-gel method, using a different citric acid : total metal amount ratio in order to
optimize this parameter. Li1.02Ni0.5Mn1.5O4 and one Fe-doped sample were synthesized using a citric
acid : total metal amount ratio of 0.3 : 1, whereas the other Fe-doped powder was synthesized using a
citric acid : total metal amount ratio of 1 : 1. The latter was the most appropriate ratio to synthesize
high-voltage cathode materials with electrochemical performances close to that showed in literature.
As seen in Figure 1.19, metals (Li, Ni, Mn and Fe) are not complexed from the three carboxylic
functions of citric acid because two of them react to give the polymeric gel and only one acts as
chelating. If citric acid is in defect, the spinel structure will be poor on metal content and hence a
consequent decline of electrochemical performances will occur. Instead, an excess of citric acid could
increase the resistance of electrochemical systems.
All the powders obtained from this synthesis were conserved under vacuum in order to avoid
contamination of moisture.
78
5.2.1 Synthetic procedure
I.
Li1.02Ni0.5Mn1.5O4 (3MNO)
A
stoichiometric
amount
of
lithium
acetate
[Li(CH3COO)],
manganese
acetate
[Mn(CH3COO)2·4H2O], and nickel acetate [Ni(CH3COO)2·4H2O], were dissolved in an appropriate
quantity of ultrapure water at room temperature. Since at the beginning the solution appears opaque
and bright green, ultrapure water was added in order to solubilize all the components.
The solution was stirred at 50 °C and the citric acid was added to the solution (citric acid :
metals 0.3 : 1). After this addition the solution becomes transparent and remains bright green. The pH
of the solution was adjusted to 7.0 by slowly dropping ammonium hydroxide drop wise and continued
stirring for 4 h. The temperature of the solution raised to 80-90 °C and continued stirring till the
solution turned into high-viscous gel (Figure 5.15). The resulted gel was dried at 80 °C for 24 h in a
temperature controlled oven.
Figure 5.15. Formation of the high-viscous gel (citric acid : Li-Ni-Mn 0.3 : 1).
After this treatment, the product was recovered and put in oven at 200 °C with a heating rate of
-1
2 °C min . From this heating process a product with a consistency very similar to that of polystyrene
was obtained: this phenomenon is probably due to water traces remained inside the structure (Figure
5.16).
Figure 5.16. The product obtained after heating process at 200 °C.
79
This precursor was ground to fine powder and calcined at 450 °C under oxygen flowing
-1
conditions with a constant heating followed by cooling rate at 4 °C min
to decompose organic
constituents. The calcined powder was grounded to fine powder and re-sintered at 800 °C for 16 h
-1
under oxygen flowing conditions and heating / cooling rate was maintained at 2 °C min .
The synthesized powder was ball-milled with agate balls at speed of 350 rpm for 2 h in order to
reduce the particle size.
The quantity of Li1.02Ni0.5Mn1.5O4 obtained was 4.50 g, with a yield of reaction equal to 75.01 %.
The low yield value was caused from a consistent lost of product during the sol-gel method.
For simplicity, the Li1.02Ni0.5Mn1.5O4 powder synthesized by using a citric acid : total metal
content ratio of 0.3 : 1 was denominated 3MNO and this name will be used in the following part of the
thesis.
II.
LiFe0.08Ni0.42Mn1.5O4 (2FNM)
The Fe
3+
precursor chosen for the substitution of Ni alone in spinel LiNi 0.5Mn1.5O4 cathode
material is the ferric ammonium citrate – red (III). This compound is completely soluble in water
88
.
Stoichiometric amounts of this iron precursor and lithium, manganese and nickel acetates were
dissolved in an appropriate quantity of ultrapure water at room temperature. Trough the addition of
iron precursor, high-viscous gel became brown (Figure 5.17): this colour suggests that a quantity of
citric acid in defect not allow to complex all the metals in solution. This synthesis follows the same
steps of the previous one, with the only exception that the gel was dried at 80 °C for 24 h in a
temperature controlled oven under vacuum. Particles of the obtained powder appeared smaller than to
that of powder 3MNO. For this reason, the powder was ground only through a mortar. The quantity of
LiFe0.08Ni0.42Mn1.5O4 obtained was 5.75 g, with a yield of reaction of 95.83 %.
For simplicity, the LiFe0.08Ni0.42Mn1.5O4 powder synthesized by using a citric acid : total metal
content ratio of 0.3 : 1 was denominated 2FNM and this name will be used in the following part of the
thesis.
Figure 5.17. Formation of the high-viscous gel (citric acid : Li-Ni-Mn-Fe 0.3 : 1).
80
III.
LiFe0.08Ni0.42Mn1.5O4 (3FNM)
The synthesis of the second LiFe0.08Ni0.42Mn1.5O4 sample was carried out following each step of
the previous ones but using a citric acid : total metal content ratio of 1 : 1. A yellow gel (Figure 5.18)
was obtained. This colour suggests that probably all metals were complexed by chelating agent. Also
for this synthetic procedure, the high-viscous gel was dried under vacuum. The synthesized powder
was ball-milled with agate balls at 350 rpm for 2 h in order to decrease the particle size.
The quantity of LiFe0.08Ni0.42Mn1.5O4 obtained was 5.70 g, with a yield of reaction of 95 %.
For simplicity, the LiFe0.08Ni0.42Mn1.5O4 powder synthesized by using a citric acid : total metal
content ratio of 1 : 1 was denominated 3FNM and this name will be used in the following part of the
thesis.
Figure 5.18. Formation of the high-viscous gel (citric acid : Li-Ni-Mn-Fe 1 : 1).
5.2.2 Structural and Morphological characterization of the powders
synthesized by sol-gel method
The structural characterization of synthesized powders was carried out by the X-ray diffraction
technique (XRD) using a Philips X-ray diffractometer with Cu Kα radiation; while their morphology and
particle size were investigated by SEM employing a Cambridge Stereoscan 360 with an electron
acceleration potential of 20 kV.
81
I.
Li1.02Ni0.5Mn1.5O4 powder (3MNO)
The synthesized powder was characterized by X-ray diffraction (XRD) between 10° and 60° with
a scan range 3.00-70.00, a step size of 0.02 and time/step of 1.50.
Figure 5.19 shows the XRD pattern of powder 3MNO. The spinel characteristic peaks at (311),
(400), (331) and (440) reflections are observed in the pattern and small traces of Li xN1-xO impurity
peaks are observed closely to the spinel characteristic peaks of the (222), (400) and (440) reflections.
The preparation of pure phase of controlled stoichiometry of nickel and oxygen content in
Li1.02Ni0.5Mn1.5O4 powder is difficult due to similarity in the ionic radii of Li and Ni ions
57
. In addition, the
3000
3MNO
2500
* LixNi1-xO impurity
20
* *
*
0
30
(331)
(220)
500
(511)
1000
40
(440)
1500
(531)
(311)
(400)
2000
(222)
Intensity (a.u.)
(111)
absence of (220) reflection indicates that no transition metal ions exist in the tetrahedral (8a-Li) sites.
*
50
60
70
2  (degree)
Figure 5.19. XRD pattern of powder 3MNO.
The morphology, particle shape and their distribution of the Li1.02Ni0.5Mn1.5O4 powder were
examined by SEM.
SEM images reveal that the powder is characterized by some aggregates and holes that could
affect its electrochemical performances. The estimated particle size ranges from 0.2 to 5 µm.
Figure 5.20. SEM images of powder 3MNO at different magnifications: a) 1000X and b) 5000X.
82
II.
LiFe0.08Ni0.42Mn1.5O4 powder (2FNM)
The synthesized powder was characterized by X-ray diffraction (XRD) between 10° and 70° with
a scan range 3.00-70.00, a step size of 0.02 and time/step of 1.50.
Figure 5.21 shows the XRD pattern of LiFe0.08Ni0.42Mn1.5O4 powder (2FNM). The spinel
characteristic peaks at (311), (400), (331) and (440) reflections are observed. For this sample, the
introduction of Fe in spinel structure did not lead no structural improvement since the reflections due to
the LixN1-xO impurity phase are still present. However, also in this case the absence of (220) reflection
3000
(111)
suggests that the tetrahedral (8a) sites are occupied only from lithium ion.
2FNM
* LixNi1-xO impurity
2000
30
40
50
(531)
(440)
*
*
0
20
*
(511)
(220)
500
(331)
1000
(400)
(311)
1500
(222)
Intensity (a.u.)
2500
60
70
2  (degree)
Figure 5.21. XRD pattern of powder 2FNM.
The morphology, particle shape and their distribution of the LiFe0.08Ni0.42Mn1.5O4 powder were
probed by (SEM). From SEM images, it is possible to observe small particles with the presence of
some larger agglomerates that increase the particle size (1-5 µm) of this powder compared with that of
powder 3MNO (0.2-5 µm). This difference depends on the fact that the sample 2FNM was not ballmilled with agate balls.
Figure 5.22. SEM images of powder 2FNM at different magnifications: a) 1000X and b) 5000X.
83
III.
LiFe0.08Ni0.42Mn1.5O4 powder (3FNM)
Figure 5.23 shows the XRD pattern of LiFe0.08Ni0.42Mn1.5O4 powder (3FNM) characterized by X-
ray diffraction (XRD) between 10° and 70° with a scan range 3.00-70.00, a step size of 0.02 and
time/step of 1.50.
The spinel characteristic peaks at (311), (400), (331) and (440) reflections are observed in the
pattern. The weak reflections observed at around 37.6°, 45.7° and 63.5° due to the LixN1-xO impurity
phase both in powders 3MNO and 2FNM are absent in this Fe-doped sample, indicating the
effectiveness of Fe in eliminating the impurity phase
63
. Also for this synthesized material, the absence
3500
(111)
of (220) reflection confirms that no transition metal ions exist in the tetrahedral (8a) sites.
3FNM
2500
(440)
(331)
(220)
500
(222)
1000
(531)
1500
(511)
(400)
2000
(311)
Intensity (a.u.)
3000
0
20
30
40
50
60
70
2  (degree)
Figure 5.23. XRD pattern of powder 3FNM.
Making a comparison between XRD analysis of the two Fe-doped powders synthesized by solgel method with different citric acid : total metal content ratio, it is possible to observe some important
differences.
Figure 5.24 shows the comparison between the XRD patterns obtained for powders 3FNM and
2FNM.
The XRD pattern of 3FNM shows more defined and intense peak reflections than the 2FNM
graph. In addition, the 3FNM has less LixN1-xO impurity phase. This suggests that the powder 3FNM
shows a better crystallinity and purity than 2FNM.
84
(111)
3500
3FNM
2FNM
3000
* LixNi1-xO impurity
500
30
40
(440)
*
*
0
20
*
(331)
(220)
(222)
1000
(511)
1500
(531)
(400)
2000
(311)
Intensity (a.u.)
2500
50
60
70
2  (degree)
Figure 5.24. Comparison between XRD patterns of powders 3FNM and 2FNM.
Figure 5.25 shows the SEM images for the synthesized powder at different magnification. The
sample is not very homogeneous. The particle size ranges from values smaller of 0.5 µm up to
reaches 20 µm.
Figure 5.25. SEM images of powder 3FNM at different magnifications: a) 1000X and b) 5000X.
Depending on the synthesis temperature, two types of space groups can be formed: the cubic
P4332 space group, with a cation ordering and the face – centered cubic Fd3m space group, with a
cation disordering
100
. It has been proved that a high-temperature synthesis (> 700 °C) usually leads to
a LiNi0.5Mn1.5O4 powder with the Fd3m structure. In this structure, Ni and Mn ions are randomly
distributed in the 16d sites, while some of the oxygen is released out of its lattice structure and a small
amount of Mn
4+
ions are reduced to Mn
3+
to balance the charge. Therefore, during charge/discharge
processes a Fd3m spinel structure can be recognized from an additional 4.0 V plateau corresponding
3+/4+
to the Mn
redox couple and from two plateaus at around 4.7 V which corresponding to the redox
reactions of Ni
2+/3+
and Ni
3+/4+
couples. The LiNi0.5Mn1.5O4 with the Fd3m structure exhibits better
85
electrochemical performances than that with the P4332 structure. In addition, as mentioned before, the
Fe-doped samples are characterized from a Fd3m structure because the metal stabilized the structure
with this symmetry
61,63
.
Since the three powders were synthesized at temperature of 800 °C, probably their crystalline
structures have a Fd3m symmetry. Rietveld refinement of the XRD data obtained must be done in
future in order to define lattice parameters.
5.2.3 Electrode preparation
Before to prepare the layers, the synthesized powders were ground to fine powders with a
mortar. The percentage composition of each layer is shown in the following tables.
Li1.02Ni0.5Mn1.5O4
82.0 %
SP
10.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0 %
NM2P
0.5 ml
Table 5.6. Percentage composition of layer 3MNO-1.
LiFe0.08Ni0.42Mn1.5O4
80.0 %
SP
12.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0 %
NM2P
0.5 ml
Table 5.7. Percentage composition of layer 2FNM-1.
LiFe0.08Ni0.42Mn1.5O4
80.0 %
SP
12.0 %
PVdF (Aldrich) (in NM2P 5% wo)
8.0 %
NM2P
0.5 ml
Table 5.8. Percentage composition of layer 3FNM-1.
The slurries were stirred for 6 h with a magnetic stirrer, then stratified on aluminium foil by
Doctor Blade technique at thickness of 200 µm. The obtained layers were dried on a hot plate at 50 °C
under hood to evaporate the NM2P solvent.
86
From each cathode film several electrodes (Ø 9 mm) were cut. These electrodes were pressed
in order to compact the electrodes and to guarantee a good contact among aluminium current
collector, active material and conductive agent. All obtained electrodes were dried overnight at 120 °C
under vacuum.
The electrodes 3MNO-1 have an average loading of active material of 3.88 mg cm
-2
and an
-3
average density of 1.30 g cm . Since the composite active material for these electrodes is
Li1.02Ni0.5Mn1.5O4, their capacities were calculated considering a specific theoretical capacity of 146.7
-1
mA h g .
The electrodes 2FNM-1 have an average loading of active material of 5.76 mg cm
-2
and an
-2
and an
-3
average density of 1.61 g cm .
The electrodes 3FNM-1 have an average loading of active material of 3.27 mg cm
-3
average density of 1.09 g cm . Both for electrodes 2FNM-1 and 3FNM-1 the capacities were
-1
calculated considering a specific theoretical capacity of 148 mA h g .
5.2.4 Electrochemical characterization
In order to characterize electrochemically the synthesized powders, several experiments were
carried out at both room and low temperatures.
The capacity fading for LiNi0.5Mn1.5O4 based high-voltage cathode materials is well known at
high temperatures, but at the best of our knowledge there are no studies in literature on the bahaviour
of these materials at low temperatures. This chapter focuses attention on the low-temperature
behaviour of pristine and Fe-doped LiNi0.5Mn1.5O4 cathode material.
This is a part of a research project between UNICAM and FAAM SpA with the aim to develop
high-voltage cathode materials for Lithium-ion batteries in automotive field.
The choice of the electrolyte solution represents a major factor influencing the performances of
Lithium-ion batteries, especially for particular applications. High specific conductivity over a wide
temperature range is required for quick discharge and recharge at low temperature. The major
approaches for developing conductive electrolyte solutions at lower temperatures than room
conditions involve the use of mixed solvents, in which high dielectric constant, to minimize ion
association, and low viscosity, to assure high conductivities, are required. In fact, some ternary and
quaternary mixtures of solvents are employed for Lithium-ion batteries technology at very low
temperatures. Use of electrolyte additives is one of the most economic and effective methods to
improve the performance of Lithium-ion battery, but their effect is still unclear when high-cathode
materials work at low temperatures
101
.
The electrochemical characterizations for electrodes obtained from the layers 3MNO-1, 2FNM-1
and 3FNM-1, were carried out through T-shaped cells using a galvanostat/potentiostat VMP2/Z by
Bio-Logic. The employed electrolytes were: a 1 M solution of LiPF6 in EC:DMC:DEC 1:1:1 (LP71,
Merck), a 1 M solution of LiPF6 in EC:DMC 1:1 (LP30, Merck) and the electrolyte FAAM, whose
87
composition cannot be disclose due to internal FAAM reasons. Galvanostatic cycles at C/10-rate and
at different C-rates (C/5, C/2, C and 2C) were performed on the different types of electrodes.
The electrochemical tests were carried out over the potential range of 3.5-5 V. Only in few cases, the
potential range was set between 3.5-4.9 V in order to avoid side reactions between active cathode
material and electrolyte
57
+
. All potentials are referred to Li /Li.
5.2.4.1 Electrochemical characterization of layer 3MNO-1
I.
Galvanostatic cycles at C/10-rate
th
Figure 5.26 shows the charge/discharge profiles obtained at C/10-rate, up to the 20 cycle, for
the electrodes 3MNO-1 by using different electrolyte systems. The discharge capacities are lower than
-1
the expected value (~ 130 mA h g ), probably due to small traces of LixN1-xO impurity and the
presence of agglomerates already observed by structural and morphological characterization. As
already observed for the sample 1FNM, the use of the electrolyte FAAM leads to a reduction of the
irreversible capacity at the first cycle. In fact, electrode 3MNO-1 cycled with electrolyte LP30 has a
-1
irreversible capacity value of 66.94 mA h g , while that for electrode 3MNO-1 cycled with electrolyte
-1
FAAM is equal to 28.46 mA h g . This can be ascribed to the use of different types of electrolytes:
probably, the electrolyte FAAM leads to the formation of a more stable SEI on cathode surface, but
further studies should be carried out in order to assess this statement. Moreover, we can observe that
the reversible capacity slightly increases over the first 15 cycles for the electrode cycled with
electrolyte FAAM, whilst decreases after 5 cycles for the electrode cycled with LP30.
5.2
5.2
a)
b)
5.0
4.8
E / V vs. Li /Li
4.6
4.4
4.2
4.6
1st cycle
5th cycle
10th cycle
15th cycle
20th cycle
+
1st cycle
5th cycle
10th cycle
15th cycle
20th cycle
+
E / V vs. Li /Li
4.8
5.0
4.0
4.4
4.2
4.0
3MNO-1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
3.8
3MNO-1
electrolyte FAA001
3.8
3.6
3.6
0
20
40
60
80
100
Specific Capacity / mA h g
120
140
0
-1
20
40
60
80
100
Specific Capacity / mA h g
120
140
-1
Figure 5.26. Charge/discharge profiles at C/10-rate for the electrodes 3MNO-1.
Figure 5.27 shows the trend of charge/discharge capacities vs. cycle number. The trend for the
electrode performed with electrolyte system LP30 shows an increasing in the reversibility of the
electrochemical process by cycling, but, as expected, the capacity slowly decreases with the cycle
88
number. On the other hand, the electrode cycled with electrolyte FAAM shows an increase in the
reversible capacity with the ageing of the system.
180
180
a)
-1
Q charge / mA h g
-1
Q discharge / mA h g
b)
-1
140
Specific Capacity / mA h g
Specific Capacity / mA h g
-1
160
120
100
80
60
3MNO-1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
40
20
-1
Q charge / mA h g
160
-1
Q discharge / mA h g
140
120
100
80
60
3MNO-1
electrolyte FAAM
40
20
0
0
0
2
4
6
8
10
12
14
16
18
20
22
0
2
Cycle number
4
6
8
10
12
14
16
18
20
22
Cycle number
Figure 5.27. Comparison of cycling performance at C/10-rate between electrodes 3MNO-1.
Due to the lack of time related to this thesis period, these data can be considered as preliminary
results in the context of the influence of the electrolyte system of the electrode performances for this
class of cathode materials. However, these results suggest that electrolyte FAAM guarantees a better
cyclability of the electrochemical system than to electrolyte LP30. The reasons of this behaviour are
still not clear and the study of the phenomena occurring at the interface between the electrode and the
electrolyte system will be the subject of a further study in the next-step collaboration between the
Unicam Research Group of Electrochemistry and FAAM R&D department.
II.
Galvanostatic cycles at different C-rates
The experimental protocol used to carry out the C-rates performances of the different electrodes
at room temperature was: 5 cycles at C/5, C/2, C and 2C. In order to verify if the change of electrolyte
system can affect the electrochemical performances of electrodes 3MNO-1, three T-shaped cells were
cycled at different C-rates by using the electrolytes LP71 (EC:DMC:DEC 1:1:1) and the electrolyte
FAAM. The ternary electrolyte LP71 is used hereafter as a commercial term of comparison since lowtemperatures measurements will be introduced. The lower freezing point of LP71 with respect binary
LP30 system allows an investigation of this temperature range without any complication related to a
+
drop of Li mobility because of solvent freezing.
In the case of LP71, we have observed that a longer Open Circuit Voltage (OCV) period, of
about 1 h, before starting the experiment, leads to a better electrochemical cycling of the cells. This
behaviour is probably due to a more complete wetting of the electrode that allows the formation of a
more stable SEI layer. Figure 5.28 highlights the improvement of the cyclability of the cell in which an
OCV period was set before cyclations.
89
3MN0-2-p-C_30cyclesC10_C03.m pr
Ew e vs. time
5
4,8
4,6
Ew e/V vs. L i+/L i
4,4
4,2
4
3,8
3,6
3,4
3,2
0
100.000
200.000
300.000
400.000
500.000
t im e /s
Ew e vs. tim e
3MNO-3-M_cyclesC-rates_01_GCPL_C16.mpr #
3MNO-3-M_cyclesC-rates_02_GCPL_C16.mpr
4,8
4,6
4,4
Ew e/V vs. L i+/L i
4,2
4
3,8
3,6
3,4
3,2
3
0
50.000
100.000
150.000
200.000
t im e /s
Figure 5.28. Comparison between a cell with an OCV of few seconds and a cell with an OCV of about 1 h, both
performed with electrolyte LP71.
Figure 5.29 shows the galvanostatic discharge profiles of the 3
rd
cycle at each C-rate for the
different electrodes. It is possible to see that the electrode cycled with LP71 delivers slightly higher
discharge capacity values at each C-rate compared to that for the electrode cycled with electrolyte
FAAM. In addition, at 2C rate both electrodes present still well-defined plateaus corresponding to the
2+/3+
redox reactions of Ni
and Ni
3+/4+
redox couples. Although the presence of impurity phase influences
the lower discharge capacities, for both cells a good combination of rate capability and capability
retention at room temperature are obtained.
90
a)
5.0
b)
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
4.8
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
4.8
4.6
+
+
E / V vs. Li /Li
4.6
E / V vs. Li /Li
5.0
4.4
4.2
4.0
3.8
4.4
4.2
4.0
3MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3MNO-1
electrolyte FAA001
3.8
3.6
3.6
0
20
40
60
80
100
Specific Capacity / mA h g
120
0
20
-1
40
60
80
Specific Capacity / mA h g
100
120
-1
rd
Figure 5.29. Comparison of discharge capacity profiles of the 3 cycle at each C-rate for a) the electrode cycled
with LP71 and b) the electrode cycled with electrolyte FAAM.
A summary of discharge capacity values of the 3
rd
cycle of each C-rate for tested
electrochemical systems is shown in Table 5.9.
Electrolyte system
LP71
(1M LiPF6 EC:DMC:DEC 1:1:1)
Electrolyte FAAM
rd
112.69 mA h g
-1
105.25 mA h g
-1
rd
110.71 mA h g
-1
107.76 mA h g
-1
105.77 mA h g
-1
103.02 mA h g
-1
3 cycle C/5
3 cycle C/2
rd
3 cycle C
rd
3 cycle 2C
97.93 mA h g
-1
93.33 mA h g
-1
rd
Table 5.9. Summary of discharge capacities for the 3 cycle at each C-rate for electrodes 3MNO-1 cycled with
different electrolytes.
Figure 5.30 shows the trends of charge/discharge capacities vs. cycle number for the different
electrode/electrolyte combinations and the variation of columbic efficiency at different C-rates.
-1
The electrode cycled with LP71 has a higher irreversible capacity value (50.82 mA h g )
-1
compared with that of electrode cycled with electrolyte FAAM (25.06 mA h g ). This behaviour has
been already observed with the same composite cathode material, but with different electrochemical
test and also for the electrode 1FNM-1. Also in this case, probably, the phenomenon can be ascribed
to the electrolyte FAAM which leads to the formation of a more stable SEI on cathode surface, but
further studies should be carried out in order to assess this statement. However, no remarkable
difference in terms of cyclability and reversibility can be evidence for both electrodes. In regard of
coulombic efficiency (the percentage ratio between discharge capacities and charge capacities), these
two electrodes maintained an efficiency, at various C-rates, close to 100 %.
91
100
160
b)
a)
140
140
-1
120
C
2C
80
40
60
-1
3MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
20
Q charge / mA h g
-1
Q discharge / mA h g
Efficiency %
20
0
2
4
6
8
10
12
14
16
18
20
80
C
2C
100
60
80
40
60
3MNO-1
electrolyte FAAM
40
-1
Q charge / mA h g
-1
Q discharge / mA h g
Efficiency %
20
0
0
C/2
120
20
0
22
Efficiency %
60
Efficiency %
100
40
C/5
80
C/2
Specific Capacity / mA h g
-1
C/5
Specific Capacity / mA h g
100
160
0
0
2
4
6
8
Cycle number
10
12
14
16
18
20
22
Cycle number
Figure 5.30. Comparison of cycling performance at various C-rates for a) the electrode cycled with LP71 and b)
the electrode cycled with electrolyte FAAM.
Based on these results, we can conclude that the electrolyte FAAM is a more suitable choice for
room-temperature cycling performances of Li1.02Ni0.5Mn1.5O4 cathode material for Lithium-ion batteries.
The electrolyte LP71 resulted a good choice if only the electrochemical system was left in an longer
OCV period to ensure a good contact between electrode surface and electrolyte. This aspect will
require further investigations to clarify the side reactions taking place at OCV on the electrode surface.
III.
Galvanostatic cycles at different C-rates at low temperatures
Galvanostatic cycles have been performed at different C-rates at low temperatures (0 °C and 20 °C) as well. At each change of temperature, the electrochemical systems were left at an OCV
period of 4 h in order to allow equilibration.
The discharge galvanostatic profiles at C/5-rate (insertion of lithium ions in the spinel structure)
at different temperatures are compared in Figure 5.31. In particular, the third cycle at C/5-rate of each
temperature was taken in consideration. As it is possible to see, the electrode shows well-defined
profiles and good discharge capacity values at each temperature.
92
5.0
T = 20 °C
T = 0 °C
T = - 20 °C
4.8
+
E / V vs. Li /Li
4.6
4.4
4.2
4.0
3.8
3MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.6
0
20
40
60
80
Specific Capacity / mA h g
100
120
-1
rd
Figure 5.31. Comparison of discharge capacities of the 3 cycle at C/5-rate at 20 °C, 0°C and -20 °C for
electrode 3MNO-1.
In order to evaluate, in more detail, which processes occur in this electrochemical system and to
estimate the polarization variation induced by the temperature, the differential analysis dQ/dE vs. E of
galvanostatic profiles at C/5-rate was performed.
In according with the relation V = I * R, if the polarization increases also the voltage separation
between anodic and cathodic peaks increases. This is mainly evidenced in Figure 5.32 by a shift of
-1
-1
dQ / dE (mA h g V )
the oxidation peaks toward more anodic potentials when the temperature is reduced to 0 and -20 °C.
1600
1400
1200
1000
800
600
400
200
0
-200
-400
-600
-800
-1000
-1200
-1400
-1600
-1800
T = 20 °C
T = 0 °C
T = - 20 °C
3MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
+
E / V vs. Li /Li
rd
Figure 5.32. dQ/dE vs. E curves at C/5-rate (3 cycle) at 20 °C, 0 °C and -20 °C for electrode 3MNO-1.
93
Figure 5.33 shows the cycling performance of electrode 3MNO at temperatures of 20 °C, 0 °C
and -20 °C and when the system is brought back at 20 °C. As it is possible to see, the reversible
capacity decreases quickly as the temperature decreases, while the whole capacity is recovered when
the T = 20 °C condition is restored. The decrease of capacity at lower temperatures and higher Crates typically described an increase of polarization. In addition, the presence of impurities could
enhance this polarization effect. However these results are only preliminary and they should be further
proved through EIS studies.
160
Q charge / mA h g
-1
Q discharge / mA h g
-1
140
Specific Capacity / mA h g
-1
C/5
120
C/5
C/2
C
100
C/2
C/5
2C
C
2C
C/2
C/5
C
80
60
2C
40
20
0
C/2
3MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
T = 20 °C
0
10
30
2C
T = 20 °C
T = -20 °C
T = 0 °C
20
C
40
50
60
70
80
Cycle number
Figure 5.33. Cycling performance for electrode 3MNO-1.
94
90
5.2.4.2 Electrochemical characterization of layer 2FNM-1
The electrochemical performances of 2FNM-1 cathode (Fe-doped LiNi0.5Mn1.5O4 prepared
according to synthesis in Section 5.2.1.II) were tested by using galvanostatic cycles at C/10-rate at
room temperature. Through these experiments also the compatibility of different electrolyte systems at
high-operating voltages was tested.
I.
Galvanostatic cycles at C/10-rate at room temperature
st
rd
th
th
Figure 5.34 compares the 1 , 3 , 5 and 10 galvanostatic charge/discharge cycles at C/10rate for electrode tested with different electrolyte systems. Even if the performances are comparable
among these electrochemical systems, the electrode 2FNM-1 cycled with electrolyte FAAM delivers
the highest discharge capacity values. Since also for this synthesized spinel active cathode material
the structural and morphological characterization revealed the presence of impurity phases, though
the presence of Fe doping material, and bad particle size distribution, the discharge capacities are
-1
lower than the expected values (~ 140 mA h g ). However, all electrodes show a good capacity
retention. The first charge profile of the electrode 2FNM-1 cycled with LP30 reveals some stability
st
issues the first charge process. Since these issues occur only during 1 cycle, it is suggested that they
are related to the formation of an unstable interfacial layer between the high-voltage cathode and the
LP30 electrolyte.
Figure 5.35 compares the cycling stabilities of the electrode 2FNM-1 in the electrolyte systems
under investigation. The electrode 2FNM-1 cycled with LP30 shows the higher irreversible capacity
-1
value (114.20 mA h g ) among tested electrodes: this confirmed that LP30 has some problems related
st
to the SEI layer formation during 1 charge process. The electrode 2FNM-1 cycled with electrolyte
-1
FAAM has an irreversible capacity value equal to 23.99 mA h g , whereas the electrode 2FNM-1
-1
cycled with LP71 has an irreversible capacity value of 30.67 mA h g . Cells cycled with electrolytes
FAAM and LP71 demonstrate a better reversibility of the lithium insertion/de-insertion mechanism over
one cycled with electrolyte LP30.
95
5.2
a)
5.0
4.6
+
E / V vs. Li /Li
4.8
1st cycle
3rd cycle
5th cycle
10th cycle
4.4
4.2
4.0
2FNM-1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
3.8
3.6
0
20
40
60
80
Specific Capacity / mA h g
100
120
-1
5.2
b)
5.0
4.6
1st cycle
3rd cycle
5th cycle
10th cycle
+
E / V vs. Li /Li
4.8
4.4
4.2
4.0
2FNM-1
electrolyte FAAM
3.8
3.6
0
20
40
60
80
Specific Capacity / mA h g
100
120
-1
5.2
c)
5.0
4.6
1st cycle
3rd cycle
5th cycle
10th cycle
+
E / V vs. Li /Li
4.8
4.4
4.2
4.0
2FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.8
3.6
0
20
40
60
80
Specific Capacity / mA h g
100
120
-1
Figure 5.34. Charge/discharge profiles at C/10-rate for a) the electrode cycled with LP30, b) the electrode cycled
with electrolyte FAAM and c) the electrode cycled with LP71.
96
220
a)
220
b)
200
Q charge / mA h g
-1
Q discharge / mA h g
-1
160
140
120
100
80
2FNM-1
electrolyte LP30
(1 M LiPF6 EC:DMC 1:1)
60
40
-1
Q charge / mA h g
-1
Q discharge / mA h g
180
160
Specific Capacity / mA h g
-1
180
Specific Capacity / mA h g
200
-1
140
120
100
80
60
2FNM-1
electrolyte FAAM
40
20
20
0
0
0
2
4
6
8
10
0
2
4
Cycle number
c)
6
8
10
Cycle number
220
200
-1
Q charge / mA h g
-1
Q discharge / mA h g
Specific Capacity / mA h g
-1
180
160
140
120
100
80
2FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
60
40
20
0
0
2
4
6
8
10
Cycle number
Figure 5.35. Comparison of cycling performance at C/10-rate for a) the electrode cycled with LP30, b) the
electrode cycled with electrolyte FAAM and c) the electrode cycled with LP71.
II.
Conclusions
For these electrodes, characterize in this section, the optimization of electrode composition (12
% vs. 10 % conductive agent in the mixture), was useless. In fact, the Fe-doped powder was
synthesized by using a quantity of citric acid in defect relative to the total metal content that did not
complex all metals. This is the cause of low discharge capacity values obtained for this material. The
same problem appeared also for the Li1.02Ni0.5Mn1.5O4 powder which was prepared following the same
synthetic route.
In order to eliminate these issues, the sol-gel synthesis for Fe-doped sample was repeated
using a citric acid : total metal content ratio of 1 : 1.
97
5.2.4.3 Electrochemical characterization of layer 3FNM-1
The electrochemical performances of the electrode 3FNM-1, preparing according to the
procedure described in Section 5.2.1.III, were tested by using galvanostatic cycles at different C-rates
both at room and lower temperatures (0 °C and -20 °C). The used protocol includes 5 cycles at C/5,
C/2, C and 2C. At room temperature, we used LP71 and electrolyte FAAM as electrolyte systems,
while at low temperatures only electrolyte LP71 was used. In addition, a comparison between pristine
and Fe-doped materials was carried out in order to evaluate the increase of electrochemical
performances due to the Fe-doping of spinel cathode materials.
I.
Galvanostatic cycles at different C-rates
A comparison between the electrochemical behaviours of 3FNM-1 electrodes in cells containing
FAA01 and LP71 electrolytes was carried out, in order to investigate the possible electrochemical
performance differences due to the change of electrolyte solution.
rd
Figure 5.36 shows the galvanostatic discharge profiles of the 3 cycle of each C-rate for both
cells. For both systems the plateaus corresponding to the redox processes are well defined, also at
high C-rates. However, the electrode 3FNM-1 cycled with LP71 demonstrates a better rate capability
and higher discharge capacity values at different C-rates than the electrode cycled with electrolyte
FAAM.
In order to highlight the different electrochemical performances of these systems, a summary of
rd
discharge capacity values of the 3 cycle at each C-rate is shown in Table 5.10. As it is possible to
observe from the values in table, at 2C the electrode cycled with LP71 delivers a discharge capacity
-1
value higher than 100 mA h g , whereas the electrode cycled with electrolyte FAAM delivers only
-1
86.69 mA h g .
5.2
5.2
a)
4.8
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
5.0
4.8
4.6
+
E / V vs. Li /Li
4.6
+
E / V vs Li /Li
b)
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
5.0
4.4
4.2
4.4
4.2
4.0
4.0
3FNM-1
electrolyte FAAM
3.8
3.8
3.6
3FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.6
0
20
40
60
80
100
Specific capacity / mA h g
120
140
0
-1
20
40
60
80
100
Specific Capacity / mA h g
120
140
-1
rd
Figure 5.36. Comparison of discharge capacity profiles of the 3 cycle at each C-rate for a) the electrode 3FNM-1
cycled with electrolyte FAAM and b) the electrode 3FNM-1 cycled with LP71.
98
Electrolyte system
rd
rd
LP71
Electrolyte FAAM
3 cycle C/5
3 cycle C/2
rd
3 cycle C
rd
(1M LiPF6 EC:DMC:DEC 1:1:1)
119.19 mA h g
-1
134.33 mA h g
-1
114.04 mA h g
-1
133.84 mA h g
103.02 mA h g
-1
131.10 mA h g
-1
-1
-1
3 cycle 2C
-1
86.69 mA h g
126.01 mA h g
rd
Table 5.10. Summary of discharge capacities for the 3 cycle at each C-rate, at room temperature, for electrode
cycled with electrolyte FAAM and with LP71.
The
table
above
points
out
that
the
active
material
3FNM,
corresponding
to
LiFe0.08Ni0.42Mn1.5O4, synthesized by sol-gel method with a citric acid : metals ratio of 1 : 1 (different
than the 3MNO and 2FNM, prepared with a citric acid : metals ratio lower than 1 : 1) delivers
-1
discharge capacity values very close to the expected values, around 140 mA h g .
Figure 5.37 shows the trends of charge/discharge capacities vs. cycle number for those
electrodes in order to highlight their cycling performance and the variation of columbic efficiency at
different C-rates.
The electrode 3FNM-1 cycled with LP71 has a higher irreversible capacity value (48.73 mA h g
1
-
-1
) compared with that of electrode cycled with electrolyte FAAM (31.17 mA h g ). This result confirms
the results previously obtained. Both cells exhibit a better electrochemical behavior than those
prepared with the 2FNM active material. However, the electrode 3FNM-1 cycled with LP71 shows a
better cyclability and reversibility of electrochemical processes than the performances of electrode
3FNM-1 cycled with electrolyte FAAM. Both cells exhibit lower efficiencies at the slowest C-rate. This
phenomenon could be explained by assuming that at lower charge/discharge rates the parasitic
reactions that lead to interfacial irreversible processes are kinetically favoured, while at higher rates
+
the charge is totally used to sustain the reversible processes related to Li ion intercalation. However,
further studies are needed to validate this hypothesis. Nevertheless, from C/2-rate to 2C-rate the
efficiency for both cells is close to 100 %.
100
a)
100
200
180
b)
-1
C
120
60
2C
100
80
40
60
-1
Q charge / mA h g
-1
Q discharge / mA h g
Efficiency %
3FNM-1
electrolyte FAA001
20
20
0
2
4
6
8
10
12
14
16
18
C/2
C
140
2C
60
120
100
40
80
60
3FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
40
-1
Q charge / mA h g
-1
20
Q discharge / mA h g
Efficiency %
20
0
0
80
C/5
160
0
20
0
0
Cycle number
Efficiency %
Specific Capacity / mA h g
C/2
40
180
80
C/5
140
Efficiency %
Specific Capacity / mA h g
-1
160
2
4
6
8
10
12
14
16
18
20
22
Cycle number
Figure 5.37. Comparison of cycling performance at various C-rates for a) the electrode 3FNM-1 cycled with
electrolyte FAAM and b) the electrode 3FNM-1 cycled with LP71.
99
The introduction of Fe in spinel LiNi0.5Mn1.5O4 increases the operating voltage of the cell. While
the stoichiometry of the prepared cathode could not be verified by elemental analysis, the XRD pattern
(Figure 5.23) suggests that the iron is present in the structure thank to the disappearance of impurity
phase (the substitution of Fe for Ni alone enhances the degree of crystallinity).
Through the differential analysis (dQ/dE vs. E) it is possible to observe small redox peak in the
5 V region corresponding to the electrochemical activity of Fe
3+/4+
couple
102
. In Figure 5.38 an
example is reported.
2000
4.76
LiFe0.08Ni0.42Mn1.5O4
-1
-1
dQ / dE (mA h g V )
1500
1000
4.69
500
Fe
3+/4+
0
-500
-1000
4.68
4.74
-1500
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
+
E / V vs. Li /Li
Figure 5.38. dQ/dE vs. E voltage curve of LiFe0.08Ni0.42Mn1.5O4.
II.
Galvanostatic cycles at different C-rates at low temperatures
In this temperature-dependent experiment, only the electrode 3FNM-1 cycled with LP71 could
operate without any issues, while the cells assembled by using electrolyte FAAM underwent repeated
electrode failure. This suggests that electrolyte FAAM is not the most suitable electrolyte system for
Fe-doped spinel cathode materials, probably because of electrode/electrolyte interface instability,
differently from that seen for pristine material.
The electrode 3FNM-1 was cycled for 5 cycles at C/5, C/2, C and 2C at the temperatures 20 °C,
0 °C, -20 °C (an OCV period of 4 h was set at each change of temperature value). After that, the
electrochemical system was brought back at room temperature condition. At room temperature, the
cycling protocol at various rates was repeated, and in addition the cell was submitted at higher Crates: 5C and 10 C-rates. Figure 5.39 shows the cycling performance of electrode 3FNM-1 cycled
with LP71. As it is possible to see, when the cell was brought back at 20 °C, the initial
charge/discharge performances are almost entirely recovered, with a slight decrease probably due to
the aging of the system. At higher C-rates (5C and 10C) the discharge capacities become very low
-1
(about 20 mA h g at the highest cycling rate).
100
The dQ/dE vs. E differential profiles for the 3
rd
cycle at C/5-rate, for each temperature, are
shown in Figure 5.40. A small separation between anodic and cathodic peaks of Ni
2+/3+
3+/4+
and Ni
redox couples is shown at each temperature. This trend suggests that at lower temperatures neither
the lithium ion mobility nor the insertion/de-insertion rate of lithium into/from the spinel structure of the
material are affected in a relevant way. Only an almost negligible shift of the reduction peaks to more
cathodic values can be observed at lower temperatures.
220
-1
Q charge / mA h g
-1
Q discharge / mA h g
200
Specific Capacity / mA h g
-1
180
C/5
C/2
160
C
140
2C
C/5
C/2
120
C
C/5
2C
C/5
C/2
C/2
C
C
2C
2C
100
80
60
40
5C
3FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
10C
20
T = 20 °C
T = 0 °C T = -20 °C
T = 20 °C
0
0
10
20
30
40
50
60
70
80
90
100
Cycle number
Figure 5.39. Cycling performance for electrode 3FNM-1.
2000
T = 20 °C
T = 0 °C
T = -20 °C
-1
-1
dQ / dE (mA h g V )
1500
1000
500
0
-500
-1000
3FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
-1500
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
+
E / V vs. Li /Li
rd
Figure 5.40. dQ/dE vs. E curves at C/5-rate (3 cycle) at 20 °C, 0 °C and -20 °C for electrode 3FNM-1.
101
Figure 5.41 shows outstanding charge/discharge performances of electrode 3FNM-1 from 20
°C to -20 °C. Excellent cyclability is retained both at lower temperatures and higher C-rates In fact, the
-1
cell can deliver a capacity higher than 100 mA h g also at 2C-rate and at -20 °C.
-1
200
Q charge / mA h g
-1
Q discharge / mA h g
Specific Capacity / mA h g
-1
180
C/5
160
C/2
140
C
C/5
2C
C/2 C
120
2C
C/5 C/2
C
2C
100
80
3FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
60
40
20
T = 20 °C
T = 0 °C
T = -20 °C
0
0
10
20
30
40
50
60
Cycle number
Figure 5.41. Cycling performance of electrode 3FNM-1 at 20 °C, 0 °C and -20 °C.
III.
Electrochemical comparison between pristine and Fe-doped 5 V spinel
cathode materials
In order to demonstrate the improvement of electrochemical performances of Fe-doped spinel
LiNi0.5Mn1.5O4 both at room and low temperatures, the main electrochemical results obtained with both
cathodes are here compared.
Figure 5.42 compares the discharge profiles (3
rd
cycle) of pristine and Fe-doped samples at
various C-rates, both cycled with electrolyte LP71. The Fe-doped sample exhibits much higher rate
capability than the pristine material. For instance, while the 3MNO-1 material exchanges 97.23 mA h
-1
-1
g at 2C, Fe-doped sample delivers a remarkable capacity of 126 mA h g at 2C.
102
5.2
5.2
a)
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
T = 20 °C
5.0
3rd cycle C/5
3rd cycle C/2
3rd cycle C
3rd cycle 2C
T = 20 °C
5.0
4.8
4.6
+
E / V vs. Li /Li
4.6
+
E / V vs. Li /Li
4.8
b)
4.4
4.2
4.0
4.2
4.0
3MNO-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.8
4.4
3FNM-1
electrolyte LP71
(1 M LiPF6 EC:DMC:DEC 1:1:1)
3.8
3.6
3.6
0
20
40
60
80
100
Specific Capacity / mA h g
120
140
0
20
40
-1
60
80
100
Specific Capacity / mA h g
120
140
-1
Figure 5.42. Discharge profiles of the pristine and Fe-substituted samples at various C-rates.
Figure 5.43 compares the dQ/dE vs. E curves of pristine and LiFe0.08Ni0.42Mn1.5O4 for the third
cycle at C/5 rate. The much smaller potential separation between the anodic and cathodic peaks of
2+/3+
Ni
3+/4+
- Ni
redox couples in LiFe0.08Ni0.42Mn1.5O4 compared to that in Li1.02Ni0.5Mn1.5O4 suggests
faster lithium insertion/de-insertion kinetics in the former, and hence a lower polarization. In addition,
small redox peaks appear around 4.9 V only in the case of Fe-substituted sample, which have been
attributed to the electrochemical activity of Fe
3+/4+
redox couple
63
.
A more detailed description of polarization phenomena for both cathode materials requires
further studies, such as electrochemical impedance spectroscopy (EIS) experiments.
2000
LiFe0.08Ni0.42Mn1.5O4
4.76 V
Li1.02Ni0.5Mn1.5O4
1500
4.72 V
-1
-1
dQ / dE (mA h g V )
4.77 V
4.69 V
1000
500
Fe
3+/4+
0
-500
T = 20 °C
-1000
4.67 V
-1500
4.70 V
3.6
3.8
4.0
4.2
4.4
4.6
4.74 V
4.74 V
4.8
5.0
5.2
+
E / V vs. Li /Li
Figure 5.43. dQ/dE vs. E voltage curve of Li1.02Ni0.5Mn1.5O4 and LiFe0.08Ni0.42Mn1.5O4.
If the dQ/dE vs. E profiles of pristine and Fe-doped material cycled with electrolyte LP71 are
compared with the dQ/dE vs. E profiles of pristine and Fe-doped material cycled with electrolyte
FAAM, we can evaluate possible differences between the two behaviours. Figure 5.44 Panel a)
103
shows the dQ/dE profiles for pristine and Fe-doped materials cycled with LP71 while Panel b) shows
the dQ/dE profiles for pristine and Fe-doped materials cycled with electrolyte FAAM. It is possible to
2+/3+
see that for both cells cycled with LP71 the peaks’ intensity for the redox reactions of Ni
3+/4+
and Ni
redox couples is higher than the cells cycled with electrolyte FAAM. However no remarkable
difference can be noted. This means that, at room temperature, the type of used electrolyte does not
affect on the kinetics of lithium ions insertion/de-insertion.
2000
a)
LiFe0.08Ni0.42Mn1.5O4
4.76 V
Li1.02Ni0.5Mn1.5O4
1500
1000
-1
dQ / dE (mA h g V )
4.69 V
T = 20 °C
-1
4.77 V
4.72 V
Fe
500
3+/4+
0
-500
-1000
4.67 V
Electrolyte LP71
(EC:DMC:DEC 1:1:1)
-1500
4.70 V
3.6
3.8
4.0
4.2
4.4
4.6
4.74 V
4.74 V
4.8
5.0
5.2
+
E / V vs. Li /Li
b)
2000
-1
-1
dQ / dE (mA h g V )
4.76 V
LiFe0.08Ni0.42Mn1.5O4
Li1.02Ni0.5Mn1.5O4
1500
1000
4.69 V
T = 20 °C
500
4.76 V
4.71 V
Fe
3+/4+
0
-500
4.69 V
-1000
Electrolyte FAAM
4.68 V
4.73 V
4.74 V
-1500
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
+
E / V vs. Li /Li
Figure 5.44. Comparison of dQ/dE vs. E curves between pristine and Fe-doped samples cycled with the
electrolyte a) LP71 and b) electrolyte FAAM.
Figure 5.45 compares the cycling performance of pristine and Fe-doped samples at different
temperatures. The Fe-substituted sample (3FNM) offers a combination of higher capacity and
excellent cycleability both at room and low temperatures in comparison with the pristine, un-doped,
material.
104
C/5
140
C/2
C
C/5
2C
C/2
C/5
C
2C
C/2
C
Specific Capacity / mA h g
-1
120
2C
100
80
60
3FNM
3MNO
40
20
T= 20 °C
T= 0 °C
T= -20 °C
0
0
5
10
15
20
25
30
35
40
45
50
55
60
Cycle number
Figure 5.45. Comparison of discharge cycling performances between pristine and Fe-substituted samples at
different temperatures.
5.2.4.4 Conclusions and future developments
Thank to the optimization of sol-gel synthesis for Fe-doped spinel cathode material, of
stoichiometry LiFe0.08Ni0.42Mn1.5O4, encouraging electrochemical results were obtained. It has been
possible to conclude that the most suitable citric acid : metal content ratio to synthesize this type of
cathode material is 1:1. Following this expedient, all metals can take part to the citrate matrix that
build-up the gel precursor, and then be introduced into the spinel structure formed after calcination
and finally participate to the redox reactions. The substitution of Fe for Ni alone seems is in agreement
with all the results reported in literature
63
: i) elimination of impurity phase, ii) stabilization of the
structure with space group Fd3m (this feature is visible from galvanostatic curves of cycled cells,
because they show two typical plateus, but this structural characteristic will have to be proved by XRD
phase analysis and refinement), iii) the reduction of polarization of the system at higher C-rates. It is
possible to asses, thank to the experimental results, that LiFe 0.08Ni0.42Mn1.5O4 composition offers a
combination of high capacity, excellent rate capability and excellent cyclability.
In regard on the electrolyte systems tested all over the thesis work, it was possible to observe a
strong dependence of charge/discharge behaviour on the different kinds of electrolyte. This is
probably related to different formation processes of electrode/electrolyte interfaces with different
stabilities, that strongly affect the reversibility of charge/discharge processes.
Particularly, it was possible to observe that the electrolyte FAAM revealed itself as a suitable
electrolyte for pristine, un-doped, spinel cathode material but not for the Fe-doped one. On the
105
contrary, the ternary electrolyte LP71 behaved as the most appropriate electrolyte system for the Fedoped cathode, both at room and low temperatures.
Finally, it should be pointed out that the results presented in this thesis work make up only a
preliminary investigation of the interactions of high-voltage spinel cathodes with different electrolyte
systems, and therefore further studies need to be carried out. In particular, EIS analysis is needed to
characterize the polarization phenomena of pristine and Fe-doped samples; XPS of both pristine and
Fe-doped samples to reveal the oxidation states of metals in spinel structure; analysis of the
components of SEI layers on cathode material to describe the interfacial electrode/electrolyte
irreversible processes.
106
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