ELECTROCHEMICAL KINETICS STUDIES OF COPPER ANODE MATERIALS
IN LITHIUM ION BATTERY ELECTROLYTE
A dissertation presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Doctor of Philosophy
Mingming Xu
June 2005
This dissertation entitled
ELECTROCHEMICAL KINETICS STUDIES OF COPPER ANODE MATERIALS
IN LITHIUM ION BATTERY ELECTROLYTE
by
MINGMING XU
has been approved for
the Department of Chemistry and Biochemistry
and the College of Arts and Sciences by
Howard D. Dewald
Professor of Chemistry
Leslie A. Flemming
Dean, College of Arts and Sciences
XU, MINGMING. Ph.D. June 2005. Analytical Chemistry.
Electrochemical Kinetics Studies of Copper Anode Materials in Lithium Ion Battery
Electrolyte (108 pp.)
Director of Dissertation: Howard D. Dewald
The kinetic processes at the interface of uncoated copper foil or graphitecoated copper (Cu-C) foil anodes and lithium ion battery electrolyte were studied in
this dissertation through different electrochemical approaches.
First, as part of an intrinsic stability study of copper foil in Li ion battery
electrolyte solutions, the effect of trace amounts of HF in the electrolyte was studied.
The open circuit voltage (OCV) results of HF doped electrolyte, along with other
OCV results from previous studies, indicate that the adsorption of HF on the graphite
coating protected the Cu foil beneath the coating from corrosion.
Controlled-potential electrolysis of copper foil and graphite-coated copper foil
electrodes in a typical lithium ion battery electrolyte was then performed in order to
construct Tafel plots to obtain values of the exchange current, i0, and charge transfer
coefficient, α. The charge transfer coefficients of both electrodes were found to be
small (α = ~0.25), which was consistent with an assumption of a dominant anodic
process in the cell. At 25 ˚C the graphite-coated copper foil was found to have a higher
exchange current than the copper foil. This can be explained by the intercalation of
lithium ions into the graphite coating which increases the electron transfer rate. In the
range of 0 ˚C to 50 ˚C, the exchange currents of both electrodes increased with
temperature, but at different rates, while the charge transfer coefficients were not
significantly affected by temperature.
Finally, the electrochemical impedance spectroscopy (EIS) of copper foil and
graphite-coated copper foil electrodes was studied. It was found that both electrodes
gave similar impedance spectra of two successive semicircular arcs. Detailed studies
of the effects of different cell parameters including overpotential (η), temperature and
graphite-coating saturation time were then performed. Based on the impedance spectra
results of the electrodes, it appeared that the high frequency response represented a
surface oxide layer. At low frequency further oxidation occurs at both electrodes, but
is kinetically controlled for bare copper, while the graphite-coated copper undergoes
diffusional blocking through the porous carbon layer. An equivalent circuit of the
impedance spectrum was also proposed.
Approved:
Howard D. Dewald
Professor of Chemistry
Acknowledgments
My deepest gratitude goes to my advisor Dr. Howard D. Dewald, not only for
his guidance through the course in preparation of this dissertation, but also for all his
help and care during all these years. His confidence in me always encourages me to do
better. His patience, accessibility and guidance are one of the best memories I have for
the time I spend in Ohio University. I feel so fortunate for choosing him as my advisor
years ago.
I also would like to express my sincere appreciation to my research committee,
Dr. Wenjia Chen, Dr. Tadeusz Malinski, Dr. Bruce McCord and Dr. Frederick Lemke
for their invaluable and professional aid to me.
I am gratitude to Ohio University and Department of Chemistry and
Biochemistry to have given me the opportunity and the financial support to pursue a
doctoral degree. The Condensed Matter and Surface Science Program of Ohio
University also provided me a quarter of scholarship.
I would like to thank all of the members of the Department. My special thanks
shall go to Dr. Frederick Lemke for the use of the dry box. Dr. Mingchuan Zhao,
Qingzhou Cui, Junfeng Huang and Xueguang Jiang are thanked for their friendship
and peer advice.
Ms. Li Wang is specially thanked for all her heart-warming help and support
through the years.
Finally, I want to thank my parents. Although we are separated by distance, I
can still feel their support every minute of every day. Without that, I would not be able
to earn this degree.
vii
Table of Contents
Abstract ........................................................................................................................ iii
Acknowledgments ........................................................................................................ v
Table of Contents ....................................................................................................... vii
List of Tables ................................................................................................................ x
List of Figures............................................................................................................... xi
Abbreviations ............................................................................................................. xv
Chapter 1 - Brief Review of Lithium-Ion Batteries and Motivation for
This Work ..................................................................................................................... 1
1.1 General Concepts ............................................................................................. 2
1.2 Review of Lithium Batteries ........................................................................... 6
1.2.1 Lithium Metal Batteries ..................................................................................... 6
1.2.2 Lithium Ion batteries ......................................................................................... 8
1.3 Research Motivation ........................................................................................... 12
Chapter 2 - HF Impurity Studies in Lithium Ion Battery Electrolyte........................... 17
2.1 Introduction ................................................................................................... 17
2.2 Materials and Reagents .................................................................................. 18
2.3 Electrochemical Cell Assembly .................................................................... 19
2.4 Instrumentation .............................................................................................. 21
2.5 Procedure ....................................................................................................... 23
viii
2.6 Results and Discussion .................................................................................... 23
2.7 Conclusions ..................................................................................................... 29
Chapter 3 - Controlled-potential Electrolysis Studies of Copper Foil and
Graphite-Coated Copper Foil Electrodes in Lithium-ion Battery Electrolyte............. 30
3.1 Introduction . ................................................................................................. . 30
3.2 Experimental Section ..................................................................................... 32
3.3 Results and Discussion .................................................................................. 33
3.4 Conclusions ................................................................................................... 41
Chapter 4 - Impedance Studies of Copper Foil and Graphite-Coated Copper
Foil Electrodes in Lithium-ion Battery Electrolyte .................................................... 42
4.1 Introduction ......................................................................................................... 42
4.2 Experimental Section .......................................................................................... 44
4.3 Results and Discussion ....................................................................................... 45
4.3.1 Impedance at Room Temperature and Open Circuit Voltage ........................... 45
4.3.2 Impedance at Different Overpotentials ............................................................ 52
4.3.3 Impedance of Graphite-coated Copper Foil Electrode with
Different Saturation Time in the Electrolyte............................................................... 66
4.3.4 Impedance at Different Temperatures............................................................... 68
4.3.5 Impedance of Graphite-coated Copper Foil Electrode at
Different Temperatures After Saturation ........................................................ 78
ix
4.4 Conclusion ..................................................................................................... 84
Chapter 5 - Summary and Future Work ...................................................................... 86
5.1 Summary and Conclusions ............................................................................ 86
5.2 Suggestions for Future Research ................................................................... 88
References .................................................................................................................. 89
x
List of Tables
Table 1-1.
Comparison of performance parameters of different batteries. ............... 5
Table 1-2.
Comparison of the performance parameters of lithium, cadmium
and zinc anodes. ....................................................................................... 7
Table 1-3.
Ionic conductivity of some organic liquid electrolytes (1M). ................ 13
Table 2-1.
Dynamic range of experimental parameters of
CHI-604A Electrochemical analyzer. ................................................... 22
Table 3-1.
Linear regression results of Tafel plot of Cu foil and
Cu-C foil electrode in 1M LiPF6 in PC:EC:DMC [1:1:3 vol]................ 38
Table 3-2.
Linear regression analysis of Tafel plots, exchange current
and transfer coefficient of Cu foil and Cu-C foil electrode in 1M
LiPF6 in PC:EC:DMC [1:1:3 vol]. ......................................................... 40
Table 4-1.
Comparison of peak impedance value of both copper foil
and graphite coated copper foil electrode in the kinetic controlled
process. .................................................................................................. 56
Table 4-2.
Comparison of peak impedance value of both copper foil
and graphite coated copper foil electrode at different temperature. ....... 72
Table 4-3.
Values of equivalent circuit parameters for both copper foil
and graphite coated copper foil electrodes at different temperatures..... 77
Table 4-4.
Comparison of peak impedance value of graphite-coated
copper foil electrode after 48 h of electrolyte saturation
at different temperatures. ....................................................................... 81
xi
List of Figures
Figure 1-1. Schematic description of the charge-discharge process of a
lithium-ion cell. ....................................................................................... 4
Figure 2-1. The schematic of the homemade cell. Electrolyte solutions
flooded just above the 1×1 cm2 portion of Cu foil when
the cell was assembled in a dry box. ..................................................... 20
Figure 2-2. OCV vs. time of Cu-C foil electrode in 1 M LiPF6/ PC:EC:DMC
(1:1:3 vol.). Reference electrode was made fresh. ................................. 25
Figure 2-3. Comparison of the initial part of the OCV of Al-C and Cu-C
in 1 M LiPF6/ PC:EC:DMC (1:1:3 vol.). ............................................... 26
Figure 2-4. OCV vs. time of Cu-C foil electrode in 1 M LiPF6/ PC:EC:DMC
(1:1:3 vol.) doped with (a) 500 ppm HF and (b) 1000 ppm HF. ........... 28
Figure 3-1. Electrolysis of copper foil electrode in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.] at 25 ˚C; Initial potential at OCV
= 3.40 V; Overpotential from 50mV to 350 mV. .................................. 35
Figure 3-2. Electrolysis of graphite-coated copper foil electrode in
1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at 25 ˚C;
Initial potential at OCV = 3.39 V; Overpotential from 50mV to
350 mV. ................................................................................................. 36
Figure 3-3. Tafel plots of (a) copper foil electrode; and (b) graphitecoated copper foil electrode in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.]. ............................................................................................ 37
xii
Figure 4-1. Nyquist plot of the impedance spectroscopy of copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]; Initial
potential at OCV = 3.30 V; Frequency range 100 kHz to
0.01 Hz. ................................................................................................. 47
Figure 4-2. Nyquist plot of the impedance spectroscopy of graphitecoated copper foil electrode in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.]; Initial potential at OCV = 3.29 V; Frequency
range 100 kHz to 0.01 Hz. ..................................................................... 48
Figure 4-3. Microscopic image of copper foil electrode surface (200×)
a) before experiment and b) 24 h after experiment.
Scale as shown in a). ............................................................................. 50
Figure 4-4. Microscopic image of graphite-coated copper foil electrode
surface (200×) a) before experiment and b) 24 h
after experiment. Scale as shown in a). ................................................. 51
Figure 4-5. Nyquist plot of the impedance spectroscopy of copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different
overpotentials (OCV–100 mV to OCV+100 mV); Frequency
range 100 kHz to 0.01 Hz.. .................................................................... 53
Figure 4-6. Nyquist plot of the impedance spectroscopy of graphitecoated copper foil electrode in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.] at different overpotentials (OCV–100 mV to
OCV+100 mV); Frequency range 100 kHz to 0.01 Hz. ........................ 54
Figure 4-7. Three dimensional plot of the impedance spectroscopy of
copper foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]
at different overpotentials (OCV–100 mV to OCV+100 mV);
Frequency range 100 kHz to 0.01 Hz. ................................................... 57
Figure 4-8. Three dimensional plot of the impedance spectroscopy of
graphite-coated copper foil electrode in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.] at different overpotentials (OCV–100 mV to
OCV+100 mV); Frequency range 100 kHz to 0.01 Hz. ........................ 58
xiii
Figure 4-9a. Bode representation of impedance spectroscopy of
copper foil electrode; in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.] log (Impedance) vs. log (Frequency). ................................. 60
Figure 4-9b. Bode representation of impedance spectroscopy of graphite-coated
copper foil electrode; in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.] log (Impedance) vs. log (Frequency). ................................. 61
Figure 4-10a. Phase angle vs. log (Frequency) of impedance spectroscopy
of copper foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]
at different overpotentials. ..................................................................... 62
Figure 4-10b. Phase angle vs. log (Frequency) of impedance spectroscopy
of graphite-coated copper foil electrode in 1M LiPF6/
PC:EC:DMC [1:1:3 vol.] at different overpotentials. ........................... 63
Figure 4-11. Proposed equivalent circuit of both copper foil and graphitecoated foil electrode; Rs: uncompensated resistance; C1:
capacitance of surface process; CPE: constant phase element
of kinetics controlled process; Rct1: charge transfer resistance of
surface process; Rct2: charge transfer resistance of kinetics
controlled process; Zwr: reflective Warburg impedance;
Zwt: transmissive Warburg impedance. ................................................. 65
Figure 4-12. Nyquist plot of the impedance spectroscopy of graphitecoated copper foil electrode after 24 h, 48 h, 60 h of saturation
in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]; Initial potential at
OCV = 3.36 V; Frequency range 100 kHz to 0.01 Hz. ......................... 67
Figure 4-13. Nyquist plot of the impedance spectroscopy of copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at
different temperatures ( from 0 ˚C to 50 ˚C ); Frequency
range 100 kHz to 0.01 Hz...................................................................... 69
Figure 4-14. Nyquist plot of the impedance spectroscopy of graphitecoated copper foil electrode in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.] at different temperatures ( from 0 ˚C to 50 ˚C );
Frequency range 100 kHz to 0.01 Hz. ................................................... 70
Figure 4-15a. Bode representation of impedance spectroscopy of copper
foil electrode at different temperature; in 1M LiPF6/ PC:EC:DMC
[1:1:3 vol.] log (Impedance) vs. log (Frequency). ................................. 73
xiv
Figure 4-15b. Bode representation of impedance spectroscopy of
graphite-coated copper foil electrode at different
temperature; in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]
log (Impedance) vs. log (Frequency). .................................................... 74
Figure 4-16a. Phase angle vs. log (Frequency) of impedance spectroscopy
of copper foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]
at different temperature. ........................................................................ 75
Figure 4-16b. Phase angle vs. log (Frequency) of impedance spectroscopy
of graphite-coated copper foil electrode in 1M LiPF6/
PC:EC:DMC [1:1:3 vol.] at different temperatures. ............................. 76
Figure 4-17. Nyquist plot of the impedance spectroscopy of graphitecoated copper foil electrode saturated for 48 h in 1M LiPF6/
PC:EC:DMC [1:1:3 vol.] at different temperatures ( from
0 °C to 50 ˚C ); Frequency range 100 kHz to 0.01 Hz. ......................... 79
Figure 4-18. Bode representation of impedance spectroscopy of graphitecoated copper foil electrode saturated for 48 h in 1M LiPF6/
PC:EC:DMC [1:1:3 vol.] at different temperature;
log (Impedance) vs. log (Frequency). .................................................... 82
Figure 4-19. Phase angle vs. log (Frequency) of impedance spectroscopy
of graphite-coated copper foil electrode saturated for 48 h in
1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different temperatures. ........... 83
xv
Abbreviations
AE ......................................... auxiliary electrode
Al-C ...................................... aluminum foil coated with graphite and binder
Cu-C ..................................... copper foil coated with graphite and binder
CPE ....................................... constant phase element
CT ......................................... charge transfer
CV ......................................... cyclic voltammetry
DEC ...................................... diethyl carbonate
DMC ..................................... dimethyl carbonate
DME ..................................... 1, 2-dimethoxyethane
EC ......................................... ethylene carbonate
EIS ........................................ electrochemical impedance spectroscopy
FT ......................................... Fourier transform
GC ......................................... glassy carbon
MCMB .................................. mesocarbon microbeads
MEC ..................................... methyl ethyl carbonate
NiMH .................................... nickel metal hydride
OCV ...................................... open circuit voltage
PC ......................................... propylene carbonate
ppm ....................................... parts per million (mg L-1)
xvi
PVDF .................................... polyvinylidene fluoride
RC ......................................... resistance-capacitance
RE ......................................... reference electrode
SEI ........................................ solid electrolyte interface
SPE ....................................... solid polymer electrolyte
WE ........................................ working electrode
1
Chapter 1 – Brief Review of Lithium-ion Batteries and Motivation for This Work
In the past two decades, with the prevalence of portable consumer electronics,
the demand for rechargeable energy storage sources of high energy density and low
weight has been growing rapidly. Currently, larger applications such as zero emission
electric vehicles and satellites put up even more stringent requirements for energy
storage devices in both energy density and power density. In all of these applications,
high performance batteries are more and more desired. Lithium secondary batteries are
a very promising fulfillment for such demands because of some outstanding
characteristics such as their high energy density, intrinsic discharge voltage, relatively
light weight and less environmental concerns. Since its first introduction into the
market by Sony Energetic, Inc. in 1991, the lithium ion battery has become dominant
in small rechargeable battery applications including cellular phones, notebook
computers, personal digital assistants (PDA), etc.1
Its excellent characteristics certainly make the lithium ion battery a favorable
candidate for large applications such as satellites and electric vehicles. For this
purpose intensive research efforts have been directed at lithium ion cells by many
universities, government agencies, and, battery companies. However, some problems
still have to be solved before complete success of the lithium ion battery is achieved
for these applications. Among the concerns are long-term stability of cell components,
heat control of the cell, higher quality and better cost-performance requirements. The
2
research work this area has resulted in hundreds of publications and symposia at
national and international conferences, including the semi-annual meetings of The
Electrochemical Society (ECS) and the annual meting of the International Meeting of
Lithium Batteries (IMLB).
In this chapter, some general concepts of lithium ion batteries are introduced,
followed by a brief review of the lithium ion battery. The motivation for the work in
this dissertation is then discussed at the end.
1.1
General Concepts
A battery consists of a group of interconnected electrochemical cells. How
many cells are packaged and how they are connected depends on the specific
application they are designed for. An electrochemical cell composed of a negative
electrode (anode), a positive electrode (cathode) and electrolyte are the elemental
building blocks of a battery.
Lithium ion cells use a solid reductant as the anode and a solid oxidant as the
cathode. A present-day lithium ion cell uses carbon-based material as the active anode
material and a lithiated transition metal oxide as the active cathode material. On
charge, the cathode material releases Li ions to the electrolyte (deintercalation) and
electrons are removed from cathode by an external field and transferred to the anode.
Charge-compensating Li ions are then attracted into the carbonaceous anode material
(intercalation). The chemical reaction at the anode and cathode of a Li ion cell is
reversible. On discharge, the anode supplies intercalated Li ions to the Li ion
3
electrolyte and electrons to the external circuit, the cathode is an electronically
conducting host into which the Li ions intercalate from the electrolyte and compensate
the charge of the electrons from the external circuit. Since both anode and cathode are
hosts for the reversible intercalation/deintercalation of Li ions from/into the electrolyte,
the electrochemical cell is known as a “rocking-chair” cell. Pending different
requirements for different applications, the common carbon material for the anode is
either graphite or coke type, or a combination of both. Common cathode materials
include LiMn2O4, LiCoO2 and LiNiO2. Electrolyte can be either liquid or solid. Liquid
electrolyte is usually a nonaqueous solution of a lithium salt and various solvents
including different types of esters, ethers and carbonates. A schematic of a typical Li
ion cell is shown in Fig 1-1.2
The reactions at both electrodes and the cell as a whole for a typical Li ion cell
are shown as:3
C h a rg e
Cathode:
LiMO2
Anode:
C + xLi+ + xe
Overall:
LiMO2 + C
D is c h a rg e
C h a rg e
D is c h a rg e
C h a rg e
D is c h a rg e
Li1-xMO2 + xLi+ + xe
(1-1)
LixC
(1-2)
LixC + Li1-xMO2
(1-3)
LiMO2 represents the lithiated transition metal oxide.
Table 1-1 listed some parameters that have been used to evaluate battery
performance of both lithium and lithium ion batteries, as well as, some traditional
rechargeable batteries including lead/acid, nickel/cadmium and nickel metal hydride
battery systems. 3 The lithium ion battery shows a superior advantage in both energy
4
Li(1-x)MO2
Aluminum
Current
Collector
LixC6
Copper
Current
Collector
Figure 1-1. Schematic description of the charge-discharge process of a lithium-ion
cell. [Adapted from: D. A. J. Rand, R. Woods, R. M. Dell, in Batteries for Electric
Vehicles, N. E. Bagshaw, Editor, p. 427, Research Studies Press Ltd., Somerset,
England (1998).]
5
Table 1-1. Comparison of performance parameters of different batteries.
[Adapted from: S. Hossain, in Handbook of Batteries, 2nd ed., D. Linden, Editor,
McGraw-Hill Inc., New York, NY (1995).]
Parameters
Battery Type
Lead Acid
Ni/Cd
NiMH
Lithium Metal
Lithium Ion
Energy
Density*
Wh/kg
30-35
30-40
60-80
150-200
100-125
Wh/L
80-90
70100
140180
300-400
260
Voltage
V
2
1.2
1.2
3.6
3.6
10-20
10-20
2-3
9-12
5001000
5001000
100-150
300-800
Self
%/month 8
Discharge
Cycle
100-500
Life**
*
**
Energy density (both gravimetric and volumetric) delivered by typical cells.
Cycle life - to 80% capacity, 100% depth of discharge.
6
density and cell voltage compare to its competitors. These excellent characteristics
brought the lithium ion battery its success in the small application market and the
interest in possible large applications.
1.2
Review of Lithium Batteries
1.2.1
Lithium Metal Batteries
Being the lightest and the most electropositive metal, lithium is the first choice
as the negative electrode material for a high power density energy storage system.
Theoretically, lithium has a specific capacity to store 3860 Ah/kg compared to 820
Ah/kg for zinc and 260 Ah/kg for lead.2 A comparison of the performance parameters
of different metal anodes is shown in Table 1-2. The concept of “lithium secondary
batteries” was first presented by Chilton and Cook at The Electrochemical Society fall
meeting in Boston in 1962.4 However, many difficulties, including finding a stable
electrolyte medium and a reversible positive electrode material, had to be overcome
before the concept could be turned into a reality.
The high reactivity of lithium made an aqueous electrolyte impossible.
Therefore, a nonaqueous electrolyte was required for the lithium metal anode. Chilton
and Cook proposed the LiCl-AlCl3 electrolyte dissolved in propylene carbonate.4 As
for the cathode, different halides, such as AgCl, CuCl, CuCl2, CuF2 and NiF2 were
investigated in early prototype cells.5 During the discharge process they form lithium
halide and the corresponding metallic phase. The formation of soluble complexes of
7
Table 1-2. Comparison of the performance parameters of lithium, cadmium and
zinc anodes. [Reproduced from: D. A. J. Rand, R. Woods, R. M. Dell, in Batteries
for Electric Vehicles, N. E. Bagshaw, Editor, p. 420, Research Studies Press Ltd.,
Somerset, England (1998).]
Parameter
Lithium
Cadmium
Zinc
Capacity density (Ah/ cm3)
2.05
4.13
5.85
Specific capacity (Ah/g)
3.86
0.48
0.82
Standard potential (V)
-3.04
-0.40
-0.76
Specific Energy (Wh/g)
11.7
0.19
0.62
8
the cathode such as CuCl2¯ is a major drawback of this system since it leads to a high
self-discharge rate.
The breakthrough in cathode material came in the early 1970s when the
reversible “topotactical” electrochemical reaction of the insertion compounds such as
chalcogenides was discovered.6 These materials could host lithium ions inside their
crystal structure while simultaneously reducing the transition metal from a higher
oxidation state. One of these compounds TiS2 was used as the cathode material in the
first commercial lithium rechargeable battery manufactured by Exxon in the mid
1980s.
Using lithium metal as the anode created a series of problems for this relatively
new technology. First, lithium corrosion and dendrite formation during cycling caused
a high volume change of the anode and resulted in a poor cycling efficiency (less than
200 cycle life).3, 5 More importantly, the high reactivity of lithium raised many safety
concerns about this type of cell. Lithium alloys with other metals such as aluminum
were studied to replace lithium metal,7 but the problem of large anode volume change
was not solved until the introduction of the concept of lithium ion cells.
1.2.2
Lithium Ion Batteries
Lazzari and Scrosati proposed the “rocking chair concept” in 1980 using two
insertion compounds based on metallic sulfides and oxides as both positive and
negative electrodes.8 By replacing the lithium metal anode with WO2, the safety
9
problem was solved. However, with only a 1.8 V whole cell voltage, the energy
density of this system was far less than lithium metal cells.
The unexpected discovery of the lithium insertion property of carbon in the
mid 1980s led to the use of carbon as the anode material in the new “Li ion cell”
concept.1 During the charging and discharging process, lithium ion can intercalate into
and deintercalate from carbon without significant change in volume and electrical
properties. Compared to lithium metal, lithiated carbon has a very close potential
(potential difference of lithium metal vs. lithiated carbon is ≤ 0.5V) which gave Li ion
cells a comparable whole cell voltage to the lithium metal cells.3 Extensive research
on a variety of carbonaceous materials also indicated that the carbon morphology
played a critical role in the lithium intercalation process.5 Hard carbon, including
glassy carbon and some carbon fibers, is a highly disordered non-crystalline carbon,
usually prepared from a polymer. It is less sensitive to electrolyte decomposition and
has a higher lithium capacity with a deep first charge.9 Using such a non-crystalline
carbon anode, Sony manufactured the first commercial rechargeable lithium ion
battery in 1990.1 However, high irreversible specific capacity during cycling makes
hard carbon an undesirable anode material. Other carbonaceous materials studied
include crystalline carbon, such as graphite, and less crystalline but highly oriented
“soft carbon”, such as petroleum coke and mesocarbon microbeads (MCMB).1-3
Graphite has a desirable lithium intercalation ratio of 1:6 (LiC6), while for coke the
ratio is 0.5:6 (Li0.5C6). The first attempts to use graphite as the anode material failed
because of severe exfoliation of the graphite resulted from gas releasing in the graphite
10
structure after lithium intercalation in some electrolyte.10 Soft carbon, despite the
lower specific capacity, has the advantage of being less sensitive to the nature of the
electrolyte used. Dahn et al. studied the dependence of the intercalation process on the
crystal structure of a variety of carbons.11 It seemed that the optimum choice of carbon
for a negative electrode should be a combination of graphite and soft carbon with a
carefully designed electrolyte composition.
Layered TiS2 stood for a long time as the standard positive electrode material
for lithium metal cells as it provided high drain capability and good reversibility.12
However, to build the lithium ion cell with pure carbon as the negative electrode, a
fully lithiated material at the discharge state is required for the positive electrode.
Sony’s first Li ion cell chose LiCoO2, which is now the most prevalent positive
electrode material. Two other materials, LiNiO2, LiMn2O4 and their derivatives, have
also been widely studied.5 LiNiO2 has the same layered structure with LiCoO2 and
excellent specific capacity. However, it is more difficult to synthesis and is
significantly less stable than the other two materials. It is generally not used in any
commercial cells. LiMn2O4 has a three-dimensional spinel structure. Compared to
LiCoO2 and LiNiO2, LiMn2O4 is less expensive because of the natural abundance of
manganese. It also has a slightly higher discharge voltage. The major difficulties in the
applications of LiMn2O4 are fading and losses of storage, expecially with increasing
temperature.13 Despite this, a limited amount of Li ion cells with LiMn2O4 as the
cathode material are now available from Moli Energy Ltd for portable equipments.
The average voltages of LiCoO2, LiNiO2 and LiMn2O4 versus Li/Li+ at low discharge
11
rate are 3.7 V, 3.5 V and 3.8 V, respectively.3 Numerous research efforts, such as
partial or multiple substitutions with other transition metals, are still ongoing
worldwide in attempts to understand and improve the behavior of these three oxide
families.5
One of the major reasons for the success of the Li ion cell is the solid
electrolyte interface (SEI). It is a stable, protective passivating layer formed on the
surface of the carbon anode during the charging process.14 While the mechanism of
SEI formation is still not fully understood; it is believed to be an insulating lithium ion
permeable layer formed by the electrolyte solution reduction.1-3, 5 This makes the
choice of electrolyte even more important. A suitable electrolyte should have
characteristics such as high ion conductivity (>10-3 S/cm), wide voltage window (0 to
5 V), thermal stability (up to 90 °C), etc. Various lithium salts and organic solvents
have been studied as the components of Li ion cell electrolyte. Such lithium salts
include LiClO4, LiAlCl4, LiBF4, LiAsF6, LiPF6, LiCF3SO3, and LiN(CF3SO2)2.15 Each
salt has its own drawbacks. LiClO4 constitutes an explosive hazard with an organic
solvent; LiBF4 has low ionic conductivity; and LiAsF6 has environmental issues
because the presence of As, etc. LiPF6 is currently the most widely used salt for Li ion
cell electrolyte and is also used in the work performed in this dissertation.
The most commonly used solvent for the electrolyte are nonaquous aprotic
organic compounds including different carbonates, esters and ethers. Currently the
carbonates such as propylene carbonate (PC) and ethylene carbonate (EC) are the most
widely used solvents since they yield double lithium alkyl carbonates with a good
12
passivating ability. Research also indicates that an electrolyte with a mixture of
carbonate solvents may be the best overall for different electrodes.15 Table 1-3 shows
the ionic conductivity of electrolytes with different lithium salts and solvents used in
Li ion battery systems.
Solid electrolytes for Li ion cells are also under extensive study. These
electrolytes include solid polymer electrolytes (SPE) and inorganic electrolytes. SPE
are produced by incorporating lithium salts into polymer matrices and casting into thin
films. Inorganic electrolytes include crystalline materials, such as lithium halides, and
glassy materials, such as lithium beta-alumina.3 Although bearing a much lower ionic
conductivity compared to liquid electrolytes, solid electrolytes offer the advantages of
a “nonliquid” battery and the flexibility of designing thin batteries with different
configurations.
Ever since its first introduction into the market, the Li ion battery has prevailed
its way to the top rapidly. In 1999 the sales volume of lithium batteries exceeded 50%
of the market total of all rechargeable batteries.5 According to Business
Communication Co.’s report GB-210N, “Lithium Batteries: Materials and Markets”,
the U.S. lithium battery market grew to over $1.7 billion in 2002 and is projected to
grow to over $2.6 billion by 2009.16
1-3
Research Motivation
Thanks to their outstanding characteristics, lithium ion rechargeable batteries
have achieved great commercial success and established dominance in small cell
13
Table 1-3. Ionic conductivity of some organic liquid electrolytes (1M). (Adapted
from: S. Hossain, in Handbook of Batteries, 2nd ed., D. Linden, Editor, p. 36.1,
McGraw-Hill Inc., New York (1995).
Salt
Solvents
Solvent
vol.%
EC/PC
Conductivity at different ˚C (mS/cm)
-40
-20
0
20
40
60
80
50/50
0.23
1.36
3.45
6.56
10.34
14.63
19.35
EC/DMC
33/67
-
1.2
5.0
10.0
-
20.0
-
EC/DME
33/67
-
8.0
13.6
18.1
25.2
31.9
-
EC/DEC
33/67
-
2.5
4.4
7.0
9.7
12.9
-
EC/DME
50/50
*
5.27
9.50
14.52
20.64
26.65
32.57
PC/DME
50/50
*
4.43
8.37
13.15
18.46
23.92
28.18
EC/PC
50/50
0.02
0.55
1.24
2.22
3.45
4.88
6.43
PC/DME
50/50
-
2.61
4.17
5.88
7.46
9.07
10.61
PC/DMC
50/50
-
*
5.32
7.41
9.43
11.44
13.20
EC/PC
50/50
0.28
1.21
2.80
5.12
7.69
10.70
13.86
EC/DME
50/50
-
*
7.87
12.08
16.58
21.25
25.97
PC/DME
50/50
-
3.92
7.19
11.23
15.51
19.88
24.30
LiPF6
LiAsF6
LiCF3SO3
LiN(CF3SO2)2
14
Table 1-3. Ionic conductivity of some organic liquid electrolytes (1M). (contd.)
EC/PC
50/50
0.19
1.11
2.41
4.25
6.27
8.51
10.79
EC/DMC
33/67
-
1.3
2.5
4.9
6.4
7.8
-
EC/DEC
33/67
-
1.2
2.0
3.2
4.4
5.5
-
EC/DME
33/67
-
6.7
9.9
12.7
15.6
18.5
-
EC/DMC
33/67
-
1.0
5.7
8.4
11.0
13.9
-
EC/DEC
33/67
-
1.8
3.5
5.2
7.3
9.4
-
EC/DME
33/67
-
8.4
12.3
16.5
20.3
23.9
-
LiBF4
LiClO4
*: Electrolyte freezes at such temperature;
EC = ethylene carbonate;
PC = propylene carbonate;
DMC = dimethyl carbonate;
DEC = diethyl carbonate;
DME = 1, 2-dimethoxyethane.
15
applications, such as batteries for portable electronic devices, since their first
emergence in 1991. Their high energy density and intrinsic discharge voltage also
make them a desirable solution for larger applications, including electronic vehicles
and satellites. Such applications also brought a higher performance standard for
lithium ion batteries. Low cycling efficiency under critical environmental conditions,
electrode decomposition related to possible over-discharge, and long term stability of
electrode and electrolyte materials are some of the difficulties this technology is facing.
As stated earlier, a copper foil current collector as the negative electrode is
used in most commercially available Li ion cells. A coating of carbonaceous material
is fabricated from a slurry of carbon, binder and an appropriate solvent onto the copper
foil by a casting or pressing procedure. The cell is usually assembled in a total
discharged state from unlithiated carbon and fully lithiated positive electrode. During
the first charge lithium ion will intercalate into the carbon and form the SEI
passivation film to protect the air sensitive lithiated electrode. The electrode is
expected to be stable. However, the shelf life (i.e. the time before the first charging of
the cell) of the cell depends on the intrinsic stability of the electrode materials.
The electrochemical behavior of copper in nonaqueous environments and as
the negative electrode current collector in Li ion batteries has not been systematically
studied. Kawakita and Kobayashi observed oxidative dissolution of copper in
LiClO4/propylene carbonate caused by anodic polarization.17 Previous open circuit
voltage (OCV) studies of both copper foil and graphite-coated copper foil electrodes
in this laboratory attributed copper dissolution to impurities in the electrolyte.18, 19
16
However, the nature of the kinetic processes of both copper and graphite-coated
copper foil has remained unclear.
The research results presented in this dissertation have been aimed at studying
the kinetic processes at the interface of the Cu anode and the Li ion battery electrolyte
through different electrochemical approaches. It will fill some of the void in the
understanding of copper stability in nonaqueous media by providing fundamental
kinetic information of the electrode process. The results from this research work
should also be helpful in practical applications such as the design of batteries and
electrodes.
17
Chapter 2 – HF Impurity Studies in Lithium Ion Battery Electrolyte
2.1
Introduction
Lithium ion batteries have proven to be a great commercial success since their
first introduction into the market in the early 1990s.1 Excellent characteristics such as
high energy density and more environmentally friendly materials have resulted in Liion battery dominance in portable cell applications. Recent studies have been focused
on larger application of lithium ion batteries, such as satellites and electric
automobiles.2, 5 Long term chemical stability of battery materials and components is
one of the major concerns in these large applications. Among these components, the
electrochemical stability of the anode material has been the focus of this study.
All current commercially-available lithium ion batteries use a carbonaceous
coating applied on a copper foil substrate as the anode. During the charging and
discharging processes lithium ions in liquid electrolyte can intercalate and
deintercalate from this coating with the copper foil as the current collector.
Throughout this work, anode materials have been studied in a simplified homemade
cell. This chapter is focused on the experimental conditions including material used,
cell assembly and instrumentation. Also, some experiments with HF impurity in
lithium ion battery electrolyte were performed to study its effects on OCV.
In order to understand the intrinsic stability of the anode materials in Li-ion
battery electrolytes, this lab has studied previously the electrochemical behavior of Cu
18
foil electrodes and Cu-C foil electrodes in different nonaqueous organic carbonate Liion battery electrolyte solutions in half-cell reactions.20, 21 Open-circuit voltage (OCV)
studies on Cu foil electrodes (without graphite coating) have also been performed.19
The OCV variation over time of Cu foil electrodes was observed and subsequently
studied in detail. Combined with previous findings of Cu dissolution, it was suggested
that impurities, such as HF, could oxidize copper foils in nonaqueous electrolyte
solutions, which resulted in the OCV variation of Cu foil electrodes over time. The
OCV studies provided some insight into the intrinsic stability of uncoated Cu foil in
the Li-ion electrolyte solutions. In the work reported here, as one of the pertinent cell
factors, the effect of trace amounts of HF in the electrolyte was studied. The OCV
results of HF doped electrolyte, along with other OCV results from previous studies,
are compared with the findings on uncoated Cu foil electrodes and summarized.
2.2
Materials and Reagents
The battery grade copper foil and the graphite coated copper foil were supplied
by Saft and used as received. The copper foil used was 12 μm thick, grade LP1/LP3
(Fukuda Metal Foil and Powder Co.). The electrodeposited copper foil had a purity of
99.9% with the major trace element Cr at ≤ 130 ppm. The graphite coating was a
blend of 50 wt % mesocarbon microbeads (MCMB 10-28) and 50 wt % Timcal SFG44 graphite using 4.5% polyvinylidene fluoride (PVDF) as a binder. The carbon
loading per electrode was ~ 26 mg/ cm2 (double side coated). The graphite-coated
copper foil was calendered (roll-pressed) between rollers after solvent evaporation
19
from the drying process. The calendered graphite-coated copper foils used in these
studies sat as small coupons and likely were subject to a relaxation process whereby
the graphite coating springs back. The extent of relaxation is unknown.
The electrolyte used in this study was 1 M LiPF6 in a ternary mixture of
propylene carbonate (PC)-ethylene carbonate (EC)-dimethyl carbonate (DMC) [1:1:3
vol.]. The electrolyte was obtained from EM Industries/Merck K. G. a. A. and was
prepared from 99.98% purity solvents (<20 ppm H2O as determined by a Karl Fischer
titration) and Stella LiPF6. The electrolyte was guaranteed at <80 ppm HF and was
analyzed at Saft as <50 ppm using an acid base titration. Electrolyte was frozen before
degassing for 30 min and then thawed. This procedure was repeated three times before
the electrolyte was transferred to a dry box and stored.
Lithium metallic foil was also obtained from Saft Research and Development
Center (Cockeysville, MD). It was manufactured by Fukuda Metal Foil and Powder
Co.
Hydrofluoric acid (HF) used as the doping reagent is a 49% HF solution
(A147-1) from Fisher Scientific Inc.
2.3
Electrochemical Cell Assembly
All of the electrochemical studies performed used a homemade three-electrode
cell (Fig. 2-1). The cell body was an end-sealed 25 mm Ace-thred glass connector
(Ace Glass, catalog No. 7644-20). The cell cap was an O-ring sealed threaded Teflon
plug with three electrode ports and a rubber septum port. The working electrodes (WE)
20
RE (Ni wire connected
to Li metal)
Glass Electrode Tube
(sealed at opening
WE (Cu foil connected
to Ni wire)
AE (isolated Pt
wire coil)
Rubber Septum
Internally Threaded
Glass Cell
Li Metal
Porous Vycor Tip
Ni Wire Connected
to Cu Foil
1×1 cm2 Cu Foil
Stirring Bar
Figure 2-1. The schematic of the homemade cell. Electrolyte solutions flooded just
above the 1×1 cm2 portion of Cu foil when the cell was assembled in a dry box.
21
were battery-grade copper foil and graphite-coated copper foil that were cut into a
“flag” with a 1 × 1 cm2 working area. The flags were then connected to a 22-gauge
nickel wire by pressing the tip of the nickel wire on to the flag pole. The reference
electrodes (RE) were made by rolling and pressing a 1 × 1 cm2 lithium foil onto the tip
of a nickel wire and assembled in a dry box in electrolyte solution in a glass tube with
a 6 mm diameter porous VycorTM tip (Bioanalytical Systems, BAS, MF-2042). The
auxiliary electrode (AE) was a 0.5 mm diameter, 23 cm long platinum wire coil (BAS,
MW-1033). All working electrodes were rinsed with acetone and air-dried before
assembly of the cell.
2.4
Instrumentation
All the cells were assembled and sealed in a glove box filled with argon. Both
water and oxygen content inside the glove box were less than 1 ppm.
All electrochemical experiments (except the open circuit voltage experiment in
the preliminary work) were performed with a CH Instruments CH-604A
electrochemical analyzer. It is a computerized electrochemical instrument with a fast
digital function generator, a high speed data acquisition circuitry and a potentiostat.
The dynamic range of experimental parameters of the CH-604A electrochemical
analyzer is listed in Table 2-1. 22
The OCV experiments in the preliminary work were performed with a Cypress
Systems CS-2000 computer-controlled electrochemical system potentiostat
/galvanostat. The input impedance of this instrument is ~ 1011 ohms.
22
Table 2-1. Dynamic range of experimental parameters of CHI-604A
Electrochemical analyzer 20
Parameters
Dynamic Range
Techniques Involved
Potential (V)
-10 to +10
all
Current (A)
0 to ± 0.25
all
Sensitivity (A/V)
1 × 10-12 to 0.1
all
Scan Rate (V/s)
1 × 10-6 to 5000
CV, LSV
Pulse Width (s)
1 × 10-4 to 1000
CA, CC
Frequency (Hz)
1 × 10-4 to 1 × 105
IMP
Measurable Current (A)
< 1 × 10-11
CV, CA
Input Impedance (Ω)
1 × 1012
OCV
23
2.5
Procedure
OCV studies of Cu-C foil electrodes were performed using the homemade
three-electrode cell and the materials as described earlier in this chapter. HF was then
introduced to the assembled cell with a GC syringe. The two concentrations of HF
studied were 500 ppm and 1000 ppm. The time of each OCV experiment was set as 40
h. During assembly of the cell, the WEs were kept above the electrolyte solution. Just
before the OCV measurement, the WEs were immersed into the electrolyte solution,
which ensured that the OCV measurements had the same starting time. Results from
several previously performed experiments were analyzed together: (1) OCV studies of
Cu-C foil electrodes in electrolyte without doping; (2) OCV measurement of a cell
consisting of Al-C foil as the working electrode for comparison to Cu-C.
2.6
Results and Discussion
In previous electrochemical studies of Cu foil electrodes in Li-ion battery
electrolytes, the OCV was shown to be an effective way to study the interfacial change
of the Cu over time. This provided some insight into the intrinsic stability of the
electrodes and the role of impurities in electrolyte solutions. Similar OCV studies were
performed on Cu-C foil electrodes by continuous measurement of the OCV for an
extensive period of time (up to 40 h) when the electrode was immersed into electrolyte
solution.
From the OCV results in Figure 2-2 one can see that the OCV appears to
follow a two step process. In the first step, the OCV decreased quickly until reaching a
24
minimum. This was followed by increasing gradually until reaching a steady state.
The initial OCV drop was 0.29 V and took about 35 min. The OCV increase from the
minimum to the steady state was about 0.20 V and took about 1200 min.
As compared with the OCV results obtained from Cu foil electrodes6, it can be
seen that the characteristic OCV signals of Cu-C electrodes are quite different from
that of Cu foil electrodes even though the OCV variation of Cu foil electrodes also
showed a two-step process over time. In the first step, the initial OCV drop of Cu-C
foil electrode was about two times larger than that of Cu foil electrode. In the second
step, the OCV of the Cu foil electrodes never achieved a real steady state, but the
OCV of the Cu-C foil electrodes did.
Figure 2-3 shows a close comparison of the OCVs of graphite coated
aluminum (Al-C) and Cu-C. It can be seen that the initial drop in the OCV of the Al-C
foil electrode was more gradual and lagged as compared with that of a Cu-C foil
electrode while the rising portion of the OCV of Cu-C is more gradual than that of AlC. Despite the difference, the magnitude of the initial OCV drop and the subsequent
OCV increase of both electrodes are close to each other. In addition, both electrodes
achieved a steady state. These results suggest that the graphite coating is the main
factor causing the characteristic two-step OCV variation over time. The foil substrate
has limited influence.
It is a well accepted concept that the OCV of an electrode is determined by the
double-layer charge formed on the interface between electrode and solution.
Investigations of interfacial structure of both Hg/PC and Hg/EC proved that the
25
Figure 2-2. OCV vs. time of Cu-C foil electrode in 1 M LiPF6/ PC:EC:DMC (1:1:3
vol.). Reference electrode was made fresh. (Reproduced from Reference 18)
26
Figure 2-3. Comparison of the initial part of the OCV of Al-C and Cu-C in 1 M
LiPF6/ PC:EC:DMC (1:1:3 vol.). (Reproduced from Reference 18)
27
capacity of the interface changed as the result of the reorientation of the solvent
molecules (PC or EC).23, 24 It appears that the adsorption of the electrolyte solvents on
the graphite surface resulted in the initial OCV drop of Cu-C and Al-C electrodes due
to the transfer of negative charge from the solvent molecules to the electrodes. The
OCV reached its minimum after a complete, highly oriented layer was formed on the
graphite surface. After the OCV minimum, the increase in the OCV denotes a gradual
discharge of the accumulated charge. It appears that the specific adsorption of an
impurity of HF into the graphite coating from the electrolyte solution resulted in the
positive shift in the OCV.
It is known that clean carbon surfaces are reactive to form surface oxides in the
form of chemisorption from the uncompensated valences.25 On acid-adsorbing carbons,
the accumulated positive charge resulted from the specific adsorption of H+ to the
graphite coating caused the positive shift of OCV. Panzer and Elving describe this
process with equation: 26
(CxO) + H+ + A− → (CxOH)+ + A−
(2-1)
Since the rate of the adsorption process is mainly controlled by the diffusion rate of H+
to the graphite coating, the OCV shift is gradually increased until the adsorption
saturation is achieved, which corresponds to the OCV steady state.
To study the influence of impurity on the OCV variation of a Cu-C foil electrode,
measurements were then performed in electrolyte doped with 500 ppm and 1000 ppm
concentrated aqueous HF (49 wt %, Fisher). The results are shown in Fig. 2-4. The
magnitude of the initial OCV drop is smaller (0.19V in the 1000 ppm electrolyte
28
(a)
(b)
Figure 2-4. OCV vs. time of Cu-C foil electrode in 1 M LiPF6/ PC:EC:DMC (1:1:3
vol.) doped with (a) 500 ppm HF and (b) 1000 ppm HF.
29
solution) and took longer (over 2 h) to reach the minimum than observed in Fig. 2-2.
The subsequent OCV increase from its minimum to the steady state was also smaller
than observed in the absence of HF. These observations further indicate that the
adsorption of HF on the graphite coating protected the Cu foil beneath the coating
from corrosion.
2.7
Conclusions
The OCV of Cu-C foil was known to undergo a two-step process over time,
which resulted from the interaction between the graphite coating and the electrolyte
solution. In the first step, the OCV dropped quickly until reaching a minimum as a
result of adsorption of electrolyte solvents on the graphite coating. In the second step,
the OCV gradually increased as a result of specific adsorption of HF in the graphite
coating, which protected the Cu substrate from oxidation. HF impurities in the
solution resulted in a smaller and longer OCV drop, as well as a smaller subsequent
OCV incensement. The foil substrate had limited influence on the OCV variation.
30
Chapter 3 – Controlled-potential Electrolysis Studies of Copper Foil and
Graphite-Coated Copper Foil Electrodes in Lithium-ion Battery Electrolyte
3.1
Introduction
Kinetic and mechanistic studies of copper electrodissolution in neutral and
acidic solutions have been performed for more than half a century 27 and continue to
attract the interest of researchers working in fields related to corrosion,
superconductivity, alloys, biochemistry, batteries, etc.. 28-32 Less systematic effort has
been directed to similar studies of copper in nonaqueous environments 27 and in
particular as negative electrode current collectors in Li ion batteries. 33, 34, 35
Owing to outstanding characteristics such as high energy density, high cell
voltage and few safety concerns over conventional rechargeable cells, Li-ion batteries
have achieved high use since first being introduced to the market in 1991 by Sony. 1
Extensive studies have been dedicated to rechargeable Li-ion batteries operating at
ambient temperature. 2 Today Li-ion batteries are the most promising energy sources
for various portable electronic devices including cell phones and notebook computers.
The total sale value of Li-ion batteries exceeded that of conventional NiCd and NiMH
rechargeable cells in 1999 and continues to expand. 3 In all commercially available Liion cells, copper foil or graphite-coated copper foil is used as the negative electrode
current collector while aluminum foil is used as positive electrode current collector.
The electrolyte is either a nonaqueous liquid electrolyte comprised of a lithium salt
31
and various carbonate, ether, or ester solvents or a solid polymer electrolyte. Recently,
higher energy density required by both commercial and military applications including
zero emission electric vehicles and satellites has drawn increasing interest in large Liion cells. 1, 3 Long term stability of all cell components is crucial in these applications.
One of the concerns is the electrochemical stability of anode materials.
Kawakita and Kobayashi found that anodic polarization in LiClO4/propylene
carbonate (PC) brought about oxidative dissolution of the copper substrate. 17 Open
circuit voltage (OCV) studies of both copper foil and graphite-coated copper foil
electrodes suggested that impurities such as HF could oxidize copper foil in
nonaqueous electrolyte solutions. 18, 19 The OCV change was attributed to copper
dissolution. However, the nature of the kinetic processes of both copper and graphitecoated copper foil remains unclear in nonaqueous liquid electrolyte.
Controlled-potential electrolysis is useful as an electrochemical technique for
metal dissolution and deposition. It is also a suitable technique for electrochemical
kinetic studies. A square-wave voltage signal is used as the excitation signal to step
the electrode potential from a value at which no Faradaic current occurs, Ei, to a
potential Es, at which oxidation of the metal proceeds. Current as a function of time is
monitored as the system response. The difference between Es and the equilibrium
potential of the electrode, Eeq is called overpotential. When the overpotential exceeds a
certain limit, where it may be assumed that the rate of one of the electrode reactions
becomes negligible, the relation between overpotential (η) and the net current (inet) is
shown in the Tafel equation:
32
inet = i0 e-αnfη
or ln inet = ln i0 - αnfη
(1)
From the ln inet versus η Tafel plot one can calculate the exchange current, i0,
which is the current on both anode and cathode at equilibrium and the charge transfer
coefficient, α, which is the symmetry factor between the potential curves of the two
electrodes.
Electrolysis experiments were performed on both copper foil and graphitecoated copper foil electrodes at different overpotentials. Tafel plots were then made to
calculate the exchange current and charge transfer coefficient. A similar set of
experiments was undertaken at different temperatures to determine the change in the
exchange current or the charge transfer coefficient.
3.2
Experimental Section
All experiments were performed using a homemade three-electrode cell. The
schematic and the details of the cell were described in Chapter 2. All electrolysis
experiments were performed with a CH Instruments CH-604A electrochemical
analyzer. A J-KEM Scientific Model-150 temperature control unit and an oil bath
were used for 50 ˚C experiment. The same temperature control unit and an ice/water
bath were used for 0 ˚C experiment.
The electrolysis studies were performed in two steps. First, experiments were
performed on both copper foil and graphite-coated copper foil WEs in the electrolyte
solution at 25 ˚C at different overpotentials (η) from 50 mV to 350mV with a 50 mV
interval. All the REs were made fresh in these cells. All the WEs were rinsed with
33
acetone and then air-dried before the cell assembly. Next the same electrolysis
experiments on the same electrodes were performed at 0 ˚C and at 50 ˚C. In these
experiments the cell body was submerged in the temperature bath for 1.5 h to ensure
that the entire cell reached the desired temperature. All electrolysis experiments were
performed with a single 30 s step width and the current was recorded with a 0.1 s
interval.
3.3
Results and Discussion
Previous studies from this laboratory showed that the open circuit voltage of
both copper foil and graphite-coated copper foil electrodes in a typical Li ion battery
electrolyte was not as stable as expected. 21 To better understand the copper
dissolution and the electrochemical processes at the electrode surface, controlledpotential electrolysis experiments were preformed on both electrodes.
Before beginning the electrolysis experiments, the open circuit voltage (OCV)
measurement was obtained on the assembled cells. The OCV was set as the initial
potential of the electrolysis experiments. The cell potential was stepped from the
initial value to the desired overpotential (η) for 30 s. The current through the cell was
recorded and plotted versus elapsed time. The 30 s experiment duration was chosen
based on two considerations. First, the length of time should be sufficient enough for
the oxidation processes to take place. Second, at high overpotentials, copper
dissolution is severe, especially for the uncoated copper foil electrode; a longer time at
the highest overpotential sometimes can cause extensive oxidation to the electrode.
34
Both the copper foil and graphite-coated copper foil electrode were in the same
cell assembly and tested under the same conditions. The results from different
overpotentials for copper foil and graphite-coated copper foil are shown in Fig. 3-1
and Fig. 3-2, respectively. Fig. 3-1 shows the electrolysis of the copper foil electrode
in 1 M LiPF6 in PC:EC:DMC [1:1:3 vol.] electrolyte at an initial potential of 3.40 V
and after each successively increasing 50 mV potential step. Fig. 3-2 shows the
electrolysis of a graphite-coated copper foil electrode in the same electrolyte at an
initial potential of 3.39 V and again after each potential step. Both sets of data show a
logarithmic increase in current in the low overpotential region (η<250 mV), the Tafel
region.
Figure 3-3 shows the Tafel plots of copper foil and the graphite-coated copper
foil electrodes, respectively. In both plots a linear relationship in the low overpotential
range (η<250 mV) was readily observed. Linear regression was performed on the
Tafel plot data to extract the slope and the intercept in order to obtain the exchange
current and the charge transfer coefficient, as shown in Table 3-1. The value of the
exchange current was 0.091 mA for copper foil and 0.11 mA for graphite-coated
copper foil electrodes. For the graphite-coated copper foil electrode, the higher
exchange current can be attributed to the intercalation of lithium ions from the
electrolyte into the graphite layer which facilitated the electron transfer. The values of
the charge transfer coefficient for the copper foil and graphite-coated copper foil were
0.24 and 0.20, respectively (using n equal to 2). The small value (α< 0.25) of the
charge transfer coefficient for both electrodes indicates that anodic current is dominant.
Figure. 3-1. Electrolysis of copper foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3
vol.] at 25 ˚C; Initial potential at OCV = 3.40 V; Overpotential from 50mV to 350
mV.
Figure. 3-2. Electrolysis of graphite-coated copper foil electrode in 1M LiPF6/
PC:EC:DMC [1:1:3 vol.] at 25 ˚C; Initial potential at OCV = 3.39 V; Overpotential from
50mV to 350 mV.
37
a)
b)
Figure. 3-3. Tafel plots of (a) copper foil electrode; and (b) graphite-coated copper
foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.].
38
Table 3-1. Linear regression results of Tafel plot of Cu foil and Cu-C foil
electrode in 1M LiPF6 in PC:EC:DMC [1:1:3 vol]
Electrode
Intercept
(log µA)
Exchange
Current Density
(mA/cm2)
Slope
(log µA / V)
Charge
Transfer
Coefficient
R2
Cu
4.51 ± 0.16
9.12 × 10-2
18.88 ± 1.19
0.24
0.992
Cu-C
4.72 ± 0.16
1.12 × 10-1
15.72 ± 0.95
0.21
0.989
39
The difference between the charge transfer coefficients of the two electrodes may be
explained by different copper dissolution rates of each electrode.
Temperature is an important factor both in the theory of and measurement of
electrode oxidation processes. A range of 0 ˚C to 50 ˚C is a common working
temperature for most commercially available lithium ion batteries. Thus the behavior
of both copper foil and graphite-coated copper foil electrodes at 0 ˚C and 50 ˚C was
studied and compared with that of 25 ˚C, as shown in Table 3-2. For the copper foil
electrode the exchange current increased with the increasing temperature. At 0 ˚C, the
exchange current was 0.016 mA. It increased to 0.091 mA at 25 ˚C, while at 50 ˚C, it
increased significantly to 0.18 mA. The exchange current of graphite-coated copper
foil also showed the same increasing values with temperature. However, the
differences in exchange current between temperatures were not as significant as the
bare copper electrode. The exchange current increased from 0.077 mA at 0 ˚C to 0.11
mA at 25 ˚C. When the temperature increased to 50 ˚C the exchange current increased
to 0.14 mA. The α value, on the other hand, does not show a significant effect of
temperature in the operating temperature range of 0 ˚C to 50 ˚C on either of the two
electrodes. Instead, the charge transfer coefficient of both electrodes was ~0.25.
The i0 and α values obtained for copper electrolysis in the nonaqueous
electrolyte compare reasonably well with values reported for dissolution of copper in
aqueous electrolyte. 29 The assessment of the effect of surface layers on anodic mass
transfer cannot be fully elucidated by steady-state electrolytic experiments. Thus
41
electrochemical impedance spectroscopy (EIS) was used to further elucidate the
effects of oxide layer coverage and the graphite coating on the copper foils. 33 These
results have been described in the next chapter.
3.4
Conclusions
Controlled-potential electrolysis of both copper foil and graphite-coated copper
foil electrodes was used to construct Tafel plots and calculate values of exchange
currents and charge transfer coefficients in a typical lithium ion battery electrolyte. At
25 ˚C the graphite-coated copper foil was found to have a higher exchange current
than copper. This may possibly be explained by the intercalation of lithium ion into
the graphite coating which increases the electron transfer rate. The small value of the
charge transfer coefficient is consistent with the assumption of a dominant anodic
current. Also, experiments at 0 ˚C and 50 ˚C showed that exchange current increases
with temperature for both electrodes, but at different rates. However, the operating
temperatures showed no significant effect on the electron transfer coefficient. The
charge transfer coefficient for both electrodes tended to be stable at ~0.25.
42
Chapter 4- Impedance Studies of Copper Foil and Graphite-Coated Copper Foil
Electrodes in Lithium-ion Battery Electrolyte
4.1
Introduction
Ambient temperature rechargeable Li-ion batteries have been under extensive
study since first introduced into the market by Sony in 1991. 1 Some outstanding
characteristics including high energy density, higher cell voltage and fewer safety
concerns over conventional rechargeable cells have made Li-ion batteries a great
success. 3 Portable Li-ion batteries are widely used in electronic devices, such as
cellular phones and notebook computers. Total sale value of Li-ion cells exceeded that
of NiMH and NiCd rechargeable cells in the portable cell market in 1999. 5 Copper
foil is used as the negative electrode current collector and aluminum foil is used as the
positive electrode current collector in all commercially available Li-ion cells. The
electrolyte can be either a solid polymer electrolyte or a nonaqueous liquid electrolyte
comprised of a lithium salt and various carbonate, ether, or ester solvents. Recently,
commercial and military applications requiring larger power density and energy
density, such as electrical vehicles and satellites has increased interest in large Li-ion
cells. 3, 5 Long-term stability of cell components is required. The electrochemical
stability of anode materials, copper foil or graphite coated copper foil, is one of the
concerns.
43
Only a few studies have been reported on the electrochemical behavior of
copper as the negative electrode current collector in Li ion batteries. 34, 35, 36 Recently,
Kawakita and Kobayashi found that anodic polarization in LiClO4/propylene
carbonate brought about oxidative dissolution of the copper substrate. 17
Previously, the electrochemical behavior of copper foil and graphite coated
copper foil electrodes in different nonaqueous Li-ion battery electrolyte solutions in
half-cell reactions has been reported from this laboratory. 21, 22 Further, open circuit
voltage studies of both copper foil and graphite-coated copper foil electrodes
suggested that impurities such as HF could oxidize copper foil in nonaqueous
electrolyte solutions. 18, 19 Thus the OCV changes in this study were concerned with
copper electrodissolution. However, owing to a lack of systematic work on the
electrochemistry of copper in nonaqueous environments, 27 the nature of the surface
and kinetic processes for both the copper and graphite-coated copper electrodes is
uncertain.
Impedance spectroscopy (EIS) is increasingly being applied to electrochemical
studies because of its ability to characterize many of the electrical properties of
materials, as well as, the electrode-electrolyte interfaces. 37, 38 EIS studies of copper in
aqueous media have been reported for over a quarter century. 30, 31, 32, 39, 40 Braithwaite,
et al. used EIS to study Al current collectors in Li ion battery electrolyte, 34 but
previous EIS studies of copper in nonaqueous media have not been reported. In the
work reported here, EIS was performed on both copper foil and graphite-coated
copper foil electrodes in 1 M LiPF6 in a ternary mixture of organic carbonates. The
44
initial potential of the impedance experiments was chosen near the open circuit
voltage since it is the voltage that the electrode operates under most of the time. An
equivalent circuit is proposed for the electrode process.
4.2
Experimental Section
All experiments were performed using a homemade three-electrode cell. The
schematic and the details of the cell were described in Chapter 2. All impedance
experiments were performed with a CH Instruments CH-604A electrochemical
analyzer. A J-KEM Scientific Model-150 temperature control unit and an oil bath
were used for 50 ˚C experiment. The same temperature control unit and an ice/water
bath were used for 0 ˚C experiment. Microscopic images were taken with an Intelplay
QX3+ computer microscope.
The impedance studies in this work can be classified into a few sets of
experiments. First, impedance studies of (1) copper foil electrode; and (2) graphitecoated copper foil electrode were performed in the electrolyte solution at open circuit
voltage (OCV). All the REs were made fresh in these cells. All the WEs were rinsed
with acetone and then air-dried before the cell assembly. Microscopic images of the
electrode surface were taken both before and after the experiment. The second set of
impedance experiments were performed on the same electrodes at different
overpotentials (η) (1) 100 mV lower than OCV; (2) 50 mV lower than OCV; (3) OCV;
(4) 50 mV higher than OCV; (5) 100 mV higher than OCV. The impedance
experiments were then performed under different temperatures. 50 ˚C and 0 ˚C were
45
chosen as the high and low temperature in this study as 0 ˚C to 50 ˚C is usually the
working temperature range for most commercially available lithium ion cells in the
market. The aging effect of the electrolyte to the graphite-coated copper foil electrode
is also studied as a part of this work through another set of experiments. The
experiments were performed after different electrode saturation time of the electrode
in the electrolyte. The frequency range investigated in all the experiments in this study
was from 100 kHz to 0.01 Hz, and the a.c. perturbation amplitude was 5 mV. The
measurement model for frequencies above 100 Hz in the impedance experiments was
the Fourier transform (FT).
4.3
Results and Discussion
4.3.1
Impedance at Room Temperature and Open Circuit Voltage
Impedance spectroscopy is widely used to investigate interfaces in
electrochemical systems. The data presented previously in Chapter 3 showed that the
exchange current of the copper foil electrode (0.091 mA) was smaller than that of the
graphite-coated copper foil electrode (0.11 mA) through controlled-potential
electrolytic experiments. Thus, to further study the copper oxidation processes at the
electrode-electrolyte interface, impedance spectroscopy experiments were performed
on both electrodes.
Open circuit voltage (OCV) experiments of freshly assembled cells were
performed before any other experiments to obtain initial voltages for the impedance
experiments. The impedance experiment was then performed on a cell with the initial
46
potential set at OCV. Both copper foil electrode and graphite-coated copper foil
electrode were in the same cell assembly and tested under same conditions. Fig. 4-1
shows the Nyquist impedance spectrum of a copper foil electrode in 1 M LiPF6 in
PC:EC:DMC [1:1:3 vol] electrolyte at an initial potential of 3.30 V. Figure 4-2 shows
the Nyquist impedance spectrum of a graphite-coated copper foil electrode in the same
electrolyte at an initial potential of 3.29 V. The offset on both spectra shows the high
frequency range impedance for both electrodes. A similar trend of a small semicircle
in the high frequency range followed by a much bigger semicircle in the low
frequency range can be seen in both spectra. It appeared that at the high frequency
range both electrodes showed a similar behavior while at the low frequency range the
graphite-coated copper foil electrode had larger impedance. The two-semicircle
structure of the impedance spectra indicates a two step electrode process, a surface
process(oxide film coverage) step in the high frequency range at the beginning of the
experiment and a kinetic controlled electrode process step in the low frequency range
later in the experiment. The graphite-coated copper foil electrode showed much higher
impedance. It is believed that the high impedance at the graphite-coated copper foil
electrodes resulted from the surface being blocked to reaction.
It was also observed that after the impedance experiments, the copper foil
electrode showed a certain degree of oxidation on the edges of the electrode. The
graphite-coated copper foil, on the other hand, showed no signs of oxidation that could
be observed by unaided eyes, but on the electrode surface there were visible spots of
the graphite layer peeled off from the copper foil inside. Fig. 4-3 shows the
47
Figure 4-1. Nyquist plot of the impedance spectroscopy of copper foil electrode in 1M
LiPF6/ PC:EC:DMC [1:1:3 vol.]; Initial potential at OCV = 3.30 V; Frequency range
100 kHz to 0.01 Hz.
48
Figure 4-2. Nyquist plot of the impedance spectroscopy of graphite-coated copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]; Initial potential at OCV = 3.29 V;
Frequency range 100 kHz to 0.01 Hz.
49
microscopic images of the copper electrode surfaces before cell assembly (a) and 24 h
after (b) the impedance experiments.The same images of the graphite-coated copper
foil electrode are shown in Fig. 4-4 (a) and (b), respectively. The electrodes were
pulled out of the electrolyte solution but still kept inside the sealed cell body for 24 h
after the experiment to allow the carbonate solvent to dry before the microscopic
images were taken. Fig. 4-3a and Fig. 4-4a show the rough surfaces of copper and
graphite in the two electrodes, respectively. A bottom light source was used with the
image in Fig. 4-3b. It can clearly be seen that there is significant copper dissolution at
the edge of the copper foil electrode. Fig. 4-4b shows a spot where the graphite peeled
from the graphite-coated copper foil electrode. A salt crystal can be seen protruding
from inside the graphite layer. The crystal is thought to be a lithium salt. The different
extent of oxidation for the two electrodes further illustrated the blocking effect of the
graphite layer.
The copper dissolution observed here is consistent with previous study in this
lab which suggested that the trace amount of HF presented in the electrolyte solution
can cause oxidation of the unprotected copper foil. 18 It is well known that clean
carbon surface are reactive to form surface oxides in the form of chemisorption from
unsatisfied valences. 25 Panzer and Elving described the acid adsorption of carbon
material to the interaction between the acid in the solution and the surface oxide by the
following equation: 26
H+ + A- + CxO → (CxOH)+ + A-
(4-1)
50
a)
0.1 mm
b)
Figure 4-3. Microscopic image of copper foil electrode surface (200×) a) before
experiment and b) 24 h after experiment. Scale as shown in a).
51
a)
b)
0.1 mm
Figure 4-4. Microscopic image of graphite-coated copper foil electrode surface (200×)
a) before experiment and b) 24 h after experiment. Scale as shown in a).
52
Thus, the observed difference of copper oxidation between copper foil and graphitecoated copper foil electrode can be attributed to the specific adsorption of HF on the
graphite coating in the latter that gives protection to the copper foil substrate from
oxidation. The peeled graphite spots are considered exfoliation caused by gas
releasing following the intercalation of lithium ion into the graphite coating.
4.3.2
Impedance at Different Overpotentials
During the charging and discharging processes, a rechargeable cell is
constantly under potentials either lower or higher than its open circuit voltage. To
better understand the electrode behavior under these circumstances, a series of
experiments were performed under different overpotentials (η) for both copper foil
and graphite-coated copper foil electrodes in the same electrolyte solution. To avoid
possible overcharge and over-discharge, overpotential values in this set of experiment
were set from 100 mV higher than OCV to 100 mV lower than OCV with a 50 mV
interval between each tested value.
Fig. 4-5 and Fig. 4-6 show the Nyquist impedance spectra of copper foil and
graphite-coated copper foil electrode at different overpotentials, respectively. The
offset on both spectra also reflected the impedance at the high frequency part (> 1 kHz)
of the experiments. It can be seen that the impedance of both electrodes shows the
same two step electrode process at all the tested η values. The surface process (oxide
film coverage) step remains the same for both electrodes at different overpotentials. In
the kinetic controlled process step, the impedance decreases with increasing
53
Figure 4-5. Nyquist plot of the impedance spectroscopy of copper foil electrode in 1M
LiPF6/ PC:EC:DMC [1:1:3 vol.] at different overpotentials (OCV–100 mV to
OCV+100 mV); Frequency range 100 kHz to 0.01 Hz.
54
Figure 4-6. Nyquist plot of the impedance spectroscopy of graphite-coated copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different overpotentials (OCV–
100 mV to OCV+100 mV); Frequency range 100 kHz to 0.01 Hz.
55
overpotential which is seen as smaller semicircles on the spectra. For copper electrode,
the maximum imaginary impedance for this process decreased from around 6000 Ω at
OCV – 100 mV to about 1200 Ω at OCV, and then to only about 100 Ω at OCV + 100
mV. As for the graphite-coated copper foil electrode, the decrease of the same value is
from about 8000 Ω at OCV – 100 mV to about 4000 Ω at OCV, and then to about 600
Ω at OCV + 100 mV. The relative real impedance values for this process are also
decreasing respectively with different overpotentials. The detailed maximum
impedance values for both processes are listed in Table 4-1. An obvious reason for the
decrease of the impedance with increasing overpotential in the kinetic controlled
process is that a higher voltage makes the electrons transfer faster in the cell system,
thus lowering the impedance of the system. The low frequency looks like depressed
semicircle which is related to a finite diffusion length.
In Figure 4-7 and Figure 4-8 the three dimensional impedance spectra were
constructed for copper foil and graphite-coated copper foil electrodes. Only the kinetic
controlled process can be observed due to the large scale of the figure. It can clearly be
seen that compared to the spectra of graphite-coated copper foil electrode, the
impedance of copper foil electrode showed a more drastic change in impedance with
the decrease of the overpotential. This observation is consistent with the discussion
earlier in this chapter that the graphite coating protected the copper foil from oxidation
during the electrode process. Higher overpotentials produced a higher oxidation
current and a more severe dissolution of the copper foil. Both of the two factors
lowered the
56
Table 4-1. Comparison of peak impedance value of both copper foil and
graphite coated copper foil electrode in the kinetic controlled process.
Electrode
Overpotential (mV)
R’ (Ω)
-R” (Ω)
OCV -100
7894
6128
OCV-50
6148
3685
OCV
1828
1192
OCV + 50
825
454
OCV + 100
145
97
OCV -100
4623
7509
OCV-50
4301
6875
OCV
3802
3975
OCV + 50
1441
1085
OCV + 100
908
572
Cu
Cu-C
R’: Real impedance
R”: Imaginary impedance
Figure 4-7. Three dimensional plot of the impedance spectroscopy of copper foil electrode in
1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different overpotentials (OCV–100 mV to OCV+100
mV); Frequency range 100 kHz to 0.01 Hz.
Figure 4-8. Three dimensional plot of the impedance spectroscopy of graphite-coated copper
foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different overpotentials (OCV–100 mV
to OCV+100 mV); Frequency range 100 kHz to 0.01 Hz.
59
impedance of the system even further that the protected graphite-coated copper foil
electrode.
To better understand the impedance, Bode representations of the impedance
spectra shown in Figure 4-5 and 4-6 are plotted in Figure 4-9 and 4-10. Figure 4-9 (a)
and (b) show the relationship between the logarithmic values of the impedance and the
logarithmic value of the frequency for the copper foil and graphite-coated copper foil
electrodes, respectively. For both electrodes the impedance increased with decreasing
frequency. The small increasing slope in the high frequency range represents the
surface process and the large increasing slope in the low frequency range represents
the kinetic controlled process. All the increasing curves tend to level off at the end of
second step which clearly indicated the kinetic controlled process transformed into a
diffusion controlled process when the frequency was low enough. The kinetic
controlled process started at ca. 10 kHz for copper foil and ca. 100 Hz for graphitecoated copper foil. Figure 4-10 shows the relationship between phase angle (φ) and log
[frequency] of (a) copper foil and (b) graphite-coated copper foil. The maximum phase
angle of copper foil ~ 72° appeared around 100 Hz and decreased with increasing η
values in the investigated range. For graphite-coated copper foil, the maximum phase
angle ~ 82° appeared around 1 Hz with the same decreasing tendency with increasing
overpotential.
Based on the experimental results, an equivalent circuit for both electrodes is
proposed in Figure 4-11. Rs is the uncompensated resistance including the resistance
of electrolyte and the Ni wires. The value of the Rs is about 2 Ω for both electrodes.
Figure 4-9 (a) Bode representation of impedance spectroscopy of copper foil electrode; in 1M
LiPF6/ PC:EC:DMC [1:1:3 vol.] log (Impedance) vs. log (Frequency).
Figure 4-9 (b) Bode representation of impedance spectroscopy of graphite-coated copper foil
electrode; in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] log (Impedance) vs. log (Frequency).
Figure 4-10 (a) Phase angle vs. log (Frequency) of impedance spectroscopy of copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different overpotentials.
Figure 4-10 (b) Phase angle vs. log (Frequency) of impedance spectroscopy of graphite-coated
copper foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different overpotentials.
64
The first resistance-capacitance (RC) circuit represents the small semicircle in the
Nyquist plot of the impedance spectra at the high frequency range. For both electrodes,
it represents an oxide film formation on the copper foil surface. The resistance for both
electrodes during this process is about 10 Ω. The second RC circuit represents the
large half circle in the Nyquist plot of the impedance spectra in the low frequency
range. The capacitance has been replaced with a CPE (constant phase element) to
represent the depressed semicircle. For the copper foil electrode, it represents a kinetic
controlled process during which charge transfer and further oxidation of the copper
occurs. The charge transfer resistance (RCT) values in this process for copper electrode
extracted from impedance spectra are about 4 kΩ at OCV, 1.6 kΩ at 50 mV above
OCV and 250 Ω at 100 mV above OCV. For potentials lower than OCV one can only
tell from the spectra that the RCT values are well over 10 kΩ. For the graphite-coated
copper foil electrode, this oxidation process is more complex related to a finite
diffusion length with a transmissive boundary condition through the porous carbon
layer which resulted in a higher impedance as compared to the copper foil electrode.
Oxidation of impurities is another contributing factor. 18, 19 RCT values extracted from
impedance spectra in this process are about 8 kΩ at OCV, 2000 Ω at 50 mV above
OCV and 1 kΩ at 100 mV. Similarly, for potentials lower than OCV the RCT values
are hard to extract and can only be reported as over 10 kΩ as well. ZWr and ZWt
represent reflective and transmissive Warburg impedances, respectively, which
dominate the extreme low frequency range.
65
C1
CPE
Rs
Rct1
Rct2
Zwr
Zwt
Figure 4-11. Proposed equivalent circuit of both copper foil and graphite-coated foil
electrode; Rs: uncompensated resistance; C1: capacitance of the surface oxidation
process; CPE: constant phase element of kinetics controlled process; R ct1: charge
transfer resistance of surface oxidation process; Rct2: charge transfer resistance of
kinetic controlled process; Zwr: reflective Warburg impedance; Zwt: transmissive
Warburg impedance.
66
4.3.3
Impedance of Graphite-coated Copper Foil with Different Saturation Time in
the Electrolyte
From discussion in 4.3.2 it is known that the graphite coating protected the
copper foil from oxidation. Since the graphite coating on the electrode is porous,
another set of impedance experiments was performed on the graphite coated electrode.
After the cell was assembled, the electrode was immersed in the electrolyte for a
certain period of time to allow for saturation before the measurement at open circuit
voltage. The Nyquist impedance spectra after 24, 48 and 60 h of saturation time are
shown in Figure 4-12. The impedance spectra at longer saturation times remain almost
identical after 60 h. It was found that the high frequency response remained the same
after saturation. On the other hand, the low frequency impedance decreased
significantly as compared to that without saturation. A reasonable explanation would
be that the electrolyte replaced the inert Ar gas in the porous graphite coating, thus
facilitating the electron transfer during the kinetic controlled process. A bare copper
electrode does not show a change in impedance spectra with different immersion times.
Figure 4-12 Nyquist plot of the impedance spectroscopy of graphite coated copper foil
electrode after 24 h, 48 h, 60 h of saturation in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.]; Initial
potential at OCV = 3.36 V; Frequency range 100 kHz to 0.01 Hz.
68
4.3.4 Impedance at Different Temperatures
Temperature is a critical factor in lithium ion battery applications. With the
controlled-potential electrolysis study earlier in Chapter 3 it was found that
temperature plays an important role in the electrode oxidation process. As the range of
0 ˚C to 50 ˚C is a common working temperature for most commercially available
lithium ion batteries, the impedance spectra of both copper foil and graphite copper
foil electrodes were also studied at 0 ˚C and 50 ˚C and compared with that of the 25 ˚C.
Before each experiment the cell body was submerged in the temperature bath for 1.5 h
to ensure that the entire cell reached the desired temperature.
Nyquist impedance spectra of copper foil and graphite-coated copper foil
electrode at different temperatures were shown in Figure 4-13 and Figure 4-14,
respectively. The impedance of the high frequency part was also shown in the offset in
each figure. The two depressed semicircle structure of the impedance spectra can still
be clearly observed for both electrodes. The overall impedance for both electrodes
seems to be decreasing with the increasing temperature. This is consistent with our
observations in Chapter 3 that the exchange current increases with the increasing
temperature. In the kinetic controlled step, the maximum imaginary impedance
decreased from around 13 kΩ at 0 ˚C to about 1200 Ω at 25 ˚C then to only about 200
Ω at 50 ˚C for copper foil electrode. The same value for graphite-coated copper foil
electrode also decreased from nearly 12k Ω at 0 ˚C to about 4 kΩ at 25 ˚C and then to
a mere 6 Ω at 50 ˚C. It is also noticed that the high frequency semicircle also
decreased with the increasing temperature. Since the high frequency semicircle
Figure 4-13. Nyquist plot of the impedance spectroscopy of copper foil electrode in 1M LiPF6/
PC:EC:DMC [1:1:3 vol.] at different temperatures ( from 0 ˚C to 50 ˚C ); Frequency range
100 kHz to 0.01 Hz.
Figure 4-14. Nyquist plot of the impedance spectroscopy of graphite-coated copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different temperatures ( from 0 ˚C to 50
˚C ); Frequency range 100 kHz to 0.01 Hz.
71
represents the surface process, an explanation for the decrease is that with the
increasing temperature, the surface oxidation of the copper is accelerated and
increases the electron flow in the circuit, thus lower the impedance of the process. The
detailed value of the impedance spectra is listed in Table 4-2.
Bode representations of the impedance spectra shown in Figure 4-13 and 4-14
are also plotted in Figure 4-15 and 4-16. Figure 4-15 shows the relationship between
the logarithmic values of the impedance and the logarithmic value of the frequency. It
can be clearly seen that for both electrodes the surface process occurs between ca. 100
kHz and ca. 10 kHz. The kinetic controlled process occurs between ca. 1 kHz and ca.
0.1 Hz for copper foil electrode. While for graphite coated copper foil electrode the
same process occurs between ca. 10 Hz and ca. 0.01 Hz. The increasing curves tend to
level off at extreme low frequency for both electrodes at all temperatures which
demonstrates the diffusive feature of the electrode process when frequency is low
enough. Figure 4-16 (a) and (b) show the relationship between phase angle (φ) and log
[frequency] of the impedance spectra for copper and graphite coated copper foil
electrodes, respectively. For both electrodes the maximum phase angle appeared at 0
˚C and decreased with increasing temperature. The maximum phase angle was 70° at
10 Hz for copper and 80° at 0.1 Hz for graphite-coated copper foil electrode.
Based on the experimental results and the equivalent circuit proposed earlier in
this chapter, some parameters of the equivalent circuit are calculated and listed in
Table 4-3. From the results it can be seen, the surface oxidation processes are almost
identical for copper foil and graphite-coated copper foil at 25 ˚C. Charge transfer
72
Table 4-2. Comparison of peak impedance value of both copper foil and
graphite coated copper foil electrode at different temperature.
High Frequency
Electrode
Cu
Cu-C
Low Frequency
Temperature (˚C)
Z’ (Ω)
-Z” (Ω)
Z’ (Ω)
-Z” (Ω)
0
18.8
16.1
16210
13260
25
10.9
4.8
1828
1192
50
8.4
5.5
468
237
0
17.2
12.4
6003
12410
25
6.0
4.1
3802
3975
50
4.9
3.7
51.8
18.5
Figure 4-15 (a) Bode representation of impedance spectroscopy of copper foil electrode at
different temperatures; in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] log (Impedance) vs. log
(Frequency)
Figure 4-15 (b) Bode representation of impedance spectroscopy of graphite-coated copper foil
electrode at different temperatures; in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] log (Impedance) vs. log
(Frequency).
Figure 4-16 (a) Phase angle vs. log (Frequency) of impedance spectroscopy of copper foil
electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different temperatures.
Figure 4-16 (b) Phase angle vs. log (Frequency) of impedance spectroscopy of graphite-coated
copper foil electrode in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different temperatures.
77
Table 4-3. Values of equivalent circuit parameters for copper foil and graphite
coated copper foil electrodes at different temperatures.
Parameters
Electrode
Cu
Cu-C
Temperature (°C)
Rs (Ω)
C1 (μF)
R ct1 (Ω)
0
2.7
0.13
32.2
25
1.9
0.26
9.4
50
2.5
0.82
10.8
0
4.8
0.17
24.8
25
2.2
0.23
8.2
50
0.2
0.46
7.4
78
resistance decreased with increasing temperature while the respective capacitance
increased, for both copper foil and graphite-coated copper foil electrode. This
corresponds with the explanation of the temperature effect in the surface oxidation
process.
4.3.5
Impedance of Graphite-coated Copper Foil Electrode at Different
Temperatures After Saturation
In 4.3.3 it was found that the impedance of the graphite-coated copper foil
electrode decreased significantly after saturation in the electrolyte for a certain time. It
was also found in 4.3.4 that temperature played an important role in the impedance
study of the electrode. In this part of the work, the impedance behavior of presaturated graphite-coated copper foil electrodes was studied under different
temperatures. After the cell assembly, the electrode was pretreated through immersion
into the electrolyte for 48 h at the desired temperatures (0 ˚C and 50 ˚C) in the
temperature bath. OCV experiments were carried out before all impedance
measurements to obtain the initial potential for each experiment. The results were then
compared with the 25 ˚C result.
Figure 4-17 shows the Nyquist impedance spectra of graphite-coated copper
foil electrode after electrolyte saturation at different temperatures. The high frequency
portion of the spectra was shown in the offset for better observation. The two
depressed semicircle structure remained the same in this set of experiments as was
expected. However, the overall impedance decreased significantly with the increasing
600
0C
25 C
500
50 C
35
30
300
-Z" [ohm]
-Z" [ohm]
400
200
25
20
15
10
5
100
0
0
10
20
30
40
50
60
Z' [ohm]
0
0
200
400
600
800
1000
1200
Z' [ohm]
Figure 4-17. Nyquist plot of the impedance spectroscopy of graphite-coated copper foil electrode
saturated for 48 h in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different temperatures ( from 0 °C to 50
˚C ); Frequency range 100 kHz to 0.01 Hz.
80
temperature in both the high frequency and low frequency portion of the experiment.
In the surface process (oxide film coverage), the maximum imaginary decreased from
about 11.6 Ω at 0 ˚C to nearly 6.3 Ω at 25 ˚C and then 3.7 Ω at 50 ˚C. The increased
temperature and the replacement of the inert Ar gas to conductive electrolyte greatly
increased the speed of the oxide film formation at the copper surface under the
graphite coating. In the kinetic controlled step, the maximum imaginary impedance
was about 400 Ω at 0 ˚C. It dropped to only about 31 Ω at 25 ˚C and then 5.5 Ω at 50
˚C. Detailed impedance values are listed in Table 4-4.
Figure 4-18 and 4-19 are the Bode representations of the impedance spectra
shown in Figure 4-17. Figure 4-18 shows the relationship between the logarithmic
values of the impedance and the logarithmic value of the frequency. Despite the
obvious decreasing of the impedance value from low temperature to high temperature,
the surface process remains to occur between ca. 100 kHz and ca. 10 kHz. The kinetic
controlled process occurs between ca. 10 Hz and ca. 0.1 Hz. It seems that the higher
the temperature is, the slower the impedance increases with frequency. It is safe to say
that with the increasing temperature this whole electrode process becomes more and
more diffusion-controlled. Figure 4-19 shows the relationship between phase angle (φ)
and log [frequency]. In the kinetic controlled process, the maximum phase angle of
60° appeared at 1 Hz for 0 ˚C. At 25 ˚C the maximum phase angle dropped to 33°
which appear between 1 Hz and 10 Hz. When the temperature increased to 50 ˚C, the
maximum phase angle decreased to only 15° at higher than 10 Hz. The low frequency
semicircle was more depressed because the increased finite length diffusion with
81
Table 4-4. Comparison of peak impedance value of graphite coated copper foil
electrode after 48 h of electrolyte saturation at different temperature
High Frequency
Electrode
Cu-C
Low Frequency
Temperature (˚C)
Z’ (Ω)
-Z” (Ω)
Z’ (Ω)
-Z” (Ω)
0
13.2
11.6
700
400
25
9.6
6.3
60.1
31.0
50
5.6
3.7
21.1
5.5
4
0C
log(Z/[ohm])
3
25 C
50 C
2
1
0
-3
-2
-1
0
1
2
3
4
5
log(frequency/[Hz])
Figure 4-18 Bode representation of impedance spectroscopy of graphite-coated copper
foil electrode saturated for 48 h in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at different
temperatures; log (Impedance) vs. log (Frequency).
6
90
80
-Phase [deg]
0 C
70
25 C
60
50 C
50
40
30
20
10
0
-3
-2
-1
0
1
2
3
4
5
log(frequency/[Hz])
Figure 4-19 Phase angle vs. log (Frequency) of impedance spectroscopy of graphitecoated copper foil electrode saturated for 48 h in 1M LiPF6/ PC:EC:DMC [1:1:3 vol.] at
different temperatures.
6
84
transmissive boundary condition through the porous carbon layer with the increasing
temperature. In the surface process, although the impedance also decreased with
increasing temperature, the phase angle change remained almost the same.
4.4
Conclusion
The impedance spectra of both copper foil and graphite-coated copper foil
electrodes in a nonaqueous Li-ion battery electrolyte were shown to undergo a twostep process. The first step for both electrodes is a surface oxidation process at the
interface of the electrode and the electrolyte at the beginning of the experiment in the
high frequency range. For the copper foil electrode the second step is a kinetic
controlled charge transfer and double layer process during which further copper
oxidation occurs. At the graphite-coated copper foil electrode the graphite coating
blocks the electrode surface area creating diffusional obstruction, which resulted in the
higher impedance value for the graphite-coated copper foil electrode in the kinetic
controlled process. The different impedances observed reflected the different
processes that occurred on the two electrodes in the low frequency range. Higher
electron transfer rate caused by increasing overpotential will decrease the impedance
of both electrode processes. The impedance of both electrodes decreased with
increasing temperature. This can be explained by the accelerated surface oxidation of
copper and increasing electron flow in the circuit. An equivalent circuit of the
electrode process for the two electrodes is proposed and some parameters are
calculated. Because of the porous nature of the graphite coating, the impedance of the
85
graphite-coated copper foil electrode will be significantly reduced by electrolyte
saturation of the coating as the inert Ar gas is replaced by the conductive lithium ion
battery electrolyte.
86
Chapter 5 –Summary and Future Work
5.1
Summary and Conclusions
Studies of the fundamental electrochemical behavior and stability of the anode
materials (battery-grade graphite-coated copper foil) of Lithium-ion batteries in Li-ion
battery electrolytes (nonaqueous organic carbonates) were performed in this
dissertation.
In Chapter 1, a brief review of the technical aspects of Li-ion cells was given
followed by a statement of the motivation of this research.
In Chapter 2, the experimental conditions of this study were introduced. Some
preliminary work for this study as part of the previous research in this laboratory was
also included in this chapter. The effect of trace amounts of HF in the electrolyte was
studied as one of the pertinent cell factors. It was found the OCV of Cu-C foil
electrode was shown to undergo a two-step process over time, which resulted from the
interaction between the graphite coating and the electrolyte solution. In the first step,
the OCV dropped quickly until reaching a minimum as a result of adsorption of
electrolyte solvents on the graphite coating. In the second step, the OCV gradually
increased as a result of specific adsorption of HF in the graphite coating, which
protected the Cu substrate from oxidation. The foil substrate was shown to have
limited influence on the OCV variation.
87
To better understand the nature of the kinetic processes of battery grade
copper and graphite-coated copper foil electrodes, controlled-potential electrolysis of
both electrodes was performed in order to construct Tafel plots to obtain values of the
exchange current and transfer coefficient in Chapter 3. The transfer coefficients of
both electrodes were found to be a small value, which was consistent with an
assumption of a dominant anodic process in the cell. At 25 ˚C the graphite-coated
copper foil was found to have a higher exchange current value than the copper foil.
This can be explained by the intercalation of lithium ion into the graphite coating
which increases the electron transfer rate. In the temperature range of 0 ˚C to 50 ˚C,
the exchange currents of both electrodes increased with the temperature, but at
different rates, while the transfer coefficients were not significantly affected by the
temperature.
Impedance spectroscopy is increasingly being applied to electrochemical
studies because of its ability to characterize many of the electrical properties of
materials and the electrode-electrolyte interfaces. In Chapter 4 the electrochemical
impedance spectroscopy of both copper foil and graphite-coated copper foil electrodes
was obtained under different conditions. Detailed studies showed that both electrodes
gave similar impedance spectra of two successive semicircular arcs. When
overpotential was increased for both electrodes, the high frequency semicircles
remained the same on each electrode, but the second semicircle increased for both
electrodes. It was also found that the impedance of both electrodes decreased with
increasing temperature which can be explained by the accelerated surface oxidation of
88
copper and increasing electron flow in the circuit. Because of the porous nature of the
graphite coating, the impedance of the graphite-coated copper foil electrode will be
significantly reduced by electrolyte saturation of the coating as the inert Ar gas is
replaced by the conductive lithium ion battery electrolyte. Based on the impedance
spectra results of the electrodes, it appeared that both electrodes went through a
surface charging process at high frequency. At low frequency oxidation occurs at both
electrodes, but is kinetically controlled for bare copper, while the graphite-coated
copper undergoes diffusional blocking through the porous carbon layer. An
equivalent circuit of the impedance spectrum was then proposed.
5.2
Suggestions for Future Research
The combination of reflective and transmissive Warburg impedance makes this
electrode process difficult to analyze quantitatively. In order to further investigate the
electrochemical stability of the anode materials in lithium ion batteries and the
reaction mechanism at the electrode/electrolyte interface, further impedance
spectroscopy studies can be performed if different lithium ion battery electrolytes and
copper foil with graphite coating at different thickness could be obtained. With the
known thickness of the graphite layer and the established equivalent circuit model,
calculation may be made to determine the diffusion coefficient of reactant through
carbon layer and also further analyze the contribution of different factors such as
electrode reactions, mass transfer or surface layers, etc. in the whole process.
89
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