DEVELOPMENT OF HIGH PERFORMANCE AIR-CATHODES
FOR SOLID STATE LITHIUM-AIR CELLS
Thesis
Submitted to
The School of Engineering of the
UNIVERSITY OF DAYTON
in Partial Fulfillment of the Requirements for
The Degree
Master of Science in Mechanical Engineering
by
Vasisht Garlapati
UNIVERSITY OF DAYTON
Dayton, Ohio
May, 2010
DEVELOPMENT OF HIGH PERFORMANCE AIR-CATHODES
FOR SOLID STATE LITHIUM-AIR CELLS
APPROVED BY:
Binod Kumar, Ph.D.
Advisory Committee Chairman
Group Leader, Electrochemical Power,
Metals and Ceramics Division
Margaret Pinnell, Ph.D.
Committee Member
Associate Professor, Mechanical and
Aerospace Engineering Department
Vinod K. Jain, Ph.D.
Committee Member
Professor, Mechanical and Aerospace
Engineering Department
Malcolm Daniels, Ph.D.
Associate Dean
School of Engineering
Tony E. Saliba, Ph.D.
Dean, School of Engineering
ii
ABSTRACT
DEVELOPMENT OF HIGH PERFORMANCE AIR-CATHODES
FOR SOLID STATE LITHIUM-AIR CELLS
Name: Garlapati, Vasisht
University of Dayton
Advisor: Dr. Binod Kumar
A solid state lithium-air battery is receiving considerable attention by the battery
community recently. A challenging part of making a solid state lithium-air battery is to
develop a solid state air-cathode. The present study relates to the development of the aircathode. The air-cathode consists of a lithium ion conducting material, electron
conducting material, metal substrate, binder and dispersant. For lithium ion conduction
Lithium aluminum germanium phosphate (LAGP) glass-ceramic powder was used. For
electron conduction two types of carbon were used. One type of carbon help in providing
better electron conductivity and the other type of carbon helps in pore formation in the
cathode. Nickel mesh/foam was used for structural support and current collection.
Polytetrafluoroethylene (PTFE) was used to bind LAGP and carbon. Dispersant was used
for preventing LAGP and carbon powders from agglomerating. The investigation
includes evaluation of processing parameters including the effect of LAGP or dispersant
concentration on the rate capacity. LAGP glass was prepared at 1350°C and crystallized
at 850°C for 12 hours to transform it into a glass- ceramic. The cathodes prepared from
the batch materials and processed were characterized to determine porosity, surface area,
iii
pore size and volume. To evaluate the electrochemical properties of these cathodes
twelve lithium-air cells were fabricated and tested in the temperature range 45 – 115°C
and were characterized using a Solartron 1260 impedance analyzer with 1287
electrochemical interface in oxygen atmosphere under 1 kPa pressure.
It was determined that the use of dispersant helped the cathode obtain a porosity of
22% which is due to the proper dispersion of LAGP in cathode. A dispersant
concentration of 5.92 wt% helped the cell discharge at a voltage of 2.5 V for 35 hours
and achieved a capacity of 14.1 mAh at 75°C. LAGP concentration of 90 wt% helped in
discharging the cell with a current of 0.50 mA for 13 hours at 75°C. The enhanced
capacity of the cell is attributed to the catalyzing effect of LAGP for oxygen reduction. A
capacity of 8.76 mAh was obtained at 67°C by having nickel foam side exposed to
oxygen flow. The capacity was obtained because of the better oxygen transport inside the
cathode.
iv
ACKNOWLEDGEMENTS
I would like to thank my advisor, Dr. Binod Kumar, for providing this valuable
opportunity for carrying out my current research and also for his support and guidance. I
would also like to thank Dr. Jitendra Kumar for his valuable time and continual support in
helping me understand the practical aspects of the research and answering my questions.
I am grateful to my committee members Dr. Margaret Pinnell and Dr. Vinod Jain for
their helpful comments. I would like to thank Mr. Robert Leese and Sheila Liskany for
their varied contributions and assistance.
v
TABLE OF CONTENTS
ABSTRACT......................................................................................................................iii
ACKNOWLEDGEMENTS..............................................................................................v
LIST OF FIGURES..........................................................................................................ix
LIST OF TABLES............................................................................................................xi
CHAPTER I
INTRODUCTION.......................................................................................................1
1.1
Advantages of Metal-air batteries....................................................3
1.2
Prior work on cathodes for lithium-air cell......................................4
CHAPTER II
EXPERIMENTAL.......................................................................................................6
2.1
Lithium-oxygen/air cell and materials for cathode..........................6
2.1.1 LAGP......................................................................................7
2.1.2 Carbon.....................................................................................7
2.1.3 Nickel mesh/nickel foam........................................................8
2.1.4 Polytetrafluoroethylene (PTFE) binder...................................9
2.1.5 Dispersant.............................................................................10
2.2
Formulation and Processing of Materials......................................10
2.3
Preparation of the cathode paste....................................................12
2.3.1 Hand mixer method...............................................................12
2.3.2 Energy mill method...............................................................12
vi
2.3.3 Modified energy mill method...............................................13
2.4
Preparation of lithium-air cell components....................................14
2.4.1 Processing of polymer-ceramic membrane...........................14
2.4.2 Processing of glass-ceramic membrane................................15
2.4.3 Cleaning of lithium metal strip.............................................15
2.4.4 Aluminum foil.......................................................................16
2.4.5 Preparation of air-cathode.....................................................16
2.5
Fabricate of Li-oxygen/air cell.......................................................16
2.5.1 Preparation of the electrolyte laminate.................................16
2.5.2 Preparation of the cap for coin cell.......................................17
2.5.3 Lithium-oxygen/Air cell assembly.......................................17
2.6
Characterization.............................................................................18
2.6.1 Porosity.................................................................................18
2.6.1.1 Water saturation technique....................................18
2.6.1.2 BET method...........................................................19
2.6.2 Electrical characterization.....................................................19
CHAPTER III
RESULTS AND DISCUSSIONS..............................................................................21
3.1
Effect of processing on physical properties of cathode.................21
3.2
Electrochemical performance of cathodes as influenced by
dispersant.......................................................................................24
3.2.1 Effect of thermal treatment on physical properties of
cathode...........................................................................................26
vii
3.2.2
Effect
of
dispersant
concentration
on
discharge
profiles...........................................................................................28
3.3
Effect of composition of cathode on electrochemical performance
of lithium-O2 cells..........................................................................31
3.4
Effect of cathode porosity, cell temperature and discharge current
on the performance of the lithium-O2 cell.....................................37
3.5
3.4.1
Effect of porosity on discharge capacity............................37
3.4.2
Effect of temperature on discharge capacity......................40
3.4.3
Effect of discharge current on capacity.............................42
3.4.4
Effect of cell resistance on discharge capacity..................44
Effect of oxygen transport on electrochemical performance of
cathode...........................................................................................46
CHAPTER IV
SUMMARY AND CONCLUSIONS........................................................................50
REFERENCES.................................................................................................................54
viii
LIST OF FIGURES
Figure 1-1: Electrochemical operation of a cell. (a) During discharge and (b) During
charge...............................................................................................................2
Figure 2-1: Schematic representation of the cell components…………………………...18
Figure 3-1: Discharge profiles of cells 1 and 2 with and without the dispersant in the
cathode at constant current (0.2 mA) and at a constant temperature
(75°C).............................................................................................................25
Figure 3-2: Optical micrograph of the cathode specimen at 50x magnification which has a
composition of 25 wt% carbon and 75 wt% LAGP with 5.92%
dispersant........................................................................................................26
Figure 3-3: Discharge profiles of two cells with cathodes containing 5.92 and 7.91 wt. %
of dispersant concentrations at constant current (0.2 mA) and temperature
(75°C).............................................................................................................29
Figure 3-4: Impedance of the cells with different dispersant concentration before
discharge at 0.2 mA at 75°C...........................................................................30
Figure 3-5: Discharge profiles of cells with different porosities.......................................39
Figure 3-6: Temperature dependence of discharge capacity.............................................40
Figure 3-7: (a) Discharge curves (voltage vs. time) for a cell at 67°C discharged with
different discharge currents of 0.50, 0.30 and 0.25 mA...............................43
ix
Figure 3-7: (b) drop in the cell voltage for the first 5 seconds with different discharge
currents of 0.50 mA, 0.30 mA and 0.25 mA..................................................44
Figure 3-8 (a): Nyquist plots for the cell before discharge................................................45
Figure 3-8 (b): The effect of cell resistance on the discharge capacity of the lithium-air
cell...........................................................................................................46
Figure 3-9: (a) Schematic representation of components in cell with nickel foam exposed
to oxygen flow and (b) Schematic representation of components in cell with
active materials exposed to oxygen flow.......................................................48
Figure 3-10: Curve (a) and (b) represents the discharge profiles for the cell orientation (a)
and (b) respectively.......................................................................................49
x
LIST OF TABLES
Table 1-1: Open circuit voltage and theoretical specific energies of metal-air batteries
(Reproduced from Ref. 2)...............................................................................3
Table 2-1: Air-cathode formulations with different LAGP and carbon concentrations....13
Table 2-2: Air-cathode formulation for the modified energy mill method........................14
Table 3-1: Summary of variables, specimens characterized and major observations…...22
Table 3-2: Properties of the cathode with different processing technique.........................23
Table 3-3: Properties of the cathode as a function of thermal treatment time...................27
Table 3-4: Air-cathode formulation with different concentrations of dispersant..............28
Table 3-5: Cathode composition and its properties...........................................................31
Table 3-6: Electrochemical data on lithium-air cell # 3 with cathode composition 10 wt%
carbon and 90 wt% LAGP...............................................................................33
Table 3-7: Electrochemical data on lithium-air cell # 4 with cathode composition 25 wt%
carbon and 75 wt% LAGP...............................................................................35
Table 3-8: Electrochemical data on lithium-air cell # 5 with cathode composition 50 wt%
carbon and 50 wt% LAGP...............................................................................36
xi
CHAPTER I
INTRODUCTION
A battery is an electrochemical device that converts the chemical energy contained in
the active material directly into electrical energy by means of oxidation-reduction
reactions.1 Advantages of a battery are many. As batteries directly convert chemical
energy into electrical energy, they are capable of having higher energy conversion
efficiencies. Unlike heat engines, batteries are not subjected to the limitations of Carnot
cycle. Batteries are very clean sources of energy as they do not produce any green house
gasses which affect the environment. Absence of moving parts makes the battery
operation noiseless.
A cell of a battery contains three major components; anode, cathode and electrolyte.
At the anode an oxidation reaction takes place leading to a generation of electrons which
are transported through the external circuit to the cathode provided through the external
circuit. A reduction reaction takes place at the cathode that requires consumption of
electrons. The electrolyte provides a medium for transfer of ions inside the cell between
the anode and cathode. The requirements of the three cell components are based on these
three basic functions.
1
Batteries are classified either as primary or secondary depending on their
rechargeability. Primary batteries are not capable of being recharged. Hence they are used
only once and then discarded. Secondary batteries can be charged electrically to their
original charged state. The secondary batteries have lower energy density compared to
primary batteries.1
The two operations that can be performed in a rechargeable metal-air cell are
discharge and charge. These two operations are explained in figure 1-1.
Discharge
The operation of a metal-air cell during discharge is shown in the figure 1-1 (a).
When a cell starts drawing current, metal anode gets oxidized forming metal ions and
electrons (M → M+ + e-). These electrons flow to the cathode through the external circuit.
These electrons are accepted by the cathode during which the oxygen molecule is
reduced (O2-/O1-) and either M2O or M2O2 is formed. The electric circuit is completed in
the electrolyte by the flow of cations to the cathode.
(a)
(b)
Figure 1-1: Electrochemical operation of a cell. (a) During discharge and (b) During
charge
Charge
The operation of a cell during charge is shown in the figure 1-1 (b). During this
process current flow is reversed. The reactant product accumulated during discharge is
2
oxidized at the cathode, during which it gets dissociated into metal ions, electrons and
oxygen. These electrons flow to the anode through the external circuit. The M+ ion is
transported to the anode through the electrolyte. This metal ion gets reduced with the
electron to form pure metal which is plated on the residual metal anode.
There are different kinds of commercially available batteries. They are lithium, zinccarbon, lead-acid, nickel-cadmium, lithium-ion and metal-air.
1.1 Advantages of Metal-air batteries
There are many advantages of metal-air batteries over commercially available
batteries.1 They possess high energy density, flat discharge, longer shelf life and
environmental compatibility. As oxygen is the unlimited cathode reactant, the capacity of
the battery is limited by the weight of metal anode.
Table 1-1: Open circuit voltage and theoretical specific energies of metal-air batteries
(Reproduced from Ref. 2)
Metal-air
battery
Li/O2
Na/O2
Calculated Theoretical specific
OCV, V
energy, Wh/kg
Including Excluding
oxygen
oxygen
2.91
5200
11140
1.94
1677
2260
Ca/O2
3.12
2990
4180
Mg/O2
2.93
2789
6462
Zn/O2
1.65
1090
1350
Table 1-1 shows the open circuit voltage and theoretical specific energy densities of
metal-air batteries which are in use or under development. From the table 1-1 it is noted
that lithium metal when coupled with air cathode has the highest theoretical specific
3
energy compared to any other metal-air cell. Hence considerable research activities are
underway to harness the chemical energy of lithium in an efficient manner.
Advantages of lithium as anode in a lithium-air cell are:1,3
1. Higher specific energy density (Theoretical 11,140 Wh/kg, Practical 3,700 Wh/kg).
2. High open circuit voltage (2.9V to 3.1V).
3. Relatively inexpensive.
4. Environmentally benign (does not emit greenhouse gas).
1.2 Prior work on cathodes for lithium-air cell
To harness the chemical energy contained in the lithium metal efficiently, many
researchers are developing air cathodes which can increase life, energy density,
rechargeability, and achieve higher current densities. To accomplish this many
researchers have used a liquid medium (organic or inorganic) for the conduction of
lithium-ions in the cathode.4-13 Kumar et al.14 suggested the use of ionic glass-ceramic
(GC) in the cathode as an alternative to ionic liquid electrolytes. An energy density of
750 Wh/kg has been reported with the use of GC powder in the cathode.14
In this research, the performance of a totally solid-state cathode for use in a lithiumair cell is presented and discussed. Processing of the materials for an air cathode is
enumerated. Different processing techniques for making an air cathode paste are
presented. The fabrication technique for making cathode is discussed in detail. To
evaluate these cathodes, a few lithium-air cells were fabricated to investigate the effect of
composition on porosity and operation temperature. The overall objective of this work is
4
to recognize the working parameters to develop an efficient solid state air cathode for
lithium air cells.
5
CHAPTER II
EXPERIMENTAL
2.1 Lithium-oxygen/air cell and materials for cathode
A lithium-air cell comprises of anode, electrolyte and cathode. These three
components are used to carry out electrochemical reactions which convert chemical
energy into electrical energy. The electrochemical reaction in a typical lithium-air cell
can be expressed by either equation 1 or 2:15
2Li + 1/2O2 → Li2O
(1)
2Li + O2 → Li2O2
(2)
During lithium-air cell reaction, lithium metal gets oxidized at anode forming lithium
ion and electron (Li→ Li
+
+ e-). The lithium ion moves through the electrolyte to air-
cathode internally and the electron moves through the metallic circuit to the air-cathode
externally. Oxygen (source: air) available at air-cathode gets reduced to oxygen anion
(O2-/O1-) by electron transfer. Subsequently, the oxygen anion (O2-/O1-) reacts with
lithium ion and form either Li2O or Li2O2 according to equations (1) or (2) respectively.
To continue the aforementioned electrochemical reactions, the reactants (viz. Li+ ion,
O2-/O1-anion and e-) must move throughout the cathode matrix and meet (all three
6
reactants) at a common point called triple phase boundary (TPB). To form the TPB in aircathode following materials were selected:
Lithium aluminum germanium phosphate (LAGP) glass ceramic – A medium for Li+
transport and a catalyst for oxygen reduction,
Carbon – A medium for transport of electrons,
Ni mesh or Ni foam – A support structure and current collector as LAGP and carbon
are mechanically very fragile,
Polytetrafloroethylene (PTFE) – A binder for LAGP and carbon,
Dispersant – To prevent LAGP/carbon powder agglomeration
The details of the materials used for making air-cathodes are presented in the
following paragraphs:
2.1.1 Lithium aluminum germanium phosphate
LAGP is a glass-ceramic originally developed by Fu16 and has been synthesized in
our laboratory.17 The LAGP is an excellent ionic conductor. In solid-state it has a
conductivity of
10-3-10-1 S/cm in the 0 to 110°C temperature range. It has been
determined that LAGP is an insulator and may prevent formation and growth of dendrites
while a battery (using LAGP as electrolyte/cathode) is operational. Moreover, recently it
was observed that LAGP catalyzes oxygen reduction (O2 +2e- → O2-) reaction (ORR).14
Thus, LAGP was selected due to the aforementioned characteristics to meet the
requirements for a solid-state air-cathode.
2.1.2 Carbon
Carbon is a very good electronic conductor. There is a wide range of commercially
available carbon powders with diverse physical and chemical properties. Certain carbon
7
materials possess large active surface area and inherently act as a catalyst for the oxygen
reduction (O2 +2e- → O2-) reaction. Carbon possesses a porous structure which is one of
the most important requirements for oxygen diffusion in air-cathode matrix to facilitate
the cathodic reaction during discharge.
Ketjen black EC-600JD is a highly branched, high surface area (1400 m2/g) and low
density (0.11 g/cc) carbon which helps in improving electrical conductivity. What makes
Ketjen black special is its low ash content (0.1% w/w max) that leads to a superior
electronic conductivity. PWA activated carbon is used for achieving highly porous
cathodes in this study. It is a high density (0.51 g/cc) carbon. The surface area of the
PWA activated carbon (900-1000 m2/g) is not as high as Ketjen black EC-600JD. The
PWA activated carbon has an ash content of 6 wt% which makes it a poor electronic
conductor compared to Ketjen black EC-600JD. The use of PWA activated carbon is the
meso-pores which help in increasing the effective porosity of the cathode by interlinking
these meso-pores through macro-pores. The high effective porosity is required to achieve
adequate oxygen diffusion in the cathode matrix.
2.1.3 Nickel mesh/nickel foam
The air-cathode materials which take part in the electrochemical reaction are
comprised of electronic and ionic conductors. In the present study carbon and LAGP
were selected as electronic and ionic conductor respectively; therefore, they constitute the
active cathode materials. By using these two materials one can fabricate an air-cathode,
but the composite of carbon and LAGP is mechanically very fragile and thus cannot be
useful as such. To provide support, Ni mesh or foam was selected. In addition to
8
providing mechanical support Ni mesh or foam acts as current collector or electron
transporter.
Ni-mesh/foam with 96% porosity and the surface density between 350-420 g/m2 was
used as a substrate for the active materials. Initially Ni mesh was used which has only 36
pores per inch (PPI). Later on Ni foam was used which has 110 PPI. The change from Ni
mesh to foam was justified in view of increasing the number of cathodic reactions. Since
nickel foam has about three times more PPI as compared to Ni mesh, the number of
TPB’s can be maximized. Hence higher cathode performance can be achieved.
2.1.4 Polytetrafluoroethylene (PTFE) binder
Due to different surface properties, carbon and LAGP do not interact with each other
in a common solvent to form a mechanically coherent bulk structure. Therefore, a
homogeneous and mechanically stable mixture of carbon and LAGP could not be
achieved. For proper mixing of both LAGP and carbon, they have to be dispersed in a
liquid medium. PTFE (Teflon TE-3859) was used to bind carbon and LAGP. Teflon TE3859 is water soluble. The PTFE (TE-3859) is a colloidal solution that contains 60 wt%
polytetrafluoroethylene (PTFE) resign particles suspended in water. The PTFE resin is a
negatively charged hydrophobic colloid. It appears as a milky white liquid which
contains 6 wt% of nonionic wetting agent and stabilizers. Deionized water (D.I. water)
was used in the cathode paste preparation for proper mixing of the LAGP powder and
carbon and this water was removed later on during a drying process. Thus, the purpose of
PTFE binder was to provide a stable bond between carbon and LAGP throughout the
matrix of cathode.
9
2.1.5 Dispersant
Initially it was found that a mixture of carbon, LAGP and PTFE binder was not very
stable. After mixing LAGP started agglomerating and separated out of the mixture.
Agglomeration of LAGP may be due to its high density, large particle size and low
affinity with PTFE binder. To prevent LAGP agglomeration a dispersant named 2propenoic acid 2-methyl ammonium salt was selected. The dispersant is available in the
market by the trade name Darvan CN. Daravn CN is a clear to amber colored liquid
which contains 75% water and 25% ammonium polymethacrylate by weight. The Darvan
CN prevented LAGP agglomeration and helped to form a stable cathode mixture.
The essential ingredients to fabricate an air-cathode are: LAGP glass-ceramic, carbon,
Ni mesh/foam, PTFE binder and dispersant.
2.2 Formulation and Processing of Materials
For
processing
LAGP
glass-ceramic
powder,
a
40gm
batch
of
19.75Li2O.6.17Al2O3.37.04GeO2.37.04P2O5 (mol %) composition was prepared by using
reagent-grade chemicals such as Li2CO3 (Alfa Aesar), Al2O3 (Aldrich, particle size < 10
mm), GeO2 (Alfa Aesar) and NH4H2PO4 (Acros Organics). The chemicals were weighed,
mixed and ground for 10 min in an agate mortar and pestle. For further homogenization,
the batch was milled in a glass jar for one hour using a roller mill. The milled batch was
contained in a platinum crucible and transferred to an electric furnace. Initially, the
furnace was heated to 350°C at the rate of 1°C/min and held at that temperature for one
hour to release the volatile components of the batch before raising the furnace
10
temperature to 1350°C at the rate of 1°C/min after which the glass was melted for two
hours. A clear, homogeneous viscous melt was poured onto a stainless steel (SS) plate at
room temperature and pressed by another SS plate to yield < 1 mm thick transparent glass
sheet. Subsequently, the cast and pressed glass sheets were annealed at 500°C for two
hours to release thermal stresses and were then allowed to cool to room temperature.
These annealed specimens remained in the glassy state as noted by visual observation.
The annealed glass specimens were subsequently crystallized at 850°C for 12 hours. The
crystallization transformed the glass to a glass-ceramic that led to a change in the
appearance of the glass from colorless transparent to bluish opaque. This crystallized
glass-ceramic was ground using a high energy mill for one hour to form a fine powder.
This powdered glass-ceramic is then sieved using a 38 micron size sieve and the sieved
powder was in the size of less than 38 micron. This sieved LAGP powder was used for
making the cathode.
Both kinds of carbon namely; PWA activated carbon (Calgon Carbon Corp.) and
ketjen black (Akzo Noble) were dried at 80°C for one hour in argon atmosphere to
remove any moisture that may have been incorporated in the carbon during processing,
shipment and storage.
Ni mesh (Dexmet Corp.) and Ni foam (Dalian Thrive Metallurgy Import and Export
Co., Ltd.) were used as received.
Teflon TE-3859 and dispersant Darvan CN were also used as-received.
11
2.3 Preparation of the cathode paste
2.3.1 Hand mixer method
The LAGP glass-ceramic powder (less than 38 μm), PWA activated carbon, ketjen
black EC600JD, Teflon and D.I. water were weighed and measured in the required
proportions as tabulated in the table 2-1. Mixing was done by hand mixer for 5 – 6 hours
to form a good paste of both carbon and LAGP glass-ceramic powder. Initially this
method was used and subsequently it was abandoned.
2.3.2 Energy mill method
The cathode paste prepared using a hand mixer consumed a lot of time during which
some amount of carbon was spilled from the beaker. Subsequently an attempt was made
to mix all the components using an energy milling machine (SPEX, 8000M) with the
same proportions that are listed in the table 2-1 for one hour. The attempt was a success
in the cathode paste preparation and therefore the use of energy mill was practiced for
making the cathode paste.
12
Table 2-1: Air-cathode formulations with different LAGP and carbon concentrations
Cathode
Materials
Composition
90% LAGP + LAGP
10% C
Carbon
Activated carbon
Ketjen black
PTFE
D.I. water
Total
75% LAGP + LAGP
25% C
Carbon
Activated carbon
Ketjen black
PTFE
D.I. water
Total
50% LAGP + LAGP
50% C
Carbon
Activated carbon
Ketjen black
PTFE
D.I. water
Total
Weight
(gm)
0.90
Weight %
0.06
0.04
0.17
5.13
3.42
14.53
1.17
0.75
100
64.10
0.15
0.10
0.17
12.82
8.55
14.53
1.17
0.50
100
42.73
0.30
0.20
0.17
25.64
17.10
14.53
1.17
100
Volume
(ml)
76.92
0.40
8.00
0.40
8.00
0.40
8.00
2.3.3 Modified energy mill method
An attempt was made to make use of the dispersant to reduce the LAGP
agglomeration which became the basis of the modified energy mill method. Table 2-2
shows the cathode formulation prepared by this method. In this method, a measured
quantity of LAGP was taken into a vial. Next a measured amount of dispersant and D.I.
water was added to the LAGP powder. This mixture was allowed for milling for one
hour. Subsequently measured amounts of carbon and PTFE were added and milled for an
additional hour to obtain a homogeneous paste.
13
Table 2-2: Air-cathode formulation for the modified energy mill method
Cathode
Materials
Composition
75% LAGP + LAGP
25% C
Carbon
Activated carbon
Ketjen black
PTFE
D.I. water
Dispersant
Total
Weight
(gm)
0.75
Weight %
0.15
0.10
0.24
11.38
7.59
18.21
0.078
1.318
5.92
100
Volume
(ml)
56.90
0.40
4.00
0.40
2.4 Preparation of lithium-air cell components
The components used for making a lithium-air cell were aluminum foil, pure lithium
metal, polymer ceramic membranes, glass-ceramic membrane and an air-cathode.
2.4.1 Processing of polymer-ceramic membrane
A polymer complex comprising of poly(ethylene) oxide (PEO) (m.w. 1,000,000,
Union Carbide) and LiN(SO2CF2CF3)2 (3M) (LiBETI), was used as lithium ion
conducting host matrix. Nanosize boron nitride (BN, 5-20 nm) and lithium oxide (Li2O)
were used as ceramic dopants. PEO and LiBETI were dried in an oven under inert
atmosphere for 48 hours at 60°C. All the materials were weighed inside a dry box
maintained with argon atmosphere (<50 ppm of oxygen and <88 ppm of moisture). The
dried PEO and LiBETI in ratio 8.5:1 were mixed with 1 wt% of each BN or Li2O. The
weighed materials were transferred into a metallic jar and taken out from the dry box for
high energy milling (Spex, 8000M) for one hour to obtain a homogeneous mixture. After
milling, 200 mg of the specimen was loaded into a die in the dry box and heated for 10
14
min at 95°C and pressed with a 1816 kg pressure to transform it into a polymer-ceramic
(PC) disc membrane. The PC membrane was removed from the die after it was cooled
down to room temperature. Thus, PC specimens in the form of discs (11 mm dia and 0.81.0 mm thick) were obtained. The PC membranes, viz., PEO:LiBETI(8.5:1)-1 wt% BN
and PEO:LiBETI(8.5:1)-1 wt% Li2O are hereafter abbreviated as PC(BN) and PC(Li2O)
respectively.
2.4.2 Processing of glass-ceramic membrane
The sieved LAGP glass-ceramic powder (1-38 μm particle size) was used for making
the electrolyte membranes. LAGP powder weighing 450 mg was loaded into a die and
pressed with 5448 kg pressure to obtain <0.450 mm thick and 18 mm dia discs. These
glass-ceramic discs were transferred onto a flat ceramic plate for sintering at 850°C for
12 hours to obtain a glass-ceramic membrane with high ionic conductivity.17 These
sintered glass-ceramic membranes were later rounded to 16 mm dia to fit into the cell
casing and ground to make the discs less than 0.350 mm thick membrane using a fine
abrasive (3M, 220 grade). Finally, the LAGP glass-ceramic membrane was cleaned using
ethanol.
2.4.3 Cleaning of lithium metal strip
The oil soaked lithium metal strip was cleaned by using naphthalene solution and
mechanically scrubbed to remove oxide deposit from the surface of lithium formed
during its storage and immediately used to make a cell.
15
2.4.4 Aluminum foil
As-received aluminum foil was used on one side of the lithium metal. It was
established earlier from the cell performance data that the use of aluminum foil to cover
lithium surface yielded reproducible data.15
2.4.5 Preparation of air-cathode
The cathode paste as prepared by the methods described in the section 2.3 was then
applied onto the both sides of the Ni mesh. On the Ni foam, the paste was applied only on
one side. This Ni mesh/foam with cathode paste was sandwiched between two steel plates
and a 2724 kg pressure was applied using a laboratory hydraulic press. Thereafter, these
cathodes were dried overnight at 100°C on a hot plate for the removal of D.I. water. Later
on, the dried cathodes were sintered in argon atmosphere at 300°C for six hour in the
cathodes with dispersant (to burn the dispersant) or for 20 minutes in the non-dispersant
cathodes to provide bonding and mechanical stability to the cathode structure. In the
sintering process, the heating and cooling rates of 3°C per minute were practiced.
2.5 Fabrication of Li-oxygen/Air cell
2.5.1 Preparation of the electrolyte laminates
The components used for making the electrolyte laminate were membranes of LAGP
glass-ceramic, PC(BN) and PC(Li2O) (polymer-ceramics). Laminate preparation was
done inside the dry box maintained with argon atmosphere (<50 ppm oxygen and <88
ppm moisture). PC(BN) and PC(Li2O) discs were placed on each side of the LAGP glass-
16
ceramic membrane. To spread the PC on both sides of the LAGP glass-ceramic evenly,
the laminate was heated to 95°C using a hot plate.
2.5.2 Preparation of the cap for coin cell
The electrochemical reaction in the cathode occurs at the (triple phase boundary) TPB
and one of the phases being a gas-phase (O2). There needs to be a provision for the
oxygen to be present at the cathode. This was achieved by drilling 48 holes using a drill
bit of size 0.50 mm diameter. These holes drilled on to the cathode side provided the
source of oxygen gas to the cathode.
2.5.3 Lithium-oxygen/Air cell assembly
The lithium-air cell was put together in a dry box maintained with argon atmosphere
(<50 ppm oxygen and <88 ppm moisture). A coin-cell casing with a diameter of 16 mm
and a depth of 2.9 mm was chosen to contain a lithium-air cell. All the lithium-air cell
components added up to a thickness of 1.06 to 2.43 mm. A spacer was used to fill the
remaining space. A wave spring was also used to keep all the lithium-air cell components
under pressure to reduce the contact resistance and to improve the lithium ion
conductivity.
Figure 2-1 shows the schematic representation of cell components. First the wave
spring was placed. Next to the wave spring the spacer and then aluminum foil were
placed. Next to the aluminum foil, a cleaned lithium metal foil was situated. The prepared
electrolyte laminate is placed with PC(Li2O) facing lithium foil and PC(BN) facing the
cathode. Then a cap was placed on the coin cell casing. Then the whole assembly was
transferred into a zip lock bag and transferred to the crimping tool. The coin cell was
fabricated using Hohsen Corporation crimping tool outside the dry box.
17
Figure 2-1: Schematic representation of the cell components
2.6 Characterization
2.6.1 Porosity
2.6.1.1 Water saturation technique
The porosity and pore structure are of a significant importance for the cathode
performance. The pore structure should allow lithium ions, oxygen molecules and
electrons to form a TPB for the cathodic reaction to proceed. The water saturation
procedure was employed to determine the porosity of the air-cathode. The water
saturation procedure is based on Archimede’s principle and illustrated as follows:
Porosity = Volume of pores / Volume of specimen
Volume of pores = WWA - WA
Volume of the specimen = WA - WW
Where
WWA = Weight in air of the water soaked sample
WA = Weight of the dry specimen in air
WW = Weight of the specimen in water
18
(3)
The porous cathode was soaked in water to determine the pore volume by calculating
the weight of water the pores have adsorbed. This pore volume accounts only for the
interconnected pores. The isolated pores remain unaccounted for.
2.6.1.2 BET method
The Brunauer, Emmett and Teller (BET) was used to determine the effective surface
area of a specimen using the adsorption isotherms. By knowing the amount of the gas
molecules (N2) adsorbed in the experiment, the effective surface area can be determined.
Porosity, pore volume and pore size distribution can be determined from this method
directly.
The number of gas molecules adsorbed on to the specimen can be computed from the
ideal gas law pV = nRT, where V is the volume which is fixed, R is universal gas
constant and T is absolute temperature which is a constant in the isothermal case. Hence
pressure (p) is directly proportional to the number of gas molecules (n) adsorbed in the
specimen pores.
2.6.2 Electrical characterization
All the lithium-air cells were placed in a cell holder and contained in an air-tight glass
jar fitted with leads for electrical characterization. The lithium-air cells were tested in the
temperature range 67°C to 115°C in oxygen atmosphere with a pressure of 1 kPag.
CorrWare and z-Plot were used for the electrical characterization of lithium-air cell.
z-Plot was used to measure the resistance of the lithium-air cell by the AC impedance
technique. For the AC impedance technique a Solartron 1260 impedance analyzer with
1287 electrochemical interface was used to obtain impedance data in the 0.1-106 Hz
19
frequency range. CorrWare was used to discharge or charge the lithium-air cell at a
constant current or voltage using the option Galvanostatic in the CorrWare.
20
CHAPTER III
RESULTS AND DISCUSSIONS
This chapter presents significant results and related discussions. Table 3-1 illustrates
important variables, specimens characterized and major observations.
3.1 Effect of processing on physical properties of cathode
Table 3-2 shows the physical properties of cathodes that play a very important role in
their performance. The cathodes were processed by different techniques and all contained
25 wt% carbon and 75 wt% LAGP. A dispersant, 2-propenoic acid 2-methyl ammonium
salt was used only in modified energy mill method to reduce the LAGP and carbon
agglomeration.
Each property listed in table 3-2 has its effect on the cell performance. Porosity
determines the amount of space available for accommodating the product (Li2O/Li2O2) in
the cathode during discharge. Surface area determines the probability of forming triple
phase boundary (TFB) within the bulk structure of cathode. Pore volume determines the
effective space available to accommodate the discharge product. Pore size determines the
relative dimension of the reaction product that can be accommodated.
21
Table 3-1: Summary of variables, specimens characterized and major observations
Variables
Effect of cathode
processing on
physical properties
Effect of dispersant
Number of
cathodes assessed
Three
Two
Cathode/cell variable
Major observations
Processing technique,
dispersant used only in
modified energy milling
One cathode with
dispersant and the other
without dispersant
Cathode processed with
modified energy mill
provided higher porosity
Cathode with dispersant
had better electrochemical
performance
Thermal treatment of six
hours is sufficient to get rid
of the dispersant
Cathode with a dispersant
concentration of 5.92 wt%
performed superior
Effect of thermal
treatment on
dispersant cathodes
Effect of dispersant
concentration
Three
Thermal treatment time of
0, 3 and 6 hours at 300°C
Two
Effect of LAGP
concentration
Three
One cathode with a
dispersant concentration
of 7.91 wt% and other
with a dispersant
concentration of 5.92 wt%
LAGP concentration of
90, 75and 50 wt%
Effect of cathode
porosity
Two
Effect of temperature
One
One cathode with a
porosity of 16% and other
with a porosity of 24%
Temperature of the cell
Effect of discharge
current
One
Discharge current in a cell
Effect of cell
resistance
Effect of oxygen
transport
One
Discharge/charge cycles
in a cell
Cathode orientation with
respect to the oxygen flow
Two
Higher concentrations of
LAGP helped the cell to
allow higher
discharge/charge currents
24% porosity cathode
outperformed 16% porosity
cathode
Capacity increases with
increase in temperature,
gradually decreases after
105°C
With increase in discharge
current, there is a decrease
in the capacity due to the
cathodes inability
Resistance is increased due
to the parasitic reactions
In orientation (a) LAGP
and carbon side provided
more TPB regions and
hence performed superior
Irrespective of the processing technique, it is noted from the table 3-2 that the
porosity value obtained by water saturation technique was higher as compared to the BET
method. Higher porosity in water saturation technique is obtained due to the dissolution
of PTFE during the measurement process. The dissolution of PTFE creates additional
voids in the bulk structure and thereby yields higher porosity value. Nonetheless, it is a
simpler technique which provides qualitative information. In BET technique, nitrogen gas
22
is adsorbed along the pores of the cathode. The BET technique always provided a lower
value of porosity, but it is more credible as compared to the water saturation technique.
Table 3-2: Properties of the cathode with different processing technique
Measurement
Technique
Property
Water saturation
technique
BrunauerEmmett-Teller
(BET)
Porosity
Porosity
Surface area
(m2/g)
Pore volume
(m3/g)
Pore size
(nm)
Processing Technique
Hand
Energy Modified energy
mixing Milling milling
12%
36%
42%
10%
76.84
21%
73.58
22%
25.80
0.104 x
10-6
5.40
0.086 x
10-6
4.70
0.0035 x 10-6
3.90
From table 3-2 it is noted that the porosity is the highest for the cathode processed by
the modified energy mill method as compared to other two processing techniques as
measured by both the measurement techniques. But, this cathode possessed the lowest
pore volume and the smallest pore size. The porosity plays a very important role in
determining electrochemical performance of cathodes. Surface area and pore volume are
greater for the cathode processed by hand mixing method compared to other two
processing techniques. The highest pore size was observed in the case of the cathode
processed by the energy mill method.
From the table 3-2 it is apparent that the properties are affected by the processing
technique. The use of dispersant allowed LAGP and carbon to be uniformly dispersed
and a high value of porosity was obtained. Therefore, this technique was employed for
processing most of the cathode pastes.
23
3.2 Electrochemical performance of cathodes as influenced by dispersant
In the section 3-1, it has been shown that the properly dispersed LAGP and carbon
cathodes prepared through the use of modified energy milling method had better physical
properties as compared to any other processing techniques. By avoiding or minimizing
LAGP and carbon agglomeration the performance of the cathode was enhanced as shown
by the results in the section 3-1.
To study the effects of the dispersant on the cathode performance two cells were
assembled by keeping all the cell parameters constant except the use of dispersant. Figure
3-1 shows the discharge profiles of the cells with (cell 2) and without (cell 1) the use of a
dispersant in the cathode. Both the cells were discharged with a current of 0.2 mA at
75°C. The cathode composition was 25 wt% carbon and 75 wt% LAGP. In the figure 3-1,
the curve that represents cell 2 had a dispersant concentration of 5.92 wt%.
24
5
Cell voltage (V)
4
3
Cell 2
2
Cell 1
1
0
0
10
20
30
40
50
60
70
80
Time (hours)
Figure 3-1: Discharge profiles of cells 1 and 2 with and without the dispersant in the
cathode at constant current (0.2 mA) and at a constant temperature (75°C).
From the figure 3-1 it is noted that the shape of the discharge profile has been
changed significantly with the use of dispersant. The discharge curve of cell 2 is much
smoother than the discharge curve of cell 1. It is noted that the cell 2 was discharged
above 2.5 volts up to 35 hours which was not the case with the cell 1. Because of the
proper dispersion of LAGP and carbon in the composite structure the cathodic losses are
minimized. It is observed that the cell 2 was discharged for slightly more time as
compared to the cell 1. The discharge capacities of cells 1 and 2 were 13.2 mAh and 14.1
mAh respectively. The role of dispersant in the cathode is significant for achieving higher
discharge voltage, energy density and power density.
25
Figure 3-2 shows the micrograph of the cathode specimen at 50x magnification which
contained 5.92 wt% dispersant. The white region in the micrograph represent LAGP and
black areas represent carbon. It is noted that even after the use of dispersant there was
some LAGP agglomeration. The blurred portion was interpreted as uneven, elevated
surface of the specimen.
Figure 3-2: Optical micrograph of the cathode specimen at 50x magnification which has a
composition of 25 wt% carbon and 75 wt% LAGP with 5.92 wt% dispersant.
3.2.1 Effect of thermal treatment on physical properties of cathode
The dispersant used in the cathode formulation should be eventually burned out.
From the data sheet provided by the supplier, the dispersant should be burnt out at 230°C.
However, a thermal treatment of the specimen in argon atmosphere at 300°C was
employed to ensure its complete removal.
Cathode paste preparation and fabrication steps were followed as discussed in
chapter 2. To analyze the effect of thermal treatment time, a cathode which had 7.91 wt%
26
dispersant was selected. The cathode was thermally treated at 300°C for three hours and
six hours in argon atmosphere. The specimens were characterized using the BET surface
area analyzer and water saturation technique. Table 3-3 shows the effect of the thermal
treatment on weight loss, porosity, surface area, pore volume and pore size.
Table 3-3: Properties of the cathode as a function of thermal treatment time
Measurement
Technique
Property
Weight loss
Water saturation Porosity
technique
BET
Porosity
Surface area (m²/g)
Pore volume (m³/g)
Pore size (nm)
Time of sintering
0 hour
3 hour
N/A
6.85 %
2%
39%
16.50%
6.10
-------
19.20%
25.55
0.0020
x 10-6
4.21
6 hour
9.6 %
53%
21.20%
25.80
0.0035
x 10-6
3.90
From the table 3-3 it is noted that the loss in the case of the six hour sample was 9.6
wt%, which was greater than the dispersant added in the cathode paste preparation. This
indicates that the dispersant was burned out completely from the six hour sample. The
excess weight loss in the specimen can be explained on the basis of removal of side
chains of the PTFE molecules.
In the three hour sample the loss was 6.85 wt% which is less than the dispersant
added suggesting that the dispersant was not burned out completely. Cathode porosity
(measured by both BET and water saturation technique), surface area and pore volume
are also greater in the six hour sample as compared to the three hour sample. As the
properties of the six hour sample are promising in all the aspects, the thermal treatment
time of six hour was used for all the cathodes whose properties will be discussed and
covered in subsequent sections.
27
3.2.2 Effect of dispersant concentration on discharge profiles
In this section the effect of dispersant concentration on discharge profiles of lithiumair cells are presented. Table 3-4 shows the cathode formulation with two different
dispersant concentrations of 7.91 wt% and 5.92 wt% which were thermally treated at
300°C for 6 hours while keeping other chemical constituents identical.
To evaluate the effect of dispersant concentration in the cathode two cells were tested
while keeping all the cell parameters constant. Figure 3-3 shows the discharge profiles of
the two cells with 5.92 and 7.91 wt% dispersant concentrations. Both the cells were
discharged with a current of 0.2 mA and cell temperature of 75°C. The cathode
composition comprised of 25 wt% C and 75 wt% LAGP. In the figure 3-2, the curves are
identified with the dispersant concentrations i.e., 7.91 and 5.92 wt%.
Table 3-4: Air-cathode formulation with different concentrations of dispersant
Materials
Specimen 1
Volume Wt
(ml)
(gm)
LAGP
--0.75
Carbon Activated
--0.15
Ketjen black --0.10
PTFE
0.40
0.28
D.I. Water
4.00
--Dispersant
0.40
0.11
Total
--1.39
wt%
53.96
10.79
7.20
20.14
--7.91
100
Specimen 2
Volume Wt
(ml)
(gm)
--0.75
--0.15
--0.10
0.40
0.24
4.00
--0.30
0.078
--1.318
wt%
56.90
11.38
7.59
18.21
--5.92
100
From the figure 3-3 it is noted that the cell with 5.92 wt% dispersant was discharged
above 2.5 volts for half of the discharge. This cell also provided a higher capacity and
discharge voltage. The discharge capacity of this cell is 5.6 times greater than the
discharge capacity of the cell containing a cathode with 7.91 wt% dispersant.
28
5
Cell voltage (V)
4
3
2
1
5.92 wt%
7.91 wt%
0
0
10
20
30
40
50
60
70
80
Time (hours)
Figure 3-3: Discharge profiles of two cells with cathodes containing 5.92 and 7.91 wt. %
of dispersant concentrations at constant current (0.2 mA) and temperature (75°C)
The reason for the use of dispersant is to improve the distribution of LAGP and
carbon within the bulk structure of the cathode by minimizing the LAGP and carbon
agglomeration. The dispersion leads to better capacity utilization of cathodic constituents.
The polymers (dispersant and PTFE) present in the cathode allow formation of a charged
layer on LAGP and carbon surfaces. The LAGP and carbon particles were uniformly
dispersed, due to the fact that they repel each other. As the amount of dispersant
increases, its elimination during the thermal treatment process becomes a cumbersome
task. The residual dispersant in cathode may influence the impedance of the cell. To
29
investigate the effect of residual dispersant the impedance of the cells containing
cathodes with 5.92 and 7.91 wt% dispersants before discharge were measured and the
data are shown in figure 3-4.
-140
-120
-100
Z''
-80
-60
-40
5.92 wt%
-20
7.91 wt%
0
20
40
60
80
100
120
140
Z'
Figure 3-4: Impedance of the cells with different dispersant concentration before
discharge at 0.2 mA at 75°C
The cell containing 5.92 wt% had a resistance of 68 ohm and the cell containing 7.91
wt% had a resistance of 70 ohm. Since the difference in resistance of these two cells was
only 2 ohm, which is almost 3% and considered to be insignificant. From this experiment
it can be inferred that the cell with lower dispersant concentration has lower impedance
and electrochemically performed superior compared to the cell with higher dispersant
concentration.
30
3.3 Effect of composition of cathode on electrochemical performance of lithium-O2
cells
In a lithium-O2 electrochemical cell the cathode comprises of ionic and electronic
conductors. In order to optimize the cell performance the composition of ionic and
electronic conductors must be in a proper proportion. The chemical proportion may
affect both physical properties (e.g., surface area, pore volume, porosity etc.,) of a
cathode and electrochemical properties (e.g., cell capacity, discharge/charge current, cell
impedance, energy density, power density, etc.) of a lithium-O2 cell. To assess the effect
of the cathode composition or chemistry three different lithium-air cells were assembled
while
keeping
other
cell
parameters
constant.
The
cells
comprised
of
Al/Li/PC(Li2O)/LAGP/PC(BN)/cathode structure. In these cells cathode compositions
were varied as presented in table 3-5. The test procedures for all the cells were similar.
Table 3-5: Cathode composition and its properties
Cell
#
3
4
5
Cathode Composition
10 wt % C + 90 wt %
LAGP
25 wt % C + 75wt %
LAGP
50 wt % C + 50 wt %
LAGP
BET
Surface
Area
(m²/g)
22.77
75.80
114.14
BET
BET
Pore
Pore
Volume size BET
(m³/g) (nm)
0.035 x 6.17 16%
10-6
0.091 x 4.81 24%
10-6
0.140 x
10-6
4.90
28%
Porosity
Water saturation
technique
19%
40%
24%
Table 3-5 shows that carbon concentrations were varied from 10 to 50 wt%. The table
3-5 also lists the physical properties such as surface area, pore volume, pore size and
porosity as determined by the BET method. Porosity of cathodes was also measured by
the water saturation technique. From the table 3-5 it is noted that the surface area, pore
31
volume and the BET porosity are highest for 50 wt% carbon cathode. Due to variation in
physical properties of cathodes used in table 3-5, it is expected that cells 3, 4 and 5 will
show different electrochemical cell properties. Furthermore, these three cells were tested
at different temperatures and discharge currents; and the obtained electrochemical data of
lithium-air cells 3, 4 and 5 are listed in the tables 3-6, 3-7 and 3-8 respectively.
The lithium-air cell with cathode composition 10 wt% carbon (cell 3) was discharged
and charged in temperature range 75 to 105°C. The data obtained from the cell 3 are
presented in table 3-6. Discharge and charge currents were varied to attain maximum
discharge/charge currents. The electrochemical properties of a cell, to some extent
depend on the cell resistance. The cell resistances, before discharge/charge are also
presented in the table 3-6. In general, the cell was discharged down to a voltage of 1.0 V
and charged up to 4.5 V. In every case an effort was made to equalize discharge capacity.
First discharge is carried out at 75°C with a discharge current of 0.50 mA. The calculated
cell capacity for the discharge is found to be 6.46 mAh at an initial cell resistance of 67
Ω. After the first discharge, the cell is charged with 0.275 mA current to a capacity of
5.32 mAh. During the first charge, the charge capacity (5.32 mAh) is less than the first
discharge (6.46 mAh). To equalize the discharge capacity the cell is charged more than
once as expressed by 1C(1) and 1C(2). It is noted that after the first discharge/charge, cell
resistance increased from 67 Ω to 92 Ω. The increase in cell resistance during first
discharge/charge may cause performance degradation of the lithium-air cell. To equalize
the discharge capacity achieved in the third (95°C) and fourth (105°C) discharges, two
consecutive charges were performed in each case. It is observed that the resistance of the
cell was increased at 105°C after one discharge/charge cycle to 1635Ω. This indicates
32
that the cell has a tendency to degrade at higher temperatures i.e., above 100°C. Because
of the cell degradation the cell is not able to sustain even lowered discharge/charge
currents for a considerable period of time. The cell is brought down to 75°C and the cell
resistance was measured to be 3749 Ω which is approximately 66 times greater than the
cell resistance before the first discharge. A maximum discharge capacity for cell 3
cathode composition is observed at 95°C and is 7.85 mAh with a discharge current 0.50
mA.
Table 3-6: Electrochemical data on lithium-air cell 3 with cathode composition 10 wt%
carbon and 90 wt% LAGP
D/C
D/C
Cell
Rcell
Voltage
current time capacity
excursion
(Ω)
(mA)
(hrs)
(mAh)
(V)
75
1D
0.50
12.92 6.46
67
3.08 - 1.00
75
1C
0.275
19.35 5.32
100
2.91 - 4.50
1C(1) 0.275
1.58
0.79
96
3.65 - 4.50
1C(2) ---4.00
0.24
92
---85
2D
0.50
6.53
3.27
62
3.60 - 1.00
85
2C
0.50
6.50
3.25
62
3.01 - 4.43
95
3D
0.50
15.70 7.85
42
3.49 - 1.00
95
3C
0.50
11.69 5.85
139
2.60 - 4.50
3C(1) 0.20
12.00 2.40
81
3.63 - 4.32
105
4D
0.50
8.60
4.30
73
3.56 - 1.00
105
4C
0.50
5.34
2.67
198
3.27 - 4.50
4C(1) 0.11
17.00 1.87
163
3.62 - 4.32
105
5D
0.20
0.92
0.18
1635 3.02 - 1.00
105
5C
0.20
0.12
0.024
---3.66 - 4.30
75
6D
0.20
0.24
0.048
---2.96 - 1.00
75
6C
0.003
15.37 0.046
3749 3.65 - 3.84
*
Notations: T- temperature, D- discharge, C- charge, Rcell- cell resistance in D/C state, V- voltage
excursion during D/C.
T(°C)
D/C
cycle
The cell with cathode composition 25 wt% carbon (cell 4) was discharged and
charged in temperature range 65 to 115°C. The data obtained from the cell are presented
in table 3-7. Discharge and charge currents were varied to attain maximum
discharge/charge currents. First discharge was carried out at 75°C with a discharge
33
current of 0.30 mA. The calculated cell capacity for discharge is found to be 4.33 mAh at
an initial cell resistance of 88
Ω. This discharge capacity is less than the discharge
capacity obtained with 10 wt% carbon cathode (cell 3). After first discharge, cell was
charged with 0.20 mA current to a capacity of 4.29 mAh. It was noted that after first
discharge/charge cell resistance increased to 954Ω. This behavior was also noted in 10
wt% carbon cathode. The increased cell resistance during first discharge/charge was a
technical issue that needs to be analyzed and understood. The second discharge was
carried out at 95°C with a discharge current of 0.30 mA. The measured discharge
capacity was 0.98 mAh with a cell resistance of 254 Ω. The charge was carried out with a
current of 0.20 mA and a capacity of 1.0 mAh was achieved. The third discharge was
carried out with a discharge current of 0.30 mA also at 95°C and the calculated discharge
capacity was found to be 1.36 mAh with a resistance of 220 Ω. In this case it wa s noted
that the capacity increased as compared to the previous discharge. The fifth discharge
was carried out at 105°C with a discharge current of 0.20 mA. The calculated cell
capacity for discharge was found to be 1.14 mAh with a cell resistance of 140Ω. The
charge was carried out with a current of 0.10 mA and a capacity of 1.20 mAh is achieved.
The next discharge was carried out at 115°C with a discharge current of 0.20 mA. The
calculated cell capacity for discharge is found to be 1.02 mAh with a cell resistance of 71
Ω. The charge was carried out with a current of 0.10 mA and a capacity of 1.20 mAh is
achieved. To assess the cell degradation the temperature was reduced to 65°C and a few
discharge/charge cycles were performed. The capacity of the cell was considerably
reduced when the cell temperature is brought down to 65°C. A maximum discharge
capacity for cell 4, 4.33 mAh is observed at 75°C. A large increase in the cell resistance
34
during discharge/charge cycle in the 75 – 115°C reinforces earlier observation about
degradation of cells at temperatures above 100°C.
Table 3-7: Electrochemical data on lithium-air cell 4 with cathode composition 25 wt%
carbon and 75 wt% LAGP
D/C
D/C
Cell
Rcell
Voltage
curren time capacity (Ω)
excursion
t(mA) (hrs)
(mAh)
(V)
75
1D
0.30
14.42 4.33
88
3.24 - 1.00
75
1C
0.20
21.47 4.29
954
2.69 - 4.50
95
2D
0.30
3.28
0.98
254
3.17 - 2.11
95
2C
0.40
2.50
1.00
222
2.79 - 3.40
95
3D
0.30
4.54
1.36
220
2.93 - 1.00
85
3C
0.10
16.00 1.60
741
2.68 - 3.80
85
4D
0.20
5.95
1.19
701
3.40 - 1.00
85
4C
0.10
12.00 1.20
---2.73 - 3.63
105
5D
0.20
5.72
1.14
140
2.99 - 1.00
105
5C
0.10
12.00 1.20
125
2.64 - 3.40
115
6D
0.20
5.11
1.02
71
2.87 - 1.00
115
6C
0.10
12.00 1.20
66
2.37 - 3.26
65
7D
0.20
0.41
0.082
2533 2.26 - 1.00
65
7C
0.05
2.00
0.10
2890 2.78 - 4.03
65
8D
0.10
3.22
0.32
2325 3.12 - 0.97
65
8C
0.10
4.00
0.40
3940 2.42 - 4.20
65
9D
0.10
1.64
0.16
2061 2.90 - 1.00
*
Notations: T- temperature, D- discharge, C- charge, Rcell- cell resistance in D/C state, V- voltage
excursion during D/C.
T(°C)
D/C
cycle
The cell with cathode composition 50 wt% carbon (cell # 5) was discharged and
charged in temperature range 45 to 105°C. The data obtained from cell # 5 with 50 wt%
carbon cathode are presented in table 3-8. Discharge and charge currents were varied to
attain maximum discharge/charge currents. First discharge was carried out at 45°C with a
discharge current of 0.20 mA. The calculated cell capacity for discharge was found to be
0.17 mAh with an initial cell resistance of 4148 Ω. After first discharge, cell was charged
with 0.20 mA current. Before charging with 0.2 mA current the cell resistance was
reduced to 818Ω. A charge capacity of 0.10 mAh is attained. It shoul d be noted that
these cells can be discharged even at lower temperature (< 65°C) although the achievable
35
capacity was low. The maximum discharge capacity for 50 wt% carbon and 50 wt%
LAGP composition cathode was observed at 85°C and was 12.61 mAh with a discharge
current 0.20 mA.
Table 3-8: Electrochemical data on lithium-air cell 5 with cathode composition 50 wt%
carbon and 50 wt% LAGP
D/C
D/C
Cell
Rcell
Voltage
current
time
capacity
excursion
(Ω)
(mA)
(hrs)
(mAh)
(V)
45
1D
0.20
0.85
0.17
4148 3.08 - 1.00
45
1C
0.20
0.51
0.10
816
3.23 - 4.50
57
4D
0.10
8.07
0.81
1585 3.09 - 1.00
57
4C
0.075
11.00 0.83
127
2.77 - 4.17
57
5D
0.20
4.18
0.84
1690 3.45 - 1.00
57
5C
0.075
12.00 0.90
185
2.98 - 4.00
57
6D
0.10
10.14 1.02
436
3.66 - 1.00
57
6C
0.075
15.00 1.12
323
3.12 - 4.05
67
7D
0.20
10.76 2.15
927
3.62 - 1.00
67
7C
0.10
22.00 2.20
123
2.63 - 4.00
75
8D
0.20
24.40 4.88
249
3.44 - 1.00
75
8C
0.125
40.00 5.00
156
2.35 - 4.04
85
9D
0.20
63.05 12.61
97
3.56 - 1.00
85
9C
0.20
64.03 12.81
92
2.76 - 4.18
95
10D
0.30
21.77 6.53
126
3.55 - 1.00
95
10C
0.30
22.00 6.60
70
2.14 - 4.09
95
11D
0.20
56.72 11.34
151
3.53 - 1.00
95
11C
0.30
38.00 11.40
107
2.80 - 4.21
105
12D
0.20
50.20 10.04
288
3.50 - 1.00
105
12C
0.30
34.00 10.20
155
2.55 - 4.15
*
Notations: T- temperature, D- discharge, C- charge, Rcell- cell resistance in D/C state, V- voltage
excursion during D/C.
T(°C)
D/C
cycle
From the data provided in the tables 3-6, 3-7 and 3-8 it is clear that the cell which had
higher concentration of LAGP is able to withstand higher discharge/charge currents for
much longer time. The reason for a superior performance of the cathode with higher
concentration of LAGP cathodes is attributed to its catalyzing effect for oxygen
reduction.
36
The electrochemical data of cell 3 having 10 wt% carbon cathode suggest that to
make the charge capacity equal to the discharge capacity multiple charge cycles are
required. It is important to note that this was not the case with higher carbon
concentration cathode cells. The cathodes which have higher concentrations of carbon
had no rechargeability problem. This kind of behavior was observed in all the cells which
had high concentrations of carbon in the cathode.
The cell resistances at 75°C were 67, 88 and 249
Ω for 10 wt% carbon, 25
wt%
carbon and 50 wt% carbon cathode cells respectively. This difference in cell resistance is
being investigated.
By analyzing all the data provided in the section it can be inferred that the cathode
composition with 10 wt% carbon and 90 wt% LAGP has advantages as compared to
other two compositions.
3.4 Effect of cathode porosity, cell temperature and discharge current on the
performance of the lithium-O2 cell
3.4.1 Effect of porosity on discharge capacity
To analyze the effect of cathode porosity two cells were assembled while keeping all
the parameters constant. The cathode comprised of 25 wt% carbon and 75 wt% LAGP
with no dispersant. The only variable was cathode porosity.
Figure 3-5 shows the effect of cathode porosity on the discharge profiles of the cells
at 105°C. The cell comprised of Al/Li/PC(Li2O)/LAGP/PC(BN)/cathode configuration.
37
In figure 3-5 the discharge curves are identified by the porosity value as measured by the
water saturation technique. The two cells were discharged with a discharge current of 0.2
mA and at 105°C.
From the figure 3-5 it is noted that the cell with cathode having the porosity of 16%
possessed a discharge capacity of 12.19 mAh corresponding to the discharge time of
60.94 hour. The cell with a porosity of 24% exhibits a discharge capacity of 16.11 mAh
corresponding to a discharge time of 80.54 hour. From the figure 3-5 it is observed that
the two discharge profiles look alike. It is also observed that more than half of the
discharge capacity was achieved below 1.5 volts in both the cells. The discharge profiles
also exhibit two plateau regions. The reason for the two plateau sections could be
explained on the basis of existence of two different types of the pores in the cathode. The
existence of two plateau regions is not desired in a cell. More work and characterization
are needed to understand and solve this technical problem.
38
5
Cell voltage (V)
4
3
24%
2
16%
1
0
0
3
6
9
12
15
18
Discharge capacity (mAh)
Figure 3-5: Discharge profiles of cells with different porosities
From the figure 3-5 it is concluded that higher cathode porosity increases discharge
voltage and capacity. Higher the cathode porosity superior the electrochemical
performance. However, increasing porosity also adversely affect the mechanical stability
of cathode. Therefore, there is an optimum value of porosity; generally in the range of 50
- 60% which can provide desired electrochemical performance. Further work will address
the porosity issue.
39
3.4.2 Effect of temperature on discharge capacity
Figure 3-6 shows the effect of temperature on the discharge capacity of a cell having
Li/PC(Li2O)/LAGP/PC(BN)/cathode configuration. The cathode was 25 wt% carbon and
75 wt% LAGP.
5
Capacity (mAh)
4
3
2
1
0
80
85
90
95
100
105
110
115
120
o
Temperature ( C)
Figure 3-6: Temperature dependence of discharge capacity
The capacities were determined in the temperature range 85 – 115°C with an
increment of 10°C at a constant discharge current of 0.5 mA. The discharge capacities
were 1.55, 2.67, 4.61 and 3.21 mAh at 85, 95, 105 and 115°C respectively. The data
show that the optimum temperature for the highest discharge current is 105°C.
40
The conductivity of the composite membrane laminate used in this cell is highly
dependent on the temperature. The higher the temperature, the higher the conductivity. At
higher temperatures, the lithium ion transport is facilitated. Furthermore, electrode
kinetics is also enhanced at elevated temperatures. These two factors collectively
contribute to the higher discharge capacity at elevated temperatures. The anodic and
cathodic activation losses are minimized at higher temperatures hence increasing the
discharge capacity. The elevated temperatures also help to improve the ability of the
cathode to accommodate the reaction product, Li2O2/Li2O in the structure.
The capacity, Q and discharge rate, dQ/dT are related by equation (4)
t
Q=∫
0
dQ
− − − (4)
dt
Where, Q = capacity, coulombs (Ah)
t = time (sec)
dQ/dt = discharge rate, coulomb/sec (A)
In an electrochemical reaction the capacity, Q is also proportional to the quantity
(weight or volume) of the discharge product. The discharge product in lithium-O2 cells
must be accommodated in the cathode structure. Therefore, the pore volume, porosity and
pore structure (open or closed) are critical properties of the cathode.
The temperature affects the cell performance in several ways. Firstly, a higher
temperature reduces cell impedance by facilitating transport of ions through the cell.
Secondly, elevated temperatures lower electrode activation losses and enhance electrode
kinetics. Thirdly, higher temperatures create more space in the cathode structure to
accommodate the discharge product. However, higher temperatures can also trigger
41
parasitic reactions within the cell and degrade its performance. Therefore, an
optimization of cell operating temperature is critical in the development of batteries.
3.4.3 Effect of discharge current on capacity
Another parameter that affects the performance of the cathode is discharge current.
To
analyze
the
effect
of
discharge
current
a
cell
with
Al/Li/PC(BN)/LAGP/PC(BN)/cathode configuration was characterized. The cathode was
25 wt% carbon and 75 wt% LAGP. Figure 3-7 (a) shows the relationship between
capacity and discharge current.
The lithium-air cell was evaluated with 0.50, 0.30 and 0.25 mA discharge currents
while keeping the cell temperature constant at 67°C.
In figure 3-7 (a) curves are
identified with the discharge currents. The discharge capacities corresponding to the 0.50,
0.30 and 0.25 mA discharge currents were 0.84, 1.76 and 1.77 mAh respectively.
Increase in discharge current reduces the capacity and is related to the structure of
cathode.
The electrochemical events taking place in a lithium-air cell are oxidation and
reduction reactions at anode and cathode respectively. For an electrochemical reaction to
take place there need to be a presence of reaction sites available at the lithium/electrolyte
and electrolyte/cathode interface. Whenever a current is drawn from a cell there is an
initial drop in the cell voltage (figure 3-7 (a)). This drop is due to the activation losses.
This comprises of both anodic and cathodic activation losses.
Activation losses are followed by the ‘iR’ drop in figure 3-7 (a). In this ‘iR’ drop, if
the cell resistance was constant for all the discharges, voltage drop is proportional to the
discharge current (i). The voltage drop is much higher for the higher discharge current. In
42
order to reduce the initial drop in the cell voltage, activation losses should be reduced for
both anodic and cathodic reactions. Activation losses result from charge transfer reactions
of lithium at the anode and oxygen at the cathode. These charge transfer reactions are
also the main sources of losses even at higher temperatures.
5
(a)
Cell voltage (V)
4
3
2
0.50 mA
0.30 mA
0.25 mA
1
0
0
1
2
3
4
5
6
7
8
Time (hours)
Figure 3-7: (a) Discharge curves (voltage vs. time) for a cell at 67°C discharged with
different discharge currents of 0.50, 0.30 and 0.25 mA
Figure 3-7 (b) shows the voltage drops in a lithium-air cell for the first 5 seconds of
the cell discharge which represent the activation losses at discharge currents of 0.50, 0.30
and 0.25 mA. The curves are marked accordingly. The activation losses are 0.51, 0.55
and 0.57 V for 0.25, 0.30 and 0.50 mA discharge currents respectively.
43
To calculate voltage drop due to the iR losses, the cell resistances before the
discharge of the cell were measured. The cell resistances were 147, 216 and 366 ohm for
0.50, 0.30 and 0.25 mA discharge currents respectively. Hence from this data, the voltage
drop in each discharge because of iR losses can be calculated. The calculated voltage
drop due to iR losses were 0.073, 0.064 and 0.091 V for 0.50, 0.30 and 0.25 mA
discharge currents respectively.
4
(b)
Cell voltage (V)
3.8
OCV
3.6
3.4
3.2
0.30 mA
0.25 mA
0.50 mA
3
0
1
2
3
4
5
Time (sec)
Figure 3-7: (b) Drop in the cell voltage for the first 5 seconds with different discharge
currents of 0.50 mA, 0.30 mA and 0.25 mA.
3.4.4 Effect of cell resistance on discharge capacity
A cell was characterized to analyze the effect of cell resistance on the discharge
capacity. The cell comprised of Al/Li/PC(Li2O)/LAGP/PC(BN)/cathode configuration.
The cathode composition consisted of 25 wt% carbon and 75 wt% LAGP with no
44
dispersant. In this experiment the discharge current was 0.1 mA and the cell temperature
was 750C.
In figure 3-8 (a), the Nyquist plots show the cell resistances before the 23rd and 29th
discharges. The cell resistances are 543 and 588
Ω before the 23
rd
and 29th discharge
respectively. In the figure 3-8 (b), the discharge curves are shown by corresponding cell
resistances. From the figure 3-8(b) the discharge capacity of the cell was calculated as
0.199 mAh and 0.184 mAh for the 23rd and 29th discharges respectively.
-800
Z''
-600
-400
-200
588 Ohm
543 Ohm
0
0
200
400
600
800
Z'
Figure 3-8 (a): Nyquist plots for the cell before discharge.
Discharge profiles corresponding to 23rd and 29th were considered because there was
a significant change in the cell resistance before discharge in these two cases. All the rest
of the data in between 23rd and 29th discharges showed a trend towards increasing
resistance. There was a drop in the discharge capacity with an increase in the cell
resistance. It was observed that with increase in the number of discharge and charge
45
cycles, the resistance of the cell increases because of the parasitic reactions and
accumulation of the irreversible discharge products. This behavior was observed for all
the cells. For a better electrochemical performance of the cell, the cell resistance should
be as low as possible.
5
Cell voltage (V)
4
3
543 ohm
2
588 ohm
1
0
0
0.05
0.1
0.15
0.2
Discharge capacity (mAh)
Figure 3-8 (b): The effect of cell resistance on the discharge capacity of the lithium-air
cell
3.5 Effect of oxygen transport on electrochemical performance of cathode
The lithium-air cell undergoes electrochemical reactions such as oxidation of lithium
metal at anode and reduction of oxygen at the cathode in to generate electrochemical
power. To achieve maximum cell performance both electrode (anode and cathode)
46
reactions should be as efficient as possible and the slowest reaction determines the
overall cell performance. Cathode reaction is a complex reaction. It will occur only if the
oxygen molecule is in the reduced form; lithium is in the oxidized form; and electrons are
present simultaneously at a point. Lithium ion and electron moves intrinsically by the
ionic conductor (LAGP) and electron conductor (nickel mesh/foam and carbon)
respectively. The oxygen molecule should also be transported in cathode matrix to
interact with lithium ion and electrons to carry out the electrochemical reaction. The
minimum criteria to move oxygen molecule from one point to another point within the
cathode is availability of tunnel having inner dimension greater than the oxygen
molecule. Also the tunnel should be inter-connected to maximize oxygen transport. In the
cathodes both carbon and LAGP help in forming such type of tunnels which allow
transport of oxygen. It has been found that the mixture of carbon and LAGP when
pressed and sintered gives a porosity value around 25%. It means that approximately 75%
space of cathode is not accessible for oxygen diffusion. Furthermore, the cathode material
is applied on only one side of nickel foam. Therefore, the oxygen partial pressure to carry
out the electrochemical reaction will depend upon the orientation of the cathode i.e.,
whether the nickel side or carbon and LAGP side is facing the oxygen inlet. Thus, there
is a possibility to achieve different cell performance based on the orientation of cathode
with respect of oxygen environment.
An experiment was designed to investigate the role of the orientation of the cathode
with respect to the oxygen environment which may affect oxygen transport in an aircathode to electrochemically active sites. Figures 3-9(a) and 3-9(b) show the schematic of
the two cells that were assembled for this purpose. The cathode used in these two cells
47
have the same cathode porosity, pore size, pore volume and thickness. The only
difference in these two cells is the side of the cathode (either carbon + LAGP side or
nickel foam side) exposed to the oxygen flow. In figure 3-9(a) the LAGP and carbon side
was placed onto the electrolyte and nickel foam side was exposed to the oxygen flow. In
figure 3-9(b) the nickel foam side was placed onto the electrolyte and carbon and LAGP
side was exposed to the oxygen flow. These two cells were assembled keeping all other
cell parameters constant.
Orientation (a)
Orientation (b)
Figure 3-9: (a) Schematic representation of components in cell with nickel foam exposed
to oxygen flow and (b) Schematic representation of components in cell with
active materials exposed to oxygen flow.
Figure 3-10 shows the discharge profiles of the cells whose configurations are shown
in figure 3-9 (a) and (b). Both the cells were discharged keeping all the test parameters
constant (discharge current = 0.2 mA and cell temperature = 67°C). The cathode
composition was 25 wt% carbon and 75 wt% LAGP. In figure 3-10, the curve (a) and (b)
represent the cathode orientation of figure 3-9 (a) and 3-9 (b) respectively.
From the discharge profiles shown in Figure 3-10, the cell with orientation (a) was
discharged for more time and for a higher voltage as compared to the cell with orientation
(b). For the orientation (a) the discharge capacity was 8.76 mAh, whereas for orientation
(b) it was 6.02 mAh.
48
5
I = 0.20 mA
D
0
Cell temperature = 67 C
Cell voltage (V)
4
3
2
(a)
(b)
1
00
10
20
30
40
50
Time (hours)
Figure 3-10: Curve (a) and (b) represents the discharge profiles for the cell orientation (a)
and (b) respectively.
The reason for better performance of orientation (a) over orientation (b) is described
on the basis of the fact that LAGP and carbon side provides more triple phase boundary
regions.
The aforementioned experiment reveals the role of oxygen availability and its
diffusion in functioning of a lithium-air cell using a solid air-cathode.
49
CHAPTER IV
SUMMARY AND CONCLUSIONS
This investigation was carried out to develop an air-cathode for a solid-state,
rechargeable lithium-air battery. The investigation included evaluation of processing
parameters and evaluation of cathodes in working lihtium-O2 cells. The cathode
components included nickel foam/mesh as a substrate, carbon and LAGP as active
materials, PTFE as a binder for active materials and a dispersant to prevent
agglomeration of carbon and LAGP powders. The active material compositions and
dispersant concentration in the cathode batch were varied. The processed cathodes were
characterized for porosity, surface area, pore size and volume. The lithium-O2 cells
containing the cathodes were evaluated for their electrochemical performance in an
oxygen atmosphere at 1 kPa pressure. Significant conclusions of the investigation are
summarized in the following paragraphs:
•
The modified energy mill method for the paste preparation provided higher cathode
porosity and therefore is recommended for the cathode processing.
•
A thermal treatment of cathode at 300°C for six hours in the argon atmosphere was
determined to be adequate to pyrolyze and remove the dispersant from air-cathode.
50
•
A dispersant concentration of 5.92 wt% was determined to better than a dispersant
concentration of 7.91 wt% for an effective cathode.
•
Three different compositions of the cathode were investigated in this work. It has
been determined that higher concentrations of LAGP in the cathode help in achieving
higher cell capacity. This has been attributed to the catalyzing effect of LAGP for
oxygen reduction. It was observed in the cells with higher concentrations of carbon
that the resistance increased rapidly from the first to second discharge. This increase
in the cell resistance was attributed to the discontinuity of the ionic path in the
cathode.
•
It was determined that higher cathode porosity leads to a better cell performance. The
determination was based on an investigation in which two cells were evaluated with
16% and 24% porosity. The discharge capacities for 16 and 24% porosity cells were
12.19 and 16.11 mAh respectively. The optimum porosity for even higher discharge
capacity is estimated to be around 50 – 60%.
•
The lithium-O2 cells were discharged in the temperature range of 85 – 115°C. It was
determined that as the temperature of the cell increases the cell performance also
increases up to 105°C. At 115°C the discharge capacity was reduced. Hence the
highest operating temperature of the lithium- O2 cells is estimated to be 105°C. The
improved performance of cells at higher temperatures is mainly because of lowered
cell resistance, reduced electrode activation energies and improved microstructure of
the cathode for accommodating the discharge product.
51
•
A cell was evaluated with discharge currents of 0.50, 0.30 and 0.25 mA while
keeping the cell temperature constant at 67°C. It was observed that the anodic and
cathodic activation losses were higher in the case of 0.50 mA compared to 0.30 and
0.25 mA currents. The discharge capacities for 0.50, 0.30 and 0.25 mA currents were
0.84, 1.76 and 1.77 mAh respectively. This decrease in discharge capacity with
increase in currents is attributed to the inability of the cathode to sustain and
accommodate the electrochemical reaction.
•
It was noted that with an increase in the cell resistance the discharge capacity of cell
is reduced. This increase in the cell resistance is because of the parasitic reactions
taking place in the cell.
•
Two cells were assembled to investigate the effect of oxygen transport in the
performance of the cathode. The cells were evaluated at the same temperature and
discharge currents. The only variable was the orientation of the cathode with respect
to the oxygen flow. It was observed that the cell in which the nickel foam side was
exposed to oxygen flow performed better than the cell in which the active material
was exposed to oxygen flow. The discharge capacity was 8.76 mAh when nickel
foam side was exposed to oxygen flow and 6.02 mAh when the active material side
was exposed to the oxygen flow. The superior performance of the cell when nickel
foam was exposed to oxygen flow is due to the ease of electrochemical reactions
taking place in the cathode.
52
Further improvements in the cathode processing may lead to even better
electrochemical performance of the lithium-O2 cells. The cathode design should also
address the issues associated with the use of air rather than oxygen.
53
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