Capacity Fade Studies of LiCoO2
Based Li-ion Cells Cycled at
Different Temperatures
Bala S. Haran, P.Ramadass,
Ralph E. White and Branko N. Popov
Center for Electrochemical Engineering
Department of Chemical Engineering,
University of South Carolina Columbia, SC 29208
Objectives
Study the change in capacity of commercially
available Sony 18650 Cells cycled at different
temperatures.
Perform rate capability studies on cells cycled
to different charge-discharge cycles.
Perform half-cell studies to analyze causes for
capacity fade.
Use impedance spectroscopy to analyze the
change in cathode and anode resistance with
SOC.
Study structural and phase changes at both
electrodes using XRD.
Characteristics of a
Sony 18650 Li-ion cell
 Cathode (positive
electrode) - LiCoO2.
 Anode (negative
electrode) - MCMB.
 Cell capacity – 1.8 Ah
Characteristics of a
Sony 18650 Li-ion cell
Characteristics
Mass of the electrode
material (g)
Geometric area (both
sides) (cm2)
Loading on one side
(mg/cm2)
Total Thickness
of the Electrode (m)
Specific Capacity
(mAh/g)
Positive
LiCoO2
Negative
Carbon
15.1
7.1
531
603
28.4
11.9
183
193
148
306
Experimental – Cycling Studies
 Cells cycled using Constant Current-Constant Potential
(CC-CV) protocol.
 Cells were discharged at a constant current of 1 A.
 Batteries were cycled at 3 different temperatures –
25oC, 45oC and 55oC.
 Experiments done on three cells for each temperature.
 Rate capability studies done after 150, 300 and 800
cycles - Cells charged at 1 A and discharged at currents
of 0.2, 0.4, 0.6, 0.8 and 1.0 A.
Experimental - Characterization
 Batteries were cut open in a glove box after 150, 300
and 800 cycles.
 Cylindrical disk electrodes (1.2 cm dia) were punched
from both the electrodes.
 Electrochemical characterization studies were done
using a three electrode setup.
 Impedance analysis - 100 kHz ~ 1 mHz ±5 mV.
 Material characterization - XRD studies and SEM,
EPMA analysis.
Experimental - Characterization
LiCoO2 or carbon
inert material
porous electrode
separator
Lithium Foil
reference electrode
-lithium foil
current collector
Swagelok TM Three Electrode Cell
Discharge Curve Comparison of Sony
18650 Cells after 800 Cycles
4.20
300-55 300-45
300-RT
Voltage (V)
3.76
3.32
Fresh
2.88
800-45
2.44
2.00
0.0
490-55
0.4
800-RT
0.8
1.2
Capacity (Ah)
1.6
2.0
Capacity Fade as a Function of Cycle Life
Percentage Capacity Fade
Temperature
RT
45
55
50
100
150
300
500
800
3.8
5.11
6.09
10.29
22.5
30.63
3.8
5.46
7
11.75
26.46
36.21
4.3
6.4
9.4
27
70.56
fail
Capacity Fade as a Function of Cycle Life
1.90
Capacity (Ah)
1.55
RT
1.20
45oC
0.85
55oC
0.50
0
100
200
300
400
500
Cycle Number
600
700
800
Charge Curves at Various Cycles
300
1.1
Current (A)
0.5
0.1
0.1
Room Temperature
2
3
0
4
1
50
Time (h)
300
0.9
150
1
0.7
0.5
0.3
0.1
55 deg C
0
2
Time (h)
45 degree-charge
RT-charge
1
0.5
0.3
1
300
800
0.7
0.3
0
150 50
0.9
1
0.7
Current (A)
Current (A)
150
50
800
0.9
1.1
1
2
T ime (h)
3
3
45 deg C
Change in Charging Times with Cycling
1
1
1.5
1
CC Time (h)
150
150
1.0
150
300
300
800
300
800
0.5
490
3
300
800
RT
Constant Current
45
55
CV Time (h)
0.0
150
2
300
800
1
300
150
150
1
1
1
0
RT
Constant Voltage
45
55
Rate Capability after 150 and 800 Cycles
Discharge Capacity (Ah)
2.00
Fresh
1.75
150-RT
1.50
150-45
150-55
800-45
800-RT
1.25
1.00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Applied Current (A)
Rate Capability comparison after 150 and 800 cycles
Nyquist Plots of Sony Cell at RT and 55oC
0.10
ZIm ()-Fresh
0.08
300-RT -0 SOC
300-55-0 SOC
Fresh-RT -0 SOC
Fresh-55-0 SOC
0.06
0.04
0.02
0.00
0.30
0.35
0.40
ZRe( )
0.45
0.50
Nyquist Plots of Sony Cell at RT and 45oC
0.20
0.4
800-RT-0 SOC
800-45-0 SOC
Fresh-RT-0 SOC
Fresh-45-0 SOC
0.3
0.12
0.2
0.08
0.1
0.04
0.00
0.3
0.4
0.5
0.6
ZRe ()
0.7
0.0
0.8
ZIm()-800 cyc
ZIm()-Fresh
0.16
Negative Electrode Resistance
(Fully Lithiated)
Resistance (cm2 )
600
RT
45 Deg C
55 Deg C
500
400
300
200
100
0
60
120
180
Cycle Number
240
300
Positive Electrode Resistance
(Fully Lithiated)
Resistance (cm2 )
500
RT
45 Deg C
55 Deg C
400
300
200
100
0
0
60
120
180
Cycle Number
240
300
Comparison of Electrode Resistances
200
Carbon
150
100
50
600
0
RT
150 Cycles
45
500
55
Resistance (ohm-cm 2 )
Resistance (ohm-cm 2 )
LiCoO2
400
300
LiCoO2
Carbon
200
100
0
RT
45
55
300 Cycles
Possible Reasons for Rapid Capacity Fade
at Elevated Temperatures
 The SEI layer formed on a graphite electrode changes in both
morphology and chemical composition during cycling at
elevated temperature.
 The R-OCO2Li phase is not stable on the surface and
decomposes readily when cycled at elevated temperatures
(55oC).
 This creates a more porous SEI layer and also partially exposes
the graphite surface, causing loss of charge on continued
cycling.
 The LiF content on the surface increases with increasing storage
temperature mainly due to decomposition of the electrolyte salt.
 SEI and electrolyte (both solvents and salt) decomposition have
a more significant influence than redox reactions on the
electrochemical performance of graphite electrodes at elevated
temperatures.
Nyquist Plot of Fresh LiCoO2
as a function of SOC at RT
250
Zim (ohm)
200
150
100
0 SOC
50 SOC
100 SOC
50
0
0
100
200
300
Zre (ohm)
400
500
600
Nyquist Plot of Fully Delithiated LiCoO2 as
a function of Storage Time at RT
Day 1
Day 3
Day 5
Day 7
Day 9
140
ZIm (ohms)
120
100
80
60
40
20
0
0
50
100
150
200
250
ZRe (ohms)
300
350
400
Nyquist Plot of Fully Lithiated LiCoO2 as
a function of Storage Time at RT
250
Day
Day
Day
Day
ZIm (ohms)
200
1
2
3
4
150
100
50
0
0
100
200
300
400
ZRe (ohms)
500
600
700
800
Specific Capacity of Positive and Negative
Electrodes at Various Cycles and Temperature
Cell
(Cycle No. –
Temperature)
Fresh
Specific capacity (mAh/g)
LiCoO2
Carbon
147.81
306.17
150-RT
144.29
2.38%
299.55
2.16%
150-45
143.12
3.17%
296.58
3.13%
150-55
141.25
4.44%
290.56
5.10%
300-RT
139.17
5.84%
283.95
7.26%
300-45
138.21
6.49%
282.17
7.84%
300-55
125.10
15.36%
246.58
19.46%
Comparison of Capacity Fade of Individual
Electrodes with Full Cell Loss
Cell
(Cycle No. –
Temperature)
Capacity Lost
(mAh)
LiCoO2
Carbon
Full Cell
Capacity
Loss
(mAh)
150-RT
53.061
46.947
107
150-45
70.744
68.046
125
150-55
98.996
110.773
168
300-RT
130.390
157.719
182
300-45
144.885
170.379
209
300-55
342.846
423.046
481
CV’s of Sony Cell
2
Scan rate: 0.1 mV/sec
Current (A)
1
Room Temperature
0
-1
Fresh
800 cycles
-2
2.0
2.5
3.0
3.5
Voltage (V)
CV-fullcell-fresh and 800 cycles-RT
4.0
4.5
CV’s of Sony Cell
2
Scan rate: 0.1 mV/sec
Current (A)
1
0
Fresh-RT
Fresh-45
800-RT
800-45
-1
-2
2.0
2.5
3.0
3.5
Voltage (V)
4.0
4.5
XRD Patterns of LiCoO2 after Different
Charge-Discharge Cycles
Cell
c/a
Fresh
5.103
150-RT
5.077
300-45
150-45
5.066
300-RT
150-55
4.995
300-RT
4.998
300-45
4.995
300-55
4.985
Intensity
300-55
150-55
150-45
150-RT
Fresh
20
30
40
50
2
60
70
Variation of Lattice Constants with
Cycling and Temperature
Decrease in c/a ratio leads to
decrease in Li stoichiometry*
c/a
5.10
5.05
RT
45 deg C
55 Deg C
5.00
0
100
200
Cycle Number
*G.
300
Ting-Kuo Fey et al., Electrochemistry Comm. 3 (2001) 234
Capacity Fade
Loss of Li
(Primary Active Material)
Degradation of C, LiCoO2
(Secondary Active Material)
SEI Formation
Electrolyte Oxidation
Salt Reduction

PF6  2e  3Li   3LiF  PF3
Overcharge
Solvent Reduction
CH3CHOCO2CH 2  2e  2Li   CH3CHCH2  Li2CO3 
Structural
Degradation
Conclusions
 Capacity fade increases with increase in temperature.
 For all cells decrease in rate capability with cycling is
associated with increased resistance at both
electrodes.
 Both primary (Li+) and secondary active material
(LiCoO2, C) are lost during cycling.
 The fade in anode capacity with cycling could be due
to repeated film formation.
 XRD reveals a decrease in Li stoichiometry at the
positive electrode with cycling.
Acknowledgements
This work was carried out under a contract with
Mr. Joe Stockel, National Reconnaissance Office
for
Hybrid Advanced Power Sources # NRO-00-C-1034.