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
Positive Negative
Characteristics
LiCoO2 Carbon
15.1 7.1
Mass of the electrode
material (g)
531 603
Geometric area (both
sides) (cm2)
28.4 11.9
Loading on one side
(mg/cm2)
Total Thickness 183 193
of the Electrode (m)
Specific Capacity
148 306
(mAh/g)
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
reference electrode
Lithium Foil -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
3.76
300-RT
Voltage (V)
3.32 Fresh
2.88
800-45
2.44 490-55
800-RT
2.00
0.0 0.4 0.8 1.2 1.6 2.0
Capacity (Ah)
Capacity Fade as a Function of Cycle Life
Percentage Capacity Fade
Temperature 50 100 150 300 500 800
RT 3.8 5.11 6.09 10.29 22.5 30.63
45 3.8 5.46 7 11.75 26.46 36.21
55 4.3 6.4 9.4 27 70.56 fail
Capacity Fade as a Function of Cycle Life
1.90
1.55
Capacity (Ah)
RT
1.20
45oC
0.85 55oC
0.50
0 100 200 300 400 500 600 700 800
Cycle Number
Charge Curves at Various Cycles
1.1 300 1.1 150 50
150 1
0.9 800 50 0.9 800 300
1
Current (A)
0.7
Current (A)
0.7
0.5 0.5
0.3 0.3
0.1 0.1
0 1 2 3 4 0 1 2 3
50 Time (h)
45 deg C
Time (h)
Room Temperature 300 150 1
0.9
45 degree-charge
RT-charge 0.7
Current (A)
0.5
0.3
0.1
55 deg C 0 1 2 3
T ime (h)
Change in Charging Times with Cycling
1
1.5 1
1
150
150 150
CC Time (h)
300 300
1.0
800
800 300
0.5
3 490
800 300
0.0 800
300
RT 45 55 150
CV Time (h)
Constant Current 2 1 300
150
150
1
1
1
0
RT 45 55
Constant Voltage
Rate Capability after 150 and 800 Cycles
2.00
Fresh
Discharge Capacity (Ah)
1.75
150-RT
150-45
1.50 150-55
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
300-RT -0 SOC
300-55-0 SOC
0.08 Fresh-RT -0 SOC
Fresh-55-0 SOC
ZIm ()-Fresh
0.06
0.04
0.02
0.00
0.30 0.35 0.40 0.45 0.50
ZRe( )
Nyquist Plots of Sony Cell at RT and 45oC
0.20 0.4
800-RT-0 SOC
800-45-0 SOC
0.16 Fresh-RT-0 SOC
0.3
Fresh-45-0 SOC
ZIm()-800 cyc
ZIm()-Fresh
0.12
0.2
0.08
0.1
0.04
0.00 0.0
0.3 0.4 0.5 0.6 0.7 0.8
ZRe ()
Negative Electrode Resistance
(Fully Lithiated)
600
RT
45 Deg C
500
55 Deg C
Resistance (cm2 )
400
300
200
100
0 60 120 180 240 300
Cycle Number
Positive Electrode Resistance
(Fully Lithiated)
500
RT
45 Deg C
Resistance (cm2 )
400 55 Deg C
300
200
100
0
0 60 120 180 240 300
Cycle Number
Comparison of Electrode Resistances
200
LiCoO2
Carbon
Resistance (ohm-cm 2 )
150
100
50
600
0
RT 45 55 500
Resistance (ohm-cm 2 )
400
150 Cycles LiCoO2
Carbon
300
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
200
Zim (ohm)
150
100
0 SOC
50 50 SOC
100 SOC
0
0 100 200 300 400 500 600
Zre (ohm)
Nyquist Plot of Fully Delithiated LiCoO2 as
a function of Storage Time at RT
140 Day 1
Day 3
120 Day 5
Day 7
100 Day 9
ZIm (ohms)
80
60
40
20
0
0 50 100 150 200 250 300 350 400
ZRe (ohms)
Nyquist Plot of Fully Lithiated LiCoO2 as
a function of Storage Time at RT
250
Day 1
Day 2
200 Day 3
Day 4
ZIm (ohms)
150
100
50
0
0 100 200 300 400 500 600 700 800
ZRe (ohms)
Specific Capacity of Positive and Negative
Electrodes at Various Cycles and Temperature
Cell
Specific capacity (mAh/g)
(Cycle No. –
Temperature)
LiCoO2 Carbon
Fresh 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
Full Cell
Cell
Capacity Lost Capacity
(Cycle No. – Loss
(mAh)
Temperature)
LiCoO2 Carbon (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
Room Temperature
1
Current (A)
0
-1
Fresh
800 cycles
-2
2.0 2.5 3.0 3.5 4.0 4.5
Voltage (V)
CV-fullcell-fresh and 800 cycles-RT
CV’s of Sony Cell
2
Scan rate: 0.1 mV/sec
1
Current (A)
0
Fresh-RT
-1 Fresh-45
800-RT
800-45
-2
2.0 2.5 3.0 3.5 4.0 4.5
Voltage (V)
XRD Patterns of LiCoO2 after Different
Charge-Discharge Cycles
Cell c/a
Fresh 5.103
300-55
150-RT 5.077
300-45 150-45 5.066
150-55 4.995
Intensity
300-RT
300-RT 4.998
150-55
300-45 4.995
150-45
300-55 4.985
150-RT
Fresh
20 30 40 50 60 70
2
Variation of Lattice Constants with
Cycling and Temperature
Decrease in c/a ratio leads to
5.10
decrease in Li stoichiometry*
c/a
5.05
5.00 RT
45 deg C
55 Deg C
0 100 200 300
Cycle Number
*G. Ting-Kuo Fey et al., Electrochemistry Comm. 3 (2001) 234
Capacity Fade
Loss of Li Degradation of C, LiCoO2
(Primary Active Material) (Secondary Active Material)
SEI Formation
Electrolyte Oxidation
Salt Reduction
PF6 2e 3Li 3LiF PF3
Overcharge Structural
Degradation
Solvent Reduction
CH 3CHOCO 2CH 2 2e 2 Li CH 3CHCH 2 Li 2CO3
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.