Capacity Fade Studies of LiCoO 2 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
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Capacity Fade Studies of LiCoO 2 Based Li-ion Cells Cycled at Different Temperatures Bala S. Haran, P.Ramadass, Ralph E. White and Branko N. Popov Center.
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Capacity Fade Studies of
LiCoO2 Based Li-ion Cells
Cycled at Different Temperatures
Bala S. Haran, P.Ramadass,
Ralph E. White and Branko N. PopovCenter for Electrochemical Engineering
Department of Chemical Engineering,
University of South Carolina Columbia, SC 29208
ObjectivesStudy 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
Positive LiCoO2
Negative Carbon
Mass of the electrode
material (g)
15.1 7.1
Geometric area (both
sides) (cm2)
531 603
Loading on one side
(mg/cm2)
28.4 11.9
Total Thickness of the Electrode (m)
183 193
Specific Capacity (mAh/g)
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 - Characterization2LiCoO or carbon inert material
reference electrode
-lithium foil
separatorporous electrode
TMSwagelok Three Electrode Cell
current collector
Lithium Foil
Discharge Curve Comparison of Sony 18650 Cells after 800 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
Nyquist Plot of Fully Delithiated LiCoO2 as a function of Storage Time at RT
0
20
40
60
80
100
120
140
0 50 100 150 200 250 300 350 400ZRe (ohms)
ZIm
(o
hm
s)
Day 1
Day 3
Day 5
Day 7
Day 9
Nyquist Plot of Fully Lithiated LiCoO2 as a function of Storage Time at RT
0
50
100
150
200
250
0 100 200 300 400 500 600 700 800
ZRe (ohms)
Z Im (
oh
ms)
Day 1
Day 2
Day 3
Day 4
Specific Capacity of Positive and Negative Electrodes at Various Cycles and Temperature
Cell
(Cycle No. – Temperature)
Specific capacity (mAh/g)
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
Cell
(Cycle No. – Temperature)
Capacity Lost
(mAh)
Full Cell Capacity
Loss
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
2.0 2.5 3.0 3.5 4.0 4.5
Voltage (V)
-2
-1
0
1
2C
urre
nt (
A)
CV-fullcell-fresh and 800 cycles-RT
Scan rate: 0.1 mV/sec
Fresh800 cycles
Room Temperature
CV’s of Sony Cell
2.0 2.5 3.0 3.5 4.0 4.5
Voltage (V)
-2
-1
0
1
2C
urre
nt (
A)
CV-fullcell-fresh-800-RT-45
Scan rate: 0.1 mV/sec
Fresh-RTFresh-45800-RT800-45
XRD Patterns of LiCoO2 after Different Charge-Discharge Cycles
20 30 40 50 60 70
Inte
nsit
y
Fresh, 150-45 and 150-55
Fresh
150-RT
300-RT
150-45
150-55
300-45
300-55
2
Cell c/aFresh 5.103
150-RT 5.077
150-45 5.066
150-55 4.995
300-RT 4.998
300-45 4.995
300-55 4.985
Variation of Lattice Constants with Cycling and Temperature
0 100 200 300
Cycle Number
5.00
5.05
5.10
c /
a
RT45 deg C55 Deg C
Variation of lattice constants for LiCoO2
electrode with cycling and temperature
*G. Ting-Kuo Fey et al., Electrochemistry Comm. 3 (2001) 234
Decrease in c/a ratio leads to decrease in Li stoichiometry*
Capacity Fade
Loss of Li(Primary Active Material)
Degradation of C, LiCoO2
(Secondary Active Material)
SEI Formation
Overcharge
3223223 22 COLiCHCHCHLieCHCHOCOCH
36 332 PFLiFLiePF
Salt Reduction
Solvent Reduction
Electrolyte Oxidation
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.