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

0.0 0.4 0.8 1.2 1.6 2.0

Capacity (Ah)

2.00

2.44

2.88

3.32

3.76

4.20

Vol

tage

(V

)

Dicharge curves after 800 cycles

Fresh

300-RT300-45300-55

800-RT

800-45

490-55

Capacity Fade as a Function of Cycle Life

Temperature 50 100 150 300 500 800RT 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

Percentage Capacity Fade

Capacity Fade as a Function of Cycle Life

0 100 200 300 400 500 600 700 800

Cycle Number

0.50

0.85

1.20

1.55

1.90C

apac

ity

(Ah)

45oC

55oC

RT

Charge Curves at Various Cycles

0 1 2 3

Time (h)

0.1

0.3

0.5

0.7

0.9

1.1

Cur

rent

(A

)

45 degree-charge

50150

1300800

45 deg C

55 deg C 0 1 2 3

Time (h)

0.1

0.3

0.5

0.7

0.9

Cur

rent

(A)

55 degree-charge

50

1150300Room Temperature

0.1

0.3

0.5

0.7

0.9

1.1

Cur

rent

(A

)

Time (h)

RT-charge

1

50

150

300

0 1 2 3 4

800

Change in Charging Times with Cycling

Constant CurrentRT 45 55

0.0

0.5

1.0

1.5

CC

Tim

e (h

)

1

150

300

1

150

300

1

150

300800

800

1

Constant Voltage

RT 45 550

1

2

3

CV

Tim

e (h

)

1150 300

1150

300

1

150

300800

800

490

1

Rate Capability after 150 and 800 Cycles

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Applied Current (A)

1.00

1.25

1.50

1.75

2.00D

isch

arge

Cap

acit

y (A

h)

Rate Capability comparison after 150 and 800 cycles

Fresh

150-RT150-45

150-55

800-RT

800-45

Nyquist Plots of Sony Cell at RT and 55oC

0.30 0.35 0.40 0.45 0.50ZRe( )

0.00

0.02

0.04

0.06

0.08

0.10Z

Im(

)-F

resh

cell cycled at RT and 45 after 800 cycles (0 soc)

300-RT-0 SOC300-55-0 SOCFresh-RT-0 SOCFresh-55-0 SOC

Nyquist Plots of Sony Cell at RT and 45oC

0.3 0.4 0.5 0.6 0.7 0.8ZRe()

0.00

0.04

0.08

0.12

0.16

0.20

ZIm

()-Fre

sh

ZIm

()-800 cyc

0.0

0.1

0.2

0.3

0.4

cell cycled at RT and 45 after 800 cycles (0 soc)

800-RT-0 SOC800-45-0 SOCFresh-RT-0 SOCFresh-45-0 SOC

Negative Electrode Resistance (Fully Lithiated)

0 60 120 180 240 300

Cycle Number

100

200

300

400

500

600

Res

ista

nce

(c

m2)

Resistances as a function of cycles

RT45 Deg C55 Deg C

Positive Electrode Resistance (Fully Lithiated)

0 60 120 180 240 300

Cycle Number

0

100

200

300

400

500

Res

ista

nce

(c

m2)

Resistances as a function of cycles

RT45 Deg C55 Deg C

Comparison of Electrode Resistances

RT 45 550

50

100

150

200

Res

ista

nce

(o

hm

-cm

2)

LiCoO2Carbon

1

RT 45 550

100

200

300

400

500

600

Res

ista

nce

(o

hm

-cm

2)

LiCoO2Carbon

1

150 Cycles

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

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.