Capacity fade studies of licoo 2 based li ion cells cycled at different temperatures
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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 for Electrochemical Engineering Department of Chemical Engineering, University of South Carolina Columbia, SC 29208. Objectives.

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Capacity fade studies of licoo 2 based li ion cells cycled at different temperatures l.jpg

Capacity Fade Studies of LiCoO2 Based Li-ion Cells Cycled at Different Temperatures

Bala S. Haran, P.Ramadass,

Ralph E. Whiteand Branko N. Popov

Center for Electrochemical Engineering

Department of Chemical Engineering,

University of South Carolina Columbia, SC 29208


Objectives l.jpg
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 l.jpg

  • Cathode (positive electrode) - LiCoO2.

  • Anode (negative electrode) - MCMB.

  • Cell capacity – 1.8 Ah

Characteristics of a Sony 18650 Li-ion cell


Characteristics of a sony 18650 li ion cell4 l.jpg

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

Characteristics of a Sony 18650 Li-ion cell


Experimental cycling studies l.jpg
Experimental – Cycling Studies

  • 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.

  • Cells cycled using Constant Current-Constant Potential (CC-CV) protocol.


Experimental characterization l.jpg
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.






Charge curves at various cycles l.jpg

45 deg C Cycles

Room Temperature

55 deg C

Charge Curves at Various Cycles


Change in charging times with cycling l.jpg

Constant Current Cycles

Constant Voltage

Change in Charging Times with Cycling





Negative electrode resistance fully lithiated l.jpg
Negative Electrode Resistance Cycles (Fully Lithiated)


Positive electrode resistance fully lithiated l.jpg
Positive Electrode Resistance Cycles(Fully Lithiated)


Comparison of electrode resistances l.jpg
Comparison of Electrode Resistances Cycles

150 Cycles

300 Cycles


Possible reasons for rapid capacity fade at elevated temperatures l.jpg
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 licoo 2 as a function of soc at rt l.jpg
Nyquist Plot of Fresh LiCoO Temperatures2 as a function of SOC at RT


Nyquist plot of fully delithiated licoo 2 as a function of storage time at rt l.jpg
Nyquist Plot of Fully Delithiated LiCoO Temperatures2 as a function of Storage Time at RT


Nyquist plot of fully lithiated licoo 2 as a function of storage time at rt l.jpg
Nyquist Plot of Fully Lithiated LiCoO Temperatures2 as a function of Storage Time at RT




Cv s of sony cell l.jpg

Room Temperature Full Cell Loss

CV’s of Sony Cell


Cv s of sony cell26 l.jpg
CV’s of Sony Cell Full Cell Loss


Xrd patterns of licoo 2 after different charge discharge cycles l.jpg
XRD Patterns of LiCoO Full Cell Loss2 after Different Charge-Discharge Cycles


Variation of lattice constants with cycling and temperature l.jpg
Variation of Lattice Constants with Full Cell LossCycling and Temperature

Decrease in c/a ratio leads to decrease in Li stoichiometry*

*G. Ting-Kuo Fey et al., Electrochemistry Comm. 3 (2001) 234


Slide29 l.jpg

Capacity Fade Full Cell Loss

Loss of Li

(Primary Active Material)

Degradation of C, LiCoO2

(Secondary Active Material)

SEI Formation

Electrolyte Oxidation

Salt Reduction

Overcharge

Structural

Degradation

Solvent Reduction


Conclusions l.jpg
Conclusions Full Cell Loss

  • 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 l.jpg
Acknowledgements Full Cell Loss

This work was carried out under a contract with Mr. Joe Stockel, National Reconnaissance Office

for

Hybrid Advanced Power Sources # NRO-00-C-1034.


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