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Capacity Fade Studies of LiCoO 2 Based Li-ion Cells Cycled at Different Temperatures PowerPoint PPT Presentation


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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

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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.


Experimental characterization7 l.jpg

Experimental - Characterization


Discharge curve comparison of sony 18650 cells after 800 cycles l.jpg

Discharge Curve Comparison of Sony 18650 Cells after 800 Cycles


Capacity fade as a function of cycle life l.jpg

Capacity Fade as a Function of Cycle Life


Capacity fade as a function of cycle life10 l.jpg

Capacity Fade as a Function of Cycle Life


Charge curves at various cycles l.jpg

45 deg C

Room Temperature

55 deg C

Charge Curves at Various Cycles


Change in charging times with cycling l.jpg

Constant Current

Constant Voltage

Change in Charging Times with Cycling


Rate capability after 150 and 800 cycles l.jpg

Rate Capability after 150 and 800 Cycles


Nyquist plots of sony cell at rt and 55 o c l.jpg

Nyquist Plots of Sony Cell at RT and 55oC


Nyquist plots of sony cell at rt and 45 o c l.jpg

Nyquist Plots of Sony Cell at RT and 45oC


Negative electrode resistance fully lithiated l.jpg

Negative Electrode Resistance (Fully Lithiated)


Positive electrode resistance fully lithiated l.jpg

Positive Electrode Resistance (Fully Lithiated)


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Comparison of Electrode Resistances

150 Cycles

300 Cycles


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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 LiCoO2 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 LiCoO2 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 LiCoO2 as a function of Storage Time at RT


Specific capacity of positive and negative electrodes at various cycles and temperature l.jpg

Specific Capacity of Positive and Negative Electrodes at Various Cycles and Temperature


Comparison of capacity fade of individual electrodes with full cell loss l.jpg

Comparison of Capacity Fade of Individual Electrodes with Full Cell Loss


Cv s of sony cell l.jpg

Room Temperature

CV’s of Sony Cell


Cv s of sony cell26 l.jpg

CV’s of Sony Cell


Xrd patterns of licoo 2 after different charge discharge cycles l.jpg

XRD Patterns of LiCoO2 after Different Charge-Discharge Cycles


Variation of lattice constants with cycling and temperature l.jpg

Variation of Lattice Constants with Cycling 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

Loss of Li

(Primary Active Material)

Degradation of C, LiCoO2

(Secondary Active Material)

SEI Formation

Electrolyte Oxidation

Salt Reduction

Overcharge

Structural

Degradation

Solvent Reduction


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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.


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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.


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