Studies on Capacity Fade of Spinel based Li-Ion Batteries

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Studies on Capacity Fade of Spinel based Li-Ion
Batteries

• by
• P. Ramadass , A. Durairajan, Bala S. Haran, R.
  E. White and B. N. Popov
• Center for Electrochemical Engineering
• Department of Chemical Engineering, University of
  South Carolina Columbia, SC 29208

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Motivation

• To characterize the capacity fade phenomena of
  Li-ion batteries.
• To decrease the capacity fade on both positive
  and negative electrode by optimizing the DC and
  pulse charging protocol.
• To develop mathematical model which will explain
  the capacity fade in the spinel system.

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Objectives

• To study the change in capacity of commercially
  available spinel based Li-ion Cells (Cellbatt?
  cells).
• Study the performance of Li-ion cells under DC
  charging at different rates.
• Use impedance spectroscopy to analyze the change
  in cathode and anode resistance with cycling.
• Determine experimentally which electrode is more
  important in contributing to capacity fade.
• Do material characterization to study changes in
  electrode structure with cycling.

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Capacity Fade may Result from

• Overcharge Phenomena
• Lithium deposition on negative electrodes
• Electrolyte oxidation on positive electrode
• Passivation (Interfacial film formation)
• Self discharge
• Electrolyte Reduction
• Active Material Dissolution
• Phase Change

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Physical Characteristics of Cellbatt? Lithium Ion
Battery Electrodes
Cellbatt? is a Prismatic type cell
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Electrode Reactions
At anode
At cathode
Non-Stoichiometric Spinel
Cell Reaction
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Charging Protocols

• Constant current - Constant voltage
• Total charging time fixed
• Constant voltage
• Charging done completely at constant voltage
• Constant current - Constant voltage
• Charging stopped when the current reaches a value
  of 50 mA during the CV part
• Charging done to different cut-off potentials

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Change in discharge capacity for Li-ion cells
charged to different potentials
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Experimental

• Full Cell studies on CellBatt Li-ion Cells
• Galvanostatic charge-discharge
• 0.25 A, 0.5 A, 0.75 A, 1 A - (3.0-4.17 V)
• Cyclic Voltammograms - 0.05 mV/s, 2.5-4.2 V
• T-cell (half cell) studies
• Glove Box - Disk electrodes 1.2 cm ?
• Counter, Reference electrodes Li metal
• Cyclic Voltammograms - 0.05, 0.1 and 0.2 mV/s,
  3-4.5 V vs. Li/Li for spinel and 0-1.2V vs.
  Li/Li for carbon
• Impedance Analysis - 100 kHz 1 mHz 5 mV.
• XRD studies of spinel electrode at various
  cycles.

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Charge curves for CC-CV Protocol
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Charge and Discharge curves for Li-ion Cell at
various Cycles
C/2 Rate
Capacity Fade 15.4 for C/2 rate
Capacity Fade 19 for 1 C rate
C/2 Rate
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Change in CC-CV Profiles with Cycling
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Nyquist plots for Cellbatt? cell charged at 0.5 A
at different states of charge
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Nyquist plots for Cellbatt? cell charged at 0.5 A
during different cycles
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Nyquist Plots for Spinel and Carbon Electrodes at
Discharged state at Various Cycles
Spinel
Carbon
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Cyclic Voltammograms of Spinel Electrode after
800 Cycles at various Scan rates
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Cyclic Voltammograms of Carbon Electrode after
800 Cycles at various Scan rates
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Cyclic Voltammograms of Spinel and Carbon
Electrodes at Different Cycles
Spinel
Carbon
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XRD Patterns of Spinel after Different
Charge-Discharge Cycles
P. G.. Bruce et al., J. Electrochem. Soc., 146,
3649 (1999).
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Conclusions

• Varying the charging rate affects the overall
  capacity of the cell.
• Impedance studies reveal no significant increase
  in resistance at both electrodes after 800
  cycles.
• XRD studies of Spinel electrode reveal the
  formation of an additional phase with cycling.
• Capacity fade in the case of Cellbatt? cells can
  be summarized as

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Capacity Fade in Cellbatt? Li-ion cells
Secondary Active Material Degradation(C6
LiMn2O4)
Structural Degradation of LiMn2O4
Mn Dissolution from Spinel
SEI layer attack on Negative Electrode
HF formation
Accumulation of ?-MnO2 with Cycling
E. Wang et al.
J.C.Hunter et al.
Electrolyte Oxidation (starts from 3.7 V)
Salt Hydrolysis
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Acknowledgements
Financial support provided in part by the
Department of Energy (DOE) is gratefully
acknowledged.
Thank you!