Degradation Behavior of Lithium-ion Cells Under Overcharge Extremes

Degradation behavior of commercial lithium-ion pouch cells containing LiCoO2 cathode and graphite anode was investigated for a cycling under continuous overcharge condition. This condition is frequently experienced in electric vehicles in an event of Battery Management System (BMS) failure. Failure of BMS results in an unbalanced module further resulting in overcharging or overdischarging the cells. Commercial cells with 5Ah capacity were continuously cycled at different upper cutoff voltages and 1C-rate to develop a better understanding of the overcharge process. The results show that as the upper cutoff voltage is extended, the cell gains a higher initial capacity. However, the cycle life of the cell diminishes significantly. The extent of overcharge was found to be an important parameter not only for the electrochemical performance but also for cell integrity. Cells overcharged beyond 4.5 V had a significant volume increase and a rapid increase in the capacity fade. The cell starts to swell at this stage and a considerable increase in the temperature and internal resistance of the cells is observed. Thermal imaging of the cell revealed non-uniform temperature distribution and localized degradation sites were identified. Evidence of lithium plating and electrolyte deposits on anode was observed in cells charged beyond 4.4 V, with SEM-EDS verifying their presence. A comparative study of various State of Health (SoH) estimation parameters is presented and the proposed parameter Φ_R based on internal resistance measurement is found to be a good indicator of aggravated degradation in cells.