Unlocking the thermal safety evolution of lithium-ion batteries under shallow over-discharge

Shallow over-discharge has a significant impact on cell performance and thermal safety. This work comprehensively investigates the impact of shallow over-discharge on the heat generation upon discharging and thermal runaway behavior under adiabatic conditions, post-mortem characterization analysis is utilized to reveal the degradation mechanisms caused by shallow over-discharge. The solid electrolyte interface film decomposition and the copper plating are the primary degradation mechanisms during the shallow over-discharge process. Solid electrolyte interface film regeneration consumes active lithium and the electrolyte. The plated copper-containing substances tightly coating the secondary particles of the cathode severely hinders the intercalation of lithium ions. The shedding of the anode active materials causes the loss of the charge storage capability. These are the major causes for the cell capacity degradation. With the depth of over-discharge deepening, the severity of cell degradation increases, which makes the cell impedance increase. Therefore, as the depth of over-discharge deepens, the heat generation becomes more significant. Besides, the cell thermal stability decreases due to the reduced thermal stability of the regenerated solid electrolyte interface film. In addition, the maximum temperature of the over-discharged cell is lower than that of fresh cell, which is mainly caused by the loss of partial active materials.