Degradation behavior and underlying mechanisms of electrochemical and thermal stability in Li(Ni0.5Mn0.3Co0.2)O2/graphite lithium-ion batteries under overcharge conditions

This paper investigates the impact of overcharge on the electrochemical performance and thermal stability of Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 /graphite lithium-ion batteries (LIBs). The results show that deep overcharging reduces the usable capacity of the battery, primarily due to reactions between lithium deposition and O 2 released from the cathode, as well as the reaction between lithium and H 2 O. The incremental capacity (IC) curve shows two-phase transition peaks at 100 %–140 % SOC, while only one peak appears at 155 % and 165 % SOC, which is attributed to the loss of active material (LAM). The internal resistance increases with SOC, and the lithium-ion diffusion coefficient decreases sharply, mainly due to the loss of lithium inventory (LLI) and LAM. At 165 % SOC, the impedance of the solid electrolyte interface (SEI) layer exceeds the ohmic resistance for the first time, and the electrode diffusion impedance reaches 3.4 Ω. Overcharging accelerates thermal runaway, occurring 23 h earlier at 165 % SOC than at 100 % SOC. This is mainly due to greater heat release from lithium deposition and the electrolyte at a lower threshold temperature, as well as decreased cathode thermal stability resulting from transition metal valence reconstruction and lattice oxygen release. • Electrochemical and thermal stability degradation mechanism after deep overcharging • ICA and EIS can effectively evaluate battery overcharge status • LLI and LAM are the main causes of electrochemical performance degradation • Thermal instability arises from lithium deposition and transition metal reconstruction