Study on the electrochemical-thermal-gas coupling characteristics of NCM811 lithium-ion batteries under overcharging conditions

This study systematically investigates the overcharging behavior of NCM811 lithium-ion batteries by analyzing electrochemical, thermal, and gas evolution characteristics. Experimental measurements track voltage, temperature, swelling force, and gas generation under different charging rates. Results show that swelling force exhibits four distinct inflection points that sensitively reflect thermal runaway (TR) mechanisms, presenting a segmented linear relationship with state of charge (SOC). During the voltage plateau phase, rapid swelling force increase coincides with internal resistance growth and temperature rise. Differential capacity analysis reveals that swelling force variations directly correspond to side reaction peaks, confirming gas generation as the primary expansion driver. Advanced characterization using in-situ gas monitoring and post-mortem analysis demonstrates that electrolyte oxidation and lithium plating-induced reduction reactions are responsible for gas accumulation. The NCM811 cathode undergoes significant structural degradation during overcharging, as verified by SEM (Scanning Electron Microscopy) observations. These findings establish a comprehensive electrochemical-thermal-gas coupling mechanism during overcharging, providing critical insights into the failure processes of high-nickel batteries. The study highlights swelling force as a more sensitive thermal runaway indicator compared to conventional voltage and temperature monitoring, offering valuable perspectives for battery safety management in electric vehicles.