State-of-charge-dependent thermal runaway in NCM333 pouch lithium-ion batteries under external heating: Linking macroscopic failure responses with multiscale material degradation

Lithium-ion batteries (LIBs) are key components in electric vehicles. However, external overheating-induced thermal abuse is a major trigger of thermal runaway (TR), which seriously limits their large-scale commercial application. In this study, the physical and electrochemical behaviors of NCM333 batteries with different states of charge (SOCs) under external heating were systematically investigated to clarify the mechanisms governing TR hazards. The results show that TR severity increases markedly with SOC. The cell at 0% SOC did not undergo TR, whereas the maximum temperature of the fully charged cell (100% SOC) reached 517.2 °C. Under high-power external heating, all five cells in the battery module experienced TR, with a peak temperature of 933.3 °C. Gas chromatography, scanning electron microscopy, and other characterization techniques were used to analyze gaseous products and to examine the structural and compositional evolution of the cathode and anode materials. The results indicate significant material degradation and partial structural collapse, both of which reduce thermal stability. This work provides practical guidance for the safety-oriented design of high energy density LIBs.