Investigation of the internal physical and chemical changes of a cylindrical lithium-ion battery during thermal runaway

This study delves into the critical safety issue of thermal runaway (TR) in lithium-ion batteries (LIBs), particularly focusing on the physical and chemical changes occurring in the electrode materials during temperature escalation. We investigate a commercial 18650 type 2.6 Ah Li[Ni₅Co₂Mn₃]O₂/graphite battery, tracing changes from room temperature to the point of TR. Our findings reveal that the negative electrode experiences gradual decomposition and regeneration of the solid electrolyte interface (SEI) film, reacting with the electrolyte to form compounds such as Li₂CO₃ and LiF on its surface. Similarly, the positive electrode also generates Li₂CO₃ on its surface as temperature rises. Notably, at 185 °C, there is a partial disintegration of the positive electrode particles, accompanied by a structural transformation from the LiMO₂ (M representing Ni, Co, and Mn) R-3m layered structure to a disordered spinel LiM₂O₄. This research contributes to a deeper understanding of the TR mechanism in LIBs, offering valuable insights for material researchers in designing safer battery systems.