Revealing the thermal stability and component heat contribution ratio of overcharged lithium-ion batteries during thermal runaway

The thermal stability of overcharged lithium-ion batteries (LIBs) and heat contribution ratio of different components during thermal runaway are unclear. This paper investigates the thermal stability changes of the full battery and components after overcharging. The degradation mechanism of thermal stability induced by overcharging is revealed. The onset temperature of exothermic reactions of LIBs decreases with the increase of state of charge (SOC) - to as low as 31.7 °C at 165% SOC - due to internal short-circuit caused by separator piercing. The activation energy of exothermic side reactions decreases with the increase of SOC. The heat contribution ratio of different battery components is revealed, showing about 80% contribution from cathode at 100% and 120% SOC during thermal runaway. However, the heat contribution ratio from anode becomes bigger (about 60%) at SOC ≥140%, because of a large amount of heat released by the reaction of the electrolyte and lithium deposited at the anode side. Overcharge accelerates the phase transition of cathode crystal structure from a layered rhombohedral structure (R-3m) to the disordered spinel (Fd-3m) phase, which reduces the thermal stability of LIBs. These findings provide a theoretical basis for material-level safe design to reduce the occurrence of thermal runaway.