An experimental study on the mechanical characteristics of Li-ion battery during overcharge-induced thermal runaway

Lithium batteries are prone to swelling during overcharge, which leads to a huge external force in modules and poses a potential threat to electric vehicles. Therefore, it is extremely important to understand the swelling mechanism of lithium batteries in-depth. In this paper, expansion force evolution during overcharge-induced thermal runaway process for large format lithium-ion battery with Li(Ni_0.5Co_0.2Mn_0.3)O₂ cathode is investigated. The rupture of the battery shell due to huge swelling is proved as the trigger of the thermal runaway event. In detail, the expansion force increases dramatically with the state of charge, which can be up to 3400 N. And it is a highly dynamic quantity with four obvious inflection points during overcharge, indicating a strong relationship with failure mechanisms. The inherent failure mechanism behind expansion force is investigated using both in-situ and ex-situ measurement techniques. The results revealing that the early abnormal expansion of the battery is caused by lithium dendrite and lithium plating. The rapid expansion mainly caused by CO, which is produced by incomplete electrolyte oxidation, leads to the rupture of the battery shell and thermal runaway events. This paper aims to unravel the mystery of lithium battery swelling during overcharge and provide valuable guidance to mitigate the overcharge risk induced by battery swelling.