A systematic investigation of internal physical and chemical changes of lithium-ion batteries during overcharge

The overcharge of lithium-ion batteries (LIBs) can not only cause irreversible battery degradation and failure but also trigger detrimental thermal runaway. This paper presents a systematic investigation of the electrical and thermal behaviors of LIBs during overcharge up to thermal runaway, and reveals the underlying physical, structural, and chemical changes at each electrode at different stages of overcharge using microscopic and spectroscopy characterizations. The overcharge process of LIBs with Li(Ni_0.6Mn_0.2Co_0.2)O₂ cathodes can be divided into four stages. Stage I involves the decomposition of LiMnO₂ and LiNiO₂ to MnO and NiO, respectively, accompanied by a slight collapse of the cathode. During stage II, the cathode forms a relatively stable hexagonal phase H3 while Li plating occurs at the anode surface. Moreover, NiO and Ni(OH)₂ decompose into metallic Ni and release a large amount of heat. During stage III, MnO₂ is formed on the cathode, which is irreversibly damaged, and unstable substance LiH is formed on the anode, which accelerates thermal runaway onset. During stage IV, the battery ruptures at approximately 174% state of charge (SOC), followed by thermal runaway in just 20 s. During overcharge, the crystallinity of the cathode decreases with the increase of SOC.