Abstract
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(Ni0.6Mn0.2Co0.2)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.
Upon charging LIBs to different SOCs, the side reactions produce different substances. The red mark represents the new substances that may be produced in each stage, which participate in the next reaction, and generate abundant heat and gas, eventually leading to thermal runaway. [Display omitted]
•A comprehensive and systematic investigation of LIB overcharge behavior is presented.•The decomposition mechanism of the NMC311 cathode during overcharge is speculated.•The evolution of the electrode crystal structure during LIB overcharge is examined.•A four-stage degradation and failure process during LIB overcharge is revealed.•The findings help to understand the thermal runaway behavior during LIB overcharge.