Morphology, structure and thermal stability analysis of cathode and anode material under overcharge

Safety behaviors of 10 Ah Li[Ni_1/3Co_1/3Mn_1/3]O₂/graphite lithium ion battery were investigated under overcharge condition by testing the morphology, structure and thermal stability with different states of charge (100%, 110%, 120%, 130% and 140%). The results showed that the volume of lithium ion battery increased quickly from 110% to 140% state of charge due to the electrodes expansion and the decomposition of solid electrolyte interphase and electrolyte. The cathode and anode material were getting powdery as the state of charge increasing. At 140% state of charge, the separator was covered by black residue, which blocked the pores and made the internal resistance increased. Due to the deep-degree delithiation process, the irreversible conversion of hexagonal phase H2 of Li[Ni_1/3Co_1/3Mn_1/3]O₂ to hexagonal phase H3 was observed on the cathode. However, the destruction of graphite structure was not observed during overcharge. The cathode released oxygen twice at about 250–400°C and 500–550°C, respectively, which accelerated the oxidation of electrolyte and heat releasing. The decomposition of solid electrolyte interphase on the anode was the onset of self-heating during overcharge, while the formation of hydrogen fluoride was viewed as the initiation of thermal runaway. It could be found that the structure and thermal stability of cathode and anode became worse with an increase in the state of charge.