Post-mortem analysis of the Li-ion battery with charge/discharge deterioration in high- and low-temperature environments

Li-ion batteries have widespread applications. However, their deterioration mechanisms at different temperature conditions remain unclear. In this study, we investigate the effect of high- and low-temperature environments on the charge–discharge performance of an 18650 Li-ion battery having a Li(Ni,Co,Al)O2-family cathode and a graphite anode. After 50 cycles under three temperature conditions, the surfaces of the cathode and anode removed from the disassembled batteries are analyzed using scanning electron microscopy. The crystal and electronic structures of the cathode and anode materials are investigated using X-ray diffraction and X-ray absorption spectroscopy. The anode after 50 cycles at 5 °C is also investigated using X-ray photoemission spectroscopy. Results of the charge–discharge experiments indicate that, under identical charge–discharge conditions, the deterioration in the battery performance at a low-temperature environment of 5 °C is greater than that at environments of 25 and 80 °C. The post-mortem analyses of the cathode and anode samples of the battery cycled at 5 °C reveal the primary reasons for the decrease in battery capacity. The generation of deposits, including metallic Li, Li2CO3, and LiF, on the anode surface is clarified. During this process, the Li ions available for reinsertion into the cathode decrease. Furthermore, the Ni3+ component is partly reduced to Ni2+ near the surface of the cathode active material, and the reactive oxygen species are generated simultaneously. The reactive oxygen species can promote Li deficiency and surface-electrolyte interface formation. The above understanding of the deterioration mechanism can contribute to developing safer battery usage methods.