Depth-dependent understanding of the degradation mechanisms for typical LiNi0.5Co0.2Mn0.3O2/artificial graphite lithium-ion batteries upon high-temperature storage

With the expanding application fields of lithium-ion batteries (LIBs), some LIBs may experience periods of higher-temperature storage, e.g., in electric vehicles (EVs) parked under sunlight in hot summers. However, reports on cell behaviors and performance degradation mechanisms during high-temperature storage have rarely been released. Thus, there are still issues requiring in-depth investigation, including the (i) effects of operating conditions on cell performance, (ii) reasons for cell swelling, (iii) substances of generated solid byproducts and gases, and (iv) causes for reversible and irreversible capacity losses. To answer these questions, the performances of ∼2 Ah LiNi0.5Co0.2Mn0.3O2/artificial graphite pouch cells stored at various temperatures and voltages are studied. Interestingly, the cell is more sensitive to temperature than voltage. After storage at 4.4 V and 60 °C for 20 days, the cell exhibits a severe voltage drop and swelling, low capacity retention and recovery, and high impedance. The mechanisms are understood through advanced characterizations. The results indicate that early cell degradation is caused by a series of chain reactions involved in electrolyte and electrode/electrolyte interphase film decomposition and structural evolution in the outermost regions of the electrodes. We believe that these findings provide a theoretical basis for developing high-temperature resistant materials and LIBs.