Degradation Mechanism of LiNi_0.5Co_0.2Mn_0.3O₂/Graphite Cells at Elevated State of Charge and High Temperature

Lithium-ion batteries (LIBs) with LiNixCoyMn1-x-yO2 (NCM) cathode are among the state-of-the-art batteries with high energy and power densities. NCM has been reported with issues of phase transition, volume change upon cycling and reacting with electrolyte. However, the possible degradation behaviors of NCM are still unclear when working at high state of charge (SOC) and elevated temperatures. In this work, life tests for commercial LiNi0.5Co0.2Mn0.3O2 (NCM523)/graphite cells are performed at high SOC (4.0-4.1 V or 4.1-4.2 V) and elevated temperature (45 C) for the first time. The post-tested cells are characterized with multiple techniques to present a clear and comprehensive image of the cell degradations. The revealed degradation mechanisms of NCM cell at high SOC and high temperature include nikel (Ni) dissolution of NCM523, the formation of the oxidation layer of LixPOyFz on cathode particles and gas release from the oxidation of solvents in the electrolyte.