Phase Behavior during Electrochemical Cycling of Ni‐Rich Cathode Materials for Li‐Ion Batteries

Although layered lithium nickel‐rich oxides have become the state‐of‐the‐art cathode materials for lithium‐ion batteries in electric vehicle (EV) applications, they can suffer from rapid performance failure—particularly when operated under conditions of stress (temperature, high voltage)‐the underlying mechanisms of which are not fully understood. This essay aims to connect electrochemical performance with changes in structure during cycling. First, structural properties of LiNiO2 are compared to the substituted Ni‐rich compounds NMCs (LiNixMnyCo1−x−yO2) and NCAs (LiNixCoyAl1−x−yO2). Particular emphasis is placed on decoupling intrinsic behavior and extrinsic “two‐phase” reactions observed during initial cycles, as well as after extensive cycling for NMC and NCA cathodes. The need to revisit the various high‐voltage structural changes that occur in LiNiO2 with modern characterization tools is highlighted to aid the understanding of the accelerated degradation for Ni‐rich cathodes at high voltages. This essay discusses the intrinsic and extrinsic phase behaviors of the state‐of‐the‐art cathode materials, namely Ni‐rich layered lithium transition metal oxides, during electrochemical cycling. A close comparison is performed with their parent material LiNiO2 to highlight their similarities as well as differences in structural properties and electrochemistry.