Decoupling Substitution Effects from Point Defects in Layered Ni‐Rich Oxide Cathode Materials for Lithium‐Ion Batteries

Abstract

Ni‐rich LiNixCoyMnzO2 cathode materials offer high practical capacities and good rate capability, but are notorious for being unstable at high state of charge. Here, a series of such layered oxides with nickel contents ranging from 88 to 100 mol% is fabricated by sodium‐to‐lithium ion exchange, yielding materials devoid of substitutional defects. Examining the initial charge/discharge cycle reveals effects that are specifically caused by transition‐metal substitution, which would otherwise be obscured by changes in lithium‐site defect concentration. Lowering the nickel content helps to stabilize the high‐voltage regime, while simultaneously negatively affecting lithium diffusion. Operando X‐ray diffraction indicates mitigation of volume variation during cycling and transition toward solid‐solution behavior with sufficiently high cobalt and manganese contents, thus providing an explanation for the increased stability. The interplay between transition‐metal substitution, kinetic hindrance, and solid‐solution behavior may be a result of local inhomogeneities due to lithium‐vacancy pinning, which is further elucidated through density functional theory calculations. Overall, this work sheds new light on the effects of manganese and cobalt incorporation into the transition‐metal layer and their conjunction with defects.