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

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 ${\mathrm{Ni}}_{{\mathrm{Li}}}^ \bullet $
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. ... mehrOverall, this work sheds new light on the effects of manganese and cobalt incorporation into the transition-metal layer and their conjunction with ${\mathrm{Ni}}_{{\mathrm{Li}}}^ \bullet $ defects.