Flash joule heating-induced spinel-phase surface in Ni-rich layered oxide positive electrodes to stabilise lattice oxygen

High-nickel layered positive electrodes suffer from progressive structural degradation arising from lattice oxygen loss and inherent lattice strain. Although surface coatings are widely used to stabilize lattice-oxygen redox and mitigate electro-chemo-mechanical degradation, achieving coatings with full continuity, robust interfacial bonding, and fast Li⁺ conductivity remains challenging. Herein, we present a fundamentally different approach to shell formation via a self-derived subtractive strategy, departing from the conventional additive-based coating methods. By accurately applying transient thermal pulses, surface lithium is selectively extracted from layered LiNi_xCo_yMn_1-x-yO₂ (x = 0.8 ~ 0.9), directly converting the outer region into a coherent spinel-phase shell with tunable thickness. This nanoscale spinel-phase skin forms a robust mortise-and-tenon-like interconnection with the layered bulk, enabling isotropic, high-rate Li⁺ extraction/insertion while maintaining electronic conductivity throughout cycling. It effectively confines active oxygen intermediates, and suppresses interfacial side reactions and strain evolution under high-potential operation. Therefore, the spinel-phase skin-encapsulated LiNi_0.8Co_0.1Mn_0.1O₂ achieves an initial Coulombic efficiency of 95.3% and enables pouch cells with 80.1% capacity retention after 2000 cycles at 180 mA g^-1. This strategy is extendable to LiNi_0.9Co_0.05Mn_0.05O₂, may open new avenues for advancing nickel-rich positive electrode technologies.