Tuning surface residual lithium into a Li-ion-conductive protective interphase toward stable Ni-rich cathodes

Nickel-Rich layered oxide cathodes (LiNi_xCo_yMn_1-x-yO₂, x ≥ 0.6) offer high energy density but suffer from rapid performance degradation arising from structural and interfacial instabilities. In particular, surface-accumulated residual lithium compounds (RLCs), act as a critical trigger for interfacial failure by impeding Li⁺ transport, disrupting surface structural order, and inducing localized stress that accelerates microcrack formation and electrolyte corrosion. Herein, we propose a simple and scalable in situ chemical conversion strategy that transforms detrimental RLCs into a robust, Li⁺-conductive biphasic protective interphase on Ni-rich cathodes. The resulting composite interphase simultaneously scavenges RLCs and provides effective interfacial shielding, thereby suppressing parasitic side reactions and structural degradations. As a result, the cathode exhibits markedly improved electrochemical performance durability, delivering a capacity retention of 81.9% after 200 cycles at 1C. This work establishes in situ residual‑lithium conversion as an effective and practical paradigm for interfacial stabilization of Ni-rich cathodes, offering a viable pathway toward their reliable application in high-energy lithium-ion batteries.