Conducting Li _x PO _y Interface Generated From Insulating Residual Lithium Compounds on LiNi _0.8 Mn _0.1 Co _0.1 O ₂ Surface Improves Cycle Life and Assists in Fast Cycling

Abstract LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811) is the most promising cathode material for future Li‐ion batteries (LIBs). However, the bulk and surface structural instabilities retard its commercial success. Surface chemical instability toward exposure to moisture (H ₂ O and CO ₂ ) leads to the formation of residual lithium compounds (RLCs: Li ₂ CO ₃ , LiOH) on the surface. The alkaline RLCs form a resistive layer on the surface of NMC811 by undergoing parasitic side reactions with electrolytes. Herein, an “Adverse‐to‐Beneficial” approach is proposed to eliminate RLCs by chemically transforming them into a Li _x PO _y (Li ₃ PO ₄ and LiPO ₃ ) interface. The interface protects the NMC811 surface from moisture attack and unwanted side reactions with electrolytes. It enhances the cycle life by retaining 70% of the initial capacity after 300 cycles at a 0.5C rate and 60% after 500 cycles, even at a 5C rate in a voltage window of 3.0–4.3 V versus Li ⁺ /Li. The coexistence of two Li‐conducting phases lowers the voltage polarization of the kinetically sluggish H1 → M phase transition to unlock fast cycling, reduces cationic disorder, improves coulombic efficiency, enhances ion diffusion kinetics, and minimizes particle crack formation after long‐term cycling. Hence, the Li _x PO _y interface yields multifaceted benefits in the storage, processing, and electrochemistry of NMC811.