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

LiNi_0.8Mn_0.1Co_0.1O₂ (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 (H₂O and CO₂) leads to the formation of residual lithium compounds (RLCs: Li₂CO₃, 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_xPO_y (Li₃PO₄ and LiPO₃) 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_xPO_y interface yields multifaceted benefits in the storage, processing, and electrochemistry of NMC811.