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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., (© 2024 Wiley‐VCH GmbH.)

LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) is the most promising cathode material for next-generation lithium-ion batteries (LIBs). However, the chemical instability of the material during air exposure leads to the formation of residual lithium compounds (RLCs: LiOH and Li 2 CO 3 ) on the surface and inhibits its practical application. Here, we propose a chemical conversion process to remove RLCs by utilizing them and forming a hybrid coating layer on the surface of NMC811 that contains Li 3 PO 4 , LiMn 2 O 4 , and LiMnPO 4 phases, yielding multifaceted benefits. The hybrid layer on the surface protects the material from undesirable side reactions. It improves the cycle life of NMC811 by retaining 80% of its initial capacity after 300 cycles and 66% after 500 cycles at a 0.5C rate in the operating voltage of 3.0-4.3 V. The process enables high-voltage (4.7 V vs Li + /Li) operation by stabilizing the electrode-electrolyte interface, reduces the degree of cationic disorder and the voltage polarization for phase transitions, improves Coulombic efficiency and ion diffusion kinetics, and minimizes the secondary particle crack formation over long-term cycling. In fact, the coating reduces the detrimental effects of RLCs, leaves the surface for better Li + transport, and hence significantly improves the electrochemical performance of NMC811.