Transforming Residual Lithium Compounds on the LiNi0.8Mn0.1Co0.1O2 Surface into a Li–Mn–P–O-Based Composite Coating for Multifaceted Improvements

LiNi_0.8Mn_0.1Co_0.1O₂ (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 Li₂CO₃) 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 Li₃PO₄, LiMn₂O₄, and LiMnPO₄ 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.