High-voltage medium-nickel low-cobalt lithium layered oxide cathode materials are becoming a popular development route for high-energy lithium-ion batteries due to their relatively high capacity, low cost, and improved safety. Unfortunately, capacity fading derived from surface lithium residue, electrode-electrolyte interfacial side reactions, and bulk structure degradation severely limits large-scale commercial utilization. In this work, an ultrathin and uniform NASICON-type Li₃V₂(PO₄)₃ (LVP) nanoscale functional coating is formed in situ by utilizing residual lithium to enhance the lithium storage performance of LiNi_0.6Co_0.05Mn_0.35O₂ (NCM) cathode. The GITT and ex-situ EIS and XPS demonstrate exceptional Li⁺ diffusion and conductivity and attenuated interfacial side reactions, improving the electrode-electrolyte interface stability. The variable temperature in-situ XRD demonstrates delayed phase transition temperature to improve thermal stability. The battery in-situ XRD displays the single-phase H1-H2 reaction and weakened harmful H3 phase transition, minimizing the bulk mechanical degradation. These improvements are attributed to the removal of surface residual lithium and the formation of NASICON-type Li₃V₂(PO₄)₃ functional coatings with stable structure and high ionic and electronic conductivity. Consequently, the obtained NCM@LVP delivers a higher capacity retention rate (97.1% vs. 79.6%) after 150 cycles and a superior rate capacity (87 mAh·g^–1 vs. 58 mAh·g^–1) at a 5 C current density than the pristine NCM under a high cut-off voltage of 4.5 V. This work suggests a clever way to utilize residual lithium to form functional coatings in situ to improve the lithium storage performance of high-voltage medium-nickel low-cobalt cathode materials.