Enhanced Electrochemical Performance and Cycling Stability of High-Nickel NCM90 Cathodes via g‑C₃N₄ Surface Coating

High-nickel layered oxide cathodes, such as LiNi_0.9Co_0.05Mn_0.05O₂ (NCM90), represent promising options for next-generation lithium-ion batteries owing to their high specific capacity and reduced cobalt content. Nevertheless, structural degradation, transition metal (TM) dissolution, and electrolyte decomposition at elevated voltages (>4.1 V vs Li/Li⁺) significantly constrain their long-term performance. In this investigation, we apply a graphitic carbon nitride (g-C₃N₄) coating on NCM90 particles through a straightforward two-step thermal annealing process, with melamine as the precursor. The nitrogen-rich g-C₃N₄ layer enhances the surface structural stability of NCM90, thereby mitigating transition metal (TM) dissolution and facilitating the formation of a stable cathode-electrolyte interphase. Electrochemical impedance spectroscopy revealed a decrease in charge transfer resistance (R _ct) from 220.2 Ω to 144.3 Ω. Following 100 charge-discharge cycles at 0.5C, the g-C₃N₄-coated NCM90 retained a specific capacity of 179.67 mAh g^- 1 (corresponding to 81% retention), compared to 74.32 mAh g^- 1 (34%) for the pristine sample. Furthermore, the coated sample demonstrated enhanced high-rate performance, with 182.96 mAh g^- 1 at 5C. Notably, in full-cell evaluations for practical cycle life, the g-C₃N₄-coated NCM90 retained a discharge capacity of 120.16 mAh g^- 1 (83% retention) after 100 cycles at 2C, whereas the pristine sample exhibited only 83.51 mAh g^- 1 (57%). These findings illustrate that g-C₃N₄ surface coatings effectively alleviate interfacial degradation and enhance electrochemical performance, providing a scalable approach for stabilizing high-voltage Ni-rich cathodes.