High-capacity nickel-rich layered oxides are promising cathode materials for high-energy-density lithium batteries. However, the poor structural stability and severe side reactions at the electrode/electrolyte interface result in unsatisfactory cycle performance. Herein, the thin layer of two-dimensional (2D) graphitic carbon-nitride (g-C₃N₄) is uniformly coated on the LiNi_0.8Co_0.1Mn_0.1O₂ (denoted as NCM811@CN) using a facile chemical vaporization-assisted synthesis method. As an ideal protective layer, the g-C₃N₄ layer effectively avoids direct contact between the NCM811 cathode and the electrolyte, preventing harmful side reactions and inhibiting secondary crystal cracking. Moreover, the unique nanopore structure and abundant nitrogen vacancy edges in g-C₃N₄ facilitate the adsorption and diffusion of lithium ions, which enhances the lithium deintercalation/intercalation kinetics of the NCM811 cathode. As a result, the NCM811@CN-3wt% cathode exhibits 161.3 mAh g⁻¹ and capacity retention of 84.6% at 0.5 C and 55 ℃ after 400 cycles and 95.7 mAh g⁻¹ at 10 C, which is greatly superior to the uncoated NCM811 (i.e. 129.3 mAh g⁻¹ and capacity retention of 67.4% at 0.5 C and 55 ℃ after 220 cycles and 28.8 mAh g⁻¹ at 10 C). The improved cycle performance of the NCM811@CN-3wt% cathode is also applicable to solid–liquid-hybrid cells composed of PVDF:LLZTO electrolyte membranes, which show 163.8 mAh g⁻¹ and the capacity retention of 88.1% at 0.1 C and 30 ℃ after 200 cycles and 95.3 mAh g⁻¹ at 1 C.