Cationic Covalent Organic Framework Coating of the Ni-Rich Cathode Surface for the Significant Improvement of the Interfacial Li⁺ Transport and Structural Stability

LiNi1–x–yMnxCoyO2 (NCM) with a high Ni content is widely used as a cathode material because of its high energy density. However, it faces challenges such as structural instability and undesired side reactions with electrolytes, resulting in capacity fading and cell failure. In this study, we utilize a cyclopropenium cationic covalent organic framework (C-COF) as a coating material of NCM with 94% Ni content. The C-COF synthesized by the interaction between cyclopropane and piperazine precursors through a strong SN2 reaction not only generates a chemically stable framework structure but also facilitates smooth Li+ transport into the coating layer. In the C-COF skeleton, cation moieties attract PF6– anions in the Li salts of the electrolyte through electrostatic interactions. The solvation between Li+ and PF6– is suppressed by the formation of a C-COF layer on the cathode surface, which results in the free movement of Li ions into the cathode lattice. It was found that the C-COF layer offers about 3 times higher Li+ diffusion coefficient of 1.51 × 10–10 cm2 s–1 than unmodified NCM (5.13 × 10–11). It was observed that the uniformly coated C-COF layer prevents not only crack formation but also direct contact between the cathode and electrolyte. The incorporation of a C-COF layer serves as a physical barrier that effectively mitigates structural degradation and chemical decomposition of the cathode surface. The C-COF-coated NCM exhibits superior cycling stability and improved cycling performance from 60.17 to 71.31% after 200 cycles.