1. Introduction
The development of portable electronics and electric vehicles has stimulated a huge demand for rechargeable lithium batteries because of their high energy density and high safety requirements.[1] To continuously improve the energy density of lithium batteries, various cathode and anode materials with high specific capacities have been explored. Among them, Ni-rich layered transition metal oxide LiNixCoyMnzO2(NCM, x ≥ 0.8, x + y + z = 1) has become a key cathode material to be developed for applications with its reasonable cost and high practical energy density.[2,3] However, several problems still need to be overcome to meet practical demands, including insufficient cycling stability and poor rate ability. These problems all stem from Li+/Ni2+ cation mixing, poor structural stability, and harmful side reactions at the electrode/electrolyte interface.[4-6] Surface side reactions might cause a significant increase in interfacial resistance as well as severe dissolution of transition metals, especially at high temperatures and increased anodic cut-off voltage.[7-9] Therefore, side reactions at the electrode/electrolyte interface remain the limiting factor for performance.
Protective particle coatings, mainly including metal oxides,[10-13] oxide solid electrolytes,[14-16]phosphates,[6,17,18]fluorides,[19-22] or graphene,[23-25] were widely utilized to address these problems. For example, Xu et al.[26] used Al2O3 and Zhou et al.[10] utilized ZnO. In recent years, two-dimensional (2D) materials, such as graphene and boron nitride, were used as electrode coating materials due to their high specific surface area, and adjustable physicochemical properties,[27] which could significantly enhance the electrochemical performance of cathode materials. However, ex-situ-added 2D materials tended to restack and aggregate in the cathode, which seriously slowed down the lithium-ion transport rate and reduced the active surface of the active materials.[28,29] Moreover, commonly used 2D materials often exhibited poor lithium-ion conductivity, which hindered the effective transfer of lithium ions inside the cathode. Selecting the 2D material with excellent lithium conductivity and achieving its uniform coating on the surface of cathode particles is crucial for improving the electrochemical performance of the cathode. g-C3N4 consists of a continuous 3-s triazine ring (C6N7) as the basic unit and is thought to possess a two-dimensional layered structure similar to graphene.[30] C and N atoms are sp2-hybridised, forming a highly delocalised p-conjugated system.[31] The unique nano-pore structure and abundant nitrogen vacancy edges in g-C3N4 facilitate the adsorption and diffusion of lithium ions.[32,33] Moreover, the mild synthesis conditions make it suitable for uniform in-situ coating of matrix electrode materials. Therefore, the utilization of 2D thin-layer g-C3N4 for uniform coating of NCM cathode particles is expected to suppress side reactions at the cathode/electrolyte interface without affecting the lithium-ion transport, thereby improving the electrochemical performance of NCM cathodes.
Herein, g-C3N4-coated LiNi0.8Co0.1Mn0.1O2(NCM811@CN) is constructed to alleviate the side reaction and improve the cycle performance. The g-C3N4coating effectively inhibits structural degradation and intergranular cracks. Meanwhile, strong chemical bonds form between g-C3N4 and the NCM cathode due to the unsaturated bonds on the surface of g-C3N4,[34,35]which not only improves the interface stability and accelerates the interfacial Li+ diffusion but also avoids the direct contact between the cathode and the electrolyte, suppressing the side reactions. Benefiting from the above advantages, the optimized NCM811@CN-3wt% cathode exhibits better rate capability and cycling stability than the uncoated NCM811 in both conventional liquid and solid-liquid-hybrid electrolyte-based cells. This work sparks new ideas on constructing ultra-thin interface layers with excellent comprehensive performance for high-nickel ternary-metal oxide cathodes.