Research progress on coating modification of lithium-rich cathode materials for lithium-ion batteries

With the rapid development of new energy vehicles and the energy storage industry, traditional cathode materials often do not meet people’s expectations of high energy output and high density for lithium-ion batteries. The layered oxide xLi₂MnO₃·(1−x)LiMO₂ (M=Ni, Mn, Co), rich in Li and Mn, is expected to be an ideal anodic material for the next generation of lithium-ion batteries owing to its high specific capacity exceeding 250 mA·h·g⁻¹. However, the Li-rich materials still suffer from high irreversible capacity loss at the first cycle, poor cycle performance, and inferior rate capacity. The voltage decay mechanism of lithium-rich manganese-based cathode materials involves factors such as surface phase transition, anion redox, transition metal migration, and oxygen release. As a commonly used modification method, the coating can effectively solve these problems. At present, the coating modification mechanism of cathode materials mainly includes the following three types. (1) Surface coatings can reduce the direct contact between lithium-rich materials and electrolytes. They stabilize the interface, prevent excessive metal dissolution, and effectively prevent the surface structure of the active material from collapsing. (2) Surface coatings can reduce oxygen activity, improve irreversible oxygen loss, inhibit solid electrolyte interphase (SEI) film growth, and improve material thermal stability. (3) Surface coatings can improve the conductivity of the positive electrode active material, which builds a conductive network on the surface to provide a fast transmission channel for electrons and lithium ions. Surface modification can optimize the surface and structure of the lithium-rich layered material, and the modified material shows a higher discharge capacity and good cycle stability, with superior rate performance and thermal stability. This paper expounded upon the lithium-rich cathode material structure and electrochemical reaction between the structure–activity relationship and discussed the influence of metal oxides, metal fluoride, carbon, conductive polymer, and lithium-ion conductors on the coating material, the electrochemical performance of lithium-ion battery cathode materials, and the mechanism of action. We also summarized the advantages and disadvantages of the abovementioned coating in the application of lithium-ion battery cathode materials. Finally, future developments in the coating modification of lithium-rich cathode materials for lithium-ion batteries were discussed.