Recent progress in coating modification of layered lithium manganese-rich cathode materials

Developing lithium-ion batteries (LIBs) with higher energy densities has been necessary in recent years to satisfy the increasing requirements of the energy storage and conversion fields. The limited performance of LIBs stems from their low available capacities and cycling stability. As one of the most important components of LIBs, the cathode material plays an essential role in determining the energy density and cycling stability of LIBs. Among the cathode materials, lithium-rich manganese-based materials are considered to be promising cathode materials for the next generation. This is due to their high specific capacity (theoretical capacity > 250 mA·h·g−1 for 1 Li+ extraction and approximately 378 mA·h·g−1 for 1.2 Li+ extractions), high energy density (approximately 900 W·h·kg−1 vs Li metal), and low cost. Despite these advantages, one major weakness of xLi2MnO3·(1–x) LiMO2 is its intrinsically poor rate capability. This has been recently verified to be associated with the retardation of mass transport of the rearranged surface after activating Li2MnO3 at >4.5 V charge. This rearrangement causes a large capacity loss. In addition, these materials exhibit fragile surface properties at high potentials, erosion from the electrolytes, and dissolution of transition metal ions. In addition, the high working voltage not only ensures high energy density but also triggers a side reaction between the electrode material and the electrolyte. This leads to problems such as transition metal dissolution, surface cracks, and laminated structure collapse during the cycling process, limiting the material’s commercial application. Surface coating can effectively alleviate the side reactions between the electrode and electrolyte, suppress the dissolution of transition metals, and thus improve the material’s coulombic efficiency and cycling stability during cycling. Until now, there have been many reports on the surface coating modification of Li-rich manganese-based materials; however, there are few review reports in this field. This paper summarizes the structure and characteristics of Li-rich manganese-based materials and their problems during the cycle process. Furthermore, it introduces the latest progress in the coating modification of Li-rich manganese-based cathode materials and systematically describes the application of different coating materials, such as electrochemically active materials, non-electrochemically active materials, and conductive polymers. Moreover, this study addresses the coating methods of single-layer coating, double-layer composite coating, and in situ coating and further analyzes the failure and enhancement mechanisms of cycling stability of the modification methods and coatings. Finally, a future development direction for Li1.2Mn0.54Ni0.13Co0.13O2 (LMNC) materials and modification methods is envisioned. This result provides a reference for the practical application of Li-rich cathode materials.