Fundamental understanding and practical challenges of lithium-rich oxide cathode materials: Layered and disordered-rocksalt structure

Lithium-rich oxides have been considered as one of the most promising cathode materials for lithium-ion batteries due to the competitively high specific capacity contributed by both anionic and cationic redox reactions. Recent years have witnessed the successful achievements of lithium-rich oxides with layered structure due to their high capacity. More recently, another new class of lithium-rich oxides with disordered-rocksalt structure has been demonstrated to possess high capacity and high energy density with moderate structural changes during cycling. A comprehensive comparison and greater fundamental understanding of these two classes of lithium-rich oxides have not been achieved, however, which has greatly hindered the further application of these lithium-rich materials. Herein, an overview of the lithium-rich oxide cathode materials, including the conventional layered and the emerging disordered structure, is provided. Their structural characteristics, mechanisms of anionic redox reactions, and lithium diffusion pathways are systematically summarized. The key challenges to practical application and related strategies to enhance the electrochemical performance of the lithium-rich oxide cathodes are also discussed. At the end, the possible directions for the future development of the lithium-rich oxide cathodes are outlined.