Structure evolution of layered transition metal oxide cathode materials for Na-ion batteries: Issues, mechanism and strategies

Due to the obvious benefits of abundant resources and low cost of sodium, sodium-ion batteries have enormous development potential and a broad market outlook in the field of large-scale energy storage and low-speed electric cars. Layered oxide cathode material is considered as one of the most promising cathode materials for sodium-ion batteries due to its high energy density, rich variety, simple synthesis method and low environmental pollution. However, the layered oxides of sodium-ion batteries suffer from instability in air storage and unstable electrochemical performance during cycling. This review summarizes surface and bulk structural evolution in air exposure and during cycling to reveal the relationship between structure evolution, charge transfer mechanisms and electrochemical performance, which is of great significance for designing advanced electrode materials. The strategies based on the degradation mechanisms for layered oxides cathode materials are also discussed for the future development and industrialized application. We hope that this review will provide a basis for an accurate understanding of the structural evolution of layered oxides and provide some inspiration for the design and development of electrochemical systems with excellent performance.