Layered oxide cathodes for sodium-ion batteries: microstructure design, local chemistry and structural unit

Because of the low price and abundant reserves of sodium compared with lithium, the research of sodium-ion batteries (SIBs) in the field of large-scale energy storage has returned to the research spotlight. Layered oxides distinguish themselves from the mains cathode materials of SIBs owing to their advantages such as high specific capacity, simple synthesis route, and environmental benignity. However, the commercial development of the layered oxides is limited by sluggish kinetics, complex phase transition and poor air stability. Based on the research ideas from macro- to micro-scale, this review systematically summarizes the current optimization strategies of sodium-ion layered oxide cathodes (SLOC) from different dimensions: microstructure design, local chemistry regulation and structural unit construction. In the dimension of microstructure design, the various structures such as the microspheres, nanoplates, nanowires and exposed active facets are prepared to improve the slow kinetics and electrochemical performance. Besides, from the view of local chemistry regulation by chemical element substitution, the intrinsic electron/ion properties of SLOC have been enhanced to strengthen the structural stability. Furthermore, the optimization idea of endeavors to regulate the physical and chemical properties of cathode materials essentially is put forward from the dimension of structural unit construction. The opinions and strategies proposed in this review will provide some inspirations for the design of new SLOC in the future.