Substitution and electrochemistry in layered oxide cathode materials for sodium-ion batteries

Substitution is a vital strategy for developing high-performance sodium layered oxides (SLOs), which demonstrates great potential for making sodium-ion batteries a viable alternative to lithium-ion batteries. Numerous studies have been conducted on substituted SLOs; however, each substitute exhibits varied effects on the structure and electrochemical performance of the SLOs, and no clear design principles have been established. Clarifying the relationship among substitution, structure and performance is therefore important to enable a rational design strategy for high-performance SLOs. In this Review, the up-to-date substitution guidelines and the current understanding of how substitution affects the structure and electrochemistry in SLOs are discussed, and the site preference and characteristic redox features of different types of substitutes are outlined. The inherent challenges and opportunities for the innovation of better-performing SLOs are summarized, paving the way for accelerating the commercialization of SLO-based sodium-ion batteries and the realization of their applications ranging from electric vehicles to grid energy storage systems.