A comprehensive understanding of the anionic redox chemistry in layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries (SIBs) have demonstrated great application prospects in large-scale energy storage systems and low-speed electric vehicles due to the cost effectiveness and abundant resources. Layered transition-metal oxides are recognized as one of the most attractive sodium-ion storage cathode candidates by virtue of their high compositional diversity, environmental friendliness, ease of synthesis, and promising theoretical capacities. The practicability, however, is still limited by the fact that the energy densities of most Na-storage layered oxide cathodes solely using the conventional cationic redox are not comparable to those of the lithium-ion storage counterparts. Recently, the strategy of activating anionic redox (O2−/On−) which is popular in Li-rich layered materials has been successfully applied in oxide cathodes of SIBs to promote the energy density to a new level. It is interesting to note that excess Na is not the prerequisite to induce anionic redox in sodium oxides, indicating a new mechanism underlying Na-ion materials. Herein, the latest advances on the anionic redox chemistry in layered oxide cathodes for SIBs, including the fundamental theories, triggering strategies, and applicable cathode materials, are comprehensively reviewed. Moreover, the challenges (mainly O2 release) facing anionic redox are discussed, and the possible remedies are outlined for future developments toward a highly reversible oxygen usage. We believe that this review can provide a valuable guidance for the exploration of high-energy layered oxide cathode materials of SIBs.