Research progress of oxygen redox in sodium-layered oxides

Sodium-ion batteries are emerging as promising alternative energy sources compared to lithium-ion batteries, due to the abundant sodium resources in Earth's crust and their low cost. Nevertheless, the larger ionic radius of sodium ions leads to minor energy density in sodium-layered oxide cathodes. To address this, anionic redox has attracted significant attention as it provides additional capacity beyond cationic redox. In this comprehensive review, the history and fundamental mechanisms of anionic redox are systematically summarized, and the recent advancements in sodium-layered oxides with anionic redox is categorized and discussed according to deficient sodium-layered oxides, stoichiometric sodium-layered oxides, and sodium-rich layered oxides. Finally, several prospects and challenges for anionic redox-layered oxide cathodes have also been proposed. This review sheds light on the potential trajectory of sodium-ion battery technology and highlights the pathways to harness the full capabilities of anionic redox for energy storage applications. Recent advancements in sodium-layered oxides with anionic redox are reviewed and discussed according to deficient sodium-layered oxides, stoichiometric sodium-layered oxides, and sodium-rich layered oxides. In addition, the fundamental mechanisms of anionic redox are systematically summarized. Finally, prospects and challenges for anionic redox-layered oxide cathodes are also proposed.image