Unlocking Energy Potential: Exploiting Anionic Redox Activity in Na-Based Layered Oxides.

Sodium-ion batteries (SIBs) are progressively recognized as a viable alternative to lithium-ion batteries due to their operational and economic viability. However, their practical application is limited by lower energy density and intermediate cycling performance, predominantly limited by the cathodes. The exploration of anionic redox in layered oxides has introduced a new approach to enhance the energy density of rechargeable Na-ion batteries. By exploiting anionic redox activity, various positive electrodes are capable of providing additional capacity beyond their theoretical capacity. This additional capacity emerges from the combined contributions of anionic and cationic redox processes. However, the anionic capacity achieved during charge is often only partially reversible upon discharge, posing a significant challenge for practical use. This review offers a comprehensive analysis of anionic redox phenomena in sodium-based layered oxides, summarizing recent research strategies aimed at improving the anionic and cationic redox performance of these cathode insertion materials. It also elucidates the relationship between structure, function, and electrochemical performance. Eventually, it provides insights into the future research directions for Na-based layered oxide cathodes for energy storage applications.