Unraveling Anionic Redox for Sodium Layered Oxide Cathodes: Breakthroughs and Perspectives.

Sodium-ion batteries (SIBs) as the next generation of sustainable energy technologies have received widespread investigations for large-scale energy storage systems (EESs) and smart grids due to the huge natural abundance and low cost of sodium. Although the great efforts have been made in exploring layered transition metal oxide cathode for SIBs, their performances have reached the bottleneck for further practical application. Nowadays, anionic redox in layered transition metal oxides have emerged as a new paradigm to increase the energy density of rechargeable batteries. Based on this point, in this review, we attempt to systematically summarize the development history of anionic redox reaction and provide an in-depth discussion on the anionic redox mechanism. Particularly, the major challenges of anionic redox and the corresponding available strategies toward triggering and stabilizing anionic redox are proposed. Subsequently, several types of sodium layered oxide cathodes are classified and comparatively discussed according to Na-rich or Na-deficient materials. We also summarize a large amount of progressive characterization techniques of anionic oxygen redox. Finally, an overview of the existing prospective and the future development directions of sodium layered transition oxide with anionic redox reaction are analyzed and suggested. This article is protected by copyright. All rights reserved.