Progress of anion redox in Na-rich layered transition metal oxides (NaMO) as cathode materials for sodium-ion batteries.

Under the background of surging global demand for batteries and scarcity of Li resources, sodium-ion batteries (SIBs) are attracting attention as a potential alternative with their unique advantages, and the layered transition metal (TM) oxides therein are considered to be one of the most promising cathode materials. In this paper, firstly, the diversity of cathode materials for SIBs is systematically introduced, as well as the layered oxide structures among them are categorized, and then it focuses on the O3-type sodium-rich NaMO, which is promising for large-scale commercial applications, illustrating the development and mechanism of anion redox. Excess Na transforms the TM layer into the mixed NaMOlayer, leading to the formation of localized configuration Na-O-Na. Thereby, isolated nonhybridized O 2p states are introduced, which participate in the charge compensation process (O/O) under high-voltage conditions and provide the battery with additional capacity beyond the cation redox reaction. Therefore, the NaMOformed by its TM element located in different periods are classified, discussed and summarized in terms of structural change characteristics, electrochemical properties and anion-redox mechanism. However, this particular redox mechanism is also accompanied by the challenges such as voltage hysteresis, irreversible oxygen loss, TM migration, capacity decay and poor air stability. Therefore, to address these challenges, various improvement strategies have been proposed, including doping of large radius metal ions, light metal ions, TM ions with high covalency with O, nonmetal ions, formation of mixed phases, and surface modification. This work is expected to provide new ways to find and design novel high-capacity Na-rich layered oxide cathode materials.