Progress of Anion Redox in Na-rich Layered Transition Metal Oxides (Na2MO3) as Cathode Materials for Sodium-ion Batteries.

Abstract

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 oxides therein are considered to be one of the most promising cathode materials. In this paper, firstly, the diversity of cathode materials for sodium-ion batteries is systematically introduced, as well as the layered oxide structures among them are categorized, and then it focuses on the O3-type sodium-rich Na₂MO₃, which is promising for large-scale commercial applications, illustrating the development and mechanism of anion redox. Excess Na transforms the transition metal layer into the mixed Na_1/3M_2/3O₂layer, 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^2-/O^n-) under high-voltage conditions and provide the battery with additional capacity beyond the cation redox reaction. Therefore, the Na2MO3 formed by its transition metal 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, transition metal 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. .