Abstract
The capacity of Li‐ion cathode materials has recently been greatly improved by the feasibility to trigger both cationic and anionic redox reactions within the same material. This concept has rapidly been implemented to Na‐ion batteries to boost their energy density. The electrochemical properties of Na3RuO4 with Li3RuO4 are reported and compared herein. Strikingly, it is found that 3 Na can be extracted from Na3RuO4 with the charge compensation mechanism enlisting first the oxidation of Ru5+ to Ru6+, leading to Na2RuO4, and then the oxidation of oxygen during the rest of the charge. This drastically contrasts with the behavior of the Li counterpart since Ru never reaches that high oxidation state during lithium removal. By comparing the phase diagrams of AxRuO4 (A being Li, Na, or K) together with density functional theory calculations, this finding is rationalized and it is demonstrated that this difference is mainly rooted in the size of the alkali cation. The bigger the alkali, the lower the coordination of Ru will be, stabilized by the same higher oxidation states. This work highlights the difference between Li and Na materials toward anionic redox and suggests the unusual coexistence of Ru6+ and (O2)n−, hence setting new challenges to theoreticians and opening opportunities for materials design.