Synthesis and Electrochemical Activity of Some Na(Li)‐Rich Ruthenium Oxides with the Feasibility to Stabilize Ru6+

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 A_xRuO₄ (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.