Uncovering the Structural Evolution in Na-Excess Layered Cathodes for Rational Use of an Anionic Redox Reaction

A new paradigm based on an anionic O2–/O n– redox reaction has been highlighted in high-energy density cathode materials for sodium-ion batteries, achieving a high voltage (∼4.2 V vs Na/Na+) with a large anionic capacity during the first charge process. The structural variations during (de)­intercalation are closely correlated with stable cyclability. To determine the rational range of the anion-based redox reaction, the structural origins of Na1–x Ru0.5O1.5 (0 ≤ x ≤ 1.0) were deduced from its vacancy (□)/Na atomic configurations, which trigger different interactions between the cations and anions. In the cation-based Ru4+/Ru5+ redox reaction, the □ solubility into fully sodiated Na2RuO3 predominantly depends on the crystallographic 4h site when 0.0 ≤ x ≤ 0.25, and the electrostatic repulsion of the linear O2––□–O2– configuration is accompanied by the increased volumetric strain. Further Na extraction (0.25 ≤ x ≤ 0.5) induces a compensation effect, leading to Na2/3[Na□Ru2/3]­O2 with the □ formation of 2b and 2c sites, which drastically reduce the volumetric strain. In the O2–/O n– anionic redox region (0.5 ≤ x ≤ 0.75), Na removal at the 4h site generates a repulsive force in O2––□–O2– that increases the interlayer distance. Finally, in the 0.75 ≤ x ≤ 1.0 region, the anionic O charges are unprotected by repulsive forces, and their consumption causes severe volumetric strain in Na1–x Ru0.5O1.5. Coupling our mechanistic understanding of the structural origin with the □- and Na-site preferences and the electrostatic interaction between lattice O and vacancies in Na1–x Ru0.5O1.5, we determined the rational range of the anionic redox reaction in layered cathode materials for rechargeable battery research.