Intercalation‐based cathodes typically rely on the cationic redox activity of transition metals to deliver capacity, but, recently, anionic redox chemistry has emerged as a way to increase the energy density of rechargeable batteries. However, the irreversible structural disorder and voltage fading accompanying oxygen release are major problems preventing commercial use. To overcome these limitations, the connection between structural stability and anionic redox activity must be understood. Here, we present a review of theoretical and experimental progress in anionic redox in sodium intercalation cathodes. First, the effects of structural factors including stacking sequences and cationic vacancies on the reversible capacity originating from anionic redox are discussed. Second, the effects on anionic redox activity of cationic substitution with alkaline earth metals (Li or Na) and the coordination environment are highlighted. Third, the progress and challenges facing materials based on 3d/4d/5d metals are reviewed. Finally, research directions for the development of anionic redox active materials are outlined.

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A new paradigm based on an anionic O 2− /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 Na 1−x Ru 0.5 O 1.5 (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 Ru 4+ /Ru 5+ redox reaction, the □ solubility into fully sodiated Na 2 RuO 3 predominantly depends on the crystallographic 4h site when 0.0 ≤ x ≤ 0.25, and the electrostatic repulsion of the linear O 2− −□− O 2− configuration is accompanied by the increased volumetric strain. Further Na extraction (0.25 ≤ x ≤ 0.5) induces a compensation effect, leading to Na 2/3 [Na □ Ru 2/3 ]O 2 with the □ formation of 2b and 2c sites, which drastically reduce the volumetric strain. In the O 2− /O n− anionic redox region (0.5 ≤ x ≤ 0.75), Na removal at the 4h site generates a repulsive force in O 2− −□−O 2− 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 Na 1−x Ru 0.5 O 1.5 . Coupling our mechanistic understanding of the structural origin with the □and Na-site preferences and the electrostatic interaction between lattice O and vacancies in Na 1−x Ru 0.5 O 1.5 , we determined the rational range of the anionic redox reaction in layered cathode materials for rechargeable battery research.

Unlike cathodes for lithium-ion batteries, oxygen redox (OR) processes at a high voltage (�4.2 V) during the first charge in sodium-ion batteries (SIBs) employ some Li-incorporated Mn oxides that is recovered during subsequent discharge. To determine the intrinsic origin, P2-type Na 0.6 [Li 0.2 Mn 0.8 ]O 2 exhibiting a reversible OR-induced two-phase reaction was investigated using experiments and first-principle calculations. First, operando X-ray diffraction results in reversible P2-Z phase transformations and thermodynamic analysis show the twophase reaction features Li migration into the tetrahedral sites from the transition-metal layer in the latter phase. Second, Liinduced decoupling of the oxygen 2p-electron led to selective anion redox activity depending on the oxygen sites that are Li-rich (redox-active) and Mn-rich (redox-inactive) environments. Third, redox-active oxygen coordinated to the Li vacancy predominantly participates in the formation of peroxo-like dimers with distortion of the MnO 6 octahedron, as observed in the reversible extended X-ray absorption fine structure spectra during the OR reaction. Considering three physicochemical perspectives, we reveal that Li ions play a role in activating OR reactions and control OR participation in the charge-compensation process. Our findings suggest that the Li/Mn ratio is a critical factor for achieving a reversible OR reaction, and broaden the possibilities of exploiting OR to reach high-energy densities in next-generation SIBs.

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