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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The catalytic oxidation of ethane using CO2 as a soft oxidant could facilitate the utilization of CO2 and ethane from the shale gas as a raw material to produce value-added ethylene via a dehydrogenation process. Pt and Ce species were supported on mesoporous zeolite containing surface framework defects, and the resulting supported catalysts were investigated for the oxidative dehydrogenation of ethane with CO2. Extended X-ray absorption fine structure and high-resolution transmission electron microscopy evidenced that Pt5Ce intermetallic nanoparticles with an average diameter of ∼2 nm and single atomic Ce species were presented in mesoporous zeolites after H2 reduction at 973 K. This supported catalyst was highly stable and selective for ethylene production compared to supported platinum and supported Pt/CeO2@SiO2 catalysts. Characterization of the fresh and spent catalysts with CO chemisorption, thermogravimetric analyses, temperature-programmed desorption of ethylene, and electron microscopy revealed that the supported Pt5Ce intermetallic catalysts exhibited a much lower affinity for ethylene than monometallic Pt, which diminishes the possibility of coke formation onto the active Pt surface due to the over-dehydrogenation reaction of ethylene. Instead, cokes were predominantly deposited on the zeolite support, which might be attributed to the olefinic polymerization by weakly acidic silanol groups at the external surface. In contrast, the monometallic Pt catalyst exhibited a high affinity for ethylene. The strongly adsorbed ethylene onto the Pt surface could be further converted into carbonaceous coke, which caused the rapid deactivation. Furthermore, density functional theory calculations revealed that single atomic Ce species closed to Pt5Ce intermetallic nanoparticles elevated the energy barrier of C–C bond rupture over C–H bond scission, which significantly suppresses the CO formation via the reforming pathway.

An intriguing redox chemistry via oxygen has emerged to achieve high-energy-density cathodes and has been intensively studied for practical use of anion-utilization oxides in A-ion batteries (A: Li or Na). However, in general, the oxygen redox disappears in the subsequent discharge with a large voltage hysteresis after the first charge process for A-excess layered oxides exhibiting anion redox. Unlike these hysteretic oxygen redox cathodes, the two Na-excess oxide models Na2IrO3 and Na2RuO3 unambiguously exhibit nonhysteretic oxygen capacities during the first cycle, with honeycomb-ordered superstructures. In this regard, the reaction mechanism in the two cathode models is elucidated to determine the origin of nonhysteretic oxygen capacities using first-principles calculations. First, the vacancy formation energies show that the thermodynamic instability in Na2IrO3 increases at a lower rate than that in Na2RuO3 upon charging. Second, considering that the strains of Ir–O and Ru–O bonding lengths are softened after the single-cation redox of Ru4+/Ru5+ and Ir4+/Ir5+, the contribution in the oxygen redox from x = 0.5 to 0.75 is larger in Na1–x Ru0.5O1.5 than that in Na1–x Ir0.5O1.5. Third, the charge variations indicate a dominant cation redox activity via Ir­(5d)–O­(2p) for x above 0.5 in Na1–x Ir0.5O1.5. Its redox participation occurred with the oxygen redox, opposite to the behavior in Na1–x Ru0.5O1.5. These three considerations imply that the chemical weakness of Ir­(5d)–O­(2p) leads to a more redox-active environment of Ir ions and reduces the oxygen redox activity, which triggers the nonhysteretic oxygen capacity during (de)­intercalation. This provides a comprehensive guideline for design of reversible oxygen redox capacities in oxide cathodes for advanced A-ion batteries.

Unlike in lithium-ion batteries (LIBs), in sodiumion batteries (SIBs), nonhysteretic oxygen redox (OR) reactions are observed in Li-excess Na-layered oxides. This necessitates an understanding of the reaction mechanism of an O3-type Li-excess Mn oxide, Na[Li 1/3 Mn 2/3 ]O 2 , a novel OR material designed for advanced SIBs. It could establish the role of Li in triggering nonhysteretic oxygen capacities during (de)sodiation. Three biphasic mechanisms were compared using first-principles calculations under the desodiation modes: (i) Na/vacancy ordering, (ii) Li migration in the NaO 2 layer, and (iii) in-plane Mn migration. The migrated Li ions generated a "physicochemical screen" effect upon electrochemical OR reactions in the oxide cathode. Thermodynamic formation energies showed different biphasic pathways upon charging in Na 1−x [Li 2/6 Mn 4/6 ]O 2 (NLMO) under the three modes. O−O bond population indicated that biphasic-reaction paths -i and -iii were derived from generating inter/ intralayer O−O dimers, and path-iii was triggered by the formation of a Mn−O 2 −Mn moiety. However, Li migration exhibited an ideal OR process (O 2− /O n− ) without forming anionic dimers. The electronic structures of Mn(3d) and O(2p) revealed that Li migration pushed lattice-based O(2p)-hole states to a high energy level, resulting in the chemical suppression of O 2 molecule formation. Selectively decoupled oxygen ordering indicated that the oxygen species coordinated with two Mn (O Mn2 ) derived from Li migration played an important role in nonhysteretic oxygen capacities during cycling. From these findings, we propose the "physicochemical screen" concept that physically suppresses interlayer O−O dimers and chemically hinders discretized O(2p)− O(2p) states formed by molecular O 2 . This could significantly impact the role of Li ions in Li-excess OR-layered oxides for SIBs.