Stabilizing Transition Metal Vacancy Induced Oxygen Redox by Co2+/Co3+ Redox and Sodium‐Site Doping for Layered Cathode Materials

Abstract Anionic redox is an effective way to boost the energy density of layer‐structured metal‐oxide cathodes for rechargeable batteries. However, inherent rigid nature of the TMO 6 (TM: transition metals) subunits in the layered materials makes it hardly tolerate the inner strains induced by lattice glide, especially at high voltage. Herein, P2‐Na 0.8 Mg 0.13 [Mn 0.6 Co 0.2 Mg 0.07 □ 0.13 ]O 2 (□: TM vacancy) is designed that contains vacancies in TM sites, and Mg ions in both TM and sodium sites. Vacancies make the rigid TMO 6 octahedron become more asymmetric and flexible. Low valence Co 2+ /Co 3+ redox couple stabilizes the electronic structure, especially at the charged state. Mg 2+ in sodium sites can tune the interlayer spacing against O‐O electrostatic repulsion. Time‐resolved in situ X‐ray diffraction confirms that irreversible structure evolution is effectively suppressed during deep desodiation benefiting from the specific configuration. X‐ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations demonstrate that, deriving from the intrinsic vacancies, multiple local configurations of “□‐O‐□”, “Na‐O‐□”, “Mg‐O‐□” are superior in facilitating the oxygen redox for charge compensation than previously reported “Na‐O‐Mg”. The resulted material delivers promising cycle stability and rate capability, with a long voltage plateau at 4.2 V contributed by oxygen, and can be well maintained even at high rates. The strategy will inspire new ideas in designing highly stable cathode materials with reversible anionic redox for sodium‐ion batteries.