Stabilizing Transition Metal Vacancy Induced Oxygen Redox by Co²⁺ /Co³⁺ Redox and Sodium-Site Doping for Layered...

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 TMO₆ (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 ]O₂ (□: TM vacancy) is designed that contains vacancies in TM sites, and Mg ions in both TM and sodium sites. Vacancies make the rigid TMO₆ octahedron become more asymmetric and flexible. Low valence Co²⁺ /Co³⁺ redox couple stabilizes the electronic structure, especially at the charged state. Mg²⁺ 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.