Tuning Bulk O₂ and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2‐Layered Cathodes

Abstract

Improving the reversibility of oxygen redox is quite significant for layered oxides cathodes in sodium-ion batteries. Herein, we for the first time simultaneously tune bulk O₂ and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na_0.67 Mn_0.5 Fe_0.5 O₂ through a synergy of Li₂ TiO₃ coating and Li/Ti co-doping. O^2- is oxidized to molecular O₂ and peroxide (O₂ )^n- (n<2) during charging. Molecular O₂ derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li₂ TiO₃ coating and Li/Ti co-doping on the two O-redox modes is revealed. Firstly, Li₂ TiO₃ coating restrains the surface O₂ and inhibits O₂ loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O^2- →(O₂ )^n- . Thirdly, Ti doping strengthens the TM-O bond which fixes lattice oxygen. The cationic redox reversibility is also enhanced by Li/Ti co-doping. The proposed insights into the oxygen redox reversibility are insightful for other oxide cathodes.