Tuning Bulk O and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2-Layered Cathodes.

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 O and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na Mn Fe O through a synergy of Li TiO coating and Li/Ti co-doping. O is oxidized to molecular O and peroxide (O ) (n<2) during charging. Molecular O derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li TiO coating and Li/Ti co-doping on the two O-redox modes is revealed. Firstly, Li TiO coating restrains the surface O and inhibits O loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O →(O ) . 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.