Tuning Bulk O2 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 O2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na0.67Mn0.5Fe0.5O2 through a synergy of Li2TiO3 coating and Li/Ti co-doping. O2- is oxidized to molecular O2 and peroxide (O-2)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 Li2TiO3 coating and Li/ Ti co-doping on the two O-redox modes is revealed. Firstly, Li2TiO3 coating restrains the surface O2 and inhibits O2 loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O2-→(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.