Tuning Bulk O 2 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 2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na 0.67 Mn 0.5 Fe 0.5 O 2 through a synergy of Li 2 TiO 3 coating and Li/Ti co-doping. O 2- is oxidized to molecular O 2 and peroxide (O 2 ) n- (n<2) during charging. Molecular O 2 derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li 2 TiO 3 coating and Li/Ti co-doping on the two O-redox modes is revealed. Firstly, Li 2 TiO 3 coating restrains the surface O 2 and inhibits O 2 loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O 2- →(O 2 ) 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.