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-Na0.67 Mn0.5 Fe0.5 O₂ through a synergy of Li₂ TiO₃ coating and Li/Ti co-doping. O²⁻ 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 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.
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Li et al. (2022) studied this question.
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