• Li, Z.; Kong, W.; Yu, Y.; Zhang, J.; Wong, D.; Xu, Z.; Chen, Z.; Schulz, C.; Bartkowiak, M.; Liu, X.: Tuning Bulk O2 and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2-Layered Cathodes. Angewandte Chemie - International Edition 61 (2022), p. e202115552/1-9

Open Access Version

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 (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 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.