Unraveling the functioning mechanism of fluorine-doping in Mn-based layered oxide cathodes toward enhanced sodium-ion storage performance

Abstract

Manganese-based layered oxides with anionic redox activity are considered as one of the most promising cathode candidates for sodium-ion batteries (SIBs) owing to their abundant resources and high theoretical specific capacities. However, the severe Jahn-Teller (J-T) effect of Mn3+ and irreversible lattice oxygen loss result in rapid structural degradation and electrochemical performance deterioration. Herein, the functioning mechanism of F-doping in regulating the local and electronic structures of Mn-based layered oxides is unraveled. The introduction of the more electronegative F ions on one hand breaks the electronic symmetry of the MnO6 octahedra and effectively alleviates the J-T distortion, on the other hand suppresses the Zn ions migration through the strong Zn-F bonds and stabilizes the oxygen redox chemistry and facilitates the Na+ diffusion. The above reaction mechanisms are systematically validated by in-situ/ex-situ analyses and theoretical computations. As a result, the optimum P2-Na0.75Zn0.28Mn0.72O1.93F0.07 cathode demonstrates significantly improved rate capability (178.6 mAh g−1 at 0.1 C with 64.4 mAh g−1 at 10 C) and enhanced cycling durability (83.1 % capacity retention over 400 cycles at 3 C) compared to the un-doped P2-Na0.75Zn0.28Mn0.72O2 material. This study clarifies the F-doping mechanism in layered oxides and provides new perspectives for designing high-energy and high-stability cathodes for SIBs.