Electronic structure regulation of Na2FePO4F cathode toward superior high-rate and high-temperature sodium-ion batteries

Abstract

The layered iron-based fluorophosphate Na2FePO4F is a potential candidate cathode material with 2D transport channels for sodium ions. However, its cycling stability and rate capability are unsatisfying due to the inferior intrinsic electronic conductivity of Na2FePO4F. Herein, a small amount of vanadium is used to substitute Fe to form a carbon-coated composition, i.e. Na2Fe0.95V0.05PO4F@C (NFVPF). The carbon coating has more sp2 component formed via an in-situ catalytic carbonization of polyvinyl alcohol. Meanwhile, the intrinsic electronic conductivities of NFVPF and particularly its de-sodiated phases are enhanced owing to the substantially reduced band gaps according to the first principle calculations. In addition, a systematic study of electrode kinetics through cyclic voltammetry and electrochemical impedance spectroscopy reveals increased sodium ion diffusion coefficient and reduced charge transfer impedance. Benefiting from synergetic contributions of facilitated Na+ diffusion dynamics and improved electronic conductivities of the surface and bulk phases, the NFVPF electrode yields a high initial discharge capacity of 110.1 mAh g−1 at 0.1C, high-rate reversible capacity of 78.3 mAh g−1 at 10C, and long-term capacity retention of 83.8% after 600 cycles. Even at 50 °C, it still delivers a capacity retention of 87.6% after 150 cycles at 10C. Furthermore, the Na-storage mechanism of NFPF and NFVPF is determined through in-situ X-ray diffraction as two sequential two-phase reactions with Na1.5Fe1-xVxPO4F as the intermediate phase. Such a novel strategy of bulk-to-surface electronic structure regulation may provide new vision for other cathode materials suffering from poor electronic conductivity.