Electrostatically Shielded Transportation Enabling Accelerated Na⁺ Diffusivity in High-Performance Fluorophosphate Cathode for Sodium-Ion Batteries

A typical polyanionic based material Na₃V₂(PO₄)₂O₂F (Na₃VPO₂F) attracts much interest as a cathode for large-scale sodium-ion batteries in consideration of its stable structure and remarkable energy density. Nevertheless, the large coulombic attraction and repulsion suffered by the mobile Na⁺ from structural anions and surrounding Na⁺, respectively, result in a torpid reaction kinetics and inferior rate capability. Herein, Br⁻-doped and Na⁺ vacancy preinstalled Na_3−yVPO_2−xBr_xF is prepared to dilute the charges on and inside the Na⁺ transportation tunnel. In virtue of density functional theory analysis, Na_3−yVPO_2−xBr_xF reveals a reduction in the bandgap and an increase in electronic conductivity. Meanwhile, the almost electrostatically shielded tunnel in Na_3−yVPO_2−xBr_xF alleviates the coulombic hindrance imposed on Na⁺ during its (de)intercalation, which demonstrates a Na⁺ diffusivity about five times higher than that of Na₃VPO₂F. Consequently, the Na_3−yVPO_2−xBr_xF cathode shows a superior rate capacity of 77.7 mAh g⁻¹ under 50 C and great cycling property corresponding to a high capacity retention of 94.4% over 800 cycles at 10 C. The assembled Na_3−yVPO_2−xBr_xF//hard-carbon sodium-ion full-cell presents excellent specific energy/power (226 Wh kg⁻¹@15424.2 W kg⁻¹) as well as outstanding long-term cyclic stability over 1000 cycles at 5 C.