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

A typical polyanionic based material Na3V2(PO4)(2)O2F (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 Na3-yVPO2-xBrxF is prepared to dilute the charges on and inside the Na+ transportation tunnel. In virtue of density functional theory analysis, Na3-yVPO2-xBrxF reveals a reduction in the bandgap and an increase in electronic conductivity. Meanwhile, the almost electrostatically shielded tunnel in Na3-yVPO2-xBrxF 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 Na3-yVPO2-xBrxF 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 Na3-yVPO2-xBrxF//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.