Sodium-Rich Fluorine-Doped Na_3.475Fe_2.4(PO₄)_1.4(P₂O₇)F_0.075 Cathode for High-Rate Performance in Sodium-Ion Batteries

Pure-phase iron-based phosphate Na_3.4Fe_2.4(PO₄)_1.4P₂O₇ (NFPP) is anticipated to emerge as a competitive candidate material for sodium-ion batteries (SIBs). Nevertheless, the low electronic conductivity and sluggish sodium ion diffusion kinetics during sodium storage present significant challenges to its electrochemical performance. Consequently, a sodium-rich fluorine-doping strategy has been proposed, and we elucidate the mechanism through which F doping influences the crystal structure and electronic conductivity of NFPP. Both experimental and theoretical calculations demonstrate that F doping expands the diffusion channels for Na⁺, reduces the band gap and Na⁺ migration energy barrier, and enhances the intrinsic electronic conductivity of NFPP. Owing to the enhanced charge transport capability, the electrochemical performance of Na_3.475Fe_2.4(PO₄)_1.4(P₂O₇)F_0.075 (NFPPF-0.075) significantly surpasses that of the undoped sample. NFPPF-0.075 demonstrates a discharge specific capacity of 113.7 mAh g^-1 at 0.1 C; even at a current density of 30 C, the discharge specific capacity is sustained at 84.1 mAh g^-1. NFPPF-0.075 also exhibits remarkable cycle stability, achieving a capacity retention of 88.7% over 2000 cycles at 10 C. Furthermore, the NFPPF-0.075||HC full cell demonstrates remarkable rate performance and cycle performance. Therefore, Na_3.475Fe_2.4(PO₄)_1.4(P₂O₇)F_0.075 has the potential to serve as a highly promising cathode material for large-scale applications in SIBs.