Sodium-Rich Fluorine-Doped NaFe(PO)(PO)F Cathode for High-Rate Performance in Sodium-Ion Batteries.

Pure-phase iron-based phosphate NaFe(PO)PO (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 NaFe(PO)(PO)F (NFPPF-0.075) significantly surpasses that of the undoped sample. NFPPF-0.075 demonstrates a discharge specific capacity of 113.7 mAh g at 0.1 C; even at a current density of 30 C, the discharge specific capacity is sustained at 84.1 mAh g. 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, NaFe(PO)(PO)F has the potential to serve as a highly promising cathode material for large-scale applications in SIBs.