Suppressing V/Na Anti‐Site Defects via Fluorine Doping Enables High‐Voltage and Stable V‐Based NASICON Cathodes for Sodium‐Ion Batteries

ABSTRACT The NASICON‐type Na 3 VAl(PO 4 ) 3 (NVAP) cathode has garnered extensive attention owing to its robust thermodynamic stability and high operating voltage enabled by multi‐electron redox reactions (V 3+ /V 4+ /V 5+ ). However, the voltage hysteresis observed in the high‐voltage region (∼4 V vs. Na + /Na), caused by derived anti‐site defects (DASDs) where V 5+ occupies Na1 vacancies (V/Na), results in sluggish Na + diffusion kinetics and poor electrochemical reversibility. To mitigate this issue, we introduced highly electronegative F doping into the NVAP structure. Density functional theory (DFT) calculations of ion migration energetics reveal that Na 2.88 VAl(PO 3.96 F 0.04 ) 3 (NVAP‐F) exhibits a higher formation energy for V/Na anti‐site defects and a higher migration barrier for V 5+ moving to the Na1 site (3.80 eV) compared to NVAP (3.15 eV). In situ XRD results further demonstrate that the F doping effectively suppresses the formation of V/Na anti‐site defects in NVAP‐F, thus alleviating voltage hysteresis during the V 4+ /V 5+ redox reaction and maintaining a low volume strain of 4.62%. Consequently, NVAP‐F delivers superior rate capability (54.4 mAh g −1 at 30 C) and exceptional long‐cycle stability (91.5% capacity retention after 5000 cycles). Comprehensive in‐/ex situ analyses confirm enhanced ion/electron diffusion kinetics in the NVAP‐F cathode. This work offers valuable insights for the development of high‐voltage and high‐rate vanadium‐based phosphate cathodes.