Rational modulation of fluorophosphate cathode by anionic groups to reduce the polarization behavior for fast-charging sodium-ion batteries

Na₃V₂(PO₄)₂O₂F (VP) is recognized as a promising cathode material for sodium-ion batteries due to its stable structural framework and high specific capacity. Density functional theory (DFT) and finite element simulations show that incorporating SO₄²⁻ into VP decreases its band gap, lowers the migration energy barrier, and ensures a uniform Na⁺ concentration gradient and stress distribution during charge and discharge cycles. Consequently, the average Na⁺ diffusion coefficient of Na₃V₂(PO₄)_1.95(SO₄)_0.05O₂F (VPS-1) is roughly double that of VP, leading to enhanced rate capability (80 C, 75.5 mAh g⁻¹) and cycling stability (111.0 mAh g⁻¹ capacity after 1000 cycles at 10 C current density) for VPS-1. VPS-1 exhibits outstanding fast-charging capabilities, achieving an 80% state of charge in just 8.1 min. The assembled VPS-1//SbSn/NPC full cell demonstrated stable cycling over 200 cycles at a high 5 C current, maintaining an average coulombic efficiency of 95.35%.