Enhanced Na⁺ diffusion in Na₃V₂(PO₄)₂F₂O cathodes via Zr⁴⁺ doping for high-rate and long-cycling sodium batteries

Sodium vanadium oxyfluorophosphate is a class of high-stability, high-capacity and high-voltage cathode materials for sodium ion batteries. But its practical performance is severely constrained by the low electric conductivity. In this paper, high-valence Zr⁴⁺ is designed and applied to improve the electrochemical performance of Na₃V₂(PO₄)₂F₂O (NVPFO). A series of Na₃V_2−xZr_x(PO₄)₂F₂O (0 ≤x < 1) are prepared by a facile solid-state method. X-ray photoelectron spectroscopy and energy dispersive spectroscopy mapping demonstrate Zr⁴⁺ is successfully doped into NVPFO. X-ray diffraction with Rietveld refinement indicates the lattice parameters and cell volume are increased due to the large ionic radius of Zr⁴⁺. Scanning electron microscopy and transmission electron microscopy images verify the particle size is slightly decreased and more uniformly distributed after Zr⁴⁺ doping, and the carbon coating layer is also well reserved. The optimal doping is found 4 at% by generating Na₃V_1.96Zr_0.04(PO₄)₂F₂O, which shows a specific capacity of 128, 106, 85 and 55 mA h g⁻¹ at 0.1, 1, 8 and 60 C, respectively. After 350 cycles at 0.5 C, the specific capacity is gradually reduced from 115 to 87 mA h g⁻¹ corresponding to a capacity retention of 75.7%. Its Na⁺ diffusion coefficient is calculated more than one magnitude higher than un-doped NVPFO.