Preventing structural degradation from Na₃V₂(PO₄)₃ to V₂(PO₄)₃: F-doped Na₃V₂(PO₄)₃/C cathode composite with stable lifetime for sodium ion batteries

A prospective NASICON-type F-doped Na₃V₂(PO₄)_2.93F_0.07/C (F-0.07-NVP/C) composite is synthesized by a solid-state reaction method. F-doping can restrain the structural degradation from Na₃V₂(PO₄)₃ to V₂(PO₄)₃ and enhance the structural stability. Meanwhile, it can decrease the particle size to diminish the pathway of Na⁺ diffusion, which can increase ionic conductivity efficiently. The kinetic behavior is significantly improved and it is beneficial to reinforcing the electrochemical performance of F-doping composites. Compared with Undoped-NVP/C sample, F-0.07-NVP/C composite delivers a 113 mAh g⁻¹ discharge capacity at 10 mA g⁻¹, which is very close to the theoretical capacity (117 mAh g⁻¹). As for cycle performance, a reversible capacity of 97.8 mAh g⁻¹ can be obtained and it retains 86% capacity after 1000 cycles at 200 mA g⁻¹. F-0.07-NVP/C composite presents the highest D_Na⁺ (2.62 × 10⁻¹⁵ cm²s⁻¹), two orders of magnitude higher than the undoped sample (4.8 × 10⁻¹⁷ cm²s⁻¹). This outstanding electrochemical performance is ascribed to the synergetic effect from improved kinetic behavior and enhanced structural stability due to F-doping. Hence, the F-doped composite would be a promising cathode material in SIB for energy storage and conversion.