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)2.93F0.07/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 DNa+ (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.