F-doping and V-defect synergetic effects on Na3V2(PO4)3/C composite: A promising cathode with high ionic conductivity for sodium ion batteries

A facile and simple solid-state reaction route is developed to synthesize F-doping and V-defect Na3V1.98(PO4)3-xF3x/C composites. F-doping is facilitated to decrease the particle size to diminish the pathway of Na+ diffusion. Beneficial by-products are generated due to F-doping and V-defect, Na3PO4 is a typical Na ion conductor and NaVO2 has the layer structure for rapid migration of Na+. The synergetic effect of F-doping and V-defect on the Na+ diffusion is significant. The kinetic behavior is dramatically enhanced and it is beneficial to reinforcing the electrochemical performance. Meanwhile, the extraction of the third Na+ at Na1 site is observed at 4.0 V corresponding to the V4+/V5+ redox couple. The optimized Na3V1.98(PO4)2.9F0.3/C composite delivers an initial charge capacity as high as 143.5 mAh g−1 and an 116.9 mAh g−1 discharge capacity at 0.1C. A reversible capacity of 100.6 mAh g−1 is obtained and it retains 89.3% capacity after 100 cycles at 1C. It presents the highest DNa+ (3.66 × 10−13 cm2s−1), close to three orders of magnitude higher than the Na3V2(PO4)3/C (7.41 × 10−16 cm2s−1). Moreover, Ex situ XRD results demonstrate that broadened channels along x and y directions are favorable to improving DNa+ and it exhibits the highest DNa+ when charging to 3.4 V.