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-x)F-3x/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)(2.9)F-0.3/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 D-Na(+) (3.66 x 10(-13) cm(2)s(-1)), close to three orders of magnitude higher than the Na3V2(PO4)(3)/C (7.41 x 10(-16) cm(2)s(-1)). Moreover, Ex situ XRD results demonstrate that broadened channels along x and y directions are favorable to improving D-Na(+) and it exhibits the highest D-Na(+) when charging to 3.4 V.