Off-stoichiometric Na3V2-x(PO4)(3)/C cathode composites with stable lifetime for sodium ion batteries

Off-stoichiometric Na3V2-x(PO4)3/C (NV2-xP/C) cathode composites with decreased V content are synthesized by a solid-state reaction method. X-ray diffraction patterns demonstrate the presence of Na3PO4 and NaVP2O7. The synergic effects between Na3PO4 (Na ion conductor) and NaVP2O7 significantly facilitate the diffusion of Na+ and improve its kinetic behavior. Comparing with stoichiometric Na3V2(PO4)(3)/C (NVP/C) sample, Na3V1.98(PO4)(3)/C (NV1.98P/C) composite produces a higher initial discharge capacity (116.6 mA h g(-1)) at 10 mA g(-1), which is very close to the theoretical capacity(117.6 mA h g(-1)). NV1.98P/C composite shows a capacity retention as high as 98% after 1000 cycles at 200 mA g(-1), and a DNa+ of 7.48 x 10(-14) cm(2) s(-1), which is one order of magnitude higher than that in stoichiometric NVP/C sample (2.00 x 10(-15) cm(2) s(-1)). The activity of V4+/V5+ redox couple at 4.0 V is also discovered in NV1.98P/C composite, indicating the feasibility of de-intercalation of the third Na+ in Na1 site. The off-stoichiometric design can enhance the electrochemical performance and induce the extra emerging V4+/V5+ redox couple. Hence, decreasing the V content is a cost-effective method to improve the electrochemical properties of Na3V2(PO4)(3) material system.