Off-stoichiometric Na₃V_2-x(PO₄)₃/C cathode composites with stable lifetime for sodium ion batteries

Off-stoichiometric Na₃V_2-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, Na₃V_1.98(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 D_Na ⁺ of 7.48 × 10^–14 cm² s⁻¹, which is one order of magnitude higher than that in stoichiometric NVP/C sample (2.00 × 10^–15 cm² 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.