Enhanced Na+/electron transport of NASICON by multiple regulations for sodium ion battery with high rate capability and ultralong lifespan

Na ₃V ₂(PO ₄) ₂F _3–2yO _2y (0 ≤ y ≤ 1, NVPOF) as a polyanionic compounds cathode material of sodium ion batteries (SIBs), features the excellent intrinsic advantages of high average operating voltage, high reversible capacity and faint structural volume change. Whereas, the intrinsically low Na ⁺ /electronic conductivity of sodium vanadium fluorophosphate leads to the unreliable electrochemical performance of the battery, thus reducing the possibility of practical application. This paper puts forward the method of hydrothermal synthesis, and carries out a series of studies on modification methods such as Al doping and composite with carbonaceous materials. The high conductivity carbon coated sodium vanadium fluorophosphate particles with high purity and crystallinity are realized, and a double carbon network is formed with carbon nanotubes with large aspect ratio, which signally enhances the kinetic and electrochemical properties of the materials. Na ₃V _1.94Al _0.06(PO ₄) ₂F _1.06O _1.94 /C/CNTs with the optimum Al doping amount can obviously decrease the band gap energy and enhance the Na ⁺ /electronic dynamic transportation. At the same time, it shows the best electrochemical performance at room temperature, with an excellent specific discharge capacity of 127.4 mAh g ⁻¹ at 0.5 C, and the capacity retention rate can achieve 93% after long lifespan 700 cycles, showing unexpected cycle stability.