Improving the Electrochemical Performance of Na3V2(PO4)3 Cathode in Na-Ion Batteries by Si-Doping

High-performance cathode materials are highly desirable for the realization of commercial Na-ion batteries. Sodium super ion conductor (NASICON)-type Na3V2(PO4)3 is one of the most promising cathode materials. In the present study, an improvement in the kinetics of the redox reactions and consequently, the electrochemical performance of Na3V2(PO4)3 cathode have been demonstrated by the partial substitution of Si for P. Nano-sized powders of Na3+xV2(PO4)3–x(SiO4)x (x = 0, 0.1, 0.2 and 0.4) are synthesized using the citric acid-assisted sol-gel route. A thin carbon layer of ∼8–16 nm thickness is formed on the calcined particles of the active material. The vanadium present in Na3V2(PO4)3 is shown to retain the trivalent state after the Si substitution. Coin cells are fabricated using an organic electrolyte, carbon nanotubes decorated Na3+xV2(PO4)3–x(SiO4)x as a cathode, and Na as an anode. Cyclic voltammetry profiles acquired on the half-cells confirmed (1) improved reversibility of the redox reactions, (2) lower polarization losses, and (3) superior sodium-ion diffusivity in the Si-substituted compounds. As a result, cells with these electrodes exhibit an enhanced electrochemical performance with a high specific capacity, excellent rate capability, and stable cycling performance. The electroactive material with Na3.1V2(PO4)2.9(SiO4)0.1 composition exhibits ∼21% higher specific capacity than that with Na3V2(PO4)3 at 5C charging/discharging rates, furnishing a Coulombic efficiency of >99%. The expansion of the crystal volume and the presence of excess Na-ion for ionic conduction are primarily responsible for the observed superior performance. The exceptional capacity of 85 mA h/g at 1C charging/discharging rates and superior cycling life achieved in Na3.1V2(PO4)2.9(SiO4)0.1/carbon nanotubes show that the strategy of expanding the crystal lattice through appropriate doping is an effective way to develop Na3V2(PO4)3 cathode for Na-ion batteries.