High-performance cathode materials are highly desirable for the realization of commercial Na-ion batteries. Sodium super ion conductor (NASICON)-type Na 3 V 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 3 cathode have been demonstrated by the partial substitution of Si for P. Nano-sized powders of Na 3+x V 2 (PO 4 ) 3−x (SiO 4 ) 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 Na 3 V 2 (PO 4 ) 3 is shown to retain the trivalent state after the Si substitution. Coin cells are fabricated using an organic electrolyte, carbon nanotubes decorated Na 3+x V 2 (PO 4 ) 3−x (SiO 4 ) 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 Na 3.1 V 2 (PO 4 ) 2.9 (SiO 4 ) 0.1 composition exhibits ∼21% higher specific capacity than that with Na 3 V 2 (PO 4 ) 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 Na 3.1 V 2 (PO 4 ) 2.9 (SiO 4 ) 0.1 /carbon nanotubes show that the strategy of expanding the crystal lattice through appropriate doping is an effective way to develop Na 3 V 2 (PO 4 ) 3 cathode for Na-ion batteries. KEYWORDS: sodium-ion battery, cathodes, NASICON, doping, Na 3 V 2 (PO 4 ) 3

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