Improving\nthe Electrochemical Performance of Na₃V₂(PO₄)₃ Cathode in Na-Ion\nBatteries by Si-Doping

Abstract

High-performance\ncathode materials are highly desirable for the\nrealization of commercial Na-ion batteries. Sodium super ion conductor\n(NASICON)-type Na₃V₂(PO₄)₃ is one of the most promising cathode materials. In the present study,\nan improvement in the kinetics of the redox reactions and consequently,\nthe electrochemical performance of Na₃V₂(PO₄)₃ cathode have been demonstrated by the partial\nsubstitution of Si for P. Nano-sized powders of Na_3+<i>x</i>V₂(PO₄)_3–<i>x</i>(SiO₄)_<i>x</i> (<i>x</i> = 0, 0.1, 0.2 and 0.4) are synthesized using the citric acid-assisted\nsol-gel route. A thin carbon layer of ∼8–16 nm thickness\nis formed on the calcined particles of the active material. The vanadium\npresent in Na₃V₂(PO₄)₃ is shown to retain the trivalent state after the Si substitution.\nCoin cells are fabricated using an organic electrolyte, carbon nanotubes\ndecorated Na_3+<i>x</i>V₂(PO₄)_3–<i>x</i>(SiO₄)_<i>x</i> as a cathode, and Na as an anode. Cyclic voltammetry\nprofiles acquired on the half-cells confirmed (1) improved reversibility\nof the redox reactions, (2) lower polarization losses, and (3) superior\nsodium-ion diffusivity in the Si-substituted compounds. As a result,\ncells with these electrodes exhibit an enhanced electrochemical performance\nwith a high specific capacity, excellent rate capability, and stable\ncycling performance. The electroactive material with Na_3.1V₂(PO₄)_2.9(SiO₄)_0.1 composition exhibits ∼21% higher specific capacity\nthan that with Na₃V₂(PO₄)₃ at 5C charging/discharging rates, furnishing a Coulombic efficiency\nof >99%. The expansion of the crystal volume and the presence of\nexcess\nNa-ion for ionic conduction are primarily responsible for the observed\nsuperior performance. The exceptional capacity of 85 mA h/g at 1C\ncharging/discharging rates and superior cycling life achieved in Na_3.1V₂(PO₄)_2.9(SiO₄)_0.1/carbon nanotubes show that the strategy of expanding\nthe crystal lattice through appropriate doping is an effective way\nto develop Na₃V₂(PO₄)₃ cathode for Na-ion batteries.