Modulating Na vacancies of Na₄FeV(PO₄)₃ via Zr-substitution: Toward a superior rate and ultrastable cathode for sodium-ion batteries

The Na Super Ionic Conductor (NASICON)-type Na₃V₂(PO₄)₃ has aroused enormous attentions as the cathode material for sodium-ion batteries. However, the scarcity of vanadium resources and toxicity of vanadium complexes limit its large-scale application. Herein, we use a reductive acid assisted sol-gel method to replace half of V³⁺ in Na₃V₂(PO₄)₃ with Fe²⁺ for synthesizing a NASICON-structure Na₄FeV(PO₄)₃/C, and substitute V³⁺ in Na₄FeV(PO₄)₃ by small amount of Zr⁴⁺ for fabricating a series of Na_4-xFeV_1-xZr_x (PO₄)₃/C (x = 0.1 and 0.2) materials. The substitution of V³⁺ by Fe²⁺ shows reduced cost, low toxicity, and increased Na⁺ concentration in per formula unit; while the substitution of V³⁺ by Zr⁴⁺ indicates expanded unit cell volume and abundant Na vacancies, which facilitate Na⁺ mobility and inherent electronic conductivity. Consequently, Na_3.9FeV_0.9Zr_0.1(PO₄)₃/C electrode exhibits high discharge capacity (114 mAh g⁻¹ at 0.1C), superior rate capability (66.7 mAh g⁻¹ at 40C), and remarkable cyclic stability (capacity retention of 82.4% after 4000 cycles at 20C). The kinetic analyses and ex-situ characterizations confirm that small volume change (5.21%) and highly reversible redox reaction (Fe^2+/3+ and V^3+/4+) occur during the electrochemical process.