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

Abstract

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 Na4-xFeV1-xZrx (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, Na3.9FeV0.9Zr0.1(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 (Fe2+/3+ and V3+/4+) occur during the electrochemical process.