Tuning the Electronic Structure of Niobium Oxyphosphate/Reduced Graphene Oxide Composites by Vanadium-Doping for High-Performance Na+ Storage Application

Abstract

Sodium-ion batteries have become a significant research focus in academia. As a novel sodium anode material, layered NbOPO₄, consisting of octahedral NbO₆ units sharing oxygen atoms with tetrahedral PO₄ units, exhibits stability due to strong phosphorus-oxygen covalent bonds that prevent oxygen loss from the framework. However, its inherently low electrical conductivity and sluggish charge transfer kinetics limit its electrochemical performance. To address these challenges, we designed and synthesized vanadium-doped niobium oxyphosphate coated with reduced graphene oxide (V-NbOPO₄@rGO) via a microwave hydrothermal method followed by calcination. Vanadium doping effectively modulated the electronic structure of NbOPO₄ and significantly enhanced its conductivity, as corroborated by density functional theory (DFT) calculations. Consequently, the V_0.15-NbOPO₄@rGO electrode demonstrated exceptional rate capability, achieving 418 mAh g⁻¹ at a low current density of 0.1 A g⁻¹ and maintaining a reversible capacity exceeding 100 mAh g⁻¹ even at an ultrahigh current density of 50 A g⁻¹. Furthermore, the reversible sodium storage mechanism of V_0.15-NbOPO₄@rGO was validated through in-situ XRD, TEM, and XPS analyses. This study provides an effective strategy for improving the electrochemical performance of NbOPO₄based anodes and deepens understanding of the sodium storage mechanism in V-doped NbOPO₄, emphasizing its potential for practical application in sodium-ion batteries.