Owing to the increasing demand for the electrochemical energy storage devices with sufficient capacity, high reliability and low cost, rechargeable sodium-ion batteries (SIBs) have recently regarded as alternative candidates to lithium-ion batteries (LIBs) because of their benefits on material abundance, cost-effectiveness and electrochemical similarities. However, the energy density delivered from the current SIBs is still not comparable with that of the LIBs, which was ascribed to the lower cell voltage. Therefore, developing the cathode materials with higher redox potentials is eagerly anticipated. As one of the most promising cathode materials, carbon-coated sodium vanadium fluorophosphates (Na₃V₂(PO₄)₂F₃/C) nanocomposites have been extensively investigated due to the merits of their sodium superionic conductor (NASICON) structures for facile ionic transportations, high voltage and high capacity. In this study, the Na₃V₂(PO₄)₂F₃/C nanoparticles were synthesized using a wet-chemistry route, followed by calcination in an argon atmosphere. The resulting powders were characterized by powder X-ray diffractometer, electron microscopes and thermogravimetric analyzer to confirm their crystalline structure, morphologies and amount of carbon loading, respectively. Benefiting from the nanostructures and continuous carbon coatings, the outstanding reversible capacity (128 mAh/g@0.1 A/g), rate capability (106 mAh/g@1.0 A/g) and capacity retention (98.2 %@100 cycles@1.0 A/g) were achieved as demonstrated by galvanostatic testing. Given the improved electrochemical properties, the NASICON-type Na₃V₂(PO₄)₂F₃/C nanoparticles synthesized in the present study were anticipated to function as efficient cathode materials for SIBs.

Acknowledgement: We gratefully acknowledge financial support from the Bureau of Energy (BOE), Ministry of Economy Affair (MOEA), Taiwan.