Mesoporous Sodium Vanadium Phosphate/Carbon As Efficient Cathode Materials for Rechargeable Sodium-Ion Batteries

Abstract

Rechargeable sodium-ion batteries (SIBs) have recently considered as alternative energy storage devices to lithium-ion batteries (LIBs) because of their benefits on material abundance, low cost and electrochemical similarities. In spite of the previously mentioned advantages for SIBs, there still remain challenges to realize the practicalities. In contrast to the smaller lithium cation, for example, the larger radius of sodium cation not only lowers its transportation ability but also causes the more severe structural impact and volume variation of electrode materials during the intercalation/de-intercalation processes. Therefore, searching the electrode materials with excellent structural stability, high reversible capacity, superior rate and cycling performances becomes one of the critical issues. Among the developed cathode materials, sodium vanadium phosphate/carbon (Na3V2(PO4)3/C) nanocomposites with sodium superionic conductor (NASICON) structure have been regarded as one of the most promising candidates for SIBs owing to the high theoretical energy density (ca. 400 Wh/kg), good thermal stability (450°C) and highly covalent three-dimensional framework with large interstitial channels for fast sodium-ion migration. In the present study, mesoporous Na3V2(PO4)3 nanoparticles with highly sp2-coordinated carbon coatings (meso-Na3V2(PO4)3/C) were successfully synthesized by employing ascorbic acid as both of the reductant and carbon source, followed by calcination at 750°C in an argon atmosphere. The crystalline structure, morphology, surface area, carbon nature and amount were systematically identified by powder X-ray diffractometer, field-emission scanning electron microscope, transmission electron microscope, nitrogen adsorption-desorption technique, Raman microscope and thermogravimetric analyzer, respectively. Given their distinctive mesostructures and continuous sp2-coordinated carbon coatings, the remarkable reversible capacity (98 mAh/g@0.1 A/g), rate capability (63 mAh/g@1 A/g) and capacity retention (76 %@450 cycles@0.4 A/g) were achieved as demonstrated by galvanostatic testing. Consequently, the resultant meso-Na3V2(PO4)3/C were anticipated to function as efficient cathode materials for SIBs. More importantly, it appears to be a facile approach towards mass production of the electrode materials because the inexpensive chemicals and mild conditions were introduced to prepare the precursor powders. Acknowledgements We gratefully appreciate the financial support from the Bureau of Energy (BOE), Ministry of Economy Affair (MOEA), Taiwan.