Carbon-Coated Sodium Vanadium Phosphate Nanocomposites as Efficient Cathode Materials for Sodium-Ion Batteries and Capacitors

Sodium vanadium phosphate (Na 3 V 2 (PO 4 ) 3 ) with continuous carbon-coating (NVP/C) has been regarded as one of the most promising cathode materials for sodium-ion batteries (SIBs) owing to the three-dimensional sodium superionic conductor (NASICON) structure for fast sodium-ion migrations, high theoretical energy density (ca. 400 Wh/ kg), and good thermal stability (450°C). In this study, NVP/C nanoparticles (NPs) were successfully synthesized by employing ascorbic acid as the reductant and carbon source simultaneously, followed by calcination at 750°C in an argon atmosphere. The NVP/C NPs possessed great crystallinity, high purity, size of 100-200 nm, coated carbon layer of 5-10 nm, and carbon amounts of 8.5 wt. % were confirmed through the X-ray diffractometer, transmission electron microscope, and thermogravimetric analyzer, respectively. The remarkable reversible capacity (75 mAh/g at 0.1 C), rate capability (62 mAh/g at 2 C), and capacity retention (~100% after 1,000 cycles at 1 C) were delivered from the NVP/C half-cell. Encouraged by these impressive results, the asymmetric sodium-ion capacitors (SICs) consisted of the NVP/C cathode, free-standing hydrogel-derived hierarchical porous activated carbon (H-HPAC) anode, and organic electrolyte were also assembled to explore the electrochemical performances. As revealed, excellent rate capabilities were also achieved, i.e. , 44 mAh/g at 0.45 A/g NVP and 28 mAh/g at 4.5 A/g NVP . Accordingly, the NVP/C NPs synthesized in the present study are reasonably believed to serve as potential cathode materials for SIBs and SICs. Figure 1