Sulfate-Substituted Na₃V₂(PO₄)₃ Nanodots Embedded in Carbon Nanotubes for Ultrafast Sodium-Ion Storage

Polyanion-substituted sodium vanadium phosphate (Na₃V₂(PO₄)₃, NVP) derivatives, including SO₄, BO₃, WO₄, and SiO₄ substitutions, were systematically synthesized and impregnated with a nanocarbon network via ultracentrifugation. Among them, nanosized (5-30 nm), highly crystalline, and well-dispersed sulfate-substituted NVP (NVPS) nanodots were directly nucleated onto multiwalled carbon nanotubes, enabling ultrafast electrochemical kinetics. This nanoscale architecture delivered exceptional rate capability, achieving 97 mAh g^-1 at 1000C (3.6 s discharge), corresponding to 83% of the theoretical capacity, outperforming conventional NVP. The electrochemical kinetics analysis using a cavity microelectrode revealed reduced polarization, enhanced capacitive charge storage, and rapid sodium ion diffusion during intercalation/deintercalation, facilitated by the conformal interface between NVPS and MWCNT, possibly by sulfate-induced surface modifications. These findings establish polyanion substitution and ultracentrifugation-assisted materials processing as a transformative strategy for overcoming intrinsic transport limitations in NASICON-type phosphates, positioning NVPS as a benchmark material for next-generation high-power sodium-ion batteries and hybrid capacitors.