Interfacial Nano-Pitting and In Situ Sulfate-Directed Growth of Na2.85V2(PO4)2.85(SO4)0.15 Nanodots on MWCNT for Ultrafast Sodium-Ion Storage.

To design nanostructured electrodes capable of sustaining both high energy and power in sodium-ion supercapacitors, we present a mechanistically defined synthesis strategy that couples interfacial nano-pitting of carbon frameworks with in situ sulfate-substituted crystallization of Na2.85V2(PO4)2.85(SO4)0.15 (NVPS) nanodots. By integrating phosphate-sulfate ultracentrifugation pretreatment with brief thermal annealing, NVPS nanodots (5 nm) are homogeneously dispersed and tightly anchored along multi-walled carbon nanotube (MWCNT) networks. Mechanistic analysis revealed that a bifunctional role of Na2SO4: it induces localized sodification-assisted etching that forms ≈5 nm nanopits serving as confined nucleation cavities, while sulfate species participate in NVPS crystallization within these domains. Concurrent redeposition of sodified carbon fragments during ultracentrifugation further reinforces interfacial stability, producing nanodots electronically bridged and mechanically stabilized by the carbon matrix. The resulting NVPS/MWCNT composites exhibit exceptional rate capability, delivering 97 mAh g-1 (83% of the theoretical capacity) at 1000C, corresponding to a full charge-discharge cycle of 3.6 s. This ultrafast behavior originates from the formation of robust, strongly coupled nanodot-carbon junctions that facilitate rapid electron transport and sodium-ion intercalation. This sulfate-directed nano-pitting approach provides a generalizable and scalable route to engineer high-density nanodot/carbon hybrid architectures that bridge the gap between battery-type and capacitor-type storage.