Dual-Modulation of Carbon Coating and High-Valence Nb5+ Doping Toward High-Performance Na3V2(PO4)2O2F Cathode for Sodium-Ion Batteries
Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries for large-scale energy storage due to sodium's abundance and low cost. Among cathode materials, polyanionic compounds like Na3V2(PO4)2O2F (NVPOF) offer high energy density and dual voltage plateaus at ~3.6 and 4.0 V, but suffer from low electronic conductivity and sluggish Na+ diffusion. Here, we report a dual-modulation strategy combining high-valence Nb5+ doping and polydopamine-derived carbon coating to synthesize Na3V1.94Nb0.06(PO4)2O2F-C (NVPOF-Nb-C) via a hydrothermal route. X-ray diffraction and Rietveld refinement confirm that Nb5+ doping induces slight lattice expansion without altering the tetragonal I4/mmm framework. Density functional theory calculations reveal that Nb5+ doping optimizes the crystal structure and reduces the Na+ diffusion barrier, while the uniform carbon coating enhances electron transport. Consequently, NVPOF-Nb-C exhibits remarkably improved electrochemical performance, including high reversible capacity, excellent rate capability, and ultralong cycling stability. In a full cell with hard carbon anode, it delivers a high energy density of 487.2 Wh kg−1 at 1C and retains 91.51% capacity after 3000 cycles at 20C. This work provides a synergistic strategy to overcome the intrinsic limitations of polyanionic cathodes for practical SIB applications.