• • Nb5+ doping (6 mol%) expands the lattice (shift of (200) reflection to lower 2θ) and reduces Na+ diffusion barrier, as confirmed by DFT, enhancing rate capability.
• • Polydopamine-derived carbon coating provides a uniform conductive framework, improving electron transport and structural stability during cycling.
• • NVPOF-Nb-C delivers a high reversible capacity and excellent rate capability; full cell achieves 487.2 Wh kg−1 at 1C, indicating high energy density for practical applications.
• • Full cell retains 91.51% capacity after 3000 cycles at 20C, demonstrating ultralong cycling stability critical for grid-scale storage.