Synergistic Ni doping and F-carbon coating of Na₃V₂(PO₄)₂F₃ for high-performance sodium-ion batteries over a wide...

The NASICON-type (sodium super ionic conductor) compound Na₃V₂(PO₄)₂F₃(NVPF) has emerged as a highly promising cathode candidate for sodium-ion batteries (SIBs) owing to its high operating potential and robust three-dimensional framework structure. However, its practical utility is hindered by the limited energy density, a restricted two-electron redox mechanism, and inherently poor electrical conductivity. Furthermore, fluorine depletion during prolonged cycling leads to the generation of a secondary Na₃V₂(PO₄)₃ (NVP) phase, resulting in structural degradation and reduced reversibility. To address these challenges, a dual-modified cathode, Na₃V_1.95Ni_0.05(PO₄)₂F₃@C@FC (NVPF-Ni5@C@FC), was synthesized via a sol-gel process integrating Ni doping with fluorocarbon surface modification. Density functional theory (DFT) calculations and structural analyses reveal that Ni incorporation effectively narrows the band gap and enhances intrinsic electronic conductivity, while the fluorocarbon coating reinforces the lattice and suppresses irreversible phase transitions under an extended voltage window. As a result, the optimized electrode delivers a high reversible capacity of 154.3 mAh g^-1 at 0.1C and retains 81.5% and 77.0% of its capacity after 1000 and 2000 cycles at 5C and 10C, respectively. The full cell (NVPF-Ni5@C@FC||HC) achieves an impressive energy density of 325 Wh kg^-1, demonstrating its great promise for practical sodium-ion energy storage systems.