Optimizing Sc-Doped Na₃V₂(PO₄)₂F₃/C as a High-Performance Cathode Material for Sodium-Ion Battery Applications

Na3V2(PO4)2F3 (NVPF) has been considered as a promising NASICON-type (sodium superionic conductor) cathode material for sodium-ion batteries (SIBs). However, their development has been limited by poor electronic conductivity. To address this issue, a series of Sc-doped carbon-coated Na3V2–xScx(PO4)2F3/C, x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1 were synthesized by the sol–gel method. Sc is found to partially stabilize the crystalline framework, reduce deformation during Na+ insertion/extraction, and reduce polarization, which in turn impacts the diffusion of Na+ ions in the crystal lattice. The optimized NVPF has good electronic conductivity and cycling stability; in particular, the NVSPF/C-0.04 electrode exhibits the highest discharge specific capacity (125 mAh g–1 at 0.2 C), excellent rate performance (87 mAh g–1 at 30 C), and outstanding cycling performance (90% capacity retention after 1000 cycles at 10 C and 94% capacity retention after 100 cycles at 1 C). Density functional theory shows that the Sc-doping effectively improves the electronic conductivity of the NVPF. In situ and operando synchrotron XRD data show that the NVSPF/C-0.04 electrode undergoes phase transitions during charge/discharge at 0.2 and 2 C and indicates the reason for the capacity decay at 5 C. Finally, the NVSPF/C-0.04//HC (hard carbon) full cell performs with a good discharge specific capacity (112 mAh g–1 at 1 C). This study presents a feasible approach to developing high-performance cathode materials that can be used in SIBs, and it holds great potential for the future advancement of SIBs.