Vanadium-enhanced Na₂FePO₄F cathodes for high-performance sodium-ion batteries

Sodium fluorophosphate Na₂FePO₄F holds great promise for sodium-ion batteries due to its high theoretical capacity, excellent structural stability, abundant resources, and affordability. However, its poor electronic and ionic conductivities limit its practical applications. Therefore, ion doping and carbon coating have been employed as synergistic strategies in this study to overcome these limitations. A one-step, energy-efficient solid-state method using sucrose as a carbon coating source was used to synthesize Na₂FePO₄F/C (NFPF/C) and its doped variant, Na₂Fe₀.₈₅V₀.₁PO₄F/C (NFVPF/C).²³Na-MAS-NMR spectra confirm the existence of two distinct sites for sodium (Na1/Na2). The ex-situ²³Na-MAS-NMR performed at different states-of-charge reveals the activity of only one sodium. The scanning electron microscopy findings reveal a reduction in the particle size with V-introduction, enhancing the energetic performances. NFVPF/C delivers higher specific capacity of 122 mAh g⁻¹compared to 116 mAh g⁻¹for NFPF/C at 0.1C. It also demonstrates improved cycling stability, retaining 81 % of its initial capacity after 120 cycles, in contrast to 46 % for the pristine material. The doped phase outperforms the pristine at higher current rates, delivering specific capacities of 81 and 55 mAh g⁻¹at 2C and 3C, respectively, compared to 35 and 17 mAh g⁻¹for NFPF/C.