Preparation Conditions of Precursor Solution for Carbon-Coated Na₃V₂(PO₄)₂F₃

Na3V2(PO4)2F3 (NVPF) is emerging as a promising positive electrode for sodium-ion batteries due to its high operating voltage. However, its inherent low electronic conductivity is the main obstacle to practical application. Although carbon incorporation could address this issue, it also accelerates fluorine loss during synthesis, forming impurities such as Na3V2(PO4)3 that compromise the high operating voltage of NVPF. Herein, the synthesis conditions are systematically investigated to obtain pure phase NVPF via a direct heating of the prepared precursor. According to the analytical results, the amounts of fluorine and carbon sources play an important role in the successful synthesis of pure phase NVPF, whereas the pH of the precursor solution has an unexpectedly limited impact on impurity suppression. The sample prepared in the optimal condition achieves excellent electrochemical performance, delivering 119.5 mAh g–1 at 0.1C and remarkable long-term cycling stability. These findings provide insights into the scalable synthesis of high-purity NVPF for sodium-ion batteries.