Engineering Fluorine-Stabilized, Phase-Pure Na ₃ V ₂ (PO ₄ ) ₂ F ₃ Cathode via In Situ Carbon Coating for High-Voltage Sodium-Ion Batteries

Na3V2(PO4)2F3 (NVPF) has recently been identified as a highly promising cathode material for sodium-ion batteries (SIBs) due to its outstanding structural stability, elevated operating voltage, and notable theoretical capacity. Despite its advantages, the electrochemical performance of this material is restricted by low electronic conductivity, and by fluorine loss during synthesis, which results in a low-voltage plateau around 3.4 V, thereby limiting its practical applications. In the present work, the stabilization of volatile fluorine was achieved by simply tuning the carbon coating on the NVPF surface. In situ carbon-coated NVPF (NVPF/C) materials were prepared through the sol–gel technique, with inexpensive citric acid as the carbon source. The carbon content was tuned by varying the molar ratios of vanadium in NVPF to citric acid; 2:1 (1 M NVPF/C), 2:1.5, 2:2, and 2:2.5. XRD patterns of the synthesized materials matched well with the reference pattern of NVPF (tetragonal, P42/mnm). The relative carbon content and elemental composition were determined using SEM-EDS; the carbon layer thickness was measured using HRTEM; and the degree of graphitization was assessed via Raman spectroscopy. Electrochemical analyses, including GCD, and differential capacity curves showed a low-voltage plateau around 3.4 V for 1 M NVPF/C. This plateau diminished with increasing carbon content, indicating fluorine stabilization at higher coating concentrations. The optimized carbon coating effectively suppressed fluorine loss, resulting in a significant reduction in charge-transfer resistance. 2 M NVPF/C exhibited discharge capacity of 113.9 mA h g–1 at 0.1 C and demonstrated excellent cycling performance, highlighting its strong potential for high-energy-density SIBs.