Deep-Eutectic Solvent as a Solvent and Precursor for the Synthesis of a Carbon-Coated Na₃V₂(PO₄)₂F_3-O Material

Deep eutectic solvents (DES) are well-known as cost-effective and environmentally friendly "designer solvents" for controlling the size and morphology of nanomaterials. In this study, we leverage DES not only as a solvent for the topochemical synthesis of Na₃V₂(PO₄)₂F_3-O (0 ≤ y ≤ 2) but also as a precursor for a uniform and thin carbon coating. After solvothermal synthesis in a green deep eutectic solvent composed of a mixture of choline chloride, citric acid, and water (3:1:3 molar ratio), XRD refinements and FTIR, XPS, and TEM analyses confirmed the obtention of a pure Na₃V₂(PO₄)₂F_3-O (0 ≤ y ≤ 2) phase encapsulated in an organic layer derived from a residual deep eutectic solvent. Subsequent sintering at 600 °C under an argon atmosphere produced a homogeneous nitrogen-doped carbon coating without the need for additional carbon sources. Electrochemical tests in galvanostatic conditions demonstrated that this material exhibits excellent performance in terms of capacity retention and rate capabilities, with specific capacities exceeding 110 mAh/g at 2C versus Na metal and 68 mAh/g at 10C in full cells versus hard carbon.