Ionothermal Synthesis of Polyanionic Electrode Material Na₃V₂(PO₄)₂FO₂ through a Topotactic Reaction

Polyanionic Na₃V₂(PO₄)₂FO₂ has been successfully prepared for the first time by ionothermal reaction in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) ionic liquid. Its structure and elemental stoichiometry are confirmed by X-ray diffraction, NMR spectroscopy, and ICP-OES, respectively. Furthermore, the scanning electron microscopy reveals that the as-obtained material possesses an original platelet-like morphology. A topochemical reaction mechanism is proposed to explain the formation of the 3D framework of Na₃V₂(PO₄)₂FO₂ from layered compound α-VOPO₄·2H₂O. Galvanostatic electrochemical tests indicate a modification of the desodiation and sodiation mechanism of the as-prepared Na₃V₂(PO₄)₂FO₂ compared to those synthesized by conventional solid-state approaches. Furthermore, the electrochemical performance of Na₃V₂(PO₄)₂FO₂ obtained at different cycling rates is also discussed.