Ionothermal Synthesis of Polyanionic Electrode Material Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>FO<sub>2</sub> through a Topotactic Reaction
Jacob Olchowka, Long H. B. Nguyen, Emmanuel Petit, Paula Sanz Camacho, Christian Masquelier, Dany Carlier, Laurence Croguennec
Abstract
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.