Stabilization of a mixed iron vanadium based hexagonal tungsten bronze hydroxyfluoride HTB-(Fe0.55V0.45)F2.67(OH)0.33 as a positive electrode for lithium-ion batteries

In our search for novel insertion compounds for Li-based batteries, we have identified a new mixed iron vanadium based Hexagonal Tungsten Bronze (HTB) type phase. Its synthesis involves two steps which consist first of preparing mixed metal hydrated fluoride Fe1.64V1.36F8(H2O)(2)by a microwave assisted thermal process, followed by thermal treatment under air to obtain metastable HTB-(Fe0.55V0.45)F-2.67(OH)(0.33)hydroxyfluoride.Fe-57 Mossbauer spectrometry demonstrates the presence of oxidation states Fe(2+)and Fe(3+)in Fe1.64V1.36F8(H2O)(2)as opposed to only Fe(3+)in HTB-(Fe0.55V0.47)F-2.67(OH)(0.33). Moreover, the Mossbauer spectra recorded at 77 K reveal that none of the compounds shows magnetic ordering owing to the presence of V(3+)distributed over the crystallographic sites of Fe3+. Complementary X-ray spectroscopy and Rietveld refinement further confirm the successful synthesis of HTB-(Fe0.55V0.45)F-2.67(OH)(0.33). Electrochemically, the new HTB-(Fe0.55V0.45)F-2.67(OH)(0.33)shows a first discharge capacity of 181 mA h g(-1)with 67% of this capacity remaining upon cycling. Unlike HTB-FeF2.66(OH)(0.34), the structure remains stable after the first discharge confirming the positive effect of vanadium in the HTB network.