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)F2.67(OH)0.33 hydroxyfluoride. 57Fe Mössbauer spectrometry demonstrates the presence of oxidation states Fe2+ and Fe3+ in Fe1.64V1.36F8(H2O)2 as opposed to only Fe3+ in HTB–(Fe0.55V0.47)F2.67(OH)0.33. Moreover, the Mössbauer spectra recorded at 77 K reveal that none of the compounds shows magnetic ordering owing to the presence of V3+ distributed over the crystallographic sites of Fe3+. Complementary X-ray spectroscopy and Rietveld refinement further confirm the successful synthesis of HTB–(Fe0.55V0.45)F2.67(OH)0.33. Electrochemically, the new HTB–(Fe0.55V0.45)F2.67(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.