Stabilization of a mixed iron vanadium based Hexagonal Tungsten Bronze hydroxyfluoride HTB-(Fe0.53V0.47)F2.67(OH)0.33 as 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 the mixed metal hydrate fluoride Fe1.64V1.36F8(H2O)2 by a microwave assisted thermal process, followed by a proper thermal treatment under air to obtain HTB-(Fe0.53V0.47)F2.67(OH)0.33 hydroxyfluoride. We have demonstrated, by 57Fe Mössbauer spectrometry, the presence of oxidation states Fe2+ and Fe3+ in Fe1.64V1.36F8(H2O)2 as opposed to only Fe3+ in HTB-(Fe0.53V0.47)F2.67(OH)0.33. Moreover we discovered through complementary X-ray spectroscopy and Rietveld refinement that in both phases, V3+ is distributed over the crystallographic sites of Fe3+. We used this mixture of sites to explain that none of the compounds shows a magnetic ordering down to 77 K, contrary to what was previously reported for the vanadium free Fe3F8(H2O)2 and HTB-FeF3 phases. Electrochemically-wise, the new HTB-(Fe0.53V0.47)F2.67(OH)0.33 shows a first discharge capacity of 181 mAh.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 the vanadium in the HTB network.