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

Abstract

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 Fe_1.64V_1.36F₈(H₂O)₂ by a microwave assisted thermal process, followed by thermal treatment under air to obtain metastable HTB–(Fe_0.55V_0.45)F_2.67(OH)_0.33 hydroxyfluoride. ⁵⁷Fe Mössbauer spectrometry demonstrates the presence of oxidation states Fe²⁺ and Fe³⁺ in Fe_1.64V_1.36F₈(H₂O)₂ as opposed to only Fe³⁺ in HTB–(Fe_0.55V_0.47)F_2.67(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–(Fe_0.55V_0.45)F_2.67(OH)_0.33. Electrochemically, the new HTB–(Fe_0.55V_0.45)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–FeF_2.66(OH)_0.34, the structure remains stable after the first discharge confirming the positive effect of vanadium in the HTB network.