The use of metal fluorides and most particularly, iron fluorides, as an intercalation host electrode for Li-metal rechargeable batteries has been considered only in the late 1990&’s.  High band gaps of fluorides inducing low electronic conductivity as well as large voltage hysteresis have impeded their utilization in commercial cells. To overcome these limitations, Badway et al proposed the use of a carbon-metal fluoride nanocomposite prepared by high energy ball milling showing their potential use as cathode materials.  Furthermore, the incorporation of oxygen within the framework has been shown to lower the polarization and improved the energy efficiency and cycle life of the cell. 
Iron fluoride compounds can adopted several crystallographic structures including the ReO3 (rhombohedra, R-3c), the Pyrochlore (cubic, Fd-3m) or the Hexagonal-Tungsten-Bronze (denoted as HTB, orthorhombic, Cmcm) type frameworks. [7-9] The latter consists of corner-shared FeF6 octahedra forming hexagonal section along the c-axis. Structural waters are located within the tunnels and can be thermally removed without any structural collapse.  Surprisingly, despite the presence of such water molecules, a reversible electrochemical intercalation of lithium within FeF3.0.33H2O has been demonstrated paving the way for new researches. 
Depending on the synthesis method used, it has been shown that during the formation of transition metal-fluorides, a competition between M-F and M-OH bond formation might occur. [12-13] This deviation from the ideal composition is likely to impact on the physic-chemical properties of the solid. Dealing with metal fluorides and especially nano-sized metal fluorides prepared by Chimie Douce required fine structural characterizations as slight changes within the composition might strongly affect the material&’s properties.
Therein, we report a detailed investigation of the structural features of an iron-based fluoride compound having the HTB type structure where fluoride anions have been partially substituted with OH groups. The impact of the later on the structural features, thermal behavior as well as electrochemistry has been thoroughly investigated. This has allowed correlating the lithium intercalation properties with the thermal behavior of this compound emphasizing an optimal oxy-hydroxyl-fluoride composition.
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