Fluoride substitution in LiBH4; destabilization and decomposition

Abstract

Fluoride substitution in LiBH₄ is studied by investigation of LiBH₄–LiBF₄ mixtures (9 : 1 and 3 : 1). Decomposition was followed by in situ synchrotron radiation X-ray diffraction (in situ SR-PXD), thermogravimetric analysis and differential scanning calorimetry with gas analysis (TGA/DSC-MS) and in situ infrared spectroscopy (in situ FTIR). Upon heating, fluoride substituted LiBH₄ forms (LiBH_4−xF_x) and decomposition occurs, releasing diborane and solid decomposition products. The decomposition temperature is reduced more than fourfold relative to the individual constituents, with decomposition commencing at T = 80 °C. The degree of fluoride substitution is quantified by sequential Rietveld refinement and shows a selective manner of substitution. In situ FTIR experiments reveal formation of bands originating from LiBH_4−xF_x. Formation of LiF and observation of diborane release implies that the decomposing materials have a composition that facilitates formation of diborane and LiF, i.e. LiBH_4−xF_x (LiBH₃F). An alternative approach for fluoride substitution was performed, by addition of Et₃N·3HF to LiBH₄, yielding extremely unstable products. Spontaneous decomposition indicates fluoride substitution to have occurred. From our point of view, this is the most significant destabilization effect seen for borohydride materials so far.