Reactivity of LiBH4:: In situ synchrotron radiation powder X-ray diffraction study

Lithium tetrahydridoboranate (LiBH4) may be a potentially interesting material for hydrogen storage, but in order to absorb and desorb hydrogen routinely and reversibly, the kinetics and thermodynamics need to be improved significantly. A priori, this material has one of the highest theoretical gravimetric hydrogen contents, 18.5 wt%, but unfortunately for practical applications, hydrogen release occurs at too high temperature in a non-reversible way. By means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), the interaction between LiBH4 and different additives-SiO2, TiCl3, LiCl, and Au - is investigated. It is found that silicon dioxide reacts with molten LiBH4 and forms Li2SiO3 or Li4SiO4 at relatively low amounts of SiO2, e.g., with 5.0 and 9.9 mol % SiO2 in LiBH4, Whereas, for higher amounts of SiO2 (e.g., 25.5 mol %), only the Li2SiO3 phase is observed. Furthermore, we demonstrate that a solid-state reaction occurs between LiBH4 and TiCl3 to form LiCl at room temperature. At elevated temperatures, more LiCl is formed simultaneously with a decrease in the diffracted intensity from TiCl3. Lithium chloride shows some solubility in solid LiBH4 at T > 100 degrees C. This is the first report of substituents that accommodate the structure of LiBH4 by a solid/solid dissolution reaction. Gold is found to react with molten LiBH4 forming a Li-Au alloy with CuAu3-type structure. These studies demonstrate that molten LiBH4 has a high reactivity, and finding a catalyst for this H-rich system may be a challenge.