Reactivity of LiBH₄: In Situ Synchrotron Radiation Powder X-ray Diffraction Study

Abstract

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 additivesSiO2, TiCl3, LiCl, and Auis 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 °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.