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

Abstract

Lithium tetrahydridoboranate (LiBH 4 ) 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 LiBH 4 and different additivessSiO 2 , TiCl 3 , LiCl, and Ausis investigated. It is found that silicon dioxide reacts with molten LiBH 4 and forms Li 2 SiO 3 or Li 4 SiO 4 at relatively low amounts of SiO 2 , e.g., with 5.0 and 9.9 mol % SiO 2 in LiBH 4 , whereas, for higher amounts of SiO 2 (e.g., 25.5 mol %), only the Li 2 SiO 3 phase is observed. Furthermore, we demonstrate that a solid-state reaction occurs between LiBH 4 and TiCl 3 to form LiCl at room temperature. At elevated temperatures, more LiCl is formed simultaneously with a decrease in the diffracted intensity from TiCl 3 . Lithium chloride shows some solubility in solid LiBH 4 at T > 100°C. This is the first report of substituents that accommodate the structure of LiBH 4 by a solid/solid dissolution reaction. Gold is found to react with molten LiBH 4 forming a Li-Au alloy with CuAu 3 -type structure. These studies demonstrate that molten LiBH 4 has a high reactivity, and finding a catalyst for this H-rich system may be a challenge.

Reversibility of Al/Ti Modified LiBH4

“…Indeed, Au et al 41 and Yang et al 42 have screened a batch of additives, and if one classifies them with respect to their Pauling electronegativities, the sequence obtained does not match with their effectiveness in destabilizing the hydride. Evidences of partial substitution came recently from Mosegaard et al 43 They found from an in situ X-ray diffraction study that LiCl, formed from the reduction reaction of TiCl 3 by LiBH 4 , is dissolved to some extent in the structure of solid lithium borohydride at temperatures above ∼100°C, giving an example of a chemical substitution in LiBH 4 . It is also possible to synthesize by wet chemistry ternary metal borohydrides, 44 and recently a computational screening study has been performed to identify which ternary metal borohydrides may form stable alloys with promising decomposition energies.…”

Recent Progress in Metal Borohydrides for Hydrogen Storage

“…Since Bogdanovic et al found that the addition of Ti-based compounds significantly promoted the dehydrogenation-rehydrogenation reactions of NaAlH 4 [193], the development of complex hydrides for hydrogen storage has significantly increased. Stimulated by this finding, a large number of additives from oxides, halides, metals, and carbon-based materials to M(BH 4 ) n have been examined [19,20,58,87,101,130,144,150,161,168,170,171,191,[194][195][196][197][198][199][200][201][202][203][204][205][206][207][208][209][210]; the corresponding dehydrogenation and rehydrogenation properties are summarized in Table 5. For instance, the most effective additive for LiBH 4 was found to be the mixture of 0.2 MgCl 2 + 0.1 TiCl 3 , in which approximately 5 mass% of hydrogen was released from 333 K and 4.5 mass% of hydrogen was rehydrogenated at 873 K in 7 MPa H 2 [195].…”

Hydrogen–fluorine exchange in NaBH4–NaBF4

“…13,14 Anion substitu-tion in metal borohydride materials was reported for LiBH 4 -LiX, where X = Cl, Br, and I in NaBH 4 -NaCl, in Ca(BH 4 ) 2 -CaX 2 , X = Cl and I, and in Mg(BH 4 ) 2 -MgX 2 , X = Cl and Br. [15][16][17][18][19][20][21][22][23][24] The change in the hydrogen storage properties of anion-substituted metal borohydride using the heavier halides, Cl, Br or I, is small and may lead to a stabilization, which tends to facilitate hydrogen absorption. 18,19,24 In contrast, calculations reveal that fluorine substitution in LiBH 4 is not thermodynamically favored but should indeed provide a destabilization of lithium borohydride.…”