Fluoride substitution in LiBH₄; destabilization and decomposition

Abstract

Fluoride substitution in LiBH is studied by investigation of LiBH-LiBF 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 LiBH forms (LiBHF) 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 LiBHF. 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. LiBHF (LiBHF). An alternative approach for fluoride substitution was performed, by addition of EtN·3HF to LiBH, 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.

Quantitative Assessment of B−B−B, B−Hb−B, and B−Ht Bonds: From BH3 to B12H122−

“…During the thermal decomposition, ( 2 ) releases hydrogen and forms many possible intermediates (see Figure ). The formation of intermediates, such as ( 4 ) and ( 5 ), facilitate the reverse reactions, due to their weak B−B−B and B−H b −B bonds ( ( 5 ): BSO(B−B−B)=0.415, k a (B−B−B)=0.824 mdyn/Å; BSO(B−H b −B)=0.188, k a (B−H b −B)=0.477 mdyn/Å and ( 4 ): BSO(B−B)=0.265, k a (B−B)=0.296 mdyn/Å; BSO(B−H b −B)=0.058, k a (B−H b −B)=0.109 mdyn/Å). The intermediates which facilitate the reverse reactions, such as ( 4 ) and ( 5 ) also show low thermodynamic stability with ΔH −24.85 and −27.42 kcal/mol, respectively, as shown in Figure .…”

“…The formation of diborane is observed during the thermal decomposition of e. g . LiBH 4 , , and . Although diborane has weak B−H b −B bonds ( k a (B−H b −B)=0.904 mdyn/Å, BSO=0.312), the formation of diborane hinder the reversibility of the dehydrogenation reaction due to the loss of boron (B 2 H 6 ( 3 ) is a volatile gas) …”

“…Thermal decomposition pathways of borohydrides ( ( 2 )) are of multi‐step character and complex (see Figure and Figure ) . Some intermediates, such as B 2 H 6 ( 3 ), ( 18 ), and ( 21 ) have been observed during the decomposition of LiBH 4 . The formation of LiB 12 H 12 has been confirmed by NMR and Raman spectroscopy .…”

Lightweight hydrides nanocomposites for hydrogen storage: Challenges, progress and prospects

“…Chong et al [161] introduced several lanthanide elements fluorides into NaBH 4 , and observed that the 3NaBH 4 -GaF 3 composites displayed a fast kinetics and remained high cycling stability even after 51 cycles. Richter et al [177] reported that milling with LiBF 4 could significantly decrease the decomposition temperature of LiBH 4 , but only diborane gas was released during this process. Zheng et al [178] ball-milled NaBH 4 and Mg(BH 4 ) 2 with fluorographene.…”

Complex Metal Hydrides for Hydrogen, Thermal and Electrochemical Energy Storage

“…The heavier halides have provided a wide range of metal borohydride halides with either fully ordered, e.g., KZn(BH 4 ) 2 Cl or Sr(BH 4 )Cl [92,186], partly ordered, e.g., NaY(BH 4 ) 2−x Cl 2+x [187,188], or disordered structures, e.g., K 2 Zn(BH 4 ) x Cl 4−x [189]. The smaller fluoride ion can substitute for the hydride ion, i.e., intramolecular anion substitution, F − → H − , in the BH 4 − complex and the composite NaBH 4 -NaBF 4 provided the first fluorine-substituted borohydride, NaBH 2.1 F 1.9 , observed in the temperature range of 200-215 • C [190,191]. Recently, a new class of Li ion conductors was discovered, LiRe(BH 4 ) 3 Cl, Re = La, Ce, Pr, Nd, Sm, Gd, with an interesting new structure type [115,[192][193][194].…”