Al₃Li₄(BH₄)₁₃: A Complex Double‐Cation Borohydride with a New Structure

Abstract

The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (approximately 70 degrees C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl(3) and LiBH(4). The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) A). The unexpected composition Al(3)Li(4)(BH(4))(13) can be rationalized on the basis of a complex cation [(BH(4))Li(4)](3+) and a complex anion [Al(BH(4))(4)](-). The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at approximately 70 degrees C, forming LiBH(4). The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane.

The First Halide-Free Bimetallic Aluminum Borohydride: Synthesis, Structure, Stability, and Decomposition Pathway

“…44,45 Similar peaks were seen at 2440 and 2480 cm −1 for Al 3 Li 4 (BH 4 ) 13 in Raman and at 2420 with 2480 cm −1 in the infrared spectrum as well as 2444 and 2503 cm −1 for Na[Al(BH 4 ,Cl) 4 ] in Raman spectra. 8,9 In the region of B−H bending modes, a band at 1249 cm Taking together with the TGA data, all the observations can be described by the following hypothetic decomposition reaction 3:…”

“…5,6 The increasing covalence in M−H bonding weakens the B−H bonding and improves thermal decomposition properties of bimetallic complexes. Among the so far published bimetallic complex borohydrides, there are several that match the temperature range of 60−120°C applicable for fuel cells: 7 "Li 4 Al 3 (BH 4 ) 13 " containing some chloride on a borohydride site, 8 Na[Al(BH 4 ) 4−x Cl x ], 9 NaZn 2 (BH 4 ) 5 , NaZn(BH 4 ) 3 , 10 KCd(BH 4 ) 3 , K 2 Cd(BH 4 ) 4 . 11 All the mentioned bimetallic borohydrides evolve diborane (B 2 H 6 ) as hydrogen desorption byproduct which prevents full reversibility and is undesirable for fuel cell applications.…”

Structure and Characterization of KSc(BH4)4

“…[5][6][7][8] Metal borohydrides are known to have a rich chemistry and can also form eutectic melting mixtures as observed for the Ca(BH 4 ) 2 -LiBH 4 system or new solids as in the Al(BH 4 ) 3 -LiBH 4 system, i.e., Li 4 Al 3 (BH 4 ) 13 . 9,10 The thermal stability of binary metal hydrides has been inversely related to the metal electronegativity (and consequently to the standard redox potential). 3 A similar relation has been postulated also for borohydrides a half-century ago, 11 based on the stability ratio theory of Sanderson,12 and has been recently analyzed theoretically and experimentally.…”

Powder diffraction methods for studies of borohydride-based energy storage materials

“…Its P " 4 4 3 n structurew as determined from synchrotron X-ray powder diffraction data supported by DFT calculations (Lindemann et al,2010 À ,w here Zn has an early flat trigonal environment. Ka tom, having 8n eighbouring anions, acts like ac ounter-ion.T he secondc ompound is polymeric (Ravnsbaek et al, 2010b).…”