Hydrogen storage properties of LiBH4

Abstract

Metal hydrides typically absorb 1–2 hydrogen atoms per metal atom and exhibit very large volumetric storage densities of up to 150 kg H2 m−3 (e.g. Mg2FeH6). However, due to the large atomic mass of the transition metals the gravimetric hydrogen density is limited to less than 5 mass%. Light weight group 3 metals, e.g. Al, B, are able to bind four hydrogen atoms and form together with an alkali metal an ionic or at least partially covalent compound. These compounds are rather stable and often desorb the hydrogen only above their melting temperature. LiBH4 has a gravimetric hydrogen density of 18.5 mass% and a volumetric hydrogen density of 121 kg H2 m−3. The compound was first synthesized by Schlesinger and Brown [J. Am. Chem. Soc. 62 (1940) 3429] in an organic solvent. According to the work of Stasinevich and Egorenko [Russian J. Inorg. Chem. 13(3) (1968) 341] hydrogen desorbs from LiBH4 at temperatures greater than 470°C. We have successfully identified the low temperature structure of LiBH4: orthorhombic, space group Pnma (#62), the unit cell contains four molecules and has the dimensions a=7.1730 Å, b=4.4340 Å, c=6.7976 Å at 25°C. A slight hydrogen desorption was observed during the structure transformation around 100°C and the major hydrogen desorption (13.5 mass%) starts at approximately 200°C when SiO2-powder is added to the LiBH4 sample.