Mechanical and Thermal Dehydrogenation of Lithium Alanate (LiAlH4) and Lithium Amide (LiNH2) Hydride Composites

Hydrogen storage properties of the (nLiAlH 4 + LiNH 2) hydride composite where n = 1, 3, 11.5 and 30, synthesized by high energy ball milling have been investigated. The composite with the molar ratio n = 1 releases large quantities of H 2 (up to ~5 wt.%) during ball milling up to 100–150 min. The quantity of released H 2 rapidly decreases for the molar ratio n = 3 and is not observed for n = 11.5 and 30. The XRD studies indicate that the H 2 release is a result of a solid state decomposition of LiAlH 4 into (1/3)Li 3AlH 6 + (2/3)Al + H 2 and subsequently decomposition of (1/3)Li 3AlH 6 into LiH + (1/3)Al + 0.5H 2. Apparently, LiAlH 4 is profoundly destabilized during ball milling by the presence of a large quantity of LiNH 2 (37.7 wt.%) in the n = 1 composite. The rate of dehydrogenation at 100–170 °C (at 1 bar H 2) is adversely affected by insufficient microstructural refinement, as observed for the n = 1 composite, which was milled for only 2 min to avoid H 2 discharge during milling. XRD studies show that isothermal dehydrogenation of (nLiAlH 4 + LiNH 2) occurs by the same LiAlH 4 decomposition reactions as those found during ball milling. The ball milled n = 1 composite stored under Ar at 80 °C slowly discharges large quantities of H 2 approaching 3.5 wt.% after 8 days of storage.