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

Hydrogen storage properties of the (nLiAlH(4) + LiNH2) 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 similar 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 LiAlH4 into (1/3) Li3AlH6 + (2/3) Al + H-2 and subsequently decomposition of (1/3) Li3AlH6 into LiH + (1/3) Al + 0.5H(2). Apparently, LiAlH4 is profoundly destabilized during ball milling by the presence of a large quantity of LiNH2 (37.7 wt.%) in the n = 1 composite. The rate of dehydrogenation at 100-170 degrees 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 (nLiAlH4 + LiNH2) occurs by the same LiAlH4 decomposition reactions as those found during ball milling. The ball milled n = 1 composite stored under Ar at 80 degrees C slowly discharges large quantities of H-2 approaching 3.5 wt.% after 8 days of storage.