Reversible hydrogen storage by a Li-Al-N-H complex

Stepwise solid-state reaction between LiNH2 and LiAlH4 at a molar ratio of 2:1 is investigated in this paper. It is observed that approximately four H atoms are evolved from a mixture of LiNH2-LiAlH4 (2:1) after mechanical ball milling. The transformation of tetrahedral [AlH4](-) in LiAlH4 to the octahedral [AlH6](3-) in Li3AlH6 is observed after ball milling LiAlH4 with LiNH2. Al-N bonding is identified by using solid-state Al-27 nuclear magnetic resonance (NMR) measurements. The NMR data, together with the results of X-ray diffraction and Fourier transform IR measurements, indicate that a Li-Al-N-H intermediate with the chemical composition of Li3AlN2H4 forms after ball milling. Heating the post-milled sample to 500 degrees C results in the liberation of an additional four H atoms and the formation of Li3AlN2. More than 5 wt % hydrogen can be reversibly stored by Li3AlN2. The hydrogenated sample contains LiNH2, LiH, and AlN. The role of AlN in the reversible hydrogen storage over Li-Al-N-H is discussed.