A novel intermediate in the LiAlH4-LiNH2 hydrogen storage system

The decomposition pathways for the composite LiAlH4-LiNH2 in different ratios of (1 : 1), (1 : 1.5), (1 : 2) and (1 : 2.5) have been systematically studied using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as well as simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectroscopy. The study reveals that LiAlH4 decomposes in two steps to LiH, Al and H-2 and, subsequently, the produced LiH reacts with LiNH2 forming Li2NH and H-2. A new intermediate, Li4-xAlx(NH)(2-2x)N-2x, is observed during the decomposition of LiAlH4-LiNH2 (1 : 1.5), (1 : 2) and (1 : 2.5), formed from Li2NH and Al prior to the formation of Li3AlN2. Li4-xAlx(NH)(2-2x)N-2x is characterized by Rietveld refinement of SR-PXD data and solid-state Al-27 MAS NMR spectroscopy (chemical shift, delta(Al) = 125 ppm) and both techniques reveal a maximum value for x of similar to 0.10, i.e., Li3.90Al0.10(NH)(1.80)N-0.20. The solid solution Li4-xAlx(NH)(2-2x)N-2x crystallizes in a cubic unit cell, a = 4.9854(7) angstrom with space group Fm (3) over barm, similar to the crystal structure for Li2NH and is a rare type with both cation and anion disorder. For LiAlH4-LiNH2 (1 : 1) 8.7 wt% of H-2 is released during heating from RT to 500 degrees C, while for LiAlH4-LiNH2 composites with molar ratios of LiNH2 higher than 0.5 the release of both H-2 and NH3 is observed.