A novel intermediate in the LiAlH4–LiNH2 hydrogen storage system

Abstract

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 H₂ and, subsequently, the produced LiH reacts with LiNH₂ forming Li₂NH and H₂. A new intermediate, Li_4−xAl_x(NH)_2–2xN_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. Li_4−xAl_x(NH)_2–2xN_2x is characterized by Rietveld refinement of SR-PXD data and solid-state ²⁷Al MAS NMR spectroscopy (chemical shift, δ(Al) = 125 ppm) and both techniques reveal a maximum value for x of ∼0.10, i.e., Li_3.90Al_0.10(NH)_1.80N_0.20. The solid solution Li_4−xAl_x(NH)_2–2xN_2x crystallizes in a cubic unit cell, a = 4.9854(7) Å with space group Fmm, 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 H₂ is released during heating from RT to 500 °C, while for LiAlH₄–LiNH₂ composites with molar ratios of LiNH₂ higher than 0.5 the release of both H₂ and NH₃ is observed.