Hydrogen storage in LiAlH ₄: Predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics

We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH ₄. First, we explore the decomposition mechanism of monoclinic LiAlH ₄ into monoclinic Li3AlH ₆ plus face-centered cubic (fee) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH ₄ and Li3AlH ₆ agree well with experimental data. On the other hand, the alternative decomposition of LiAlH ₄ into LiAlH ₂ plus H ₂ is predicted to be unstable with respect to that through Li3AlH ₆. Next, we investigate thermal decomposition of Li3AlH ₆ into fee LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH ₆ to form the stable Li-H-Li-H complex. Then, two sequential H ₂ desorption steps are followed, which eventually result in fee LiH plus fee Al and hydrogen: Li3AlH ₆(monoclinic) → 3 LiH(fcc) + Al(fcc) + 3/2 H ₂ is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s) + Al(s) → LiAl(s) + 1/2H ₂(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.