Hydrogen storage in LiAlH4:: 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 LiAlH4. First, we explore the decomposition mechanism of monoclinic LiAlH4 into monoclinic Li3AlH6 plus face-centered cubic (fcc) 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 LiAlH4 and Li3AlH6 agree well with experimental data. On the other hand, the alternative decomposition of LiAlH4 into LiAlH2 plus H-2 is predicted to be unstable with respect to that through Li3AlH6. Next, we investigate thermal decomposition of Li3AlH6 into fcc 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 AlH6 to form the stable Li-H-Li-H complex. Then, two sequential H-2 desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li3AlH6(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. (C) 2004 American Institute of Physics.