First-principles determination of crystal structures, phase stability, and reaction thermodynamics in the Li-Mg-Al-H hydrogen storage system

Abstract

First-principles density-functional theory (DFT) calculations have been used to investigate the crystal structures, thermodynamic stability, and decomposition pathways of Li-Mg-Al-H hydrogen storage compounds. We find that the recently discovered $\text{LiMg}{({\text{AlH}}_{4})}_{3}$ compound is stable with respect to solid-state decomposition into ${\text{LiAlH}}_{4}$ and $\text{Mg}{({\text{AlH}}_{4})}_{2}$; however, we also find that $\text{LiMg}{({\text{AlH}}_{4})}_{3}$ is unstable with respect to hydrogen release and decomposes exothermically into ${\text{LiMgAlH}}_{6}$, Al, and ${\text{H}}_{2}$ with a calculated $T=300\text{ }\text{K}$ enthalpy of $\ensuremath{-}7.3\text{ }\text{kJ}/(\text{mol}\text{ }{\text{H}}_{2})$, in excellent agreement with the weakly exothermic value of $\ensuremath{-}5\text{ }\text{kJ}/(\text{mol}\text{ }{\text{H}}_{2})$ obtained from differential scanning calorimetry measurements [M. Mamatha et al., J. Alloys Compd. 407, 78 (2006)]. ${\text{LiMgAlH}}_{6}$ is a stable intermediate, which has two competing endothermic decomposition pathways for ${\text{H}}_{2}$ release: one going directly into the binary hydrides of Li and Mg and the other proceeding via the formation of an intermediate ${\text{Li}}_{3}{\text{AlH}}_{6}$ phase, with room-temperature enthalpies of $+18.6$ and $+16.6\text{ }\text{kJ}/(\text{mol}\text{ }{\text{H}}_{2})$, respectively. Using database searching based on known crystal structures from the inorganic crystal structure database, we predict that the hypothetical ${\text{MgAlH}}_{5}$ compound should assume the orthorhombic ${\text{BaGaF}}_{5}$ prototype structure, in contrast to a previous DFT study of ${\text{MgAlH}}_{5}$, [A. Klaveness et al., Phys. Rev. B 73, 094122 (2006)]. However, the decomposition enthalpy of ${\text{MgAlH}}_{5}$ is only weakly endothermic, $+1.1\text{ }\text{kJ}/(\text{mol}\text{ }{\text{H}}_{2})$, and therefore this compound is not expected to occur in the high-temperature decomposition sequence of Mg alanate. We also present a comprehensive investigation of the phonon spectra and vibrational thermodynamics of Li-Mg-Al-H compounds, finding that vibrations typically decrease reaction enthalpies by up to $10\text{ }\text{kJ}/\text{mol}\text{ }{\text{H}}_{2}$ at ambient temperatures and significantly lower reaction entropies.