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 LiMg (AlH4) 3 compound is stable with respect to solid-state decomposition into LiAlH4 and Mg (AlH4) 2; however, we also find that LiMg (AlH4) 3 is unstable with respect to hydrogen release and decomposes exothermically into LiMgAlH6, Al, and H2 with a calculated T=300 K enthalpy of -7.3 kJ/ (mol H2), in excellent agreement with the weakly exothermic value of -5 kJ/ (mol H2) obtained from differential scanning calorimetry measurements. LiMgAlH6 is a stable intermediate, which has two competing endothermic decomposition pathways for H2 release: one going directly into the binary hydrides of Li and Mg and the other proceeding via the formation of an intermediate Li3 AlH6 phase, with room-temperature enthalpies of +18.6 and +16.6 kJ/ (mol H2), respectively. Using database searching based on known crystal structures from the inorganic crystal structure database, we predict that the hypothetical MgAlH5 compound should assume the orthorhombic BaGaF5 prototype structure, in contrast to a previous DFT study of MgAlH5,. However, the decomposition enthalpy of MgAlH5 is only weakly endothermic, +1.1 kJ/ (mol H2), 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 kJ/mol H2 at ambient temperatures and significantly lower reaction entropies.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - May 1 2009|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics