Electronic structure and optical properties of lightweight metal hydrides

We study the electronic structures and dielectric functions of the simple hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6, LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory and GW calculations. All these compounds are large gap insulators with GW single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4 compounds. The valence bands are dominated by the hydrogen atoms, whereas the conduction bands have mixed contributions from the hydrogens and the metal cations. The electronic structure of the aluminium compounds is determined mainly by aluminium hydride complexes and their mutual interactions. Despite considerable differences between the band structures and the band gaps of the various compounds, their optical responses are qualitatively similar. In most of the spectra the optical absorption rises sharply above 6 eV and has a strong peak around 8 eV. The quantitative differences in the optical spectra are interpreted in terms of the structure and the electronic structure of the compounds.Comment: 13 pages, 10 figure

Electronic Structure of the Complex Hydride NaAlH4

Density functional calculations of the electronic structure of the complex hydride NaAlH4 and the reference systems NaH and AlH3 are reported. We find a substantially ionic electronic structure for NaAlH4, which emphasizes the importance of solid state effects in this material. The relaxed hydrogen positions in NaAlH4 are in good agreement with recent experiment. The electronic structure of AlH3 is also ionic. Implications for the binding of complex hydrides are discussed.Comment: 4 pages, 5 figure

Comparative Investigation of MgMnSiO₄ and Olivine-Type MgMnSiS₄ as Cathode Materials for Mg Batteries

The identification of potential cathode materials is necessary for the development of a new magnesium-based battery technology. Most attempts focus on oxide and sulfide materials, which in general suffer, respectively, of poor Mg mobility and low intercalation voltage. New chemistries should be explored. In this work, we investigate the basic electrode characteristics of olivine-type thiosilicates MgMSiS₄ (M = Fe, Mn) with the double challenge of (1) raising the low intercalation voltage of transition metal sulfides and (2) improving the poor Mg diffusion of the oxosilicate counterparts MgMSiO₄ (M = Fe, Mn). Density functional theory (DFT) calculations corroborate both expectations. The calculated average Mg deintercalation voltage for MgMnSiS₄ (2.31 V) is above that of virtual MgMnS₂ compounds (around 1.8 V), accounting for the inductive effect of the Si⁴⁺ ion on the transition metal. The calculated energy barriers for Mg diffusion in Mg_<i>x</i>MnSiO₄ are 0.75 eV at <i>x</i> ∼ 1 and higher than 1.1 eV at <i>x</i> ∼ 0 and <i>x</i> = 0.5. The energy barriers decrease to 0.68 and 0.76 eV in Mg_<i>x</i>MnSiS₄ (<i>x</i> ∼ 0, 1), thanks to the more covalent Mn–S bond (compared to the Mn–O bond) that renders less oxidized Mn ions, therefore favoring Mg²⁺ mobility. Although these results are promising, more work is needed to ensure the potential application of thiosilicates as cathode materials for Mg batteries