Decomposition of lithium magnesium aluminum hydride

Abstract

The quaternary aluminum hydride LiMg(AlH 4) 3 contains 9.7 wt% hydrogen, of which 7.2 wt% can be released in a two-step decomposition reaction via first formation of LiMgAlH 6 and then the binary hydrides MgH 2 and LiH. In-situ synchrotron radiation powder X–ray diffraction and thermal desorption spectroscopy measurements were performed to analyze the product distributions formed during the thermal decomposition of LiMg(AlD 4) 3. The first decomposition step occurs at about 120 °C and the second at about 160 °C for the as-milled sample, while for a purified sample of LiMg(AlD 4) 3, the decomposition temperatures involving release of hydrogen increase to 140 and 190 °C, respectively, suggesting that pure samples of LiMg(AlD 4) 3 are kinetically stabilized. Studies of the purified LiMg(AlD 4) 3 also showed that the second decomposition step can be divided into two reactions: 3LiMgAlD 6 → Li 3AlD 6 + 3MgD 2 + 2Al + 3D 2 and Li 3AlD 6 → 3LiD + Al + 3/2D 2. Addition of TiCl 3 to LiMg(AlD 4) 3 under a variety of ball milling conditions consistently led to decomposition of LiMg(AlD 4) 3 during milling. Correspondingly, all attempts to rehydrogenate the (completely or partially) decomposed samples at up to 200 bar hydrogen pressure failed. Decomposition of MgD 2 was observed at relatively low temperatures. This is ascribed to thermodynamic destabilization due to the formation of different Al xMg y phases, and to kinetic destabilization by addition of TiCl 3. A thermodynamic assessment was established for the calculation of phase stability and decomposition reaction relationships within the Li–Mg–Al−H system. The calculations confirmed the metastability of the LiMg(AlH 4) 3 phase and the irreversibility of the Li–Mg alanate phase decomposition reactions. The Li–Mg alanate decomposition pathways followed experimentally could be explained by the endothermicity of the calculated decomposition enthalpies, in that an impure or catalyzed LiMgAlH 6 intermediate phase could more directly access an endothermic decomposition reaction at lower temperatures, while a kinetically-hindered, purified LiMgAlH 6 would require higher temperatures to initiate the two-step decomposition through an exothermic reaction.