Hydrogenation Behavior of Mg–Li Alloys

Magnesium-based alloys have been investigated for many years as potential hydrogen storage materials. Owing to the different natures (phase compositions) of magnesium alloys and the significant number of alloying/catalytic additives used, gaps in the basic knowledge regarding the interaction of hydrogen with magnesium alloys still exist. The “solid solution case” is a quite rare example since magnesium does not have wide solid solubility regions with many elements. However, in the Mg–Li system, only cubic Mg in Li or hexagonal Li in Mg solid solutions and their mixtures are present. Moreover, these two elements form stable ionic and ionic–covalent hydrides. In such cases, whether the product of the hydrogenation of such solid solution-based alloys is a mixture of basic hydrides or a mixture of hydrides with different properties due to interactions between them is not obvious. To answer this question, we conducted a systematic study of the hydrogenation of magnesium–lithium alloys with significantly different chemical and phase compositions. The results of our research show that the kinetics of the reaction of alloys with hydrogen in the gas phase are significantly influenced by the phase composition of the alloys, but the final product (within the sensitivity range of our measurements) of hydrogenation is always a mixture of magnesium and lithium hydrides. From such a mixture, MgH2 can be thermally decomposed under reasonable conditions (up to 450 °C), while LiH remains stable. The thermal decomposition of the hydrogenated samples demonstrated significant differences in the temperatures of desorption of magnesium hydride for each sample. The desorbed amount of hydrogen decreases with increasing lithium content in the samples. Despite the interesting findings of this study, from a practical point of view, magnesium–lithium alloys are not good candidates for reversible hydrogen storage because their reversibility and stability are very similar to those of pure magnesium.