Reaction Pathways in the Reactive Composite Mg(NH$_{2})_{2}$ + LiH

Chen \textit{et al} [1] reported reversible hydrogen storage in a mixture of LiH + LiNH$_{2}$ with a storage capacity of 6.5 wt {\%}. However, this system requires an operating temperature in excess of 250 C to achieve a hydrogen pressure of 1 bar. Several efforts including cation substitution have been considered in order to improve the operating conditions, which is necessary for onboard applications. For instance, replacing LiH with MgH$_{2}$ markedly reduces the operating temperature through the reaction MgH$_{2 }$+ 2LiNH$_{2} \quad \to $Li$_{2}$Mg(NH)$_{2 }$+ 2H$_{2} \quad \leftrightarrow $Mg(NH$_{2})_{2}$ + 2LiH. Recent experimental results however indicate that the latter is not a simple one-step reaction and full hydrogenation of Li$_{2}$Mg(NH)$_{2}$ occurs in a two-step sequence via an intermediate Li$_{2}$Mg$_{2}$(NH)$_{3}$ [2]. In this work we examine the stability and structure of possible intermediates compounds, namely Li$_{2-2x}$Mg$_{x}$NH, Li$_{1-2x}$Mg$_{x}$NH$_{2}$, and Li$_{2-x}$Mg(NH)$_{2-x}$(NH$_{2})_{x}$, by means of first-principles DFT calculations. All intermediate compounds are thermodynamically stable with respect to the elements. The hydrogenation reaction of Li$_{2}$Mg(NH)$_{2}$ via the intermediate imides Li$_{2-2x}$Mg$_{x}$NH is energetically favorable compared to other intermediates.\\[0pt] Ref~: [1] Nature \textbf{420}, 302 (2002). [2] J. Phys. Chem. C \textbf{113}, 15772 (2009).