Electronic structure, stability and bonding of the Li-N-H hydrogen storage system
Abstract
The Li-N-H system holds great promise for on-board hydrogen storage applications, particularly due to reversible interactions among lithium amide $(\mathrm{Li}\mathrm{N}{\mathrm{H}}_{2})$, imide $({\mathrm{Li}}_{2}\mathrm{N}\mathrm{H})$, and hydride (LiH). However, practical applications of the system are hindered by the relatively high stabilities of the compounds and uncertainty of their reaction paths. Understanding the mechanism of hydrogen interactions with the host structures is essential for further development. Here, we calculated the electronic structures and total energies of lithium hydride (LiH), lithium imide $({\mathrm{Li}}_{2}\mathrm{N}\mathrm{H})$, and lithium amide $(\mathrm{Li}\mathrm{N}{\mathrm{H}}_{2})$ using a first-principles full potential approach. The estimated formation enthalpies for the two-step reactions, ${\mathrm{Li}}_{3}\mathrm{N}+2{\mathrm{H}}_{2}\ensuremath{\leftrightarrow}{\mathrm{Li}}_{2}\mathrm{N}\mathrm{H}+\mathrm{Li}\mathrm{H}+{\mathrm{H}}_{2}\ensuremath{\leftrightarrow}\mathrm{Li}\mathrm{N}{\mathrm{H}}_{2}+2\mathrm{Li}\mathrm{H}$ are $\ensuremath{-}162.05$ and $\ensuremath{-}40.94\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$, comparable to the experimental values of $\ensuremath{-}165$ and $\ensuremath{-}45.5\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$, respectively. The bonding interaction characteristics and the stability of these materials were further analyzed from the electronic structures. It is noted that the N atom bonds unequally with the two H atoms in lithium amide. As a result, the amide $\mathrm{Li}\mathrm{N}{\mathrm{H}}_{2}$ can dissociate in two almost equivalent transient steps: ${\mathrm{Li}}^{+}+{(\mathrm{N}{\mathrm{H}}_{2})}^{\ensuremath{-}}$; and ${(\mathrm{Li}\mathrm{N}\mathrm{H})}^{\ensuremath{-}}+{\mathrm{H}}^{+}$. The reaction of the relevant species may evolve $\mathrm{N}{\mathrm{H}}_{3}$ as a transient gas in the $(\mathrm{Li}\mathrm{N}{\mathrm{H}}_{2}+\mathrm{Li}\mathrm{H})$ system.