Theoretical prediction of low-energy crystal structures and hydrogen storage energetics inLi2NH

Abstract

We illustrate a systematic search method for determining hydrogen atomic arrangements in the hydrogen storage material, ${\mathrm{Li}}_{2}\mathrm{N}\mathrm{H}$. Our method relies on total-energy density-functional calculations, and yields a minimum-energy crystal structure for ${\mathrm{Li}}_{2}\mathrm{N}\mathrm{H}$ as well as several low-energy metastable structures. Linear-response calculated Born effective charges show strong ionic interactions between the Li and N-H dimers, while the bonding between the N and H has covalent character. Including vibrational contributions, our orthorhombic $Pnma$ structure yields a hydrogen storage ${\mathrm{Li}}_{2}\mathrm{N}\mathrm{H}∕\mathrm{Li}\mathrm{N}{\mathrm{H}}_{2}$ reaction enthalpy of $63.7\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}{\mathrm{H}}_{2}$ at $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $74.8\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}{\mathrm{H}}_{2}$ at $T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in good agreement with experimental reports of $\ensuremath{\sim}66\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}\phantom{\rule{0.2em}{0ex}}{\mathrm{H}}_{2}$ for this reaction.