First-principles study on lithium amide for hydrogen storage

Abstract

The fundamental properties of lithium amide $\mathrm{Li}\mathrm{N}{\mathrm{H}}_2$, which is fully hydrogenated phase of lithium nitride ${\mathrm{Li}}_3\mathrm{N}$, have been investigated by the first-principles calculations using the ultrasoft pseudopotential method, including the structural, electronic, dielectric, and vibrational properties. The calculated structural parameters agree well with the experimental data except for hydrogen positions. The analyses for the electronic structure and the Born effective charge tensors indicate an ionic feature between ${\mathrm{Li}}^{+}$ and ${\left[\mathrm{N}{\mathrm{H}}_2\right]}^{-}$. The internal bonding of ${\left[\mathrm{N}{\mathrm{H}}_2\right]}^{-}$ anions is primarily covalent. The internal $\mathrm{N}-\mathrm{H}$ bending and stretching vibrations of ${\left[\mathrm{N}{\mathrm{H}}_2\right]}^{-}$ anions yield $\Gamma$-phonon modes around 1500 and $3400{\mathrm{cm}}^{-1}$, respectively. These can be fairly reproduced by the molecular approximation, suggesting a strong internal bonding of ${\left[\mathrm{N}{\mathrm{H}}_2\right]}^{-}$ anions. The heat of formation for the fully hydriding reaction of ${\mathrm{Li}}_3\mathrm{N}$ is predicted as $-85\mathrm{kJ}∕\mathrm{mol}$ ${\mathrm{H}}_2$ which agrees well with the experimental value. Some discussions are also presented for the properties of ${\mathrm{Li}}_3\mathrm{N}$.