Native defects in LiNH2: A first-principles study

Abstract

Native defects in lithium amide (LiNH${}_{2}$), a promising candidate for hydrogen storage, are investigated by first-principles calculations based on density functional theory. We examine the structural properties and formation energies of H-, Li-, and N-related defects in all possible states. We find that the dominant H- and Li-related defects are in charged states, i.e., negatively charged H vacancy (V${}_{\mathrm{H}}$${}^{\ensuremath{-}}$), positively charged H interstitial (H${}_{i}$${}^{+}$), negatively charged Li vacancy (V${}_{\mathrm{Li}}$${}^{\ensuremath{-}}$), and positively charged Li interstitial (I${}_{\mathrm{Li}}$${}^{+}$). ${\mathrm{V}}_{\mathrm{Li}}$${}^{\ensuremath{-}}$ and ${\mathrm{I}}_{\mathrm{Li}}$${}^{+}$ are present in the highest concentration. The positively charged NH${}_{2}$ vacancy has the lowest formation energy among N-related defects. Furthermore, migration processes of the dominant defects are investigated. ${\mathrm{V}}_{\mathrm{Li}}$${}^{\ensuremath{-}}$ diffuses most rapidly with the lowest migration energy of 0.20 eV. Both formation and migration energies of Li-related dominant defects are found to be lower than those of H-related dominant defects. With an activation energy of 0.72 eV, ${\mathrm{V}}_{\mathrm{Li}}$${}^{\ensuremath{-}}$ is the major diffusive species in LiNH${}_{2}$. Our results further indicate that the formation of ${\mathrm{H}}_{i}$ is the bottleneck for H transport.