First-Principles Study of the Hydrogenation Process of Li2NH

On the basis of ab initio simulations, we identified possible microscopic pathways for the Li2NH + H-2 -> LiNH2 + LiH rehydrogenation reaction of Li imide. Dissociative chemisorption of H-2 at the Li2NH(001) surface is an activated process with an energy barrier of 0.5 eV. The heterolytic splitting of H-2 leads to the formation of a subsurface NH2- group and a H- ion coordinated by Li+ surface ions. Surface diffusion of H- to form LiH is rate-limiting, whereas H+ (NH2-) and Li+ diffusion in the bulk to sustain the rehydrogenation reaction is easier. Heterolytic splitting of H-2 is possible also in the bulk, but insertion of H-2 in bulk sites is more costly than the activation energy for dissociative chemisorption at the surface, which suggests that rehydrogenation proceeds from the surface into the bulk. The large difference in lattice parameters between LiNH2/Li2NH and LiH suggests that LiH cannot grow epitaxially on Li imide/amide but flakes off, providing a free surface of the imide for hydrogen adsorption and dissociation.