Intrinsic defects and dopants in LiNH2: a first-principles study

Abstract

The lithium amide (LiNH₂) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH₂ using density functional theory. We find that both Li-related defects (the positive interstitial Li⁺_i and the negative vacancy V⁻_Li) and H-related defects (H⁺_i and V⁻_H) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3–0.5 eV, they diffuse rapidly at moderate temperatures. V⁻_H corresponds to the [NH]²⁻ ion. It is the dominant species available for proton transport with a diffusion barrier of ∼0.7 eV. The equilibrium concentration of H⁺_i, which corresponds to the NH₃ molecule, is negligible in bulk LiNH₂. Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH₂ lattice, which may affect the crystal morphology on the nano-scale.