Role of native defects in the Li amide/Li imide hydrogen storage reaction

Reversible reaction involving Li amide/Li imide (LiNH$_{2}$ + LiH $\leftrightarrow $ Li$_{2}$NH + H$_{2})$ has been shown to be a potential mechanism for hydrogen storage [1]. Recent synchrotron x-ray diffraction refinement suggests that the transformation between LiNH$_{2}$ and Li$_{2}$NH is a bulk reaction that occurs through non-stoichiometric processes [2]. To build a deeper understanding of these processes, we have carried out first-principles studies based on density functional theory of native point defects and defect complexes in LiNH$_{2}$ and Li$_{2}$NH. Among the native defects, we find that positively and negatively charged Li and H interstitials and vacancies have the lowest formation energies. Some of the Li-related defects are found to be very mobile, and should be the dominant migratory species in the systems. Our first-principles results suggest specific mechanisms for the role of native defects in the Li amide/Li imide reaction. [1] P. Chen \textit{et al.}, Nature \textbf{420}, 302 (2002). [2] W. I. F. David \textit{et al.}, J. Am. Chem. Soc. \textbf{129}, 1594 (2007).