Ab-initio Investigations of Li and Mg Amide-Imide Systems for Hydrogen Storage

Reversible hydrogen storage in light-element materials has been recognized as one of the most practical approaches for on-board application. Lithium nitride Li$_3$N can reversibly store large amount of hydrogen in the two-step reversible reaction composed of lithium amide LiNH$_2$ and imide Li$_2$NH[1]. Quite recently, in an effort to reach further performance, several types of magnesium substitutions in Li-N-H system have been investigated. For instance, Leng $et$ $al$. have examined a composite material made by ball milling of 3:8 molar mixture of magnesium amide Mg(NH$_2$)$_2$ and lithium hydride LiH[2]. The hydrogenating and dehydrogenating reaction mechanism and fundamental properties of these hydrides still remain as a matter to be investigated. In particular, crystal structures of some metel imides such as Li$_2$NH, MgNH and Li$_2$Mg(NH)$_2$ are not fully determined yet. In this paper, we discuss structural stability and heats of formation of these hydrides from first-principles calculations based on the all-electron FLAPW method. [1] P. Chen Z. Xiong, J. Luo, J. Lin and K.L. Tan, Nature 420, 302 (2002). [2] H. Y. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe and H. Fujii, J. Phys. Chem. B 108, 8763 (2004).