Electronic structure, stability and bonding of the Li-N-H hydrogen storage system

Abstract

The Li-N-H system holds great promise for on-board hydrogen storage applications, particularly due to reversible interactions among lithium amide (LiN H2), imide (Li2 NH), and hydride (LiH). However, practical applications of the system are hindered by the relatively high stabilities of the compounds and uncertainty of their reaction paths. Understanding the mechanism of hydrogen interactions with the host structures is essential for further development. Here, we calculated the electronic structures and total energies of lithium hydride (LiH), lithium imide (Li2 NH), and lithium amide (LiN H2) using a first-principles full potential approach. The estimated formation enthalpies for the two-step reactions, Li3 N+2 H2 ↔ Li2 NH+LiH+ H2 ↔LiN H2 +2LiH are -162.05 and -40.94 kJ/mol, comparable to the experimental values of -165 and -45.5 kJ/mol, respectively. The bonding interaction characteristics and the stability of these materials were further analyzed from the electronic structures. It is noted that the N atom bonds unequally with the two H atoms in lithium amide. As a result, the amide LiN H2 can dissociate in two almost equivalent transient steps: Li+ + (N H2) -; and (LiNH)- + H+. The reaction of the relevant species may evolve N H3 as a transient gas in the (LiN H2 +LiH) system. © 2006 The American Physical Society.