Bonding mechanism and stability of lithium hydrides

Abstract

Li-N-H system holds great promise for hydrogen storage applications. Particularly, reversible hydrogenation/dehydrogenation has been realized from the interactions between lithium amide, LiNH2, lithium imide, Li2NH, and lithium hydride, LiH. However, the temperature for effective "sorption" of hydrogen is still relatively high and the reversibility is poor for practical considerations. Understanding of the bonding mechanism is essential to develop and design a desirable material system. Here we calculate the electronic structures and total energies of lithium imide (Li2NH), lithium amide (LiNH2), and lithium hydride (LiH) with the first principles full potential method to benchmark the energetics of the system. The estimated formation enthalpy for the reactions Li2NH + LiH + H2↔ LiNH2+ 2LiH is -43.396 kJ/mol, which is very close to the experimental values of -45.5 kJ/mol. The bonding mechanism and the stability of these materials were studied based on the electronic structures. The bonding between nitrogen and hydrogen atoms is noted to be much stronger in lithium imide than in lithium amide. The implication of the bonding characteristics for hydrogen sorption is discussed.