Hydrogen desorption mechanism of a Li–N–H hydrogen storage system

Alkali metal amides may exist in solution, the solid phase, and even the gas phase. Based on a theoretical model of a Li₃N system which adsorbs and desorbs two hydrogen molecules, we examine the possible pathways of the ${\text{Li}}_3\text{N}+2{\text{H}}_2\leftrightarrow {\text{LiNH}}_2+2\text{LiH}$ reversible reaction. The dehydrogenation process can be separated into two-step reactions, ${\text{Li}}_2\text{NH}+\text{LiH}\to {\text{Li}}_3\text{N}+{\text{H}}_2$ (−9.5 kcal/mol exothermic) and ${\text{LiNH}}_2+\text{LiH}\to {\text{Li}}_2\text{NH}+{\text{H}}_2$ (+0.7 kcal/mol endothermic). Along the reaction pathway, two intermediates and a transition state for each reaction were found in our ab initio molecular orbital calculations at the MP2 and CCSD(T) levels of theory. A total of two H₂ molecules can be stored and released at normal temperature and pressure if there are means to substantially raise the energy of the two stable intermediates. Reaction energy profiles from our calculations support the much higher temperature of the first step reaction in experiment.