The particle-size dependence of the activation energy for decomposition of lithium amide.

thus suggesting that the thermodynamics and kinetics of the decomposition depend on the particle size. The high surfaceto-volume ratio of nanoparticles that result from the ballmilling process not only leads to an increase in the number of surface-active sites for reaction, but may also affect the actual reaction mechanisms. Based on results of ab initio calculations for native point defects and defect complexes in LiNH2, we propose herein that the decomposition of LiNH2 into lithium imide (Li2NH) and ammonia (NH3) occurs through two competing mechanisms, one that involves the formation of native defects in the interior of the material and the other at the surface. As a result, the prevailing mechanism and hence the activation energy depend on the surface-to-volume ratio, or the specific surface area (SSA), which changes with the particle size. We explain the observed variation of activation energy with ball milling, and address the role played by LiH in the dehydrogenation of (LiNH2+LiH) mixtures. At temperatures below 300 8C, LiNH2 reversibly stores approximately 6.5 wt% hydrogen during absorption under 20 bar followed by desorption under 0.04 bar, according to the following reaction [Eq. (1)]: