Hydrogen release and structural transformations in LiNH2-MgH2 systems

Reactive hydride composites are good candidates for solid hydrogen storage due to their high gravimetric capacity, cyclability, and suitable thermodynamic properties. The LiNH2-MgH2 system is promising as changes in stoichiometry and milling conditions may result in tailoring of these properties. In this work, LiNH2-MgH2 with different ratios (Li2:Mg, Li:Mg) and ball milling conditions (100, 600 rpm) were investigated. Thermal desorption profiles shows hydrogen release starting at 125 degrees C for Li2:Mg 600 sample and at 225 degrees C for Li:Mg 600 sample, while for Li:Mg 100 sample simultaneous hydrogen and ammonia release at 175 degrees C is observed. In-situ synchrotron X-ray diffraction shows the related structural transformations, such as formation of Mg(NH2)(2) and allotropic transformation of alpha into beta-Li2Mg(NH)(2) for Li2:Mg 600 sample at 350 degrees C or direct formation of beta-Li2Mg(NH)(2) for Li:Mg 100 sample at 370 degrees C. Different polymorphs of the LiMgN phase were also observed during cooling for these two samples. For the Li:Mg 600 sample, transformation occurs in a unique reaction from an unknown phase into beta-Li2Mg(NH)(2) at 290 degrees C.