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 °C for Li2:Mg 600 sample and at 225 °C for Li:Mg 600 sample, while for Li:Mg 100 sample simultaneous hydrogen and ammonia release at 175 °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 α into β-Li2Mg(NH)2 for Li2:Mg 600 sample at 350 °C or direct formation of β-Li2Mg(NH)2 for Li:Mg 100 sample at 370 °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 β-Li2Mg(NH)2 at 290 °C. The unknown phase is indexed as a Fm3m cubic similar to the high temperature γ-Li2Mg(NH)2.

Research highlights▶ H2 release in LiNH2–MgH2 system depends on Li:Mg ratio and ball milling conditions (rpm). ▶ Li:Mg (600 rpm) forms an unknown phase (Fm3m cubic) during ball milling. ▶ Li:Mg (100 rpm) releases simultaneously H2 and NH3. ▶ Li2:Mg (600 rpm) and Li:Mg (100 rpm) decompose into Li2Mg(NH)2 and LiMgN through intermediate Mg(NH2)2. ▶ Li:Mg at 600 rpm presents a unique deydrogenation reaction into Li2Mg(NH)2.