Investigation of the processes for reversible hydrogen storage in the Li–Mg–N–H system

The hydrogen storage performances of the Li–Mg–N–H system are investigated starting either from 1:2 Mg(NH2)2–LiH or 1:2 MgH2–LiNH2 ball-milled mixtures. It is shown that, for 1:2 MgH2–LiNH2, an ammonia release occurs if the first heating is conducted under a dynamic vacuum, leading to a fast degradation of the material. The positive role of LiH, if initially present in the mixture, is therefore emphasized as LiH rapidly reacts with ammonia and avoids the contamination of the hydrogen desorbing flow. The desorption kinetics of the ball-milled 1:2 Mg(NH2)2–LiH mixture are fast: a total amount of 5.0 wt.% of hydrogen is desorbed in 25 min at 220 °C. This material exhibits a nice reversibility at 200 °C with an experimental capacity around 4.8 wt.%. Preliminary results are given on the structure of Li2Mg(NH)2, formed upon desorption: this phase crystallizes in a cubic unit cell with a lattice parameter of 10.06(1) Å. In addition, by plotting an absorption isotherm of the Li2Mg(NH)2 phase at 200 °C, two pressure plateaus are observed revealing the existence of an intermediary phase between Li2Mg(NH)2 and the rehydrided material, which is the 1:2 Mg(NH2)2–LiH mixture.