Improved hydrogen storage kinetics of the Li-Mg-N-H system by addition of Mg(BH sub(4)) sub(2)

A Mg(BH sub(4)) sub(2)-added Mg(NH sub(2)) sub(2)-2LiH system was prepared by ball milling the corresponding chemicals. The hydrogen storage properties of the Mg(NH sub(2)) sub(2)-2LiH-xMg(BH sub(4)) sub(2) (x= 0, 0.1, 0.2, 0.3) samples and the role played by Mg(BH sub(4)) sub(2) were systematically investigated. The results show that the onset and peak temperatures for hydrogen desorption from the Mg(BH sub(4)) sub(2)-added Mg(NH sub(2)) sub(2)-2LiH sample shifted to lower temperatures. In particular, the Mg(NH sub(2)) sub(2)-2LiH-0.1Mg(BH sub(4)) sub(2) sample could reversibly absorb similar to 4.5 wt% of hydrogen in the temperature range of 120-150 degree C, which is superior to the pristine sample. During ball milling, a metathesis reaction between Mg(BH sub(4)) sub(2) and LiH readily occurred to form LiBH sub(4) and MgH sub(2) and subsequently, the newly formed MgH sub(2) reacted with Mg(NH sub(2)) sub(2) to generate MgNH. Upon heating, the presence of LiBH sub(4) not only decreased the recrystallization temperature of Mg(NH sub(2)) sub(2) but also reacted with LiNH sub(2) to form the Li sub(4)(BH sub(4))(NH sub(2)) sub(3) intermediate, which weakens the N-H bonding and enhances the ion conductivity. Meanwhile, MgNH may act as the nucleation center for the dehydrogenation product of Li sub(2)MgN sub(2)H sub(2) due to the structural similarity. Thus, the in situ formed LiBH sub(4) and MgNH provide a synergetic effect to improve the hydrogen storage performances of the Mg(NH sub(2)) sub(2)-2LiH system.