Effect and mechanism of Mg on crystal structures and electrochemical cyclic stability of Ce2Ni7-type LaMgNi-based hydrogen storage alloys

LaNi3.50 and La0.80Mg0.20Ni3.50 hydrogen storage alloys with single-phase Ce2Ni7-type structure are prepared, respectively. It is found that the expansion of [A2B4] subunits volumes of Mg-containing alloy is bigger than that of Mg-free alloy, while the expansion of [AB5] subunits volumes of the two alloys are almost the same, hence the deviation value of [A2B4] and [AB5] subunits volumes and strain value of super stacking structure for Mg-containing alloy are obviously higher than those of Mg-free alloy, and Mg-containing alloy is easier to be pulverized during charge/discharge cycles. Mg-containing alloy has more apparent cracks on the surface of alloy at the same cycle numbers, which demonstrate that Mg-containing alloy has poorer anti-pulverization. Besides, the presence of Mg hydroxides with a regular hexagon shape makes the surface of Mg-containing alloy become loose enough to be easily oxidized after charge/discharge cycles. The corrosion current density of Mg-containing alloy is larger than that of Mg-free alloy, so Mg-containing alloy is easier to be oxidized. Therefore, reducing the deviation value of [A2B4] and [AB5] subunits of super stacking structure and the corrosion of alloying element Mg is the basic solution to enhance the electrochemical cycling stability of LaMgNi alloy.