Abstract: Compared with traditional AB5-type alloys, La-Mg-Ni-based superlattice hydrogen storage alloys have superior electrochemical capacity, which has attracted extensive attention in recent years. In this work, an A2B7-type La0.75Mg0.25Ni3.5 alloy containing only (La,Mg)2Ni7 and (La,Mg)5Ni19 superlattice structure was prepared by annealing the as-cast alloy at 1 203 K for 40 h, and the effects of phase transformation on the electrochemical properties of the alloy has been studied. The results showed that the as-cast alloy contained (La,Mg)Ni3, (La,Mg)2Ni7 and (La,Mg)5Ni9 superlattice phases and LaNi5 phase. During annealing process, LaNi5 and (La,Mg)Ni3 phases at the relative edge of the phase diagram disappeared, resulting in the alloy containing only (La,Mg)2Ni7 and (La,Mg)5Ni9 superlattice phases. The increase of the superlattice phase amount was accompanied by the homogenization of the element composition and the reduction of stress, which not only effectively improved the hydrogen storage capacity of the alloy, but also reduced the pulverization and oxidation during charge/discharge cycling, thus significantly improving the cycling stability of the alloy electrode. The maximum discharge capacity of the alloy electrode increased from 355 mAh·g-1 of the as-cast alloy to 367 mAh·g-1 of the annealed alloy, and the cyclic stability significantly increased from 57.97% to 81.47%. However, because of the reduction of the defects and grain boundaries in the alloy, the high-rate discharge capacity of the alloy was slightly reduced.
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