Effect of Nd on Subunits Structure and Electrochemical Properties of Super-Stacking PuNi3-Type La–Mg–Ni-Based Alloys

Abstract

The super-stacking PuNi3-type La0.67-xNdxMg0.33Ni3.0 (x = 0, 0.12) alloys have been prepared using induction melting followed by annealing treatment method. The alloys are composed of PuNi3-type main phase and CaCu5-, MgCu4Sn-, Ce5Co19- and Gd2Co7-type minor phases in as-cast alloy. As annealing temperature increases from 850 to 950°C, the minor phases gradually transform to the PuNi3-type phase via peritectic reactions, forming PuNi3-type single-phase La0.67Mg0.33Ni3.0 alloy. Partial substitution of Nd for La has enhanced the phase stability of Gd2Co7-type phase, resulting in the existence of Gd2Co7-type phase as a secondary phase with the PuNi3-type main phase in the La0.55Nd0.12Mg0.33Ni3.0 alloy after annealing treatment. It is found that Nd mainly replace La in [AB5] slabs in the PuNi3-type structure, which increases the volume ratio of [A2B4] slabs in the cell and decreases the volume change of [A2B4] slabs during hydrogenation/dehydrogenation process, thus contributing to the improvement in cycling stability at the 100th cycle of the alloy, from 76.3% to 80.3%. Moreover, the cell volume contraction due to Nd substitution heightens the discharge plateau pressure and potential of the alloy electrode, and helps to the enhancement in high rate dischargeability (HRD) from 55.7% to 68.1% at a 1200 mA g−1 discharge current density.