The super-stacking PuNi 3 -type La 0.67-x Nd x Mg 0.33 Ni 3.0 (x = 0, 0.12) alloys have been prepared using induction melting followed by annealing treatment method. The alloys are composed of PuNi 3 -type main phase and CaCu 5 -, MgCu 4 Sn-, Ce 5 Co 19 -and Gd 2 Co 7 -type minor phases in as-cast alloy. As annealing temperature increases from 850 to 950 • C, the minor phases gradually transform to the PuNi 3 -type phase via peritectic reactions, forming PuNi 3 -type single-phase La 0.67 Mg 0.33 Ni 3.0 alloy. Partial substitution of Nd for La has enhanced the phase stability of Gd 2 Co 7 -type phase, resulting in the existence of Gd 2 Co 7 -type phase as a secondary phase with the PuNi 3 -type main phase in the La 0.55 Nd 0.12 Mg 0.33 Ni 3.0 alloy after annealing treatment. It is found that Nd mainly replace La in [AB 5 ] slabs in the PuNi 3 -type structure, which increases the volume ratio of [A 2 B 4 ] slabs in the cell and decreases the volume change of [A 2 B 4 ] slabs during hydrogenation/dehydrogenation process, thus contributing to the improvement in cycling stability at the 100 th 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.In recent years, although lithium ion battery has been rapidly developed in the secondary battery market, the nickel/metal hydride (Ni/MH) battery still occupies a larger market and has a broad potential application because it can offer significant advantages of high capacity, environmental friendly and good safety features. 1-3 As negative electrode materials for the Ni/MH battery, La-Mg-Ni-based hydrogen storage with super-stacking structures have been on focus due to their higher discharge capacities than those of the commercially available AB 5 -type alloys. [4][5][6] La-Mg-Ni-based alloys have super-stacking structures with [AB 5 ] and [A 2 B 4 ] subunits stacking along c-axis in certain ratios forming various AB x phase, where x = (5n + 4)/(n + 2) and n is an integer (n = 1,2,3. . . ), with A = La, Mg and B = Ni, as shown in Fig. 1. Each AB x phase structure has two kinds of configurations: the hexagonal (2H) and rhombohedral (3R) structures, crystallized in space groups of P6 3 /mmc (no. 194) and R-3m (no. 166), respectively. 7 For n = 1, the structure stands for AB 3 -type phase. AB 3 -type La-Mg-Ni-based alloys show higher discharge capacities than those of the other superstacking AB x -type alloys, and have also applied as the Ni/MH battery negative electrodes. 8 However, the cycling stability and high rate dischargeability of these new Mg-containing alloys are still unsatisfied and need to be further improved.Partial substitution of La by Nd is reported an effective way to improve the electrochemical properties of La-Mg-Ni-based alloys. 9-11 Li et al. found that the discharge capacity of the low-Co La 0.80−x Nd ...