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 ...

In recent years, ternary La-Mg-Ni-based hydrogen storage alloys have attracted attention as promising electrode materials for nickel-metal hydride batteries. A La 0.77 Mg 0.23 Ni 3.72 alloy with the both rhombohedral (3R) Gd 2 Co 7 -type and Ce 5 Co 19 -type phases was successfully prepared by induction melting followed by annealing treatment method at 950 • C. Via annealing treatment for 54 h, the CaCu 5 -type, Ce 2 Ni 7 -type, Gd 2 Co 7 -type, and Ce 5 Co 19 -type phases of the as-cast alloy converted into the rhombohedral Gd 2 Co 7 -type and Ce 5 Co 19 -type phases. The electrochemical properties of the rhombohedral-phases alloy electrode exhibited an excellent maximum discharge capacity (388.8 mAh g -1 ) and high rate discharge ability (53.1% at a 1440 mA g -1 discharge current density). The discharge capacity retention at the 100 th cycle is particularly heightened to 86.2%. X-ray diffraction and scanning electron microscope analyses verify that the annealed alloy comprised of rhombohedral-phases exhibits a larger crystallite size and smaller strain after 100 th charge/discharge cycles than those of the alloys comprised of a mixture of hexagonal and rhombohedra polymorphic phases. Moreover, the corrosion current density of the alloy is reduced (23.53 mA cm −2 ) in an alkaline electrolyte. The superior properties of the alloy may be attributed to the R-3m space group Gd 2 Co 7 -type and Ce 5 Co 19 -type phases possessing high structural matching.Nickel metal hydride (Ni/MH) batteries have been extensively used as high capacity secondary batteries due to their favorable resistance to overcharge and overdischarge performance, good safety properties and environmental benignity. 1,2 As negative electrode materials for the Ni/MH batteries, the AB 5 -type rare earth-based hydrogen storage alloys have been commercialized, but they suffer from limited discharge capacity in the range 310-330 mAh g −1 . 2 In recent years, ternary rare earth (RE)-Mg-Ni-based hydrogen storage alloys have attracted much attention as promising candidates to replace AB 5 -type alloys owing to their remarkable discharge capacity and high-rate dischargeability. [3][4][5] The ternary RE-Mg-Ni x (2 < x < 4) alloys have layered AB 3.0 , AB 3.5 , and AB 3.8 structures, where [AB 5 ] and [A 2 B 4 ] units are alternatively stacked along the c axis with different proportions. 6,7 Each structure has an allotrope with either a hexagonal (2H) structure with MgZn 2 -type Laves subunits or a rhombohedral (3R) structure with MgCu 2 -type Laves subunits, e.g., CeNi 3 -type (2H) and PuNi 3 -type (3R), Ce 2 Ni 7 -type (2H) and Gd 2 Co 7 -type (3R), and Pr 5 Co 19 -type (2H) and Ce 5 Co 19 -type (3R) structures. 8,9 The 2H and 3R structures in each allotrope possess the same proportions of [AB 5 ] and [A 2 B 4 ] units, but the stacking sequence of these units along c axis is different. Therefore, the 2H-type and 3R-type phases may have different effects on the electrochemical performance of La-Mg-Ni-based alloys. Zhang et al. found that the equilibrium pressu...

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