Effect of Magnesium on the Crystal Transformation and Electrochemical Properties of A₂B₇-Type Metal Hydride Alloys

Abstract

The effect of Mg content on crystal transformation and electrochemical characteristics of La 2-x Mg x Ni 7 (x = 0.40-0.60) alloys has been studied. It is found that alloys with allotropic structures Ce 2 Ni 7 -type (2H) and Gd 2 Co 7 -type (3R) phases exist in a range of Mg concentration from 0.48 to 0.50. Beyond that range, further increasing Mg to 0.60 is favorable for the formation of PuNi 3 -type phase due to the rising ratio of [A 2 B 4 ] to [AB 5 ] slabs in regard to the demand of La and Ni atoms for Mg to assemble [A 2 B 4 ] slabs. Conversely, the surplus of La and Ni atoms for Mg to assemble [A 2 B 4 ] slabs lowers the ratio of [A 2 B 4 ] to [AB 5 ] slabs, which helps the formation of CaCu 5 -type phase as Mg decreases to 0.40. Electrochemical studies show that the electrochemical properties of the alloy electrodes with allotropic phases are superior in discharge capacity (388 mAh g −1 ) and cycling stability (78% capacity retaining rate at 100 th cycle). Appearance of PuNi 3 -type or CaCu 5 -type phase reduces the cycling stability, while CaCu 5 -type phase can significantly improve the high rate dischargeablity (HRD) of the alloy electrode.Metal hydride/nickel (MH/Ni) batteries have been widely used due to their high energy density, durability to over charge/discharge, and environmental friendliness. 1-5 As the battery cathode materials, the commercialized AB 5 -type alloys, however, are subjected to low discharge capacity, restricting the development of MH/Ni batteries. 6,7 Recently, La-Mg-Ni-based hydrogen storage alloys with AB 3 -type or A 2 B 7 -type superlattice structures have been on focus for their higher discharge capacities than AB 5 -type alloys (reaching to 410 mAh g −1 , 30% superior compared to that of the AB 5 -type alloy electrodes), especially the A 2 B 7 -type La-Mg-Ni-based alloys of which studies have shown that their cycling stabilities are superior than that of AB 3 -type alloys. [8][9][10][11] Ternary A 2 B 7 -type La-Mg-Ni alloys relevant for electrochemical applications are related to the binary La-Ni intermetallic alloys, possessing two allotropes of hexagonal (2H, Ce 2 Ni 7 -type) and rhombohedral (3R, Gd 2 Co 7 -type) superlattice structures, which both are composed of a layered structure of Laves-type and CaCu 5 -type subunits stacking along c-axis by 1: 2 ratio. The Laves unit of 2H-type crystal structure is MgZn 2 -type, and that of 3R-type crystal structure is MgCu 2 -type. When Mg atoms are added to the La-Ni system, they preferentially occupy La sites in Laves-type subunit rather than CaCu 5 -type subunit, forming ternary (La,Mg) 2 Ni 7 alloys.The introduction of Mg is important, which can improve hydrogen absorption/desorption pressures and enhance hydrogen storage capacities of the alloys, and the alloys possess superior electrochemical discharge capacities compared to binary La-Ni alloys, making them preferable for using as the electrode materials. 4,12 Therefore, people have focused on studying the effect of amount of Mg on the phase structures and overall proper...

Study on solid solubility of Mg in Pr 3−x Mg x Ni 9 and electrochemical properties of PuNi 3 -type single-phase RE–Mg–Ni (RE = La, Pr, Nd) hydrogen storage alloys

“…The XRD profiles of the samples were analyzed by Rietveld refinement program RIETICA [24]. The degree of the refinement was judged by S: S = R wp /R e , where R wp was the residue of the weighted pattern and R e was the statistically expected residue value.…”

The Influence of Pr and Nd Substitution on Hydrogen Storage Properties of Mechanically Alloyed (La,Mg)2Ni7-Type Alloys

“…( Ref 1,[3][4][5]. For example, the effect of the Mg content on the properties of the La 2Àx Mg x Ni 7 system was investigated (Ref [6][7][8]. The concentration of Mg in the basic system is very impactful on the final phase composition.…”

Phase Structure and Electrochemical Characteristics of Rhombohedral Super-Stacking La0.77Mg0.23Ni3.72Hydrogen Storage Alloy

“…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.…”

“…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.…”