Phase Structure and Electrochemical Characteristics of Rhombohedral Super-Stacking La_0.77Mg_0.23Ni_3.72Hydrogen Storage Alloy

Abstract

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...

Phase Structure, Electrochemical Properties and Cyclic Characteristic of a Rhombohedral-Type Single-Phase Nd2MgNi9Hydrogen Storage Alloy

“…5 Since then, the neotype La-Mg-Ni-based alloys, as negative electrode material of the Ni/MH batteries, have attracted much attention and been extensively studied. 6,7 The ternary (La,Mg)Ni 3 alloys have AB 3 -type layered structure, in which [AB 5 ] and [A 2 B 4 ] units alternative stacking along c axis with 1:1 proportion. 8,9 Other similar to A 2 B 4 /AB 5 layered structure alloys, such as A 2 B 7 -type (Ce 2 Ni 7 -type or Gd 2 Co 7 -type) and A 5 B 19type (Pr 5 Co 19 -type or Ce 5 Co 19 -type) alloys are also studied as metal hydride alloys.…”