Phase Structure and Electrochemical Characteristics of Rhombohedral Super-Stacking La0.77Mg0.23Ni3.72 Hydrogen 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 La0.77Mg0.23Ni3.72 alloy with the both rhombohedral (3R) Gd2Co7-type and Ce5Co19-type phases was successfully prepared by induction melting followed by annealing treatment method at 950°C. Via annealing treatment for 54 h, the CaCu5-type, Ce2Ni7-type, Gd2Co7-type, and Ce5Co19-type phases of the as-cast alloy converted into the rhombohedral Gd2Co7-type and Ce5Co19-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 100th 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 100th 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 Gd2Co7-type and Ce5Co19-type phases possessing high structural matching.