Effects of stoichiometric ratio La/Mg on structures and electrochemical performances of as-cast and annealed La–Mg–Ni-based A2B7-type electrode alloys

Abstract

In order to investigate the influences of the stoichiometric ratio of La/Mg (increasing La and decreasing Mg on the same mole ratio) on the structure and electrochemical performances of the La–Mg–Ni-based A2B7-type electrode alloy, the as-cast and the annealed ternary La0.8+xMg0.2−xNi3.5 (x=0−0.05) electrode alloys were prepared. The characterization of electrode alloys by X-ray diffraction (XRD) and scanning electron microscopy (SEM) shows that all the as-cast and the annealed alloys hold two major phases of (La,Mg)2Ni7 and LaNi5 as well as a residual phase of LaNi3. Moreover, the increase of La/Mg ratio brings on a decline of (La,Mg)2Ni7 phase and a rise of LaNi5 and LaNi3 phases. The variation of La/Mg ratio gives rise to an evident change of the electrochemical performances of the alloys. The discharge capacities of the as-cast and the annealed alloys evidently decrease with growing the La/Mg ratio, while the cycle stabilities of the alloys visibly augment under the same condition. Furthermore, the high rate discharge ability (HRD), the electrochemical impedance spectrum (EIS), the Tafel polarization curves, and the potential step measurements all indicate that the electrochemical kinetic properties of the alloy electrodes increase with the La/Mg ratio rising.