Cycling durability and degradation behavior of La-Mg-Ni-Co-type metal hydride electrodes

The cycling durability and degradation behavior of the La-Mg-Ni-based hydrogen storage alloys La0.7Mg0.3Ni3.4−xCoxMn0.1 (x = 0, 0.75, 1.3) during charge/discharge cycling has been systematically studied by XRD, SEM, EIS, XPS and AES measurements. The reasons for the improvement of the cycling stability of the alloy electrodes with increasing Co content have also been analyzed and discussed. The results show that the pulverization of the alloy particles and the oxidation/corrosion of the active components of the alloys during charge/discharge cycling in the alkaline electrolyte are the two main factors responsible for the fast capacity degradation of the La-Mg-Ni-based alloy electrodes, and the capacity degradation mechanism can be decomposed into three consequent stages, i.e., the pulverization and Mg oxidation stage, the Mg and La oxidation stage and the oxidation and passivation stage. With the increase in Co content, the cell volume expansion ratio ΔV/V of the two main phases during hydrogenation/dehydrogenation was obviously decreased, which results in a reduction of the pulverization of the alloy particles and, consequently, in an increase in the charge and discharge efficiency and a decrease in the rate of contact of the fresh alloy surface with alkaline electrolyte and a subsequent lower rate of oxidation/corrosion. It is believed to be the most important reason responsible for the improvement of the cycling stability of the alloy electrodes with increasing Co content.