XRD study on the electrochemical hydriding/dehydriding behavior of the La–Mg–Ni–Co-type hydrogen storage alloys

Electrochemical hydriding/dehydriding behavior of the La–Mg–Ni-based alloy La 0.7Mg 0.3(Ni 0.85Co 0.15) 3.5 consisting of a (La,Mg)Ni 3 phase with the PuNi 3-type rhombohedral structure and a LaNi 5 phase with the CaCu 5-type hexagonal structure was systematically investigated by means of X-ray diffraction (XRD) analyses. The results indicate that the PuNi 3-type rhombohedral structure and the CaCu 5-type hexagonal structure of the alloy are still preserved during hydriding/dehydriding process. For the alloy studied, the LaNi 5 phase was first hydrided due to good surface electrochemical activity for decomposing water and provides hydrogen to the (La,Mg)Ni 3 phase in the very initial electrochemical charging process. Moreover, it is clearly noted that, for both the (La,Mg)Ni 3 phase and LaNi 5 phase, there is a large difference in the cell volume expansion between the α- and β-phase during hydriding/dehydriding process, that is, a discrete cell volume expansion, which covers most of the total cell volume expansion. Therefore, the discrete cell volume expansion rather than the total cell volume expansion is believed to be essential for the formation of defect structures and, subsequently, for the pulverization of the alloy particles, which leads to the larger capacity degradation during electrochemical charge/discharge cycling. A schematic model is proposed to illustrate in detail the α- to β-phase transition during hydriding/dehydriding process.