Effect of Co content on the structure and electrochemical properties of La1.5Mg0.5Ni7−xCox (x = 0, 1.2, 1.8) hydrogen storage alloys

La1.5Mg0.5Ni7−xCox (x = 0-1.8) hydrogen storage alloys have been prepared by induction melting followed by an annealing treatment. X-ray diffraction (XRD) and electron probe microanalysis (EPMA) show that all of the alloys consist of a main phase with Ce2Ni7-type structure with small impurities, such as LaNi5-type and PuNi3-type phases. The combination of X-ray and neutron diffraction data shows that Mg atoms are located only at the Laves unit and that Co atoms are located only at the CaCu5 unit of the Ce2Ni7-type unit cell. As Co content increased, the lattice parameters and cell volume increase and the hydrides become more stable. Electrochemical analyses show that all the alloys have a large discharge capacity (above 390 mAh/g) and can be easily activated by three cycles. However, the cyclic stability of the alloy electrodes becomes worse as Co content increased, which could be mainly attributed to the distribution of Co atoms in the Ce2Ni7-type unit cell. The La1.5Mg0.5Ni7.0 alloy exhibits better cyclic stability (S70% = 84.7) and a higher rate dischargeability (HRD900% = 92.32). Electrochemical analyses show that the control process of alloy electrode reaction is hydrogen diffusion in bulk of alloy.