Effect of Co substitution for Ni and magnetic-heat treatment on the structures and electrochemical properties of La-Mg-Ni-type hydrogen storage alloys

In this paper, the phase microstructures and electrochemical performances of La0.67Mg0.33Ni3.0 and La0.67Mg0.33Ni2.5Co0.5 hydrogen storage alloys treated by casting, heat treatment and magnetic-heat treatment were studied. The influence of the partial substitution of Co for Ni and heat treatment with and without an external magnetic field on the microstructures and electrochemical performances of La0.67Mg0.33Ni2.5Co0.5 alloys was investigated in detail. The microstructures and morphology of alloys analyzed by XRD and optical microscopy and SEM and EDX showed that the as-cast La0.67Mg0.33Ni3.0 and La0.67Mg0.33Ni2.5Co0.5 alloys in different states had a multiphase structure which is composed of two major phases (La,Mg)2Ni7 and LaNi5 as well as a residual phase (La,Mg)Ni3. The partial substitution of Co for Ni dose not change the phase compositions, but the magnetic-heat treatment is helpful to a transformation of (La,Mg)Ni3 phase to (La,Mg)2Ni7 main phase, and enlarges the cell volumes of alloys. The electrochemical measurements showed that the La0.67Mg0.33Ni2.5Co0.5 alloys obtained by magnetic-heat treatment exhibited the best discharge capacity of 324.8 mAh/g with a good activation property and a large capacity retention of 83.07% after 50 charge-discharge cycles.