Studies On Phase Transformation And Electrochemical Properties Of(La,Nd)–Mg–Ni-based Hydrogen Storage Alloys

In the paper, the 3R-type La0.77Mg0.23Ni3.72, La0.68Mg0.32M3.01, and Ndo.69Mg0.31Ni3.0 alloys were prepared through induction melting followed by zoning annealing method, and then the effect of phase transformation on electrochemical properties of alloy electrodes during the annealing process was studied. On the basis of the Rietveld and XRD analyses, the phase constitutes and the lattice parameters of the alloys are investigated. The electrochemical and kinetic properties of the alloy electrodes were tested by the constant current charge/discharge method.The analyses of the Rietveld and XRD show that the as-cast Lao.74Mg0.26M3.59 alloy comprises of the CaCu5-type, Ce2Ni7-type (2H), Gd2Co7-type (3R), and Ce5Co19-type (3R) phases, which converted into the rhombohedral Gd2Co7-type and Ce5Co19-type phases via annealing treatment at 950℃ for 54 h. The electrochemical measurements show that the annealed alloy comprised of rhombohedral-phases exhibits better overall electrochemical properties than those of the alloys comprised of a mixture of hexagonal and rhombohedra polymorphic phases. The high rate dischargeability and the cyclic life after 100 charge/discharge cycles of the 3R-type alloy electrode reach the maximum (53.1% and 86.2%, respectively).The superior properties of the alloy may be attributed to the R-3m space group Gd2Co7-type and Ce5Co19-type phases possessing high structural matching.The single-phase PuNi3-type Lao.67Mgo.33Ni3.0 and Nd0.67Mg0.33M2.95 were prepared through induction melting followed by zoning annealing at 950℃ for 12h and 1025℃ for 24h, respectively. It is found that the Nd0.69Mg0.31Ni3.0 alloy electrode exhibits excellent electrochemical properties. The discharge capacity of the alloy electrode is 370.6 mAh g-1 and its high rate dischargeability reaches up to 49.6% at a 1440 mA g-1 discharge current density. Especially, the cyclic stability is heightened to 92.1% at the 100th charge/discharge cycle and retains 79% at the 300th charge/discharge cycle, which are significantly better than those of the PuNi3-type Lao.68Mg0.32M3.01 alloy. On the basis of the R-3m crystal structure calculation results, we found that the single-phase Nd0.69Mg0.31Ni3.0 alloy has the smaller volume deformation rate ΔV/V(+1.19%) and lower lattice strain (0.32%) after hydrogenation/dehydrogenation, resulting in the weaker pulverization degree of the alloy particle and minor oxidization/corrosion of active material.