Investigation on structures and electrochemical performances of the as-cast and -quenched electrode alloys

The La–Mg–Ni system PuNi₃-type hydrogen storage alloys La_0.7Mg_0.3Co_0.45Ni_2.55-xFe_x (x=0, 0.1, 0.2, 0.3, 0.4) were prepared by casting and rapid quenching. Ni in the alloy was partially substituted by Fe in order to improve the cycle stability of the alloys. The effects of the substitution of Fe for Ni combining rapid quenching on the microstructures and electrochemical performances of the as-cast and -quenched alloys were investigated in detail. The results indicate that the substitution of Fe for Ni obviously decreases the discharge capacity and high rate discharge (HRD) capability as well as the discharge potential of the as-cast and -quenched alloys, but it significantly improves their cycle stabilities. The microstructure of the alloys analyzed by XRD, SEM and TEM shows that the as-cast and -quenched alloys have a multiphase structure which is composed of two major phases (La, Mg)Ni₃ and LaNi₅ as well as a residual phase LaNi₂. The substitution of Fe for Ni leads to an obvious increase in the LaNi₂ phase in the as-cast alloys, and it also helps the formation of a like amorphous structure in the as-quenched alloy. With the increase of Fe content, the grain sizes of the as-quenched alloys significantly reduce, and the lattice constants and cell volumes of the alloys obviously enlarge.