Phase structure and cycling stability of A2B7 superlattice La0.60Sm0.15Mg0.25Ni3.4 metal hydride alloy

Abstract

Improvement in cycling stability is a key issue for La–Mg–Ni-based alloys to be widely applied as anode electrode materials of nickel metal hydride batteries. In this paper, we report the phase structure and melioration of cycling stability of a Sm-containing A2B7-type La–Mg–Ni-based alloy. Sm element promotes the Gd2Co7-type phase formation with increasing annealing period, which the Ce5Co19-, CaCu5- and MgCu4Sn-type phases in as-cast La0.60Sm0.15Mg0.25Ni3.4 alloy are successively eliminated as the annealing time increases from 6 to 24 h at 950 °C and forms a single A2B7-type alloy with coexistence of 73.2 wt.% Gd2Co7- and 26.8 wt.% Ce2Ni7-type phases. The single A2B7-type La0.60Sm0.15Mg0.25Ni3.4 alloy is superior in discharge capacity (382 mAh g−1) and high rate dischargeability (40% at a 1500 mA g−1 discharge current density). Especially, the cycling stability of the alloy is enhanced to 87.7% at the 100th cycle. The improved cycling stability of the single A2B7-type alloy is mainly attributed to its slower growth of inner strain during charging/discharging, which improves anti-pulverization/amorphization ability, and drops oxidation degree of the alloy. Therefore, Sm is a positive element in improvement of the cycling stability property of the La–Mg–Ni-based alloys.