Effects of Mg on the structures and cycling properties of the LaNi3.8 hydrogen storage alloy for negative electrode in Ni/MH battery

The LaNi3.8-type LaNi3.8 and La0.8Mg0.2Ni3.8 hydrogen storage alloys were prepared by high frequency induction melting (HFIM) under helium atmosphere followed by annealing treatment. Either alloy mainly consists of a rhombohedral Ce5Co19-type phase (R-3 m) and a hexagonal Pr5Co19-type (P6(3)/mmc) phase. The unit cell volume of either Ce5Co19-type or Pr5Co19-type phase in the La0.8Mg0.2Ni3.8 alloy is smaller than that in LaNi3.8. The Mg-containing La0.8Mg0.2Ni3.8 alloy has a lower absorption/desorption plateau, smaller hysteresis and a higher hydrogen storage capacity than those of LaNi3.8. The La0.8Mg0.2Ni3.8 alloy electrode shows an excellent activation capability and a higher discharge capacity (373.1 mA h g (1)) than those of the LaNi3.8 alloy electrode (184.0 mA h g (1)). The capacity decay rate of the La0.8Mg0.2Ni3.8 alloy electrode (0.88 mA h (g cycle) (1)) is much lower than that of the LaNi3.8 alloy electrode (1.32 mA h (g cycle) (1)), which is attributed to that the addition of Mg suppresses the hydrogen-induced amorphization of the LaNi3.8 alloy. Moreover, during the charge-discharge cycling of the Mg-containing alloy electrode, the dissolutions of La and Ni in the alkaline solution can be suppressed remarkably due to the sacrifice of Mg, which correspondingly results in preferable cycling stability and an anti-corrosion capability of the La0.8Mg0.2Ni3.8 alloy electrode. (C) 2014 Elsevier B.V. All rights reserved.