Phase transformation and cycling characteristics of a Ce2Ni7-type single-phase La0.78Mg0.22Ni3.45 metal hydride alloy

Abstract

A Ce₂Ni₇-type single-phase La_0.78Mg_0.22Ni_3.45 alloy has been prepared by zoning annealing of the as-cast sample. It is found that at temperatures below 890 °C, non-super-stacking CaCu₅- and MgCu₄Sn-type phases disappear and super-stacking Ce₅Co₁₉-, Gd₂Co₇- and Ce₂Ni₇-type phases remain. The Ce₅Co₁₉-type phase can totally transform into the Ce₂Ni₇-type phase via a peritectic reaction at temperatures of 890–900 °C. At temperatures of 900–950 °C, the Gd₂Co₇-type phase melts and decomposes into the Ce₅Co₁₉-type phase, and the newly formed Ce₅Co₁₉-type phase subsequently reacts to form the Ce₂Ni₇-type phase. The Ce₂Ni₇-type single phase remains stable even at higher temperatures of 950–975 °C. The single-phase alloy shows a superior discharge capacity, close to 394 mA h g⁻¹, and high electrochemical cycling stability, which can achieve 413 cycles as its discharge capacity reduces to 60% of the maximum value. We found that the capacity attenuation of the single-phase alloy is mainly due to the loss of active material at the alloy surface caused by oxidization of La and Mg, and the pulverization of the alloy is not severe with 100 charge/discharge cycles. The crystal structure of the single-phase alloy can be preserved well. Oxidation of La occurs prior to that of Mg. La hydroxide grows from nano-structured needles to larger-scaled rods then to unformed lamellar hydroxide, whereas the precipitation of Mg forms as irregular lamellae inlaid with hexagonal flakes.