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

A Ce2Ni7-type single-phase La0.78Mg0.22Ni3.45 alloy has been prepared by zoning annealing of the as-cast sample. It is found that at temperatures below 890 degrees 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 degrees C. At temperatures of 900-950 degrees 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 degrees 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.