Electrochemical performance of nanocrystalline and amorphous Mg-Nd-Ni-Cu-based Mg2Ni-type alloy electrodes used in Ni-MH batteries

Abstract

Nanocrystalline and amorphous Mg2Ni-type (Mg24Ni10Cu2)100-xNdx (x = 0, 5, 10, 15, 20) alloys were prepared by melt-spinning technology. The structures of as-cast and spun alloys were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Electrochemical performance of the alloy electrodes was measured using an automatic galvanostatic system. The electrochemical impedance spectra and Tafel polarisation curves of the alloy electrodes were plotted using an electrochemical work station. The hydrogen diffusion coefficients were calculated using the potential step method. Results indicate that all the as-cast alloys present a multiphase structure with Mg2Ni type as the major phase with Mg6Ni, Nd5Mg41 and NdNi as secondary phases. The secondary phases increased with the increasing Nd content. The as-spun Nd-free alloy exhibited nanocrystalline structure, whereas the as-spun Nd-doped alloys exhibited nanocrystalline and amorphous structures. These results suggest that adding Nd facilitates glass formation of Mg2Ni-type alloys. Melt spinning and Nd addition improved alloy electrochemical performance, which includes discharge potential characteristics, discharge capacity, electrochemical cycle stability and high-rate discharge ability.