Influence of Annealing Treatment on Microstructure and Cycling Stability of La-Rich Ml(NiCoMnAl)_5 Alloy Electrode for Ni/MH Batteries

摘要

The crystallographic structure, microstructure, composition homogeneity and electrode charge-discharge cycling stability were investigated of the as-cast and annealed La-rich mischmetal （designating Ml）-based hydrogen storage alloy with a composition of MlNi3.55Co0.75Mn0.4Al0.3. X-ray diffraction analysis shows that the MlNi3.55Co0.75Mn0.4Al0.3 alloy is composed of the dominant phase with a CaCu5-type hexagonal structure and small amounts of the second phase with a La2Ni7-type structure. The annealing heat treatment conducted at 1273 K for 10 h results in decrease of the crystal lattice strain and composition segregation, disappearance of the dendrite structure and growth of the crystal grain of the MlNi3.55Co0.75Mn0.4Al0.3 alloy. The annealing causes the cycle life to be increased by about 30% over the as-cast alloy electrode. The cycling stability of the alloy electrode is improved significantly upon annealing. The cause of the improvement in the cycling stability was discussed based on the alloy composition distribution and microstructure changes due to annealing.