Microstructure and electrochemical characteristics of nonstoichiometric La-rich AB_5-type alloy for Ni/MH battery electrode

摘要

The nonstoichiometric La-rich mischmetal (designated by Ml)-based hydrogen storage alloy with a composition of Ml(Ni_(0.64)Co_(0.20)Mn_(0.12)Al_(0.04))_(4.76) was prepared by arc melting and annealed at 1173 K for 10 h to investigate the effect of annealing treatment on the microstructure and electrochemical characteristics of the alloy. X-ray diffraction analysis showed that annealing can cause a release of the crystal lattice strain and an increase in amounts of the La_2Ni_7-type second phase in Ml(Ni_(0.64)Co_(0.20)Mn_(0.12)Al_(0.04))_(4.76) alloy- Scanning electron microscopy and electron probe microanalysis examinations indicated that annealing leads to disappearance of the dendrite structure in the as-cast alloy, growth of crystal grain, and decrease of composition segregation. The annealing at 1173 K for 10 h flattened and extended the potential plateau and increased the maximum discharge capacity to 328 mA centre dot h/g from 310 mA centre dot h/g and the cycling life. The mechanism of the improvement in electrochemical characteristics was discussed based on the alloy microstracture change induced by annealing.