Microstructure and Electrochemical Characteristics of Step-Wise Annealed La0.75Mg0.25Ni3.5 Alloy with A2B7- and A5B19-type Super-Stacking Structure

Abstract

La−Mg−Ni-based alloys with A2B7- and A5B19-type main phases have been prepared by step-wise annealing method at various temperatures and times. When annealing at 1173 K, the alloy is composed of (La,Mg)2Ni7 and (La,Mg)5Ni19 super-stacking phases. Increasing annealing time from 15 to 25 hours at 1173 K causes the formation of LaNi5 phase at the cost of (La,Mg)2Ni7 phase, while the (La,Mg)5Ni19 phase abundance changed very little. When the annealing temperature rises to 1223 K, (La,Mg)5Ni19 phase abundance increases drastically, indicating that (La,Mg)5Ni19 phase abundance is more affected by annealing temperature than annealing time. Electrochemical P-C isotherms show that the alloys with only super-stacking phases have single plateau. The appearance of LaNi5 phase leads to an additional plateau which becomes more obvious as LaNi5 phase abundance increases. Electrochemical measurements show that higher (La,Mg)5Ni19 to (La,Mg)2Ni7 phase ratio benefits high rate dischargeablity (HRD) and cycling stability but suppresses the discharge capacity of the alloy electrodes. Besides, with increasing LaNi5 phase from zero to 15.1 wt.% in expense of (La,Mg)2Ni7 phase, the maximum discharge capacity and cycling stability is deteriorated while the HRD, exchange current density, limiting current density and hydrogen diffusion coefficient are promoted.