Phase Structure And Electrochemical Behavoiur Of La-Mg-Ni Based A₂B₇Type Hydrogen Storage Alloy

Re-Mg-Ni hydrogen storage alloy is a new generation of high performance rareearth hydrogen storage material since last ten years, and has been preliminarily appliedto the Ni/H batteries. In order to improve the market competitiveness of the hydrogenmaterials, the exploration of new type rare earth hydrogen storage and the applicationtechnology in the Ni/H batteries have been become the focus. But as so far, it still donotget breakthrough progress, especially slugging in the alloy industrialization and batteryapplication technology. This paper is mainly focusing on the La-Mg-Ni A₂B₇typehydrogen storage alloys, aiming to better the poor cycling stability by thecombination of elements substitution and improvement of heat treatment technology.The microstructure and phase structure were investigated by the X-ray diffraction（XRD）, scanning electron microscope （SEM）. The Land test system and electrochemicalsystem were applied to analyze the electrochemical property and dynamics performance.The research results can provide certain theoretical basis for the key technology of theindustrialization and maybe guiding significance in the battery application.The vacuum induction melting was used in this paper to prepared theLa_0.6Nd_0.1Re_0.1Mg_0.2Ni_3.35Al_0.15（Re=Pr, Sm, Ce, Y） hydrogen storage alloys. And theanalysis of XRD and SEM reveals that the alloys hold a multiphase structure, containingtwo main phase （La, Mg）₂Ni₇with Ce₂Ni₇type and LaNi₅with CaCu₅type as well as atrace of other residual phase. It was founded that the influence of A-side substitution isjust on the difference of the phase abundance of the main phase, the partial replacementof La by Sm makes an increase of （La, Mg）₂Ni₇phase, whereas the Y replacement canfacilite the LaNi₅phase, and there are little difference between the ratio of two phaseswhen Pr and Ce replacing. It is indicated from the electrochemical measurement that theelements substitution plays a small role in the activation performance of the hydrogenstorage alloys, but the substitution of Sm for La increases the maximum dischargecapacity, and the replacement La by Y can improve the cycling stability of alloys. Thedynamics properties were investigated by electrochemical impedance spectra （EIS）,potentiodynamic polarization, potential step method and cyclic voltammetry. Thelimiting current density （IL） and hydrogen diffusion coefficient （D） of the alloy have a good coincidence with the analysis results of the electrochemical measurement. Theanalysis about the variation of working electrode and charge transfer resistance fromEIS measurement disagree with the variation of limiting current density （IL） andhydrogen diffusion coefficient （D）, it can ascribed to that the high rate discharge abilityof the alloy is determined by both the surface charge transfer rate and the internalhydrogen diffusion ability, so the sort order of the high rate discharge ability of thealloys after substitution is Ce, Pr, Y, Sm.The annealing treatment was carried for the La_0.6Nd_0.1Re_0.1Mg_0.2Ni_3.35Al_0.15（Re=Pr,Sm, Ce, Y） hydrogen storage alloys. In this paper, the abundance of the （La, Mg）₂Ni₇phase with Ce₂Ni₇type is increased with the appropriate annealing treatment, but thephase transition process,（La, Mg）₂Ni₇phase with Ce₂Ni₇type turning to LaNi₅phasewith CaCu₅type, occurs when the annealing temperature is too high about1273K. Andthe discharge capacity and cycle stability can be improved at the appropriate annealingtemperature. The maximum discharge capacity of the alloys with the best annealingtemperature are as flow: Pr:377.3mAh/g（1173K）, Sm:372.0mAh/g（1223K）,Ce:376.2mAh/g（1223K）, Y:420.2mAh/g（1223K）. The cycle stability of the alloys alsoimproved by annealing treatment, the best capacity retaining rate S100are:Pr:95.56%（1223K）, Sm:94.96%（1173K）, Ce:88.80%（1223K）, Y:92.46%（1173K）respectively. But when the annealing temperature is too high, the discharge capacitydecreased, while the cycle stability is also significantly reduced. In addition, the highrate discharge ability is also improved to some extent.