Effects Of Phase Transformation On Electrochemical Properties Of La-Mg-Ni-based Hydrogen Storage Alloys

In this paper, the La0.7Mg0.3Ni3.3alloy was prepared by inductive melting method.And a powder sintering method was applied to prepare a series of La-Mg-Ni-basedhydrogen storage alloys using different molar ratios of LaNi5to LaMgNi4precursors (x).The effects of phase transformation on electrochemical performances during annealingtreatment and powder sintering process were investigated systematically.XRD and Rietveld analysis indicate that the as-cast La0.7Mg0.3Ni3.3alloy consists ofLaNi5,(La,Mg)Ni3(2H) and (La,Mg)2Ni7(2H and3R) phases and a few (La,Mg)Ni2impurity phase.(La,Mg)Ni2phase whose melting point is below1073K becomes liquidphase during annealing treatment, while LaNi5phase still remains solid at this temperature.Hence,(La,Mg)5Ni19phase is formed through peritectic reaction between LaNi5and liquidphase. Increasing annealing temperature to1123K causes the phase transformation ofpartial (La,Mg)5Ni19into (La,Mg)2Ni7phase to occur by absorbing La and Mg atoms of(La,Mg)Ni3liquid phase. When annealed at1223K, the (La,Mg)5Ni19phase completelydecomposes into (La,Mg)2Ni7phase and LaNi5phase which can further form (La,Mg)2Ni7phase at cost of liquid phase. Consequently,(La,Mg)2Ni7phase abundance increasesdrastically. Phase transformation makes a significant improvement to the dischargecapacity and cycling stability of La0.7Mg0.3Ni3.3alloy electrodes, and the alloy annealed at1223K shows a large discharge capacity of401mAh/g. The high rate dischargeability(HRD1800) first increases to36.1%(1073K), then reduces to23.6%(1223K). This is dueto the reduction of (La,Mg)5Ni19phase who possesses superior high rate dischargeabilityand to the reduction of phase boundary available for the diffusion of hydrogen atoms.As for the process of preparing La-Mg-Ni-based alloys by powder sintering method,when the molar ratio of LaNi5to LaMgNi4(x) is0.56, the alloy has a PuNi3-type(La,Mg)Ni3single phase structure. The LaNi5phase reacts with (La,Mg)Ni3liquid phaseto become (La,Mg)2Ni7phase whose abundance increases with increasing x from0.56to0.9. While LaNi5or (La,Mg)Ni2impurity phase is observed respectively when x is below0.56or more than0.9. Electrochemical studies show that the increasing content of(La,Mg)2Ni7phase benefits the discharge capacity and cycling stability of alloy electrode. Besides, the appearance of a few LaNi5phase is responsible for the better HRD, but itaccelerates cycling degradation of alloy electrode.