The Phase Structures And Electrochemical Properties Of La-Mg-Ni Based AB₃-type Hydrogen Storage Alloys

In this thesis, previous research works on RE-based non-AB₅ type hydrogenstorage alloys have been extensively reviewed. On this basis, the La-Mg-Ni basedAB₃-type hydrogen storage electrode alloys were selected as the subject of this study.By means of XRD analysis and the electrochemical test methods such as galvanostaticcharge-discharge, linear polarization, potentialstatic discharge etc., the phase structureand electrochemical properties of La₂MgNi_x(x=8.7-9.9) alloys andLa₂Mg(Ni_0.85Co_0.15)_x(x=9.0-10.5) alloys were studied systematically, and from whichthe AB_3.2-type alloy with a better overall electrode properties was selected for furtherstudy. Based on these works, the effect of the substitutions of Co, Mn, Fe, Cu, Al, Snon the phase structure and electrochemical properties of the AB_3.2-typeLa₂Mg(Ni_0.90M_0.10)_9.6 alloys were studied in order to improve the overallelectrochemical properties of the alloys.For the La₂MgNi_x(x=8.7-9.9) alloys, the effects of Ni content and stoichiometricratio(x) on the phase structure and electrochemical properties of all the alloys weresystemically investigated. It is found that all the alloys mainly consist of the(La,Mg)Ni₃ phase with the PuNi₃-type structure and a few impurity phases (LaNi₅ andLaNi₂) . But the increase of x value leads to a decrease in cell volume expansion rate(â–³V/V) on hydriding(24.4% as x=8.7 to 22.3% as x=9.9). With increasing of x, thedesorption plateau pressure of the alloys increases noticeably, the discharge capacityof the alloys increases slightly and passes through a maximum of 411.5mAh/g atx=9.0 and then decreases with the further increase of x. The high-ratedischargeability(HRD) of the alloy electrodes improves greatly as x increases, theHRD₈₀₀ of the alloys increases from 38.8% (x=8.7) to 84.4% (x=9.9). The increase ofhigh-rate dischargeability of the Ni-rich alloys is ascribed to the increase of both theelectrocatalytic activity of the alloy electrode and the diffusion rate of hydrogen in thealloy bulk. As x increases, the cycling stability (S₁₀₀) of the alloys increases slightly,increasing from 54.1% (x=8.7) to 63.4 (x=9.6), Among the alloys studied, theLa₂MgNi_9.6(AB_3.2) alloy shows a relatively good overall properties with C_max=396.8mAh/g, HRD₈₀₀=74.5%and S₁₀₀=63.4%. The high-rate dischargeabilityand the cycling stability of the alloys are needed to be further improved.The study on La₂Mg(Ni_0.85Co_0.15)_x (x=9.0-10.5) alloys shows that all the alloysconsist of the (La,Mg)Ni₃ main phase with the PuNi₃-type structure and a fewimpurity phases(LaNi₅ and La₂Ni₇), and the increase of x leads to noticeable increaseof the content of impurity phases. The increase of x leads to a decrease in both cellvolume and cell volume expansion rate (â–³V/V) on hydriding(23.2% as x=9.0 to 20.2%as x=10.5). With increasing x value, the discharge capacity of the alloys increasesslightly and shows a maximum (405mAh/g at x=9.3) and then decreases with thefurther increase of x. The high-rate dischargeability(HRD) of the alloy electrodesincreases noticeably as x increases, the high-rate dischargeability of the alloyelectrodes(HRD₈₀₀) improves greatly from 85.7% as x=9.0 to 94.8% as x=10.5. Onthe other hand, the cycling stability(S₁₀₀) of the alloys decreases with the increase of x,decreasing from 69.5% as x=9.0 to 38.5 as x=10.5. The decrease in stability of thealloys is mainly attributed to that the alloys contain more impurity phases(LaNi₅,La₂Ni₇) as x increases and the difference between the main phase and impurity phasesin cell volume expansion rate would lead to a higher degree of pulverization andcorrosion of the alloys during cycling. Among the alloys studied, theLa₂Mg(Ni_0.85Co(0.15))_9.6 alloy shows a relatively good overall properties withC_max=390.5mAh/g, HRD₈₀₀=91.4%and S₁₀₀=66.0%. The cycling stability of thealloys is needed to be further improved.For the AB_3.2 type La₂Mg(Ni_0.9M_0.1)_9.6(M=Co, Mn, Fe, Cu, Al, Sn) alloys, it isfound all the alloys still consist of the (La,Mg)Ni₃ main phase with the PuNi₃-typestructure and a few impurity phases (LaNi₅, LaNi and LaNiSn), while a portion of theAl substituted alloy became amorphous on hydriding. The part substitution of Melement for Ni leads to an increase in cell volume of the alloys, but leads to somedecrease in cell volume expansion rate (â–³V/V) on hydriding. The substitution leads tosome decrease in the discharge capacity of the alloys (C_max=195-305mAh/g as M=Mn,Fe ,Cu, Al, Sn), and leads to a large reduction in high-rate dischargeability(HRD) ofthe alloys(HRD₈₀₀=33.9-58.8%) decrease greatly except the Sn substitution one. While the substitution improved the improvement in cycling stability (S₁₀₀) of thealloys to a different extent. Among the alloys studied, the La₂Mg(Ni_0.90Co(0.10))_9.6 alloyshows a relatively good overall properties with C_max=380mAh/g, HRD₈₀₀=43.3%andS₁₀₀=66.8%. The high-rate dischargeability and the cycling stability of the alloys areneeded to be further improved.