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Based on the review of the research and development of the Mg-based hydrogen storage alloys, La2Mg17 hydrogen storage alloy was selected as the study object of this work due to its higher storage capacity than Mg-Ni system alloy. However, the application of La2Mg17 alloy in the electrochemical hydrogen storage can hardly be performed because of its poor kinetic properties and high hydride stability. The purpose in this thesis is to make La2Mg17 alloy charge/discharge reversibly by ball-milling with certain amounts of Ni or Co powders. By means of XRD, SEM, XPS and AES analyses and the electrochemical test methods including the galvanostatic charge-discharge, EIS, linear polarization, anodic polarization and cyclic voltammogram etc., the microstructure and electrochemical properties of the ball-milled composites were systematically studied. The study includes the followings: the effect of Ni/Co amounts and ball-milling time on the microstructure and electrochemical properties of the ball-milledLa2 Mg17 composite alloys, the effect of small addition of TiO2, Bi2O3, MnO2, Fe2O3 and mechanical microencapsulation with Ti, graphite, Al, Co, Ni on the microstructure and electrochemical properties of amorphousLa2Mg17+200wt.%Ni composites, the influence of Ni substitution for Mg in La2Mg17-xNix （x=1, 3, 5）, Al substitution for Ni in La2Mg14Ni3-xAlx（x=0.5, 1, 1.5, 2） and Ti substitution for Mg in La2Mg14-xTixNi3 （x=1, 2） on the phase structure of the as-cast alloy and electrochemical properties of the ball-milled composites with 200wt.%Ni, and the cycling capacity degradation mechanism of the amorphousLa2Mg17-Ni alloy.The study of the influence of Ni/Co amounts and ball-milling time on the microstructure and electrochemical properties of La2Mg17 composite alloys revealed that ball-milling with certain amount of Ni/Co can make La2Mg17 alloy charge/dishcharge reversibly at room temperature. The maximum discharge capacity of amorphous La2Mg17+200wt.%Ni and La2Mg17+200wt.%Co composites is 929.5 and 748.2mAh/g, respectively. It is found that ball-milling with adequate amount ofNi/Co powders is advantageous for the formation of amorphous structure, thus leading to the improvement of thermodynamic and kinetic properties of the ball-milled composites. The discharge capacity of the amorphous composite is influnced by both the amorphization degree and particle size. There exist an optimal Ni amount and ball-milling time for the achievement of the highest discharge capacity. Before the optimal value, the discharge capacity is mainly influenced by the amorphization degree, and beyond the optimal one, the particle size becomes the dominating factor.In order to improve the overall electrochemical properties of the amorphous La2Mg17+200wt.%Ni composite, the effects of adding small amounts of TiO2, Bi2O3, Fe2O3, MnO2 and mechanical microencapsulation with Ti, graphite, Al, Co, Ni were investigated. The results indicate that the addition of small amounts of metal oxides can improve the amorphization degreee of the composites, thus leading to the improvement of discharge capacity and high-rate dischargeability. The maximum discharge capacity of the amorphous La2Mg17+200wt.%Ni composite with addition of 3wt.%TiO2 is 979.7mAh/g. The improvement of discharge capacity and high-rate dischargeability can be attributed to the following two factors. On the one hand, metal oxides have certain catalytic effect on the electrochemical hydriding/dehydriding of the Ln-Mg-based amorphous alloys, thus leading to the decrease of activation energy and electrochemical reaction resistance and the increase of exchange current density. On the other hand, metal oxides are very bittle and helpful for the refinement of alloy particle and the augmentation of specific surface areas, which make the hydrogen adsorption and oxidation much easier. But the addition of metal oxides has little effect on the cycling stability. The effect of mechanical microencapsulation with different elements on the electrochemical properties of the amorphous La2Mg17 + 200wt.%Ni composite was thoroughly invetigated. The results show that Ti and Al microencapsulation decreases the discharge capacity and high-rate dischargeability but improves the cycling stability. While Co and graphite microencapsulation improves the discharge capacity and high-rate dischargeability but degrades the cycling stability. Ni element is an exception, which displays the above twofoldfuntions. The maximum discharge capacity Cmax and cycling retention rate after 60 cycles S60 of the amorphous La2Mg17+200wt.%Ni composite are 861.9mAh/g and 34.4%, respectively. While the Cmax and S60 of the composites microencapsulated with 5wt.% Ti, Al, Co, graphite, Ni are 690.4, 823.7, 936.4, 918.9, 921.6mAh/g and 53.6, 38.0, 34.3,32.9,35.5%, respectively.In order to improve the overall electrochemical properties of the amorphous alloy, Mg in the La2Mg17 alloy was first partly substituted by Ni, and the phase structure of the as-cast La2Mg17-xNix（x=1, 3, 5） alloy and electrochemical properties of the ball-milled composites with 200wt.%Ni were investigated. The XRD results reveal that as-cast La2Mg17-xNix（x=1, 3, 5） alloy has a multiphase structure, including La2Mg17 and Mg2Ni. And with the increase of Ni amount, the amount of Mg2Ni increases but that of La2Mg17 decreases. The Ni substitution can also accelerate the amorphization process of the ball-milled composites. Electrochemical studies reveal that Ni substitution leads to the improvement of high-rate dischargeability and cycling stability, but to the decrease of discharge capacity. The ball-milled La2Mg14Ni3+200wt.%Ni amorphous composite displays better overall electrochemical properties, e.g., its maximum discharge capacity is 704.8mAh/g, the HRD1440 value reaches 72.4%, S20 is as high as 63.0%. Based on these results, Al and Ti were selected in addition as the substitution elements for Ni and Mg in La2Mg14Ni3 alloy, and the phase structure of the as-cast La2Mg14Ni3-xAlx（x=0.5, 1, 1.5, 2） and La2Mg14-xTi,Ni3（x=1, 2） alloys and the electrochemical properties of the ball-milled composites with 200wt.%Ni were investigated. The XRD results indicate that the substitution of Ni by Al and Mg by Ti leads to the appearance of AINi and TiNi phase besides La2Mg17 and Mg2Ni. Electrochemical results reveal that substitution of Al for Ni and Ti for Mg leads to the improvement of discharge capacity, among which the maximum discharge capacities of La2Mg14Ni1.5Al1.5+200wt.%Ni and La2Mg12Ti2Ni3 + 200wt.%Ni amorphuos alloys are 827.2 and 843.1mAh/g, respectively. This is mainly due to the existence of catalytic phase AINi or TiNi, which plays an important role in improving the electrocatalytic activity and increasing the discharge capacity. But the substitution of Al or Ti leads to the worsening of high-rate dischargeability andcycling performance.Based on the study of the cycling capacity degradation mechanism of the amorphous La2Mg17-Ni system alloys, it is found that the structure stability of the alloys is good during cycling and has no effect on the cycling capacity degradation of the alloys. The main reason of the fast cycling capacity degradation of the alloys is the corrosion of La and Mg on the alloy surface, especially the corrosion of Mg, which decreases the amounts of hydrogen absorption elements. During the initial cycling period, the pulverization of the alloy particles accelerates the corrosion process, thus leading to the fast degradation of discharge capacity at initial period. And afterwards, the particle pulverization tends to terminate and the surface corrosion layer becomes thicker, leading to the decrease of capacity degradation rate.
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