| In this paper, Ce2Ni7-type hydrogen storage alloy of La-Mg-Ni system is determined to improve the electrochemical cycle stability as the researth target. Firstly, the hydrogenation behavior and electrochemical properties of ternary La-Mg-Ni Ce2Ni7-type alloy were investigated. Then, the effects of the substitution of rare earth and Mg element for A site were systematically studied to improve the overall electrochemical properties of the alloys, especially the cycle stability and high rate dischange capacity. On the basis of element substitution of A2B7-type alloys, it is found that the A2B7(Ce2Ni7)-type alloy of Pr, Nd, Sm, Gd, Y and Mg substitution show much better overall electrochemical properties than non-substitution alloy, especially that of Gd-substitution alloy for matching with proper Mg content is the best. Consequently A2B7-type alloys of low Mg and RE substitution became the research object. The influences of Ce2Ni7-type phase structure and R element substitution on hydride and electrochemical capacity degradation were investigated systemically, and also clarified alloying mechanism of rare earth elements, and revealed the nature of the best cyclic stability of La0.63Gd0.2Mg0.17Ni3.1Co0.3Al0.1alloys.La1-xMgxNi3.5alloy was as the research beginning in this paper. The hydrogenation and electrochemical behavior of alloys were investigated systemically. It was found that the annealing treatment for a long time could gain a single Ce2Ni7-type phase. The stability of Ce2Ni7-type phase was closely related with the size of La side. The La side atomic radius was smaller, it was easy to cause Ce2Ni7structure unstable; the La side atomic radius was larger, it was easy to form Ce2Ni7-type phase under low Mg, but lead to the amorphous phenomenon for hydrogenation. The isotropous change of microstructure of unit cell was happened after hydrogen absorption. The hydrogenation leaded to an even distribution hydrogen through all the slabs of the Ce2Ni7-type structure, and Laves unit was the weak link for particle pulverization. The discharge capacity and kinetic properties of Ce2Ni7-type electrode alloy were low and the cycle stability of that, viz, capacity retention rate after100charge/discharge cycles was up to80%, owing to the amorphous phenomenon for hydrogenation and particle pulverization.Based on Ce2Ni7-type alloys, the influences of rare earth elements and Mg content on alloy structure and electrochemical properties were systematically investigated. La0.83Mg0.17Ni3.1Co0.3Al0.1alloy was as the basic component for study. Experiment results showed that the introduction of Pr, Nd, Sm, Gd and Y elements could improve the Ce2Ni7-type phase aboundance and electrochemical properties of the alloy electrodes. Lao.63Gd0.2Mg0.17Ni3.1Co0.3Al0.1alloy was the best overall electrochemical properties (Cmax=389.94mAhg-1, S100=92.5%). Misch metal could increase HRD of alloy electrodes remarkably. On the basic of La0.8Gd0.2Ni3.1Co0.3Al0.1alloy, Mg content had an important influence on the phase structure of A2B7-type alloys. According to the different Mg content, alloys would present two different kinds of crystal structure with Ce2Ni7-type and Gd2Co7-type. High Mg content in alloys would lead to the decomposition of A2B7-type phase into PuNi3-type phase and LaNi5phase. Because the ratio of CaCu5unit and Laves unit in A2B7-type alloy and PuNi3-type alloy is different, the microstructure of both kinds of alloys exhibited different change rule. Mg atoms were distributed mainly at Laves unit of A2B7-type alloy. The effect of Mg content on hydrogenation and electrochemical properties of alloys was distinct. When the Mg content was low, alloys would show hydrogen induced amorphous phenomenon with low discharge capacity, but cyclic stability was better; when Mg content increased, amorphous phenomenon disappeared, alloy electrodes exhibited high discharge capacity and good kinetic properties; when Mg content increased further, PuNi3-type phase appeared and the overall electrochemical properties of alloy electrodes would be deteriorated. When Mg content was0.1～0.2, La-Gd-Mg-Ni-based alloy exhibit the perfect overall electrochemical properties.Compared to the La0.83Mg0.17Ni3.5alloy, intruduction of RE elements could not change the Ce2Ni7-type phase structure stability of La0.63R0.2Mgo.17Ni3.1Co0.3Al0.1alloy. Experiment of hydrogen absorption and desorption showed four aspect change. Firstly, intruduction of RE elements could improve hydrogenation properties of alloys remarkably; secondly, intruduction of RE elements could restain the amorphous phenomenon and disproportionation reaction of alloys; thirdly, the mechanism of hydrogenation changed from an "isotropy" hydride to an "anisotropism" one, then to an "isotropy" hydride; finally, intruduction of RE elements could effectively improve cell volume expansion of the Laves unit, also lead to restrain particle pulverization remarkably. During electrochemical charge and descharge cycle, the alloy containing Gd exhibited the slightest corrosion. In contrast to other Ce2Ni7-type alloy, La0.63Gd0.2Mg0.17Ni3.1Co0.3Al0.1alloy exhibited the minimum particle pulverization and the slightest corrosion, which was the main reason of better cyclic stability. |
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