Investigation On The Phase Structure And Electrochemical Performances Of La-Mg-Ni Hydrogen Storage Alloys

High capacity Ni/MH battery is more and more needed to meet the requirements of advanced and mobile electric appliance. It is well know the AB5 typed alloy, essential part of the most widely used Ni/MH battery, is approaching its theoretical capacity limit because of its CaCu5 structure. Thus the searching for higher capacity hydrogen storage alloy has remained the task for scientists. Among all the potential new alloys, La-Mg-Ni system alloy is the most promisting candidate. But the main weakness of La-Mg-Ni system alloy is its poor cycle stability. Therefore, the studying on La-Mg-Ni system alloy holds significant academic value and application potential.In order to improve its cyclic stability, the La-Mg-Ni-Co hydrogen storage alloy was selected as the object of this work based on the review of the research and development of the La-Mg-Ni system alloys both from home and abroad. By means of XRD, SEM and electrochemical testing, the influence of element substitution, rapid quenching and annealing treatment on the phase structure and electrochemical properties of alloys were systemically studied to discover the relationship between composition, phase structure and edectrochemical properties of alloys. Some important conclusions were obtained.For the La_0.75-xPr_xMg_0.25Ni_2.9Co_0.4Al_0.2 (x=0,0.1,0.2,0.3,0.4)alloys, with the increasing of Pr content, the main phases of the alloys transformed from LaNi3 phase to LaNi5 and LaNi₂ phases. The maximum discharge capacity, HRD ability and discharge voltage characteristic of the alloys decreased but the cyclic stability of the alloys increased with the increasing Pr content. When x was 0.4, the alloy exhibited the best cyclic stability. The vacuum rapid quenching was in favor of the LaNi₅ phase which decreased the maximum discharge capacity but improved cyclic stability.The reason for capacity decay was analyzed and the results showed that the main reasons for capacity degradation of the alloys were pulverization and oxidation during charging-discharging cycles.Zr was selected as a substitution element for La in La_0.75-xZr_xMg_0.25Ni_2.9Co_0.4Al_0.2 (x=0,0.05,0.1,0.15,0.2) alloys and the effects of Zr content x on the structure and electrochemical properties of alloys were investigated in detail. The results showed that the maximum discharge capacities decreased, but the HRD ability and cycle stability first increased and then decreased with the gradual increasement of Zr content. And the alloy exhibited the best cyclic stability and HRD ability when x=0.2 and x=0.1, respectively. Pulverization and oxidation of the alloys during charging-discharging cycles were the main reasons for cycle stability degradation.The La_0.75Mg_0.25Ni_3.1-xCo_0.4Al_x (x=0,0.05,0.1,0.15,0.2) alloys were researched systemically. The obtained results showed that Al element decreased HRD ability but promoted the activation performance and discharge voltage plateau pressure, especially the cyclic stability. The alloy with x=0.2 Al content exhibited the best cyclic stability. A compact film of Al₂O₃ was formed on the surface of the alloy, which could reduce the degree of oxidation/corrosion and enhance the cycle stability. The annealing treatment was in favor of LaNi3 and microstructure homogenization, resulting in the increasement of maximum discharge capacity, discharge voltage pleatau and cyclic stability.