The Effects Of Annealing Treatment On The Phase Structure And Electrochemical Properties Of La-Mg-Ni-Co-based Hydrogen Storage Electrode Alloys

In this thesis, based on the review of the research and development of the non-AB5 type rare earth-based hydrogen storage alloys, the La-Mg-Ni-Co-based hydrogen storage electrode alloy was selected as the study object of this work. By means of XRD/Rietveld analyses and the electrochemical tests including the galvanostatic charge-discharge, EIS, linear polarization, anodic polarization, potentialstatic discharge and etc., the relationship among compositions, phase structure and electrochemical properties was systemically studied for developing the new type rare earth-based hydrogen storage electrode alloys with high discharge capacity and long cycling life. The study includes the effect of annealing temperature on the structure and electrochemical properties of La₀.7Mg₀.3Ni₂.45Co₀.75Mn₀.1Al₀.2 hydrogen storage alloy electrode, the effect of annealing holding time on the structure and electrochemical properties of La₀.7Mg₀.3Ni₂.45Co₀.75Mn₀.1Al₀.2 hydrogen storage alloy electrode and the effect of annealing treatment on the structure and electrochemical properties of La₀.7Mg₀.3Ni₂.45-xCo₀.75+xMn₀.1Al₀.2(x=0.00, 0.15, 0.30) hydrogen storage alloy electrodes.The study of the effect of annealing temperature on the structure and electrochemical properties of La₀.7Mg₀.3Ni₂.45Co₀.75Mn₀.1Al₀.2 hydrogen storage alloy shows that all the alloys mainly consist of the (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure and the LaNi₅ phase with the hexagonal CaCu₅-type structure. The lattice parameters and cell volumes of two phases are all increased with increasing annealing temperature by Rietveld analyses. The relative phase abundance of the (La,Mg)Ni3 phase and the LaNi₅ phase varies and leads to the discharge capacity increase first from 350.9 mAh/g (as-cast) to 370.0 mAh/g (T=1173K), and then decrease to 359.8 mAh/g(T=1373K). Owing to composition homogenization, the cycle life of the alloy improves after annealing treatment. However, high rate dischargeability (HRD), electrochemical impedance spectra (EIS), linear polarization, anode polarization and potential-step studies indicate that the electrochemical kinetics of the alloy electrodes deteriorate with increasing annealing temperature. Electrochemical studies reveal that the overall electrochemical properties of the alloy annealing at 1173K are better, the alloy electrode can be completely activated within 2 cycles and its maximum discharge capacity is 370.0 mAh/g, the HRD₁000 value reaches 77.7%, S₁00 is 67.5%.Based on these results, the effect of annealing holding time on the structure and electrochemical properties of La₀.7Mg₀.3Ni₂.45Co₀.75Mn₀.1Al₀.2 hydrogen storage alloyelectrode has been studied systematically. The results indicate that all the alloys still consist of the (La,Mg)Ni3 phase and LaNis phase, and the lattice parameters and cell volumes of two phases are all increased with prolongation of holding time. With prolongation of holding time, the abundance of the (La,Mg)Ni3 phase in the alloy increases but the LaNis phase abundance decreases. Electrochemical studies show that the maximum discharge capacity and the cyclic stability of the alloy electrode increase dramatically with increasing annealing time owing to the phase abundance variation and the composition homogenization, respectively. However, the high rate dischargeability of the alloy electrode decreases with increasing annealing time. To summarize the results of the investigation, the optimum annealing holding time is found to be t=8h.In order to reveal the relationship among compositions, phase structure and electrochemical properties, the effect of annealing treatment on the structure and electrochemical properties of La₀.7Mg₀.3Ni₂.45-xCo₀.75+xMn₀.1Al₀.2(x=0.00, 0.15, 0.30) hydrogen storage alloy electrodes were investigated. It is found that all the above Co-containing alloys still contain both the (La,Mg)Ni3 phase and the LaNis phase. The lattice parameters and cell volume of these two phases all increase with increasing Co content. With increasing...