The Effects Of Chemical Composition And Rapid Quenching On The Phase Structure And Electrochemical Properties Of Hydrogen Storage Electrode Alloys

In the thesis, based on the review of the research and development of the PuNi₃ type La-Mg-Ni based hydrogen storage electrode alloys, the La₂Mg(Ni_0.85Co_0.15)₉ alloy with high discharge capacity was selected as the subject of this study. By means of XRD, SEM, EDS analysis and the electrochemical test methods including the galvanostatic charge-discharge, liner polarization etc., the effects of added elements (Cu and Si) on the phase structure and electrochemical properties of La₂Mg(Ni_0.85Co_0.15)₉M_x(M=Cu, Si; x= 0, 0.1, 0.3, 0.5) alloys, and the effects of rapid quenching on the phase structure and electrochemical properties of La₂Mg(Ni_0.85Co_0.15)₉M_0.3 (M=Cu, Si) alloys were investigated for improving the overall electrochemical properties of the alloys.The study on the phase structure and electrochemical properties of the Cu-added La₂Mg(Ni_0.85Co_0.15)₉Cu_x(x= 0-0.5) alloys shows that all La₂Mg(Ni_0.85Co_0.15)₉Cu_x(x=0-0.5) alloys consist of a main (La,Mg)Ni3 phase with PuNi₃-type structure, a secondary LaNi₅ phase and a little LaNi₂ phase. The abundance of LaNi₂ phase increases with the addition of Cu increases. Electrochemical measurements show that the increase of Cu content leads to some decrease in both the discharge capacity and the high-rate dischargeability (HRD), but leads to a significant improvement in cycling stability of the alloys. The discharge capacity decreases from 388.1 mAh/g (x=0) to 355.4 mAh/g (x=0.5), while the cycle stability S₅₀ increase from 50.4% (x=0) to 58.7% (x=0.5).Based on the above work, the effects of rapid quenching (melt-spinning with different cooling rates: 10, 20 and 30 m/s) on the phase structure and electrochemical properties of La₂Mg(Ni_0.85Co_0.15)₉Cu_0.3 alloy were investigated for improving the cycling stability of the alloy. It is found that all rapid quenching alloys consist of a main (La,Mg)Ni₃ phase with PuNi₃-type structure, a secondary LaNi₅ phase and a trace LaNi₂ phase. The abundance of LaNi₂ phase decreases with the increase of rapid quenching rate. Electrochemical measurements show that the increase of the rapid quenching rate leads to some decrease in both the discharge capacity and the high-rate dischargeability (HRD), but leads to a significant improvement in cycling stability of the alloy. The discharge capacity decreases from 369.1 mAh/g (as cast) to 349.4 mAh/g (30 m/s), while the cycle stability increases from 55.9% (as cast) to 64.6% (30m/s).The study on the phase structure and electrochemical properties of the Si-added La₂Mg(Ni_0.85Co_0.15)₉Si_x(x=0-0.5) alloys shows that all La₂Mg(Ni_0.85Co_0.15)₉Si_x(x=0-0.5) alloys consist of a main (La,Mg)Ni₃ phase with PuNi₃-type structure, a secondary LaNi₅ phase and a little LaNi₂ phase. The abundance of LaNi₂ phase increases with the addition of Si increases. Electrochemical measurements show that the increase of Si content leads to some decrease in both the discharge capacity and the high-rate dischargeability (HRD), but leads to a significant improvement in cycling stability of the alloys. The discharge capacity decreases from 388.1 mAh/g (x=0) to 293.4 mAh/g (x=0.5), while the cycle stability increase from 50.4% (x=0) to 73.9% (x=0.5).For improving the cycling stability of the alloys, the effects of rapid quenching on the phase structure and electrochemical properties of La₂Mg(Ni_0.85Co_0.15)₉Si_0.3 alloy were investigated. It is found that all rapid quenching La₂Mg(Ni_0.85Co_0.15)₉Cu_0.3 alloys consist of a main (La,Mg)Ni₃ phase with PuNi₃-type structure, a secondary LaNi₅ phase and a trace LaNi₂ phase. The abundance of LaNi₂ phase decreases with the increase of rapid quenching rate. Electrochemical measurements show that the increase of the rapid quenching rate leads to some decrease in both the discharge capacity and the high-rate dischargeability (HRD), but leads to a significant improvement in cycling stability of the alloy. The discharge capacity decreases from 335.1 mAh/g (as cast) to 297.8 mAh/g (30 m/s), while the cycle stability increase from 62.1% (as cast) to 84.2% (30 m/s).