Microstructure And Electrochemical Properties Of Rare Earth-Mg-Ni-Based Hydrogen Storage Alloys

The as-prepared rare earth-Mg-Ni-based hydrogen storage alloys are high capacity electrode materials which have been developed in recent years, and they are considered as promising negative electrode materials for nickel-metal hydride (Ni-MH) batteries. However, these alloys are subject to poor cycling stability which is a key drawback for their industrial application. In order to reveal micromechanisms and changes in cycling stability, and to provide methods for improving electrochemical properties of alloy electrodes, series of rare earth-Mg-Ni-based hydrogen storage alloys were prepared and studied. XRD and SEM-EDS methods were used to thoroughly characterize phase structure and morphology. Galvanostatic method was used to measure charge/discharge characteristics. Electrochemical kinetic parameters for electrode reactions, such as exchange current density, limiting current density and hydrogen diffusion coefficient were also tested. Effects of stoichiommetry, elemental Mg, rare earths and CuO addition on microstructure and electrochemical properties of alloys were studied.The as-prepared Ml_0.70Mg_0.30(Ni_3.95Co_0.75Mn_0.15Al_0.15)_x (x = 0.60, 0.64, 0.68, 0.70, 0.76) (Ml denotes La-rich rare earths) alloys with Ml/Mg = 0.70/0.30 and Ml_0.80Mg_0.20- (Ni_3.95Co_0.75Mn_0.15Al_0.15)_x (x = 0.68, 0.70, 0.72, 0.74, 0.76) alloys with Ml/Mg = 0.80/0.20 consist of LaNi₅ phase, LaNi₃ phase and minor La₂Ni₇ phase. The content of LaNi₅ phase increases and that of LaNi₃ phase decreases with increasing B/A. Electrochemical P-C-T results show that metal hydride stability of the alloys decreases with increasing B/A. The maximum discharge capacity, electrochemical kinetics, high rate dischargeability and low temperature dischargeability all increase and then decrease with increasing stoichio- mmetry. In case of B/A = 3.5, the alloy electrode shows better high rate dischargeability and low temperature dischargeability. For alloy electrodes with Ml/Mg = 0.70/0.30, cycling stability increases with stoichiommetry, the capacity retention rate at the 140th cycle increases from 71.5% (x = 0.60) to 85.8% (x = 0.76).It is found that the rare earth-Mg-Ni-based hydrogen storage alloy electrodes with B/A = 3.5 show excellent overall electrochemical properties. Therefore, on basis of B/A = 3.5 Ml_1-xMg_xNi_2.80Co₍0.50_Mn₍0.10_Al_0.10 (x = 0.08, 0.12, 0.20, 0.24, 0.28) alloys were prepared. The content of LaNi₅ phase decreases and that of LaNi₃ phase increases with increasing Mg content. The maximum discharge capacity increases from 322 mAh/g (x = 0.08) to 375 mAh/g (x = 0.12) and then decreases to 351 mAh/g (x = 0.28). As Mg content increases, exchange current density, limiting current density, hydrogen diffusion coefficient increase, and then decrease, showing that charge transfer rate at the surface and hydrogen diffusion rate in the bulk increase, and then decrease. Consequetly, when x is 0.20, the alloy electrode shows better high rate dischargeability and low temperature dischargeability. The high rate dischargeability at 1200 mA/g is 41.7% and the discharge capacity at 233 K is 256 mAh/g. However, the cycling stability needs further improvement.The low-Co and Mn-free alloys La_0.80-xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) and La_0.20Nd_0.60-xPr_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0, 0.10, 0.20, 0.30, 0.40) consist of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. In case of partial Nd substitution for La, the maximum discharge capacity and high rate dischargeability increase, and then decrease with increasing Nd content. The maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The high rate dischargebility at 1200 mA/g increases from 34.0% (x = 0.20) to 45.3% (x = 0.50) and then decreases to 44.1% (x = 0.60). The low temperature dischargeability and self-dischrge rate increase as Nd content increases. When x is 0.50, the alloy elecreode shows good cycling stability, with capacity retention rate at the 100th cycle of 82.7%. In case of partial Pr substitution for Nd, the lattice parameters and cell volume increase with increasing Pr content. The metal hydride stability is enhanced by increase in Pr content, suppressing self-discharge. When Pr/Nd is 1:1, the alloy electrode exhibits better high rate dischargeability and low temperature dischargeability, as well as better cycling stability, with a capacity retention rate at the 100th cycle of 88.7%.Some fine Cu particles are reduced upon charging from CuO, micro-capsulating at surface of alloy particles in case of adding CuO powders into the rare earth-Mg-Ni-based alloy electrodes. These Cu particles ameliorate electrical conductivity, heating conductivity and corrosion resistance. Consequently, the maximum discharge capacity, high rate dischargeability, cycling stability and high temperature dischargeability are all improved. The degradation in capacity becomes slower when the electrodes are charged / discharged for more cycles.