Preparation And Electrochemical Properties Of Rare-earth Based Hydrogen Storage Electrode Alloys Used In Special MH/Ni Battery

In order to improve the overall performance of MH/Ni battery, the key point is the negative electrode materials with excellent electrochemical properties. AB5-type hydrogen storage alloys are main negative electrode materials in MH/Ni battery. However, the cobalt is very expensive and shares the40-50%of the total cost of the alloys, which limits their extensive applications in MH/Ni battery. So research and development of low-Co or Co-free alloy has been the focus of the study on AB5-type hydrogen storage alloys. The rare earth-Mg-Ni-based hydrogen storage alloys with high capacity and power are considered as the promising negative electrode materials for MH-Ni battery. However, these alloys are subject to poor cycling stability which is a key drawback for their industrial application. Furthermore, to date, the discharge performance of commercial MH/Ni battery at low temperature is poor and generally operating temperature is limited to above253K. It is urgent to improve the electrochemical performance of MH/Ni batteries at low emperature, especially below253K. In order to obtain negative electrode materials with excellent overall performance, the relationships among the preparation technology, composition, crystal structure and electrochemical peformance of several hydrogen storage alloys were studied in details. This thesis is composed of three parts. The first is a study on the element substitution of low-Co and Co-free AB5-type alloys. The second part is concerned with the effect factors of low-temperature performance. The third part is related to the rare earth-based hydrogen storage alloys with high low-temperature properties.Firstly, the electrochemical properties of LaNi4.1-xCo0.6Mn0.3Mx(M=Cu、Al;0≤x≤0.45) alloys in the temperature range of238to323K were studied systematically to aim at the inferior low-temperature discharge performance of hydrogen storage alloys. The key factors and mechanism to affect the low-temperature performance of hydrogen storage alloy were investigated, including their elements and temperatures.The Cu-containing alloy electrodes exhibit good low-temperature performance. Where x=0.15and0.3, the capacity of alloys are319.28mAh/g and316.24mAh/g at238K, while the discharge capacities of205.5mAh/g and154.52mAh/g are obtained at the discharge current of300mA/g, respectively. With the increasing of Cu content, the high rate dischargeability decrease for the temperature above273K, and both the enthalpy of hydride formations and exchange current density first increase then decrease. However, the activation energy of reaction on the surface first decrease then increase, and the hydrogen diffusion coefficients decrease gradually.The Al-containing alloy electrodes show high high-temperature capacity. The capacity at323K increases from266.04mAh/g (x=0) to302.04mAh/g (x=0.15), and then decreases to299.88mAh/g (x=0.45). With the increasing of Al content, the high rate dischargeability, exchange current density and hydrogen diffusion coefficient decrease in the temperature range of238-323K.Based on the results of low-temperature performance, thermodynamic and kinetic characteristics, it can be confirmed that the low-temperature properties of the hydrogen storage alloys with B-site substitution are mainly related to surface reaction activation energies and hydrogen diffusion behavior, other than the enthalpy of hydride-generation.Secondly, the effects of Mg addition and heat treatment on the electrochemical properties of La-Mg-Ni system alloys were investigated, and the factors and action mechanism of the effects on the cycle stability of hydrogen storage alloys were discovered. The effects of Mm substitution of La on the electrochemical performance in the temperature range from238K to303K were also studied.The as-prepared La0.7MgxNi2.5gCo0.5Mn0.3Al0.12(x=0.15,0.3) alloys consist of LaNis phase and minor La2Ni7phase. With the increasing of Mg content, the maximum capacities and high-rate dischargeabilities of as-cast La0.7MgxNi2.58Co0.5Mn0.3Al0.12alloys at room temperature increase gradually. The discharge efficiency increases from52.51%of Mg-free alloy to65.61of alloy with x=0.3tested at the discharge current of1800mA/g. The electrochemical properties are mainly attributed to the abundance of La2Ni7phase. With the increase of heat treatment temperature, the maximum capacity and high-rate dischargeability of La0.7MgxNi2.58Co0.5Mn0.3Al0.12alloys first increase then decrease. After heat treated at1123K, the La0.7Mg0.3Ni2.58Co0.5Mn0.3Al0.12alloy exhibits higher capacity and better high-rate dischargeability. The capacity is340mAh/g, and the discharge efficiency is59.67%at the discharge current of1800mA/g. When tested at238K, the capacity measured at the discharge current of60mA/g increases from247.9mAh/g (x=0) to286.6mAh/g (x=0.3). After heat treatment at1123K, the capacities are obviously improved, and the capacity is304.6mAh/g for the alloy with x=0.3. However, the rate capabilities of the electreods are deteriorated under low temperature, which indicates that heat treatment can’t improve the low-temperature rate capability of alloy electrodes.After heat treatment at1123K for12h, the Mm0.7MgxNi2.58Co0.5Mn0.3Al0.12(x=0,0.3) shows a good performance in the temperature range from238K to303K. As temperature increases, the capacities of Mg-free alloy electrode are335mAh/g,329.6mAh/g,325.8mAh/g, and320.1mAh/g,350.4mAh/g,347.mAh/g for the Mg-content alloy electrodes at238K,273K and303K, respectively. When tested at238K, the capacities of Mm0.7Mg0.3Ni2.58Co0.5Mn0.3Al0.12alloy electrode are241mAh/g and130mAh/g at the discharge current of150mA/g and300mA/g, much higher than the Mg-free alloy electrode. The test of electrochemical performance on MH-Ni AA1300mAh batteries has been performed, in which the Mmo.7Mgo.3Ni2.58Coo.5Mno.3Alo.12alloy is used as the negative electrode. The internal resistance, discharge characteristic and the capacity retention match the standard of Q/BS-001-2009. With good properties and high capacity of5C at both high and low temperatures, the batteries show a prospect in practical use. The capacity retention rate is higher than85%after150charge-discharge cycles.Finally, to improve the electrochemical properties of low-Co and Co-free AB5-type alloys the element substitution and rapidly quenched technique were used.LaNi4.5Co0.4-xAl0.1+x(x=0.00-0.30) alloys are prepared by arc melting method. With the increase of Al content, the high-rate dischargeability and exchange current density first increase and then decrease, the electrochemical reaction impedance first reduce then increase. The cycle stability and high-rate dischargeability of low-Co AB5-type hydrogen storage alloys are improved by the optimal replacement of Al (0.10≤x≤0.15). Good cycle stability and high-rate dischargeability have been observed for the alloys with Al content of x=0.15, on which the discharge efficiency is70.21%measured at the discharge current of1800mA/g.The effect of quenching rate on the microstructure and electrochemical properties of LaNi4.5Co0.25Al0.25alloy at303K has been investigated. With the increasing of quenching rate, the cycling stability of the alloy electrodes are markedly improved, whereas the high-rate dischargeability decrease. In summary, the alloy with10m/s quenching rate exhibits best cycle life, the capacity retention rate is85.20%after100charge-discharge cycles. Although the rapid solidification technology with appropriate quenching rate is benefit for cycle life, it is unsuited to prepare the high-power electrode materials.The effects of Pr replacement on the electrochemical properties of Co-free La1-xPrxNi4.2Mn0.3Al0.3Cu0.15Fe0.05(x=0-0.3) alloys are investigated in details. With the increase of Pr content, the cycle performance and high-rate dischargeability increase gradually, whereas the capacities decrease from318mAh/g to292.5mAh/g. The overall performance of La0.7Pr0.3Ni4.2Mn0.3Al0.3Cn0.15Fe0.05alloy is superior to that of low-co LaNi4.2Mn0.3Co0.2Al0.3alloy. For La0.7Pr0.3Ni4.2Mn0.3Al0.3Cu0.15Fe0.05alloy, the capacity retention rate is54.47%after100charge-discharge cycles, and the discharge efficiency of56.2%are obtained at the discharge current of1800mA/g.