La1.7+xMg1.3-x(NiCoMn)9.3(x=0～0.4)贮氢合金的相结构与电化学性能

摘要

The phase structure and electrochemical properties of La1.7+xMg1.3-x(NiCoMn)9.3(x=0-0.4) alloys were investigated.The XRD analysis reveals that the alloys consist of LaNi5 phase and other phases,such as LaMg2Ni9 phase (PuNi3 structure) and La4MgNi19 phases (Ce5Co19+Pr5Co19 structure,namely A5B19 type).With the increase of the x value,the LaMg2Ni9 phase fades away and La4MgNi19 phases appear,while the abundance of LaNi5 phase firstly increases and then decreases.At the same time,the cell volume of LaNi5 phase and LaMg2Ni9 phase decreases.The electrochemical measurement shows that alloy electrodes could be activated in 4-5 cycles,and with the increase of the x value,the maximum discharge capacity gradually increases from 330.9 mA·h/g (x=0) to 366.8 mA.h/g (x=0.4),but the high-rate dischargeability (HRD) and cyclic stability (S) decrease somewhat (x=0.4,HRD600=82.32％,S100=73.8％).It is found that the HRD is mainly controlled by the electrocatalytic activity on the alloy electrode surface,and the decline of cyclic stability is due to the appearance of A5B19 type phase with larger hydrogen storage capacity,which leads to larger volume expansion and more intercrystalline stress and then easier pulverization during charging/discharging.%对La1.7+xMg1.3-x(NiCoMn)9.3(x=0～0.4)贮氢合金相结构和电化学性能进行研究.结构分析表明,合金主要由LaNi5相(CaCu5结构)和其他相组成,如LaMg2Ni9相(PuNi3结构)或La4MgNi19相(Ce5Co19+Pr5Co19结构).随着x的增加,LaMg2Ni9相消失并出现La4MgNi19相,而LaNi5相的含量则先增加后减小,且晶胞体积下降.电化学分析表明,合金电极只需4、5次循环即可活化；随着x的增加,最大放电容量逐渐增大,从x=0的330.9 mA.h/g增加到x=0.4的366.8 mA·h/g,但高倍率放电性能(HRD)和循环稳定性(S)则有所下降(x=0.4,HRD600=82.32％,S100=73.8％).研究认为,HRD主要由合金电极表面的电催化活性控制,而循环稳定性的下降则是由于x=0.4合金中出现了具有较大吸氢量的Ce5C019和Pr5C019型结构相,导致吸氢膨胀率和晶间应力增大,使合金颗粒在吸放氢过程中较易粉化所致.