In order to improve the electrochemical cycle stability of the RE–Mg–Ni-based A2B7-type electrode alloys, a small amount of Si has been added into the alloys.The casting and annealing technologies were adopted to fabricate the La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) electrode alloys. The impacts of the addition of Si and annealing treatment on the structures and electrochemical performances of the alloys were investigated systematically. The results obtained by XRD and SEM show that all the as-cast and annealed alloys are of a multiphase structure, involving two main phases(La, Mg)2Ni7and La Ni5 as well as a residual phase La Ni3. Both adding Si and the annealing treatment lead to an evident change in the phase abundance and cell parameters of(La, Mg)2Ni7and La Ni5 major phases of the alloy without altering its main phase component. Moreover, the annealing treatment has the composition of the alloy distributed more homogeneously overall and simultaneously causes the grain of the alloy to be coarsened obviously. The electrochemical measurements indicate that adding Si and the annealing treatment give a significant rise to the influence on the electrochemical performances of the alloys. In brief, the cycle stability of the as-cast and annealed alloys evidently increases with the rising of Si content, while their discharge capacities obviously decrease under the same circumstances. Furthermore, the electrochemical kineticproperties of the electrode alloys, including the high rate discharge ability, the limiting current density(IL), hydrogen diffusion coefficient(D), and the charge-transfer resistance, first augment and then decline with the rising of Si content. Similarly, it is found that the above-mentioned electrochemical properties first mount up and then go down with the rising annealing temperature.
以综合电化学性能较佳的低镁含量合金La0.64Gd0.2Mg0.16Ni3.1Co0.3Al0.1为基础,通过改变Mg元素含量的添加方式,用感应熔炼方法与热处理制备了La0.64Gd0.2Mg0.16（1＋x）Ni3.1Co0.3Al0.1系列合金,系统研究了该条件下镁元素成分波动（Mg过量值x）对La-Mg-Ni系A2B7型合金微观结构和电化学性能的影响规律。合金相结构分析表明,合金退火组织由主相Ce2Ni7（Gd2Co7）型以及Pr5Co19型、Pu Ni3型和Ca Cu5型多相组成,随Mg过量值x增加,合金中主相Ce2Ni7型相丰度呈现先增加后减小的趋势;当Mg过量值0〈x≤50%时,合金组织的Ce2Ni7型主相相丰度达到81.04%~87.18%;x=0,80%时,Ce2Ni7型主相丰度减小至76.3%以下。电化学测试结果表明,随Mg过量值x增加,合金电极最大放电容量呈先增加后降低趋势,x=10%时合金具有最高电化学放电容量（384.6 m Ah·g-1）;当Mg过量值x在5%~50%范围内变化时,其电极循环稳定性均保持在S100≥90%,此时镁元素成分波动变化对合金电极循环稳定性的影响不敏感。合金电极的高倍率放电性能（HRD）随Mg过量值的增加呈先增大后减小趋势,其中电极表面的电荷转移速率是影响合金电化学反应动力学性能的主要控制步骤。
The La-Mg-Ni-based A2B7-type Lao.8_xNdx Mgo.2Ni3.35Alo.lSio.o5 （x = 0, 0.1, 0.2, 0.3, and 0.4） electrode alloys were prepared by casting and annealing. The influence of the partial substitution of Nd for La on the structure and electrochemical performances of the alloys was investigated. The structural analysis of X-ray diffraction and scanning electron microscopy reveals that the experimental alloys consist of two major phases： （La,Mg）2Ni7 with the hexagonal Ce2Ni7-type structure and LaNi5 with the hexagonal CaCus-type structure as well as some residual phases of LaNi3 and NdNis. The electrochemical measurements indicate that an evident change of the electrochemical performance of the alloys is associated with the substitution of Nd for La. The discharge capacity of the alloy first increases then decreases with the growing Nd content, whereas their cycle stability clearly grows all the time. Furthermore, the measurements of the high rate discharge ability, the limiting current density, and hydrogen diffusion coefficient all demonstrate that the electrochemical kinetic properties of the alloy electrodes first augment then decline with the rising amount of Nd substitution.
用XRD、SEM、EDS和电化学测试方法研究了退火温度对A_2B_7型La_(0.33)Y_(0.67)Ni_(3.25)Mn_(0.15)Al_(0.1)储氢合金微观组织和电化学性能的影响规律。结果表明,合金铸态组织由2H-Ce_2Ni_7、3R-Gd_2Co_7、CaCu_5和3R-Ce_5Co_(19)型相组成;随退火温度(850~950℃)升高,Ce_2Ni_7型主相丰度和晶胞体积逐渐增加,至950℃退火后,CaCu_5和Gd_2Co_7型相基本消失,主相Ce_2Ni_7型相丰度和晶胞体积均达到最大值;退火温度≥950℃时,Ce_2Ni_7型和Ce_5Co_(19)型相丰度分别又有所减少和增加。950℃退火合金具有较低的放氢平台压(1.92~8.70 k Pa)和较高的电化学放电容量(371 mAh/g),经100次充放电循环后其容量保持率S100达到89%。退火合金电极的HRD性能均得到不同程度的提高,其中950℃退火合金具有最佳的大电流放电性能(HRD900=83.4%)。氢在合金中的扩散是影响其高倍率放电性能的控制因素。
