过化学计量比La(NiMnFe)_(6.0)贮氢合金的晶体结构和电化学性能

Structural and Electrochemical Properties of Super-Stoichiometric La(NiMnFe)_(6.0) Hydrogen Storage Alloys

详细研究了无Co过化学计量比合金LaNi(4.75-x)FexMn1.25及LaNi4.75Mn(1.25-x)Fex(x=0.2～0.7)在1000℃168h固溶及均匀化退火条件下合金的相结构及电化学性能。X射线衍射(XRD)及能谱分析(EDS)表明,La(NiMnFe)6.0退火合金由过化学计量比CaCu5型结构吸氢主相和含Ni Fe Mn三元组分少量的第二相组成。当Fe元素替代合金中的Ni时,合金晶胞体积增大,主相的化学计量比及Mn的固溶度有所增加。而当Fe元素替代合金中的Mn时,合金的晶胞体积减小,主相的化学计量比减小。电化学测试表明,Fe替代合金中的Ni时合金电极的放电容量较低但循环稳定性好;Fe替代Mn时合金电极放电容量较高但循环稳定性差。分析原因可能是由于两系列合金中Mn原子哑铃对数量上的差别所引起的,并基于B端双原子哑铃占位模型分析了合金B端组成和结构对合金电极性能的影响,提出了改善合金综合电化学性能的方法。

The microstructure and electrochemical properties of Co-free super-stoichiometric hydrogen storage alloy LaNi_((4.75-x))Fe_xMn_(1.25) and LaNi_(4.75)Mn_((1.25-x))Fe_x (x=0.2～0.7) were investigated in detail. The alloys were annealed homogeneously at 1000 ℃ for 168 h. XRD and EDS analysis show that the alloys are composed of hydrogen absorption CaCu_5-type phase and little second phase containing ternary ingredient Ni-Mn-Fe. The substitution of Fe for Ni in the alloy enlarges the unit cell volume, the stoichiometry of main phase and the solid solubility of element Mn in the main phase, whereas substitution of Fe for Mn in the alloy diminishes the unit cell volume and the stoichiometry of main phase. The electrochemical analysis shows that the discharge capacity of electrodes is decreased by the partial substitution of Fe for Ni, while the cycle stability was improved. In the case of the substitution of Fe for Mn, the discharge capacity is increased while the cycle stability is deteriorated. This may be attributed to the difference of amount in sites occupied by dumbbell pair of element Mn in the two series of alloys. The effects of composition and microstructure of substitutional element B on the properties of alloy electrodes were discussed based on the microstructure of dumbbell pairs. From the results an approach was proposed to improve the comprehensive electrochemical properties.