La(Ni,Sn)_(5+x)(x=0～0.35)贮氢合金的晶体结构与电化学性能

Crystal structure and electrochemical properties of La(Ni,Sn)_(5+x)(x=0～0.35) hydrogen storage alloys

系统研究了La（Ni,Sn）5＋x（x=0-0.35）无Co贮氢合金的化学计量比对其晶体结构及电化学性能的影响.X射线衍射分析仪（XRD）分析表明,上述合金均为单相CaCu5结构.但在过计量比（x〉0）合金的结构中,有部分1a位置元素（La）的原子被沿c轴定向排列的Ni-Ni“哑铃”对所替代,且其替代La原子的分数随x的增大而增多,从而导致合金晶胞的c轴及c/a比值明显增大,晶胞体积有所减小,并显著降低了合金的吸氢体积膨胀率.电化学测试表明,增大x值可使合金的循环稳定性得到显著提高,但也使合金的最大放电容量和高倍率放电性能有所降低.研究发现,由于合金的化学计量比增大会使其结构中含有较多的Ni-Ni“哑铃”对,合金的抗吸氢粉化能力得到了明显的改善,从而使合金在充放电过程中的反应比表面积有所减小、腐蚀速率得到抑制、循环稳定性得到显著提高.但合金反应比表面积的减小也导致了电极反应的速率的减小,从而使其高倍率放电性能有所降低.

The effect of stoichiometric ratio on the crystal structure and electrochemical properties of Co-free La（Ni,Sn）5＋x（x=0~0.35） alloys was systematically studied.x-ray diffraction（XRD） analysis shows that all the alloys are single-phase with CaCu5-type structure.In the CaCu5-type structure of over-stoichiometric（x〉0） alloys,La atoms on 1a sites are partially substituted by the dumbbell pairs of Ni atoms aligned along the c axis,and the fraction of Ni-Ni dumbbells increases with the increase of x value,which leads to a noticeable increase in the c axis and the c/a ratio but a decrease in the cell volume,and also leads to a noticeable decrease in volume expansion rate of the alloys on hydriding.Electrochemical test shows that with the increase of x,the cycling stability of the alloys is greatly improved,while the maximum discharge capacity and high-rate dischargeability of the alloys are somewhat lowered.The increase of stoichiometric ratio of the alloys leads to a larger fraction of Ni-Ni dumbbells in their structure,and improves greatly the pulverization resistance of the alloys on hydriding,and then decreases the reaction surface area of the alloy particles during cycling,which leads to higher corrosion resistance and improved cycling stability.But the decrease of the reaction surface area of the alloys also lowers the electrode reaction rate,resulting in a lower high-rate dischargeability.