Abstract:The phase structure and electrochemical properties of La_1.7+xMg_1.3-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.

Key words: hydrogen storage alloy; A₅B₁₉ type; crystal structure; electrochemical property; La-Mg-Ni system