Phase structure and electrochemical properties of La1.7+xMg1.3-x(NiCoMn)9.3(x=0-0.4) hydrogen storage alloys

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 A(5)B(19) 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%, S-100=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 A(5)B(19) type phase with larger hydrogen storage capacity, which leads to larger volume expansion and more intercrystalline stress and then easier pulverization during charging/discharging.