Effect of Pr on phase structure and cycling stability of La-Mg-Ni-based alloys with A2B7- and A5B19-type superlattice structures

Fast capacity degradation is a significant drawback hindering La-Mg-Ni-based alloys from wide application as anode electrode materials of nickel metal hydride batteries. Herein, the effect of Pr element on the phase structure and cycling stability of La0.8−xPrxMg0.2Ni3.4Al0.1 (x = 0, 0.1, 0.2 and 0.3) alloys is investigated. All the alloys contain (La,Mg)2Ni7 and (La,Mg)5Ni19 main phases as well as LaNi5 minor phase. It is found that Pr tends to form [AB5] subunits more than [A2B4] subunits, thus increasing the content of (La,Mg)5Ni19 phase with higher [AB5]/[A2B4] ratio in sacrifice of (La,Mg)2Ni7 phase. The increased (La,Mg)5Ni19 phase network with good structural stability increases the anti-pulverization and anti-amorphization resistance of the alloys. After 100 charge/discharge cycles, part of the superlattice phases in La0.6Mg0.2Ni3.4Al0.1 alloy decomposes into amorphous phase and LaNi5 phase, leading to the decrease in superlattce phase abundance; while little amorphization is observed for La0.6Pr0.2Mg0.2Ni3.4Al0.1 alloy and its phase contents remain almost unchanged. Correspondingly La0.6Pr0.2Mg0.2Ni3.4Al0.1 alloy electrode has a slighter oxidation degree after 100 cycles in alkaline electrolyte and exhibits good electrochemical cycling stability with a discharge capacity of 340 mAh g−1 and a cycling stability of 90.7% at the 100th cycle.