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

Abstract 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 La 0.8− x Pr x Mg 0.2 Ni 3.4 Al 0.1 ( x  = 0, 0.1, 0.2 and 0.3) alloys is investigated. All the alloys contain (La,Mg) 2 Ni 7 and (La,Mg) 5 Ni 19 main phases as well as LaNi 5 minor phase. It is found that Pr tends to form [AB 5 ] subunits more than [A 2 B 4 ] subunits, thus increasing the content of (La,Mg) 5 Ni 19 phase with higher [AB 5 ]/[A 2 B 4 ] ratio in sacrifice of (La,Mg) 2 Ni 7 phase. The increased (La,Mg) 5 Ni 19 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 La 0.6 Mg 0.2 Ni 3.4 Al 0.1 alloy decomposes into amorphous phase and LaNi 5 phase, leading to the decrease in superlattce phase abundance; while little amorphization is observed for La 0.6 Pr 0.2 Mg 0.2 Ni 3.4 Al 0.1 alloy and its phase contents remain almost unchanged. Correspondingly La 0.6 Pr 0.2 Mg 0.2 Ni 3.4 Al 0.1 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 100 th cycle.