Effect of crystal transformation on electrochemical characteristics of La–Mg–Ni-based alloys with A2B7-type super-stacking structures

Abstract

La0.75Mg0.25Ni3.5 alloys with hexagonal (2H-) and rhombohedral (3R-) (La,Mg)2Ni7 phase were created by powder metallurgy. Partial crystal transformation of 2H- into 3R-type allotropes was realized by heat treatment and introducing LaNi5 compound. It was found that the alloy annealed within 1073–1223 K kept (La,Mg)2Ni7 phase and obvious crystal transformation of 2H- into 3R-type occurred as annealing temperature reached 1223 K. Electrochemical study showed similar discharge capacity and degradation behavior for La0.75Mg0.25Ni3.5 alloys with different amounts of 2H- and 3R-type allotropes while HRD was promoted by increasing 3R-type phase abundance. Introducing LaNi5 into La0.75Mg0.25Ni3.5 alloy increased 3R- to 2H-type phase ratio and led to an additional plateau in P–C isotherms. LaNi5 introduction improved HRD, however it accelerated cycling degradation. Rietveld analysis indicated that after hydrogenation, the cell expansion of 2H- and 3R-type (La,Mg)2Ni7 phase was similar while the cell expansion of LaNi5 phase was smaller than that of (La,Mg)2Ni7 phase. This caused discrete cell expansion between (La,Mg)2Ni7 and LaNi5 phases, leading to severe pulverization and oxidation.