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

La0.75Mg0.25Ni3.5 alloys with hexagonal (2H-) and rhombohedral (3R-) (La,Mg)(2)Ni-7 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)(2)Ni-7 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)(2)Ni-7 phase was similar while the cell expansion of LaNi5 phase was smaller than that of (La,Mg)(2)Ni-7 phase. This caused discrete cell expansion between (La,Mg)(2)Ni-7 and LaNi5 phases, leading to severe pulverization and oxidation. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.