La 0.75 Mg 0.25 Ni 3.5 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 LaNi 5 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 La 0.75 Mg 0.25 Ni 3.5 alloys with different amounts of 2H- and 3R-type allotropes while HRD was promoted by increasing 3R-type phase abundance. Introducing LaNi 5 into La 0.75 Mg 0.25 Ni 3.5 alloy increased 3R- to 2H-type phase ratio and led to an additional plateau in P–C isotherms. LaNi 5 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 LaNi 5 phase was smaller than that of (La,Mg) 2 Ni 7 phase. This caused discrete cell expansion between (La,Mg) 2 Ni 7 and LaNi 5 phases, leading to severe pulverization and oxidation.