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The MmNi3.55Co0.75Mn0.4Al0.3/5 wt.% Mg2Ni composite hydrogen storage alloys were prepared by two-step remelting, and the as-cast alloys were annealed at 1023, 1123 and 1223 K for 10 h under vacuum.Phase structures and electrochemical properties of the alloys were investigated systematically by X-ray diffraction (XRD) and electrochemical experiments.The results show that the as-cast composite alloy is composed of the LaNi5 phase and a small amount of the Mg2Ni phase.For the annealed alloys, however, the LaNi5 phase is the main phase and a (La,Mg)Ni3 new phase appears obviously only when the annealed temperature is 1023 K.The maximum discharge capacity, discharge potential characteristic and high rate dischargeability of composite alloys increase firstly and then decrease with increasing annealing temperature.The alloy annealed at 1023 K exhibits the best the maximum discharge capacity, discharge potential characteristic and high rate dischargeability.The capacity retention increases monotonically with the increase of annealing temperature, and the capacity retention of the alloy increases from 86.6 % (As-cast) to 92.4 % (annealed at 1223 K) after 60 charge-discharge cycles.