Single Gd2Co7-type phase La0.65Gd0.10Mg0.25Ni3.50 alloy is prepared with induction melting followed by annealing treatment. Rietveld refinements show that the as-cast alloy consists of CaCu5-type phase, Gd2Co7-type phase and Ce5Co19-type phase. At temperature of 1123-1248 K, the CaCu5-type and Ce5Co19-type phases transform into Gd2Co7-type phase via peritectic reactions. As annealed at 1248 K for 36 h, the CaCu5-type and Ce5Co19-type phases totally transform into Gd2Co7-type phase and the alloy shows a single phase structure. The increasing abundance of Gd2Co7-type phase makes a significant improvement on the discharge capacity and the Gd2Co7-type single phase alloy shows a superior discharge capacity of 386 mAh g(-1). Moreover, the Gd2Co7-type single phase alloy exhibits preferable capacity retention, which is up to 88.4% at the 160th cycles. The pulverization of the single phase alloy mainly occurs at earlier stage and the average particle size is not significantly reduced after 50 charge/discharge cycles. It is found that the oxidation of Mg mainly occurs at an early stage, while the oxidation of La happens in the whole process of charge/discharge cycling. Moreover, the oxidation of La gets aggravated after 100 cycles as the hydroxides of La is observed growing from nanoscale (< 100 nm) needles (100th cycles) to micrometer-size (> 2 mu m) rods (150th cycles). (c) 2017 The Electrochemical Society. All rights reserved.