The effect of Mg content on crystal transformation and electrochemical characteristics of La2-xMgxNi7 (x = 0.40-0.60) alloys has been studied. It is found that alloys with allotropic structures Ce2Ni7-type (2H) and Gd2Co7-type (3R) phases exist in a range of Mg concentration from 0.48 to 0.50. Beyond that range, further increasing Mg to 0.60 is favorable for the formation of PuNi3-type phase due to the rising ratio of [A(2)B(4)] to [AB(5)] slabs in regard to the demand of La and Ni atoms for Mg to assemble [A(2)B(4)] slabs. Conversely, the surplus of La and Ni atoms for Mg to assemble [A(2)B(4)] slabs lowers the ratio of [A(2)B(4)] to [AB(5)] slabs, which helps the formation of CaCu5-type phase as Mg decreases to 0.40. Electrochemical studies show that the electrochemical properties of the alloy electrodes with allotropic phases are superior in discharge capacity (388 mAh g(-1)) and cycling stability (78% capacity retaining rate at 100th cycle). Appearance of PuNi3-type or CaCu5-type phase reduces the cycling stability, while CaCu5-type phase can significantly improve the high rate dischargeablity (HRD) of the alloy electrode. (C) 2014 The Electrochemical Society. All rights reserved.