Physical properties and electrochemical behavior of LiNi1−x Co x O2 (0.1≤x≤0.9) were investigated for use as positive electrode material in lithium secondary batteries with long cycle life. LiNi1−x Co x O2 are characterized by an α-NaFeO2 type structure. In a three-electrode test cell, LiNi0.7Co0.3O2 showed a large discharge capacity of 159Ahkg−1, and good cycle performance, with a discharge capacity deterioration ratio of 0.73% per cycle. In a 2Wh-class cell, LiNi0.7Co0.3O2 exhibited a discharge capacity of 520mAh, and good cyclability, with a discharge capacity deterioration ratio of 0.026% per cycle. LiNi0.7Co0.3O2 is a promising positive electrode material, having the largest discharge capacity and the best cycle performance in the series of LiNi1−x Co x O2 compounds.