LiNi 1-x Co x O 2 is being considered for use as a cathode material in commercial lithium-ion batteries because of its higher capacity and lower material cost than its end member LiCoO 2 . On the other hand, LiNi 1-x Co x O 2 has better thermal stability than its other end member LiNiO 2 . In this paper we examine the influence of particle size and particle morphology on the electrochemical behavior of LiNi 1-x Co x O 2 and the dependence of the irreversible capacity on Co content, temperature, and particle size. Coin cell performance at room temperature and at 40°C indicates that LiNi 1-x Co x O 2 with a large crystal particle size (P c ) has a low capacity fade rate. The irreversible capacity is reduced at high temperature. By optimizing particle size and cobalt content in LiNi 1-x Co x O 2 , it is possible to achieve cycle performance comparable with LiCoO 2 at room temperature and at 40°C, and have a reversible capacity (between 150 and 200 mAh/g ) substantially higher than LiCoO 2 (140 mAh/g). LiNi 1-x Co x O 2 (0.1 < x < 0.3) tested at C/1 discharge and at 40°C also shows a low capacity fade rate comparable to that of LiCoO 2 material