Cobalt-exchanged LiCoyMn2-yO4 (y = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) is prepared at 800 degrees C in air. The crystal symmetry of the LiCoyMn2-yO4 is determined as cubic spinel with space group Fd3m. The lattice parameter and the discharge capacity decrease with increase in substituted Co content, but the cycle performance is enhanced. The first discharge capacity of LiCo0.2Mn1.8O4 is 96 mAh g(-1) in the 3.7-4.3 V range and 109 mAh g(-1) in the 3.7-5.1 V range. A Mn(II) peak is observed in the cyclic voltammogram for spinel LiMn2O4. It is difficult to remove this Mn(II) with conventional preparation methods. The peak disappears in cobalt-exchanged spinel sample, LiCoyMn2-yO4 (y > 0.1), and the cycle performance is enhanced.