The synthesis and the structural and morphological characterization of micro-sized, spherical LiNi0.5−x Co2x Mn1.5−x O4 powders are reported. The results demonstrate that these materials have a high electronic conductivity as well as a high tap density. We show that when tested in a lithium cell the Co-substituted LiNi0.5−x Co2x Mn1.5−x O4 electrodes exhibited a better cycling performance and a better rate capability than the Co-free LiNi0.5Mn1.5O4 parent cells. The enhanced electrochemical performances are attributed to the increase in electrical conductivity and in structural stability caused by Co substitution for Ni and Mn and the mixed valence effect of Mn3+/Mn4+. Due to these properties, the LiNi0.5−x Co2x Mn1.5−x O4 electrodes are believed to be of importance for the progress of the lithium battery technology.