The effects of conducting carbon additive on the performance of LiCoO2 and LiMn2O4 cathodes in lithium-ion batteries have been investigated. While 2–5wt.% of carbon are adequate for the LiMn2O4 cathode to deliver good rate capability and maintain cyclability, the LiCoO2 cathode requires more carbon (10wt.%) to achieve likewise. The sensitivity of the LiCoO2 electrode for carbon content is particularly pronounced in high rate charging. Cyclic voltammetry and electrochemical impedance spectroscopy measurements indicate that deteriorating particle contacts leading to the incomplete charging of the active materials is the primary cause of material non-performance.