In order to enhance the specific capacity of a sonochemically synthesized manganese oxide/carbon nanocomposite as the cathode material of a high-power lithium-ion battery, we optimized the synthesis conditions, such as the reaction temperature and specific surface area of the carbon. Through the reaction temperature control, the initial discharge capacity increased in keeping with the active material content and then decreased above a certain active material content. This degradation was caused by an increase in the active material content that was not able to specifically contact the carbon. In contrast, the use of a carbon with a higher specific surface area was found to lead to the higher specific capacity and suffered no capacity drop; the initial discharge capacities of 126 and 99.9 mAh g-1 were obtained at current densities of 1 and 10 A g-1, respectively. 2005 The Electrochemical Society.