In order to investigate the influence of the dispersion state of electro-conductive additives on the properties of lithium ion batteries, we examined the charge–discharge properties of cathodes prepared with suspensions of ketjen black (KB) dispersed in an organic solvent with different particle sizes. Discharge capacity, durability and resistance to high-current discharge of the cathode prepared from a well-dispersed suspension was higher than those of the cathode prepared with ketjen black powder. Because ketjen black particles in the cathode prepared from a well-dispersed suspension formed a conductive dense-network uniformly, despite the low amount of additive, we concluded that an electro-conductive network made of ketjen black, which can transmit electrons smoothly from active materials to the collector, formed in the cathode when the cathode was prepared from a well-dispersed suspension.