To improve the safety of lithium-ion batteries, a new type of positive electrode, which contains a positive temperature coefficient (PTC) compound consisting of a carbon black/polyethylene composite as the conductive material, was fabricated. The relation between the positive electrode composition and both the discharge characteristics and safety was investigated. The discharge capacity of PTC cells increased as the PTC compound ratio decreased or the acetylene black (AB) ratio increased. In an external short-circuit test, the maximum short-circuit current decreased with an increase in the PTC compound ratio, or with a decrease in the AB ratio. Moreover, in an overcharge test, the voltage of the PTC cells with any composition increased earlier than that in a conventional cell and a cell with a high PTC compound ratio showed high overcharge tolerance. Based on the results of X-ray diffraction (XRD) measurements for the positive electrodes of PTC cells, it was clear that there was inadequate oxidation of the active material, since the Li+ extraction reaction from Li x CoO2 hardly progress at the part of the active material which contacts the PTC compound due to an increase in the resistance of the PTC compound at the high temperature.