To improve the safety of lithium-ion battery, a new conceptual cathode, which contains a positive temperature coefficient (PTC) compound consisting of a carbon black/polyethylene composite as the conductive material, was fabricated. Cells that incorporated PTC cathodes not had only good discharge characteristics but also high safety performance. To investigate the safety mechanism of PTC cathodes, alternating current (ac) impedance spectra were measured and analyzed. Based on the results of fitting, the resistance of a PTC cell which corresponds to ohmic resistance increased several fold and the resistance which corresponds to charge transfer resistance increased more than one order of magnitude at 140 degrees C, because the electrical resistance of PTC cathodes increased at high temperature. Moreover, an overcharge test was performed for laminated prismatic PTC cells under a charge rate of 1.5 C to 10 V. The cell temperature did not increase after the short circuit, because the cell voltage reached the set voltage early and the short-circuit current barely flowed due to an increase in the cell impedance. These results indicate that cells which incorporate PTC cathodes are safer than conventional cells. (C) 2006 The Electrochemical Society.