A conceptual positive temperature coefficient (PTC) cathode has been proposed to improve the safety of large-scale lithium-ion batteries. The PTC cathode contains the PTC compound as the conductive material, which increases its resistivity at temperatures above its melting point. In this paper, to improve the performance of cells using PTC cathodes, acetylene black (AB), which supports conductivity in the cathode as a secondary conductive material, was added. Cells using PTC cathodes containing a small amount of AB (PTC-AB cell) had better discharge characteristics and a longer cycle life than a cell using a PTC cathode without AB (PTC cell). For a basic evaluation of battery safety, an external short-circuit test and a discharge test were performed at a temperature higher than the melting point of the PTC compound. The short-circuit current of the PTC-AB cells was lower than 1 A at 140 degrees C, which was almost the same as the current of the PTC cell. Moreover, under a discharge rate of 3C, the voltage of PTC-AB cells dropped sharply at 135 degrees C due to a drastic increase in PTC cathode resistivity. These results indicate that the addition of AB to PTC cathodes improves cell performance while maintaining battery safety. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1943594] All rights reserved.