An electroactive polytriphenylamine (PTPAn) was synthesized and used as separator material for providing a self-activating overcharge protection of rechargeable lithium batteries. The experimental results from the Li-LiFePO4 cells demonstrated that the electroactive separator could transform from an electronically isolating state to a conductive state at overcharge, producing an resistive internal short circuit to maintain the cell's voltage at the safety value of similar to 3.75 V. In addition, the electroactive PTPAn separator works reversibly and has no negative influences on the normal charge-discharge behaviors of the Li-LiFePO4 cells. (c) 2006 Elsevier B.V. All rights reserved.