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 ∼3.75V. In addition, the electroactive PTPAn separator works reversibly and has no negative influences on the normal charge–discharge behaviors of the Li–LiFePO4 cells.