By an oxidative-coupling copolymerization with coumarin, the oxidation potential of polytriphenylamine was improved to 3.75 V from 3.60 V. The copolymer exhibited excellent electrochemical activity in redox cycling. The separator used in the experiment was prepared by roll pressing the copolymer powder with 30% (wt.) polytetrafluoroethylene into a similar to 80 mu m thick sheet, which was incorporated into a LiFePO4-Li cell. Tests showed that when the cell was overcharged to 3.789 V, the copolymer separator was oxidized and became electronically conducting, which caused short circuit formation and prevented the organic electrolyte from being oxidized. Although the cell was overcharged at 0.3 C for 5 h, the voltage was stable at 3.789 V. When discharged, the cell released all its capacity at normal charge. The cell was overcharged seven times with identical results obtained for each cycle. Overcharging at different rates demonstrated that the separator could exhibit reversible, self-activating protection for LiFePO4-based batteries, even at 1.3 mA/cm(2) (3.2 C to the cell). (C) 2009 Elsevier Ltd. All rights reserved.