A polytriphenylamine (PTPAn)-modified separator was prepared simply by impregnating triphenylamine monomers into a commercial Celgard separator and in situ polymerizing the monomers into electroactive phase by oxidant ozone. This type of electroactive separator can transform from an insulating state to a conductive state at overcharged voltage of ∼3.7V (vs. Li+/Li) and act as a self-actuating potential-switchable separator for overcharge protection of LiFePO4/C Li-ion batteries. The experimental results demonstrated that this electroactive separator can reversibly control the cell's voltage at the safe value less than 4.15V at high rate overcharge of 2C current without obvious negative impact on the normal charge–discharge performances of the commercial LiFePO4/C batteries even at prolonged overcharge cycling, showing a potential application in 3.6V-class lithium-ion batteries.