A voltage-sensitive separator is prepared simply by impregnating electroactive poly(3-decylthiophene) (P3DT) polymer into a commercial porous separator and tested for a self-actuating control of overcharge voltage of LiFePO4/C lithium-ion batteries. The experimental results demonstrate that this type of separator can be reversibly p-doped and dedoped to maintain the cell's voltage at a safe value of ≤4V even at high rate overcharge of 3C current, effectively protecting the batteries from voltage runaway. Since this P3DT-modified separator has no obvious negative impact on the normal charge–discharge performance of the batteries, it may be adopted for practical application in commercial lithium ion batteries.