The thermal shrinkage of the widely adopted polyethylene (PE) separators has become a very critical issue for safety in lithium ion batteries, especially for electrical vehicle applications. As an effort to minimize the thermal shrinkage of PE separators, in this study, we coat PE separators with a substance from the polyimide family by engaging a simple dipping method. A polymer coating at the optimal concentration endows PE separators with improved mechanical strength and thus remarkably increased resistance against thermal shrinkage at elevated temperatures. Despite the coating layers, the cells with the polyimide-coated separators exhibit almost identical electrochemical properties to those of the cells with the bare PE separators, thus proving that the polymer coating does not affect the battery performance. The preserved electrochemical properties can be explained by morphology and porosity characterization.