As the secondary battery market expands to the electric vehicle and renewable energy fields and in particular as demand for high capacity and high safety lithium-ion batteries continues to grow, the function of the separator is becoming more important than ever. In this report, we propose an inorganic-organic hybrid coating strategy that enhances the thermal and dimensional stability of polymer battery separators without increasing the overall separator thickness. Ultrathin coating layers prepared using a combination of atomic layer deposition and polydopamine treatment cover fibrils of a porous polyethylene separator more uniformly and endow polyethylene separators with superior thermal properties compared to those coated with only one layer, which is linked to the safety of battery cells. In addition, dual coating enhances the electrolyte uptake and wettability of a conventional polyethylene separator, which ensures improved power capability and stable cycle performance. The dual coating strategy via organic/inorganic hybridization is an easy and effective way to overcome the inherent disadvantages of conventional polyethylene separators and can be a practical approach to safer and higher capacity batteries.