Polyolefin separators are inherently hydrophobic and thermally unstable, contributing to poor cycle performance and high thermal shrinkage, respectively, which can shorten cycle life. Herein, a high-performance supercapacitor based on a composite separator made from nano-Al2O3/PVDF-coated on polyethylene (PE) polyolefin substrate was prepared using a low-cost casting (stir-dip-coat-dry) technique and an electrolyte containing 1M EMI-BF4 salt in EC : EMC:DMC (1 : 1 : 2 vol%) is reported. The results show that integration of nano-Al2O3 in the PVDF matrix contributes to a large interactive surface area that attenuates interfacial energy at the separator-electrolyte boundary and improves porosity as well as the overall performance. The filler also enhances high mechanical anchoring onto the PE substrate, contributing to the overall physical and electrochemical properties of the separator. These modified PE separators with porous microstructure demonstrate superior electrolyte wettability (88%), stable electrochemical performance, and high cycle stability superior to analogous cells with commercial separators. The pair of coated modified separators with the 1M EMI-BF4 modified electrolyte registered a high ionic conductivity value of 2.23mS/cm. This facile technique is scalable for separator-electrolyte design and is attractive for low-cost supercapacitor manufacturing which is safe and fast charging.