In an effort to improve thermal shrinkage and electrochemical performance of a separator for a lithium-ion battery, we develop a new composite separator by introducing ceramic coating layers onto both sides of a polyethylene (PE) separator. The ceramic coating layers are comprised of SiO2 nanoparticles and polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) binders. In comparison to the dense structure of conventional nanocomposite coating layers, the ceramic coating layers are featured with close-packed SiO2 nanoparticles, which affords a well-developed porous structure, i.e. highly connected interstitial voids formed between the nanoparticles. On the basis of this structural understanding of the composite separators, the effects of ceramic coating layers on the separator properties are investigated as a function of SiO2 powder size. Owing to the existence of the heat-resistant SiO2 coating layers, the composite separators show significant reduction in thermal shrinkage than the pristine PE separator. More intriguingly, in comparison to the large-sized (=530nm) SiO2, the small-sized (=40nm) SiO2 offers a large number of SiO2 nanoparticles in the ceramic coating layers, high porosity contributing to facile ion transport, and small increase in the cell impedance, which consequently allows substantial improvements in cell performances as well as thermal shrinkage of the separator.