In a bid to develop a separator with improved thermal shrinkage that critically affects internal short-circuit failures of lithium-ion batteries, we demonstrate a new approach, which is based on introducing microporous composite coating layers onto both sides of a polyethylene (PE) separator. The composite coating layers consist of alumina (Al2O3) nanoparticles and polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) gel polymer electrolytes. The microporous structure of the composite coating layers is considered a crucial factor governing the thermal shrinkage and electrochemical performance of the separators. The microporous structure is determined by controlling the phase inversion, specifically the solvent-nonsolvent miscibility, in the coating solutions. To quantitatively identify the effect of the solvent-nonsolvent miscibility, three different nonsolvents are chosen in the decreasing order of solubility parameter (delta) difference against the solvent (acetone, delta = 20 MPa1/2), which are respectively water (delta = 48 MPa1/2) butyl alcohol (delta = 29 MPa1/2), and isopropyl alcohol (delta = 24 MPa1/2). The microporous structure of the composite coating layers becomes more developed with the increase of not only the nonsolvent content but also the solubility parameter difference between acetone and nonsolvent. Based on this observation, we investigate the influence of the morphological variations on the thermal shrinkage and electrochemical performance of the composite separators. (C) 2010 Elsevier B.V. All rights reserved.