We demonstrate a novel and facile approach to fabrication of a new composite nonwoven separator for a lithium-ion battery, which comprises a phase inversion-controlled, microporous polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) gel polymer electrolyte and a polyethylene terephthalate (PET) nonwoven support. The thermally stable PET nonwoven is chosen as a mechanical support and contributes to improving the thermal shrinkage of the composite nonwoven separator. The microporous PVdF-HFP gel polymer electrolyte serves as a pore size controller for the composite nonwoven separator. In order to provide a theoretical basis for this approach, an investigation of the phase diagram for coating solutions consisting of PVdF-HFP, solvent (acetone), and nonsolvent (water) is preceded. Based on this understanding of the phase behavior, the effects of phase inversion, more specifically, the water content in the coating solutions, on the morphology evolution of the composite nonwoven separators are identified. The phase inversion-governed microporous structures of the composite nonwoven separators play a crucial role in determining electrochemical performances of cells. The composite nonwoven separator is expected to be a promising alternative to a commercialized polyethylene (PE) separator, particularly in next-generation lithium-ion batteries necessitating superior battery safety and performance.