A porous thin-film polymer separator for use in a lithium ion battery may be formed by a phase separation method in which hydrophobic-treated ceramic particles are used to help induce the formation of a tortuous, interconnected network of pores coextensively across the thickness of the separator. As part of the phase separation method, a wet thin-film layer is formed from a polymer slurry that comprises a polymer solvent in which a polymer material is dissolved and the hydrophobic-treated ceramic particles are dispersed. The wet thin-film layer is subsequently exposed to a polymer non-solvent to form a solvent-exchanged thin-film precipitated polymer layer which is then heated to produce the separator.