Separators play a vital role in the battery industry, particularly Li-ion battery separators, and should be thermally and chemically stable to prevent internal short circuits for safety and ionic conductivity. In this study, a polyimides-film separator (PI-FS) was synthesized using a simple one-pot method, and its thermal and chemical stabilities were studied. The results of this study were compared with those of Celgard 2400 and PP-PE-PP separators. The PI-FS exhibited superior thermal stability in the EC-DMC electrolyte than in the EC-DEC and other commercial separators.