Ionic liquid (IL) electrolytes offer thermal stability and non-flammability for lithium-ion batteries (LIBs); however, suitable separator materials that can effectively harness these advantages remain limited. Polyimide-based separators, known for their excellent thermal stability and mechanical robustness, have emerged as promising alternatives for harsh operating conditions. In this work, a porous polyimide foam (PIF) separator fabricated via an emulsion-templated poly(amic acid) salt (PAAS) precursor followed by thermal imidization is proposed for IL–based LIBs. The PIF separator is compatible with IL electrolytes owing to the polar nature of the polyimide framework, which enhances electrolyte wettability. LIBs fabricated using a PIF separator swollen with an IL salt successfully exhibit stable charge–discharge behavior over a wide temperature range from −20 °C to 100 °C. These results demonstrate that PIF separators are a promising alternative for IL electrolytes in LIBs.