Porosity Controlled Polyimide Separators via NIPS for Improved Electrolyte Affinity and Ionic Conductivity

The rapid development of high-energy-density lithium-ion batteries demands separator membranes that combine outstanding thermal stability with rapid ion transport. Conventional polyolefin separators often suffer from thermal shrinkage at elevated temperatures, raising safety concerns. To address this, porous polyimide (PI) membranes are prepared via non-solvent induced phase separation (NIPS) of poly(amic acid) (PAA) precursors, followed by thermal imidization. The PAA precursor solutions are synthesized from equimolar dianhydride and diamine monomers in N,N-dimethylacetamide (DMAc) or N-methyl-2-pyrrolidone (NMP). Once synthesized, the PAA solutions are cast onto glass substrates and immersed in coagulation baths of deionized water, ethanol, or mixed non-solvents to induce phase inversion and initiate pore formation. In this study, we systematically tune the polymer concentration and coagulation bath composition to precisely tailor the membrane porosity, tortuosity, and pore size distribution. By controlling the thermodynamic and kinetic parameters of the demixing process, we aim to suppress macrovoids and fabricate PI separators with highly interconnected pore structures. This structural optimization significantly increases electrolyte uptake, improves surface wettability, and enhances ionic conductivity. Consequently, this scalable approach provides microstructure-engineered PI separators that offer a highly effective strategy for improving both the thermal safety and electrochemical performance of high-power battery systems.