Rheological and light scattering analyses for characterizing phase separation of polymer solutions in lithium-ion battery separator coating system

Nonsolvent-induced phase separation is a widely used technique in the manufacture of polymeric separators. This method involves fine-tuning porous structures through the phase separation of polymer solutions in Li-ion secondary battery systems. The phase separation properties and kinetics of heat-resistant poly(vinylidene fluoride-co-hexafluoropropylene) polymer solutions were characterized by adjusting the weight ratio of N-methyl-2-pyrrolidone (NMP) as a solvent and water as a nonsolvent using both macro- and micro-rheological techniques. The viscoelastic moduli, measured with a rotational rheometer as a macro-rheological technique, and autocorrelation functions describing the fast movements of tracer ceria particles within polymer solutions—quickly detected using the micro-rheological light scattering technique of multi-speckle diffusing wave spectroscopy—offered a comprehensive assessment of the phase separation status and its kinetics during changes in the NMP/water ratio. These results are expected to play a fundamental role in understanding and controlling the pore structures of actual separator membranes applied in Li-ion battery systems. Graphical abstract: (Figure presented.)