Effect of solvent-nonsolvent miscibility on morphology and electrochemical performance of SiO₂/PVdF-HFP-based composite separator membranes for safer lithium-ion batteries

As a new approach to improve the thermal stability of separator membranes crucially affecting the internal short-circuit failures of lithium-ion batteries, we develop a new composite separator membrane. The composite separator membrane is prepared by introducing microporous composite coating layers onto both sides of a polyethylene (PE) separator membrane. The composite coating layers consist of silica (SiO₂) nanoparticles and gel-type polymer electrolytes (PVdF-HFP, polyvinylidene fluoride-hexafluoropropylene). The microporous morphology of composite coating layers is determined by controlling the phase inversion, more specifically the solvent-nonsolvent miscibility in the coating solutions. To induce the phase inversion, three different nonsolvents are chosen in the decreasing order of solubility parameter (d) difference against the solvent (acetone, δ=20 MPa^1/2); the nonsolvents are water (δ=48 MPa^1/2), ethanol (δ=26 MPa^1/2), and isopropyl alcohol (δ=24 MPa^1/2). The microporous structure of composite coating layers becomes more developed with the increase of not only the nonsolvent content, but also the solubility parameter difference between acetone and the nonsolvent. Based on this understanding of the phase inversion, the influence of the morphological variation on the thermal shrinkage and electrochemical performance of the composite separator membranes is quantitatively identified.