For the purpose of improving the safety of lithium-ion batteries

polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) porous layers were coated onto both sides of polyethylene (PE) separators by immersion-precipitation process.The effect of differences in solubility parameters () between solvent and composite non-solvent on the structure of the coating layer was studied by varying the strong non-solvent/weak non-solvent ratio in order to prepare a coating layer with porous structure

which is beneficial to reduce thermal shrinkage of composite separator and to improve electrochemical properties of the lithium-ion batteries.As between the solvent (DMAc) and composite non-solvent (ethanol/water composite non-solvent

different in ratio between ethanol and water) increases

the microporous structure of coating layer becomes more developed.Taking the conceptual image of phase diagram into consideration

this is suggested to be caused by different phase separation mechanisms.Ethanol

a weak non-solvent which is against the phase separation process

just develops a dense structure.As the content of strong non-solvent (water) increases

solid-liquid phase separation takes place and it finally develops a porous layer with spherical PVDF-HFP crystallites of roughly identical size.Obvious differences in thermal shrinkage and electrochemical properties of the composite separators were observed with different structures in the coating layers.Sandwiched separators with reduced thermal shrinkage

increased conductivity

enhanced cycle and C-rate performance were prepared by this immersion-precipitation process with ethanol/water ratios 3:2 and 2:3.