Prediction of Swelling of Polypropylene Separators and Its Effect on the Lithium-Ion Battery Performance

The safety and reliability of lithium-ion batteries depend on porous separators, many of which are made of polymer materials, such as polypropylene. Favorable interactions between a separator and organic electrolyte solvents used in the batteries often induce separator swelling. Swelling increases the electric resistance of the cell and is accompanied by plasticization of the separator, which also affects the battery's performance. Here we propose a model based on Flory's theory of polymer solutions which can predict the swelling of a porous polymer separator based on the Flory–Huggins parameter for polymer–solvent interactions. Despite the complexity of the polymer structure, introducing only two additional parameters provides predictive capability for the model. These two parameters can be obtained based on experimental measurements of separator swelling in two different solvents; the model also requires the Flory–Huggins parameter as an input, which can be calculated based on the UNIFAC-FV group contribution method for a given polymer–solvent pair. We illustrated the applicability of this model to recent experimental data on the swelling of polypropylene separators in various solvents. We also showed a simple relation between the separator swelling and an increase in cell resistance. Our model can be used for a quick assessment of various polymer–solvent pairs for application in lithium-ion batteries.