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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.

Gas adsorption is a standard method for measuring pore-size distributions of nanoporous materials. This method is often based on assuming the pores as separate entities of a certain simple shape: slit-like, cylindrical, or spherical. Here, we study the effect of interconnections on gas adsorption in materials with spherical pores, such as three-dimensionally ordered mesoporous (3DOm) carbons. We consider interconnected systems with two, four, and six windows of various sizes. We propose a simple method based on the integration of solid–fluid interactions to take into account these windows. We used Monte Carlo simulations to model argon adsorption at the normal boiling point and obtained adsorption isotherms for the range of systems. For a system with two windows, we obtained a remarkably smooth transition from the spherical to cylindrical isotherm. Depending on the size and number of windows, our system resembles both spherical and cylindrical pores. These windows can drastically shift the point of capillary condensation and result in pore-size distributions that are very different from the ones based on a spherical pore model. Our results can be further used for modeling fluids in a system of interconnected pores using Monte Carlo and density functional theory methods.