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

Abstract

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.

Solvent Selection Guided by Self-Consistent Field Theory for Improved Dispersion of Metal–Organic Frameworks in Polymers

“…Additionally, fitting group activity parameters from the work presented here in a framework like the Universal Functional-Group Activity Coefficient (UNIFAC) , model may be beneficial to consider. This would potentially allow for approximate Flory–Huggins parameters to be derived for systems involving novel MOFs without having to experimentally determine Hansen solubility parameters, analogous to previous polymer/solvent calculations . While this approach has not been used to our knowledge for MOFs in the fashion suggested here, this may allow for the use of SCFT to guide solution compositions for a wide variety of MOF-polymer–solvent combinations.…”

“…This would potentially allow for approximate Flory−Huggins parameters to be derived for systems involving novel MOFs without having to experimentally determine Hansen solubility parameters, analogous to previous polymer/solvent calculations. 53 While this approach has not been used to our knowledge for MOFs in the fashion suggested here, this may allow for the use of SCFT to guide solution compositions for a wide variety of MOF-polymer− solvent combinations. Some important practical considerations need to be accounted for to produce high-quality films using this method.…”