The mechanical stability of inactive polymeric components (e.g. separator and binder) can play an
important role in the long term performance of lithium-ion batteries. Here we investigate the effects of
electrolyte solvents on the mechanical properties of a polypropylene battery separator through experimental
measurements of thickness and elastic modulus of separator samples immersed in different
solvent environments. We find that certain electrolyte solvents such as dimethyl carbonate, diethyl
carbonate, and ethyl acetate cause noticeable softening of the separator. However, in other solvent environments
such as propylene carbonate and acetonitrile, the separator retains the mechanical properties
of a dry material. We show that the mechanical property reduction can be attributed to polymer
swelling and explain these observations in the context of the Hildebrand solubility and FloryeHuggins
interaction parameters. The solubility/interaction parameter analysis provides a straightforward method
for predicting the in situ mechanical behavior of polymer separators in solvent environments. The relationships
discussed herein can be used to screen and identify mechanically-stable polymer and electrolyte
solvent pairs for use in lithium-ion batteries designed for long life