Factors Determining Ionic Mobility in Ion Migration Pathways of Polypropylene (PP) Separator for Lithium Secondary Batteries

To clarify the primary ion-transport factors in separator membranes, we evaluated the effects of pathway tortuosity and width on the mobilities (Dca and Dan) and microviscosities (η, α, and βca) of ions in a PP separator. Ionic diffusivities in different directions, along and inclined to the straight pathway composed of aligned pores, were compared to understand the effect of the geometrical pathway tortuosity. For cations in a specific solution moving along the straight pathway (tortuosity ∼ 1), the βca value attributable to cation–membrane interaction was around zero, which was different than the positive values of βca of cations of the same solution in random-network pathways with higher tortuosity. When cations diffuse in the direction inclined to the straight pathway, which corresponds to ion migration in a pathway of tortuosity > 1, βca was larger than both η and α attributed to the interactions between the ions and surrounding species. On the other hand, α and η were almost independent of pathway t...