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

• Published in: Journal of physical chemistry. C Vol. 123; no. 36; pp. 21888 - 21895
• Main Authors: Saito, Yuria, Takeda, Sahori, Nakadate, Junichi, Sasaki, Tomoya, Cho, Taehyung
• Format: Journal Article
• Language: English
• Published: American Chemical Society 12.09.2019
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b04742

To clarify the primary ion-transport factors in separator membranes, we evaluated the effects of pathway tortuosity and width on the mobilities (D ca and D an) 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 tortuosity and depended on the pathway width instead. These results suggest that reducing the geometrical pathway tortuosity of the separator, which assumes an ideal form with straight pathways, is key to designing separators for high ionic mobility in a high-power battery system.