Effect of Cross-Sectional Shape of Pathway on Ion Migration in Polyethylene Separators for Lithium-Ion Batteries

Battery power performance is dominated by ionic mobility, i.e., the rate of ion transport in the separator between the electrodes. To elucidate the correlation between the separator pathway morphology and ionic mobility, porous polyethylene membranes were prepared with different stretching ratios to obtain pathways of different cross-sectional shapes. The mode pore size (the most frequent pore size) increased with an increasing stretching ratio without significant changes in the membrane porosity. This indicates that pore size was increased by the connection of several pores, which were originally formed by the volatilization of the grains of the dispersed oil, in the stretching direction. A large difference in the stretching ratio between the machine direction (MD) and transverse direction (TD) led to the anisotropic cross-sectional shape of pathways, characterized by anisotropy of the elastic moduli in MD and TD (f₍TD/MD₎). The cross-sectional anisotropy of the pathway affected the ionic mobility. When f₍TD/MD₎ exceeded f ≈ 0.8, the diffusion coefficients of the cations and anions decreased drastically owing to increases in cation–anion interactions and cation–membrane wall interactions in the pathways. These results suggest that pathways with an isotropic cross-section are appropriate for ion movement, with lower resistance and lower tortuosity of the pathways composed of the connected pores in the separator membrane.