Characterization of Tortuosity in Polyetherimide Membranes Based on Gurley and Electrochemical Impedance Spectroscopy

Both porosity and tortuosity are important measures for liquid-permeable membranes. Porosity is easily calculated from the membrane's basis weight, density and dimensions. Tortuosity cannot be assessed directly, and is commonly estimated for by the air permeability using the Gurley number. Yet tortuosity may also be determined by measuring the ionic conductivity of an electrolyte-impregnated membrane. In the present work, the resistance of stacked membrane layers impregnated with a lithium-ion electrolyte is measured using a custom designed electrochemical cell optimized for electrochemical impedance spectroscopy (EIS). Linear regression of the resistance as a function of the number of layers provides the resistance of a single membrane layer. Analysis of the linear fit provides an error-corrected ionic resistance value per membrane layer, which is subsequently used to calculate the MacMullin number and tortuosity. After validating this approach with a commercial polyolefin battery separator, polyetherimide membranes having different pore morphologies were analyzed. The tortuosity values measured through EIS (2.5-3.9) showed a poor correlation to the measured Gurley numbers (34-879 s/100cc), which demonstrates that, depending on membrane structure and composition, Gurley numbers cannot simply be used as a means to predict the effective ionic conduction properties of an electrolyte-wetted membrane.