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4.4 Tortuosity The tortuosity (τ) is defined as the ratio between mean effective capillarity length and the thickness of the membrane and can be determined by: τ = ls d where ls is the ion path through the membrane and d is the thickness. The tortuosity is called a long-range property of a porous material describing qualitatively the average pore conductivity of the material and can be also determined from the electrical measurements and porosity through the equation: τ=√σ0 ε σi where σ0 is liquid electrolyte conductivity, σi is the conductivity of the membrane soaked in the electrolyte at room temperature and ε is the membrane porosity. This parameter is often used to characterize the ionic transport providing information about the pore blockage of the membrane. A τ = 1 corresponds to an ideal porous material with cylindrical and parallel pores, while τ > 1 corresponds to a hindered porous material [15, 98]. 4.5 Permeability The permeability of a membrane is typically described by the MacMullin number and the Gurley value. The MacMullin number is the ratio between the resistivities of the separator soaked with the electrolyte and in the dry state and should be as low as possible, numbers between 10-12 are usually found in commercial cells. The Gurley value is defined as the time required for a specific amount of air to pass through a specific area of the membrane under a specific pressure, the typical values being below 0.025 s/μm. A lower Gurley value indicates 17PDF Image | Synthetic Polymer-based Membrane for Lithium Ion
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