Battery separators Gurley number

The measurement of separator resistance is very important to the art of battery manufacture because of the influence the separator has on electrical performance. Electrical resistance is a more comprehensive measure of permeability than the Gurley number in that the measurement is carried out in the actual electrolyte solution. The ionic resistivity of the porous membrane is essentially the resistivity of the electrolyte that is embedded in the pores of the separator. Typically, a micropo-rous separator, immersed in an electrolyte, has an electrical resistivity about six to seven times that of a comparable volume of electrolyte, which it displaces. It is a function of the membrane s porosity, tortuosity, the resistivity of the electrolyte, the thickness of the membrane, and the extent to which the electrolyte wets the pores of the membrane.The ER of the separator is the true performance indicator of the cell. It describes a predictable voltage loss within the ceU during discharge and allows one to estimate rate limitations. 

The separator suppliers often provide their products with Gurley numbers which characterize air permeability (Chap 1 Table 1.10) of the films. The battery industry, however, is interested in the electric resistance of the separators wetted with the electrolyte solutions as discussed earlier. The electric resistance can be expressed in the units of specific area resistance (in 2 cm ), or via MacMullin number which is the ratio of the resistance of the separator filled with electrolyte divided by the resistance of the electrolyte alone. Although, generally, the thinner... 

The ER is a more comprehensive measure of permeability than the Gurley number (air permeability) in that the measurement can be carried out in the actual electrolyte solution. Thus, the measurement reflects the compatibility of the separator with the electrolyte (e.g., the value of the ER will reflect pores that are nonwetting). The magnitude of the ER can be used to estimate ohmic losses in a cell. Unfortunately, ER is significantly more difficult to measure than air permeability. Battery makers may find evaluating the electrical performance of a spirally wound cell made with dummy electrodes and a particular separator an efficient means of measuring ER [60]. 

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