Gurley number, separators

Ideally, separators would present no resistance to ion transport. In practice, some resistance must be tolerated. Still, the resistance of the separator is usually insignificant relative to the transport limitations in the electrodes. Separator permeability is typically characterized by air permeability. The Gurley number expresses the time required for a specific amount of air to pass through a specific area of separator under a specific pressure (e.g., 10 mL through 1 in2 (6.45 cm2) at 2.3 cm Hg). This measurement depends on porosity, pore size, thickness, and tortuosity according to Eq. (1) [17] ... 

The electrical resistivity (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... 

Gurley. Separator permeability is usually characterized by air permeability. The gurley number expresses the time required for a specific amount of air to pass through a specific area of separator under a specific pressure. The standard test method is described in ASTM-D726 (B). 

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 design to decrease R is straightforward in that the electronic or ion conductivity of the components has a direct influence on the value of For instance, separators with different porosity and pore tortuosity lead to different values of apparent electrolyte resistance, since these two factors play a critical role in the polarization inside the separator. (In fact, air permeability and the Gurley number are more frequently used in industry to evaluate the degree of penetrability of active species through the separator than the terminologies such as porosity, tortuosity, and polarization [9].) The separators with more porous and less tortuous pore structure result in smaller Ro (or polarization). 

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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