The separator should not limit the electrical performance of the battery. Usually the presence of a polymer separator will increase the resistance of the electrolyte by a factor of four to five. The ratio of the resistance of the separator filled with electrolyte divided by the resistance of the electrolyte alone is called the MacMullin number. Air permeability can be used indirectly to estimate the MacMullin number. Air permeability is expressed in terms of the Gurley value, which is defined as the time required for a specific amount of air to pass through a specific area of the separator under a specific pressure. The Gurley value reflects the tortuosity of the pores, when the porosity and thickness of the separators are fixed. A separator with uniform porousness is vital to the long life cycle of a battery. Deviations from uniform permeability will result in uneven current density distribution, which causes the formation of Li crystals on the graphite anode.
Labthink International, Inc.
200 River’s Edge Drive, Medford, Massachusetts, 02155, U.S.A.