A valuable way to compare battery separators is to monitor the difference between the resistivity of the separator soaked in the electrolyte and the resistivity of the free electrolyte. The ratio of these two values is the MacMullin number.
This Application Note shows how to perform the necessary measurements and how to calculate the MacMullin number.
The main components of a battery are the positive and negative electrodes, together with the electrolyte. While both ionic and electronic conductivities take place at the electrodes, the electrolyte provides only the ionic conductivity.
The most common electrolytes are in the liquid state. Therefore, they cannot be employed without a solid supporting material, which provides physical separation between the electrodes, necessary to avoid short circuits.
A typical separator is composed of a porous sheet of a polymeric material. Before assembling the battery, the separator is soaked with the electrolyte.
The polymeric structure provides the physical separation between the electrodes, while the pores allow the electrolyte to be in contact with both electrodes, providing the desired ionic conductivity.
Refer to the Metrohm Application Note below for further details.