One important component of lithium ion batteries is the separator foil – a thin electronically insulating, porous membrane that prevents the electrodes from being in direct contact with each other while being permeable for ionic current. To maintain a good lithium ion battery performance, the resistance to the ionic current of a separator foil filled with electrolyte should be low. Consequently, information about the effective ion conductivity of these systems is an important parameter for the performance of a separator material.
Usually, one does not focus on the effective ionic conductivity, but on another quantity: the so-called MacMullin number NM. The MacMullin number is the ratio of the ionic conductivity of the pure electrolyte σelectrolyte and ionic conductivity of the separator foil filled with electrolyte σseparator.
Therefore, the MacMullin number is a quantity for the decrease of the effective conductivity by the presence of a separator foil. For lithium ion battery separators, often a value between 4 and 20 is found. Theoretically, the MacMullin number is linked to the separator foil’s porosity and tortuosity.
In this application note, we want to focus on showing how to determine the MacMullin number following the so-called stacking-method approach introduced by Landesfeind et al.
To cite this application note, please use: “Drüschler et al., rhd instruments GmbH & Co. KG, Application Note: Determination of MacMullin numbers: The stacking-method, May 2021”.