The MacMullin number
Separators are used in battery cells to avoid electrical contact between the electrodes, while also allowing fast ion transport. The MacMullin number (NM) describes how much the ionic conductivity through a separator is limited compared to the pure electrolyte:
The MacMullin number is related to the porosity and tortuosity of the separator, and is in the range 4 – 20 for common lithium ion battery separator materials. Clearly this is an important property, since decreasing NM can lead to a significantly lower internal cell resistance.
Decide which method works best for your application
In this application note, we describe the determination of the MacMullin number of a commercially available separator by measuring electrochemical impedance spectra (EIS) in a TCE Cell One test cell and fitting the resulting data to an equivalent circuit. A separate application note will demonstrate a alternative DC method that does not require EIS. Additionally, a previous application note shows how to use the TSC Battery cell for this purpose, using the “stacking method”.
To cite this application note, please use: “Karlsson et al, rhd instruments GmbH & Co. KG, Application Note: MacMullin Number Determination for Lithium-ion Battery Separators by EIS – an Alternative Approach”, January 2023, https://docs.rhd-instruments.de/appnotes/application-note_TCE_Separator_EIS.pdf”.