Performance and Properties of Laboratory and Commercial Separators under Compression and Varying Temperature

Lithium-ion battery separators contribute to the ionic resistance as their microstructure influences the transport of lithium ions through the electrolyte. This effect is commonly characterized by the MacMullin number and tortuosity. Since lithium-ion batteries are usually operated under mechanical pressure, structural changes in the separator due to compression must be considered, as they can significantly affect ionic transport properties. Moreover, different manufacturing processes and material compositions influence the separators’ compressibility and, consequently, ion transport. Temperature may further impact the separator’s mechanical behavior and the electrolyte’s conductivity. Understanding the combined effects of pressure, temperature, and material-specific microstructure is essential for accurate simulation and optimization of lithium-ion battery performance. In this work, pressure is applied onto a symmetrical single-layer pouch cell to determine the ionic resistance through various separators via electrochemical impedance spectroscopy. Our findings demonstrate that the investigated wet-processed separator exhibits a more pronounced temperature-dependent increase in the MacMullin number under pressure than the examined dry-processed separators. Furthermore, the glass fiber separator, commonly used in lab-scale cells, shows approximately twice the relative increase in MacMullin number compared to microporous separators. However, unlike the microporous separators, the ionic resistance of the glass fiber separator decreases under pressure.