Elia Zonta

The porous microstructure of electrodes influences the ionic resistance in lithium-ion cells and is commonly described by the porosity and the tortuosity. Changes in microstructure due to external compression can have a substantial impact on the ionic resistance and therefore the performance of the battery. Properties such as particle size, particle shape, and varying binder composition can affect the microstructure’s impact on ion transport and the compressibility of the electrode. Temperature may also influence mechanical behavior and electrode wettability. Consequently, a combined understanding of mechanical and thermal effects on transport parameters of electrodes is essential for improving electrode design and model parametrization. This study examines the effect of pressure and temperature on the ionic resistance of artificial graphite (AG), lithium-iron-phosphate (LFP), and lithium-nickel-manganese-cobalt-oxide (NMC) electrodes using electrochemical impedance spectroscopy (EIS). The results show that temperature does not significantly affect compressibility. However, AG and NMC exhibit additional temperature-dependent changes in ionic resistance beyond the influence of electrolyte conductivity. This may result from variations in the contact angle between electrode and electrolyte. The MacMullin number increases with higher compression for all observed electrodes, while the tortuosity increases only for the AG and LFP electrodes.

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.