Wetting and Inductivity in the Impedance Behavior of Large Lithium-Ion Cells

Lithium-ion cells find applications in various fields, especially automobile and consumer electronics. Despite the increased commercialization and widespread application of lithium-ion cells, considerable efforts have been devoted to enhancing their energy density and specific energy. To accomplish this, the emerging trend is toward large-format lithium-ion cells. While these high-capacity cells have numerous benefits, their economic production remains the biggest challenge for the industry as it demands a reduction in cost while increasing or maintaining the quality of the product.

The cost of the lithium-ion cell highly depends on filling the cell with electrolyte liquid, a process characterized by dosing the electrolyte in the cells and wetting the porous electrodes and the separator. In particular, the wetting of the porous media plays a vital role in the production of lithium-ion cells. Although many methods for measuring wetting have been developed, most are less effective due to several limitations.

Recently, electrochemical impedance spectroscopy (EIS) has been identified as a promising approach for measuring, characterizing and describing the wetting behavior of lithium-ion cells. However, only the intercept resistance or the whole spectrum could be analyzed without modeling the cell impedance. Moreover, an equivalent circuit based on the transmission line model (TLM) has also proved useful in determining the in-plane and through-plane tortuosity as well as the binder migration in electrodes. Unfortunately, current wetting visualization methods fail to distinguish the wetting of electrodes and separator or whether the liquid is on the pores or macroscopic surfaces. Thus, developing effective methods for describing wetting is highly desirable.

Herein, German scientists, Mr. Florian Günter, Mr. Josef Keilhofer, Mr. Viktor Böhm, Professor Rüdiger Daub and Professor Gunther Reinhart from the Technical University of Munich designed an equivalent circuit model for high-capacity large-format full cells and demonstrated a more effective method for analyzing and calculating their wetting. The equivalent circuit was derived considering external inductance, the wetting process boundary conditions and the TLM for describing the pore impedance, contact resistance and substrate foil inductance impedance. The superposition of the impedance was detailed based on symmetric and full laboratory cells. Finally, the feasibility of this impedance adjustment and wetting analyzing method was validated on plug-in hybrid electric vehicle cells with a capacity of 22 Ah. Their work is currently published in the journal, Journal of Electrochemical Society.

The research team showed that besides inductance for electrode-external contacts and cables, the high-capacity cells could build up inductance due to the combination of the substrate foils and the electrode area. Together with the contact resistance, this inductance resulted in a characteristic hook in the Nyquist plot. Following the adjustments, the Nyquist plot of the high-capacity cell corresponded to the laboratory cell in blocking conditions at a lower impedance. As a result, the separator wetting and electrode wetting were separated for easy evaluation of the wetting. By clearly distinguishing the electrode and separator wetting, the slower wetting of the electrode was observed especially for the anodes.

In summary, Technical University of Munich researchers developed a novel TLM-based circuit model considering the inductances of high-capacity cells. Distinguishing between the electrodes required half-cell measurements. In addition, displaying the superimposed electrodes in the full cell allowed complete evaluation of the wetting of both electrodes. The presented scheme improved the identification and characterization of wetting. In a statement to Advances in Engineering, the authors explained that their findings would advance the prediction models used to improve the performance of lithium-ion cells.