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

The wetting of the porous electrodes and the separator is crucial in the production of lithium-ion cells. Electrochemical impedance spectroscopy is able to measure and characterize the wetting. This paper p resents an equivalent circuit for commercial highcapacity cells and shows a method to analyze the wetting of these cells. The equivalent circuit includes an external inductance, a transmission line model (TLM) for the description of the pore impedance and, additionally, a TLM for the impedance of substrate foil inductance and contact resistance. Based on symmetric and full laboratory cells, the superposition of the impedance is discussed. Furthermore, the method to adjust the impedance and analyze the wetting is demonstrated on hard case cells with a capacity of 22 Ah. It is shown that, in addition to inductance for cables and electrode-external contacts, high-capacity lithium-ion cells build up inductance due to the electrode area in combination with the substrate foil. This inductance, together with the contact resistance, result in a characteristic hook in the Nyquist plot. A TLM describes and explains this behavior quite well. Additionally, the impedance of the cell is adjustable so that it corresponds to a laboratory cell in blocking conditions. Thus, the wetting of the separator and the wetting of the electrode become separately evaluable and calculable.