Influence of the Cell Casing on the Impedance of the Lithium Metal Electrode

Abstract

The impedance response of a Li electrode enclosed in a pouch cell casing is compared with the response of the same electrode in coin cell and Swagelok cell casing. A significant difference is observed in the high-to-medium-frequency part of the response. The difference is explained using a simple equivalent circuit. The explanation is further confirmed by a series of experiments in which the electrodes are either transferred from one cell type to the other, or the surface area of lithium metal electrode and contactingmetal from the cell casing is varied. Six different electrolytes are used to demonstrate the generality of the phenomenon due to (inadvertent) wetting of the stainless steel case in coin cells and Swagelok cells; such wetting results in distortion of the main impedance arc, which may even split into two separate arcs. A similar situation can occur in pouch cells when a significant surface area of the metal used for contacting the lithium metal electrode is in the direct electrochemical field of the cell. Solutions to this problem are briefly presented.