Measurements and Simulations of Electrochemical Impedance Spectroscopy of a Three-Electrode Coin Cell Design for Li-Ion Cell Testing

A 2-D mathematical model of the secondary current distribution in a three-electrode cell is used in order to understand distortion of impedance spectra influenced by the position of the reference electrode (RE) in a cell when one electrode extends past the other. The cell setup consists of a modified coin cell, i.e., working and counter electrodes face each other, and a RE is positioned behind the counter electrode (CE) that has a small hole in it to allow for electrolyte access. This configuration shows large distortion of WE and CE impedance data when taken against the RE. The WE/RE impedance is underestimated and contains a negative impedance contribution from the CE whereas the CE/RE impedance is overestimated and contains a positive impedance contribution from the WE. As supported by simulations, the distortions arise from a radial ionic current flowing from the WE toward the edge of the CE hole, and are minimized through an increase of the electrolyte conductivity, an increase of the separator thickness, or a decrease of the hole size in the CE. The distortions nearly disappear when an additional hole is included in the WE that is perfectly aligned with the hole in the CE. These results should be considered whenever designing a 3-electrode cell where the RE is not sandwiched between the two other electrodes.