Three-Electrode Coin Cell with Gold Micro-Reference Electrode
Authors: Bakhshi Mahla , Sivasengaran Ashvini , Landesfeind Johannes 
Published: February 2024

As the demand for Li-ion batteries continues to grow, accelerating their development is crucial. An essential aspect of enhancing battery performance involves obtaining a deeper understanding of fundamental electrochemical processes. This understanding can be achieved by applying advanced measurement techniques such as electrochemical impedance spectroscopy (EIS), a promising method for investigating batteries, providing insights into reactions, material properties, and interphases.1 However, when EIS is conducted on a full cell, the resulting spectra combine information from both electrodes, limiting the characterization of each electrode.2 To deconvolute the influence of the electrodes, a three-electrode setup can be used. This setup introduces a reference electrode (RE) which allows deconvolution of the full cell impedance into the impedance of the separate electrodes.
The choice and placement of the RE significantly impact impedance spectra.3 Ideally, the RE should maintain a stable potential, allow ion flow, and not obstruct any part of the other electrodes.4 Micro-REs are often favored for EIS due to these considerations. Additionally, the RE placement can distort impedance spectra if positioned in a geometrically or electrically asymmetric region of the cell.3 Several cell designs have been proposed for three-electrode configurations.4 An innovative approach incorporates a gold micro-RE into a Swagelok T-cell setup.5 This work involves a 50 µm thick insulated gold wire sandwiched between two separators which could be lithiated to form a gold-Li alloy at its tip. This lithiated tip shows a stable potential for extended periods, resulting in reliable impedance spectra of the electrodes. While the proposed setup has been instrumental in facilitating valuable research, the cell format has room for improvement. Even though effective, Swagelok T-cells come with high costs and require intricate assembly. Furthermore, disassembly, cleaning, and drying of cell components for re-use substantially prolong the preparation time. Other studies have explored the coin cell format for the three-electrode setup4, offering several advantages, including simplified and rapid assembly procedures. Moreover, the disposable nature of this cell format significantly reduces unit costs and shortens preparation times. Consequently, some researchers have sought to leverage these benefits by incorporating the RE through a coaxial configuration within a hole in the electrodes.4 However, these adaptations often necessitate substantial modifications to cell components and electrodes and are not well-suited for reliable impedance measurements. In some other approaches that involve placing the RE between separators and electrodes4, achieving consistent cell assembly appears to be a persistent challenge, and cell sealing issues may arise in these setups as well.
In this work, we have devised a reliable, cost-efficient approach for three-electrode coin cell construction that employs common coin cell parts. Our approach eliminates the need for specialized tools, relying instead on conventional laboratory-scale equipment and accessible commercial materials. In this configuration, as depicted in the inset of Fig.1, the assembly process is carefully adapted so that the micro-RE wire remains straight and lies as flat as possible throughout the cell, positioned between the electrodes and separators. The sealing in our setup is ensured with a special resin compatible with the micro-RE and the components inside the cell, effectively preventing the penetration of moisture and/or air into the cell. We have validated the effectiveness of this sealing method by electrochemical tests that are sensitive to the water and oxygen content within the cell (see Fig. 1). Here, we present comprehensive details about the assembly modifications and custom-designed cell holder. This study also includes examples of measurement results, especially impedance spectra. The proposed setup holds significant promise for various applications in research laboratories and industrial settings, enabling fast, reliable, and cost-efficient setup for impedance characterization tests.

References

[1] L. Andrzej, Electrochemical Impedance Spectroscopy and its Applications, Springer, New York, NY, (2014).
[2] M. Dollé, F. Orsini, A. S. Gozdz, and J.-M. Tarascon, J. Electrochem. Soc., 148, A851–A857 (2001).
[3] M. Ender, A. Weber, and I.-T. Ellen, J. Electrochem. Soc., 159, A128 (2011).
[4] R. Raccichini, M. Amores, and G. Hinds, Batteries, 5, 12 (2019).
[5] S. Solchenbach, D. Pritzl, E. Jia Yi Kong, J. Landesfeind, and H. Gasteiger, J. Electrochem. Soc., 163, A2265–A2272 (2016).
Fig. 1. Electrochemical sealing test results from a lithium graphite half-cell coin cell in a three-electrode setup. The graphite electrode lithiated to 500 mV vs. Li/Li+ maintains its stable potential in the absence of current over extended periods, demonstrating reliable sealing and no water and or oxygen penetration in the cell. Inset: a simplified sketch of the three-electrode coin cell design and components with micro-RE placed between two separators, allowing to detangle impedance contributions from the anode and cathode side individually.

Figure 1