Electrochemical impedance spectroscopy (EIS) is a powerful and versatile tool to investigate interfaces in batteries. In order to disentangle the anode and cathode contributions from the full-cell impedance, a reference electrode (RE) is required. In the field of batteries based on liquid electrolytes, the concept of a RE has become a widespread tool for the EIS analysis of small-scale cells [1]. However, there are only very few reports on the use of a three-electrode setup with a reference electrode for all-solid-state batteries (ASSBs) [2,3], which is due to the complexity of integrating a RE with a suitable geometry in the typical ASSB test cells that are based on a compressed electrolyte pellet (further on referred to as bulk-type ASSB cells), since for artifact-free single-electrode impedance spectra, the RE should be placed between the electrodes and should be thin compared to the thickness of the pellet. In contrast to the widely used bulk-type ASSB cells, a recently available alternative construction is offered by the use of free-standing separator sheets based on a solid electrolyte / polymer binder composite (further on referred to as sheet-type ASSB cells),[4,5] in which case a micro‑RE can be placed between two separator sheets, in analogy to the micro-RE concept used with batteries based on liquid electrolytes [1].

In this study, we use sheet-type separators based on a composite consisting of Li₆PS₅Cl (LPSCl) solid electrolyte and a hydrogenated nitrile butadiene rubber (HNBR) binder to build ASSB pouch cells that include a gold wire micro-RE (µ-GWRE). We show that upon in‑situ lithiation of the µ‑GWRE a stable reference potential is obtained and that artifact-free single-electrode impedance spectra can be obtained, analogous to what we had found previously for a µ‑GWRE in a lithium ion battery with liquid electrolyte.[1] Figure 1 shows both half-cell impedance spectra of an InLi | separator sheet | Li cell. The sum of both half-cell impedances (blue) is identical to the full-cell impedance (green) and now impedance loops or other common artefacts are observed for the InLi (black) and the Li (red) electrodes, indicating the viability of this setup to determine single-electrode impedances. Since the InLi electrode is commonly used as counter electrode (CE) for ASSB testing cells, we will also use this setup to investigate the potential stability of InLi alloys and their impedance evolution upon lithiation and delithiation.

Acknowledgements:

This work was carried out as part of the research project “Industrialisierbarkeit Festkörperelektrolytzellen”, funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy.

References:

• [1] Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind and H. A. Gasteiger, J. Electrochem. Soc., 163 (10) A2265-A2272 (2016).
• [2] Dougas, Y. Dupraz, E. Quemin, T. Koc, and J.-M. Tarascon, J. Electrochem. Soc., 168 (9), 090508 (2021).
• [3] J. Nam, K. H. Park, D. Y. Oh, W. H. An and Y. S. Jung, J. Mater. Chem., 6, 14867‑14875 (2018).
• [4] Riphaus, P. Strobl, B. Stiaszny, T. Zinkevich, M. Yavuz, J. Schnell, S. Indris, H. A. Gasteiger and S. Sedlmaier, J. Electrochem. Soc., 165 (16) A3993-A3999 (2018).
• [5] Sakuda, K. Kuratani, M. Yamamoto, M. Takahasi, T. Takeuchi and H. Kobayashi, J. Electrochem. Soc., 164 (12) A2474-A2478 (2017).

Figure 1: Impedance Spectra of an InLi | separator sheet | Li pouch cell (4 cm² electrode area) with a µ-GWRE recorded at open circuit voltage (OCV) with a voltage amplitude of 10 mV between 100 kHz and 100 mHz. The half-cell impedance spectra of the InLi working electrode (WE) and of the Li counter electrode (CE) are displayed in black and red, respectively. The sum of the individual half-cell impedance spectra (blue) matches with the full-cell impedance spectrum (green). The separator sheet is a » 400 µm thick composite of LPSCl and HNBR and no impedance loops are observed, demonstrating the successful implementation of the RE.