Combined Impedance and Electron Paramagnetic Resonance Spectroscopy for Investigating the Dynamics of Li/Solid Li‐ion Conductor Interfaces

Electrochemical impedance spectroscopy (EIS) is a widely used tool for the electrochemical characterization of all‐solid‐state batteries (ASSBs) with Li‐metal anodes. However, an unambiguous interpretation of the observed impedance response often requires additional independent information on the actual interfacial phenomena obtained. The measurement methodology presented in this study allows to conduct electron paramagnetic resonance (EPR) spectroscopy and EIS concurrently. Therefore, the informative power of EIS experiments can be significantly improved via monitoring of structural changes of paramagnetic lithium at the electrochemical interface. As the solid‐electrolyte‐lithium interface is a critical part of all‐solid‐state batteries, this study employs a model oxide solid electrolyte in contact with lithium metal. During the polarization of the cell with thin evaporated lithium electrodes, the ratio between positive and negative peaks ( a/b ) of the EPR signal momentarily rises, which indicates an accumulation of lithium on one side of the electrolyte. The peak ratio a/b then drops abruptly, accompanied by current irregularities. Both are indicative of a diminishing contact area, and as a result, finer lithium morphologies form. Shortly after that, a contact loss is observed. The change of the EPR signal shape before cell breakdown can hence be associated with the worsening Li‐electrolyte contact, providing a tool for physical in‐situ cell diagnostics.