Determination of the Exchange Current Density at Lithium │ Polymer Electrolyte Interfaces

While interfacial processes can dominate the internal resistance in solid‐state batteries, the (electro‐)chemical reactions occurring at the lithium│polymer interface are complex and dynamic upon cycling. A central factor for evaluating such reactions is the exchange current density j 0 that characterizes the kinetics of the interfacial charge transfer. However, its determination is challenging due to superimposed impedance contributions from the solid electrolyte interphase and interfacial charge transfer reactions. Moreover, different methodologies for determining j 0 can lead to different j 0 values. Herein, a carefully validated method to determine j 0 for polymer electrolytes is reported, using the example of polyethylene oxide‐based systems, by combining electrochemical impedance spectroscopy, Bayesian inference analysis, and distribution of relaxation times analysis. These impedance‐based methodologies are validated via the determination of j 0 using DC polarization measurements that are fit to a modified Butler–Volmer model, enabling the reliable determination of j 0 for polymer electrolytes and, thus, the analysis of the interfacial processes and reactions occurring in such systems.