Electrochemical Impedance Spectroscopy Investigation of the SEI Formed on Lithium Metal Anodes

Electrochemical impedance spectroscopy (EIS) is widely used to probe the solid electrolyte interphase (SEI) under realistic conditions, without causing damage to its structure. However, the models and experimental conditions often raise concerns about the reliability of the results. In this work, we present an extensive EIS study of lithium metal in the model electrolyte lithium bis-(fluorosulfonyl)-imide in tetraglyme, analyzing the system at equilibrium as a function of time, temperature, and salt concentration using a setup designed to minimize artifacts. We apply information theory to determine the number of independent degrees of freedom and constrain the number of Voigt elements used in fitting. Our analysis reveals strong correlations among processes, warranting caution when assigning physical meaning. X-ray photoelectron spectroscopy and 4D-scanning transmission electron microscopy measurements are used to support the interpretation and provide complementary insights into the chemical nature of the interphase. The unique and extensive dataset we have collected, comprising over 12000 highly reproducible impedance spectra, will serve as a valuable resource to the community for further analysis and for supporting additional modeling and experimental efforts.