SEI Formation in Sulfide-Based Solid-State Batteries: Influence of Contact Conditions on Impedance-Derived Interphase...

The temporal evolution of the solid-electrolyte interphase (SEI) resistance in sulfide-based solid-state batteries with lithium metal anode has been shown to be well described by diffusion-controlled interphase growth. Yet, recent studies reveal that the extracted SEI rate constant estimates are highly sensitive to experimental conditions, such as stack pressure, and vary significantly depending on the electrochemical characterization method used. In this study, we evaluate SEI growth kinetics derived from symmetric cell-level impedance measurements. Through comprehensive transport simulations and experiments with the argyrodite solid-electrolyte Li₆PS₅Cl, we investigate how the characteristic contact conditions encountered in typical impedance studies affect the corresponding SEI rate constant estimates. We find that increasing stack and joining pressure leads to decreasing rate constant estimates, driven by an increase in the true contact area. Apparent saturation of SEI growth may originate from the presence of native surface passivation layers on lithium metal foil. Crucially, we highlight important uncertainties in experimental impedance data analysis and contextualize our findings by comparison with coulometric titration time analysis (CTTA). Overall, our findings contribute to the in-depth understanding of interphase growth kinetics in solid-state batteries and their quantification using impedance spectroscopy.