Modelling Transport Mechanisms in the Solid Electrolyte Interphase
Authors: Horstmann Birger , von Kolzenberg Lars , Single Fabian , Schmitt Tobias , Bolay Linda , Latz Arnulf 
Published: February 2020

On negative carbon electrodes of lithium-ion batteries, a thin layer of electrolyte reduction products forms. Even though this solid-electrolyte interphase (SEI) suppresses further electrolyte reduction, continued SEI growth is a main contributor to lithium-ion loss, capacity fade, and limited battery lifetime [1]. Our theory-based continuum models and its experimental validation give insights into the relevant transport mechanisms for electrons and lithium ions in the SEI.
Standard SEI models study capacity fade as a function of storage time or cycle number. We developed a series of models to predict additional dependencies, i.e., SEI morphology [2,3], potential dependence of SEI growth [4], dependence of SEI growth on cycling conditions [5], and lithium-ion concentration polarization inside SEI. We gain novel insights into transport mechanisms by comparing with dedicated experiments, e.g., battery storage at various state-of-charge [6], differential capacity analysis during cycling [7], and impedance spectroscopy on model electrodes [8].
Going beyond the standard SEI models, we consider electron transport and solvent diffusion in a single model. Therefore, we can discuss the morphology of SEI by taking into account the interplay of structure, reaction kinetics, and transport mechanisms [2,3].
From the potential dependence of capacity fade [6], we predict the mechanism behind its continued growth [4]. We present the first indirect experimental evidence that neutral radicals carry a negative charge and diffuse through the SEI. Bazant et al. have recently measured and simulated the differential capacity loss during cycling [5,7]. They find that SEI grows mainly while lithium is intercalating. In this talk, we will combine these models for calendar-life and cycle-life. Our model naturally predicts the transition from square-root-of-time growth to linear growth of SEI observed during fast cycling [9]. We analyze with 3D micro-structure resolved simulations how the potential dependence of SEI growth results in SEI inhomogeneity throughout the negative electrode.
In this talk, we discuss a physics-based model for impedance spectroscopy of lithium batteries with SEI as porous surface film. Our consistent model results in an analytic expression for the cell impedance including bulk and surface processes. Validating our model with experiments of lithium metal electrodes [8], we find large transference numbers for lithium ions. This analysis reveals that lithium-ion transport through the SEI has solid electrolyte character.
The figure is reprinted from Ref. 4 with permission from Wiley. This work is supported by the German Research Foundation (DFG) via the research training group SIMET.

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Figure 1