Theory of Impedance Spectroscopy for Lithium Batteries

In this article, we derive and discuss a physics-based model for impedance spectroscopy of lithium batteries. Our model for electrochemical cells with planar electrodes takes into account the solid-electrolyte interphase (SEI) as a porous surface film. We present two improvements over standard impedance models. First, our model is based on a consistent description of lithium transport through electrolyte and the SEI. We use well-defined transport parameters, e.g., transference numbers, and consider convection of the center-of-mass. Second, we solve our model equations analytically and state the full transport parameter dependence of the impedance signals. Our consistent model results in an analytic expression for the cell impedance including bulk and surface processes. The impedance signals due to concentration polarizations highlight the importance of electrolyte convection in concentrated electrolytes. We simplify our expression for the complex impedance and compare it to common equivalent circuit models. Such simplified models are good approximations in concise parameter ranges. Finally, we compare our model with experiments of lithium metal electrodes and find large transference numbers for lithium ions. This analysis reveals that lithium-ion transport through the SEI has solid-electrolyte character.