Evaluating Passivated Battery Electrodes using AC Impedance Spectroscopy

Abstract

Battery electrodes are often covered with passive films, also called solid-electrolyte interphase (SEI) layers, which impact the device’s rate capability, efficiency, and lifetime. Electrochemical impedance spectroscopy is widely used to study electrode passivation, but most battery studies only report impedance around open-circuit conditions and assume an ohmic film resistance. The present work derives a general expression for the AC impedance of passivated electrodes and shows that resistances associated with the film and defect-mediated ion diffusion are highly dependent on steady-state operating conditions. By integrating these resistances with respect to steady-state current, the overpotential corresponding to each process is obtained. Near open-circuit conditions, the film exhibits ohmic behavior, whereas a Tafel-like relationship is expected at larger current densities. For oxidizing currents, mass transport overpotential grows as the limiting current is approached. On the other hand, reducing conditions generate ion vacancies near the electrode/film interface, resulting in Nyquist plots with a low-frequency inductive loop. Based on these trends, straightforward analyses can be applied to experimental data to determine key parameters associated with the passive film and ion transport. The approach illustrated herein may be used to gain new insights on SEI formation and guide optimization of electrode/electrolyte formulations.