E.C. Self
Oak Ridge National Laboratory,
United States
Keywords: electrochemical impedance spectroscopy, redox flow batteries, solid-state batteries
Summary:
Batteries are used to power a range of electrical loads including portable electronics, electric vehicles, and utility grids. The energy efficiency of these systems decreases with increasing current load due to energy losses (in the form of heat) associated with different processes. AC impedance spectroscopy, also called electrochemical impedance spectroscopy (EIS), is a nondestructive technique commonly used to assess rate-limiting steps in electrochemical devices. Notably, most EIS measurements on batteries are only performed around open-circuit and do not probe system behavior under non-zero biases where these devices operate. Despite its widespread use, this approach does not provide information about the relative overpotentials associated with coupled nonlinear processes (e.g., diffusion-limited redox reactions) commonly encountered in these systems. Furthermore, ambiguous terms such as equivalent series resistance and interfacial resistance can lead to confusion and data misinterpretation. For example, Ohm’s law cannot be applied to calculate nonlinear overpotentials using a resistance that was measured from a locally linear response. To determine nonlinear overpotentials using EIS, one must integrate the resistive elements of the equivalent circuit as a function of steady-state current. While this has proven to be a powerful experimental approach, a rigorous mathematical framework describing the technique is lacking. To address this knowledge gap, recent efforts have derived impedance and steady-state polarization relationships for various phenomena including: (i) charge-transfer reactions where transport of redox-active species is limited by finite diffusion and (ii) solid-electrolyte interphase (SEI) layers on Li-ion electrodes where ion transport is enabled by lattice point defects (e.g., Schottky pairs). By combining theoretical predictions with experimental EIS measurements, one can deduce key system parameters related to reaction kinetics, mass transport, and electrode passivation. This presentation will be broken into two parts including: (i) an overview of EIS theory to probe nonlinear systems and (ii) success stories and challenges using the method to study performance limitations in various battery technologies. Experimental demonstrations will focus on redox flow batteries and solid-state batteries which are of interest for various applications including grid storage and electric vehicles. The presentation’s broader aim is to demonstrate the value of this novel EIS approach to guide component/system design and optimize device performance. Acknowledgements This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE). This material is based upon work supported by the U.S. Department of Energy, Office of Electricity (OE), Energy Storage Division.