Controllable Electrochemical Impedance Spectroscopy: From Circuit Design to Control and Data Analysis

This article describes fundamentals of controllable electrochemical impedance spectroscopy (cEIS), from circuit design to control and data analysis. In cEIS, a feedback system controls the process of injecting the excitation signal. We design a two degree-of-freedom robust control system, which guarantees tracking and stability of cEIS in the presence of model uncertainties. This article also addresses the concept of persistently exciting signals. cEIS using current driving mode (CDM) and voltage driving mode, and their differences are highlighted. An online cEIS device is designed and built based on the dc-dc buck converter for batteries online applications, where the excitation signal is superimposed on a dc level. The performance of the fabricated cEIS is evaluated through extensive experiments in CDM. The accuracy of the fabricated cEIS is tested, which results in $\text{0.002}\;\Omega$ root mean square error in the impedance spectra computation of a three-parameter Randles equivalent circuit model (ECM). The performance of the fabricated cEIS is practically verified on a battery cell at different C-rates. First-, second-, and fractional-order Randles ECMs are estimated by using system identification methods, and their impedance spectra are compared with those obtained through the fast Fourier transform.