Guidance to solid state electrochemical impedance spectroscopy

• Proper definition of impedance and of complex valued graphics. • Importance of Kramers-Kronig impedance data validation. • Complex Nonlinear Least Squares procedure revisited. • Importance of basic statistical evaluation of results. • Proper evaluation of apparent low- and mid-frequency ‘inductive effects’. • Discussion of limitations in Distribution Function of Relaxation Times analysis. Electrochemical Impedance Spectroscopy (EIS) is a much valued research tool for studying electrochemical cells, systems and devises. With this non-invasive technique mass and charge transport and storage is studied by measuring the response to a sinusoidal perturbation signal (voltage or current) over a wide frequency range. The frequency dependent relation between perturbation and response can be presented as a complex valued impedance, admittance, dielectric, etc. Acquisition of the spectra is relatively easy with modern automated Frequency Response Analysis (FRA) systems, but the interpretation is often complicated. Lack of experience can lead to misinterpretation and inadequate presentation of results. Data validation with a Kramers-Kronig consistency check is an essential starting point for the data analysis process. The Complex Nonlinear Least Squares (CNLS) analysis method, based on an Equivalent Circuit Model (ECM) or a special transfer function is still the most used procedure for data interpretation. Inversion of impedance data to a Distribution Function of Relaxation Times (DFRT) is gaining significant attention. Sufficient knowledge of, and experience with these techniques is required for arriving at a trustworthy result. Unfortunately, many manuscripts lack a thorough evaluation of the obtained results. In several cases these problems pass uncorrected the peer review system. This contribution provides a brief overview of mistakes quite often made in EIS analysis. It tries to provide guidance for the inexperienced scientists. Simultaneously, to support prospective reviewers of EIS related manuscripts, a checklist is presented with common EIS analysis and interpretation errors.