Identifying Non-Stationarity in Non-Linear Harmonics of EIS Measurements of Li Batteries

Abstract

Non-linear Harmonic Analysis is a technique used to investigate the distortions in Electrochemical Impedance Spectroscopy (EIS) measurements. The distortions are in both non-linear and non-stationary nature that appear as high order harmonics in the frequency domain. Differentiating the origin of the harmonics is important in understanding their nature and relating them to electrochemical phenomena. Here, we will show how the harmonics are in direct relation to the initial non-stationary response (initial transient) of the excited system [1]. We will show by both experimental measurements and simulations of dummy cell circuits, representing the electrochemical interface, that the initial transient exhibits high order harmonics that show the highest value at the characteristic frequency of the process [1]. We also extend the analysis to include more complex systems of primary Lithium batteries [2]. It is observed that the same non-stationary initial transient behavior is persistent in the time constants of the electrochemical processes. It is concluded that the capacitive components of the electrochemical processes are responsible for such non-stationary behavior in electrochemical systems [2].[1] G. Katırcı, M. A. Zabara, and B. Ülgüt, “Methods—Unexpected Effects in Galvanostatic EIS of Randles’ Cells: Initial Transients and Harmonics Generated,” J Electrochem Soc, vol. 169, no. 3, p. 030527, Mar. 2022, doi: 10.1149/1945-7111/ac5ad7.[2] M. A. Zabara, G. Katırcı, and B. Ülgüt, “Non-linear harmonics in EIS of batteries with lithium anodes: Proper controls and analysis,” Electrochim Acta, vol. 429, Oct. 2022, doi: 10.1016/j.electacta.2022.140969.