Non-linear harmonics in EIS of batteries with lithium anodes: Proper controls and analysis

Non-linear Harmonic Analysis is used to investigate the distortions accompanying the Electrochemical Impedance Spectroscopy (EIS) measurements. The distortions emerge from non-linearity and non-stationarity due to the underlying redox reactions as well as the capacitance. Identifying each response and its source is crucial to correlate the obtained harmonics to their corresponding electrochemical processes. In this study, we identify the distortions of Galvanostatic-EIS measurements of Lithium Primary Batteries by means of comparing mathematical simulations and experimental measurements. The simulations were performed on RC-based equivalent circuit models, which showed the presence of certain distortions. The experimental measurements displayed distortions in the form of both non-linearity and non-stationarity in the case of batteries. The level of the harmonics in the measurements is either similar or higher than the harmonics simulated in the RC equivalent circuits at different frequencies. The obtained harmonics compared to simulations highlights the importance of control measurements before the analysis of non-linear harmonics of EIS measurements galvanostatically excited.