Electrochemical Impedance Spectroscopy (EIS) is a powerful tool for characterizing Lithium-Ion batteries (LIB). However, it relies on strict adherence to Linear Time-Invariant (LTI) conditions for valid interpretation. A violation of the LTI criteria can significantly impact the reproducibility and accuracy of impedance spectra.
One influential yet often overlooked factor is a superimposed DC current. Many studies in the context of superimposed DC currents rely on common knowledge, that the measurement time needs to be as low as possible to reduce State of Charge variations during the measurement. But this mostly limits the measurement frequency range and gives no indication for the excitation amplitude. Investigations of the boundaries of validity and a closer examination of the LTI criteria are scarce.
In this study, the influence of varying excitation amplitudes with a superimposed DC current on the LTI criteria is investigated. The goal is to increase the understanding on how to choose excitation amplitudes and on the interaction between DC current, excitation amplitude and the validity of the LTI criteria. To evaluate the impact, standard validity criteria, including Total Harmonic Distortion (THD) and Kramers-Kronig (KK) analysis are used.
For this investigation, a baseline is first established with known relationships in order to validate them on the measurement setup. Initially, variations in excitation amplitude and DC currents are considered separately.
As expected, an increase in excitation amplitude (at zero DC current) results in more pronounced deviations in validity criteria. Similarly, an increase in DC current, while maintaining a constant excitation amplitude, leads to greater deviations.
Further, and as a contribution of this study, the interaction of variations in excitation amplitude with a superimposed DC current is examined. The observations reveal that increasing the excitation amplitude results in reduced deviations in the validity criteria. Contrary to expectations, the correlation between higher excitation amplitudes and the resulting deviations in the validity criteria reversed compared to the measurements with zero DC current.
To further investigate these findings, electrochemical simulations based on a P2D model are conducted. The simulations confirm the observed behavior, indicating that this phenomenon is not an artifact of the measurement but is attributed to the intrinsic electrochemical behavior of the LIB.