Methods—Unexpected Effects in Galvanostatic EIS of Randles’ Cells: Initial Transients and Harmonics Generated

Abstract

Non-linear EIS analysis is gaining wider attention and interest due to the deeper understanding that is provided especially when combined with linear EIS. The nonlinear part of the data can get corrupted due to a number of effects. One of these is the initial transient, which is the response right after excitation signal is applied before a steady-state is reached. In this study, we demonstrate this phenomenon through analyzing simplified Randles’ cells via Kirchoff’s laws. To get rid of the effects of the initial transient, instrument manufacturers typically discard some fraction of the response, the effectiveness of which, as demonstrated here has to be checked.

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

“…Further, any electrode/electrolyte interface leads to a double-layer capacitance. As shown in an earlier study [14], the existence of this capacitance leads to non-linear behavior during the startup phase of any frequency sine wave excitation.…”

“…which shows harmonic spectra obtained from analyzing the voltage The harmonic spectrum shows increase in the higher order harmonics at the time constant of the electrochemical response. We have shown through simulating with Randles' cell that this behavior is from the capacitive contribution of the electrochemical process, which shows highest harmonic response at the time constant of the cell [14]. The second is the result of the continuous change in the voltage level of the system (Voltage drift) see Fig.…”

“…Simulations: The simulations were done by using Spyder IDE of Anaconda with importing NumPy, Matplotlib, and SciPy libraries [32][33][34]. This simulation is a modified version of the code we used in our previous publication [14]. The code builds a class for generating the simulations with all the resistance and capacitance values.…”

“…This behavior is the result of the distortion caused by the large AC amplitude which causes initial drift at the first cycles of the G-EIS measurement that vanishes for the rest of cycles. This is related to the double layer capacitance and can be expected to appear in every electrochemical system [14]. The second, is the voltage-dependent capacitive behavior which is expected to increase the value of the higher harmonics as the frequency decreases.…”

“…-Simulation mathematics: The simulations were performed on an equivalent circuit model of three RC elements connected in series. In a previous publication, we employed the same fundamentals for NHA of simplified Randles' cells [14].…”