Method to Increase the Speed of EIS Series Measurements on Battery Stacks during Cycling While Avoiding Dramatic Loss of Measurement Accuracy
Authors: Feihl Sebastian , Sholin Mark , Schiller Carl-Albrecht , Fröba Steffen , Multerer Michael , Strunz Werner 
Published: February 2020

The amount of time expended while running experiments is a crucial factor to consider when screening new materials as candidates for improving battery performance. Electrochemical impedance spectroscopy (EIS) is among the most appropriate evaluation tools for collecting in-situ information about electrochemical processes occurring inside battery systems. However, during charging and discharging cycles, the electrodes, electrolytes, and related materials comprising battery systems are continuously undergoing changes in state over time, even at timescales shorter than the amount of time required to run a single charge-discharge experiment. For instance, voltage is changing during constant current load. Here, the application of EIS can become restricted because the prerequisite of steady state as a foundational principle to interpret the resulting EIS spectra is violated due to changes of state in between the time it takes to sweep through frequencies of interest. Trying to yield significant results accounting for drift effects requires implementing particular modeling measures.
This abstract presents an ensemble of solutions to combat this issue. Parallel EIS, automatic drift correction of the measured time dependent signals, time interpolation by series measurements, and the application of the Z-HIT algorithm as post-processing of the data are the tools of choice. Simultaneous EIS is an effective time saving procedure. Otherwise, sequential EIS evaluation of n single cells in a stack increases time consumption by the factor n. Also, comparing the results for each individual cell to other cells is no longer directly comparable to an exact degree because the measurement drifts due to time effects are difficult to correct for. Second, precautions must be provided during the data acquisition phase in order to suppress the erroneous assignment of drift signals to the system response [1]. Furthermore, “non causal” spectra in the meaning of the Kramers-Kronig relations can be converted into correct spectra by interpolation of spectra series vs. time and, by removal of residual non-causalities with the help of the Z-HIT transform, which has the capability to reconstruct drift-affected data [2]. In many cases, the user would favor the comfortable possibility to significantly reduce the time for recording the EIS spectra by the application of multi-sine excitation. However, conventional multi-sine excitation normally is accompanied by significant losses of accuracy. An advanced technique provides the features of automatic drift correction and error detection known from single-sine excitation [3]. As a result, substantial loss in accuracy can be avoided.
Experimental data obtained from a four-cell Li-FePO4 stack (13.2 V / 40 Ah) using this new multi-sine algorithm will be presented upon approval of this abstract for presentation.
Keywords: Multi-sine, Battery stacks, EIS, True parallel measurements, Z-HIT
References:
[1] C. A. Schiller, F. Richter, E. Gülzow 3, N. Wagner; J. Phys. Chem. Chem. Phys. 3 (2001) 374
[2] W. Ehm, R. Kaus, C. A. Schiller, W. Strunz, New Trends in Electrochemical Impedance Spectroscopy and Electrochemical Noise Analysis, ed. F. Mansfeld, F. Huet, O. R. Mattos, Electrochemical Society Inc., Pennington, NJ, 2001, vol. 2000-24, 1
[3] C.-A. Schiller, R. Kaus; Bulgarian Chemical Communications, Volume 41, Number 2 (2009), pp. 192-198