Deconvolution of an Soc-Dependent Contact Resistance in a Multilayer Pouch Cell Using Impedance Spectroscopy
Authors: Helmer Simon , Morasch Robert , Friedrich Sven , Jossen Andreas , Gasteiger Hubert A. 
Published: September 2023

Electrochemical impedance spectroscopy (EIS) can be used as an effective non-destructive and easy-to-implement measurement method to determine the electrochemical response and state-of-health of battery cells. However, the analysis of the individual impedance contributions of the anode and cathode in a conventional two-electrode setup is quite complex due to several overlapping processes and the dominating inductive contributions at high frequencies (f ≥ 1 kHz) [1]. Therefore, in the absence of prior knowledge at the electrode level, statements about specific SoC-dependent characteristics may lead to an erroneous analysis of the impedance data. As an example, in a study by Kollmeyer et al. [2], a SoC-dependent change of the high-frequency x-axis intercept - in the literature also often defined as the high-frequency ohmic resistance Rhf [3] - is observed, which in this case, however, rather indicates superimposed impedance contributions of an interfacial contact impedance Rcont/Qcont due to dominating inductances.
To address the latter point, we simulate impedance spectra of a model electrode in blocking conditions with a contact impedance Rcont/Qcont and series-connected inductance, scaling the parameter for different geometric electrode areas Ageo. We show how, for larger electrodes, the decreasing high-frequency impedance contributions of Rcont/Qcont are increasingly superimposed by the inductance. To validate our theoretical finding, we compare laboratory-scale impedance spectra (Ageom= 0.94 cm²) acquired with a gold wire microreference (GWRE) T-cell setup with those of a large-scale cell (Ageom= 1475 cm², Fig. 1), both built with the same anode and cathode electrodes. For this, we measure SoC-dependent EIS of the NCA cathode in a 3.6 mAh NCA/Si T-cell via the GWRE, which shows an SoC-dependent contact resistance at high frequencies that is clearly visible as a high-frequency feature in the 50% SoC full-cell spectrum (f ≥ 1 kHz, Fig. 1b). In contrast, for the 5.6 Ah multilayer pouch cell with the same electrodes we see an SoC-dependent apparent high-frequency resistance Rhf app, as observed by Kollmeyer et al. [2], but, as shown in the 50% SoC impedance spectrum depicted in Fig. 1a, we do not observe the high-frequency semicircle (f ≥ 1 kHz) that was observed for the 3.6 mAh T-cell (Fig. 1b). Based on the simulation results and T-cell analysis, we can attribute the SoC dependence of the Rhf app of the multilayer pouch cell spectra to the SoC-dependent cathode contact resistance superimposed by inductances.
Acknowledgments:
S.H. gratefully acknowledges the financial support from the BMBF (Federal Ministry of Education and Research, Germany), under the auspices of the ExZellTUM III project (grant number 03XP0255). The authors want to thank the research battery production team at the iwb (TUM) for manufacturing the large-format pouch cells.
References:
[1] N. Meddings, M. Heinrich, F. Overney, J. S. Lee, V. Ruiz, E. Napolitano, S. Seitz, G. Hinds, R. Raccichini, M. Gaberšček,
J. Park, Journal of Power Sources 2020, 480, 228742.
[2] P. Kollmeyer, A. Hackl, & A. Emadi, IEEE transportation electrification conference and expo (ITEC) 2017. IEEE.
[3] X. Wang, X. Wei, J. Zhu, H. Dai, Y. Zheng, X. Xu & Q. Chen, ETransportation 2021, 7, 100093.
Figure 1: Nyquist plot of the impedance at 50% SoC of NCA/Si full-cells assembled either as 5.6 Ah multilayer pouch cells (MLP-cell) or as 3.6 mAh T-cells, built with the same electrodes and the same electrolyte. a) MLP-cell impedance measured in the frequency range from 3 kHz–0.1 Hz with an excitation amplitude of 5 mV (cell built with Celgard 2500 separator); b) T-cell impedance measured in the frequency range from 5 MHz – 0.1 Hz with an excitation amplitude of 10 mV (cell built with glass fiber separator).

Figure 1