The conflict between high charging rates and battery lifetime is still a challenge in the development of electric vehicles and power-tools. It is necessary to identify operating conditions that trigger degradation processes to be able to combine fast charging with a long battery lifetime. Therefore, non-invasive diagnostic methods to monitor real-time dynamics within the battery are crucial.

Electrochemical impedance spectroscopy (EIS) serves as a method to investigate the kinetics occurring in lithium-ion batteries. EIS can be used as a diagnostic indicator for the batteries’ state of charge (SOC) and state of health (SOH) or as a mean to estimate the internal temperature [1]. For EIS measurements in the conventional sense, a linear and stationary state is required [2]. Whereas with dynamic electrochemical impedance spectroscopy (DEIS), impedance data is recorded under charge or discharge to gain information about the battery under operation. By examining impedance anomalies, evidence of competing parasitic reactions and battery degradation can be found during operation. A drop of the impedance at the characteristic SEI and charge transfer excitation frequency shows the onset of metallic lithium deposition [3].

In our study, we compare the time-varying impedance data of two commercially available lithium ion-batteries at different temperatures and initial SOC. Impedance data are recorded during fast-charging experiments at different C-rates with a commercial potentiostat. We evaluate the data at eight frequency points between 2 kHz and 0,5 Hz. The frequency range was chosen so that a full spectrum can be mapped in a short measurement period. Both battery types are 18650 cylindrical cells but have different power-to-energy ratios. The high-energy cell has a graphite/SiOx anode, while the anode active material of the high power cell is graphite. The impedance characteristics of the cell incorporating graphite/SiOx are contrasted with those of the cell containing graphite as anode active material, highlighting the differences between the two cells. Observed impedance anomalies give an indication of occurring parasitic reactions. Thus, the charge limitation, dependent on the initial state-of-charge, the temperature, and the C-rate can be set. The occurrence of metallic lithium deposition and cell degradation is validated via complementary detection methods like the differential voltage analysis.

Literature
[1] Meddings, N., Heinrich, M., Overney, F., Lee, J.-S., Ruiz, V., Napolitano, E., Seitz, S., Hinds, G., Raccichini, R., Gaberšček, M. u. Park, J.: Application of electrochemical impedance spectroscopy to commercial Li-ion cells: A review. Journal of Power Sources 480 (2020), S. 228742
[2] Barsoukov, E. u. Macdonald: Impedance spectroscopy. Theory, experiment, and applications. Hoboken N.J.: Wiley-Interscience 2005
[3] Katzer, F., Rüther, T., Plank, C., Roth, F. u. Danzer, M. A.: Analyses of polarisation effects and operando detection of lithium deposition in experimental half- and commercial full-cells. Electrochimica Acta 436 (2022), S. 141401