Nawfal Al-Zubaidi-R-Smith

Electrochemical Impedance Spectroscopy (EIS) measurements are highly sensitive to the fixturing, temperature, and state of charge (SoC) of batteries. For 10 kWh automotive battery modules, we show that variations in SoC and temperature introduce significant errors at low-to-medium frequencies (<100 Hz), while improper fixture wiring affects mainly higher-frequency accuracy, with errors up to 100% in the imaginary part at 1 kHz. In addition, we study repeatability across various tester-module configurations. EIS results remain highly consistent (±100 µΩ) across three different modules. Comparing the same module across two different testers, deviations are even lower (±30 µΩ up to 1 kHz). The EIS evolution is studied with respect to the cycle numbers, where a strong correlation of low-frequency impedance features is demonstrated. A new combined quotient feature is introduced and suggested as a reliable and efficient state of health (SoH) indicator, solely based on a model-free and phenomenological approach. The study demonstrates the potential of EIS as a powerful tool for battery module characterization, provided that its requirements and limitations are carefully addressed through well-defined experimental setups. Accurate and repeatable EIS measurements are particularly important for obtaining accurate electrochemical insights, especially in the low-to-mid frequency domain, where impedance variations are most sensitive to battery states and ageing effects.

We demonstrate the reconstruction of battery electrochemical impedance spectroscopy (EIS) curves from time-domain pulse testing and the distribution of relaxation times (DRT) analysis. In the proposed approach, the DRT directly utilizes measured current data instead of simulated current patterns, thereby enhancing robustness against current variations and data anomalies. The method is demonstrated with a simulation, a single cylindrical battery cell experiment, and an experimental EIS of a completely assembled module of 448 cells. For the 3.7 kWh battery module, we applied a transient current pulse and analyzed the dynamic voltage responses. The EIS curves were reconstructed with DRT and compared to experiments across different states of charge (SoC). The experimental EIS data were corrected by a multistep calibration workflow in a frequency range from 50 mHz to 1 kHz, achieving error corrections of up to 80% at 1 kHz. The reconstructed impedances from the pulse test data are in good agreement with the EIS experiments in a broad frequency range, delivering relevant electrochemical information including the ohmic resistance and dynamic time constants of a battery module and its corresponding submodules. With the proposed workflow, rapid pulse tests can be used for extracting electrochemical information faster than standard EIS, with a 67% reduction in measurement time. This time-domain pulsing approach provides an alternative to EIS characterization, making it particularly valuable for battery monitoring, the classification of battery packs upon their return to the manufacturer, second-life applications, and recycling.

The electrochemical performance of carbon-based supercapacitors containing ionic liquid electrolytes was investigated through calibrated impedance spectroscopy and finite element modeling (FEM). To ensure precisely calibrated complex impedance measurements over a wide frequency range the measured pouch cells were mounted in a pressure fixture with stable terminal contacts, and a two-term impedance calibration workflow was applied. For the physical interpretation of the measurement results, FEM was used. Experimental findings demonstrated a clear dependency of the capacitive behavior on the electrode material, where cells with activated carbon electrodes showed lower impedance compared to cells with graphene electrodes. For FEM, we used a volume-averaged approach to study the effect of the electrode structure on the EIS response of the cells. The simulated impedance results showed a good agreement with experimental data in the middle- to high-frequency regions, ranging from 10 Hz to 10 kHz. Deviations from the ideal Warburg impedance were observed at lower frequencies, suggesting nonlinearity effects of the porous structure on ion transport mechanisms. FEM analysis was performed for both graphene and activated carbon electrodes showing a steeper transition region for activated carbon electrodes, indicating a reduced diffusion resistance for electrolyte ions.

We present an interoperable hierarchical data representation for battery tests, leading to improved scalability of data transmission and enhanced data accessibility and comprehensibility for both human interpretation and machine processing. The hierarchical data format includes the raw trace electrical measurement data, the metrological calibration and uncertainty data, the metadata such as experimental settings, instruments and software versions, as well as post‐processed data such as electrochemical model fit parameters. This data representation allows repetition of the battery test under the exact same conditions such that identical results are achieved within defined error bounds. This is in line with the general F. A. I. R. data approach and provides repeatability and traceability in the battery value chain. As an application of the hierarchical data representation, we show the classification of cells as pass/fail being performed with quantitative confidence levels. We demonstrate the complete workflow of establishing the hierarchical data structure for electrochemical impedance spectroscopy (EIS), starting from metrological traceability of the calibration and uncertainty analysis towards the storage of the structured data as a single integrated file that preserves the hierarchical data format. The structured data file is provided in JSON format in the Zenodo repository, as well as the program scripts to generate and read the JSON EIS files.

A 7 kWh automotive battery module with 396 interconnected cells was tested with electrochemical impedance spectroscopy (EIS) and time‐domain pulsing over 260 charge‐discharge cycles. An EIS calibration workflow was developed for low complex impedance values in a frequency range of 1 kHz to 50 mHz. Significant corrections on the resistance and the reactance were obtained from the calibration, particularly at frequencies above 100 Hz. Equivalent circuit parameters were extracted from the EIS spectra and the pulse response and investigated with respect to the cycle number and state‐of‐charge (SoC). Fit parameters were robustly extracted including R_sol, R_ct, and L from EIS, and R₀, τ₁ and τ₂ from time‐domain pulsing. The ohmic resistance decreased over the cycling number indicating an enhanced wetting of the electrodes. Charge transfer resistance R_ct showed a monotonic increase over the cycles related to cell ageing. From the charge and discharge pulses, the ohmic resistance R₀ was determined from the instantaneous voltage step of the recovery pulse, while the two time constants τ₁ and τ₂ correspond to the slower exponential recovery phase. R₀ from the time‐domain showed a similar trend as R_sol plus a contribution of R_ct from EIS. Overall, we show that calibrated EIS and time‐domain pulsing are efficient methods to gain insights into the electrochemical processes related to different SoCs and the cycling ageing of battery modules and packs.

Advanced electrochemical impedance spectroscopy (EIS) was applied to characterize industrial Ni-Cd batteries and to investigate the electrochemical redox processes. A two-term calibration workflow was used for accurate complex impedance measurements across a broad frequency range of 10 mHz to 2 kHz, resulting in calibrated resistance and reactance values. The EIS calibration significantly improved the measurements, particularly at high frequencies above 200 Hz, with differences of 6–8% to the uncalibrated impedance. With an electromagnetic finite element method (FEM) model, we showed that the impedance is strongly influenced by the cable fixturing and the self-inductance of the wire conductors due to alternating currents, which are efficiently removed by the proposed calibration workflow. For single cells, we measured the resistance and the reactance with respect to the state-of-charge (SoC) at different frequencies and a given rest period. For Ni-Cd blocks that include two cells in series, we found good agreement of EIS curves with single cells. As such, EIS can be used as a fast and reliable method to estimate the cell or block capacity status. For electrochemical interpretation, we used an equivalent electric circuit (EEC) model to fit the impedance spectra and to extract the main electrochemical parameters based on calibrated EIS, including charge-transfer kinetics, mass transport, and ohmic resistances. From the charge-transfer resistance, we computed the exchange current density, resulting in 0.23 A/cm², reflecting high intrinsic rates of the redox electron transfer processes in Ni-Cd cells.