Advanced Electrochemical Impedance Spectroscopy of Industrial Ni-Cd Batteries

Abstract

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.