Electrochemical-Impedance Spectroscopy (EIS) is a method of obtaining detailed information on the performance characteristics of electro-chemical cells. During EIS, the cell is exposed to an excitation signal at a given frequency. This signal can be described in voltage terms (potentiostatic) or current terms (galvanostatic). For example, during galvanostatic EIS the resulting voltage signal is measured for the given current excitation. For a fully resistive load, the voltage would be completely in-phase with the current. When the load has capacitance, the voltage will experience a phase shift in proportion to the capacitive impedance. When the frequency of the excitation is varied, electrochemical cells exhibit different impedance characteristics which can be used to analyse performance and durability.
Figure 1: Galvanostatic Voltage Response
The data obtained during an EIS sweep is typically processed into a Nyquist plot, which separates real impedance (Z’) and imaginary impedance (Z’’). The phase shift represents the angle between the in-phase real impedance and out-of-phase imaginary impedance. Figure 2 shows a typical Nyquist plot for a lithium-ion battery. As the frequency of excitation changes, the process which dominates the characteristics of the response also changes.
At low frequencies (typically on the right hand side of the plot) ion diffusion in the electrodes are dominant which is recognised as the slowest process in the battery. As the test frequency rises, processes which take longer to occur than the period of the excitation are excluded from having an effect on the response. At the highest frequencies (left hand side), the excitation signal reaches a speed which is faster than any of the chemical processes within the battery, so the response become purely resistive and is a measure of ohmic resistance of system.
Each individual time constant within the system will create a semi-circle figure in a Nyquist plot.
Figure 2: Typical Nyquist Plot
Data from EIS testing can be used to create and parameterize battery models, understand and track state of health during battery lifetime and improve battery technology by improving our understanding of battery processes.
CamMotive has expert knowledge of electric vehicle and battery testing as well as decades of experience in the automotive testing industry. To discuss your testing requirements, or for more information, contact CamMotive to connect to an expert.