Electrochemical impedance spectroscopy (EIS) can be used for estimating the power delivery capability and state of health (SoH) of Li-ion batteries. It is important because it has the potential to improve rapid and accurate SoH monitoring of Li-ion batteries and support more sustainable battery storage systems for electric vehicles (EVs) and grid-scale energy storage systems. EIS is implemented without disassembly of the battery and can be performed under actual operating conditions. Today, EIS is mainly used for battery research and development projects. That may change in the future.

This FAQ briefly reviews various nondestructive methods to estimate the SoH of a battery, looks at an impedance model for a battery cell that supports EIS analysis, compares the use of Nyquist versus Bode plots for EIS, and closes with a brief look at efforts to develop EIS as a practical tool for real-time SoH measurements.

Currently, battery SoH is measured using nondestructive methods like series impedance measurements and voltammetry in addition to EIS. Measuring battery impedance at a single frequency or a range of frequencies can provide limited information. Certain degradation mechanisms can be measured only at specific frequencies depending on the battery. That often necessitates using low frequencies and can take time to implement.

Voltammetry measures current as a function of applied voltage. It’s a dynamic electrochemical measurement method where the voltage varies over time, and changes in current are measured. It’s primarily useful in laboratory settings. Of the nondestructive SoH testing methods, ampere hour (Ah) counting is currently the most popular. It’s also time-consuming and challenging to implement when the battery is only partially discharged before being recharged.

Implementing EIS
EIS can address the shortcomings of other battery SoH measurement technologies. It’s implemented with a small ac signal applied over a wide frequency range and measuring the response. EIS is especially useful for systems like Li batteries that include several impedance elements like resistances, capacitances, and materials interfaces. Because the various processes and impedance elements in a Li battery have different time constants, they can be separated and measured using EIS. EIS requires an accurate equivalent circuit model of a Li battery half-cell and the related Nyquist plot, including (Figure 1):

• The bulk resistance of the cell (R_b) is the total resistance of the electrodes, electrolyte, and separator and is the initial x-axis intercept value of the Nyquist plot.
• The resistance and capacitance of the solid electrolyte interphase layer, R_SEI, and CPE_SEI, respectively, form the first semicircle in the Nyquist plot and are associated with the deposition of the interfacial layer on the electrode. They are used to measure the formation of the solid electrolyte interphase (SEI) layer formed from the decomposition materials from the electrolyte.
• The second semicircle in the Nyquist plot is the charge-transfer resistance (R_ct) and double-layer capacitance (CPE_electrode). These values are related to the electrochemical reaction kinetics, which change based on the surface coating, phase transition, band gap structure, and particle sizes. It corresponds to the faradic charge-transfer resistance and can help measure temperature and reaction-dependent characteristics of the battery.
• The Warburg impedance (W_Warburg) relates to the diffusion of ions in the battery and forms the straight line at the right side of the Nyquist plot.

Bode plots vs. Nyquist plots
Impedance spectrums can be visualized in two ways, using a Bode plot, or using a Nyquist plot. Bode plots show the phase shift and magnitude changes with applied frequency and are generally used to measure the performance and stability of electronic circuits. A Bode plot does not show imaginary impedance information. A Nyquist plot represents an impedance spectrum’s real and imaginary parts in cartesian coordinates. To get a Nyquist plot, the negative imaginary impedance is plotted versus the real part of the impedance. Nyquist plots can provide insights into reaction mechanisms and related phenomena using an equivalent circuit model and are favored for electrochemical analysis (Figure 2).

Why not always use EIS?
Unfortunately, EIS is not easily implemented in real-world situations, especially with Li-ion batteries. The circuit model reviewed above applies to a theoretical and basic Li-ion battery half-cell. It’s useful for reviewing the principles of EIS but not for implementing EIS in a real battery. In physical batteries, the electrode characteristics are more complex, cell chemistries vary, cell constructions vary, and cycling conditions vary. EIS-based battery testers are available on a limited basis for use with lead acid batteries in warranty and repair depots (Figure 3). Work is underway to make the use of EIS practical with Li-ion batteries.

Figure 3: Lead acid battery tester that uses multi-model EIS. (Image: Cadex)

The key challenge for using EIS with Li-ion batteries is the development of custom circuit models by adding or subtracting electronic elements to match the specific characteristics of each electrochemical cell. It’s an iterative process that requires testing the accuracy of the models as they evolve. It’s not necessarily that simple or quick. In one instance, an EIS curve fitting algorithm is implemented using a digital signal processing front end connected to a personal computer. The curve-fitting algorithm repeatedly runs until a consistent result is obtained. That can produce an EIS test that works initially but becomes inaccurate as the batteries age.

Batteries suffer from lower performance as they are cycled. Common aging mechanisms result in decreased power and/or battery capacity. The primary causes of degraded performance in Li-ion batteries are:

• Loss of Li metal that reduces the battery capacity
• Loss of active anode and/or cathode materials that also reduces the battery capacity
• Lower ionic transport through the various battery components and interfaces that increases internal cell impedances and reduces power capability.

The usual process of parameter optimization for EIS measurements required a good starting point and intensive signal processing. Determination of an adequate starting point is not always possible. Research is underway to use EIS technology without relying on an equivalent circuit model to determine Li ion SoH. One strategy being investigated involves changing the shape and characteristics of the EIS scan.

To simplify the analysis, the initial EIS development work was performed assuming that the battery was in a steady state condition, at room temperature, and fully charged. Any variation away from that initial steady-state condition negatively affects the results of the EIS analysis.

Improved algorithms are being developed to adjust for less-than-optimal steady-state starting conditions. They are expected to contribute to more robust and sustainable energy storage systems for EVs and grid-scale energy storage to support green energy technologies if they become available.

Summary
Li-ion batteries are important parts of sustainable transportation and green energy systems. EIS can quickly and non-invasively measure the SoH of Li-ion batteries and help improve the performance of energy storage systems. EIS uses a Nyquist plot to represent an impedance spectrum’s real and imaginary parts and provide insights into reaction mechanisms and battery SoH. Due to the complexities of applying EIS with commercial Li-ion batteries, its use is currently limited to lead-acid batteries. Work is underway to develop EIS implementations that can be quickly and easily applied to commercial Li-ion batteries.

References

A Deeper Look at Lithium-Ion Cell Internal Resistance Measurements, Keysight
A Practical Beginner’s Guide to Cyclic Voltammetry, Journal of Chemical Education
Investigation and comparison of the electrochemical impedance spectroscopy and internal resistance indicators for early-stage internal short circuit detection through battery aging, Journal of Energy Storage
Modeling and Applications of Electrochemical Impedance Spectroscopy (EIS) for Lithium-ion Batteries, Journal of Electrochemical Science and Technology
State of Health Estimation of Li-Ion batteries using Electrochemical Impedance Spectroscopy, Cadex