Entering Electrochemistry | Testing Methods, Advantages, and Applications of Dynamic EIS

1. Introduction

Electrochemical Impedance Spectroscopy (EIS) is one of the most important techniques for studying electrochemical interface issues and is widely used to investigate the electrochemical behavior within lithium-ion batteries. Its principle involves treating the lithium battery as a “black box.” By applying a sinusoidal AC signal with a specific amplitude across a range of frequencies, the corresponding electrical signal response in the frequency domain is acquired. Subsequently, by analyzing the feedback signals at different frequencies within the EIS spectrum, the goal is to diagnose whether abnormalities exist in the internal electrochemical behavior of the cell.

Traditional static EIS testing requires satisfying three prerequisites: 1) Causality; 2) Linearity; 3) Stability. Consequently, EIS testing can only commence after the lithium-ion battery has equilibrated for a prolonged period. However, this approach risks missing transient dynamic changes occurring within the battery. Therefore, Dynamic EIS (DEIS) testing has gradually become a key focus for lithium battery engineers.

2. Significance of Dynamic EIS Testing

As the name suggests, Dynamic EIS involves performing EIS testing during battery charging or discharging. Its significance lies primarily in the following aspects:

2.1 Real-time Monitoring and Capturing Dynamic Processes

Dynamic EIS can reflect the instantaneous state information of a battery in real-time. For instance, it can capture the rapid growth process of lithium dendrites or fast electrochemical reactions during fast charging. If one waits for a long rest period before conducting EIS testing and monitoring, lithium plating behavior may gradually weaken or even disappear. Therefore, Dynamic EIS possesses a unique advantage in capturing rapid electrochemical reaction behaviors.

2.2 Analysis of Non-linear and Non-steady-state Processes

Actual battery systems often exhibit non-linear or non-steady-state characteristics. For example, batteries in practical use frequently operate under non-steady-state conditions, constantly subject to disturbances like temperature fluctuations and load variations. Dynamic EIS enables testing under these non-steady-state conditions, acquiring impedance information of the battery in different states. This provides more comprehensive data support for battery performance evaluation and fault diagnosis.

2.3 Early Fault Diagnosis and Advantages for Data Modeling

Since Dynamic EIS eliminates the need for lengthy rest periods, it enables earlier detection of potential battery faults. For instance, when lithium plating or abnormal thermal runaway occurs internally, the mid-frequency impedance measured by Dynamic EIS may decrease or increase abnormally, providing timely warning information to users. Furthermore, the volume of data acquired in Dynamic EIS mode is typically much larger than in static EIS. This data can be used to establish more accurate battery models, providing more reliable data support for battery performance prediction, lifetime assessment, and optimized design.

3. Innovative Solution from IEST Instrument

IEST Instrument newly launched Battery Impedance Tester (BIT6000) can be integrated with any third-party charge/discharge equipment. This enables Dynamic EIS testing of lithium-ion batteries during the charging or discharging process. Continuous testing mode can be configured in the software, allowing for uninterrupted DEIS testing and monitoring, facilitating battery performance monitoring and failure mechanism analysis under non-steady-state conditions. Additionally, another key advantage of this instrument is its capability to perform EIS testing on large-capacity, low-internal-resistance batteries (e.g., power batteries or energy storage batteries) without requiring an external amplifier. This effectively addresses the current challenge of EIS testing large-capacity cells.

Figure 1. Schematic diagram of the Battery Impedance Tester (BIT6000) integrated with an arbitrary third-party charge/discharge device for DEIS testing.

4. Continuous Dynamic EIS Monitoring During Charging

Next, taking a 40Ah power battery (LFP//C) as an example, the battery was first fully discharged. Subsequently, under a constant current (CC) charging condition of 0.3C, DEIS testing was performed continuously while charging, until the battery reached approximately 70% State of Charge (SOC). The results are shown in Figure 2. These test results can be further analyzed by leveraging established electrochemical models to obtain the evolution of dynamic parameters (including resistance and capacitive reactance corresponding to electrochemical processes with different time constants) as the cell charges to different SOC levels. This assists in understanding the instantaneous electrochemical behaviors occurring during the charging process.

Figure 2. Continuous Dynamic EIS monitoring of an LFP battery under 0.3C constant current charging

Figure 3(a) illustrates the micro-model proposed by Barsoukov et al. [1] for the intercalation and deintercalation processes of lithium ions within the electrode active material. Figure 3(b) shows the typical EIS spectrum for the deintercalation and intercalation of lithium ions in the electrode active material. It can be observed that conventional EIS test results generally encompass five electrochemical processes with distinct time constants [1, 2]:

4.1 Ultra-High Frequency Region:

The ohmic resistance associated with lithium-ion and electron transport through the electrolyte, porous separator, and current collectors. This appears as a single point intersecting the real axis (Z’) on the EIS plot, typically denoted as Rs.

4.2 High Frequency Region:

A semicircle related to lithium-ion diffusion through the Solid Electrolyte Interphase (SEI) film. This process can be represented in an equivalent circuit by a parallel combination of R~SEI~ and C~SEI~ (R~SEI~ // C~SEI~).

4.3 Mid-Frequency Region:

A semicircle corresponding to the charge transfer process. This process can be represented in an equivalent circuit by a parallel combination of charge transfer resistance (R~ct~) and double-layer capacitance (C~dl~) (R~ct~ // C~dl~).

4.4 Low Frequency Region:

Related to solid-state diffusion of lithium ions within the active material particles. This appears as a straight line with a 45° slope on the EIS plot. In the equivalent circuit, this process is represented by a Warburg impedance (Z~W~) describing diffusion.

4.5 Very Low Frequency Region (<0.01 Hz):

Comprises a semicircle potentially related to changes in the active material particle crystal structure or new phase formation, and a near-vertical line related to lithium-ion accumulation/depletion in the active material. Lithium batteries are typically not analyzed at such low frequencies. Therefore, the common equivalent circuit used for analyzing lithium battery EIS spectra is shown in Figure 4(a).

Figure 4(b) shows the variation of the dynamic R~ct~ with SOC, obtained by fitting the continuous DEIS test results (shown in Figure 2) using the equivalent circuit. It can be observed that within the 0% to 70% SOC range, the dynamic R~ct~ first decreases rapidly (0%-30% SOC) and then decreases more slowly (30%-70% SOC). It is known that for LFP batteries, due to their relatively flat charge/discharge voltage plateau, using the Open Circuit Voltage (OCV) curve to estimate SOC in practical applications results in significantly larger errors compared to other cell chemistries [3]. However, DEIS analysis reveals that the change in dynamic R~ct~ exhibits a strong correlation with SOC. Therefore, it can serve as an important parameter for estimating the SOC of LFP batteries.

Figure 3. (a) Micro-model for Li-ion intercalation/deintercalation in electrode active materials proposed by Barsoukov et al.; (b) Typical EIS spectrum for Li-ion deintercalation/intercalation in electrode active materials.

Figure 4. (a) Common equivalent circuit model for lithium batteries; (b) Variation curve of dynamic R~ct~ with SOC derived from continuous DEIS testing of a 40Ah LFP power battery.

5. Summary

IEST Battery Impedance Tester (BIT6000, shown in Figure 5) can be paired with any third-party charge/discharge equipment to achieve continuous dynamic EIS testing on large-capacity, low-internal-resistance batteries. It effectively captures transient electrochemical behaviors during charge/discharge processes, facilitating research into the non-steady-state kinetics of lithium-ion batteries. Furthermore, continuous Dynamic EIS testing on a 40Ah LFP power battery demonstrated a strong correlation between dynamic R~ct~ and SOC, indicating its potential as an important parameter for estimating the SOC of LFP cells.

Figure 5. Appearance of the Battery Impedance Tester (BIT6000)

6. References

[1] E. Barsokov, D.H. kim, H.-S. Lee, H. Lee, M. Yakovleva, Y. Gao and J.F. Engel, Comparison of kinetic properties of LiCoO₂ and LiTi_0.05Mg_0.05Ni_0.7Co_0.2O₂ by impedance spectroscopy. Solid State Ionics 161 (2003) 19-29.

[2] E. Barsoukov and J.R. Macdonald. Impedance spectroscopy theory, experiment, and applications. Second Edition. New Jersey: John Wiley & Sons, Inc., Hoboken, 2005.

[3] P.P. Xu, J.Q. Li, Q. Xue and F.C. Sun. A syncretic state-of-charge estimator for LiFePO₄ batteries leveraging expansion force. J Energy Storage 50 (2022) 104559.