Entering Electrochemistry | Application of Broadband Dynamic EIS Testing in Battery Cell

Within the battery testing field, Electrochemical Impedance Spectroscopy (EIS) is an indispensable analytical technique. Functioning like an “X-ray machine,” it enables non-destructive interrogation of a cell’s internal state, providing insights into battery health, aging mechanisms, and material properties. This raises a critical question: Among the diverse EIS testing equipment available on the market, which offers superior performance? Today, we conduct a direct comparison between our IEST ERT7008 System and a renowned international electrochemical workstation to evaluate which delivers more precise and stable EIS measurements!

1. What is EIS Testing?

EIS, an acronym for Electrochemical Impedance Spectroscopy, fundamentally involves applying a small-amplitude sinusoidal alternating current (AC) signal to a battery and measuring its complex AC impedance as a function of frequency. The data is typically presented as a Nyquist plot (Z’ vs Z”). This technique non-destructively resolves the impedance characteristics of various interfaces and bulk phases within the cell, encompassing electron/ion conduction, interfacial reaction kinetics, and mass transport processes. By analyzing the response features within distinct frequency regions of the EIS spectrum, key parameters such as SEI layer stability, charge transfer efficiency, and lithium-ion diffusion coefficients can be quantitatively assessed. This provides critical insights for optimizing battery material design, diagnosing degradation mechanisms, and enhancing electrochemical performance.

2. Applications of EIS in Lithium-Ion Battery Research

First, trace the lithium-ion transport pathway within the cell to identify the corresponding semicircular features on the Nyquist plot for each process. This facilitates the deduction of the underlying electrochemical mechanisms from the measured impedance curve.

Figure 1. Lithium-Ion Transport Pathway in a Lithium-Ion Battery

2.1 The diagram illustrates the Li⁺ transport sequence:

① External Circuit: Electron conduction (Cu current collector → Graphite anode).

② Graphite Anode: Li⁺ intercalation into graphite, traversing the SEI (Solid Electrolyte Interphase) layer.

③ Electrolyte: Li⁺ solvation (binding with electrolyte solvent), migration through the porous separator.

④ Cathode (LiCoO₂): Li⁺ desolvation, passage through the CEI (Cathode Electrolyte Interphase) layer (if present), intercalation into the cathode active material.

⑤ External Circuit: Electron conduction via the Al current collector.

Figure 2. Physical and Chemical Properties of Electrochemical Systems Characterized by EIS in Different Frequency Ranges

The high-frequency region reflects the ionic transport characteristics across the SEI layer, the mid-frequency region corresponds to the charge transfer reaction, while the low-frequency region characterizes solid-state diffusion kinetics within the electrode material. By deconvoluting the impedance response across distinct EIS frequency domains, key electrochemical parameters governing interfacial and bulk processes can be quantitatively evaluated, including SEI film resistance (R_SEI), charge transfer resistance (R_ct), and diffusion impedance (R_w).

2.2 High-Frequency Region (First Semicircle)

•  Corresponding Process: Ionic transport impedance of Li⁺ through the SEI layer.

• Electrochemical Parameters:

  • R_SEI: Resistance of the SEI layer.

  • C_SEI: Capacitance of the SEI layer (reflecting its double-layer characteristics).

2.3 Mid-Frequency Region (Second Semicircle)

• Corresponding Process: Charge transfer reaction (electrochemical reaction) at the electrode/electrolyte interface, including:

• Anode: Li⁺ intercalation/deintercalation in graphite.

• Cathode: Li⁺ intercalation/deintercalation in LiCoO₂.

• Electrochemical Parameters:

  • R_ct: Charge transfer resistance (indicates kinetics of electrochemical reactions).

  • C_dl: Double-layer capacitance (charge accumulation at the electrode/electrolyte interface).

2.4 Low-Frequency Region (Sloping Line)

• Corresponding Process: Solid-state diffusion of Li⁺ within the bulk electrode material (e.g., inside graphite or LiCoO₂ particles).

• Electrochemical Parameters:

  • R_w: Warburg impedance (diffusion impedance, inversely proportional to the square root of frequency).

3. Experimental Comparison

3.1 Experimental Setup：

Two distinct cell configurations were selected for EIS testing:

• Cell Comparison: Coin cell vs. Pouch cell
• Equipment Comparison: International electrochemical workstation vs. IEST ERT7008

Coin Cell (4.15V, 25°C), Test Parameters: 100 kHz ~ 0.1 Hz, 10 mV sinusoidal signal amplitude (PEIS).

Pouch Cell (2Ah, 3.19V, 25°C), Test Parameters: 100 kHz ~ 0.1 Hz, 80 mA AC current amplitude (GEIS).

Figure 3. EIS Analysis–Different Cell Impedance Analysis

3.2 Results

The EIS data for the Coin Cell demonstrates exceptional congruence between the IEST ERT7008 (green curve) and the competitor equipment (red curve), indicating a very high degree of impedance data matching. Similarly, EIS results for the Pouch Cell show only minor discrepancies between the IEST system and the international electrochemical workstation, with deviations consistently below 5%.

4. IEST ERT 7 Series Specifications

4.1 Wide Frequency Range: 0.01 Hz ~ 100 kHz

• Frequency range is a critical EIS specification. The low-frequency segment (<1 Hz) analyzes diffusion processes, while the high-frequency segment (>10 kHz) reflects contact resistance. The IEST system covers the broad range of 0.01 Hz to 100 kHz, suitable for diverse cell types.

4.2 High-Precision Signal Control: Accommodates cells from 10 mΩ to kΩ range

• Cell impedance spans milliohm (mΩ) to kiloohm (kΩ) levels. The IEST system employs a high-precision signal source and low-noise measurement system, ensuring accuracy and enabling detection of subtle impedance variations.

4.3 Support for Multiple EIS Modes: PEIS & GEIS

• Potentiostatic EIS (PEIS) is suitable for micro-current systems like coin cells and lab-scale cells. Galvanostatic EIS (GEIS) is better suited for pouch cells undergoing low-current discharge. The IEST system supports both modes, catering to varied application requirements.

4.4 Why Choose IEST Instruments Solutions?