Entering Electrochemistry | PEIS vs GEIS, Dynamic EIS, and EIS Analyzer Validation

1. Introduction

EIS functions as a non-destructive diagnostic tool for lithium-ion batteries: by decomposing the cell’s internal electrochemical and transport processes across different time scales, it reveals information that voltage or capacity measurements alone cannot provide. This report compares the performance of the IEST ERT7008 (ERT7 series) against a well-known international electrochemical workstation to evaluate how closely their EIS measurements agree across common cell formats and test modes. The evaluation covers static EIS battery testing in PEIS and GEIS modes, as well as dynamic EIS measurement during active cycling.

2. How EIS Frequency Regions Reveal Battery Internal Processes

EIS characterizes a battery’s AC impedance by applying a small-amplitude sinusoidal signal across a frequency spectrum, with results typically presented as a Nyquist plot (Z’ vs −Z”). Each frequency region of the EIS spectrum corresponds to a distinct electrochemical process within the cell, enabling quantitative assessment of SEI film stability, charge transfer efficiency, and lithium-ion diffusion rates.

2.1 Li⁺ Transport Pathway and Nyquist Plot Interpretation

Understanding the Li⁺ transport sequence within a cell enables direct mapping of each step to a specific feature on the Nyquist plot. The transport sequence proceeds as follows:

• External circuit: Electron conduction from Cu current collector into the graphite anode.
• Graphite anode: Li⁺ intercalation into graphite, passing through the SEI film.
• Electrolyte: Li⁺ solvation and migration through the porous separator.
• Cathode (e.g., LiCoO₂): Li⁺ de-solvation, passage through any surface film, and intercalation into the cathode active material.
• External circuit: Electron conduction via the Al current collector.

Figure 1. Li⁺ transport pathway in a lithium-ion battery and the corresponding electrochemical processes, each associated with a distinct time constant in the EIS spectrum.

Figure 2. Typical EIS Nyquist plot for a lithium-ion battery with characteristic frequency regions identified: high-frequency SEI semicircle, mid-frequency charge transfer semicircle, and low-frequency Warburg diffusion tail.

By deconvoluting the impedance response across distinct EIS frequency domains, three key electrochemical parameters can be quantitatively resolved: SEI film resistance (\(R_{SEI}\)), charge transfer resistance (\(R_{ct}\)), and diffusion impedance (\(Z_W\)).

2.2 High-Frequency Region — SEI Film Resistance (\(R_{SEI}\))

• Process: Ionic transport impedance of Li⁺ through the SEI layer.
• \(R_{SEI}\): Resistance of the SEI film — increases with film thickening during aging.
• \(C_{SEI}\): Capacitance of the SEI layer, reflecting its double-layer characteristics.

2.3 Mid-Frequency Region — Charge Transfer Resistance (\(R_{ct}\))

• Process: Charge transfer reaction at the electrode/electrolyte interface — Li⁺ intercalation/deintercalation in graphite (anode) and LiCoO₂ (cathode).
• \(R_{ct}\): Charge transfer resistance, indicating the kinetics of the electrochemical reaction.
• (C_{dl}\): Double-layer capacitance at the electrode/electrolyte interface.

2.4 Low-Frequency Region — Solid-State Diffusion (Warburg Impedance \(Z_W\))

• Process: Solid-state diffusion of Li⁺ within the bulk electrode active material (inside graphite or LiCoO₂ particles).
• \(Z_W\)): Warburg diffusion impedance, inversely proportional to the square root of frequency, appearing as a 45° slope on the Nyquist plot.

3. PEIS vs GEIS: Choosing the Right EIS Mode for Battery Testing

The choice between PEIS and GEIS is the most critical EIS battery test configuration decision — selecting the wrong mode for a given cell format produces unreliable impedance data and can damage the cell. The two modes differ in how the AC excitation signal is applied and which parameter is controlled.

PEIS applies a fixed small-amplitude AC voltage across the cell and measures the resulting current response. Because the voltage amplitude is fixed regardless of cell impedance, PEIS is suited to high-impedance systems such as coin cells and small lab cells where a 10 mV excitation produces a well-defined, linearity-compliant current response. Applying PEIS to a large, low-impedance pouch cell risks driving excessive currents.

GEIS applies a fixed small-amplitude AC current and measures the voltage response. For large-format, low-impedance cells — where the impedance may be in the single-digit milliohm range — a controlled current amplitude of, for example, 80 mA produces a well-defined millivolt-scale voltage response that complies with linearity requirements. GEIS is therefore the appropriate mode for production-format pouch and cylindrical cells.

4. EIS Battery Test Validation: IEST ERT7008 vs International Electrochemical Workstation

Two cell formats were selected to evaluate EIS data agreement between the IEST ERT7008 and a well-known international electrochemical workstation across both PEIS and GEIS modes.

4.1 Coin Cell EIS — PEIS Mode (100 kHz–0.1 Hz, 10 mV, 4.15 V, 25°C)

EIS data for the coin cell in PEIS mode demonstrates exceptional congruence between the IEST ERT7008 (green curve) and the international workstation (red curve) across the full frequency range from 100 kHz to 0.1 Hz. The Nyquist plots overlay with a high degree of impedance data matching — the high-frequency SEI semicircle, mid-frequency charge transfer semicircle, and low-frequency Warburg diffusion tail are all faithfully reproduced by the ERT7008.

4.2 Pouch Cell EIS — GEIS Mode (100 kHz–0.1 Hz, 80 mA AC, 3.19 V, 25°C)

EIS results for the 2 Ah pouch cell in GEIS mode show only minor discrepancies between the IEST ERT7008 and the international electrochemical workstation, with deviations consistently below 5% across all impedance features. The low-impedance, large-format cell format represents the more demanding validation condition for an EIS analyzer — the sub-5% agreement confirms that the ERT7008’s galvanostatic signal control and current measurement precision are adequate for production-format cell characterization.

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%.

5. IEST ERT7 Series EIS Analyzer: Key Specifications

5.1 Frequency Range: 0.01 Hz to 100 kHz

The low-frequency segment below 1 Hz probes solid-state diffusion processes (Warburg impedance), while the high-frequency segment above 10 kHz captures ohmic and contact resistance. The 0.01 Hz lower bound enables characterization of slow diffusion processes that would be invisible to analyzers with a 0.1 Hz floor — relevant for thick electrodes and solid electrolyte systems. The 100 kHz upper bound ensures accurate capture of the SEI semicircle in its entirety.

5.2 Impedance Range: 10 mΩ to kΩ

The ERT7008 accommodates cell impedances from 10 mΩ (production-format large pouch cells) to kΩ (lab-scale half-cells and coin cells) within a single instrument. A high-precision signal source and low-noise measurement architecture ensure accurate resolution of subtle impedance changes across this six-order-of-magnitude range — without requiring external boosters or switching between multiple measurement heads.

5.3 PEIS and GEIS Mode Support

The ERT7 series natively supports both PEIS (potentiostatic EIS) for high-impedance coin and lab cells, and GEIS (galvanostatic EIS) for low-impedance production-format cells, within the same instrument and software environment. This eliminates the need to maintain separate instruments for different cell formats or test protocols.

5.4 Comparison with Standalone Electrochemical Workstations

8. FAQs