Entering Electrochemistry | Electrochemical Impedance Spectroscopy (EIS) Guide for Battery Testing

Electrochemical Impedance Spectroscopy (EIS) is a non-destructive frequency-domain technique for characterizing the internal electrochemical processes of lithium-ion batteries. A small-amplitude sinusoidal voltage signal (typically 5 mV) is applied across a battery across a frequency range of 0.1 Hz to 100 kHz, and the resulting current response is recorded. The ratio of voltage to current at each frequency yields the impedance spectrum Z(ω), which is visualized as a Nyquist plot or Bode plot. Battery EIS analysis separates distinct electrochemical processes — ohmic resistance (Rs), SEI layer resistance (Rsei), charge transfer resistance (Rct), and Warburg diffusion impedance (Zw) — based on the frequency region in which each process occurs, providing a complete picture of battery state-of-health, aging mechanisms, and internal kinetics.

1. Preface

Electrochemical measurement methods assess electrical quantities such as potential, conductivity, and current to analyze the system under study — enabling qualitative and quantitative characterization of its components. Common electrochemical measurement techniques include constant current/potential methods, chronoamperometry/chronopotentiometry, voltammetry, and electrochemical impedance spectroscopy.

EIS imposes minimal disturbance on the system under study and provides comprehensive insight into internal electrochemical processes across a wide frequency range — advantages that distinguish it from other electrochemical characterization techniques. Battery EIS has been widely applied in lithium-ion battery research, where accurate acquisition and analysis of impedance spectra have led to significant advances in understanding degradation mechanisms, ionic transport, and interfacial behavior. This article presents the principles, EIS graph interpretation, equivalent circuit modeling, and application scenarios of battery EIS analysis.

2. Basic Principles of Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS), also known as electrochemical AC impedance spectroscopy, is a frequency-domain characterization technique in which a small-amplitude sinusoidal signal is applied to the battery and the impedance response is measured across a sweep of frequencies. Given that lithium-ion batteries behave as linear, stable, and causal systems, applying a series of sinusoidal voltage signals — with amplitudes of 5 mV and frequencies ranging from 0.1 Hz to 100 kHz — generates corresponding sinusoidal current responses. The frequency-domain response function Z(ω) = V/I represents the impedance at each frequency, and this complete series of impedance values constitutes the battery’s EIS spectrum.

A Nyquist plot is an EIS graph that uses the real part of impedance (Z_Re) as the x-axis and the negative imaginary part (−Z_Im) as the y-axis, providing a direct visual representation of the time constants for different electrochemical processes.  The Bode plot displays the phase shift and magnitude as functions of the applied frequency, often used to assess the performance and stability of electronic circuits. Figure 1 shows Nyquist and Bode plots for a 2500mAh lithium iron phosphate (LiFePO4) battery. The impedance spectrum ranges from 0.1 Hz to 1 kHz, with frequency decreasing from left to right. At 1 kHz, the imaginary part of the impedance is nearly zero. As the frequency decreases, the real part of the impedance increases, and the negative imaginary part exhibits an initial increase, followed by a decrease, and then another increase. The EIS curve comprises three segments: two irregular semicircles in the high and mid-frequency regions, and a straight line in the low-frequency region. By analyzing the electrochemical processes within the lithium-ion battery, it is evident that different processes correspond to distinct curves: the high-frequency region represents the migration and diffusion of lithium ions through the SEI layer, the mid-frequency region reflects the charge transfer process, and the low-frequency region depicts the solid-state diffusion of lithium ions within the active electrode material.

Figure 1. Battery EIS graph showing Nyquist plot (a) and Bode plot (b) for a 2500 mAh LFP cell 

3. EIS Analysis: Components and Equivalent Circuit Model

3.1 EIS Spectrum Components by Frequency Region

Frequency Region	Frequency Range	Physical Process (Electrochemical Mechanism)	Equivalent Circuit Element
Ultrahigh-Frequency	> 10 kHz	Ohmic resistance (Transport of Li+ & electrons in electrolyte/separator)	Rs (Ohmic Resistance)
High-Frequency	(MHz – kHz range)	Li+ diffusion through the SEI layer	Rsei || Csei (SEI Resistance & Capacitance)
Mid-Frequency	(kHz – Hz range)	Charge transfer process (Electrochemical reaction)	Rct || Cdl (Charge Transfer Resistance & Double-layer Capacitance)
Low-Frequency	(Hz – mHz range)	Solid-state diffusion of Li+ within active material	Zw (Warburg Impedance)
Extremely Low-Frequency	< 0.01 Hz	Phase change / Crystal structure change & Li+ accumulation	(Rb || Cb) + Cint (Bulk Resistance/Capacitance & Integration Capacitance)

A typical lithium battery EIS spectrum can be divided into five regions, as illustrated in Figure 2:

Figure 2. Typical EIS spectrum Nyquist plot for Lit intercalation and deintercalation in compound electrodes frequency regions labeled- five frequency regions labeled

How to Read an EIS Graph: Nyquist Plot Interpretation

When interpreting a battery EIS graph in Nyquist format, three features carry diagnostic significance:

• X-intercept at high frequency (Z_Re minimum): The point where the EIS curve crosses the real axis at high frequency represents the ohmic resistance Rs — the combined resistance of the electrolyte, separator, and current collectors. A rising Rs over cycles indicates electrolyte depletion or contact degradation.

• High-frequency semicircle diameter: The first semicircle in the EIS graph corresponds to the SEI layer resistance (Rsei). Growth of this semicircle over cycling indicates SEI thickening — a key aging indicator.

• Mid-frequency semicircle diameter: The second semicircle represents charge transfer resistance (Rct) at the electrode–electrolyte interface. An increasing Rct is the primary indicator of active material degradation and capacity fade.

• Low-frequency slope (Warburg region): The 45° straight line in the low-frequency EIS curve is the Warburg impedance (Zw), reflecting solid-state Li⁺ diffusion within the active material. Steeper slopes indicate slower diffusion kinetics.

3.2 Equivalent Circuit Model and EIS Data Analysis

Equivalent circuit modeling is the standard method for quantitative EIS analysis — it translates the measured impedance spectrum into specific resistance and capacitance values for each internal process. A lithium-ion battery is modeled as an electrical circuit comprising resistors, capacitors, and distributed elements. The equivalent circuit model simplifies the battery into a circuit system that reproduces the measured EIS spectrum through simulation.

The common equivalent circuit model for lithium-ion batteries, shown in Figure 3, assigns each element to a specific frequency region: Rs represents ohmic resistance; Rsei and Csei model the SEI layer (high-frequency semicircle); Rct and Cdl model charge transfer (mid-frequency semicircle); and the Warburg element W represents solid-state Li⁺ diffusion, appearing as a 45° line relative to the real axis in the low-frequency EIS graph.

EIS analysis workflow: Once the impedance spectrum is acquired, data fitting is performed using equivalent circuit software — commonly Zview, ZSimpWin, EIS300, LEVMW, or Autolab Nova. The analyst selects the appropriate equivalent circuit topology for the system, then performs a complex nonlinear least-squares (CNLS) fit to extract the values of Rs, Rsei, Rct, and Zw from the experimental EIS data. The quality of the fit is assessed by the chi-squared residual; values below 10⁻³ indicate a reliable fit. Extracted impedance parameters are then tracked over cycling, temperature, or state-of-charge (SOC) to quantify aging trends and identify rate-limiting processes.

Figure 3. Lithium-Ion Battery EIS Spectrum and Equivalent Circuit Model

4. Application Scenarios

EIS testing of battery cells and electrode materials enables dynamic, non-destructive characterization of internal electrochemical processes across the full frequency range — from ohmic transport at high frequency to solid-state diffusion at low frequency. Specific applications of battery EIS testing include:

• Electrode Material Characterization: EIS measurement of electrode materials in different electrolytes assesses ionic conductivity, interfacial reaction activity, and the effect of formulation or coating changes — supporting electrode material optimization.
• Study of Internal Electrochemical Processes: Battery EIS analysis provides direct quantification of ion migration in electrolytes, charge transfer kinetics at the electrode–electrolyte interface, and solid-state diffusion coefficients within active materials.
• State-of-Health (SOH) Diagnosis and Fault Detection: Tracking impedance changes (particularly Rsei and Rct) over cycling enables quantitative SOH assessment and early detection of degradation modes — including SEI growth, electrolyte decomposition, and lithium plating.
• Cycle Life and Aging Assessment: Periodic EIS measurement during cycling captures the evolution of individual impedance components, providing mechanistic data for predicting remaining useful life and formulating battery management strategies.
• Thermal Runaway Characterization: EIS testing at elevated temperatures reveals how temperature, current density, and capacity affect battery impedance — supporting safety assessment and the design of thermally resilient cell architectures.
• Electrolyte Property Evaluation: EIS measurement quantifies the ionic conductivity, ion mobility, and diffusion coefficients of electrolyte systems — including liquid, gel polymer, and solid electrolytes.

In summary, battery EIS testing provides researchers with a single, non-destructive measurement that simultaneously characterizes ohmic resistance, interfacial kinetics, and diffusion behavior — making it an indispensable tool for battery R&D, quality control, and performance modeling.

5. In-Situ EIS Battery Testing with IEST ERT7008

The IEST ERT7008 Electrochemical Performance Analyzer integrates battery cycling, EIS measurement, and cyclic voltammetry (CV) in a single instrument, enabling in-situ impedance spectroscopy during long-term cycling protocols without cell disassembly or transfer. In the test configuration shown in Figure 4, EIS is inserted as an independent measurement step within the cycling sequence — triggered after every N cycles or after the cell reaches a defined SOC during charge or discharge. This approach preserves consistent cell contact conditions and eliminates the variability introduced by repeated connection and disconnection in ex-situ setups.

Figure 4. IEST Electrochemical Performance Analyzer ERT7008 (a) and Cyclic In-Situ EIS Testing Step Configuration (b)

Figure 5. EIS Nyquist Plot Data Acquired During Battery Cell Cycling (In-Situ)

6. References

[1] Ning B,Cao B,Wang B,et al. Adaptive Sliding Mode Observers for Lithium-ion Battery State Estimation Based on Parameters Identified Online[J].

[2] Zhang Jinlong, Tong Wei, Qi Hanhong, et al. Application of Square Root Sigma Point Kalman Filter to SOC Estimation of Li FePO_4 Battery Pack[J].Proceedings of the CSEE,2016,36(22):6246-6253.

[3] Barsoukov E , Macdonald R J .Impedance Spectroscopy: Theory, Experiment, and Applications[M].Wiley-Interscience, 2005.

[4] Zhang S S, Xu K, Jow T R. Electrochemical impedance study on the low temperature of Li-ion batteries［J］. Electro-chim Acta, 2004, 49 ( 7) : 1057-1061．

 7. FAQ About Battery EIS Testing