EIS can track SEI formation by measuring how a sodium-ion cell’s impedance spectrum changes over cycling, storage, or formulation changes. A frequency sweep—commonly from about 100 kHz to 0.01 Hz—separates the high-frequency ohmic resistance from interfacial charge-transfer and low-frequency diffusion behavior. Growth of an interfacial semicircle, especially when supported by equivalent-circuit fitting and galvanostatic data, indicates increasing resistance associated with SEI evolution and other electrode–electrolyte reactions.
Core takeaway: The high-frequency real-axis intercept estimates the cell’s bulk and contact resistance, while the interfacial semicircle tracks SEI-related and charge-transfer resistance. Because these features can overlap, SEI conclusions should be based on controlled measurements, model fitting, and comparison with cycling behavior—not on a single Nyquist feature alone.
What EIS Measures in a Sodium-Ion Cell
Applying a small AC perturbation
EIS applies a small sinusoidal voltage or current perturbation at multiple frequencies and measures the resulting amplitude and phase response.
Because the perturbation is small, the cell can be examined near a selected state of charge without imposing the large disturbances associated with conventional DC resistance tests.
Reading the Nyquist plot
A Nyquist plot commonly displays the real impedance, (Z'), on the horizontal axis and the negative imaginary impedance, (-Z''), on the vertical axis.
Different frequency regions correspond to processes with different characteristic time scales. The exact frequency boundaries depend on electrode chemistry, cell format, temperature, state of charge, and measurement configuration.
How to Identify Internal Resistance Changes
High-frequency real-axis intercept
The first intercept with the real axis provides an estimate of the cell’s ohmic or series resistance.
This value can include:
- Electrolyte ionic resistance
- Resistance of active materials and current collectors
- Electrode and current-collector contact resistance
- Tabs, welds, leads, and other fixture contributions
An increase in this intercept during aging indicates a rise in the cell’s overall series resistance, but it does not by itself prove that the SEI has thickened.
High-frequency inductive artifacts
At the highest frequencies, wiring, tabs, connectors, and metal current paths can produce an inductive loop or distortion.
These features should be separated from the electrochemical response where possible. Shorter leads, proper shielding, open- and short-circuit compensation, and consistent fixtures help reduce measurement artifacts.
Interfacial semicircles
A high-to-medium-frequency semicircle generally represents interfacial processes, including charge transfer and interfacial film effects.
If the SEI produces a resolvable time constant, it may appear as a separate higher-frequency semicircle or as part of a combined interfacial arc. In many practical sodium-ion cells, however, SEI resistance and charge-transfer resistance overlap, so the entire semicircle should not automatically be labeled “SEI resistance.”
Low-frequency diffusion response
At lower frequencies, the spectrum often develops a sloping or capacitive tail associated with ion transport and diffusion within the porous electrode and active particles.
This region is commonly represented using a Warburg-type element, although real porous electrodes may require more distributed or empirical models.
How EIS Reveals SEI Formation
Establishing a baseline
Measure the fresh cell after formation or an equivalent initial conditioning procedure.
Record the spectrum at a defined state of charge, temperature, rest period, AC amplitude, and frequency range. This baseline provides the reference for later changes.
Tracking resistance after cycling
Repeat EIS measurements at defined cycle intervals, such as after formation, periodically during cycling, and at end of life.
A typical aging pattern is:
- High-frequency intercept increases: bulk, contact, or series resistance has risen.
- Interfacial semicircle expands: charge-transfer and/or interfacial-film resistance has increased.
- Low-frequency tail changes: ion diffusion, electrode utilization, or porous-electrode transport has changed.
An increase in the interfacial resistance after repeated cycling is consistent with SEI formation, SEI thickening, electrolyte degradation, or loss of interfacial contact.
Using capacitance as supporting evidence
Equivalent-circuit fitting can estimate interfacial capacitance or a constant-phase-element response.
Changes in this parameter may indicate changes in interfacial area, film properties, surface heterogeneity, or cell capacitance. It is useful supporting evidence, but it is not a unique chemical measurement of SEI composition.
Comparing electrolyte and electrode formulations
Run the same EIS protocol for cells that differ only in electrolyte additive, electrode coating, pressing density, or assembly condition.
A formulation that produces slower growth of interfacial resistance over equivalent cycling conditions may provide better interfacial stability. The comparison is meaningful only when cell construction, test temperature, state of charge, and cycling protocol are controlled.
Building an Equivalent-Circuit Interpretation
A practical starting model
A simplified model may contain:
- A series ohmic resistance, (R_s)
- An interfacial branch containing (R_{SEI}) and (C_{SEI}), or a constant phase element
- Charge-transfer resistance, (R_{ct})
- Double-layer or interfacial capacitance
- A diffusion element, (Z_W)
A possible representation is an interfacial film branch ((R_{SEI} \parallel C_{SEI})) in series with charge-transfer and diffusion elements.
Interpreting fitted parameters
The fitted (R_s) estimates bulk and contact contributions, while (R_{ct}) reflects the difficulty of the electrode reaction under the tested conditions.
A fitted (R_{SEI}) can quantify the resistance of a distinguishable interfacial-film time constant. If the film and charge-transfer responses overlap, the fitted values become model-dependent and should be reported as combined or apparent interfacial resistance unless independently validated.
Validating the model
Do not select a circuit solely because it produces a visually good curve.
Use physically plausible elements, examine residuals, compare replicate cells, and verify that extracted parameters change consistently with cycling, temperature, state of charge, and known formulation differences.
Combining EIS With Galvanostatic Cycling
Linking impedance to practical performance
EIS explains why a cell’s voltage response changes, while galvanostatic cycling shows how those changes affect capacity, energy efficiency, rate capability, and voltage polarization.
For example, rising interfacial resistance accompanied by increasing polarization during charge and discharge provides stronger evidence of kinetic degradation than either observation alone.
Separating formation from degradation
Early-cycle impedance growth may reflect beneficial or necessary SEI formation and stabilization.
Continued growth later in life is more concerning because it can indicate ongoing electrolyte consumption, unstable SEI renewal, loss of active contact, or increasing transport limitations.
Using periodic diagnostic measurements
A practical workflow is to:
- Measure fresh-cell impedance.
- Apply a controlled formation protocol.
- Measure EIS again at the same state of charge and temperature.
- Cycle the cell under defined conditions.
- Repeat EIS at scheduled intervals.
- Correlate fitted resistance changes with capacity retention and voltage polarization.
Controlling the Measurement Conditions
State of charge and relaxation
Impedance depends strongly on sodium concentration and electrode potential.
Measure cells at consistent states of charge and allow a defined relaxation period before each spectrum. Otherwise, apparent resistance changes may reflect different electrochemical states rather than aging.
Temperature
Temperature affects electrolyte conductivity, charge-transfer kinetics, and diffusion.
All comparisons should use controlled and recorded temperatures. Storage-temperature studies are particularly sensitive because elevated temperature can accelerate interfacial reactions and increase resistance over calendar life.
Perturbation amplitude and linearity
Use a sufficiently small AC perturbation to keep the cell near its operating point.
A perturbation that is too large can produce nonlinear behavior and distort the interpretation of the fitted resistances.
Cell assembly and contact pressure
Contact resistance can obscure SEI-related changes, particularly in laboratory coin or pouch cells.
Consistent electrode coating, pressing density, current-collector contact, stack pressure, tab configuration, and fixture assembly are essential for distinguishing true interfacial evolution from assembly variability.
Understanding the Trade-offs
EIS is powerful but not chemically specific
EIS detects electrical consequences of interfacial change; it does not directly identify SEI composition or thickness.
An increase in a semicircle can result from SEI growth, charge-transfer kinetics, electrolyte degradation, active-material changes, contact loss, or several mechanisms at once.
Equivalent circuits are not unique
Different circuit configurations can fit the same spectrum adequately.
Reported parameters should therefore be treated as model-based estimates rather than direct measurements of isolated physical quantities unless the model is supported by complementary evidence.
Low-frequency measurements take time
Measurements near 0.01 Hz or below require long acquisition times.
During a long sweep, the cell may drift in state of charge, temperature, or equilibrium condition. Shorter frequency ranges can be used when justified, but the selected range must still capture the processes being studied.
Reproducibility limits conclusions
One cell can show an apparent trend that is actually caused by assembly variation or measurement noise.
Use replicate cells, identical test histories, and consistent fitting procedures. Compare changes in resistance with capacity retention, rate performance, and—when needed—post-mortem or complementary surface analysis.
How to Apply This to Your Project
Use EIS as a controlled, repeated diagnostic rather than as a one-time fingerprint.
- If your primary focus is SEI stability: Track the interfacial semicircle and fitted film or apparent interfacial resistance at fixed state of charge, temperature, and rest time, then compare the trend with cycling and capacity retention.
- If your primary focus is total internal resistance: Monitor the high-frequency real-axis intercept and separately control electrolyte, contact, tab, and fixture contributions.
- If your primary focus is electrolyte additives: Compare otherwise identical cells and evaluate the rate of interfacial-resistance growth across formation and extended cycling.
- If your primary focus is electrode or cell assembly quality: Use fresh-cell (R_s) and interfacial resistance comparisons to identify contact, coating, pressing, and current-collector inconsistencies before aging tests.
- If your primary focus is degradation modeling: Fit the spectrum across cycle life using a consistent equivalent circuit and combine the resistance parameters with galvanostatic polarization and capacity data.
With controlled testing and cautious interpretation, EIS can turn changes in sodium-ion cell impedance into actionable evidence about SEI evolution, internal resistance, and long-term interfacial stability.
Summary Table:
| Feature | What It Measures | Relevance to SEI/Resistance |
|---|---|---|
| High-frequency intercept | Series resistance (Rs) | Rising Rs indicates bulk/contact resistance increase, not SEI alone |
| Interfacial semicircle | Charge-transfer and/or SEI resistance | Growth suggests SEI formation or thickening, but overlaps possible |
| Low-frequency tail | Diffusion/Warburg impedance | Changes imply electrode transport limitations |
| Equivalent-circuit fitting | Model-based Rs, Rct, RSEI, etc. | Provides quantifiable parameters if model validated |
| Capacitance/CPE | Interfacial capacitance | Supports interfacial area or film property changes |
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