How can electrochemical feedback imaging and approach curve analysis be applied in battery research to evaluate Cathode-Electrolyte Interphase (CEI) behavior on high-voltage cathodes?

Electrochemical feedback imaging and approach curve analysis can reveal whether a CEI on a high-voltage cathode is electronically passivating, heterogeneous, or still electrochemically active. In SECM feedback mode, a redox mediator at the probe is regenerated by the cathode surface, and the resulting probe-current response indicates local interfacial electron-transfer behavior. Spatial feedback maps identify nonuniform CEI regions, while approach curves provide quantitative information about the effective electronic conductivity and passivation of the cycled surface.

A successful CEI should suppress unwanted electrolyte oxidation and transition-metal dissolution without becoming so resistive that it obstructs useful charge transfer. SECM distinguishes these outcomes by comparing local feedback signals and approach-curve responses before and after high-voltage cycling.

What SECM Measures at a High-Voltage Cathode

Feedback imaging as a local conductivity probe

In SECM feedback mode, the ultramicroelectrode is positioned above the cathode in an electrolyte containing a suitable reversible redox mediator. The mediator is oxidized or reduced at the probe and then interacts with the cathode surface.

If the cathode can efficiently regenerate the mediator, the probe current increases relative to the response over an insulating region. This is known as positive feedback and generally indicates accessible electronic conductivity or rapid heterogeneous electron transfer at the surface.

If the CEI blocks electron transfer, mediator regeneration is suppressed and the probe current decreases as the probe approaches the surface. This produces negative feedback, characteristic of an electronically insulating interface.

Why cathode behavior differs from typical anode SEI behavior

A conventional anode SEI is often strongly electronically insulating. Consequently, an uncoated conductive anode may show positive feedback, while formation of a well-passivating SEI causes the signal to move toward negative feedback.

High-voltage cathodes can behave differently. Some cycled cathodes retain positive feedback or even show increased apparent electronic activity after cycling, indicating that their CEI is not a fully blocking electronic film.

This does not automatically mean the CEI is beneficial or harmful. Partial electronic conductivity can reflect a thin, defective, compositionally heterogeneous, or electrochemically active CEI that allows continued electrolyte decomposition.

Applying Feedback Imaging During Battery Research

Establish the uncycled reference state

First, map the pristine cathode under controlled potential and electrolyte conditions. The baseline image establishes the surface’s initial electronic activity and reveals heterogeneity caused by active-material particles, conductive carbon, binder, pores, or pre-existing surface films.

The same region should ideally be examined before and after cycling. If relocating to the identical area is not practical, statistically comparable regions and multiple electrodes should be measured.

Cycle the cathode to the relevant upper cutoff voltage

The electrode should be tested under the intended high-voltage protocol, including the selected upper cutoff potential, charge rate, hold period, electrolyte composition, and number of cycles. These variables strongly influence CEI formation.

For lithium-ion systems, measurements may be designed around upper cutoffs near 4.3–4.5 V, while sodium-ion cathodes can experience severe interfacial oxidation at similarly high cathode potentials. The experiment should distinguish voltage effects from cycle-number effects.

Re-image the cathode after cycling

After cycling, repeat the SECM feedback map under the same mediator concentration, probe geometry, bias, temperature, and electrode potential. Changes in local probe current can then be interpreted as changes in CEI-mediated electron transfer.

Useful imaging outputs include:

• Average feedback current: Indicates the overall change in surface activity.
• Current distribution: Shows whether the CEI is spatially uniform.
• High-feedback regions: May indicate exposed conductive material, thin CEI, cracks, or electronically active decomposition products.
• Low-feedback regions: May indicate thicker or more insulating CEI coverage.
• Evolution with cycling: Shows whether passivation improves, remains stable, or progressively fails.

Interpret spatial heterogeneity carefully

A high-voltage cathode is rarely an ideal flat electrode. Particle curvature, crystallographic orientation, conductive-carbon contact, porosity, and local delamination can all affect the feedback signal.

Therefore, a local positive-feedback feature should not be assigned to CEI composition alone. Correlation with particle morphology, topography, optical imaging, or complementary surface analysis is necessary.

Using Approach Curves for Quantitative CEI Evaluation

What an approach curve reveals

In an SECM approach curve, the probe current is recorded while the probe moves toward the cathode. The current-distance relationship contains information about mediator diffusion, surface electron-transfer kinetics, probe-surface separation, and the effective blocking character of the CEI.

For an electronically active surface, the current can increase as the probe approaches because the mediator is regenerated more efficiently. For an insulating CEI, the current typically decreases or deviates toward negative-feedback behavior.

Approach curves are more quantitative than a single feedback image because they track the response over a controlled range of distances.

Compare pristine and cycled electrodes

Approach curves should be collected from pristine cathodes, cathodes cycled to moderate voltage, and cathodes exposed to the full high-voltage protocol. Comparing these curves can show whether cycling causes:

• A transition toward negative feedback, consistent with increasing electronic blocking.
• Persistent positive feedback, consistent with incomplete electronic passivation.
• Greater curve-to-curve variation, indicating CEI heterogeneity.
• A progressive loss of feedback response, which may indicate thickening, delamination, or severe surface blockage.

The same comparisons can be made across electrolyte additives, surface coatings, cathode compositions, and charging protocols.

Use model fitting with appropriate controls

Approach curves can be compared with theoretical feedback models to estimate effective surface behavior. Depending on the experimental configuration, the analysis may include the mediator diffusion coefficient, probe radius, normalized probe-surface distance, and an apparent heterogeneous electron-transfer parameter.

These parameters should be treated as effective interfacial properties, not as a direct measurement of CEI thickness or a single intrinsic film conductivity. Roughness, porosity, mixed conductivity, and uncertainty in probe distance can distort the fitted values.

Distinguish electronic insulation from chemical inactivity

A negative-feedback response indicates restricted mediator regeneration, but it does not prove that the CEI is chemically stable. A film can be electronically insulating while still undergoing electrolyte-driven chemical growth or allowing ionic transport.

Conversely, positive feedback may indicate exposed cathode, conductive CEI components, surface defects, or mediator reactions unrelated to the desired CEI pathway. Approach curves should therefore be interpreted alongside cycling data and chemical characterization.

Connecting SECM Results to CEI Function

Evaluating electrolyte oxidation suppression

High-voltage charging can drive electrolyte oxidation at the cathode. A CEI that becomes more electronically blocking should reduce the ability of the cathode to participate in further mediator regeneration and, by analogy, may help suppress continued interfacial electron-transfer reactions.

However, SECM does not directly measure electrolyte oxidation rate unless the experiment is specifically configured around an oxidation or reduction product. It primarily reports local electron-transfer accessibility under the chosen mediator conditions.

Assessing the risk of continued CEI growth

Persistent positive feedback after high-voltage cycling suggests that the surface remains at least partly electronically accessible. This can be a warning sign for ongoing electrolyte decomposition, particularly if the positive-feedback regions expand with cycling or coincide with capacity fade.

The result should be considered a screening indicator rather than definitive proof of continuous CEI growth. Direct evidence can be obtained by combining SECM with surface-sensitive methods such as XPS, HAXPES, time-of-flight secondary-ion mass spectrometry, or cryo-TEM.

Investigating transition-metal dissolution

A stable CEI should help limit transition-metal dissolution from high-voltage cathodes. SECM can identify surface regions where the CEI is electronically defective or unusually active, which may help locate sites vulnerable to degradation.

SECM alone does not quantify dissolved nickel, manganese, cobalt, or other metals. Dissolution measurements should be performed using complementary techniques such as electrolyte analysis, ICP-OES, ICP-MS, or post-cycling surface and cross-sectional microscopy.

Comparing protective strategies

The method is particularly useful for comparing:

• Electrolyte additives.
• Surface coatings such as ceramic or carbon layers.
• Different cathode compositions.
• Upper cutoff voltages.
• Charge rates and voltage-hold protocols.
• Electrode processing conditions.

The strongest candidate is not necessarily the sample with the most negative feedback. The target is a CEI that produces stable, spatially consistent interfacial behavior while preserving acceptable battery impedance and capacity retention.

Designing a Reliable SECM Experiment

Select a mediator compatible with the battery electrolyte

The mediator must be electrochemically reversible at the probe and sufficiently stable in the nonaqueous electrolyte. It should not chemically attack the CEI, extract transition metals, or react irreversibly with electrolyte decomposition products.

Mediator concentration and probe potential should be selected to avoid altering the cathode surface during measurement. A control experiment without the mediator can help determine whether the probe or electrolyte introduces parasitic currents.

Control electrode potential and state of charge

The cathode’s electronic structure and surface reactivity vary with state of charge. Measurements performed at different cathode potentials may therefore produce different feedback signals even when the CEI is unchanged.

For meaningful comparisons, record the cathode potential, state of charge, rest time, temperature, and atmosphere. Measurements should also be conducted under conditions that minimize post-cycling relaxation or air exposure.

Account for probe distance and topography

Approach curves are highly sensitive to the probe-surface separation. On rough porous electrodes, the apparent distance may not correspond to the true local separation, and the probe can sample both active material and surrounding electrolyte-filled pores.

Topographic feedback, optical tracking, or independent surface profiling can improve distance control. The probe radius and geometry should be calibrated before quantitative comparison.

Use replicate locations and replicate electrodes

A single feedback image can be misleading because cathode surfaces are heterogeneous. Measurements should cover multiple particles and regions, with replicate electrodes from independent cells whenever possible.

Statistical descriptors such as mean feedback current, variance, fraction of high-feedback area, and distribution of approach-curve parameters are more informative than a representative image alone.

Understanding the Trade-offs

Negative feedback is not automatically a successful CEI

Strong negative feedback indicates electronic blocking, but an excessively resistive CEI can increase interfacial impedance and hinder lithium-ion or sodium-ion transfer. It may also mask poor electrochemical performance behind an apparently passivating surface.

The correct interpretation is therefore controlled passivation, not maximum insulation.

Positive feedback is not automatically evidence of failure

Positive feedback can arise from a thin and still-functional surface film, exposed conductive regions, mediator regeneration at defects, or changes in surface catalytic activity. It becomes more concerning when it is spatially heterogeneous, increases with cycling, or correlates with electrolyte consumption and capacity loss.

SECM cannot uniquely identify CEI chemistry

Feedback imaging reports electrochemical behavior, not chemical composition. It cannot by itself distinguish carbonate-rich, fluoride-rich, phosphate-derived, fluorophosphate, or inorganic oxide components of the CEI.

Chemical identification requires complementary analysis. For example, high-voltage electrolyte degradation may produce different phosphate or fluorophosphate species depending on the operating voltage and salt chemistry, but SECM would register their combined effect on local electron transfer rather than identify them individually.

Avoid overinterpreting a single voltage or cycle count

CEI behavior depends on formation history. A cathode tested only after one high-voltage hold may show a different response from one cycled repeatedly under high-rate conditions.

A useful study maps the response as a function of upper cutoff voltage, cycle number, hold time, and charge rate rather than treating one post-cycling measurement as representative.

Making the Right Choice for Your Goal

Use SECM as a local electrochemical screening tool and combine it with battery performance and chemical characterization.

• If your primary focus is CEI passivation: Compare pristine and cycled approach curves, looking for stable movement toward electronically blocking behavior without a disproportionate impedance increase.
• If your primary focus is spatial uniformity: Use feedback imaging across many particles and quantify the distribution of high- and low-feedback regions rather than relying on the average current.
• If your primary focus is electrolyte or additive screening: Rank formulations by the stability of their feedback response over cycling, then verify capacity retention, Coulombic efficiency, impedance, and electrolyte decomposition products.
• If your primary focus is transition-metal dissolution: Use SECM to locate electronically active or defective regions, but quantify dissolution independently with electrolyte analysis and post-cycling surface characterization.
• If your primary focus is high-rate or high-voltage operation: Measure at controlled states of charge and correlate feedback changes with the applied voltage, charge rate, and duration of the high-voltage hold.

A well-designed SECM study identifies whether a high-voltage CEI is uniform, electronically passivating, and stable enough to protect the cathode without imposing excessive transport resistance.

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