Ex situ FTIR and Raman analysis can reveal interphase composition, but only after the battery has been stopped, opened, and transferred for measurement. That process can alter air-sensitive species, erase transient electrochemical states, and create uncertainty about the electrode’s true state of charge. In situ spectroelectrochemical cells address these limitations by collecting spectra from the electrode during controlled electrochemical operation, without removing it from its environment.
Core takeaway: Ex situ analysis provides valuable chemical and structural snapshots, but those snapshots may not represent the interphase under operating conditions. Properly designed in situ cells preserve the electrochemical state and enable real-time observation of SEI, CEI, electrolyte, and electrode evolution.
Why Ex Situ Spectroscopy Can Misrepresent Battery Interphases
Atmospheric contamination changes the sample
Disassembling a cell exposes the electrode and its interphase to moisture, oxygen, and carbon dioxide. These species can react with highly sensitive components, particularly lithium-metal electrodes and materials used in high-capacity conversion or alloying electrodes.
Washing or transferring the electrode can introduce additional surface chemistry changes. The measured FTIR or Raman spectrum may therefore contain products formed during handling rather than only those generated during battery operation.
Removing the electrochemical bias changes metastable phases
Interphase chemistry is often potential-dependent. Once cycling stops and the electrode is removed from the cell, the electrochemical bias disappears.
Metastable phases can then relax, transform, or disappear, making the ex situ spectrum different from the state that existed during charge or discharge. This is especially important when studying transient SEI or CEI formation mechanisms.
The measured state of charge may not be the intended state
An electrode recovered at a nominal SOC or DOD may not retain that exact condition during disassembly and transfer. Self-discharge, delithiation, and spatially nonuniform reaction states can shift the local or average composition.
Consequently, two samples labeled with the same SOC may produce different spectra because they experienced different relaxation or storage histories before measurement.
Sample-to-sample variation limits reproducibility
Ex situ studies generally require multiple cells or electrodes when researchers need measurements at several cycling stages or across multiple techniques. This introduces variation in electrode morphology, interphase coverage, cycling history, and fabrication quality.
The resulting differences can be mistaken for electrochemical trends. In practice, the experiment may compare both different SOC conditions and different physical samples.
Offline analysis loses temporal information
Ex situ measurements provide a sequence of snapshots rather than a continuous record. They cannot directly show when a specific band, vibrational feature, phase, or surface species first appears during a charge or discharge process.
This limits the ability to distinguish formation, transformation, and disappearance of interphase components and transient electrode phases.
How In Situ Spectroelectrochemical Cells Address These Problems
They preserve the operating chemical environment
An in situ cell keeps the electrode, electrolyte, and counter-electrode assembled while the spectrum is collected. A properly sealed design reduces exposure to atmospheric moisture, oxygen, and carbon dioxide.
This preserves air-sensitive interphase chemistry and avoids the surface reactions that can occur during electrode recovery, washing, or transfer.
They maintain electrochemical control
The electrode remains under a controlled potential or active cycling condition during measurement. FTIR or Raman spectra can therefore be correlated directly with applied potential, current, charge, and discharge behavior.
This allows researchers to observe interphase formation and transformation as they occur, rather than inferring them from post-cycling samples.
They capture transient and metastable species
Because measurement occurs without removing the electrochemical bias, in situ methods can detect short-lived or metastable states that might relax during ex situ preparation.
This is particularly valuable for tracking the dynamic development of the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI).
They improve state-of-charge correlation
Spectra are acquired from the same electrode while its electrochemical state is controlled and recorded. This creates a more direct relationship between spectral changes and the actual SOC or DOD.
The approach reduces ambiguity caused by self-discharge or uncontrolled changes between cell shutdown and external analysis, although local reaction inhomogeneity still needs to be considered.
They reduce destructive sampling
A single dedicated cell can be monitored through multiple electrochemical stages without harvesting a new electrode for every condition. This reduces the sample-to-sample variability associated with using separate cells.
It also supports correlation of chemical, structural, and electrochemical changes over the same cycling history.
What In Situ Measurements Can Reveal
SEI and CEI formation
In situ FTIR and Raman can follow changes in vibrational features associated with electrolyte decomposition and interphase development. The main advantage is not simply identifying the final products, but determining when and under what electrochemical conditions they form.
This helps separate initial interphase formation from later growth, restructuring, or degradation.
Electrolyte solvation behavior
Spectral changes can be used to monitor electrolyte solvation and coordination behavior during operation. These changes may help connect electrolyte structure with interphase formation and electrode reactions.
Because the electrolyte remains in the cell, the analysis can observe its response under the same potential and concentration conditions affecting the electrode.
Lattice and phase changes
In situ Raman and related optical measurements can track changes in electrode structure and lattice state during cycling. They can also help identify phase transitions that may relax after the cell is stopped.
The same approach can reveal mechanical or structural evolution, including substantial volume changes in high-capacity materials such as silicon or tin anodes.
Cell Design Must Match the Spectroscopic Geometry
ATR-FTIR requires chemically compatible optical interfaces
Germanium ATR crystals provide a high refractive index and good signal-to-noise performance. However, germanium can react with alkaline-metal counter-electrodes and form alloys.
For reactive metal anodes, the cell may therefore require an alternative window, a protective configuration, or a different electrode arrangement. Optical performance alone is not sufficient; the crystal must also remain electrochemically and chemically compatible.
Transmission FTIR requires a short optical path
Bulk electrolyte and cell components can absorb infrared radiation strongly. Excessive electrolyte thickness can overwhelm the interfacial signal and reduce spectral resolution.
Transmission cells should minimize the liquid path through the beam, for example by positioning the anode away from the infrared beam line and using suitable IR-transparent windows such as potassium bromide (KBr).
Optical windows must withstand electrochemical hazards
Lithium or sodium dendrites can puncture soft optical windows such as KBr and cause an internal short circuit. This creates both a measurement failure and a cell-safety problem.
Possible design responses include increasing internal clearance or replacing a pure-metal anode with an intercalation anode such as graphite when compatible with the research objective.
Hermetic sealing and stable assembly are essential
The cell must maintain a controlled atmosphere while also providing reliable electrical contact, stable electrode positioning, and an unobstructed optical path. Small assembly defects can cause leakage, drift, poor reproducibility, or loss of electrochemical performance.
Reliable cell-assembly equipment and specialized optical fixtures are therefore part of the measurement method, not merely auxiliary laboratory hardware.
Understanding the Trade-offs
In situ cells are more complex than ex situ workflows
Ex situ analysis is comparatively flexible: a conventional cycled electrode can be transferred to many established FTIR or Raman instruments. In situ experiments require a purpose-built cell whose materials, geometry, sealing, and electrical configuration are compatible with the optical technique.
That added complexity increases setup and validation requirements.
Optical design can limit the electrochemical design
The best optical configuration may not be the most representative battery configuration. A window, shortened electrolyte path, modified electrode spacing, or alternative counter-electrode can change the cell geometry and potentially influence transport or dendrite behavior.
Researchers must distinguish effects caused by the material system from effects introduced by the spectroelectrochemical cell.
Signal quality and cell integrity can conflict
Increasing optical access may require thinner windows or reduced spacing, while robust electrochemical operation may favor greater mechanical clearance and stronger protective structures. Transmission FTIR also faces interference from electrolyte absorption.
The cell should therefore be optimized for the specific measurement mode rather than treated as a universal platform.
In situ does not eliminate interpretation challenges
A real-time spectrum is still an indirect measurement of chemical and structural change. Overlapping bands, limited penetration depth, background contributions, and changes in optical alignment can complicate assignment.
In situ results are strongest when spectral data are correlated with electrochemical measurements and, where appropriate, validated against carefully controlled complementary analyses.
Making the Right Choice for Your Goal
Use the method according to whether the priority is identifying final products, understanding dynamic mechanisms, or preserving sensitive operating states.
- If your primary focus is final interphase composition: Use ex situ FTIR or Raman as a practical chemical snapshot, but control atmosphere, transfer, washing, and SOC history rigorously.
- If your primary focus is interphase formation mechanisms: Use an in situ spectroelectrochemical cell to correlate spectral evolution with potential, current, and cycling time.
- If your primary focus is air-sensitive chemistry: Prioritize hermetic sealing and minimized handling, because atmospheric exposure can change the very species being measured.
- If your primary focus is transient phases or structural evolution: Prefer in situ monitoring so metastable states and lattice changes are observed before electrochemical relaxation.
- If your primary focus is high-quality FTIR data: Match the cell to the optical mode—use chemically compatible ATR components or minimize electrolyte thickness and optical interference in transmission mode.
- If your primary focus is reliable long-term cycling: Validate electrode spacing, window strength, dendrite clearance, electrical stability, and reproducibility before interpreting spectra.
The most reliable interphase conclusions come from preserving the battery’s operating state while deliberately matching cell design to the spectroscopy and electrochemistry.
Summary Table:
| Limitation of Ex Situ | How In Situ Overcomes It |
|---|---|
| Atmospheric contamination | Hermetic sealing preserves air-sensitive species |
| Loss of electrochemical bias | Maintains controlled potential during measurement |
| Metastable phase relaxation | Captures transient states in real time |
| SOC mismatch | Direct correlation with actual SOC |
| Sample variability | Same cell used for multiple stages |
| No temporal info | Continuous monitoring of formation processes |
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