Electrochemical impedance spectroscopy (EIS) is a critical tool for analyzing battery systems, providing insights into kinetic and transport processes. Two primary methodologies exist for conducting EIS measurements: in-situ (operando) and ex-situ approaches. These methods differ significantly in ecological validity, experimental complexity, and data interpretation challenges, particularly when applied to half-cell and full-cell configurations.

In-situ EIS involves measuring impedance while the battery is under operational conditions, such as during charge-discharge cycles or under load. This approach preserves the electrochemical environment of the cell, offering high ecological validity. The data reflects real-time processes, including interfacial reactions, charge transfer resistance, and diffusion limitations as they occur. However, in-situ measurements introduce experimental complexity due to the need for precise control over operating conditions. External factors such as temperature fluctuations, current interruptions, or voltage drift can distort impedance spectra. Additionally, the dynamic nature of battery operation means that impedance data may vary with state of charge (SOC), state of health (SOH), and cycling history, complicating interpretation.

In full-cell configurations, in-situ EIS captures interactions between electrodes and electrolyte under realistic conditions. For example, lithium-ion batteries exhibit evolving solid-electrolyte interphase (SEI) layers during cycling, which directly influence charge transfer resistance. Measuring these changes in-situ provides a more accurate representation of cell behavior but requires careful synchronization with cycling protocols to avoid artifacts. Half-cell studies, such as those using lithium metal as a counter electrode, benefit from in-situ EIS by revealing electrode-specific degradation mechanisms. However, the presence of a non-equilibrium reference electrode can introduce additional uncertainties in data analysis.

Ex-situ EIS, in contrast, involves measurements on cells or components removed from operational conditions. This method simplifies experimental setup by eliminating dynamic electrochemical variables. Researchers can isolate specific components, such as individual electrodes or separators, and study them under controlled environments. The reduced complexity allows for more precise impedance measurements, free from interference caused by ongoing electrochemical reactions. However, ex-situ measurements suffer from lower ecological validity because they do not account for interactions present during actual battery operation.

In full-cell ex-situ studies, disassembling the cell risks altering electrode-electrolyte interfaces, leading to misleading impedance data. For instance, SEI layers may degrade upon exposure to air, artificially inflating charge transfer resistance values. Half-cell ex-situ measurements, while more controlled, still face challenges in extrapolating results to practical battery systems. A common application is studying aged electrodes harvested from cycled cells to identify degradation modes. While this provides valuable post-mortem insights, it cannot capture transient processes that occur during cycling.

Data interpretation differs substantially between the two methods. In-situ EIS spectra often contain overlapping time-dependent processes, necessitating advanced equivalent circuit models or distribution of relaxation times (DRT) analysis to deconvolute contributions. The dynamic nature of battery systems means that traditional static models may not fully capture impedance behavior. Ex-situ EIS, being more stable, allows for cleaner spectra with fewer overlapping processes. However, the absence of operational conditions means that certain phenomena, such as lithium plating or electrolyte depletion, may not be observable.

Examples from research highlight these tradeoffs. In lithium-sulfur batteries, in-situ EIS has been used to track polysulfide shuttling and precipitation during cycling, revealing real-time resistance changes linked to capacity fade. Ex-situ studies of sulfur cathodes, while useful for characterizing material properties, miss the dynamic electrolyte interactions critical to cell performance. Similarly, in sodium-ion batteries, in-situ EIS helps identify sodium metal deposition in half-cells, whereas ex-situ analysis of cycled electrodes provides only indirect evidence of such phenomena.

Experimental constraints also influence method selection. In-situ EIS requires specialized equipment capable of synchronized impedance and cycling measurements, increasing cost and complexity. Ex-situ setups are more accessible but may require glovebox environments to prevent air exposure of sensitive materials. The choice between methods ultimately depends on research objectives: in-situ for studying operational mechanisms, ex-situ for controlled material characterization.

A comparison of key aspects can be summarized as follows:

| Aspect | In-situ EIS | Ex-situ EIS |
|----------------------|--------------------------------------|--------------------------------------|
| Ecological validity | High (real-time conditions) | Low (static environment) |
| Experimental complexity | High (synchronized measurements) | Low (isolated components) |
| Data interpretation | Complex (dynamic processes) | Simpler (controlled conditions) |
| Full-cell relevance | Directly applicable | Limited by disassembly effects |
| Half-cell utility | Reveals transient behavior | Focuses on intrinsic properties |

Both approaches have complementary strengths. In-situ EIS is indispensable for understanding batteries under working conditions, despite its analytical challenges. Ex-situ EIS provides foundational insights into material properties but may lack direct applicability to real-world systems. Researchers must carefully balance these factors when designing experiments, selecting the method that best aligns with their investigative goals.

The evolution of battery technologies continues to drive methodological refinements in EIS. Advanced in-situ techniques now incorporate multi-frequency perturbations to minimize measurement artifacts, while ex-situ methods benefit from improved environmental controls. Future developments may bridge the gap between these approaches, enabling more comprehensive impedance analysis across scales and conditions. Until then, the judicious use of both in-situ and ex-situ EIS remains essential for advancing battery research.