In-situ and operando electrochemical impedance spectroscopy (EIS) methods have emerged as powerful tools for real-time monitoring of batteries during charge and discharge cycles. These techniques provide dynamic insights into ion transport, interfacial reactions, and degradation mechanisms while the battery is under operational conditions. Unlike ex-situ EIS, which measures impedance in a static, non-operating state, in-situ and operando EIS capture transient behaviors that are critical for understanding performance and failure modes in practical applications.

The experimental setup for operando EIS involves integrating impedance measurement capabilities into a functioning battery cell. A typical configuration includes a potentiostat or frequency response analyzer connected to the working, counter, and reference electrodes of the cell. The cell must be designed to minimize artifacts such as inductive effects from cables or parasitic resistances from current collectors. Specialized cell designs, such as symmetric cells or three-electrode setups, are often employed to isolate the impedance contributions of individual components like the anode, cathode, or electrolyte.

One of the primary challenges in operando EIS is signal interference from the DC current during charge or discharge. The AC perturbation used for EIS must be sufficiently small to avoid distorting the battery’s state while large enough to produce a measurable response. Advanced signal processing techniques, such as frequency domain multiplexing or digital filtering, are used to separate the AC impedance response from the DC current. Additionally, the non-stationary nature of battery systems during cycling requires rapid data acquisition to capture fast-evolving processes like solid-electrolyte interphase (SEI) growth or lithium plating.

Operando EIS reveals dynamic processes that are inaccessible to ex-situ methods. For example, it can track the evolution of charge transfer resistance at the electrode-electrolyte interface, which varies with state of charge (SOC) and state of health (SOH). Diffusion-related impedance, observable in the low-frequency region of the EIS spectrum, provides information on ion transport limitations in the electrolyte and active materials. By analyzing these features over multiple cycles, researchers can identify degradation pathways such as electrode cracking, electrolyte depletion, or metallic dendrite formation.

In contrast, ex-situ EIS measures impedance in a controlled, equilibrium state, eliminating the complications of dynamic operation. While ex-situ data is easier to interpret and useful for baseline characterization, it fails to capture the transient behaviors that dominate real-world battery performance. For instance, ex-situ EIS cannot detect the impedance changes caused by phase transitions in electrode materials during cycling or the temporary formation of resistive intermediates in redox reactions.

Innovations in cell design have advanced operando EIS capabilities. For example, reference electrode integration allows impedance measurements of individual electrodes rather than the full cell, resolving ambiguities in data interpretation. Microfabricated cells with minimized ohmic losses enable high-frequency measurements, which are critical for studying fast kinetic processes. Additionally, pouch cells with optimized tab placement reduce inductive artifacts, improving signal fidelity.

Another key development is the use of multi-sine excitation signals, which accelerate data acquisition by measuring multiple frequencies simultaneously. This is particularly valuable for operando studies, where time resolution is essential to track rapid changes. Coupling EIS with other real-time diagnostics, such as temperature or pressure monitoring, further enriches the understanding of coupled electrochemical-thermal-mechanical behaviors.

Operando EIS has provided critical insights into battery degradation mechanisms. For example, it has been used to study the growth of the SEI layer on graphite anodes, revealing how its composition and resistivity evolve with cycling. In lithium-metal batteries, operando EIS has identified the onset of dendritic growth through changes in interfacial impedance. Similarly, in high-voltage cathodes, it has elucidated the role of surface films in increasing charge transfer resistance over time.

Despite its advantages, operando EIS faces limitations. The technique assumes linearity and stationarity, which may not hold under high currents or extreme temperatures. Additionally, interpreting EIS data requires robust equivalent circuit models, which can become overly complex for multi-phase electrode materials. Advances in distribution of relaxation times (DRT) analysis have improved the deconvolution of overlapping impedance processes, but challenges remain in assigning physical meaning to every circuit element.

In summary, in-situ and operando EIS offer unparalleled insights into the dynamic processes governing battery performance. By enabling real-time monitoring of impedance under operational conditions, these methods bridge the gap between fundamental research and practical applications. While challenges like signal interference and data interpretation persist, innovations in cell design and measurement techniques continue to expand the capabilities of operando EIS. Compared to ex-situ methods, operando EIS provides a more realistic and comprehensive view of battery behavior, making it indispensable for advancing next-generation energy storage technologies.