Dynamic EIS Reveals Real-Time Battery Impedance Measurement During Charge and Discharge

1. Background: Why Static EIS Falls Short

Traditional electrochemical impedance spectroscopy (EIS) is a powerful tool for deciphering battery mechanisms. However, its requirement for steady-state measurement conditions creates a fundamental disconnect from the dynamic conditions of real battery operation. This gap between “offline” characterization and “in-operation” reality prevents researchers from capturing the transient evolution of electrochemical reactions during actual charge and discharge processes.

Static EIS refers to impedance measurement performed after charging or discharging the battery to a specific state of charge (SOC) and allowing it to rest until electrochemical equilibrium is reached. While static EIS yields highly reproducible spectra under well-defined thermodynamic conditions, it measures the state after the process has stopped — not the dynamic behavior during the process. Transient failures, micro-crack formation, and short-lived impedance anomalies that occur under load are invisible to static EIS.

In contrast, Dynamic EIS (DEIS) introduces the AC perturbation concurrently with the ongoing charge or discharge process, enabling continuous impedance monitoring under realistic operating conditions. Dynamic EIS testing advances EIS from a post-hoc characterization step to an in-situ diagnostic tool.

Figure 1. Schematic comparison of static EIS vs. dynamic EIS measurement principles

2. Principle

Dynamic EIS captures battery impedance by superimposing a small-amplitude AC perturbation onto the ongoing charge or discharge current, then measuring the amplitude ratio and phase shift between the response signal and the perturbation signal across a defined frequency range. Each frequency point probes a different internal physical or electrochemical process — from ohmic resistance at high frequencies to diffusion-limited responses at low frequencies — yielding a complete impedance spectrum as a “snapshot” of the cell’s internal state at that moment during cycling.

Figure 2. Dynamic EIS Signal During Battery Charging

2.1 Static EIS: Principles and Limitations

Static EIS is performed by first charging or discharging the battery to a specific SOC, allowing it to rest until equilibrium is reached, and then acquiring an impedance spectrum. Key characteristics:

• Advantage: Yields highly reproducible spectra under well-defined, stable thermodynamic conditions, making static EIS suitable for precise mechanistic analysis and inter-laboratory comparison.

• Fundamental limitation: Measures the cell state after the process has stopped, not the dynamic behavior during operation. Any transient phenomena — such as SEI micro-cracking under high-rate discharge — may self-heal during the rest period, making them invisible to the subsequent static EIS measurement.

2.2 Dynamic EIS (DEIS): Continuous In-Situ Impedance Monitoring

Dynamic EIS introduces the AC perturbation concurrently with the ongoing charge or discharge process. This approach directly captures a continuous evolution of impedance spectra under realistic operating conditions — a sequence of time-resolved snapshots rather than a single static measurement. Key operational parameters:

• Trigger modes: The AC signal can be triggered at specific voltage thresholds, time intervals, or applied continuously throughout the entire charge/discharge cycle.

• Recommended frequency range: 100 kHz to 0.1 Hz (approximately 1 minute per full spectrum), providing a balance between measurement speed and frequency coverage.

• Extended range (0.01 Hz): Extends acquisition time to approximately 15 minutes per spectrum — during which the cell’s SOC and potential will have shifted substantially, making the spectrum representative of a different electrochemical state.

As cycling progresses, the charge‑transfer resistance (\( R_{\text{CT}} \)) gradually increases due to aging. DEIS captures this evolution in real time, revealing when and under what conditions the degradation begins. It also enables precise localization of the root cause—for instance, how impedance changes during high‑rate charge/discharge—a capability beyond the reach of static EIS.

3. Case Study 1: Tracking Impedance Evolution During Cycling Degradation

Dynamic EIS enables continuous, cycle-resolved monitoring of electrochemical degradation — capturing exactly when and how resistance components evolve during aging. DEIS was performed on a cell undergoing 50 cycles to monitor electrochemical impedance changes throughout the degradation process. Impedance spectra were extracted at cycles 20, 30, and 40, covering the 10–90% SOC range during both charge and discharge.

4. Case Study 2: Identifying Operational Failure Modes in Real Time

Dynamic EIS during active cycling exposes transient failure mechanisms that static EIS systematically misses — particularly SEI cracking and non-uniform formation that self-heal during conventional rest-based measurements. A batch of cells was tested at a 2C charge/discharge rate using DEIS to track impedance variations across the batch. Equivalent circuit model (ECM) fitting was applied to deconvolute the SEI resistance (R_SEI) from the total impedance. The analysis precisely identified that uneven SEI formation — caused by:

• Temperature gradients or pressure non-uniformity during the formation protocol

• Variations in electrolyte filling volume and wetting time between cells

— was responsible for the observed inter-cell impedance differences. This distinction is only possible under dynamic conditions.

A batch of cells was tested at a 2 C charge/discharge rate using DEIS to track impedance variations. Equivalent circuit model (ECM) fitting was applied to deconvolute the SEI resistance (\( R_{\text{SEI}} \)). The analysis precisely identified that uneven SEI formation—caused by temperature gradients or pressure non‑uniformity during formation, or by variations in electrolyte filling and wetting time—was responsible for the observed impedance differences.

During charge and discharge, the anode (graphite or silicon‑based) undergoes rapid, non‑uniform volume expansion and contraction. The rate of volume change increases with higher C‑rates, leading to larger local strain gradients. If the SEI layer lacks sufficient flexibility or adhesion, these dynamic strains can induce localized micro‑cracking or delamination.
With static EIS, the cell would first be discharged and rested. By the time the measurement begins, the anode volume would have recovered, and any micro‑cracks would have been promptly re‑wetted by electrolyte, potentially self‑healing. The impedance would then return to a “normal” value, masking the underlying issue. DEIS, by contrast, detects the anomaly during the dynamic event, revealing the transient failure that static methods miss.

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