Dynamic EIS Reveals Impedance Evolution During Charge and Discharge

1. Background

Traditional electrochemical impedance spectroscopy (EIS) is a powerful tool for deciphering battery mechanisms. However, its requirement for steady‑state measurement conditions creates a disconnect from the dynamic conditions of real battery operation. This gap between “offline” characterization and “in‑operation” reality prevents us from capturing the transient evolution of electrochemical reactions during actual charge and discharge processes. In this work, we employ dynamic EIS (DEIS) to capture impedance in real time during cycling, advancing EIS from an offline dissection technique to an online monitoring methodology.

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

2. Principle

The fundamental difference between static EIS and dynamic EIS lies in the state of the battery system when the AC perturbation is applied. Static EIS requires the system to be at equilibrium, whereas DEIS allows measurements while the system is undergoing a dynamic process—such as charging or discharging. The core principle and the distinction between the two approaches are illustrated below.

Figure 2. Dynamic EIS Signal During Battery Charging

When a voltage or current perturbation is applied to a battery, different internal physical or electrochemical processes respond at different rates, each contributing differently to the overall impedance. By measuring the amplitude ratio and phase shift between the response signal and the perturbation signal across a range of frequencies, the complex impedance at each frequency can be calculated.

2.1 Static EIS

Static EIS is performed by first charging or discharging the battery to a specific state‑of‑charge (SOC), allowing it to rest until equilibrium is reached, and then acquiring an impedance spectrum.
Advantage: It yields highly reproducible spectra under well‑defined, stable thermodynamic conditions, making it suitable for precise mechanistic analysis.

Fundamental limitation: It measures the state after the process has stopped, not the dynamic behavior during the process.

Figure 3. Timing diagram of synchronous chargedischarge and EIS acquisition for dynamic impedance monitoring

2.2 Dynamic EIS (DEIS)

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 “snapshots” rather than a single static measurement. The AC signal can be triggered at specific voltage or time thresholds, or can be applied throughout the entire charge/discharge cycle.

Figure 4. Dynamic EIS measurement sequence constant current charge step followed by an impedance scan from 100 kHz to 0.01 Hz

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

DEIS was performed on a cell undergoing 50 cycles to monitor electrochemical impedance changes throughout the process. The figure extracts EIS spectra recorded at 20, 30, and 40 cycles, covering the 10–90% SOC range during cycling.

Figure 5. Dynamic EIS parameter configuration and representative Nyquist plots across cycles 2–4 during charge and discharge

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.

4. Case Study 2: Pinpointing Operational Issues in Cells

Figure 6. Tabulated impedance evolution data across multiple chargedischarge cycles during Dynamic EIS monitoring

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.

Conventional cell testing often requires lengthy cycling protocols. Neither a handheld internal resistance meter nor traditional static EIS can probe a cell’s internal mechanisms under actual operating conditions. The IEST ERT7008 Electrochemical Performance Analyzer, equipped with DEIS capability, overcomes this limitation, enabling researchers to investigate internal mechanistic changes in real‑world usage scenarios.