The EIS measurements often require the conditions of stable state for batteries. So the perturbation signals for EIS measurement are low, such as 5-10 mV for the perturbation voltage. However, the EIS measurement time is usually long, such as 10 min or even 30 min. In this case, the battery may not locate at stable states. For example, the battery temperature or SOC is changed. Thereby, the measured EIS under the unstable state is not the true EIS at the setting point. Exactly, many factors would influence the EIS measurement. Which one is dominant? Which one is inherent?
Dear Haijun Ruan
These papers might be useful, have a look:
1. https://www.sciencedirect.com/science/article/abs/pii/S0378775320306650
2. https://www.palmsens.com/knowledgebase-article/electrochemical-impedance-spectroscopy-eis/
3. https://www.gamry.com/application-notes/EIS/basics-of-electrochemical-impedance-spectroscopy/
Kind Regards
Qamar Ul Islam
Hi Qamar Ul Islam
Thanks for your suggested materials. They are useful to understand the base theory of EIS. But it remains unclear whether the measured EIS is the true impedance of batteries and which factors influence the EIS measurement.
Best wishes
Haijun
Dear Haijun Ruan,
what you refer to as being "stability" is actually more accurately named, in systems theory, "time invariance" or "stationarity".
The use of a small perturbation has to do with linearity, that is to say the perturbation to the system must be small enough such that the system behaves linearly.
Anyway, when using very low frequencies, it is certainly possible that the system is in a different state and has varied with time. There are a few tools that can be used to check the stationarity of the system, the linearity of the behaviour and correct all these effects.
Please see the following references and papers:
https://www.pub.iapchem.org/ojs/index.php/JESE/article/view/725/713
https://www.biologic.net/documents/eis-quality-indicators-thd-nsd-nsr-electrochemistry-battery-corrosion-application-note-64/
https://www.biologic.net/documents/eis-quality-indicators-thd-electrochemistry-battery-corrosion-application-note-65/
https://www.biologic.net/documents/how-to-check-and-correct-non-stationary-eis-measurements-using-ec-lab-battery-cell-an69-2/
https://www.biologic.net/topics/how-to-check-and-correct-the-time-variance-of-your-system-under-eis-measurements/
Let me know if it helps and if you have more questions,
Best regards,
Nicolas
For a detailed knowledge about factors influencing the EIS response in different frequency regime, I will refer to an comprehensive article by Kant et al. (J. Power Sources, 473 (2020) 228467.)
Dear Nicolas Murer
Thanks for your explanation and suggested materials. They are quite useful. Nicolas Murer
Dear Haijun Ruan
The impedance of a battery depends on its state-of-charge (SOC) and state-of-health (SOH), also the temperature will also cause an effect. Even the SOC, SOH, and temperature remain the same, the measurement uncertainty and error of measuring instruments will also result in a non-negligible effect in some applications. The influence of the first 3 factors (SOC, SOH, Temperature) will be dominant in cases that the battery impedance change (due to these factors) is not considered during the measurement. However, if we take into account the impedance change and rely on an advanced impedance measurement setup for monitoring the state-related impedance continuously and dynamically, this issue can be solved. In addition, the measurement uncertainty and error can be minimized by choosing suitable measuring instruments.
Dear Zhenyu
Thanks for your answer. I agree with you. The EIS is often different under different SOC, temperatures and SOH. However, the AC is used so the SOC is kept unchanged. The EIS testing current is low so both the temperature and SOH are almost kept the same, respectively. The EIS measurement setup and instruments are important and should be carefully considered.
One key point we should take care of is the OCV change. Although the average SOC is kept the same under AC, I find the SOC is changed during AC excitation and the SOC lags by pi/2 than the AC. As the SOC changes with the sine function, the OCV would change like the sine function. Thereby the OCV variation lags by pi/2 than the AC. If the OCV variation amplitude is high, the OCV will significantly influence the EIS and the measured EIS would not be true EIS.
What is all about this[1] 'very interesting' case study, dear Haijun Ruan ?
1. Partial quotation from last answer of Haijun Ruan : "... I find the SOC is changed during AC excitation and the SOC lags by pi/2 than the AC. As the SOC changes with the sine function, the OCV would change like the sine function. Thereby the OCV variation lags by pi/2 than the AC. ... "
Hi Ioannis,
Let me explain it. The AC current can be assumed as I*sin(wt) and so the capacity change should be I/w*(1-cos(wt)). So the SOC lags by pi/2 than the AC. More details can be found in the material Article A modified-electrochemical impedance spectroscopy-based mult...
Dear Haijun Ruan,
It was not possible to find this equation: "... the capacity change should be I/w*(1-cos(wt)).", in your existing paper[1]. Maybe, It will be shown in your next, paper, plans.
1. Article A modified-electrochemical impedance spectroscopy-based mult...
Thanks Ioannis
I am sorry. Perhaps it is not so clear. In Eq. (3), qe=I/w*[cosΦ-cos(wt-Φ)] shows the relationship between the AC and capacity change. Yeah, I plan to formulate another paper to clearly explain it. Thanks
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