I'm trying to analyze my EIS data about Li-ion battery. But I noticed there is a strange data in high frequency region(~100k - 10k) that the Z(im) decreases as measurement frequency decreases.
It seems that it could be related to an inductance, but I learned inductance term is represented at the negative imaginary region. Since their values are in the positive region, I don't know what that is. Please help me understand.
This could be the inductive effect of the magnetic coupling in the cabling. Magnetic field in the power leads can induce a voltage in the voltage sensing leads. This translates into a fake positive or negative inductance. My recommendation is to reduce as much as possible the length of the cables, place the power cables side by side and twist the sensing leads. For more details see the “Low impedance samples” in the following document: https://www.bio-logic.net/wp-content/uploads/20101105-application-note-5.pdf
It could be that you're measuring your potentiostat's internal circuitry. This happens when the potentiostat's impedance is close to the cell impedance.
@Tuomas Vainikka Hi, Thank you for answering. Could you explain the detail about your opinion? What causes this to happen in terms of the potentiostat's impedance?
Dear Donghoon Lee,
1) what is your Li-ion battery and your EIS setup ?
2) Could you show/send, if possible, a photo of your battery, with the terminal-wires on this battery, e.g. during such an EIS-measurement ?
@ Ioannis Samaras,
1) The chemical of Li-ion battery is NCM622/PE Separator/Graphite with 1M LiPF6 dissolved in EC/DEC electrolyte. It is a quite large pouch cell, capacity of which is about 60 Ah.
EIS setup was as follows:
I did galvanostatic EIS, which applies current sine wave and measures voltage for open-circuit condition.
Current amplitude was 1.5 A
2) Although I cannot share the picture because of a confidential issue, but terminal-wires connected to the battery were crocodile-type connector.
My EIS equipment has 5 wires; Working, Working sense, Counter, Reference and Earth. Except earth cable, 4 wires were used for the experiment.
Working and Working sense cable were connected to a current collector of the cathode and Counter and Reference cables were connected to a current collector of the anode.
Each wires were directly connected to the correspond current collector, I mean, there was no direct connection between wires.
I think your current amplitude is too high. If the resistance is of the order of mΩ, the potential fluctuates kV's.
Dear Donghoon Lee ,
try to apply a much lower current amplitude, in the range : 10mA[1], 100mA[2]
Also, this (Li-ion battery, GEIS-) measurement is about a:
1) commercial(?) or a home-made cell ?
or about an
2) old (or/and highly cycled[3]) cell ?
1. For some new EIS-systems having a high measuring (voltage) resolution;(16-bit or higher). as an example: AC amplitude 0 - 6 V with 18 bit resolution http://zahner.de/products/electrochemistry/zennium-x.html
2. Recommended for old EIS-systems having a low measuring (voltage) resolution (12-bit or lower). The same (100mA) is good, also, for very old (mainly-)analog EIS-systems.
3. Very old cells with nanosize (NCM*) cathode materials start to present such a (or similar, upon cycling and/or aging!) behavior, but in a higher frequency region (~100kHz or higher).
This could be the inductive effect of the magnetic coupling in the cabling. Magnetic field in the power leads can induce a voltage in the voltage sensing leads. This translates into a fake positive or negative inductance. My recommendation is to reduce as much as possible the length of the cables, place the power cables side by side and twist the sensing leads. For more details see the “Low impedance samples” in the following document: https://www.bio-logic.net/wp-content/uploads/20101105-application-note-5.pdf
Dear all,
Thank you very much for answering.
1) It is a commercial cell for EV.
2) I'll apply a much lower current for the next experiment. Since my EIS equipment is 16-bit system, I would try 10 mA first.
Just wondering, why should I use a much lower current? The total resistance of the cell is at mΩ scale and applying current is at Ampere scale, voltage fluctuation would be at mV scale which is in detectable range.
Is this due to suppress the change of the cell during EIS?
Dear Donghoon Lee ,
did you notice, again, those "strange" data (in the high frequency region), using, now, a low(er) current ?
Dear Donghoon Lee,
1) About "... why should I use a much lower current ?"
Yes, it is, mainly, to follow through the hardware specs[1] of your EIS-system.
So, the strange data in the high frequency region (~100k - 10k) should be due to the "Illegal"[2] dispatch(/"interference" in Zphase) of the EIS-system's internal impedance[3].
2) About "... internal changes of (inside) the cell ?"
No, practically, no, major, changes[4] : Donghoon Lee's - Question [Is this due to suppress the change of the cell during EIS?]
1. The (maximum) values of current are much lower at high(er) frequency range(s) in (hardware specs) of most commercial EIS-systems.
2. The high frequency regimes (~1MHz - 100kHz, also yours ~100kHz - 10kHz) are, usually, calibrated with smaller loads, i.e. compared to lower frequency region(s): ~1mHz - 1kHz.
3. See the hardware specs of the your own EIS-system at the "high" frequency regime (~100kHz - 10kHz). What is your EIS-system (EIS-model, or hardware specs) ?
4. At least, for "short" time (commonly ~< 1min, and t(Max) < < 1 day) EIS-measurements, at the high frequency regime (~100kHz - 10kHz).
I have a similar behavior, my GEIS current amplitude is 1mV, it has to do with the inductive effect of wires or cables around cell. Since my cell impendence is so low (~Rct 30ohms). My cell set up is 1080 Steel coupon in 5%Nacl water.
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