I have an EIS spectrum as illustrated in the images and I am trying to fit it by Randles equivalent circuit. My fit is not perfect but the question is which value should I use to calculate the conductivity of my solid electrolyte in the image? Usually, I was extrapolating the linear part of the curve down to X axis and take the value as resistance to calculate ionic conductivity according to the formula:
1/q=(1/R)*(l/S)
I recently read in this website that diameter of the semicircle can also be used to determine the resistance to calculate ionic conductivity. However, in my case both values are significantly different from each other. Therefore, I am looking for a more solid approach. How can I calculate ionic conductivity of my sample according to output of the Zfit? The value in the red marking is Warburg factor. Should I somehow convert it to resistance?
My sample is a lithium containing amorphous silicate thin film.
Dear sir,
Regarding the ionic conductivity, I would give you the same answer as for your previous question.
I have one question here... I see in your spectrum that the ohmic resistance is close to zero. Did you make a correction/shift of your spectra? If not, this very low value is a good sign for the health of your battery.
Dear Philippe,
I did not make any corrections to my spectrum. Well, I am quite new to the area and it is good to hear that at least my SSEs are healthy :) After I asked this question I read and played with Zfit more so I realize something. R2 value in Zfit output is giving pretty much the same value with the linear extrapolation. I wonder can I also use that value to calculate ionic conductivity?
I don't understand your point.
As I told you in my answer to your other question and considering my understanding of the physics, the ohmic resistance (that you obtain from the intercept of the curve with the x-axis at very high frequencies) is almost exclusively due to the ionic resistance, the electronic resistance contribution beeing negligible.
My point is to calculate ionic conductivity by using the ZFit ouput without taken the intercept. As you told in the previous answer I increased my frequency range up to 1MHz-100mHz so now I have more data in the low frequency range which allows me to take the intercept more accurately. I am also trying to understand how can I use the values that I obtain from ZFit. For example, Rct value which comes from ZFit is almost equal to the value that I calculate via extrapolation. In this case, if I have a good ZFit, can I use the Rct value to calculate ionic conductivity? They are basically the same thing, right?
Let's go back to physics: everything stands there!
Ohmic resistance is, in my definition (and also the usual one), related to the Li+ transfer between both electrodes under the force linked with the gradient of electric potential due to the applied current. In this case, as I told you before, the ohmic resistance (and in 1st approximation the ionic resistance) IS the intercept of the spectrum with the x-axis at high frequency. Therefore, there is nothing to fit and you can set at as a fixed value for the ionic resistance the average of Re(Z) in the highest frequency range in your Zfit model. Looking at your EC model, the ionic resistance is R1. Now, at lower frequencies, you have charge transfer and mass transfer resistances. Charge transfer resistance (R2 in your model) is linked to the kinetics of the electrochemical reaction and is in parallel with the double layer capacitance (C in your model). Mass transfer resistance is linked to the straight line that you observe in the lowest frequency range (the Warburg element of your circuit). I guess that impedance of the Li electrode is negligible vs the one of the positive electrode and the loop and straight line that you observe are linked to the positive electrode?...
Therefore, I don't understand what you really try to do with your Zfit software to determine the ohmic resistance....
By the way, to catch the whole characteristics of the mass transfer limitations, you should decrease the lower limit of your impedance spectra down to 10 mHz. Unless the time of the experiment is so long that the SoC of your battery has significantly changed? In this case, might be reducing the number of points/decade (let's say 5 points/decade) would be relevant.
Best regards.
Dear Orhan Kıbrıslı ,
the temperature and the DC-polarization(s) help to discriminate the (electronic vs ionic) conductivity character. How much your (EIS-)spectra change with the parameter of the DC-polarization, VDC , value(s) ?
Also say, please, what are
1) your (2 or 4?) electrodes (' materials ?),
and what is
2) the temperature and the DC-polarization(s), VDC (= ?) in this ('EIS curve.png') EIS study.
- Badges
- Science topic
- Similar topics
- Mathematical Sciences
- Graphs