when performing EIS for lithium ion batteries (whether it's LCO/LFP/NMC), you're going to get semi-circles followed by a straight line in low frequency regions.
there could be 2-3 semi-circles. particularly, how do you know which semi-circle is the SEI and which corresponds to the CEI?
additionally, i wanted to know how at the start of a scan during high frequencies before the line crosses the x-axis, sometimes it's loopy, what does that mean or how could that be interpreted? what if it's perfectly straight and THEN forms the semi circles?
just trying to learn the thought process you folks might have, thank you.
Dear Ahmer,
according to the EIS test which is based on monitoring impedance Vs frequency (time), it is noteworthy to consider that, while you are going from higher frequency toward lower frequency, the impedance corresponding to faradic and non faradic reaction in the cell will be monitored sequentially. in the other words, reaction with higher kinetic happens in higher frequency. that is why mass transfer impedance (highest impediment) occurs at the lowest frequency as Warburg impedance and you can see R solution in high frequency as the start point of semi-circle.
usually the kinetic of producing SEI layer is higher than electron transfer reaction in cathode, so SEI will monitor in the higher frequencies.
if you have more than 2 semi-circle, it might be as a result of your electrode preparation method or your nano-particles properties. some resources consider the first semi-circle as the impedance for grain boundaries, then the second semi-circle would be considered for SEI, and finally the CEI.
Regarding the second section of your question, I have had the same experience about loopy start. at first I thought it might happen because of inductive effect of adsorption and desorption of ions in the surface of electrode. However, gradually I found that if I give my cell more time to reach equilibrium it wont happen. In this regard, you should check the OCP of your cell and when its OCP does not fluctuate in a wide rang, it means your cell is ready for test and hopefully will not see that loopy start.
Regards
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