The reason for the appearance of two semicircles in the alternating impedance spectroscopy (EIS) diagram of a lithium-symmetrical battery is due to the presence of two different charge transport processes inside the battery.

First semicircle: This semicircle is usually located in the high-frequency region and represents the charge transport process inside the battery, also known as the charge transfer resistance. This semicircle reflects the speed at which charges are transported on the surface of the electrodes in the battery, and the speed at which ions in the electrolyte are transported between the electrodes and the electrolyte.

Second semicircle: This semicircle is usually located in the low-frequency region and represents the diffusion process inside the cell, also known as the charge diffusion resistance. This semicircle reflects the rate at which ions in the battery diffuse in the electrolyte, as well as the rate at which charges are transported at the electrode surface.

The diameter and shape of these two semicircles can provide information about the performance and interface characteristics of the battery. By analyzing the position, size, and frequency response of these semicircles, the internal resistance of the battery, the rate of charge transfer, and the diffusion capacity of the ions in the electrolyte can be evaluated.

It is common to observe two semi-circles in EIS for battery cells. I have discussed this in a recent publication. Please proceed with this information.

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Huang Liu

But for polymer solid electrolyte, we find only one semi-circle in the lithium symmetric cell for testing lithium ion transfer number. Why for liquid it showed two semi-cycles?

The reason why liquids show two half-cycles is due to the fact that the ion transport mechanism in liquid electrolytes is different from that in solid electrolytes. Ion transport in liquid electrolytes is mainly achieved by the diffusion of solute ions and the flow of solvent molecules, so two half-cycles are observed in cyclic voltammetry tests.

The lower freq. (2nd) nominal "semi-circle" seems to be even more complex (multiple 'mixed semi-circles'). Try to (re)measure some more experimental points for this regime.

If possible, show, also, the corresponding Bode plot(s), that might illuminate more the explication about the origin(s) of the lower freq. (2nd) "semi-circle".

What are the VDC polarization(s) values for your plots, dear Ching-Hsun Wu ?

Also, can you show plots for the EIS measurements applying the additional (common) VDC values: a) VDC= 0 mV, and b) VDC= OCV ?