I did EIS on a full cell (graphite vs NMC) with NMC as working electrode in coin cell format. The result shows decreasing trend of 2nd semi-circle (refers to charge transfer resistance) over the cycle. However, according to literature, charge transfer resistance must increase with respect to cycle. I repeated the test, but the trend is always decreasing.
The frequency range used was EIS = 100kHz-10mHz with 10mV/s amplitude. The impedance was taken after 1st cycle and then after each 10 cycle interval until 100 cycles. EIS was done on bio-logic VMP3. All the EIS data has been taken after the end of discharge of the cell. The cycling rate of the cell is C/2 which corresponds to 1.3mA current.
The attached image shows the Nyquist plot in which the the blue line is after 1st cycle discharge end, the red line is after 10th cycle and the green lines are taken after each 10 cycle interval of until 100 cycles.
I guess the content of the remaining Li in the electrodes after the cycling affects a lot. If your NMC has fewer Li ions in its structure as the cycle number increases, the charge transfer resistance will be lower.
Maybe that is because of formation surface layer on NMC, as well graphite with smaller charge transfer resistance. As mentioned by Hong, you should also make sure the cell is at identical SOC when doing the impedance. Small variations in SOC specifically in the end of discharge have large effect on impedance spectra.
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