*4.2. Aging Study*

## 4.2.1. EIS Measurement Evaluation

In Figure 7 shows the measured spectra and the corresponding DRT against frequency of an examined cell as well as the development during aging. The measurements from one cell from aging scenario *Sc*<sup>1</sup> were selected for the exemplary representation. The upper diagrams show the values determined at 20% and the lower part at 80% SOC.

**Figure 7.** Measured impedance spectra and corresponding DRTs over aging (**a**) At 20 % SOC; (**b**) At 80 % SOC.

In the Nyquist diagram, two major changes can be seen as the cell ages:


According to the second bullet point, the DRT at 20% reveals distinct changes in the polarization contribution and the time constant for a process at moderate frequencies. This peak can be assigned to the second semicircle in the spectrum because it occurs particularly when the SOC is low and the cell is new. As already mentioned, this peak can be assigned to a charge transfer process due to the range of the time constant.

In contrast to the observation of the spectra, the analysis of the DRT shows a slight increase in the contribution of this process during the last cycles. Furthermore, slight increases in the polarization of the charge transfer processes are visible at moderate frequencies and 80%. In addition, small changes in the parameters of the process with the smallest time constant are visible, but they do not follow a clear trend. Due to the frequency range, this process can be assigned to the transfer through the SEI of the anode [4,34].

For frequencies <0.1 Hz, a single process is visible. At low SOC, the polarization of the process remains almost constant during the first 4950 cycles. After the first 4950 cycles, a continuous increase can be observed. At 80% SOC, this continuous increase begins with the start of the long-term test.

The mentioned changes in the process parameters were recognizable for all cells, except for the increase in polarization contribution at low frequencies. This behavior was not pronounced in the cells of scenario *Sc*2. Changes of the process parameters were made at a similar rate within a scenario. However, there were deviations in the rate of change between the scenarios, especially with regard to the increase in polarization contribution at frequencies below 0.1 Hz.
