*2.2. Cell Degradation Experiments*

Cell degradation experiments were conducted using EL-CELL PAT-Cell cases manufactured by EL-Cell GmbH (Hamburg, Germany). Before cell assembly, Cu current collectors were dried under vacuum for *t*dry = 16 h at *T*dry = 120 ◦C and were transferred into the glove box. Celgard 2500 from Celgard (North Carolina, USA) was used as a separator and was also dried under vacuum for *t*dry = 16 h at *T*dry = 80 ◦C prior to use. Prepared electrolytes were poured on both sides of the separator and the amount was determined depending on the measurement temperature and applied current density. For the measurements performed at *T*Cell = 25 ◦C, an electrolyte amount of *V*elec = 50 μL was used for the current densities of *<sup>J</sup>* = 0.5 mAh · cm−<sup>2</sup> and *<sup>J</sup>* = 1 mAh·cm−2. The amount of electrolyte was increased to *V*elec = 100 μL for the experiments with an applied current density of *<sup>J</sup>* = 2 mAh·cm<sup>−</sup>2. Regardless of current density, *<sup>V</sup>*elec = <sup>100</sup> μL was used for the measurements at *T*Cell = 40 ◦C and *T*Cell = 60 ◦C.

In order to evaluate the degradation behavior, cycling tests were conducted. A segment of the measurement procedure is visualized in Figure 1 with a C-rate of *I*Cell = 1 C. The current is presented in Figure 1a and the corresponding voltage in Figure 1b.

**Figure 1.** Segment of the measurement procedure of the conducted cycling tests example at *I*Cell = 1 C. The current data are presented in (**a**) and the corresponding voltage data are visualized in (**b**). Before the first cycle and after each 20 full charge/discharge cycles, EIS measurements were conducted.

The tests were conducted using a battery cell tester from Basytec GmbH (Asselfingen, Germany) in combination with a Reference 3000 from Gamry Instruments (Warminster, PA, USA) in a climate chamber manufactured by Memmert GmbH (Schwabach, Germany). Prior to cycling, cells were relaxed at the considered measurement temperature for *t* = 4 h to achieve a homogeneous electrolyte distribution and a steady state temperature. The Cu/Li cells showed an open circuit potential (OCP) of *U*OCP ≈ 2.7 V which is observable during the relaxation period in the first 4 h of the procedure.

The cells were continuously charged and discharged at the considered C-rate with the maximum voltage limit of *U*max = 1.5 V. In this set of experiments, no minimum voltage limit (*U*min) was defined and instead the specified time period based on the applied current density limited the discharge process. The cells were not relaxed between the charge and discharge cycles. After each 20th full charge/discharge cycle the impedance of the cells was evaluated via EIS measurements. To consider the initial impedance behavior of the cells, an EIS was also conducted after the first deposition period (discharge process) prior to 20 full cycles repetition. The segment including the 20 full cycles and the EIS was repeated up to 20 times depending on the degradation level of the cells.

The cell current and the cell voltage were controlled and captured by the mentioned battery tester. The EIS measurements were conducted using the Reference 3000 from Gamry. The frequency of the EIS was varied between *f*EIS, min = 0.1 Hz and *f*EIS, max = 100 kHz.

The purpose of this article is to investigate the influence of the C-rate, cell temperature, used salt and salt concentration on the aging behavior of the cells. The set boundary conditions of the conducted experiments are given by the measurement matrix in Figure 2. The different salts used are separated by color; orange measurement points correspond to experiments with LiTFSI and for the measurement points colored blue, LiFSI was used as the salt.

**Figure 2.** Matrix of the conducted measurements. For the salt LiFSI the c-rate was varied with *I*{0.5, 1, 2} C at a salt concentration of *c* = 2 M and a cell temperature of *T*Cell = 25 ◦C. The temperature was varied with *T*Cell = {25, 40, 60} ◦C at the same concentration and *I*{1, 2} C. At a concentration of *c* = 1 M measurements at *I* = 1 C and *T*Cell = {25, 40, 60} ◦C for two salts: LifTSI and LiFSI.

As presented in the measurement matrix, the C-rate was varied with *I*Cell{0.5, 1, 2} C in order to evaluate the influence of the C-rate. For the current stress of *I*Cell{1, 2} C the cell temperature was varied with *T*Cell = {25, 40, 60} ◦C. All mentioned measurements were conducted with LiFSI and a salt concentration of *c* = 2 M. To evaluate the impact of the salt and salt concentration experiments at a molar concentration of *c* = 1 M and two different salts—LiFSI (blue) and LiTFSI (orange)—were performed. Some measurement points were repeated in order to check the reproducibility and to determine the standard deviation of the experimental data due to the measurement setup, procedure and cell production. The number of measurements for each measurement point is listed in Table 1.


**Table 1.** Number of measurements for each condition considered.
