**4. Discussion**

It has been realized that the type of lithium salt as well as its concentration can strongly influence the performance and cycle life of Li-metal cells. To further investigate this issue, EIS measurements were performed on the cells with different electrolytes of (1) LiFSI 2M in DME, (2) LiFSI 1M in DME, (3) LiTFSI 1M in DME, different measurement temperatures of *T*Cell{25, 40, 60} ◦C and different C-rates of *I*Cell{0.5, 1, 2} C.

As explained in Figure 1, the first EIS measurement was carried out after the first Li deposition on Cu and then was repeated every 20 cycles until the Coulombic efficiency of the cell reached the value of 0.95. The spectra of cells with different electrolytes after the first Li plating performed at *<sup>T</sup>*Cell = <sup>25</sup> ◦C and an applied current density of *<sup>j</sup>* = 1 mAh·cm−<sup>2</sup> (C-rate = 1 C) are presented in Figure 7a. The EIS spectra of cells after the first Li plating performed at different measurement temperatures, having LiFSI 2M in DME electrolyte and a C-rate of *I*Cell = 1 C, are presented in Figure 7b. The influence of aging on the EIS spectra of a Cu/Li cell with LiFSI 2M in DME electrolyte, performed at *T*Cell = 25 ◦C and a C-rate of *I*Cell = 1 C, is visualized in Figure 7c. The spectrum #1 is the first EIS performed after the first Li deposition, #2 is assigned to the second EIS performed after 20 full cycles, #3 is the third EIS after 40 full cycles and so on.

**Figure 7.** EIS spectra of Cu/Li cells: (**a**) Three cells with different electrolytes after the first Li plating. Orange represents LiFSI 2M in DME, black is LiTFSI 1M in DME, and blue shows LiFSI 1M in DME at *<sup>T</sup>*Cell <sup>=</sup> <sup>25</sup> ◦C and *<sup>j</sup>* <sup>=</sup> 1 mAh·cm<sup>−</sup>2. (**b**) Three cells at different temperatures *<sup>T</sup>*Cell{25, 40, 60} ◦C after the first Li plating. Electrolyte: LiFSI 2M in DME with an applied current density of *<sup>j</sup>* <sup>=</sup> 1 mAh·cm<sup>−</sup>2. (**c**) EIS spectra of a single cell, performed every 20th cycle during the degradation test with LiFSI 2M in DME, *<sup>T</sup>*Cell <sup>=</sup> <sup>25</sup> ◦C and *<sup>j</sup>* <sup>=</sup> 1 mAh·cm<sup>−</sup>2. EIS # 1 is after first plating, EIS # 2 is after 21 cycles and EIS # 7 is after 121 cycles.

The correlation of EIS measurements with cycling results could be better realized by considering the induced overpotentials of Li deposition nucleation *μ*nucleation and particle growth *μ*growth during one full cycle. The initial voltage drop at the beginning of plating on Cu is considered to be nucleation overpotential and the steady potential during the rest of deposition period is considered to be growth overpotential [13]. These values are illustrated in Figure 8. The influence of the electrolyte variation is presented in Figure 8a, the influence of temperature variation is visualized in Figure 8b, and at last the impact of the applied current density is displayed in Figure 8c. The cells presented in Figure 8a,b are identical to the cells shown in Figure 7a,b.

**Figure 8.** Potential–Capacity profile at cycle number 10 for Li/Cu cells, (**a**) with different electrolytes of LiFSI 1M in DME (orange), LiTFSI 1M in DME (black), and LiFSI 2M in DME (blue). Measurements are performed at *<sup>T</sup>*Cell <sup>=</sup> <sup>25</sup> ◦C and with an applied current density of *<sup>j</sup>* <sup>=</sup> 1 mAh·cm−2. (**b**) performed with different C-rates of *I*Cell = 0.5 C (orange), *I*Cell = 1 C (black) and *I*Cell = 2 C (blue), with LiFSI 2M in DME as electrolyte at *T*Cell = 25 ◦C. (**c**) performed at different temperature of *T*Cell = 25 ◦C (orange), *T*Cell = 40 ◦C (black) and *T*Cell = 60 ◦C (blue), using LiFSI 2M in DME as electrolyte and a current density of *<sup>j</sup>* <sup>=</sup> 1 mAh·cm<sup>−</sup>2.

All three cells presented in Figure 7a show a comparable ohmic resistance (*R*ohmic), which is mainly correlated to electrolyte and contact resistances. This was expected as the cells consist of the same electrodes and the cells are still too fresh to be influenced by different aging rates due to different used electrolytes. The typical semi-circle is easily noticeable in the EIS data of all cells. Additionally, all three cells in Figure 7a show a second semi-circle at lower frequencies, which are partly overlapped with the first ones. The semi-circles at higher frequencies are formed in a similar frequency range for the cells having an LiFSI based electrolyte (see Figure 7a). The frequency values are different for the cells containing the LiTFSI based electrolyte.

These results indicate that the first semi-circle is based on the interface or SEI related impedance. The second semi-circle shows the impedance related to the charge transfer *R*ct. The consequence of a different impedance behavior of the cells due to the used electrolyte can also be seen in the overpotential of cells during cycling. The voltage behavior of one charge and discharge process (at cycle number #10) is illustrated in Figure 8a; these are the cells identical to those presented in Figure 7a. As expected, the cell with the LiTFSI based electrolyte shows the highest overpotentials (*μ*nucleation = −6.5 mV and *μ*growth = −2.6 mV) and irreversible capacity among the rest. The two LiFSI based cells show comparable overpotentials of *μ*nucleation = −5.2 mV and *μ*growth = −1.4 mV for LiFSI 2M and of *μ*nucleation = −4.5 mV and *μ*growth = −1.5 mV for LiFSI 1M. The high concentrated LiFSI based cell shows the minimum of irreversible capacity.

By varying the temperature it can be noticed that the cells have the most stable performance at *T* = 25 ◦C (see Figure 7b). By increasing the temperature, the frequency which corresponds to a maximum of the semi-circle moves to higher values (from 5 kHz at *T*Cell = 25 ◦C to 12.5 kHz at *T*Cell = 40 ◦C and to 20 kHz at *T*Cell = 60 ◦C). The second semi-circle at elevated temperatures is not distinct anymore and is hardly noticeable at *T*Cell ≥ 40 ◦C. This effect can be explained by the fact that with increasing temperature the charge transfer resistance decreases and consequently the correlated semi-circle is more overlapped to the SEI based semi-circle. The ohmic resistances (*R*ohmic) are comparable in both cells at elevated temperatures and are smaller compared to the resistance of the cell performed at *T*Cell = 25 ◦C. The cell cycled at *T*Cell = 40 ◦C has the smallest length of the SEI related semi-circle. This is a consequence of the improved kinetic at *T*Cell = 40 ◦C in comparison to *T*Cell = 25 ◦C. This is in line with the seen voltage profile of the cells presented in Figure 8b. The data at *T*Cell = 40 ◦C show the lowest overpotentials of *μ*nucleation = −4 mV and *μ*growth = −0.7 mV. The cell cycled at *T*Cell = 60 ◦C also has a low nucleation overpotential of *μ*nucleation = −4 mV; however, *μ*growth in contrast to the rest of the cells is increasing during the plating period and approaches bigger values (*μ*growth = −2.8 mV). Cycling at a temperature of *T*Cell = 60 ◦C has the maximum growth overpotential and the worst cycling stability among all considered temperatures.

Another influencing parameter on the kinetics of Li deposition is the applied current density. We have noticed that cells with a higher C-rate achieve a longer cycle life. This can be seen in Figure 8c, which shows the data extracted from three Li/Cu cells containing LiFSI 2M in DME electrolyte at *T*Cell = 25 ◦C with different current densities. As expected, the overpotential increases with increasing current density which causes a higher deviation from equilibrium. However, the better cycling performance of the cells cycled at a higher C-rate could be due to fewer interfacial side reactions as the cycling time is shorter. On the other hand, however, the higher current density could be a trigger to side reactions. More interfacial investigation (EIS and *insitu* observation) is needed for a better understanding of this process. EIS measurements on the cell with LiFSI 2M in the DME electrolyte performed at *T*Cell = 25 ◦C show that, after 20 cycles, the interface resistance significantly decreased. This could be due to the SEI layer not being formed homogeneously and completely after the first deposition. By continuing the cycling, however, the layer becomes denser and more uniform and therefore after 20 cycles the semi-circle is significantly smaller than that after the first deposition. By continuing the cycling, the second semi-circle at low frequencies is still noticeable. This was not the case for the cells cycled at higher temperatures. By cycling the frequency corresponding to the maximum of the first, the semi-circles move towards higher values (similar to the high temperature behavior). The *R*ct increases by cycling but is still smaller than that of the very first cycle. The ohmic resistance is also slightly increased by cycling. A relatively sharp increase of *R*ohmic is noticeable between cycle 100 and cycle 120. It is worth mentioning that, based on cycle performance results, the cell starts to show slight instabilities in the CE trend from the 110th cycle. It can be concluded that both *R*ohmic and *R*ct influence the cycle behavior and the cycle life of the Li-metal cells.
