**3. Results**

In this section, the results of the experimental data will be presented. First, the reproducibility will be analyzed. Thereafter, the influence of the C-rate, temperature and electrolyte composition will be shown.

#### *3.1. Reproducibility of Measurements*

To validate the reproducibility of the galvanostatic cycling measurements on Cu/Li cells for selected points in the measurement matrix, up to three different EL-CELLs with exactly the same cell chemistry were assembled and the cycling results of the conducted measurements were compared (see Table 1). As an example, the cycling results of three cells at one specific measurement point are illustrated in Figure 3, with the areal capacity in Figure 3a and the Coulombic efficiency in Figure 3b. The cells contained LiFSI based electrolyte with a concentration of *c* = 2 M in DME and the measurements were performed at a current density of *<sup>J</sup>* = 1 mAh·cm−<sup>2</sup> and a temperature of *<sup>T</sup>*Cell = <sup>25</sup> ◦C.

**Figure 3.** Galvanostatic cycling performance of three identical cells with Cu/Li structure. Electrolyte used in all cells is LiFSI 2M in DME. (**a**) areal capacity and (**b**) Coulombic efficiency over life time is shown for cells running at *<sup>T</sup>*Cell <sup>=</sup> <sup>25</sup> ◦C with the current density of *<sup>J</sup>* <sup>=</sup> 1 mA·cm<sup>−</sup>2.

Both the areal capacity (Figure 3a) and the Coulombic efficiency (Figure 3b) of all three cells show a high comparability at the first 100 cycles. This is indicated by the calculated standard deviation of *σ*CE ± 0.005 for the Coulombic efficiency and the corresponding value of *<sup>σ</sup>*A.Cap ± 0.011 mAh · cm−<sup>2</sup> for the areal capacity. Only cell 1 shows an instability from cycle number 80 onward, which is not the case for cell 2 and cell 3. The ideal discharge capacity that the cells could reach is 1 mAh·cm−2. The average Coulombic efficiency of *<sup>η</sup>*CE-mean ≈ 0.99 indicates the average areal capacity with a value of 0.99 mAh·cm−<sup>2</sup> as well. For cell 1 there were four cycles which had a areal capacity and Coulombic efficiency above the theoretical maximum: *<sup>η</sup>*CE > 1. It has been reported that micro shorts can happen during the Li deposition period using Li metal electrodes, which could be seen as tiny fluctuations in the voltage behavior of the cell [23]. The micro shorts happen when local dendrites can penetrate through the separator and contact the other electrode. In these cases, the cell locally experiences the occurrence of charge and discharge processes simultaneously. Due to this effect, the cell could show a longer deposition period, as under normal conditions, causing values above the theoretical maximum. In the following, only the Coulombic efficiency is presented and discussed as the areal capacity and the Coulombic efficiency correlate to each other.
