**5. Future Work**

Another method for investigating lithium deposition during cycling is *insitu* observation using special cell holders. *Insitu* observations in the ECC-Opto Stud, manufactured by EL-Cells GmbH (Hamburg, Germany), on battery cells were made in [26–33] using Raman spectroscopy, X-ray diffraction (XRD) and optical analytics. Observations were made using Raman spectroscopy [27,31,32] and XRD [26,28,33] to observe changes in electrodes and SEI during aging. Merryweather et al. [29] used the above-mentioned cell housing for optical interferometric scattering measurements to detect single-particle ion dynamics, and Rittweger et al. [30] observed the reflectivity of cathodes during charge and discharge.

In future work, the above-mentioned cell holder ECC-Opto-Std cell, manufactured by EL-Cell, will be used to conduct *insitu* measurements. The cell setups, like those described in this article, consisting of Cu/Li, will be used. The experiments will also investigate the influence of C-rate, temperature, used salt and salt concentration on the morphology of lithium deposition. Additionally, in order to better understand the behavior of the lithium at different conditions, a fully developed model will be included.

#### **6. Conclusions**

In summary, the electrochemical performance of Cu/Li cells was investigated with the motivation of understanding the kinetics of the deposition mechanism of Li metal electrodes. The influence of temperature *T*Cell{25, 40, 60} ◦C and C-rate *I*Cell{0.5, 1, 2} C was examined. Additionally, the variety of electrolytes, including LiFSI 2M in DME, LiFSI 1M in DME, and LiTFSI 1M in DME, were utilized to address the impact of type and concentration of Li salt on the electrolyte. Coulombic efficiency and induced depositions overpotential, as well as EIS measurements, were used to evaluate the aging behavior of cells under different conditions. Based on our results, it is confirmed that cycling at a temperature of *T*Cell = 40 ◦C has the best kinetics in comparison to the cycling data with *T*Cell = 25 ◦C and *T*Cell = 60 ◦C, as it shows minimum deposition overpotentials and impedance. However, the best performance regarding the stability and long cycle life is achieved at *T*Cell = 25 ◦C. The LiFSI in general showed better cyclability in Cu/Li cells compared to LiTFSI and the best performance could be gained by a high concentration of the LiFSI-DME electrolyte.

**Author Contributions:** Conceptualization, S.M.B., A.F. and A.R.; methodology, S.M.B.; validation, S.M.B.; investigation, S.M.B.; data curation, S.M.B.; writing—original draft preparation, S.M.B.; writing—review and editing, A.F.; visualization, S.M.B., A.F.; supervision, K.P.B.; project administration, K.P.B.; funding acquisition, K.P.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partly funded by Robert Bosch GmbH.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author

**Acknowledgments:** The authors thankfully acknowledge Leonie Wolf for helping with the preparation of the cell assemblies. The authors would also like to thank Robert Bosch GmbH for their support during the Bosch Doctoral College.

**Conflicts of Interest:** The authors declare no conflict of interest.
