**2. Materials and Methods**

In this section, the preparation of the cell structure consisting of the electrolyte and the electrodes will be discussed first. Thereafter, the measurement procedure of the cycling tests and the boundary conditions of the conducted experiments will be presented.

#### *2.1. Material and Cell Preparation*

**Electrolytes preparation:** LiTFSI and LiFSI (with a chemical purity of *<sup>x</sup>*LiTFSI,LiFSI > 99% , manufactured by Ionic Liquid Technologies GmbH (Heilbronn, Germany), were purchased and used as received. The electrolyte DME with a chemical purity of *<sup>x</sup>*DME > 99% manufactured by Thermo Fisher GmbH (Kandel, Germany) was used as the solvent. Three different

ether-based electrolytes were prepared as follows: (1) LiTFSI 2M in DME, (2) LiFSI 2M in DME and (3) LiFSI 1M in DME.

**Electrode preparation:** An oxygen-free copper (Cu) foil from SCHLENK Metal Foils GmbH & Co. KG. (Roth, Germany) with a thickness of *h*Cu = 6 μm was purchased and used as received. Current collector (CC) disks with a diameter of *d*CC = 18 mm were punched out of the foil to be used as working electrodes. Lithium metal foil manufactured by Sigma Aldrich (St. Louis, MO, USA) with a thickness of *h*Li foil = 320 μm was used as the counter electrode. The lithium surface was cleaned of an oxide layer before punching. Disks with a diameter of *d*Li foil = 18 mm were made. All handling was performed inside an argon-filled glove box from M. Braun Inertgas-Systeme GmbH (Garching, Germany) with an oxygen (O2) and water (H2O) concentration *<sup>c</sup>*O2,H2O <sup>&</sup>lt; 0.5 ppm. Both counter and working electrodes had an effective surface area of *A* = 2.54 cm2.
