*2.1. Materials*

Tributyl acetylcitrate (TBAC, purity > 98% Sigma Aldrich, Germany) was dried over a molecular sieve (mesh size ≈ 0.3 nm) inside a glovebox exposed to argon atmosphere over night (moisture content of less than 0.5 ppm). Figure 1 shows the chemical structure of TBAC [24]. Ethylene carbonate (EC), ethyl methyl carbonate (EMC, purity > 99.9%), propylene carbonate (PC, purity > 99.7%), diethyl carbonate (DEC, purity > 99.9%), lithium tetrafluoroborate (LiBF4 purity > 99%, all from Sigma Aldrich), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, purity > 99%), lithium bis(fluorosulfonyl)imide (LiFSI, both fromIOLITEC GmbH, Germany), and lithium hexafluorophosphate (LiPF6, BASF) were opened in the glovebox and were used as received.

Graphite electrodes (3 mAh/cm2, provided by Varta AG) and lithium nickel manganese cobalt oxide electrodes (NCM 622, 1.3 mAh/cm2, provided by Münster Electrochemical Energy Technology) were punched to 18 mm coins and dried in a vacuum oven (Buechi Labortechnik AG B-585, *p* < 50 mbar, *T* = 120 ◦C). The separator Freudenberg 2190 (thickness = 176 μm) was punched to coins with 21 mm in diameter and dried in a vacuum oven (*p* < 50 mbar, *T* = 120 ◦C).

**Figure 1.** Chemical structure of tributyl acetylcitrate [24].

#### *2.2. Electrolyte Preparation*

The electrolytes were prepared in an argon-filled glovebox (H2 and O2 content lower than 0.5 ppm). TBAC was combined with different well-known solvents like EC and EMC. The formulated solvent mixtures and the amount of each solvent (in wt) is shown in Table 1a. Table 1b shows the combination of the solvent mixtures and the investigated lithium salts with a salt concentration of 1 M.


**Table 1.** (**a**) Formulated solvent-mixtures wt. (**b**) Investigated combinations of lithium salts and solvent-mixtures (concentration of 1 M).
