**3. Results**

Table 2 shows the melting, flash, and boiling point of TBAC, EC, EMC, PC, and DEC. The boiling point as well as the flash point *T*FP = 217 ◦C of TBAC is almost 200 K higher than that of EMC and DEC. The melting point is more than 100 K lower than that of EC. The high boiling point and the high flash point are the main advantages of TBAC as a solvent for lithium-ion batteries. However, during cell preparation, TBAC showed a higher viscosity than the other solvents, which could cause a lower conductivity. Therefore, the solvent-mixtures shown in Table 1 were investigated to find a composition with adequate cycling behavior and a high amount of TBAC.

**Table 2.** Physical properties of TBAC [24], EC [28], EMC [4], PC [29], and DEC [5]. Symbols used: *T*MP, melting point; *T*FP, flash point; *T*BP, boiling point.


The following discussion is structured by presenting the results for each investigated conducting salt in combination with TBAC-based solvent mixtures from Table 1.

The main focus lies in combining TBAC with LiTFSI since other conducting salts are thermally less stable [30]. However, other possible combinations of salts are presented to give an overview of further possibilities.

#### *3.1. TBAC Solvent-Mixtures with LiTFSI*

The lithium salt concentration of 1 M LiTFSI seems to be completely dissolvable in F1 = TBAC:EC (85:15 wt). However, in a cell with NCM as positive electrode and graphite as a negative electrode with LiTFSI in F1, the coulombic efficiency of the life cycle test was only about *η*coul = 80% and the ionic conductivity limits the current to a maximum of *C*/10 at *T* = 25 ◦C. By adding EMC to the solvent-mixture, the coulombic efficiency increased above *η*coul = 99%. To increase the life cycle performance, DEC was added to the electrolyte. This resulted in an increase of conductivity that allowed to cycle the cells with *C*/4. Figure 2 shows the results of the cycling test of a graphite/NCM cell with the electrolyte formulation with 1 M LiTFSI in the solvent mixture F4 = TBAC:EC:EMC:DEC (60:15:5:20 wt) at a C-rate of *C*/4 at *T* = 25 ◦C.

**Figure 2.** Cycling performance of a graphite/NCM cell with 1 M LiTFSI in TBAC:EC:EMC:DEC (60:15:5:20 wt) electrolyte at *T* = 25 ◦C. The cell was charged and discharged at C−rate *C*/4.

The coulombic efficiency achieved was *η*coul = 99.6%. After 100 cycles there was about 88% of the initial capacity left. Different lithium salt concentrations between 0.8 M and 1.2 M were investigated. The best cycling performance was achieved with a concentration of 1 M. Therefore, all further tests were done with a salt concentration of 1 M.

*C*-rate tests were performed to investigate the electrode performance in combination with the novel electrolyte composition for different currents. Figure 3 shows the capacity obtained at different *C*-rates for a graphite/NCM cell with 1 M LiTFSI in the solventmixture F4.

**Figure 3.** C-rate performance of a graphite/NCM cell with 1 M LiTFSI in TBAC:EC:EMC:DEC (60:15:5:20 wt) electrolyte at *T* = 25 ◦C in the potential range of 2.5–4.2 V.

The applied currents were: *C*/10, *C*/5, *C*/3, *C*/2, 0.75*C*, 1*C*, 1.5*C*, and 2*C*. At a current rate of 1*C*, the usable capacity dropped to 70% of the initial capacity. Higher *C*-rates decrease the usable capacity significantly.

Figure 4 shows the discharge capacity and the potential profiles at different temperatures between *T* = 30 ◦C and *T* = 70 ◦C. Until 60 ◦C the voltage profile seems to be nearly independent of temperature. At 70 ◦C there was a small drop in voltage observed.

**Figure 4.** Voltage profiles at different temperatures of a graphite/NCM cell with 1 M LiTFSI in TBAC:EC:EMC:DEC (60:15:5:20 wt) electrolyte.

The solvent mixture F3 = TBAC:EC:EMC:PC (60:15:5:20 wt) was investigated in combination with LiTFSI. However, as expected from the literature, the combination of PC and LiTFSI is not compatible and it is not possible to cycle a lithium-ion cell with this electrolyte [31].
