*3.4. TBAC Solvent-Mixtures with LiPF*<sup>6</sup>

Similar to LiTFSI and LiFSI, the conducting salt LiPF6 does dissolve in F2. The coulombic efficiency of 1 M LiPF6 in F2 in a graphite/NCM cell was about *ηcoul* = 98%. It was necessary to add DEC or PC to improve the ionic conductivity to cycle cells at C-rate *C*/4. Figure 6 shows the cycling results for a graphite/NCM cell with 1 M LiPF6 in F4 at *T* = 25 ◦C with C-rate of *C*/4. The results of the electrolyte formulation with PC (F3) are comparable to the DEC electrolyte. Therefore, we will discuss only the DEC containing electrolyte..

The coulombic efficiency after 100 cycles is above *ηcoul* = 99.5%, the remaining capacity is about 93% of the initial capacity. Compared to TBAC-LiTFSI electrolytes, the coulombic efficiency and the remaining total capacity are increased.

**Figure 6.** Cycling performance of a graphite/NCM cell with 1 M LiPF6 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.

#### *3.5. Cyclic Voltammetry*

To determine the electrochemical stability window, CV measurements between electrochemical inert stainless steel electrodes were observed. Figure 7a shows the electrochemical stability window of LP40 electrolyte as reference. Therefore, the potential of a stainless steel working electrode connected to the electrolyte LP40 against stainless steel was scanned at 1 mVs<sup>−</sup>1. The electrolyte is stable up to 3.5 V vs. stainless steel.

The stability window of LiPF6 dissolved in TBAC:EC:EMC:DEC (60:15:5:20 wt) is shown in Figure 7b. For the second cycle, the TBAC based electrolyte seems to be stable over the total potential range. Compared to the LP40 reference electrolyte the stability is improved. At 3.5 V vs. stainless steel, the current begins to rise but is still below 3 μA/cm2.

Figure 7c shows the electrochemical stability window for LiTFSI dissolved in TBAC:EC:EMC:DEC (60:15:5:20 wt) between two stainless steel electrodes and Figure 7d between aluminum as working electrode and stainless steel as the counter electrode. Between two stainless steel electrodes, the stability window of the LiTFSI-TBAC based electrolyte is comparable to the LP40 reference. The electrolyte decomposes above 3.5 V vs. stainless steel. However, after the second cycle the decomposition current is still below 4 μA/cm2.

Since aluminum is used as a current collector in commercial lithium-ion cells the electrochemical stability window was investigated for an aluminum working electrode. CV measurements with LP40 and LiPF6-TBAC based electrolytes in contact with aluminum were comparable to CV measurements without aluminum. Figure 7d shows the result for the LiTFSI-TBAC based electrolyte in contact with aluminum. Above 3 V vs. stainless steel, the decomposition current is increased by an order of one magnitude compared to the sample without aluminum. This effect is described in the literature as aluminum corrosion by the LiTFSI conducting salt [32,33].

LiFSI-TBAC based electrolytes were not further investigated since the tendency to allow aluminum current collector corrosion beyond 3.8 V vs. Li+/Li<sup>0</sup> is also described in literature [34].

**Figure 7.** Electrochemical stability window of (**a**) LP40, (**b**) 1 M LiPF6 dissolved in TBAC:EC:EMC: DEC (60:15:5:20 wt), (**c**) 1 M LiTFSI in TBAC:EC:EMC:DEC (60:15:5:20 wt) (all with stainless steel as working and counter electrodes), and (**d**) 1 M LiTFSI in TBAC:EC:EMC:DEC (60:15:5:20 wt) (with aluminum as working electrode). Scan rate 1 mVs<sup>−</sup>1. The electrochemical stability of LiPF6 in TBAC seems to be improved compared to LP40. LiTFSI−TBAC based electrolytes in contact with aluminum show higher decomposition rates, which is assumed to be linked to aluminum corrosion by LiTFSI.

#### **4. Discussion**

TBAC in combination with EC and EMC dissolves the conducting salts LiTFSI, LiFSI, and LiPF6. The main focus of this study lays in the combination of TBAC with LiTFSI to create an electrolyte with high thermal stability and high flash point, since the thermal stability and the ionic conductivity of LiTFSI are improved compared to LiPF6. By dissolving 1 M LiTFSI in TBAC:EC:EMC:DEC (60:15:5:20 wt) a coulombic efficiency of 99% at a C-rate of *C*/4 was achieved. However, the coulombic efficiency of LiTFSI in life cycle tests was slightly reduced compared to LiPF6. To determine the reason for the efficiency reduction CV measurements were performed. They showed that the electrochemical stability window of LiPF6-TBAC based electrolytes is improved compared to the LP40 reference. Furthermore, the electrochemical stability window of LiTFSI-TBAC based electrolyte between stainless steel is comparable to LP40. CV measurements of the LiTFSI-TBAC based electrolyte connected to an aluminum working electrode show decomposition reactions beyond 3.5 V vs. stainless steel. In literature this is referred to as aluminum corrosion by LiTFSI.

TBAC itself seems to be electrochemically stable in the same window as LP40.
