Investigation of a Novel Ecofriendly Electrolyte-Solvent for Lithium-Ion Batteries with Increased Thermal Stability

: This study presents tributyl acetylcitrate (TBAC) as a novel ecofriendly high ﬂash point and high boiling point solvent for electrolytes in lithium-ion batteries. The ﬂash point ( T FP = 217 ◦ C) and the boiling point ( T BP = 331 ◦ C) of TBAC are approximately 200 K greater than that of conventional linear carbonate components, such as ethyl methyl carbonate (EMC) or diethyl carbonate (DEC). The melting point ( T MP = − 80 ◦ C) is more than 100 K lower than that of ethylene carbonate (EC). Furthermore, TBAC is known as an ecofriendly solvent from other industrial sectors. A life cycle test of a graphite/NCM cell with 1 M lithium hexaﬂuorophosphate (LiPF 6 ) in TBAC:EC:EMC:DEC (60:15:5:20 wt) achieved a coulombic efﬁciency of above 99% and the remaining capacity resulted in 90 percent after 100 cycles ( C /4) of testing. As a result, TBAC is considered a viable option for improving the thermal stability of lithium-ion batteries.


Introduction
Lithium-ion cells are the technological standard for portable devices such as smartphones, notebooks, and electric vehicles, and as a result, they are viewed as a key for the global transition to electro-mobility. In terms of thermal stability, the electrolyte of a lithium-ion cell is considered a critical component, which is responsible for ionic conductivity. The electrolyte is mainly composed of solvents, lithium conducting salts, and various additives [1,2]. The usage of conventional solvents for electrolytes with low boiling points and flash points (T FP ) like dimethyl carbonate (DMC, T FP < 20 • C [3]), EMC (T FP ≈ 22 • C [4]), or DEC (T FP ≈ 25 • C [5]) bear an increased risk to ignite lithium-ion cells [6][7][8]. The low boiling point generates high pressure gradients at moderate temperatures (<100 • C), which can lead to the explosion of the cell. The chemical products of burned fluorine-containing electrolytes are highly toxic [9][10][11][12]. Therefore, an expensive thermal management system and a massive casing for lithium-ion batteries are required. This heavy casing for battery packs for electric vehicles lowers the gravimetric density of the pack and increases the weight of the vehicles. Increasing the intrinsic thermal stability is a key factor to lower costs for cell protection and increase the gravimetric density of the battery pack. Investigations on new electrolyte formulations have been considered before, for example by using flame retardant additives like organic phosphates [13] or phosphonates [14]. However, using these additives to improve the thermal stability reduces the cell performance [15]. Investigations show that the use of ionic liquids can increase the flash point of electrolytes however, these ionic liquids are linked with high costs [16][17][18][19]. Another promising approach is to investigate co-solvents with higher flash points like adiponitrile (ADN, T FP ≈ 163 • C [20]) by Isken et al [21]. Co-solvents were able to increase the flash point significantly from T FP,EC:DEC ≈ 36 • C of the EC:DEC (3:7 wt) mixture to T FP,EC:ADN ≈ 149 • C of the EC:ADN (1:1 wt) mixture. This indicates that it is possible to formulate electrolytes with higher flash points by replacing volatile carbonates. However, the melting points (MP) of EC T MP,EC ≈ 36 • C and ADN T MP,ADN ≈ 2 • C are extremely high for low temperature applications. To lower the working temperature applicability T < 0 • C research has shown many different classes of solvents like sulfones [22,23]. Unfortunately, most of these solvents are ecologically harmful.
Therefore, we investigated tributyl acetylcitrate as a solvent to formulate an electrolyte composition with a high flash point and a wide operating range from very low to high temperature. TBAC has a high flash point of T FP ≈ 217 • C [24], its melting point is very low with T MP ≈ −80 • C and the boiling point is at T BP ≈ 330 • C. Another noteworthy advantage of TBAC is that it is ecofriendly [25], nontoxic [26], and therefore safe to handle. For example, it is well known as a plasticizer in the nail polish industry [27].
This study presents tributyl acetylcitrate as a novel solvent for lithium-ion cells. The combination of conventional solvents like EC and DEC with TBAC creates an electrolyte with improved thermal stability.

Electrolyte Preparation
The electrolytes were prepared in an argon-filled glovebox (H 2 and O 2 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.

Cell Preparation
Cells were assembled as coin cells in PAT-Cell or ECC-Ref cells from EL Cell. For constant current cycling and C-rate tests, cells were assembled with NCM as the working electrode and graphite as the counter electrode. The polypropylene separator (Freudenberg FS2190) was moistened on both sides with a total of 120 µL of the selected electrolyte before the electrodes were added. For cyclic voltammetry measurements, two electrode cells were assembled with stainless steel or aluminum as working electrode and stainless steel as counter and reference electrode.

Electrochemical Characterization
All tests were done in a Memmert IPP110 thermal chamber at a constant temperature of T = 25 ± 0.1 • C. The cell performance was evaluated by cycling tests with constant current charge and discharge with the battery tester CTS from BaSyTec. At the beginning, three preliminary formation cycles were performed at I = C/10 in a potential range from U = 2.5 V to U = 4.2 V. Subsequently lifecycle tests were performed at C/4.
The cyclic voltammetry measurements were performed with a reference 3000 AE from Gamry. The electrochemical stability was measured by linear sweep voltammetry. The scan rate was set to 1 mVs −1 , the potential limits were set from −4.3 V to 4.3 V vs. stainless steel. 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.

Results
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.

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.  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. The applied currents were: C/10, C/5, C/3, C/2, 0.75C, 1C, 1.5C, and 2C. At a current rate of 1C, 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. 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].

TBAC Solvent-Mixtures with LiBF 4
A total of1 M LiBF 4 was investigated in TBAC:EC:EMC (80:15:5 wt). However, LiBF 4 did not dissolve. It was not possible to cycle the electrolyte formulation in a graphite/NCM cell. We assume that TBAC is incompatible with LiBF 4 .

TBAC Solvent-Mixtures with LiFSI
The results for the electrolytes with LiFSI as conducting salt are comparable to LiTFSI. Graphite/NCM cells with 1 M LiFSI dissolved in F2 showed a coloumbic efficiency of about η coul = 98%. The results of the cycling tests are shown in Figure 5 with three formation cycles at C/10 and 100 cycles at C/4. However, it was not possible to achieve a durable coulombic efficiency above η coul = 99%. Therefore TBAC-LiFSI electrolytes were not further investigated.

TBAC Solvent-Mixtures with LiPF 6
Similar to LiTFSI and LiFSI, the conducting salt LiPF 6 does dissolve in F2. The coulombic efficiency of 1 M LiPF 6 in F2 in a graphite/NCM cell was about η coul = 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 LiPF 6 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 η coul = 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.

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 −1 . The electrolyte is stable up to 3.5 V vs. stainless steel.
The stability window of LiPF 6 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/cm 2 . 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/cm 2 .
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 LiPF 6 -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 0 is also described in literature [34]. The electrochemical stability of LiPF 6 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.

Discussion
TBAC in combination with EC and EMC dissolves the conducting salts LiTFSI, LiFSI, and LiPF 6 . 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 LiPF 6 . 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 LiPF 6 . To determine the reason for the efficiency reduction CV measurements were performed. They showed that the electrochemical stability window of LiPF 6 -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.

Conclusions
In this paper we have investigated tributyl acetylcitrate as a novel and ecofriendly electrolyte-solvent for lithium-ion cells. Compared to conventional solvents, the flash point and the boiling point are almost 200 K higher than that of EMC or DEC. The melting point is more than 100 K lower than that of EC. In combination with LiPF 6 and other solvents (TBAC:EC:EMC:DEC, 60:15:5:20 wt), TBAC shows good electrochemical stability between −4 V and 3 V vs. stainless steel. In graphite/NCM cells the coulombic efficiency is above η coul = 99%. After 100 cycles the remaining capacity is still above 90%. Therefore, TBAC when compared to other solvents is a promising alternative to improve the intrinsic thermal stability of lithium-ion cells.