Non-Flammable Dual-Salt Deep Eutectic Electrolyte for High-Voltage Lithium Metal Battery

Abstract

The application of high voltage cathode electrode materials is an effective way to increase the energy density of batteries. However, the development and design of a stable electrolyte at high voltages needs to be further addressed. Herein, we developed a non-flammable dual-salt deep eutectic solvent (DES) as a safe electrolyte containing LiTFSI, LiDFOB, and succinonitrile in different molar ratios. This non-flammable DES provides high ionic conductivity (4.23 mS cm⁻¹) at 25 °C, high Li⁺ transference number (0.75), and wide electrochemical stability (>5.5 V). When using the designed DES electrolytes in high voltage LiCoO₂||Li cells, superior electrochemical performance was achieved at cut-off voltages of 3.0–4.45 V and 3.0–4.6 V, even at a high current density of 2 C. This work offers an in-depth understanding of the critical role of dual-salts in DES and provides an approach to designing safe electrolytes for high voltage LiCoO₂||Li cells.

1. Introduction

With the rapid development of information technology, electronic products have become more concerned with the issues of lightness and long lifespan. The higher demands are being made on the volume energy density of batteries. LiCoO₂ (LCO), the first commercially available cathode material, continues to occupy an important position in portable electronics and consumer electronics, despite the fact that the price of cobalt has been rising, because LCO has the advantages of high theoretical capacity (275 mAh g⁻¹), high operating voltage, high bulk energy density, and excellent electronic conductivity [1,2]. However, the LCO has a discharge capacity of only 140 mAh g⁻¹ when the charge cut-off voltage is at 4.2 V in practical applications. When the charging cut-off voltage is increased, the discharge capacity also rises, rising to 180 mAh g⁻¹ at the 4.5 V charging cut-off voltage and even to 220 mAh g⁻¹ at the 4.6 V cut-off voltage [3,4]. However, conventional electrolytes such as carbonates and ethers cannot withstand high voltages above 4.5 V and will decompose, failing to allow stable cycling of the battery. In addition, increasing the charge cut-off voltage will deteriorate the cathode/electrolyte interface and cause irreversible phase transitions, resulting in rapid capacity decay [5,6,7,8].

Researchers have devoted considerable effort to stabilizing the cathode structure and cathode/electrolyte interface at high voltages, including element doping [9,10,11,12,13], surface coating [14,15,16], binder improvement [5,17], separator modification [18,19,20], and electrolyte optimization [21,22,23,24]. Among these strategies, elemental doping obviously adjusts the basic physical properties of the electrode material. However, some inactive elements are also introduced, lowering the energy density of the cathode. Although the coating method is an effective way to improve the electrochemical performance of high-voltage LCO, the coating layer can also lead to high interfacial resistance. Based on cost and procedure considerations, electrolyte modification is a convenient and promising method. Traditional organic carbonate electrolytes, although preferred for commercial lithium-ion batteries, are not suitable for lithium metal batteries. This is because carbonate electrolytes cannot form a stable electrode/electrolyte interface, resulting in a continuous reaction between electrolyte and lithium metal, leading to low Coulombic efficiency and rapid capacity decay. Although ether electrolytes have good compatibility with lithium metal, the low oxidation decomposition potential (<4 V vs. Li⁺/Li) limits their applications in high-voltage cathodes. More importantly, both carbonate and ether electrolytes have low flash points and are highly flammable, leading to safety hazards in the event of thermal runaway of the battery. The strategies for electrolyte modification in high voltage include additives [22], fluorinated solvents [25], localized high concentration electrolytes [26], deep eutectic solvents [27], and solid-state electrolytes [28] have been investigated. Among these, deep eutectic solvents have attracted a lot of attention because of their advantages, such as non-flammability, low cost, and ease of preparation. Fu’s group reported a new deep eutectic electrolyte consisting of 2,2′-dipyridyl disulfide and LiTFSI with a relatively low ionic conductivity of 1.5 × 10⁻⁴ S cm⁻¹ at 50 °C [29]. Therefore, the issues of low ionic conductivity and high viscosity still need to be addressed. Moreover, there are few reports regarding the use of deep eutectic electrolytes in high voltage LCO cathodes, and the optimization of electrolytes compatible with high voltage cathodes is still required.

According to some previous studies, LiDFOB as an additive can facilitate the formation of a stable cathode electrolyte interface (CEI) due to its high HOMO, which can be preferentially oxidized and involved in the formation of CEI film [30,31]. Furthermore, LiDFOB can be preferentially reduced to form stable SEI films, which effectively inhibits the side-reaction between SN and lithium metal [27,32]. In this work, we reported a non-flammable dual salt deep eutectic solvent electrolyte (DES) containing LiTFSI, LiDFOB, and SN in different molar ratios. This well-designed electrolyte possesses high thermal stability, high ionic conductivity (4.23 mS cm⁻¹, 25 °C), a wide electrochemical window (>5.5 V), and a high Li⁺ transport number (0.75). We also investigated the effect of different LiTFSI and LiDFOB molar ratios on the electrochemical performance of high voltage LCO cathodes. By benefitting from the favorable characteristics of the dual salt DES electrolyte, the LCO||Li cells demonstrated superior cycling performance. The capacity is 111.5 mAh g⁻¹ with a cut-off voltage of 4.45 V after 600 cycles at 0.5 C and 91.1 mAh g⁻¹ with a cut-off voltage of 4.6 V after 500 cycles at 1 C.

2. Materials and Methods

2.1. Material

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium difluoro(oxalato)borate (LiDFOB) and succinonitrile (SN) were all obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). LiCoO₂ powder was purchased from Guangdong Canrd New Energy Technology Co., Ltd. (Zhaoqing, China).

2.2. Preparation of Electrolytes

The dual salt deep eutectic solvent (DES) was prepared by mixing LiTFSI, LiDFOB, and SN at 50 °C until the solution became clarified. The molar ratios of LiTFSI, LiDFOB, and SN are 0.7:0.3:10, 0.8:0.2:10, and 0.9:0.1:10, respectively, and are defined as DES-1, DES-2, and DES-3, respectively. All DES electrolytes were prepared in an argon-filled glove box.

2.3. Characterization

The viscosity of electrolytes was tested at 25 °C using an A&D SV-1A vibrational viscometer. The ionic conductivity of the DES electrolytes was carried out on a Leici 308F conductivity meter at 25 °C. Thermogravimetric analyses were recorded from 30 °C to 600 °C in a nitrogen atmosphere with a heating rate of 10 °C/min.

2.4. Electrochemical Measurements

The LiCoO₂ cathode was prepared by dispersing LCO material, Super P, and polyvinylidene fluoride in a mass ratio of 8:1:1 through an FA25 superfine homogenizer with N-methyl-2-pyrrolidone as solvent. The coated electrodes were dried in a vacuum at 80 °C for 12 h. The mass loading of active material was controlled at 2.0~2.5 mg cm⁻². Lithium metal (450 µm) was used as the anode, and Glass fiber (Whatman GF/A) was used as the separator. A total of 80 µL of deep eutectic electrolyte was used for cell testing. The carbonate electrolyte (1 M LiPF₆ EC-DEC) was used as a comparison. All the cells were assembled in the Ar-filled glove box (Vigor, H₂O/O₂ < 0.2 ppm) and tested at 25 °C. Linear scanning voltammetry was measured by Li||SS cells from 1 V to 6 V with a sweep rate of 1 mV s⁻¹. The cyclic voltammetry (CV) was carried out by LCO||Li cells from 3.0–4.6 V with a scan rate of 0.1 mV s⁻¹. The rate performance and cycling performance of LCO||Li cells were tested on the NEWARE BTS-51 battery test system. The EIS test was performed on a CHI 760E electrochemical workstation from 10 MHz to 0.1 Hz. The Li⁺ transference number of deep eutectic electrolytes was performed by Li||Li cells and calculated by the following equation:

$$t_{L{i}^{+}}=\frac{I_s{R}_b\left(s\right)\left[\Delta V-I_0{R}_{ct}\left(0\right)\right]}{I_0{R}_b\left(0\right)\left[\Delta V-I_s{R}_{ct}\left(s\right)\right]}$$

where $\Delta V$ is the applied potential of 5 mV, R_b(0) and R_b(s) stand for the initial and final.

Resistance of deep eutectic electrolytes, respectively. The R_ct(0) and R_ct(s) are the initial and after polarization interfacial resistance of the cell, respectively. I₀ and I_s are the initial and the steady-state currents after polarization, respectively.

3. Results and Discussion

In this work, we prepared three dual-salt DES electrolytes containing LiTFSI, LiDFOB, and SN in different molar ratios (Figure 1). The composition of the different DES electrolytes is shown in

Table 1. The composition of different DES electrolytes.

Name	LiTFSI (Molar)	LiDFOB (Molar)	SN (Molar)
DES-1	0.7	0.3	10
DES-2	0.8	0.2	10
DES-3	0.9	0.1	10

. The physicochemical properties of different DES electrolytes are shown in Figure 2. The ionic conductivity increased with decreasing LiDFOB content. The ionic conductivity is 3.91, 4.23, and 4.35 mS cm⁻¹ for DES-1, DES-2, and DES-3, respectively, which is higher than some reported DES electrolytes [29,33,34]. As illustrated in Figure 2b, the viscosity of three DES electrolytes is not significantly different, which range between 23 and 27 mPa s. SN containing C≡N bond has a stronger coordination ability with Li+, which weakens the intermolecular forces of Li⁺ and TFSI⁻ and DFOB⁻ thus lowering the melting point and forming a eutectic liquid. Furthermore, the high polarity of SN facilitates the dissociation of lithium salts. These results contribute to the lower viscosity of dual-salt deep eutectic electrolytes. The high conductivity and low viscosity of DES electrolytes can be attributed to the good ability of SN to dissociate lithium salts.

The electrochemical window is a key factor in evaluating the properties of an electrolyte. An electrolyte with high electrochemical stability at high voltages matched with high voltage electrode materials can effectively improve the energy density of the battery. Figure 2c presents the LSV curves for three DES electrolytes. The oxidation resistance of all electrolytes exceeds 5.5 V (vs. Li/Li⁺), which allows matching with high voltage cathodes, such as high voltage LCO electrodes, even at operating voltages up to 4.6 V.

The ion transference number of an electrolyte is as important as its ionic conductivity. We further measured the Li⁺ transference numbers for three DES electrolytes by using Li||Li symmetric cells. As depicted in Figure 3a–c, DES-2 exhibits the highest Li⁺ transference number of 0.75, which is higher than DES-1 of 0.59 and DES-3 of 0.54. In contrast, the Li⁺ transference number of carbonate-based electrolytes is only 0.2~0.4 [35,36,37,38]. The high lithium-ion transference number reduces polarization during charging and discharging, enabling the cell to cycle stably at higher current densities.

The high thermal stability and non-flammability of the electrolyte can reduce the risk of battery in case of thermal runaway or short circuit. To further verify the potential of DES as a safe electrolyte, flammability tests were carried out on conventional electrolytes and three DES electrolytes. As shown in Figure 3d–g, the conventional electrolyte is easily ignited and poses a significant safety hazard in practical applications (Figure 3d). In contrast, all DES electrolytes cannot be combusted, which is attributed to the inherent non-flammability of SN. This result demonstrates the remarkable safety of DES electrolytes.

The above experimental results demonstrate the advantages of the DES electrolyte with its non-flammability, high ionic conductivity, high Li⁺ transference number, and oxidation potential of up to 5.5 V. To take advantage of the DES electrolyte, we assembled cells with a high voltage LCO cathode and lithium metal as the anode to evaluate its electrochemical performance at high voltages. The cyclic voltammetry (CV) was performed to understand the electrochemical characteristics of high voltage LCO||Li cells with DES electrolytes. As depicted in Figure 4a–c, the three DES electrolytes exhibit similar oxidation and reduction peaks in the first three cycles. The good reversible redox peaks demonstrate that the phase transition of LCO at high voltage (3.0–4.6 V) is highly reversible in the DES electrolyte. Furthermore, EIS measurements indicate that all three DES electrolytes possess a low interfacial resistance (Figure 4d).

Figure 5a,b demonstrates the rate performance of three DES electrolytes; the DES-1 delivers a reversible capacity of 170.9, 168.6, 162.8, 157.9, and 152.2 mAh g⁻¹ at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively, and the capacity recovers to 170.3 mAh g⁻¹ after returning to 0.1 C. DES-2 exhibits better rate performance than DES-1, with a relatively high capacity of 188.3, 181.1, 171.9, 165.8, and 159.2 mAh g⁻¹, at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively, and the high capacity recovers to 187.8 mAh g⁻¹ after returning to 0.1 C. In addition, the DES-3 delivers a reversible capacity of 178.8, 175.4, 169.8, 164.8, and 158.3 mAh g⁻¹ at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively, and the capacity recovers to 177.7 mAh g⁻¹ after returning to 0.1 C. By contrast, carbonate-based electrolyte only delivers a reversible capacity of 174.8, 170.4, 166.5, 161.1, and 153.6 mAh g⁻¹ at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. The better rate performance of DES-2 is mainly attributed to the higher Li⁺ transference number, which facilitates the rapid Li⁺ migration and reduces cell polarization.

The long cycling performance of DES electrolytes was further evaluated in LCO||Li cells under 3.0–4.45 V and 3.0–4.6 V. As shown in Figure 5c, the cell with DES-2 delivers a high capacity of 111.5 mAh g⁻¹ after 600 cycles at 0.5 C under 3.0–4.45 V, which is higher than DES-1 of 97.9 mAh g⁻¹ and DES-3 of 77.5 mAh g−1. In contrast, carbonate-based electrolytes exhibit a relatively low initial capacity of 153.9 mAh g⁻¹ and suffer from rapid capacity decay. Figure 5f–h shows the voltage–capacity curve of three DES electrolytes with different cycles under 3.0–4.45 V. The results demonstrate that the initial Coulombic efficiencies of DES-1, DES-2, and DES-3 are 87.9%, 91.2%, and 63.1%, respectively. Moreover, DES-2 exhibits a lower polarization during cycling. Moreover, the cells display outstanding cycling stability, with the reversible capacity of 112.1, 110.3, and 105.0 mAh g⁻¹ for DES-1, DES-2, and DES-3 after 700 cycles at 2 C, respectively (Figure 5d), which is much higher than carbonate-based of 54.8 mAh g⁻¹. Notably, DES-2 exhibited the highest average Coulomb efficiency of 99.6%, higher than DES-1 at 97.6% and DES-3 at 99.4%. To further demonstrate the good oxidation resistance of DES, LCO||Li cells were cycled at 3.0–4.6 V. The DES-3 delivers the highest initial discharge capacity of 208.9 mAh g⁻¹ at 1 C. In addition, all DES electrolytes can be stably cycled for 500 cycles at 1 C and under the voltage of 3.0–4.6 V (Figure 5e,i–k). The carbonate-based only delivers a reversible capacity of 89.7 mAh g⁻¹ after 200 cycles at 3.0–4.6 V and 1 C. Consequently, the dual-salt DES electrolyte exhibits excellent cycling performance in high voltage LCO||Li cells (

Table 2. A comparison with some previous reports on cycling performance.

Strategy	Voltage	Rate	Cycling	Retention	Ref.
PVDF/PVAC-Based CPE	3.0–4.5	0.5 C	200	85%	[39]
Sulfonamide-based electrolyte	3.0–4.55	0.3 C	200	89%	[40]
0.2% ATCN	3.0–4.5	1 C	200	91%	[41]
LiTFSI-P13FSI-TTE	3.0–4.3	0.5 C	350	80%	[26]
SN-DLi-FEC	3.0–4.4	0.5 C	200	85%	[27]
10% MSM + DMC/FEC/HFE	2.75–4.45	200 mA/g	300	87.1%	[25]
Dual-salt Deep Eutectic Electrolyte	3.0–4.45	0.5 C	200	94%	This work
	3.0–4.45	2 C	500	72%
	3.0–4.6	1 C	200	70%

).

4. Conclusions

In summary, a non-flammable dual-salt DES has been designed as a safe electrolyte for high voltage LCO||Li cells owing to the inherent non-flammability of SN. Comparative and comprehensive studies were carried out on the ion conductivity, viscosity, electrochemical window, Li⁺ transference number, rate performance, cycling performance, and Coulombic efficiency for different molar ratios of DES. As a result, DES-2 possesses a better rate performance and stable cycling performance with high Coulombic efficiency owing to its high ion conductivity and higher Li⁺ transference number. In detail, the LCO||Li cell with DES-2 delivers a high reversible capacity of 110.3 mAh g⁻¹ with high Coulombic efficiency of 99.6% at 2 C under 3.0–4.45 V. This work broadens the application of deep eutectic solvent as a safe electrolyte in high safety and high voltage lithium metal batteries.