Effects of Li Salt and Additive Content on the Electrochemical Performance of [C₄C₁mim]-Based Ionic Liquid Electrolytes

Abstract

Ionic liquids based on imidazolium cations have attracted attention due to their high safety and exceptional ionic conductivity. However, imidazole-based ionic liquids exhibit poor electrochemical stability due to the strong reactivity of hydrogen atoms at the C-2 position of imidazole cations. In this work, an ionic liquid 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([C₄C₁mim][TFSA]), characterized by a methyl-substituted C-2 position and a butyl chain, was investigated in various Li⁺ environments created by different lithium salt concentrations and fluoroethylene carbonate (FEC) additives. Both optimal Li⁺ concentrations and the addition of reasonable FEC enable the improvement of ionic conductivity to 3.32 mS cm⁻¹ at 25 °C and a maximum electrochemical window of 5.21 V. The ionic liquid electrolyte Li[TFSA]-[C₄C₁mim][TFSA] at a molar ratio of 2:8 with 5 wt% FEC addition demonstrates excellent thermal stability. The corresponding Li/LiFePO₄ cell exhibits a mitigated polarization growth (increasing from 0.12 V to 0.25 V over 10 cycles) with a high initial discharge capacity of 169.3 mAh g⁻¹.

1. Introduction

Ionic liquids (ILs), a class of liquid materials that melt near room temperature, are composed of asymmetric salts formed from positive and negative ions [1]. Due to their distinctive physical and chemical properties—such as low volatility, high flame retardancy, and superior thermal stability, which are not typically found in molecular liquids—ionic liquids have garnered significant interest in recent years as potential next-generation electrolytes with enhanced safety features [2,3]. Numerous studies have demonstrated that the physical and chemical properties of ionic liquid electrolytes—such as lithium diffusion coefficient, ionic conductivity, and viscosity—are significantly affected by the choice of cations [4,5]. Among the various types of ionic liquids utilized as electrolytes in lithium batteries, imidazolium cations have emerged as the most promising candidates for developing high-performance ionic liquid electrolytes. This is largely due to the fact that ionic liquids based on imidazolium cations generally exhibit lower viscosity, higher ionic conductivity, and enhanced permeability [6,7]. These attributes contribute to wide electrochemical steady windows and improved specific capacity in batteries, thereby establishing a strong foundation for the advancement of high-energy density lithium metal batteries.

However, the carbon atom at the C-2 position of the imidazole cation is prone to protonation by acids, and the hydrogen atom at this position is highly reactive, leading to poor electrochemical stability in imidazole-based ionic liquids [8]. To enhance the electrochemical stability, strategies such as introducing substituents or increasing the length of alkyl chains have been employed [9,10,11,12,13]. A type of ionic liquid, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([C₄C₁mim][TFSA]), featuring the cation with a methyl substitution at the C-2 position and a butyl chain, has been synthesized and reported with enhanced safety compared to conventional carbonate-based electrolytes [14,15,16]. Further studies have demonstrated that substituting the C-2 hydrogen atoms on imidazolium rings can broaden the electrochemical window to approximately 5 V while also achieving high ionic conductivity. These attributes position it as an ideal candidate for further exploration as a potential electrolyte for Li-ion power sources [8,17]. The ionic liquid [C₄C₁mim][TFSA] has been studied by groups interested in their applications in batteries [18] and supercapacitors [19]. In addition to its high electrochemical activity, it has also garnered significant interest for synthesizing Li metal to create artificial LiF-rich SEI layers [20]. However, there are currently very few reports on the effects of changes in the Li⁺ environment of IL [C₄C₁mim][TFSA] on the stable electrochemical performance of lithium-ion systems.

Additionally, recent research has extensively focused on IL-based electrolyte systems with varying lithium salt concentrations to enhance the stability of lithium metal batteries. Specifically, high concentrations of lithium salts enhance the electrochemical stability of the electrolyte by reducing the availability of reaction solvents and facilitating anion reduction, particularly at medium to high temperatures [21,22]. In this context, in order to achieve high performance of ionic liquids (such as reducing the viscosity of ionic liquids, improving ionic conductivity, and high stability of ionic liquids), methyl substitution is introduced at the C-2 position of imidazole cations with a butyl chain ([C₄C₁mim]⁺). On this basis, investigating the conductivity and other electrochemical properties of ILs at different lithium salt concentrations holds significant scientific value, which will provide a theoretical basis for the development of a new generation of highly safe IL electrolytes. Additionally, incorporating additives into ILs can significantly enhance ion transport performance and improve the properties of the solid electrolyte interface (SEI) [23]. However, there is currently no literature addressing the effects of additives in IL [C₄C₁mim][TFSA] on specific electrochemical properties. Therefore, this study will also discuss the impact of additives on the properties of IL [C₄C₁mim][TFSA] to address this gap.

Herein, the ILs system with different Li⁺ environments based on [C₄C₁mim][TFSA] was investigated, where IL [C₄C₁mim][TFSA] was synthesized using a solvent-free and catalyst-free ion exchange method. The influence of the Li⁺ environment generated by varying concentrations of the lithium salt Li[TFSA] and the additive fluoroethylene carbonate (FEC) on the electrochemical performance of Li battery was examined using several techniques, including thermogravimetric analysis (TG), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and charge–discharge tests.

2. Materials and Methods

2.1. Synthesis of [C₄C₁mim][TFSA] IL

The synthesis of [C₄C₁mim][TFSA] IL was performed via a solvent-free and catalyst-free ion exchange method, building on previous work [14,15]. This approach significantly minimizes the environmental footprint compared to traditional organic solvent-based methods. Firstly, 0.046 mol of 1,2-dimethylimidazole (Aladdin, Shanghai, China) was mixed with an equimolar amount of n-butyl bromide (Aladdin, Shanghai, China) in a round-bottom flask at 60 °C for 24 h to synthesize the reaction intermediate [C₄C₁mim]Br with white crystals. The reaction mixture was then treated with 10 mL of water and heated until all the crystals were completely dissolved, followed by mixing with 0.05 mol of Li[TFSA] (Picasso, Shanghai, China) in 10.0 mL of water. The mixture was then stirred at 35 °C for 24 h to promote the replacement of Br⁻, resulting in the formation of two distinct phases that were easily separated. The phase containing the ionic liquid was washed several times with water to remove LiBr. Finally, the ionic liquid was dried at −55 °C for 12 h, yielding [C₄C₁mim][TFSA] as a colorless liquid.

2.2. Synthesis of IL-Based Electrolytes

After freeze-drying, the synthesized [C₄C₁mim]-based ILs were transferred to a glove box filled with Ar to prepare the electrolytes. Li[TFSA] powder and IL [C₄C₁mim][TFSA] were weighed in molar ratios of 2:8 and 3:7, respectively, and then mixed with stirring at 35 °C for 12 h to prepare Li[TFSA]-[C₄C₁mim][TFSA] IL electrolytes with different Li salt concentrations (labeled as IL2:8 and IL3:7). Afterward, FEC additives were weighed at concentrations of 3 wt%, 5 wt%, and 9 wt%, and added to the above-prepared IL2:8 electrolyte to generate different [C₄C₁mim]-based IL comparison systems, labeled as IL2:8+3%, IL2:8+5%, and IL2:8+9%. Additionally, an IL3:7+5 wt% FEC electrolyte was prepared as a reference.

2.3. Physicochemical and Electrochemical Characterization

A thermal analysis instrument (NETZSCH TG 209 F3, Selb, Germany) was used to evaluate the thermal stability of the [C₄C₁mim]-based IL electrolytes from 25 to 600 °C at a rate of 10 °C min⁻¹ under a nitrogen flow of 20 mL min⁻¹. Differential scanning calorimetry (DSC, NETZSCH DSC 214 Polyma, Selb, Germany) was conducted between 25 to 550 °C under a nitrogen flow at a rate of 20 °C min⁻¹. The chemical structure of [C₄C₁mim]-based IL electrolytes was determined with the Fourier transform infrared spectrometry (FTIR, Thermo Fisher Nicolet iS50, Waltham, MA, USA) method. The electrochemical impedance spectroscopy (EIS) from 0.1 MHz to 0.01 Hz was performed to measure the conductivities of various IL electrolytes on a CHI760E electrochemical workstation, where the electrolytes were sealed in an airtight T-shaped cell equipped with two stainless-steel (ss) disk electrodes (symmetrical SS/ILs/SS) under an atmosphere of dry Ar. The cell constant was determined using 0.1 M KCl aqueous solution. The ionic conductivity (σ) of the IL electrolytes was calculated based on the following Equation (1).

σ = L/(R × S),

(1)

where L is the thickness of the electrolyte, R is the impedance of the symmetrical SS/ILs/SS cells, and S is the electrode area.

Linear sweep voltammetry (LSV) was conducted using the CHI760E (Chinstruments, Shanghai, China) electrochemical workstation with a Li/ILs/SS asymmetrical cell over a potential range of 2.5−6.5 V (Li/Li⁺) at a scanning rate of 0.001 V s⁻¹. The electrochemical performance of the prepared IL systems was evaluated by assembling a 2032-type coin cell (NEWARE, Shenzhen, China) with Li metal and LiFePO₄ as electrodes. The cell was galvanostatically cycled within a voltage range of 2.5–4.2 V at a rate of 0.025C (where 1C = 170 mAh g⁻¹) using a battery testing system (Neware CT4008T, Shenzhen, China).

3. Results and Discussion

To investigate the thermal stability of [C₄C₁mim][TFSA]-based ILs (the basic structure of IL [C₄C₁mim][TFSA] is shown in Figure 1) and the effect of adding FEC on its thermal properties, TG analysis was performed on Li[TFSA]-[C₄C₁mim][TFSA] IL electrolytes at a molar ratio of 2:8 and those containing 5 wt% FEC (Figure 2a). The results indicated a decomposition temperature of 167 °C, at which an approximate 5 wt.% loss of the IL electrolyte comprising FEC occurred, fully corresponding to the loss of the FEC [24]. In addition, Li[TFSA]-[C₄C₁mim][TFSA] ILs demonstrate high thermal stability and typical two-step decomposition behavior, with the maximum derivative mass loss (DTG) at 489 °C and a second DTG at 569 °C. This temperature range and decomposition behavior of [C₄C₁mim]-based ILs during thermal decomposition are consistent with previous reports [18,19]. These TG results indicated [C₄C₁mim]-based ILs can be used as electrolytes in a wider range of temperatures. The thermal properties of these two ILs were further examined by DSC over a temperature range of 25 to 550 °C. As illustrated in Figure 2b, both IL2:8 and IL2:8+5% exhibit a thermal decomposition temperature of approximately 400 °C, which aligns with the TG results, indicating the minor impact of FEC on the thermal properties of [C₄C₁mim]-based ILs. Additionally, the presence of the alkyl group at C-2 in [C₄C₁mim][TFSA] raises the decomposition temperature from 423 to 430 °C while lowering the melting point from −3 to −13 °C, attributed to the geometric packing constraints of the planar imidazolium rings [7,12,25].

FTIR spectroscopy was employed to investigate the interaction that occurred between IL pure [C₄C₁mim][TFSA] IL, Li[TFSA] salt, and FEC additive. The FTIR spectra obtained in the case of pure IL [C₄C₁mim][TFSA], IL2:8 composed of Li[TFSA]-[C₄C₁mim][TFSA], and IL2:8+5% electrolytes are shown in Figure 3, respectively. For the pure IL [C₄C₁mim][TFSA], the bands appearing between 700 and 800 cm⁻¹ may be mainly ascribed to contributions from ring bending modes of the imidazolium cation [26]. The spectral range of 1000–1400 cm⁻¹ shows the most intense IR bands, which are dominated by vibrations of the [TFSA]⁻ anion [27]. Moreover, the bonds at 571 and 617 cm⁻¹ correspond to the cis- and trans-conformers of the [TFSA]⁻ anion, respectively [27]. The peaks appearing between 1400–1600 cm⁻¹ are likely to be the ring bending modes in the spectra of [C₄C₁mim]⁺ [26]. The CH₂/CH₃ stretching vibrations of [C₄C₁mim]⁺ are observed at 2880, 2937, and 2968 cm⁻¹ [28], while the C–H stretching vibrations of the imidazolium cation ring are observed at 3152 and 3190 cm⁻¹ [27]. Most of these bands show no obvious changes in their spectral feature when the lithium salt Li[TFSA] is dissolved in [C₄C₁mim][TFSA] ionic liquid solution. This result indicates minimal structural perturbation of the [C₄C₁mim]⁺ cation and [TFSA]⁻ anion by Li⁺ coordination. However, a slight broadening of the [TFSA]⁻ conformer peaks (571/617 cm⁻¹) suggests weak Li⁺–[TFSA]⁻ interactions. Upon dissolving FEC in IL2:8, several notable differences in the spectral characteristics emerge. The first significant change between the spectra of ILs with FEC and without FEC is observed at 866 cm⁻¹, where a new vibrational mode appears in the spectrum of IL2:8+5%FEC. This mode is likely attributed to the stretching vibrations of the carbonate (CO₃) group [29]. Additionally, two new high-intensity bands appear at 1809 and 1834 cm⁻¹, corresponding to the stretching vibrations of C=O, likely representing the carbonyl stretch of the free FEC solvent [30]. The existence of the surface-absorbed H₂O in both IL2:8 and IL2:8+5% is monitored by the water bending mode band, the O–H stretching and H–O–H bending vibrations observed at 3472 and 1634 cm⁻¹, respectively [27,31]. These signals are likely due to residual moisture introduced by the hygroscopic Li[TFSA] salt. These spectral changes demonstrate that FEC interacts with the IL/Li salt system primarily through physical mixing rather than chemical bonding, while Li⁺ coordination weakly affects the [TFSA]⁻ conformation.

The impedance spectroscopy of the symmetrical blocking cells (SS/ILs/SS) was used to study the conductivity of various ionic liquid electrolytes at 25 °C, as shown in Figure 4, in which the ionic conductivity (Figure 4b) was calculated based on the Equation (1) according to the Nyquist plot shown in Figure 4a [32,33,34]. The calculation results indicate that the ionic conductivity of pure IL [C₄C₁mim][TFSA] without Li[TFSA] salt is 4.02 mS cm⁻¹. The addition of Li[TFSA] salt with a molar ratio of 2:8 decreases the ionic conductivity to 2.30 mS cm⁻¹. Further increasing the lithium salt concentration to a molar ratio of 3:7 results in an additional drop in ionic conductivity to 1.22 mS cm⁻¹. The observed decrease in ionic conductivity upon the addition of Li salt (Li[TFSA]) to [C₄C₁mim][TFSA] is primarily attributed to an increase in viscosity, which in turn lowers ion mobility and leads to decreased conductivity [35]. This drawback can be compensated by introducing the FEC additive, which enables the operation of ILs at high Li⁺ contents. Particularly, the addition of 5% FEC significantly boosts ionic conductivity to 3.32 mS cm⁻¹, while 3% FEC yields a value of 2.35 mS cm⁻¹, which is nearly equivalent to that of the IL2:8 electrolyte. Similarly, adding 5% FEC to IL3:7 electrolyte also has a positive effect on ionic conductivity, reaching a value of 2.11 mS cm⁻¹. Furthermore, increasing the FEC concentration to 9% leads to a substantial rise in ionic conductivity to 4.21 mS cm⁻¹. The increase in ionic conductivity resulting from the addition of FEC can be attributed to the lowering of electrolyte viscosity, which facilitates freer movement of ions. This improved ion mobility is directly linked to the increased ionic conductivity, facilitating easier ion migration [36,37]. The increase in ionic conductivity will significantly enhance the performance of Li-ion batteries. Additionally, the impact of FEC concentration on conductivity was investigated in the IL2:8 system. As illustrated in Figure S1 (in Supplementary Materials), an excessive addition of FEC reveals a peak trend in ionic conductivity, evidenced by a lower conductivity of 3.53 mS cm⁻¹ at IL2:8+11% concentration. This phenomenon is likely attributable to the high viscosity of FEC, where excessive amounts increase overall viscosity, thereby impeding ion migration.

The electrochemical stability window of various IL-based electrolytes was assessed using linear sweep voltammetry (LSV) within a voltage range of 2.5–6.5 V (vs. Li⁺/Li). The onset decomposition voltage is defined as the voltage corresponding to a current density of 5 μA cm⁻², consistent with previous studies [35,38]. As shown in Figure 5, the IL3:7 electrolyte exhibits a low oxidation onset voltage of 3.18 V, whereas the IL2:8 electrolyte demonstrates significantly improved stability with an onset stability of 4.26 V. This indicates that reducing the Li salt concentration (from 3:7 to 2:8 molar ratio) enhances the oxidative stability of the IL electrolyte. Further improvement is achieved by introducing FEC additives. For the IL3:7 electrolyte, adding 5 wt% FEC increases the oxidation onset voltage to 4.51 V. Similarly, for the IL2:8 electrolyte, the oxidation onset voltage progressively rises with increasing FEC content: 4.58 V (3 wt% FEC), 4.67 V (5 wt%), and 4.68 V (9 wt%). Notably, the IL2:8 electrolytes with 5% and 9% FEC exhibit oxidation stability close to the reported value of 4.7 V for imidazolium-based ILs with [TFSA]⁻ anions under identical conditions [38]. The enhanced oxidation window (≥4.5 V) of these optimized electrolytes exceeds the operational voltage range of LiFePO₄ cathodes (2.5–4.0 V), confirming their compatibility with high-voltage cathodes and potential to mitigate oxidative degradation during cycling.

The electrochemical performance of the prepared IL-based system was evaluated by assembling 2032-type coin cells with Li metal anodes and LiFePO₄ cathodes. Figure 6 presents the charge–discharge curves and cycling performance of Li/LiFePO₄ batteries assembled with IL2:8 and IL3:7 electrolytes. For the IL2:8 electrolytes (Figure 6a), the charge–discharge curves exhibit distinct plateaus at 3.49 V/charge and 3.15 V/discharge (Figure 6(a-i)), corresponding to the Li⁺ extraction/insertion process in the olivine structure (space group: Pnma) (charging: LiFePO₄ − xLi⁺ − xe⁻ → xFePO₄ + (1 − x)LiFePO₄; discharging: FePO₄ + xLi⁺ + xe⁻ → xLiFePO₄ + (1 − x)LiFePO₄) [39]. The first cycle delivers an initial discharge capacity of 170.0 mAh g⁻¹, matching the theoretical capacity of LiFePO₄, with a high Coulombic efficiency of 96.8%. The initial charge capacity exceeds the theoretical capacity of the LiFePO₄ positive electrode material. This may result from side reactions occurring between LiFePO₄ material and the IL electrolyte, as well as the formation of a stable interfacial layer between them. With the repeated cycling of the Galvanostatic charge–discharge process, the discharge capacity shows a slight variation (Figure 6(a-ii)), but the voltage difference ΔV between the charge–discharge plateaus, which reflects the polarization of the battery system, exhibits a significant change, increasing from 0.13 V in the first cycle to 0.47 V in the 10th cycle [40,41]. This result indicates that the polarization of Li/LiFePO₄ with IL2:8 in the lithium-ion battery gradually intensifies with an increasing number of cycles. To investigate the influence of Li⁺ concentrations on the electrochemical behavior, charge–discharge tests were conducted on Li/LiFePO₄ batteries assembled with Li[TFSA]:[C₄C₁mim][TFSA] at a 3:7 ratio. These tests were performed under the same conditions as those for IL2:8 electrolytes, as shown in Figure 6b. Compared to the results obtained within the IL2:8 electrolyte, the IL3:7 electrolyte shows inferior performance. The initial discharge capacity is reduced to 161.9 mAh g⁻¹, with a lower Coulombic efficiency of 95.1% (Figure 6(b-i)). These values consistently exhibit a trend of being lower than those of IL2:8 during repeated charge–discharge testing experiments (Figure 6(b-ii)). Additionally, the voltage difference ΔV in the first cycle is measured as 0.18 V, reflecting greater polarization compared to that measured in the IL2:8 electrolyte. This voltage difference increases significantly with the number of cycles, rising from 0.18 V to 1.00 V after 10 cycles. The performance degradation in IL3:7 is primarily attributed to the higher Li salt concentration, which increases electrolyte viscosity, reduces ionic conductivity, and narrows the electrochemical stability window. These factors collectively exacerbate polarization and hinder Li⁺ transport, leading to capacity fading and lower efficiency. The optimization of Li[TFSA] salt concentration was performed by further evaluating the charge–discharge test in IL1:9 and IL4:6 electrolytes. As depicted in Figure S2 (Supporting Information), IL4:6 exhibits extremely low reversible capacity (about 50 mAh g⁻¹) and severe polarization (ΔV~0.37 V over three cycles), likely due to excessive viscosity and Li⁺-[TFSA]⁻ aggregation. IL1:9 electrolyte shows higher capacity of over 100 mAh g⁻¹ compared to IL4:6, but it still underperforms relative to IL2:8 and IL3:7. Therefore, IL2:8 was selected as the optimal Li salt concentration for further studies on the effects of the FEC additive in [C₄C₁mim]-based IL electrolytes.

The impact of the FEC additive on the polarization of the LiFePO₄ electrode in the BMMI-based IL electrolyte was further examined in the IL2:8 electrolyte. Figure 7a–c depicts the charge–discharge curves and cycling performance obtained with the addition of 3 wt%, 5 wt%, and 9 wt% of FEC, respectively. The charge–discharge curves of the Li/LiFePO₄ cell in IL2:8+3 wt% electrolyte (Figure 7(a-i)) exhibit clear plateaus; however, the initial discharge capacity is only 79.8% of the theoretical capacity of LiFePO₄, with an initial Coulombic efficiency of 91.8%, both significantly lower than those measured in IL2:8 electrolyte without additives. Additionally, as the number of cycles increases, the voltage difference ΔV rises significantly from 0.24 V in the first cycle to 1.43 V by the 8th cycle, while the discharge capacity shows a marked decline (Figure 7(a-ii)). This trend signifies a notable decline in performance compared to that observed in IL2:8 electrolyte, suggesting that the addition of 3% FEC in IL2:8 electrolyte has no positive effect on the electrochemical behavior. The Li/LiFePO₄ battery assembled with the IL2:8+5% FEC electrolyte exhibits a higher discharge capacity of 169.3 mAh g⁻¹, which is close to the theoretical capacity of the LiFePO₄ positive electrode (Figure 7(b-i)). The first charge capacity exceeds the theoretical capacity of the LiFePO₄ positive electrode material, with an initial Coulombic efficiency of 97.4%. This phenomenon may be due to side reactions occurring in the IL-based electrolyte on the surface of the LiFePO₄ electrode, leading to the formation of a stable interfacial layer. The discharge capacity during 10 cycles exhibits minimal fluctuation, with Coulombic efficiency exceeding 99% (Figure 7(b-ii)), revealing that FEC effectively reduces the irreversible capacity loss of the LiFePO₄ electrode. Additionally, the voltage difference ΔV of the first charge–discharge curve is 0.12 V, indicating that its initial polarization is significantly lower than that measured in the IL2:8 electrolyte. Most importantly, this voltage difference shows minimal variation during the cycling test, increasing only to 0.25 V by the 10th cycle. This increasing trend is significantly lower than that observed during the cycling of the IL2:8 electrolyte, indicating that the addition of 5% FEC effectively alleviates polarization in Li/LiFePO₄ batteries when cycling with the [C₄C₁mim]-based IL electrolyte. This improvement may be attributed to the significant increase in the oxidation window and ionic conductivity resulting from the addition of 5% FEC. The extended cycling performance of IL2:8+5% FEC electrolyte is exhibited in Figure S3 (in Supplementary Materials). The results show excellent capacity retention of 93.9% (159.0 mAh g⁻¹ at the 40th cycle) with a Coulombic efficiency of 101.5%. The slightly elevated Coulombic efficiency may arise from lithium inventory fluctuations caused by partial Li dendrite dissolution during stripping. However, a severe side reaction between the electrolyte and LiFePO₄/Li electrodes occurs at the 42nd cycle, which might be caused by cumulative electrolyte decomposition at high voltages or mechanical stress from repeated Li plating/stripping. These findings highlight the need for further optimization, such as incorporating SEI−stabilizing additives (e.g., LiNO₃) or enhancing Li salt dissociation through anion receptor engineering. While the current 40-cycle data validate the short-term feasibility of the IL2:8+5% FEC electrolyte, we are actively pursuing strategies to mitigate degradation mechanisms and achieve >100 cycles. Future work will focus on electrolyte formulation refinement and advanced characterization to unlock long-term cycling stability. The beneficial effects of adding FEC to IL2:8 are comparable to those of typical carbonate-based and advanced IL-based electrolytes (Table S1 in Supplementary Materials), demonstrating its potential as a stable electrolyte [2,32,42,43,44]. The significant improvement effect of adding 5% FEC was also verified in the IL3:7 electrolyte, as evidenced by the initial voltage difference ΔV of 0.14 V, which slightly increased to 0.25 V after 10 cycles (Figure S4 in Supplementary Materials). Figure 7(c-i) shows the charge–discharge curves of the Li/LiFePO₄ battery in IL2:8+9% FEC. In this electrolyte, the discharge capacity of the Li/LiFePO₄ battery is 165.9 mAh g⁻¹ with an initial Coulombic efficiency of 95.2%. This Coulombic efficiency increases a lot during the following test, while the capacity remains stable at a low trend compared to the IL2:8 electrolyte without additives (Figure 7(c-ii)). In addition, the voltage difference ΔV of the first charge–discharge cycle is 0.13 V, and its polarization is close to that measured in the IL2:8 electrolyte. Furthermore, this voltage difference ΔV slightly increases as the charge–discharge test repeated to 0.26 V in the 5th cycle, which is close to the voltage difference ΔV of the 10th cycle measured in IL2:8+5% electrolyte. This higher growth trend indicates that excessive addition of FEC in [C₄C₁mim]-based IL electrolyte cannot further alleviate the polarization of the battery system during the cycling process, which may be related to the decrease in oxidation voltage.

4. Conclusions

In summary, a series of imidazole-based IL electrolyte systems with different Li⁺ environments was investigated based on [C₄C₁mim] cation with methyl substitution at C-2 position, as well as reasonable Li salt concentration and addition of FEC. Benefiting from the appropriate concentration of Li⁺ and the addition of FEC, the [C₄C₁mim]-based IL2:8+5% FEC electrolyte presents improved ionic conductivity (3.32 mS cm⁻¹ at 25 °C) and the highest electrochemical stability window (5.21 V). Consequently, the Li/LiFePO₄ battery assembled with the IL2:8+5% FEC electrolyte demonstrated enhanced electrochemical performance, featuring a high initial discharge capacity of 169.3 mAh g⁻¹ and reduced polarization growth, rising from 0.12 V to 0.25 V. It is expected that this work can be extended to design [C₄C₁mim]-based electrolytes for high safety lithium metal batteries.