Electrolyte suitable for electric double layer capacitor and preparation method and application thereof

Abstract

This invention discloses an electrolyte suitable for electric double-layer capacitors, its preparation method, and its application, belonging to the field of electrochemical energy storage technology. The electrolyte comprises an organic solvent, an electrolyte salt, and additives, wherein the additives are fluorinated acrylate additives. The electrolyte of this invention, applied to 2.5~2.8V electric double-layer capacitors, can suppress hydrolysis gas generation and electrode micropore blockage induced by trace amounts of moisture under high-temperature float charging through the synergistic effects of ion solvation sheath reconstruction, interface property regulation, and steric hindrance polymerization. This results in a reduction of more than half in cell volume expansion, a reduction of more than 27.9% in internal resistance increase, an improvement of capacity retention of more than 6.4%, and a reduction of more than 59.7% in electrolyte free acid content after 8 weeks of float charging at 65℃ and 2.9V. Furthermore, only trace amounts are needed to significantly extend the high-temperature service life of the cell without sacrificing the initial conductivity and capacity of the electrolyte, greatly improving the high-temperature degradation problem of existing AN-based electric double-layer capacitors.

Description

Technical Field

[0001] This invention relates to the field of electrochemical capacitor technology, specifically to an electrolyte suitable for electric double-layer capacitors, its preparation method and application, and more particularly to an electrolyte including fluorinated acrylate electrolyte additives suitable for acetonitrile-based electrolyte systems and an electric double-layer capacitor cell containing the same. Background Technology

[0002] Electric double-layer capacitors (EDLCs) rely on the physical adsorption-desorption of ions at the electrode / electrolyte interface for energy storage. They do not undergo a Radie reaction and possess advantages such as high power density and long cycle life, making them widely used in renewable energy grid connection and rail transportation. The mainstream commercial electrolyte is a 1 mol / L dimethylpyrrolidone tetrafluoroborate (DMP BF4) / acetonitrile (AN) system, suitable for a rated voltage of 2.9V. However, it has a significant drawback under high-temperature float charging conditions: trace amounts of moisture in the electrolyte (at the ppm level) can induce BF4 degradation. - Hydrolysis produces hydrogen fluoride and small molecule gases; the polar oxygen-containing functional groups on the surface of activated carbon easily adsorb water molecules and accumulate in micropores, accelerating hydrolysis and by-product deposition, leading to cell volume expansion, increased internal resistance, and capacity decay.

[0003] Existing physical dehydration methods cannot completely remove residual water, while chemical dehydration agents are prone to side reactions. Ordinary hydrophobic additives have poor compatibility with acetonitrile, require high dosages, and reduce conductivity, making it difficult to balance high-temperature stability and basic electrochemical performance.

[0004] Therefore, developing a specialized additive with excellent compatibility, requiring only trace amounts and modified by purely physical means has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] To address the problems existing in the prior art, one objective of this invention is to provide an electrolyte suitable for electric double-layer capacitors. By adding a small amount of a fluorinated acrylate additive with uniform structure, high thermal stability, and strong hydrophobicity, multiple physical synergistic effects can be achieved to suppress water-induced hydrolysis and micropore blockage under high-temperature float charging, reduce the increase in internal resistance and volume expansion rate after cell aging, and improve long-term stability at high temperatures.

[0006] Another object of the present invention is to provide a method for preparing such an electrolyte containing the additive and its application in electric double-layer capacitors, which is simple in process and suitable for industrial production.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] An electrolyte suitable for electric double-layer capacitors, the electrolyte comprising an organic solvent, an electrolyte salt, and an additive; wherein the additive is a fluorinated acrylate additive.

[0009] In some specific embodiments, the fluorinated acrylate additive has the following general formula structure:

[0010] CH2=CH-COO-CH(RF)2

[0011] RF is selected from any one of trifluoromethyl (-CF3), pentafluoroethyl (-C2F5), heptafluoropropyl (-C3F7), and difluoromethyl (-CHF2), and the two RF groups may be the same or different from each other.

[0012] In a preferred embodiment, the fluorinated acrylate additive is bis(pentafluoroethyl) methyl acrylate (DFEMA).

[0013] In some specific implementations, the amount of the additive is 0.1wt% to 2.0wt% of the total mass of the organic solvent and electrolyte salt.

[0014] In some specific implementations, the organic solvent is a nitrile, acid ester, or amide organic solvent.

[0015] In some preferred embodiments, the organic solvent is selected from at least one of acetonitrile (AN), propylene carbonate (PC), ethylene carbonate (EC), N,N-dimethylformamide (DMF), and 3-methoxypropionitrile (MPN).

[0016] In some specific embodiments, the electrolyte salt is selected from one or more of tetraethylamine tetrafluoroborate (TEA BF4), spiro-(1,1')-bispyrrolidine tetrafluoroborate (SBP BF4), 1,1-dimethylpyrrolidine tetrafluoroborate (DMP BF4), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm BF4), and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIm FSI), preferably 1,1-dimethylpyrrolidine tetrafluoroborate (DMP BF4).

[0017] In some specific implementations, the concentration of the electrolyte salt is 0.5 mol / L to 2.0 mol / L.

[0018] On the other hand, the aforementioned method for preparing an electrolyte suitable for double-layer capacitors includes the following steps:

[0019] (1) Mix at least two of the organic solvents, namely carbonates, nitriles, and amides, in a certain proportion, and purify them by removing impurities and water to obtain a mixed solvent;

[0020] (2) At room temperature, the electrolyte salt is added to the mixed solvent obtained in step (1) and allowed to stand to dissolve, thus obtaining electrolyte I;

[0021] (3) Add 0.1 to 2.0 wt% of the additives to the electrolyte I obtained in step (2) and let it stand to dissolve, so as to obtain the electrolyte suitable for double-layer capacitors.

[0022] On the other hand, a double-layer capacitor includes the aforementioned electrolyte suitable for a double-layer capacitor or the electrolyte suitable for a double-layer capacitor prepared by the aforementioned preparation method.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] After 8 weeks of float charging at 65°C and 2.9V, the EDLC containing the electrolyte of this invention exhibits the following improvements: cell volume expansion rate decreases from 3.13% to 0.945-1.51%, a reduction of more than half; internal resistance increase decreases from 72.0% to 30.75-44.1%, a reduction of more than 27.9%; capacity retention rate increases from 82.3% to 88.7%-92.5%, an increase of more than 6.4%; and free acid content in the electrolyte decreases from 186ppm to 62-98ppm, a reduction of more than 59.7%. Moreover, only a trace amount of 0.1wt%-2.0wt% is required to significantly extend the high-temperature service life of the cell without sacrificing the initial conductivity of the electrolyte (reduction ≤0.12%) and capacity (difference ≤0.5%), greatly improving the high-temperature degradation problem of existing AN-based double-layer capacitors. Detailed Implementation

[0025] To make the technical problem to be solved by the present invention, the technical solution, and the beneficial effects clearer, the present invention will be further described in detail below. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0026] The current mainstream commercial electrolyte is a 1 mol / L dimethylpyrrolidine tetrafluoroborate (DMP BF4) / AN system, which is suitable for a rated voltage of 2.9V. However, it has a significant drawback under high-temperature float charging conditions: trace amounts of water in the electrolyte at the ppm level can induce BF4 oxidation. - Hydrolysis produces hydrogen fluoride and small molecule gases. The polar oxygen-containing functional groups on the surface of activated carbon easily adsorb water molecules and accumulate in the micropores, accelerating hydrolysis and by-product deposition, leading to cell volume expansion, increased internal resistance, and capacity decay. Existing physical dehydration methods cannot completely remove residual water, while chemical dehydrating agents are prone to side reactions. Ordinary hydrophobic additives have poor compatibility with acetonitrile, require high dosages, and reduce conductivity, making it difficult to balance high-temperature stability and basic electrochemical performance.

[0027] This invention designs an electrolyte suitable for electric double-layer capacitors by adding a fluorinated acrylate additive to the existing dimethylpyrrolidone tetrafluoroborate (DMP BF4) / AN electrolyte system; specifically, the fluorinated acrylate additive has the following general formula structure:

[0028] CH2=CH-COO-CH(RF)2,

[0029] RF is selected from any one of trifluoromethyl (-CF3), pentafluoroethyl (-C2F5), heptafluoropropyl (-C3F7), and difluoromethyl (-CHF2), and the two RF groups may be the same or different from each other. This additive is added at 0.1wt%~2.0wt% to an electrolyte composed of an organic solvent (acetonitrile, etc.) and a conductive salt (DMP, BF4, etc.) and applied to a 2.5~2.8V double-layer capacitor. Through the synergistic effect of ion solvation sheath reconstruction, interface property regulation, and steric hindrance polymerization, it can suppress hydrolysis gas generation and electrode micropore blockage induced by trace amounts of moisture under high-temperature float charging.

[0030] Preferably, RF is selected from pentafluoroethyl (-C2F5), and the two RF groups are identical to each other. In this case, the fluorinated acrylate additive is bis(pentafluoroethyl) methyl acrylate (DFEMA).

[0031] In this invention, the amount of the additive is 0.1 wt% to 2.0 wt% of the total mass of the organic solvent and electrolyte salt, for example, including but not limited to 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, etc.

[0032] In this invention, the organic solvent is a nitrile, acid ester, or amide organic solvent. For example, it is selected from acetonitrile (AN), and may also be any one or more of the following: propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), γ-butyrolactone (GBL), γ-valerolactone (GVL), propionitrile (PN), methoxypropionitrile (MPN), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,2-dimethoxyethane (DME), sulfolane (SL), dimethyl sulfoxide (DMSO), methyl isopropyl sulfone (MIPS), and N,N-dimethylformamide (DMF). Preferably, the organic solvent is selected from at least one of acetonitrile (AN), propylene carbonate (PC), ethylene carbonate (EC), N,N-dimethylformamide (DMF), and 3-methoxypropionitrile (MPN), and more preferably acetonitrile (AN).

[0033] In this invention, the electrolyte salt is selected from one or more of tetraethylamine tetrafluoroborate (TEA BF4), spiro-(1,1')-bispyrrolidine tetrafluoroborate (SBP BF4), 1,1-dimethylpyrrolidine tetrafluoroborate (DMP BF4), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm BF4), and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIm FSI), preferably 1,1-dimethylpyrrolidine tetrafluoroborate (DMP BF4). The concentration of the electrolyte salt is 0.5 mol / L to 2.0 mol / L, including but not limited to 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, and 2.0 mol / L.

[0034] In another aspect of the present invention, the aforementioned method for preparing an electrolyte suitable for electric double-layer capacitors includes the following steps:

[0035] (1) Mix at least two of the organic solvents, namely carbonates, nitriles, and amides, in a certain proportion, and purify them by removing impurities and water to obtain a mixed solvent;

[0036] (2) At room temperature, the electrolyte salt is added to the mixed solvent obtained in step (1) and allowed to stand to dissolve, thus obtaining electrolyte I;

[0037] (3) Add 0.1 to 2.0 wt% of the additives to the electrolyte I obtained in step (2) and let it stand to dissolve, so as to obtain the electrolyte suitable for double-layer capacitors.

[0038] The key to this invention lies in adding a certain amount of the fluorinated acrylate additive of this invention to the existing dimethylpyrrolidone tetrafluoroborate (DMP BF4) / acetonitrile (AN) system to form a new electrolyte. For example, in the actual electrolyte preparation process, the electrolyte salt and organic solvent are first pretreated to remove water and impurities, and then the two are mixed in a certain proportion to prepare an electrolyte salt solution with a certain molar concentration. On this basis, a certain amount of the fluorinated acrylate additive of this invention is added and mixed evenly. Where this invention does not specifically describe a particular aspect, reference can be made to the prior art, which will not be elaborated upon here.

[0039] This invention also relates to the application of fluorinated acrylate additives in electrolytes. Specifically, a double-layer capacitor includes the aforementioned electrolyte suitable for double-layer capacitors or the electrolyte prepared by the aforementioned method suitable for double-layer capacitors. The process of this invention is simple and suitable for industrial production.

[0040] The present invention will be further explained and illustrated below through more specific embodiments, but these do not constitute any limitation.

[0041] Example 1

[0042] (1) Electrolyte preparation: Acetonitrile (AN) was dehydrated using 3A molecular sieves until the water content was ≤10ppm; DMP BF4 salt was vacuum dried at 100℃ for 12h for later use. Under an inert atmosphere, the conductive salt DMP BF4 was added to the dehydrated organic solvent acetonitrile (AN) and stirred to dissolve, thus preparing a basic electrolyte with a salt concentration of 1mol / L; under an inert atmosphere, 0.5wt% of bis(pentafluoroethyl) methyl acrylate (DFEMA) was added based on the total mass of the basic electrolyte, and stirred at room temperature for 30min until completely dissolved and homogeneous;

[0043] (2) Electrode preparation: Both positive and negative electrode sheets are made by coating aluminum foil with a mixture of activated carbon YP-50, conductive agent Super P and binder PAA in a mass ratio of 90:5:5.

[0044] (3) Cell assembly: The symmetrical electrode and cellulose membrane are wound and assembled into the shell and injected with the electrolyte from step (1) to form a 900F cylindrical cell.

[0045] Example 2

[0046] Similar to Example 1, except that the additive was replaced with 0.5 wt% of bis(heptafluoropropyl)methyl acrylate (TFPMA).

[0047] Example 3

[0048] The experiment was largely the same as in Example 1, except that the additive was replaced with 0.5 wt% of difluoromethyl methacrylate (DFMMA).

[0049] Example 4

[0050] Similar to Example 1, except that the additive was replaced with 0.5 wt% of trifluoromethyl pentafluoroethyl acrylate (TFEMMA).

[0051] Example 5

[0052] The experiment was largely the same as in Example 1, except that the additive was replaced with 0.1 wt% of bis(pentafluoroethyl) methyl acrylate (DFEMA).

[0053] Example 6

[0054] The experiment was largely the same as in Example 1, except that the additive was replaced with 1.5 wt% of bis(pentafluoroethyl) methyl acrylate (DFEMA).

[0055] Example 7

[0056] The experiment was largely the same as in Example 1, except that the additive was replaced with 2.0 wt% of bis(pentafluoroethyl) methyl acrylate (DFEMA).

[0057] Comparative Example 1

[0058] (1) Electrolyte preparation: Acetonitrile (AN) was dehydrated by 3A molecular sieve until the water content was ≤10ppm; DMP BF4 salt was vacuum dried at 100℃ for 12h for later use. Under an inert atmosphere, the conductive salt DMP BF4 was added to the dehydrated organic solvent acetonitrile and stirred to dissolve, thus preparing a basic electrolyte with a salt concentration of 1mol / L;

[0059] (2) Electrode preparation: Both positive and negative electrode sheets are made by coating aluminum foil with a mixture of activated carbon YP-50, conductive agent SuperP and binder PAA in a mass ratio of 90:5:5.

[0060] (3) Cell assembly: The symmetrical electrode and cellulose membrane are wound and assembled into the shell and injected with the electrolyte of Example (1) to make a 900F cylindrical cell.

[0061] Performance Testing

[0062] Test method:

[0063] Electrolyte physical characterization: Ionic conductivity (DS3070 conductivity meter), electrolyte viscosity (Ubbelohde viscometer, 25℃), moisture content (Karl Fischer moisture meter), and surface tension (ring method, 25℃) were tested at room temperature.

[0064] Electrochemical performance: AC internal resistance, Faraday capacity, and cyclic voltammetry curves were measured at room temperature at 1 kHz (scan rate 10 mV / s, voltage range 0~2.9 V).

[0065] High-temperature float charging aging: 2.9V float charging for 8 weeks in a 65℃ constant temperature chamber, and testing the capacity retention rate, internal resistance increase rate, and volume expansion rate (thickness change rate) after aging.

[0066] Verification of gas generation inhibition by hydrolysis: A sealed pressure-resistant container was used, 5 mL of electrolyte was injected, and the container was kept at 65℃ for 8 weeks. The pressure change inside the container (including the pressure sensor line) was tested.

[0067] Microporous structure stability: After aging, the battery cell was disassembled and the changes in the specific surface area and micropore size distribution of the activated carbon electrode were tested using a BET adsorption instrument.

[0068] Electrolyte stability: After aging, the free acid content of the electrolyte is tested (potentiometric titration).

[0069] The physical characterization results of the electrolytes prepared in the above embodiments and comparative examples are shown in Table 1.

[0070] Table 1. Physical characterization results of the electrolyte

[0071] Group additive Amount added / wt% Ionic conductivity / mS / cm Viscosity / mPa・s Surface tension / mN / m Initial moisture / ppm Free acid after aging / ppm Comparative Example 1 none 0 50.06 0.68 29.1 8 186 Example 1 DFEMA 0.5 50.91 0.7 26.3 12 75 Example 2 TFPMA 0.5 50.17 0.71 25.8 9 68 Example 3 DFMMA 0.5 50.05 0.69 27.5 12 98 Example 4 TFEMMA 0.5 50.03 0.7 26.1 10 82 Example 5 DFEMA 0.1 50.20 0.68 27.2 11 89 Example 6 DFEMA 1.5 51.91 0.72 25.9 13 72 Example 7 DFEMA 2.0 53.26 0.78 24.5 18 65

[0072] As shown in Table 1, the fluorinated acrylate additives (DFEMA / TFPMA / DFMMA / TFEMMA) of this invention have minimal impact on the basic physicochemical properties of the acetonitrile (AN)-based electrolyte and can significantly optimize interfacial characteristics and stability. Table 1 shows that the initial ionic conductivity of all examples is in the range of 50.03~53.26 mS / cm, and the conductivity increases slightly with increasing DFEMA concentration. Regarding viscosity, the examples are all controlled within the range of 0.68~0.78 mPa·s, without a significant increase, proving that the additives have excellent compatibility with the acetonitrile (AN) system and will not cause deterioration of electrolyte flow due to molecular introduction. More importantly, the fluorinated alkyl branches of the additives can effectively reduce the surface tension of the electrolyte. The surface tension of Examples 1-7 is reduced to 24.5~27.5 mN / m compared to the comparative example (29.1 mN / m), laying the foundation for subsequent improvement of microporous wettability. Meanwhile, the change in free acid content in the electrolyte after cell aging directly reflects the inhibitory effect of the additives on the hydrolysis reaction: the free acid content in the comparative example is as high as 186 ppm, while that in the examples is only 65~98 ppm. Moreover, as the length of the fluorinated alkyl chain increases (TFPMA > DFEMA > DFMMA), the free acid content gradually decreases. This trend confirms the construction logic of the hydrophobic barrier: longer perfluorinated chains have stronger hydrophobicity and can more effectively block moisture and BF4. - Contact with the source inhibits hydrolysis and acid production.

[0073] The initial electrochemical performance characterization results of the cells prepared in the above examples and comparative examples are shown in Table 2.

[0074] Table 2 Initial Electrochemical Performance Results of the Cell

[0075] Group additive Amount added / wt% Faraday capacity / F Internal resistance / mΩ Comparative Example 1 none 0 950.2 1.498 Example 1 DFEMA 0.5 969.5 1.472 Example 2 TFPMA 0.5 965.3 1.480 Example 3 DFMMA 0.5 965.9 1.496 Example 4 TFEMMA 0.5 964.4 1.464 Example 5 DFEMA 0.1 962.3 1.480 Example 6 DFEMA 1.5 958.4 1.481 Example 7 DFEMA 2.0 976.8 1.506

[0076] The initial electrochemical performance test results of the cells above further verify the compatibility and safety of the additives. Table 2 shows that the Faraday capacity of all examples remained between 950 and 970 F, slightly higher than the control ratio, and the internal resistance was also stable between 1.464 and 1.506 mΩ, with no increase in internal resistance due to the introduction of additives.

[0077] The results of high-temperature aging stability characterization of the battery cells prepared in the above embodiments and comparative examples are shown in Table 3.

[0078] Table 3. Results of high-temperature aging stability of battery cells

[0079] Group additive Amount added / wt% 8-week capacity retention rate / % Internal resistance growth rate / % Volume expansion rate / % Comparative Example 1 none 0 82.3 72.90 3.130 Example 1 DFEMA 0.5 91.5 33.45 1.090 Example 2 TFPMA 0.5 92.1 30.75 0.980 Example 3 DFMMA 0.5 88.7 44.10 1.510 Example 4 TFEMMA 0.5 90.8 35.40 1.175 Example 5 DFEMA 0.1 89.2 40.65 1.335 Example 6 DFEMA 1.5 91.8 31.80 1.025 Example 7 DFEMA 2.0 92.3 31.20 0.945

[0080] The high-temperature aging stability test results clearly demonstrate the core technical advantages of this invention, and the performance improvements show a clear logical correlation. The hydrophobic barrier reduces BF4. - Contact with moisture significantly reduces the generation of small molecule gases, directly leading to an optimized volume expansion rate: the volume expansion rate in the comparative example is 3.13%, while that in the examples is reduced to 0.945~1.51%, effectively solving the cell bulging problem under high-temperature float charging. The unobstructed microporous channels ensure efficient ion transport, effectively controlling the increase in internal resistance: the increase in internal resistance in the examples is only 30.75~44.1%, nearly half that of the comparative example (72.0%). Simultaneously, the stable microporous structure and efficient ion transport also ensure long-term capacity retention; the capacity retention rate of the examples is ≥88.7%, an improvement of 6.4~10.2% compared to the comparative example (82.3%).

[0081] Further analysis of the effects of molecular structure and addition amount on performance reveals that the modification effect of perfluorinated long-chain substituted additives (TFPMA, DFEMA) is better than that of low-fluorinated substituted (DFMMA) and asymmetric substituted (TFEMMA). This is because the longer perfluorinated chain not only has stronger hydrophobicity, but also more significant steric hindrance, which can simultaneously enhance the hydrophobic barrier and anti-clogging effect.

[0082] In summary, the fluorinated acrylate additives (DFEMA / TFPMA / DFMMA / TFEMMA) of this invention achieve a comprehensive improvement in electrolyte stability, microporous structure stability, and cell electrochemical stability through the triple physical synergistic effect of ion solvation sheath hydrophobic reconstruction, electrolyte interface property regulation, and steric hindrance to prevent pore blockage. This provides a reliable technical solution for the long-term stable operation of acetonitrile (AN)-based EDLCs at high temperatures.

[0083] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. An electrolyte suitable for electric double-layer capacitors, characterized in that, The electrolyte comprises an organic solvent, an electrolyte salt, and an additive; wherein the additive is a fluorinated acrylate additive.

2. The electrolyte for double-layer capacitors according to claim 1, characterized in that, The fluorinated acrylate additives have the following general formula structure: CH2=CH-COO-CH(RF)2 RF is selected from any one of trifluoromethyl (-CF3), pentafluoroethyl (-C2F5), heptafluoropropyl (-C3F7), and difluoromethyl (-CHF2), and the two RF groups may be the same or different from each other.

3. The electrolyte for double-layer capacitors according to claim 2, characterized in that, The fluorinated acrylate additive mentioned is bis(pentafluoroethyl) methyl acrylate (DFEMA).

4. The electrolyte suitable for double-layer capacitors according to claim 1, 2, or 3, characterized in that, The amount of the additive added is 0.1wt% to 2.0wt% of the total mass of the organic solvent and electrolyte salt.

5. The electrolyte for double-layer capacitors according to claim 4, characterized in that, The organic solvent is a nitrile, acid ester, or amide organic solvent.

6. The electrolyte for double-layer capacitors according to claim 5, characterized in that, The organic solvent is selected from at least one of acetonitrile (AN), propylene carbonate (PC), ethylene carbonate (EC), N,N-dimethylformamide (DMF), and 3-methoxypropionitrile (MPN).

7. The electrolyte for double-layer capacitors according to claim 4, characterized in that, The electrolyte salt is selected from one or more of tetraethylamine tetrafluoroborate (TEA BF4), spiro-(1,1')-bispyrrolidine tetrafluoroborate (SBP BF4), 1,1-dimethylpyrrolidine tetrafluoroborate (DMP BF4), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm BF4), and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIm FSI).

8. The electrolyte for a double-layer capacitor according to claim 7, characterized in that, The concentration of the electrolyte salt is 0.5 mol / L to 2.0 mol / L.

9. The method for preparing an electrolyte suitable for a double-layer capacitor according to any one of claims 1 to 8, characterized in that, Includes the following steps: (1) Mix at least two of the organic solvents, namely carbonates, nitriles, and amides, in a certain proportion, and purify them by removing impurities and water to obtain a mixed solvent; (2) At room temperature, the electrolyte salt is added to the mixed solvent obtained in step (1) and allowed to stand to dissolve, thus obtaining electrolyte I; (3) Add 0.1 to 2.0 wt% of the additives to the electrolyte I obtained in step (2) and let it stand to dissolve, so as to obtain the electrolyte suitable for double-layer capacitors.

10. A double-layer capacitor, characterized in that, Includes the electrolyte suitable for double-layer capacitors as described in any one of claims 1 to 8, or the electrolyte suitable for double-layer capacitors prepared by the preparation method described in claim 9.