Super capacitor ionic liquid mixed electrolyte with wide potential window and preparation method thereof

Abstract

The invention discloses a supercapacitor ionic liquid mixed electrolyte with a wide potential window, which is composed of an organic solvent and two ionic liquids, the two ionic liquids are dissolved in the organic solvent, the organic solvent is triethyl phosphate, and the ionic liquids are mixed uniformly. The two kinds of ionic liquid are 1-ethyl-3-methylimidazole bis (trifluoromethanesulfonyl) imide salt and 1-ethyl-3-methylimidazole tetrafluoroborate respectively, and the two kinds of ionic liquid are mixed according to different proportions. The preparation method has the beneficial effects that an ion rearrangement system is constructed, the electrolyte with a wide potential window, high energy density and low self-discharge is obtained, and meanwhile, the electrolyte has excellent rate capability and stable cycle performance. By optimizing electrolyte components, the use temperature range of the supercapacitor is effectively widened, and compared with pure ionic liquid and common carbonic ester electrolyte, the electrolyte has wider use temperature range, can normally work in the temperature range of-20 DEG C to 80 DEG C, and can meet the use requirements of the supercapacitor under the extreme temperature condition.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy materials and devices, and particularly to a supercapacitor ionic liquid mixed electrolyte with a wide potential window and a preparation method thereof. Background Art

[0002] As an efficient energy storage device, the double-layer energy storage mechanism of a supercapacitor realizes energy storage through the electrostatic charge adsorption at the interface between the electrode and the electrolyte. It has outstanding advantages such as high power density, long cycle life, and fast charge and discharge speed, and plays an irreplaceable role in fields such as smart grid frequency modulation, rail transit braking energy recovery, and instantaneous power compensation of industrial equipment.

[0003] However, the existing technical system faces three core bottlenecks: First, the adaptability to working temperature is poor. Conventional aqueous electrolytes are prone to freezing in low-temperature environments, resulting in hindered ion migration. Organic electrolytes are prone to decomposition at high temperatures, generating gases and causing expansion failure. Although ionic liquids have high thermal stability, their viscosity is too high at low temperatures, leading to a sharp drop in the interfacial charge transfer efficiency. Second, the energy density is low. Limited by the double-layer physical energy storage mechanism and the electrochemical window of the electrolyte, it is difficult to break through the order-of-magnitude gap with the energy density of the battery system. More prominent is the self-discharge problem. Due to the thermal motion of ions in the electrolyte and the internal microcurrent caused by redox impurities, the voltage decays severely when the device is stationary, restricting long-term energy storage applications. These defects essentially stem from the inherent contradiction between the thermodynamic properties and electrochemical stability of the electrolyte system. For example, improving the voltage window requires electrolytes with high decomposition potentials, but such materials often have problems such as high viscosity and poor wettability.

[0004] Therefore, constructing a new electrolyte system with a wide electrochemical window, low viscosity temperature coefficient, and high interfacial stability, and fundamentally coordinating the competition relationship between energy density, temperature adaptability, and self-discharge characteristics, is of great significance for the development of supercapacitors. Summary of the Invention

[0005] The purpose of the present invention is to propose a supercapacitor ionic liquid mixed electrolyte with a wide potential window and a preparation method thereof for the deficiencies of the existing technology. Through the ion component regulation and organic solvent co-optimization strategy, the performance of the supercapacitor is finally improved, and the application boundary is expanded.

[0006] The specific technical solution adopted by the present invention is as follows: A supercapacitor ionic liquid mixed electrolyte with a wide potential window is composed of an organic solvent and two ionic liquids. The two ionic liquids are dissolved in the organic solvent, and the total molar concentration is 0.25 - 4 mol / L.

[0007] Preferably, the total molar concentration of the two ionic liquids is 1 mol / L.

[0008] Preferably, the organic solvent is triethyl phosphate (TEP).

[0009] Preferably, the two ionic liquids are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]) respectively. The two ionic liquids are mixed in different ratios to form a binary liquid phase system, namely A / B. Taking the mole fraction as the regulation parameter, isogradient adjustment is carried out in the range of 0-100%, and 11 characteristic samples are prepared at 10% mole fraction intervals, that is, A:B = 10:0, 9:1,..., 0:10.

[0010] Preferably, the molar ratio of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) to 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]) is 7:3.

[0011] Preferably, the preparation method of the ionic liquid mixed electrolyte for the supercapacitor with a wide potential window is as follows: In a glove box filled with a protective gas, use a pipette to transfer the ionic liquid into a volumetric flask, transfer the organic solvent to make up the volume, take it out and stir it on a magnetic stirrer until it is evenly mixed to obtain the electrolyte.

[0012] Preferably, the protective gas is high-purity argon.

[0013] Preferably, the present invention provides a supercapacitor, and the electrolyte of the supercapacitor is the above-mentioned preferred electrolyte.

[0014] Preferably, the supercapacitor is an electric double layer symmetric supercapacitor in the form of a 2032 button battery, including a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. Both the positive electrode sheet and the negative electrode sheet are YP50F activated carbon materials, and the separator is a Whatman GF / D glass fiber separator.

[0015] Preferably, the operating temperature of the supercapacitor is -20°C to 80°C; the operating voltage of the supercapacitor is 0 to 3.2V.

[0016] The beneficial effects of the present invention are as follows: (1) The present invention constructs an ion rearrangement system, and obtains an electrolyte with both a wide potential window, high energy density and low self-discharge, and at the same time has excellent rate performance and stable cycle performance. (2) By optimizing the components of the electrolyte, the present invention effectively broadens the operating temperature range of the supercapacitor. Compared with pure ionic liquids and common carbonate electrolytes, it has a wider operating temperature and can work normally in the temperature range of -20 to 80°C, and can meet the usage requirements of the supercapacitor under extreme temperature conditions. Description of the Drawings

[0017] Figure 1 Ragone curves of the coin-type supercapacitors assembled with the electrolytes of Examples 1, 2, and 3 at room temperature and a current density of 0.2 - 10 A / g, showing the schematic diagrams of the energy density and power density of the supercapacitors.

[0018] Figure 2 Raman test diagrams of the electrolytes of Examples 1, 2, and 3 and Comparative Examples 2 and 3.

[0019] Figure 3 Cyclic voltammograms of the coin-type supercapacitor assembled with the electrolyte of Example 2 at room temperature and a scan rate of 10 - 200 mV / s.

[0020] Figure 4 Galvanostatic charge-discharge curves of the coin-type supercapacitor assembled with the electrolyte of Example 2 at room temperature and a current density of 0.2 - 10 A / g.

[0021] Figure 5 Cyclic voltammograms of the coin-type supercapacitor assembled with the electrolyte of Example 2 at a working temperature of -20 - 80°C and a scan rate of 50 mV / s.

[0022] Figure 6 Cyclic voltammograms of the coin-type supercapacitor assembled with the electrolyte of Comparative Example 4 at a working temperature of -20 - 80°C and a scan rate of 50 mV / s.

[0023] Figure 7 Cyclic voltammograms of the coin-type supercapacitor assembled with the electrolyte of Comparative Example 5 at a working temperature of -20 - 80°C and a scan rate of 50 mV / s.

[0024] Figure 8 Galvanostatic charge-discharge curves of the coin-type supercapacitor assembled with the electrolyte of Example 2 at a working temperature of -20 - 80°C and a current density of 1 A / g.

[0025] Figure 9 Long-term cyclic curves of 10,000 cycles of the coin-type supercapacitor assembled with the electrolyte of Example 2 at a working temperature of -20 - 80°C and a current density of 5 A / g.

[0026] Figure 10 Schematic diagrams of the leakage current of the coin-type supercapacitors assembled with the electrolytes of Examples 1, 2, and 3 and Comparative Example 1 after being charged to the working voltage and maintaining a constant potential for 2 h. Among them, Examples 1, 2, and 3 were charged to 3.2 V, and Comparative Example 1 was charged to 2.5 V.

[0027] Figure 11Schematic diagram of voltage decay of coin-type supercapacitors assembled with the electrolytes of Examples 1, 2, 3 and Comparative Example 1 after being charged to the working voltage, maintaining a constant potential for 2 h, and then maintaining an open-circuit potential for 20 h. Among them, Examples 1, 2, and 3 were charged to 3.2 V, and Comparative Example 1 was charged to 2.5 V. Detailed implementation mode

[0028] The present invention provides a method for preparing an ionic liquid mixed electrolyte for a supercapacitor with a wide potential window. Without specific special instructions, the materials or reagents used in the present invention are common materials or reagents in the art and can be obtained from commercial products in the art. The following details the specific implementation process of the present invention to more clearly illustrate the present invention, but the protection scope of the present invention is not limited by it.

[0029] As Figures 1 - 11 shown, the following examples and comparative examples are all electrolytes, which are filled into a capillary for Raman testing and assembled into a supercapacitor for electrochemical testing. The supercapacitor is a double-layer symmetric supercapacitor in the form of a 2032 button battery, including a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. Both the positive electrode sheet and the negative electrode sheet are YP50F activated carbon materials, and the separator is a Whatman GF / D glass fiber separator. The electrochemical testing is carried out in a DH7000 electrochemical workstation.

[0030] The innovative parts of the present invention are two points: 1. Ion component regulation: Select [EMIM][BF4] with the theoretically smallest-sized anion and [EMIM][TFSI] with a large-sized anion to construct an ion rearrangement system, which promotes the spontaneous formation of a double-layer structure, provides an efficient charge storage architecture, optimizes the ion migration rate by regulating the spatial arrangement of anions, breaks through the traditional concentration gradient-driven adsorption mode, and enhances the adsorption ability of anions to the graphite electrode, providing theoretical guidance for obtaining supercapacitors with high specific capacitance and low self-discharge. 2. Organic solvent synergistic optimization: On the basis of the ion rearrangement system, use non-flammable and low-toxic triethyl phosphate to dilute the ionic liquid, successfully develop an intrinsically non-flammable electrolyte, obtain an electrolyte system with both low-temperature and high-temperature adaptability, and systematically verify its application potential in wide-temperature-range supercapacitors.

[0031] Example 1:

[0032] This example provides a single ionic liquid electrolyte based on triethyl phosphate, and its preparation method is as follows:

[0033] Take 2.576 mL of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI), add it to a 10 mL volumetric flask, make up the volume with triethyl phosphate (TEP), take it out and stir it on a magnetic stirrer until it is evenly mixed to obtain the electrolyte solution, and the ionic liquid concentration is 1 mol / L;

[0034] Example 2:

[0035] This example provides a binary ionic liquid electrolyte based on triethyl phosphate, and its preparation method is as follows:

[0036] Take 1.803 mL of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) and 0.459 mL of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]), add them to a 10 mL volumetric flask, make up the volume with triethyl phosphate (TEP), take it out and stir it on a magnetic stirrer until it is evenly mixed to obtain the electrolyte solution, the molar ratio of the two ionic liquids is 7:3, and the total concentration is 1 mol / L;

[0037] Example 3:

[0038] This example provides a single ionic liquid electrolyte based on triethyl phosphate, and its preparation method is as follows:

[0039] Take 1.530 mL of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]), add it to a 10 mL volumetric flask, make up the volume with triethyl phosphate (TEP), take it out and stir it on a magnetic stirrer until it is evenly mixed to obtain the electrolyte solution, and the ionic liquid concentration is 1 mol / L;

[0040] Comparative Example 1:

[0041] This comparative example provides a commercial electrolyte solution, and its preparation method is as follows:

[0042] Weigh 2.171 g of tetraethylammonium tetrafluoroborate (TEATFB), add it to a 10 mL volumetric flask, make up the volume with acetonitrile (ACN), take it out and stir it on a magnetic stirrer until it is evenly mixed to obtain the electrolyte solution, and the concentration is 1 mol / L;

[0043] Comparative Example 2:

[0044] This comparative example provides a pure ionic liquid electrolyte solution, and its preparation method is as follows:

[0045] Directly use the pure ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) as the electrolyte solution;

[0046] Comparative Example 3:

[0047] This comparative example provides a pure ionic liquid electrolyte, and its preparation method is as follows:

[0048] Directly use pure ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]) as the electrolyte;

[0049] Comparative Example 4:

[0050] This comparative example provides a binary ionic liquid electrolyte, and its preparation method is as follows:

[0051] Transfer 1.803 mL of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) and 0.459 mL of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]), place them on a magnetic stirrer and stir until evenly mixed to obtain the electrolyte. The molar ratio of the two ionic liquids is 7:3;

[0052] Comparative Example 5:

[0053] This comparative example provides an acetonitrile-based binary ionic liquid electrolyte, and its preparation method is as follows:

[0054] Transfer 1.803 mL of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) and 0.459 mL of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF4]), add them to a 10 mL volumetric flask, make up the volume with acetonitrile (ACN), take it out and place it on a magnetic stirrer and stir until evenly mixed to obtain the electrolyte. The molar ratio of the two ionic liquids is 7:3, and the total concentration is 1 mol / L;

[0055] Test 1:

[0056] Compare the energy density and power density of the supercapacitors assembled in the above Examples 1, 2, and 3 through constant current charge and discharge tests. The working voltage is 0 to 3.2 V, the current density is 0.2 to 10 A / g, and the working temperature is room temperature. The comparison graph is as Figure 1 shown. The results show that Example 2 exhibits an energy density as high as 42.43 Wh kg -1 at a power density of 302.1 W kg -1 Even at a power density of 16.2 kW kg -1 it still has an energy density as high as 28.71 Wh kg -1 Compared with Examples 1 and 3, the energy density of Example 2 has been significantly improved.

[0057] Test 2:

[0058] The electrolytes of Examples 1, 2, 3 and Comparative Examples 2, 3 were analyzed by Raman test. The results showed that the addition of TEP dissociated the ion pairs of the ionic liquid to form solvated ions, which would improve the ionic conductivity and reduce the viscosity of the electrolyte.

[0059] Test 3:

[0060] The coin-type supercapacitor assembled with the electrolyte of Example 2 was analyzed by cyclic voltammetry test. The scanning rate was 10 - 200 mV / s and the working temperature was room temperature, as Figure 3 shown. These curves clearly showed a quasi-rectangular shape and negligible distortion when the scanning rate increased from 10 mV s -1 to 200 mV s -1 , indicating good capacitive characteristics and fast rate capability.

[0061] Test 4:

[0062] The coin-type supercapacitor assembled with the electrolyte of Example 2 was analyzed by galvanostatic charge-discharge test. The current density was 0.2 - 10 A / g and the working temperature was room temperature, as Figure 4 shown. The results showed that at different current densities, the curves presented a capacitive behavior similar to an approximate symmetric triangle, showing a high Coulomb efficiency.

[0063] Test 5:

[0064] The coin-type supercapacitor assembled with the electrolyte of Example 2 was analyzed by cyclic voltammetry test. The scanning rate was 50 mV / s and the working temperature was -20 - 80 °C, as Figure 5 shown. The results showed that whether in a low-temperature (-20 °C) or high-temperature (80 °C) environment, the curves maintained an approximately rectangular shape, showing ideal capacitive behavior and very little change in current density, indicating that the electrolyte had good wide-temperature adaptability.

[0065] Test 6:

[0066] The coin-type supercapacitor assembled with the electrolyte of Comparative Example 4 was analyzed by cyclic voltammetry test. The scanning rate was 50 mV / s and the working temperature was -20 - 80 °C, as Figure 6 shown. The results showed that for the supercapacitor based on pure ionic liquid, due to the too high viscosity of the electrolyte resulting in slow ion transport, the current density decreased sharply at -5 °C and it could not work properly at -20 °C.

[0067] Test 7:

[0068] The coin-type supercapacitor assembled with the electrolyte of Comparative Example 5 was analyzed by cyclic voltammetry test. The scanning rate was 50 mV / s and the working temperature was -20 - 80 °C, asFigure 7 As shown. The results show that its low-temperature and room-temperature performance is good. When the temperature is below 60 °C, all curves maintain a good rectangular shape. However, as the temperature continues to rise to 80 °C, obvious polarization of the current occurs, indicating that the electrolyte is unstable and decomposes at high temperatures, resulting in an increase in current.

[0069] Test 8:

[0070] The coin-type supercapacitor assembled with the electrolyte of Example 2 was analyzed by constant current charge-discharge tests at a current density of 1 A / g and a working temperature of -20 to 80 °C, as Figure 8 shown. The results show that at different working temperatures, the curves exhibit a capacitance behavior similar to an approximately symmetric triangle, showing a high Coulomb efficiency, indicating that the electrolyte has good wide-temperature adaptability.

[0071] Test 9:

[0072] The coin-type supercapacitor assembled with the electrolyte of Example 2 was analyzed by constant current charge-discharge tests at a current density of 5 A / g, a working temperature of -20 to 80 °C, and 10,000 cycles, as Figure 9 shown. The results show that at temperatures of 25 °C, -20 °C, and 80 °C, the capacitance retention rates after 10,000 cycles are 87.4%, 85.6%, and 78.4% respectively, showing excellent long-cycle stability. And during the long-cycle process at different temperatures, the Coulomb efficiency of the supercapacitor is always close to 100%, indicating its good reversibility.

[0073] Test 10:

[0074] The leakage current of the coin-type supercapacitors assembled with the electrolytes of Examples 1, 2, 3 and Comparative Example 1 was analyzed by self-discharge tests after charging to the working voltage and maintaining a constant potential for 2 h, and the voltage decay after maintaining an open-circuit potential for 20 h, as Figure 10 , 11 shown, and the working temperature was room temperature. Among them, Examples 1, 2, 3 were charged to 3.2 V, and Comparative Example 1 was charged to 2.5 V. The results show that after charging to 3.2 V, the open-circuit potential of the supercapacitor based on the electrolyte of Example 2 dropped to 2.16 V after 20 h, and the corresponding voltage loss was about 32.5%, which is better than that of the supercapacitors using Example 1 (dropping to 1.83 V, voltage loss 42.8%) and Example 3 (dropping to 1.36 V, voltage loss 57.5%). At the same time, the leakage currents corresponding to the supercapacitors based on Example 2, Example 1, and Example 3 were stably at 5.9, 8.4, and 13.5 μA g -1, which is consistent with the voltage loss of their self-discharge. The self-discharge test results of Examples 1, 2, and 3 are significantly better than those of the supercapacitor of Comparative Example 1 using commercial electrolyte (dropping to 1.05 V, voltage loss 58.0%, leakage current 15.1 μA g -1 ), proving its practical application value.

Claims

1. A supercapacitor ionic liquid mixed electrolyte with a wide potential window, characterized in that: It consists of an organic solvent and two ionic liquids. The two ionic liquids are dissolved in the organic solvent, and the total molar concentration is 0.25-4 mol / L.

2. The ionic liquid mixed electrolyte for supercapacitors with a wide potential window according to claim 1, characterized in that: The total molar concentration of the two ionic liquids is 1 mol / L.

3. The ionic liquid mixed electrolyte for supercapacitors with a wide potential window according to claim 2, characterized in that: The organic solvent is triethyl phosphate.

4. The ionic liquid mixed electrolyte for supercapacitors with a wide potential window according to claim 3, characterized in that: The two ionic liquids are 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt and 1-ethyl-3-methylimidazolium tetrafluoroborate, respectively. The two ionic liquids are mixed in different proportions to form a two-component liquid phase system, namely A / B. The mole fraction is used as a control parameter, and isogradient adjustment is performed in the range of 0 to 100%. 11 characteristic samples are prepared at 10% mole fraction intervals, namely A:B=10:0, 9:1,..., 0:

10.

5. The ionic liquid mixed electrolyte for supercapacitors with a wide potential window according to claim 4, characterized in that: The molar ratio of the 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt to the 1-ethyl-3-methylimidazolium tetrafluoroborate salt is 7:

3.

6. A method for preparing the ionic liquid mixed electrolyte for supercapacitors with a wide potential window according to claim 5, characterized in that: In a glove box filled with protective gas, use a pipette to transfer the ionic liquid into a volumetric flask, transfer the organic solvent to make up the volume, take it out and stir it on a magnetic stirrer until it is evenly mixed to obtain an electrolyte.

7. The method for preparing a supercapacitor ionic liquid mixed electrolyte with a wide potential window according to claim 6, characterized in that: The protective gas is high-purity argon.

8. An application of the ionic liquid mixed electrolyte for supercapacitors with a wide potential window according to claim 5, characterized in that: Provided is a supercapacitor, wherein the electrolyte of the supercapacitor is a supercapacitor ion liquid mixed electrolyte with a wide potential window.

9. The use of a supercapacitor ionic liquid mixed electrolyte with a wide potential window according to claim 8, characterized in that: The supercapacitor is a double-layer symmetrical supercapacitor in the form of a 2032 button cell, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a diaphragm. The positive electrode sheet and the negative electrode sheet are both made of YP50F activated carbon material, and the diaphragm is a Whatman GF / D glass fiber diaphragm.

10. The use of a supercapacitor ionic liquid mixed electrolyte with a wide potential window according to claim 9, characterized in that: The working temperature of the super capacitor is -20°C to 80°C; the working voltage of the super capacitor is 0 to 3.2V.

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