An electrolyte for supercapacitors suitable for ultralow temperatures, a supercapacitor

Abstract

The application discloses an electrolyte suitable for supercapacitors in ultralow temperature, which comprises a main solvent, a cosolvent and an organic salt, wherein the main solvent is acetonitrile, and the cosolvent is selected from 2-pentanone or acetone. The application also discloses a supercapacitor using the electrolyte. The electrolyte provided by the application has a higher temperature capacity retention rate, higher rate performance, lower impedance characteristics, higher energy density and power density and long cycle life under low temperature conditions.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of supercapacitors, in particular to an electrolyte for a supercapacitor suitable for ultra-low temperature and a supercapacitor. BACKGROUND

[0002] Supercapacitors have outstanding advantages such as super-high power density, super-long cycle stability and second-level instantaneous response, and have good application prospects in intermittent energy storage of wind and photovoltaic, power system frequency modulation, heavy machinery potential energy recovery and the like. Supercapacitors are usually composed of an electrolyte, an electrode sheet, a separator and a shell and the like. As one of the most critical components, the electrolyte is usually composed of a solvent and a solute (salt), which determines the working temperature range, power density, energy density, cycle stability and transmission impedance of the supercapacitor, and deeply affects the application and development prospects of the supercapacitor. For example, a lithium ion battery suitable for low temperature and containing the electrolyte is disclosed in Chinese Patent No. CN115642308A, the electrolyte comprises a solvent, a film-forming additive and a lithium salt; the solvent comprises a nitrile solvent and a carbonate solvent; the nitrile solvent is any one or more of butyronitrile, isobutyronitrile, valeronitrile and isovaleronitrile. And a low-temperature flame-retardant organic electrolyte for a double-layer capacitor and a preparation method are disclosed in Chinese Patent No. CN113035586A, which comprises an electrolyte and an organic solvent, the organic solvent comprises a main solvent, a low-temperature cosolvent and a flame-retardant additive, the electrolyte is a quaternary ammonium salt, the main solvent is acetonitrile, and the low-temperature cosolvent is one or a combination of several of carbonic ester, gamma-butyrolactone, propionic ester and 1,3-dioxolane.

[0003] Ultra-low temperature application scenarios are particularly important in many professional fields (for example, the lowest working temperature of energy storage devices required in the field of polar scientific exploration needs to reach-60 DEG C, the lowest working temperature of military equipment needs to reach-50 DEG C, and the lowest working temperature of electric vehicles needs to reach-40 DEG C). However, current commercial supercapacitors are usually difficult to meet the above application requirements. They adopt 1 mol / L tetraethylammonium tetrafluoroborate salt (TEA-BF4) dissolved in acetonitrile (AN) or propylene carbonate (PC) solution to form an electrolyte, wherein the dielectric constant of the AN solvent and the PC solvent is relatively high, and the solvation between the ions and the solvent molecules is relatively strong, which makes it difficult for the ions to complete the desolvation process, thereby bringing higher ion pore impedance and poorer capacity retention rate and rate performance. In addition, the viscosity of the PC solvent is relatively high, which leads to poor molecular dynamics characteristics of the electrolyte. The above two reasons cause the lowest working temperature of the commercial supercapacitor to be limited to-40 DEG C or higher. Therefore, reasonable selection of the solvent and the solute (salt) of the electrolyte will be conducive to reducing the desolvation energy of the ions and reducing the viscosity of the electrolyte, thereby improving the low-temperature working characteristics of the supercapacitor. SUMMARY

[0004] The electrolyte for supercapacitors suitable for ultra-low temperature can effectively improve the low-temperature working characteristics of supercapacitors.

[0005] The present application provides the following technical solutions:

[0006] The electrolyte for supercapacitors suitable for ultra-low temperature comprises a main solvent, a co-solvent and an organic salt, the main solvent is acetonitrile, and the co-solvent is selected from 2-pentanone or acetone.

[0007] In the electrolyte provided by the present application, acetonitrile with high dielectric constant is used as the main solvent to promote ion dissociation and thus improve the solution conductivity; one of 2-pentanone and acetone with different dielectric constants is used as the co-solvent. The electrolyte based on the acetone / 2-pentanone system (medium dielectric constant) has a lower ion desolvation energy and lower viscosity, and thus has a higher temperature capacity retention rate, higher rate performance, lower impedance characteristics, higher energy density and power density, and long cycle life under low-temperature conditions, can effectively improve the low-temperature working characteristics of supercapacitors, and has potential for low-temperature energy storage applications.

[0008] The volume ratio of the main solvent to the co-solvent is (0.5-4):1, and the electrolyte has small impedance characteristics at the ratio of the main solvent to the co-solvent. The supercapacitor assembled based on the electrolyte has a high capacity retention rate of more than 65% under a scan rate of 100 mV / s in a cyclic voltammetry method.

[0009] The organic salt is selected from triethylmethylammonium tetrafluoroborate (TEMA-BF4).

[0010] The concentration of the organic salt in the electrolyte is 0.5-1 mol / L.

[0011] The lower limit of the working temperature of the electrolyte is -70 DEG C.

[0012] Preferably, the co-solvent is acetone, the volume ratio of the main solvent to the co-solvent is (0.5-4):1, and the electrolyte simultaneously exhibits excellent capacity retention rate, excellent energy density and power density, and low-temperature cycle performance.

[0013] Further preferably, the electrolyte is TEMA-BF4-(acetonitrile / acetone (1:1)), the supercapacitor of which exhibits excellent capacitance retention (89% after the capacitance retention test at a scan rate of 10 mV / s and a temperature drop from 20 DEG C to -70 DEG C); the supercapacitor of the electrolyte reaches a high energy density of 22.28 Wh / kg at a power density of 141.22 W / kg and a high power density of 6849.35 W / kg at an energy density of 11.79 Wh / kg; the supercapacitor of the electrolyte has an ultra-long cycle stability and can reach a capacity retention of 94.1% after 30000 cycles at a current density of 5 A / g in a -70 DEG C environment.

[0014] The preparation method of the electrolyte is as follows: the cosolvent and the main solvent are mixed under an absolute pressure of 101-110 kPa in an oxygen and water free room temperature environment to form a homogeneous solvent, and then the organic salt is added.

[0015] Specifically, one of the cosolvents 2-pentanone (MPK, dielectric constant ε=15.4) and acetone (ACT, dielectric constant ε=21.4) (purity ≥99.0%) with different dielectric constants is mixed with acetonitrile (purity ≥99.0%) which can promote ion dissociation and thus increase the conductivity of the solution, according to the above-mentioned proportion, under an absolute pressure of 101-110 kPa in an oxygen and water free room temperature environment (oxygen ≤1 ppm, water ≤1 ppm) until a clear and transparent homogeneous solvent (denoted as acetonitrile / X (y:1), y represents the proportion of the main solvent, and X represents one of the above-mentioned cosolvents) is formed. Then, the TEMA-BF4 salt with the above-mentioned concentration is added to the binary solvent to obtain the electrolyte for supercapacitors, denoted as TEMA-BF4-(acetonitrile / X (y:1)).

[0016] The application further provides a supercapacitor using the above-mentioned electrolyte.

[0017] Compared with the prior art, the application has the following advantages:

[0018] 1. The electrolyte system provided by the application has an extremely low lower limit of working temperature (-70 DEG C), and the supercapacitor of the electrolyte exhibits excellent capacitance retention.

[0019] 2. The electrolyte system provided by the application has excellent energy density and power density at -70 DEG C.

[0020] 3. The electrolyte system provided by the application has outstanding low-temperature cycle performance at -70 DEG C. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1Cyclic voltammetry curves of the supercapacitor based on TEMA-BF4- (acetonitrile / acetone (1 : 1)) electrolyte in Example 1 at 20 and -70 °C at a scan rate of 100 mV / s.

[0022] Figure 2 Cyclic voltammetry curves of the supercapacitor based on TEMA-BF4- (acetonitrile / acetone (1 : 1)) electrolyte in Example 1 at -70 °C at scan rates of 10, 50, 100 mV / s.

[0023] Figure 3 Electrochemical impedance spectroscopy curves of the supercapacitor based on TEMA-BF4- (acetonitrile / acetone (1 : 1)) electrolyte in Example 1 at 20 and -70 °C.

[0024] Figure 4 Capacitance retention and coulombic efficiency curves of the supercapacitor based on TEMA-BF4- (acetonitrile / acetone (1 : 1)) electrolyte in Example 1 at -70 °C for 30000 cycles.

[0025] Figure 5 Energy density-power density curves of the supercapacitor based on TEMA-BF4- (acetonitrile / acetone (1 : 1)) electrolyte in Example 1 at -60 and -70 °C, respectively.

[0026] Figure 6 Cyclic voltammetry curves of the supercapacitor based on TEMA-BF4- (acetonitrile / acetone (0.5: 1)) electrolyte in Example 2 at 20 and -70 °C at a scan rate of 100 mV / s.

[0027] Figure 7 Cyclic voltammetry curves of the supercapacitor based on TEMA-BF4- (acetonitrile / acetone (4: 1)) electrolyte in Example 3 at 20 and -70 °C at a scan rate of 100 mV / s.

[0028] Figure 8 Cyclic voltammetry curves of the supercapacitor based on TEMA-BF4- (acetonitrile / 2-pentanone (1 : 1)) electrolyte in Example 4 at 20 and -70 °C at a scan rate of 100 mV / s.

[0029] Figure 9 Electrochemical impedance spectroscopy curves of the supercapacitor based on TEMA-BF4- (acetonitrile / 2-pentanone (1 : 1)) electrolyte in Example 4 at 20 and -70 °C.

[0030] Figure 10 Cyclic voltammetry curves of the supercapacitor based on TEMA-BF4- (acetonitrile / dioxolane) electrolyte in Comparative Example 1 at 20 and -70 °C at a scan rate of 100 mV / s.

[0031] Figure 11 The electrochemical impedance spectra of the supercapacitor based on TEMA-BF4- (acetonitrile / propylene carbonate) electrolyte in Example 2 at 20 and -70°C.

[0032] Figure 12 The cyclic voltammetry test curves of the supercapacitor based on TEMA-BF4- (acetonitrile / propylene carbonate) electrolyte in Example 2 at 20 and -70°C at a scan rate of 100 mV / s.

[0033] Figure 13 The electrochemical impedance spectra of the supercapacitor based on TEMA-BF4- (acetonitrile / propylene carbonate) electrolyte in Example 2 at 20 and -70°C. DETAILED DESCRIPTION

[0034] In order to describe the present application in a more detailed and comprehensible way, the technical solutions of the present application are described in detail below in combination with the drawings and specific embodiments. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present application, but not for limiting the claims of the present application.

[0035] Example 1

[0036] A co-solvent of acetone (ACT, ε = 21.4, purity ≥ 99.0%) with a medium dielectric constant was mixed with a main solvent of acetonitrile (purity ≥ 99.0%) which can promote ion dissociation and thus improve the conductivity of the solution, in a volume ratio of 1:1, in an oxygen and water isolated room temperature environment (oxygen ≤ 1 ppm, water ≤ 1 ppm) under an absolute pressure of 101-110 kPa, until a clear and transparent homogeneous phase solvent was formed (denoted as: acetonitrile / acetone (1:1)). Then 0.5M TEMA-BF4 salt was added to the binary solvent to obtain an electrolyte for supercapacitors, denoted as: TEMA-BF4- (acetonitrile / acetone (1:1)). The electrolyte was stored in a sealed manner in an oxygen and water isolated room temperature environment.

[0037] When the TEMA-BF4-(acetonitrile / acetone (1 : 1)) electrolyte obtained in this example is used in a supercapacitor (using symmetric commercial YP-50 activated carbon electrodes), due to the weak solvation structure of the acetone molecules (medium dielectric constant, ε = 21.4) and ions in the electrolyte, which will bring lower ion desolvation energy and smaller pore entry impedance, and its lower viscosity (0.316 mPa-s) will bring faster molecular dynamics characteristics, therefore its temperature capacity retention rate, low temperature rate performance and impedance characteristics are all relatively excellent. When the temperature decreases from 20 °C to -70 °C, even at a high scan rate of 100 mV / s, the cyclic voltammetry curve of the supercapacitor based on TEMA-BF4-(acetonitrile / acetone (1 : 1)) electrolyte still remains rectangular, with a capacity retention rate as high as 76% Figure 1 ). The cyclic voltammetry curve was tested at different scan rates at -70 °C, as shown in Figure 2 . At a scan rate of 50, 100 mV / s, the specific capacitance decreased to 88.1% (19.3 F / g) and 78.3% (16.9 F / g) at 10 mV / s, respectively, indicating excellent rate performance of the supercapacitor. The impedance of the electrolyte at -70 °C was only 4.52 Ω, and the charge transfer impedance was only 3.35 Ω Figure 3 , indicating the lower impedance characteristics of the supercapacitor. After 30,000 cycles of long cycle test at -70 °C, the capacity retention rate of the supercapacitor was 94.1% Figure 4 . The supercapacitor has a high energy density of 22.28 Wh / kg at a power density of 141.22 W / kg, and a high power density of 6849.35 W / kg at an energy density of 11.79 Wh / kg Figure 5 . In summary, the acetone-based electrolyte exhibits good low-temperature electrochemical characteristics.

[0038] Example 2

[0039] A co-solvent acetone (ACT, ε = 21.4, purity ≥ 99.0%) with a medium dielectric constant was mixed with a main solvent acetonitrile (purity ≥ 99.0%) which can promote ion dissociation and thus improve the solution conductivity, in a volume ratio of 0.5: 1, in an oxygen and water free room temperature environment (oxygen ≤ 1 ppm, water ≤ 1 ppm) under an absolute pressure of 101-110 kPa, until a clear and transparent homogeneous solvent was formed (denoted as: acetonitrile / acetone (0.5: 1)). Then 0.5M TEMA-BF4 salt was added to the binary solvent to obtain an electrolyte for supercapacitors, denoted as: TEMA-BF4-(acetonitrile / acetone (0.5: 1)). The electrolyte was stored in a sealed oxygen and water free room temperature environment.

[0040] When the TEMA-BF4-(acetonitrile / acetone) electrolyte obtained in this embodiment is used in a supercapacitor (using a symmetrical commercial YP-50 activated carbon electrode), the weak solvation structure formed by acetone molecules (medium dielectric constant, ε = 21.4) and ions in the electrolyte results in lower ion desolvation energy and smaller pore impedance. Furthermore, its low viscosity (0.316 mPa·s) leads to faster molecular dynamics. Therefore, its temperature-dependent capacity retention, low-temperature rate performance, and impedance characteristics are all excellent. When the temperature drops from 20℃ to -70℃, even at a high scan rate of 100 mV / s, the cyclic voltammetry curve of the supercapacitor based on the TEMA-BF4-(acetonitrile / acetone (0.5:1)) electrolyte remains rectangular, with a capacity retention rate as high as 73%. Figure 6 This indicates that the supercapacitor has excellent capacity retention.

[0041] Example 3

[0042] Acetone (ACT, ε = 21.4, purity ≥ 99.0%), a co-solvent with a moderate dielectric constant, is mixed with acetonitrile (purity ≥ 99.0%), a main solvent that promotes ion dissociation and thus increases solution conductivity, at a volume ratio of 4:1. The mixture is kept at room temperature (oxygen ≤ 1 ppm, water ≤ 1 ppm) under an absolute pressure of 101-110 kPa, in the absence of oxygen and water, until a clear and transparent homogeneous solvent is formed (denoted as: acetonitrile / acetone (4:1)). Then, 0.5 M of TEMA-BF4 salt is added to this binary solvent to obtain the electrolyte for supercapacitors, denoted as: TEMA-BF4-(acetonitrile / acetone (4:1)). The electrolyte is sealed and stored at room temperature in the absence of oxygen and water.

[0043] When the TEMA-BF4-(acetonitrile / acetone) electrolyte obtained in this embodiment is used in a supercapacitor (using a symmetrical commercial YP-50 activated carbon electrode), the weak solvation structure formed by acetone molecules (medium dielectric constant, ε=21.4) and ions in the electrolyte results in lower ion desolvation energy and smaller pore impedance. Furthermore, its low viscosity (0.316 mPa·s) leads to faster molecular dynamics. Therefore, its temperature-dependent capacity retention, low-temperature rate performance, and impedance characteristics are all excellent. When the temperature drops from 20℃ to -70℃, even at a high cyclic voltammetry scan rate of 100 mV / s, the cyclic voltammetry curve of the supercapacitor based on the TEMA-BF4-(acetonitrile / acetone (4:1)) electrolyte remains rectangular, with a capacity retention rate as high as 68%. Figure 7 This indicates that the supercapacitor has excellent capacity retention.

[0044] Example 4

[0045] A co-solvent with low dielectric constant, 2-pentanone (MPK, ε = 7.3, purity > 99.0%), and a main solvent, acetonitrile (purity > 99.0%), which can promote ion dissociation and thus improve the conductivity of the solution, were mixed in the above-mentioned proportions in an oxygen- and water-free room temperature environment (oxygen < 1 ppm, water < 1 ppm) at an absolute pressure of 101-110 kPa until a clear and transparent homogeneous solvent was formed (denoted as: acetonitrile / 2-pentanone (1:1)). Then, 0.5M TEMA-BF4 salt was added to the binary solvent to obtain an electrolyte for supercapacitors, denoted as: TEMA-BF4-(acetonitrile / 2-pentanone (1:1)). The electrolyte was stored in a sealed form in an oxygen- and water-free room temperature environment.

[0046] When the TEMA-BF4-(acetonitrile / 2-pentanone (1:1)) electrolyte obtained in this example was used in a supercapacitor (using symmetric commercial YP-50 activated carbon electrodes), the temperature-dependent capacity retention, low-temperature rate performance and impedance characteristics were all relatively poor due to the 2-pentanone molecules (low dielectric constant, ε = 15.4) in the electrolyte not participating in the ion solvation structure, which would result in higher ion desolvation energy and higher pore entry impedance. The capacity retention at -60°C relative to that at 20°C was only 50.4% ( Figure 8 ) and the electrolyte impedance at -60°C was as high as 8.32Ω and the charge transfer impedance was as high as 8.58Ω Figure 9 ). In summary, the electrolyte based on 2-pentanone exhibited poor low-temperature electrochemical characteristics.

[0047] Comparative Example 1

[0048] A co-solvent with low dielectric constant, 2-pentanone (MPK, ε = 7.3, purity > 99.0%), and a main solvent, acetonitrile (purity > 99.0%), which can promote ion dissociation and thus improve the conductivity of the solution, were mixed in the above-mentioned proportions in an oxygen- and water-free room temperature environment (oxygen < 1 ppm, water < 1 ppm) at an absolute pressure of 101-110 kPa until a clear and transparent homogeneous solvent was formed (denoted as: acetonitrile / 2-pentanone (1:1)). Then, 0.5M TEMA-BF4 salt was added to the binary solvent to obtain an electrolyte for supercapacitors, denoted as: TEMA-BF4-(acetonitrile / 2-pentanone (1:1)). The electrolyte was stored in a sealed form in an oxygen- and water-free room temperature environment.

[0049] When the TEMA-BF4-(acetonitrile / dioxolane) electrolyte obtained in this comparative example is used in a supercapacitor (using symmetric commercial YP-50 activated carbon electrodes), the temperature-dependent capacity retention, low-temperature rate performance and impedance characteristics are all relatively poor because the dioxolane molecules (low dielectric constant, ε = 7.3) in the electrolyte do not participate in the ion solvation structure, which will result in higher ion desolvation energy and higher pore entry impedance. The capacity retention at -60°C relative to that at 20°C is only 43.1% ( Figure 10 ). The electrolyte impedance at -60°C is as high as 8.03 Ω, and the charge transfer impedance is as high as 8.53 Ω ( Figure 11 ). In summary, the dioxolane-based electrolyte exhibits poor low-temperature electrochemical characteristics.

[0050] Comparative Example 2

[0051] A co-solvent propylene carbonate (PC, ε = 64.9, purity ≥ 99.0%) with a high dielectric constant and a main solvent acetonitrile (purity ≥ 99.0%) capable of promoting ion dissociation and thus increasing the solution conductivity are mixed in the above-mentioned proportions in an oxygen- and water-free room temperature environment (oxygen ≤ 1 ppm, water ≤ 1 ppm) at an absolute pressure of 101-110 kPa until a clear and transparent homogeneous solvent (denoted: acetonitrile / propylene carbonate) is formed. Then 0.5 M TEMA-BF4 salt is added to the binary solvent to obtain an electrolyte for supercapacitors, denoted: TEMA-BF4-(acetonitrile / propylene carbonate). The electrolyte is stored in a sealed form in an oxygen- and water-free room temperature environment.

[0052] When the TEMA-BF4-(acetonitrile / propylene carbonate) electrolyte obtained in this comparative example is used in a supercapacitor (using symmetric commercial YP-50 activated carbon electrodes), the temperature-dependent capacity retention, low-temperature rate performance and impedance characteristics are all relatively poor because the propylene carbonate molecules (high dielectric constant, ε = 64.9) in the electrolyte form strong solvation structures with the ions, which will result in higher ion desolvation energy and higher pore entry impedance. The capacity retention at -60°C relative to that at 20°C is only 40.8% ( Figure 12 ). The electrolyte impedance at -60°C is as high as 10.12 Ω, and the charge transfer impedance is as high as 9.52 Ω ( Figure 13 ). In summary, the propylene carbonate-based electrolyte exhibits poor low-temperature electrochemical characteristics.

[0053] The above examples are merely specific embodiments of the present application, which are described in more detail and in a more specific manner, but should not be construed as limiting the scope of the patent of the present application Making . It should be noted that, for those skilled in the art, a number of modifications and improvements can be made without departing from the concept of the present application, and these alternative forms all belong to the protection scope of the present application.

Claims

1. An electrolyte for supercapacitors suitable for ultra-low temperature applications, characterized in that, The electrolyte comprises a main solvent, a co-solvent, and an organic salt, wherein the main solvent is acetonitrile and the co-solvent is acetone; The volume ratio of the main solvent to the co-solvent is 1:1; The organic salt is selected from triethylmethylammonium tetrafluoroborate (TEMA-BF4); the concentration of the organic salt in the electrolyte is 0.5 mol / L.

2. The electrolyte for supercapacitors suitable for ultra-low temperatures according to claim 1, characterized in that, The lower limit of the operating temperature of the electrolyte is -70℃.

3. The electrolyte for ultra-low temperature supercapacitors according to any one of claims 1-2, characterized in that, The electrolyte is prepared by mixing the co-solvent and the main solvent at an absolute pressure of 101-110 kPa and at room temperature in the absence of oxygen and water to form a homogeneous solvent, and then adding an organic salt.

4. A supercapacitor, characterized in that, The electrolyte described in any one of claims 1-2 is used.

Images