Super capacitor electrolyte and preparation method and application thereof

Abstract

The invention discloses a supercapacitor electrolyte and a preparation method and application thereof. The preparation method of the electrolyte comprises the following steps: dissolving dissolving pulp by using a lithium bromide aqueous solution; adding an acrylamide solution; and adding a cross-linking agent and an initiator to initiate polymerization reaction to form the membrane material with an interpenetrating network structure of cellulose and polyacrylamide molecules. The cellulose and polyacrylamide composite membrane obtained by the invention has an obvious pore structure, cellulose and polyacrylamide molecules form an interpenetrating network, the formed porous network structure is beneficial to electrolyte ion transmission and rapid diffusion and has good ionic conductivity, and the polyacrylamide has good tensile property, so that the composite membrane has good mechanical properties and good mechanical properties. And the cellulose has certain mechanical strength, so that the membrane material has good mechanical properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrolyte materials, and particularly relates to a supercapacitor electrolyte and its preparation method and application. Specifically, it relates to a technology of treating dissolving pulp fibers with an aqueous lithium bromide solution, preparing a composite membrane material with polyacrylamide, and assembling a supercapacitor using this composite membrane as an electrolyte. Background Art

[0002] Disclosing the information of this background art section is only intended to enhance the overall understanding of the present invention, and it is not necessarily regarded as an admission or an implication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] Natural polysaccharide cellulose and its derivatives have a high aspect ratio, high specific surface area, high porosity, excellent mechanical properties and excellent flexibility, and are considered to be very promising polymers for preparing flexible capacitor materials. Based on these composite materials, scientific researchers have conducted a large number of studies on the development of electrodes, diaphragms, current collectors and electrolytes for capacitors. Researchers use various micron- and nanostructured fibers as well as nanostructured inorganic materials to produce cellulose-based bionanocomposites for energy devices. There have been relatively many studies on flexible electrolytes and membrane materials prepared from cellulose. Its rich functional groups enable it to combine with other capacitive materials to prepare cellulose gels, which have the advantages of good flexibility, high mechanical strength and special capacitive properties, and can be used as working electrodes or electrolytes for flexible supercapacitors.

[0004] Cellulose has a highly crystalline structure and contains a large number of intermolecular and intramolecular hydrogen bonds, so it is extremely difficult to dissolve in water and most conventional organic solvents, which brings difficulties to the preparation of cellulose hydrogels. It is difficult to prepare a cellulose matrix with high ionic conductivity and good mechanical properties. The discovered dissolution systems include the DMAc / LiCl system, N 2 O 4 / DMF system, DMSO / TBAF system, alkali / urea system and ionic liquids, etc. Molten salt hydrates are also an effective method for dissolving cellulose. In addition, in practical applications, cellulose-based electrolytes also have the problem of poor adaptability to extreme environments, which limits the expansion of their applications. Summary of the Invention

[0005] In view of the deficiencies of the prior art, the present invention provides a supercapacitor electrolyte and its preparation method and application. In the present invention, dissolving pulp is dissolved in an aqueous lithium bromide solution, and a composite membrane with conductive and tensile properties is prepared with polyacrylamide. The lithium ions contained in the solution serve as conductive ions. This composite membrane can be used as an electrolyte to assemble a supercapacitor with common electrodes such as activated carbon electrodes, and has good electrochemical performance at low temperature and in a bent state. To achieve the above technical objectives, the technical solutions implemented by the present invention are as follows:

[0006] As the first aspect of the present invention, there is provided a method for preparing a supercapacitor electrolyte, comprising the following steps:

[0007] S1, dissolving dissolving pulp with an aqueous lithium bromide solution;

[0008] S2, adding an acrylamide solution;

[0009] S3, adding a crosslinking agent and an initiator to initiate a polymerization reaction to form a membrane material with an interpenetrating network structure of cellulose and polyacrylamide molecules.

[0010] In an embodiment of the present invention, in step S1, the dissolving pulp is dissolving pulp fibers with a degree of polymerization of 500 - 900 and an α-cellulose content of more than 91%. The influence of the degree of polymerization on the present invention is reflected in the dissolution difficulty in the lithium bromide solution. The higher the degree of polymerization, the more difficult the dissolution, the less mass that can be added, which may affect the electrochemical and mechanical properties. Although a too low degree of polymerization is easier to dissolve, it will affect the mechanical strength of the composite membrane.

[0011] It is found that lithium chloride needs to be combined with DMAc in a certain proportion to dissolve cellulose, and the organic solvent DMAc is required, while the aqueous solution of lithium bromide can dissolve cellulose, reducing the use of organic reagents and being more environmentally friendly.

[0012] In an embodiment of the present invention, in step S1, the molar ratio of lithium bromide to water in the aqueous lithium bromide solution is 1:3 - 1:3.5. For different types of cellulose, 1:3 can basically dissolve within a certain range. For cellulose with a particularly high degree of polymerization or insufficient purity, lithium bromide solutions with other molar ratios may not be able to dissolve.

[0013] In one or more embodiments of the present invention, the mass fraction of the dissolving pulp when dissolved in the aqueous lithium bromide solution is 0.38% - 2.26%, preferably 0.75% - 1.5%, the dissolution temperature is 120°C - 130°C, the cellulose dissolution time is 10 - 30 min, preferably 15 - 20 min, the acrylamide polymerization time is 20 - 60 min, and the polymerization temperature is 70°C - 90°C, preferably 80°C.

[0014] The crosslinking agent is N,N'-methylenebisacrylamide (MBA).

[0015] The initiator is ammonium persulfate (APS) or potassium persulfate.

[0016] The mass of the crosslinking agent MBA is equivalent to 5% of the mass of acrylamide, and the mass of the initiator APS is equivalent to 0.5% of the mass of acrylamide.

[0017] As a second aspect of the present invention, a supercapacitor electrolyte is provided, comprising 0.25%-1.5% w / v of dissolving slurry, 30%-40% w / v of acrylamide, and 30%-40% w / v of lithium bromide.

[0018] Considering the dissolution of cellulose, the electrochemical and mechanical properties of the composite membrane and the stability of the preparation process, the preferred component contents are 0.5%-1% w / v of dissolving pulp, 30%-40% w / v of acrylamide and 40% w / v of lithium bromide.

[0019] As a third aspect of the present invention, there is provided an application of a supercapacitor electrolyte in preparing a supercapacitor.

[0020] As a fourth aspect of the present invention, a supercapacitor is provided, comprising the supercapacitor electrolyte composite membrane and an activated carbon electrode, wherein the supercapacitor electrolyte composite membrane and the activated carbon electrode are assembled into a sandwich structure.

[0021] The preparation method of the activated carbon electrode is as follows: the activated carbon electrode uses nickel foam as a current collector, the mass ratio of activated carbon (AC), acetylene black (AB) and polyvinylidene fluoride (PVDF) powder is 8-7:2-1:1, after the powders are mixed, a proper amount of N-methylpyrrolidone (NMP) is dropped into the powders for thorough grinding to obtain a conductive material, the uniformly mixed conductive material is evenly coated on the nickel foam, and the electrode sheet is obtained after drying and pressing.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The present invention provides a method for dissolving dissolving pulp fibers with a lithium bromide solution and then compounding polyacrylamide. The lithium bromide aqueous solution can fully dissolve the dissolving pulp to form a uniform cellulose solution. The acrylamide is dispersed in the cellulose solution to form a uniform mixture of cellulose and acrylamide. The acrylamide is polymerized by adding a crosslinking agent and an initiator to obtain a composite membrane of cellulose and polyacrylamide having a distinct pore structure. The cellulose and polyacrylamide molecules form an interpenetrating network. The formed porous network structure is conducive to the transmission and rapid diffusion of electrolyte ions and has good ion conductivity. Since polyacrylamide has good tensile properties and cellulose has a certain mechanical strength, the membrane material has good mechanical properties.

[0024] (2) The present invention has found through research and comparison that the composite membrane material prepared based on high-concentration lithium bromide provided by the present invention can prevent the membrane material from dehydrating, and can maintain good electrochemical properties for a long time, especially at low temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings of the specification, which form a part of the present invention, are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0026] Figure 1 Microscopic morphologies with dissolving pulp ratios of 0.5% and 1.0% respectively; among them, the left figure is the microscopic morphology of 0.5% in Example 3, and the right figure is the microscopic morphology of 1.0% in Example 2.

[0027] Figure 2 Effects of different amounts of added dissolving pulp on electrochemical performance; among them, the left figure is the current density graph, and the right figure is the capacitance graph.

[0028] Figure 3 Effects of different amounts of added dissolving pulp on mechanical properties. Detailed Description of the Invention

[0029] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0030] The present invention provides a method for effectively dissolving dissolving pulp with a lithium bromide solution, polymerizing acrylamide in a cellulose solution to form an interpenetrating network of two polymers, cellulose and polyacrylamide. The obtained membrane material has a three-dimensional pore structure, can form a conductive path, and has excellent electrochemical performance and mechanical properties. The technology of the present invention can realize the preparation of a supercapacitor electrolyte material with excellent electrochemical performance using degradable and renewable dissolving pulp raw materials.

[0031] In the embodiments of the present invention, the dissolving pulp is sappi dissolving pulp provided by a South African pulp mill, and the α-cellulose content is more than 91%.

[0032] Example 1, 1.5% w / v dissolving pulp test

[0033] A method for preparing a dissolving pulp / polyacrylamide composite membrane with a lithium bromide solution and assembling a supercapacitor, comprising the following steps:

[0034] (1) Add 0.188 g of dissolving pulp with a degree of polymerization of 520 to 4.5 ml of an aqueous lithium bromide solution. The molar ratio of lithium bromide to water is 1:3. The process conditions are: temperature 120 °C, dissolution time 15 min, 800 rpm.

[0035] (2) Dissolve 5 g of acrylamide in 8 ml of deionized water. After complete dissolution, add the acrylamide solution to the cellulose solution in step (1), add 0.25 g of crosslinking agent (MBA) and 0.025 g of initiator (APS). After complete dissolution, pour the mixture into a mold for polymerization. The process conditions are: polymerization temperature 80 °C, polymerization time 20 min.

[0036] (3) Prepare an activated carbon electrode: Mix the activated carbon (AC), acetylene black (AB) and polyvinylidene fluoride (PVDF) powders with a mass ratio of 8:1:1 evenly, then drop an appropriate amount of N-methylpyrrolidone (NMP) and grind thoroughly. Coat the conductive material in the form of slurry evenly on the nickel foam.

[0037] (4) Place the electrode prepared in step (3) in a vacuum drying oven and dry at 80 °C for 4 h, then press it under 10 MPa for 30 s using a tablet press. Assemble this electrode sheet with the dissolved pulp / polyacrylamide composite film in step (2) into a supercapacitor with a sandwich structure.

[0038] (5) Perform electrochemical performance tests on the supercapacitor obtained in (4). Through GCD tests, when the current density is 2 mA / cm 2 , the areal capacitance at room temperature is 125 mF / cm 2 , and the capacitance at -5 °C is 120 mF / cm 2 .

[0039] Example 2, 1.0% w / v dissolved pulp test

[0040] A method for preparing a dissolved pulp / polyacrylamide composite film using a lithium bromide solution and assembling a supercapacitor, comprising the following steps:

[0041] (1) Add 0.125 g of dissolved pulp with a polymerization degree of 700 to 4.5 ml of an aqueous lithium bromide solution. The molar ratio of lithium bromide to water is 1:3. The process conditions are: temperature 130 °C, dissolution time 15 min, 800 rpm.

[0042] (2) Dissolve 5 g of acrylamide in 8 ml of deionized water. After complete dissolution, add the acrylamide solution to the cellulose solution in step (1), add 0.25 g of crosslinking agent (MBA) and 0.025 g of initiator (APS). After complete dissolution, pour the mixture into a mold for polymerization. The process conditions are: polymerization temperature 80 °C, polymerization time 30 min.

[0043] (3) Prepare an activated carbon electrode: Mix the activated carbon (AC), acetylene black (AB) and polyvinylidene fluoride (PVDF) powders with a mass ratio of 7:2:1 evenly, then drop an appropriate amount of N-methylpyrrolidone (NMP) and grind thoroughly. Coat the conductive material in the form of slurry evenly on the nickel foam.

[0044] (4) The electrode prepared in step (3) was placed in a vacuum drying oven at 80°C for 4 h, and pressed at 10 MPa for 30 s using a tablet press. The electrode sheet was assembled with the dissolving pulp / polyacrylamide composite film in step (2) into a sandwich structure supercapacitor.

[0045] (5) The electrochemical performance of the supercapacitor obtained in (4) was tested. The current density was 2 mA / cm 2 When the area specific capacitance is 156mF / cm 2 , the capacitance is 149mF / cm at -5℃ 2 .

[0046] Example 3, 0.5% w / v dissolving pulp test

[0047] A method for preparing a dissolving pulp / carbon conductive material / polypyrrole composite supercapacitor electrode using a lithium bromide solution system comprises the following steps:

[0048] (1) A dissolving pulp with a degree of polymerization of 700 and a mass of 0.0625 g was added to 4.5 ml of lithium bromide aqueous solution, the molar ratio of lithium bromide to water was 1:3, and the process conditions were: temperature 125°C, dissolution time 15 min, and 800 rpm.

[0049] (2) Dissolve 5g of acrylamide in 8ml of deionized water. After fully dissolved, add the acrylamide solution to the cellulose solution in step (1), add 0.25g of crosslinking agent (MBA) and 0.025g of initiator (APS), and after fully dissolved, pour into a mold for polymerization. The process conditions are: polymerization temperature 80°C, polymerization time 40min.

[0050] (3) Preparation of activated carbon electrode: After mixing activated carbon (AC), acetylene black (AB) and polyvinylidene fluoride (PVDF) powder in a mass ratio of 8:1:1, an appropriate amount of N-methylpyrrolidone (NMP) is dropped into the mixture and the mixture is fully ground. The conductive material ground into a slurry is evenly coated on the nickel foam.

[0051] (4) The electrode prepared in step (3) was placed in a vacuum drying oven at 80°C for 4 h, and pressed at 10 MPa for 30 s using a tablet press. The electrode sheet was assembled with the dissolving pulp / polyacrylamide composite film in step (2) into a sandwich structure supercapacitor.

[0052] (5) The electrochemical performance of the supercapacitor obtained in (4) was tested. The current density was 2 mA / cm 2 When the area specific capacitance is 140mF / cm 2 , the capacitance is 133mF / cm at -5℃2 。

[0053] In another embodiment, in step (1), 0.03125 g of dissolving pulp with a degree of polymerization of 700 was added to 4.5 ml of lithium bromide aqueous solution, and other preparation methods were the same as those in Example 2.

[0054] In another embodiment, in step (1), 0.09375 g of dissolving pulp with a degree of polymerization of 700 was added to 4.5 ml of lithium bromide aqueous solution, and other preparation methods were the same as those in Example 2.

[0055] In other embodiments, in step (1), the mass fraction of the dissolving pulp when dissolved in the lithium bromide aqueous solution was 0.38%, 0.75%, 1.5%, 2.26% or any mass fraction between 0.38% and 2.26%.

[0056] Test Example

[0057] The assembled supercapacitor was tested for its electrochemical performance under normal temperature and low temperature environments. As shown by Figure 1 the scanning electron microscope pictures of Figure 2 and the conductive effect results of Figure 2 After cyclic voltammetry and constant current charge-discharge tests were carried out on the supercapacitors assembled with electrolytes with an addition amount ranging from 0.25% to 1.0%, as the addition amount of the dissolving pulp increased, the pore structure became more uniform and the electrochemical performance was better.

[0058] As shown by Figure 3 the mechanical property results of

[0059] By comparing Examples 1 to 3, it can be seen that the electrolyte material provided by the present invention can maintain good electrical properties at low temperatures. The principle is that lithium bromide, as an inorganic salt, has a mass-volume ratio of 40% (w / v) in the finally prepared composite membrane material. A high concentration of inorganic salts can inhibit the formation of ice crystals at low temperatures, and halogen ions have water absorption properties, which can prevent the membrane material from dehydrating. Therefore, good electrochemical performance can be maintained for a long time at low temperatures.

[0060] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a supercapacitor electrolyte, characterized in that: The steps include: S1, dissolving the dissolving slurry with a lithium bromide aqueous solution; S2, adding acrylamide solution; S3, adding a cross-linking agent and an initiator to initiate a polymerization reaction to form a membrane material with an interpenetrating network structure of cellulose and polyacrylamide molecules.

2. The method for preparing a supercapacitor electrolyte according to claim 1, characterized in that: In step S1, the dissolving pulp is dissolving pulp fiber with a degree of polymerization of 500-900 and an α-cellulose content of more than 91%.

3. The method for preparing a supercapacitor electrolyte according to claim 1, characterized in that: In step S1, the molar ratio of lithium bromide to water in the lithium bromide aqueous solution is 1:3 to 1:3.

5.

4. The method for preparing a supercapacitor electrolyte according to claim 1, characterized in that: The mass fraction of the dissolving pulp when dissolved in the lithium bromide aqueous solution is 0.38%-2.26%, the dissolving temperature is 120° C.-130° C., and the cellulose dissolving time is 15-30 minutes.

5. The method for preparing a supercapacitor electrolyte according to claim 1, characterized in that: The polymerization time of acrylamide is 20-60min, and the polymerization temperature is 70℃-90℃.

6. The method for preparing a supercapacitor electrolyte according to claim 1, characterized in that: The cross-linking agent is N,N-methylenebisacrylamide, and the initiator is ammonium persulfate or potassium persulfate.

7. A supercapacitor electrolyte, characterized in that: The invention comprises dissolving pulp 0.25%-1.5% w / v, acrylamide 30%-40% w / v, and lithium bromide 30%-40% w / v.

8. Use of the supercapacitor electrolyte according to claim 7 in preparing a supercapacitor.

9. A supercapacitor, comprising the composite film of the supercapacitor electrolyte according to claim 7, and an activated carbon electrode, wherein the composite film of the supercapacitor electrolyte and the activated carbon electrode are assembled into a sandwich structure.

10. The supercapacitor according to claim 9, characterized in that: The preparation method of the activated carbon electrode is as follows: the activated carbon electrode uses nickel foam as a current collector, the mass ratio of activated carbon, acetylene black and polyvinylidene fluoride powder is 8-7:2-1:1, the powders are mixed, N-methylpyrrolidone is dropped into the powders for thorough grinding to obtain a conductive substance, the uniformly mixed conductive substance is evenly coated on the nickel foam, and the electrode sheet is obtained after drying and pressing.

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