Preparation method for lignin-based binder, and use of lignin-based binder in double-layer supercapacitor

Abstract

Disclosed in the present invention are a preparation method for a lignin-based binder, and the use of the lignin-based binder in a double-layer supercapacitor. PVDF is grafted to water-insoluble lignin molecules to prepare a composite binder, such that the electrochemical performance of a supercapacitor can be improved, and the high-value utilization of waste lignocellulose resources can also be achieved.

Description

A method for preparing a lignin-based binder and its application in electric double-layer supercapacitors Technical fields:

[0001] This invention relates to the fields of high-value utilization of lignin and supercapacitor technology, specifically to a method for preparing a lignin-based binder and its application in electric double-layer supercapacitors. Background technology:

[0002] Supercapacitors, also known as electrochemical capacitors, are mainly composed of carbon electrodes, electrolytes, separators, binders, conductive agents, and current collectors. Based on their energy storage mechanisms, they are primarily classified into symmetrical double-layer supercapacitors and pseudocapacitors. Their energy storage capacity falls between that of batteries and capacitors, and they possess numerous advantages such as fast charging and discharging speeds, high power density, and good cycle stability. They have immense application potential in emerging industries represented by electric vehicles and electronic devices, and are of great significance in promoting the rapid development of new productive forces in my country. Currently, research on supercapacitors mainly focuses on the development of novel carbon materials and dielectrics, with relatively little research on binders. However, binders are crucial for maintaining the structure of conductive active materials. Among the many binders used in batteries / supercapacitors, polyvinylidene fluoride (PVDF) is considered a highly efficient polymer binder due to its good electrical, mechanical, and chemical stability, and has been widely used. However, PVDF also suffers from extremely poor conductivity, easy swelling by electrolyte solutions (leading to loose electrode structures), and poor cycle stability. Furthermore, the preparation of PVDF requires the consumption of large amounts of petrochemical-based chemical products such as hydrogen fluoride and ethylene, which is detrimental to achieving the dual-carbon target.

[0003] Patent CN106025283A discloses a lignin-based aqueous binder for the negative electrode of lithium-ion batteries. This composite binder is prepared from 100 parts of water-soluble lignin (e.g., lignin sulfonate, sulfonated alkali lignin, sulfonated enzymatically hydrolyzed lignin, carboxylated alkali lignin, carboxylated enzymatically hydrolyzed lignin, ammonium-modified alkali lignin, ammonium-modified alkali lignin, ammonium-modified enzymatically hydrolyzed lignin, and ammonium-modified enzymatically hydrolyzed lignin) and 20-1000 parts of styrene-butadiene rubber. It is mainly used in lithium-ion batteries using 1M LiPF6 EC / DMC / DEC (v / v / v = 1 / 1 / 1) as the organic solvent electrolyte. Patent CN110061239A discloses a lignin-based binder, its preparation method, and a lithium-ion battery. This composite binder, prepared from lignin, an initiator, and a prepolymer, is mainly used in lithium-ion batteries using 1mol / L LiPF6 EC and EMC (v / v = 1:1) as the organic solvent electrolyte. The lignin is at least one of alkali lignin, lignin sulfonate, sulfonated alkali lignin, sulfonated enzymatically hydrolyzed lignin, carboxylated alkali lignin, carboxylated enzymatically hydrolyzed lignin, ammonium-modified alkali lignin, and ammonium-modified enzymatically hydrolyzed lignin. The lignin is also water-soluble. However, the lignin-based aqueous binder prepared by the above patent is suitable for lithium-ion batteries with non-aqueous electrolytes. If the disclosed composite binder is used in aqueous electrolyte supercapacitors, it poses a risk of absorbing water and swelling, leading to a loose electrode structure and consequently reducing the electrochemical performance and cycle life of the supercapacitor. Summary of the Invention:

[0004] The purpose of this invention is to provide a method for preparing a lignin-based binder for supercapacitors and its application in electric double-layer supercapacitors.

[0005] This invention is achieved through the following technical solutions:

[0006] A method for preparing a lignin-based binder for supercapacitors involves grafting PVDF onto water-insoluble lignin macromolecules in a solvent medium to prepare a novel composite binder for double-layer supercapacitors. The method includes the following steps: adding water-insoluble lignin and PVDF in a mass ratio of 2:3 to 3:2 to a solvent soluble in water-insoluble lignin and PVDF, and stirring the mixture at 55-65℃ for 20-24 hours.

[0007] The water-insoluble lignins include industrial lignin (sulfate lignin, Kraft, CAS: 8068-05-1), organic solvent lignin, ground wood lignin, enzymatically hydrolyzed lignin, and other water-insoluble lignins.

[0008] The organic solvent lignin includes acidic TEG lignin and basic TEG lignin, which are lignin obtained after separating sugarcane bagasse from the pretreated components using acidic or basic triethylene glycol (TEG) solvent systems. The specific preparation method for acidic TEG lignin is as follows: sugarcane bagasse is pretreated for 120 min at 90°C with 100% TEG (v / v), 0.1 M H₂SO₄, a liquid-to-solid ratio of 20:1. Then, three times the volume of water is added to the pretreated black liquor as a reverse solvent. The mixture is centrifuged, and the precipitate is washed until neutral to obtain acidic TEG lignin. The specific preparation method for basic TEG lignin is as follows: sugarcane bagasse is pretreated for 120 min at 90°C with 20% TEG (v / v), 1% NaOH, a liquid-to-solid ratio of 20:1. Then, three times the volume of water is added to the pretreated black liquor, the pH is adjusted to 2 with dilute hydrochloric acid, and the mixture is centrifuged, and the precipitate is washed until neutral to obtain basic TEG lignin.

[0009] The water-insoluble lignin and PVDF soluble solvent are selected from one or more of N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), methyl vinyl ketone, tetrahydrofuran (THF), and dichloromethane.

[0010] This invention also protects the application of the lignin-based binder in double-layer supercapacitors. The process includes the following steps: mixing lignin porous carbon, conductive carbon black, and the aforementioned lignin-based binder at a mass ratio of 82:10:8 to 80:10:10 to obtain a viscous slurry; adding an organic solvent and stirring until homogeneous; adjusting the solvent content to achieve a semi-dry state; repeatedly and evenly rolling the slurry with a glass rod; cutting out circular electrode films; then bonding them to a nickel foam current collector; and finally, vacuum drying and pressing to obtain electrode sheets.

[0011] The organic solvent is one or both of ethanol and acetone.

[0012] The preparation method of the lignin porous carbon is as follows: using industrial lignin (Kraft lignin, CAS: 8068-05-1) as a precursor, the industrial lignin and KOH are first dissolved in deionized water at a mass ratio of 1:2, and dried in an oven at 105℃ for 12h to obtain a mixed solid; then, in a tube furnace under N2 atmosphere, the temperature is raised from room temperature to 700℃ at a rate of 5℃ / min, held for 2h, and after cooling, the black powder is soaked in 1M HCl aqueous solution to remove excess KOH and impurities, and washed with deionized water until neutral to obtain lignin porous carbon material.

[0013] The present invention also protects a double-layer supercapacitor electrode sheet, comprising a nickel current collector and a material layer attached to the surface of the nickel current collector, characterized in that: the material layer comprises lignin porous carbon, conductive carbon black and the above-mentioned lignin-based binder.

[0014] The double-layer supercapacitor includes a positive electrode shell, a negative electrode shell, electrode plates, an aqueous PP diaphragm, an electrolyte, and a sealing ring. The electrolyte is a KOH electrolyte solution.

[0015] The beneficial effects of this invention are as follows:

[0016] 1) Using renewable water-insoluble lignin as raw material is not only inexpensive, but also helps to realize the high-value utilization of waste materials.

[0017] 2) By replacing 40-50% of the traditional polyvinylidene fluoride (PVDF) binder with water-insoluble lignin, and using water-insoluble lignin and PVDF soluble solvent as a medium, PVDF is grafted onto water-insoluble lignin molecules to form a novel lignin@PVDF composite binder. The process of preparing composite binders by modifying PVDF with water-insoluble lignin is simple, effective, and low-cost, significantly reducing the use of petrochemical resources and contributing to the achievement of dual carbon targets.

[0018] 3) The novel lignin@PVDF composite binder obtained can be assembled with lignin porous carbon materials to prepare double-layer supercapacitors. Water-insoluble lignin molecules can be strengthened through covalent bonds (CO and CC bonds) and hydrogen bonds. Compared to traditional PVDF binders and existing water-based lignin-based composite binders, the low-water-absorption composite binder disclosed in this invention enhances the bonding performance to lignin porous carbon materials, significantly improving the electrochemical performance of water-based electrolyte double-layer supercapacitors.

[0019] In summary, water-insoluble lignin is a major byproduct in the pulp and paper or cellulosic ethanol biorefining industry. By grafting PVDF onto water-insoluble lignin molecules using this method to prepare composite binders, not only can the electrochemical performance of supercapacitors be improved, but also the high-value utilization of waste lignocellulosic resources can be achieved. Attached image description:

[0020] Figure 1 shows the electrode sheets prepared in the comparative examples, Example 1, Example 2, and Example 3 of this invention at 200 mV·s. -1 Cyclic voltammetry curves at scan rate.

[0021] Figure 2 shows the specific capacitance of the electrode sheets prepared in the comparative examples, Example 1, Example 2 and Example 3 of the present invention as a function of current density.

[0022] Figure 3 shows the infrared spectra of the adhesives and lignin raw materials prepared in the comparative examples, Example 2 and Example 3 of the present invention, where aL@PVDF refers to the acidic TEG lignin@PVDF composite adhesive prepared in Example 2, and bL@PVDF refers to the alkaline TEG lignin@PVDF composite adhesive prepared in Example 3. Detailed implementation method:

[0023] The following is a further description of the invention, but not a limitation thereof.

[0024] Comparative example:

[0025] 0.5 g of PVDF was added to 20 mL of NMP solvent and stirred at 60 °C for 24 h to obtain a viscous solution containing PVDF binder. Kraft lignin porous carbon, conductive carbon black, and PVDF binder were mixed in the above viscous solution at a mass ratio of 8:1:1 to obtain a viscous slurry. An appropriate amount of ethanol was added to the slurry and stirred until homogeneous. The ethanol content was adjusted to make the slurry semi-dry. The slurry was repeatedly and evenly rolled with a glass rod for 15 min, and a circular electrode film with a diameter of 14 mm was cut out using a punch. The electrode film was bonded to a nickel foam current collector, vacuum dried at 70 °C for 12 h, and then pressed into a sheet under 10 MPa and 60 s to obtain an electrode sheet. Its electrochemical performance was measured in a three-electrode system using 6 M KOH solution as the electrolyte.

[0026] The CV curve of the Kraft lignin-based porous carbon electrode sheet prepared with PVDF binder, as shown in Figure 1, is rectangular, indicating that it exhibits standard double-layer capacitance behavior. As shown in Figure 2, the specific capacitance can reach 166.3 F / g at a current density of 0.5 A / g.

[0027] Example 1

[0028] 0.2 g of Kraft lignin and 0.3 g of PVDF were added to 20 mL of NMP solvent and stirred at 60 °C for 24 h to obtain a viscous solution containing Kraft lignin@PVDF composite binder. Kraft lignin porous carbon, conductive carbon black, and Kraft lignin@PVDF composite binder were mixed in the above viscous solution at a mass ratio of 8:1:1 to obtain a viscous slurry. An appropriate amount of ethanol was added to the slurry and stirred until homogeneous. The ethanol content was adjusted to make the slurry semi-dry. The slurry was repeatedly and evenly rolled with a glass rod for about 15 min. A circular electrode film with a diameter of 14 mm was cut out using a punch. The electrode film was bonded to a nickel foam current collector, vacuum dried at 70 °C for 12 h, and then pressed into a sheet under 10 MPa and 60 s conditions to obtain an electrode sheet. Its electrochemical performance was measured in a three-electrode system using 6 M KOH solution as the electrolyte.

[0029] The CV curve of the Kraft lignin-based porous carbon electrode sheet prepared with Kraft lignin@PVDF composite binder, as shown in Figure 1, is rectangular, indicating that it exhibits standard double-layer capacitance behavior. As shown in Figure 2, at a current density of 0.5 A / g, the specific capacitance can reach 170.1 F / g, which is better than the comparative example (166.3 F / g).

[0030] Example 2

[0031] 0.2 g of acidic TEG lignin and 0.3 g of PVDF were added to 20 mL of NMP solvent and stirred at 60 °C for 24 h to obtain a viscous solution containing acidic TEG lignin@PVDF composite binder. Kraft porous lignin carbon, conductive carbon black, and acidic TEG lignin@PVDF composite binder were mixed in the above viscous solution at a mass ratio of 8:1:1 to obtain a viscous slurry. An appropriate amount of ethanol was added to the slurry and stirred until homogeneous. The ethanol content was adjusted to make the slurry semi-dry. The slurry was repeatedly and evenly rolled with a glass rod for ~15 min, and a circular electrode film with a diameter of 14 mm was cut out using a punch. The electrode film was bonded to a nickel foam current collector, vacuum dried at 70 °C for 12 h, and pressed into a sheet under 10 MPa and 60 s to obtain an electrode sheet. The electrochemical performance was measured in a three-electrode system using 6 M KOH solution as the electrolyte.

[0032] The CV curves of the Kraft lignin-based porous carbon electrode sheet prepared using acidic TEG lignin@PVDF composite binder, as shown in Figure 1, are rectangular, indicating standard double-layer capacitance behavior. As shown in Figure 2, at a current density of 0.5 A / g, the specific capacitance reaches 171.9 F / g, superior to the comparative example (166.3 F / g). As shown in Figure 3, comparison of the infrared spectra reveals that acidic TEG lignin and PVDF were successfully grafted via covalent bonds.

[0033] Example 3

[0034] 0.25 g of alkaline TEG lignin and 0.25 g of PVDF were added to 20 mL of NMP solvent and stirred at 60 °C for 24 h to obtain a viscous solution containing alkaline TEG lignin@PVDF composite binder. Kraft lignin porous carbon, conductive carbon black, and alkaline TEG lignin@PVDF composite binder were mixed in the above viscous solution at a mass ratio of 8:1:1 to obtain a viscous slurry. An appropriate amount of ethanol was added to the slurry and stirred until homogeneous. The ethanol content was adjusted to make the slurry semi-dry. The slurry was repeatedly and evenly rolled with a glass rod for ~15 min, and a circular electrode film with a diameter of 14 mm was cut out using a punch. The electrode film was bonded to a nickel foam current collector, vacuum dried at 70 °C for 12 h, and then pressed into a sheet under 10 MPa and 60 s to obtain an electrode sheet. The electrochemical performance was measured in a three-electrode system using 6 M KOH solution as the electrolyte.

[0035] The CV curves of the Kraft lignin-based porous carbon electrode sheet prepared using alkaline TEG lignin@PVDF composite binder, as shown in Figure 1, are rectangular, indicating standard double-layer capacitance behavior. As shown in Figure 2, at a current density of 0.5 A / g, the specific capacitance reaches 194.5 F / g, superior to the comparative example (166.3 F / g). As shown in Figure 3, comparison of the infrared spectra reveals that alkaline TEG lignin and PVDF were successfully grafted via covalent bonds. The analysis of the infrared spectra in Figure 3 is shown in Table 1.

[0036] Table 1

[0037] The applicant declares that the present invention is illustrated through the above embodiments to demonstrate its detailed features and methods, but the present invention is not limited to the above detailed features and methods, that is, it does not mean that the present invention must rely on the above detailed features and methods to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the materials and steps used in the present invention, additions of auxiliary materials and steps, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for the preparation of a lignin-based binder for supercapacitors, characterized by, A novel composite binder for electric double-layer supercapacitors is prepared by grafting PVDF onto water-insoluble lignin macromolecules in a solvent medium. The process includes the following steps: adding water-insoluble lignin and PVDF to a solvent soluble in water-insoluble lignin and PVDF at a mass ratio of 2:3 to 3:2, and stirring the mixture at 55-65℃ for 20-24 hours.

2. The production method according to claim 1, characterized by, The water-insoluble lignin includes industrial lignin, organic solvent lignin, ground wood lignin, and enzymatically hydrolyzed lignin.

3. The production method according to claim 2, characterized by, The organic solvent lignin includes acidic TEG lignin and alkaline TEG lignin, which is the lignin obtained after separating sugarcane bagasse from the pretreated components of the acidic or alkaline triethylene glycol solvent system.

4. The production method according to claim 3, characterized by, The specific preparation method of acidic TEG lignin is as follows: 100% vol. TEG, 0.1 M H2SO4, liquid-solid ratio 20:1, sugarcane bagasse pretreated for 120 min at 90℃. Then, 3 times the volume of water was added to the pretreated black liquor as a reverse solvent. After centrifugation, the precipitate was washed until neutral to obtain acidic TEG lignin. The specific preparation method of alkaline TEG lignin is as follows: 20 vol.% TEG, 1 wt% NaOH, liquid-solid ratio 20:1, sugarcane bagasse pretreated for 120 min at 90℃. Then, 3 times the volume of water was added to the pretreated black liquor, the pH was adjusted to 2 with dilute hydrochloric acid, after centrifugation, the precipitate was washed until neutral to obtain alkaline TEG lignin.

5. The preparation method according to claim 1, characterized in that, The water-insoluble lignin and PVDF soluble solvent are selected from one or more of N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, methyl vinyl ketone, tetrahydrofuran, and dichloromethane.

6. Use of the lignin-based binder obtained by the production process according to claim 1, characterized in that, Used in electric double-layer supercapacitors.

7. Use according to claim 6, characterized in that, Includes the following steps: The lignin porous carbon, conductive carbon black and the above-mentioned lignin-based binder are mixed in a mass ratio of 82:10:8 to 80:10:10 to obtain a viscous slurry. An organic solvent is added and stirred evenly. The slurry is made into a semi-dry state by adjusting the solvent content. The slurry is repeatedly and evenly rolled with a glass rod to cut out a circular electrode film. Then it is bonded to a nickel foam current collector. After vacuum drying and pressing, an electrode sheet is obtained.

8. Use according to claim 7, characterized in that, The organic solvent is one or both of ethanol and acetone.

9. Use according to claim 7, characterized in that, The preparation method of the lignin porous carbon is as follows: using industrial lignin as a precursor, the industrial lignin and KOH are first dissolved in deionized water at a mass ratio of 1:2, and dried in an oven at 105℃ for 12h to obtain a mixed solid; then, in a tube furnace under N2 atmosphere, the temperature is raised from room temperature to 700℃ at a rate of 5℃ / min, held for 2h, and after cooling, the black powder is soaked in 1M HCl aqueous solution to remove excess KOH and impurities, and washed with deionized water until neutral to obtain lignin porous carbon material.

10. A double layer supercapacitor electrode sheet comprising a nickel current collector and a layer of material attached to the surface of the nickel current collector, characterized in that: The material layer includes lignin porous carbon, conductive carbon black, and the aforementioned lignin-based binder.

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