A self-supporting electrode, a preparation method thereof, and a supercapacitor

Abstract

The application belongs to the technical field of electrochemical energy storage, and specifically discloses a self-supporting electrode, a preparation method thereof and a supercapacitor, which comprises the following steps: S1, adding graphene oxide into deionized water and performing first ultrasonic emulsification, then adding a nanocellulose solution, performing second ultrasonic emulsification and stirring to obtain a colloidal solution; S2, performing suction filtration on the colloidal solution obtained in S1 and drying to obtain a composite film; and S3, uniformly laying polyvinylidene fluoride in a crucible, placing the composite film obtained in S2 on the polyvinylidene fluoride, placing the crucible in a tube furnace, performing high-temperature calcination in an inert atmosphere, and taking out after cooling to obtain the self-supporting electrode.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, and in particular to a self-supporting electrode and its preparation method, and a supercapacitor. Background Technology

[0002] Supercapacitors, as high-power-density, long-cycle-life, fast-charge-discharge energy storage devices, have demonstrated significant application value in fields such as start-stop systems for new energy vehicles, energy recovery in rail transit, and frequency regulation in smart grids. However, current mainstream super-activated carbon-based materials are limited by low specific surface area utilization; the actual specific surface area of ​​commercially available products is currently less than 2000 m². 2 The pore connectivity is less than 30%, resulting in a lack of intrinsic electrochemical activity. Furthermore, current traditional supercapacitor electrodes use aluminum foil as the current collector, and the preparation process requires the addition of 10%-30% insulating polymer binder. These inactive components not only block ion transport channels but also reduce the electrode specific capacitance (calculated by the entire electrode mass, the supercapacitor's mass specific capacitance is <90F / g). These combined effects lead to low mass and volumetric specific capacitances in supercapacitors, severely limiting their application range.

[0003] To address the aforementioned issues, current research focuses on: 1) introducing heteroatoms such as nitrogen, sulfur, and oxygen for doping. These heteroatoms can improve pseudocapacitance, but this is only applicable to aqueous supercapacitors. In commercial organic systems (such as EMIM-BF4 / AN) supercapacitors, they react with the electrolyte, leading to gas buildup or failure; 2) increasing the specific surface area and pore volume of commercially available super-activated carbon. However, research has found that increasing the specific surface area leads to an increased micropore ratio, reducing the electrolyte wetting rate, extending ion transport distance, and imposing stringent requirements on the preparation process and equipment, significantly increasing production costs; 3) improving supercapacitor performance by incorporating novel materials, such as graphene and MOF materials, with super-activated carbon. However, these methods have limited effectiveness in improving the specific capacitance of supercapacitors, and issues such as poor component interface compatibility and discontinuous conductive networks often result in significant deterioration of electrode rate performance and cycle stability. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a self-supporting electrode and its preparation method, as well as a supercapacitor. This solves the problem that existing technologies require the addition of 10%-30% insulating polymer binder to the electrode and rely on current collectors such as aluminum foil, resulting in a high proportion of inactive components and thus a low specific capacity calculated based on the entire electrode mass.

[0005] The technical solution adopted by this invention to solve its technical problem is: A method for fabricating a self-supporting electrode includes the following steps: S1, add graphene oxide to deionized water and perform a first ultrasonic emulsification, then add nanocellulose solution, perform a second ultrasonic emulsification and stir to obtain a colloidal solution; S2, the colloidal solution obtained in S1 is filtered and dried to obtain a composite film; S3. Polyvinylidene fluoride is evenly spread in a crucible. The composite film obtained in S2 is placed on top of the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at high temperature in an inert atmosphere. After cooling, it is taken out to obtain a self-supporting electrode.

[0006] As a further approach, in S1, the mass ratio of graphene oxide to cellulose nanoparticles is 10:2-5. Specifically, if the cellulose nanoparticle content is too low, the cellulose nanoparticles are insufficient to effectively insert between the graphene oxide sheets, failing to form a sufficient physical barrier. After high-temperature calcination, the graphene sheets will still undergo severe stacking and densification, leading to a sharp reduction in the electrolyte's accessible active interface and a decrease in specific capacitance. If the cellulose nanoparticle content is too high, although the barrier effect is further enhanced, the cellulose nanoparticles themselves become amorphous carbon after high-temperature carbonization, and their conductivity is far lower than that of reduced graphene oxide. Excessive introduction will dilute the proportion of highly conductive components in the electrode, leading to a decrease in overall electronic conductivity. At the same time, the increase in inactive carbon components will also reduce the specific capacity.

[0007] As a further option, preferably, in S1, the mass ratio of graphene oxide to deionized water is 1:5-10, thereby achieving a suitable concentration of graphene oxide.

[0008] As a further embodiment, the concentration of the nanocellulose solution is 10-20 wt%.

[0009] Specifically, concentrations below 10 wt% result in insufficient nanocellulose content, leading to inadequate hydrogen-bonded cross-linking with graphene oxide after mixing. During ultrasonic emulsification and stirring, the lack of sufficient nanocellulose interpenetration between graphene oxide sheets makes aggregation or sedimentation likely, reducing the stability of the colloidal solution and causing structural inhomogeneity during subsequent filtration and film formation. Concentrations above 20 wt% result in excessively high nanocellulose concentration, causing a sharp increase in solution viscosity and decreased fluidity. Uniform dispersion is difficult to achieve when mixing with graphene oxide, easily forming localized enrichment areas, leading to uneven distribution of graphene oxide and nanocellulose in the composite system and affecting the structural consistency of the final film. A concentration range of 10-20 wt% ensures good dispersibility of nanocellulose in water while allowing it to fully interact with graphene oxide after mixing, forming a stable and homogeneous colloidal system.

[0010] As a further embodiment, the mass ratio of the composite film to polyvinylidene fluoride is 1-3:1.

[0011] Specifically, if the amount of polyvinylidene fluoride (PVDF) is too small, the concentration of fluorine radicals generated at high temperatures will be insufficient to effectively replace the unstable oxygen-containing functional groups (such as carboxyl and carbonyl groups) on the surface of graphene oxide, resulting in insufficient fluorine doping. This will weaken the conductivity-enhancing effect of the semi-ionic CF bond and make it difficult to effectively reduce the risk of gas generation from side reactions between the electrode and the organic electrolyte at high potentials. If the amount of PVDF is too large, excessive fluorine radicals will over-etch the carbon skeleton, destroying the two-dimensional conductive network of graphene and the structure after carbonization of cellulose, leading to a decrease in the mechanical strength of the electrode, discontinuity of the conductive network, and even breakage of the self-supporting film, resulting in the loss of the morphological integrity of the integrated electrode.

[0012] As a further embodiment, the graphene oxide sheet diameter is 5-20 μm.

[0013] Specifically, if the sheet size is too small (<5μm): the small size of graphene oxide sheets, while easy to disperse, results in fewer hydrogen bond crosslinking points with nanocellulose, making it difficult to construct a continuous network structure. Simultaneously, small-sized sheets are easily penetrated by water flow during filtration, leading to uneven film distribution. Nanocellulose cannot effectively penetrate between graphene sheets, weakening the barrier effect on graphene stacking, and densification still easily occurs after high-temperature calcination. If the sheet size is too large (>20μm): the large size of graphene oxide sheets easily causes curling or sedimentation in colloidal solutions, making it difficult to form a uniformly dispersed composite system when mixed with nanocellulose. Large-sized sheets tend to form tight stacks, making it difficult for nanocellulose to penetrate into the sheet interior, resulting in uneven distribution of graphene and nanocellulose in the composite film, affecting structural uniformity. Within the 5-20μm range, the graphene oxide sheet size is moderate, allowing for sufficient hydrogen bond network formation with nanocellulose while enabling effective penetration of nanocellulose between sheets, achieving good physical barrier and structural uniformity.

[0014] As a further option, in S1, the time for the first ultrasonic emulsification is 60-120 min, the time for the second ultrasonic emulsification is 20-60 min, and the stirring time is 60-120 min; in S2, the drying temperature is 60-90℃, and the drying time is 8-12 h.

[0015] As a further option, in S3, during the high-temperature calcination process, the temperature is 800-950℃, the calcination time is 2-3 hours, and the heating rate is 5-10℃ / min.

[0016] In addition, the present invention also provides a self-supporting electrode, which is prepared by the above-described method for preparing a self-supporting electrode.

[0017] In addition, the present invention also provides a supercapacitor, comprising: The positive electrode uses the self-supporting electrode described above; The negative electrode uses the self-supporting electrode described above; A membrane is disposed between the positive electrode and the negative electrode to separate the positive electrode from the negative electrode while allowing ions to pass through; The electrolyte fills the pores formed by the positive electrode, the negative electrode, and the separator; The outer casing is used to house the positive electrode, the negative electrode, the separator, and the electrolyte, and to lead out the positive and negative terminals.

[0018] As a further embodiment, the diaphragm is a cellulose diaphragm; and the electrolyte is an acetonitrile / tetraethylamine tetrafluoroborate electrolyte.

[0019] This invention has at least one of the following beneficial effects: (1) The self-supporting electrode prepared by this invention does not require the addition of insulating polymer binders and conductive carbon black necessary in traditional electrodes, nor does it require the use of metal current collectors such as aluminum foil or copper foil, thus completely eliminating the weight ratio of inactive components in the electrode mass. At the same time, the integrated structure avoids multi-interface contact impedance between the active material and the current collector, and between the active material and the binder, which greatly improves the overall electronic conduction efficiency of the electrode. This structural design enables the supercapacitor to achieve significant improvements in both mass specific capacitance and volume specific capacitance, effectively overcoming the technical bottlenecks of low specific capacitance and high internal resistance of traditional electrodes.

[0020] (2) By utilizing the hydrogen bond network formed between the oxygen-containing groups on the surface of graphene oxide and the hydrophilic groups of nanocellulose, and combining the complementary structure of rigidity and flexibility of the two, nanocellulose is uniformly interspersed between graphene sheets. On the one hand, this structure effectively inhibits the densification caused by the recombination and lattice reconstruction of graphene during high-temperature calcination, thus preserving abundant accessible active interfaces for electrolyte ions; on the other hand, the composite film can dissipate energy through its own deformation under external force, significantly improving the mechanical flexibility and structural integrity of the material, enabling it to be used directly as a self-supporting electrode.

[0021] (3) Using polyvinylidene fluoride (PVDF) as the fluorine source, PVDF undergoes carbon-hydrogen bond cleavage during high-temperature calcination to generate highly active fluorine radicals, which are then used for in-situ fluorine doping of the composite film. Fluorine atoms have strong electronegativity (4.0) and can form semi-ionic CF bonds within the carbon material plane, improving the intrinsic conductivity of the material. At the same time, fluorine doping significantly improves the wettability of the electrode surface to the organic electrolyte, reduces the contact angle, and accelerates the adsorption / desorption kinetics of ions at the electrode interface, thereby effectively reducing the internal resistance of the supercapacitor. More importantly, fluorine radicals can replace unstable oxygen-containing functional groups (such as carboxyl and carbonyl groups) on the surface of graphene oxide, reducing the risk of side reactions and gas generation between the electrode and commercial organic electrolytes (such as EMIM-BF4 / AN, AN / TEABF4, etc.) at high potentials, avoiding device swelling failure, and solving the key problem of poor compatibility of traditional heteroatom doping in organic systems. Attached Figure Description

[0022] Figure 1 These are the XPS full spectrum analysis diagrams of Example 4 and Comparative Example 2 of the present invention; Figure 2 This is a comparison chart of the electrolyte wetting angle analysis of Example 4 and Comparative Example 2 of the present invention. Detailed Implementation

[0023] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0024] It should be noted that the self-supporting electrode of the present invention is prepared by one of the self-supporting electrode preparation methods in the following embodiments.

[0025] In addition, the present invention also provides a supercapacitor, comprising: The positive electrode uses the self-supporting electrode described above; The negative electrode uses the self-supporting electrode described above; The membrane, made of cellulose, is placed between the positive and negative electrodes to separate them while allowing ions to pass through. The electrolyte is an acetonitrile / tetraethylamine tetrafluoroborate electrolyte, which fills the pores formed by the positive electrode, negative electrode and the separator; The outer casing is used to house the positive electrode, negative electrode, separator, and electrolyte, and to bring out the positive and negative terminals.

[0026] The present invention will be further illustrated by the following examples.

[0027] Example 1 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 5μm was added to 20g of deionized water and ultrasonically emulsified for 90min using an ultrasonic disperser. Then, 10g of 10 wt% nanocellulose solution was added and ultrasonically emulsified for 30min using an ultrasonic disperser, and stirred for 120min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 60℃ for 12h to obtain a composite film; S3. 1.5g of polyvinylidene fluoride is evenly spread in the crucible. The composite film obtained in S2 is placed on the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at 850℃ for 2 h in an argon atmosphere with a heating rate of 5℃ / min. After cooling, the self-supporting electrode is obtained.

[0028] Example 2 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 5μm was added to 20g of deionized water and ultrasonically emulsified for 90min using an ultrasonic disperser. Then, 6g of 10 wt% nanocellulose solution was added and ultrasonically emulsified for 20min using an ultrasonic disperser, and stirred for 120min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 80℃ for 12h to obtain a composite film; S3. 1.3g of polyvinylidene fluoride is evenly spread in the crucible. The composite film obtained in S2 is placed on the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at 850℃ for 2 hours in an argon atmosphere with a heating rate of 5℃ / min. After cooling, the self-supporting electrode is obtained.

[0029] Example 3 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 15μm was added to 20g of deionized water and ultrasonically emulsified for 60min using an ultrasonic disperser. Then, 4g of 10 wt% nanocellulose solution was added and ultrasonically emulsified for 30min using an ultrasonic disperser, and stirred for 60min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 90℃ for 10h to obtain a composite film; S3. 1.2g of polyvinylidene fluoride is evenly spread in the crucible. The composite film obtained in S2 is placed on the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at 850℃ for 2 h in an argon atmosphere with a heating rate of 5℃ / min. After cooling, the self-supporting electrode is obtained.

[0030] Example 4 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 15μm was added to 20g of deionized water and ultrasonically emulsified for 90min using an ultrasonic disperser. Then, 6g of 10 wt% nanocellulose solution was added and ultrasonically emulsified for 60min using an ultrasonic disperser, and stirred for 90min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 90℃ for 8 hours to obtain a composite film; S3. 0.87g of polyvinylidene fluoride is evenly spread in the crucible. The composite film obtained in S2 is placed on the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at 900℃ for 2 h in an argon atmosphere with a heating rate of 10℃ / min. After cooling, the self-supporting electrode is obtained.

[0031] Example 5 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 20μm was added to 20g of deionized water and ultrasonically emulsified for 120min using an ultrasonic disperser. Then, 3g of 20 wt% nanocellulose solution was added and ultrasonically emulsified for 60min using an ultrasonic disperser, and stirred for 90min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 60℃ for 12h to obtain a composite film; S3. 2.6g of polyvinylidene fluoride is evenly spread in the crucible. The composite film obtained in S2 is placed on the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at 950℃ for 2 h in an argon atmosphere with a heating rate of 10℃ / min. After cooling, the self-supporting electrode is obtained.

[0032] Example 6 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 20μm was added to 10g of deionized water and ultrasonically emulsified for 120min using an ultrasonic disperser. Then, 4g of 15 wt% nanocellulose solution was added and ultrasonically emulsified for 20min using an ultrasonic disperser, and stirred for 60min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 90℃ for 8 hours to obtain a composite film; S3. 1.3g of polyvinylidene fluoride is evenly spread in the crucible. The composite film obtained in S2 is placed on the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at 800℃ for 3 h in an argon atmosphere with a heating rate of 5℃ / min. After cooling, the self-supporting electrode is obtained.

[0033] Example 7 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 20μm was added to 15g of deionized water and ultrasonically emulsified for 60min using an ultrasonic disperser. Then, 4g of 10wt% nanocellulose solution was added and ultrasonically emulsified for 40min using an ultrasonic disperser, and stirred for 90min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 90℃ for 10h to obtain a composite film; S3. 1.2g of polyvinylidene fluoride is evenly spread in the crucible. The composite film obtained in S2 is placed on the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at 850℃ for 3 h in an argon atmosphere with a heating rate of 5℃ / min. After cooling, the self-supporting electrode is obtained.

[0034] Comparative Example 1 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 15μm was added to 20g of deionized water and ultrasonically emulsified for 90min using an ultrasonic disperser to obtain a colloidal solution; S2, the colloidal solution obtained in S1 is filtered and dried at 90℃ for 8 hours to obtain a thin film; S3. Place the thin film obtained in S2 in a crucible, then place the crucible in a tube furnace, and calcine it at 900℃ for 2 h in an argon atmosphere with a heating rate of 10℃ / min. After cooling, remove the film to obtain a self-supporting electrode.

[0035] Comparative Example 2 This embodiment describes a method for fabricating a self-supporting electrode, comprising the following steps: S1, 2g of graphene oxide with an average sheet diameter of 15μm was added to 20g of deionized water and ultrasonically emulsified for 90min using an ultrasonic disperser. Then, 6g of 10 wt% nanocellulose solution was added and ultrasonically emulsified for 60min using an ultrasonic disperser, and stirred for 90min to obtain a colloidal solution. S2, the colloidal solution obtained in S1 is filtered and dried at 90℃ for 8 hours to obtain a composite film; S3. Place the composite film obtained in S2 in a crucible, then place the crucible in a tube furnace, and calcine it at 900℃ for 2 h in an argon atmosphere with a heating rate of 10℃ / min. After cooling, remove the film to obtain a self-supporting electrode.

[0036] Performance testing Example and comparative performance testing methods: (1) Supercapacitor assembly: The self-supporting electrode is punched into a Φ14mm round piece, and the separator is cut into a Φ16mm round piece; the electrode and separator are immersed in 1M TEABF4 / PC electrolyte and treated under vacuum conditions (-0.1MPa) for 15min; in an argon glove box (H2O<0.1ppm), they are sequentially packaged as follows: negative electrode shell → spring sheet → gasket → negative electrode sheet (add 50μL electrolyte) → separator (add 50μL electrolyte) → positive electrode sheet (add 50μL electrolyte) → positive electrode shell, and the CR2032 button capacitor is formed by maintaining a pressure of 25±2 MPa for 30s using a hydraulic sealing machine. Internal resistance test method: The test is performed using an electrochemical workstation with a frequency of 0.05-100000HZ; (2) Capacitance test method: The test was conducted using an electrochemical workstation. Based on the electrode quality, the currents were set sequentially to 0.5A / g, 1A / g, 2A / g, 5A / g, and 10A / g. The potential window was 0-2.7V. The specific capacitance was calculated based on the discharge time.

[0037] Calculation formula (single electrode): Cm = 4 I×Δt / (m×ΔV) in: Cm: Specific capacitance (F / g) I: Discharge current (A) Δt: Discharge time (s) m: Total mass of active material in a single electrode (g) ΔV: Discharge voltage window (V) The performance test results of the supercapacitors corresponding to Examples 1-7 and Comparative Examples 1-2 are shown in Table 1 below: Table 1. Performance Test Comparison of Examples and Comparative Examples As shown in Table 1 above, the supercapacitor made from the self-supporting electrode material of the present invention has an internal resistance of 0.50-0.74 Ω and a specific capacitance of 125-141 F / g. The specific capacitance values ​​in Examples 1-7 are greater than those in Comparative Example 2, but the performance improvement is limited. However, the internal resistance values ​​are much lower than those in Comparative Example 2, indicating that fluorine doping resulting from polyvinylidene fluoride treatment can significantly reduce the internal resistance of the supercapacitor.

[0038] The specific capacitance of Comparative Example 2 is greater than that of Comparative Example 1, indicating that the addition of nanocellulose can improve the performance of graphene oxide, increase the specific capacitance calculated based on the entire electrode mass, significantly improve the specific capacitance of the supercapacitor, and thus improve the performance of the supercapacitor.

[0039] Appendix Figure 1 The XPS spectra of Example 4 and Comparative Example 2 are shown in the figures. It can be seen from the figures that after high-temperature treatment with added polyvinylidene fluoride, a distinct F1s peak appears in the XPS spectra, indicating successful F (fluorine) doping. (See attached figures.) Figure 2 The diagram shows a comparison of the electrolyte wetting angle analysis between Example 4 and Comparative Example 2, illustrating that the electrolyte contact angle decreases and the wettability of the electrolyte improves after F (fluorine) doping.

[0040] In summary, this electrode, based on a synergistic composite design of graphene oxide and cellulose nanoparticles, constructs a three-dimensional network framework with both high specific surface area and mechanical stability, and features an optimized gradient pore distribution. The directional introduction of fluorine atoms into the carbon substrate significantly enhances the electrode's intrinsic affinity for organic electrolytes, effectively improving the contact efficiency between the electrolyte and the active surface. Furthermore, this three-dimensional network framework completely eliminates the need for aluminum foil current collectors and polymeric binders, comprehensively improving both the electrode's mass specific capacitance and volumetric specific capacitance while simultaneously achieving high volumetric energy density and excellent cycle durability, providing a new technological platform for advancing the practical application of organic supercapacitors.

[0041] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing a self-supporting electrode, characterized in that: Includes the following steps: S1, add graphene oxide to deionized water and perform a first ultrasonic emulsification, then add nanocellulose solution, perform a second ultrasonic emulsification and stir to obtain a colloidal solution; S2, the colloidal solution obtained in S1 is filtered and dried to obtain a composite film; S3. Polyvinylidene fluoride is evenly spread in a crucible. The composite film obtained in S2 is placed on top of the polyvinylidene fluoride. The crucible is then placed in a tube furnace and calcined at high temperature in an inert atmosphere. After cooling, it is taken out to obtain a self-supporting electrode.

2. The method for preparing a self-supporting electrode according to claim 1, characterized in that: In S1, the mass ratio of graphene oxide to cellulose nanoparticles is 10:2-5.

3. The method for preparing a self-supporting electrode according to claim 1, characterized in that: The concentration of the nanocellulose solution is 10-20 wt%.

4. The method for preparing a self-supporting electrode according to claim 1, characterized in that: The mass ratio of the composite film to polyvinylidene fluoride is 1-3:

1.

5. The method for preparing a self-supporting electrode according to claim 1, characterized in that: The graphene oxide sheets have a diameter of 5-20 μm.

6. The method for preparing a self-supporting electrode according to claim 1, characterized in that: In S1, the first ultrasonic emulsification takes 60-120 min, the second ultrasonic emulsification takes 20-60 min, and the stirring time is 60-120 min; in S2, the drying temperature is 60-90℃, and the drying time is 8-12 h.

7. The method for preparing a self-supporting electrode according to claim 1, characterized in that: In S3, during the high-temperature calcination process, the temperature is 800-950℃, the calcination time is 2-3 hours, and the heating rate is 5-10℃ / min.

8. A self-supporting electrode, characterized in that: It is prepared by the method of any one of claims 1-7.

9. A supercapacitor, characterized in that: include: The positive electrode is a self-supporting electrode as described in claim 8; The negative electrode is a self-supporting electrode as described in claim 8; A membrane is disposed between the positive electrode and the negative electrode to separate the positive electrode from the negative electrode while allowing ions to pass through; The electrolyte fills the pores formed by the positive electrode, the negative electrode, and the separator; The outer casing is used to house the positive electrode, the negative electrode, the separator, and the electrolyte, and to lead out the positive and negative terminals.

10. The supercapacitor according to claim 9, characterized in that: The diaphragm is a cellulose diaphragm; The electrolyte is an acetonitrile / tetraethylamine tetrafluoroborate electrolyte.

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