High-performance flexible graphene-based supercapacitor and preparation method thereof

Abstract

The invention provides a flexible supercapacitor. The flexible supercapacitor comprises a graphene-polyimide composite electrode with conductive networks connected with one another, a gel electrolyte and a flexible packaging layer. The problems that the performance of an existing flexible supercapacitor is reduced and the thermal stability is insufficient under repeated bending, folding or stretching are solved, and preparation of the flexible supercapacitor with high performance, high flexibility and high thermal stability is achieved. Experiments prove that the specific capacitance of the flexible supercapacitor obtained by the invention is high and is greater than 200F / g; the rate capability is extremely high and is greater than 70%; the flexibility (mechanical stability) is very good and is greater than 80%; the thermal stability is very good and is greater than 80%; the cycling stability (10,000 times) is high and is greater than 85%; and the energy / power density is extremely high and is greater than 9 Wh / kg.

Description

Technical Field

[0001] The present invention relates to the technical field of supercapacitors, and in particular to a high-performance flexible graphene-based supercapacitor and a preparation method thereof. Background Art

[0002] Flexible supercapacitors have received great attention as the demand for wearable electronics, portable devices, and flexible energy storage systems continues to grow. Traditional rigid supercapacitors lack the mechanical adaptability required for integration into flexible and wearable devices. Flexible supercapacitors address this limitation by combining materials and designs that allow bending, stretching, and folding while maintaining excellent electrochemical performance. Flexible supercapacitors need to achieve uniform and stable integration of active materials with flexible substrates and maintain electrochemical performance under repeated bending, folding, or stretching; at the same time, they need to increase energy storage capacity without compromising flexibility; finally, in high-power applications or extreme environmental conditions, flexible supercapacitors require a certain high temperature resistance to prevent thermal degradation.

[0003] Chinese patent document CN118969516A discloses a flexible solid-state supercapacitor and a method for inhibiting self-discharge behavior, which uses the following two methods to jointly achieve the inhibition of self-discharge behavior of flexible solid-state supercapacitors: ① using piezoelectric ion gel prepared by freeze-thaw method as electrolyte, wherein the number of freeze-thaw cycles is 3 to 5 times; ② using external pressure to generate ion piezoelectric potential to apply pressure to the supercapacitor, and the applied pressure is at least 100 Pa. The flexible solid-state supercapacitor with self-discharge inhibition function in the present invention has a dense cross-linked network and piezoelectric potential generated by the ion gel when subjected to pressure; the combined action of the two hinders the movement of ions and reduces the self-discharge caused by diffusion caused by the ion concentration gradient; the flexible supercapacitor has excellent flexibility and toughness and a low self-discharge rate, and the present invention provides a new strategy for the practical application of flexible supercapacitors.

[0004] Chinese patent document CN118263039A discloses a method for preparing a carbon nanotube / chemically converted graphene / polyaniline composite film for a flexible solid-state supercapacitor. First, graphene oxide is moderately reduced to prepare partially reduced chemically converted graphene (CCG) with high oxidation activity. Subsequently, a three-dimensional network structure is constructed by adding carbon nanotubes (CNT) to intercalate CCG to obtain a composite material CNT / CCG with a larger specific surface area and more active sites. Polyaniline is introduced into the CNT / CCG composite material by in-situ polymerization to obtain a carbon nanotube / chemically converted graphene / polyaniline composite film (CNT / CCG / PANi). The electrochemical test results show that at 0.1 mA cm 2 At a current density of 1.3 Å, the specific capacitance of CNT / CCG / PANi is 706 mF cm 2, at 1mA cm 2 After 3000 cycles, the specific capacitance is still 78% of the original value. The energy density of the flexible supercapacitor prepared by CNT / CCG / PANi composite electrode material is as high as 245mW cm 2 , power density is 124mWh cm 2 Flexible solid-state supercapacitors with CNT / CCG / PANi as electrodes have potential application prospects as energy storage devices.

[0005] The inventors have found through research that the flexible supercapacitors disclosed in the above prior art are still prone to losing performance under repeated bending, folding or stretching, and may still suffer from thermal degradation under high-power applications or extreme environmental conditions. Therefore, it is of great significance to develop a flexible supercapacitor that can maintain high performance and high thermal stability. Summary of the invention

[0006] The present invention provides a high-performance flexible graphene-based supercapacitor and a preparation method thereof, aiming to solve the problem that the existing flexible supercapacitors have performance degradation and insufficient thermal stability under repeated bending, folding or stretching. The flexible supercapacitor of the present invention uses a graphene-polyimide composite material as an electrode, combined with a rare earth-doped gel electrolyte, to achieve the goals of high performance, high flexibility and high thermal stability.

[0007] The present invention specifically proposes a flexible supercapacitor, which comprises: a graphene-polyimide composite electrode with an interconnected conductive network, a gel electrolyte and a flexible packaging layer.

[0008] Preferably, the graphene-polyimide composite electrode is prepared by mixing a graphene suspension with a polyamic acid solution at a graphene:polyamic acid mass ratio of 1:5-5:1, and then heating and performing an imidization reaction to form a composite material.

[0009] More preferably, the graphene suspension is composed of a graphene having a specific surface area of ​​400-600m 2 The graphene powder of 500 g (w / v) is dispersed in N-methyl-2-pyrrolidone at a concentration of 5-15% (w / v) by ultrasonic dispersion at an output power of 100-300 w for 1-2 hours.

[0010] More preferably, the polyamic acid solution is formed by reacting 4,4'-diaminodiphenyl ether and phthalic anhydride at a molar ratio of 1:2-2:1 under an inert gas atmosphere.

[0011] Preferably, the gel electrolyte is an ion rare earth doped gel electrolyte or a liquid gel electrolyte.

[0012] More preferably, the rare earth element doped in the rare earth-doped gel electrolyte is selected from lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium or gadolinium.

[0013] More preferably, the rare earth doped gel electrolyte is formed by mixing polyvinyl alcohol, phosphoric acid and cerium nitrate.

[0014] Preferably, the encapsulation layer is formed by curing polydimethylsiloxane and a curing agent after mixing.

[0015] The present invention also provides a method for preparing the flexible supercapacitor of the present invention, comprising the steps of preparing electrodes, preparing gel electrolytes and assembling capacitors.

[0016] Preferably, the step of preparing the electrode includes mixing a pre-dispersed graphene suspension with a polyamic acid solution, casting the mixture on a metal substrate, heating the mixture to perform an imidization reaction to form a graphene-polyimide composite material, hot pressing the mixture to obtain a graphene-polyimide composite film, and cutting the composite film into electrodes.

[0017] More preferably, the imidization reaction comprises multi-step heating and heat preservation in the order of 70-90°C 10-30min, 130-170°C 10-30min, 200-300°C 10-30min, and 300-400°C 10-30min.

[0018] More preferably, the hot pressing molding comprises controlling the temperature in the mold at 300-350° C., the pressure at 8-15 MPa, and the molding time at 20-40 minutes.

[0019] Preferably, the step of preparing the gel electrolyte comprises dissolving polyvinyl alcohol in deionized water, adding phosphoric acid and cerium nitrate, mixing thoroughly and cooling to room temperature, wherein the mass ratio of polyvinyl alcohol to phosphoric acid is 1:2-2:1, and the added cerium nitrate accounts for 0.1-0.2% of the total mass of polyvinyl alcohol-phosphoric acid-cerium nitrate.

[0020] Also preferably, the step of assembling the capacitor includes cutting the graphene-polyimide composite film into electrodes, coating a gel electrolyte and then superimposing another electrode, using a mixture of polydimethylsiloxane and a curing agent as an encapsulation layer, pouring it on the capacitor structure and curing it.

[0021] Beneficial Effects

[0022] The present invention can ensure the dispersion of graphene to avoid aggregation by precisely controlling the graphene content and the molding process; at the same time, the flexibility of the composite material is maintained, giving full play to the synergy between the high conductivity of graphene and the mechanical robustness and thermal stability of polyimide. The present invention reduces internal resistance and promotes effective charge transfer by creating a conductive network at the graphene-polyimide interface; and the matrix of polyimide can promote uniform distribution and penetration of electrolytes, improve the accessibility of ions to the graphene surface, and thus improve power density.

[0023] The in-situ polymerization process of the present invention produces a strong interfacial bond between graphene and polyimide, so that the composite material maintains high flexibility and mechanical robustness without affecting the electrical properties, so that the flexible capacitor prepared by the present invention can withstand repeated bending, twisting and stretching without reducing the electrical properties, and can maintain >80% of the performance under repeated bending at an angle of 180°. At the same time, the strong interaction between graphene and polyimide at the molecular level reduces the contact resistance, improves the electron transfer kinetics, and prevents delamination or cracking during thermal cycling.

[0024] The flexibility of the polyimide used in the present invention prevents mechanical failure, while the stability of graphene ensures consistent electrochemical performance, and the functional groups on the graphene promote stable interaction with the electrolyte, reducing degradation during cycling. The porous structure of the product of the present invention minimizes stress accumulation during ion insertion and deinsertion, so that the capacitance retention rate of the capacitor after 10,000 cycles is >85%.

[0025] The formula of the electrolyte used in the present invention and the rare earth doping improve the ionic conductivity, reduce thermal degradation, and further supplement the stability and energy density of the composite material. The rare earth doped gel electrolyte improves the ionic conductivity, reduces thermal degradation, and further supplements the stability of the composite material. The integration of rare earth elements in the electrolyte further improves the ion mobility, reduces the charge transfer resistance, and supplements the performance of the electrode.

[0026] The experimental results show that the specific capacitance of the flexible supercapacitor obtained by the present invention is high, all greater than 200F / g; the rate capability is extremely high, all greater than 70%; the flexibility (mechanical stability) is very good, all greater than 80%; the thermal stability is very good, all greater than 80%; the cycle stability (10,000 times) is high, all greater than 85%; the energy / power density is extremely high, all greater than 9Wh / kg. It proves that the present invention solves the problems of performance degradation and insufficient thermal stability of existing flexible supercapacitors under repeated bending, folding or stretching, and realizes the preparation of flexible supercapacitors with high performance, high flexibility and high thermal stability. DETAILED DESCRIPTION

[0027] The following is further described in detail through specific implementation methods. However, it should be pointed out that the following embodiments of the present invention are only for better illustrating the content of the present invention, but do not mean that the content of the present invention is limited to the examples. Therefore, those skilled in the art can make non-essential improvements and adjustments to the implementation scheme according to the above invention content, which still belongs to the protection scope of the present invention and is subject to the protection scope of the attached claims.

[0028] It is clear to those skilled in the art that the supercapacitor involved in the present invention, also known as supercapacitor and ultracapacitor, is a new type of energy storage device between traditional capacitors and rechargeable batteries, which has both the characteristics of fast charging and discharging of capacitors and the energy storage characteristics of batteries. Its capacitance value is much higher than that of ordinary capacitors, usually 10 to 100 times the energy that can be stored per unit volume or mass of electrolytic capacitors. Supercapacitors are new components that store energy through the interfacial double layer (also known as double electric layer) formed between electrodes and electrolytes. When the electrode is in contact with the electrolyte, due to the action of Coulomb force, intermolecular force and interatomic force, a stable double layer of charges with opposite signs appears at the solid-liquid interface. Depending on the electrode material and surface shape, some ions may penetrate the double electric layer to become specific adsorbed ions, and provide the entire capacitance of the supercapacitor together with the pseudocapacitor. Supercapacitors can be classified according to the electrode material, the type of active material and the state of the electrolyte. According to the different electrode materials, they can be divided into carbon electrode double layer supercapacitors, metal oxide electrode supercapacitors and organic polymer electrode supercapacitors, etc. According to whether the type of active material is the same, they can be divided into symmetric supercapacitors and asymmetric supercapacitors. According to the state of the electrolyte, supercapacitors can be divided into two categories: solid electrolyte supercapacitors and liquid electrolyte supercapacitors. Supercapacitors are mainly used in scenarios that require fast charge and discharge cycles, such as braking energy recovery, short-term energy storage, and power transmission in emergencies in cars, buses, rail trains, cranes, elevators, etc. In the power sector, supercapacitors can be used as a backup power source for feeder terminal equipment FTU, or provide voltage, frequency, and power stabilization services in microgrids and public power grids. Supercapacitors have many advantages, such as strong environmental adaptability, good temperature characteristics, long service life, and fast charging.

[0029] Flexible supercapacitors are a classification of supercapacitors. They are also an emerging type of electronic energy storage device in recent years. They have attracted much attention due to their unique physical properties and superior performance. Flexible supercapacitors are high-performance energy storage devices used in flexible electronic devices and wearable technologies. Their working principle is based on the ion migration and charge distribution between electrode materials, and energy storage is achieved through the process of charge storage and release formed by the double electric layer. At the same time, the pseudocapacitive effect also plays a role in enhancing the capacitance and energy density of the capacitor. Flexible supercapacitors are composed of flexible substrates, electrode materials, and solid electrolytes. The electrode materials can simultaneously play the role of energy storage and current collector, and the solid electrolyte can simultaneously play the role of electrolyte and diaphragm. Flexible supercapacitors can be bent, folded, and even stretched to a certain extent without losing their performance. This flexibility makes flexible supercapacitors have great application potential in wearable devices, smartphones, tablets, and other fields that require thin, flexible power sources. Flexible supercapacitors have the characteristics of high power density and can be quickly charged and discharged in a short time. This feature makes them very suitable for occasions that require instantaneous high power output, such as rapid start-up and acceleration of electric vehicles, or rapid start-up of emergency equipment. Flexible supercapacitors usually have a longer life than traditional batteries. Since there is no chemical reaction during the charging and discharging process, there is no performance degradation due to chemical reactions like batteries. This gives flexible supercapacitors significant advantages in devices that require long-term stable operation. Compared with lithium-ion batteries, flexible supercapacitors excel in safety performance. They will not explode or catch fire due to overheating, overcharging or overdischarging, so flexible supercapacitors have significant advantages in application scenarios with high safety requirements, such as medical equipment, aerospace and other fields. Flexible supercapacitors are widely used in many fields due to their unique advantages. In wearable devices such as smart watches, smart clothing, and biomedical sensors, flexible supercapacitors can provide long-lasting and stable power support. In addition, in smart home devices such as smart door locks and smart cameras, flexible supercapacitors also play a role as important energy storage components. With the advancement of science and technology and the increasing demand for flexible electronic devices, the application prospects of flexible supercapacitors will be broader. With the continuous research and development and application of new materials such as graphene, the performance of flexible supercapacitors will be further improved.

[0030] Those skilled in the art know that graphene electrodes are electrodes prepared with graphene as the matrix, formed by tightly stacking a single layer of carbon atoms in a two-dimensional honeycomb lattice. Graphene has the advantages of high specific surface area (total area per unit mass of material), high conductivity, chemical inertness and high mechanical strength. As an electrode material, it is very advantageous and has been widely used in supercapacitors. Due to its high specific surface area and high conductivity, graphene can significantly improve the energy density and power density of supercapacitors, while achieving good charge and discharge cycle stability.

[0031] The rare earth involved in the present invention refers to lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), scandium (Sc) and yttrium (Y), a total of 17 elements, and the English name is Rare Earth. The present invention preferably uses light rare earth, including lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, and gadolinium, and cerium is most preferably used.

[0032] The present invention uses polydimethylsiloxane (PDMS) as the packaging material of the flexible supercapacitor. PDMS has good flexibility, chemical stability and thermal stability. It can effectively protect the electrodes, electrolytes and other components inside the capacitor and prevent them from being damaged by the external environment. The transparency of PDMS also makes it advantageous in certain situations where it is necessary to observe the internal state of the capacitor.

[0033] Curing agent is a key component in the PDMS curing process. By mixing with PDMS prepolymer and heating, curing agent can initiate polymerization reaction, so that PDMS is converted from liquid to solid. The type and amount of curing agent have an important influence on the curing speed, curing degree and curing performance of PDMS. Those skilled in the art need to prepare according to specific application requirements when selecting and using curing agent. The present invention preferably uses amine curing agent, especially aromatic amine curing agent (such as m-phenylenediamine, diaminodiphenylmethane, etc.), which has high thermal stability and mechanical strength, but toxicity is also large, and the price is relatively high, so aliphatic amine curing agent (such as ethylenediamine, diethylenetriamine, triethylenetetramine, etc.) with lower toxicity and cost can also be selected, which has lower viscosity, is easy to mix with PDMS, and can be cured at room temperature. Of course, it is also possible to improve some properties of amine curing agent by introducing other functional groups or structures, such as reducing toxicity, increasing application period, etc. The present invention also preferably uses anhydride curing agent, and the cured product generated by its reaction with PDMS has high thermal stability and electrical properties. They usually cure at elevated temperatures and are slower to cure, but have a longer pot life.

[0034] The basic method steps for preparing the flexible supercapacitor of the present invention are as follows:

[0035] 1. Preparation of graphene-polyimide composite electrode

[0036] (1) Preparation of graphene suspension: weigh a graphene suspension with a specific surface area of ​​400-600 m 2 / g high-purity few-layer graphene powder is ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 5-15% (w / v) at an output power of 100-300W for 1-2 hours to obtain a graphene suspension after achieving stable suspension.

[0037] (2) Preparation of polyamic acid (PAA) precursor: 4,4'-diaminodiphenyl ether (ODA) is dissolved in an NMP solvent under an inert gas atmosphere such as nitrogen or argon at a concentration of 5-20% (w / v), and then phthalic anhydride (PMDA) is slowly added in a molar ratio of 1:2-2:1 to ODA, and stirred at room temperature for 12-24 hours to form a polyamic acid precursor.

[0038] (3) Mixing polyamic acid and graphene: adding the graphene suspension pre-dispersed in step (1) to the polyamic acid precursor in step (2) at a graphene:polyamic acid mass ratio of 1:5-5:1, stirring for 4-6 hours to make it uniformly distributed. Then, the obtained graphene-polyamic acid mixture is cast on a clean metal substrate, heated in an oven with nitrogen, and subjected to multi-step heating (70-90° C. 10-30 min, 130-170° C. 10-30 min, 200-300° C. 10-30 min, 300-400° C. 10-30 min) and heat preservation treatment to make it undergo imidization reaction to form a graphene-polyimide composite material.

[0039] (4) Hot pressing to make a composite film: The graphene-polyimide composite material obtained in step (3) is dried and then ground into a fine powder, which is then placed in a mold for hot pressing at a temperature of 300-350° C. and a pressure of 8-15 MPa for 20-40 minutes to obtain a graphene-polyimide composite film.

[0040] 2. Preparation of rare earth doped gel electrolyte

[0041] Dissolve polyvinyl alcohol (PVA) in deionized water (5-15 wt%) at 60-100°C with continuous stirring. Then, add phosphoric acid (H 3 PO 4 ) and ensure PVA:H 3 PO 4 The mass ratio of PVA-H is 1:2-2:1. 3 PO 40.1-0.2 wt% (total weight percentage of polyvinyl alcohol-phosphoric acid-cerium nitrate) of cerium nitrate (Ce(NO 3 ) 3 6H 2 O), mix thoroughly. During the above process, keep the temperature at 60-100℃ and continue stirring until PVA-H 3 PO 4 -Ce(NO 3 ) 3 6H 2 O are fully mixed. Finally, the mixture is cooled to room temperature to form a rare earth doped gel electrolyte.

[0042] 3. Equipment manufacturing

[0043] The graphene-polyimide composite film obtained in step 1 (4) is cut into two identical electrodes, and the rare earth doped gel electrolyte obtained in step 2 is coated on one of the electrodes. Then, the second electrode is placed on top of the first gel-coated electrode, ensuring proper alignment and gently pressing to establish contact. Finally, polydimethylsiloxane (PDMS) and a curing agent are mixed in a ratio of 5:1-15:1 (w / w), poured on the device, and cured at 60-100°C for 1-3 hours to form the flexible supercapacitor of the present invention.

[0044] Example 1

[0045] 1. Preparation of graphene-polyimide composite electrode

[0046] (1) Preparation of graphene suspension: Weigh a graphene suspension with a specific surface area of ​​500 m 2 / g high-purity few-layer graphene powder was ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 10% (w / v) at an output power of 200 W for 1.5 hours to obtain a graphene suspension after achieving stable suspension.

[0047] (2) Preparation of polyamic acid (PAA) precursor: 4,4'-diaminodiphenyl ether (ODA) was dissolved in an NMP solvent under an inert gas atmosphere such as nitrogen or argon at a concentration of 12% (w / v), and then phthalic anhydride (PMDA) was slowly added at a molar ratio of 1:1 to ODA, and stirred at room temperature for 20 hours to form a polyamic acid precursor.

[0048] (3) Mixing polyamic acid and graphene: adding the graphene suspension pre-dispersed in step (1) to the polyamic acid precursor in step (2) at a graphene:polyamic acid mass ratio of 1:1, stirring for 5 hours to make it uniformly distributed. Then, the obtained graphene-polyamic acid mixture is cast on a clean metal substrate, heated in an oven with nitrogen, and subjected to multi-step heating (80°C 20min, 150°C 20min, 250°C 10min, 350°C 30min) and heat preservation treatment to make it undergo imidization reaction to form a graphene-polyimide composite material.

[0049] (4) Hot pressing to make a composite film: The graphene-polyimide composite material obtained in step (3) is dried and then ground into a fine powder, which is then placed in a mold for hot pressing at a temperature of 330° C. and a pressure of 15 MPa for 30 minutes to obtain a graphene-polyimide composite film.

[0050] 2. Preparation of rare earth doped gel electrolyte

[0051] Polyvinyl alcohol (PVA) was dissolved in deionized water (10 wt%) at 80 °C with continuous stirring. Then, phosphoric acid (H 3 PO 4 ) and ensure PVA:H 3 PO 4 The mass ratio of PVA-H is 1:1. 3 PO 4 0.2 wt% cerium nitrate (Ce(NO 3 ) 3 6H 2 O), and mix thoroughly. During the above process, the temperature was maintained at 80°C and stirring was continued until PVA-H 3 PO 4 -Ce(NO 3 ) 3 6H 2 O are all mixed thoroughly. Finally, the mixture is cooled to room temperature to form a rare earth doped gel electrolyte.

[0052] 3. Equipment manufacturing

[0053] The graphene-polyimide composite film obtained in step 1 (4) is cut into two identical electrodes, and the rare earth doped gel electrolyte obtained in step 2 is coated on one of the electrodes. Then, the second electrode is placed on top of the first gel-coated electrode, ensuring proper alignment and gently pressing to establish contact. Finally, polydimethylsiloxane (PDMS) and curing agent diaminodiphenylmethane are mixed in a ratio of 10:1 (w / w), poured on the device, and cured at 80°C for 2 hours to form the flexible supercapacitor of the present invention.

[0054] Example 2

[0055] 1. Preparation of graphene-polyimide composite electrode

[0056] (1) Preparation of graphene suspension: Weigh a graphene suspension with a specific surface area of ​​600 m 2 / g high-purity few-layer graphene powder was ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 5% (w / v) at an output power of 100 W for 1 hour to obtain a graphene suspension after achieving stable suspension.

[0057] (2) Preparation of polyamic acid (PAA) precursor: 4,4'-diaminodiphenyl ether (ODA) was dissolved in an NMP solvent under an inert gas atmosphere such as nitrogen or argon at a concentration of 8% (w / v), and then phthalic anhydride (PMDA) was slowly added at a molar ratio of 1:1.5 to ODA, and stirred at room temperature for 15 hours to form a polyamic acid precursor.

[0058] (3) Mixing polyamic acid and graphene: adding the graphene suspension pre-dispersed in step (1) to the polyamic acid precursor in step (2) at a graphene:polyamic acid mass ratio of 3:1, stirring for 4 hours to make it uniformly distributed. Then, the obtained graphene-polyamic acid mixture is cast on a clean metal substrate, heated in an oven with nitrogen, and subjected to multi-step heating (70°C 30min, 140°C 25min, 200°C 30min, 300°C 30min) and heat preservation treatment to make it undergo imidization reaction to form a graphene-polyimide composite material.

[0059] (4) Hot pressing to make a composite film: The graphene-polyimide composite material obtained in step (3) is dried and then ground into a fine powder, which is then placed in a mold for hot pressing at a temperature of 300° C. and a pressure of 10 MPa for 20 minutes to obtain a graphene-polyimide composite film.

[0060] 2. Preparation of rare earth doped gel electrolyte

[0061] Polyvinyl alcohol (PVA) was dissolved in deionized water (15 wt%) at 100 °C with continuous stirring. Then, phosphoric acid (H 3 PO 4 ), and ensure PVA:H 3 PO 4 The mass ratio of PVA-H is 2:1. 3 PO 4 0.2 wt% cerium nitrate (Ce(NO 3 ) 3 6H2 O) and mix thoroughly. During the above process, the temperature was maintained at 100°C and stirring was continued until PVA-H 3 PO 4 -Ce(NO 3 ) 3 6H 2 O are fully mixed. Finally, the mixture is cooled to room temperature to form a rare earth doped gel electrolyte.

[0062] 3. Equipment manufacturing

[0063] The graphene-polyimide composite film obtained in step 1 (4) is cut into two identical electrodes, and the rare earth doped gel electrolyte obtained in step 2 is coated on one of the electrodes. Then, the second electrode is placed on top of the first gel-coated electrode, ensuring proper alignment and gently pressing to establish contact. Finally, polydimethylsiloxane (PDMS) and curing agent diaminodiphenylmethane are mixed in a ratio of 15:1 (w / w), poured on the device, and cured at 100°C for 3 hours to form the flexible supercapacitor of the present invention.

[0064] Example 3

[0065] 1. Preparation of graphene-polyimide composite electrode

[0066] (1) Preparation of graphene suspension: Weigh a graphene suspension with a specific surface area of ​​400 m 2 / g high-purity few-layer graphene powder was ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 12% (w / v) at an output power of 160 W for 1.5 hours to obtain a graphene suspension after achieving stable suspension.

[0067] (2) Preparation of polyamic acid (PAA) precursor: 4,4'-diaminodiphenyl ether (ODA) was dissolved in an NMP solvent under an inert gas atmosphere such as nitrogen or argon at a concentration of 10% (w / v), and then phthalic anhydride (PMDA) was slowly added at a molar ratio of 2:1 to ODA, and stirred at room temperature for 20 hours to form a polyamic acid precursor.

[0068] (3) Mixing polyamic acid and graphene: adding the graphene suspension pre-dispersed in step (1) to the polyamic acid precursor in step (2) at a graphene:polyamic acid mass ratio of 2:1, stirring for 6 hours to make it uniformly distributed. Then, the obtained graphene-polyamic acid mixture is cast on a clean metal substrate, heated in an oven with nitrogen, and subjected to multi-step heating (90°C 10min, 170°C 15min, 300°C 10min, 400°C 10min) and heat preservation treatment to make it undergo imidization reaction to form a graphene-polyimide composite material.

[0069] (4) Hot pressing to make a composite film: The graphene-polyimide composite material obtained in step (3) is dried and then ground into a fine powder, which is then placed in a mold for hot pressing at a temperature of 350° C. and a pressure of 20 MPa for 30 minutes to obtain a graphene-polyimide composite film.

[0070] 2. Preparation of rare earth doped gel electrolyte

[0071] Polyvinyl alcohol (PVA) was dissolved in deionized water (5 wt%) at 60°C with continuous stirring. Then, phosphoric acid (H 3 PO 4 ) and ensure PVA:H 3 PO 4 The mass ratio of PVA-H is 1:2. 3 PO 4 0.1 wt% cerium nitrate (Ce(NO 3 ) 3 6H 2 O), and mix thoroughly. During the above process, the temperature was maintained at 60°C and stirring was continued until PVA-H 3 PO 4 -Ce(NO 3 ) 3 6H 2 O are fully mixed. Finally, the mixture is cooled to room temperature to form a rare earth doped gel electrolyte.

[0072] 3. Equipment manufacturing

[0073] The graphene-polyimide composite film obtained in step 1 (4) was cut into two identical electrodes, and the rare earth doped gel electrolyte obtained in step 2 was coated on one of the electrodes. Then, the second electrode was placed on top of the first gel-coated electrode, ensuring proper alignment and gently pressing to establish contact. Finally, polydimethylsiloxane (PDMS) and curing agent were mixed in a ratio of 5:1 (w / w), poured on the device, and cured at 60°C for 3 hours to form the flexible supercapacitor of the present invention.

[0074] The specific capacitance of a capacitor is the capacitance per unit mass. The capacitance measurement of the present invention is carried out in accordance with the provisions of Section 6.4.1.3.1 of the national standard "GB / T34870.1-2017 Supercapacitor Part 1: General Provisions".

[0075] The rate capability of the present invention is determined by charging and discharging the capacitor at a constant current of 10A / g (for example, if the mass of the capacitor is 1g, the charge and discharge current is 10A), and the actual released energy is calculated by integrating the charge and discharge curve, and then converted into the actual capacity, and the formula is:

[0076]

[0077] The capacity measurement is carried out in accordance with the national standard "GB / T 34870.1-2017 Supercapacitors Part 1: General Principles" Section 6.4.1.3.2 Energy Storage and IEC 62576-2009 "Test Methods for Electrical Characteristics of Double-Layer Capacitors for Hybrid Electric Vehicles".

[0078] The method for determining the flexibility (mechanical stability) of the present invention is carried out in accordance with the provisions of the extrusion test in Section 6.4.1.17 of the national standard "GB / T 34870.1-2017 Supercapacitor Part 1: General Provisions", and the retention rate of the capacitance after the test is measured.

[0079] The thermal stability of the present invention is determined in accordance with the provisions of Section 6.4.1.18 Heating Test of the national standard GB / T 34870.1-2017 Supercapacitor Part 1: General Provisions, and the capacitance retention rate after the test is measured.

[0080] The determination of the cycle stability (10,000 times) of the present invention is carried out in accordance with the provisions of the cycle life test in Section 6.4.1.12 of the national standard "GB / T 34870.1-2017 Supercapacitor Part 1: General Provisions", and the cycle is 10,000 times.

[0081] The energy / power density of the present invention is measured in accordance with the provisions of Section 6.4.1.3.2 on energy storage of the national standard GB / T 34870.1-2017 Supercapacitors Part 1: General Principles.

[0082] The technical parameters of the flexible supercapacitors obtained in Examples 1-3 are shown in Table 1 below.

[0083] Table 1 Technical parameters of the flexible supercapacitor of the present invention

[0084] Example 1 Example 2 Example 3 Traditional activated carbon supercapacitor Specific capacitance (F / g) 200 235 285 100-150 Rate capability (%@10A / g) 75％ 82％ 71％ 60-70％ Flexibility (mechanical stability) 95％ 88％ 82％ <60％ Thermal stability (%@85℃) 92％ 87％ 83％ ≤80％ Cycle stability (10,000 times) 89％ 87％ 85％ 80-85％ Energy density (Wh / kg) 9.1 10.5 13.3±1.0 4-6

[0085] As shown in Table 1, the specific capacitance of the flexible supercapacitor obtained by the present invention is high, all greater than 200F / g; the rate capability is extremely high, all greater than 70%; the flexibility (mechanical stability) is very good, all greater than 80%; the thermal stability is very good, all greater than 80%; the cycle stability (10,000 times) is high, all greater than 85%; the energy / power density is extremely high, all greater than 9Wh / kg. It proves that the present invention solves the problem of performance degradation and insufficient thermal stability of existing flexible supercapacitors under repeated bending, folding or stretching, and realizes the preparation of flexible supercapacitors with high performance, high flexibility and high thermal stability.

[0086] The above is only an embodiment of the present invention. The commonly known technical common sense in the scheme is not described in detail here. The ordinary technicians in the relevant field are aware of all the common technical knowledge in the technical field to which the invention belongs before the application date, can obtain all the existing technologies in the field, and have the ability to apply the conventional experimental means before that date. The ordinary technicians in the relevant field can improve and implement this scheme in combination with their own abilities under the enlightenment given by this application. Some typical commonly known technologies should not become obstacles for ordinary technicians in the relevant field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, they can also make some adjustments and improvements, which should also be regarded as the scope of protection of the present invention, which will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection claimed by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A flexible supercapacitor, characterized in that: The capacitor includes: a graphene-polyimide composite electrode having an interconnected conductive network, a gel electrolyte, and a flexible packaging layer.

2. The flexible supercapacitor according to claim 1, characterized in that: The graphene-polyimide composite electrode is prepared by mixing a graphene suspension with a polyamic acid solution in a graphene:polyamic acid mass ratio of 1:5-5:1, and then heating and performing an imidization reaction to form a composite material.

3. The flexible supercapacitor according to claim 1, characterized in that: The gel electrolyte is an ion rare earth doped gel electrolyte or a liquid gel electrolyte.

4. The flexible supercapacitor according to claim 3, characterized in that: The rare earth element doped in the rare earth doped gel electrolyte is selected from lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium or gadolinium.

5. The flexible supercapacitor according to claim 4, characterized in that: The rare earth doped gel electrolyte is formed by mixing polyvinyl alcohol, phosphoric acid and cerium nitrate.

6. The flexible graphene-based supercapacitor according to claim 1, characterized in that: The encapsulation layer is formed by mixing polydimethylsiloxane and a curing agent and then curing.

7. A method for preparing the flexible supercapacitor according to any one of claims 1 to 6, characterized in that: The method includes steps of preparing electrodes, preparing gel electrolyte and assembling capacitors.

8. The preparation method according to claim 7, characterized in that: The steps of preparing the electrode include mixing a pre-dispersed graphene suspension with a polyamic acid solution, casting the mixture on a metal substrate, heating the mixture to perform an imidization reaction to form a graphene-polyimide composite material, hot pressing the mixture to obtain a graphene-polyimide composite film, and cutting the composite film into electrodes.

9. The preparation method according to claim 7, characterized in that: The step of preparing the gel electrolyte comprises dissolving polyvinyl alcohol in deionized water, adding phosphoric acid and cerium nitrate, mixing thoroughly and cooling to room temperature, wherein the mass ratio of polyvinyl alcohol to phosphoric acid is 1:2-2:1, and the added cerium nitrate accounts for 0.1-0.2% of the total mass of polyvinyl alcohol-phosphoric acid-cerium nitrate.

10. The preparation method according to claim 7, characterized in that: The steps of assembling the capacitor include cutting the graphene-polyimide composite film into electrodes, coating a gel electrolyte and then superimposing another electrode, using a mixture of polydimethylsiloxane and a curing agent as a packaging layer, pouring it on the capacitor structure and curing it.

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