High performance flexible graphene-based supercapacitors and methods of making the same

Abstract

The application provides a flexible supercapacitor, comprising: a graphene-polyimide composite electrode with a conductive network connected to each other, a gel electrolyte and a flexible packaging layer. The application solves the problems of performance decline and insufficient thermal stability of the existing flexible supercapacitor under repeated bending, folding or stretching, and realizes the preparation of a flexible supercapacitor with high performance, high flexibility and high thermal stability. The experimental verification shows that the specific capacitance of the flexible supercapacitor obtained by the application is high, all greater than 200 F / 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 (10000 times) is high, all greater than 85%; and the energy / power density is extremely high, all greater than 9 Wh / kg.

Description

Technical Field

[0001] This invention relates to the field of supercapacitor technology, specifically a high-performance flexible graphene-based supercapacitor and its preparation method. Background Technology

[0002] With the growing demand for wearable electronics, portable devices, and flexible energy storage systems, flexible supercapacitors have attracted significant attention. Traditional rigid supercapacitors lack the mechanical adaptability required for integration into flexible and wearable devices. Flexible supercapacitors overcome this limitation by combining materials and designs that allow bending, stretching, and folding, while maintaining excellent electrochemical performance. Flexible supercapacitors require uniform and stable integration of active materials with a flexible substrate, maintaining electrochemical performance under repeated bending, folding, or stretching; simultaneously, they need to increase energy storage capacity without compromising flexibility; and finally, in high-power applications or extreme environmental conditions, flexible supercapacitors need a certain degree of high-temperature resistance to prevent thermal degradation.

[0003] Chinese patent document CN118969516A discloses a flexible solid-state supercapacitor and a method for suppressing self-discharge behavior. The method employs two methods to suppress the self-discharge behavior of the flexible solid-state supercapacitor: ① using a piezoelectric ion gel prepared by a freeze-thaw method as the electrolyte, wherein the freeze-thaw cycles are 3-5 times; ② applying pressure to the supercapacitor using an external pressure to generate an ion piezoelectric potential, with the applied pressure being at least 100 Pa. The flexible solid-state supercapacitor with self-discharge suppression function in this invention utilizes the dense cross-linked network and piezoelectric potential generated by the ion gel under pressure; the combined effect of these two factors hinders ion movement and reduces self-discharge caused by diffusion due to ion concentration gradients. This flexible supercapacitor exhibits excellent flexibility and toughness as well as a low self-discharge rate. This 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 flexible solid-state supercapacitors. First, graphene oxide is moderately reduced to prepare partially reduced chemically converted graphene (CCG) with high oxidation activity. Then, carbon nanotubes (CNTs) are added to intercalate the CCG to construct a three-dimensional network structure, resulting in a composite material CNT / CCG with a larger specific surface area and more active sites. Polyaniline is then introduced into the CNT / CCG composite material through in-situ polymerization, thereby obtaining a carbon nanotube / chemically converted graphene / polyaniline composite film (CNT / CCG / PANi). Electrochemical testing results show that at 0.1 mA / cm², [the desired effect is achieved]. 2 At a current density of 706 mF / cm, the specific capacitance of CNT / CCG / PANi is 706 mF / cm. 2At 1mA cm 2 After 3000 cycles, the specific capacitance remains at 78% of its original value. The flexible supercapacitor fabricated with the CNT / CCG / PANi composite electrode material achieves an energy density as high as 245 mW / cm². 2 The power density is 124 mWh / cm³. 2 Flexible solid-state supercapacitors using CNT / CCG / PANi electrodes have potential applications as energy storage devices.

[0005] Through research, the inventors have discovered that the flexible supercapacitors disclosed in the prior art are still prone to performance loss under repeated bending, folding, or stretching, and may still suffer thermal degradation under high-power applications or extreme environmental conditions. Therefore, developing a flexible supercapacitor that can maintain high performance and high thermal stability is of great significance. Summary of the Invention

[0006] This invention provides a high-performance flexible graphene-based supercapacitor and its preparation method, aiming to solve the problems of performance degradation and insufficient thermal stability of existing flexible supercapacitors under repeated bending, folding, or stretching. The flexible supercapacitor of this invention uses graphene-polyimide composite materials as electrodes, combined with a rare-earth-doped gel electrolyte, achieving the goals of high performance, high flexibility, and high thermal stability.

[0007] The present invention specifically provides a flexible supercapacitor, the capacitor comprising: a graphene-polyimide composite electrode having an interconnected conductive network, a gel electrolyte, and a flexible encapsulation layer.

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

[0009] More preferably, the graphene suspension is composed of materials with a specific surface area of ​​400-600 m². 2 The graphene powder was formed by ultrasonically dispersing / g of graphene powder in N-methyl-2-pyrrolidone at a concentration of 5-15% (w / v) for 1-2 hours with an output power of 100-300w.

[0010] More preferably, the polyamic acid solution is formed by reacting 4,4'-diaminodiphenyl ether and phthalic dianhydride in a molar ratio of 1:2 to 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 elements doped in the rare earth-doped gel electrolyte are 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 a mixture of polydimethylsiloxane and a curing agent.

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

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

[0017] More preferably, the imidization reaction includes multiple heating and holding steps in the order of 70-90℃ for 10-30 min, 130-170℃ for 10-30 min, 200-300℃ for 10-30 min, and 300-400℃ for 10-30 min.

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

[0019] Preferably, the step of preparing the gel electrolyte includes 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] Preferably, the step of assembling the capacitor includes cutting a graphene-polyimide composite film into electrodes, coating it with a gel electrolyte, superimposing another electrode, and then using a mixture of polydimethylsiloxane and a curing agent as an encapsulation layer poured onto the capacitor structure and cured.

[0021] Beneficial effects

[0022] This invention, through precise control of graphene content and molding process, ensures graphene dispersion to avoid aggregation; simultaneously, it maintains the flexibility of the composite material, fully leveraging the synergistic effect between the high conductivity of graphene and the mechanical robustness and thermal stability of polyimide. By creating a conductive network at the graphene-polyimide interface, this invention reduces internal resistance and promotes efficient charge transport; furthermore, the polyimide matrix promotes uniform electrolyte distribution and penetration, increasing ion accessibility to the graphene surface, thereby improving power density.

[0023] The in-situ polymerization process of this invention generates strong interfacial bonds between graphene and polyimide, thereby enabling the composite material to maintain high flexibility and mechanical strength without affecting its electrical properties. This allows the flexible capacitor prepared by this invention to withstand repeated bending, twisting, and stretching without degrading its electrical performance, maintaining >80% of its performance under repeated bending at 180° angles. Simultaneously, the strong molecular-level interaction between graphene and polyimide reduces contact resistance, improves electron transfer kinetics, and prevents delamination or cracking during thermal cycling.

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

[0025] The electrolyte formulation and rare-earth doping used in this invention improve ionic conductivity, reduce thermal degradation, and further enhance the stability and energy density of the composite material. The rare-earth-doped gel electrolyte further improves ionic conductivity, reduces thermal degradation, and enhances the stability of the composite material. The integration of rare-earth elements into the electrolyte further improves ion mobility, reduces charge transfer resistance, and complements the electrode performance.

[0026] Experimental verification shows that the flexible supercapacitors obtained by this invention have high specific capacitance, all greater than 200 F / g; extremely high rate capability, all greater than 70%; excellent flexibility (mechanical stability), all greater than 80%; excellent thermal stability, all greater than 80%; high cycle stability (10,000 cycles), all greater than 85%; and extremely high energy / power density, all greater than 9 Wh / kg. This demonstrates that this invention solves the problems of performance degradation and insufficient thermal stability of existing flexible supercapacitors under repeated bending, folding, or stretching, and achieves the fabrication of high-performance, highly flexible, and highly thermally stable flexible supercapacitors. Detailed Implementation

[0027] The following detailed description provides further details through specific embodiments. However, it should be noted that the embodiments described below are merely for illustrating the content of the invention and do not represent that the invention is limited to the described embodiments. Therefore, non-essential improvements and adjustments made to the implementation schemes by those skilled in the art based on the above-described invention still fall within the protection scope of the invention, and the scope of protection of the appended claims shall prevail.

[0028] Those skilled in the art will understand that the supercapacitor involved in this invention, also known as a supercapacitor or supercapacitor, is a novel energy storage device that falls between traditional capacitors and rechargeable batteries. It possesses both the rapid charging and discharging characteristics of capacitors and the energy storage characteristics of batteries. Its capacitance is far higher than that of ordinary capacitors, typically 10 to 100 times the energy stored per unit volume or mass of an electrolytic capacitor. A supercapacitor is a novel device that stores energy through a double layer (also called an electric double layer) formed between the electrode and the electrolyte. When the electrode comes into contact with the electrolyte, stable double-layered charges of opposite signs appear at the solid-liquid interface due to the effects of Coulomb forces, intermolecular forces, and interatomic forces. Depending on the electrode material and surface shape, some ions may permeate the electric double layer, becoming specific adsorbed ions, and contributing the full capacitance of the supercapacitor along with pseudocapacitance. Supercapacitors can be classified according to the electrode material, the type of active material, and the state of the electrolyte. Based on the electrode material, they can be classified as carbon electrode double-layer supercapacitors, metal oxide electrode supercapacitors, and organic polymer electrode supercapacitors, etc. Based on whether the active materials are the same, they can be classified as symmetrical supercapacitors and asymmetrical supercapacitors. Based on the state of the electrolyte, supercapacitors can be divided into two main categories: solid electrolyte supercapacitors and liquid electrolyte supercapacitors. Supercapacitors are primarily used in applications requiring rapid charge-discharge cycles, such as regenerative braking in automobiles, buses, trams, cranes, and elevators, short-term energy storage, and power transmission in emergency situations. In the power sector, supercapacitors can serve as backup power for feeder terminal units (FTUs) or provide voltage, frequency, and power stabilization services within microgrids and public power grids. Supercapacitors offer numerous advantages, including strong environmental adaptability, excellent temperature characteristics, long lifespan, and rapid charging capability.

[0029] Flexible supercapacitors are a type of supercapacitor and a relatively new electronic energy storage device that has attracted much attention in recent years due to their unique physical characteristics 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 ion migration and charge distribution between electrode materials, achieving energy storage through the process of charge storage and release via the formation of an electric double layer. Simultaneously, the pseudocapacitive effect also enhances the capacitance and energy density of the capacitor. Flexible supercapacitors consist of a flexible substrate, electrode materials, and a solid electrolyte. The electrode materials simultaneously function as energy storage and current collectors, while the solid electrolyte functions as both an electrolyte and a separator. Flexible supercapacitors can be bent, folded, and even stretched to a certain extent without losing performance. This flexibility makes them highly promising for applications requiring thin, flexible power sources, such as wearable devices, smartphones, and tablets. Flexible supercapacitors feature high power density and can be charged and discharged rapidly in a short time. This characteristic makes them ideal for applications requiring instantaneous high-power output, such as the rapid start-up and acceleration of electric vehicles or the rapid start-up of emergency equipment. The lifespan of flexible supercapacitors is typically longer than that of traditional batteries. Because there are no chemical reactions during charging and discharging, flexible supercapacitors do not experience performance degradation due to chemical reactions like batteries. This gives them a significant advantage in devices requiring long-term stable operation. Compared to lithium-ion batteries, flexible supercapacitors excel in safety performance. They will not explode or catch fire due to overheating, overcharging, or over-discharging, making them a significant advantage in safety-critical applications such as medical devices and aerospace. Due to their unique advantages, flexible supercapacitors are widely used in various fields. In wearable devices such as smartwatches, smart clothing, and biomedical sensors, flexible supercapacitors provide long-lasting and stable power support. Furthermore, they play a crucial role as energy storage components in smart home devices such as smart locks and smart cameras. With technological advancements and increasing demand for flexible electronic devices, the application prospects of flexible supercapacitors will become even broader. With the continuous research and application of new materials such as graphene, the performance of flexible supercapacitors will be further improved.

[0030] Those skilled in the art will understand that graphene electrodes are electrodes fabricated using graphene as a substrate, formed by the close packing of a single layer of carbon atoms within a two-dimensional honeycomb lattice. Graphene possesses advantages such as high specific surface area (the total area per unit mass of material), high conductivity, chemical inertness, and high mechanical strength, making it highly advantageous as an electrode material and leading to its widespread application 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-discharge cycle stability.

[0031] The rare earth elements involved in this invention refer to a collective term for 17 elements: 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). The English name is Rare Earth. This invention preferably uses light rare earth elements, including lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, and gadolinium, with cerium being the most preferred.

[0032] This invention uses polydimethylsiloxane (PDMS) as the encapsulation material for flexible supercapacitors. PDMS has good flexibility, chemical stability and thermal stability, which can effectively protect the internal components of the capacitor, such as electrodes and electrolytes, from damage by the external environment. The transparency of PDMS also gives it an advantage in some situations where it is necessary to observe the internal state of the capacitor.

[0033] Curing agents are key components in the PDMS curing process. By mixing with the PDMS prepolymer and heating, the curing agent initiates a polymerization reaction, transforming PDMS from a liquid to a solid state. The type and amount of curing agent significantly affect the curing speed, degree of curing, and post-curing properties of PDMS. Those skilled in the art need to formulate curing agents according to specific application requirements when selecting and using them. This invention preferably uses amine curing agents, especially aromatic amine curing agents (such as m-phenylenediamine, diaminodiphenylmethane, etc.), which have high thermal stability and mechanical strength, but are also highly toxic and relatively expensive. Therefore, aliphatic amine curing agents (such as ethylenediamine, diethylenetriamine, triethylenetetramine, etc.), which have lower toxicity and lower cost, can also be used. They have lower viscosity, are easy to mix with PDMS, and can cure at room temperature. Of course, some properties of amine curing agents can be improved by introducing other functional groups or structures, such as reducing toxicity and increasing pot life. This invention also preferably uses acid anhydride curing agents, whose cured products reacting with PDMS have high thermal stability and electrical properties. They typically require heating to cure, and the curing speed is relatively slow, but the pot life is relatively long.

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

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

[0036] (1) Preparation of graphene suspension: Weigh out a sample with a specific surface area of ​​400-600 m². 2 High-purity few-layer graphene powder of / g was ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 5-15% (w / v) for 1-2 hours with an output power of 100-300W to achieve stable suspension, thus obtaining a graphene suspension.

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

[0038] (3) Mixing polyamic acid and graphene: The pre-dispersed graphene suspension from step (1) is added to the polyamic acid precursor from step (2) at a graphene:polyamic acid mass ratio of 1:5-5:1, and stirred for 4-6 hours to ensure uniform distribution. Then, the resulting graphene-polyamic acid mixture is cast onto a clean metal substrate and heated in an oven with nitrogen. After multiple heating steps (70-90℃ for 10-30 min, 130-170℃ for 10-30 min, 200-300℃ for 10-30 min, 300-400℃ for 10-30 min) and heat preservation treatment, an imidization reaction is carried out to form a graphene-polyimide composite material.

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

[0040] 2. Preparation of rare earth-doped gel electrolytes

[0041] Polyvinyl alcohol (PVA) is dissolved in deionized water (5-15 wt%) at 60-100°C with continuous stirring. Then, phosphoric acid (H3PO4) is added to the PVA solution, ensuring a PVA:H3PO4 mass ratio of 1:2-2:1. Next, 0.1-0.2 wt% (percentage of the total mass of PVA-phosphoric acid-cerium nitrate) of cerium nitrate (Ce(NO3)3·6H2O) is added to the PVA-H3PO4 solution and mixed thoroughly. Throughout the entire process, the temperature is maintained at 60-100°C with continuous stirring until PVA-H3PO4-Ce(NO3)3·6H2O is completely and thoroughly 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 pressed to establish contact. Finally, polydimethylsiloxane (PDMS) and a curing agent are mixed in a ratio of 5:1-15:1 (w / w), poured onto the device, and cured at 60-100°C for 1-3 hours to form the flexible supercapacitor of this invention.

[0044] Example 1

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

[0046] (1) Preparation of graphene suspension: Weigh out a sample with a specific surface area of ​​500 m². 2 High-purity few-layer graphene powder of / g was ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 10% (w / v) for 1.5 hours with an output power of 200W to achieve stable suspension, thus obtaining a graphene suspension.

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

[0048] (3) Mixing polyamic acid and graphene: The pre-dispersed graphene suspension from step (1) was added to the polyamic acid precursor from step (2) at a graphene:polyamic acid mass ratio of 1:1, and stirred for 5 hours to ensure uniform distribution. Then, the resulting graphene-polyamic acid mixture was cast onto a clean metal substrate and heated in an oven with nitrogen. After multiple heating steps (80℃ for 20 min, 150℃ for 20 min, 250℃ for 10 min, and 350℃ for 30 min) and heat preservation treatment, an imidization reaction was carried out to form a graphene-polyimide composite material.

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

[0050] 2. Preparation of rare earth-doped gel electrolytes

[0051] Polyvinyl alcohol (PVA) was dissolved in deionized water (10 wt%) at 80°C with continuous stirring. Then, phosphoric acid (H3PO4) was added to the PVA solution, ensuring a PVA:H3PO4 mass ratio of 1:1. Next, 0.2 wt% cerium nitrate (Ce(NO3)3·6H2O) was added to the PVA-H3PO4 solution and mixed thoroughly. Throughout the entire process, the temperature was maintained at 80°C with continuous stirring until the PVA-H3PO4-Ce(NO3)3·6H2O was completely and thoroughly mixed. Finally, the mixture was 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) 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 pressed to establish contact. Finally, a mixture of polydimethylsiloxane (PDMS) and curing agent diaminodiphenylmethane at a ratio of 10:1 (w / w) was poured onto the device and cured at 80°C for 2 hours to form the flexible supercapacitor of this invention.

[0054] Example 2

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

[0056] (1) Preparation of graphene suspension: Weigh out a sample with a specific surface area of ​​600 m². 2High-purity few-layer graphene powder of / g was ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 5% (w / v) for 1 hour at an output power of 100W to obtain a graphene suspension after reaching a stable suspension.

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

[0058] (3) Mixing polyamic acid and graphene: The pre-dispersed graphene suspension from step (1) was added to the polyamic acid precursor from step (2) at a graphene:polyamic acid mass ratio of 3:1, and stirred for 4 hours to ensure uniform distribution. Then, the resulting graphene-polyamic acid mixture was cast onto a clean metal substrate and heated in an oven with nitrogen. After multiple heating steps (70℃ for 30 min, 140℃ for 25 min, 200℃ for 30 min, 300℃ for 30 min) and heat preservation treatment, an imidization reaction was carried out to form a graphene-polyimide composite material.

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

[0060] 2. Preparation of rare earth-doped gel electrolytes

[0061] Polyvinyl alcohol (PVA) was dissolved in deionized water (15 wt%) at 100°C with continuous stirring. Then, phosphoric acid (H3PO4) was added to the PVA solution, ensuring a PVA:H3PO4 mass ratio of 2:1. Next, 0.2 wt% cerium nitrate (Ce(NO3)3·6H2O) was added to the PVA-H3PO4 solution and mixed thoroughly. Throughout the entire process, the temperature was maintained at 100°C with continuous stirring until the PVA-H3PO4-Ce(NO3)3·6H2O was completely and thoroughly mixed. Finally, the mixture was 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) 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 pressed to establish contact. Finally, a mixture of polydimethylsiloxane (PDMS) and curing agent diaminodiphenylmethane at a ratio of 15:1 (w / w) was poured onto 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 out a sample with a specific surface area of ​​400 m². 2 High-purity few-layer graphene powder of / g was ultrasonically dispersed in N-methyl-2-pyrrolidone (NMP) solvent at a concentration of 12% (w / v) for 1.5 hours with an output power of 160W to achieve stable suspension, thus obtaining a graphene suspension.

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

[0068] (3) Mixing polyamic acid and graphene: The pre-dispersed graphene suspension from step (1) was added to the polyamic acid precursor from step (2) at a graphene:polyamic acid mass ratio of 2:1, and stirred for 6 hours to ensure uniform distribution. Then, the resulting graphene-polyamic acid mixture was cast onto a clean metal substrate and heated in an oven with nitrogen. After multiple heating steps (90℃ for 10 min, 170℃ for 15 min, 300℃ for 10 min, 400℃ for 10 min) and heat preservation treatment, an imidization reaction was carried out to form a graphene-polyimide composite material.

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

[0070] 2. Preparation of rare earth-doped gel electrolytes

[0071] Polyvinyl alcohol (PVA) was dissolved in deionized water (5 wt%) at 60°C with continuous stirring. Then, phosphoric acid (H3PO4) was added to the PVA solution, ensuring a PVA:H3PO4 mass ratio of 1:2. Next, 0.1 wt% cerium nitrate (Ce(NO3)3·6H2O) was added to the PVA-H3PO4 solution and mixed thoroughly. Throughout the entire process, the temperature was maintained at 60°C with continuous stirring until PVA-H3PO4-Ce(NO3)3·6H2O was completely and thoroughly mixed. Finally, the mixture was 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 pressed to establish contact. Finally, a mixture of polydimethylsiloxane (PDMS) and curing agent at a ratio of 5:1 (w / w) was poured onto the device and cured at 60°C for 3 hours to form the flexible supercapacitor of this invention.

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

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

[0076]

[0077] The capacity determination was carried out in accordance with the provisions of Section 6.4.1.3.2 of the national standard GB / T 34870.1-2017 Supercapacitors Part 1: General Rules for Stored Energy and IEC 62576-2009 Test Method for Electrical Characteristics of Double-Layer Capacitors for Hybrid Electric Vehicles.

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

[0079] The thermal stability of this invention was determined in accordance with the heating test provisions of Section 6.4.1.18 of the national standard GB / T 34870.1-2017 Supercapacitors Part 1: General Rules, and the retention rate of the capacitance after the test was measured.

[0080] The cyclic stability (10,000 cycles) of this invention was determined in accordance with the provisions of Section 6.4.1.12 of the national standard GB / T 34870.1-2017 Supercapacitors Part 1: General Rules for Cyclic Life Test, with a cycle count of 10,000 cycles.

[0081] The energy / power density of this invention is determined in accordance with the provisions of Section 6.4.1.3.2 of the national standard GB / T 34870.1-2017 Supercapacitors Part 1: General Rules for Energy Storage.

[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] 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％ Cyclic stability (10,000 cycles) 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 flexible supercapacitors obtained by this invention exhibit high specific capacitance, all exceeding 200 F / g; extremely high rate capability, all exceeding 70%; excellent flexibility (mechanical stability), all exceeding 80%; excellent thermal stability, all exceeding 80%; high cycle stability (10,000 cycles), all exceeding 85%; and extremely high energy / power density, all exceeding 9 Wh / kg. This demonstrates that this invention solves the problems of performance degradation and insufficient thermal stability in existing flexible supercapacitors under repeated bending, folding, or stretching, achieving the fabrication of high-performance, highly flexible, and highly thermally stable flexible supercapacitors.

[0086] The above descriptions are merely embodiments of the present invention. Commonly known technical knowledge in the solutions is not described in detail here. Those skilled in the art are aware of all common technical knowledge in the field prior to the filing date, are aware of all existing technologies in that field, and have the ability to apply conventional experimental methods prior to that date. Those skilled in the art can, under the guidance of this application, improve and implement this solution in combination with their own capabilities. Some typical well-known technologies should not be obstacles for those skilled in the art to implement this application. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the structure of the present invention. These should also be considered within the scope of protection of the present invention, and will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A flexible supercapacitor, characterized in that, The capacitor comprises: a graphene-polyimide composite electrode having an interconnected conductive network, a gel electrolyte, and a flexible encapsulation layer; the graphene-polyimide composite electrode is made of a composite film formed by hot pressing of graphene-polyimide composite material; the graphene-polyimide composite material is prepared by heating and imidizing a mixture of graphene suspension and polyamic acid precursor at a graphene:polyamic acid mass ratio of 1:1; wherein, the graphene suspension is prepared by dispersing graphene at a concentration of 10% (w / v) in N-methyl-2-pyrrolidone solvent; the polyamic acid precursor is prepared by dissolving 4,4'-diaminodiphenyl ether in N-methyl-2-pyrrolidone solvent and reacting it with phthalic anhydride at a molar ratio of 1:

1.

2. 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.

3. The flexible supercapacitor according to claim 2, characterized in that, The rare earth elements doped in the rare earth-doped gel electrolyte are selected from lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, or gadolinium.

4. The flexible supercapacitor according to claim 3, characterized in that, The rare earth-doped gel electrolyte is formed by mixing polyvinyl alcohol, phosphoric acid and cerium nitrate.

5. The flexible supercapacitor according to claim 1, characterized in that, The encapsulation layer is formed by curing a mixture of polydimethylsiloxane and a curing agent.

6. A method for preparing the flexible supercapacitor according to any one of claims 1-5, characterized in that, It includes the steps of preparing electrodes, preparing gel electrolytes, and assembling capacitors.

7. The preparation method according to claim 6, characterized in that, The steps for preparing the electrode include mixing a pre-dispersed graphene suspension with a polyamic acid solution, casting the mixture onto a metal substrate, heating it to undergo an imidization reaction to form a graphene-polyimide composite material, hot-pressing it to obtain a graphene-polyimide composite film, and cutting the composite film into electrodes.

8. The preparation method according to claim 6, characterized in that, The steps for preparing the gel electrolyte include dissolving polyvinyl alcohol in deionized water, adding phosphoric acid and cerium nitrate, mixing thoroughly, and cooling to room temperature. 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.

9. The preparation method according to claim 6, characterized in that, The steps of assembling the capacitor include cutting a graphene-polyimide composite film into electrodes, coating it with a gel electrolyte, stacking another electrode on top, and using a mixture of polydimethylsiloxane and a curing agent as an encapsulation layer poured onto the capacitor structure and cured.

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