Self-supporting diaphragm as well as preparation method and application thereof

Abstract

The invention relates to a self-supporting diaphragm as well as a preparation method and application thereof. The self-supporting diaphragm comprises an electrode active material, a conductive agent, polytetrafluoroethylene and a polymethylsilane-olefine acid grafted copolymer in a mass ratio of (88-99): (0.1-10): (0.5-10): (0.1-10); the polymethylsilane-olefine acid grafted copolymer is of a flexible elastic structure and is used for forming a continuous three-dimensional network structure in the self-supporting diaphragm, so that the flexibility and the self-supporting property of the diaphragm are improved; the polytetrafluoroethylene is fiberized in the processing process to form a fiberized three-dimensional structure which is used for enhancing the mechanical strength and the structural stability of the self-supporting membrane; the conductive agent is uniformly distributed in the self-supporting membrane under the dispersion effect of the polymethylsilane-olefine acid graft copolymer to form conductive paths, and the fiberized three-dimensional structure formed by polytetrafluoroethylene is used as a'support 'to connect the conductive paths to construct a conductive network.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a self-supporting membrane, its preparation method, and its application. Background Technology

[0002] With the increasing global demand for clean energy, large-scale energy storage technology has become crucial. Sodium-ion batteries, with their abundant sodium resources and low cost, are considered an important alternative to lithium-ion batteries, showing great potential in smart grids and renewable energy storage. However, to achieve large-scale commercial application of sodium-ion batteries, many technical challenges still need to be addressed, among which electrode fabrication technology is a key aspect. Dry electrode technology has significant advantages over traditional wet electrode technology. Firstly, it avoids the use of large amounts of organic solvents, reducing environmental pollution and lowering production costs. Secondly, dry electrode technology can increase the compaction density of the electrode, thereby improving the energy density of the battery. However, the dry electrode process faces two major challenges in the application of sodium-ion battery positive and negative electrode materials:

[0003] 1. Insufficient membrane flexibility: Traditional binders cannot construct a three-dimensional network structure with both flexibility and mechanical strength during the dry film formation process, resulting in high brittleness of the self-supporting membrane. It is prone to cracking and peeling during preparation, handling and battery assembly, which significantly reduces the battery cycle life and reliability.

[0004] 2. Conductive network defects: In the dry process, the conductive agent is not evenly dispersed, making it difficult to form a continuous low-impedance path. This results in high electrode resistivity, which limits the charge and discharge efficiency and rate performance of sodium-ion batteries, hindering their application in high-power scenarios.

[0005] In summary, there is a significant conflict between the environmentally friendly and low-cost advantages of dry electrodes and the stringent requirements of sodium-ion battery positive and negative electrode materials for flexible and conductive structures. Therefore, how to improve the flexibility and conductivity of self-supporting films remains a key technical issue in dry electrode research. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a self-supporting diaphragm, its preparation method, and its application. By optimizing the material composition of the self-supporting diaphragm, the problems of poor flexibility and high resistivity of the self-supporting diaphragm in electrode sheets are effectively solved, thereby improving the mechanical properties and conductivity of the electrode sheets.

[0007] To achieve the above objectives, in a first aspect, embodiments of the present invention provide a self-supporting membrane, the self-supporting membrane comprising an electrode active material, a conductive agent, polytetrafluoroethylene, and a polymethylsilane-acrylic acid graft copolymer in a mass ratio of 88-97:1-10:1-10:1-10;

[0008] The polymethylsilane-acrylic acid graft copolymer is a flexible and elastic structure used to form a continuous three-dimensional network structure inside the self-supporting membrane, thereby improving the membrane's flexibility and self-support. The polytetrafluoroethylene undergoes fibrosis during processing, forming a fibrous three-dimensional structure, which enhances the mechanical strength and structural stability of the self-supporting membrane. The conductive agent, under the dispersion effect of the polymethylsilane-acrylic acid graft copolymer, is uniformly distributed in the self-supporting membrane to form conductive pathways. The fibrous three-dimensional structure formed by the polytetrafluoroethylene acts as a "scaffold" to connect the conductive pathways and construct a conductive network.

[0009] Preferably, the electrode active material is a positive electrode active material, including one or more of the following: polyanionic positive electrode materials and layered oxide positive electrode materials; or,

[0010] The electrode active material is a negative electrode active material, including one or more of hard carbon and soft carbon.

[0011] Preferably, the polymethylsilane-acrylic acid graft copolymer is a copolymer of polymethylsilane and acrylic acid grafted together, wherein the acrylic acid includes one or more of acrylic acid, butenoic acid, pentenoic acid, and hexenoic acid.

[0012] Preferably, the conductive agent includes one or more of the following: conductive carbon black, carbon nanotubes, graphene, Ketjen black, acetylene black, graphite, and conductive carbon fiber.

[0013] Preferably, the thickness of the self-supporting membrane is 50μm-400μm.

[0014] In a second aspect, embodiments of the present invention provide a method for preparing the self-supporting membrane described in the first aspect above, the method comprising:

[0015] The electrode active material, conductive agent, and polymethylsilane-acrylic acid graft copolymer are mixed evenly in a high-speed shear mill to obtain the first mixture.

[0016] While maintaining stirring at 70℃-120℃, polytetrafluoroethylene is added to the first mixture, causing the polytetrafluoroethylene to fibroinate under shear force and temperature, resulting in a second mixture with a fibrous three-dimensional structure.

[0017] The second mixture is preheated and then calendered into a self-supporting film using a twin-roll calender.

[0018] In the self-supporting membrane, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 88-99:0.1-10:0.5-10:0.1-10.

[0019] Preferably, when preparing the first mixture, the high-speed shear machine operates at a speed of 500 rpm to 5000 rpm, a time of 5 min to 60 min, and a temperature of 10℃ to 150℃.

[0020] Preferably, the temperature of the preheating treatment is 50℃-150℃.

[0021] Thirdly, embodiments of the present invention provide an electrode sheet, the electrode sheet comprising the self-supporting film described in the first aspect above, or comprising the self-supporting film prepared by the preparation method described in the second aspect above.

[0022] Fourthly, embodiments of the present invention provide an energy storage device, the energy storage device comprising: a sodium-ion battery or a sodium-ion capacitor; the energy storage device comprising the self-supporting membrane described in the first aspect above, or comprising the self-supporting membrane prepared by the preparation method described in the second aspect above, or comprising the electrode sheet described in the third aspect above.

[0023] The self-supporting diaphragm provided in this invention uses a polymethylsilane-acrylic acid grafted polymer as a binder. On one hand, the flexible segments of polymethylsilane and the rigid segments of acrylic acid work synergistically, enabling the binder to maintain toughness while improving tensile strength. The acrylic acid grafted chains are connected to the polymethylsilane main chain through chemical bonds, forming stable grafting points. These grafting points act as "fixed nodes" within the self-supporting diaphragm, restricting the free movement of the chain segments, thereby constructing a continuous three-dimensional network structure. This ensures that stress is evenly distributed within the diaphragm, avoiding crack propagation caused by localized stress concentration, thus solving the problem of poor flexibility in the self-supporting diaphragm. On the other hand, the polymethylsilane-acrylic acid grafted copolymer acts as a "bridge" to connect the conductive agent and the electrode active material, allowing the conductive agent to be evenly distributed within the self-supporting diaphragm. Under the synergistic effect of the fibrous three-dimensional structure formed by polytetrafluoroethylene, a more complete and effective conductive network is formed, reducing the resistivity of the electrode sheet. The self-supporting diaphragm provided in this invention effectively solves the problems of poor flexibility and high resistivity of the electrode sheet, improves the mechanical and electrical properties of the electrode sheet, and lays a solid foundation for the large-scale commercial application of sodium-ion batteries and sodium-ion capacitors. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the preparation method of a self-supporting membrane provided in an embodiment of the present invention. Detailed Implementation

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0026] The reagents and materials used in the following examples and comparative examples are all commercially available conventional reagent products, or can be prepared by conventional methods. Where specific experimental steps or conditions are not specified in the examples, they were performed according to conventional experimental steps and conditions in the art. Unless otherwise specified, all equipment used is conventional equipment currently available in the art.

[0027] This invention provides a self-supporting membrane comprising an electrode active material, a conductive agent, polytetrafluoroethylene, and a polymethylsilane-acrylic acid graft copolymer in a mass ratio of 88-99:0.1-10:0.5-10:0.1-10.

[0028] Among them, the polymethylsilane-acrylic acid graft copolymer is a flexible elastic structure used to form a continuous three-dimensional network structure inside the self-supporting membrane, thereby improving the membrane's flexibility and self-support. Polytetrafluoroethylene undergoes fibrosis during processing, forming a fibrous three-dimensional structure, which is used to enhance the mechanical strength and structural stability of the self-supporting membrane. Under the dispersion effect of the polymethylsilane-acrylic acid graft copolymer, the conductive agent is uniformly distributed in the self-supporting membrane to form conductive pathways. The fibrous three-dimensional structure formed by polytetrafluoroethylene acts as a "scaffold" to connect the conductive pathways and construct a conductive network.

[0029] Specifically, the electrode active material is the positive electrode active material, including one or more of the following: polyanionic positive electrode materials and layered oxide positive electrode materials. Polyanionic positive electrode materials are one of the key materials for sodium-ion batteries, possessing advantages such as high stability, long cycle life, and high safety. They mainly include phosphate, pyrophosphate, and sulfate types, such as sodium iron phosphate, sodium iron sulfate, and sodium vanadium phosphate. Layered oxide positive electrode materials include layered oxides composed of various transition metal elements such as copper-based, nickel-based, cobalt-based, and iron-based materials, such as sodium nickel iron manganese oxide, sodium cobalt oxide, sodium nickel oxide, and sodium manganese oxide.

[0030] Polyanionic cathode materials have a stable framework structure, which can provide high operating voltage and good cycle stability, while layered oxide cathode materials have high specific capacity and can be flexibly combined according to the needs of different lithium-ion batteries to meet diverse requirements.

[0031] Alternatively, the electrode active material is a negative electrode active material, including one or more of hard carbon and soft carbon.

[0032] Hard carbon has a rich microporous structure, which can provide a high specific capacity, while soft carbon has good conductivity and a layered structure, which is conducive to the insertion and extraction of sodium ions. Both of these materials are low in cost, widely available, and suitable for large-scale applications, and can meet the basic requirements of sodium-ion battery anode materials.

[0033] Conductive agents include one or more of the following: conductive carbon black, carbon nanotubes, graphene, Ketjen black, acetylene black, graphite, and conductive carbon fiber.

[0034] The polymethylsilane-acrylic acid graft copolymer used in this application is a copolymer of polymethylsilane and acrylic acid grafted together. Acrylic acid includes one or more of acrylic acid, butenoic acid, pentenoic acid, and hexenoic acid.

[0035] The introduction of olefin groups improves the surface properties and chemical activity of polymethylsilane, giving the polymethylsilane-olefin graft copolymer better flexibility, adhesion and thermal stability, which helps to improve the overall performance of the self-supporting membrane and enhance the bonding force between the self-supporting membrane and other electrode components.

[0036] In this application, the above-mentioned material components are mixed evenly, then fiberized, and finally hot-pressed into a film to obtain a self-supporting membrane. After fiberization, polytetrafluoroethylene will form a fiberized three-dimensional structure. On the one hand, the fiberized three-dimensional structure can disperse the stress in the self-supporting membrane and enhance its flexibility. On the other hand, the fiberized three-dimensional structure helps to coat the conductive agent particles and fix their distribution position, so that the conductive agent particles do not agglomerate or migrate during the hot-pressing process, thereby giving the self-supporting membrane good electronic conductivity.

[0037] The polymethylsilane-acrylic acid graft copolymer further crosslinks the fibers, further improving the flexibility and tear resistance of the self-supporting membrane.

[0038] Specifically, firstly, the acrylic acid grafted chains are connected to the polymethylsilane backbone through chemical bonds, forming stable grafting points. These grafting points act as "fixed nodes" within the self-supporting membrane, restricting the free movement of the chain segments, thereby constructing a continuous three-dimensional network structure. This allows stress to be evenly distributed within the membrane, avoiding crack propagation caused by local stress concentration, thus solving the problem of poor flexibility in self-supporting membranes.

[0039] Secondly, the polymethylsilane backbone is composed of silicon-silicon bonds, and its electron delocalization characteristics endow the molecular bonds with high flexibility. When olefinic acid is grafted onto the polymethylsilane backbone, the introduced rigid groups such as carboxylate groups can form physical cross-linking points. These cross-linking points limit excessive slippage of chain segments during stretching, prevent permanent deformation of the material, and maintain the integrity of the three-dimensional network structure. Therefore, the flexible chain segments of polymethylsilane and the rigid chain segments of olefinic acid work synergistically to improve the tensile strength of the adhesive while maintaining its toughness.

[0040] In addition, the polymethylsilane-acrylic acid graft copolymer acts as a "bridge" to connect the conductive agent and the electrode active material, so that the conductive agent is evenly distributed in the self-supporting film. Under the synergistic effect of the fibrous three-dimensional structure formed by polytetrafluoroethylene, a more complete and effective conductive network is formed, reducing the resistivity of the electrode sheet.

[0041] In other words, this application uses two binders: polytetrafluoroethylene (PTFE) and polymethylsilane-acrylic acid graft copolymer. On the one hand, this improves the flexibility and mechanical properties of the self-supporting membrane. On the other hand, because the conductive agent is dispersed in the polymethylsilane-acrylic acid graft copolymer, it is uniformly distributed in the self-supporting membrane to form conductive channels. The fibrous three-dimensional structure formed by PTFE acts as a "scaffold" to connect the conductive channels and construct a conductive network, thus reducing the resistivity of the electrode sheet.

[0042] The self-supporting diaphragm provided in this invention effectively solves the problems of poor flexibility and high resistivity of the electrode sheets, improves the mechanical and electrical properties of the electrode sheets, and thus enhances the overall performance of sodium-ion batteries, laying a solid foundation for the large-scale commercial application of sodium-ion batteries and sodium-ion capacitors.

[0043] A suitable thickness helps balance the energy density, functional density, and mechanical properties of the self-supporting membrane, ensuring sufficient mechanical strength and flexibility while achieving high energy storage and rapid charge / discharge capabilities. Preferably, the thickness of the self-supporting membrane in this application is 50 μm-200 μm.

[0044] In addition to this, the embodiments of this application also provide a method for preparing the above-mentioned self-supporting membrane, the main steps of which are as follows: Figure 1 As shown, it includes the following steps:

[0045] Step 110: The electrode active material, conductive agent, and polymethylsilane-acrylic acid graft copolymer are mixed evenly in a high-speed shearing machine to obtain the first mixture.

[0046] The mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 88-99:0.1-10:0.5-10:0.1-10.

[0047] The electrode active material is a positive electrode active material, including one or more of polyanionic positive electrode materials and layered oxide positive electrode materials. Polyanionic positive electrode materials mainly include phosphate, pyrophosphate, and sulfate types, such as sodium iron phosphate, sodium iron sulfate, and sodium vanadium phosphate. Layered oxide positive electrode materials include layered oxides composed of various transition metal elements such as copper-based, nickel-based, cobalt-based, and iron-based materials, such as sodium nickel iron manganese oxide, sodium cobalt oxide, sodium nickel oxide, and sodium manganese oxide. Alternatively, the electrode active material is a negative electrode active material, including one or more of hard carbon and soft carbon.

[0048] Conductive agents include one or more of the following: conductive carbon black, carbon nanotubes, graphene, Ketjen black, acetylene black, graphite, and conductive carbon fiber.

[0049] The polymethylsilane-acrylic acid graft copolymer used in this application is a copolymer of polymethylsilane and acrylic acid grafted together. Acrylic acid includes one or more of acrylic acid, butenoic acid, pentenoic acid, and hexenoic acid.

[0050] Shear force can achieve nanoscale dispersion of the mixture, ensuring that the conductive agent can uniformly coat the electrode active material, avoiding local agglomeration and forming a uniform conductive network. However, excessive rotation speed can damage the material structure due to excessive shearing, while insufficient rotation speed can lead to insufficient dispersion of components and local agglomeration. Appropriate mixing time ensures sufficient contact between the components. Low-temperature control can prevent premature fiberization of polytetrafluoroethylene, prevent the first mixture from clumping, and ensure process stability.

[0051] In other words, appropriate rotation speed and time ensure thorough mixing of the components while avoiding material structure damage due to excessive shearing, and suitable temperature helps maintain material stability, providing a guarantee for the subsequent preparation of high-performance self-supporting membranes. Preferably, in this step, the rotation speed of the high-speed shearing machine is 500 rpm-5000 rpm, the time is 5 min-60 min, and the temperature is 0℃-150℃.

[0052] Step 120: While maintaining stirring at 70℃-120℃, polytetrafluoroethylene (PTFE) is added to the first mixture, causing the PTFE to fibrose under shear force and temperature, resulting in a second mixture with a fibrous three-dimensional structure.

[0053] Polytetrafluoroethylene (PTFE) possesses a strong ability to form molecular entanglements. Under certain temperature and shear force, PTFE undergoes fibrillation, being drawn into filamentous structures to form a fibrous three-dimensional structure. Firstly, during the fibrillation process, if the fibrillation temperature is too low, the PTFE will not be sufficiently fibrillated, resulting in a brittle self-supporting membrane. Conversely, if the temperature is too high, it will trigger the decomposition of the polymethylsilane-acrylic acid graft copolymer, disrupting ion transport channels. Secondly, the stirring speed also needs to be moderate to prevent the PTFE fibers from agglomerating around the high-speed shearing machine due to centrifugal force, or to prevent the formation of a fibrous three-dimensional structure due to insufficient shear force. Thirdly, the fibrillation time should not be too long, otherwise the PTFE may deorient or thermally decompose, leading to carbonization defects. If the fibrillation time is too short, the uniformity of the PTFE fibers will deteriorate. Furthermore, fibers are prone to longitudinal splitting or breakage under shear force in a fixed direction. To prevent fiber breakage from causing a decrease in the mechanical properties of the composite material, this application selects an alternating forward and reverse stirring method during the fiberization stage. When the rotation direction changes, the direction of the shear force changes accordingly, and the stress on the fiber changes from a single direction to alternating forces in multiple directions. This "dynamic shearing" can disperse local stress concentration and reduce the risk of fiber breakage. Therefore, preferably, in this step, the temperature during fiberization is controlled at 70℃-120℃, the stirring speed of the high-speed shear machine is controlled at a forward rotation speed of 10rpm-500rpm, a reverse rotation speed of 10rpm-500rpm, and a stirring time of 5min-10min. Within this temperature and shear force range, polytetrafluoroethylene can be fiberized better.

[0054] Specifically, heating the first mixture to 70℃-120℃ and adding polytetrafluoroethylene (PTFE) while stirring creates a second mixture with a specific structure. This is because the fibrous PTFE can entwine and crosslink to form a fibrous three-dimensional structure. This fibrous three-dimensional structure acts like "tendons" to fix the active material and conductive agent together, enabling film formation without current collectors. This gives the entire self-supporting membrane mechanical strength, while its good elasticity and flexibility enhance the self-supporting membrane's resistance to bending and tearing, thus improving its brittleness.

[0055] In other words, after polytetrafluoroethylene is fiberized, it can form a fibrous three-dimensional structure in the self-supporting membrane, which enhances the bonding force between the components inside the membrane, improves the mechanical properties and structural stability of the self-supporting membrane, and makes the self-supporting membrane less prone to breakage or deformation during subsequent use.

[0056] In addition, the fibrous three-dimensional structure helps to coat the conductive agent particles and fix their distribution position, making them less likely to agglomerate or migrate during calendering, thus giving the self-supporting film good electronic conductivity.

[0057] Polytetrafluoroethylene (PTFE) forms self-supporting membranes through mechanical dispersion and hot-pressing fiberization, eliminating the need for organic solvents and promoting environmentally friendly and large-scale dry membrane production.

[0058] The obtained second mixture can be sieved as needed to remove large particles and obtain uniformly drawn agglomerates, which are then subjected to calendering and film formation in the subsequent step 130. Preferably, the obtained second mixture is sieved through a 100-200 mesh sieve to obtain uniformly drawn agglomerates.

[0059] Step 130: The second mixture is preheated and then calendered into a self-supporting film using a two-roll calender.

[0060] The fibrous three-dimensional structure formed by polytetrafluoroethylene after fiberization helps to coat conductive agent particles and fix their distribution position, improves the stability of conductive agent dispersion, and makes it less likely to agglomerate or migrate during calendering. This allows the prepared self-supporting film to maintain good electronic conductivity and reduce the resistivity of the electrode sheet.

[0061] Preheating reduces internal stress in the mixture caused by mechanical stirring or storage, preventing cracking or deformation of the self-supporting membrane during calendering due to stress release. A suitable preheating temperature softens the PTFE fibers, promoting the density of the self-supporting membrane during calendering, and prevents structural damage to the active material, such as phase transitions in layered oxide cathode materials. Appropriate pressure compresses the pores in the self-supporting membrane, increasing density and mechanical strength, and reducing cracks or defects. Preferably, in this step, the preheating temperature is 50℃-150℃, and the pressure is 5MPa-15MPa. Within this temperature and pressure range, the second mixture is sufficiently softened, improving its flowability and plasticity, facilitating calendering in a twin-roll calender, while preventing material decomposition or performance degradation due to excessively high temperatures, thus ensuring the quality and performance stability of the obtained self-supporting membrane.

[0062] The self-supporting membrane provided by the present invention or the self-supporting membrane prepared by the preparation method of the present invention can be applied to electrode sheets. Specifically, the self-supporting membrane is hot-pressed with an aluminum foil current collector to obtain an electrode sheet. Preferably, the hot-pressing conditions are: temperature of 120℃-150℃, pressure of 10MPa-20MPa, and time of 1min-5min.

[0063] In addition, the preparation method provided in this application also uses polymethylsilane-acrylic acid graft copolymer as a binder. On the one hand, the unique grafting structure of this binder gives it good flexibility and elasticity, and it can form a continuous three-dimensional network structure in the self-supporting membrane, which can effectively disperse stress and solve the problem of poor flexibility of the self-supporting membrane. On the other hand, this binder has a good dispersing effect on the conductive agent, so that the conductive agent is evenly distributed in the self-supporting membrane. Under the synergistic effect of the fibrous three-dimensional structure formed by polytetrafluoroethylene, a more perfect and effective conductive network is formed, which reduces the resistivity of the electrode sheet.

[0064] The preparation method of this invention uses polytetrafluoroethylene (PTFE) fiberization and polymethylsilane-acrylic acid graft copolymer as binders. On the one hand, it improves the flexibility of the self-supporting membrane; on the other hand, because the polymethylsilane-acrylic acid graft copolymer also has a dispersing effect, the conductive agent is uniformly distributed in the self-supporting membrane under its dispersion effect to form conductive channels. The fibrous three-dimensional structure formed by PTFE acts as a "scaffold" to connect the conductive channels and build a conductive network, thus reducing the resistivity of the electrode plate.

[0065] The method for preparing the self-supporting membrane provided in this application omits steps such as coating and drying, reduces solvent use and consumption, simplifies the preparation process, significantly reduces costs, and is beneficial for industrial applications.

[0066] The self-supporting membrane of this invention can be applied to electrode sheets, and further to sodium-ion batteries or sodium-ion capacitors. This is because the self-supporting membrane in this application effectively solves the problems of poor flexibility of self-supporting membranes and high resistivity of electrode sheets, improves the mechanical and electrical properties of electrode sheets, and thus enhances the overall performance of sodium-ion batteries or sodium-ion capacitors, including charging efficiency, rate performance and cycle stability, laying a solid foundation for the large-scale commercial application of sodium-ion batteries or sodium-ion capacitors.

[0067] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0068] Example 1

[0069] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0070] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 95:1.5:2:1.5.

[0071] (1) Sodium iron phosphate, conductive carbon black and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear machine at 4500 rpm for 15 min at 40°C to disperse and mix the materials evenly by mechanical force to obtain the first mixture.

[0072] (2) Polytetrafluoroethylene fiberization:

[0073] The first mixture was heated to 100°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 100-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 8 minutes, with a forward rotation speed of 200 rpm and a reverse rotation speed of 200 rpm.

[0074] (3) Hot pressing film formation:

[0075] The second mixture after sieving is preheated at 100°C and then calendered at a pressure of 5MPa using a two-roll calender to form a self-supporting film with a thickness of 50μm.

[0076] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 120℃, pressure 10MPa, and time 3min.

[0077] Example 2

[0078] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0079] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 88:10:0.5:1.5.

[0080] (1) Sodium iron phosphate, conductive carbon black, graphene, and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear mill at 3000 rpm for 20 min at 10°C to disperse and mix the materials evenly by mechanical force, thus obtaining the first mixture. The mass ratio of conductive carbon black to graphene was 5:5.

[0081] (2) Polytetrafluoroethylene fiberization:

[0082] The first mixture was heated to 70°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 100-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 10 minutes, with a forward rotation speed of 10 rpm and a reverse rotation speed of 10 rpm.

[0083] (3) Hot pressing film formation:

[0084] The second mixture after sieving is preheated at 110°C and then calendered at a pressure of 10 MPa by a two-roll calender into a self-supporting film with a thickness of 100 μm.

[0085] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 130℃, pressure 15MPa, and time 2min.

[0086] Example 3

[0087] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0088] In this embodiment, the mass ratio of the electrode active material, conductive agent, polytetrafluoroethylene, and polymethylsilane-acrylic acid graft copolymer is 90:1:4:5.

[0089] (1) Sodium vanadium phosphate, conductive carbon black and polymethylsilane-acrylic acid graft copolymer were stirred at 50°C in a high-speed shear machine at 500 rpm for 60 min to disperse and mix the materials evenly by mechanical force to obtain the first mixture.

[0090] (2) Polytetrafluoroethylene fiberization:

[0091] The first mixture was heated to 110°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 150-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 5 minutes, with a forward rotation speed of 500 rpm and a reverse rotation speed of 500 rpm.

[0092] (3) Hot pressing film formation:

[0093] The second mixture after sieving is preheated at 100°C and then calendered at a pressure of 5MPa using a two-roll calender to form a self-supporting film with a thickness of 50μm.

[0094] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 120℃, pressure 10MPa, and time 5min.

[0095] Example 4

[0096] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0097] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 99:0.5:0.4:0.1.

[0098] (1) Sodium ferric sulfate, sodium ferric phosphate, carbon nanotubes, and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear press at 2000 rpm for 30 min at 20°C to disperse and mix the materials evenly by mechanical force, thus obtaining the first mixture. The mass ratio of sodium ferric sulfate to sodium ferric phosphate was 45:44.

[0099] (2) Polytetrafluoroethylene fiberization:

[0100] The first mixture was heated to 80°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 200-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 7 minutes, with a forward rotation speed of 300 rpm and a reverse rotation speed of 350 rpm.

[0101] (3) Hot pressing film formation:

[0102] The second mixture after sieving is preheated at 50°C and then calendered at a pressure of 5MPa using a two-roll calender to form a self-supporting film with a thickness of 400μm.

[0103] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 150℃, pressure 10MPa, and time 4min.

[0104] Example 5

[0105] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 95:0.1:0.9:4.

[0106] (1) Hard carbon, conductive carbon black and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear machine at 100°C for 45 minutes at 1000 rpm to disperse and mix the materials evenly by mechanical force to obtain the first mixture.

[0107] (2) Polytetrafluoroethylene fiberization:

[0108] The first mixture was heated to 100°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 150-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 6 minutes, with a forward rotation speed of 150 rpm and a reverse rotation speed of 120 rpm.

[0109] (3) Hot pressing film formation:

[0110] The second mixture after sieving is preheated at 150°C and then calendered at a pressure of 5MPa using a twin-roll calender to form a self-supporting film with a thickness of 300μm.

[0111] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 120℃, pressure 10MPa, and time 3min.

[0112] Example 6

[0113] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0114] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 88:5:2.5:4.5.

[0115] (1) Sodium nickel oxide, conductive carbon black and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear machine at 2500 rpm for 15 min at 15°C to disperse and mix the materials evenly by mechanical force to obtain the first mixture.

[0116] (2) Polytetrafluoroethylene fiberization:

[0117] The first mixture was heated to 90°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 200-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 9 minutes, with a forward rotation speed of 50 rpm and a reverse rotation speed of 100 rpm.

[0118] (3) Hot pressing film formation:

[0119] The second mixture after sieving is preheated at 90°C and then calendered at a pressure of 15MPa using a two-roll calender to form a self-supporting film with a thickness of 90μm.

[0120] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 130℃, pressure 10MPa, and time 5min.

[0121] Example 7

[0122] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0123] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 93:2:2:3.

[0124] (1) Hard carbon, acetylene black and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear machine at 3500 rpm for 20 min at 120℃ to disperse and mix the materials evenly by mechanical force to obtain the first mixture.

[0125] (2) Polytetrafluoroethylene fiberization:

[0126] The first mixture was heated to 100°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 100-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 6 minutes, with a forward rotation speed of 400 rpm and a reverse rotation speed of 400 rpm.

[0127] (3) Hot pressing film formation:

[0128] The second mixture after sieving is preheated at 120°C and then calendered at a pressure of 5MPa using a two-roll calender to form a self-supporting film with a thickness of 150μm.

[0129] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 140℃, pressure 20MPa, and time 2min.

[0130] Example 8

[0131] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0132] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 88:1:1:10.

[0133] (1) Hard carbon, conductive carbon black and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear machine at 4000 rpm for 25 min at 25°C to disperse and mix the materials evenly by mechanical force to obtain the first mixture.

[0134] (2) Polytetrafluoroethylene fiberization:

[0135] The first mixture was heated to 110°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 150-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 8 minutes, with a forward rotation speed of 250 rpm and a reverse rotation speed of 250 rpm.

[0136] (3) Hot pressing film formation:

[0137] The second mixture after sieving is preheated at 120°C and then calendered at a pressure of 15 MPa using a two-roll calender to form a self-supporting film with a thickness of 200 μm.

[0138] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 120℃, pressure 10MPa, and time 3min.

[0139] Example 9

[0140] In this embodiment, a self-supporting membrane was prepared and then fabricated into an electrode plate.

[0141] In this embodiment, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 88:1:10:1.

[0142] (1) Soft carbon, graphene and polymethylsilane-acrylic acid graft copolymer were stirred in a high-speed shear machine at 5000 rpm for 5 min at 60°C to disperse and mix the materials evenly by mechanical force to obtain the first mixture.

[0143] (2) Polytetrafluoroethylene fiberization:

[0144] The first mixture was heated to 120°C, and polytetrafluoroethylene was added while stirring to obtain the second mixture. The second mixture was then passed through a 100-mesh sieve. The stirring conditions were as follows: the forward and reverse stirring modes were started simultaneously for 8 minutes, with a forward rotation speed of 200 rpm and a reverse rotation speed of 200 rpm.

[0145] (3) Hot pressing film formation:

[0146] The second mixture after sieving is preheated at 100°C and then calendered at a pressure of 10MPa using a two-roll calender to form a self-supporting film with a thickness of 100μm.

[0147] (4) The self-supporting diaphragm and the aluminum foil current collector are hot-pressed together to obtain the electrode plate. The hot-pressing conditions are: temperature 120℃, pressure 15MPa, and time 5min.

[0148] Comparative Example 1

[0149] The difference from Example 1 is that no polymethylsilane-olefin copolymer is added.

[0150] Comparative Example 2

[0151] The difference from Example 2 is that no polymethylsilane-olefin copolymer is added.

[0152] Comparative Example 3

[0153] The difference from Example 3 is that no polymethylsilane-olefin copolymer is added.

[0154] Comparative Example 4

[0155] The difference from Example 4 is that no polymethylsilane-olefin copolymer is added.

[0156] Comparative Example 5

[0157] The difference from Example 5 is that no polymethylsilane-olefin copolymer is added.

[0158] Comparative Example 6

[0159] The difference from Example 6 is that no polymethylsilane-olefin copolymer is added.

[0160] Comparative Example 7

[0161] The difference from Example 7 is that no polymethylsilane-olefin copolymer is added.

[0162] Comparative Example 8

[0163] The difference from Example 8 is that no polymethylsilane-olefin copolymer is added.

[0164] Comparative Example 9

[0165] The difference from Example 9 is that no polymethylsilane-olefin copolymer is added.

[0166] The performance of the self-supporting films and electrode sheets prepared in the above embodiments and comparative examples was characterized.

[0167] The electrode sheets prepared in the above embodiments and comparative examples were subjected to flexibility and resistivity tests.

[0168] The flexibility test method has the following steps:

[0169] Fold the electrode sheet 180° in half, then observe the bent electrode sheet against a light source to check for any light-transmitting areas and powder shedding.

[0170] The resistivity testing method and operating steps are as follows:

[0171] 1. Sample cutting:

[0172] Use a circular die to cut the electrode sheets, ensuring that each sheet has an area of ​​10 cm². 2 The error shall not exceed ±0.5cm 2 , to be used as samples for testing.

[0173] 2. Testing: Place the sample to be tested on the metal terminals of the resistivity meter. Apply a pressure of 5 MPa to both sides of the sample using the upper metal terminals and maintain the pressure for 5 seconds. Click "Start Resistivity Test" to measure the resistance R and thickness d. Calculate the resistivity using the following formula:

[0174] ρ = R·A / d, where R is the measured resistance value, A is the contact area, i.e. the area of ​​the sample to be tested, and d is the thickness of the electrode sheet.

[0175] The measured d is actually the sum of the thicknesses of the self-supporting diaphragm and the aluminum foil. Therefore, the method for measuring the thickness of the self-supporting diaphragm prepared in the examples and comparative examples is: d minus the difference in the thickness of the aluminum foil, wherein the aluminum foil in this application is a conventional 12μm.

[0176] The data on the flexibility and resistivity of the electrode sheets obtained through the above tests are recorded in Table 1.

[0177] Example resistivity / Ω·m Electrode plate flexibility / 180° folding Example 1 1.2 No light transmission or powder shedding after bending. Example 2 1.5 No light transmission or powder shedding after bending. Example 3 1.0 No light transmission or powder shedding after bending. Example 4 1.5 No light transmission or powder shedding after bending. Example 5 1.0 No light transmission or powder shedding after bending. Example 6 1.3 No light transmission or powder shedding after bending. Example 7 1.2 No light transmission or powder shedding after bending. Example 8 1.1 No light transmission or powder shedding after bending. Example 9 1.0 No light transmission or powder shedding after bending. Comparative Example 1 0.3 The electrode sheet sheds powder after bending. Comparative Example 2 0.4 The electrode sheet sheds powder after bending. Comparative Example 3 0.4 The electrode sheet sheds powder after bending. Comparative Example 4 0.2 The electrode sheet sheds powder after bending. Comparative Example 5 0.2 The electrode sheet sheds powder after bending. Comparative Example 6 0.4 The electrode sheet sheds powder after bending. Comparative Example 7 0.3 The electrode sheet sheds powder after bending. Comparative Example 8 0.2 The electrode sheet sheds powder after bending. Comparative Example 9 0.2 The electrode sheet sheds powder after bending.

[0178] Table 1

[0179] First, a comparison between Examples 1-9 and Comparative Examples 1-9 shows that the self-supporting membrane with added polymethylsilane-acrylic acid graft copolymer showed no light transmission or powder shedding after being bent at 180°. This indicates that the self-supporting membrane with added polymethylsilane-acrylic acid graft copolymer has better flexibility. This is because, on the one hand, the acrylic acid graft chain is connected to the polymethylsilane main chain through chemical bonds, forming stable grafting points. These grafting points act as "fixed nodes" within the self-supporting membrane, restricting the free movement of chain segments, thereby constructing a continuous three-dimensional network structure. This allows the stress to be evenly distributed within the membrane, avoiding crack propagation caused by local stress concentration and improving the flexibility of the self-supporting membrane. On the other hand, the polymethylsilane backbone is composed of silicon-silicon bonds, and its electron delocalization characteristics endow the molecular bonds with high flexibility. When olefinic acid is grafted onto the polymethylsilane backbone, the introduced rigid groups such as carboxylate groups can form physical cross-linking points. These cross-linking points limit excessive slippage of chain segments during stretching, prevent permanent deformation of the material, and maintain the integrity of the three-dimensional network structure. Therefore, the flexible chain segments of polymethylsilane and the rigid chain segments of olefinic acid work synergistically to improve the tensile strength of the adhesive while maintaining its toughness.

[0180] Secondly, a comparison between Examples 1-9 and Comparative Examples 1-9 shows that the electrode sheet prepared using the self-supporting membrane with added polymethylsilane-acrylic acid graft copolymer has a lower resistivity. This is because the polymethylsilane-acrylic acid graft copolymer acts as a "bridge" to connect the conductive agent and the electrode active material, allowing the conductive agent to be evenly distributed within the self-supporting membrane. Under the synergistic effect of the fibrous three-dimensional structure formed by polytetrafluoroethylene, a more complete and effective conductive network is formed, reducing the resistivity of the electrode sheet.

[0181] In summary, this invention incorporates a polymethylsilane-acrylic acid graft copolymer as a binder during the preparation of the self-supporting membrane. On one hand, the unique grafting structure of this binder gives it excellent flexibility and elasticity, enabling it to form a continuous three-dimensional network structure within the self-supporting membrane, effectively dispersing stress and solving the problem of poor flexibility in the self-supporting membrane. On the other hand, this binder has a good dispersing effect on the conductive agent, allowing it to be evenly distributed within the self-supporting membrane to form conductive pathways. The fibrous three-dimensional structure formed by polytetrafluoroethylene acts as a "scaffold" to connect the conductive pathways and construct a conductive network, thereby reducing the resistivity of the electrode sheets.

[0182] The self-supporting diaphragm provided in this invention effectively solves the problems of poor flexibility and high resistivity of the electrode sheet, improves the mechanical and electrical properties of the electrode sheet, and lays a solid foundation for the large-scale commercial application of sodium-ion batteries and sodium-ion capacitors.

[0183] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A self-supporting diaphragm, characterized in that, The self-supporting membrane comprises an electrode active material, a conductive agent, polytetrafluoroethylene, and a polymethylsilane-acrylic acid graft copolymer in a mass ratio of 88-99:0.1-10:0.5-10:0.1-10. The polymethylsilane-acrylic acid graft copolymer is a flexible and elastic structure used to form a continuous three-dimensional network structure inside the self-supporting membrane, thereby improving the membrane's flexibility and self-support. The polytetrafluoroethylene undergoes fibrosis during processing, forming a fibrous three-dimensional structure, which enhances the mechanical strength and structural stability of the self-supporting membrane. The conductive agent, under the dispersion effect of the polymethylsilane-acrylic acid graft copolymer, is uniformly distributed in the self-supporting membrane to form conductive pathways. The fibrous three-dimensional structure formed by the polytetrafluoroethylene acts as a "scaffold" to connect the conductive pathways and construct a conductive network.

2. The self-supporting diaphragm according to claim 1, characterized in that, The electrode active material is a positive electrode active material, including one or more of polyanionic positive electrode materials and layered oxide positive electrode materials; or, the electrode active material is a negative electrode active material, including one or more of hard carbon and soft carbon.

3. The self-supporting diaphragm according to claim 1, characterized in that, The polymethylsilane-acrylic acid graft copolymer is a copolymer of polymethylsilane and acrylic acid grafted together, wherein the acrylic acid includes one or more of acrylic acid, butenoic acid, pentenoic acid, and hexenoic acid.

4. The self-supporting diaphragm according to claim 1, characterized in that, The conductive agent includes one or more of the following: conductive carbon black, carbon nanotubes, graphene, Ketjen black, acetylene black, graphite, and conductive carbon fiber.

5. The self-supporting diaphragm according to claim 1, characterized in that, The thickness of the self-supporting membrane is 50μm-400μm.

6. A method for preparing a self-supporting membrane as described in any one of claims 1-5, characterized in that, The preparation method includes: The electrode active material, conductive agent, and polymethylsilane-acrylic acid graft copolymer are mixed evenly in a high-speed shear mill to obtain the first mixture. While maintaining stirring at 70℃-120℃, polytetrafluoroethylene is added to the first mixture, causing the polytetrafluoroethylene to fibrose under shear force and temperature, resulting in a second mixture with a fibrous three-dimensional structure. The second mixture is preheated and then calendered into a self-supporting film using a twin-roll calender. In the self-supporting membrane, the mass ratio of electrode active material, conductive agent, polytetrafluoroethylene and polymethylsilane-acrylic acid graft copolymer is 88-99:0.1-10:0.5-10:0.1-10.

7. The preparation method according to claim 6, characterized in that, The high-speed shearing machine operates at a speed of 500 rpm to 5000 rpm, a time of 5 min to 60 min, and a temperature of 10℃ to 150℃.

8. The preparation method according to claim 6, characterized in that, The preheating temperature is 50℃-150℃.

9. An electrode sheet, characterized in that, The electrode sheet includes the self-supporting film as described in any one of claims 1-5, or the self-supporting film prepared by the preparation method as described in any one of claims 6-8.

10. An energy storage device, characterized in that, The energy storage device includes: a sodium-ion battery or a sodium-ion capacitor; The energy storage device includes the self-supporting membrane as described in any one of claims 1-5, or the self-supporting membrane prepared by the preparation method described in any one of claims 6-8, or the electrode sheet as described in claim 9.

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