Fiber membrane and preparation method thereof

Abstract

The present application relates to the field of lithium battery technology, and discloses a fiber membrane and a method for preparing the same. According to an embodiment of the present application, the fiber membrane comprises a plurality of nanofiber filaments, and the nanofiber filaments comprise: a core layer, the core layer comprises a carbon material and silver particles; a shell layer, the shell layer comprises a carbon material; the shell layer is sleeved on the outer surface of the core layer; wherein the carbon material is selected from at least one of carbon particles and continuous carbon materials. The fiber membrane according to the embodiment of the present application can improve the rate performance and cycle performance of lithium-ion batteries.

Description

Technical Field

[0001] The present application relates to the field of lithium battery technology, and in particular to a fiber membrane and a method for preparing the same, and further to a lithium ion battery and an electrical device. Background Art

[0002] With the increasing global energy demand and the increasingly serious environmental pollution problem, the development of new energy technology, especially battery technology for electric vehicles, has received widespread attention. Lithium-ion batteries have become a hot spot for research and application due to their high energy density, long cycle life and good environmental adaptability. Among them, the negative electrode material of lithium-ion batteries plays a vital role in improving battery performance and increasing energy density. However, the existing negative electrode materials of lithium-ion batteries have the problem of low stability.

[0003] Therefore, there is an urgent need to improve the negative electrode materials of existing lithium-ion batteries in order to enhance the stability of the negative electrode materials and thus enhance the performance of lithium batteries. Summary of the invention

[0004] The present application aims to solve one of the technical problems in the related art at least to a certain extent. To this end, the purpose of the present application is to propose a fiber membrane.

[0005] In one aspect of the present application, a fiber membrane is proposed. According to an embodiment of the present application, the fiber membrane includes a plurality of nanofiber filaments, and the nanofiber filaments include: a core layer, the core layer includes a carbon material and silver particles; a shell layer, the shell layer includes a carbon material; the shell layer is sleeved on the outer surface of the core layer; wherein the carbon material is selected from at least one of carbon particles and continuous carbon materials. The fiber membrane according to the embodiment of the present application can improve the rate performance and cycle performance of lithium-ion batteries.

[0006] In some embodiments of the present application, the diameter of the nanofiber filaments is 100 nm to 1500 nm. According to the fiber membrane of the embodiment of the present application, the rate performance and cycle performance of the lithium-ion battery can be further improved.

[0007] In some embodiments of the present application, the fiber membrane has a thickness of 1 μm to 50 μm. According to the fiber membrane of the embodiment of the present application, the rate performance and cycle performance of the lithium-ion battery can be further improved.

[0008] In some embodiments of the present application, the core layer includes the continuous carbon material and the silver particles, and the shell layer includes the continuous carbon material; or, the core layer includes the carbon particles and the silver particles, and the shell layer includes the continuous carbon material.

[0009] In some embodiments of the present application, the continuous carbon material of the core layer forms a core carbon layer, the continuous carbon material of the shell layer forms a shell carbon layer, and at least a portion of the silver particles are disposed between the core carbon layer and the shell carbon layer.

[0010] In some embodiments of the present application, the core layer and the shell layer are coaxially arranged.

[0011] In some embodiments of the present application, the thickness of the core layer is 10 nm to 500 nm.

[0012] In some embodiments of the present application, the thickness of the shell layer is 10nm~500nm.

[0013] In some embodiments of the present application, the diameter of the silver particles is 2nm~100nm.

[0014] In some embodiments of the present application, the carbon materials of the core layer and the shell layer are formed by carbonizing different carbon-containing compounds.

[0015] In some embodiments of the present application, the carbon-containing compound of the core layer is formed by carbonizing at least one selected from the following: an organic small molecule mixture and a first polymer, wherein the first polymer includes at least one of polyacrylic acid, polyamic acid, polyvinylidene fluoride and polymethacrylate.

[0016] In some embodiments of the present application, the carbon-containing compound of the shell layer is formed by carbonizing a second polymer, and the second polymer includes at least one of polyacrylonitrile, polyvinylidene fluoride, polyamic acid and polyetherimide.

[0017] In some embodiments of the present application, the organic small molecule mixture includes at least one of kerosene, naphtha, paraffin, mineral oil, turpentine, pine oil and camphor oil.

[0018] In some embodiments of the present application, the silver particles are formed by reducing at least one of the following silver salts: silver nitrate, silver fluoride, silver fluoroborate, silver acetate, silver trifluoroacetylacetonate and silver acetylacetonate.

[0019] In some embodiments of the present application, the mass fraction of silver element in the fiber membrane is 2%~30%.

[0020] In the second aspect of the present application, the present application proposes a method for preparing the fiber membrane described in the first aspect of the present application. According to an embodiment of the present application, the method comprises: spinning a core layer spinning solution containing a core layer carbon source precursor and a silver salt with a shell layer spinning solution containing a second polymer solution to obtain nanofiber filaments, and carbonizing the nanofiber filaments to form the fiber membrane described in the first aspect of the present application. The fiber membrane prepared by the method of the embodiment of the present application can improve the rate performance and cycle performance of lithium-ion batteries.

[0021] According to an embodiment of the present application, the mass fraction of the silver salt in the core layer carbon source precursor is 4% to 60%.

[0022] According to an embodiment of the present application, the spinning process is achieved by at least one of the following technologies: coaxial electrospinning technology and air-blowing spinning technology.

[0023] According to an embodiment of the present application, the mass concentration of the shell layer spinning solution is 6% to 20%.

[0024] According to an embodiment of the present application, the mass concentration of the core layer spinning solution is 10% to 65%.

[0025] According to an embodiment of the present application, the solvent of the shell spinning solution includes at least one of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dimethyl carbonate, N-methylpyrrolidone, propylene carbonate, vinyl acetate, butylene carbonate and γ-butyrolactone.

[0026] According to an embodiment of the present application, the solvent of the core layer spinning solution includes at least one of dichloromethane, chloroform, benzene, toluene, xylene, petroleum ether, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and N-methylpyrrolidone.

[0027] According to an embodiment of the present application, the carbonization treatment is achieved by at least one of the following conditions: carried out in at least one of nitrogen, argon or helium, at a temperature of 600°C~2800°C, for 0.5h~48h, a heating rate of 1°C / min~20°C / min, and a cooling rate of 1°C / min~30°C / min.

[0028] According to an embodiment of the present application, the spinning process using coaxial electrospinning technology is achieved in the following manner: under the conditions of an electrostatic voltage of 8~30KV and a receiving distance of 10~40cm, the core layer spinning liquid and the shell layer spinning liquid are discharged using a coaxial needle, the coaxial needle includes a core layer spinning hole and a shell layer spinning hole, the core layer spinning hole discharges the core layer spinning liquid, and the shell layer spinning liquid is discharged using the shell layer spinning hole.

[0029] According to an embodiment of the present application, the diameter of the core layer spinning holes is 0.2 mm to 1.0 mm.

[0030] According to an embodiment of the present application, the diameter of the shell layer spinning holes is 0.5 mm to 2 mm.

[0031] According to an embodiment of the present application, the single-hole flow rate of the core layer spinning solution is 0.1 mL / h to 0.6 mL / h.

[0032] According to an embodiment of the present application, the single-hole flow rate of the shell spinning solution is 0.3 mL / h to 3 mL / h.

[0033] According to an embodiment of the present application, when the second polymer contains polyacrylonitrile, before the carbonization treatment, the method further includes pre-oxidizing the nanofiber filaments in an air atmosphere at a temperature of 200° C. to 400° C. for 30 min to 60 min.

[0034] In the third aspect of the present application, the present application proposes a method for preparing the fiber membrane described in the first aspect of the present application. According to an embodiment of the present application, the method includes: providing a core layer spinning solution, which contains a core layer carbon source precursor and a silver salt, and the mass fraction of the silver salt in the core layer carbon source precursor is 5% to 60%; at the same time, providing a shell layer spinning solution containing polyacrylonitrile; under the conditions of an electrostatic voltage of 8KV to 30KV, a core layer spinning hole diameter of 0.2mm to 1.0mm, and a shell layer spinning hole diameter of 0.5mm to 2mm, the core layer spinning solution is discharged through the core layer spinning hole through the coaxial electrospinning technology, and the shell layer spinning solution is discharged through the shell layer spinning hole to form a nanofiber filament with a coaxial structure; adjusting the single hole flow rate of the core layer spinning solution to 0.1 mL / h to 0.6mL / h, the single hole flow rate of the shell spinning solution to 0.3mL / h to 3mL / h, and the receiving distance is set to 10cm~40cm; the nanofiber filaments are pre-oxidized in an air atmosphere at a temperature of 200℃~400℃ for 30min~60min; the pre-oxidized nanofiber filaments are carbonized in an atmosphere of nitrogen, argon or helium, the treatment temperature is controlled at 600℃ to 2800℃, the heating rate is 1℃ / min to 20℃ / min, the cooling rate is 1℃ / min to 30℃ / min, and the carbonization time is 0.5h~48h, so as to form the fiber membrane described in the first aspect of the present application. The fiber membrane prepared according to the method of the embodiment of the present application can improve the rate performance and cycle performance of lithium-ion batteries.

[0035] In the fourth aspect of the present application, the present application proposes a lithium-ion battery. According to an embodiment of the present application, the lithium-ion battery contains: the fiber membrane described in the first aspect of the present application as a negative electrode material. According to the lithium-ion battery of the embodiment of the present application, the rate performance and cycle performance are improved.

[0036] In a fifth aspect of the present application, the present application proposes an electric device. According to an embodiment of the present application, the electric device comprises: the lithium-ion battery described in the fourth aspect of the present application.

[0037] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0039] Figure 1 Schematic diagram of the structure of nanofiber filaments according to an embodiment of the present application.

[0040] Figure 2 Schematic diagram of the structure of nanofiber filaments according to an embodiment of the present application.

[0041] Figure 3 Schematic diagram of the structure of nanofiber filaments according to an embodiment of the present application.

[0042] Figure 4 Schematic diagram of the structure of a coaxial needle according to an embodiment of the present application.

[0043] Figure 5 This is a TEM image of nanofiber filaments of a negative electrode material for a lithium-ion battery according to an embodiment of the present application.

[0044] Figure 6 This is a SEM image of the fiber membrane surface of the lithium-ion battery negative electrode material according to an embodiment of the present application.

[0045] Description of reference numerals:

[0046] Nanofiber filament 100; core layer 10; shell layer 20; carbon particle 5; silver particle 8; core carbon layer 30; shell carbon layer 40; axis 50. DETAILED DESCRIPTION

[0047] The embodiments of the present application are described in detail below. Examples of the embodiments are shown in the drawings, but unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repeated descriptions of actually identical structures may be omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0048] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by technicians in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; unless otherwise specified, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0049] The terms "including" and "having" in the specification and claims of the present application and any modifications thereof are open expressions, that is, including the contents specified in the present application but not excluding other contents.

[0050] In the description of the present application, it should be understood that the terms "width", "thickness", etc. indicate orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0051] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. "First feature" and "second feature" may include one or more of the features.

[0052] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0053] In the description of the present application, unless otherwise clearly specified and limited, a first feature being “above” or “below” a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being “above”, “above”, and “above” a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being “below”, “below”, and “below” a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0054] In the description of the present application, “plurality” means two or more.

[0055] In the description of the present application, "A and / or B" may include the case of A alone, the case of B alone, or any of the cases of A and B, where A and B are only used for examples and may be any technical feature connected by "and / or" in the present application.

[0056] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0057] With the growth of global energy demand and the intensification of environmental problems, society has an increasingly urgent need for new energy vehicles to reduce dependence on fossil fuels and reduce pollution. The promotion of new energy vehicles is largely limited by the energy density of their power source, the battery. In order to compete with traditional vehicles, battery technology must achieve higher energy density, with the goal of reaching 350Wh / kg or higher. In the field of battery technology, the selection of positive and negative electrode materials is crucial to improving battery performance. At present, silicon-based anodes and lithium metal anodes are favored by researchers because of their high theoretical capacity. However, these materials have some challenges in practical applications, such as the volume expansion problem of silicon-based anodes during charging and discharging, and the dendrites that are easily formed during charging and discharging of lithium metal anodes, which may affect the safety and cycle life of the battery. Lithium-ion batteries prepared with existing anode materials have problems such as low rate performance, cycle performance, energy density and high manufacturing cost. In order to overcome this problem, the present application provides a fiber membrane, which includes a shell layer and a core layer, the core layer includes a carbon material and silver particles, the shell layer includes a carbon material, and the shell layer is sleeved on the outer surface of the core layer. The fiber membrane with the above structure can optimize the distribution of silver particles, so that the silver particles and the carbon material form a core layer together and are distributed in the core layer, or the silver particles are distributed between the core carbon layer and the shell carbon layer, thereby forming a stable conductive network, thereby improving the electrochemical performance of the lithium-ion battery. In addition, the fiber membrane can be directly used as a negative electrode material, which simplifies the preparation process of the battery and helps to reduce manufacturing costs.

[0058] In one aspect of the present application, the present application proposes a fiber membrane. According to an embodiment of the present application, the fiber membrane comprises a plurality of nanofiber filaments, Figure 1The nanofiber filament 100 comprises: a core layer 10, the core layer 10 comprises carbon material and silver particles; a shell layer 20, the shell layer 20 comprises carbon material; the shell layer 20 is sleeved on the outer surface of the core layer 10; it should be explained that the carbon material and silver particles in the core layer 10 are physically mixed, the carbon material in the core layer 10 is selected from at least one of carbon particles and continuous carbon materials, and the carbon material in the shell layer 20 comprises continuous carbon materials. The fiber membrane according to the embodiment of the present application can improve the rate performance and cycle performance of lithium-ion batteries.

[0059] The “continuous carbon material” in the present application refers to a carbonaceous material forming a continuous phase, including but not limited to graphene, carbon nanotubes, carbon fibers, amorphous carbon and graphite.

[0060] It should be explained that the fiber membrane of the embodiment of the present application is a nanocomposite fiber membrane, which is a high-performance material. It is composed of nanoscale fibers and other nanoscale materials or components are integrated into the fiber structure to form a composite material with specific functions. Among them, the fiber diameter in the fiber membrane is usually at the nanometer level, which makes the material have an extremely high specific surface area and enhances its physical and chemical activity. The nanocomposite fiber membrane not only contains the fiber itself, but also combines other nanomaterials, such as nanoparticles, nanowires or nanosheets, which can be metals, oxides, polymers or other compounds.

[0061] In some embodiments of the present application, the diameter of the nanofiber filament is 100nm to 1500nm, for example, 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1000nm, 1100nm, 1200nm, 1300nm, 1400nm and 1500nm and any range value therebetween. Within this range, according to the embodiments of the present application, the moderate diameter of the nanofiber filament not only helps to build a stable shell, but also ensures the overall stability of the material, so that the structure of the nanofiber filament is more complete. This optimized morphology reduces the path length of lithium ions during the embedding and extraction process, and reduces the resistance to ion transmission. Thus, the nanofiber filament with a moderate diameter can further improve the rate performance and cycle performance of lithium-ion batteries.

[0062] In some embodiments of the present application, the thickness of the fiber membrane is 1 μm to 50 μm, for example, 1 μm, 5 μm, 10 μm, 15 μm, 20 nm, 25 μm, 30 nm, 35 μm, 40 nm, 45 μm and 50 nm and any range of values ​​therebetween. Within this range, according to the embodiments of the present application, a fiber membrane with moderate thickness is not only conducive to inducing uniform deposition of lithium ions on the surface of the negative electrode, but also significantly improves the cycle stability and durability of the battery. By optimizing the thickness of the fiber membrane, the weight of the negative electrode can be effectively reduced. As a result, a fiber membrane with moderate thickness can further improve the rate performance and cycle performance of lithium-ion batteries.

[0063] In some embodiments of the present application, the core layer includes a continuous carbon material and silver particles, and the shell layer includes a continuous carbon material.

[0064] In some embodiments of the present application, reference Figure 2 The structure of the nanofiber filament 100 includes a core layer 10 and a shell layer 20, wherein the core layer 10 includes carbon particles 5 and silver particles 8, and the shell layer 20 includes a continuous carbon material.

[0065] In some embodiments of the present application, reference Figure 3 The structure of the nanofiber filament 100 includes a core layer 10 and a shell layer 20, the continuous carbon material of the core layer 10 forms a core carbon layer 30, the continuous carbon material of the shell layer 20 forms a shell carbon layer 40, and at least a portion of the silver particles 8 are disposed between the core carbon layer 30 and the shell carbon layer 40. The core layer 10 includes the core carbon layer 30 and the silver particles 8, and the shell layer 20 is the shell carbon layer 40.

[0066] In certain embodiments of the present application, the core layer and the shell layer are coaxially arranged, and the axis 50 is as follows: Figure 1 However, those skilled in the art should understand that even if the axes of the core layer and the shell layer are not completely coincident, that is, there is a certain deviation or offset between their axes, this situation also belongs to a part of the technical solution to be protected by the present application.

[0067] In some embodiments of the present application, the thickness of the core layer is 10nm~500nm, such as 10nm, 20nm, 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm and 500nm and any range of values ​​therebetween. The moderate thickness of the core layer means that the silver-carbon content in the core layer is moderate, which is conducive to forming a stable coaxial structure, reducing the path length of lithium ions during the insertion and extraction process, and reducing the resistance to ion transmission. Therefore, the moderate thickness of the core layer can further improve the rate performance and cycle performance of the lithium-ion battery.

[0068] In some embodiments of the present application, the thickness of the shell layer is 10nm to 500nm, such as 10nm, 20nm, 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm and 500nm and any range of values ​​therebetween. The thickness of the shell layer is moderate, which is conducive to forming a stable coaxial structure and ensuring the stability of the material structure. Thus, the thickness of the shell layer is moderate, which can further improve the rate performance and cycle performance of the lithium-ion battery.

[0069] In some embodiments of the present application, the diameter of the silver particles is 2nm to 100nm, such as 2nm, 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm and 100nm and any range of values ​​therebetween. The diameter of the silver particles is moderate, and they can stably exist inside the coaxial fiber, reduce the weight of the negative electrode, and effectively induce lithium deposition. Therefore, the diameter of the silver particles is moderate, which can further improve the rate performance and cycle performance of the lithium-ion battery.

[0070] In some embodiments of the present application, the carbon materials of the core layer and the shell layer are formed by carbonizing different carbon-containing compounds.

[0071] In some embodiments of the present application, the carbon-containing compound of the core layer is formed by carbonizing at least one selected from the following: an organic small molecule mixture and a first polymer, wherein the first polymer includes at least one of polyacrylic acid, polyamic acid, polyvinylidene fluoride and polymethacrylate.

[0072] In some embodiments of the present application, a "small molecule mixture of organic matter" refers to a substance composed of two or more small organic molecules with relatively low molecular weights containing carbon and hydrogen elements.

[0073] In some embodiments of the present application, the carbon-containing compound of the shell layer is formed by carbonizing a second polymer, and the second polymer includes at least one of polyacrylonitrile, polyvinylidene fluoride, polyamic acid and polyetherimide.

[0074] In some embodiments of the present application, the second polymer includes at least one of polyacrylonitrile and its copolymers, polyvinylidene fluoride and its copolymers, polyamic acid and polyetherimide. In some embodiments of the present application, "copolymer" refers to a polymer formed by copolymerization of two or more different monomers, and in the present application, "polyacrylonitrile and its copolymers" refers to a copolymer formed by copolymerization of polyacrylonitrile and other monomers, such as a copolymer formed by copolymerization of polyacrylonitrile with acrylic acid, styrene, and butadiene.

[0075] In some embodiments of the present application, the organic small molecule mixture includes at least one of kerosene, naphtha, paraffin, mineral oil, turpentine, pine oil and camphor oil.

[0076] In some embodiments of the present application, the silver particles are formed by reducing at least one of the following silver salts: silver nitrate, silver fluoride, silver fluoroborate, silver acetate, silver trifluoroacetylacetonate and silver acetylacetonate.

[0077] In some embodiments of the present application, the mass fraction of the silver element in the fiber membrane is 2% to 30%, for example, 2%, 3%, 5%, 10%, 13%, 15%, 17%, 19%, 21%, 23%, 25%, 27%, 30% or a range thereof. Within this range, the mass fraction of the silver element is moderate, and it can stably exist inside the coaxial fiber, reduce the weight of the negative electrode, and effectively induce lithium deposition. Thus, the mass fraction of the silver element is moderate, which can further improve the rate performance and cycle performance of the lithium-ion battery.

[0078] In the second aspect of the present application, the present application proposes a method for preparing the fiber membrane described in the first aspect of the present application. According to an embodiment of the present application, the method comprises: spinning a core layer spinning solution containing a core layer carbon source precursor and a silver salt with a shell layer spinning solution containing a second polymer solution to obtain nanofiber filaments, and carbonizing the nanofiber filaments to form the fiber membrane described in the first aspect of the present application. The fiber membrane prepared by the method of the embodiment of the present application can improve the rate performance and cycle performance of lithium-ion batteries.

[0079] According to an embodiment of the present application, the mass fraction of the silver salt in the core carbon source precursor is 4% to 60%. The mass fraction of the silver salt in the core carbon source precursor is moderate, and it can stably exist inside the coaxial fiber, reduce the weight of the negative electrode, and effectively induce lithium deposition. Therefore, the mass fraction of the silver salt in the core carbon source precursor is moderate, which can further improve the rate performance and cycle performance of the lithium-ion battery.

[0080] According to an embodiment of the present application, the spinning process is achieved by at least one of the following technologies: coaxial electrospinning technology and air-blowing spinning technology.

[0081] According to an embodiment of the present application, the mass concentration of the shell layer spinning solution is 6% to 20%.

[0082] According to an embodiment of the present application, the mass concentration of the core layer spinning solution is 10% to 65%.

[0083] According to an embodiment of the present application, the solvent of the shell spinning solution includes at least one of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dimethyl carbonate, N-methylpyrrolidone, propylene carbonate, vinyl acetate, butylene carbonate and γ-butyrolactone.

[0084] According to an embodiment of the present application, the solvent of the core layer spinning solution includes at least one of dichloromethane, chloroform, benzene, toluene, xylene, petroleum ether, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and N-methylpyrrolidone.

[0085] According to an embodiment of the present application, the carbonization treatment is achieved by at least one of the following conditions: carried out in at least one of nitrogen, argon or helium, at a temperature of 600°C~2800°C, for 0.5h~48h, a heating rate of 1°C / min~20°C / min, and a cooling rate of 1°C / min~30°C / min.

[0086] According to the embodiment of the present application, the spinning process using coaxial electrospinning technology is achieved by: under the conditions of an electrostatic voltage of 8 to 30 kV and a receiving distance of 10 to 40 cm, the core layer spinning solution and the shell layer spinning solution are discharged using a coaxial needle, Figure 4 The coaxial needle head includes a core layer spinning hole and a shell layer spinning hole. The core layer spinning hole discharges the core layer spinning solution, and the shell layer spinning hole discharges the shell layer spinning solution.

[0087] According to an embodiment of the present application, the pore size of the core layer spinning hole is 0.2mm to 1.0mm, for example, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1.0mm or any range of values ​​therebetween. The moderate diameter of the core layer spinning hole means that the silver-carbon content in the core layer is moderate, which is conducive to forming a stable coaxial structure, reducing the path length of lithium ions during the embedding and extraction process, and reducing the resistance to ion transmission. As a result, the moderate pore size of the core layer spinning hole can further improve the rate performance and cycle performance of lithium-ion batteries.

[0088] According to an embodiment of the present application, the pore size of the shell spinning hole is 0.5mm to 2mm, for example, 0.5mm, 0.7mm, 0.9mm, 1.0mm, 1.1mm, 1.2mm, 1.3mm, 1.4mm, 1.5mm, 1.6mm, 1.7mm, 1.8mm, 1.9mm, 2.0mm or any range of values ​​therebetween. The shell spinning hole has a moderate diameter, which is conducive to forming a stable coaxial structure, reducing the path length of lithium ions during the embedding and extraction process, and reducing the resistance to ion transmission. As a result, the shell spinning hole has a moderate pore size, which can further improve the rate performance and cycle performance of lithium-ion batteries.

[0089] According to an embodiment of the present application, the single-hole flow rate of the core layer spinning solution is 0.1mL / h to 0.6mL / h, for example, 0.1mL / h, 0.2mL / h, 0.3mL / h, 0.4mL / h, 0.5mL / h, 0.6mL / h or any range of values ​​therebetween. The moderate single-hole flow rate of the core layer spinning hole means that the silver-carbon content in the core layer is moderate, which is conducive to forming a stable coaxial structure, reducing the path length of lithium ions during the embedding and extraction process, and reducing the resistance to ion transmission. As a result, the moderate single-hole flow rate of the core layer spinning solution can further improve the rate performance and cycle performance of lithium-ion batteries.

[0090] According to an embodiment of the present application, the single hole flow rate of the shell spinning solution is 0.3mL / h to 3mL / h, for example, 0.3mL / h, 0.5mL / h, 0.8mL / h, 1.1mL / h, 1.3mL / h, 1.5mL / h, 1.7mL / h, 1.9mL / h, 2.1mL / h, 2.3mL / h, 2.5mL / h, 2.7mL / h, 3.0mL / h or any range value therebetween. The single hole flow rate of the shell spinning hole is moderate, which is conducive to forming a stable coaxial structure, reducing the path length of lithium ions during the embedding and extraction process, and reducing the resistance to ion transmission. As a result, the single hole flow rate of the shell spinning solution is moderate, which can further improve the rate performance and cycle performance of the lithium-ion battery.

[0091] According to an embodiment of the present application, when the second polymer contains polyacrylonitrile, before the carbonization treatment, the nanofiber filaments are further pre-oxidized in an air atmosphere at a temperature of 200°C to 400°C for 30min to 60min. Pre-oxidation of polyacrylonitrile can improve the stability and mechanical properties of polyacrylonitrile after carbonization. During the pre-oxidation process, the stress inside the polyacrylonitrile molecular chain is released, the chain structure is rearranged, and some groups in the polyacrylonitrile are oxidized into functional groups such as carbonyl and hydroxyl groups. The formation and mutual cross-linking of these functional groups further improve the mechanical strength of the fiber membrane.

[0092] In the third aspect of the present application, the present application proposes a method for preparing the fiber membrane described in the first aspect of the present application. According to an embodiment of the present application, the method includes: providing a core layer spinning solution, which contains a core layer carbon source precursor and a silver salt, and the mass fraction of the silver salt in the core layer carbon source precursor is 5% to 60%; at the same time, providing a shell layer spinning solution containing polyacrylonitrile; under the conditions of an electrostatic voltage of 8KV to 30KV, a core layer spinning hole diameter of 0.2mm to 1.0mm, and a shell layer spinning hole diameter of 0.5mm to 2mm, the core layer spinning solution is discharged through the core layer spinning hole through the coaxial electrospinning technology, and the shell layer spinning solution is discharged through the shell layer spinning hole to form a nanofiber filament with a coaxial structure; adjusting the single hole flow rate of the core layer spinning solution to 0.1 mL / h to 0.6mL / h, the single-hole flow rate of the shell spinning solution is increased to 0.3mL / h to 3mL / h, and the receiving distance is set to 10cm~40cm; the nanofiber filaments are pre-oxidized in an air atmosphere at a temperature of 200℃~400℃ for 30min~60min; the pre-oxidized nanofiber filaments are carbonized in an atmosphere of nitrogen, argon or helium, the treatment temperature is controlled at 600℃ to 2800℃, the heating rate is 1℃ / min to 20℃ / min, the cooling rate is 1℃ / min to 30℃ / min, and the carbonization time is 0.5h~48h, so as to form the fiber membrane described in the first aspect of the present application. The fiber membrane prepared according to the method of the embodiment of the present application has a coaxial structure of carbon / silver / carbon. By embedding silver at the interface between the core carbon and the shell carbon, the dispersibility and stability of nanosilver can be greatly improved, which is beneficial to avoid the migration of nanosilver. The silver particles can be evenly distributed in the material, thereby inducing the uniform deposition of lithium ions by forming a silver-lithium alloy, thereby improving the rate performance and cycle performance of the lithium-ion battery.

[0093] In the fourth aspect of the present application, the present application proposes a lithium-ion battery. According to an embodiment of the present application, the lithium-ion battery contains: the fiber membrane described in the first aspect of the present application as a negative electrode material. According to the lithium-ion battery of the embodiment of the present application, the rate performance and cycle performance are improved.

[0094] In a fifth aspect of the present application, the present application proposes an electric device. According to an embodiment of the present application, the electric device comprises: the lithium-ion battery described in the fourth aspect of the present application.

[0095] The embodiments of the present application are described in detail below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. In addition, unless otherwise explicitly stated, all reagents used in the following embodiments are commercially available or can be synthesized according to this article or known methods, and the reaction conditions not listed are also easily available to those skilled in the art.

[0096] Example 1

[0097] This embodiment relates to a method for preparing a fiber membrane, which is suitable for the negative electrode material of a lithium-ion battery. The preparation process is as follows: (1) Preparation of core layer spinning solution: a dimethylacetamide solution containing polyacrylic acid is used as a core layer carbon source precursor and mixed with silver trifluoroacetylacetonate to form a core layer spinning solution, wherein the mass fraction of silver element in the silver trifluoroacetylacetonate solution to polyacrylic acid is 15%. (2) Preparation of shell layer spinning solution: polyacrylonitrile and dimethylformamide are mixed at a ratio of 0.6 g / 10 mL to form a shell layer spinning solution. (3) Coaxial spinning: the core layer spinning solution and the shell layer spinning solution are respectively loaded into the syringe of the coaxial spinning equipment for electrospinning. The process parameters were set as follows: core layer flow rate 0.4 mL / h, shell layer flow rate 1.5 mL / h, core layer spinneret inner diameter 0.5 mm, shell layer spinneret inner diameter 2.5 mm, electrostatic voltage 30 KV, receiving mode drum receiving, receiving distance 35 cm, receiving time 10 hours, the diameter of the electrospun nanofibers was 300-600 nm, and the structure of the nanofibers under a transmission electron microscope can be seen in Figure 5 . (4) Pre-oxidation treatment: The nanofiber filaments obtained by electrospinning are heated to 350°C in air at a rate of 3°C / min and pre-oxidized for 30 minutes, wherein the oxygen content of the air is 21% (V / V). (5) Carbonization treatment: The pre-oxidized nanofiber filaments are placed in a carbonization furnace and carbonized at a temperature of 2800°C for 0.5 hours under a nitrogen atmosphere. The heating rate is controlled at 20°C / min. After the carbonization is completed, the nanofiber filaments are cooled to room temperature at a rate of 30°C / min. (6) Final product: Through the above steps, a fiber membrane is prepared. The thickness of the fiber membrane is 30μm. The structure of the fiber membrane under a scanning electron microscope can be seen in Figure 6 The fiber membrane can be used as a negative electrode material for lithium-ion batteries. Thermogravimetry (TG) test showed that the mass fraction of silver in the fiber membrane was 6.5%.

[0098] Example 2

[0099] The only difference between Example 2 and Example 1 is that mineral oil is used as a core layer carbon source precursor and mixed with a silver trifluoroacetylacetone solution to form a core layer spinning solution, and the solvent is dichloromethane. The other contents are the same as Example 1.

[0100] Example 3

[0101] The only difference between Example 3 and Example 1 is that naphtha is used as a core layer carbon source precursor and mixed with a silver trifluoroacetylacetone solution to form a core layer spinning solution, and the solvent is dichloromethane. The other contents are the same as Example 1.

[0102] Example 4

[0103] The only difference between Example 4 and Example 1 is that silver acetate is used as the silver salt and mixed with polyacrylic acid to form the core layer spinning solution. The other contents are the same as those in Example 1.

[0104] Example 5

[0105] The only difference between Example 5 and Example 1 is that polyvinylidene fluoride is used as the second polymer and mixed with dimethylformamide at a ratio of 0.6 g / 10 mL to form a shell spinning solution. Other contents are the same as Example 1.

[0106] Example 6

[0107] The only difference between Example 6 and Example 1 is that polyacrylonitrile is used as the second polymer and mixed with dimethylacetamide at a ratio of 0.8 g / 10 mL to form a shell spinning solution. Other contents are the same as Example 1.

[0108] Example 7

[0109] The only difference between Example 7 and Example 1 is that polyacrylic acid is used as a core layer carbon source precursor and mixed with a silver trifluoroacetylacetonate solution to form a core layer spinning solution, wherein the solvent of the silver trifluoroacetylacetonate solution is replaced by toluene instead of dimethylacetamide, and the other contents are the same as Example 1.

[0110] Example 8

[0111] The only difference between Example 8 and Example 1 is that polyacrylic acid is used as a core layer carbon source precursor and mixed with a silver trifluoroacetylacetonate solution to form a core layer spinning solution, wherein the solvent of the silver trifluoroacetylacetonate solution is replaced by chloroform instead of dimethylacetamide, and the other contents are the same as Example 1.

[0112] Example 9

[0113] The only difference between Example 9 and Example 1 is that the diameter of the electrospun nanofibers is 1000-1500 nm, and the other contents are the same as those of Example 1.

[0114] Example 10

[0115] The only difference between Example 10 and Example 1 is that the diameter of the electrospun nanofibers is 100-200 nm, and the other contents are the same as Example 1.

[0116] Embodiment 11

[0117] The only difference between Example 11 and Example 1 is that the diameter of the electrospun nanofiber filaments is 2000-3000 nm, and the other contents are the same as Example 1.

[0118] Example 12

[0119] The only difference between Example 12 and Example 1 is that the diameter of the electrospun nanofibers is 50-100 nm, and the other contents are the same as those of Example 1.

[0120] Embodiment 13

[0121] The only difference between Example 13 and Example 1 is that the thickness of the prepared fiber membrane is 50 μm, and the other contents are the same as Example 1.

[0122] Embodiment 14

[0123] The only difference between Example 14 and Example 1 is that the thickness of the prepared fiber membrane is 1 μm, and the other contents are the same as Example 1.

[0124] Embodiment 15

[0125] The only difference between Example 15 and Example 1 is that the thickness of the prepared fiber membrane is 80 μm, and the other contents are the same as Example 1.

[0126] Example 16

[0127] The only difference between Example 16 and Example 1 is that the thickness of the prepared fiber membrane is 0.9 μm, and the other contents are the same as Example 1.

[0128] Comparative Example 1

[0129] The difference between Comparative Example 1 and Example 1 is that the core layer spinning solution in Example 1 is not prepared, and only the shell layer spinning solution in Example 1 is used for electrospinning. Other contents are the same as Example 1.

[0130] Comparative Example 2

[0131] The difference between Comparative Example 2 and Example 1 is that the shell layer spinning solution in Example 1 is not prepared, and only the core layer spinning solution in Example 1 is used for electrospinning. Other contents are the same as Example 1.

[0132] Comparative Example 3

[0133] (1) Preparation of nanosilver sol: Weigh 100g of silver nitrate and dissolve it in 5000g of deionized water to form liquid A; weigh 100g of NaBH4 and dissolve it in 5000g of deionized water to form liquid B; add 30g of sodium citrate to liquid A, and after dissolving evenly, mix the two solutions evenly under stirring to obtain a uniform and stable yellow nanosilver sol. (2) Activated carbon adsorption: Weigh 20g of polyvinyl alcohol and add it to the nanosilver sol prepared in step (1) and mix evenly to obtain liquid C; weigh 100g of activated carbon and add it to liquid C under stirring, and continue stirring for 1h to allow the nanosilver to be fully adsorbed on the surface of the activated carbon to form a mixed solution D.

[0134] (3) Artificial graphite composite: Weigh 4800g of artificial graphite and add it to mixed solution D under stirring. Continue stirring at 1000rpm for 2h to obtain mixed solution E. Filter and wash the mixed solution E, then take the filter cake and put it into a blast oven to dry out the moisture.

[0135] (3) Nanosilver reduction: The dried filter cake is crushed and placed in a tubular furnace, heated to 400°C in a hydrogen atmosphere for 5 h, and then taken out after cooling to obtain nanosilver carbon.

[0136] The high nickel ternary material: conductive agent (SP): binder (PVDF) was evenly dispersed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 96:2:2 to form a stable slurry, which was then coated on both sides of the aluminum foil. The surface density after drying was 20 mg / cm 2 , cut into 58*78mm pole pieces for standby use;

[0137] Cut 8 μm copper foil into 60*80 mm, cut the fiber membranes prepared in Examples 1 to 16 and Comparative Examples 1 to 3 into 60*80 mm, and composite the non-woven fabric to both sides of the copper foil to prepare a composite negative electrode for use.

[0138] The composite negative electrode, 12μm wet PE separator, and positive electrode are stacked up in sequence, with 10 layers of positive electrode, 20 layers of separator, and 11 layers of negative electrode, packaged into a soft-pack battery, injected with liquid, and set aside;

[0139] The above soft pack battery was tested for rate performance and cycle performance in a blue dot electrochemical tester. The first charge was charged and discharged at 0.1C, the discharge cut-off voltage was 2.5V, and the charge cut-off voltage was 4.3V. After the first charge, it was left to stand for 10 minutes, and then the charge and discharge tests were performed twice at current densities of 0.2C, 1C, and 5C respectively; the battery cycle performance was obtained by charging and discharging the battery 100 times at a constant current at 1C. The above test temperature was room temperature, and the voltage range was 2.8~4.3V. The experimental results are shown in Table 1.

[0140] Table 1

[0141]

[0142] It can be seen from Example 1 and Comparative Example 1 that the first discharge specific capacity, first efficiency, 5C discharge specific capacity, and 1C cycle 500 capacity retention rate of the lithium ion battery prepared using the fiber membrane with a core layer structure in the present application are improved; the first discharge specific capacity, first efficiency, 5C discharge specific capacity, and 1C cycle 500 capacity retention rate of the lithium ion battery prepared using the fiber membrane without a core layer structure are reduced. This shows that the core layer structure in the fiber membrane of the present application is conducive to improving the electrical performance of the lithium ion battery.

[0143] It can be seen from Example 1 and Comparative Example 2 that the first discharge specific capacity, first efficiency, 5C discharge specific capacity, and 1C cycle 500 capacity retention rate of the lithium ion battery prepared using the fiber membrane with a shell structure in the present application are improved; the first discharge specific capacity, first efficiency, 5C discharge specific capacity, and 1C cycle 500 capacity retention rate of the lithium ion battery prepared using the fiber membrane without a shell structure are reduced. This shows that the shell layer in the fiber membrane of the present application is beneficial to improving the electrical performance of the lithium ion battery.

[0144] It can be seen from Example 1 and Comparative Example 3 that the first discharge specific capacity, first efficiency, 5C discharge specific capacity, and 1C cycle 500 capacity retention rate of the lithium ion battery prepared using the fiber membrane in this application are improved, and the first discharge specific capacity, first efficiency, 5C discharge specific capacity, and 1C cycle 500 capacity retention rate of the lithium ion battery prepared by the nano silver carbon prepared in Comparative Example 3 are reduced. This shows that the fiber membrane of this application is beneficial to improving the electrical performance of lithium ion batteries.

[0145] By comparing Examples 1, 9, 10 with Examples 11, 12, it can be seen that only when the diameter of the nanofiber filaments is between 100 and 1500 nm, the first discharge specific capacity, first efficiency, 5C discharge specific capacity and 500 cycles (1C charge and discharge) capacity retention rate are improved. When the diameter of the nanofiber filaments exceeds 1500 nm or is less than 100 nm, the first discharge specific capacity, first efficiency, 5C discharge specific capacity and 500 cycles (1C charge and discharge) capacity retention rate decrease to varying degrees. This shows that when the diameter of the nanofiber filaments is 100-1500 nm, it is beneficial to improve the electrical performance of the lithium-ion battery.

[0146] It can be seen from Examples 1, 13, 14 and Examples 15 and 16 that only when the thickness of the fiber membrane is between 1 and 50 μm, the first discharge specific capacity, first efficiency, 5C discharge specific capacity and 1C cycle 500 cycle capacity retention rate are improved; when the thickness of the fiber membrane exceeds 50 μm or is less than 1 μm, the first discharge specific capacity, first efficiency, 5C discharge specific capacity and 1C cycle 500 cycle capacity retention rate decrease to varying degrees. This shows that when the thickness of the fiber membrane is 1 to 50 μm, it is beneficial to improve the electrical performance of the lithium-ion battery.

[0147] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0148] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A fiber membrane for a negative electrode material of a lithium ion battery, characterized in that: Comprising a plurality of nanofiber filaments, the nanofiber filaments comprising: A core carbon layer, the core carbon layer comprises a carbon material; a shell carbon layer, the shell carbon layer comprises a carbon material; the core carbon layer and the shell carbon layer are coaxially arranged, and the shell carbon layer is sleeved on the outer surface of the core carbon layer; Silver nanoparticles, the silver nanoparticles being disposed between the core carbon layer and the shell carbon layer; Wherein, the carbon material is selected from at least one of carbon particles and continuous carbon materials; And / or, the thickness of the core carbon layer is 10nm~500nm; And / or, the thickness of the shell carbon layer is 10nm~500nm; And / or, the diameter of the silver nanoparticles is 2nm~100nm; The mass fraction of silver element in the fiber membrane is 2% to 30%; The silver nanoparticles are formed by reducing at least one of the following silver salts: silver acetate, silver trifluoroacetylacetonate and silver acetylacetonate; the mass fraction of the silver salt in the core carbon layer carbon source precursor is 5% to 60%.

2. The fiber membrane according to claim 1, characterized in that The diameter of the nanofiber filaments is 100nm to 1500nm; And / or, the fiber membrane has a thickness of 1 μm to 50 μm.

3. The fiber membrane according to claim 1, characterized in that The core carbon layer comprises the continuous carbon material, and the shell carbon layer comprises the continuous carbon material; or, The core carbon layer includes the carbon particles, and the shell carbon layer includes the continuous carbon material.

4. The fiber membrane according to any one of claims 1 to 3, characterized in that The carbon materials of the core carbon layer and the shell carbon layer are formed by carbonizing different carbon-containing compounds.

5. The fiber membrane according to claim 4, characterized in that The carbon-containing compound of the core carbon layer is formed by carbonizing at least one selected from the following: an organic small molecule mixture and a first polymer, wherein the first polymer includes at least one of polyacrylic acid, polyamic acid, polyvinylidene fluoride and polymethacrylate; And / or, the carbon-containing compound of the shell carbon layer is formed by carbonizing a second polymer, and the second polymer includes at least one of polyacrylonitrile, polyvinylidene fluoride, polyamic acid and polyetherimide.

6. The fiber membrane according to claim 5, characterized in that The organic small molecule mixture includes at least one of kerosene, naphtha, paraffin, mineral oil, turpentine, pine oil and camphor oil.

7. A method for preparing the fiber membrane according to any one of claims 1 to 6, characterized in that: include: Spinning a core carbon layer spinning solution containing a core carbon layer carbon source precursor and a silver salt and a shell carbon layer spinning solution containing a second polymer solution to obtain nanofibers; The nanofiber filaments are carbonized to form the fiber membrane according to any one of claims 1 to 6.

8. The method according to claim 7, characterized in that The spinning process is achieved by at least one of the following technologies: coaxial electrostatic spinning technology and air-blowing spinning technology.

9. The method according to claim 7, characterized in that: The mass concentration of the shell carbon layer spinning solution is 6% to 20%; and / or, the mass concentration of the core carbon layer spinning solution is 10% to 65%; and / or, the solvent of the shell carbon layer spinning solution comprises at least one of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dimethyl carbonate, N-methylpyrrolidone, propylene carbonate, vinyl acetate, butylene carbonate and γ-butyrolactone; And / or, the solvent of the core carbon layer spinning solution includes at least one of dichloromethane, chloroform, benzene, toluene, xylene, petroleum ether, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and N-methylpyrrolidone.

10. The method according to claim 7, characterized in that The carbonization treatment is achieved by at least one of the following conditions: carried out in at least one of nitrogen, argon or helium, at a temperature of 600°C to 2800°C, for 0.5h to 48h, at a heating rate of 1°C / min to 20°C / min, and at a cooling rate of 1°C / min to 30°C / min.

11. The method according to claim 8, characterized in that The spinning process using coaxial electrospinning technology is achieved in the following manner: under the conditions of an electrostatic voltage of 8~30KV and a receiving distance of 10~40cm, the core carbon layer spinning solution and the shell layer spinning solution are discharged using a coaxial needle, the coaxial needle includes a core carbon layer spinning hole and a shell carbon layer spinning hole, the core carbon layer spinning hole discharges the core carbon layer spinning solution, and the shell layer spinning hole is used to discharge the shell carbon layer spinning solution.

12. The method according to claim 11, characterized in that The diameter of the spinning holes of the core carbon layer is 0.2 mm to 1.0 mm; and / or, the shell carbon layer spinning holes have a diameter of 0.5 mm to 2 mm; and / or, the single-hole flow rate of the core carbon layer spinning solution is 0.1 mL / h to 0.6 mL / h; And / or, the single-hole flow rate of the shell carbon layer spinning solution is 0.3 mL / h to 3 mL / h.

13. The method according to claim 7, characterized in that When the second polymer contains polyacrylonitrile, before the carbonization treatment, the method further comprises pre-oxidizing the nanofiber filaments in an air atmosphere at a temperature of 200° C. to 400° C. for 30 min to 60 min.

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