A silicon carbon fiber material, its preparation method, application and battery

Abstract

This invention discloses a silicon carbon fiber material, its preparation method, applications, and batteries. The preparation method includes the following steps: electrospun fibers are obtained by coaxial electrospinning of a core layer spinning solution and a shell layer spinning solution; the electrospun fibers are then subjected to pre-oxidation and carbonization to obtain the silicon carbon fiber material. The core layer spinning solution comprises Si nanoparticles and a hard carbon precursor A, with a mass ratio of Si nanoparticles to hard carbon precursor A of (0.25-0.5):1; the shell layer spinning solution comprises a soft carbon precursor C and a hard carbon precursor B, with a mass ratio of soft carbon precursor C to hard carbon precursor B of (0.1-0.4):1; during the coaxial electrospinning process, the core layer spinning solution is encapsulated by the shell layer spinning solution. The silicon carbon fiber material prepared by this invention possesses excellent flexibility, mechanical strength, and electrical conductivity, and can be used alone as a negative electrode.

Description

Technical Field

[0001] This invention relates to a silicon carbon fiber material, its preparation method, applications, and batteries. Background Technology

[0002] Lithium-ion batteries are important energy storage devices with advantages such as high energy density, low self-discharge rate, and good cycle performance, and are widely used in electric vehicles, mobile communications, and portable electronic devices. However, as the functions of electronic products continue to improve, the performance requirements for batteries are also increasing. Therefore, developing new high-capacity and high-stability electrode materials is key to the development of lithium-ion battery technology. Silicon is an ideal anode material for lithium-ion batteries, with a theoretical specific capacity as high as 4200 mAh / g, more than 10 times that of currently commercially available graphite anode materials. Moreover, its lithium intercalation potential is below 0.4V, which is beneficial to improving battery safety. In addition, silicon is one of the most abundant elements in the Earth's crust, with abundant resources, low price, and is non-toxic and harmless, which aligns with the concept of sustainable development. However, silicon anode materials also have some serious problems, mainly their poor conductivity, requiring the addition of a large amount of conductive agent, reducing the effective capacity of the electrode, and the huge volume expansion (about 300%) that occurs during lithium intercalation, leading to structural damage to the electrode, detachment from the current collector, and continuous reconstruction of the solid electrolyte intercalation (SEI) film, resulting in capacity decay and reduced cycle life.

[0003] Chinese patent document CN116314630A discloses a method for preparing a silicon-carbon anode. This method involves mixing an organosilicon source, a polymer, and a solvent to obtain a spinning solution, then electrospinning the spinning solution to obtain electrospun fibers. The coating solution and the electrospun fibers are then mixed and carbonized to obtain the silicon-carbon anode material. However, the silicon-carbon anode material prepared by this method lacks flexibility, is prone to breakage, and exhibits poor coating uniformity and difficulty in controlling the coating process. Summary of the Invention

[0004] The technical problem solved by this invention is to overcome the defects of insufficient flexibility and easy breakage of silicon-carbon anode materials in the prior art, and to provide a silicon-carbon fiber material, its preparation method, application, and battery. The silicon-carbon fiber material prepared by this invention has excellent flexibility, mechanical strength, and conductivity, and can be used alone as a negative electrode sheet without the need for current collectors, binders, or other substances, thereby improving the volumetric energy density of the battery and effectively suppressing the volume expansion and pulverization problems of silicon-based anode materials.

[0005] The present invention solves the above-mentioned technical problems through the following technical solutions:

[0006] This invention provides a method for preparing silicon carbon fiber material, which includes the following steps:

[0007] The core spinning solution and the shell spinning solution are coaxially electrospun to obtain electrospun fibers. The electrospun fibers are then pre-oxidized and carbonized to obtain the silicon carbon fiber material.

[0008] The core spinning solution comprises Si nanoparticles and hard carbon precursor A, wherein the mass ratio of Si nanoparticles to hard carbon precursor A is (0.25-0.5):1; the shell spinning solution comprises soft carbon precursor C and hard carbon precursor B, wherein the mass ratio of soft carbon precursor C to hard carbon precursor B is (0.1-0.4):1.

[0009] During the coaxial electrospinning process, the core spinning solution is encapsulated by the shell spinning solution.

[0010] In this invention, the particle size of the Si nanoparticles can be 10-50 nm, preferably 20-40 nm, for example 30 nm or 40 nm.

[0011] In this invention, the hard carbon precursor A can be one or more of polyacrylonitrile, polymethyl methacrylate, and polyvinylpyrrolidone, preferably polyvinylpyrrolidone. Preferably, when the hard carbon precursor A is polyvinylpyrrolidone, the hard carbon component in the core layer of the prepared silicon carbon fiber material can provide more sufficient strain space for the expansion of silicon nanoparticles and material deformation, resulting in superior electrolyte wettability.

[0012] In this invention, the hard carbon precursor B may be one or more of polyacrylonitrile, polymethyl methacrylate, and polyvinylpyrrolidone.

[0013] In this invention, the weight-average molecular weight of the hard carbon precursor A or the hard carbon precursor B is preferably 80,000 to 200,000 g / mol, for example 160,000 g / mol.

[0014] In this invention, the hard carbon precursor A and the hard carbon precursor B may be the same or different.

[0015] In this invention, the soft carbon precursor C can be one or more of petroleum asphalt, mesophase asphalt, and spinnable asphalt.

[0016] In this invention, the softening point of the soft carbon precursor C is preferably not higher than 300°C, for example, 220-250°C, 215-285°C or 250-280°C, and more preferably not higher than 250°C.

[0017] In one specific implementation scheme, the hard carbon precursor B is polyacrylonitrile, and the soft carbon precursor C is petroleum asphalt. The petroleum asphalt can be conventional in the art, generally referring to the residue after crude oil distillation, such as petroleum asphalt of model MQ-250 purchased from Dalian Mingqiang Chemical Materials Co., Ltd.

[0018] In one specific implementation, the hard carbon precursor B is polyacrylonitrile, and the soft carbon precursor C is spinnable pitch. The spinnable pitch can be a conventional spinnable pitch produced from ethylene cracking tar as raw material through a process of primary distillation, primary polymerization, air oxidation, secondary distillation, and secondary polymerization. For example, it can be the spinnable pitch prepared according to Chinese patent document CN111925818A, or, for example, the spinnable pitch of model XD270 purchased from Liaoning Xinde New Material Technology Co., Ltd.

[0019] In one specific implementation scheme, the hard carbon precursor B is polyvinylpyrrolidone, and the soft carbon precursor C is mesophase pitch. The mesophase pitch generally refers to a nematic liquid crystal material composed of disk-shaped or rod-shaped molecules, generated during the heat treatment of heavy aromatic hydrocarbons, such as mesophase pitch of model ZLQ-601 purchased from Guangzhou Tongda Chemical Engineering Co., Ltd.

[0020] In this invention, the preferred mass ratio of the Si nanoparticles to the hard carbon precursor A is (0.3-0.5):1, for example, 0.4:1 or 0.44:1. If the mass ratio of the Si nanoparticles to the hard carbon precursor A is low, the prepared silicon carbon fiber material will have a low capacity when used as a negative electrode material in lithium-ion batteries; if the mass ratio is high, the core layer spinning viscosity will be low, and the spun fibers will not form properly.

[0021] In this invention, the preferred mass ratio of the soft carbon precursor C to the hard carbon precursor B is (0.1-0.3):1, for example, 0.14:1, 0.18:1, or 0.3:1. If the mass ratio of the soft carbon precursor C to the hard carbon precursor B is low, the resulting shell-layer non-block soft carbon enriched phase has a small proportion, the material is easily broken when bent, and the elongation at break is low; if the mass ratio is high, the resulting solution has a low viscosity, and it is easily broken when stretched by electrostatic force during electrospinning.

[0022] In this invention, the mass ratio of the shell spinning solution to the core spinning solution can be (1.2-8):1, preferably (1.3-3):1, for example 1.5:1, 1.6:1 or 1.77:1.

[0023] In this invention, the core spinning solution preferably further includes solvent A.

[0024] Solvent A can be a polar solvent that is conventional in the art, capable of dissolving the hard carbon precursor A, and has a low boiling point and is easily volatile, preferably ethanol.

[0025] In the core spinning solution, the mass ratio of the hard carbon precursor A to the solvent A can be (0.05-0.15):1, for example, 0.09:1.

[0026] The method for preparing the core spinning solution preferably includes the following steps:

[0027] The mixture of the Si nanoparticles and the hard carbon precursor A is added to solvent A for mixing;

[0028] Alternatively, the Si nanoparticles can be premixed with solvent A before being mixed with the hard carbon precursor A. The mixing is generally performed at room temperature. The premixing and mixing methods can be conventional in the art, such as stirring or ultrasonication. The mixing time can be 6-12 hours, for example, 8 hours. The mixing speed can be 400-600 rpm, for example, 500 rpm. The premixing time can be 1-3 hours, for example, 2 hours.

[0029] In this invention, the shell spinning solution preferably further includes solvent B and solvent C.

[0030] The solvent B may be an organic solvent that is conventional in the art capable of dissolving the hard carbon precursor B, preferably one or more of N,N-dimethylformamide, N,N-dimethylacetamide and dimethyl sulfoxide.

[0031] The solvent C can be any organic solvent that is conventional in the art capable of dissolving the soft carbon precursor C, preferably tetrahydrofuran and / or dichloromethane.

[0032] The mass ratio of the hard carbon precursor B to the solvent B can be (0.1-0.2):1, for example, 0.11:1, 0.13:1 or 0.15:1.

[0033] The mass ratio of the soft carbon precursor C to the solvent C can be (0.06-0.18):1, for example, 0.08:1, 0.1:1 or 0.12:1.

[0034] The preferred method for preparing the shell spinning solution includes the following steps: preparing shell spinning solution A and shell spinning solution B separately, and then mixing shell spinning solution A and shell spinning solution B; the raw materials for shell spinning solution A are the hard carbon precursor B and the solvent B; the raw materials for shell spinning solution B are the soft carbon precursor C and the solvent C. The mixing is generally carried out at room temperature. The mixing method can be conventional in the art, such as stirring. The mixing time can be 10-20 hours, for example, 12 hours. The mixing speed can be 400-600 rpm, for example, 300 rpm or 500 rpm. The preparation process of shell spinning solution A may include: adding the hard carbon precursor B to the solvent B and mixing. The preparation process of shell spinning solution B may include: adding the soft carbon precursor C to the solvent C and mixing. During the preparation of the shell spinning solution A or the shell spinning solution B, the mixing speed can be 400-600 rpm, for example, 300 rpm or 500 rpm. During the preparation of the shell spinning solution A or the shell spinning solution B, the mixing time can be 5-10 hours, for example, 8 hours.

[0035] In this invention, according to conventional practice in the art, during the coaxial electrospinning process, the core spinning solution is completely enveloped by the shell spinning solution.

[0036] In this invention, during the coaxial electrospinning process, the core spinning solution serves as the inner layer solution, and the shell spinning solution serves as the outer layer solution.

[0037] In this invention, the electrospinning equipment used in the coaxial electrospinning process can be conventional in the field. Generally speaking, the electrospinning equipment includes a nozzle, a receiving plate, and a controller. The nozzle includes an inner needle and an outer needle disposed inside and outside. The controller can control the propulsion rate of the inner needle and the outer needle of the nozzle respectively.

[0038] In this invention, during the coaxial electrospinning process, the core layer spinning solution enters through the inner needle, and the inner diameter of the inner needle can be 0.3-0.6 mm, for example, 0.4 mm or 0.5 mm.

[0039] In this invention, during the coaxial electrospinning process, the flow rate of the core layer spinning solution can be 0.6-1 mL / h, for example, 0.6 mL / h.

[0040] In this invention, during the coaxial electrospinning process, the shell spinning solution enters from the outer needle, and the inner diameter of the outer needle can be 0.5-1.2 mm, for example, 0.7 mm.

[0041] In this invention, during the coaxial electrospinning process, the flow rate of the shell spinning solution can be 0.9-1.5 mL / h, for example, 0.9 mL / h or 1.2 mL / h.

[0042] In this invention, during the coaxial electrospinning process, the flow rate of the core spinning solution is preferably lower than that of the shell spinning solution.

[0043] In this invention, during the coaxial electrospinning process, the inner diameter of the outer needle is preferably 1.3-2 times, for example 1.4 times, 1.5 times or 1.75 times, the inner diameter of the inner needle.

[0044] In one specific implementation, the inner diameter of the inner needle is 0.4 mm, the inner diameter of the outer needle is 0.7 mm, the flow rate of the core spinning solution is 0.6 mL / h, and the flow rate of the shell spinning solution is 1.2 mL / h.

[0045] In one specific implementation, the inner diameter of the inner needle is 0.5 mm, the inner diameter of the outer needle is 0.7 mm, the flow rate of the core spinning solution is 0.6 mL / h, and the flow rate of the shell spinning solution is 0.9 mL / h.

[0046] In a preferred embodiment, during the coaxial electrospinning process, the inner diameter of the outer needle is 1.3-2 times the inner diameter of the inner needle, and the propulsion rate of the core spinning solution is less than that of the shell spinning solution.

[0047] In this invention, during the coaxial electrospinning process, the spinning voltage can be 13-17kV, for example, 15kV or 17kV.

[0048] In this invention, during the coaxial electrospinning process, the receiving distance can be 15-18cm, for example, 16cm.

[0049] In this invention, during the coaxial electrospinning process, the receiving roller speed can be 300-500 rpm, for example, 500 rpm.

[0050] In this invention, the environmental parameters during the coaxial electrospinning process can be: temperature 30±5℃, humidity 45±10%.

[0051] In this invention, the coaxial electrospinning time can be 12-18 hours, for example, 12 hours.

[0052] In some specific embodiments of the present invention, during the coaxial electrospinning process, the spinning voltage is 13-17kV, the receiving distance is 15-18cm, the receiving roller speed is 300-500rpm, and the environmental parameters for electrospinning are: temperature 30±5℃ and humidity 45±10%.

[0053] In this invention, during the coaxial electrospinning process, the soft carbon precursor C and the hard carbon precursor B in the shell spinning solution undergo phase separation to form a hard carbon enriched phase that provides support and mechanical strength, and a soft carbon enriched phase that provides flexible connection and high electronic conductivity.

[0054] In this invention, the pre-oxidation is generally carried out in a muffle furnace.

[0055] In this invention, the pre-oxidation is generally carried out in an oxidizing gas atmosphere. The oxidizing gas generally refers to a gas containing oxygen, such as air.

[0056] In this invention, the pre-oxidation temperature can be 220-310℃, preferably 250-280℃, for example 250℃.

[0057] In this invention, the pre-oxidation time can be 1-3 hours, for example, 90 minutes or 2 hours.

[0058] In this invention, the rate of heating to the pre-oxidation temperature can be 1-4°C / min, preferably 2-3°C / min, for example 2°C / min.

[0059] In this invention, the pre-oxidation process transforms the hard carbon precursor A and hard carbon precursor B in the electrospun fiber from a linear chain structure to a heat-resistant trapezoidal structure, thereby maintaining good mechanical strength, high stability, and high carbon yield during the carbonization process.

[0060] In this invention, the carbonization is generally carried out in a tube furnace.

[0061] In this invention, the carbonization is generally carried out under a protective atmosphere that does not react with the reaction system, such as nitrogen or an inert gas. The inert gas is, for example, argon.

[0062] In this invention, the carbonization temperature can be 800-1300℃, for example 1100℃.

[0063] In this invention, the carbonization time can be 1.5-5 hours, for example, 2 hours or 3 hours.

[0064] In this invention, the rate of heating to the carbonization temperature can be 5-7°C / min, for example, 5°C / min or 7°C / min.

[0065] In this invention, the carbonization process is generally followed by natural cooling to room temperature.

[0066] The present invention also provides a silicon carbon fiber material prepared by the method described above.

[0067] In this invention, the silicon carbon fiber material includes a core layer and a shell layer covering the surface of the core layer; wherein the core layer is composed of Si particles and carbon fibers, and the shell layer is a carbon layer.

[0068] In the core layer, the Si particles are uniformly dispersed within the carbon fibers. The carbon fibers provide a buffer space for the volume expansion of the Si particles.

[0069] Preferably, the shell has a scaly structure formed by clusters of pitch aromatic molecules, which helps to improve the electrical conductivity of the silicon carbon fiber material.

[0070] In this invention, the diameter of the silicon carbon fiber material can be 300-900 nm.

[0071] In this invention, the silicon carbon fiber material preferably has a hybrid matrix membrane structure composed of a three-dimensional fiber network.

[0072] This invention also provides an application of the aforementioned silicon carbon fiber material in the preparation of a battery. The battery is preferably a lithium-ion battery.

[0073] The present invention also provides a battery comprising the silicon carbon fiber material as described above. The battery is preferably a lithium-ion battery.

[0074] This invention involves pre-dispersing silicon nanoparticles in a core spinning solution. During electrospinning, the core spinning solution is completely enveloped by the shell spinning solution. After the solvent in the shell spinning solution evaporates, the two components with different solubilities separate, resulting in a flake-like shell structure. After pre-oxidation and carbonization treatment, a uniformly carbon-coated silicon carbon fiber material is obtained. The highly graphitized soft carbon in the shell structure acts as a "hinge," providing deformation space for the material to bend. The thickness of the carbon coating layer can be controlled by changing the inner diameter of the inner and outer needles or the flow rate.

[0075] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0076] The reagents and raw materials used in this invention are all commercially available.

[0077] The positive and progressive effects of this invention are as follows:

[0078] (1) The preparation method of the present invention can make the shell coating of the prepared silicon carbon fiber material have good uniformity and can effectively control the thickness of the coating layer.

[0079] (2) The silicon carbon fiber material prepared by the present invention has excellent flexibility, mechanical strength and conductivity, and can be used as a negative electrode sheet alone without the need for current collectors, binders and other substances, which improves the volume energy density of the battery, effectively suppresses the volume expansion and pulverization problem of silicon-based negative electrode materials, and has excellent electrochemical performance, especially high cycle stability. Attached Figure Description

[0080] Figure 1 This is an optical image of the silicon carbon fiber material prepared in Example 1 in a curled state.

[0081] Figure 2 The image shows a 10000x magnification SEM image of the silicon carbon fiber material prepared in Example 1.

[0082] Figure 3 The image shows a 50,000x magnification SEM image of the silicon carbon fiber material prepared in Example 1.

[0083] Figure 4 The stress-strain curves are those of the silicon carbon fiber materials prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0084] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0085] The reagents and equipment used in the following examples and comparative examples are shown in Table 1:

[0086] Table 1

[0087]

[0088] The electrospinning environment in the following examples and comparative examples was as follows: temperature 30±5℃, humidity 45±10%; weight-average molecular weight of polyvinylpyrrolidone was 160,000 g / mol; softening point of petroleum asphalt was 220℃-250℃, softening point of mesophase asphalt was 215℃-285℃, and softening point of spinnable asphalt was 250℃-280℃. The softening point was tested using the ring and ball method. The sample was placed in a copper ring of a specified size (16 mm in diameter and 6 mm in height), and a standard steel ball (9.5 mm in diameter and weighing approximately 3.5 g) was placed on the sample. The sample was then immersed in water and heated at a specified rate (5℃ / min) until it softened and sagged. The temperature at which the sag reached 25.4 mm was taken as the softening point.

[0089] Example 1

[0090] (1) After mixing 0.4g of Si nanoparticles with a particle size of 30nm with 0.9g of polyvinylpyrrolidone evenly, it was slowly added to 10g of ethanol solvent. After stirring magnetically at 500rpm for 8h at room temperature, the core layer spinning solution was obtained.

[0091] 0.3g of soft carbon precursor petroleum pitch was dissolved in 3g of tetrahydrofuran solvent to obtain solution A, and 1.7g of hard carbon precursor polyacrylonitrile was dissolved in 15g of N,N-dimethylformamide solvent to obtain solution B. Solution A and solution B were magnetically stirred at 300 rpm for 8 hours at room temperature, and then solution A and solution B were mixed and magnetically stirred at 300 rpm for 12 hours at room temperature to obtain a homogeneous shell spinning solution.

[0092] (2) The core spinning solution and shell spinning solution prepared in step (1) are injected into the push pump respectively. The core spinning solution enters from the inner needle and the shell spinning solution enters from the outer needle. The electrospun fiber membrane is obtained by coaxial electrospinning. The inner diameter of the inner and outer needles are 0.4 mm and 0.7 mm respectively. The push flow rates of the core layer and shell layer are 0.6 mL / h and 1.2 mL / h respectively. The spinning voltage is 17 kV, the receiving distance is 16 cm, and the receiving drum speed is 500 rpm.

[0093] (3) Place the electrospun fiber membrane obtained in step (2) in a muffle furnace, raise the temperature to 250°C in air at a heating rate of 2°C / min and hold the temperature for 90 min to obtain a pre-oxidized electrospun fiber membrane.

[0094] (4) The pre-oxidized electrospun fiber membrane obtained in step (3) is placed in an atmosphere tube furnace, heated to 1100°C at a heating rate of 5°C / min in a nitrogen atmosphere and kept at the temperature for 2 hours, and then cooled to room temperature by self-heating to obtain a flexible self-supporting fiber membrane with a surface scale structure (i.e., silicon carbon fiber material).

[0095] Example 2

[0096] (1) 0.4g of Si nanoparticles with a particle size of 40nm were added to 10g of ethanol solvent and ultrasonically dispersed for 2h. Then, 0.9g of polyvinylpyrrolidone was added and magnetically stirred at 500rpm for 8h at room temperature to obtain the core spinning solution.

[0097] 0.2g of soft carbon precursor spinnable pitch was dissolved in 2.4g of tetrahydrofuran solvent to obtain solution A, and 1.4g of hard carbon precursor polyacrylonitrile was dissolved in 13g of N,N-dimethylformamide solvent to obtain solution B. Solution A and solution B were magnetically stirred at 300 rpm for 8 h at room temperature, respectively. Then, solution A and solution B were mixed and magnetically stirred at 300 rpm for 12 h at room temperature to obtain a homogeneous shell spinning solution.

[0098] (2) The core spinning solution and shell spinning solution prepared in step (1) are injected into the push pump respectively. The core spinning solution enters from the inner needle and the shell spinning solution enters from the outer needle. The electrospun fiber membrane is obtained by coaxial electrospinning. The inner diameter of the inner and outer needles are 0.5 mm and 0.7 mm respectively. The push flow rates of the core layer and shell layer are 0.6 mL / h and 0.9 mL / h respectively. The spinning voltage is 15 kV, the receiving distance is 16 cm, and the receiving drum speed is 500 rpm.

[0099] (3) Place the electrospun fiber membrane obtained in step (2) in a muffle furnace, raise the temperature to 250°C in air at a heating rate of 2°C / min and hold the temperature for 2 hours to obtain a pre-oxidized electrospun fiber membrane.

[0100] (4) The pre-oxidized electrospun fiber membrane obtained in step (3) is placed in an atmosphere tube furnace, heated to 1100°C at a heating rate of 7°C / min in a nitrogen atmosphere and kept at the temperature for 3 hours, and then cooled to room temperature by self-heating to obtain a flexible self-supporting fiber membrane with a surface scale structure (i.e., silicon carbon fiber material).

[0101] Example 3

[0102] Except for replacing the shell hard carbon precursor with polyvinylpyrrolidone and the soft carbon precursor with mesophase pitch in step (1), everything else is the same as in Example 1.

[0103] Example 4

[0104] Except for replacing the shell hard carbon precursor with polyvinylpyrrolidone and the soft carbon precursor with mesophase pitch in step (1), everything else is the same as in Example 2.

[0105] Comparative Example 1

[0106] Except for the steps used to prepare the shell spinning solution, all other steps are the same as in Example 1:

[0107] 2g of polyacrylonitrile was dissolved in 3g of tetrahydrofuran solvent and 15g of N,N-dimethylformamide solvent to obtain a solution. The solution was magnetically stirred at 300rpm for 20h at room temperature to obtain a homogeneous shell spinning solution.

[0108] Comparative Example 2

[0109] Except for the steps used to prepare the shell spinning solution, everything else is the same as in Example 2:

[0110] 1.6 g of polyacrylonitrile was dissolved in 2.4 g of tetrahydrofuran solvent and 13 g of N,N-dimethylformamide solvent to obtain a solution. The solution was magnetically stirred at 300 rpm for 20 h at room temperature to obtain a homogeneous shell spinning solution.

[0111] Effect Example

[0112] (1) Morphological characteristics

[0113] Figure 1 This is an optical image of the silicon carbon fiber material prepared in Example 1 in a curled state. Figures 2-3 All images are SEM images of the silicon carbon fiber material prepared in Example 1.

[0114] (2) Electrochemical performance testing

[0115] The electrochemical performance of the flexible self-supporting fiber membranes prepared in Examples 1-4 and Comparative Examples 1-2 was tested.

[0116] The specific steps are as follows:

[0117] Preparation of negative electrode sheets in Examples 1-4 and Comparative Examples 1-2: The silicon carbon fiber materials prepared in Examples 1-4 and Comparative Examples 1-2 were cut into circular sheets with a diameter of 13.5 mm as negative electrode sheets;

[0118] Preparation of positive electrode sheets in Examples 1-4 and Comparative Examples 1-2: Positive electrode sheets were prepared by coating lithium cobalt oxide, an active material, onto aluminum foil, and the areal density of the positive electrode coating was controlled to be 12 mg / cm³. 2 The aluminum foil thickness is 15μm.

[0119] Electrolyte preparation: NaPF6 is used as solute, ethylene carbonate (EC) and diethyl carbonate (DEC) are used as solvents, the volume ratio of EC to DEC is 1:1, the concentration of NaPF6 is 0.8M, and fluoroethylene carbonate (FEC) is used as an additive, with FEC accounting for 2% of the total mass of the electrolyte.

[0120] The negative electrode sheets prepared in Examples 1-4 and Comparative Examples 1-2 were assembled into CR2430 button batteries with lithium cobalt oxide positive electrode, electrolyte, polypropylene separator, positive electrode shell, negative electrode shell, and positive electrode sheet, respectively. The batteries were charged to 4.2V at a constant current and constant voltage of 0.1C and discharged at a constant current of 0.1C. The capacity, initial efficiency, and capacity retention after 100 cycles were tested. The test results are shown in Table 2.

[0121] (3) Stress characterization

[0122] (a) Testing standard: ISO 2008;

[0123] (b) Testing instrument: Etnaln YTN-S02;

[0124] (c) Basic definition:

[0125] Tensile strength: The maximum tensile stress that can be applied to a material before it breaks under tension;

[0126] Elongation at break: The percentage elongation of a material at break;

[0127] (d) Test steps:

[0128] The silicon carbon fiber materials prepared in Examples 1-4 and Comparative Examples 1-2 were used as test materials, and the following tests were performed:

[0129] According to the requirements of the test standard, the cutting error of the test material shall not exceed 2%, and the test material shall be placed in an environment of 27℃±2℃ and (65±5)%RH for more than 12 hours before the test.

[0130] The length was measured by marking two reference lines on the test material. The lines were marked in parallel with a marker, and the distance between the reference lines (i.e., the initial length L0) was 25 mm.

[0131] Zero the load indicator, then place the test material into the clamps of the tensile testing machine, symmetrically mounting the specimen to ensure the tensile force is evenly distributed across its cross-section. Apply a maximum preload of 0.1 kPa, and using a tensile timer, zero the timer after preloading. Start the machine at a separation speed of 500 mm / min ± 50 mm / min, and record the maximum stress (±1%) before specimen fracture and the distance between the inner edges of the two reference lines (±1.25 mm).

[0132] Tensile strength calculation:

[0133] Based on the average thickness of the test material and the width of the center portion of the specimen (13 mm), the average initial cross-sectional area (A) of the test material is calculated. The formula for calculating the tensile strength (TS) of the test material is as follows:

[0134]

[0135] In the formula, F is the maximum fracture strength in N; A is the average initial cross-sectional area in mm. 2 .

[0136] Calculation of elongation at break:

[0137] Elongation at break (E) is expressed as a percentage of the original length, and its formula is:

[0138]

[0139] In the formula, L is the actual measured fracture length in mm; L0 is the initial length in mm.

[0140] Figure 4 The images show the stress-strain curves of the silicon carbon fiber materials prepared in Example 1 and Comparative Example 1. From... Figure 4It can be clearly seen that the elongation at break of the silicon carbon fiber material prepared in Example 1 is much higher than that of the silicon carbon fiber material prepared in Comparative Example 1, indicating that the silicon carbon fiber material prepared in Example 1 has better flexibility.

[0141] Table 2

[0142]

[0143] According to the data in Table 2, the silicon carbon fiber materials prepared in Examples 1-4 can be used as negative electrode sheets on their own without the need to coat the silicon carbon fiber material onto the negative electrode current collector. This saves production costs and improves the volumetric energy density of the battery cell. Even without using the negative electrode current collector, good electrochemical performance can be achieved.

[0144] The shell spinning solutions in Comparative Examples 1-2 used only the hard carbon precursor polyacrylonitrile and did not use the soft carbon precursor. The resulting silicon carbon fiber materials not only had poorer electrochemical performance, but also had a much lower elongation at break than those in Examples 1-4 and poorer flexibility.

Claims

1. A method for preparing silicon carbon fiber material, characterized in that, It includes the following steps: The core spinning solution and the shell spinning solution are coaxially electrospun to obtain electrospun fibers. The electrospun fibers are then pre-oxidized and carbonized to obtain the silicon carbon fiber material. The core spinning solution comprises Si nanoparticles and hard carbon precursor A, wherein the mass ratio of Si nanoparticles to hard carbon precursor A is (0.25-0.5):1; the shell spinning solution comprises soft carbon precursor C and hard carbon precursor B, wherein the mass ratio of soft carbon precursor C to hard carbon precursor B is (0.1-0.4):

1. During the coaxial electrospinning process, the core spinning solution is encapsulated by the shell spinning solution. The soft carbon precursor C is one or more of petroleum asphalt, mesophase asphalt, and spinnable asphalt.

2. The method for preparing silicon carbon fiber material as described in claim 1, characterized in that, The preparation method satisfies one or more of the following conditions: (1) The particle size of the Si nanoparticles is 10-50 nm; (2) The hard carbon precursor A is one or more of polyacrylonitrile, polymethyl methacrylate and polyvinylpyrrolidone; (3) The hard carbon precursor B is one or more of polyacrylonitrile, polymethyl methacrylate and polyvinylpyrrolidone; (4) The weight-average molecular weight of the hard carbon precursor A or the hard carbon precursor B is 80,000~200,000 g / mol; (5) The softening point of the soft carbon precursor C is not higher than 300℃.

3. The method for preparing silicon carbon fiber material as described in claim 2, characterized in that, The preparation method satisfies one or more of the following conditions: (1) The particle size of the Si nanoparticles is 20-40 nm; (2) The hard carbon precursor A is polyvinylpyrrolidone; (3) The softening point of the soft carbon precursor C is not higher than 250℃.

4. The method for preparing silicon carbon fiber material as described in claim 2, characterized in that, The hard carbon precursor B is polyacrylonitrile, and the soft carbon precursor C is petroleum asphalt.

5. The method for preparing silicon carbon fiber material as described in claim 2, characterized in that, The hard carbon precursor B is polyacrylonitrile, and the soft carbon precursor C is spinnable pitch.

6. The method for preparing silicon carbon fiber material as described in claim 2, characterized in that, The hard carbon precursor B is polyvinylpyrrolidone, and the soft carbon precursor C is mesophase pitch.

7. The method for preparing silicon carbon fiber material according to any one of claims 1-6, characterized in that, The preparation method satisfies one or more of the following conditions: (1) The mass ratio of the Si nanoparticles to the hard carbon precursor A is (0.3-0.5):1; (2) The mass ratio of the soft carbon precursor C to the hard carbon precursor B is (0.1-0.3):1; (3) The mass ratio of the shell spinning solution to the core spinning solution is (1.2-8):

1.

8. The method for preparing silicon carbon fiber material as described in claim 7, characterized in that, The mass ratio of the shell spinning solution to the core spinning solution is (1.3-3):

1.

9. The method for preparing silicon carbon fiber material according to any one of claims 1-6, characterized in that, The core spinning solution also includes solvent A.

10. The method for preparing silicon carbon fiber material as described in claim 9, characterized in that, Solvent A is ethanol.

11. The method for preparing silicon carbon fiber material as described in claim 9, characterized in that, In the core spinning solution, the mass ratio of the hard carbon precursor A to the solvent A is (0.05-0.15):

1.

12. The method for preparing silicon carbon fiber material as described in claim 9, characterized in that, The preparation method of the core spinning solution includes the following steps: The mixture of the Si nanoparticles and the hard carbon precursor A is added to solvent A for mixing; Alternatively, the Si nanoparticles can be premixed with solvent A before being mixed with the hard carbon precursor A.

13. The method for preparing silicon carbon fiber material as described in claim 12, characterized in that, The mixing time is 6-12 hours.

14. The method for preparing silicon carbon fiber material as described in claim 12, characterized in that, The premixing time is 1-3 hours.

15. The method for preparing silicon carbon fiber material according to any one of claims 1-6, characterized in that, The shell spinning solution also includes solvent B and solvent C.

16. The method for preparing silicon carbon fiber material as described in claim 15, characterized in that, The solvent B is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

17. The method for preparing silicon carbon fiber material as described in claim 15, characterized in that, The solvent C is tetrahydrofuran and / or dichloromethane.

18. The method for preparing silicon carbon fiber material as described in claim 15, characterized in that, The mass ratio of the hard carbon precursor B to the solvent B is (0.1-0.2):

1.

19. The method for preparing silicon carbon fiber material as described in claim 15, characterized in that, The mass ratio of the soft carbon precursor C to the solvent C is (0.06-0.18):

1.

20. The method for preparing silicon carbon fiber material as described in claim 15, characterized in that, The preparation method of the shell spinning solution includes the following steps: preparing shell spinning solution A and shell spinning solution B respectively, and mixing shell spinning solution A and shell spinning solution B; the raw materials of shell spinning solution A are the hard carbon precursor B and the solvent B; the raw materials of shell spinning solution B are the soft carbon precursor C and the solvent C.

21. The method for preparing silicon carbon fiber material as described in claim 20, characterized in that, The mixing time is 10-20 hours.

22. The method for preparing silicon carbon fiber material as described in claim 20, characterized in that, The preparation process of the shell spinning solution A includes: adding the hard carbon precursor B to the solvent B and mixing them to obtain the solution.

23. The method for preparing silicon carbon fiber material as described in claim 20, characterized in that, The preparation process of the shell spinning solution B includes: adding the soft carbon precursor C to the solvent C and mixing them to obtain the solution.

24. The method for preparing silicon carbon fiber material according to any one of claims 1-6, characterized in that, The preparation method satisfies one or more of the following conditions: (1) During the coaxial electrospinning process, the core layer spinning solution enters from the inner needle, and the inner diameter of the inner needle is 0.3-0.6 mm; (2) During the coaxial electrospinning process, the flow rate of the core spinning solution is 0.6-1 mL / h; (3) During the coaxial electrospinning process, the shell spinning solution enters from the outer needle, and the inner diameter of the outer needle is 0.5-1.2 mm; (4) During the coaxial electrospinning process, the flow rate of the shell spinning solution is 0.9-1.5 mL / h; (5) During the coaxial electrospinning process, the flow rate of the core spinning solution is lower than that of the shell spinning solution. (6) During the coaxial electrospinning process, the inner diameter of the outer needle is 1.3-2 times the inner diameter of the inner needle; (7) During the coaxial electrospinning process, the spinning voltage is 13-17kV; (8) During the coaxial electrospinning process, the receiving distance is 15-18cm; (9) During the coaxial electrospinning process, the receiving roller speed is 300-500 rpm; (10) During the coaxial electrospinning process, the environmental parameters are: temperature 30±5℃, humidity 45±10%.

25. The method for preparing silicon carbon fiber material as described in claim 24, characterized in that, The inner diameter of the inner needle is 0.4 mm, the inner diameter of the outer needle is 0.7 mm, the flow rate of the core spinning solution is 0.6 mL / h, and the flow rate of the shell spinning solution is 1.2 mL / h.

26. The method for preparing silicon carbon fiber material as described in claim 24, characterized in that, The inner diameter of the inner needle is 0.5 mm, the inner diameter of the outer needle is 0.7 mm, the flow rate of the core spinning solution is 0.6 mL / h, and the flow rate of the shell spinning solution is 0.9 mL / h.

27. The method for preparing silicon carbon fiber material as described in claim 24, characterized in that, During the coaxial electrospinning process, the inner diameter of the outer needle is 1.3-2 times the inner diameter of the inner needle, and the propulsion rate of the core spinning solution is less than that of the shell spinning solution.

28. The method for preparing silicon carbon fiber material as described in claim 24, characterized in that, During the coaxial electrospinning process, the spinning voltage is 13-17kV, the receiving distance is 15-18cm, the receiving roller speed is 300-500rpm, and the environmental parameters for electrospinning are: temperature 30±5℃ and humidity 45±10%.

29. The method for preparing silicon carbon fiber material according to any one of claims 1-6, characterized in that, The preparation method satisfies one or more of the following conditions: (1) The pre-oxidation is carried out in an oxidizing gas atmosphere; (2) The pre-oxidation temperature is 220-310℃; (3) The pre-oxidation time is 1-3 hours; (4) The rate of heating to the pre-oxidation temperature is 1-4℃ / min; (5) The carbonization temperature is 800-1300℃; (6) The carbonization time is 1.5-5 hours; (7) The rate of heating to the carbonization temperature is 5-7℃ / min.

30. The method for preparing silicon carbon fiber material as described in claim 29, characterized in that, The preparation method satisfies one or two of the following conditions: (1) The pre-oxidation temperature is 250-280℃; (2) The rate of heating to the pre-oxidation temperature is 2-3℃ / min.

31. A silicon carbon fiber material, characterized in that, It is prepared by the method for preparing silicon carbon fiber material according to any one of claims 1-30.

32. The silicon carbon fiber material as described in claim 31, characterized in that, The silicon carbon fiber material includes a core layer and a shell layer covering the surface of the core layer; wherein, the core layer is composed of Si particles and carbon fibers, and the shell layer is a carbon layer.

33. The silicon carbon fiber material as described in claim 32, characterized in that, The Si particles are uniformly dispersed in the carbon fibers.

34. The silicon carbon fiber material as described in claim 32, characterized in that, The shell has a scaly structure formed by clusters of pitch aromatic molecules.

35. The silicon carbon fiber material as described in claim 31, characterized in that, The diameter of the silicon carbon fiber material is 300-900 nm.

36. The use of silicon carbon fiber material as described in any one of claims 31-35 in the preparation of batteries.

37. A battery, characterized in that, It includes silicon carbon fiber material as described in any one of claims 31-35.

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