Carbon fiber material, silicon-carbon composite negative electrode and preparation method thereof, and secondary battery

Abstract

The present application relates to the field of material preparation, and discloses a carbon fiber material, a silicon-carbon composite electrode, a preparation method thereof and a secondary battery. The preparation of the carbon fiber material comprises: S100, pretreating a biomass carbon source, heat treating and carbonizing, and then performing post-treatment after carbonization; S200, using a compound containing a hetero element as a hetero atom source, and doping the biomass carbon fiber material obtained in S100 with a hetero atom by a chemical vapor deposition method to obtain a biomass carbon fiber material with high conductivity. Further, the present application improves the conductivity of the biomass carbon fiber by doping with a hetero atom, and controls the length of the carbon fiber material, so that after the carbon fiber material is combined with silicon nanoparticles to prepare a negative electrode material, a full-range continuous conductive network can be formed. The full-range continuous conductive network structure not only can reduce the electrochemical polarization of the battery in the cycle, but also can effectively alleviate the failure problem of the electrode, and improve the cycle stability of the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chemical technology, in particular to a carbon fiber material, a silicon-carbon composite negative electrode and a preparation method thereof and a secondary battery. BACKGROUND

[0002] Silicon is considered to be the most promising next-generation negative electrode material due to its high theoretical capacity and abundant reserves in nature, and is the best choice for high-energy-density lithium-ion batteries. However, due to the intrinsic properties of silicon, silicon materials still face many challenges, including the intrinsic poor electrical conductivity and the high volume expansion of up to 300% during charge and discharge cycles, which limit the application of silicon materials. In order to alleviate the above problems, researchers widely adopt the construction of three-dimensional network structure of the electrode to enhance the electrical conductivity of the electrode and inhibit the failure of the negative electrode.

[0003] Although the construction of three-dimensional network can alleviate the problems of electrical conductivity and electrode failure of silicon-based negative electrodes to some extent, the structure design at the molecular level and the simple mixing of conductive materials with silicon nanoparticles cannot guarantee the designed three-dimensional conductive structure. Moreover, the existing methods also have non-negligible problems, including: ① the conductive additives that cannot be mass-produced usually have high cost, such as carbon nanotubes and graphene; ② most structure designs require complex preparation procedures, reducing the feasibility. Therefore, a low-cost, environmentally friendly and simple preparation method can well solve the problems of negative electrode materials and has good prospects for industrial development and application.

[0004] Based on this, the present application provides a solution, which adopts biomass carbon sources with fiber structure such as tree leaves, wood, cotton and rice husk and the like to obtain fibrous carbon materials after carbonization and specific treatment. This type of material has the advantages of being cheap and easy to obtain, convenient to produce and having good physical and chemical properties. The obtained carbon fiber material is further doped with heteroatoms to further enhance the electrical conductivity of the carbon material. At the same time, the surface of the biomass material after carbonization also has many functional functional groups, which can better interact with silicon particles and binders to form a more stable and stable electrode plate. SUMMARY

[0005] In view of the above problems, the purpose of the present application is to provide an economical and environmentally friendly preparation of carbon fiber material and a preparation of silicon-carbon composite negative electrode material to form a three-dimensional continuous conductive network to improve the polarization phenomenon of lithium-ion battery negative electrode material during the cycle process and to improve the cycle stability of the negative electrode plate.

[0006] The technical scheme adopted by the present application is as follows:

[0007] The application provides a carbon fiber material for preparing a silicon-carbon composite negative electrode, and a preparation method of the carbon fiber material.

[0008] S100, a biomass carbon source is used as a raw material, and the biomass carbon source is carbonized by a heat treatment method to obtain a carbon fiber material.

[0009] S200, a heteroatom precursor is used, and the carbon fiber material prepared by S100 is doped with the heteroatom by a chemical vapor deposition method to obtain a high-conductivity biomass carbon fiber material.

[0010] In the above technical solution, further, the biomass carbon source includes at least one of the following or a combination thereof: leaves, cottonwood, sorghum, cotton, rice husk, ramie, and soybeans.

[0011] The heteroatom precursor includes at least one of the following or a combination thereof: boron tribromide, boron fluoride, boric acid, sodium borohydride, urea, sodium azide, ammonium nitrate, sodium hypophosphite, phosphoric acid, phosphine, thiourea, ammonium sulfide, thioacetamide, sodium thiosulfate, ammonium fluoride, trifluoroacetic acid, and hydrofluoric acid.

[0012] Further, S100 specifically includes:

[0013] S110, the biomass carbon source is subjected to one-stage heat treatment in a protective gas, and the carbon fiber is obtained after cooling after the one-stage heat treatment;

[0014] S120, the carbon fiber material obtained in S110 is subjected to physical treatment, and the physical treatment includes grinding, i.e., a micronized fiber material is obtained;

[0015] The one-stage heat treatment carbonizes the biomass carbon source into a carbon fiber material, and the physical treatment micronizes the long fiber obtained after the one-stage heat treatment. The diameter of the carbon fiber material ranges from 5 to 20 microns, and the micronized length ranges from 10 to 150 microns. The grinding specifically includes: the obtained carbon fiber material is put into a grinding machine, and the carbon fiber material is ground and micronized at a speed of 50-150 r / min. The operation program specifically includes: ① starting the machine in forward rotation at a speed of 50 r / min for 5 min; ② stopping the machine for 30 s, and then starting the machine in reverse rotation at a speed of 50 r / min for 5 min; ③ stopping the machine for 30 s, and then starting the machine in forward rotation at a speed of 150 r / min for 2 min; ④ stopping the machine for 30 s, and then starting the machine in reverse rotation at a speed of 150 r / min for 2 min; and ⑤ repeating steps ③ and ④ three times, and then the carbon material is taken out to obtain the micronized fiber material.

[0016] Further, the temperature of the one-stage heat treatment ranges from 500 to 1500 degrees Celsius.

[0017] Further, between S110 and S120, the carbon fiber material obtained after one-stage heat treatment can be washed and dried.

[0018] Further, S200 further comprises:

[0019] The micronized carbon fiber prepared in S100 is fed into a fluidized bed reactor, and chemical vapor deposition is performed on the micronized carbon fiber under the condition of a pressure of 0.2-0.3 MPa by using a heteroatom precursor to obtain the heteroatom-doped carbon fiber material.

[0020] The carbon fiber material doped with the heteroatom has a high defect concentration, and the electrical conductivity is greatly improved.

[0021] The application further provides a carbon fiber material, which is obtained by using the method according to any one of the preceding technical solutions.

[0022] The application further provides a silicon-carbon composite negative electrode material, which is a full-range long-range conductive structure and is prepared by coating a silicon-carbon composite slurry containing silicon nanoparticles and the carbon fiber material on an electrode current collector. The mass fraction of the silicon nanoparticles in the silicon-carbon composite slurry is not less than 50%, and the mass fraction of the carbon fiber material is not less than 10%.

[0023] The application further provides a secondary battery containing the silicon-carbon composite negative electrode material.

[0024] The application has the following beneficial effects:

[0025] In the preparation process of the carbon fiber material, a heteroatom precursor is used as a heteroatom source to dope the specific carbon fiber prepared, so as to increase the defect concentration of the carbon fiber material and improve the electrical conductivity of the carbon fiber material. The carbon fiber material can be used as a raw material for preparing a silicon-carbon negative electrode material. Unlike the simple mixing of silicon particles and particulate or layered conductive agents to form local conductive sites of the electrode, the carbon fiber material treated in the application can form a long-range full-range conductive network after being combined with the silicon particles. On the one hand, the conductive network can reduce the resistance of the electrode, thereby reducing the polarization of the battery during charging and discharging, improving the charging and discharging efficiency of the battery, and preventing the safety problems such as heating and gas production of the battery due to high resistance; on the other hand, the carbon fiber network in the electrode sheet can also act as a high-strength skeleton in the case of being combined with the silicon particles and the binder, and can better alleviate the failure of the battery sheet under the stress in the battery, thereby ensuring the long cycle stability of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1SEM images of carbon fiber materials (a-c) prepared in Example 1, and the obtained silicon-carbon composite material electrode (d) and the electrode (e) of Comparative Example 1.

[0027] Figure 2 Elemental content and distribution characterization of the carbon fiber material prepared in Example 1.

[0028] Figure 3 XRD test results of the carbon fiber material prepared in Example 1.

[0029] Figure 4 Cycle-capacity comparison chart of the first 100 cycles obtained by battery test of Example 1 and Comparative Example 1.

[0030] Figure 5 Cycle-capacity comparison chart obtained by battery test of Example 1 and Comparative Examples 2 and 3. DETAILED DESCRIPTION

[0031] In order to enable a more complete understanding of the above-mentioned objects, advantages and features of the present application, the present application will be further described in detail below with specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0032] In the following description, many specific details of the present application are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced in other different manners that are not specifically described. Therefore, the scope of the present application is not limited by the specific embodiments disclosed below. The following technical features of the embodiments of the present application can be combined with each other without conflict.

[0033] The present application provides a preparation method of a carbon fiber material. The carbon fiber material is used for preparing a silicon-carbon composite negative electrode material, and the preparation method of the carbon fiber material comprises:

[0034] S100, using a biomass carbon source as a raw material, carbonizing the biomass carbon source by a heat treatment method, and then performing physical treatment to shorten the carbon fiber material to obtain a micronized fiber material.

[0035] S200, using a heteroatom precursor, doping the micronized fiber material prepared by S100 with a heteroatom by a chemical vapor deposition method to obtain the high-conductivity biomass carbon fiber material.

[0036] In the above embodiments, the biomass carbon source has the advantages of environmental friendliness, low cost, easy availability and simple preparation. Using the biomass carbon source as the carbon source in the silicon-carbon material can obtain economic and ecological benefits.

[0037] In some embodiments of the present application, the biomass carbon source comprises at least one of the following or a combination thereof: leaves, cottonwood, sorghum, cotton, rice husk, ramie, soybean.

[0038] It is easily understood that the heat treatment can carbonize the biomass carbon source.

[0039] In some embodiments of the present application, the temperature range of the heat treatment is 650-950℃.

[0040] In some embodiments of the present application, preferably, the temperature range of the heat treatment is 700-900℃.

[0041] In some embodiments of the present application, further preferably, the temperature of the heat treatment is 800℃.

[0042] In some embodiments of the present application, S100 specifically comprises:

[0043] S110, subjecting the biomass carbon source to one-stage heat treatment in a protective gas, and obtaining carbon fibers after cooling after the one-stage heat treatment;

[0044] S120, subjecting the carbon fiber material prepared in S110 to physical treatment (including grinding treatment), i.e., preparing micronized fiber material.

[0045] In the one-stage heat treatment, the biomass carbon source is carbonized, and in the physical treatment, the prepared carbon fiber material is micronized.

[0046] The most critical physical treatment is grinding treatment, which specifically comprises: putting the obtained carbon fiber material into a grinding machine, and grinding and micronizing the carbon fiber material at a rotating speed of 50-150 r / min, with the following running program: ① starting the machine in forward rotation, grinding at a rotating speed of 50 r / min for 5 min; ② stopping the machine after 30 s, and starting the machine in reverse rotation, grinding at a rotating speed of 50 r / min for 5 min; ③ stopping the machine after 30 s, and starting the machine in forward rotation, grinding at a rotating speed of 150 r / min for 2 min; ④ stopping the machine after 30 s, and starting the machine in reverse rotation, grinding at a rotating speed of 150 r / min for 2 min; ⑤ repeating steps ③ and ④ three times, and taking out the carbon material to obtain the micronized fiber material. It should be noted that: steps ① and ② can make the original long fibers be preliminarily ground and mixed uniformly, and the subsequent steps mainly make the fiber length further reduced. If steps ① and ② are absent, the fiber length will be uneven, and the length distribution will not meet the requirements; if the rotating speed is too fast or the single grinding time is too long, the fiber will be powdered, and otherwise the fiber cannot be sufficiently ground.

[0047] Generally, after the one-stage heat treatment, the biomass carbon source is carbonized into carbon fibers with a length greater than 300 μm, and after the physical treatment, the length of the carbon fiber material ranges from 10 μm to 150 μm. During the preparation of the electrode plate, the short fibers combine with the silicon particles to form several local conductive sites, and the long fibers connect several local conductive sites to form a long-range global conductive network in the electrode. If the fibers are excessively pulverized during the grinding process, a long-range continuous conductive network cannot be formed during the preparation of the electrode plate. If the fibers are not sufficiently ground and are too long, the silicon-fiber composite cannot be tightly connected, which reduces the compaction density and overall conductivity of the electrode plate. Therefore, the length distribution of the fibers is critical for the preparation of the electrode plate.

[0048] In some embodiments of the present application, between S110 and S120, the carbon fiber material obtained after the one-stage heat treatment is further washed and dried.

[0049] In some embodiments of the present application, the heteroatom precursor used in the chemical vapor deposition process is one or a combination of the following: boron tribromide, boron fluoride, boric acid, sodium borohydride, urea, sodium azide, ammonium nitrate, sodium hypophosphite, phosphoric acid, phosphine, thiourea, ammonium sulfide, thioacetamide, sodium thiosulfate, ammonium fluoride, trifluoroacetic acid, and hydrofluoric acid. During the heteroatom doping process, the solid-state heteroatom precursor (such as sodium hypophosphite) is decomposed by heat to form phosphine, which enters the lumen with the gas flow and settles in the carbon fiber material. After settling, the phosphine reacts with the carbon fiber, and the phosphorus atoms enter the interstitial sites of the carbon fiber lattice or replace the carbon atoms as heteroatoms distributed in the structure of the carbon crystal. If a liquid precursor is used, the pressure in the reaction container can increase under high-temperature conditions during the reaction process, which is dangerous. At the same time, after the reaction, the liquid substance remains in the carbon fiber and cannot be removed, which increases the impurity concentration and the difficulty of impurity removal, and it is not easy to obtain ideal results.

[0050] In some embodiments of the present application, the temperature range during the chemical vapor deposition process is 150°C to 500°C.

[0051] In some embodiments of the present application, preferably, the temperature range during the chemical vapor deposition process is 300°C to 400°C.

[0052] In some embodiments of the present application, further preferably, the temperature range during the chemical vapor deposition process is 350°C.

[0053] After the one-stage heat treatment, the carbonized material still contains a large amount of ash and metal and metal salt impurities, so it is necessary to remove the ash and metal and metal salt impurities by washing and drying.

[0054] It can be understood that the carbon fiber material prepared is heat treated by using the vapor deposition method to dope the carbon material with heteroatoms and improve the conductivity of the carbon material by increasing the defect concentration in the material.

[0055] Example 1

[0056] The carbon fiber material is prepared by using cotton as the raw material for preparing the carbon fiber, carbonizing the cotton fiber in a converter under an argon atmosphere by heating the converter to 800℃, and shortening the length of the carbon fiber to be in the range of 10 μm to 150 μm by using a physical treatment method, i.e., the aforementioned grinding operation program. The prepared micron-level carbon fiber is added into a fluidized bed, the reaction pressure is controlled to be in the range of 0.2-0.3 MPa, the reaction temperature is controlled to be in the range of 340℃-350℃, sodium hypophosphite is used as the heteroatom doping source, and the prepared carbon fiber is doped with heteroatoms by introducing sodium hypophosphite decomposition gas.

[0057] The prepared carbon fiber is mixed with silicon particles, a binder and deionized water are added, the prepared slurry is fully mixed in a homogenizer, the prepared uniform slurry is coated on a copper foil to prepare a pole piece, and the pole piece is dried to obtain the silicon-carbon composite material pole piece.

[0058] The prepared carbon fiber material and the silicon-carbon composite electrode are characterized by using a scanning electron microscope, and the results are shown in Figure 1 .

[0059] The prepared carbon fiber material is characterized by using a scanning electron microscope for element content and distribution, and the results are shown in Figure 2 .

[0060] The prepared carbon fiber material is characterized by using XRD testing for crystal structure, and the results are shown in Figure 3 .

[0061] The silicon-carbon composite electrode is assembled into a battery for testing, and the charge-discharge curve is shown in Figure 4 .

[0062] As shown in Figure 1 , the diameter of the carbonized cotton fiber is about 10 μm, and the fiber has a hollow structure. Figure 1 -b and Figure 1 -c are the morphology diagrams of a single carbon fiber before and after heteroatom doping, respectively, and it can be seen that the doping process does not affect the basic morphology of the fiber, but only changes the surface structure and state of the fiber.

[0063] As shown in Figure 2It can be seen that the carbon fiber after doping of the heteroatom mainly contains carbon, oxygen and phosphorus elements, and the phosphorus elements are uniformly distributed in the carbon fiber material.

[0064] Example 2

[0065] The carbon fiber material is prepared by using sorghum as the raw material for preparing the carbon fiber, heating the converter to 800 DEG C for carbonization under argon atmosphere, and shortening the length of the carbon fiber to 10-150 microns by physical treatment method, i.e., the aforementioned grinding operation program.

[0066] Example 3

[0067] The carbon fiber material is prepared by using sorghum as the raw material for preparing the carbon fiber, heating the converter to 800 DEG C for carbonization under argon atmosphere, and shortening the length of the carbon fiber to 10-150 microns by physical treatment method, i.e., the aforementioned grinding operation program.

[0068] Comparative Example 1

[0069] The carbon in the silicon-carbon composite material in the present embodiment is derived from commercial graphite. The graphite is mixed with silicon particles, a binder and deionized water is added, and the obtained slurry is mixed thoroughly in a homogenizer. The obtained uniform slurry is coated on a copper foil to obtain an electrode sheet, and the electrode sheet is dried to obtain the silicon-carbon composite material electrode sheet.

[0070] The obtained electrode sheet is characterized by scanning electron microscopy, and the results are shown in Figure 1 .

[0071] The obtained silicon-carbon composite material electrode is assembled into a battery using copper foil as the negative electrode material, and the charge-discharge curve is shown in Figure 4 .

[0072] The cycle-capacity comparison chart of the first 100 cycles of the battery assembled in Example 1 and the battery assembled in Comparative Example 1 is shown in Figure 5 .

[0073] As can be seen from Figure 1 , the carbon fiber material in Example 1 forms a three-dimensional conductive network structure with silicon particles, and the structure has the characteristics of continuous conduction and also serves as a skeleton structure for maintaining the stability of the electrode. In Comparative Example 1, the graphite and silicon particles are only partially conductive, which has limited effect on the conductivity of the electrode and cannot maintain the stability of the electrode.

[0074] As can be seen from Figure 4 , the cycle stability of the battery assembled in Example 1 is higher than that of the battery assembled in Comparative Example 1, so it can be confirmed that the silicon-carbon composite electrode obtained by compounding the carbon fiber material and silicon particles has beneficial effects.

[0075] Comparative Example 2

[0076] In the present embodiment, a carbon fiber material is prepared as follows: cotton is used as the raw material for preparing carbon fibers, and the carbonization is carried out in a converter under an argon atmosphere by heating the converter to 800℃. At this time, the original carbon fibers obtained have a relatively long length and contain a large amount of ash and metal and metal salt impurities. After ultrasonic cleaning and drying, the length of the carbon fibers is shortened by physical treatment, i.e. grinding, at a grinding intensity higher than a proper intensity. Specifically, the rotation speed in the aforementioned running procedures ③ and ④ is adjusted to 180 r / min, and the time is 5 min. The length of the fibers is mainly in the range of 10 μm to 100 μm. The micron-sized carbon fibers obtained are added to a fluidized bed, the reaction pressure is controlled at 0.2-0.3 MPa, the reaction temperature is controlled at 340-350℃, and sodium hypophosphite is used as a heteroatom doping source. The sodium hypophosphite decomposition gas is introduced to dope the carbon fibers with heteroatoms.

[0077] The carbon fibers obtained by the preparation are mixed with silicon particles, a binder and deionized water are added, and the obtained slurry is mixed thoroughly in a homogenizer. The obtained uniform slurry is coated on a copper foil to obtain an electrode sheet, and the electrode sheet is dried to obtain the silicon-carbon composite electrode sheet.

[0078] The electrode sheet obtained by the preparation is assembled into a battery for testing, and the performance is as shown in Table 1. Figure 5

[0079] Comparative Example 3

[0080] In the present embodiment, a carbon fiber material is prepared as follows: cotton is used as a raw material for preparing carbon fibers, and the carbon fibers are carbonized in a converter under an argon atmosphere by heating the converter to 800°C. The obtained raw carbon fibers have a relatively long length and contain a large amount of ash and metal and metal salt impurities. After ultrasonic cleaning and drying, the length of the carbon fibers is shortened by physical treatment, i.e. grinding, at a grinding intensity lower than a proper intensity. Specifically, step ② in the above procedure is omitted, and the fiber length is mainly in the range of 50 μm to 200 μm. The obtained micron-sized carbon fibers are added to a fluidized bed, the reaction pressure is controlled in the range of 0.2-0.3 MPa, the reaction temperature is controlled in the range of 340-350°C, and sodium hypophosphite is used as a heteroatom doping source. Sodium hypophosphite decomposition gas is introduced to dope the obtained carbon fibers with heteroatoms.

[0081] The carbon fibers obtained by the preparation are mixed with silicon particles, a binder and deionized water are added, and the obtained slurry is mixed thoroughly in a homogenizer. The obtained uniform slurry is coated on a copper foil to obtain an electrode sheet, and the electrode sheet is dried to obtain the silicon-carbon composite electrode sheet.

[0082] The electrode sheet obtained by the preparation is assembled into a battery for testing, and the performance is as shown in Table 1. Figure 5 Figure 5 It can be seen that a grinding intensity higher than a proper intensity will result in excessive pulverization of the fiber material, which is not conducive to the formation of a long-range continuous conductive network and reduces the cycle performance of the battery; a grinding intensity lower than a proper intensity will result in a fiber material that is too long, which reduces the compaction density of the electrode material and the contact between materials, thereby increasing the overall resistance of the electrode and reducing the cycle performance of the battery.

[0083] The above merely describes preferred embodiments of the present application, but should not be construed to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.​​

Claims

1. A method for producing a carbon fiber material, characterized by, The carbon fiber material is used for preparing a silicon-carbon negative electrode material, and a preparation method of the carbon fiber material comprises the following steps: S100, using a biomass carbon source as a raw material, carbonizing the biomass carbon source material through heat treatment, and then performing physical treatment to shorten the carbon fiber material to obtain a micronized fiber material with a length ranging from 10 μm to 150 μm; S200, using a heteroatom precursor, and performing heteroatom doping on the carbon fiber material prepared through S100 by a chemical vapor deposition method to obtain a high-conductivity biomass carbon fiber material; The steps of S100 are specifically as follows: S110, performing heat treatment on the biomass carbon source in a protective gas, and obtaining carbon fiber after cooling; S120, performing grinding treatment on the carbon fiber material prepared in S110 to obtain a micronized fiber material; the grinding is specifically as follows: the obtained carbon fiber material is put into a grinding machine, and the carbon fiber material is ground and micronized at a rotating speed of 50-150 r / min, and the running program is ① starting the machine in forward rotation, the rotating speed is 50 r / min, and the grinding time is 5 min; ② stopping the machine after 30 s, starting the machine in reverse rotation, the rotating speed is 50 r / min, and the grinding time is 5 min; ③ stopping the machine after 30 s, starting the machine in forward rotation, the rotating speed is 150 r / min, and the grinding time is 2 min; ④ stopping the machine after 30 s, starting the machine in reverse rotation, the rotating speed is 150 r / min, and the grinding time is 2 min; ⑤ repeating steps ③ and ④ three times, and then the carbon material is taken out to obtain the micronized fiber material; The steps of S200 are specifically as follows: S210, sending the prepared micronized fiber material into a fluidized bed reactor, using a heteroatom precursor, and performing chemical vapor deposition under a pressure of 0.2 Mpa to 0.3 Mpa to obtain a heteroatom-doped carbon fiber material.

2. The preparation method of the carbon fiber material according to claim 1, wherein the biomass carbon source comprises at least one of the following or a combination thereof: tree leaves, cottonwood fluff, sorghum, cotton, rice husk, ramie, and soybean; the heteroatom precursor comprises at least one of the following or a combination thereof: boron tribromide, boron fluoride, boric acid, sodium borohydride, urea, sodium azide, ammonium nitrate, sodium hypophosphite, phosphoric acid, phosphine, thiourea, ammonium sulfide, thioacetamide, sodium thiosulfate, ammonium fluoride, trifluoroacetic acid, and hydrofluoric acid. The diameter of the carbon fiber prepared in S110 ranges from 5 μm to 20 μm.

3. The method of claim 1, wherein the carbon fiber material is prepared by a process comprising:

4. The preparation method of the carbon fiber material according to claim 1, wherein the heat treatment temperature ranges from 500 ℃ to 1500 ℃, and the chemical vapor deposition temperature ranges from 150 ℃ to 500 ℃. The carbon fiber material is obtained by the method according to any one of claims 1 to 4. The silicon-carbon composite slurry is coated on an electrode current collector, and the silicon-carbon composite slurry contains silicon nanoparticles and the carbon fiber material according to claim 5.

5. A carbon fiber material, characterized by, The mass fraction of the silicon nanoparticles in the silicon-carbon composite slurry is not less than 50 %, and the mass fraction of the carbon fiber material is not less than 10 %.

6. A global long-range conductive silicon-carbon composite negative electrode structure, characterized by, The silicon-carbon composite negative electrode structure according to claim 6 is contained.

7. The silicon-carbon composite negative electrode structure of claim 6, wherein, ​ 8. A secondary battery characterized by comprising: ​

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