Fibrous silicon-carbon composite material and method for preparing the same

Abstract

The present invention discloses a fibrous silicon-carbon composite material having a core-shell structure, an inner core of the core-shell structure being composed of porous carbon fiber and nanosilicon, and an outer shell of the core-shell structure being composed of inorganic lithium salt and amorphous carbon, and a preparation method thereof. The present invention has high electronic conductivity, and a lithium ion battery using the same has excellent rate and cycle performance.

Description

Technical Field

[0001] This application claims the priority of Chinese Patent Application No. 2023101521471, titled "Fibrous Silicon Carbon Composite Material and Its Preparation Method", filed with the Patent Office of the State Intellectual Property Office of China on February 22, 2023, and all of its contents are incorporated herein by reference.

[0002] The present invention belongs to the field of preparing lithium-ion battery materials, and specifically relates to a fibrous silicon carbon composite material and its preparation method.

Background Art

[0003] Silicon carbon composite materials are widely used in lithium-ion batteries with high specific energy density due to their advantages such as high specific capacity and wide material sources. Currently, commercially available silicon carbon composite materials mainly include two types: silicon carbon by the grinding method and silicon carbon by the silane thermal decomposition method. Here, for silicon carbon by the grinding method, its silicon crystal grains are large, prone to aggregation and large expansion, resulting in poor cycle performance, and at the same time, the specific capacity and initial efficiency of the material are low (1250 mAh / g, 86%). On the other hand, for silicon carbon by the silane thermal decomposition method, its silicon crystal grains are small, with high specific capacity and initial efficiency (1900 mAh / g, 90%), low expansion, and good cycle performance, etc., making it a new material. However, since silicon carbon by the silane thermal decomposition method exhibits a porous structure, although the expansion is reduced, the electronic conductivity of the material is reduced due to the porous structure, and the rate performance is reduced. Therefore, in current technological development, one of the main means to improve silicon carbon materials is the doping and coating of the materials, that is, by coating materials with high electronic conductivity and high ionic conductivity, the impedance of the material is reduced.

[0004] Based on the above technical background, the technical team of the present applicant, based on its dedicated research and development experience in the field of silicon carbon composite materials, eagerly hopes to explore a new technical route to obtain a silicon carbon composite material with high-efficiency performance. [Overview of the Initiative]

[0005] In view of this, the object of the present invention is to provide a fibrous silicon-carbon composite material and a method for preparing the same, thereby providing a lithium-ion battery with high electronic conductivity and excellent rate and cycle performance.

[0006] The technical solutions used in this invention are as follows:

[0007] A fibrous silicon-carbon composite material, wherein the fibrous silicon-carbon composite material includes a core-shell structure, where the internal core of the core-shell structure is composed of porous carbon fibers and nanosilicon, and the external shell of the core-shell structure is composed of an inorganic lithium salt and amorphous carbon.

[0008] Preferably, the mass percentage of the outer shell in the fibrous silicon-carbon composite material is 3 to 20 wt%, more preferably 5 to 15 wt%.

[0009] Preferably, the method for preparing the fibrous silicon-carbon composite material described above is used. Step S1) involves dissolving polyacrylonitrile in an N,N-dimethylformamide solvent to obtain a spinning solution, and then electrospinning to obtain nanocarbon fibers. Step S2) involves adding the nanocarbon fibers obtained in step S1) to the alkaline solution and mixing them uniformly, spray-drying them, and then heating and sintering them in an inert atmosphere for at least 1 hour to obtain porous carbon fibers, wherein the mass ratio of the nanocarbon fibers to the alkaline solution is 100:100~300. Step S3) involves transferring the porous carbon fibers obtained in step S2) to a reaction chamber, evacuating the chamber, passing chlorosilane through it, maintaining the pressure in the chamber at 0.1~1 MPa, and thermally decomposing it at a temperature of 200~400°C for at least 1 hour, and then lowering the temperature under an inert atmosphere to obtain a nanosilicon-porous carbon fiber composite material. Step S4) includes at least the following steps: transferring the nanosilicon-porous carbon fiber composite material obtained in step S3) to a vacuum reaction chamber, first depositing lithium silicate on the surface by atomic layer deposition, then transferring it to a carbonization apparatus, and performing carbonization treatment by vapor phase deposition at a temperature of 700-900°C through a carbon source for at least 1 hour to obtain a nanosilicon-porous carbon fiber composite material coated with two layers of lithium silicate and amorphous carbon as the fibrous silicon-carbon composite material.

[0010] Preferably, in step S1), the mass concentration of the spinning solution is 10 to 40 wt%, in step S2), the heating and sintering includes sintering at a temperature of 750 to 850°C for 1 to 6 hours, and / or in step S3), the thermal decomposition time is 1 to 6 hours, and / or in step S4), the carbonization treatment time is 1 to 6 hours.

[0011] Preferably, in step S1), the spinning voltage of the electrospinning is 15-20kV, the supply speed is 0.1-0.5mm / min, and the receiving distance is 15-20cm.

[0012] Preferably, in step S1), the alkaline solution is a mixture of one or more of sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate, and / or the mass concentration of the alkaline solution is 1 to 5 wt%.

[0013] Preferably, in step S3), the chlorosilane is a mixture of one or more of trichlorosilane, tetrachlorosilane, dimethylchlorosilane, propyltrichlorosilane, dimethylchlorosilane, allyltrichlorosilane, and phenyldichlorosilane.

[0014] Preferably, in step S4), the atomic layer deposition operation step is at least:

[0015] Step S3) involves transferring the nanosilicon-porous carbon fiber composite material obtained in step S3) to a vacuum chamber, using lithium silicate as the target material, evacuating the vacuum chamber to maintain a pressure range of 0.05 to 0.5 Torr, raising the temperature to 280 to 350°C, and then performing circulating deposition by passing lithium silicate and an oxygen source through the reaction chamber according to a set procedure, wherein the set procedure involves passing lithium silicate through, purging with nitrogen, passing the oxygen source through, purging with nitrogen, passing water through, and purging with nitrogen, and the process is repeated for 10 to 100 cycles according to this procedure.

[0016] Preferably, the set procedure involves passing lithium silicate through for 0.2 to 1 second, purging with nitrogen for at least 30 seconds, passing an oxygen source through for 2 to 8 seconds, purging with nitrogen for at least 30 seconds, passing water through for 0.01 to 0.06 seconds, and purging with nitrogen for at least 30 seconds.

[0017] Preferably, in step S4), the carbon source is one of methane, acetylene, ethylene, and ethane.

[0018] In one aspect, this application obtains a nanosilicon-porous carbon fiber composite material by depositing nanosilicon onto specific porous carbon fibers. Based on the porous structure of the porous carbon fibers and the fibrous carbon structure, experimental verification revealed that by using this specific nanosilicon-porous carbon fiber composite material as the internal core structure of a silicon-carbon composite material, the expansion of nanosilicon can be significantly reduced. Furthermore, the fibrous nanosilicon-porous carbon fiber composite material has high electronic conductivity, significantly improving rate performance.

[0019] In other respects, this application further involves depositing lithium silicate onto the surface of a nanosilicon-porous carbon fiber composite material using atomic layer deposition. Due to the high ionic conductivity of lithium silicate, the amorphous carbon located in the outer layer further improves the electronic conductivity of the silicon-carbon composite material. At the same time, direct contact between lithium silicate and the electrolyte is avoided, reducing side reactions. As a result, initial efficiency and storage performance are improved, and a synergistic effect is exerted between the lithium silicate and amorphous carbon, which constitute the outer shell structure, improving rate, cycle, and high-temperature storage performance. [Brief explanation of the drawing]

[0020] [Figure 1] This is an SEM image of the silicon-carbon composite material prepared in Example 1 of the present invention. [Modes for carrying out the invention]

[0021] This embodiment provides a fibrous silicon-carbon composite material, which includes a core-shell structure, where the inner core of the core-shell structure is composed of porous carbon fibers and nanosilicon, and the outer shell of the core-shell structure is composed of an inorganic lithium salt and amorphous carbon. Preferably, in this embodiment, the mass percentage of the outer shell in the fibrous silicon-carbon composite material is 3 to 20 wt%, more preferably 5 to 15 wt%.

[0022] Preferably, this embodiment further provides a method for preparing the fibrous silicon-carbon composite material as described above, and includes at least the following steps S1) to S4).

[0023] In step S1), polyacrylonitrile is dissolved in an N,N-dimethylformamide solvent to obtain a spinning solution, and then electrospinning is performed to obtain nanocarbon fibers. Preferably, in this step S1), the mass concentration of the spinning solution is 10-40 wt%, more preferably 10-30 wt%. The electrospinning voltage is 15-20 kV, the feeding speed is 0.1-0.5 mm / min, the receiving distance is 15-20 cm, the alkali solution is one or a mixture of any of sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate. Of course, other suitable alkali solutions may be selected, and the mass concentration of the alkali solution is 1-5 wt%.

[0024] In step S2), the nanocarbon fibers obtained in step S1) are added to an alkali solution and uniformly mixed. After spray drying, heat sintering is performed in an inert atmosphere for at least 1 hour (more preferably sintering at a temperature of 750-850 °C for 1-6 hours) to obtain porous carbon fibers. Here, the mass ratio of nanocarbon fibers:alkali solution is 100:100-300.

[0025] In step S3), the porous carbon fibers obtained in step S2) are transferred to a reaction chamber. After evacuation, chlorosilane is passed through, the pressure in the chamber is maintained at 0.1-1 Mpa, and thermal decomposition is performed at a temperature of 200-400 °C for at least 1 hour (more preferably, the thermal decomposition time is 1-6 hours). Then, the temperature is lowered in an inert atmosphere (preferably lowered to room temperature) to obtain a nanosilicon-porous carbon fiber composite material. Preferably, in this step S3), the chlorosilane is one or a mixture of any of trichlorosilane, tetrachlorosilane, dimethylchlorosilane, propyltrichlorosilane, dimethylchlorosilane, allyltrichlorosilane, and phenyldichlorosilane. In this example, the method of depositing nanosilicon by thermal decomposition has the advantage that the activity is weak at low temperatures, the silicon crystal grains are small, and uniform deposition can be expected.

[0026] In step S4), the nano-silicon - porous carbon fiber composite material obtained in step S3) is transferred to a vacuum reaction chamber. First, lithium silicate is deposited on the surface by atomic layer deposition, and then it is transferred to a carbonization device. By a vapor deposition method (a known process), at a temperature of 700 - 900 °C, a carbon source (a suitable carbon source is any one of methane, acetylene, ethylene, and ethane) is passed through for at least 1 hour (more preferably 1 - 6 hours) for carbonization treatment, and a nano-silicon - porous carbon fiber composite material coated with a two-layer of lithium silicate and amorphous carbon is obtained as a fibrous silicon carbon composite material. Preferably, in this step S4), the operation steps of the atomic layer deposition method include at least the following.

[0027] The nano-silicon - porous carbon fiber composite material obtained in step S3) is transferred to a vacuum chamber. Using lithium silicate as the target material, the vacuum chamber is evacuated to maintain a pressure range of 0.05 - 0.5 Torr. After heating up to 280 - 350 °C, according to the set procedure, lithium silicate and an oxygen source are respectively passed through the reaction chamber for cyclic deposition. Here, the set procedure is to pass through lithium silicate, purge with nitrogen, pass through the oxygen source, purge with nitrogen, pass through water, and purge with nitrogen, and cycle 10 - 100 times according to this procedure. More preferably, in the set procedure, lithium silicate is passed through for 0.2 - 1 second, purged with nitrogen for at least 30 seconds, the oxygen source is passed through for 2 - 8 seconds, purged with nitrogen for at least 30 seconds, water is passed through for 0.01 - 0.06 second, purged with nitrogen for at least 30 seconds. Specifically, preferably, in the set procedure, lithium silicate is passed through for 0.5 second, purged with nitrogen for 60 seconds, the oxygen source is passed through for 5 seconds, purged with nitrogen for 5 seconds, water is passed through for 0.03 second, purged with nitrogen for 50 seconds. Of course, according to the actual implementation conditions, it is also possible to select specific time parameters from the range of the preferred set procedure, and this application is not uniquely limited in implementation.

[0028] To enable those skilled in the art to better understand the technical solutions of the present invention, the following, together with the drawings of the embodiments of the present invention, will clearly and completely describe the technical solutions in the embodiments of the present invention. Naturally, the embodiments described are only a part of, not all, of the embodiments of the present invention. All other embodiments that can be obtained by those skilled in the art without requiring any creative effort based on the embodiments of the present invention are included within the scope of the protection of the present invention.

[0029] Based on the embodiments described above, this application further provides the following specific embodiments.

[0030] Example 1: Follow the steps below.

[0031] In step S1), 20 g of polyacrylonitrile was dissolved in 100 g of N,N-dimethylformamide solvent to obtain a spinning solution with a mass concentration of 20 wt%, and then electrospinned (spinning voltage of 20 kV, supply rate of 0.3 mm / min, receiving distance of 20 cm) to obtain nanocarbon fibers.

[0032] In step S2), 100 g of nanocarbon fibers obtained in step S1) were added to 200 g of a 2 wt% sodium carbonate aqueous solution and mixed uniformly. After spray drying, the mixture was sintered at 800°C for 3 hours under an argon inert atmosphere to obtain porous carbon fibers.

[0033] In step S3), the porous carbon fibers obtained in step S2) were transferred to a reaction chamber, first vacuumed to 0.1 Torr, then trichlorosilane was passed through, the pressure in the chamber was maintained at 0.5 MPa, and the material was thermally decomposed at a temperature of 300°C for 3 hours. Subsequently, the temperature was lowered to room temperature under an argon atmosphere to obtain a nanosilicon-porous carbon fiber composite material.

[0034] In step S4), the nanosilicon-porous carbon fiber composite material obtained in step S3) is transferred to a vacuum reaction chamber, and first, atomic layer deposition is performed (specifically, the nanosilicon-porous carbon fiber composite material is transferred to a vacuum chamber, lithium silicate is used as the target material, the vacuum chamber is evacuated and a pressure of 0.1 Torr is maintained, and after heating to 300°C, lithium silicate and an oxygen source are passed through the reaction chamber separately according to the set procedure to perform circulating deposition, and here the set procedure is to transfer lithium silicate to 0.5 The process involves passing a vapor deposition device through a vapor source for 5 seconds, purging with nitrogen for 60 seconds, passing an oxygen source through the vapor source for 5 seconds, purging with nitrogen for 5 seconds, passing water through the vapor source for 0.03 seconds, and purging with nitrogen for 50 seconds, and repeating this procedure for 50 cycles. Lithium silicate is then deposited onto the surface by this method, which is then transferred to a tubular furnace (as a carbonization device), and carbonized by vapor deposition using methane at a temperature of 800°C for 3 hours. A nanosilicon-porous carbon fiber composite material coated with two layers of lithium silicate and amorphous carbon is obtained as the fibrous silicon-carbon composite material of this Example 1.

[0035] This application describes a scanning electron microscope (SEM) test performed on the silicon-carbon composite material prepared in Example 1 described above, and the test results are shown in Figure 1. As can be seen from Figure 1, the silicon-carbon composite material provided in Example 1 exhibits a fibrous structure, with a diameter of approximately 1 μm and a length in the range of 20 to 100 μm.

[0036] Example 2: Follow the steps below.

[0037] In step S1), 10 g of polyacrylonitrile was dissolved in 100 g of N,N-dimethylformamide solvent to obtain a spinning solution with a mass concentration of 10 wt%, which was then electrospinned (spinning voltage of 20 kV, supply rate of 0.3 mm / min, receiving distance of 20 cm) to obtain nanocarbon fibers.

[0038] In step S2), 100 g of nanocarbon fibers obtained in step S1) were added to 100 g of a 5 wt% sodium bicarbonate aqueous solution and mixed uniformly. After spray drying, the mixture was sintered at 750°C for 6 hours under an argon inert atmosphere to obtain porous carbon fibers.

[0039] In step S3), the porous carbon fibers obtained in step S2) were transferred to a reaction chamber, first vacuumed to 0.1 Torr, then tetrachlorosilane was passed through, maintaining the pressure in the chamber at 0.1 MPa, and thermal decomposition was carried out at a temperature of 200°C for 6 hours. Subsequently, the temperature was lowered to room temperature under an argon atmosphere to obtain a nanosilicon-porous carbon fiber composite material.

[0040] In step S4), the nanosilicon-porous carbon fiber composite material obtained in step S3) is transferred to a vacuum reaction chamber, and first, atomic layer deposition is performed (specifically, the nanosilicon-porous carbon fiber composite material is transferred to a vacuum chamber, lithium silicate is used as the target material, the vacuum chamber is evacuated and a pressure of 0.1 Torr is maintained, and after heating to 300°C, lithium silicate and an oxygen source are passed through the reaction chamber separately according to the set procedure to perform circulating deposition, and here the set procedure is to transfer lithium silicate to 0.5 The process involves passing a vapor deposition device through a vapor source for 5 seconds, purging with nitrogen for 60 seconds, passing an oxygen source through the device for 5 seconds, purging with nitrogen for 5 seconds, passing water through the device for 0.03 seconds, and purging with nitrogen for 50 seconds, and repeating this procedure for 10 cycles to deposit lithium silicate onto the surface. The material is then transferred to a tubular furnace (as a carbonization device), and carbonization is performed by passing acetylene through the material at a temperature of 900°C for 6 hours using a vapor deposition method. A nanosilicon-porous carbon fiber composite material coated with two layers of lithium silicate and amorphous carbon is obtained as the fibrous silicon-carbon composite material of this Example 2.

[0041] Example 3: Follow the steps below.

[0042] In step S1), 30 g of polyacrylonitrile was dissolved in 100 g of N,N-dimethylformamide solvent to obtain a spinning solution with a mass concentration of 30 wt%, and then electrospinned (spinning voltage of 20 kV, supply rate of 0.1 mm / min, receiving distance of 20 cm) to obtain nanocarbon fibers.

[0043] In step S2), 100 g of nanocarbon fibers obtained in step S1) were added to 300 g of a 1 wt% sodium carbonate aqueous solution and mixed uniformly. After spray drying, the mixture was sintered at 850°C for 1 hour under an argon inert atmosphere to obtain porous carbon fibers.

[0044] In step S3), the porous carbon fibers obtained in step S2) were transferred to a reaction chamber, first vacuumed to 0.1 Torr, then trichlorosilane was passed through, the pressure in the chamber was maintained at 1 MPa, and the material was thermally decomposed at a temperature of 400°C for 1 hour. Subsequently, the temperature was lowered to room temperature under an argon atmosphere to obtain a nanosilicon-porous carbon fiber composite material.

[0045] In step S4), the nanosilicon-porous carbon fiber composite material obtained in step S3) is transferred to a vacuum reaction chamber, and first, atomic layer deposition is performed (specifically, the nanosilicon-porous carbon fiber composite material is transferred to a vacuum chamber, lithium silicate is used as the target material, the vacuum chamber is evacuated and a pressure of 0.1 Torr is maintained, and after heating to 300°C, lithium silicate and an oxygen source are passed through the reaction chamber separately according to the set procedure to perform circulating deposition, and here the set procedure is to transfer lithium silicate to 0.5 The process involves passing a vapor deposition device through a vapor source for 5 seconds, purging with nitrogen for 60 seconds, passing an oxygen source through the vapor source for 5 seconds, purging with nitrogen for 5 seconds, passing water through the vapor source for 0.03 seconds, and purging with nitrogen for 50 seconds, and repeating this procedure for 100 cycles. Lithium silicate is then deposited onto the surface by this process, which is then transferred to a tubular furnace (as a carbonization device), and carbonization is performed by passing ethylene through the vapor at a temperature of 900°C for 1 hour using a vapor deposition method. A nanosilicon-porous carbon fiber composite material coated with two layers of lithium silicate and amorphous carbon is obtained as the fibrous silicon-carbon composite material of this Example 3.

[0046] Comparative Example 1: The other technical solutions in Comparative Example 1 are the same as in Example 1, but the difference is that in Comparative Example 1, steps S1) and S2) are omitted, and known carbon fibers are used instead of the porous carbon fibers used in step S3).

[0047] Comparative Example 2: The other technical solutions in Comparative Example 2 are the same as in Example 1, but the difference is that in step S4) of Comparative Example 2, instead of depositing lithium silicate on the surface by atomic layer deposition, the nanosilicon-porous carbon fiber composite material obtained in step S3) is directly transferred to a tubular furnace (as a carbonization device), and carbonized for 3 hours by passing methane through it at a temperature of 800°C using vapor phase deposition.

[0048] Comparative Example 3: The other technical solutions in Comparative Example 3 are the same as in Example 1, but the difference is that in step S4) of Comparative Example 3, the deposition of amorphous carbon by vapor deposition, specifically, the nanosilicon-porous carbon fiber composite material obtained in step S3) is transferred to a vacuum reaction chamber, and the deposition of lithium silicate on the surface by atomic layer deposition (specifically, the method used involves transferring the nanosilicon-porous carbon fiber composite material to a vacuum chamber, using lithium silicate as the target material, evacuating the vacuum chamber to maintain a pressure of 0.1 Torr, raising the temperature to 300°C, and then performing circulating deposition by passing lithium silicate and an oxygen source through the reaction chamber according to a set procedure, where the set procedure involves passing lithium silicate for 0.5 seconds, purging nitrogen for 60 seconds, passing the oxygen source for 5 seconds, purging nitrogen for 5 seconds, passing water for 0.03 seconds, and purging nitrogen for 50 seconds, and this procedure is repeated for 50 cycles) is omitted.

[0049] Comparative Example 4: This example uses the silicon-carbon composite material provided in Example 1 of the prior disclosure patent CN103305965A.

[0050] Comparative Example 5: Steps S1), S2), and S3) were omitted, and step S4) was directly performed using a silicon-carbon composite material having a nanomicroporous structure provided in Example 1 of prior disclosure patent CN103305965A, instead of the nanosilicon-porous carbon fiber composite material obtained after going through steps S1), S2), and S3) in Example 1.

[0051] To compare and verify the effects of the above examples and comparative examples, the following physicochemical tests were performed on the silicon-carbon composite materials obtained in Examples 1-3 and Comparative Examples 1-5 in this application.

[0052] 1. Button battery test: The silicon-carbon composite materials obtained in Examples 1-3 and Comparative Examples 1-5 were used as the negative electrode material for a lithium-ion battery and assembled into a button cell. The button cell preparation process was as follows: The present invention provides a method for obtaining a negative electrode sheet for a lithium-ion battery, comprising the steps of adding a binder, a conductive agent, and a solvent to the negative electrode material of a lithium-ion battery, stirring to form a paste, applying it to copper foil, drying, and rolling, wherein the binder used is polyvinylidene fluoride (PVDF), the conductive agent is conductive carbon black (SP), the solvent is N-methylpyrrolidone (NMP), and the ratio of silicon carbon composite material, SP, PVDF, and NMP used is 95g:1g:4g:220mL; using LiPF6 as the electrolyte in the electrolyte solution and using a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1 as the solvent; using a metallic lithium sheet as the counter electrode and a polypropylene (PP) film as the diaphragm; and assembling the button battery in an argon-filled glove box.

[0053] The button batteries prepared in accordance with Examples 1-3 and Comparative Examples 1-5 underwent the following electrochemical performance tests. The tests were conducted using a Wuhan Land CT2001A battery tester, with a charge / discharge voltage range of 0.005V to 2.0V and a charge / discharge rate of 0.1C. In addition, the powder conductivity of the corresponding silicon-carbon composite materials in Examples 1-3 and Comparative Examples 1-5 was tested using a four-probe tester. Particle size, specific surface area, and tap density were tested according to the Chinese national standard GBT-38823-2020 "Silicon Carbon," and the test results are shown in Table 1 below.

[0054] TIFF0007840412000001.tif98170

[0055] As can be seen from the data in Table 1 above, the specific capacity and initial efficiency of the silicon-carbon composite materials prepared in Examples 1 to 3 of this application are significantly better than those of Comparative Examples 1 to 5. The main reason for this is that porous carbon fibers have the characteristics of lower expansion and a larger specific surface area compared to carbon fiber materials. This reduces expansion, and the coating of lithium silicate on its surface further reduces irreversible capacity loss, thereby improving the initial efficiency of the material.

[0056] 2. Pouch battery test: In Examples 1-3 and Comparative Examples 1-5, the corresponding silicon-carbon composite materials were doped with 90% artificial graphite as the negative electrode material to prepare negative electrode sheets. NCM532 was used as the positive electrode material, LiPF6 was used as the electrolyte in the electrolyte solution, a 1:1 volume ratio mixture of EC and DEC was used as the solvent, and a Celgard 2400 membrane was used as the diaphragm to prepare a 5Ah pouch battery. Subsequently, the liquid absorption and retention rate of each negative electrode sheet, the rebound rate of the sheet when fully charged, and the cycle performance were tested, and the results are as follows.

[0057] a. Liquid absorption test: Using a 1 mL burette, 1 mL of electrolyte was drawn up and dropped once onto the surface of the negative electrode sheet. The time until the electrolyte was completely absorbed was measured as time t, and the liquid absorption rate V / t of the negative electrode sheet was calculated. The test results are shown in Table 2 below.

[0058] b. Liquid retention rate test: The theoretical liquid absorption capacity m1 of the negative electrode sheet is calculated from the parameters of the negative electrode sheet, and the weight of the negative electrode sheet is weighed as m2. Then the negative electrode sheet is immersed in the electrolyte for 24 hours, and the weight of the negative electrode sheet is weighed as m3. The liquid absorption capacity m3-m2 of the negative electrode sheet is calculated, and the following formula is used: The liquid retention rate was calculated according to the formula (m³-m²)*100% / m¹, and the test results are shown in Table 2 below.

[0059] TIFF0007840412000002.tif50170

[0060] As can be seen from Table 2, the liquid absorption and liquid retention rates of the silicon-carbon composite materials obtained in Examples 1 to 3 were significantly higher than those of Comparative Examples 1 to 5. These experimental results indicate that the silicon-carbon composite material provided in this application has high liquid absorption and liquid retention rates. A possible reason for this is that the silicon-carbon composite material provided in this application has a high specific surface area, which improves the liquid absorption and liquid retention rates of the material.

[0061] c. Testing the rebound rate of the electrode sheet in a fully charged state: First, the average roll thickness of the electrode sheet is tested as D1 using a thickness gauge, then the pouch battery is fully charged to 4.2V, and then the battery is disassembled to remove the negative electrode sheet, and its thickness is tested as D2, and, The rebound rate of the electrode sheet in a fully charged state was calculated according to the formula = (D2-D1)*100% / D1, and the test results are shown in Table 3 below.

[0062] TIFF0007840412000003.tif50170

[0063] As can be seen from the data in Table 3, the rebound rate of the silicon-carbon composite negative electrode sheets obtained in Examples 1-3 in the fully charged state is lower than that of Comparative Examples 1-3. In other words, the negative electrode sheets made from the silicon-carbon composite material of the present invention have a relatively low rebound rate in the fully charged state. A possible reason for this is that the porous structure of the porous carbon fibers and the fibrous carbon structure reduce expansion.

[0064] d. Cycle performance test: The battery's cycle performance was tested with a charge / discharge rate of 1C / 1C, a voltage range of 2.5V to 4.2V, and a temperature of 25±3℃. The test results are shown in Table 4 below.

[0065] TIFF0007840412000004.tif50170

[0066] As can be seen from Table 4 above, the cycle performance of batteries made using the silicon-carbon composite material provided in this application is significantly better than that of Comparative Examples 1 to 5. The main reasons for this are that the electrode sheets made from the silicon-carbon composite material provided in this application have a low expansion rate, resulting in a more stable structure of the electrode sheets during the charge-discharge process, thereby improving their cycle performance. Furthermore, by depositing lithium silicate on the surface of the nanosilicon-carbon fibers using atomic layer deposition, the high ionic conductivity of the lithium silicate further enhances its cycle performance.

[0067] It will be obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be carried out in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered illustrative and non-limiting in any respect, and the scope of the invention is limited not by the above description but by the appended claims; thus, all variations falling within the meaning and scope of equivalent elements of the claims are intended to be included in the invention. Any reference number assigned to the claims should not be considered to limit the scope of those claims.

[0068] Furthermore, as can be understood, although this specification is described according to embodiments, each embodiment does not consist of only one distinct technical solution, and such description in this specification is merely for the purpose of clarifying the explanation, and those skilled in the art should view this specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that will be understood by those skilled in the art.

Claims

1. A fibrous silicon-carbon composite material comprising a core-shell structure, wherein the internal core of the core-shell structure is a nanosilicon-porous carbon fiber composite material composed of porous carbon fibers and nanosilicon, and the external shell of the core-shell structure is composed of a lithium silicate layer and an amorphous carbon layer, wherein the lithium silicate layer is coated on the surface of the nanosilicon-porous carbon fiber composite material, and the amorphous carbon layer is coated on the surface of the lithium silicate layer. A fibrous silicon-carbon composite material characterized in that the mass percentage of the outer shell in the fibrous silicon-carbon composite material is 3 to 20 wt%.

2. The fibrous silicon-carbon composite material according to claim 1, characterized in that the mass percentage of the outer shell in the fibrous silicon-carbon composite material is 5 to 15 wt%.

3. Step S1) involves dissolving polyacrylonitrile in an N,N-dimethylformamide solvent to obtain a spinning solution, and then electrospinning to obtain nanocarbon fibers. Step S2) involves adding the nanocarbon fibers obtained in step S1) to an alkaline solution and mixing them uniformly, spray-drying them, and then heating and sintering them in an inert atmosphere for at least one hour to obtain porous carbon fibers, wherein the mass ratio of the nanocarbon fibers to the alkaline solution is 100:100 to 300. Step S3) involves transferring the porous carbon fibers obtained in step S2) to a reaction chamber, evacuating the chamber, passing chlorosilane through it, maintaining the pressure in the chamber at 0.1 to 1 MPa, and thermally decomposing it at a temperature of 200 to 400°C for at least 1 hour, and then lowering the temperature under an inert atmosphere to obtain a nanosilicon-porous carbon fiber composite material. A method for preparing a fibrous silicon-carbon composite material according to any one of claims 1 to 2, characterized by comprising at least step S4) transferring the nanosilicon-porous carbon fiber composite material obtained in step S3) to a vacuum reaction chamber, first depositing lithium silicate on the surface by atomic layer deposition, then transferring to a carbonization apparatus, and performing carbonization treatment by vapor phase deposition at a temperature of 700 to 900°C through a carbon source for at least 1 hour to obtain a nanosilicon-porous carbon fiber composite material coated with two layers of lithium silicate and amorphous carbon as the fibrous silicon-carbon composite material.

4. The preparation method according to claim 3, characterized in that in step S1), the mass concentration of the spinning solution is 10 to 40 wt%, and / or in step S2), the heating and sintering includes sintering at a temperature of 750 to 850°C for 1 to 6 hours, and / or in step S3), the thermal decomposition time is 1 to 6 hours, and / or in step S4), the carbonization treatment time is 1 to 6 hours.

5. The preparation method according to claim 3, characterized in that, in step S1), the alkaline solution is a mixture of one or more of sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate, and / or the mass concentration of the alkaline solution is 1 to 5 wt%.

6. The preparation method according to claim 3, characterized in that in step S3), the chlorosilane is a mixture of one or more of trichlorosilane, tetrachlorosilane, dimethylchlorosilane, propyltrichlorosilane, dimethylchlorosilane, allyltrichlorosilane, and phenyldichlorosilane.

7. In step S4), the operation steps of the atomic layer deposition method are at least: The preparation method according to claim 3, characterized in that it includes the step of transferring the nanosilicon-porous carbon fiber composite material obtained in step S3) to a vacuum chamber, using lithium silicate as the target material, evacuating the vacuum chamber and maintaining a pressure range of 0.05 to 0.5 Torr, raising the temperature to 280 to 350°C, and then performing circulating deposition by passing lithium silicate and an oxygen source through the reaction chamber according to a set procedure, wherein the set procedure is to pass lithium silicate through, purge with nitrogen, pass the oxygen source through, purge with nitrogen, pass water through, and purge with nitrogen, and the process is repeated for 10 to 100 cycles according to this procedure.

8. The preparation method according to claim 7, characterized in that the set procedure involves passing lithium silicate through for 0.2 to 1 second, purging with nitrogen for at least 30 seconds, passing an oxygen source through for 2 to 8 seconds, purging with nitrogen for at least 30 seconds, passing water through for 0.01 to 0.06 seconds, and purging with nitrogen for at least 30 seconds.

9. The preparation method according to claim 3, characterized in that in step S4), the carbon source is one of methane, acetylene, ethylene, and ethane.

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