Silicon-carbon composite material, preparation method thereof and lithium ion battery

Abstract

The embodiment of the invention discloses a silicon-carbon composite material, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: S1, obtaining a porous carbon precursor; s2, placing the porous carbon precursor in a fluidized bed, introducing metal lithium gas, performing lithium doping treatment, and introducing carbon dioxide gas for surface passivation treatment to obtain a lithium-doped porous carbon material; and S3, introducing a silicon source and a carbon source into the lithium-doped porous carbon material by using a silane cracking method, and treating to obtain the silicon-carbon composite material. According to the silicon-carbon composite material and the preparation method thereof disclosed by the embodiment of the invention, the electronic conductivity can be effectively improved, the loss of charge-discharge irreversible capacity is reduced, and the first efficiency and the storage performance are improved.

Description

Technical Field

[0001] The present invention relates to the field of material preparation, and in particular to a silicon-carbon composite material and a preparation method thereof, and a lithium ion battery. Background Art

[0002] As the market's requirements for the energy density of lithium-ion batteries increase, the negative electrode materials used in lithium-ion batteries are required to have high energy density while also improving the fast-charging power performance of the materials. Existing silicon-carbon materials composed of porous carbon and nano-silicon deposited in the pores have the characteristics of high specific capacity, high initial efficiency and excellent cycle performance. However, the porous carbon itself has poor electronic conductivity and many defects, resulting in deviations in the fast-charging performance of the material and poor high-temperature storage. The method for improving the fast-charging and high-temperature storage of the material is mainly to improve the electronic conductivity of the material and reduce the defects of the material to improve the initial efficiency. Although some researchers have made some improvements to the structure or preparation method of the material, the improvement in the rate of the material is not large, and the defects of the material have not been improved, resulting in no improvement in its initial efficiency and storage performance. Summary of the invention

[0003] Therefore, in order to overcome at least some of the defects and shortcomings in the prior art, the embodiments of the present invention provide a silicon-carbon composite material and a preparation method thereof and a lithium-ion battery, which can effectively improve electronic conductivity, reduce the loss of irreversible capacity during charge and discharge, and improve initial efficiency and storage performance.

[0004] An embodiment of the present invention provides a method for preparing a silicon-carbon composite material, comprising the following steps: step S1, obtaining a porous carbon precursor; step S2, placing the porous carbon precursor in a fluidized bed and introducing metallic lithium gas to perform lithium doping treatment, and introducing carbon dioxide gas to perform surface passivation treatment to obtain a lithium-doped porous carbon material; step S3, using a silane cracking method to introduce a silicon source and a carbon source into the lithium-doped porous carbon material for treatment to obtain the silicon-carbon composite material.

[0005] In some embodiments, the temperature of the lithium doping treatment in step S2 is 1400-1600°C.

[0006] In some embodiments, the flow rate of the metallic lithium gas in step S2 is 10 to 100 ml / min, and the introduction time is 30 to 300 min.

[0007] In some embodiments, the step S1 specifically includes: mixing a phenolic compound, an aldehyde compound, a carboxylated carbon nanotube and a pore-forming agent in a preset mass ratio and sequentially performing a heating curing treatment and a temperature-raising carbonization treatment; after the temperature-raising carbonization treatment, cooling the mixture and introducing water vapor for water vapor activation treatment to obtain carbon nanotube-doped porous carbon as the porous carbon precursor.

[0008] In some embodiments, the reaction temperature of the water vapor activation treatment is 1000-1200° C., and the water vapor introduction time is 60-600 min.

[0009] In some embodiments, the preset mass ratio is phenolic compound: aldehyde compound: carboxylated carbon nanotube: pore-forming agent = 100: 100-200: 1-5: 1-5; and / or, the phenolic compound in step S1 is any one or more of phenol, resorcinol, cresol, and naphthol; and / or, the aldehyde compound is any one or more of formaldehyde, acetaldehyde, and benzaldehyde; and / or, the pore-forming agent is any one or more of zinc chloride and zinc bromide.

[0010] In some embodiments, the step S3 specifically includes: placing the lithium-doped porous carbon material in a fluidized bed, heating it for the first time and introducing monosilane gas for a first preset time, then heating it for a second time and introducing carbon source gas for a second preset time to obtain the silicon-carbon composite material.

[0011] In some embodiments, the carbon source gas in step S3 is any one or more of acetylene, propyne, ethylene, and propylene.

[0012] An embodiment of the present invention further provides a silicon-carbon composite material, which is prepared by any one of the preparation methods described above.

[0013] An embodiment of the present invention further provides a lithium-ion battery, comprising the aforementioned silicon-carbon composite material.

[0014] It can be seen from the above that the above technical solution has at least one or more of the following beneficial effects:

[0015] (1) Lithium doping can improve the electronic conductivity of the material and reduce the loss of irreversible capacity during discharge, thereby improving the initial efficiency and storage performance. In step S2, after the lithium doping treatment, carbon dioxide gas is used for surface passivation treatment, so that the metallic lithium doped on the surface of the porous carbon reacts with carbon dioxide to form a lithium carbonate (LiCO3) passivation film, which can reduce the activity of the nano-silicon in the core, reduce the contact with the electrolyte, and thus reduce gas production. Compared with metallic lithium, the lithium carbonate protective film has better compatibility with the electrolyte, which can improve the initial efficiency, improve its processing performance and feasibility of mass production, and improve safety performance.

[0016] (2) By doping carbon nanotubes, the electronic conductivity and compressive strength of porous carbon can be improved by relying on the high electronic conductivity and high mechanical strength of carbon nanotubes. At the same time, a uniform micron pore structure can be formed by using a pore-forming agent, and nanopores can be formed by combining with water vapor. The reasonable distribution of nano-micron pores can further improve the specific capacity of the material and reduce expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The specific implementation modes of the present invention will be described in detail below with reference to the accompanying drawings.

[0018] Figure 1 A schematic flow chart of a method for preparing a silicon-carbon composite material provided in one embodiment of the present invention.

[0019] Figure 2 This is the SEM image of the silicon-carbon composite material prepared in Example 1. DETAILED DESCRIPTION

[0020] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0021] In order to enable those skilled in the art to better understand the technical solution of the present invention, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of the present invention.

[0022] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0023] It should also be noted that the division of multiple embodiments in the present invention is only for the convenience of description and should not constitute a special limitation. The features in various embodiments can be combined and referenced to each other without contradiction.

[0024] Reference Figure 1 The present invention provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0025] Step S1, obtaining a porous carbon precursor;

[0026] Step S2, placing the porous carbon precursor in a fluidized bed and introducing metallic lithium gas to perform lithium doping treatment, and introducing carbon dioxide gas to perform surface passivation treatment to obtain a lithium-doped porous carbon material;

[0027] Step S3, introducing a silicon source and a carbon source into the lithium-doped porous carbon material by a silane cracking method for treatment to obtain the silicon-carbon composite material.

[0028] The porous carbon precursor in step S1 may also be referred to as a resin precursor, which can be prepared by using a resin material and a pore-forming agent. The use of the porous carbon precursor can ensure a higher specific capacity and a lower expansion rate.

[0029] The lithium doping treatment in step S2 can improve the electronic conductivity of the material and reduce the loss of irreversible discharge capacity, thereby improving the initial efficiency and storage performance. After the lithium doping treatment, step S2 uses carbon dioxide gas to perform surface passivation treatment, so that the metal lithium gas is deposited in the porous carbon to reduce the impedance, and the metal lithium is deposited on the porous carbon surface to react with carbon dioxide to generate lithium carbonate (LiCO 3 ) passivation film can reduce the activity of the nano silicon in the core, reduce the contact with the electrolyte and thus reduce the gas production. Compared with metallic lithium, the lithium carbonate protective film has better compatibility with the electrolyte, which can improve the initial efficiency, improve its processing performance and feasibility of mass production, and improve safety performance. In addition, lithium carbonate has a higher density and the impedance of the obtained material is lower.

[0030] In step S3, the silane cracking method is adopted and a silicon source is introduced to deposit nano-silicon in the lithium-doped porous carbon material to increase the specific capacity; the introduction of the carbon source can coat the outer layer of the material with amorphous carbon, which can prevent the inner core nano-silicon from directly contacting the electrolyte when it is subsequently prepared into a lithium-ion battery, reduce side reactions, and improve the initial efficiency.

[0031] Specifically, the temperature of the lithium doping treatment in step S2 is 1400-1600°C. This temperature can ensure that the metallic lithium is gasified and evenly deposited on the core and surface of the porous carbon, thereby reducing impedance. In some embodiments, the flow rate of the metallic lithium gas in step S2 is 10-100ml / min, and the introduction time is 30-300min. This can ensure that the metallic lithium gas is fully doped and reacted. The flow rate of the carbon dioxide gas introduced in step S2 is 100ml / min, and the time for the surface passivation treatment is 60-600min, which can ensure that the carbon dioxide gas fully reacts with the metallic lithium gas and prevents the problem of poor density caused by incomplete lithium carbonate. It can also avoid the waste of carbon dioxide caused by too long a reaction time.

[0032] In some embodiments, step S1 specifically includes:

[0033] The phenolic compound, the aldehyde compound, the carboxylated carbon nanotubes and the pore-forming agent are mixed according to a preset mass ratio and then subjected to a heating curing treatment and a temperature-raising carbonization treatment in sequence;

[0034] After the heating carbonization treatment, the temperature is lowered and water vapor is introduced to perform water vapor activation treatment to obtain carbon nanotube-doped porous carbon as the porous carbon precursor.

[0035] Specifically, the temperature of the heat curing treatment is 400-600°C, and the curing time is 1-6h. After the heat curing treatment, the phenolic compounds and the aldehyde compounds can be reacted to form the carbon skeleton structure of the phenolic resin, and the hydrogen and oxygen in the carbon skeleton are removed in the subsequent temperature carbonization treatment to form a pure carbon material (porous carbon). Among them, the carboxyl content of the carboxylated carbon nanotubes is 0.5-2wt%. The surface of the carboxylated carbon nanotubes contains electron-withdrawing groups, which are more easily doped inside the phenolic resin to achieve uniform distribution and reduce impedance. The temperature of the temperature carbonization treatment is 1200-1400°C, and the carbonization time is 1-6h. In this embodiment, the electronic conductivity and compressive strength of the porous carbon can be improved by doping carbon nanotubes, relying on the high electronic conductivity and high mechanical strength of the carbon nanotubes. At the same time, a uniform micron pore structure can be formed by a pore-forming agent, and nanopores can be formed by combining water vapor to achieve a reasonable distribution of nano-micron pores, which can further improve the specific capacity of the material and reduce expansion. Specifically, according to the pore size classification, the proportion of pores ≤2nm is ≤10%, the proportion of pores 2-100nm is 10-80%, and the proportion of pores 100-1000nm is ≤10%. Among them, the pores with a pore size of ≤2nm are closed pores and cannot store lithium, but can reduce expansion during charging and discharging. If there are too many micron pores, that is, the pores are large (the proportion of pores with a diameter of 100-1000nm is too large), nano-silicon will agglomerate in the pores, resulting in large expansion and high impedance.

[0036] In some embodiments, the pore-forming agent is an inorganic pore-forming agent, so that smaller pores can be obtained. Specifically, the pore-forming agent is any one or more of zinc chloride and zinc bromide.

[0037] In some embodiments, the reaction temperature of the water vapor activation treatment is 1000-1200° C., and the water vapor introduction time is 60-600 min.

[0038] In some embodiments, the preset mass ratio is phenolic compound: aldehyde compound: carboxylated carbon nanotube: pore former = 100: 100-200: 1-5: 1-5. This ratio can ensure that the phenolic compound reacts completely, and the proportion of carboxylated carbon nanotubes can ensure effective improvement of electronic conductivity, while preventing the carboxylated carbon nanotubes from agglomerating due to excessive proportion. The proportion of pore former can ensure the formation of micropores with appropriate pore size and number, ensure the subsequent deposition of nano-silicon to reduce specific capacity, and avoid excessive impedance caused by excessive micropore content.

[0039] In some embodiments, the phenolic compound is any one or more of phenol, resorcinol, cresol, and naphthol. The aldehyde compound is any one or more of formaldehyde, acetaldehyde, and benzaldehyde.

[0040] In some embodiments, the step S3 specifically includes: placing the lithium-doped porous carbon material in a fluidized bed, heating it for the first time and introducing monosilane gas for a first preset time, then heating it for a second time and introducing carbon source gas for a second preset time to obtain the silicon-carbon composite material.

[0041] Specifically, the temperature range of the first heating in step S3 is 450-650° C., the flow rate of monosilane gas is 100-500 ml / min, and the first preset time is 60-600 min. The temperature range of the second heating is 650-750° C., the flow rate of carbon source gas is 100-500 ml, and the second preset time is 60-600 min.

[0042] Specifically, the carbon source gas is any one or more of acetylene, propyne, ethylene, and propylene.

[0043] An embodiment of the present invention provides a silicon-carbon composite material, which can be prepared by the above-mentioned preparation method. And an embodiment of the present invention also provides a lithium-ion battery, including the above-mentioned silicon-carbon composite material. The lithium-ion battery can be a power battery.

[0044] The following describes the method for preparing the silicon-carbon composite material provided by the embodiment of the present invention and the effects and performances of the prepared silicon-carbon composite material and lithium ions in combination with Examples 1 to 3 and Comparative Examples 1 to 3 and related experiments.

[0045] Example 1

[0046] Step S1:

[0047] 100g of phenol, 150g of formaldehyde, 3g of carboxylated carbon nanotubes and 3g of zinc chloride were mixed evenly, and then heated to 500°C for curing for 3h (heat curing treatment), and then continued to heat to 1300°C for carbonization for 3h (heating carbonization treatment), and then cooled to 1100°C, and water vapor was introduced at a flow rate of 300ml / min for 300min (water vapor activation treatment) to obtain carbon nanotube-doped porous carbon (i.e., porous carbon precursor);

[0048] Step S2:

[0049] The carbon nanotube-doped porous carbon is transferred to a fluidized bed, heated to 1500° C., and a metallic lithium gas is introduced at a flow rate of 50 ml / min for 180 min (lithium doping treatment), and then a carbon dioxide gas is introduced, and the carbon dioxide gas is introduced at a flow rate of 100 ml / min for 180 min at a room temperature of 25° C. for surface passivation (surface passivation treatment), to obtain a lithium carbon nanotube-doped porous carbon (i.e., a lithium-doped porous carbon material);

[0050] Step S3:

[0051] The lithium carbon nanotube-doped porous carbon is transferred to a fluidized bed, and the temperature is raised to 550°C (first preset temperature) through a silane cracking method, and monosilane gas is introduced at a flow rate of 300 ml / min for 300 min (first preset time), and then the temperature is raised to 700°C (second preset temperature), and ethylene gas is introduced at a flow rate of 300 ml / min for 300 min (second preset time) to obtain a silicon-carbon composite material.

[0052] Example 2

[0053] Step S1:

[0054] 100g of resorcinol, 100g of acetaldehyde, 1g of carboxylated carbon nanotubes and 1g of zinc bromide were mixed evenly, and then heated to 400°C for curing for 6h, and then continued to heat to 1200°C for carbonization for 6h, and then cooled to 1000°C, and water vapor was introduced at a flow rate of 100ml / min for 600min to obtain carbon nanotube-doped porous carbon;

[0055] Step S2:

[0056] The carbon nanotube-doped porous carbon was transferred to a fluidized bed, heated to 1400° C., and introduced with metallic lithium gas at a flow rate of 10 ml / min for 300 min, and then introduced with carbon dioxide gas at a flow rate of 50 ml / min for 300 min at room temperature of 25° C. for surface passivation to obtain lithium carbon nanotube-doped porous carbon;

[0057] Step S3:

[0058] The lithium carbon nanotube-doped porous carbon was transferred to a fluidized bed, and the temperature was raised to 450°C through a silane cracking method. Monosilane gas was introduced at a flow rate of 100 ml / min for 600 min, and then the temperature was raised to 650°C. Acetylene gas was introduced at a flow rate of 100 ml / min for 600 min to obtain a silicon-carbon composite material.

[0059] Example 3

[0060] Step S1:

[0061] 100g of cresol, 200g of benzaldehyde, 5g of carboxylated carbon nanotubes and 5g of zinc chloride were mixed evenly, and then heated to 600°C for curing for 1h, and then continued to heat to 1400°C for carbonization for 1h, and then cooled to 1200°C, and water vapor was introduced at a flow rate of 500ml / min for 60min to obtain carbon nanotube-doped porous carbon;

[0062] Step S2:

[0063] The carbon nanotube-doped porous carbon was transferred to a fluidized bed, heated to 1600° C., and introduced with metallic lithium gas at a flow rate of 100 ml / min for 30 min, and then introduced with carbon dioxide gas at a flow rate of 200 ml / min for 30 min at room temperature of 25° C. for surface passivation to obtain lithium carbon nanotube-doped porous carbon;

[0064] Step S3:

[0065] The lithium carbon nanotube-doped porous carbon was transferred to a fluidized bed, and the temperature was raised to 650°C through a silane cracking method. Monosilane gas was introduced at a flow rate of 500 ml / min for 60 minutes. The temperature was then raised to 750°C, and methane gas was introduced at a flow rate of 500 ml / min for 60 minutes to obtain a silicon-carbon composite material.

[0066] Comparative Example 1

[0067] The difference from Example 1 is that carboxylated carbon nanotubes and zinc chloride are not added in step S1 , and the other steps are the same as Example 1.

[0068] Comparative Example 2

[0069] The difference from the first embodiment is that step S2 is not performed, and the rest is the same as the first embodiment.

[0070] Comparative Example 3

[0071] The difference from Example 1 is that water vapor is not introduced in step S1 , and the rest is the same as Example 1.

[0072] Test Case

[0073] 1. Appearance

[0074] The silicon-carbon composite material in Example 1 was subjected to SEM testing, and the test results are as follows: Figure 2 As shown. Figure 2 It can be seen that the material has a granular structure with a small amount of fine powder on the surface and the particle size is between 8-12μm.

[0075] 2. Button battery test

[0076] The silicon-carbon composite materials in Examples 1-3 and Comparative Examples 1-3 were used as negative electrode materials for lithium-ion batteries to prepare button batteries, and the preparation method was as follows:

[0077] Add a binder, a conductive agent and a solvent to a silicon-carbon composite material, stir to make a slurry, coat it on a copper foil, dry it, and roll it to obtain a negative electrode sheet; the binder used is polyvinylidene fluoride (PVDF), the conductive agent is conductive carbon black (SP), the solvent is N-methylpyrrolidone (NMP), and the amount ratio of the silicon-carbon composite material, SP, PVDF, and NMP is 90g:4g:6g:250mL; the electrolyte is lithium hexafluorophosphate (LiPF 6 ) as the electrolyte, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1 as the solvent, and the electrolyte concentration is 1 mol / L; the metal lithium sheet is the counter electrode, and the diaphragm is a polypropylene (PP) film. The button cell assembly is carried out in an argon-filled glove box.

[0078] The electrochemical performance of button cells comprising the silicon-carbon composite materials of Examples 1-3 and Comparative Examples 1-3 was tested using a Wuhan Blue Power CT2001A battery tester with a charge and discharge voltage range of 0.005 V to 2.0 V and a charge and discharge rate of 0.1 C. The charging DCR (50% SOC) of the material was also tested.

[0079] The thickness D1 of the negative electrode sheet of the button cell containing the silicon-carbon composite materials of Examples 1-3 and Comparative Examples 1-3 after rolling was tested, and then the full-charge thickness D2 of the negative electrode sheet of the button cell fully charged to 100% SOC was dissected, and then the expansion rate was calculated:

[0080]

[0081] The specific surface area of ​​the silicon-carbon composite material was tested with reference to GB / T 38823-2020 "Silicon Carbon", and the powder resistivity of the silicon-carbon composite material was tested using a four-probe tester.

[0082] The test results are shown in Table 1.

[0083] Table 1

[0084]

[0085] It can be seen from the data in Table 1 that the specific capacity, initial efficiency, powder conductivity, full charge expansion and DCR of the silicon-carbon composite materials prepared in Examples 1 to 3 of the present invention are significantly better than those in the comparative example. The reason is that the core materials of the materials in Examples 1 to 3 are doped with carbon nanotubes to improve the electronic conductivity of the materials, reduce polarization, and improve the specific capacity; at the same time, metallic lithium is deposited in the materials to reduce the irreversible capacity of the materials and improve the initial efficiency. On the other hand, the metal reacts with carbon dioxide on the porous carbon surface to generate lithium carbonate (LiCO 3 ) passivation film can reduce the activity of the nano-silicon in the core, reduce the contact with the electrolyte and thus reduce gas production. Compared with metallic lithium, the lithium carbonate protective film has better compatibility with the electrolyte, which can improve the initial efficiency, improve its processing performance and feasibility of mass production, and improve safety performance. Lithium carbonate has a higher density and the impedance of the obtained material is lower; at the same time, carbon nanotubes are doped in the core, and the high mechanical strength of the carbon nanotubes is used to reduce the expansion of silicon during charging and discharging.

[0086] According to the comparison of the results of Example 1 and Comparative Example 1 in Table 1, the specific surface area, powder resistivity, full charge expansion rate and DCR are quite different. The reason is that in Example 1, the addition of carboxylated carbon nanotubes and pore-forming agents can improve the electronic conductivity of porous carbon and reduce expansion.

[0087] According to the comparison of the results of Example 1 and Comparative Example 2 in Table 1, the first discharge specific capacity and the first efficiency are quite different. The reason is that in Example 1, lithium doping can improve the electronic conductivity of the material and reduce the loss of irreversible discharge capacity, thereby improving the first efficiency. In addition, Example 1 can form a lithium carbonate protective film, which has better compatibility with the electrolyte than metallic lithium, thereby improving the first efficiency.

[0088] According to the comparison of the results of Example 1 and Comparative Example 3 in Table 1, the powder resistivity and full-charge expansion are quite different. The reason is that in Example 1, nanopores can be formed by water vapor activation, and the nano-micron pores can be reasonably distributed. The nanopores can reduce expansion during the charge and discharge process. And fewer micron pores can prevent nano-silicon from agglomerating in the pores, reducing impedance and expansion.

[0089] 3. Soft pack battery test

[0090] The silicon-carbon composite materials of Examples 1-3 and Comparative Examples 1-3 were mixed with 95% artificial graphite as negative electrode materials to prepare negative electrode sheets. 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 ) is the positive electrode; the electrolyte is lithium hexafluorophosphate (LiPF 6) as electrolyte, a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (DEC) in a volume ratio of 1:1 as solvent, and an electrolyte concentration of 1.1 mol / L; Celgard 2400 membrane as separator, a 5Ah soft pack battery was prepared. The liquid absorption and retention capacity, rate and high temperature storage performance of the negative electrode were tested respectively.

[0091] a. Liquid absorption capacity test

[0092] Use a 1mL burette and draw 1mL of electrolyte, add a drop on the surface of the electrode, and count until the electrolyte is completely absorbed, and record the time t. The test results are shown in Table 2.

[0093] b. Liquid retention rate test

[0094] According to the electrode parameters, the theoretical liquid absorption of the electrode m1 is calculated, and the weight of the electrode m2 is weighed. Then the electrode is placed in the electrolyte and soaked for 24 hours. The weight of the electrode is weighed as m3, and the liquid absorption of the electrode m3-m2 is calculated, and calculated according to the following formula: Liquid retention rate = (m3-m2)*100% / m1. The test results are shown in Table 2.

[0095] Table 2

[0096] Material Liquid absorption speed (t) Fluid retention rate Example 1 87 93.1％ Example 2 79 93.9％ Example 3 107 91.6％ Comparative Example 1 136 86.1％ Comparative Example 2 123 87.4％ Comparative Example 3 156 84.8％

[0097] As can be seen from Table 2, the liquid absorption and liquid retention capacity of the silicon-carbon composite materials obtained in Examples 1 to 3 is significantly higher than that of the comparative example, indicating that the silicon-carbon composite materials provided by the embodiments of the present invention have a higher liquid absorption and liquid retention capacity. The reason may be that the specific surface area of ​​the composite materials in the embodiments is larger, which improves the liquid absorption and liquid retention capacity of the materials.

[0098] c.HPPC test:

[0099] HPPC test was performed on the soft pack batteries including the composite materials of Examples 1-3 and Comparative Examples 1-3.

[0100] HPPC test: The DCR test at 3C charging rate with different SOC (10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%) was tested. The test results are shown in Table 3.

[0101] Table 3

[0102]

[0103] It can be seen from Table 3 that the HPPC (charging DCR) of the soft-pack lithium-ion batteries prepared using the silicon-carbon composite materials of Examples 1 to 3 of the present invention is better than that of Comparative Examples 1-3. The reason is that the silicon-carbon composite materials of the present invention have high powder electronic conductivity, which can reduce DCR and improve rate performance.

[0104] In summary, the preparation method of the silicon-carbon composite material provided by the embodiment of the present invention improves the electronic conductivity and compressive strength of the porous carbon by doping carbon nanotubes in the resin precursor, relying on the high electronic conductivity and high mechanical strength of the carbon nanotubes; at the same time, an inorganic pore-forming agent is added to the resin precursor to form a uniform micron pore structure, and the nanopores formed by water vapor pore formation are combined to achieve a reasonable distribution of nano-micron pores of the porous carbon pores, thereby improving the specific capacity of the material and reducing its expansion. By depositing metallic lithium in the carbon nanotube-doped porous carbon, its electronic conductivity is improved and the loss of irreversible capacity of charge and discharge is reduced, thereby improving the initial efficiency and its storage performance. And by using the silane cracking method to deposit nano-silicon on the lithium carbon nanotube-doped porous carbon, the specific capacity is improved, and amorphous carbon is coated on its outer layer to avoid direct contact between the inner core nano-silicon and the electrolyte, thereby reducing side reactions and improving the initial efficiency.

[0105] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a silicon-carbon composite material, characterized in that: The steps include: Step S1, obtaining a porous carbon precursor; Step S2, placing the porous carbon precursor in a fluidized bed and introducing metallic lithium gas to perform lithium doping treatment, and introducing carbon dioxide gas to perform surface passivation treatment to obtain a lithium-doped porous carbon material; Step S3, introducing a silicon source and a carbon source into the lithium-doped porous carbon material by a silane cracking method for treatment to obtain the silicon-carbon composite material.

2. The method for preparing the silicon-carbon composite material according to claim 1, characterized in that: The temperature of the lithium doping treatment in step S2 is 1400-1600°C.

3. The method for preparing the silicon-carbon composite material according to claim 1, characterized in that: In step S2, the flow rate of the metallic lithium gas is 10-100 ml / min, and the introduction time is 30-300 min.

4. The method for preparing the silicon-carbon composite material according to claim 1, characterized in that: The step S1 specifically includes: The phenolic compound, the aldehyde compound, the carboxylated carbon nanotubes and the pore-forming agent are mixed according to a preset mass ratio and then subjected to a heating curing treatment and a temperature-raising carbonization treatment in sequence; After the heating carbonization treatment, the temperature is lowered and water vapor is introduced to perform water vapor activation treatment to obtain carbon nanotube-doped porous carbon as the porous carbon precursor.

5. The method for preparing the silicon-carbon composite material according to claim 4, characterized in that: The reaction temperature of the water vapor activation treatment is 1000-1200° C., and the water vapor introduction time is 60-600 minutes.

6. The method for preparing the silicon-carbon composite material according to claim 4, characterized in that: The preset mass ratio is phenolic compound: aldehyde compound: carboxylated carbon nanotube: pore-forming agent = 100: 100-200: 1-5: 1-5; and / or, the phenolic compound in step S1 is any one or more of phenol, resorcinol, cresol, and naphthol; and / or, the aldehyde compound is any one or more of formaldehyde, acetaldehyde, and benzaldehyde; and / or, the pore-forming agent is any one or more of zinc chloride and zinc bromide.

7. The method for preparing the silicon-carbon composite material according to claim 1, characterized in that: The step S3 specifically includes: placing the lithium-doped porous carbon material in a fluidized bed, heating it for the first time and introducing monosilane gas for a first preset time, then heating it for the second time and introducing carbon source gas for a second preset time, to obtain the silicon-carbon composite material.

8. The method for preparing the silicon-carbon composite material according to claim 7, characterized in that: In step S3, the carbon source gas is any one or more of acetylene, propyne, ethylene, and propylene.

9. A silicon-carbon composite material, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: Comprising the silicon-carbon composite material as claimed in claim 9.

Images