Waste graphite-based fibrous silicon-carbon composite material as well as preparation and application thereof

Abstract

The invention belongs to the field of waste material recovery, and particularly relates to a waste graphite-based fibrous silicon-carbon composite material and preparation and application thereof.The waste graphite material is dispersed in a modified solution and then subjected to variable-pressure treatment; carrying out double-frequency ultrasonic treatment on the system subjected to the variable-pressure treatment, and then carrying out solid-liquid separation to obtain modified graphite; slurrying the modified graphite and a polymer to form a graphite polymer solution, and then performing fibrosis, curing, carbonization and activation treatment to prepare waste graphite modified carbon fibers; and carrying out silicon deposition and surface carbon coating treatment by taking the waste graphite modified carbon fiber as a substrate to prepare the waste graphite-based fibrous silicon-carbon composite material. The method provided by the invention is beneficial to regeneration of the waste graphite to obtain the negative electrode active material with super fast charge and wide temperature range stability.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and particularly to the technical field of regenerating waste graphite materials to prepare anode active materials. Background Art

[0002] With the large-scale production and use of lithium batteries, the number of retired lithium-ion batteries has shown an explosive growth trend. Retired lithium-ion batteries not only contain valuable metal resources such as lithium, nickel, cobalt, and manganese, but also components such as organic electrolytes therein, if not properly treated, will cause serious environmental pollution. Therefore, how to efficiently recycle and reuse the materials in these retired lithium batteries has become an urgent and important environmental protection and economic issue.

[0003] In the composition of lithium-ion batteries, graphite has long been widely used as the main anode material due to its excellent electrochemical performance and stable structural characteristics, and its mass ratio in lithium-ion batteries is usually 12% to 21%. However, compared with the cathode material, the recycling problem of anode graphite has not received enough attention. Currently, most graphite anode recycling processes can only use the recycled graphite for low-end products, such as low-quality graphite anodes, manufacturing refractory materials, or directly burning as fuel, which not only fails to fully utilize the potential value of graphite, but may also cause environmental pollution problems, such as particulate matter pollution and the exacerbation of the greenhouse effect.

[0004] In recent years, in order to further improve the energy density of lithium-ion batteries, researchers have turned their attention to silicon-based anode materials, especially silicon-carbon composite materials. Among silicon-carbon composite materials, a new type of silicon-carbon anode material prepared by chemical vapor deposition technology of silane is favored due to its high theoretical capacity and first Coulomb efficiency. Porous carbon materials are often used as the supporting framework in silicon-carbon composite materials to alleviate the volume change generated by silicon during charge and discharge and avoid the rupture of silicon particles. However, the defects existing in porous carbon materials themselves, such as insufficient compressive strength and poor electrical conductivity, limit their application effects in silicon-carbon anode materials, thereby affecting the overall cycle stability and rate performance. Existing solutions usually involve coating a layer of conductive polymer on the surface of porous carbon or silicon-carbon composite materials (as shown in patents CN116072845 A and CN202410410857.4). Although this method can improve the electrical conductivity to a certain extent, it does not completely solve the inherent physical property problems of the porous carbon matrix itself, and the regenerated materials are difficult to adapt to the requirements of high-end application scenarios. For example, it is difficult to meet the application requirements such as super-fast charging and wide-temperature range cycling. Summary of the Invention

[0005] Aiming at the problem that it is difficult to regenerate high-performance silicon-carbon materials from existing waste graphite materials, the first object of the present invention is to provide a preparation method of a waste graphite-based fibrous silicon-carbon composite material, aiming to prepare a negative electrode active material adaptable to the requirements of fast charging and wide-temperature stability applications based on waste graphite materials.

[0006] The second object of the present invention is to provide a waste graphite-based fibrous silicon-carbon composite material prepared by the above preparation method and its application in preparing a lithium secondary battery as a negative electrode active material.

[0007] The third object of the present invention is to provide a lithium secondary battery containing the waste graphite-based fibrous silicon-carbon composite material, its negative electrode and negative electrode material.

[0008] There are many cyclic impurities on the surface and between the layers of waste graphite materials. In addition, there are also many structural defects. Using it as a substrate to prepare silicon-carbon materials is difficult to achieve the expected effect. To solve this problem, the present invention proposes a modification idea of dispersing waste graphite in carbon fiber and then compounding it with silicon. Early research shows that this idea can improve the performance of the regenerated material to a certain extent, but it is still necessary to properly solve the problems caused by impurities between the layers and structural defects of waste graphite, as well as the entanglement and agglomeration caused by the mismatch of the physical and chemical properties between waste graphite and carbon fiber. In this way, it is still difficult to effectively exert the performance of the material and make it adapt to high-demand application scenarios such as fast charging and wide-temperature stability. To solve this problem, the present invention has conducted in-depth research and provides the following improvement scheme:

[0009] A preparation method of a waste graphite-based fibrous silicon-carbon composite material, the steps include:

[0010] Step 1:

[0011] Disperse the waste graphite material in a modified solution and then perform variable-pressure treatment; the variable-pressure treatment includes a process of pressurizing with gas to a positive pressure, and a process of switching the system pressure to normal pressure or negative pressure after maintaining the positive pressure; the modified solution is an aqueous solution containing auxiliary agent A and auxiliary agent B; wherein, the auxiliary agent A is an oxidizing component and / or an acidic component; the auxiliary agent B is a component with surface activity;

[0012] Perform dual-frequency ultrasonic treatment on the system after variable-pressure treatment, and then perform solid-liquid separation to obtain modified graphite;

[0013] Step 2:

[0014] Pulverize the modified graphite prepared in Step 1 and a polymer to form a graphite-polymer solution, and then perform fibrillation, curing, carbonization, and activation treatments to obtain waste graphite-modified carbon fiber;

[0015] Step 3:

[0016] The waste graphite-modified carbon fiber is used as a substrate for silicon deposition and carbon coating to obtain the waste graphite-based fibrous silicon-carbon composite material.

[0017] In response to the problems faced in the regeneration of waste graphite materials, the present invention innovatively subjects the waste graphite to the voltage change treatment and dual-frequency ultrasonic treatment in the modified solution, which is conducive to obtaining modified graphite with a thin layer and excellent surface properties based on the waste graphite material, and further subjecting it to subsequent fiberization, carbonization, activation and silicon precipitation treatments, which can improve the dispersed distribution morphology and uniformity of graphite in the carbon substrate, facilitate the construction of a fast electron and ion transmission network, and facilitate the regeneration of waste graphite to obtain negative electrode active materials that have both super-fast charging and wide temperature range stability.

[0018] In the present invention, the waste graphite material is a graphite-containing material recovered from the negative electrode of waste batteries.

[0019] Preferably, the waste graphite material is also allowed to contain at least one of a conductive agent, a binder, and a current collector.

[0020] Preferably, the content of graphite in the waste graphite material is above 80wt.%.

[0021] In the present invention, through the modified solution containing additives A and additives B and the pressure change treatment, synergy can be achieved. Based on the effects of gas-solid-liquid three-phase and gas pressure transformation, the graphite layers can be improved, the interlayer surface formation and defects can be optimized, and thus the dispersion distribution morphology and interface bonding of waste graphite materials in the carbon fiber matrix can be improved, thereby synergistically improving the super-fast charging and wide temperature range stability of the prepared materials.

[0022] In the present invention, in the modified solution, the auxiliary agent A comprises at least one of nitric acid, sulfuric acid and potassium permanganate.

[0023] The auxiliary agent B includes at least one of hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, hexadecylpyridinium chloride, sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, sodium stearate, polyethylene glycol, polyvinylpyrrolidone, Tween20, Tween 80, F-127, sodium polyacrylate, and polyacrylamide. Preferably, the auxiliary agent B includes at least one of hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, hexadecylpyridinium chloride, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfate. Studies have shown that the preferred auxiliary agent B, combined with the process of the present invention, helps to further synergistically improve the fast charging and wide temperature range performance of the recycled material.

[0024] Preferably, in the modified solution, the concentration of additive A is 0.5 to 5 mol / L; the concentration of additive B is 0.05 to 5 wt.%. Further, the concentration of additive A is 1 to 2.5 mol / L; the concentration of additive B is 1 to 2 wt.%.

[0025] Preferably, in the mixed solution, the liquid-solid ratio is 5 to 100 mL / g, and further can be 30 to 55 mL / g.

[0026] In the present invention, the solvent in the modified solution contains water. In addition, an auxiliary solvent is also allowed to be included. The auxiliary solvent includes at least one of methanol, ethanol, isopropanol, acetone, methyl ketone, dimethyl sulfoxide, and N,N-dimethylformamide. Research shows that through the polar system constructed by the auxiliary solvent and water, further in combination with the additive A, additive B, and the variable pressure technique, it helps to further optimize the interlayer and surface structures of graphite, helps to further synergistically improve the dispersion distribution form and interfacial bonding effect of waste graphite materials in the carbon fiber matrix, and thus is beneficial to synergistically improving the fast charging and wide temperature range stability of the prepared materials.

[0027] Preferably, in the modified solution, the volume ratio of water to the auxiliary solvent is 1 to 20:1; further can be 5 to 15:1.

[0028] In the present invention, in step 1, the gas includes carbon dioxide.

[0029] Preferably, the gas further includes at least one of oxygen and air. Research in the present invention shows that by using the composite atmosphere, further in combination with the modified solution and variable gas combination, it can further optimize the super fast charging and wide temperature range stability of the prepared materials.

[0030] In the present invention, in the composite atmosphere, the content of carbon dioxide is above 50 wt.%, and further can be 80 to 95 wt.%.

[0031] Preferably, the positive pressure is 0.5 to 6 MPa, and further can be 2 to 5.5 MPa; further can be 3 to 5 MPa.

[0032] Preferably, the holding time under positive pressure is 5 to 60 min, and further can be 20 to 40 min.

[0033] Preferably, the negative pressure is below -0.01 MPa.

[0034] Preferably, the time from positive pressure to negative pressure is within 1 min.

[0035] Preferably, the number of gas change treatments is 1 to 5 times. The positive pressure - negative pressure step is one gas change treatment process. In the present invention, multiple gas change treatment processes can be carried out as needed.

[0036] Preferably, the temperature of the gas change treatment is 0°C to 80°C, and further can be 20 to 50°C.

[0037] In the present invention, in step 1, the dual - frequency ultrasonic treatment process includes a pre - carried out low - frequency ultrasonic process with a power of 10 to 50 kHz; and a subsequent high - frequency ultrasonic treatment process with a power of 60 to 100 kHz.

[0038] Further, the power of the low - frequency ultrasonic is 20 to 40 kHz. The power of the high - frequency ultrasonic is 75 to 95 kHz.

[0039] In the present invention, the treatment times of the low - frequency ultrasonic and the high - frequency ultrasonic can be more than 10 minutes respectively, further can be 20 to 80 minutes; further can be 40 to 70 min.

[0040] In the present invention, the sequence of low - frequency ultrasonic first and then high - frequency ultrasonic is one dual - frequency ultrasonic process. In the present invention, 1 to 5 times, preferably 3 to 5 times of dual - frequency ultrasonic treatment processes can be carried out as needed. Research shows that on the premise of the same total treatment time, multi - stage short - time dual - frequency ultrasonic treatment can help to further optimize the physical and chemical structure of the material, and help to further optimize the high - rate and wide - temperature - range performance of the prepared material.

[0041] In the present invention, the temperature in the dual - frequency ultrasonic treatment stage can be 0°C to 80°C, and further can be 20 to 60°C.

[0042] Research in the present invention shows that under the combination of the modified solution and the gas change technology, further combined with the dual - frequency ultrasonic treatment, the interlayer and physical and chemical structure of waste graphite can be further optimized, which helps to further optimize the distribution and morphology of regenerated graphite in carbon fiber, and further can optimize the fast - charging and wide - temperature - range stability of the prepared material.

[0043] In the present invention, the modified graphite and the polymer are slurried to obtain a polymer solution, and then fiberization, carbonization and activation treatments are carried out to prepare carbon fiber modified with waste graphite.

[0044] In the present invention, in step 2, the polymer includes at least one of phenolic resin, epoxy resin, furan resin, polyethylene oxide, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinylidene fluoride, polyacrylonitrile, polystyrene, polycaprolactone, polyamide, polylactic acid, polyethersulfone, polyurethane, cellulose acetate, chitosan, silk fibroin, collagen. There is no special requirement for the molecular weight of the polymer, as long as it meets the application requirements in the battery field.

[0045] In Step 2, the weight ratio of the modified graphite to the polymer is 1:5 to 50, preferably 1:10 to 25.

[0046] Preferably, in the graphite polymer solution, the solid content is 5 to 30 wt.%, and further can be 15 to 25 wt.%.

[0047] Preferably, the fibrillating process is an electrostatic method.

[0048] Both the means and parameters of the electrostatic method can be well-known. For example, its voltage can be 10 kV to 30 kV. The collection distance can be 10 cm to 20 cm. The injection rate can be between 0.1 ml / h and 10 ml / h. The temperature is maintained at room temperature.

[0049] Preferably, the curing temperature is 50 to 250 °C, and further can be 150 to 250 °C;

[0050] The curing time can be 0.5 to 72 h, and further can be 10 to 50 h.

[0051] Preferably, the carbonization temperature is 500 to 1000 °C, and further can be 750 to 850 °C.

[0052] The carbonization time can be 0.5 to 5 h, and further can be 1 to 3 h.

[0053] Preferably, the activator in the activation stage includes at least one of water vapor, CO2, KOH, alkali metal salts, phosphoric acid, and zinc chloride.

[0054] Preferably, the weight ratio of the carbonized product to the activator is 1:1 to 5; further can be 1:1 to 3.

[0055] Preferably, the activation time is 1 to 8 h, and further can be 1 to 3 h.

[0056] Preferably, the activation temperature is 700 °C to 1000 °C; further can be 800 to 950 °C.

[0057] In the present invention, the carbon fiber is subjected to silicon deposition treatment in a silane atmosphere to obtain the waste graphite-based fibrous silicon carbide composite material;

[0058] Preferably, the silane includes at least one of silane, disilane, trisilane, tetrasilane, chlorosilane, and hexachloroethylsilane;

[0059] Preferably, the temperature of silicon deposition is 400 to 1000 °C, and further can be 450 to 650 °C.

[0060] The time of silicon deposition can be adjusted as needed, for example, it can be 1 to 10 h.

[0061] In the present invention, after vapor deposition of silicon, carbon coating treatment can be further carried out by conventional processes, and further heat treatment can be carried out in a vapor-phase carbon source for vapor-phase carbon deposition treatment.

[0062] The vapor-phase carbon source described in the present invention can be an alkane, an alkene, an alkyne, etc.

[0063] In the present invention, in the vapor deposition of silicon stage, the carbon source content in the atmosphere can be adjusted as needed, that is, in its atmosphere system, a diluent gas is also allowed to exist, which can specifically be an inert gas such as nitrogen, argon, etc. In the deposition stage, the content of the vapor-phase carbon source atmosphere can be 10-90 v%, and further can be 40-60 v%.

[0064] In the present invention, the time for vapor-phase carbon deposition can be 0.5-3 h, and further can be 1-2 h.

[0065] In the present invention, the temperature in the vapor-phase carbon deposition stage can be 500-1000 °C, and further can be 550-650 °C.

[0066] The present invention also includes the waste graphite-based fibrous silicon-carbon composite material prepared by the described preparation method.

[0067] In the present invention, the described preparation method can endow the prepared material with special physical and chemical characteristics, and the waste graphite-based fibrous silicon-carbon composite material prepared by the described preparation method can significantly improve the performance advantages such as fast charging and wide-temperature stability of the material.

[0068] The present invention also provides an application of using the waste graphite-based fibrous silicon-carbon composite material as a negative electrode active material to prepare a lithium-ion battery.

[0069] The present invention also includes a negative electrode material for a lithium-ion battery, wherein the negative electrode active material contains the waste graphite-based fibrous silicon-carbon composite material. In addition, components such as a binder and a conductive agent are also allowed to exist, and the weight of each component can be reasonably controlled based on conventional means.

[0070] The present invention also provides a negative electrode for a lithium-ion battery, including a current collector and a negative electrode material compounded on its surface, wherein the negative electrode material can be the negative electrode material containing the waste graphite-based fibrous silicon-carbon composite material described in the present invention.

[0071] The present invention also provides a lithium-ion battery, including the negative electrode containing the waste graphite-based fibrous silicon-carbon composite material described in the present invention.

[0072] The lithium-ion battery, negative electrode and negative electrode material described in the present invention, except for containing the waste graphite-based fibrous silicon-carbon composite material described in the present invention, other components, structures and contents can be well-known or reasonably controlled based on well-known means.

[0073] Beneficial effects

[0074] In the present invention, the waste graphite is innovatively subjected to the variable pressure treatment and dual-frequency ultrasonic treatment in the modified solution, so as to facilitate the obtaining of modified graphite with a thin layer and excellent surface properties based on the waste graphite material. Further, it is subjected to subsequent fibrillation, carbonization, activation and silicon deposition treatment, so that the dispersion morphology and uniformity of graphite in the carbon substrate can be improved, which is conducive to constructing a fast electron and ion transport network, and is conducive to regenerating the waste graphite to obtain a negative electrode active material that takes into account fast charging and wide temperature range stability.

[0075] The research of the present invention also shows that the combined control of the variable pressure atmosphere, solvent and dual-frequency ultrasonic and other processes in the modification process of waste graphite can further optimize the interlayer and surface physical and chemical structures of waste graphite, which helps to further improve the dispersion state of waste graphite-based modified graphite in carbon fibers and the interface with the carbon matrix, helps to reconstruct an efficient transport network, and can obtain better performances such as fast charging and wide temperature range cycle stability. Description of the drawings

[0076] Figure 1 SEM diagram of the modified graphite prepared in step 1 of Example 1;

[0077] Figure 2 Diagram of the waste graphite-based fibrous silicon-carbon composite material prepared in Example 1. Detailed implementation manners

[0078] The waste graphite is sourced from the negative electrode sheets of retired lithium-ion batteries. After pretreatment to remove most of the non-active substances such as electrode sheets, conductive agents and binders, a waste graphite material with a graphite content > 80 wt.% is obtained.

[0079] Example 1:

[0080] Step 1:

[0081] First, take waste graphite materials and disperse them in a modified solution (liquid-solid ratio is 50 mL / g). The modified solution is prepared with deionized water and contains 2 mol / L nitric acid as additive A and 1 wt.% (based on the weight of waste graphite) cetyltrimethylammonium bromide as additive B. Place the mixture in a high-pressure reactor and pressurize it to 3 MPa using pure carbon dioxide gas (variable pressure atmosphere or pressurized atmosphere), and maintain this pressure for 30 minutes. Subsequently, quickly release the pressure to -0.05 MPa, and complete the switching process within 30 seconds to achieve variable pressure treatment, and control the temperature of the variable pressure treatment to be 40 °C.

[0082] Next, transfer the system after variable pressure treatment to an ultrasonic device for dual-frequency ultrasonic treatment: first treat it with low-frequency ultrasonic waves of 30 kHz for 60 minutes, and then treat it with high-frequency ultrasonic waves of 80 kHz for 60 minutes, and control the temperature of the ultrasonic treatment to be 50 °C. After the ultrasonic treatment is completed, separate the solid and liquid by a centrifuge at 8000 rpm. The obtained solid is washed 3 times with deionized water and then vacuum dried at 80 °C for 12 hours to obtain modified graphite. SEM is shown in Figure 1 .

[0083] Step 2:

[0084] Mix the obtained modified graphite with a thermosetting phenolic resin ethanol solution (molecular weight is about 3000) at a weight ratio of 1:15 (the weight ratio of modified graphite to thermosetting phenolic resin is 1:15), add a spinning aid polyvinyl butyral (molecular weight is 300,000), and stir for 24 hours to form a uniform graphite-polymer solution. The concentration of polyvinyl butyral in the solution is controlled at 0.5%, and the solid content of the graphite-polymer solution is controlled at 20 wt.%. Subsequently, use an electrospinning device for fibrillation: set the spinning voltage to 20 kV, the distance between the nozzle and the collection plate to 15 cm, the injection rate to 5 mL / h, maintain the ambient temperature at 25 °C, and collect a fibrous precursor. Cure the precursor at 150 °C for 12 h, then place the cured precursor in a tubular furnace, heat it to 800 °C at a rate of 5 °C / min under nitrogen protection, and hold for 2 hours for carbonization. Mix the carbonized product with an activator KOH at a weight ratio of 1:2, heat it to 900 °C at a rate of 5 °C / min in a nitrogen atmosphere, activate for 2 hours, cool it, wash it with 1 mol / L hydrochloric acid until neutral, then wash it 3 times with deionized water, and dry it at 80 °C to obtain waste graphite modified porous carbon fiber.

[0085] Step 3:

[0086] The modified porous carbon fiber was placed in a chemical vapor deposition furnace, and silane gas (60 v% silane, 40 v% nitrogen) was introduced, and silicon was deposited at 500 °C for 6 hours. Subsequently, the gas was switched to acetylene gas (50 v% carbon source acetylene, 50 v% nitrogen) (carbon source content 50 v%, diluent gas is nitrogen), and gas-phase carbon deposition was carried out at 600 °C for 1.5 hours. After cooling, a waste graphite-based fibrous silicon-carbon composite material was obtained. The structure is shown in Figure 2 。

[0087] Test:

[0088] The waste graphite-based fibrous silicon-carbon composite material powder, acetylene black, and PAA in Example 1 were mixed and slurried in a mass ratio of 7:1.5:1.5, and then uniformly coated on a copper foil current collector. After the coated copper foil was dried at a constant temperature of 120 °C for 720 min under vacuum conditions, it was cut into circular pieces with the same size and a diameter of 12 mm using a mold, and the electrode sheet to be tested was obtained. Using metallic lithium as the negative electrode, 1 M LiPF6 was dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1, and 5 wt% fluoroethylene carbonate (FEC) was added as an additive to enhance the stability of the SEI film, and a PE-PP composite film was used as the separator. A CR2025 type coin cell was assembled in a glove box filled with argon for electrochemical performance testing. Among them, at 25 °C, 500 charge-discharge cycles were carried out at 3C to evaluate fast charging and cycle stability. At 60 °C and 0 °C, 200 charge-discharge cycles were carried out at 0.1C respectively to evaluate the wide-temperature stability.

[0089] In the examples:

[0090] At 25 °C, 500 charge-discharge cycles were carried out at 3C: the initial capacity was 1200 mAh / g, and the capacity retention rate after 500 cycles was 68%;

[0091] At 60 °C, 200 charge-discharge cycles were carried out at 0.1C: the initial capacity was 1940 mAh / g, and the capacity retention rate after 200 cycles was 80%;

[0092] At 0 °C, 200 charge-discharge cycles were carried out at 0.1C: the initial capacity was 1388 mAh / g, and the capacity retention rate after 200 cycles was 81%.

[0093] Example 2:

[0094] Compared with Example 1, the difference is only that the graphite modification scheme is changed. The experimental groups are as follows:

[0095] Group A:

[0096] The auxiliary agent B is Tween 80, and other operations, steps, and parameters are the same as those in Example 1.

[0097] Group B:

[0098] Ethanol was added as an auxiliary solvent to the modified solution, with a volume ratio of water to ethanol of 9:1. Other operations, steps, and parameters were the same as those in Example 1.

[0099] Group C:

[0100] The graphite was subjected to variable pressure treatment under a composite gas, and the pressurizing atmosphere was a composite atmosphere of CO2 and O2 with a volume ratio of 9:1. Other operations, steps, and parameters were the same as those in Example 1.

[0101] Other operations, parameters, and tests were the same as those in Example 1.

[0102] Example 3:

[0103] Compared with Example 1, the only difference was that after the variable gas treatment, an ultrasonic treatment scheme was carried out. The specific groups were as follows:

[0104] Group A: Case of high-frequency single-segment ultrasound

[0105] The dual-frequency ultrasonic treatment was modified to high-frequency ultrasonic treatment. The steps were to treat with 80 kHz high-frequency ultrasonic waves for 120 minutes, and other steps were the same as those in Example 1.

[0106] Group B: Case of low-frequency single-segment ultrasound

[0107] The dual-frequency ultrasonic treatment was modified to low-frequency ultrasonic treatment. The steps were to treat with 30 kHz low-frequency ultrasonic waves for 120 minutes, and other steps were the same as those in Example 1.

[0108] Group C:

[0109] During the dual-frequency ultrasonic treatment, the low-frequency treatment time was 15 min, the high-frequency treatment time was 15 min, and the low-frequency - high-frequency treatment was repeated four times. Other operations and parameters were the same as those in Example 1.

[0110] Other operations, parameters, and tests were the same as those in Example 1.

[0111] Example 4:

[0112] Step 1:

[0113] First, take waste graphite materials and disperse them in a modified solution (liquid-solid ratio of 40 mL / g). The modified solution was prepared with deionized water and contained 1.5 mol / L nitric acid as auxiliary agent A and 1.5 wt.% (based on the weight of the waste graphite) cetyltrimethylammonium bromide as auxiliary agent B. The mixture was placed in a high-pressure reactor and pressurized to 5 MPa using pure carbon dioxide gas (atmosphere) and maintained at this pressure for 25 minutes. Subsequently, the pressure was rapidly released to -0.03 MPa, and the switching process was completed within 60 seconds to achieve variable pressure treatment, and the temperature of the variable pressure treatment was controlled at 50 °C.

[0114] Next, transfer the system after voltage transformation to an ultrasonic device for dual-frequency ultrasonic treatment: first, treat it with low-frequency ultrasonic waves of 25 kHz for 50 minutes, then treat it with high-frequency ultrasonic waves of 90 kHz for 50 minutes, and control the temperature of the ultrasonic treatment at 45 °C. After the ultrasonic treatment is completed, separate the solid and liquid by a centrifuge at 7000 rpm. The obtained solid is washed 3 times with deionized water and then vacuum-dried at 80 °C for 12 hours to obtain modified graphite.

[0115] Step 2:

[0116] Mix the obtained modified graphite with a thermosetting phenolic resin ethanol solution (molecular weight of about 3000) at a weight ratio of 1:20 (the weight ratio of modified graphite to thermosetting phenolic resin is 1:20), add a spinning aid polyvinyl butyral (molecular weight of 300,000), and stir for 24 hours to form a uniform graphite polymer solution. The concentration of polyvinyl butyral in the solution is controlled at 0.5%, and the solid content of the graphite polymer solution is controlled at 25 wt.%. Subsequently, use an electrospinning device for fibrillation: set the spinning voltage at 25 kV, the distance between the nozzle and the collecting plate at 18 cm, the injection rate at 4.5 mL / h, maintain the ambient temperature at 28 °C, and collect a fibrous precursor. Cure the precursor at 200 °C for 10 h, then place the cured precursor in a tubular furnace, heat it to 900 °C at a rate of 4 °C / min under nitrogen protection, and hold for 1.5 hours for carbonization. Mix the carbonized product with ZnCl2 at a weight ratio of 1:2.5, heat it to 850 °C at a rate of 4 °C / min in a nitrogen atmosphere, activate for 2.5 hours, cool it, wash it with 1.5 mol / L hydrochloric acid until neutral, then wash it 3 times with deionized water, and dry it at 80 °C to obtain waste graphite-modified porous carbon fiber.

[0117] Step 3:

[0118] Place the modified porous carbon fiber in a chemical vapor deposition furnace, introduce silane gas (50 v% silane, 50 v% nitrogen), and deposit silicon at 450 °C for 8 hours. Subsequently, switch to a carbon source acetylene gas (40 v% acetylene, 60 v% nitrogen) (the carbon source content is 40%, and the diluent gas is nitrogen), and carry out gas-phase carbon deposition at 550 °C for 2 hours. After cooling, obtain a waste graphite-based fibrous silicon carbide composite material. The test is shown in Example 1.

[0119] Comparative Example 1

[0120] Compared with Example 1, the difference is only that the waste graphite does not undergo the treatment in Step 1, directly proceeds to Step 2 and subsequent treatments, and other operations, parameters, and tests are the same as those in Example 1.

[0121] Comparative Example 2

[0122] Compared with Example 1, the only difference is that in step 1, the gas change treatment is not performed, that is, the original positive pressure of the treatment process and the pressure of the negative pressure treatment process are maintained at normal pressure, and other operations, parameters and tests are the same as in Example 1.

[0123] Comparative Example 3

[0124] Compared with Example 1, the only difference is that in step 1, no additive A and additive B are added to the modified solution, which is pure water. Other operations, parameters and tests are the same as those in Example 1.

[0125] Comparative Example 4

[0126] Compared with Example 1, the only difference is that in step 1, no auxiliary agent A and auxiliary agent B, specifically water, are added to the modified solution. After the gas change treatment is completed, the auxiliary agent A and auxiliary agent B equal to those in Example 1 are added to the system after the gas change treatment for stirring and modification treatment, and then the obtained product is subjected to step 2 and subsequent treatments. Other operations, parameters, and tests are the same as those in Example 1.

[0127] Comparative Example 5

[0128] Compared with Example 1, the only difference is that in step 2, the activator is omitted, and other operations, parameters and tests are the same as in Example 1.

[0129] Table 1

[0130]

[0131] It can be seen from Example 1 and the comparative example that the waste graphite is innovatively subjected to the voltage change treatment and the dual-frequency ultrasonic treatment in the modified solution, which is conducive to obtaining modified graphite with a thin layer and excellent surface properties based on the waste graphite material, and further subjected to subsequent fiberization, carbonization, activation and silicon precipitation treatment, which can improve the dispersed distribution morphology and uniformity of graphite in the carbon substrate, facilitate the construction of a fast electron and ion transmission network, and facilitate the regeneration of waste graphite to obtain a negative electrode active material that takes into account fast charging and stability in a wide temperature range.

[0132] In the present invention, it can be seen from Examples 1 and 2B to 2C that during the gas change treatment stage, the combined control of the solvent and atmosphere of the system can unexpectedly further enhance the process synergy, which helps to further improve the fast charging and wide temperature range performance of the material.

[0133] By comparing Examples 1 and 3, it can be seen that the dual-frequency gas-changing treatment described in the present invention, especially the multi-stage dual-frequency treatment (such as Example 3C), can achieve synergy and enhance the fast charging and wide temperature range stability of the recovered graphite.

Claims

1. A preparation method of a waste graphite-based fibrous silicon-carbon composite material, characterized in that the steps Comprising: Step 1: Disperse the waste graphite material in a modified solution and then perform variable-pressure treatment; the variable-pressure treatment includes a process of pressurizing with a gas to a positive pressure, and a process of maintaining the positive pressure and then switching the system pressure to normal pressure or negative pressure; the modified solution is an aqueous solution in which auxiliary agent A and auxiliary agent B are dissolved; wherein, the auxiliary agent A is an oxidizing component and / or an acidic component; the auxiliary agent B is a component with surface activity; Perform dual-frequency ultrasonic treatment on the system after the variable-pressure treatment, and then perform solid-liquid separation to obtain modified graphite; Step 2: Slurry the modified graphite prepared in Step 1 and a polymer to form a graphite-polymer solution, and then perform fibrillation, curing, carbonization, and activation treatments to obtain waste graphite-modified carbon fiber; Step 3: Use the waste graphite-modified carbon fiber as a substrate to perform silicon deposition and surface carbon coating treatment to obtain the waste graphite-based fibrous silicon-carbon composite material.

2. The preparation method of the waste graphite-based fibrous silicon-carbon composite material according to claim 1, characterized in that The waste graphite material is a graphite-containing material recovered from the negative electrode of a waste battery; Preferably, in the waste graphite material, at least one of a conductive agent, a binder, and a current collector is also allowed to be included; Preferably, the content of graphite in the waste graphite material is 80 wt.% or more.

3. The preparation method of the waste graphite-based fibrous silicon-carbon composite material according to claim 1, characterized in that, In the modified solution, the auxiliary agent A includes at least one of nitric acid, sulfuric acid, and potassium permanganate; The auxiliary agent B includes at least one of cetyltrimethylammonium bromide, dodecyltrimethylammonium bromide, cetylpyridinium chloride, sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, sodium stearate, polyethylene glycol, polyvinylpyrrolidone, Tween 20, Tween 80, F-127, sodium polyacrylate, and polyacrylamide; Preferably, in the modified solution, the concentration of auxiliary agent A is 0.5 to 5 mol / L; the concentration of auxiliary agent B is 0.05 to 5 wt.%; Preferably, in the mixed solution, the liquid-solid ratio is 5 to 100 mL / g.

4. The preparation method of the waste graphite-based fibrous silicon-carbon composite material according to claim 1 or 3, characterized in that, In the modified solution, an auxiliary solvent is further included, and the auxiliary solvent includes at least one of methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, dimethyl sulfoxide, and N,N-dimethylformamide; Preferably, in the modified solution, the volume ratio of water to the auxiliary solvent is 1 to 20:

1.

5. The preparation method of the waste graphite-based fibrous silicon-carbon composite material according to claim 1, characterized in that, In Step 1, the gas includes carbon dioxide; Preferably, in the gas, at least one of oxygen and air is also included; Preferably, the positive pressure is 0.5 to 6 MPa; Preferably, the holding time under positive pressure is 5 to 60 min; Preferably, the negative pressure is below -0.01 MPa; Preferably, the time from positive pressure to negative pressure is within 1 min; Preferably, the number of variable-gas treatment times is 1 to 5 times; Preferably, the variable-gas treatment temperature is 0°C to 80°C.

6. The preparation method of the waste graphite-based fibrous silicon carbon composite material according to claim 1, wherein, In Step 1, the dual-frequency ultrasonic treatment process includes a low-frequency ultrasonic process with a power of 10 to 50 kHz performed in advance; and a high-frequency ultrasonic treatment process with a power of 60 to 100 kHz performed subsequently.

7. The preparation method of the waste graphite-based fibrous silicon carbon composite material according to claim 1, wherein, In step 2, the polymer includes at least one of phenolic resin, epoxy resin, furan resin, polyethylene oxide, polyvinyl alcohol, polyvinylpyrrolidone, polyvinylidene fluoride, polyacrylonitrile, polystyrene, polycaprolactone, polyamide, polylactic acid, polyethersulfone, polyurethane, cellulose acetate, chitosan, silk fibroin, and collagen; In step 2, the weight ratio of the modified graphite to the polymer is 1:5 to 50, preferably 1:10 to 25; Preferably, in the graphite polymer solution, the solid content is 5 to 30 wt.%; Preferably, the process of fibrillation is electrospinning; Preferably, the curing temperature is 50 to 250 °C; Preferably, the carbonization temperature is 500 to 1000 °C; Preferably, the activator in the activation stage includes at least one of water vapor, CO2, KOH, alkali metal salts, phosphoric acid, and zinc chloride; Preferably, the weight ratio of the carbonized product to the activator is 1:1 to 5; Preferably, the activation temperature is 700 °C to 1000 °C.

8. The preparation method of the waste graphite-based fibrous silicon-carbon composite material according to claim 1, wherein, The waste graphite-based fibrous silicon-carbon composite material is prepared by subjecting carbon fiber to silicon deposition treatment in a silane atmosphere and then performing surface carbon coating; Preferably, the silane includes at least one of silane, disilane, trisilane, tetrasilane, chlorosilane, and hexachloroethylsilane; Preferably, the temperature of silicon deposition is 400 to 1000 °C, and further can be 450 to 650 °C.

9. A waste graphite-based fibrous silicon-carbon composite material prepared by the preparation method according to any one of claims 1 to 8.

10. A lithium-ion battery, characterized in that, Comprising the negative electrode according to claim 9.

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