Crimped carbon material coated silicon composite negative electrode material, negative electrode plate, secondary battery and preparation method

Abstract

The invention provides a crimped carbon material coated silicon composite negative electrode material, a negative electrode plate, a secondary battery and a preparation method, and relates to the technical field of negative electrode material preparation. The curled carbon material coated silicon composite negative electrode material comprises an inner-layer core structure, a secondary outer-layer shell structure coated outside the inner-layer core structure and an outer-layer shell structure coated outside the secondary outer-layer shell structure, the inner core structure is a silicon-based particle; the secondary outer shell structure is a curled carbon shell which is formed by ball-milling a layered carbon material, freeze-drying and freely curling; the outer shell structure is a pyrolytic carbon coating layer. The crimped carbon material coated silicon composite negative electrode material provided by the invention can absorb stress generated by volume expansion of a silicon-based material, improve material interface ion and electron transmission, and improve dynamics.

Description

Technical Field

[0001] The present invention relates to the technical field of negative electrode material preparation, and in particular to a coiled carbon material-coated silicon composite negative electrode material, a negative electrode sheet, a secondary battery, and a preparation method thereof. Background Art

[0002] Silicon-based negative electrode materials for lithium-ion batteries have advantages such as relatively low lithium intercalation potential, high theoretical specific capacity, and abundant reserves in nature, and are deeply concerned by the academic and industrial circles. They are considered to be one of the most potential ideal materials to replace commercial graphite negative electrodes in the future; however, during the process of lithium intercalation and deintercalation of silicon-based negative electrode materials, the material undergoes a large volume change (~400%). During repeated cycling, the SEI on the material surface will be continuously damaged and grown, consuming a large amount of active lithium, resulting in attenuation of electrical performance, which seriously hinders the large-scale commercial application of silicon-based negative electrode materials in lithium-ion batteries.

[0003] In order to accelerate the practical use of silicon negative electrode materials, those skilled in the art have improved the volume expansion during lithium intercalation of silicon by regulating the material morphology and constructing a buffer structure. Such solutions can improve the lithium intercalation expansion of silicon negative electrode materials to a certain extent. However, such silicon materials with special morphologies and structures often have characteristics such as high specific surface area and low tap density, and there is a risk of deterioration of high-temperature performance, and the industrial feasibility is low; therefore, while solving the problem of silicon negative electrode expansion, it is still necessary to consider the industrial feasibility of the material, the balance of material physical and chemical indexes and electrical performance;

[0004] Chinese Patent CN111333063B discloses a natural graphite-based silicon-carbon composite negative electrode material, its preparation method and application. Nanoscale silicon is deposited on the surface of natural graphite, and then mixed with molten asphalt, cooled and crushed and shaped. During the shaping process, natural flake graphite curls, and the nanoscale silicon and asphalt attached to the surface of natural flake graphite are wrapped on its inner surface by the curled graphite, and finally acidified treatment is carried out; the structure of this material has the effect of alleviating lithiation expansion and can improve the cycle performance, but the material preparation process is cumbersome and requires complex processes of gas-phase deposition of silicon and acidification treatment, and the industrial feasibility is low. On the one hand, the process of gas-phase deposition of silicon requires the use of high-risk chemical silane gas. Silane is chemically active, explosive and flammable in air, there are safety risks, the energy consumption of the silane deposition process is high, and tail waste gas is discharged; on the other hand, acidification treatment has safety hazards and hazardous waste liquid discharge, which violates the concept of low-carbon and green development.

[0005] In view of this, the present invention is specifically proposed. Summary of the Invention

[0006] The object of the present invention is to provide a silicon composite anode material wrapped with a curly carbon material, an anode sheet, a secondary battery and a preparation method thereof. The silicon composite anode material has a stress buffering effect, can absorb the stress and strain generated by the volume change during the lithium insertion and extraction process of silicon, and also has a stable and efficient conductive path, which can improve the electron and ion transport at the interface of silicon anode particles. The preparation method is simple, without the use of hazardous chemicals and the generation of hazardous wastes, and conforms to the concept of low-carbon and green development.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a silicon composite anode material wrapped with a curly carbon material, including an inner core structure, a secondary outer shell structure coated on the outside of the inner core structure, and an outer shell structure coated on the outside of the secondary outer shell structure;

[0009] The inner core structure is a silicon-based particle;

[0010] The secondary outer shell structure is a curly carbon shell formed by ball milling a layered carbon material and then freeze-drying it to curl freely;

[0011] The outer shell structure is a pyrolytic carbon coating layer.

[0012] Further, on the basis of the above technical solution, the inner core structure is a silicon-based particle formed by crushing silicon material by a ball milling method;

[0013] The outer shell structure is a pyrolytic carbon coating layer formed by depositing a carbon source material on the outside of the secondary outer shell structure by chemical vapor deposition.

[0014] Further, on the basis of the above technical solution, the median particle size Dv50 of the silicon-based particles is 0.03 - 1 μm;

[0015] and / or, the median particle size Dv50 of the curly carbon shell is 3 - 15 μm;

[0016] and / or, the thickness of the pyrolytic carbon coating layer is 3 - 500 nm, and the pyrolytic carbon coating layer is in an amorphous state.

[0017] Further, on the basis of the above technical solution, the tap density of the silicon composite anode material wrapped with the curly carbon material > 0.8 g / cm 3 ;

[0018] The specific surface area of the silicon composite anode material wrapped with the curly carbon material < 6 m 2 / g.

[0019] The present invention also provides a preparation method of the silicon composite anode material wrapped with the curly carbon material as described above, including the following steps:

[0020] S1. Ball milling: Mix silicon material, layered carbon material, binder and solvent evenly and perform ball milling to obtain a mixed slurry;

[0021] S2. Freeze drying: Transfer the mixed slurry obtained in step S1 to a freeze dryer, first perform pre-freezing and then perform sublimation drying to obtain a silicon composite material wrapped by a curly carbon shell;

[0022] S3. Surface coating: Place the composite material obtained in S2 in a chemical vapor deposition furnace. After heating, introduce a carbon source gas for pyrolytic carbonization to obtain a composite anode material with a pyrolytic carbon coating layer as the outer shell, a curly carbon shell as the sub-outer shell, and a silicon-based particle as the inner core.

[0023] Further, based on the above technical solution, in step S1, the mass ratio of the silicon material to the layered carbon material is 0.03 - 1:1;

[0024] And / or, the silicon material includes one or more of SiO x , silicon, and silicon alloy, where 0 < x ≤ 2;

[0025] And / or, the median particle size Dv50 of the silicon material is 0.5 - 1500 μm;

[0026] And / or, the layered carbon material includes one or more of expanded graphite, natural graphite, and graphene;

[0027] And / or, the median particle size Dv50 of the layered carbon material is 10 - 30 μm;

[0028] And / or, in step S1, the mass ratio of the binder to the layered carbon material is 0.005 - 0.03:1;

[0029] And / or, the binder includes one or more of sodium polyacrylate, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, guar gum, polyvinylidene fluoride, polytetrafluoroethylene, and asphalt;

[0030] And / or, in step S1, the solid content of the mixed slurry is 10 - 40%;

[0031] And / or, the solvent includes one or more of water, 50% ethanol aqueous solution, and 50% isopropanol aqueous solution.

[0032] Further, based on the above technical solution, in step S1, the conditions of the ball milling include:

[0033] The equipment is a planetary ball mill; the ball milling method is wet ball milling; the ball milling speed is 200 - 1500 r / min; the ball milling time is 30 - 60 min; the ball-to-material ratio is 5 - 15:1;

[0034] In step S2, the conditions for pre-freezing include:

[0035] The pre-freezing temperature is -20 to -60 °C, and the pre-freezing time is 1 - 5 h;

[0036] And / or, the conditions for sublimation drying include:

[0037] The temperature is -20 to -60 °C, the vacuum degree is -5 Pa to -2 Pa, and the sublimation drying time is 8 - 24 h.

[0038] Furthermore, on the basis of the above technical solution, in step S3, the mass ratio of the obtained pyrolytic carbon coating layer to the coiled carbon shell is 0.01 - 0.06:1;

[0039] And / or, in step S3, the conditions of the chemical vapor deposition furnace include:

[0040] The protective atmosphere includes one or more of nitrogen and argon;

[0041] The pyrolysis temperature is 550 - 750 °C; the deposition time is 30 - 180 min;

[0042] And / or, the carbon source gas includes one or more of methane, acetylene, ethylene, propane, n-butane, and isobutane;

[0043] And / or, the gas flow rate of the carbon source gas is 0.1 - 5 mL / min.

[0044] The present invention also provides a negative electrode sheet, which includes the coiled carbon material-coated silicon composite negative electrode material prepared by the preparation method of the coiled carbon material-coated silicon composite negative electrode material as described above or the coiled carbon material-coated silicon composite negative electrode material as described above.

[0045] The present invention also provides a secondary battery, which includes the coiled carbon material-coated silicon composite negative electrode material prepared by the preparation method of the coiled carbon material-coated silicon composite negative electrode material as described above or the coiled carbon material-coated silicon composite negative electrode material as described above or the negative electrode sheet as described above.

[0046] A coiled carbon material-coated silicon composite negative electrode material, a negative electrode sheet, a secondary battery and a preparation method provided by the present invention have the following beneficial effects:

[0047] 1. By the ball milling process, the silicon material is crushed and the carbon material layers are peeled off from each other, which helps the silicon particles to adhere to the surface of the carbon layer.

[0048] 2. Through the freeze-drying sublimation process, the layered carbon material is coiled and wraps the silicon particles to form a structure of irregular coiled carbon shell wrapping the silicon particles, and this structure serves as a stress buffer structure to absorb the stress generated by the volume expansion of the silicon-based material.

[0049] 3. The surface of the curly carbon shell is coated with pyrolytic carbon by chemical vapor deposition. The coating layer is dense and uniform, and parameters such as the specific surface area and the tapped density of the material are controllable, which helps to improve the ion and electron transport at the material interface and enhance the kinetics.

[0050] 4. The secondary battery prepared from the silicon composite anode material wrapped with the curly carbon material provided by the present invention can maintain the stable structure and good conductive stability of the material during the charge / discharge process, with a high first charge specific capacity, a high first Coulomb efficiency, a low first lithium intercalation expansion, a high 50-week cycle capacity retention rate, and a low lithium intercalation expansion. Description of the Drawings

[0051] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0052] Figure 1 SEM photograph of the silicon composite anode material coated with a curly carbon shell provided in Embodiment 1 of the present invention. Detailed Embodiments

[0053] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in combination with the embodiments of the present invention. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0054] In the ranges disclosed in the present invention, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, between the endpoints of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0055] According to the first aspect of the present invention, a silicon composite anode material wrapped with a curly carbon material is provided, including an inner core structure, a secondary outer shell structure coated outside the inner core structure, and an outer shell structure coated outside the secondary outer shell structure;

[0056] The inner core structure is a silicon-based particle;

[0057] The second outermost shell structure is a coiled carbon shell formed by freeze-drying and freely coiling the layered carbon material after ball milling;

[0058] The outermost shell structure is a pyrolytic carbon coating layer.

[0059] As an alternative embodiment of the present invention, the inner core structure is silicon-based particles formed by crushing large particle size silicon materials by ball milling;

[0060] The outermost shell structure is a pyrolytic carbon coating layer formed by depositing a carbon source material on the outside of the second outermost shell structure by chemical vapor deposition.

[0061] As an alternative embodiment of the present invention, the median particle size Dv50 of the silicon-based particles is 0.03 - 1 μm;

[0062] The median particle size Dv50 of the coiled carbon shell is 3 - 15 μm.

[0063] As an alternative embodiment of the present invention, the thickness of the pyrolytic carbon coating layer is 3 - 500 nm (such as 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, etc.), and the pyrolytic carbon coating layer is in an amorphous state.

[0064] As an alternative embodiment of the present invention, the tap density of the coiled carbon material-wrapped silicon composite anode material > 0.8 g / cm 3 (such as 0.83 g / cm 3 , 0.85 g / cm 3 , 0.87 g / cm 3 , 0.9 g / cm 3 , 0.95 g / cm 3 , 1 g / cm 3 , 1.2 g / cm 3 etc.); the specific surface area of the coiled carbon material-wrapped silicon composite anode material < 6 m 2 / g (such as 4 m 2 / g, 4.5 m 2 / g, 5 m 2 / g, 5.2 m 2 / g, 5.4 m 2 / g, 5.6 m 2 / g, 5.8 m 2 / g, etc.). A lower tap density and a high specific surface area can easily deteriorate the performance at high temperatures, leading to a decline in battery performance or potential safety hazards.

[0065] The present invention also provides a method for preparing the coiled carbon material-wrapped silicon composite anode material as described above, comprising the following steps:

[0066] S1. Ball milling: Mix silicon material, layered carbon material, binder and solvent evenly and perform ball milling to obtain a mixed slurry.

[0067] S2. Freeze-drying: Transfer the mixed slurry obtained in step S1 to a freeze-dryer, first perform pre-freezing and then sublimation drying to obtain a silicon composite material wrapped by a curly carbon shell.

[0068] S3. Surface coating: Place the composite material obtained in S2 in a chemical vapor deposition furnace. After heating, introduce a carbon source gas for pyrolytic carbonization to obtain a composite anode material with a pyrolytic carbon coating layer as the outer shell, a curly carbon shell as the sub-outer shell, and silicon-based particles as the inner core.

[0069] Specifically, in step S2 of the present invention, freeze-drying can, on the one hand, remove liquid components by low-temperature sublimation under vacuum conditions to achieve the drying purpose; on the other hand, freeze-drying can form a curly carbon shell from thinner layered carbon materials, thus more completely wrapping silicon-based particles, and the silicon-based particles wrapped by the curly carbon shell obtained after freeze-drying have good dispersibility and do not require complex processes such as granulation; the use of freeze-drying in the present invention can not only avoid the risk of material structure and component variation, but also is simple to operate and thoroughly dried.

[0070] In step S3 of the present invention, a gas-phase carbon source is used to coat a carbon layer on the material surface by chemical vapor deposition, and the advantages are as follows:

[0071] 1. High film-forming quality: The film formed by uniformly coating the material surface with a carbon source gas through chemical vapor deposition has excellent uniform density, purity and grain structure, and can precisely control the coating amount, thickness, composition and performance of the film. By adjusting process parameters such as temperature, precise control of film characteristics can be achieved.

[0072] 2. Wide application range: The chemical vapor deposition technology is applicable to substrates with various complex shapes and is suitable for large-area deposition. This technology can uniformly cover substrates with complex surface morphologies and has excellent conformal properties.

[0073] 3. High-temperature deposition: Using a high-temperature environment can promote the reaction rate and improve the crystallization integrity. It is usually carried out in a vacuum environment, which helps to improve the quality of the film.

[0074] 4. Composition control: The film components and doping levels can be accurately controlled.

[0075] 5. Simple process: Coating by chemical vapor deposition is more uniform and dense, and the coating-carbonization is completed in one step with high efficiency.

[0076] As an alternative embodiment of the present invention, in step S1, the mass ratio of the silicon material to the layered carbon material is 0.03 to 1:1, such as 0.05:1, 0.08:1, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, etc.;

[0077] Specifically, the present invention defines the mass ratio of the silicon material to the layered carbon material as 0.03 to 1:1, even if the mass of the crimped carbon shell is greater than or equal to that of the silicon-based particles, in order to avoid the expansion of the silicon negative electrode. Since silicon has a high energy storage density, when it is used as the negative electrode material of a lithium-ion battery, it can significantly improve the overall energy capacity of the battery. However, a significant disadvantage of silicon is that it will undergo significant volume expansion during charge and discharge. If the mass ratio of the silicon material to the layered carbon material is 2:1, that is, the content of silicon is relatively high, from a performance perspective, it will not only exacerbate the volume expansion problem of the silicon negative electrode during charge and discharge cycles, affecting the insertion and extraction efficiency of lithium ions, but also pose a risk of deteriorating the overall electrical performance of the prepared lithium-ion battery.

[0078] The silicon material is a conventional silicon material in the art, typically but not limited to including SiO x , silicon, one or more of silicon alloys, where 0 < x ≤ 2;

[0079] The median particle size Dv50 of the silicon material is 0.5 - 1500 μm (such as 100 μm, 500 μm, 800 μm, 1000 μm, etc.);

[0080] The layered carbon material typically but not limited to includes one or more of expanded graphite, natural graphite, graphene;

[0081] The median particle size Dv50 of the layered carbon material is 10 - 30 μm (such as 15 μm, 20 μm, 25 μm, etc.).

[0082] As an alternative embodiment of the present invention, in step S1, the mass ratio of the binder to the layered carbon material is 0.005 - 0.03:1 (such as 0.01:1, 0.015:1, 0.02:1, 0.025:1, etc.);

[0083] The binder typically but not limited to includes one or more of sodium polyacrylate, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, guar gum, polyvinylidene fluoride, polytetrafluoroethylene, asphalt;

[0084] In step S1, the solid content of the mixed slurry is 10 - 40% (such as 15%, 20%, 25%, 30%, 35%, etc.);

[0085] The solvent typically includes, but is not limited to, one or more of water, 50% aqueous ethanol solution, and 50% aqueous isopropanol solution;

[0086] As an alternative embodiment of the present invention, in step S1, the purpose of ball milling is to attach the silicon material to the surface of the layered carbon material, and the conditions include:

[0087] The equipment is a planetary ball mill; the ball milling method is wet ball milling; the ball milling speed is 200 - 1500 r / min; the ball milling time is 30 - 60 min; the ball-to-material ratio is 5 - 15:1.

[0088] As an alternative embodiment of the present invention, in step S2, the conditions for pre-freezing include:

[0089] The pre-freezing temperature is -20 to -60 °C (such as -30 °C, -40 °C, -50 °C, etc.), and the pre-freezing time is 1 - 5 h (such as 2 h, 3 h, 4 h, etc.);

[0090] Specifically, the purpose of pre-freezing is to lower the temperature of the material-solution mixture below the eutectic point of the solution and completely freeze it into a solid in advance, thereby ensuring the solidification of the liquid in the material and preparing for subsequent sublimation drying and curling.

[0091] The purpose of freeze-drying sublimation is to curl the layered carbon material into a curled carbon shell for wrapping the silicon-based particles. The conditions for sublimation drying include:

[0092] The temperature is -20 to -60 °C (such as -30 °C, -40 °C, -50 °C, etc.), the vacuum degree is -5 Pa to -2 Pa (such as -4 Pa, -3 Pa, etc.), and the sublimation drying time is 8 - 24 h (such as 10 h, 15 h, 20 h, 22 h, etc.).

[0093] As an alternative embodiment of the present invention, in step S3, the mass ratio of the obtained pyrolytic carbon shell to the curled carbon shell is 0.01 - 0.06:1, such as 0.02:1, 0.03:1, 0.04:1, 0.05:1, etc.

[0094] Specifically, the present invention limits the mass ratio of the pyrolytic carbon coating layer to the curled carbon shell to 0.01 - 0.06:1, aiming to find a balance point that can not only ensure the uniformity and integrity of the coating layer, effectively improve the structural stability and electronic conductivity of the material, but also avoid the loss of kinetic performance caused by an overly thick coating layer, thereby comprehensively improving the comprehensive performance of the anode material for lithium-ion batteries.

[0095] The main function of the pyrolytic carbon coating layer is to enhance the structural stability of the coiled carbon shell and provide a certain degree of electronic conductivity to facilitate the rapid transport of lithium ions. If the mass of the pyrolytic carbon coating layer is excessive, such as 0.1:1, and the coating layer is relatively thick, this not only increases the overall mass of the anode material and reduces the energy density, but also the overly thick coating layer will significantly extend the migration path of lithium ions. During the charge and discharge process of the battery, lithium ions need to migrate frequently between the electrolyte and the anode material. The extension of the path will increase the migration resistance, thereby reducing the kinetic performance of the material and affecting the charge and discharge rate and efficiency of the battery;

[0096] If the mass of the pyrolytic carbon coating layer is too small, such as 0.005:1, the overly thin coating layer poses extremely high requirements for process control. In actual production, it is difficult to achieve a thin and uniform coating layer, and high-precision coating equipment and technologies often need to be used, increasing the production cost. More importantly, the thin coating layer may not be able to completely cover the surface of the coiled carbon shell, resulting in incomplete coating and exposed areas. These exposed areas not only lose the structural support and electronic conductivity provided by the pyrolytic carbon coating layer, but may also become active sites during the battery cycling process, accelerating the decomposition of the electrolyte and side reactions, thereby damaging the cycle stability and overall performance of the material.

[0097] As an optional implementation manner of the present invention, in step S3, the conditions of the chemical vapor deposition furnace include:

[0098] The protective atmosphere typically includes, but is not limited to, one or more of nitrogen and argon; the pyrolysis temperature is 550 - 750 °C; the deposition time is 30 - 180 min;

[0099] The carbon source gas typically includes, but is not limited to, one or more of methane, acetylene, ethylene, propane, n-butane, and isobutane;

[0100] The gas flow rate of the carbon source gas is 0.1 - 5 mL / min (such as 0.5 mL / min, 1 mL / min, 2 mL / min, 3 mL / min, 4 mL / min, 4.5 mL / min, etc.).

[0101] Specifically, the present invention limits the gas flow rate to 0.1 - 5 mL / min, aiming to control the quality of the coating layer. If the gas flow rate of the carbon source gas is too low, it will affect the uniformity and thickness of the deposited layer; while if the flow rate is too high, it may lead to an overly intense reaction process, a rough surface of the deposited layer, or the formation of independent carbon materials, affecting the material yield.

[0102] According to the third aspect of the present invention, a negative electrode sheet is provided, including the coiled carbon material-wrapped silicon composite anode material as described above or the coiled carbon material-wrapped silicon composite anode material prepared by the preparation method as described above.

[0103] According to the fourth aspect of the present invention, a secondary battery is provided, which includes a crimped carbon material-coated silicon composite anode material prepared by the preparation method of the crimped carbon material-coated silicon composite anode material as described above, or the crimped carbon material-coated silicon composite anode material as described above, or the anode sheet as described above.

[0104] In an embodiment of the present invention, the initial de-lithiation specific capacity of the secondary battery prepared by using the crimped carbon material-coated silicon composite anode material provided by the present invention is > 400 mAh / g;

[0105] The initial Coulombic efficiency of the secondary battery is > 88%;

[0106] The initial lithium insertion expansion of the processed electrode sheet of the secondary battery is < 35%;

[0107] The lithium insertion expansion of the secondary battery is < 45% after 50 cycles of constant current charge and discharge at 45 °C with 1C / 1C.

[0108] The present invention will be further described in detail below with specific embodiments and comparative examples.

[0109] Example 1

[0110] S1 Ball milling: 500 g of 2-μm silicon monoxide particles, 1000 g of graphene material (median particle size Dv50 is 10 μm), 20 g of sodium carboxymethyl cellulose, 1013.5 g of deionized water, and 1013.5 g of 50% ethanol aqueous solution are mixed evenly, placed in a planetary ball mill, and wet ball milling is carried out to obtain a mixed slurry with nanoscale silicon monoxide attached to the surface of graphene;

[0111] Among them, the rotation speed of the ball milling is 500 r / min, the mass ratio of the ball to the total mass of the silicon material, layered carbon material, and binder is 10:1, and the ball milling time is 40 min.

[0112] S2 Freeze drying: The mixed slurry obtained in step S1 is pre-frozen in a freeze drying device at -40 °C for 3 h, and then vacuumed to -4 Pa for sublimation drying for 12 h to obtain a crimped graphene-coated nanoscale silicon monoxide particle composite anode material;

[0113] S3 Surface coating: The composite anode material obtained in S2 is placed in a chemical vapor deposition furnace, nitrogen gas as a protective gas is introduced, the temperature is raised to 650 °C, acetylene gas is introduced, the gas flow rate is 0.15 mL / min, vapor deposition of carbon material is carried out on the surface of the material, and the deposition ends after 180 min to obtain a crimped graphene-coated nanoscale silicon monoxide composite anode material with pyrolytic carbon material coated on the surface;

[0114] As Figure 1 shown, it is the scanning electron microscope image of the crimped carbon shell-coated silicon composite anode material prepared in this example;

[0115] Among them, the thickness of the pyrolytic carbon coating layer is 150 nm, and the tap density of the negative electrode material is 0.96 g / cm 3 , and the specific surface area is 2.9 m 2 / g;

[0116] The mass ratio of the obtained pyrolytic carbon coating layer to the crumpled graphene shell is 0.01:1.

[0117] Example 2

[0118] S1 Ball milling: 600 g of 2-μm silicon particles, 1200 g of graphene material (median particle size Dv50 is 10 μm), 24 g of sodium carboxymethyl cellulose, 1216 g of deionized water, and 1216 g of 50% ethanol aqueous solution are mixed evenly and placed in a planetary ball mill for wet ball milling to obtain a mixed slurry with nano-silicon attached to the surface of graphene;

[0119] Among them, the rotation speed of the ball milling is 500 r / min, the mass ratio of the balls to the total mass of the silicon material, layered carbon material, and binder is 10:1, and the ball milling time is 40 min.

[0120] S2 Freeze drying: The slurry in step S1 is pre-frozen in a freeze drying equipment at -40 °C for 3 h, and then vacuumed to -4 Pa for sublimation drying for 12 h to obtain a composite material of crumpled graphene-wrapped nano-silicon particles.

[0121] S3 Surface coating: The composite negative electrode material obtained in S2 is placed in a chemical vapor deposition furnace, nitrogen gas as the protective gas is introduced, the temperature is raised to 650 °C, acetylene gas is introduced, the gas flow rate is 0.27 mL / min, and gas phase deposition coating is carried out on the material surface for 180 min to obtain a composite negative electrode material of crumpled graphene-wrapped nano-silicon coated with pyrolytic carbon material.

[0122] Among them, the thickness of the pyrolytic carbon coating layer is 200 nm, and the tap density of the negative electrode material is 1.03 g / cm 3 , and the specific surface area is 2.1 m 2 / g;

[0123] The mass ratio of the obtained pyrolytic carbon coating layer to the crumpled graphene shell is 0.018:1.

[0124] Example 3

[0125] S1 Ball milling: 45 g of 5-μm silicon particles, 1500 g of expanded graphite (median particle size Dv50 is 13 μm), 7.5 g of sodium alginate, and 1725 g of deionized water are mixed evenly and placed in a planetary ball mill for wet ball milling to obtain a mixed slurry with nano-silicon attached to the surface of the graphite layer.

[0126] Among them, the rotation speed of the ball milling is 500 r / min, the mass ratio of the balls to the total mass of the silicon material, the layered carbon material and the binder is 10:1, and the ball milling time is 40 min.

[0127] S2 Freeze-drying: The slurry in step S1 is pre-frozen in a freeze-drying device at -20 °C for 1 h, and vacuumized to -5 Pa for sublimation drying for 8 h to obtain a composite material of nanosilicon particles wrapped by curly expanded graphite layers.

[0128] S3 Surface coating: The composite anode material obtained in S2 is placed in a chemical vapor deposition furnace, nitrogen gas as the protective gas is introduced, the temperature is raised to 550 °C, methane gas is introduced, the gas flow rate is 0.72 mL / min, and carbon material is deposited on the surface of the material. The deposition ends after 150 min to obtain a composite anode material of nanosilicon wrapped by curly expanded graphite layers coated with pyrolytic carbon material.

[0129] Among them, the thickness of the pyrolytic carbon coating layer is 250 nm, the tap density of the anode material is 1.17 g / cm 3 and the specific surface area is 2.8 m 2 / g;

[0130] The mass ratio of the obtained pyrolytic carbon coating layer to the curly expanded graphite shell is 0.026:1.

[0131] Example 4

[0132] S1 Ball milling: 800 g of silicon particles with a median particle size of 20 μm, 800 g of natural graphite (the median particle size Dv50 is 17 μm), 24 g of sodium polyacrylate, 1353.3 g of deionized water and 1353.3 g of 50% isopropyl alcohol aqueous solution are mixed evenly and placed in a ball mill for wet ball milling to obtain a mixed slurry with nanosilicon attached to the surface of the graphite layer;

[0133] Among them, the rotation speed of the ball milling is 500 r / min, the mass ratio of the balls to the total mass of the silicon material, the layered carbon material and the binder is 10:1, and the ball milling time is 40 min.

[0134] S2 Freeze-drying: The slurry in step S1 is pre-frozen in a freeze-drying device at -60 °C for 5 h, and then vacuumized to -2 Pa for sublimation drying for 24 h to obtain a composite material of nanosilicon particles wrapped by curly natural graphite layers.

[0135] S3 Surface coating: The composite anode material obtained in S2 is placed in a chemical vapor deposition furnace, nitrogen gas as the protective gas is introduced, the temperature is raised to 750 °C, ethylene gas is introduced, the gas flow rate is 1.17 mL / min, and carbon material is deposited on the surface of the material. The deposition ends after 180 min to obtain a composite anode material of nanosilicon wrapped by curly natural graphite layers coated with pyrolytic carbon material.

[0136] Among them, the thickness of the pyrolytic carbon coating layer is 400 nm, and the tap density of the negative electrode material is 1.10 g / cm 3 , and the specific surface area is 3.2 m 2 / g;

[0137] The mass ratio of the obtained pyrolytic carbon coating layer to the coiled natural graphite shell is 0.06:1.

[0138] Example 5

[0139] S1 Ball milling: Mix 600 g of silicon particles with a median particle size of 50 μm, 1200 g of expanded graphite (median particle size Dv50 is 14 μm), 15 g of sodium polyacrylate, 903.5 g of deionized water, and 903.5 g of 50% ethanol aqueous solution evenly, and place them in a planetary ball mill for wet ball milling to obtain a mixed slurry with nano-silicon attached to the surface of the graphite layer;

[0140] Among them, the rotation speed of the ball milling is 500 r / min, the mass ratio of the balls to the total mass of the silicon material, layered carbon material, and binder is 10:1, and the ball milling time is 40 min.

[0141] S2 Freeze-drying: Pre-freeze the slurry in step S1 in a freeze-drying equipment at -40 °C for 3 h, and then evacuate to -4 Pa for sublimation drying for 12 h to obtain a composite material with nano-silicon particles wrapped by coiled expanded graphite layers.

[0142] S3 Surface coating: Place the composite negative electrode material obtained in S2 in a chemical vapor deposition furnace, introduce the protective gas argon, heat up to 650 °C, and at the same time introduce acetylene gas and methane gas, and the gas flow rates are both 0.92 mL / min. Deposit carbon materials on the surface of the material, and end the deposition after 240 min to obtain a composite negative electrode material with coiled expanded graphite layers wrapped with pyrolytic carbon materials;

[0143] Among them, the thickness of the pyrolytic carbon coating layer is 300 nm, and the tap density of the negative electrode material is 1.05 g / cm 3 , and the specific surface area is 3.6 m 2 / g;

[0144] The mass ratio of the obtained pyrolytic carbon coating layer to the coiled expanded graphite shell is 0.045:1.

[0145] Comparative Example 1

[0146] Directly use the silicon particle material with a median particle size of 2 μm used in Example 2 as the negative electrode active material without any other treatment.

[0147] Among them, the tap density of the negative electrode material is 1.02 g / cm 3 , and the specific surface area is 4.31 m2 / g.

[0148] Comparative Example 2

[0149] S1 Ball milling: Mix 600 g of silicon particles with a median particle size of 50 μm, 1200 g of expanded graphite (median particle size Dv50 is 14 μm), 15 g of sodium polyacrylate, 903.5 g of deionized water, and 903.5 g of 50% ethanol aqueous solution evenly, and place them in a planetary ball mill for wet ball milling to obtain a mixed slurry with nano-silicon attached to the surface of the graphite layer.

[0150] Among them, the rotation speed of the ball milling is 500 r / min, the mass ratio of the balls to the total mass of the silicon material, layered carbon material, and binder is 10:1, and the ball milling time is 40 min.

[0151] S2 Drying: Place the slurry in step S1 in a vacuum drying oven and dry it under vacuum at 120 °C for 12 h to obtain a composite anode material with nano-silicon particles attached to the surface of the expanded graphite.

[0152] S3 Surface coating: Place the composite material obtained in S2 in a chemical vapor deposition furnace, introduce the inert protective gas argon, heat up to 650 °C, and at the same time introduce acetylene gas and methane gas, with the gas flow rate of both being 0.92 mL / min, and perform vapor deposition coating of carbon material on the surface of the material for 240 min to obtain an expanded and nano-silicon composite anode material with a pyrolytic carbon material coated on the surface.

[0153] Among them, the thickness of the pyrolytic carbon coating layer is 300 nm, the tapped density is 1.14 g / cm 3 , and the specific surface area is 2.7 m 2 / g.

[0154] Comparative Example 3

[0155] S1. Ball milling: Mix flaky expanded graphite, silicon monoxide particles with a median particle size of 3 mm, sodium carboxymethyl cellulose, and absolute ethanol evenly according to the mass ratio of 10:1:0.01:33, and use a high-energy ball mill for wet milling to obtain a primary silicon-based ball; among them, the median particle size of the primary silicon-based ball is 5 μm;

[0156] Among them, the rotation speed of the ball milling is 500 r / min, the mass ratio of the balls to the total mass of the silicon material, layered carbon material, and binder is 10:1, and the ball milling time is 40 min.

[0157] S2. Granulation: Mix the primary silicon-based ball slurry, graphene, and citric acid obtained in step S1 evenly according to the mass ratio of 4.44:0.05:0.4 and make it into a slurry, and pump it into a spray drying device at a rate of 50 g / min for granulation to obtain a secondary silicon-based ball with D50 of 1 μm;

[0158] S3. Coating: Place the material obtained in step S2 in a fluidized bed, and pump a dopamine solution with a solid content of 15% into the fluidized bed at a rate of 0.1 g / min for surface coating; wherein, the mass ratio of the material obtained in step S2 to the dopamine solution is 1.56:1.67, and the solvent of the dopamine solution is deionized water.

[0159] S4. High-temperature sintering: Place the material obtained in step S3 in a tube furnace, and heat-treat it at 700 °C for 3 h under nitrogen to obtain the silicon-based anode material.

[0160] Among them, the tap density of the anode material is 0.96 g / cm 3 , and the specific surface area is 4.69 m 2 / g.

[0161] Comparative Example 4

[0162] Compared with Example 1, the main difference in this comparative example is that in step S3, the gas flow rate is 10 mL / min, and the deposition time is appropriately controlled. The remaining steps and technical parameters are the same as those in Example 1.

[0163] Among them, the thickness of the pyrolytic carbon coating layer is 150 nm, the tap density is 0.89 g / cm 3 , and the specific surface area is 4.5 m 2 / g.

[0164] Electrochemical performance test:

[0165] Half-cell test method: The formula of the button cell slurry is, according to the solid mass ratio, the composite anode material prepared in each example or comparative example: conductive carbon black (SP): sodium polyacrylate (PAA): sodium carboxymethyl cellulose (CMC): conductive slurry (SWCNT) = 85.0%: 9.9%: 4.0%: 1.0%: 0.1%. Stir evenly. The current collector uses a 6-μm copper foil. Coat the evenly stirred slurry on the copper foil. The thickness of the copper foil is 8 μm, and the thickness of the slurry coating is 125 μm. Put the coated electrode into a vacuum drying oven at 120 °C and bake for 12 h. The coating areal density is 6 mg / cm 2 , assemble a 2032-type button cell in a glove box. The counter electrode uses a lithium metal sheet with a thickness of 0.25 mm. The electrolyte is LiPF 6 / EC + DMC + DEC (volume ratio 1:1:1), where the concentration of LiPF 6 is 1.0 mol / L; the separator is a 12-μm pp separator.

[0166] The charge-discharge test was carried out at a working voltage of 0.005V - 2V and a constant current of 0.1C / 0.1C at 45°C. The first charge specific capacity, the first Coulombic efficiency, the discharge capacity retention rate after 50 cycles, the first lithium intercalation expansion, and the lithium intercalation expansion after 50 cycles were tested. The results are shown in Table 1. Among them,

[0167] The calculation of the first lithium intercalation expansion is as follows:

[0168] y 1 =(h 1 -h 0 ) / (h 0 -d)×100%, where y 1 is the first lithium intercalation expansion ratio, h 1 is the thickness of the electrode sheet after the first lithium intercalation, h 0 is the initial thickness of the electrode sheet, and d is the thickness of the copper foil;

[0169] The calculation of the lithium intercalation expansion after 50 cycles is as follows:

[0170] y 2 =(h 2 -h 0 ) / (h 0 -d)×100%, where y 2 is the lithium intercalation expansion ratio after 50 cycles, h 2 is the thickness of the electrode sheet after lithium intercalation in 50 cycles, h 0 is the initial thickness of the electrode sheet, and d is the thickness of the copper foil.

[0171] Effect data

[0172] Table 1 Comparison table of the first charge specific capacity, the first Coulombic efficiency, the first lithium intercalation expansion, and the lithium intercalation expansion after 50 cycles

[0173]

[0174]

[0175] It can be seen from the result comparison in Table 1 that for the silicon-based composite anode materials prepared in the five embodiments provided by the present invention, compared with the samples in Comparative Examples 1-4, the first Coulombic efficiency is significantly improved, the first lithium intercalation expansion is significantly reduced, and the lithium intercalation expansion after 50 cycles is significantly reduced. It shows that for the silicon-based composite anode material of the present invention, the curly structure of the layered carbon material can effectively absorb the stress generated by the increased volume of the silicon-based material during lithium intercalation, can inhibit the expansion of the silicon anode, and maintain the structural stability of the composite anode material and the electrode sheet structure.

[0176] According to Table 1, compared with Example 2, in Comparative Example 1, since only silicon particle material is used as the anode active material, the first lithium intercalation expansion is significantly increased, indicating that obvious volume expansion has occurred in the silicon anode.

[0177] As shown in Table 1, compared with Example 5, in Comparative Example 2, since pre-freeze drying was not carried out, the degree of curling of the layered carbon material was such that it could not fully wrap the silicon-based particles, and the silicon particles led to a significant increase in the first lithiation expansion, causing obvious volume expansion of the negative electrode.

[0178] As shown in Table 1, compared with Examples 1-5, in Comparative Example 3, since the mass ratio of flaky expanded graphite to silicon monoxide particles was 10:1, the mass of flaky expanded graphite was significantly higher than that of silicon monoxide particles, resulting in a significant decrease in the first charge specific capacity. Moreover, the coating process used in Comparative Example 3 was different from that of the present invention. The coating process in Comparative Example 3 was complex, and the coating thickness was uncontrollable, with poor material stability, thus affecting the battery performance.

[0179] As shown in Table 1, compared with Example 1, in Comparative Example 4, in step S3, the gas flow rate was 10 mL / min. When depositing a pyrolytic carbon coating layer of the same thickness by chemical vapor deposition, too high a flow rate led to an overly intense reaction process, deteriorating the quality of the carbon material, further making the surface of the deposition layer rough. Moreover, compared with the negative electrode material obtained in Example 1, the negative electrode material obtained in Comparative Example 4 had a larger specific surface area and a lower tap density, with more side reactions during the charge-discharge process, ultimately affecting the battery performance.

[0180] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A curled carbon material wrapped with a silicon composite negative electrode material, characterized in that: It comprises an inner core structure, a sub-outer shell structure coated on the outside of the inner core structure, and an outer shell structure coated on the outside of the sub-outer shell structure; The inner core structure is silicon-based particles; The sub-outer shell structure is a curled carbon shell formed by ball-milling a layered carbon material and then freeze-drying and freely curling it; The outer shell structure is a pyrolytic carbon coating layer.

2. The curled carbon material wrapped silicon composite negative electrode material according to claim 1, characterized in that: The inner core structure is silicon-based particles formed by crushing silicon material by ball milling; The outer shell structure is a pyrolytic carbon coating layer formed by depositing a carbon source material onto the outside of the secondary outer shell structure by chemical vapor deposition.

3. The curled carbon material wrapped silicon composite negative electrode material according to claim 1, characterized in that: The median particle size Dv50 of the silicon-based particles is 0.03 to 1 μm; and / or, the median particle size Dv50 of the curled carbon shell is 3-15 μm; And / or, the pyrolytic carbon coating layer has a thickness of 3 to 500 nm, and the pyrolytic carbon coating layer is amorphous.

4. The curled carbon material wrapped silicon composite negative electrode material according to claim 1, characterized in that: The tap density of the rolled carbon material wrapped silicon composite negative electrode material is greater than 0.8 g / cm 3 ; The specific surface area of ​​the rolled carbon material wrapped silicon composite negative electrode material is less than 6m 2 / g.

5. A method for preparing a silicon composite negative electrode material wrapped with a curled carbon material as claimed in any one of claims 1 to 4, characterized in that: The steps include: S1. Ball milling: mixing the silicon material, the layered carbon material, the binder and the solvent uniformly, and ball milling to obtain a mixed slurry; S2, freeze drying: transferring the mixed slurry obtained in step S1 to a freeze dryer, pre-freezing it, and then performing sublimation drying to obtain a curled carbon shell-wrapped silicon composite material; S3. Surface coating: The composite material obtained in S2 is placed in a chemical vapor deposition furnace. After heating, a carbon source gas is introduced for pyrolysis and carbonization to obtain a composite negative electrode material with an outer shell of a pyrolytic carbon coating layer, a secondary outer shell of a curled carbon shell, and an inner core of silicon-based particles.

6. The method for preparing a curled carbon material wrapped silicon composite negative electrode material according to claim 5, characterized in that: In step S1, the mass ratio of the silicon material to the layered carbon material is 0.03 to 1:1; And / or, the silicon material comprises SiO x , silicon, silicon alloy or more, wherein 0<x≤2; And / or, the silicon material has a median particle size Dv50 of 0.5-1500 μm; And / or, the layered carbon material includes one or more of expanded graphite, natural graphite, and graphene; and / or, the median particle size Dv50 of the layered carbon material is 10 to 30 μm; and / or, in step S1, the mass ratio of the adhesive to the layered carbon material is 0.005 to 0.03:1; And / or, the adhesive includes one or more of sodium polyacrylate, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, guar gum, polyvinylidene fluoride, polytetrafluoroethylene, and asphalt; And / or, in step S1, the solid content of the mixed slurry is 10-40%; And / or, the solvent includes one or more of water, 50% ethanol aqueous solution, and 50% isopropanol aqueous solution.

7. The method for preparing a curled carbon material wrapped silicon composite negative electrode material according to claim 5, characterized in that: In step S1, the ball milling conditions include: The equipment is a planetary ball mill; the ball milling method is wet ball milling; the ball milling speed is 200-1500r / min; the ball milling time is 30-60min; the ball-to-material ratio is 5-15:1; In step S2, the pre-freezing conditions include: The pre-freezing temperature is -20 to -60°C, and the pre-freezing time is 1 to 5 hours; And / or, the conditions for sublimation drying include: The temperature is -20~-60℃, the vacuum degree is -5Pa~-2Pa, and the sublimation drying time is 8-24h.

8. The method for preparing a curled carbon material wrapped silicon composite negative electrode material according to claim 5, characterized in that: In step S3, the mass ratio of the obtained pyrolytic carbon coating layer to the curled carbon shell is 0.01 to 0.06:1; And / or, in step S3, the conditions of the chemical vapor deposition furnace include: The protective atmosphere includes one or more of nitrogen and argon; The pyrolysis temperature is 550-750°C; the deposition time is 30-180 min; and / or, the carbon source gas includes one or more of methane, acetylene, ethylene, propane, n-butane, and isobutane; And / or, the gas flow rate of the carbon source gas is 0.1 to 5 mL / min.

9. A negative electrode sheet, characterized in that: It includes a curled carbon material wrapped silicon composite negative electrode material obtained by the preparation method of a curled carbon material wrapped silicon composite negative electrode material as described in any one of claims 5-8 or a curled carbon material wrapped silicon composite negative electrode material as described in any one of claims 1-4.

10. A secondary battery, characterized in that: The secondary battery comprises a curled carbon material wrapped silicon composite negative electrode material obtained by the preparation method of a curled carbon material wrapped silicon composite negative electrode material as described in any one of claims 5-8, or a curled carbon material wrapped silicon composite negative electrode material as described in any one of claims 1-4, or a negative electrode sheet as described in claim 9.

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