Silicon-carbon composite material as well as preparation method, detection method and application thereof

Abstract

The invention provides a silicon-carbon composite material as well as a preparation method, a detection method and application thereof, and relates to the technical field of battery negative electrode materials. The silicon-carbon composite material comprises a porous carbon substrate and nano silicon particles deposited in the carbon substrate, and the strength of the silicon-carbon composite material is subjected to characterization test by adopting gas production rates before and after compaction. The strength is improved on the premise that the electrochemical performance is not sacrificed. The quality and performance of the silicon-carbon composite material can be accurately evaluated by introducing the characterization strength of the gas production rate, and an important reference is provided for application of the material in batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of battery materials and related performance detection and analysis, and particularly relates to a silicon-carbon composite material, a preparation method thereof, a strength detection method of the silicon-carbon composite material, and an electrode and a battery comprising the material. BACKGROUND

[0002] The silicon-carbon negative electrode material is obtained by combining nanosilicon with carbon material through gas deposition and other means, and utilizes the pore structure of the porous carbon to alleviate the volume expansion of the nanosilicon. The silicon-carbon negative electrode material is expected to become a commercialized negative electrode material for high specific energy density lithium batteries. The carbon acts as a carrier of the silicon, significantly alleviates the volume expansion of the silicon particles during the charging and discharging process, and forms a conductive network to further improve the conductivity of the negative electrode material. However, the silicon expansion is still the biggest problem faced by the silicon-carbon negative electrode material. The silicon expansion often squeezes the porous carbon material and causes the rupture of the porous carbon material, thereby leading to the performance degradation of the negative electrode material.

[0003] Therefore, it is necessary to study a porous carbon material with higher strength as a substrate, so as to ensure the stability of the silicon-carbon composite material and further improve the performance of the silicon-carbon composite negative electrode material. SUMMARY

[0004] The silicon-carbon composite material can be regarded as a composite material of the silicon active material and the carbon substrate. Due to the serious volume expansion effect of the silicon during the charging and discharging process, it is often required that the elemental silicon is distributed in the carbon substrate material in the smallest size (<10 nm) as possible, but a substrate with higher strength is still needed to inhibit the expansion of the silicon. The inventors of the present application found that in the silicon-carbon composite material, the Si mainly exists in the form of nanoparticles and amorphous state, and after being contacted with water, the Si is easily reacted with water: Si + H2O = Si(OH)4+ H2↑. After the silicon-carbon negative electrode material is pressed under a certain pressure, the particles will be ruptured, the silicon particles will be exposed, and then reacted with water, and the amount of the generated gas will be increased. Therefore, it is reasonable to use the change amount of the generated gas before and after the pressing to measure the strength of the silicon-carbon negative electrode material. In addition, there may be closed pores or large pores not filled in the silicon-carbon negative electrode material, which will increase the amount of the gas caused by the internal defects of the carbon substrate after the pressing (before the pressing, the part has a certain volume space, and the drainage volume is increased). Therefore, it is necessary to consider the error of this part, and introduce the true density to remove the influence of the closed pores in the material. The present application provides a new detection method for the strength of the silicon-carbon composite material, and the introduction of the gas generation amount to characterize the strength can accurately evaluate the quality and performance of the silicon-carbon composite material, and provides an important reference for the application of the material in the battery.

[0005] In one aspect, the present application provides a silicon-carbon composite material, which comprises a porous carbon substrate and nanosilicon particles distributed in the substrate; the strength of the silicon-carbon composite material is calculated according to the following formula,

[0006]

[0007] wherein P is the strength of the silicon-carbon composite material, %; L1, L2 are the gas production before and after compaction, mL; p is the true density of the silicon-carbon composite material, g / cm 3 .

[0008] Further, the strength of the silicon-carbon composite material is ≥ 85%, preferably ≥ 89%. Further, the strength of the silicon-carbon composite material is ≥ 85%, preferably ≥ 89% under the pressure of 450-500 MPa for 30 s or more.

[0009] Further, the nano-silicon particles are deposited in the pores of the porous carbon substrate.

[0010] Further, the mass ratio of the nano-silicon particles to the porous carbon substrate is (0.4-1.5):1, preferably, the silicon content of the silicon-carbon composite material is 40%-60%.

[0011] Further, the true density p of the silicon-carbon composite material is 1.87-2.16 g / cm 3 .

[0012] Further, the gas production L1 before compaction is 0.65-0.85 mL, and the difference in gas production L2-L1 before and after compaction is 1.0-6.5 mL.

[0013] Further, the sphericity of the silicon-carbon composite material is ≥ 80%.

[0014] Further, the particle size of the silicon-carbon composite material is 1-60 μm, wherein the D10 particle size is 3-7 μm, the D50 particle size is 7-12 μm, and the D90 particle size is 15-40 μm.

[0015] Further, the silicon-carbon composite material further comprises a carbon coating layer, which at least coats part of the surface of the porous carbon substrate and the nano-silicon particles.

[0016] Further, the porous carbon substrate is spherical, spheroidal or irregular in shape, preferably, the porous carbon substrate is spherical.

[0017] Further, the sphericity of the porous carbon substrate is ≥ 80%.

[0018] Further, the porous carbon substrate is uniformly distributed with micropores and / or mesopores. Preferably, the porous carbon substrate is uniformly distributed with micropores and mesopores, the micropore rate is ≥ 70%, and the mesopore rate is < 30%.

[0019] Further, the total pore volume of the porous carbon substrate is ≥ 0.7 cm 3 / g.

[0020] Further, the porous carbon substrate comprises doped N and / or P elements.

[0021] Further, the porous carbon substrate comprises a porous carbon skeleton and nanometer carbon fibers interpenetrated in the porous carbon skeleton. Preferably, the diameter of the nanometer carbon fibers is 10-50 nm. Preferably, the length of the nanometer carbon fibers is 0.5-2 um. Preferably, the length-diameter ratio of the nanometer carbon fibers is 10-200:1.

[0022] Further, the Young's modulus of the nanometer carbon fibers is ≥200 Gpa.

[0023] Further, the nanometer carbon fibers are nanometer carbon fibers after surface modification treatment, preferably, the nanometer carbon fibers are nanometer carbon fibers after silanization surface modification treatment.

[0024] Further, the porous carbon substrate further comprises carbon nanotubes interpenetrated in the porous carbon skeleton.

[0025] Further, the porous carbon substrate comprises a spherical porous carbon skeleton and nanometer carbon fibers interpenetrated in the porous carbon skeleton.

[0026] In another aspect, the present application provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0027] S1) generating a spherical porous carbon substrate;

[0028] S2) depositing nanometer silicon in the pores of the porous carbon substrate by vapor deposition to form the silicon-carbon composite material, the strength P of the silicon-carbon composite material being ≥85%.

[0029] Further, the step S1 comprises:

[0030] S11) mixing nanometer carbon fibers with a resin solution to obtain a suspension;

[0031] S12) performing spray drying treatment on the suspension to obtain carbon particles;

[0032] S13) sequentially performing carbonization treatment and activation treatment on the carbon particles to obtain a porous carbon substrate.

[0033] Further, the mass ratio of the nanometer carbon fibers to the resin solution is 1:(1-5).

[0034] Further, the resin is at least one or several of phenolic resin, polyimide resin or polystyrene resin.

[0035] Further, the diameter of the nanometer carbon fiber is 10-50 nm. The length of the nanometer carbon fiber is 0.5-2 um. The length-diameter ratio of the nanometer carbon fiber is (10-200):1.

[0036] Further, the Young's modulus of the nanometer carbon fiber is ≥200 Gpa.

[0037] Further, step S11 further comprises surface modification treatment of the nanometer carbon fiber, and then mixing with the resin liquid to obtain a suspension; preferably, the surface modification treatment can be silanization surface modification treatment.

[0038] The silanization surface modification treatment is: dispersing the nanometer carbon fiber in a solvent, stirring, adding a silane coupling agent, continuing to stir, and drying.

[0039] Further, step S11 further comprises adding carbon nanotubes to the suspension.

[0040] The amount of the carbon nanotubes is 5-10 wt%.

[0041] Further, step S11 further comprises adding a heteroelement resin to the suspension; the heteroelement resin is a resin containing N and / or P elements.

[0042] Further, before step S12, dispersing the suspension in a solvent to form a spray liquid, and then performing a spray drying treatment; the solvent includes an alcohol solvent, preferably ethanol.

[0043] Further, the solid content of the spray liquid is 30%-50%.

[0044] Further, the rotation speed of the spray drying treatment is 27000-32000 rpm.

[0045] Further, the inlet air temperature of the spray drying treatment is 180-250°C.

[0046] Further, the feeding speed of the spray drying treatment is 1.2-3 L / h.

[0047] Further, in step S13, the carbonization temperature is 400°C-800°C.

[0048] Further, the carbonization time is 3h-8h.

[0049] Further, the activation is physical method activation or chemical method activation.

[0050] The physical method activation is: after carbonization, an activation gas is introduced for physical activation; the activation gas is carbon dioxide and / or water vapor; the activation temperature is 800°C-1000°C, and the time is 9h-20h.

[0051] Further, the step S2 comprises introducing a mixed gas of the first protective gas and the silicon source gas into the porous carbon substrate, and performing a vapor deposition treatment to obtain the silicon-carbon composite negative electrode material.

[0052] Further, the first protective gas is any one or a combination of at least two of nitrogen, neon, argon, krypton, xenon or radon.

[0053] Further, the silicon source gas is any one or a combination of at least two of monosilane, disilane, dichlorosilane or trichlorosilane.

[0054] Further, the content of the silicon source gas in the mixed gas is 70-90 vol%.

[0055] Further, the temperature of the vapor deposition is 400-900°C.

[0056] Further, the time of the vapor deposition is 2-8h.

[0057] Further, before the vapor deposition treatment, the material obtained after activation is sequentially subjected to acid washing, water washing and drying treatment.

[0058] Further, the acid washing uses a 2-5wt% hydrochloric acid solution.

[0059] Further, the temperature of the acid washing is 20-100°C.

[0060] Further, the time of the acid washing is 1-24h.

[0061] Further, the temperature of the drying is 70-90°C.

[0062] Further, after the step S2, there is further a step S3 of carbon-coating the silicon-carbon composite material, and the step S3 satisfies one or more of the following:

[0063] 1) introducing a carbon source gas to deposit a carbon layer on the surface of the silicon-carbon composite material by chemical vapor deposition;

[0064] 2) the flow rate of the carbon source gas is 5-70L / min;

[0065] 3) the carbon source gas is a mixed gas of a carbon-containing gas and a second protective gas, and the flow rate ratio of the carbon-containing gas to the protective gas is (50-80):(20-50);

[0066] 4) the carbon-containing gas comprises any one or a combination of at least two of methane, ethane, acetylene or ethylene, and the second protective gas comprises any one or a combination of at least two of nitrogen, neon, argon, krypton, xenon or radon;

[0067] 5) the deposition temperature is 300-800 DEG C, and the deposition time is 5-10 h.

[0068] In another aspect, the present application provides a method for detecting the strength of a silicon-carbon composite material, comprising the following steps:

[0069] measuring the gas production L1 before compaction;

[0070] compacting the sample to be detected;

[0071] measuring the gas production L2 after compaction;

[0072] substituting L1 and L2 into the formula to calculate the strength value;

[0073]

[0074] wherein P is the strength of the silicon-carbon composite material, %; L1 and L2 are the gas production before and after compaction, mL; and p is the true density of the silicon-carbon composite material, g / cm 3 .

[0075] In another aspect, the present application provides a negative electrode sheet comprising any of the above silicon-carbon composite materials or the silicon-carbon composite material prepared by any of the above preparation methods.

[0076] In another aspect, the present application provides a lithium ion battery comprising any of the above negative electrode sheets or the negative electrode sheet prepared by any of the above silicon-carbon composite materials.

[0077] Compared with the prior art, the present application has the following beneficial effects:

[0078] 1) The present application utilizes high-strength nanocarbon fibers combined with resin to generate a porous carbon substrate. Compared with biomass-based silicon-carbon composite materials, the obtained silicon-carbon composite material has higher strength, and there are no large pores in the internal biomass-based natural pores, and the pore structure is mainly microporous (<2 nm), so that the nanosilicon is uniformly distributed in the pores of the porous carbon in the form of an element, further dispersing the stress caused by silicon expansion. And the high electrical conductivity of nanocarbon fibers can enhance the electrochemical performance of the silicon-carbon composite material.

[0079] 2) The present application deposits silicon particles into the pores of the porous carbon substrate by CVD chemical deposition technology, and then coats a carbon layer to prepare a high-strength silicon-carbon composite silicon-carbon negative electrode material. The sample has good sphericity, adjustable particle size and pore size, and simple operation steps.

[0080] 3、The application utilizes high-strength nanometer carbon fibers combined with resin to generate a spherical porous carbon substrate. The resin carbonizes to form a primary spherical carbon skeleton, imparting toughness, and the nanometer carbon fibers penetrate the primary spherical carbon skeleton to form a three-dimensional network spherical skeleton, imparting rigidity to the skeleton.

[0081] 4、The silicon-carbon composite material of the application: the unique spherical porous carbon substrate and nanometer silicon particle composite structure effectively alleviate the volume expansion problem of silicon during charging and discharging. The high microporosity and large total pore volume of the porous carbon substrate provide sufficient buffer space for silicon volume expansion, reducing the pulverization of silicon particles and the separation of the silicon particles from the current collector, and improving the structural stability of the material. The porous carbon substrate formed by the combination of nanometer carbon fibers and organic resin liquid, as well as the specific material sphericity and true density, optimizes the internal structure of the material, which is beneficial to the transmission and storage of lithium ions and improves the energy density of the battery. The reasonable strength calculation method and the higher strength standard ensure the reliability and stability of the material in actual application, prolonging the cycle life of the battery.

[0082] 5、After the silicon-carbon composite material is pressed under a certain pressure, the particles will break, the silicon particles will be exposed, and then react with water, resulting in an increase in the amount of gas produced. The application uses the gas production rate before and after pressing to characterize the strength. The application uses a compaction density instrument to press, and finds that the compaction density and the pressing force have a good linear correlation with the gas production value after pressing. The strength is characterized by the gas production rate before and after pressing. Since there may be closed pores or large pores that are not filled in the silicon-carbon negative electrode material, the amount of gas caused by defects in the carbon substrate will increase after pressing (before pressing, this part has a certain volume space, and the drainage volume increases), so it is necessary to consider this part of the error. The true density is introduced to remove the influence of the closed pores in the material. The application eliminates the influence of the error caused by the possible closed pores or large pores that are not filled in the silicon-carbon composite material.

[0083] 6、The strength detection method of the application can accurately evaluate the quality and performance of the silicon-carbon composite material. By testing the gas production before and after pressing, and calculating the strength value according to a specific formula, the structural stability of the material can be intuitively reflected. The specific conditions of the gas production test, as well as the setting of the test pressure and holding time, make the test results more consistent with the actual application scenario. The relationship between the material strength value and the cycle capacity retention rate provides an important reference for the application of the material in the battery, which helps to select materials with excellent performance. BRIEF DESCRIPTION OF DRAWINGS

[0084] Figure 1 A flowchart showing the generation process of the porous carbon substrate of the silicon-carbon composite material described in Example 1 of the application is shown.

[0085] Figure 2 A scanning electron microscope (SEM) image of the carbon substrate of the silicon-carbon composite material described in Comparative Example 3 of the application is shown.

[0086] Figure 3 A scanning electron microscope (SEM) image of the carbon substrate of the silicon-carbon composite material described in Inventive Comparative Example 4 is shown.

[0087] Figure 4 A scanning electron microscope (SEM) image of the carbon substrate of the silicon-carbon composite material described in Inventive Comparative Example 5 is shown.

[0088] Figure 5 A N2 adsorption-desorption isotherm of the porous carbon substrate of the silicon-carbon composite material described in Inventive Example 1 is shown. DETAILED DESCRIPTION

[0089] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0090] Silicon-carbon composite material

[0091] The silicon-carbon composite material provided by the embodiments of the present application comprises a porous carbon substrate and nano-silicon particles distributed in the porous carbon substrate, and the porous carbon substrate comprises a porous carbon skeleton and nano-carbon fibers interpenetrated in the porous carbon skeleton.

[0092] The silicon-carbon composite material of the present application comprises a porous carbon substrate and nano-silicon particles uniformly deposited in the carbon substrate. The carbon substrate generated by combining nano-carbon fibers with a resin with better toughness integrates the high strength and high conductivity of the nano-carbon fibers, and improves the strength of the silicon-carbon composite material without sacrificing the electrochemical performance.

[0093] Another embodiment of the present application provides a silicon-carbon composite material comprising a porous carbon substrate and nano-silicon particles deposited in the porous carbon substrate; the strength of the silicon-carbon composite material is calculated according to the following formula I,

[0094]

[0095] wherein P is the strength of the silicon-carbon composite material, %; L1 is the gas production before compaction, mL; L2 is the gas production after compaction, mL; and p is the true density of the silicon-carbon composite material, g / cm 3 .

[0096] The silicon-carbon composite material of the present application has a strength ≥85% under a pressure of 450-500 MPa for 30 s or more, preferably ≥89%.

[0097] The present application uses the compaction densitometer to press, finds that the compactness and the pressure and the linear correlation degree of the gas production value after pressing are good, and uses the gas production change rate before and after pressing to represent the strength.The present application eliminates the influence of the part of error caused by the possible closed pores or unfilled large pores in the silicon-carbon composite material.

[0098] The carbon base refers to a structural carrier or matrix based on carbon material in the field of battery materials, used for loading active substances.

[0099] In an embodiment, the mass ratio of the nanosilicon particles to the porous carbon base is (0.4-1.5):1, preferably, the silicon content of the silicon-carbon composite material is 40%-60%, for example, 40%, 45%, 50%, 55%, 60%, etc.When the silicon content is >40%, the strength reduction rate of the traditional carbon base is >35%.The carbon base in the present application is combined with the resin with high toughness, and the nanocarbon fiber has high specific surface area, which provides certain help for the subsequent activation to generate micropores;compared with the biomass-based silicon-carbon composite material, there is no natural large pore in the internal, and the silicon-carbon material has higher strength.

[0100] In another embodiment, the true density p of the silicon-carbon composite material is 1.87-2.16g / cm 3 , for example, 1.87g / cm 3 , 1.90g / cm 3 , 1.93g / cm 3 , 1.95g / cm 3 , 1.98g / cm 3 , 2.00g / cm 3 , 2.03g / cm 3 , 2.05g / cm 3 , 2.08g / cm 3 , 2.10g / cm 3 , 2.13g / cm 3 , 2.16g / cm 3 .

[0101] Further, the gas production L1 before compaction is 0.65-0.85mL, and the difference L2-L1 between the gas production before and after compaction is 1.0-6.5mL.The true density is the density of the material in the completely compacted (basically no pore) state, reflecting the tightness of the substance itself, which is usually measured by helium replacement method, excluding the influence of pores.The present application introduces the gas production to represent the strength, which can accurately evaluate the quality and performance of the silicon-carbon composite material, and provides an important reference for the application of the material in the battery.

[0102] In another embodiment, the sphericity of the silicon-carbon composite material is ≥80%, for example, 80%, 85%, 90%, 95%, etc.

[0103] In another embodiment, the particle size of the silicon-carbon composite material is 1-60 μm, wherein the D10 particle size is 3-7 μm, the D50 particle size is 7-12 μm, and the D90 particle size is 15-40 μm.

[0104] In another embodiment, the silicon-carbon composite material further comprises a carbon coating layer, which coats at least part of the surface of the porous carbon substrate and the nano-silicon particles.

[0105] In another embodiment, the porous carbon substrate is spherical, spheroid or irregular in shape. Preferably, the porous carbon substrate is spherical.

[0106] In another embodiment, the sphericity of the porous carbon substrate is ≥80%. For example, the sphericity can be ≥85%, ≥90%, ≥95%, etc.

[0107] In another embodiment, the porous carbon substrate is uniformly distributed with micropores and mesopores.

[0108] In another embodiment, the microporosity of the porous carbon substrate is ≥70%, for example, 70%, 75%, 80%, 85%, 90%, 95%, etc.

[0109] In another embodiment, the mesoporosity of the porous carbon substrate is <30%, for example, 15%, 20%, 25%, etc.

[0110] In another embodiment, the total pore volume of the porous carbon substrate is ≥0.7 cm 3 / g, for example, 0.7 cm 3 / g, 0.75 cm 3 / g, 0.80 cm 3 / g, 0.85 cm 3 / g, etc.

[0111] In another embodiment, the porous carbon substrate comprises a porous carbon skeleton and nano-carbon fibers interpenetrated in the porous carbon skeleton.

[0112] In another embodiment, the diameter of the nano-carbon fibers is 10-50 nm; for example, 10-30 nm, 20-40 nm, 30-50 nm, 10-45 nm, etc., for example, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, etc.

[0113] The length of the nano-carbon fibers is 0.5-2 μm. For example, 0.5 μm, 0.75 μm, 0.9 μm, 1.0 μm, 1.5 μm, 2 μm, etc.

[0114] The aspect ratio of the nanocarbon fiber is 10-200:1. For example, it can be 10:1, 50:1, 80:1, 100:1, 120:1, 150:1, 180:1, 200:1, etc.

[0115] The porous carbon substrate suitable for the application includes a porous carbon skeleton and a three-dimensional network skeleton in which nanocarbon fibers are interpenetrated into the porous carbon skeleton. The diameter of the nanocarbon fiber is precisely controlled to be 10-50 nm, and the aspect ratio is in the range of 10-200. Such nanoscale size and specific aspect ratio enable the construction of a unique three-dimensional network skeleton structure when the nanocarbon fiber is interpenetrated into the spherical porous carbon skeleton to form the porous carbon substrate, which not only provides an ideal support framework for the subsequent deposition of nanosilicon particles, but also to a certain extent, buffers the volume change of silicon during the charging and discharging process.

[0116] In another embodiment, the Young's modulus of the nanocarbon fiber is ≥200 Gp.

[0117] In another embodiment, the porous carbon substrate is spherical. The spherical porous carbon substrate includes a spherical porous carbon skeleton and nanocarbon fibers distributed in the spherical porous carbon skeleton. Compared with irregular shapes, the silicon-carbon material of the application includes a spherical carbon substrate, which is stronger and avoids poor compression resistance caused by more corners during rolling.

[0118] In another embodiment, the nanocarbon fiber is a nanocarbon fiber after surface modification treatment, such as a nanocarbon fiber after silanization surface modification treatment. The silanization surface modification treatment introduces active groups on the surface of the nanocarbon fiber, enhances the interfacial bonding force between the nanocarbon fiber and the resin liquid, and improves the interfacial shear strength.

[0119] In another embodiment, the porous carbon substrate further includes carbon nanotubes interpenetrated into the porous carbon skeleton. The porous carbon substrate can further introduce carbon tubes to further increase the electrical conductivity. The amount of carbon tubes can be 5-10 wt.%.

[0120] In another embodiment, the porous carbon substrate includes doped N and / or P elements. N and / or P and other impurity elements can further enter the porous carbon substrate to increase active defects, thereby increasing the electrical conductivity of the silicon-carbon product.

[0121] In another embodiment, the spherical porous carbon substrate is formed by carbonizing and activating nanocarbon fibers and a resin solution. The resin is carbonized to form a primary spherical carbon framework, which imparts toughness, and the nanocarbon fibers are interpenetrated in the primary spherical carbon framework to form a three-dimensional network spherical framework, which imparts rigidity to the framework. The resin is at least one of a phenolic resin, a polyimide resin, or a polystyrene resin. Compared with the method of using carbon microspheres prepared by ball milling as a carbon substrate, the resin solution itself serves as a carbon source and an adhesive, and forms a continuous carbon framework after carbonization. The nanocarbon fibers are uniformly dispersed and interpenetrated in the continuous carbon framework to form a three-dimensional network framework mechanism, which enhances the strength and toughness of the framework, avoids fiber breakage caused by mechanical crushing (ball milling), and thus will not be randomly stacked into microspheres, resulting in uneven pore distribution, a large number of closed pores, and defects. The spherical porous carbon substrate of the present application can maintain the aspect ratio of the nanocarbon fibers intact, and the conductive path is continuous.

[0122] In another embodiment, nanosilicon is deposited in the pores of the porous carbon substrate.

[0123] In the silicon-carbon composite material of the present application, the porous carbon substrate has an excellent pore structure, a microporosity greater than 70%, a total pore volume greater than or equal to 0.7 cm 3 / g with an average pore diameter less than 3 nm. A high microporosity means that the material has a large number of small pores inside, which can accommodate more nanosilicon particles, and at the same time provide sufficient buffer space when the silicon volume expands; a larger total pore volume ensures the material's ability to store lithium ions, which is beneficial to improve the capacity of the battery. The sphericity of the silicon-carbon composite material is greater than or equal to 80%, and the true density is in the range of 1.87-2.16 g / cm 3 A higher sphericity allows the material to be more tightly packed during electrode preparation, increasing the compaction density of the electrode and reducing the voids inside the electrode, thereby improving the energy density of the battery; and a specific true density range reflects the tightness of the internal structure of the material and the rationality of the composition.

[0124] Compared with the silicon-carbon composite material using a silicon matrix and carbon nanotubes or nanocarbon fibers grown thereon, the carbon nanotubes or nanocarbon fibers grown on the surface of the material provide a larger specific surface area and a larger porosity, but the silicon particles cannot be uniformly distributed on the surface. The silicon-carbon composite material of the present embodiment has uniform micropores and mesopores, and nanosilicon is deposited in the pores of the spherical porous carbon substrate, and the nanosilicon is uniformly distributed in the carbon substrate.

[0125] In another embodiment, the strength of the silicon-carbon composite material is calculated according to the following formula I,

[0126]

[0127] wherein L1 and L2 are the gas production amounts before and after compaction, respectively, in mL; and p is the true density of the silicon-carbon composite material, in g / cm3 The true density p of the silicon-carbon composite material is 1.87-2.16 g / cm 3 For example, 1.87 g / cm3, 1.90 g / cm 3 , 1.93 g / cm 3 , 1.95 g / cm 3 , 1.98 g / cm 3 , 2.00 g / cm 3 , 2.03 g / cm 3 , 2.05 g / cm 3 , 2.08 g / cm 3 , 2.10 g / cm 3 , 2.13 g / cm 3 , 2.16 g / cm 3 The gas production L1 before compaction is 0.65-0.85 mL, and the difference between the gas production before and after compaction L2-L1 is 1.0-6.5 mL.

[0128] The strength value calculated in this way can intuitively reflect the change of gas production before and after the material bears pressure, and then evaluate the stability of the material structure. The strength of the silicon-carbon composite material of the embodiment of the present application is ≥85% under the pressure of 450-500 MPa for 30 s or more, preferably ≥89%, which means that the material has good performance in actual application. Higher strength means that the material can better maintain its structural integrity during charging and discharging, reducing structural damage caused by volume change, thereby improving the cycle life and stability of the battery.

[0129] The specific capacity of the silicon-carbon composite material of the present application is greater than 1500 mAh / g, and the initial efficiency is greater than 70%. In further embodiments, the specific capacity is greater than 1600 mAh / g, and the initial efficiency is greater than 80%.

[0130] Negative electrode sheet

[0131] The silicon-carbon composite material described in the present application is used as a negative electrode material. The embodiment of the present application provides a negative electrode sheet made of the above-mentioned silicon-carbon composite material.

[0132] Battery

[0133] The embodiment of the present application provides a battery, especially a lithium ion battery, which comprises a negative electrode sheet made of the above-mentioned silicon-carbon composite material or a negative electrode of the battery using the silicon-carbon composite material of the embodiment of the present application.

[0134] The excellent performance of the silicon-carbon composite negative electrode material can effectively improve the overall performance of the lithium ion battery. In the battery charging and discharging process, the nano-silicon particles in the silicon-carbon composite negative electrode material provide high specific capacity, realizing high energy density of the battery; the porous carbon substrate buffers the volume expansion of silicon, ensuring the stability of the material structure and improving the cycle life of the battery; at the same time, the good conductivity of the material also helps to improve the charging and discharging efficiency of the battery.

[0135] Preparation method

[0136] An embodiment of the present application provides a preparation method of a silicon-carbon composite material, comprising the following steps:

[0137] S1) generating a porous carbon substrate;

[0138] S2) depositing nano-silicon in the pores of the porous carbon substrate by vapor deposition to form the silicon-carbon composite material, wherein the strength P of the silicon-carbon composite material is greater than or equal to 85%.

[0139] Step S1 can further comprise:

[0140] S11) mixing nano-carbon fibers with a resin solution to obtain a suspension;

[0141] S12) performing spray drying treatment on the suspension to obtain carbon particles;

[0142] S13) sequentially performing carbonization treatment and activation treatment on the carbon particles to obtain the porous carbon substrate.

[0143] In another embodiment, the mass ratio of the nano-carbon fibers to the resin solution is 1:(1-5).

[0144] The resin is at least one of phenolic resin, polyimide resin or polystyrene resin. Different types of organic resin solutions have their own unique chemical structure and physical properties. Selecting a suitable organic resin solution and precisely controlling the solid-liquid ratio can regulate the structure and performance of the porous carbon substrate, so that it better meets the needs of the silicon-carbon composite negative electrode material.

[0145] In order to better mix the resin solution and the nano-carbon fibers, the nano-carbon fibers can be subjected to surface modification treatment, such as silanization surface modification treatment. In another embodiment, step S11 further comprises subjecting the nano-carbon fibers to silanization surface modification treatment, and then mixing the surface-modified nano-carbon fibers with the resin solution to obtain a suspension.

[0146] The silanization surface modification treatment comprises dispersing the nano-carbon fibers in a solvent (alcohol solvent, preferably ethanol), stirring, adding a silane coupling agent, continuing to stir, and drying.

[0147] The amount of ethanol can be, the mass volume ratio of nanometer carbon fiber and ethanol can be 1g:30-100ml. The volume ratio of silane coupling agent and anhydrous ethanol is 1:50-100.

[0148] The nanometer carbon fiber is dispersed in the silane coupling agent, and dried at 80-120℃, so that the silane coupling agent forms a chemical bond on the surface of the nanometer carbon fiber, which can promote the better mixing of the resin liquid and the nanometer carbon fiber. The silane coupling agent treatment can introduce active groups on the surface of the nanometer carbon fiber, enhance the interfacial bonding force between the nanometer carbon fiber and the organic resin liquid, make the two form a more compact and stable structure in the composite process, and thus improve the mechanical properties and stability of the porous carbon substrate.

[0149] In another embodiment, 5-10wt% carbon tubes or heteroelement resins can also be introduced in this step. For example, nanometer carbon fiber, carbon tube, resin liquid are mixed to obtain a suspension; or nanometer carbon fiber, heteroelement resin, resin liquid are mixed to obtain a suspension; the heteroelement resin is a resin doped with N and / or P elements, such as polyimide resin.

[0150] The carbon tube has excellent electrical conductivity and mechanical properties, the addition of carbon tube can significantly improve the electrical conductivity of the material, enhance the transmission ability of electrons in the material, and is beneficial to improve the charge and discharge efficiency of the battery; the addition of heteroelement resin can introduce N and / or P elements into the carbon substrate, which can change the electronic structure of the material and optimize the electrochemical performance of the material, further improve the performance of the battery.

[0151] In another embodiment, the suspension obtained in step S11 is dispersed in a solvent (such as an alcohol solvent, preferably anhydrous ethanol) to form a spray liquid, and then a spray drying process is carried out. The solid content of the spray liquid is 30%-50%; for example, it can be 30%, 35%, 45%, 50% and the like.

[0152] The nanometer carbon fiber and the resin liquid are fully mixed to form a spray liquid with a solid content of 30%-50%. In this step, the accurate control of the solid content has a great influence on the subsequent material performance. If the solid content is too low, the spray liquid is too thin, and it is difficult to form an ideal particle morphology in the spray drying process, resulting in loose material structure; if the solid content is too high, the spray liquid is too thick, which may cause the spray equipment to be blocked, affecting the production efficiency, and there may be many defects in the prepared material. The rotation speed of the spray drying process is 27000-32000rpm.

[0153] The inlet temperature of the spray drying process is 180-250℃; for example, it can be 180℃, 190℃, 200℃, 210℃, 230℃, 240℃ and the like.

[0154] The feeding speed of the spray drying process can be 1.2-3L / h.

[0155] In the embodiments of the present application, the rotation speed of the atomizing disc during spray drying is controlled to be 27000-32000 rpm, and the inlet air temperature is 180-250℃. The rotation speed of the atomizing disc determines the size and uniformity of the liquid droplets formed by the spray liquid, and a suitable rotation speed can make the liquid droplets uniform in size, so that particles with uniform particle size are formed during the drying process, which is conducive to the uniformity of the structure of the subsequent material and the maintenance of the complete aspect ratio structure and the conductive network; the inlet air temperature affects the drying speed of the liquid droplets and the formation quality of the particles, and if the temperature is too low, the drying speed is slow, the production efficiency is low, and the particles may not be completely dried, which affects the material performance; if the temperature is too high, the surface of the particles may be overheated, which may cause damage to the surface structure and even carbonization, which also affects the material performance.

[0156] The carbonization treatment can convert the organic resin liquid into carbon, and form a stable porous carbon base structure with the nanometer carbon fibers, thereby improving the thermal stability and chemical stability of the material.

[0157] In another embodiment, the carbonization temperature is 400℃-800℃; for example, it can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃, etc.

[0158] The carbonization time is 3h-8h; for example, it can be 3h, 4h, 4.5h, 5h, 6h, 6.5h, 7h, 8h, etc.

[0159] The control of the carbonization temperature and time is crucial, and if the temperature is too low or the time is too short, the resin liquid cannot be completely carbonized, and an ideal porous carbon structure cannot be formed; if the temperature is too high or the time is too long, the porous carbon structure may be excessively shrunk, the porosity is reduced, and the loading capacity and buffering performance of the material for the nanometer silicon particles are affected.

[0160] The carbonization treatment is carried out under the protection of a protective gas. The protective gas can be nitrogen or an inert gas.

[0161] The purpose of the activation treatment is to further increase the porosity and specific surface area of the porous carbon base, and to improve the adsorption and transmission capacity of the material for lithium ions. The activation treatment suitable for the embodiments of the present application can include physical activation or chemical activation.

[0162] The physical activation is: after carbonization, an activation gas is introduced for physical activation; the activation gas is carbon dioxide and / or water vapor; the activation temperature is 800℃-1000℃, and the time is 9h-20h. The use of CO2 or water vapor for activation treatment at 800-1000℃ for 9-20 hours can ensure that the activation gas is uniformly distributed in the material, so that the activation reaction is fully and uniformly carried out, and the situation of excessive or insufficient local activation is avoided, thereby ensuring the consistency of the material performance.

[0163] In one embodiment, the suspension is subjected to spray drying in step S12 to obtain carbon particles. The carbon particles are subjected to carbonization and activation in sequence in step S13 to obtain the porous carbon substrate.

[0164] In another embodiment, the suspension is subjected to spray drying in step S12 to obtain carbon microsphere particles. The carbon microsphere particles are subjected to carbonization and activation in sequence in step S13 to obtain the spherical porous carbon substrate.

[0165] Step S2 can further include introducing a mixed gas of the first protective gas and a silicon source gas into the porous carbon substrate to perform vapor deposition to obtain the silicon-carbon composite negative electrode material.

[0166] In another embodiment, the first protective gas is nitrogen or inert gas.

[0167] In another embodiment, the silicon source gas is any one or a combination of at least two of monosilane, disilane, dichlorosilane, or trichlorosilane.

[0168] In another embodiment, the flow rate of the mixed gas is, for example, 5-70 L / min.

[0169] In another embodiment, the content of the silicon source gas in the mixed gas is 70-90%.

[0170] In another embodiment, the deposition amount (Si) of the vapor deposition to the amount of the material (porous carbon substrate) is 0.6-2:1.

[0171] In another embodiment, the temperature of the vapor deposition is 400-900°C.

[0172] In another embodiment, the time of the vapor deposition is 2-8 h.

[0173] Chemical vapor deposition is a method that can precisely control the deposition position and growth morphology of the nano-silicon particles. In this step, by precisely controlling the deposition temperature and time, the nano-silicon particles can be uniformly deposited in the pores of the porous carbon substrate to form a stable silicon-carbon composite structure. If the deposition temperature is too low or the time is too short, the deposition amount of the nano-silicon particles is insufficient, and the high specific capacity advantage of silicon cannot be fully utilized. If the temperature is too high or the time is too long, the nano-silicon particles can grow excessively, and the agglomeration phenomenon is intensified, which affects the material performance.

[0174] In another embodiment, the material obtained after activation is subjected to acid washing, water washing, and drying in sequence before the vapor deposition treatment. In another embodiment, the acid washing uses a 2-5 wt% hydrochloric acid solution.

[0175] In another embodiment, the temperature of the acid washing is 20-100℃. For example, it can be 20℃, 25℃, 30℃, 40℃, 50℃, 60℃, 70℃, 100℃, etc.

[0176] In another embodiment, the time of the acid washing is 1-24h. For example, it can be 1h, 2h, 5h, 8h, 10h, 12h, 16h, 18h, 24h, etc.

[0177] In another embodiment, the temperature of the drying is 70-90℃. For example, it can be 70℃, 80℃, 90℃, etc.

[0178] After activation, the spherical porous carbon material is placed in a 2-5wt% hydrochloric acid solution and cooked at 20-100℃ for 1-24h, and stirred overnight, then washed with pure water until the pH is 6.5-7.5, and then the filtered material is placed in an oven at 70℃-90℃ to dry until the moisture is <3% for standby. In this step, the impurities of the porous carbon base are removed by acid washing.

[0179] In another embodiment, after step S2, step S3 is further included, that is, the silicon-carbon composite material is coated with carbon, a carbon layer is coated on the surface of the silicon-carbon composite material, specifically, for example, a carbon source gas is introduced, and a carbon coating layer is deposited on the surface of the silicon-carbon composite material (preferably, the carbon coating layer is deposited on the surface of the silicon-carbon composite material by chemical vapor deposition), and the carbon coating layer at least coats part of the surface of the porous carbon base and the nano-silicon particles.

[0180] The flow rate of the carbon source gas is, for example, 5-70L / min.

[0181] The carbon source gas is a mixed gas of a carbon-containing gas and a second protective gas, and the flow rate ratio of the carbon-containing gas to the second protective gas is, for example, (50-80):(20-50).

[0182] The carbon-containing gas includes any one or a combination of at least two of methane, ethane, acetylene or ethylene. The second protective gas includes any one or a combination of at least two of nitrogen, neon, argon, krypton, xenon or radon.

[0183] The deposition temperature is, for example, 300-800℃, and the deposition time is 5-10h.

[0184] Strength detection

[0185] The embodiment of the present application provides a detection method for the strength of a silicon-carbon composite material, which comprises the following steps:

[0186] Measuring the gas production L1 before compaction;

[0187] Compacting the silicon-carbon composite material sample;

[0188] Measuring the gas production L2 after compaction;

[0189] L1, L2 are substituted into the following formula I to calculate the strength value;

[0190]

[0191] wherein p is the true density of the silicon-carbon composite material.

[0192] In another embodiment, the gas production L1 or L2 is the 48-hour gas production; specifically, the 48-hour gas production at 50-60°C.

[0193] In another embodiment, the compaction condition is a pressure of 450-500 MPa and a holding time of > 30 seconds.

[0194] In another embodiment, the detection method can specifically include the following steps:

[0195] The in-situ multi-channel storage gas production test system is used to measure the gas production before pressing (compaction) as L1 / ml, and the 48-hour gas production L1 at 60°C.

[0196] The silicon-carbon composite material sample is placed in the sample chamber of the compaction density instrument, pressed, and the sample is compacted.

[0197] The compaction condition is set so that the particles of the sample are broken under the same pressure (ensure that the pressing time and holding time can crush the sample).

[0198] After completion, the rod is withdrawn using the rod withdrawal device, and the sample after pressing is collected.

[0199] The in-situ multi-channel storage gas production test system is used to measure the gas production after pressing as L2 / ml, and the 48-hour gas production L2 at 60°C.

[0200] L1, L2 are substituted into the above formula I to calculate the strength value;

[0201] wherein p is the true density of the material.

[0202] The test conditions for measuring the gas production can be: 3 g of silicon-carbon material powder plus 9 g of water, the test amount is 10 ml, the temperature is 60°C, and the time is 48 hours; or 7 g of silicon-carbon material powder plus 40 g of water.

[0203] In the silicon-carbon composite material, Si mainly exists in the form of nanoparticles and amorphous state, and is easy to react with water after contacting with water: Si + H2O = Si(OH)4+ H2↑; The in-situ multi-channel storage gas production test system GVM2200 is adopted. The mass change of the sample slurry with time is monitored in real time by the drainage method combined with a high-precision sensor, and then the volume change of the produced gas is converted. The Newton theorem and Archimedes buoyancy law are used to calculate the amount of gas produced by measuring the volume of water displaced by the sample in water. The temperature can be controlled between 20℃ and 85℃. The sample is packaged with an aluminum plastic film and a counterweight is added. The mass change of the sample slurry with time is measured in real time by the drainage method combined with a high-precision sensor, and the volume change of the produced gas is further converted (generally, the volume change rate is collected for 48 hours). After the silicon-carbon negative electrode material is pressed under a certain pressure, the particles will be broken and the silicon particles will be exposed, and then react with water to produce more gas. It is reasonable to use the change in gas production before and after pressing to measure the strength of the silicon-carbon negative electrode material. Because there may be closed pores or large pores not filled in the silicon-carbon negative electrode material, the amount of gas caused by defects in the carbon matrix will increase after pressing (before pressing, this part has a certain volume space, and the drainage volume increases), so the true density is introduced to remove the influence of the closed pores in the material.

[0204] The strength detection method of the silicon-carbon composite material of the present application first measures the gas production L1 and L2 before and after compaction. Then the strength value is calculated according to the above formula I, wherein p is the true density of the material. The strength value calculated by the formula can accurately reflect the change in structural stability of the material before and after compaction. The gas production L1 and L2 before and after compaction is the gas production at 60℃ for 48 hours. The material is fully gassed under this condition, and the gassing process is stable, which is convenient for accurate measurement of the gas production. The compaction pressure is controlled at 450-500MPa, and the holding time is greater than or equal to 30 seconds. The appropriate test pressure and holding time can simulate the pressure conditions that the material bears in actual application, so that the test results have more practical reference value. When the material strength value P is greater than 90%, the capacity retention rate after 100 cycles is greater than 93%. This relationship shows that the strength value obtained by the strength detection method can effectively predict the capacity retention ability of the material in the battery cycle charging and discharging process, and provides an important basis for the quality evaluation and application of the material.

[0205] The silicon-carbon composite material, its preparation method and strength detection method of the present application are further described below in combination with specific examples and comparative examples.

[0206] Example 1

[0207] Step 1: The nano-carbon fiber (fiber diameter 10nm, length: 0.5um, aspect ratio 50:1) is ultrasonically cleaned three times with deionized water, dried and added to anhydrous ethanol, and then ultrasonically cleaned three times and dried to remove surface oil and impurities to obtain pure nano-carbon fiber.

[0208] Step 2: Take 8g of nano-carbon fibers and disperse them in 240ml of anhydrous ethanol, stir at 80℃ for 1h, then add silane coupling agent (using silane coupling agent KH550), the ratio of silane coupling agent to anhydrous ethanol is 1:100, continue stirring for 5h, then centrifuge and purify, and then dry in a vacuum drying oven at 60℃ for 15h.

[0209] Step 3: The silanized carbon nanofibers and the phenolic resin liquid are stirred and mixed in a ratio of 1:5 to form a suspension, and then dispersed in anhydrous ethanol to form a spray liquid with a solid content of 30%.

[0210] Step 4: The above-mentioned spray liquid is subjected to a spraying experiment: spray curing conditions: atomizing disk speed: 28000rpm, air inlet temperature: 175℃, feed rate: 0.8L / h, induced draft fan frequency: 50HZ, and small balls (carbon microsphere particles) with a particle size of 5um to 20um can be sprayed.

[0211] Step 5: The carbon microsphere particles are heat-treated in a nitrogen atmosphere at a heating rate of 10°C min-1 and a heat treatment temperature of 400°C for 5 hours for carbonization.

[0212] Step 6: Place the carbonized carbon microspheres into a well-sealed rotary kiln, rotate the kiln at a frequency of 30 Hz, introduce water vapor, and a gas flow rate of 12 L / min. The heating rate of the rotary kiln is 10°C / min, and the temperature is raised to 900°C, kept warm for 8 hours, and then cooled to room temperature to obtain the material.

[0213] Step 7: Heat and cook the material obtained in step 6 with 2-5wt% hydrochloric acid solution for 1-2h and stir overnight, then rinse with pure water until the pH is 6.5-7.5, and then place the filtered material in an oven at 80℃~90℃ to dry until the moisture is <3% for later use.

[0214] Step 8: Place the material obtained in step 7 in a well-sealed rotary kiln, introduce a mixed gas of inert gas and silicon source, and obtain a silicon-carbon composite negative electrode material after chemical vapor deposition.

[0215] The mass ratio of the deposited amount of silicon to the material obtained in the step is 0.4-1.5:1, the deposition temperature is 400-900° C., the deposition time is 2-8 hours, and then the silicon-carbon composite material is obtained by cooling to room temperature.

[0216] The porous carbon substrate of the embodiment 1 of the present application presents abundant pores, and the nano-silicon particles are uniformly distributed inside the pores. Figure 1 The spherical porous carbon substrate of the silicon-carbon composite material of the embodiment 1 of the present application is shown. After the resin liquid is carbonized, the spherical porous carbon skeleton is formed, and the nano-carbon fibers penetrate the spherical porous carbon skeleton to form a three-dimensional network spherical skeleton.

[0217] Figure 5 The N2 adsorption-desorption isotherm curve of the porous carbon substrate of the silicon-carbon composite material of the embodiment 1 of the present application is shown. The single-point BET specific surface area of the porous carbon substrate of the silicon-carbon composite material of the embodiment 1 of the present application is 1856.22 m2 / g; the total pore volume is 0.91 cm3 / g; the mesopore volume is 0.23 cm3 / g; the average pore size is 2.99 nm; the mesopore ratio is 25%; and the micropore ratio is 75%. 3 3

[0218] Embodiment 2

[0219] Step one: a proper amount of nano-carbon fiber (fiber diameter 10 nm, length: 0.5 um, aspect ratio 50:1) is cleaned with deionized water by ultrasonic treatment for three times, dried, added to anhydrous ethanol, and then cleaned with ultrasonic treatment for three times, dried, and treated to remove surface oil stains and impurities to obtain pure nano-carbon fiber.

[0220] Step two: the pure nano-carbon fiber is mixed with phenolic resin liquid at a ratio of 1:5 to form a suspension, and then dispersed in anhydrous ethanol to form a spray liquid with a solid content of 30%.

[0221] Step three: the spray liquid is subjected to a spray experiment: spray solidification conditions: atomizing disc rotation speed: 28000 rpm, air inlet temperature: 175℃, feeding speed: 0.8 L / h, air blower frequency: 50 HZ, and particles with a particle size of 5 um-20 um can be sprayed out.

[0222] Step four: the carbon microsphere particles after solidification are heat treated under a nitrogen atmosphere, the heating rate is 10℃ / min, the heat treatment temperature is 400℃, and the treatment time is 5 h for carbonization.

[0223] Step five: the carbonized carbon microspheres are placed in a well-sealed rotary kiln, the kiln is rotated at a frequency of 30 Hz, water vapor is introduced at a flow rate of 12 L / min, the heating rate of the rotary kiln is 10℃ / min, the temperature is raised to 900℃, and the temperature is maintained for 8 h, and then the material is cooled to room temperature.

[0224] Step six: the material obtained in step six is heated and boiled in a 2-5% hydrogen chloride solution for 1-2 h, and stirred overnight, then washed with pure water until the pH is 6.5-7.5, and then the filtered material is placed in an oven at 80℃-90℃ for drying until the moisture content is less than 3% for standby use.​​

[0225] Step seven: Put the material obtained in step six into a rotary kiln (secondary rotary) with good sealing, and pass the mixed gas of inert gas and silicon source into the rotary kiln, to obtain the silicon-carbon composite negative electrode material by chemical vapor deposition.

[0226] The deposition amount of silicon is 0.6-2 times the mass of the material obtained in step six, the deposition temperature is 400-900°C, the deposition time is 2-8 hours, and then the temperature is cooled to room temperature to obtain the final silicon-carbon composite material.

[0227] Example 3

[0228] The same as example 1, except that the fiber diameter of the nanometer carbon fiber in step one is 10 nm, the length is 1 um, and the aspect ratio is 100:1.

[0229] Example 4

[0230] The same as example 1, except that the fiber diameter of the nanometer carbon fiber in step one is 10 nm, the length is 1.5 um, and the aspect ratio is 150:1.

[0231] Example 5

[0232] The same as example 1, except that the fiber diameter of the nanometer carbon fiber in step one is 10 nm, the length is 2 um, and the aspect ratio is 200:1.

[0233] Example 6

[0234] The same as example 1, except that in step three, the silanized nanometer carbon fiber and 5 wt.% carbon tube are mixed with the phenolic resin liquid in a ratio of 1:5 to form a suspension, which is then dispersed in anhydrous ethanol to form a spray liquid with a solid content of 30%.

[0235] Example 7

[0236] The same as example 1, except that in step three, the silanized nanometer carbon fiber and the phenolic resin liquid are mixed in a ratio of 1:5, and then a hetero-element (N and / or P) resin liquid is added and mixed to form a suspension, which is then dispersed in anhydrous ethanol to form a spray liquid with a solid content of 30%.

[0237] Comparative Example 1

[0238] The same as example 1, except that the aspect ratio of the nanometer carbon fiber in step one is 5:1.

[0239] Comparative Example 2

[0240] The same as example 1, except that the aspect ratio of the nanometer carbon fiber in step one is 250:1.

[0241] Comparative Example 3

[0242] Step one: phenolic resin liquid is dispersed in anhydrous ethanol to form a spraying liquid with solid content of 30%.

[0243] Step two: the spraying liquid is sprayed to form particles with particle size of 5-20 um.

[0244] Step three: the carbon microsphere particles after solidification are heat treated under nitrogen atmosphere at a heating rate of 10 ℃ / min, and the heat treatment temperature is 400 ℃ for 5 h for carbonization.

[0245] Step four: the carbonized carbon microspheres are placed in a good sealing rotary kiln, the kiln is rotated at a frequency of 30 Hz, water vapor is introduced at a flow rate of 12 L / min, the rotary kiln is heated at a rate of 10 ℃ / min, and then heated to 900 ℃ and kept for 8 h, and then cooled to room temperature to obtain the material.

[0246] Step five: the material of step four is soaked in 5%-8% HCl and heated and stirred for more than 1 h, then washed with pure water until neutral, and dried at 80 ℃ to obtain the spherical porous silicon-carbon negative electrode carbon base material.

[0247] Step six: the material of step five is placed in a good sealing rotary kiln, the kiln is rotated at a frequency of 40 Hz, and a deposition gas source is introduced, which is a mixture of inert gas and silicon source: a mixture of acetylene and monosilane, the deposition amount is 0.6 times the amount of the material, the deposition temperature is 500 ℃, and the deposition time is 6 h. The spherical silicon-carbon negative electrode material is obtained.

[0248] Figure 2 The scanning electron microscope (SEM) image of the silicon-carbon negative electrode material prepared in Comparative Example 3 is shown.

[0249] Comparative Example 4

[0250] Step one: the biomass raw material is placed in a good sealing rotary kiln, and inert gas (nitrogen, neon, argon) is introduced for pre-carbonization at 300-700 ℃.

[0251] Step three: the carbonized sample is crushed into uniform particles of 3-5 mm by a crusher.

[0252] Step four: the material obtained in step three is placed in a good sealing rotary kiln, and a mixture of inert gas and pore-forming gas is introduced, and the rotary kiln is heated to 600-950 ℃ at a rate of 5-10 ℃ / min, and then cooled to room temperature to obtain the material. The pore-forming gas includes but is not limited to one or more of water vapor, carbon dioxide, flue gas or air.

[0253] Step five: the material obtained in step six is heated and boiled with 2-5% hydrogen chloride solution for 1-2h, and stirred overnight, then washed with pure water to pH 6.5-7.5, and then the filtered material is placed in an oven at 80-90°C to dry to moisture <3% for standby.

[0254] Step six: the obtained material is crushed to D50 4-10um for standby.

[0255] Step seven: the obtained material is placed in a well-sealed rotary kiln, and a mixed gas of inert gas and silicon source is introduced, and after chemical vapor deposition, a silicon-carbon composite negative electrode material is obtained.

[0256] The deposition amount of silicon is 0.8-2 times the mass of the material obtained in step, the deposition temperature is 400-800°C, the deposition time is 2-6h, and then cooled to room temperature to obtain the final silicon-carbon composite material.

[0257] Figure 3 The scanning electron microscope SEM of the silicon-carbon negative electrode material prepared in Comparative Example 4 is shown.

[0258] Comparative Example 5

[0259] Step one: cut the carbonized chopped carbon fibers of a certain length: 8um, diameter 5-7um into a well-sealed rotary kiln, rotate the kiln at a frequency of 30Hz, introduce water vapor, the flow rate of the gas is 12L / min, the heating rate of the rotary kiln is 10°C / min, heat to 900°C, and keep the temperature for 8h, then cool to room temperature to obtain the material.

[0260] Step two: soak the material of step four with 5%-8% HCl and heat and stir for more than 1 hour, then wash with pure water to neutral, and dry at 80°C to obtain a spherical porous silicon-carbon negative electrode carbon base material.

[0261] Step three: place the material in step five into a well-sealed rotary kiln, rotate the kiln at a frequency of 40Hz, introduce a deposition gas source which is a mixed gas of inert gas and silicon source: a mixed gas of acetylene and monosilane, the deposition amount is 0.6 times the amount of the material, the deposition temperature is 500°C, and the deposition time is 6h. A spherical silicon-carbon negative electrode material is obtained.

[0262] Figure 4 The scanning electron microscope SEM of the silicon-carbon negative electrode material prepared in Comparative Example 5 is shown.

[0263] Strength detection

[0264] Step a: use an in-situ multi-channel storage gas production test system, 3g of silicon-carbon powder is added to 9g of water, the test volume is 10ml, the temperature is 60°C, and the time is 48 hours, the gas production before pressing is L1 / ml;

[0265] Step b: Weigh the appropriate amount of silicon-carbon powder sample into the sample chamber of the compaction density instrument;

[0266] Step c: Set the compaction test conditions: set the appropriate pressurization and unloading displacement, pressure holding pressure and pressure holding time, so that the sample particles appear to be broken under the same pressure (ensure that the pressurization time and pressure holding time can crush the sample)

[0267] Step d: After the test is completed, use the rod retractor to exit, and collect the sample after the press.

[0268] Step e: Mix the collected sample for testing.

[0269] Step f: Use the in-situ multi-channel storage gas production test system, according to 3g silicon-carbon powder plus 9g water, test volume is 10ml, temperature is 60℃, time is 48 hours, test the gas production of the sample after the press is L2 / ml.

[0270] Step g: The overall strength of the silicon-carbon sample is represented by the change rate of gas production before and after the press.

[0271] Strength detection formula - formula I:

[0272]

[0273] P - the strength of the silicon-carbon sample, %;

[0274] L1 - the gas production before the press (compaction), mL;

[0275] L2 - the gas production after the press (compaction) mL;

[0276] P - the true density of the silicon-carbon material, g / cm 3 ;

[0277] The compaction conditions of step c are: 10mm / min, pressurization and holding time: 30S, test pressure: 450MPa-500Mpa.

[0278] Electrochemical test method: the negative electrode material prepared in each example and comparative example is assembled into a battery, specifically including the following steps. (1) Preparation of positive electrode sheet: the positive electrode active material lithium nickel cobalt manganese oxide (NCM811), conductive agent Super P, carbon nanotube, and binder polyvinylidene fluoride (PVDF) are uniformly mixed with N-methyl pyrrolidone (NMP) at a mass ratio of 97:1:0.5:1.5 to prepare a positive electrode slurry (solid content of 70%), which is coated on the positive and negative sides of the current collector aluminum foil, dried at 100°C, cold-pressed at 4 MPa at room temperature, then edge cut, sheet cutting, striping, and tab welding to prepare a positive electrode sheet. (2) Preparation of negative electrode sheet: under a nitrogen protective atmosphere, the solvent N-methyl pyrrolidone (NMP) is uniformly stirred with the binder PVDF, then the conductive agent Super P is uniformly stirred and mixed, followed by the addition of the silicon-carbon composite material for uniform stirring and mixing to prepare a negative electrode slurry (solid content of 50%).

[0279] The above negative electrode slurry is coated on the positive and negative sides of the current collector copper foil, dried at 100°C, cold-pressed at 4 MPa at room temperature, then edge cut, sheet cutting, striping, and tab welding to prepare a negative electrode sheet.

[0280] Assembly of lithium ion battery

[0281] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order with the separator between the positive and negative electrode sheets, and wound to obtain a bare cell; the bare cell is placed in an aluminum plastic shell package, dried at 100°C under a relative vacuum pressure of -0.95x105Pa to a moisture content of less than 100 ppm. The electrolyte is injected into the dried bare cell, wherein the electrolyte is composed of ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) (EC: EMC: DEC volume ratio = 1:1:1) and LiPF6 (1.0M), and the package is sealed, rested, formed (0.02C constant current charging for 2h, 0.1C constant current charging for 2h), shaped, and capacity tested (splitting), to prepare a soft-packaged liquid lithium ion battery.

[0282] During battery assembly, five batteries are prepared for each test, a total of five sets of data are tested, and the average value of the five sets of data is taken as the final performance.

[0283] The battery cycle performance is tested on a newwei device, specifically:

[0284] First cycle: discharge at 0.1C to 0.005V, then discharge at 0.08C to 0.001V, discharge at 0.05C to 0.001V, discharge at 0.02C to 0.001V, rest for 10min; charge at 0.1C to 1.5V, rest for 10min, record the charge-discharge capacity after the first cycle, calculate the first coulombic efficiency; cycle 100 times according to the above method, record the charge-discharge capacity after 100 times, and calculate the capacity retention rate after 100 cycles.

[0285] Table 1 performance test results

[0286]

[0287] As can be seen from Table 1, Comparative Example 3 is a spherical silicon-carbon composite material directly prepared from phenolic resin without adding carbon fibers, Comparative Example 4 is a silicon-carbon composite material directly prepared from biomass silicon-carbon, and Comparative Example 5 is a silicon-carbon composite material prepared by using only carbon fibers as a porous substrate. Compared with Comparative Examples 3-5, the silicon-carbon composite material (such as Examples 1-7) of the present application uses high-strength nanocarbon fibers to combine with resin to form a spherical porous carbon substrate. Compared with the biomass-based silicon-carbon composite material, the obtained silicon-carbon composite material has higher strength and does not have the natural macropore inside the biomass-based silicon-carbon composite material. In addition, the high conductivity of the nanocarbon fibers can enhance the electrochemical performance of the silicon-carbon composite material. The method of the present application improves the strength without sacrificing the electrochemical performance, and the strength is more than 89% and the first efficiency is more than 90%.

[0288] As can be seen from Table 1, the aspect ratio of the nanocarbon fibers used in Comparative Example 1 is 5:1, and the aspect ratio of the nanocarbon fibers used in Comparative Example 2 is 250:1. Compared with Comparative Examples 1 and 2, the aspect ratio of the nanocarbon fibers in the silicon-carbon composite material (such as Examples 1-7) of the present application is 10-200:1. The capacity, first efficiency and 100-cycle capacity retention rate of the silicon-carbon composite material prepared in this way are all improved.

[0289] The nanocarbon fibers have this specific aspect ratio, so that when the nanocarbon fibers are inserted into the porous carbon skeleton to form a porous carbon substrate, a three-dimensional conductive network skeleton that is interconnected can be constructed, which not only provides an ideal support framework for the subsequent deposition of nanosilicon particles, but also to some extent buffers the volume change of silicon during the charging and discharging process, thereby improving the electrochemical performance. If the aspect ratio of the nanocarbon fibers is low, a network structure that is interconnected cannot be formed inside the porous carbon substrate, which affects the conductivity of the composite material. If the aspect ratio of the nanocarbon fibers is high, the nanocarbon fibers will be too dense inside the porous carbon substrate, and due to the high strength, the subsequent activation porosity will be reduced, and a uniform porous structure cannot be formed, thereby affecting the deposition of nanosilicon particles.

[0290] Although the embodiments of the present application have been shown and described above, it should be understood by those having ordinary skill in the art that the above embodiments are exemplary, and cannot be construed as limiting the present application, and those having ordinary skill in the art can make changes, modifications, replacements and variations to the above embodiments without departing from the principles and spirit of the present application, and the scope of the present application. The scope of the present application is defined by the appended claims and their equivalents.

[0291] The above description is merely a specific implementation of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A silicon-carbon composite material, characterized in that: Comprising a porous carbon substrate and nano-silicon particles distributed on the porous carbon substrate; the strength P of the silicon-carbon composite material is ≥85%; The strength P of the silicon-carbon composite material is calculated according to the following formula: Wherein, P is the strength of the silicon-carbon composite material, %; L1 and L2 are the gas production of the silicon-carbon composite material before and after compaction, mL; ρ is the true density of the silicon-carbon composite material, g / cm 3 .

2. The silicon-carbon composite material according to claim 1, wherein The silicon-carbon composite material meets at least one or more of the following requirements: 1) The strength P of the silicon-carbon composite material is ≥89%; 2) the nano-silicon particles are deposited in the pores of the porous carbon substrate; 3) The mass ratio of the nano-silicon particles to the porous carbon substrate is (0.4-1.5):

1. Preferably, the silicon content of the silicon-carbon composite material is 40% to 60%; 4) The true density ρ of the silicon-carbon composite material is 1.87-2.16 g / cm 3 ; 5) The sphericity of the silicon-carbon composite material is ≥80%; 6) The particle size of the silicon-carbon composite material is 1 to 60 μm, wherein the D10 particle size is 3 μm to 7 μm, the D50 particle size is 7 μm to 12 μm, and the D90 particle size is 15 to 40 μm; 7) The silicon-carbon composite material further includes a carbon coating layer, which covers at least a portion of the surface of the porous carbon substrate and the nano-silicon particles.

3. The silicon-carbon composite material according to claim 1, wherein The porous carbon substrate meets at least one or more of the following requirements: 1) The porous carbon substrate is spherical, quasi-spherical or irregular in shape. Preferably, the porous carbon substrate is spherical; 2) The sphericity of the porous carbon substrate is ≥80%; 3) The porous carbon substrate has uniformly distributed micropores and / or mesopores; 4) The porous carbon substrate has uniformly distributed micropores and mesopores, with a microporosity of ≥70% and a mesoporosity of <30%; 5) The total pore volume of the porous carbon substrate is ≥ 0.7 cm 3 / g; 6) The porous carbon substrate includes doped N and / or P elements.

4. The silicon-carbon composite material according to claim 1, wherein The porous carbon substrate comprises a porous carbon skeleton and carbon nanofibers interspersed in the porous carbon skeleton, and the porous carbon substrate meets at least one or more of the following conditions: 1) The diameter of the carbon nanofiber is 10 to 50 nm; 2) The length of the carbon nanofiber is 0.5 to 2 μm; 3) The aspect ratio of the carbon nanofiber is 10 to 200:1; 4) the Young's modulus of the nano-carbon fiber is ≥200 GPa; 5) The carbon nanofibers are surface-modified carbon nanofibers, preferably, the carbon nanofibers are silanized surface-modified carbon nanofibers; 6) The porous carbon substrate further comprises carbon nanotubes interspersed within the porous carbon skeleton.

5. A method for preparing the silicon-carbon composite material according to any one of claims 1 to 4, characterized in that: Including steps: S1) generating a porous carbon substrate; S2) depositing nano-silicon particles in the pores of the porous carbon substrate by vapor deposition to form the silicon-carbon composite material, wherein the strength P of the silicon-carbon composite material is ≥85%.

6. The preparation method according to claim 5, wherein The step S1 comprises: S11) mixing the carbon nanofibers with the resin solution to obtain a suspension; S12) spray drying the suspension to obtain carbon particles; S13) sequentially subjecting the carbon particles to carbonization treatment and activation treatment to obtain a porous carbon substrate.

7. The preparation method according to claim 6, wherein The step S11 satisfies one or more of the following conditions: 1) The mass ratio of the carbon nanofiber to the resin liquid is 1:(1-5); 2) The resin is at least one or more of phenolic resin, polyimide resin or polystyrene resin; 3) The diameter of the carbon nanofiber is 10 to 50 nm; 4) The length of the carbon nanofiber is 0.5 to 2 μm; 5) The aspect ratio of the carbon nanofiber is 10 to 200; 6) The Young's modulus of the nano-carbon fiber is ≥200 GPa; 7) Step S11 further includes performing surface modification treatment on the carbon nanofibers, and then mixing with the resin liquid to obtain a suspension; preferably, the surface modification treatment is a silanization surface modification treatment; The silanization surface modification treatment comprises: dispersing the carbon nanofibers in a solvent, stirring, adding a silane coupling agent, continuing stirring, and drying; 8) Step S11 further includes adding carbon nanotubes to the suspension; The amount of the carbon nanotubes is 5 to 10 wt%; 9) Step S11 further includes adding a heteroelement resin into the suspension; the heteroelement resin is a resin containing N and / or P elements.

8. The preparation method according to claim 6, wherein Step S12 satisfies one or more of the following: 1) Before step S12, the suspension is dispersed in a solvent to form a spray liquid, and then spray-dried; 2) The solvent includes an alcohol solvent, preferably ethanol; 3) The solid content of the spray liquid is 30%-50%; 4) The rotation speed of the spray drying process is 27000-32000 rpm; 5) The inlet air temperature of the spray drying process is 180-250°C; 6) The feed rate of the spray drying process is 1.2 to 3 L / h.

9. The preparation method according to claim 6, wherein Step S13 satisfies one or more of the following: The carbonization temperature is 400°C to 800°C; The carbonization time is 3h to 8h; The activation is physical activation or chemical activation; The physical activation method comprises the following steps: introducing activation gas after carbonization for physical activation; the activation gas is carbon dioxide and / or water vapor; the activation temperature is 800° C. to 1000° C., and the activation time is 9 hours to 20 hours.

10. The preparation method according to claim 5, characterized in that The step S2 comprises: Passing a mixed gas of a first protective gas and a silicon source gas into the porous carbon substrate to perform a vapor deposition process to obtain the silicon-carbon composite material; Step S2 complies with one or more of the following: The first protective gas is any one of nitrogen, neon, argon, krypton, xenon or radon, or a combination of at least two thereof; The silicon source gas is any one of monosilane, disilane, dichlorosilane or trichlorosilane, or a combination of at least two thereof; The content of silicon source gas in the mixed gas is 70-90 vol%; The temperature of the vapor deposition is 400° C. to 900° C.; The vapor deposition time is 2 to 8 hours; Before the vapor deposition process, the activated material is sequentially subjected to acid washing, water washing, and drying processes; The pickling adopts 2-5wt% hydrochloric acid solution; The pickling temperature is 20-100°C; The pickling time is 1 to 24 hours; The drying temperature is 70-90°C.

11. The preparation method according to claim 5, wherein After step S2, the method further includes step S3 of carbon coating the silicon-carbon composite material, wherein step S3 satisfies one or more of the following conditions: 1) introducing a carbon source gas to deposit a carbon layer on the surface of the silicon-carbon composite material by chemical vapor deposition; 2) The flow rate of the carbon source gas is 5-70 L / min; 3) The carbon source gas is a mixed gas of a carbon-containing gas and a second protective gas, and the flow ratio of the carbon-containing gas to the protective gas is (50-80): (20-50); 4) the carbon-containing gas comprises any one of methane, ethane, acetylene or ethylene, or a combination of at least two thereof; and the second protective gas comprises any one of nitrogen, neon, argon, krypton, xenon or radon, or a combination of at least two thereof; 5) The deposition temperature is 300-800°C and the deposition time is 5-10 hours.

12. A method for detecting the strength of a silicon-carbon composite material, characterized in that: include: Measure the gas production L1 before compaction; Compacting the silicon-carbon composite material sample to be tested; Measure the gas production L2 after compaction; Substitute L1 and L2 into the formula to calculate the intensity value; P is the strength of the silicon-carbon composite material, %; L1 and L2 are the gas production before and after compaction, mL; ρ is the true density of the silicon-carbon composite material, g / cm 3 .

13. The detection method according to claim 12, wherein The compaction conditions are a pressure of 450-500 MPa and a holding time of ≥30 seconds; The gas production is measured at a temperature of 50-60°C.

14. A negative electrode sheet, characterized in that: comprising the silicon-carbon composite material according to any one of claims 1 to 4, Or the silicon-carbon composite material prepared by the preparation method according to any one of claims 5 to 11.

15. A lithium ion battery, characterized in that: The negative electrode sheet according to claim 14 is included.

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