Carbon material, silicon-carbon composite material and application thereof

Abstract

The invention relates to a carbon material and a silicon-carbon composite material, which are especially suitable for the field of lithium ion battery negative electrode materials. The carbon material particles are provided with compact outer surfaces and internal holes, the internal holes show that compact parts and hole parts can be observed on the internal cross sections of the carbon material particles, and the compact parts are continuous and partition and surround the hole parts; the hole parts are not communicated and are isolated and dispersed holes, the size of the holes is 10 nm to 4 microns, and the area of the holes accounts for 0.0001 to 20 percent of the area of the cross section. The invention also provides a silicon-carbon composite material which comprises the carbon material and silicon nanoparticles with the size of 2-5nm, and the internal pore size of the silicon-carbon composite material is 10nm-4mu m. The silicon-carbon composite material disclosed by the invention can be used as a negative electrode active material for a negative electrode of a lithium ion battery, and has excellent electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical fields of carbon material preparation and lithium-ion batteries, and in particular to a carbon material, a silicon-carbon composite material and their applications. Background Art

[0002] During the charge and discharge process, silicon undergoes a huge volume expansion (about 300%), resulting in pulverization of the electrode material, capacity attenuation, and shortened cycle life. The volume expansion of silicon-based materials particularly limits their applications in the consumer electronics field, such as mobile phones. Silicon-carbon composite materials combine the high capacity of silicon and the high conductivity of carbon, and have been favored by the industrial community in the field of lithium-ion batteries in recent years, with CVD silicon-carbon negative electrodes being the leading ones. CVD silicon-carbon negative electrodes use porous carbon as the framework, and silicon is deposited in the pores of the porous carbon. The pore size of the porous carbon determines the upper limit of the size of silicon particles. By regulating the pore size distribution and porosity of the carbon material, the size of silicon particles can be controlled. Nanoscale silicon particles help reduce the lithium intercalation expansion of silicon; however, since the pore sizes of most carbon framework materials are micropores or mesopores and are usually connected pore channels, silicon nanoparticles may also be continuous in the composite material, and their volume expansion will produce a chain effect, leading to increased expansion of the composite material and ultimately being difficult to control and resulting in particle pulverization failure; therefore, how to limit the lithium intercalation volume expansion of silicon particles within a limited range and avoid overall failure is an urgent problem to be solved, and there is no related disclosure in the prior art.

[0003] In view of this, the present invention is specifically proposed. Summary of the Invention

[0004] One object of the present invention is to provide a carbon material to solve the problem in the prior art that it is difficult to control the volume change of the content in the connected pore channels.

[0005] Another object of the present invention is to provide a silicon-carbon composite material, the internal discontinuous pores of which can limit the lithium intercalation volume expansion of silicon particles within a limited range, effectively relieve the volume expansion during the cycling of the silicon-carbon negative electrode, and solve the problem of high expansion rate of the existing silicon-carbon composite materials.

[0006] A third object of the present invention is to provide a negative electrode to improve the structural stability of the silicon-carbon negative electrode material, enhance the first Coulomb efficiency, and improve the cycle life and rate performance.

[0007] A fourth object of the present invention is to provide a battery to optimize the overall performance of the battery by using the negative electrode, and improve the energy density, cycle stability and service life.

[0008] In order to achieve the above objects of the present invention, the following technical solutions are specifically adopted: In a first aspect, the present invention provides a carbon material, which comprises carbon material particles. The carbon material particles have a dense outer surface and internal pores. The internal pores are manifested as a dense part and a pore part observable in the internal cross-section of the carbon material particles. The dense part is continuous and separates and surrounds the pore part. The pore parts are not connected and are isolated and dispersed pores.

[0009] Further, the size of the internal pores of the carbon material particles is 10 nm to 4 μm; and / or, on the internal cross-section of the carbon material particles, the ratio of the area of the pores to the area of the cross-section is 0.0001 to 20%.

[0010] Further, the carbon material is prepared from a carbon precursor by carbonization and / or activation. The carbon precursor includes resin, polysaccharide, biomass material, pitch, etc. Preferably, the resin includes one or more of phenolic resin, epoxy resin, polyester resin, polyurethane, polyethylene, and polypropylene.

[0011] Further, the carbon precursor is prepared by suspension polymerization and / or spray method. Preferably, the slurry of the suspension polymerization and / or spray method includes one or more of resin, polysaccharide, biomass material, pitch, etc. More preferably, the slurry includes a cross-linking agent and / or a pore-forming agent.

[0012] Further, the specific surface area of the carbon material is 200 to 2000 m 2 / g; and / or, the pore volume of the carbon material is 0.2 to 1.5 cm 3 / g; and / or, the average pore diameter of the carbon material is 1.6 to 4.0 nm; and / or, the proportion of mesopores in the carbon material is 5 to 50%.

[0013] In a second aspect, the present invention provides a silicon-carbon composite material. The silicon-carbon composite material particles comprise a carbon material and silicon nanoparticles. The silicon-carbon composite material particles have a dense outer surface and internal pores. The internal pores are manifested as a dense part and a pore part observable in the internal cross-section of the silicon-carbon composite material particles. The dense part is continuous and separates and surrounds the pore part. The pore parts are not connected and are isolated and dispersed pores.

[0014] Further, the size of the internal pores of the silicon-carbon composite material particles is 10 nm to 4 μm; and / or, on the internal cross-section of the silicon-carbon composite material particles, the ratio of the area of the pores to the area of the cross-section is 0.0001 to 20%; and / or, the size of the silicon nanoparticles is 2 to 5 nm.

[0015] Furthermore, the silicon-carbon composite material comprises silicon-carbon composite material particles, and the carbon material is the carbon material provided in the first aspect of the present invention, comprising the resin sphere carbon material and silicon nanoparticles provided in the first aspect of the present invention.

[0016] Furthermore, the silicon-carbon composite material is obtained by chemical vapor deposition of a silicon-containing precursor on the porous carbon material at 150-1000 °C; preferably, the silicon-containing precursor is selected from one or more of silane, disilane, trisilane, halogenated silane, polysilane, silole and its derivatives, silafluorene and its derivatives.

[0017] Furthermore, the silicon-carbon composite material comprises a coating layer on the surface of the silicon-carbon composite material; preferably, the material of the coating layer is selected from at least one of a solid electrolyte, a conductive polymer, a carbonaceous material, a metal, an alloy, a metal oxide, a metal hydroxide, a halogen-containing compound, a nitrogen-containing compound, a phosphorus-containing compound, a boron-containing compound, and a sulfur-containing compound; more preferably, the material of the coating layer is a carbonaceous material.

[0018] In a third aspect, the present invention provides a negative electrode, characterized in that the negative electrode comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material provided in the second aspect of the present invention.

[0019] In a fourth aspect, the present invention provides a battery, characterized in that the battery comprises a positive electrode, a negative electrode, a separator, an electrolyte, etc., the negative electrode comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material provided in the second aspect of the present invention.

[0020] Compared with the prior art, the present invention has at least the following beneficial effects: The present invention provides a carbon material, in which a certain number of pores are present inside the carbon material particles, and the internal pores inside the carbon material particles provide a buffer space for the volume expansion of silicon, effectively reducing the pulverization and capacity attenuation of the electrode material; and by regulating the number and size of the pores, the cycle stability of the silicon-carbon negative electrode is significantly improved.

[0021] The present invention provides a silicon-carbon composite material, in which the silicon nanoparticles are controlled to be 2-5 nm, and a certain number of pores are present inside the carbon material. The silicon nanoparticles in the CVD silicon-carbon composite material can be uniformly loaded, avoiding agglomeration. Controlling the size of the silicon nanoparticles to 2-5 nm greatly reduces the silicon particle expansion during the lithium insertion process, improving its cycle stability; on the other hand, by controlling the pore size and distribution optimization, the volume expansion of silicon is effectively buffered, and better long cycle stability can be obtained in the application of lithium-ion batteries. Description of the Drawings

[0022] 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 drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0023] Figure 1 This is the electron microscope scanning image of the carbon material provided in Embodiment 1 of the present invention.

[0024] Figure 2 This is the electron microscope scanning image of the carbon material provided in Embodiment 5 of the present invention.

[0025] Figure 3 This is the electron microscope scanning image of the carbon material provided in Comparative Example 1 of the present invention. Specific Embodiments

[0026] The following will describe the implementation solutions of the present invention in detail in combination with the embodiments and examples. However, those skilled in the art will understand that the following embodiments and examples are only used to illustrate the present invention and should not be regarded as limiting the scope 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. It should be noted that, without conflict, the embodiments and features in the embodiments of the present application can be combined with each other.

[0027] In the first typical embodiment of the present invention, a carbon material is provided. The carbon material includes carbon material particles. The carbon material particles have a dense outer surface and internal pores. The internal pores are manifested as a dense part and a pore part that can be observed in the internal cross-section of the carbon material particles. The dense part is continuous and separates and surrounds the pore part. The pore parts are not connected and are isolated and dispersed pores.

[0028] For porous materials, the so-called pores and voids usually refer to the pores that can be contacted by gas molecules such as N 2 etc. These pores can be obtained by performing a static adsorption test on the porous material with N 2 The DFT method can be used to analyze N 2The adsorption-desorption isotherm is fitted to analyze the pore size and pore distribution of the porous material; in the first typical embodiment of the present invention, the internal pores of the carbon material particles are essentially different from it. The internal pores of the present invention are enclosed inside the dense outer surface of the carbon material particles and can only be seen by crushing or cutting the particles. For dense materials, ordinary crushing methods may not be able to effectively break the particles. Only by means of Ar ion polishing / cutting, the atoms above the pores are peeled off layer by layer to expose the internal cross-section of the carbon material particles, and the existence of the internal pores can be observed through an electron microscope.

[0029] The carbon material provided by the present invention, the carbon material particles can be spherical or non-spherical, which does not affect the formation and existence of the internal pores. For example, spherical particles can be obtained by suspension polymerization or spraying methods, and non-spherical particles can be obtained by curing and carbonizing non-spherical carbon precursors, and processes such as crushing may be introduced. These methods only change the appearance morphology and do not affect the internal pores. The formation of the internal pores is related to the nature of the carbon precursor, the crosslinking network, whether a pore-forming agent is added, and the volatilization of organic matter. Having a relatively loose crosslinking network and adding a suitable pore-forming agent can achieve the formation of internal pores.

[0030] The cavities or pores inside the carbon material particles can provide a buffer space for the volume change of the filler and limit its volume change within a limited space, preventing damage to the overall material particles. In some embodiments, the size of the pores can be but is not limited to 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 500 nm, 1 μm or 4 μm, and can also be any value between 10 nm and 4 μm. 10 - 50 nm, 50 - 100 nm, 50 - 4 μm; if the pore size is too large, the structural stability of the material decreases.

[0031] In some embodiments, on the cross-section of the carbon material particles, the ratio of the area of the pores to the cross-sectional area is 0.0001% - 20%, and the ratio of the cross-sectional area can be but is not limited to 0.0001%, 0.001%, 0.01%, 0.02%, 0.04%, 0.06%, 0.08%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20% or any value between them.

[0032] In some embodiments, the carbon material is prepared by carbonizing and / or activating a carbon precursor, and the carbon precursor includes resin, polysaccharide, biomass material, pitch, etc.; preferably, the resin includes one or more of phenolic resin, epoxy resin, polyester resin, polyurethane, polyethylene, and polypropylene.

[0033] In some embodiments, the carbon precursor is prepared by suspension polymerization and / or spray method; preferably, the slurry of the suspension polymerization and / or spray method contains one or more of resin, polysaccharide, biomass material, asphalt, etc., and is obtained by precisely controlling the stirring rate, solid content of the slurry, heating rate, etc.; more preferably, the slurry contains a cross-linking agent and / or a pore-forming agent to ensure that the carbon precursor after solid-liquid separation has a fixed morphology and controllable internal pores.

[0034] In some embodiments, the specific surface area of the carbon material is 200 - 2000 m 2 / g. Among them, the specific surface area of the carbon material can be, but is not limited to, 200 m 2 / g, 500 m 2 / g, 700 m 2 / g, 800 m 2 / g, 1000 m 2 / g, 1200 m 2 / g, 1500 m 2 / g, 1700 m 2 / g or 2000 m 2 / g, and can also be any value between 200 - 2000 m 2 / g; in some embodiments, the pore volume of the carbon material is 0.2 - 1.5 cm 3 / g. Among them, the pore volume of the carbon material can be, but is not limited to, 0.2 cm 3 / g, 0.4 cm 3 / g, 0.6 cm 3 / g, 0.8 cm 3 / g, 1.0 cm 3 / g, 1.2 cm 3 / g, 1.4 cm 3 / g or 1.5 cm 3 / g, and can also be any value between 0.2 - 1.5 cm 3 / g; in some embodiments, the average pore diameter of the carbon material is 1.6 - 4 nm; in some embodiments, the mesopore ratio of the carbon material is 5 - 50%.

[0035] In the second typical embodiment of the present invention, a silicon-carbon composite material is further provided. The silicon-carbon composite material includes silicon-carbon composite material particles. The silicon-carbon composite material particles include a carbon material and silicon nanoparticles. The silicon-carbon composite material particles have a dense outer surface and internal pores. The internal pores are manifested as a dense part and a pore part observable in the internal cross-section of the silicon-carbon composite material particles. The dense part is continuous and separates and surrounds the pore part. The pore parts are not connected and are isolated and dispersed pores.

[0036] As mentioned in the first typical embodiment of the present invention, the internal pores of the silicon-carbon composite material particles in the present invention are also enclosed by their dense surfaces and can only be detected by means such as Ar polishing.

[0037] For the silicon-carbon composite material provided by the present invention, the internal pores in the silicon-carbon composite material particles can provide a buffer space for the volume expansion of silicon and limit its volume change within a limited space, preventing damage to the overall material particles, thereby effectively reducing the volume expansion of the composite material during the lithium intercalation process.

[0038] In some embodiments, the size of the internal pores of the silicon-carbon composite material particles is 10 nm to 4 μm.

[0039] In some embodiments, on the internal cross-section of the silicon-carbon composite material particles, the ratio of the area of the pores to the area of the cross-section is 0.0001 to 20%.

[0040] In some embodiments, the size of the silicon nanoparticles is 2 to 5 nm.

[0041] In some embodiments, the carbon material is the carbon material described in the first typical embodiment, including the resin sphere carbon material and silicon nanoparticles described in the first typical embodiment.

[0042] In some embodiments, the silicon-carbon composite material is obtained by chemical vapor deposition of a silicon-containing precursor on the porous carbon material at 150 to 1000 °C. Preferably, the silicon-containing precursor is selected from one or more of silane, disilane, trisilane, halogenated silane, polysilane, silafluorene and its derivatives, silole and its derivatives.

[0043] In some embodiments, the silicon-carbon composite material includes a coating layer on the surface of the silicon-carbon composite material. Preferably, the material of the coating layer is selected from at least one of a solid electrolyte, a conductive polymer, a carbonaceous material, a metal, an alloy, a metal oxide, a metal hydroxide, a halogen-containing compound, a nitrogen-containing compound, a phosphorus-containing compound, a boron-containing compound, and a sulfur-containing compound. Further preferably, the material of the coating layer is a carbonaceous material.

[0044] In the third typical embodiment of the present invention, a negative electrode is further provided. The negative electrode includes a negative electrode active material, and the negative electrode active material includes the silicon-carbon composite material in the second typical embodiment of the present invention.

[0045] In the fourth typical embodiment of the present invention, a battery is further provided. The battery includes a positive electrode, a negative electrode, a separator, an electrolyte, etc. The negative electrode includes a negative electrode active material, and the negative electrode active material includes the silicon-carbon composite material in the second typical embodiment of the present invention.

[0046] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work shall fall within the protection scope of the present invention.

[0047] Example 1

[0048] A preparation method of a carbon material is specifically operated according to the following steps: Step S1, preparing a carbon precursor: Add 100 g of deionized water to a beaker, heat it to 95 °C, add 0.4 g of polyvinyl alcohol (PVA), stir for 2 h to fully dissolve it, and then add 0.5 g of curing agent hexamethylenetetramine and stir for 0.5 h until fully dissolved; continue to add an ethanol solution containing 5 g of thermoplastic resin, and stir and react at 97 °C for 4 h. Separate the solid-liquid of the above mixed solution, and the carbon precursor can be obtained after drying.

[0049] Step S2, carbonization: The carbon precursor obtained in the above step S1 is heated to 700 °C at a rate of 2 °C / min in an N 2 atmosphere and held for 2 h.

[0050] Step S3, activation: The carbonized material is subjected to water activation, and heated to 900 °C at a rate of 2 °C / min in an N 2 and H 2 O(g) mixed atmosphere and held for 2 h; the carbon material is obtained.

[0051] The internal cross-section of the obtained carbon material particles contains pores, the size of the pores is in the range of 10 nm to 100 nm, including 10 nm and 100 nm; the ratio of the area of the pores to the cross-sectional area is 0.0004%. The electron microscope scan diagram of the carbon material in this Example 1 is as shown in the appendix Figure 1 as follows.

[0052] Example 2

[0053] This embodiment provides a carbon material. The difference in the preparation method from Embodiment 1 is that the reaction temperature is changed from 97 °C to 93 °C.

[0054] Embodiment 3

[0055] Step S1, prepare a carbon precursor: Add 100 g of deionized water to a beaker, heat it to 95 °C, add 0.4 g of polyvinyl alcohol (PVA) and 0.1 mL of pore-forming agent toluene, stir for 2 h to fully dissolve it, and then add 0.5 g of curing agent hexamethylenetetramine and stir for 0.5 h until fully dissolved; continue to add 5 g of an ethanol solution of thermoplastic resin and stir and react for 4 h. Separate the solid from the liquid of the above mixture to obtain the carbon precursor.

[0056] Step S2, carbonization: Heat the carbon precursor obtained in Step S1 above to 700 °C at a rate of 2 °C / min under a N 2 atmosphere and hold for 2 h.

[0057] Step S3, activation: Activate the carbonized material by water activation. Heat it to 900 °C at a rate of 2 °C / min under a mixed atmosphere of N 2 and H 2 O(g) and hold for 2 h; obtain the carbon material.

[0058] Embodiment 4

[0059] This embodiment provides a carbon material. The difference in the preparation method from Embodiment 3 is that 0.1 mL of pore-forming agent toluene is replaced by 2 g of cyclohexane, and other conditions remain unchanged.

[0060] Embodiment 5

[0061] A preparation method of a carbon material is specifically operated according to the following steps: Step S1, prepare a carbon precursor: Mix 10 g of starch with 100 mL of deionized water and stir into a uniform suspension. Use 1 mol / L NaOH solution to adjust the pH of the solution to 9 - 11. Then mix the starch slurry with liquid paraffin in a volume ratio of 1:3, add 1 g of emulsifier (Span 80), stir at high speed to form a water / oil emulsion, and heat to 80 °C to solidify the starch spheres. Then centrifuge and wash with ethanol to remove the oil phase, and dry at 50 °C for 6 h.

[0062] Step S2, carbonization: Heat the carbon precursor obtained in Step S1 above to 700 °C at a rate of 2 °C / min under a N 2 atmosphere and hold for 2 h.

[0063] Step S3, activation: Activate the carbonized material by water activation. Heat it to 900 °C at a rate of 2 °C / min under a mixed atmosphere of N 2 and H 2Under an O(g) mixed atmosphere, it is heated to 900 °C at a rate of 2 °C / min and held for 2 h; a carbon material is obtained.

[0064] Comparative Example 1

[0065] Step S1, carbonization: directly carbonize the thermoplastic resin, and heat it to 700 °C at a rate of 2 °C / min in an N 2 atmosphere and hold for 2 h.

[0066] Step S2, activation: water-activate the carbonized material, and heat it to 900 °C at a rate of 2 °C / min in an N 2 and H 2 O(g) mixed atmosphere and hold for 2 h; a carbon material is obtained.

[0067] Example 6

[0068] This example provides a silicon-carbon composite material, and its preparation method is as follows: Step S1, provide a carbon material, and the carbon material is the carbon material obtained in Example 1; Step S2, crush and classify the carbon in Step S1 to obtain d V50 carbon powder with a size of 8 μm; Step S3, place the mesoporous carbon powder obtained in Step S2 in a tube furnace, and heat it from room temperature to 550 °C at a rate of 2 °C / min in an N 2 atmosphere; then change to a 20% SiH 4 -N 2 mixed gas, and hold at 550 °C for 10 h in a 20% SiH 4 -N 2 mixed atmosphere; change to introduce a 10% C 2 H 2 -N 2 mixed gas, and hold at 600 °C for 1 h; naturally cool down in an N 2 atmosphere to obtain a silicon-carbon composite material.

[0069] Example 7

[0070] This example provides a silicon-carbon composite material, and its preparation method is as follows: Step S1, provide a carbon material, and the carbon material is the carbon material obtained in Example 2; Steps S2 and S3 are the same as Steps S2 and S3 in Example 6.

[0071] Example 8

[0072] This example provides a silicon-carbon composite material, and its preparation method is as follows: Step S1, provide a carbon material, and the carbon material is the carbon material obtained in Example 3; Steps S2 and S3 are the same as Steps S2 and S3 in Example 6.

[0073] Example 9

[0074] This example provides a silicon-carbon composite material, and its preparation method is as follows: Step S1: Provide a carbon material, which is the carbon material obtained in Example 4; Steps S2 and S3 are the same as Steps S2 and S3 in Example 6.

[0075] Example 10

[0076] This example provides a silicon-carbon composite material, and its preparation method is as follows: Step S1: Provide a carbon material, which is the carbon material obtained in Example 5; Steps S2 and S3 are the same as Steps S2 and S3 in Example 6.

[0077] Comparative Example 2

[0078] This example provides a silicon-carbon composite material, and its preparation method is as follows: Step S1: Provide a carbon material, which is the carbon material obtained in Comparative Example 1; Steps S2 and S3 are the same as Steps S2 and S3 in Example 6.

[0079] (1) N 2 The adsorption test is a well-known material characterization method for those skilled in the art, and its specific test conditions will not be elaborated here. The specific surface area A of the carbon material is obtained by multi-point BET calculation, and the calculation result satisfies the intercept C value > 0 and the correlation coefficient R2 ≥ 0.9999. The pore volume V is obtained according to the maximum adsorption amount at p / p 2 >0.99 in the N 0 adsorption test, and the average pore diameter D = 4V / A * 1000 (nm). The pore distribution of the carbon material can be estimated through the average pore diameter. When the average pore diameter D < 2 nm, the micropore proportion is usually greater than 80%. The DFT method is used to fit the N 2 adsorption-desorption isotherm, analyze the pore distribution of different pore diameters in the carbon material, and obtain the content information of each level of pore diameter.

[0080] (2) Swelling rate test in the silicon-carbon composite material: Use a BlueTEC (LAND) battery test system to perform charge and discharge tests on the battery.

[0081] After the CR2032 coin cell stands still for 6 h, it is discharged at 0.05 C to 0.005 V and then discharged at 0.01 C to 0.005 V. After standing still for 5 min, it is charged at a constant current of 0.05 C to 15 V. The ratio of the first de-lithiation capacity to the first lithium intercalation capacity is the first Coulombic efficiency of the battery.

[0082] After standing still for 5 min, repeat the above charge-discharge steps twice. Then discharge at 0.25 C to 0.005 V. After standing still for 5 min, charge at a constant current of 0.25 C to 1.5 V for 50 cycles. Calculate the capacity retention rate by (charge specific capacity of the 50th cycle / charge capacity of the 1st cycle) x 100%.

[0083] The following method is used to test the expansion rate of the electrode sheet: After the CR2032 coin cell stands still for 6 h, it is discharged at 0.05 C to 0.005 V and then discharged at 0.01 C to 0.005 V. Then disassemble the coin cell in the glove box, clean the electrode sheet with DEC and measure the thickness of the electrode sheet. The calculation formula for the expansion rate is: (thickness of the electrode sheet in the first fully charged state - thickness of the fresh electrode sheet) / thickness of the fresh electrode sheet x 100%.

[0084] (3)Testing the Si content in the silicon-carbon composite material: Place 3.0 g of the silicon-carbon composite material in an oven at 150 °C and dry it to a constant weight, record the mass m. 1 ; Place the dried composite material in a muffle furnace, heat it to 1100 °C and keep it for 2 h, cool down and weigh it, record the mass m. 2 . The calculation formula for the Si content is: .

[0085] (4)The following gives a brief description of the battery tests.

[0086] Half-cell test method: Electrochemical performance tests are carried out on the silicon-carbon composite materials obtained in Examples 6 - 10 and Comparative Example 2. The results are shown in Table 2. Using the silicon-carbon composite materials obtained in Examples 6 - 10 and Comparative Example 2 as the negative electrode active materials, negative electrode sheets are prepared respectively. The negative electrode sheets are used to prepare CR2032 coin cells by conventional methods, and electrical performance tests are carried out on the cells. Use a Blue-Energy (LAND) battery test system to carry out charge-discharge tests on the cells.

[0087] (1)Half-cell assembly: Assemble a CR2032 coin cell in a glove box, using a lithium metal sheet as the counter electrode, a polypropylene microporous membrane as the separator, and an electrolyte of LiPF6 dissolved in a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio EC:DEC = 1:1), where the concentration of LiPF6 is 1 mol / L.

[0088] (2)Cyclic specific capacity and first efficiency test: After the CR2032 coin cell stands still for 6 h, it is discharged at 0.05 C to 0.005 V, and then discharged at 0.01 C to 0.005 V; after standing still for 5 min, it is charged at a constant current of 0.05 C to 1.5 V; the first lithium deintercalation specific capacity at 0.8 V is the 0.8 V specific capacity (or mass specific capacity) of the electrode material, and the ratio of the first lithium deintercalation capacity at 0.8 V to the first lithium intercalation capacity at 1.5 V is the first Coulombic efficiency of the battery at 0.8 V.

[0089] High-pressure dense half-cell test method: After the negative electrode sheet is prepared by the conventional method, it is further roll-pressed to prepare a high-pressure dense negative electrode sheet. The compaction of the conventional electrode sheet is 0.8 g / cm 3 , and the compaction of the high-pressure dense electrode sheet is about 1.0 g / cm 3 .

[0090] Full-cell test method: Using the silicon-carbon composite materials obtained in Examples 6 to 10 and Comparative Example 2 as the negative electrode active material, the electrode sheet containing the negative electrode active material is used to prepare a soft-pack battery by the conventional method and its electrical properties are tested.

[0091] (1)Fabrication of the positive electrode sheet: The positive electrode active material LiCoO 2 , conductive agent Super P, binder PVDF and solvent NMP are stirred and mixed evenly according to the mass ratio of 92:3:5:150, and then evenly coated on the positive electrode current collector, and then dried at 80 °C to obtain the positive electrode sheet.

[0092] (2)Fabrication of the negative electrode sheet: The negative electrode active material, conductive agent Super P, binder polyacrylic acid and solvent deionized water are stirred and mixed evenly according to the mass ratio of 95:1:4:120, and then evenly coated on the negative electrode current collector, and then dried at 100 °C to obtain the negative electrode sheet.

[0093] (3)The positive electrode sheet and the negative electrode sheet are stacked in a square and separated by a polypropylene separator to form a battery core, which is then encapsulated in an aluminum-plastic bag. The corresponding capacity of the electrolyte is injected into the aluminum-plastic bag, and after vacuum sealing, the soft-pack battery is obtained. The electrolyte is a mixed solution of EC and DEC of LiPF6, where the concentration of LiPF6 is 1 mol / L and the volume ratio of EC to DEC is 1:1.

[0094] (4)Formation and grading: The battery after injection and sealing starts the formation process. It is left standing in an incubator at 25 °C for 12 h, then charged at a constant current of 0.02C to 3.3 V, left standing for 30 min, charged at a constant current of 0.025 C to 3.8 V, left standing for 10 min, and charged at a constant current of 0.33 C to 4.2 V. After formation, the battery is evacuated and the airbag is cut, and then graded. It is charged at a constant current of 0.33 C to 4.45 V, left standing for 10 min, discharged at a constant current of 1 C to 3 V, left standing for 10 min, and discharged at a constant current of 0.33 C to 3 V, and the grading ends. The ratio of the discharge capacity to the charge capacity during the formation and grading of the soft-pack battery is the first efficiency of the battery.

[0095] (5)25 °C cycle test: The battery is placed in an incubator at 25 °C, charged at a constant current of 1 C to 4.45 V, and then charged at a constant voltage of 4.45 V until the current is 0.1 C. After standing for 10 min, it is discharged at a constant current of 1 C to 3.0 V, left standing for 10 min, and the above charging and discharging steps are repeated until the discharge capacity is lower than 80% of the discharge capacity of the first cycle and then stopped. At this time, the number of cycles obtained is the cycle life of the soft-pack battery; record the capacity retention rate of 100 cycles.

[0096] Table 1 shows the physical and chemical properties of the carbon materials obtained in Examples 1 to 5 and Comparative Example 1, and Table 2 shows the physical and chemical properties and electrochemical performance of the silicon-carbon composite materials obtained in Examples 6 to 10 and Comparative Example 2.

[0097] Table 1 Physical and Chemical Properties of Carbon Materials Obtained in Examples 1 to 5 and Comparative Example 1 Average pore diameter (nm) <![CDATA[Pore volume (cm 3 / g)]]> Mesopore ratio (%) Pore size range (nm) Percentage of pore area % Example 1 1.75 0.88 7.7 10~100 0.0004 Example 2 1.70 0.79 6.3 50~500 0.01 Example 3 1.67 0.73 5.9 300~2000 0.245 Example 4 1.64 0.63 5.0 200~3000 2.5 Example 5 1.65 0.58 5.9 150~4000 20.0 Comparative Example 1 1.75 0.81 7.4 0 0

[0098] Table 2 Physical and Chemical Properties and Electrochemical Performance of Silicon-Carbon Composite Materials Obtained in Examples 6 to 10 and Comparative Example 2 Silicon content (wt.%) <![CDATA[Specific surface area (m 2 / g)]]> 0.8 V specific capacity (mAh / g) 0.8 V initial efficiency (%) Capacity retention rate at 25°C after 100 cycles (%) Silicon-carbon expansion rate (%) Example 6 52.6 1.1 1847 86.8 99.3 120 Example 7 50.0 3.3 1763 85.2 99.7 100 Example 8 48.0 1.7 1701 85.7 99.5 78 Example 9 44.3 0.9 1585 84.6 99.1 65 Example 10 42.3 2.7 1520 83.9 99.0 40 Comparative Example 2 50.6 2.1 1783 85.2 97.4 150

[0099] The data in Table 1 show that the proportion of the pore area in Examples 1 to 5 gradually increases, indicating that different preparation methods (such as resin monomer emulsion polymerization, pore-forming agents toluene and cyclohexane, and starch emulsion method) can regulate the pore structure. There are no pores in Comparative Example 1 by directly carbonizing the resin, indicating that the treatment of the precursor is crucial for the internal pore structure of the material. Example 5 (starch-based) has the highest pore area proportion (20%), but the lowest pore volume (0.58 cm³ / g), because the highest proportion of internal pores in the material leads to the lowest pore volume. When preparing the carbon precursor, pore-forming agents (toluene, cyclohexane) and the starch emulsion method can significantly increase the proportion of the pore area (0.245% - 20%).

[0100] As shown in the data in Table 2, the capacity retention rates of the silicon-carbon materials obtained in Examples 6 to 10 after 100 cycles are all greater than 99%, showing the best performance. This is due to the reasonable pore structure (pore proportion: 0.0004% - 20%) that alleviates the expansion of silicon nanoparticles during the cycling process. The capacity retention rate of the silicon-carbon material obtained in Comparative Example 2 is only 97.4% at the lowest, and the expansion rate is as high as 150%, verifying the importance of pores in buffering the expansion of silicon. The expansion rate of Example 10 (starch-based carbon, pore proportion: 20%) is only 40%, indicating that the macroporous structure is more effective in alleviating volume changes. Comparative Example 1 (without pores) has the worst performance, proving that pores are the key to improving the performance of silicon-carbon composites and demonstrating the necessity of the pore structure for the long-cycle performance of silicon-carbon composites.

[0101] 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 for some or all of the technical features. However, such modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A carbon material, characterized in that The carbon material comprises carbon material particles, which have a dense outer surface and internal pores. The internal pores are manifested in that a dense part and a pore part can be observed in the internal cross-section of the carbon material particles, the dense part is continuous and separates and surrounds the pore part; the pore part is not connected and is isolated and dispersed.

2. The carbon material according to claim 1, characterized in that The size of the internal pores of the carbon material particles is 10 nm to 4 μm; and / or, on the internal cross section of the carbon material particles, the ratio of the area of ​​the pores to the cross section area is 0.0001 to 20%.

3. The carbon material according to claim 1, characterized in that The carbon material is obtained by carbonizing and / or activating a carbon precursor, and the carbon precursor includes resin, polysaccharide, biomass material, asphalt, etc.; preferably, the resin includes one or more of phenolic resin, epoxy resin, polyester resin, polyurethane, polyethylene, and polypropylene.

4. The carbon material according to claim 3, characterized in that The carbon precursor is prepared by suspension polymerization and / or spraying; preferably, the slurry of the suspension polymerization and / or spraying method contains one or more of resin, polysaccharide, biomass material, asphalt, etc. Further preferably, the slurry contains a cross-linking agent and / or a porogen.

5. The carbon material according to claim 1, characterized in that The specific surface area of ​​the carbon material is 200-2000 m 2 / g; And / or, the pore volume of the carbon material is 0.2-1.5 cm 3 / g; And / or, the average pore size of the carbon material is 1.6-4.0 nm; And / or, the mesopore ratio of the carbon material is 5-50%.

6. A silicon-carbon composite material, characterized in that: The silicon-carbon composite material comprises silicon-carbon composite material particles, wherein the silicon-carbon composite material particles comprise carbon material and silicon nanoparticles, wherein the silicon-carbon composite material particles have a dense outer surface and internal pores, wherein the internal pores are manifested in that a dense portion and a pore portion can be observed in the internal cross section of the silicon-carbon composite material particle, wherein the dense portion is continuous and separates and surrounds the pore portion; and the pore portion is not connected and is isolated and dispersed.

7. The silicon-carbon composite material according to claim 6, characterized in that: The size of the internal pores of the silicon-carbon composite material particles is 10 nm to 4 μm; and / or, on the internal cross-section of the silicon-carbon composite material particles, the ratio of the area of ​​the pores to the cross-sectional area is 0.0001 to 20%; and / or, the size of the silicon nanoparticles is 2 to 5 nm.

8. The silicon-carbon composite material according to claim 6, characterized in that: The carbon material is the carbon material according to claims 1 to 5, comprising the resin ball carbon material according to claim 1 and silicon nanoparticles.

9. The silicon-carbon composite material according to claim 6, characterized in that: The silicon-carbon composite material is obtained by chemical vapor deposition of a silicon-containing precursor on the porous carbon material at 150-1000°C; preferably, the silicon-containing precursor is selected from one or more of monosilane, disilane, trisilane, halosilanes, polysilanes, silole and its derivatives, silanol and its derivatives.

10. The silicon-carbon composite material according to claim 6, characterized in that: The silicon-carbon composite material comprises a coating layer located on the surface of the silicon-carbon composite material; preferably, the material of the coating layer is selected from at least one of solid electrolytes, conductive polymers, carbonaceous materials, metals, alloys, metal oxides, metal hydroxides, halogen-containing compounds, nitrogen-containing compounds, phosphorus-containing compounds, boron-containing compounds, and sulfur-containing compounds; further preferably, the material of the coating layer is a carbonaceous material.

11. A negative electrode, characterized in that The negative electrode comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material according to any one of claims 6 to 10.

12. A battery, characterized in that The battery comprises a positive electrode, a negative electrode, a separator, an electrolyte, etc., and the negative electrode comprises the silicon-carbon composite material according to any one of claims 6 to 10.

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