Porous carbon material, silicon-based composite material, preparation methods of porous carbon material and silicon-based composite material, negative electrode and secondary battery

Abstract

The invention provides a porous carbon material, a silicon-based composite material, a preparation method of the silicon-based composite material, a negative electrode and a secondary battery, and particularly relates to the technical field of secondary batteries. The porous carbon material comprises nano carbon dot microcrystals; and the size of the nano carbon dot microcrystal is 2-8 nm. The nano carbon dot microcrystals comprise graphene quantum dot microcrystals; the porous carbon material comprises an amorphous continuous region, and the nano carbon dot microcrystals are dispersed in the amorphous continuous region; the specific surface area of the porous carbon material is 200-3000m < 2 > / g, and the pore volume is 0.2-3.0 cm < 3 > / g. The porous carbon material provided by the invention contains 2-8nm carbon dot microcrystals, so that the electrical conductivity and ionic conductivity of the porous carbon material are improved, poor contact between silicon and carbon is avoided, the contact performance of silicon and carbon is improved, and the rate capability and cycling stability of a silicon-based composite material are improved; and thus, the capacity stability and coulombic efficiency of the secondary battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to a porous carbon material, a silicon-based composite material and a preparation method thereof, a negative electrode and a secondary battery. Background Art

[0002] In recent years, Si / C composite negative electrode materials are facing industrialization, but their various performances still need to be further improved.

[0003] Silicon-based materials have an extremely high theoretical lithium storage capacity (4200 mAh / g), but significant volume effects also occur during charge and discharge processes, resulting in problems such as damage to the electrode structure, attenuation of battery capacity, and reduction of cycle efficiency. The conductivity of silicon is poor, and its electronic conductivity is only 10 -5 ~10 -3 S / cm. The composite carbon material can not only improve the conductivity of the silicon-based negative electrode, but also relieve the volume expansion of silicon during lithium intercalation, greatly improving the initial Coulomb efficiency and cycle stability of the silicon negative electrode. However, the composite carbon material has a defect: the volume of silicon in the silicon-carbon composite material expands and shrinks repeatedly, and it is easy to have poor contact between silicon and carbon during charge and discharge cycles, affecting the transmission of electrons and lithium ions, resulting in poor rate performance of the silicon-carbon composite material.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] One of the purposes of the present invention is to provide a porous carbon material to solve the technical problem in the prior art that there is poor contact between silicon and carbon, affecting the transmission of electrons and lithium ions.

[0006] Another purpose of the present invention is to provide a silicon-based composite material and a preparation method thereof.

[0007] The third purpose of the present invention is to provide a negative electrode and a secondary battery including the negative electrode.

[0008] In order to achieve the above purposes of the present invention, the following technical solutions are specifically adopted:

[0009] The first aspect of the present invention provides a porous carbon material, which includes nano-carbon dot microcrystals; the size of the nano-carbon dot microcrystals is 2-8 nm.

[0010] Further, the nano-carbon dot microcrystals include graphene quantum dot microcrystals, and the planar size of the graphene quantum dots is 2-8 nm.

[0011] Preferably, the porous carbon material includes an amorphous continuous region, and the nano-carbon dot microcrystals are dispersed in the amorphous continuous region.

[0012] Preferably, the specific surface area of the porous carbon material is 200-3000 m2 / g, the pore volume is 0.2 - 3.0 cm 3 / g.

[0013] In the second aspect of the present invention, a silicon-based composite material is provided, which mainly consists of silicon-based composite material particles;

[0014] The silicon-based composite material particles comprise silicon nanoparticles and the porous carbon material described in the first aspect;

[0015] The silicon nanoparticles are located in the pores of the porous carbon material.

[0016] Furthermore, the silicon content is 5 - 90 wt.%, preferably 30 - 70 wt.%.

[0017] And / or, the specific surface area of the silicon-based composite material is 0.1 - 50 m 2 / g, the pore volume is 0.001 - 0.1 cm 3 / g; preferably, the specific surface area of the silicon-based composite material is 0.5 - 10 m 2 / g, the pore volume is 0.001 - 0.05 cm 3 / g.

[0018] And / or, the true density of the silicon-based composite material measured by helium pycnometry is 1.3 - 2.0 g / cm 3 , the closed pore volume is 0.01 - 0.25 cm 3 / g.

[0019] And / or, the median particle size d V,50 of the silicon-based composite material particles is 5 - 20 μm, and the diameter interval (d V,90 -d V,10 ) / d V,50 is 0.7 - 2.0. Preferably, the median particle size d V,50 of the silicon-based composite material is 6 - 12 μm, and the diameter interval (d V,90 -d V,10 ) / d V,50 is 0.7 - 1.2.

[0020] Furthermore, the surface of the silicon-based composite material particles has a coating layer;

[0021] Preferably, the surface of the silicon-based composite material particles has a coating layer;

[0022] Preferably, the material of the coating layer includes at least one of solid electrolytes, conductive polymers, artificial SEI, etc.;

[0023] Alternatively, the material of the coating layer includes at least one of carbonaceous materials, metals, alloys, metal oxides, nitrogen-containing compounds, phosphorus-containing compounds, boron-containing compounds, halogen-containing compounds, and sulfur-containing compounds.

[0024] In the third aspect of the present invention, there is provided a method for preparing the silicon-based composite material, which comprises contacting a silicon precursor with a porous carbon material and performing chemical vapor deposition to obtain the silicon-based composite material.

[0025] Further, the temperature of the chemical vapor deposition is 150 to 1000 °C, and the time is 1 to 100 h.

[0026] And / or, the silicon precursor includes at least one of silane, disilane, trisilane, halogenated silane, polysilane, polysiloxane, polycarbosilane, silafluorene and its derivatives, and silole and its derivatives.

[0027] Further, it further includes introducing a heteroatom-containing precursor during the chemical vapor deposition.

[0028] Preferably, the heteroatom-containing precursor includes at least one of a nitrogen-containing precursor, a phosphorus-containing precursor, a sulfur-containing precursor, or a boron-containing precursor.

[0029] In the fourth aspect of the present invention, there is provided a negative electrode, which includes a negative electrode active material;

[0030] The negative electrode active material includes the silicon-based composite material described in the second aspect and the silicon-based composite material obtained according to the preparation method described in the third aspect.

[0031] In the fifth aspect of the present invention, there is provided a battery, which includes a positive electrode, a separator, an electrolyte, and the negative electrode described in the fourth aspect.

[0032] Compared with the prior art, the present invention has at least the following beneficial effects:

[0033] The porous carbon material provided by the present invention contains carbon dot microcrystals with a size of 2 to 8 nm. The nano carbon dot microcrystals have a significant quantum effect, which can effectively enhance the conductivity and ion conductivity of the porous carbon material. The silicon-based composite material made of this porous carbon material also has significantly higher conductivity and ion conductivity than ordinary porous carbon materials, improving the Coulomb efficiency and rate performance of the material; on the other hand, in this silicon-based composite material, the presence of nano carbon dot microcrystals improves the electrical contact between the porous carbon material and silicon nanoparticles, improving the cycle stability of the composite material; thereby improving the performance of secondary batteries and promoting the development of downstream industries. Description of the Drawings

[0034] 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 also be obtained based on these drawings.

[0035] Figure 1 HRTEM image of the silicon-based composite material obtained in Example 4. Specific embodiments

[0036] 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 belong to the scope of protection of the present invention.

[0037] The first aspect of the present invention provides a porous carbon material, which includes nano-carbon dot microcrystals; the size of the nano-carbon dot microcrystals is 2 - 8 nm.

[0038] The porous carbon material provided by the present invention contains nano-carbon dot microcrystals with a size of 2 - 8 nm. These nano-carbon dot microcrystals have significant quantum effects, which can effectively enhance conductivity and ion conductivity. When used as a supported material, the nano-carbon dot microcrystals in the bulk phase of the porous carbon material can improve the conductivity and ion conductivity of the composite material and enhance the electrical contact between the porous carbon matrix and the loaded substance.

[0039] Since the nano-carbon dot microcrystals are dispersed inside the amorphous porous carbon matrix and the content is low, the porous carbon material does not have high long-range order and its crystallization situation cannot be detected by XRD. The lattice fringes of the nano-carbon dot microcrystals can be observed by high-resolution TEM.

[0040] Further, the nano-carbon dot microcrystals include graphene quantum dot microcrystals, and the planar size of the graphene quantum dots is 2 - 8 nm.

[0041] Compared with nano-graphene, the graphene quantum dot microcrystals have stronger quantum confinement effects and boundary effects, which can further enhance conductivity and ion conductivity, thereby improving the Coulomb efficiency and rate performance of the material.

[0042] Preferably, the porous carbon material includes an amorphous continuous region, and the nano-carbon dot microcrystals are dispersed in the amorphous continuous region.

[0043] Preferably, the specific surface area of the porous carbon material is 200 - 3000 m 2 / g, with a pore volume of 0.2 - 3.0 cm 3 / g.

[0044] The preparation method of the porous carbon material is as follows: a carbon material precursor and well-dispersed nano-graphene are mixed evenly and then carbonized. Because the size of nano-graphene is small, graphene quantum dot microcrystals are easily formed during carbonization and are included in the porous carbon material, improving the conductivity and ion conductivity.

[0045] The second aspect of the present invention provides a silicon-based composite material, which is mainly composed of silicon-based composite material particles;

[0046] The silicon-based composite material particles include silicon nanoparticles and the porous carbon material described in the first aspect;

[0047] The silicon nanoparticles are located in the pores of the porous carbon material.

[0048] Since the porous carbon material contains carbon dot microcrystals with a size of 2 - 8 nm, the nano-carbon dot microcrystals have significant quantum effects, thus having higher conductivity and improved ion conductivity. The silicon-based composite material prepared from this porous carbon material also has significantly higher conductivity and ion conductivity than that prepared from ordinary porous carbon materials, improving the Coulomb efficiency and rate performance of the material; on the other hand, in this silicon-based composite material, the presence of nano-carbon dot microcrystals improves the electrical contact between the porous carbon material and the silicon nanoparticles, improving the cycle stability of the composite material.

[0049] In some embodiments, the porous carbon material contains graphene nano-microcrystals, and the planar size of the graphene quantum dots is 2 - 8 nm, having stronger quantum confinement effects and boundary effects, which can further enhance the conductivity and ion conductivity, thereby improving the Coulomb efficiency and rate performance of the material. In the silicon-based composite material, the presence of graphene quantum dot microcrystals can also further improve the electrical contact between the porous carbon material and the silicon nanoparticles, improving the cycle stability of the composite material.

[0050] Furthermore, the silicon content is 5 - 90 wt.%, preferably 30 - 70 wt.%.

[0051] In some embodiments of the present invention, the silicon content is typically but not limited to 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.%, 60 wt.%, 70 wt.%, 80 wt.% or 90 wt.%.

[0052] And / or, the specific surface area of the silicon-based composite material is 0.1 - 50 m 2 / g, with a pore volume of 0.001 - 0.1 cm 3 / g; preferably, the specific surface area of the silicon-based composite material is 0.5 - 10 m 2 / g, and the pore volume is 0.001 - 0.05 cm 3 / g.

[0053] In some embodiments of the present invention, the specific surface area is typically but not limited to 0.1 m 2 / g, 1 m 2 / g, 10 m 2 / g, 20 m 2 / g, 30 m 2 / g, 40 m 2 / g or 50 m 2 / g; the pore volume is typically but not limited to 0.001 cm 3 / g, 0.005 cm 3 / g, 0.01 cm 3 / g, 0.05 cm 3 / g or 0.1 cm 3 / g.

[0054] And / or, the true density of the silicon-based composite material measured by helium pycnometry is 1.3 - 2.0 g / cm 3 , and the closed pore volume is 0.01 - 0.25 cm 3 / g.

[0055] In some embodiments of the present invention, the true density of the silicon-based composite material measured by helium pycnometry is typically but not limited to 1.3 g / cm 3 , 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 , 1.8 g / cm 3 , 1.9 g / cm 3 or 2.0 g / cm 3 ; the closed pore volume is typically but not limited to 0.01 g / cm 3 , 0.05 g / cm 3 , 0.1 g / cm 3 , 0.15 g / cm 3 , 0.2 g / cm 3 or 0.25 g / cm 3 .

[0056] And / or, the median particle size d V,50 of the silicon-based composite material particles is 5 - 20 μm, and the span (d V,90 -d V,10 ) / d V,50 is 0.7 - 2.0. Preferably, the median particle size d V,50 of the silicon-based composite material is 6 - 12 μm, and the span (dV,90 -d V,10 ) / d V,50 is 0.7 to 1.2.

[0057] In some embodiments of the present invention, the median particle size d of the silicon-based composite material particles V,50 Typical but non-limiting are 5 μm, 6 μm, 10 μm, 12 μm, 15 μm or 20 μm; the distance between diameters (d V,90 -d V,10 ) / d V,50 Typical but non-limiting are 0.8, 1.0, 1.2, 1.5, 1.8 or 2.0.

[0058] Furthermore, the surface of the silicon-based composite material particles has a coating layer;

[0059] Preferably, the material of the coating layer includes at least one of solid electrolytes, conductive polymers, artificial SEI films, etc.;

[0060] Or, the material of the coating layer includes at least one of carbonaceous materials, metals, alloys, metal oxides, nitrogen-containing compounds, phosphorus-containing compounds, boron-containing compounds, halogen-containing compounds, sulfur-containing compounds.

[0061] Preferably, the solid electrolyte includes at least one of LATP, Al2O3, ZrO2, TiO2, LiTi2O4, Li3InCl6, etc.;

[0062] Preferably, the conductive polymer includes at least one of PEO (polyethylene oxide), PEG (polyethylene glycol), PPy (polypyrrole), PTh (polythiophene), PZ (polycarbazole), etc.;

[0063] Preferably, the artificial SEI film includes at least one of Li2S, Li2Se, LiF, Li3N, LiAlO2, etc.;

[0064] Preferably, the carbonaceous material includes at least one of carbon nanotubes, nanographene, carbon black, amorphous carbon, carbon fiber, etc.;

[0065] Preferably, the metal includes at least one of iron, copper, silver, platinum, aluminum, etc.;

[0066] Preferably, the alloy includes at least one of aluminum alloy, magnesium alloy, titanium alloy, nickel alloy, etc.;

[0067] Preferably, the metal oxide includes at least one of Al2O3, MgO, TiO2, WO3, Nb2O5, etc.;

[0068] Preferably, the nitrogen-containing compound includes at least one of Li3N, Mg3N2, AlN, TiN, PN, etc.;

[0069] Preferably, the phosphorus-containing compound includes at least one of AlPO4, Ti3(PO4)4, Na3PO4, LATP, etc.;

[0070] Preferably, the boron-containing compound includes at least one of vinylphenylboronic acid, 3-fluorophenylboronic acid, magnesium borate, carboxylphenylboronic acid, 4-(trifluoromethyl)phenylboronic acid, lithium borate, sodium borate, etc.;

[0071] Preferably, the halogen-containing compound includes at least one of Li3YBr6, Li3ScCl6, Li3InCl6, VCl3, Li2ZrCl6, etc.;

[0072] Preferably, the sulfur-containing compound includes at least one of Li2S, SiS2, Na3SbS4, MoS2, CoS, etc.

[0073] In the third aspect of the present invention, there is provided a method for preparing the silicon-based composite material, wherein a silicon-containing precursor is deposited in the porous carbon material by chemical vapor deposition to obtain the silicon-based composite material.

[0074] Further, the temperature of the chemical vapor deposition is 150-1000 °C, and the time is 1-100 h.

[0075] Preferably, the silicon-containing precursor includes at least one of silane, disilane, trisilane, halogenated silane, polysilane, polysiloxane, polycarbosilane, silafluorene and its derivatives, silole and its derivatives.

[0076] Further, a heteroatom-containing precursor is introduced during the chemical vapor deposition process.

[0077] Preferably, the heteroatom-containing precursor includes at least one of a nitrogen-containing precursor, a phosphorus-containing precursor, a sulfur-containing precursor or a boron-containing precursor.

[0078] In the fourth aspect of the present invention, there is provided a negative electrode, which includes a negative electrode active material;

[0079] The negative electrode active material includes the silicon-based composite material described in the second aspect and the silicon-based composite material obtained according to the preparation method described in the third aspect.

[0080] In the fifth aspect of the present invention, there is provided a battery, which includes a positive electrode, a separator, an electrolyte and the negative electrode described in the fourth aspect.

[0081] The present invention will be further illustrated below by specific examples and comparative examples. However, it should be understood that these examples are only for more detailed illustration and should not be construed as limiting the present invention in any way. For the raw materials used in the examples and comparative examples of the present invention, unless otherwise specified, they are carried out under conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments used, unless the manufacturer is specified, they are all conventional products that can be obtained by commercial purchase.

[0082] Example 1

[0083] This example provides a porous carbon material, and the preparation method is as follows:

[0084] The commercially available graphene oxide was coated with sodium polystyrene sulfonate (PSS) to obtain well-dispersed nano-graphene; sucrose and monolayer graphene were mixed at a mass ratio of 100:5, pre-stabilized at 200 °C for 2 h, then crushed, carbonized at 850 °C for 2 h in an inert atmosphere, and then changed to CO2 gas for continuous high-temperature treatment for 2 h. The obtained material was crushed and classified to obtain a porous carbon material with a specific surface area of 1420 m 2 / g, a pore volume of 0.62 cm 3 / g, and containing nano-carbon dot microcrystals.

[0085] Example 2

[0086] This example provides a porous carbon material, and the preparation method is as follows:

[0087] The commercially available graphene oxide was coated with sodium polystyrene sulfonate (PSS) to obtain well-dispersed monolayer graphene; sucrose and monolayer graphene were mixed at a mass ratio of 100:2, pre-stabilized at 200 °C for 2 h, then crushed, carbonized at 850 °C for 2 h in an inert atmosphere, and then changed to CO2 gas for continuous high-temperature treatment for 2 h. The obtained material was crushed and classified to obtain a porous carbon material with a specific surface area of 1425 m 2 / g, a pore volume of 0.65 cm 3 / g, and containing nano-carbon dot microcrystals.

[0088] Example 3

[0089] This example provides a porous carbon material, and the preparation method is as follows:

[0090] The commercially available graphene oxide was coated with sodium polystyrene sulfonate (PSS) to obtain well-dispersed monolayer graphene; using starch as the raw material, starch and monolayer graphene were mixed at a mass ratio of 100:20, pre-stabilized at 200 °C for 2 h, then crushed, carbonized at 850 °C for 2 h in an inert atmosphere, and then changed to CO2 gas for continuous high-temperature treatment for 2 h. The obtained material was crushed and classified to obtain a porous carbon material with a specific surface area of 1415 m 2 / g, a pore volume of 0.64 cm 3 / g, porous carbon material containing nano-carbon dot microcrystals.

[0091] Examples 4 to 6

[0092] These examples provide silicon-based composites, and the preparation process is as follows: correspondingly, the porous carbon materials obtained in Examples 1 to 3 are placed in a tubular furnace, and heated from room temperature to 600 °C at a rate of 2 °C / min in an N2 atmosphere; then it is changed to a mixed gas of SiH4 and N2 (the volume content of SiH4 is 20%), and silicon deposition is carried out at 600 °C in this mixed atmosphere for 30 h. After changing to an N2 atmosphere, it is cooled naturally, crushed and classified to obtain the silicon-based composite.

[0093] The silicon content in the silicon-based composite obtained in Example 4 is 48.2 wt.%, the specific surface area is 3.5 m 2 / g, the pore volume is 0.008 cm 2 / g, the median particle size d V,50 is 9.5 μm, and the diameter distance (d V,90 -d V,10 ) / d V,50 is 0.98.

[0094] Figure 1 is the HRTEM image of the silicon-based composite obtained in Example 4. From Figure 1 it can be seen that the composite contains nano-carbon dot microcrystals with an average size of about 4 - 5 nm.

[0095] The silicon content in the silicon-based composite obtained in Example 5 is 48.5 wt.%, the specific surface area is 3.5 m 2 / g, the pore volume is 0.008 cm 2 / g, the median particle size d V,50 is 9.5 μm, and the diameter distance (d V,90 -d V,10 ) / d V,50 is 0.98.

[0096] The silicon content in the silicon-based composite obtained in Example 6 is 48.3 wt.%, the specific surface area is 3.5 m 2 / g, the pore volume is 0.008 cm 2 / g, the median particle size d V,50 is 9.5 μm, and the diameter distance (d V,90 -d V,10 ) / d V,50 is 0.98.

[0097] Example 7

[0098] This embodiment provides a CNT-coated silicon-based composite material. The silicon-based composite material obtained in Example 4 is uniformly dispersed in pure water, and then 0.4 wt.% of single-walled CNTs (outer diameter range 1.6 ± 0.4 nm, length ≥ 5 μm) is added. After stirring evenly, it is spray-dried to obtain the CNT-coated silicon-based composite material.

[0099] The silicon-based composite material obtained in Example 7 has a silicon content of 48.1 wt.%, a specific surface area of 3.5 m 2 / g, a pore volume of 0.008 cm 2 / g, a median particle size d V,50 of 9.5 μm, and a diameter distance (d V,90 -d V,10 ) / d V,50 of 0.98.

[0100] Comparative Example 1

[0101] This comparative example provides a silicon-based composite material, including the following steps:

[0102] 1. After pre-stabilizing sucrose at 200 °C for 2 h, it is crushed, carbonized at 850 °C in an inert atmosphere for 2 h, and then changed to CO2 gas for continuous high-temperature treatment for 2 h. The obtained material is crushed and classified to obtain a porous matrix with a specific surface area of 1468 m 2 / g and a pore volume of 0.65 cm 3 / g.

[0103] 2. The porous matrix is placed in a tubular furnace and heated from room temperature to 600 °C at a rate of 2 °C / min in a N2 atmosphere; then it is changed to a mixture of SiH4 and N2 (the volume content of SiH4 is 20%), and silicon deposition is carried out at 600 °C in this mixed atmosphere for 30 h. After changing to a N2 atmosphere, it is cooled naturally, crushed and classified to obtain the silicon-based composite material.

[0104] The silicon-based composite material obtained in Comparative Example 1 has a silicon content of 48.5 wt.%, a specific surface area of 3.2 m 2 / g, a pore volume of 0.008 cm 2 / g, a median particle size d V,,50 of 9.5 μm, and a diameter distance (d V,90 -d V,10 ) / d V,50 of 0.98.

[0105] Test Example 1

[0106] Electronic conductivity test: The four-wire two-terminal method is adopted. The resistance is determined by measuring the voltage across the resistance to be measured and the current flowing through it, and the conductivity is calculated by combining the height and bottom area of the resistance to be measured.

[0107] Take a certain amount of powder and add it to the test mold. After gently leveling it, place the gasket on the mold on the sample. After loading the sample, place the mold on the workbench of the electronic pressure testing machine, raise it to 500 kg (159 Mpa) at a rate of 5 mm / min, keep the pressure constant for 60 s, and then relieve the pressure to 0. When the sample is under constant pressure of 5000 ± 2 kg (about 15 - 25 s after the pressure rises to 5000 kg), record the sample pressure, read the deformation height of the sample, and record the value displayed by the resistance tester at this time. Then, the electronic conductivity can be calculated using the following formula (1):

[0108]

[0109] The data obtained are shown in Table 1.

[0110] Test Example 2

[0111] Using the silicon-based composite materials obtained in the above examples and comparative examples as the negative electrode active material, prepare negative electrode sheets respectively, prepare CR2032 type button cells by conventional methods, and conduct electrical performance tests on the cells. The specific test method is as follows:

[0112] (1) Half-cell assembly: Assemble a CR2032 type button cell in a glove box, using a lithium metal sheet as the counter electrode, a polypropylene microporous membrane as the separator, and the electrolyte is 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.

[0113] Use a Blue Electric (LAND) battery test system to conduct charge and discharge tests on the battery.

[0114] (2) Specific capacity and first efficiency test: After the CR2032 type button cell stands still for 6 h, discharge it to 0.005 V at 0.05 C, and then discharge it to 0.005 V at 0.01 C; after standing still for 5 min, charge it at a constant current of 0.05 C to 1.5 V. The first lithium deintercalation specific capacity is the specific capacity of the electrode material (or mass specific capacity).

[0115] (3) Lithium ion conductivity test: After the CR2032 type button cell stands still for 6 h, discharge it to 0.005 V at 0.05 C, and then discharge it to 0.005 V at 0.01 C; after standing still for 5 min, charge it at a constant current of 0.05 C to 1.5 V; after standing still for 5 min, repeat the above steps twice, and then use an electrochemical workstation to conduct electrochemical impedance spectroscopy tests on the button cell. The frequency range is 0.01 - 105 Hz, the voltage amplitude is 0.005 V, and the last 5 data points are taken from the data points obtained by the test to obtain Z' versus the rotation speed ω at low frequency -1 / 2The linear slope, and this slope value is the Warburg parameter. The obtained data is shown in Table 1 below. Under the same test conditions, the smaller the Warburg parameter, the higher the lithium-ion conductivity.

[0116] Rate performance test: The prepared coin cell was left standing at room temperature for 6 h and then tested on a Blue Current test system. In the first charge-discharge cycle, it was discharged at 0.05C to 0.005V and then at 0.01C to 0.005V; after standing for 5 min, it was charged at a constant current of 0.05C to 1.5V and then left standing for 5 min; the second charge-discharge cycle was carried out, with the same procedure as the first cycle; then a constant current charge-discharge test was carried out, with the charge-discharge cut-off voltage of 0.005 - 1.5V. First, it was charged and discharged at a current of 0.1C for 3 cycles; then it was charged and discharged at a current of 0.2C for 3 cycles; finally, it was charged and discharged at a current of 0.5C for 3 cycles. The capacity retention rate was calculated as (discharge capacity of the 11th cycle / discharge capacity of the 3rd cycle) × 100%. The higher the value, the better the rate performance is considered.

[0117] Test Example 3

[0118] Using the silicon-based composite materials obtained in the above examples and comparative examples as the negative electrode active material, a soft-pack battery was prepared from the electrode sheet containing the negative electrode active material by a conventional method and its electrical properties were tested. The soft-pack battery was prepared in a dehumidifying chamber with a dew point of -45°C. A Blue Battery (LANBTS) test system was used to conduct a charge-discharge cycle test on the battery. The specific test method was as follows:

[0119] (1) Preparation of the positive electrode sheet: The positive electrode active material NCM811, the conductive agent Super P, the binder PVDF, and the solvent NMP were stirred and mixed evenly according to a 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.

[0120] (2) Preparation of the negative electrode sheet: The silicon-carbon composite materials obtained in the examples and comparative examples were mixed with graphite to obtain the negative electrode active material. The negative electrode active material, the conductive agent Super P, the binder polyacrylic acid, and the solvent deionized water were stirred and mixed evenly according to a 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.

[0121] (3) The positive electrode sheet and the negative electrode sheet were stacked in a square shape and separated by a polypropylene separator to form a battery core, which was then encapsulated in an aluminum-plastic bag. An electrolyte corresponding to its capacity was injected into the aluminum-plastic bag, and after vacuum sealing, the soft-pack battery was obtained. The electrolyte was a mixed solution of EC and DEC of LiPF6, where the concentration of LiPF6 was 1 mol / L and the volume ratio of EC and DEC was 1:1.

[0122] (4) Capacity division: After the battery is filled and sealed, it begins to form, and is placed in a 25°C constant temperature box for 12 hours, then charged to 3.3V at 0.02C constant current, placed for 30 minutes, charged to 3.8V at 0.025C constant current, placed for 10 minutes, and charged to 4.2V at 0.33C constant current; the formed battery is vacuumed and the air bag is cut, and then capacity division is performed, charged to 4.45V at 0.33C constant current, placed for 10 minutes, discharged to 3V at 1C constant current, placed for 10 minutes, and discharged to 3V at 0.33C constant current, and the capacity division is completed. The ratio of the discharge capacity to the charge capacity in the capacity division of the soft pack battery is the first coulombic efficiency of the battery.

[0123] (5) 25℃ cycle test: The battery was placed in a 25℃ constant temperature box, charged to 4.45V at 1C constant current, and then charged to 0.1C at 4.45V constant voltage; after standing for 10 minutes, discharged to 3.0V at 1C constant current, and stood for 10 minutes. The above charging and discharging steps were repeated until the discharge capacity was lower than 80% of the first cycle discharge capacity. The number of cycles obtained at this time is the cycle life of the soft-pack battery; the capacity retention rate after 100 cycles is recorded.

[0124] The obtained data are shown in Table 1.

[0125] Table 1

[0126]

[0127] As can be seen from Table 1, compared with Comparative Example 1, the 1.5V first coulomb efficiency, 100-cycle capacity retention rate and rate performance of the silicon-based composite materials obtained in Examples 4 to 7 have been significantly improved, and the resistivity has also been significantly reduced. This is because the addition of nanographene allows the porous carbon material to form nanocarbon dot crystallites in the amorphous continuous region, which has a significant quantum effect and can effectively enhance the electrical conductivity and ion conductivity, and improve the electrical contact between the porous carbon material and the silicon nanoparticles. Therefore, the silicon-based composite material made of the porous carbon material improves the rate performance and cycle stability of the material; thereby improving the capacity stability and coulomb efficiency of the secondary battery and improving the performance of the secondary battery.

[0128] In Examples 4 to 6, the proportion of nanographene added is different. With the increase of the amount of nanographene, the 1.5V first coulomb efficiency, 100 cycle capacity retention rate and rate performance of the silicon-based composite material are significantly improved, and the resistivity is also significantly reduced. This is due to the formation of more nano-carbon dot crystallites in the amorphous continuous region of the porous carbon material.

[0129] Example 7 is based on Example 4, with single-walled CNTs coated. The single-walled CNTs form a three-dimensional conductive network structure among the silicon-based composites, which can further improve the conductivity and ion conductivity of the silicon-based composites. Therefore, the silicon-based composites obtained in Example 7 have more excellent cycle stability, rate performance, and lower resistivity.

[0130] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than 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; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A porous carbon material, characterized in that, It contains nanocarbon dot microcrystals; the size of the nanocarbon dot microcrystals is 2 to 8 nm.

2. The porous carbon material according to claim 1, characterized in that, The nanocarbon dot microcrystals include graphene quantum dot microcrystals, and the planar size of the graphene quantum dots is 2 to 8 nm; and / or, the porous carbon material contains an amorphous continuous region, and the nanocarbon dot microcrystals are dispersed in the amorphous continuous region; And / or, the specific surface area of the porous carbon material is 200 to 3000 m 2 / g, and the pore volume is 0.2 to 3.0 cm 3 / g.

3. A silicon-based composite material, characterized in that, It is mainly composed of silicon-based composite material particles; The silicon-based composite material particles contain silicon nanoparticles and the porous carbon material described in claim 1 or 2; The silicon nanoparticles are located in the pores of the porous carbon material.

4. The silicon-based composite material according to claim 3, characterized in that, The silicon content is 5 to 90 wt.%, preferably 30 to 70 wt.%; And / or, the specific surface area of the silicon-based composite material is 0.1 to 50 m 2 / g, and the pore volume is 0.001 to 0.1 cm 3 / g; preferably, the specific surface area of the silicon-based composite material is 0.5 to 10 m 2 / g, and the pore volume is 0.001 to 0.05 cm 3 / g; And / or, the true density of the silicon-based composite material measured by helium pycnometry is 1.3 to 2.0 g / cm 3 , and the closed pore volume is 0.01 to 0.25 cm 3 / g; And / or, the median particle size d of the silicon-based composite material particles V,50 is 5 - 20 μm, and the diameter distance (d V,90 - d V,10 ) / d V,50 is 0.7 - 2.0; preferably, the median particle size d of the silicon-based composite material V,50 is 6 - 12 μm, and the diameter distance (d V,90 - d V,10 ) / d V,50 is 0.7 - 1.

2.

5. The silicon-based composite material according to claim 3, characterized in that, The surface of the silicon-based composite material particles has a coating layer; Preferably, the material of the coating layer includes at least one of a solid electrolyte, a conductive polymer, and an artificial SEI; or, the material of the coating layer includes at least one of a carbonaceous material, a metal, an alloy, a metal oxide, a nitrogen-containing compound, a phosphorus-containing compound, a boron-containing compound, a halogen-containing compound, and a sulfur-containing compound.

6. A method for preparing the silicon-based composite material according to any one of claims 3 to 5, characterized in that, Contact a silicon precursor with a porous carbon material and perform chemical vapor deposition to obtain the silicon-based composite material.

7. The preparation method according to claim 6, characterized in that, The temperature of the chemical vapor deposition is 150 to 1000 °C, and the time is 1 to 100 h; and / or, the silicon precursor includes at least one of silane, disilane, trisilane, halogenated silane, polysilane, polysiloxane, polycarbosilane, silafluorene and its derivatives, and silole and its derivatives; 8. The preparation method according to claim 6 or 7, characterized in that, It also includes introducing a heteroatom-containing precursor during the chemical vapor deposition process; Preferably, the heteroatom-containing precursor includes at least one of a nitrogen-containing precursor, a phosphorus-containing precursor, a sulfur-containing precursor, or a boron-containing precursor.

9. A negative electrode, characterized in that, It contains a negative electrode active material; The negative electrode active material contains the silicon-based composite material described in any one of claims 3 to 5 and the silicon-based composite material obtained by the preparation method described in any one of claims 6 to 8.

10. A battery, characterized in that, It includes a positive electrode, a separator, an electrolyte, and the negative electrode described in claim 9.

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