Silicon-carbon negative electrode material and preparation method thereof, negative electrode plate and secondary battery

Abstract

The invention discloses a silicon-carbon negative electrode material and a preparation method thereof, a negative electrode plate and a secondary battery, the silicon-carbon negative electrode material comprises a nano-silicon-loaded graphitized porous carbon matrix, in an XRD pattern of the silicon-carbon negative electrode material, a (002) crystal face diffraction peak is located at a diffraction angle 2theta of 27.8 degrees to 28.8 degrees, the half-peak width of the (002) crystal face diffraction peak is FWMH degrees, and 2.20 < = FWMH < = 3.0. The lithium-intercalation expansion of the silicon-carbon material is effectively reduced, and meanwhile, the capacity and the first effect of the material are improved.

Description

Technical Field

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

[0002] Due to its rich reserves, theoretical specific capacity and suitable voltage platform, silicon-based materials are gradually becoming the preferred choice for battery companies and lithium battery material companies to improve the performance of negative electrodes, and are one of the next-generation lithium-ion battery negative electrode materials with the greatest potential to replace graphite negative electrodes. Currently, the severe volume effect during lithiation of silicon is the biggest limitation for the commercialization of silicon-based materials. One of the solutions is to compound silicon with carbon materials having better conductivity and higher porosity in order to solve the disadvantages of silicon materials.

[0003] However, it is difficult to suppress the expansion of the silicon-based material at present, so that the expanded lithium-silicon alloy during lithiation will overflow from the pores of the porous carbon, and the silicon formed during delithiation reacts with the electrolyte outside the silicon-carbon particles, resulting in a sharp increase in by-products, reducing the service life of the battery cell. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a silicon-carbon negative electrode material, a preparation method thereof, a negative electrode sheet and a secondary battery, aiming to solve the problem that the porous carbon in the silicon-carbon negative electrode material is difficult to suppress the expansion of the silicon-based material.

[0005] To achieve the above object, the present invention provides a silicon-carbon negative electrode material. In the XRD pattern of the silicon-carbon negative electrode material, the diffraction peak of the (002) crystal plane is located at a diffraction angle 2θ of 27.8° to 28.8°, and the full width at half maximum of the diffraction peak of the (002) crystal plane is FWMH°, and 2.20 ≤ FWMH ≤ 3.0.

[0006] In some embodiments, the silicon-carbon negative electrode material includes a graphitized porous carbon matrix, and the graphitization degree of the graphitized porous carbon matrix is 25% to 45%; and / or, the pore volume of the graphitized porous carbon matrix is 0.9 g / cm 3 ~1.2 g / cm 3 ; and / or, the pore size distribution of the graphitized porous carbon matrix is: the proportion of micropores is 50% to 70%, the proportion of mesopores is 30% to 45%, and the proportion of macropores is 0 to 10%.

[0007] In some embodiments, the silicon-carbon negative electrode material includes loaded nano-silicon, and the loading amount of the nano-silicon is 46 wt% to 52 wt%; and / or, the particle size Dv50 of the nano-silicon is 6 nm to 10 nm.

[0008] In some embodiments, the silicon-carbon negative electrode material includes a carbon coating layer provided on the surface of the graphitized porous carbon matrix, and the coating amount of the carbon coating layer is 10 wt% to 15 wt%.

[0009] In some embodiments, the silicon-carbon negative electrode material includes an artificial SEI film coating layer provided on the surface of the carbon coating layer, and the coating amount of the artificial SEI film coating layer is 0.5 wt% to 4 wt%.

[0010] In some embodiments, the types of coating agents for the artificial SEI film coating layer include at least one of lithium fluoride, aluminum fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0011] In some embodiments, the graphitized porous carbon matrix is obtained by catalytic graphitization of a petroleum coke-based porous carbon material; and / or, the graphitization degree of the petroleum coke-based porous carbon material is 1% to 30%; and / or, the pore volume of the petroleum coke-based porous carbon material is 1.2 g / cm 3 ~1.6 g / cm 3 .

[0012] To achieve the above object, the present invention also provides a preparation method of the above silicon-carbon negative electrode material, including the following steps:

[0013] The petroleum coke is crushed and shaped to obtain crushed petroleum coke, and the crushed petroleum coke is mixed with a pore-forming agent and placed in a constant-temperature device for pore formation to obtain a petroleum coke-based porous carbon material;

[0014] The petroleum coke-based porous carbon material is catalytically graphitized and then purified to obtain a graphitized porous carbon matrix;

[0015] Chemical vapor deposition is performed on the graphitized porous carbon matrix to achieve silane deposition and prepare a carbon coating layer, and then magnetic separation and screening are performed to obtain the silicon-carbon negative electrode material.

[0016] In some embodiments, the mass ratio of the pore-forming agent to the crushed petroleum coke is 30 wt% to 40 wt%; and / or, the volume concentration of the pore-forming agent is 5 mol / L to 10 mol / L.

[0017] In some embodiments, during the catalytic graphitization process, the catalyst accounts for 0.5% to 3% of the mass of the petroleum coke-based porous carbon material; and / or, the temperature of the catalytic graphitization is 500 °C to 1000 °C.

[0018] In some embodiments, before the magnetic separation and screening, it includes preparing an artificial SEI film coating layer using a coating agent, and the coating agent includes at least one of lithium fluoride, aluminum fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0019] In some embodiments, the deposition temperature of the artificial SEI film coating layer is 200 °C to 500 °C.

[0020] To achieve the above object, the present invention further provides a negative electrode tab, which includes the above silicon-carbon negative electrode material, or includes the silicon-carbon negative electrode material prepared by the above preparation method.

[0021] To achieve the above object, the present invention further provides a secondary battery, and the battery includes the above negative electrode tab.

[0022] Advantages of the present invention:

[0023] The silicon-carbon negative electrode material provided by the present invention uses a graphitized porous carbon matrix loaded with nano-silicon. When the diffraction peak of its (002) crystal plane is located at a diffraction angle 2θ of 27.8° to 28.8°, it has higher toughness. In the actual application of the battery cell, it can withstand a greater compaction density and has a smaller degree of fragmentation, which is beneficial to the improvement of the energy density of the battery cell. At the same time, the graphitization degree is increased, and the full width at half maximum of the diffraction peak of the (002) crystal plane is controlled within the range of 2.2 to 3.0. Through the synergistic effect, the lithium intercalation expansion of the silicon-carbon material is effectively reduced, and the capacity and initial efficiency of the material itself are improved. Description of the Drawings

[0024] Figure 1 is an XRD (X-ray diffraction method) diagram of a conventional phenolic resin-based silicon negative electrode material before graphitization. The horizontal axis represents the diffraction angle, and the vertical axis represents the diffraction intensity;

[0025] Figure 2 is an XRD (X-ray diffraction method) diagram of the silicon-carbon negative electrode material of an embodiment of the present invention. The horizontal axis represents the diffraction angle, and the vertical axis represents the diffraction intensity;

[0026] The realization of the purpose, functional characteristics and advantages of this application will be further described in conjunction with the embodiments with reference to the drawings. Detailed Embodiments

[0027] To make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be further described in detail below in conjunction with the specific embodiments. The drawings and the following description are provided for those skilled in the art to fully understand this application, and are not intended to limit the subject matter recited in the claims.

[0028] The "ranges" disclosed in this application are defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a specific parameter, ranges of 60 to 110 and 80 to 120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise specified, the numerical range "a to b" represents an abbreviated representation of any real number combination between a and b, where both a and b are real numbers. For example, the numerical range "0 to 5" means that all real numbers between "0 to 5" have been fully listed herein, and "0 to 5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0029] If there is no special instruction, all embodiments and optional embodiments of this application can be combined with each other to form new technical solutions.

[0030] If there is no special instruction, all technical features and optional technical features of this application can be combined with each other to form new technical solutions.

[0031] If there is no special instruction, all steps of this application can be carried out sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out sequentially, or can also include steps (b) and (a) carried out sequentially. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can also include steps (a), (c), and (b), or can also include steps (c), (a), and (b), etc.

[0032] Due to its rich reserves, theoretical specific capacity, and suitable voltage platform, silicon-based materials are gradually becoming the preferred choice for battery companies and lithium battery material companies to improve the performance of the negative electrode, and are one of the most promising next-generation lithium-ion battery negative electrode materials to replace graphite negative electrodes. Currently, the severe volume effect during lithiation of silicon is the biggest limitation for the commercialization of silicon-based materials. One of the solutions is to compound silicon with carbon materials that have better conductivity and higher porosity in order to solve the disadvantages of silicon materials.

[0033] However, it is difficult for current porous carbon to inhibit the expansion of silicon-based materials. During lithiation, the expanding lithium-silicon alloy will overflow from the pores of the porous carbon. During delithiation, the formed silicon reacts with the electrolyte outside the silicon-carbon particles, resulting in a sharp increase in by-products and reducing the service life of the battery cell.

[0034] In view of this, the present invention provides a silicon-carbon negative electrode material. In the XRD pattern of the silicon-carbon negative electrode material, the diffraction peak of the (002) crystal plane is located at a diffraction angle 2θ of 27.8° to 28.8°, and the full width at half maximum (FWMH) of the diffraction peak of the (002) crystal plane is FWMH°, where 2.20 ≤ FWMH ≤ 3.0.

[0035] Using a graphitized porous carbon matrix loaded with nano-silicon, the diffraction peak of the (002) crystal plane is located at a diffraction angle 2θ of 27.8° to 28.8°, with higher toughness. In actual battery cell applications, it can withstand a greater compaction density with less fragmentation, which is beneficial to improving the energy density of the battery cell. At the same time, the graphitization degree is increased, and the full width at half maximum of the diffraction peak of the (002) crystal plane is controlled within the range of 2.2 to 3.0. Through the synergistic effect, the lithiation expansion of the silicon-carbon material is effectively reduced, and at the same time, the capacity and initial efficiency of the material itself are improved.

[0036] In some embodiments, the diffraction peak of the (002) crystal plane is located at any value of 27.8° to 28.8°, such as 27.8°, 27.9°, 28.0°, 28.1°, 28.2°, 28.3°, 28.4°, 28.5°, 28.6°, 28.7°, 28.8°. In some embodiments, the value of FWMH is any value of 2.20 to 3.0, such as 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0.

[0037] In some embodiments, the silicon-carbon negative electrode material includes a graphitized porous carbon matrix, and the graphitization degree of the graphitized porous carbon matrix is 25% to 45%; the pore volume of the graphitized porous carbon matrix is 0.9 g / cm 3 ~1.2 g / cm 3 ; the pore size distribution of the graphitized porous carbon matrix is: the proportion of micropores is 50% to 70%, the proportion of mesopores is 30% to 45%, and the proportion of macropores is 0 to 10%.

[0038] The degree of graphitization refers to the degree to which the crystal structure of a carbon material is close to the ideal graphite crystal structure. Carbon materials with a high degree of graphitization have a more complete crystal structure and fewer lattice defects, and the migration path of electrons in them is smoother, so the electronic conductivity is better. Good electronic conductivity helps to reduce the charge transfer impedance of the battery and improve the electron transfer efficiency during the charge and discharge process. The interlayer spacing of graphite materials with a high degree of graphitization is closer to the ideal value (0.3354nm), the diffusion path of lithium ions between graphite layers is shorter, and the diffusion kinetics performance is better. This helps to reduce the diffusion resistance of lithium ions during the insertion / extraction process, reduce the charge transfer impedance of the battery, and increase the battery life.

[0039] Carbon materials with high graphitization have a more stable crystal structure and can better resist the mechanical stress caused by volume changes during charging and discharging. This structural stability helps to maintain the integrity of the electrode material and reduce the capacity decay and shortened life caused by structural damage during the cycle. In addition, the surface of carbon materials with high graphitization is smoother, which is conducive to the formation of a uniform and stable SEI film. However, if the graphitization is too high, pore collapse is likely to occur. Therefore, the range of graphitization in this scheme is 25.0%, 26.5%, 29.0%, 30.0%, 32.0%, 33.3%, 37.0%, 40.0%, 41.0%, 43.0%, 44.5%, etc. Any value between 25% and 45%.

[0040] The pore volume value depends on the preparation method and the choice of precursor, and is affected by preparation conditions such as temperature, time, catalyst and other factors. In general, graphitization will cause some pore structures to change, resulting in a decrease in pore volume, because the carbon atoms are rearranged during the graphitization process, which may block some pores. Although graphitization may reduce the pore volume, it can significantly improve the conductivity and structural stability of the material. Since it is necessary to consider the problem of pore collapse and shrinkage during catalytic graphitization, this solution needs to further increase the pore distribution compared to conventional porous carbon, and finally obtain the pores of the above-mentioned ideal graphitized porous carbon matrix. According to the definition of pore size by the International Union of Pure and Applied Chemistry (IUPAC), the pores in porous materials can be divided into micropores (pore size <2nm), mesopores (pore size 2-50nm) and macropores (pore size >50nm). In some embodiments, the pore volume of the graphitized porous carbon matrix is ​​0.9g / cm 3 , 1.0g / cm 3 , 1.1g / cm 3 , 1.2g / cm 3 Equivalent to 0.9g / cm 3 ~1.2g / cm 3 Any value in .

[0041] In some embodiments, the carbon particle size of the graphitized porous carbon matrix has a Dv50 of 7 μm to 9 μm. If the carbon particle size is too large, the diffusion impedance of lithium ions in the particles is large, and the rate performance is poor. In addition, when the particle size is too large and Dv100 is greater than the roll gap size of the coater, scratches may occur during coating, resulting in a low yield of the electrode sheet; if the carbon particle size is too small, the yield is low and the cost increases.

[0042] In some embodiments, the silicon-carbon negative electrode material includes loaded nano-silicon with a loading amount of 46 wt% to 52 wt%; and / or, the particle size Dv50 of the nano-silicon is 6 nm to 10 nm.

[0043] Silicon itself has poor conductivity. Appropriate silicon deposition can form a good conductive network with carbon materials. Silicon has a high theoretical capacity, but its volume changes greatly during charge and discharge, which easily leads to capacity attenuation and shortened life. By controlling the silicon deposition amount, the capacity, cycle stability, and conductivity can be balanced. In this solution, too low a silicon loading amount results in a low first efficiency of the material, low specific capacity of the anode, and increased cost of the battery cell; too high a silicon loading amount causes large expansion of the material, high stress during expansion, serious damage to the porous carbon matrix, and easier pulverization and failure of the negative electrode material.

[0044] The particle size of the nano-silicon deposited on the carbon matrix by CVD (chemical vapor deposition) technology is usually between 10 nm and 20 nm. Such nano-scale silicon particles help improve the specific capacity and cycle stability of the material while reducing side reactions with the electrolyte. However, in this solution, the particle size of the nano-silicon needs to be coordinated with the pore size of the porous carbon and the carbon particle size of the porous carbon matrix. If the deposited nano-silicon particle size is too small, there will be more interfaces between silicon and carbon, more lithium-ion transfer interfaces, greater impedance, and greater polarization of the material during use; if the nano-silicon size is too large, the uniformity of lithium intercalation inside the silicon is worse, it is more likely to break, and the expansion increases. Therefore, better electrical properties of the material can be achieved at 6 nm to 10 nm. In some embodiments, the particle size Dv50 of the nano-silicon is any value in the range of 6 nm to 10 nm, such as 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, etc.

[0045] In some embodiments, the silicon-carbon negative electrode material includes a carbon coating layer provided on the surface of the graphitized porous carbon matrix, and the coating amount of the carbon coating layer is 10 wt% to 15 wt%. Too large a carbon coating amount results in a low first efficiency of the material and an increase in the diffusion impedance of lithium ions; too small a carbon coating amount leads to a large specific surface area of the material, resulting in a low first efficiency, and a large contact area with water during homogenization, which easily generates gas.

[0046] In some embodiments, the silicon-carbon negative electrode material includes an artificial SEI film coating layer disposed on the surface of the carbon coating layer, and the coating amount of the artificial SEI film coating layer is 0.5 wt% to 4 wt%. During charge and discharge, silicon undergoes significant volume expansion, which can lead to the rupture and reconstruction of the natural SEI film, thereby increasing the consumption of the electrolyte and the loss of active lithium. The artificial SEI film can provide better flexibility and structural stability, effectively alleviating the volume expansion of silicon. By constructing an artificial SEI film on the surface of the silicon-carbon negative electrode, the side reaction between silicon and the electrolyte is reduced, and the capacity attenuation of the battery during cycling is decreased. At the same time, the material structure of the highly graphitized porous carbon matrix is more inclined to soft carbon (the structural toughness is higher than that of the hard carbon resin-based matrix), which can limit the movement of silicon particles and reduce the stress concentration caused by expansion. The flexibility of the artificial SEI film may further disperse these stresses, and the two work together to reduce structural damage.

[0047] In some embodiments, the types of coating agents for the artificial SEI film coating layer include at least one of lithium fluoride, aluminum fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes. Among them, lithium fluoride as a coating agent for the negative electrode material of a lithium-ion battery helps to improve the cycle stability and Coulomb efficiency of the negative electrode material; the aluminum fluoride coating layer can improve the cycle stability of the negative electrode material; single-walled carbon nanotubes and multi-walled carbon nanotubes have excellent electrical conductivity and mechanical strength. Coating them on the surface of the negative electrode material can improve the electrical conductivity of the material, alleviate volume expansion, and thus improve the electrochemical performance.

[0048] In some embodiments, the coating agent is preferably lithium fluoride. When lithium fluoride is coated on the silicon-carbon surface, it is beneficial to improve the kinetic performance and SEI film stability when forming the SEI film; and the matrix structure of the petroleum coke-based porous carbon after graphitization tends to a more stable lamellar structure, which is beneficial to the embedding of lithium fluoride, making the structure and chemical properties of the artificial SEI film formed during subsequent use more stable.

[0049] In some embodiments, the graphitized porous carbon matrix is obtained by catalytic graphitization of a petroleum coke-based porous carbon material; the graphitization degree of the petroleum coke-based porous carbon material is 1% to 30%; the pore volume of the petroleum coke-based porous carbon material is 1.2 g / cm 3 ~1.6 g / cm 3 ; the pore size distribution of the petroleum coke-based porous carbon material is: the proportion of micropores is 50% to 65%, the proportion of mesopores is 30% to 45%, and the proportion of macropores is 5% to 10%.

[0050] Petroleum coke is a by-product of the petroleum refining process, mainly composed of carbon, usually containing more than 80% carbon, and other elements also include hydrogen, oxygen, nitrogen, sulfur, and metal elements. Compared with conventional resin and biomass materials, which are hard carbon materials, using petroleum coke as the raw material for porous carbon has better electrical conductivity and pressure resistance than conventional hard carbon materials.

[0051] In this solution, the silicon-carbon anode material uses petroleum coke with an easily graphitizable carbon soft carbon structure as the raw material, which has higher self-strength. In actual battery cell applications, it can withstand a greater compaction density and has a smaller degree of fragmentation, which is beneficial to the improvement of the energy density of the battery cell.

[0052] To solve the above problems, the present invention also proposes a preparation method for the above silicon-carbon anode material, which includes the following steps:

[0053] S1: After crushing and shaping petroleum coke, the crushed petroleum coke is obtained. The crushed petroleum coke is mixed with a pore-forming agent and placed in a constant-temperature device for pore formation to obtain a petroleum coke-based porous carbon material;

[0054] In some embodiments, after the rough crushing of petroleum coke, it is crushed by ball milling and then shaped by jet milling to modify the particle edges and corners to make the particles approach spheres.

[0055] In some embodiments, the Dv50 of the crushed petroleum coke is 10 μm to 12 μm. Too small particle size affects the particle strength and the subsequent amount of silane deposition, and at the same time, a margin for particle size shrinkage during subsequent high-temperature treatment needs to be reserved; too large particle size will affect the deposition uniformity and the rate performance of the material.

[0056] S2: The petroleum coke-based porous carbon material is catalytically graphitized and then purified to obtain a graphitized porous carbon matrix;

[0057] In some embodiments, the catalytic graphitization is carried out in a box furnace. Under heating conditions, the product is mixed evenly with a catalyst and then subjected to catalytic graphitization treatment. The catalytic graphitization can also be carried out in a carbon tube furnace, an internal series graphitization furnace, or other equipment that can achieve the graphitization process of this solution.

[0058] In some embodiments, during the catalytic graphitization process, the catalyst accounts for 0.5% to 3% of the mass of the petroleum coke-based porous carbon material. The catalysts for catalytic graphitization include elemental catalysts (such as B, Al, Ti, V, Cr, Fe, Co, Ni, W, Y, La, etc.), compound catalysts (such as FeO, Fe2O3, Fe3O4, and FeSiO3, etc.), and alloy catalysts (such as Fe-Si alloy and P-Ni alloy, etc.), and the mass ratio of the catalyst to the petroleum coke-based porous carbon material is 0.5% to 3%.

[0059] In some embodiments, the purification includes pickling and calcination.

[0060] In some embodiments, the pickling includes pickling with hydrochloric acid followed by washing with water until neutral and then drying. Since a relatively large amount of impurity magnetic substances and heteroatoms are introduced during the pore formation and catalytic graphitization processes, the obtained product is dissolved and purified with acid, and the excess residual alkali is washed and neutralized. Then, the obtained product is placed in a tube furnace (inert atmosphere) for calcination. The inert atmosphere uses gases that do not cause side reactions, including nitrogen, noble gases, etc.

[0061] S3: Perform chemical vapor deposition on the graphitized porous carbon matrix to achieve silane deposition and prepare a carbon coating layer, and then perform demagnetization screening to obtain a silicon-carbon negative electrode material.

[0062] In some embodiments, the graphitized porous carbon matrix is placed in a rotary furnace / fluidized bed, and a mixed gas of a protective gas (inert gas) and a silane gas is introduced for deposition.

[0063] In some embodiments, the protective gas accounts for 10% - 30% of the volume ratio of the mixed gas. If the proportion of the protective gas is too high, the production efficiency is low; if it is too low, the safety is poor, that is, reducing the silane concentration can improve production safety.

[0064] The gas flow rate of the mixed gas is related to the size of the rotary furnace. The rotary furnace can be divided into production capacities of 20 kg, 50 kg, and 300 kg. If the gas flow rate is too large, silicon is likely to deposit on the surface. If the gas flow rate is too small, the production capacity is low but silicon can be better deposited inside, and the pores of the porous carbon can be blocked, reducing the possibility of silicon overflowing from the porous carbon during lithium insertion and reducing the generation of by-products during the charge and discharge of the battery cell. During the preparation, the gas flow rate can be adjusted according to the actual situation.

[0065] In some embodiments, the product after silane deposition is placed in a rotary furnace / fluidized bed, and chemical vapor deposition is performed in a mixed atmosphere of a protective gas (nitrogen or argon) and an organic gas source (acetylene, ethylene, methane, etc.) to carbon coat the porous carbon matrix with growing nanosilicon. By reducing the gas flow rate, the rate of reduction of the organic gas into carbon is decreased, and the pores in the porous carbon are gradually deposited and blocked, enhancing the protection of the silicon in the porous carbon and suppressing the expansion during lithium insertion. Finally, a silicon-based negative electrode material with a large pore volume, a large proportion of mesopores, and unblocked pores is obtained.

[0066] In some embodiments, the mass ratio of the pore-forming agent to the pulverized petroleum coke is 30 wt% - 40 wt%; the volume concentration of the pore-forming agent is 5 mol / L - 10 mol / L.

[0067] In some embodiments, the pore-forming agent is an alkaline compound, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, etc. By controlling the amount of the strong base and the corrosion time, the carbon source is corroded to obtain porous carbon pores that meet the requirements.

[0068] If the concentration and addition amount of the pore-forming agent alkali are too small or the etching time is too short, the pore distribution of the material is uneven, the porosity is low, and most of the obtained pores are micropores; if the concentration and addition amount of the pore-forming agent alkali are too large or the etching time is too long, most of the obtained pores are mesopores, and even macropores, and the structure is more loose, the pressure resistance is poor, while increasing the pore volume, the proportion of mesopores increases and the proportion of micropores decreases.

[0069] In some embodiments, the temperature for pore formation is 95°C to 105°C; the temperature for catalytic graphitization is 500°C to 1000°C; the purification includes pickling and calcination, and the temperature for calcination is 400°C to 600°C; the temperature for silane deposition is 350°C to 450°C; the deposition temperature for carbon coating is 500°C to 700°C.

[0070] During the pore formation process, etch for 2 - 4 hours at 95°C to 105°C.

[0071] During the catalytic graphitization process, the temperature required for graphitization is reduced by the catalyst, which is beneficial to preserving the micro-mesopore ratio of the porous carbon itself.

[0072] During the calcination of purification, the carbon matrix is purified by high-temperature heating to reduce heteroatoms, especially to reduce the content of oxygen atoms to <2%, thereby improving the initial efficiency of the material in battery applications.

[0073] During the silane deposition process, if the deposition temperature is too low, the silane is difficult to be reduced and the production capacity is low; if it is too high, the nano-silicon is converted from amorphous silicon to crystalline silicon, resulting in anisotropic expansion and increasing the expansion rate of the material; and if it is too high, chemically inert silicon carbide is easily formed. Preferably, the silane deposition time is generally 1 - 3 hours. If the time is too short, the silicon cannot be completely deposited inside; if the time is too long, the silicon is likely to accumulate on the surface, increasing the material expansion and shortening the service life.

[0074] During the deposition process of carbon coating, if the deposition temperature is too high, chemically inert silicon carbide is easily formed; if the deposition temperature is too low, the organic gas source is difficult to be reduced. The same is true for too long and too short deposition times.

[0075] In some embodiments, the method for preparing the above-mentioned silicon-carbon negative electrode material includes the following steps:

[0076] S1: After crushing and shaping petroleum coke, the crushed petroleum coke is obtained, and the crushed petroleum coke is mixed with a pore-forming agent and placed in a constant-temperature device for pore formation to obtain a petroleum coke-based porous carbon material;

[0077] S2: The petroleum coke-based porous carbon material is catalytically graphitized and then purified to obtain a graphitized porous carbon matrix;

[0078] S3: Perform chemical vapor deposition on the graphitized porous carbon matrix to achieve silane deposition and prepare a carbon coating layer. Then, use a coating agent to prepare an artificial SEI film coating layer, and then perform demagnetization screening to obtain a silicon-carbon negative electrode material.

[0079] In some embodiments, the method for preparing an artificial SEI film coating layer using a coating agent includes techniques such as layer-by-layer assembly and spray drying. In some embodiments, after mixing the intermediate product with the carbon coating layer with a coating agent in a certain mass ratio (0.5% - 4%) to form a slurry, the slurry is spray-dried to obtain a powder, and then under the protection of an inert gas such as nitrogen, high-temperature treatment is carried out to obtain an artificial SEI film coating layer.

[0080] In some embodiments, the coating agent includes at least one of lithium fluoride, aluminum fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes; the deposition temperature of the artificial SEI film coating layer is 200°C - 500°C.

[0081] Setting an artificial SEI film coating layer with high conductivity outside the carbon coating layer is beneficial to improving the conduction of lithium ions in silicon-carbon during charge and discharge, enhancing the ionic conductivity of the material, reducing the polarization of the material during use, and further extending the service life of the battery cell. Under the physical constraint of the artificial SEI film coating layer, the damage to the carbon skeleton and SEI film caused by the lithium insertion expansion of silicon can be alleviated, playing a role in reducing expansion.

[0082] In some embodiments, the deposition temperature of the artificial SEI film coating layer is any value in 200°C - 500°C such as 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, etc. If the deposition temperature is too high, it is easy for carbon and silicon to undergo an irreversible reaction to generate silicon carbide with ineffective capacity; if the deposition temperature is too low, the reaction is incomplete, resulting in insufficient activity of the generated artificial SEI film. Preferably, the deposition temperature of the artificial SEI film coating layer is 330°C - 450°C.

[0083] To solve the above problems, the present invention also proposes a negative electrode tab, which includes the above-mentioned silicon-carbon negative electrode material, and also includes a negative electrode current collector, a binder, and an additive. It should be noted that regardless of how the negative electrode current collector, binder, and additive are conventionally selected, as long as the silicon-carbon negative electrode material of the present solution is used in the negative electrode tab, the related beneficial effects of the silicon-carbon negative electrode material can be obtained.

[0084] To solve the above problems, the present invention also proposes a secondary battery, the battery includes the above-mentioned silicon-carbon negative electrode material, and the battery includes any one of a battery cell, a battery module, and a battery pack.

[0085] Example 1

[0086] I. Preparation of negative electrode material

[0087] S1: Preparation of petroleum coke-based porous carbon material

[0088] After the initial crushing of petroleum coke, it is pulverized by ball milling and then shaped by jet milling to modify the particle edges and corners, making the particles approach a spherical shape. The Dv50 of the pulverized petroleum coke is 11 μm. The pulverized petroleum coke is mixed with a pore-forming agent, where the pore-forming agent accounts for 35 wt% of the pulverized petroleum coke and the volume concentration of the pore-forming agent is 8 mol / L. The mixture is placed in a constant-temperature device at 100 °C and etched for 3 hours to obtain a petroleum coke-based porous carbon material. The graphitization degree of the petroleum coke-based porous carbon material is 4%, and the pore distribution is a pore volume of 1.2 - 1.6 g / cm 3 ;

[0089] S2: Preparation of graphitized porous carbon matrix

[0090] The petroleum coke-based porous carbon material is placed in a box furnace and mixed evenly with a cobalt catalyst (the catalyst accounts for 2 wt%) at 600 °C, followed by catalytic graphitization treatment. Then, it is washed with hydrochloric acid and then with water until neutral and dried. The obtained product is dissolved with acid and the excess residual alkali is washed and neutralized. The obtained product is placed in a tube furnace (nitrogen atmosphere) and calcined at 500 °C to obtain a graphitized porous carbon matrix. The graphitized porous carbon matrix has a pore volume of 0.9 - 1.2 g / cm 3 ; a pore distribution structure with a micropore proportion of 50% - 70%, a mesopore proportion of 30% - 45%, and a macropore proportion of 0 - 10%, and the obtained carbon matrix has a particle size range Dv50 of 8 μm;

[0091] S3: Preparation of silicon-carbon anode material

[0092] Silane deposition: The graphitized porous carbon matrix material is placed in a rotary furnace / fluidized bed, and a mixed gas of protective gas nitrogen and silane gas is introduced. The protective gas accounts for 20%. The deposition temperature is set at 400 °C and the deposition time is 2 hours. The particle size is 8 nanometers and the silicon deposition mass is 47.2%. Nanoscale silicon is deposited inside the porous carbon matrix, and the particle size of the nanoscale silicon is 10 nm to obtain a petroleum coke-based porous carbon matrix with grown nanoscale silicon;

[0093] Carbon coating: The petroleum coke-based porous carbon matrix with grown nanoscale silicon is placed in a rotary furnace / fluidized bed, and chemical vapor deposition is carried out in a mixed atmosphere of protective gas nitrogen and organic gas sources (acetylene, ethylene, methane, etc.) to perform soft carbon coating on the porous carbon matrix with grown nanoscale silicon. During this process, the deposition temperature is 500 °C, the deposition time is 2 hours, and the soft carbon deposition amount (coating amount) is 14.5 wt%. After magnetic separation, a silicon-carbon anode material is obtained.

[0094] Example 2

[0095] In Example 2, after the step of preparing the carbon coating layer, a step of preparing an artificial SEI film coating layer is further included: after adding a lithium fluoride coating agent to the intermediate product of the carbon coating layer and mixing to form a slurry, the deposition amount of the lithium fluoride coating agent is 3 wt%, and after obtaining a powder by spray drying, under the protection of an inert gas, a deposition treatment is carried out at 400 °C, and the deposition amount of the lithium fluoride coating agent is 3 wt% to obtain an artificial SEI film coating layer. For other aspects, refer to Example 1.

[0096] Example 3

[0097] In Example 3, the silicon deposition amount of the silicon-carbon negative electrode material is 46.6 wt%, the particle size of the nano-silicon is 6 nm, the deposition amount of the soft carbon material of the carbon coating layer is 10 wt%, and the deposition amount of the lithium fluoride coating agent of the artificial SEI film coating layer is 0.5 wt%. For other aspects, refer to Example 2.

[0098] Example 4

[0099] In Example 4, the silicon deposition amount of the silicon-carbon negative electrode material is 48 wt%, the particle size of the nano-silicon is 10 nm, the deposition amount of the soft carbon material of the carbon coating layer is 12 wt%, the deposition temperature of the artificial SEI film coating layer is 200 °C, and the deposition amount of the lithium fluoride coating agent is 2.4 wt%. For other aspects, refer to Example 2.

[0100] Example 5

[0101] In Example 5, the deposition temperature of the artificial SEI film coating layer is 400 °C. For other aspects, refer to Example 2.

[0102] Example 6

[0103] In Example 6, the deposition amount of the artificial SEI film coating layer is 2.2 wt%. For other aspects, refer to Example 2.

[0104] Example 7

[0105] In Example 7, the lithium fluoride is replaced with single-walled carbon nanotubes. For other aspects, refer to Example 2.

[0106] Example 8

[0107] In Example 8, in the process of preparing the graphitized porous carbon matrix of S2, after mixing evenly with a cobalt catalyst at 700 °C (the catalyst accounts for 3 wt%) and then carrying out catalytic graphitization treatment. For other aspects, refer to Example 2.

[0108] Example 9

[0109] In Example 9, in the process of preparing the graphitized porous carbon matrix of S2, after mixing evenly with a cobalt catalyst at 500 °C (the catalyst accounts for 1.5 wt%) and then carrying out catalytic graphitization treatment. For other aspects, refer to Example 2.

[0110] Comparative Example 1:

[0111] In Comparative Example 1, step S2 is not included, and the rest refers to Example 1; that is, catalytic graphitization is not performed, and the petroleum coke-based porous carbon material obtained by step S1 is used for silane deposition and carbon coating in step S3.

[0112] Comparative Example 2:

[0113] In Comparative Example 2, the raw materials are purchased from BETRAY New Materials Group Co., Ltd., a carbon source of biomass carbonization is used, a petroleum coke carbon source is not used, catalytic graphitization is not performed, and the other refers to Example 2.

[0114] Comparative Example 3:

[0115] In Comparative Example 3, the raw materials are purchased from BETRAY New Materials Group Co., Ltd., a carbon source of biomass carbonization is used, a petroleum coke carbon source is not used, and the other refers to Example 2.

[0116] Comparative Example 4:

[0117] In Comparative Example 4, the raw materials are purchased from BETRAY New Materials Group Co., Ltd., a carbon source of phenolic resin carbonization is used, a petroleum coke carbon source is not used, catalytic graphitization is not performed, and the other refers to Example 2. Comparative Example 5:

[0118] In Comparative Example 5, the particle size of the nano-silicon is 15 nm, and the other refers to Example 2.

[0119] Comparative Example 6:

[0120] In Comparative Example 6, after being uniformly mixed with a cobalt catalyst at 450 °C (the catalyst accounts for 1.3 wt%), catalytic graphitization treatment is carried out, and the other refers to Example 2.

[0121] Comparative Example 7:

[0122] In Comparative Example 6, it is uniformly mixed with a cobalt catalyst at 760 °C (the catalyst accounts for 3.3 wt%), and the other refers to Example 2.

[0123] Performance Test

[0124] (1) XRD Test Method

[0125] The silicon-carbon anode material is ground into powder and pressed into tablets. A target material such as Cu Kα is selected. ) The scanning range is set to 10° to 50°, the step size is 0.02°, and the scanning speed range is 4° / min to 8° / min. The diffraction pattern is recorded by a detector.

[0126] (2) Graphitization Degree Test Method

[0127] Calculating the interlayer spacing (d 002 ) through the diffraction peak of the (002) crystal plane, and calculating and evaluating the graphitization degree of the graphitized porous carbon matrix using the Bragg equation.

[0128] (3) Porosity test method

[0129] Using the instrument Micromeritics: AutoPore IV 9500, forcing mercury into the pores under high pressure, and calculating the pore size distribution according to the relationship between pressure and pore size. After the sample is dried, it is placed in the sample chamber, evacuated, gradually pressurized, and the mercury intrusion volume is recorded. The pore size distribution of the graphitized porous carbon matrix is calculated using the Washburn equation.

[0130] (4) Pressure resistance test

[0131] Put the silicon-carbon materials in the comparative examples and examples into a compaction density meter, keep pressing under a pressure of 5 tons for 5 minutes, take out the particles and test the specific surface area with a specific surface area meter, and calculate the difference compared with the specific surface area of the silicon-carbon material before pressing.

[0132] (5) Specific capacity test

[0133] Assemble the silicon-carbon materials in the comparative examples and examples into electrode sheets to make button cells, charge them at a constant current rate of 0.1C at room temperature until the voltage reaches 4.3V, and further charge them at a constant voltage of 4.3V until the current is lower than 0.05C to make them in a fully charged state of 4.3V. Subsequently, discharge them at a constant current rate of 0.1C until the voltage reaches 3.0V and stop.

[0134] (6) Swelling test

[0135] Take 5 lithium-ion secondary batteries prepared in the comparative examples and examples each. When the batteries are fully charged, disassemble the electrode sheets and measure the thickness of the fully lithiated electrode sheets with a micrometer. The swelling rate = (the thickness of the fully lithiated disassembled electrode sheet - the thickness of the electrode sheet when assembling the battery) / the thickness of the electrode sheet when assembling the battery * 100%.

[0136] (7) Powder resistivity

[0137] Put the powder into a mold, apply a constant pressure of 10 - 30 MPa to press it into a sheet to ensure close contact between particles. Use a four-probe resistivity tester, and the four probes contact the sample surface at equal intervals. Input current through a constant current source, measure the voltage between the two inner probes, and calculate the resistivity using LabVIEW.

[0138] (8) Cycle performance test

[0139] Take 5 lithium-ion secondary batteries prepared in the comparative examples and examples each, and repeat charging and discharging the lithium-ion secondary batteries through the following steps, and calculate the cycle capacity retention rate of the lithium-ion secondary batteries.

[0140] First, in an environment of 25 °C, the first charge and discharge are carried out. Constant current and constant voltage charging are carried out at a charging current of 0.1C (i.e., the current value for completely discharging the theoretical capacity within 10h) until the upper limit voltage is 4.3V, and then constant current discharge is carried out at a discharge current of 1C until the final voltage is 3V. Record the discharge capacity of the first cycle; then carry out 100 charge and discharge cycles and record the discharge capacity of the 100th cycle.

[0141] Cycle capacity retention rate = (discharge capacity of the 100th cycle / discharge capacity of the first cycle) ×

[0142] 100%. See Table 1 for test results.

[0143] Table 1

[0144]

[0145]

[0146]

[0147] It can be seen from the data in Table 1 that, compared with the comparative examples, Examples 1-9 have less specific surface area loss after the pressure resistance test. Since the catalytic graphitization improves the strength of the carbon matrix, the pressure resistance of the material is improved.

[0148] The expansion rate of Examples 1-9 decreases significantly, indicating that it can better inhibit the large expansion of the material caused by high silicon deposition quality and large stress during expansion, and prevent the damage of the porous carbon matrix by silicon expansion.

[0149] Examples 1-9 have lower powder resistivity and better electrical conductivity, and can conduct current more smoothly. The powder resistivity of Comparative Examples 1-7 is higher, and the current will be greatly hindered when conducting in them, resulting in an increase in energy loss.

[0150] Examples 1-9 have higher specific capacity compared with the comparative examples, indicating that they can store more electrical energy per unit mass. This means that the battery can provide a longer battery life or a greater energy output under the same mass.

[0151] Examples 1-9 have higher efficiency during the first charge and discharge process compared with the comparative examples, can better convert the input energy into available energy, reduce the energy loss in the initial stage, and are beneficial to the battery performance and service life.

[0152] Examples 1-9 have a relatively high capacity retention rate, showing good cycle stability. They can still maintain a relatively high capacity after 500 cycles, have a relatively long service life, can maintain good performance for a long time, and reduce the replacement frequency and usage cost. Comparative Examples 1-7 have a relatively low capacity retention rate and poor cycle stability. As the number of cycles increases, the capacity decreases significantly, resulting in significant performance degradation during long-term use.

[0153] Comparative Example 1 has no SEI film. From the perspective of the expansion rate, resistivity, and cycle retention rate, it shows that the absence of the coated SEI film leads to intense interfacial side reactions and serious structural damage. In Comparative Example 4, the phenolic resin matrix has poor conductivity and cannot inhibit the silicon expansion. Comparative Example 7 has a relatively high graphitization degree, too low half-peak width, a specific capacity of only 1584 mAh / g, and too large silicon particle size, resulting in a decrease in pore utilization rate and a significant reduction in capacity.

[0154] In summary, for the silicon-carbon anode material provided by the present invention, using petroleum coke with an easily graphitizable carbon soft carbon structure as the raw material, it has higher self-strength. In actual battery cell applications, it can withstand a greater compaction density with less fragmentation, which is beneficial to the improvement of the energy density of the battery cell. The catalytic graphitization of the present invention greatly improves the graphitization degree of the petroleum coke-based porous carbon material. At the same time, due to graphitization at a lower temperature, the micro-mesopore ratio of the porous carbon itself is preserved, improving the problem of pore collapse and shrinkage of the porous carbon under traditional high-temperature graphitization, effectively reducing the lithium intercalation expansion of the silicon-carbon material, and at the same time improving the capacity and initial efficiency of the material itself.

[0155] Without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0156] The above are only partial or preferred embodiments of the present invention. Neither the text nor the drawings can limit the scope of protection of the present invention. Any equivalent structural transformation made using the content of the specification and drawings of the present invention under the overall concept of the present invention, or any direct / indirect application in other related technical fields, is included in the scope of protection of the present invention.

Claims

1. A silicon-carbon negative electrode material, characterized in that: In the XRD spectrum of the silicon-carbon negative electrode material, the (002) crystal plane diffraction peak is located at a diffraction angle 2θ of 27.8° to 28.8°, and the half-peak width of the (002) crystal plane diffraction peak is FWMH°, 2.20≤FWMH≤3.

0.

2. The silicon-carbon negative electrode material according to claim 1, characterized in that: The silicon-carbon negative electrode material comprises a graphitized porous carbon matrix, and the graphitization degree of the graphitized porous carbon matrix is ​​25% to 45%; And / or, the pore volume of the graphitized porous carbon substrate is 0.9 g / cm 3 ~1.2g / cm 3 ; And / or, the pore distribution of the graphitized porous carbon matrix is ​​as follows: micropores account for 50% to 70%, mesopores account for 30% to 45%, and macropores account for 0% to 10%.

3. The silicon-carbon negative electrode material according to claim 1, characterized in that: The silicon-carbon negative electrode material includes loaded nano-silicon, and the loaded amount of the nano-silicon is 46wt% to 52wt%; And / or, the particle size Dv50 of the nano-silicon is 6nm-10nm.

4. The silicon-carbon negative electrode material according to claim 2, characterized in that: The silicon-carbon negative electrode material comprises a carbon coating layer disposed on the surface of the graphitized porous carbon substrate, and the coating amount of the carbon coating layer is 10wt% to 15wt%.

5. The silicon-carbon negative electrode material according to claim 4, characterized in that: The silicon-carbon negative electrode material comprises an artificial SEI film coating layer arranged on the surface of the carbon coating layer, and the coating amount of the artificial SEI film coating layer is 0.5wt% to 4wt%.

6. The silicon-carbon negative electrode material according to claim 5, characterized in that: The coating agent of the artificial SEI film coating layer includes at least one of lithium fluoride, aluminum fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

7. The silicon-carbon negative electrode material according to claim 2, characterized in that: The graphitized porous carbon matrix is ​​obtained by catalytic graphitization of a petroleum coke-based porous carbon material; And / or, the graphitization degree of the petroleum coke-based porous carbon material is 1% to 30%; And / or, the pore volume of the petroleum coke-based porous carbon material is 1.2 g / cm 3 ~1.6g / cm 3 .

8. A method for preparing a silicon-carbon negative electrode material according to any one of claims 1 to 7, characterized in that: The steps include: The petroleum coke is crushed and shaped to obtain crushed petroleum coke, and the crushed petroleum coke is mixed with a pore-forming agent to form pores to obtain a petroleum coke-based porous carbon material; catalytically graphitizing the petroleum coke-based porous carbon material and then purifying it to obtain a graphitized porous carbon matrix; The graphitized porous carbon substrate is subjected to chemical vapor deposition of silane, and then a carbon coating layer is provided, followed by demagnetization and screening to obtain a silicon-carbon negative electrode material.

9. The method for preparing the silicon-carbon negative electrode material according to claim 8, characterized in that: The mass ratio of the pore-forming agent to the crushed petroleum coke is 30wt% to 40wt%; And / or, the molar concentration of the pore-forming agent is 5 mol / L to 10 mol / L.

10. The method for preparing the silicon-carbon negative electrode material according to claim 8, characterized in that: In the catalytic graphitization process, the catalyst accounts for 0.5% to 3% of the mass of the petroleum coke-based porous carbon material; And / or, the temperature of the catalytic graphitization is 500°C to 1000°C.

11. The method for preparing the silicon-carbon negative electrode material according to claim 8, characterized in that: Before the demagnetization and screening, an artificial SEI film coating layer is prepared by using a coating agent, and the coating agent includes at least one of lithium fluoride, aluminum fluoride, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

12. The method for preparing the silicon-carbon negative electrode material according to claim 11, characterized in that: The deposition temperature of the artificial SEI film coating layer is 200°C to 500°C.

13. A negative electrode plate, characterized in that: It includes the silicon-carbon negative electrode material according to any one of claims 1 to 7, or includes the silicon-carbon negative electrode material prepared according to any one of the preparation methods according to claims 8 to 12.

14. A secondary battery, characterized in that: The battery comprises the negative electrode sheet as claimed in claim 13.

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