Silicon-carbon composite material, preparation method therefor, secondary battery and electronic device

Abstract

The present application relates to a silicon-carbon composite material, a preparation method therefor, a secondary battery and an electronic device, and belongs to the technical field of electrochemistry. The silicon-carbon composite material of the present application comprises a porous carbon support and silicon particles located in pores of the porous carbon support; at least a partial region on the surface of the porous carbon support is provided with a carbon coating layer. The true density of the silicon-carbon composite material in an atmosphere of N2 is ρ1 g / cm3, and the true density of the silicon-carbon composite material in an atmosphere of He is ρ2 g / cm3, 1-ρ1 / ρ2≥10％. The silicon-carbon composite material can effectively alleviate the volume expansion of silicon inside the silicon-carbon composite material, thereby reducing the swelling of negative electrode sheets, and further significantly improving the swelling and cycle performance of secondary batteries.

Description

A silicon-carbon composite material and its preparation method, a secondary battery, and electronic devices. Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a silicon-carbon composite material and its preparation method, a secondary battery, and an electronic device. Background Technology

[0002] Currently, the main anode material for traditional commercial lithium-ion batteries is graphite. Graphite possesses high chemical stability and good cycle performance, but its specific capacity is limited (theoretical specific capacity is 372 mAh / g), severely hindering the improvement of lithium-ion battery energy density. Silicon-based materials, on the other hand, have a theoretical specific capacity as high as 4200 mAh / g, making them suitable as anode materials to significantly improve the energy density of lithium-ion batteries. However, silicon-based materials experience volume expansion and contraction exceeding 300% during lithium insertion / extraction, resulting in significant mechanical stress. After multiple cycles, this causes silicon particles to fracture and pulverize, severely hindering lithium-ion transport within the anode, thus leading to poor cycle performance of lithium-ion batteries. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the prior art and provide a silicon-carbon composite material, a method for preparing the same, a secondary battery, and an electronic device.

[0004] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0005] The first aspect of this application provides a silicon-carbon composite material comprising a porous carbon support and silicon particles located within the pores of the porous carbon support, wherein at least a portion of the surface of the porous carbon support is provided with a carbon coating layer; the true density of the silicon-carbon composite material in an N2 atmosphere is ρ1 g / cm³. 3 The true density of the silicon-carbon composite material in a He atmosphere is ρ² g / cm³. 3 The condition 1-ρ1 / ρ2≥10% can be achieved by ensuring that the secondary battery meets this requirement, thereby improving its expansion and cycle performance.

[0006] In some embodiments of this application, 10% ≤ 1 - ρ1 / ρ2 ≤ 40%. Ensuring that the secondary battery meets the 10% ≤ 1 - ρ1 / ρ2 ≤ 40% requirement can further improve the expansion and cycle performance of the secondary battery, while also taking into account the kinetic performance of the secondary battery.

[0007] In some embodiments of this application, 15% ≤ 1 - ρ1 / ρ2 ≤ 40%. Ensuring that the secondary battery meets the 15% ≤ 1 - ρ1 / ρ2 ≤ 40% requirement can further improve the expansion and cycle performance of the secondary battery, while also taking into account the kinetic performance of the secondary battery.

[0008] In some embodiments of this application, 1.32≤ρ1≤1.75; and / or, 1.95≤ρ2≤2.19. Ensuring that the secondary battery satisfies 1.32≤ρ1≤1.75 and / or 1.95≤ρ2≤2.19 can further improve the expansion and cycle performance of the secondary battery.

[0009] In some embodiments of this application, the average pore size of the porous carbon support is 3 nm to 5.8 nm. Setting the average pore size of the porous carbon support to 3 nm to 5.8 nm can further improve the expansion and cycle performance of the secondary battery.

[0010] In some embodiments of this application, the specific surface area of ​​the porous carbon support is 1000 m². 2 / g to 1800m 2 / g. This results in a specific surface area of ​​1000 m² for the porous carbon support. 2 / g to 1800m 2 / g can further improve the expansion and cycle performance of secondary batteries.

[0011] In some embodiments of this application, the pore volume of the porous carbon support is 0.6 cm³. 3 / g to 1.4cm 3 / g. This makes the pore volume of the porous carbon support 0.6cm³. 3 / g to 1.4cm 3 / g can further improve the expansion and cycle performance of secondary batteries.

[0012] In some embodiments of this application, the silicon particles comprise silicon grains with an average grain size of 1.1 nm to 4.7 nm. Having an average grain size of 1.1 nm to 4.7 nm for the silicon grains can further improve the expansion and cycle performance of the secondary battery.

[0013] In some embodiments of this application, the average thickness of the carbon coating layer is from 2.1 nm to 10.5 nm. Having an average carbon coating layer thickness of 2.1 nm to 10.5 nm can further improve the expansion and cycle performance of the secondary battery.

[0014] A second aspect of this application provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0015] S1. Silicon deposition: Using silicon-containing gas as the silicon source, silicon particles are deposited in the pores of a porous carbon support by chemical vapor deposition to obtain a pre-porous silicon-carbon material.

[0016] S2, Sulfur Deposition: Under an inert gas atmosphere, sulfur vapor is deposited to fill the remaining pores of the silicon-carbon material with pre-reserved pores in S1 to obtain an intermediate.

[0017] S3, Carbon Coating: The intermediate in S2 is carbon coated to obtain carbon-coated silicon-carbon material;

[0018] S4, Sulfur Sublimation: Under an inert gas atmosphere, the sulfur in the carbon-coated silicon-carbon material in S3 is sublimated to obtain a silicon-carbon composite material.

[0019] When the silicon-carbon composite material obtained by the above preparation method is used in the negative electrode of a secondary battery, it can improve the expansion and cycle performance of the secondary battery.

[0020] In some embodiments of this application, step S1 specifically involves: heating a porous carbon support to 400°C to 550°C at a heating rate of 1°C / min to 10°C / min under an inert gas atmosphere, holding at that temperature, and then depositing it in a silane mixture for 1 hour to 20 hours to obtain a pre-porous silicon-carbon material; wherein the silane mixture consists of silane and an inert gas; based on the volume of the silane mixture, the volume percentage of silane is 10% to 50%, and the volume percentage of the inert gas is 50% to 90%. When the silicon-carbon composite material obtained by the preparation method including the above step S1 is applied to the negative electrode of a secondary battery, it can improve the expansion and cycle performance of the secondary battery.

[0021] In some embodiments of this application, step S2 specifically involves: depositing sulfur vapor for 0.5 to 5 hours in an inert gas atmosphere at 300 to 500°C to fill the remaining pores of the silicon-carbon material with pre-reserved pores in S1 with elemental sulfur, thereby obtaining an intermediate. When the silicon-carbon composite material obtained by the preparation method including the above step S2 is applied to the negative electrode of a secondary battery, it can improve the expansion and cycle performance of the secondary battery.

[0022] In some embodiments of this application, step S4 specifically involves sublimating the elemental sulfur in the carbon-coated silicon-carbon material from step S3 at 500°C to 700°C in an inert gas atmosphere to obtain a silicon-carbon composite material. When the silicon-carbon composite material obtained by the preparation method including step S4 is applied to the negative electrode of a secondary battery, it can improve the expansion and cycle performance of the secondary battery.

[0023] A third aspect of this application provides a secondary battery comprising a negative electrode, a positive electrode, and an electrolyte, wherein the negative electrode comprises the silicon-carbon composite material provided in the first aspect of this application. The secondary battery provided in the third aspect of this application exhibits a low expansion rate and good cycle performance.

[0024] In some embodiments of this application, the electrolyte comprises lithium tetrafluoroborate and lithium difluorophosphate. Combining lithium tetrafluoroborate and lithium difluorophosphate in the electrolyte can improve the expansion and cycle performance of the secondary battery.

[0025] In some embodiments of this application, the electrolyte includes ethylene carbonate, propylene carbonate, and fluoroethylene carbonate. Including ethylene carbonate, propylene carbonate, and fluoroethylene carbonate in the electrolyte can improve the expansion and cycle performance of the secondary battery.

[0026] The fourth aspect of this application provides an electronic device that includes the secondary battery provided in the third aspect of this application.

[0027] Compared with the prior art, the beneficial effects of this application are as follows:

[0028] This application forms reserved pores occupying a certain space by controlling the volume ratio of silicon particles in the pores of the porous carbon support and the volume ratio of the remaining pores. The reserved pores can effectively buffer the volume expansion of silicon during the lithiation / delithiation process to release the stress generated by silicon expansion, which is conducive to maintaining the structural integrity of silicon-carbon composite materials, thereby improving the expansion of composite silicon-carbon particles and negative electrode sheets, and thus significantly improving the expansion and cycle performance of secondary batteries. Detailed Implementation

[0029] To better illustrate the purpose, technical solution, and advantages of this application, the following description, in conjunction with specific embodiments and comparative examples, aims to provide a detailed understanding of the content of this application, rather than limiting it. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this application. Unless otherwise specified, the experimental reagents and instruments involved in the implementation of this application are all commonly used reagents and instruments.

[0030] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0031] The first aspect of this application provides a silicon-carbon composite material comprising a porous carbon support and silicon particles located within the pores of the porous carbon support.

[0032] At least a portion of the surface of the porous carbon support is provided with a carbon coating layer;

[0033] The true density of the silicon-carbon composite material in a N2 atmosphere (25℃) is ρ1 g / cm³. 3 The true density of the silicon-carbon composite material in a He atmosphere (25℃) is ρ2 g / cm³. 3 1-ρ1 / ρ2≥10% (for example, 1-ρ1 / ρ2 can be 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, or a range of any two of these values).

[0034] This application forms reserved pores that occupy a certain space by controlling the volume ratio of silicon particles and the volume ratio of remaining pores in the porous carbon carrier. (Large molecular gases such as nitrogen and argon cannot enter the reserved pores, while helium can enter the reserved pores). The reserved pores can effectively buffer the volume expansion of silicon during the lithiation / delithiation process to release the stress generated by silicon expansion. This is beneficial to maintaining the structural integrity of the silicon-carbon composite material, thereby alleviating the expansion of the silicon-carbon composite material and the negative electrode sheet, and thus significantly improving the cycle performance and expansion performance of the secondary battery.

[0035] It should be noted that true density refers to the actual mass of a solid substance per unit volume in an absolutely dense state, that is, the density after removing internal pores or voids between particles. 1-ρ1 / ρ2 represents the volume ratio of the pre-reserved pores within the silicon-carbon composite particle, based on the total volume of the silicon-carbon composite particle. If the value of 1-ρ1 / ρ2 is less than 10%, it indicates that the volume of the pre-reserved pores within the silicon-carbon composite is too small, making it difficult to effectively buffer the expansion of silicon, increasing the pulverization of the silicon-carbon composite, and even causing electrode cracking.

[0036] In some embodiments of this application, 10% ≤ 1-ρ1 / ρ2 ≤ 40%; preferably, 15% ≤ 1-ρ1 / ρ2 ≤ 40%. For example, 1-ρ1 / ρ2 can be 15%, 17%, 19%, 21%, 23%, 25%, 27%, 29%, 31%, 33%, 35%, 40%, or a range of any two of these values. When 1-ρ1 / ρ2 ≥ 15%, the volume ratio of the pre-reserved holes can be further increased, further utilizing the pre-reserved holes to buffer the volume expansion of silicon during the lithiation / delithiation process and maintain the structural integrity of the silicon-carbon composite material. When 1-ρ1 / ρ2 ≤ 40%, the possibility of restricted ion transport inside the silicon-carbon composite material due to an excessively large volume ratio of the pre-reserved holes can be reduced. When 15% ≤ 1-ρ1 / ρ2 ≤ 40%, the cycle performance and expansion performance of the secondary battery can be further improved. Simultaneously, the kinetic performance of the secondary battery can also be considered. Preferably, 20% ≤ 1 - ρ1 / ρ2 ≤ 40%.

[0037] In some embodiments of this application, 1.32 ≤ ρ1 ≤ 1.75; and / or, 1.95 ≤ ρ2 ≤ 2.19. For example, ρ1 can be 1.32, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75 or a range of any two of these values; ρ2 can be 1.95, 1.98, 2, 2.03, 2.05, 2.08, 2.1, 2.12, 2.15, 2.17, 2.19 or a range of any two of these values.

[0038] In some embodiments of this application, the average pore size of the porous carbon support is from 3 nm to 5.8 nm. For example, the average pore size of the porous carbon support can be 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 5.8 nm, or a range of any two values ​​therein. For example, the average pore size of the porous carbon support is from 3 nm to 4.5 nm. For example, the average pore size of the porous carbon support is from 4.5 nm to 5.8 nm. The average pore size of the porous carbon support affects its adsorption performance on silanes during the preparation of silicon-carbon composite materials, thereby affecting the volume ratio of silicon particles formed by silicon deposition within the pores of the porous carbon support, and consequently affecting the volume ratio of the remaining pores within the pores of the porous carbon support. Studies have found that by controlling the average pore size of the porous carbon support within the above range, after the pores of the porous carbon support reach a preset silicon content, reserved pores with a suitable volume ratio can be formed. These pore structures can serve as reserved space to buffer the volume expansion of silicon during lithiation / delithiation, thereby better reducing the volume expansion of the silicon-carbon composite material.

[0039] In some embodiments of this application, the specific surface area of ​​the porous carbon support is 1000 m². 2 / g to 1800m 2 / g. For example, the specific surface area of ​​porous carbon supports can be 1000 m². 2 / g、1100m 2 / g、1200m 2 / g、1300m 2 / g, 1400m 2 / g, 1500m 2 / g, 1600m 2 / g, 1700m 2 / g、1800m 2 / g or a range consisting of any two of these values. For example, the specific surface area of ​​a porous carbon support is 1000 m². 2 / g to 1400m 2 / g. For example, the specific surface area of ​​porous carbon supports is 1400 m². 2 / g to 1800m 2 / g. By adjusting the specific surface area of ​​the porous carbon support within the above range, the volume ratio of silicon particles in the pores of the porous carbon support can be better controlled to ensure the specific capacity / first-time efficiency of the silicon-carbon composite material, while also taking into account the volume ratio of the reserved pores to better alleviate the volume expansion of the silicon-carbon composite material.

[0040] In some embodiments of this application, the pore volume of the porous carbon support is 0.6 cm³. 3 / g to 1.4cm 3 / g. For example, the pore volume of a porous carbon support can be 0.6 cm³. 3 / g, 0.7cm3 / g, 0.8cm 3 / g, 0.9cm 3 / g, 1cm 3 / g, 1.1cm 3 / g, 1.2cm 3 / g, 1.3cm 3 / g, 1.4cm 3 / g or a range consisting of any two of these values. The utilization of the pore volume of the porous carbon support is divided into the pore volume occupied by silicon particles and the pore volume occupied by reserved pores. By adjusting the pore volume value of the porous carbon support within the above range, it is possible to better ensure that the reserved pores have a suitable volume ratio after achieving the preset silicon content, so as to balance the specific capacity / first-time efficiency and volume expansion of the silicon-carbon composite material.

[0041] In some embodiments of this application, the silicon particles comprise silicon grains with an average grain size of 1.1 nm to 4.7 nm. For example, the average grain size of the silicon grains can be 1.1 nm, 1.3 nm, 1.5 nm, 1.7 nm, 1.9 nm, 2.1 nm, 2.3 nm, 2.5 nm, 2.7 nm, 2.9 nm, 3.1 nm, 3.3 nm, 3.5 nm, 3.7 nm, 3.9 nm, 4.1 nm, 4.3 nm, 4.5 nm, 4.7 nm, or a range of any two of these values. For example, the average grain size of the silicon grains is 1.1 nm to 2.9 nm. For example, the average grain size of the silicon grains is 1.1 nm to 2.5 nm. For example, the average grain size of the silicon grains is 1.1 nm to 2 nm. The size of silicon particles mainly depends on the size of silicon grains. Larger average grain diameters result in a larger proportion of silicon particles within the pores of the porous carbon support, leading to a smaller proportion of remaining pores and thus weakening the buffering effect against silicon volume expansion. Conversely, smaller average grain diameters mean a smaller proportion of silicon particles within the pores of the porous carbon support, resulting in a larger proportion of remaining pores. While this effectively mitigates silicon volume expansion and reduces the overall volume expansion of silicon-carbon composites, it also leads to lower kinetic properties and specific capacity / first coulombic efficiency in the composites. By controlling the average grain diameter within the aforementioned range, it is possible to balance the kinetic properties and specific capacity / first coulombic efficiency of silicon-carbon composites while further improving their volume expansion performance.

[0042] It should be noted that the average grain size of silicon grains mentioned above refers to the average grain size (D) of silicon grains in silicon-carbon composite materials calculated using the Debye-Scherrer formula (D = Kλ / βcosθ) at 2θ = 28.4° after obtaining the XRD pattern of silicon-carbon composite materials using Highscore diffraction pattern analysis. In Kλ / βcosθ, K is the Scherrer constant, B is the measured half-width at half maximum (FWHM) of the diffraction peak of the sample, θ is the Bragg diffraction angle, and γ is the X-ray wavelength.

[0043] In some embodiments of this application, the particle size D of the silicon-carbon composite material v50 The particle size ranges from 5 μm to 11 μm. For example, the particle size D of silicon-carbon composites... v50 The particle size can be 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 10.5 μm, 11 μm, or any combination of two of these values. For example, the particle size D of a silicon-carbon composite material... v50 The particle size ranges from 5 μm to 8 μm. Among them, the particle size D... v50 This refers to the particle size distribution in a material's volumetric metric, from the smallest particle size to the 50% of the total volumetric particle size. This is achieved by controlling the particle size D of the silicon-carbon composite material. v50 When the value is within the above range, it can better balance the side reactions between the surface of silicon-carbon anode material particles and the electrolyte, as well as the kinetic performance of silicon-carbon anode material particles.

[0044] In some embodiments of this application, the average thickness of the carbon coating layer is from 2.1 nm to 10.5 nm. For example, the average thickness of the carbon coating layer can be 2.1 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 10.5 nm, or a range of any two of these values. For example, the average thickness of the carbon coating layer is from 2.1 nm to 6.4 nm. The carbon coating layer affects the structural stability of the silicon-carbon composite material. If the carbon coating layer is thin, it is easy for the carbon coating layer on the surface of the silicon-carbon composite material to break during electrode processing or silicon expansion, causing the pre-reserved pores to be exposed to air or electrolyte, which in turn leads to a reduction in the initial efficiency of the silicon-carbon composite material and an increase in electrolyte consumption during subsequent cycles. The carbon coating layer also affects the specific capacity and lithium-ion transport of the silicon-carbon composite material. If the carbon coating layer is thick, it will reduce the specific capacity of the silicon-carbon composite material and hinder the lithium-ion mass transfer into the interior of the silicon-carbon composite material particles. By adjusting the average thickness of the carbon coating layer within the above range, not only can the silicon-carbon composite material have high structural stability, but it also has high specific capacity and ensures good lithium-ion transport.

[0045] Furthermore, the coating form in the carbon coating layer can include a film-like distribution or a particulate distribution. The film-like distribution can make the particle surface of the silicon-carbon composite material uniform, thereby promoting the homogenization of SEI film formation and reducing the risk of silicon-lithium alloy contacting the electrolyte due to cracks caused by insufficient thickness in a certain part of the carbon coating layer. The particulate distribution can give the particle surface of the silicon-carbon composite material a certain roughness, thereby improving the bonding between the particle surface of the silicon-carbon composite material and the negative electrode binder, thus enhancing the stability and adhesion of the SEI film on the particle surface of the silicon-carbon composite material.

[0046] Furthermore, the carbon coating layer can be located in a partial area or the entire surface of the porous carbon support. When the carbon coating layer is located in the entire surface of the porous carbon support, the structural stability of the silicon-carbon composite material can be further improved.

[0047] A second aspect of this application provides a method for preparing a silicon-carbon composite material, comprising the following steps:

[0048] S1. Silicon deposition: Using silicon-containing gas as the silicon source, silicon particles are deposited in the pores of a porous carbon support by chemical vapor deposition to obtain a pre-porous silicon-carbon material.

[0049] S2, Sulfur Deposition: Under an inert gas atmosphere, sulfur vapor is deposited to fill the remaining pores of the silicon-carbon material with pre-reserved pores in S1 to obtain an intermediate.

[0050] S3, Carbon Coating: The intermediate in S2 is carbon coated to obtain carbon-coated silicon-carbon material;

[0051] S4, Sulfur Sublimation: Under an inert gas atmosphere, the sulfur in the carbon-coated silicon-carbon material in S3 is sublimated to obtain a silicon-carbon composite material.

[0052] In some embodiments of this application, step S1 specifically involves: heating the porous carbon support to 400°C to 550°C in an inert gas atmosphere at a heating rate of 1°C / min to 10°C / min (e.g., the heating rate can be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, or any two of these values), and then holding it at that temperature (e.g., the holding temperature can be 400°C, 420°C, 440°C, 460°C, 480°C, 500°C, 520°C, etc.). The temperature range is 0℃, 550℃, or any two of these values; the holding time is 60 min to 180 min, specifically 60 min, 80 min, 100 min, 120 min, 140 min, 160 min, 180 min, or any two of these values. Then, it is deposited in a silane mixture for 1 h to 20 h (for example, the deposition time in the silane mixture can be 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, or any two of these values) to obtain a pre-pored silicon-carbon material.

[0053] The heating rate of the aforementioned porous carbon support in an inert gas atmosphere mainly affects the consistency of the internal and external temperatures of the porous carbon support, thereby affecting the uniformity of silicon deposition. By adjusting the heating rate within the above range, the silane deposition reaction can be better promoted to form uniformly distributed silicon particles while ensuring production efficiency.

[0054] The aforementioned insulation temperature and deposition temperature in the silane mixture primarily affect the decomposition of silane in the mixture. Higher temperatures accelerate silane decomposition, causing silane to begin decomposing and depositing before it can penetrate the pores of the porous carbon support. This results in a higher concentration of silicon particles on the surface of the porous carbon support, which is detrimental to mitigating silicon expansion. Conversely, lower temperatures reduce silane utilization. By controlling the insulation and silicon deposition temperatures within the aforementioned ranges, a high silane utilization rate can be maintained while simultaneously promoting sufficient silane penetration into the pores of the porous carbon support before decomposition and deposition of silicon particles within the pores.

[0055] The deposition time of the aforementioned porous carbon support in the silane mixture mainly affects the amount of silicon particles generated. By adjusting the silicon deposition time within the above range, the silicon particles in the pores of the porous carbon support can reach a suitable silicon content. This not only ensures that the reserved pores have a suitable volume ratio to effectively buffer the expansion of silicon and improve cycle performance, but also enables the silicon-carbon composite material to have a high specific capacity / first coulombic efficiency.

[0056] The silane mixture consists of silane and an inert gas. Based on the volume of the silane mixture, the volume percentage of silane is 10% to 50%, and the volume percentage of the inert gas is 50% to 90%. For example, the volume percentage of silane can be 10%, 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, 33%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, or any range of two such values; the volume percentage of the inert gas can be 50%, 53%, 55%, 58%, 60%, 63%, 65%, 68%, 70%, 73%, 75%, 78%, 80%, 83%, 85%, 88%, 90%, or any range of two such values. By adjusting the volume ratio of silane and inert gas in the silane mixture to within the above range, it is not only beneficial to improve the fluidization state of the silane mixture in the fluidized bed equipment and the uniformity of its distribution in the hot zone, but also to maintain high production efficiency.

[0057] In some embodiments of this application, step S2 specifically involves: in an inert gas atmosphere, at a temperature of 300°C to 500°C (e.g., the temperature can be 300°C, 330°C, 350°C, 380°C, 400°C, 420°C, 450°C, 470°C, 500°C, or any two of these values), sulfur vapor is introduced and deposited for 0.5h to 5h (e.g., the deposition time can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, or any two of these values), to fill the remaining pores of the silicon-carbon material with pre-reserved holes in S1 with elemental sulfur, thereby obtaining an intermediate.

[0058] In some embodiments of this application, the carbon coating treatment method in step S3 includes any one of liquid-phase method, gas-phase method, or gas-liquid combined method; preferably, it is a gas-phase method. For example, the intermediate obtained in S2 is subjected to carbon coating treatment in a reducing atmosphere to obtain carbon-coated silicon-carbon material; wherein, the reducing atmosphere includes at least one of acetylene, propylene, natural gas, ethylene, or cyclohexane; the carbon coating treatment temperature is 500°C to 650°C, and the carbon coating treatment time is 2h to 6h. For example, the carbon coating treatment temperature can be 500°C, 530°C, 550°C, 570°C, 600°C, 630°C, 650°C, or any two of these values, and the carbon coating treatment time can be 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, or any two of these values. The thickness of the carbon coating layer is mainly related to the carbon coating treatment time. As the carbon coating treatment time increases, the average thickness of the formed carbon coating layer gradually increases.

[0059] In some embodiments of this application, step S4 specifically involves sublimating the elemental sulfur in the carbon-coated silicon-carbon material from step S3 at a temperature of 500°C to 700°C in an inert gas atmosphere (e.g., the temperature can be 500°C, 530°C, 550°C, 580°C, 600°C, 630°C, 650°C, 680°C, 700°C, or any two of these values), to obtain a silicon-carbon composite material. The sublimation temperature of elemental sulfur in step S4 affects the average particle size of silicon grains in the porous carbon support. The average particle size of silicon grains increases with increasing sublimation temperature of elemental sulfur, thereby reducing the volume ratio of pre-reserved pores in the porous carbon support. By controlling the sublimation temperature of elemental sulfur within the above range, not only can the elemental sulfur filling the pre-reserved pores be fully sublimated, but the size of the silicon grains can also be better maintained, so that the pre-reserved pores in the porous carbon support have a larger volume ratio, thus better buffering silicon expansion.

[0060] A third aspect of this application provides a secondary battery comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material, and the negative electrode active material comprising the silicon-carbon composite material provided in the first aspect of this application. This application does not particularly limit the type of secondary battery, which may include any device in which an electrochemical reaction occurs. The secondary battery in this application may include, but is not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, lithium-ion polymer secondary batteries (lithium-ion polymer batteries), etc.

[0061] The negative electrode active material of this application may also include carbon materials such as graphite or mesophase carbon microspheres, wherein the graphite may include, but is not limited to, at least one of artificial graphite or natural graphite. In some embodiments, the negative electrode active material includes silicon-carbon composite material and graphite; in some embodiments, the negative electrode active material includes silicon-carbon composite material and mesophase carbon microspheres. This application does not impose any particular limitation on the mass ratio of silicon-carbon composite material to the aforementioned carbon material, as long as the purpose of this application can be achieved. For example, the mass ratio of silicon-carbon composite material to the aforementioned carbon material can be from 1:1 to 1:9.

[0062] In some embodiments of this application, the compaction density of the negative electrode sheet is 1.1 g / cm³. 3 Up to 1.75 g / cm 3 For example, the compaction density of the negative electrode sheet can be 1.1 g / cm³. 3 1.2g / cm 3 1.3g / cm 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 3 1.75g / cm 3This can be a range consisting of any two of these values. By adjusting the compaction density of the negative electrode sheet within the above range, the negative electrode sheet achieves both high compaction density and high porosity, which helps to further mitigate the expansion of the silicon negative electrode sheet, thereby improving the cycle performance and expansion performance of the secondary battery. Typically, the compaction density of the negative electrode sheet can be adjusted by changing the cold pressing pressure; increasing the cold pressing pressure increases the compaction density of the negative electrode sheet, while decreasing the cold pressing pressure decreases the compaction density.

[0063] The negative electrode of the secondary battery of this application includes a negative electrode current collector and a negative electrode material layer located on at least one side surface of the negative electrode current collector. The negative electrode material layer comprises a negative electrode active material, a negative electrode binder, and a negative electrode dispersant. This application does not impose any particular limitation on the mass ratio of the negative electrode active material, the negative electrode binder, and the negative electrode dispersant in the negative electrode material layer. Those skilled in the art can select them according to actual needs, as long as the purpose of this application can be achieved.

[0064] The aforementioned "negative electrode material layer located on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be located on one surface of the negative electrode current collector along its own thickness direction, or it can be located on two surfaces of the negative electrode current collector along its own thickness direction. "Surface" can be the entire area of ​​the negative electrode current collector surface, or it can be a part of the negative electrode current collector surface. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0065] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, carbon-coated aluminum foil, carbon-coated copper foil, lithium-copper composite current collector, carbon-copper composite current collector, nickel-copper composite current collector, titanium-copper composite current collector, etc. This application does not impose any particular limitation on the thickness of the negative electrode current collector and the negative electrode material layer on the surface of the negative electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector may be 5 μm to 10 μm, and the thickness of the single-sided negative electrode material layer may be 50 μm to 75 μm.

[0066] This application does not impose any particular restrictions on the types of negative electrode binders and negative electrode dispersants, as long as they can achieve the purpose of this application. For example, negative electrode binders may include, but are not limited to, polyacrylates, polyimides, polyamides, polyamide-imides, polyvinylidene fluoride, styrene-butadiene rubber, sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium hydroxymethyl cellulose, potassium hydroxymethyl cellulose, etc.; negative electrode dispersants may include, but are not limited to, carboxymethyl cellulose, sodium carboxymethyl cellulose, etc.

[0067] The secondary battery of this application also includes a positive electrode sheet, which comprises a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector. The positive electrode material layer includes a positive active material, a positive binder, and a positive conductive agent. This application does not impose any particular limitation on the mass ratio of the positive active material, the positive binder, and the positive conductive agent in the positive electrode material layer. Those skilled in the art can select them according to actual needs, as long as the purpose of this application can be achieved.

[0068] The aforementioned "positive electrode material layer located on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector along its own thickness direction, or it can be located on two surfaces of the positive electrode current collector along its own thickness direction. "Surface" can be the entire area of ​​the surface of the positive electrode current collector, or it can be a part of the surface of the positive electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0069] This application does not impose any particular limitation on the positive electrode current collector, as long as it achieves the purpose of this application. For example, the positive electrode current collector may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, carbon-coated aluminum foil, carbon-coated copper foil, lithium-copper composite current collector, carbon-copper composite current collector, nickel-copper composite current collector, titanium-copper composite current collector, etc. This application does not impose any particular limitation on the thickness of the positive electrode current collector and the positive electrode material layer on the surface of the positive electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector may be 7 μm to 10 μm, and the thickness of the single-sided positive electrode active material layer may be 80 μm to 120 μm.

[0070] This application does not specifically limit the type of positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide (NCM811, NCM622, NCM523, NCM111), lithium nickel manganese aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganese oxide, spinel-type lithium nickel manganese oxide, and lithium titanate.

[0071] This application does not impose any particular restrictions on the types of positive electrode binders and positive electrode conductive agents, as long as they can achieve the purpose of this application. For example, positive electrode binders may include, but are not limited to, polyacrylates, polyimides, polyamides, polyamide-imides, polyvinylidene fluoride, styrene-butadiene rubber, sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium hydroxymethyl cellulose, potassium hydroxymethyl cellulose, etc.; positive electrode conductive agents may include, but are not limited to, conductive carbon black, carbon nanotubes, carbon fibers, flake graphite, Ketjen black, graphene, metallic materials (copper, nickel, aluminum, or silver), conductive polymers (polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole), etc.

[0072] The secondary battery of this application also includes a separator, which separates the positive electrode and the negative electrode to prevent internal short circuits in the secondary battery. The separator allows electrolyte ions to pass freely without affecting the electrochemical charging and discharging process. This application does not impose any particular limitations on the separator, as long as it can achieve the purpose of this application. For example, the material of the separator can include, but is not limited to, polyethylene, polyolefins mainly composed of polypropylene, polyesters (such as polyethylene terephthalate membranes), cellulose, polyimide, polyamide, spandex, aramid, etc.; the type of separator can include woven membranes, nonwoven membranes, microporous membranes, composite membranes, rolled membranes, spun membranes, etc.

[0073] The diaphragm of this application may include a base membrane and a coating on at least one surface of the base membrane. The base membrane may be a nonwoven fabric or composite membrane with a porous structure. For example, the base membrane may be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, a polypropylene-polyethylene-polypropylene porous composite membrane, etc. The coating may be a polymer layer, an inorganic layer, or a mixture layer formed by a polymer and an inorganic material. The inorganic particles in the coating may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate.

[0074] The secondary battery of this application also includes an electrolyte.

[0075] In some embodiments of this application, the electrolyte includes both lithium tetrafluoroborate and lithium difluorophosphate. Based on the mass of the electrolyte, the mass percentage of lithium tetrafluoroborate can be 0.01% to 2%. In some embodiments, the mass percentage of lithium tetrafluoroborate can be 0.2% to 1.5%. In some embodiments, the mass percentage of lithium tetrafluoroborate can be 0.2% to 1%. Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate can be 0.01% to 2%. In some embodiments, the mass percentage of lithium difluorophosphate can be 0.2% to 1%. In some embodiments, the mass percentage of lithium difluorophosphate can be 0.2% to 0.8%. When the electrolyte includes both lithium tetrafluoroborate and lithium difluorophosphate, a solid electrolyte interface film with strong ion transport capability can be formed on the surface of the silicon-carbon composite material. This is beneficial for adapting to silicon-carbon composite materials with a large pre-reserved pore volume ratio, further improving the electrochemical performance of the secondary battery, such as kinetic performance, expansion performance, and cycle performance.

[0076] In some embodiments of this application, the electrolyte simultaneously comprises ethylene carbonate, propylene carbonate, and fluoroethylene carbonate. Based on the mass of the electrolyte, the mass percentage of ethylene carbonate can be 1% to 20%. In some embodiments, the mass percentage of ethylene carbonate can be 5% to 15%. In some embodiments, the mass percentage of ethylene carbonate can be 11% to 15%. Based on the mass of the electrolyte, the mass percentage of propylene carbonate can be 1% to 20%. In some embodiments, the mass percentage of propylene carbonate can be 5% to 15%. In some embodiments, the mass percentage of propylene carbonate can be 10% to 20%. Based on the mass of the electrolyte, the mass percentage of fluoroethylene carbonate can be 1% to 20%. In some embodiments, the mass percentage of fluoroethylene carbonate can be 5% to 15%. In some embodiments, the mass percentage of fluoroethylene carbonate can be 10% to 20%. When the electrolyte simultaneously comprises ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, a solid electrolyte interface film with strong ion transport capability can be formed on the surface of the silicon-carbon composite material. This is beneficial for adapting to silicon-carbon composite materials with a large pre-reserved pore volume ratio, further improving the electrochemical performance of the secondary battery, such as kinetic performance, expansion performance, and cycle performance.

[0077] In some embodiments of this application, the electrolyte simultaneously comprises lithium tetrafluoroborate, lithium difluorophosphate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate. When the electrolyte simultaneously comprises lithium tetrafluoroborate, lithium difluorophosphate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, the electrochemical performance of the secondary battery is further improved, such as kinetic performance, expansion performance, and cycle performance.

[0078] The electrolyte also includes other lithium salts and other organic solvents. This application does not impose any particular limitation on the mass percentage of other lithium salts and other organic solvents in the electrolyte, as long as the purpose of this application is achieved. For example, based on the mass of the electrolyte, the mass percentage of other lithium salts may be 10% to 25%, and the mass percentage of other organic solvents may be 75% to 90%.

[0079] This application does not impose any particular restrictions on the types of lithium salts used, as long as they achieve the purpose of this application. Lithium salts known in the art can be used, such as lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), lithium difluorooxalatoborate, etc. This application does not impose any particular restrictions on organic solvents, as long as they achieve the purpose of this application. For example, organic solvents can include carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents (such as dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or phosphate esters).

[0080] The secondary battery of this application also includes a packaging bag for containing the positive electrode, separator, negative electrode, electrolyte, and other components known in the art. This application does not limit the scope of these other components. This application does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it achieves the purpose of this application. For example, an aluminum-plastic film packaging bag can be used.

[0081] The fourth aspect of this application provides an electronic device that includes the secondary battery provided in the third aspect of this application. This application does not particularly limit the specific types of electronic devices; for example, electronic devices may include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0082] To clearly understand the technical solution of this application, the following detailed description of this application is provided in conjunction with specific embodiments and comparative examples. These embodiments should not be construed as limiting the scope of protection claimed in this application.

[0083] Test methods and equipment

[0084] 1. Pre-reserved hole ratio (α) test

[0085] The true density of the silicon-carbon composite material was tested using an Accupyc II 1340 true density tester under a He atmosphere and at 25°C, and the density ρ(He) was obtained. At the same time, the true density of the silicon-carbon composite material was tested using an Accupyc II 1340 true density tester under a N2 atmosphere and at 25°C, and the density ρ(N2) was obtained.

[0086] The percentage of reserved holes α = 1 - ρ(N2) / ρ(He) represents the volume ratio of reserved holes in the silicon-carbon composite material particles.

[0087] The specific derivation process of the reserved hole ratio α is as follows:

[0088] V(pre-drilled hole) + V(He) = V(N2);

[0089] V(He)=m / ρ(He);

[0090] V(N2)=m / ρ(N2);

[0091] V(He) / V(N2)=ρ(N2) / ρ(He);

[0092] V(He)=ρ(N2) / ρ(He)×V(N2);

[0093] V(reserved hole) + ρ(N2) / ρ(He) × V(N2) = V(N2);

[0094] α = V(reserved hole) / V(N2) = 1 - ρ(N2) / ρ(He);

[0095] The above V (reserved hole) refers to the volume of the reserved hole in the silicon-carbon composite material, in cm³. 3 V(He) is the volume of the silicon-carbon composite material measured under He conditions, in cm³. 3 V(N2) is the volume of the silicon-carbon composite material measured under N2, in cm³. 3 m is the mass of the silicon-carbon composite material, in g; ρ(He) is the true density of the silicon-carbon composite material in a He atmosphere, in g / cm³. 3 ρ(N2) is the true density of the silicon-carbon composite material in an N2 atmosphere, in g / cm³. 3 .

[0096] 2. Testing of pore volume, average pore size, and specific surface area of ​​porous carbon supports.

[0097] The porous carbon support was determined by argon gas adsorption. After obtaining adsorption / desorption data, the pore structure was fitted using the NLDFT model to obtain pore volume data for <2nm and >2nm and DFT specific surface area data, respectively.

[0098] 3. Silicon grain size testing

[0099] Using the D8Advance equipment with a Cu target Using the target material, tests were conducted at 60 kV, with an angle range from 10° to 80° at 2θ. Highscore diffraction pattern analysis was performed to obtain the XRD pattern of the silicon-carbon composite material. Then, the Debye-Scherrer formula, D = Kλ / βcosθ, was applied at 2θ = 28.4° to calculate the average grain size of silicon in the composite material. Where K is the Scherrer constant, D is the average grain size of silicon, B is the measured half-width at half-maximum (FWHM) of the diffraction peak, θ is the Bragg diffraction angle, and γ is the X-ray wavelength.

[0100] 4. Average thickness test of carbon coating layer

[0101] The silicon-carbon composite material was sliced ​​using focused ion beam (FIB) and characterized by high-resolution transmission electron microscopy (HRTEM, model TalosF200X). Within the same selected area, the thickness of the carbon coating layer in the silicon-carbon composite material particles was measured at five randomly selected locations. The arithmetic mean of the carbon coating layer thickness at the five different locations was calculated to obtain the average thickness of the carbon coating layer.

[0102] 5. Particle size testing of silicon-carbon composite materials

[0103] The particle size distribution of the silicon-based composite material was tested using a Malvern particle size analyzer (MasterSizer2000). The sample preparation method was as follows: Approximately 0.02 g of powder sample was added to a 50 mL clean beaker, followed by approximately 20 mL of deionized water. Then, 3 drops of sodium dodecyl sulfate surfactant were added to uniformly disperse the powder in the water. The mixture was then ultrasonically cleaned for 5 minutes using a 120 W ultrasonic cleaner to obtain the powder particle size test sample. In the volumetric particle size distribution of the material, the particle size D is defined as the particle size that reaches 50% of the volumetric cumulative distribution, starting from the smallest particle size. v50 .

[0104] 6. Compacted density test of negative electrode sheet

[0105] Cut the negative electrode sheet into 10 pieces with an area of ​​1540.25 mm² using a cutting machine. 2 Weigh the small discs and take the average value M; then measure the thickness of the small discs with a micrometer and take the average value H. Then wipe off the negative electrode material layer on 10 small discs, weigh them, and take the average value m; then measure the thickness of the negative electrode current collector with a micrometer and take the average value h. The compaction density of the negative electrode material layer = (Mm) / (2×1540.25) / [(Hh) / 2].

[0106] 7. Cyclic performance test

[0107] The soft-pack lithium-ion battery was placed in a constant temperature chamber at 25℃±1℃ for 30 minutes, then charged at a constant current of 0.5C to 4.53V, and then charged at a constant voltage of 4.53V to 0.025C. After being placed in a constant temperature chamber for 5 minutes, it was discharged at 0.5C to 3.0V. This constitutes one charge-discharge cycle. The initial cycle discharge capacity C0 of the lithium-ion battery was recorded. This cycle was then repeated 500 times. The cycle discharge capacity C1 of the 500th cycle was recorded. The 500-cycle capacity retention rate = C1 / C0 × 100%.

[0108] 8. Expansion performance test

[0109] The pouch lithium-ion battery was charged at a constant current of 0.5C to 3.95V, representing the initial half-charge state. The thickness of the lithium-ion battery at this initial half-charge state was measured using a micrometer and was identified as H0. After 500 cycles of the above cycle performance test, the lithium-ion battery was charged at a constant current of 0.5C to 4.53V, and then charged at a constant voltage of 4.53V to 0.025C, representing the full charge state. The thickness of the lithium-ion battery at this point was measured using a micrometer again and was identified as H1. The expansion rate after 500 cycles = (H1 - H0) / H0 × 100%.

[0110] Example 1

[0111] <Preparation of Silicon-Carbon Composite Materials>

[0112] 100g of porous carbon support was heated to 400℃ at a rate of 10℃ / min under a nitrogen atmosphere. The nitrogen atmosphere was then switched to a silane mixture (50% silane and 50% nitrogen by volume). Deposition was carried out at 400℃ for 5 hours to form a nucleus. The temperature was then raised to 500℃ under nitrogen, and sulfur vapor was introduced for deposition for 1 hour to seal the remaining surface pores. Next, an acetylene mixture (50% acetylene and 50% nitrogen by volume) was introduced for surface carbon coating for 2 hours. The temperature was then raised to 700℃ under nitrogen, and the sulfur was sublimated to form pre-existing pores, thus obtaining the silicon-carbon composite material.

[0113] <Preparation of Negative Electrode Sheets>

[0114] The negative electrode active material (composed of artificial graphite and the aforementioned silicon-carbon composite material in a mass ratio of 80:20), polymethyl acrylate, and sodium carboxymethyl cellulose (CMC) in a weight ratio of 97:2:1 were thoroughly mixed in an appropriate amount of deionized water to form a uniform negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was coated onto one side of the copper foil of the negative electrode current collector and dried at 85°C. After cold pressing, cutting, and slitting, it was dried under vacuum at 120°C for 12 hours to obtain a single-sided coated negative electrode sheet. The above steps were repeated on the other side of the copper foil in the single-sided coated negative electrode sheet to obtain a double-sided coated negative electrode sheet. After cold pressing, cutting, slitting, drying, and welding of tabs, a negative electrode sheet with a size of 78 mm × 875 mm was obtained for use. The compaction density of the negative electrode material layer after cold pressing was 1.7 g / cm³. 3 .

[0115] <Preparation of the positive electrode>

[0116] Lithium cobalt oxide (CCO), conductive carbon black (SuperP), and CCO binder (polyvinylidene fluoride (PVDF)) were mixed at a mass ratio of 97:1.4:1.6. N-methylpyrrolidone (NMP) was added as a solvent to prepare a CCO slurry with a solid content of 72 wt%. The slurry was then uniformly stirred under vacuum. The CCO slurry was then uniformly coated onto one side of a 12 μm thick aluminum foil used as a CCO current collector and dried at 85 °C to obtain a single-sided coated CCO electrode sheet. The coating weight of the CCO material layer was 19 mg / cm³. 2 Then, the above steps are repeated on the other side of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After cold pressing, cutting, slitting, drying, and welding of tabs, a positive electrode sheet with a size of 74mm × 867mm is obtained for use. The compaction density of the positive electrode material layer after cold pressing is 4.2 g / cm³. 3 .

[0117] <Preparation of Electrolyte>

[0118] In an argon-atmospheric glove box with a water content of less than 10 ppm, propyl propionate, ethyl propionate, ethylene carbonate, diethyl carbonate, 1,3-propanesulfonate lactone, and vinylene carbonate were mixed uniformly in a weight ratio of 10:40:20:20:5:5 to obtain a base solvent. Lithium hexafluorophosphate (LiPF6) was then added to the base solvent and mixed uniformly to obtain the electrolyte. The lithium salt content was 12.5% ​​by mass, with the remainder being the base solvent.

[0119] <Preparation of the diaphragm>

[0120] A porous polyethylene (PE) film with a thickness of 7 μm was used as the separator.

[0121] <Preparation of Lithium-ion Full Batteries>

[0122] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide insulation. This is then wound to form the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried at 80°C, and then injected with electrolyte. After vacuum sealing, settling, formation, shaping, and capacity testing, a soft-pack lithium-ion battery is obtained, which is a lithium-ion full battery.

[0123] Examples 2 to 3

[0124] Except for the difference in adjusting the average pore size, specific surface area, and pore volume of the porous carbon support in <Preparation of Silicon-Carbon Composite Materials> compared to Example 1, the rest is the same as in Example 1.

[0125] Examples 4 to 6

[0126] Except for the difference in the preparation of silicon-carbon composite materials, where the average particle size of silicon grains differs from that in Example 1 by adjusting the sulfur sublimation temperature, the rest of the process is the same as in Example 1. The average particle size of silicon grains increases with increasing sulfur sublimation temperature.

[0127] Examples 7 to 8

[0128] Except for the difference in the preparation of silicon-carbon composite materials, where the average thickness of the carbon coating layer differs from that in Example 1 by adjusting the carbon coating treatment time, the rest of the process is the same as in Example 1. The average thickness of the carbon coating layer increases with increasing carbon coating treatment time.

[0129] Example 9

[0130] In addition to the grading and grinding of the silicon-carbon composite material prepared in "Preparation of Silicon-Carbon Composite Materials", the particle size D of the silicon-carbon composite material is also adjusted. v50 Unlike Example 1, the rest is the same as Example 1.

[0131] Example 10

[0132] Except for the difference in the compaction density of the negative electrode sheet compared to Example 1, which is achieved by adjusting the cold pressing pressure in the <Preparation of Negative Electrode Sheet>, the rest is the same as Example 1.

[0133] Example 11

[0134] Except for the difference between Example 1 and Example 2, which is that the mass percentage of silicon-carbon composite material and the mass percentage of graphite in the negative electrode active material are different by adjusting the mass ratio of artificial graphite to silicon-carbon composite material in the <Preparation of Negative Electrode Sheet>, the rest is the same as Example 1.

[0135] Example 12

[0136] Except for the addition of lithium tetrafluoroborate and lithium difluorophosphate to the electrolyte, everything else is the same as in Example 4. Based on the mass of the electrolyte, the mass percentage of lithium tetrafluoroborate is 1%, and the mass percentage of lithium difluorophosphate is 1%. At the same time, based on the mass of the electrolyte, the mass percentage of the base solvent is reduced by 2%, while the weight ratio of each component in the base solvent remains unchanged.

[0137] Example 13

[0138] Except for the addition of propylene carbonate and fluoroethylene carbonate to the electrolyte, everything else was the same as in Example 4. Based on the mass of the electrolyte, the mass percentage of propylene carbonate was 10%, and the mass percentage of fluoroethylene carbonate was 10%. Meanwhile, based on the mass of the electrolyte, the mass percentage of the base solvent was reduced by 20%, while the weight ratio of each component in the base solvent remained unchanged.

[0139] Example 14

[0140] Except for the addition of lithium tetrafluoroborate, lithium difluorophosphate, propylene carbonate, and fluoroethylene carbonate to the electrolyte, everything else was the same as in Example 4. Based on the mass of the electrolyte, the mass percentages of lithium tetrafluoroborate, lithium difluorophosphate, propylene carbonate, and fluoroethylene carbonate were 0.2%, 0.2%, 5%, and 5%, respectively. Meanwhile, based on the mass of the electrolyte, the mass percentage of the base solvent was reduced by 10.4%, while the weight ratios of the components in the base solvent remained unchanged.

[0141] Example 15

[0142] Except for the addition of lithium tetrafluoroborate, lithium difluorophosphate, propylene carbonate, and fluoroethylene carbonate to the electrolyte, everything else was the same as in Example 4. Based on the mass of the electrolyte, the mass percentages of lithium tetrafluoroborate were 1.5%, lithium difluorophosphate 1%, propylene carbonate 15%, and fluoroethylene carbonate 15%. Meanwhile, based on the mass of the electrolyte, the mass percentage of the base solvent was reduced by 32.5%, while the weight ratios of the components in the base solvent remained unchanged.

[0143] Comparative Example 1

[0144] Except for the differences in the average pore size, specific surface area, and pore volume of the porous carbon carrier in the <Preparation of Silicon-Carbon Composite Material> compared to Example 1, and the difference in the average particle size of silicon grains in the silicon-carbon composite material caused by adjusting the removal temperature of sublimated sulfur, the rest are the same as in Example 1.

[0145] According to the data in Table 1, the capacity retention rate of the secondary batteries after 500 cycles is ≥91% and the expansion rate is ≤7.30%, indicating that the silicon-carbon composite material can effectively alleviate the volume expansion of silicon inside, thereby reducing the expansion of the negative electrode and significantly improving the cycle performance and expansion performance of the secondary batteries. Comparative Example 1 shows that when the value of 1-ρ1 / ρ2 is too small, the cycle performance and expansion performance of the secondary batteries will be poor.

[0146] As shown in Table 1, when the electrolyte simultaneously includes lithium tetrafluoroborate and lithium difluorophosphate, or simultaneously includes propylene carbonate and fluoroethylene carbonate, the capacity retention and cycle expansion rate of the secondary battery are improved. When the electrolyte simultaneously includes lithium tetrafluoroborate, lithium difluorophosphate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, the capacity retention and cycle expansion rate of the secondary battery are further improved.

[0147] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A silicon-carbon composite material, characterized in that, It includes porous carbon supports and silicon particles located within the pores of the porous carbon supports; At least a portion of the surface of the porous carbon support is provided with a carbon coating layer; The true density of the silicon-carbon composite material under N2 atmosphere is ρ1g / cm³. 3 The true density of the silicon-carbon composite material in a He atmosphere is ρ2 g / cm³. 3 , 1-ρ1 / ρ2≥10%.

2. The silicon-carbon composite material as described in claim 1, characterized in that, 10% ≤ 1 - ρ1 / ρ2 ≤ 40%, preferably 15% ≤ 1 - ρ1 / ρ2 ≤ 40%.

3. The silicon-carbon composite material as described in claim 1, characterized in that, 1.32≤ρ1≤1.75; and / or, 1.95≤ρ2≤2.

19.

4. The silicon-carbon composite material as described in claim 1, characterized in that, The porous carbon support satisfies at least one of the following: (1) The average pore size of the porous carbon support is 3 nm to 5.8 nm; (2) The specific surface area of ​​the porous carbon support is 1000 m². 2 / g to 1800m 2 / g; (3) The porous carbon support has a pore volume of 0.6 cm³. 3 / g to 1.4cm 3 / g.

5. The silicon-carbon composite material as described in claim 1, characterized in that, The silicon particles comprise silicon grains, the average particle size of which is 1.1 nm to 4.7 nm.

6. The silicon-carbon composite material as described in claim 1, characterized in that, The average thickness of the carbon coating layer is 2.1 nm to 10.5 nm.

7. A method for preparing the silicon-carbon composite material according to any one of claims 1 to 6, comprising the following steps: S1. Silicon deposition: Using silicon-containing gas as the silicon source, silicon particles are deposited in the pores of a porous carbon support by chemical vapor deposition to obtain a pre-porous silicon-carbon material. S2, Sulfur Deposition: Under an inert gas atmosphere, sulfur vapor is deposited to fill the remaining pores of the pre-reserved porous silicon-carbon material described in S1 to obtain an intermediate. S3, Carbon Coating: The intermediate described in S2 is subjected to carbon coating treatment to obtain carbon-coated silicon-carbon material; S4. Sulfur sublimation: Under an inert gas atmosphere, the sulfur in the carbon-coated silicon-carbon material described in S3 is sublimated to obtain a silicon-carbon composite material.

8. The preparation method according to claim 7, characterized in that, The preparation method satisfies at least one of the following: (1) Step S1 is as follows: The porous carbon support is heated to 400°C to 550°C in an inert gas atmosphere at a heating rate of 1°C / min to 10°C / min and then kept at that temperature. Then it is deposited in a silane mixed gas for 1h to 20h to obtain a pre-pored silicon carbon material. The silane mixture is composed of silane and an inert gas; based on the volume of the silane mixture, the volume percentage of silane is 10% to 50%, and the volume percentage of the inert gas is 50% to 90%. (2) Step S2 is as follows: in an inert gas atmosphere at 300°C to 500°C, sulfur vapor is introduced for deposition for 0.5h to 5h to fill the remaining pores of the pre-reserved porous silicon-carbon material in S1 with elemental sulfur to obtain an intermediate. (3) Step S4 is specifically: in an inert gas atmosphere at 500°C to 700°C, the elemental sulfur in the carbon-coated silicon-carbon material described in S3 is sublimated to obtain a silicon-carbon composite material.

9. A secondary battery, comprising a negative electrode, a positive electrode, and an electrolyte, characterized in that, The negative electrode sheet comprises the silicon-carbon composite material according to any one of claims 1 to 6.

10. The secondary battery according to claim 9, wherein the electrolyte comprises lithium tetrafluoroborate and lithium difluorophosphate.

11. The secondary battery according to claim 9, wherein the electrolyte comprises ethylene carbonate, propylene carbonate, and fluoroethylene carbonate.

12. An electronic device, characterized in that, Includes the secondary battery as described in any one of claims 9 to 11.

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