A silicon-carbon anode material, its preparation method and application

Abstract

The present invention relates to a silicon-carbon negative electrode material, a preparation method thereof, and an application thereof. The silicon-carbon negative electrode material comprises porous silicon, a carbon layer filled in pores of the porous silicon and coated outside the porous silicon. Wherein, the initial true density of the silicon-carbon negative electrode material is 1.7-2.1 g / cm<supgt;3< / supgt;, and the change value of the true density after cold pressing at 500 MPa compared with the initial true density is 4-10%. The present invention can effectively solve the problem of volume expansion (>300%) of the silicon negative electrode material during the charge and discharge process of the lithium-ion battery, greatly improve the first discharge capacity and the first Coulomb efficiency of the silicon-carbon negative electrode material, and the lithium-ion battery using the silicon-carbon negative electrode material of the present invention has better cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a silicon-carbon anode material, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid growth of electric vehicles (EVs) in the global market and the continuous improvement of the cruising range requirements, it has become crucial to further improve the energy density of lithium-ion batteries (LIBs) used in electric vehicles. A rechargeable lithium-ion battery mainly consists of four parts: two electrodes (a positive electrode and a negative electrode), a separator, and an electrolyte. Among them, the positive and negative electrodes are separated by the separator and the liquid electrolyte, allowing lithium ions to shuttle back and forth between the two electrodes. When the battery is charged, the lithium ions released from the positive electrode will move towards the negative electrode. When discharging, the lithium ions will move from the negative electrode to the positive electrode. At the same time, the electrons released by the lithium atoms in the negative electrode will reach the positive electrode through the external circuit, thereby providing electricity through chemical energy.

[0003] The negative electrode materials of traditional commercial lithium-ion batteries are mainly graphite materials, with a theoretical specific capacity of 372 mAh / g (only one lithium atom can be accommodated per six carbon atoms). At present, its actual specific energy density is gradually approaching its theoretical limit, but it is still difficult to meet the market demand. Therefore, it is urgent to develop a new generation of high specific capacity lithium-ion battery negative electrode materials. Silicon is widely regarded as the most promising negative electrode material for high energy density lithium-ion batteries due to its high specific capacity (>3500 mAh / g, each silicon atom can combine with 4.4 lithium ions) and abundant reserves (the second most abundant element in the earth's crust). However, although the specific capacity of silicon is very high, the serious problems caused by its huge volume change (>300%) during the lithiation / delithiation process, such as pulverization caused by the repeated fracture of silicon, unstable solid electrolyte interface (SEI), and deformation of the electrode structure, will lead to rapid attenuation of the battery capacity and decline of the cycle performance. In addition, the electrode expansion will also squeeze the electrolyte filled in the pores of the separator, thereby hindering the transmission of lithium ions. Severe electrode expansion will cause the battery package to bulge, leading to potential safety hazards. If these challenges cannot be successfully overcome, silicon can only be used as a limited additive to the graphite-based negative electrode to gradually increase the energy density of lithium-ion batteries.

[0004] To solve the severe volume expansion problem of silicon, researchers have currently proposed some strategies: (1) Nanostructuring of silicon, which overcomes the above problems by reducing volume changes and providing short lithium-ion transport paths. However, silicon nanocomposite anodes face more fundamental problems, such as a higher specific surface area leading to more irreversible reactions and a low tap density; (2) Silicon / carbon composites, where carbon materials have good electrical conductivity and cycling stability and can be used as a substrate for silicon materials to effectively inhibit the volume expansion change of silicon materials and improve the cycling performance of the battery. However, it is a great challenge to achieve the uniformity of silicon particle dispersion in carbon, the uniformity of carbon coating on the surface of silicon particles, and the high electrical conductivity of the carbon substrate; (3) Porosification of silicon, which alleviates the volume change of silicon materials by introducing pores. However, the presence of pores may lead to a decrease in the mechanical properties of the material. For example, the material may collapse or break during the homogenization and pole piece rolling processes.

[0005] For example, CN106935834A provides a composite carbon-coated porous silicon anode material. The first layer of this anode material is coated with pyrolytic carbon or graphene of low carbon content organic matter, which can well coat or fill the porous silicon. Thereafter, the second layer on its surface is coated with pyrolytic carbon of high carbon content polymer, which has a dense characteristic and can well improve the electrical conductivity and prevent the electrolyte from entering the interior of the whole particle. Although this solution solves the problems of low Coulomb efficiency, poor electrical conductivity, and silicon volume expansion in the application of silicon in the anode to a certain extent, the maximum discharge capacity of the composite carbon-coated porous silicon anode material of this solution is only 1000 mAh / g.

[0006] Therefore, it is necessary to further improve it and develop a more excellent silicon-carbon anode material. Summary of the Invention

[0007] The object of the present invention is to provide a new silicon-carbon anode material and its preparation method. The lithium-ion battery using this silicon-carbon anode material has a better initial discharge capacity, a higher initial Coulomb efficiency, and a more excellent cycle capacity retention rate.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] In a first aspect of the present invention, a silicon-carbon anode material is provided, which includes porous silicon and a carbon layer filled in the pores of the porous silicon and coated outside the porous silicon; wherein, the initial true density of the silicon-carbon anode material is 1.7~2.1 g / cm 3 , and the change value of the true density after cold pressing at 500 MPa compared to the initial true density is 4~10%.

[0010] Among them, the initial true density of the silicon-carbon anode material refers to the true density directly measured without cold pressing of the silicon-carbon anode material product.

[0011] The calculation method for the change value of the true density after cold pressing at 500 MPa compared to the initial true density is (true density after cold pressing - initial true density) / initial true density × 100%.

[0012] According to some specific embodiments, the change value of the true density after cold pressing at 500 MPa compared to the initial true density is 4%, 4.1%, 4.2%, 4.3%, 4.4%, 4.5%, 4.6%, 4.7%, 4.8%, 4.9%, 5%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, 6%, 6.1%, 6.2%, 6.3%, 6.4%, 6.5%, 6.6%, 6.7%, 6.8%, 6.9%, 7%, 7.1%, 7.2%, 7.3%, 7.4%, 7.5%, 7.6%, 7.7%, 7.8%, 7.9%, 8%, 8.1%, 8.2%, 8.3%, 8.4%, 8.5%, 8.6%, 8.7%, 8.8%, 8.9%, 9%, 9.1%, 9.2%, 9.3%, 9.4%, 9.5%, 9.6%, 9.7%, 9.8%, 9.9% or 10%.

[0013] According to some specific embodiments, the change value of the true density after cold pressing at 500 MPa compared to the initial true density is 4 - 6%.

[0014] According to some specific embodiments, the specific surface area of the silicon - carbon negative electrode material is 1 - 20 m 2 / g. Further, the specific surface area of the silicon - carbon negative electrode material is 1 - 10 m 2 / g, such as 1 m 2 / g, 1.1 m 2 / g, 1.2 m 2 / g, 1.3 m 2 / g, 1.4m 2 / g, 1.5 m 2 / g, 1.6 m 2 / g, 1.7 m 2 / g, 1.8 m 2 / g, 1.9 m 2 / g, 2 m 2 / g, 2.1 m 2 / g, 2.2 m 2 / g, 2.3m 2 / g, 2.4 m 2 / g, 2.5 m 2 / g, 2.6 m 2 / g, 2.7 m 2 / g, 2.8 m2 / g, 2.9 m 2 / g, 3 m 2 / g, 3.1 m 2 / g, 3.2 m 2 / g, 3.3 m 2 / g, 3.4 m 2 / g, 3.5 m 2 / g, 3.6 m 2 / g, 3.7 m 2 / g, 3.8 m 2 / g, 3.9 m 2 / g, 4 m 2 / g, 4.1 m 2 / g, 4.2 m 2 / g, 4.3 m 2 / g, 4.4 m 2 / g, 4.5 m 2 / g, 4.6 m 2 / g, 4.7 m 2 / g, 4.8 m 2 / g, 4.9 m 2 / g, 5 m 2 / g, 5.1 m 2 / g, 5.2 m 2 / g, 5.3 m 2 / g, 5.4 m 2 / g, 5.5 m 2 / g, 5.6 m 2 / g, 5.7 m 2 / g, 5.8 m 2 / g, 5.9 m 2 / g, 6 m 2 / g, 6.1 m 2 / g, 6.2 m 2 / g, 6.3 m 2 / g, 6.4 m 2 / g, 6.5 m 2 / g, 6.6 m 2 / g, 6.7 m 2 / g, 6.8 m 2 / g, 6.9 m 2 / g, 7 m 2 / g, 7.1 m 2 / g, 7.2 m 2 / g, 7.3 m 2 / g, 7.4 m 2 / g, 7.5 m 2 / g, 7.6 m2 / g, 7.7 m 2 / g, 7.8 m 2 / g, 7.9 m 2 / g, 8 m 2 / g, 8.1 m 2 / g, 8.2 m 2 / g, 8.3 m 2 / g, 8.4 m 2 / g, 8.5 m 2 / g, 8.6 m 2 / g, 8.7 m 2 / g, 8.8 m 2 / g, 8.9 m 2 / g, 9 m 2 / g, 9.1 m 2 / g, 9.2 m 2 / g, 9.3 m 2 / g, 9.4 m 2 / g, 9.5 m 2 / g, 9.6 m 2 / g, 9.7 m 2 / g, 9.8 m 2 / g, 9.9 m 2 / g or 10 m 2 / g.

[0015] According to some specific embodiments, the carbon content of the silicon-carbon negative electrode material is 20-60 wt%, for example, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, 30 wt%, 31 wt%, 32 wt%, 33 wt%, 34 wt%, 35 wt%, 36 wt%, 37 wt%, 38 wt%, 39 wt%, 40 wt%, 41 wt%, 42 wt%, 43 wt%, 44 wt%, 45 wt%, 46 wt%, 47 wt%, 48 wt%, 49 wt%, 50 wt%, 51 wt%, 52 wt%, 53 wt%, 54 wt%, 55 wt%, 56 wt%, 57 wt%, 58 wt%, 59 wt% or 60 wt%. Further, the carbon content of the silicon-carbon negative electrode material is 40-50 wt%.

[0016] According to some specific embodiments, the oxygen content of the silicon-carbon negative electrode material is 10 wt% or less, for example 10 wt%, 9 wt%, 8 wt%, 7 wt%, 6 wt%, 5 wt%, 4 wt%, 3 wt%, 2 wt%, 1 wt%, and even the oxygen content of the silicon-carbon negative electrode material is 0.

[0017] According to some specific embodiments, the silicon carbide content of the silicon-carbon negative electrode material is 10 wt% or less, such as 10 wt%, 9 wt%, 8 wt%, 7 wt%, 6 wt%, 5 wt%, 4 wt%, 3 wt%, 2 wt%, 1 wt%, or even the silicon carbide content of the silicon-carbon negative electrode material is 0.

[0018] According to some specific embodiments, the porosity of the silicon-carbon negative electrode material is 5-30%. Among them, the porosity can be inferred from the ratio of the measured true density and the theoretical true density of the material, that is, porosity = (1 - measured true density value / theoretical true density) × 100%. The theoretical true density of the material can be calculated from the carbon content, silicon content of the material, and the theoretical densities of carbon and silicon. Due to the existence of internal pores, the measured true density is less than the theoretical true density of the material. Further, the porosity of the silicon-carbon negative electrode material is 13-20%, such as 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5% or 20%.

[0019] According to some specific embodiments, the pores of the silicon-carbon negative electrode material are discrete closed pores. Discrete closed pores refer to the fact that multiple pores are separated from each other and not connected. Discrete closed pores are relative to through holes, and the two can be judged by the change in true density before and after the material is broken. After the material with through holes is broken, the internal pores can be detected by gas, and the true density will increase significantly. For example, when detected with nitrogen, since the exposed pores will adsorb nitrogen, the result may even be negative. And after the material with discrete closed pores is broken, since only part of the pore structure is exposed, the change degree of its true density after breaking is small.

[0020] According to some specific embodiments, the D50 particle size of the silicon-carbon negative electrode material is 1-20 microns.

[0021] According to some specific embodiments, the pore diameter of the internal closed pores of the silicon-carbon negative electrode material is 0.1-5 nm.

[0022] The second aspect of the present invention provides a preparation method of the above-mentioned silicon-carbon negative electrode material, including the following steps:

[0023] (1) Mix porous silicon and a carbon source to obtain a silicon-carbon precursor, and carbonize the silicon-carbon precursor to obtain a silicon-carbon intermediate; wherein, the carbon source is a solid-phase carbon source or a liquid-phase carbon source;

[0024] (2) Perform secondary or multiple carbon coatings on the silicon-carbon intermediate to obtain the silicon-carbon negative electrode material.

[0025] The function of the first carbon coating on the porous silicon in step (1) of the present invention is to maintain the structural stability of the porous silicon during the high-temperature carbonization process and form the basic structure of discrete closed pores. The function of the secondary or multiple carbon coatings in step (2) is to form a dense coating layer on the outside of the material, further improve the structural stability of the material, and prevent the entry of the electrolyte.

[0026] According to some specific embodiments, the peak pore diameter of the porous silicon is 3 to 10 nanometers, such as 3 nanometers, 3.5 nanometers, 4 nanometers, 4.5 nanometers, 5 nanometers, 5.5 nanometers, 6 nanometers, 6.5 nanometers, 7 nanometers, 7.5 nanometers, 8 nanometers, 8.5 nanometers, 9 nanometers, 9.5 nanometers or 10 nanometers. The pore volume of the porous silicon is 0.45 to 0.75 cm 3 / g, such as 0.45 cm 3 / g, 0.5 cm 3 / g, 0.55 cm 3 / g, 0.6 cm 3 / g, 0.65 cm 3 / g, 0.7 cm 3 / g or 0.75 cm 3 / g. Among them, the peak pore diameter refers to the pore diameter value with the largest proportion in the pore diameter numerical distribution. By using porous silicon with a suitable peak pore diameter and pore volume as raw materials, the silicon-carbon negative electrode material prepared according to the above method can obtain an initial true density of 1.7 to 2.1 g / cm 3 , and the change value of the true density after cold pressing at 500 MPa compared to the initial true density is 4 to 10% of the silicon-carbon negative electrode material. The silicon-carbon negative electrode material under this characteristic has discrete closed pores, which can not only provide buffering for the volume expansion of silicon, thereby improving the cycle capacity retention rate and rate performance of the silicon-carbon negative electrode material, but also improve the first discharge capacity and first Coulomb efficiency of the silicon-carbon negative electrode material.

[0027] According to some specific embodiments, the particle size D50 of the primary particles of the porous silicon is 1 to 10 nanometers, and the particle size D50 of the secondary particles of the porous silicon is 1 to 10 micrometers.

[0028] According to some specific embodiments, the porous silicon is obtained by high-temperature disproportionation of silicon oxide and etching with a fluorine-containing reagent, or by etching aluminum-silicon alloy with hydrochloric acid, or by reducing silicon dioxide with magnesium.

[0029] Further, the specific method for obtaining the porous silicon by high-temperature disproportionation of silicon oxide and etching with a fluorine-containing reagent is as follows: First, silicon oxide (SiOx, 0 < x < 2) is placed in an atmospheric tube furnace for high-temperature disproportionation reaction to obtain a mixture of silicon and silicon oxide (SiOy, 0 < y <= 2, y > x); the obtained mixture is etched with a fluorine-containing reagent to obtain porous silicon.

[0030] Furthermore, the fluorine-containing reagent can be a liquid reagent such as hydrofluoric acid, or a gaseous reagent such as hydrogen fluoride gas.

[0031] Furthermore, the temperature of the high-temperature disproportionation reaction is 800-1200 °C.

[0032] Further, the specific method for obtaining the porous silicon by etching an aluminum-silicon alloy with hydrochloric acid is as follows: The aluminum-silicon alloy powder with a silicon content of 15 wt% - 25 wt% is pickled with hydrochloric acid at a concentration of 15 wt% - 25 wt% for 4 - 6 h to obtain the porous silicon.

[0033] According to some specific embodiments, the solid-phase carbon source or the liquid-phase carbon source is selected from one or more of tar, asphalt, glucose, sucrose, citric acid, and resin.

[0034] According to some specific embodiments, in step (1), the feeding mass ratio of the porous silicon to the carbon source is 1:0.1 - 2. For example, 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2. Further, the feeding mass ratio of the porous silicon to the carbon source is 1:0.1 - 1.5. Further, the feeding mass ratio of the porous silicon to the carbon source is 1:0.3 - 0.8.

[0035] According to some specific embodiments, in step (1), the temperature of carbonization is 700 - 900 °C, and the carbonization time is 3 - 8 hours.

[0036] According to some specific embodiments, the carbon coating in step (2) is the same as the method in step (1), or carbon coating is carried out by decomposing a gaseous carbon source at a high temperature, wherein the flow rate of the gaseous carbon source is 100 - 300 sccm.

[0037] According to some specific embodiments, the gaseous carbon source is a gaseous hydrocarbon. Among them, gaseous hydrocarbons such as acetylene and ethylene.

[0038] According to some specific embodiments, when carbon coating is carried out using a gaseous carbon source, the flow rate ratio of the gaseous carbon source to the inert gas in the introduced mixed gas is 1:2 - 3.

[0039] Further, the inert gas is one or a combination of nitrogen, helium, neon, or argon.

[0040] The third aspect of the present invention provides an application of the above silicon-carbon anode material in a lithium-ion battery.

[0041] According to some specific embodiments, the method for preparing the negative electrode of the lithium-ion battery includes: First, mixing the silicon-carbon negative electrode material, binder, conductive agent, and solvent according to a certain mass ratio to prepare a slurry with a certain viscosity, and then applying the obtained slurry onto the current collector by doctor blade coating with a coating thickness of 20-60 microns. The obtained electrode sheet is transferred to a vacuum oven for drying, and the dried electrode sheet is cut into circular sheets with a diameter of 9-15 mm by a cutting machine for the assembly of the lithium-ion battery.

[0042] Further, the binder is CMC (carboxymethyl cellulose), the conductive agent is conductive carbon black (C), and the solvent is water; the current collector is copper foil; the temperature of the vacuum oven is 80-120 °C.

[0043] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0044] The silicon-carbon negative electrode material of the present invention has a plurality of discrete closed-pore structures and a uniformly dense carbon coating layer, which can effectively solve the volume expansion problem (>300%) of the silicon negative electrode material during the charge and discharge process of the lithium-ion battery, greatly improving the first discharge capacity, first Coulombic efficiency, and battery capacity retention rate of the silicon-carbon negative electrode material. The lithium-ion battery using the silicon-carbon negative electrode material of the present invention has better cycle performance. The preparation method of the silicon-carbon negative electrode material of the present invention has a simple process, rich raw material reserves, and wide sources. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0046] Figure 1 Schematic structural diagram of the silicon-carbon negative electrode material prepared for the embodiment (with a discrete closed-pore structure);

[0047] Figure 2 Scanning electron microscope image of the cross-section of the silicon-carbon negative electrode material prepared for Example 1;

[0048] Figure 3 Schematic structural diagram of the silicon-carbon negative electrode material prepared for Comparative Example 3 (with a through-hole structure and silicon carbide);

[0049] Figure 4 Schematic structural diagram of the silicon-carbon negative electrode material prepared for Comparative Example 6 (without a pore structure);

[0050] Among them, 1, hole; 2, silicon; 3, carbon layer; 4, silicon carbide. Detailed implementation manners

[0051] In the following text, only some exemplary embodiments are simply described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the embodiments of the present invention. Therefore, the drawings and the description are considered to be exemplary in nature rather than restrictive.

[0052] All features disclosed in the present invention, or steps in all methods or processes disclosed, except for mutually exclusive features or steps, can be combined in any manner.

[0053] The technical solutions of the present invention will be further described below in conjunction with specific embodiments. However, the present invention should not be limited to these embodiments. Unless otherwise stated, they can all be replaced by other equivalent or alternative features with similar purposes. Unless otherwise stated, each feature is only an example in a series of equivalent or similar features. The terms used in the present invention generally have the meanings commonly understood by those of ordinary skill in the art unless otherwise specified. The implementation conditions adopted in the embodiments can be further adjusted according to different requirements of specific uses, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in each embodiment of the present invention can be combined with each other as long as they do not conflict with each other.

[0054] In the present invention, operations without special instructions are carried out at room temperature. The raw materials in this application can be obtained commercially or prepared by conventional methods in the prior art. In the present invention, unless otherwise specified, the contents are all mass contents, and "%" is the mass percentage.

[0055] In the design and selection of the anode material of the present invention, the serious influence of the volume expansion problem (>300%) of the silicon anode material during the charge and discharge process of the lithium-ion battery is considered, as well as the problem that the electrical properties of the carbon-coated porous silicon material still need to be improved. It is unexpectedly found that using porous silicon with a specific size as the raw material and by adjusting the dosage ratio of the solid-phase carbon source or the liquid-phase carbon source to the porous silicon in the primary carbon coating process, a silicon-carbon anode material intermediate with a discrete closed-pore structure inside can be prepared, and then secondary carbon coating is carried out to form a uniform and dense carbon coating layer, thereby effectively solving the problems of material pulverization and electrode deformation caused by the volume change of the silicon material during the charge and discharge process of the lithium-ion battery, improving the electrochemical performance of the silicon-carbon anode material, and moreover, the silicon-carbon anode material of this application has good rate performance and can meet the requirements of rapid charge and discharge of the battery. Among them, the porous silicon with a specific size is porous silicon with a peak pore diameter of 3 to 10 nanometers and a pore volume of 0.45 to 0.75 cm 3 / g.

[0056] Further, control the pore diameter of the discrete closed pores inside the silicon-carbon anode material to be 0.1 to 5 nanometers to further improve the mechanical properties and electrochemical properties of the material. If the silicon-carbon anode material does not have discrete closed pores or has more through holes that are interconnected inside, it is not conducive to the electrochemical properties of the material.

[0057] Among them, as Figure 2 shown, from the scanning electron microscope image, it is impossible to clearly see the discrete closed pores inside the silicon-carbon anode material because the pore diameter of the closed pores is too small. However, the true density of the silicon-carbon anode material after cold pressing is increased compared with that before cold pressing, and the increased value is between 4% and 10%, indicating that there are pores inside the silicon-carbon anode material. Combining with the scanning electron microscope images of multiple different cross-sections of the silicon-carbon anode material by this applicant and no obvious pores are observed, it can be inferred that the silicon-carbon anode material of this application has discrete closed pores with a pore diameter less than 5 nm inside.

[0058] Further, the particle size D50 of the primary particles of the porous silicon is 1 to 10 nanometers, and the particle size D50 of the secondary particles of the porous silicon is 1 to 10 micrometers. Among them, the primary particles of the porous silicon refer to the size of a single solid particle inside the porous silicon, that is, the size of the silicon particle numbered 2 as Figure 1 shown. The pore diameter of the porous silicon refers to the pore diameter in the secondary particles formed by connecting multiple primary particles together, that is, the gap between multiple primary particles. The secondary particles of the porous silicon refer to individual porous silicon particles that are separated from each other. Example 1

[0059] 1. Preparation of the silicon-carbon anode material:

[0060] (1) Place 10 g of silicon monoxide in an atmosphere tube furnace, introduce argon protective gas into the tube furnace, and perform high-temperature treatment at 900 °C for 5 h. The silicon monoxide undergoes a disproportionation reaction at high temperature to generate a mixture of silicon and silicon dioxide;

[0061] (2) Add the above mixture to a 49% hydrofluoric acid aqueous solution and stir to remove silicon dioxide, and then wash and dry with deionized water to obtain porous silicon. The particle size D50 of the secondary particles of the porous silicon is 5.2 micrometers, the particle size D50 of the primary particles is 3.8 nanometers, the peak pore diameter of the porous silicon is 3.7 nanometers, and the pore volume of the porous silicon is 0.7 cm 3 / g;

[0062] (3) Place the porous silicon and the asphalt acetone solution in a stirring kettle, where the mass ratio of the porous silicon to the asphalt is 1:0.5, stir well and dry to obtain a silicon-carbon precursor;

[0063] (4) Place the silicon-carbon precursor in an atmospheric tube furnace, introduce argon protective gas into the tube furnace, and perform high-temperature treatment at 800 °C for 5 h. The high-temperature carbonization of the asphalt material forms a carbon layer on the porous silicon, thereby obtaining a silicon-carbon intermediate;

[0064] (5) Repeat steps (3) and (4) for the silicon-carbon intermediate obtained in the previous step to obtain the final product, the silicon-carbon negative electrode material.

[0065] 2. Preparation of lithium-ion half cells:

[0066] Add the above-mentioned silicon-carbon negative electrode material, conductive carbon black, and binder to an appropriate amount of deionized water according to a mass ratio of 8:1:1 to make a slurry. Coat the obtained slurry on a copper foil through an adjustable scraper to make a battery electrode sheet. The coated electrode sheet is dried in an oven at 80 °C for 2 h. After taking it out, the electrode sheet is cut into a negative electrode sheet with a diameter of 15 mm through an electrode sheet cutter. The cut electrode sheet is placed in a vacuum drying oven at 120 °C and dried for 5 h;

[0067] Quickly transfer the dried electrode sheet to a glove box filled with argon (the water content and oxygen content are lower than 0.1 ppm). Use a lithium metal sheet as the counter electrode, and the electrolyte is a mixed solvent containing 1 M LiPF 6 (EC / EMC 3:7 v / v), and a polypropylene (PP) membrane is used as the separator to assemble a lithium-ion half cell;

[0068] Let the assembled battery stand for 12 h, and then perform a 0.1 C constant current charge-discharge test on a Blue Electric Battery Test System, with the voltage range being 0.01 - 1.5 V;

[0069] Rate test: Discharge the half cell at 0.1 C, charge it at 0.1 C and 2 C respectively, and use the ratio of the capacity obtained at 2 C to the capacity obtained at 0.1 C to represent the rate performance of the material.

[0070] 3. Preparation of lithium-ion full cells:

[0071] Use commercial ternary NMC532 as the positive electrode, the prepared silicon-based material as the negative electrode, the electrolyte is a mixed solvent containing 1 M LiPF 6 (EC / EMC 3:7 v / v, 10% FEC), and a polypropylene (PP) membrane is used as the separator to assemble a lithium-ion full cell. Let the assembled battery stand for 12 h, and then perform a 0.33 C constant current and constant voltage charge-discharge test on a Blue Electric Battery Test System, with the voltage range being 2.5 - 4.2 V. Example 2

[0072] Other steps are the same as those in Example 1, except that the mass ratio of porous silicon to asphalt is 1:1. Example 3

[0073] Other steps are the same as those in Example 1, except that the secondary carbon coating is carried out in a mixed gas of acetylene (200 sccm) and nitrogen (500 sccm) at a temperature of 800 °C for 3 hours. Example 4

[0074] Other steps are the same as those in Example 1, except that in step (1), the heat treatment temperature of silicon monoxide is 1000 °C. The D50 of the secondary particles of the prepared porous silicon is 5.2 microns, the D50 of the primary particles is 8.0 nanometers, the peak pore diameter of the porous silicon is 5.9 nanometers, and the pore volume of the porous silicon is 0.5 cm 3 / g. Example 5

[0075] Other steps are the same as those in Example 1, except that the porous silicon is obtained by pickling an aluminum-silicon alloy powder with 20 wt% silicon content with 20 wt% hydrochloric acid for 5 h. The D50 of the secondary particles of the prepared porous silicon is 7.2 microns, the D50 of the primary particles is 5.8 nanometers, the peak pore diameter of the porous silicon is 8.1 nanometers, and the pore volume of the porous silicon is 0.7 cm 3 / g. Example 6

[0076] Other steps are the same as those in Example 1, except that the carbon source for carbon coating is glucose. Comparative Example 1

[0077] Other steps are the same as those in Example 1, except that the material is neither etched with hydrofluoric acid nor subjected to secondary carbon coating, that is, steps (2) and (5) in Example 1 are omitted. Comparative Example 2

[0078] Other steps are the same as those in Example 1, except that the material is not subjected to secondary carbon coating, that is, step (5) in Example 1 is omitted. Comparative Example 3

[0079] Other steps are the same as those in Example 1, except that the porous silicon is only carbon-coated once and is carried out in a mixed gas of acetylene (200 sccm) and nitrogen (500 sccm) at a temperature of 800 °C for 5 hours. Comparative Example 4

[0080] Other steps are the same as those in Example 1, except that the porous silicon is only carbon-coated once and is carried out in a mixed gas of acetylene (200 sccm) and nitrogen (500 sccm) at a temperature of 600 °C for 5 hours. Comparative Example 5

[0081] The other steps are the same as those in Example 1, except that the primary carbon coating of the porous silicon is carried out in a mixed gas of acetylene (200 sccm) and nitrogen (500 sccm) at a temperature of 700 °C for 1 hour. The secondary carbon coating uses pitch, where the mass ratio of the porous silicon to the pitch is 1:0.5, the temperature is 800 °C, and the time is 3 hours. Comparative Example 6

[0082] The other steps are the same as those in Example 1, except that the precursor of the porous silicon is SiO 1.5 and the mass ratio of the porous silicon to the pitch is 1:2.5. Among them, the particle size D50 of the secondary particles of the prepared porous silicon is 6.7 μm, the particle size D50 of the primary particles is 3.5 nm, the peak pore diameter of the porous silicon is 6.3 nm, and the pore volume of the porous silicon is 0.8 cm 3 / g. Comparative Example 7

[0083] The other steps are the same as those in Example 5, except that the silicon content of the silicon-aluminum alloy powder is 10 wt%. The particle size D50 of the secondary particles of the prepared porous silicon is 6.2 μm, the particle size D50 of the primary particles is 4.5 nm, the peak pore diameter of the porous silicon is 20.3 nm, and the pore volume of the porous silicon is 0.9 cm 3 / g.

[0084] Performance testing:

[0085] The test methods for specific surface area, pore diameter, and pore volume are as follows: Measured using Guoyi Quantum V-SORP 2800P.

[0086] The test method for carbon content is as follows: Heat the sample in a muffle furnace to oxidize the carbon in the sample to carbon dioxide and the silicon to silicon dioxide. The content of silicon in the original sample can be calculated from the mass of silicon dioxide, and then the content of carbon can be calculated.

[0087] The test method for silicon carbide content: The silicon carbide content is determined from the peak of silicon carbide in the X-ray diffraction pattern.

[0088] The test method for oxygen content: The oxygen content is obtained by EDS energy spectrum analysis.

[0089] The test method for primary particle size: Measured using Dandong BETTERSIZE2600 laser particle size analyzer.

[0090] The test method for secondary particle size: Calculated from the intensity of the silicon diffraction peak in the X-ray diffraction pattern by the Scherrer formula.

[0091] Calculation method of porosity: The porosity can be inferred from the ratio of the measured true density of the material to the theoretical true density, i.e., porosity = (1 - measured true density value / theoretical true density) × 100%. The theoretical true density of the material can be calculated from the carbon content, silicon content of the material, and the theoretical densities of carbon and silicon.

[0092] The test method for the true density of the raw material is as follows: Use Guoyi Quantum G-DenPyc X900 to measure the true density of the material.

[0093] The test method for the true density of the material after cold pressing at 500 MPa is as follows: Put 0.5 g of the material into a mold with a diameter of 1.5 cm, apply a pressure of 500 MPa, keep it for 30 s and then release the pressure. Take out the material and conduct the true density test according to the above-mentioned true density test method.

[0094] The test results of the performance parameters of the above-mentioned examples and comparative examples are shown in Table 1. The first discharge (delithiation) capacity, first Coulombic efficiency, and cycle capacity retention rate of the above-mentioned examples and comparative examples are shown in Table 2. The rate performance test results of some examples and comparative examples are shown in Table 3.

[0095]

[0096]

[0097]

[0098] Compared with Example 1, the asphalt content in Example 2 increases, and the carbon content in the obtained product increases accordingly. Both the first discharge capacity and efficiency decrease, but the cycle capacity retention rate increases. In Example 3, the secondary carbon coating uses a gaseous carbon source, and similar effects to those of Example 1 can be achieved. The obtained product has similar capacity, efficiency, and a discrete closed-pore structure. In Example 4, the treatment temperature of silicon monoxide increases, resulting in changes in the pore structure of the generated porous silicon, which can regulate the true density of the final product. However, the porosity of the negative electrode material prepared in Example 4 is smaller than that of other examples. Therefore, it cannot provide enough space for the volume expansion of the silicon material, resulting in a lower cycle capacity retention rate of Example 4 than that of other examples. Examples 5 and 6 use different silicon sources and carbon sources respectively, and similar effects to those of Example 1 can be achieved.

[0099] In Comparative Example 1, neither hydrofluoric acid etching nor secondary carbon coating was carried out. The obtained product was a mixture of carbon-coated silicon and silicon dioxide. The presence of silicon dioxide led to lower capacity and efficiency. In Comparative Example 2, secondary carbon coating was not carried out. The carbon layer of the obtained product was loose and porous, resulting in an increased specific surface area, a decrease in the first Coulombic efficiency, and the electrolyte was prone to contact with the internal silicon to occur side reactions, thus leading to a significant decrease in the cycle capacity retention rate and rate performance. In Comparative Example 3, acetylene was used as the single carbon source. Since the decomposition of acetylene is a strong exothermic reaction and the decomposition products of acetylene have high activity, the formation of silicon carbide was caused; in addition, during the carbon coating process, the decomposition products of acetylene entered the porous silicon structure from the outside to the inside, resulting in more carbon deposition on the outside, blocking the pores of the porous silicon in the early stage of carbon coating, thus forming a pore structure with through holes. The negative electrode material with such a pore structure has low mechanical strength. After the material breaks, its specific surface area will increase significantly, resulting in problems such as a decrease in efficiency. In Comparative Example 4, acetylene was used as the single carbon source and a lower carbon coating temperature was used. Although low-temperature carbon coating avoided the formation of a large amount of silicon carbide, the coating layer formed at low temperature had a high degree of amorphousness and low conductivity, resulting in a decrease in material performance. In Comparative Example 5, acetylene was used for primary carbon coating at 700 °C. The reaction activity of acetylene was high, and the primary particles of silicon would grow significantly during primary carbon coating, resulting in a decrease in the performance of the material. In Comparative Example 6, the asphalt content was further increased, and the carbon content of the obtained product increased, resulting in the pores inside the porous silicon being filled. The material did not have a discrete closed-pore structure, thus leading to a decrease in the first discharge capacity, first Coulombic efficiency, and cycle capacity retention rate of the negative electrode material. In Comparative Example 7, the peak pore diameter of the porous silicon was too large, resulting in a still large internal pore diameter after carbon coating, and a connected structure was formed between the pores. After cold pressing, it broke, resulting in the exposure of the internal pores, thus leading to a decrease in the first Coulombic efficiency and cycle capacity retention rate of the negative electrode material.

[0100] In the silicon-carbon negative electrode materials of Examples 1 to 6, there is a discrete closed-pore structure. Therefore, after the material is cold-pressed at 500 MPa, only some of the closed pores are opened, and most of the closed pores still exist. The change in the true density after cold pressing is less than 10%, and more preferably less than 6%. Comparative Example 1 and Comparative Example 6 do not have a porous structure, and Comparative Example 2 itself is loose and porous. Therefore, the true density of Comparative Example 1 and Comparative Example 2 does not change before and after cold pressing. Comparative Examples 3 to 5 and Comparative Example 7 are all pore structures with through holes. Therefore, after cold pressing, the pores are opened, resulting in a large change in the true density after cold pressing.

[0101] The above embodiments are only used to illustrate the technical concept and characteristics of the present invention. The purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized in that: The steps include: (1) Mixing porous silicon with a carbon source to obtain a silicon-carbon precursor, and carbonizing the silicon-carbon precursor to obtain a silicon-carbon intermediate; wherein the carbon source is a solid carbon source or a liquid carbon source; the solid carbon source or the liquid carbon source is selected from one or more of tar, asphalt, glucose, sucrose, citric acid, and resin; the mass ratio of the porous silicon to the carbon source is 1:0.1-2; (2) performing carbon coating on the silicon-carbon intermediate twice or more to obtain the silicon-carbon negative electrode material; The carbon coating in step (2) is the same as the method in step (1), or the carbon coating is performed by decomposing a gaseous carbon source at high temperature, wherein the gaseous carbon source is a gaseous hydrocarbon; The silicon-carbon negative electrode material comprises porous silicon, a carbon layer filling the pores of the porous silicon and coating the outside of the porous silicon; wherein the initial true density of the silicon-carbon negative electrode material is 1.7-2.1 g / cm 3 The change value of the true density after cold pressing at 500 MPa compared with the initial true density is 4-10%; the specific surface area of ​​the silicon-carbon negative electrode material is 1-20 m 2 / g; the carbon content of the silicon-carbon negative electrode material is 40~50wt%; the pores of the silicon-carbon negative electrode material are discrete closed pores.

2. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The change in true density after cold pressing at 500 MPa compared to the initial true density is 4-6%.

3. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The specific surface area of ​​the silicon-carbon negative electrode material is 1-10 m 2 / g.

4. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The porosity of the silicon-carbon negative electrode material is 5-30%.

5. The method for preparing the silicon-carbon negative electrode material according to claim 4, characterized in that: The porosity of the silicon-carbon negative electrode material is 13-20%.

6. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The oxygen content of the silicon-carbon negative electrode material is less than 10 wt %, and the silicon carbide content of the silicon-carbon negative electrode material is less than 10 wt %.

7. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The particle size D50 of the silicon-carbon negative electrode material is 1-20 micrometers, and the pore size of the internal closed pores of the silicon-carbon negative electrode material is 0.1-5 nanometers.

8. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The peak pore size of the porous silicon is 3-10 nanometers, and the pore volume of the porous silicon is 0.45-0.75 cm 3 / g.

9. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The particle size D50 of the primary particles of the porous silicon is 1 to 10 nanometers, and the particle size D50 of the secondary particles of the porous silicon is 1 to 10 micrometers.

10. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The porous silicon is obtained by high-temperature disproportionation of silicon oxide and etching with a fluorine-containing reagent, or by etching an aluminum-silicon alloy with hydrochloric acid, or by reducing silicon dioxide with magnesium.

11. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: In step (1), the carbonization temperature is 700-900° C., and the carbonization time is 3-8 hours.

12. The method for preparing the silicon-carbon negative electrode material according to claim 1, characterized in that: The flow rate of the gas phase carbon source is 100-300 sccm.

13. Use of the silicon-carbon negative electrode material prepared by the preparation method according to any one of claims 1 to 12 in a lithium-ion battery.

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