Green low-energy-consumption low-expansion silicon-carbon negative electrode as well as preparation method and application thereof

Abstract

The invention belongs to a lithium ion battery silicon-carbon negative electrode material, and discloses a green low-energy-consumption low-expansion silicon-carbon negative electrode and a preparation method and application thereof.The preparation method comprises the following steps that an organic silicon source is added into an organic carbon source solution A, after hydrolysis reaction, drying is conducted, and a coprecipitate H2SiO3-C of colloidal silicic acid and an organic carbon source is obtained; carrying out two-stage heat treatment on the co-precipitate H2SiO3-C, cooling, and grinding to obtain a porous Si-C compound; and coating the Si-C compound with carbon, pyrolyzing, cooling and grinding to obtain the low-expansion silicon-carbon negative electrode Si-C-C. The low-expansion silicon-carbon negative electrode prepared by the preparation method provided by the invention and the preparation method thereof have the advantages of stable structure, excellent cycle performance, commercial application prospect and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery silicon-carbon negative electrode materials, and particularly relates to a green low-energy-consumption low-expansion silicon-carbon negative electrode and a preparation method and application thereof. BACKGROUND

[0002] With the vigorous development of the new energy industry, electric vehicles and energy storage systems have put forward higher requirements for battery performance. The current energy density of commercial lithium ion batteries has approached the theoretical upper limit of 300 Wh / kg, which makes the development of new electrode materials an urgent need in the industry. Under this trend, silicon-based negative electrode materials have rapidly become a research hotspot due to their advantages of ultra-high theoretical specific capacity of 4200 mAh / g, suitable charge-discharge potential, and abundant crustal content.

[0003] However, there are still key bottlenecks in the industrial application of silicon negative electrodes: the insufficient conductivity of the material itself restricts the speed of electron transmission; the huge volume expansion (about 300%) during charging and discharging easily causes the failure of the electrode structure; and the material pulverization caused by repeated cycles seriously affects the service life of the battery. To solve these problems, researchers have adopted three major innovative strategies: (1) adopting silicon nanometer size design to reduce the absolute deformation of the material; (2) constructing a porous structure with buffer space; and (3) developing a composite system with carbon material as the matrix - through the synergistic effect of silicon and carbon, the high capacity characteristics of silicon are maintained, and the conductivity, prevention of particle aggregation, and relief of mechanical stress are improved by using the carbon skeleton. These breakthroughs have greatly improved the cycle performance of the electrode and laid an important foundation for the industrialization of the next generation of high-energy-density silicon-carbon negative electrodes.

[0004] Patent No. CN106058207A passes a mixed gas composed of silicon tetrachloride gas and a reducing carrier gas into a reaction chamber in which carbon materials are placed, reduces the silicon tetrachloride gas to elemental silicon, and deposits the elemental silicon on the carbon materials to form a silicon-carbon composite material. Although this method is simple and the raw materials are inexpensive, the pores of the carbon matrix are difficult to control, resulting in uneven silicon deposition, uneven stress distribution in the material during charging and discharging, easy pulverization of the material structure, and poor cycle performance. Moreover, the required temperature for directly reducing silicon tetrachloride with a reducing gas is too high, leading to increased energy consumption and difficulty in large-scale application. Patent No. CN107394176A solves the technical problem of battery performance decline caused by the volume change of silicon material and the detachment of the silicon material from the current collector by adopting an electroplating form, using silicon tetrachloride as the silicon source, and electrodepositing silicon onto the surface of three-dimensional carbon materials, and finally obtaining a silicon-carbon negative electrode sheet through direct heat treatment. Although this invention alleviates the problem of structure pulverization of silicon-based negative electrodes during cycling to some extent, the direct contact of silicon particles with the electrolyte and the large specific surface area of the material result in low initial efficiency, many side reactions, low cycle capacity retention rate, and low practical application value.

[0005] Therefore, it is particularly important to develop a silicon-carbon negative electrode material with stable structure, excellent cycle performance and commercial application prospect to solve the problems of poor structural stability, easy expansion and pulverization and low capacity retention rate of the silicon-carbon negative electrode material. SUMMARY

[0006] In view of the above-mentioned deficiencies, the present application provides a green low-energy low-expansion silicon-carbon negative electrode and a preparation method and application thereof.

[0007] To achieve the above-mentioned purpose, the present application provides a preparation method of a green low-energy low-expansion silicon-carbon negative electrode, comprising the following steps:

[0008] S1, adding an organic silicon source to an organic carbon source solution A, and obtaining a common precipitate H2SiO3-C of colloidal silicon acid and organic carbon source after hydrolysis reaction and drying;

[0009] S2, performing two-stage heat treatment on the common precipitate H2SiO3-C, and grinding after cooling to obtain a porous Si-C composite;

[0010] S3, performing carbon coating on the Si-C composite, and grinding after pyrolysis and cooling to obtain a low-expansion silicon-carbon negative electrode Si-C@C.

[0011] According to an aspect of the present application, in step S1, the organic silicon source comprises at least one of silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, silicon tetraiodide and hexachloroethyldisilane; the organic carbon source solution A comprises an organic carbon source A and a solvent A, and the mass fraction of the organic carbon source A in the organic carbon source solution A is 5-60wt%; the organic carbon source A comprises at least one of glucose, sucrose, citric acid, urea, polyethylene glycol, pitch, phenol formaldehyde resin, carboxymethyl cellulose, polyvinylpyrrolidone, polyaniline, polypyrrole and polyvinylidene fluoride; the molar ratio of the organic silicon source to the organic carbon source is 1-10:2-15; and the solvent A comprises deionized water.

[0012] According to an aspect of the present application, the solvent A further comprises at least one of ethanol, acetone, NMP and toluene.

[0013] According to an aspect of the present application, in step S1, the drying temperature is 40-100℃, and the drying time is 1-6h.

[0014] According to one aspect of the present application, in step S2, the two-stage heat treatment is performed in a protective atmosphere, the protective atmosphere comprising at least one of nitrogen, helium, argon, and hydrogen-argon mixture; the two-stage heat treatment comprises a low-temperature holding treatment and a high-temperature holding treatment; the low-temperature holding treatment is performed at a temperature of 300-700℃ for 1-4h; the high-temperature holding treatment is performed at a temperature of 750-1350℃ for 6-48h.

[0015] According to one aspect of the present application, in step S3, the carbon coating method comprises at least one of chemical vapor deposition, physical vapor deposition, atomic layer deposition, liquid-phase wet coating of organic carbon source, and solid-phase dry coating of organic carbon source; the organic carbon source solution B for the liquid-phase wet coating of organic carbon source comprises an organic carbon source B and a solvent B, the mass fraction of the organic carbon source B in the organic carbon source solution B is 20-75wt%, and the viscosity of the organic carbon source solution B is 100-4000mPa; the organic carbon source B comprises at least one of glucose, sucrose, citric acid, cellulose, dopamine salt, formaldehyde, phenol formaldehyde resin, pitch, coal tar, dimethyl phenol, resorcinol, polyacrylonitrile, polypyrrole, polyaniline, and polythiophene; and the solvent B comprises at least one of water, ethanol, acetone, and NMP.

[0016] According to one aspect of the present application, in step S3, the carbon coating process comprises: uniformly mixing the Si-C composite with the organic carbon source solution B to obtain a slurry; and drying after removing the solvent B from the slurry; wherein the solid content of the slurry is 10-80wt%.

[0017] According to one aspect of the present application, in step S3, the pyrolysis is performed at a temperature of 500-1100℃ for 1-24h; and the protective atmosphere for the pyrolysis comprises at least one of nitrogen, helium, and argon.

[0018] Based on the same inventive concept, the present application further provides a green low-energy-consumption low-expansion silicon-carbon negative electrode prepared by any of the above preparation methods, wherein the carbon content in the low-expansion silicon-carbon negative electrode Si-C@C is 20wt%-80wt%; the outer carbon coating layer of the low-expansion silicon-carbon negative electrode Si-C@C is uniform and dense, the thickness of the carbon coating is 0.1-3μm, and the specific surface area is 1-20m 2 / g.

[0019] Based on the same inventive concept, the present application further provides an application of the above green low-energy-consumption low-expansion silicon-carbon negative electrode in a lithium ion battery.

[0020] The present application has the following advantages:

[0021] (1) In the preparation method of the present application, the silicic acid is mixed with the organic carbon source at the molecular level. Due to the steric hindrance effect of the organic carbon, the silica formed by dehydration condensation has extremely small particle size and is highly dispersed in the carbon matrix; the amorphous state and small particle size of the silica endow it with high reactivity, so that carbon reduction can be carried out at a lower temperature, and the silica is converted into nano Si particles, while avoiding the problem of simultaneous generation of silicon carbide at a higher temperature;

[0022] (2) In the preparation method of the present application, when the silicic acid is dehydrated and condensed to form silica, the organic carbon source is simultaneously pyrolyzed, and the water vapor generated by dehydration is overflowed to realize the pore formation of the carbon matrix; the temperature is continuously increased subsequently, the carbon reduces the silica in situ to silicon, the volume shrinks, and the carbon around the particles is consumed at the same time, so there are sufficient pores between the silicon particles and the carbon for the expansion and contraction of the silicon, thereby improving the cycle stability; the silicon obtained by carbon reduction of silica is highly dispersed in the carbon matrix, has extremely small particle size, small self-volume effect, high reversible capacity, and excellent structural stability; at the same time, the carbon matrix in the Si-C composite has a porous structure, similar to the function of a sponge, which can further alleviate the volume effect of silicon, limit the volume change within large particles, and ensure that the overall silicon-carbon particles do not expand; the outer coating layer reduces the specific surface area of the material, isolates the silicon from the electrolyte, significantly improves the initial efficiency, reduces the side reaction, and improves the cycle capacity retention rate;

[0023] (3) In the preparation method of the present application, the organic silicon source such as silicon tetrachloride and silicon tetrafluoride is dropped into the organic carbon source solution, hydrolysis consumes the solvent, and the organic carbon source is simultaneously precipitated with silicic acid, thereby reducing the energy consumption of solvent drying; hydrogen chloride and hydrogen fluoride gas can be recycled and used to react with low-cost metallurgical micron silicon powder to prepare the process raw material organic silicon source-silicon tetrachloride and silicon tetrafluoride. The production process of the present application is emission-free, environmentally friendly, and low-cost. The low-expansion silicon-carbon negative electrode material proposed in the present application has a unique structural design, which endows the material with high reversible capacity, excellent cycle performance, excellent rate performance, and stable structure. The Si-C@C can achieve similar structure and performance to CVD silicon-carbon, has a porous elastic structure inside, and Si nanoparticles are embedded therein, which can internally relieve the stress effect of silicon. However, the cost is much lower than that of CVD silicon-carbon negative electrode, and it is a silicon-carbon negative electrode material with excellent application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 FIG. 1 is a structural schematic diagram of the low-expansion silicon-carbon negative electrode of the present application;

[0025] Figure 2 FIG. 6 is a cycle performance comparison diagram of Example 1 and Comparative Example 1 of the present application.

[0026] BRIEF DESCRIPTION OF DRAWINGS

[0027] 1, carbon coating layer; 2, amorphous carbon; 3, nano Si particles; 4, void; 5, void in carbon matrix; DETAILED DESCRIPTION

[0028] To make the present invention easier to understand, the present invention is further described below with reference to specific examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Unless otherwise defined, the professional terms used below are consistent with the meanings understood by professional and technical personnel in this field; unless otherwise specified, the raw materials and reagents involved in this article can be purchased from the market or prepared by known methods.

[0029] In order to solve the problems of poor structural stability, easy expansion and pulverization, and low capacity retention of current silicon-carbon negative electrode materials, the inventors of this application provide a green, low-energy consumption, low-expansion silicon-carbon negative electrode preparation method, comprising the following steps:

[0030] S1, adding an organic silicon source to an organic carbon source solution A, performing a hydrolysis reaction, and drying to obtain a co-precipitate H2SiO3-C of colloidal silicic acid and the organic carbon source;

[0031] S2, subjecting the co-precipitate H2SiO3-C to two-stage heat treatment, and then grinding it after cooling to obtain a porous Si-C composite;

[0032] S3. The Si-C composite is coated with carbon, pyrolyzed, cooled, and then ground to obtain a low-expansion silicon-carbon negative electrode Si-C@C.

[0033] It should be noted that in step S1, the mixing method of the organosilicon source into the organic carbon source solution A varies depending on the organosilicon source, including at least one of gas introduction, slow liquid addition, direct liquid mixing, and direct addition of solid powder.

[0034] It should be noted that when the organic silicon source is added to the organic carbon source solution A, it will spontaneously undergo a hydrolysis reaction with water to generate a colloidal silica precipitate. The hydrolysis reaction consumes water and releases a large amount of heat, so all or most of the solvent is consumed during the hydrolysis reaction. At this time, the organic carbon source will be synchronously precipitated and coexist with the colloidal silica precipitate, and the two can reach a molecular-level mixed state.

[0035] In some specific embodiments, in step S1, the organic silicon source comprises at least one of silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, silicon tetraiodide, hexachloroethyldisilane; the organic carbon source solution A comprises an organic carbon source A and a solvent A, the mass fraction of the organic carbon source A in the organic carbon source solution A is 5-60 wt%; the organic carbon source A comprises at least one of glucose, sucrose, citric acid, urea, polyethylene glycol, pitch, phenol formaldehyde resin, carboxymethyl cellulose, polyvinylpyrrolidone, polyaniline, polypyrrole, polyvinylidene fluoride; the molar ratio of the organic silicon source to the organic carbon source is 1-10:2-15; the solvent A comprises deionized water.

[0036] In some specific embodiments, the solvent A further comprises at least one of ethanol, acetone, NMP, toluene.

[0037] It should be noted that the deionized water in the solvent A is necessary, while the ethanol, acetone, NMP and toluene are not necessary.

[0038] In some specific embodiments, in step S1, the drying temperature is 40-100℃, and the drying time is 1-6h.

[0039] It should be noted that, in step S1, since most of the solvent has been consumed in the hydrolysis process, the above drying temperature is taken as a lower temperature of 40-100℃, and the drying time is taken as a shorter time of 1-6h, which greatly reduces the energy consumption; and the lower drying temperature can prevent the agglomeration of silicic acid, facilitating the subsequent preparation of small-particle-size nanosilicon.

[0040] In some specific embodiments, in step S2, the two-stage heat treatment is carried out in a protective atmosphere, the protective atmosphere comprises at least one of nitrogen, helium, argon, hydrogen-argon mixed gas; the two-stage heat treatment comprises low-temperature holding treatment and high-temperature holding treatment; the temperature of the low-temperature holding treatment is 300-700℃, and the time is 1-4h; the temperature of the high-temperature holding treatment is 750-1350℃, and the time is 6-48h.

[0041] It should be noted that, as Figure 1As shown, the precipitate H2SiO3-C changes as follows during the low-temperature holding process: H2SiO3 is dehydrated and condensed into amorphous SiO2, and the organic carbon source is pyrolyzed into amorphous carbon 2; since the two are in a molecular level mixed state in the precipitate, the generated SiO2 is uniformly dispersed in the amorphous carbon 2, and, thanks to the steric hindrance effect of the organic carbon source or amorphous carbon on the reaction of H2SiO3 into SiO2, the SiO2 is difficult to grow after nucleation, so the particle size of SiO2 is extremely small, being 20-100 nm; the large amount of water molecules escaping during the dehydration of colloidal H2SiO3 plays a pore-forming role in the pyrolysis process of the organic carbon source, and the amorphous carbon obtained by pyrolysis is of a porous structure, and the SiO2-C composite obtained during the low-temperature holding process is porous.

[0042] It should be noted that, as shown in Figure 1 As shown, the SiO2-C composite changes as follows during the high-temperature holding process: since the SiO2 particle size is small and amorphous, the reaction activity is high, and the amorphous carbon reduces the SiO2 at high temperature, converting it into nano-Si particles 3; the reduction process consumes the amorphous carbon 2 in contact with the SiO2, and the volume of the SiO2 shrinks when it is converted into Si particles 3, so a gap 4 is generated between the amorphous carbon 2 and the Si particles 3; the volume of the gap around the Si is 2-4 times the volume of the Si particle, reserving space for the expansion of the Si; the Si-C composite obtained during the high-temperature holding process is porous (with internal gaps 5 in the carbon matrix), and the specific surface area of the material is 100-1000 m 2 / g.

[0043] In some specific embodiments, in step S3, the carbon coating method includes at least one of chemical vapor deposition, physical vapor deposition, atomic layer deposition, liquid-phase wet coating of an organic carbon source, and solid-phase dry coating of an organic carbon source.

[0044] It should be noted that, as shown in Figure 1 The purpose of the carbon coating in step S3 is to form a dense carbon coating layer 1 on the surface of the porous Si-C composite, reduce the specific surface area of the material, and at the same time preserve the porous structure of the Si-C composite.

[0045] In some specific embodiments, the organic carbon source solution B for liquid-phase wet coating of an organic carbon source includes an organic carbon source B and a solvent B, the mass fraction of the organic carbon source B in the organic carbon source solution B is 20-75 wt%, and the viscosity of the organic carbon source solution B is 100-4000 mPa; the organic carbon source B includes at least one of glucose, sucrose, citric acid, cellulose, dopamine salt, formaldehyde, phenol formaldehyde resin, pitch, coal tar, dimethyl phenol, resorcinol, polyacrylonitrile, polypyrrole, polyaniline, and polythiophene; and the solvent B includes at least one of water, ethanol, acetone, and NMP.

[0046] In some specific embodiments, in step S3, the carbon-coating process is: mixing the Si-C composite with the organic carbon source solution B to obtain a slurry; after removing the solvent B in the slurry, drying; wherein the solid (Si-C composite + organic carbon source) content of the slurry is 10-80 wt%.

[0047] In some specific embodiments, in step S3, the pyrolysis temperature is 500-1100℃, and the time is 1-24h; the protective atmosphere of the pyrolysis includes at least one of nitrogen, helium, and argon.

[0048] The application also provides a green low-energy low-expansion silicon-carbon negative electrode prepared by any of the above preparation methods, wherein the carbon content in the low-expansion silicon-carbon negative electrode Si-C@C is 20 wt%-80 wt%; the outer carbon coating layer of the low-expansion silicon-carbon negative electrode Si-C@C is uniform and dense, the carbon-coating thickness is 0.1-3μm, and the specific surface area is 1-20m 2 / g.

[0049] The application also provides an application of the above green low-energy low-expansion silicon-carbon negative electrode in a lithium ion battery.

[0050] The application is further described below in combination with specific examples and comparative examples.

[0051] Example 1

[0052] A preparation method of a green low-energy low-expansion silicon-carbon negative electrode, comprising the following steps:

[0053] (1) slowly passing silicon tetrafluoride gas into a glucose aqueous solution (45 wt%) to perform a hydrolysis reaction, drying the product at 60℃ for 4h to obtain a common precipitate H2SiO3-C of colloidal silicic acid and organic carbon source; wherein the molar ratio of silicon tetrafluoride to glucose is 2:5;

[0054] (2) grinding the precipitate H2SiO3-C after heat preservation at 600℃ for 2h and then heat preservation at 1100℃ for 4h under a nitrogen atmosphere to obtain a porous Si-C composite with a specific surface area of 354 m 2 / g;

[0055] (3) performing liquid-phase carbon coating on the Si-C composite by using a sucrose solution with a mass fraction of 55 wt%, the viscosity of the sucrose solution is 550 mPas, the solid (Si-C composite + organic carbon source) content after adding the Si-C composite into the sucrose solution is 65 wt%, spray drying the uniformly mixed solution, and then performing pyrolysis at 700℃ for 6h under an argon atmosphere to obtain a low-expansion silicon-carbon negative electrode material Si-C@C, wherein the carbon content is 52 wt%, and the specific surface area of the material is 5.6 m2 / g.

[0056] Example 2

[0057] A green and low-energy-consumption method for preparing a low-swelling silicon-carbon negative electrode, comprising the following steps:

[0058] (1) Slowly drop silicon tetrachloride liquid into a sucrose aqueous solution (35 wt%), and perform a hydrolysis reaction, dry the product at 55°C for 5h to obtain a colloidal silica and organic carbon source co-precipitate H2SiO3-C; wherein the molar ratio of silicon tetrachloride to sucrose is 5:4;

[0059] (2) Heat the co-precipitate H2SiO3-C to 660°C under a nitrogen atmosphere for 3h, and then heat to 1150°C for 6h, and after cooling, grind to obtain a porous Si-C composite with a specific surface area of 257 m 2 / g;

[0060] (3) Perform liquid-phase carbon coating on the Si-C composite using a 45 wt% pitch solution, the solution has a viscosity of 350 mPas, and the solid (Si-C composite + organic carbon source) content after adding the Si-C composite to the pitch solution is 60 wt%, spray dry the uniformly mixed solution, and then heat to 750°C under an argon atmosphere for 3h, and after cooling, grind to obtain a low-swelling silicon-carbon negative electrode material Si-C@C, which contains 46 wt% carbon, and the material has a specific surface area of 4.3 m 2 / g.

[0061] Example 3

[0062] A green and low-energy-consumption method for preparing a low-swelling silicon-carbon negative electrode, comprising the following steps:

[0063] (1) Slowly add silicon tetraiodide solid powder to a citric acid aqueous solution (35 wt%), and stir to perform a hydrolysis reaction, dry the product at 75°C for 8h to obtain a colloidal silica and organic carbon source co-precipitate H2SiO3-C; wherein the molar ratio of silicon tetraiodide to citric acid is 3:8;

[0064] (2) Heat the co-precipitate H2SiO3-C to 550°C under a nitrogen atmosphere for 6h, and then heat to 1250°C for 8h, and after cooling, grind to obtain a porous Si-C composite with a specific surface area of 406 m 2 / g;

[0065] (3) The Si-C composite was coated with a 35 wt% glucose solution by liquid phase carbon coating. The solution viscosity was 204 mPas, and the solid (Si-C composite + organic carbon source) content after adding the Si-C composite to the glucose solution was 50 wt%. The uniformly mixed solution was spray dried, and then heat treated at 800°C for 2h under a nitrogen atmosphere. After cooling, the low-expansion silicon-carbon negative electrode material Si-C@C was obtained, in which the carbon content was 57 wt%, and the specific surface area of the material was 3.8 m 2 / g.

[0066] Comparative Example 1

[0067] The difference between this comparative example and Example 1 is that step (1) directly uses nanosilicon instead of silicon tetrafluoride, without a hydrolysis step. The nanosilicon and the organic carbon source solution are mixed uniformly and then directly dried under the same conditions. The other steps and parameters are the same as in Example 1.

[0068] Comparative Example 2

[0069] The difference between this comparative example and Example 2 is that the concentration of the pitch solution in step (3) is 15 wt%, and the viscosity is 75 mPa. The other steps and parameters are the same as in Example 1.

[0070] Comparative Example 3

[0071] The difference between this comparative example and Example 3 is that step (2) only performs high-temperature heat treatment, and the heat treatment time is correspondingly extended to 14h. The other steps and parameters are the same as in Example 1.

[0072] Performance testing and result analysis:

[0073] The negative electrode materials prepared in Examples 1-3 and Comparative Example 103 were subjected to discharge performance tests, with a pole piece surface density of 2 mg cm-2, an electrolyte solute of LiPF6, a solvent of EC, DMC, and DEC in a volume ratio of 0.3:0.3:0.4, and a solute concentration of 1 mol L-1. Half-cell tests were performed, and the charge and discharge tests were performed between 0.01~1.5 V at a current density of 50 mA g -1 The first three charge and discharge tests were performed at a current density of 200 mA g -1 The cycle tests were performed, and the electrochemical performance is shown in Table 1. The cycle performance comparison between Example 1 and Comparative Example 1 is shown in Figure 1 .

[0074] Table 1:

[0075] From Figure 1Compared with Example 1 and Comparative Example 1 in Table 1, although the two materials of Example 1 and Comparative Example 1 have the same silicon content and the same first reversible capacity, the cycle capacity retention rate of the silicon-carbon negative electrode material prepared from nano-silicon is sharply reduced due to the structural defects. Moreover, the energy consumption of the drying process is greatly increased due to the consumption of solvent by the hydrolysis reaction, which is not conducive to reducing the production cost. It is shown that when using nano-silicon as the raw material, compared with using silicon tetrafluoride as the raw material, the deficiency of the prepared silicon-carbon negative electrode is that there is no gap between the nano-silicon and the amorphous carbon matrix in the particle, and there is no porous structure in the amorphous carbon due to the water vapor overflow; during the charging and discharging process, the nano-silicon expands, leading to the crushing and pulverization of the particle, and the cycle performance is sharply reduced.

[0076] It is shown that due to the reduction of the viscosity of the organic carbon source solution, a large amount of organic carbon enters the pores of the amorphous carbon, destroying the porous structure in the material, leading to the weakening of the ability of the material to resist the expansion of the nano-silicon particles, and thus the cycle performance is reduced.

[0077] It is shown that the material only with high-temperature heat treatment lacks the low-temperature holding process, and the dehydration condensation of silicic acid to SiO2, the water vapor overflow and the pyrolysis of the organic carbon source are completed under the condition of temperature rise, and the process is too violent, so the particle size of the formed SiO2 is large (leading to the large particle size of the nano-silicon in the final material), and the pore of the pyrolyzed amorphous carbon is too large, and the specific surface is low; therefore, the material only with high-temperature heat treatment is less stable than the material with two-stage heat treatment in the cycle process.

[0078] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any changes or replacements easily thought by those skilled in the art within the technical range disclosed by the present application should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode, characterized in that: The following steps are involved: S1, adding an organic silicon source to an organic carbon source solution A, performing a hydrolysis reaction, and drying to obtain a co-precipitate H2SiO3-C of colloidal silicic acid and the organic carbon source; S2, subjecting the co-precipitate H2SiO3-C to two-stage heat treatment, and then grinding it after cooling to obtain a porous Si-C composite; S3. The Si-C composite is coated with carbon, pyrolyzed, cooled, and then ground to obtain a low-expansion silicon-carbon negative electrode Si-C@C.

2. The method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode according to claim 1, characterized in that: In step S1, the organosilicon source includes at least one of silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, silicon tetraiodide, and hexachlorodisilane; the organic carbon source solution A includes an organic carbon source A and a solvent A, and the mass fraction of the organic carbon source A in the organic carbon source solution A is 5-60wt%; the organic carbon source A includes at least one of glucose, sucrose, citric acid, urea, polyethylene glycol, asphalt, phenolic resin, carboxymethyl cellulose, polyvinyl pyrrolidone, polyaniline, polypyrrole, and polyvinylidene fluoride; the molar ratio of the organosilicon source to the organic carbon source is 1-10:2-15; and the solvent A includes deionized water.

3. The method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode according to claim 2, characterized in that: The solvent A further comprises at least one of ethanol, acetone, NMP and toluene.

4. The method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode according to claim 1, characterized in that: In step S1, the drying temperature is 40-100° C. and the drying time is 1-6 hours.

5. The method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode according to claim 1, characterized in that: In step S2, the two-stage heat treatment is carried out under a protective atmosphere, and the protective atmosphere includes at least one of nitrogen, helium, argon, and a hydrogen-argon mixture; the two-stage heat treatment includes a low-temperature insulation treatment and a high-temperature insulation treatment; the temperature of the low-temperature insulation treatment is 300-700°C, and the time is 1-4 hours; the temperature of the high-temperature insulation treatment is 750-1350°C, and the time is 6-48 hours.

6. The method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode according to claim 1, characterized in that: In step S3, the carbon coating method includes at least one of chemical vapor deposition, physical vapor deposition, atomic layer deposition, organic carbon source liquid phase wet coating, and organic carbon source solid phase dry coating; the organic carbon source solution B in the organic carbon source liquid phase wet coating includes an organic carbon source B and a solvent B, the mass fraction of the organic carbon source B in the organic carbon source solution B is 20-75wt%, and the viscosity of the organic carbon source solution B is 100-4000mPa; the organic carbon source B includes at least one of glucose, sucrose, citric acid, cellulose, dopamine salt, formaldehyde, phenolic resin, asphalt, coal tar, xylenol, resorcinol, polyacrylonitrile, polypyrrole, polyaniline, and polythiophene; the solvent B includes at least one of water, ethanol, acetone, and NMP.

7. The method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode according to claim 6, characterized in that: In step S3, the carbon coating process is as follows: uniformly mixing the Si-C composite with the organic carbon source solution B to obtain a slurry; removing the solvent B from the slurry, and drying; wherein the solid content of the slurry is 10-80 wt%.

8. The method for preparing a green, low-energy, low-expansion silicon-carbon negative electrode according to claim 1, characterized in that: In step S3, the pyrolysis temperature is 500-1100° C., and the time is 1-24 hours; the protective atmosphere of the pyrolysis includes at least one of nitrogen, helium, and argon.

9. A green, low-energy, low-expansion silicon-carbon negative electrode prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The carbon content of the low expansion silicon carbon negative electrode Si-C@C is 20wt%-80wt%; the outer carbon coating layer of the low expansion silicon carbon negative electrode Si-C@C is uniform and dense, the thickness of the carbon coating is 0.1-3μm, and the specific surface area is 1-20m 2 / g.

10. Use of the green, low-energy, low-expansion silicon-carbon negative electrode prepared by the preparation method according to any one of claims 1 to 8 or the green, low-energy, low-expansion silicon-carbon negative electrode according to claim 9 in lithium-ion batteries.

Images