Preparation method of silicon-carbon negative electrode material

Abstract

According to the preparation method of the silicon-carbon negative electrode material disclosed by the invention, by improving the types and the dosage of raw materials in the sanding process of silicon powder, the silicon powder is prevented from being in excessive contact with oxygen while ensuring that the silicon powder has excellent dispersity, so that the probability that the silicon powder is oxidized is greatly reduced; meanwhile, the slurry prepared from the silicon powder and the organic alcohol is directly used for preparing the silicon-carbon negative electrode material, so that the drying step is reduced, the waste of the dispersing agent can be avoided, and the silicon powder can be prevented from being oxidized in the drying process. According to the preparation method, the dispersion is optimized, and the gel formula is matched, so that the coating structure of Si-coated Si-coated C is more uniform, the particle size of the finished negative electrode powder is more uniform, the morphology is more granulated, the formation of a conductive network with higher quality is facilitated, and the charge-discharge performance of the negative electrode is improved.

Description

Technical Field

[0001] The present application relates to the technical field of electrode materials, and in particular to a method for preparing a silicon-carbon negative electrode material. Background Art

[0002] Lithium-ion batteries have the advantages of high specific energy, small size, high stability, and environmental friendliness. Therefore, they have shown extremely broad application prospects in portable electronic devices, new energy vehicles and other fields. With the growing demand for high-capacity and high-stability batteries, people have put forward higher requirements on the performance of lithium-ion batteries. Among them, the negative electrode material is one of the important factors to improve the performance of lithium-ion batteries.

[0003] In the prior art, silane is used as raw material to prepare silicon-carbon negative electrode materials through chemical or physical vapor deposition methods, but silane is expensive and its active nature makes storage, transportation and use highly risky. In addition to the deposition method, the sand milling process has become increasingly mature, and silicon powder with a particle size D90 less than 100nm can be obtained. After coating and other process methods, it has met the needs of preparing mid-to-high-end silicon-carbon negative electrodes.

[0004] However, as the particle size of silicon powder decreases, new technical problems begin to emerge. The existing technology often obtains nano-scale silicon powder through traditional sand grinding. However, the silicon powder obtained by this method is easy to agglomerate, which makes it difficult to disperse the nano-silicon powder during the coating process and forms a homogeneous aggregate, resulting in uneven powder during the coating process. Moreover, the silicon powder is extremely easy to oxidize during the crushing process, which will affect the performance of the negative electrode material. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present application is to provide a method for preparing a silicon-carbon negative electrode material, so as to solve the problems of easy agglomeration when obtaining nano silicon powder by the sand grinding method in the prior art, easy formation of homogeneous aggregates during the coating process due to poor dispersion, easy oxidation during the silicon powder crushing process, and uneven powder coating process.

[0006] In order to solve the above technical problems, this application adopts the following technical solutions:

[0007] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0008] Step 1: using organic alcohol as a dispersion medium, adding micron or submicron silicon powder to the organic alcohol, and then adding CTAB to prepare slurry V1; placing slurry V1 into a sand milling device, and sand milling it for 4 to 24 hours under specific conditions to obtain slurry V2; wherein the content of CTAB in the organic alcohol is 2wt%-6wt%, so that the Si content in slurry V1 is 10 to 20wt%; the specific conditions are: adding organic acid to the slurry V1 to make its pH <7 before sand milling; and / or: the sand milling process is carried out in an inert atmosphere;

[0009] Alternatively, step 1: using organic alcohol as a dispersion medium, adding silicon powder to the organic alcohol to prepare slurry V1, so that the Si content in the slurry V1 is 10-20wt%; loading the slurry V1 into a sand mill, sand milling it for 4-24h under specific conditions, drying the slurry obtained after sand milling at 50-70°C under vacuum conditions until a dry powder is obtained, adding the dry powder to methanol and / or ethanol, adding CTAB, the content of CTAB in methanol and / or ethanol is 2wt%-6wt%; performing ultrasonic dispersion for 30-120min under sealed conditions to obtain slurry V2; wherein the specific conditions are: adding an organic acid to the slurry V1 to make its pH <7 before sand milling; and / or: the sand milling process is carried out in an inert atmosphere;

[0010] Step 2: adding nitric acid, orthosilicate, pure water and dimethyldimethoxysilane to the slurry V2, stirring at 25-60° C. for 2-8 hours to obtain sol B; wherein, in sol B, the mass ratio of nitric acid, orthosilicate, pure water and dimethyldimethoxysilane is (12-20):36:(50-65):1, and the concentration of nitric acid is 0.2 mol / L;

[0011] Step 3: Prepare Sol A, mix methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane, and stir at 25-60° C. for 2-8 hours; wherein, in Sol A, the mass ratio of methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane is (25-35):(8-15):(5-10):14:1, and the concentration of nitric acid is 0.2 mol / L;

[0012] Step 4: Stir and mix sol A and sol B at 25-60°C, add ammonia water, stir slowly, stop stirring after gelation begins, let stand for 20-120 minutes after gelation is completed, add organic alcohol, and continue aging the gel system for 4-48 hours; wherein, the concentration of ammonia water is 0.3-1.2 mol / L, and the amount of ammonia water added is determined according to the pre-gelation test, and the amount of ammonia water added should be able to control the gelation time within 1-6 minutes, that is, the time from sol to gelation is controlled within 1-6 minutes;

[0013] Step 5: supercritically drying the gel obtained in step 4, and crushing the solid to obtain powder P1; mixing graphite with powder P1 to obtain powder P2; subjecting powder P2 to high temperature treatment at 1300 to 1600° C. in an inert atmosphere for 60 to 240 min to obtain powder P3; screening powder P3 to obtain powder P4;

[0014] Step 6: Mix the powder P4 with the high-temperature asphalt powder, add an organic solvent to ensure that the asphalt is completely dissolved, and dry at 60-120°C to obtain the powder P5; the powder P5 is heated to 1000°C-1200°C at a heating rate of 2-9°C / min, and kept warm for 2-12h to obtain the powder P6, which is sieved to obtain the powder Si@SiC@pitC; wherein, in the mixture obtained by mixing the asphalt with the powder P4, the mass proportion of the asphalt is 20wt%-50wt%, and the asphalt can be selected from one of coal tar, petroleum asphalt, and resin asphalt. More preferably, the high-rate negative electrode is preferably resin asphalt.

[0015] Preferably, in step 1, the organic alcohol includes one or a mixture of ethanol, methanol or isopropanol.

[0016] Preferably, in step 1, calculated by mass percentage, the particle size of 90wt% of the silicon powder as a raw material is controlled at 15-30μm; and the particle size of the silicon powder after sand grinding in step 1 is 50-1000nm, and the oxygen content is ≤5wt%.

[0017] Preferably, in step 1, in the slurry V2, 90wt% of the silicon powder has a particle size of ≤500nm.

[0018] Preferably, in step 1, the organic acid is acetic acid or citric acid.

[0019] Preferably, in step 2, the following steps are performed to confirm whether the silicon content in the slurry V2 meets the requirements:

[0020] The slurry V2 with a mass of m1 was weighed and dried to obtain a silicon content of C1, and C1×m1=94.5 g.

[0021] Preferably, in step 4, the organic alcohol is one or a mixture of methanol, ethanol or isopropanol.

[0022] Preferably, in step 5, the particle size of the graphite is controlled to be 20 to 1000 nm.

[0023] Preferably, in step 5, calculated as a percentage by mass, 90 wt% of the powder P4 has a particle size of 1 to 15 μm.

[0024] Preferably, in step 6, calculated by mass percentage, 90 wt% of the powder P6 has a particle size of 5 to 20 μm.

[0025] Compared with the prior art, this application has the following beneficial effects:

[0026] 1. When preparing silicon-carbon negative electrode materials, the present applicant found that storing silicon powder in an organic alcohol solvent from the beginning of sand grinding can ensure its dispersibility while avoiding excessive contact with oxygen, greatly reducing the probability of oxidation of nano-silicon powder; at the same time, the present applicant also found that adding an appropriate amount of organic acid to the slurry V1 to make its pH value acidic can also reduce the oxidation of nano-silicon powder. This method, combined with sand grinding under an inert atmosphere, can further reduce the occurrence of oxidation of nano-silicon powder.

[0027] 2. In the present application, the slurry V2 prepared by silicon powder and organic alcohol is directly used for the preparation of silicon-carbon negative electrode materials, which reduces the drying step, avoids the waste of dispersant, and avoids the oxidation of silicon powder during the drying process.

[0028] 3. After research, the present applicant found that monomethyltrimethoxysilane and dimethyldimethoxysilane are difficult to gel, so while reducing the input amount of the two, the addition amount of methyl orthosilicate is increased to ensure that the content of the intermediate coating silica in the gel will not change after the gel is dried. At the same time, with the effect of the hexadecyltrimethylammonium bromide (CTAB) stabilizer in the sand milling process, the suspension and dispersion of the silicon powder are better during the gel process, reducing the number and particle size of homogeneous aggregates. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the EDX spectrum of the negative electrode material prepared in Example 1.

[0030] Figure 2 This is a curve chart of the charge and discharge cycle performance of a lithium ion battery made from the negative electrode material prepared in Example 1.

[0031] Figure 3 This is a curve diagram of the charge and discharge cycle performance of a lithium-ion battery made from the negative electrode material prepared in Example 4 DETAILED DESCRIPTION

[0032] This application will combine the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the described embodiments are only part of the embodiments of this application, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on this application belong to the scope of protection of this application.

[0033] Unless otherwise specified in specific circumstances in this application, the numerical ranges listed herein include upper and lower limits, as well as all integers and fractions within the range, and are not limited to the specific values ​​listed when the range is limited. "And / or" referred to herein is inclusive, for example, "A and / or B" means only A, or only B, or both A and B.

[0034] 1. A method for preparing a silicon-carbon negative electrode material

[0035] Step 1: using organic alcohol as a dispersion medium, adding micron or submicron silicon powder to the organic alcohol, and then adding CTAB to prepare slurry V1; placing slurry V1 into a sand milling device, and sand milling it for 4 to 24 hours under specific conditions to obtain slurry V2; wherein the content of CTAB in the organic alcohol is 2wt%-6wt%, so that the Si content in slurry V1 is 10 to 20wt%; the specific conditions are: adding organic acid to the slurry V1 to make its pH <7 before sand milling; and / or: the sand milling process is carried out in an inert atmosphere;

[0036] Alternatively, step 1: using organic alcohol as a dispersion medium, adding silicon powder to the organic alcohol to prepare slurry V1, so that the Si content in the slurry V1 is 10-20wt%; loading the slurry V1 into a sand mill, sand milling it for 4-24h under specific conditions, drying the slurry obtained after sand milling at 50-70°C under vacuum conditions until a dry powder is obtained, adding the dry powder to methanol and / or ethanol, adding 2wt%-6wt% of CTAB, and ultrasonically dispersing it for 30-120min under sealed conditions to obtain slurry V2; wherein the specific conditions are: adding an organic acid to the slurry V1 to make its pH <7 before sand milling; and / or: the sand milling process is carried out in an inert atmosphere;

[0037] Step 2: adding nitric acid, orthosilicate, pure water and dimethyldimethoxysilane to the slurry V2, stirring at 25-60° C. for 2-8 hours to obtain sol B; wherein, in sol B, the mass ratio of nitric acid, orthosilicate, pure water and dimethyldimethoxysilane is (12-20):36:(50-65):1, and the concentration of nitric acid is 0.2 mol / L;

[0038] Step 3: Prepare Sol A, mix methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane, and stir at 25-60° C. for 2-8 hours; wherein, in Sol A, the mass ratio of methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane is (25-35):(8-15):(5-10):14:1, and the concentration of nitric acid is 0.2 mol / L;

[0039] Step 4: Stir and mix sol A and sol B at 25-60°C, add ammonia water, stir slowly, stop stirring after gelation begins, let stand for 20-120 minutes after gelation is completed, add organic alcohol, and allow the gel system to continue aging for 4-48 hours; wherein, the concentration of ammonia water is 0.3-1.2 mol / L, and the amount of ammonia water input is determined according to the pre-gel test. The amount of ammonia water input should be able to control the gelation time within 1-6 minutes, and the time from sol to gel should be controlled within 1-6 minutes.

[0040] Step 5: supercritically drying the gel obtained in step 4, and crushing the solid to obtain powder P1; mixing graphite with powder P1 to obtain powder P2; subjecting powder P2 to high temperature treatment at 1300 to 1600° C. in an inert atmosphere for 60 to 240 min to obtain powder P3; screening powder P3 to obtain powder P4;

[0041] Step 6: Mix powder P4 with asphalt powder, add an organic solvent to ensure that the asphalt is completely dissolved, and dry at 60-120°C to obtain powder P5; the powder P5 is heated to 1000°C-1200°C at a heating rate of 2-9°C / min, and kept warm for 2-12h to obtain powder P6, which is sieved to obtain powder Si@SiC@pitC; wherein, in the mixture obtained by mixing asphalt and powder P4, the mass proportion of asphalt is 20wt%-50wt%.

[0042] When studying the preparation method of silicon-carbon negative electrode materials, the present application found that the silicon-carbon negative electrode materials prepared by the prior art often have poor performance. After in-depth research, it was found that the reason for this problem is the use of nano-scale silicon powder. Because the prior art often obtains nano-scale silicon powder by traditional sand grinding, but the silicon powder obtained by this method is very easy to agglomerate, which will cause the nano-silicon powder to be difficult to disperse during the coating process and form a homogeneous aggregate, that is: the large silicon powder particles that are agglomerated together are coated, which ultimately leads to the problem of uneven powder during the coating process. Moreover, since the silicon powder particle size has reached the nanometer level, the surface of the silicon powder has a very large specific surface area, which means that more surface atoms are exposed to the air, which increases the chance of silicon powder contacting oxygen; at the same time, the atoms on the surface of the silicon powder have high chemical activity and are easy to react with oxygen to form silicon oxide (SiO 2). This oxide layer will not only reduce the electrochemical performance of silicon powder, but may also change its surface properties, thereby further weakening its dispersion effect. Therefore, the present application considers improving the preparation method of silicon-carbon negative electrode materials, protecting the silicon powder with an organic alcohol solvent, which can ensure the dispersibility of the silicon powder. At the same time, the organic alcohol isolates the silicon powder from excessive contact with oxygen, greatly reducing the amount of oxidation of nano silicon powder. Furthermore, the present application considers directly using the sand-milled slurry V2 for the preparation of silicon-carbon negative electrode materials. Compared with the traditional preparation method, the present application reduces the drying step, which can avoid the waste of dispersant and the oxidation of silicon powder during the drying process.

[0043] In some embodiments of the present application, in step 1, an organic alcohol is added as a dispersion medium so that the silicon powder can be better dispersed in the organic alcohol. At the same time, the organic alcohol can protect the silicon powder during the subsequent sanding process. The type of organic alcohol will not affect the preparation of the final silicon-carbon negative electrode material, so the organic alcohol includes one or more of ethanol, methanol or isopropanol.

[0044] In some embodiments of the present application, the orthosilicate is methyl orthosilicate or ethyl orthosilicate.

[0045] In some embodiments of the present application, the present application considers to provide certain protection for silicon powder during the sand milling process. Therefore, the present application finds that the oxidation effect of nano silicon powder during the sand milling process can be reduced by two methods, namely: adjusting the pH value of the slurry V1 to make it acidic; or, sand milling in an inert atmosphere. These two methods can be used alone. Compared with the traditional sand milling process, when used alone, they can significantly improve the oxidation of nano silicon powder during the sand milling process. Of course, these two methods can also be used at the same time, namely: adjusting the pH value of the slurry V1 to make it acidic, and then sand milling in an inert atmosphere. The combined use of these two methods can further reduce the occurrence of oxidation of nano silicon powder. Therefore, the specific conditions are: adding an organic acid to the slurry V1 to make its pH <7 before sand milling; and / or: the sand milling process is carried out in an inert atmosphere.

[0046] In some embodiments of the present application, the present application has found after research that monomethyltrimethoxysilane and dimethyldimethoxysilane are difficult to gel, so while reducing the input amount of the two, the amount of methyl orthosilicate added is increased to ensure that after the gel is dried, the content of the intermediate coating silicon dioxide will not change, although this will make the intermediate layer SiO 2The skeleton pore size is slightly reduced, but this application adopts the role of CTAB stabilizer in the sand milling process. In the gel process, the suspension and dispersion of silicon powder are better, and the number and particle size of homogeneous aggregates are reduced. Therefore, the adjustment of the raw material dosage has little effect on the performance of the product. In sol A, the mass ratio of methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane is (25-35): (8-15): (5-10): 14: 1, which can be 25: 8: 5: 14: 1, 30: 10: 8: 14: 1, 35: 15: 10: 14: 1, etc., and all ranges and sub-ranges between the above values; in sol B, the mass ratio of nitric acid, orthosilicate, pure water and dimethyldimethoxysilane is (12-20): 36: (50-65): 1, which can be 12: 36: 50: 1, 15: 36: 55: 1, 20: 36: 65: 1, etc., and all ranges and sub-ranges between the above values. It should be understood that in the embodiment, any of the above ranges can be combined with any other ranges.

[0047] In some embodiments of the present application, in step 1, 90 wt% of the silicon powder has a particle size of 15-30 μm calculated by mass percentage. After sand grinding in step 1, the particle size of the silicon powder is 50-1000 nm, and the oxygen content is ≤ 4 wt% (the oxygen content is calculated by mass percentage).

[0048] In some embodiments of the present application, in step 1, in the slurry V2, 90 wt% of the silicon powder has a powder particle size of ≤500 nm.

[0049] In some embodiments of the present application, the organic acid is acetic acid or citric acid.

[0050] In some embodiments of the present application, in step 2, the following steps are performed to confirm whether the silicon content in the slurry V2 meets the requirements:

[0051] The slurry V2 with a mass of m1 was weighed and dried to obtain a silicon content of C1, and C1×m1=94.5 g.

[0052] In some embodiments of the present application, in step 4, the organic alcohol is one or a mixture of methanol, ethanol or isopropanol.

[0053] In some embodiments of the present application, in step 5, the particle size of the graphite is controlled to be 20 to 1000 nm.

[0054] In some embodiments of the present application, in step 5, calculated as a percentage by mass, 90 wt% of the powder P4 has a particle size controlled within a range of 1 to 15 μm.

[0055] In some embodiments of the present application, in step 6, calculated as a percentage by mass, 90 wt% of the powder P6 has a particle size controlled within the range of 5 to 20 μm.

[0056] 2. Examples and Comparative Examples

[0057] Embodiment 1:

[0058] S1: Prepare silicon powder with D90=15μm, use ethanol as the dispersion medium, add 5g CTAB to make slurry V1, with Si content of 10wt%, add 10wt% acetic acid to make its pH <7; put slurry V1 into the closed tank of the sand mill, evacuate, and pass nitrogen inert gas to maintain a slight positive pressure in the stirring tank chamber. Use 0.1mm zirconium ball as abrasive, sand mill for 12h, and obtain slurry V2, in which silicon powder D90=100 nanometers.

[0059] S2 Take a small amount of slurry, weigh it and dry it completely to obtain its silicon content C1 after sand grinding. According to the final Si content, weigh the mass of the slurry as m1, so that m1×C1=94.5g; add 0.2M HNO 3 12g, 36g methyl orthosilicate, 50g pure water, 1g dimethyldimethoxysilane, stirred and heated in a 25℃ water bath for 2h to obtain sol B; the nitric acid concentration is 0.2mol / L.

[0060] S3 Sol A: 25g methanol, 8g pure water, 5g 0.2M HNO 3 , 14g monomethyltrimethoxysilane, 1g dimethyldimethoxysilane, stirred and heated in a 25℃ water bath for 2h. The concentration of nitric acid is 0.2mol / L.

[0061] S4: Stir and mix Sol A and Sol B in a 25°C water bath, add 40 ml of 0.85 mol / L ammonia water, stir slowly, stop stirring after initial gelation, let stand for 20 minutes after gelation is completed, add methanol / ethanol / isopropanol, and allow the entire gel system to continue aging.

[0062] After S5 is aged for 4 hours, a dry block is obtained by supercritical drying, and after simple crushing, powder P1 is obtained. Take 10g of 20-500nm graphite, and thoroughly mix powder P1 and graphite through a V-type mixer to obtain mixed powder P2. The powder is placed in a rotary furnace filled with inert gas such as nitrogen / argon, and subjected to high-temperature treatment at 1500°C for 60 minutes to obtain powder P3. Use an ultrasonic vibrating screen or an airflow sieving machine to sieve out uniform particles; the particle size D90 of the obtained powder P4 is controlled between 1-15μm.

[0063] S6 Take 10g of powder P4 and 6.7g of high-temperature asphalt powder, mix them thoroughly, add a small amount of carbon tetrachloride / n-hexane / xylene / carbon disulfide, ensure that the asphalt is completely dissolved, put it in a 60℃ oven, and stir it intermittently during the drying process. After complete drying, powder P5 is obtained. Powder P5 is placed in a tubular furnace for carbonization at a heating rate of 2℃ / min. After it is heated to 1000℃, it is kept warm for 2h to obtain powder P6. Use an ultrasonic vibrating screen or an airflow screen to sieve out uniform particles. The particle size D90 is controlled within 5-20μm, which is the final finished powder Si@SiC@pitC-1.

[0064] Example 2

[0065] The method is adjusted based on Example 1, except that: in S1, 10 wt% acetic acid is added to the slurry V1 to make its pH less than 7, but nitrogen protection is not performed. The remaining steps are exactly the same as those in Example 1.

[0066] Example 3

[0067] The adjustment is made on the basis of Example 1, except that: in S1, only nitrogen protection is performed, and acetic acid is not added to adjust the pH. The remaining steps are exactly the same as in Example 1.

[0068] Example 4

[0069] Adjustments were made based on Example 2, except that: in step 1, silicon powder with D90 = 15 μm was prepared, ethanol was used as the dispersion medium, slurry V1 was prepared, Si content was 10 wt%, 10 wt% acetic acid was added to make its pH < 7; slurry V1 was loaded into the sand mill tank, and the stirred tank cavity was kept at a slightly positive pressure. 0.1 mm zirconium balls were used as abrasives, and after sand milling for 4 hours, the obtained slurry was dried at 60 ° C under vacuum conditions until powder was obtained, the dried powder was added to methanol, and then 5 g CTAB was added, and ultrasonic dispersion was carried out under sealed conditions for 30 minutes to obtain slurry V2. The remaining steps are exactly the same as Example 1.

[0070] Example 5

[0071] The method is adjusted based on Example 1, except that the mass ratio of methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane in Sol A is 30:10:8:14:1. The mass ratio of nitric acid, orthosilicate, pure water and dimethyldimethoxysilane in Sol B is 15:36:55:1. The remaining steps are exactly the same as those in Example 1.

[0072] Example 6

[0073] The method is adjusted based on Example 1, except that the mass ratio of methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane in Sol A is 35:15:10:14:1. The mass ratio of nitric acid, orthosilicate, pure water and dimethyldimethoxysilane in Sol B is 20:36:65:1. The remaining steps are exactly the same as those in Example 1.

[0074] Comparative Example 1

[0075] The method is adjusted based on Example 1, except that the sand mill is not evacuated, inert gas is not introduced, and acid adjustment is not performed. The remaining steps are exactly the same as those in Example 1.

[0076] Comparative Example 2

[0077] The silicon powder was not sand-milled, and 100 nm fumed silicon was directly used as the silicon core, which was directly dispersed in organic alcohol by sealed ultrasonication, and 4 wt % CTAB was added to obtain slurry V2c. The remaining steps were exactly the same as those in Example 1.

[0078] The present application tests the oxygen content of silicon powder obtained after treatment by different treatment methods, so as to analyze the influence of different treatment methods on the oxidation of silicon powder, and the test results are shown in Table 1. In Table 1, gas phase silicon refers to silicon powder prepared by vapor deposition in the prior art; sand-milled silicon refers to silicon powder obtained by sand-milling silicon powder by a sand mill; the treatment method of No. 5 is the treatment method of silicon powder in Comparative Example 1; the treatment method of No. 6 is the treatment method of silicon powder in Example 2; No. 7 is the treatment method of silicon powder in Example 3; No. 8 is the treatment method of silicon powder in Example 1; No. 9 is based on Comparative Example 1, but the dispersion medium is replaced by water, and the sand mill is replaced by a ball mill.

[0079] Table 1

[0080] Serial number Silica powder particle size and sand grinding method Oxygen content 1 Unsanded silica powder D90 = 15-30 μm 0.944％ 2 100nm fumed silica non-sanded sample 1.53％ 3 500nm fumed silica non-sanded sample 1.93％ 4 1000nm fumed silica non-sanded sample 1.62％ 5 100nm sand-milled silicon without nitrogen protection and acid adjustment (the dispersion medium used is the same as in Example 1) 12.91％ 6 100nm sand grinding silica acidity adjustment 4.61％ 7 100nm sanded silicon nitrogen protection 1.54％ 8 100nm sand grinding silicon nitrogen protection + acid adjustment 1.41％ 9 500nm ball-milled silica water as dispersion medium, without nitrogen protection and acid adjustment 29.01％

[0081] From Table 1 we can see that:

[0082] (1) The oxygen content of silicon powder as raw material is not high. However, the oxygen content of silicon powder prepared by vapor deposition or by ball milling or sand milling has increased significantly, which indicates that the raw silicon powder is easily oxidized in these different treatment methods.

[0083] (2) The present application uses acid adjustment and / or nitrogen protection to treat silicon powder, which can reduce the amount of silicon powder oxidized, so that the oxygen content in the silicon powder obtained after treatment is significantly lower than the oxygen content in the silicon powder obtained by direct sand milling and ball milling;

[0084] (3) The present application uses organic alcohol as the dispersion medium. Compared with water as the dispersion medium, organic alcohol can protect silicon powder, and its oxygen content is significantly reduced compared with water as the dispersion medium.

[0085] (4) Under the conditions of preparing silicon powder with the same particle size, the oxygen content in the silicon powder obtained by the method described in the present application is very close to or even better than the quality of vapor-phase silicon; and in the case of preparing silicon powder with a particle size of 500nm, 1000nm, etc., the oxygen content in the silicon powder obtained by the method described in the present application is better than the quality of vapor-phase silicon under the conditions of the same size.

[0086] The content of each element in the silicon-carbon negative electrode material prepared in Example 1 was tested, and the results were as follows: Figure 1 And as shown in Table 2.

[0087] Table 2 Content of each element in the silicon-carbon negative electrode material prepared in Example 1

[0088] element wt% Wt%Sigma At% C 36.81 0.35 56.90 O 2.68 0.09 3.11 Si 60.50 0.34 39.99 Total 100.00 100.00

[0089] The negative electrode powders prepared in Examples 1 to 6 and Comparative Examples 1 to 2 were made into lithium-ion batteries with exactly the same specifications, and the prepared lithium-ion batteries were subjected to performance tests. The test conditions were: the examples and comparative examples prepared in this application were used as samples, CR2032 half-cells were made for cycle testing, and the rate was: 0.05C (first 3 cycles) + 0.1C process step, and the following data were obtained:

[0090] Table 3

[0091] Negative electrode powder First effect After 200 cycles, capacity retention rate Example 1 88％ 96％ Example 2 82％ 87％ Example 3 85％ 92％ Example 4 84％ 85％ Example 5 87％ 97％ Example 6 88％ 94％ Comparative Example 1 64％ 34％ Comparative Example 2 83％ 94％

[0092] From Table 3 we can see that:

[0093] (1) Compared with Comparative Example 2, the performance of the negative electrode powder material obtained by the preparation method described in the present application is not much different from that of the negative electrode material prepared by using silicon powder obtained by the gas phase method as raw material, and can even be better than the negative electrode material prepared by using silicon powder obtained by the gas phase method as raw material. This shows that the preparation method described in the present application can significantly reduce the oxygen content of the silicon core in the coating structure, further improve the first efficiency and other performance of the lithium-ion battery, and the small amount of oxygen in the coating layer will continue to benefit the cycle performance; however, the cost of silicon powder obtained by the gas phase method is extremely high, and the cost of silicon powder obtained by the method described in the present application is only 50-70% of that of gas phase silicon powder, which is more advantageous than the gas phase method.

[0094] (2) In Comparative Example 1, the silicon powder was not protected, resulting in an excessively high oxygen content in the silicon powder obtained after sand grinding, which ultimately led to a decrease in the first efficiency and capacity of the negative electrode, and its performance was far inferior to that of the embodiment.

[0095] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application rather than to limit the technical solution. Ordinary technicians in this field should understand that those modifications or equivalent substitutions of the technical solution of the present application that do not depart from the purpose and scope of the technical solution of the present application should be included in the scope of the claims of the present application.

Claims

1. A method for preparing a silicon-carbon negative electrode material, characterized in that: The steps include: Step 1: using organic alcohol as a dispersion medium, adding silicon powder to the organic alcohol, and then adding CTAB to prepare slurry V1; placing slurry V1 into a sand milling device, and sand milling it for 4 to 24 hours under specific conditions to obtain slurry V2; wherein the content of CTAB in the organic alcohol is 2wt%-6wt%, so that the Si content in slurry V1 is 10 to 20wt%; the specific conditions are: adding organic acid to the slurry V1 to make its pH <7 before sand milling; and / or: the sand milling process is carried out in an inert atmosphere; Alternatively, step 1: using organic alcohol as a dispersion medium, adding silicon powder to the organic alcohol to prepare slurry V1, so that the Si content in the slurry V1 is 10-20wt%; loading the slurry V1 into a sand mill, sand milling it for 4-24h under specific conditions, drying the slurry obtained after sand milling at 50-70°C under vacuum conditions until a dry powder is obtained, adding the dry powder to methanol and / or ethanol, adding CTAB, the content of CTAB in methanol and / or ethanol is 2wt%-6wt%; performing ultrasonic dispersion for 30-120min under sealed conditions to obtain slurry V2; wherein the specific conditions are: adding an organic acid to the slurry V1 to make its pH <7 before sand milling; and or: the sand milling process is carried out in an inert atmosphere; Step 2: adding nitric acid, orthosilicate, pure water and dimethyldimethoxysilane to the slurry V2, stirring at 25-60° C. for 2-8 hours to obtain sol B; wherein, in sol B, the mass ratio of nitric acid, orthosilicate, pure water and dimethyldimethoxysilane is (12-20):36:(50-65):1, and the concentration of nitric acid is 0.2 mol / L; Step 3: Prepare Sol A, mix methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane, and stir at 25-60° C. for 2-8 hours; wherein, in Sol A, the mass ratio of methanol, pure water, nitric acid, monomethyltrimethoxysilane and dimethyldimethoxysilane is (25-35):(8-15):(5-10):14:1, and the concentration of nitric acid is 0.2 mol / L; Step 4: Stir and mix Sol A and Sol B at 25-60° C., add ammonia water, stir slowly, stop stirring after gelation begins, let stand for 20-120 minutes after gelation is completed, then add organic alcohol to continue aging the gel system for 4-48 hours; wherein, the concentration of ammonia water is 0.3-1.2 mol / L, and the amount of ammonia water added can control the gelation time within 1-6 minutes; Step 5: supercritically drying the gel obtained in step 4, and crushing the solid to obtain powder P1; mixing graphite with powder P1 to obtain powder P2; subjecting powder P2 to high temperature treatment at 1300 to 1600° C. in an inert atmosphere for 60 to 240 min to obtain powder P3; screening powder P3 to obtain powder P4; Step 6: Mix powder P4 with asphalt powder, add an organic solvent to ensure that the asphalt is completely dissolved, and dry at 60-120°C to obtain powder P5; the powder P5 is heated to 1000°C-1200°C at a heating rate of 2-9°C / min, and kept warm for 2-12h to obtain powder P6, which is sieved to obtain powder Si@SiC@pitC; wherein, in the mixture obtained by mixing asphalt and powder P4, the mass proportion of asphalt is 20wt%-50wt%.

2. The preparation method according to claim 1, characterized in that: In step 1, the organic alcohol includes one or a mixture of ethanol, methanol or isopropanol.

3. The preparation method according to claim 1, characterized in that: In step 1, according to the mass percentage, the particle size of 90wt% of the silicon powder is controlled to be 15-30μm; after sand grinding in step 1, the particle size of the silicon powder is 50-1000nm, and the oxygen content is ≤4wt%.

4. The preparation method according to claim 1, characterized in that: In step 1, in the slurry V2, 90 wt% of the silicon powder has a particle size of ≤500 nm.

5. The preparation method according to claim 1, characterized in that: In step 1, the organic acid is acetic acid or citric acid.

6. The preparation method according to claim 1, characterized in that: In step 2, the following steps are performed to confirm whether the silicon content in the slurry V2 meets the requirements: The slurry V2 with a mass of m1 was weighed and dried to obtain a silicon content of C1, and C1×m1=94.5g.

7. The preparation method according to claim 1, characterized in that: In step 4, the organic alcohol is one or a mixture of methanol, ethanol or isopropanol.

8. The preparation method according to claim 1, characterized in that: In step 5, the particle size of the graphite is controlled to be 20 to 1000 nm.

9. The preparation method according to claim 1, characterized in that: In step 5, according to the mass percentage calculation, 90wt% of the powder P4 has a particle size of 1-15 μm.

10. The preparation method according to claim 1, characterized in that: In step 6, according to the mass percentage calculation, 90wt% of the powder P6 has a particle size of 5-20 μm.

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