Preparation method and application of micron silicon negative electrode slurry

Abstract

The invention discloses a preparation method and application of micron silicon negative electrode slurry, and relates to the technical field of lithium batteries. The preparation method of the micron silicon negative electrode slurry comprises the following steps: dissolving boric acid, adding carboxymethyl cellulose, uniformly stirring, and adding citric acid for continuous crosslinking to obtain a micron silicon negative electrode binder; the preparation method comprises the following steps: mixing micron silicon powder, artificial graphite and primary asphalt, performing high-temperature carbonization, mixing with secondary asphalt, and performing high-temperature carbonization to obtain a micron silicon negative electrode material; the prepared micron silicon negative electrode binder is mixed with a micron silicon negative electrode material and conductive carbon black to prepare micron silicon negative electrode slurry, the surface of a copper foil is coated with the micron silicon negative electrode slurry, drying is performed to obtain an electrode plate, and the electrode plate and a counter electrode metal lithium sheet are assembled to obtain the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery, in particular to a preparation method of micron silicon negative electrode slurry and application thereof. BACKGROUND

[0002] As the core of modern energy storage technology, the improvement of energy density and cycle life of lithium ion battery has always been the focus of scientific research and industry. At present, the specific capacity of high-end graphite negative electrode has reached 360-365mAh / g, close to the theoretical specific capacity of 372mAh / g. It is difficult to meet the demand for high energy density of electric vehicles, portable electronic devices and the like; therefore, from the perspective of negative electrode material, it is necessary to develop new negative electrode materials with higher specific capacity. Based on this, silicon-based materials with extremely high theoretical specific capacity have become the focus of research. However, the huge volume expansion effect of silicon negative electrode will cause particle breakage, electrode structure collapse and continuous reconstruction of solid electrolyte interface (SEI) film, thereby causing rapid capacity decay and insufficient cycle life, which hinders the industrialization process. The binder, as the "skeleton" of the electrode, is responsible for bonding with the current collector and fixing the active material and conductive agent, and the bonding performance directly affects the mechanical integrity and electrochemical stability of the electrode; introducing reactive functional groups into the binder is a method to strengthen the interfacial bonding force, and the common functional groups are hydroxyl and carboxyl groups; carboxymethyl cellulose, polyacrylic acid and other binders have rich hydroxyl and carboxyl functional groups, which can improve the interfacial bonding strength and cycle stability of the electrode through hydrogen bonding, ionic dipole interaction and controllable chemical bonding, and bring excellent mechanical properties and stability, and can also prevent the occurrence of side reactions and improve the structural stability of the electrode material. However, some of these binders will have weak adhesion with silicon particles due to their linear single-chain structure, and will easily slide off and fail after multiple cycles; some are difficult to adapt to large-scale production due to high research cost and relatively immature technology. Therefore, it is urgent to find a kind of binder to optimize the negative electrode material and solve the above problems. SUMMARY

[0003] The purpose of the present application is to provide a preparation method of micron silicon negative electrode slurry and its application, to solve the problems in the prior art.

[0004] To achieve the above-mentioned purpose, the present application provides the following technical scheme: A preparation method of micron silicon negative electrode slurry, the specific steps are as follows: Step 1: take boric acid and put it in deionized water, stir and then add carboxymethyl cellulose, continue to stir for 24h, to obtain a boric acid cross-linked hydroxymethyl cellulose solution; take citric acid and put it in an ethanol solution, stir and then pour into the boric acid cross-linked carboxymethyl cellulose solution, to obtain a micron silicon negative electrode binder; Step 2: micron silicon powder, artificial graphite and primary pitch are mixed at 60 r / min for 2 h, and then molded, high-temperature carbonized and crushed to obtain a mixed powder; the mixed powder and secondary pitch are ball-milled and high-temperature carbonized to obtain a micron silicon-carbon composite material; Step 3: the micron silicon-carbon composite material prepared in Step 2, conductive carbon black and the micron silicon negative electrode binder prepared in Step 1 are placed in deionized water and ground and mixed for 30 min to obtain a micron silicon negative electrode slurry; Preferably, in Step 2, the amount ratio of boric acid, deionized water and carboxymethyl cellulose is 1 g:80 mL:1 g; the amount ratio of citric acid and ethanol solution is (1-3) g:80 mL; the ratio of boric acid and citric acid is 1:(0.5-1.5); and the water bath heating is at 45-55°C during stirring; Preferably, in Step 1, the mixed powder includes the following components by mass fraction: 55-60 parts of micron silicon powder, 10-15 parts of artificial graphite and 20-35 parts of primary pitch; and the mass of secondary pitch is 5% of the mixed powder; Preferably, the process parameters of high-temperature carbonization are 300°C for 2 h and 1000°C for 2 h, and the heating rate is 5°C / min; Preferably, the particle size D50 of the micron silicon powder is 4-8 μm; Preferably, in Step 3, the micron silicon negative electrode slurry includes the following components by mass fraction: 80-90 parts of the micron silicon-carbon composite material, 5-10 parts of conductive carbon black and 5-10 parts of the micron silicon negative electrode binder; More preferably, in Step 3, the mass ratio of the micron silicon-carbon composite material, conductive carbon black and micron silicon negative electrode binder is 8:1:1; Preferably, the micron silicon negative electrode slurry further includes water-soluble polyimide, and the preparation steps of the water-soluble polyimide are as follows: under a nitrogen atmosphere, p-phenylenediamine, 3,5-diaminobenzoic acid and 1,2-dimethylimidazole are placed in deionized water, stirred at 25-30°C for 1 h, then diphthalic anhydride is added, and the mixture is stirred and kept at 70-80°C for 18 h to obtain the water-soluble polyimide; Preferably, the molar ratio of diphthalic anhydride, p-phenylenediamine, 3,5-diaminobenzoic acid and 1,2-dimethylimidazole is (2-3):(1-1.6):(1-1.6):5; Preferably, the amount of the water-soluble polyimide is 2-5 wt% of the micron silicon negative electrode binder; Preferably, the preparation steps of the negative electrode sheet include: the micron silicon negative electrode slurry is coated on the surface of a copper foil, vacuum dried for 12 h and then sliced to obtain the negative electrode sheet; and the coating thickness is 50-300 μm; Preferably, after adding water-soluble polyimide in the micron silicon negative electrode slurry, the negative electrode sheet needs to be placed in vacuum heating at 140-150 DEG C for 6-8h to enhance the bonding effect when the negative electrode sheet is obtained; Compared with the prior art, the present application has the following advantages: 1、The present application uses micron silicon powder with high energy density and artificial graphite with good conductivity as the base material, forms micron carbon silicon balls by coating the silicon particles with asphalt once, enhances the connection between the graphite and the current collector by high-temperature carbonization, and then adds asphalt again to fill the gaps between the high-temperature carbonized graphite, further increase the adhesion and conductivity, and form a silicon-carbon composite material with high energy density, good conductivity and structural stability; and in the assistance of carboxymethyl cellulose and boric acid, the first cross-linking network is constructed by using the hydroxyl and carboxyl groups, the strong water solubility and the environment-friendly citric acid to generate hydrogen bond cross-linking, increase the anchor points, enhance the strength of the binder, effectively resist the volume strain effect of silicon, reduce the consumption of irreversible lithium, and thus improve the performance of the silicon-based battery; 2、In a further scheme, water-soluble polyimide is added to compensate for the adhesion decay of the micron silicon negative electrode binder in long-term cycling; since the common water-based binder and oil-based binder are difficult to coexist in the market, the oil-based binder is changed to water-based synthesis to form a water-soluble salt, which retains the good mechanical properties and chemical resistance of the oil-based binder itself, and realizes the synergistic effect between the water-based binder; wherein the water-soluble polyimide improves the water solubility by carboxylate and 1,2-dimethyl imidazole to form a weak ionic polyimide, which effectively stabilizes the dispersion of the micron silicon-carbon composite material and the conductive agent; and the rigid backbone formed by the molecular chain of the water-soluble polyimide effectively inhibits the deformation of the first cross-linking network, reduces the volume expansion, and reduces the powder shedding in long-term cycling; 3、The above cross-linking network combined with porous asphalt coating effectively resists the volume strain effect of silicon, helps to form a thin and dense SEI film, greatly reduces the irreversible capacity loss of the SEI film, and ultimately improves the performance of the silicon-based battery. DETAILED DESCRIPTION

[0005] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0006] In the experiment, the carboxymethyl cellulose was battery grade, purchased from Japan Daiseluo Co., Ltd.; the artificial graphite was battery grade, purchased from Shanggao County Rong Carbon Technology Co., Ltd.; the asphalt was battery grade, purchased from Jiangxi Zhengtuo; and the conductive carbon black was acetylene black, purchased from Hefei Kexing. Embodiment 1: The embodiment provides a preparation method of a negative electrode sheet for a battery, and the specific steps are as follows: Step 1: 5 g of boric acid was placed in 400 mL of deionized water, stirred in a water bath at 50°C for 30 min to fully dissolve, then 5 g of carboxymethyl cellulose was added, and stirring was continued for 24 h to obtain a boric acid cross-linked hydroxymethyl cellulose solution; 5 g of citric acid was placed in 200 mL of ethanol solution, stirred in a water bath at 50°C for 30 min, then poured into the boric acid cross-linked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: The micron silicon powder, artificial graphite and primary pitch were mixed in a mass ratio of 58:11:31, and mixed at 60 r / min for 2 h to obtain a shaped sample, which was then placed in a lifting furnace for high-temperature carbonization, and in an argon atmosphere, the temperature was raised at a rate of 5°C / min, and the temperature was kept at 300°C for 2 h, then the temperature was raised to 1000°C for 2 h until carbonization was completed, and the mixture was broken to obtain a mixed powder; the mixed powder and secondary pitch were mixed in a mass ratio of 1:0.05, and ball milled at 300 r / min for 1 h, and then high-temperature carbonization was carried out, and in an argon atmosphere, the temperature was raised at a rate of 5°C / min, and the temperature was kept at 300°C for 2 h, then the temperature was raised to 1000°C for 2 h to obtain a micron silicon-carbon composite material; Step 3: The micron silicon-carbon composite material prepared in step 2, conductive carbon black and the micron silicon negative electrode binder prepared in step 1 were placed in deionized water, and ground and mixed for 30 min to obtain a micron silicon negative electrode slurry, which was coated on the surface of a copper foil using a 200 μm film applicator, and then vacuum dried at 80°C for 12 h, and then the dried copper foil was cut and pressed into a sheet using a manual slicer to obtain a circular electrode sheet with a diameter of 11 mm, i.e. a negative electrode sheet for a battery; wherein the particle size D50 of the micron silicon powder is 6 μm; The mass ratio of the micron silicon-carbon composite material, conductive carbon black, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0007] Embodiment 2: Referring to Embodiment 1, the mass ratio of boric acid, carboxymethyl cellulose and citric acid is 1:1:0.5, and the specific steps are as follows: Step 1: 5 g of boric acid was placed in 400 mL of deionized water, stirred in a water bath at 50°C for 30 min to fully dissolve, then 5 g of carboxymethyl cellulose was added, and stirring was continued for 24 h to obtain a boric acid cross-linked hydroxymethyl cellulose solution; 5 g of citric acid was placed in 200 mL of ethanol solution, stirred in a water bath at 50°C for 30 min, then poured into the boric acid cross-linked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: micron silicon powder, artificial graphite and primary pitch were mixed according to a mass ratio of 58:11:31, mixed at 60 r / min for 2 h, and molded to obtain a molded sample, which was transferred to a lifting furnace for high-temperature carbonization, and was kept at 300℃ for 2 h at a temperature increasing rate of 5℃ / min in an argon atmosphere, and then was kept at 1000℃ for 2 h to complete carbonization, and was broken to obtain a mixed powder; the mixed powder and secondary pitch were mixed according to a mass ratio of 1:0.05, ball-milled at 300 r / min for 1 h, and high-temperature carbonization was performed after ball-milling, and was kept at 300℃ for 2 h at a temperature increasing rate of 5℃ / min in an argon atmosphere, and then was kept at 1000℃ for 2 h to obtain a micron silicon-carbon composite material; Step 3: the micron silicon-carbon composite material prepared in step 2, conductive carbon black and the micron silicon negative electrode binder prepared in step 1 were placed in deionized water, and were ground and mixed for 30 min to obtain a micron silicon negative electrode slurry, which was coated on the surface of a copper foil using a 200 μm film coater, vacuum dried at 80℃ for 12 h, and then the dried copper foil was cut and pressed into a sheet using a manual slicer to obtain a circular electrode sheet with a diameter of 11 mm, i.e. a negative electrode sheet for a battery; wherein the particle size D50 of the micron silicon powder is 6 μm; The mass ratio of the micron silicon-carbon composite material, conductive carbon black, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0008] Example 3: referring to Example 1, the mass ratio of boric acid, carboxymethyl cellulose and citric acid was 1:1:1.5, and the specific steps were as follows: Step 1: 5 g of boric acid was placed in 400 mL of deionized water, and was stirred in a water bath at 50℃ for 30 min to fully dissolve, and then 5 g of carboxymethyl cellulose was added, and stirring was continued for 24 h to obtain a boric acid cross-linked hydroxymethyl cellulose solution; 7.5 g of citric acid was placed in 200 mL of an ethanol solution, and was stirred in a water bath at 50℃ for 30 min, and then was poured into the boric acid cross-linked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: micron silicon powder, artificial graphite and primary pitch were mixed according to a mass ratio of 58:11:31, mixed at 60 r / min for 2 h, and molded to obtain a molded sample, which was transferred to a lifting furnace for high-temperature carbonization, and was kept at 300℃ for 2 h at a temperature increasing rate of 5℃ / min in an argon atmosphere, and then was kept at 1000℃ for 2 h to complete carbonization, and was broken to obtain a mixed powder; the mixed powder and secondary pitch were mixed according to a mass ratio of 1:0.05, ball-milled at 300 r / min for 1 h, and high-temperature carbonization was performed after ball-milling, and was kept at 300℃ for 2 h at a temperature increasing rate of 5℃ / min in an argon atmosphere, and then was kept at 1000℃ for 2 h to obtain a micron silicon-carbon composite material; Step 3: Take the micron silicon carbon composite material, conductive carbon black and micron silicon negative electrode binder prepared in step 1 in deionized water, grind and mix for 30 min to obtain a micron silicon negative electrode slurry, coat the surface of the copper foil with a 200 μm film applicator, vacuum dry at 80°C for 12 h, then use a manual slicer to cut and press the dried copper foil to obtain a circular electrode sheet with a diameter of 11 mm, which is used as the negative electrode sheet of the battery; wherein the particle size D50 of the micron silicon powder is 6 μm; The mass ratio of the micron silicon carbon composite material, conductive carbon black, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0009] Example 4, referring to Example 1, the micron silicon negative electrode binder is replaced with a micron silicon negative electrode binder and water-soluble polyimide with a mass ratio of 98:2, and the specific steps are as follows: Step 1: Take 5 g of boric acid in 400 mL of deionized water, stir in a water bath at 50°C for 30 min to fully dissolve, then add 5 g of carboxymethyl cellulose, continue to stir for 24 h to obtain a boric acid cross-linked hydroxymethyl cellulose solution; take 5 g of citric acid in 200 mL of ethanol solution, stir in a water bath at 50°C for 30 min, then pour into the boric acid cross-linked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: Mix the micron silicon powder, artificial graphite and primary pitch according to a mass ratio of 58:11:31, mix at 60 r / min for 2 h, mold into a shaped sample, and then place it in a lifting furnace for high-temperature carbonization, under an argon atmosphere, at a heating rate of 5°C / min, at 300°C for 2 h, then heat to 1000°C for 2 h until carbonization is complete, and then crush to obtain a mixed powder; mix the mixed powder and secondary pitch according to a mass ratio of 1:0.05, ball mill at 300 r / min for 1 h, and then perform high-temperature carbonization, under an argon atmosphere, at a heating rate of 5°C / min, at 300°C for 2 h, then heat to 1000°C for 2 h to obtain a micron silicon carbon composite material; Step 3: Take the micron silicon carbon composite material, conductive carbon black, water-soluble polyimide and micron silicon negative electrode binder prepared in step 1 in deionized water, grind and mix for 30 min to obtain a micron silicon negative electrode slurry, coat the surface of the copper foil with a 200 μm film applicator, vacuum dry at 80°C for 12 h, then use a manual slicer to cut and press the dried copper foil to obtain a circular electrode sheet with a diameter of 11 mm, which is used as the negative electrode sheet of the battery; wherein the particle size D50 of the micron silicon powder is 6 μm; The mass ratio of the micron silicon carbon composite material, conductive carbon black, water-soluble polyimide, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:0.18:8.82:10; The preparation method of the water-soluble polyimide is as follows: 0.32 g of p-phenylenediamine, 1.07 g of 3,5-diaminobenzoic acid and 2.4 g of 1,2-dimethylimidazole are placed in deionized water, 2.94 g of biphenyl tetracarboxylic dianhydride is added after stirring at 25°C for 1 h, and the mixture is incubated and stirred at 70°C for 18 h to obtain the water-soluble polyimide.

[0010] In example 5, the particle size D50 of the micron silicon powder is adjusted to 4 μm, and the specific steps are as follows: Step 1: 5 g of boric acid is placed in 400 mL of deionized water, and after being fully dissolved by stirring in a water bath at 50°C for 30 min, 5 g of carboxymethyl cellulose is added, and the mixture is continuously stirred for 24 h to obtain a boric acid cross-linked hydroxymethyl cellulose solution; 5 g of citric acid is placed in 200 mL of an ethanol solution, stirred in a water bath at 50°C for 30 min, and then poured into the boric acid cross-linked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: The micron silicon powder, artificial graphite and primary pitch are mixed in a mass ratio of 58:11:31, and the mixture is mixed at 60 r / min for 2 h to obtain a molded sample, which is then placed in a lifting furnace for high-temperature carbonization. In an argon atmosphere, the temperature is raised at a rate of 5°C / min to 300°C and incubated for 2 h, and then raised to 1000°C and incubated for 2 h until carbonization is complete. The mixture is broken to obtain a mixed powder; the mixed powder and secondary pitch are mixed in a mass ratio of 1:0.05, and ball milling is performed at 300 r / min for 1 h. After ball milling, high-temperature carbonization is performed in an argon atmosphere at a rate of 5°C / min to 300°C for 2 h, and then raised to 1000°C for 2 h to obtain a micron silicon carbon composite material; Step 3: The micron silicon carbon composite material prepared in step 2, conductive carbon black and the micron silicon negative electrode binder prepared in step 1 are placed in deionized water, and the mixture is ground and mixed for 30 min to obtain a micron silicon negative electrode slurry. The slurry is coated on the surface of a copper foil using a 200 μm coating film applicator, vacuum dried at 80°C for 12 h, and then cut and pressed into a sheet using a manual slicer to obtain a circular electrode sheet with a diameter of 11 mm, which is used as the negative electrode sheet of the battery; The particle size of the micron silicon powder is D50 of 4 μm; The mass ratio of the micron silicon carbon composite material, conductive carbon black, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0011] In example 6, the particle size D50 of the micron silicon powder is adjusted to 8 μm, and the specific steps are as follows: Step 1: 5 g of boric acid was taken and dissolved in 400 mL of deionized water in a water bath at 50°C for 30 min, then 5 g of carboxymethyl cellulose was added and stirred for 24 h to obtain a boric acid cross-linked hydroxymethyl cellulose solution; 5 g of citric acid was taken and dissolved in 200 mL of ethanol solution in a water bath at 50°C for 30 min, then poured into the boric acid cross-linked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: micron silicon powder, artificial graphite and primary pitch were mixed in a mass ratio of 58:11:31, and mixed at 60 r / min for 2 h, then molded to obtain a molded sample, which was transferred to a lifting furnace for high-temperature carbonization, and carbonized at 300°C for 2 h and then at 1000°C for 2 h at a heating rate of 5°C / min in an argon atmosphere, then broken to obtain a mixed powder; the mixed powder and secondary pitch were mixed in a mass ratio of 1:0.05, and ball milled at 300 r / min for 1 h, then high-temperature carbonized at 300°C for 2 h and then at 1000°C for 2 h at a heating rate of 5°C / min in an argon atmosphere to obtain a micron silicon-carbon composite material; Step 3: the micron silicon-carbon composite material prepared in step 2, conductive carbon black and the micron silicon negative electrode binder prepared in step 1 were taken and mixed in deionized water by grinding for 30 min to obtain a micron silicon negative electrode slurry, which was coated on the surface of a copper foil using a 200 μm film applicator, and then vacuum dried at 80°C for 12 h, and then the dried copper foil was cut and pressed into a sheet using a manual slicer to obtain a circular electrode sheet with a diameter of 11 mm, which was used as a negative electrode sheet for the battery; wherein the particle size D50 of the micron silicon powder is 8 μm; The mass ratio of the micron silicon-carbon composite material, conductive carbon black, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0012] Comparative Example 1: As a control experiment of Example 1, the micron silicon negative electrode binder was not cross-linked with boric acid and citric acid, and the specific steps were as follows: Step 1: micron silicon powder, artificial graphite and primary pitch were weighed in a mass ratio of 58:11:31, mixed at a rotation speed of 60 r / min for 2 h, and then molded to obtain a molded sample, which was transferred to a lifting furnace for high-temperature carbonization, and carbonized at 300°C for 2 h and then at 1000°C for 2 h at a heating rate of 5°C / min in an argon atmosphere, then broken to obtain a mixed powder; the mixed powder and secondary pitch were mixed in a mass ratio of 1:0.05, and ball milled at a rotation speed of 300 r / min for 1 h, then high-temperature carbonized at 300°C for 2 h and then at 1000°C for 2 h at a heating rate of 5°C / min in an argon atmosphere, and then screened after carbonization to obtain a micron silicon-carbon composite material; Step 2: Take the micron silicon carbon composite material, conductive carbon black and carboxymethyl cellulose in deionized water, grind and mix thoroughly for 30 min to obtain a micron silicon negative electrode slurry, coat the surface of the copper foil with a 200 µm film applicator, vacuum dry at 80°C for 12 h, and cut and press the dried copper foil with a manual slicer to obtain a circular electrode sheet with a diameter of 11 mm, which is used as the negative electrode sheet of the battery; wherein the particle size D50 of the micron silicon powder is 6 µm; The mass ratio of the micron silicon carbon composite material, conductive carbon black, carboxymethyl cellulose and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0013] Comparative Example 2: As a control experiment of Example 1, the micron silicon negative electrode binder is not crosslinked with citric acid, and the specific steps are as follows: Step 1: Take 5 g of boric acid in 400 mL of deionized water, stir in a water bath at 50°C for 30 min to fully dissolve to obtain a boric acid aqueous solution, add 5 g of carboxymethyl cellulose, continue to stir for 24 h to obtain a micron silicon negative electrode binder; Step 2: Take the micron silicon powder, artificial graphite and primary pitch according to a mass ratio of 58:11:31, mix at a speed of 60 r / min for 2 h, then mold into a shape to obtain a molded sample, and then perform high-temperature carbonization in a lifting furnace, in an argon atmosphere, at a heating rate of 5°C / min, at 300°C for 2 h and at 1000°C for 2 h, and then crush to obtain a mixed powder; mix the mixed powder and secondary pitch according to a mass ratio of 1:0.05, ball mill at a speed of 300 r / min for 1 h, then take out and perform high-temperature carbonization, in an argon atmosphere, at a heating rate of 5°C / min, at 300°C for 2 h and at 1000°C for 2 h, then take out and sieve to obtain the micron silicon carbon composite material; Step 3: Take the micron silicon carbon composite material, conductive carbon black and micron silicon negative electrode binder in deionized water, grind and mix thoroughly for 30 min to obtain a micron silicon negative electrode slurry, coat the surface of the copper foil with a 200 µm film applicator, vacuum dry at 80°C for 12 h, and cut and press the dried copper foil with a manual slicer to obtain a circular electrode sheet with a diameter of 11 mm, which is used as the negative electrode sheet of the battery; wherein the particle size D50 of the micron silicon powder is 6 µm; The mass ratio of the micron silicon carbon composite material, conductive carbon black, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0014] Comparative Example 3: As a control experiment of Example 1, no secondary pitch is mixed, and the specific steps are as follows: Step 1: Take 5g of boric acid and place it in 400mL of deionized water. Stir it in a water bath at 50°C for 30min to fully dissolve it, then add 5g of carboxymethyl cellulose and continue stirring for 24h to obtain a boric acid-crosslinked hydroxymethyl cellulose solution; take 5g of citric acid and place it in 200mL of ethanol solution. Stir it in a water bath at 50°C for 30min and then pour it into the boric acid-crosslinked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: Micron silicon powder, artificial graphite and primary asphalt were mixed in a mass ratio of 58:11:31, mixed at 60 r / min for 2 h, and compression molded to obtain a molded sample. The molded sample was transferred to a lifting furnace for high-temperature carbonization. The temperature was kept at 300°C for 2 h at a heating rate of 5°C / min under an argon atmosphere, and then heated to 1000°C for 2 h until carbonization was completed. The sample was crushed to obtain a micron silicon-carbon composite material. Step 3: Place the micron silicon-carbon composite material prepared in step 2, the conductive carbon black, and the micron silicon negative electrode binder prepared in step 1 in deionized water, grind and mix for 30 minutes to obtain a micron silicon negative electrode slurry, apply it on the surface of the copper foil with a 200μm applicator, vacuum dry it at 80°C for 12 hours, and then use a manual slicer to cut and press the dried copper foil into a circular electrode sheet with a diameter of 11mm, i.e., the negative electrode sheet used in the battery; Among them, the particle size D50 of micron silicon powder is 6μm; The mass ratio of micron silicon-carbon composite material, conductive carbon black, micron silicon negative electrode binder and deionized water in the micron silicon negative electrode slurry is 72:9:9:10.

[0015] Application example: Using the negative electrode sheets prepared in Examples 1-6 and Comparative Examples 1-3, a metal Li sheet with a diameter of 15.6 mm and a thickness of 0.45 mm as the counter electrode, a polypropylene film with a diameter of 16 mm purchased from Celgard2400 as the diaphragm, and a 5V high-voltage electrolyte LB111 in which LiPF6 is dissolved in a mixed solvent of EC:DMC with a ratio of 1:1 as the electrolyte, LIR2477 button batteries are assembled in an argon-filled glove box. The assembly order is: negative electrode shell, a small amount of electrolyte, diaphragm, electrolyte, positive electrode sheet, gasket, spring, and positive electrode shell. After the battery is assembled, it is sealed using a hydraulic press. The battery is left to stand for 12 hours to allow the electrolyte to fully infiltrate the diaphragm and the electrode sheet. They are recorded as battery samples 1-9 in sequence.

[0016] Detection experiment 1. Peeling test: The electrode prepared in Example 1-6 and Comparative Example 1-3 was cut into a 40*25 mm strip, then 3M double-sided tape was attached to the back of the electrode, the other side of the double-sided tape was attached to a prepared glass slide, and 3M double-sided tape with a size of 9*1 mm was attached to the surface of the electrode. After turning over the tape, one side of the tape was attached to the upper clamp of a microcomputer-controlled electronic universal testing machine, and the other end of the glass slide was attached to the lower clamp of the testing machine. The mode was selected as tension-displacement mode, and the stretching speed was set to 5 mm / min. The peeling strength of the electrode was recorded in Table 1. Table 1

[0017] 2. First discharge specific capacity test: The assembled button cell samples 1-9 were tested for cycle performance in a constant current charge-discharge mode using a CT2001A type blue battery test system and a CT-4008 type Xinyi battery test system. Voltage range: 5mV-2V, discharge is 0.05C constant current discharge to 5mV, stand for 5min; 200μA constant current discharge to 5mV, stand for 5min; 50μA constant current discharge to 5mV cutoff; charge is 0.1C constant current charge to 2V; calculate the first discharge specific capacity, the data is recorded in Table 2. 3. Reversible capacity test: The assembled button cell samples 1-9 were tested for charge-discharge cycle at 25℃ under a current density of 0.05C, and the capacity after 50 cycles was recorded, the data is recorded in Table 2. 4. Volume expansion rate test: First measure the initial thickness of the electrode h1, then measure the thickness of the negative electrode after lithium extraction or after cycle charge-discharge h2, then calculate the expansion rate according to the formula (h2-h1) / h1*100%, the data is recorded in Table 2.

[0018] Table 2

[0019] Conclusion: 1. As can be seen from the data of Comparative Examples 1 and 2, the CMC binder is successfully cross-linked by BA to form a binder network, increasing the contact points between the foil and stabilizing the contact between the silicon particles and the conductive agent during the cycle process, effectively relieving the volume strain effect of the silicon negative electrode material, generating a stable SEI layer, and reducing the consumption of irreversible lithium. 2. The cross-linked modified binders prepared in Examples 1, 2, and 3 also have large differences in performance due to the component ratios. In Example 2, when the mass ratio of BA, CMC, and CA is 1:1:0.5, the CA content is low and the cross-linking degree is low, resulting in insufficient rigidity of the binder network and slightly poor inhibition of the volume expansion effect of the active component during the cycle. In Example 3, when the mass ratio of BA, CMC, and CA is 1:1:1.5, the excessive degree of cross-linking reaction is not conducive to the transport of electrons, and excessive cross-linking consumes more reactive functional groups, which is not conducive to the subsequent molecular interaction between the binder and the silicon particles, resulting in poor technical effects. 3. Use water-soluble polyimide and carboxymethyl cellulose binders to act together on the silicon negative electrode material. Due to the rigid skeleton formed by polyimide, it has a certain supporting effect and can work synergistically with the carboxymethyl cellulose binder to withstand the expansion and contraction of the volume of silicon particles during the cycle, reduce the pulverization and shedding of silicon particles, and improve the long-cycle stability of the silicon-carbon negative electrode.

[0020] 4. Changing the particle size of micron silicon powder effectively improves the peeling strength of the negative electrode slurry, while slowing down the volume expansion of silicon particles and improving the long-term stability of the negative electrode slurry.

[0021] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. A method for preparing micron silicon negative electrode slurry, characterized in that: The specific steps are as follows: Step 1: Boric acid is placed in deionized water, stirred, and then carboxymethyl cellulose is added. The stirring is continued for 24 hours to obtain a boric acid cross-linked hydroxymethyl cellulose solution; citric acid is placed in an ethanol solution, stirred, and then poured into the boric acid cross-linked carboxymethyl cellulose solution to obtain a micron silicon negative electrode binder; Step 2: Mix the micron silicon powder, artificial graphite and primary asphalt, mix at 60 r / min for 2 hours, press-form, high-temperature carbonize and crush to obtain a mixed powder; ball-mill the mixed powder and secondary asphalt, and carbonize at high temperature to obtain a micron silicon-carbon composite material; Step 3: Place the micron silicon-carbon composite material prepared in step 2, the conductive carbon black, and the micron silicon negative electrode binder prepared in step 1 in deionized water, grind and mix for 30 minutes to obtain a micron silicon negative electrode slurry.

2. The method for preparing a micron silicon negative electrode slurry according to claim 1, characterized in that: In step 1, the usage ratio of boric acid, deionized water, and carboxymethyl cellulose is 1 g:80 mL:1 g; the usage ratio of citric acid and ethanol solution is (1-3) g:80 mL; the ratio of boric acid to citric acid is 1:(0.5-1.5); and heating in a water bath at 45-55° C. is used during stirring.

3. The method for preparing a micron silicon negative electrode slurry according to claim 1, characterized in that: The mixed powder in step 2 includes the following components by mass: 55-60 parts of micron silicon powder, 10-15 parts of artificial graphite and 20-35 parts of primary asphalt; the mass of the secondary asphalt is 5% of the mixed powder.

4. The method for preparing a micron silicon negative electrode slurry according to claim 1, characterized in that: The particle size D50 of micron silicon powder is 4-8μm.

5. The method for preparing a micron silicon negative electrode slurry according to claim 1, characterized in that: In step 3, the mass ratio of the micron silicon-carbon composite material, the conductive carbon black and the micron silicon negative electrode binder is 8:1:

1.

6. The method for preparing a micron silicon negative electrode slurry according to claim 1, characterized in that: The micron silicon negative electrode slurry also includes water-soluble polyimide. The preparation steps of the water-soluble polyimide are as follows: under a nitrogen atmosphere, p-phenylenediamine, 3,5-diaminobenzoic acid and 1,2-dimethylimidazole are placed in deionized water, stirred at 25-30°C for 1 hour, and then biphenyltetracarboxylic dianhydride is added. The mixture is kept warm and stirred at 70-80°C for 18 hours to obtain water-soluble polyimide.

7. The method for preparing a micron silicon negative electrode slurry according to claim 6, characterized in that: The molar ratio of biphenyltetracarboxylic dianhydride, p-phenylenediamine, 3,5-diaminobenzoic acid and 1,2-dimethylimidazole is (2-3): (1-1.6): (1-1.6): 5; the amount of the water-soluble polyimide is 2-5wt% of the micron silicon negative electrode binder.

8. A micron silicon negative electrode slurry, characterized in that: The micron silicon negative electrode slurry is prepared by the preparation method according to any one of claims 1 to 7.

9. An application of micron silicon negative electrode slurry, characterized in that: Used for preparing negative electrode sheets in batteries; the preparation steps of the negative electrode sheets include: coating micron silicon negative electrode slurry on the surface of copper foil, baking and vacuum drying for 12 hours, and slicing to obtain negative electrode sheets; wherein the coating thickness is 50-300μm.

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