Composite conductive agent for silicon-based material and silicon-based negative plate and lithium battery using same

Abstract

The invention relates to a composite conductive agent for a silicon-based material. The composite conductive agent comprises conductive carbon black and a modified single-walled carbon nanotube, the modified single-walled carbon nanotubes contain modified groups, and the modified groups comprise carboxyl and sulfonic acid groups. In the composite conductive agent provided by the invention, the conductive carbon black provides a short-range conductive path, the modified SWCNT provides a long-range conductive channel, and the conductive carbon black and the modified SWCNT are matched to form a perfect multi-dimensional conductive network, so that rapid transmission of electrons in a silicon-based material is promoted, and the transmission rate of lithium ions at high rate is increased; moreover, the composite conductive agent can be in full contact and tight connection with the silicon-based material, endows the silicon-based negative electrode plate with excellent conductive performance, and relieves volume expansion of the silicon-based material, so that the structural stability and the cycling stability of the silicon-based negative electrode plate are improved, and the lithium battery applying the silicon-based negative electrode plate has excellent fast charging performance and dynamic characteristics.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery conductive agents, and in particular relates to a composite conductive agent for silicon-based materials and a silicon-based negative electrode sheet and a lithium battery using the composite conductive agent. Background Art

[0002] Silicon-based materials have the advantages of high theoretical specific capacity, low lithium insertion potential, and abundant reserves, and are currently a hot research topic for negative electrode materials for lithium batteries. However, the drastic volume changes and low electrical conductivity of silicon-based materials during electrochemical cycles limit their practical applications. Therefore, how to select a conductive agent for silicon-based materials is crucial.

[0003] Currently, the commonly used silicon-based conductive agents mainly include conductive carbon black (SP) and carbon nanotubes (CNTs). Among them, SP is a point-like conductive agent. Due to the volume expansion of silicon-based materials during the cycle, it is easy to lose contact with the silicon-based materials, and its large addition amount is not conducive to improving the energy density of lithium batteries; CNT is a linear conductive agent with good electronic conductivity, but due to its small tube diameter and large aspect ratio, it is easy to agglomerate under the action of van der Waals force, which limits its conductive effect. Summary of the invention

[0004] The object of the present invention is to provide a composite conductive agent for silicon-based materials, which has excellent conductivity and can promote rapid transmission of electrons in silicon-based materials.

[0005] According to one aspect of the present invention, a composite conductive agent for silicon-based materials is provided, comprising conductive carbon black and modified single-walled carbon nanotubes; the modified single-walled carbon nanotubes contain modified groups, and the modified groups include carboxyl groups and sulfonic acid groups.

[0006] The present invention compounds single-walled carbon nanotubes (modified SWCNT) with conductive carbon black (SP). On the one hand, the modified group carboxyl group contained in the modified single-walled carbon nanotube can form a covalent ester bond between the modified SWCNT and the hydroxyl group on the surface of the silicon-based material, thereby promoting a firm connection between the modified single-walled carbon nanotube and the silicon-based material; at the same time, the modified group sulfonic acid group contained in the modified SWCNT has hydrophilicity, thereby improving the dispersibility and stability of the modified SWCNT in an aqueous medium, promoting its contact with the silicon-based material, and further promoting the carboxyl group and the hydroxyl group on the surface of the silicon-based material. The combination of the sulfonic acid group and the silicon-based material increases the bonding strength between the composite conductive agent and the silicon-based material, which is beneficial to increase the migration rate of electrons in the silicon-based negative electrode sheet and improve the conductivity of the silicon-based negative electrode sheet; in addition, the sulfonic acid group also has a flame retardant function, which can improve the safety performance of the silicon-based negative electrode sheet; on the other hand, SP has a good dispersion effect, which is beneficial to further promote the contact between the composite conductive agent and the silicon-based material, so that the conductivity of the silicon-based negative electrode sheet is further improved. At the same time, SP has a strong liquid absorption ability, which is beneficial to promote the transmission of lithium ions in the silicon-based material and improve the rate performance of the silicon-based negative electrode sheet.

[0007] In summary, in the composite conductive agent provided by the present invention, the conductive carbon black provides a short-range conductive path, and the modified SWCNT provides a long-range conductive channel. The combination of the two forms a complete multi-dimensional conductive network, which promotes the rapid transmission of electrons in silicon-based materials and improves the transmission rate of lithium ions at high rates; moreover, the composite conductive agent can fully contact and tightly connect with the silicon-based material, giving the silicon-based negative electrode sheet excellent conductive properties, and alleviating the volume expansion of the silicon-based material, thereby improving the structural stability and cycle stability of the silicon-based negative electrode sheet, so that the lithium battery using it has excellent fast charging performance and dynamic characteristics.

[0008] Preferably, the molar ratio of carboxyl group to sulfonic acid group is (2.5-7.5):1. The molar ratio of carboxyl group to sulfonic acid group can be 2.5:1, 3.5:1, 4.5:1, 5.5:1, 6.5:1, 7.5:1, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable. By reasonably setting the molar ratio of carboxyl group to sulfonic acid group in the modified SWCNT, the carboxyl group and the sulfonic acid group can fully play a synergistic role, promote the full contact and firm bonding between the modified SWCNT and the silicon-based material, improve the conductivity of the silicon-based negative electrode sheet, and alleviate the volume change of the silicon-based material, so that the rate performance, structural stability and cycle stability of the silicon-based negative electrode sheet are further improved.

[0009] Preferably, the total content of carboxyl and sulfonic acid groups in the modified single-walled carbon nanotubes is 3wt% to 4.5wt%. The total content of carboxyl and sulfonic acid groups in the modified single-walled carbon nanotubes can be 3wt%, 3.5wt%, 4wt%, 4.5wt%, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable. By controlling the content of the modified group in the modified SWCNT, it is beneficial to maintain a suitable bonding force between the modified SWCNT and the silicon-based material, thereby enabling the composite conductive agent to be firmly bonded to the silicon-based material while maintaining good mechanical strength and flexibility of the negative electrode sheet, avoiding the problem that the modified SWCNT contains more modified groups, resulting in more covalent bonds between the composite conductive agent and the silicon-based material, making the negative electrode sheet brittle and having poor stability.

[0010] Preferably, the ratio of the length of the modified single-walled carbon nanotube to the particle size of the conductive carbon black is (100-800):1, and the aspect ratio of the modified single-walled carbon nanotube is (800-1000):1. The ratio of the length of the modified single-walled carbon nanotube to the particle size of the conductive carbon black can be 100:1, 200:1, 300:1, 400:1, 500:1, 600:1, 700:1, 800:1, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable. The aspect ratio of the modified single-walled carbon nanotube can be 800:1, 900:1, 1000:1, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable. By reasonably setting the ratio of the length of the modified single-walled carbon nanotubes to the particle size of the conductive carbon black and the aspect ratio of the modified single-walled carbon nanotubes, it is beneficial to fully disperse the SP and the modified SWCNT, and effectively reduce the risk of entanglement and agglomeration of the components of the composite conductive agent. As a result, the SP and the modified SWCNT are coordinated to form a complete multi-dimensional conductive network, which increases the contact area between the multi-dimensional conductive network and the silicon-based material, promotes the rapid transmission of electrons in the silicon-based material, and improves the conductivity of the silicon-based negative electrode sheet.

[0011] Preferably, the outer diameter of the modified single-walled carbon nanotube is 1 to 2 nm, and the length is 1 to 4 μm.

[0012] Preferably, the mass ratio of conductive carbon black to modified single-walled carbon nanotubes is (10-20):1. The mass ratio of conductive carbon black to modified single-walled carbon nanotubes can be 10:1, 15:1, 20:1, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable. By reasonably setting the ratio of SP to modified SWCNT, it is beneficial to fully disperse SP and modified SWCNT, thereby making SP and modified SWCNT cooperate to form a complete multi-dimensional conductive network, increase the contact area between the multi-dimensional conductive network and the silicon-based material, promote the rapid transmission of electrons in the silicon-based material, and improve the conductivity of the silicon-based negative electrode sheet.

[0013] Preferably, the preparation method of modified single-walled carbon nanotubes comprises the following operations: dispersing carboxylated single-walled carbon nanotubes in a solvent to obtain a dispersion; then adding sulfur trioxide to the dispersion for sulfonation treatment to prepare modified single-walled carbon nanotubes, wherein the temperature of the sulfonation treatment is 60 to 80° C. Sulfonation treatment of carboxylated single-walled carbon nanotubes with sulfur trioxide is conducive to uniformly and firmly grafting sulfonic acid groups to the carboxylated single-walled carbon nanotubes, so that the conductivity of the modified single-walled carbon nanotubes obtained is improved.

[0014] Preferably, the solvent is a carboxymethyl cellulose aqueous solution (CMC aqueous solution), and the solid content of the carboxymethyl cellulose aqueous solution is 0.4% to 0.8%. The carboxymethyl cellulose aqueous solution (CMC aqueous solution) can play a certain bonding role and promote the dispersion of carboxylated single-walled carbon nanotubes.

[0015] According to another aspect of the present invention, a silicon-based negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material and the above-mentioned composite conductive agent, and the negative electrode active material comprises a silicon-based material. The composite conductive agent provided by the present invention is used as a conductive agent for the silicon-based negative electrode sheet, which effectively improves the problem of poor conductivity of the silicon-based material, improves the transmission rate of lithium ions at high rates, and enables the silicon-based negative electrode sheet and lithium battery to have good cycle characteristics and rate performance, and avoids the polarization phenomenon of the battery due to low conductivity and slow lithium ion transmission rate of the silicon-based material during high-rate discharge.

[0016] Preferably, the mass of the composite conductive agent accounts for 1.5-3% of the total mass of the negative electrode active material layer. Using a small amount of the composite conductive agent can significantly enhance the conductivity of the silicon-based negative electrode sheet, and can increase the content of silicon-based materials in the negative electrode sheet, so that the energy density of the battery is further improved.

[0017] Preferably, the method for preparing a silicon-based negative electrode sheet comprises the following operations: mixing a silicon-based material, a composite conductive agent and a binder to prepare a negative electrode slurry, coating the negative electrode slurry on a negative electrode current collector and performing a baking treatment, thereby preparing a silicon-based negative electrode sheet, wherein the baking treatment temperature is 80 to 120° C. Under the above baking treatment temperature conditions, it is beneficial to promote the formation of ester bonds between the carboxyl groups contained in the modified SWCNT in the composite conductive agent and the hydroxyl groups on the surface of the silicon-based material.

[0018] According to another aspect of the present invention, there is provided a lithium battery comprising the above-mentioned silicon-based negative electrode sheet. DETAILED DESCRIPTION

[0019] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present invention, rather than all embodiments.

[0020] Example 1

[0021] This embodiment provides a lithium battery, and the preparation method thereof includes the following operations:

[0022] (1) Preparation of modified single-walled carbon nanotubes

[0023] S1. Ultrasonic dispersion of commercial carboxylated single-walled carbon nanotubes (the diameter of the carboxylated single-walled carbon nanotubes is 1 nm, the length is 1 μm, and the content of carboxyl groups in the carboxylated single-walled carbon nanotubes is 2 wt%) in a 0.6% CMC aqueous solution to obtain a dispersion;

[0024] S2, slowly adding sulfur trioxide to the above dispersion, heating to 70° C. and stirring for 10 h to perform sulfonation treatment to obtain modified single-walled carbon nanotubes;

[0025] The modified single-walled carbon nanotubes prepared in this embodiment contain modified groups, including carboxyl groups and sulfonic acid groups, wherein the molar ratio of carboxyl groups to sulfonic acid groups is 3.5:1, and the total content of carboxyl groups and sulfonic acid groups in the modified single-walled carbon nanotubes is 3.5wt%; the diameter of the modified single-walled carbon nanotubes is 1nm and the length is 1μm (the aspect ratio of the modified single-walled carbon nanotubes is 1000:1).

[0026] (2) Preparation of composite conductive agents for silicon-based materials

[0027] Conductive carbon black (SP) and the modified single-walled carbon nanotubes (modified SWCNT) were mixed at a mass ratio of 15:1 and subjected to conventional planetary stirring to obtain a composite conductive agent; the particle size of SP was 4 nm (the ratio of the length of the modified single-walled carbon nanotubes to the particle size of the conductive carbon black was 250:1).

[0028] 2. Preparation of silicon-based negative electrode sheet

[0029] The silicon-based material (silicon-carbon material), the composite conductive agent, and the binder PAA are mixed in a mass ratio of 80:10:10 and subjected to conventional planetary stirring to obtain a negative electrode slurry. The negative electrode slurry is coated on the current collector and then baked at 100°C for 12 hours to allow the carboxyl groups in the composite conductive agent to form ester groups with the hydroxyl groups on the surface of the silicon-based material. At the same time, the negative electrode slurry is dried and die-cut to obtain a silicon-based negative electrode sheet.

[0030] The silicon-based negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector. In this embodiment, the mass of the composite conductive agent accounts for 2% of the total mass of the negative electrode active material layer.

[0031] 3. Preparation of lithium batteries

[0032] The above-mentioned silicon-based negative electrode sheet, the ternary positive electrode sheet prepared by traditional mature technology (the positive electrode active material is NCM811, the mass ratio of NCM811, conductive SP, and binder PVDF is 93:3:4), 1mol / L LiPF6 / EC+DMC+EMC (v / v=1:1:1)+10% FEC electrolyte, PE+alumina separator, and the shell are assembled into a soft-pack battery using conventional production technology.

[0033] Example 2

[0034] This embodiment prepares lithium batteries with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the modified single-walled carbon nanotubes, the content of carboxyl groups in the carboxylated single-walled carbon nanotubes and the feed ratio of carboxylated single-walled carbon nanotubes to sulfur trioxide are controlled so that the molar ratio of carboxyl groups to sulfonic acid groups in the prepared modified single-walled carbon nanotubes is 1.5:1. Except for the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0035] Example 3

[0036] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the modified single-walled carbon nanotubes, the content of carboxyl groups in the carboxylated single-walled carbon nanotubes and the feed ratio of carboxylated single-walled carbon nanotubes to sulfur trioxide are controlled so that the molar ratio of carboxyl groups to sulfonic acid groups in the prepared modified single-walled carbon nanotubes is 8.5:1. Except for the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0037] Example 4

[0038] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the diameter of the modified single-walled carbon nanotube prepared in this embodiment is 1 nm and the length is 2 μm (the aspect ratio of the modified single-walled carbon nanotube is 2000:1). Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0039] Example 5

[0040] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that the diameter of the modified single-walled carbon nanotube prepared in this example is 2 nm and the length is 1 μm (the aspect ratio of the modified single-walled carbon nanotube is 500:1). Except for the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0041] Example 6

[0042] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the composite conductive agent in this embodiment, the particle size of the conductive carbon black used is 0.8 nm (the ratio of the length of the modified single-walled carbon nanotube to the particle size of the conductive carbon black is 1250:1). Except for the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0043] Example 7

[0044] This embodiment refers to Example 1 to prepare a lithium battery. The difference between this embodiment and Example 1 is that in the process of preparing the composite conductive agent in this embodiment, the particle size of the conductive carbon black used is 12.5 nm (the ratio of the length of the modified single-walled carbon nanotube to the particle size of the conductive carbon black is 80:1). Except for the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0045] Example 8

[0046] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the composite conductive agent in this embodiment, the mass ratio of SP to the modified single-walled carbon nanotube is 8:1. Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0047] Example 9

[0048] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the composite conductive agent in this embodiment, the mass ratio of SP to the modified single-walled carbon nanotube is 23:1. Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0049] Example 10

[0050] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the modified single-walled carbon nanotubes in this embodiment, sulfuric acid with the same sulfur content is used to replace the sulfur trioxide in the embodiment 1 to prepare the modified single-walled carbon nanotubes. Except for the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0051] Comparative Example 1

[0052] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the silicon-based negative electrode sheet, the modified single-walled carbon nanotubes in Example 1 are replaced by conventional single-walled carbon nanotubes of the same mass. Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0053] Comparative Example 2

[0054] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the composite conductive agent, the modified single-walled carbon nanotubes in Example 1 are replaced with the same mass fraction of carboxylated single-walled carbon nanotubes. Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0055] Comparative Example 3

[0056] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the composite conductive agent, the modified single-walled carbon nanotubes in Example 1 are replaced with the same mass fraction of sulfonated single-walled carbon nanotubes. Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0057] The preparation method of sulfonated single-walled carbon nanotubes is as follows:

[0058] S1, ultrasonically dispersing commercial single-walled carbon nanotubes (single-walled carbon nanotubes with a diameter of 1 nm and a length of 3 μm) in a 0.6% CMC aqueous solution to obtain a dispersion;

[0059] S2. Slowly add sulfur trioxide to the above dispersion at a mass ratio of single-walled carbon nanotubes to sulfur trioxide of 100:1, raise the temperature to 70°C and stir for 10 hours for sulfonation treatment to obtain sulfonated single-walled carbon nanotubes; the sulfonated single-walled carbon nanotubes obtained in this comparative example contain modified groups, which only include sulfonic acid groups, and the content of sulfonic acid groups in the modified single-walled carbon nanotubes is 1.5wt%.

[0060] Comparative Example 4

[0061] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the composite conductive agent, the composite conductive agent of Example 1 is replaced by the modified single-walled carbon nanotubes of the same mass. Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those of Example 1.

[0062] Comparative Example 5

[0063] This comparative example refers to Example 1 to prepare a lithium battery. The difference between this comparative example and Example 1 is that in the process of preparing the silicon-based negative electrode sheet, the composite conductive agent of Example 1 is replaced by the same mass fraction of SP. Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those of Example 1.

[0064] Test Example 1

[0065] 1. Test object:

[0066] The lithium batteries prepared in Examples 1 to 10 and Comparative Examples 1 to 5 were used as test objects in this test example.

[0067] 2. Test items:

[0068] (1) First efficiency: First efficiency = total capacity of battery discharged at 0.33C for the first time / total capacity of battery charged at 0.33C for the first time * 100%.

[0069] (2) 2C / 2C cycle 500-cycle capacity retention rate: At 25°C, charge at 2C constant current and constant voltage to 4.2 V, cut off at 0.05C, let stand for 10 min, discharge at 2C constant current to 2.5 V, let stand for 10 min, cycle for 500 weeks, and record the battery 2C / 2C cycle 500-cycle capacity retention rate.

[0070] (3) 6C rate performance at room temperature: At 25°C, discharge at 1C constant current to 2.5V, let stand for 10 min, charge at 6C constant current and constant voltage to 4.2V, cut off at 0.05C, record the battery constant current charging capacity, constant current and constant voltage charging total capacity and the highest temperature during fast charging, constant current charging ratio = constant current charging capacity / constant current and constant voltage charging total capacity.

[0071] 3. Test results:

[0072] Table 1. Performance test results of Examples 1 to 10 and Comparative Examples 1 to 5

[0073]

[0074] The test results are shown in Table 1. By comparing the performance test results corresponding to Example 1 and Comparative Examples 1 to 3, it can be found that the first efficiency, capacity retention rate of 500 cycles of 2C / 2C at room temperature, and constant current charging ratio of 6C rate charging of the batteries prepared in Comparative Examples 1 to 3 are significantly lower than those in Example 1, and the maximum temperature of 6C rate charging is significantly higher than that in Example 1. The reason is that Comparative Examples 1 to 3 respectively use conventional single-walled carbon nanotubes, carboxylated single-walled carbon nanotubes, sulfonated single-walled carbon nanotubes and conductive carbon black as composite conductive agents, and the composite conductive agents used in Comparative Examples 1 to 3 have low bonding strength with silicon-based materials, which is not conducive to the rapid transmission of electrons and ions at high rates, thereby significantly deteriorating the rate performance and cycle characteristics of the battery.

[0075] By comparing the performance test results of Example 1 with those of Comparative Examples 4-5, it can be found that the initial efficiency, capacity retention rate of 2C / 2C cycle 500 cycles at room temperature, and constant current charging ratio of 6C rate charging of the batteries prepared in Comparative Examples 4-5 are significantly lower than those in Example 1, and the maximum temperature of 6C rate charging is significantly higher than that in Example 1. The reason is that Comparative Example 4 only uses modified single-walled carbon nanotubes as a conductive agent, and Comparative Example 5 only uses SP as a conductive agent, which is not conducive to forming a complete multi-dimensional conductive network, reducing the electronic conductivity and lithium ion transmission efficiency in the silicon-based negative electrode sheet, and deteriorating the cycle characteristics and rate performance of the battery.

[0076] The performance test results of Example 1 are compared with those of Examples 2 to 3. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, the molar ratio of carboxyl group to sulfonic acid group in the modified single-walled carbon nanotubes used in Examples 2 and 3 is lower than 2.5:1 and higher than 7.5:1, respectively. The first efficiency, capacity retention rate of 2C / 2C cycle 500 cycles at room temperature, and constant current charging ratio of 6C rate charging of the battery prepared therefrom are lower than those of Example 1, and the maximum temperature of 6C rate charging is higher than that of Example 1. This shows that, relative to Examples 2 to 3, Example 1 can make the carboxyl group and the sulfonic acid group fully play a synergistic role by reasonably setting the molar ratio of carboxyl group to sulfonic acid group in the modified SWCNT, promote full contact and firm bonding between the modified SWCNT and the silicon-based material, improve the conductivity of the silicon-based negative electrode sheet, and alleviate the volume change of the silicon-based material, thereby further improving the rate performance and cycle stability of the silicon-based negative electrode sheet.

[0077] The performance test results of Example 1 are compared with those of Examples 4 to 7. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, the aspect ratio of the modified single-walled carbon nanotubes in Examples 4 and 5 is greater than 1000:1 and less than 800:1, respectively, and the ratio of the length of the modified single-walled carbon nanotubes to the particle size of the conductive carbon black in Examples 6 and 7 is greater than 800:1 and less than 100:1, respectively, and the first efficiency, capacity retention rate of 500 cycles of 2C / 2C cycles at room temperature, and constant current charging ratio of 6C rate charging of the battery prepared therefrom are lower than those of Example 1, and the maximum temperature of 6C rate charging is higher than that of Example 1. This shows that, compared with Examples 4 to 7, Example 1 is beneficial to fully disperse SP and modified SWCNT by reasonably setting the ratio of the length of the modified single-walled carbon nanotube to the particle size of the conductive carbon black and the aspect ratio of the modified single-walled carbon nanotube, and effectively reduces the risk of entanglement and agglomeration of the components of the composite conductive agent. As a result, SP and modified SWCNT are coordinated to form a complete multi-dimensional conductive network, increase the contact area between the multi-dimensional conductive network and the silicon-based material, promote the rapid transmission of electrons in the silicon-based material, improve the conductivity of the silicon-based negative electrode sheet, and make the rate performance and cycle characteristics of the prepared battery better.

[0078] The performance test results of Example 1 are compared with those of Examples 8 to 9. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, the mass ratio of SP to modified single-walled carbon nanotubes in Examples 8 and 9 is lower than 10:1 and higher than 20:1, respectively, and the first efficiency, capacity retention rate of 2C / 2C cycle 500 cycles at room temperature, and constant current charging ratio of 6C rate charging of the battery prepared thereby are lower than those of Example 1, and the maximum temperature of 6C rate charging is higher than that of Example 1. This shows that, relative to Examples 8 to 9, Example 1 is conducive to fully dispersing SP and modified SWCNT by reasonably setting the ratio of SP to modified SWCNT, thereby making SP and modified SWCNT cooperate to form a perfect multi-dimensional conductive network, increase the contact area between the multi-dimensional conductive network and the silicon-based material, promote the rapid transmission of electrons in the silicon-based material, improve the conductivity of the silicon-based negative electrode sheet, and make the rate performance and cycle characteristics of the prepared battery better.

[0079] The performance test results of Example 1 and Example 10 are compared. As shown in Table 1, under the same conditions of other materials and operations for preparing the battery, sulfuric acid is used as the sulfonating agent to prepare the modified single-walled carbon nanotubes in Example 10, and the first efficiency, capacity retention rate of 2C / 2C cycle 500 cycles at room temperature, and constant current charging ratio of 6C rate charging of the battery prepared thereby are lower than those of Example 1, and the maximum temperature of 6C rate charging is higher than that of Example 1. This shows that, relative to Example 10, Example 1 uses sulfur trioxide to sulfonate the carboxylated single-walled carbon nanotubes, which is conducive to uniformly and firmly grafting the sulfonic acid groups to the carboxylated single-walled carbon nanotubes, so that the conductivity of the modified single-walled carbon nanotubes prepared thereby is improved, and the rate performance and cycle characteristics of the prepared battery are better.

[0080] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solutions of the present invention can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present invention.

Claims

1. A composite conductive agent for silicon-based materials, characterized in that: The invention comprises conductive carbon black and modified single-walled carbon nanotubes; the modified single-walled carbon nanotubes contain modified groups, and the modified groups include carboxyl groups and sulfonic acid groups.

2. The composite conductive agent for silicon-based materials according to claim 1, characterized in that: The molar ratio of the carboxyl group to the sulfonic acid group is (2.5-7.5):

1.

3. The composite conductive agent for silicon-based materials according to claim 2, characterized in that: The total content of the carboxyl group and the sulfonic acid group in the modified single-walled carbon nanotube is 3 wt % to 4.5 wt %.

4. The composite conductive agent for silicon-based materials according to claim 1, characterized in that: The ratio of the length of the modified single-walled carbon nanotube to the particle size of the conductive carbon black is (100-800):1, and the aspect ratio of the modified single-walled carbon nanotube is (800-1000):

1.

5. The composite conductive agent for silicon-based materials according to claim 4, characterized in that: The mass ratio of the conductive carbon black to the modified single-walled carbon nanotube is (10-20):

1.

6. The composite conductive agent for silicon-based materials according to any one of claims 1 to 5, characterized in that: The preparation method of the modified single-walled carbon nanotubes comprises the following operations: dispersing carboxylated single-walled carbon nanotubes in a solvent to obtain a dispersion; then adding sulfur trioxide to the dispersion for sulfonation treatment to prepare the modified single-walled carbon nanotubes, wherein the temperature of the sulfonation treatment is 60 to 80°C.

7. A silicon-based negative electrode sheet, characterized in that: The invention comprises a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material and the composite conductive agent according to any one of claims 1 to 6, and the negative electrode active material comprises a silicon-based material.

8. The silicon-based negative electrode sheet according to claim 7, characterized in that: The mass of the composite conductive agent accounts for 1.5-3% of the total mass of the negative electrode active material layer.

9. The silicon-based negative electrode sheet according to claim 7, characterized in that: The method for preparing the silicon-based negative electrode sheet comprises the following operations: The silicon-based material, the composite conductive agent and the binder are mixed to prepare a negative electrode slurry, and the negative electrode slurry is coated on a negative electrode collector and baked to prepare the silicon-based negative electrode sheet, wherein the baking temperature is 80 to 120°C.

10. A lithium battery, characterized in that: It comprises the silicon-based negative electrode sheet as described in any one of claims 7 to 9.

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