A three-dimensional conductive binder, a negative electrode sheet, a secondary battery and a preparation method thereof

Abstract

The present invention provides a three-dimensional conductive binder, a negative electrode sheet, a secondary battery and a preparation method thereof, relating to the technical field of secondary batteries. The three-dimensional conductive binder comprises a polymer and a carbon-based conductive agent; the polymer comprises a polyester amide; the polyester amide has a three-dimensional bonding network; the polyester amide is prepared by vacuum in-situ polymerization of polyamic acid and polyol; the number of hydroxyl groups in the polyol is 3-6; the number of carbon atoms between the two terminal hydroxyl groups of the polyol is 3-8; the carbon-based conductive agent is grafted onto the polyester amide through a chemical bond. The three-dimensional conductive binder provided by the present invention restrains the volume effect during the lithium intercalation process of the silicon-based material through actions such as hydrogen bonding, thereby effectively inhibiting the expansion of the silicon-based negative electrode material and improving the cycle performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a three-dimensional conductive binder, a negative electrode sheet, a secondary battery and a preparation method thereof. Background Art

[0002] Due to its high energy density, long life and good environmental performance, lithium-ion batteries have become an indispensable energy storage method in mobile electronic devices, electric vehicles and energy storage systems. However, the performance of lithium-ion batteries is affected by the battery negative electrode material. Conventional graphite negative electrode materials have approached their theoretical capacity limit. Therefore, researchers are exploring high-capacity silicon-based materials as the negative electrode materials for lithium-ion batteries.

[0003] During the use of existing silicon-based negative electrode materials, due to the expansion and contraction of silicon materials during lithium insertion and extraction, the active materials will be detached from the current collector, resulting in a decrease in conductivity and a deterioration of cycling performance. Existing silicon-based negative electrode materials usually use the SBR (styrene-butadiene rubber) + CMC (carboxymethyl cellulose) system or the PAA (polyacrylic acid) system as the conductive binder. The SBR + CMC system is a point bonding, suitable for low-capacity silicon negative electrode materials. The PAA system is a linear bonding, suitable for high-capacity silicon negative electrode materials. The effect of these binders on suppressing the expansion of silicon materials is limited. Therefore, existing silicon-based negative electrode materials still face many technical and performance challenges in practical applications.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a three-dimensional conductive binder, a negative electrode sheet, a secondary battery and a preparation method thereof. The polymer in the three-dimensional conductive binder can form a three-dimensional bonding network, graft carbon black onto the polymer structure, which not only has a bonding function but also has conductivity, and restrains the volume effect during the lithium insertion process of the silicon-based material through hydrogen bonding and other interactions, thereby suppressing the expansion of the silicon-based negative electrode material. After the irreversible expansion of the material, the broken hydrogen bonds can self-heal to a certain extent, thereby enhancing the force between the active materials and between the active materials and the current collector, and improving the cycling performance.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] The present invention provides a three-dimensional conductive binder, and the three-dimensional conductive binder includes a polymer and a carbon-based conductive agent;

[0008] The polymer includes polyester amide;

[0009] The polyester amide is prepared by vacuum in-situ polymerization of polyamic acid and polyol;

[0010] The number of hydroxyl groups in the polyol is 3 - 6; the number of carbon atoms between the hydroxyl groups at both ends of the polyol is 3 - 8;

[0011] The polyester amide has a three-dimensional bonding network;

[0012] The carbon-based conductive agent is grafted onto the polyester amide through chemical bonds.

[0013] Further, on the basis of the above technical solution, the carbon-based conductive agent is grafted onto the polyester amide through an ester bond or an amide bond.

[0014] The present invention also provides a preparation method of a three-dimensional conductive binder as described above, comprising the following steps:

[0015] S1: Add aromatic diamine and aromatic dianhydride into a solvent and stir for polymerization to obtain a polyamic acid solution;

[0016] S2: Add the polyol into the polyamic acid solution and carry out vacuum in-situ polymerization to obtain a polymer solution with a three-dimensional bonding network;

[0017] Wherein, the mass ratio of the polyol to the polyamic acid solution is 0.3 - 0.5:1;

[0018] S3: Add the carbon-based conductive agent and a catalyst into the polymer solution and react to obtain the three-dimensional conductive binder.

[0019] Further, on the basis of the above technical solution, in step S1, the molar ratio of the aromatic diamine to the aromatic dianhydride is (0.9~0.95):(1~1.2);

[0020] And / or, in step S1, the solid content of the polyamic acid solution is 10~30wt%;

[0021] And / or, the aromatic diamine includes at least one of p-phenylenediamine, 4,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxyphenyl)]hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 1,3,5-tris(4-aminophenoxy)benzene;

[0022] And / or, in step S1, the aromatic dianhydride includes at least one of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenylsulfone tetracarboxylic dianhydride, 1,2,4,5-pyromellitic dianhydride, 3-bromopyromellitic dianhydride, 3,6-bis(trifluoromethyl)-pyromellitic dianhydride, 3,6-bis(methoxy)-pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-p-terphenyltetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, benzophenone tetracarboxylic dianhydride, 1,3-bis(3,4-dicarboxybenzoyl)benzene dianhydride, 3,5-bis(3,4-dicarboxyphenyl)biphenyl dianhydride, 4,4'-m-phenylenediamine dianhydride, 4,4'-(p-phenylene)ether dianhydride, 4,4'-(4,4'-biphenoxy)dianhydride, 3,3',4,4'-dimethyl diphenylsilane tetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, binaphthalene dianhydride, cyclobutane dianhydride, and 1,4-bis(phenylmaleic anhydride)benzene;

[0023] And / or, the solvent includes one or more of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone;

[0024] And / or, in step S1, the temperature of the stirring polymerization is 25°C - 120°C, and the time is 1h - 12h.

[0025] Further, on the basis of the above technical solution, in step S2, the polyol includes one of glycerol, pentaerythritol, sorbitol, and trimethylolpropane;

[0026] And / or, in step S2, the temperature of the vacuum in-situ polymerization is 100°C - 200°C, the time is 1h - 5h, and the vacuum degree is 10 -2 ~10 -5 Pa.

[0027] Further, on the basis of the above technical solution, the mass ratio of the total mass of the aromatic diamine, the aromatic dianhydride, and the polyol to the mass of the carbon-based conductive agent is 5 - 15:1;

[0028] And / or, in step S3, the carbon-based conductive agent includes one or more of carbon black sp, acetylene black, Ketjen black, carbon nanotubes, and carbon nanofibers;

[0029] And / or, in step S3, the catalyst includes one or more of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide, and thionyl chloride;

[0030] And / or, in step S3, the reaction temperature is -5°C to 0°C, and the reaction time is 1 h - 10 h.

[0031] The present invention also provides a negative electrode sheet, which includes the three-dimensional conductive binder prepared by the preparation method of the three-dimensional conductive binder as described above or the three-dimensional conductive binder as described above.

[0032] The present invention also provides a preparation method of a negative electrode sheet, including the following steps:

[0033] Mix the silicon-carbon negative electrode material and the three-dimensional conductive binder prepared by the preparation method of the three-dimensional conductive binder as described above or the three-dimensional conductive binder as described above according to a mass ratio of (94 - 97):(3 - 6) to obtain a negative electrode active material; subject the negative electrode active material to high-speed shear stirring, add deionized water to adjust the solid content to 45 - 55%, uniformly scrape the slurry on a copper foil, and obtain the negative electrode sheet through drying, rolling, and slitting.

[0034] Further, on the basis of the above technical solution, the silicon-carbon negative electrode material includes one or more of carbon-coated nano-silicon, carbon-coated porous silicon, porous carbon CVD-deposited silicon, and carbon-coated silicon nanowires.

[0035] The present invention also provides a secondary battery, which is prepared by winding the negative electrode sheet prepared by the preparation method of the negative electrode sheet as described above or the negative electrode sheet as described above, a positive electrode sheet, and a separator into an electrode core, packaging it into a dry electrode core with an aluminum-plastic film, and performing liquid injection, formation, and aging.

[0036] A three-dimensional conductive binder, a negative electrode sheet, a secondary battery, and a preparation method thereof provided by the present invention have the following beneficial effects:

[0037] 1. The conductive binder of the present invention uses a polymer to form a three-dimensional conductive binder network. This structure can not only effectively inhibit the expansion of silicon materials but also improve the conductivity of the silicon negative electrode material. Compared with the existing SBR + CMC system and PAA system, the conductive binder of the present invention has a better effect on inhibiting the expansion of silicon materials.

[0038] 2. The three-dimensional conductive binder provided by the present invention restricts the volume effect during the lithium intercalation process of the silicon-based material through hydrogen bond interaction, thereby effectively inhibiting the expansion of the silicon-based negative electrode material and improving the cycle performance of the battery. In contrast, the existing binders have a weaker binding effect on the silicon-based material.

[0039] 3. After the volume of the material expands, the broken hydrogen bonds of the conductive binder of the present invention can self-heal to a certain extent. This self-healing ability can enhance the force between the active materials and between the active material and the current collector, thereby further improving the cycle performance of the battery.

[0040] 4. The polymer can be adjusted by the ratio of polymer monomers to adapt to different silicon-based anode materials and battery design requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0042] Figure 1 Schematic diagram of the three-dimensional conductive binder acting on the anode material provided by the present invention;

[0043] Figure 2 Chemical reaction formula of the polyamic acid solution provided in Example 1 of the present invention;

[0044] Figure 3 Schematic diagram of the formation mechanism of the polymer solution provided in Example 1 of the present invention;

[0045] Figure 4 Schematic diagram of the polymer fragment with a three-dimensional bonding network provided in Example 1 of the present invention;

[0046] Figure 5 3D structure diagram of the polymer fragment with a three-dimensional bonding network provided in Example 1 of the present invention;

[0047] Description of the reference numerals:

[0048] 1. Silicon anode active material; 2. Three-dimensional conductive binder. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] To make the purpose, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in combination with the embodiments of the present invention. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention. The process parameters not specified in the following embodiments are usually in accordance with conventional conditions.

[0050] The endpoints and any values within the ranges disclosed in the present invention are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0051] According to the first aspect of the present invention, there is provided a three-dimensional conductive binder, and the three-dimensional conductive binder includes a polymer and a carbon-based conductive agent;

[0052] The polymer includes polyester amide;

[0053] The polyester amide has a three-dimensional bonding network;

[0054] The polyester amide is prepared by in-situ vacuum polymerization of polyamic acid and polyol;

[0055] The number of hydroxyl groups in the polyol is 3 - 6 (such as 4, 5, etc.); the number of carbon atoms between the two terminal hydroxyl groups of the polyol is 3 - 8 (such as 4, 5, 6, 7, etc.);

[0056] The carbon-based conductive agent is grafted onto the polyester amide through chemical bonds.

[0057] Specifically, as Figure 1 shown, the three-dimensional network structure formed by the three-dimensional conductive binder 2 provided by the present invention wraps around the surface of the silicon anode active material 1, and can resist the stress caused by the huge volume expansion during lithium insertion of the active material.

[0058] Specifically, the polyamic acid is formed by the reaction of aromatic diamine and aromatic dianhydride.

[0059] Specifically, the purpose of adding polyol in this application is to connect the chain-like reactants in the polyamic acid solution to each other through the esterification reaction between the polyol and the carboxyl group on the tetravalent aromatic group of the polyamic acid, forming a stable three-dimensional network polymer. The crosslinking points in the three-dimensional network polymer are dense and evenly distributed, and the crosslinking points limit the movement of the chain segments, making the overall structure of the polymer tend to be rigid, and can effectively inhibit the expansion effect of the silicon anode.

[0060] Furthermore, the present invention defines the number of hydroxyl groups in the polyol to be 3-6 because crosslinking reactions generally require at least three functional groups. Monomers with two functional groups can only form linear polymers or cyclic structures, while three or more are required to form crosslinks. Therefore, alcohols with only one hydroxyl group such as methanol cannot undergo crosslinking and can only be used as capping agents to terminate chain growth; diols (such as 1,4-butanediol, etc.) can be used as chain extenders to connect two PAA chains, but it cannot be guaranteed that a three-dimensional network structure can be formed. On the contrary, if there are too many hydroxyl groups (more than 6) in the crosslinking reaction, the unreacted hydroxyl groups may absorb water and cause swelling (as water molecules continuously enter, the distance between the molecular chains of the substance is stretched, the molecular chains stretch and move, resulting in an increase in the volume of the substance, and this phenomenon is called the water absorption swelling of hydroxyl groups). The water absorption swelling of hydroxyl groups will change the internal structure of the adhesive. The excessive stretching and movement of the molecular chains may cause the physical structure of the adhesive to become loose, reducing its own cohesive strength. When the strength of the adhesive itself decreases, it is more likely to be damaged when subjected to external forces, resulting in a decrease in adhesion, which further affects the swelling inhibition effect of the three-dimensional conductive adhesive on the silicon negative electrode.

[0061] Furthermore, the present invention also defines the number of carbon atoms between the two terminal hydroxyl groups of the polyol to be 3-8 because if the number of carbon atoms between the two terminal hydroxyl groups of the polyol is less than 3, it will lead to a large steric hindrance effect and difficult reaction. On the contrary, if the number of carbon atoms between the two terminal hydroxyl groups of the polyol is greater than 8, the crosslinking point spacing will increase significantly due to the too long carbon chain, and the equivalent crosslinking point density will decrease, making the three-dimensional bonding network sparse and the strength decrease.

[0062] And the present invention defines the number of hydroxyl groups in the polyol to be 3-6 and the number of carbon atoms between the two terminal hydroxyl groups to be 3-8, which can not only avoid the generation of more unreacted hydroxyl groups due to the steric hindrance effect on the same molecule during the reaction, but also avoid the problem that due to the too long carbon chain, the steric hindrance effect is large, resulting in a significant increase in the crosslinking point spacing, a decrease in the equivalent crosslinking point density, a sparse three-dimensional bonding network, and a decrease in strength.

[0063] As an alternative embodiment of the present invention, the carbon-based conductive agent is grafted onto the poly(ester amide) through an ester bond or an amide bond.

[0064] Specifically, the oxygen-containing functional groups on the surface of the carbon-based conductive agent, such as carboxyl groups, can react with the hydroxyl groups and amine groups in the poly(ester amide) to form ester bonds or amide bonds for connection.

[0065] According to the second aspect of the present invention, a method for preparing a three-dimensional conductive adhesive is provided, including the following steps:

[0066] S1: Add aromatic diamine and aromatic dianhydride to a solvent and stir for polymerization to obtain a polyamic acid solution;

[0067] S2: Add the polyol to the polyamic acid solution and conduct in-situ polymerization under vacuum to obtain a polymer solution with a three-dimensional bonding network.

[0068] Wherein, the mass ratio of the polyol to the polyamic acid solution is 0.3 - 0.5:1 (such as 0.35:1, 0.4:1, 0.45:1, 0.47:1, etc.).

[0069] S3: Add the carbon-based conductive agent and catalyst to the polymer solution and react to obtain the three-dimensional conductive binder.

[0070] Specifically, the three-dimensional conductive binder provided by the present invention has carboxyl groups on its surface, which can form hydrogen bonds with the hydroxyl groups on the surface of the active material, and restrain the volume effect during the lithium intercalation process of the silicon-based material through hydrogen bond interaction, thereby suppressing the expansion of the silicon-based anode material to a certain extent.

[0071] Furthermore, the carboxyl groups of the polymer are prone to form hydrogen bond interactions with the active substance and the surface of the current collector, enhancing the adhesion between the materials and between the material and the foil; at the same time, the three-dimensional network structure polymer firmly wraps the surface of the active material and can resist the stress caused by the huge volume expansion of the active substance during lithium intercalation.

[0072] Furthermore, if the three-dimensional conductive binder provided by the present invention undergoes volume expansion of the active anode material, the hydrogen bonds therein will break. Due to the dynamic reversibility of hydrogen bonds, a large number of high-energy unpaired hydrogen bonds will be formed after breaking. Through close contact, the mobility of the polymer chains enables the hydrogen bond interactions between the molecular chains to re-form, thereby reconnecting the broken surfaces and realizing the self-healing function, further alleviating the volume expansion of the active anode material. This self-healing ability can enhance the force between the active materials and between the active material and the current collector, thereby further improving the cycle performance of the battery.

[0073] Furthermore, the present invention defines the mass ratio of the polyol to the polyamic acid solution as 0.3 - 0.5:1, aiming to provide a suitable three-dimensional network structure, so that the three-dimensional network polymer can play a role in optimizing the battery performance when applied to the active anode material; if the content of the polyol is too small, insufficient cross-linking points will be provided, resulting in insufficient cross-linking density of the polymer network structure, affecting the strength and stability of the three-dimensional network polymer and making the polymer network not tight enough; if the content of the polyol is too large, it will lead to over-crosslinking, making the polymer network too dense, reducing the penetration space of the electrolyte, affecting the ion transport efficiency of the battery, and further reducing the battery performance.

[0074] As an alternative embodiment of the present invention, in step S1, the molar ratio of the aromatic diamine to the aromatic dianhydride is (0.9 to 0.95):(1 to 1.2), such as 0.91:1.1, 0.92:1.15, 0.93:1.16, 0.94:1.17, 0.91:1.18, etc.

[0075] Specifically, the present invention defines the molar ratio of the aromatic diamine to the aromatic dianhydride as (0.9 to 0.95):(1 to 1.2), aiming to enable the chain polymers in the polyamic acid solution to exhibit good adhesiveness and conductivity; if the content of the aromatic diamine is too high or the content of the aromatic dianhydride is too low, the carboxyl group content of the polymer will decrease. The carboxyl group is the key functional group for forming adhesiveness, and the decrease in its content will directly affect the adhesion performance of the polymer, resulting in a decrease in adhesiveness; and by adjusting the proportion of the aromatic dianhydride monomer, the carboxyl group content of the polymer can be controlled to adapt to the capacity and expansion characteristics of the silicon negative electrode material.

[0076] As an alternative embodiment of the present invention, in step S1, the solid content of the polyamic acid solution is 10 to 30 wt%, such as 15 wt%, 20 wt%, 25 wt%, 27 wt%, etc.

[0077] Specifically, the present invention defines the solid content of the polyamic acid solution as 10 to 30 wt%, which can not only ensure the rate of the polymerization reaction but also avoid the precipitation of reaction substances, thereby optimizing the entire vacuum polymerization process; if the solid content is too low, the concentration of the reactants will be insufficient, resulting in a decrease in the collision frequency of the chain reactants of the polyol and the polyamic acid during the vacuum polymerization process, thus slowing down the reaction rate of step S2; if the solid content is too high, it is easy to cause the precipitation of reaction substances, affecting the uniformity of the polymerization reaction.

[0078] As an alternative embodiment of the present invention, in step S1, the aromatic diamine includes at least one of p-phenylenediamine, 4,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxyphenyl)]hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 1,3,5-tris(4-aminophenoxy)benzene.

[0079] As an optional embodiment of the present invention, in step S1, the aromatic dianhydride includes at least one of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenylsulfone tetracarboxylic dianhydride, 1,2,4,5-pyromellitic dianhydride, 3-bromopyromellitic dianhydride, 3,6-bis(trifluoromethyl)-pyromellitic dianhydride, 3,6-bis(methoxy)-pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-p-terphenyltetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, benzophenone tetracarboxylic dianhydride, 1,3-bis(3,4-dicarboxybenzoyl)benzene dianhydride, 3,5-bis(3,4-dicarboxyphenyl)biphenyl dianhydride, 4,4'-m-phenylenediamine dianhydride, 4,4'-(p-phenylene)ether dianhydride, 4,4'-(4,4'-biphenoxy)dianhydride, 3,3',4,4'-dimethyl diphenylsilane tetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, binaphthalene dianhydride, cyclobutane dianhydride, and 1,4-bis(phenylmaleic anhydride)benzene.

[0080] As an optional embodiment of the present invention, in step S1, the solvent includes one or more of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0081] As an optional embodiment of the present invention, in step S1, the temperature of the stirring polymerization is 25°C - 120°C (such as 50°C, 70°C, 100°C, 110°C, 115°C, etc.), and the time is 1 h - 12 h (such as 2 h, 4 h, 6 h, 8 h, 10 h, etc.).

[0082] As an optional embodiment of the present invention, the polyol includes one of glycerol, pentaerythritol, sorbitol, and trimethylolpropane.

[0083] As an optional embodiment of the present invention, in step S2, the temperature of the vacuum polymerization is 100°C - 200°C (such as 120°C, 140°C, 160°C, 180°C, etc.), the time is 1 h - 5 h (such as 2 h, 3 h, 4 h, etc.), and the vacuum degree is 10 -2 ~10 - 5 Pa (such as 10 -3 Pa, 10 -4 Pa, etc.).

[0084] As an optional embodiment of the present invention, in step S3, the carbon-based conductive agent includes one or more of carbon black sp, acetylene black, Ketjen black, carbon nanotubes, and carbon nanofibers.

[0085] Specifically, in step S3 of the present invention, a conductive agent is added to the binder. Compared with the prior art where the conductive agent is added to the active slurry, the single-functional binder can be changed into a multi-functional conductive binder, and the conductive agent can be reduced or even replaced in the subsequent pulping process, reducing the proportion of auxiliary materials in the whole system, and correspondingly improving the energy density and capacity performance of the system.

[0086] As an optional implementation manner of the present invention, the mass ratio of the total mass of the aromatic diamine, the aromatic dianhydride and the polyol to the mass of the carbon-based conductive agent is 5-15:1 (such as 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, etc.). Under this mass ratio condition, the three-dimensional conductive binder can exhibit good adhesiveness and conductivity.

[0087] As an optional implementation manner of the present invention, in step S3, the catalyst includes one or more of EDS (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride), NHS (N-hydroxysuccinimide), and SOCL2 (thionyl chloride).

[0088] As an optional implementation manner of the present invention, in step S3, the reaction temperature is -5°C to 0°C (such as -4°C, -3°C, -2°C, -1°C, etc.), and the reaction time is 1h - 10h (such as 2h, 4h, 6h, 8h, etc.).

[0089] According to the third aspect of the present invention, a negative electrode sheet is provided, and the negative electrode sheet includes the three-dimensional conductive binder prepared by the preparation method of the three-dimensional conductive binder as described above or the three-dimensional conductive binder as described above.

[0090] According to the fourth aspect of the present invention, a preparation method of the negative electrode sheet as described above is provided, including the following steps:

[0091] Mix the silicon-carbon negative electrode material and the three-dimensional conductive binder prepared by the preparation method as described above or the three-dimensional conductive binder as described above according to a mass ratio of (94-97):(3-6) (such as 95:5, 96:4, etc.) to obtain the negative electrode active material; perform high-speed shear stirring on the negative electrode active material, add deionized water to adjust the solid content to 45-55%, evenly scrape the slurry on the copper foil, and obtain the negative electrode sheet through drying, rolling, and slitting.

[0092] Specifically, the present invention defines the mass ratio of the silicon-carbon negative electrode material to the three-dimensional conductive binder as (94-97):(3-6). This is to ensure that while the silicon-carbon negative electrode material can provide high energy density, the binder provides sufficient adhesion force to effectively inhibit the expansion of the electrode material, thus taking into account both the energy density and comprehensive performance of the battery; the content of the silicon-carbon negative electrode material directly affects the energy density of the battery. If the content of the silicon-carbon negative electrode material is too low, it will lead to a decrease in the energy density of the battery and an inability to provide sufficient power and battery life; the content of the binder is related to the adhesiveness of the material and the stability of the battery structure. If the content of the binder is too low, the adhesion effect will be reduced, making the electrode material prone to detachment during the charge and discharge process of the battery, affecting the cycle performance and expansion control of the battery.

[0093] As an alternative embodiment of the present invention, the silicon-carbon negative electrode material includes one or more of carbon-coated nano-silicon, carbon-coated porous silicon, porous carbon CVD-deposited silicon, and carbon-coated silicon nanowires.

[0094] According to the fifth aspect of the present invention, a secondary battery is provided. The secondary battery is prepared by winding the negative electrode sheet prepared by the method for preparing a negative electrode sheet as described above, or the negative electrode sheet, positive electrode sheet, and separator as described above into an electrode core, encapsulating it into a dry electrode core with an aluminum-plastic film, and then performing liquid injection, formation, and aging.

[0095] The present invention will be further described in detail below with reference to specific examples and comparative examples.

[0096] Example 1

[0097] Preparation of three-dimensional conductive binder:

[0098] S1: Weigh 20 g of 4,4'-diaminodiphenyl ether and 21.8 g of pyromellitic dianhydride, add them to 200 mL of N-methylpyrrolidone, and stir at 25 °C for 2 hours to obtain 240 g of polyamic acid solution;

[0099] As Figure 2 shown, it is the reaction mechanism diagram of 4,4'-diaminodiphenyl ether and pyromellitic dianhydride to form polyamic acid;

[0100] S2: Add 72 g of glycerol to the polyamic acid solution, with a vacuum degree of 10 -2 Pa, and at 150 °C, as Figure 3 shown, carry out a vacuum polymerization reaction for 2 h to obtain a polymer solution with a three-dimensional bonding network;

[0101] Among them, the mass ratio of glycerol to the polyamic acid solution is 0.3:1;

[0102] As Figure 4As shown, it is a schematic diagram of a fragment of a polymer with a three-dimensional bonding network prepared in this embodiment;

[0103] As Figure 5 shown, it is a 3D structural schematic diagram of a fragment of a polymer with a three-dimensional bonding network prepared in this embodiment. The polymer chains of polyesters and amides can extend in multiple directions. Combining with the rotation and torsion of chemical bonds, an ideal three-dimensional network structure is finally formed;

[0104] S3: Weigh 10 g of conductive carbon black and 0.005 g of EDC and add them to the above polymer solution. Mix evenly, stir at 0 °C for 5 h, pour the obtained solution into a large amount of ethanol to precipitate, and then centrifuge the precipitate and dry it in vacuum at 80 °C for 12 hours to obtain a three-dimensional conductive binder.

[0105] As Figure 1 shown, the three-dimensional conductive binder prepared in this embodiment forms a three-dimensional conductive bonding network. This structure can not only effectively inhibit the expansion of silicon materials, but also improve the conductivity of silicon negative electrode materials.

[0106] Preparation of negative electrode sheet:

[0107] Mix the silicon-carbon negative electrode material and the three-dimensional conductive binder prepared by the above preparation method or the three-dimensional conductive binder as described above in a mass ratio of 95:5 to obtain the negative electrode active material; subject the negative electrode active material to high-speed shear stirring, add deionized water to adjust the solid content to 45 - 55%, uniformly scrape the slurry on the copper foil, and obtain the negative electrode sheet after drying, rolling, and slitting. The surface density of the negative electrode sheet is 184 g / m 2 , and the thickness is 124 μm.

[0108] Preparation of positive electrode sheet:

[0109] The components of the positive electrode material are mixed in a mass ratio of ternary material NCM811: conductive agent SP + CNT: binder PVDF of 96:2:2, and then subjected to high-speed shear stirring. Add NMP to adjust the solid content to 70 - 75%, uniformly scrape the slurry on the aluminum foil, and obtain the positive electrode sheet to be processed after drying, rolling, and slitting. The surface density of the positive electrode sheet is 534 g / m 2 , and the thickness is 167 μm.

[0110] Preparation of secondary battery: Wind the prepared positive and negative electrode sheets and the separator into an electrode core, and encapsulate it into a dry electrode core with an aluminum-plastic film. After processes such as injection of electrolyte (the electrolyte is a solution with a concentration of 1 moI / L obtained by dissolving LiPF6 in ethylene carbonate (EC) / dimethyl carbonate (DMC) with a mass ratio of 1:1), formation, and aging, a secondary battery is prepared.

[0111] Example 2

[0112] The difference between this embodiment and Embodiment 1 lies in that in the negative electrode active material, the mass ratio of the silicon-carbon negative electrode material to the three-dimensional conductive binder prepared in Embodiment 1 is 97:3, and the remaining steps and technical parameters are the same as those in Embodiment 1.

[0113] Embodiment 3

[0114] The difference between this embodiment and Embodiment 1 lies in that in the negative electrode active material, the mass ratio of the silicon-carbon negative electrode material to the three-dimensional conductive binder prepared in Embodiment 1 is 94:6, and the remaining steps and technical parameters are the same as those in Embodiment 1.

[0115] Embodiment 4

[0116] The difference between this embodiment and Embodiment 1 lies in the preparation of the three-dimensional conductive binder:

[0117] S1: Weigh 10.8 g of p-phenylenediamine and 29.5 g of 1,4,5,8-naphthalenetetracarboxylic dianhydride, add them to 200 mL of N-methylpyrrolidone, and stir at 25 °C for 2 hours to obtain a polyamic acid solution;

[0118] The remaining steps and technical parameters are the same as those in Embodiment 1.

[0119] Embodiment 5

[0120] The difference between this embodiment and Embodiment 1 lies in the preparation of the three-dimensional conductive binder:

[0121] S1: Weigh 29.2 g of 1,4-bis(4-aminophenoxy)benzene and 29.0 g of biphenyltetracarboxylic dianhydride, add them to 200 mL of N-methylpyrrolidone, and stir at 25 °C for 2 hours to obtain a polyamic acid solution;

[0122] The remaining steps and technical parameters are the same as those in Embodiment 1.

[0123] Comparative Example 1

[0124] The main difference between this comparative example and Embodiment 1 lies in that in the negative electrode active material, the mass ratio of the silicon-carbon negative electrode material: conductive agent sp: binder (PAA+SBR) is 95:2:3, and the remaining steps and technical parameters are the same as those in Embodiment 1.

[0125] Comparative Example 2

[0126] The main difference between this comparative example and Embodiment 1 lies in that in the negative electrode active material, the mass ratio of the silicon-carbon negative electrode material: conductive agent sp: binder (PAA+SBR) is 97:1:2, and the remaining steps and technical parameters are the same as those in Embodiment 1.

[0127] Comparative Example 3

[0128] The main difference between this comparative example and Example 1 lies in that in the negative electrode active material, the mass ratio of the silicon-carbon negative electrode material: conductive agent sp: binder (PAA+SBR) is 94:3:3, and the remaining steps and technical parameters are the same as those in Example 1.

[0129] Comparative Example 4

[0130] The main difference between this comparative example and Example 1 lies in that in the negative electrode active material, the mass ratio of the silicon-carbon negative electrode material: conductive agent sp: binder (CMC+SBR) is 95:2:3, and the remaining steps and technical parameters are the same as those in Example 1.

[0131] Comparative Example 5

[0132] The main difference between this comparative example and Example 1 lies in that the mass ratio of the polyol to the polyamic acid solution is 0.1:1, and the remaining steps and technical parameters are the same as those in Example 1.

[0133] Comparative Example 6

[0134] The main difference between this comparative example and Example 1 lies in that the mass ratio of the polyol to the polyamic acid solution is 0.8:1, and the remaining steps and technical parameters are the same as those in Example 1.

[0135] Comparative Example 7

[0136] The main difference between this comparative example and Example 1 is that instead of adding conductive carbon black in step S3, the conductive carbon black is added to the active slurry of the negative electrode sheet;

[0137] The preparation of the three-dimensional conductive binder in this comparative example includes the following steps:

[0138] S1: Weigh 20 g of 4,4'-diaminodiphenyl ether and 21.8 g of pyromellitic dianhydride and add them to 200 mL of N-methylpyrrolidone, stir at 25 °C for 2 hours to obtain 240 g of polyamic acid solution;

[0139] S2: Add 72 g of glycerol to the polyamic acid solution, with a vacuum degree of 10 -2 Pa, vacuum polymerization at 150 °C for 2 h to obtain a polymer solution with a three-dimensional bonding network;

[0140] Among them, the mass ratio of glycerol to the polyamic acid solution is 0.3:1;

[0141] S3: Weigh 0.005 g of EDC and add it to the above polymer solution, mix evenly, stir at 0 °C for 5 h, pour the obtained solution into a large amount of ethanol to precipitate, and then centrifuge the precipitate and vacuum dry it at 80 °C for 12 hours to obtain a three-dimensional binder.

[0142] In the negative electrode active paste prepared in this comparative example, the silicon-carbon negative electrode material: the three-dimensional binder: the conductive carbon black prepared in this comparative example = 95:4:1, and the remaining steps and technical parameters are the same as those in Example 1.

[0143] Comparative Example 8

[0144] The main difference between this comparative example and Example 1 is that glycerol is not added in step S2. The preparation of the conductive binder in this comparative example includes the following steps:

[0145] S1: Weigh 20 g of 4,4'-diaminodiphenyl ether and 21.8 g of pyromellitic dianhydride and add them to 200 mL of N-methylpyrrolidone. Stir at 25 °C for 2 hours to obtain 240 g of polyamic acid solution;

[0146] S2: Weigh 10 g of conductive carbon black and 0.005 g of EDC and add them to the above polyamic acid solution. Mix evenly, stir at 0 °C for 5 h, pour the obtained solution into a large amount of ethanol to precipitate, then centrifuge the precipitate and vacuum dry it at 80 °C for 12 hours, then raise the temperature to 300 °C for high-temperature thermal curing for 30 min, and then crush it to obtain the conductive binder.

[0147] The remaining steps and technical parameters are the same as those in Example 1.

[0148] Comparative Example 9

[0149] The main difference between this comparative example and Example 1 is that the number of hydroxyl groups in the polyol is greater than 6, and 1,2,3,4,5,6,7-heptanheptol (containing 7 hydroxyl groups, and the number of carbon atoms between the two terminal hydroxyl groups is 6) is used for the reaction. The remaining steps and technical parameters are the same as those in Example 1.

[0150] Comparative Example 10

[0151] The main difference between this comparative example and Example 1 is that the number of carbon atoms between the two terminal hydroxyl groups in the polyol is greater than 8, and 1,7,13-tridecanetriol (containing 3 hydroxyl groups, and the number of carbon atoms between the two terminal hydroxyl groups is 11) is used for the reaction. The remaining steps and technical parameters are the same as those in Example 1.

[0152] Performance Test

[0153] Negative electrode sheet peel strength test

[0154] Take the electrode sheet and cut it to obtain a test sample with a length of 100 mm and a width of 10 mm. Take a stainless steel plate with a width of 25 mm, stick double-sided tape (width 11 mm), and paste the side of the test sample coated with the negative electrode material on the double-sided tape on the stainless steel plate. Use a 2000 g pressure roller to roll back and forth on its surface three times at a speed of 300 mm / min. Bend one end of the test sample by 180 degrees, manually peel the negative electrode material layer of the test sample from the current collector along the length direction by 25 mm, and then fix the test sample on an INSTRON 336 testing machine so that the peeling surface is consistent with the force line of the testing machine (that is, parallel to the moving direction when peeling with the testing machine). Continue to use the testing machine to continuously peel the test sample at a speed of 30 mm / min to obtain the peeling force curve. Take the average value at the stable section within the range of 10 - 50 mm on the curve (that is, the section where the peeling force curve no longer increases monotonically) as the peeling force F0. Then, the adhesion force F between the negative electrode material layer and the current collector in the test sample is F = F0 / the width of the test sample (the measurement unit of F: N / m).

[0155] Ionic conductivity of the negative electrode sheet

[0156] The ionic conductivity of each example and comparative example is determined by testing the electrochemical impedance spectroscopy (EIS) of the negative electrode sheet sandwiched between two fluorine-doped tin oxide (FTO) electrodes. The area of the negative electrode sheet is 1×1 cm 2 , and the corresponding ionic conductivity is calculated according to the actual thickness.

[0157] Full charge negative electrode sheet expansion rate

[0158] At 25°C, charge the secondary battery to be tested at a constant current of 1C until the charge cut-off voltage of 4.30V, then charge at a constant voltage until the current < 0.05C, let it stand for 10 min and then disassemble it. Measure the thickness of the negative electrode sheet with a micrometer and take the average value. Calculate the expansion rate according to the initial thickness of the negative electrode sheet.

[0159] Battery cycle performance test

[0160] At 25°C, charge the secondary battery to be tested at a constant current of 1C until the charge cut-off voltage of 4.30V, then charge at a constant voltage until the current < 0.05C, let it stand for 10 min, and then discharge at a constant current of 1C until the discharge cut-off voltage of 3.3V, let it stand for 10 min. This is one charge-discharge cycle (that is, one circle (cls)). Perform charge-discharge cycle tests on the battery 100 times according to this method. The percentage of the discharge capacity of the last cycle to the discharge capacity of the third cycle is the cycle capacity retention rate.

[0161] 2C discharge capacity ratio

[0162] At 25 °C, the secondary battery to be tested is charged at a constant current of 1C to the charging cut-off voltage of 4.30V, then charged at a constant voltage until the current is <0.05C, left standing for 10 min, and then discharged at a constant current of 2C to the discharge cut-off voltage of 3.3V and left standing for 10 min. The percentage of the discharge capacity to the initial capacity is the 2C discharge capacity ratio.

[0163] Effect data

[0164] Table 1

[0165]

[0166] As shown in Table 1, compared with Example 1, in Comparative Examples 1-4, since the binders used in the negative active slurries of Comparative Examples 1-4 are PAA+SBR binder or CMC+SBR binder, which are linear binder or dot binder respectively, the effect of suppressing the expansion of silicon materials is significantly inferior to that of the three-dimensional conductive binder provided in Example 1. Moreover, the binders provided in Comparative Examples 1-4 are difficult to achieve self-repair after irreversible expansion of the materials, so they cannot enhance the force between the active materials and between the active materials and the current collector, and thus cannot improve the cycle performance of the battery.

[0167] As shown in Table 1, compared with Example 1, in Comparative Example 5, since the mass ratio of polyol to the polyamic acid solution in Comparative Example 5 is 0.1:1 and the content of polyol is too small to provide enough cross-linking points, the cross-linking density of the polymer network structure is insufficient, which affects the strength and stability of the three-dimensional network polymer, and further affects the effect of suppressing the expansion of silicon materials.

[0168] As shown in Table 1, compared with Example 1, in Comparative Example 6, since the mass ratio of polyol to the polyamic acid solution in Comparative Example 6 is 0.8:1 and the content of polyol is too large, excessive cross-linking occurs, making the polymer network too dense, reducing the penetration space of the electrolyte, affecting the ion transport efficiency of the battery, and thus reducing the performance of the battery.

[0169] As shown in Table 1, compared with Example 1, when the main material and auxiliary material ratios are the same, in Comparative Example 7, since conductive carbon black is added to the negative active slurry, the binder has a single function and acts separately from the conductive agent, and the performance of each part of the battery cell deteriorates significantly. If Comparative Example 7 wants to achieve the performance indicators such as the peel strength and ionic conductivity of the negative electrode sheet in Example 1, it is necessary to correspondingly increase the ratios of auxiliary materials such as the binder and the conductive agent, which will lead to a lower content of the main material and further reduce the energy density of the system.

[0170] As shown in Table 1, compared with Example 1, in Comparative Example 8, since no polyol is added and conductive agents etc. are directly added to the polyamic acid solution, a three-dimensional network structure cannot be formed, the effect of suppressing the expansion of silicon materials is poor, and the performance of the battery is reduced.

[0171] As shown in Table 1, in Comparative Example 9, 1,2,3,4,5,6,7-heptanol (containing 7 hydroxyl groups, and the number of carbon atoms between the hydroxyl groups at both ends is 6) was used to react with polyamic acid. The excessive number of hydroxyl groups may cause swelling due to water absorption of the unreacted hydroxyl groups, which will not only affect the bonding performance of the three-dimensional conductive binder, but also reduce the strength of the three-dimensional conductive binder, thereby affecting the swelling inhibition effect of the three-dimensional conductive binder on the silicon negative electrode.

[0172] As shown in Table 1, in Comparative Example 10, 1,7,13-tridecanetriol (containing 3 hydroxyl groups, and the number of carbon atoms between the hydroxyl groups at both ends is 11) was used to react with polyamic acid. When the number of carbon atoms between the hydroxyl groups at both ends of the polyol is greater than 8, the distance between crosslinking points will increase significantly due to the too long carbon chain, and the equivalent crosslinking point density will decrease, making the three-dimensional bonding network sparse and the strength decrease, thereby affecting the swelling inhibition effect on silicon materials.

[0173] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A three-dimensional conductive binder, characterized in that, The three-dimensional conductive binder includes a polymer and a carbon-based conductive agent; The polymer includes polyester amide; The polyester amide has a three-dimensional bonding network; The polyester amide is prepared by vacuum in-situ polymerization of polyamic acid and polyol; The number of hydroxyl groups in the polyol is 3-6; the number of carbon atoms between the two terminal hydroxyl groups of the polyol is 3-8; The carbon-based conductive agent is grafted onto the polyester amide through chemical bonds; The preparation method of the three-dimensional conductive binder includes the following steps: S1: Add aromatic diamine and aromatic dianhydride into a solvent and stir for polymerization to obtain a polyamic acid solution; S2: Add the polyol into the polyamic acid solution and carry out vacuum in-situ polymerization to obtain a polymer solution with a three-dimensional bonding network; Wherein, the mass ratio of the polyol to the polyamic acid solution is 0.3-0.5:1; S3: Add the carbon-based conductive agent and a catalyst into the polymer solution and react to obtain the three-dimensional conductive binder; The catalyst includes one or more of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide, and thionyl chloride.

2. The three-dimensional conductive binder according to claim 1, wherein The carbon-based conductive agent is grafted onto the polyester amide through an ester bond or an amide bond.

3. The three-dimensional conductive binder according to claim 1, wherein In step S1, the molar ratio of the aromatic diamine to the aromatic dianhydride is (0.9~0.95):(1~1.2); And / or, in step S1, the solid content of the polyamic acid solution is 10~30wt%; And / or, the aromatic diamine includes at least one of p-phenylenediamine, 4,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)benzene]hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenyl ether, 1,3,5-tris(4-aminophenoxy)benzene; And / or, in step S1, the aromatic dianhydride includes at least one of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenylsulfone tetracarboxylic dianhydride, 1,2,4,5-pyromellitic dianhydride, 3-bromopyromellitic dianhydride, 3,6-bis(trifluoromethyl)-pyromellitic dianhydride, 3,6-bis(methoxy)-pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-p-terphenyltetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, benzophenone tetracarboxylic dianhydride, 1,3-bis(3,4-dicarboxybenzoyl)benzene dianhydride, 3,5-bis(3,4-dicarboxyphenyl)biphenyl dianhydride, 4,4'-m-phenylenediamine diphthalic anhydride, 4,4'-(p-phenylene)ether dianhydride, 4,4'-(4,4'-biphenoxy)dianhydride, 3,3',4,4'-dimethyl diphenylsilane tetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, binaphthalene dianhydride, cyclobutane dianhydride, and 1,4-bis(phenylmaleic anhydride)benzene; And / or, the solvent includes one or more of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone; And / or, in step S1, the temperature of the stirring polymerization is 25°C - 120°C, and the time is 1h - 12h.

4. The three-dimensional conductive binder according to claim 1, wherein In step S2, the polyol includes one of glycerol, pentaerythritol, sorbitol, and trimethylolpropane; And / or, in step S2, the temperature of the vacuum in-situ polymerization is 100°C - 200°C, the time is 1h - 5h, and the vacuum degree is 10 -2 ~10 -5 Pa.

5. The three-dimensional conductive binder according to claim 1, wherein The mass ratio of the total mass of the aromatic diamine, the aromatic dianhydride, and the polyol to the mass of the carbon-based conductive agent is 5 - 15:1; And / or, in step S3, the carbon-based conductive agent includes one or more of carbon black sp, acetylene black, Ketjen black, carbon nanotubes, and carbon nanofibers; And / or, in step S3, the reaction temperature is -5°C to 0°C, and the reaction time is 1h - 10h.

6. A negative electrode sheet, characterized in that, The negative electrode sheet includes the three-dimensional conductive binder as described in any one of claims 1 - 5.

7. A method for preparing a negative electrode sheet as described in claim 6, characterized in that, Comprises the following steps: Mix the silicon-carbon negative electrode material and the three-dimensional conductive binder as described in any one of claims 1 - 5 according to a mass ratio of (94 - 97):(3 - 6) to obtain the negative electrode active material; subject the negative electrode active material to high-speed shear stirring, add deionized water to adjust the solid content to 45 - 55%, uniformly scrape the slurry on the copper foil, and obtain the negative electrode sheet through drying, rolling, and slitting.

8. The method for preparing a negative electrode sheet according to claim 7, wherein The silicon-carbon negative electrode material includes one or more of carbon-coated nano-silicon, carbon-coated porous silicon, porous carbon CVD-deposited silicon, and carbon-coated silicon nanowires.

9. A secondary battery, characterized in that, The secondary battery is prepared by winding the negative electrode sheet prepared by the method for preparing a negative electrode sheet as described in any one of claims 7 - 8 or the negative electrode sheet as described in claim 6, a positive electrode sheet, and a separator into an electrode core, encapsulating it with an aluminum-plastic film to form a dry electrode core, and then performing liquid injection, formation, and aging.

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