Three-dimensional conductive binder, negative plate, secondary battery and preparation method

Abstract

The invention provides a three-dimensional conductive binder, a negative plate, a secondary battery and a preparation method, and relates to the technical field of secondary batteries, the three-dimensional conductive binder comprises a high-molecular polymer and a carbon-based conductive agent; the high-molecular polymer comprises polyesteramide; the polyesteramide has a three-dimensional bonding network; the polyesteramide is prepared from polyamide acid and polyhydric alcohol through vacuum in-situ polymerization; the number of hydroxyl groups in the polyhydric alcohol is 3 to 6; the number of carbon atoms between hydroxyl groups at two ends of the polyol is 3-8; the carbon-based conductive agent is grafted on the polyesteramide through a chemical bond. According to the three-dimensional conductive binder provided by the invention, the volume effect of the silicon-based material in the lithium intercalation process is bound through the effects of hydrogen bonds and the like, so that the expansion of the silicon-based negative electrode material is effectively inhibited, and the cycle performance of the battery is improved.

Description

Technical Field

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

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

[0003] During the use of existing silicon-based negative electrode materials, the expansion and contraction of silicon materials when lithium is inserted and extracted will cause the active materials to separate from the current collector, resulting in a decrease in conductivity and poor cycle performance. Existing silicon-based negative electrode materials usually use SBR (styrene-butadiene rubber) + CMC (carboxymethyl cellulose) system or PAA (polyacrylic acid) system as conductive binders. The SBR+CMC system is a point-like bonding, which is suitable for low-capacity silicon negative electrode materials. The PAA system is a linear bonding, which is suitable for high-capacity silicon negative electrode materials. These binders have limited effects on inhibiting the expansion of silicon materials. 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 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. The high molecular polymer in the three-dimensional conductive binder can form a three-dimensional bonding network, and the conductive carbon black is grafted into the polymer structure, which has both bonding function and conductivity, and restrains the volume effect of the silicon-based material during lithium insertion through hydrogen bonds and other effects, thereby inhibiting the expansion of the silicon-based negative electrode material. After the material undergoes irreversible expansion, the broken hydrogen bonds of the binder can self-repair to a certain extent, thereby enhancing the interaction between the active materials and between the active materials and the current collector, and improving the cycle performance.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: The present invention provides a three-dimensional conductive adhesive, which comprises a high molecular polymer and a carbon-based conductive agent; The high molecular polymer includes polyester amide; 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 end hydroxyl groups of the polyol is 3-8; The polyester amide has a three-dimensional bonding network; The carbon-based conductive agent is grafted onto the polyester amide through chemical bonds.

[0007] Furthermore, based on the above technical solution, the carbon-based conductive agent is grafted onto the polyester amide via an ester bond or an amide bond.

[0008] The present invention also provides a method for preparing the three-dimensional conductive adhesive as described above, comprising the following steps: S1: adding aromatic diamine and aromatic dianhydride into a solvent and stirring and polymerizing to obtain a polyamic acid solution; S2: adding the polyol to the polyamic acid solution, and performing vacuum in-situ polymerization to obtain a high molecular polymer solution having a three-dimensional bonding network; Wherein, the mass ratio of the polyol to the polyamic acid solution is 0.3-0.5:1; S3: adding the carbon-based conductive agent and the catalyst into the high molecular polymer solution to react and obtain the three-dimensional conductive adhesive.

[0009] 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); 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-aminophenoxybenzene)]hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 2,2'-bistrifluoromethyl-4,4'-diaminodiphenyl ether, and 1,3,5-tris(4-aminophenoxy)benzene; And / or, in step S1, the aromatic dianhydride includes pyromellitic dianhydride, biphenyl tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-bisphenyl sulfone 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'-biphenyl tetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyl tetracarboxylic dianhydride, 3,3',4,4'-terphenyl tetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyl tetracarboxylic dianhydride, At least one of (3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, benzophenone tetracarboxylic dianhydride, 1,3-bis(3,4-dicarboxybenzoyl)phthalic dianhydride, 3,5-bis(3,4-dicarboxyphenyl)biphenyl dianhydride, 4,4'-m-phenylenediamine diphenyl anhydride, 4,4'-(terephthalene) diether dianhydride, 4,4'-(4,4'-biphenyloxy) dianhydride, 3,3',4,4'-dimethyldiphenylsilane tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, binaphthene dianhydride, cyclobutane dianhydride and 1,4-bis(phenylmaleic anhydride)benzene; and / or, the solvent comprises one or more of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone; And / or, in step S1, the stirring polymerization is carried out at a temperature of 25°C-120°C and for a time of 1h-12h.

[0010] Further, on the basis of the above technical solution, in step S2, the polyol includes one of glycerol, pentaerythritol, sorbitol, and trimethylolpropane; And / or, in step S2, the vacuum in-situ polymerization temperature is 100°C-200°C, the time is 1h-5h, and the vacuum degree is 10 -2 ~10 -5 Pa.

[0011] 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 ratio 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 catalyst includes one or more of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide, and thionyl chloride; And / or, in step S3, the reaction temperature is -5°C to 0°C, and the reaction time is 1h to 10h.

[0012] The present invention also provides a negative electrode sheet, which comprises the three-dimensional conductive adhesive prepared by the preparation method of the three-dimensional conductive adhesive as described above or the three-dimensional conductive adhesive as described above.

[0013] The present invention also provides a method for preparing a negative electrode sheet, comprising the following steps: The silicon-carbon negative electrode material and the three-dimensional conductive binder obtained by the preparation method of the three-dimensional conductive binder as described above or the three-dimensional conductive binder as described above are mixed in a mass ratio of (94-97): (3-6) to obtain a negative electrode active material; the negative electrode active material is subjected to high-speed shear stirring, deionized water is added to adjust the solid content to 45-55%, the slurry is evenly scraped on a copper foil, and a negative electrode sheet is obtained after drying, rolling and slitting.

[0014] Furthermore, based on 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.

[0015] The present invention also provides a secondary battery, which is prepared by winding a negative electrode sheet obtained by the negative electrode sheet preparation method as described above, or a negative electrode sheet, a positive electrode sheet and a separator as described above into a battery core, and encapsulating it into a dry battery core with an aluminum-plastic film, and then being prepared by liquid injection, formation and aging.

[0016] The present invention provides a three-dimensional conductive adhesive, a negative electrode sheet, a secondary battery and a preparation method thereof, and the beneficial effects are as follows: 1. The conductive binder of the present invention uses a high molecular polymer to form 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. 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.

[0017] 2. The three-dimensional conductive binder provided by the present invention restrains the volume effect of silicon-based materials during lithium insertion through hydrogen bonding, thereby effectively inhibiting the expansion of silicon-based negative electrode materials and improving the cycle performance of the battery. In contrast, the binding effect of existing binders on silicon-based materials is relatively weak.

[0018] 3. After the volume expansion of the conductive adhesive of the present invention, the broken hydrogen bonds can self-repair to a certain extent. This self-repairing ability can enhance the interaction between active materials and between active materials and current collectors, thereby further improving the cycle performance of the battery.

[0019] 4. The polymer can adapt to different silicon-based negative electrode materials and battery design requirements by adjusting the ratio of polymer monomers. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 A schematic diagram of the three-dimensional conductive binder provided by the present invention acting on the negative electrode material; Figure 2 The chemical reaction formula of the polyamic acid solution provided in Example 1 of the present invention; Figure 3 A schematic diagram of the formation mechanism of the polymer solution provided in Example 1 of the present invention; Figure 4 A schematic diagram of a polymer fragment having a three-dimensional bonding network provided in Example 1 of the present invention; Figure 5 A schematic diagram of the 3D structure of a polymer fragment having a three-dimensional bonding network provided in Example 1 of the present invention; Description of reference numerals: 1. Silicon negative electrode active material; 2. Three-dimensional conductive binder. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0023] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and 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, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0024] According to a first aspect of the present invention, there is provided a three-dimensional conductive adhesive, the three-dimensional conductive adhesive comprising a high molecular polymer and a carbon-based conductive agent; The high molecular 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 (e.g., 4, 5, etc.); the number of carbon atoms between the two hydroxyl groups of the polyol is 3-8 (e.g., 4, 5, 6, 7, etc.); The carbon-based conductive agent is grafted onto the polyester amide through chemical bonds.

[0025] Specifically, Figure 1 As shown, the three-dimensional network structure formed by the three-dimensional conductive binder 2 provided by the present invention is wrapped on the surface of the silicon negative electrode active material 1, which can resist the stress caused by the huge volume expansion of the active material when lithium is inserted.

[0026] Specifically, the polyamic acid is produced by the reaction of aromatic diamine and aromatic dianhydride.

[0027] Specifically, the purpose of adding polyols in the present application is to connect the chain reactants in the polyamic acid solution to each other through an esterification reaction between the polyols and the carboxyl groups on the tetravalent aromatic groups of the polyamic acid to form a stable three-dimensional network polymer. The cross-linking points in the three-dimensional network polymer are dense and evenly distributed, and the cross-linking points limit the movement of the chain segments, making the overall structure of the polymer rigid, which can effectively inhibit the expansion effect of the silicon negative electrode.

[0028] Furthermore, the present invention limits the number of hydroxyl groups in the polyol to 3-6 because the cross-linking reaction usually requires at least three functional groups, and monomers with two functional groups can only form linear polymers or cyclic structures, while three or more can form cross-links. Therefore, alcohols such as methanol that contain only one hydroxyl group cannot be cross-linked and can only be used as end-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 there is no guarantee that a three-dimensional network structure can be formed. On the contrary, if there are too many hydroxyl groups in the cross-linking reaction (greater than 6), the unreacted hydroxyl groups may absorb water and cause swelling (as water molecules continue to enter, the distance between the molecular chains of the substance is stretched, the molecular chains stretch and move, causing the volume of the substance to increase. This phenomenon is called hydroxyl swelling). Hydroxyl swelling will change the internal structure of the adhesive. Excessive stretching and movement of the molecular chains may cause the physical structure of the adhesive to become loose, reducing its own cohesive strength. The strength of the adhesive itself decreases, and it is more likely to be damaged when subjected to external forces, resulting in a decrease in adhesion, which further affects the effect of the three-dimensional conductive adhesive in inhibiting the expansion of the silicon negative electrode.

[0029] Furthermore, the present invention also limits the number of carbon atoms between the two hydroxyl groups of the polyol to 3-8, because if the number of carbon atoms between the two 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 hydroxyl groups of the polyol is greater than 8, the distance between the crosslinking points will increase significantly due to the long carbon chain, and the density of the equivalent crosslinking points will decrease, making the three-dimensional bonding network sparse and the strength decreased.

[0030] In addition, the present invention limits the number of hydroxyl groups in the polyol to 3-6, and the number of carbon atoms between the two end hydroxyl groups to 3-8, which can not only avoid the generation of more unreacted hydroxyl groups on the same molecule due to the steric hindrance effect during the reaction, but also avoid the problem that the steric hindrance effect is large due to the carbon chain being too long, thereby significantly increasing the distance between cross-linking points, reducing the density of equivalent cross-linking points, making the three-dimensional bonding network sparse, and reducing the strength.

[0031] As an optional embodiment of the present invention, the carbon-based conductive agent is grafted onto the polyester amide via an ester bond or an amide bond.

[0032] Specifically, oxygen-containing functional groups on the surface of the carbon-based conductive agent, such as carboxyl groups, can react with hydroxyl groups and amine groups in polyester amide to form ester bonds or amide bonds.

[0033] According to a second aspect of the present invention, there is provided a method for preparing a three-dimensional conductive adhesive, comprising the following steps: S1: adding aromatic diamine and aromatic dianhydride into a solvent and stirring and polymerizing to obtain a polyamic acid solution; S2: adding the polyol to the polyamic acid solution, and performing vacuum in-situ polymerization to obtain a high molecular polymer solution having a three-dimensional bonding network; 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.); S3: adding the carbon-based conductive agent and the catalyst into the high molecular polymer solution to react and obtain the three-dimensional conductive adhesive.

[0034] Specifically, the surface of the three-dimensional conductive binder provided by the present invention contains carboxyl groups, which can form hydrogen bonds with the hydroxyl groups on the surface of the active material, and restrain the volume effect of the silicon-based material during lithium insertion through hydrogen bonding, thereby inhibiting the expansion of the silicon-based negative electrode material to a certain extent; Furthermore, the carboxyl groups of the polymer easily form hydrogen bonds with the active material and the surface of the current collector, thus enhancing the bonding between the materials and between the materials and the foil. At the same time, the three-dimensional network structure polymer is firmly wrapped on the surface of the active material, which can resist the stress caused by the huge volume expansion of the active material when lithium is inserted. Furthermore, if the hydrogen bonds in the three-dimensional conductive binder provided by the present invention are broken after the volume expansion of the active negative electrode material, a large number of unpaired hydrogen bonds in high energy states will be formed after the break due to the dynamic reversibility of the hydrogen bonds. Through close contact, the mobility of the polymer chain allows the hydrogen bonds between the molecular chains to be re-formed, thereby reconnecting the broken surfaces together, achieving a self-repairing function, and further alleviating the volume expansion of the active negative electrode material. This self-repairing ability can enhance the interaction between the active materials and between the active materials and the current collector, thereby further improving the cycle performance of the battery.

[0035] Furthermore, the present invention limits the mass ratio of the polyol and the polyamic acid solution to 0.3-0.5:1, in order to provide a suitable three-dimensional network structure so that the three-dimensional network polymer can be applied to the active negative electrode material to optimize the battery performance; if the polyol content is too little, it will not be able to provide enough cross-linking points, 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 polyol content is too much, it will lead to excessive cross-linking, making the polymer network too dense, reducing the penetration space of the electrolyte, affecting the ion transmission efficiency of the battery, and thus reducing the performance of the battery.

[0036] As an optional embodiment of the present invention, in step S1, the molar ratio of the aromatic diamine to the aromatic dianhydride is (0.9-0.95):(1-1.2), such as 0.91:1.1, 0.92:1.15, 0.93:1.16, 0.94:1.17, 0.91:1.18, etc.

[0037] Specifically, the present invention limits the molar ratio of aromatic diamine to aromatic dianhydride to (0.9-0.95): (1-1.2), in order to make the chain polymer in the polyamic acid solution exhibit good adhesion and conductivity; excessive content of aromatic diamine or too little content of aromatic dianhydride will lead to a decrease in the carboxyl content of the polymer. The carboxyl group is a key functional group for forming adhesion, and a reduction in its content will directly affect the adhesion performance of the polymer, resulting in a decrease in adhesion; and by adjusting the ratio of aromatic dianhydride monomers, the carboxyl content of the polymer can be controlled to adapt to the capacity and expansion characteristics of the silicon negative electrode material.

[0038] As an optional embodiment of the present invention, in step S1, the solid content of the polyamic acid solution is 10-30wt%, such as 15wt%, 20wt%, 25wt%, 27wt%, etc.

[0039] Specifically, the present invention limits the solid content of the polyamic acid solution to 10-30wt%, which can not only ensure the rate of the polymerization reaction, but also avoid the precipitation of the 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 frequency of collisions between the chain reactants of the polyol and the polyamic acid during the vacuum polymerization process, thereby slowing down the reaction rate of step S2; if the solid content is too high, it is easy to cause the precipitation of the reaction substances, affecting the uniformity of the polymerization reaction.

[0040] As an optional 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-aminophenoxybenzene)]hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 2,2'-bistrifluoromethyl-4,4'-diaminodiphenyl ether, and 1,3,5-tris(4-aminophenoxy)benzene.

[0041] As an optional embodiment of the present invention, in step S1, the aromatic dianhydride includes pyromellitic dianhydride, biphenyl tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-bisphenyl sulfone 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'-biphenyl tetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyl tetracarboxylic dianhydride, 3,3',4,4'-terphenyl tetracarboxylic dianhydride, At least one of 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)phthalic dianhydride, 3,5-bis(3,4-dicarboxyphenyl)biphenyl dianhydride, 4,4'-m-phenylenediamine diphenyl anhydride, 4,4'-(terephthalene) diether dianhydride, 4,4'-(4,4'-biphenyloxy) dianhydride, 3,3',4,4'-dimethyldiphenylsilane tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, binaphthene dianhydride, cyclobutane dianhydride and 1,4-bis(phenylmaleic anhydride)benzene.

[0042] 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.

[0043] As an optional embodiment of the present invention, in step S1, the stirring polymerization temperature is 25°C-120°C (such as 50°C, 70°C, 100°C, 110°C, 115°C, etc.), and the time is 1h-12h (such as 2h, 4h, 6h, 8h, 10h, etc.).

[0044] As an optional embodiment of the present invention, the polyol includes one of glycerol, pentaerythritol, sorbitol, and trimethylolpropane.

[0045] 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 1h-5h (such as 2h, 3h, 4h, etc.), and the vacuum degree is 10 -2 ~10 - 5 Pa (e.g. 10 -3 Pa, 10 -4 Pa, etc.).

[0046] 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.

[0047] Specifically, compared with the prior art of adding the conductive agent to the active slurry, the conductive agent is added to the binder in step S3 of the present invention, which can transform the single-function binder into a multifunctional conductive binder. The conductive agent can be reduced or even replaced in the subsequent pulping process, thereby reducing the ratio of auxiliary materials in the entire system and correspondingly improving the energy density and capacity of the system.

[0048] As an optional embodiment of the present invention, the mass ratio of the total mass of the aromatic diamine, the aromatic dianhydride and the polyol to the carbon-based conductive agent is 5-15:1 (for example, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, etc.). Under this mass ratio, the three-dimensional conductive adhesive can exhibit good adhesion and conductivity.

[0049] As an optional embodiment 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 (sulfonyl chloride).

[0050] As an optional embodiment of the present invention, in step S3, the reaction temperature is -5°C~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.).

[0051] According to a third aspect of the present invention, a negative electrode sheet is provided, wherein the negative electrode sheet comprises the three-dimensional conductive binder prepared by the method for preparing the three-dimensional conductive binder as described above or the three-dimensional conductive binder as described above.

[0052] According to a fourth aspect of the present invention, there is provided a method for preparing the negative electrode sheet as described above, comprising the following steps: 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 are mixed in a mass ratio of (94-97): (3-6) (such as 95:5, 96:4, etc.) to obtain a negative electrode active material; the negative electrode active material is subjected to high-speed shear stirring, deionized water is added to adjust the solid content to 45-55%, the slurry is evenly scraped on a copper foil, and a negative electrode sheet is obtained after drying, rolling and slitting.

[0053] Specifically, the present invention defines the mass ratio of the silicon-carbon negative electrode material and the three-dimensional conductive binder as (94-97): (3-6), which ensures that the binder provides sufficient bonding force and effectively inhibits the expansion of the electrode material while the silicon-carbon negative electrode material can provide high energy density, thereby taking into account 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, the energy density of the battery will decrease, and it will not be able to provide sufficient power and endurance; the content of the binder is related to the bonding of the material and the stability of the battery structure. If the content of the binder is too low, the bonding effect will be reduced, making the electrode material easy to fall off during the battery charging and discharging process, affecting the battery's cycle performance and expansion control.

[0054] As an optional 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.

[0055] According to a fifth aspect of the present invention, a secondary battery is provided, wherein the secondary battery is prepared by a negative electrode sheet prepared by the method for preparing a negative electrode sheet as described above, or by winding the negative electrode sheet, positive electrode sheet and separator as described above into a battery cell, and encapsulating the battery cell into a dry battery cell with an aluminum-plastic film, and then undergoing liquid injection, formation and aging.

[0056] The present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0057] Example 1 Preparation of three-dimensional conductive adhesive: S1: Weigh 20 g of 4,4'-diaminodiphenyl ether and 21.8 g of pyromellitic anhydride, add them to 200 mL of N-methylpyrrolidone, stir at 25°C for 2 hours, and obtain 240 g of polyamic acid solution; like Figure 2 As shown, it is a reaction mechanism diagram of 4,4'-diaminodiphenyl ether and pyromellitic acid dianhydride to generate polyamic acid; S2: Add 72g of glycerol to the polyamic acid solution and set the vacuum degree to 10 -2Pa, at 150℃, Figure 3 As shown, vacuum polymerization reaction takes place for 2 hours to obtain a high molecular polymer solution having a three-dimensional bonding network; Wherein, the mass ratio of glycerol to polyamic acid solution is 0.3:1; like Figure 4 , which is a schematic diagram of a fragment of a high molecular polymer having a three-dimensional bonding network prepared in this embodiment; like Figure 5 As shown, it is a schematic diagram of the 3D structure of a fragment of a high molecular polymer having a three-dimensional bonding network prepared in this embodiment. The high molecular chain of polyester amide can be extended in multiple directions, combined with the rotation and torsion of chemical bonds, and finally form an ideal three-dimensional network structure; S3: Weigh 10g of conductive carbon black and 0.005g of EDC, add them to the above polymer solution, mix well, stir at 0℃ for 5h, pour the resulting solution into a large amount of ethanol to precipitate, then centrifuge the precipitate and vacuum dry it at 80℃ for 12 hours to obtain a three-dimensional conductive adhesive.

[0058] like Figure 1 As shown, the three-dimensional conductive adhesive 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.

[0059] Negative electrode preparation: According to the silicon-carbon negative electrode material: the three-dimensional conductive binder prepared by the preparation method described above or the three-dimensional conductive binder described above are mixed in a mass ratio of 95:5 to obtain a negative electrode active material; the negative electrode active material is subjected to high-speed shear stirring, deionized water is added to adjust the solid content to 45-55%, the slurry is evenly scraped on the copper foil, and the negative electrode sheet is obtained after drying, rolling and slitting. The surface density of the negative electrode sheet is 184 g / m 2 , thickness is 124μm.

[0060] Positive electrode preparation: The composition of the positive electrode material is mixed in a mass ratio of 96:2:2 of ternary material NCM811: conductive agent SP+CNT: binder PVDF, and then high-speed shear stirring is performed. NMP is added to adjust the solid content to 70-75%. The slurry is evenly scraped on aluminum foil, and then dried, rolled, and cut to obtain the positive electrode sheet to be processed. The surface density of the positive electrode sheet is 534 g / m 2 , thickness is 167μm.

[0061] Preparation of secondary batteries: The prepared positive and negative electrode sheets and separators are wound into battery cells, and packaged into dry battery cells with aluminum-plastic film. After liquid injection (the electrolyte is a solution with a concentration of 1 mol / L obtained by dissolving LiPF6 in ethylene carbonate (EC) / dimethyl carbonate (DMC) with a mass ratio of 1:1), formation, aging and other processes, a secondary battery is prepared.

[0062] Example 2 The difference between this embodiment and embodiment 1 is 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.

[0063] Example 3 The difference between this embodiment and embodiment 1 is 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.

[0064] Example 4 The difference between this embodiment and embodiment 1 is that the three-dimensional conductive adhesive is prepared as follows: S1: 10.8 g of p-phenylenediamine and 29.5 g of 1,4,5,8-naphthalenetetracarboxylic dianhydride were weighed and added to 200 mL of N-methylpyrrolidone, and stirred at 25° C. for 2 hours to obtain a polyamic acid solution; The remaining steps and technical parameters are the same as those in Example 1.

[0065] Example 5 The difference between this embodiment and embodiment 1 is that the three-dimensional conductive adhesive is prepared as follows: 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; The remaining steps and technical parameters are the same as those in Example 1.

[0066] Comparative Example 1 The main difference between this comparative example and Example 1 is that in the negative electrode active material, the mass ratio of 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 Example 1.

[0067] Comparative Example 2 The main difference between this comparative example and Example 1 is that in the negative electrode active material, the mass ratio of 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 Example 1.

[0068] Comparative Example 3 The main difference between this comparative example and Example 1 is that in the negative electrode active material, the mass ratio of 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.

[0069] Comparative Example 4 The main difference between this comparative example and Example 1 is that in the negative electrode active material, the mass ratio of 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.

[0070] Comparative Example 5 The main difference between this comparative example and Example 1 is 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.

[0071] Comparative Example 6 The main difference between this comparative example and Example 1 is 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.

[0072] Comparative Example 7 The main difference between this comparative example and Example 1 is that the conductive carbon black is not added in step S3, but is added to the active slurry of the negative electrode sheet; The preparation of the three-dimensional conductive adhesive in this comparative example includes the following steps: S1: Weigh 20 g of 4,4'-diaminodiphenyl ether and 21.8 g of pyromellitic anhydride, add them to 200 mL of N-methylpyrrolidone, stir at 25°C for 2 hours, and obtain 240 g of polyamic acid solution; S2: Add 72g of glycerol to the polyamic acid solution and set the vacuum degree to 10 -2 Pa, 150 ° C vacuum polymerization for 2 h, to obtain a high molecular polymer solution with a three-dimensional bonding network; Wherein, the mass ratio of glycerol to polyamic acid solution is 0.3:1; S3: Weigh 0.005g EDC and add it to the above polymer solution, mix well, stir at 0℃ for 5h, pour the obtained solution into a large amount of ethanol to precipitate, and then centrifuge the precipitate and vacuum dry it at 80℃ for 12 hours to obtain a three-dimensional adhesive.

[0073] The ratio of silicon-carbon negative electrode material in the negative electrode active slurry prepared in this comparative example to the three-dimensional binder prepared in this comparative example to the conductive carbon black is 95:4:1, and the remaining steps and technical parameters are the same as those in Example 1.

[0074] Comparative Example 8 The main difference between this comparative example and Example 1 is that glycerol is not added in step S2. The preparation of the conductive adhesive in this comparative example includes the following steps: S1: Weigh 20 g of 4,4'-diaminodiphenyl ether and 21.8 g of pyromellitic anhydride, add them to 200 mL of N-methylpyrrolidone, stir at 25°C for 2 hours, and obtain 240 g of polyamic acid solution; S2: Weigh 10g of conductive carbon black and 0.005g of EDC, add them to the above polyamic acid solution, mix well, stir at 0℃ for 5h, pour the obtained solution into a large amount of ethanol to precipitate, then centrifuge the precipitate and vacuum dry it at 80℃ for 12h, then heat it to 300℃ for high-temperature thermal curing for 30min, and then crush it to obtain a conductive adhesive.

[0075] The remaining steps and technical parameters are the same as those in Example 1.

[0076] Comparative Example 9 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-heptanol (containing 7 hydroxyl groups and the number of carbon atoms between the hydroxyl groups at both ends is 6) is used for the reaction, and the remaining steps and technical parameters are the same as those in Example 1.

[0077] Comparative Example 10 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-tridecantriol (containing 3 hydroxyl groups and the number of carbon atoms between the two terminal hydroxyl groups is 11) is used for the reaction, and the remaining steps and technical parameters are the same as those in Example 1.

[0078] Performance Testing Negative electrode peel strength test Take the electrode sheet and cut it to get a test sample with a length of 100mm and a width of 10mm. Take a stainless steel plate with a width of 25mm, stick double-sided tape (width 11mm), stick the side of the test sample coated with the negative electrode material on the double-sided tape on the stainless steel plate, and use a 2000g roller to roll back and forth on its surface three times at a speed of 300mm / min. Bend one end of the test sample 180 degrees, manually peel off the negative electrode material layer and the current collector of the test sample along the length direction by 25mm, and then fix the test sample on the INSTRON 336 test machine so that the peeling surface is consistent with the force line of the test machine (that is, parallel to the movement direction of the test machine when peeling). Continue to use the testing machine to continuously peel the test sample at a speed of 30mm / min. The peeling force curve obtained is taken as the average value of the stable section within the range of 10-50mm on the curve (that is, the section on the peeling force curve that no longer increases monotonically) as the peeling force F0. The bonding force F between the negative electrode material layer and the current collector in the test sample = F0 / width of the test sample (the unit of F: N / m).

[0079] Ionic conductivity of the negative electrode The ionic conductivity of each embodiment and comparative example was determined by measuring the electrochemical impedance spectroscopy (EIS) of the negative electrode sheet sandwiched between two fluorine-doped tin oxide (FTO) electrodes. The negative electrode sheet area is 1 x 1 cm 2 , the corresponding ionic conductivity is calculated based on the actual thickness.

[0080] Fully charged negative electrode expansion rate At 25°C, the secondary battery to be tested was charged at a constant current of 1C to a charge cut-off voltage of 4.30V, and then charged at a constant voltage to a current <0.05C. After standing for 10 minutes, it was disassembled and the thickness of the negative electrode sheet was measured with a micrometer. The average value was taken and the expansion rate was calculated based on the initial thickness of the negative electrode sheet.

[0081] Battery cycle performance test At 25°C, the secondary battery to be tested is charged at a constant current of 1C to a charge cut-off voltage of 4.30V, then charged at a constant voltage to a current <0.05C, left to stand for 10 minutes, and then discharged at a constant current of 1C to a discharge cut-off voltage of 3.3V, left to stand for 10 minutes. This is a charge and discharge cycle (i.e., one circle (cls)). According to this method, the battery is tested for 100 cycles of charge and discharge cycles. The percentage of the discharge capacity of the last circle to the discharge capacity of the third circle is the cycle capacity retention rate.

[0082] 2C discharge capacity ratio At 25°C, the secondary battery to be tested is charged at a constant current of 1C to a charging cut-off voltage of 4.30V, then charged at a constant voltage to a current <0.05C, left standing for 10 minutes, and then discharged at a constant current of 2C to a discharge cut-off voltage of 3.3V, left standing for 10 minutes. The percentage of discharge capacity to initial capacity is the 2C discharge capacity ratio.

[0083] Performance data Table 1 As shown in Table 1, compared with Example 1, since the binders used in the negative electrode active slurries of Comparative Examples 1-4 are PAA+SBR binders or CMC+SBR binders, which are linear binders or point binders, respectively, the expansion inhibition effect of the silicon material is obviously not as good as the three-dimensional conductive binder provided in Example 1, and the binders provided in Comparative Examples 1-4 are difficult to achieve self-repair after the material undergoes irreversible expansion, and it is also impossible to enhance the interaction between the active materials and between the active materials and the current collector, and improve the cycle performance of the battery.

[0084] As shown in Table 1, compared with Example 1, since the mass ratio of the polyol to the polyamic acid solution in Comparative Example 5 is 0.1:1, the content of the polyol is too small to provide sufficient cross-linking points, resulting in insufficient cross-linking density of the polymer network structure, affecting the strength and stability of the three-dimensional network polymer, and further affecting the expansion inhibition effect on the silicon material.

[0085] As shown in Table 1, compared with Example 1, since the mass ratio of the polyol to the polyamic acid solution in Comparative Example 6 is 0.8:1, the content of the polyol is too much, resulting in excessive cross-linking, making the polymer network too dense, reducing the penetration space of the electrolyte, affecting the ion transmission efficiency of the battery, and thus reducing the performance of the battery.

[0086] As shown in Table 1, compared with Example 1, when the ratio of main materials to auxiliary materials is the same, since conductive carbon black is added to the negative electrode active slurry in Comparative Example 7, the binder has a single function and acts separately from the conductive agent, and the performance of the battery cell is significantly deteriorated. If Comparative Example 7 wants to achieve the performance indicators of Example 1 such as the peeling strength and ionic conductivity of the negative electrode sheet, it is necessary to increase the ratio of auxiliary materials such as the binder and the conductive agent accordingly, which will lead to a lower content of the main material and further reduce the energy density of the system.

[0087] As shown in Table 1, compared with Example 1, Comparative Example 8 does not add polyol, and directly adds conductive agent to the polyamic acid solution, so that a three-dimensional network structure cannot be formed, the expansion inhibition effect of silicon material is poor, and the performance of the battery is reduced.

[0088] As shown in Table 1, in Comparative Example 9, 1,2,3,4,5,6,7-heptahedral (containing 7 hydroxyl groups, and the number of carbon atoms between the hydroxyl groups at both ends is 6) is used to react with polyamic acid. The number of hydroxyl groups is too large, so the unreacted hydroxyl groups may absorb water and cause swelling, which will not only affect the bonding performance of the three-dimensional conductive adhesive, but also reduce the strength of the three-dimensional conductive adhesive, thereby affecting the three-dimensional conductive adhesive. The effect of inhibiting the expansion of the silicon negative electrode.

[0089] 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 two end hydroxyl groups is 11) is used to react with polyamic acid. The number of carbon atoms between the two end hydroxyl groups of the polyol is greater than 8, which will significantly increase the distance between crosslinking points due to the long carbon chain, reduce the density of equivalent crosslinking points, make the three-dimensional bonding network sparse, and reduce the strength, thereby affecting the expansion inhibition effect of silicon materials.

[0090] 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A three-dimensional conductive adhesive, characterized in that: The three-dimensional conductive adhesive comprises a high molecular polymer and a carbon-based conductive agent; The high molecular 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 end hydroxyl groups of the polyol is 3-8; The carbon-based conductive agent is grafted onto the polyester amide through chemical bonds.

2. The three-dimensional conductive adhesive according to claim 1, characterized in that: The carbon-based conductive agent is grafted onto the polyester amide via an ester bond or an amide bond.

3. A method for preparing a three-dimensional conductive adhesive according to any one of claims 1 to 2, characterized in that: The steps include: S1: adding aromatic diamine and aromatic dianhydride into a solvent and stirring and polymerizing to obtain a polyamic acid solution; S2: adding the polyol to the polyamic acid solution, and performing vacuum in-situ polymerization to obtain a high molecular polymer solution having a three-dimensional bonding network; Wherein, the mass ratio of the polyol to the polyamic acid solution is 0.3-0.5:1; S3: adding the carbon-based conductive agent and the catalyst into the high molecular polymer solution to react and obtain the three-dimensional conductive adhesive.

4. The method for preparing a three-dimensional conductive adhesive according to claim 3, characterized in that: 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-aminophenoxybenzene)]hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 2,2'-bistrifluoromethyl-4,4'-diaminodiphenyl ether, and 1,3,5-tris(4-aminophenoxy)benzene; And / or, in step S1, the aromatic dianhydride includes pyromellitic dianhydride, biphenyl tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-bisphenyl sulfone 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'-biphenyl tetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyl tetracarboxylic dianhydride, 3,3',4,4'-terphenyl tetracarboxylic dianhydride, 2,2-diphenyl-4,4',5,5'-biphenyl tetracarboxylic dianhydride, At least one of (3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, benzophenone tetracarboxylic dianhydride, 1,3-bis(3,4-dicarboxybenzoyl)phthalic dianhydride, 3,5-bis(3,4-dicarboxyphenyl)biphenyl dianhydride, 4,4'-m-phenylenediamine diphenyl anhydride, 4,4'-(terephthalene) diether dianhydride, 4,4'-(4,4'-biphenyloxy) dianhydride, 3,3',4,4'-dimethyldiphenylsilane tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, binaphthene dianhydride, cyclobutane dianhydride and 1,4-bis(phenylmaleic anhydride)benzene; and / or, the solvent comprises one or more of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methylpyrrolidone; And / or, in step S1, the stirring polymerization is carried out at a temperature of 25°C-120°C and for a time of 1h-12h.

5. The method for preparing a three-dimensional conductive adhesive according to claim 3, characterized in that: 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.

6. The method for preparing a three-dimensional conductive adhesive according to claim 3, characterized in that: The mass ratio of the total mass of the aromatic diamine, the aromatic dianhydride and the polyol to 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 catalyst includes one or more of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide, and thionyl chloride; And / or, in step S3, the reaction temperature is -5°C to 0°C, and the reaction time is 1h to 10h.

7. A negative electrode sheet, characterized in that: The negative electrode sheet comprises a three-dimensional conductive adhesive prepared by the method for preparing a three-dimensional conductive adhesive as described in any one of claims 3 to 6 or a three-dimensional conductive adhesive as described in any one of claims 1 to 2.

8. A method for preparing a negative electrode sheet as claimed in claim 7, characterized in that: The steps include: The silicon-carbon negative electrode material and the three-dimensional conductive adhesive obtained by the preparation method of the three-dimensional conductive adhesive as described in any one of claims 3 to 6 or the three-dimensional conductive adhesive as described in any one of claims 1 to 2 are mixed in a mass ratio of (94-97): (3-6) to obtain a negative electrode active material; the negative electrode active material is subjected to high-speed shear stirring, deionized water is added to adjust the solid content to 45-55%, the slurry is evenly scraped on a copper foil, and a negative electrode sheet is obtained after drying, rolling and slitting.

9. The method for preparing a negative electrode sheet according to claim 8, characterized in that: 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.

10. A secondary battery, characterized in that: The secondary battery is prepared by winding a negative electrode sheet obtained by the method for preparing a negative electrode sheet according to any one of claims 8 to 9, or a negative electrode sheet according to claim 7, a positive electrode sheet and a separator into a battery cell, and encapsulating the battery cell with an aluminum-plastic film into a dry battery cell, and then undergoing liquid injection, formation and aging.

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