Negative electrode binder, negative electrode sheet, and battery

Abstract

This application relates to the technical field of silicon-based negative electrode binders for lithium-ion batteries, specifically disclosing a negative electrode binder, a negative electrode sheet, and a battery. The negative electrode binder comprises modified polyimide, which includes rigid dianhydride structural units and diamine structural units. The diamine structural units include flexible diamine structural units, co-solubilizing diamine structural units, and nitrogen-containing heterocyclic diamine structural units. The weight-average molecular weight of the modified polyimide is 10 w Da to 50 w Da. The polyimide binder of this application exhibits good solubility in negative electrode slurry, while maintaining high elongation at break and excellent ion conductivity after swelling in the electrolyte.

Description

Technical Field

[0001] This application relates to the technical field of silicon-based negative electrode binders for lithium batteries, and in particular to a negative electrode binder, a negative electrode sheet, and a battery. Background Technology

[0002] Polyimide (PI) binders are widely studied for use in silicon-based anodes due to their excellent thermal stability, mechanical strength, and chemical stability. In the fabrication of silicon-based anode sheets, a coating process is typically used to coat a cathode slurry containing binders and solvents onto a current collector. N-methylpyrrolidone is usually used as the main solvent in the cathode slurry. However, existing polyimide binders suffer from the following key technical challenges, limiting their industrial application: a. Conventional polyimide is insoluble in N-methylpyrrolidone (NMP) solvent and is often used in the form of precursor polyamic acid. It requires an additional high-temperature imidization process after the electrode coating is completed. It cannot be adapted to the existing coating process of lithium battery electrodes. It requires additional adjustment of process parameters, which increases production costs. In addition, silicon-based anodes are prone to oxidation during this process, and the production energy consumption is high, which does not meet the requirements of green and low-carbon production. b. Silicon anodes can expand by more than 300% during charge-discharge cycles. The binder should primarily have high elongation and moderate modulus for synergistic buffering, while also considering post-swelling retention and interfacial adhesion. Conventional polyimide has excessively high modulus and low elongation, making it unsuitable for the volume expansion of silicon-based anodes. c. Ordinary polyimide binders do not have ion conduction capabilities, resulting in high resistance to lithium ion transport inside the electrode, which limits the rate performance and fast charging performance of lithium batteries. Therefore, there is an urgent need to develop a polyimide binder that can simultaneously solve the above-mentioned pain points, achieve good solubility, maintain high elongation at break after electrolyte swelling, and possess excellent ion conductivity, so as to support the industrial application of silicon-based anodes. Summary of the Invention

[0003] In order to improve the solubility of polyimide binder in negative electrode slurry, and at the same time achieve the effect of maintaining high elongation at break and excellent ion conduction after swelling in electrolyte, this application provides a negative electrode binder, a negative electrode sheet and a battery.

[0004] In a first aspect, this application provides a negative electrode binder comprising a modified polyimide, wherein the modified polyimide comprises a rigid dianhydride structural unit and a diamine structural unit, wherein the diamine structural unit comprises a flexible diamine structural unit, a solubilizing diamine structural unit and a nitrogen-containing heterocyclic diamine structural unit; and the modified polyimide has a weight-average molecular weight of 10 w Da to 50 w Da.

[0005] Optionally, the molar ratio of the rigid dianhydride structural unit to the diamine structural unit is 1:(0.97~1.03).

[0006] Optionally, the modified polyimide has an elongation at break of 30% to 200% after being immersed in the electrolyte; And / or, the modified polyimide has an elastic modulus of 5 MPa to 200 MPa after being soaked in the electrolyte.

[0007] Optionally, based on the total molar amount of diamine structural units being 100%, the molar percentage of the flexible diamine structural units is 20% to 65%, the molar percentage of the solubilizing diamine structural units is 30% to 50%, and the molar percentage of the nitrogen-containing heterocyclic diamine structural units is 5% to 30%.

[0008] Optionally, the rigid dianhydride structural unit is selected from one or more of aromatic dianhydride structural units and alicyclic dianhydride structural units; Optionally, the nitrogen-containing heterocyclic diamine structural unit includes one or more of the following: pyridine diamine structural unit, imidazole diamine structural unit, nitrazole diamine structural unit, pyrazine diamine structural unit, and quinoxaline diamine structural unit.

[0009] Optionally, the flexible diamine structural unit includes one or more of the following: ether-bonded diamine structural units and siloxane-containing diamine structural units.

[0010] Optionally, the solubilizing diamine structural unit includes one or more of the following: asymmetric diamine structural units and diamine structural units containing side groups.

[0011] Secondly, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode binder described above.

[0012] Thirdly, this application provides a battery including the negative electrode sheet described above.

[0013] In summary, this application includes at least one of the following beneficial technical effects: In this application, the rigid dianhydride structural units and nitrogen-containing heterocyclic diamine structural units in the binder molecular chain jointly construct a stable molecular framework, endowing the binder with excellent resistance to electrolyte swelling. The rigid framework restricts excessive penetration of electrolyte molecules, while the nitrogen-containing heterocyclic diamine structural units further inhibit swelling by increasing polarity, allowing the binder to maintain structural integrity after immersion in electrolyte, avoiding softening or collapse. The synergistic effect of the flexible diamine structural units and the nitrogen-containing heterocyclic diamine structural units enables the binder to possess efficient ion conduction performance while maintaining good solubility. The flexible diamine structural units effectively disrupt the tight packing of the molecular chain, reducing intermolecular forces and ensuring that the binder has good solubility and film-forming properties in conventional solvents, making it directly compatible with existing coating processes. The nitrogen atoms in the nitrogen-containing heterocyclic diamine structural units coordinate with lithium ions through lone pair electrons, constructing a continuous ion conduction pathway within the molecular chain, significantly reducing lithium ion transport resistance. The flexible diamine structural unit and the solubilizing diamine structural unit work together to disrupt the tight packing of molecular chains and reduce intermolecular forces.

[0014] The weight-average molecular weight of the binder in this application is controlled within the range of 10 w Da to 50 w Da. This molecular weight range ensures that the molecular chains have sufficient entanglement density to provide cohesive force, while avoiding processing difficulties caused by excessively high molecular weight. At the same time, combined with the synergistic effect of the above-mentioned structural units, the binder can achieve comprehensive optimization of solubility, ionic conductivity, resistance to electrolyte swelling, and mechanical toughness over a wide range of molecular weights. Detailed Implementation

[0015] To make the technical problems solved, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the embodiments described herein are only some, not all, of the embodiments of this invention, and are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the protection scope of this invention.

[0016] It should be noted that, in this invention, as is known to those skilled in the art of chemical synthesis, each structural unit represents the structural portion of the corresponding monomer present in the resulting polymer after the monomer participates in the polymerization reaction. The molar ratio of each structural unit is the molar ratio of the monomers providing each structural unit.

[0017] In a first aspect, this application provides a negative electrode binder comprising a modified polyimide, wherein the modified polyimide comprises rigid dianhydride structural units and diamine structural units, the rigid dianhydride structural units and diamine structural units being linked by imide bonds, and the diamine structural units comprising flexible diamine structural units, solubilizing diamine structural units and nitrogen-containing heterocyclic diamine structural units; the modified polyimide has a weight-average molecular weight of 10 w Da to 50 w Da.

[0018] The introduction of rigid dianhydride structural units provides the binder with high modulus and excellent cohesive force, effectively suppressing the volume expansion of silicon-based anodes and preventing electrode structure collapse. The stable molecular framework provided by the rigid dianhydride structural units, combined with appropriate molecular crosslinking density, reduces the penetration of electrolyte molecules into the binder. At the same time, the introduction of nitrogen-containing heterocyclic diamine structural units can optimize the polarity of the molecular chain, matching it with the polarity of the electrolyte, reducing the amount of electrolyte absorbed by the binder. Ultimately, the electrolyte swelling rate is controlled within a reasonable range, and the structure remains stable after swelling, without softening or collapse, thus achieving the effect of resistance to electrolyte swelling.

[0019] The introduction of nitrogen-containing heterocyclic diamine structural units has two advantages. First, the nitrogen atom in the nitrogen heterocycle is basic, enabling intramolecular autocatalysis and imine formation during synthesis. Second, the nitrogen atom in the nitrogen-containing heterocyclic diamine structural unit contains lone pairs of electrons, which can react with Li in the lithium battery electrolyte. + Coordination occurs, realizing Li + The dissociation and transport of lithium ions enhance the ion conductivity of the binder. Simultaneously, the nitrogen-containing heterocyclic diamine structural units form continuous ion conduction pathways within the molecular chain, reducing lithium ion transport resistance and significantly improving the ion conductivity of the binder, thereby improving the rate performance and fast-charging performance of the lithium battery. Furthermore, the nitrogen-containing heterocyclic diamine structural units can form strong hydrogen bonds and coordination interactions with the hydroxyl groups on the silicon-based anode surface, enhancing the interfacial adhesion between the binder and the silicon anode and copper foil, preventing debonding. Since the nitrogen-containing heterocyclic diamine structural units themselves have a high density, they are prone to excessive molecular polarity and decreased solubility. Therefore, this application also introduces flexible diamine and co-solubilizing diamine structural units to work synergistically, disrupting the tight packing of the molecular chains and reducing intermolecular forces.

[0020] The introduction of flexible diamine structural units endows the binder with high toughness and high elongation at break. The synergistic effect of the flexible diamine structural units and the nitrogen-containing heterocyclic diamine structural units enables it to maintain good solubility while possessing efficient ion conductivity. The flexible diamine structural units effectively disrupt the tight packing of molecular chains, reducing intermolecular forces and ensuring that the binder has good solubility and film-forming properties in common solvents, making it directly compatible with existing coating processes. By precisely introducing nitrogen-containing heterocyclic diamine structural units, a "rigid-flexible" balance is achieved, allowing the binder to maintain excellent elongation at break even after swelling in the electrolyte.

[0021] The introduction of solubilizing diamine structural units improves the solubility of the material, enabling it to dissolve in conventional solvents such as NMP.

[0022] The weight-average molecular weight of the negative electrode binder in this application is set in the range of 10 w Da to 50 w Da. Optionally, the weight-average molecular weight of the negative electrode binder is 10 w Da, 15 w Da, 20 w Da, 25 w Da, 30 w Da, 35 w Da, 40 w Da, 45 w Da, 50 w Da, etc. The above-mentioned weight-average molecular weight range ensures that the molecular chains have sufficient entanglement density to provide cohesive force, while avoiding processing difficulties caused by excessively high molecular weight.

[0023] In some specific embodiments, the elongation at break after the modified polyimide is immersed in the electrolyte is 30%~200%. Optionally, the elongation at break after the modified polyimide is immersed in the electrolyte can be 30%, 50%, 70%, 100%, 150%, 200%, etc. If the above elongation at break is too high, the binder is too soft in the wet state and is prone to irreversible plastic creep. When the silicon expands, the binder is overstretched, and after the silicon shrinks, the binder cannot spring back to its original shape, resulting in permanent voids inside the electrode and the conductive network gradually loosening. If the above elongation at break is too low, the silicon particles expand violently during charging and discharging, the binder cannot follow the deformation, the binder film is prone to cracking, and microcracks appear inside the electrode; the active material particles lose contact with the conductive network, and the reversible capacity drops rapidly.

[0024] In some specific embodiments, the elastic modulus of the modified polyimide after immersion in the electrolyte is 5 MPa to 200 MPa. Optionally, the elastic modulus of the modified polyimide after immersion in the electrolyte can be 5 MPa, 10 MPa, 20 MPa, 40 MPa, 60 MPa, 80 MPa, 100 MPa, 120 MPa, 140 MPa, 160 MPa, 180 MPa, 200 MPa, etc. If the above elastic modulus is too high, the binder is too rigid, the deformation resistance is large, and the huge internal stress generated by silicon expansion cannot be buffered and released, causing stress concentration, cracking of the binder layer, and debonding of the particle-binder interface. If the above elastic modulus is too low, the binder stiffness is insufficient, and during the silicon expansion-contraction cycle, the binder cannot maintain the tight contact between particles, the electrode pores continue to increase, and the electronic conductivity path is broken.

[0025] The binder of this application exhibits an elongation at break ranging from 30% to 200% after immersion in the electrolyte, and an elastic modulus ranging from 5 MPa to 200 MPa. This effectively resists the volume expansion of the silicon-based anode exceeding 300%, preventing electrode pulverization and debonding, thus achieving a high elongation at break after binder swelling. Simultaneously, it effectively buffers the volume expansion stress during the charging and discharging process of the silicon-based anode, preventing electrode pulverization and debonding, and ensuring cycle stability.

[0026] In some specific embodiments, the molar ratio of the rigid dianhydride structural unit to the diamine structural unit is 1:(0.97~1.03). Optionally, the molar ratio of the rigid dianhydride structural unit to the diamine structural unit can be 1:0.97, 1:0.98, 1:0.99, 1:1.00, 1:1.01, 1:1.02, 1:1.03, etc.

[0027] By adopting the above specific embodiments, when the molar ratio of rigid dianhydride structural units to diamine structural units in the modified polyimide of the negative electrode binder in this application embodiment is 1:(0.97~1.03), a higher molecular weight product can be obtained; when the molar ratio of the two is higher or lower than this range, the molecular weight will be lower and the performance will be worse.

[0028] In some specific embodiments, with the total molar amount of diamine structural units being 100%, the molar percentage of the flexible diamine structural units is 20% to 65%, the molar percentage of the solubilizing diamine structural units is 30% to 50%, and the molar percentage of the nitrogen-containing heterocyclic diamine structural units is 5% to 30%.

[0029] By adopting the above specific implementation method, and controlling the molar ratio of flexible diamine structural units to diamine structural units to be 20%~65%, the molar ratio of solubilizing diamine structural units to diamine structural units to be 30%~50%, and the molar ratio of nitrogen-containing heterocyclic diamine structural units to diamine structural units to be 5%~30%, this dosage range can achieve synergistic optimization of "soluble in NMP solvent, excellent ion conduction, swelling resistance, and high elongation at break," avoiding the problems of insufficient ion conduction due to too low a content of nitrogen-containing heterocyclic diamine structural units, and the impact of too high a content on the solubility and mechanical properties of the binder. At the same time, controlling the content of nitrogen-containing heterocyclic diamine structural units can avoid the situation where the molecular polarity is too strong and the solubility decreases due to excessively high nitrogen heterocycle density. Ultimately, the binder achieves a solubility of ≥10 wt% in NMP, with a clear solution without gelation, which can be directly applied to existing coating processes.

[0030] Specifically, the molar percentage of the flexible diamine structural unit can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or 65%, etc.; the molar percentage of the solubilizing diamine structural unit can be 30%, 35%, 40%, 45%, or 50%, etc.; and the molar percentage of the nitrogen-containing heterocyclic diamine structural unit can be 5%, 10%, 15%, 20%, 25%, or 30%, etc.

[0031] By adjusting the molar ratio of rigid dianhydride structural units, flexible diamine units, and nitrogen-containing heterocyclic diamine units, it is helpful to control the polarity of the polyimide molecular chain, which ensures the solubility of the binder in NMP, reduces its swelling rate in the electrolyte, and ensures that the mechanical properties do not decrease after swelling.

[0032] In some specific embodiments, the rigid dianhydride structural unit includes one or more of aromatic dianhydride structural units and alicyclic dianhydride structural units.

[0033] The rigid dianhydride structural unit is derived from a rigid dianhydride monomer. Specifically, the rigid dianhydride monomer is selected from the following: pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, 4,4'-oxophthalic anhydride, 4,4'-(hexafluoroisopropene)phthalic anhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 3, One or more of the following: 3',4,4'-diphenylsulfone tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride, hydroquinone diphthalic anhydride, perylene-3,4,9,10-tetracarboxylic dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,2,4,5-cyclohexane tetracarboxylic dianhydride, hydroquinone diphthalic anhydride, and dimethylbiphenyl tetracarboxylic dianhydride.

[0034] In some specific embodiments, the nitrogen-containing heterocyclic diamine structural unit includes one or more of the following: pyridine diamine structural unit, imidazole diamine structural unit, nitrazole diamine structural unit, pyrazine diamine structural unit, and quinoxaline diamine structural unit.

[0035] The nitrogen-containing heterocyclic diamine structural unit is derived from a nitrogen-containing heterocyclic diamine monomer. Specifically, the nitrogen-containing heterocyclic diamine monomer is selected from one or more of the following: 4,4'-diamino-2,2'-bipyridine, 3,5-diamino-1,2,4-triazole, 2-(4-aminophenyl)-5-aminobenzimidazole, 2,2'-bis(4-aminophenyl)-5,5'-bibenzimidazole, 5,6-diaminobenzimidazole, 2-(3-aminophenyl)-5-aminobenzimidazole, 2,3-diaminopyrazine, 2,5-diaminopyrazine, 2,6-diaminopyrazine, 2,5-bis(4-aminophenyl)pyrazine, 3,5-bis(4-aminophenyl)-1,2,4-triazole, and 2,3-bis(4-aminophenyl)quinoxaline.

[0036] In some specific embodiments, the flexible diamine structural unit includes one or more of ether-bonded diamine structural units and siloxane-containing diamine structural units. The flexible diamine structural unit imparts good toughness and high elongation at break to the binder, adapting to the volume changes of the silicon-based anode and preventing binder cracking.

[0037] The flexible diamine structural unit is derived from a flexible diamine monomer, specifically, the flexible diamine monomer is selected from one or more of 2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenyl ether, bis(4-aminophenoxy)propane, aminopropyl-terminated polydimethylsiloxane, and polyetheramine.

[0038] In some specific embodiments, the solubilizing diamine structural unit includes one or more of asymmetric diamine structural units and side-group-containing diamine structural units. Using asymmetric or side-group-containing diamine structural units can improve the solubility of the imidized polymer and meet process requirements. Specifically, the solubilizing diamine structural unit derives a solubilizing diamine monomer, which includes asymmetric diamine monomers and / or side-group-containing diamine monomers.

[0039] The asymmetric diamine structural unit is derived from an asymmetric diamine monomer, specifically, the asymmetric diamine monomer is selected from at least one or more of 3,4'-diaminodiphenyl ether, 3,4'-diaminodiphenylmethane, 3-amino-4-hydroxydiphenylamine, 4-(3-aminophenoxy)aniline, 3,4'-diaminodiphenyl sulfide, 3-aminophenyl-4-aminobenzoic acid ester, 4-amino-N-(3-aminophenyl)benzamide, 3,4'-diaminobenzophenone, and 4-amino-N-[4-carbamoylphenyl]benzamide.

[0040] The side-chain-containing diamine structural unit is derived from a side-chain-containing diamine monomer. Specifically, the side-chain-containing diamine monomer is selected from at least one or more of the following: 9,9-bis(3-fluoro-4-aminophenyl)fluorene, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 1,3-bis(4-aminophenoxy)naphthalene, 9,9-bis(4-aminophenyl)fluorene, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 1,4-bis(4-aminophenoxy)naphthalene, 4,4'-diamino-2,2'-dimethylbiphenyl, 9,9-bis(3-methyl-4-aminophenyl)fluorene, 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane, 1,5-bis(4-aminophenoxy)naphthalene, and 4,4'-diamino-3,3'-ditert-butylbiphenyl.

[0041] It should be noted that in this invention, each structural unit in the rigid dianhydride structural unit and the diamine structural unit originates from its corresponding monomer. For example, the nitrogen-containing heterocyclic structural unit of the nitrogen-containing heterocyclic diamine structural unit originates from a monomer such as a nitrogen-containing heterocyclic diamine, which is polymerized to become a component of the diamine structural unit. It can be understood that the molar ratio between the structural units in the rigid dianhydride structural unit and the diamine structural unit can be the molar ratio between the monomers corresponding to each structural unit.

[0042] The negative electrode binder provided in the specific embodiments of the present invention can be prepared by the following example preparation method, including the following steps: (1) In a dry reaction vessel, add 4 to 10 times the total mass of NMP solvent to control the reaction system temperature at -10℃ to 40℃, add flexible diamine monomer, co-solubilizing diamine monomer and nitrogen-containing heterocyclic diamine monomer, and stir until completely dissolved to obtain a diamine solution; wherein, the molar ratio of flexible diamine monomer to the total amount of diamine monomer is 20% to 65%, the molar ratio of co-solubilizing diamine monomer to the total amount of diamine monomer is 30% to 50%, and the molar ratio of nitrogen-containing heterocyclic diamine monomer to the total amount of diamine monomer is 5% to 30%.

[0043] (2) Polymerization reaction: The rigid dianhydride monomer was added to the above diamine solution in four portions, and the amount added each time was controlled to be 1 / 4 of the total designed amount to avoid the reaction being too exothermic. The reaction was stirred at room temperature for 8 h to 12 h to obtain a polyamic acid solution. The molar ratio of the added rigid dianhydride monomer to the total molar ratio of the above flexible diamine monomer, solubilizing diamine monomer and nitrogen heterocyclic diamine monomer was 1:(0.97~1.03).

[0044] (3) Thermal imidization: The above solution was subjected to imidization reaction by gradient heating and reflux to remove water. The heating program was as follows: 70℃~90℃ for 0.5 h~1 h, 110℃~130℃ for 0.5 h~1.5 h, 170℃~190℃ for 2 h~3 h, nitrogen protection was provided, and water was drained in time (imidization byproducts). After cooling to room temperature, nitrogen heterocyclic functionalized modified polyimide negative electrode binder was obtained.

[0045] It should be noted that the specific substances used in the above preparation method do not mean that only the above substances can be used in this invention, but should be selected according to the description of each raw material and monomer in the specific embodiments section of this invention.

[0046] Secondly, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode binder described above.

[0047] As one specific implementation method, the negative electrode sheet of this application is manufactured according to the following steps: A negative electrode slurry is coated onto a copper foil, dried to form a negative electrode active material layer, and then rolled and cut to obtain the negative electrode sheet. The negative electrode slurry comprises a mixture of NMP solvent and solid components, with a solid content of 50%. The solid components include the following components by mass percentage: graphite: silicon carbon: conductive agent: modified polyimide binder = (40-80): (20-60): (0.3-2.0): (2.5-5.0); wherein the modified polyimide binder is prepared through the above steps, and the conductive agent is carbon nanotubes (CNTs).

[0048] Thirdly, this application provides a battery including the negative electrode sheet described above.

[0049] As one specific embodiment, the battery of this application is manufactured according to the following steps: The negative electrode sheet, separator, and positive electrode sheet prepared above are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as an separator, forming an electrode assembly. The electrode assembly is then placed in an outer package, filled with electrolyte, and sealed. After processes such as electrolyte injection, formation, and venting, a lithium-ion secondary battery is obtained. The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate, with a ethylene carbonate:methyl ethyl carbonate:diethyl carbonate ratio of 3:2:5. The lithium hexafluorophosphate concentration is 1 mol / L. The positive electrode material is selected from one or more of lithium iron phosphate, ternary nickel-cobalt-manganese, nickel-cobalt-aluminum, lithium manganese oxide, lithium cobalt oxide, and lithium-rich manganese-based positive electrode materials. The separator is selected from any one of polyethylene separator, polypropylene separator, polyethylene / polypropylene composite separator, and ceramic-coated modified separator. The battery form can be button cell, pouch cell, or cylindrical cell.

[0050] The specific implementation methods of this application will be further explained and illustrated below through examples and comparative examples.

[0051] Unless otherwise specified, all reagents, materials, and instruments used in the following description are conventional reagents, materials, and instruments, all of which are commercially available. The reagents involved can also be synthesized using conventional synthetic methods. Unless otherwise specified, the methods in the examples are conventional methods in the art.

[0052] Example 1 (1) Preparation of negative electrode binder A negative electrode binder includes modified polyimide. The main raw materials used in the modified polyimide include rigid dianhydride structural units and diamine structural units in a molar ratio of 1:1, connected by imide bonds. The rigid dianhydride is 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA). The diamine structural units include flexible diamine structural units, co-solubilizing diamine structural units, and nitrogen-containing heterocyclic diamine structural units. Based on the total molar amount of diamine structural units being 100%, the flexible diamine structural units account for 50% of the molar amount of diamine structural units, the co-solubilizing diamine structural units account for 35% of the molar amount of diamine structural units, and the nitrogen-containing heterocyclic diamine structural units account for 15% of the molar amount of diamine structural units.

[0053] Specifically, the flexible diamine structural unit is a 2,2'-bis[4-(4-aminophenoxy)phenyl]propane structural unit, the cosolubilizing diamine structural unit is a 4,4'-diamino-3,3'-di-tert-butylbiphenyl structural unit (containing a side-chain diamine structure), and the nitrogen-containing heterocyclic diamine structural unit is a 4,4'-diamino-2,2'-bipyridine structural unit.

[0054] Its preparation method includes the following steps: S1. In a dry reaction vessel, add NMP solvent at a ratio of 5 times the total mass of the monomers. Maintain the reaction temperature at 0°C. Then, sequentially add 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP, a flexible diamine monomer), 4,4'-diamino-3,3'-di-tert-butylbiphenyl (DTBB, a solubilizing diamine monomer), and 4,4'-diamino-2,2'-bipyridine (DAPY, a nitrogen-containing heterocyclic diamine monomer). Stir until completely dissolved to obtain a diamine solution. The molar ratio of the flexible diamine monomer to the total molar ratio of the diamine monomers is 50%, the solubilizing diamine monomer is 35%, and the nitrogen-containing heterocyclic diamine monomer is 15%. S2. Polymerization reaction: Add rigid dianhydride monomer to the above diamine solution in four batches, controlling the feeding interval to be 1 hour to avoid excessively rapid exothermic reaction. Stir the reaction at room temperature for 10 hours to obtain polyamic acid (PAA) solution. S3. Thermal imidization: The above solution is subjected to imidization reaction by gradient heating and reflux to remove water. The heating program is as follows: 80℃ for 1 hour, 120℃ for 1 hour, 180℃ for 2 hours, nitrogen protection, timely drainage (imidization byproducts), and cooling to room temperature to obtain nitrogen heterocyclic functionalized modified polyimide anode binder.

[0055] (2) Preparation of negative electrode A negative electrode sheet includes a copper foil and a negative electrode active material layer. The negative electrode slurry is coated on the copper foil and dried at 120°C to form the negative electrode active material layer. The negative electrode sheet is obtained by rolling and cutting into strips of 20mm×100mm.

[0056] The negative electrode slurry is obtained by mixing NMP solvent and solid components, with a solid content of 50%. The solid components include the following components by mass percentage: graphite: silicon carbon: conductive agent: modified polyimide binder = 48:48:1:3. The modified polyimide binder is prepared by the above steps, and the conductive agent is carbon nanotubes (CNTs). The negative electrode slurry is obtained by stirring and dispersing.

[0057] (3) Battery preparation A battery comprises a negative electrode and a separator prepared as described above, with the positive electrode stacked sequentially, the separator positioned between the positive and negative electrodes to act as a separator, forming an electrode assembly. The electrode assembly is placed in an outer package, an electrolyte is injected, and the package is sealed. After processes such as electrolyte injection, formation, and venting, a lithium-ion secondary battery is obtained. The electrolyte comprises lithium hexafluorophosphate, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate, wherein the ratio of ethylene carbonate:methyl ethyl carbonate:diethyl carbonate is 3:2:5, and the concentration of lithium hexafluorophosphate is 1 mol / L. The positive electrode material is lithium cobalt oxide, and the separator is a ceramic-coated modified separator.

[0058] Example 2 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that the monomer selection and molar ratio are different in the preparation of the negative electrode binder.

[0059] Specifically: The main raw materials used in the modified polyimide adhesive include rigid dianhydride monomers and diamine monomers in a molar ratio of 1:1. The rigid dianhydride monomer is 1,2,4,5-phenyltetracarboxylic dianhydride (PMDA). Based on the total molar amount of diamine monomers as 100%, the molar amount of flexible diamine monomers is 50% of the total molar amount of diamine, the molar amount of cosolvent diamine monomers is 45% of the total molar amount of diamine, and the molar amount of nitrogen-containing heterocyclic diamine monomers is 5% of the total molar amount of diamine.

[0060] The flexible diamine monomer is 4,4'-diaminodiphenyl ether (ODA), the solubilizing diamine monomer is 4,4'-diamino-3,3'-di-tert-butylbiphenyl DTBB (containing a side-chain diamine structure), and the nitrogen-containing heterocyclic diamine monomer is 3,5-diamino-1,2,4-triazole.

[0061] The rest is the same as in Example 1.

[0062] Example 3 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that the monomer selection and molar ratio are different in the preparation of the negative electrode binder.

[0063] Specifically: The main raw materials used in the modified polyimide adhesive include rigid dianhydride monomers and diamine monomers in a molar ratio of 1:1. The rigid dianhydride monomer is 4,4'-oxydiphthalic anhydride (ODPA). Based on the total molar amount of diamine monomers as 100%, the molar amount of nitrogen-containing heterocyclic diamine monomers is 30% of the total molar amount of diamine, the molar amount of flexible diamine monomers is 30% of the total molar amount of diamine, and the molar amount of cosolvent diamine monomers is 40% of the total molar amount of diamine.

[0064] The flexible diamine monomer is bis(4-aminophenoxy)propane (BAPO), the solubilizing diamine monomer is 4-amino-N-[4-carbamoylphenyl]benzamide PABABA (an asymmetric diamine monomer), and the nitrogen-containing heterocyclic diamine monomer is 4,4'-diamino-2,2'-bipyridine (DAPY).

[0065] The rest is the same as in Example 1.

[0066] Example 4 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that the monomers are selected differently in the preparation of the negative electrode binder.

[0067] Specifically, the main raw materials used in the polyimide adhesive include rigid dianhydride monomers and diamine monomers in a molar ratio of 1:0.97. The rigid dianhydride monomer is 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride (DSDA). Based on the total molar amount of diamine monomers as 100%, the molar amount of nitrogen-containing heterocyclic diamine monomers is 30%, the molar amount of flexible diamine monomers is 20%, and the molar amount of cosolvent diamine monomers is 50%.

[0068] The flexible diamine monomer is aminopropyl-terminated polydimethylsiloxane (PDMS), the solubilizing diamine monomer is 3-aminophenyl-4-aminobenzoate (asymmetric diamine monomer), and the nitrogen-containing heterocyclic diamine monomer is 5,6-diaminobenzimidazole (ABIA).

[0069] The rest is the same as in Example 1.

[0070] Example 5 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that the monomers are selected differently in the preparation of the negative electrode binder.

[0071] Specifically, the main raw materials used in the polyimide adhesive include rigid dianhydride monomers and diamine monomers in a molar ratio of 1:1.03. The rigid dianhydride monomer is dimethylbiphenyltetracarboxylic dianhydride (DMCBDA). Based on the total molar amount of diamine monomers as 100%, the molar amount of nitrogen-containing heterocyclic diamine monomers is 5%, the molar amount of flexible diamine monomers is 65%, and the molar amount of cosolvent diamine monomers is 30%.

[0072] The flexible diamine monomer is 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), the solubilizing diamine monomer is 4,4'-diamino-2,2'-dimethylbiphenyl DMB (containing a side-chain diamine monomer), and the nitrogen-containing heterocyclic diamine monomer is 2,5-bis(4-aminophenyl)pyrazine (BAPZ).

[0073] The rest is the same as in Example 1.

[0074] Example 6 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that in the preparation of the negative electrode binder, the molar ratio of the rigid dianhydride monomer and the diamine monomer is 1:0.8.

[0075] Example 7 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that in the preparation of the negative electrode binder, the molar ratio of the rigid dianhydride monomer and the diamine monomer is 1:1.1.

[0076] Example 8 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that in the preparation of the negative electrode binder, the total molar amount of the diamine monomer is 100%, the molar proportion of the flexible diamine monomer is 38%, the molar proportion of the cosolvent diamine monomer is 60%, and the molar proportion of the nitrogen-containing heterocyclic diamine monomer is 2%.

[0077] Example 9 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry and negative electrode sheet. The main difference from Example 1 is that in the preparation of the negative electrode binder, the total molar amount of diamine monomer is 100%, the molar proportion of flexible diamine monomer is 15%, the molar proportion of cosolubilizing diamine monomer is 50%, and the molar proportion of nitrogen-containing heterocyclic diamine monomer is 35%.

[0078] Example 10 This embodiment uses most of the operating steps in Example 1 to prepare the negative electrode binder, negative electrode slurry, and negative electrode sheet. The main difference from Example 1 is that in the preparation of the negative electrode binder, based on the total molar amount of diamine structural units as 100%, the molar proportion of flexible diamine structural units is 70%, the molar proportion of cosolubilizing diamine structural units is 20%, and the molar proportion of nitrogen-containing heterocyclic diamine structural units is 10%.

[0079] Comparative Example 1 This embodiment uses most of the operating steps of Example 1 to prepare the negative electrode binder, negative electrode slurry, and negative electrode sheet. The main difference from Example 1 is that no nitrogen-containing heterocyclic diamine monomer is added in the preparation of the negative electrode binder. All other steps are the same as in Example 1.

[0080] Comparative Example 2 This embodiment uses most of the operating steps of Example 1 to prepare the negative electrode binder, negative electrode slurry, and negative electrode sheet. The main difference from Example 1 is that no solubilizing diamine monomer is added in the preparation of the negative electrode binder. All other steps are the same as in Example 1.

[0081] Comparative Example 3 This embodiment uses most of the operating steps of Example 1 to prepare the negative electrode binder, negative electrode slurry, and negative electrode sheet. The main difference from Example 1 is that no flexible diamine monomer is added in the preparation of the negative electrode binder. All other steps are the same as in Example 1.

[0082] Comparative Example 4 This embodiment uses most of the operating steps of Example 1 to prepare the negative electrode binder, negative electrode slurry, and negative electrode sheet. The main difference from Example 1 is that in the preparation of the negative electrode binder, an equal amount of a diamine monomer without flexible ether bonds is used to replace the flexible diamine monomer, that is, an equal amount of 4,4'-diaminobiphenyl-2,2'-disulfonic acid (BDSA) is used to replace 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP). All other aspects are the same as in Example 1.

[0083] Comparative Example 5 This embodiment uses most of the operating steps of Example 1 to prepare the negative electrode binder, negative electrode slurry, and negative electrode sheet. The main difference from Example 1 is that in the preparation of the negative electrode binder, an equal amount of diamine monomer with a symmetrical structure is used to replace the solubilizing diamine monomer containing a large side group; that is, an equal amount of 4,4'-diaminodiphenyl ether-2,2'-disulfonic acid (ODADS) is used to replace 4,4'-diamino-3,3'-di-tert-butylbiphenyl (DTBB). All other steps are the same as in Example 1.

[0084] Performance testing Sample processing: The negative electrode binders prepared in each embodiment and comparative example were poured into a square mold and baked into a film according to the heating program of 100℃ for 1h, 120℃ for 1h, 160℃ for 1h, and 200℃ for 2h to prepare a PI film. The film thickness was controlled at 200±20μm.

[0085] (1) Electrolyte swelling rate test: Cut PI films into 20 mm × 20 mm samples, vacuum dry at 80 °C for 12 h until constant weight, and record the dry weight (W). _0 The samples were completely immersed in commercial lithium battery electrolyte (the electrolyte included lithium hexafluorophosphate, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate, with a ethylene carbonate:methyl ethyl carbonate:diethyl carbonate ratio of 3:2:5 and a lithium hexafluorophosphate concentration of 1 mol / L), sealed, and placed in a constant temperature environment of 60°C for 72 hours. After the immersion time was reached, the samples were removed, the surface electrolyte was gently wiped off with filter paper, and the wet weight (W) was quickly obtained. _t The electrolyte mass swelling ratio is given by the formula (S). _w =(W _t -W _0 ) / W _0 The result is calculated as (×100%), and each group of samples is tested in parallel 3 times and the average value is taken.

[0086] (2) Tensile strength, elongation at break and modulus of elasticity tests: PI films were cut into dumbbell-shaped tensile strips and vacuum-dried at 80℃ for 12 h. The strips were then completely immersed in lithium battery electrolyte (containing lithium hexafluorophosphate, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate, with a ethylene carbonate:methyl ethyl carbonate:diethyl carbonate ratio of 3:2:5 and a lithium hexafluorophosphate concentration of 1 mol / L) and sealed at 60℃ for 72 h. After removal, the surface electrolyte was gently wiped off with filter paper, and tensile tests were performed within 1 min using a universal testing machine. Five strips were tested in parallel for each group, and the average value was taken. The changes in elongation at break and elastic modulus of the immersed strips were obtained.

[0087] (3) Ionic conductivity test: Stainless steel sheet (SS) / film / stainless steel sheet (SS) blocked cells were assembled in an Ar atmosphere glove box. After the cells were allowed to stand for 12 hours, a 10mV perturbation voltage was applied to them using a Chenhua CHI660E electrochemical workstation with a frequency range of 1Hz-1MHz. The intercept of the measured AC impedance spectrum curve with the X-axis is the bulk resistance R. b The ionic conductivity at each temperature is calculated using the following formula: Ionic conductivity = L / (R) b A) Wherein, ionic conductivity is in units of S / cm; L is the thickness of the sample electrolyte membrane in units of cm; and A is the effective contact area between the stainless steel sheet and the sample electrolyte membrane in units of cm². 2 .

[0088] The negative electrode sheets prepared in the above examples and comparative examples were characterized and their performance was tested. The test results are shown in Table 1.

[0089] Table 1 The negative electrode sheets and batteries prepared in the above examples and comparative examples were characterized and their performance was tested. The test results are shown in Table 2.

[0090] (4) Peel strength test: The bonding strength of the positive electrode was tested using a GBH-1 tensile testing machine with a 180° tape peel method. The test specimen size was 20mm×100mm, and the tensile rate was 50mm / min. The specific test method referred to GB / T2792-2014 Test Method for Peel Strength of Adhesive Tape.

[0091] (5) Capacity retention test: The capacity of the pouch battery was tested at a temperature of 25℃±2℃ and recorded as the capacity before storage. The pouch battery was discharged at a constant current of 0.5C to a voltage of 2.7V. Then, it was charged at a constant current of 1C to a voltage of 4.3V, followed by constant voltage charging to a cutoff current of 0.05C. After charging, it was discharged again at a constant current of 1C to 2.75V. The discharge capacity C1 of the first cycle was recorded. This cycle was repeated for 1000 cycles, and the discharge capacity C of the 1000th cycle was recorded. 1000 Cyclic capacity retention rate = (C 1000 / C1)×100%.

[0092] (6) Full charge thickness rebound test: The above-mentioned battery cells are activated at 0.1C for 2-3 cycles to form SEI, then charged at 0.1C to full charge. The negative electrode is removed inside the glove box, lightly washed with DMC, and slightly dried at low temperature. The coating thickness (average of 10 pieces) is measured with a micrometer. The electrode of the same batch of unassembled batteries is used as a benchmark (average of 10 pieces) to obtain the full charge expansion thickness. Full charge thickness rebound = full charge thickness / uncharged electrode thickness × 100%.

[0093] Table 2 Combining Examples 1-5 and Comparative Examples 1-4 with Table 1-2, it can be seen that the overall performance of Examples 1-5 is superior to that of Comparative Examples 1-4. The reason for this is that this application controls the polarity of the polyimide molecular chain by adjusting the ratio of rigid dianhydride structural units, flexible diamine structural units, and nitrogen-containing heterocyclic diamine structural units. This ensures the solubility of the binder in NMP while reducing its swelling rate in the electrolyte, and simultaneously ensures that the mechanical properties do not decrease after swelling. The introduction of rigid dianhydride structural units provides the binder with high modulus and excellent cohesive force, effectively suppressing the volume expansion of the silicon-based anode and preventing electrode structure collapse. The stable molecular framework provided by the rigid dianhydride structural units, combined with an appropriate molecular crosslinking density, reduces the penetration of electrolyte molecules into the binder. At the same time, the introduction of nitrogen-containing heterocyclic diamine structural units optimizes the polarity of the molecular chain, matching it with the polarity of the electrolyte, reducing the amount of electrolyte absorbed by the binder. Ultimately, this achieves electrolyte swelling rate control within a reasonable range, and the structure remains stable after swelling, without softening or collapse. The nitrogen atom in the nitrogen-containing heterocyclic diamine structural unit contains a lone pair of electrons, which can react with the Li in the lithium battery electrolyte. + Coordination occurs, realizing Li +The dissociation and transport of nitrogen-containing heterocyclic structures enhance the ion conductivity of the binder. The nitrogen-containing heterocyclic structure forms a continuous ion conduction pathway in the molecular chain, reducing lithium-ion transport resistance and significantly improving the ion conductivity of the binder, thereby improving the rate performance and fast-charging performance of lithium batteries. Flexible diamine structural units endow the binder with high toughness and high elongation at break. Rigid dianhydride structural units and nitrogen-containing heterocyclic diamine structural units synergistically provide sufficient cohesive force, preventing mechanical property degradation after swelling. Comparative Example 1, lacking a nitrogen-containing heterocyclic diamine monomer, exhibited a sharp decrease in ionic conductivity, and significantly reduced peel strength and cycle capacity retention. Comparative Example 2, lacking a co-solubilizing diamine monomer or using a symmetrical diamine monomer, was insoluble in NMP solvent and could not be prepared into a slurry. Comparative Examples 3 and 4, lacking flexible diamine structural units or flexible ether bonds, showed a significant decrease in elongation at break and a significantly higher elastic modulus after swelling, resulting in poor peel strength and cycle capacity retention.

[0094] Combining Examples 1 and 6-7 with Table 1, it can be seen that Example 1 is superior to Examples 6-7. The reason for this is that the polyimide binder of this application is obtained by polymerization of rigid dianhydride structural units and diamine structural units. During polymerization, when the molar ratio of rigid dianhydride structural units to diamine structural units is 1:(0.97~1.03), a higher molecular weight product can be obtained. When the molar ratio is higher than (as in Example 7) or lower than (as in Example 6) this range, the molecular weight will be lower and the performance will be worse.

[0095] Combining Examples 1 and 8-10 with Tables 1-2, it can be seen that Example 1 is superior to Examples 8-10. The reason for this is that by controlling the content of nitrogen-containing heterocyclic diamine structural units to 5%~30%, this application achieves a "rigid-flexible" balance, so that the binder can still maintain an excellent elongation at break of 30%~200% after swelling in the electrolyte. This can effectively resist the volume expansion of silicon-based anodes of more than 300%, avoid electrode pulverization and debonding, and thus achieve the effect of high elongation at break after the binder swells. In Example 8, the content of nitrogen-containing heterocyclic diamine structural units was too low, while the content of co-solvent diamine was too high, resulting in insufficient construction of ion conduction pathways, a significant decrease in ionic conductivity, an increase in swelling ratio, and a significant reduction in peel strength and cycle capacity retention. In Example 9, the content of nitrogen-containing heterocyclic diamine structural units was too high. Although the ionic conductivity was improved, the elongation at break after swelling decreased sharply, the elastic modulus increased significantly, the binder became more brittle, and the peel strength and cycle capacity retention were significantly lower than in Example 1. In Example 10, the content of flexible diamine structural units was too high, while the content of co-solvent diamine was too low, resulting in a significant increase in swelling ratio and an extremely low elastic modulus. Although the elongation at break was very high, the binder was too soft in the wet state, with insufficient cohesion, poor peel strength and cycle capacity retention, and decreased cycle stability.

[0096] Combining Example 1 and Comparative Examples 4-5 with Table 1-2, it can be seen that Example 1 is superior to Comparative Examples 4-5. The reason for this is that the flexible diamine used in this application is a diamine monomer containing flexible ether bonds, which imparts good toughness and high elongation at break to the binder, adapting to the volume changes of the silicon-based anode and preventing the binder from cracking. The solubilizing diamine used in this application is an asymmetric or side-group-containing diamine monomer, which can improve the solubility of the polymer after imidization and adapt to process requirements. Comparative Example 4 uses a diamine monomer without flexible ether bonds instead of a flexible diamine monomer, and Comparative Example 5 uses a diamine monomer with a symmetrical structure instead of a side-group-containing solubilizing diamine monomer; the solubility and overall performance of both are significantly inferior to Example 1.

[0097] This invention's binder controls electrolyte swelling behavior by regulating molecular weight and structure, ensuring the film possesses suitable elastic modulus and elongation at break after electrolyte immersion. Higher modulus inhibits polymer plastic creep and reduces full-charge thickness rebound; sufficient elongation at break accommodates the volume expansion of lithium-intercalated silicon particles; and high peel strength is maintained to ensure interface stability between the binder, active particles, and current collector, inhibiting interface debonding and SEI growth during cycling, resulting in excellent long-cycle performance. In existing technologies, simply increasing binder elasticity and pursuing high elongation at break leads to increased electrolyte swelling, a significant decrease in modulus after swelling, severe plastic creep during electrode cycling, increased full-charge thickness rebound, decreased interface peel strength, and rapid cycle decay. While simply increasing modulus and reducing swelling rate can reduce full-charge thickness rebound, the material becomes brittle and lacks ductility, leading to binder breakage during silicon particle expansion, interface failure, and still poor cycle performance. This invention achieves a synergistic balance between mechanics, swelling, and interfacial bonding within the aforementioned parameter range.

[0098] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the foregoing application concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions claimed in this application.

Claims

1. A negative electrode binder, characterized in that, The invention includes modified polyimide, wherein the modified polyimide comprises rigid dianhydride structural units and diamine structural units; the diamine structural units include flexible diamine structural units, solubilizing diamine structural units, and nitrogen-containing heterocyclic diamine structural units; and the weight-average molecular weight of the modified polyimide is 10 w Da to 50 w Da.

2. The negative electrode binder according to claim 1, characterized in that, The molar ratio of the rigid dianhydride structural unit to the diamine structural unit is 1:(0.97~1.03).

3. A negative electrode binder according to claim 1 or 2, characterized in that, The modified polyimide has an elongation at break of 30% to 200% after being immersed in the electrolyte. And / or, the modified polyimide has an elastic modulus of 5 MPa to 200 MPa after being soaked in the electrolyte.

4. The negative electrode binder according to claim 3, characterized in that, Based on the total molar amount of diamine structural units being 100%, the molar percentage of the flexible diamine structural units is 20% to 65%, the molar percentage of the solubilizing diamine structural units is 30% to 50%, and the molar percentage of the nitrogen-containing heterocyclic diamine structural units is 5% to 30%.

5. The negative electrode binder according to claim 3, characterized in that, The rigid dianhydride structural unit includes one or more of aromatic dianhydride structural units and alicyclic dianhydride structural units.

6. The negative electrode binder according to claim 4, characterized in that, The nitrogen-containing heterocyclic diamine structural unit includes one or more of the following: pyridine diamine structural unit, imidazole diamine structural unit, nitrogen azole diamine structural unit, pyrazine diamine structural unit, and quinoxaline diamine structural unit.

7. The negative electrode binder according to claim 4, characterized in that, The flexible diamine structural unit includes one or more of the following: diamine structural units containing ether bonds and diamine structural units containing siloxanes.

8. The negative electrode binder according to claim 4, characterized in that, The solubilizing diamine structural unit includes one or more of the following: asymmetric diamine structural units and diamine structural units containing side groups.

9. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode binder according to any one of claims 1-8.

10. A battery, characterized in that, Includes the negative electrode sheet as described in claim 9.

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