Thickening agent, negative electrode slurry, negative electrode sheet, and secondary battery

Abstract

The application discloses a thickening agent, a negative electrode slurry, a negative electrode sheet and a secondary battery. The thickening agent comprises a copolymer and a branched long-chain fatty alcohol polyether, the copolymer comprises acrylic structural units, acrylonitrile structural units, flexible cross-linking functional structural units and polyoxyethylene ether structural units containing polyalkyl, and the flexible cross-linking functional structural units comprise hydroxyl groups and ester groups. The thickening agent has good mechanical properties, has excellent thickening effect and dispersion capacity when used in the negative electrode slurry, has high adhesion and good flexibility when used in the negative electrode sheet, guarantees the structural stability of the negative electrode sheet, and thus improves the safety and cycle life of the secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of battery material technology, specifically relating to a thickener, a negative electrode slurry, a negative electrode sheet, and a secondary battery. Background Technology

[0002] Thickeners, as key functional components in secondary battery electrode slurries, mainly play the role of adjusting slurry viscosity, maintaining slurry dispersion stability, and improving the bonding strength between electrode active materials and current collectors, as well as between active materials. Their performance directly affects the processing performance of the slurry, the forming quality of the electrode sheets, and the final electrochemical performance and structural stability of the battery.

[0003] Currently, sodium carboxymethyl cellulose (CMC) is the most widely used thickener in the secondary battery field. However, it has significant performance bottlenecks: On the one hand, the solid content of the negative electrode slurry in the CMC system has limited room for improvement. CMC aqueous solution exhibits strong shear-thinning characteristics and has limited affinity for particle surfaces. To ensure the slurry has a suitable viscosity to meet the coating process requirements, the solid content of the negative electrode slurry is usually 50%~53%, which restricts further increases in electrode energy density. On the other hand, it is difficult to balance the bonding performance and flexibility of CMC. The CMC molecular chain is relatively rigid, and the film is brittle after formation. Electrodes prepared using CMC as a thickener have low peel strength and poor flexibility. Cracking and curling are prone to occur during electrode rolling, cutting, battery assembly, and cycling, affecting battery safety and cycle life.

[0004] To address the aforementioned shortcomings of existing CMC thickeners, the industry urgently needs to develop a novel thickener that combines high adhesion, excellent thickening effect, good flexibility, and high solids content dispersion capability. This would meet the development needs of high-performance and efficient processes in secondary batteries, break through existing technological bottlenecks, and improve the overall performance and production efficiency of secondary batteries. Summary of the Invention

[0005] To address the problems of existing thickeners, this invention provides a thickener, a negative electrode slurry, a negative electrode sheet, and a secondary battery.

[0006] The objective of this invention is achieved through the following technical solution.

[0007] In a first aspect, the present invention provides a thickener comprising a copolymer and a branched long-chain fatty alcohol polyether, wherein the copolymer comprises acrylic structural units, acrylonitrile structural units, flexible crosslinking functional structural units, and polyalkyl-containing polyoxyethylene ether structural units, and the flexible crosslinking functional structural units comprise hydroxyl groups and ester groups.

[0008] Furthermore, the branched long-chain fatty alcohol polyether includes isostearyl alcohol ether, wherein the isostearyl alcohol ether has a mass percentage of 6% to 14% in the solids of the thickener.

[0009] Furthermore, the mass ratio of the acrylic structural unit, acrylonitrile structural unit, flexible cross-linking functional structural unit, polyoxyethylene ether structural unit containing polyalkyl groups to branched long-chain fatty alcohol polyether is (22~32):(16~24):(14.4~21.6):(21~29):(6~14).

[0010] Furthermore, the acrylic structural unit includes acrylic structural units and / or methacrylic structural units.

[0011] Furthermore, the acrylonitrile structural unit includes an acrylonitrile structural unit and / or a methacrylonitrile structural unit.

[0012] Furthermore, the polyalkyl-containing polyoxyethylene ether structural unit includes at least one of isobutylene polyoxyethylene ether structural unit, tert-butylallyl polyoxyethylene ether structural unit, and 2-methylbutenyl polyoxyethylene ether structural unit.

[0013] Furthermore, the flexible cross-linked functional structural unit includes hydroxyethyl acrylate structural units and / or pentaerythritol triacrylate structural units.

[0014] Furthermore, the mass ratio of the hydroxyethyl acrylate structural unit to the pentaerythritol triacrylate structural unit is (4~6):1.

[0015] In a second aspect, the present invention provides a negative electrode slurry, the negative electrode slurry comprising a thickener as described in the first aspect, wherein the amount of thickener added is 0.5% to 1% of the total mass of the solids in the negative electrode slurry, and the solid content of the negative electrode slurry is 50% to 65%.

[0016] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector and an active material layer disposed on at least one side surface of the current collector, the active material layer comprising a thickener as described in the first aspect.

[0017] Fourthly, the present invention provides a secondary battery comprising a negative electrode as described in the first aspect.

[0018] Compared with the prior art, the present invention has the following beneficial effects.

[0019] This invention achieves multi-dimensional functional coverage of "bonding, rigidity, flexibility, dispersion, and thickening" through polymers and various structural units. Acrylic structural units provide core bonding, acrylonitrile structural units enhance mechanical strength, flexible cross-linking functional structural units construct cross-linking sites and balance flexibility, polyalkylene oxide ether structural units optimize interfacial dispersion, and branched long-chain fatty alcohol polyethers achieve efficient targeted thickening, forming a multi-dimensional functional synergy. The thickener of this invention has good mechanical properties, exhibiting excellent thickening and dispersing effects when used in negative electrode slurries, and combining high adhesion and good flexibility when used in negative electrode sheets, thus overcoming the performance shortcomings of traditional CMC thickeners.

[0020] This invention also constructs a dual synergistic network of "chemical crosslinking + physical association." The hydroxyl groups of the flexible crosslinked functional structural units and the carboxyl groups of the acrylic structural units form chemical crosslinking points, while the branched long-chain fatty alcohol polyether forms a reversible physically hydrophobic associative network. This dual network ensures structural stability and possesses dynamic rheological adjustment capabilities, buffering the stress caused by changes in the volume of the electrode material and balancing the rigidity of the acrylonitrile structural units with the flexibility of the system. Moreover, the branched long-chain alkyl structure of the branched long-chain fatty alcohol polyether can disrupt the regular stacking and crystallization behavior of the hydrophobic segments, and the polyether structure forms a uniform and flexible hydrophobic microphase separation structure on the copolymer side chains. Under the high shear forces of slurry stirring, conveying, and coating, this flexible associative structure can achieve reversible dissociation and recombination, avoiding the collapse of the thickener's physical crosslinking network and significantly improving the shear stability and coating processability of the negative electrode slurry.

[0021] This invention achieves a synergistic effect of "low dosage, high thickening, high adhesion, and excellent dispersion" without the need for additional auxiliary reagents, simplifying the negative electrode slurry formulation. The thickener of this invention can directly replace CMC with a relatively reduced dosage, increasing the solid content of the negative electrode slurry at the same viscosity, and significantly improving the dispersion uniformity of the negative electrode slurry, eliminating particle agglomeration and contributing to increased electrode energy density. The negative electrode sheet prepared from this slurry exhibits significantly improved peel strength compared to the traditional CMC system, while also showing significantly optimized flexibility. This effectively avoids cracking and curling problems during processing and use, ensuring the structural stability of the negative electrode sheet, thereby improving the safety, cycle life, and rate performance of the secondary battery. Detailed Implementation

[0022] 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.

[0023] 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 copolymer after the monomer participates in the polymerization reaction. The mass ratio of each structural unit is the mass ratio of the monomers providing each structural unit.

[0024] In a first aspect, the present invention provides a thickener comprising a copolymer and a branched long-chain fatty alcohol polyether, wherein the copolymer comprises acrylic structural units, acrylonitrile structural units, flexible crosslinking functional structural units, and polyalkyl-containing polyoxyethylene ether structural units, and the flexible crosslinking functional structural units comprise hydroxyl groups and ester groups.

[0025] Specifically, the acrylic structural unit provides core adhesion. Its side chain carboxyl groups can form hydrogen bonds with hydroxyl and oxygen-containing functional groups introduced through oxidation and modification on the surface of carbon-based (graphite, hard carbon / soft carbon) and silicon-based (Si / SiOx) negative electrode active materials. On the other hand, it can form interfacial bonds with the hydroxyl groups of the oxide layer on the surface of the copper foil current collector. At the same time, weak self-crosslinking can occur between carboxyl groups, which synergistically improves the interfacial bonding strength between the thickener and the negative electrode active material and the current collector, thereby significantly improving the peel strength of the negative electrode sheet. In addition, the strong hydrophilicity of the carboxyl groups can improve the solubility and dispersion performance of the copolymer in the aqueous negative electrode slurry, reduce slurry agglomeration and sedimentation, and further optimize the static stability of the slurry.

[0026] Acrylonitrile structural units can effectively improve the mechanical strength and structural density of copolymers, prevent the thickener layer from undergoing plastic deformation during electrode rolling and cycling, and enhance the compatibility of copolymers with conductive agents and active substances, reducing interfacial voids.

[0027] Flexible cross-linking functional structural units construct cross-linking sites and balance flexibility. Hydroxyl groups can undergo esterification with the carboxyl groups of acrylic acid to form chemical cross-linking points, constructing a three-dimensional network structure, balancing the rigidity and flexibility of the copolymer, and solving the brittleness problem caused by the introduction of acrylonitrile structural units. Ester groups can improve the interfacial wettability of the copolymer and promote the uniform coating of the slurry on the current collector. At the same time, the hydrophilicity of hydroxyl groups can help improve the dispersion stability of the slurry.

[0028] Polyalkyl-containing polyoxyethylene ether structural units, due to the presence of multiple alkyl groups, exhibit greater steric hindrance. This steric hindrance expands the main chain and hydrophilic segments of the polyoxyethylene ether, forcing these segments to fully extend in water. This allows for the binding of a large number of water molecules, forming intermolecular physical entanglement. At the same addition amount, the thickening effect is more stable, aiding in the construction of a thickening network. Furthermore, it forms a steric barrier between copolymer molecules, preventing excessive entanglement and flocculation, thus preventing a sudden increase in viscosity and stratification of the slurry, resulting in better dispersion and thickening effects. Polyalkyl-containing polyoxyethylene ether structural units can also adapt to the interfacial characteristics of active materials and conductive agents in lithium battery slurries, improving compatibility and dispersion stability.

[0029] In some specific embodiments, the branched long-chain fatty alcohol polyether includes isostearyl alcohol ether, and the isostearyl alcohol ether accounts for 6% to 14% of the solid content of the thickener by mass. Isostearyl alcohol ether is a branched long-chain fatty alcohol polyether. Compared to linear polyethers, its branched alkyl structure can disrupt the regular stacking and crystallization behavior of hydrophobic segments, forming a uniform and flexible hydrophobic microphase separation structure on the copolymer side chains. This flexible associative structure achieves reversible dissociation and recombination under the high shear forces of slurry stirring, conveying, and coating, avoiding the collapse of the physical crosslinking network of the thickener, significantly improving the shear stability and coating processability of the negative electrode slurry, increasing the slurry viscosity, and achieving a highly efficient thickening effect. When the isostearyl alcohol ether accounts for less than 6% of the solid content of the thickener by mass, the thickening effect is insufficient; when it accounts for more than 14%, the viscosity of the negative electrode slurry becomes too high, making it difficult to stir and coat.

[0030] In some specific embodiments, the mass ratio of the acrylic structural unit, acrylonitrile structural unit, flexible cross-linking functional structural unit, polyalkyl-containing polyoxyethylene ether structural unit to branched long-chain fatty alcohol polyether is (22~32):(16~24):(14.4~21.6):(21~29):(6~14).

[0031] When the mass percentage of acrylic structural units is too low, adhesion is insufficient; when it is too high, the slurry viscosity increases abnormally, affecting the coating process. When the mass percentage of acrylonitrile structural units is too low, mechanical strength is insufficient; when it is too high, the thickener flexibility is reduced, leading to electrode cracking. When the mass percentage of flexible crosslinking functional structural units is too low, crosslinking effect is insufficient and flexibility is poor; when it is too high, over-crosslinking occurs, resulting in poor slurry flowability and dispersion stability. When the mass percentage of polyalkyl polyoxyethylene ether structural units is too low, dispersion effect is poor; when it is too high, the concentration of binding functional groups is diluted, reducing electrode peel strength. When the ratio of the four structural units to the branched long-chain fatty alcohol polyether is within a suitable range, the carboxyl concentration of the acrylic structural unit can achieve better adhesion, the acrylonitrile structural unit and the flexible cross-linking functional structural unit balance the rigidity and flexibility of the thickener, and the polyalkyl polyoxyethylene ether structural unit and the branched long-chain fatty alcohol polyether are suitable for the dispersion and thickening requirements of water-based negative electrode slurry, with no functional antagonism, and the copolymer has a narrow weight-average molecular weight distribution.

[0032] In some specific embodiments, the weight-average molecular weight M of the copolymer w The molecular weight average (MWA) is 80,000–150,000 g / mol, with a molecular weight distribution index (PDI) of 1.8–2.2. The preferred MWA is 90,000–120,000 g / mol. If the MWA is too low (<80,000 g / mol), the molecular chain segments are too short, resulting in weak intermolecular physical entanglement, significantly reduced thickening efficiency, insufficient copolymer cohesion, poor mechanical strength of the film, and a marked decrease in peel strength. The electrode is also prone to powdering. Furthermore, the dispersion and stability of graphite and conductive carbon black deteriorates, and the slurry is prone to sedimentation and stratification. If the MWA is too high (>150,000 g / mol), the overall entanglement of the system is too high, the initial viscosity of the slurry is too high, the slurry fluidity is poor, processing is hindered, and molecular chain agglomeration and localized gelation easily occur under high shear conditions, resulting in particle and streak defects during processing. The increased rigidity of the macromolecular chain segments increases the brittleness of the film, decreases the elongation at break, and reduces the flexibility of the electrode, making it prone to cracking during rolling and subsequent winding processes. A molecular weight distribution coefficient (PDI) within the above range helps to achieve uniform cross-linking network and particle dispersion, balance the mechanical flexibility of the electrode with the interfacial adhesion, and reduce system viscosity fluctuations while improving the electrochemical cycle stability of the battery.

[0033] In some specific embodiments, the acrylic structural unit includes acrylic acid structural units and / or methacrylic acid structural units, derived from acrylic acid and / or methacrylic acid. Acrylic acid contains a strongly polar carboxyl group (-COOH), which can form hydrogen bonds and coordination bonds with electrode active materials and current collectors, serving as the source of the core adhesion functional group while ensuring the water solubility of the thickener and the dispersion stability of the slurry. Methacrylic acid has slightly larger steric hindrance of the carboxyl group, improving adhesion durability. Therefore, acrylic acid and / or methacrylic acid are selected as the core adhesion functional monomers to provide carboxyl-enhanced adhesion strength.

[0034] In some specific embodiments, the acrylonitrile structural unit includes an acrylonitrile structural unit and / or a methacrylonitrile structural unit, derived from acrylonitrile and / or methacrylonitrile. Acrylonitrile contains a strong electron-withdrawing cyano group (-CN), which enhances the mechanical strength and structural compactness of the copolymer through dipole-dipole interactions, while also improving interfacial compatibility with conductive agents and active substances. Methacrylonitrile has low steric hindrance of the cyano substituent, high polymerization activity, and a more significant increase in rigidity. Therefore, using acrylonitrile and / or methacrylonitrile can enhance intermolecular forces to strengthen mechanical strength.

[0035] In some specific embodiments, the flexible cross-linked functional structural unit includes hydroxyethyl acrylate structural units and / or pentaerythritol triacrylate structural units, derived from hydroxyethyl acrylate and / or pentaerythritol triacrylate. The flexible cross-linked functional structural unit in this application is a composite system of "main chain cross-linking + side chain cross-linking". The hydroxyethyl acrylate structural unit provides the main chain flexibility and hydroxyl group bonding sites, while the pentaerythritol triacrylate structural unit provides multifunctional cross-linking, improving network density and constructing a nanoscale micro-cross-linked network, which can significantly improve the peel strength of the electrode, thereby improving the cycle life of the battery.

[0036] In some specific embodiments, the flexible crosslinking functional structural unit includes hydroxyethyl acrylate structural units and pentaerythritol triacrylate structural units. The mass ratio of the hydroxyethyl acrylate structural units to the pentaerythritol triacrylate structural units is (4~6):1. Within a suitable mass ratio range, better flexibility and crosslinking effect can be achieved.

[0037] In some specific embodiments, the polyalkyl-containing polyoxyethylene ether structural unit includes at least one of isobutylene polyoxyethylene ether structural unit, isopentenyl polyoxyethylene ether structural unit, tert-butylallyl polyoxyethylene ether structural unit, and 2-methylbutenyl polyoxyethylene ether structural unit. This type of polyalkyl-containing polyoxyethylene ether structural unit, with its sterically hindered alkyl backbone, can suppress the orderly stacking and excessive entanglement of molecular chains, and optimize the amphiphilic balance of the copolymer. It can not only enhance the dispersion and anti-agglomeration ability of negative electrode particles by relying on steric hindrance, but also improve the shear stability and storage viscosity stability of the slurry through dynamic association.

[0038] The thickener of this invention can be obtained by conventional polymerization methods, including emulsion polymerization, solution polymerization, precipitation polymerization, and suspension polymerization, with solution polymerization being preferred. Exemplarily, this invention employs an aqueous free radical polymerization process, using deionized water as the reaction medium, avoiding the introduction of impurities by organic solvents, and is suitable for the high purity requirements of lithium batteries.

[0039] The preparation method of the thickener in this invention includes: weighing each monomer raw material and deionized water according to a certain ratio; adding the deionized water to a reaction vessel; adding polyalkyl polyoxyethylene ether, branched long-chain fatty alcohol polyether, acrylic monomer, acrylonitrile monomer and initiator dropwise at 40-45°C with stirring; maintaining the temperature for 1 hour to ensure uniform dispersion of monomers and avoid uneven local polymerization; raising the temperature to 65-70°C and adding hydroxyethyl acrylate and pentaerythritol triacrylate; reacting for 2-4 hours to construct a stable copolymer skeleton and ensure the structural stability of the thickener; raising the temperature to 80-85°C and adding a trace amount of chain transfer agent to adjust the weight-average molecular weight (Mw) of the copolymer to 80,000-150,000 g / mol and the molecular weight distribution coefficient (PDI) to 1.8-2.2 to improve the reaction conversion rate and further optimize the thickening and bonding properties; cooling to room temperature, adjusting the pH value to 7-8, and adjusting the solid content to obtain the thickener of this invention.

[0040] In the above preparation method, the mass ratio of acrylic monomers, acrylonitrile monomers, flexible crosslinking functional monomers, polyalkyl-containing polyoxyethylene ethers and branched long-chain fatty alcohol polyethers is (22~32):(16~24):(14.4~21.6):(21~29):(6~14).

[0041] The mass ratio of the hydroxyethyl acrylate structural unit to the pentaerythritol triacrylate structural unit is (4~6):1. The chain transfer agent used includes mercaptoethanol, and the amount of chain transfer agent is 0.05%~0.1% of the total mass of the monomers. The initiator used includes ammonium persulfate and / or sodium bisulfite, and the amount of initiator is 0.3%~0.8% of the total weight of the monomers.

[0042] The solid content of the thickener of the present invention can be adjusted to 1% to 5% according to the coating process and slurry requirements, preferably 1.2% to 2.2%.

[0043] Secondly, the present invention provides a negative electrode slurry, the negative electrode slurry comprising a thickener as described in the first aspect, wherein the amount of thickener added is 0.5% to 1% of the total solid content of the negative electrode slurry, and the solid content of the negative electrode slurry is 50% to 65%. Preferably, the solid content of the negative electrode slurry is 55% to 65%.

[0044] The amount of thickener added in this invention is 60% to 80% of that used in traditional CMC. While maintaining the same viscosity to meet process requirements, the amount of thickener used is reduced, yet the solid content of the negative electrode slurry is increased, and the slurry dispersion uniformity is significantly improved, with no particle agglomeration. Furthermore, it ensures excellent thickening effect at low dosages while significantly improving electrode peel strength (more than 30% higher than the CMC system), thus meeting the preparation requirements of high-performance lithium battery slurries.

[0045] The negative electrode slurry of this invention also includes a negative electrode active material, a conductive agent, a binder, and a solvent. For example, the negative electrode active material can be graphite, the conductive agent can be at least one of conductive carbon black, Ketjen black, acetylene black, carbon nanotubes, carbon fibers, graphene, and conductive polymers, the binder can be styrene-butadiene rubber (SBR) or polyacrylic acid (PAA), and the solvent can be deionized water. The mass ratio of the negative electrode active material, conductive agent, thickener, and binder can be (94~96): (1.5~2.5): (0.5~1): (2~3), and an appropriate amount of deionized water is used. Specifically, the mass ratio of the negative electrode active material, conductive agent, thickener, and binder can be 94:2.5:1:2.5, 94:2:1:3, 94.5:2:1:2.5, 94.5:1.5:1:3, 95:1.5:1:2.5, 95:2:1:2, 95.5:1.5:0.7:2.3, 96:1.2:0.8:2, or 96:1:1:2, etc.

[0046] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector and an active material layer disposed on at least one side surface of the current collector, the active material layer comprising a thickener as described in the first aspect.

[0047] First, the thickener and deionized water are pre-dispersed at a ratio of 1:50 at 300 rpm for 30 minutes, followed by stirring for 20 minutes. Then, the negative electrode active material and conductive agent are added, and the mixture is dispersed at a high speed of 800 rpm for 60 minutes. Finally, the binder is added, and the mixture is stirred at a low speed of 250 rpm for 30 minutes to remove bubbles. The coating process uses a comma-shaped doctor blade to achieve a wet film thickness of 120 μm. The film is then dried with hot air at 70–80℃ to ensure residual moisture is <0.5%. A roller pressing pressure of 8–16 MPa is applied to achieve a compaction density of 1.5–1.6 g / cm³ for the negative electrode sheet. 3 .

[0048] Fourthly, the present invention provides a secondary battery comprising a negative electrode as described in the third aspect.

[0049] The specific embodiments of the present invention will be further explained and described below through examples and comparative examples.

[0050] 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. Monomers conforming to this invention are commercially available.

[0051] Example 1 1) Preparation of thickener: Weigh out each monomer raw material according to the following weight proportions: 27 parts acrylic acid, 20 parts acrylonitrile, 15 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, 25 parts isobutylene polyoxyethylene ether, 10 parts isostearyl alcohol ether, and 0.5 parts ammonium persulfate as the initiator.

[0052] Deionized water was added to the reactor. At 42°C, with stirring, isobutylene polyoxyethylene ether, isostearyl ether, acrylic acid, acrylonitrile, and an initiator were added dropwise, and the mixture was kept at this temperature for 1 hour. The temperature was then raised to 67°C, and hydroxyethyl acrylate and pentaerythritol triacrylate were added. The reaction was continued for 2 hours to construct a stable copolymer backbone, ensuring the structural stability of the thickener. The temperature was then raised to 82°C, and 0.05% (by weight of the monomers) of a chain transfer agent (mercaptoethanol) was added. The weight-average molecular weight M of the copolymer was... w The concentration was 105,000 g / mol, and the molecular weight distribution coefficient (PDI) was 1.82. The mixture was cooled to room temperature (25°C), the pH was adjusted to 7.5, and the solid content was adjusted to 2% to obtain the thickener.

[0053] 2) Preparation of the negative electrode: The negative electrode slurry contains 96 parts graphite, 2 parts conductive carbon black (Super P), 0.7 parts thickener of this invention, 1.3 parts styrene-butadiene rubber (SBR), and an appropriate amount of deionized water. The solid content of the negative electrode slurry is 55%.

[0054] First, the thickener and deionized water were pre-dispersed at a ratio of 1:50 at 300 rpm for 30 minutes, followed by stirring for 20 minutes. Then, graphite and Super P were added, and the mixture was dispersed at a high speed of 800 rpm for 60 minutes. Finally, SBR was added, and the mixture was stirred at a low speed of 250 rpm for 30 minutes to remove bubbles. The coating process used a comma-shaped doctor blade, achieving a wet film thickness of 120 μm. The film was then dried in hot air at 80℃ for 30 minutes to ensure residual moisture of <0.5%. A rolling pressure of 12 MPa was applied to achieve a compaction density of 1.6 g / cm³ for the negative electrode sheet. 3 .

[0055] 3) Preparation of lithium-ion secondary batteries: The prepared negative electrode sheet, separator, and positive electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as an separator, thus forming an electrode assembly. The electrode assembly is placed in an outer package, injected with commercially available electrolyte, and sealed. After processes such as electrolyte injection, formation, and degassing, the electrodes are assembled into a CR2032 coin cell, with lithium metal as the counter electrode and 1 mol / L LiPF6-EC / DEC as the electrolyte.

[0056] Example 2 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 22 parts acrylic acid, 24 parts acrylonitrile, 18 parts hydroxyethyl acrylate, 3.6 parts pentaerythritol triacrylate, 23 parts isobutylene polyoxyethylene ether, and 9.4 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The concentration is 102,000 g / mol, and the molecular weight distribution coefficient (PDI) is 1.85.

[0057] Example 3 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 32 parts acrylic acid, 16 parts acrylonitrile, 12 parts hydroxyethyl acrylate, 2.4 parts pentaerythritol triacrylate, 27 parts isobutylene polyoxyethylene ether, and 10.6 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The value is 108000 g / mol, and the molecular weight distribution coefficient (PDI) is 1.83.

[0058] Example 4 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, negative electrode sheet, and lithium-ion secondary battery. The difference lies in the preparation of the thickener: 27 parts methacrylic acid, 20 parts methacrylonitrile, 15 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, 25 parts isobutylene polyoxyethylene ether, and 10 parts isostearyl alcohol ether are used to adjust the weight-average molecular weight M of the copolymer. w The value is 103000 g / mol, and the molecular weight distribution coefficient (PDI) is 1.86.

[0059] Example 5 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 25 parts methacrylic acid, 18 parts methacrylonitrile, 14 parts hydroxyethyl acrylate, 3.5 parts pentaerythritol triacrylate, 26.5 parts isobutylene polyoxyethylene ether, and 13 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... wIt has a concentration of 98,000 g / mol and a molecular weight distribution coefficient (PDI) of 1.88.

[0060] Example 6 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 22 parts methacrylic acid, 22 parts methacrylonitrile, 18 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, 25 parts isobutylene polyoxyethylene ether, and 10 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The concentration is 100,000 g / mol, and the molecular weight distribution coefficient (PDI) is 1.90.

[0061] Example 7 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare the thickener, negative electrode sheet, and lithium-ion secondary battery. The difference lies in the preparation of the negative electrode sheet. The negative electrode slurry contains 95 parts graphite, 2 parts conductive carbon black (Super P), 0.5 parts thickener from Example 1, 2.5 parts styrene-butadiene rubber (SBR), and an appropriate amount of deionized water. The solid content of the negative electrode slurry is 50%, and the weight-average molecular weight M of the copolymer is... w The value is 105,000 g / mol, and the molecular weight distribution coefficient (PDI) is 1.84.

[0062] Example 8 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare the thickener, negative electrode sheet, and lithium-ion secondary battery. The difference lies in the preparation of the negative electrode sheet. The negative electrode slurry contains 95 parts graphite, 2 parts conductive carbon black (Super P), 1 part thickener from Example 1 of this invention, 2 parts styrene-butadiene rubber (SBR), and an appropriate amount of deionized water. The solid content of the negative electrode slurry is 65%, and the weight-average molecular weight M of the copolymer is... w The value is 105,000 g / mol, and the molecular weight distribution coefficient (PDI) is 1.87.

[0063] Example 9 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 17 parts acrylic acid, 28 parts acrylonitrile, 25 parts hydroxyethyl acrylate, 5 parts pentaerythritol triacrylate, 25 parts isobutylene polyoxyethylene ether, and 10 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w It has a concentration of 78,000 g / mol and a molecular weight distribution coefficient (PDI) of 2.05.

[0064] Example 10 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 41 parts acrylic acid, 12 parts acrylonitrile, 10 parts hydroxyethyl acrylate, 2 parts pentaerythritol triacrylate, 25 parts isobutylene polyoxyethylene ether, and 10 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The value is 132,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.10.

[0065] Example 11 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 27 parts acrylic acid, 20 parts acrylonitrile, 15 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, 18 parts isobutylene polyoxyethylene ether, and 17 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The value is 128,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.08.

[0066] Example 12 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 27 parts acrylic acid, 20 parts acrylonitrile, 15 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, 32 parts isobutylene polyoxyethylene ether, and 3 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The molecular weight distribution (PDI) is 82000 g / mol, and the molecular weight distribution coefficient is 2.02.

[0067] Example 13 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 27 parts acrylic acid, 20 parts acrylonitrile, 18 parts hydroxyethyl acrylate, 25 parts isobutylene polyoxyethylene ether, and 10 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The concentration is 130,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.12.

[0068] Example 14 This embodiment uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a negative electrode sheet, and a lithium-ion secondary battery. The difference lies in the preparation of the thickener: 27 parts acrylic acid, 20 parts acrylonitrile, 18 parts pentaerythritol triacrylate, 25 parts isobutylene polyoxyethylene ether, and 10 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... wThe concentration is 72000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.18.

[0069] Comparative Example 1 This comparative example uses existing CMC as the thickener. The negative electrode slurry contains 96 parts graphite, 2 parts conductive carbon black (SuperP), 1 part CMC, 1 part PAA, and an appropriate amount of deionized water. The solid content of the negative electrode slurry is 55%. Subsequent coating processes and the preparation of the lithium-ion secondary battery are the same as in Example 1. The weight-average molecular weight M of the copolymer is... w The concentration is 180,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.35.

[0070] Comparative Example 2 This comparative example uses existing CMC as the thickener. The negative electrode slurry contains 96 parts graphite, 2 parts conductive carbon black (SuperP), 1 part CMC, 1 part SBR, and an appropriate amount of deionized water. The solid content of the negative electrode slurry is 55%. Subsequent coating processes and the preparation of the lithium-ion secondary battery are the same as in Example 1. The weight-average molecular weight M of the copolymer is... w The concentration is 165,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.40.

[0071] Comparative Example 3 This comparative example uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that the monomer raw material for the thickener preparation lacks isostearyl alcohol ether; specifically, the monomer raw materials are: 27 parts acrylic acid, 20 parts acrylonitrile, 15 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, and 35 parts isobutylene polyoxyethylene ether. The weight-average molecular weight M of the copolymer is... w It has a concentration of 85,000 g / mol and a molecular weight distribution coefficient (PDI) of 2.15.

[0072] Comparative Example 4 This comparative example uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that the monomer raw material for the thickener preparation lacks isobutylene polyoxyethylene ether; specifically, the monomer raw materials are 27 parts acrylic acid, 20 parts acrylonitrile, 15 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, and 35 parts isostearyl alcohol ether, and the weight average molecular weight M of the copolymer is... w The value is 125,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.13.

[0073] Comparative Example 5 This comparative example uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a positive electrode sheet, and a lithium-ion secondary battery. The difference lies in the absence of hydroxyethyl acrylate and pentaerythritol triacrylate in the preparation of the thickener. Specifically, the monomer raw materials are: 32 parts acrylic acid, 25 parts acrylonitrile, 28 parts isobutylene polyoxyethylene ether, and 15 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w The value is 138,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.20.

[0074] Comparative Example 6 This comparative example uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that the thickener preparation lacks acrylic acid as a monomer raw material; specifically, it contains 27 parts acrylonitrile, 25 parts hydroxyethyl acrylate, 5 parts pentaerythritol triacrylate, 29 parts isobutylene polyoxyethylene ether, and 14 parts isostearyl alcohol ether. The weight-average molecular weight M of the copolymer is... w It has a concentration of 76,000 g / mol and a molecular weight distribution coefficient (PDI) of 2.16.

[0075] Comparative Example 7 This comparative example uses most of the operating steps and monomer raw materials from Example 1 to prepare a thickener, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that acrylonitrile is missing in the preparation of the thickener monomer raw material; specifically, the monomer raw materials are 49 parts acrylic acid, 15 parts hydroxyethyl acrylate, 3 parts pentaerythritol triacrylate, 29 parts isobutylene polyoxyethylene ether, and 14 parts isostearyl alcohol ether. The weight average molecular weight M of the copolymer is... w The concentration is 122,000 g / mol, and the molecular weight distribution coefficient (PDI) is 2.14.

[0076] Performance testing: To better understand the present invention, the thickener, negative electrode slurry and negative electrode sheet prepared in the above embodiments and comparative examples were tested as follows, and the test results are shown in Table 1.

[0077] [Mechanical Properties of Thickener]: The thickener adhesive was poured into a polytetrafluoroethylene mold and dried in an oven at 80°C for 2 hours. The thickener film was then cut into test strips of fixed specifications (10 mm wide and 0.25 mm thick) using a cutting die. The tensile strength, Young's modulus, and elongation at break of the thickener were measured using a universal testing machine according to GB / T 1040-1992 at a tensile rate of 1 mm / min.

[0078] Tensile strength characterizes the maximum stress a thickener can withstand before fracture. Higher strength indicates a more robust and less likely material to break. Young's modulus characterizes the stiffness of a thickener during elastic deformation—its ability to resist elastic deformation. Higher modulus indicates a harder material that deforms less under stress. Elongation at break is the percentage of the thickener's elongation at fracture relative to its original gauge length. A higher value indicates better flexibility, while a lower value indicates greater brittleness.

[0079] [Viscosity]: The viscosity of the negative electrode slurry was tested using a viscometer. The testing procedure was as follows: the viscometer probe was inserted into the slurry, and the viscosity value at 25°C was recorded. The initial viscosity and the viscosity after standing for 24 hours were also measured. Viscosity increase rate = Viscosity after 24 hours of standing / Initial viscosity 100%.

[0080] [Flexibility]: The test was conducted according to GB / T 1731-2020 "Determination of Coating Film Flexibility" (Extended Method for Electrode Flexibility Test); the negative electrode sheet was tightly wound around cylindrical needles of different diameters, and the coating of the electrode sheet was observed for cracking or peeling. The smallest needle diameter at which the coating of the electrode sheet did not crack or peel was recorded, in mm. The smaller the value, the better the flexibility of the negative electrode sheet.

[0081] [Peel Strength]: A universal tensile testing machine was used, referring to GB / T 2792-2014 "Test Method for Peel Strength of Adhesive Tapes", and the 180° peel method was used to test the bonding strength of the negative electrode sheet. The negative electrode sheet was cut into 10mm×100mm pieces, the tensile rate was set to 100mm / min, and the data acquisition frequency was not less than 10Hz. The negative electrode sheet was flatly fixed on a stainless steel plate, and the copper foil current collector of the negative electrode sheet was tightly attached to the stainless steel back plate. The coating of the negative electrode sheet was tightly attached with special adhesive tape, and the coating was peeled off from the copper foil substrate at a 180° angle. The stable peel force was recorded, and the peel strength was calculated, with the unit being N / m.

[0082] Table 1 The lithium-ion secondary batteries prepared in the above examples and comparative examples were tested for electrochemical cycle performance and rate performance. The test results are shown in Table 2.

[0083] Charge-discharge cycle tests were conducted at 25°C and 1C rate, and the initial capacity and capacity retention after 500 cycles were recorded. Rate performance tests were conducted at 25°C (0.1C-0.5C-1C-2C-3C-4C-0.5C), and the discharge capacity at different rates was recorded.

[0084] Table 2 As can be seen from the test results of Examples 1-14 and Comparative Examples 1-7 in Tables 1 and 2, the thickener of the present invention has good mechanical properties, excellent thickening effect and dispersion ability when used in negative electrode slurry, and high adhesion and good flexibility when used in negative electrode sheet, thus making up for the performance shortcomings of traditional CMC thickeners. Specifically, the thickener of this invention possesses high strength, rigidity, and flexibility; the viscosity of the negative electrode slurry can be stably controlled between 2000 and 3300 mPa·s, and the viscosity increase rate after 24 hours of standing is less than 10%, demonstrating excellent slurry dispersion stability and solving the problems of slurry sedimentation and stratification; the peel strength and bonding performance of the negative electrode sheet are significantly improved by 15% to 20%, effectively solving the problems of electrode sheet powder and particle shedding, ensuring battery cycle stability, and exhibiting good flexibility, making it less prone to cracking and curling during electrode sheet rolling, cutting, battery assembly, and cycling, thus ensuring the structural stability of the negative electrode sheet and improving battery safety, cycle life, and rate performance; after 500 cycles at 1C rate, the battery capacity retention rate can reach over 92%, demonstrating excellent bonding durability and electrochemical stability.

[0085] The test results from Examples 1, 7, and 8 and Comparative Examples 1 and 2 also show that: with a solid content of 55% in the negative electrode slurry, only 0.7 wt% thickener is added, resulting in a higher initial viscosity than existing technologies. This means that the present invention uses a relatively small amount of thickener to achieve the same or higher solid content and viscosity as the negative electrode slurry in the CMC system, indicating that the thickener added in the present invention has a low addition amount but high thickening efficiency. At the same addition amount, the solid content of the negative electrode slurry can be increased at the same viscosity, and the dispersion uniformity of the negative electrode slurry is significantly improved, with no particle agglomeration, which helps to improve the electrode energy density.

[0086] The test results of Examples 1-8 and Comparative Examples 3-7 also show that the four structural units and the branched long-chain fatty alcohol polyether are indispensable. If any one of them is missing, the overall performance of the thickener, negative electrode slurry, negative electrode sheet and battery will be significantly reduced.

[0087] The test results from Examples 1-8 and Examples 9-14 also show that when the ratio of the four structural units to the branched long-chain fatty alcohol polyether is within the preferred range, the strength, stiffness, and flexibility of the thickener are optimally balanced, the dispersion stability and thickening effect of the negative electrode slurry are better, and the bonding strength and flexibility of the negative electrode sheet are also better, which helps to improve the safety and cycle life of the battery. Moreover, when the mass ratio of hydroxyethyl acrylate structural units to pentaerythritol triacrylate structural units is within the preferred range, the flexibility and crosslinking effect are better, the flexibility of the thickener and the negative electrode sheet is better, and the viscosity and dispersion stability of the negative electrode slurry are higher.

[0088] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A thickener, characterized in that, The thickener comprises copolymers and branched long-chain fatty alcohol polyethers. The copolymers comprise acrylic structural units, acrylonitrile structural units, flexible crosslinking functional structural units, and polyalkyl-containing polyoxyethylene ether structural units. The flexible crosslinking functional structural units comprise hydroxyl and ester groups.

2. The thickener according to claim 1, characterized in that, The branched long-chain fatty alcohol polyether includes isostearyl alcohol ether, wherein the isostearyl alcohol ether has a mass percentage of 6% to 14% in the solids of the thickener.

3. The thickener according to claim 1 or 2, characterized in that, The mass ratio of the acrylic structural unit, acrylonitrile structural unit, flexible cross-linking functional structural unit, polyoxyethylene ether structural unit containing polyalkyl groups to branched long-chain fatty alcohol polyether is (22~32):(16~24):(14.4~21.6):(21~29):(6~14).

4. The thickener according to claim 1, characterized in that, The acrylic structural unit includes an acrylic structural unit and / or a methacrylic structural unit; And / or, the acrylonitrile structural unit includes an acrylonitrile structural unit and / or a methacrylonitrile structural unit.

5. The thickener according to claim 1, characterized in that, The polyalkyl-containing polyoxyethylene ether structural unit includes at least one of isobutylene polyoxyethylene ether structural unit, isopentenyl polyoxyethylene ether structural unit, tert-butylallyl polyoxyethylene ether structural unit, and 2-methylbutenyl polyoxyethylene ether structural unit.

6. The thickener according to claim 1, characterized in that, The flexible cross-linked functional structural unit includes hydroxyethyl acrylate structural units and / or pentaerythritol triacrylate structural units.

7. The thickener according to claim 6, characterized in that, The mass ratio of the hydroxyethyl acrylate structural unit to the pentaerythritol triacrylate structural unit is (4~6):

1.

8. A negative electrode slurry, characterized in that, The negative electrode slurry includes a thickener as described in any one of claims 1 to 7, wherein the amount of thickener added is 0.5% to 1% of the total mass of the solids in the negative electrode slurry, and the solid content of the negative electrode slurry is 50% to 65%.

9. A negative electrode sheet, characterized in that, The negative electrode includes a current collector and an active material layer disposed on at least one side of the current collector, the active material layer including a thickener as described in any one of claims 1 to 7.

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

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