Diaphragm wet adhesion enhanced binder and preparation method thereof

Abstract

The invention relates to the technical field of lithium battery diaphragms, in particular to a diaphragm wet adhesion enhanced binder and a preparation method thereof. The core of the binder is that a core-shell structure polymer design is adopted, an inner core is composed of a monomer with a high glass transition temperature, and the inner core is endowed with the capacity of moderately swelling in an electrolyte and releasing cohesiveness; the shell is composed of monomers with low glass transition temperature, and excellent initial dry-state binding power is provided. Meanwhile, active groups such as carboxyl and hydroxyl are introduced into a molecular chain, and a three-dimensional cross-linked network can be formed under specific conditions. According to the invention, the problems of bonding strength reduction and coating layer stripping caused by swelling of a traditional binder in an electrolyte environment are effectively solved, the safety performance and the cycle life of the battery are remarkably improved, the process compatibility is good, and industrial application is easy.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator technology, and more specifically, to a separator wet-bonding reinforced adhesive and its preparation method. Background Technology

[0002] Lithium-ion batteries, as highly efficient energy storage devices, have been widely used in consumer electronics, new energy vehicles, and energy storage systems. In the structure of a lithium-ion battery, the separator is one of the key inner components, its function being to isolate the positive and negative electrodes to prevent short circuits while allowing lithium ions to pass freely. To improve the battery's thermal stability, safety, and electrochemical performance, ceramic or other functional materials are typically coated onto a polyolefin-based separator.

[0003] In this coating process, the binder is a crucial component, its function being to firmly bond the coating particles to the substrate membrane surface. Currently, the industry commonly uses water-based binders (such as polyacrylate binders), but this has certain limitations in practical applications: when the battery is injected with electrolyte, the membrane is in a wet environment, and the bonding strength of some binders decreases significantly, leading to delamination between the coating layer and the substrate membrane. This not only increases the battery's internal resistance and affects cycle life, but the detached ceramic or other functional materials may also cause internal short circuits in the battery, posing a safety hazard.

[0004] Therefore, the industry urgently needs a wet-bonding reinforced adhesive for separators. There is a pressing need to develop a new type of adhesive that not only meets the requirements of conventional coating processes but also significantly improves the wet-bonding peel strength of the separator after electrolyte immersion, thereby ensuring the structural integrity and reliability of the coating throughout the battery's entire lifespan. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a wet-bonding reinforced adhesive for separators, which solves the technical problem that the bonding strength of traditional aqueous adhesives decreases in the electrolyte environment, leading to easy peeling of the battery separator coating and affecting battery safety and cycle life.

[0006] To achieve the above objectives, the present invention is implemented through the following solutions; A method for preparing a diaphragm wet-bonding reinforced adhesive includes the following steps: (1) Preparation of premix: Mix 15-30 parts of deionized water and 0.5-1 parts of emulsifier at 50-60℃, then add 1-5 parts of hydroxyethyl methacrylate monomer, 20-25 parts of methacrylate monomer, and 0.5-1 parts of crosslinking monomer in sequence, and stir to form a homogeneous premix; (2) Core layer polymerization: Add 30-40 parts of deionized water, 0.5-1 parts of emulsifier, and 0.01-0.05 parts of pH adjuster to the reactor, and heat to 60-90℃; add 2-20% of the total amount of premix and 0.01-0.05 parts of initiator, maintain the stirring speed at 100-200 rpm, and react for 10-30 minutes; (3) Shell gradient addition: The remaining premixed solution is added in two stages, as follows: First stage: At 70-90℃, add the premixed solution dropwise over 1-3 hours until it reaches 50% of its total volume; Second stage: Add 5-10 parts of (meth)acrylate monomers to the remaining premix, stir and disperse at high speed, and after half an hour, add the remaining premix and 0.1-0.5 parts of initiator dropwise; (4) Post-treatment: Keep warm at 75-90℃ for 1-3 hours; cool down to 65-80℃ for oxidation-reduction treatment; cool down to below 45℃, add neutralizing agent, filter and discharge to obtain a membrane wet bonding reinforced adhesive.

[0007] Preferably, the (meth)acrylate monomer is selected from at least one of n-butyl methacrylate, isobornyl methacrylate, methyl methacrylate, and ethyl acetoacetate methacrylate.

[0008] Preferably, the crosslinking monomer is at least one of diacetone acrylamide, adipic acid dihydrazide, and glycidyl methacrylate.

[0009] Preferably, 1-5 parts of styrene are added in step (1).

[0010] Preferably, the emulsifier is composed of anionic emulsifier and nonionic emulsifier mixed in a weight ratio of 2:1.

[0011] Preferably, the anionic emulsifier is at least one of alkyl sulfonate, alkyl sulfate, alkylbenzene sulfonate, and alkyl succinate; and the nonionic emulsifier is at least one of alkylphenol polyoxyethylene ether and fatty alcohol polyoxyethylene ether.

[0012] Preferably, the initiator is selected from at least one of sodium persulfate, ammonium persulfate, and potassium persulfate.

[0013] Another object of the present invention is to provide a diaphragm wet-bonding reinforced adhesive, which is prepared by the aforementioned preparation method.

[0014] This application offers the following advantages: By precisely introducing active reactive groups such as carboxyl groups, epoxy groups, and carbon-carbon double bonds into the polymer chain of the binder through molecular design, covalent cross-linking reactions are achieved under specific conditions, thereby improving performance. Specifically: First, it significantly enhances wet bonding strength. The active groups can form a stable three-dimensional cross-linked network in drying or electrolyte environments. This network effectively resists the swelling and penetration of the electrolyte, fundamentally solving the coating peeling problem caused by molecular chain slippage in traditional binders, thus significantly improving wet bonding peel strength. Second, it improves the safety and lifespan of lithium batteries. The robust coating effectively prevents the risk of internal short circuits, ensuring the interface stability of the battery during long-term cycling, thereby extending its service life. Third, it possesses excellent process adaptability. This binder system is compatible with aqueous formulations, its cross-linking reaction conditions are mild, and it seamlessly integrates with existing coating production lines. It can achieve efficient and stable industrial applications without complex modifications, combining excellent performance with economic benefits. Detailed Implementation

[0015] The present invention will be further described below with reference to embodiments, but the scope of protection of the present invention is not limited to the scope described in the embodiments. Example 1

[0016] A method for preparing a diaphragm wet-bonding reinforced adhesive includes the following steps: (1) Preparation of premix: 20 parts of deionized water and 0.8 parts of emulsifier were stirred and mixed at 55±2℃, and 3 parts of hydroxyethyl methacrylate, 25 parts of methyl methacrylate, and 0.8 parts of glycidyl methacrylate were added in sequence and stirred to form a homogeneous premix; (2) Core layer polymerization: Add 35 parts of deionized water, 0.7 parts of emulsifier, and 0.02 parts of pH adjuster to the reactor, and heat to 65±2℃; add 10% (about 5 parts) of the total amount of premix and 0.03 parts of initiator, stir at 150 rpm, and react for 20 minutes; (3) Shell gradient addition: The remaining premixed solution is added in two stages, as follows: First stage: At 75±5℃, add the premixed solution dropwise over 1.5-2 hours until it reaches 50% of its total volume (approximately 19.8 parts). Second stage: Add 8 parts of n-butyl methacrylate to the premix, stir at 1000 rpm for 30 minutes, and then add the remaining premix (about 24.8 parts) and 0.2 parts of initiator dropwise over 1 hour; (4) Post-treatment: Keep warm at 85±5℃ for 2 hours; cool down to 70±5℃ and add 0.2 parts of oxidant, then add 0.2 parts of reducing agent for oxidation-reduction treatment; cool down to 45℃ and add 0.1 parts of neutralizer, filter and discharge to obtain a membrane wet bonding reinforced adhesive.

[0017] In this embodiment, the emulsifier is obtained by uniformly mixing alkyl sulfonate and alkylphenol polyoxyethylene ether at a weight ratio of 2:1; the pH adjuster is triethanolamine; the initiator is potassium persulfate; the oxidant is a mixture of tert-butyl hydroperoxide and hydrogen peroxide; the reducing agent is a mixture of sodium bisulfite and sodium metabisulfite; and the neutralizing agent is triethylamine. Example 2

[0018] The preparation method of a membrane wet-adhesive reinforced adhesive differs from that of Example 1 only in that: during the preparation of the premixed liquid, the methyl methacrylate is reduced to 20 parts and 5 parts of styrene are added, that is, styrene is used to replace part of the methyl methacrylate. Example 3

[0019] The preparation method of a membrane wet-adhesive reinforced adhesive differs from that of Example 1 only in that: in the preparation of the premix, diacetone acrylamide is used instead of glycidyl methacrylate; and 0.2 parts of adipic acid dihydrazide are added in the second stage of the shell gradient addition, that is, after high-speed stirring and dispersion for 30 minutes, the remaining premix, 0.2 parts of initiator and 0.2 parts of adipic acid dihydrazide are added dropwise within 1 hour.

[0020] Comparative Example 1 Compared to Example 1, the preparation method using conventional emulsion polymerization does not form a core-shell structure, and includes the following steps: (1) Preparation of pre-emulsion: Add 40 parts of deionized water and 1.5 parts of emulsifier (alkyl sulfonate and alkylphenol polyoxyethylene ether are mixed at a weight ratio of 2:1) to the reaction vessel. At room temperature, turn on the stirrer and add 3 parts of hydroxyethyl methacrylate, 25 parts of methyl methacrylate, 8 parts of n-butyl methacrylate and 0.8 parts of glycidyl methacrylate in sequence. Stir at 300 rpm for 30 minutes to form monomer pre-emulsion.

[0021] (2) Preparation of initiator solution: Dissolve 0.23 parts of potassium persulfate in 5 parts of deionized water to prepare an initiator aqueous solution.

[0022] (3) Polymerization reaction: Add 10 parts of deionized water to the reactor, heat to 75°C, add about 10% of the above initiator solution, keep warm for 5 minutes, and then add the monomer pre-emulsion and the remaining initiator solution to the reactor at a uniform rate over 4 hours through different constant pressure funnels. During the addition process, maintain a stirring speed of 150 rpm and control the temperature at 75±2°C.

[0023] (4) Post-processing: Same as Example 1.

[0024] Comparative Example 2 Compared with Example 1, the difference lies in the fact that an equal amount of deionized water is used instead of glycidyl methacrylate in the components for preparing the premix. The preparation method of the premix in this comparative example is as follows: 20.8 parts of deionized water and 0.8 parts of emulsifier are stirred and mixed at 55±2℃, and then 3 parts of hydroxyethyl methacrylate and 25 parts of methyl methacrylate are added sequentially and stirred to form a homogeneous premix.

[0025] Comparative Example 3 Compared with Example 1, the difference is that in the second stage of the shell gradient addition, an equal amount of methyl methacrylate is used instead of n-butyl methacrylate. That is, after the first stage is completed, 8 parts of methyl methacrylate are added to the premix, and the mixture is dispersed by high-speed stirring at 1000 rpm for 30 minutes. Then, the remaining premix and 0.2 parts of initiator are added dropwise within 1 hour.

[0026] To test the performance of the binders prepared in Examples 1-3 and Comparative Examples 1-3, they were applied to the lithium battery separator manufacturing process to prepare the corresponding lithium battery separators. The lithium battery separator manufacturing process was as follows: the binder emulsion was uniformly coated onto the surface of the PE separator using a wire bar coater, controlling the dry weight coating amount to be 2-4 mg / cm²; subsequently, the coated separator was pre-dried in a forced-air drying oven at 60°C for 15 minutes, and then transferred to a vacuum oven at 60°C for 3 hours; after that, the coated surface of the dried separator was aligned and bonded with a clean ceramic membrane, and hot-pressed using a roller press at 80°C and 5 MPa; the composite sample was then cut into strips of 25 mm * 200 mm for peel strength testing.

[0027] The test samples prepared above were immersed in the electrolyte and left at room temperature for 24 hours. After being taken out, the surface electrolyte was washed off and the samples were air-dried. The peel strength was tested at a tensile speed of 50 mm / min. 10 PCS of each sample were tested and the average value was calculated. The data are shown in Table 1.

[0028] Table 1 Test Data

[0029] As shown in Table 1, the peel strength of the three embodiments provided by this invention not only did not decrease after immersion in the electrolyte, but also significantly improved compared to before immersion (Example 1 improved by 14.2%, Example 2 by 15.3%, and Example 3 by 22.8%). This anomalous phenomenon is due to the activation of the active crosslinking groups in the adhesive of this invention in the electrolyte environment, forming a denser three-dimensional crosslinking network, thereby achieving the "wet-state enhancement" effect. Comparative Example 1 (without core-shell structure) showed a sharp drop in peel strength to 45.2 N / m after immersion in electrolyte, a decrease of 49%. This indicates that without a core-shell structure design, the adhesive molecular chains undergo disordered swelling in the electrolyte, leading to severe failure of the adhesive properties. This, in turn, proves that the core-shell structure is the basic framework for maintaining stable wet performance.

[0030] Comparative Example 2 (without crosslinked monomers) showed a strength decrease to 58.9 N / m (a decrease of 35.1%) after immersion in electrolyte. Although its performance was better than Comparative Example 1, it was far inferior to the Example. This demonstrates that without active crosslinking groups, even with a core-shell structure, it is impossible to achieve an increase in wet strength. The active crosslinking network is the direct cause of the "wet enhancement" effect.

[0031] Comparative Example 3 (using a high glass transition temperature monomer for the outer shell) had an initial strength (before immersion in electrolyte) of 86.2 N / m, which was comparable to the example, but its strength after immersion in electrolyte (51.4 N / m) deteriorated significantly. This indicates that a rigid outer shell severely affects the release of the adhesive's bonding performance in the electrolyte environment, proving that using a low glass transition temperature monomer for the outer shell to provide good flexibility and wettability is a prerequisite for achieving effective bonding.

[0032] In summary, the technical effect of this invention is the result of the synergistic effect of the core-shell structure and the active crosslinking: the core-shell structure provides a physical framework to resist electrolyte swelling, while the active crosslinking further strengthens the bonding network in a wet environment.

[0033] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method of preparing a separator wet adhesion enhanced adhesive, characterized by, The method comprises the following steps: (1) Preparation of a premix: mix deionized water 15-30 parts and emulsifier 0.5-1 part at 50-60°C under stirring, then add (meth) acrylic acid hydroxyethyl ester monomer 1-5 parts, (meth) acrylic acid ester monomer 20-25 parts, crosslinking monomer 0.5-1 part, and stir to form a uniform premix; (2) Core layer polymerization: add deionized water 30-40 parts, emulsifier 0.5-1 part, and pH regulator 0.01-0.05 part to a reaction kettle, heat to 60-90°C; add 2-20% of the total amount of the premix and initiator 0.01-0.05 part, and maintain the stirring speed at 100-200 rpm, and react for 10-30 minutes; (3) Shell layer gradient dropping: drop the remaining premix in two stages as follows: First stage: drop the premix to 50% of the total amount at 70-90°C within 1-3 hours; Second stage: add (meth) acrylic acid ester monomer 5-10 parts to the remaining premix, disperse under high-speed stirring, and then drop the remaining premix and initiator 0.1-0.5 part after half an hour; (4) Post-treatment: maintain the temperature at 75-90°C for 1-3 hours, reduce the temperature to 65-80°C for redox treatment, and then add neutralizing agent when the temperature is below 45°C, and filter the product to obtain a wet adhesion enhanced separator.

2. The production method according to claim 1, characterized by, The (meth) acrylic acid ester monomer is at least one of n-butyl methacrylate, isobornyl methacrylate, methyl methacrylate, and acetoacetoxy methyl methacrylate.

3. The preparation method according to claim 1, characterized in that, The crosslinking monomer is at least one of diacetone acrylamide, adipic acid dihydrazide, and glycidyl methacrylate.

4. The production method according to claim 1, characterized by, In step (1), 1-5 parts of styrene is further added.

5. The method of claim 1, wherein, The emulsifier is composed of anionic emulsifier and non-ionic emulsifier at a weight ratio of 2:

1.

6. The production method according to claim 5, wherein The anionic emulsifier is at least one of alkyl sulfonate, alkyl sulfate, alkyl benzene sulfonate, and alkyl succinate; and the non-ionic emulsifier is at least one of alkyl phenol polyoxyethylene ether and fatty alcohol polyoxyethylene ether.

7. The preparation method according to claim 1, characterized in that, The initiator is at least one of sodium persulfate, ammonium persulfate, and potassium persulfate.

8. A separator wet adhesion enhanced adhesive characterized by, The product is prepared by the method of any one of claims 1-8.

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