An acid-free acrylic pressure-sensitive adhesive for lithium batteries, a pi tab adhesive tape, and a preparation method thereof

Abstract

The application relates to the technical field of lithium battery tab adhesive tape, in particular to an acid-free acrylic pressure-sensitive adhesive for lithium batteries, a PI tab adhesive tape and a preparation method thereof. The pressure-sensitive adhesive comprises an acid-free acrylic copolymer, amino-modified nano-silica and a polyisocyanate crosslinking agent; the copolymer adopts a polymerization monomer system without free carboxyl groups, and polymerizable tertiary amine type functional monomers, epoxy group-containing functional monomers and hydroxyl group-containing branched functional monomers are introduced; the epoxy groups in the copolymer are subjected to ring-opening pre-grafting with the amino-modified nano-silica, and the polyisocyanate is added for crosslinking, so as to form an organic-inorganic crosslinking network; the obtained pressure-sensitive adhesive is coated on a PI base material subjected to surface activation treatment, and the PI tab adhesive tape is prepared through segmented drying and curing. The application can reduce the migration of acidic structures and small molecule amines, improve the electrolyte resistance, heat resistance and tab interface stability of the adhesive layer.

Description

Technical Field

[0001] This application relates to the field of lithium battery tab tape technology, and in particular to an acid-free acrylic pressure-sensitive adhesive for lithium batteries, a PI tab tape, and a method for preparing the same. Background Technology

[0002] The tabs of lithium-ion batteries typically require PI tab tape for insulation, fixation, and interface protection. Existing PI tab tapes mostly use acrylic pressure-sensitive adhesives. To improve the adhesion of the adhesive to the PI substrate and metal tabs, carboxyl-containing monomers such as acrylic acid and methacrylic acid are usually introduced into the acrylic copolymer system. However, the carboxyl groups in the polymer and any residual acidic substances that may be present in the system can adversely affect the aluminum tab interface under long-term high temperatures and electrolyte conditions, causing blackening, corrosion, and deterioration of the interface contact.

[0003] Existing methods for reducing acid value mainly include reducing the amount of acidic monomers or adding amine neutralizers. However, reducing the proportion of acidic monomers can easily lead to a decrease in the polarity and adhesion of the adhesive layer, while free small-molecule amine neutralizers pose a migration risk under high temperature and electrolyte environments. On the other hand, existing technologies also improve the cohesiveness and dielectric resistance of the adhesive layer by adding nano-silica and isocyanate crosslinking agents. However, ordinary nanofillers mainly exist in a physically dispersed form, and the interfacial bonding between the filler and the polymer is limited. Furthermore, directly increasing the amount of crosslinking agent may lead to a decrease in initial tack performance. Therefore, how to reduce the acidic structure and the content of migratable small molecules in the adhesive layer while simultaneously ensuring the adhesion performance, high-temperature stability, electrolyte resistance, and aluminum tab interface protection of the PI substrate remains a technical problem that lithium battery PI tab tapes need to solve. Summary of the Invention

[0004] This application provides an acid-free acrylic pressure-sensitive adhesive for lithium batteries, a PI tab tape, and a method for preparing the same, to solve the above-mentioned problems.

[0005] In a first aspect, this application provides an acid-free acrylic pressure-sensitive adhesive for lithium batteries, wherein the acid-free acrylic pressure-sensitive adhesive comprises an acid-free acrylic copolymer, amino-modified nano-silica, and a polyisocyanate crosslinking agent; The acid-free acrylic copolymer is formed by free radical copolymerization of a monomer system including soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers and hydroxyl-branched functional monomers, wherein the monomer system does not contain polymerizable acidic monomers with free carboxyl groups. The polymerizable tertiary amine functional monomer has both a carbon-carbon double bond and a tertiary amine group that can be polymerized by free radicals, so that the tertiary amine group is fixed in the molecular chain of the acid-free acrylic copolymer in a covalent bond form. The epoxy groups in the acid-free acrylic copolymer form an epoxy ring-opening graft structure with the amino groups on the surface of the amino-modified nano silica, and hydroxyl groups are formed in the epoxy ring-opening graft structure. The polyisocyanate crosslinking agent forms a crosslinking structure with the hydroxyl groups in the epoxy ring-opening graft structure and the hydroxyl groups provided by the hydroxyl-containing branched functional monomer, thereby enabling the acid-free acrylic copolymer and the amino-modified nano silica to form an organic-inorganic crosslinking network.

[0006] Optionally, the soft monomer includes isooctyl acrylate and butyl acrylate, the hard monomer is methyl methacrylate, the polymerizable tertiary amine functional monomer is N,N-dimethylacrylamide, the epoxy-containing functional monomer is glycidyl methacrylate, and the hydroxyl-containing branched functional monomer is trimethylolpropane diallyl ether; the amino-modified nano silica is nano-fumed silica surface-modified with γ-aminopropyltriethoxysilane, and the polyisocyanate crosslinking agent is TDI trimer isocyanate.

[0007] Optionally, based on a total mass of 100 parts for the soft monomer, hard monomer, polymerizable tertiary amine functional monomer, epoxy-containing functional monomer, and hydroxyl-containing branched functional monomer, the soft monomer comprises 84-87 parts, the hard monomer comprises 9-10 parts, the polymerizable tertiary amine functional monomer comprises 2-3 parts, the epoxy-containing functional monomer comprises 0.5-2 parts, and the hydroxyl-containing branched functional monomer comprises 0.5-2 parts. Based on the effective solid content of 100 parts by weight of the acid-free acrylic copolymer, the amount of amino-modified nano-silica is 0.5 to 1 part by weight, and the amount of TDI trimer isocyanate is 0.5 to 1 part by weight.

[0008] Secondly, this application provides an acid-free acrylic PI tab tape for lithium batteries, the acid-free acrylic PI tab tape comprising a PI substrate and an acid-free acrylic pressure-sensitive adhesive layer disposed on at least one surface of the PI substrate; The acid-free acrylic pressure-sensitive adhesive layer comprises an acid-free acrylic copolymer, amino-modified nano-silica, and a polyisocyanate crosslinking agent; The acid-free acrylic copolymer is formed by free radical copolymerization of a monomer system including soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers and hydroxyl-branched functional monomers. The monomer system does not contain polymerizable acidic monomers with free carboxyl groups. The tertiary amine groups in the polymerizable tertiary amine functional monomers are fixed in the molecular chain of the acid-free acrylic copolymer in a covalent bond. The epoxy groups in the acid-free acrylic copolymer form an epoxy ring-opening graft structure with the amino groups on the surface of the amino-modified nano silica. The polyisocyanate crosslinking agent forms a crosslinking structure with the hydroxyl groups formed by the epoxy ring-opening grafting structure and the hydroxyl groups provided by the hydroxyl-containing branched functional monomer, thereby forming an organic-inorganic crosslinking network containing the acid-free acrylic copolymer and the amino-modified nano silica in the acid-free acrylic pressure-sensitive adhesive layer.

[0009] Optionally, the amino-modified nano-silica is nano-fumed silica that has been surface-modified with γ-aminopropyltriethoxysilane; Based on 100 parts by mass of the effective solid content of the acid-free acrylic copolymer, the amount of amino-modified nano silica is 0.5 to 1 part by mass, the amount of polyisocyanate crosslinking agent is 0.5 to 1 part by mass, and the polyisocyanate crosslinking agent is TDI trimer isocyanate.

[0010] Optionally, the acid-free acrylic copolymer has an acid value of less than 0.01 mg KOH / g, a number-average molecular weight of 300,000 to 500,000, and a glass transition temperature of -55℃ to -45℃. The thickness of the PI substrate is 25 μm, and the dry adhesive thickness of the acid-free acrylic pressure-sensitive adhesive layer is 6–10 μm.

[0011] Thirdly, this application provides a method for preparing an acid-free acrylic pressure-sensitive adhesive and a PI electrode tab tape for lithium batteries, the method comprising: S1. Soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers, and hydroxyl-containing branched functional monomers are copolymerized under the action of a free radical initiator to obtain an acid-free acrylic copolymer. The monomer system used in the copolymerization reaction does not contain polymerizable acidic monomers with free carboxyl groups, and the polymerizable tertiary amine functional monomers simultaneously possess carbon-carbon double bonds and tertiary amine groups that can be polymerized by free radicals. S2. Nano-silica with surface modification by amino-containing silane coupling agent is added to the acid-free acrylic copolymer for dispersion, so that the epoxy groups in the acid-free acrylic copolymer react with the amino groups on the surface of the nano-silica to form an epoxy ring-opening graft structure connecting the acid-free acrylic copolymer and the nano-silica, and hydroxyl groups are formed in the epoxy ring-opening graft structure. S3. After completing the ring-opening reaction, reduce the temperature of the adhesive solution and add a polyisocyanate crosslinking agent to allow the polyisocyanate crosslinking agent to crosslink with the hydroxyl groups in the epoxy ring-opening graft structure and the hydroxyl groups provided by the hydroxyl-containing branched functional monomer to obtain an acid-free acrylic pressure-sensitive adhesive coating solution. S4. Perform surface activation treatment on the PI substrate surface to be coated with adhesive, and apply the acid-free acrylic pressure-sensitive adhesive coating liquid to the surface of the PI substrate after surface activation treatment. S5. The coated PI substrate is subjected to segmented heating, drying and curing to form an acid-free acrylic pressure-sensitive adhesive layer from the acid-free acrylic pressure-sensitive adhesive coating liquid, thereby obtaining the acid-free acrylic PI tab tape for lithium batteries.

[0012] Optionally, in step S1, dilauryl peroxide is used as the free radical initiator; After adding the monomer and part of the solvent to the polymerization reactor, nitrogen gas is introduced from the bottom of the reactor at a flow rate of 60 L / min for 60 min, and the mixture is stirred at a frequency of 15 Hz. After heating the reaction system to 80-82℃, the first initiator solution was added, and the mixture was refluxed at 80-86℃ for 60 min. The second initiator solution was then added dropwise over 120 minutes, with the temperature of the reaction system maintained at 78–82 °C during the addition process. After the addition is complete, the mixture is kept at 76–85°C and refluxed for 180 min. After cooling, the remaining polymerization solvent is added to adjust the solid content of the resulting acid-free acrylic copolymer to 38%–42%.

[0013] Optionally, in step S2, based on 100 parts by mass of the effective solid component of the acid-free acrylic copolymer, 0.5 to 1 part by mass of amino-modified nano-silica is added, and dispersed at a speed of 1000 to 1600 r / min for 10 to 20 min, and then the adhesive is kept at 40 to 55°C for 20 to 60 min to react. In step S3, after the ring-opening reaction in step S2 is completed, the temperature of the adhesive solution is reduced to below 30°C, and 0.5 to 1 part by mass of TDI trimer isocyanate is added based on 100 parts by mass of the effective solid component of the acid-free acrylic copolymer, and stirred for 3 to 10 minutes.

[0014] Optionally, in step S4, a PI substrate with a thickness of 25 μm is used, the surface of the PI substrate to be coated is subjected to corona treatment, and the acid-free acrylic pressure-sensitive adhesive coating liquid is applied to the corona-treated PI substrate surface with a wet adhesive thickness of 30 to 50 μm. In step S5, the coated PI substrate is sequentially passed through six drying zones at temperatures of 80℃, 100℃, 120℃, 140℃, 160℃ and 120℃, with a total drying time of 1 to 2 minutes, so that the dry adhesive thickness of the resulting acid-free acrylic pressure-sensitive adhesive layer is 6 to 10 μm, and then cured at 40 to 60℃ for 24 to 72 hours.

[0015] Through the above technical solution, this application has the following beneficial effects: The acidity of the polymer structure is reduced. The monomer system of this application does not introduce polymerizable acidic monomers with free carboxyl groups, so that the resulting acid-free acrylic copolymer can maintain a low acid value, thereby reducing the adverse effects of traditional carboxyl-based acrylic pressure-sensitive adhesives on the aluminum tab interface from the source.

[0016] Reduce the migration risk of small molecule amines. Use functional monomers that simultaneously possess polymerizable carbon-carbon double bonds and tertiary amine groups, so that the tertiary amine groups are fixed in the polymer molecular chain by free radical copolymerization, rather than existing in the form of free amines. This balances the role of nitrogen-containing polar structures with the requirement of low migration in long-term electrolyte environments.

[0017] A chemically grafted organic-inorganic network is formed. The epoxy groups on the side chains of the copolymer first undergo a ring-opening reaction with the amino groups on the surface of the amino-modified nano-silica, transforming the nano-silica from a common physical filler into cross-linked network nodes, thereby improving the interfacial bonding stability between the inorganic particles and the acrylic polymer.

[0018] The dielectric resistance is improved by "pre-grafting followed by secondary crosslinking". After the epoxy-amino pre-grafting is completed, polyisocyanate is added to allow it to react further with the hydroxyl groups generated by ring opening and the hydroxyl groups provided by branched functional monomers, forming a more continuous organic-inorganic crosslinking structure, thereby improving the anti-swelling, high-temperature tack, and electrolyte resistance of the adhesive layer.

[0019] Balancing processability and long-term reliability, the PI tab tape utilizes PI surface activation, segmented temperature drying, and post-curing to achieve good structural stability while maintaining suitable initial tack and peel performance. This reduces the risk of adhesive layer delamination and aluminum tab corrosion under long-term electrolyte exposure, making it suitable for insulation, fixation, and interface protection of lithium battery tabs. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic flowchart illustrating the preparation method of the acid-free acrylic pressure-sensitive adhesive and PI electrode tape of this application. Figure 2 This is a schematic diagram of the organic-inorganic crosslinking network in the acid-free acrylic pressure-sensitive adhesive of this application; Figure 3 This is a schematic diagram of the epoxy-amino pre-grafting and isocyanate secondary crosslinking process of this application; Figure 4 This is a schematic diagram of the layered structure of the PI electrode tab tape of this application; Figure 5 This is a schematic diagram of the segmented drying and curing process of the PI electrode tape in this application; Figure 6 This is a schematic diagram of the application structure of the PI electrode tab tape applied to the surface of the aluminum electrode tab in this application. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0023] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0024] The present invention will be further described below with reference to specific embodiments. These embodiments are used to illustrate the technical concept and implementation of the present invention and do not constitute a limitation on the scope of protection of the present invention. Without departing from the technical concept of the present invention, equivalent substitutions or conventional adjustments made by those skilled in the art to the types of raw materials, dosage ratios, and process parameters are all applicable to the present invention.

[0025] The "acid-free acrylic pressure-sensitive adhesive" described in this invention refers to a polymerizable acidic monomer with free carboxyl groups that is not actively added to the monomer system used to prepare the acrylic copolymer. The polymerizable acidic monomer with free carboxyl groups includes one or more of acrylic acid, methacrylic acid, itaconic acid, maleic acid, and fumaric acid. The resulting acrylic copolymer preferably has an acid value of less than 0.05 mgKOH / g, more preferably less than 0.01 mgKOH / g.

[0026] This invention does not reduce the apparent acid value by adding a free alkaline neutralizing agent after polymerization. Instead, it introduces a tertiary amine functional monomer capable of participating in free radical copolymerization into the acrylic copolymer, fixing the tertiary amine groups to the polymer molecular chain via covalent bonds. Simultaneously, epoxy and hydroxyl reaction sites are introduced into the polymer molecular chain, and an organic-inorganic crosslinking network is constructed using the ring-opening reaction between epoxy groups and amino-modified nano-silica, as well as subsequent isocyanate crosslinking reactions. The overall preparation process is as follows: Figure 1 As shown.

[0027] General Implementation I. General Preparation Methods for Acid-Free Acrylic Acid Copolymers The polymerizable monomers used to prepare acid-free acrylic copolymers include soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers, and hydroxyl-branched functional monomers.

[0028] The soft monomer is used to impart room temperature adhesion and chain segment mobility to the pressure-sensitive adhesive layer, and preferably includes one or both of butyl acrylate and isooctyl acrylate, more preferably using both butyl acrylate and isooctyl acrylate.

[0029] The hard monomer is used to improve the cohesive strength and high-temperature dimensional stability of the copolymer, and is preferably methyl methacrylate.

[0030] The polymerizable tertiary amine functional monomer possesses both carbon-carbon double bonds and tertiary amine groups capable of participating in free radical polymerization. After the carbon-carbon double bonds participate in copolymerization, the tertiary amine groups are fixed within the acrylic copolymer molecular chain, thereby reducing the migration risk present when using free small-molecule amines.

[0031] The epoxy-containing functional monomer is preferably glycidyl methacrylate, whose carbon-carbon double bond participates in the copolymerization reaction, and the epoxy group is retained in the side chain of the copolymer for ring-opening reaction with the subsequently added amino-modified nano silica.

[0032] The hydroxyl-containing branched functional monomer is preferably trimethylolpropane diallyl ether, in which the allyl group participates in polymerization to form a branched structure, and the hydroxyl group retained after polymerization serves as the reaction site for subsequent polyisocyanate crosslinking reaction.

[0033] The polymer raw materials preferably include 84-87 parts of soft monomers, 9-11 parts of hard monomers, 1-3 parts of polymerizable tertiary amine functional monomers, 0.5-2 parts of epoxy-containing functional monomers, and 0.5-2.5 parts of hydroxyl-branched functional monomers by weight.

[0034] The free radical initiator is preferably dilauryl peroxide, and its amount is preferably 0.1 to 0.5 parts by mass relative to 100 parts by mass of the polymerizing monomer. The polymerization solvent is selected from one or more of ethyl acetate, toluene, butanone, xylene, and acetone, and preferably a mixture of ethyl acetate and toluene.

[0035] In the specific preparation process, soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers, hydroxyl-containing branched functional monomers, and a portion of the polymerization solvent are added to the polymerization reactor, and the stirring device is started.

[0036] Subsequently, nitrogen gas was continuously introduced from the bottom of the reactor to replace oxygen. The nitrogen flow rate was controlled at 60 L / min, the nitrogen replacement time was 60 min, and the stirring motor frequency was controlled at 15 Hz during the nitrogen replacement stage.

[0037] After nitrogen purging, the reaction system is heated. When the temperature of the reaction system reaches 80°C, active heating is stopped, and the temperature of the reaction system is stabilized at 80–82°C. Then, the first portion of dilauryl peroxide, which has been pre-dissolved in the solvent, is added.

[0038] When the reaction system exhibits temperature rise and reflux, the stirring motor frequency is increased to 30 Hz, and the polymerization is carried out under reflux conditions of 80–86℃ for 60 min.

[0039] After the initial polymerization is completed, the remaining dilauryl peroxide solution is continuously added dropwise over 120 min, with the temperature of the reaction system controlled at 78–82 °C during the dropwise addition stage.

[0040] After the initiator is added, the reaction system is kept under reflux at 76–85°C for 180 min to allow the remaining polymerizable monomers to further participate in polymerization.

[0041] After the heat preservation is completed, heating is stopped and the material is cooled. When the material temperature drops to no higher than 50°C, the remaining polymerization solvent is added to adjust the copolymer solid content to 38%–42%. The mixture is then filtered to obtain an acid-free acrylic copolymer solution. The remaining polymerization solvent refers to the additional solvent added after polymerization to adjust the solid content. The amount of mixed solvent used in the polymerization stage listed in the formulation table of the examples does not include this additional portion.

[0042] The number-average molecular weight of the obtained acid-free acrylic copolymer is preferably 300,000 to 500,000, the glass transition temperature is -55℃ to -45℃, and the viscosity at 23℃ is 1500 to 4000 mPa·s.

[0043] For residual monomer content, the absolute statement "residual monomer is zero" is not used. Instead, chromatographic methods are used to quantify each target monomer. Quantitatively detectable values ​​are reported as measured values. Values ​​lower than the corresponding method's limit of quantitation are indicated as "<limit of quantitation".

[0044] II. General Preparation Methods for Amino-Modified Nano-Silica This invention employs nano-fumed silica with a surface modified by an amino-silane coupling agent to introduce amino groups on the surface of the nano-silica that can undergo ring-opening reactions with the side chain epoxy groups of the acid-free acrylic copolymer.

[0045] The primary particle size of the nano-vaporized silica is preferably 7–30 nm.

[0046] The preferred aminosilane coupling agent is γ-aminopropyltriethoxysilane, i.e., KH-550.

[0047] In the specific preparation process, KH-550 is added to a mixed medium of ethanol and water for hydrolysis. The preferred mass ratio of ethanol to water is 85:15 to 95:5. Acetic acid is used to adjust the pH of the system to 4.5 to 5.5. Hydrolysis is carried out at 20 to 30°C for 30 to 60 minutes, so that the alkoxy groups in KH-550 are converted into silanol structures that can react with the hydroxyl groups on the surface of silica.

[0048] Subsequently, nano-fumed silica was added to a hydrolyzed KH-550 solution, with the mass fraction of nano-silica in the mixed medium controlled at 5%–10%. The mixture was stirred at 50–70°C for 1–3 h to ensure sufficient contact between KH-550 and nano-silica, and to allow the silanol groups to undergo a condensation reaction with the hydroxyl groups on the surface of nano-silica, thereby forming an amino-containing organosilicon modified layer on the surface of nano-silica.

[0049] The preferred amount of KH-550 added is 2% to 8% of the mass of nano-silica.

[0050] After modification, the liquid medium is removed, and the obtained modified nano silica is dried at 100-130°C until its moisture content is not higher than 0.20 wt%, and more preferably not higher than 0.10 wt%.

[0051] By controlling the water content of modified nano-silica, the risk of introducing additional moisture into the pressure-sensitive adhesive layer can be reduced.

[0052] III. General Preparation Method of Acid-Free Acrylic Pressure-Sensitive Adhesive Coating Solution Take 100 parts by weight of the aforementioned acid-free acrylic copolymer solution and add 0.5 to 1 part by weight of amino-modified nano-silica based on the effective components of the copolymer.

[0053] Depending on the viscosity of the adhesive, add ethyl acetate or a mixture of ethyl acetate and toluene as a solvent to keep the adhesive in a coatable state.

[0054] After adding amino-modified nano-silica to the acid-free acrylic copolymer, disperse it at a speed of 1000-1600 r / min for 10-20 min to ensure that the nano-silica is uniformly dispersed in the adhesive solution.

[0055] After dispersion, the temperature of the adhesive solution is controlled at 40-55℃, and the reaction is maintained at this temperature for 20-60 minutes.

[0056] In this process, the epoxy groups on the side chains of the acid-free acrylic copolymer undergo a ring-opening reaction with the primary amino groups on the surface of the amino-modified nano silica, causing the nano silica to be chemically bonded to the acrylic copolymer segments and forming hydroxyl groups at the ring-opening positions of the epoxy.

[0057] Through the above pre-grafting steps, nano-silica is transformed from a simple physical filler into a reaction node in an organic-inorganic network, thereby reducing the possibility of electrolyte penetration along the interface between polymer and inorganic particles.

[0058] After the pre-grafting reaction is completed, the temperature of the adhesive solution is lowered to below 30°C, and then a polyisocyanate crosslinking agent is added.

[0059] The polyisocyanate crosslinking agent is preferably TDI trimer isocyanate, and its addition amount is 0.5 to 1 part by mass relative to 100 parts by mass of the effective solid content of the acid-free acrylic copolymer; the addition amount is based on the effective solid content of TDI trimer isocyanate, and preferably a crosslinking agent with an NCO mass fraction of 12% to 14% is used.

[0060] Before adding TDI trimer isocyanate, dilute it with a solvent and then add it to the adhesive solution and stir for 3 to 10 minutes. After mixing, add ethyl acetate or a mixture of ethyl acetate and toluene according to the target dry adhesive thickness to adjust the total solid content of the acid-free acrylic pressure-sensitive adhesive coating solution to 20% to 25%, so that the 30 to 50 μm wet adhesive layer will form a 6 to 10 μm dry adhesive layer after drying.

[0061] In this embodiment, the feeding sequence of "pre-grafting epoxy-amino groups first, then adding isocyanate crosslinking agent" is adopted, so that the amino-modified nano-silica preferentially participates in the epoxy ring-opening reaction, reducing the premature consumption of amino groups on the surface of nano-silica by polyisocyanates. The above-mentioned pre-grafting and secondary crosslinking process is as follows: Figure 3 As shown.

[0062] Subsequently, the polyisocyanate crosslinking agent undergoes a crosslinking reaction with the hydroxyl groups formed by the ring-opening of the epoxy resin and the hydroxyl groups provided by the hydroxyl-branched functional monomers, thereby forming an organic-inorganic crosslinked structure with the acrylic copolymer as the continuous phase and chemically grafted nano-silica as the crosslinking nodes. This organic-inorganic crosslinked network structure is as follows: Figure 2 As shown.

[0063] IV. General Preparation Method of PI Electrode Ear Tape A PI film with a thickness of 25 μm was used as the substrate. The resulting PI tab tape has a layered structure as shown below. Figure 4 As shown.

[0064] Before coating, the surface of the PI film to be coated is subjected to corona treatment to improve the wettability of the PI surface and the interfacial bonding ability between the pressure-sensitive adhesive layer and the PI substrate.

[0065] In one implementation, the corona treatment power is 0.5 kW.

[0066] The aforementioned acid-free acrylic pressure-sensitive adhesive coating solution was uniformly coated onto the corona-treated PI film surface using a comma-shaped doctor blade coating head or a slit-type coating head, with the wet adhesive layer thickness controlled to be 30–50 μm.

[0067] The coated PI film passes through six drying zones sequentially, with the temperature of each zone set as follows: 80℃, 100℃, 120℃, 140℃, 160℃ and 120℃.

[0068] The front temperature zone is mainly used to gradually remove organic solvents from the adhesive solution, the medium and high temperature zone is used to further promote the cross-linking reaction inside the adhesive layer and complete the shaping of the adhesive layer, and the final section reduces the heat load when the film material leaves the oven.

[0069] The dry thickness of the acid-free acrylic pressure-sensitive adhesive layer formed after drying is controlled to be 6–10 μm.

[0070] After drying, the obtained PI tape is placed in an environment of 40-60℃ for 24-72 h to allow the remaining isocyanate groups in the adhesive layer to continue to react with the hydroxyl groups, thereby stabilizing the cross-linking structure of the adhesive layer.

[0071] After curing, the material is wound and slit to obtain acid-free acrylic PI electrode tab tape for lithium batteries. The above-mentioned segmented drying and curing process is as follows: Figure 5 As shown.

[0072] V. General Testing Methods To evaluate the performance of the acid-free acrylic copolymer and PI tab tape, the following methods were used for testing.

[0073] Acid value was determined by potassium hydroxide potentiometric titration with blank correction, and the test result was expressed as mgKOH / g. For samples with low acid value, when the test result is below the reliable quantitative level of 0.01 mgKOH / g, it is expressed as "<0.01 mgKOH / g", and values ​​below this level are not reported with excessive refinement.

[0074] The number-average molecular weight (Mn) and molecular weight distribution (PDI) were determined by gel permeation chromatography, calibrated with polystyrene standards; the glass transition temperature (Tg) was determined by differential scanning calorimetry, taking the midpoint temperature of the glass transition on the second heating curve.

[0075] The viscosity of the adhesive solution was measured using a rotational viscometer at 23°C.

[0076] The solid content was determined by quantitative sampling and heating and drying to constant weight.

[0077] Residual polymerizable monomers are quantitatively detected by gas chromatography, and calibration curves are established using the corresponding monomer standards. The measured concentrations that can be quantitatively detected are reported, and those below the method limit of quantitation are indicated as "< limit of quantitation".

[0078] The 180° peel strength of the PI tab tape was tested according to GB / T 2792; the initial tack was tested according to the rolling ball method specified in GB / T 4852; and the holding power was tested according to GB / T 4851. All adhesive performance tests were conducted after equilibration at 23℃ and 50%RH, using the same batch of substrate and the same bonding pressure.

[0079] The heat resistance was evaluated by observing whether there was any overflow, deformation, or peeling of the adhesive layer after heat treatment at 150℃ for 1 hour.

[0080] Electrolyte resistance was tested using a 1 mol / L LiPF6 carbonate electrolyte. The electrolyte solvent consisted of a mixture of EC, EMC, and DEC in a volume ratio of 3:5:2, with the moisture content controlled below 20 mg / kg. PI tab tape samples of specified dimensions were immersed in the electrolyte and treated at a set temperature and time. After treatment, the appearance, quality changes, and peel strength changes of the adhesive layer were tested.

[0081] For the electrolyte resistance of the adhesive layer, the peel force retention rate before and after immersion is used as a quantitative evaluation index: ; In the formula, Peel strength retention rate, in percentages (%) The peel strength at 180° before immersion in electrolyte; The peel strength at 180° after immersion in electrolyte.

[0082] To reduce the interference with substrate quality caused by direct weighing of the 6–10 μm thin adhesive layer, the adhesive layer swelling rate was tested using a free adhesive film prepared with the same formulation and curing conditions as the tape under test. The dry film thickness of the free adhesive film was controlled at 80 ± 5 μm. After soaking, the electrolyte adhering to the surface was quickly absorbed with lint-free paper, and weighing was completed within 2 minutes. The adhesive layer swelling rate was calculated using the following formula: ; In the formula, The swelling ratio of the adhesive layer; The quality of the adhesive layer before immersion in electrolyte; The quality of the adhesive layer after removing the electrolyte adhering to the sample surface and treating it under uniform conditions after soaking is determined.

[0083] For the electrode tab corrosion performance, a 25 mm × 50 mm tape sample was applied to the surface of an aluminum electrode tab of the same specification. After standing at 23°C for 24 h, it was placed in 20 mL of the aforementioned lithium-ion battery electrolyte for accelerated treatment. After the test, images of the applied area were acquired under uniform lighting and shooting distance. The percentage of the area with obvious gray-black discoloration, pitting, or localized corrosion to the total area covered by the tape was used as the corrosion area rate. The Al element content in the electrolyte after blank correction was determined by ICP-OES to evaluate the inhibitory effect of the adhesive layer on the corrosion of the aluminum electrode tab. The above application and interface protection conditions are as follows. Figure 6 As shown.

[0084] The breakdown voltage was evaluated using a power frequency withstand voltage test. The samples were tested after equilibration at 23℃ and 50%RH for 24 hours. The voltage at which electrical breakdown occurred was recorded during the voltage rise process, and the average value of no less than 5 parallel samples was taken.

[0085] The D50 and D90 of nano-silica in the adhesive solution were determined by dynamic light scattering method. Before the test, the solution was diluted by a uniform multiple and fully dispersed using a mixed solvent compatible with the adhesive solution. The gel fraction of the adhesive layer was determined by extracting the free adhesive film of the same formulation in ethyl acetate for 24 h and drying it to constant weight, and the contribution of inorganic silica mass to the insoluble matter was deducted.

[0086] For free tertiary amines in the electrolyte, gas chromatography-mass spectrometry was used for detection. An external standard calibration curve was established using triethylamine, and the method limit of quantitation was set to 5 mg / L. Results below this limit of quantitation were expressed as "<5 mg / L".

[0087] Example 1 This embodiment provides an acid-free acrylic PI tab tape for lithium batteries, using a formulation located in the middle of the preferred range of the present invention, to verify the synergistic effect of polymerizable tertiary amine functional monomers, epoxy-containing functional monomers, amino-modified nano-silica, and isocyanate secondary crosslinking system.

[0088] 1. Preparation of acid-free acrylic copolymers The following polymerization formulation is used in this embodiment, based on parts by weight: Table 1 Formulation information of acid-free acrylic copolymer in Example 1

[0089] Among them, isooctyl acrylate and butyl acrylate are soft monomers, methyl methacrylate is a hard monomer, N,N-dimethylacrylamide is a polymerizable tertiary amine functional monomer, glycidyl methacrylate is an epoxy-containing functional monomer, and trimethylolpropane diallyl ether is a hydroxyl-branched functional monomer.

[0090] Free radical solution polymerization was carried out according to the aforementioned general implementation method. The nitrogen displacement flow rate was 60 L / min, and the displacement time was 60 min. The initial initiation temperature was 82 °C. After adding the first portion of dilauryl peroxide, the reaction was refluxed at 80–86 °C for 60 min. Subsequently, the remaining dilauryl peroxide solution was added dropwise over 120 min, and the reaction was maintained at 78–82 °C. After the addition was completed, the reaction was held at 80 ± 3 °C for another 180 min.

[0091] After the reaction was completed, the temperature was lowered to 45°C. The 120 parts by mass of mixed solvent listed in Table 1 were used as solvents for the polymerization stage and initiator preparation, excluding the solvents used for adjusting the solids after the reaction. Without considering the small amount of sampling and evaporation loss, about 28 parts by mass of ethyl acetate were added to adjust the measured solid content of the copolymer solution to 40.4%. The solution was then filtered through a 100-mesh filter to obtain a transparent, acid-free acrylic copolymer solution without obvious gel particles.

[0092] 2. Preparation of amino-modified nano-silica KH-550 modified nano-fumed silica was prepared according to the aforementioned general preparation method. In this example, fumed silica with a primary particle size of approximately 15 nm was used. 5 wt% KH-550 was added based on the mass of the nano-silica for surface treatment. KH-550 was hydrolyzed in an ethanol / water mixture at pH 5.0 for 40 min. After adding the nano-silica, the mixture was stirred at 60°C for 2 h and then dried at 110°C to constant weight. The moisture content of the modified nano-silica after treatment and drying was 0.08 wt%. After surface treatment, the surface of the nano-silica possessed amino groups that could react with the side-chain epoxy groups of the acid-free acrylic copolymer.

[0093] 3. Preparation of Acid-Free Acrylic Pressure-Sensitive Adhesive Coating Solution Weigh 100 parts by weight of the above-mentioned acid-free acrylic copolymer based on solid effective components, add 20 parts by weight of ethyl acetate as a regulating solvent, and then add 0.5 parts by weight of the above-mentioned amino-modified nano silica. Disperse the mixture at a speed of 1450 r / min for 15 min.

[0094] After dispersion, the adhesive solution temperature was adjusted to 50℃ and kept at this temperature for 40 min under continuous stirring to allow the epoxy groups in the side chains of the acid-free acrylic copolymer to undergo a pre-grafting reaction with the amino groups on the surface of the amino-modified nano-silica. After pre-grafting, the adhesive solution was cooled to 27℃, and then 0.5 parts by mass of TDI trimer isocyanate was pre-diluted with ethyl acetate at a mass ratio of 1:4, added to the adhesive solution, and stirred at 500 r / min for 5 min. Ethyl acetate was then added to adjust the total solids content of the resulting coating solution to 22.0 ± 1.0 wt%, yielding the acid-free acrylic pressure-sensitive adhesive coating solution. The above pre-grafting and secondary crosslinking processes are described in [reference needed]. Figure 3 .

[0095] 4. Preparation of PI electrode ear tape A 25 μm thick PI film was selected as the substrate, and the surface to be coated was subjected to corona treatment with a power of 0.5 kW. The pressure-sensitive adhesive solution was uniformly coated onto the PI film surface using a slit-type coating method, with the wet adhesive layer thickness controlled at 40 μm. The layered structure of the resulting PI tab tape is described in [reference needed]. Figure 4 .

[0096] The coated PI film was sequentially dried in six zones at 80℃, 100℃, 120℃, 140℃, 160℃, and 120℃, with a total drying time of 90 seconds. The average dry adhesive layer thickness after drying was 8.2 μm. The tape was then cured at 50℃ for 48 hours, followed by 23℃ and 50%RH for 24 hours. The six-stage drying and curing process is detailed below. Figure 5 .

[0097] 5. Performance Testing Table 2 Properties of the acid-free acrylic copolymer in Example 1

[0098] The resulting copolymer was a transparent and homogeneous liquid, with no obvious gel particles observed; its acid value was less than 0.01 mgKOH / g, and its molecular weight, viscosity, and glass transition temperature were all within the applicable range defined in this application.

[0099] Table 3 Performance of PI tab tape in Example 1

[0100] The PI tab tape prepared in this embodiment was further applied to the surface of aluminum tabs from the same batch. After standing at 23°C for 24 h, it was completely immersed in 1 mol / L LiPF6 carbonate electrolyte and maintained at 60°C for 7 days. After the test, no obvious interfacial delamination was observed between the tape and the aluminum tab, and no continuous blackening or obvious pitting occurred in the adhesive-coated area of ​​the aluminum tab. The Al content in the electrolyte after immersion was determined by ICP-OES to be 0.62 mg / L. For the application structure of the tape applied to the surface of the aluminum tab, please refer to [link to relevant documentation]. Figure 6 .

[0101] Example 2 This embodiment provides another type of acid-free acrylic PI tab tape for lithium batteries. Compared with Example 1, this embodiment reduces the amount of epoxy-containing functional monomers and hydroxyl-containing branched functional monomers, and adopts a lower degree of pre-grafting conditions to verify the implementation effect of the technical solution of this application under a lower degree of crosslinking.

[0102] 1. Preparation of acid-free acrylic copolymers Table 4 Formulation information for acid-free acrylic copolymer in Example 2

[0103] Polymerization was carried out according to the aforementioned general implementation method. The 120 parts by weight of mixed solvent listed in Table 4 were used for the polymerization stage and initiator preparation. After the reaction, approximately 29 parts by weight of ethyl acetate were added to adjust the solid content. The resulting acid-free acrylic copolymer was a transparent, homogeneous liquid with a solid content of 40.1%, a viscosity of 2480 mPa·s at 23°C, a number-average molecular weight (Mn) of 365,000, a PDI of 1.67, a Tg of -51.2°C, and an acid value of <0.01 mgKOH / g.

[0104] 2. Preparation of pressure-sensitive adhesive coating solution and PI electrode tab tape KH-550 modified nano-fumed silica was prepared using the same method as in Example 1. 0.5 parts by weight of the modified nano-silica were added based on 100 parts by weight of the effective solids component of the acid-free acrylic copolymer. The mixture was dispersed at 1450 r / min for 15 min, and then reacted at 40°C for 20 min. After pre-grafting, the mixture was cooled to 28°C, and 0.5 parts by weight of TDI trimer isocyanate were added and stirred for 5 min. Ethyl acetate was then added to adjust the total solids content of the coating solution to 22.0 ± 1.0 wt%. The PI substrate, corona treatment, coating, and drying conditions were the same as in Example 1. The resulting dry adhesive layer had an average thickness of 8.0 μm and was cured at 50°C for 48 h.

[0105] Table 5 Performance of PI electrode ear tape in Example 2

[0106] Compared with Example 1, the initial tack of this example is slightly improved, while the peel strength retention rate and long-term tack performance after electrolyte resistance are slightly reduced. This indicates that with a lower epoxy functional monomer content and pre-grafting degree, the adhesive layer still has good adhesion and electrolyte resistance, but the network density is relatively reduced.

[0107] Example 3 This embodiment provides a highly functionalized, highly cross-linked acid-free acrylic PI tab tape for lithium batteries. By increasing the amount of polymerizable tertiary amine functional monomers, epoxy-containing functional monomers, hydroxyl-containing branched functional monomers, and amino-modified nano-silica, and by adopting a higher degree of pre-grafting conditions, the implementation effect of the technical solution of this application under a higher degree of cross-linking is verified.

[0108] 1. Preparation of acid-free acrylic copolymers Table 6 Formulation information of acid-free acrylic copolymer in Example 3

[0109] Polymerization was carried out according to the aforementioned general implementation method. The 125 parts by weight of mixed solvent listed in Table 6 were used for the polymerization stage and initiator preparation. After the reaction, approximately 21 parts by weight of ethyl acetate were added to adjust the solid content. The resulting copolymer had a solid content of 40.6%, a viscosity of 3270 mPa·s at 23°C, a number-average molecular weight (Mn) of 436,000, a PDI of 1.48, a Tg of -47.8°C, and an acid value of <0.01 mgKOH / g.

[0110] 2. Preparation of pressure-sensitive adhesive coating solution and PI electrode tab tape Based on 100 parts by weight of the effective solids component of the acid-free acrylic copolymer, 1.0 part by weight of KH-550 modified nano-fumed silica was added. After dispersion at 1450 r / min for 15 min, the adhesive solution was maintained at 55℃ for 60 min. After pre-grafting, the adhesive solution was cooled to 26℃, 1.0 part by weight of TDI trimer isocyanate was added and stirred for 5 min, and then ethyl acetate was added to adjust the total solids content of the coating solution to 22.0 ± 1.0 wt%. The PI substrate, corona treatment, coating, and six-stage drying conditions were the same as in Example 1. The average thickness of the resulting dry adhesive layer was 8.3 μm, and it was cured at 50℃ for 48 h.

[0111] Table 7 Performance of PI electrode ear tape in Example 3

[0112] The initial peel strength and initial tack of this embodiment are slightly lower than those of Example 1, but the holding power, heat resistance, peel retention rate after electrolyte immersion and aluminum tab interface protection ability are further improved, indicating that increasing the number of epoxy-amino grafting points and isocyanate crosslinking points can enhance the long-term structural stability of the adhesive layer.

[0113] Comparative Example 1 This comparative example was used to investigate the effect of introducing a polymerizable acidic monomer containing a free carboxyl group into the polymerizable monomer system on the acid value, electrolyte resistance, and aluminum tab interface corrosion behavior of the pressure-sensitive adhesive. Except for replacing 1 part by mass of isooctyl acrylate with 1 part by mass of acrylic acid in Example 1, the other functional monomers, amino-modified nano-silica, TDI trimer, and preparation process were the same as in Example 1.

[0114] The obtained carboxylated acrylic acid copolymer had a solid content of 40.2%, a viscosity of 3010 mPa·s at 23℃, a Mn of 396,000, a PDI of 1.61, a Tg of -48.9℃, an acid value of 7.24 mgKOH / g, and a residual acrylic acid content of 32 mg / kg.

[0115] Table 8 Performance Comparison between Comparative Example 1 and Example 1

[0116] It can be seen that the initial peel strength of the tape was slightly improved after the introduction of acrylic acid, but after soaking in electrolyte at 60℃ for 7 days, the peel strength retention rate decreased significantly and the Al dissolution rate increased significantly. This indicates that the improvement of the initial adhesive strength is not equivalent to the improvement of the long-term interface reliability of the lithium battery.

[0117] Comparative Example 2 This comparative example was used to examine the differences between free small-molecule tertiary amines and polymerizable tertiary amine functional monomers. During polymerization, 2 parts by mass of N,N-dimethylacrylamide in Example 1 were removed, and an equal amount of isooctyl acrylate was added. After polymerization was complete and the temperature was lowered to below 35°C, 1.3 parts by mass of triethylamine were added to make the molar number of tertiary amine nitrogen essentially equivalent to the molar number of tertiary amine nitrogen provided by the polymerizable tertiary amine functional monomer in Example 1. The remaining components and preparation conditions were the same as in Example 1.

[0118] The resulting system had a solid content of 40.3%, a viscosity of 2590 mPa·s at 23℃, a Mn content of 378,000, a PDI of 1.64, a Tg of -50.7℃, and an acid value of 0.006 mgKOH / g.

[0119] Table 9 Performance Comparison between Comparative Example 2 and Example 1

[0120] The initial acid values ​​of the two groups of samples were basically the same, but Comparative Example 2 showed obvious small molecule amine migration in the high-temperature electrolyte environment, and the adhesion, peeling retention rate after electrolyte immersion and tab interface stability were all lower than those of Example 1. This indicates that fixing the tertiary amine sites to the polymer molecular chain in a covalent manner is beneficial to reducing small molecule migration.

[0121] Comparative Example 3 This comparative example was used to investigate the effect of unmodified amino-based nano-fumed silica on tape performance. Except for replacing the KH-550 amino-modified nano-fumed silica in Example 1 with unmodified nano-fumed silica of the same particle size and dosage, the formulation and process were identical to those in Example 1.

[0122] After the unmodified nano-silica was added to the adhesive, it was also dispersed at 1450 r / min for 15 min and kept at 50℃ for 40 min. Since there are no primary amino groups introduced by KH-550 on the particle surface, this heat preservation process cannot form the epoxy-amino ring-opening graft structure in Example 1.

[0123] Table 10 Performance Comparison between Comparative Example 3 and Example 1

[0124] The initial adhesion performance of Comparative Example 3 was not significantly different from that of Example 1, but the long-term adhesion, gel fraction, peel retention rate after electrolyte immersion and tab protection performance were significantly reduced, indicating that nano-silica does not only play a physical filling role in this application.

[0125] Comparative Example 4 This comparative example was used to examine the process sequence of "epoxy-amino pre-grafting followed by TDI addition". The types and amounts of the acid-free acrylic copolymer, KH-550 modified nano-silica, and TDI trimer used were exactly the same as in Example 1, but the independent pre-grafting stage was omitted. 0.5 parts by mass of amino-modified nano-silica and 0.5 parts by mass of TDI trimer were added to the adhesive simultaneously, dispersed at 1450 r / min for 15 min, and then kept at 50°C for 40 min.

[0126] Table 11 Performance Comparison between Comparative Example 4 and Example 1

[0127] With identical components, the initial peel performance of Comparative Example 4 was similar to that of Example 1, but the gel fraction, long-term tack, and electrolyte resistance all decreased. This indicates that the process sequence of pre-grafting epoxy groups and amino-modified nano-silica before adding polyisocyanate crosslinking agent can improve the integrity of the resulting organic-inorganic crosslinked network.

[0128] Test Example 1 To compare the effects of different formulation structures and preparation methods on the overall performance of PI tab tapes, PI tab tapes prepared in Examples 1-3 and Comparative Examples 1-4 were used as test samples. All samples were placed at 23°C and 50%RH for 24 hours before being tested according to the aforementioned general testing methods.

[0129] Table 12 Basic Adhesive Properties of PI Tape Groups

[0130] The initial 180° peel strength of each embodiment is not necessarily higher than that of the comparative example. For example, the initial peel strength of Comparative Example 1 is higher than that of Example 1 due to the introduction of carboxyl groups, indicating that the technical effect of this application does not simply come from improving the initial tack of the adhesive layer.

[0131] Table 13 Electrolyte and tab corrosion resistance of each group of PI tab tapes

[0132] The corrosion area ratio is the percentage of the area of ​​obvious gray-black discoloration, pitting, or localized corrosion in the aluminum tab adhesive area to the total area covered by the tape. In Examples 1-3, the peeling retention rate after immersion in 60°C electrolyte for 7 days was close to or higher than 90%, and the Al dissolution and corrosion area ratio were lower than those of the comparative examples.

[0133] Table 14 represents the gel fraction of the sample gel layer.

[0134] Table 14 shows that a high gel fraction can be obtained by using amino-modified nano-silica and preparing the adhesive layer by pre-grafting followed by isocyanate crosslinking. Combining the results in Tables 12 and 13, it can be seen that there is a synergistic effect between the carboxylic acid monomer-free system, the covalent fixation of polymerizable tertiary amines, the pre-grafting of epoxy groups and amino-modified nano-silica, and the subsequent isocyanate crosslinking.

[0135] Test Example 2 To further verify the applicability of the PI tab tape in the actual working environment of lithium batteries, Example 1, Comparative Example 1, and Comparative Example 4 were selected for whole-cell verification. Except for the PI tape used at the tab position, the positive and negative electrode materials, electrode specifications, separator, electrolyte, electrolyte injection volume, packaging conditions, and formation conditions of each group of cells were kept consistent. Each group had 10 parallel cells, of which 5 were used for high-temperature full-charge storage and disassembly observation, and the other 5 were used for accelerated cycling and internal resistance testing.

[0136] 1. High-temperature full-charge storage test The formed battery was fully charged and stored at 60°C for 30 days. After storage, the battery was disassembled, and the tab tape and aluminum tab interface were observed.

[0137] Table 15 High-Temperature Fully Charged Storage Test Results

[0138] No obvious tape delamination or continuous blackening area was observed in Example 1 after being stored at 60°C with full charge for 30 days; Comparative Example 1 showed a high proportion of interface separation and tab blackening, while the overall performance of Comparative Example 4 was between the two.

[0139] 2. Accelerate loop testing The other five parallel cells were subjected to accelerated cycling at 45°C. They were charged at a constant current of 1C to 4.0V, then switched to constant voltage charging with a cutoff current of 0.05C, and then discharged at a constant current of 1C to 2.65V, with an 80°C high-temperature cutoff protection setting. Using the initial discharge capacity before the start of cycling as 100%, the capacity retention rate after different number of cycles was calculated. The results listed in Table 16 are the arithmetic mean of the five parallel cells in each group.

[0140] Table 16 Capacity retention rate at different cycle counts

[0141] In the early stages of cycling, the differences between the three groups of batteries were small. As the number of cycles increased, the capacity decay rate of Comparative Example 1 accelerated significantly, while the capacity retention rate of Comparative Example 4 was between that of Example 1 and Comparative Example 1. This indicates that the differences in the long-term electrolyte resistance of the adhesive layer and the stability of the tab interface will gradually accumulate during the cycling process.

[0142] 3. Changes in battery internal resistance Before and after 700 cycles, each group of cells was adjusted to 50% SOC and left to stand at 25°C for 2 hours. The internal resistance was measured using a 1 kHz AC internal resistance tester. The initial internal resistance and the internal resistance after 700 cycles listed in Table 17 are the arithmetic average of the five parallel cells in each group, and the internal resistance growth rate was calculated accordingly.

[0143] Table 17 Changes in battery internal resistance before and after 700 cycles

[0144] The initial internal resistances of the three battery groups were similar, but after 700 cycles, the internal resistance growth rates of Comparative Example 1 and Comparative Example 4 were significantly higher than those of Example 1. Considering high-temperature storage, capacity retention, and the state of the tab interface after disassembly, it can be seen that this application, by eliminating carboxylic acid-type polymer monomers, covalently fixing tertiary amine sites to the polymer molecular chain, and constructing an organic-inorganic crosslinking network through epoxy-amino pre-grafting and isocyanate secondary crosslinking, can reduce the swelling and interface failure of the adhesive layer in the electrolyte, and slow down aluminum tab corrosion and internal resistance growth during long-term cycling.

Claims

1. An acid-free acrylic pressure-sensitive adhesive for lithium batteries, characterized in that, Including acid-free acrylic copolymers, amino-modified nano-silica, and polyisocyanate crosslinking agents; The acid-free acrylic copolymer is formed by free radical copolymerization of a monomer system including soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers and hydroxyl-branched functional monomers, wherein the monomer system does not contain polymerizable acidic monomers with free carboxyl groups. The polymerizable tertiary amine functional monomer has both a carbon-carbon double bond and a tertiary amine group that can be polymerized by free radicals, so that the tertiary amine group is fixed in the molecular chain of the acid-free acrylic copolymer in a covalent bond form. The epoxy groups in the acid-free acrylic copolymer form an epoxy ring-opening graft structure with the amino groups on the surface of the amino-modified nano silica, and hydroxyl groups are formed in the epoxy ring-opening graft structure. The polyisocyanate crosslinking agent forms a crosslinking structure with the hydroxyl groups in the epoxy ring-opening graft structure and the hydroxyl groups provided by the hydroxyl-containing branched functional monomer, thereby enabling the acid-free acrylic copolymer and the amino-modified nano silica to form an organic-inorganic crosslinking network.

2. The acid-free acrylic pressure-sensitive adhesive for lithium batteries according to claim 1, characterized in that, The soft monomers include isooctyl acrylate and butyl acrylate; the hard monomer is methyl methacrylate; the polymerizable tertiary amine functional monomer is N,N-dimethylacrylamide; the epoxy-containing functional monomer is glycidyl methacrylate; and the hydroxyl-containing branched functional monomer is trimethylolpropane diallyl ether. The amino-modified nano-silica is nano-fumed silica modified with γ-aminopropyltriethoxysilane; and the polyisocyanate crosslinking agent is TDI trimer isocyanate.

3. The acid-free acrylic pressure-sensitive adhesive for lithium batteries according to claim 2, characterized in that, Based on a total mass of 100 parts for the soft monomer, hard monomer, polymerizable tertiary amine functional monomer, epoxy-containing functional monomer, and hydroxyl-containing branched functional monomer, the soft monomer comprises 84-87 parts, the hard monomer comprises 9-10 parts, the polymerizable tertiary amine functional monomer comprises 2-3 parts, the epoxy-containing functional monomer comprises 0.5-2 parts, and the hydroxyl-containing branched functional monomer comprises 0.5-2 parts. Based on the effective solid content of 100 parts by weight of the acid-free acrylic copolymer, the amount of amino-modified nano-silica is 0.5 to 1 part by weight, and the amount of TDI trimer isocyanate is 0.5 to 1 part by weight.

4. An acid-free acrylic PI electrode tab tape for lithium batteries, characterized in that, Includes a PI substrate and an acid-free acrylic pressure-sensitive adhesive layer disposed on at least one surface of the PI substrate; The acid-free acrylic pressure-sensitive adhesive layer comprises an acid-free acrylic copolymer, amino-modified nano-silica, and a polyisocyanate crosslinking agent; The acid-free acrylic copolymer is formed by free radical copolymerization of a monomer system including soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers and hydroxyl-branched functional monomers. The monomer system does not contain polymerizable acidic monomers with free carboxyl groups. The tertiary amine groups in the polymerizable tertiary amine functional monomers are fixed in the molecular chain of the acid-free acrylic copolymer in a covalent bond. The epoxy groups in the acid-free acrylic copolymer form an epoxy ring-opening graft structure with the amino groups on the surface of the amino-modified nano silica. The polyisocyanate crosslinking agent forms a crosslinking structure with the hydroxyl groups formed by the epoxy ring-opening grafting structure and the hydroxyl groups provided by the hydroxyl-containing branched functional monomer, thereby forming an organic-inorganic crosslinking network containing the acid-free acrylic copolymer and the amino-modified nano silica in the acid-free acrylic pressure-sensitive adhesive layer.

5. The acid-free acrylic PI electrode tab tape for lithium batteries according to claim 4, characterized in that, The amino-modified nano-silica is nano-fumed silica that has been surface-modified with γ-aminopropyltriethoxysilane; Based on 100 parts by mass of the effective solid content of the acid-free acrylic copolymer, the amount of amino-modified nano silica is 0.5 to 1 part by mass, the amount of polyisocyanate crosslinking agent is 0.5 to 1 part by mass, and the polyisocyanate crosslinking agent is TDI trimer isocyanate.

6. The acid-free acrylic PI electrode tab tape for lithium batteries according to claim 4 or 5, characterized in that, The acid-free acrylic copolymer has an acid value of less than 0.01 mg KOH / g, a number-average molecular weight of 300,000 to 500,000, and a glass transition temperature of -55℃ to -45℃. The thickness of the PI substrate is 25 μm, and the dry adhesive thickness of the acid-free acrylic pressure-sensitive adhesive layer is 6–10 μm.

7. A method for preparing an acid-free acrylic pressure-sensitive adhesive and a PI electrode tab tape for lithium batteries, characterized in that, Includes the following steps: S1. Soft monomers, hard monomers, polymerizable tertiary amine functional monomers, epoxy-containing functional monomers, and hydroxyl-containing branched functional monomers are copolymerized under the action of a free radical initiator to obtain an acid-free acrylic copolymer. The monomer system used in the copolymerization reaction does not contain polymerizable acidic monomers with free carboxyl groups, and the polymerizable tertiary amine functional monomers simultaneously possess carbon-carbon double bonds and tertiary amine groups that can be polymerized by free radicals. S2. Nano-silica with surface modification by amino-containing silane coupling agent is added to the acid-free acrylic copolymer for dispersion, so that the epoxy groups in the acid-free acrylic copolymer react with the amino groups on the surface of the nano-silica to form an epoxy ring-opening graft structure connecting the acid-free acrylic copolymer and the nano-silica, and hydroxyl groups are formed in the epoxy ring-opening graft structure. S3. After completing the ring-opening reaction, reduce the temperature of the adhesive solution and add a polyisocyanate crosslinking agent to allow the polyisocyanate crosslinking agent to crosslink with the hydroxyl groups in the epoxy ring-opening graft structure and the hydroxyl groups provided by the hydroxyl-containing branched functional monomer to obtain an acid-free acrylic pressure-sensitive adhesive coating solution. S4. Perform surface activation treatment on the PI substrate surface to be coated with adhesive, and apply the acid-free acrylic pressure-sensitive adhesive coating liquid to the surface of the PI substrate after surface activation treatment. S5. The coated PI substrate is subjected to segmented heating, drying and curing to form an acid-free acrylic pressure-sensitive adhesive layer from the acid-free acrylic pressure-sensitive adhesive coating liquid, thereby obtaining the acid-free acrylic PI tab tape for lithium batteries.

8. The preparation method according to claim 7, characterized in that, In step S1, dilauryl peroxide is used as the free radical initiator; After adding the monomer and part of the solvent to the polymerization reactor, nitrogen gas is introduced from the bottom of the reactor at a flow rate of 60 L / min for 60 min, and the mixture is stirred at a frequency of 15 Hz. After heating the reaction system to 80-82℃, the first initiator solution was added, and the mixture was refluxed at 80-86℃ for 60 min. The second initiator solution was then added dropwise over 120 minutes, with the temperature of the reaction system maintained at 78–82 °C during the addition process. After the addition is complete, the mixture is kept at 76–85°C and refluxed for 180 min. After cooling, the remaining polymerization solvent is added to adjust the solid content of the resulting acid-free acrylic copolymer to 38%–42%.

9. The preparation method according to claim 7, characterized in that, In step S2, based on 100 parts by mass of the effective solid component of the acid-free acrylic copolymer, 0.5 to 1 part by mass of amino-modified nano silica is added and dispersed at a speed of 1000 to 1600 r / min for 10 to 20 min. Then, the adhesive is kept at 40 to 55°C and reacted for 20 to 60 min. In step S3, after the ring-opening reaction in step S2 is completed, the temperature of the adhesive solution is reduced to below 30°C, and 0.5 to 1 part by mass of TDI trimer isocyanate is added based on 100 parts by mass of the effective solid component of the acid-free acrylic copolymer, and stirred for 3 to 10 minutes.

10. The preparation method according to claim 9, characterized in that, In step S4, a PI substrate with a thickness of 25 μm is used, and the surface of the PI substrate to be coated with adhesive is subjected to corona treatment. The acid-free acrylic pressure-sensitive adhesive coating liquid is then applied to the corona-treated PI substrate surface with a wet adhesive thickness of 30-50 μm. In step S5, the coated PI substrate is sequentially passed through six drying zones at temperatures of 80℃, 100℃, 120℃, 140℃, 160℃ and 120℃, with a total drying time of 1 to 2 minutes, so that the dry adhesive thickness of the resulting acid-free acrylic pressure-sensitive adhesive layer is 6 to 10 μm, and then cured at 40 to 60℃ for 24 to 72 hours.

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