A polymer emulsion for battery separator and its preparation method and application

Abstract

The present invention discloses a polymer emulsion for battery separators, a preparation method, and applications thereof. The latex particles of the polymer emulsion include a three-layer structure from the inside to the outside, with the inner layer being a core layer, the middle layer being a first shell layer, and the outer layer being a second shell layer. The core layer serves as the core of a functional polymer, is electrolyte-resistant, and stabilizes the emulsion particles. The first shell layer is an intermediate layer of the functional polymer, imparting elasticity and lithium ion conductivity to the emulsion particles. The second shell layer is the outermost layer of the functional polymer, composed of a functional monomer copolymer, and primarily serves as a bonding agent, capable of simultaneously bonding the separator substrate and the positive and negative electrode materials of the battery. The use of the functional coating separator of the present invention can ensure good battery capacity, rate, and long cycle life for soft-pack batteries, and ensure good shaping effects and long battery cycle life for square batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separators, and in particular relates to a polymer emulsion for battery separators, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries currently used in the 3C, power, and energy storage markets, particularly prismatic and pouch cells, require separators coated with organic functional materials to meet the requirements of battery shaping and integration, increase battery energy density, and improve battery hardness and cycle life. Typically, a PVDF organic material is coated on one or both sides of the separator substrate. During the battery's pressure formation process, the PVDF material is swelled by the organic electrolyte and physically cross-linked with the PVDF binder used in the positive electrode material, thereby bonding the separator and positive electrode materials together to form a positive electrode / diaphragm / negative electrode sandwich structure, thereby improving battery performance.

[0003] PVDF coated membranes can be prepared using water-based or oil-based processes. The oil-based process involves the use of organic solvents in the manufacturing process, which will produce environmental problems such as three wastes. In addition, organic solvents and materials can easily penetrate into the pores of the membrane, causing pore blockage. When used in batteries, it leads to performance problems such as large internal resistance and poor rate. Therefore, it is not preferred. The water-based process generally prepares water-based PVDF slurry first, and then uses micro-gravure coating to produce PVDF coated membranes. However, the water-based PVDF coated diaphragms currently used in the market generally have the following problems: First, PVDF material is insoluble in water and can only be dispersed and emulsified in water by additives. PVDF particles exist in the form of agglomerates in water, which is uneven and has poor slurry stability; second, due to the unevenness and poor stability of the slurry, the slurry is unevenly coated on the surface of the diaphragm, and the coating thickness is poorly uniform; third, the water-based PVDF slurry is coated on the surface of the diaphragm, and the coating adhesion is poor after drying, which is prone to powdering and coating peeling problems; fourth, when the PVDF coated diaphragm is made into a battery cell, the battery becomes soft after hot pressing. The main reason is that the PVDF coating has poor adhesion to the electrode, especially the poor adhesion to the negative electrode material, which leads to many problems such as low battery hardness, deformation, and poor battery cycle performance; fifth, PVDF material is a special chemical product, and high-end products are monopolized by foreign oligarchs, with high prices and insufficient production capacity. The cost of domestic PVDF has also risen, which is not conducive to the market-scale application of the product.

[0004] Therefore, there is an urgent need to find an organic functional adhesive material with good performance and low cost. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention provides a polymer emulsion for battery separators, a preparation method thereof, and an application thereof.

[0006] According to one aspect of the present invention, a polymer emulsion is proposed, wherein the latex particles of the polymer emulsion include a three-layer structure from the inside to the outside, the inner layer being the core layer, the middle layer being the first shell layer, and the outer layer being the second shell layer; the core layer is composed of a hard monomer copolymer, and the homopolymer Tg value of the monomer used in the core layer is greater than 100°C; the first shell layer is composed of a moderately soft and hard monomer copolymer and a lithium-containing compound, the homopolymer Tg value of the monomer used in the first shell layer is -20°C to 120°C, and the monomer used in the first shell layer contains at least one functional group selected from carboxyl, hydroxyl or amino groups; the second shell layer is composed of a functional monomer copolymer, and the homopolymer Tg value of the monomer used in the second shell layer is less than 120°C.

[0007] The core layer serves as the core of the polymer, is resistant to electrolyte, and plays the role of stabilizing the emulsion particles. It can keep the main shape of the coating particles (coated on the surface of the diaphragm) unchanged during the coating process, the battery hot pressing process, and the long-term use of the battery, and will not be excessively swollen or dissolved by the electrolyte, thereby ensuring the long cycle life of the battery.

[0008] The first shell layer is the middle layer of the polymer, which gives the emulsion particles elasticity and lithium ion conductivity: (1) The middle layer exhibits high elastic properties during the coating process, battery manufacturing process and battery use. During the charging and discharging process of the battery, the positive and negative electrodes will expand and contract. The high elastic properties of the diaphragm coating particles can play a buffering role, keeping the diaphragm in close contact with the positive and negative electrodes, and maintaining a good interface, thereby helping to improve the battery cycle life; (2) The middle layer includes a lithium-containing compound. The lithium-containing compound reacts with the residual active groups, carboxyl groups, hydroxyl groups and amino groups on the surface of the first shell polymer to generate an organic lithium-containing compound, which can replenish lithium or improve the transmission efficiency of lithium ions and improve the conductivity of lithium ions.

[0009] The second shell is the outermost layer of the polymer, which is composed of functional monomer copolymers and mainly plays a bonding role. On the one hand, when the slurry containing the functional high molecular polymer is coated on the surface of the diaphragm and dried, the functional polymer material can bond itself to the diaphragm substrate to ensure that the coating does not fall off; on the other hand, when the high-adhesion functional coating diaphragm is applied to the lithium-ion battery, after dry pressing or hot pressing, the functional coating can be bonded to the positive and negative electrodes of the battery, thereby forming a tight sandwich structure, thereby increasing the battery hardness and improving the battery cycle life.

[0010] In some embodiments of the present invention, the thickness ratio of the core layer, first shell layer, and second shell layer is (2-5):(1-2):(1-3); the primary particle size of the latex particles of the polymer emulsion is 0.3-0.6 μm, and the secondary particle size is 0.6-6.0 μm. It should be noted that the thickness of the core layer refers to the radius of the core layer particles.

[0011] In some embodiments of the present invention, the weight average molecular weight (Mw) of the core layer is 30,000-100,000; the weight average molecular weight (Mw) of the first shell layer is 20,000-80,000; and the weight average molecular weight (Mw) of the second shell layer is 10,000-60,000.

[0012] In some embodiments of the present invention, the solid content of the polymer emulsion is 5%-50%.

[0013] In some embodiments of the present invention, the monomers of the core layer contain at least one of a methyl group, a benzene ring, an amide group, or a carboxyl group. The core layer is made of polar monomers with excellent electrolyte resistance.

[0014] In some embodiments of the present invention, the monomer of the core layer is at least one of succinic acid, maleic acid, acrylic acid, methacrylic acid, hydroxymethyl acrylamide, acrylamide, styrene, α-methylstyrene, 2-methylstyrene, 2,4-dimethylstyrene, tetrahydrofuran methacrylate or propylene methacrylate.

[0015] In some embodiments of the present invention, the monomer of the first shell layer is at least one of acrylic acid, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, vinyl acetate, dimethylaminoethyl methacrylate or diethylaminoethyl methacrylate.

[0016] In some embodiments of the present invention, the lithium-containing compound is at least one of a lithium salt, a lithium oxide, or a lithium hydroxide.

[0017] In some preferred embodiments of the present invention, the homopolymer Tg value of the monomer used in the second shell layer is ≤105°C.

[0018] In some preferred embodiments of the present invention, the homopolymer Tg value of the monomer used in the second shell layer is -70°C to 105°C.

[0019] In some embodiments of the present invention, the monomer of the second shell layer is at least one of methyl acrylate, ethyl acrylate, butyl acrylate, isooctyl acrylate, methyl methacrylate, ethyl methacrylate, isooctyl methacrylate, lauryl methacrylate, allyl glycidyl ether, allyl methoxyethylene ether, 3-glycidyloxypropyltrimethoxysilane, glycidyl methacrylate, or dicyclopentenylethoxy methacrylate. The monomer of the second shell layer contains at least one functional group selected from the group consisting of an ester group and an ether group.

[0020] The present invention also provides a method for preparing the polymer emulsion, comprising the following steps:

[0021] S1: mixing the core layer monomer and an emulsifier to prepare a core layer monomer pre-emulsion, and adding the core layer monomer pre-emulsion dropwise to an initiator solution to carry out a polymerization reaction to obtain a core layer emulsion;

[0022] S2: mixing the monomer of the first shell layer with an emulsifier to prepare a first shell layer monomer pre-emulsion, adding an initiator solution to the core layer emulsion, and then dropwise adding the first shell layer monomer pre-emulsion to carry out a polymerization reaction. After the reaction is completed, a lithium-containing compound is added and the reaction is stirred to obtain a first shell layer emulsion;

[0023] S3: Mixing the monomer of the second shell layer with an emulsifier to prepare a second shell layer monomer pre-emulsion, adding an initiator solution to the first shell layer emulsion, and then dropwise adding the second shell layer monomer pre-emulsion to carry out polymerization reaction to obtain the polymer emulsion.

[0024] The synthesis technology of polymer emulsion: a multi-layer structure is formed by a step-by-step emulsion polymerization method; the inner layer polymer emulsion is prepared by a soap-free emulsion polymerization process as a seed emulsion (core layer polymerization); then the first shell layer is added with a moderately soft and hard monomer raw material to continue polymerization (first shell layer polymerization) to initially form a concentric double-layer structure polymer material, and a lithium-containing compound is added to react with the residual active groups, carboxyl groups, hydroxyl groups and amino groups on the surface of the first shell layer polymer; then the polymer composed of the core layer and the first shell layer is used as a seed emulsion to continue the second shell layer polymerization, thereby synthesizing a functional polymer emulsion with a core-shell shell three-layer structure, and the shape of the latex particles is spherical or elliptical.

[0025] In some embodiments of the present invention, in step S1, the concentration of the core layer monomer pre-emulsion is 10%-50%.

[0026] In some embodiments of the present invention, in step S1, the concentration of the initiator solution is 1%-3%.

[0027] In some embodiments of the present invention, in step S1, the polymerization reaction temperature is 50-95° C., and the reaction time is 1.0-5.0 h.

[0028] In some embodiments of the present invention, in step S1, the emulsifier used in the core layer monomer pre-emulsion is a nonionic emulsifier. Furthermore, the emulsifier is selected from at least one of fatty alcohol polyoxyethylene ethers, fatty alcohol polyoxypropylene ethers, ethylene oxide and propylene oxide block copolymers, polyol fatty acid esters, or polyvinyl alcohol. Furthermore, the amount of emulsifier used in the core layer monomer pre-emulsion is 0.1%-2% by weight of the core layer monomers. This process utilizes soap-free emulsion polymerization, requiring minimal emulsifier usage.

[0029] In some embodiments of the present invention, in step S1, the amount of the initiator is 0.5%-3% of the mass of the core layer monomers. Further, the initiator is at least one of potassium persulfate, ammonium persulfate, dibenzoyl peroxide, hydrogen peroxide, or an azo compound.

[0030] In some embodiments of the present invention, in step S1, the solvent used for the core layer monomer pre-emulsion and the initiator solution is at least one of methanol, water, ethanol or isopropanol.

[0031] In some embodiments of the present invention, in step S2, the concentration of the initiator solution is 1%-5%.

[0032] In some embodiments of the present invention, in step S2, the concentration of the first shell layer monomer pre-emulsion is 10%-50%.

[0033] In some embodiments of the present invention, in step S2, the polymerization reaction temperature is 50-95° C., and the reaction time is 0.5-4.0 h.

[0034] In some embodiments of the present invention, in step S2, the stirring reaction is carried out at a stirring speed of 500-2000 rpm, the reaction temperature is 40-80° C., and the reaction time is 0.5-2.0 h.

[0035] In some embodiments of the present invention, in step S2, the emulsifier used in the first shell layer monomer pre-emulsion is a nonionic emulsifier. Furthermore, the emulsifier is selected from at least one of fatty alcohol polyoxyethylene ethers, fatty alcohol polyoxypropylene ethers, ethylene oxide and propylene oxide block copolymers, polyol fatty acid esters, or polyvinyl alcohol. Furthermore, the amount of the emulsifier used in the first shell layer monomer pre-emulsion is 1% to 5% by weight of the first shell layer monomers.

[0036] In some embodiments of the present invention, in step S2, the amount of the initiator used is 0.5%-3% of the mass of the first shell monomer. Further, the initiator is at least one of potassium persulfate, ammonium persulfate, dibenzoyl peroxide, hydrogen peroxide, or an azo compound.

[0037] In some embodiments of the present invention, in step S2, the solvent used for the first shell layer monomer pre-emulsion and the initiator solution is at least one of methanol, water, ethanol or isopropanol.

[0038] In some embodiments of the present invention, in step S3, the concentration of the initiator solution is 1%-3%.

[0039] In some embodiments of the present invention, in step S3, the concentration of the second shell layer monomer pre-emulsion is 10%-50%.

[0040] In some embodiments of the present invention, in step S3, the polymerization reaction temperature is 50-95° C., and the reaction time is 1.0-5.0 h.

[0041] In some embodiments of the present invention, in step S3, the emulsifier used in the second shell layer monomer pre-emulsion is a nonionic emulsifier. Furthermore, the emulsifier is selected from at least one of fatty alcohol polyoxyethylene ethers, fatty alcohol polyoxypropylene ethers, ethylene oxide and propylene oxide block copolymers, polyol fatty acid esters, or polyvinyl alcohol. Furthermore, the amount of the emulsifier used in the second shell layer monomer pre-emulsion is 1% to 5% by weight of the second shell layer monomers.

[0042] In some embodiments of the present invention, in step S3, the amount of the initiator used is 0.5%-3% of the mass of the second shell monomer. Further, the initiator is at least one of potassium persulfate, ammonium persulfate, dibenzoyl peroxide, hydrogen peroxide, or an azo compound.

[0043] In some embodiments of the present invention, in step S3, the solvent used for the second shell layer monomer pre-emulsion and the initiator solution is at least one of methanol, water, ethanol or isopropanol.

[0044] The present invention also provides application of the polymer emulsion in battery separators.

[0045] The present invention also provides a functional coating membrane, wherein at least one surface of the functional coating membrane is coated with a functional coating, and the functional coating is made of a functional polymer slurry, and the functional polymer slurry comprises the polymer emulsion, a flocculant, a thickener, an additive and a solvent.

[0046] In some embodiments of the present invention, the flocculant contains at least one group selected from -COO-, -NH-, -CO-NH-, -SO3 or -OH, and has a chain or / and cyclic structure. Preferably, the flocculant is a polymer material with a branched structure. Further preferably, the flocculant is at least one selected from polyacrylic acid, polyacrylonitrile-acrylic acid copolymer, polyacrylate and its derivatives, polyacrylamide, polyvinyl alcohol or polyurethane. Organic flocculants can promote the agglomeration of polymer emulsion particles to form secondary particle size, and the particle aggregation increases. According to the addition amount of different flocculants, the particle size can be controlled, which is beneficial to improve the increase in coating thickness of the slurry after diaphragm coating, the improvement in porosity and the improvement in the adhesion between the coating and the electrode.

[0047] In some embodiments of the present invention, the amount of the flocculant used is 1% to 15% of the solid content in the polymer emulsion.

[0048] In some embodiments of the present invention, the thickener is at least one of sodium carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, polyethylene oxide, polyacrylate, or polyurethane with a degree of substitution (DS) of 0.6-0.9. The thickener can increase the viscosity of the functional polymer slurry, prevent sedimentation of polymer particles during prolonged storage and transportation, and ensure the stability and uniformity of the slurry.

[0049] In some embodiments of the present invention, the amount of the thickener is 0.5% to 15% of the solid content in the polymer emulsion.

[0050] In some embodiments of the present invention, the additive is a nonionic surfactant or an organosilicon wetting agent. Furthermore, the additive is at least one of polydimethylsiloxane, an organosiloxane copolymer, a fatty alcohol polyether compound, or an acetylenic diol compound. The additive primarily improves the wetting and leveling properties of the functional polymer slurry on the diaphragm surface.

[0051] In some embodiments of the present invention, the amount of the auxiliary agent is 0.5%-10% of the solid content in the polymer emulsion.

[0052] In some embodiments of the present invention, the solvent used to prepare the functional polymer slurry is at least one of water, a lower alcohol solvent (C1-C5), or a carbonate solvent. Furthermore, the amount of solvent used is 1-13 times the weight of the solids in the polymer emulsion. The primary function of the solvent is to adjust the solids content of the functional polymer slurry to suit different coating process requirements.

[0053] In some embodiments of the present invention, the functional coating membrane is prepared by the following method: mixing the polymer emulsion, flocculant, thickener, additive and solvent to obtain a functional polymer slurry, applying the functional polymer slurry on one or both sides of the membrane substrate, and drying to obtain the functional coating membrane.

[0054] In some embodiments of the present invention, the separator substrate is a PE or PP lithium-ion battery separator produced by a wet or dry process, with a thickness of 5-25 μm and a porosity of 30%-55%.

[0055] In some embodiments of the present invention, the coating thickness on the functional coating membrane is 0.3-3 μm, and the gram weight is 0.1-1.0 g / m 2 Preferably, the coating thickness is 0.3-2 μm and the weight is 0.3-0.6 g / m 2 .

[0056] In some embodiments of the present invention, the coating method is one of dip coating, roller coating, spot coating or spray coating.

[0057] According to a preferred embodiment of the present invention, there are at least the following beneficial effects:

[0058] 1. The present invention creatively develops an organic functional adhesive material. Through an original synthesis and formulation process, a functional polymer emulsion with a core-shell three-layer structure is developed. The material has a uniform primary particle size, a narrow particle size distribution, and good stability. After being coated on the diaphragm by adding the formulated material to make a slurry, the coating amount is small, the gram weight is low, and the ionic conductivity is high. After the battery is made, the hot pressing conditions are mild, and the adhesion between the diaphragm and the electrode is excellent. The use of the functional coating diaphragm of the present invention can ensure the good battery capacity, rate and long cycle life of the soft-pack battery, ensure the good shaping effect of the square battery, and long battery cycle life.

[0059] 2. The innovative functional polymer material of the present invention has a suitable particle size and is not prone to clogging the diaphragm micropores; moderate electrolyte swelling and excellent long-term electrolyte stability; a unique core-shell structure, with the core layer as the core of the functional polymer, electrolyte-resistant, and stabilizing the emulsion particles; the first shell layer, the middle layer of the functional polymer, imparts elasticity and lithium ion conductivity to the emulsion particles; the second shell layer, the outermost layer of the functional polymer, is composed of functional monomer copolymers and mainly acts as a bonding agent, capable of simultaneously bonding the diaphragm substrate and the battery's positive and negative electrode materials. The functional polymer material of the present invention can simultaneously meet the requirements of both wet-pressing (electrolyte + temperature + pressure) and dry-pressing (temperature + pressure) battery processes, has good compatibility, wide applicability, and excellent performance, and can replace PVDF materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0061] Figure 1 Schematic diagram of the structure of the polymer emulsion latex particles of the present invention. DETAILED DESCRIPTION

[0062] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0063] Example 1

[0064] This example prepares a polymer emulsion and a functional coating membrane, and the specific process is as follows:

[0065] Synthesis formula of polymer emulsion:

[0066] The core layer monomer is a mixture of 30g styrene and 70g methacrylic acid emulsified and dispersed in 100g water to form a core layer monomer emulsion with a concentration of 50%, and the core layer emulsifier is 1g polyvinyl alcohol; the core layer initiator solution is a solution with a concentration of 2% formed by dissolving 2g ammonium persulfate in 98g water, that is, the core layer solvent is 198g water.

[0067] The first shell layer uses a monomer mixture of 40g of 2-hydroxyethyl acrylate, 40g of vinyl acetate and 20g of acrylic acid, which is emulsified, dispersed or dissolved in 100g of water and 50g of ethanol to form a first shell layer monomer emulsion with a concentration of 40%. The first shell layer emulsifier uses 5g of fatty alcohol polyoxyethylene ether. The first shell layer initiator solution is a 1% solution formed by dissolving 1g of potassium persulfate in 99g of water, that is, the first shell layer solvent uses 199g of water and 50g of ethanol. The first shell layer uses 5g of lithium hydroxide monohydrate as the lithium-containing compound.

[0068] The second shell layer monomers are a mixture of 20 g of isooctyl acrylate, 60 g of methyl methacrylate, and 20 g of butyl acrylate, which is emulsified, dispersed, or dissolved in 150 g of water and 150 g of ethanol to form a second shell layer monomer emulsion with a concentration of 25%. The second shell layer emulsifier is 5 g of fatty alcohol polyoxyethylene ether, and the second shell layer initiator solution is 100 g of a 3% concentration hydrogen peroxide solution, that is, the second shell layer solvent is 250 g of water and 150 g of ethanol.

[0069] The synthesis process of the polymer emulsion comprises the following steps:

[0070] 1) The pre-emulsified core layer monomer emulsion was added dropwise to the core layer initiator solution in a quantitative and continuous manner over 8 minutes, with stirring at 500 rpm. The mixture was reacted at 80°C for 2 hours, and then cooled to 40°C to obtain a milky white seed emulsion (core layer emulsion).

[0071] 2) adding the first shell layer initiator solution to the core layer emulsion obtained in step 1), and then dropping the pre-emulsified first shell layer monomer emulsion, and completing the dropwise addition over 10 minutes. The mixture was reacted at 75° C. for 1.5 hours. Maintaining the temperature, lithium hydroxide monohydrate was added to the above polymer emulsion, and the mixture was stirred at 800 rpm for 1.0 hour. After the reaction was completed, the mixture was cooled to 40° C. to obtain an emulsion-like lithium-containing intermediate layer core-shell emulsion.

[0072] 3) Adding the second shell layer initiator solution to the lithium-containing intermediate layer core-shell emulsion obtained in step 2), and then adding the pre-emulsified second shell layer monomer emulsion dropwise, the dripping is completed over 15 minutes, the stirring speed is 600 rpm, and the reaction is carried out at 90° C. for 1.0 hour; after the reaction is completed, the temperature is lowered to 40° C. to obtain a core-shell three-layer polymer emulsion.

[0073] In this embodiment, the thickness ratio of the core layer, first shell layer, and second shell layer is 3.2:1.3:1, and the structure is shown in Table 1. The core layer has a weight-average molecular weight (Mw) of 40,000, the first shell layer has a weight-average molecular weight (Mw) of 25,000, and the second shell layer has a weight-average molecular weight (Mw) of 20,000. The polymer latex particles have a primary particle size of 0.5 μm, a secondary particle size of 1.0 μm, and a solids content of 26%.

[0074] The method for preparing a functional coating membrane comprises the following steps:

[0075] 1150g of the above polymer emulsion was added one by one according to the ratio of 3g polyacrylic acid flocculant, 3g sodium carboxymethyl cellulose thickener (DS = 0.75), 1.5g fatty alcohol polyether (AEO-7) additive and 350g water solvent, and stirred thoroughly to prepare a functional polymer slurry with a solid content of about 20%. The slurry was then coated on one side of a PE substrate with a thickness of 9μm and a porosity of 42% by roller coating, and then dried, rolled and packaged to obtain a functional coating diaphragm with a coating thickness of 1μm and a gram weight of 0.6g / m 2 .

[0076] Example 2

[0077] This example prepares a polymer emulsion and a functional coating membrane, and the specific process is as follows:

[0078] Synthesis formula of polymer emulsion:

[0079] The core layer monomer is a mixture of 20g succinic acid and 60g α-methylstyrene emulsified and dispersed in 120g water to form a core layer monomer emulsion with a concentration of 40%, and the core layer emulsifier is 0.4g fatty alcohol polyoxyethylene ether; the core layer initiator solution is a solution with a concentration of 1% formed by dissolving 0.8g dibenzoyl peroxide in 79.2g water, that is, the core layer solvent is 199.2g water.

[0080] The first shell layer uses a monomer mixture of 20g acrylic acid, 20g 2-hydroxyethyl methacrylate and 10g diethylaminoethyl methacrylate, which is emulsified, dispersed or dissolved in 50g water and 150g ethanol to form a first shell layer monomer emulsion with a concentration of 20%. The first shell layer emulsifier uses 2g fatty alcohol polyoxypropylene ether. The first shell layer initiator solution is 1.5g azobisisobutyronitrile dissolved in 48.5g methanol to form a 3% solution, that is, the first shell layer solvent uses 50g water, 150g ethanol and 48.5g methanol. The first shell layer uses 2g lithium carbonate as the lithium-containing compound.

[0081] The second shell layer monomers are a mixture of 10 g methyl acrylate, 30 g isooctyl methacrylate, and 10 g lauryl methacrylate, which is emulsified, dispersed, or dissolved in 200 g water and 160 g isopropyl alcohol to form a 10% second shell monomer emulsion. The second shell emulsifier is 1.2 g of a block copolymer of ethylene oxide and propylene oxide. The second shell initiator solution is 100 g of a 1% hydrogen peroxide solution. That is, the second shell solvent is 300 g water and 160 g isopropyl alcohol.

[0082] The synthesis process of the polymer emulsion comprises the following steps:

[0083] 1) The pre-emulsified core layer monomer emulsion was added dropwise to the core layer initiator solution in a quantitative and continuous manner over 12 minutes with stirring at 400 rpm. The mixture was reacted at 70°C for 3 hours and then cooled to 30°C to obtain a milky white seed emulsion (core layer emulsion).

[0084] 2) adding the first shell layer initiator solution to the core layer emulsion obtained in step 1), and then dropping the pre-emulsified first shell layer monomer emulsion, and completing the dropwise addition over 15 minutes. The mixture was reacted at 85° C. for 2.5 hours. Maintaining the temperature, lithium carbonate was added to the polymer emulsion, and the mixture was stirred at 1000 rpm for 0.5 hours. After the reaction was completed, the temperature was lowered to 30° C. to obtain an emulsion-like lithium-containing intermediate layer core-shell emulsion.

[0085] 3) adding the second shell layer initiator solution to the lithium-containing intermediate layer core-shell emulsion obtained in step 2), and then adding the pre-emulsified second shell layer monomer emulsion dropwise over 20 minutes, stirring at 800 rpm, reacting at 95° C. for 1.5 hours; after the reaction is completed and the temperature is lowered to 30° C., a core-shell polymer emulsion with a three-layer structure is obtained.

[0086] In this embodiment, the thickness ratio of the core layer, first shell layer, and second shell layer is 4.4:1.2:1; the core layer has a weight-average molecular weight (Mw) of 60,000, the first shell layer has a weight-average molecular weight (Mw) of 20,000, and the second shell layer has a weight-average molecular weight (Mw) of 10,000. The polymer latex particles have a primary particle size of 0.30 μm, a secondary particle size of 0.6 μm, and a solids content of 16.5%.

[0087] The method for preparing a functional coating membrane comprises the following steps:

[0088] 1000g of the above polymer emulsion was added with 9g of polyacrylonitrile-acrylic acid copolymer flocculant, 1.8g of polyacrylate, 1.8g of acetylene glycol additive, and 710g of water solvent in a proportioned manner, and stirred thoroughly to prepare a functional polymer slurry with a solid content of about 10%. The slurry was then coated on one side of a PE substrate with a thickness of 9μm and a porosity of 42% by roller coating, followed by drying, winding, and packaging to obtain a functional coating diaphragm with a coating thickness of 0.5μm and a gram weight of 0.3g / m 2 .

[0089] Example 3

[0090] This example prepares a polymer emulsion and a functional coating membrane, and the specific process is as follows:

[0091] Synthesis formula of polymer emulsion:

[0092] The core layer monomer is a mixture of 60g methacrylate and 40g 2,4-dimethylstyrene emulsified and dispersed in 900g water to form a core layer monomer emulsion with a concentration of 10%, and the core layer emulsifier is 1g polyol fatty acid ester; the core layer initiator solution is a solution with a concentration of 3% formed by dissolving 3g potassium persulfate in 97g water, that is, the core layer solvent is 997g water.

[0093] The first shell layer uses a monomer mixture of 10g acrylic acid, 10g dimethylaminoethyl methacrylate and 70g 2-hydroxypropyl methacrylate, which is emulsified, dispersed or dissolved in 110g water and 100g isopropyl alcohol to form a first shell layer monomer emulsion with a concentration of 30%. The first shell layer emulsifier uses 4.5g of ethylene oxide and propylene oxide block copolymer. The first shell layer initiator solution is a 5% solution formed by dissolving 5g of ammonium persulfate in 45g of water, that is, the first shell layer solvent is 155g water and 100g isopropyl alcohol. The first shell layer uses 3g of lithium chloride as the lithium-containing compound.

[0094] The second shell layer uses a monomer mixture of 20 g of ethyl acrylate, 30 g of ethyl methacrylate, and 10 g of glycidyl methacrylate, which is emulsified, dispersed, or dissolved in 120 g of ethanol and 120 g of water to form a second shell layer monomer emulsion with a concentration of 20%. The second shell layer emulsifier uses 1.2 g of fatty alcohol polyoxypropylene ether, and the second shell layer initiator solution is 100 g of a 1.5% concentration hydrogen peroxide solution, that is, the second shell layer solvent uses 120 g of ethanol and 220 g of water.

[0095] The synthesis process of the polymer emulsion comprises the following steps:

[0096] 1) The pre-emulsified core layer monomer emulsion was added dropwise to the core layer initiator solution in a quantitative and continuous manner over 20 minutes with stirring at 600 rpm. The mixture was reacted at 85°C for 2 hours and then cooled to 30°C to obtain a milky white seed emulsion (core layer emulsion).

[0097] 2) adding the first shell layer initiator solution to the core layer emulsion obtained in step 1), and then adding the pre-emulsified first shell layer monomer emulsion dropwise over 15 minutes, reacting at 85° C. for 2.5 hours; maintaining the temperature, adding lithium chloride to the above polymer emulsion, stirring at 1000 rpm for 0.5 hours, and cooling to 30° C. after the reaction is completed to obtain an emulsion-like lithium-containing intermediate layer core-shell emulsion.

[0098] 3) adding the second shell layer initiator solution to the lithium-containing intermediate layer core-shell emulsion obtained in step 2), and then adding the pre-emulsified second shell layer monomer emulsion dropwise over 20 minutes, stirring at 800 rpm, reacting at 95° C. for 1.5 hours; after the reaction is completed and the temperature is lowered to 30° C., a core-shell polymer emulsion with a three-layer structure is obtained.

[0099] In this embodiment, the thickness ratio of the core layer, first shell layer, and second shell layer is 5:2:1; the core layer has a weight-average molecular weight (Mw) of 100,000, the first shell layer has a weight-average molecular weight (Mw) of 40,000, and the second shell layer has a weight-average molecular weight (Mw) of 20,000. The polymer latex particles have a primary particle size of 0.40 μm, a secondary particle size of 0.8 μm, and a solids content of 13.5%.

[0100] The method for preparing a functional coating membrane comprises the following steps:

[0101] 1850g of the above polymer emulsion was added with 5g of polyurethane flocculant, 2.5g of hydroxyethyl cellulose, 5g of polydimethylsiloxane additive and 3150g of water solvent in a proportion, and stirred thoroughly to prepare a functional polymer slurry with a solid content of about 5%. The slurry was then coated on one side of a PE substrate with a thickness of 9μm and a porosity of 42% by roller coating, and then dried, rolled and packaged to obtain a functional coating diaphragm with a coating thickness of 0.5μm and a gram weight of 0.4g / m 2 .

[0102] Comparative Example 1

[0103] In this comparative example, a polymer emulsion was prepared. The difference from Example 1 is that the core layer structure was omitted, the first shell layer monomer was first synthesized to form a seed emulsion, and then the second shell layer monomer was synthesized to form a double-layer structure. The remaining components and preparation method were the same as those in Example 1.

[0104] Comparative Example 2

[0105] In this comparative example, a polymer emulsion was prepared. The difference from Example 1 is that the first shell structure was omitted, the core layer monomer was first synthesized to form a seed emulsion, and then the second shell layer monomer was synthesized to form a double-layer structure. The remaining components and preparation method were the same as those in Example 1.

[0106] Comparative Example 3

[0107] In this comparative example, a polymer emulsion was prepared. The difference from Example 1 is that the second shell structure was omitted, and the remaining components and preparation method were the same as those in Example 1.

[0108] Comparative Example 4

[0109] In this comparative example, a conventional PVDF coated diaphragm was prepared. The difference from Example 1 is that no polymer emulsion was synthesized. The polymer emulsion was replaced with 300g of PVDF powder in the slurry. 3g of polyacrylic acid flocculant, 3g of sodium carboxymethyl cellulose thickener (DS=0.75), 1.5g of fatty alcohol polyether (AEO-7) additive and 1200g of water solvent were added one by one according to the ratio, and stirred sufficiently to prepare a PVDF slurry with a solid content of about 20%. The slurry was coated on one side of a PE substrate with a thickness of 9μm and a porosity of 42% by roller coating, and then dried, rolled and packaged to obtain a coated diaphragm with a coating thickness of 2μm and a gram weight of 0.8g / m 2 .

[0110] Performance Testing

[0111] The separators prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to basic physical property tests, including air permeability, liquid absorption and retention, and swelling. Each separator set was then fabricated into soft-pack lithium-ion batteries: model 506090, with an NCM523 cathode and natural graphite anode; and an electrolyte consisting of EC / PC / DMC in a ratio of 2:3:5 (v / v / v), 1M LiPF6, and 2% VC. The separator-to-battery adhesion, cell resistance, and cycling parameters were measured. The adhesion between the separator and the electrode was measured under both wet pressing (electrolyte: EC / PC / DMC in a ratio of 2:3:5 (v / v / v), 1M LiPF6, and 2% VC; temperature: 75°C, pressure: 1.0 MPa) and dry pressing (temperature: 80°C, pressure: 1.5 MPa). The test results are shown in Table 1.

[0112] Table 1 Test results of membrane performance of Examples 1 to 3 and Comparative Examples 1 to 4

[0113]

[0114]

[0115] As shown in Table 1, the wet and dry peel strengths of the separators made from the slurries of Examples 1-3 are higher than those of Comparative Examples 1-4, indicating that the separators of Examples 1-3 can meet both wet pressing (electrolyte + temperature + pressure) and dry pressing (temperature + pressure) battery processes and have good compatibility. After using the separators of Examples 1-3 to make batteries, the capacity, 3C rate, and conventional 800 cycle capacity retention rate are all higher than those of Comparative Examples 1-4, indicating that the separators of Examples 1-3 can ensure good battery hardness and long cycle life for soft-pack batteries.

[0116] As can be seen from Table 1, the functional polymer coating membrane manufactured by Example 1 through a better formula combination and process has the best physical and chemical properties: appropriate coating amount, low air permeability, moderate liquid absorption rate, and high ionic conductivity; the battery prepared by the functional polymer coating membrane manufactured in Example 1 has the best battery performance: strong adhesion between the membrane and the electrode after wet pressing and dry pressing, small battery internal resistance, high capacity, good 3C rate performance, long cycle life, and high capacity retention rate, which is the best solution among the three embodiments.

[0117] Compared with Example 1, Comparative Example 1 removes the core layer structure. This is because the core layer serves as the core of the functional polymer material, is resistant to electrolyte, and plays a role in stabilizing the emulsion particles. Without the core layer, the coating weight of the functional polymer coating diaphragm manufactured is also reduced, the liquid absorption rate increases sharply (large swelling, not resistant to electrolyte), the air permeability increases, and the ionic conductivity decreases; when manufactured into a soft-pack battery, the adhesion between the diaphragm and the electrode decreases after wet pressing and dry pressing, the internal resistance increases significantly, the capacity is significantly smaller, the 3C rate deteriorates significantly, the cycle life is short, and the capacity retention rate is low.

[0118] Compared to Example 1, Comparative Example 2 removed the first shell structure. The first shell serves as an intermediate layer of functional material, imparting elasticity and lithium ion conductivity to the emulsion particles. The lack of this first shell also reduced the coating weight of the functional polymer-coated separator, with minimal change in liquid absorption and a slight increase in air permeability, but significantly reduced ionic conductivity. Fabricated into soft-pack batteries, wet and dry pressing revealed decreased adhesion between the separator and the electrode, a slight increase in internal resistance, a decrease in capacity, a significant decrease in 3C rate, a shortened cycle life, and low capacity retention.

[0119] Compared with Example 1, Comparative Example 3 removed the second shell structure. The second shell is the outermost layer of the functional polymer material and primarily serves as an adhesive, simultaneously bonding the separator substrate and the battery's positive and negative electrode materials. Without this second shell, the functional polymer-coated separator produced also exhibited a reduced coating weight, minimal change in liquid absorption, a slight increase in air permeability, and decreased ionic conductivity. While the internal resistance of the resulting soft-pack battery remained unchanged, the adhesion between the separator and the electrode sheet decreased significantly after both wet and dry pressing, resulting in reduced battery capacity, significantly reduced 3C rate, a shortened cycle life, and low capacity retention.

[0120] Comparative Example 4 directly uses PVDF powder instead of functional polymer emulsion to prepare a PVDF slurry with a solid content of about 20%, which is coated on one side of a 9μm thick PE substrate to produce a conventional PVDF coated diaphragm with large thickness, high gram weight, high air permeability and low ionic conductivity; after being made into a soft-pack battery, the adhesion between the diaphragm and the electrode is low after wet pressing and dry pressing, the battery internal resistance is large, the capacity is small, the 3C rate is poor, the cycle life is short, the capacity retention rate is the lowest, and the overall performance is poor.

[0121] From the test results of the diaphragms prepared in the above embodiments and comparative examples, it can be seen that the original functional polymer material of the product of the present invention has a core-shell three-layer structure. The first shell layer is the middle layer of the functional material, which gives the emulsion particles elasticity and lithium ion conductivity function; the second shell layer is the outermost layer of the functional polymer material, mainly composed of functional monomer copolymers, which mainly plays a bonding role and can simultaneously bond the diaphragm substrate and the positive and negative electrode materials of the battery; each layer of the structure is indispensable. Comparative Example 1 lacks the core layer, resulting in poor electrolyte resistance of the coating, large swelling, large internal resistance of the manufactured battery, low capacity and short cycle life; Comparative Example 2 lacks the first shell layer, resulting in a decrease in the ion conductivity of the coating, low ionic conductivity, and a significant deterioration in the 3C rate of the manufactured battery and a short cycle life; Comparative Example 3 lacks the second shell layer, resulting in a significant decrease in the adhesion between the coating and the electrode, and a significant reduction in the battery cycle life.

[0122] In summary, the present invention combines a unique formula with a unique process to produce a functional polymer coating diaphragm with a small coating amount, low gram weight and good adhesion. After the battery is made, the hot pressing conditions are mild and the adhesion between the diaphragm and the electrode is excellent. The use of the functional polymer coating diaphragm of the present invention can ensure good battery hardness and long cycle life of soft-pack batteries, good square battery shaping effect and long battery cycle life.

[0123] While the embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof may be combined with one another unless there is a conflict.

Claims

1. A polymer emulsion, characterized in that The latex particles of the polymer emulsion include a three-layer structure from the inside to the outside, the inner layer being the core layer, the middle layer being the first shell layer, and the outer layer being the second shell layer; the core layer is composed of a hard monomer copolymer; the first shell layer is composed of a monomer copolymer with moderate hardness and softness and a lithium-containing compound; the second shell layer is composed of a functional monomer copolymer, and the homopolymer Tg value of the monomer used in the second shell layer is less than 120°C; the monomers of the core layer are more than one of maleic acid, methacrylic acid, hydroxymethyl acrylamide, acrylamide, styrene, α-methylstyrene, 2-methylstyrene, 2,4-dimethylstyrene, tetrahydrofuran methacrylate, or propylene methacrylate; the monomers of the first shell layer are more than one of acrylic acid, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, vinyl acetate, dimethylaminoethyl methacrylate, or diethylaminoethyl methacrylate.

2. The polymer emulsion according to claim 1, characterized in that The thickness ratio of the core layer, the first shell layer and the second shell layer is (2-5): (1-2): (1-3); the primary particle size of the latex particles of the polymer emulsion is 0.3-0.6 μm, and the secondary particle size is 0.6-6.0 μm.

3. The polymer emulsion according to claim 1, characterized in that The lithium-containing compound is at least one of a lithium salt, a lithium oxide or a lithium hydroxide.

4. The polymer emulsion according to claim 1, characterized in that The monomers of the second shell layer are more than one of methyl acrylate, ethyl acrylate, butyl acrylate, isooctyl acrylate, methyl methacrylate, ethyl methacrylate, isooctyl methacrylate, lauryl methacrylate, glycidyl methacrylate or dicyclopentenylethoxy methacrylate.

5. The method for preparing the polymer emulsion according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1: mixing the core layer monomer and an emulsifier to prepare a core layer monomer pre-emulsion, and adding the core layer monomer pre-emulsion dropwise to an initiator solution to carry out a polymerization reaction to obtain a core layer emulsion; S2: mixing the monomer of the first shell layer with an emulsifier to prepare a first shell layer monomer pre-emulsion, adding an initiator solution to the core layer emulsion, and then dropwise adding the first shell layer monomer pre-emulsion to carry out a polymerization reaction. After the reaction is completed, a lithium-containing compound is added and the reaction is stirred to obtain a first shell layer emulsion; S3: Mixing the monomer of the second shell layer with an emulsifier to prepare a second shell layer monomer pre-emulsion, adding an initiator solution to the first shell layer emulsion, and then dropwise adding the second shell layer monomer pre-emulsion to carry out polymerization reaction to obtain the polymer emulsion.

6. Use of the polymer emulsion according to any one of claims 1 to 4 in battery separators.

7. A functional coating membrane, characterized in that: At least one surface of the functional coating membrane is coated with a functional coating, and the functional coating is made of a functional polymer slurry, and the functional polymer slurry comprises the polymer emulsion according to any one of claims 1 to 4, a flocculant, a thickener, an additive and a solvent.

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