Diaphragm binder, diaphragm and battery

Abstract

The invention provides a diaphragm binder, a diaphragm and a battery in order to solve the problem of low cohesiveness of a diaphragm and a pole piece in the prior art. The diaphragm binder is of a core-shell structure and comprises a core body and a shell layer which is arranged on the outer surface of the core body and at least partially wraps the core body, the core body comprises a first polymer, and the shell layer comprises a second polymer; the glass transition temperature of the first polymer is-60 to 20 DEG C, and the glass transition temperature of the second polymer is 40 to 110 DEG C; the first polymer comprises a first structural unit containing a polar functional group, and the polar functional group is selected from at least one of carboxyl, sulfonic acid group, phosphate group, amide group and cyano group; the mass percentage content of the first structural unit in the first polymer is 1%-10%. When the diaphragm binder provided by the invention is applied to a diaphragm, after the diaphragm and a pole piece are pressurized at normal temperature, a shell layer of the diaphragm binder is broken, and a core body is exposed, so that the diaphragm is bonded with the pole piece under normal-temperature pressurization, but the diaphragm does not generate self-adhesion after being rolled.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a separator adhesive, a separator and a battery. BACKGROUND

[0002] Lithium ion batteries have the advantages of high voltage, large specific energy, stable discharge voltage, good cycle performance, good safety performance and long service life, and are widely used in the fields of power automobiles and energy storage.

[0003] A lithium ion battery is usually mainly composed of a positive electrode, a negative electrode, a separator, an electrolyte and a battery shell. The separator, as one of the important inner components, mainly functions to separate the positive electrode and the negative electrode of the battery to prevent the positive electrode and the negative electrode from contacting each other to cause short circuit. At present, the separators used in the prior art are mostly polyolefin porous membranes. Since the melting point of the polyolefin porous membrane is lower than 200 DEG C, when the temperature of the battery is increased due to internal or external factors, the polyolefin porous membrane shrinks or melts, so that the positive and negative electrodes directly contact each other, resulting in battery short circuit, and further causing the occurrence of accidents such as battery combustion and explosion.

[0004] In order to solve the above problems, the prior art attempts to use an adhesive to coat ceramic particles on the surface of a separator substrate to make a ceramic / polymer composite separator, so as to reduce the thermal shrinkage rate of the separator by using the heat resistance of the ceramic particles, and to improve the adhesion between the separator and the electrode interface by using the polymer, so as to achieve the purposes of avoiding battery short circuit and improving the hardness of the battery cell. However, the adhesion between the above composite separator and the electrode is low, and the separator and the electrode are prone to misalignment.

[0005] Therefore, there is an urgent need for an adhesive capable of improving the adhesion between the separator and the electrode. SUMMARY

[0006] The technical problem to be solved by the application is the low adhesion between the existing separator and the electrode. The application provides a separator adhesive, a separator and a battery.

[0007] To solve the above technical problem, the application provides a separator adhesive, which is a core-shell structure and comprises a core and a shell layer arranged on the outer surface of the core and at least partially covering the core. The core comprises a first polymer, and the shell layer comprises a second polymer. The glass transition temperature of the first polymer is-60-20 DEG C, and the glass transition temperature of the second polymer is 40-110 DEG C. The first polymer comprises a first structural unit containing a polar functional group, and the polar functional group is selected from at least one of carboxyl, sulfonic acid group, phosphate group, hydroxyl group and cyano group. The mass percentage content of the first structural unit in the first polymer is 1%-10%.

[0008] Preferably, the first structural unit is a structural unit formed by polymerization of a first monomer, the first monomer comprising at least one of acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, β-carboxyethyl acrylate, cinnamic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, alkenyl sulfonic acid compound, sodium allyloxy hydroxypropyl methanesulfonate, acrylic sulfonate, acrylamido alkyl sulfonic acid compound, acryloyloxy ethyl phosphate compound, allyl phosphate, acrylamide compound, methacrylamide compound, acrylonitrile compound, methacrylonitrile compound, 2-cyanoethyl acrylate.

[0009] Preferably, the first polymer further comprises a cross-linking structural unit, the mass percentage of the cross-linking structural unit in the first polymer being 0.1% to 3%.

[0010] Preferably, the cross-linking structural unit is a structural unit obtained by polymerization of a cross-linking monomer, the cross-linking monomer comprising at least two polymerizable double bonds. The cross-linking monomer comprises at least one of acrylate compound, methacrylate compound, aromatic vinyl compound, acrylamide compound and allyl compound.

[0011] Preferably, the first polymer further comprises a second structural unit, the second structural unit comprising at least one of vinyl structural unit, acrylate structural unit, methacrylate structural unit, maleate structural unit, itaconate structural unit, maleimide structural unit, acrylamide structural unit, methacrylamide structural unit. And / or, the second polymer comprises at least one of vinyl structural unit, acrylate structural unit, methacrylate structural unit, maleate structural unit, itaconate structural unit, maleimide structural unit, acrylamide structural unit, methacrylamide structural unit.

[0012] Preferably, the mass ratio of the first polymer to the second polymer is (30:70) to (80:20).

[0013] Preferably, the dissolution rate of the separator binder in electrolyte is less than 3%. And / or, the average particle size of the separator binder is 0.2 to 6 μm.

[0014] In a second aspect, the application provides a separator comprising a base film and a bonding layer, the bonding layer being arranged on at least one side of the base film, the bonding layer comprising the separator binder according to any one of the above.

[0015] Preferably, the coating area density of the separator adhesive of each of the adhesive layers is 0.1-0.6 g / m 2 ; Preferably, the peeling strength of the adhesive layer from the negative electrode sheet is ≥1 N / m at 25-30℃ and 2-10 MPa.

[0016] In a third aspect, the present application provides a lithium ion battery comprising the separator as described above.

[0017] The separator adhesive provided by the present application has a core-shell structure. When the separator adhesive is applied to the separator, the shell layer of the separator adhesive is broken after the separator is pressed at room temperature, and the core body is exposed. The glass transition temperature of the first polymer of the core body is between -60 and 20℃, so that the separator is bonded to the electrode sheet at room temperature and pressure, and has good bonding force with the electrode sheet, which can reduce energy consumption and save costs. At the same time, the separator will not be self-bonded after being wound. The carboxyl, sulfonic acid group and phosphate group in the first polymer have strong intermolecular forces with the negative electrode sheet, so that the separator and the electrode sheet have good bonding force. The cyano group or amide group can form hydrogen bonds and polar effects with elements such as hydroxyl and N in the negative electrode sheet, thereby increasing the bonding force between the separator and the electrode sheet. By limiting the mass percentage content of the first structural unit in the first polymer to 1%-10%, the bonding force between the separator adhesive and the electrode sheet is regulated, and the glass transition temperature of the first polymer is regulated to be between -60 and 20℃. DETAILED DESCRIPTION

[0018] In order to make the technical problems, technical solutions and beneficial effects of the present application clearer, the present application will be further described in detail below with reference to examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0019] An embodiment of the present application provides a separator adhesive, which has a core-shell structure and includes a core body and a shell layer arranged on the outer surface of the core body and at least partially covering the core body. The core body includes a first polymer, and the shell layer includes a second polymer. The glass transition temperature (Tg) of the first polymer is -60-20℃, and the glass transition temperature of the second polymer is 40-110℃. The first polymer includes a first structural unit containing a polar functional group, and the polar functional group is selected from at least one of carboxyl, sulfonic acid group, phosphate group, amide group and cyano group. The mass percentage content of the first structural unit in the first polymer is 1%-10%.

[0020] The separator adhesive provided in this application has a core-shell structure. When applied to the separator, the shell of the separator adhesive ruptures upon pressurization of the separator and the electrode at room temperature (25-30°C), revealing the core. The glass transition temperature of the first polymer in the core is between -60°C and 20°C. This allows the separator to bond to the electrode under pressure at room temperature, resulting in strong adhesion, reducing energy consumption and saving costs. Furthermore, the separator does not self-adhere after winding. The carboxyl, sulfonic, and phosphate groups in the first polymer form strong intermolecular interactions with the negative electrode, resulting in strong adhesion between the separator and the electrode. The cyano or amide groups can form hydrogen bonds and polar interactions with elements such as hydroxyl groups and nitrogen in the negative electrode, enhancing adhesion between the separator and the electrode. By limiting the mass percentage of the first structural unit in the first polymer to 1% to 10%, the adhesion of the separator adhesive to the electrode is regulated, while the glass transition temperature of the first polymer is also regulated between -60°C and 20°C.

[0021] The intermolecular interactions between polar functional groups can enhance the cohesive strength of the first polymer. However, if the content of the first structural unit exceeds 10%, the cohesive strength of the first polymer is too high and the toughness is insufficient, which increases the glass transition temperature of the first polymer and reduces its elasticity, making it impossible for the diaphragm to bond to the electrode under normal temperature and pressure. If the content of the first structural unit is less than 1%, the intermolecular forces between the polar functional groups in the diaphragm binder and the electrode are weakened, resulting in a decrease in the bonding strength between the diaphragm and the electrode.

[0022] Specifically, the mass percentage of the first structural unit in the first polymer includes but is not limited to 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0023] The glass transition temperatures of the first polymer and the second polymer are both obtained by differential scanning calorimetry (DSC) testing.

[0024] In some embodiments, the first structural unit is a structural unit formed by the polymerization of a first monomer, and the first monomer includes at least one of acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, β-carboxyethyl acrylate, cinnamic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, alkenyl sulfonic acid compounds, sodium allyloxyhydroxypropyl methanesulfonate, acrylic acid sulfonate salts, acrylamide alkyl sulfonic acid compounds, acryloyloxyethyl phosphate compounds, allyl phosphoric acid, acrylamide compounds, methacrylamide compounds, acrylonitrile compounds, methacrylonitrile compounds, and 2-cyanoethyl acrylate.

[0025] Specifically, the alkenyl sulfonic acid compound includes at least one of vinyl sulfonic acid, allyl sulfonic acid, methyl allyl sulfonic acid, methyl vinyl sulfonic acid, styrene sulfonic acid, and α-methyl styrene sulfonic acid.

[0026] The acrylic sulfonate ester salt includes at least one of 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate, 2-ethyl sulfonate acrylate, and 2-ethyl sulfonate methacrylate.

[0027] The acrylamido alkyl sulfonic acid compound includes at least one of acrylamido-2-methyl propane sulfonic acid, acrylamido-2-hydroxy propane sulfonic acid, methacrylamido-2-methyl propane sulfonic acid, and methacrylamido-2-hydroxy propane sulfonic acid.

[0028] The acryloyloxyethyl phosphate compound includes at least one of acryloyloxyethyl phosphate, methacryloyloxyethyl phosphate, 2-hydroxyethyl acryloyl phosphate, 2-hydroxyethyl methacryloyl phosphate, phenyl-2-acryloyloxyethyl phosphate, and di[2-(methacryloyloxy)ethyl] phosphate.

[0029] The acrylamide compound and the methacrylamide compound include at least one of acrylamide, methacrylamide, C1-C12 alkyl-substituted acrylamide, C1-C12 alkyl-substituted methacrylamide, C6-C18 aryl-substituted acrylamide, and C6-C18 aryl-substituted methacrylamide.

[0030] The acrylonitrile compound includes at least one of acrylonitrile and methacrylonitrile.

[0031] In some embodiments, the first polymer further includes a cross-linking structural unit, and a mass percentage content of the cross-linking structural unit in the first polymer is 0.1% to 3%.

[0032] The first polymer has a low glass transition temperature, and may be dissolved in a high-temperature electrolyte. Once the first polymer is dissolved out, an electrochemical reaction occurs at the positive electrode or the negative electrode, causing a decrease in the cycle performance of the battery. Therefore, to reduce the electrolyte dissolution rate of the first polymer, a cross-linking structural unit is introduced into the first polymer to avoid the flow of the core during use, thereby improving the cycle performance of the battery.

[0033] In addition, if the content of the cross-linking monomer is less than 0.1%, the flow of the first polymer during use cannot be completely inhibited, thereby causing the blocking of the pores of the separator, affecting the lithium ion transmission during use of the battery, and causing a decrease in the cycle performance of the battery. If the content of the cross-linking monomer is more than 3%, the cohesive strength of the first polymer is too high and the toughness is insufficient, that is, the elasticity decreases, and the separator cannot be bonded to the electrode tab under normal temperature and pressure.

[0034] In some embodiments, the cross-linking structural unit is a structural unit obtained by polymerization of a cross-linking monomer, and the cross-linking monomer contains at least two polymerizable double bonds. The cross-linking monomer includes at least one of an acrylate compound, a methacrylate compound, an aromatic vinyl compound, an acrylamide compound, and an allyl compound.

[0035] Specifically, the acrylate compound and the methacrylate compound include at least one of ethylene glycol diacrylate, polyethylene glycol diacrylate, propylene glycol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, trimethylolpropane triacrylate, glycerol triacrylate, pentaerythritol triacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, propylene glycol dimethacrylate, dipropylene glycol dimethacrylate, tripropylene glycol dimethacrylate, trimethylolpropane trimethacrylate, glycerol trimethacrylate, pentaerythritol trimethacrylate, allyl acrylate, and allyl methacrylate.

[0036] The aromatic vinyl compound includes divinylbenzene.

[0037] The acrylamide compound includes N,N-methylenebisacrylamide.

[0038] The allyl compound includes at least one of diallyl phthalate, glycerol diallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, diallyl adipate, allyl acrylate, and allyl methacrylate.

[0039] To inhibit the first polymer from dissolving in the electrolyte, in a preferred embodiment, the cross-linking monomer is selected from at least one of N,N-methylenebisacrylamide, polyethylene glycol diacrylate, and polyethylene glycol dimethacrylate.

[0040] In some embodiments, the first polymer further includes a second structural unit including at least one of a vinyl structural unit, an acrylate structural unit, a methacrylate structural unit, a maleate structural unit, an itaconate structural unit, a maleimide structural unit, an acrylamide structural unit, and a methacrylamide structural unit. The glass transition temperature and the adhesion of the first polymer are regulated by the cooperation of the above-mentioned structural units with the cross-linking unit and the first structural unit.

[0041] In some embodiments, the second polymer includes at least one of a vinyl structural unit, an acrylate structural unit, a methacrylate structural unit, a maleate structural unit, an itaconate structural unit, a maleimide structural unit, an acrylamide structural unit, and a methacrylamide structural unit.

[0042] Specifically, the vinyl-based structural unit in the second structural unit and the second polymer is a structural unit derived from polymerization of a vinyl-based monomer, and the vinyl-based monomer includes at least one of an aliphatic vinyl-based compound, an alicyclic vinyl-based compound, an aromatic vinyl-based compound, an allyl-based compound, and a heteroatom-containing vinyl-based compound.

[0043] The aliphatic vinyl-based compound includes, but is not limited to, at least one of a C2-C12 alkene, a C3-C24 α-olefin, and a C4-C12 diene. The C2-C12 alkene includes, but is not limited to, at least one of ethylene, propylene, butylene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, and octadecene. The C4-C12 diene includes, but is not limited to, at least one of butadiene, isoprene, 1,4-pentadiene, 1,6-hexadiene, and 1,7-octadiene.

[0044] The alicyclic vinyl-based compound includes, but is not limited to, at least one of a C6-C15 monocyclic or bicyclic alkene, a C5-C12 monocyclic or bicyclic diene, and a terpene compound. The C6-C15 monocyclic or bicyclic alkene includes, but is not limited to, at least one of cyclohexene, vinylcyclohexene, and ethylidene bicycloheptene. The C5-C12 monocyclic or bicyclic diene includes, but is not limited to, at least one of cyclopentadiene, cycloheptadiene, bicyclo[2.2.1]pentadiene, and bicyclo[2.2.2]octadiene. The terpene compound includes, but is not limited to, at least one of limonene and indene.

[0045] The aromatic vinyl-based compound includes, but is not limited to, at least one of styrene and substituted styrene, and the substituted styrene includes, but is not limited to, at least one of α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, crotylstyrene, divinylbenzene, divinyltoluene, divinylxylene, trivinylbenzene, vinylnaphthalene, and p-t-butylstyrene.

[0046] The heteroatom-containing vinyl-based compound includes, but is not limited to, at least one of a vinyl (thio) ether, a vinyl sulfone, acrylonitrile, methacrylonitrile, cyanostyrene, 4-vinylpyridine, 2-vinylpyridine, vinylimidazole, N-vinylpyrrole, and N-vinylthiopyrrolidone.

[0047] The acrylate structural unit or methacrylate structural unit in the second structural unit and the second polymer is a structural unit obtained by polymerization of acrylate monomers or methacrylate monomers, including but not limited to methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-pentyl acrylate, isopentyl acrylate, n-hexyl acrylate, n-octyl acrylate, isooctyl acrylate, isobornyl acrylate, phenoxyethyl acrylate, dicyclopentenyl acrylate, cyclohexyl acrylate, benzyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, carboxyethyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, n-hexyl methacrylate, isooctyl methacrylate, isobornyl methacrylate, phenoxyethyl meth ... Cyclopentenyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, polyethylene glycol mono[(meth)acrylate] ester, glycidyl (meth)acrylate, tetrahydrofuran (meth)acrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, ethoxylated ethylene glycol diacrylate, ethoxylated ethylene glycol dimethacrylate, allyl methacrylate, diallyl phthalate, diallyl adipate At least one of ester, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol diacrylate, pentaerythritol dimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate, aminoethyl acrylate, aminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate, diethylaminoethyl methacrylate, and tert-butylaminoethyl methacrylate.

[0048] The maleate structural units in the second structural unit and the second polymer are structural units obtained by polymerization of maleate monomers, wherein the maleate monomers include at least one of C1-C12 monoalkyl maleate or C1-C12 dialkyl maleate. The C1-C12 above indicates a carbon number of 1-12. Specifically, the maleate monomers include at least one of monomethyl maleate, dimethyl maleate, monoethyl maleate, diethyl maleate, monopropyl maleate, dipropyl maleate, monobutyl maleate, dibutyl maleate, monooctyl maleate, and dioctyl maleate.

[0049] The itaconate structural unit in the second structural unit and the second polymer is a structural unit obtained by polymerization of an itaconate monomer, and the itaconate monomer includes at least one of C1-C12 mono-itaconate ester and C1-C12 di-itaconate ester. Specifically, the itaconate monomer includes at least one of monomethyl itaconate, dimethyl itaconate, monoethyl itaconate, diethyl itaconate, monobutyl itaconate, dibutyl itaconate, monooctyl itaconate, and dioctyl itaconate.

[0050] The maleimide structural unit in the second structural unit and the second polymer is a structural unit obtained by polymerization of a maleimide monomer, and the maleimide monomer includes at least one of maleimide, C1-C12 alkyl-substituted maleimide, and C6-C16 aryl-substituted maleimide.

[0051] The acrylamide structural unit or methacrylamide structural unit in the second structural unit and the second polymer is a structural unit obtained by polymerization of an acrylamide monomer or a methacrylamide monomer, and the acrylamide monomer or the methacrylamide monomer includes at least one of (meth)acrylamide, N-methyl(meth)acrylamide, N-butyl acrylamide, acetylacetone acrylamide, N-hydroxymethyl(meth)acrylamide, N,N'-methylenebis[(meth)acrylamide], cinnamamide, N,N-dimethyl acrylamide, N,N-dibenzyl acrylamide, methacryloyl formamide, N-methyl N-vinyl acetamide, and N-vinyl pyrrolidone.

[0052] In some embodiments, the mass ratio of the first polymer to the second polymer is (30:70) to (80:20). By limiting the mass ratio of the first polymer to the second polymer to be within the above range, the thickness of the shell layer is limited, so that the thickness of the shell layer of the separator binder can be broken under pressure, while ensuring the integrity of the shell layer during winding.

[0053] Specifically, the mass ratio of the first polymer to the second polymer includes but is not limited to 30:70, 40:60, 50:50, 60:40, 70:30, or 80:20.

[0054] Further, the mass content of the first polymer in the core-shell structure is 30% to 80%, and the mass content of the second polymer is 20% to 70%.

[0055] In some embodiments, the dissolution rate of the separator binder in the electrolyte is <3%. By limiting the dissolution rate of the separator binder in the electrolyte, the occurrence of electrochemical reaction of the separator binder at the positive electrode or negative electrode position is avoided, which causes the cycle performance of the battery to decrease.

[0056] In some embodiments, the average particle size of the separator binder is 0.2-6 pm. By limiting the average particle size of the separator binder, the thickness of the adhesive layer formed by the separator binder on the separator is limited, so that the separator has a higher adhesive strength with the pole piece. Specifically, the average particle size of the separator binder of the embodiments of the present application can be obtained by measuring the separator binder using a laser particle size analyzer or the like. When the separator is prepared, the average particle size of the separator binder in the adhesive layer of the separator can be tested by the following steps: using SEM to test the separator, testing the average particle size of at least 20 particles, and then taking the average value as the final test result.

[0057] The average particle size of the separator binder includes, but is not limited to, 0.2 pm, 0.5 pm, 1 pm, 1.5 pm, 2 pm, 2.5 pm, 3 pm, 3.5 pm, 4 pm, 4.5 pm, 5 pm, 5.5 pm, or 6 pm.

[0058] The particle size is the volume average particle size. When the particle size is calculated using SEM, the particle can be calculated as a sphere.

[0059] Further, an embodiment of the present application provides a preparation method of a separator binder, comprising the following steps: Preparation of the core: In an emulsification kettle, deionized water, an emulsifier, and a monomer mixture of the first polymer are added, and the monomer pre-emulsion of the first polymer is obtained by stirring and emulsifying. The amount of the emulsifier is 0.01%-1% of the weight of the monomer mixture.

[0060] In another reaction kettle, deionized water is added, and after warming with nitrogen, an initiator is added, and then the monomer pre-emulsion of the first polymer is added dropwise, and the reaction is carried out to obtain the first polymer emulsion.

[0061] If the particle size of the prepared first polymer is greater than 1 pm, the suspension polymerization, dispersion polymerization, and precipitation polymerization methods can be used for preparation.

[0062] Taking suspension polymerization as an example: in a reaction kettle, a monomer mixture of the first polymer, an initiator, and an aqueous solution containing a dispersant are added to obtain an oil-water mixture, and the mixture is homogenized to the desired particle size using a homogenizer, and then polymerization is carried out to obtain the first polymer with the desired particle size.

[0063] When prepared by suspension polymerization, the particle size of the polymer particles is mainly related to the rotation speed of the homogenizer, and the particle size can be adjusted by adjusting the rotation speed of the homogenizer. The specific preparation process can refer to patent CN117060010A.

[0064] Preparation of the separator binder: The first polymer emulsion is heated, an initiator is added, and then a monomer mixture of the second polymer is added dropwise to react to obtain a core-shell structured granular polymer, namely, a diaphragm binder.

[0065] As known to those skilled in the art, the reactions in the above steps are conventional free radical polymerization, etc., and the specific methods and reaction conditions are free radical polymerization methods commonly used in the prior art, which will not be described in detail in the present invention.

[0066] In this application, there is no limitation on the mass fractions of the monomers of the core and the shell, as long as the glass transition temperature of the first polymer finally formed is -60~20°C, the glass transition temperature of the second polymer is 40~110°C, and the mass ratio of the first polymer to the second polymer is (30:70)~(80:20).

[0067] The initiator is not particularly limited and can be a commonly used initiator for free radical polymerization, which is usually a persulfate, azo and peroxide initiator, including but not limited to one or more of sodium persulfate, ammonium persulfate, potassium persulfate, tert-butyl hydroperoxide, azobisisobutyronitrile, benzoyl peroxide / sucrose, tert-butyl hydroperoxide / rongalite, tert-butyl hydroperoxide / sodium metabisulfite, benzoyl peroxide / N,N-dimethylaniline, ammonium persulfate / sodium bisulfite, potassium persulfate / sodium bisulfite, hydrogen peroxide / tartaric acid, hydrogen peroxide / diaobaikuai, ammonium persulfate / ferrous sulfate, hydrogen peroxide / ferrous sulfate, benzoyl peroxide / N,N-diethylaniline, benzoyl peroxide / ferrous pyrophosphate, potassium persulfate / silver nitrate, persulfate / thiol, cumene hydroperoxide / ferrous chloride, potassium persulfate / ferrous chloride, hydrogen peroxide / ferrous chloride, cumene hydroperoxide / tetraethyleneimine, and lauroyl peroxide.

[0068] The emulsifier may be any of the various emulsifiers currently available on the market.

[0069] In the present invention, each of the aforementioned structural units represents the structural moiety present in the resulting polymer after the corresponding monomer participates in the polymerization reaction. The mass ratio of each structural unit is based on the mass content of the corresponding monomer in the total amount of monomers participating in the polymerization.

[0070] An embodiment of the present application further provides a diaphragm, comprising a base film and an adhesive layer, wherein the adhesive layer is disposed on at least one side of the base film or the coated diaphragm, and the adhesive layer comprises the diaphragm adhesive as described above.

[0071] In some embodiments, the coating area density of the membrane adhesive of each adhesive layer is 0.1-0.6 g / m 2By limiting the coating area density of the separator adhesive, the amount of the separator adhesive in the adhesive layer is limited to ensure a high bonding strength between the separator and the pole piece.

[0072] Specifically, the coating area density of the separator adhesive in the adhesive layer on one side of the separator includes but is not limited to 0.1 g / m 2 , 0.2 g / m 2 , 0.3 g / m 2 , 0.4 g / m 2 , 0.5 g / m 2 or 0.6 g / m 2 .

[0073] In some embodiments, the separator further comprises a non-conductive particle coating layer disposed on at least one side of the base film, and the thickness of the non-conductive particle coating layer is 0.5-4 μm. When the thickness of the non-conductive particle coating layer is less than 0.5 μm, the heat resistance of the separator will be reduced, and the prepared battery has a high safety risk. When the thickness of the non-conductive particle coating layer is greater than 4 μm, the thickness of the separator is increased, and the energy density of the prepared battery is low.

[0074] Preferably, the thickness of the non-conductive particle coating layer is 0.5-3 μm.

[0075] Specifically, the non-conductive particle coating layer and the adhesive layer exist at the same time, and the adhesive layer is located on the side of the separator facing the pole piece, and the adhesive layer can be disposed on the non-conductive particle coating layer.

[0076] In some embodiments, the non-conductive particle coating layer comprises non-conductive particles, a ceramic binder, and optionally added rheological modifiers, dispersants and wetting agents.

[0077] The non-conductive particles include but are not limited to at least one of hydrated aluminum oxide, diatomic aluminum oxide, boehmite, silicon dioxide, titanium dioxide, zirconium dioxide, calcium oxide, magnesium oxide, magnesium hydroxide, calcium carbonate, barium titanate, barium sulfate, and heat-resistant organic polymer particles. When the non-conductive particles are selected from the above materials, the non-conductive particles can better improve the mechanical strength and thermal stability of the separator, further reduce the thermal shrinkage of the separator, and further improve the safety of the battery.

[0078] The ceramic binder in the present embodiment can use various existing ceramic binders on the market, and the ceramic binder with the required particle size and glass transition temperature can be directly purchased.

[0079] The wetting agent can use various existing wetting agents, such as at least one of alkyl phenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, fatty acid polyoxyethylene ether, and polyether modified polysiloxane.

[0080] The dispersant can be any of the various dispersants available, such as sodium polyacrylate or sodium carboxymethyl cellulose.

[0081] In some embodiments, the weight ratio of the non-conductive particles, ceramic binder, sodium carboxymethyl cellulose, dispersant, and wetting agent in the non-conductive particle coating is (88-96.3):(3-15):(0-3):(0-2):(0-2).

[0082] Preferably, the weight ratio of the non-conductive particles, ceramic binder, rheology modifier, dispersant, and wetting agent in the non-conductive particle coating is (88-96.3):(3-15):(0.5-3):(0.1-2):(0.1-2).

[0083] In some embodiments, the average particle size of the non-conductive particles is 0.1-2 μm, and the average particle size of the non-conductive particles is less than the thickness of the non-conductive particle coating. In the case where the thickness of the non-conductive particle coating is 0.5-4 μm, if the average particle size of the non-conductive particles exceeds 2 μm, the thermal shrinkage of the separator will increase significantly.

[0084] Specifically, the average particle size of the non-conductive particles includes but is not limited to 0.1 μm, 0.2 μm, 0.5 μm, 0.8 μm, 1.1 μm, 1.4 μm, 1.7 μm, or 2 μm.

[0085] In some embodiments, the base film is selected from at least one of a polyethylene-based film, a polypropylene-based film, a polypropylene-polyethylene-polypropylene laminated base film, or a non-woven fabric base film.

[0086] In some embodiments, the thickness of the base film is 5-12 μm.

[0087] In some embodiments, the peel strength of the adhesive layer to the negative electrode sheet is ≥1 N / m at 25-30°C and 2-10 MPa, so as to avoid relative displacement between the separator and the adhesive.

[0088] An embodiment of the present application also provides a lithium ion battery comprising the separator as described above.

[0089] The present application is further illustrated by the following examples.

[0090] The separator adhesive, positive electrode composition, negative electrode sheet, and lithium ion battery disclosed by the present application are specifically described.

[0091] Example 1 1) Preparation of the separator adhesive The monomers of the first polymer include the following components: 368 parts of butyl acrylate, 16 parts of styrene, 4 parts of divinylbenzene, and 12 parts of acrylic acid.

[0092] The second polymer comprises 95 parts of styrene and 5 parts of butyl acrylate.

[0093] The preparation of the separator adhesive comprises the following steps: (1) adding the monomers of the core, deionized water and an emulsifier into an emulsifier, stirring and emulsifying to obtain a monomer pre-emulsion of the first polymer. The amount of the emulsifier is 0.1% of the weight of the monomers.

[0094] (2) adding deionized water into another emulsifier, heating to 80°C under nitrogen, adding the initiator sodium persulfate, and then adding the monomer pre-emulsion of the first polymer dropwise. After 3 hours of dropwise addition, the reaction is continued for 3 hours to obtain a first polymer emulsion with an average particle size of 0.21 μm. (3) heating the first polymer emulsion to 80°C, adding the ammonium persulfate initiator, and then adding the monomer mixture of the second polymer dropwise. After 3 hours of dropwise addition, the reaction is continued for 3 hours to obtain a separator adhesive with a core-shell structure and an average particle size of 0.23 μm.

[0095] The solid weight ratio of the first polymer to the second polymer is 80:20.

[0096] In the separator adhesive, the glass transition temperature of the first polymer is -48°C, and the glass transition temperature of the second polymer is 86°C. The dissolution rate of the separator adhesive in the electrolyte is 1.5%.

[0097] 2) Preparation of the separator: Non-conductive particles boehmite powder 93.4 parts, ceramic adhesive (HD2126 of Shenzhen Haodianshi Technology Co., Ltd.) 6 parts, dispersant sodium polyacrylate 0.3 parts, sodium carboxymethyl cellulose 1 part, wetting agent alkyl phenol polyoxyethylene ether 0.3 parts are prepared into non-conductive particle slurry. The non-conductive particle slurry is coated on one surface of a polyvinyl film with a thickness of 9 μm, dried in a 50°C oven, and then a non-conductive particle coating layer is formed on the surface of the base film.

[0098] The separator adhesive is coated on the side of the base film without non-conductive particles by roll coating, and the coating surface density is 0.2 g / m 2 . After drying, a bonding layer is obtained.

[0099] 3) Preparation of the negative electrode sheet: 1.5% of butadiene-styrene rubber (SBR), 96% of negative electrode active material graphite, 1% of negative electrode conductive agent conductive carbon black, and 1.5% of thickening agent sodium carboxymethyl cellulose (CMC) are mixed, and then deionized water is added for stirring to prepare a negative electrode composition. The negative electrode composition is then coated on both surfaces of the negative electrode current collector Cu foil, and after drying, cold pressing, and slitting processes, a negative electrode sheet is obtained.

[0100] The particle size of the negative electrode active material graphite is 20 μm.

[0101] 4) Preparation of the positive electrode sheet: 97.8% of the positive electrode active material lithium cobaltate, 1% of the positive electrode conductive agent conductive carbon black, and 1.2% of the binder PVDF were added to N-methylpyrrolidone and stirred to prepare a positive electrode slurry. Then, the positive electrode slurry was coated on both surfaces of the positive electrode current collector Al foil, and after drying, cold pressing, and slitting processes, a separator was obtained.

[0102] 5) Preparation of the lithium ion battery: The separator, the positive electrode sheet, and the negative electrode sheet were stacked in order, with the separator between the positive electrode and the negative electrode to serve as a separator, to prepare an electrode assembly. The electrode assembly was placed in an outer package, injected with a commercially available electrolyte, and packaged. After the injection, formation, and degassing processes, a lithium ion battery was obtained.

[0103] Examples 2 to 11 Examples 2 to 11 and Example 1 are the same in most steps, except that the monomers of the first polymer in the separator binder are different, as shown in Table 1.

[0104] Table 1 Example 12 Example 12 and Example 1 are the same in most steps, except that the second polymer includes 75 parts of styrene and 25 parts of butyl acrylate.

[0105] The Tg of the second polymer is 45℃.

[0106] Example 13 Example 1 and Example 1 are the same in most steps, except that the second polymer includes 100 parts of styrene.

[0107] The Tg of the second polymer is 104℃.

[0108] Example 14 Example 14 and Example 1 are the same in most steps, except that the mass ratio of the first polymer to the second polymer is 30:70.

[0109] Example 15 Example 15 and Example 1 are the same in most steps, except that the mass ratio of the first polymer to the second polymer is 60:40.

[0110] Example 16 Example 16 and Example 1 are the same in most steps, except that the mass ratio of the first polymer to the second polymer is 20:80.

[0111] Example 17 Most of the steps of Example 17 are the same as those of Example 1, except that the mass ratio of the first polymer to the second polymer is 90:10.

[0112] Example 18 Most of the steps of Example 18 are the same as those of Example 1, except that the particle size of the diaphragm binder is 3 μm.

[0113] Example 19 Most of the steps of Example 19 are the same as those of Example 1, except that the particle size of the diaphragm binder is 0.1 μm.

[0114] Example 20 Most of the steps in Example 20 are the same as those in Example 1, except that the particle size of the diaphragm binder is 6 μm.

[0115] Example 21 Most of the steps in Example 21 are the same as those in Example 1, except that the particle size of the diaphragm binder is 10 μm.

[0116] Example 22 Most of the steps in Example 22 are the same as those in Example 1, except that when preparing the diaphragm, the diaphragm adhesive coating surface density is 0.1 g / m 2 .

[0117] Example 23 Most of the steps in Example 23 are the same as those in Example 1, except that when preparing the diaphragm, the diaphragm adhesive coating surface density is 0.3 g / m 2 .

[0118] Example 24 Most of the steps in Example 24 are the same as those in Example 1, except that when preparing the diaphragm, the diaphragm adhesive coating surface density is 0.6 g / m 2 .

[0119] Example 25 Most of the steps in Example 25 are the same as those in Example 1, except that when preparing the diaphragm, the diaphragm adhesive coating surface density is 0.7 g / m 2 .

[0120] Example 26 Most of the steps in Example 26 are the same as those in Example 1, except that when preparing the diaphragm, the diaphragm adhesive coating surface density is 0.08 g / m 2 .

[0121] Comparative Example 1 to Comparative Example 3 Comparative Example 1 to Comparative Example 3 and Example 1 are the same in most steps, except that the composition of the monomers of the first polymer added in the separator binder is different, as shown in Table 1.

[0122] Comparative Example 4 Comparative Example 4 and Example 1 are the same in most steps, except that the separator binder does not have a core-shell structure, and the specific preparation method is as follows: (1) Mix the monomers of the core, add the auxiliary agent, and stir under heating to obtain the first polymer; (2) After the monomers of the shell layer are uniformly mixed, add potassium persulfate and stir under heating to obtain the second polymer; (3) Mix the first polymer and the second polymer, and then spray dry to obtain the separator binder.

[0123] Comparative Example 5 Comparative Example 5 and Example 1 are the same in most steps, except that the separator binder uses existing PVDF.

[0124] Comparative Example 6 Comparative Example 6 and Example 1 are the same in most steps, except that the second polymer includes 70 parts of styrene and 30 parts of α-methylstyrene. (Tg is 119°C) Comparative Example 7 Comparative Example 7 and Example 1 are the same in most steps, except that the second polymer includes 70 parts of styrene and 30 parts of butyl acrylate. (Tg is 35°C) Electrical performance test: The separator and lithium ion battery prepared in the above examples and comparative examples were tested as follows.

[0125] 1), 25°C room temperature peel strength test: cut the separator and negative electrode sheet into 20mm*100mm strips respectively, press the side of the separator containing the separator binder against the negative electrode sheet under the condition of 25°C, 2MPa for 60s, and use an electronic tensile testing machine to test the 180° peel strength. The test result is the average value of 3 samples.

[0126] 2), electrolyte dissolution rate test: dry the separator binder at 70°C to obtain a film with a weight of m1; soak in a carbonate solvent without lithium salt at 60°C for 72 hours (EC:DEC:EMC=3:2:5), and the weight after drying is m2. The electrolyte dissolution rate is [(m1-m2) / m1]*100%.

[0127] 3), separator unwinding self-adhesion verification: use a 9μm base film, coat 2μm ceramic coating on both sides, and then coat 0.6g / m 2The face density of the double-side coated separator adhesive is 2000 m, and the separator is unwound after being stored at 25°C for 3 days. Observe whether there is self-adhesion.

[0128] 4) 30°C 1 MPa separator self-adhesion simulation test: The double-side coated separator adhesive is pressed at 30°C and 1 MPa for 60 s, and the peel strength between the separators is tested. Adhesion ≤0.5 N / m is considered as no self-adhesion between the separators.

[0129] 5) Normal temperature cycle test In the constant temperature test room at 25°C±2°C, the battery is placed for 1 h, then charged at 1C constant current and constant voltage to 4.45V, with a cutoff current of 0.05C; discharged at 1C constant current to 3V, and the discharge capacity is recorded; repeat the above steps 500 times, and calculate the capacity retention rate.

[0130] Fill the test results of the examples and comparative examples into Table 2.

[0131] Table 2 Table 2 (continued) From the test results of Table 2, it can be seen that the first structural unit in the first polymer has a significant effect on the adhesion. From the test results of the examples and Comparative Examples 1-3, it can be seen that when the first polymer does not contain the first structural unit, the adhesion between the separator and the pole piece is extremely low. Since the first structural unit in the first polymer has good water solubility, when the content of the first structural unit exceeds the upper limit, the polymerization process becomes unstable or even breaks, and it is more appropriate to control the first structural unit within 10%. From the test results of Example 1, Examples 4-7, it can be seen that the cross-linking structural unit in the first polymer has a significant effect on the dissolution and adhesion. When there is no cross-linking structural unit, the dissolution exceeds 3%, thereby negatively affecting the battery cycle performance. When the cross-linking structural unit exceeds 3%, the room temperature adhesion of the separator adhesive is significantly reduced. The glass transition temperature of the first polymer is also related to the room temperature adhesion of the adhesive, and the lower the glass transition temperature of the first polymer, the better the room temperature adhesion. From the test results of Example 1, Example 12, Example 13, Comparative Example 6 and Comparative Example 7, it can be seen that the glass transition temperature of the second polymer mainly affects the room temperature adhesion and the self-adhesion of the separator. When the glass transition temperature of the second polymer is too high, the core of the first polymer cannot contact the pole piece, resulting in insufficient adhesion. When the glass transition temperature of the second polymer is too low, the separator is prone to self-adhesion. From the test results of Example 1, Examples 14-17, it can be seen that the ratio of the first polymer to the second polymer has a significant effect on the room temperature adhesion and the self-adhesion of the separator. The smaller the proportion of the first polymer, the worse the room temperature adhesion, and when it is lower than the lower limit, the adhesion is almost not shown. The larger the proportion of the first polymer, the worse the coating effect of the second polymer on the first polymer, thereby causing the separator to produce self-adhesion. From the test results of Example 1, Examples 18-21, it can be seen that the particle size of the separator adhesive mainly affects the adhesion and the battery performance. The smaller the particle size, the larger the contact area, and the better the adhesion. When the particle size is less than the lower limit, the separator adhesive blocks the pores of the separator, resulting in a decrease in the battery cycle performance. When the particle size is greater than the upper limit, the separator adhesive is difficult to adhere to the separator and falls off, resulting in a decrease in the adhesion. When the separator adhesive has no core-shell structure and is simply mixed, since the first polymer is directly exposed, although the separator has good room temperature adhesion, the separator also produces obvious self-adhesion. From the test results of Example 1, Examples 22-26, it can be seen that the coating surface density of the separator adhesive in the separator mainly affects the adhesion, and the higher the coating surface density, the stronger the adhesion, and the lower the coating surface density, the weaker the adhesion. When the coating surface density is too high, the solution in the pores of the separator is blocked, resulting in a decrease in the battery performance. When the coating surface density is too low, the adhesion is insufficient, which also easily leads to a decrease in the battery performance.

[0132] The above merely describes preferred embodiments of the present application, and is not used to limit the present application, any modification, equivalent replacement and improvement within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A diaphragm adhesive, characterized in that: The membrane binder is a core-shell structure, comprising a core and a shell layer disposed on the outer surface of the core and at least partially covering the core, wherein the core comprises a first polymer and the shell comprises a second polymer; The glass transition temperature of the first polymer is -60 to 20°C, and the glass transition temperature of the second polymer is 40 to 110°C; The first polymer includes a first structural unit containing a polar functional group, wherein the polar functional group is selected from at least one of a carboxyl group, a sulfonic acid group, a phosphate group, an amide group, and a cyano group; the mass percentage of the first structural unit in the first polymer is 1% to 10%.

2. The diaphragm adhesive according to claim 1, characterized in that The first structural unit is a structural unit formed by the polymerization of the first monomer, and the first monomer includes at least one of acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, β-carboxyethyl acrylate, cinnamic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, alkenyl sulfonic acid compounds, sodium allyloxyhydroxypropyl methanesulfonate, acrylic acid sulfonate salts, acrylamide alkyl sulfonic acid compounds, acryloyloxyethyl phosphate compounds, allyl phosphoric acid, acrylamide compounds, methacrylamide compounds, acrylonitrile compounds, methacrylonitrile compounds, and 2-cyanoethyl acrylate.

3. The diaphragm adhesive according to claim 1, characterized in that The first polymer further includes a cross-linked structural unit. In the first polymer, the mass percentage of the cross-linked structural unit is 0.1% to 3%.

4. The diaphragm adhesive according to claim 3, characterized in that The cross-linking structural unit is a structural unit obtained by polymerization of a cross-linking monomer, wherein the cross-linking monomer contains at least two polymerizable double bonds; The cross-linking monomer includes at least one of an acrylate compound, a methacrylate compound, an aromatic vinyl compound, an acrylamide compound and an allyl compound.

5. The diaphragm adhesive according to claim 1, characterized in that The first polymer further includes a second structural unit, wherein the second structural unit includes at least one of a vinyl structural unit, an acrylate structural unit, a methacrylate structural unit, a maleate structural unit, an itaconate structural unit, a maleimide structural unit, an acrylamide structural unit, and a methacrylamide structural unit; And / or, the second polymer includes at least one of vinyl structural units, acrylate structural units, methacrylate structural units, maleate structural units, itaconate structural units, maleimide structural units, acrylamide structural units, and methacrylamide structural units.

6. The diaphragm adhesive according to claim 1, characterized in that The mass ratio of the first polymer to the second polymer is (30:70) to (80:20).

7. The diaphragm adhesive according to claim 1, characterized in that The dissolution rate of the separator binder in the electrolyte is less than 3%; And / or, the average particle size of the separator binder is 0.2-6 μm.

8. A diaphragm, characterized in that: The diaphragm adhesive comprises a base film and an adhesive layer, wherein the adhesive layer is provided on at least one side of the base film, and the adhesive layer comprises the diaphragm adhesive according to any one of claims 1 to 7.

9. The diaphragm according to claim 8, characterized in that The coating surface density of the membrane adhesive of each adhesive layer is 0.1-0.6 g / m 2 ; And / or, at 25-30° C. and 2-10 MPa, the peel strength between the adhesive layer and the negative electrode sheet is ≥1 N / m.

10. A lithium ion battery, characterized in that: The diaphragm according to claim 8 or 9 is included.

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