Composite separator and lithium ion battery

Abstract

In order to overcome the problem that the adhesion of the non-fluorine separator adhesive in the prior art is low, resulting in that the composite separator prepared by using the separator adhesive has low adhesion strength with the pole piece, the application provides a composite separator and a lithium ion battery.The composite separator comprises a base film, a non-conductive particle coating on the surface of the base film, and a bonding layer on the surface of the non-conductive particle coating; the bonding layer comprises a separator adhesive, the separator adhesive comprises a particulate polymer, the average particle size D1 of the particulate polymer is 0.2-6 μm; the non-conductive particle coating comprises non-conductive particles, the average particle size D2 of the non-conductive particles is 0.1-2 μm; the average particle size D1 of the particulate polymer and the average particle size D2 of the non-conductive particles satisfy: D1 / D2≥0.4.The composite separator provided in the application has excellent air permeability and adhesion strength with the pole piece.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a composite diaphragm and a lithium ion battery adopting the same. BACKGROUND

[0002] Lithium ion batteries are rechargeable batteries with broad application prospects, and are favored by various industries due to their high energy density, long service life, small size, no maintenance, environmental friendliness and other characteristics. They have gone from mobile phones, notebook computers and other fields to electric bicycles, electric vehicles, energy storage and various portable device fields, and are an ideal mobile power source.

[0003] A lithium ion battery is usually composed of a positive electrode, a negative electrode, a diaphragm, an electrolyte and a battery shell. The diaphragm is one of the key inner components, which separates the positive and negative electrodes of the battery to prevent the positive and negative electrodes from directly contacting and short-circuiting.

[0004] At present, the diaphragm used in lithium ion batteries is generally a polyolefin porous membrane. Since the melting point of the polyolefin porous membrane is relatively low, when the temperature of the battery rises due to internal or external factors, the polyolefin porous membrane will shrink or melt, causing the positive and negative electrodes to directly contact, leading to battery short-circuiting, and further causing accidents such as battery combustion and explosion.

[0005] In order to solve these problems, inorganic particles are usually coated on the surface of the diaphragm substrate using a polymer adhesive to make a composite diaphragm, such as coating ceramic particles on the surface of the diaphragm substrate to make a ceramic / polymer composite diaphragm, hoping to use the heat resistance of the ceramic particles to reduce the thermal shrinkage of the diaphragm.

[0006] At the same time, a layer of diaphragm adhesive is coated on the surface of the ceramic coating, which is used to bond the diaphragm and the positive and negative electrode sheets, and plays a role in fixing the structure of the battery.

[0007] At present, the most commonly used diaphragm adhesive on the market is mainly polyvinylidene fluoride (PVDF).

[0008] However, the PVDF adhesive has problems such as upstream fluorine resource shortage, recycling difficulty, and serious environmental pollution of decomposition products.

[0009] Therefore, it is the trend of the industry to use non-fluorine diaphragm adhesives to bond the diaphragm and the electrode sheet.

[0010] However, the existing non-fluorine diaphragm adhesives (such as polyacrylate adhesives) have low adhesion, and the composite diaphragm prepared using the diaphragm adhesive has the problem of low bonding strength between the diaphragm and the electrode sheet. SUMMARY

[0011] The technical problem to be solved by the present application is that the existing non-fluorine separator adhesive has low adhesion, and the composite separator prepared using the separator adhesive has low adhesion strength with the pole piece.

[0012] To solve the above technical problem, the present application provides a composite separator, comprising a base film, a non-conductive particle coating layer on the surface of the base film, and a bonding layer on the surface of the non-conductive particle coating layer.

[0013] The bonding layer comprises a separator adhesive, and the separator adhesive comprises a particulate polymer, wherein the average particle size D1 of the particulate polymer is 0.2-6 μm.

[0014] The non-conductive particle coating layer comprises non-conductive particles, and the average particle size D2 of the non-conductive particles is 0.1-2 μm.

[0015] The average particle size D1 of the particulate polymer and the average particle size D2 of the non-conductive particles satisfy:

[0016] D1 / D2≥0.4.

[0017] Preferably, the average particle size D1 of the particulate polymer and the average particle size D2 of the non-conductive particles satisfy: 0.4≤D1 / D2≤100.

[0018] Preferably, the glass transition temperature Tg of the particulate polymer is 10-90℃.

[0019] Preferably, the particulate polymer is a polymer comprising at least one of a vinyl monomer unit, a (meth)acrylate monomer unit, a maleate monomer unit, an itaconate monomer unit, a maleimide monomer unit, and a (meth)acrylamide monomer unit.

[0020] Preferably, the vinyl monomer unit is at least one selected from the group consisting of an aliphatic vinyl hydrocarbon compound unit, an alicyclic vinyl hydrocarbon compound unit, an aromatic vinyl hydrocarbon compound unit, an olefin, a propyl compound unit, and a vinyl compound unit containing a heteroatom.

[0021] The (meth)acrylate monomer unit is at least one selected from the group consisting of a methacrylate monomer unit and an acrylate monomer unit.

[0022] The maleate monomer unit comprises at least one of a C1-C12 monoalkyl maleate or a C1-C12 dialkyl maleate.

[0023] The itaconate monomer unit comprises at least one of a C1-C12 monoester of itaconic acid and a C1-C12 diester of itaconic acid.

[0024] The maleimide monomer unit includes at least one of a maleimide monomer unit, a C1-C12 alkyl-substituted maleimide monomer unit, and a C6-C16 aryl-substituted maleimide monomer unit.

[0025] The (meth)acrylamide monomer unit includes at least one of a (meth)acrylamide monomer unit, an N-methyl (meth)acrylamide monomer unit, an N-butyl acrylamide monomer unit, an acetylacetone acrylamide monomer unit, an N-hydroxymethyl (meth)acrylamide monomer unit, an N,N'-methylenebis[(meth)acrylamide] monomer unit, a cinnamamide monomer unit, an N,N-dimethyl acrylamide monomer unit, an N,N-dibenzyl acrylamide monomer unit, a methacryloylcarboxamide monomer unit, an N-methyl N-vinylacetamide monomer unit, and an N-vinylpyrrolidone monomer unit.

[0026] Preferably, the non-conductive particle coating layer has a thickness of 0.5-4 μm.

[0027] Preferably, the adhesive layer has an area density of 0.1-0.6 g / m 2 .

[0028] Preferably, the non-conductive particle coating layer includes non-conductive particles, a ceramic binder, and optionally added rheological modifiers, dispersants, and wetting agents.

[0029] In the non-conductive particle coating layer, the weight ratio of the non-conductive particles, the ceramic binder, the sodium carboxymethyl cellulose, the dispersant, and the wetting agent is (88-96.3):(3-15):(0-3):(0-2):(0-2).

[0030] Preferably, the base film is selected from at least one of a polyvinyl base film, a polypropylene base film, a polypropylene-polyethylene-polypropylene laminated base film, or a non-woven fabric base film.

[0031] Preferably, the base film has a thickness of 3-12 μm.

[0032] In a second aspect, the present application provides a lithium ion battery including the composite separator as described above.

[0033] Through extensive research, the applicant found that the granular polymer in the adhesive layer formed on the surface of the non-conductive particle coating layer is extremely easy to embed into the gap between the non-conductive particles in the non-conductive particle coating layer. During the subsequent hot pressing of the composite separator and the pole piece, the granular polymer in the adhesive layer is easy to contact insufficiently with the pole piece, thereby resulting in low adhesive strength between the composite separator and the pole piece.

[0034] In view of the above research, the applicant sets the adhesive layer of the composite separator at 0.1-0.6 g / m2 Within the range, in combination with using the particulate polymer with an average particle size D1 of 0.4-3 μm and the non-conductive particles with an average particle size D2 of 0.1-2 μm, and defining D1 / D2≥0.4, the composite separator can have both excellent air permeability and the bonding strength between the separator and the pole piece. DETAILED DESCRIPTION

[0035] In order to make the technical problems, technical solutions and beneficial effects solved by the present application clearer, the present application will be further described in detail below in combination with embodiments.

[0036] It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

[0037] In the description of the present application, the term "monomer unit" refers to the corresponding unit of the structure of a monomer formed on the polymer segment after the monomer participates in polymerization.

[0038] The term "average particle size" refers to the volume average particle size D V 50.

[0039] The composite separator provided by the present application comprises a base film, a non-conductive particle coating layer located on the surface of the base film, and a bonding layer located on the surface of the non-conductive particle coating layer.

[0040] The bonding layer comprises a separator adhesive, and the separator adhesive comprises a particulate polymer, wherein the average particle size D1 of the particulate polymer is 0.4-3 μm.

[0041] The non-conductive particle coating layer comprises non-conductive particles, and the average particle size D2 of the non-conductive particles is 0.1-2 μm.

[0042] The average particle size D1 of the particulate polymer and the average particle size D2 of the non-conductive particles satisfy:

[0043] D1 / D2≥0.4.

[0044] According to the present application, in the bonding layer, the average particle size D1 of the particulate polymer is 0.2-6 μm, and specifically, the average particle size D1 of the particulate polymer can be 0.1 μm, 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, 2.1 μm, 2.4 μm, 2.5 μm, 2.8 μm, 3 μm, 4 μm, 5 μm, or 6 μm.

[0045] Preferably, the average particle size D1 of the particulate polymer is 0.2-5 μm.

[0046] In the composite separator provided by the present application, if the average particle size D1 of the granular polymer is greater than 6 μm or less than 0.1 μm, the adhesion strength between the composite separator and the pole piece will be significantly reduced.

[0047] It is speculated that if D1 is greater than 6 μm, the granular polymer formed on the surface of the non-conductive particle coating is easy to fall off, thereby resulting in a decrease in the adhesion strength; if D1 is less than 0.1 μm, the granular polymer is more likely to be embedded in the non-conductive particle coating, thereby resulting in a decrease in the adhesion strength.

[0048] Meanwhile, for the composite separator provided by the present application, the surface density of the adhesion layer on the surface thereof is 0.1-0.6 g / m 2 .

[0049] Specifically, the surface density of the adhesion layer can be 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 .

[0050] Preferably, the surface density of the adhesion layer is 0.1-0.3 g / m 2 .

[0051] In the present application, if the surface density of the adhesion layer is not within the above range, the air permeability of the composite separator will be reduced or the adhesion strength between the composite separator and the pole piece will be insufficient.

[0052] According to the present application, the glass transition temperature of the granular polymer can vary within a relatively large range, and preferably, the glass transition temperature Tg of the granular polymer is 10-90 °C.

[0053] Further preferably, it is 20-80 °C.

[0054] The granular polymer in the present application can use an existing non-fluorine adhesion agent for bonding the separator and the pole piece, such as a polyacrylate adhesion agent, and the preparation method thereof is known to those skilled in the art.

[0055] In the present application, preferably, the granular polymer is a polymer comprising at least one of a vinyl monomer unit, a (meth)acrylate monomer unit, a maleate monomer unit, an itaconate monomer unit, a maleimide monomer unit, and a (meth)acrylamide monomer unit.

[0056] Further, the vinyl monomer units are selected from at least one of aliphatic vinyl hydrocarbon compound units, alicyclic vinyl hydrocarbon compound units, aromatic vinyl hydrocarbon compound units, allyl compound units, and heteroatom-containing vinyl compound units.

[0057] wherein the aliphatic vinyl hydrocarbon compound includes, but is not limited to, alkenes having 2 to 12 carbon atoms (exemplarily including, but not limited to, ethylene, propylene, butylene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene), alpha-olefins having 3 to 24 carbon atoms, dienes having 4 to 12 carbon atoms (exemplarily including, but not limited to, butadiene, isoprene, 1,4-pentadiene, 1,6-hexadiene, and 1,7-octadiene).

[0058] The alicyclic vinyl hydrocarbon compound includes, but is not limited to, monocyclic or bicyclic alkenes having 6 to 15 carbon atoms (exemplarily including, but not limited to, cyclohexene, vinylcyclohexene, and ethylidenebicycloheptene), monocyclic or bicyclic dienes having 5 to 12 carbon atoms (exemplarily including, but not limited to, cyclopentadiene, cycloheptadiene, bicyclopentadiene, bicycloheptadiene), terpenes (exemplarily including, but not limited to, limonene and indene).

[0059] The aromatic vinyl hydrocarbon compound includes, but is not limited to, styrene and substituted styrenes, which exemplarily include, but are not limited to, alpha-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, crotylstyrene, divinylbenzene, divinyltoluene, divinylxylene, trivinylbenzene, vinylnaphthalene, p-t-butylstyrene.

[0060] The heteroatom-containing vinyl compound includes, but is not limited to, vinyl (thio) ether, vinyl sulfone, acrylonitrile, methacrylonitrile, cyano styrene, 4-vinylpyridine, 2-vinylpyridine, vinyl imidazole, N-vinylpyrrole, N-vinylthiopyrrolidone.

[0061] The (meth)acrylate monomer unit is selected from at least one of methacrylate monomer units and acrylate monomer units.

[0062] The (meth)acrylate monomer exemplarily includes, but is not limited to, at least one of methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-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, t-butyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, n-hexyl methacrylate, isooctyl methacrylate, isobornyl methacrylate, phenoxyethyl methacrylate, dicyclopentenyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, polyethylene glycol mono[(meth)acrylate], (meth)acrylic acid aminoethyl ester, (meth)acrylic acid dimethylaminoethyl ester, (meth)acrylic acid diethylaminoethyl ester, (meth)acrylic acid t-butylaminoethyl ester, (meth)acrylic acid glycidyl ester, (meth)acrylic acid tetrahydrofurfuryl ester, ethylene glycol diacrylate, ethylene glycol dimethacrylate, ethoxylated ethylene glycol diacrylate, ethoxylated ethylene glycol dimethacrylate, allyl methacrylate, diallyl phthalate, diallyl adipate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol diacrylate, pentaerythritol dimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate, acrylic acid aminoethyl ester, methacrylic acid aminoethyl ester, acrylic acid dimethylaminoethyl ester, methacrylic acid dimethylaminoethyl ester, acrylic acid diethylaminoethyl ester, methacrylic acid diethylaminoethyl ester, and (meth)acrylic acid t-butylaminoethyl ester.

[0063] The maleate monomer unit includes at least one of a C1-C12 monoalkyl maleate or a C1-C12 dialkyl maleate.

[0064] As is well known to those skilled in the art, the C1-C12 above means that the number of carbon atoms is 1 to 12.

[0065] Specifically, the maleate monomer can include at least one of monomethyl maleate, dimethyl maleate, monoethyl maleate, diethyl maleate, monopropyl maleate, dipropyl maleate, monobutyl maleate, dibutyl maleate, monooctyl maleate, dioctyl maleate.

[0066] The itaconate monomer unit includes at least one of itaconic acid C1-C12 monoester, itaconic acid C1-C12 diester.

[0067] Specifically, the itaconate monomer can include at least one of itaconic acid monomethyl ester, itaconic acid dimethyl ester, itaconic acid monoethyl ester, itaconic acid diethyl ester, itaconic acid monobutyl ester, itaconic acid dibutyl ester, itaconic acid monooctyl ester, itaconic acid dioctyl ester.

[0068] The maleimide monomer unit includes at least one of maleimide, C1-C12 alkyl-substituted maleimide, C6-C16 aryl-substituted maleimide.

[0069] The (meth)acrylamide monomer unit includes at least one of (meth)acrylamide monomer unit, N-methyl (meth)acrylamide monomer unit, N-butyl acrylamide monomer unit, acetylacetone acrylamide monomer unit, N-hydroxymethyl (meth)acrylamide monomer unit, N,N'-methylenebis[(meth)acrylamide] monomer unit, cinnamamide monomer unit, N,N-dimethyl acrylamide monomer unit, N,N-dibenzyl acrylamide monomer unit, methacryloylcarboxamide monomer unit, N-methyl N-vinylacetamide monomer unit, N-vinylpyrrolidone monomer unit.

[0070] In the present application, the method for preparing the particulate polymer is not particularly limited, and can be any one of emulsion polymerization, suspension polymerization, dispersion polymerization, and precipitation polymerization, as long as the particle size and the glass transition temperature of the prepared particulate polymer satisfy the particle size range.

[0071] The particulate polymer in the present application can also be obtained by commercial purchase, and a particulate polymer product with a desired average particle size D1 and glass transition temperature can be selected.

[0072] According to the present application, the bonding layer in the composite separator is formed by a separator binder containing the above-mentioned particulate polymer.

[0073] The method for forming the above-mentioned bonding layer can use the existing technology, for example, the separator binder can be coated on the non-conductive particle coating layer by a conventional coating process.

[0074] It should be noted that, as mentioned above, the areal density of the bonding layer formed by coating must be in the range of 0.1-0.6 g / m 2 .

[0075] In the present application, the non-conductive particle coating layer includes non-conductive particles, a ceramic binder, and optionally added rheology modifiers, dispersants, wetting agents.

[0076] The rheology modifier can include at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium polyacrylate, and lithium polyacrylate.

[0077] The non-conductive particles are well known to those skilled in the art, and specifically, the non-conductive particles are particles that are non-conductive, do not dissolve in water used as a dispersion medium for the slurry and a non-aqueous electrolyte used for the secondary battery, and can maintain their shapes.

[0078] Further, the non-conductive particles are electrochemically stable, and thus stably exist in the separator under the use environment of the secondary battery.

[0079] As the non-conductive particles, various inorganic particles, heat-resistant organic particles, for example, can be used.

[0080] As the inorganic particles, oxide particles such as hydrated aluminum oxide, diatomic aluminum oxide, boehmite, silicon dioxide, titanium dioxide, zirconium dioxide, calcium oxide, magnesium oxide, magnesium hydroxide, and aluminum oxide-silicon dioxide composite oxide; nitride particles such as aluminum nitride and boron nitride; covalent bond crystal particles such as silicon and diamond; and insoluble ionic crystal particles such as calcium carbonate, barium titanate, barium sulfate, calcium fluoride, and barium fluoride; clay particles such as talc and montmorillonite; and the like can be given.

[0081] As the heat-resistant organic particles, various cross-linked polymer particles such as polyethylene, polystyrene, polydivinylbenzene, styrene-divinylbenzene copolymer cross-linking product, and various cross-linked polymer particles such as polyimide, polyamide, polyamide-imide, melamine resin, phenol formaldehyde resin, benzoguanamine-formaldehyde condensate; heat-resistant polymer particles such as polysulfone, polyacrylonitrile, polyaramide, polyacetal, and thermoplastic polyimide; and the like can be given.

[0082] Here, the heat-resistant organic particles differ from the above-described particulate polymers in that the particulate polymers have a cohesive ability, whereas the heat-resistant organic particles do not have a cohesive ability.

[0083] In the present application, as the non-conductive particles, inorganic particles are preferable, and boehmite, hydrated aluminum oxide, and diatomic aluminum oxide are more preferable.

[0084] According to the present application, the average particle diameter D2 of the non-conductive particles is 0.1 to 2 μm.

[0085] Specifically, the average particle diameter D2 of the non-conductive particles can be 0.1 μm, 0.2 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, or 2 μm.

[0086] Preferably, the average particle diameter D2 of the non-conductive particles is 0.2 to 1.5 μm.

[0087] In the composite separator provided by the present application, if the average particle size D2 of the non-conductive particles is greater than 2 μm, it is not conducive to balancing the heat resistance and the volumetric energy density of the battery.

[0088] If the average particle size D2 of the non-conductive particles is less than 0.1 μm, it will cause the air permeability of the composite separator to increase too much, the air permeability to decrease, the internal resistance of the battery to be too high, and the battery prepared to have poor performance.

[0089] Importantly, in the present application, the average particle size D1 of the particulate polymer and the average particle size D2 of the non-conductive particles satisfy D1 / D2≥0.4.

[0090] The applicant of the present application has creatively used the particulate polymer and the non-conductive particles with the above specific average particle sizes together, and kept the area density of the adhesive layer at 0.1-0.6 g / m 2 within the range, which can effectively inhibit the particulate polymer in the adhesive layer formed on the surface of the non-conductive particle coating from being embedded into the gaps between the non-conductive particles in the non-conductive particle coating, thereby ensuring that the particulate polymer in the adhesive layer fully contacts the pole piece during the subsequent hot pressing of the composite separator and the pole piece, and improving the adhesive strength between the composite separator and the pole piece.

[0091] Moreover, the above composite separator has excellent air permeability.

[0092] Preferably, the average particle size D1 of the particulate polymer and the average particle size D2 of the non-conductive particles satisfy 0.4≤D1 / D2≤100, and more preferably 0.6≤D1 / D2≤60.

[0093] According to the present application, the thickness of the non-conductive particle coating is 0.5-4 μm.

[0094] Preferably, it is 0.5-3 μm.

[0095] When the thickness of the non-conductive particle coating is less than 0.5 μm, it will cause the heat resistance of the composite separator to decrease, and the battery prepared to have a large safety risk.

[0096] When the thickness of the non-conductive particle coating is greater than 4 μm, it will cause the composite separator to be too thick, and the energy density of the battery prepared to be low.

[0097] The ceramic adhesive in the non-conductive particle coating serves to bond the non-conductive particles to the base film.

[0098] The ceramic adhesive in the present application can adopt various existing ceramic adhesives, and the ceramic adhesive with the required particle size and glass transition temperature can be directly obtained by commercial purchase.

[0099] The wetting agent can be any of the various wetting agents known in the art, for example, any one or a mixture of several of alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, fatty acid polyoxyethylene ether, and polyether-modified polysiloxane.

[0100] The dispersing agent can be any of the various dispersing agents known in the art, for example, sodium polyacrylate.

[0101] In the present application, the weight ratio of the non-conductive particles, the ceramic binder, the rheology modifier, the dispersing agent, and the wetting agent in the non-conductive particle coating is (88-96.3):(3-15):(0-3):(0-2):(0-2).

[0102] In the present application, the above weight ratio represents the weight ratio between each substance, and is not a percentage.

[0103] Preferably, the weight ratio of the non-conductive particles, the ceramic binder, the rheology modifier, the dispersing agent, and the wetting agent in the non-conductive particle coating is (88-96.3):(3-15):(0.5-3):(0.1-2):(0.1-2).

[0104] As known to those skilled in the art, the non-conductive particle coating is prepared from a non-conductive particle slurry.

[0105] For example, the non-conductive particle slurry is coated on the base film, and dried in an oven.

[0106] The base film is not particularly limited in the present application, and can be a polymer separator commonly used in the art. Illustratively, the base film can be selected from a polyethylene base film, a polypropylene base film, a polypropylene-polyethylene composite base film, or a non-woven fabric base film. The polypropylene-polyethylene composite base film refers to a base film formed by stacking polypropylene and polyethylene layers. The present application does not limit the stacking order and number of the polypropylene and polyethylene layers. The base film can be a polypropylene-polyethylene double-layer base film, a polypropylene-polyethylene-polypropylene triple-layer base film, or a polypropylene-polyethylene-polyethylene-polypropylene four-layer base film.

[0107] Preferably, the base film is selected from at least one of a polyethylene base film, a polypropylene base film, a polypropylene-polyethylene-polypropylene stacked base film, or a non-woven fabric base film.

[0108] The thickness of the base film can vary within a wide range. Preferably, the thickness of the base film is 3-12 μm.

[0109] In a second aspect, the present application provides a lithium ion battery comprising the composite separator as described above.

[0110] The composite separator provided by the present application has excellent air permeability and adhesion strength with the pole piece.

[0111] The application is further illustrated by the following examples.

[0112] Example 1

[0113] The battery of this example is prepared by a method comprising the following steps:

[0114] 1) Preparation of the separator binder

[0115] SL814 from Shenzhen Haoguang Technology Co., Ltd. is used.

[0116] 2) Preparation of the separator

[0117] Non-conductive particles boehmite powder 93.4 parts, ceramic binder (SWA610 from Shenzhen Haoguang Technology Co., Ltd.) 6 parts, dispersant sodium polyacrylate 0.3 parts, sodium carboxymethyl cellulose 1 part, wetting agent alkylphenol polyoxyethylene ether 0.3 parts are prepared into a non-conductive particle slurry, the non-conductive particle slurry is coated on one surface of a polyethylene-based film with a thickness of 9 μm, and after drying in a 50°C oven, a non-conductive particle coating layer is formed on the surface of the base film.

[0118] The separator binder of step 1) is coated on the surface of the non-conductive particle coating layer, and after drying, a bonding layer is obtained.

[0119] The specific parameters of the particulate polymer, the bonding layer, the non-conductive particles, the non-conductive particle coating layer and the base film are shown in Table 1.

[0120] 3) Preparation of the positive electrode

[0121] N-methylpyrrolidone is used to disperse the positive electrode active particles lithium nickel cobalt manganese oxide NCM523, conductive carbon black Super P and the binder polyvinylidene fluoride to form a positive electrode slurry; the positive electrode slurry is uniformly coated on an Al foil by coating, and after baking and rolling processes, a positive electrode including a positive electrode active layer is obtained.

[0122] The mass ratio of the positive electrode active particles, the conductive agent and the binder is 97.5:1.0:1.5.

[0123] 4) Preparation of the negative electrode

[0124] Deionized water is used to disperse the negative electrode active particles artificial graphite, conductive carbon black Super P, the binder sodium carboxymethyl cellulose and butadiene-styrene latex to form a negative electrode slurry; then the coating, baking and rolling processes are used to form a negative electrode active layer on the surface of the negative electrode current collector to obtain a negative electrode.

[0125] The mass ratio of the negative electrode active particles, the conductive agent and the binder (the mass ratio of sodium carboxymethyl cellulose to butadiene-styrene latex is 1:1.5) is 96.5:1.0:2.5.

[0126] 5)Battery preparation

[0127] The positive electrode, the aforementioned composite separator, and the negative electrode are sequentially stacked and wound to obtain a battery cell. The battery cell is placed in an aluminum plastic film, and an electrolyte is injected into the battery cell. The battery cell is subjected to vacuum packaging, standing, formation, shaping, and capacity testing to obtain a battery.

[0128] The electrolyte includes lithium hexafluorophosphate, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate. The ratio of ethylene carbonate to methyl ethyl carbonate to diethyl carbonate is 3:2:5, and the concentration of lithium hexafluorophosphate is 1 mol / L.

[0129] Examples 2-7 and Comparative Examples 1-6

[0130] The composite separator and the battery were prepared according to the method of Example 1, except that the parameters of the particulate polymer, the adhesive layer, the non-conductive particles, and the non-conductive particle coating were adjusted according to the contents of Table 1 (wherein the particulate polymers and non-conductive particles of different particle sizes were selected and purchased as shown in Table 1).

[0131] Table 1

[0132]

[0133] Performance test

[0134] The composite separators in the examples and comparative examples were subjected to the following performance tests, and the results are shown in Table 2.

[0135] 1. Air permeability value test of the separator

[0136] The air permeability test was performed in accordance with GB / T 36363-2018 "Polyolefin separators for lithium ion batteries".

[0137] Three pieces of the separator were cut from the film roll at a longitudinal interval of 150 mm. If the width of the separator is ≥100 mm, the sample size was 100 mm x 100 mm. If the width of the separator is <100 mm, the sample size was 100 mm x the width of the separator.

[0138] The separator was placed in the test head of the air permeability tester suitable for the test range to perform the air permeation time test. The average value of three test results was taken as the air permeation time of the separator, with the unit being s / 100cc.

[0139] The air permeability value of the separator was the difference between the air permeation time of the separator coated with non-conductive particles and the adhesive and the air permeation time of the polyolefin-based film, with the unit being s / 100cc.

[0140] 2. Peel strength test

[0141] The composite separator and the positive electrode tab prepared by the scheme of the application are cut into 20mm*100mm strips respectively, cold-pressed at 95℃ and 2MPa for 60s, and then tested for 180° peeling strength by an electronic tensile testing machine, and the test results are the average values of 3 samples.

[0142] Table 2

[0143]

[0144] As can be seen from the test results in Table 2, the composite separator provided by the application can effectively improve the air permeability and the bonding strength.

[0145] When the average particle size D1 of the particulate polymer, the area density of the bonding layer, the average particle size D2 of the non-conductive particles, and the ratio of D1 / D2 are not within the ranges disclosed in the application, the air permeability and the bonding strength of the composite separator will decrease to different extents.

[0146] Specifically, as can be seen from the test results of Comparative Examples 1-7 and Comparative Example 1, if the ratio of D1 / D2 is too small, the peeling strength of the composite separator and the tab will decrease significantly, and the air permeability will also decrease to a certain extent.

[0147] As can be seen from the test results of Comparative Examples 1-7 and Comparative Examples 2 and 3, under the scheme of the application, if the average particle size D1 of the particulate polymer is too small, the air permeability of the composite separator will increase significantly, and the air permeability will decrease obviously.

[0148] If the average particle size D1 of the particulate polymer is too large, the peeling strength of the composite separator and the tab will decrease significantly.

[0149] As can be seen from the test results of Comparative Examples 1-7 and Comparative Example 4, under the scheme of the application, if the average particle size D2 of the non-conductive particles is too small, the air permeability of the composite separator will increase significantly, and the air permeability will decrease obviously.

[0150] As can be seen from the test results of Comparative Examples 1-7 and Comparative Examples 5 and 6, if the area density of the bonding layer is too small, the peeling strength of the composite separator and the tab will decrease significantly; if the area density of the bonding layer is too large, the air permeability of the composite separator will increase significantly, and the air permeability will decrease obviously.

[0151] The above only describes the preferred embodiments of the application and is not intended to limit the application, and any modifications, equivalent replacements and improvements made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. A composite separator, characterized by, The separator includes a base film, a non-conductive particle coating layer on the surface of the base film, and a bonding layer on the surface of the non-conductive particle coating layer. The bonding layer includes a separator binder, and the separator binder includes a particulate polymer with an average particle size D1 of 0.2-6 μm. The non-conductive particle coating layer includes non-conductive particles with an average particle size D2 of 0.2-1.5 μm. The areal density of the adhesive layer is 0.1-0.3 g / m 2 ; The average particle size D1 of the particulate polymer and the average particle size D2 of the non-conductive particles satisfy 0.6≤D1 / D2≤60. The particulate polymer has a glass transition temperature Tg of 10-90℃.

2. The composite separator of claim 1, wherein The particulate polymer is a polymer including at least one of a vinyl monomer unit, a (meth)acrylate monomer unit, a maleate monomer unit, an itaconate monomer unit, a maleimide monomer unit, and a (meth)acrylamide monomer unit.

3. The composite separator of claim 2, wherein, The vinyl monomer unit is selected from at least one of an aliphatic vinyl hydrocarbon compound unit, an alicyclic vinyl hydrocarbon compound unit, an aromatic vinyl hydrocarbon compound unit, an allyl compound unit, and a heteroatom-containing vinyl compound unit. The (meth)acrylate monomer unit is selected from at least one of a methacrylate monomer unit and an acrylate monomer unit. The maleate monomer unit includes at least one of a C1-C12 monoalkyl maleate or a C1-C12 dialkyl maleate. The itaconate monomer unit includes at least one of a C1-C12 monoester of itaconic acid and a C1-C12 diester of itaconic acid. The maleimide monomer unit includes at least one of a maleimide monomer unit, a C1-C12 alkyl-substituted maleimide monomer unit, and a C6-C16 aryl-substituted maleimide monomer unit. The (meth)acrylamide monomer unit includes at least one of a (meth)acrylamide monomer unit, an N-methyl (meth)acrylamide monomer unit, an N-butyl acrylamide monomer unit, an acetoacetone acrylamide monomer unit, an N-hydroxymethyl (meth)acrylamide monomer unit, an N,N'-methylenebis[(meth)acrylamide] monomer unit, a cinnamamide monomer unit, an N,N-dimethyl acrylamide monomer unit, an N,N-dibenzyl acrylamide monomer unit, a methacryloylcarboxamide monomer unit, an N-methyl N-vinylacetamide monomer unit, and an N-vinylpyrrolidone monomer unit.

4. The composite separator of claim 1, wherein The non-conductive particle coating layer has a thickness of 0.5-4 μm.

5. The composite separator according to any one of claims 1, 4, wherein The non-conductive particle coating layer includes non-conductive particles, a ceramic binder, and optionally added rheological modifiers, dispersants, and wetting agents. In the non-conductive particle coating layer, the weight ratio of the non-conductive particles, the ceramic binder, the rheological modifiers, the dispersants, and the wetting agents is (88-96.3):(3-15):(0-3):(0-2):(0-2).

6. The composite separator of claim 1, wherein 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. And / or, the base film has a thickness of 3-12 μm.

7. A lithium-ion battery, characterized by The composite separator of any one of claims 1-6.

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