Positive electrode edge coating, positive electrode plate, lithium ion secondary battery and electronic device

Abstract

The invention discloses a positive electrode edge coating, a positive electrode plate, a lithium ion secondary battery and an electronic device. The positive electrode edge coating comprises the following components: 15-35 wt% of a main adhesive and 65-85 wt% of ceramic particles, the main adhesive is a modified olefin copolymer; and the substitution degree of the polar branch chain in the main adhesive is 10%-25%. According to the present invention, the adhesion between the positive electrode edge coating and the current collector is improved, and when the positive electrode plate containing the positive electrode edge coating is used for the lithium ion secondary battery, the phenomenon that the positive electrode edge coating wrinkles or even falls off after the battery cell is fully charged can be effectively improved so as to improve the storage performance of the obtained battery.

Description

Technical Field

[0001] This invention relates to a positive electrode edge coating, a positive electrode sheet, a lithium-ion secondary battery, and an electronic device. Background Technology

[0002] Currently, the edges of the positive electrode of lithium batteries on the market are coated with a layer of ceramic edge coating material AT9 to protect the burrs generated by the die cutting of the positive electrode, reduce the possibility of the positive aluminum directly contacting the negative electrode, and improve the safety and reliability of the cell. Like the positive electrode formulation, the mainstream AT9 is an oil-based formulation, and the main materials used are ceramic particles, PVDF and solvent NMP.

[0003] However, PVDF has a large swelling capacity and low adhesion in organic electrolytes. After PVDF swells in the electrolyte, its adhesion to the current collector further decreases. After the cell is fully charged, AT9 wrinkling or even falling off may occur, which may deteriorate the cell performance.

[0004] Therefore, there is an urgent need for a positive electrode edge coating that can effectively improve the wrinkling or even peeling of the positive electrode edge coating, thereby improving the performance of the battery cell. Summary of the Invention

[0005] To address the shortcomings of existing lithium-ion battery positive electrode edge coatings, such as easy wrinkling and even peeling, which degrades cell performance, this invention provides a positive electrode edge coating, a positive electrode sheet, a lithium-ion secondary battery, and an electronic device. The adhesion between the positive electrode edge coating and the current collector in this invention is significantly improved compared to existing formulations, effectively reducing the wrinkling and even peeling of the positive electrode edge coating after the cell is fully charged, thereby improving the storage performance of the resulting lithium-ion secondary battery.

[0006] In a first aspect, the present invention provides a positive electrode edge coating comprising the following components: 15wt%-35wt% of a main adhesive and 65wt%-85wt% of ceramic particles;

[0007] The main adhesive is a modified olefin copolymer; the polar branching degree of the main adhesive is 10%-25%.

[0008] Secondly, the present invention provides a positive electrode sheet comprising the positive electrode edge coating as described above.

[0009] Thirdly, the present invention provides a lithium-ion secondary battery comprising, as described above, a positive electrode, a negative electrode, an electrolyte, and a separator.

[0010] Fourthly, the present invention provides an electronic device comprising a lithium-ion secondary battery as described above.

[0011] The positive and progressive effects of this invention are as follows:

[0012] This invention provides a positive electrode edge coating that simultaneously contains modified olefin copolymers and ceramic particles. The modified olefin copolymers have a specific degree of polar branch substitution, and with their specific contents, the adhesion between the positive electrode edge coating and the current collector is improved. This effectively improves the phenomenon of wrinkling or even peeling of the positive electrode edge coating after the battery cell is fully charged, thereby improving the storage performance of the resulting battery and maintaining excellent capacity retention even after long-term storage. Attached Figure Description

[0013] Figure 1 This is the cell structure of the lithium-ion secondary battery of the present invention. Detailed Implementation

[0014] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0015] Positive electrode edge coating

[0016] The positive electrode edge coating described in the first aspect of the present invention comprises the following components: 15wt%-35wt% of main adhesive and 65wt%-85wt% of ceramic particles;

[0017] The main adhesive is a modified olefin copolymer; the polar branching degree of the main adhesive is 10%-25%.

[0018] In this invention, the modified olefin copolymer is defined as follows: it is obtained by modifying a copolymer obtained by polymerizing a conventional olefin with an alkenyl acid or an alkenyl ester. The modification is achieved by introducing a specific modifying monomer to copolymerize it with the monomers of the conventional olefin copolymer. During this process, the degree of substitution of polar branches on the main chain of the resulting copolymer can be controlled by adjusting the ratio of each monomer to the raw materials. Those skilled in the art can adjust the ratio of each monomer to the raw materials according to the specific degree of substitution of polar branches, in combination with conventional polymerization methods in the art, based on actual needs.

[0019] In this invention, the polar branched chain may be selected from one or more of hydroxyl, carboxyl, ester, amino, and amide groups.

[0020] The degree of polar branch substitution of the main adhesive can be determined by the following method: measuring the oxygen and nitrogen content using an X-ray energy dispersive spectroscopy (EDS) instrument, measuring the functional groups using infrared spectroscopy, and calculating the degree of substitution by dividing the molecular weight of the polar functional groups by the total molecular weight.

[0021] In this invention, the modified olefin copolymer may be a modified ethylene copolymer, and the monomers of the modified ethylene copolymer include ethylene, comonomers, and modifying monomers. The comonomers may be selected from one or more of olefin monomers, olefin ester monomers, and olefinic ester monomers. The modifying monomers may be selected from one or more of α-olefin monomers, phenylcycloene-containing monomers, and aminoene-containing monomers.

[0022] The acrylic acid monomer may include acrylic acid monomer and / or methacrylic acid monomer.

[0023] The acrylate monomer may include ethyl acrylate and / or butyl acrylate.

[0024] The olefin monomer may include vinyl acetate monomer.

[0025] The α-olefin monomer may include one or more of 1-butene, 1-hexene, and 1-octene.

[0026] The phenylcycloene monomer may include one or more of alkenylphenol, alkenylbenzoic acid and styrene.

[0027] The inventors discovered that adding monomers containing benzene rings can reduce the swelling degree of the copolymer in the electrolyte; and adding monomers containing amino olefins can, to some extent, regulate the adhesive strength of the resulting copolymer.

[0028] When conventional monomers such as ethylene, olefin monomers, olefin ester monomers, and acid olefin ester monomers are polymerized, the resulting copolymers exhibit a significant decrease in melting point and an increase in water solubility (soluble in water) and a decrease in oil solubility (soluble in NMP and electrolyte) when the AA or EV content is too high, which is detrimental to slurry preparation. Furthermore, their melting point and electrolyte resistance are difficult to control. Compared to copolymers obtained from conventional monomer polymerization, the modified olefin copolymer of this invention can further balance the adhesion, melting point, and swelling degree in electrolyte of the resulting positive electrode edge coating.

[0029] In some embodiments, the modified olefin copolymer is an ethylene-acrylic acid-styrene copolymer; wherein the mass ratio of ethylene, acrylic acid and styrene is (30-85):(10-40):(5-30), preferably 65:20:15.

[0030] In some embodiments, the modified olefin copolymer is an ethylene-acrylic acid-1-butene copolymer; wherein the mass ratio of ethylene, acrylic acid and 1-butene is preferably 70:20:10; and the molecular weight of the modified olefin copolymer is preferably 200,000 to 500,000 Daltons.

[0031] In some embodiments, the modified olefin copolymer is an ethylene-acrylic acid-1-hexene-styrene copolymer; wherein the mass ratio of ethylene, acrylic acid, 1-hexene and styrene is preferably 50:35:5:10; and the molecular weight of the modified olefin copolymer is preferably 200,000-500,000 Daltons.

[0032] In some embodiments, the modified olefin copolymer is an ethylene-vinyl acetate-styrene copolymer; wherein the mass ratio of ethylene, vinyl acetate and styrene is preferably 65:23:12; and the molecular weight of the modified olefin copolymer is preferably 200,000-500,000 Daltons.

[0033] In some embodiments, the modified olefin copolymer is an ethylene-methacrylic acid-phenol copolymer; wherein the mass ratio of ethylene, methacrylic acid and phenol is preferably 55:30:15; and the molecular weight of the modified olefin copolymer is preferably 200,000-500,000 Daltons.

[0034] In some embodiments, the modified olefin copolymer is an ethylene-methacrylic acid-phenol copolymer; wherein the mass ratio of ethylene, methacrylic acid and phenol is preferably 55:30:15; and the molecular weight of the modified olefin copolymer is preferably 200,000-500,000 Daltons.

[0035] In some embodiments, the modified olefin copolymer is an ethylene-ethyl acrylate-phenol copolymer; wherein the mass ratio of ethylene, ethyl acrylate and phenol copolymer is preferably 60:25:15; and the molecular weight of the modified olefin copolymer is preferably 200,000-500,000 Daltons.

[0036] In some embodiments, the modified olefin copolymer is an ethylene-butyl acrylate-phenol copolymer copolymer; wherein, the mass ratio of ethylene, butyl acrylate and phenol copolymer is preferably 60:25:15; and the molecular weight of the modified olefin copolymer is preferably 200,000-500,000 Daltons.

[0037] In this invention, the content of the main adhesive is preferably 15wt%-30wt%, for example 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, or 29wt%.

[0038] In this invention, the content of the ceramic particles is preferably 70wt%-85wt%, for example 71wt%, 72wt%, 73wt%, 74wt%, 75wt%, 76wt%, 77wt%, 78wt%, 79wt%, 80wt%, 81wt%, 82wt%, 83wt%, or 84wt%.

[0039] In this invention, the polar branch substitution degree of the main adhesive is preferably 12%-22%, for example 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or 21%.

[0040] In this invention, the ceramic particles can be conventional ceramic particles in the art, generally selected from one or more of alumina, boehmite, zirconium oxide and aluminum hydroxide, such as alumina.

[0041] In this invention, the positive electrode edge coating may further include an auxiliary adhesive, which is an adhesive polymer different from the main adhesive; the content of the auxiliary adhesive is 0-2 wt%, and not 0.

[0042] Preferably, the content of the auxiliary adhesive is 0.5wt%-2wt%, for example 1wt% or 1.5wt%.

[0043] The adhesive polymer may be conventional in the art, such as one or more selected from polyimide, polyvinylidene fluoride (PVDF), polyvinyl alcohol, polyvinyl ester, polyacrylonitrile, polyacrylic acid, polyacrylate, carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR).

[0044] In some embodiments, the auxiliary adhesive is selected from one or both of polyimide and PVDF, such as PVDF 5130.

[0045] In this invention, the molecular weight of the auxiliary adhesive can be 1 million to 3 million Daltons, preferably 1.5 million to 2.5 million Daltons, for example 2 million Daltons.

[0046] The molecular weight of the auxiliary adhesive can be determined by the following method: The auxiliary adhesive is dissolved in the organic solvent N,N-dimethylformamide (DMF) to prepare a DMF solution (0.1%). The molecular weight is measured using a gel permeation chromatography (Agilent 1260GPC) instrument. (The 0.1% polymer and DMF solution are filtered through a 45µm filter. 10ml of the filtrate is taken, and another 10ml of chromatographic grade DMF solution is prepared. The column temperature is set to 40℃, and baseline measurements are performed for approximately 30-40 minutes. Then, the sample is added using the syringe, and the test is performed to obtain the molecular weight.) The inventors believe that if the molecular weight of the auxiliary adhesive is too high, the dissolution rate is slow, affecting production efficiency; if the molecular weight is too low, the stability of the positive electrode edge coating slurry will deteriorate, and the shelf life of the positive electrode edge coating slurry will be shortened.

[0047] In one specific embodiment, the positive electrode edge coating comprises the following components: 20 wt% ethylene-acrylic acid-styrene copolymer, 1 wt% PVDF 5130, and 79% alumina; the polar branching degree of the ethylene-acrylic acid-styrene copolymer is 15%, and the molecular weight of the PVDF 5130 is 2 million Daltons.

[0048] In one specific embodiment, the positive electrode edge coating comprises the following components: 35 wt% ethylene-1-butene acrylic acid copolymer and 65 wt% alumina; the polar branching degree of the ethylene-1-butene acrylic acid copolymer is 10%.

[0049] In one specific embodiment, the positive electrode edge coating comprises the following components: 15 wt% ethylene-acrylic acid-1-hexene-styrene copolymer, 0.5 wt% PVDF 5130, and 84.5 wt% alumina; the polar branching degree of the ethylene-acrylic acid-1-hexene-styrene copolymer is 25%, and the molecular weight of the PVDF 5130 is 2 million Daltons.

[0050] In one specific embodiment, the positive electrode edge coating comprises the following components: 20 wt% ethylene-vinyl acetate-styrene copolymer, 1 wt% PVDF 5130, and 79 wt% alumina; the polar branching degree of the ethylene-vinyl acetate-styrene copolymer is 15%, and the molecular weight of the PVDF 5130 is 2 million Daltons.

[0051] In one specific embodiment, the positive electrode edge coating comprises the following components: 20 wt% ethylene-methacrylic acid-phenol copolymer, 1 wt% PVDF 5130, and 79 wt% alumina; the polar branching degree of the ethylene-methacrylic acid-phenol copolymer is 15%, and the molecular weight of the PVDF 5130 is 2 million Daltons.

[0052] In one specific embodiment, the positive electrode edge coating comprises the following components: 20 wt% ethylene-ethyl acrylate-phenol copolymer, 1 wt% PVDF 5130, and 79 wt% alumina; the polar branching degree of the ethylene-ethyl acrylate-phenol copolymer is 15%, and the molecular weight of the PVDF 5130 is 2 million Daltons.

[0053] In one specific embodiment, the positive electrode edge coating comprises the following components: 20 wt% ethylene-butyl acrylate-phenol copolymer, 1 wt% PVDF 5130, and 79 wt% alumina; the polar branching degree of the ethylene-butyl acrylate-phenol copolymer is 15%, and the molecular weight of the PVDF 5130 is 2 million Daltons.

[0054] In one specific embodiment, the positive electrode edge coating comprises the following components: 20 wt% ethylene-acrylic acid-styrene copolymer, 2 wt% PVDF 5130, and 78 wt% alumina; the polar branching degree of the ethylene-acrylic acid-styrene copolymer is 15%, and the molecular weight of the PVDF 5130 is 2 million Daltons.

[0055] In one specific embodiment, the positive electrode edge coating comprises the following components: 20 wt% ethylene-acrylic styrene copolymer, 1 wt% polyimide, and 79 wt% alumina; the polar branching degree of the ethylene-acrylic styrene copolymer is 15%, and the molecular weight of the polyimide is 2 million Daltons.

[0056] The present invention also provides a method for preparing a positive electrode edge coating as described in the first aspect, comprising the following steps:

[0057] The main adhesive and organic solvent are dispersed at low speed, the ceramic particles are added, and then dispersed at high speed to obtain a positive electrode edge coating slurry. This slurry is then applied to the edges of both sides of the positive electrode sheet and dried.

[0058] The organic solvent can be conventional in the art, and is generally one or more of N-methylpyrrolidone, N,N-dimethylacetamide, dimethyl sulfoxide and dihydroglucuronide, such as N-methylpyrrolidone (NMP).

[0059] The low-speed dispersion is, for example, a stirring dispersion.

[0060] The high-speed dispersion is, for example, high-speed dispersion using a high-speed disperser.

[0061] The drying process, for example, is oven drying.

[0062] When the positive electrode edge coating further comprises the auxiliary adhesive, the preparation method preferably includes:

[0063] The main adhesive, auxiliary adhesive and organic solvent are dispersed at low speed, the ceramic particles are added, and then dispersed at high speed to obtain the positive electrode edge coating slurry. The slurry is then applied to the edges of both sides of the positive electrode sheet and dried.

[0064] Positive electrode sheet

[0065] The positive electrode sheet described in the second aspect of the present invention includes the positive electrode edge coating as described above.

[0066] In this invention, the positive electrode sheet before the positive electrode edge coating is conventional in the art, generally comprising a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector; the positive electrode material layer comprises a positive electrode active material, and conductive agents and binders may be further added as needed.

[0067] The positive electrode active material can be a conventional positive electrode active material used in the art to prepare positive electrode sheets, such as a lithium nickel cobalt manganese oxide composite material; the molecular formula of the lithium nickel cobalt manganese oxide composite material is Li(Ni) a Co b Mn c O2, where a+b+c=1.

[0068] In some embodiments, the positive electrode active material is Li(Ni) 0.8 Co 0.1 Mn 0.1 O2 (NCM811), wherein the content of NCM811 in the cathode material layer is, for example, 96.5 wt%.

[0069] The conductive agent is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. For example, specific materials that can be used include: graphite, such as natural or artificial graphite; carbon-based materials, such as carbon black (Super P), acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium dioxide; or conductive polymers, such as polyphenylene derivatives, etc.

[0070] The binder serves to improve the adhesion between the positive electrode active materials and the adhesion between the positive electrode active materials and the positive electrode current collector. There are no particular limitations on the type of binder. Specific examples of binders may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or various copolymers thereof, and any one or a mixture of two or more thereof may be used.

[0071] In some embodiments, the positive electrode active material is NCM811, the conductive agent is Super P, and the binder is PVDF; the mass ratio of NCM811, Super P, and PVDF in the positive electrode material layer is preferably 96.5:1.5:2.

[0072] In this invention, the positive electrode sheet before the positive electrode edge coating is applied can be prepared by conventional methods in the art, such as including:

[0073] The positive electrode active material, conductive agent and binder are mixed in a certain mass ratio, and then a solvent is added and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on the positive electrode current collector, and then dried, cold pressed and cut to obtain the final product.

[0074] The solvent may be an organic solvent commonly used in the art for preparing positive electrode slurry, such as one or more selected from NMP, DMAC and acetone.

[0075] Lithium-ion secondary batteries

[0076] The lithium-ion secondary battery described in the third aspect of the present invention comprises a positive electrode, a negative electrode, an electrolyte, and a separator as described above.

[0077] In this invention, the negative electrode sheet can be a conventional negative electrode sheet in the art, generally comprising a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector; the negative electrode material layer contains a negative electrode active material, and one or more of conductive agents, thickeners and binders can be further added as needed.

[0078] The negative electrode active material can be a negative electrode active material conventionally used in the field to prepare negative electrode sheets, such as natural graphite, artificial graphite, soft carbon, hard carbon, lithium metal, silicon-based materials, etc.

[0079] The conductive agent is a reagent used to ensure that the electrode has good charge and discharge performance. It can be selected from graphite materials such as natural graphite and artificial graphite, carbon black materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black, conductive fibers such as carbon fiber and metal fiber, metal powders such as fluorinated carbon powder, aluminum powder, and nickel powder, conductive whiskers such as zinc oxide and potassium titanate, and conductive metal oxides or polyphenylene derivatives such as titanium dioxide.

[0080] The addition of the thickener can increase the viscosity of the system of each component in the negative electrode material layer. It can be a thickener commonly used in the art to prepare negative electrode sheets, such as sodium carboxymethyl cellulose (CMC).

[0081] The binder is a component that facilitates the bonding between the negative electrode active material and the conductive agent, and also facilitates the bonding between the negative electrode active material and the current collector. It can typically be selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers.

[0082] In some embodiments, the negative electrode material is graphite, the conductive agent is acetylene black, the thickener is CMC, and the binder is SBR; the preferred mass ratio of graphite, acetylene black, CMC, and SBR in the negative electrode material layer is 95:1:1.8:2.2.

[0083] In this invention, the negative electrode sheet can be prepared using conventional methods in the art, such as the following method: mixing the negative electrode active material, conductive agent, thickener and binder in a certain mass ratio, adding solvent and mixing evenly to obtain a negative electrode slurry; uniformly coating the negative electrode slurry onto the negative electrode current collector, and then preparing the negative electrode sheet through processes such as drying, cold pressing and slitting.

[0084] The solvent may be conventional in the art, such as deionized water.

[0085] In this invention, both the positive and negative current collectors can be made of materials that do not cause chemical changes and have high conductivity, without restriction, as needed. Commonly used materials include stainless steel, aluminum, nickel, titanium, or calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. To enhance the adhesion of the positive and negative electrode active materials, micro-embossing can be formed on the surfaces of both the positive and negative current collectors. The positive and negative current collectors can be used in various forms, such as films, sheets, foils, meshes, or porous bodies.

[0086] In this invention, the electrolyte can be a conventional electrolyte used in lithium-ion secondary batteries, generally including a non-aqueous solvent and a lithium salt.

[0087] The non-aqueous solvent is selected, for example, from carbonate solvents. Specifically, the carbonate solvent may be selected from one or more of polyvinyl carbonate (PEC), fluorovinyl carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC).

[0088] In some embodiments, the electrolyte comprises PEC, EMC, EC, PC and FEC; the volume ratio of PEC, EMC, EC, PC and FEC is (30-50):(10-30):(15-35):(1-10):(1-15), preferably 40:20:25:5:10.

[0089] The lithium salt can be a conventional lithium salt in the art, such as a mixed lithium salt of LiPF6 and LiODFB.

[0090] In this invention, the preparation method of the electrolyte can be conventional in the art, for example including:

[0091] (1) Under dry conditions, PEC, EMC, EC, PC and FEC are mixed in a volume ratio of (30-50):(10-30):(15-35):(1-10):(1-15) to form a non-aqueous solvent;

[0092] (2) Under a dry argon environment, the mixed lithium salt of LiPF6:LiODFB=(90-97):(3-10) is dissolved in the non-aqueous solvent to prepare an electrolyte with a lithium salt concentration of 1-2 mol / L.

[0093] In this invention, the diaphragm can be conventional in the art, such as polypropylene (PP) diaphragm or polyethylene (PE) diaphragm.

[0094] The thickness of the membrane can be 9-18 μm, for example 11 μm. The air permeability of the membrane can be 180s / 100mL-380s / 100mL. The porosity of the membrane can be 30%-50%.

[0095] In this invention, the cell structure of the lithium-ion secondary battery may include, but is not limited to, cylindrical, prismatic, and pouch cell structures.

[0096] In some embodiments, the cell structure includes a cell body and a top cover plate, the top cover plate including one or more of a positive terminal, a negative terminal, an explosion-proof valve, and a liquid injection port. This cell structure is as follows... Figure 1 As shown.

[0097] In this invention, the method for preparing the lithium-ion secondary battery can be conventional in the art, for example including:

[0098] The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. Then, an aluminum-plastic film is wrapped around the separator, dried, and electrolyte is injected. After encapsulation, settling, and formation processes, a lithium-ion secondary battery is finally produced.

[0099] Electronic devices

[0100] The electronic device described in the fourth aspect of the present invention includes a lithium-ion secondary battery as described above.

[0101] In this invention, the electronic device may include, but is not limited to, mobile devices (e.g., mobile phones, tablets, laptops, video recorders, portable printers / copiers, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, and backup power supplies.

[0102] Based on common knowledge in the field, the above optional conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0103] The reagents and raw materials used in this invention are conventional reagents and raw materials, all of which are commercially available.

[0104] Examples 1-9 and Comparative Examples 1-8

[0105] Preparation of positive electrode sheet

[0106] (1) The positive electrode active material NCM811, the conductive agent Super P and the binder PVDF are mixed in a weight ratio of 96.5:1.5:2, and the solvent NMP is added. The mixture is stirred and mixed thoroughly to obtain a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector aluminum foil, and after drying, cold pressing and slitting, a positive electrode sheet without edge coating is prepared.

[0107] (2) As shown in Table 1, the main adhesive, auxiliary adhesive, color developer and organic solvent (N-methylpyrrolidone) with different performance parameters are stirred and dispersed in different proportions, and then dispersed by a high-speed disperser to obtain the corresponding positive electrode edge coating slurry (the contents in the table are all mass percentages). The positive electrode sheet containing the positive electrode edge coating is obtained by simultaneously drying on the above-mentioned positive electrode sheet without edge coating.

[0108] The degree of substitution of polar branched chains in the main adhesive was measured by the following method:

[0109] The oxygen and nitrogen content is measured using an X-ray energy dispersive spectroscopy (EDS) instrument, and the functional groups are measured using infrared spectroscopy. The result is calculated by dividing the molecular weight of the polar functional groups by the total molecular weight.

[0110] The molecular weight of the auxiliary adhesive was determined by the following method:

[0111] The auxiliary adhesive was dissolved in the organic solvent N,N-dimethylformamide (DMF) to prepare a DMF solution (0.1%). The molecular weight was measured using a gel permeation chromatography (Agilent 1260GPC) instrument. (The 0.1% polymer and DMF solution were filtered through a 45µm filter. 10 ml of the filtrate was taken, and another 10 ml of chromatographic grade DMF solution was prepared. The column temperature was set to 40℃, and baseline measurements were performed for approximately 30–40 minutes. Then, the injector was opened, and the sample to be tested was added. The results were then obtained.)

[0112] The main adhesive is obtained by emulsion polymerization of different monomers listed in Table 1; the molecular weight of the obtained main adhesive is 200,000 to 500,000 Daltons; for the purposes of this invention, the main adhesive meeting the above molecular weight is sufficient to meet the application requirements and achieve the technical effects involved in this invention.

[0113] Table 1. Composition of the positive electrode edge coating in Examples 1-9 and Comparative Examples 1-8

[0114]

[0115]

[0116] Example 1

[0117] Peel force test of positive electrode edge coating

[0118] The positive electrode sheets of Examples 1-9 and Comparative Examples 1-8 were cut into strips of 15mm*200mm. Using a Vantage tensile testing machine, the strips were peeled 180° at a test rate of 10mm / min and a gauge length of 50mm to obtain the peel strength of the coating. The results are shown in Table 2.

[0119] Example 2

[0120] Electrolyte immersion test of positive electrode edge coating

[0121] (1) Preparation of electrolyte

[0122] In a dry environment, PEC, EMC, EC, PC and FEC are mixed in a volume ratio of 40:20:25:5:10 to form a non-aqueous solvent.

[0123] In a dry argon environment, a mixed lithium salt of LiPF6:LiODFB = 97:3 was dissolved in the non-aqueous solvent to prepare an electrolyte with a lithium salt concentration of 1.1 mol / L.

[0124] (2) Electrolyte immersion test

[0125] The positive electrode sheets from Examples 1-9 and Comparative Examples 1-8 were placed in the above electrolyte at a weight ratio of 1:5. After being sealed and stored at room temperature for 30 days, they were taken out, the electrolyte was dried, and the coating peeling was observed. The results are shown in Table 2.

[0126] Example 3

[0127] DC resistance (DCR) of lithium-ion secondary batteries—capacity retention test

[0128] 1. Preparation of negative electrode sheet

[0129] The negative electrode active material graphite, conductive agent acetylene black, thickener CMC and binder SBR are mixed in a mass ratio of 95:1:1.8:2.2, and then deionized water is added as a solvent. The mixture is stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, and after drying, cold pressing and slitting, a negative electrode sheet is prepared.

[0130] 2. Preparation of electrolyte

[0131] (1) Under dry conditions, PEC, EMC, EC, PC and FEC are mixed in a volume ratio of 40:20:25:5:10 to form a non-aqueous solvent;

[0132] (2) Under a dry argon environment, the mixed lithium salt of LiPF6:LiODFB = 97:3 is dissolved in the non-aqueous solvent to prepare an electrolyte with a lithium salt concentration of 1.1 mol / L.

[0133] 3. Preparation of the diaphragm

[0134] A porous PE membrane with a thickness of 11 μm was used as the diaphragm (air permeability of 180s / 100mL-380s / 100mL; porosity of 30%-50%).

[0135] 4. Preparation of lithium-ion secondary batteries

[0136] The positive electrode, separator, and negative electrode of Examples 1-9 and Comparative Examples 1-8 are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. Then, an aluminum-plastic film is wrapped around the separator, dried, and the electrolyte prepared above is injected. After encapsulation, settling, and formation processes, a 1Ah soft-pack battery (i.e., a lithium-ion secondary battery) is finally prepared.

[0137] (2) DC resistance (DCR) test

[0138] For the different lithium-ion secondary batteries mentioned above, at room temperature, the cell is fully charged using a current of 0.3C, and then fully discharged using a current of 0.3C. This is the base capacity. After fully charging the cell again using a current of 0.3C, the cell is placed in an oven at 60°C for 60 days. After 60 days, the cell is taken out and fully discharged → fully charged → fully discharged using a current of 0.3C. The capacity retention rate is the ratio of the second full discharge capacity to the base capacity. The results are shown in Table 2.

[0139] Table 2. Relevant performance of positive electrode sheets and resulting lithium-ion secondary batteries in Examples 1-9 and Comparative Examples 1-8

[0140]

[0141]

[0142] To reflect the performance of the positive electrode edge coating of this invention after it is coated on the positive electrode sheet and actually applied to a lithium-ion secondary battery, this invention was first characterized by the peel force test and electrolyte immersion test as described above. Combined with the DC resistance (DCR)-capacity retention rate test in actual application, the effect of the positive electrode edge coating is reflected.

[0143] According to the data in Table 2, the peel force between the positive electrode edge coating and the aluminum foil in Examples 1-9 is not less than 100 N / m, and no abnormalities such as coating peeling or wrinkling occurred after immersion in electrolyte for 30 days. This indicates that the adhesion between the positive electrode edge coating and the current collector of the present invention is significantly improved, which can significantly improve the phenomenon of edge coating wrinkling or even peeling. When the positive electrode sheet containing the positive electrode edge coating is used in lithium-ion secondary batteries, the capacity retention rate of the cell is not less than 95% after storage at 60°C for 60 days, which effectively improves the storage performance of the obtained battery.

[0144] Examples 1 and 4-7 show that the type of modified olefin copolymer in the main adhesive has a significant impact on peel strength. Copolymers obtained by adding modifying monomers to the copolymer of ethylene and acrylic acid show better performance than copolymers obtained by adding modifying monomers to the copolymer of ethylene and other acrylates (or acid acrylates). Furthermore, according to Examples 1-3, even without auxiliary adhesives, the performance remains significantly better as the degree of polar branching substitution and content of the copolymers obtained by copolymerizing ethylene, acrylic acid, and modifying monomers change.

[0145] The difference between Comparative Examples 1 and 6 and the Examples lies in the absence of a primary adhesive (modified olefin copolymer). The results show that, without the modified olefin copolymer and containing only an auxiliary adhesive at a content as high as 18%, the peel force between the positive electrode edge coating and the aluminum foil is only 40 N / m. After immersion in the electrolyte, the coating wrinkles, and the final battery capacity retention rate is only 90%. Therefore, the key to this invention lies in the addition of the modified olefin copolymer. Compared to copolymers obtained through conventional monomer polymerization, the modified olefin copolymer of this invention can further balance the adhesion, melting point, and swelling degree of the obtained positive electrode edge coating in the electrolyte, ultimately improving the relevant performance of the resulting lithium-ion secondary battery.

[0146] Furthermore, even when modified olefin copolymers are present, they must meet specific content ranges. The main difference between Comparative Examples 4, 5, and 8 and Example 1 lies in the content of the modified olefin copolymer. The results show that when the modified olefin copolymer content is too high (above 35%, Comparative Examples 4 and 8), the viscosity of the resulting positive electrode edge coating slurry becomes too high, making production impossible and affecting the coating effect. When the modified olefin copolymer content is too low (below 15%, Comparative Example 5), the peel strength decreases, the coating wrinkles after immersion in the electrolyte, and the capacity retention of the final battery also decreases.

[0147] Furthermore, even when modified olefin copolymers are present, they must meet specific polar branching substitution requirements. The main difference between Comparative Examples 2-3 and 7 and Examples 1-2 lies in the polar branching substitution degree of the modified olefin copolymers. The results show that excessively high polar branching substitution (above 25%, Comparative Example 2) leads to abnormal dissolution of the main adhesive, making it impossible to prepare the slurry and, moreover, to obtain the edge coating. Conversely, excessively low polar branching substitution (below 10%, Comparative Example 7) results in reduced peel strength, causing the coating to wrinkle or even peel off after immersion in the electrolyte, ultimately reducing the capacity retention of the resulting battery. Even with excessively low polar branching substitution, increasing the content of the modified olefin copolymer (Comparative Example 3) yields significantly poorer results.

[0148] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and the scope of protection of the present invention is defined by the appended claims. Those skilled in the art can make various changes or modifications to these embodiments without departing from the principles and essence of the present invention, but all such changes and modifications fall within the scope of protection of the present invention.

Claims

1. A positive electrode edge coating, characterized in that, It contains the following components: 15wt%-35wt% main adhesive and 65wt%-85wt% ceramic particles; The main adhesive is a modified olefin copolymer; The polar branched substitution degree of the main adhesive is 10%-25%.

2. The positive electrode edge coating as described in claim 1, characterized in that, The polar side chain is selected from one or more of hydroxyl, carboxyl, ester, amino, and amide groups.

3. The positive electrode edge coating as described in claim 2, characterized in that, The modified olefin copolymer is a modified ethylene copolymer; the monomers of the modified ethylene copolymer include ethylene, comonomers and modifying monomers; the comonomers are selected from one or more of olefinic acid monomers, olefinic acid ester monomers and acidic olefinic acid ester monomers; the modifying monomers are selected from one or more of α-olefin monomers, phenylcycloene-containing monomers and aminoene-containing monomers.

4. The positive electrode edge coating as described in claim 3, characterized in that, The modified olefin copolymer satisfies one or more of the following conditions (a)-(e): (a) The acrylic monomers include acrylic acid monomers and / or methacrylic acid monomers; (b) The acrylate monomers include ethyl acrylate and / or butyl acrylate; (c) The acid olefin monomers include vinyl acetate monomers; (d) The α-olefin monomer includes one or more of 1-butene, 1-hexene and 1-octene; (e) The benzene-containing monomers include one or more of alkenylphenol, alkenylbenzoic acid and styrene.

5. The positive electrode edge coating as described in claim 1, characterized in that, The positive electrode edge coating satisfies one or more of the following conditions (a)-(c): (a) The content of the main adhesive is 15wt%-30wt%; (b) The content of the ceramic particles is 70wt%-85wt%; (c) The polar branching degree of the main adhesive is 12%-22%.

6. The positive electrode edge coating as described in claim 1, characterized in that, The positive electrode edge coating further comprises an auxiliary adhesive; the auxiliary adhesive is an adhesive polymer; the content of the auxiliary adhesive is 0-2 wt%, and not 0.

7. The positive electrode edge coating as described in claim 6, characterized in that, The auxiliary adhesive B satisfies one or both of the following conditions (a)-(b): (a) The auxiliary adhesive B is selected from one or more of polyimide, polyvinylidene fluoride, polyvinyl alcohol, polyvinyl ester, polyacrylonitrile, polyacrylic acid, polyacrylate, carboxymethyl cellulose and styrene-butadiene rubber; (b) The molecular weight of the auxiliary adhesive B is 1,000,000 to 3,000,000 Daltons.

8. A positive electrode sheet, characterized in that, It comprises a positive electrode edge coating as described in any one of claims 1-7.

9. A lithium-ion secondary battery, characterized in that, It comprises the positive electrode, negative electrode, electrolyte, and separator as described in claim 8.

10. An electronic device, characterized in that, It comprises the lithium-ion secondary battery as described in claim 9.

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