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: 60-93 wt% of an adhesive A, 5-30 wt% of an adhesive B and 2-15 wt% of a color developing agent, the adhesive A is an adhesive polymer; the adhesive B is a modified olefin copolymer; and the crystallinity of the adhesive B is 10%-40%. According to the positive electrode edge coating, the edge identification performance of the positive electrode edge coating can be improved, meanwhile, the adhesive force of the positive electrode edge coating and a current collector is improved, and the phenomena that the positive electrode edge coating wrinkles and even falls off after a battery cell is fully charged are effectively improved.

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] In general, ceramic formulations contain about 85% ceramic particles. Ceramic particles have a high density and are prone to powder shedding during the slitting process. This powder can easily get caught in the battery cell, affecting the yield of the finished product. The remaining 15% is PVDF as an adhesive. However, PVDF has a high swelling capacity in organic electrolytes and low adhesion. After swelling in the electrolyte, the adhesion between PVDF and the current collector further decreases, making the battery cell prone to wrinkling or even detachment at AT9 after full charging, thus deteriorating the battery cell performance. Even identifiable edge coating materials have the same defects.

[0004] Therefore, there is an urgent need for a positive electrode edge coating that can identify and effectively improve phenomena such as easy powdering, wrinkling, and even peeling. Summary of the Invention

[0005] To address the shortcomings of existing positive electrode edge coatings for lithium batteries, such as easy powder shedding, wrinkling, and even peeling, this invention provides a positive electrode edge coating, a positive electrode sheet, a lithium-ion secondary battery, and an electronic device. The positive electrode edge coating of this invention possesses excellent identifiability while significantly improving powder shedding and exhibiting significantly enhanced adhesion to the current collector compared to existing formulations. This effectively reduces the wrinkling and even peeling of the positive electrode edge coating after the battery cell is fully charged.

[0006] In a first aspect, the present invention provides a positive electrode edge coating comprising the following components: 60wt%-93wt% adhesive A, 5wt%-30wt% adhesive B, and 2wt%-15wt% color developer;

[0007] The adhesive A is an adhesive polymer;

[0008] The adhesive B is a modified olefin copolymer; the crystallinity of the adhesive B is 10%-40%.

[0009] Secondly, the present invention provides a positive electrode sheet comprising the positive electrode edge coating as described above.

[0010] 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.

[0011] Fourthly, the present invention provides an electronic device comprising a lithium-ion secondary battery as described above.

[0012] The positive and progressive effects of this invention are as follows:

[0013] This invention provides a positive electrode edge coating that simultaneously contains an adhesive polymer, a modified olefin copolymer, and a color developer. The modified olefin copolymer has a specific degree of crystallinity, and with the specific content of each component, it improves the edge recognition performance of the obtained positive electrode edge coating while enhancing the adhesion between the positive electrode edge coating and the current collector, effectively improving the phenomenon of wrinkling or even peeling of the positive electrode edge coating after the battery cell is fully charged. Attached Figure Description

[0014] Figure 1 This is the cell structure of the lithium-ion secondary battery of the present invention.

[0015] Figure 2 This is an overall appearance view of the positive electrode edge coating in Example 2 of the present invention.

[0016] 1-Coated area; 2-Foil area; 3-Boundary line. Detailed Implementation

[0017] 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.

[0018] Positive electrode edge coating

[0019] The positive electrode edge coating described in the first aspect of the present invention comprises the following components: 60wt%-93wt% adhesive A, 5wt%-30wt% adhesive B and 2wt%-15wt% color developer;

[0020] The adhesive A is an adhesive polymer;

[0021] The adhesive B is a modified olefin copolymer; the crystallinity of the adhesive B is 10%-40%.

[0022] 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 crystallinity and melting point of the resulting copolymer can be controlled by adjusting the raw material ratio of each monomer. Those skilled in the art can combine conventional polymerization methods in the field according to actual needs, that is, adjust the raw material ratio of each monomer according to the specific crystallinity.

[0023] The crystallinity of adhesive B can be determined by differential scanning calorimetry (DSC). Maintaining a heating rate of 10℃ / min, the sample is heated from 25℃ to 200℃. The area enclosed by the melt peak curve and the baseline represents the enthalpy of melting of the crystalline portion of the polymer, ΔHf. Integrating the melt peak area, the enthalpy of melting of the crystalline portion, ΔHf, is calculated. The crystallinity is then calculated using the following formula:

[0024]

[0025] This invention utilizes a modified olefin copolymer with a specific degree of crystallinity, which reduces the transparency of the positive electrode edge coating, facilitating edge identification by CCD (online microscope), while simultaneously improving the peel strength of the positive electrode edge coating. However, its melting point is relatively low, and its content must be controlled within a specific range to avoid severe shrinkage during laser die-cutting. The inventors discovered that if adhesive A is used alone, the resulting positive electrode edge coating is transparent, and even with the addition of more than 10 wt% pigment, it is still difficult to identify. However, by further adding adhesive B to adhesive A, the positive electrode edge coating can be "fogged," becoming an opaque gel-like substance. In this case, adhesive B and the color developer work together, making it easier for machines and the human eye to identify.

[0026] In this invention, the modified olefin copolymer may have polar branches on its CC backbone, and the crystallinity can be controlled by adjusting the degree of substitution of the polar branches; the polar branches are preferably selected from one or more of hydroxyl, carboxyl, ester, amino and amide groups; the degree of substitution of the polar branches of the modified olefin copolymer is preferably 10%-25%.

[0027] The degree of polar branch substitution of the modified olefin copolymer 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.

[0028] 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.

[0029] The acrylic acid monomer may include acrylic acid monomer and / or methacrylic acid monomer.

[0030] The acrylate monomer may include ethyl acrylate and / or butyl acrylate.

[0031] The olefin monomer may include vinyl acetate monomer.

[0032] The α-olefin monomer may include one or more of 1-butene, 1-hexene, and 1-octene.

[0033] The phenylcycloene monomer may include one or more of alkenylphenol, alkenylbenzoic acid and styrene.

[0034] 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.

[0035] 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.

[0036] In some embodiments, the modified olefin copolymer is obtained by copolymerizing ethylene, acrylic acid, and styrene; the mass ratio of ethylene, acrylic acid, and styrene is preferably 65:25:10; and the molecular weight of the modified olefin copolymer is preferably 200,000 to 500,000 Daltons.

[0037] In some embodiments, the modified olefin copolymer is obtained by copolymerizing ethylene, acrylic acid, and phenol; the mass ratio of ethylene, acrylic acid, and phenol is preferably 70:20:10; and the molecular weight of the modified olefin copolymer is preferably 200,000 to 500,000 Daltons.

[0038] In some embodiments, the modified olefin copolymer is obtained by copolymerizing ethylene, vinyl acetate and 1-butene; the mass ratio of ethylene, vinyl acetate and 1-butene is preferably 60:30:10; and the molecular weight of the modified olefin copolymer is preferably 200,000 to 500,000 Daltons.

[0039] In some embodiments, the modified olefin copolymer is obtained by copolymerizing ethylene, methacrylic acid and 1-butene; the mass ratio of ethylene, methacrylic acid and 1-butene is preferably 60:30:5; and the molecular weight of the modified olefin copolymer is preferably 200,000 to 500,000 Daltons.

[0040] In some embodiments, the modified olefin copolymer is obtained by copolymerizing ethylene, acrylic acid, and styrene; the mass ratio of ethylene, acrylic acid, and styrene is preferably 55:30:15; and the molecular weight of the modified olefin copolymer is preferably 200,000 to 500,000 Daltons.

[0041] In this invention, the content of adhesive A is preferably 65wt%-85wt%, for example 66wt%, 67wt%, 68wt%, 69wt%, 70wt%, 71wt%, 72wt%, 73wt%, 74wt%, 75wt%, 76wt%, 77wt%, 78wt%, 79wt%, 80wt%, 81wt%, 82wt%, 83wt%, or 84wt%.

[0042] In this invention, the content of adhesive B is preferably 15wt%-30wt%, for example 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, or 29wt%.

[0043] In this invention, the content of the color developer is preferably 3wt%-10wt%, for example 4wt%, 5wt%, 6wt%, 7wt%, 8wt% or 9wt%.

[0044] In this invention, the crystallinity of adhesive B is preferably 10%-30%, for example 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, or 29%.

[0045] In this invention, the adhesive A can be conventional in the art, generally selected from one or more adhesive polymers used in oil-based solvents such as polyimide, polyphenylene sulfide, polyarylsulfone, chlorinated polyether, polyvinyl alcohol, polyvinyl ester and polyacrylonitrile, for example, polyimide and / or polyphenylene sulfide.

[0046] In this invention, the colorimetric agent can be conventional in the art, generally selected from one or more of bismuth yellow, cobalt blue, cobalt green, titanium nickel yellow, phthalocyanine, insulating carbon black and naphthol, for example, one or more of bismuth yellow, cobalt blue and phthalocyanine.

[0047] In one specific embodiment, the positive electrode edge coating comprises the following components: 80 wt% adhesive A, 15 wt% adhesive B, and 5 wt% color developer; adhesive A is polyimide, adhesive B is ethylene-acrylic acid-styrene copolymer, and the color developer is bismuth yellow; the crystallinity of adhesive B is 25%.

[0048] In one specific embodiment, the positive electrode edge coating comprises the following components: 67 wt% adhesive A, 30 wt% adhesive B, and 3 wt% color developer; adhesive A is polyimide, adhesive B is ethylene-acrylic acid-styrene copolymer, and the color developer is cobalt blue; the crystallinity of adhesive B is 25%.

[0049] In one specific embodiment, the positive electrode edge coating comprises the following components: 85 wt% adhesive A, 13 wt% adhesive B, and 2 wt% color developer; adhesive A is polyimide, adhesive B is an ethylene-acrylic acid-phenol copolymer, and the color developer is bismuth yellow; the crystallinity of adhesive B is 30%.

[0050] In one specific embodiment, the positive electrode edge coating comprises the following components: 90 wt% adhesive A, 5 wt% adhesive B, and 5 wt% color developer; adhesive A is polyimide, adhesive B is ethylene-vinyl acetate-1-butene copolymer, and the color developer is phthalocyanine; the crystallinity of adhesive B is 15%.

[0051] In one specific embodiment, the positive electrode edge coating comprises the following components: 80 wt% adhesive A, 15 wt% adhesive B, and 5 wt% color developer; adhesive A is polyimide, adhesive B is ethylene-methacrylate-1-butene copolymer, and the color developer is phthalocyanine; the crystallinity of adhesive B is 25%.

[0052] In one specific embodiment, the positive electrode edge coating comprises the following components: 75 wt% adhesive A, 15 wt% adhesive B, and 10 wt% color developer; adhesive A is polyphenylene sulfide, adhesive B is ethylene-acrylic acid-styrene copolymer, and the color developer is bismuth yellow; the crystallinity of adhesive B is 10%.

[0053] The present invention also provides a method for preparing the positive electrode edge coating as described above, which includes the following steps:

[0054] The adhesive A, adhesive B, color developer, and organic solvent are dispersed in a certain proportion to prepare a positive electrode edge coating slurry. This slurry is then applied to the edges of both sides of the positive electrode sheet and dried.

[0055] 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).

[0056] The dispersion may include first dispersing at low speed and then dispersing at high speed; the low-speed dispersion may be, for example, stirring dispersion; the high-speed dispersion may be, for example, high-speed disperser dispersion.

[0057] The drying operation can be conventional in the art, such as oven drying.

[0058] Positive electrode sheet

[0059] The positive electrode sheet described in the second aspect of the present invention includes the positive electrode edge coating as described above.

[0060] In this invention, the positive electrode sheet before the application of the positive electrode edge coating can be 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.

[0061] 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.

[0062] 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%.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] In this invention, the positive electrode sheet before the application of the positive electrode edge coating can be prepared by conventional methods in the art, such as including:

[0067] The positive electrode active material, conductive agent and binder are mixed in a certain proportion, 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.

[0068] 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.

[0069] Lithium-ion secondary batteries

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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).

[0075] 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.

[0076] 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.

[0077] 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 proportion, 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.

[0078] The solvent may be conventional in the art, such as deionized water.

[0079] 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.

[0080] 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.

[0081] 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).

[0082] 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.

[0083] The lithium salt can be a conventional lithium salt in the art, such as a mixed lithium salt of LiPF6 and LiODFB.

[0084] In this invention, the preparation method of the electrolyte can be conventional in the art, for example including:

[0085] (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;

[0086] (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.

[0087] In this invention, the diaphragm can be conventional in the art, such as polypropylene (PP) diaphragm or polyethylene (PE) diaphragm.

[0088] 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%.

[0089] 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.

[0090] 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.

[0091] In this invention, the method for preparing the lithium-ion secondary battery can be conventional in the art, for example including:

[0092] The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to act as a separator. Then, an aluminum-plastic film is wrapped around the separator, dried, and electrolyte is injected. After processes such as encapsulation, settling, and formation, a lithium-ion secondary battery is finally prepared.

[0093] Electronic devices

[0094] The electronic device described in the fourth aspect of the present invention includes a lithium-ion secondary battery as described above.

[0095] 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.

[0096] Based on common knowledge in the field, the above optional conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0097] The reagents and raw materials used in this invention are conventional reagents and raw materials, all of which are commercially available.

[0098] Examples 1-6 and Comparative Examples 1-7

[0099] Preparation of positive electrode sheet

[0100] (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.

[0101] (2) As shown in Table 1, adhesive A, adhesive B, 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.

[0102] The crystallinity of adhesive B was determined by differential scanning calorimetry (DSC). The sample was heated from 25°C to 200°C at a rate of 10°C / min. The area enclosed by the melt peak curve and the baseline represents the enthalpy of melting (ΔHf) of the crystalline portion of the polymer. The enthalpy of melting (ΔHf) of the crystalline portion was calculated by integrating the melt peak area, and the crystallinity was then calculated using the following formula:

[0103]

[0104] Adhesive B is obtained by emulsion polymerization of different monomers listed in Table 1; the molecular weight of the resulting adhesive B is 200,000 to 500,000 Daltons; for the purposes of this invention, adhesive B meeting the above molecular weight is sufficient to meet the usage requirements and achieve the technical effects involved in this invention.

[0105] Table 1. Composition of the positive electrode edge coating in Examples 1-6 and Comparative Examples 1-7

[0106]

[0107] Application Example 1

[0108] Preparation of lithium-ion secondary batteries

[0109] 1. Preparation of negative electrode sheet

[0110] 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.

[0111] 2. Preparation of electrolyte

[0112] (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;

[0113] (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.

[0114] 3. Preparation of the diaphragm

[0115] 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%).

[0116] 4. Preparation of lithium-ion secondary batteries

[0117] The positive electrode, separator, and negative electrode of Examples 1-6 and Comparative Examples 1-7 were stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. Then, an aluminum-plastic film was wrapped around the separator, dried, and the electrolyte prepared above was injected. After encapsulation, settling, and formation processes, a soft-pack battery (i.e., a lithium-ion secondary battery) with a capacity of 1Ah was finally prepared.

[0118] Example 1

[0119] Peel force test of positive electrode edge coating

[0120] The positive electrode sheets of Examples 1-6 and Comparative Examples 1-7 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.

[0121] Example 2

[0122] CCD edge recognition test

[0123] A CCD is an online microscope device that automatically measures and identifies edges based on color difference comparisons of materials. For example... Figure 2 As shown, for adhesives A and B of the present invention, only adhesive A (Comparative Example 1) is added. Figure 2 The left side of the positive electrode edge coating (coating area 1) is generally "transparent". Under light irradiation, it is directly transparent through the substrate (foil area 2), and the boundary line 3 between coating area 1 and foil area 2 is not clear; while after adding adhesives A and B at the same time (Example 2, Figure 2 The right side portion of the positive electrode edge coating (coating area 1) exhibits a frosted (atomized) effect, clearly distinguishing it from the substrate (foil area 2). The boundary line 3 between coating area 1 and foil area 2 is clear. The specific edge recognition effects of the positive electrode sheets in Examples 1-6 and Comparative Examples 1-7 are shown in Table 2.

[0124] Example 3

[0125] Laser die-cutting edge shrinkage test

[0126] The positive electrode sheets of Examples 1-6 and Comparative Examples 1-7 were removed after laser slitting. The distance from the edge coating of the positive electrode to the edge of the aluminum foil was measured using an imaging instrument, which is the shrinkage distance. The results are shown in Table 2.

[0127] Example 4

[0128] Electrolyte immersion test of positive electrode edge coating

[0129] 1. Preparation of electrolyte

[0130] (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;

[0131] (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.

[0132] 2. Electrolyte immersion test

[0133] The positive electrode sheets of Examples 1-6 and Comparative Examples 1-7 were placed in the above electrolyte at a weight ratio of 1:5. After being sealed and stored at room temperature for 7 days, the pigment dissolution was observed. After being sealed and stored for 30 days, the coating was removed, dried, and the coating peeling was observed. The results are shown in Table 2.

[0134] Table 2. Relevant performance of the positive electrode edge coating in Examples 1-6 and Comparative Examples 1-7

[0135]

[0136] (In the table, "OK" means CCD edge recognition can be performed, and "NG" means CCD edge recognition failed.)

[0137] According to the data in Table 2, the positive electrode edge coatings in Examples 1-6 can all be identified by CCD, and no powder loss occurred during the slitting process. The shrinkage of the laser die-cut edge is relatively small, and the peel force with aluminum foil is not less than 15 N / m. Furthermore, no dissolution occurred after soaking in electrolyte for 7 days, and no abnormalities such as coating wrinkling occurred after soaking for 30 days. This indicates that the positive electrode edge coating of the present invention has good edge identification performance, no powder is generated during slitting, and the adhesion to the current collector is significantly improved. It can significantly improve the phenomenon of edge coating wrinkling or even peeling, and can improve the yield of battery cells.

[0138] Examples 1-2 show that, within a certain range, as the content of adhesive A increases and the content of adhesive B decreases, the shrinkage of the laser-cut edge of the resulting positive electrode edge coating is reduced, but the peel strength with the aluminum foil decreases. Furthermore, according to Examples 3-5, when the content of adhesive A increases and the content of adhesive B decreases, even with changes in the type and crystallinity of adhesive B, the shrinkage of the laser-cut edge of the resulting positive electrode edge coating is reduced, and the peel strength with the aluminum foil also decreases, indicating that the relative content of the two has a significant impact.

[0139] Comparative Example 1 represents a commonly used ceramic edge coating formulation in the prior art. It is evident that significant powder shedding occurs during the slitting process, and the coating wrinkles after immersion in the electrolyte for 30 days. In contrast, the pure adhesive insulating coating of the present invention overcomes the powder shedding issue during slitting.

[0140] The main difference between Comparative Examples 2-3 and Example 2 is that the content of adhesive A is too high (or too low), and the content of adhesive B is too low (or too high). As can be seen from the results, the relative amounts of the two need to be within a specific range in order to simultaneously meet a certain degree of laser die-cutting edge shrinkage and peeling force.

[0141] The main difference between Comparative Examples 4-5 and Example 2 is that the content of the color developer is too high (or too low). As can be seen from the results, if the content of the color developer is too low, the CCD cannot recognize the edge, and if the content of the color developer is too high, the pigment will dissolve in the electrolyte.

[0142] The main difference between Comparative Examples 6-7 and Example 1 is that the crystallinity of adhesive B is too high (or too low). As can be seen from the results, if the crystallinity of adhesive B is too low, the light transmittance of the material increases and the CCD cannot recognize the edge; if the crystallinity of adhesive B is too high, the material cannot be prepared.

Claims

1. A positive electrode edge coating characterized by, It comprises the following components: adhesive A 60wt%-93wt%, adhesive B 5wt%-30wt%, and color developer 2wt%-15wt%; The adhesive A is a cohesive polymer; The adhesive B is a modified olefin copolymer; the crystallinity of the adhesive B is 10%-40%.

2. The positive electrode edge coating of claim 1, wherein The modified olefin copolymer has polar branches; the polar branches are selected from one or more of hydroxyl, carboxyl, ester, amino, and amide groups; the polar branch substitution degree of the modified olefin copolymer is 10%-25%.

3. The positive electrode edge coating of claim 2, wherein The modified olefin copolymer is a modified ethylene copolymer, the monomers of which include ethylene, a comonomer, and a modifying monomer; the comonomer is selected from one or more of an enoic acid monomer, an enoic acid ester monomer, and an acid enol ester monomer; the modifying monomer is selected from one or more of an α-olefin monomer, a benzene ring-containing olefin monomer, and an amino-containing olefin monomer.

4. The positive electrode edge coating of claim 3, wherein The modified olefin copolymer satisfies one or more of the following conditions (a)-(e): (a) the enoic acid monomer includes an acrylic acid monomer and / or a methacrylic acid monomer; (b) the enoic acid ester monomer includes an ethyl acrylate and / or a butyl acrylate; (c) the acid enol ester monomer includes a vinyl acetate monomer; (d) the α-olefin monomer includes one or more of 1-butene, 1-hexene, and 1-octene; (e) the benzene ring-containing olefin monomer includes one or more of an alkenyl phenol, an alkenyl benzoic acid, and styrene.

5. The positive electrode edge coating of claim 1, wherein The positive electrode edge coating satisfies one or more of the following conditions (a)-(d): (a) the content of the adhesive A is 65wt%-85wt%; (b) the content of the adhesive B is 15wt%-30wt%; (c) the content of the color developer is 3wt%-10wt%; (d) the crystallinity of the adhesive B is 10%-30%.

6. The positive electrode edge coating of claim 1, wherein The adhesive A is selected from one or more of polyimide, polyphenylene sulfide, polyaryl sulfone, chlorinated polyether, polyvinyl alcohol, polyvinyl acetate, and polyacrylonitrile.

7. The positive electrode edge coating of claim 1, wherein The color developer is selected from one or more of bismuth yellow, cobalt blue, cobalt green, titanium nickel yellow, phthalocyanine, insulating carbon black, and naphthol.

8. A positive electrode sheet characterized by comprising: It comprises the positive electrode edge coating as claimed in any one of claims 1-7.

9. A lithium-ion secondary battery characterized by comprising: It comprises the positive electrode tab, the negative electrode tab, the electrolyte, and the separator as claimed in claim 8.

10. An electronic device, comprising: It comprises the lithium ion secondary battery as claimed in claim 9.

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