Positive electrode slurry, positive electrode sheet, and electrochemical device containing same

Abstract

Disclosed are a positive electrode slurry, a positive electrode sheet, and an electrochemical device containing same. The positive electrode slurry comprises a positive electrode active material, a binder, and a dispersant. The dispersant comprises component A and component B. Component A is an acidic dispersant, component B is an alkaline dispersant, and the mass ratio of component A to component B is (1-30):1. The positive electrode active material comprises carbon-coated lithium iron phosphate. The binder comprises polyvinylidene fluoride. The positive electrode slurry uses a composite dispersant comprising two different components, and has good coating performance and excellent bonding performance. A positive electrode sheet prepared using the positive electrode slurry has high peel strength, and the resulting electrochemical device (especially a lithium-ion battery) has excellent capacity retention.

Description

Positive electrode slurry, positive electrode sheet and electrochemical device comprising same TECHNICAL FIELD

[0001] The present application relates to a positive electrode slurry, a positive electrode sheet and an electrochemical device comprising same. BACKGROUND

[0002] In the positive electrode slurry used in lithium ion batteries, the particles of the active material often have a certain size and aggregation state. If the particles are aggregated together, it will lead to a decrease in the specific surface area of the positive electrode material, limiting the insertion and extraction rate of lithium ions, thereby reducing the discharge capacity of the battery. Therefore, in order to improve the dispersibility of the positive electrode material, a dispersant needs to be added. The addition of a dispersant not only improves the dispersibility of the slurry, but also reduces the viscosity of the slurry and improves its flowability. The appropriate viscosity helps the uniform distribution of the positive electrode slurry in the coating process and the coating performance, thereby improving the consistency and reliability of battery manufacturing. However, the dispersants in the prior art cannot improve the coating performance while ensuring the adhesion performance of the positive electrode slurry. SUMMARY

[0003] In order to ensure that the positive electrode slurry has excellent coating performance and adhesion performance, the present application provides a positive electrode slurry, a positive electrode sheet and an electrochemical device comprising same. The positive electrode slurry uses a composite dispersant comprising two different components, which has good coating performance and excellent adhesion performance. The positive electrode sheet prepared using the positive electrode slurry has high peeling force, and the capacity retention rate of the electrochemical device (especially lithium ion battery) obtained is excellent.

[0004] To achieve the above object, the technical scheme adopted by the present application is as follows.

[0005] In a first aspect, the present application provides a positive electrode slurry comprising a positive electrode active material, a binder and a dispersant; wherein the dispersant comprises component A and component B; the component A is an acidic dispersant; the component B is an alkaline dispersant; the mass ratio of the component A to the component B is (1-30): 1; the positive electrode active material comprises carbon-coated lithium iron phosphate; the binder comprises polyvinylidene fluoride.

[0006] In a second aspect, the present application provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode material layer on at least one surface of the positive electrode current collector, wherein the positive electrode material layer is formed by the positive electrode slurry as described above.

[0007] In a third aspect, the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode material layer on at least one surface of the positive electrode current collector, wherein the positive electrode material layer comprises a positive electrode active material, a binder and a dispersant; the dispersant comprises component A and component B; the component A is an acidic dispersant; the component B is an alkaline dispersant; the mass ratio of the component A to the component B is (1-30):1; the positive electrode active material comprises carbon-coated lithium iron phosphate; and the binder comprises polyvinylidene fluoride.

[0008] In a fourth aspect, the present application provides an electrochemical device, comprising the positive electrode sheet as described above.

[0009] The positive progress effect of the present application is that:

[0010] The present application provides a positive electrode slurry, which uses a composite dispersant comprising component A and component B with different acid and alkali properties, and realizes the balance of excellent coating performance and excellent adhesion performance. The positive electrode sheet prepared by using the positive electrode slurry has high peeling force, good consistency and reliability, and the electrochemical device (especially lithium ion battery) prepared further has good cycle stability. BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 is a graph showing the change of viscosity of the positive electrode slurry in Example 1 and Comparative Examples 1-2 with standing time. DETAILED DESCRIPTION

[0012] The present application will be further described by way of examples, but the present application is not limited to the examples. In the following examples, the experimental methods not specified in the specific conditions are selected according to the conventional methods and conditions, or according to the instructions of the commodity.

[0013] Positive electrode slurry

[0014] In the positive electrode slurry provided in the first aspect of the present application, it comprises a positive electrode active material, a binder and a dispersant; the dispersant comprises component A and component B; the component A is an acidic dispersant; the component B is an alkaline dispersant; the mass ratio of the component A to the component B is (1-30):1; the positive electrode active material comprises carbon-coated lithium iron phosphate; and the binder comprises polyvinylidene fluoride.

[0015] Dispersant

[0016] In the present application, the component A can be an acidic dispersant commonly used in the art.

[0017] In the present application, the acidic dispersant can comprise one or more of phosphoric acid dispersants, carboxylic acid dispersants, enoic acid dispersants and phenolic dispersants.

[0018] The phosphoric acid dispersant refers to a dispersant containing a phosphoric acid or a phosphoric acid ester group. For example, the phosphoric acid dispersant can be selected from one or more of trialkyl phosphate, stearic acid phosphate, heterocyclic phosphoric acid ester, catechol diphosphate, and pyridine phosphoric acid ester (O, O-dimethyl-O-(2, 2-dichlorovinyl) phosphate, English name: Dichlorvos).

[0019] The carboxylic acid dispersant refers to a dispersant containing a carboxylic acid or a carboxylic acid ester group. For example, the carboxylic acid dispersant can be selected from one or both of carboxylic acid polyester and maleic acid.

[0020] The enoic acid dispersant refers to a dispersant containing an enoic acid or an enoic acid ester group. For example, the enoic acid dispersant can be selected from one or more of polyacrylate, polyacrylic acid, methacrylic acid, and acrylic acid.

[0021] The phenolic dispersant refers to a dispersant containing a phenolic hydroxyl group, such as sorbitan monolaurate.

[0022] In some preferred embodiments, the component A comprises a phosphoric acid dispersant.

[0023] In some alternative embodiments, the component A comprises a phosphoric acid dispersant and a carboxylic acid dispersant.

[0024] In some preferred embodiments, the component A comprises one or more of trialkyl phosphate, stearic acid phosphate, heterocyclic phosphoric acid ester, catechol diphosphate, pyridine phosphoric acid ester, carboxylic acid polyester, maleic acid, polyacrylate, polyacrylic acid, methacrylic acid, acrylic acid, and sorbitan monolaurate.

[0025] In some specific embodiments, the component A is one or more of trialkyl phosphate, carboxylic acid polyester, polyacrylic acid, and sorbitan monolaurate.

[0026] In a specific embodiment, the component A is trialkyl phosphate and carboxylic acid polyester, and the mass ratio of the trialkyl phosphate to the carboxylic acid polyester is preferably 1:1.

[0027] In the present application, the component B can be a basic dispersant commonly used in the art.

[0028] In the present application, the basic dispersant can comprise one or more of an amine dispersant, an ammonia dispersant, an ammonium dispersant, a heterocyclic dispersant, an amide dispersant, and a cyano dispersant.

[0029] The amine dispersant can be selected from one or more of polyethyleneimine, triethanolamine, tripropanolamine, polyetheramine, and polyetheramine methyl ether.

[0030] The ammonium dispersant is, for example, hexadecyltrimethylammonium bromide (CTAB).

[0031] The heterocyclic dispersant may be selected from one or more of pyrrole dispersants, pyridine dispersants, furan dispersants, and imidazole dispersants. The pyrrole dispersant may be selected from one or more of pyrrole, polypyrrole, indole, and 4-pyrrolidone.

[0032] The amide dispersant may be selected from one or more of polyvinylpyrrolidone (PVP) and benzamide.

[0033] The cyano-based dispersant may be selected from one or more of polyacrylonitrile, polycyanate, and cyano-tert-butyloxypropyltrimethoxysilane.

[0034] In some preferred embodiments, component B includes one or more of polyethyleneimine, triethanolamine, tripropanolamine, polyetheramine, polyetheramine methyl ether, hexadecyltrimethylammonium bromide, pyrrole, polypyrrole, 4-pyrrolidone, indole, polyvinylpyrrolidone, benzamide, polyacrylonitrile, polycyanate, and cyanotert-butyloxypropyltrimethoxysilane.

[0035] In some specific embodiments, component B is one or more of polyvinylpyrrolidone, hexadecyltrimethylammonium bromide, polyacrylonitrile, and benzamide.

[0036] In some preferred embodiments, the dispersant comprises trialkyl phosphate and polyvinylpyrrolidone.

[0037] In some preferred embodiments, the dispersant includes carboxylic acid polyesters, trialkyl phosphates, and polyvinylpyrrolidone.

[0038] In some preferred embodiments, the dispersant comprises carboxylic acid polyester and polyvinylpyrrolidone.

[0039] In some preferred embodiments, the dispersant comprises polyacrylic acid and polyvinylpyrrolidone.

[0040] In some preferred embodiments, the dispersant comprises sorbitan monolaurate and polyvinylpyrrolidone.

[0041] In some preferred embodiments, the dispersant comprises trialkyl phosphate and benzamide.

[0042] In some preferred embodiments, the dispersant comprises trialkyl phosphate and hexadecyltrimethylammonium bromide.

[0043] In some preferred embodiments, the dispersant comprises trialkyl phosphate and polyacrylonitrile.

[0044] In some alternative embodiments, the content of component A is 0.005%-1.5%, preferably 0.01%-1%, for example 0.01%, 0.05%, 0.1%, 0.12%, 0.2% or 1%, the percentage being the percentage of the mass of component A in the total mass of the components in the positive electrode slurry excluding the solvent.

[0045] In some alternative embodiments, the content of component B is 0.005%-1%, preferably 0.01%-1%, for example 0.01%, 0.03%, 0.05% or 0.15%, the percentage being the percentage of the mass of component B in the total mass of the components in the positive electrode slurry excluding the solvent.

[0046] In some alternative embodiments, the content of the dispersant is 0.03%-2%, for example 0.055%, 0.06%, 0.10%, 0.105%, 0.11%, 0.13%, 0.15%, 0.25%, 1.05%, 1.55%, the percentage being the percentage of the mass of the dispersant in the total mass of the components in the positive electrode slurry excluding the solvent.

[0047] In some alternative embodiments, the mass ratio of component A to component B is (1-30):1, for example 2:1, 3.3:1, 4:1, 10:1 or 20:1.

[0048] Positive electrode active material

[0049] In the present application, the lithium iron phosphate in the carbon-coated lithium iron phosphate can be a conventional lithium iron phosphate positive electrode active material in the art, which is generally LiFeP04(LFP), but is not limited to LiFeP04, and can be doped with other metal elements, the metal elements including one or more of Zr, Mg, Mn, Al, Ti, Zn, Cr, Ni, Co and Nb, the doping amount of the metal elements being 0.1%-10%, the percentage being the percentage of the mass of the metal elements in the mass of LFP. When the metal elements are Mn, the lithium iron phosphate in the carbon-coated lithium iron phosphate is lithium manganese iron phosphate.

[0050] In some preferred embodiments, the positive electrode active material is carbon-coated lithium iron phosphate. The carbon coating amount is generally 0.05%-5%, preferably 0.5%-5%, for example 1.3% or 0.7%, the percentage being the percentage of the mass of carbon in the mass of carbon-coated lithium iron phosphate.

[0051] In some alternative embodiments, the content of the positive electrode active material is 85% to 98%, for example, 95.95%, 96.45%, 96.75%, 97.25%, 97.35%, 97.37%, 97.39%, 97.395%, 97.4%, 97.44%, 97.445%, or 98.05%, the percentage being the percentage of the mass of the positive electrode active material with respect to the total mass of the components other than the solvent in the positive electrode slurry.

[0052] Binder

[0053] In the present application, the positive electrode slurry must contain a binder, which includes polyvinylidene fluoride (PVDF).

[0054] In some alternative embodiments, the content of the binder is 0.5% to 10%, preferably 1% to 10%, for example, 0.9% or 1.8%, the percentage being the percentage of the mass of the binder with respect to the total mass of the components other than the solvent in the positive electrode slurry.

[0055] In some alternative embodiments, the binder further includes one or more of polytetrafluoroethylene, hexafluoropropylene, polyacrylonitrile, polyimide, and perfluorosulfonic acid ionomer.

[0056] In some alternative embodiments, the binder further includes one or more of sodium carboxymethyl cellulose, polyolefin, polyacrylic acid (PAA), and polymethyl methacrylate.

[0057] Conductive agent

[0058] In some alternative embodiments, the positive electrode slurry further includes a conductive agent.

[0059] The kind of the conductive agent is not particularly limited and is an agent for ensuring that the electrode has good charge and discharge performance. It can be optionally selected from graphite-based materials such as natural graphite and artificial graphite, carbon black-based materials such as carbon black SP, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal-cracking carbon black, conductive fibers such as carbon fibers and metal fibers, 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 such as titanium dioxide, or polyphenylene derivatives.

[0060] In a specific embodiment, the conductive agent is carbon black SP (SP).

[0061] In some alternative embodiments, the content of the conductive agent is 0.2% to 3%, for example, 0.7%, the percentage being the percentage of the mass of the conductive agent with respect to the total mass of the components other than the solvent in the positive electrode slurry.

[0062] Solvent

[0063] In the present application, the positive electrode slurry can include a solvent that is conventional in the art.

[0064] In some alternative embodiments, the solvent includes one or more of N-methyl pyrrolidone (NMP), dimethyl carbonate, ethylene carbonate, and diethylene carbonate.

[0065] In some alternative embodiments, the solid content of the positive electrode slurry is 45-80%, for example, 62.12%, 62.13%, 63.11%, 63.37%, 63.87%, 64.32%, 64.37%, 64.57%, 64.73%, 64.87%, 65.02%, 65.11%, 65.17%, 65.21%, 65.34%, 65.56%, 65.57%, 65.88%, 67.23%, or 67.32%. The solid content refers to the percentage of the total mass of components other than the solvent in the positive electrode slurry with respect to the mass of the positive electrode slurry.

[0066] In some alternative embodiments, the viscosity of the positive electrode slurry is 3000-20000 mPa·s, for example, 4200 mPa·s, 5100 mPa·s, 5200 mPa·s, 5400 mPa·s, 5600 mPa·s, 5700 mPa·s, 5900 mPa·s, 6150 mPa·s, 6200 mPa·s, 6300 mPa·s, 6400 mPa·s, 6700 mPa·s, 6900 mPa·s, 7200 mPa·s, or 7300 mPa·s.

[0067] In some alternative embodiments, the pH of the positive electrode slurry is 4-11, preferably 4-10, for example, 5, 6, 7, 8, 9, or 10.

[0068] In some preferred embodiments, the second mixing time is less than the third mixing time.

[0069] In some preferred embodiments, before the first mixing, a pre-mixing of components other than the dispersant in the raw composition of the positive electrode slurry is further included, and the pre-mixing is by stirring.

[0070] In a specific embodiment, the positive electrode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1%, and the content of polyvinylpyrrolidone is 0.05%.

[0071] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.03%.

[0072] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, carboxylic acid polyester, and polyvinylpyrrolidone, wherein the content of carboxylic acid polyester is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0073] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, carboxylic acid polyester, trialkyl phosphate, and polyvinylpyrrolidone, wherein the sum of the content of carboxylic acid polyester and trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0074] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, polyacrylic acid, and polyvinylpyrrolidone, wherein the content of polyacrylic acid is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0075] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, sorbitan monolaurate, and polyvinylpyrrolidone, wherein the content of sorbitan monolaurate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0076] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and benzamide, wherein the content of trialkyl phosphate is 0.1% and the content of benzamide is 0.05%.

[0077] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and benzamide, wherein the content of trialkyl phosphate is 0.12% and the content of benzamide is 0.03%.

[0078] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and cetyltrimethylammonium bromide, wherein the content of trialkyl phosphate is 0.1% and the content of cetyltrimethylammonium bromide is 0.05%.

[0079] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1% and the content of polyacrylonitrile is 0.05%.

[0080] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 1% and the content of polyvinylpyrrolidone is 0.05%.

[0081] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.05% and the content of polyvinylpyrrolidone is 0.05%.

[0082] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.2% and the content of polyvinylpyrrolidone is 0.05%.

[0083] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 1.5% and the content of polyvinylpyrrolidone is 0.05%.

[0084] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.01%.

[0085] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.005%.

[0086] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, polyacrylic acid, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0087] In one embodiment, the cathode slurry includes carbon-coated lithium iron phosphate, Li5FeO4, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0088] In a specific embodiment, the cathode slurry comprises carbon-coated lithium iron phosphate, Li5FeO4, polyvinylidene fluoride, trialkyl phosphate and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0089] In some preferred embodiments, the method for preparing the cathode slurry comprises the following steps:

[0090] S1, first mixing the cathode active material, the binder and the solvent in the cathode slurry to obtain a first mixture;

[0091] S2, second mixing the first mixture and the component A to obtain a second mixture;

[0092] S3, third mixing the second mixture and the component B.

[0093] In some preferred embodiments, in step S1, the first mixing is performed by kneading.

[0094] In some preferred embodiments, in step S2, the second mixing is performed by stirring; in step S3, the third mixing is performed by stirring; and the stirring speed of the second mixing is greater than or equal to the stirring speed of the third mixing.

[0095] In some preferred embodiments, in step S3, after the third mixing, an operation of adding a solvent to adjust the viscosity is further included.

[0096] Cathode sheet

[0097] In the cathode sheet provided in the second aspect of the present application, it comprises a cathode current collector and a cathode material layer located on at least one surface of the cathode current collector, and the cathode material layer is formed by the cathode slurry as described above.

[0098] In the cathode sheet provided in the third aspect of the present application, it comprises a cathode current collector and a cathode material layer located on at least one surface of the cathode current collector, and the cathode material layer comprises a cathode active material, a binder and a dispersant; the dispersant comprises a component A and a component B; the component A is an acidic dispersant; the component B is an alkaline dispersant; the mass ratio of the component A to the component B is (1-30):1; the cathode active material comprises carbon-coated lithium iron phosphate; and the binder comprises polyvinylidene fluoride.

[0099] In the cathode sheet of the third aspect, the types of the dispersant, the component A, the component B and the binder are as described above, and the contents of the components are the mass percentages of the cathode material layer.

[0100] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0101] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.03%.

[0102] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, carboxylic acid polyester, and polyvinylpyrrolidone, wherein the content of carboxylic acid polyester is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0103] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, carboxylic acid polyester, trialkyl phosphate, and polyvinylpyrrolidone, wherein the sum of the content of carboxylic acid polyester and trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0104] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, polyacrylic acid, and polyvinylpyrrolidone, wherein the content of polyacrylic acid is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0105] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, sorbitan monolaurate, and polyvinylpyrrolidone, wherein the content of sorbitan monolaurate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0106] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and benzamide, wherein the content of trialkyl phosphate is 0.1% and the content of benzamide is 0.05%.

[0107] In one embodiment, the positive electrode material layer includes carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and benzamide, wherein the content of trialkyl phosphate is 0.12% and the content of benzamide is 0.03%.

[0108] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and cetyl trimethyl ammonium bromide, wherein the content of trialkyl phosphate is 0.1%, and the content of cetyl trimethyl ammonium bromide is 0.05%.

[0109] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1%, and the content of polyacrylonitrile is 0.05%.

[0110] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1%, and the content of polyacrylonitrile is 0.05%.

[0111] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1%, and the content of polyacrylonitrile is 0.05%.

[0112] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1%, and the content of polyacrylonitrile is 0.05%.

[0113] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1%, and the content of polyacrylonitrile is 0.05%.

[0114] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1%, and the content of polyacrylonitrile is 0.05%.

[0115] In one embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, carbon black SP, trialkyl phosphate, and polyacrylonitrile, wherein the content of trialkyl phosphate is 0.1%, and the content of polyacrylonitrile is 0.05%.

[0116] In a specific embodiment, the positive electrode material layer comprises carbon-coated lithium iron phosphate, polyvinylidene fluoride, polyacrylic acid, carbon black SP, trialkyl phosphate and polyvinylpyrrolidone, wherein the content of trialkyl phosphate is 0.1% and the content of polyvinylpyrrolidone is 0.05%.

[0117] In the present application, the positive electrode current collector can be a conventional positive electrode current collector in the art. The positive electrode current collector serves as a substrate to support the positive electrode material layer and is usually a metal foil having a thickness of 3-500 microns. There is no particular limitation on the material as long as it has high electrical conductivity and does not chemically react in the system of the secondary battery. For example, it can be a foil formed after surface treatment of nickel, titanium, aluminum, nickel, silver, stainless steel, carbon, etc. The positive electrode current collector usually has a smooth surface, but fine lines or the like can be formed on the surface to improve the adhesion between the positive electrode active material and the current collector. In addition to the foil, the positive electrode current collector can also take any one or a combination of multiple forms of film, mesh, porous, foam or non-woven fabric, etc. Generally, the positive electrode current collector is an aluminum foil.

[0118] In some embodiments, the method for preparing the positive electrode sheet comprises: coating the positive electrode slurry obtained by thoroughly mixing the components of the positive electrode material layer in a solvent on at least one surface of the positive electrode current collector, drying, roll pressing and compacting to obtain the positive electrode sheet.

[0119] Electrochemical device

[0120] In the fourth aspect of the present application, the electrochemical device comprises the positive electrode sheet as described above.

[0121] In the present application, the electrochemical device is preferably a battery.

[0122] In an alternative embodiment, the electrochemical device is a lithium ion battery; the lithium ion battery further comprises a negative electrode sheet, a separator and an electrolyte.

[0123] Negative electrode sheet

[0124] In the present application, the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector.

[0125] In the present application, the negative active material in the negative electrode material layer can be a negative active material of a lithium ion battery in the art, and the negative active material of the lithium ion battery can use a compound capable of reversibly intercalating and deintercalating lithium, and specific examples include, but are not limited to, carbon materials such as crystalline carbon (natural graphite and artificial graphite, etc.), amorphous carbon, carbon-coated graphite, and resin-coated graphite, or oxide materials such as indium oxide, silicon oxide, tin oxide, lithium titanate, zinc oxide, lithium oxide, and the like, and can also be lithium metal or a metal material capable of forming an alloy with lithium; wherein the metal material capable of forming an alloy with lithium is, for example, Cu, Sn, Si, Co, Mn, Fe, Sb, and Ag. Alloys containing two or three of these metals and lithium can also be used as negative active materials. These negative active materials can be used alone or in combination of two or more. From the perspective of high energy density, carbon materials such as graphite can also be used in combination with Si-based materials such as Si, Si alloy, and Si oxide.

[0126] In some embodiments, the negative active material includes any one of or a combination of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, a silicon oxide compound, a silicon carbon compound, or lithium titanate.

[0127] In a specific embodiment, the negative active material is artificial graphite.

[0128] In some embodiments, the negative electrode material layer further includes a thickening agent.

[0129] The addition of the thickening agent can increase the system viscosity of the components in the negative electrode slurry, and the thickening agent can be a thickening agent commonly used in the art for preparing negative electrode sheets, such as sodium carboxymethyl cellulose (CMC).

[0130] In some embodiments, the negative electrode material layer further includes a conductive agent.

[0131] The conductive agent is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black (Super P), acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal carbon black, or carbon fibers; 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, and the like.

[0132] In some embodiments, the negative electrode material layer further includes a binder.

[0133] The binder is not particularly limited and can be selected from polyvinylidene fluoride, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid (PAA), ethylene-propylene-diene terpolymer and sulfonated products thereof, styrene butadiene rubber (SBR), fluororubber, and various copolymers.

[0134] In some embodiments, the mass ratio of the negative active material, the conductive agent, the binder, and the thickening agent in the negative electrode material layer is 97.3:0.4:1.8:0.5.

[0135] In the present application, the negative current collector can be a conventional negative current collector in the art. The negative current collector serves as a substrate to support the negative active material and is typically a metal foil having a thickness of 3-500 microns. The material is not particularly limited as long as it has high electrical conductivity and does not chemically react in the system of the secondary battery. For example, it can be a foil formed after surface treatment of nickel, titanium, aluminum, nickel, silver, stainless steel, carbon, etc. The negative current collector typically has a smooth surface, but can also have fine lines or the like formed on the surface to improve the adhesion between the positive active material and the current collector. In addition to the foil, the negative current collector can also take any one or a combination of multiple forms such as a film, a mesh, a porous material, a foam, or a non-woven fabric. Generally, the negative current collector is a copper foil. The thickness of the negative current collector is, for example, 10 μm.

[0136] In some embodiments, the method for preparing the negative electrode sheet comprises the following steps: coating the negative electrode slurry obtained by thoroughly stirring and mixing the components of the negative electrode material layer in a solvent on at least one surface of the negative current collector, drying, cold pressing, and slitting to obtain the negative electrode sheet.

[0137] Separator

[0138] In some alternative embodiments, the separator can be a polypropylene film or a polyethylene film.

[0139] The air permeability of the separator can be 180-380 s / 100 mL.

[0140] The porosity of the separator can be 30%-50%.

[0141] The thickness of the separator can be 9-18 μm.

[0142] In one embodiment, the separator is a polyethylene film; the thickness of the separator is 11 μm; the air permeability of the separator is 230 s / 100 mL; and the porosity of the separator is 40%.

[0143] Electrolyte

[0144] In some embodiments, the electrolyte can be an electrolyte commonly used in the art for batteries, generally comprising a non-aqueous solvent, a lithium salt and an additive.

[0145] In some embodiments, the method for preparing the lithium ion battery comprises the following steps: sequentially laminating the positive electrode sheet, the separator and the negative electrode sheet, so that the separator is between the positive electrode sheet and the negative electrode sheet to play a separating role; then wrapping an aluminum plastic film, injecting the electrolyte after drying, and finally preparing a soft package battery through processes such as packaging, standing and formation.

[0146] On the basis of common sense in the art, the above-mentioned preferred conditions can be combined arbitrarily, i.e. to obtain each preferred example of the present application.

[0147] The reagents and raw materials used in the present application are commercially available.

[0148] The main reagents used in the following examples and comparative examples are as follows:

[0149] Polyvinylidene fluoride (PVDF) was purchased from Arkema (Changshu) Special Material Co., Ltd., with a product model of HSV900;

[0150] Polyacrylic acid (PAA) was purchased from Yingdel Technology Co., Ltd., with a product model of LA133;

[0151] Trialkyl phosphate was purchased from Nantong Gaoke Chemical Co., Ltd., with a product model of C1618 phosphate;

[0152] Polyvinylpyrrolidone was purchased from Huzhou Shenhua High Polymer Material Co., Ltd., with a product model of K90;

[0153] Polyacrylonitrile was purchased from Dongguan Hengtai Plastic Co., Ltd., in the form of a powder;

[0154] Benzamide was purchased from Weifang Qianjin Fine Chemical Co., Ltd., with a product model of 55-21-0;

[0155] Cetyltrimethylammonium bromide was purchased from Shandong Duohe Chemical Co., Ltd., with a product model of 57-09-0.

[0156] Example 1

[0157] (1) Preparation of positive electrode slurry

[0158] S1, first mixing:

[0159] S1-1, the positive active material, PVDF and SP (contents are 97.35%, 1.8% and 0.7% respectively) are added into a stirring device for pre-dispersion treatment at a speed of 800 rpm for 30 min to obtain a premix; wherein the positive active material is carbon-coated LFP (carbon-coated amount is 1.3%);

[0160] S1-2, the solvent NMP is added into the premix for kneading treatment at a speed of 500 rpm for 30 min to obtain a first mixture;

[0161] S2, second mixing: trialkyl phosphate (content is 0.1%) is added into the first mixture for high-speed dispersion treatment at a speed of 3000 rpm for 60 min to obtain a second mixture;

[0162] S3, third mixing: PVP (content is 0.05%) is added into the second mixture for high-speed dispersion treatment at a speed of 3000 rpm for 180 min, and then appropriate NMP is added to adjust the viscosity to 4000-10000 mPa·s.

[0163] (2) Preparation of the positive electrode sheet

[0164] The obtained positive electrode slurry is coated on at least one surface of the positive electrode current collector, dried, roll-pressed and compacted to obtain the positive electrode sheet.

[0165] Example 2

[0166] The difference between Example 2 and Example 1 is that in the preparation process of the positive electrode slurry, carboxylic acid polyester and trialkyl phosphate (mass ratio is 1:1, content of component A is still 0.1%) are used in step S2. The rest of the conditions remain the same as in Example 1.

[0167] Example 3

[0168] The difference between Example 3 and Example 1 is that in the preparation process of the positive electrode slurry, carboxylic acid polyester is used instead of trialkyl phosphate in step S2. The rest of the conditions remain the same as in Example 1.

[0169] Example 4

[0170] The difference between Example 4 and Example 1 is that in the preparation process of the positive electrode slurry, polyacrylic acid is used instead of trialkyl phosphate in step S2. The rest of the conditions remain the same as in Example 1.

[0171] Example 5

[0172] The difference between Example 5 and Example 1 is that in the preparation process of the positive electrode slurry, sorbitan monolaurate is used instead of trialkyl phosphate in step S2. The rest of the conditions remain the same as in Example 1.

[0173] Example 6

[0174] Example 6 and Example 1 differ in that benzamide is used instead of PVP in step S3 during the preparation of the positive electrode slurry. The rest of the conditions and Example 1 remain unchanged.

[0175] Example 7

[0176] Example 7 and Example 6 differ in that the content of trialkyl phosphate is 0.12% in step S2 and the content of benzamide is 0.03% in step S3 during the preparation of the positive electrode slurry. The rest of the conditions and Example 6 remain unchanged.

[0177] Example 8

[0178] Example 8 and Example 1 differ in that CTAB is used instead of PVP in step S3 during the preparation of the positive electrode slurry. The rest of the conditions and Example 1 remain unchanged.

[0179] Example 9

[0180] Example 9 and Example 1 differ in that polyacrylonitrile is used instead of PVP in step S3 during the preparation of the positive electrode slurry. The rest of the conditions and Example 1 remain unchanged.

[0181] Example 10

[0182] Example 10 and Example 1 differ in that the content of carbon-coated LFP is 96.45% in step S1-1 and the content of trialkyl phosphate is 1% in step S2 during the preparation of the positive electrode slurry. The rest of the conditions and Example 1 remain unchanged.

[0183] Example 11

[0184] Example 11 and Example 1 differ in that the content of carbon-coated LFP is 97.4% in step S1-1 and the content of trialkyl phosphate is 0.05% in step S2 during the preparation of the positive electrode slurry. The rest of the conditions and Example 1 remain unchanged.

[0185] Example 12

[0186] Example 12 and Example 1 differ in that the content of carbon-coated LFP is 97.25% in step S1-1 and the content of trialkyl phosphate is 0.2% in step S2 during the preparation of the positive electrode slurry. The rest of the conditions and Example 1 remain unchanged.

[0187] Example 13

[0188] Example 13 and Example 1 differ in that the content of carbon-coated LFP in step S1-1 is 95.95% and the content of trialkyl phosphate in step S2 is 1.5% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0189] Example 14

[0190] Example 14 and Example 1 differ in that the content of carbon-coated LFP in step S1-1 is 97.37% and the content of PVP in step S3 is 0.03% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0191] Example 15

[0192] Example 15 and Example 1 differ in that the content of carbon-coated LFP in step S1-1 is 97.39% and the content of PVP in step S3 is 0.01% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0193] Example 16

[0194] Example 16 and Example 1 differ in that the content of carbon-coated LFP in step S1-1 is 97.395% and the content of PVP in step S3 is 0.005% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0195] Example 17

[0196] Example 17 and Example 1 differ in that the content of carbon-coated LFP in step S1-1 is 0.7% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0197] Example 18

[0198] Example 18 and Example 1 differ in that the binder in step S1-1 is PVDF and PAA (mass ratio of 1:1, and the content of the binder is still 1.8%) during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0199] Example 19

[0200] Example 19 and Example 1 differ in that no conductive agent SP is used in step S1-1 during the preparation of the positive electrode slurry (the content of the positive electrode active material and PVDF is 98.05% and 1.8%, respectively), and the rest of the conditions remain the same as in Example 1.

[0201] Example 20

[0202] Example 20 and Example 1 differ in that the content of carbon-coated LFP in step S1-1 is 0.05% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0203] Example 21

[0204] Example 21 and Example 1 differ in that the content of carbon-coated LFP in step S1-1 is 5% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0205] Comparative Example 1

[0206] (1) Preparation of the positive electrode slurry

[0207] S1, First mixing:

[0208] The content of carbon-coated LFP in step S1-1 is 97.3%, and the rest of the conditions remain the same as in Example 1.

[0209] S1-2 is the same as in Example 1.

[0210] S2, Second mixing: Add trialkyl phosphate (content is 0.2%) to the first mixture, and perform high-speed dispersion treatment at a speed of 3000 rpm for 240 min. After the dispersion treatment, add appropriate NMP to adjust the viscosity to mPa·s, and it is obtained.

[0211] (2) Preparation of the positive electrode sheet and Example 1 remains the same.

[0212] Comparative Example 2

[0213] (1) Preparation of the positive electrode slurry

[0214] S1, First mixing:

[0215] The content of carbon-coated LFP in step S1-1 is 97.42%, and the rest of the conditions remain the same as in Example 1.

[0216] S1-2 is the same as in Example 1.

[0217] S2, Second mixing: Add PVP (content is 0.08%) to the first mixture, and perform high-speed dispersion treatment at a speed of 3000 rpm for 240 min. After the dispersion treatment, add appropriate NMP to adjust the viscosity to 4000-10000 mPa·s, and it is obtained.

[0218] (2) Preparation of the positive electrode sheet and Example 1 remains the same.

[0219] Comparative Example 3

[0220] The difference between Comparative Example 3 and Example 1 is that trialkyl phosphate is used instead of PVP in step S3 during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0221] Comparative Example 4

[0222] The difference between Comparative Example 4 and Example 1 is that PVP is used instead of trialkyl phosphate in step S2 during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0223] Comparative Example 5

[0224] The difference between Comparative Example 5 and Example 1 is that the content of carbon-coated LFP in step S1-1 is 97.44% and the content of trialkyl phosphate in step S2 is 0.01% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0225] Comparative Example 6

[0226] The difference between Comparative Example 6 and Example 1 is that the content of carbon-coated LFP in step S1-1 is 97.445% and the content of trialkyl phosphate in step S2 is 0.005% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0227] Comparative Example 7

[0228] The difference between Comparative Example 7 and Example 1 is that the content of carbon-coated LFP in step S1-1 is 97.25% and the content of PVP in step S3 is 0.15% during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 1.

[0229] Comparative Example 8

[0230] The difference between Comparative Example 8 and Example 19 is that trialkyl phosphate is used instead of PVP in step S3 during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 19.

[0231] Comparative Example 9

[0232] The difference between Comparative Example 9 and Example 19 is that PVP is used instead of trialkyl phosphate in step S2 during the preparation of the positive electrode slurry. The rest of the conditions remain the same as in Example 19.

[0233] Comparative Example 10

[0234] The difference between Comparative Example 10 and Example 1 is that carbon-coated LFP in step S1-1 is replaced by NCM811 (Li(Ni 0.8 Co 0.1 Mn 0.1LFP replaced by NCM811 (Li(Ni0.6Co0.2Mn0.2)O2) without conductive agent SP (the content of positive active material and PVDF is 98.05% and 1.8% respectively), PVP replaced by trialkyl phosphate in step S3, the rest conditions are the same as example 1.

[0235] Comparative example 11

[0236] Comparative example 11 and example 1 are different in that: in the preparation process of the positive electrode slurry, the carbon-coated LFP in step S1-1 is replaced by NCM811 (Li(Ni0.6Co0.2Mn0.2)O2) without CNT, and conductive agent SP is not used (the content of positive active material and PVDF is 98.05% and 1.8% respectively), PVP is replaced by trialkyl phosphate in step S2, and the rest conditions are the same as example 1. 0.8 Co 0.1 Mn 0.1 )O2), and without using conductive agent SP (the content of positive active material and PVDF is 98.05% and 1.8% respectively), PVP is replaced by trialkyl phosphate in step S2, and the rest conditions are the same as example 1.

[0237] Comparative example 12

[0238] Comparative example 12 and example 1 are different in that: in the preparation process of the positive electrode slurry, the carbon-coated LFP in step S1-1 is replaced by NCM811 (Li(Ni0.6Co0.2Mn0.2)O2) without CNT, and conductive agent SP is not used (the content of positive active material and PVDF is 98.05% and 1.8% respectively), and the rest conditions are the same as example 1. 0.8 Co 0.1 Mn 0.1 )O2), and without using conductive agent SP (the content of positive active material and PVDF is 98.05% and 1.8% respectively), and the rest conditions are the same as example 1.

[0239] Comparative example 13

[0240] Comparative example 13 and example 1 are different in that: in the preparation process of the positive electrode slurry, the carbon-coated LFP in step S1-1 is replaced by NCM811 (Li(Ni0.6Co0.2Mn0.2)O2) containing CNT (the addition amount of CNT is 0.3%), and conductive agent SP is not used (the content of positive active material and PVDF is 98.05% and 1.8% respectively), and the rest conditions are the same as example 1.

[0241] Comparative example 14

[0242] Comparative example 14 and example 1 are different in that: in the preparation process of the positive electrode slurry, the carbon-coated LFP in step S1-1 is replaced by NCM811 (Li(Ni0.6Co0.2Mn0.2)O2) containing CNT (the addition amount of CNT is 0.3%, which is the mass percentage of CNT in NCM811), and the rest conditions are the same as example 1.

[0243] Comparative example 15

[0244] The difference between Comparative Example 15 and Example 1 is that in the preparation process of the positive electrode slurry, the carbon-coated LFP in step S1-1 is replaced by carbon-coated NCM811 (carbon-coated amount is 0.3%, NCM811 contains CNT, and the addition amount of CNT is 0.3%). The rest of the conditions and Example 1 remain unchanged.

[0245] Comparative Example 16

[0246] The difference between Comparative Example 16 and Example 1 is that in the preparation process of the positive electrode slurry, the carbon-coated LFP in step S1-1 is replaced by carbon-coated NCM811 (carbon-coated amount is 0.3%, NCM811 does not contain CNT). The rest of the conditions and Example 1 remain unchanged.

[0247] Effect Example 1

[0248] The positive electrode slurry prepared in Examples 1-21 and Comparative Examples 1-16 is tested for the following pH value, viscosity, solid content, and coating speed:

[0249] (1) pH value of the positive electrode slurry

[0250] The pH value of the positive electrode slurry is tested using a pH meter (manufacturer: Mettler Toledo Technology (China) Co., Ltd., model: S-400K). The test results are shown in Tables 1-3.

[0251] (2) Viscosity of the positive electrode slurry

[0252] The viscosity test is tested using a viscosity tester (manufacturer: AMETEK Brookfield, model: DV2T). The test results are shown in Tables 1-3 and Figure 1.

[0253] (3) Solid content of the positive electrode slurry

[0254] The solid content is tested using a solid content detector (manufacturer: Xiamen Lesder Scientific Instrument Co., Ltd., model: QL-720D). The test results are shown in Tables 1-3.

[0255] (4) Coating speed of the positive electrode slurry

[0256] The coating speed is the maximum coating speed on the coating machine at a temperature of 95°C, a wind frequency of 15Hz, and a coating surface density of 24.67mg / cm 2 . The test results are shown in Tables 1-3.

[0257] Effect Example 2

[0258] The positive electrode sheet prepared in Examples 1-21 and Comparative Examples 1-16 is tested for the peeling force, and the specific test conditions are as follows:

[0259] The peeling force was tested using a peeling force tester (universal material testing machine, Instron 5940).

[0260] The test electrode was cut into a long strip (length x width = 10 cm x 3 cm) and attached to a long strip of quartz glass plate (slightly larger than the electrode) with VHB tape. The electrode was repeatedly rolled with a special small roller three times to ensure that the electrode was fully adhered to the VHB. The standard method for measuring the peeling force at 180° was selected to test the peeling force of the electrode, and each sample was tested three times to obtain the average value. The test results are shown in Tables 1-3.

[0261] Example 3

[0262] Lithium ion batteries were prepared using the positive electrode prepared in Examples 1-21 and Comparative Examples 1-16. The preparation method of the lithium ion battery included the following steps:

[0263] The positive electrode, negative electrode and separator were wound to obtain a battery core, which was then packaged in a package shell and injected with electrolyte.

[0264] The preparation of the negative electrode included the following steps: the negative electrode active material (artificial graphite), conductive agent (SP), binder (PAA and SBR, mass ratio 1.3:0.5) and thickening agent carboxymethyl cellulose (CMC) were mixed in a mass ratio of 97.3:0.4:1.8:0.5 (total 100 parts by mass), then 82 parts by mass of deionized water was added and mixed uniformly to obtain a negative electrode slurry; then the negative electrode slurry was uniformly coated on the negative electrode current collector (copper foil with a thickness of 10 μm); and then the negative electrode was prepared by drying, rolling, cutting and other processes.

[0265] The separator was a polyethylene film; the thickness of the separator was 11 μm; the air permeability of the separator was 230 s / 100 mL; and the porosity of the separator was 40%.

[0266] The electrolyte was a commercial electrolyte (manufacturer: Xinyashanshan New Material Technology (Quzhou) Co., Ltd., model: E3).

[0267] The capacity retention rate of the lithium ion battery prepared above was tested, and the test procedure was as follows:

[0268] At 25°C, after 0.33C charge-discharge cycle activation for 3 cycles, 1C charging to 80% SOC, 0.33C charging 80%-100% SOC (3.65V), 1C discharging to 2.5V; the capacity of the first cycle using this process is C0, and the capacity after 500 cycles using this process is C1; the capacity retention rate is C1 / C0 x 100%. The test results are shown in Tables 1-3.

[0269] Table 1

[0270] Table 2

[0271] Table 3

[0272] Note: " / " in Tables 2-3 represents that the condition parameter is not involved in the specific experiment.

[0273] In the positive electrode slurry of embodiments 1-21 of the present application, the carbon-coated LFP is used as the positive electrode active material, and the composite dispersant of the present application is used in combination. Within the scope of the present application, whether the amount of component A or component B is changed, or the type of component A or component B is changed, or the amount of carbon coating is changed, or the type of binder in the positive electrode slurry is changed, the coating performance of the obtained positive electrode slurry is excellent, which is conducive to coating during preparation, and the adhesion performance is good. The positive electrode slurry obtained has high peeling force, and the capacity retention rate of the obtained electrochemical device (especially lithium ion battery) is excellent.

[0274] Component A (acidic dispersant) has an anchoring effect on the positive electrode active material, and the dispersion effect relies on the steric hindrance effect produced by the dissolution of the solvent chain segment in the solvent to inhibit particle agglomeration and produce dispersion effect; component B (basic dispersant) realizes anchoring with coated carbon to increase the anchoring ability of the dispersant, and at the same time, the amount of component B is controlled to reduce the influence of basic dispersant on the binder as much as possible. Among them, according to embodiments 1 and 3-5, under the condition that other conditions are the same, the slurry coating speed and the peeling force of the electrode sheet are both better when trialkyl phosphate is used as the acidic dispersant, which may be because the phosphate group in the phosphate ester dispersant is complexed with the surface of lithium iron phosphate to form complex functional groups, so that the anchoring effect is stronger.

[0275] Comparative Examples 1-4 are cases where a single component dispersant is used. In Comparative Example 1, only trialkyl phosphate (component A) is added in the first mixture for the second mixing, and the third mixing using PVP (component B) is not performed, and the coating speed of the positive electrode slurry is greatly reduced, and the coating performance is poor. In Comparative Example 2, only PVP (component B) is added in the first mixture for the second mixing, and the third mixing using trialkyl phosphate (component A) is not performed, and the coating speed of the positive electrode slurry is greatly reduced, and the coating performance is poor. Moreover, as shown in FIG. 1, compared with Comparative Examples 1-2, the positive electrode slurry in Example 1 has a lower viscosity, and the viscosity can still be maintained at a low level as the standing time is prolonged, and the viscosity stabilization effect is good. In Comparative Example 3, the second mixing and the third mixing are performed, but trialkyl phosphate is added in steps S2 and S3, and compared with Example 1, although the peeling force of the positive electrode sheet is improved, the coating speed of the positive electrode slurry is greatly reduced, and the coating performance is poor. In Comparative Example 4, the second mixing and the third mixing are performed, but PVP is added in steps S2 and S3, and compared with Example 1, the coating performance of the positive electrode slurry is obviously poor, the adhesion performance of the positive electrode slurry is too poor, even large-area cracking occurs, the coating cannot be performed, the electrode sheet cannot be obtained, and the peeling force test cannot be performed.

[0276] It can be seen that the simple acidic dispersant has limited dispersion effect on lithium iron phosphate, and the viscosity reduction and viscosity stabilization effect is general. The use of a simple basic dispersant makes the alkalinity of the positive electrode slurry stronger, and PVDF is more prone to gelation and irreversible alkaline decomposition reaction, leading to the breakage of polymer chains and the destruction of the structure, so that the adhesion performance of the positive electrode slurry cannot be guaranteed.

[0277] In Comparative Examples 5 and 6, the content of component A (acidic dispersant) in the second mixing of step S2 is relatively low, and the coating speed of the slurry is poorer than that of Example 1. In Comparative Example 7, the amount of component B (basic dispersant) in the third mixing of step S3 is relatively high, and the coating speed of the slurry is poorer than that of Example 1.

[0278] When the positive electrode slurry does not use a conductive agent, the use of the composite dispersant of the present application is also superior to the use of a single dispersant component in terms of coating performance and adhesion performance. In Comparative Example 8, the positive electrode slurry does not use a conductive agent, and the dispersant only uses a single component A, and compared with Example 19, although the peeling force of the positive electrode sheet is improved, the coating speed of the positive electrode slurry is obviously reduced, and the coating is not conducive in the preparation process. In Comparative Example 9, the positive electrode slurry does not use a conductive agent, and the dispersant only uses a single component B, and compared with Example 19, the coating speed of the positive electrode slurry and the peeling force of the positive electrode sheet are greatly reduced.

[0279] Comparative Examples 10-16 illustrate the use of NCM as the positive electrode active material. In Comparative Examples 12-13, the positive electrode active material was either CNT-free or CNT-containing NCM811, and no conductive agent was used. Although the coating speed of the positive electrode slurry was improved compared to Comparative Example 10 (using only trialkyl phosphate) or Comparative Example 11 (using only PVP), and the coating performance of the resulting positive electrode slurry was improved, it was still significantly worse than that of the positive electrode slurry obtained in Example 19 (using carbon-coated lithium iron phosphate as the positive electrode active material). In Comparative Example 14, the positive electrode slurry contained a conductive agent and used CNT-containing NCM811 as the positive electrode active material. Compared to Example 1 (using carbon-coated lithium iron phosphate as the positive electrode active material), the coating speed of the resulting positive electrode slurry was significantly reduced, and the coating performance was poor. In Comparative Examples 15-16, the cathode slurry contained a conductive agent, and carbon-coated NCM811 containing CNTs and carbon-coated NCM811 without CNTs were used as cathode active materials, respectively. Although two dispersants were used, compared with the carbon-coated lithium iron phosphate used as the cathode active material in Example 1, the coating speed of the resulting cathode slurry was significantly reduced, and the coating performance was poor. It is evident that the composite dispersant of this invention, used in conjunction with carbon-coated lithium iron phosphate cathode active materials, can simultaneously achieve excellent coating and adhesion properties.

[0280] 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 slurry, characterized by, It includes a positive electrode active material, a binder and a dispersant; wherein the dispersant includes component A and component B; the component A is an acidic dispersant; the component B is an alkaline dispersant; the mass ratio of the component A and the component B is (1-30):1; the positive electrode active material includes carbon-coated lithium iron phosphate; the binder includes polyvinylidene fluoride.

2. The positive electrode slurry of claim 1, wherein, The dispersant meets one or more of the following conditions a-e: a. The component A includes one or more of trialkyl phosphate, stearic acid phosphate, heterocyclic phosphate, catechol diphosphate, azulene phosphate, carboxylic acid polyester, maleic acid, polyacrylate, polyacrylic acid, methacrylic acid, and sorbitan monolaurate; b. The component B includes one or more of polyethyleneimine, triethanolamine, tripropylamine, polyetheramine, polyetheramine methyl ether, cetyltrimethylammonium bromide, pyrrole, polypyrrone, 4-pyrrolidone, indole, polyvinylpyrrolidone, benzamide, polyacrylonitrile, polyurethane, and cyanotertiarybutyl oxpropyl trimethoxysilane; c. The content of the component A is 0.01%-1%, the percentage being the percentage of the mass of the component A in the total mass of the components in the positive electrode slurry except the solvent; d. The content of the component B is 0.01%-1%, the percentage being the percentage of the mass of the component B in the total mass of the components in the positive electrode slurry except the solvent; e. The content of the dispersant is 0.03%-2%, the percentage being the percentage of the mass of the dispersant in the total mass of the components in the positive electrode slurry except the solvent.

3. The positive electrode slurry of claim 1, wherein the lithium transition metal phosphate is LiFePO4. The positive electrode slurry meets one or more of the following conditions a-f: a. The carbon-coated amount in the carbon-coated lithium iron phosphate is 0.05%-5%, the percentage being the percentage of the mass of carbon in the mass percentage of carbon-coated lithium iron phosphate; b. The content of the positive electrode active material is 85%-98%, the percentage being the percentage of the mass of the positive electrode active material in the total mass of the components in the positive electrode slurry except the solvent; c. The content of the binder is 1%-10%, the percentage being the percentage of the mass of the binder in the total mass of the components in the positive electrode slurry except the solvent; d. The binder further includes one or more of polytetrafluoroethylene, hexafluoropropylene, polyacrylonitrile, polyimide, perfluorosulfonic acid ionomer, sodium carboxymethyl cellulose, polyolefin, polyacrylic acid, and polymethyl methacrylate; e. The positive electrode slurry further includes a conductive agent; the content of the conductive agent is 0.2%-3%, the percentage being the percentage of the mass of the conductive agent in the total mass of the components in the positive electrode slurry except the solvent; f. The positive electrode slurry further includes a solvent; the solvent includes one or more of N-methylpyrrolidone, dimethyl carbonate, ethylene carbonate, and diethylene carbonate.

4. The positive electrode slurry of claim 1, wherein the lithium transition metal phosphate is LiFePO4. It meets one or more of the following conditions a-c: a. The solid content of the positive electrode slurry is 45%-80%; b. The viscosity of the positive electrode slurry is 3000-20000 mPa·s; c. The pH value of the positive electrode slurry is 4-10.

5. The positive electrode slurry of claim 1, wherein the lithium transition metal phosphate is LiFePO4. The preparation method of the positive electrode slurry includes the following steps: S1, mixing the positive active material, the binder and the solvent in the positive slurry to obtain a first mixture; S2, mixing the first mixture and the component A to obtain a second mixture; S3, mixing the second mixture and the component B.

6. The positive electrode slurry of claim 5, wherein the lithium transition metal phosphate is LiFePO4. The preparation method of the positive slurry satisfies one or more of the following conditions a-e: a. In step S1, the first mixing is kneading treatment; b. In step S2, the second mixing is stirring; in step S3, the third mixing is stirring; the stirring speed of the second mixing is greater than or equal to the stirring speed of the third mixing; c. In step S3, after the third mixing, an operation of adding a solvent to adjust the viscosity is further included; d. The second mixing time is less than the third mixing time; e. Before the first mixing, a pre-mixing of the positive active material and the binder in the positive slurry is further included, and the pre-mixing is stirring.

7. A positive electrode sheet comprising a positive electrode current collector and a positive electrode material layer on at least one surface of the positive electrode current collector, characterized by, The positive material layer is formed by the positive slurry of any one of claims 1-6.

8. A positive electrode sheet comprising a positive electrode current collector and a positive electrode material layer on at least one surface of the positive electrode current collector, characterized by, The positive material layer includes a positive active material, a binder and a dispersant; the dispersant includes a component A and a component B; the component A is an acidic dispersant; the component B is an alkaline dispersant; the mass ratio of the component A to the component B is (1-30):1; the positive active material includes carbon-coated lithium iron phosphate; the binder includes polyvinylidene fluoride.

9. The positive electrode sheet according to claim 8, wherein The dispersant satisfies one or both of the following conditions a-b: a. The content of the component A is 0.01%-1%, and the percentage is the percentage of the mass of the component A in the total mass of the positive material layer; b. The content of the component B is 0.01%-1%, and the percentage is the percentage of the mass of the component B in the total mass of the positive material layer.

10. An electrochemical device, characterized by, It includes the positive sheet of any one of claims 7-9.

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