A positive electrode binder, a positive electrode composition, a positive electrode sheet, and a battery

Abstract

To overcome the problem that the positive electrode binder PVDF in the prior art is unstable in a strong alkaline environment, the present invention provides a positive electrode binder, a positive electrode composition, a positive electrode sheet, and a battery. The positive electrode binder has a core-shell structure, including a core body and a shell layer disposed on the outer surface of the core body and at least partially covering the core body. The core body includes a first polymer, and the shell layer includes a second polymer. The glass transition temperature of the first polymer is lower than that of the second polymer. The second polymer includes a first structural unit containing a polar functional group, and the polar functional group is selected from at least one of a carboxyl group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, and a cyano group; the mass percentage content of the first structural unit in the second polymer is more than 20%. The positive electrode binder provided by the present invention is used in a positive electrode sheet mainly composed of lithium cobaltate or ternary positive electrode active material, and has excellent adhesion and dispersibility.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to a cathode binder, a cathode composition, a cathode sheet and a battery. Background Art

[0002] Lithium-ion batteries have advantages such as high voltage, high specific energy, stable discharge voltage, good cycle performance, good safety performance, and long working life, and are widely used in fields such as electric vehicles and energy storage.

[0003] Lithium-ion batteries usually consist of a positive electrode, a negative electrode, a separator, an electrolyte, and a battery casing. Among them, the positive electrode usually uses a metal as a substrate, and the positive electrode active material is attached to the metal surface through a binder. Currently, the commonly used cathode binder is polyvinylidene fluoride (PVDF), which has excellent performance in aspects such as electrochemical stability, adhesiveness, lithium ion migration ability, and thermal stability, and can basically meet the requirements of lithium-ion battery cathode binders. However, with the increasing requirement for the energy density of lithium-ion batteries, high-nickel ternary cathode active materials have become a development trend. The higher the nickel content in the ternary cathode active material lithium nickel cobalt manganese oxide, the stronger the basicity of the material. Since the PVDF molecular chain will remove HF molecules in a strong alkaline environment and form continuous double bonds on the molecular chain, and there may be a situation where the double bonds break and crosslink with other molecular chains, ultimately leading to gelation of the slurry, affecting normal batching, coating and subsequent processes. In addition, PVDF belongs to fluorochemical materials, with high production costs and serious environmental pollution. Therefore, developing a new fluorine-free cathode binder is an urgent problem to be solved. Summary of the Invention

[0004] The technical problem to be solved by the present invention is aimed at the problem that the existing cathode binder PVDF is unstable in a strong alkaline environment. The present invention provides a cathode binder, a cathode composition, a cathode sheet and a battery.

[0005] To solve the above technical problem, the present invention provides a cathode binder. The cathode binder has a core-shell structure, including a core body and a shell layer disposed on the outer surface of the core body and at least partially covering the core body. The core body includes a first polymer, and the shell layer includes a second polymer. The glass transition temperature of the first polymer is lower than the glass transition temperature of the second polymer. The second polymer includes a first structural unit containing a polar functional group, and the polar functional group is selected from at least one of carboxyl, sulfonic acid group, phosphoric acid group, hydroxyl group, and cyano group; the mass percentage content of the first structural unit in the second polymer is more than 20%.

[0006] Preferably, the second polymer further includes a second structural unit, and the second structural unit includes at least one of acrylate structural units, aromatic vinyl structural units, and conjugated diene structural units.

[0007] Preferably, the second polymer further includes a crosslinked structural unit. In the second polymer, the mass percentage content of the crosslinked structural unit is 0.1% - 2%.

[0008] Preferably, the crosslinked structural unit includes one or more of acrylate crosslinked structural units, acrylamide crosslinked structural units, and allyl crosslinked structural units.

[0009] Preferably, the first polymer includes at least one of acrylate copolymers, polyurethanes, butyl rubbers, styrene-butadiene rubbers, hydrogenated styrene-butadiene rubbers, nitrile rubbers, hydrogenated nitrile rubbers, and ethylene-propylene rubbers.

[0010] Preferably, the mass ratio of the first polymer to the second polymer is (20:80) - (80:20).

[0011] Preferably, the mass swelling degree of the positive electrode binder electrolyte is less than 100%;

[0012] And / or, the glass transition temperature of the first polymer is -60°C to 10°C, and the glass transition temperature of the second polymer is above 50°C.

[0013] In a second aspect, the present application provides a positive electrode composition, including a positive electrode active material, a conductive agent, and the above-mentioned positive electrode binder.

[0014] Preferably, the positive electrode active material includes at least one of lithium cobaltate, nickel cobalt aluminum ternary positive electrode active materials, and nickel cobalt manganese ternary positive electrode active materials.

[0015] In a third aspect, the present application provides a positive electrode sheet, including a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the above-mentioned positive electrode composition.

[0016] In a fourth aspect, the present application provides a lithium-ion battery, including the above-mentioned positive electrode sheet.

[0017] The cathode binder provided by this application has a core-shell structure, with excellent adhesion and good flexibility. The glass transition temperature of the first polymer is lower than that of the second polymer, so that the prepared cathode electrode sheet has good flexibility and adhesion by introducing the first polymer. When the glass transition temperature of the first polymer is controlled between -60°C and 10°C, the cathode electrode sheet prepared with this binder has better flexibility and cohesive strength. When the glass transition temperature is lower than -60°C, the cohesive strength is relatively low. When the glass transition temperature is higher than 10°C, the flexibility of the cathode electrode sheet is poor. The surfaces of lithium cobaltate and ternary cathode active materials have many metal elements and oxygen elements. By introducing more than 20% of the first structural units with polar functional groups into the second polymer, the polar functional groups can produce effective polar interactions and charge adsorption effects with the metal elements in the cathode active material, and hydrogen bond interactions with the oxygen elements in the cathode active material, so that the cathode binder and the cathode active material are firmly adsorbed to produce excellent adhesion and dispersibility. At the same time, the cathode binder in this application does not contain fluorine elements, avoiding the generation of HF in an alkaline environment, so that the cathode binder can stably exist in the cathode slurry, ensuring good bonding strength between the cathode active material layer and the cathode current collector. Detailed Embodiments

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

[0019] An embodiment of this application provides a cathode binder. The cathode binder has a core-shell structure and includes a core body and a shell layer disposed on the outer surface of the core body and at least partially covering the core body. The core body includes a first polymer, and the shell layer includes a second polymer. The glass transition temperature of the first polymer is lower than that of the second polymer. The second polymer includes a first structural unit containing a polar functional group, and the polar functional group is selected from at least one of carboxyl group, sulfonic acid group, phosphoric acid group, hydroxyl group, and cyano group; the mass percentage content of the first structural unit in the second polymer is more than 20%.

[0020] The cathode binder provided by this application has a core-shell structure, with excellent adhesion and good flexibility. The glass transition temperature of the first polymer is lower than that of the second polymer, enabling the cathode binder to endow the prepared cathode sheet with good flexibility and adhesion by introducing the first polymer. When the glass transition temperature of the first polymer is controlled between -60°C and 10°C, the cathode sheet prepared with this binder has better flexibility and cohesive strength. When the glass transition temperature is lower than -60°C, the cohesive strength is relatively low. When the glass transition temperature is higher than 10°C, the flexibility of the cathode sheet is poor. The surfaces of lithium cobaltate and ternary cathode active materials have a relatively large number of metal elements and oxygen elements. By introducing more than 20% of the first structural units with polar functional groups, the second polymer enables the polar functional groups to have effective polar interactions and charge adsorption with the metal elements in the cathode active material, as well as hydrogen bonding with the oxygen elements in the cathode active material, so that the cathode binder can be firmly adsorbed to the cathode active material, generating excellent adhesion and dispersibility. At the same time, the cathode binder in this application does not contain fluorine elements, avoiding the generation of HF in an alkaline environment, enabling the cathode binder to stably exist in the cathode slurry, and ensuring good bonding strength between the cathode active material layer and the cathode current collector.

[0021] In some embodiments, the first structural unit is formed by the polymerization of a first monomer;

[0022] When the polar functional group is a carboxyl group, the first monomer is selected from at least one of monocarboxylic acids, C3-C30 unsaturated dicarboxylic acids, C3-C30 unsaturated dicarboxylic anhydrides, and monoalkyl esters of C3-C30 unsaturated dicarboxylic acids.

[0023] Specifically, the monocarboxylic acids include, but are not limited to, acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, β-carboxyethyl acrylate, and cinnamic acid.

[0024] The C3-C30 unsaturated dicarboxylic acids include, but are not limited to, maleic acid, fumaric acid, itaconic acid, citraconic acid, and mesaconic acid.

[0025] The C3-C30 unsaturated dicarboxylic anhydrides include, but are not limited to, maleic anhydride and citraconic anhydride.

[0026] In the monoalkyl esters of C3-C30 unsaturated dicarboxylic acids, the number of carbon atoms in the alkyl group is 1-24, and the monoalkyl esters of C3-C30 unsaturated dicarboxylic acids include, but are not limited to, monomethyl maleate, monooctadecyl maleate, monoethyl fumarate, monobutyl itaconate, ethylene glycol monoether itaconate, and monoeicosyl citraconate.

[0027] In some embodiments, when the polar functional group is a sulfonic acid group, the first monomer is selected from at least one of olefin sulfonic acids having 2 to 14 carbon atoms, styrene sulfonic acid, alkyl-substituted styrene sulfonic acids having 7 to 24 carbon atoms, sulfopropyl acrylates having 5 to 18 carbon atoms, sulfopropyl acrylamides having 5 to 18 carbon atoms, and alkyl-substituted allyl sulfosuccinic acids having 3 to 18 carbon atoms.

[0028] Specifically, the olefin sulfonic acids having 2 to 14 carbon atoms include, but are not limited to, vinyl sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, methyl vinyl sulfonic acid, and sodium allyloxyhydroxypropyl sulfonate.

[0029] The alkyl-substituted styrene sulfonic acids having 7 to 24 carbon atoms include, but are not limited to, α-methylstyrene sulfonic acid.

[0030] The sulfopropyl acrylates having 5 to 18 carbon atoms include, but are not limited to, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate, 2-ethylsulfonate acrylate, and 2-ethylsulfonate methacrylate.

[0031] The sulfopropyl acrylamides having 5 to 18 carbon atoms include, but are not limited to, acrylamido-2-methylpropane sulfonic acid, acrylamido-2-hydroxypropane sulfonic acid, methacrylamido-2-methylpropane sulfonic acid, and methacrylamido-2-hydroxypropane sulfonic acid. [[ID=Y]]

[0032] The alkyl-substituted allyl sulfosuccinic acids having 3 to 18 carbon atoms include, but are not limited to, propyl allyl sulfosuccinic acid, butyl allyl sulfosuccinic acid, and 2-ethylhexyl allyl sulfosuccinic acid.

[0033] In some embodiments, when the polar functional group is a phosphoric acid group, the first monomer is selected from at least one of acryloyloxyalkyl phosphate monoester compounds and allyl phosphate compounds.

[0034] Specifically, in the acryloyloxyalkyl phosphate monoester compounds, the number of carbon atoms of the monoester group is 1 to 24, and the acryloyloxyalkyl phosphate monoester compounds include, but are not limited to, acryloyloxyethyl phosphate, methacryloyloxyethyl phosphate, 2-hydroxyethyl acryloyl phosphate, 2-hydroxyethyl methacryloyl phosphate, phenyl-2-acryloyloxyethyl phosphate, and bis[2-(methacryloyloxy)ethyl] phosphate.

[0035] The allyl phosphate compounds include, but are not limited to, allyl phosphate containing phosphoric acid.

[0036] In some embodiments, when the polar functional group is a hydroxyl group, the first monomer is selected from at least one of hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, and hydroxybutyl methacrylate.

[0037] In some embodiments, when the polar functional group is a cyano group, the first monomer is selected from at least one of acrylonitrile, methacrylonitrile, α - cyanostyrene, N - cyanomethylacrylamide, and cyano vinyl ether.

[0038] In a further preferred case, the first structural unit is acrylonitrile.

[0039] It can be understood that when the first structural unit includes a plurality of different structural units, the mass percentage content of the first structural unit in the second polymer is the total amount of the plurality of different structural units.

[0040] In the present invention, preferably, the mass percentage content of the first structural unit in the second polymer is 20% - 98%, and more preferably 30% - 90%.

[0041] In some embodiments, the second polymer further includes a second structural unit, and the second structural unit includes at least one of an acrylate structural unit, an aromatic vinyl structural unit, a conjugated diene structural unit, and a nitrile structural unit.

[0042] The above - mentioned acrylate structural unit is obtained by polymerizing acrylate monomers, and the acrylate monomers include, but are not limited to, methyl acrylate, ethyl acrylate, n - propyl acrylate, isopropyl acrylate, n - butyl acrylate, isobutyl acrylate, tert - butyl acrylate, n - pentyl acrylate, isopentyl acrylate, n - hexyl acrylate, n - octyl acrylate, isooctyl acrylate, isobornyl acrylate, phenoxyethyl acrylate, dicyclopentenyl acrylate, cyclohexyl acrylate, benzyl acrylate, methyl methacrylate, ethyl methacrylate, n - propyl methacrylate, isopropyl methacrylate, n - butyl methacrylate, isobutyl methacrylate, tert - butyl methacrylate, n - pentyl methacrylate, isopentyl methacrylate, n - hexyl methacrylate, isooctyl methacrylate, isobornyl methacrylate, phenoxyethyl methacrylate, dicyclopentenyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, polyethylene glycol mono[methacrylate] ester, 2 - (dimethylamino)ethyl methacrylate, 2 - (diethylamino)ethyl methacrylate, 2 - (tert - butylamino)ethyl methacrylate, glycidyl methacrylate, tetrahydrofurfuryl methacrylate, ethoxylated ethylene glycol diacrylate, ethoxylated ethylene glycol dimethacrylate, allyl methacrylate, diallyl phthalate, pentaerythritol diacrylate, and pentaerythritol dimethacrylate.

[0043] The aromatic vinyl-based structural unit is obtained by polymerizing aromatic vinyl hydrocarbon compounds, which include but are not limited to styrene and substituted styrenes. The substituted styrenes include but are not limited to α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, crotylstyrene, divinylbenzene, divinyltoluene, divinyldimethylbenzene, trivinylbenzene, vinylnaphthalene, and p-tert-butylstyrene.

[0044] The above-mentioned conjugated diene-based structural unit is obtained by polymerizing conjugated diene monomers, which include but are not limited to butadiene, pentadiene, and isoprene.

[0045] In the present invention, preferably, the second polymer further includes a crosslinked structural unit. In the second polymer, the mass percentage content of the crosslinked structural unit is 0.1% - 2%, more preferably 0.5% - 1.5%.

[0046] Specifically, the crosslinked structural unit includes one or more of acrylate crosslinked structural units, acrylamide crosslinked structural units, and allyl crosslinked structural units. The acrylate crosslinked structural unit is selected from structural monomers including multiple acrylate structures. The crosslinked structural unit is selected from one or more of ethylene glycol diacrylate structural unit, polyethylene glycol diacrylate structural unit, propylene glycol diacrylate structural unit, dipropylene glycol diacrylate structural unit, dipropylene glycol triacrylate structural unit, trimethylolpropane triacrylate structural unit, glycerol triacrylate structural unit, pentaerythritol triacrylate structural unit, ethylene glycol dimethacrylate structural unit, polyethylene glycol dimethacrylate structural unit, propylene glycol dimethacrylate structural unit, dipropylene glycol dimethacrylate structural unit, dipropylene glycol trimethacrylate structural unit, trimethylolpropane trimethacrylate structural unit, glycerol trimethacrylate structural unit, pentaerythritol trimethacrylate structural unit, divinylbenzene structural unit, diallyl phthalate structural unit, glycerol diallyl ether structural unit, pentaerythritol diallyl ether structural unit, pentaerythritol triallyl ether structural unit, diallyl adipate structural unit, N,N-methylenebisacrylamide structural unit, allyl acrylate ether structural unit, and allyl methacrylate ether structural unit. More preferably, it is at least one of N,N-methylenebisacrylamide structural unit, polyethylene glycol diacrylate structural unit, and polyethylene glycol dimethacrylate structural unit.

[0047] When the second polymer further includes the above crosslinked structural unit, after the above positive electrode binder is used to prepare a positive electrode paste, the positive electrode binder can more stably maintain its structure and morphology, which is beneficial to further improving the bonding strength.

[0048] It can be understood that for the above second polymer, in addition to the first structural unit and the optionally present crosslinked structural unit, the rest are second structural units.

[0049] In some embodiments, the first polymer includes at least one of acrylate copolymers, polyurethanes, butyl rubbers, styrene-butadiene rubbers, hydrogenated styrene-butadiene rubbers, nitrile rubbers, hydrogenated nitrile rubbers, and ethylene-propylene rubbers.

[0050] Among them, the acrylate copolymer in the first polymer is formed by polymerizing at least one acrylate monomer or methacrylate monomer with other monomers, and the other monomers can be at least one of methacrylate monomers, acrylate monomers, vinyl monomers, acrylamide monomers, and allyl monomers.

[0051] Acrylate monomers include but are not limited to methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-pentyl acrylate, isopentyl acrylate, n-hexyl acrylate, n-octyl acrylate, isooctyl acrylate, isobornyl acrylate, phenoxyethyl acrylate, dicyclopentenyl acrylate, cyclohexyl acrylate, benzyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, carboxyethyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, n-hexyl methacrylate, isooctyl methacrylate, isobornyl methacrylate, phenoxyethyl methacrylate, dicyclopentenyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, polyethylene glycol mono-[methacrylic acid] ester, aminoethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, glycidyl methacrylate, tetrahydrofurfuryl methacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, ethoxylated ethylene glycol diacrylate, ethoxylated ethylene glycol dimethacrylate, allyl methacrylate, diallyl phthalate, diallyl adipate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol diacrylate, pentaerythritol dimethacrylate, pentaerythritol triacrylate, pentaerythritol trimethacrylate.

[0052] In some embodiments, the polyurethane is a polymer obtained by reacting a diisocyanate with a polymer diol or a polymer diamine, or a chain extender is further included in the reaction raw materials of the polyurethane.

[0053] The diisocyanate includes but is not limited to at least one of toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, and dicyclohexylmethane diisocyanate.

[0054] The polymer diol includes but is not limited to polyether diol, polyester diol, and polyolefin diol.

[0055] The polyether diol includes but is not limited to polytetrahydrofuran ether diol, polypropylene oxide ether diol, polyethylene oxide ether diol, and polypropylene oxide / polyethylene oxide ether diol.

[0056] The polyester diol includes but is not limited to polycarbonate diol, polycaprolactone diol, neopentyl glycol adipate diol, hexanediol adipate diol, and butanediol adipate diol.

[0057] The polyolefin diol includes but is not limited to polybutadiene diol, hydrogenated polybutadiene diol, and polyisobutylene diol.

[0058] The chain extender includes but is not limited to dimethylolpropane, dimethylolbutane, ethylene glycol, 1,4-butanediol, 1,3-propanediol, neopentyl glycol, hexanediol, ethylenediamine, hexamethylenediamine, and isophoronediamine.

[0059] In some embodiments, the mass ratio of the first polymer to the second polymer is (20:80) to (80:20). Calculated based on 100 parts by weight of the positive electrode binder, when the proportion of the first polymer is less than 20 parts, the flexibility effect of the electrode sheet is poor and the binding force is insufficient. When the proportion of the first polymer is higher than 80 parts, the dispersibility of the positive electrode binder to the positive electrode active material is poor.

[0060] Specifically, the mass ratio of the first polymer to the second polymer includes but is not limited to 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, or 80:20.

[0061] In some embodiments, the glass transition temperature Tg of the first polymer is -60 to 10 °C, and the glass transition temperature Tg of the second polymer is greater than or equal to 50 °C. The glass transition temperature of the second polymer is higher than 50 °C, making it easier to form the polymer into powder and facilitating transportation. When the glass transition temperature of the first polymer is within the above range, the positive electrode binder has flexibility and cohesive strength, further making the prepared positive electrode sheet have good flexibility and binding force.

[0062] In some preferred embodiments, the first polymer is selected from at least one of methacrylate copolymers, acrylate copolymers, and polyurethanes.

[0063] In the present invention, each of the foregoing structural units represents the structural part corresponding to the monomer in the resulting polymer after the monomer participates in the polymerization reaction. Among them, the mass content of each structural unit is based on the mass content of the corresponding monomer in the total amount of monomers participating in the polymerization.

[0064] In some embodiments, the mass swelling ratio of the positive electrode binder electrolyte is less than 100%. When the mass swelling ratio of the positive electrode binder electrolyte is higher than 100%, the battery is prone to swelling during use, resulting in poor contact between the positive electrode active material and the positive electrode current collector in the positive electrode sheet, increasing the battery impedance and degrading the battery performance.

[0065] Furthermore, an embodiment of the present application provides a method for preparing a positive electrode binder, including the following steps:

[0066] (1) Mix the monomers of the core body, add additives, and stir under heating to obtain a first polymer;

[0067] (2) After uniformly mixing the monomers of the shell layer, add the first polymer and stir under heating to obtain Reactant 2.

[0068] (3) Prepare the positive electrode binder powder by spray drying Reactant 2.

[0069] As is known to those skilled in the art, the reactions of the above steps are conventional radical polymerizations, etc. The specific methods and reaction conditions are the commonly used radical polymerization methods in the prior art and will not be elaborated in the present invention.

[0070] If the polymer composition contains a polyurethane structure, the preparation method of polyurethane well-known in the art is adopted.

[0071] In the present application, the mass fractions of the monomers of the core body and the monomers of the shell layer are not limited, as long as the glass transition temperature of the finally formed first polymer is -60 to 10 °C and the glass transition temperature of the second polymer is greater than or equal to 50 °C.

[0072] The additives include one or more of sodium persulfate, ammonium persulfate, potassium persulfate, tert-butyl hydroperoxide, azobisisobutyronitrile, benzoyl peroxide / sucrose, tert-butyl hydroperoxide / rongalite, tert-butyl hydroperoxide / sodium metabisulfite, benzoyl peroxide / N,N-dimethylaniline, ammonium persulfate / sodium bisulfite, potassium persulfate / sodium bisulfite, hydrogen peroxide / tartaric acid, hydrogen peroxide / rongalite, ammonium persulfate / ferrous sulfate, hydrogen peroxide / ferrous sulfate, benzoyl peroxide / N,N-diethylaniline, benzoyl peroxide / ferrous pyrophosphate, potassium persulfate / silver nitrate, persulfate / thiol, cumene hydroperoxide / ferrous chloride, potassium persulfate / ferrous chloride, hydrogen peroxide / ferrous chloride, cumene hydroperoxide / tetraethylenimine.

[0073] An embodiment of the present application further provides a positive electrode composition, which includes a positive electrode active material, a conductive agent, and the positive electrode binder described above.

[0074] In some embodiments, the positive electrode active material includes at least one of lithium cobaltate, nickel cobalt aluminum ternary positive electrode active material, and nickel cobalt manganese ternary positive electrode active material (such as NCM523, NCM622, NCM822). By selecting the above positive electrode active material and the above positive electrode binder for use, the polar functional groups in the positive electrode binder have polar interactions and charge adsorption effects with the metal elements in the positive electrode active material, and have hydrogen bond effects with the oxygen elements in the positive electrode active material, so that the positive binder and the positive electrode active material are firmly adsorbed to each other, generating excellent adhesion and dispersibility.

[0075] In some embodiments, the conductive agent includes at least one of, but is not limited to, conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, carbon fibers, graphene, and conductive polymers.

[0076] An embodiment of the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes the positive electrode composition described above. The positive electrode sheet provided by the present application contains the above positive electrode composition, and the positive electrode binder with the above core-shell structure contained in the positive electrode composition improves the adhesion between the positive electrode active material and the positive electrode current collector and improves the peel strength of the positive electrode sheet.

[0077] An embodiment of the present application provides a lithium-ion battery, which includes the positive electrode sheet described above. The lithium-ion battery provided by the present application uses a positive electrode sheet containing the above binder to avoid the shedding of the positive electrode active material in the positive electrode sheet and improve the electrical performance of the battery.

[0078] The present invention will be further described below through examples.

[0079] Specifically describe the positive electrode binder, positive electrode composition, negative electrode sheet and lithium-ion battery disclosed by the present invention.

[0080] Example 1

[0081] 1) Preparation of positive electrode binder

[0082] The comonomers of the first polymer include the following components: 70 parts of butyl acrylate, 10 parts of N,N-dimethylacrylamide.

[0083] The second polymer includes 120 parts of acrylonitrile copolymer, and the comonomers of the acrylonitrile copolymer include the following components: 110 parts of acrylonitrile, 5 parts of isooctyl acrylate and 5 parts of styrene.

[0084] Prepare the positive electrode binder, including the following steps: (1) Stir and emulsify the monomers of the core with deionized water and emulsifier, add ammonium persulfate, and stir and polymerize under heating to obtain the first polymer;

[0085] (2) Mix the monomers of the shell layer evenly and add them dropwise to the first polymer, and continue to stir and polymerize under heating to obtain reactant 2.

[0086] (3) Obtain the positive electrode binder powder by spray drying reactant 2.

[0087] In the positive electrode binder, the mass ratio of the first polymer to the second polymer is 40:60; the glass transition temperature of the first polymer is -45°C, and the glass transition temperature of the shell layer is 85°C; the electrolyte mass swelling rate of the positive electrode binder is 45%.

[0088] Specifically, the test method for the electrolyte mass swelling rate is as follows: Under standard atmospheric pressure and at 25°C, add 1M lithium hexafluorophosphate to a mixed solution of ethylene carbonate and ethyl methyl carbonate with a mass ratio of 1:1 as the electrolyte. Take two containers and add the electrolyte. Make the positive electrode binder into a film and dry it and weigh it to get Winitial. The film specification is a circular sheet with a diameter of 20 mm and a thickness of 20 μm. Immerse the positive electrode binder film in the electrolyte, take it out after 72 h, wipe the electrolyte on the surface dry and weigh it to get Wequilibrium. The mass swelling rate = (Wequilibrium - Winitial) / Winitial * 100%.

[0089] 2) Preparation of the negative electrode sheet:

[0090] Mix 1.5% styrene-butadiene rubber latex (SBR), 96% of the negative electrode active material graphite, 1% of the negative electrode conductive agent conductive carbon black, and 1.5% of the thickener sodium carboxymethyl cellulose (CMC), and then add deionized water and stir to make a negative electrode composition. Then coat the negative electrode composition on both surfaces of the negative electrode current collector Cu foil, and obtain the negative electrode sheet after processes such as drying, cold pressing, and slitting.

[0091] Among them, the particle size of the graphite as the negative electrode active material is 20 μm.

[0092] 3) Preparation of the positive electrode sheet:

[0093] 97.8% of the positive electrode active material lithium cobaltate, 1% of the positive electrode conductive agent conductive carbon black, and 1.2% of the positive electrode binder are added to N-methylpyrrolidone and stirred to prepare a positive electrode paste. Then, the positive electrode paste is coated on both surfaces of the positive electrode current collector Al foil, and after processes such as drying, cold pressing, and slitting, the positive electrode sheet is obtained.

[0094] 4) Preparation of the lithium-ion battery:

[0095] The positive electrode sheet, the separator (PE film), and the negative electrode sheet are stacked in sequence, with the separator placed in the middle between the positive electrode and the negative electrode to play a role in isolation, forming an electrode assembly. The electrode assembly is placed in an outer package, a commercially available electrolyte is injected and sealed, and then after processes such as liquid injection, formation, and exhaust, the lithium-ion battery is obtained.

[0096] Examples 2 to 11

[0097] Examples 2 to 11 and Example 1 have most of the steps in common. The difference lies in that different shell monomers are added to the positive electrode binder, as shown in Table 1 specifically.

[0098] Table 1

[0099]

[0100] Example 12

[0101] Preparation of the positive electrode binder

[0102] (1) The monomer of the first polymer is added to an acetone solvent. After heating and reacting, an aqueous NaOH solution is added dropwise while stirring vigorously to obtain a dispersion. The acetone solvent in the dispersion is removed by vacuum to obtain a first polymer dispersion.

[0103] (2) An auxiliary agent is added to the first polymer dispersion and heated. Then, the shell monomers are mixed evenly and added dropwise to the first polymer dispersion, and stirring polymerization is continued under heating to obtain Reactant 2.

[0104] (3) Reactant 2 is made into a positive electrode binder powder by spray drying.

[0105] Examples 12 to 15

[0106] Examples 12 to 15 and Example 1 have most of the steps in common. The difference lies in that the components of the core monomers added to the positive electrode binder are different, as shown in Table 2 specifically.

[0107] Table 2

[0108]

[0109] Example 18

[0110] Most of the steps of Example 18 are the same as those of Example 1, except that the mass ratio of the first polymer to the second polymer is 20:80.

[0111] Example 19

[0112] Most of the steps of Example 19 are the same as those of Example 1, except that the mass ratio of the first polymer to the second polymer is 80:20.

[0113] Example 20

[0114] Most of the steps of Example 20 are the same as those of Example 1, except that the mass ratio of the first polymer to the second polymer is 10:90.

[0115] Example 21

[0116] Most of the steps of Example 21 are the same as those of Example 1, except that the mass ratio of the first polymer to the second polymer is 90:10.

[0117] Example 22

[0118] Most of the steps of Example 22 are the same as those of Example 1, except that the positive electrode active material is NCM622.

[0119] Example 23

[0120] Most of the steps of Example 23 are the same as those of Example 1, except that the positive electrode active material is lithium iron phosphate.

[0121] Comparative Example 1

[0122] Most of the steps of Comparative Example 1 are the same as those of Example 1, except that the components of the shell monomer added to the positive electrode binder are different, as shown in Table 1 for details.

[0123] Comparative Example 2

[0124] Most of the steps of Comparative Example 2 are the same as those of Example 1, except that the positive electrode binder does not have a core-shell structure, and the specific preparation method is as follows:

[0125] (1) Mix the monomers of the core body, add the auxiliary agent, and stir under heating to obtain the first polymer;

[0126] (2) After mixing the monomers of the shell layer evenly, add potassium persulfate, and stir under heating to obtain the second polymer;

[0127] (3) Mix the first polymer and the second polymer and obtain the positive electrode binder by spray drying.

[0128] Comparative Example 3

[0129] Most steps of Comparative Example 3 are the same as those of Example 1, except that the existing PVDF is used as the positive electrode binder in Comparative Example 3.

[0130] Comparative Example 4

[0131] Most steps of Comparative Example 4 are the same as those of Example 3, except that in the comonomer of the second polymer of the positive electrode binder in Comparative Example 4, the content of methacrylic acid is adjusted to 10 parts, the content of isooctyl acrylate is adjusted to 20 parts, and the addition amount of styrene is adjusted to 90 parts.

[0132] Electrical property test:

[0133] Perform the following tests on the positive electrode sheets and lithium-ion batteries prepared in the above examples and comparative examples.

[0134] 1) Test for the peel strength and cohesive strength of the positive electrode sheet:

[0135] After the positive electrode sheet is compacted, use a tensile machine to measure the peel strength of the positive electrode sheet, and the test method refers to GB2792-2014.

[0136] Peel strength: Fix the positive electrode coating on a stainless steel plate by the method described in the national standard, and use a tape to peel the Al foil at 180 °C to obtain the peel strength.

[0137] Cohesive strength: Fix the Al foil on a stainless steel plate by the method described in the national standard, and use a tape to peel the positive electrode coating at 180 °C to obtain the cohesive strength.

[0138] 2) Room temperature cycle test

[0139] Lithium cobalt oxide battery: After leaving the battery in a constant temperature test chamber at 25 °C ± 2 °C for 1 h, charge it at a constant current and constant voltage of 1C to 4.45 V, with a cut-off current of 0.05C; discharge it at a constant current of 1C to 3V, and record the discharge capacity; repeat the above steps 500 times, and calculate the capacity retention rate.

[0140] NCM622 battery: After leaving the battery in a constant temperature test chamber at 25 °C ± 2 °C for 1 h, charge it at a constant current and constant voltage of 1C to 4.2 V, with a cut-off current of 0.05C; discharge it at a constant current of 1C to 3V, and record the discharge capacity; repeat the above steps 500 times, and calculate the capacity retention rate.

[0141] Lithium iron phosphate battery: After leaving the battery to stand for 1 h at 25°C ± 2°C in a constant-temperature test chamber, charge it at a constant current and constant voltage of 1C to 3.65V, with a cut-off current of 0.05C; discharge it at a constant current of 1C to 2V, and record the discharge capacity; repeat the above steps 500 times and calculate the capacity retention rate.

[0142] 3) Flexibility test of the positive electrode sheet

[0143] The specific method is as follows: After winding with different-diameter winding needles, observe the winding needle diameter corresponding to cracks and powder shedding on the surface of the electrode sheet. The larger the winding needle diameter, the worse the flexibility, in units of mm.

[0144] Fill the test results obtained from the examples and comparative examples into Table 3.

[0145] Table 3

[0146]

[0147]

[0148] It can be seen from this that when the glass transition temperature of the first polymer is lower than -60°C, the cohesive strength of the electrode sheet will decrease significantly. When the glass transition temperature of the first polymer is higher than 10°C, the softness of the electrode sheet will decrease significantly. When the glass transition temperature of the second polymer is lower than 50°C, it is difficult to prepare the binder powder by spray drying the prepared binder liquid, and the peel strength of the electrode sheet will also decrease significantly. When the swelling of the second polymer is higher than 100%, the volume change of the prepared battery during the cycle is too large, resulting in poor contact between the positive active particles, and the cycle performance of the battery decreases. When the ratio of the first polymer to the second polymer is lower than 20:80, the peel strength of the electrode sheet decreases somewhat, and the softness of the electrode sheet decreases significantly. When the ratio of the first polymer to the second polymer is higher than 20:80, the peel strength and cycle performance of the electrode sheet decrease significantly. When the positive active material is lithium iron phosphate, due to the weak interaction between the binder structure of this patent and lithium iron phosphate, the peel strength of the electrode sheet is low, and the cycle performance of the battery also decreases significantly. This may be because part of the active material falls off during the cycle of the battery. Therefore, when the binder composition is within the range described in the claims, the positive electrode sheet prepared with the prepared binder has good peel strength, cohesive strength, and softness at the same time, and the prepared battery also has good cycle performance.

[0149] When the second polymer contains a crosslinked structural unit, it is very beneficial to further improve the peel strength and cohesive strength of the positive electrode sheet.

[0150] If the content of the first structural unit in the second polymer is too low, the peel strength and cohesive strength of the high positive electrode sheet will decrease significantly.

[0151] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A cathode binder, characterized in that, The positive electrode binder has a core-shell structure, including a core body and a shell layer disposed on the outer surface of the core body and at least partially covering the core body. The core body includes a first polymer, and the shell layer includes a second polymer. The glass transition temperature of the first polymer is lower than that of the second polymer. The glass transition temperature of the first polymer is -60 to 10 °C, and the glass transition temperature of the second polymer is above 50 °C; The first polymer includes at least one of acrylate copolymers, polyurethanes, styrene-butadiene rubbers, hydrogenated styrene-butadiene rubbers, nitrile rubbers, and hydrogenated nitrile rubbers; The second polymer includes a first structural unit containing a polar functional group, and the polar functional group is selected from at least one of carboxyl group, sulfonic acid group, phosphoric acid group, hydroxyl group, and cyano group; the mass percentage content of the first structural unit in the second polymer is above 20%; the second polymer further includes a second structural unit, and the second structural unit includes at least one of acrylate structural units, aromatic vinyl structural units, and conjugated diene structural units; The second polymer further includes a crosslinked structural unit, and in the second polymer, the mass percentage content of the crosslinked structural unit is ​ 2. The cathode binder according to claim 1, wherein ​ 3. The positive electrode binder according to claim 1, wherein ​ 4. A positive electrode composition, characterized in that, ​ 5. A positive electrode sheet, characterized in that, ​ 6. A lithium-ion battery, characterized in that, ​

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