Positive electrode binder, positive electrode composition, positive electrode plate and battery

Abstract

The invention provides a positive electrode binder, a positive electrode composition, a positive plate and a battery in order to solve the problem that a positive electrode binder PVDF is unstable in a strong alkali environment in the prior art. The positive electrode binder is of a core-shell structure and comprises a core body and a shell layer which is arranged on the outer surface of the core body and at least partially wraps the core body, the core body comprises a first polymer, the shell layer comprises a second polymer, and the glass transition temperature of the first polymer is lower than that of the second polymer. The second polymer comprises 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, phosphate group, hydroxyl and cyano; the mass percentage content of the first structural unit in the second polymer is more than 20%. The positive electrode binder provided by the invention is used in a positive electrode sheet using lithium cobalt oxide or a ternary positive electrode active material as a main material, and has excellent binding power and dispersibility.

Description

Technical Field

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

[0002] Lithium-ion batteries have the advantages of high voltage, high specific energy, stable discharge voltage, good cycle performance, good safety performance, and long service life. They are widely used in power vehicles, energy storage and other fields.

[0003] Lithium-ion batteries are usually composed of a positive electrode, a negative electrode, a separator, an electrolyte, and a battery shell. The positive electrode is usually made of a metal substrate, and the positive electrode active material is attached to the metal surface by a binder. The commonly used positive electrode binder is polyvinylidene fluoride (PVDF). PVDF has excellent performance in electrochemical stability, adhesion, lithium ion migration ability and thermal stability, and can basically meet the needs of positive electrode binders for lithium-ion batteries. However, with the increasing requirements for the energy density of lithium-ion batteries, high-nickel ternary positive electrode active materials have become a development trend. The higher the nickel content in the ternary positive electrode active material lithium nickel cobalt manganese oxygen, the stronger the alkalinity of the material. Because the PVDF molecular chain will remove the HF molecule in a strong alkaline environment and form a continuous double bond on the molecular chain, and there may be a situation where the double bond breaks and cross-links with other molecular chains, the slurry is prone to gelation, which affects the normal batching, coating and subsequent processes. In addition, PVDF is a fluorine chemical material with high production costs and serious environmental pollution. Therefore, the development of new fluorine-free positive electrode binders is an urgent problem to be solved. Summary of the invention

[0004] The technical problem to be solved by the present invention is to solve the problem that the existing positive electrode binder PVDF 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.

[0005] In order to solve the above technical problems, the present invention provides a positive electrode binder, which is a core-shell structure, including a core body and a shell layer arranged on the outer surface of the core body and at least partially covering the core body, the core body includes a first polymer, 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 a carboxyl group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, and a cyano group; the mass percentage 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 an acrylate structural unit, an aromatic vinyl structural unit, and a conjugated diene structural unit.

[0007] Preferably, the second polymer further comprises a cross-linked structural unit, and in the second polymer, the mass percentage of the cross-linked structural unit is 0.1%-2%.

[0008] Preferably, the cross-linked structural unit includes one or more of an acrylate cross-linked structural unit, an acrylamide cross-linked structural unit, and an allyl cross-linked structural unit.

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

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

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

[0012] In a second aspect, the present application provides a positive electrode composition, comprising a positive electrode active material, a conductive agent and the positive electrode binder described above.

[0013] Preferably, the positive electrode active material includes at least one of lithium cobalt oxide, nickel-cobalt-aluminum ternary positive electrode active material, and nickel-cobalt-manganese ternary positive electrode active material.

[0014] In a third aspect, the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one side surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode composition as described above.

[0015] In a fourth aspect, the present application provides a lithium-ion battery, comprising the positive electrode sheet as described above.

[0016] The positive electrode binder provided in the present application is a core-shell structure with excellent bonding strength and good softness. The glass transition temperature of the first polymer is lower than the glass transition temperature of the second polymer, so that the positive electrode binder introduces the first polymer to make the prepared positive electrode sheet have good softness and bonding strength. When the glass transition temperature of the first polymer is controlled between -60°C and 10°C, the positive electrode sheet prepared by the binder has good softness and cohesive strength. When the glass transition temperature is lower than -60°C, the cohesive strength is low. When the glass transition temperature is higher than 10°C, the softness of the positive electrode sheet is poor. Lithium cobalt oxide and ternary positive active materials have more metal elements and oxygen elements on their surfaces. The second polymer introduces more than 20% of the first structural unit with polar functional groups, so that the polar functional groups and the metal elements in the positive active material produce effective polar interactions and charge adsorption, and produce hydrogen bonds with the oxygen elements in the positive active material, so that the positive electrode binder and the positive active material are firmly adsorbed, resulting in excellent bonding strength and dispersibility. At the same time, the positive electrode binder in the present application does not contain fluorine element, which avoids the generation of HF in an alkaline environment, so that the positive electrode binder can exist stably in the positive electrode slurry, ensuring that the positive electrode active material layer and the positive electrode current collector have good bonding strength. DETAILED DESCRIPTION

[0017] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the 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.

[0018] An embodiment of the present application provides a positive electrode binder, which is a core-shell structure, including a core body and a shell layer arranged on the outer surface of the core body and at least partially covering the core body, the core body includes a first polymer, 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 a carboxyl group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, and a cyano group; the mass percentage of the first structural unit in the second polymer is greater than 20%.

[0019] The positive electrode binder provided in the present application is a core-shell structure with excellent bonding strength and good softness. The glass transition temperature of the first polymer is lower than the glass transition temperature of the second polymer, so that the positive electrode binder introduces the first polymer to make the prepared positive electrode sheet have good softness and bonding strength. When the glass transition temperature of the first polymer is controlled between -60°C and 10°C, the positive electrode sheet prepared by the binder has good softness and cohesive strength. When the glass transition temperature is lower than -60°C, the cohesive strength is low. When the glass transition temperature is higher than 10°C, the softness of the positive electrode sheet is poor. Lithium cobalt oxide and ternary positive active materials have more metal elements and oxygen elements on their surfaces. The second polymer introduces more than 20% of the first structural unit with polar functional groups, so that the polar functional groups and the metal elements in the positive active material produce effective polar interactions and charge adsorption, and produce hydrogen bonds with the oxygen elements in the positive active material, so that the positive electrode binder and the positive active material are firmly adsorbed, resulting in excellent bonding strength and dispersibility. At the same time, the positive electrode binder in the present application does not contain fluorine element, which avoids the generation of HF in an alkaline environment, so that the positive electrode binder can exist stably in the positive electrode slurry, ensuring that the positive electrode active material layer and the positive electrode current collector have good bonding strength.

[0020] In some embodiments, the first structural unit is formed by polymerization of a first monomer; When the polar functional group is a carboxyl group, the first monomer is selected from at least one of a monocarboxylic acid, a C3-C30 unsaturated dicarboxylic acid, a C3-C30 unsaturated dicarboxylic anhydride, and a monoalkyl ester of a C3-C30 unsaturated dicarboxylic acid.

[0021] Specifically, the monocarboxylic acid includes, but is not limited to, acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, β-carboxyethyl acrylate and cinnamic acid.

[0022] The C3-C30 unsaturated dicarboxylic acid includes but is not limited to maleic acid, fumaric acid, itaconic acid, citraconic acid and mesaconic acid.

[0023] The C3-C30 unsaturated dicarboxylic anhydrides include but are not limited to maleic anhydride and citraconic anhydride.

[0024] The number of carbon atoms of the alkyl group in the monoalkyl ester of the C3-C30 unsaturated dicarboxylic acid is 1-24, and the monoalkyl ester of the C3-C30 unsaturated dicarboxylic acid includes but is not limited to monomethyl maleate, monooctadecyl maleate, monoethyl fumarate, monobutyl itaconate, ethylene glycol monoether itaconate, and monoeicosyl citrate.

[0025] In some embodiments, when the polar functional group is a sulfonic acid group, the first monomer is selected from at least one of C2~C14 olefin sulfonic acid, styrene sulfonic acid, C7~C24 alkyl-substituted styrene sulfonic acid, C5~C18 sulfoalkyl acrylate, C5~C18 sulfoalkyl acrylamide, and C3~C18 alkyl-substituted allyl sulfosuccinic acid.

[0026] Specifically, the C2-C14 olefin sulfonic acid includes but is not limited to vinyl sulfonic acid, allyl sulfonic acid, methyl allyl sulfonic acid and methyl vinyl sulfonic acid, and sodium allyloxyhydroxypropyl sulfonate.

[0027] The C7-C24 alkyl-substituted styrene sulfonic acid includes but is not limited to α-methylstyrene sulfonic acid.

[0028] The C5-C18 sulfoalkyl acrylates include, but are not limited to, 3-sulfonyl propyl acrylate, 3-sulfonyl propyl methacrylate, 2-ethanesulfonate acrylate, and 2-ethanesulfonate methacrylate.

[0029] The C5-C18 sulfoalkyl acrylamide includes, but is not limited to, acrylamido-2-methylpropane sulfonic acid, acrylamido-2-hydroxypropane sulfonic acid, methacrylamido-2-methylpropane sulfonic acid and methacrylamido-2-hydroxypropane sulfonic acid.

[0030] The C3-C18 alkyl-substituted allyl sulfosuccinic acid includes, but is not limited to, propyl allyl sulfosuccinic acid, butyl allyl sulfosuccinic acid and 2-ethylhexyl allyl sulfosuccinic acid.

[0031] In some embodiments, when the polar functional group is a phosphate group, the first monomer is selected from at least one of an acryloxyalkyl phosphate monoester compound and an allyl phosphate compound.

[0032] Specifically, the number of carbon atoms in the monoester group of the acryloxyalkyl phosphate monoester compound is 1 to 24, and the acryloxyalkyl phosphate monoester compound includes but is not limited to acryloxyethyl phosphate, methacryloyloxyethyl phosphate, 2-hydroxyethyl acryloxy phosphate, 2-hydroxyethyl methacryloyl phosphate, phenyl-2-acryloyloxyethyl phosphate, and di[2-(methacryloyloxy)ethyl] phosphate.

[0033] The allyl phosphate compounds include but are not limited to allyl phosphate phosphoric acid.

[0034] 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.

[0035] 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.

[0036] In a further preferred case, the first structural unit is acrylonitrile.

[0037] It can be understood that when the first structural unit includes a plurality of different structural units, the mass percentage of the first structural unit in the second polymer is the total amount of the plurality of different structural units.

[0038] In the present invention, preferably, the mass percentage of the first structural unit in the second polymer is 20%-98%, more preferably 30%-90%.

[0039] 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.

[0040] The above-mentioned acrylic acid ester structural unit is obtained by polymerization of acrylic acid ester monomers, and the acrylic acid ester 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 , isoamyl methacrylate, n-hexyl methacrylate, isooctyl methacrylate, isobornyl methacrylate, phenoxyethyl methacrylate, dicyclopentenyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, polyethylene glycol mono[methacrylate] ester, aminoethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, glycidyl methacrylate, tetrahydrofuran methacrylate, ethoxylated ethylene glycol diacrylate, ethoxylated ethylene glycol dimethacrylate, allyl methacrylate, diallyl phthalate, pentaerythritol diacrylate, pentaerythritol dimethacrylate.

[0041] The aromatic vinyl structural units are obtained by polymerization of aromatic vinyl hydrocarbon compounds, wherein the aromatic vinyl hydrocarbon compounds include but are not limited to styrene and substituted styrenes, wherein the substituted styrenes include but are not limited to α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, crotonylstyrene, divinylbenzene, divinyltoluene, divinylxylene, trivinylbenzene, vinylnaphthalene, and p-tert-butylstyrene.

[0042] The conjugated diene structural units are obtained by polymerizing conjugated diene monomers, and the conjugated diene monomers include but are not limited to butadiene, pentadiene, and isoprene.

[0043] In the present invention, preferably, the second polymer further comprises a cross-linked structural unit, and the mass percentage of the cross-linked structural unit in the second polymer is 0.1%-2%, more preferably 0.5%-1.5%.

[0044] Specifically, the cross-linked structural unit includes one or more of an acrylate cross-linked structural unit, an acrylamide cross-linked structural unit, and an allyl cross-linked structural unit. The acrylate cross-linked structural unit is selected from a structural monomer including multiple acrylate structures. The cross-linked structural unit is selected from an ethylene glycol diacrylate structural unit, a polyethylene glycol diacrylate structural unit, a propylene glycol diacrylate structural unit, a dipropylene glycol diacrylate structural unit, a tripropylene glycol diacrylate structural unit, a trimethylolpropane triacrylate structural unit, a glycerol triacrylate structural unit, a pentaerythritol triacrylate structural unit, an ethylene glycol dimethacrylate structural unit, a polyethylene glycol dimethacrylate structural unit, a propylene glycol dimethacrylate structural unit, a dipropylene glycol dimeth ... One or more of glycol dimethacrylate 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, acrylic acid allyl ether structural unit, methacrylic acid allyl ether structural unit. More preferably, at least one of N,N-methylenebisacrylamide structural unit, polyethylene glycol diacrylate structural unit and polyethylene glycol dimethacrylate structural unit.

[0045] When the second polymer further includes the cross-linked structural unit, after the positive electrode binder is used to prepare positive electrode slurry, the positive electrode binder can more stably maintain its structure and morphology, which is beneficial to further improve the bonding strength.

[0046] It can be understood that, for the above-mentioned second polymer, except for the first structural unit and the selectively existing cross-linking structural unit, the rest are second structural units.

[0047] In some embodiments, the first polymer includes at least one of acrylic copolymers, polyurethane, butyl rubber, styrene butadiene rubber, hydrogenated styrene butadiene rubber, nitrile rubber, hydrogenated nitrile butadiene rubber, and ethylene propylene rubber.

[0048] The acrylic copolymer in the first polymer is formed by polymerizing at least one acrylic ester monomer or methacrylic ester monomer with other monomers, and the other monomers may be at least one of methacrylic ester monomers, acrylic ester monomers, vinyl monomers, acrylamide monomers, and allyl monomers.

[0049] 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, hydroxyethyl acrylate, hydroxypropyl acrylate, 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 acrylate, cyclohexyl acrylate, benzyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, carboxyethyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, n-hexyl methacrylate, isooctyl methacrylate, isobornyl methacrylate, Cyclopentenyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, polyethylene glycol mono[methacrylate] ester, aminoethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, glycidyl methacrylate, tetrahydrofuran 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.

[0050] In some embodiments, the polyurethane is a polymer obtained by reacting a diisocyanate with a polymer diol or a polymer diamine, or the reaction raw materials of the polyurethane further include a chain extender.

[0051] The diisocyanate includes, but is not limited to, at least one of toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, and dicyclohexylmethane diisocyanate.

[0052] The polymer diols include but are not limited to polyether diols, polyester diols, and polyolefin diols.

[0053] The polyether diols include, but are not limited to, polytetramethylene oxide diol, polypropylene oxide diol, polyethylene oxide diol, and polypropylene oxide / polyethylene oxide diol.

[0054] The polyester diols include, but are not limited to, polycarbonate diol, polycaprolactone diol, polyneopentyl adipate diol, polyhexane adipate diol, and polybutylene adipate diol.

[0055] The polyolefin diols include, but are not limited to, polybutadiene diol, hydrogenated polybutadiene diol, polyisobutylene diol.

[0056] The chain extender includes, but is not limited to, dimethylolpropane, dimethylolbutane, ethylene glycol, 1,4-butanediol, 1,3-propylene glycol, neopentyl glycol, hexylene glycol, ethylenediamine, hexamethylenediamine, and isophoronediamine.

[0057] In some embodiments, the mass ratio of the first polymer to the second polymer is (20:80) to (80:20). When the weight of the positive electrode binder is 100 parts, if the first polymer accounts for less than 20 parts, the flexibility of the electrode sheet is poor and the bonding force is insufficient. When the first polymer accounts for more than 80 parts, the dispersion of the positive electrode binder to the positive electrode active material is poor.

[0058] 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.

[0059] 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, which makes it easier to make the polymer into powder for transportation. When the glass transition temperature of the first polymer is within the above range, the positive electrode binder has flexibility and cohesive strength, and further the prepared positive electrode sheet has good flexibility and adhesion.

[0060] In some preferred embodiments, the first polymer is selected from at least one of methacrylate copolymers, acrylate copolymers and polyurethane.

[0061] In the present invention, the aforementioned structural units represent the structural parts corresponding to the monomers in the obtained polymer after the corresponding monomers participate in the polymerization reaction. 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.

[0062] In some embodiments, the positive electrode binder electrolyte mass swelling rate is less than 100%. When the positive electrode binder electrolyte mass swelling rate is higher than 100%, the battery is prone to expansion during use, resulting in poor contact between the positive electrode active material in the positive electrode sheet and the positive electrode current collector, increasing the battery impedance and reducing the battery performance.

[0063] Furthermore, an embodiment of the present application provides a method for preparing a positive electrode binder, comprising the following steps: (1) Mixing monomers of the core body, adding an auxiliary agent, and stirring under heating to obtain a first polymer; (2) After the monomers of the shell layer are uniformly mixed, the first polymer is added and stirred under heating to obtain reactant 2.

[0064] (3) The reactant 2 is spray-dried to obtain a positive electrode binder powder.

[0065] As known to those skilled in the art, the reactions in the above steps are conventional free radical polymerization, etc., and the specific methods and reaction conditions are the free radical polymerization methods commonly used in the prior art, which will not be described in detail in the present invention.

[0066] If the polymer composition contains a polyurethane structure, a polyurethane preparation method known in the art is used.

[0067] The mass fractions of the monomers of the core and shell are not limited in the present application, as long as the glass transition temperature of the first polymer finally formed is -60~10°C and the glass transition temperature of the second polymer is greater than or equal to 50°C.

[0068] The auxiliary agents include one or more of sodium persulfate, ammonium persulfate, potassium persulfate, tert-butyl hydroperoxide, azobisisobutyronitrile, benzoyl peroxide / sucrose, tert-butyl hydroperoxide / carbalite, tert-butyl hydroperoxide / sodium metabisulfite, benzoyl peroxide / N,N-dimethylaniline, ammonium persulfate / sodium bisulfite, potassium persulfate / sodium bisulfite, hydrogen peroxide / tartaric acid, hydrogen peroxide / carbalite, 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 / tetraethyleneimine.

[0069] An embodiment of the present application further provides a positive electrode composition, comprising a positive electrode active material, a conductive agent and the positive electrode binder described above.

[0070] In some embodiments, the positive electrode active material includes at least one of lithium cobalt oxide, nickel-cobalt-aluminum ternary positive electrode active material, and nickel-cobalt-manganese ternary positive electrode active material (such as NCM523, NCM622, and NCM822). By selecting the above positive electrode active material and using it in combination with the above positive electrode binder, the polar functional groups in the positive electrode binder produce polar interactions and charge adsorption with the metal elements in the positive electrode active material, and produce hydrogen bonds with the oxygen elements in the positive electrode active material, so that the positive binder and the positive electrode active material are firmly adsorbed, resulting in excellent bonding force and dispersibility.

[0071] In some embodiments, the conductive agent includes but is not limited to at least one of conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, carbon fibers, graphene, and conductive polymers.

[0072] An embodiment of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode composition as described above. The positive electrode sheet provided by the present application comprises the positive electrode composition as described above, and the positive electrode binder with the 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.

[0073] An embodiment of the present application provides a lithium ion battery, comprising the positive electrode sheet as described above. The lithium ion battery provided by the present application uses the positive electrode sheet containing the above binder to prevent the positive electrode active material in the positive electrode sheet from falling off, thereby improving the electrical performance of the battery.

[0074] The present invention is further described below by way of examples.

[0075] The positive electrode binder, positive electrode composition, negative electrode sheet and lithium ion battery disclosed in the present invention are specifically described.

[0076] Example 1 1) Preparation of positive electrode binder The comonomers of the first polymer include the following components: 70 parts of butyl acrylate and 10 parts of N,N-dimethylacrylamide.

[0077] The second polymer includes 120 parts of an 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.

[0078] The preparation of a positive electrode binder comprises the following steps: (1) stirring and emulsifying a core monomer with deionized water and an emulsifier, adding ammonium persulfate, and stirring and polymerizing under heating to obtain a first polymer; (2) The monomers of the shell layer are mixed uniformly and then added dropwise to the first polymer, and the polymerization is continued by stirring under heating to obtain reactant 2.

[0079] (3) The reactant 2 is spray-dried to obtain a positive electrode binder powder.

[0080] 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 is 85°C; the mass swelling rate of the positive electrode binder electrolyte is 75%.

[0081] Specifically, the test method for the mass swelling rate of the electrolyte is as follows: under standard atmospheric pressure and 25°C, 1M lithium hexafluorophosphate is added to a mixed solution of ethylene carbonate and ethyl methyl carbonate in 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 and weigh it to obtain W initial, 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, and after leaving it for 72 hours, take it out and wipe the surface of the electrolyte to weigh it to obtain W balance, and the mass swelling rate = (W balance - W initial) / W initial * 100%.

[0082] 2) Preparation of negative electrode sheet: 1.5% styrene butadiene latex (SBR), 96% negative electrode active material graphite, 1% negative electrode conductive agent conductive carbon black, 1.5% thickener sodium carboxymethyl cellulose (CMC) were mixed, and deionized water was added for stirring to prepare a negative electrode composition. The negative electrode composition was then coated on both surfaces of the negative electrode current collector Cu foil, and after drying, cold pressing, slitting and other processes, a negative electrode sheet was obtained.

[0083] The particle size of graphite, the negative electrode active material, is 20 μm.

[0084] 3) Preparation of positive electrode: 97.8% of the positive electrode active material lithium cobalt oxide, 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 slurry. The positive electrode slurry is then coated on both surfaces of the positive electrode current collector Al foil, and after drying, cold pressing, slitting and other processes, a positive electrode sheet is obtained.

[0085] 4) Lithium-ion battery preparation: The positive electrode sheet, the isolation film (PE film), and the negative electrode sheet are stacked in order, with the isolation film placed between the positive and negative electrodes to play an isolating role, to form an electrode assembly, which is placed in an outer package, injected with commercially available electrolyte and packaged, and then subjected to processes such as liquid injection, formation, and exhaust to obtain a lithium-ion battery.

[0086] Embodiments 2 to 11 Most of the steps of Examples 2 to 11 are the same as those of Example 1, except that different shell monomers are added to the positive electrode binder, as shown in Table 1.

[0087] Table 1

[0088] Example 12 Cathode binder preparation (1) The monomer of the first polymer is added to an acetone solvent, and after the temperature is raised for reaction, an aqueous solution of NaOH is added dropwise while vigorously stirring to obtain a dispersion. The acetone solvent in the dispersion is removed by vacuum to obtain a first polymer dispersion.

[0089] (2) Adding an auxiliary agent to the first polymer dispersion and heating the mixture, then uniformly mixing the monomers of the shell layer and adding the mixture dropwise to the first polymer dispersion, and continuing to stir and polymerize the mixture under heating to obtain reactant 2.

[0090] (3) The reactant 2 is spray-dried to obtain a positive electrode binder powder.

[0091] Embodiments 12 to 17 Most of the steps of Examples 12 to 17 are the same as those of Example 1, except that the components of the core monomers added to the positive electrode binder are different, as shown in Table 2.

[0092] Table 2

[0093] Embodiment 18 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.

[0094] Embodiment 19 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.

[0095] Embodiment 20 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.

[0096] Embodiment 21 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.

[0097] Embodiment 22 Most of the steps of Example 22 are the same as those of Example 1, except that the positive electrode active material is NCM622.

[0098] Embodiment 23 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.

[0099] Comparative Example 1 Most of the steps of Comparative Example 1 and Example 1 are the same, except that the components of the shell monomer added to the positive electrode binder are different, as shown in Table 1.

[0100] Comparative Example 2 Most of the steps of Comparative Example 2 and Example 1 are the same, except that the positive electrode binder does not have a core-shell structure. The specific preparation method is as follows: (1) Mixing monomers of the core body, adding an auxiliary agent, and stirring under heating to obtain a first polymer; (2) After the monomers of the shell layer are uniformly mixed, potassium persulfate is added, and the mixture is stirred under heating to obtain a second polymer; (3) The first polymer and the second polymer are mixed and then spray-dried to obtain a positive electrode binder.

[0101] Comparative Example 3 Most of the steps of Comparative Example 3 are the same as those of Example 1, except that the positive electrode binder of Comparative Example 3 adopts the existing PVDF.

[0102] Comparative Example 4 Most of the steps of Comparative Example 4 are the same as those of Example 3, except that in the copolymerized monomer of the second polymer of the positive electrode binder of 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 amount of styrene added is adjusted to 90 parts.

[0103] Electrical performance test: The positive electrode sheets and lithium-ion batteries prepared in the above examples and comparative examples were subjected to the following tests.

[0104] 1) Positive electrode peel strength and cohesive strength test: After the positive electrode sheet is compacted, a tensile testing machine is used to measure the peel strength of the positive electrode sheet. The test method refers to GB2792-2014.

[0105] Peel strength: The positive electrode coating was fixed on a stainless steel plate using the method described in the national standard, and the Al foil was peeled off at 180°C using an adhesive tape to obtain the peel strength.

[0106] Peel strength: The Al foil was fixed on a stainless steel plate using the method described in the national standard, and the positive electrode coating was peeled off at 180°C using tape to obtain the cohesive strength.

[0107] 2) Normal temperature cycle test Lithium cobalt oxide battery: After placing the battery in a constant temperature test room at 25℃±2℃ for 1h, charge it to 4.45V at 1C constant current and constant voltage, with a cut-off current of 0.05C; discharge it to 3V at 1C constant current, and record the discharge capacity; repeat the above steps 500 times and calculate the capacity retention rate.

[0108] NCM622 battery: After placing the battery in a constant temperature test room at 25℃±2℃ for 1h, charge it to 4.2V at 1C constant current and constant voltage, with a cut-off current of 0.05C; discharge it to 3V at 1C constant current, and record the discharge capacity; repeat the above steps 500 times and calculate the capacity retention rate.

[0109] Lithium iron phosphate battery: After placing the battery in a constant temperature test room at 25℃±2℃ for 1h, charge it to 3.65V at 1C constant current and constant voltage, with a cut-off current of 0.05C; discharge it to 2V at 1C constant current, and record the discharge capacity; repeat the above steps 500 times and calculate the capacity retention rate.

[0110] 3) Positive electrode flexibility test The specific method is: use winding needles of different particle sizes to wind and observe the corresponding winding needle diameter when there are cracks and powder falling on the surface of the electrode. The larger the winding needle diameter, the worse the flexibility, unit: mm.

[0111] The test results obtained from the embodiments and comparative examples are entered into Table 3.

[0112] Table 3

[0113] It can be seen that when the glass transition temperature of the first polymer is lower than -60°C, the cohesive strength of the pole piece will decrease significantly. When the glass transition temperature of the first polymer is higher than 10°C, the softness of the pole piece 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 from the prepared binder liquid, and the peel strength of the pole piece 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 electrode active particles, which reduces the cycle performance of the battery. When the ratio of the first polymer to the second polymer is lower than 20:80, the pole piece peeling strength decreases and the softness of the pole piece decreases significantly. When the ratio of the first polymer to the second polymer is higher than 20:80, the pole piece peeling strength and cycle performance decrease significantly. When the positive electrode active material is lithium iron phosphate, due to the weak interaction between the binder structure of this patent and lithium iron phosphate, the pole piece peeling strength is low and the battery cycle performance also decreases significantly, which may be caused by the partial shedding of the active material in the battery during the cycle. Therefore, when the binder composition is within the range described in the claims, the positive electrode sheet prepared using the prepared binder has good peel strength, cohesive strength, and flexibility, and the prepared battery also has good cycle performance.

[0114] When the second polymer contains a cross-linked structural unit, it is very beneficial to further improve the peel strength and cohesive strength of the positive electrode sheet.

[0115] If the content of the first structural unit in the second polymer is too low, the peel strength and cohesive strength of the positive electrode sheet will be significantly reduced.

[0116] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A positive electrode binder, characterized in that: The positive electrode binder is a core-shell structure, including a core body and a shell layer arranged on the outer surface of the core body and at least partially covering the core body, the core body includes a first polymer, 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 a carboxyl group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, and a cyano group; the mass percentage of the first structural unit in the second polymer is greater than 20%.

2. The positive electrode binder according to claim 1, characterized in that 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, and a conjugated diene structural unit.

3. The positive electrode binder according to claim 1, characterized in that The second polymer further includes a cross-linked structural unit. In the second polymer, the mass percentage of the cross-linked structural unit is 0.1%-2%.

4. The positive electrode binder according to claim 3, characterized in that The cross-linked structural unit includes one or more of an acrylate cross-linked structural unit, an acrylamide cross-linked structural unit, and an allyl cross-linked structural unit.

5. The positive electrode binder according to claim 1, characterized in that The first polymer includes at least one of acrylic copolymers, polyurethane, butyl rubber, styrene-butadiene rubber, hydrogenated styrene-butadiene rubber, nitrile rubber, hydrogenated nitrile rubber, and ethylene-propylene rubber.

6. The positive electrode binder according to claim 1, characterized in that The mass ratio of the first polymer to the second polymer is (20:80) to (80:20).

7. The positive electrode binder according to claim 1, characterized in that The mass swelling degree of the positive electrode binder electrolyte is less than 100%; And / or, 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.

8. A positive electrode composition, characterized in that: The invention comprises a positive electrode active material, a conductive agent and a positive electrode binder as claimed in any one of claims 1 to 7.

9. A positive electrode sheet, characterized in that: It comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode composition according to claim 8.

10. A lithium ion battery, characterized in that: Including the positive electrode sheet as described in claim 9.

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