KR102586146B1 - Method for producing slurry composition for secondary battery positive electrode, positive electrode for secondary battery, and secondary battery - Google Patents

Abstract

The purpose of the present invention is to provide a method for producing a slurry composition for a secondary battery positive electrode that exhibits excellent high potential durability in the positive electrode. The method for producing a slurry composition for a positive electrode of the present invention includes the steps of dry mixing polymer particles for a secondary battery positive electrode, a positive electrode active material, and a conductive material to obtain a dry mixture, and mixing the dry mixture and the dispersion medium to obtain a dry mixture for a secondary battery positive electrode. It includes a process for obtaining a slurry composition. Here, the polymer particle contains a copolymer containing a nitrile group-containing monomer unit and a hydrophilic group-containing monomer unit, and its volume average particle diameter D50 is 1 μm or more. And, the ratio of the volume average particle diameter D50 of the polymer particles to the volume average particle diameter D50 of the positive electrode active material is 0.1 or more.

Description

Method for producing slurry composition for secondary battery positive electrode, positive electrode for secondary battery, and secondary battery

The present invention relates to a method for producing a slurry composition for secondary battery positive electrodes, a positive electrode for secondary batteries, and secondary batteries.

Secondary batteries such as lithium ion secondary batteries have the characteristics of being small and lightweight, have high energy density, and can be repeatedly charged and discharged, and are used in a wide range of applications. Therefore, in recent years, improvements in battery members such as electrodes have been studied for the purpose of further improving the performance of secondary batteries.

Here, the positive electrode used in secondary batteries such as lithium ion secondary batteries usually includes a current collector and an electrode mixture layer (positive electrode mixture layer) formed on the current collector. Then, this positive electrode mixture layer is formed, for example, by dispersing and/or dissolving the positive electrode active material and the binder in a dispersion medium to prepare a slurry composition, applying the slurry composition onto a current collector, and drying it.

Therefore, in recent years, attempts have been made to improve the method of forming a positive electrode mixture layer on a current collector in order to achieve further performance improvement of secondary batteries.

For example, in Patent Document 1, a positive electrode mixture layer is formed on a current collector by dry mixing a positive electrode active material and a particulate binder each having a predetermined average particle size at a predetermined amount ratio and fixing the resulting mixed powder on the current collector. A method of forming is proposed. It has been reported that by forming a positive electrode mixture layer using this method, it is possible to increase the density of the active material in the positive electrode mixture layer, increase the capacity of the secondary battery, and suppress cycle deterioration.

Japanese Patent Publication No. 4778034

Here, in accordance with the diversification of its uses in recent years, secondary batteries are required to exhibit battery characteristics such as excellent lifespan characteristics even when exposed to high potential for a long period of time. However, when a positive electrode formed by forming a positive electrode mixture layer on a current collector using the above conventional method is exposed to a high potential over a long period of time, battery characteristics sometimes deteriorate significantly due to deterioration of the surface of the positive electrode active material. Therefore, there is a demand for a new technology that increases the high potential durability of the positive electrode and improves the battery characteristics of the secondary battery.

Therefore, the purpose of the present invention is to provide a method for producing a slurry composition for a secondary battery positive electrode that exhibits excellent high potential durability in the positive electrode.

Additionally, the present invention aims to provide a positive electrode for a secondary battery excellent in high-potential durability, and a secondary battery comprising the positive electrode for a secondary battery and excellent in battery characteristics.

The present inventor conducted intensive studies to achieve the above object. Then, the present inventor dry mixed polymer particles having a predetermined composition and properties with a positive electrode active material and a conductive material, and then mixed the obtained mixture again with a dispersion medium to prepare a slurry composition for a secondary battery positive electrode, the resulting slurry It was found that it was possible to improve the high potential durability of a positive electrode formed from the composition, and the present invention was completed.

That is, the purpose of this invention is to advantageously solve the above problems, and the method for producing a slurry composition for a secondary battery positive electrode of the present invention is to dry polymer particles for a secondary battery positive electrode, a positive electrode active material, and a conductive material. A step of mixing to obtain a dry mixture, and mixing the dry mixture and the dispersion medium to obtain a slurry composition for a secondary battery positive electrode, wherein the polymer particles for a secondary battery positive electrode include a nitrile group-containing monomer unit and a hydrophilic group-containing monomer. Containing a copolymer containing both units, the volume average particle diameter D50 is 1 μm or more, and the ratio of the volume average particle diameter D50 of the polymer particles for secondary battery positive electrode to the volume average particle diameter D50 of the positive electrode active material is 0.1 or more. It is characterized by In this way, by using a slurry composition for a secondary battery positive electrode prepared by dry mixing polymer particles, a positive electrode active material, and a conductive material having the above-described composition and properties, and then mixing a dispersion medium, a positive electrode having excellent high potential durability can be obtained. can be manufactured.

In addition, in the present invention, the phrase “contains a monomer unit” means that “a structural unit derived from the monomer is contained in the polymer obtained using the monomer.”

In addition, in the present invention, the “volume average particle diameter D50” is calculated as the particle size at which the cumulative volume calculated from the small diameter side is 50% in the particle size distribution measured dry using a laser diffraction/scattering type particle size distribution measuring device. You can.

In the present invention, “dry mixing” refers to mixing so that the solid content concentration of the mixture at the time of mixing is more than 90% by mass.

Here, in the method for producing a slurry composition for a secondary battery positive electrode of the present invention, the volume average particle diameter D50 of the polymer particles for a secondary battery positive electrode is 2000 μm or less, and the volume average particle diameter D50 of the positive electrode active material is It is preferable that the ratio of the volume average particle diameter D50 of the polymer particles is 200 or less. This is because the internal resistance of the secondary battery can be reduced if the ratio of the volume average particle diameter D50 of the polymer particles to the volume average particle diameter D50 of the positive electrode active material is each less than the above-mentioned value.

In the method for producing a slurry composition for a secondary battery positive electrode of the present invention, the copolymer contains 80% by mass or more and 99.9% by mass or less of the nitrile group-containing monomer units, and 0.1% by mass or more of the hydrophilic group-containing monomer units. It is preferable to contain 20% by mass or less. By using a copolymer containing a nitrile group-containing monomer unit and a hydrophilic group-containing monomer unit at the above-mentioned content ratios, the peeling strength of the positive electrode can be increased, the high potential durability can be further improved, and the internal resistance of the secondary battery can be reduced. This is because it can be reduced.

Furthermore, in the manufacturing method of the slurry composition for secondary battery positive electrode of this invention, it is preferable that the molecular weight distribution (Mw/Mn) of the said copolymer is 10 or less. This is because if a copolymer with a molecular weight distribution of 10 or less is used, the internal resistance of the secondary battery can be reduced.

In addition, in the present invention, “molecular weight distribution (Mw/Mn)” refers to the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn). And in the present invention, “number average molecular weight (Mn)” and “weight average molecular weight (Mw)” can be measured using gel permeation chromatography.

Moreover, in the manufacturing method of the slurry composition for secondary battery positive electrodes of this invention, it is preferable that the glass transition temperature of the said copolymer is 60 degreeC or more and 170 degrees C or less. This is because when a copolymer with a glass transition temperature of 60°C or higher and 170°C or lower is used, high potential durability can be further improved while increasing the peeling strength of the positive electrode, and the internal resistance of the secondary battery can be reduced.

In addition, in this invention, "glass transition temperature" can be measured based on JIS K7121.

Moreover, the purpose of this invention is to solve the above problems advantageously, and the positive electrode for a secondary battery of the present invention includes a current collector and a positive electrode mixture layer formed on at least one surface of the current collector, The positive electrode mixture layer is characterized in that it is formed from a slurry composition for secondary battery positive electrodes obtained by the manufacturing method of any of the above-mentioned slurry compositions for secondary battery positive electrodes. The positive electrode formed from the slurry composition for secondary battery positive electrodes obtained by the manufacturing method of the slurry composition for secondary battery positive electrodes described above is excellent in high potential durability.

In addition, this invention aims to advantageously solve the above problems, and the secondary battery of the present invention is provided with a positive electrode, a negative electrode, an electrolyte, and a separator, and the positive electrode is the above-mentioned positive electrode for secondary batteries. Do it as By using the positive electrode for a secondary battery described above, a secondary battery with excellent battery characteristics such as life characteristics can be obtained even when exposed to high potential for a long time.

According to the present invention, a method for producing a slurry composition for a secondary battery positive electrode that exhibits excellent high potential durability in the positive electrode can be provided.

Moreover, according to the present invention, it is possible to provide a positive electrode for a secondary battery excellent in high potential durability, and a secondary battery comprising the positive electrode for a secondary battery and excellent in battery characteristics.

Hereinafter, embodiments of the present invention will be described in detail.

Here, the manufacturing method of the slurry composition for secondary battery positive electrodes of this invention is used for preparation of the slurry composition for secondary battery positive electrodes. Moreover, the positive electrode for secondary batteries of this invention is characterized by being produced using the slurry composition for secondary battery positive electrodes obtained by the manufacturing method of the slurry composition for secondary battery positive electrodes of this invention. And the secondary battery of the present invention is characterized by comprising the positive electrode for secondary batteries of the present invention.

(Method for producing slurry composition for secondary battery positive electrode)

The method for producing a slurry composition for a secondary battery positive electrode of the present invention is to produce a slurry composition for a secondary battery positive electrode by mixing at least polymer particles for a secondary battery positive electrode, a positive electrode active material, a conductive material, and a dispersion medium in a predetermined order. This is how to do it.

Here, the polymer particles for secondary battery positive electrodes used in the manufacturing method of the slurry composition for secondary battery positive electrodes of the present invention contain a copolymer containing both a nitrile group-containing monomer unit and a hydrophilic group-containing monomer unit, and the volume is The average particle diameter D50 is 1 μm or more, and the ratio of the volume average particle diameter D50 of the positive electrode active material (particle diameter ratio of the polymer particles to the positive electrode active material) is 0.1 or more. And, the method for producing a slurry composition for a secondary battery positive electrode of the present invention includes the steps of dry mixing the above-described polymer particles for a secondary battery positive electrode, the positive electrode active material, and the conductive material to obtain a dry mixture, and the dry mixture and the dispersion medium. It is characterized by including a step of mixing to obtain a slurry composition for a secondary battery positive electrode.

Here, when the production method of the slurry composition for secondary battery positive electrode of the present invention is used, the polymer particles for secondary battery positive electrode described above are dry mixed with the positive electrode active material and the conductive material, and the obtained dry mixture is then mixed with the dispersion medium. Therefore, a slurry composition for a secondary battery positive electrode that can sufficiently increase the high potential durability of the positive electrode can be prepared. And by using the positive electrode, it is possible to obtain a secondary battery that exhibits battery characteristics such as excellent lifespan characteristics even when exposed to high potential for a long time.

In addition, by dry premixing the polymer particles for the positive electrode of the secondary battery with the positive electrode active material and the conductive material before adding the dispersion medium, the high potential durability of the positive electrode can be sufficiently increased and excellent battery characteristics can be exhibited in the secondary battery. The reason for this is not clear, but it is assumed to be due to the following reasons. That is, by premixing the polymer particles having the above-described composition and properties, the positive electrode active material, and the conductive material, the conductive material is dispersed appropriately, thereby forming a good conductive path in the positive electrode mixture layer, and the polymer particles are homogeneous during premixing. By adsorbing to the surface of the positive electrode active material, sufficient protection of the surface of the positive electrode active material by the copolymer becomes possible in the resulting positive electrode. It is presumed that this is because deterioration of the surface of the positive electrode active material at high potential is suppressed.

Hereinafter, the components mixed in the slurry composition and the procedure for preparing the slurry composition using those components will be explained.

<Polymer particles for secondary battery positive electrode>

The polymer particle for a secondary battery positive electrode is a component containing a copolymer that functions as a binder. This copolymer maintains the components contained in the positive electrode mixture layer from being separated from the positive electrode mixture layer in the positive electrode produced using the slurry composition obtained using the slurry composition production method of the present invention. And the copolymer contained in the polymer particle for a secondary battery positive electrode must contain both a nitrile group-containing monomer unit and a hydrophilic group-containing monomer unit. Additionally, the copolymer may optionally contain monomer units other than the nitrile group-containing monomer unit and the hydrophilic group-containing monomer unit, as long as the effect of the present invention is not impaired.

[Composition of copolymer]

[[Nitrile group-containing monomer unit]]

The nitrile group-containing monomer unit is a repeating unit derived from a nitrile group-containing monomer.

Here, examples of the nitrile group-containing monomer that can form the nitrile group-containing monomer unit include α,β-ethylenically unsaturated nitrile monomer. Specifically, the α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, but examples include acrylonitrile; α-chloroacrylonitrile; α-bro α-halogenoacrylonitrile such as moacrylonitrile; α-alkylacrylonitrile such as methacrylonitrile and α-ethylacrylonitrile; and the like. Among these, from the viewpoint of increasing the binding force of the copolymer, acrylonitrile and methacrylonitrile are preferable as nitrile group-containing monomers, and acrylonitrile is more preferable.

These can be used individually or in combination of two or more types.

The content ratio of the nitrile group-containing monomer unit in the copolymer is preferably 80% by mass or more, more preferably 82% by mass or more, and 85% by mass, when the total repeating units in the copolymer are 100% by mass. Above is more preferable, 90 mass% or more is particularly preferable, 99.9 mass% or less is preferable, 99 mass% or less is more preferable, and 98.5 mass% or less is still more preferable. When the content ratio of the nitrile group-containing monomer unit in the copolymer is within the above-mentioned range, the binding force of the copolymer and the dispersibility of the resulting slurry composition are improved, the peeling strength of the positive electrode can be sufficiently increased, and the high potential of the copolymer itself can be improved. Durability can also be increased. Accordingly, peeling of the positive electrode mixture layer is prevented, the internal resistance of the secondary battery is reduced, and a secondary battery with excellent battery characteristics such as life characteristics can be obtained even when exposed to high potential for a long time.

[[Monomer unit containing hydrophilic group]]

The hydrophilic group-containing monomer unit is a repeating unit derived from a hydrophilic group-containing monomer.

Here, examples of the hydrophilic group-containing monomer include a monomer having a carboxylic acid group, a monomer having a sulfonic acid group, a monomer having a phosphoric acid group, and a monomer having a hydroxyl group.

Monomers having a carboxylic acid group include monocarboxylic acids and their derivatives, dicarboxylic acids and their acid anhydrides, and their derivatives.

Examples of monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid.

Examples of monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, and β-diaminoacrylic acid. You can.

Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid.

Dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, methylallyl maleate, diphenyl maleate, nonyl maleate, decyl maleate, Examples include maleic acid esters such as dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate.

Examples of acid anhydrides of dicarboxylic acid include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride.

Moreover, as a monomer having a carboxylic acid group, an acid anhydride that generates a carboxyl group by hydrolysis can also be used.

In addition, monoethyl maleate, diethyl maleate, monobutyl maleate, dibutyl maleate, monoethyl fumarate, diethyl fumarate, monobutyl fumarate, dibutyl fumarate, monocyclohexyl fumarate, dicyclohexyl fumarate, ita Monoesters and diesters of α,β-ethylenically unsaturated polyhydric carboxylic acids, such as monoethyl itaconate, diethyl itaconate, monobutyl itaconate, and dibutyl itaconate, are also included.

Monomers having a sulfonic acid group include vinylsulfonic acid, methylvinylsulfonic acid, (meth)allylsulfonic acid, styrenesulfonic acid, ethyl (meth)acrylic acid-2-sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, and 3-allyloxy-2. －Hydroxypropanesulfonic acid, etc. can be mentioned.

In addition, in the present invention, “(meth)allyl” means allyl and/or methallyl. In addition, in the present invention, “(meth)acryl” means acrylic and/or methacryl.

Examples of monomers having a phosphoric acid group include -2-(meth)acryloyloxyethyl phosphoric acid, methyl-2-(meth)acryloyloxyethyl phosphate, and ethyl-(meth)acryloyloxyethyl phosphate.

In addition, in the present invention, “(meth)acryloyl” means acryloyl and/or methacryloyl.

Examples of monomers having a hydroxyl group include ethylenically unsaturated alcohols such as (meth)allyl alcohol, 3-buten-1-ol, and 5-hexen-1-ol; -2-hydroxyethyl acrylate, -2-hydroxypropyl acrylate, Ethylene such as -2-hydroxyethyl methacrylate, -2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconic acid. Alkanol esters of unsaturated carboxylic acids; General formula CH ₂ =CR ¹ -COO -(C _n H _2n O) _m -H (where m is an integer of 2 to 9 and n is an integer of 2 to 4 , R ¹ represents hydrogen or a methyl group), esters of polyalkylene glycol and (meth)acrylic acid; 2-hydroxyethyl-2'-(meth)acryloyloxyphthalate, 2-hydroxyethyl- Mono(meth)acrylic acid esters of dihydroxy esters of dicarboxylic acids such as 2'-(meth)acryloyloxysuccinate; vinyl such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether Ethers; (meth)allyl-2-hydroxyethyl ether, (meth)allyl-2-hydroxypropyl ether, (meth)allyl-3-hydroxypropyl ether, (meth)allyl-2-hydroxybutyl ether , mono(meth)allyl ethers of alkylene glycols such as (meth)allyl-3-hydroxybutyl ether, (meth)allyl-4-hydroxybutyl ether, and (meth)allyl-6-hydroxyhexyl ether; Polyoxyalkylene glycol mono(meth)allyl ethers such as diethylene glycol mono(meth)allyl ether, dipropylene glycol mono(meth)allyl ether; glycerol mono(meth)allyl ether, (meth)allyl-2-chloro -3-hydroxypropyl ether, (meth)allyl-2-hydroxy-3-chloropropyl ether, etc., mono(meth)allyl ether of halogen and hydroxy substituent of (poly)alkylene glycol; Eugenol, Mono(meth)allyl ethers of polyhydric phenols such as isoeugenol and their halogen-substituted products; alkylene glycols such as (meth)allyl-2-hydroxyethylthioether and (meth)allyl-2-hydroxypropylthioether (meth)allylthioethers; and the like.

The above-mentioned hydrophilic group-containing monomers can be used individually or in combination of two or more types. And among these, methacrylic acid, acrylic acid, and itaconic acid are more preferable from the viewpoint of increasing the peeling strength and high potential durability of the positive electrode. That is, it is more preferable that the copolymer contains a monomer unit derived from at least one type selected from the group consisting of methacrylic acid, acrylic acid, and itaconic acid as the hydrophilic group-containing monomer unit.

The content ratio of the hydrophilic group-containing monomer unit in the copolymer is preferably 0.1% by mass or more, more preferably 1% by mass or more, and 2% by mass or more when the total repeating units in the copolymer are 100% by mass. It is more preferable that it is 20 mass% or less, more preferably 17 mass% or less, even more preferably 15 mass% or less, and especially preferably 10 mass% or less. When the content ratio of the hydrophilic group-containing monomer unit in the copolymer is 0.1% by mass or more, the adhesive strength of the copolymer is improved, thereby increasing the peeling strength of the positive electrode, and the copolymer can better cover the positive electrode active material in the positive electrode mixture layer. Therefore, the high potential durability of the positive electrode can be increased. On the other hand, if the content ratio of the hydrophilic group-containing monomer unit in the copolymer is 20% by mass or less, the positive electrode active material in the positive electrode mixture layer will not be excessively covered by the copolymer, and an increase in internal resistance can be suppressed.

－Other monomer units－

The copolymer may contain monomer units other than the nitrile group-containing monomer unit and the hydrophilic group-containing monomer unit. Examples of such other monomer units include (meth)acrylic acid ester monomer units.

Here, examples of the (meth)acrylic acid ester monomer that can form the (meth)acrylic acid ester monomer unit include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, and t-butyl. Acrylate, isobutyl acrylate, n-pentyl acrylate, isopentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate , alkyl acrylic esters such as n-tetradecyl acrylate and stearyl acrylate; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t- Butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate Methacrylic acid alkyl esters such as methyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate; and the like.

Monomers capable of forming other monomer units can be used individually or in combination of two or more types.

The content ratio of other monomer units is not particularly limited, but from the viewpoint of sufficiently obtaining the desired effect of the present invention, when the total repeating units in the copolymer is 100% by mass, it is preferably 20% by mass or less, It is more preferable that it is 15 mass% or less, and even more preferably it is 10 mass% or less.

[Properties of copolymer]

［[Weight average molecular weight (Mw)]］

The copolymer preferably has a weight average molecular weight of 500,000 or more, more preferably 1,000,000 or more, preferably 2,500,000 or less, and more preferably 2,000,000 or less. If the weight average molecular weight of the copolymer is 500,000 or more, the positive electrode active material will not be excessively covered by the copolymer due to an increase in the low molecular weight component, and the internal resistance of the secondary battery can be reduced. Additionally, if the weight average molecular weight of the polymer is 2,500,000 or less, the slurry composition obtained using the polymer particles does not thicken excessively, and a positive electrode mixture layer with a uniform thickness can be formed. Therefore, the internal resistance of the secondary battery can be reduced while increasing the high potential durability of the positive electrode.

［[Molecular weight distribution (Mw/Mn)]］

The copolymer has a molecular weight distribution of preferably 10 or less, more preferably 9 or less, further preferably 8 or less, and especially preferably 7 or less. If the molecular weight distribution of the copolymer is 10 or less, the positive electrode active material will not be excessively covered by the copolymer due to an increase in the low molecular weight component, and the internal resistance of the secondary battery can be reduced.

［［Glass transition temperature (Tg)］］

The copolymer preferably has a glass transition temperature of 60°C or higher, more preferably 80°C or higher, more preferably 100°C or higher, preferably 170°C or lower, more preferably 160°C or lower, and 150°C or lower. It is more desirable. If the glass transition temperature of the copolymer is 60°C or higher, blocking between particles during dry mixing is suppressed, and the polymer particles, positive electrode active material, and conductive material can be uniformly dispersed. Therefore, the copolymer in the positive electrode mixture layer can better cover the positive electrode active material, thereby increasing the high potential durability of the positive electrode and reducing the internal resistance of the secondary battery. Moreover, if the glass transition temperature of the copolymer is 170°C or lower, the flexibility of the positive electrode can be ensured and the peeling strength can be increased.

[Preparation of polymer particles containing copolymer]

As long as the polymer particles contain the above-mentioned copolymer, the preparation method is not particularly limited. Polymer particles containing a copolymer can be obtained, for example, by polymerizing a monomer composition containing the above-described monomers in an aqueous solvent to form the copolymer in the form of particles, and then coagulating, filtering, and drying under reduced pressure as necessary. Alternatively, it may be obtained by polymerizing a monomer composition containing the above-mentioned monomers in any polymerization solvent to obtain a copolymer and then spray drying it.

Here, in the present invention, the content ratio of each monomer in the monomer composition can be determined based on the content ratio of each monomer unit in the copolymer constituting the polymer particle.

The polymerization method is not particularly limited, and any method such as solution polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization can be used. Additionally, as the polymerization reaction, any reaction such as ionic polymerization, radical polymerization, or living radical polymerization can be used.

Among them, from the viewpoint of narrowing the molecular weight distribution while increasing the weight average molecular weight of the copolymer and suitably producing polymer particles having a predetermined volume average particle diameter D50, monomers are polymerized by radical polymerization using suspension polymerization as the polymerization mode. It is preferred to polymerize the composition. In this radical polymerization, known additives such as dispersants and polymerization initiators can be used. Examples of such known additives include those described in Japanese Patent No. 5573966.

[Volume average particle diameter of polymer particles D50]

The volume average particle diameter D50 of the polymer particles obtained as described above must be 1 μm or more, preferably 10 μm or more, more preferably 50 μm or more, further preferably 100 μm or more, particularly preferably 200 μm or more, and 2000 μm or less. It is preferable, it is more preferable that it is 1800 micrometers or less, it is still more preferable that it is 1000 micrometers or less, and it is especially preferable that it is 500 micrometers or less. If the volume average particle diameter D50 of the polymer particles is less than 1 μm, the copolymer layer covering the positive electrode active material in the positive electrode mixture layer becomes too thin, so the high potential durability of the positive electrode cannot be secured, and the peeling strength of the positive electrode also decreases. It deteriorates. On the other hand, if the volume average particle diameter D50 of the polymer particles is 2000 μm or less, the copolymer layer covering the positive electrode active material in the positive electrode mixture layer does not become excessively thick, and the internal resistance of the secondary battery can be reduced.

In addition, the volume average particle diameter D50 of the polymer particles is determined by the preparation conditions of the polymer particles (polymerization concentration, polymerization temperature and stirring speed, type and amount of dispersant, polymerization initiator and chain transfer agent, and spray speed and drying temperature of spray drying, etc.). You can change it by adjusting it.

In addition, the ratio of the volume average particle diameter D50 of the polymer particles to the volume average particle diameter D50 of the positive electrode active material is required to be 0.1 or more, preferably 1 or more, more preferably 5 or more, still more preferably 10 or more, and especially 20 or more. It is preferable that it is 200 or less, more preferably 100 or less, and even more preferably 50 or less. If the particle diameter ratio of the polymer particles to the positive electrode active material is less than 0.1, the copolymer layer covering the positive electrode active material in the positive electrode mixture layer becomes excessively thin, so the high potential durability of the positive electrode cannot be secured, and the peeling strength of the positive electrode cannot be secured. also deteriorates. On the other hand, if the particle diameter ratio of the polymer particles to the positive electrode active material is 200 or less, the copolymer layer covering the positive electrode active material in the positive electrode mixture layer does not become excessively thick, and the internal resistance of the secondary battery can be reduced.

＜Positive electrode active material＞

A positive electrode active material is a material that exchanges electrons in the positive electrode of a secondary battery. And, for example, as a positive electrode active material for a lithium ion secondary battery, a material that can occlude and release lithium is usually used.

Specifically, the positive electrode active material for lithium ion secondary batteries is not particularly limited and includes lithium-containing cobalt oxide (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium-containing nickel oxide (LiNiO ₂ ), Co-Ni- Lithium-containing complex oxide of Mn (Li(CoMnNi)O ₂ ), lithium-containing complex oxide of Ni-Mn-Al, lithium-containing complex oxide of Ni-Co-Al, olivine-type lithium iron phosphate (LiFePO ₄ ), olivine-type phosphoric acid _Lithium manganese (LiMnPO ₄ ), _{Li 1+x Mn 2-x O 4 ( 0 <} and base positive electrode active materials such as ₄ .

Among the above, from the viewpoint of improving the battery capacity of the secondary battery, etc., positive electrode active materials include lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), lithium-containing composite oxide of Co-Ni-Mn, It is preferable to use Li[Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ or LiNi _0.5 Mn _1.5 O ₄ , and lithium-containing cobalt oxide (LiCoO ₂ ), Li[Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ or It is more preferable to use LiNi _0.5 Mn _1.5 O ₄ .

These can be used individually or in combination of two or more types.

The volume average particle diameter D50 of the positive electrode active material is not particularly limited as long as the particle diameter ratio of the above-mentioned polymer particles to the positive electrode active material is within a predetermined range, but is preferably 0.1 μm or more and 100 μm or less.

＜Conductive material＞

The conductive material is used to ensure electrical contact between the positive electrode active materials. And, as the conductive material, carbon black (e.g., acetylene black, Ketjen black (registered trademark), furnace black, etc.), graphite, carbon fiber, carbon flake, and carbon ultrashort fiber (e.g., carbon nanotubes or vapor phase Conductive carbon materials such as grown carbon fiber, etc.; Fibers of various metals, foil, etc. can be used. Among these, as a conductive material, carbon black is preferable and acetylene black is more preferable.

These can be used individually or in combination of two or more types.

＜Dispersion medium＞

After dry mixing the above-mentioned polymer particles, positive electrode active material, and conductive material, the dispersion medium added to the resulting dry mixture is not particularly limited, and an organic solvent can be used. And, as organic solvents, for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, and amyl. Alcohols such as alcohol, ketones such as acetone, methyl ethyl ketone, and cyclohexanone, esters such as ethyl acetate and butyl acetate, ethers such as diethyl ether, dioxane, and tetrahydrofuran, and N,N-dimethylform. Amide, amide-based polar organic solvents such as N-methyl-2-pyrrolidone (NMP), and aromatic hydrocarbons such as toluene, xylene, chlorobenzene, orthodichlorobenzene, and paradichlorobenzene. These may be used individually, or two or more types may be mixed and used.

Among them, NMP is preferable as a dispersion medium.

＜Other ingredients＞

In the method for producing the slurry composition of the present invention, components other than those described above may be used. Examples of such other components include polymer powder containing a binder other than the above-mentioned copolymer, and known additives such as reinforcing materials, leveling agents, viscosity modifiers, and electrolyte additives described in International Publication No. 2012/115096. I can hear it. These components may be used individually, or two or more types may be used in combination at any ratio. The timing of addition of these other components is not particularly limited, and may be added appropriately in any one process depending on their properties. Additionally, the properties of the binder (polymer) constituting the above-mentioned polymer powder are arbitrary, and for example, a polymer having a glass transition temperature of 40°C or lower may be used.

＜Preparation procedure＞

In the slurry composition for secondary battery positive electrodes of this invention, the slurry composition is prepared using the components mentioned above. Specifically, a process for obtaining a dry mixture by dry mixing polymer particles for a secondary battery positive electrode, a positive electrode active material, and a conductive material (dry mixing process), and mixing the dry mixture obtained in the dry mixing process with a dispersion medium to obtain a secondary battery positive electrode. A slurry composition is prepared through a process for obtaining a slurry composition (dispersion medium mixing process).

［Dry mixing process］

First, the polymer particles, the positive electrode active material, and the conductive material are mixed (dry mixing) in a state where the solid content concentration of the mixture at the time of mixing is more than 90 mass% to obtain a dry mixture. Additionally, the solid content concentration of the mixture during dry mixing is preferably 95% by mass or more, more preferably 97% by mass or more. In dry mixing, it is preferable to mix each in powder form without intentionally adding water or organic solvent. In addition, in the present invention, the solid content concentration (%) of the mixture during dry mixing is obtained by transferring the mixture before mixing, for example, about 3 g of the measurement object at a temperature of 25°C to an aluminum dish and accurately weighing it (mass before drying). ), the moisture is volatilized by drying in a dryer at 105℃ for 24 hours, and then cooled to 25℃ in a desiccator. The mass of the dried product (mass after drying) is precisely measured, and the mass after drying is calculated as the mass before drying. It can be obtained by dividing by and multiplying by 100.

The mixing method for dry mixing is not particularly limited, but mixing using a mixer is preferable. Mixers used for dry mixing include dry tumbler, super mixer, Henschel mixer, flash mixer, air blender, flow jet mixer, drum mixer, ribocon mixer, pug mixer, Nauta mixer, ribbon mixer, Spartan Leuser, and Rodige mixer. , planetary mixers, and further devices such as screw-type kneaders, defoaming kneaders, and paint shakers, and kneaders such as pressure kneaders and two rolls. Among the above-mentioned devices, mixers that can be used relatively easily, such as a planetary mixer that allows dispersion by stirring, are preferable, and planetary mixers and Henschel mixers are especially preferable.

The mixing time for dry mixing is not particularly limited as long as each component is mixed uniformly, but for example, it is preferably 1 minute or more, more preferably 2 minutes or more, even more preferably 10 minutes or more, and preferably 60 minutes or more. minutes or less, more preferably 30 minutes or less, even more preferably 20 minutes or less.

Additionally, in dry mixing, the addition amount ratio of the polymer particles, positive electrode active material, and conductive material is not particularly limited.

For example, the amount of polymer particles added is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, preferably 5 parts by mass or less, and more preferably 4 parts by mass or less per 100 parts by mass of the positive electrode active material. If the amount of polymer particles added per 100 parts by mass of the positive electrode active material is 0.5 parts by mass or more, the copolymer will suitably cover the positive electrode active material in the positive electrode mixture layer formed using the resulting slurry composition, and the high potential durability and peeling strength of the positive electrode will be maintained. You can secure enough. On the other hand, if the blending amount of polymer particles per 100 parts by mass of the positive electrode active material is 5 parts by mass or less, the internal resistance of the secondary battery does not increase excessively.

The amount of the conductive material added is preferably 1 part by mass or more, more preferably 1.5 parts by mass or more, preferably 7 parts by mass or less, and more preferably 5 parts by mass or less per 100 parts by mass of the positive electrode active material. If the amount of the conductive material added is within the above-mentioned range, a conductive path is favorably formed in the positive electrode mixture layer formed using the resulting slurry composition, thereby reducing the internal resistance of the secondary battery and sufficiently ensuring high potential durability of the positive electrode. .

［Dispersion medium mixing process］

Next, the above-mentioned dispersion medium is added to the dry mixture obtained through the dry mixing process and mixed to prepare a slurry composition.

The method of adding the dispersion medium to the dry mixture is not particularly limited, and may be added all at once or sequentially. However, sequential addition is preferable from the viewpoint of obtaining a slurry composition in which each component is uniformly dispersed.

In addition, the amount of the dispersion medium added in the dispersion medium mixing step is not particularly limited. For example, the solid content concentration of the resulting slurry composition is preferably 50% by mass or more, more preferably 60% by mass or more, and preferably 80% by mass. Hereinafter, the dispersion medium is added in an amount that is more preferably 75% by mass or less.

The method of mixing the dry mixture and the dispersion medium is not particularly limited, and for example, mixers such as ball mill, sand mill, bead mill, pigment disperser, brain mill, ultrasonic disperser, homogenizer, planetary mixer, fill mix, etc. A slurry composition can be prepared by mixing the dry mixture and the dispersion medium.

The mixing time in the dispersion medium mixing step is not particularly limited as long as the dry mixture and the dispersion medium are uniformly mixed, but is preferably 1 minute or more, more preferably 2 minutes or more, and still more preferably 10 minutes or more, Preferably it is 120 minutes or less, more preferably 90 minutes or less, and even more preferably 60 minutes or less.

(Positive electrode for secondary battery)

The positive electrode for a secondary battery of the present invention has a current collector and a positive electrode mixture layer formed on the current collector, and the positive electrode mixture layer is a slurry composition for a secondary battery positive electrode obtained by the manufacturing method of the slurry composition for a secondary battery positive electrode. It is formed using .

And, since the positive electrode for secondary batteries of the present invention is produced using the slurry composition for secondary battery positive electrodes obtained by the manufacturing method of the slurry composition for secondary battery positive electrodes of the present invention, it is excellent in high potential durability and peeling strength. . And, the positive electrode for a secondary battery of the present invention can reduce the internal resistance of the secondary battery and enable the secondary battery to exhibit excellent life characteristics even when exposed to high potential for a long time.

＜Manufacturing method of positive electrode＞

In addition, the positive electrode for a secondary battery of the present invention includes, for example, a step of applying the above-described slurry composition on a current collector (application step), drying the slurry composition applied on the current collector, and forming a positive electrode mixture on the current collector. It is manufactured through a layer forming process (drying process).

In addition, the positive electrode for a secondary battery of the present invention can also be manufactured by drying and granulating the above-described slurry composition to prepare composite particles, and using the composite particles to form a positive electrode mixture layer on a current collector.

［Application process］

The method for applying the slurry composition onto the current collector is not particularly limited and any known method can be used. Specifically, as an application method, a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush application method, etc. can be used. At this time, the slurry composition may be applied to only one side of the current collector or to both sides. The thickness of the slurry film on the current collector after application and before drying can be appropriately set according to the thickness of the positive electrode mixture layer obtained by drying.

Here, as the current collector to which the slurry composition is applied, a material that has electrical conductivity and is electrochemically durable is used. Specifically, as the current collector, for example, a current collector made of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. can be used. Among these, aluminum foil is particularly preferable as a current collector used as a positive electrode. In addition, the above-mentioned materials may be used individually, or two or more types may be used in combination at any ratio.

［Drying process］

The method of drying the slurry composition on the current collector is not particularly limited and known methods can be used, for example, drying with warm air, hot air, or low humidity air, vacuum drying, and drying by irradiation with infrared rays or electron beams. You can. By drying the slurry composition on the current collector in this way, a positive electrode mixture layer can be formed on the current collector, and a positive electrode for a secondary battery including the current collector and the positive electrode mixture layer can be obtained.

Additionally, after the drying process, pressure treatment may be applied to the positive electrode mixture layer using a mold press, roll press, etc. By pressure treatment, the adhesion between the positive electrode mixture layer and the current collector can be improved.

Furthermore, when the positive electrode mixture layer contains a curable polymer, it is preferable to cure the polymer after forming the positive electrode mixture layer.

(secondary battery)

The secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolyte, and a separator, and uses the positive electrode for secondary batteries of the present invention as the positive electrode. In addition, since the secondary battery of the present invention is provided with the positive electrode for secondary batteries of the present invention, the increase in internal resistance is suppressed, and even when exposed to a high potential (for example, 4.4 V or more) for a long period of time, Excellent lifespan characteristics.

＜Negative pole＞

As the negative electrode, a known negative electrode can be used. Specifically, as the negative electrode, for example, a negative electrode made of a thin plate of metallic lithium or a negative electrode formed by forming a negative electrode mixture layer on a current collector can be used.

Additionally, as the current collector, one made of metal materials such as iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum can be used. Additionally, as the negative electrode mixture layer, a layer containing a negative electrode active material and a binder can be used. Furthermore, the binder is not particularly limited, and any known material can be used.

＜Electrolyte＞

As the electrolyte solution, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. As a supporting electrolyte for a lithium ion secondary battery, for example, lithium salt is used. Examples of lithium salts include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, etc. can be mentioned. Among them, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li are preferable because they are easily soluble in solvents and exhibit a high degree of dissociation, and LiPF ₆ is particularly preferable. In addition, one type of electrolyte may be used individually, or two or more types may be used in combination in an arbitrary ratio. In general, lithium ion conductivity tends to increase as a supporting electrolyte with a higher degree of dissociation is used, so lithium ion conductivity can be adjusted depending on the type of supporting electrolyte.

The organic solvent used in the electrolyte solution is not particularly limited as long as it can dissolve the supporting electrolyte, but examples include dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), Carbonates such as butylene carbonate (BC) and ethylmethyl carbonate (EMC); Esters such as γ-butyrolactone and methyl formate; Ethers such as 1,2-dimethoxyethane and tetrahydrofuran; Sulfolane, Sulfur-containing compounds such as dimethyl sulfoxide; etc. are suitably used. Additionally, a mixed solution of these solvents may be used. Among them, it is preferable to use carbonates because the dielectric constant is high and the stable potential region is wide, and it is more preferable to use a mixture of ethylene carbonate and ethylmethyl carbonate.

In addition, the concentration of the electrolyte in the electrolyte solution can be adjusted arbitrarily, and for example, it is preferably 0.5 to 15 mass%, more preferably 2 to 13 mass%, and even more preferably 5 to 10 mass%. . Additionally, known additives such as fluoroethylene carbonate or ethylmethyl sulfone can be added to the electrolyte solution.

＜Separator＞

The separator is not particularly limited, and for example, those described in Japanese Patent Application Publication No. 2012-204303 can be used. Among these, polyolefin-based (polyethylene, polypropylene, polybutene, poly A microporous membrane made of a resin (vinyl chloride) is preferable.

＜Manufacturing method of secondary battery＞

In the secondary battery of the present invention, for example, a positive electrode and a negative electrode are overlapped with a separator interposed, this is wound, folded, etc. according to the shape of the battery as needed, placed in a battery container, and an electrolyte solution is injected into the battery container. It can be manufactured by sealing. In order to prevent an increase in the internal pressure of the secondary battery, overcharge and discharge, etc., overcurrent prevention elements such as fuses and PTC elements, expanded metal, lead plates, etc. may be installed as necessary. The shape of the secondary battery may be, for example, a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, or a flat shape.

Example

Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited to these examples. In addition, in the following description, “%” and “part” indicating quantities are based on mass, unless otherwise specified.

In addition, in a polymer manufactured by copolymerizing multiple types of monomers, unless otherwise specified, the ratio in the polymer of the structural unit formed by polymerizing a certain monomer is usually the total monomer used in the polymerization of the polymer. It corresponds to the ratio (input ratio) of a certain monomer in .

In the examples and comparative examples, the volume average particle diameter D50 of each particle, the molecular weight (weight average molecular weight, number average molecular weight, molecular weight distribution) and glass transition temperature of the copolymer, the peel strength and high potential durability of the positive electrode, and 2 The internal resistance of the car battery was measured and evaluated by the following method.

＜Volume average particle diameter D50＞

In the particle size distribution measured dry using a laser diffraction/scattering type particle size distribution measuring device (“Microtruck MT3200II” manufactured by Nikkiso Co., Ltd.), the particle size at which the cumulative volume calculated from the small diameter side is 50% was determined.

＜Molecular weight＞

The weight average molecular weight (Mw) and number average molecular weight (Mn) of the copolymer contained in the polymer particles were measured by gel permeation chromatography (GPC) under the following measurement conditions using a LiBr-DMF solution with a concentration of 10mM. , and also calculated the molecular weight distribution (Mw/Mn).

· Separation column: Shodex KD-806M (made by Showa Electric Co., Ltd.)

・Detector: Differential refractometer detector RID-10A (manufactured by Shimadzu Corporation)

· Flow rate of eluent: 0.3mL/min

· Column temperature: 40℃

· Standard polymer: TSK standard polystyrene (manufactured by Tosoh Corporation)

＜Glass transition temperature＞

The polymer particles were molded to obtain a film with a thickness of 1 ± 0.3 mm. This film was dried in a hot air oven at 120°C for 1 hour. Thereafter, using the dried film as a sample, DSC6220SII (differential scanning calorimeter, manufactured by Nanotechnology) was used under the conditions of a measurement temperature of -100°C or more and 180°C or less and a temperature increase rate of 5°C/min in accordance with JIS K7121. The glass transition temperature (°C) of the copolymer contained in the polymer particles was measured.

＜Peel strength＞

The positive electrodes produced in the examples and comparative examples were cut into squares with a width of 1.0 cm and a length of 10 cm to prepare test pieces. Then, a cellophane tape was attached to the surface of the test piece on the positive electrode mixture layer side. At this time, the cellophane tape specified in JIS Z1522 was used. After that, the stress was measured when the test piece was peeled from one end toward the other end at a speed of 50 mm/min while the cellophane tape was fixed to the test table. The measurement was performed 10 times, the average value of the stress was obtained, and this was used as the peel strength and evaluated according to the following standards. The greater the peeling strength, the better the adhesion of the positive electrode mixture layer to the current collector.

A: Peel strength is 50 N/m or more

B: Peel strength is 10 N/m or more but less than 50 N/m

C: Peel strength less than 10 N/m

＜High potential durability＞

The slurry composition for secondary battery positive electrode prepared in the examples and comparative examples was applied with a comma coater on an aluminum foil (thickness 20 μm) as a current collector so that the weight per unit area after drying was 20 mg/cm 2 , and heated for 20 mg/cm ² at 90°C. After drying at 120°C for 20 minutes, heat treatment was performed again at 60°C under vacuum for 10 hours to obtain a positive electrode including a positive electrode mixture layer on a current collector.

This positive electrode was cut into a circle with a diameter of 12 mm, and on the positive electrode mixture layer side of the cut positive electrode, a circular polypropylene porous film (diameter 18 mm, thickness 25 μm), metallic lithium (diameter 14 mm), and expanded metal were placed in this order. was laminated to obtain a laminate. This laminate was stored in a coin-shaped external container made of stainless steel (diameter 20 mm, height 1.8 mm, stainless steel thickness 0.25 mm) equipped with polypropylene packing. An electrolyte solution (LiPF ₆ solution with a concentration of 1.0M (the solvent is a mixed solvent of ethylene carbonate/ethylmethyl carbonate = 3/7 (weight ratio))) was injected into this container so that no air remained. After injection of the electrolyte, the external container was secured with a stainless steel cap with a thickness of 0.2 mm through polypropylene packing, and the battery can was sealed to produce a coin cell with a diameter of 20 mm and a thickness of about 2 mm.

A voltage of 4.4 V was applied to the obtained coin cell in an atmosphere of 25°C for 10 hours. The current density per unit mass (mA/g) of the positive electrode mixture layer flowing after 10 hours was determined and set as the oxidation current density. The smaller the oxidation current density, the better the copolymer with high high potential durability is coated with the positive electrode active material, and the oxidation reaction on the surface of the positive electrode active material when high voltage is applied and the oxidation reaction of the electrolyte near the surface of the positive electrode active material are suppressed. This means that the positive electrode has excellent high potential durability.

A: Oxidation current density is less than 0.2mA/g

B: Oxidation current density is 0.2 mA/g or more and less than 0.3 mA/g

C: Oxidation current density 0.3 mA/g or more but less than 0.4 mA/g

D: Oxidation current density is 0.4 mA/g or more and less than 0.5 mA/g

E: Oxidation current density is 0.5mA/g or more

＜Internal resistance＞

In order to evaluate the internal resistance of the secondary battery, the IV resistance was measured as follows. In an atmosphere of 25°C, charge to 50% of SOC (State of Charge: depth of charge) at 1C (C is a value expressed as rated capacity (mA)/1h (time)), then charge with 50% of SOC as the center. Charging for 20 seconds and discharging for 20 seconds were performed at 0.5C, 1.0C, 1.5C, and 2.0C, respectively. The battery voltage after 20 seconds in each case (charging side and discharging side) was plotted against the current value, and the slope was obtained as IV resistance (Ω) (IV resistance during charging and IV resistance during discharging). The obtained IV resistance value (Ω) was evaluated based on the following standards. The smaller the value of IV resistance, the smaller the internal resistance.

A: IV resistance is 2.0Ω or less

B: IV resistance exceeds 2.0Ω but does not exceed 2.3Ω

C：IV resistance is more than 2.3Ω and less than 2.5Ω

D: IV resistance exceeds 2.5Ω but does not exceed 3.0Ω

E：IV resistance exceeds 3.0Ω

(Example 1)

<Preparation of polymer particles for secondary battery positive electrode>

Put 400 parts of ion-exchanged water into a pressure-resistant vessel equipped with a stirrer, thermometer, cooling pipe, and nitrogen gas introduction pipe, and repeat three times the pressure reduction (-600 mmHg) and normal pressure with nitrogen gas while gently rotating the stirrer. It was confirmed using a dissolved oxygen meter that the oxygen concentration in the gas phase portion of the reaction vessel was 1% or less and that the dissolved oxygen in water was 1 ppm or less. After that, 0.2 part of partially saponified polyvinyl alcohol (manufactured by Nippon Chemical Industry Co., Ltd., “Gosenol GH-20” (degree of saponification 86.5 mol% to 89.0 mol%)) was slowly added as a dispersant and dispersed well, and then the temperature was raised to 60°C. Stirring was continued for 30 minutes to dissolve the partially saponified polyvinyl alcohol.

Subsequently, under the condition of a nitrogen gas ventilation rate of 0.5 ml/min, 85 parts of acrylonitrile as a nitrile group-containing monomer, 5 parts of methacrylic acid as a hydrophilic group-containing monomer, and 0.2 parts of t-dodecyl mercaptan as a chain transfer agent were added, It was stirred and mixed and maintained at 60 ± 2°C. Here, 0.4 parts of 1,1-azobis (1-acetoxy-1-phenylethane) (Otsuka Chemical Co., Ltd., “OTAZO-15”; abbreviated name OTazo-15), which is an oil-soluble polymerization initiator, contains a nitrile group. The reaction was started by adding a solution dissolved in 10 parts of acrylonitrile as a monomer. After the reaction was performed at 60 ± 2°C for 3 hours, the reaction was further continued at 70 ± 2°C for 2 hours, and then again at 80 ± 2°C for 2 hours. After that, it was cooled to 40°C or lower to obtain polymer particles containing the copolymer. The obtained polymer particles were collected in a 200-mesh filter cloth, washed three times with 100 parts of ion-exchanged water, and then dried under reduced pressure at 70°C for 12 hours to isolate and purify (recovery rate: 70%). The volume average particle diameter D50 of the polymer particles after this isolation and purification, and the molecular weight (weight average molecular weight, number average molecular weight, molecular weight distribution) and glass transition temperature of the copolymer constituting the polymer particles were measured. The results are shown in Table 1.

<Preparation of slurry composition for secondary battery positive electrode>

100 parts of a ternary active material (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) with a layered structure as a positive electrode active material (volume average particle diameter D50: 10 μm), and acetylene black as a conductive material (Denka black powder: Denki Chemical Industry, specific surface area: 68 m ² 3.0 parts (/g, number average particle diameter: 35 nm) and 2.0 parts of the above-described polymer particles were placed in a planetary mixer and mixed dry for 20 minutes at a stirring blade rotation speed of 5 rpm and a solid content concentration exceeding 90% to obtain a dry mixture ( dry mixing process). A slurry composition for secondary battery positive electrodes was obtained by sequentially adding an appropriate amount of NMP as a dispersion medium to the obtained dry mixture and mixing for 20 minutes (dispersion medium mixing step). Additionally, the solid content concentration of the obtained slurry composition was 70 mass%, and the viscosity at 60 rpm measured with a B-type viscometer according to JIS Z8803:1991 was 4400 mPa·s (25°C, spindle shape: 4). Then, the high potential durability of the positive electrode was evaluated using this slurry composition. The results are shown in Table 1.

<Production of positive electrode for secondary battery>

An aluminum foil with a thickness of 20 μm was prepared as a current collector. And the slurry composition for secondary battery positive electrodes mentioned above was applied to one side of the aluminum foil so that the application amount after drying was 20 mg/cm ² , and dried at 90°C for 20 minutes and at 120°C for 20 minutes. After that, heat treatment was performed again at 60°C for 10 hours to obtain a positive electrode fabric. This positive electrode fabric was rolled with a roll press to produce a positive electrode with a thickness of 70 μm consisting of a positive electrode mixture layer with a density of 3.2 g/cm ³ and aluminum foil. Using the obtained positive electrode, the peeling strength of the positive electrode was evaluated. The results are shown in Table 1.

<Production of negative electrode for secondary battery>

In a planetary mixer equipped with a disper, 100 parts of artificial graphite (volume average particle diameter D50: 24.5 μm, specific surface area 4 m ² /g) as a negative electrode active material and a 1% aqueous solution of carboxymethyl cellulose as a dispersant (Daiichi Kogyo Pharmaceutical Co., Ltd.) , "BSH-12") was added in an amount equivalent to the solid content, and the solid content concentration was adjusted to 55% with ion-exchanged water, followed by mixing at 25°C for 60 minutes. Then, after adjusting the solid content concentration to 52% with ion-exchanged water, the mixture was mixed again at 25°C for 15 minutes to obtain a mixed solution. To this mixed solution, 1.0 parts of a 40% aqueous dispersion of styrene-butadiene copolymer (glass transition temperature: -15°C) as a binder (equivalent to solid content) and ion-exchanged water were added to adjust the final solid concentration to 50%, and 10% Mixed for another minute. This was degassed under reduced pressure to obtain a slurry composition for secondary battery negative electrodes with good fluidity.

The slurry composition for secondary battery negative electrode was applied using a comma coater on a copper foil having a thickness of 20 μm as a current collector so that the film thickness after drying was about 150 μm and dried. This drying was performed by transporting the copper foil through an oven at 60°C for 2 minutes at a speed of 0.5 m/min. After that, heat treatment was performed at 120°C for 2 minutes to obtain a negative electrode fabric. This negative electrode fabric was rolled with a roll press to produce a negative electrode whose negative electrode mixture layer had a thickness of 80 μm.

＜Preparation of separator＞

A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 25 μm, manufactured by dry method, porosity 55%) was cut into a 5 cm × 5 cm square.

＜Manufacture of secondary batteries＞

As the exterior of the battery, an aluminum wrapping exterior was prepared. The positive electrode obtained above was cut into a 4 cm x 4 cm square and placed so that the surface on the current collector side was in contact with the aluminum wrapping exterior. Next, the 5 cm x 5 cm square separator obtained above was placed on the surface of the positive electrode mixture layer of the positive electrode. Furthermore, the negative electrode obtained above was cut into a square of 4.2 cm x 4.2 cm, and this was placed on the separator so that the surface on the negative electrode mixture layer side faced the separator. Then, an electrolyte (LiPF ₆ solution with a concentration of 1.0 M (the solvent is a mixed solvent of ethylene carbonate/ethylmethyl carbonate = 3/7 (weight ratio), and 1.5 volume% (solvent ratio) of vinylene carbonate is added as an additive))) Charged. Furthermore, in order to seal the opening of the aluminum wrapping material exterior, heat sealing was performed at 150°C to close the aluminum wrapping material exterior, and a lithium ion secondary battery was manufactured. Using the obtained lithium ion secondary battery, internal resistance was evaluated. The results are shown in Table 1.

(Example 2, 3)

When preparing the polymer particles, polymer particles for secondary battery positive electrode and slurry for secondary battery positive electrode were prepared in the same manner as in Example 1, except that the amount of partially saponified polyvinyl alcohol as a dispersant was changed to 0.1 part and 2.0 part, respectively. A composition, a positive electrode for a secondary battery, a negative electrode for a secondary battery, and a secondary battery were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Examples 4 to 6)

Polymer particles for secondary battery positive electrode, slurry composition for secondary battery positive electrode, and secondary battery positive electrode were prepared in the same manner as in Example 1, except that when preparing the polymer particles, the monomers shown in Table 1 were used in the ratios shown in the table. A positive electrode for a battery, a negative electrode for a secondary battery, and a secondary battery were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Additionally, in Example 6, 2-ethylhexyl acrylate was used as the (meth)acrylic acid ester monomer.

(Example 7)

Polymer particles for secondary battery positive electrode, slurry composition for secondary battery positive electrode, A positive electrode for a secondary battery, a negative electrode for a secondary battery, and a secondary battery were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 1)

Polymer particles for secondary battery positive electrode, slurry composition for secondary battery positive electrode, and secondary battery positive electrode were prepared in the same manner as in Example 1, except that particles made of polyvinylidene fluoride (volume average particle diameter D50: 300 μm) were used as polymer particles. A positive electrode for a battery, a negative electrode for a secondary battery, and a secondary battery were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 2)

<Preparation of varnish for secondary battery positive electrode>

100 parts of the polymer particles obtained in Example 1 and 1800 parts of NMP were placed in a pressure-resistant container equipped with a stirrer, thermometer, cooling tube, and nitrogen gas introduction tube, and the mixture was stirred for 80 minutes while a very small amount (200 ml/min) of nitrogen gas was ventilated. The temperature was raised to ±2°C and maintained for 3 hours. In order to remove the contained moisture, stirring and dissolution was performed at 85 ± 2°C and under reduced pressure (25 torr or less) until the moisture content became 1000 ppm or less. After that, it was cooled to 40°C or lower and filtered using a 100 μm filtration filter to obtain varnish for secondary battery positive electrode (solid content: 6%).

<Preparation of slurry composition for secondary battery positive electrode>

100 parts of a ternary active material (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) with a layered structure as a positive electrode active material (volume average particle diameter D50: 10 μm), and acetylene black as a conductive material (Denka black powder: Denki Chemical Industry, specific surface area: 68 m ² 3.0 parts (/g, number average particle diameter 35 nm) were put into a planetary mixer and mixed for 20 minutes at a rotation speed of the stirring blade of 5 rpm to obtain a mixture. A slurry composition for secondary battery positive electrodes was obtained by mixing for 20 minutes while sequentially adding the above-mentioned varnish (2.0 parts in solid content) and an appropriate amount of NMP as a dispersion medium to the obtained mixture. Additionally, the solid content concentration of the obtained slurry composition was 70 mass%, and the viscosity at 60 rpm measured with a B-type viscometer according to JIS Z8803:1991 was 4400 mPa·s (25°C, spindle shape: 4). Using this slurry composition, the high potential durability of the positive electrode was evaluated. The results are shown in Table 1.

Then, except that the slurry composition for a secondary battery positive electrode obtained in this way was used, a positive electrode for a secondary battery, a negative electrode for a secondary battery, and a secondary battery were produced in the same manner as in Example 1. An evaluation was conducted. The results are shown in Table 1.

(Comparative Example 3)

When preparing the polymer particles, polymer particles for secondary battery positive electrode, slurry composition for secondary battery positive electrode, and secondary battery positive electrode were prepared in the same manner as in Example 1, except that the amount of partially saponified polyvinyl alcohol as a dispersant was changed to 3.0 parts. A positive electrode for a battery, a negative electrode for a secondary battery, and a secondary battery were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 4)

When preparing the polymer particles, the polymer particles for secondary battery positive electrode, slurry composition for secondary battery positive electrode, and secondary battery positive electrode were prepared in the same manner as in Example 1, except that the monomers shown in Table 1 were used in the ratio shown in the table. A positive electrode, a negative electrode for a secondary battery, and a secondary battery were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Additionally, in Comparative Example 4, 2-ethylhexyl acrylate was used as the (meth)acrylic acid ester monomer.

Additionally, in Table 1 shown below,

“AN” represents acrylonitrile,

“MAA” represents methacrylic acid,

“2EHA” represents 2-ethylhexyl acrylate,

“PVDF” represents polyvinylidene fluoride.

From Examples 1 to 7 and Comparative Examples 1 to 4 in Table 1, it can be seen that in Examples 1 to 7, positive electrodes excellent in high potential durability were obtained. Moreover, in Examples 1 to 7, it can be seen that the positive electrode is excellent in peeling strength and that the internal resistance of the secondary battery can be sufficiently reduced.

Here, from Examples 1 to 3 in Table 1, by adjusting the volume average particle diameter D50 of the polymer particles, the peeling strength and high potential durability of the positive electrode can be further improved, and the internal resistance of the secondary battery can be further reduced. You can see what can be done.

And, from Examples 1, 4 to 6 in Table 1, by changing the composition of the copolymer contained in the polymer particles, the peeling strength and high potential durability of the positive electrode can be further improved, and the internal resistance of the secondary battery can be further improved. It can be seen that it can be further reduced.

In addition, from Examples 1 and 7 in Table 1, by adjusting the molecular weight distribution of the copolymer contained in the polymer particles, the high potential durability of the positive electrode can be further improved, and the internal resistance of the secondary battery can be further reduced. You can see what can be done.

According to the present invention, a method for producing a slurry composition for a secondary battery positive electrode that exhibits excellent high potential durability in the positive electrode can be provided.

Moreover, according to the present invention, it is possible to provide a positive electrode for a secondary battery excellent in high potential durability, and a secondary battery comprising the positive electrode for a secondary battery and excellent in battery characteristics.

Claims (7)

A process of dry mixing polymer particles for a secondary battery positive electrode, a positive electrode active material, and a conductive material to obtain a dry mixture;
A step of mixing the dry mixture and the dispersion medium to obtain a slurry composition for a secondary battery positive electrode,
The polymer particles for secondary battery positive electrode contain a copolymer and have a volume average particle diameter D50 of 1 μm or more,
The copolymer contains 80 to 99.9% by mass of monomer units containing a nitrile group, and 0.1 to 20% by mass of monomer units having a carboxylic acid group,
And the method for producing a slurry composition for secondary battery positive electrodes, wherein the ratio of the volume average particle diameter D50 of the polymer particles for secondary battery positive electrodes to the volume average particle diameter D50 of the positive electrode active material is 0.1 or more. According to claim 1,
A secondary battery positive electrode, wherein the volume average particle diameter D50 of the polymer particles for the secondary battery positive electrode is 2000 μm or less, and the ratio of the volume average particle diameter D50 of the polymer particles for the secondary battery positive electrode to the volume average particle diameter D50 of the positive electrode active material is 200 or less. Method for producing a slurry composition. According to claim 1,
A method for producing a slurry composition for a secondary battery positive electrode, wherein the molecular weight distribution (Mw/Mn) of the copolymer is 10 or less. According to claim 1,
A method for producing a slurry composition for a secondary battery positive electrode, wherein the glass transition temperature of the copolymer is 60°C or more and 170°C or less. Equipped with a current collector and a positive electrode mixture layer formed on at least one surface of the current collector,
A positive electrode for a secondary battery, wherein the positive electrode mixture layer is formed from a slurry composition for a secondary battery positive electrode obtained by the method for producing a slurry composition for a secondary battery positive electrode according to any one of claims 1 to 4. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the positive electrode is the positive electrode for a secondary battery according to claim 5. delete