CN110061187B - Binder composition for negative electrode of lithium ion secondary battery, slurry composition for negative electrode, and lithium ion secondary battery - Google Patents

Abstract

The invention relates to a binder composition for a negative electrode of a lithium ion secondary battery, a slurry composition for a negative electrode, and a lithium ion secondary battery. The binder composition for a negative electrode of a lithium ion secondary battery can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics. The binder composition for a negative electrode of a lithium ion secondary battery of the present invention comprises a particulate polymer and water, wherein the particulate polymer comprises: 50 to 80% by mass of an aromatic vinyl monomer unit, 20 to 40% by mass of an aliphatic conjugated diene monomer unit, 0.5 to 10% by mass of an ethylenically unsaturated carboxylic acid monomer unit, and 0.1 to 3% by mass of a (meth) acrylate monomer unit, wherein the particle polymer has a THF swelling degree of 3 to 10 times, and the (meth) acrylate monomer unit contains a hydroxyl group-containing (meth) acrylate monomer unit.

Description

Binder composition for negative electrode of lithium ion secondary battery, slurry composition for negative electrode, and lithium ion secondary battery

The present invention is a divisional application of chinese patent application having application number 201480063662.1, application date 2014, 12/8, entitled "binder composition for negative electrode of lithium ion secondary battery, slurry composition for negative electrode, and lithium ion secondary battery".

Technical Field

The present invention relates to a binder composition for a negative electrode of a lithium ion secondary battery, a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

Background

Lithium ion secondary batteries are small in size, light in weight, high in energy density, and capable of repeated charge and discharge, and have been used in a wide range of applications. Therefore, in recent years, for the purpose of higher performance of lithium ion secondary batteries, improvement of battery members such as electrodes has been studied.

Here, battery members such as electrodes (positive electrode and negative electrode) of a lithium ion secondary battery are formed by binding components contained in these battery members to each other or to a base material (for example, a current collector or the like) with a binder. Specifically, for example, a negative electrode of a lithium ion secondary battery generally includes a current collector and a negative electrode mixture layer (also referred to as a "negative electrode active material layer") formed on the current collector. The negative electrode mixture layer is formed by, for example, applying a slurry composition in which a binder composition containing a particulate polymer and a negative electrode active material or the like are dispersed in a dispersion medium to a current collector and drying the applied slurry composition to bind the negative electrode active material or the like with the particulate polymer.

In addition, with respect to such a slurry composition, in recent years, attention has been increasingly paid to an aqueous slurry composition using an aqueous medium as a dispersion medium from the viewpoint of reducing environmental load and the like.

Therefore, in order to achieve further performance improvement of the lithium ion secondary battery, improvement of a binder composition and a slurry composition using an aqueous medium as a dispersion medium for forming an electrode has been attempted.

For example, patent document 1 reports the following: the stability of a slurry composition for an electrode, which is an aqueous dispersion containing an electrode active material and a binder, can be improved so that the adhesion of an electrode composite layer (also referred to as an "electrode active material layer") to a current collector is good by using a binder for a secondary battery electrode formed from a copolymer latex obtained by emulsion polymerization of monomers including: 2 to 30 mass% of a hydroxyl group-containing (meth) acrylate monomer, 10 to 50 mass% of an aliphatic conjugated diene monomer, 0.1 to 10 mass% of an ethylenically unsaturated carboxylic acid monomer, and 10 to 87.9 mass% of another monomer copolymerizable with these monomers.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2010-140841

Disclosure of Invention

Problems to be solved by the invention

Here, in the lithium ion secondary battery, the negative electrode active material may expand and contract with charge and discharge. As described above, if the expansion and contraction of the negative electrode active material are repeated, the binder cannot sufficiently follow the expansion and contraction, and the electrical characteristics such as the cycle characteristics may be degraded. Therefore, in order to obtain a lithium ion secondary battery having excellent electrical characteristics such as cycle characteristics, it is required that a binder used for a negative electrode of the lithium ion secondary battery can sufficiently follow expansion and contraction of a negative electrode active material accompanying charge and discharge.

In addition, in the lithium ion secondary battery, gas is generated by decomposition of an electrolyte additive or the like under high-temperature storage, and the battery cell expands, thereby reducing the battery capacity, that is, the high-temperature storage characteristics may be impaired. Therefore, lithium ion secondary batteries are required to ensure high-temperature storage characteristics by suppressing the swelling of the battery cells during high-temperature storage.

However, the above conventional adhesives cannot achieve all of the following properties at a sufficiently high level: has sufficient followability to expansion and contraction of the negative electrode active material accompanying charge and discharge, suppresses expansion of the battery cell during high-temperature storage, and has high-temperature storage characteristics. Therefore, there is room for improvement in the negative electrode formed using the conventional binder and in the lithium ion secondary battery using the negative electrode, in order to ensure excellent cycle characteristics and also to ensure high-temperature storage characteristics by suppressing the swelling of the battery cell during high-temperature storage.

Accordingly, an object of the present invention is to provide a binder composition for a secondary battery negative electrode, which, when used for forming a negative electrode, can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics.

It is another object of the present invention to provide a slurry composition for a secondary battery negative electrode, which, when used for forming a negative electrode, can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell at high temperatures and ensure high-temperature storage characteristics.

Further, an object of the present invention is to provide a negative electrode for a lithium ion secondary battery, which can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high temperature storage characteristics.

Further, an object of the present invention is to provide a lithium ion secondary battery having excellent cycle characteristics and high-temperature storage characteristics.

Means for solving the problems

The present inventors have conducted intensive studies with a view to solving the above problems. The present inventors have found that a particulate polymer containing an aromatic vinyl monomer unit, an aliphatic conjugated diene monomer unit, an ethylenically unsaturated carboxylic acid monomer unit, and a (meth) acrylate monomer unit in a predetermined ratio and having a swelling degree with respect to Tetrahydrofuran (THF) within a predetermined range has appropriate elasticity and exhibits good followability to expansion and contraction of a negative electrode active material. Further, the present inventors have also found that a binder composition containing the particulate polymer ensures good cycle characteristics and high-temperature storage characteristics, and have completed the present invention.

That is, an object of the present invention is to effectively solve the above problems, and the binder composition for a negative electrode of a lithium ion secondary battery according to the present invention contains a particulate polymer and water, the particulate polymer containing: 50 to 80% by mass of an aromatic vinyl monomer unit, 20 to 40% by mass of an aliphatic conjugated diene monomer unit, 0.5 to 10% by mass of an ethylenically unsaturated carboxylic acid monomer unit, and 0.1 to 3% by mass of a (meth) acrylate monomer unit, wherein the THF swelling degree of the particulate polymer is 3 to 10 times. Thus, if the particulate polymer containing the monomer units in the predetermined ratio and having the THF swelling degree within the predetermined range is used, a lithium ion secondary battery having excellent cycle characteristics can be provided when used for forming a negative electrode, and swelling of the battery cell at high temperatures can be suppressed to ensure high-temperature storage characteristics.

Here, in the binder composition for a negative electrode of a lithium ion secondary battery of the present invention, the particulate polymer preferably has an electrolyte swelling degree of 1 to 2 times. If a particulate polymer having an electrolyte swelling degree within the above-mentioned predetermined range is used, the particulate polymer is moderately swollen in the electrolyte of the lithium ion secondary battery, and therefore, it is possible to ensure lithium ion conductivity and to ensure charge and discharge characteristics such as cycle characteristics. Further, if the particulate polymer is used, the negative electrode active material and other particles in the negative electrode binder layer are appropriately bonded, and these substances can be sufficiently suppressed from falling off from the current collector, so that the adhesion strength between the negative electrode binder layer and the current collector can be improved.

In the binder composition for a negative electrode of a lithium ion secondary battery of the present invention, it is preferable that the amount of surface acid of the particulate polymer is 0.20mmol/g or more, and a value obtained by dividing the amount of surface acid (mmol/g) of the particulate polymer by the amount of acid (mmol/g) of the particulate polymer in an aqueous phase is 1.0 or more. When the surface acid amount of the particulate polymer is set to the above value or more and the relationship between the surface acid amount and the acid amount of the particulate polymer in the aqueous phase is set to the above relationship, the stability of the particulate polymer can be secured and the viscosity stability of the slurry composition using the binder composition containing the particulate polymer can be improved. In addition, the adhesion between the negative electrode mixture layer obtained from the binder composition containing the particulate polymer and the current collector can be improved, and electrical characteristics such as cycle characteristics of the lithium ion secondary battery can be ensured.

Further, in the binder composition for a negative electrode of a lithium ion secondary battery of the present invention, it is preferable that the ethylenically unsaturated carboxylic acid monomer unit of the particulate polymer contains an itaconic acid monomer unit. If the particulate polymer contains a monomer unit derived from itaconic acid, the viscosity stability of a slurry composition using a binder composition containing the particulate polymer can be improved.

In the binder composition for a negative electrode of a lithium ion secondary battery of the present invention, the (meth) acrylate monomer unit of the particulate polymer preferably contains a 2-hydroxyethyl acrylate monomer unit. If the particulate polymer contains a monomer unit derived from 2-hydroxyethyl acrylate, the viscosity stability of a slurry composition using a binder composition containing the particulate polymer can be improved.

The present invention is also directed to effectively solve the above problems, and is characterized in that the slurry composition for a negative electrode of a lithium ion secondary battery of the present invention contains a negative electrode active material and any one of the above binder compositions for a negative electrode of a lithium ion secondary battery. Thus, if a slurry composition containing a negative electrode active material and any one of the above-described binder compositions for a negative electrode of a lithium ion secondary battery is used for forming a negative electrode, a lithium ion secondary battery having excellent cycle characteristics can be provided, and swelling of a battery cell at high temperatures can be suppressed to ensure high-temperature storage characteristics.

In addition, the present invention is directed to effectively solve the above problems, and is characterized in that the negative electrode for a lithium ion secondary battery of the present invention has a negative electrode binder layer obtained by using the slurry composition for a negative electrode for a lithium ion secondary battery. By using the negative electrode having the negative electrode mixture layer obtained from the slurry composition, a lithium ion secondary battery having excellent cycle characteristics and high-temperature storage characteristics can be provided.

In addition, the present invention is directed to effectively solve the above problems, and a lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, an electrolyte solution, and a separator, wherein the negative electrode is a negative electrode for a lithium ion secondary battery manufactured by the above method for manufacturing a negative electrode for a lithium ion secondary battery. The lithium ion secondary battery of the present invention is excellent in cycle characteristics and high-temperature storage characteristics.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a binder composition for a secondary battery negative electrode can be provided, which, when used for forming a negative electrode, can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics.

Further, according to the present invention, there can be provided a slurry composition for a secondary battery negative electrode, which, when used for forming a negative electrode, can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics.

Further, according to the present invention, it is possible to provide a negative electrode for a lithium ion secondary battery, which can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics.

Further, according to the present invention, a lithium ion secondary battery having excellent cycle characteristics and high-temperature storage characteristics can be provided.

Drawings

FIG. 1 is a graph obtained by plotting the electrical conductivity (ms) against the cumulative amount (mmol) of hydrochloric acid added when calculating the surface acid amount of a particulate polymer and the acid amount in an aqueous phase.

Detailed description of the invention

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

Here, the binder composition for a negative electrode of a lithium ion secondary battery of the present invention is used for preparation of a slurry composition for a negative electrode of a lithium ion secondary battery. The slurry composition for a negative electrode of a lithium ion secondary battery of the present invention is used for forming a negative electrode of a lithium ion secondary battery. Further, the negative electrode for a lithium ion secondary battery of the present invention is characterized by having a negative electrode binder layer formed from the slurry composition for a negative electrode for a lithium ion secondary battery of the present invention. Further, the lithium ion secondary battery of the present invention is characterized by using the negative electrode for a lithium ion secondary battery of the present invention.

(Binder composition for negative electrode of lithium ion Secondary Battery)

The binder composition for a negative electrode of a lithium ion secondary battery of the present invention contains a particulate polymer and water. The binder composition for a negative electrode of a lithium ion secondary battery of the present invention contains 50 to 80 mass% of an aromatic vinyl monomer unit, 20 to 40 mass% of an aliphatic conjugated diene monomer unit, 0.5 to 10 mass% of an ethylenically unsaturated carboxylic acid monomer unit, and 0.1 to 3 mass% of a (meth) acrylate monomer unit, and the particle polymer has a THF swelling degree of 3 to 10 times.

According to the binder composition for a negative electrode of a lithium ion secondary battery of the present invention, since a particulate polymer containing an aromatic vinyl monomer unit, an aliphatic conjugated diene monomer unit, an ethylenically unsaturated carboxylic acid monomer unit, and a (meth) acrylate monomer unit at a predetermined ratio and having a THF swelling degree within a predetermined range is used, the cycle characteristics of the lithium ion secondary battery can be improved, and the swelling of the battery cell at high temperature can be suppressed to ensure high-temperature storage characteristics.

The following description will be made of the particulate polymer contained in the binder composition for a negative electrode of a lithium ion secondary battery.

< particulate Polymer >

The particulate polymer is composed of the following components: when a negative electrode is formed using the binder composition for a secondary battery negative electrode of the present invention, a component contained in the negative electrode binder layer (for example, a negative electrode active material) can be retained in the manufactured negative electrode without being separated from the negative electrode member. Here, in general, when the particulate polymer in the negative electrode mixture layer is impregnated with the electrolyte solution, the particulate polymer can maintain the particulate shape even though it absorbs the electrolyte solution and swells, and the negative electrode active materials are bonded to each other, thereby preventing the negative electrode active materials from falling off from the current collector. The particulate polymer also serves to bind particles other than the negative electrode active material contained in the negative electrode binder layer and to maintain the strength of the negative electrode binder layer.

The "particulate polymer" refers to a polymer that can be dispersed in an aqueous medium such as water, and exists in the form of particles in the aqueous medium. In general, when 0.5g of the particulate polymer is dissolved in 100g of water at 25 ℃, the insoluble content of the particulate polymer is 90 mass% or more.

[ composition of particulate Polymer ]

The particulate polymer used in the present invention has a proportion of an aromatic vinyl monomer unit of 50 to 80% by mass, a proportion of an aliphatic conjugated diene monomer unit of 20 to 40% by mass, a proportion of an ethylenically unsaturated carboxylic acid monomer unit of 0.5 to 10% by mass, and a proportion of a (meth) acrylate monomer unit of 0.1 to 3% by mass, based on the total monomer units. The particulate polymer may contain a monomer unit other than the above-described monomer units (aromatic vinyl monomer unit, aliphatic conjugated diene monomer unit, ethylenically unsaturated carboxylic acid monomer unit, and (meth) acrylate monomer unit).

The phrase "comprising … … monomer units" as used herein means that "a constitutional unit derived from the monomer is contained in a polymer obtained by using the monomer".

In the present invention, "(meth) acrylic acid" means acrylic acid and/or methacrylic acid.

The following will describe monomers that can be used for producing a particulate polymer used in the present invention.

[ [ aromatic vinyl monomer ] ]

The aromatic vinyl monomer that can form the aromatic vinyl monomer unit of the particulate polymer is not particularly limited, and examples thereof include: styrene, α -methylstyrene, vinyltoluene, divinylbenzene, etc., and among them, styrene is preferable. These may be used alone in 1 kind, or may be used in combination in 2 or more kinds at an arbitrary ratio.

In the particulate polymer, the content of the aromatic vinyl monomer unit is necessarily 50% by mass or more, preferably 56% by mass or more, more preferably 62% by mass or more, and is necessarily 80% by mass or less, preferably 79.4% by mass or less, more preferably 74% by mass or less, and particularly preferably 68% by mass or less. If the content ratio of the aromatic vinyl monomer unit is not within the above range, adhesion between the negative electrode mixture layer and the current collector cannot be secured, and cycle characteristics deteriorate.

[ [ aliphatic conjugated diene monomer ] ]

The aliphatic conjugated diene monomer that can form the aliphatic conjugated diene monomer unit of the particulate polymer is not particularly limited, and examples thereof include: 1, 3-butadiene, 2-methyl-1, 3-butadiene, 2, 3-dimethyl-1, 3-butadiene, 2-chloro-1, 3-butadiene, substituted straight-chain conjugated pentadienes, substituted and side-chain conjugated hexadienes, of which 1, 3-butadiene is preferred. These may be used alone in 1 kind, or may be used in combination in 2 or more kinds at an arbitrary ratio.

In the particulate polymer, the content ratio of the aliphatic conjugated diene monomer unit is 20% by mass or more, preferably 26% by mass or more, more preferably 32% by mass or more, and necessarily 40% by mass or less, preferably 38% by mass or less, more preferably 35% by mass or less. If the content ratio of the aliphatic conjugated diene monomer unit is less than 20% by mass, the flexibility of the particulate polymer cannot be ensured, and it becomes difficult to follow the expansion and contraction of the negative electrode active material, and the cycle characteristics cannot be ensured. On the other hand, if the content ratio of the aliphatic conjugated diene monomer unit exceeds 40 mass%, adhesion between the negative electrode mixture layer and the current collector cannot be secured, and cycle characteristics and high-temperature storage characteristics deteriorate.

[ [ ethylenically unsaturated carboxylic acid monomer ] ]

Examples of the ethylenically unsaturated carboxylic acid monomer that can form the ethylenically unsaturated carboxylic acid monomer unit of the particulate polymer include ethylenically unsaturated monocarboxylic acids and derivatives thereof, ethylenically unsaturated dicarboxylic acids and anhydrides thereof, and derivatives thereof.

Examples of the ethylenically unsaturated monocarboxylic acid include acrylic acid, methacrylic acid, crotonic acid, and the like. Further, as examples of the derivative of the ethylenically unsaturated monocarboxylic acid, 2-ethacrylic acid, isocrotonic acid, α -acetoxyacrylic acid, β -trans-aryloxyacrylic acid, α -chloro- β -E-methoxyacrylic acid, β -diaminoacrylic acid, and the like can be cited.

Examples of the ethylenically unsaturated dicarboxylic acid include maleic acid, fumaric acid, and itaconic acid. Examples of the acid anhydride of the ethylenically unsaturated dicarboxylic acid include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride. Further, examples of the derivative of the ethylenically unsaturated dicarboxylic acid include methyl maleic acid, dimethyl maleic acid, phenyl maleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, fluoroalkyl maleate and the like.

These can be used alone in 1 kind, also can be more than 2 kinds in any ratio combination use. Among these, from the viewpoint of viscosity stability of a slurry composition using a binder composition containing a particulate polymer, ethylenically unsaturated dicarboxylic acids, anhydrides thereof, and derivatives thereof are preferable, and itaconic acid is more preferable. That is, the particulate polymer preferably contains a monomer unit derived from itaconic acid (ethylenically unsaturated carboxylic acid monomer unit).

In the particulate polymer, the content of the ethylenically unsaturated carboxylic acid monomer unit is necessarily 0.5% by mass or more, preferably 2% by mass or more, more preferably 3% by mass or more, and necessarily 10% by mass or less, preferably 8% by mass or less, more preferably 6% by mass or less, and particularly preferably 4% by mass or less. If the content ratio of the ethylenically unsaturated carboxylic acid monomer unit is less than 0.5 mass%, viscosity stability of the slurry composition using the binder composition containing the particulate polymer cannot be ensured, adhesion between the negative electrode binder layer and the current collector is reduced, and cycle characteristics cannot be ensured. On the other hand, if the content ratio of the ethylenically unsaturated carboxylic acid monomer unit exceeds 10 mass%, the viscosity of the binder composition increases, making handling difficult, and the viscosity of the slurry composition also changes drastically, making it sometimes difficult to produce an electrode plate. In addition, the adhesion between the negative electrode mixture layer and the current collector is reduced, and the cycle characteristics are reduced.

[ [ (meth) acrylate monomer units ] ]

Examples of the (meth) acrylate monomer capable of forming the (meth) acrylate monomer unit of the particulate polymer include: alkyl acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, tert-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, and 2-ethylhexyl acrylate; alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, and 2-ethylhexyl methacrylate; hydroxyl group-containing (meth) acrylates such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate and 3-chloro-2-hydroxypropyl methacrylate. These may be used alone in 1 kind, or may be used in combination in an arbitrary ratio in 2 or more kinds.

Among these, from the viewpoint of viscosity stability of a slurry composition using a binder composition containing a particulate polymer, a hydroxyl group-containing (meth) acrylate is preferable, and 2-hydroxyethyl acrylate is more preferable. That is, the particulate polymer preferably contains a monomer unit derived from 2-hydroxyethyl acrylate (hydroxyl group-containing (meth) acrylate monomer unit).

The content of the (meth) acrylate monomer unit in the particulate polymer is required to be 0.1% by mass or more, preferably 0.3% by mass or more, more preferably 0.5% by mass or more, particularly preferably 0.6% by mass or more, and is required to be 3% by mass or less, preferably less than 2% by mass, more preferably 1.5% by mass or less. If the content ratio of the (meth) acrylate monomer unit is less than 0.1% by mass, the cycle characteristics and the high-temperature storage characteristics cannot be ensured. On the other hand, if the content ratio of the (meth) acrylate monomer unit exceeds 3 mass%, the cycle characteristics and the high-temperature storage characteristics cannot be ensured, and the viscosity stability of the slurry composition and the adhesion strength between the negative electrode mixture layer and the current collector are also deteriorated.

[ [ other monomers ] ]

The particulate polymer may further contain any monomer unit other than the above. Examples of other monomers that can form any of the above-mentioned monomer units include: vinyl cyanide monomers, unsaturated carboxylic acid amide monomers, and the like.

Examples of the vinyl cyanide monomer include: acrylonitrile, methacrylonitrile, α -chloroacrylonitrile, α -ethylacrylonitrile, and the like. These may be used alone in 1 kind, or may be used in combination in an arbitrary ratio in 2 or more kinds.

In the particulate polymer used in the present invention, the content ratio of the vinyl cyanide monomer unit is preferably 4% by mass or less, and more preferably 2% by mass or less. The particulate polymer preferably contains substantially no vinyl cyanide monomer units. This is because if the particulate polymer contains a large amount of vinyl cyanide monomer units, the degree of swelling of the particulate polymer in an electrolyte increases, and it is difficult to design the particulate polymer to have an appropriate degree of swelling in an electrolyte, which will be described later.

Examples of the unsaturated carboxylic acid amide monomer include: acrylamide, methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, N-dimethylacrylamide and the like. Among them, acrylamide and methacrylamide are preferable. These may be used alone in 1 kind, or may be used in combination in an arbitrary ratio in 2 or more kinds.

[ Process for producing particulate Polymer ]

The particulate polymer can be produced by polymerizing a monomer composition containing the above-mentioned monomer in an aqueous solvent.

Here, in the present invention, the content ratio of each monomer in the monomer composition may be determined based on the content ratio of the monomer unit (repeating unit) in the particulate polymer.

The aqueous medium is not particularly limited as long as the particulate polymer can be dispersed in a particulate state, and water is particularly preferable from the viewpoint of not having flammability and easily obtaining a dispersion of particles of the particulate polymer. An aqueous medium other than water may be used as the main solvent, and may be mixed within a range in which the dispersion state of the particles of the particulate polymer is ensured.

The polymerization method is not particularly limited, and any method of, for example, solution polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, and the like can be used. As the polymerization method, for example, any of ionic polymerization, radical polymerization, living radical polymerization, and the like can be used. The emulsion polymerization method is particularly preferable from the viewpoint of production efficiency, such as production of the binder composition of the present invention and the slurry composition of the present invention, because a high molecular weight material is easily obtained and a polymer can be directly obtained in a state of being dispersed in water, and thus no treatment for redispersion is required. The emulsion polymerization may be carried out by a conventional method.

Further, as the emulsifier, dispersant, polymerization initiator, polymerization auxiliary and the like used in the polymerization, those generally used can be used, and the amount thereof is also the amount generally used.

In addition, for producing the particulate polymer used in the present invention, batch polymerization or semi-batch polymerization may be employed, and semi-batch polymerization in which a monomer is continuously or intermittently added to a reaction system is preferably employed. By using the semi-batch polymerization, the THF swelling degree of the particulate polymer described later can be effectively controlled, that is, the THF swelling degree of the particulate polymer specified in the present invention can be easily achieved, as compared with the case of using the batch polymerization. Further, the surface acid amount of the particulate polymer described later can be increased by using the semi-batch polymerization.

A preferred embodiment of the method for producing a particulate polymer by semi-batch polymerization comprises the steps of: a step of obtaining seed particles from the primary monomer composition in the reaction system, and a step of adding a secondary monomer composition to the obtained reaction system containing the seed particles to obtain a particulate polymer. The preferred embodiment will be described in detail below.

First, seed particles are obtained from a primary monomer composition.

The term "primary monomer composition" as used herein means a monomer composition which is initially added to the reaction system in order to obtain seed particles by polymerization, and the primary monomer composition is preferably contained in an amount of 1 to 10% by mass, more preferably 3 to 7% by mass, based on the total monomer composition used for polymerization. The primary monomer composition is not particularly limited, but preferably contains an aromatic vinyl monomer, an aliphatic conjugated diene monomer, and an ethylenically unsaturated carboxylic acid monomer, and further preferably contains substantially no hydroxyl group-containing (meth) acrylate.

The seed particles are obtained by appropriately adding an emulsifier, a chain transfer agent, water, a polymerization initiator, and the like to the primary monomer composition and initiating a polymerization reaction. The reaction conditions for obtaining the seed particles are not particularly limited, and the reaction temperature is preferably 40 to 80 ℃, more preferably 50 to 70 ℃, and the reaction time is preferably 1 to 20 hours, more preferably 3 to 10 hours.

Then, a secondary monomer composition is continuously or intermittently added to the obtained reaction system containing the seed particles to obtain a particulate polymer (stage 2 polymerization).

Here, the "secondary monomer composition" refers to a monomer composition that is not added as a primary monomer composition to the reaction system in the entire monomer composition used for polymerization.

Further, "continuously or intermittently added" means that the secondary monomer composition is not added to the reaction system at the same time, but is added for a certain period of time (for example, at least 30 minutes or more).

In the reaction system containing the seed particles, an emulsifier, a chain transfer agent, water, and a polymerization initiator are appropriately added in addition to the secondary monomer composition to initiate the polymerization in the 2 nd stage, thereby obtaining a particulate polymer.

The reaction conditions for the 2 nd stage polymerization are not particularly limited, and the reaction temperature is preferably 60 to 95 ℃ and the reaction time is preferably 3 to 15 hours.

In the 2 nd-stage polymerization, it is preferable that the addition of the hydroxyl group-containing (meth) acrylate is started after the addition rate of the monomer composition becomes 70% or more (that is, after the addition of 70% by mass of the total monomer composition used for the polymerization to the reaction system is completed). When such an addition procedure is adopted, it is preferable that the reaction is started at 60 to 80 ℃ from the start of the addition of the secondary monomer composition (start of polymerization in stage 2) other than the hydroxyl group-containing (meth) acrylate, the hydroxyl group-containing (meth) acrylate monomer is started after 2 to 6 hours have elapsed, and then the reaction is started at 80 to 90 ℃ for 3 to 9 hours after the completion of the addition of the secondary monomer composition. Thus, by adding the hydroxyl group-containing (meth) acrylate in the subsequent step, the amount of acid in the aqueous phase can be controlled.

Then, when the polymerization conversion rate becomes sufficient (for example, 95% or more), the reaction is stopped by cooling.

Here, the aqueous dispersion of the particulate polymer obtained by the above polymerization method or the like may be obtained by using a hydroxide containing an alkali metal (e.g., Li, Na, K, Rb, Cs), ammonia, or an inorganic ammonium compound (e.g., NH)₄Cl, etc.), an organic amine compound (e.g., ethanolamine, diethylamine, etc.), etc., to a pH of usually 5 or more, usually 10 or less, preferably 9 or less. Among these, pH adjustment by an alkali metal hydroxide is preferable because adhesion between the negative electrode mixture layer and the current collector can be improved.

[ Properties of particulate Polymer ]

Hereinafter, the THF swelling degree, THF insoluble matter, electrolyte swelling degree, surface acid amount, and acid amount in the aqueous phase of the particulate polymer used in the present invention will be described in detail.

[ [ degree of swelling by THF ] ]

In the present invention, the THF swelling degree of the particulate polymer means the degree of swelling of a THF-insoluble portion when a film obtained by drying an aqueous dispersion of the particulate polymer is immersed in THF. The THF swelling degree is an index indicating the properties of the polymer chain constituting the particulate polymer, and is an index unrelated to the THF insoluble matter content, which is mainly indicative of the properties of the particles formed of the polymer chain, and the swelling degree of the electrolyte solution described later.

The THF swelling degree of the particulate polymer can be calculated specifically by the following method

An aqueous dispersion containing a particulate polymer is prepared, and the aqueous dispersion is dried at room temperature to form a film having a thickness of 0.2 to 0.5 mm. The film was cut into 2.5mm squares and about 1g was accurately weighed. The mass of the film piece obtained by cutting was set to W0.

The obtained membrane sheet was immersed in 100g of THF (tetrahydrofuran) at 25 ℃ for 48 hours. Then, the mass W1 of the film pieces fished out of THF was measured. The membrane taken out of THF was vacuum-dried at 105 ℃ for 3 hours, and the mass W2 of the THF-insoluble matter was measured. The change in mass was calculated according to the following formula and taken as the THF swelling degree.

THF swelling degree (times) ═ W1/W2

The THF swelling degree of the particulate polymer is required to be 3 times or more, preferably 4 times or more, particularly preferably 5 times or more, and 10 times or less, preferably 8 times or less, and more preferably 7 times or less. If the THF swelling degree is less than 3 times, the adhesion between the negative electrode mixture layer and the current collector is reduced. Further, a particulate polymer having a THF swelling degree of less than 3 times is not easy to produce and is rigid, and thus, for example, the processability of a molded article (negative electrode mixture layer or the like) containing the particulate polymer cannot be ensured. On the other hand, if the THF swelling degree exceeds 10 times, the cycle characteristics and the high-temperature storage characteristics cannot be ensured in a well-balanced manner.

The THF swelling degree of the particulate polymer can be controlled by changing, for example, the kind and the ratio of the monomer units constituting the particulate polymer, the polymerization method, and the polymerization conditions (polymerization temperature, amount of the molecular weight modifier, etc.).

More specifically, for example, the THF swelling degree can be reduced by increasing the ratio of the conjugated diene monomer unit to the crosslinkable monomer unit. Further, by using the above-mentioned semi-batch polymerization, the THF swelling degree can be reduced. Further, in the semi-batch polymerization, the THF swelling degree can be reduced by increasing the reaction temperature at the time of the 2 nd stage polymerization.

[ [ THF-insoluble component ] ]

In the present invention, the THF-insoluble content of the particulate polymer means the proportion of the insoluble portion when a film obtained by drying an aqueous dispersion of the particulate polymer is immersed in THF.

The THF-insoluble content of the particulate polymer can be calculated specifically by the following method.

The THF insoluble content (mass%) was calculated from the mass W0 of the cut membrane used in the measurement of the THF swelling degree and the mass W2 of the THF insoluble content of the membrane taken out of the THF after vacuum drying at 105 ℃ for 3 hours according to the following formula.

THF-insoluble matter (% by mass) W2/W0 × 100

The THF-insoluble content of the particulate polymer is preferably 70% by mass or more, more preferably 80% by mass or more, and particularly preferably 90% by mass or more. When the THF-insoluble content of the particulate polymer is 70 mass% or more, the particulate polymer is less likely to dissolve in the electrolyte solution, and a decrease in adhesion between the negative electrode material layer and the current collector due to the electrolyte solution can be suppressed. Therefore, the cycle characteristics of the lithium ion secondary battery can be improved. Further, by making the THF-insoluble matter 70 mass% or more, the breaking strength of the particulate polymer can be increased, and the adhesion between the current collector and the negative electrode material layer can be improved.

The THF-insoluble content of the particulate polymer can be controlled by, for example, the molecular weight of the particulate polymer. More specifically, the value of the THF-insoluble matter can be increased by increasing the weight average molecular weight of the particulate polymer.

[ [ degree of swelling by electrolyte ] ]

In the present invention, the electrolyte swelling degree of the particulate polymer means a swelling degree when a film obtained by drying an aqueous dispersion of the particulate polymer is immersed in a specific electrolyte. Here, the degree of swelling in the electrolyte of the particulate polymer can be calculated by the following method.

An aqueous dispersion containing a particulate polymer is prepared, and the aqueous dispersion is dried at room temperature to form a film having a thickness of 0.2 to 0.5 mm. The film was cut into 4cm²Size, mass (pre-impregnation quality)Amount a). The mass-measured membrane was immersed in an electrolyte solution (ethylene carbonate (EC) and diethyl carbonate (DEC) mixed in such a manner that the volume ratio of these was EC: DEC ═ 1:2 at 20 ℃) at a temperature of 60 ℃, and LiPF was dissolved in a concentration of 1.0mol/L₆Solution of (b). The immersed film was taken out after 72 hours, and the mass (mass B after immersion) was measured immediately after wiping off the electrolyte with a facial tissue. The mass change was calculated as follows, and this was taken as the swelling degree of the electrolyte.

Swelling degree of electrolyte B/A

The particulate polymer preferably has an electrolyte swelling degree of 1 time or more, more preferably 1.2 times or more, particularly preferably 1.4 times or more, preferably 2 times or less, more preferably 1.8 times or less, and particularly preferably 1.6 times or less. By setting the swelling degree of the electrolyte to 1 time or more, the conductivity of lithium ions and electrical characteristics such as cycle characteristics can be ensured. On the other hand, when the swelling degree of the electrolytic solution is 2 times or less, the negative electrode active material and other particles in the negative electrode mixture layer can be appropriately bonded to each other, and the falling of these materials from the current collector can be sufficiently suppressed, thereby ensuring the strength of the negative electrode mixture layer in the electrolytic solution.

The degree of swelling in the electrolyte solution of the particulate polymer can be controlled by, for example, the type and the ratio of the monomer units constituting the particulate polymer.

More specifically, for example, the degree of swelling in the electrolyte solution can be reduced by decreasing the proportion of the vinyl cyanide monomer or increasing the proportion of the crosslinkable monomer unit. In addition, the degree of swelling of the electrolyte can be reduced by selecting a substance in which the solubility parameter of the monomer forming the monomer unit is greatly different from that of the electrolyte.

[ [ amount of surface acid and amount of acid in aqueous phase ] ]

In the present invention, the surface acid amount of the particulate polymer means the amount of the surface acid per 1g of the solid content of the particulate polymer, and the acid amount of the particulate polymer in the aqueous phase means the amount of the acid present in the aqueous phase in the aqueous dispersion containing the particulate polymer, and is the amount of the acid per 1g of the solid content of the particulate polymer. Here, the surface acid amount of the particulate polymer and the acid amount in the aqueous phase can be calculated by the following method.

First, an aqueous dispersion containing a particulate polymer (solid content concentration: 4%) was prepared. 50g of the aqueous dispersion containing the particulate polymer was put into a 150-ml glass vessel washed with distilled water, and the vessel was set in a solution conductivity meter and stirred. The stirring was continued until the addition of hydrochloric acid described later was completed.

0.1 equivalent of an aqueous sodium hydroxide solution is added to the aqueous dispersion containing the particulate polymer so that the conductivity of the aqueous dispersion containing the particulate polymer becomes 2.5 to 3.0 mS. Then, after 5 minutes, the conductivity was measured. This value was taken as the conductivity at the start of the measurement.

Further, 0.1 equivalent of 0.5ml of hydrochloric acid was added to the aqueous dispersion containing the particulate polymer, and the electric conductivity was measured after 30 seconds. Then, 0.1 equivalent of 0.5ml of hydrochloric acid was added again, and the conductivity was measured after 30 seconds. This operation was repeated at intervals of 30 seconds until the electrical conductivity of the aqueous dispersion containing the particulate polymer became equal to or higher than the electrical conductivity at the start of the measurement.

The resulting conductivity data are plotted on a graph having the conductivity (in units "mS") as the vertical axis (Y axis) and the cumulative amount of hydrochloric acid added (in units "mmol") as the horizontal axis (X axis). Thus, a hydrochloric acid amount-conductivity curve having 3 inflection points as shown in fig. 1 can be obtained. The X-coordinate of the 3 inflection points and the X-coordinate at the end of hydrochloric acid addition were designated as P1, P2, P3, and P4, respectively, in order from the smaller one. For data in 4 sections of the X coordinate from zero to the coordinate P1, the coordinate P1 to the coordinate P2, the coordinate P2 to the coordinate P3, and the coordinate P3 to the coordinate P4, the approximate straight lines L1, L2, L3, and L4 are obtained by the least square method, respectively. The X coordinate of the intersection of the approximate straight line L1 and the approximate straight line L2 is a1(mmol), the X coordinate of the intersection of the approximate straight line L2 and the approximate straight line L3 is a2(mmol), and the X coordinate of the intersection of the approximate straight line L3 and the approximate straight line L4 is A3 (mmol).

The surface acid amount per 1g of the particulate polymer and the acid amount in the aqueous phase per 1g of the particulate polymer were obtained as the following formulae (a) and (b) and a value (mmol/g) in terms of hydrochloric acid, respectively. The total amount of acid per 1g of the particulate polymer dispersed in water is the sum of the formula (a) and the formula (b), as shown in the formula (c).

(a) The amount of surface acid per 1g of particulate polymer is A2-A1

(b) The acid content of the particulate polymer per 1g in the aqueous phase is A3-A2

(c) Total acid group amount per 1g of particulate polymer dispersed in water A3-a1

The amount of surface acid of the particulate polymer is preferably 0.20mmol/g or more, more preferably 0.25mmol/g or more, and particularly preferably 0.27mmol/g or more. By setting the surface acid amount to 0.20mmol/g or more, the viscosity stability of the slurry composition can be improved. Further, since the coating property of the slurry composition can be improved, and the negative electrode mixture layer with less defects can be produced, the low-temperature output characteristics of the lithium ion secondary battery can be improved. Further, if the amount of the surface acid of the particulate polymer is 0.20mmol/g or more, migration at the time of applying the slurry composition to the current collector is suppressed, adhesion between the negative electrode mixture layer and the current collector can be improved, and cycle characteristics of the lithium ion secondary battery can be improved.

The upper limit of the amount of the surface acid of the particulate polymer is not particularly limited, but is, for example, 0.8mmol/g or less.

The amount of acid in the aqueous phase of the particulate polymer is preferably 0.25mmol/g or less, more preferably 0.2mmol/g or less, and still more preferably 0.15mmol/g or less. When the amount of acid in the aqueous phase is 0.25mmol/g or less, it is possible to suppress a decrease in adhesion between the negative electrode material layer and the current collector and a decrease in electrical characteristics such as cycle characteristics due to the influence of the monomer having an acidic group mixed into the hydrophilic oligomer formed in the production of the particulate polymer.

The lower limit of the amount of acid in the aqueous phase of the particulate polymer is not particularly limited, and is, for example, 0.01mmol/g or more.

The value obtained by dividing the surface acid amount of the particulate polymer by the acid amount of the particulate polymer in the aqueous phase is preferably 1.0 or more, more preferably 1.1 or more, and particularly preferably 1.2 or more. When the value obtained by dividing the surface acid amount of the particulate polymer by the acid amount of the particulate polymer in the aqueous phase is 1.0 or more, the electrical characteristics such as the adhesion between the negative electrode mixture layer and the current collector, the cycle characteristics, the low-temperature output characteristics, and the like, and the dispersion stability of the slurry composition can be made excellent.

The upper limit of the value obtained by dividing the surface acid amount of the particulate polymer by the acid amount of the particulate polymer in the aqueous phase is not particularly limited, and is, for example, 10 or less.

The amount of the surface acid of the particulate polymer can be controlled by, for example, changing the kind and ratio of the monomer units constituting the particulate polymer and the polymerization method.

More specifically, for example, the amount of the surface acid can be effectively controlled by adjusting the kind of the ethylenically unsaturated carboxylic acid monomer unit and the ratio thereof. In general, if a monomer (itaconic acid or the like) having a large difference in reactivity with other monomers among the ethylenically unsaturated carboxylic acid monomers is used, the ethylenically unsaturated carboxylic acid monomer is easily copolymerized on the surface of the particulate polymer, and thus the amount of surface acid tends to increase. Further, by using the semi-batch polymerization described above, the surface acid amount of the particulate polymer can be increased.

On the other hand, the acid amount of the particulate polymer in the aqueous phase can be reduced by adding a hydroxyl group-containing monomer (containing a monomer unit derived from a hydroxyl group-containing (meth) acrylate) in the latter half of the polymerization reaction to improve the copolymerizability of the ethylenically unsaturated carboxylic acid monomer with other monomers.

[ [ Properties of other particulate polymers ]

The glass transition temperature of the particulate polymer is preferably-30 ℃ or higher, more preferably-20 ℃ or higher, particularly preferably-5 ℃ or higher, preferably 40 ℃ or lower, more preferably 25 ℃ or lower, and particularly preferably 15 ℃ or lower. When the glass transition temperature of the particulate polymer is in the above range, the characteristics such as flexibility and winding property of the negative electrode and adhesion between the negative electrode material layer and the current collector are well balanced, and therefore, it is preferable.

The glass transition temperature of the particulate polymer can be measured by the method described in the section of examples in the present specification.

The number average particle diameter of the particulate polymer is preferably 50nm or more, more preferably 80nm or more, further preferably 110nm or more, preferably 500nm or less, more preferably 300nm or less, further preferably 200nm or less. The standard deviation of the number average particle diameter is preferably 30nm or less, more preferably 15nm or less. When the number average particle diameter and the standard deviation are within the above ranges, the strength and flexibility of the obtained negative electrode can be improved. The number average particle diameter can be easily measured by a transmission electron microscopy. The particle size and its distribution can be controlled according to the number and particle size of the seed particles.

< preparation of Binder composition for negative electrode of lithium ion Secondary Battery >

The pressure-sensitive adhesive composition of the present invention can be prepared by adding water to an aqueous dispersion of a particulate polymer obtained by polymerizing a monomer composition and adding any other component within a range not impairing the effect of the present invention. The aqueous dispersion of the particulate polymer obtained may be used as it is as the binder composition for a negative electrode of a lithium ion secondary battery of the present invention.

(slurry composition for negative electrode of lithium ion Secondary Battery)

The slurry composition for a lithium ion battery negative electrode of the present invention is an aqueous slurry composition containing a negative electrode active material and the binder composition for a lithium ion secondary battery negative electrode of the present invention. The slurry composition for a negative electrode of a lithium ion secondary battery of the present invention may contain other components described later in addition to the negative electrode active material and the binder composition described above.

Further, according to the slurry composition for a negative electrode of a lithium ion secondary battery of the present invention, since the binder composition of the present invention is contained, the lithium ion secondary battery can be made excellent in cycle characteristics, and the swelling of the battery cell due to high temperature can be suppressed to ensure high temperature storage characteristics.

Hereinafter, each component contained in the slurry composition for a negative electrode of a lithium ion secondary battery will be described.

< negative electrode active Material >

The negative electrode active material is a material that transfers electrons to and from the negative electrode of the lithium ion secondary battery. In addition, as a negative electrode active material of a lithium ion secondary battery, a material capable of occluding and releasing lithium is generally used. Examples of the substance capable of occluding and releasing lithium include: carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials composed of a combination thereof.

[ carbon-based negative electrode active material ]

Here, the carbon-based negative electrode active material refers to an active material having carbon as a main skeleton, which is capable of intercalating (also referred to as "doping") lithium, and examples of the carbon-based negative electrode active material include carbonaceous materials and graphitic materials.

The carbonaceous material is a material having a low degree of graphitization (i.e., low crystallinity) obtained by carbonizing a carbon precursor by heat treatment at 2000 ℃ or lower. The lower limit of the heat treatment temperature at the time of carbonization is not particularly limited, and may be, for example, 500 ℃.

Examples of the carbonaceous material include: easily graphitizable carbon whose carbon structure is easily changed by the heat treatment temperature, hardly graphitizable carbon having a structure close to an amorphous structure represented by glassy carbon, and the like.

The graphite material is obtained by heat-treating graphite-susceptible carbon at 2000 ℃ or higher and has high crystallinity close to graphite. The upper limit of the heat treatment temperature is not particularly limited, and may be, for example, 5000 ℃.

Examples of the graphite material include: natural graphite, artificial graphite, and the like.

[ Metal-based negative electrode active Material ]

The metal-based negative electrode active material is an active material containing a metal, and generally refers to an active material that contains an element capable of intercalating lithium in its structure, and has a theoretical capacity per unit mass of 500mAh/g or more when intercalating lithium. As the metal-based active material, for example: lithium metal, elemental metals that can form lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.) and alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. thereof.

Among the metal-based negative electrode active materials, an active material containing silicon (silicon-based negative electrode active material) is preferable. The reason for this is that the use of a silicon-based negative electrode active material enables the lithium ion secondary battery to have a high capacity.

Examples of the silicon-based negative electrode active material include: silicon (Si), silicon-containing alloy, SiO_xAnd a composite of a Si-containing material and conductive carbon, which is formed by coating or compositing a Si-containing material with conductive carbon. These silicon-based negative electrode active materials may be used alone in 1 kind, or in combination of 2 or more kinds.

Examples of the silicon-containing alloy include alloy compositions containing a transition metal such as silicon, aluminum, or iron, and further containing a rare earth element such as tin or yttrium. SiO 2_xIs composed of SiO and SiO₂And (3) at least one of (1) and (b), wherein x is usually 0.01 or more and less than 2.

Here, the particle diameter and specific surface area of the negative electrode active material are not particularly limited, and may be the same as those of conventionally used negative electrode active materials.

< Binder composition for negative electrode of lithium ion Secondary Battery >

The binder composition used in the slurry composition of the present invention is a binder composition for a negative electrode of a lithium ion secondary battery, which contains the particulate polymer of the present invention. In addition, the slurry composition of the present invention contains the particulate polymer in an amount of preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, particularly preferably 1 part by mass or more, preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and particularly preferably 5 parts by mass or less, per 100 parts by mass of the negative electrode active material. By making the slurry composition contain the particulate polymer in the above amount, the amount of the particulate polymer is sufficient to follow the expansion and contraction of the negative electrode active material well, and the cycle characteristics of the lithium ion secondary battery can be made excellent.

< other ingredients >

The slurry composition for a negative electrode of a lithium ion secondary battery of the present invention may contain, in addition to the above components, components such as a water-soluble polymer such as carboxymethyl cellulose and polyacrylic acid, a conductive material, a reinforcing material, a leveling agent, and an electrolyte additive. These components are not particularly limited as long as they do not affect the battery reaction, and known materials, for example, the materials described in international publication No. 2012/115096, can be used. These components can be used alone in 1 kind, also can be used in any ratio of combination of 2 or more. In addition, these other components can be contained in the slurry composition of the present invention by using the binder composition of the present invention in which the components are blended.

< method for producing slurry composition for negative electrode of lithium ion Secondary Battery >

The slurry composition for a negative electrode of a lithium ion secondary battery of the present invention can be prepared by dispersing the above-mentioned respective components in an aqueous medium as a dispersion medium. Specifically, a slurry composition can be prepared by mixing the above-described respective ingredients and an aqueous medium using a mixer such as a ball mill, a sand mill, a bead mill, a pigment dispersing machine, an attritor, an ultrasonic dispersing machine, a homogenizer, a planetary mixer, a fillmix, or the like.

Here, water is generally used as the aqueous medium, but an aqueous solution of an arbitrary compound, a mixed solution of a small amount of an organic medium and water, or the like may be used. The slurry composition may also be prepared by adding a negative electrode active material or the like to the binder composition after the binder composition is prepared. The aqueous medium in the slurry composition may be a medium derived from the binder composition.

(negative electrode for lithium ion Secondary Battery)

The negative electrode for a lithium ion secondary battery of the present invention includes a negative electrode mixture layer formed from the slurry composition for a negative electrode for a lithium ion secondary battery of the present invention. The specific production method is described in detail in the following "production method of a negative electrode for a lithium ion secondary battery".

The negative electrode for a lithium ion secondary battery comprises a current collector and a negative electrode mixture layer formed on the current collector, wherein the negative electrode mixture layer contains at least a negative electrode active material and the particulate polymer. The components contained in the negative electrode mixture layer are the components contained in the slurry composition for a negative electrode of a lithium ion secondary battery of the present invention, and the preferred presence ratio of these components is the same as the preferred presence ratio of the components in the slurry composition for a negative electrode.

The negative electrode is excellent in cycle characteristics of a lithium ion secondary battery by using the slurry composition of the present invention, and can ensure high-temperature storage characteristics by suppressing expansion of a battery cell at high temperature.

(method for producing negative electrode for lithium ion Secondary Battery)

The negative electrode for a lithium ion secondary battery of the present invention can be produced, for example, by the following steps: the method for producing a lithium ion secondary battery negative electrode includes a step (coating step) of applying the slurry composition for a lithium ion secondary battery negative electrode to a current collector, and a step (drying step) of drying the slurry composition for a lithium ion secondary battery negative electrode applied to the current collector to form a negative electrode mixture layer on the current collector.

[ coating Process ]

The method for applying the slurry composition for a negative electrode of a lithium ion secondary battery to a current collector is not particularly limited, and a known method can be used. Specifically, as the coating method, a blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush coating method, or the like can be used. In this case, the slurry composition for a negative electrode may be applied to only one surface of the current collector, or may be applied to both surfaces. The thickness of the slurry film on the current collector after coating and before drying can be appropriately set according to the thickness of the negative electrode mixture layer obtained by drying.

Here, as the current collector to be coated with the slurry composition for a negative electrode, a material having conductivity and electrochemical durability can be used. Specifically, as the current collector, for example, a current collector made of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, or the like can be used. Among these, copper foil is particularly preferable as the current collector for the negative electrode. The above materials may be used alone in 1 kind, or may be used in combination in 2 or more kinds at an arbitrary ratio.

[ drying Process ]

The method for drying the slurry composition for a negative electrode on a current collector is not particularly limited, and known methods can be used, and examples thereof include drying with warm air, hot air, or low-humidity air, vacuum drying, and drying with irradiation of infrared rays, electron beams, or the like. By drying the slurry composition for a negative electrode on the current collector in this manner, a negative electrode mixture layer can be formed on the current collector, and a negative electrode for a lithium ion secondary battery including the current collector and the negative electrode mixture layer can be obtained.

After the drying step, the negative electrode mixture layer may be subjected to a pressing treatment using a press, a roll press, or the like. By the pressure treatment, the adhesion between the negative electrode mixture layer and the current collector can be improved.

(lithium ion secondary battery)

The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolyte solution, and a separator, and the negative electrode of the present invention is used as the negative electrode. In addition, since the lithium ion secondary battery of the present invention uses the negative electrode for a lithium ion secondary battery of the present invention, the lithium ion secondary battery has excellent cycle characteristics and high-temperature storage characteristics.

< Positive electrode >

As the positive electrode of the lithium ion secondary battery, a known positive electrode that can be used as a positive electrode for a lithium ion secondary battery can be used. Specifically, for example, a positive electrode in which a positive electrode material layer (also referred to as a "positive electrode active material layer") is formed on a current collector can be used as the positive electrode.

As the current collector, a current collector made of a metal material such as aluminum can be used. As the positive electrode mixture layer, a layer containing a known positive electrode active material, a conductive material, and a binder can be used.

< electrolyte solution >

As the electrolytic solution, an electrolytic solution in which an electrolyte is dissolved in a solvent can be used.

Here, as the solvent, an organic solvent capable of dissolving the electrolyte may be used. Specifically, as the solvent, a viscosity adjusting solvent such as 2, 5-dimethyltetrahydrofuran, tetrahydrofuran, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, methyl acetate, dimethoxyethane, dioxolane, methyl propionate, or methyl formate is added to an alkyl carbonate solvent such as ethylene carbonate, propylene carbonate, or γ -butyrolactone to obtain a solvent.

As the electrolyte, a lithium salt may be used. As the lithium salt, for example, the lithium salt described in Japanese patent laid-open No. 2012-204303 can be used. Among these lithium salts, LiPF is preferable as the electrolyte from the viewpoint of being easily dissolved in an organic solvent and showing a high dissociation degree₆、LiClO₄、CF₃SO₃Li。

< separator >

As the separator, for example, the separator described in japanese patent laid-open No. 2012-204303 can be used. Among these separators, a microporous membrane formed of a polyolefin-based resin (polyethylene, polypropylene, polybutylene, polyvinyl chloride) is preferable from the viewpoint that the thickness of the entire separator can be reduced, and thus the ratio of an electrode active material in a lithium ion secondary battery can be increased to increase the capacity per unit volume.

(method for manufacturing lithium ion Secondary Battery)

The lithium ion secondary battery of the present invention can be manufactured, for example, by the following method: the positive electrode and the negative electrode were stacked with a separator interposed therebetween, and the stack was rolled, bent, and the like according to the shape of the battery, if necessary, and then placed in a battery container, vacuum-dried at 105 ℃ for 2 hours in a nitrogen atmosphere, and then an electrolyte was injected into the battery container and sealed. In order to prevent the occurrence of pressure rise, overcharge, discharge, and the like in the lithium ion secondary battery, an overcurrent prevention element such as a fuse or a PTC element, an expansion alloy, a lead plate, and the like may be provided as necessary. The shape of the lithium ion secondary battery may be any of, for example, a coin type, a button type, a sheet type, a cylindrical type, a square type, a flat type, and the like.

Examples

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. In the following description, "%" and "part" representing amounts are based on mass unless otherwise specified.

The THF swelling degree, THF insoluble matter, electrolyte swelling degree, surface acid amount, and acid amount in the aqueous phase of the particulate polymer were calculated and measured by the above-mentioned methods. In the measurement of the surface acid amount and the acid amount in the aqueous phase, a solution conductivity meter (CM-117, cell type: K-121, manufactured by Kyoto electronics industries, Ltd.) was used as the solution conductivity meter, and a reagent grade material manufactured by Wako pure chemical industries and Wako pure chemical industries were used as 0.1 equivalent of sodium hydroxide and 0.1 equivalent of hydrochloric acid, respectively.

The glass transition temperature of the particulate polymer, the adhesion strength between the negative electrode material layer and the current collector, the viscosity stability of the slurry composition, the cycle characteristics and high-temperature storage characteristics of the lithium ion secondary battery, and the volume change rate of the battery cell after high-temperature storage were evaluated by the following methods.

< glass transition temperature of particulate Polymer >

The aqueous dispersion containing the particulate polymer was dried at a temperature of 23 to 26 ℃ for 3 days at a humidity of 50% to obtain a film having a thickness of about 1 mm. Then, the dried film was used as a sample, and the glass transition temperature was measured by using a differential scanning calorimetry analyzer (DSC 6220SII, manufactured by Nanotecology) under the conditions of a measurement temperature of-100 to 180 ℃ and a temperature rise rate of 5 ℃/min according to JIS K7121. In the measurement using the differential scanning calorimetry, when 2 or more peaks appear, the peak on the high temperature side is regarded as the glass transition temperature.

< adhesion strength between negative electrode mixture layer and current collector >

A rectangular shape having a width of 1cm X a length of 10cm was cut out from the negative electrode for a secondary battery as a test piece. A transparent tape (transparent tape specified in JIS Z1522) was attached to the surface of the negative electrode composite layer with the surface having the negative electrode composite layer facing downward, and the stress when one end of the current collector was pulled in the 180 ° direction at a pulling rate of 50 mm/min and peeled off was measured (wherein the transparent tape was fixed to a test stand). The average value of the results of the measurements was obtained 3 times, and the average value was evaluated as the peel strength according to the following criteria. The larger the value, the more excellent the adhesion between the negative electrode mixture layer and the current collector.

< viscosity stability of slurry composition >

After the viscosity (. eta.0) of the slurry composition was measured by a B-type viscometer (25 ℃ C., rotation speed 60rpm), it was stirred at 40 ℃ for 4 days at 10rpm using a stirrer (mix rotor). After stirring, the mixture was naturally cooled to 25 ℃ and the viscosity (. eta.1) was measured again by a B-type viscometer (25 ℃ C., rotation speed 60 rpm). Then, the degree of change in viscosity was calculated according to the following equation.

Degree of viscosity change ═ η 1/η 0

The closer the value is to 1, the more excellent the viscosity stability of the slurry composition.

< cycle characteristics of lithium ion Secondary Battery >

For the laminated cell type lithium ion secondary battery, after the electrolyte injection, the battery was allowed to stand at 25 ℃ for 24 hours, and then, charge and discharge operations were performed by a constant current method of 0.1C until the battery voltage reached 4.25V and discharge until the battery voltage reached 3.0V, and the initial capacity C was measured₀. Further, the charge and discharge operation was repeated in an environment of 60 ℃ by a constant current method of 0.1C until the battery voltage reached 4.25V and the battery voltage reached 3.0V, and the capacity C after 100 cycles was measured₂. Then, the capacity retention rate Δ Cc was calculated according to the following formula.

△Cc(％)＝(C₂/C₀)×100

The larger the value, the more excellent the high-temperature cycle characteristics.

< high temperature storage characteristics of lithium ion Secondary Battery >

For a lithium ion secondary battery of a laminated cell type, inAfter the electrolyte was injected, the mixture was allowed to stand at 25 ℃ for 24 hours, and then charged and discharged by a constant current method of 0.1C until the cell voltage reached 4.25V and 3.0V, and the initial capacity C was measured₀. Further, charging was performed in an environment of 25 ℃ by a constant current method of 0.1C until the battery voltage reached 4.25V, and then storage was performed in an environment of 60 ℃ for 7 days (high-temperature storage). Then, in an environment of 25 ℃, charge and discharge operations were performed by a constant current method of 0.1C until the battery voltage reached 4.25V and discharge until the battery voltage reached 3.0V, and the capacity C after high-temperature storage was measured₁. Then, the capacity retention rate Δ Cs was calculated according to the following equation.

△Cs(％)＝(C₁/C₀)×100

The larger the value, the more excellent the high-temperature storage property.

< percentage change in volume of Battery after storage at high temperature >

For the laminated cell type lithium ion secondary battery, after being charged with an electrolyte, it was allowed to stand at 25 ℃ for 24 hours, and then, charging and discharging operations were performed by a constant current method of 0.1C until the battery voltage reached 4.25V and discharging until the battery voltage reached 3.0V. Then, the battery cell of the battery was immersed in liquid paraffin, and the initial volume V0 was measured.

Further, the battery cell of the battery after the evaluation of the high-temperature storage characteristics of the lithium ion secondary battery was immersed in liquid paraffin, and the volume thereof was measured. Then, the volume change rate Δ V was calculated according to the following equation.

ΔV(％)＝(V1-V0)/V0×100

The smaller the value, the more excellent the ability to suppress the generation of gas and the expansion of the battery cell after high-temperature storage.

(example 1)

< preparation of particulate Polymer (semi-batch polymerization) >

To a 5MPa pressure-resistant vessel a equipped with a stirrer were charged 3.15 parts of styrene as an aromatic vinyl monomer, 1, 3-butadiene as an aliphatic conjugated diene monomer, 0.19 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer (5 parts of the above primary monomer composition), 0.2 parts of sodium lauryl sulfate as an emulsifier, 20 parts of ion exchange water, and 0.03 parts of potassium persulfate as a polymerization initiator, and after sufficient stirring, the mixture was heated to 60 ℃ to initiate polymerization, and reacted for 6 hours to obtain seed particles.

After the above reaction, heating to 75 ℃ started an operation of adding these mixtures to pressure resistant vessel a from another vessel B to which 58.85 parts of styrene as an aromatic vinyl monomer, 32.34 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 0.81 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, 2 parts of methyl methacrylate as a (meth) acrylic acid ester monomer, 0.25 parts of t-dodecyl mercaptan as a chain transfer agent, and 0.35 parts of sodium lauryl sulfate as an emulsifier were added, and at the same time, an operation of adding 1 part of potassium persulfate as a polymerization initiator to pressure resistant vessel a was started, thereby initiating polymerization in stage 2.

Further, 4 hours after the start of the 2 nd stage polymerization (after 70% of the total monomer composition was added), 1 part of 2-hydroxyethyl acrylate as a (meth) acrylic acid ester monomer was added to the pressure resistant vessel a over 1.5 hours.

That is, as the whole monomer composition, 62 parts of styrene as an aromatic vinyl monomer, 34 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 1 part of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, 1 part of 2-hydroxyethyl acrylate as a (meth) acrylic acid ester monomer, and 2 parts of methyl methacrylate were used.

After 5.5 hours from the start of the 2 nd stage polymerization, the addition of the mixture containing the whole amount of these monomer compositions was terminated, and then, the reaction was further heated to 85 ℃ for 6 hours.

The reaction was stopped by cooling at a point of time when the polymerization conversion rate reached 97%, to obtain a mixture containing a particulate polymer. To the mixture containing the particulate polymer, a 5% aqueous sodium hydroxide solution was added to adjust the pH to 8. Then, the unreacted monomers were removed by distillation under reduced pressure by heating. Further, the resultant was cooled to obtain an aqueous dispersion containing a desired particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery). The aqueous dispersion containing the particulate polymer was used to measure the THF swelling degree, electrolyte swelling degree, surface acid amount, acid amount in the aqueous phase, and THF-insoluble matter of the particulate polymer in the above-described manner. The results are shown in Table 1.

< production of slurry composition for negative electrode of lithium ion Secondary Battery >

The specific surface area of 4m as a negative electrode active material was added to a planetary mixer with a disperser²100 parts/g of artificial graphite (volume average particle diameter: 24.5 μm) and 0.90 parts by weight of a 1% aqueous solution of carboxymethylcellulose (Nippon PAPER Chemicals, "MAC-350 HC", 1% aqueous solution viscosity: 4000 mPas) serving as a water-soluble polymer and serving as a dispersant were adjusted to a solid content concentration of 55% by ion-exchanged water and mixed at room temperature for 60 minutes. Then, the solid content was adjusted to 52% with ion-exchanged water, and the mixture was further mixed for 15 minutes to obtain a mixed solution.

To the mixed solution, 1.8 parts of the aqueous dispersion containing the particulate polymer was added per 100 parts of the negative electrode active material, based on the solid content corresponding to the particulate polymer. Further, ion-exchanged water was added to adjust the final solid content concentration to 50%, and the mixture was mixed for 10 minutes. This was subjected to defoaming treatment under reduced pressure to obtain a slurry composition for a negative electrode of a lithium ion secondary battery having good fluidity.

< production of negative electrode >

The slurry composition for a negative electrode of a lithium ion secondary battery was applied to a copper foil (current collector) having a thickness of 18 μm by a gap roll coater so that the dried film thickness was about 150 μm. The copper foil coated with the slurry composition for a negative electrode of a lithium ion secondary battery was conveyed at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 75 ℃ and further for 2 minutes in an oven at a temperature of 120 ℃, whereby the slurry composition on the copper foil was dried to obtain a negative electrode base film. The negative electrode raw film was rolled by a roll press, and a negative electrode having a thickness of a negative electrode mixture layer of 80 μm was obtained.

The adhesion strength between the negative electrode material layer and the copper foil (current collector) was measured for the obtained negative electrode in the same manner as described above.

< production of Positive electrode >

LiCoO having a spinel structure as a positive electrode active material was added to a planetary mixer₂95 parts, 3 parts by solid content equivalent of PVDF (polyvinylidene fluoride) as a binder for the positive electrode mixture layer, 2 parts by conductive material acetylene black, and 20 parts by solvent N-methylpyrrolidone were mixed to obtain a slurry composition for a positive electrode of a lithium ion secondary battery.

The obtained slurry composition for a positive electrode of a lithium ion secondary battery was applied to an aluminum foil having a thickness of 20 μm by a die coater so that the dried film thickness was about 100 μm. The aluminum foil coated with the slurry composition for a positive electrode of a lithium ion secondary battery was conveyed at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 60 ℃ and further for 2 minutes in an oven at a temperature of 120 ℃, whereby the slurry composition on the aluminum foil was dried to obtain a raw positive electrode film. The raw positive electrode film was rolled by a roll press to obtain a positive electrode having a thickness of a positive electrode material layer of 70 μm.

< preparation of separator >

A single-layer polypropylene separator (65 mm in width, 500mm in length, 25 μm in thickness; dry-process production; porosity: 55%) was prepared. The separator was die-cut into a square of 5cm × 5cm, and used in a lithium ion secondary battery described below.

< lithium ion Secondary Battery >

An aluminum exterior material was prepared as an exterior of the battery. The positive electrode was cut into a square of 4cm × 4cm, and was disposed so that the surface on the collector side was in contact with the aluminum exterior material. Then, the above-described square separator was disposed on the surface of the positive electrode material layer of the positive electrode. The negative electrode was cut into a square of 4.2cm × 4.2cm, and was disposed on the separator so that the surface on the negative electrode mixture layer side faced the separator. Then, LiPF with a concentration of 1.0M as an electrolyte was filled₆Solution (solvent is ethylene carbonate)(EC)/diethyl carbonate (DEC) ═ 1/2 (volume ratio) of the mixed solvent, and vinylene carbonate 2 volume% (solvent ratio) was contained as an additive. Further, in order to seal the opening of the aluminum packaging material, heat sealing was performed at 150 ℃. The obtained lithium ion secondary battery was evaluated for cycle characteristics, high-temperature storage characteristics, and a volume change rate of the battery after high-temperature storage in the same manner as described above.

(example 2)

An aqueous dispersion containing a particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery), a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in example 1, except that 65 parts of styrene as an aromatic vinyl monomer, 30 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 4 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, and 1 part of 2-hydroxyethyl acrylate as a (meth) acrylate monomer were used as the whole monomer composition, the temperature at the time of monomer addition in the 2 nd-stage polymerization was changed to 70 ℃, and the amount of t-dodecyl mercaptan as a chain transfer agent was changed to 0.3 part, without changing the composition of the monomers used as the primary monomer composition. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

The particle diameter of the particulate polymer was measured by a transmission electron microscope. Specifically, the particle size of 1000 particles arbitrarily selected after staining the particles with osmium tetroxide by a conventional method was measured. The average particle diameter (number average particle diameter) was 160nm, and the standard deviation was 11 nm.

(example 3)

An aqueous dispersion containing a particulate polymer (binder composition for negative electrodes of lithium ion secondary batteries), a slurry composition for negative electrodes of lithium ion secondary batteries, a negative electrode, a positive electrode, a negative electrode active material, and a negative electrode active material were prepared in the same manner as in example 1 except that 57.1 parts of styrene as an aromatic vinyl monomer, 38 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 4 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, and 0.9 parts of 2-hydroxyethyl acrylate as a (meth) acrylate monomer were used as the entire monomer compositions, the temperature at the time of monomer addition in the 2-stage polymerization was changed to 70 ℃, the temperature after monomer addition in the 2-stage polymerization was changed to 88 ℃, and the amount of t-dodecyl mercaptan as a chain transfer agent was changed to 0.2 parts, A positive electrode and a lithium ion secondary battery. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

The particle size of the particulate polymer was measured in the same manner as in example 2. The average particle size was 156nm with a standard deviation of 11 nm.

(example 4)

An aqueous dispersion containing a particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery), a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in example 1, except that 66 parts of styrene as an aromatic vinyl monomer, 29.8 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 3 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, and 1.2 parts of 2-hydroxyethyl acrylate as a (meth) acrylate monomer were used as the whole monomer composition, the temperature after the addition of the monomers in the 2 nd-stage polymerization was changed to 90 ℃ and the amount of t-dodecyl mercaptan as a chain transfer agent was changed to 0.3 part. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

The particle size of the particulate polymer was measured in the same manner as in example 2. The average particle size was 155nm with a standard deviation of 11 nm.

(example 5)

An aqueous dispersion containing a particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery), a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in example 1, except that 69.4 parts of styrene as an aromatic vinyl monomer, 27 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 3 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, and 0.6 part of 2-hydroxyethyl acrylate as a (meth) acrylate monomer were used as the whole monomer composition, the temperature at the time of monomer addition in the 2 nd-stage polymerization was changed to 73 ℃, and the amount of t-dodecyl mercaptan as a chain transfer agent was changed to 0.4 part. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

(example 6)

An aqueous dispersion containing a particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery), a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in example 1, except that 60 parts of styrene as an aromatic vinyl monomer, 35.6 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 3 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, and 1.4 parts of 2-hydroxyethyl acrylate as a (meth) acrylate monomer were used as the whole monomer composition, the temperature at the time of monomer addition in the 2 nd-stage polymerization was changed to 70 ℃, and the amount of t-dodecyl mercaptan as a chain transfer agent was changed to 0.8 part. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

(example 7)

An aqueous dispersion containing a particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery), a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in example 3, except that 0.9 part of 2-hydroxyethyl acrylate was added to the pressure resistant vessel a over 3.5 hours after 2 hours from the start of the 2 nd stage polymerization (after 40% of the total monomer composition was added). Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

The particle size of the particulate polymer was measured in the same manner as in example 2. The average particle size was 157nm with a standard deviation of 12 nm.

Comparative example 1

An aqueous dispersion containing a particulate polymer (binder composition for negative electrodes of lithium ion secondary batteries), a slurry composition for negative electrodes of lithium ion secondary batteries, a slurry composition for positive electrodes of lithium ion secondary batteries, and a slurry composition were prepared in the same manner as in example 1 except that 57 parts of styrene as an aromatic vinyl monomer, 31 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 1 part of itaconic acid and 1 part of acrylic acid as ethylenically unsaturated carboxylic acid monomers, 6 parts of 2-hydroxyethyl acrylate as a (meth) acrylic acid ester monomer, and 4 parts of methyl methacrylate were used as the whole monomer composition, and the temperature at the time of monomer addition in the 2-stage polymerization was changed to 70 ℃ A negative electrode, a positive electrode, and a lithium ion secondary battery. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

Comparative example 2

An aqueous dispersion containing a particulate polymer (binder composition for negative electrodes of lithium ion secondary batteries) was prepared in the same manner as in example 1, except that 18 parts of styrene as an aromatic vinyl monomer, 43.5 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 1.5 parts of itaconic acid and 2 parts of acrylic acid as ethylenically unsaturated carboxylic acid monomers, 15 parts of methyl methacrylate as a (meth) acrylate monomer, and 20 parts of acrylonitrile as a vinyl cyanide monomer were used as the entire monomer composition, the temperature at the time of monomer addition in the 2-stage polymerization was changed to 73 ℃, the temperature after monomer addition in the 2-stage polymerization was changed to 88 ℃, and 0.4 part of t-dodecyl mercaptan and 0.6 part of α -methyl styrene dimer were used as chain transfer agents, without changing the composition of the monomers used as the primary monomer composition, Slurry composition for negative electrode of lithium ion secondary battery, negative electrode, positive electrode, and lithium ion secondary battery. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

Comparative example 3

< production of particulate Polymer (batch polymerization) >

57 parts of styrene as an aromatic vinyl monomer, 39 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 1 part of acrylic acid and 3 parts of methacrylic acid as ethylenically unsaturated carboxylic acid monomers, 2.0 parts of sodium lauryl sulfate as an emulsifier, 0.5 parts of t-dodecyl mercaptan as a chain transfer agent, 150 parts of ion exchange water, and 0.4 parts of potassium persulfate as a polymerization initiator were charged into a 5MPa pressure resistant vessel equipped with a stirrer, and after sufficient stirring, the mixture was heated to 70 ℃ to initiate polymerization.

The reaction was stopped by cooling at a point of time when the polymerization conversion rate reached 97%, to obtain a mixture containing a particulate polymer. To the mixture containing the particulate polymer, a 5% aqueous sodium hydroxide solution was added to adjust the pH to 8. Then, the unreacted monomers were removed by distillation under reduced pressure by heating. Further, the resultant was cooled to obtain an aqueous dispersion containing a desired particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery). The aqueous dispersion containing the particulate polymer was used to measure the THF swelling degree, electrolyte swelling degree, surface acid amount, acid amount in the aqueous phase, and THF-insoluble matter of the particulate polymer in the above-described manner. The results are shown in Table 1.

Further, a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode and a lithium ion secondary battery were produced in the same manner as in example 1, except that the particulate polymer obtained by the batch polymerization was used. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

Comparative example 4

The composition of the monomers used as the primary monomer composition was not changed, and 39 parts of styrene as an aromatic vinyl monomer, 43 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer, 3 parts of itaconic acid as an ethylenically unsaturated carboxylic acid monomer, 10 parts of methyl methacrylate as a (meth) acrylate monomer, and 5 parts of acrylonitrile as a cyanated vinyl monomer were used as the whole monomer composition, and the temperature after the addition of the monomers in the 2 nd stage polymerization was changed to 90 ℃, further, the amount of t-dodecylmercaptan as a chain transfer agent was 0.4 part, and in addition, an aqueous dispersion containing a particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery), a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in example 1. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

Comparative example 5

An aqueous dispersion containing a particulate polymer (binder composition for a negative electrode of a lithium ion secondary battery), a slurry composition for a negative electrode of a lithium ion secondary battery, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared in the same manner as in example 2, except that the temperature at the time of monomer addition in the polymerization in the stage 2 was changed to 60 ℃. Then, evaluation was performed in the same manner as in example 1. The results are shown in Table 1.

In table 1, ST represents styrene, BD represents 1, 3-butadiene, IA represents itaconic acid, AA represents acrylic acid, MAA represents methacrylic acid, AN represents acrylonitrile, 2-HEA represents 2-hydroxyethyl acrylate, MMA represents methyl methacrylate, TDM represents t-dodecyl mercaptan, and MSD represents α -methylstyrene dimer.

As is clear from table 1, in examples 1 to 7, the cycle characteristics were excellent, and the expansion of the battery cells after high-temperature storage was sufficiently suppressed, so that the high-temperature storage characteristics were excellent.

On the other hand, as is clear from table 1, in comparative examples 1,3 and 4, the cycle characteristics were reduced as compared with examples 1 to 7, and the suppression of swelling of the battery cells after high-temperature storage was significantly deteriorated, resulting in a reduction in high-temperature storage characteristics. In comparative examples 2 and 5, although the cycle characteristics were maintained to some extent, the suppression of swelling of the battery cells after high-temperature storage was insufficient, and the high-temperature storage characteristics were significantly deteriorated. That is, in comparative examples 2 and 5, excellent cycle characteristics and high-temperature storage characteristics could not be obtained in a well-balanced manner.

Industrial applicability

According to the present invention, it is possible to provide a binder composition for a secondary battery negative electrode, which, when used for forming a negative electrode, can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics.

Further, according to the present invention, it is possible to provide a slurry composition for a secondary battery negative electrode, which, when used for forming a negative electrode, can provide a lithium ion secondary battery having excellent cycle characteristics, and can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics.

Further, according to the present invention, it is possible to provide a negative electrode for a lithium ion secondary battery, which can provide a lithium ion secondary battery having excellent cycle characteristics, and which can suppress swelling of a battery cell due to high temperature and ensure high-temperature storage characteristics.

Further, according to the present invention, a lithium ion secondary battery having excellent cycle characteristics and high-temperature storage characteristics can be provided.

Claims (8)

1. A binder composition for a negative electrode of a lithium ion secondary battery, comprising a particulate polymer and water,

the particulate polymer contains 50 to 80 mass% of aromatic vinyl monomer units, 20 to 40 mass% of aliphatic conjugated diene monomer units, 0.5 to 10 mass% of ethylenically unsaturated carboxylic acid monomer units, and 0.1 mass% or more and less than 2 mass% of (meth) acrylate monomer units,

the particle-like polymer has a THF swelling degree of 3 to 10 times,

the (meth) acrylate monomer unit is a hydroxyl-containing (meth) acrylate monomer unit.

2. The binder composition for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the particulate polymer has an electrolyte swelling degree of 1 to 2 times.

3. The binder composition for a negative electrode of a lithium ion secondary battery according to claim 1 or 2, wherein,

the amount of surface acid of the particulate polymer is 0.20mmol/g or more, and,

the amount of the surface acid of the particulate polymer divided by the amount of the acid in the aqueous phase is 1.0 or more, and the unit of the amount of the surface acid and the amount of the acid in the aqueous phase is mmol/g.

4. The binder composition for a negative electrode of a lithium-ion secondary battery according to claim 1 or 2, wherein the ethylenically unsaturated carboxylic acid monomer unit of the particulate polymer contains an itaconic acid monomer unit.

5. The binder composition for a negative electrode of a lithium-ion secondary battery according to claim 1 or 2, wherein the hydroxyl group-containing (meth) acrylate monomer unit of the particulate polymer contains a 2-hydroxyethyl acrylate monomer unit.

6. A slurry composition for a negative electrode of a lithium ion secondary battery, comprising:

a negative electrode active material, and

the binder composition for a negative electrode of a lithium ion secondary battery according to any one of claims 1 to 5.

7. A negative electrode for a lithium ion secondary battery, comprising a negative electrode mixture layer obtained by using the slurry composition for a negative electrode for a lithium ion secondary battery according to claim 6.

8. A lithium ion secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a separator,

the negative electrode is the negative electrode for a lithium ion secondary battery according to claim 7.

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