JP7493912B2 - Binder composition for power storage device, slurry for power storage device electrode, power storage device electrode, and power storage device - Google Patents

Description

The present invention relates to a binder composition for an electricity storage device, a slurry for an electricity storage device electrode, an electricity storage device electrode, and an electricity storage device.

In recent years, there has been a demand for high-voltage, high-energy-density power storage devices as power sources for electronic devices. Lithium-ion batteries and lithium-ion capacitors are expected to be such power storage devices.

Electrodes used in such electricity storage devices are manufactured by applying a composition (slurry for electricity storage device electrodes) containing an active material and a polymer that functions as a binder to the surface of a current collector and drying it. The properties required of the polymer used as a binder include the ability to bind active materials together and to adhere to the current collector, abrasion resistance during the process of winding up the electrode, and resistance to powder shedding so that fine powder of the active material does not fall off from the applied and dried composition coating (hereinafter also referred to as the "active material layer") even when cut afterwards.

It has been empirically demonstrated that the performance of the above-mentioned active materials is roughly proportional to their bonding ability, adhesion ability between the active materials and the current collector, and resistance to powder falling off. Therefore, in the following specification, these are sometimes collectively referred to as "adhesion."

In recent years, in order to achieve even higher output and higher energy density in power storage devices, research has been conducted into the use of materials with large lithium storage capacities as active materials. For example, as disclosed in Patent Document 1, a promising method is to use silicon materials, which have a theoretical lithium storage capacity of up to about 4,200 mAh/g, as active materials.

However, active materials that use materials with a large lithium storage capacity undergo large volume changes due to the absorption and release of lithium. For this reason, when conventional electrode binders are used with materials with a large lithium storage capacity, they are unable to maintain adhesion, causing the active material to peel off, resulting in a significant decrease in capacity during charging and discharging.

Technologies proposed to improve the adhesion of electrode binders include a technique for controlling the amount of surface acid on particulate binder particles (see Patent Documents 2 and 3) and a technique for improving the above-mentioned properties by using binders having epoxy or hydroxyl groups (see Patent Documents 4 and 5). In addition, a technique has been proposed that uses the rigid molecular structure of polyimide to bind the active material and suppress volumetric changes in the active material (see Patent Document 6). A technique has also been proposed that uses a water-soluble polymer such as polyacrylic acid (see Patent Document 7).

JP 2004-185810 A International Publication No. 2011/096463 International Publication No. 2013/191080 JP 2010-205722 A JP 2010-3703 A JP 2011-204592 A International Publication No. 2015/098050

However, the electrode binders disclosed in the above Patent Documents 1 to 7 do not have sufficient adhesion for practical use of new active materials, such as silicon materials, which have a large lithium occlusion capacity and a large volume change associated with the occlusion and release of lithium. When such electrode binders are used, the active material falls off due to repeated charging and discharging, causing the electrode to deteriorate, and there is a problem that the charge-discharge durability characteristics required for practical use cannot be sufficiently obtained.

Some aspects of the present invention provide a binder composition for an electricity storage device that can produce an electricity storage device electrode that is excellent in flexibility and adhesion and exhibits good charge/discharge durability characteristics. Some aspects of the present invention also provide a slurry for an electricity storage device electrode that contains the composition. Some aspects of the present invention also provide an electricity storage device electrode that is excellent in flexibility and adhesion and exhibits good charge/discharge durability characteristics. Furthermore, some aspects of the present invention provide an electricity storage device that has excellent charge/discharge durability characteristics.

The present invention has been made to solve at least some of the above problems, and can be realized in any of the following forms:

One aspect of the binder composition for an electrical storage device according to the present invention is
Contains a polymer (A) and a liquid medium (B),
The polymer (A) is
30 to 65 parts by mass of a repeating unit (a1) derived from a conjugated diene compound;
5 to 40 parts by mass of a repeating unit (a2) derived from an unsaturated carboxylic acid;
and 10 to 40 parts by mass of a repeating unit (a3) derived from an α,β-unsaturated nitrile compound, and the following condition is satisfied:
<Conditions>
1.0 g of a film obtained by drying the polymer (A) in a thermostatic bath at 40° C. is immersed in 20 mL of an aqueous sodium hydroxide solution having a pH of 9, and shaken for 1 hour in an ultrasonic cleaner. In this case, the value of (Mb/Ma)×100 is 90% or more, where Ma is the weight of the film before immersion and Mb is the weight of the dried film after immersion.

In one embodiment of the binder composition for an electricity storage device,
The polymer (A) may have a swelling ratio of 150% or more and 350% or less when immersed in a solvent consisting of propylene carbonate and diethyl carbonate in a volume fraction of 1:1 at 70° C. for 24 hours.

In any one of the embodiments of the binder composition for an electricity storage device,
The polymer (A) is a polymer particle,
The polymer particles may have a surface acid amount of 0.5 mmol/g or more and 3 mmol/g or less.

In any one of the embodiments of the binder composition for an electricity storage device,
The number average particle size of the polymer particles may be 50 nm or more and 500 nm or less.

In any one of the embodiments of the binder composition for an electricity storage device,
The liquid medium (B) may be water.

One embodiment of a slurry for an electric storage device electrode according to the present invention is
The binder composition for an electricity storage device according to any one of the above aspects and an active material are contained.

In one embodiment of the slurry for an electricity storage device electrode,
The active material may contain a silicon material.

In any one of the embodiments of the slurry for an electricity storage device electrode,
It may further contain cellulose fibers.

One aspect of the electricity storage device electrode according to the present invention is
The electrode assembly includes a current collector and an active material layer formed by applying the slurry for an electricity storage device electrode according to any one of the above aspects to the surface of the current collector and drying the applied active material layer.

One aspect of the electricity storage device according to the present invention is
The power storage device includes the electrode according to the above aspect.

The binder composition for an electricity storage device according to the present invention can improve flexibility and adhesion, making it possible to manufacture an electricity storage device electrode that exhibits good charge/discharge durability characteristics. The binder composition for an electricity storage device according to the present invention exerts the above-mentioned effect particularly when the electricity storage device electrode contains a material with a large lithium occlusion capacity as the active material, for example, a carbon material such as graphite or a silicon material. In this way, a material with a large lithium occlusion capacity can be used as the active material of the electricity storage device electrode, and therefore battery performance is also improved.

The following describes in detail preferred embodiments of the present invention. Note that the present invention is not limited to the embodiments described below, and should be understood to include various modifications that are implemented within the scope of the present invention.

In this specification, "(meth)acrylic acid..." is a term that encompasses both "acrylic acid..." and "methacrylic acid...". Similarly, "(meth)acrylate" is a term that encompasses both "acrylate" and "methacrylate". Similarly, "(meth)acrylamide" is a term that encompasses both "acrylamide" and "methacrylamide".

In this specification, a numerical range described using "~" means that the numerical values before and after "~" are included as the lower and upper limits.

1. Binder composition for power storage device The binder composition for power storage device according to this embodiment contains a polymer (A) and a liquid medium (B). The binder composition for power storage device according to this embodiment can be used as a material for producing a power storage device electrode (active material layer) having improved binding ability between active materials, adhesion ability between the active material and the current collector, and powder fall resistance, and can also be used as a material for producing a protective film for suppressing short circuits caused by dendrites that occur during charging and discharging. Hereinafter, each component contained in the binder composition for power storage device according to this embodiment will be described in detail.

1.1. Polymer (A)
The binder composition for an electrical storage device according to this embodiment contains a polymer (A). The polymer (A) contains, when the total of the repeating units contained in the polymer (A) is taken as 100 parts by mass, 30 to 65 parts by mass of a repeating unit (a1) (hereinafter also simply referred to as "repeating unit (a1)") derived from a conjugated diene compound, 5 to 40 parts by mass of a repeating unit (a2) (hereinafter also simply referred to as "repeating unit (a2)") derived from an unsaturated carboxylic acid, and 10 to 40 parts by mass of a repeating unit (a3) (hereinafter also simply referred to as "repeating unit (a3)") derived from an α,β-unsaturated nitrile compound. In addition to the repeating units, the polymer (A) may contain a repeating unit derived from another monomer that is copolymerizable therewith.

Polymer (A) also satisfies the following condition: 1.0 g of a film obtained by drying the polymer (A) in a thermostatic bath at 40°C is immersed in 20 mL of an aqueous sodium hydroxide solution of pH 9, shaken in an ultrasonic cleaner for 1 hour, and separated into a liquid component and a solid component by centrifugation. In this case, if the weight of the film before immersion is Ma and the weight of the solid component after drying after centrifugation is Mb, the value of (Mb/Ma) x 100 is 90% or more.

The polymer (A) contained in the binder composition for a storage battery device according to this embodiment may be in a latex form dispersed in the liquid medium (B) or dissolved in the liquid medium (B), but is preferably in a latex form dispersed in the liquid medium (B). When the polymer (A) is in a latex form dispersed in the liquid medium (B), the stability of the slurry for a storage battery device electrode (hereinafter also simply referred to as "slurry") prepared by mixing the polymer (A) with the active material is improved, and the coating property of the slurry on a current collector is also improved, which is preferable.

The repeating units constituting polymer (A), the physical properties of polymer (A), and the manufacturing method will be described below.

1.1.1. Repeating units constituting polymer (A) 1.1.1.1. Repeating unit (a1) derived from a conjugated diene compound
The content of the repeating unit (a1) derived from the conjugated diene compound is 30 to 65 parts by mass when the total of the repeating units contained in the polymer (A) is 100 parts by mass. The lower limit of the content of the repeating unit (a1) is preferably 33 parts by mass, more preferably 35 parts by mass. The upper limit of the content of the repeating unit (a1) is preferably 60 parts by mass, more preferably 55 parts by mass. By containing the repeating unit (a1) in the above range, the dispersibility of the active material and the filler is improved, and a uniform active material layer and a protective film can be prepared, so that the structural defects of the electrode plate are eliminated and good charge and discharge characteristics are exhibited. In addition, the polymer (A) covering the surface of the active material can be given elasticity, and the polymer (A) can be stretched and stretched to improve adhesion, so that good charge and discharge durability characteristics are exhibited.

The conjugated diene compound is not particularly limited, but examples thereof include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and may be one or more selected from these. Among these, 1,3-butadiene is particularly preferred.

1.1.1.2. Repeating unit (a2) derived from unsaturated carboxylic acid>
The content of the repeating unit (a2) derived from the unsaturated carboxylic acid is 5 to 40 parts by mass when the total of the repeating units contained in the polymer (A) is 100 parts by mass. The lower limit of the content of the repeating unit (a2) is preferably 7 parts by mass, more preferably 10 parts by mass. The upper limit of the content of the repeating unit (a2) is preferably 35 parts by mass, more preferably 30 parts by mass. By containing the repeating unit (a2) in the above range, the dispersibility of the active material and the filler is improved. Furthermore, the affinity with the silicon material as the active material is improved, and the swelling of the silicon material is suppressed, thereby exhibiting good charge and discharge durability characteristics.

The unsaturated carboxylic acid is not particularly limited, but may be one or more selected from monocarboxylic acids and dicarboxylic acids (including anhydrides) such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid. As the unsaturated carboxylic acid, it is preferable to use one or more selected from acrylic acid, methacrylic acid, and itaconic acid.

1.1.1.3. Repeating unit (a3) derived from an α,β-unsaturated nitrile compound
The content ratio of the repeating unit derived from the α,β-unsaturated nitrile compound is 10 to 40 parts by mass when the total of the repeating units contained in the polymer (A) is 100 parts by mass. The lower limit of the content ratio of the repeating unit (a3) is preferably 12 parts by mass, more preferably 15 parts by mass. The upper limit of the content ratio of the repeating unit (a3) is preferably 35 parts by mass, more preferably 30 parts by mass. By containing the repeating unit (a3) in the above range, it is possible to reduce the dissolution of the polymer (A) in the electrolyte solution, and the decrease in adhesion due to the electrolyte solution can be suppressed. In addition, the increase in internal resistance caused by the polymer component dissolved in the electricity storage device becoming an electric resistance component can be suppressed.

The α,β-unsaturated nitrile compound is not particularly limited, but includes acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile, vinylidene cyanide, etc., and one or more selected from these can be used. Among these, one or more selected from the group consisting of acrylonitrile and methacrylonitrile are preferred, and acrylonitrile is particularly preferred.

1.1.1.4. Other repeating units In addition to the repeating units (a1) to (a3), the polymer (A) may contain repeating units derived from other monomers copolymerizable therewith. Examples of such repeating units include a repeating unit (a4) derived from (meth)acrylamide (hereinafter also simply referred to as "repeating unit (a4)"); a repeating unit (a5) derived from an unsaturated carboxylate ester having a hydroxyl group (hereinafter also simply referred to as "repeating unit (a5)"); a repeating unit (a6) derived from an unsaturated carboxylate ester (hereinafter also simply referred to as "repeating unit (a6)"); a repeating unit (a7) derived from an aromatic vinyl compound (hereinafter also simply referred to as "repeating unit (a7)"); a repeating unit (a8) derived from a compound having a sulfonic acid group (hereinafter also simply referred to as "repeating unit (a8)"); and a repeating unit derived from a cationic monomer.

<Repeating Unit (a4) Derived from (Meth)acrylamide>
The polymer (A) may contain a repeating unit (a4) derived from (meth)acrylamide. The content ratio of the repeating unit (a4) is preferably 0.1 to 10 parts by mass when the total of the repeating units contained in the polymer (A) is 100 parts by mass. When the polymer (A) contains the repeating unit (a4) in the above range, the dispersibility of the active material and the filler in the slurry may be improved. In addition, the flexibility of the obtained active material layer becomes appropriate, and the adhesion between the current collector and the active material layer becomes good. Furthermore, since the bonding ability between the active materials containing a carbon material such as graphite and a silicon material can be increased, an active material layer having better flexibility and adhesion to the current collector may be obtained.

(Meth)acrylamides include, but are not limited to, acrylamide, methacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide, N,N-diethylacrylamide, N,N-diethylmethacrylamide, N,N-dimethylaminopropylacrylamide, N,N-dimethylaminopropylmethacrylamide, N-methylolmethacrylamide, N-methylolacrylamide, diacetoneacrylamide, maleic acid amide, acrylamido tert-butylsulfonic acid, and the like, and may be one or more selected from these.

<Repeating Unit (a5) Derived from Unsaturated Carboxylic Acid Ester Having a Hydroxyl Group>
The polymer (A) may contain a repeating unit (a5) derived from an unsaturated carboxylic acid ester having a hydroxyl group. The content ratio of the repeating unit (a5) is preferably 0.1 to 10 parts by mass when the total of the repeating units contained in the polymer (A) is 100 parts by mass. When the polymer (A) contains the repeating unit (a5) in the above range, it may be easier to prepare a slurry in which the active material is well dispersed without agglomerating the active material when preparing a slurry described later. As a result, the polymer (A) in the active material layer prepared by applying and drying the slurry is distributed nearly uniformly, so that an electric storage device electrode with very few binding defects may be prepared. That is, the binding ability between active materials and the adhesion ability between the active material layer and the current collector may be dramatically improved.

The unsaturated carboxylate ester having a hydroxyl group is not particularly limited, but may be one or more selected from 2-hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 5-hydroxypentyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, glycerin mono(meth)acrylate, glycerin di(meth)acrylate, etc. Among these, 2-hydroxyethyl (meth)acrylate and glycerin mono(meth)acrylate are preferred.

<Repeating Unit (a6) Derived from Unsaturated Carboxylic Acid Ester>
The polymer (A) may contain a repeating unit (a6) derived from an unsaturated carboxylate (excluding the unsaturated carboxylate having a hydroxyl group). The content of the repeating unit (a6) is preferably 0.1 to 30 parts by mass when the total of the repeating units contained in the polymer (A) is taken as 100 parts by mass. When the polymer (A) contains the repeating unit (a6) in the above range, the affinity between the polymer (A) and the electrolyte is improved, and an increase in internal resistance due to the binder becoming an electric resistance component in the electricity storage device is suppressed, and a decrease in adhesion due to excessive absorption of the electrolyte may be prevented.

Among the unsaturated carboxylic acid esters, (meth)acrylic acid esters can be preferably used. Specific examples of (meth)acrylic acid esters include, for example, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, n-amyl (meth)acrylate, i-amyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and allyl (meth)acrylate, and may be one or more selected from these. Among these, one or more selected from methyl (meth)acrylate, ethyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate are preferred, with methyl (meth)acrylate being particularly preferred.

<Repeating Unit (a7) Derived from Aromatic Vinyl Compound>
The polymer (A) may contain a repeating unit (a7) derived from an aromatic vinyl compound. The content of the repeating unit (a7) is preferably 0.1 to 30 parts by mass when the total of the repeating units contained in the polymer (A) is taken as 100 parts by mass. When the polymer (A) contains the repeating unit (a7) in the above range, the polymer (A) has an appropriate binding strength with respect to graphite used as an active material, and an electricity storage device electrode having excellent flexibility and adhesion may be obtained.

Aromatic vinyl compounds include, but are not limited to, styrene, α-methylstyrene, p-methylstyrene, vinyltoluene, chlorostyrene, divinylbenzene, etc., and may be one or more selected from these.

<Repeating Unit (a8) Derived from Compound Having Sulfonic Acid Group>
The polymer (A) may contain a repeating unit (a8) derived from a compound having a sulfonic acid group. The content of the repeating unit (a8) is preferably 0.1 to 10 parts by mass when the total amount of the repeating units contained in the polymer (A) is taken as 100 parts by mass.

The compound having a sulfonic acid group is not particularly limited, but may be vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, sulfoethyl (meth)acrylate, sulfopropyl (meth)acrylate, sulfobutyl (meth)acrylate, 2-acrylamido-2-methylpropanesulfonic acid, 2-hydroxy-3-acrylamidopropanesulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, or other compounds having a sulfonic acid group, as well as alkali salts thereof.

<Repeating Unit Derived from Cationic Monomer>
The polymer (A) may contain a repeating unit derived from a cationic monomer. The cationic monomer is not particularly limited, but is preferably at least one monomer selected from the group consisting of secondary amines (salts), tertiary amines (salts), and quaternary ammonium salts. Specific examples of these cationic monomers are not particularly limited, but include 2-(dimethylamino)ethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate methyl chloride quaternary salt, 2-(diethylamino)ethyl (meth)acrylate, 3-(dimethylamino)propyl (meth)acrylate, 3-(diethylamino)propyl (meth)acrylate, 4-(dimethylamino)phenyl (meth)acrylate, 2-[(3,5-dimethylpyrazolyl)carbonylamino]ethyl (meth)acrylate, 2-(0-[1'-methylpropylideneamino]carboxyamino)ethyl (meth)acrylate, 2-(1-aziridinyl)ethyl (meth)acrylate, methacryloylcholine chloride, tris(2- acryloyloxyethyl), 2-vinylpyridine, quinaldine red, 1,2-di(2-pyridyl)ethylene, 4'-hydrazino-2-stilbazole dihydrochloride hydrate, 4-(4-dimethylaminostyryl)quinoline, 1-vinylimidazole, diallylamine, diallylamine hydrochloride, triallylamine, diallyldimethylammonium chloride, dichlormid, N-allylbenzylamine, N-allylaniline, 2,4-diamino-6-diallylamino-1,3,5-triazine, N-trans-cinnamyl-N-methyl-(1-naphthylmethyl)amine hydrochloride, trans-N-(6,6-dimethyl-2-hepten-4-ynyl)-N-methyl-1-naphthylmethylamine hydrochloride, etc. These monomers may be used alone or in combination of two or more.

1.1.2. Physical properties of polymer (A) <Water-insoluble content>
The polymer (A) has a value of (Mb/Ma)×100 measured under the following conditions (hereinafter also referred to as "water-insoluble content") of 90% or more. This water-insoluble content is preferably 91% or more, and more preferably 92% or more.
(conditions)
1.0 g of a film obtained by drying the polymer (A) in a thermostatic chamber at 40° C. is immersed in 20 mL of an aqueous sodium hydroxide solution of pH 9, shaken for 1 hour in an ultrasonic cleaner, and separated into a liquid component and a solid component by centrifugation. The weight of the film before immersion is Ma, and the weight of the solid component after drying after centrifugation is Mb, and the value of (Mb/Ma)×100 is calculated.

When the water-insoluble content is within the above range, it indicates that a large proportion of hydrophilic monomers are chemically bonded to the polymer, and the surface of polymer (A) is likely to have a high hydrophilicity. As a result, the amount of adsorption to active materials, particularly silicon-based active materials, is improved, and the adhesion between active materials and between the current collector and the active material is improved, which is presumably responsible for improved charge/discharge durability.

<Electrolyte Swelling Rate>
The swelling ratio (hereinafter also referred to as "electrolyte swelling ratio") when the polymer (A) is immersed in a solvent consisting of propylene carbonate and diethyl carbonate at a volume fraction of 1:1 at 70 ° C. for 24 hours is preferably 150% or more and 350% or less, more preferably 155% or more and 330% or less, and particularly preferably 160% or more and 300% or less. When the electrolyte swelling ratio of the polymer (A) is within the above range, the polymer (A) can swell moderately by absorbing the electrolyte. As a result, the solvated lithium ions can easily reach the active material, so that the electrode resistance can be effectively reduced and better charge and discharge characteristics can be realized. In addition, if the electrolyte swelling ratio is within the above range, the polymer (A) does not undergo a large volume change even when it absorbs the electrolyte, and therefore has excellent adhesion.

<Number average particle size>
When the polymer (A) is a particle, the number average particle size of the particles is preferably 50 nm or more and 500 nm or less, more preferably 60 nm or more and 450 nm or less, and particularly preferably 70 nm or more and 400 nm or less. When the number average particle size of the particles of the polymer (A) is within the above range, the particles of the polymer (A) are easily adsorbed on the surface of the active material, so that the particles of the polymer (A) can move along with the movement of the active material. As a result, it is possible to suppress the migration of only one of the two particles alone, thereby reducing the deterioration of electrical properties.

The number-average particle size of the particles of polymer (A) can be calculated from the average particle size of 50 particles obtained from images of the particles observed with a transmission electron microscope (TEM). An example of a transmission electron microscope is the "H-7650" manufactured by Hitachi High-Technologies Corporation.

<Amount of surface acid>
When the polymer (A) is a particle, the surface acid amount of the particle is preferably 0.5 mmol/g or more and 3 mmol/g or less, more preferably 0.6 mmol/g or more and 2.8 mmol/g or less, and particularly preferably 0.7 mmol/g or more and 2.5 mmol/g or less. When the surface acid amount of the polymer (A) particles is within the above range, a stable and homogeneous slurry can be prepared. When an active material layer is prepared using such a homogeneous slurry, an active material layer with a small thickness variation in which the active material and the polymer (A) particles are uniformly dispersed can be obtained. As a result, the variation in the charge/discharge characteristics in the electrode can be suppressed, and therefore an electricity storage device exhibiting good charge/discharge characteristics can be obtained.

<Glass transition temperature (Tg)>
The polymer (A) preferably has only one endothermic peak in the temperature range of −100° C. to 150° C. when measured by differential scanning calorimetry (DSC) in accordance with JIS K7121. The temperature of this endothermic peak (i.e., the glass transition temperature (Tg)) is more preferably in the range of −50° C. to 50° C. When the polymer (A) has only one endothermic peak in the DSC analysis and the peak temperature is in the above range, the polymer (A) is preferred because it shows good adhesion and can impart good flexibility and adhesion to the active material layer.

1.1.3. Method for Producing Polymer (A) The method for producing the polymer (A) is not particularly limited, but may be, for example, an emulsion polymerization method carried out in the presence of a known emulsifier (surfactant), chain transfer agent, polymerization initiator, etc. As the emulsifier (surfactant), chain transfer agent, and polymerization initiator, compounds described in Japanese Patent No. 5999399 and the like can be used.

The emulsion polymerization method for synthesizing polymer (A) may be carried out in a single-stage polymerization or in a multi-stage polymerization of two or more stages.

When polymer (A) is synthesized by single-stage polymerization, the mixture of the above monomers can be subjected to emulsion polymerization in the presence of a suitable emulsifier, chain transfer agent, polymerization initiator, etc., preferably at 40 to 80°C, for preferably 4 to 36 hours.

When polymer (A) is synthesized by two-stage polymerization, it is preferable to set the polymerization in each stage as follows.

The proportion of the monomer used in the first-stage polymerization is preferably in the range of 20 to 99% by mass, more preferably 25 to 99% by mass, based on the total mass of the monomers (the sum of the mass of the monomers used in the first-stage polymerization and the mass of the monomers used in the second-stage polymerization). By carrying out the first-stage polymerization at such a proportion of the monomers, it is possible to obtain particles of polymer (A) that are excellent in dispersion stability and are less likely to form aggregates, and it is also preferable because the increase in viscosity over time of the binder composition for electrical storage devices is suppressed.

The type and proportion of monomers used in the second-stage polymerization may be the same as or different from the type and proportion of monomers used in the first-stage polymerization.

The polymerization conditions in each stage are preferably as follows, from the viewpoint of the dispersibility of the particles of the resulting polymer (A).
First-stage polymerization: a temperature of preferably 40 to 80° C.; a polymerization time of preferably 2 to 36 hours; a polymerization conversion rate of preferably 50% by mass or more, more preferably 60% by mass or more.
Second stage polymerization: a temperature preferably between 40 and 80° C.; a polymerization time preferably between 2 and 18 hours.

By setting the total solids concentration in the emulsion polymerization to 50% by mass or less, the polymerization reaction can be carried out in a state in which the dispersion stability of the particles of the resulting polymer (A) is good. This total solids concentration is preferably 45% by mass or less, and more preferably 42% by mass or less.

Whether the synthesis of the polymer (A) is carried out as a single-stage polymerization or a two-stage polymerization method, it is preferable to adjust the pH to about 3.5 to 10, preferably 4 to 9, more preferably 4.5 to 8, by adding a neutralizing agent to the polymerization mixture after the completion of emulsion polymerization. The neutralizing agent used here is not particularly limited, but examples thereof include metal hydroxides such as sodium hydroxide and potassium hydroxide; ammonia, etc. By setting the pH in the above range, the stability of the polymer (A) is improved. By concentrating the polymerization mixture after the neutralization treatment, the solid content concentration can be increased while maintaining the good stability of the polymer (A).

The content of the polymer (A) in the binder composition for an electrical storage device according to this embodiment is preferably 10 to 100 parts by mass, more preferably 20 to 95 parts by mass, and particularly preferably 25 to 90 parts by mass, per 100 parts by mass of the polymer component. Here, the polymer component includes the polymer (A), a polymer other than the polymer (A) described below, a thickener, and the like.

1.2. Liquid medium (B)
The binder composition for a storage battery device according to this embodiment contains a liquid medium (B). The liquid medium (B) is preferably an aqueous medium containing water, and more preferably water. The aqueous medium can contain a non-aqueous medium other than water. Examples of the non-aqueous medium include amide compounds, hydrocarbons, alcohols, ketones, esters, amine compounds, lactones, sulfoxides, and sulfone compounds, and one or more selected from these can be used. The binder composition for a storage battery device according to this embodiment uses an aqueous medium as the liquid medium (B), which reduces the degree of adverse effects on the environment and increases the safety for workers who handle the binder composition.

The content of the non-aqueous medium in the aqueous medium is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and particularly preferably substantially free of the non-aqueous medium, per 100 parts by mass of the aqueous medium. Here, "substantially free of the non-aqueous medium" means that the non-aqueous medium is not intentionally added as a liquid medium, and may include a non-aqueous medium that is inevitably mixed in when preparing the binder composition for an electrical storage device.

1.3 Other Additives The binder composition for an electrical storage device according to this embodiment may contain additives other than the above-mentioned components as necessary. Examples of such additives include polymers other than the polymer (A), preservatives, thickeners, etc.

<Polymers other than polymer (A)>
The binder composition for a storage battery device according to this embodiment may contain a polymer other than the polymer (A). Such a polymer is not particularly limited, but may be an acrylic polymer containing an unsaturated carboxylic acid ester or a derivative thereof as a structural unit, a fluorine-based polymer such as PVDF (polyvinylidene fluoride), etc. These polymers may be used alone or in combination of two or more. By containing these polymers, flexibility and adhesion may be further improved.

<Preservatives>
The binder composition for a storage battery device according to the present embodiment may contain a preservative. By containing a preservative, it may be possible to suppress the proliferation of bacteria, mold, and the like and the generation of foreign matter when the binder composition for a storage battery device is stored. Specific examples of the preservative include compounds described in Japanese Patent No. 5477610 and the like.

<Thickener>
The binder composition for an electricity storage device according to this embodiment may contain a thickener. By containing a thickener, it may be possible to further improve the coatability of the slurry and the charge/discharge characteristics of the resulting electricity storage device.

Specific examples of thickeners include cellulose compounds such as carboxymethylcellulose, methylcellulose, and hydroxypropylcellulose; poly(meth)acrylic acid; ammonium salts or alkali metal salts of the cellulose compounds or the poly(meth)acrylic acid; polyvinyl alcohol (co)polymers such as polyvinyl alcohol, modified polyvinyl alcohol, and ethylene-vinyl alcohol copolymers; and water-soluble polymers such as saponified copolymers of vinyl esters and unsaturated carboxylic acids such as (meth)acrylic acid, maleic acid, and fumaric acid. Among these, alkali metal salts of carboxymethylcellulose and alkali metal salts of poly(meth)acrylic acid are preferred.

Commercially available thickeners include alkali metal salts of carboxymethylcellulose such as CMC1120, CMC1150, CMC2200, CMC2280, and CMC2450 (all manufactured by Daicel Corporation).

When the binder composition for an electrical storage device according to this embodiment contains a thickener, the content of the thickener is preferably 5 parts by mass or less, and more preferably 0.1 to 3 parts by mass, per 100 parts by mass of the total solid content of the binder composition for an electrical storage device.

1.4. pH of the binder composition for an electricity storage device
The pH of the binder composition for an electricity storage device according to this embodiment is preferably 3.5 to 10, more preferably 4 to 9, and particularly preferably 4.5 to 8. When the pH is within this range, the occurrence of problems such as insufficient leveling and dripping can be suppressed, and it becomes easy to produce an electricity storage device electrode that has both good electrical properties and good adhesion.

In this specification, "pH" refers to a physical property measured as follows: It is a value measured at 25°C using a pH meter that uses a glass electrode calibrated with neutral phosphate standard solution and borate standard solution as pH standard solutions, in accordance with JIS Z8802:2011. Examples of such pH meters include the "HM-7J" manufactured by DKK-TOA Corporation and the "D-51" manufactured by Horiba, Ltd.

It should be noted that although it is not denied that the pH of the binder composition for electrical storage devices is affected by the monomer composition constituting the polymer (A), it is not determined solely by the monomer composition. In other words, it is generally known that even with the same monomer composition, the pH of the binder composition for electrical storage devices changes depending on the polymerization conditions, etc., and the examples in this specification are merely examples of such cases.

For example, even with the same monomer composition, the amount of carboxyl groups derived from unsaturated carboxylic acid exposed on the surface of the resulting polymer will differ if all the unsaturated carboxylic acid is added to the polymerization reaction solution from the beginning and then other monomers are added sequentially, compared with if monomers other than the unsaturated carboxylic acid are added to the polymerization reaction solution and the unsaturated carboxylic acid is added last. In this way, it is believed that the pH of the binder composition for electricity storage devices will differ significantly just by changing the order in which the monomers are added in the polymerization method.

2. Slurry for Electricity Storage Device The slurry for the electricity storage device according to this embodiment contains the above-mentioned binder composition for the electricity storage device. The above-mentioned binder composition for the electricity storage device can be used as a material for producing a protective film for suppressing short circuits caused by dendrites that occur with charging and discharging, and can also be used as a material for producing an electricity storage device electrode (active material layer) with improved binding ability between active materials, adhesion ability between the active material and the current collector, and powder fall resistance. Therefore, the slurry for the electricity storage device for producing a protective film (hereinafter also referred to as "slurry for protective film") and the slurry for the electricity storage device for forming an active material layer of the electricity storage device electrode (hereinafter also referred to as "slurry for electricity storage device electrode") will be described separately.

2.1. Slurry for protective film The term "slurry for protective film" refers to a dispersion liquid used to prepare a protective film on the surface of an electrode or a separator or both by applying the slurry to the surface of an electrode or a separator or both, and then drying the slurry. The slurry for protective film according to this embodiment may be composed only of the binder composition for a storage battery device described above, or may further contain an inorganic filler. Hereinafter, each component contained in the slurry for protective film according to this embodiment will be described in detail. Note that the binder composition for a storage battery device is as described above, so a description thereof will be omitted.

2.1.1 Inorganic filler The slurry for the protective film according to this embodiment contains an inorganic filler, thereby improving the toughness of the protective film. As the inorganic filler, it is preferable to use at least one type of particle selected from the group consisting of silica, titanium oxide (titania), aluminum oxide (alumina), zirconium oxide (zirconia), and magnesium oxide (magnesia). Among these, titanium oxide or aluminum oxide is preferable from the viewpoint of further improving the toughness of the protective film. Moreover, as the titanium oxide, rutile type titanium oxide is more preferable.

The average particle size of the inorganic filler is preferably 1 μm or less, and more preferably 0.1 to 0.8 μm. The average particle size of the inorganic filler is preferably larger than the average pore size of the separator, which is a porous membrane. This reduces damage to the separator and prevents the inorganic filler from clogging the micropores of the separator.

The protective film slurry according to this embodiment preferably contains 0.1 to 20 parts by mass, and more preferably 1 to 10 parts by mass, of the binder composition for an electricity storage device described above, calculated as solid content, per 100 parts by mass of inorganic filler. When the content ratio of the binder composition for an electricity storage device is within the above range, a good balance between the toughness of the protective film and the permeability of lithium ions is achieved, and as a result, the resistance increase rate of the obtained electricity storage device can be reduced.

2.1.2. Liquid medium In addition to the liquid medium carried over from the binder composition for a storage battery device, the slurry for the protective film according to this embodiment may further contain a liquid medium. The amount of the liquid medium added can be adjusted as necessary so as to obtain an optimal slurry viscosity depending on the coating method, etc. Examples of such a liquid medium include the materials described in "1.2. Liquid medium (B)" above.

2.1.3. Other Components The slurry for the overcoat film according to this embodiment may contain the materials described in "1.3. Other additives" above in appropriate amounts as necessary.

2.2. Slurry for Electricity Storage Device Electrode The "slurry for electricity storage device electrode" refers to a dispersion liquid used for preparing an active material layer on the surface of a current collector by applying the dispersion liquid to the surface of a current collector and then drying the dispersion liquid. The slurry for electricity storage device electrode according to this embodiment contains the above-mentioned binder composition for electricity storage devices and an active material.

Generally, a slurry for an electric storage device electrode often contains a binder component such as an SBR-based copolymer and a thickener such as carboxymethyl cellulose in order to improve adhesion. On the other hand, the slurry for an electric storage device electrode according to this embodiment can improve flexibility and adhesion even when it contains only the above-mentioned polymer (A) as the polymer component. Of course, the slurry for an electric storage device electrode according to this embodiment may contain a polymer other than the polymer (A) or a thickener in order to further improve adhesion.

The components contained in the slurry for the power storage device electrode according to this embodiment are described below.

2.2.1. Polymer (A)
The composition, characteristics and production method of the polymer (A) are as described above, and therefore description thereof will be omitted.

The content of the polymer component in the slurry for the electric storage device electrode according to this embodiment is preferably 1 to 8 parts by mass, more preferably 1 to 7 parts by mass, and particularly preferably 1.5 to 6 parts by mass, relative to 100 parts by mass of the active material. When the content of the polymer component is within the above range, the dispersibility of the active material in the slurry is good, and the coating properties of the slurry are also excellent. Here, the polymer component includes the polymer (A), polymers other than the polymer (A) that are added as necessary, and a thickener, etc.

2.2.2. Active material Examples of active materials used in the slurry for the electric storage device electrode according to this embodiment include carbon materials, silicon materials, oxides containing lithium atoms, lead compounds, tin compounds, arsenic compounds, antimony compounds, aluminum compounds, conductive polymers such as polyacene, composite metal oxides represented by A _x B _y O _z (wherein A is an alkali metal or transition metal, B is at least one selected from transition metals such as cobalt, nickel, aluminum, tin, and manganese, O is an oxygen atom, and X, Y, and Z are numbers in the ranges of 1.10>X>0.05, 4.00>Y>0.85, and 5.00>Z>1.5, respectively), and other metal oxides. Specific examples of these include compounds described in Japanese Patent No. 5,999,399 and the like.

The slurry for electricity storage device electrodes according to this embodiment can be used when producing either positive or negative electricity storage device electrodes, and is preferably used for both positive and negative electrodes.

When lithium iron phosphate is used as the positive electrode active material, there are issues such as insufficient charge/discharge characteristics and poor adhesion. Lithium iron phosphate has a fine primary particle size and is known to be in the form of secondary aggregates. When repeatedly charged and discharged, the aggregates break down in the active material layer, causing the active material to separate from each other, which is thought to be one of the reasons for this.

However, in the case of an electricity storage device electrode produced using the electricity storage device electrode slurry according to this embodiment, the above-mentioned problems do not occur even when lithium iron phosphate is used, and good electrical properties can be exhibited. The reason for this is believed to be that the polymer (A) can firmly bind the lithium iron phosphate and at the same time, can maintain the state in which the lithium iron phosphate is firmly bound even during charging and discharging.

On the other hand, when preparing a negative electrode, it is preferable to use an active material containing a silicon material among the above-mentioned examples. Silicon materials have a larger amount of lithium absorbed per unit weight than other active materials, so by containing a silicon material as a negative electrode active material, the storage capacity of the resulting electricity storage device can be increased, and as a result, the output and energy density of the electricity storage device can be increased.

The negative electrode active material is preferably a mixture of a silicon material and a carbon material. The volume change of a carbon material during charging and discharging is smaller than that of a silicon material. Therefore, by using a mixture of a silicon material and a carbon material as the negative electrode active material, the effect of the volume change of the silicon material can be mitigated, and the adhesion ability between the active material layer and the current collector can be further improved.

When silicon (Si) is used as an active material, while it has a high capacity, it undergoes a large volume change when it absorbs lithium. This causes silicon materials to pulverize due to repeated expansion and contraction, which can lead to peeling from the current collector and separation of active materials from each other, and the conductive network inside the active material layer is easily disrupted. This causes the charge/discharge durability characteristics to deteriorate drastically in a short period of time.

However, the electricity storage device electrode produced using the electricity storage device electrode slurry according to this embodiment can exhibit good electrical properties even when a silicon material is used, without the above-mentioned problems. This is thought to be because the polymer (A) can firmly bind the silicon material, and at the same time, even if the silicon material expands in volume due to lithium absorption, the polymer (A) can expand and contract, thereby maintaining the state in which the silicon material is firmly bound.

The content of silicon material in 100% by mass of active material is preferably 1% by mass or more, more preferably 1 to 50% by mass, even more preferably 5 to 45% by mass, and particularly preferably 10 to 40% by mass. When the content of silicon material in 100% by mass of active material is within the above range, an electricity storage device can be obtained that has an excellent balance between improved output and energy density of the electricity storage device and the charge/discharge durability characteristics.

The active material is preferably in the form of particles. The average particle size of the active material is preferably 0.1 to 100 μm, and more preferably 1 to 20 μm. Here, the average particle size of the active material refers to the volume average particle size calculated from the particle size distribution measured using a particle size distribution measuring device that uses laser diffraction as the measurement principle. Examples of such laser diffraction particle size distribution measuring devices include the HORIBA LA-300 series and the HORIBA LA-920 series (both manufactured by Horiba, Ltd.).

2.2.3. Other components In addition to the above-mentioned components, other components may be added to the slurry for the electric storage device electrode according to this embodiment as necessary. Examples of such components include polymers other than the polymer (A), thickeners, liquid media, conductive agents, pH adjusters, corrosion inhibitors, cellulose fibers, etc. The polymers other than the polymer (A) and thickeners can be selected from the compounds exemplified in "1.3. Other additives" above and used for the same purpose and in the same content ratio.

<Liquid medium>
A liquid medium may be added to the slurry for an electric storage device electrode according to this embodiment in addition to the liquid medium carried over from the binder composition for an electric storage device. The liquid medium added may be the same type as the liquid medium (B) contained in the binder composition for an electric storage device,
Although they may be different, it is preferable to select and use one from the liquid media exemplified in "1.2. Liquid medium (B)" above.

The content of the liquid medium (including the amount carried over from the binder composition for the electricity storage device) in the slurry for the electricity storage device electrode according to this embodiment is preferably such that the solids concentration in the slurry (meaning the proportion of the total mass of the components in the slurry other than the liquid medium to the total mass of the slurry; the same applies below) is 30 to 70 mass%, and more preferably 40 to 60 mass%.

<Conductivity imparting agent>
The slurry for an electricity storage device electrode according to this embodiment may contain a conductivity imparting agent for the purposes of imparting electrical conductivity and buffering the volume change of the active material caused by the ingress and egress of lithium ions.

Specific examples of the conductive agent include carbon such as activated carbon, acetylene black, ketjen black, furnace black, graphite, carbon fiber, and fullerene. Among these, acetylene black or ketjen black can be preferably used. The content of the conductive agent is preferably 20 parts by mass or less, more preferably 1 to 15 parts by mass, and particularly preferably 2 to 10 parts by mass, relative to 100 parts by mass of the active material.

<pH adjuster/corrosion inhibitor>
The slurry for an electricity storage device electrode according to this embodiment may contain a pH adjuster and/or a corrosion inhibitor for the purpose of suppressing corrosion of the current collector depending on the type of active material.

Examples of pH adjusters include hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid, formic acid, ammonium phosphate, ammonium sulfate, ammonium acetate, ammonium formate, ammonium chloride, sodium hydroxide, potassium hydroxide, etc., and among these, sulfuric acid, ammonium sulfate, sodium hydroxide, and potassium hydroxide are preferred. In addition, a neutralizing agent selected from those described in the production method for polymer (A) can also be used.

Examples of corrosion inhibitors include ammonium metavanadate, sodium metavanadate, potassium metavanadate, ammonium metatungstate, sodium metatungstate, potassium metatungstate, ammonium paratungstate, sodium paratungstate, potassium paratungstate, ammonium molybdate, sodium molybdate, potassium molybdate, etc., and among these, ammonium paratungstate, ammonium metavanadate, sodium metavanadate, potassium metavanadate, and ammonium molybdate are preferred.

<Cellulose fiber>
The slurry for an electricity storage device electrode according to this embodiment may contain cellulose fibers. By containing cellulose fibers, the adhesion of the active material to the current collector may be improved. It is considered that the fibrous cellulose fibers bond adjacent active materials to each other in a fibrous form by linear adhesion or linear contact, thereby preventing the active material from falling off and improving the adhesion to the current collector.

The average fiber length of the cellulose fiber can be selected from a wide range of 0.1 to 1000 μm, and is, for example, preferably 1 to 750 μm, more preferably 1.3 to 500 μm, even more preferably 1.4 to 250 μm, and particularly preferably 1.8 to 25 μm. When the average fiber length is within the above range, the surface smoothness (coating film uniformity) becomes good, and the adhesion of the active material to the current collector may be improved.

The fiber length of the cellulose fibers may be uniform, and the coefficient of variation of the fiber length ([standard deviation of fiber length/average fiber length] x 100) is, for example, preferably 0.1 to 100, more preferably 0.5 to 50, and particularly preferably 1 to 30. The maximum fiber length of the cellulose fibers is, for example, preferably 500 μm or less, more preferably 300 μm or less, even more preferably 200 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less.

It is advantageous to set the average fiber length of the cellulose fibers to 5 times or less the average thickness of the active material layer, since this further improves the surface smoothness (uniformity of the coating film) and the adhesion of the active material to the current collector. The average fiber length of the cellulose fibers is preferably 0.01 to 5 times, more preferably 0.02 to 3 times, and particularly preferably 0.03 to 2 times the average thickness of the active material layer.

The average fiber diameter of the cellulose fibers is preferably 1 nm to 10 μm, more preferably 5 nm to 2.5 μm, even more preferably 20 to 700 nm, and particularly preferably 30 to 200 nm. When the average fiber diameter is within the above range, the volume occupied by the fibers does not become too large, and the packing density of the active material may be increased. Therefore, it is preferable that the cellulose fibers are cellulose nanofibers with an average fiber diameter of nanometer size (for example, cellulose nanofibers with an average fiber diameter of about 10 to 500 nm, preferably about 25 to 250 nm).

The fiber diameter of the cellulose fibers is also uniform, and the coefficient of variation of the fiber diameter ([standard deviation of fiber diameter/average fiber diameter] x 100) is preferably 1 to 80, more preferably 5 to 60, and particularly preferably 10 to 50. The maximum fiber diameter of the cellulose fibers is preferably 30 μm or less, more preferably 5 μm or less, and particularly preferably 1 μm or less.

The ratio of the average fiber length to the average fiber diameter of the cellulose fibers (aspect ratio) is, for example, preferably 10 to 5,000, more preferably 20 to 3,000, and particularly preferably 50 to 2,000. When the aspect ratio is within this range, the adhesion of the active material to the current collector is good, and the surface smoothness of the electrode (uniformity of the coating film) may be good without weakening the breaking strength of the fibers.

In the present invention, the average fiber length, the standard deviation of the fiber length distribution, the maximum fiber length, the average fiber diameter, the standard deviation of the fiber diameter distribution, and the maximum fiber diameter may be values calculated from fibers (n=about 20) measured based on electron micrographs.

The material of the cellulose fiber may be formed of a polysaccharide having a β-1,4-glucan structure. Examples of cellulose fibers include cellulose fibers derived from higher plants (e.g., wood fibers (wood pulp of conifers, broad-leaved trees, etc.), bamboo fibers, sugarcane fibers, seed hair fibers (e.g., cotton linters, bombax cotton, kapok, etc.), gin bark fibers (e.g., hemp, paper mulberry, Mitsumata, etc.), leaf fibers (e.g., Manila hemp, New Zealand hemp, etc.), and other natural cellulose fibers (pulp fibers), etc.), cellulose fibers derived from animals (e.g., sea squirt cellulose, etc.), cellulose fibers derived from bacteria (e.g., cellulose contained in nata de coco, etc.), chemically synthesized cellulose fibers (e.g., rayon, cellulose esters (cellulose acetate, etc.), cellulose ethers (e.g., hydroxyethyl cellulose (HEC), hydroxyalkyl cellulose such as hydroxypropyl cellulose, methyl cellulose, ethyl cellulose, and other alkyl celluloses, etc.), and other cellulose derivatives. These cellulose fibers may be used alone or in combination of two or more types.

Among these cellulose fibers, cellulose fibers derived from higher plants, such as pulp-derived cellulose fibers (such as wood pulp from conifers and broad-leaved trees) and seed hair fibers (such as cotton linter pulp), are preferred because they are easy to prepare into nanofibers with an appropriate aspect ratio.

The method for producing cellulose fiber is not particularly limited, and depending on the desired fiber length and fiber diameter, conventional methods such as those described in JP-B-60-19921, JP-A-2011-26760, JP-A-2012-25833, JP-A-2012-36517, JP-A-2012-36518, and JP-A-2014-181421 may be used.

2.2.4. Method for preparing slurry for power storage device electrode The slurry for power storage device electrode according to this embodiment may be produced by any method as long as it contains the above-mentioned binder composition for power storage device and active material. From the viewpoint of producing a slurry having better dispersibility and stability more efficiently and inexpensively, it is preferable to produce the slurry by adding the active material and optional additive components used as necessary to the binder composition for power storage device and mixing them. Specific examples of the production method include the method described in Japanese Patent No. 5999399 and the like.

3. Electricity storage device electrode The electricity storage device electrode according to this embodiment includes a current collector and an active material layer formed by applying and drying the above-mentioned slurry for electricity storage device electrodes on the surface of the current collector. Such an electricity storage device electrode can be manufactured by applying the above-mentioned slurry for electricity storage device electrodes to the surface of a current collector such as a metal foil to form a coating film, and then drying the coating film to form an active material layer. The electricity storage device electrode manufactured in this manner is formed by binding an active material layer containing the above-mentioned polymer (A) and active material, and further optional components added as necessary, on a current collector, and therefore has excellent flexibility and adhesion and exhibits good charge/discharge durability characteristics.

There are no particular limitations on the current collector as long as it is made of a conductive material, but examples of the current collector include those described in Japanese Patent No. 5,999,399, etc.

There are no particular limitations on the method of applying the slurry for the electric storage device electrode to the current collector, and it can be applied by the method described in, for example, Japanese Patent No. 5,999,399.

When a silicon material is used as the active material in the electricity storage device electrode according to this embodiment, the content of silicon elements in 100 parts by mass of the active material layer is preferably 2 to 30 parts by mass, more preferably 2 to 20 parts by mass, and particularly preferably 3 to 10 parts by mass. When the content of silicon elements in the active material layer is within the above range, the electricity storage capacity of the electricity storage device produced using it is improved, and an active material layer with a uniform distribution of silicon elements is obtained.

In the present invention, the content of silicon element in the active material layer can be measured, for example, by the method described in Japanese Patent No. 5,999,399.

4. Electricity storage device The electricity storage device according to this embodiment includes the above-mentioned electricity storage device electrode, further contains an electrolyte, and can be manufactured by a conventional method using components such as a separator. A specific manufacturing method includes, for example, stacking a negative electrode and a positive electrode via a separator, and storing the stack in a battery container by rolling or folding the stack according to the battery shape, and injecting an electrolyte into the battery container and sealing it. The shape of the battery can be any appropriate shape, such as a coin type, a cylindrical type, a square type, or a laminate type.

The electrolyte may be liquid or gel-like, and may be selected from among known electrolytes used in power storage devices that effectively exhibit the functions of a battery, depending on the type of active material. The electrolyte may be a solution in which an electrolyte is dissolved in a suitable solvent. Examples of such electrolytes and solvents include the compounds described in Japanese Patent No. 5,999,399 and the like.

The above-mentioned electricity storage device can be applied to lithium ion secondary batteries, electric double layer capacitors, lithium ion capacitors, etc., which require discharge at a high current density. Among these, lithium ion secondary batteries are particularly preferred. In the electricity storage device electrode and electricity storage device according to this embodiment, the components other than the binder composition for electricity storage devices can be publicly known components for lithium ion secondary batteries, electric double layer capacitors, and lithium ion capacitors.

5. Examples The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. "Parts" and "%" in the examples and comparative examples are based on mass unless otherwise specified.

Example 1
5.1.1. Preparation and evaluation of binder composition for electric storage device (1) Preparation of binder composition for electric storage device A binder composition for electric storage device containing polymer (A) was obtained by one-stage polymerization as shown below . A monomer mixture consisting of 400 parts by mass of water, 50 parts by mass of 1,3-butadiene, 15 parts by mass of styrene, 10 parts by mass of acrylic acid, and 25 parts by mass of acrylonitrile, 0.1 parts by mass of tert-dodecyl mercaptan as a chain transfer agent, 1 part by mass of sodium alkyl diphenyl ether disulfonate as an emulsifier, and 0.2 parts by mass of potassium persulfate as a polymerization initiator was charged into a reactor, and polymerization was performed for 24 hours at 70 ° C. while stirring. The reaction was terminated at a polymerization conversion rate of 98% at this time. Unreacted monomers were removed from the particle dispersion of polymer (A) obtained in this way, and after concentration, a 10% aqueous sodium hydroxide solution and water were added to obtain a binder composition for electric storage device containing 20% by mass of polymer (A) particles and having a pH of 6.0.

(2) Measurement of water-insoluble content The polymer (A) obtained above was dried in a thermostatic chamber at 40° C. to prepare a film. 1 g of this film was immersed in 20 mL of an aqueous sodium hydroxide solution of pH 9 and shaken in an ultrasonic cleaner for 1 hour. The film was centrifuged to separate into a liquid component and a solid component. When the weight of the film before immersion was Ma and the weight of the solid component after drying and centrifugation was Mb, the value calculated by (Mb/Ma)×100 was taken as the water-insoluble content ratio. As a result, the water-insoluble content ratio of the polymer (A) was 96%.

(3) Measurement of electrolyte swelling rate (electrolyte immersion test)
1 g of the film of the polymer (A) obtained in (2) above was immersed in 20 mL of a mixed solution (PC/DEC = 1/1 (volume ratio), hereinafter, this mixed solution will be referred to as "PC/DEC") consisting of propylene carbonate (PC) and diethyl carbonate (DEC) used as an electrolyte in the production of an electricity storage device described below, and shaken at 70 ° C. for 24 hours. Next, the insoluble matter was separated by filtration through a 300 mesh wire net, and the weight (Y (g)) of the residue obtained by evaporating and removing the soluble PC/DEC was measured. In addition, the PC/DEC attached to the surface of the insoluble matter (film) separated by the above filtration was absorbed into paper and removed, and the weight (Z (g)) of the insoluble matter (film) was measured. The electrolyte swelling ratio was measured according to the following formula, and the electrolyte swelling ratio of the polymer (A) was 270 wt%.
Electrolyte swelling rate (mass%)=(Z/(1−Y))×100

(4) Measurement of surface acid amount The surface acid amount of the particles of the polymer (A) contained in the binder composition for a storage battery device obtained above was measured as follows. First, it was confirmed that 0.005 mol/L of sulfuric acid was filled in the titration burette of the potentiometric titration device (manufactured by Kyoto Electronics Manufacturing Co., Ltd., model "AT-510") and the reagent bottle at the top of the main body, and it was confirmed that the conductivity of the ultrapure water was 2 μS or less. Next, purging was performed to remove air from the burette, and further bubbles were removed from the nozzle. After that, about 1 g of the binder composition for a storage battery device obtained above was collected in a 300 mL beaker in terms of solid content, and the sample weight was recorded. Ultrapure water was added thereto to dilute it to 200 mL, and then 1 mol/L aqueous sodium hydroxide solution was dropped. When the end point was reached, it was stirred for about 30 seconds, and it was confirmed that the conductivity had settled. The RESET button of the measurement program was pressed to put it into a measurement standby state. The START button of the measurement program was pressed to start the measurement using 0.005 mol/L sulfuric acid. When the end point was reached, the process was automatically terminated and the file was saved, and the obtained curve was analyzed to determine the surface acid amount from the amount of sulfuric acid used according to the following formula: As a result, the surface acid amount of the polymer (A) was 1.2 mmol/g.
Surface acid amount (mmol/g)=Amount of acid used in the carboxylic acid region on the particle surface [mL]×Acid concentration [mol/L]×Ionization degree/Sample weight [g]/1000

(5) Measurement of number average particle size The binder composition for electric storage devices obtained above was diluted to 0.1 wt % to form a latex, which was dropped onto a collodion support film with a pipette, and then a drop of 0.02 wt % osmium tetroxide solution was dropped onto the collodion support film with a pipette, and the sample was air-dried for 12 hours to prepare a sample. The sample thus prepared was observed at a magnification of 10K (magnification) using a transmission electron microscope (TEM, Hitachi High-Technologies Corporation, model number "H-7650"), and image analysis was performed using a HITACH EMIP program. The number average particle size of 50 randomly selected polymer (A) particles was calculated to be 200 nm.

(6) pH
The pH of the binder composition for an electricity storage device obtained above was measured at 25° C. using a pH meter (manufactured by Horiba, Ltd.), and it was confirmed to be pH 6.0.

5.1.2 Preparation and Evaluation of Slurry for Electricity Storage Device Electrodes (1) Synthesis of Silicon Material (Active Material) A mixture of pulverized silicon dioxide powder (average particle size 10 μm) and carbon powder (average particle size 35 μm) was heat-treated for 10 hours under a nitrogen gas flow (0.5 NL/min) in an electric furnace adjusted to a temperature range of 1100 to 1600° C. to obtain silicon oxide powder (average particle size 8 μm) represented by the composition formula SiO _x (x = 0.5 to 1.1). 300 g of this silicon oxide powder was placed in a batch-type heating furnace, and the temperature was increased from room temperature (25° C.) to 1100° C. at a heating rate of 300° C./h while maintaining a reduced pressure of 100 Pa absolute using a vacuum pump. Next, while maintaining the pressure in the heating furnace at 2000 Pa, methane gas was introduced at a flow rate of 0.5 NL/min, and a heat treatment (graphite coating treatment) was performed at 1100°C for 5 hours. After the graphite coating treatment was completed, the mixture was cooled to room temperature at a temperature drop rate of 50°C/h, thereby obtaining about 330 g of graphite-coated silicon oxide powder. This graphite-coated silicon oxide is a conductive powder (active material) in which the surface of silicon oxide is coated with graphite, and has an average particle size of 10.5 µm. The proportion of the graphite coating was 2 mass% when the entire graphite-coated silicon oxide obtained was 100 mass%.

(2) Preparation of Slurry for Electricity Storage Device Electrode Into a two-shaft planetary mixer (manufactured by Primix Corporation, product name "TK Hivismix 2P-03"), 1 part by mass (solid content equivalent, added as an aqueous solution with a concentration of 2% by mass) of a thickener (product name "CMC2200", manufactured by Daicel Corporation), 4 parts by mass of a polymer (A) (solid content equivalent, added as the binder composition for electricity storage devices obtained above), 76 parts by mass (solid content equivalent) of artificial graphite (manufactured by Hitachi Chemical Co., Ltd., product name "MAG") which is a highly crystalline graphite as a negative electrode active material, 19 parts by mass (solid content equivalent) of the graphite-coated silicon oxide powder obtained above, and 1 part by mass of carbon (manufactured by Denka Co., Ltd., acetylene black) which is a conductivity imparting agent were added and stirred at 60 rpm for 1 hour to obtain a paste. Water was added to the obtained paste to adjust the solid content concentration to 48 mass %, and then the mixture was stirred and mixed using a stirring degassing machine (manufactured by Thinky Corporation, product name "Awatori Rentaro") at 200 rpm for 2 minutes, at 1800 rpm for 5 minutes, and further at 1800 rpm for 1.5 minutes under reduced pressure (approximately 2.5 × 10 ⁴ Pa) to prepare a slurry for an electricity storage device electrode (C/Si (20%)) containing 20 mass % Si in the negative electrode active material.

5.1.3. Production and Evaluation of Electricity Storage Device (1) Production of Electricity Storage Device Electrode (Negative Electrode) The slurry for the electricity storage device electrode (C/Si (20%)) obtained above was uniformly applied by a doctor blade method to a surface of a current collector made of copper foil with a thickness of 20 μm so that the film thickness after drying would be 80 μm, and the slurry was dried at 60° C. for 10 minutes, and then dried at 120° C. for 10 minutes. Thereafter, the slurry was pressed with a roll press machine so that the density of the active material layer became 1.5 g/cm ³ , thereby obtaining an electricity storage device electrode (negative electrode).

(2) Evaluation of Adhesion Strength of Negative Electrode Coating Layer On the surface of the obtained electricity storage device electrode, 10 incisions were made vertically and horizontally at 2 mm intervals from the active material layer to a depth reaching the current collector, creating a grid pattern. An adhesive tape with a width of 18 mm (manufactured by Nichiban Co., Ltd., product name "Cellotape" (registered trademark), as specified in JIS Z1522) was attached to the incisions and immediately peeled off, and the degree of detachment of the active material was evaluated by visual inspection. The evaluation criteria were as follows. The evaluation results are shown in Table 1.
(Evaluation criteria)
5 points: No active material layer fell off.
4 points: 1 to 5 pieces of the active material layer fell off.
3 points: 6 to 20 pieces of the active material layer fell off.
2 points: 21 to 40 pieces of the active material layer fell off.
Score 1: 41 or more pieces of the active material layer fell off.

(3) Production of Counter Electrode (Positive Electrode) 4 parts by mass (solid content equivalent) of electrochemical device electrode binder (Kureha Corporation, product name "KF Polymer #1120"), 3.0 parts by mass of conductive assistant (Denka Company, product name "Denka Black 50% Pressed Product"), 100 parts by mass (solid content equivalent) of _LiCoO2 (Hayashi Chemical Industry Co., Ltd.) having an average particle size of 5 μm as a positive electrode active material, and 36 parts by mass of N-methylpyrrolidone (NMP) were added to a biaxial planetary mixer (Primix Corporation, product name "TK Hivismix 2P-03") and stirred at 60 rpm for 2 hours. NMP was added to the obtained paste, and the solid content concentration was adjusted to 65% by mass. Then, using a stirring and defoaming machine (manufactured by Thinky Corporation, trade name "Awatori Rentaro"), the mixture was stirred and mixed at 200 rpm for 2 minutes, 1,800 rpm for 5 minutes, and further at 1,800 rpm for 1.5 minutes under reduced pressure (about 2.5 × 10 ⁴ Pa), to prepare a positive electrode slurry. The positive electrode slurry was uniformly applied to the surface of a current collector made of aluminum foil by the doctor blade method so that the film thickness after solvent removal was 80 μm, and the solvent was removed by heating at 120 ° C. for 20 minutes. Thereafter, the active material layer was pressed with a roll press machine so that the density of the active material layer was 3.0 g / cm ³ , thereby obtaining a counter electrode (positive electrode).

(4) Assembly of a Lithium-Ion Battery Cell In a glove box substituted with Ar so that the dew point is −80° C. or less, the negative electrode produced above was punched out to a diameter of 15.95 mm and placed on a two-electrode coin cell (manufactured by Hosen Co., Ltd., product name “HS Flat Cell”). Next, a separator made of a polypropylene porous film punched out to a diameter of 24 mm (manufactured by Celgard Co., Ltd., product name “Celgard #2400”) was placed on the cell, and 500 μL of electrolyte was poured so as to prevent air from entering. The positive electrode produced above was punched out to a diameter of 16.16 mm and placed on the cell, and the exterior body of the two-electrode coin cell was sealed by tightening the screws to assemble a lithium-ion battery cell (electricity storage device). The electrolyte used here was a solution in which LiPF ₆ was dissolved at a concentration of 1 mol/L in a solvent of ethylene carbonate/ethyl methyl carbonate = 1/1 (mass ratio).

(5) Evaluation of charge/discharge cycle characteristics The storage device manufactured above was charged at a constant current (1.0 C) in a thermostatic chamber adjusted to 25° C., and when the voltage reached 4.2 V, charging was continued at a constant voltage (4.2 V), and charging was completed (cut-off) when the current value reached 0.01 C. Thereafter, discharging was started at a constant current (1.0 C), and discharging was completed (cut-off) when the voltage reached 3.0 V, and the discharge capacity of the first cycle was calculated. In this manner, charging and discharging were repeated 100 times. The capacity retention rate was calculated using the following formula and evaluated according to the following criteria. The evaluation results are shown in Table 1.
Capacity retention rate (%)=(discharge capacity at 100th cycle)/(discharge capacity at 1st cycle)
(Evaluation criteria)
・5 points: Capacity retention is 95% or more.
4 points: Capacity retention is between 90% and 95%.
3 points: Capacity retention is between 85% and 90%.
・2 points: Capacity retention is between 80% and 85%.
・1 point: Capacity retention is between 75% and 80%.
0 points: Capacity retention is less than 75%.

In the measurement conditions, "1C" refers to the current value at which a cell with a certain capacitance is discharged at a constant current and the discharge is completed in 1 hour. For example, "0.1C" refers to the current value at which the discharge is completed in 10 hours, and "10C" refers to the current value at which the discharge is completed in 0.1 hours.

5.2. Examples 2 to 20 and Comparative Examples 1 to 8
A binder composition for an electricity storage device containing 20 mass% of polymer particles was obtained in the same manner as in the above "5.1.1. Preparation and evaluation of binder composition for an electricity storage device (1) Preparation of binder composition for an electricity storage device", except that the type and amount of each monomer were as shown in Table 1 or Table 2 below, respectively.

Furthermore, slurries for the electricity storage device electrodes were prepared in the same manner as in Example 1 above, except that the binder composition for the electricity storage device prepared above was used, and electricity storage device electrodes and electricity storage devices were fabricated and evaluated in the same manner as in Example 1 above.

5.3 Example 21
In Example 20, except that 0.9 parts by mass of CMC (product name "CMC2200", manufactured by Daicel Corporation) and 0.1 parts by mass of CNF (product name "Celish KY-100G", manufactured by Daicel Corporation, fiber diameter 0.07 μm) were used as the thickener, a slurry for an electricity storage device electrode was prepared in the same manner as in Example 20, and an electricity storage device electrode and an electricity storage device were each produced and evaluated in the same manner as in Example 1 above. The results are shown in Table 3.

5.4 Example 22
In Example 20, except that 0.8 parts by mass of CMC (product name "CMC2200", manufactured by Daicel Corporation) and 0.2 parts by mass of CNF (manufactured by Daicel Corporation, product name "Celish KY-100G", fiber diameter 0.07 μm) were used as the thickener, a slurry for an electricity storage device electrode was prepared in the same manner as in Example 20, and an electricity storage device electrode and an electricity storage device were each produced and evaluated in the same manner as in Example 1 above. The results are shown in Table 3.

5.5. Evaluation Results Tables 1 and 2 below show the polymer compositions, physical properties, and evaluation results used in Examples 1 to 20 and Comparative Examples 1 to 8. Table 3 below shows the evaluation results of Examples 21 and 22.

The abbreviations of the monomers in Tables 1 and 2 above represent the following compounds.
<Conjugated diene compound>
BD: 1,3-butadiene <unsaturated carboxylic acid>
TA: Itaconic acid AA: Acrylic acid MAA: Methacrylic acid <α,β-unsaturated nitrile compound>
・AN: Acrylonitrile <(meth)acrylamide>
・AAM: Acrylamide ・MAM: Methacrylamide <Unsaturated carboxylic acid ester with a hydroxyl group>
・HEMA: 2-hydroxyethyl methacrylate ・HEA: 2-hydroxyethyl acrylate <unsaturated carboxylic acid ester>
MMA: methyl methacrylate BA: butyl acrylate 2EHA: 2-ethylhexyl acrylate CHMA: cyclohexyl methacrylate EDMA: ethylene glycol dimethacrylate <aromatic vinyl compound>
ST: styrene DVB: divinylbenzene <compounds having sulfonic acid groups>
・NASS: Sodium styrene sulfonate

As is clear from Tables 1 and 2, it was found that the slurry for the electrode of the electric storage device prepared using the binder composition for the electric storage device according to the present invention shown in Examples 1 to 20 can favorably bind active materials that undergo large volume changes due to charging and discharging, and can maintain good adhesion between the active material layer and the current collector, compared to Comparative Examples 1 to 8. As a result, despite the active material repeatedly expanding and contracting in volume due to repeated charging and discharging, peeling of the active material layer was suppressed, and an electric storage device electrode having good charge and discharge durability characteristics was obtained. The reason for this is presumably that the electric storage device electrodes according to Examples 1 to 20 shown in Tables 1 and 2 can maintain the conductive network inside the active material layer by reducing the change in film thickness of the active material layer due to charging and discharging, compared to Comparative Examples 1 to 8.

Furthermore, as is clear from the results of Table 3 above, it was found that the slurry for an electrode of a storage battery device prepared using the binder composition for a storage battery device according to the present invention shown in Examples 21 and 22 was able to effectively bind active materials that undergo large volume changes during charging and discharging, even when CNF was used in combination as a thickener, and was also able to maintain good adhesion between the active material layer and the current collector.

The present invention is not limited to the above-described embodiments, and various modifications are possible. The present invention includes configurations that are substantially the same as those described in the embodiments (for example, configurations with the same functions, methods, and results, or configurations with the same purpose and effect). The present invention also includes configurations in which non-essential parts of the configurations described in the above embodiments are replaced with other configurations. Furthermore, the present invention also includes configurations that achieve the same effects as the configurations described in the above embodiments, or that can achieve the same purpose. Furthermore, the present invention also includes configurations in which publicly known technology is added to the configurations described in the above embodiments.

Claims (10)

Contains a polymer (A) and a liquid medium (B),
When the total amount of repeating units contained in the polymer (A) is 100 parts by mass, the polymer (A) has
30 to 65 parts by mass of a repeating unit (a1) derived from a conjugated diene compound;
5 to 40 parts by mass of a repeating unit (a2) derived from an unsaturated carboxylic acid;
and 15 to 40 parts by mass of a repeating unit (a3) derived from an α,β-unsaturated nitrile compound, and the binder composition for an electrical storage device satisfies the following conditions:
<Conditions>
1.0 g of a film obtained by drying the polymer (A) in a thermostatic bath at 40° C. is immersed in 20 mL of an aqueous sodium hydroxide solution having a pH of 9, and shaken for 1 hour in an ultrasonic cleaner. In this case, the value of (Mb/Ma)×100 is 90% or more, where Ma is the weight of the film before immersion and Mb is the weight of the dried film after immersion. The binder composition for an electric storage device according to claim 1, wherein the swelling ratio of the polymer (A) when immersed in a solvent consisting of propylene carbonate and diethyl carbonate in a volume fraction of 1:1 at 70°C for 24 hours is 150% or more and 350% or less. The polymer (A) is a polymer particle,
The binder composition for an electrical storage device according to claim 1 or 2, wherein the polymer particles have a surface acid amount of 0.5 mmol/g or more and 3 mmol/g or less. The binder composition for an electrical storage device according to claim 3, wherein the number average particle diameter of the polymer particles is 50 nm or more and 500 nm or less. The binder composition for an electrical storage device according to any one of claims 1 to 4, wherein the liquid medium (B) is water. The binder composition for an electrical storage device according to any one of claims 1 to 5,
A slurry for an electrode of an electricity storage device, comprising: The slurry for an electric storage device electrode according to claim 6, which contains a silicon material as the active material. The slurry for an electric storage device electrode according to claim 6 or claim 7 further contains cellulose fiber. An electric storage device electrode comprising a current collector and an active material layer formed by applying and drying the slurry for an electric storage device electrode according to any one of claims 6 to 8 to the surface of the current collector. An electricity storage device comprising the electricity storage device electrode according to claim 9.