Binder composition for energy storage device, slurry for energy storage device electrode, energy storage device electrode, and energy storage device

Abstract

Provided is a binder composition for an energy storage device that makes it possible to achieve quick charging in a short time and maintain capacity at a high level even if charging and discharging are repeated, by achieving both adhesion in a dry state and adhesion in an electrolyte-immersed state while reducing the internal resistance of a battery.　A composition for an energy storage device according to the present invention contains a polymer (A) and a liquid medium (B). With the total amount of repeating units contained in the polymer (A) being taken as 100 mass%, the polymer (A) contains 0.1 to 30 mass% of a repeating unit (a1) derived from a conjugated diene compound, 5 to 75 mass% of a repeating unit (a2) derived from an aromatic vinyl compound, and 20 to 80 mass% of a repeating unit (a3) derived from an ethylenically unsaturated carboxylic acid ester. The total amount of the repeating unit (a2) and the repeating unit (a3) is 60 mass% or more, and the composition satisfies the following condition. <Condition> The relationship represented by formula (1) is satisfied, where Shear Modulus 1 is the shear storage modulus of a film before immersion and Shear Modulus 2 is the shear storage modulus of a film after immersion, as measured with a nanoindenter.　(1): Log10\{(Shear Modulus 1) / (Shear Modulus 2)\} < 2.5

Description

Binder composition for energy storage devices, slurry for energy storage device electrodes, energy storage device electrodes and energy storage devices

[0001] The present invention relates to a binder composition for energy storage devices, a slurry for electrodes of energy storage devices, electrodes for energy storage devices, and energy storage devices.

[0002] In recent years, lithium-ion batteries and lithium-ion capacitors, which possess high voltage and high energy density, have been attracting attention as power sources for electronic devices. In particular, development trends for lithium-ion batteries for automotive applications show a growing need for rapid charging performance in a short time and extended driving range per charge, requiring the development of new materials to achieve higher battery performance.

[0003] Electrodes used in such energy storage devices are manufactured by applying and drying a composition (slurry for energy storage device electrodes) containing an active material and a polymer that functions as a binder onto the surface of a current collector. The polymer used as a binder is required to have improved adhesion. The required adhesion in the electrode manufacturing process includes the ability to bond between active materials and the adhesion between the active material and the current collector, resistance to powder shedding when the coated and dried composition film (hereinafter also referred to as the "active material layer") is cut, and flexibility to prevent cracking due to insufficient flexibility when the electrode is wound and bent on a roll. Furthermore, the required adhesion for improving battery performance includes the ability to bond between active materials and the adhesion between the active material and the current collector when immersed in the electrolyte inside the battery, and the ability to maintain the electrode structure even after repeated charging and discharging. The binder material exhibits excellent adhesion in the electrode fabrication process and improves battery performance, enabling the manufacture of defect-free electrode plates and, consequently, the realization of energy storage devices with minimal degradation due to repeated charging and discharging.

[0004] Furthermore, empirically, it has become clear that the bonding ability between the active materials, the adhesion ability between the active materials and the current collector, the resistance to powder shedding, and the flexibility are roughly proportional to the quality of the performance. Therefore, in this specification, these may be collectively referred to as "adhesion in a dry state."

[0005] Furthermore, empirically, it has become clear that the performance of the active materials in the state of being immersed in the electrolyte, including their bonding ability, the adhesion ability between the active materials and the current collector, and the ability to maintain the electrode structure even after repeated charging and discharging, is roughly proportional to the quality of the performance. Therefore, in this specification, these may be collectively referred to as "adhesion in the state of electrolyte immersion."

[0006] As mentioned above, the development of lithium-ion batteries for automotive applications that enable rapid charging in a short time is being considered. In such batteries, rapid charging in a short time causes a large current to flow, resulting in a large voltage increase. If conventional electrode binders are used in such batteries, there are concerns that the separator may be damaged due to the deposition of lithium metal, causing a short circuit, and that the repeated charge-discharge performance will be significantly reduced due to the degradation of the active material due to the voltage increase. To achieve rapid charging in a short time, techniques have been proposed to reduce the internal resistance of the battery caused by the binder material (see, for example, Patent Documents 1 and 2).

[0007] International Publication No. 2013 / 191080, International Publication No. 2016 / 039067

[0008] However, electrode binders such as those disclosed in the above-mentioned Patent Documents 1 and 2 cannot achieve both adhesion in a dry state and adhesion in an electrolyte immersion state while reducing the internal resistance of the battery. This leads to problems such as powder shedding and cracking of the electrode composite layer, a decrease in electrode manufacturing yield, and the formation of defects in the composite layer due to repeated charging and discharging, causing a decrease in capacity.

[0009] Some aspects of the present invention provide a binder composition for energy storage devices that reduces the internal resistance of the battery while achieving both adhesion in a dry state and adhesion in an electrolyte immersion state, thereby enabling rapid charging in a short time and maintaining a high level of capacity even after repeated charging and discharging.

[0010] The present invention has been made to solve at least some of the above-mentioned problems and can be realized in any of the following embodiments.

[0011] One embodiment of the binder composition for energy storage devices according to the present invention comprises a polymer (A) and a liquid medium (B), wherein when the total amount of repeating units contained in the polymer (A) is 100% by mass, the polymer (A) contains: 0.1 to 30% by mass of repeating units (a1) derived from a conjugated diene compound, 5 to 75% by mass of repeating units (a2) derived from an aromatic vinyl compound, and 20 to 80% by mass of repeating units (a3) ​​derived from an ethylenically unsaturated carboxylic acid ester, wherein the total amount of repeating units (a2) and repeating units (a3) ​​is 60% by mass or more, and satisfies the following conditions. <Conditions> The polymer (A) is placed in a petri dish with a diameter of 7.5 cm in an amount equivalent to 4.0 g of solids, dried at 25°C for 7 days, and then dried further in a vacuum dryer at 100°C for 30 minutes, and the obtained film is cut to a width of 5 mm x 5 mm. The obtained film is immersed in a solvent consisting of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume fraction of 1:3:1 at 70°C for 24 hours. When the shear storage modulus of the film before immersion, measured by a nanoindenter, is denoted as Shear Modulus 1, and the shear storage modulus of the film after immersion is denoted as Shear Modulus 2, the following relationship (1) is satisfied. Log １０ {(Shear Modulus 1) / (Shear Modulus 2)}<2.5 (1)

[0012] In one embodiment of the binder composition for the energy storage device, the polymer (A) may further contain 0.1 to 10% by mass of repeating units (a4) derived from an unsaturated carboxylic acid.

[0013] In any embodiment of the binder composition for the energy storage device, the polymer (A) may be polymer particles, and the number-average particle diameter of the polymer particles may be 50 nm or more and 500 nm or less.

[0014] In any embodiment of the binder composition for the energy storage device, the liquid medium (B) may be water.

[0015] One embodiment of the slurry for energy storage device electrodes according to the present invention contains the binder composition for energy storage devices according to any of the above embodiments and an active material.

[0016] In one embodiment of the slurry for the electrodes of the energy storage device, the active material may contain graphite and silicon material.

[0017] In any embodiment of the slurry for the electrodes of the energy storage device, a thickening agent may be further contained.

[0018] One embodiment of the energy storage device electrode according to the present invention comprises a current collector and an active material layer formed by applying and drying a slurry for energy storage device electrodes according to any of the above embodiments on the surface of the current collector.

[0019] One embodiment of the energy storage device according to the present invention comprises an electrode of the energy storage device according to the above embodiment.

[0020] The binder composition for energy storage devices according to the present invention can reduce the internal resistance of the battery, and because it has sufficient flexibility in a dry electrode, it has excellent adhesion in the dry state. Furthermore, because it does not become too soft in an electrolyte-immersed electrode, it also has excellent adhesion in the electrolyte-immersed state. As a result, a energy storage device can be obtained that enables rapid charging in a short time and can maintain a high level of capacity even after repeated charging and discharging.

[0021] Preferred embodiments of the present invention will be described in detail below. It should be understood that the present invention is not limited to the embodiments described below, but also includes various modifications that do not alter the essence of the invention.

[0022] In this specification, "(meth)acrylic acid" means "acrylic acid" or "methacrylic acid," "(meth)acrylate" means "acrylate" or "methacrylate," and "(meth)acrylamide" means "acrylamide" or "methacrylamide."

[0023] In this specification, numerical ranges described as "X to Y" are interpreted as including the numerical value X as the lower limit and the numerical value Y as the upper limit.

[0024] 1. Binder composition for energy storage devices A binder composition for energy storage devices according to one embodiment of the present invention contains a polymer (A) and a liquid medium (B). The polymer (A) contains, when the total amount of repeating units contained in the polymer (A) is 100% by mass, 0.1 to 30% by mass of repeating units (a1) derived from a conjugated diene compound, 5 to 75% by mass of repeating units (a2) derived from an aromatic vinyl compound, and 20 to 80% by mass of repeating units (a3) ​​derived from an ethylenically unsaturated carboxylic acid ester, and the total amount of the repeating units (a2) and the repeating units (a3) ​​is 60% by mass or more.

[0025] Furthermore, polymer (A) satisfies the following conditions. Specifically, polymer (A) is placed in a 7.5 cm diameter petri dish to a solid content of 4.0 g, dried at 25°C for 7 days, and then further dried in a vacuum dryer at 100°C for 30 minutes. The resulting film is then cut into 5 mm x 5 mm strips. The resulting film is immersed in a solvent consisting of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume fraction of 1:3:1 at 70°C for 24 hours. When the shear storage modulus of the film before immersion, measured by a nanoindenter, is denoted as Shear Modulus 1, and the shear storage modulus of the film after immersion is denoted as Shear Modulus 2, the following relationship (1) is satisfied. Log １０ {(Shear Modulus 1) / (Shear Modulus 2)}<2.5 (1)

[0026] The binder composition for energy storage devices according to this embodiment can be used as a material for producing an electrode (active material layer) for an energy storage device that achieves both adhesion in a dry state and adhesion in an electrolyte immersion state, or it can be used as a material for forming a protective film to suppress short circuits caused by dendrites generated during charging and discharging. The components contained in the binder composition for energy storage devices according to this embodiment will be described in detail below.

[0027] 1.1. Polymer (A) The binder composition for the energy storage device according to this embodiment contains polymer (A). It is preferable that polymer (A) is dispersed in particulate form in a liquid medium (B) described later. When polymer (A) is dispersed in particulate form in the liquid medium (B), the stability of the composition (hereinafter also referred to as "slurry") made by mixing it with the active material is improved, and the applicability of the slurry to the current collector is also improved, which is preferable. In addition, the presence of particulate binder makes it easier for the active material to slide during pressing and to increase density. Furthermore, it is possible to suppress excessive coating of the conductive additive and to form a good conductive path.

[0028] The following will explain the repeating units that make up polymer (A), the physical properties of polymer (A), and the manufacturing method, in that order.

[0029] 1.1.1. When the total of the repeating units constituting the polymer (A) is 100% by mass in the polymer (A), the polymer (A) contains 0.1 to 30% by mass of a repeating unit (a1) derived from a conjugated diene compound (also referred to as "repeating unit (a1)" in this specification), 5 to 75% by mass of a repeating unit (a2) derived from an aromatic vinyl compound (also referred to as "repeating unit (a2)" in this specification), and 30 to 70% by mass of a repeating unit (a3) derived from an ethylenically unsaturated carboxylic acid ester (also referred to as "repeating unit (a3)" in this specification). Further, the total amount of the repeating unit (a2) and the repeating unit (a3) is 60% by mass or more. In addition to the repeating unit (a1), the repeating unit (a2), and the repeating unit (a3), the polymer (A) may contain a repeating unit derived from another monomer copolymerizable with these.

[0030] 1.1.1.1. Repeating unit (a1) derived from a conjugated diene compound The content ratio of the repeating unit (a1) derived from a conjugated diene compound is 0.1 to 30% by mass when the total of the repeating units contained in the polymer (A) is 100% by mass. The content ratio of the repeating unit (a1) is preferably 0.5% by mass or more, more preferably 1% by mass or more, still more preferably 2% by mass or more, and particularly preferably 4% by mass or more. The content ratio of the repeating unit (a1) is preferably 29% by mass or less, more preferably 28% by mass or less, still more preferably 25% by mass or less, and particularly preferably 20% by mass or less. The content ratio of the repeating unit (a1) is preferably 0.5 to 29% by mass, more preferably 1 to 28% by mass, still more preferably 2 to 25% by mass, and particularly preferably 4 to 20% by mass. By containing the repeating unit (a1) within the above range in the polymer (A), the polymer (A) can be imparted with elasticity. Since the adhesion is improved by the stretching of the polymer (A), the flexibility of the electrode can be improved. Also, since the electrolyte component is less likely to be incorporated into the voids of the polymer chain, the polymer (A) does not become too soft in the electrolyte, and the adhesion can be improved.

[0031] The conjugated diene compound 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, 1,3-pentadiene, 1,3-hexadiene, 1,3-heptadiene, etc. One or more selected from these can be used. Among these, 1,3-butadiene is particularly preferred.

[0032] 1.1.1.2. Repeating unit (a2) derived from an aromatic vinyl compound The content ratio of the repeating unit (a2) derived from the aromatic vinyl compound is 5 to 75% by mass when the total of the repeating units contained in the polymer (A) is 100% by mass. The content ratio of the repeating unit (a2) is preferably 15% by mass or more, more preferably 25% by mass or more, still more preferably 30% by mass or more, and particularly preferably 36% by mass or more. The content ratio of the repeating unit (a2) is preferably 70% by mass or less, more preferably 60% by mass or less, still more preferably 55% by mass or less, and particularly preferably 52% by mass or less. The content ratio of the repeating unit (a2) is preferably 15 to 70% by mass, more preferably 25 to 60% by mass, still more preferably 30 to 55% by mass, and particularly preferably 36 to 52% by mass. When the polymer (A) contains the repeating unit (a2) within the above range, the fusion of the polymers (A) dispersed in the active material layer is suppressed, and good slurry characteristics are exhibited, and the coating property can be improved. In addition, since the permeability of the electrolytic solution can be improved, good repeated charge and discharge characteristics may be exhibited.

[0033] The aromatic vinyl compound is not particularly limited, and examples thereof include styrene, α-methylstyrene, p-methylstyrene, chlorostyrene, divinylbenzene, etc. One or more selected from these can be used. Among these, styrene is particularly preferred.

[0034] 1.1.1.3. Repeating units (a3) ​​derived from ethylenically unsaturated carboxylic acid esters The content of repeating units (a3) ​​derived from ethylenically unsaturated carboxylic acid esters is 20 to 80% by mass when the total amount of repeating units contained in polymer (A) is 100% by mass. The content of repeating units (a3) ​​is preferably 25% by mass or more, more preferably 30% by mass or more, even more preferably 35% by mass, and particularly preferably 40% by mass or more. The content of repeating units (a3) ​​is preferably 75% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, and particularly preferably 50% by mass or less. The content of repeating units (a3) ​​is preferably 25 to 75% by mass, more preferably 30 to 70% by mass, even more preferably 35 to 60% by mass, and particularly preferably 40 to 50% by mass. By containing repeating units (a3) ​​within the aforementioned range in polymer (A), the affinity between polymer (A) and the electrolyte is improved, thereby suppressing the increase in internal resistance caused by polymer (A) becoming an electrical resistance component in the energy storage device. Furthermore, during the battery charging process, the desolvation of lithium ions is promoted on the surface of polymer (A), reducing the energy required for the insertion of lithium ions into the negative electrode active material, thereby suppressing the increase in internal resistance.

[0035] Among ethylenically 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, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, n-amyl (meth)acrylate, isoamyl (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, and tri(meth)acrylic acid. Examples include trimethylolpropane tetraacrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, allyl (meth)acrylate, 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, and glycerin di(meth)acrylate, and one or more selected from these can be used. Among these, it is preferable that one or more are selected from methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, ethylene glycol di(meth)acrylate, and 2-hydroxyethyl (meth)acrylate, more preferably one or more are selected from 2-ethylhexyl acrylate, n-butyl acrylate, and ethyl acrylate, and 2-ethylhexyl acrylate is particularly preferred.

[0036] When the total amount of repeating units contained in polymer (A) is taken as 100% by mass, the total amount of repeating units (a2) and (a3) ​​is 60% by mass or more, preferably 65% ​​by mass or more, more preferably 70% by mass or more, and particularly preferably 75% by mass or more. When the total amount of repeating units (a2) and (a3) ​​is within the above range, the dispersibility of the binder material in the electrode is improved, which reduces battery resistance and suppresses the phenomenon of lithium metal deposition on the surface of the negative electrode active material when the battery is rapidly charged.

[0037] 1.1.1.4. Other repeating unit polymers (A) may contain repeating units derived from other monomers copolymerizable with repeating units (a1), (a2), and (a3). Examples of such repeating units include repeating units derived from unsaturated carboxylic acids (a4) (hereinafter also referred to as "repeating unit (a4)"), repeating units derived from α,β-unsaturated nitrile compounds (a5) (hereinafter also referred to as "repeating unit (a5)"), repeating units derived from (meth)acrylamide (a6) (hereinafter also referred to as "repeating unit (a6)"), and repeating units derived from compounds having sulfonic acid groups (a7) (hereinafter also referred to as "repeating unit (a7)").

[0038] <Repeating units (a4) derived from unsaturated carboxylic acids> Polymer (A) may contain repeating units (a4) derived from unsaturated carboxylic acids. The content of repeating units (a4) derived from unsaturated carboxylic acids is preferably 0.1 to 10% by mass when the total amount of repeating units contained in polymer (A) is 100% by mass. The content of repeating units (a4) is more preferably 0.5% by mass or more, and particularly preferably 1% by mass or more. The content of repeating units (a4) is more preferably 9% by mass or less, and particularly preferably 8% by mass or less. The content of repeating units (a4) is more preferably 0.5 to 9% by mass, and particularly preferably 1 to 8% by mass. By polymer (A) containing repeating units (a4) within the above ranges, the dispersibility of polymer (A) particles in water and slurry is improved, which can eliminate structural defects in the electrode plate and potentially result in good repeated charge-discharge performance.

[0039] Examples of unsaturated carboxylic acids include monocarboxylic acids and dicarboxylic acids (including anhydrides) such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid, and one or more selected from these can be used. Among these, it is preferable to use one or more selected from acrylic acid, methacrylic acid, and itaconic acid.

[0040] <Repeating units (a5) derived from α,β-unsaturated nitrile compounds> The content of repeating units (a5) derived from α,β-unsaturated nitrile compounds is preferably 0.1 to 10% by mass when the total amount of repeating units contained in polymer (A) is 100% by mass. The content of repeating units (a5) is more preferably 0.5% by mass or more, and particularly preferably 1% by mass or more. The content of repeating units (a5) is more preferably 8% by mass or less, and particularly preferably 5% by mass or less. The content of repeating units (a5) is more preferably 0.5 to 8% by mass, and particularly preferably 1 to 5% by mass. When polymer (A) contains repeating units (a5) within the above ranges, the affinity between polymer (A) and the liquid medium (B) becomes good, and polymer (A) may be able to maintain its particle shape more easily. In addition, the affinity between polymer (A) and the electrolyte becomes good, and the increase in internal resistance due to polymer (A) becoming an electrical resistance component in the energy storage device may be suppressed.

[0041] Examples of α,β-unsaturated nitrile compounds include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile, and vinylidene cyanide, and one or more selected from these can be used. Among these, one or more selected from acrylonitrile and methacrylonitrile are preferred, and acrylonitrile is particularly preferred.

[0042] <Repeating units (a6) derived from (meth)acrylamide> Polymer (A) may contain repeating units (a6) derived from (meth)acrylamide. The content of repeating units (a6) derived from (meth)acrylamide is preferably 0.1 to 10% by mass when the total amount of repeating units contained in polymer (A) is 100% by mass. The content of repeating units (a6) is more preferably 0.5% by mass or more, and particularly preferably 1% by mass or more. The content of repeating units (a6) is more preferably 9% by mass or less, and particularly preferably 8% by mass or less. The content of repeating units (a6) is more preferably 0.5 to 9% by mass, and particularly preferably 1 to 8% by mass. When polymer (A) contains repeating units (a6) within the above ranges, the dispersibility of the active material and filler in the slurry may be improved. In addition, the flexibility of the resulting active material layer may be appropriate, and the adhesion between the current collector and the active material layer may be improved.

[0043] Examples of (meth)acrylamides include acrylamide, methacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide, N,N-diethylacrylamide, N,N-diethylmethacrylamide, N,N-dimethylaminopropylacrylamide, N,N-dimethylaminopropylmethacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, diacetoneacrylamide, maleic acid amide, etc., and one or more selected from these can be used. Among these, acrylamide and methacrylamide are particularly preferred.

[0044] <Repeating units (a7) derived from compounds having sulfonic acid groups> Polymer (A) may contain repeating units (a7) derived from compounds having sulfonic acid groups. The content of repeating units (a7) derived from compounds having sulfonic acid groups is preferably 0.1 to 10% by mass when the total amount of repeating units contained in polymer (A) is 100% by mass. The content of repeating units (a7) is more preferably 0.5% by mass or more, and particularly preferably 1% by mass or more. The content of repeating units (a7) is more preferably 9% by mass or less, and particularly preferably 8% by mass or less. The content of repeating units (a7) is more preferably 0.5 to 9% by mass, and particularly preferably 1 to 8% by mass. By polymer (A) containing repeating units (a7) within the above ranges, the dispersibility of the active material and filler is improved, and a homogeneous active material layer or protective film may be obtained. Furthermore, good adhesion between the current collector and the active material layer can eliminate structural defects in the electrode plate, sometimes resulting in excellent charge-discharge characteristics.

[0045] Examples of compounds having a sulfonic acid group include vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, sulfoethyl (meth)acrylate, sulfopropyl (meth)acrylate, sulfobutyl (meth)acrylate, 2-acrylamido-2-methylpropanesulfonic acid, acrylamide tert-butylsulfonic acid, 2-hydroxy-3-acrylamidopropanesulfonic acid, 3-alyloxy-2-hydroxypropanesulfonic acid, and alkali salts thereof. One or more selected from these can be used. Among these, styrene sulfonic acid is particularly preferred.

[0046] 1.1.2. Physical Properties of Polymer (A) 1.1.2.1. Number Average Particle Size The polymer (A) is preferably dispersed in particulate form in a liquid medium (B) described later. When the polymer (A) is in the form of particles, the number average particle size of the particles is preferably 50 nm to 1,000 nm, more preferably 70 nm to 950 nm, and particularly preferably 90 nm to 900 nm. When the number average particle size of the polymer (A) particles is within the above range, the polymer (A) particles are more easily adsorbed onto the surface of the active material, so that the polymer (A) particles can move along with the movement of the active material. As a result, migration can be suppressed, and in some cases the deterioration of electrical properties can be reduced.

[0047] The number-average particle size of polymer (A) is the average value of particle sizes measured using a concentrated particle size analyzer employing dynamic light scattering (DLS). Examples of concentrated particle size analyzers include the "FPAR-1000" manufactured by Otsuka Electronics Co., Ltd.

[0048] 1.1.2.2. Shear Storage Modulus Measured with a Nanoindenter A film of polymer (A) prepared by the following method satisfies the following conditions. That is, when the shear storage modulus of the film before immersion in the electrolyte, measured with a nanoindenter, is denoted as "Shear Modulus 1," and the shear storage modulus of the film after immersion in the electrolyte is denoted as "Shear Modulus 2," the following relationship (1) is satisfied. Log １０ {(Shear Modulus 1) / (Shear Modulus 2)}<2.5 (1)

[0049] The measurement samples used in the shear storage modulus measurement using this nanoindenter are a film of polymer (A) and a film obtained by immersing the polymer (A) in an electrolyte solution and swelling it. The polymer (A) film is prepared by placing a binder composition for energy storage devices containing polymer (A) into a Petri dish with a diameter of 7.5 cm, adding polymer (A) in an amount equivalent to 4.0 g of solids, and drying it at 25°C for 7 days to produce a uniform film with a thickness of 1.0 ± 0.3 mm. This film is dried in a vacuum dryer at 100°C for 30 minutes and then cut into strips of 5 mm × 5 mm. On the other hand, the film obtained by immersing the polymer (A) film in an electrolyte solution and swelling it is prepared by immersing the polymer (A) film in a solvent consisting of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume fraction of 1:3:1 (hereinafter also referred to as the "specific solvent") at 70°C for 24 hours.

[0050] These measurement samples are fixed to a cylindrical SUS stage measuring φ310 mm and 240 mm in height. The top of the stage has a cylindrical hole in the center measuring φ210 mm and 60 mm in depth, which can hold a specific solvent. The size of the stage and the hole are not limited as long as they can be introduced into the apparatus. After immersion in the specific solvent, the film is fixed inside this hole, and the shear storage modulus is measured while it is immersed in the electrolyte. The amount of the specific solvent is not particularly limited as long as the film is completely immersed during the measurement. The method of fixing is not particularly limited, but examples include chemical-resistant double-sided tape and SUS clips. Next, the shear storage modulus of the film is measured at 25°C using the following nanoindenter measuring device: • Nanoindenter device: iNano, manufactured by Toyo Technica Co., Ltd. • Desktop active vibration isolation table mini: manufactured by Kurashiki Kako Co., Ltd.

[0051] A flat punch indenter is used for nanoindenter measurements. The diameter of the flat punch indenter is φ100 μm. The conditions for pressing the flat punch indenter into the material are an indentation depth of 10 μm and an indentation speed of 0.5 μm / s. The measurement conditions are a vibration load of 20 μN and a vibration amplitude of 10 Hz. The shear modulus is calculated from the following formula (2). Poisson's ratio is assumed to be 0.5. E' = {(1 - ν ２) / d}・{F / z}・cosδ (2) In equation (2), each symbol represents the following parameters: E': shear storage modulus F: excitation frequency during measurement z: vibration amplitude during measurement δ: phase shift d: contact area between the indenter and the material ν: Poisson's ratio

[0052] The “Log” of the film of polymer (A) used in this embodiment １０ The value of {(Shear Modulus 1) / (Shear Modulus 2)}" is less than 2.5, preferably 2.4 or less, and more preferably 2.0 or less. The “Log” of the film of polymer (A) １０ When the value of {(Shear Modulus 1) / (Shear Modulus 2)} is within the above range, it indicates that there is little change in the shear storage modulus between the dry film and the film immersed in the electrolyte, and that the electrode composite layer has the ability to maintain its structure in each state.

[0053] The Shear Module 1 of the polymer (A) film is preferably 150 MPa or less, more preferably 120 MPa or less, and particularly preferably 80 MPa or less. The fact that the Shear Module 1 of polymer (A) is within the above range indicates that polymer (A) is not too hard, is flexible, and has sufficient dry adhesion to maintain the electrode structure, mainly in the electrode manufacturing process.

[0054] Methods for preparing Shear Modulus 1 include adjusting the monomer composition during polymerization of polymer (A).

[0055] The Shear Modulus 2 of polymer (A) is preferably 10 kPa or more, more preferably 50 kPa or more, and particularly preferably 80 kPa or more. The fact that the Shear Modulus 2 of polymer (A) is within the above range indicates that polymer (A) is not too soft in the electrolyte and has sufficient adhesion to maintain the electrode structure in the electrolyte immersion state inside the battery, mainly when repeated charging and discharging occurs.

[0056] Methods for preparing Shear Modulus 2 include adjusting the monomer composition during polymerization of polymer (A) and changing the method of adding conjugated diene monomers during polymerization.

[0057] 1.1.3. Method for producing polymer (A) <Polymerization step> The method for producing polymer (A) is not particularly limited, but can be carried out by emulsion polymerization in the presence of known emulsifiers (surfactants), chain transfer agents, polymerization initiators, etc. Compounds described in Japanese Patent Publication No. 5999399, etc., can be used as emulsifiers (surfactants), chain transfer agents, and polymerization initiators.

[0058] The emulsion polymerization method for synthesizing polymer (A) may be carried out as a single-step polymerization or as a multi-step polymerization of two or more steps, but it is preferable to carry it out as a two-step polymerization.

[0059] When polymer (A) is synthesized by one-step polymerization, the monomer mixture can be emulsion-polymerized in the presence of a liquid medium (B), other emulsifiers, chain transfer agents, polymerization initiators, etc., preferably at a temperature of 0 to 80°C and preferably for a polymerization time of 4 to 36 hours.

[0060] When polymer (A) is synthesized by two-step polymerization, it is preferable to set the polymerization steps as follows.

[0061] The proportion of monomers used in the first stage polymerization is preferably in the range of 20 to 99% by mass, and more preferably in the range of 25 to 99% by mass, relative to the total mass of monomers (the sum of the mass of monomers used in the first stage polymerization and the mass of monomers used in the second stage polymerization). Performing the first stage polymerization with such a proportion of monomers allows for the production of polymer (A) particles with excellent dispersion stability and less tendency to form aggregates, and also suppresses the increase in viscosity of the binder composition for energy storage devices over time after solidification, which is preferable.

[0062] The types and proportions of monomers used in the second polymerization step may be the same as, or different from, the types and proportions of monomers used in the first polymerization step.

[0063] The polymerization conditions for each stage are preferably as follows, from the viewpoint of the dispersibility of the particles of the resulting polymer (A): • First stage polymerization: Preferably a temperature of 0 to 80°C, preferably a polymerization time of 2 to 36 hours, preferably a polymerization conversion rate of 50% by mass or more, more preferably 60% by mass or more. • Second stage polymerization: Preferably a temperature of 0 to 80°C, preferably a polymerization time of 5 to 18 hours.

[0064] Furthermore, in this embodiment, the structure of the crosslinked layer formed on the surface of the polymer particles changes depending on whether or not 1,3-butadiene is added in the second polymerization step and the duration of the second polymerization step, which can result in significant differences in the elastic modulus properties after immersion in the electrolyte. The examples and comparative examples described later are clearly differentiated by the process differences shown below.

[0065] (1) With or without addition of 1,3-butadiene in the second stage polymerization In the example, 1,3-butadiene is added in the second stage polymerization, and the repolymerization reaction on the surface of the polymer particles is promoted, forming a crosslinked structure derived from 1,3-butadiene. As a result, the film is less likely to soften even after immersion in the electrolyte, the shear storage modulus (Shear Modulus 2) is maintained at a high level, and elastic modulus properties that satisfy the relationship in formula (1) are obtained. On the other hand, in comparative examples 1, 2, and 5, 1,3-butadiene is not added in the second stage polymerization, so the crosslinking reaction does not proceed sufficiently. Therefore, the crosslinking density on the particle surface is low, the film is more likely to soften in the electrolyte, and the Shear Modulus 2 decreases, resulting in a failure to satisfy the relationship in formula (1).

[0066] (2) Differences in second-stage polymerization time In the example, by carrying out the second-stage polymerization at 75°C for 12 hours, the crosslinked layer derived from 1,3-butadiene on the surface of the polymer particles can be sufficiently grown. On the other hand, in comparative examples 2 and 4, the second-stage polymerization was carried out at 75°C for 3 hours, resulting in insufficient formation of the crosslinked layer derived from 1,3-butadiene on the surface of the polymer particles. As a result, the elastic modulus of the film after immersion in the electrolyte becomes low, and the relationship in equation (1) is not satisfied.

[0067] By setting the total solids content concentration in emulsion polymerization to 50% by mass or less, the polymerization reaction can proceed while maintaining good dispersion stability of the resulting polymer (A) particles. This total solids content concentration is preferably 45% by mass or less, and more preferably 40% by mass or less.

[0068] Whether the synthesis of polymer (A) is carried out by single-step polymerization or multi-step polymerization, it is preferable to neutralize the polymerization mixture by adding a neutralizing agent after the emulsion polymerization is completed. The neutralizing agent used here is not particularly limited, but examples include metal hydroxides such as sodium hydroxide and potassium hydroxide; and ammonia.

[0069] 1.1.4. Content ratio of polymer (A) The content ratio of polymer (A) in the binder composition for energy storage devices according to this embodiment is preferably 10 to 100% by mass, more preferably 20 to 95% by mass, and particularly preferably 25 to 90% by mass, of 100% by mass of the polymer component. Here, the polymer component includes polymer (A), polymers other than polymer (A) described later, and thickeners, etc.

[0070] 1.2. Liquid Medium (B) The binder composition for energy storage devices 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 may contain a non-aqueous medium other than water. Examples of this non-aqueous medium include amide compounds, hydrocarbons, alcohols, ketones, esters, amine compounds, lactones, sulfoxides, sulfone compounds, etc., and one or more selected from these can be used. By using an aqueous medium as the liquid medium (B) in the binder composition for energy storage devices according to this embodiment, the degree of adverse impact on the environment is reduced, and the safety for handling workers is also increased.

[0071] The proportion of the non-aqueous medium contained in the aqueous medium is preferably 10% by mass or less, more preferably 5% by mass or less, and particularly preferably substantially absent, based on 100% by mass of the aqueous medium. Here, "substantially absent" means that the non-aqueous medium is not intentionally added as a liquid medium.

[0072] 1.3. Other Additives The binder composition for energy storage devices according to this embodiment may contain additives other than those described above, as needed. Examples of such additives include polymers other than polymer (A), preservatives, thickeners, and the like.

[0073] <Polymers other than polymer (A)> The binder composition for energy storage devices according to this embodiment may contain polymers other than polymer (A). Such polymers are not particularly limited, but examples include styrene-butadiene rubber polymers containing aromatic vinyl compounds and conjugated diene compounds or derivatives thereof as constituent units, acrylic polymers containing unsaturated carboxylic acid esters or derivatives thereof as constituent units, and fluorine-based polymers such as PVDF (polyvinylidene fluoride). These polymers may be used individually or in combination of two or more. The inclusion of these polymers may further improve flexibility and adhesion.

[0074] <Preservatives> The binder composition for energy storage devices according to this embodiment may contain preservatives. By containing preservatives, it may be possible to suppress the growth of bacteria, mold, and other foreign substances when the binder composition for energy storage devices is stored. Specific examples of preservatives include compounds described in Japanese Patent Publication No. 5477610, etc.

[0075] <Thickening Agent> The binder composition for energy storage devices according to this embodiment may contain a thickening agent. By including a thickening agent, it may be possible to further improve the applicability of the slurry and the charge / discharge characteristics of the resulting energy storage device.

[0076] Examples of thickeners include cellulosic polymers such as carboxymethylcellulose, methylcellulose, ethylcellulose, and hydroxypropylcellulose; poly(meth)acrylic acid; ammonium salts or alkali metal salts of the aforementioned cellulose compounds or poly(meth)acrylic acid; modified polyvinyl alcohol, polyethylene oxide; polyvinylpyrrolidone, polycarboxylic acid, starch oxide, starch phosphate, casein, various modified starches, chitin, and chitosan derivatives. Among these, cellulosic polymers are preferred.

[0077] Examples of commercially available thickeners include alkali metal salts of carboxymethylcellulose such as CMC1120, CMC1150, CMC2200, CMC2280, and CMC2450 (all manufactured by Daicel Corporation).

[0078] If the binder composition for energy storage devices according to this embodiment contains a thickening agent, the proportion of the thickening agent is preferably 5% by mass or less, and more preferably 0.1 to 3% by mass, based on 100% by mass of the total solid content of the binder composition for energy storage devices.

[0079] 1.4. pH of the Binder Composition for Energy Storage Devices The pH of the binder composition for energy storage devices according to this embodiment is preferably 2.5 to 8.5, more preferably 2.7 to 8.2, and particularly preferably 3.0 to 8.0. When the pH is within the above range, it is possible to suppress the occurrence of insufficient viscosity and leveling properties and dripping of the binder composition for energy storage devices, making it easy to manufacture energy storage device electrodes that achieve both good electrical properties and adhesion.

[0080] In this specification, "pH" refers to a physical property measured as follows: a value measured at 25°C using a pH meter with 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 Toa DKK Corporation and the "D-51" manufactured by Horiba, Ltd.

[0081] While it cannot be denied that the pH of a binder composition for energy storage devices is affected by the monomer composition constituting 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 a binder composition for energy storage devices can change depending on polymerization conditions, etc., and the examples in this specification merely illustrate one such example.

[0082] 2. Slurry for Energy Storage Devices The slurry for energy storage devices according to one embodiment of the present invention contains the above-described binder composition for energy storage devices. The above-described binder composition for energy storage devices can be used as a material for producing a protective film to suppress short circuits caused by dendrites generated during charging and discharging, or as a material for producing an energy storage device electrode (active material layer) that achieves both adhesion in a dry state and adhesion in an electrolyte immersion state. The following will be explained separately as a slurry for energy storage devices for producing a protective film (hereinafter also referred to as "slurry for protective film") and a slurry for energy storage devices for producing an active material layer for an energy storage device electrode (hereinafter also referred to as "slurry for energy storage device electrode").

[0083] 2.1. Slurry for Protective Film The term "slurry for protective film" refers to a dispersion used to create a protective film on the surface of an electrode or separator, or both, by applying it to the surface of the electrode or separator, or both, and then drying it. The slurry for protective film according to this embodiment may consist only of the binder composition for energy storage devices described above, or it may further contain an inorganic filler. An example of an inorganic filler is the inorganic filler described in Japanese Patent Application Publication No. 2020-184461.

[0084] 2.2. Slurry for Energy Storage Device Electrodes "Slurry for energy storage device electrodes" refers to a dispersion used to create an active material layer on the surface of a current collector by applying it to the surface of the current collector and then drying it. The slurry for energy storage device electrodes according to this embodiment contains the above-mentioned binder composition for energy storage devices and an active material.

[0085] Generally, slurries for power storage device electrodes often contain a binder component such as an SBR copolymer and a thickener such as carboxymethyl cellulose in order to improve adhesion. On the other hand, the slurry for power storage device electrodes according to the present embodiment can improve the adhesion in the dry state and the adhesion in the electrolyte immersion state even when it contains only the polymer (A) described above as the polymer component. Of course, the slurry for power storage device electrodes according to the present embodiment may further contain polymers or thickeners other than the polymer (A) in order to further improve these adhesions. Hereinafter, the components included in the slurry for power storage device electrodes according to the present embodiment will be described.

[0086] 2.2.1. Polymer (A) The composition, physical properties, production method, etc. of the polymer (A) are as described above, so the description will be omitted.

[0087] The content ratio of the polymer component in the slurry for power storage device electrodes according to the present embodiment is preferably 0.1 to 8 parts by mass, more preferably 0.5 to 6 parts by mass, and particularly preferably 1 to 5 parts by mass with respect to 100 parts by mass of the active material. When the content ratio of the polymer component is within the above range, the dispersibility of the active material in the slurry becomes good, and the coating property of the slurry is also excellent. Here, the polymer component includes the polymer (A), polymers other than the polymer (A) added as necessary, and thickeners.

[0088] 2.2.2. Active Material Examples of the active material used in the slurry for power storage device electrodes according to the present embodiment include a positive electrode active material and a negative electrode active material. Specific examples thereof include, for example, carbon materials, silicon materials, oxides containing lithium atoms, sulfur compounds, lead compounds, tin compounds, arsenic compounds, antimony compounds, aluminum compounds, conductive polymers such as polyacene, A Ｘ B Ｙ O ＺExamples include composite metal oxides represented by (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 represents an oxygen atom, and X, Y, and Z are numbers in the range of 1.10 > X > 0.05, 4.00 > Y > 0.85, and 5.00 > Z > 1.50, respectively), and other metal oxides. Examples of the composite metal oxides include lithium cobaltate, lithium nickelate, lithium manganeseate, and ternary nickel-cobalt-manganate lithium.

[0089] The slurry for the electrodes of the energy storage device according to this embodiment can be used when manufacturing either the positive electrode or the negative electrode, but it is particularly preferable to use it for the negative electrode.

[0090] The negative electrode active material is preferably one that contains silicon and / or carbon among the active materials exemplified above, and more preferably a mixture of silicon and carbon materials. Since silicon materials have a larger lithium storage capacity per unit weight compared to other active materials, the energy storage capacity of the resulting energy storage device can be increased, and as a result, the output and energy density of the energy storage device can be increased. On the other hand, since carbon materials have a smaller volume change with charging and discharging than silicon materials, using a mixture of silicon and carbon materials as the negative electrode active material can mitigate the effect of volume change of silicon materials, and the adhesion ability between the active material layer and the current collector can be further improved.

[0091] On the other hand, the positive electrode active material preferably contains at least one selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, ternary nickel-cobalt-manganate lithium, and olivine-type lithium-containing phosphate compounds; more preferably contains at least one selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, and ternary nickel-cobalt-manganate lithium; and particularly preferably contains ternary nickel-cobalt-manganate lithium.

[0092] Energy storage device electrodes fabricated using the slurry for energy storage device electrodes according to this embodiment can exhibit good electrical characteristics even when using such positive electrode active materials. This is thought to be because the polymer (A) can firmly bind the positive electrode active material and at the same time maintain a state in which the positive electrode active material is firmly bound even during charging and discharging.

[0093] The average particle size of the positive electrode active material is preferably in the range of 0.5 to 30 μm, more preferably in the range of 0.5 to 25 μm, and particularly preferably in the range of 0.5 to 20 μm.

[0094] 2.2.3. Other Components In addition to the components described above, the slurry for the energy storage device electrode according to this embodiment may contain, as necessary, polymers other than polymer (A), thickeners, liquid media, conductive additives, pH adjusters, corrosion inhibitors, antioxidants, cellulose fibers, and other components. As polymers other than polymer (A) and thickeners, appropriate selections can be made from the compounds exemplified in the "1.3. Other Additives" section above and used for the same purposes and in the same proportions.

[0095] <Liquid Medium> In addition to the liquid medium brought in from the binder composition for the energy storage device, a liquid medium may be further added to the slurry for the energy storage device electrode according to this embodiment. The added liquid medium may be the same type as the liquid medium (B) contained in the binder composition for the energy storage device, or it may be different, but it is preferable to select and use from the liquid mediums exemplified in the section "1.2. Liquid Medium (B)" above.

[0096] In the slurry for the electrodes of the energy storage device according to this embodiment, the content ratio of the liquid medium (including the portion brought in from the binder composition for the energy storage device) is preferably such that the solid content concentration in the slurry (meaning the ratio of the total mass of components other than the liquid medium in the slurry to the total mass of the slurry; the same applies hereinafter) is 30 to 70% by mass, and more preferably 40 to 60% by mass.

[0097] <Conductive additive> A conductive additive may be further added to the slurry for the electrodes of the energy storage device according to this embodiment, for the purpose of imparting conductivity and buffering the volume change of the active material due to the movement of lithium ions.

[0098] Specific examples of conductive additives include activated carbon, acetylene black, Ketjen black, furnace black, graphite, carbon fiber, fullerene, and carbon nanotubes. Among these, acetylene black, Ketjen black, or carbon nanotubes can be preferably used. The content ratio of the conductive additive 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, per 100 parts by mass of the active material.

[0099] <pH adjuster> A pH adjuster may be further added to the slurry for the electrodes of the energy storage device according to this embodiment, for the purpose of suppressing corrosion of the current collector, depending on the type of active material.

[0100] Examples of pH adjusting agents include hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid, formic acid, ammonium phosphate, ammonium sulfate, ammonium acetate, ammonium formate, ammonium chloride, sodium hydroxide, and potassium hydroxide. Among these, sulfuric acid, ammonium sulfate, sodium hydroxide, and potassium hydroxide are preferred. Alternatively, a neutralizing agent can be selected and used from those described in the method for producing polymer (A).

[0101] <Corrosion Inhibitor> Corrosion inhibitors may be further added to the slurry for the electrodes of the energy storage device according to this embodiment, for the purpose of suppressing corrosion of the current collector, depending on the type of active material.

[0102] 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. Among these, ammonium paratungstate, ammonium metavanadate, sodium metavanadate, potassium metavanadate, and ammonium molybdate are preferred.

[0103] <Cellulose Fibers> Cellulose fibers may be further added to the slurry for the electrodes of the energy storage device according to this embodiment. Known cellulose fibers can be used. Adding cellulose fibers may improve the adhesion of the active material to the current collector. It is thought that the fibrous cellulose fibers bind adjacent active materials together in a fibrous manner through linear adhesion or linear contact, thereby preventing the active material from falling off and improving adhesion to the current collector.

[0104] 2.2.4. Method for Preparing Slurry for Energy Storage Device Electrodes The slurry for energy storage device electrodes according to this embodiment may be manufactured by any method, as long as it contains the above-described binder composition for energy storage devices and active material. From the viewpoint of producing a slurry with better dispersibility and stability more efficiently and inexpensively, it is preferable to manufacture it by adding the active material and optional additives used as needed to the binder composition for energy storage devices and mixing them. Specific manufacturing methods include, for example, the method described in Japanese Patent Publication No. 5999399.

[0105] 3. Energy Storage Device Electrode An energy storage device electrode according to one embodiment of the present invention comprises a current collector and an active material layer formed by applying and drying the above-mentioned slurry for energy storage device electrodes on the surface of the current collector. Such an energy storage device electrode can be manufactured by applying the above-mentioned slurry for energy storage device electrodes to the surface of a current collector such as metal foil to form a coating film, and then drying the coating film to form an active material layer. The energy storage device electrode manufactured in this manner has an active material layer containing the above-mentioned polymer (A), active material, and optionally added components bonded to the surface of the current collector, and therefore exhibits excellent adhesion in both the dry state and the electrolyte immersion state. As a result, rapid charging can be achieved in a short time, and the expansion of the electrode due to repeated charging and discharging is suppressed, resulting in excellent charge-discharge durability characteristics.

[0106] The current collector is not particularly limited as long as it is made of a conductive material, but examples include the current collector described in Japanese Patent Publication No. 5999399.

[0107] In the energy storage device electrode according to this embodiment, when a silicon material is used as the active material, the content of silicon elements in 100% by mass of the active material layer is preferably 1 to 30% by mass, more preferably 2 to 20% by mass, and particularly preferably 3 to 10% by mass. When the content of silicon elements in the active material layer is within the above range, the energy storage capacity of the energy storage device made using it is improved, and an active material layer with a uniform distribution of silicon elements is obtained. The content of silicon elements in the active material layer can be measured by a method described, for example, in Japanese Patent Publication No. 5999399.

[0108] 4. Energy Storage Device An energy storage device according to one embodiment of the present invention is equipped with the above-mentioned energy storage device electrodes, further contains an electrolyte, and can be manufactured by conventional methods using components such as separators. Specific manufacturing methods include, for example, stacking a negative electrode and a positive electrode via a separator, winding or folding them according to the battery shape, housing them in a battery container, and then injecting the electrolyte into the battery container and sealing it. The shape of the battery can be any suitable shape, such as coin-type, cylindrical, prismatic, or laminate-type.

[0109] The electrolyte can be in liquid or gel form, and depending on the type of active material, one can select from known electrolytes used in energy storage devices that effectively exhibits battery function. The electrolyte can be a solution in which an electrolyte is dissolved in a suitable solvent. Examples of such electrolytes and solvents include compounds described in Japanese Patent Publication No. 5999399, etc.

[0110] The energy storage device described above is applicable to lithium-ion secondary batteries, electric double-layer capacitors, and lithium-ion capacitors that require high current density discharge. Among these, lithium-ion secondary batteries are particularly preferred. In the energy storage device electrodes and energy storage device according to this embodiment, components other than the binder composition for the energy storage device can be those of known lithium-ion secondary batteries, electric double-layer capacitors, and lithium-ion capacitors.

[0111] 5. Examples The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. In the examples and comparative examples, "parts" and "%" are based on mass unless otherwise specified.

[0112] 5.1. Example 1 5.1.1. Preparation of Binder Composition for Energy Storage Devices A binder composition for energy storage devices containing polymer (A1) was obtained by the following two-stage polymerization. 300 parts by mass of water, a monomer mixture consisting of 10 parts by mass of 1,3-butadiene, 37 parts by mass of styrene, 30 parts by mass of 2-ethylhexyl acrylate, 10 parts by mass of ethyl acrylate, and 2 parts by mass of acrylic acid was charged into a reactor, along with 0.1 parts by mass of tert-dodecyl mercaptan as a chain transfer agent, 0.1 parts by mass of sodium alkyldiphenyl ether disulfonate as an emulsifier, and 1.0 part by mass of potassium persulfate as a polymerization initiator. Polymerization was carried out at 70°C for 18 hours with stirring, and the polymerization conversion rate was confirmed to be 95%. Next, 2 parts by mass of acrylic acid, 4 parts by mass of styrene, and 5 parts by mass of 1,3-butadiene were further added to the reactor, and polymerization was carried out at 75°C for 12 hours, and the polymerization conversion rate was confirmed to be 98%. Unreacted monomers were removed from the particle dispersion of polymer (A1) obtained in this manner, and the dispersion was concentrated. After adding a 5% sodium hydroxide aqueous solution, water was removed using an evaporator to obtain a binder composition for energy storage devices containing polymer (A1) particles with a solid content of 45% by mass and a pH of 7.0.

[0113] 5.1.2. Physical Properties Evaluation of Binder Composition for Energy Storage Devices <Method for Measuring Polymerization Conversion Rate> The polymerization conversion rate in the synthesis of the above polymer was measured as follows: The reaction solution polymerized for a predetermined time was removed and placed in an aluminum dish (X (g)) whose weight had been measured in advance, and the weight of the reaction solution (Y (g)) was weighed. This was dried in a hot air dryer at 155°C for 15 minutes. The aluminum dish was removed, allowed to cool, and then the weight of the aluminum dish (Z (g)) was weighed. From the weights X, Y, and Z measured in this way, the polymerization conversion rate (%) was calculated using the following formula (3). Polymerization conversion rate (%) = ((Z - X) / Y) × 100 (3)

[0114] <Shear Storage Modulus Measured with Nanoindenter> A binder composition for energy storage devices containing polymer (A1) particles was placed in a 7.5 cm diameter petri dish so that the amount of polymer (A1) was 4.0 g in terms of solid content, and dried at 25°C for 7 days to produce a uniform film with a thickness of 1.0 ± 0.3 mm. This film was dried in a vacuum dryer at 100°C for 30 minutes, and then cut into 5 mm x 5 mm strips, which were used as measurement sample D. On the other hand, the polymer (A1) film was immersed in a solvent (specific solvent) consisting of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume fraction of 1:3:1 at 70°C for 24 hours, and this was used as measurement sample W. These measurement samples were fixed to a cylindrical SUS stage with a diameter of 310 mm and a height of 240 mm. The top of the stage has a cylindrical hole in the center with a diameter of 210 mm and a depth of 60 mm, which can hold a specific solvent. Next, the shear storage modulus of measurement samples D and W at 25°C was measured using a nanoindenter measuring device (manufactured by Toyo Technica Co., Ltd., product name "iNano"). Measurement sample W was fixed inside the hole and its shear storage modulus was measured while immersed in the specific solvent. The shear storage modulus of measurement sample D, measured with the nanoindenter, was designated "Shear Modulus 1," and the shear storage modulus of measurement sample W was designated "Shear Modulus 2," and the logarithm was used. １０ The value of {(Shear Module 1) / (Shear Module 2)} was calculated. The results are shown in Table 1 below.

[0115] <Measurement of Number-Average Particle Size> The binder composition for energy storage devices containing polymer (A1) particles, obtained above with a solid content concentration of 45% by mass and pH 7.0, was diluted to 0.1 wt% by adding water and used as a sample. The number-average particle size of polymer (A1) was measured using a concentrated particle size analyzer (Otsuka Electronics Co., Ltd., model "FPAR-1000") with dynamic light scattering (DLS) under the following conditions. The results are shown in Table 1 below. (Conditions) ・Measurement temperature: 25°C ・Scattering angle: 160° ・Light source laser wavelength: 632.8 nm

[0116] 5.1.3. Preparation of Slurry for Energy Storage Device Electrodes <Synthesis of Silicon-Containing Active Material (Active Material)> A mixture of pulverized silicon dioxide powder (average particle size 10 μm) and carbon powder (average particle size 35 μm) was heated in an electric furnace with the temperature adjusted to the range of 1100°C to 1600°C under a nitrogen atmosphere (0.5 NL / min) for 10 hours to obtain silicon oxide powder (average particle size 8 μm) represented by the compositional formula SiOx (x = 0.5 to 1.1). 300 g of this silicon oxide powder was placed in a batch heating furnace and heated from room temperature (25°C) to 1100°C at a heating rate of 300°C / h while maintaining a reduced absolute pressure of 100 Pa 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 material was cooled to room temperature at a cooling rate of 50°C / h to obtain approximately 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 its average particle size is 10.5 μm. The proportion of the graphite coating when the total amount of the obtained graphite-coated silicon oxide is considered to be 100% by mass was 2% by mass.

[0117] <Preparation of Slurry for Energy Storage Device Electrodes> In a twin-shaft planetary mixer (Primix Corporation, product name "TK Hibiscus Mix 2P-03"), 1.2 parts by mass of a thickener (product name "CMC2200", manufactured by Daicel Corporation) (solid content equivalent, added as powder), 95 parts by mass of artificial graphite (Zichen Corporation, product name "QC7L") as a negative electrode active material (solid content equivalent), 5 parts by mass of the silicon-containing active material prepared above, and 0.5 parts by mass of carbon (Denka Corporation, acetylene black) as a conductive additive were added and stirred at a rotational speed of 20 rpm for 1 hour to obtain a powder mixture. Water was added to the obtained powder mixture to adjust the solid content concentration to 65% by mass, and then stirred at a rotational speed of 35 rpm for 30 minutes to obtain a paste. Water was added to the obtained paste to adjust the solid content concentration to 61%, and then the mixture was stirred for 30 minutes at an orbital speed of 35 rpm and a disperser speed of 1000 rpm to obtain a paste. To the obtained paste, 2 parts by mass of polymer (A1) (calculated as solid content, added as the binder composition for energy storage devices obtained above) and water were added to adjust the solid content concentration to 50%, and then the mixture was stirred under reduced pressure (approximately 2.5 × 10) using a stirring and defoaming machine (manufactured by Thinky Co., Ltd., product name "Awatori Rentaro") ４ A slurry for energy storage device electrodes was prepared by stirring and mixing at 900 rpm for 6 minutes in Pa.

[0118] 5.1.4. Manufacturing and Evaluation of Energy Storage Devices <Manufacturing of Energy Storage Device Electrode (Negative Electrode)> The slurry for the energy storage device electrode obtained above was uniformly applied to the surface of a current collector made of copper foil with a thickness of 20 μm using the doctor blade method so that the film thickness after drying was 100 μm. It was dried at 80°C for 5 minutes, and then dried at 120°C for 5 minutes. After that, the density of the active material layer was 1.65 g / cm³. ３ By press-forming the material using a roll press machine in this manner, an electrode (negative electrode) for an energy storage device was obtained.

[0119] <Evaluation of Adhesion Strength of Negative Electrode Coating Layer> The energy storage device electrodes obtained above were punched out into strips 20 mm wide and 100 mm long, and the composite layer surface was fixed to a horizontal SUS plate with double-sided adhesive tape. The peel strength (N / m) required to peel off the copper foil at a 90° angle was then measured. (Evaluation Criteria) When the peel strength is rounded to the second decimal place: ・5 points: Peel strength is 15.0 N / m or more. ・4 points: Peel strength is 12.0 to 14.9 N / m. ・3 points: Peel strength is 9.0 to 11.9 N / m. ・2 points: Peel strength is 6.0 to 8.9 N / m. ・1 point: Peel strength is 5.9 N / m or less.

[0120] <Manufacturing of Counter Electrode (Positive Electrode)> A twin-shaft planetary mixer (manufactured by Primix Corporation, product name "TK Hibiscus Mix 2P-03") is mixed with 3.0 parts by mass (solid content equivalent) of electrochemical device electrode binder (manufactured by Kureha Corporation, product name "KF Polymer #1120"), 3.0 parts by mass of conductive graphite powder (manufactured by Nippon Graphite Industries Co., Ltd., J-SP-α), 3.0 parts by mass of conductive additive (manufactured by Denka Co., Ltd., Li-400), and LiNi with an average particle size of 13 μm as the positive electrode active material. ０．８ Co ０．１０ Mn ０．１０ O ２ 100 parts by mass (based on solid content) of (Beijing Dangsheng Co., Ltd., ME-83SC) and 15 parts by mass of N-methylpyrrolidone (NMP) were added and stirred at 60 rpm for 2 hours. NMP was added to the resulting paste to adjust the solid content concentration to 55% by mass, and then the mixture was stirred using a stirring and defoaming machine (Sinky Co., Ltd., product name "Awatori Rentaro") at 200 rpm for 2 minutes, 1800 rpm for 5 minutes, and then under reduced pressure (approximately 2.5 × 10⁻⁶). ４ A cathode slurry was prepared by stirring and mixing at 1800 rpm for 1.5 minutes in Pa. This cathode slurry was uniformly applied to the surface of a current collector made of aluminum foil using the doctor blade method so that the film thickness after solvent removal was 80 μm, and then heated and dried at 120°C for 20 minutes. After that, the density of the active material layer was 3.0 g / cm³. ３ The opposite electrode (positive electrode) was obtained by pressing it using a roll press machine in the manner described above.

[0121] <Assembly of Lithium-ion Battery Cell> In a glove box substituted with Ar to maintain a dew point of -80°C or lower, the negative electrode manufactured above was punched out to a diameter of 16.16 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 membrane punched out to a diameter of 24 mm (manufactured by Cellguard Co., Ltd., product name "Cellguard #2400") was placed on top, and then 300 μL of electrolyte was injected to prevent air from entering. Finally, the positive electrode manufactured above was punched out to a diameter of 15.95 mm and placed on top, and the outer body of the two-electrode coin cell was sealed by closing it with screws to assemble the lithium-ion battery cell (energy storage device). The electrolyte used here was an ethylene carbonate / ethyl methyl carbonate / diethyl carbonate = 1 / 3 / 1 (volume ratio) solvent with LiPF ６ This is a solution obtained by dissolving [the substance] at a concentration of 1 mole / L.

[0122] <Evaluation of Internal Resistance> The energy storage device manufactured above was placed in a constant temperature bath at 25°C, and charging was started with a constant current (0.1C). When the voltage reached 4.25V, charging was continued at a constant voltage (4.25V), and charging was completed (cutoff) when the current value reached 0.01C. Then, discharging was started with a constant current (0.1C), and discharging was completed (cutoff) when the voltage reached 3.0V. Furthermore, charging was started with a constant current (0.1C) until the capacity reached 50% of the positive electrode capacity. After that, the temperature of the constant temperature bath was set to -10°C, and the device was discharged for 10 seconds with a constant current (0.1C). The internal resistance was calculated from the voltage change ΔV (difference between the voltage before and after 10 seconds of discharge) using the following formula (4), and evaluated according to the following criteria. The results are shown in Table 1 below. Internal Resistance (Ω・cm) ２ ) = (Voltage change (ΔV)) / (Current value during discharge (A)) × 2.0 (cm ２ ) (4) (Evaluation criteria) ・5 points: Internal resistance is 60 Ω・cm ２ Less than . • 4 points: Internal resistance is 60 Ω·cm ２ 65Ω・cm or more ２ Less than . • 3 points: Internal resistance is 65 Ω·cm ２ 70Ω・cm or more ２Less than . • 2 points: Internal resistance is 70 Ω·cm ２ 80Ω・cm or more ２ Less than . • 1 point: Internal resistance is 80 Ω·cm ２ That's all.

[0123] <Evaluation of Cycle Characteristics> For the energy storage device fabricated above, charging was started with a constant current (1.5C) in a constant temperature bath heated to 25°C. When the voltage reached 4.25V, charging was continued at a constant voltage (4.25V), and charging was completed (cutoff) when the current value reached 0.01C. After that, discharging was started with a constant current (1.0C), and discharging was completed (cutoff) when the voltage reached 3.0V, and the discharge capacity of the first cycle was calculated. This charge-discharge cycle was repeated 100 times. The capacity retention rate was calculated using the following formula (5) and evaluated according to the following criteria. The results are shown in Table 1 below. Capacity Retention Rate (%) = (Discharge Capacity at 100 Cycles / Discharge Capacity at 1 Cycle) × 100 (5) (Evaluation Criteria) ・5 points: Capacity retention rate of 95% or more. ・4 points: Capacity retention rate of 90% or more and less than 95%. ・3 points: Capacity retention rate of 85% or more and less than 90%. - 2 points: Volume retention rate is 80% or more but less than 85%. - 1 point: Volume retention rate is less than 80%.

[0124] <Evaluation of electrode expansion rate> The film thickness of the negative electrode manufactured as described above was measured and defined as the initial film thickness. Next, the energy storage device manufactured as described above was charged at a constant current (0.2C) in a constant temperature bath controlled at 25°C. When the voltage reached 4.25V, charging was continued at a constant voltage (4.25V), and the charging was completed (cutoff) when the current value reached 0.01C. After that, discharge was started at a constant current (0.2C) in a constant temperature bath controlled at 25°C, and the discharge was completed (cutoff) when the voltage reached 3.0V, thus completing the chemical conversion charge and discharge. After that, a contact sensor (manufactured by Keyence Corporation, product name "GT2-H12KLF") was attached to the energy storage device, and the film thickness at that time was defined as the film thickness after chemical conversion. Then, charging was started at a constant current (0.2C), and when the voltage reached 4.25V, charging was continued at a constant voltage (4.25V), and the charging was completed (cutoff) when the current value reached 0.01C. After that, discharge was started at a constant current (0.2C) in a constant temperature bath heated to 25°C, and the discharge was completed (cutoff) when the voltage reached 3.0V, and the film thickness at the 10th discharge cycle was measured, and the electrode expansion rate was calculated using the following formula (6) and evaluated according to the following criteria. The results are shown in Table 1 below. Electrode expansion rate (%) = (Film thickness at the 10th discharge cycle - Film thickness after chemical conversion) / (Initial film thickness) × 100 (6) (Evaluation criteria) ・5 points: Electrode expansion rate is 10% or less. ・4 points: Electrode expansion rate is greater than 10% to 13% or less. ・3 points: Electrode expansion rate is greater than 13% to 16% or less. ・2 points: Electrode expansion rate is greater than 16% to 20% or less. - 1 point: Plate expansion rate exceeds 20%.

[0125] 5.2. Examples 2-11, Comparative Examples 1, 3, 5 In Examples 2-11 and Comparative Examples 1, 3, 5, the types and amounts of monomers were as shown in Table 1 or Table 2 below, and each polymer was synthesized by two-step polymerization similar to that in Example 1 to obtain binder compositions for energy storage devices. Otherwise, the procedure was carried out in the same manner as in Example 1.

[0126] 5.3. Comparative Example 2 A binder composition for energy storage devices containing a polymer was obtained by the following two-stage polymerization. 300 parts by mass of water, a monomer mixture consisting of 5 parts by mass of 1,3-butadiene, 59 parts by mass of styrene, 28 parts by mass of 2-ethylhexyl acrylate, and 2 parts by mass of acrylic acid was charged into a reactor, along with 0.1 parts by mass of tert-dodecyl mercaptan as a chain transfer agent, 0.1 parts by mass of sodium alkyldiphenyl ether disulfonate as an emulsifier, and 1.0 part by mass of potassium persulfate as a polymerization initiator. Polymerization was carried out at 70°C for 18 hours with stirring, and the polymerization conversion rate was confirmed to be 95%. Next, 4 parts by mass of styrene and 2 parts by mass of acrylic acid were further added to the reactor, and polymerization was carried out at 75°C for 3 hours (shorter than in Example 1) to complete the reaction. The polymer particle dispersion obtained in this way was concentrated by removing unreacted monomers, and after adding a 5% sodium hydroxide aqueous solution, water was removed using an evaporator to obtain a binder composition for energy storage devices containing polymer particles with a solid content of 45% by mass and a pH of 7.0. In Comparative Example 2, since 1,3-butadiene was not added in the second stage of polymerization, a 1,3-butadiene crosslinking layer was not formed on the particle surface, resulting in a polymer with a low crosslinking density. Also, because the polymerization time in the second stage of polymerization was short, the crosslinking reaction did not proceed sufficiently on the particle surface. As a result, the obtained polymer had a low crosslinking density and was prone to softening after immersion in the electrolyte, and did not satisfy the relationship in formula (1). Except for using the binder composition for energy storage devices obtained in this way, everything was carried out in the same manner as in Example 1.

[0127] 5.4. Comparative Example 4 A binder composition for energy storage devices containing a polymer was obtained by the following two-stage polymerization. 300 parts by mass of water, a monomer mixture consisting of 15 parts by mass of 1,3-butadiene, 19.5 parts by mass of styrene, 53 parts by mass of 2-ethylhexyl acrylate, and 2 parts by mass of acrylic acid was charged into a reactor, along with 0.1 parts by mass of tert-dodecyl mercaptan as a chain transfer agent, 0.1 parts by mass of sodium alkyldiphenyl ether disulfonate as an emulsifier, and 1.0 part by mass of potassium persulfate as a polymerization initiator. Polymerization was carried out at 70°C for 18 hours with stirring, and the polymerization conversion rate was confirmed to be 95%. Next, 4.5 parts by mass of 1,3-butadiene, 4 parts by mass of styrene, and 2 parts by mass of acrylic acid were further added to the reactor, and polymerization was carried out at 75°C for 3 hours (shorter than in Example 1) to complete the reaction. The polymer particle dispersion obtained in this manner was concentrated by removing unreacted monomers, and after adding a 5% sodium hydroxide aqueous solution, water was removed using an evaporator to obtain a binder composition for energy storage devices containing polymer particles with a solid content of 45% by mass and a pH of 7.0. In Comparative Example 4, the polymerization time in the second stage of polymerization was short, so the crosslinking reaction did not proceed sufficiently on the particle surface. As a result, the obtained polymer had a low crosslinking density and was prone to softening after immersion in the electrolyte, and did not satisfy the relationship in formula (1). Except for using the binder composition for energy storage devices obtained in this manner, everything was carried out in the same manner as in Example 1.

[0128] 5.5. Evaluation Results Tables 1 and 2 below show the polymer compositions used in Examples 1 to 11 and Comparative Examples 1 to 5, the results of each physical property measurement, and the evaluation results. The numerical values ​​representing the polymer composition shown in Tables 1 and 2 below represent parts by mass.

[0129]

[0130]

[0131] Note that the monomers in Tables 1 and 2 above represent the following compounds, respectively: <Conjugated diene compounds> ・BD: 1,3-butadiene <Aromatic vinyl compounds> ・ST: Styrene <Ethylene unsaturated carboxylic acid esters> ・2EHA: 2-ethylhexyl acrylate ・BA: butyl acrylate ・BMA: butyl methacrylate ・MMA: methyl methacrylate ・EA: ethyl acrylate <Unsaturated carboxylic acids> ・AA: acrylic acid ・MAA: methacrylic acid

[0132] It was found that by including the binder compositions for energy storage devices of Examples 1 to 11, the binder particles, which are sufficiently flexible in a dry state, suitably bond the active materials together, making it possible to produce electrodes that do not experience powder shedding or cracking from the composite layer. Furthermore, it was found that by including the binder compositions for energy storage devices of Examples 1 to 11, the binder particles do not become too soft in the electrolyte, thus suitably bonding the active materials together inside the battery, suppressing electrode expansion due to repeated charge and discharge, and resulting in excellent cycle characteristics. Moreover, it is presumed that because polymer (A) sufficiently contains repeating units (a2) derived from aromatic vinyl compounds and repeating units (a3) ​​derived from ethylenically unsaturated carboxylic acid esters, desolvation of lithium ions is promoted on the surface of polymer (A) during the battery charging process, reducing the energy required for the insertion of lithium ions into the negative electrode active material. As a result, the increase in internal resistance is suppressed, and rapid charging in a short time becomes possible.

[0133] In Examples 1 to 11, 1,3-butadiene was added during the second polymerization stage, and the polymerization time was extended to 12 hours to form a crosslinked layer derived from 1,3-butadiene on the surface of the polymer particles. This crosslinked layer formation makes the film less prone to softening even after immersion in the electrolyte, and results in a high Shear Modulus 2 value. This process is effective in achieving the elastic modulus characteristics satisfying formula (1), which are a feature of the present invention.

[0134] On the other hand, although the polymers obtained in Comparative Examples 2 and 4 satisfy the monomer composition of the present invention, as described above, the crosslinking density on the particle surface and the thickness of the crosslinked layer differ from those of the examples due to the presence or absence of 1,3-butadiene addition in the second stage polymerization and the difference in the second stage polymerization time. Therefore, even with the same composition ratio, the elastic modulus (Shear Modulus 2) of the film after immersion in the electrolyte solution decreases, and the relationship in formula (1) is not satisfied.

[0135] The present invention is not limited to the embodiments described above, and various modifications are possible. The present invention encompasses configurations that are substantially identical to those described in the embodiments (for example, configurations with the same function, method, and result, or configurations with the same purpose and effect). The present invention also encompasses configurations in which non-essential parts of the configurations described in the embodiments are replaced with other configurations. Furthermore, the present invention also encompasses configurations that produce the same effects or achieve the same purpose as the configurations described in the embodiments. Furthermore, the present invention also encompasses configurations that add known technology to the configurations described in the embodiments.

Claims

1. A binder composition for energy storage devices comprising a polymer (A) and a liquid medium (B), wherein, when the total amount of repeating units contained in the polymer (A) is 100% by mass, the polymer (A) contains: 0.1 to 30% by mass of repeating units (a1) derived from a conjugated diene compound, 5 to 75% by mass of repeating units (a2) derived from an aromatic vinyl compound, and 20 to 80% by mass of repeating units (a3) ​​derived from an ethylenically unsaturated carboxylic acid ester, wherein the total amount of repeating units (a2) and repeating units (a3) ​​is 60% by mass or more, and satisfies the following conditions. <Conditions> The polymer (A) is placed in a petri dish with a diameter of 7.5 cm in an amount equivalent to 4.0 g of solids, dried at 25°C for 7 days, then further dried in a vacuum dryer at 100°C for 30 minutes, and the resulting film is cut to a width of 5 mm x 5 mm. The obtained film is immersed in a solvent consisting of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume fraction of 1:3:1 at 70°C for 24 hours. When the shear storage modulus of the film before immersion, measured by a nanoindenter, is denoted as Shear Modulus 1, and the shear storage modulus of the film after immersion is denoted as Shear Modulus 2, the following relationship (1) is satisfied. Log １０ {(Shear Modulus 1) / (Shear Modulus 2)}<2.5 (1) 2. The binder composition for energy storage devices according to claim 1, wherein the polymer (A) further contains 0.1 to 10% by mass of repeating units (a4) derived from an unsaturated carboxylic acid.

3. The binder composition for energy storage devices according to claim 1 or claim 2, wherein the polymer (A) is polymer particles, and the number-average particle diameter of the polymer particles is 50 nm or more and 500 nm or less.

4. The binder composition for energy storage devices according to claim 1 or claim 2, wherein the liquid medium (B) is water.

5. A slurry for an electrode of an energy storage device, comprising the binder composition for an energy storage device described in claim 1 and an active material.

6. The slurry for an energy storage device electrode according to claim 5, wherein the active material contains graphite and silicon material.

7. The slurry for energy storage device electrodes according to claim 5 or claim 6, further comprising a thickening agent.

8. An energy storage device electrode comprising a current collector and an active material layer formed by applying and drying the slurry for energy storage device electrodes described in claim 5 on the surface of the current collector.

9. An energy storage device comprising the energy storage device electrodes described in claim 8.

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