Binder polymer, binder composition, electrode slurry, electrode, and nonaqueous secondary battery

Abstract

Provided is a binder polymer which is a polymer of a monomer having an ethylenically unsaturated bond, wherein the monomer contains a styrene compound and an alkyl (meth)acrylate, and the swelling ratio with respect to ethylene carbonate is 6% or less.

Description

Binder polymer, binder composition, electrode slurry, electrode, and non-aqueous secondary battery

[0001] This disclosure relates to binder polymers, binder compositions, electrode slurries, electrodes, and non-aqueous secondary batteries.

[0002] Non-aqueous secondary batteries are widely used as power sources for laptop computers, mobile phones, power tools, and electronic communication equipment because they can be made smaller and lighter. In recent years, non-aqueous secondary batteries have also been used as power sources for electric vehicles and hybrid vehicles. A typical example of a non-aqueous secondary battery is the lithium-ion secondary battery.

[0003] A non-aqueous secondary battery comprises a positive electrode and a negative electrode as electrodes, and an electrolyte. The positive electrode and the negative electrode each include a current collector and an electrode active material layer formed on the current collector. The electrode active material layer usually contains a binder, which binds the active materials to each other and to the current collector. Various resins are used as binders in non-aqueous secondary batteries. For example, Patent Documents 1 to 3 describe the use of a monomer polymer having ethylenically unsaturated bonds as a binder.

[0004] Patent Document 1: Japanese Unexamined Patent Publication No. 2014-239070 Patent Document 2: Japanese Unexamined Patent Publication No. 2011-243464 Patent Document 3: International Publication No. 2023-053863

[0005] Non-aqueous secondary batteries may generate gas inside the battery due to the decomposition of the electrolyte. This gas generation inside the battery may cause the battery to expand in volume (hereinafter also referred to as battery swelling).

[0006] Therefore, the present disclosure aims to provide a binder polymer, a binder composition, an electrode slurry, and an electrode that can be used to obtain a non-aqueous secondary battery in which battery swelling is suppressed.

[0007] This disclosure includes the following aspects: <1> A binder polymer having an ethylenically unsaturated bond, wherein the monomer comprises a styrene compound and an alkyl (meth)acrylate, and the swelling rate relative to ethylene carbonate is 6% or less. <2> The binder polymer according to <1>, wherein the proportion of the styrene compound in the total monomer is 20 mol% or more and 80 mol% or less. <3> The binder polymer according to <1> or <2>, wherein the proportion of the alkyl (meth)acrylate in the total monomer is 20 mol% or more and 80 mol% or less. <4> The binder polymer according to any one of <1> to <3> for use as a component of an electrode for a non-aqueous secondary battery. <5> A binder composition comprising the binder polymer according to any one of <1> to <4> and an aqueous medium. <6> The binder composition according to <5>, wherein particles containing the binder polymer are dispersed in the aqueous medium. An electrode slurry comprising a binder polymer according to any one of items <7>, <1> to <4>, an electrode active material, and an aqueous medium. An electrode comprising a binder polymer according to any one of items <8>, <1> to <4>, and an electrode active material. A non-aqueous secondary battery comprising the electrode according to item <9>, <8>.

[0008] This disclosure provides a binder polymer, a binder composition, an electrode slurry, and an electrode that can be used to obtain a non-aqueous secondary battery in which battery swelling is suppressed. Furthermore, this disclosure provides a non-aqueous secondary battery in which battery swelling is suppressed.

[0009] The embodiments are described in detail below. However, this disclosure is not limited to the embodiments described below. In the embodiments described below, the components (including elemental steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit this disclosure.

[0010] In this disclosure, numerical ranges indicated using "~" include the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described in stages in this disclosure, the upper or lower limit of one numerical range may be replaced by the upper or lower limit of another numerical range described in stages. Also, in numerical ranges described in this disclosure, the upper or lower limit of that numerical range may be replaced by the values ​​shown in the examples. In this disclosure, each component may contain multiple types of the corresponding substance. When multiple types of substances corresponding to each component exist in a composition, the content or amount of each component means the total content or amount of the multiple types of substances present in the composition, unless otherwise specified. In this disclosure, the term "layer" includes cases where the layer is formed in the entire region when the region in which the layer exists is observed, as well as cases where it is formed in only a part of the region.

[0011] In this disclosure, "(meth)acrylic" is a general term for acrylic and methacrylic. "(meth)acrylate" is a general term for acrylate and methacrylate. In this disclosure, "ethylenically unsaturated bond" means an ethylenically unsaturated bond that is radically polymerizable.

[0012] <Binder Polymer> The binder polymer of this disclosure is a monomer polymer having an ethylenically unsaturated bond, wherein the monomer comprises a styrene compound and an alkyl (meth)acrylate, and the swelling rate relative to ethylene carbonate is 6% or less.

[0013] As shown in the examples described later, non-aqueous secondary batteries using the binder polymer of this disclosure as an electrode binder exhibit less battery swelling compared to non-aqueous secondary batteries using a binder polymer other than the binder polymer of this disclosure as an electrode binder. The reason for this effect is not clear, but it is presumed to be as follows: Ethylene carbonate is an organic compound used as a component of the electrolyte in non-aqueous secondary batteries. The generation of gas due to the decomposition of ethylene carbonate is considered to be a major cause of battery swelling. Therefore, it is presumed that using a binder polymer with a low swelling rate relative to ethylene carbonate as an electrode binder is involved in suppressing battery swelling.

[0014] The binder polymers of this disclosure are polymers of monomers having ethylenically unsaturated bonds. That is, the binder polymers of this disclosure are compounds having a molecular structure as their main chain, which is formed by the bonding of multiple structural units derived from monomers having ethylenically unsaturated bonds. In this disclosure, "structural unit derived from monomers having ethylenically unsaturated bonds" means a site in the main chain of the polymer that is formed by the reaction of the ethylenically unsaturated bonds of the monomer. In this disclosure, "monomer having ethylenically unsaturated bonds" corresponding to a certain structural unit in the polymer is a compound having a structure in which the bond between the two carbon atoms forming the main chain of the polymer is replaced with an ethylenically unsaturated bond and separated from other structural units. The chemical structure of a structural unit derived from monomers having ethylenically unsaturated bonds includes the chemical structure of the portion of the monomer from which the ethylenically unsaturated bond contributing to the formation of the main chain of the polymer has been removed. For example, a structural unit derived from styrene includes the chemical structure of the portion of styrene from which the ethylenically unsaturated bond has been removed (i.e., the phenyl group). In this disclosure, the chemical structure of a "structural unit derived from a monomer having an ethylenically unsaturated bond" may differ from the chemical structure of the monomer used in the synthesis of the polymer. For example, when a polymer obtained using vinyl acetate as the monomer is saponified, the chemical structure of the structural unit derived from vinyl acetate changes to the chemical structure of vinyl alcohol. In this case, the structural unit is considered to be a "structural unit derived from vinyl alcohol."

[0015] (Swelling rate with respect to ethylene carbonate) The swelling rate of the binder polymer with respect to ethylene carbonate is not particularly limited as long as it is 6% or less. From the viewpoint of more effectively suppressing battery swelling, the swelling rate of the binder polymer with respect to ethylene carbonate is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. The lower limit of the swelling rate of the binder polymer with respect to ethylene carbonate may be 0%, 1%, or 2%.

[0016] In this disclosure, the swelling rate of the binder polymer with respect to ethylene carbonate (EC) (hereinafter also referred to as the EC swelling rate) is measured by performing the following test. A test specimen (circular, 16 mm in diameter, 0.8 mm thick) made of the binder polymer is prepared. This test specimen is immersed in EC (4 g) heated to 60°C and left to stand at 60°C for 24 hours. After that, the test specimen is removed from the EC and the EC adhering to the test specimen is thoroughly wiped off. The mass of the test specimen after immersion (W1) is measured. Next, the test specimen is vacuum dried (100°C, 24 hours) and the mass of the test specimen after drying (W2) is measured. The EC swelling rate is calculated using the following formula: EC swelling rate (%) = 100 × (W1 - W2) / W2

[0017] (Styrene Compounds) The binder polymer of this disclosure is a monomer polymer containing a styrene compound. By using a styrene compound as the monomer of the polymer, a binder polymer with excellent durability and dispersibility of electrode active materials can be obtained.

[0018] In the present disclosure, examples of the styrene compound used as a monomer constituting the polymer include styrene or a styrene derivative. Examples of the styrene derivative include compounds in which a substituent (excluding a vinyl group) is bonded to the benzene ring contained in α-methylstyrene and styrene. Examples of the substituent bonded to the benzene ring contained in styrene include an alkyl group, an alkoxy group, an acetyl group, an acetoxy group, an amino group, a halogen atom, and the like. As the substituent bonded to the benzene ring, an alkyl group is preferable, an alkyl group having 1 to 5 carbon atoms is more preferable, and a methyl group is even more preferable. Specific examples of the styrene derivative include t-butylstyrene, methylstyrene, chlorostyrene, fluorostyrene, acetoxystyrene, aminostyrene, and the like. The bonding position of the substituent on the benzene ring may be any of the para-position, meta-position, or ortho-position. The number of substituents on the benzene ring is not particularly limited, and is preferably 1 to 3, and more preferably 1.

[0019] In the present disclosure, compounds forming salts such as sodium p-styrenesulfonate and compounds having a plurality of vinyl groups such as divinylbenzene are not regarded as "styrene compounds".

[0020] In the present disclosure, the styrene compound contained in the monomer constituting the polymer may be only one kind or two or more kinds. From the viewpoint of more effectively suppressing battery swelling, the styrene compound contained in the monomer having an ethylenic unsaturated bond preferably contains styrene.

[0021] From the viewpoint of the balance of the properties as a binder for a non-aqueous secondary battery, the proportion of the styrene compound in all the monomers of the polymer is preferably 20 mol% or more, more preferably 30 mol% or more, and even more preferably 40 mol% or more.

[0022] From the perspective of the balance of properties as a binder for non-aqueous secondary batteries, the proportion of the styrene compound in all the monomers of the polymer is preferably 80 mol% or less, more preferably 70 mol% or less, and even more preferably 60 mol% or less.

[0023] (Alkyl (meth)acrylate) The binder polymer of the present disclosure is a polymer of a monomer containing alkyl (meth)acrylate. By using alkyl (meth)acrylate as the monomer of the polymer, a binder polymer with a low swelling ratio with respect to ethylene carbonate can be obtained.

[0024] In the present disclosure, the alkyl (meth)acrylate contained in the monomer may be only one kind or two or more kinds. From the perspective of reducing the swelling ratio of the binder polymer with respect to ethylene carbonate, the alkyl (meth)acrylate preferably contains a monofunctional alkyl (meth)acrylate having an alkyl group with 8 or more carbon atoms, and more preferably contains a monofunctional alkyl (meth)acrylate that satisfies the following condition A or condition B. Condition A: A methacrylate having an alkyl group with 8 to 14 carbon atoms Condition B: An acrylate or methacrylate having an alkyl group with 15 or more carbon atoms

[0025] Hereinafter, the alkyl (meth)acrylate that satisfies condition A is also referred to as alkyl (meth)acrylate A, and the alkyl (meth)acrylate that satisfies condition B is also referred to as alkyl (meth)acrylate B.

[0026] Alkyl (meth)acrylate A is a compound having a structure in which R Ａ１ is a methyl group and R Ａ２ is an alkyl group having 8 to 14 carbon atoms. Alkyl (meth)acrylate B is a compound having a structure in which R Ａ１ is a hydrogen atom or a methyl group and R Ａ２ is an alkyl group having 15 or more carbon atoms.

[0027]

[0028] The structure of the alkyl group in alkyl (meth)acrylate A is not particularly limited as long as it has 8 to 14 carbon atoms. The alkyl group having 8 to 14 carbon atoms may be acyclic or cyclic, and is preferably acyclic. The acyclic alkyl group may or may not have branches. The alkyl group having 8 to 14 carbon atoms may or may not have a double bond, and is preferably not having a double bond. The alkyl group having 8 to 14 carbon atoms may or may not have substituents, and is preferably not having substituents.

[0029] Specific examples of alkyl groups having 4 to 14 carbon atoms include 2-ethylhexyl group (8 carbon atoms), n-octyl group (8 carbon atoms), isooctyl group (8 carbon atoms), n-nonyl group (9 carbon atoms), isononyl group (9 carbon atoms), isobornyl group (9 carbon atoms), adamantyl group (10 carbon atoms), n-decyl group (10 carbon atoms), isodecyl group (10 carbon atoms), n-undecyl group (11 carbon atoms), isoundecyl group (11 carbon atoms), dodecyl (lauryl) group (12 carbon atoms), tetradecyl group (14 carbon atoms), and the like.

[0030] The alkyl group of alkyl (meth)acrylate B is not particularly limited as long as it has 15 or more carbon atoms. The number of carbon atoms in the alkyl group of alkyl (meth)acrylate B may be 25 or less, 20 or less, or 18 or less. The alkyl group with 15 or more carbon atoms may be acyclic or cyclic, and is preferably acyclic. The acyclic alkyl group may or may not have branches. The alkyl group with 15 or more carbon atoms may or may not have a double bond, and is preferably not a double bond. The alkyl group with 15 or more carbon atoms may or may not have substituents, and is preferably not a substituent.

[0031] Specific examples of alkyl groups with 15 or more carbon atoms include the hexadecyl group (16 carbon atoms), the octadecyl (stearyl) group (18 carbon atoms), and the octadecyl (isostearyl) group (18 carbon atoms).

[0032] From the viewpoint of more effectively suppressing battery swelling, the alkyl (meth)acrylate contained in the monomer having an ethylenically unsaturated bond preferably contains alkyl (meth)acrylate B, more preferably contains alkyl (meth)acrylate B having 16 or more carbon atoms, even more preferably contains alkyl (meth)acrylate B having 17 or more carbon atoms, and even more preferably contains alkyl (meth)acrylate B having 19 or more carbon atoms.

[0033] From the viewpoint of reducing the swelling rate of the binder polymer with respect to ethylene carbonate, it is preferable that the alkyl (meth)acrylate includes an alkyl (meth)acrylate with an SP value of 9.1 or less.

[0034] In this disclosure, the SP value of a monomer is calculated using the following Fedors formula (unit: (cal / cm³) ３ ) ０．５ ) is Fedors' formula: δ ２ = ΣE / ΣV (where δ represents the SP value, E represents the evaporation energy, and V represents the molar volume)

[0035] In this disclosure, the SP value of a monomer is calculated based on the molecular structure of the monomer in the state in which it forms a polymer. That is, the SP value is calculated based on the molecular structure in which the double bonds contributing to the polymerization reaction of the monomer are replaced with single bonds. The method for calculating the SP value is described, for example, in the "Journal of the Adhesion Society of Japan," 1986, Vol. 22, p. 566.

[0036] From the viewpoint of more effectively suppressing battery swelling, the SP value of the specific alkyl (meth)acrylate is preferably 9.05 or less, more preferably 9.03 or less, and even more preferably 9.0 or less. From the viewpoint of polymerization stability, the SP value of the specific alkyl (meth)acrylate is preferably 7.5 or more, more preferably 8.0 or more, and even more preferably 8.5 or more.

[0037] Specific examples of specific alkyl (meth)acrylates include 2-ethylhexyl methacrylate (SP value: 9.04), lauryl methacrylate (SP value: 9.02), isostearyl acrylate (SP value: 8.92), stearyl methacrylate (SP value: 8.90), isobornyl methacrylate (SP value: 8.89), and others.

[0038] From the viewpoint of balancing the properties as a binder for non-aqueous secondary batteries, the proportion of alkyl (meth)acrylate in the total monomers of the polymer is preferably 20 mol% or more, more preferably 30 mol% or more, and even more preferably 40 mol% or more.

[0039] From the viewpoint of balancing the properties as a binder for non-aqueous secondary batteries, the proportion of alkyl (meth)acrylate in the total monomers of the polymer is preferably 80 mol% or less, more preferably 70 mol% or less, and even more preferably 60 mol% or less.

[0040] From the viewpoint of fully exhibiting the effect of suppressing battery swelling, the total proportion of styrene compound and alkyl (meth)acrylate in the total monomer of the polymer is preferably 70 mol% to 100 mol%, more preferably 80 mol% to 100 mol%, and even more preferably 85 mol% to 100 mol%.

[0041] From the viewpoint of more effectively suppressing battery swelling, it is preferable that the proportion of monofunctional alkyl (meth)acrylates having an alkyl group with 8 or more carbon atoms among the alkyl (meth)acrylates contained in the monomer is 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more.

[0042] (Other monomers) If necessary, the polymerization components of the binder polymer (monomers having ethylenically unsaturated bonds) may include other monomers in addition to styrene compounds and alkyl (meth)acrylates. The types of other monomers are not particularly limited and can be selected according to the desired properties of the binder polymer.

[0043] Other monomers include carboxylic acids having ethylenically unsaturated bonds, such as (meth)acrylic acid and itaconic acid; (meth)acrylamide; (meth)acrylonitrile; vinyl acetate; and styrene-related compounds that are not styrene compounds, such as sodium p-styrenesulfonate and divinylbenzene.

[0044] From the viewpoint of fully exhibiting the effect of suppressing battery swelling, the proportion of other monomers to the total monomers of the polymer is preferably less than 30 mol%, more preferably less than 20 mol%, and even more preferably less than 15 mol%. The proportion of other monomers to the total monomers of the polymer may be 1 mol% or more, 2 mol% or more, or 5 mol% or more.

[0045] [Method for synthesizing binder polymers] The method for synthesizing binder polymers is not particularly limited as long as monomer (a) as the polymerization component can be polymerized, and can be selected from known methods. A preferred example of a method for synthesizing binder polymers is a method in which monomer (a) is polymerized in the presence of an aqueous medium (b) (hereinafter also referred to as the emulsion polymerization method). By synthesizing binder polymers by the emulsion polymerization method, a binder polymer (emulsion) can be obtained in which particulate binder polymer is dispersed in an aqueous medium.

[0046] One method for synthesizing a binder polymer by emulsion polymerization involves supplying each component used for polymerization into a reaction vessel to induce a polymerization reaction of monomer (a). It is preferable to continuously supply monomer (a) and radical polymerization initiator (e) to the reaction vessel so that the concentrations of monomer (a) and radical polymerization initiator (e) in the reaction vessel become uniform. It is preferable to carry out emulsion polymerization while stirring the reaction vessel. The temperature inside the reaction vessel is not particularly limited and may be, for example, 30°C to 90°C, 50°C to 85°C, or 55°C to 80°C.

[0047] When producing binder polymers by emulsion polymerization, in addition to monomers (a) and aqueous media (b) as polymerization components, components such as surfactants (c), basic substances (d), radical polymerization initiators (e), and chain transfer agents (f) can be used.

[0048] The aqueous medium (b) is preferably at least one selected from the group consisting of water and hydrophilic organic solvents. From the viewpoint of polymerization stability, the aqueous medium (b) preferably contains water, and more preferably is water. The aqueous medium (b) may also be a mixture of water and a hydrophilic solvent. Specific examples of hydrophilic organic solvents include polar solvents such as methanol, ethanol, isopropyl alcohol, and N-methylpyrrolidone. The hydrophilic organic solvent (c) used in the synthesis of the binder polymer may be one type or two or more types.

[0049] When synthesizing a binder polymer by emulsion polymerization, a surfactant (c) may be added to the monomer (a) before polymerization or to the aqueous medium (b) containing the binder polymer after polymerization. The surfactant (c) contributes to improving the dispersion stability of the binder polymer. It is preferable to use an anionic surfactant or a nonionic surfactant as the surfactant (c). It is preferable that the surfactant (c) does not have ethylenically unsaturated bonds (i.e., does not exhibit polymerizability).

[0050] Examples of anionic surfactants include alkylbenzene sulfonates, alkyl sulfates, polyoxyethylene alkyl ether sulfates, and fatty acid salts. Examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polycyclic phenyl ethers, polyoxyalkylene alkyl ethers, sorbitan fatty acid esters, and polyoxyethylene sorbitan fatty acid esters. The surfactant (c) used in the synthesis of the binder polymer may be one type or two or more types.

[0051] When synthesizing a binder polymer by emulsion polymerization, a basic substance (d) may be added to the aqueous medium (b) containing the monomer (a) before polymerization or the binder polymer after polymerization. The basic substance (d) neutralizes the acidic component if the monomer (a) contains an acidic component. As a result, the pH of the solution containing the binder polymer becomes within an appropriate range, and the stability of the solution is improved.

[0052] Examples of basic substances (d) include ammonia, triethylamine, sodium hydroxide, and lithium hydroxide. The basic substances (d) used in the synthesis of the binder polymer may be one or two or more.

[0053] When synthesizing binder polymers by emulsion polymerization, a radical polymerization initiator (e) may be added to the aqueous medium (b) containing the monomer (a) before polymerization. The radical polymerization initiator (e) promotes the polymerization reaction of monomer (a). Specific examples of radical polymerization initiators (e) include persulfates such as ammonium persulfate and potassium persulfate; hydrogen peroxide; azo compounds; and organic peroxides such as tert-butyl hydroperoxide, tert-butyl peroxybenzoate, and cumene hydroperoxide.

[0054] When synthesizing binder polymers by emulsion polymerization, redox polymerization may be carried out using a reducing agent together with a radical polymerization initiator (e). Specific examples of reducing agents include sodium bisulfite, rongalit (sodium formaldehyde sulfoxylate dihydrate), and ascorbic acid.

[0055] The amount of radical polymerization initiator (e) added (including the reducing agent if used in combination) is preferably 0.001 parts by mass or more, more preferably 0.002 parts by mass or more, even more preferably 0.005 parts by mass or more, and particularly preferably 0.01 parts by mass or more, per 100 parts by mass of monomer (a). When the amount of radical polymerization initiator (e) added is within the above range, the conversion rate of monomer (a) to binder polymer tends to improve. The amount of radical polymerization initiator (e) added is preferably 10 parts by mass or less, more preferably 5.0 parts by mass or less, and even more preferably 2.0 parts by mass or less, per 100 parts by mass of monomer (a). When the amount of radical polymerization initiator (e) added is within the above range, the molecular weight of the binder polymer tends to increase, and the swelling rate of the binder polymer in the electrolyte tends to decrease. The amount of radical polymerization initiator (e) added may be 0.001 to 10 parts by mass, 0.002 to 5.0 parts by mass, 0.005 to 2.0 parts by mass, or 0.01 to 1.0 parts by mass per 100 parts by mass of monomer (a).

[0056] When synthesizing a binder polymer by emulsion polymerization, a chain transfer agent (f) may be added to an aqueous medium (b) containing the monomer (a) before polymerization. The chain transfer agent (f) adjusts the molecular weight of the binder polymer obtained by polymerization of monomer (a). Specific examples of chain transfer agents (f) include n-dodecyl mercaptan, tert-dodecyl mercaptan, n-butyl mercaptan, 2-ethylhexyl thioglycolate, 2-mercaptoethanol, β-mercaptopropionic acid, methyl alcohol, n-propyl alcohol, isopropyl alcohol, t-butyl alcohol, benzyl alcohol, α-methylstyrene dimer, and the like.

[0057] <Binder Composition> The binder composition of the present disclosure comprises the binder polymer of the present disclosure and an aqueous medium. The binder composition of the present disclosure is used, for example, in the manufacture of electrodes for non-aqueous secondary batteries.

[0058] The binder composition may contain other components along with the binder polymer and aqueous medium. For example, the binder composition may contain components used during the synthesis of the binder polymer, additives added after the synthesis of the binder polymer, and so on.

[0059] The binder polymer contained in the binder composition may undergo partial structural changes due to reactions with other components in the binder composition. In this case, the binder polymer in the state reacted with other components in the binder composition is also considered to be the binder polymer of this disclosure.

[0060] The binder composition is preferably in the form of an emulsion in which particles containing the binder polymer are dispersed in an aqueous medium. The particles containing the binder polymer may consist solely of the binder polymer, or they may contain the binder polymer along with other components (such as surfactants). The binder composition may contain both particles containing the binder polymer and particles that do not contain the binder polymer.

[0061] A binder composition in which particles containing a binder polymer are dispersed in an aqueous medium may contain particulate binder polymers synthesized by emulsion polymerization, or it may be obtained by dispersing binder polymers obtained by methods other than emulsion polymerization in an aqueous medium.

[0062] The aqueous medium included in the binder composition is preferably at least one selected from the group consisting of water and hydrophilic organic solvents, and is preferably water. The aqueous medium may be a mixture of water and a hydrophilic organic solvent. If the binder composition contains a hydrophilic organic solvent, the binder composition may contain only one or two or more types of hydrophilic organic solvents. The type of hydrophilic organic solvent in the binder composition is not particularly limited and may be selected from the hydrophilic organic solvents exemplified as the aqueous medium (b) used in the synthesis of the binder polymer.

[0063] The aqueous medium included in the binder composition may be the aqueous medium used during the synthesis of the binder polymer, a different aqueous solvent from the aqueous medium used during the synthesis of the binder polymer, or a combination thereof.

[0064] The non-volatile content concentration of the binder composition is not particularly limited and can be selected according to the method of use of the binder composition. From the viewpoint of increasing the amount of active ingredients contained in the binder composition, the non-volatile content concentration of the binder composition is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 30% by mass or more. From the viewpoint of adjusting the viscosity of the binder composition to a viscosity suitable for electrode fabrication, the non-volatile content concentration of the binder composition is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less.

[0065] In this disclosure, the nonvolatile content contained in the binder composition may be a liquid, a solid, or a combination thereof. The nonvolatile content concentration of the binder composition is calculated from the mass of the nonvolatile content measured by the following method: Weigh 1 g of the binder composition and place it on a 5 cm diameter aluminum dish and put it in a drying oven. Then, dry the binder composition while circulating the air in the drying oven (1 atmosphere (1013 hPa), 105°C, 1 hour). The mass of the substance remaining as a solid or liquid after drying is taken as the mass of the nonvolatile content.

[0066] <Electrode Slurry> The electrode slurry of the present disclosure comprises the binder polymer of the present disclosure, an electrode active material, and an aqueous medium. The electrode slurry of the present disclosure is used, for example, to prepare electrodes for non-aqueous secondary batteries.

[0067] The electrode slurry may contain other components along with the binder polymer, electrode active material, and aqueous medium. For example, the electrode slurry may contain components used in the synthesis of the binder polymer, thickeners, conductive additives, etc.

[0068] The binder polymer contained in the electrode slurry may undergo partial structural changes due to reactions with other components in the electrode slurry. In this case, the binder polymer in the state reacted with other components in the electrode slurry is also considered to be the binder polymer of this disclosure.

[0069] The electrode slurry is preferably in which the binder polymer and electrode active material are dispersed in an aqueous medium.

[0070] The aqueous medium contained in the electrode slurry may be the aqueous medium used during the synthesis of the binder polymer, a different aqueous solvent from the aqueous medium used during the synthesis of the binder polymer, or a combination thereof. Examples of aqueous solvents contained in the electrode slurry include water, hydrophilic organic solvents, and combinations thereof. The type of hydrophilic organic solvent is not particularly limited and may be selected from the hydrophilic organic solvents exemplified as the aqueous medium (b) used in the synthesis of the binder polymer.

[0071] The binder polymer content in the electrode slurry is preferably 0.5 parts by mass or more, and more preferably 1.0 part by mass or more, per 100 parts by mass of the electrode active material. When the binder polymer content is within the above range, the effects of including the binder polymer tend to be fully expressed.

[0072] The binder polymer content in the electrode slurry is preferably 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, and even more preferably 3.0 parts by mass or less, per 100 parts by mass of electrode active material. When the binder polymer content is within the above range, the content of electrode active material in the electrode slurry is sufficiently ensured, and the electrode obtained using the electrode slurry tends to have excellent charge-discharge characteristics.

[0073] The electrode active material contained in the electrode slurry is a material in which ions serving as charge carriers such as lithium ions can be inserted (Intercalation) and desorbed (Deintercalation). The ions serving as charge carriers are preferably alkali metal ions, more preferably lithium ions, sodium ions or potassium ions, and even more preferably lithium ions. The electrode active material contained in the electrode slurry may be only one kind or two or more kinds.

[0074] When the electrode manufactured using the electrode slurry is a negative electrode, the electrode slurry contains a negative electrode active material as the electrode active material. As the negative electrode active material, it is preferable to contain at least one selected from the group consisting of a carbon material, a material containing silicon, and a material containing titanium.

[0075] Examples of the carbon material used as the negative electrode active material include cokes such as petroleum coke, pitch coke, and coal coke, carbonized products of organic polymers, and graphites such as artificial graphite and natural graphite. Examples of the material containing silicon used as the negative electrode active material include elemental silicon and silicon compounds such as silicon oxide. Examples of the material containing titanium used as the negative electrode active material include lithium titanate and the like.

[0076] When the electrode for a non-aqueous secondary battery manufactured using the electrode slurry is a positive electrode, the electrode active material is a positive electrode active material. As the positive electrode active material, a material having a more noble standard electrode potential than the negative electrode active material is used. Specifically, as the positive electrode active material, lithium composite oxides containing nickel such as Ni-Co-Mn-based lithium composite oxides, Ni-Mn-Al-based lithium composite oxides, and Ni-Co-Al-based lithium composite oxides, lithium cobaltate (LiCoO ２ ), spinel-type lithium manganate (LiMn ２ O ４ ), olivine-type lithium iron phosphate, TiS ２ , MnO ２ , MoO ３ , V ２ O ５Examples include chalcogen compounds such as those mentioned above. These substances used as positive electrode active materials may be used individually or in combination of two or more.

[0077] The electrode slurry preferably contains at least one electrode active material selected from the group consisting of carbon materials and silicon-containing materials. Compared to other electrode active materials, carbon materials and silicon-containing materials tend to have larger volume changes during ion insertion and deinsertion, and repeated charging and discharging of the battery tends to reduce the bonding between electrode active materials or between the electrode active materials and the current collector. Therefore, if the electrode slurry contains these electrode active materials, the effect of maintaining good bonding between electrode active materials and between the electrode active materials and the current collector by the binder polymer is more effectively exhibited.

[0078] The electrode slurry may contain a thickening agent. Specific examples of thickening agents include cellulose derivatives such as carboxymethylcellulose, hydroxyethylcellulose, and hydroxypropylcellulose; ammonium salts of cellulose derivatives; alkali metal salts of cellulose derivatives; polyvinyl alcohol; polyvinylpyrrolidone; poly(meth)acrylates; poly(meth)acrylamide; and poly(meth)N-hydroxyalkylacrylamide.

[0079] The electrode slurry may contain a conductive additive. Specific examples of conductive additives include conductive substances such as carbon black and carbon fibers.

[0080] [Method for manufacturing electrode slurry] The method for manufacturing the electrode slurry of this disclosure is not particularly limited. For example, the electrode slurry may be manufactured by mixing the components contained in the electrode slurry by known methods.

[0081] <Electrode> The electrode of the present disclosure comprises the binder polymer of the present disclosure and an electrode active material. The electrode of the present disclosure is used, for example, as an electrode in a non-aqueous secondary battery.

[0082] The electrode of this disclosure may comprise a current collector and an electrode active material layer. In this case, the electrode active material layer contains a binder polymer together with the electrode active material.

[0083] In the electrode of this disclosure, the electrode active material layer is in contact with at least a portion of the surface of the current collector. The electrode active material layer may be formed on the entire surface of the current collector, or on only a portion of the surface of the current collector. If the current collector is in the shape of a sheet or the like, the electrode active material layer may be formed on both sides of the current collector, or on only one side. In this disclosure, "sheet-like" means the shape of an object (including plates, foils, films, membranes, etc.) in which the thickness dimension relative to the main surface is relatively small. The thickness of a sheet-like current collector may be, for example, in the range of 0.001 mm to 0.5 mm.

[0084] The current collector preferably contains a metal. Examples of metals include iron, copper, aluminum, nickel, and stainless steel. When the electrode of this disclosure is the negative electrode, the current collector may contain copper. When the electrode of this disclosure is the positive electrode, the current collector may contain aluminum.

[0085] The electrode active material layer comprises the binder polymer and electrode active material of this disclosure. The electrode active material layer may also contain components other than the binder polymer and electrode active material. For example, the electrode active material layer may contain conductive additives, thickeners, etc. The electrode active material, conductive additives, and thickeners contained in the electrode active material layer may be selected from the electrode active material, conductive additives, and thickeners that can be contained in the electrode slurry described above.

[0086] The binder polymer contained in the electrode may undergo partial structural changes due to reactions with other components in the electrode. In this case, the binder polymer in the state reacted with other components in the electrode is also considered to be the binder polymer of this disclosure.

[0087] The method for manufacturing the electrodes of this disclosure is not particularly limited and can be selected according to the type of non-aqueous secondary battery to which the electrodes are applied. The electrodes of this disclosure are manufactured, for example, by applying the slurry of this disclosure described above to both sides or one side of a sheet-like current collector, and then removing volatile components such as aqueous solvents from the electrode slurry to form an electrode active material layer. The method for applying the electrode slurry is not particularly limited and can be selected from known methods. If necessary, a pressing process may be performed to increase the density of the electrode active material layer formed on both sides or one side of the current collector. The method for performing the pressing process is not particularly limited and can be selected from known methods.

[0088] <Non-aqueous secondary battery> The non-aqueous secondary battery of this disclosure comprises the electrodes of this disclosure. The non-aqueous secondary battery includes, for example, a positive electrode, a negative electrode, and an electrolyte. The non-aqueous secondary battery may include a separator between the positive electrode and the negative electrode, or other components.

[0089] In the non-aqueous secondary battery of the present disclosure, it is preferable that at least one of the positive electrode and the negative electrode is an electrode of the present disclosure, and at least the negative electrode is an electrode of the present disclosure. In the non-aqueous secondary battery of the present disclosure, if either the positive electrode or the negative electrode is an electrode of the present disclosure, the other electrode may contain a binder different from the binder polymer of the present disclosure.

[0090] For non-aqueous secondary batteries, it is preferable to use a non-aqueous liquid with ionic conductivity as the electrolyte. Specific examples of electrolytes include solutions obtained by dissolving the electrolyte in an organic solvent, and ionic liquids.

[0091] The organic solvent used to dissolve the electrolyte is not particularly limited and includes carbonate ester compounds such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC), as well as carboxylic acid esters such as ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The organic solvent may be used alone or in combination of two or more. Among these, it is preferable to use two or more linear carbonate solvents in combination as the organic solvent.

[0092] Specifically, the electrolyte in the case where the non-aqueous secondary battery of this disclosure is a lithium-ion secondary battery is LiClO ４ LiBF ６ LiPF ６ LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiSbF ６ LiB １０ Cl １０ LiAlCl ４ , LiCl, LiBr, LiB(C ２ H ５ ) ４ CF ３ SO ３ Li, CH ３ SO ３ Li, LiCF ３ SO ３ LiC ４ F ９ SO ３ Li(CF ３ SO ２ ) ２ Examples include nitrogen (N) and lithium aliphatic carboxylate.

[0093] The electrolyte may contain additives. The type of additive is not particularly limited and can be selected according to its purpose. Specific examples of additives for the electrolyte include nitrile compounds, sulfur-containing compounds, and boron-containing compounds. Examples of nitrile compounds include succinonitrile and acetonitrile. Examples of sulfur-containing compounds include methyl ethyl sulfone and 1,3-propanesultone, which are compounds having a sulfonyl group, a sulfonate group, or a sultone structure. Examples of boron-containing compounds include boric acid esters.

[0094] Non-aqueous secondary batteries may have a positive electrode, a negative electrode, and an electrolyte housed in an outer casing. The type of outer casing is not particularly limited and can be selected according to the type of non-aqueous secondary battery. In one embodiment, an aluminum laminate material consisting of aluminum foil and a resin film may be used as the outer casing. Since the non-aqueous secondary battery of this disclosure suppresses battery swelling, it is also suitable when using an outer casing that is easily deformable, such as an aluminum laminate material.

[0095] Examples of embodiments of this disclosure will be described below with reference to these examples, but this disclosure is not limited to these embodiments.

[0096] In the following embodiments, the binder polymer of the present disclosure is used as a binder for the negative electrode of a lithium-ion secondary battery. In the following embodiments, deionized water is used as the water.

[0097] (Synthesis of Binder Polymer) The monomers shown in Table 1 were mixed in the proportions (mol%) shown in Table 1 to obtain a monomer mixture. This mixture (395 parts by mass), Aqualon KH-10 (1.7 parts by mass) manufactured by Daiichi Kogyo Seiyaku Co., Ltd. as a surfactant, and water (212 parts by mass) were mixed to obtain an emulsion. Next, aqueous solutions were prepared by dissolving potassium persulfate (1.6 parts by mass) as a polymerization initiator and Longalit SFS (0.6 parts by mass) manufactured by Sumitomo Seika Co., Ltd. as a reducing agent in 50 parts by mass of water. 150 parts by mass of water were placed in a separable flask equipped with a condenser, thermometer, stirrer, and dropping funnel, and the temperature was raised to 80°C. Emulsion polymerization was carried out by continuously supplying the emulsion and the aqueous solutions of the polymerization initiator and reducing agent to this separable flask while stirring at 80°C for 3 hours. Through the above steps, an emulsion was obtained in which particles containing the monomer polymers shown in Table 1 were dispersed in water. The resulting emulsion was cooled to room temperature (25°C), and the pH was adjusted with ammonia water.

[0098] (Calculation of SP value of alkyl (meth)acrylate) The SP value of the alkyl (meth)acrylate used in the synthesis of the binder polymer was calculated using Fedors' formula shown below. The results are shown in Table 1. Fedors' formula: δ ２ = ΣE / ΣV (wherein δ is the SP value, E is the evaporation energy, and V is the molar volume) In this disclosure, the SP value of a monomer is calculated based on the molecular structure of the monomer in the state in which it forms a polymer. That is, the SP value (unit: (cal / cm)) is calculated based on the molecular structure in which the double bonds contributing to the polymerization reaction of the monomer are replaced with single bonds. ３ ) ０．５ ) is calculated. The method for calculating the SP value is described, for example, in the "Journal of the Japan Adhesion Association," 1986, Vol. 22, page 566.

[0099] (Measurement of EC swelling rate) The swelling rate of the binder polymer relative to ethylene carbonate (EC swelling rate) was measured by the method described below. The emulsion (4 g) obtained by the method described above was placed in a circular container with a diameter of 60 mm and left to stand at 50°C for 6 hours, and then vacuum dried (60°C, 12 hours) to form a film containing the binder polymer. A circular test specimen with a diameter of 16 mm was prepared from the obtained film. This test specimen was immersed in EC (4 g) that had been heated to 60°C and melted, and left to stand at 60°C for 24 hours. After that, the test specimen was removed from the EC and the EC adhering to the test specimen was thoroughly wiped off. The mass of the test specimen after immersion (W1) was measured. Next, the test specimen was vacuum dried (100°C, 24 hours), and the mass of the test specimen after drying (W2) was measured. The EC swelling rate was calculated using the following formula. The results are shown in Table 1. EC swelling rate (%) = 100 × (W1 - W2) / W2

[0100] (Measurement of EMC swelling rate) The swelling rate of the binder polymer relative to ethyl methyl carbonate (EMC swelling rate) was measured in the same manner as the EC swelling rate measurement method, except that ethylene carbonate was changed to ethyl methyl carbonate and the immersion temperature of the test specimen was changed to 23°C. The results are shown in Table 1.

[0101] (Fabrication of lithium-ion secondary batteries) Using the emulsion obtained by the method described above, lithium-ion secondary batteries were fabricated by the method shown below.

[0102] LiNi as a positive electrode active material ０．６ Mn ０．２ Co ０．２ O ２ (94 parts by mass) was mixed with acetylene black (3 parts by mass) as a conductive additive and polyvinylidene fluoride (3 parts by mass) as a binder to obtain a mixture. N-methylpyrrolidone (50 parts by mass) was added to the obtained mixture and mixed further to obtain a positive electrode slurry.

[0103] A 15 μm thick aluminum foil was prepared as the positive electrode current collector. The positive electrode slurry was applied to both sides of the positive electrode current collector using the direct roll method. The positive electrode slurry applied to both sides of the positive electrode current collector was dried (120°C, 5 minutes), and roll pressing (5 t / cm) was performed to obtain a laminate in which a positive electrode layer with a thickness of 125 μm per side was formed on both sides of the positive electrode current collector. This laminate was cut into a rectangle with dimensions of 50 mm in length and 40 mm in width, and conductive tabs were attached to obtain the positive electrode.

[0104] The emulsion obtained by the method described above (3.6 parts by mass, non-volatile content: 1.4 parts by mass) was mixed with artificial graphite (G49, manufactured by Jiangxi Zichen Technology Co., Ltd.) (96.9 parts by mass) as a negative electrode active material and a 2% by mass aqueous solution (60 parts by mass) of sodium salt of CMC (manufactured by Nippon Paper Chemical Co., Ltd., Sunrose MAC500LC) to obtain a mixture. Water (16 parts by mass) was added to this mixture and mixed further to obtain a negative electrode slurry.

[0105] A copper foil with a thickness of 10 μm was prepared as the negative electrode current collector. Negative electrode slurry was applied to both sides of the negative electrode current collector using the direct roll method. The negative electrode slurry applied to both sides of the negative electrode current collector was dried (90°C, 10 minutes), and roll pressing (8 t / cm) was performed to obtain a laminate in which a negative electrode layer with a thickness of 170 μm per side was formed on both sides of the negative electrode current collector. This laminate was cut into a rectangle with dimensions of 52 mm in length and 42 mm in width, and conductive tabs were attached to obtain the negative electrode.

[0106] A laminate (layer structure: negative electrode / separator / positive electrode / separator / negative electrode) was fabricated using one positive electrode and two negative electrodes obtained by the method described above, along with a separator (porous polyethylene film, 25 μm thick). This laminate was housed in an outer casing (dimensions: 90 mm x 70 mm) made of aluminum laminate. Subsequently, an electrolyte was poured into the outer casing, vacuum impregnation was performed, and the assembly was packed using a vacuum heat sealer to obtain a lithium-ion secondary battery. As the electrolyte, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of EC:EMC:DEC = 30:50:20 was mixed with LiPF at a concentration of 1.0 mol / L. ６A mixture of a solution containing (99 parts by mass) and vinylene carbonate (1 part by mass) was used.

[0107] (Evaluation of Battery Swelling) Using the lithium-ion secondary battery obtained by the method described above, the battery swelling was evaluated using the method shown below. The results are shown in Table 1. The volume (V1) of the lithium-ion secondary battery obtained by the method described above in a fully charged state was measured. Next, this lithium-ion secondary battery was stored at 60°C for 4 weeks, and the volume (V2) of the lithium-ion secondary battery in a fully charged state after storage was measured. The volume of the lithium-ion secondary battery was measured using Method A (water displacement method) described in JIS K7112:1999 (ISO 1183). The measurement was performed at 23°C using an electronic hydrometer (Alpha Mirage Co., Ltd., MDS-300). From the measured volume of the lithium-ion secondary battery, the battery swelling (unit: cm) was calculated using the following formula. ３ ) was calculated. Battery swelling = V2 - V1

[0108]

[0109] The abbreviations shown in Table 1 represent the following compounds, respectively: St: Styrene 2EHMA: 2-ethylhexyl methacrylate (number of carbon atoms in the alkyl group: 8) LMA: Lauryl methacrylate (number of carbon atoms in the alkyl group: 12) ISTA: Isostearyl acrylate (number of carbon atoms in the alkyl group: 18) 2EHA: 2-ethylhexyl acrylate (number of carbon atoms in the alkyl group: 8) nOcA: n-octyl acrylate (number of carbon atoms in the alkyl group: 8) LA: Lauryl acrylate (number of carbon atoms in the alkyl group: 12)

[0110] The other monomers shown in Table 1 are mixtures of the following monomers: 2-hydroxyethyl methacrylate: 7.5 parts by mass Divinylbenzene: 0.3 parts by mass Itaconic acid: 7.5 parts by mass Acrylic acid: 15.0 parts by mass Sodium p-styrenesulfonate: 2.5 parts by mass

[0111] As shown in Table 1, lithium-ion batteries prepared using the binder polymer of the Examples, which has a swelling rate of 6% or less with respect to ethylene carbonate, exhibited less battery swelling compared to lithium-ion batteries prepared using the binder polymer of the Comparative Example, which has a swelling rate of more than 6% with respect to ethylene carbonate. Furthermore, as shown in the results of Comparative Example 3, battery swelling is not necessarily suppressed even when using a binder polymer with a low swelling rate with respect to ethyl methyl carbonate. These results suggest that using a binder polymer with a low swelling rate with respect to ethylene carbonate is an effective means of suppressing battery swelling in non-aqueous secondary batteries.

[0112] The disclosure of Japanese Patent Application No. 2024-231066 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

Claims

1. A binder polymer comprising monomers having ethylenically unsaturated bonds, wherein the monomers comprise a styrene compound and an alkyl (meth)acrylate, and the swelling rate relative to ethylene carbonate is 6% or less.

2. The binder polymer according to claim 1, wherein the proportion of the styrene compound in the total monomer is 20 mol% or more and 80 mol% or less.

3. The binder polymer according to claim 1, wherein the proportion of the alkyl (meth)acrylate in the total monomer is 20 mol% or more and 80 mol% or less.

4. The binder polymer according to claim 1, for use as a component of electrodes for a non-aqueous secondary battery.

5. A binder composition comprising a binder polymer according to any one of claims 1 to 4 and an aqueous medium.

6. The binder composition according to claim 5, wherein particles containing the binder polymer are dispersed in the aqueous medium.

7. An electrode slurry comprising a binder polymer according to any one of claims 1 to 4, an electrode active material, and an aqueous medium.

8. An electrode comprising a binder polymer according to any one of claims 1 to 4 and an electrode active material.

9. A non-aqueous secondary battery comprising the electrode described in claim 8.

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