Composition for forming electrode, additive, and gelling inhibitor

Abstract

This composition for forming an electrode contains a hetero ring-containing compound that is solid at room temperature, an acid (excluding acids corresponding to the heterocycle-containing compound), a positive electrode active material, a binder, and a solvent.

Description

Electrode-forming composition, additive, and gelation inhibitor

[0001] The present invention relates to an electrode-forming composition, an additive, and a gelation inhibitor. The present invention also relates to an electrode layer and a secondary battery.

[0002] Lithium-ion secondary batteries have a high energy density per unit weight and volume, which contributes to the miniaturization and weight reduction of electronic devices. In recent years, the spread of electric vehicles has accelerated as part of efforts to achieve zero-emissions automobiles, and there is a demand for batteries with even lower resistance, longer life, higher capacity, safety, and lower cost.

[0003] Lithium-ion secondary batteries generally have a three-layer structure consisting of a positive electrode, a separator, and a negative electrode, each containing an electrolyte. The positive electrode and the negative electrode are manufactured, for example, by coating a current collector with an electrode slurry, which is a mixture of an active material, a conductive material, and a binder. Currently, the mainstream method for manufacturing a negative electrode is to coat a copper foil current collector with the negative electrode slurry and dry it, while the mainstream method for manufacturing a positive electrode is to prepare a positive electrode slurry using an organic solvent such as N-methyl-2-pyrrolidone as a solvent, and then coat the resulting positive electrode slurry on an aluminum foil current collector.

[0004] As positive electrode active materials for lithium ion secondary batteries, inorganic compounds such as transition metal oxides and transition metal chalcogens containing alkali metals are known as materials capable of obtaining a battery voltage of around 4 V. Among these, highly alkaline positive electrode active materials containing large amounts of nickel and manganese are used in order to obtain high-capacity lithium ion secondary batteries.

[0005] For example, Li ｘ NiO ２ High nickel positive electrode active materials, such as those listed above, have a high discharge capacity and are attractive positive electrode materials. However, on the surface, there are proton exchange reactions with the residue of the raw material or moisture, and LiOH and Li ２ O, LiHCO ３ , Li ２ CO ３ There are alkaline components such as

[0006] When such a positive electrode active material is used, the electrode slurry may thicken or gel, gradually losing its fluidity, which not only makes it difficult to achieve a uniform coating thickness but also makes it impossible to apply the electrode slurry, resulting in waste of material.

[0007] The main cause of this is thought to be that, in the process of producing the positive electrode, alkaline components present on the surface of the positive electrode active material, in the presence of a trace amount of moisture, promote the dehydrofluorination reaction of the fluorine-based binder, such as polyvinylidene fluoride (PVdF) having a vinylidene fluoride structure, used as a binder.

[0008] Furthermore, alkaline components corrode the aluminum foil commonly used as a current collector for the positive electrode, thereby increasing the resistance of the battery, and may also react with the electrolyte in the battery, increasing the resistance of the battery and shortening its lifespan.

[0009] The thickening and gelation described above can be suppressed by handling the raw materials and the electrode slurry in a dry environment and controlling the water content. However, this requires large-scale facilities for a series of mass production processes from the preparation of the electrode slurry to the manufacture of the battery, and also poses problems of increased costs and increased environmental load due to the use of large amounts of electricity.

[0010] To solve this problem, for example, Patent Document 1 discloses a technique for suppressing gelation of an electrode slurry by preparing an electrode slurry (cathode material slurry) so that it does not exhibit strong alkalinity even when dispersed in water. However, preparing an electrode slurry so that it does not exhibit strong alkalinity using the method described in Patent Document 1 not only requires strict pH control, but also requires a process in which the cathode active material is first dispersed in water, filtered from the dispersion to extract the cathode active material, and then dried. As a result, this process leads to cumbersome operations and reduced yield. Furthermore, such a process may cause a decrease in the performance of the cathode active material itself.

[0011] Furthermore, Patent Document 2 reports a technology that uses a compound such as ultra-high molecular weight (weight average molecular weight of 2.2 million or more) polyethylene oxide to bind water through interactions with water (e.g., hydrogen bonding), thereby suppressing the reaction between the alkaline component of the positive electrode active material and water, thereby suppressing thickening and gelation. However, ultra-high molecular weight polymers with strong thickening effects have handling issues, such as the time and cost required for uniform dissolution in a solvent and the difficulty of producing a high-concentration solution. Furthermore, because the above-mentioned ultra-high molecular weight polymers have a high water-binding ability, there is a concern that the polymer itself may bring in water, and to prevent this, strict control over pre-drying is required.

[0012] Patent Document 3 reports a method of treating a positive electrode active material with fluorine gas and immobilizing the remaining LiOH as LiF, thereby preventing gelation and suppressing gas generation. However, fluorine gas is highly toxic and difficult to handle, and LiF produced as a by-product increases the internal resistance of the battery, reducing capacity. Furthermore, corrosion of the positive electrode active material by fluorine gas also reduces capacity. Furthermore, there is a problem in that the residual fluorine reacts with traces of moisture present in the active material and electrolyte to produce hydrogen fluoride, which easily causes cycle deterioration.

[0013] Patent Document 4 reports that unreacted lithium hydroxide and impurities derived from raw materials are removed by washing with an aqueous solution containing a lithium salt. However, there are issues with this method, such as increased environmental load due to wastewater generated during washing and the cost associated with treating the wastewater.

[0014] Patent Documents 5 to 8 propose adding an organic acid or inorganic acid to the positive electrode of a lithium ion secondary battery to suppress gelation of the electrode slurry (positive electrode mixture slurry). Patent Document 5 uses maleic acid, citraconic acid, and malonic acid in the positive electrode mixture, Patent Document 6 uses oxalic acid and succinic acid in the electrode mixture, Patent Document 7 uses tartaric acid or the like in the electrode mixture, and Patent Document 8 uses acetic acid, phosphoric acid, sulfuric acid, or the like in the electrode slurry (positive electrode paste).

[0015] JP 2000-90917 A JP 2019-121471 A JP 2006-286240 A International Publication No. 2017 / 034001 JP 9-306502 A JP 2005-11594 A JP 2022-173106 A JP 10-79244 A

[0016] However, when the present inventors investigated the suppression of gelation using organic or inorganic acids, they found that Li ｘ NiO ２ In electrode slurries containing high-nickel positive electrode active materials, such as those listed above, gelation has not been sufficiently suppressed even when organic or inorganic acids are used.

[0017] In view of the above, an object of the present invention is to provide an electrode-forming composition that is capable of suppressing thickening and gelation and improving storage stability by a simple method, as well as an additive and a gelation inhibitor for the electrode-forming composition. Another object of the present invention is to provide an electrode layer and a secondary battery that use the electrode-forming composition.

[0018] The present inventors have conducted extensive research to solve the above problems, and as a result have found that the above problems can be solved, and have completed the present invention having the following gist.

[0019] That is, the present invention encompasses the following: [1] An electrode-forming composition comprising a heterocycle-containing compound that is solid at room temperature, an acid (excluding acids that fall under the category of the heterocycle-containing compound), a positive electrode active material, a binder, and a solvent. [2] The electrode-forming composition according to [1], wherein the heterocycle-containing compound has a nitrogen-containing five-membered ring and the nitrogen-containing five-membered ring does not contain an oxygen atom. [3] The electrode-forming composition according to [1], wherein the acid is an organic acid. [4] The electrode-forming composition according to [1], wherein the acid is an inorganic acid. [5] The electrode-forming composition according to any one of [2] to [4], wherein the only heteroatom contained in the nitrogen-containing five-membered ring is a nitrogen atom. [6] The electrode-forming composition according to any one of [2] to [5], wherein the number of nitrogen atoms contained in the nitrogen-containing five-membered ring is two or three. [7] The electrode-forming composition according to any one of [1] to [6], wherein the heterocycle-containing compound is a non-fused nitrogen-containing five-membered ring compound. [8] The electrode-forming composition according to any one of [1] to [4], wherein the heterocycle-containing compound is represented by any one of the following formulas (1) to (4): (In formula (1), R ａ and R ｂ are each independently a hydrogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂ may be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent, each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond, Z is N or C(-L-R ｃ ) and R ｃ is a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, and X ａare each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms. ａ ~R ｃ are each independently a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂ may be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent; each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond; X ａ and X ｂ are each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms. ａ ~R ｃ , X ａ and X ｂThe electrode-forming composition according to any one of [1] to [4], wherein the heterocycle-containing compound is represented by any one of the following formulas (3) to (4): (In formulas (3) to (4), R ａ ~R ｃ are each independently a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂ may be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent; each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond; X ａ and X ｂ are each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms. ａ ~R ｃ , X ａ and X ｂ

[12] The electrode-forming composition according to

[10] , wherein the heterocycle-containing compound is represented by the following formula (4b): (In formula (4b), R ａ ~R ｃ is the same as above, and R ｅ is a hydrogen atom, a carboxy group, a hydroxy group, an aldehyde group, an ester group, a ketone group, an amino group, a phenyl group, a halogen atom, an alkoxysilyl group, an epoxy group, a carboxylic acid chloride group, or a thiol group.)

[13] The electrode-forming composition according to any one of [1] to

[12] , further comprising a conductive additive.

[14] The positive electrode active material is lithium-containing transition metal oxide particles having a layered rock salt structure, and the lithium-containing transition metal oxide particles having the layered rock salt structure are represented by the general formula Li ａ Ni （１－ｘ－ｙ） Co ｘ M １ ｙ M ２ ｚ O ２ (In the formula, M １ is at least one selected from the group consisting of Mn and Al; ２represents at least one element selected from the group consisting of Zr, Ti, Mg, B, W, and V, and 1.00≦a≦1.50, 0.00≦x≦0.50, 0.00≦y≦0.50, and 0.000≦z≦0.020, with the proviso that the Ni content is 30 mass% or more.

[15] The electrode-forming composition according to any one of [1] to

[14] , wherein the positive electrode active material includes a first positive electrode active material that is a polycrystalline body and a second positive electrode active material that is a single crystal.

[16] The electrode-forming composition according to any one of [1] to

[15] , further including a dispersant.

[17] The electrode-forming composition according to

[16] , wherein the dispersant is a polymer having a pyrrolidone structure or a nitrile group.

[18] The electrode-forming composition according to

[16] , wherein the dispersant is at least one selected from the group consisting of polyvinylpyrrolidone and polyacrylonitrile.

[19] The electrode-forming composition according to any one of [1] to

[18] , wherein the content of the heterocycle-containing compound is 0.001% by mass to 0.5% by mass based on the solid content.

[20] The electrode-forming composition according to any one of [1] to

[19] , wherein the content of the acid is 0.0001% by mass to 0.5% by mass based on the solid content.

[21] The electrode-forming composition according to any one of [1] to

[20] , wherein the content of the acid relative to the content of the heterocycle-containing compound is 0.01 to 2 by mass ratio.

[22] An electrode layer obtained from the electrode-forming composition according to any one of [1] to

[21] .

[23] A secondary battery comprising the electrode layer according to

[22] .

[24] An additive for an electrode-forming composition comprising a positive electrode active material, a binder, and a solvent, the additive comprising a heterocycle-containing compound that is solid at room temperature and an acid (excluding acids that fall under the category of the heterocycle-containing compound).

[25] A gelation inhibitor for an electrode-forming composition comprising a positive electrode active material, a binder, and a solvent, the additive comprising a heterocycle-containing compound that is solid at room temperature and an acid (excluding acids that fall under the category of the heterocycle-containing compound).

[0020] According to the present invention, it is possible to provide an electrode-forming composition that is inhibited from thickening and gelling and has improved storage stability by a simple method, as well as an additive and a gelling inhibitor for the electrode-forming composition. Furthermore, according to the present invention, it is possible to provide an electrode layer and a secondary battery that use the electrode-forming composition.

[0021] (Electrode-forming composition) The electrode-forming composition of the present invention contains at least a heterocycle-containing compound, an acid, a positive electrode active material, a binder, and a solvent. The electrode-forming composition may contain other components.

[0022] The heterocycle-containing compound is solid at room temperature. In this specification, a heterocycle-containing compound that is solid at room temperature may be simply referred to as a "heterocycle-containing compound." The acid does not include acids that fall under the category of heterocycle-containing compounds.

[0023] By including a heterocycle-containing compound and an acid in the electrode-forming composition, thickening and gelation can be suppressed, and storage stability can be improved, compared to when the electrode-forming composition does not include these or when it contains only an acid.

[0024] The present inventors have discovered that heterocycle-containing compounds are effective in suppressing thickening and gelation of electrode-forming compositions (electrode slurries), and have filed applications [(1) Japanese Patent Application No. 2023-003534, (2) PCT / JP2023 / 017367, (3) PCT / JP2023 / 022271, (4) PCT / JP2023 / 035576, and (5) PCT / JP2024 / 000558 (WO2024 / 150812). (5) is a PCT application claiming priority to (1) to (4)]. The contents of these applications are incorporated herein to the same extent as if expressly set forth in their entirety.

[0025] The electrode-forming composition of the present invention is resistant to thickening and gelation and has high storage stability, making it suitable for use in forming positive electrodes for secondary batteries. When a secondary battery equipped with an electrode made using this composition is manufactured, benefits such as improved quality and yield due to the improved storage stability of the composition, cost reduction and reduced environmental impact due to a high solids concentration, and suppression of deterioration within the battery caused by alkaline components can be expected, contributing to reduced manufacturing costs of the secondary battery and improved battery characteristics.

[0026] <Heterocycle-Containing Compound> The heterocycle-containing compound is different from the solvent in the present invention. In this respect, the heterocycle-containing compound is solid at room temperature. In the present invention, "solid at room temperature" means that the melting point at 1 atmosphere is 25°C or higher.

[0027] Examples of the heterocycle-containing compound include the heterocycle-containing compounds described in the above applications (1) to (5) filed by the present inventors.

[0028] The heterocycle-containing compound has, for example, a nitrogen-containing five-membered ring. The heterocycle-containing compound has, for example, a nitrogen-containing five-membered ring, and the nitrogen-containing five-membered ring does not contain an oxygen atom. The heteroatom contained in the nitrogen-containing five-membered ring is not particularly limited, and may be, for example, only a nitrogen atom. "The nitrogen-containing five-membered ring does not contain an oxygen atom" means that none of the five atoms constituting the nitrogen-containing five-membered ring are oxygen atoms. "The only heteroatoms contained in the nitrogen-containing five-membered ring are nitrogen atoms" means that the five atoms constituting the nitrogen-containing five-membered ring are carbon atoms or nitrogen atoms.

[0029] The heterocycle-containing compound may have a carbonyl structure (carbonyl group) on the heterocycle. Having a carbonyl structure (carbonyl group) on the ring means, for example, that at least one carbon atom among the five atoms constituting the five-membered heterocycle is bonded to an oxygen atom by a double bond. For example, furan contains an oxygen atom in the five-membered ring, but does not have a carbonyl structure (carbonyl group) on the ring. For example, pyrazolone contains two nitrogen atoms in the five-membered ring, does not contain an oxygen atom in the five-membered ring, and has a carbonyl structure (carbonyl group) on the ring. For example, oxazoline contains one oxygen atom and one nitrogen atom in the five-membered ring, but does not have a carbonyl structure (carbonyl group) on the ring.

[0030] The number of nitrogen atoms contained in the nitrogen-containing five-membered ring is preferably two or three.

[0031] The heterocycle-containing compound is preferably one in which the only heteroatom contained in the nitrogen-containing five-membered ring is a nitrogen atom, and more preferably one in which the number of nitrogen atoms contained in the nitrogen-containing five-membered ring is two or three.

[0032] When the heterocycle-containing compound has a nitrogen-containing five-membered ring, the nitrogen-containing five-membered ring may or may not be fused. In other words, the heterocycle-containing compound may be a fused nitrogen-containing five-membered ring compound or a non-fused nitrogen-containing five-membered ring compound.

[0033] The heterocycle-containing compound is preferably one having a carbonyl structure (carbonyl group) on a nitrogen-containing five-membered ring, and in this case, it is more preferable that the nitrogen-containing five-membered ring is not condensed.

[0034] The carbonyl structure preferably exhibits proton tautomerism, and more preferably keto-enol tautomerism, which refers to the tautomerism between keto (ketone, aldehyde) and enol.

[0035] Specific examples of the heterocycle-containing compound include those represented by any of the following formulas (1) to (4). Note that specific examples of formulas (1) to (4) may overlap.

[0036] (In formula (1), R ａ and R ｂ are each independently a hydrogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂ may be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent, each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond, Z is N or C(-L-R ｃ ) and R ｃ is a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, and X ａ are each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms. ａ ~R ｃ are each independently a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂmay be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent; each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond; X ａ and X ｂ are each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms.

[0037] The above formulas (2), (3), and (4) may be structural isomers. Specifically, isomers of the heterocycle-containing compound represented by formula (2) include, for example, heterocycle-containing compounds represented by the following formulas (2-A) to (2-C), isomers of the heterocycle-containing compound represented by formula (3) include, for example, heterocycle-containing compounds represented by the following formulas (3-A) to (3-E), and isomers of the heterocycle-containing compound represented by formula (4) include, for example, heterocycle-containing compounds represented by the following formulas (4-A) to (4-G).

[0038] X ａ and X ｂ are each a hydrogen atom, a lithium atom, or a sodium atom, the tautomers are preferred among the structural isomers. ａ and X ｂ are hydrogen atoms, the tautomers are also called proton tautomers.

[0039] (In the formula, R ａ ~R ｃ , L, X ａ , X ｂ is the same as above.)

[0040] R ａ ~R ｃThe alkyl group having 1 to 6 carbon atoms in the "alkyl group having 1 to 6 carbon atoms which may have a substituent" may be linear, branched, or cyclic, and specific examples thereof include linear or branched alkyl groups having 1 to 6 carbon atoms such as a methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, s-butyl group, t-butyl group, n-pentyl group, and n-hexyl group; and cyclic alkyl groups having 3 to 6 carbon atoms such as a cyclopropyl group, cyclobutyl group, cyclopentyl group, and cyclohexyl group.

[0041] R ａ ~R ｃ Examples of the alkenyl group having 2 to 6 carbon atoms in the "alkenyl group having 2 to 6 carbon atoms which may have a substituent" include ethenyl group, n-1-propenyl group, n-2-propenyl group, 1-methylethenyl group, n-1-butenyl group, n-2-butenyl group, n-3-butenyl group, 2-methyl-1-propenyl group, 2-methyl-2-propenyl group, 1-ethylethenyl group, 1-methyl-1-propenyl group, 1-methyl-2-propenyl group, n-1-pentenyl group, and the like.

[0042] R ａ ~R ｃ Examples of the aryl group having 6 to 12 carbon atoms in the "aryl group having 6 to 12 carbon atoms which may have a substituent" include a phenyl group, a tolyl group, a 1-naphthyl group, and a 2-naphthyl group.

[0043] R ａ and R ｂ Examples of the ring having 4 to 12 carbon atoms in the "ring having 4 to 12 carbon atoms which may have a substituent" formed by bonding together include a cyclopentane ring, a cyclohexane ring, a benzene ring, a naphthalene ring, a triazole ring, a pyridine ring, and a pyrazine ring. The ring having 4 to 12 carbon atoms may be an aromatic ring or a non-aromatic ring. Examples of non-aromatic rings include aliphatic rings. As the ring having 4 to 12 carbon atoms, an aromatic ring having 4 to 12 carbon atoms is preferred, and an aromatic ring having 6 to 10 carbon atoms is more preferred.

[0044] R ａ ~R ｃmay have a substituent. Examples of the substituent include a carboxy group, a hydroxy group, an aldehyde group, an ester group, a ketone group, an amino group, a phenyl group, a halogen atom, an alkoxysilyl group, an epoxy group, a carboxylic acid chloride group, and a thiol group. Examples of the alkoxysilyl group include a trimethoxysilyl group, a dimethoxymethylsilyl group, a methoxydimethylsilyl group, a triethoxysilyl group, a diethoxymethylsilyl group, and an ethoxydimethylsilyl group. In the present invention, a carboxy group is preferred. R ａ ~R ｃ When has a substituent, the number of the substituents is preferably 1 to 6, and more preferably 1 to 3.

[0045] R ａ ~R ｃ Examples of the alkyl group include a hydrogen atom, a carboxy group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, and R ａ and R ｂ are preferably bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent.

[0046] Also, R ａ ~R ｃ Examples of the alkyl group include a hydrogen atom, a carboxy group, an alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, and R ａ and R ｂ More preferably, these groups are bonded to each other to form an aromatic ring having 4 to 12 carbon atoms which may have a substituent.

[0047] Furthermore, R ａ ~R ｃ Examples of the alkyl group include a hydrogen atom, a carboxy group, an alkyl group having 1 to 3 carbon atoms, an aryl group having 6 to 10 carbon atoms, and R ａ and R ｂ are bonded to each other to form an aromatic ring having 6 to 10 carbon atoms which may have a substituent, and is even more preferred.

[0048] Furthermore, R ａ ~R ｃ Examples of the aryl group include a hydrogen atom, a carboxy group, a methyl group, a phenyl group, and R ａ and R ｂMore preferred is an optionally substituted benzene ring formed by bonding together:

[0049] L is preferably a single bond, an ester bond or an amide bond, and more preferably a single bond.

[0050] Z is preferably N.

[0051] X ａ and X ｂ The alkyl group having 1 to 6 carbon atoms in the "alkyl group having 1 to 6 carbon atoms which may have a substituent" may be linear, branched, or cyclic, and specific examples thereof include linear or branched alkyl groups having 1 to 6 carbon atoms such as a methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, s-butyl group, t-butyl group, n-pentyl group, and n-hexyl group; and cyclic alkyl groups having 3 to 6 carbon atoms such as a cyclopropyl group, cyclobutyl group, cyclopentyl group, and cyclohexyl group.

[0052] X ａ and X ｂ Examples of the aryl group having 6 to 12 carbon atoms in the "aryl group having 6 to 12 carbon atoms which may have a substituent" include a phenyl group, a tolyl group, a 1-naphthyl group, and a 2-naphthyl group.

[0053] X ａ and X ｂ may have a substituent. Examples of the substituent include a carboxy group, a hydroxy group, an aldehyde group, an ester group, a ketone group, an amino group, a phenyl group, a halogen atom, an alkoxysilyl group, an epoxy group, a carboxylic acid chloride group, and a thiol group. Examples of the alkoxysilyl group include a trimethoxysilyl group, a dimethoxymethylsilyl group, a methoxydimethylsilyl group, a triethoxysilyl group, a diethoxymethylsilyl group, and an ethoxydimethylsilyl group. In the present invention, a carboxy group and an alkoxysilyl group are preferred, and a carboxy group and a trimethoxysilyl group are more preferred.

[0054] R ｄThe alkyl group having 1 to 10 carbon atoms in the formula (I) may be linear, branched, or cyclic, and specific examples thereof include linear or branched alkyl groups having 1 to 10 carbon atoms such as methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, isobutyl, s-butyl, t-butyl, n-pentyl, neopentyl, n-hexyl, n-octyl, n-nonyl, and n-decyl; and cyclic alkyl groups having 3 to 10 carbon atoms such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and 1-adamantyl.

[0055] R ｄ Examples of the alkanol group having 1 to 10 carbon atoms in the formula include a hydroxymethyl group, a hydroxyethyl group, a hydroxypropyl group, a hydroxybutyl group, a hydroxypentyl group, a hydroxyhexyl group, a hydroxyoctyl group, a hydroxynonyl group, and a hydroxydecyl group.

[0056] R ｄ Examples of the alkenyl group having 2 to 10 carbon atoms in the formula (I) include ethenyl, n-1-propenyl, n-2-propenyl, 1-methylethenyl, n-1-butenyl, n-2-butenyl, n-3-butenyl, 2-methyl-1-propenyl, 2-methyl-2-propenyl, 1-ethylethenyl, 1-methyl-1-propenyl, 1-methyl-2-propenyl, n-1-pentenyl, and n-1-decenyl.

[0057] R ｄ Examples of the aryl group having 6 to 12 carbon atoms in the formula (I) include a phenyl group, a tolyl group, a 1-naphthyl group, and a 2-naphthyl group.

[0058] X ａ and X ｂ Examples of the alkyl group include a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, and —CH ２ NR ｄ ２ is preferred.

[0059] Also, X ａ and X ｂExamples of the alkyl group include a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 4 carbon atoms, an aryl group having 6 to 10 carbon atoms, and —CH ２ NR ｄ ２ is preferred.

[0060] Furthermore, X ａ and X ｂ Examples of the alkyl group include a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 3 carbon atoms, an aryl group having 6 to 8 carbon atoms, and —CH ２ NR ｄ ２ is more preferred.

[0061] Furthermore, X ａ and X ｂ Examples of the group include a hydrogen atom, a lithium atom, a sodium atom, a methyl group, a phenyl group, and —CH ２ NR ｄ ２ is more preferable.

[0062] R ｄ As the alkyl group, an alkyl group having 1 to 3 carbon atoms and an aryl group having 6 to 10 carbon atoms are preferred, and a methyl group and a phenyl group are more preferred.

[0063] As the heterocycle-containing compounds represented by the above formulas (1) to (4), heterocycle-containing compounds represented by the following formulas (1a) to (4a) are preferred.

[0064] (In the formula, R ａ ~R ｃ , Z, X ａ , X ｂ is the same as above.)

[0065] Furthermore, the heterocycle-containing compound is more preferably a heterocycle-containing compound represented by the following formula (5).

[0066] (In the formula, Ar １ Z, X are an aromatic ring having 4 to 12 carbon atoms which may have a substituent, or an aliphatic ring having 4 to 10 carbon atoms which may have a substituent. ａ is the same as above.)

[0067] The heterocycle-containing compound is more preferably a heterocycle-containing compound represented by any one of the following formulas (6) to (7).

[0068] (Wherein Z and X ａ is the same as above.)

[0069] In addition to the heterocycle-containing compound represented by the above formula (5), the heterocycle-containing compound represented by the following formula (4b) is more preferred.

[0070] (In the formula, R ａ ~R ｃ is the same as above, and R ｅ is a hydrogen atom, a carboxy group, a hydroxy group, an aldehyde group, an ester group, a ketone group, an amino group, a phenyl group, a halogen atom, an alkoxysilyl group, an epoxy group, a carboxylic acid chloride group, or a thiol group.

[0071] In the above formula (4b), R ｅ When is a hydrogen atom, X in the above formula (4) ａ corresponds to an embodiment in which no substituent is present. ｅ is other than a hydrogen atom, ｅ is X in the above formula (4). ａ The substituent corresponds to the substituent possessed by X ａ and X ｂ The substituents are the same as those described in the description of the substituents possessed by the group.

[0072] Furthermore, the heterocycle-containing compound is more preferably a heterocycle-containing compound represented by the following formula (8).

[0073] (In the formula, R ａ ~R ｃ is the same as above.)

[0074] Specific examples of the heterocycle-containing compound represented by formula (1) include heterocycle-containing compounds represented by the following formulae (1-1) to (1-15).

[0075] *The structure of the heterocycle-containing compound represented by formula (1-6) is the structure of the compound X-12-1214A manufactured by Shin-Etsu Chemical Co., Ltd. and described in the company's catalog. Me represents a methyl group.

[0076] Specific examples of the heterocycle-containing compound represented by formula (2) include heterocycle-containing compounds represented by the following formulas (2-1) to (2-7).

[0077]

[0078] Specific examples of the heterocycle-containing compound represented by formula (3) include heterocycle-containing compounds represented by the following formulas (3-1) to (3-13).

[0079]

[0080] Specific examples of the heterocycle-containing compound represented by formula (4) include heterocycle-containing compounds represented by the following formulae (4-1) to (4-14).

[0081]

[0082] The content of the heterocycle-containing compound in the electrode-forming composition is not particularly limited, but is preferably 0.001 to 4 mass% of the solid content, more preferably 0.001 to 2 mass%, even more preferably 0.001 to 0.5 mass%, even more preferably 0.001 to 0.3 mass%, and particularly preferably 0.001 to 0.2 mass%. An even more preferable lower limit of the content of the heterocycle-containing compound is 0.01 mass% of the solid content. By keeping the content of the heterocycle-containing compound within the above range, gelation of the electrode-forming composition can be effectively suppressed, and the battery characteristics of the resulting battery can be maintained. In this invention, the solid content refers to the components other than the solvent that constitute the composition (the same applies hereinafter).

[0083] The content of the heterocycle-containing compound in the electrode-forming composition is preferably 0.001 to 4 parts by mass, more preferably 0.001 to 2 parts by mass, even more preferably 0.001 to 0.5 parts by mass, even more preferably 0.001 to 0.3 parts by mass, and particularly preferably 0.001 to 0.2 parts by mass, relative to 100 parts by mass of the positive electrode active material. By setting the content of the heterocycle-containing compound within the above range, gelation of the electrode-forming composition can be effectively suppressed, and the battery characteristics of the resulting battery can be maintained.

[0084] The content of the heterocycle-containing compound in the electrode-forming composition is preferably 0.1 to 50 parts by mass, more preferably 0.1 to 30 parts by mass, and even more preferably 0.1 to 20 parts by mass, relative to 100 parts by mass of the binder. By setting the content of the heterocycle-containing compound within the above range, gelation of the electrode-forming composition can be effectively suppressed, and the battery characteristics of the resulting battery can be maintained.

[0085] Furthermore, when the electrode-forming composition contains a conductive auxiliary, the content of the heterocycle-containing compound in the electrode-forming composition is preferably 0.1 to 50 parts by mass, more preferably 0.1 to 30 parts by mass, and even more preferably 0.1 to 20 parts by mass, per 100 parts by mass of the conductive auxiliary.

[0086] <Acid> The acid is not particularly limited. However, in the present invention, if a compound corresponds to both an acid and a heterocycle-containing compound, it is considered to be a heterocycle-containing compound. For example, in the present invention, a heterocycle-containing compound having a carboxy group is not an acid but a heterocycle-containing compound.

[0087] The acid may be an organic acid or an inorganic acid.

[0088] The organic acid has, for example, an acidic group. Examples of the acidic group include a carboxy group, a phenolic hydroxy group, a mercapto group, a sulfo group, and a phosphate group. The number of acidic groups contained in the organic acid is not particularly limited, but is preferably 1 to 4. The number of carbon atoms contained in the organic acid is, for example, 1 to 10. The molecular weight of the organic acid is not particularly limited, but is, for example, 46 to 300. The molecular weight of formic acid is 46.

[0089] Examples of organic acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, acrylic acid, methacrylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, sebacic acid, fumaric acid, maleic acid, citraconic acid, phthalic acid, isophthalic acid, terephthalic acid, glutamic acid, aspartic acid, glycolic acid, malic acid, tartaric acid, citric acid, lactic acid, hydroxyacrylic acid, α-oxybutyric acid, glyceric acid, tartronic acid, salicylic acid, gallic acid, tropic acid, asformic acid, benzoic acid, trimellitic acid, pyromellitic acid, trifluoroacetic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, and trifluoromethanephosphonic acid.

[0090] Examples of inorganic acids include hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, boric acid, and hydrofluoric acid.

[0091] These acids may be used alone or in combination of two or more.

[0092] The acid content in the electrode-forming composition is not particularly limited, but is preferably 0.00001 to 4 mass % of the solid content, more preferably 0.00001 to 2 mass %, even more preferably 0.00001 to 0.5 mass %, even more preferably 0.00001 to 0.3 mass %, and particularly preferably 0.00001 to 0.2 mass %. An even more preferable lower limit of the acid content is 0.0001 mass % of the solid content. By setting the acid content within the above range, gelation of the electrode-forming composition can be effectively suppressed, and the battery characteristics of the resulting battery can be maintained.

[0093] The content of the acid relative to the content of the heterocycle-containing compound in the electrode-forming composition is not particularly limited, but from the viewpoint of suitably obtaining the effects of the present invention, the mass ratio is preferably 0.01 to 2, more preferably 0.01 to 0.5, and particularly preferably 0.01 to 0.1.

[0094] <Positive Electrode Active Material> The positive electrode active material is not particularly limited.

[0095] As the positive electrode active material, those containing S, Fe, or Ni are preferred, and those containing 30% by mass or more of S, Fe, or Ni are more preferred, in order to further improve the battery capacity, reduce the amount of rare metal used, and reduce costs. In order to further reduce the amount of rare metal used and obtain a battery with a longer life, those containing 35% by mass or more of Fe or Ni are even more preferred, and those containing 45% by mass or more are even more preferred. The upper limit is not particularly limited, but is usually 61% by mass or less.

[0096] An electrode-forming composition containing a positive electrode active material with a high nickel content has a strong tendency to thicken and gel. Therefore, in order to suppress thickening and gelling in an electrode-forming composition containing a positive electrode active material with a high nickel content, the positive electrode active material preferably contains Ni, more preferably has a Ni content of 30 mass% or more, and particularly preferably has a Ni content of 40 mass% or more. The upper limit of the Ni content in the positive electrode active material is not particularly limited, but is, for example, 61 mass% or less.

[0097] The positive electrode active material can be appropriately selected from various active materials conventionally used in electrodes for secondary batteries. For example, in the case of a lithium secondary battery or a lithium ion secondary battery, a chalcogen compound or a lithium ion-containing chalcogen compound capable of adsorbing and releasing lithium ions, a polyanionic compound, elemental sulfur and its compounds, etc. can be used.

[0098] Examples of lithium ion-containing chalcogen compounds include LiNiO ２ , Li ｘ Ni ｙ M １－ｙ O ２(M represents at least one metal element selected from Co, Mn, Ti, Cr, V, Al, Sn, Pb, and Zn, and 0.05≦x≦1.10, 0.3≦y≦1.0), Li ａ Ni （１－ｘ－ｙ） Co ｘ M １ ｙ M ２ ｚ O ２ (In the formula, M １ is at least one selected from the group consisting of Mn and Al; ２ represents at least one element selected from the group consisting of Zr, Ti, Mg, B, Zr, Si, W, and V, and 1.00≦a≦1.50, 0.00≦x≦0.50, 0.00≦y≦0.50, 0.000≦z≦0.020). Examples of polyanion compounds include LiFePO ４ , Li ａ Mn ｂ Fe ｃ D ｄ P.O. ４ (wherein 1.00≦a≦1.15, 0.01≦b≦0.99, 0.01≦c≦0.99, 0.00≦d≦0.10, and D is selected from Co, Mn, Ti, Cr, V, Al, Sn, Pb, and Zn, and at least a part of the metal has an olivine structure). Examples of sulfur compounds include sulfur, Li, ２ S, FeS ２ , TiS ２ , MoS ２ These positive electrode active materials may be used alone or in combination of two or more.

[0099] Among the above positive electrode active materials, the general formula Li ａ Ni （１－ｘ－ｙ） Co ｘ M １ ｙ M ２ ｚ O ２ (In the formula, M １ is at least one selected from the group consisting of Mn and Al; ２represents at least one element selected from the group consisting of Zr, Ti, Mg, B, W, and V, and preferably 1.00≦a≦1.50, 0.00≦x≦0.50, 0.00≦y≦0.50, 0.000≦z≦0.020). x may be 0.01≦x≦0.30 or 0.03≦x≦0.20. y may be 0.01≦x≦0.30 or 0.03≦x≦0.20. x+y may be 0.02≦(x+y)≦0.40 or 0.05≦(x+y)≦0.30. Li ａ Ni （１－ｘ－ｙ） Co ｘ M １ ｙ M ２ ｚ O ２ The Ni content is preferably 30% by mass or more, more preferably 40% by mass or more, more preferably 45% by mass or more, and particularly preferably 47% by mass or more. The upper limit of the Ni content is not particularly limited, but is, for example, 61% by mass or less.

[0100] The positive electrode active material is preferably lithium-containing transition metal oxide particles, more preferably lithium-containing transition metal oxide particles having a layered rock salt structure.

[0101] These active materials can be used alone or in combination of two or more.

[0102] An electrode-forming composition is more likely to thicken and gel when it contains two types of positive electrode active materials (particularly, a first positive electrode active material that is polycrystalline and a second positive electrode active material that is single crystalline). By adding a heterocycle-containing compound and an acid to an electrode-forming composition containing two types of positive electrode active materials (particularly, a first positive electrode active material that is polycrystalline and a second positive electrode active material that is single crystalline), it is possible to suppress thickening and gelling of the composition, which is more likely to thicken and gel. In this regard, it is preferable that the positive electrode active material contains a first positive electrode active material that is polycrystalline and a second positive electrode active material that is single crystalline.

[0103] When the positive electrode active material includes a first positive electrode active material that is a polycrystalline body and a second positive electrode active material that is a single crystal body, the mass ratio of the first positive electrode active material to the second positive electrode active material (first positive electrode active material:second positive electrode active material) in the electrode-forming composition is not particularly limited, but is preferably 2:8 to 8:2, more preferably 4:6 to 8:2, and particularly preferably 4:6 to 7:3.

[0104] The content of the positive electrode active material in the electrode-forming composition is not particularly limited, but is preferably 88.0 to 99.949 mass % of the solid content, more preferably 88.0 to 99.899 mass %, and even more preferably 95.0 to 99.0 mass %.

[0105] <Binder> The binder can be appropriately selected from known materials and is not particularly limited, but examples thereof include fluorine-based binders, polyimide, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, polyethylene, polypropylene, etc. These are non-aqueous binders. Examples of fluorine-based binders include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and copolymers containing at least one monomer selected from the group consisting of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene.

[0106] From the viewpoint of improving the storage stability of the electrode-forming composition, it is preferable to use a fluorine-based binder. Furthermore, the fluorine-based binder is preferably modified with a polar functional group such as a carboxyl group or a hydroxyl group. The polar functional group can be confirmed by the presence or absence of a clear peak detected in the range of 10 to 15 ppm in measurement using a nuclear magnetic resonance (NMR) device.

[0107] The binders can be used alone or in combination of two or more.

[0108] The weight-average molecular weight (Mw) of the binder is not particularly limited, but from the viewpoint of improving the adhesion between the current collector and the electrode layer, it is 600,000 to 3,000,000, preferably 700,000 to 2,000,000, and more preferably 700,000 to 1,500,000. The weight-average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0109] The content of the binder in the electrode-forming composition is not particularly limited, but from the viewpoint of reducing costs and obtaining a high energy density, it is preferably 0.05 to 8 mass % of the solid content, more preferably 0.05 to 5 mass %, even more preferably 0.05 to 4 mass %, still more preferably 0.1 to 3 mass %, particularly preferably 0.2 to 2 mass %, and most preferably 0.3 to 1.5 mass %.

[0110] <Solvent> The solvent is not particularly limited, and examples thereof include solvents that have conventionally been used in preparing electrode-forming compositions, such as water and organic solvents.

[0111] Examples of the organic solvent include ethers, halogenated hydrocarbons, amides, ketones, alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, glycol ethers, glycols, carbonates, and other organic solvents.

[0112] Examples of ethers include tetrahydrofuran (THF), diethyl ether, and 1,2-dimethoxyethane (DME). Examples of halogenated hydrocarbons include methylene chloride, chloroform, and 1,2-dichloroethane. Examples of amides include N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methyl-2-pyrrolidone (NMP). Examples of ketones include acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. Examples of alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, and t-butanol. Examples of aliphatic hydrocarbons include n-heptane, n-hexane, and cyclohexane. Examples of aromatic hydrocarbons include benzene, toluene, xylene, and ethylbenzene. Examples of glycol ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and propylene glycol monomethyl ether. Examples of glycols include ethylene glycol and propylene glycol. Examples of carbonates include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of other organic solvents include γ-butyrolactone, dimethyl sulfoxide (DMSO), dioxolane, and sulfolane.

[0113] The organic solvent may be a protic solvent or an aprotic solvent, but an aprotic solvent is preferred. The aprotic solvent may be, for example, polar or non-polar. Preferred aprotic solvents are amides, ketones, and carbonates, and more preferred are amides.

[0114] These solvents can be used alone or in combination of two or more.

[0115] The binder may be used by dissolving or dispersing it in these solvents as needed. Suitable solvents in this case include water, NMP, DMSO, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, THF, dioxolane, sulfolane, DMF, DMAc, etc. The solvent may be selected appropriately depending on the type of binder, but NMP is suitable for water-insoluble binders such as PVdF, and water is suitable for water-soluble binders.

[0116] The solid content concentration of the electrode-forming composition is set appropriately taking into consideration the coatability of the composition, the thickness of the electrode to be formed, and the like, but is usually 60 to 92 mass %, preferably 65 to 90 mass %, and more preferably 70 to 85 mass %.

[0117] <Other Components> Examples of other components that may be contained in the electrode-forming composition include a conductive assistant and a dispersant.

[0118] <<Conductive Aid>> The conductive aid is used, for example, to improve electrical conductivity. The conductive aid is not particularly limited, and examples thereof include carbon materials and conductive polymers. Examples of carbon materials include graphite, carbon black, acetylene black (AB), vapor-grown carbon fiber, carbon nanotubes (CNT), carbon nanohorns, and graphene. Examples of conductive polymers include polyaniline, polypyrrole, polythiophene, polyacetylene, and polyacene. The conductive aid may be used alone or in combination of two or more.

[0119] From the viewpoints of electrical conductivity and ease of preparation of the electrode-forming composition, it is preferable that the composition does not contain graphene, and it is preferable to use carbon black, acetylene black, vapor-grown carbon fiber, carbon nanotubes, or carbon nanohorns, and it is more preferable to use carbon black, acetylene black, or carbon nanotubes.

[0120] The content of the conductive additive in the electrode-forming composition is not particularly limited, but is preferably 0.05 to 5 mass %, more preferably 0.05 to 4 mass %, even more preferably 0.1 to 3 mass %, and even more preferably 0.2 to 2 mass % of the solid content. By setting the content of the conductive additive within the above range, good electrical conductivity can be obtained.

[0121] <<Dispersant>> Dispersants are used to improve the dispersibility of substances such as positive electrode active materials and conductive additives. Dispersants can be appropriately selected from those conventionally used as dispersants for conductive carbon materials such as CNTs. Nonionic polymers are preferred from the perspective of stability within the battery. Examples of nonionic polymers include polyvinylpyrrolidone (PVP) and polymers containing at least one functional group selected from the group consisting of nitrile groups, hydroxy groups, carbonyl groups, amino groups, sulfonyl groups, and ether groups. Examples of functional group-containing polymers include polyvinyl alcohol, polyacrylonitrile, polylactic acid, polyester, polyimide, polyphenyl ether, polyphenylsulfone, polyethyleneimine, and polyaniline. Polymers containing a pyrrolidone structure or a nitrile group are preferred as dispersants, with polyvinylpyrrolidone and polyacrylonitrile being more preferred. Dispersants can be used alone or in combination.

[0122] The content of the dispersant in the electrode-forming composition is not particularly limited, but is preferably 0.001 to 0.5 mass %, more preferably 0.001 to 0.3 mass %, and even more preferably 0.001 to 0.2 mass % based on the solid content. An even more preferable lower limit of the dispersant content is 0.01 mass % based on the solid content. Furthermore, in consideration of the adhesion between the resulting electrode layer and the current collector, the total amount of the heterocycle-containing compound, acid, and dispersant is preferably 0.001 to 1 mass %, more preferably 0.01 to 1 mass %, based on the solid content.

[0123] The viscosity of the electrode-forming composition is set appropriately taking into consideration the coating method, the thickness of the electrode to be formed, and the like, but is typically about 100 to 2,000,000 mPa·s, preferably about 300 to 1,000,000 mPa·s, and more preferably about 400 to 800,000 mPa·s. The above viscosity is a value measured at 25°C using an E-type viscometer.

[0124] The electrode-forming composition of the present invention can be obtained by mixing the above-mentioned components. When the composition contains optional components other than the gelation inhibitor (heterocycle-containing compound and acid) as an additive of the present invention, the positive electrode active material, and the binder, the gelation inhibitor as an additive and the positive electrode active material may be mixed together with the optional components, or both components may be mixed in advance and then mixed with the optional components. Either method can achieve the effects of the present invention.

[0125] (Electrode Layer) The electrode layer of the present invention is obtained from the electrode-forming composition of the present invention. Examples of methods for forming the electrode layer include a method in which the electrode-forming composition is applied to a substrate to form a coating film, which is then dried. This method is not particularly limited, and various conventionally known methods can be used. Specific examples of coating methods include various printing methods such as offset printing and screen printing, blade coating, dip coating, spin coating, bar coating, slit coating, inkjet printing, and die coating.

[0126] When drying the coating film, either natural drying or heat drying may be used, but heat drying is preferred from the viewpoint of production efficiency. When heat drying is performed, the temperature is preferably 50 to 400°C, more preferably 70 to 150°C.

[0127] The thickness of the electrode layer is not particularly limited, but is preferably 0.01 to 1,000 μm, more preferably 5 to 300 μm. In the case where the electrode layer is used solely as an electrode in a secondary battery, the thickness is preferably 10 μm or more.

[0128] (Electrode) The electrode of the present invention comprises, for example, an electrode layer of the present invention on at least one surface of a substrate serving as a current collector. Examples of substrates used for electrodes include metal substrates such as platinum, gold, iron, stainless steel, copper, aluminum, and lithium; alloy substrates made of any combination of these metals; oxide substrates such as indium tin oxide (ITO), indium zinc oxide (IZO), and antimony tin oxide (ATO); and carbon substrates such as glassy carbon, pyrolytic graphite, and carbon felt. The thickness of the substrate is not particularly limited, but is preferably 1 to 100 μm, more preferably 3 to 30 μm, and particularly preferably 5 to 25 μm.

[0129] The electrode may be pressed as necessary. A commonly used pressing method can be used, but mold pressing and roll pressing are particularly preferred. The pressing pressure is not particularly limited, but is preferably 1 kN / cm or more, more preferably 2 kN / cm or more, and particularly preferably 5 kN / cm or more. The upper limit of the pressing pressure is not particularly limited, but is preferably 50 kN / cm or less.

[0130] (Secondary Battery) The secondary battery of the present invention comprises the electrode layer of the present invention. The secondary battery of the present invention comprises, for example, the electrode of the present invention. The secondary battery comprises, for example, at least one pair of a positive electrode and a negative electrode, a separator interposed between these electrodes, and an electrolyte. The positive electrode is the electrode of the present invention.

[0131] The material used for the separator is not particularly limited, and examples thereof include glass fiber, cellulose, porous polyolefin, polyamide, polyester, and the like.

[0132] The electrolyte may be either liquid or solid, and may be either aqueous or non-aqueous. From the viewpoint of easily achieving practically sufficient performance, however, an electrolytic solution composed of an electrolyte salt, a solvent, etc. can be suitably used.

[0133] The electrolyte salt is, for example, LiPF ６ , LiBF ４ , LiN(SO ２ F) ２, LiN(C ２ F ５ SO ２ ) ２ , LiAsF ６ , LiSbF ６ , LiAlF ４ , LiGaF ４ , LiInF ４ , LiClO ４ , LiN(CF ３ SO ２ ) ２ , LiCF ３ SO ３ , LiSiF ６ , LiN(CF ３ SO ２ ), (C ４ F ９ SO ２ Lithium salts such as LiI, NaI, KI, CsI, and CaI ２ metal iodides such as those mentioned above; iodide salts of quaternary imidazolium compounds; iodide salts and perchlorates of tetraalkylammonium compounds; LiBr, NaBr, KBr, CsBr, and CaBr ２ These electrolyte salts may be used alone or in combination of two or more.

[0134] The solvent is not particularly limited as long as it does not corrode or decompose the materials constituting the battery, thereby deteriorating performance, and dissolves the electrolyte salt. Examples of the solvent include non-aqueous solvents such as cyclic esters such as ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone; ethers such as tetrahydrofuran and dimethoxyethane; linear esters such as methyl acetate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and nitriles such as acetonitrile. These solvents may be used alone or in combination of two or more.

[0135] In addition, as the solid electrolyte, inorganic solid electrolytes such as sulfide-based solid electrolytes and oxide-based solid electrolytes, and organic solid electrolytes such as polymer-based electrolytes can be suitably used. By using these solid electrolytes, an all-solid-state battery can be obtained that does not require an electrolytic solution.

[0136] Examples of sulfide-based solid electrolytes include Li ２ S-SiS ２ - lithium compounds (wherein the lithium compounds are Li ３ P.O. ４ , LiI and Li ４ SiO ４ At least one selected from the group consisting of ２ S-P ２ O ５ , Li ２ S-B ２ S ５ , Li ２ S-P ２ S ５ -GeS ２ and the like.

[0137] As the oxide-based solid electrolyte, for example, Li, which is an oxide having a garnet structure, ５ La ３ M ２ O １２ (M=Nb, Ta) or Li ７ La ３ Zr ２ O １２ , γ-Li, collectively known as LISICON ３ P.O. ４ Oxygen acid salt compounds based on the structure, perovskite type, collectively known as LiPON ３．３ P.O. ３．８ N ０．２２ , sodium / alumina, etc.

[0138] Examples of polymer-based solid electrolytes include polyethylene oxide-based materials and polymer compounds obtained by polymerizing or copolymerizing monomers such as hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, ethylene, propylene, acrylonitrile, vinylidene chloride, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, styrene, and vinylidene fluoride. The polymer-based solid electrolyte may contain a supporting salt and a plasticizer. Examples of supporting salts include lithium (fluorosulfonylimide). Examples of plasticizers include succinonitrile.

[0139] A battery manufactured using the electrode-forming composition of the present invention has high battery characteristics even if it contains less binder (for example, fluorine binder) compared to a general secondary battery.

[0140] The form of the secondary battery and the type of electrolyte are not particularly limited, and any form such as a lithium ion secondary battery, a nickel-metal hydride battery, a manganese battery, or an air battery may be used, but a lithium ion secondary battery is preferred. The lamination method and production method of the secondary battery are also not particularly limited.

[0141] For example, a lithium-ion secondary battery can be produced by placing one electrode (negative electrode) on a coin cell lid to which a washer and spacer are welded, placing a separator of the same shape impregnated with an electrolyte solution on top of the negative electrode, placing the electrode (positive electrode) of the present invention on top of that with the electrode layer facing downwards, placing a case and a gasket on top, and sealing the battery with a coin cell crimping machine.

[0142] (Additive and Gelling Inhibitor) The additive of the present invention is an additive for an electrode-forming composition containing a positive electrode active material, a binder, and a solvent. The gelation inhibitor of the present invention is a gelation inhibitor for an electrode-forming composition containing a positive electrode active material, a binder, and a solvent. The gelation inhibitor is added to an electrode-forming composition containing a positive electrode active material, a binder, and a solvent to inhibit gelation of the electrode-forming composition. The additive and gelation inhibitor are preferably a combination of a heterocycle-containing compound and an acid, and examples and preferred examples of each include the examples and preferred examples given in the above-mentioned descriptions of the heterocycle-containing compound and the acid. Examples and preferred examples of the positive electrode active material include the examples and preferred examples given in the description of the positive electrode active material as a component of the electrode-forming composition of the present invention. Examples and preferred examples of the binder include the examples and preferred examples given in the description of the binder as a component of the electrode-forming composition of the present invention. Examples and preferred examples of the solvent include the examples and preferred examples given in the description of the solvent as a component contained in the electrode-forming composition of the present invention. The electrode-forming composition in which the additive and the gelation inhibitor are used may contain other components. Examples and preferred examples of the other components include the examples and preferred examples given in the description of the other components contained in the electrode-forming composition of the present invention.

[0143] The additive and gelation inhibitor preferably contain a heterocycle-containing compound and an acid. The additive and gelation inhibitor may or may not contain a solvent. The solvent is not particularly limited. Examples and preferred examples of the solvent include those listed in the description of the solvent as a component of the electrode-forming composition of the present invention. The additive and gelation inhibitor are, for example, compositions containing a heterocycle-containing compound and an acid. As long as the additive and gelation inhibitor achieve the effects of the present invention, they do not necessarily need to be compositions containing a heterocycle-containing compound and an acid; embodiments in which a heterocycle-containing compound and an acid are added separately are also within the scope of the present invention. The additive and gelation inhibitor are, for example, compositions containing a heterocycle-containing compound and an acid, but not containing a positive electrode active material or a binder. The content of the acid relative to the content of the heterocycle-containing compound in the additive and gelation inhibitor is not particularly limited, but from the viewpoint of optimally achieving the effects of the present invention, a mass ratio of 0.01 to 2 is preferred, 0.01 to 0.05 is more preferred, and 0.01 to 0.1 is particularly preferred. When the additive and the gelation inhibitor contain a solvent, the solids concentration of the additive and the gelation inhibitor is appropriately set taking into consideration the saturated solubility in the solvent, storage stability, etc., but is preferably 1% by mass to 60% by mass, more preferably 3% by mass to 55% by mass, and particularly preferably 3% by mass to 50% by mass of the solids.

[0144] (Method and Use for Inhibiting Gelling) The method for inhibiting gelation of the present invention is a method for inhibiting gelation of an electrode-forming composition containing a positive electrode active material, a binder, and a solvent. In the method for inhibiting gelation, the electrode-forming composition is made to contain a heterocycle-containing compound and an acid. The use of the present invention is the use of a heterocycle-containing compound and an acid to inhibit gelation of an electrode-forming composition containing a positive electrode active material, a binder, and a solvent. Examples and preferred examples of the heterocycle-containing compound and the acid include the examples and preferred examples given in the above descriptions of the heterocycle-containing compound and the acid. Examples and preferred examples of the positive electrode active material include the examples and preferred examples given in the description of the positive electrode active material as a component of the electrode-forming composition of the present invention. Examples and preferred examples of the binder include the examples and preferred examples given in the description of the binder as a component of the electrode-forming composition of the present invention. Examples and preferred examples of the solvent include the examples and preferred examples given in the description of the solvent as a component of the electrode-forming composition of the present invention. The electrode-forming composition used in the method for suppressing gelation may contain other components. Examples and preferred examples of the other components include the examples and preferred examples given in the description of the other components contained in the electrode-forming composition of the present invention.

[0145] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples. The apparatus used is as follows.

[0146] (1) Rotation / revolution type mixer: Thinky Corporation, Awatori Rentaro, atmospheric pressure type, ARE-310 (2) Dry mixer: Nippon Spindle Mfg. Co., Ltd. (3) E-type viscometer: Toki Sangyo Co., Ltd., VISCOMETER TV-25H, measurement temperature: 25°C, rotor: 1°34' x R24. The viscosity was measured 5 minutes after the start of measurement under the following measurement conditions. The rotor rotation speed was selected from 0.1 to 10 rpm, and an appropriate value was selected for the viscosity of the sample to be measured. The viscosity on the day of slurry preparation was measured within 0.5 to 6 hours after preparation.

[0147] The raw materials used are as follows: <Active material> NCM-1: Lithium nickel manganese cobalt oxide (LiNi ０．８８ Co ０．０７ Mn ０．０５ O ２ , manufactured by Ningbo Ronbay New Energy Technology Co., Ltd., S90F, Ni content: 53 mass%, polycrystalline type) NCM-2: Lithium nickel manganese cobalt oxide (LiNi ０．９１ Co ０．０７ Mn ０．０２ O ２ , manufactured by Ningbo Ronbay New Energy Technology Co., Ltd., SC92Y, Ni content: 55 mass%, single crystal type)

[0148] <Binder> Solef-5140: Polyvinylidene fluoride (PVdF), manufactured by SOLVAY

[0149] <Conductive additive> AB: Denka Black (registered trademark) Li100 (high-purity acetylene black), manufactured by Denka Co., Ltd.

[0150] <Solvent> NMP: N-methyl-2-pyrrolidone, manufactured by Nippon Refine Co., Ltd.

[0151] [Additives used in Examples or Comparative Examples] <Additive A> A1: 3-Methyl-1-phenyl-5-pyrazole, manufactured by Tokyo Chemical Industry Co., Ltd. (corresponding to formula (4-6)) A2: 1H-1,2,3-Benzotriazole-5-carboxylic acid monohydrate (CBT-1), manufactured by Johoku Chemical Industry Co., Ltd. (corresponding to the monohydrate of formula (1-1)) A3: 1H-1,2,4-Triazole, manufactured by Tokyo Chemical Industry Co., Ltd. (corresponding to formula (2-1)) A4: 3-Methyl-1-p-tolyl-5-pyrazole, manufactured by Tokyo Chemical Industry Co., Ltd. (corresponding to formula (3-9)) <Additive B> B1: Oxalic B1: Acetic acid, manufactured by Kanto Chemical Co., Ltd. B2: Acetic acid, manufactured by Kanto Chemical Co., Ltd. B3: Acrylic acid, manufactured by Fujifilm Wako Pure Chemical Co., Ltd. B4: Malonic acid, manufactured by Tokyo Chemical Industry Co., Ltd. B5: Benzoic acid, manufactured by Fujifilm Wako Pure Chemical Co., Ltd. B6: Adipic acid, manufactured by Tokyo Chemical Industry Co., Ltd. B7: Phosphoric acid, manufactured by Fujifilm Wako Pure Chemical Co., Ltd. B8: Sulfuric acid, manufactured by Junsei Chemical Co., Ltd.

[0152] <Other additives> PVP: Polyvinylpyrrolidone, manufactured by Nippon Shokubai Co., Ltd., K-90, dispersant

[0153] Preparation of Positive Electrode Composition (Electrode Slurry) [Example 1-1, Comparative Examples 1-1 to 1-5] A 5% by mass NMP solution (additive A solution, additive B solution) was prepared for each additive. The positive electrode active material and binder powder were weighed in a dry mixer and mixed with a spatula to achieve the composition ratios shown in Tables 1 and 2. NMP was added to the resulting solution, and the mixture was mixed for 30 seconds at 2,000 rpm using a rotation-revolution mixer. After mixing with the spatula, the prepared additive A solution and additive B solution were added, and the mixture was mixed for 1 minute at 2,000 rpm. The conductive additive, NMP, and water were added to the resulting solution, and the mixture was mixed for 1 minute at 2,000 rpm. Finally, the mixture was degassed for 20 seconds at 2,200 rpm to obtain an electrode slurry. The total amount of the prepared slurries was 100 g, the solid content was 73.5% by mass, and the solvent composition of the slurries was NMP / H ２ The mass ratio of NCM-1 to NCM-2 was adjusted to 8:2. The water was added to the slurry to intentionally create a high water content. In Tables 1 and 2, the mixed NCM refers to a mixed active material of NCM-1 and NCM-2. The mass ratio (NCM-1:NCM-2) in the mixed active material was 8:2.

[0154] Examples 1-2 to 1-13, Comparative Examples 1-6 to 1-14: Five mass % NMP solutions (additive A solution, additive B solution) were prepared for each additive. The positive electrode active material and binder powder were weighed and mixed with a spatula in a dry mixer to achieve the composition ratios shown in Tables 1 and 2. NMP and a dispersant solution were added to the resulting solution, and the mixture was mixed for 30 seconds at 2,000 rpm using a rotation-revolution mixer. After mixing with the spatula, the prepared additive A solution and additive B solution were added, and the mixture was mixed for 1 minute at 2,000 rpm. The conductive additive, NMP, and water were added to the resulting solution, and the mixture was mixed for 1 minute at 2,000 rpm. Finally, the mixture was degassed for 20 seconds at 2,200 rpm to obtain electrode slurries. The total amount of each slurry prepared was 100 g, the solids content was 76.5 mass %, and the solvent composition of the slurry was NMP / H ２ The mass ratio of the water to the slurry was adjusted to 97 / 3. The water was added to intentionally create a state in which the water content in the slurry was high.

[0155] [Example 1-14] Regarding the additives, two components, Additive A1 and Additive B1, were mixed in a 1:1 mass ratio to prepare a 5 mass% NMP solution (mixed solution). The positive electrode active material and binder powder were weighed in a dry mixer and mixed with a spatula to obtain the composition ratio shown in Table 1. NMP and a dispersant solution were added to the mixture, and the mixture was mixed for 30 seconds at 2,000 rpm using a rotation / revolution mixer. After mixing with the spatula, the prepared mixed solution was added, and the mixture was mixed for 1 minute at 2,000 rpm. The conductive additive, NMP, and water were added to the mixture, and the mixture was mixed for 1 minute at 2,000 rpm. Finally, a degassing treatment was performed for 20 seconds at 2,200 rpm to obtain an electrode slurry. The total amount of the prepared slurries was 100 g each, the solid content was 76.5 mass%, and the solvent composition of the slurries was NMP / H ２ The mass ratio of the water to the slurry was adjusted to 97 / 3. The water was added to intentionally create a state in which the water content in the slurry was high.

[0156] The viscosity of the slurries obtained above was measured using an E-type viscometer immediately after preparation. In addition, after storage at 40°C for 24 hours, the presence or absence of gelation was confirmed visually. For those that did not gel, the viscosity was similarly measured using an E-type viscometer. Each table also lists the viscosity values ​​immediately after preparation and after storage. For those that gelled, the "gelled" status was recorded.

[0157]

[0158]

[0159] The results in Tables 1 and 2 above confirm that gelation is suppressed and the storage stability of the electrode slurry is improved in the electrode-forming composition of the present invention, which uses both an acidic compound and a heterocycle-containing compound. Therefore, the electrode-forming composition does not lose its coatability even after a long time has passed since its preparation, and can be suitably used in the industrial production of lithium-ion secondary batteries.

Claims

1. An electrode-forming composition comprising a heterocycle-containing compound that is solid at room temperature, an acid (excluding acids that fall under the category of the heterocycle-containing compound), a positive electrode active material, a binder, and a solvent.

2. The electrode-forming composition according to claim 1, wherein the heterocycle-containing compound has a nitrogen-containing five-membered ring, and the nitrogen-containing five-membered ring does not contain an oxygen atom.

3. The electrode-forming composition according to claim 1, wherein the acid is an organic acid.

4. The electrode-forming composition according to claim 1, wherein the acid is an inorganic acid.

5. The electrode-forming composition according to claim 2, wherein the nitrogen-containing five-membered ring contains only nitrogen atoms as heteroatoms.

6. The electrode-forming composition according to claim 2, wherein the number of nitrogen atoms contained in the nitrogen-containing five-membered ring is two or three.

7. The electrode-forming composition according to claim 1, wherein the heterocycle-containing compound is a non-fused nitrogen-containing five-membered ring-containing compound.

8. The electrode-forming composition according to claim 1, wherein the heterocycle-containing compound is represented by any one of the following formulas (1) to (4): (In formula (1), R ａ and R ｂ are each independently a hydrogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂ may be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent, each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond, Z is N or C(-L-R ｃ ) and R ｃ is a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, and X ａ are each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms. ａ ~R ｃ are each independently a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂ may be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent; each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond; X ａ and X ｂ are each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms.

9. The above R ａ ~R ｃ , X ａ and X ｂ 9. The electrode-forming composition according to claim 8, wherein the substituent of is at least one selected from the group consisting of a carboxy group, a hydroxy group, an aldehyde group, an ester group, a ketone group, an amino group, a phenyl group, a halogen atom, an alkoxysilyl group, an epoxy group, a carboxylic acid chloride group, and a thiol group.

10. The electrode-forming composition according to claim 1, wherein the heterocycle-containing compound is represented by any one of the following formulas (3) to (4): (In formulas (3) to (4), R ａ ~R ｃ are each independently a hydrogen atom, a halogen atom, a carboxy group, a hydroxy group, a thiol group, an amino group, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an alkenyl group having 2 to 6 carbon atoms which may have a substituent, or an aryl group having 6 to 12 carbon atoms which may have a substituent, ａ and R ｂ may be bonded to each other to form a ring having 4 to 12 carbon atoms which may have a substituent; each L is independently a single bond, a carbonyl group, an ether bond, an ester bond, or an amide bond; X ａ and X ｂ are each independently a hydrogen atom, a lithium atom, a sodium atom, an alkyl group having 1 to 6 carbon atoms which may have a substituent, an aryl group having 6 to 12 carbon atoms which may have a substituent, or —CH ２ NR ｄ ２ and R ｄ are each independently an alkyl group having 1 to 10 carbon atoms, an alkanol group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, or an aryl group having 6 to 12 carbon atoms.

11. The above R ａ ~R ｃ , X ａ and X ｂ 11. The electrode-forming composition according to claim 10, wherein the substituent of is at least one selected from the group consisting of a carboxy group, a hydroxy group, an aldehyde group, an ester group, a ketone group, an amino group, a phenyl group, a halogen atom, an alkoxysilyl group, an epoxy group, a carboxylic acid chloride group, and a thiol group.

12. The electrode-forming composition according to claim 10, wherein the heterocycle-containing compound is represented by the following formula (4b): (In formula (4b), R ａ ~R ｃ is the same as above, and R ｅ is a hydrogen atom, a carboxy group, a hydroxy group, an aldehyde group, an ester group, a ketone group, an amino group, a phenyl group, a halogen atom, an alkoxysilyl group, an epoxy group, a carboxylic acid chloride group, or a thiol group.

13. The electrode-forming composition according to claim 1, further comprising a conductive additive.

14. The positive electrode active material is lithium-containing transition metal oxide particles having a layered rock salt structure, and the lithium-containing transition metal oxide particles having a layered rock salt structure are represented by the general formula Li ａ Ni （１－ｘ－ｙ） Co ｘ M １ ｙ M ２ ｚ O ２ (In the formula, M １ is at least one selected from the group consisting of Mn and Al, M ２ represents at least one element selected from the group consisting of Zr, Ti, Mg, B, W, and V, and 1.00≦a≦1.50, 0.00≦x≦0.50, 0.00≦y≦0.50, and 0.000≦z≦0.020, with the proviso that the Ni content is 30 mass% or more.

15. The electrode-forming composition according to claim 1, wherein the positive electrode active material comprises a first positive electrode active material that is a polycrystalline body and a second positive electrode active material that is a single crystal body.

16. The electrode-forming composition according to claim 1, further comprising a dispersant.

17. The electrode-forming composition according to claim 16, wherein the dispersant is a polymer containing a pyrrolidone structure or a nitrile group.

18. The electrode-forming composition according to claim 16, wherein the dispersant is at least one selected from the group consisting of polyvinylpyrrolidone and polyacrylonitrile.

19. The electrode-forming composition according to claim 1, wherein the content of the heterocycle-containing compound is 0.001% by mass to 0.5% by mass of the solid content.

20. The electrode-forming composition according to claim 1, wherein the content of the acid is 0.0001% by mass to 0.5% by mass based on the solid content.

21. The electrode-forming composition according to claim 1, wherein the content of said acid relative to the content of said heterocycle-containing compound is 0.01 to 2 in mass ratio.

22. An electrode layer obtained from the electrode-forming composition according to any one of claims 1 to 21.

23. A secondary battery comprising the electrode layer according to claim 22.

24. An additive for an electrode-forming composition containing a positive electrode active material, a binder, and a solvent, the additive containing a heterocycle-containing compound that is solid at room temperature and an acid (excluding acids that fall under the category of the heterocycle-containing compound).

25. A gelation inhibitor for an electrode-forming composition containing a positive electrode active material, a binder, and a solvent, the gelation inhibitor comprising a heterocycle-containing compound that is solid at room temperature, and an acid (excluding acids that fall under the category of the heterocycle-containing compound).

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