Composition for electrode formation, additive, and gelation inhibitor

Abstract

The present invention provides a composition for electrode formation. In the composition, thickening and gelation of electrode slurry are inhibited by a simple method. The composition exhibits improved storage stability and comprises: an organic acid containing an optionally substituted aromatic ring and a linker; a positive electrode active material; a conductive additive; 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 most common method for manufacturing a negative electrode is to coat a copper foil current collector with the negative electrode slurry and then dry it. The most common 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 High-nickel positive electrode active materials, such as NiO2, have a high discharge capacity and are attractive positive electrode materials. However, alkaline components such as LiOH, Li2O, LiHCO3, and Li2CO3 are generated on the surface through proton exchange reactions with raw material residues or moisture, and reactions with moisture and carbon dioxide in the air.

[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 ability to bind water, there is a concern that the polymer itself may bring in water, and strict control over pre-drying is required to prevent this.

[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 gelation inhibition of various organic acids and inorganic acids that have been conventionally used, they found that Li x In electrode slurries containing high-nickel positive electrode active materials such as NiO2, gelation was not sufficiently suppressed.

[0017] In view of the above, an object of the present invention is to provide an electrode-forming composition that can suppress thickening and gelation of an electrode slurry by a simple method and improve storage stability, as well as an additive and a gelation inhibitor for an electrode slurry containing a positive electrode active material, a conductive additive, a binder, and a solvent. Another object of the present invention is to provide an electrode layer using the electrode-forming composition, and an electrode and a secondary battery that include the electrode layer.

[0018] The present inventors have conducted extensive research to achieve the above object, and have found that adding a specific organic acid having an aromatic ring and a linker moiety, which may have a substituent, to an electrode slurry containing at least a positive electrode active material, a conductive additive, a binder, and a solvent can suppress thickening and gelation of the composition and improve storage stability. Furthermore, they have found that an electrode fabricated using the electrode-forming composition of the present invention can suppress deterioration in a battery caused by alkaline components and can also improve battery characteristics, thereby completing the present invention.

[0019] That is, the present invention provides the following electrode-forming composition, additive, and gelation inhibitor: 1. An electrode-forming composition comprising an organic acid having an aromatic ring and a linker moiety, which may have a substituent, a positive electrode active material, a conductive assistant, a binder, and a solvent. 2. An electrode-forming composition 1, wherein the organic acid is represented by the following general formula (I) or (II): A-L-CO2-B 1 (I) (wherein A represents an aromatic ring which may have a substituent, L represents a linker portion, B1 represents a monovalent organic group having a hydrogen atom or a carboxy group. 2 (II) (wherein A represents an aromatic ring which may have a substituent, L represents a linker portion, B 2 represents a monovalent organic group having a carboxy group. 3. The electrode-forming composition according to 1 or 2, wherein the aromatic ring is represented by the following formula (a): [(wherein, R 1 ~R 5 are each independently a hydrogen atom, a hydroxy group, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a phenyl group, R a -COO-, or R a represents —OCO—, and R a represents an alkyl group having 1 to 5 carbon atoms, or R 1 ~R 5 represents a group represented by the following formula (X) formed by bonding two adjacent groups to each other, and * represents a site bonding to the linker portion. (wherein X represents an alkylene group having 1 to 6 carbon atoms, and *1 and *2 represent a bonding site to a benzene ring.) 4. An electrode-forming composition according to any one of 1 to 3, wherein the aromatic ring has two or more groups selected from the group consisting of a hydroxy group and an alkoxy group having 1 to 5 carbon atoms. 5. An electrode-forming composition according to any one of 1 to 4, wherein the linker portion contains an unsaturated bond. 6. An electrode-forming composition according to 5, wherein the unsaturated bond is a carbon-carbon double bond. 7. An electrode-forming composition according to any one of 1 to 6, further comprising a dispersant. 8. The positive electrode active material is lithium-containing transition metal oxide particles, wherein the lithium-containing transition metal oxide particles are represented by the general formula Li a Ni (1-x-y) Co x M 1 y M 2 z O2 (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% by mass or more. 9. An electrode-forming composition according to any one of 1 to 8, wherein the organic acid content is 0.001 to 0.5% by mass of the solid content. 10. An electrode layer obtained from the electrode-forming composition according to any one of 1 to 9. 11. An electrode comprising the electrode layer of 10. 12. A secondary battery comprising the electrode of 11. 13. An additive for an electrode slurry comprising a positive electrode active material, a conductive additive, a binder, and a solvent, the additive comprising an organic acid having an aromatic ring which may have a substituent and a linker moiety. 14. An additive composition for electrode slurry comprising a positive electrode active material, a conductive aid, a binder, and a solvent, the additive composition comprising the additive of 13. 15. The additive composition of 14, further comprising a dispersant. 16. A gelation inhibitor for electrode slurry comprising a positive electrode active material, a conductive aid, a binder, and a solvent, the gelation inhibitor comprising an organic acid having an aromatic ring which may have a substituent and a linker moiety. 17. A gelation inhibitor composition for electrode slurry comprising a positive electrode active material, a conductive aid, a binder, and a solvent, the gelation inhibitor of 16. 18. The gelation inhibitor composition of 17, further comprising a dispersant.

[0020] According to the present invention, it is possible to provide an electrode-forming composition that suppresses thickening and gelation of an electrode slurry and improves storage stability by a simple method, as well as an additive and a gelation inhibitor for an electrode slurry that includes a positive electrode active material, a conductive additive, a binder, and a solvent. Furthermore, according to the present invention, it is possible to provide an electrode layer using the electrode-forming composition, and an electrode and a secondary battery that include the electrode layer.

[0021] In the present invention, the above-described effect of suppressing thickening and gelation is achieved by adding an organic acid having an aromatic ring and a linker moiety, which may have a substituent, to an electrode slurry containing a positive electrode active material, a binder, and a solvent. This organic acid adheres to the positive electrode active material, forming a protective coating on the surface of the positive electrode active material. This protective coating suppresses the proton exchange reaction between the active material and water, and also suppresses the reaction between alkaline components derived from the positive electrode active material and the binder, particularly a fluorine-based binder. As a result, thickening and gelation of the composition can be suppressed, improving storage stability. It is known that electrode slurries containing a positive electrode active material, a binder, and a solvent are prone to gelation when a positive electrode active material containing a high amount of Ni, for example, a positive electrode active material containing 30% by mass or more of Ni, is used. Adding the additive or gelation inhibitor of the present invention to the electrode slurry significantly suppresses gelation of the electrode slurry. According to the present invention, thickening and gelation of the electrode slurry are suppressed, thereby enabling the formation of a homogeneous positive electrode layer. Furthermore, it is possible to increase the solids concentration in the electrode slurry, thereby reducing the cost and environmental impact of producing an energy storage device. Furthermore, it is possible to suppress corrosion of aluminum foil, which is commonly used as a current collector foil, caused by alkaline components, and deterioration of battery characteristics due to reaction with the electrolyte. Therefore, according to the present invention, gelation of the electrode slurry can be suppressed regardless of the type of conductive additive or binder. However, the present invention is not limited to these mechanisms.

[0022] [Electrode-forming composition] The present invention is characterized in that when an additive containing an organic acid having an aromatic ring and a linker moiety, which may be substituted, or a gelation inhibitor containing an organic acid having an aromatic ring and a linker moiety, which may be substituted, is added to an electrode slurry containing a positive electrode active material, a binder, and a solvent, a majority of the additive or the gelation inhibitor adheres to the positive electrode active material. The form of adhesion is not limited here, and adhesion may occur via physical adsorption, chemical adsorption, or chemical bonding such as ionic bonding or hydrogen bonding. In the present invention, terms related to gelation inhibition, such as "gelation inhibition," mean that gelation of the electrode-forming composition is suppressed compared to when the additive or gelation inhibitor of the present invention is not included.

[0023] <Organic Acid Having an Aromatic Ring Which May Have a Substituent and a Linker Moiety> The electrode-forming composition of the present invention contains a positive electrode active material, a binder, a solvent, and an organic acid which has an aromatic ring which may have a substituent and a linker moiety (hereinafter, sometimes simply referred to as "organic acid").

[0024] Preferred embodiments of the organic acid include those represented by the following general formula (I) or (II): A-L-CO2-B 1 (I) AL-CONH-B 2 (II)

[0025] In the formula, A represents an aromatic ring which may have a substituent, L represents a linker portion, and B 1 represents a monovalent organic group having a hydrogen atom or a carboxy group, B 2 represents a carboxy group or a monovalent organic group having a carboxy group.

[0026] Both an aromatic hydrocarbon ring and an aromatic heterocyclic ring can be employed as the aromatic ring represented by A. Examples of the aromatic hydrocarbon ring include a benzene ring as a monocyclic aromatic hydrocarbon ring, and a naphthalene ring, an anthracene ring, a phenanthrene ring, a fluorene ring, a pyrene ring, and a triphenylene ring as a polycyclic aromatic hydrocarbon ring.

[0027] Examples of the aromatic heterocyclic ring include a monocyclic heteroaromatic ring such as a pyrrole ring, a thiophene ring, a furan ring, an imidazole ring, a pyrazole ring, a thiazole ring, an oxazole ring, a pyridine ring, and a pyrazine ring, and examples of the polycyclic heteroaromatic ring include an indole ring, an isoindole ring, a benzimidazole ring, a quinoline ring, an isoquinoline ring, and a quinoxaline ring.

[0028] In the present invention, an aromatic hydrocarbon ring is preferred, and a benzene ring is more preferred.

[0029] The aromatic ring represented by A may have a substituent. Examples of the substituent include a hydroxy group, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a phenyl group, an R a -COO-, R a -OCO-(R a In the present invention, from the viewpoint of peel strength between the electrode layer and the current collector, a hydroxy group, an alkoxy group having 1 to 5 carbon atoms, R a -COO- and R a —OCO— is preferred, and a hydroxy group and an alkoxy group having 1 to 5 carbon atoms are more preferred.

[0030] The number of substituents varies depending on the type of aromatic ring, but is, for example, preferably 0 to 6, and more preferably 0 to 3. When the aromatic ring has two or more substituents, it preferably has two or more groups selected from the group consisting of hydroxy groups and alkoxy groups having 1 to 5 carbon atoms, and more preferably has one or more hydroxy groups and one or more alkoxy groups having 1 to 5 carbon atoms.

[0031] The alkyl group having 1 to 5 carbon atoms may be linear, branched, or cyclic, and examples thereof include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, a c-propyl group, an n-butyl group, an i-butyl group, an s-butyl group, a t-butyl group, a c-butyl group, an n-pentyl group, a 1-methyl-n-butyl group, a 2-methyl-n-butyl group, a 3-methyl-n-butyl group, a 1,1-dimethyl-n-propyl group, a 2,2-dimethyl-n-propyl group, a c-pentyl group, and a 2-methyl-c-butyl group.

[0032] The alkoxy group having 1 to 5 carbon atoms may be linear, branched or cyclic, and examples thereof include a methoxy group, an ethoxy group, an isopropoxy group and an n-pentyloxy group.

[0033] When the aromatic ring has two or more substituents, two adjacent substituents may be bonded to each other to form a ring.

[0034] A preferred embodiment of the aromatic ring is represented by the following formula (a).

[0035]

[0036] In the formula, R 1 ~R 5 are each independently a hydrogen atom, a hydroxy group, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a phenyl group, R a -COO-, or R a represents —OCO—, and R a represents an alkyl group having 1 to 5 carbon atoms, or R 1 ~R 5 represents a group represented by the following formula (X) formed by bonding two adjacent groups to each other, and * represents a site bonding to the linker portion.

[0037]

[0038] In the formula, X represents an alkylene group having 1 to 6 carbon atoms, and *1 and *2 represent the bonding sites to the benzene ring.

[0039] Specific examples of the alkylene group having 1 to 6 carbon atoms include a methylene group, a methylmethylene group, a dimethylmethylene group, an ethylene group, a propylene group, a 1,2-dimethylethylene group, a tetramethylethylene group, etc. In the present invention, a methylene group is preferred from the viewpoint of availability.

[0040] A preferred embodiment of the aromatic ring represented by A is represented by the following formula (a1).

[0041] (In the formula, R 2 ~R 4 and * have the same meaning as above.)

[0042] In addition, a preferred embodiment in which two adjacent substituents of the aromatic ring represented by A are bonded to each other to form a ring is an embodiment represented by the following formula (a2).

[0043] (In the formula, R 1 , R 4 , R 5 , X and * have the same meanings as above.)

[0044] Further, a preferred embodiment of the group represented by the above formula (a2) is an embodiment represented by the following formula (a2-1).

[0045] (In the formula, * has the same meaning as above.)

[0046] The linker moiety represented by L preferably contains an unsaturated bond, more preferably a carbon-carbon double bond.

[0047] Examples of the linker moiety represented by L include alkylene groups having 1 to 5 carbon atoms, alkenylene groups having 2 to 5 carbon atoms, and arylene groups having 6 to 10 carbon atoms.

[0048] Specific examples of alkylene groups having 1 to 5 carbon atoms include methylene, methylmethylene, dimethylmethylene, ethylene, 1,2-dimethylethylene, trimethylene, propylene, tetramethylene, and pentamethylene.

[0049] Specific examples of the alkenylene group having 2 to 5 carbon atoms include an ethenylene group, a propenylene group, a butenylene group, and a pentenylene group.

[0050] Specific examples of the arylene group having 6 to 10 carbon atoms include a 1,2-phenylene group, a 1,3-phenylene group, a 1,4-phenylene group, a 1,2-naphthalene-diyl group, a 2,3-naphthalenediyl group, a 1,4-naphthalenediyl group, a 1,5-naphthalenediyl group, a 2,6-naphthalenediyl group, a 2,7-naphthalenediyl group, and a 1,8-naphthalenediyl group.

[0051] Above B 1 and B 2Examples of the organic group having a carboxy group represented by the formula (c1) include a benzenecarboxylic acid group and a group represented by the following formula (c1).

[0052] (In the formula, *3 represents a site for bonding to another group.)

[0053] In addition, preferred embodiments of the organic acids represented by the above general formulas (I) and (II) include, but are not limited to, embodiments represented by the following formulas (I-1) and (II-1):

[0054] (In the formula, R 1 ~R 5 , B 1 and B 2 has the same meaning as above.)

[0055] Specific examples of the organic acid include, but are not limited to, those represented by any of the following formulas (A1) to (A14).

[0056]

[0057] The content of the organic acid is preferably 0.001 to 0.5 mass% of the solid content, more preferably 0.001 to 0.4 mass%, even more preferably 0.01 to 0.4 mass%, even more preferably 0.01 to 0.3 mass%, and particularly preferably 0.01 to 0.25 mass%. An even more preferable lower limit of the content of the organic acid is 0.01 mass% of the solid content. By keeping the content of the organic acid within this range, gelation of the electrode-forming composition can be effectively suppressed, and the battery characteristics of the resulting battery can be maintained. In the present invention, the solid content refers to the components other than the solvent that constitute the composition (the same applies hereinafter).

[0058] <Positive Electrode Active Material> The positive electrode active material is not particularly limited.

[0059] 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 lithium secondary batteries or lithium ion secondary batteries, lithium-containing chalcogen compounds or lithium-containing oxides capable of absorbing and releasing lithium ions, elemental sulfur and its compounds, etc. can be used.

[0060] Examples of lithium-containing oxides include LiNiO2, Li x Ni y M 1-y O2 (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 O2 (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, where 1.00≦a≦1.50, 0.00≦x≦0.50, 0.00≦y≦0.50, 0.000≦z≦0.020). Examples of sulfur compounds include sulfur, Li2S, FeS2, TiS2, MoS2, and rubeanic acid. These positive electrode active materials can be used alone or in combination of two or more.

[0061] As the positive electrode active material, a lithium-containing oxide containing Fe or Ni is preferred because it further improves battery capacity, uses less rare metals, and is low cost. From the viewpoint of further increasing battery capacity, a lithium-containing oxide containing Ni is more preferred. The content of Fe or Ni in the lithium-containing oxide is preferably 30% by mass or more. In consideration of further reducing the amount of rare metals used and obtaining a battery with a longer life, 35% by mass or more is more preferred, 40% by mass or more is even more preferred, 45% by mass or more is even more preferred, and 47% by mass or more is particularly preferred. The upper limit is not particularly limited, but is usually 61% by mass or less.

[0062] Among the above positive electrode active materials, the general formula Li a Ni (1-x-y) Co x M 1 y M 2 z O2 (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, W, and V, and is preferably a lithium-containing transition metal oxide represented by the following formula: 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.

[0063] The positive electrode active material is Li x Ni y M 1-y O2 (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 yM 2 z O2 (in the formula, M 1 is at least one selected from the group consisting of Mn and Al; 2 represents at least one selected from the group consisting of Zr, Ti, Mg, B, Zr, Si, W, and V, and more preferably lithium-containing transition metal oxide particles represented by 1.00≦a≦1.50, 0.00≦x≦0.50, 0.00≦y≦0.50, 0.000≦z≦0.020), and Li a Ni (1-x-y) Co x M 1 y M 2 z O2 (in the formula, M 1 is at least one selected from the group consisting of Mn and Al; 2 represents at least one 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) are even more preferred.

[0064] 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 the organic 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), thickening and gelation of the composition that is more likely to thicken and gel can be suppressed. 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.

[0065] 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.

[0066] The particle diameters of the positive electrode active materials can be measured, for example, by a laser diffraction scattering method and expressed as a volume-based particle diameter D50. The particle diameters of the first positive electrode active material are preferably 8 to 20 μm and the second positive electrode active material are preferably 2 to 6 μm, and more preferably 9 to 15 μm and 3 to 5 μm, respectively.

[0067] The content of the positive electrode active material is not particularly limited, but is preferably 88.0 to 99.899 mass % of the solid content, more preferably 88.0 to 99.899 mass %, and even more preferably 95.0 to 99.0 mass %.

[0068] <Conductive Aid> The electrode-forming composition of the present invention may further contain a conductive aid to improve electrical conductivity. Examples of the conductive aid include carbon materials such as graphite, carbon black, acetylene black (AB), vapor-grown carbon fiber, carbon nanotubes (CNT), carbon nanohorns, and graphene, and conductive polymers such as polyaniline, polypyrrole, polythiophene, polyacetylene, and polyacene. From the viewpoint of conductivity and ease of preparation of the electrode-forming composition, it is preferable to use carbon black, acetylene black, vapor-grown carbon fiber, carbon nanotubes, and carbon nanohorns, and it is more preferable to use carbon black, acetylene black, and carbon nanotubes. The conductive aids can be used alone or in combination of two or more.

[0069] When the conductive additive is contained, its content is not particularly limited, but is preferably 0.05 to 5 mass % of the solid content, more preferably 0.05 to 4 mass %, even more preferably 0.1 to 3 mass %, and even more preferably 0.2 to 2 mass %. By setting the content of the conductive additive within the above range, good electrical conductivity can be obtained.

[0070] <Binder> The binder can be appropriately selected from known materials and is not particularly limited. Specific examples include fluorine-based binders such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, copolymers containing at least one monomer selected from the group consisting of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene, and non-aqueous binders such as polyimide, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, polyethylene, and polypropylene. In the present invention, the use of a fluorine-based binder is preferable from the viewpoint of improving the storage stability of the composition. Furthermore, the fluorine-based binder is preferably modified with a polar functional group such as a carboxy 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) spectrometer. The binders can be used alone or in combination of two or more.

[0071] The weight-average molecular weight (Mw) of the binder is 600,000 to 3,000,000, preferably 700,000 to 2,000,000, and more preferably 700,000 to 1,500,000, from the viewpoint of improving the adhesion between the current collector and the electrode layer. The weight-average molecular weight is a polystyrene-equivalent value determined by gel permeation chromatography (GPC).

[0072] From the viewpoints of reducing costs and obtaining a high energy density, the content of the binder is preferably 0.05 to 8 mass %, 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 %, based on the solid content.

[0073] <Other Components> The electrode-forming composition of the present invention may further contain other components. For example, a dispersant may be included to improve the dispersibility of the active material and conductive additive. The dispersant can be selected from those conventionally used as dispersants for conductive carbon materials such as CNTs. However, from the viewpoint of stability within the battery, a nonionic polymer is preferred. Examples of the nonionic polymer include polyvinylpyrrolidone (PVP) and polymers having at least one group selected from the group consisting of a nitrile group, a hydroxy group, a carbonyl group, an amino group, a sulfonyl group, and an ether group. Specific examples of the polymer include polyvinyl alcohol, polyacrylonitrile, polylactic acid, polyester, polyimide, polyphenyl ether, polyphenylsulfone, polyethyleneimine, and polyaniline. In the present invention, polymers having a pyrrolidone structure or a nitrile group are preferred, and polyvinylpyrrolidone and polyacrylonitrile are more preferred. The dispersants may be used alone or in combination of two or more. The dispersant may be added simultaneously with the organic acid additive or gelation inhibitor, or the organic acid additive or gelation inhibitor and the dispersant may be added separately. In addition, the addition of a nonionic polymer is expected to improve the dispersibility of the active material and conductive additive, as well as the adhesion to the current collector foil.

[0074] When the dispersant is contained, its content 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 % of the solid content. An even more preferable lower limit of the content of the dispersant is 0.01 mass % of the solid content. Furthermore, in consideration of the adhesion between the resulting electrode layer and the current collector, the total amount of the organic acid and the dispersant is preferably 0.001 to 1 mass %, more preferably 0.01 to 1 mass %, of the solid content.

[0075] <Solvent> The electrode-forming composition of the present invention contains a solvent. The solvent is not particularly limited as long as it is one that has conventionally been used in preparing electrode-forming compositions, and examples thereof include water; ethers such as tetrahydrofuran (THF), diethyl ether, and 1,2-dimethoxyethane (DME); halogenated hydrocarbons such as methylene chloride, chloroform, and 1,2-dichloroethane; amides such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methyl-2-pyrrolidone (NMP); ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; and alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, and t-butanol. Examples of suitable solvents include alcohols, aliphatic hydrocarbons such as n-heptane, n-hexane, and cyclohexane, aromatic hydrocarbons such as benzene, toluene, xylene, and ethylbenzene, glycol ethers such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and propylene glycol monomethyl ether, glycols such as ethylene glycol and propylene glycol, carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, and organic solvents such as γ-butyrolactone, dimethyl sulfoxide (DMSO), dioxolane, and sulfolane. These solvents may be used alone or in combination of two or more.

[0076] 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., and may be appropriately selected depending on the type of binder. NMP is suitable for water-insoluble binders such as PVdF, and water is suitable for water-soluble binders.

[0077] The solids concentration of the electrode-forming composition of the present invention is appropriately set taking into consideration the coatability of the composition, the thickness of the electrode to be formed, and the like, but is usually about 60 to 92 mass %, preferably about 65 to 90 mass %, and more preferably about 70 to 85 mass %.

[0078] The viscosity of the electrode-forming composition of the present invention is set appropriately taking into consideration the coating method, the thickness of the electrode to be formed, and the like, but is usually 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.

[0079] 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 additive (the organic acid), the positive electrode active material, and the binder of the present invention, the 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.

[0080] [Method for producing an electrode-forming composition] Examples of a method for producing an electrode-forming composition include, but are not limited to, a method including a step of mixing a positive electrode active material, a binder, and a solvent to prepare a dispersion, and a step of mixing the dispersion with the organic acid. In addition, when a conductive additive is used, it is preferable to mix it in the step of preparing the dispersion.

[0081] Here, the organic acid may be mixed alone, or may be mixed as an additive in which the organic acid is combined with at least one selected from the group consisting of a nonionic polymer and a solvent.

[0082] Examples of the nonionic polymer include the same ones as those exemplified in the description of the dispersant, and polymers having a pyrrolidone structure or a nitrile group are preferred, with polyvinylpyrrolidone and polyacrylonitrile being more preferred.

[0083] When the organic acid is mixed with a solvent, examples of the solvent that can be used include the same solvents as those exemplified above. Among these, NMP, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate are particularly suitable for use in the present invention.

[0084] [Electrode] The electrode of the present invention comprises an electrode layer made of the electrode-forming composition described above on at least one surface of a substrate, which is a current collector. Examples of methods for forming the electrode layer on the substrate include a method in which the prepared electrode-forming composition is applied to the 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.

[0085] 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 about 50 to 400°C, and more preferably about 70 to 150°C.

[0086] Examples of substrates used for the 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. In particular, the thickness of the substrate is not particularly limited, but in the present invention, it is preferably 1 to 100 μm, more preferably 3 to 30 μm, and most preferably 5 to 25 μm.

[0087] The thickness of the electrode layer is not particularly limited, but is preferably about 0.01 to 1,000 μm, more preferably about 5 to 300 μm. When the electrode layer is used alone as an electrode, the thickness is preferably 10 μm or more.

[0088] 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 more 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.

[0089] [Secondary Battery] The secondary battery of the present invention is provided with the above-described electrodes, more specifically, is provided with at least one pair of positive and negative electrodes, a separator interposed between the electrodes, and an electrolyte, and the positive electrode is formed from the above-described electrode. Other constituent members of the battery element may be appropriately selected from conventionally known components.

[0090] Examples of materials used for the separator include glass fiber, cellulose, porous polyolefin, polyamide, and polyester.

[0091] 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. may be preferably used.

[0092] Examples of the electrolyte salt include LiPF, LiBF, and LiN(SOF). 2、 Examples of the electrolyte salt include lithium salts such as LiN(CFSO), LiAsF, LiSbF, LiAlF, LiGaF, LiInF, LiClO, LiN(CFSO), LiCFSO, LiSiF, and LiN(CFSO)(CFSO), metal iodides such as LiI, NaI, KI, CsI, and CaI, iodide salts of quaternary imidazolium compounds, iodide salts and perchlorates of tetraalkylammonium compounds, and metal bromides such as LiBr, NaBr, KBr, CsBr, and CaBr. These electrolyte salts can be used alone or in combination of two or more.

[0093] 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. For example, non-aqueous solvents include cyclic esters such as ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone; ethers such as tetrahydrofuran and dimethoxyethane; chain esters such as methyl acetate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and nitriles such as acetonitrile. These solvents can be used alone or in combination of two or more.

[0094] 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.

[0095] The sulfide-based solid electrolyte may be a Li2S-SiS2-lithium compound (wherein the lithium compound is Li3PO 4、 At least one selected from the group consisting of LiI and LiSiO 、 Li2S-PO 5、 Li2S-B2S 5、 Examples include thiolithium-based materials such as Li2S-P2S5-GeS2.

[0096] The oxide-based solid electrolyte is Li5La3M2O, an oxide having a garnet structure. 12 (M = Nb, Ta) and Li7La3Zr2O 12 , oxyacid salt compounds based on the γ-Li3PO4 structure, collectively known as LISICON, perovskite-type, Li, 3.3 P.O. 3.8 N 0.22, sodium / alumina, etc. Examples of the polymer solid electrolyte include polyethylene oxide 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 solid electrolyte may contain a supporting salt and a plasticizer.

[0097] The supporting salt contained in the polymer solid electrolyte may include lithium (fluorosulfonylimide), and the plasticizer may include succinonitrile.

[0098] A battery manufactured using the electrode-forming composition of the present invention has high battery characteristics even though it contains less fluorine binder than a typical secondary battery.

[0099] The type of secondary battery and the type of electrolyte are not particularly limited, and any type of battery 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 are also not particularly limited.

[0100] When applied to a coin cell, the electrode of the present invention described above may be punched into a predetermined disk shape for use. For example, a lithium-ion secondary battery can be produced by placing one 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 that, placing the electrode of the present invention on top with the electrode layer facing downwards, placing a case and a gasket on top, and sealing the battery with a coin cell crimping machine.

[0101] [Additive] The present invention also provides an additive for an electrode slurry containing a positive electrode active material, a conductive additive, a binder, and a solvent, the additive comprising an organic acid having an aromatic ring and a linker moiety, which may have a substituent. The additive can be suitably used as a gelation inhibitor for an electrode slurry containing a positive electrode active material, a conductive additive, a binder, and a solvent. Furthermore, the present invention can be suitably applied as a method for suppressing gelation of an electrode slurry by adding the organic acid to the electrode slurry.

[0102] As the organic acid having an aromatic ring which may have a substituent and a linker portion, the same ones as those described in the description of the electrode-forming composition can be used, and the same applies to preferred specific examples.

[0103] The additive may further include a non-ionic polymer.

[0104] Examples of the nonionic polymer include the same dispersants as those exemplified in the description of other components of the electrode-forming composition, and polymers having a pyrrolidone structure or a nitrile group are preferred, with polyvinylpyrrolidone and polyacrylonitrile being more preferred.

[0105] When the additive contains the polymer, its content is not particularly limited, but is preferably 0.001 to 0.5 mass % of the solid content, more preferably 0.001 to 0.3 mass %, and even more preferably 0.001 to 0.2 mass %. An even more preferable lower limit of the polymer content is 0.01 mass % of the solid content.

[0106] The additive may further contain a solvent. Examples of the solvent include those exemplified in the description of the electrode-forming composition. In the present invention, NMP, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate are particularly suitable.

[0107] When the additive contains a solvent, the organic acid is preferably dissolved or dispersed in the solvent, and more preferably dissolved in the solvent.

[0108] When the additive of the present invention contains a solvent, the solids concentration of the additive is appropriately set taking into consideration the saturated solubility in the solvent, storage stability, etc., but is usually about 1 to 60 mass %, preferably about 3 to 55 mass %, and more preferably about 3 to 50 mass %.

[0109] The positive electrode active material and binder of the electrode slurry are also the same as those described in the explanation of the electrode-forming composition.

[0110] [Gelation Inhibitor] The present invention further provides a gelation inhibitor for electrode slurries containing a positive electrode active material, a conductive additive, a binder, and a solvent, the gelation inhibitor comprising an organic acid having an aromatic ring, which may have a substituent, and a linker moiety. The gelation inhibitor of the present invention can more effectively inhibit gelation of electrode slurries containing a positive electrode active material, a binder, and a solvent.

[0111] As the organic acid having an aromatic ring which may have a substituent and a linker portion, the same ones as those described in the description of the electrode-forming composition can be used, and the same applies to preferred specific examples.

[0112] The gelation inhibitor may further contain a nonionic polymer.

[0113] Examples of the nonionic polymer include the same dispersants as those exemplified in the description of other components of the electrode-forming composition, and polymers having a pyrrolidone structure or a nitrile group are preferred, with polyvinylpyrrolidone and polyacrylonitrile being more preferred.

[0114] When the gelation inhibitor contains the polymer, its content 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 % of the solid content. An even more preferable lower limit of the polymer content is 0.01 mass % of the solid content.

[0115] The gelation inhibitor may further contain a solvent. Examples of the solvent include those exemplified in the description of the electrode-forming composition. In the present invention, NMP, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate are particularly suitable.

[0116] When the gelation inhibitor contains a solvent, the organic acid is preferably dissolved or dispersed in the solvent, and more preferably dissolved in the solvent.

[0117] When the gelation inhibitor of the present invention contains a solvent, the solids concentration of the gelation inhibitor is appropriately set taking into consideration the saturated solubility in the solvent, storage stability, etc., but is usually about 1 to 60 mass %, preferably about 3 to 55 mass %, and more preferably about 3 to 50 mass %.

[0118] The positive electrode active material and binder of the electrode slurry are also the same as those described in the explanation of the electrode-forming composition.

[0119] EXAMPLES The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0120] The raw materials used in the examples and comparative examples are as follows: <Active material> NCM-1: Lithium nickel manganese cobalt oxide (LiNi 0.88 Co 0.07 Mn 0.05 O2, manufactured by Ningbo Ronbay New Energy Technology Co., Ltd., S90F, Ni content: 55 mass%, particle size (D50) measured by laser diffraction scattering method: 9.3 μm, polycrystalline type) NCM-2: lithium nickel manganese cobalt oxide (LiNi 0.91 Co 0.07 Mn 0.02O2, Ningbo Ronbay NewEnergy Technology Co. , Ltd. SC92Y, Ni content: 53% by mass, particle size (D50) measured by laser diffraction scattering method: 3.8 μm, single crystal type) <Binder> Solef-5140: polyvinylidene fluoride (PVdF), manufactured by Solvay <Conductive additive> AB: Denka Black (registered trademark) Li100 (high-purity acetylene black), manufactured by Denka Company, Ltd. <Solvent> NMP: N-methyl-2-pyrrolidone, manufactured by Nippon Refine Co., Ltd. <Dispersant> PVP: polyvinylpyrrolidone, manufactured by Nippon Shokubai Co., Ltd., K-90 <Additives> A1: trans-Cinnamic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A2: p-Coumarin acid, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. A3: trans-Ferrulic A4: Isoferulic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A5: Caffeic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A6: Sinapinic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A7: Chlorogenic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A8: 4-Methylcinnamic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A9: 4-Methoxycinnamic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A10: 3-Phenylpropionic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A11: 3,4-Methylenedioxycinnamic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A12: trans-4-Acetoxycinnamic acid, manufactured by Tokyo Chemical Industry Co., Ltd. B1: trans-Cinnamamide, manufactured by Tokyo Chemical Industry Co., Ltd. B2: trans-Cinnamalaldehyde, manufactured by Kanto Chemical Co., Ltd. B3: (E)-Cinnamyl alcohol, manufactured by Tokyo Chemical Industry Co., Ltd. B4: β-Methylstyrene, manufactured by Tokyo Chemical Industry Co., Ltd. B5: Benzoic acid, manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd. B6: Adipic acid, manufactured by Tokyo Chemical Industry Co., Ltd.

[0121] (1) Preparation of Positive Electrode Composition (Electrode Slurry) for Viscosity Measurement [Example 1-1] An NMP solution containing 5% by mass of additive was prepared. The positive electrode active material, binder powder, conductive additive, additive solution, NMP, and water were mixed in a dry mixer to obtain the composition shown in Table 1, and mixed using a rotation-revolution mixer (Thinky Corporation, Awatori Rentaro Atmospheric Pressure Type ARE-310) to obtain an electrode slurry. The total amount of each slurry prepared was 50 g, the solid content was 76.5% by mass, and the solvent composition of the slurry was adjusted to NMP / HO = 97 / 3 (mass ratio). Note that the water was added to intentionally create a high water content in the slurry. In Tables 1 and 2, "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 materials was 8:2.

[0122] [Comparative Example 1-1] A 5% by mass NMP solution of PVP was prepared as a dispersant. The positive electrode active material, binder powder, conductive additive, dispersant solution, NMP, and water were mixed in a dry mixer to obtain the composition ratios shown in Table 2, and mixed using a rotation-revolution mixer (Thinky Corporation, Awatori Rentaro Atmospheric Pressure Type ARE-310) to obtain an electrode slurry. The total amount of the prepared slurries was 50 g each, the solid content was 76.5% by mass, and the solvent composition of the slurry was adjusted to NMP / HO = 97 / 3 (mass ratio). Note that the water was added to intentionally create a high water content in the slurry.

[0123] [Examples 1-2 to 1-16, Comparative Examples 1-2 to 1-7] A 5% by mass NMP solution was prepared for each additive and dispersant. The positive electrode active material, binder powder, conductive additive, dispersant solution, additive solution, NMP, and water were mixed in a dry mixer to obtain the composition ratios shown in Tables 1 and 2, and then mixed using a rotation / revolution mixer to obtain an electrode slurry. The total amount of each slurry prepared was 50 g, the solids content was 76.5% by mass, and the solvent composition of the slurry was adjusted to NMP / HO = 97 / 3 (mass ratio). Note that the water was added to intentionally create a high moisture content in the slurry.

[0124] [Examples 1-17] The positive electrode active material, binder powder, conductive additive, dispersant solution, additive powder, NMP, and water were mixed in a dry mixer so as to obtain the composition ratios shown in Table 1, and mixed using a rotation-revolution mixer (Thinky Corporation, Awatori Rentaro Atmospheric Pressure Type ARE-310) to obtain electrode slurries. The total amount of the prepared slurries was 50 g each, the solid content was 76.5 mass%, and the solvent composition of the slurries was adjusted to NMP / HO = 97 / 3 (mass ratio). The water was added to intentionally create a state in which the slurry had a high moisture content.

[0125] <Viscosity Measurement> Immediately after preparation, the slurries obtained above were measured for viscosity at a shear rate of 1 m / s using a rheometer (MCR302e, manufactured by Anton Paar K.K.) with a parallel plate as the measuring jig and a set temperature of 25°C. Evaluation after storage was also performed by allowing the slurries to stand at a temperature of 35°C for 24 hours and then measuring the viscosity under the same conditions. The presence or absence of gelation was confirmed visually. Each table also lists the viscosity values ​​immediately after preparation and after storage. Gelled slurries are described as "gelled."

[0126]

[0127]

[0128] (2) Preparation of Positive Electrode Composition (Electrode Slurry) for Adhesion Measurement [Example 2-1] An NMP solution containing 5% by mass of PVP was prepared as a dispersant. The positive electrode active material, binder powder, conductive additive, additive solution, and NMP were mixed in a dry mixer to obtain the composition shown in Table 3, and the mixture was mixed using a rotation-revolution mixer (Thinky Corporation, Awatori Rentaro Atmospheric Pressure Type ARE-310) to obtain an electrode slurry. The total amount of each slurry prepared was 50 g, with a solids content of 76.5% by mass. In Tables 3 and 4, "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.

[0129] Comparative Example 2-1: A 5% by mass NMP solution of PVP was prepared as a dispersant. The positive electrode active material, binder powder, conductive additive, dispersant solution, and NMP were mixed in a dry mixer to obtain the composition shown in Table 4, and the mixture was mixed using a rotation-revolution mixer (Thinky Corporation, Awatori Rentaro atmospheric pressure type ARE-310) to obtain an electrode slurry. The total amount of the prepared slurries was 50 g, and the solid content was 76.5% by mass.

[0130] [Examples 2-2 to 2-16, Comparative Examples 2-2 to 2-7] Five mass % NMP solutions were prepared for each additive and dispersant. A 5 mass % NMP solution of PVP was prepared as a dispersant. The positive electrode active material, binder powder, conductive additive, dispersant solution, additive solution, and NMP were mixed in a dry mixer to obtain the composition ratios shown in Tables 3 and 4, and then mixed using a rotation / revolution mixer to obtain electrode slurries. The total amount of each slurry prepared was 50 g, and the solids content was 76.5 mass %.

[0131] [Example 2-17] A 5% by mass NMP solution of PVP was prepared as a dispersant. The positive electrode active material, binder powder, conductive additive, dispersant solution, additive powder, and NMP were mixed in a dry mixer to obtain the composition ratios shown in Table 3, and the mixture was mixed using a rotation-revolution mixer (Thinky Corporation, Awatori Rentaro atmospheric pressure type ARE-310) to obtain electrode slurries. The total amount of the prepared slurries was 50 g, and the solid content was 76.5% by mass.

[0132] (3) Preparation of Electrodes [Examples 3-1 to 3-17, Comparative Examples 3-1 to 3-7] Electrodes (positive electrodes) using the above-described electrode slurries were prepared according to the following procedure. The electrode slurry was applied to a current collector made of aluminum foil (thickness: 15 μm, width: 8 cm, manufactured by UACJ Corporation) using a desktop wire bar coater (manufactured by SMT Corporation, PM-9050MC), and then dried using a vacuum constant temperature dryer (manufactured by Tokyo Rikakikai Co., Ltd., VOS-210C) to form an electrode layer. The coating conditions were a coating thickness of 170 to 175 μm and a conveying speed of 0.4 m / min. The drying conditions were an internal temperature of the drying oven of 80°C and a drying time of 30 minutes. The resulting current collector with the electrode layer was then compressed using a desktop roll press (manufactured by Takumi Giken Co., Ltd., SA-602) to prepare an electrode.

[0133] <Peel test> Each of the electrodes obtained above was cut into a 25 mm width, and a 20 mm wide double-sided tape was attached to the electrode layer surface to fix it to a glass substrate. This was fixed to an adhesive / film peeling analyzer (VPA-3, manufactured by Kyowa Interface Science Co., Ltd.) and a peel test was performed at a peel angle of 90° and a peel rate of 100 mm / min, and the peel strength was calculated using the following formula. The results are shown in Tables 3 and 4. Peel strength (N / m) = measured value * (N) / (sample measurement width (mm) x 10 -3 *The measured value was the average value from a peel distance of 10 mm to 35 mm.

[0134]

[0135]

[0136] The results in Tables 1 to 4 above confirm that the electrode-forming composition of the present invention, to which an organic acid having an aromatic ring and a linker moiety, which may have a substituent, was added, suppressed gelation due to moisture and improved the storage stability of the electrode slurry. In particular, the organic acid having an aromatic ring and a linker moiety, which may have a substituent having multiple hydroxyl groups or alkoxy groups as the substituent on the aromatic ring, was confirmed to have excellent storage stability and also to improve the peel strength between the electrode layer and the current collector. 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 an organic acid having an aromatic ring which may have a substituent and a linker moiety, a positive electrode active material, a conductive additive, a binder, and a solvent.

2. The electrode-forming composition according to claim 1, wherein the organic acid is represented by the following general formula (I) or (II): A-L-CO2-B 1 (I) (wherein A represents an aromatic ring which may have a substituent, L represents a linker portion, B 1 represents a monovalent organic group having a hydrogen atom or a carboxy group. 2 (II) (wherein A represents an aromatic ring which may have a substituent, L represents a linker portion, B 2 represents a monovalent organic group having a carboxy group.

3. The electrode-forming composition according to claim 1, wherein the aromatic ring is represented by the following formula (a): [(wherein, R 1 ~R 5 are each independently a hydrogen atom, a hydroxy group, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, a phenyl group, R a -COO-, or R a represents —OCO—, and R a represents an alkyl group having 1 to 5 carbon atoms, or R 1 ~R 5 represents a group represented by the following formula (X) formed by bonding two adjacent groups to each other, and * represents a site bonding to the linker portion. (wherein X represents an alkylene group having 1 to 6 carbon atoms, and *1 and *2 represent the bonding site to the benzene ring.) 4. The electrode-forming composition according to claim 1, wherein said aromatic ring has two or more groups selected from the group consisting of hydroxy groups and alkoxy groups having 1 to 5 carbon atoms.

5. The electrode-forming composition according to claim 1, wherein the linker portion contains an unsaturated bond.

6. The electrode-forming composition according to claim 5, wherein the unsaturated bond is a carbon-carbon double bond.

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

8. The positive electrode active material is lithium-containing transition metal oxide particles, and the lithium-containing transition metal oxide particles are represented by the general formula Li a Ni (1-x-y) Co x M 1 y M 2 z O2 (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, 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.

9. The electrode-forming composition according to claim 1, wherein the content of said organic acid is 0.001 to 0.5% by mass based on the solid content.

10. An electrode layer obtained from the electrode-forming composition according to any one of claims 1 to 9.

11. An electrode comprising the electrode layer according to claim 10.

12. A secondary battery comprising the electrode according to claim 11.

13. An additive for an electrode slurry containing a positive electrode active material, a conductive additive, a binder, and a solvent, the additive comprising an organic acid having an aromatic ring which may have a substituent and a linker portion.

14. An additive composition for electrode slurry, comprising a positive electrode active material, a conductive additive, a binder, and a solvent, the additive composition comprising the additive according to claim 13.

15. The additive composition of claim 14, further comprising a dispersant.

16. A gelation inhibitor for an electrode slurry containing a positive electrode active material, a conductive additive, a binder, and a solvent, the gelation inhibitor comprising an organic acid having an aromatic ring which may have a substituent and a linker portion.

17. A gelation-inhibiting composition for electrode slurry comprising a positive electrode active material, a conductive aid, a binder, and a solvent, the gelation-inhibiting composition comprising the gelation inhibitor according to claim 16.

18. The gelation-inhibiting composition according to claim 17, further comprising a dispersing agent.

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