Composition for forming electrode and gelation inhibitor

Abstract

As a composition for forming an electrode, which has improved storage stability by being suppressed in thickening and gelation by means of a simple method and can have a higher concentration of a solid content, the present invention provides a composition for forming an electrode, the composition containing a gelation inhibitor, a positive electrode active material, a binder, and a solvent, wherein the gelation inhibitor is at least one substance that is selected from the group consisting of benzoic acid, pyrrolecarboxylic acids, and tannic acid, and has a solid content concentration of 75% by mass or more.

Description

Electrode-forming composition and gelation inhibitor

[0001] The present invention relates to an electrode-forming composition and a gelation inhibitor.

[0002] Lithium-ion secondary batteries have a high energy density per unit weight and volume, which contributes to the miniaturization and lightening 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 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 dry it, while the most common method for manufacturing a positive electrode is to prepare a positive electrode slurry using an organic solvent such as N-methylpyrrolidone as a solvent, and coat the aluminum foil current collector with the positive electrode slurry.

[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 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 Documents 3 and 4 propose adding an organic 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 3 uses maleic acid, citraconic acid, and malonic acid in the positive electrode mixture, while Patent Document 4 uses acetic acid, phosphoric acid, sulfuric acid, or the like in the electrode slurry (positive electrode paste). However, neutralizing the alkali with an acid requires the addition of a large amount of acid, which may result in a decrease in the energy density of the battery or an increase in the battery resistance. Another problem is that the acid corrodes the device used to fabricate the electrode.

[0013] Patent Document 5 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.

[0014] Patent Document 6 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.

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

[0016] In light of the above circumstances, 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, thereby improving storage stability, increasing the solids concentration, and suppressing battery deterioration, as well as a gelation inhibitor that can be suitably used in an electrode-forming composition.

[0017] The present inventors have conducted extensive research to achieve the above object, and have found that adding a gelation inhibitor consisting of a specific compound to an electrode-forming composition containing at least a positive electrode active material, a binder, and a solvent can suppress thickening and gelation of the composition and improve storage stability. Furthermore, an electrode fabricated using the electrode-forming composition of the present invention can suppress deterioration in a battery caused by alkaline components, thereby improving battery performance.

[0018] That is, the present invention provides the following electrode-forming compositions and gelation inhibitors. 1. An electrode-forming composition comprising a gelation inhibitor, a positive electrode active material, a binder, and a solvent, wherein the gelation inhibitor is at least one selected from the group consisting of benzoic acid, pyrrolecarboxylic acid, and tannic acid, and wherein the electrode-forming composition has a solids concentration of 75% by mass or more. 2. The electrode-forming composition of 1, wherein the gelation inhibitor is at least one selected from the group consisting of benzoic acid and tannic acid. 3. The electrode-forming composition of 1 or 2, further comprising a conductive assistant. 4. The electrode-forming composition of any of 1 to 3, wherein the positive electrode active material contains 30% by mass or more of S, Fe, or Ni. 5. The electrode-forming composition of any of 1 to 4, wherein the content of the gelation inhibitor is 0.001 to 0.5% by mass of the solids. 6. 6. A gelation inhibitor for an electrode-forming composition comprising a positive electrode active material, a binder, and a solvent, the gelation inhibitor comprising at least one compound selected from the group consisting of benzoic acid, pyrrolecarboxylic acid, and tannic acid. 7. A gelation inhibitor solution for an electrode-forming composition comprising a positive electrode active material, a binder, and a solvent, the gelation inhibitor solution comprising a solvent and at least one compound selected from the group consisting of benzoic acid, pyrrolecarboxylic acid, and tannic acid.

[0019] 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 energy storage devices. When an energy storage device is manufactured using the composition, 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 for the energy storage device and improved battery characteristics.

[0020] While the mechanisms of the thickening and gelation and the suppression effect are unclear, it is believed that the addition of a gelation inhibitor consisting of a specific compound to the electrode-forming composition forms a protective film on the surface of the alkaline component. This protective film inhibits the reaction between the alkaline component and the binder, particularly the fluorine-based binder, thereby suppressing the thickening and gelation of the composition and improving its storage stability. Suppressing the thickening and gelation of the electrode-forming composition improves the dispersibility of solid components such as the positive electrode active material and conductive additive, enabling the formation of a homogeneous positive electrode layer. It also improves the solid concentration in the electrode slurry, reducing the cost and environmental impact of producing energy storage devices. Furthermore, it can suppress corrosion of aluminum foil, commonly used as a current collector foil, and the degradation of battery performance due to reaction with the electrolyte, which are caused by alkaline components. However, these are speculations, and the present invention should not be interpreted as being limited to these mechanisms.

[0021] The electrode-forming composition of the present invention is an electrode-forming composition comprising a gelation inhibitor, a positive electrode active material, a binder, and a solvent, wherein the gelation inhibitor is at least one selected from the group consisting of benzoic acid, pyrrolecarboxylic acid, and tannic acid, and the solids concentration is 75 mass% or more.

[0022] As the gelation inhibitor, benzoic acid and tannic acid are preferred from the viewpoints of their high gelation inhibitory effect and ease of availability.

[0023] The content of the gelation inhibitor 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%. Furthermore, the lower limit of the content of the gelation inhibitor is more preferably 0.01 mass% of the solid content, and even more preferably 0.06 mass% of the solid content. By keeping the content of the gelation inhibitor within the above range, the adhesion between the current collector and the electrode layer can be improved, 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).

[0024] As the positive electrode active material, one containing 30% by mass or more of S, Fe, or Ni is preferred because it further improves battery capacity, uses less rare metals, and is low cost. In the present invention, considering further reducing the amount of rare metals used and obtaining a battery with an even longer life, one containing 35% by mass or more of Fe or Ni is more preferred, and one containing 45% by mass or more is even more preferred. The upper limit is not particularly limited, but is typically 65% ​​by mass or less. As such a positive electrode active material, a material that satisfies the above conditions can be appropriately selected from various active materials conventionally used in electrodes for energy storage devices such as secondary batteries. For example, in the case of lithium secondary batteries and lithium ion secondary batteries, chalcogen compounds or lithium ion-containing chalcogen compounds capable of adsorbing and desorbing lithium ions, polyanionic compounds, sulfur elemental material and compounds thereof, etc. can be used.

[0025] Examples of lithium ion-containing chalcogen compounds 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 2z X w O2 (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, W, and V, and 1.00≦a≦1.50, 0.00≦x≦0.50, 0≦y≦0.50, 0.000≦z≦0.020, 0.000≦w≦0.020). Examples of polyanion compounds include LiFePO 4、 Li a Mn b Fe c D d PO4 (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 portion of the positive electrode active material has an olivine structure). 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.

[0026] In the present invention, among the above positive electrode active materials, Li a Ni (1-x-y) Co x M 1 y M 2 z X w O2 (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, W, and V, and preferably 1.00≦a≦1.50, 0.00≦x≦0.50, 0≦y≦0.50, 0.000≦z≦0.020, 0.000≦w≦0.020). These active materials can be used singly or in combination of two or more.

[0027] The content of the positive electrode active material 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 %.

[0028] 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.

[0029] The weight-average molecular weight (Mw) of the binder is usually about 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).

[0030] 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.

[0031] The electrode-forming composition of the present invention may further contain a conductive additive to improve electrical conductivity. Examples of the conductive additive 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. The conductive additives may be used alone or in combination of two or more.

[0032] 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.

[0033] The electrode-forming composition of the present invention may further contain a dispersant 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, it is preferable to include a nonionic polymer. 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 containing a pyrrolidone structure or a nitrile group are preferred, with polyvinylpyrrolidone and polyacrylonitrile being more preferred. The dispersants can be used alone or in combination.

[0034] When the dispersant is contained, 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 dispersant content 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 gelation inhibitor and the dispersant is preferably 0.001 to 1 mass%, more preferably 0.01 to 1 mass%, of the solid content.

[0035] 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.

[0036] 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.

[0037] The solids concentration of the electrode-forming composition of the present invention is 75% by mass or more, and taking into consideration the coatability of the composition, the thickness of the electrode to be formed, and the like, it is preferably 75 to 95% by mass, more preferably about 78 to 90% by mass, and more preferably about 80 to 85% by mass.

[0038] 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.

[0039] 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 of the present invention, the positive electrode active material, and the binder, the gelation inhibitor and the 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.

[0040] 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 serving as 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] The secondary battery of the present invention is provided with the above-described electrodes, and more specifically, is configured with at least one pair of positive and negative electrodes, a separator interposed between the electrodes, and an electrolyte, and the positive electrode is configured with the above-described electrode. Other constituent members of the battery element may be appropriately selected from conventionally known components.

[0046] Examples of materials used for the separator include glass fiber, cellulose, porous polyolefin, polyamide, and polyester.

[0047] 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.

[0048] 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, 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] The supporting salt contained in the polymer solid electrolyte may include lithium (fluorosulfonylimide), and the plasticizer may include succinonitrile.

[0054] 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.

[0055] The type of secondary battery and the type of electrolyte are not particularly limited, and any type of battery such as a lithium ion battery, nickel-metal hydride battery, manganese battery, or air battery may be used, but a lithium ion battery is preferred. The lamination method and production method are also not particularly limited.

[0056] 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.

[0057] The present invention also provides a gelation inhibitor for an electrode-forming composition containing a positive electrode active material, a binder, and a solvent, the gelation inhibitor comprising at least one compound selected from the group consisting of benzoic acid, pyrrolecarboxylic acid, and tannic acid. The gelation inhibitor is suitable as an additive for inhibiting gelation of the electrode-forming composition containing a positive electrode active material, a binder, and a solvent.

[0058] The positive electrode active material, binder, and solvent of the electrode-forming composition are the same as those described above.

[0059] Furthermore, the present invention provides a gelation inhibitor solution for an electrode-forming composition containing a positive electrode active material, a binder, and a solvent, the gelation inhibitor solution comprising a gelation inhibitor comprising at least one compound selected from the group consisting of benzoic acid, pyrrolecarboxylic acid, and tannic acid, and a solvent. Use of the gelation inhibitor solution of the present invention makes it easier to mix the gelation inhibitor with the electrode-forming composition.

[0060] The gelation inhibitor solution is preferably one in which each of the compounds is dissolved or dispersed in a solvent, and more preferably one in which each compound is dissolved in a solvent.

[0061] Usable solvents include those exemplified in the description of the electrode-forming composition, of which NMP, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate are particularly suitable for use in the present invention.

[0062] The positive electrode active material and binder of the electrode-forming composition are also the same as those described above.

[0063] The solids concentration of the gelation inhibitor solution of the present invention 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 %.

[0064] 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.

[0065] (1) Rotation / revolution type mixer: Thinky Corporation's Awatori Rentaro Atmospheric Pressure Type ARE-310 (2) Dry chamber: Nippon Spindle Mfg. Co., Ltd. (3) E-type viscometer: Toki Sangyo Co., Ltd.'s VISCOMETER TV-22, measurement temperature: 25°C, rotor: 1°34' x R24, and the viscosity was measured 5 minutes after the start of measurement under the following measurement conditions.

[0066] The raw materials used are as follows: <Active material> NCA: Lithium nickel oxide (LiNi 0.88 Co 0.11 Al 0.01 O2, NCA-034H, Ni ratio: 55% by mass, manufactured by Ecopro <Fluorine-based binder> Solef-5130: Polyvinylidene fluoride (PVdF), manufactured by Solvay <Conductive additive> AB: Denka Black (registered trademark) Li100 (high-purity acetylene black), manufactured by Denka Co., Ltd. <Solvent> NMP: manufactured by Nippon Refine Co., Ltd. <Additive A> [Additives (gelling inhibitors) used in the examples] A1: Pyrrole-2-carboxylic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A2: Benzoic acid, manufactured by Tokyo Chemical Industry Co., Ltd. A3: Tannic acid, manufactured by Tokyo Chemical Industry Co., Ltd. [Additives used in Comparative Examples] a1: Oxalic acid, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. a2: Piperonylic Acid, manufactured by Tokyo Chemical Industry Co., Ltd. a3: 8-Quinolinol, manufactured by Tokyo Chemical Industry Co., Ltd. a4: Anthranil, manufactured by Tokyo Chemical Industry Co., Ltd. a5: Phthalazone, manufactured by Tokyo Chemical Industry Co., Ltd. a6: Pyridine, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. a7: Nitrilotris(methylenephosphonic acid), manufactured by Johoku Chemical Industry Co., Ltd.

[0067] Preparation of Positive Electrode Composition (Electrode Slurry) [Examples 1-1 to 1-3, Comparative Examples 1-1 to 1-8] A 5% by mass NMP solution was prepared for each additive. A 7% by mass NMP solution of PVdF was prepared. The positive electrode active material, binder solution, conductive additive, additive solution, NMP, and water were mixed in a dry mixer to obtain the composition ratios shown in Table 1, and mixed using a rotation / revolution mixer to obtain an electrode slurry. The total amount of the prepared slurries was 20 g each, the solid content was 80% 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.

[0068] The viscosity of the slurry obtained above was measured using an E-type viscometer immediately after preparation. 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 to check for thickening and a tendency to gel, and the results were judged based on the following criteria. Table 1 also summarizes these evaluations. <Judgment Criteria> A: No gelation B: The composition gelled and could not be used to form electrodes.

[0069]

[0070] The results in Table 1 above confirm that the electrode-forming composition of the present invention, to which a gelation inhibitor consisting of a specific compound was added, suppressed thickening and gelation, and had improved storage stability.

Claims

1. An electrode-forming composition comprising a gelation inhibitor, a positive electrode active material, a binder and a solvent, wherein the gelation inhibitor is at least one selected from the group consisting of benzoic acid, pyrrole carboxylic acid and tannic acid, and the electrode-forming composition has a solids concentration of 75 mass% or more.

2. The electrode-forming composition according to claim 1, wherein said gelation inhibitor is at least one member selected from the group consisting of benzoic acid and tannic acid.

3. The electrode-forming composition according to claim 1, further comprising a conductive assistant.

4. The electrode-forming composition according to claim 1, wherein the positive electrode active material contains 30 mass % or more of S, Fe or Ni.

5. The electrode-forming composition according to any one of claims 1 to 4, wherein the content of the gelation inhibitor is 0.001 to 0.5 mass % of the solid content.

6. A gelation inhibitor for an electrode-forming composition comprising a positive electrode active material, a binder and a solvent, the gelation inhibitor comprising at least one compound selected from the group consisting of benzoic acid, pyrrole carboxylic acid and tannic acid.

7. A gelation inhibitor solution for an electrode-forming composition containing a positive electrode active material, a binder, and a solvent, the gelation inhibitor solution comprising a gelation inhibitor consisting of at least one compound selected from the group consisting of benzoic acid, pyrrole carboxylic acid, and tannic acid, and a solvent.