Base forming composition, current collector with base, electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

To provide a base forming composition capable of forming a base having a large content of a conductive carbon material.SOLUTION: The composition for forming the substrate contains a conductive carbon material and hydroxyalkyl cellulose, and a solvent is water.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a composition for forming an undercoat, an undercoated current collector, an electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are small, lightweight, have high energy density, and can be repeatedly charged and discharged, and are widely used as power sources for portable electronic devices such as portable personal computers, handheld video cameras, and information terminals. From the perspective of environmental concerns, electric vehicles using non-aqueous electrolyte secondary batteries and hybrid vehicles that use electricity as part of their power source are being put to practical use. Non-aqueous electrolyte secondary batteries are being demanded to have even higher output, higher capacity, and longer life.

[0003] Non-aqueous electrolyte secondary batteries use a positive electrode in which a positive electrode mixture layer containing a positive electrode active material, a conductive additive, and a binder is formed on a metal foil current collector, and a negative electrode in which a negative electrode mixture layer containing a negative electrode active material, a conductive additive, and a binder is formed on a metal foil current collector. Non-aqueous electrolyte secondary batteries have been developed in which a base containing a carbon source such as acetylene black is provided between the current collector and the positive electrode mixture layer or between the current collector and the negative electrode mixture layer (see, for example, Patent Document 1). A base-forming composition containing a conductive carbon material and an additive such as CMC is used to form the base. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-174809 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to reduce the interfacial resistance between the current collector and the base containing a carbon source or between the base containing a carbon source and the positive electrode mixture layer or the like, it is preferable to increase the content of the carbon source in the base. However, in the case of the underlayer containing the carbon source described in Patent Document 1, when the content of the carbon source is increased, a uniform underlayer cannot be formed, and the interface resistance cannot be reduced, which is a problem. Therefore, an object of the present disclosure is to provide a base-forming composition that can form a base having low interfacial resistance with a current collector. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the inventors conducted extensive research into the causes of the above-mentioned problems and found that a base-forming composition containing water as a solvent and a specified compound can solve the above-mentioned problems.

[0007] That is, the present disclosure provides a base-forming composition characterized by including a conductive carbon material and a hydroxyalkyl cellulose, and using water as a solvent. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to provide a base-forming composition that can form a base having low interfacial resistance with a current collector. DETAILED DESCRIPTION OF THE INVENTION

[0009] A. Base forming composition The base-forming composition of the present disclosure is a base-forming composition containing a conductive carbon material and a hydroxyalkyl cellulose, and the solvent is water. The base-forming composition of the present disclosure has a high content of conductive carbon material and can form a base having low interfacial resistance with a current collector. In the present disclosure, the content of water is not particularly limited as long as it allows the base-forming composition to be applied to a current collector. However, from the viewpoint of forming a base having a high content of conductive carbon material, the content of water is preferably 200 to 1500 parts by mass, more preferably 300 to 1000 parts by mass, even more preferably 300 to 800 parts by mass, and most preferably 350 to 800 parts by mass per 100 parts by mass of the total of exfoliated graphite, hydroxyalkyl cellulose, and carboxyalkyl cellulose (if contained).

[0010] The elements constituting the base-forming composition of the present disclosure will be described in detail below. 1.Conductive carbon materials The base-forming composition of the present disclosure contains a conductive carbon material. Examples of conductive carbon materials include carbon blacks such as furnace black, channel black, lamp black, acetylene black, thermal black, graphitized black, and Ketjen Black (registered trademark), as well as carbon fibers such as exfoliated graphite, carbon nanotubes, and carbon nanofibers. Graphitized black is carbon black obtained by heat-treating low-crystalline carbon black at 2,000 to 3,000°C to promote surface crystallization. In the present disclosure, the conductive carbon material may be used alone or in combination.

[0011] Among the conductive carbon materials, from the viewpoint of being able to form a base having a large content of conductive carbon material, the conductive carbon material preferably contains exfoliated graphite, and more preferably consists of exfoliated graphite.

[0012] In the present disclosure, from the viewpoint of being able to form a base with a high content of conductive carbon material, the content of the conductive carbon material is preferably 80 parts by mass to 99.9 parts by mass, more preferably 90 parts by mass to 99.9 parts by mass, even more preferably 90 parts by mass to 99.3 parts by mass, even more preferably 93 parts by mass to 99.3 parts by mass, even more preferably 96 parts by mass to 99.3 parts by mass, and most preferably 95 parts by mass to 99.3 parts by mass, relative to 100 parts by mass of the total of the conductive carbon material, hydroxyalkyl cellulose, and carboxyalkyl cellulose (if contained).

[0013] 1-1. Exfoliated graphite In the present disclosure, exfoliated graphite refers to a material obtained by exfoliating graphite and having a layered structure in which one to several thousand graphite unit layers are stacked. Graphite is a layered compound having unit layers made of carbon. Graphite includes not only graphite, but also expanded graphite in which the spaces between graphite layers are expanded, and graphite oxide in which graphite is oxidized with an oxidizing agent.

[0014] The thickness of the exfoliated graphite is not particularly limited, but from the viewpoint of being able to form a base with a high content of conductive carbon material, the average thickness of the exfoliated graphite is preferably 10 nm to 200 nm, more preferably 10 nm to 100 nm, even more preferably 10 nm to 70 nm, and most preferably 10 nm to 60 nm.

[0015] In the present disclosure, the thickness of exfoliated graphite refers to the thickness in the direction perpendicular to the stacking plane of the exfoliated graphite, and the average thickness refers to the average value of the thicknesses of any 30 or more pieces of exfoliated graphite. The thickness of exfoliated graphite can be measured, for example, using an SEM image of the exfoliated graphite taken with a scanning electron microscope. Note that exfoliated graphite consisting of one unit layer is called graphene, and its theoretical thickness is approximately 0.335 nm.

[0016] In the present disclosure, the method for producing exfoliated graphite is not particularly limited, and can be carried out by applying shear force, ultrasonic vibration, cavitation, microwaves, or the like to graphite using a known device to exfoliate it until the average thickness falls within the above-mentioned range. Examples of devices used for exfoliating graphite include media-agitating mills such as sand mills, attritors, and bead mills; container-driven mills using balls or rods as media, such as rotary mills, vibration mills, and planetary mills; jet mills, roll mills, hammer mills, pin mills, high-pressure emulsifiers, ultrasonic emulsifiers, and microwave ovens. Examples of high-pressure emulsifiers include penetration-type high-pressure emulsifiers and collision-type high-pressure emulsifiers. Examples of penetration-type penetration-type high-pressure emulsifiers include single-nozzle types and slit-nozzle types. Examples of collision-type collision-type high-pressure emulsifiers include a type in which a liquid containing raw materials is collided with a flat surface such as a valve or a spherical surface such as a ball, and a type in which liquids containing raw materials are collided with each other.

[0017] When exfoliating graphite, either a wet exfoliation method using a solvent or a dry exfoliation method not using a solvent may be used, and the method may be selected according to the exfoliation method of each device.

[0018] 2. Hydroxyalkyl cellulose In the present disclosure, hydroxyalkyl cellulose is a cellulose in which some of the hydrogen atoms of the hydroxyl groups are substituted with hydroxyalkyl groups. In the present disclosure, the number of carbon atoms in the hydroxyalkyl group in the hydroxyalkyl cellulose is 1 to 10. From the viewpoint of being able to form a base with a high content of conductive carbon material, the number of carbon atoms in the hydroxyalkyl group in the hydroxyalkyl cellulose is preferably 1 to 8, more preferably 1 to 6, and even more preferably 1 to 3. Furthermore, in the present disclosure, the molecular weight of the hydroxyalkyl cellulose is not particularly limited, but is preferably 100,000 to 1,300,000, and more preferably 500,000 to 800,000.

[0019] Specific examples of hydroxyalkyl cellulose include hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, etc. Among the hydroxyalkyl celluloses, hydroxyethyl cellulose and hydroxypropyl cellulose are preferred from the viewpoint of being able to form a base having a high content of conductive carbon material.

[0020] As the hydroxyalkyl cellulose, commercially available products may be used. Examples of hydroxyethyl cellulose include Hydroxyethyl Cellulose SP200, SP400, SP500, SP600, SP850, and SP900 manufactured by Daicel FineChem Co., Ltd. Examples of hydroxypropyl cellulose include HPC-SSL, HPC-SL, HPC-L, HPC-M, and HPC-H manufactured by Nippon Soda Co., Ltd.

[0021] In the present disclosure, from the viewpoint of being able to form a base with a high content of conductive carbon material, the content of hydroxyalkyl cellulose is preferably 0.1 to 20 parts by mass, more preferably 0.1 to 10 parts by mass, even more preferably 0.5 to 10 parts by mass, even more preferably 0.5 to 8 parts by mass, and most preferably 0.5 to 7 parts by mass, relative to 100 parts by mass of the total of exfoliated graphite and hydroxyalkyl cellulose.

[0022] 3. Other additives The base-forming composition of the present disclosure may contain other additives such as carboxyalkyl cellulose and a binder. From the viewpoint of being able to form a base with a high content of conductive carbon material, the base-forming composition of the present disclosure preferably further contains carboxyalkyl cellulose.

[0023] 3-1.Carboxyalkyl cellulose The carboxyalkyl cellulose of the present disclosure is obtained by bonding carboxyalkyl groups to some of the hydroxyl groups of cellulose. The number of carbon atoms in the carboxyalkyl group in the carboxyalkyl cellulose is 1 to 10. From the viewpoint of forming a base with a high content of conductive carbon material, the number of carbon atoms in the carboxyalkyl group in the carboxyalkyl cellulose is preferably 1 to 8, more preferably 1 to 6, and even more preferably 1 to 3. Furthermore, the molecular weight of the carboxyalkyl cellulose is not particularly limited, but is preferably 100,000 to 1,300,000, and more preferably 500,000 to 800,000.

[0024] Specific examples of the carboxyalkyl cellulose include carboxymethyl cellulose, carboxypropyl cellulose, etc. Among the carboxyalkyl celluloses, carboxymethyl cellulose and carboxypropyl cellulose are preferred, and carboxymethyl cellulose is more preferred, from the viewpoint of being able to form a base having a high content of conductive carbon material.

[0025] When the base-forming composition of the present disclosure contains carboxyalkyl cellulose, from the viewpoint of being able to form a base with a high content of conductive carbon material, the total content of carboxyalkyl cellulose and hydroxyalkyl cellulose is preferably 0.1 to 20 parts by mass, more preferably 0.1 to 10 parts by mass, even more preferably 0.5 to 10 parts by mass, even more preferably 0.5 to 8 parts by mass, even more preferably 0.5 to 7 parts by mass, and most preferably 0.5 to 4 parts by mass, per 100 parts by mass of the total of exfoliated graphite, carboxyalkyl cellulose, and hydroxyalkyl cellulose.

[0026] When the base-forming composition of the present disclosure contains carboxyalkyl cellulose, the content of carboxyalkyl cellulose is preferably 3 to 60 parts by mass, more preferably 5 to 50 parts by mass, even more preferably 5 to 40 parts by mass, and most preferably 5 to 35 parts by mass, per 100 parts by mass of the total of carboxyalkyl cellulose and hydroxyalkyl cellulose, from the viewpoint of being able to form a base with a high content of conductive carbon material.

[0027] 3-2. Binder In the present disclosure, examples of binders include styrene-butadiene rubber, butadiene rubber, polyethylene, polypropylene, polyamide, polyamideimide, polyimide, polyacrylonitrile, polyurethane, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-propylene-diene rubber, fluororubber, styrene-acrylic acid ester copolymer, ethylene-vinyl alcohol copolymer, acrylonitrile-butadiene rubber, styrene-isoprene rubber, polymethyl methacrylate, polyacrylate, polyacrylamide, polyvinyl alcohol, polyvinyl ether, polyethylene oxide, starch, polyvinylpyrrolidone, polyvinyl chloride, polyacrylic acid, cellulose nanofibers, and the like. In the case of binders having carboxy groups such as polymethyl methacrylate, polyacrylate, and polyacrylic acid, some or all of the carboxy groups may be neutralized with lithium, sodium, or the like. Only one binder may be used, or two or more binders may be used in combination. In the present disclosure, the binder is preferably a binder containing at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, styrene-butadiene rubber, cellulose nanofiber, and styrene-butadiene rubber, from the viewpoint of being able to form a base with excellent adhesion to the current collector.

[0028] 4.Applications The base-forming composition of the present disclosure can be suitably used as a composition for forming a base on a current collector in an electrode for a non-aqueous electrolyte secondary battery. An undercoated current collector including a base formed from the base-forming composition of the present disclosure on the current collector is characterized by low interfacial resistance with the current collector.

[0029] 5. Manufacturing method In the present disclosure, the method for producing the base-forming composition is not particularly limited, but examples thereof include a method of mixing the components and dispersing them using a conventional ball mill, sand mill, bead mill, cyclone mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, rotation-revolution mixer, planetary mixer, Filmix, jet paster, or the like.

[0030] B. Current collector with substrate In the present disclosure, a current collector with an undercoat includes a undercoat formed on a current collector from a composition for forming an undercoat. As described above, the current collector with an undercoat of the present disclosure is characterized by low interfacial resistance with the current collector. Therefore, when a positive electrode composite layer containing a positive electrode active material or a negative electrode composite layer containing a negative electrode active material or the like is further formed on the undercoat of the current collector with an undercoat of the present disclosure, the interfacial resistance between the current collector and the positive electrode active material layer or the negative electrode composite layer can be reduced while preventing corrosion of the current collector.

[0031] In the present disclosure, the current collector is made of a conductive material such as titanium, a titanium alloy, aluminum, an aluminum alloy, copper, nickel, stainless steel, nickel-plated steel, or a conductive resin. The current collector may be in the form of a foil, a plate, a mesh, or a porous material. Among these, aluminum is preferred from the viewpoints of conductivity and cost, and aluminum foil is more preferred. When the current collector is in the form of a foil, its thickness is preferably 1 μm to 100 μm from the viewpoints of increasing discharge capacity and ease of manufacture.

[0032] The thickness of the underlayer formed on the current collector is preferably thin, but if it is too thin, it is difficult to obtain a sufficient effect, so the thickness of the underlayer is preferably 0.1 μm to 20 μm, more preferably 0.3 μm to 10 μm, and even more preferably 0.5 μm to 5 μm.

[0033] Examples of methods for applying the base-forming composition of the present disclosure to a current collector include gravure, reverse roll, direct roll, doctor blade, knife, extrusion, curtain, bar, dip, squeeze, die coating, spray, and wire coating, and the gravure method is preferred because it is easy to obtain a coating film with a uniform thickness.

[0034] After the base-forming composition is applied to the current collector, the base is formed on the current collector by drying the base-forming composition, and the base is pressed or baked as necessary to obtain a current collector with an undercoat. The drying method may be any known method, such as drying with warm air, hot air, or low-humidity air, vacuum drying, leaving the composition in a heating furnace, or drying by irradiation with far-infrared rays, infrared rays, or electron beams.

[0035] C. Electrode for non-aqueous electrolyte secondary batteries The electrode for a non-aqueous electrolyte secondary battery according to the present disclosure includes the current collector with an underlayer as described above, and has a positive electrode mixture layer or a negative electrode mixture layer on the underlayer. In the present disclosure, a positive electrode mixture layer includes at least a positive electrode active material, and a negative electrode mixture layer includes at least a negative electrode active material.

[0036] When the electrode for a nonaqueous electrolyte secondary battery according to the present disclosure is used as a positive electrode, examples of the positive electrode active material include lithium transition metal composite oxides, lithium-containing transition metal phosphate compounds, lithium-containing silicate compounds, sulfur-based active materials, etc. Only one type of positive electrode active material may be used, or two or more types may be used in combination.

[0037] When the electrode for a nonaqueous electrolyte secondary battery of the present disclosure is used as a negative electrode, examples of the negative electrode active material include carbon-based active materials such as natural graphite, artificial graphite, non-graphitizable carbon, and graphitizable carbon; lithium-based active materials such as lithium and lithium alloys; silicon-based active materials such as silicon, silicon alloys, and silicon oxide; sulfur-based active materials, tin-based active materials such as tin, tin alloys, and tin oxide; and metal oxides such as copper oxide, antimony sulfide, titanium oxide, iron oxide, manganese oxide, cobalt oxide, nickel oxide, lead oxide, ruthenium oxide, tungsten oxide, and zinc oxide. Only one type of negative electrode active material may be used, or two or more types may be used in combination.

[0038] In the present disclosure, the positive electrode mixture layer and the negative electrode mixture layer may contain a binder, a conductive additive, etc. As the binder and the conductive additive, the binder and the conductive additive described in WO 2022 / 004696 may be used.

[0039] D. Nonaqueous electrolyte secondary battery The nonaqueous electrolyte secondary battery of the present disclosure includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. At least one of the positive electrode and the negative electrode is the electrode for a nonaqueous electrolyte secondary battery described above. The nonaqueous electrolyte may be any known nonaqueous electrolyte used in nonaqueous electrolyte secondary batteries.

[0040] E. Other The present disclosure includes the following aspects.

[0041] [1] A composition for forming a base, comprising a conductive carbon material and a hydroxyalkyl cellulose, and wherein the solvent is water.

[0042] [2] The composition for forming a base according to [1], wherein the conductive carbon material contains exfoliated graphite.

[0043] [3] The base-forming composition according to [1] or [2], further comprising carboxyalkyl cellulose.

[0044] [4] The composition for forming a base according to any one of [1] to [3], wherein the hydroxyalkyl cellulose is one or more selected from the group consisting of hydroxyethyl cellulose and hydroxypropyl cellulose.

[0045] [5] The base-forming composition according to [3], wherein the carboxyalkyl cellulose is carboxymethyl cellulose.

[0046] [6] The composition for forming a base according to [2], wherein the exfoliated graphite has an average thickness of 10 nm to 200 nm.

[0047] [7] A current collector with an underlayer, comprising, on a current collector, an underlayer formed from the composition for forming an underlayer according to any one of [1] to [6].

[0048] [8] An electrode for a non-aqueous electrolyte secondary battery, comprising the undercoated current collector according to [7].

[0049] [9] A non-aqueous electrolyte secondary battery comprising the electrode for a non-aqueous electrolyte secondary battery according to [8].

[0050] The present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any embodiment that has substantially the same configuration as the technical idea described in the claims and exhibits similar effects is included within the technical scope of the present disclosure. [Example]

[0051] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the following examples in any way.

[0052] <Production of exfoliated graphite> According to Experimental Example 1 in the pamphlet of International Publication No. 2016 / 148252, flake graphite (average thickness 250 nm, specific surface area 11 m) was used. 2 / g, bulk density 0.35g / cm 3 Exfoliated graphite A1 was prepared from a mixture of 74 parts by mass of 1-butyl-3-methylimidazolium hexafluorophosphate and 26 parts by mass of polyethylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, product name: Polyethylene Glycol 20000), and the mixture was heated and dissolved, and 10 parts by mass of flake graphite was dispersed therein. 0.6 g of this dispersion was added to 0.5 cm 3 The dispersion was collected in a vial and capped, and then irradiated with 2450 MHz microwaves at 175°C for 60 minutes using a microwave synthesizer (Biotage Japan Initiator+). The dispersion was then washed with acetone, filtered, and dried in an oven to obtain exfoliated graphite A1. The average thickness of exfoliated graphite A1 was 25 nm.

[0053] <Preparation of base-forming composition> The components were mixed according to the formulation shown in Table 1 below, and mixed for 60 minutes using a planetary / revolution mixer at a revolution speed of 1000 rpm and a planetary rotation speed of 500 rpm to prepare base compositions for the Examples and Comparative Examples.

[0054] <Preparation of current collector with undercoat> An undercoated current collector was prepared by the following procedure. The undercoat-forming compositions of Examples 1 to 3 and Comparative Examples 1 and 2 were applied with an applicator to one side of an aluminum foil (thickness 15 μm) serving as a current collector, dried at 90°C, and then press-molded to obtain an undercoated current collector. The thickness of the undercoated current collector was 3 μm.

[0055] <Evaluation of Coatability> In producing the above-described undercoated current collector, the applicability of the undercoat-forming composition when applied to aluminum foil with an applicator was evaluated according to the following criteria. Regarding the shape of the coated edge, a case in which the composition was highly linear and no liquid backflow was observed was rated "good (A)." Regarding the shape of the coated edge, a case in which the composition was poorly linear and liquid backflow was observed was rated "poor (B)."

[0056] <Measurement of interface resistance of undercoated current collector> Using an electrode resistance measurement system RM2610 manufactured by Hioki E.E. Corporation, a measurement probe was brought into contact with the surface of the above-mentioned undercoated current collector at normal pressure, a constant current (1 mA) was passed through the surface of the undercoated current collector, and the potential distribution generated on the surface was measured at multiple points to determine the interfacial resistance (Ωcm) between the undercoat and the aluminum foil. 2 ) was calculated.

[0057] [Table 1]

[0058] The above results show that, compared to the base-forming compositions of Comparative Examples 1 and 2, which do not contain hydroxyalkyl cellulose, the base-forming compositions of Examples 1, 2, and 3, which contain exfoliated graphite and hydroxyalkyl cellulose, have good coatability and can form bases with low interfacial resistance.

Claims

1. A composition for forming a base, comprising a conductive carbon material and a hydroxyalkyl cellulose, and wherein the solvent is water.

2. The base-forming composition according to claim 1 , wherein the conductive carbon material contains exfoliated graphite.

3. The base forming composition according to claim 2, further comprising carboxyalkyl cellulose.

4. The base-forming composition according to claim 2, wherein the hydroxyalkyl cellulose is at least one selected from the group consisting of hydroxyethyl cellulose and hydroxypropyl cellulose.

5. 4. The base forming composition according to claim 3, wherein the carboxyalkyl cellulose is carboxymethyl cellulose.

6. 3. The base-forming composition according to claim 2, wherein the exfoliated graphite has an average thickness of 10 nm to 200 nm.

7. A current collector with an undercoat, comprising an undercoat formed from the undercoat-forming composition according to any one of claims 1 to 6 on a current collector.

8. An electrode for a non-aqueous electrolyte secondary battery, comprising the undercoated current collector according to claim 7 .

9. A non-aqueous electrolyte secondary battery comprising the electrode for a non-aqueous electrolyte secondary battery according to claim 8.

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