Current collector for power storage device, method for manufacturing the same, and coating liquid used in the manufacturing

Abstract

Provided is a current collector for a power storage device for obtaining a low-resistance power storage device. A current collector for a power storage device, characterized by having a coating layer formed on one or both surfaces of a sheet-shaped conductive substrate, the coating layer containing a powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone, the content of the polyvinylpyrrolidone in the coating layer being 0.099 to 5.0 mass%, and the content of the powdered carbon material in the coating layer being 15.0 to 45.0 mass%.

Description

[0001] This application is a divisional application of patent application No. 201880003610.3, filed on September 10, 2018, entitled "Current collector for energy storage device, method of manufacturing thereof and coating liquid used in manufacturing thereof". Technical Field

[0002] This invention relates to a current collector for an energy storage device, a method for manufacturing the same, and a coating liquid used in the manufacturing process. More specifically, it relates to a current collector for an energy storage device having a resin layer containing powdered carbon material on the surface of a metal foil, a method for manufacturing the same, and a coating liquid used in the manufacturing process.

[0003] Furthermore, the energy storage device in this invention refers to a lithium-ion secondary battery in the context of batteries, and to an electric double-layer capacitor and a lithium-ion capacitor in the context of electrochemical capacitors. Background Technology

[0004] In recent years, lithium-ion rechargeable batteries and double-layer capacitors have received significant attention as energy storage devices. Lithium-ion rechargeable batteries, due to their small size and light weight, are used as power sources for laptops, mobile phones, power tools, and electronic communication devices. Recently, from the perspective of environmentally friendly vehicle applications, lithium-ion rechargeable batteries have also been used in electric vehicles and hybrid vehicles. Furthermore, double-layer capacitors, due to their extremely high energy storage capacity, have the potential to replace battery packs and are receiving considerable attention as backup power sources, automotive idling stop systems, and large-scale energy storage systems such as ESS (Energy Saving System). Moreover, redox flow batteries, due to their high cycle life, are increasingly being put into practical use as large-scale power devices in the 1000kW range.

[0005] Lithium-ion rechargeable batteries and electric double-layer capacitors share some similar structures. One example of these similarities is the electrode. Reducing the resistivity of the electrode is a common challenge for both lithium-ion rechargeable batteries and electric double-layer capacitors, and various studies have been conducted on this topic.

[0006] For example, a lithium-ion secondary battery consists of a positive electrode with metal oxides such as lithium cobalt oxide as the positive electrode active material, a negative electrode with carbon materials such as graphite as the negative electrode active material, and an electrolyte with carbonates as the solvent. In a lithium-ion secondary battery, charging and discharging occur through the movement of lithium ions between the positive and negative electrodes.

[0007] The positive electrode is obtained by coating a slurry containing positive electrode active material and binder onto the surface of a positive current collector such as aluminum foil, drying it, and then cutting it into appropriate sizes. The negative electrode is obtained by coating a slurry containing negative electrode active material and binder onto the surface of a negative current collector such as copper foil, drying it, and then cutting it into appropriate sizes. The positive electrode typically uses an organic solvent-based slurry containing binders such as poly(1,1-difluoroethylene) or polytetrafluoroethylene (PTFE), while the negative electrode typically uses an aqueous slurry containing binders such as styrene-butadiene rubber (SBR) or acrylic resins.

[0008] However, in recent years, with the increasing demand for higher capacity in energy storage devices, attempts have been made to increase voltage using high-voltage active materials. For example, in lithium-ion secondary batteries, positive electrode active materials with a high nickel ratio are used, and charging is performed at voltages above 4.2V to achieve higher capacity. Furthermore, as a means to reduce the resistance and extend the lifespan of energy storage devices, carbon-coated foils are used, where carbon microparticles and binder resins are coated onto aluminum foil or similar materials used as electrodes and current collectors, to reduce the interfacial resistance between the electrodes and current collectors and to reduce the resistance of the energy storage device itself. However, when increasing the voltage, the withstand voltage of the binder resins (acrylic resins, polysaccharide resins, etc.) used in typical carbon-coated foils is exceeded, resulting in their oxidation and decomposition, and deterioration of the energy storage device's function. Functional deterioration refers to an increase in the interfacial resistance between the electrodes and current collectors, and a decrease in the seal between the electrodes and current collectors. As a result, normal charging and discharging of the energy storage device cannot occur, leading to increased internal resistance, reduced capacity, and shortened lifespan, affecting all characteristics that are important performance indicators of secondary batteries.

[0009] As a means of solving this problem, it is conceivable to use PVDF, which has high voltage resistance, as the adhesive resin used in the carbon-coated foil of the current collector for energy storage devices. For example, Patent Documents 1 to 4 describe the use of PVDF as an adhesive, and these energy storage devices are expected to improve capacity by increasing voltage.

[0010] Patent Document 1 discloses a current collector having a conductive resin layer on at least one side of a conductive substrate. The resin layer contains a fluorinated resin and conductive particles, and has a thickness of 0.3 to 20 μm. PVDF and acrylic-modified PVDF are preferred as the fluorinated resin. It is described that lithium-ion batteries using this current collector can achieve current blocking function and high-speed charging characteristics.

[0011] Patent Document 2 discloses that in at least one of the positive and negative electrodes of a non-aqueous secondary battery, a conductive layer is sandwiched between a current collector and an electrode binder layer. The conductive layer contains a conductive agent and a binder, PVDF. Based on nuclear magnetic resonance spectroscopy, the mass ratio (α crystal / β crystal) of the PVDF in the conductive layer is 0.35 to 0.56. It is described that with this structure, the internal resistance of the battery can be increased and overheating can be suppressed when the temperature rises due to overcharging or other reasons.

[0012] Patent document 3 discloses that in a secondary battery electrode, a conductive intermediate layer containing conductive particles and a thermoplastic polymer is sandwiched between the electrode active material layer and the current collector, wherein the number average molecular weight of the thermoplastic polymer is 6.3 × 10⁻⁶. 5 Above and less than 1.0 × 10 6 PVDF is preferred as a thermoplastic polymer. This structure improves the stability and cycle characteristics of the conductive interlayer in the electrode for secondary batteries, and also effectively utilizes the current blocking effect of the conductive interlayer.

[0013] Patent document 4 discloses a lithium-ion polymer secondary battery having a positive electrode with a positive active material layer formed on the surface of a positive current collector and a negative electrode with a negative active material layer formed on the surface of a negative current collector. The positive active material contains a first binder, and the negative active material contains a second binder that is different from or the same as the first binder. There is a first sealing layer between the positive current collector and the positive active material layer, and a second sealing layer between the negative current collector and the negative active material layer. Both the first and second sealing layers contain a third binder and a conductive material. The third binder is a polymer compound modified by modifying the first or second binder with a modifying material. PVDF can be used as an example of the first and second binders. The first or second adhesive layer contains graphite, modified PVDF, and 0.1–20% by mass of a dispersant, such as an acidic polymeric dispersant, an alkaline polymeric dispersant, or a neutral polymeric dispersant. This increases the adhesion between the current collector and the active material layer, preventing dissolution by the electrolyte, resulting in excellent long-term shelf life and cycle performance. Even in the event of hydrofluoric acid formation inside the battery, the adhesive layer acts as a protective layer, inhibiting current collector corrosion.

[0014] Patent document 5 discloses a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator. The positive electrode has a positive electrode flux layer containing positive electrode active material and a positive electrode current collector. The positive electrode current collector is composed of a metal foil and a conductive layer containing carbon particles formed on the surface of the metal foil. The positive electrode flux layer is formed on the positive electrode current collector, and the porosity of the positive electrode flux layer is 25-40%. With this structure, large current charging and discharging can be performed, providing a non-aqueous electrolyte secondary battery with good load characteristics.

[0015] Existing technical documents

[0016] Patent documents

[0017] Patent Document 1: International Publication No. 2013 / 151046

[0018] Patent Document 2: Japanese Patent No. 5553165

[0019] Patent Document 3: Japanese Patent No. 5578370

[0020] Patent Document 4: Japanese Patent No. 3982221

[0021] Patent Document 5: Japanese Patent Application Publication No. 2015-88465 Summary of the Invention

[0022] The problem that the invention aims to solve

[0023] PVDF is commonly known as a binder used in electrodes of lithium-ion secondary batteries. However, in slurries containing conductive additives such as carbon microparticles and PVDF, the dispersion of carbon microparticles is very poor, especially in cases like carbon-coated foils where the particle size of the carbon microparticles used is small. Poor carbon microparticle dispersion prevents uniform coating of the slurry onto the substrate, resulting in uneven coating and areas with poor conductivity, making it undesirable. Furthermore, when using gravure printing to apply the slurry thinly and evenly, rib-like voids (areas where the substrate is exposed in rib-like patterns) can occur, or sometimes the perforations of the gravure printing plate can become clogged. This leads to poor adhesion between the coating liquid containing carbon microparticles and PVDF and the substrate, causing the coating to peel off from the substrate during the electrode layer formation process. In other words, many challenges remain in using PVDF as a binder for suitable carbon-coated foils.

[0024] Adding dispersants and other additives to improve dispersibility is an effective method for solving this problem. However, the amount added is also important; excessive addition increases the viscosity of the slurry, while insufficient addition fails to improve dispersibility. Specific research is required for its application. However, Patent Document 1 does not mention any additives. Patent Document 2 describes the inclusion of any component other than PVDF, listing polymers other than PVDF as examples, but does not specify the specifics. Patent Document 3 describes mixing various additives such as dispersants and thickeners as needed when mixing conductive intermediate layer materials (conductive particles) and thermoplastic polymers in a solvent, but does not provide details. Patent Document 4 lists acidic, basic, or neutral polymeric dispersants as dispersants, but also lacks detailed research. Patent Document 5 lists poly(1,1-difluoroethylene) as a binder and polyvinylpyrrolidone as a dispersant, but also lacks detailed research.

[0025] The object of the present invention is to provide a coating liquid for manufacturing a current collector for an energy storage device that forms a carbon coating on one or both sides of a conductive substrate, wherein the dispersibility of powdered carbon materials such as carbon microparticles in the liquid is improved, and the adhesion to the substrate is improved, thereby providing a current collector for an energy storage device for obtaining a low-resistance energy storage device, and a method thereof.

[0026] Methods for solving problems

[0027] In other words, in order to solve the above-mentioned problems, the present invention provides the following technical solution.

[0028] [1]. A current collector for an energy storage device, characterized in that a coating layer is formed on one or both sides of a sheet-like conductive substrate, said coating layer containing powdered carbon material, acid-modified poly(1,1-difluoroethylene) and polyvinylpyrrolidone.

[0029] The content of polyvinylpyrrolidone in the coating layer is 0.099–5.0% by mass.

[0030] The content of the powdered carbon material in the coating layer is 15.0 to 45.0% by mass.

[0031] [2]. The current collector for the energy storage device as described in [1] above, wherein the surface roughness Ra of the coating layer is less than 1.0 μm.

[0032] [3]. The current collector for the energy storage device as described in [1] or [2] above, wherein the average particle size of the primary particles of the powdered carbon material in the coating layer is 10 to 100 nm.

[0033] [4]. The current collector for the energy storage device as described in any of [1] to [3] above, wherein the BET specific surface area of ​​the powdered carbon material is 100 m². 2 / g and below

[0034] [5]. The current collector for an energy storage device as described in any of [1] to [4] above, wherein the weight of the coating layer on one side of the conductive substrate is 0.1 to 5.0 g / m. 2 .

[0035] [6]. A coating liquid for manufacturing current collectors for energy storage devices, characterized in that it contains powdered carbon material, acid-modified poly(1,1-difluoroethylene), polyvinylpyrrolidone, and a solvent.

[0036] The total content of the powdered carbon material, the acid-modified poly(1,1-difluoroethylene), and the polyvinylpyrrolidone in the coating solution is 2-20% by mass.

[0037] The content ratio of the powdered carbon material and the acid-modified poly1,1-difluoroethylene, i.e., the mass ratio of the powdered carbon material to the acid-modified poly1,1-difluoroethylene, is 20:80 to 40:60.

[0038] Relative to 100 parts by weight of the powdered carbon material and the acid-modified poly(1,1-difluoroethylene), the content of the polyvinylpyrrolidone is 0.099 to 5.0 parts by weight.

[0039] The solvent contains N-methyl-2-pyrrolidone.

[0040] [7]. The coating liquid described above [6] further contains alkyl alcohols having 1 to 6 carbon atoms.

[0041] [8]. The coating liquid described above [7] contains only the powdered carbon material, the acid-modified poly(1,1-difluoroethylene), the polyvinylpyrrolidone, the alkyl alcohol and N-methyl-2-pyrrolidone.

[0042] [9]. The coating liquid described in any of [6] to [8] above has a viscosity of 20 to 200 mPa·s at 25°C.

[0043]

[10] . The coating liquid described in any of [6] to [9] above, wherein the average particle size of the primary particles of the powdered carbon material is 10 to 100 nm.

[0044]

[11] . The coating liquid as described in any of [6] to

[10] above, wherein the BET specific surface area of ​​the powdered carbon material is 100 m². 2 / g or less.

[0045]

[12] . The coating liquid as described in any of [6] to

[11] above, wherein the powdered carbon material is carbon black, and the resistivity of the carbon black in powder form, as determined according to JIS K 1469:2003, is 3 × 10⁻⁶ when it is 100% compressed powder. -1 Below Ω·cm.

[0046]

[13] . The coating liquid as described in any of [6] to

[12] above, wherein the acid-modified poly(1,1-difluoroethylene) has a weight-average molecular weight of 2.0 × 10⁻⁶. 5 ~1.4×10 6 The polyvinylpyrrolidone has a weight-average molecular weight of 5.0 × 10⁻⁶. 3 ~1.4×10 6 .

[0047]

[14] . A method for manufacturing a current collector for an energy storage device, characterized in that it comprises the following steps:

[0048] The process of preparing a coating solution containing powdered carbon material, acid-modified poly1,1-difluoroethylene, polyvinylpyrrolidone, and a solvent.

[0049] The process of applying the prepared coating liquid to one or both sides of a sheet-like conductive substrate, and

[0050] The process of drying the coating liquid on the coating.

[0051] The total content of the powdered carbon material, the acid-modified poly(1,1-difluoroethylene), and the polyvinylpyrrolidone in the coating solution is 2-20% by mass.

[0052] The content ratio of the powdered carbon material and the acid-modified poly1,1-difluoroethylene, i.e., the mass ratio of the powdered carbon material to the acid-modified poly1,1-difluoroethylene, is 20:80 to 40:60.

[0053] Relative to 100 parts by weight of the powdered carbon material and the acid-modified poly(1,1-difluoroethylene), the content of the polyvinylpyrrolidone is 0.099 to 5.0 parts by weight.

[0054] The solvent contains N-methyl-2-pyrrolidone.

[0055]

[15] In the manufacturing method described above

[14] , the process of coating the prepared coating liquid is a process of gravure coating.

[0056]

[16] In the manufacturing method described above in

[14] or

[15] , the solvent further contains an alkyl alcohol having 1 to 6 carbon atoms.

[0057]

[17] . The manufacturing method described in any of

[14] to

[16] above has a viscosity of 20 to 200 mPa·s at 25°C.

[0058]

[18] . The manufacturing method described in any of

[14] to

[17] above, wherein the average particle size of the primary particles of the powdered carbon material in the coating liquid is 10 to 100 nm.

[0059]

[19] . The manufacturing method described in any of

[14] to

[18] above, wherein the BET specific surface area of ​​the powdered carbon material is 100 m². 2 / g or less.

[0060]

[20] . The manufacturing method described in any of

[14] to

[19] above, wherein the powdered carbon material is carbon black, and the resistivity of the carbon black in powder form, as measured according to JIS K 1469:2003, is 1 × 10⁻⁶ when it is 100% compressed powder. -1 Below Ω·cm.

[0061] Invention Effects

[0062] The current collector for the energy storage device according to the present invention has low resistance. Furthermore, the powdered carbon material in the coating liquid used to form the coating layer of the current collector for the energy storage device according to the present invention exhibits improved dispersibility. When using the coating liquid according to the present invention, uniform coating is achieved, adhesion to the substrate is improved, and a low-resistance energy storage device can be provided.

[0063] When using the method for manufacturing a current collector for an energy storage device according to the present invention, a conventional coating method can be selected to easily manufacture the current collector. Attached Figure Description

[0064] Figure 1 The observation results of the dispersibility of carbon black in the coating liquid prepared in Example 1-1 are shown.

[0065] Figure 2 The observation results of the dispersibility of carbon black in the coating liquid prepared in Comparative Example 1-1 are shown.

[0066] Figure 3 The appearance of the coating liquid prepared in Example 1-1 when it is gravure coated on aluminum foil is shown.

[0067] Figure 4 The appearance of the coating liquid prepared in Comparative Example 1-1 when gravure coated on aluminum foil is shown.

[0068] Figure 5 The observation results of the adhesion evaluation 1 of the coating layer formed in Example 1-1 are shown.

[0069] Figure 6 The observation results of the adhesion evaluation 1 of the coating layer formed in Comparative Example 5 are shown.

[0070] Figure 7 The observation results of the adhesion evaluation 2 of the coating layer formed in Example 1-1 are shown.

[0071] Figure 8 The observation results of the adhesion evaluation 2 of the coating layer formed in Comparative Example 5 are shown. Detailed Implementation

[0072] The following describes in detail the current collector for the energy storage device, its manufacturing method, and the coating liquid used to manufacture the current collector. The materials and methods illustrated in the following description are merely examples, and the present invention is not limited to these; appropriate modifications can be made without altering the technical concept of the invention.

[0073] [Current collector for energy storage devices]

[0074] In the current collector for the energy storage device according to the present invention, a coating layer is formed on one or both sides of a sheet-like conductive substrate. The coating layer contains powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone.

[0075] (Conductive substrate)

[0076] The material of the sheet-like conductive substrate for the current collector in an energy storage device is not particularly limited as long as it is metal, but foil-shaped substrates are preferred due to their excellent processability. For example, for the current collector of a lithium-ion secondary battery, aluminum foil can be used as the positive electrode current collector, and copper foil can be used as the negative electrode current collector.

[0077] There are no special restrictions on the material of the aluminum foil, but pure aluminum foil or aluminum alloy foil containing more than 95% aluminum by mass is preferred. As an example of pure aluminum foil, A1085 material can be listed, and as an example of aluminum alloy foil, A3003 material (Mn-added series) can be listed.

[0078] There are no particular limitations on the material of the copper foil, but electrolytic copper foil with a rust-proof surface treatment is preferred. Furthermore, substrates used in energy storage devices can be used, such as nickel foil, titanium foil, and stainless steel foil.

[0079] There is no particular limitation on the thickness of the substrate. From the perspective of miniaturization and operability of the energy storage device, a substrate thickness of 3μm to 100μm is usually preferred. In the case of roller-to-roll manufacturing, a substrate thickness of 5μm to 50μm is preferred.

[0080] The substrate can be a non-perforated foil, or a two-dimensional porous foil, a three-dimensional mesh foil, a perforated metal foil, or other perforated foil.

[0081] The surface of the substrate can be subjected to known surface treatments, such as mechanical surface processing, etching, chemical conversion treatment, anodizing, phosphating primer treatment, corona discharge, glow discharge, etc.

[0082] (Covering layer)

[0083] A coating containing powdered carbon material, acid-modified poly(1,1-difluoroethylene) and polyvinylpyrrolidone is formed on one or both sides of a sheet-like conductive substrate.

[0084] The thickness of the coating layer is preferably 0.1 μm or more and 15.0 μm or less, more preferably 0.2 μm or more and 10.0 μm or less, and even more preferably 0.3 μm or more and 5.0 μm or less. If the thickness of the coating layer is 0.1 μm or more, the conductivity between the conductive substrate and the electrode active material can be ensured by the powdered carbon material, which is therefore preferred. On the other hand, if the thickness is 15.0 μm or less, the increase in resistance caused by the layer thickness will not be significant, which is also preferred from the perspective of fabrication.

[0085] The approximate weight (coating weight per unit area) of the coating layer on one side of the conductive substrate is preferably 0.1–5.0 g / m². 2 More preferably, it is 0.2–3.0 g / m 2 The allowable weight of the coating is 0.1 g / m³. 2 In the above cases, the conductivity between the conductive substrate and the electrode active material can be ensured by using powdered carbon materials. If the allowable weight of the coating is 5.0 g / m²... 2 Therefore, compared to the case where no coating is formed on the conductive substrate, the resistance value can be reduced to less than 1 / 10, which is also preferable from a production perspective. Furthermore, for example, in lithium-ion secondary batteries, when forming electrodes by coating the current collector of the present invention with an aqueous slurry containing a positive electrode active material, from the viewpoint of suppressing corrosion of the conductive substrate by the aqueous slurry, a coating weight of 0.2 g / m² is preferred. 2 That's all. Furthermore, when a coating is formed on both sides of the conductive substrate, the amount of coating is approximately twice that mentioned above. The surface and back sides can also have different amounts of coating.

[0086] (Powdered carbon materials)

[0087] Powdered carbon materials are acceptable as long as they impart conductivity to the coating layer; there are no particular limitations. Preferred materials include carbon nanofibers, carbon nanotubes, carbon microparticles such as carbon black, and graphite microparticles. Examples of carbon black include acetylene black, furnace black, and Ketjen black. Specifically, from the viewpoint of conductivity in imparting conductivity to the coating layer, the resistivity of the powder form, as measured according to JIS K 1469:2003, is preferably 3 × 10⁻⁶ when it is in 100% compressed powder form. -1 Below Ω·cm, more preferably 2×10 -1 Below Ω·cm, the above-mentioned carbon materials can also be used in combination as needed. Among these, carbon black is preferred from the viewpoint of dispersibility in the coating liquid and adhesion of the coating layer to the substrate, and acetylene black is more preferred from the viewpoint of coatability when forming the coating layer by gravure coating.

[0088] The preferred BET specific surface area of ​​powdered carbon materials is 100 m². 2 / g or less, more preferably 75m 2 / g or less. In 100m 2 The coating solution exhibits good dispersibility when the concentration is below a certain level (e.g.), enabling it to form a uniform coating layer on a conductive substrate via gravure coating. Furthermore, it maintains good adhesion between the powdered carbon materials or between the conductive substrate and the coating layer.

[0089] The average primary particle size of carbon microparticles used as powdered carbon materials is not particularly limited, but is preferably 10–100 nm. The average primary particle size of the carbon microparticles can be obtained by measuring the primary particle size of 100–1000 carbon microparticles using an electron microscope and then arithmetically averaging them. In the case of spherical particles, the converted diameter of the sphere is used as the particle size; in the case of amorphous particles, the longest axis is used as the particle size.

[0090] There are no particular limitations on the shape of the carbon microparticles, but it is preferable that the particles are connected in a beaded shape to form a large number of conductive paths and are uniformly dispersed on a conductive substrate. The reason for this is that the electronically conductive carbon microparticles share the electron movement between the active material of the electrode and the substrate, and a large contact area between the coating layer and the active material is preferred. Furthermore, it is preferable that the carbon microparticles agglomerate into fewer lumps. This is because with less agglomeration, the coating layer thickness is uniform, allowing for a more uniform design of the thickness of the energy storage device, resulting in less thickness variation. Therefore, the surface roughness of the coating layer is preferably 1.0 μm or less, more preferably 0.9 μm or less, and even more preferably 0.8 μm or less. The lower limit of the surface roughness Ra is preferably 0.1 μm or more, and even more preferably 0.2 μm or more. If the surface roughness Ra is 0.1 μm or more, an anchoring effect with the electrode obtained using the positive or negative electrode active material can achieve a reduction in interfacial resistance.

[0091] The content of powdered carbon material in the coating layer is 15.0 to 45.0% by mass, preferably 17.5 to 42.5% by mass, and more preferably 20.0 to 40.0% by mass.

[0092] If the content of powdered carbon material in the coating is 15.0% by mass or more, sufficient conductivity can be exhibited. Furthermore, if the content of powdered carbon material is 45.0% by mass or less, the adhesion between the powdered carbon materials and between the conductive substrate and the coating can be maintained due to the sufficient presence of the binder.

[0093] (Acid-modified poly(1,1-difluoroethylene))

[0094] Acid-modified poly(1,1-difluoroethylene) is included in the coating as an adhesive.

[0095] Acid modification refers to the addition of a new acid or acid derivative to the unsaturated bonds at the defluorinated sites of poly(1,1-difluoroethylene). Defluorination can be carried out by heating poly(1,1-difluoroethylene). The newly added acid is an organic acid or similar acid. Acid-modified poly(1,1-difluoroethylene) can improve its adhesion to metal foils by adding the acid.

[0096] As acids and acid derivatives used in acid modification, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, monomethyl maleate, monoethyl maleate, maleic anhydride, 2-carboxyethyl acrylate, 2-carboxyethyl methacrylate, acryloyloxyethyl succinic acid, methacryloyloxyethyl succinic acid, acryloyloxyethyl phthalic acid, methacryloyloxyethyl phthalic acid, trifluoroacrylic acid, trifluoromethacrylic acid, 1,1-bis(acryloyloxymethyl)ethyl isocyanate, 2-acryloyloxyethyl isocyanate, 2-methacryloyloxyethyl isocyanate, etc., can be used. As acid-modified poly1,1-difluoroethylene, from the viewpoint of adhesion to metal foil, poly1,1-difluoroethylene monomethyl maleate and poly1,1-difluoroethylene modified by maleic anhydride, methyl acrylate, or methyl methacrylate can be used well.

[0097] When the total amount of structural units contained in acid-modified poly(1,1-difluoroethylene) is set to 100 mol%, the proportion of structural units from unmodified 1,1-difluoroethylene is preferably 90 to 99.9 mol%, more preferably 95 to 99.9 mol%. If the proportion of structural units from unmodified 1,1-difluoroethylene is 90 mol% or more, the coating layer exhibits good swelling resistance relative to the electrolyte in the energy storage device using the current collector of the present invention. Furthermore, the proportion of structural units from modified 1,1-difluoroethylene is preferably 0.1 to 10 mol%, more preferably 0.1 to 5 mol%. If the proportion of structural units from modified 1,1-difluoroethylene is 0.1 mol% or more, sufficient adhesion of the coating layer to the conductive substrate can be ensured.

[0098] The weight-average molecular weight of acid-modified poly1,1-difluoroethylene is not particularly limited, but is preferably 2.0 × 10⁻⁶. 5 ~1.4×10 6 More preferably 2.5×10 5 ~1.3×10 6 If the weight-average molecular weight is 2.0 × 10⁻⁶ 5 The above indicates good adhesion to the conductive substrate; if it is 1.4 × 10⁻⁶, then... 6 The following method yields a coating solution with a viscosity suitable for gravure coating. Furthermore, the "weight-average molecular weight" refers to the value determined using gel permeation chromatography (trade name: Shodex GPC-101, manufactured by Showa Denko Corporation) under the following conditions, and calculated using a standard pullulanose calibration curve.

[0099] Analytical columns: (1) OHpak SB-803HQ, (2) OHpak SB-804HQ, manufactured by Showa Denko Corporation.

[0100] Reference column: OHpak SB-800RL, manufactured by Showa Denko Co., Ltd.

[0101] Column temperature: 40℃

[0102] Sample: The concentration of the sample was determined to be 0.1% by mass.

[0103] Flow rate: 1 mL / min

[0104] Eluent: 0.1M sodium nitrate aqueous solution

[0105] Detector: RI-71S

[0106] The content of acid-modified poly(1,1-difluoroethylene) in the coating layer is preferably 55.0 to 85.0% by mass, more preferably 57.5 to 82.5% by mass, and even more preferably 60.0 to 80.0% by mass. The content ratio of powdered carbon material to acid-modified poly(1,1-difluoroethylene), i.e., the mass of powdered carbon material to the mass of acid-modified poly(1,1-difluoroethylene), is preferably 20:80 to 40:60, more preferably 25:75 to 35:65.

[0107] If the content of acid-modified poly(1,1-difluoroethylene) in the coating layer is 55.0% by mass or more, it can ensure a tight bond with the conductive substrate and prevent carbon particles from falling off the coating layer. If the content of acid-modified poly(1,1-difluoroethylene) is less than 85.0% by mass, the proportion of powdered carbon material is sufficient to maintain high conductivity.

[0108] (Polyvinylpyrrolidone)

[0109] Polyvinylpyrrolidone is included in the coating layer for the purpose of uniformly dispersing powdered carbon materials.

[0110] The preferred weight-average molecular weight of polyvinylpyrrolidone is 5.0 × 10⁻⁶. 3 ~1.4×10 6 More preferably 1.0×10 4 ~1.3×10 6 Furthermore, the "weight-average molecular weight" refers to the value determined using gel permeation chromatography (trade name: Shodex GPC-101, manufactured by Showa Denko Co., Ltd.) under the aforementioned conditions, and calculated using a standard pullulanose calibration curve. When the weight-average molecular weight is within the aforementioned range, the powdered carbon material in the coating solution of the fluorine-free vinyl polymers described later exhibits good dispersibility, preventing thickening and carbon particle aggregation during coating. This is presumably because these fluorine-free vinyl polymers are well adsorbed onto the surface of the powdered carbon material, inhibiting the aggregation of the powdered carbon material through electrostatic repulsion and steric hindrance.

[0111] The content of polyvinylpyrrolidone in the coating layer is 0.099–5.0% by mass, preferably 0.2–4.0% by mass, and more preferably 0.3–3.0% by mass.

[0112] When the polyvinylpyrrolidone (PVP) content in the coating layer is in the range of 0.099–5.0% by mass, the powdered carbon material in the coating solution used to make the coating layer disperses well, thus forming a uniform coating layer. When the PPVP content is less than 0.099% by mass, the dispersion of the powdered carbon material in the coating solution deteriorates, resulting in agglomerates, which hinders precise thickness control during electrode layer coating. When the PPVP content is greater than 5.0% by mass, the resistance value of the energy storage device increases, making it undesirable. The reason for the increase in resistance value is unclear, but it is speculated that because acid-modified poly(1,1-difluoroethylene) tends to make point contact with carbon particles and conductive substrates, while PPVP is more easily coated on the surface of carbon particles, the electrical contact between carbon particles and between carbon particles and conductive substrates deteriorates, leading to an increase in resistance value.

[0113] The content of polyvinylpyrrolidone (PVP) in the coating layer was determined by thermal decomposition gas chromatography / mass spectrometry (GC / MS). The thermal decomposition temperature of the coating solution or coating foil containing PPVP was set to 550°C, and the flow rate within the column was set to 1 mL / min. The resulting chromatographic column and mass spectra were compared with known data to determine the PPVP content. A calibration curve for calculating the PPVP content based on the determined peak area was prepared by measuring at three points, for example, PPVP content of 0.1, 1.0, and 5.0 parts by mass.

[0114] In addition to acid-modified poly(1,1-difluoroethylene) and polyvinylpyrrolidone, the coating layer may contain other resin components, provided that the effects of the present invention are not compromised. These other resins can be any type of resin, and for example, resin compounds cross-linked from polysaccharide polymers or their derivatives by a cross-linking agent can be used. Furthermore, in addition to these, polyacrylic acid resins, polyolefin resins, polyether resins, polyamides, polyimides, polyamide-imides, epoxy resins, etc., can also be used. From the viewpoint of reducing the resistance of the energy storage device, it is preferable that no additional resin components are present.

[0115] [Coating solution used in the manufacture of current collectors for energy storage devices]

[0116] The current collector coating liquid of the preferred embodiment of the present invention contains powdered carbon material, acid-modified poly(1,1-difluoroethylene), polyvinylpyrrolidone, and solvent.

[0117] When N-methyl-2-pyrrolidone (NMP) is used as a solvent, the concentration of the coating solution is less likely to change during the coating process, making it easier to obtain a coating with a uniform thickness. Furthermore, the solvent can be thoroughly removed through heat treatment.

[0118] The coating liquid may also contain alkyl alcohols with 1 to 6 carbon atoms as a solvent. Examples include methanol, ethanol, isopropanol, and hexanol, with isopropanol being preferred. By using alkyl alcohols in combination, it is expected that the dispersibility of the coating liquid will be improved, resulting in a homogenized and smoothed coating surface. When using alkyl alcohols in combination, it is preferable to use N-methyl-2-pyrrolidone and the alkyl alcohol in a ratio of N-methyl-2-pyrrolidone (parts by mass) to alkyl alcohol (parts by mass) of 95:5 to 50:50. When the proportion of alkyl alcohol is 5 parts by mass or more, the surface of the resulting coating is homogenized; when it is 50 parts by mass or less, the resin component can be used without agglomeration.

[0119] Also for the purpose of improving dispersibility, organic pigments are sometimes used, but from the viewpoint of light durability relative to indoor light, it is preferable not to use them.

[0120] The total content of powdered carbon material, acid-modified poly(1,1-difluoroethylene) and polyvinylpyrrolidone in the coating solution is 2-20% by mass, preferably 2-17.5% by mass, and more preferably 2-15% by mass.

[0121] Relative to the total mass of powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone contained in the coating liquid, the mass percentage of powdered carbon material is preferably 15.0 to 45.0% by mass, more preferably 17.5 to 42.5% by mass, and even more preferably 20.0 to 40.0% by mass. If the mass percentage of powdered carbon material is 15.0% by mass or more, a coating layer exhibiting sufficient conductivity can be formed. Furthermore, if the mass percentage of powdered carbon material is 45.0% by mass or less, sufficient binder is present, thus enabling the formation of a coating layer that maintains adhesion between the powdered carbon materials and between the conductive substrate and the coating layer.

[0122] Furthermore, relative to the total mass of powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone contained in the coating liquid, the mass ratio of acid-modified poly(1,1-difluoroethylene) is preferably 55.0–85.0% by mass, more preferably 57.5–82.5% by mass, and even more preferably 60.0–80.0% by mass. The content ratio of powdered carbon material to acid-modified poly(1,1-difluoroethylene), i.e., the mass of powdered carbon material to the mass of acid-modified poly(1,1-difluoroethylene), is 20:80–40:60, preferably 25:75–35:65. If the above-mentioned mass ratio of acid-modified poly(1,1-difluoroethylene) is 55.0% by mass or more, it is possible to ensure adhesion to the conductive substrate and form a coating layer that prevents carbon particles from falling off from the coating layer. Furthermore, if the mass ratio of acid-modified poly(1,1-difluoroethylene) is 85.0% or less, the proportion of powdered carbon material is sufficient to form a coating layer that maintains high conductivity.

[0123] Furthermore, relative to 100 parts by mass of the total powdered carbon material and acid-modified poly(1,1-difluoroethylene) contained in the coating solution, the content of polyvinylpyrrolidone is 0.099 to 5.0 parts by mass, preferably 0.2 to 4.0 parts by mass, and more preferably 0.3 to 3.0 parts by mass. When the content of polyvinylpyrrolidone is in the range of 0.099 to 5.0 parts by mass, a slurry with good dispersion of the powdered carbon material can be obtained, forming a uniform coating layer. If the content of polyvinylpyrrolidone is less than 0.099 parts by mass, the dispersion of the powdered carbon material deteriorates, resulting in agglomerates, and thus the coating properties become poor. On the other hand, when the content of polyvinylpyrrolidone is greater than 5.0 parts by mass, the slurry viscosity increases, the coating properties deteriorate, and the resistance value of the energy storage device increases, which is also undesirable.

[0124] When the total content of powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone in the coating solution is within the above-mentioned range, and the proportion of powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone in the coating solution is within the above-mentioned range, and N-methyl-2-pyrrolidone (NMP) is used as the solvent, the powdered carbon material has good dispersibility, and the liquid viscosity is suitable, allowing for the selection of conventional coating methods and easy fabrication of current collectors for energy storage devices. In this case, the viscosity of the coating solution at the coating temperature is preferably 20–200 mPa·s, more preferably 20–150 mPa·s, and even more preferably 20–100 mPa·s. When the viscosity of the coating solution is 200 mPa·s or less, coating on the substrate is easily performed. Furthermore, if the viscosity of the coating solution is 20 mPa·s or more, a sufficient film thickness can be formed on the substrate.

[0125] Viscosity was measured using a Type B viscometer, selecting a rotor and rotation speed suitable for the resin with the viscosity to be measured. For example, when measuring the viscosity of a coating liquid at 100 mPa·s, a No. 2 rotor was used, with a rotation speed of 60 rpm and a measurement temperature of 20–25 degrees Celsius.

[0126] [Manufacturing method for current collectors in energy storage devices]

[0127] The method for manufacturing a current collector for an energy storage device according to the present invention includes the following steps: preparing a coating liquid containing powdered carbon material, acid-modified poly(1,1-difluoroethylene), polyvinylpyrrolidone, and a solvent; applying the prepared coating liquid to one or both sides of a sheet-like conductive substrate; and drying the coated liquid. The aforementioned powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone can be used as the powdered carbon material, acid-modified poly(1,1-difluoroethylene), and polyvinylpyrrolidone.

[0128] There are no particular limitations on the method of applying the coating liquid to one or both sides of a conductive substrate; common coating methods such as gravure coating, die coating, rod coating, spin coating, and clamp coating can be used. From the viewpoint of high production volume, gravure coating is preferred.

[0129] In the process of drying the coating liquid on the coating, the coating liquid on the coating is dried to form a coating layer on the substrate. Drying is preferably carried out at a temperature of 50°C or higher in order to allow the solvent to evaporate completely.

[0130] When the coating liquid contains a thermosetting resin component, it is preferable to cure that resin component. When it contains a thermosetting resin, it is more preferable to dry it above the resin's curing temperature (crosslinking reaction temperature). The coating liquid may also contain catalysts, polymerizers, crosslinking agents, etc., that can promote such a curing reaction.

[0131] [electrode]

[0132] The following description will use a lithium-ion secondary battery made with the current collector for an energy storage device according to the present invention as an example. The current collector for an energy storage device of the present invention is expected to be effective when used with electrodes employing positive electrode active materials using a high-voltage mode, but it is not limited to a specific positive electrode current collector; it can also be used with a negative electrode current collector. When using the current collector for an energy storage device according to the present invention, a reduction in the interfacial resistance between the current collector and the electrode can be achieved at both the positive and negative electrodes, thus enabling the acquisition of a low-resistance energy storage device.

[0133] The positive electrode can be formed by dissolving or dispersing a positive electrode active material, a positive electrode conductive additive, and a binder in a solvent, coating the resulting slurry onto the current collector of the energy storage device of the present invention, and then drying it. Here, PVDF or the like, which can be dissolved in organic solvents, is typically used as the binder. Alternatively, an aqueous slurry containing SBR or acrylic resins may also be used.

[0134] The positive electrode active material and the positive electrode conductive additive can be of known quality.

[0135] Preferred positive electrode active materials include, for example, lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel oxide (LiNiO2), and ternary lithium compounds (Li(Co)O2) in which a portion of the Co in lithium cobalt oxide is replaced by Mn and Ni. x Mn y Ni z (O2), a portion of the Ni in lithium nickelate is replaced by Co and Al (Li(Ni) x Co y Al z O2), olivine-based (LiFePO4, LiMnPO4), etc. Preferred conductive additives for positive electrodes include, for example, acetylene black, furnace black, Ketjen black, fumed carbon fibers, and graphite powder.

[0136] The negative electrode is formed by dissolving or dispersing a negative electrode active material, a negative electrode conductive additive, and a binder in a solvent, coating the resulting slurry onto the current collector of the energy storage device of the present invention, and then drying it. Here, as a binder, organic solvents such as PVDF are typically used, and aqueous solvents such as SBR or acrylic resins are typically used.

[0137] The negative electrode active material and the conductive additive for the negative electrode can be of known quality.

[0138] As the negative electrode active material, graphite-based materials such as natural graphite and artificial graphite, alloys containing silicon or tin, titanium oxide-based materials such as lithium titanate, or mixtures thereof are preferred. As the conductive additive for the negative electrode, carbon black such as acetylene black, furnace black, and Ketjen black, as well as fumed carbon fibers, are preferred.

[0139] [Lithium-ion rechargeable battery]

[0140] One aspect of the present invention relates to a lithium-ion secondary battery having the aforementioned electrode. The electrode is formed by forming a coating layer on a conductive substrate to become a current collector, and the coating layer has an electrode active material layer containing a positive electrode active material or a negative electrode active material, a conductive additive, and a binder. The positive and negative electrodes are joined together by a separator, and the electrode is filled with an electrolyte and has an outer packaging material.

[0141] The electrolyte, separator, and outer packaging material, which are components of the energy storage device other than the electrodes, can all be of known quality. The electrolyte is not limited to liquid; it can also be gel-like or solid. The separator can be made of membranes such as polypropylene or polyethylene.

[0142] Lithium-ion rechargeable batteries can discharge by connecting their positive and negative terminals to loads such as motors and light sources, and can be charged by connecting them to a power source.

[0143] If a current collector having the coating layer of the present invention on the surface of a conductive substrate is used in the electrode of a lithium-ion secondary battery, the electrode resistance can be reduced compared to conventional current collectors. That is, the internal resistance of the lithium-ion secondary battery can be reduced. Furthermore, by using the current collector for energy storage devices of the present invention, high-voltage charging of lithium-ion secondary batteries using high-voltage active materials can be performed, achieving a high-capacity lithium-ion secondary battery.

[0144] Example

[0145] The materials used in the examples and comparative examples are as follows.

[0146] • Acetylene Black: Manufactured by Denka Co., Ltd., product name: Denka Black (registered trademark) HS-100, average primary particle size 48nm, BET specific surface area 39m² 2 / g, resistivity 0.14Ω·cm

[0147] • Ketchen Black: Manufactured by Lion Special Ketchen Co., Ltd., product name: Ketchen Black EC300J, average primary particle size 39.5nm, BET specific surface area 800m² 2 / g

[0148] • Poly(1,1-difluorovinyl chloride) modified PVDF: manufactured by SOLVAY, product name: SOLEF5120, Mw = 6.7 × 10⁻⁶ 5 Acrylic modification

[0149] • Poly(1,1-difluorovinyl chloride) modified PVDF: manufactured by SOLVAY, product name: SOLEF5130, Mw = 1.2 × 10⁻⁶ 6 Acrylic modification

[0150] • Poly(1,1-difluorovinyl chloride) modified PVDF: Kreha Co., Ltd., Product Name: KF Polymer W#9100, Mw = 2.8 × 10⁻⁶ 5 Acrylic modification

[0151] • Poly(1,1-difluoroethylene), unmodified homopolymer PVDF: manufactured by Arkema, product name: KynarHSV-900, Mw = 7.2 × 10⁻⁶5

[0152] • Poly(1,1-difluoroethylene), unmodified copolymer PVDF: manufactured by Clehwa Co., Ltd., product name: KF Polymer W#9300, Mw = 1.0 × 10⁻⁶ 6 Copolymer of 1,1-difluoroethylene and hexafluoropropylene

[0153] • Poly(1,1-difluorovinyl chloride) modified PVDF aqueous dispersion: SOLVAY Co., Ltd., Product Name: SOLVAY XPH-838

[0154] • Unmodified homopolymer PVDF of poly1,1-difluorovinyl acetate: KF Polymer W#1100, manufactured by Clehwa Co., Ltd., Mw = 2.8 × 10⁻⁶ 5

[0155] • Polyvinylpyrrolidone (PVP): Manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd., Product Name: Pitchcor K-30, Mw = 4.5 × 10⁻⁶ 4

[0156] • Polyvinylpyrrolidone (PVP): Manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd., Product Name: Pitchcor K-90, Mw = 1.2 × 10⁻⁶ 6

[0157] ·Polyvinyl acetate PVAc: manufactured by Nippon Polyvinyl Acetate Co., Ltd.

[0158] ·Polyvinyl alcohol PVA: Made by Nippon Nihon Polyvinyl Co., Ltd.

[0159] • Poly-N-vinylacetamide: Manufactured by Showa Denko Co., Ltd., Product Name: PNVA

[0160] • Vinyl alcohol / vinylpyrrolidone graft polymerization P (VA-VP), manufactured by Daiichi Kogyo Pharmaceutical Co., Ltd., product name: Pitzcort V-7154

[0161] ·N-Methyl-2-pyrrolidone (NMP): manufactured by Mitsubishi Micron Co., Ltd.

[0162] Isopropanol: Manufactured by Yamaichi Chemical Industry Co., Ltd.

[0163] [Evaluation of the coating solution]

[0164] <Dispersibility of Powdered Carbon Materials>

[0165] The dispersibility of powdered carbon material in the coating solution was evaluated as follows: 5 mL of coating solution was poured vertically into a 50 mL glass test tube held vertically along its wall. After 15 minutes, the appearance of the wall surface was observed visually. No agglomerates were observed on the wall surface, which was considered good dispersibility (marked as "○" in Tables 1 and 2). Agglomerates were observed, which was considered poor dispersibility (marked as "×" in Tables 1 and 2).

[0166] Evaluation of current collectors for energy storage devices

[0167] <Adhesion of the coating layer to the conductive substrate>

[0168] The adhesion of the coating layer to the conductive substrate is evaluated using the following two methods. If the results of both methods are good, it is marked as "○" in Tables 1 and 2. If the result of either method is poor, it is marked as "×" in Tables 1 and 2.

[0169] (Seamlessness Evaluation 1 (Using Celluloid Tape Method))

[0170] A 5cm layer of celluloid tape (product name N.29) manufactured by Nitto Denko Corporation was attached to the current collector produced in the following examples and comparative examples. The celluloid tape was rolled back and forth 5 times with a 1kg roller. Then, the top of the celluloid tape was immediately lifted and peeled off. A condition in which the coating layer was not peeled off and the conductive substrate was not exposed was evaluated as good and marked "○" in Tables 1 and 2. A condition in which the coating layer was peeled off and the conductive substrate was exposed was evaluated as bad and marked "×" in Tables 1 and 2.

[0171] (Seamlessness Evaluation 2 (Using a dry cotton swab))

[0172] The surface of the current collector coating produced in the following examples and comparative examples will be scratched 10 times with a dry cotton swab (manufactured by Kawamoto Sangyosha, product name: antibacterial paper roll cotton swab) at a load of 100g, a speed of 5cm / s, and a width of 5cm (one single pass is counted as 1 stroke). If the coating is not peeled off and the conductive substrate is not exposed, it will be evaluated as good and marked as "○" in Tables 1 and 2. If the coating is peeled off and the conductive substrate is exposed, it will be evaluated as bad and marked as "×" in Tables 1 and 2.

[0173] <Surface roughness Ra of the coating formed on the conductive substrate>

[0174] The surface roughness Ra of the coating layer formed on the conductive substrate was measured using a shape measuring laser microscope (VK-200) manufactured by Key Industries Co., Ltd. at a magnification of 20x. The change curve of the measurement line relative to a straight line parallel to the coating direction of the gravure coating was measured in accordance with JIS B 0601:2001.

[0175] [Evaluation of Lithium-ion Secondary Batteries]

[0176] <The Production of Positive Electrode Tablets>

[0177] An appropriate amount of N-methyl-2-pyrrolidone was added to 90 parts by weight of LiFePO4 (M121 manufactured by Aleees Co., Ltd.) as the positive electrode active material, 5 parts by weight of conductive carbon black (SUPER P manufactured by Imerys Co., Ltd.) as a conductive additive, and 5 parts by weight of poly(1,1-difluoroethylene manufactured by Arkema Co., Ltd., Kynar HSV-900) as a binder, and the mixture was stirred to prepare a slurry-like dispersion. The prepared dispersion was coated onto the coating layer of the current collector prepared in the following examples and comparative examples using a blade coater with a 200 μm slit, and then dried and pressed to obtain a positive electrode sheet.

[0178] <The Production of Negative Electrode Sheets>

[0179] A slurry-like dispersion was prepared by adding an appropriate amount of water to 95 parts by weight of artificial graphite (manufactured by Showa Denko Co., Ltd., SCMG (registered trademark)-AR) as the negative electrode active material, 1 part by weight of conductive carbon black (manufactured by Imerys Co., Ltd., SUPERP) as a conductive additive, 3 parts by weight of styrene-butadiene rubber (manufactured by Japan Zen Co., Ltd., BM-400B) as a binder, and 1 part by weight of carboxymethyl cellulose (manufactured by Daicel Fenke Co., Ltd., #1380) as a thickener. The dispersion was then coated onto a 20 μm thick copper foil using a 200 μm slit coater, dried, and press-molded to obtain the negative electrode sheet.

[0180] <Evaluation of the fabrication of laminated batteries>

[0181] The positive and negative electrode sheets, prepared as described above, are sandwiched together with a polypropylene separator (Celgard, Celgard 2500). The resulting laminate is placed inside an aluminum laminate packaging material, electrolyte is injected, and the aluminum laminate packaging material is heat-sealed under vacuum to obtain a laminated battery for evaluation.

[0182] As an electrolyte, it is used as follows: 1 mol / L of LiPF6 as the electrolyte and 1% by mass of vinylene carbonate as an additive are dissolved in a solvent prepared by mixing ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7 to obtain a liquid.

[0183] As described above, a battery with a rated capacity of 100mAh (1C = 100mA) was manufactured.

[0184] <Evaluation of the DC internal resistance (DC-IR) of a battery>

[0185] The DC internal resistance (DC-IR) of the battery was measured as follows: After initial charge and discharge, the battery was adjusted to 50% depth of charge (SOC). Then, at room temperature, it was discharged for 5 seconds at each of five points between 0.1C and 2C. The voltage change before and after the discharge was measured using a charge-discharge device (manufactured by Toyo System Co., Ltd., TOSCAT-3000). The average value of the voltage change / current at the five points was calculated as the DC internal resistance.

[0186] (Example 1-1)

[0187] Prepare acid-modified PVDF (Mw = 6.7 × 10⁻⁶). 5 70 parts by weight of acetylene black (HS-100), 30 parts by weight of polyvinylpyrrolidone (K-30), and 0.1 parts by weight of N-methyl-2-pyrrolidone and isopropanol were added to bring the solid content concentration to 5% by weight. The amount of N-methyl-2-pyrrolidone (parts by weight): the amount of isopropanol (parts by weight) = 84:16. The mixture was mixed at 4000 rpm for 30 minutes using a dispersion mixer (Nippon Seiki Co., Ltd., Eclipse Auto Homogenes). Table 1 shows the amount (parts by weight) of powdered carbon material, acid-modified PVDF, and polyvinylpyrrolidone added, and the content (%) of each component in the coating layer formed by the coating solution. The dispersibility of carbon black in the obtained coating solution was evaluated, and no agglomerates were found, indicating good dispersibility. Figure 1 The viscosity of the resulting coating solution was 78 mPa·s (Table 1).

[0188] Next, aluminum foil of material A1N30 with a thickness of 15μm is prepared. The prepared coating liquid is placed in the liquid collection section (tray) of a gravure coating machine (manufactured by Nakajima Seiki Engineering Co., Ltd. (now Union Tech Co., Ltd.)). The gravure roll is rotated at a certain speed, bringing the aluminum foil into contact with the roll. The aluminum foil is then moved in the opposite direction of rotation to apply the coating, allowing the coating liquid to dry. At this point, no ribs are observed on the un-engraved section (corresponding to the portion without a coating layer) and the engraved section (corresponding to the portion with a coating layer) of the gravure roll. Figure 3 Therefore, it can be concluded that the carbon black in this coating solution has good dispersibility and is suitable for gravure coating.

[0189] The measured mass (mass per unit area) of the current collector coating is 0.34 g / m². 2 The adhesion of the current collector's coating was evaluated. Adhesion evaluation 1 was performed, and no aluminum foil exposure was found. Figure 5Next, a seal evaluation 2 was performed, and no aluminum foil exposure was found. Figure 7 The surface roughness Ra is 0.55 μm (Table 1). Using the obtained current collector, a secondary cell was fabricated using the aforementioned method, and the internal resistance was determined to be 312 mΩ (Table 1).

[0190] (Examples 1-2 to 1-4)

[0191] Except that the amounts of each component added were as described in Table 1, the coating solution was prepared in the same manner as in Example 1-1, and the coating was applied using a gravure coating machine. The coating solution was then dried to obtain the current collector. The evaluations were performed in the same manner as in Example 1-1, and the results are shown in Table 1.

[0192] (Comparative Example 1-1)

[0193] Except that the amount of polyvinylpyrrolidone added was 0 parts by weight, the coating solution was prepared in the same manner as in Examples 1-1. Figure 2 The observation results of the coating liquid dispersibility shown in the figure revealed the presence of agglomerates and poor dispersibility. Here, an addition amount of 0 parts by mass indicates no addition (the same applies below). The viscosity of the obtained coating liquid was 146 mPa·s (Table 1). The coating liquid was placed in the liquid accumulation section of the above-mentioned gravure coating machine, and the gravure roller was rotated at a certain speed in the same manner as in Example 1-1. At this time (when the aluminum foil is not in contact with the gravure roller), ribs were observed to be generated in the unengraved part of the gravure roller. Figure 4 ).

[0194] It is speculated that the formation of these ridges is due to the poor dispersibility of acetylene black, leading to the formation of agglomerates. Furthermore, no ridges were observed in the engraving section of the gravure roller. This can be attributed to the fact that the agglomerates entered the tiny pits (the part holding the coating liquid) within the engraving section, making it difficult for ridges to form and thus difficult to observe with the naked eye. Therefore, this coating liquid is unsuitable for gravure coating and cannot produce the desired current collector.

[0195] (Compare Examples 1-2 and 1-3)

[0196] Except that the amount of polyvinylpyrrolidone added was 6.0 and 10.0 parts by mass, respectively, the coating solution was prepared in the same manner as in Examples 1-1. The carbon black dispersibility of the obtained coating solution was evaluated, and the viscosity was measured (Table 1) to obtain the current collector. The adhesion and surface roughness Ra of the obtained current collector were measured (Table 1). A secondary battery was fabricated, and the internal resistance was evaluated. The internal resistance was higher than that of Examples 1-1 to 4 (Table 1).

[0197] It is speculated that the high internal resistance of the secondary battery is due to the fact that when a large amount of polyvinylpyrrolidone is added, it acts as an insulating component, which deteriorates the conductivity.

[0198] (Examples 2-1 to 4)

[0199] Acid-modified PVDF (Mw = 1.2 × 10⁻⁶) was used. 6 ) to replace acid-modified PVDF (Mw=6.7×10 5 N-methyl-2-pyrrolidone and isopropanol were added to the mixture to make the acid-modified PVDF concentration 3% by mass (N-methyl-2-pyrrolidone:isopropanol = 85:15). Except for these steps, the coating solution was prepared in the same manner as in Examples 1-1 to 4. Dispersibility was evaluated, viscosity was measured, the adhesion of the resulting current collector was evaluated, surface roughness Ra was measured, a secondary battery was fabricated, and internal resistance was evaluated. The evaluation results are shown in Table 1.

[0200] (Comparative Examples 2-1 to 3)

[0201] Except that the amount of polyvinylpyrrolidone added was 0, 6.0, and 10.0 parts by weight, the evaluation was performed in the same manner as in Example 2-1. The evaluation results are shown in Table 1.

[0202] (Examples 3-1 to 4)

[0203] Acid-modified PVDF (Mw = 2.8 × 10⁻⁶) was used. 5 ) to replace acid-modified PVDF (Mw=6.7×10 5 N-methyl-2-pyrrolidone and isopropanol were added to bring the concentration of the solid component to 7% by mass. Otherwise, the coating solution was prepared in the same manner as in Examples 1-1 to 4. The dispersibility was evaluated, the viscosity was measured, the adhesion of the obtained current collector was evaluated, the surface roughness Ra was measured, a secondary battery was fabricated, and the internal resistance was evaluated. The evaluation results are shown in Table 1.

[0204] (Comparative Examples 3-1 to 3)

[0205] Except that the amount of polyvinylpyrrolidone added was 0, 6.0, and 10.0 parts by weight, the evaluation was performed in the same manner as in Example 3-1. The evaluation results are shown in Table 1.

[0206] (Examples 4-1 to 4)

[0207] Polyvinylpyrrolidone (K-90) was used instead of polyvinylpyrrolidone (K-30). Otherwise, the coating solution was prepared in the same manner as in Examples 1-1 to 4. The dispersibility was evaluated, the viscosity was measured, the adhesion of the obtained current collector was evaluated, the surface roughness Ra was measured, and a secondary battery was fabricated to evaluate the internal resistance. The evaluation results are shown in Table 1.

[0208] (Comparative Examples 4-1 to 3)

[0209] Except that the amount of polyvinylpyrrolidone added was 0, 6.0, and 10.0 parts by weight, the evaluation was performed in the same manner as in Example 4-1. The evaluation results are shown in Table 1.

[0210] (Comparative Example 5)

[0211] In addition to using homopolymer PVDF (Mw = 7.2 × 10⁻⁶), 5 ) to replace acid-modified PVDF (Mw=6.7×10 5 ), and N-methyl-2-pyrrolidone and isopropanol were added to make the solid content concentration 6.5% by mass, and the coating solution was prepared in the same manner as in Examples 1-3. The dispersibility was evaluated, and the surface roughness Ra of the resulting current collector was measured (Table 1). Next, the adhesion evaluation 1 was performed, and the results showed that the aluminum foil was exposed ( Figure 6 A seal assessment was conducted, and the results showed that the aluminum foil was exposed. Figure 8 ).

[0212] The poor adhesion is presumably due to insufficient adhesion of the homopolymer PVDF to the aluminum foil. This causes the coating layer to peel off from the aluminum foil during the process of forming an electrode layer for the purpose of fabricating a secondary battery and evaluating its internal resistance. Therefore, it is impossible to fabricate a secondary battery or evaluate its internal resistance.

[0213] (Comparative Example 6)

[0214] Using copolymer PVDF (Mw = 1.0 × 10⁻⁶) 6 ) to replace acid-modified PVDF (Mw=6.7×10 5 N-methyl-2-pyrrolidone and isopropanol were added to the mixture to bring the concentration of the solid component to 4% by mass. Otherwise, the coating solution was prepared in the same manner as in Examples 1-3 to evaluate dispersibility, assess the adhesion of the resulting current collector, and measure the surface roughness Ra (Table 1). Adhesion evaluation 1 was performed, and no aluminum foil exposure was found. However, adhesion evaluation 2 revealed aluminum foil exposure. Because of this, although an electrode layer was desired for fabricating a secondary battery and evaluating internal resistance, the coating layer peeled off from the aluminum foil substrate during this process. Therefore, it was impossible to fabricate a secondary battery to evaluate internal resistance.

[0215] (Comparative Example 7)

[0216] Aqueous PVDF dispersion was used instead of acid-modified PVDF (Mw = 6.7 × 10⁻⁶). 5 Water was added to bring the concentration of the solid components to 6% by mass. Otherwise, the coating solution was prepared in the same manner as in Examples 1-3, but agglomerates and poor dispersibility were found (Table 1). Similar to Comparative Examples 1-1, the desired sample could not be obtained.

[0217] (Comparative Example 8)

[0218] Except for using polyvinyl acetate instead of polyvinylpyrrolidone (K-30), the coating solution was prepared in the same manner as in Examples 1-3, but agglomerates and poor dispersibility were found (Table 1). Similar to Comparative Examples 1-1, the desired sample could not be obtained.

[0219] (Comparative Example 9)

[0220] Except for using polyvinyl alcohol instead of polyvinylpyrrolidone (K-30), the coating solution was prepared in the same manner as in Examples 1-3, but agglomerates and poor dispersibility were found (Table 1). Similar to Comparative Examples 1-1, the desired sample could not be obtained.

[0221] (Comparative Example 10)

[0222] Except for using poly(N-vinylacetamide) instead of polyvinylpyrrolidone (K-30), the coating solution was prepared in the same manner as in Examples 1-3, but agglomerates and poor dispersibility were found (Table 1). Similar to Comparative Examples 1-1, the desired sample could not be obtained.

[0223] (Comparative Example 11)

[0224] Except for using a vinyl alcohol / vinylpyrrolidone graft polymer instead of polyvinylpyrrolidone (K-30), the coating solution was prepared in the same manner as in Examples 1-3, but agglomerates and poor dispersibility were found (Table 1). Similar to Comparative Examples 1-1, the desired sample could not be obtained.

[0225] (Comparative Example 12)

[0226] Prepare homopolymer PVDF (Mw = 2.8 × 10⁻⁶) 5 44.2 parts by weight of Ketjen black (EC300J), 46.5 parts by weight of polyvinylpyrrolidone (K-30), and 9.3 parts by weight of polyvinylpyrrolidone (K-30) were used to prepare the coating solution in the same manner as in Examples 1-1. The dispersibility of carbon black in the obtained coating solution was evaluated and the viscosity was measured (Table 1) to obtain the current collector.

[0227] The surface roughness Ra of the obtained current collector was 1.6 μm, which is greater than 1.0 μm. It is speculated that the high surface roughness Ra is due to the agglomeration of Ketjen black, preventing the formation of a uniformly thick coating layer. Next, an adhesion evaluation 1 was performed, and aluminum foil exposure was found. An adhesion evaluation 2 was performed, confirming aluminum foil exposure. It is speculated that the poor adhesion evaluation is due to insufficient adhesion of the homopolymer PVDF to the aluminum foil. Because of this, although an electrode layer needs to be formed for fabricating a secondary battery and evaluating its internal resistance, the same problem as in Comparative Example 5 occurred, preventing the fabrication of a secondary battery for evaluating its internal resistance.

[0228] (Example 13-1)

[0229] Prepare acid-modified PVDF (Mw = 6.7 × 10⁻⁶). 5 80 parts by weight of acetylene black (HS-100), 20 parts by weight of polyvinylpyrrolidone (K-30), and N-methyl-2-pyrrolidone and isopropanol were added to make the concentration of solid components 6% by weight. The coating solution was prepared in the same manner as in Examples 1-1. The dispersibility of carbon black in the resulting coating solution was evaluated and the viscosity was measured. The adhesion of the resulting current collector was evaluated and the surface roughness Ra was measured. A secondary battery was fabricated, and the internal resistance was evaluated. The evaluation results are shown in Table 2.

[0230] (Example 13-2)

[0231] Prepare acid-modified PVDF (Mw = 6.7 × 10⁻⁶). 5 60 parts by weight of carbon black (HS-100), 40 parts by weight of acetylene black (HS-100), and 0.1 parts by weight of polyvinylpyrrolidone (K-30) were used. The coating solution was prepared in the same manner as in Examples 1-1. The dispersibility of carbon black in the resulting coating solution was evaluated and the viscosity was measured. The adhesion of the current collector was evaluated and the surface roughness Ra was measured. A secondary battery was fabricated to evaluate the internal resistance. The evaluation results are shown in Table 2.

[0232] (Comparative Example 13-1)

[0233] Prepare acid-modified PVDF (Mw = 6.7 × 10⁻⁶). 5 90 parts by weight of N-methyl-2-pyrrolidone (HS-100), 10 parts by weight of acetylene black (HS-100), and 0.1 parts by weight of polyvinylpyrrolidone (K-30) were added to the coating solution in a ratio of N-methyl-2-pyrrolidone to isopropanol of 85:15 to bring the concentration of the solid components to 3% by weight. Otherwise, the coating solution was prepared in the same manner as in Examples 1-1. The dispersibility of carbon black in the resulting coating solution was evaluated, the adhesion of the current collector was evaluated, and the surface roughness Ra was measured. A secondary battery was fabricated, and the internal resistance was evaluated. The evaluation results are shown in Table 2.

[0234] It is speculated that the high internal resistance of the secondary battery is due to the excessive addition of acid-modified PVDF, which hinders the conductivity of acetylene black.

[0235] (Comparative Example 13-2)

[0236] Prepare acid-modified PVDF (Mw = 6.7 × 10⁻⁶). 530 parts by weight of acetylene black (HS-100), 70 parts by weight of polyvinylpyrrolidone (K-30), and 0.1 parts by weight of N-methyl-2-pyrrolidone and isopropanol were added to make the concentration of solid components 4.5% by weight. Otherwise, the coating solution was prepared in the same manner as in Examples 1-1. The dispersibility of carbon black in the obtained coating solution was evaluated, the adhesion of the obtained current collector was evaluated, and the surface roughness Ra was measured (Table 2).

[0237] Exposed aluminum foil was found in both the adhesion evaluation 1 and adhesion evaluation 2. It is presumed that the poor adhesion was due to excessive acetylene black addition, resulting in insufficient adhesion of the acid-modified PVDF to the aluminum foil. Consequently, although an electrode layer needs to be formed for fabricating a secondary battery and evaluating internal resistance, the same problem as in Comparative Example 5 occurred, preventing the fabrication of a secondary battery and the evaluation of internal resistance.

[0238] (Comparative Example 13-3)

[0239] Prepare acid-modified PVDF (Mw = 6.7 × 10⁻⁶). 5 10 parts by weight of acetylene black (HS-100), 90 parts by weight of polyvinylpyrrolidone (K-30), and 5 parts by weight of N-methyl-2-pyrrolidone and isopropanol were added to make the concentration of solid components 7% by weight. Otherwise, the coating solution was prepared in the same manner as in Examples 1-4. Agglomerates and poor dispersibility were found in the coating solution (Table 2).

[0240] The poor dispersibility evaluation is presumably due to excessive addition of acetylene black, which worsened the dispersibility. As a result, the desired sample could not be obtained, similar to Comparative Example 1-1.

[0241]

[0242]

Claims

1. A coating liquid for manufacturing current collectors for energy storage devices, characterized in that, The mixture contains powdered carbon material, acid-modified poly1,1-difluoroethylene, polyvinylpyrrolidone, and a solvent. The powdered carbon material is uniformly dispersed in a connected bead-like structure. The total content of the powdered carbon material, the acid-modified poly(1,1-difluoroethylene), and the polyvinylpyrrolidone in the coating solution is 2-20% by mass. The content ratio of the powdered carbon material and the acid-modified poly1,1-difluoroethylene, i.e., the mass ratio of the powdered carbon material to the acid-modified poly1,1-difluoroethylene, is 20:80 to 40:

60. Relative to 100 parts by weight of the powdered carbon material and the acid-modified poly(1,1-difluoroethylene), the content of polyvinylpyrrolidone is 0.099 to 3.0 parts by weight. The powdered carbon material contains carbon black, and the BET specific surface area of ​​the powdered carbon material is 100 m². 2 The particles are below / g, and the average particle size of the primary particles is 10–100 nm. The solvent contains N-methyl-2-pyrrolidone and an alkyl alcohol having 1 to 6 carbon atoms, and the ratio of N-methyl-2-pyrrolidone to the alkyl alcohol is 95:5 to 50:50 by mass.

2. The coating liquid according to claim 1, comprising only the powdered carbon material, the acid-modified poly(1,1-difluoroethylene), the polyvinylpyrrolidone, the alkyl alcohol, and N-methyl-2-pyrrolidone.

3. The coating liquid as described in claim 1 or 2, wherein the powdered carbon material further comprises carbon nanofibers or carbon nanotubes.

4. The coating liquid as described in claim 1 or 2, wherein the powdered carbon material is carbon black, and the resistivity of the carbon black in powder form, as measured according to JIS K1469:2003, is 3 × 10⁻⁶ when it is 100% compressed powder. -1 Below Ω·cm.

5. The coating liquid according to claim 1 or 2, wherein the acid-modified poly1,1-difluoroethylene has a weight-average molecular weight of 2.0 × 10⁻⁶. 5 ~1.4×10 6 The polyvinylpyrrolidone has a weight-average molecular weight of 5.0 × 10⁻⁶. 3 ~1.4×10 6 .

6. A method for manufacturing a current collector for an energy storage device, characterized in that, It includes the following processes: The process of preparing a coating liquid containing powdered carbon material, acid-modified poly1,1-difluoroethylene, polyvinylpyrrolidone, and a solvent, wherein the powdered carbon material is uniformly dispersed in a connected bead-like state. The process of applying the prepared coating liquid to one or both sides of a sheet-like conductive substrate, and The process of drying the coating liquid on the coating. The total content of the powdered carbon material, the acid-modified poly(1,1-difluoroethylene), and the polyvinylpyrrolidone in the coating solution is 2-20% by mass. The content ratio of the powdered carbon material and the acid-modified poly1,1-difluoroethylene, i.e., the mass ratio of the powdered carbon material to the acid-modified poly1,1-difluoroethylene, is 20:80 to 40:

60. Relative to 100 parts by weight of the powdered carbon material and the acid-modified poly(1,1-difluoroethylene), the content of polyvinylpyrrolidone is 0.099 to 3.0 parts by weight. The powdered carbon material contains carbon black. The BET specific surface area of ​​the powdered carbon material is 100 m². 2 The particles are below / g, and the average particle size of the primary particles is 10–100 nm. The solvent contains N-methyl-2-pyrrolidone and an alkyl alcohol having 1 to 6 carbon atoms, and the ratio of N-methyl-2-pyrrolidone to the alkyl alcohol is 95:5 to 50:50 by mass.

7. In the method for manufacturing a current collector for an energy storage device as described in claim 6, the step of coating the prepared coating liquid is a gravure coating step.

8. A current collector for an energy storage device, characterized in that, It is obtained by the manufacturing method of the current collector for the energy storage device according to claim 6 or 7, wherein a coating layer is formed on one or both sides of a sheet-like conductive substrate, said coating layer containing powdered carbon material, acid-modified poly1,1-difluoroethylene and polyvinylpyrrolidone, said powdered carbon material containing carbon black. The BET specific surface area of ​​the powdered carbon material is 100 m². 2 The particles are below / g, and the average particle size of the primary particles is 10–100 nm. The content of polyvinylpyrrolidone in the coating layer is 0.099–5.0% by mass. The content of the powdered carbon material in the coating layer is 15.0 to 45.0% by mass.

9. The current collector for an energy storage device as described in claim 8, wherein the surface roughness Ra of the coating layer is 1.0 μm or less.

10. The current collector for an energy storage device as described in claim 8 or 9, wherein the powdered carbon material further comprises carbon nanofibers or carbon nanotubes.

11. The current collector for an energy storage device as described in claim 8 or 9, wherein the allowable weight of the coating layer on one side of the conductive substrate is 0.1 to 5.0 g / m². 2 .

Images