Electrode sheet and method for manufacturing electrode sheet

Abstract

To provide an electrode sheet with reduced residual stress and a method for manufacturing the electrode sheet.SOLUTION: The electrode sheet includes: a metal foil having a thickness of 15 μm at a maximum; and a coating film formed on both surfaces of the metal foil, the radius of curvature of curling of the electrode sheet being at least 2000 mm, and the radius of curvature of curling after the coating film formed on one surface of the metal foil is removed from the electrode sheet being at least 350 mm or more. The application of the electrode sheet is also provided.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an electrode sheet and a method for manufacturing an electrode sheet. [Background technology]

[0002] A method is known in which a desired coating film is produced on a support by a continuous process using a roll-to-roll system. A method for producing a coating film includes, for example, applying a coating liquid to a support to obtain a desired coating film and drying the resulting coating liquid film. An electrode sheet is produced, for example, by applying a coating liquid containing an electrode material to both sides of a metal foil.

[0003] For example, Patent Document 1 describes a method for manufacturing an electrode, which includes a pressing step in which a current collecting foil, a first electrode composite layer formed on one surface of the current collecting foil, and a second electrode composite layer formed on the other surface of the current collecting foil are pressed to manufacture an electrode having the current collecting foil, a first electrode active material layer formed by compressing the first electrode composite layer, and a second electrode active material layer formed by compressing the second electrode composite layer at a higher compression rate than the first electrode active material layer, and a winding step in which, after the pressing step, the electrode is wound up with the second electrode active material layer facing toward a roll core. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 210542 Summary of the Invention [Problem to be solved by the invention]

[0005] In electrode sheets, there are cases where it is desired to reduce residual stress. If residual stress exists in the electrode sheet, the release of the residual stress over time can cause dimensional changes within the battery, leading to a decrease in battery performance. Furthermore, if residual stress exists in the electrode sheet, stress corrosion occurs, which makes it easier for side reactions to occur during charging and discharging of the battery, and may result in a decrease in the repeated charging and discharging characteristics of the battery.

[0006] Therefore, the problem to be solved by one embodiment of the present disclosure has been made in consideration of the above circumstances, and is to provide an electrode sheet with little residual stress and a method for manufacturing the electrode sheet. [Means for solving the problem]

[0007] The present disclosure includes the following aspects. <1> A metal foil having a thickness of 15 μm or less; A coating film formed on both sides of the metal foil, The radius of curvature of the electrode sheet curl is 2000 mm or more, An electrode sheet in which the radius of curvature of the curl after removing the coating film formed on one side of the metal foil from the electrode sheet is 350 mm or more. <2> The coating thickness is 30 μm to 200 μm. <1> The electrode sheet according to claim 1. <3> The metal foil is an aluminum foil. <1> or <2> The electrode sheet according to claim 1. <4> The thickness of the metal foil is 12 μm or less. <1> ~ <3> 10. An electrode sheet according to any one of the preceding items. <5> The coating film contains carbon nanotubes. <1> ~ <4> 10. An electrode sheet according to any one of the preceding items. <6> Used in the negative electrode of secondary batteries, <1> ~ <5> 10. An electrode sheet according to any one of the preceding items. <7> The method includes a step of forming a coating film on both sides of a metal foil having a thickness of 15 μm or less to produce an electrode sheet, The radius of curvature of the electrode sheet curl is 2000 mm or more, A method for producing an electrode sheet, wherein the radius of curvature of the curl after removing a coating film formed on one side of the metal foil from the electrode sheet is 350 mm or more. [Effects of the Invention]

[0008] According to one embodiment of the present disclosure, an electrode sheet with reduced residual stress and a method for manufacturing the electrode sheet are provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing an example of a method for forming a coating film on a metal foil. [Figure 2] FIG. 2 is a schematic side view illustrating an example of the curl regulating means. [Figure 3] FIG. 3 is a schematic side view for explaining another example of the curl regulating means. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the method for producing a coating film will be described. However, the present disclosure is not limited to the following embodiments and can be implemented with appropriate modifications within the scope of the object of the present disclosure.

[0011] In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples. The elements in the drawings shown in this disclosure are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. In addition, in each drawing, components having the same functions are given the same reference numerals, and duplicated explanations will be omitted. In the present disclosure, the "width direction" refers to a direction perpendicular to the longitudinal direction of a long metal foil, a coating liquid film, and a coating film. In the present disclosure, a combination of two or more preferred aspects or embodiments is a more preferred aspect or embodiment.

[0012] <Electrode sheet> The electrode sheet according to the present disclosure is an electrode sheet including a metal foil having a thickness of 15 μm or less and a coating film formed on both sides of the metal foil, wherein the radius of curvature of the curl of the electrode sheet is 2000 mm or more, and the radius of curvature of the curl after removing the coating film formed on one side of the metal foil from the electrode sheet is 350 mm or more.

[0013] A curl radius of 2000 mm or more of the electrode sheet means high flatness. Since the radius of curvature of the curl after removing the coating film formed on one surface of the metal foil is 350 mm or more, the electrode sheet of the present disclosure has little residual stress.

[0014] <Metal foil> The electrode sheet of the present disclosure includes a metal foil.

[0015] The thickness of the metal foil is 15 μm or less, preferably 12 μm or less. The lower limit of the thickness of the metal foil is not particularly limited, and is, for example, 8 μm. The thickness of the metal foil of 15 μm or less indicates the range of thickness of the metal foil at which curling is likely to occur in practice.

[0016] The thickness of the metal foil is measured by the following method. A portion of the metal foil is cut out and measured using a micrometer (product name "High Precision Digimatic Micrometer MDH-25MC", manufactured by Mitutoyo Corporation).

[0017] The width and length of the metal foil may be appropriately determined from the viewpoint of application to the roll-to-roll method and the width and length of the intended electrode sheet.

[0018] Examples of metals that can be used to form the metal foil include copper, aluminum, silver, gold, and alloys thereof. The metal constituting the metal foil may be stainless steel, nickel, titanium, or an invar alloy.

[0019] Among these, copper foil or aluminum foil is preferred, and aluminum foil is more preferred, in terms of shape stability and usage history as a metal foil.

[0020] <Coating film> The electrode sheet of the present disclosure includes a coating film formed on both sides of a metal foil.

[0021] To increase the battery output, it is desirable to increase the specific surface area, so a thinner coating is preferable. On the other hand, to increase the battery capacity, it is desirable to increase the amount of coating per volume, so a thicker coating is preferable. To achieve both battery output and capacity, the coating thickness is preferably 30 μm to 200 μm, and more preferably 40 μm to 100 μm.

[0022] The metal foil thickness of 30 μm to 200 μm indicates the range of thickness of the metal foil in which curling is likely to occur in practice.

[0023] The thickness of the coating film is measured by the following method. A portion of the electrode sheet is cut out, and the thickness of the entire electrode sheet is measured using a micrometer (product name "High Precision Digimatic Micrometer MDH-25MC", manufactured by Mitutoyo Corporation). The thickness of the metal foil is then subtracted from the thickness of the electrode sheet to obtain the thickness of the coating film. The method for measuring the thickness of the metal foil is as described above.

[0024] The coating film is formed, for example, by applying a coating liquid (described later) onto a metal foil and drying it. The coating film is only required to contain components necessary for functioning as an electrode sheet, and the components that make up the coating film are not particularly limited. The coating film contains, for example, an electrode active material and a conductive aid.

[0025] (electrode active material) The electrode active material is a substance capable of inserting and releasing ions of a metal element belonging to Group 1 or Group 2 of the periodic table. Examples of the electrode active material include a positive electrode active material and a negative electrode active material.

[0026] -Positive electrode active material- The positive electrode active material is not limited and may be any known electrode active material used for positive electrodes. The positive electrode active material is preferably a positive electrode active material that can reversibly insert and release lithium ions.

[0027] Specific examples of the positive electrode active material include transition metal oxides and elements that can be composited with lithium (e.g., sulfur). Among the above, the positive electrode active material is preferably a transition metal oxide.

[0028] The transition metal oxide is preferably a transition metal oxide containing at least one transition metal element (hereinafter referred to as "element Ma") selected from the group consisting of Co (cobalt), Ni (nickel), Fe (iron), Mn (manganese), Cu (copper), and V (vanadium).

[0029] When the transition metal oxide contains Li and the element Ma, the molar ratio of Li to Ma (Li / Ma) is preferably 0.3 to 2.2.

[0030] The transition metal oxide may also contain at least one transition metal element (hereinafter referred to as "element Mb") selected from the group consisting of Group 1 elements other than lithium, Group 2 elements, Al (aluminum), Ga (gallium), In (indium), Ge (germanium), Sn (tin), Pb (lead), Sb (antimony), Bi (bismuth), Si (silicon), P (phosphorus), and B (boron). The content of element Mb is preferably 0 mol % to 30 mol % relative to the amount of element Ma.

[0031] Examples of transition metal oxides include transition metal oxides having a layered rock salt structure, transition metal oxides having a spinel structure, lithium-containing transition metal phosphate compounds, lithium-containing transition metal halide phosphate compounds, and lithium-containing transition metal silicate compounds.

[0032] Examples of transition metal oxides having a layered rock salt structure include LiCoO2 (lithium cobalt oxide [LCO]), LiNi2O2 (lithium nickel oxide), LiNi 0.85 Co 0.10 Al 0.05 O2 (nickel cobalt lithium aluminum oxide [NCA]), LiNi1 / 3Co 1 / 3 Mn 1 / 3 O2 (lithium nickel manganese cobalt oxide [NMC]), and LiNi 0.5 One example is Mn0.5O2 (lithium manganese nickel oxide).

[0033] Examples of transition metal oxides having a spinel structure include LiCoMnO4, Li2FeMn3O8, Li2CuMn3O8, Li2CrMn3O8, and Li2NiMn3O8.

[0034] Examples of lithium-containing transition metal phosphate compounds include olivine-type iron phosphate salts (e.g., LiFePO4 and Li3Fe2(PO4)3), iron pyrophosphate salts (e.g., LiFeP2O7), cobalt phosphate salts (e.g., LiCoPO4), and monoclinic Nasicon-type vanadium phosphate salts (e.g., Li3V2(PO4)3 (lithium vanadium phosphate)).

[0035] Examples of lithium-containing transition metal halophosphate compounds include iron fluorophosphates (e.g., LiFePOF), manganese fluorophosphates (e.g., LiMnPOF), and cobalt fluorophosphates (e.g., LiCoPOF).

[0036] Examples of lithium-containing transition metal silicate compounds include Li2FeSiO4, Li2MnSiO4, and Li2CoSiO4.

[0037] The transition metal oxide is preferably a transition metal oxide having a layered rock salt structure, such as LiCoO2 (lithium cobalt oxide [LCO]), LiNi 0.85 Co 0.10 Al 0.05 O2 (nickel cobalt lithium aluminum oxide [NCA]), and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (lithium nickel manganese cobalt oxide [NMC]) is more preferred.

[0038] The positive electrode active material may be a commercially available product or a synthetic product produced by a known method (e.g., a calcination method). For example, the positive electrode active material obtained by the calcination method may be washed with water, an acidic aqueous solution, an alkaline aqueous solution, or an organic solvent. The positive electrode active material may have a carbon coating on its surface.

[0039] The positive electrode active material may be used alone or in combination of two or more. Furthermore, even when one type of positive electrode active material is used, positive electrode active materials having different particle sizes may be used in combination.

[0040] -Negative electrode active material- The negative electrode active material is not limited and may be any known electrode active material used for negative electrodes. The negative electrode active material is preferably a negative electrode active material that can reversibly insert and release lithium ions.

[0041] Examples of the negative electrode active material include carbonaceous materials, metal oxides (e.g., tin oxide), silicon oxide, metal composite oxides, lithium alone, lithium alloys (e.g., lithium-aluminum alloys), and metals capable of forming alloys with lithium (e.g., Sn, Si, and In). Among these, the negative electrode active material is preferably a carbonaceous material or a lithium composite oxide from the viewpoint of reliability.

[0042] A carbonaceous material is a material that consists essentially of carbon. Examples of carbonaceous materials include petroleum pitch, carbon black (e.g., acetylene black), graphite (e.g., natural graphite and artificial graphite (e.g., vapor-grown graphite)), hard carbon, and carbonaceous materials obtained by calcining synthetic resins (e.g., polyacrylonitrile (PAN) and furfuryl alcohol resin). Examples of carbonaceous materials include carbon fibers (e.g., polyacrylonitrile-based carbon fibers, cellulose-based carbon fibers, pitch-based carbon fibers, vapor-grown carbon fibers, dehydrated PVA (polyvinyl alcohol)-based carbon fibers, lignin carbon fibers, glassy carbon fibers, and activated carbon fibers). Examples of graphite include mesophase microspheres, graphite whiskers, and tabular graphite. In this disclosure, "flat" means a shape having two major planes facing in opposite directions.

[0043] The metal composite oxide is preferably a metal composite oxide capable of absorbing and releasing lithium. From the viewpoint of high current density charge / discharge characteristics, the metal composite oxide capable of absorbing and desorbing lithium preferably contains at least one element selected from the group consisting of titanium and lithium.

[0044] The metal oxide and metal composite oxide are particularly preferably amorphous oxides.

[0045] The metal oxides and composite metal oxides are also preferably chalcogenides, which are reaction products of metal elements and elements of Group 16 of the periodic table.

[0046] Among the compound group consisting of amorphous oxides and chalcogenides, amorphous oxides and chalcogenides of metalloid elements are preferred, and oxides and chalcogenides containing at least one element selected from the group consisting of elements of Groups 13 to 15 of the periodic table, Al, Ga, Si, Sn, Ge, Pb, Sb, and Bi are more preferred.

[0047] It is also preferable that the negative electrode active material further contains titanium. From the viewpoints of excellent rapid charge / discharge characteristics due to small volume fluctuations during absorption and release of lithium ions and of the ability to extend the life of the lithium ion secondary battery by suppressing electrode deterioration, the negative electrode active material containing titanium is preferably Li4Ti5O 12 (lithium titanate [LTO]) is preferred.

[0048] The negative electrode active material may be a commercially available product or a synthetic product produced by a known method (e.g., a calcination method). For example, the negative electrode active material obtained by the calcination method may be washed with water, an acidic aqueous solution, an alkaline aqueous solution, or an organic solvent.

[0049] The negative electrode active material is available, for example, as CGB20 (Nippon Graphite Industries Co., Ltd.).

[0050] The composition of the negative electrode active material is measured using inductively coupled plasma (ICP) emission spectroscopy.

[0051] The negative electrode active material may be used alone or in combination of two or more kinds. Furthermore, even when one type of negative electrode active material is used, negative electrode active materials having different particle sizes may be used in combination.

[0052] The surfaces of the positive electrode active material and the negative electrode active material may each be coated with a surface coating agent. Examples of the surface coating agent include metal oxides containing Ti, Nb, Ta, W, Zr, Si, or Li. Examples of the metal oxide include titanate spinel, tantalum-based oxides, niobium-based oxides, and lithium niobate-based compounds.

[0053] (Conductive additive) The conductive aid is not particularly limited, and known conductive aids can be used.

[0054] Examples of conductive additives include graphite (e.g., natural graphite and artificial graphite), carbon black (e.g., acetylene black, ketjen black, and furnace black), amorphous carbon (e.g., needle coke), carbon fibers (e.g., vapor-grown carbon fibers and carbon nanotubes), other carbonaceous materials (e.g., graphene and fullerene), metal powders (e.g., copper powder and nickel powder), metal fibers (e.g., copper fibers and nickel fibers), and conductive polymers (e.g., polyaniline, polypyrrole, polythiophene, polyacetylene, and polyphenylene derivatives).

[0055] The conductive assistant may be used alone or in combination of two or more kinds.

[0056] <Curl curvature radius> The electrode sheet according to the present disclosure has a radius of curvature of 2000 mm or more, preferably 5000 mm or more. The upper limit of the radius of curvature is not particularly limited, and is, for example, 50000 mm.

[0057] The electrode sheet according to the present disclosure has a radius of curvature of 2000 mm or more, and therefore it can be said that coating films are formed symmetrically on both sides of the metal foil.

[0058] In this disclosure, the radius of curvature is measured in the following manner.

[0059] First, a sample is obtained by cutting the electrode sheet so that the length to width ratio is 10:1. For example, a sample having a length of 100 mm and a width of 10 mm is obtained. The sample is placed vertically and the upper end is fixed in an environment of 25°C and 50% relative humidity. The amount of curl at the bottom end of the sample is measured, and the radius of curvature is calculated from the relationship with the width of the sample.

[0060] In the electrode sheet according to the present disclosure, after removing the coating film formed on one side of the metal foil from the electrode sheet, the radius of curvature of the curl is 350 mm or more, preferably 500 mm or more. The upper limit of the radius of curvature is not particularly limited, and is, for example, 50,000 mm.

[0061] In the present disclosure, the radius of curvature of the curl after removing the coating film formed on one surface of the metal foil is measured by the following method.

[0062] The electrode sheet is left in an environment of 25°C and relative humidity of 50% for 10 minutes or more. The electrode sheet is cut out to obtain a sample having a length of 180 mm and a width of 100 mm. For the obtained sample, a solvent at 25°C is applied to the coating film formed on one side of the metal foil to moisten it. The amount of solvent is 20 g per 150 mm x 20 mm coating area. The type of solvent is not particularly limited, and water is an example. After applying the solvent to the coating, the coating is left for at least one minute. The sample is placed on a base, and the moistened coating film is removed from the metal foil by rubbing it with a nonwoven fabric without scratching the surface of the metal foil. When rubbing the coating film, the sample is not fixed to the base, or the four corners of the sample are fixed to the base with tape with enough strength to prevent the radius of curvature of the curl from changing. The base may be any smooth base. The operation of wetting the coating film with a solvent and rubbing the wet coating film with a nonwoven fabric is repeated until the entire coating film formed on one side of the metal foil is removed. Whether the metal foil is not damaged by rubbing with the nonwoven fabric can be determined by visually observing the surface of the metal foil and determining whether the gloss is maintained. Furthermore, whether the metal foil is not defective by rubbing with the nonwoven fabric can be determined by shining a light on the sample from the unrubbed coating side and determining whether there is no transmitted light. After removing all of the coating film formed on one side of the metal foil, the surface of the metal foil is wiped and the sample is dried by exposing it to air in an environment of 25°C and 50% relative humidity. After drying, the coating film formed on one side of the metal foil is removed, and the radius of curvature of the curl is measured in the same manner as in the above-mentioned method for measuring the radius of curvature. The method for measuring the radius of curvature is the same as the method for measuring the radius of curvature of the electrode sheet described above.

[0063] The electrode sheet according to the present disclosure has a curl radius of 350 mm or more after removing the coating film formed on one side of the metal foil from the electrode sheet, resulting in low residual stress. Therefore, even if the residual stress is released over time, dimensional changes within the battery are small, and the occurrence of folds and wrinkles is suppressed. This makes it less likely that the battery performance will deteriorate. Furthermore, stress corrosion is less likely to occur, and deterioration of the repeated charge / discharge characteristics of the battery can be suppressed.

[0064] Therefore, the electrode sheet according to the present disclosure is preferably used for the negative electrode of a secondary battery. Examples of secondary batteries include lithium ion batteries, lead batteries, nickel cadmium batteries, and nickel metal hydride batteries.

[0065] The method for manufacturing an electrode sheet according to the present disclosure includes a step of forming a coating film on both sides of a metal foil having a thickness of 15 μm or less to manufacture an electrode sheet, wherein the radius of curvature of the curl of the electrode sheet is 2000 mm or more, and the radius of curvature of the curl after removing the coating film formed on one side of the metal foil from the electrode sheet is 350 mm or more.

[0066] The details of the radius of curvature of the curl of the metal foil and coating film, and the electrode sheet, and the radius of curvature of the curl after removing the coating film formed on one side of the metal foil from the electrode sheet, are as described above.

[0067] A method for forming a coating film on both sides of a metal foil having a thickness of 15 μm or less will be described below.

[0068] The process of producing an electrode sheet by forming coating films on both sides of a metal foil includes, for example, a process of applying a coating liquid to the metal foil to obtain a coating liquid film, and a process of drying the coating liquid film. It is preferable to first form a coating film on one side of the metal foil, and then form a coating film on the other side of the metal foil. From the viewpoint of forming symmetrical coating films on both sides of the metal foil, it is preferable that the method of forming the coating film on one side of the metal foil and the method of forming the coating film on the other side of the metal foil are the same.

[0069] An example of a method for forming a coating film on a metal foil will be described with reference to FIG. As shown in Fig. 1, when the leading end of a wound long metal foil 10 is fed out and continuous conveyance begins, a coating liquid is applied by a coating means 20. As a result, a coating liquid film made of the coating liquid is formed on the long metal foil. Next, the laminate 12 of the formed coating liquid film and the metal foil 10 is continuously transported through the drying means 30, thereby drying the coating liquid film on the metal foil 10. As a result, the coating liquid film on the long metal foil is dried, and a coating film is formed.

[0070] The coating liquid is not particularly limited as long as it is a liquid containing a solvent (or dispersion medium) and a solid content. The solid content of the coating solution includes components for obtaining the desired coating film, as well as components for improving coating suitability. The solid content refers to components excluding the solvent (or dispersion medium).

[0071] The solvent may be water, an organic solvent, or a mixed solvent of water and an organic solvent.

[0072] That is, the coating liquid may be a water-based coating liquid or a solvent-based coating liquid. Here, the aqueous coating liquid refers to a coating liquid in which the solvent (or dispersion medium) contained therein is substantially water. The phrase "the solvent (or dispersion medium) is substantially water" means that the inclusion of solvents other than water that are introduced when using solid components is permitted, and refers to the proportion of water in all solvents (or all dispersion mediums) being 90% by mass or more, preferably the proportion of water in all solvents (or all dispersion mediums) being 95% by mass or more, and particularly preferably all solvents (or all dispersion mediums) being water.

[0073] Examples of water contained in the aqueous coating liquid include natural water, purified water, distilled water, ion-exchanged water, pure water, and ultrapure water (e.g., Milli-Q water). Milli-Q water is ultrapure water obtained using a Milli-Q water production system manufactured by Merck Millipore, a company owned by Merck Ltd.

[0074] The water content in the aqueous coating liquid is not particularly limited, and is, for example, preferably 40% by mass or more, more preferably 50% by mass or more, based on the total mass of the aqueous coating liquid. The upper limit of the water content is sufficient as long as it is less than 100% by mass, but from the viewpoint of coating suitability, it is, for example, 80% by mass relative to the total mass of the water-based coating liquid.

[0075] The solid content contained in the coating liquid is not particularly limited, and includes various components used to obtain the desired coating film. Specific examples of the solid content contained in the coating liquid include the components contained in the coating film.

[0076] The solid content of the coating liquid is not particularly limited, but is preferably less than 70% by mass, and more preferably 30% to 60% by mass.

[0077] The coating liquid is applied by a known coating means. Specific examples of the coating means (for example, coating means 20 in FIG. 1) include coating devices that use a curtain coating method, a dip coating method, a spin coating method, a print coating method, a spray coating method, a slot coating method, a roll coating method, a slide coating method, a blade coating method, a gravure coating method, a wire bar method, etc.

[0078] It is preferable that the metal foil is continuously transported and the coating liquid is applied onto the continuously transported metal foil.

[0079] Then, it is preferable to dry the coating liquid film on the metal foil which is being continuously transported. In the step of drying the coating liquid film, it is preferable to start curl control for the laminate consisting of the metal foil and the coating liquid film while the solid content concentration of the coating liquid film is 90 mass % or less, and to maintain the temperature of the laminate at 40°C to 100°C during curl control.

[0080] Here, the solid content concentration of the coating liquid film can be measured using Keyence's SI-T80 infrared spectroscopic interference film thickness meter by measuring the non-contact thickness from the time the coating liquid is applied to the metal foil until the coating liquid becomes a dry film. Specifically, first, the non-contact thickness is measured from the time when the coating liquid is applied onto the metal foil until the coating liquid becomes a dry film. Next, the thickness of the film after drying (dry film) is measured using a contact thickness meter. The measured dry film thickness is subtracted from the previously measured non-contact thickness to calculate the thickness of the solvent (or dispersion medium) in the coating liquid film at each measurement point. The obtained thickness of the dry film and the thickness of the solvent (or dispersion medium) are multiplied by their respective densities (density of the dry film and density of the solvent), which are converted into the dry film weight and solvent weight per unit area of ​​the coating liquid film at the measurement point, thereby determining the solid content value.

[0081] When the solid content of the coating liquid film is 90% by mass or less, the components contained in the coating liquid film are fluid in the coating liquid film, and the residual stress is low. By starting curl control in a state where the residual stress is low and maintaining the temperature of the laminate at 40°C to 100°C during curl control, the occurrence of folds and wrinkles can be suppressed.

[0082] From the viewpoint of further suppressing the occurrence of folds and wrinkles, the solid content concentration of the coating liquid film at the start of curl control is preferably 70% by mass or less, and from the viewpoint of improving drying efficiency, the solid content concentration of the coating liquid film at the start of curl control is preferably 50% by mass or more.

[0083] The curling control is not particularly limited as long as it is a means capable of preventing the widthwise ends of the laminate of the coating liquid film and the metal foil from curling (ie, warping) toward the coating liquid film.

[0084] The curling may be controlled by a means that comes into contact with the coating liquid film, or by a means that does not come into contact with the coating liquid film.

[0085] In order to further prevent the occurrence of folds and wrinkles, the curl regulation is preferably a non-contact curl regulation.

[0086] The non-contact curl control is not particularly limited as long as it is a means that can prevent the widthwise end of the laminate of the coating liquid film and the metal foil from curling (i.e., warping) toward the coating liquid film without coming into contact with the coating liquid film. From the viewpoint of excellent curl control ability, non-contact curl control is preferably performed by a means (hereinafter also referred to as a curl control means) that sprays gas onto one or both sides of the laminate and continuously transports the laminate while curving it in the thickness direction by the gas wind pressure. The curl control means also functions as a part of the drying means, since it expels gas onto the laminate to promote drying (that is, an increase in the solid content concentration).

[0087] Curl control means The curl regulating means will be described with reference to Figures 2 and 3. Figures 2 and 3 are schematic side views for explaining the curl regulating means. 2 and 3, reference numeral 32 denotes a pre-curl regulation area, and reference numeral 34 denotes a curl regulation area. In the drying means 30A shown in FIG. 2, the curl control area 34 employs a curl control means that sprays gas onto one side of the laminate 12 (i.e., the surface on which the coating liquid film is formed) and continuously transports the laminate while curving it in the thickness direction by the air pressure of the gas. In the drying means 30B shown in FIG. 3, the curl control area 34 employs a curl control means that blows gas onto both sides of the laminate 12 (i.e., the surface on which the coating liquid film is formed and the exposed surface of the metal foil) and continuously transports the laminate while curving it in the thickness direction by the air pressure of the gas. Such curl control means allows the laminate 12 to be conveyed while undulating in a wave-like manner, as shown in the schematic side views of Figures 2 and 3. By conveying the laminate 12 while undulating in this manner, curl control is effectively exerted, and the curl suppression effect can be enhanced. The drying speed of the coating liquid film can also be controlled by adjusting the type of gas ejected from the curl control means, the air pressure, the temperature, the humidity, and the like.

[0088] From the viewpoint of further suppressing the occurrence of folds and wrinkles, the non-contact curl regulation is preferably the curl regulation means shown in FIG. That is, non-contact curl regulation is preferably performed by blowing gas onto both sides of the laminate and continuously conveying the laminate while curving it in the thickness direction by the gas pressure.

[0089] The drying means 30A shown in FIG. 2 will be described. As shown in FIG. 2, the laminate 12 of the coating liquid film and the metal foil is transported through a drying means 30A, whereby the coating liquid film is dried. In Figure 2, in the pre-curl control region 32, the solid content concentration of the coating liquid film in the laminate 12 is increased, and in the curl control region 34, curl control begins while the solid content concentration of the coating liquid film on the metal foil 10 is 90 mass% or less.

[0090] In the pre-curl control region 32 in FIG. 2, a drying means (for example, a hot air blower) as described later is used to increase the solid content concentration of the coating liquid film in the laminate 12.

[0091] In the curl control area 34 in Figure 2, multiple transport rolls 36 are arranged side by side on the same plane on the metal foil side, and multiple gas ejection sections 38 are arranged side by side on the same plane between the installation positions of the transport rolls 36 on the coating liquid film side. Gas (for example, air) is ejected from the ejection section 38 toward the laminate 12, and the transport roll 36 rotates, whereby the laminate 12 is transported while being curved in its thickness direction by the wind pressure of the gas.

[0092] The drying means 30B shown in FIG. 3 will be described. As shown in FIG. 3, the laminate 12 of the coating liquid film and the metal foil is transported through drying means 30B, whereby the coating liquid film is dried. In Figure 3, in the pre-curl control region 32, the solid content concentration of the coating liquid film in the laminate 12 is increased, and in the curl control region 34, curl control begins while the solid content concentration of the coating liquid film on the metal foil 10 is 90 mass% or less.

[0093] In the pre-curl control region 32 in FIG. 3, a drying means (for example, a hot air blower) as described below is used to increase the solid content concentration of the coating liquid film in the laminate 12.

[0094] In the curl control area 34 in Figure 3, multiple ejection sections 38a that eject gas onto the metal foil side are arranged side by side on the same plane, and multiple ejection sections 38b that eject gas are arranged side by side on the same plane on the coating liquid film side between the installation positions of the ejection sections 38a. Gas (e.g., air) is ejected from ejection portion 38a toward stack 12, and gas (e.g., air) is ejected from ejection portion 38b toward stack 12, whereby stack 12 is curved in its thickness direction by the wind pressure of the gas while being transported.

[0095] The curl control region 34 in FIGS. 2 and 3 may be located at a position where curl control begins while the solid content of the coating liquid film of the transported laminate 12 is 90% by mass or less. The installation position of the curl control area 34 can be determined based on the results of a study conducted in advance on the transition of the solid content concentration of the coating liquid film. In addition, the installation position of the curl control region 34 may be determined in advance, and the conveying speed of the laminate 12, the drying conditions of the pre-curl control region 32, etc. may be appropriately adjusted to control the drying state of the coating liquid film so that the solid content concentration of the coating liquid film is in the range of 90 mass% or less when it reaches the curl control region 34. Furthermore, it is preferable that the curl control region 34 terminates at the exit of the drying means 30A or 30B, for example.

[0096] In the curl control region 34 in FIGS. 2 and 3, the gas ejected toward the laminate 12 may be, for example, air. The temperature of the gas to be ejected is preferably, for example, 60°C to 140°C, and more preferably 90°C to 130°C. Furthermore, the wind speed of the gas to be ejected is preferably, for example, 1.5 m / sec to 50 m / sec. Furthermore, the amount of deformation of the laminate 12 in the curl control region 34 may be adjusted. 2 and 3, when the laminate 12 is viewed from the side, the deformation amount of the laminate 12 includes the distance p between adjacent peaks in the wavy laminate 12 and the height difference h between the peaks and valleys in the laminate 12. The distance p is equivalent to the distance between adjacent transport rolls 36 or the distance between the ejection portions 38b. The distance p is, for example, preferably 100 mm to 1500 mm, and more preferably 200 mm to 1000 mm. The height difference h is preferably 10 mm to 500 mm, and more preferably 20 mm to 200 mm. Furthermore, since the smaller the value of distance p / height difference h, the stronger the curl-regulating force, it is preferable that the value be 10 or less, and more preferably 5 or less. Since a smaller value of distance p / height difference h increases the number of components in the curl-regulating area 34 or increases its size, it is preferable to optimally design the lower limit of distance p / height difference h taking into account factors such as equipment installation space, air intake capacity, and cost. An example of the lower limit of distance p / height difference h is 2.

[0097] -Drying- The laminate can be dried by a known drying method. Specific examples of the drying means (for example, part of the drying means 30 in FIG. 1, drying in the pre-curl control area in FIGS. 2 and 3) include an oven, a hot air blower, and an infrared (IR) heater.

[0098] From the viewpoint of further suppressing the occurrence of folds and wrinkles, it is preferable to heat both sides of the laminate in this step. Both sides of the laminate mean both the side on which the coating liquid is applied and the side on which the metal foil is applied. Conventionally, only the surface on which the coating liquid is applied is heated in order to dry the coating liquid applied to the metal foil. In this process, it is preferable to heat both sides of the laminate in order to maintain the temperature of the laminate at 40°C to 100°C during curl control.

[0099] -Temperature of laminate- During curl regulation, the temperature of the laminate is preferably maintained at 40° C. to 100° C. The temperature of the laminate is measured as the surface temperature of the coating liquid film (film surface temperature). The film surface temperature is measured with an infrared thermometer.

[0100] During curl control, the metal foil stretches by maintaining the temperature of the laminate at 40°C to 100°C. After drying is complete, the metal foil shrinks when the laminate returns to room temperature, preventing folds and wrinkles.

[0101] From the viewpoint of further suppressing the occurrence of folds and wrinkles, the temperature of the laminate is preferably 60°C to 80°C during curl regulation. [Example]

[0102] The present invention will be described in more detail below with reference to examples. The materials, amounts used, ratios, details of each step, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. All "parts" are based on mass.

[0103] <Preparing the metal foil> Aluminum foil: 1000 mm wide, 12 μm thick, 1000 m long aluminum foil (product name "1N30", manufactured by UACJ Foil Co., Ltd.) Copper foil: 1000 mm wide, 10 μm thick, and 1000 m long copper foil (product name "NC-WS", manufactured by Furukawa Electric Co., Ltd.)

[0104] <Preparing the coating liquid> [Preparation of Water-Based Coating Solution A] The following components were mixed and then diluted with pure water to prepare a water-based coating solution A with a solids concentration of 60% by mass.

[0105] Polyvinyl alcohol: 58 parts (CKS-50: Saponification degree 99 mol%, polymerization degree 300, Nippon Synthetic Chemical Industry Co., Ltd.) Daiichi Kogyo Seiyaku Co., Ltd. Cellogen PR: 24 copies Surfactant (Nihon Emulsion Co., Ltd., Emalex 710): 5 parts 913 parts of Art Pearl J-7P water dispersion prepared using the following method:

[0106] (Art Pearl J-7P water dispersion) Three parts of Emalex 710 (a nonionic surfactant manufactured by Nippon Emulsion Co., Ltd.) and three parts of sodium carboxymethylcellulose (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were dissolved in 74 parts of pure water. 20 parts of Art Pearl (registered trademark) J-7P (silica composite cross-linked acrylic resin microparticles manufactured by Negami Chemical Industrial Co., Ltd.) were added to the resulting aqueous solution, and the mixture was dispersed for 15 minutes at 10,000 revolutions per minute (rpm) using an Ace Homogenizer (manufactured by Nippon Seiki Seisakusho Co., Ltd.) to obtain an aqueous dispersion of Art Pearl J-7P (particle concentration: 20% by mass). The silica composite crosslinked acrylic resin fine particles in the resulting aqueous dispersion had a true specific gravity of 1.20 and an average particle size of 6.5 μm.

[0107] [Preparation of solvent-based coating solution B] LiNi 0.5 Co 0.2 Mn 0.3 95 parts by mass of O2 (NCM523) and 2.5 parts by mass of acetylene black were mixed, and 2.5 parts by mass of polyvinylidene fluoride powder (product name "PVDF5130", manufactured by Solvay) was gradually added, followed by kneading with a planetary mixer. 54 parts by mass of N-methylpyrrolidone was added, yielding a solvent-based coating solution B with a solids concentration of 65% by mass.

[0108] [Preparation of solvent-based coating solution C] Graphite (C) as the negative electrode active material, styrene butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) and polyvinylpyrrolidone (PVP) as viscosity modifiers, and graphite (GF) and carbon black (CB) as conductive additives were blended in a mass ratio of C:SBR:CMC:PVP:GF:CB=94:2:1:1:1:1, and kneaded with ion-exchanged water to obtain a solvent-based coating solution C with a solids concentration of 60 mass%.

[0109] [Example 1] Using an apparatus configured as shown in FIG. 1, the aqueous coating liquid A was applied to one surface of a metal foil (referred to as "front surface" in Table 1) to form a coating liquid film, and the formed coating liquid film was dried to obtain a coating film. Specifically, the water-based coating liquid A was applied onto a metal foil that was being continuously conveyed to form a coating liquid film. Next, while the obtained laminate 12 of the coating liquid film and metal foil was transported through the drying means 30B shown in Figure 3, the coating liquid film was dried using a non-contact curl control means to obtain a coating film. In the pre-curl control region 32 of the drying means 30B shown in Fig. 3, the gas temperature and air velocity were adjusted so that the solid content concentration of the coating liquid film was 65 mass % when curl control started. The solid content concentration of the coating liquid film was measured using an infrared spectroscopic interference film thickness meter SI-T80 manufactured by Keyence Corporation. 3, gas was blown onto both sides of the laminate 12, and the laminate was continuously conveyed while being curved in the thickness direction by the gas pressure. The conditions for curl regulation in the curl regulation area 34 were as follows: Gas type: Air Gas temperature: 120℃ -Air pressure of gas jetted onto the coating liquid film surface: 1.3kPa -Air velocity of the gas jetted onto the coating liquid film surface: 5m 3 / min Deformation of laminate: Distance p in Figure 3: 300 mm, Height difference h in Figure 3: 60 mm, Distance p / Height difference h: 5

[0110] The solid content concentration of the coating liquid film when curl regulation started was 65% by mass, and the solid content concentration of the coating liquid film when curl regulation ended was 99% by mass. During the curl regulation, the temperature of the laminate was 55° C. The temperature of the laminate was measured as the surface temperature of the coating liquid film (film surface temperature). The film surface temperature was measured with an infrared thermometer. The conveying speed of the metal foil was 3.0 m / min.

[0111] Furthermore, the water-based coating liquid A was applied to the other surface of the metal foil (rear surface in Table 1) to form a coating liquid film, and the formed coating liquid film was dried to obtain a coating film. In the same manner as in the method for forming a coating film on one surface of the metal foil ("front surface" in Table 1), the coating liquid film was dried using a non-contact curl control means to obtain a coating film. This resulted in an electrode sheet in which coating films were formed on both sides of the metal foil.

[0112] [Examples 2 to 14] A coating film was formed in the same manner as in Example 1, except that the type of metal foil, the type of coating liquid, and the film surface temperature were appropriately changed as shown in Table 1.

[0113] [Comparative Examples 1 to 4] The type of coating liquid and the film surface temperature were changed as appropriate as shown in Table 1. Furthermore, when drying the coating liquid film, a two-dimensional slit nozzle was used without using a curl control means.

[0114] When a curl control means was used in the step of drying the coating liquid film, a "Y" was entered in the curl control means column in Table 1. When a two-dimensional slit nozzle was used in the step of drying the coating liquid film, an "N" was entered in the curl control means column. When a curl control means is used, the film surface temperature shown in Table 1 means the temperature of the laminate during curl control. When a two-dimensional slit nozzle is used, the film surface temperature listed in Table 1 means the temperature of the laminate during gas ejection. The conditions when using a two-dimensional slit nozzle are as follows: Gas type: Air Gas temperature: 120℃ -Air pressure of gas jetted onto the coating liquid film surface: 1.3kPa Volume of gas sprayed onto the coating liquid film surface: 5m 3 / min

[0115] The radius of curvature of the obtained electrode sheet and the radius of curvature of the curl after removing the coating film formed on one side of the metal foil were measured by the following method. Table 1 shows the surface (ie, front surface or back surface) from which the coating film was removed.

[0116] (curvature radius) The electrode sheet was cut out to obtain a sample having a length of 100 mm and a width of 10 mm. The sample was placed vertically and fixed at the top end in an environment of 25°C and 50% relative humidity. The amount of curl at the bottom end of the sample was measured, and the radius of curvature was calculated from the relationship with the width of the sample.

[0117] (The radius of curvature of the curl after removing the coating film formed on one side of the metal foil) The electrode sheet was left in an environment of 25°C and relative humidity of 50% for 10 minutes or more. The electrode sheet was cut out to obtain a sample. The locations and sizes of the cut-out portions of the electrode sheet are as shown in Table 1. In Table 1, the "center" of the measurement point refers to the center of the cut-out central portion of the electrode sheet. In Table 1, the "edge" of the measurement point refers to the edge of the part of the electrode sheet where no coating film is formed (metal foil part) that is cut out. In Table 1, "100 mm" at the measurement point means that the long side length of the cut-out portion is 100 mm. In Table 1, "30 mm" at the measurement point means that the long side length of the cut-out portion is 30 mm. For the obtained samples, the coating film formed on the removal surface described in Table 1 was moistened with water at 25°C. The amount of water was 20 g per 150 mm x 20 mm coating area. After the water was applied to the coating film, it was left to stand for 10 minutes. The sample was placed on a base, and the moistened coating film was removed from the metal foil by rubbing it with a nonwoven fabric (product name "Bencotto", manufactured by Asahi Kasei Corporation) so as not to scratch the surface of the metal foil. When rubbing the coating film, the sample was not fixed to the base. The operation of wetting the coating film with a solvent and rubbing the wet coating film with a nonwoven fabric was repeated three times until the entire coating film formed on one side of the metal foil was removed. The absence of damage to the metal foil due to rubbing with the nonwoven fabric was judged by visually observing the surface of the metal foil and determining whether the gloss was maintained. The absence of defects in the metal foil due to rubbing with the nonwoven fabric was judged by shining a light on the sample from the unrubbed coating side and determining whether there was no transmitted light. After removing all of the coating film formed on one side of the metal foil, the surface of the metal foil was wiped and then the sample was dried by blowing air at 25°C and a relative humidity of 50%. After drying, the coating film formed on one side of the metal foil was removed, and the radius of curvature of the curl was measured in the same manner as in the above-mentioned method for measuring the radius of curvature.

[0118] Using the obtained electrode sheet, the processability of the battery and the occurrence of folds and wrinkles were evaluated.

[0119] [Battery processability] The electrode sheet was set in a roll state on a continuous cutter (slitter). The electrode sheet was cut into 1-meter lengths, which were the lengths required for incorporation into batteries. The electrode sheet in a roll state was evaluated based on the ease of setting it on the continuous cutter and the deviation of the running position on the continuous cutter. The deviation of the running position was visually confirmed. A: The electrode sheet could be set into the continuous cutting machine without being held down by hand, and no deviation in the running position was observed. B: The electrode sheet could be set into the continuous cutting machine by holding it down with one hand, and no deviation in the running position was observed. C: The electrode sheet can be set into the continuous cutting machine by holding it down with your hand, but the running position was found to be misaligned.

[0120] [Folds and wrinkles] Generally, in a continuous process using the roll-to-roll method, a coating film is not formed at both ends of the electrode sheet in the longitudinal direction. The vicinity of the boundary between the portion of the electrode sheet where no coating film is formed and the portion where a coating film is formed is prone to deformation due to the concentration of internal stress in the coating film. Therefore, folding and wrinkling near this boundary were evaluated. Samples were cut out from the electrode sheet, including a portion at both ends in the longitudinal direction where no coating film was formed (1 m long) and a portion where a coating film was formed (1 m long), and the presence or absence of folds and wrinkles was visually observed. A: No creases or wrinkles. B: There are wrinkles, but they are not completely broken (not plastically deformed). C: Wrinkled and completely broken (plastically deformed).

[0121] The evaluation results are shown in Table 1. In Table 1, radius of curvature A means the radius of curvature of the curl of the electrode sheet, and radius of curvature B means the radius of curvature of the curl after removing the coating film formed on one side of the metal foil.

[0122] [Table 1]

[0123] As shown in Table 1, in Examples 1 to 14, the electrode sheets included a metal foil having a thickness of 15 μm or less and a coating film formed on both sides of the metal foil, and the radius of curvature of the curl of the electrode sheet was 2000 mm or more, and the radius of curvature of the curl after removing the coating film formed on one side of the metal foil from the electrode sheet was 350 mm or more, which demonstrated that the occurrence of folds and wrinkles was suppressed. In other words, it was found that the residual stress in the electrode sheet was low.

[0124] On the other hand, in Comparative Examples 1 to 4, after the coating film formed on one side of the metal foil was removed from the electrode sheet, the radius of curvature of the curl was less than 350 mm, and folds and wrinkles occurred.

[0125] In Example 4, the metal foil is an aluminum foil, which is lighter than that in Example 5. When a secondary battery is made, performance per unit mass is one of the evaluation indexes, so it is desirable to use a lightweight aluminum foil.

[0126] In Example 1, the thickness of the metal foil was 12 μm or less, and it was found that the battery processability was superior compared to Example 4. [Explanation of symbols]

[0127] 10 Metal foil 12 Laminate of coating liquid film and metal foil 20 Application means 30, 30A, 30B drying means 32 Pre-curl area 34 Curl Control Area 36 Transport roll 38, 38a, 38b spout part h Height difference between peaks and valleys (height difference of swell) p Distance between adjacent peaks (spacing of undulations)

Claims

1. A metal foil having a thickness of 15 μm or less; A coating film formed on both sides of the metal foil, The radius of curvature of the curl of the electrode sheet is 2000 mm or more, An electrode sheet, wherein the radius of curvature of curl after removing the coating film formed on one surface of the metal foil from the electrode sheet is 350 mm or more.

2. The electrode sheet according to claim 1, wherein the coating film has a thickness of 30 μm to 200 μm.

3. The electrode sheet according to claim 1 or 2, wherein the metal foil is an aluminum foil.

4. The electrode sheet according to claim 1 or 2, wherein the metal foil has a thickness of 12 μm or less.

5. The electrode sheet according to claim 1 or 2, wherein the coating film contains carbon nanotubes.

6. The electrode sheet according to claim 1 or 2, which is used for a negative electrode of a secondary battery.

7. The method includes a step of forming a coating film on both sides of a metal foil having a thickness of 15 μm or less to produce an electrode sheet, The radius of curvature of the curl of the electrode sheet is 2000 mm or more, the electrode sheet has a curl radius of curvature of 350 mm or more after removing the coating film formed on one surface of the metal foil from the electrode sheet.

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