Electrode, power storage device, and method for manufacturing electrode

Abstract

To reduce the resistance of an electrode in a power storage device.SOLUTION: A power storage device used in a power storage device includes: a current collector; and an electrode composite layer formed on the current collector and including active material, a bonding material, and carbon fibers with a fiber diameter D of greater than or equal to 1 μm and less than or equal to 20 μm and a fiber length L in a range of greater than or equal to 5 μm and less than or equal to 500 μm. The electrode composite layer has a degree of orientation of the carbon fibers oriented in a thickness direction of the electrode determined on the basis of the carbon fibers exposed on the electrode surface of 50% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present specification discloses an electrode, an electricity storage device, and a method for manufacturing the electrode. [Background technology]

[0002] Conventionally, an electrode for an electricity storage device has been proposed that includes, for example, a current collector, an active material layer containing an active material formed on the surface of the current collector, a conductive additive containing carbon nanofibers, and a binder, and that the electrode surface is measured by X-ray diffraction to determine an orientation ratio within a predetermined range, calculated from the sum of the intensities of the diffraction peaks derived from each crystal plane of the carbon nanofiber and the intensity of the diffraction peak derived from the (002) crystal plane of the carbon nanofiber (see, for example, Patent Document 1).It is claimed that this electrode can provide an electrode and electricity storage device with low electrical resistance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-69928 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the electrode of the above-mentioned Patent Document 1 is manufactured using a slurry containing carbon nanofibers, but carbon nanofibers have a high tendency to aggregate, making it difficult to manage the slurry, and it is not easy to uniformly disperse the carbon nanofibers or to maintain that state. As such, the reduction in electrical resistance of electrodes used in electricity storage devices is still insufficient, and there is a need to produce electrodes with low electrical resistance.

[0005] The present disclosure has been made in consideration of such problems, and a main object of the present disclosure is to provide an electrode, an electricity storage device, and a method for manufacturing an electrode that can further reduce the resistance of the electrode. [Means for solving the problem]

[0006] As a result of intensive research to achieve the above-mentioned object, the inventors discovered that the resistance of the electrode can be further reduced by using carbon fibers that are not nanoparticles, such as carbon nanofibers, and orienting these carbon fibers using an electrostatic field, and have completed the invention disclosed in this specification.

[0007] That is, the electrode disclosed in the present specification is An electrode used in an electricity storage device, A current collector; an electrode mixture layer formed on a current collector and including an active material, a binder, and carbon fibers having a fiber diameter D of 1 μm or more and 20 μm or less and a fiber length L of 5 μm or more and 500 μm or less; The electrode mixture layer has a degree of orientation of carbon fibers oriented in the thickness direction of the electrode of 50% or more, determined based on the carbon fibers exposed on the electrode surface.

[0008] The electricity storage device disclosed in this specification includes the above-described electrodes.

[0009] The method for producing an electrode disclosed herein includes the steps of: A method for manufacturing an electrode used in an electricity storage device, comprising: an application step of generating an electrostatic field between the surface of a current collector and the active material, a binder, and carbon fibers having a fiber diameter D of 1 μm or more and 20 μm or less and a fiber length L of 5 μm or more and 500 μm or less, and applying the raw material to the surface of the current collector; a fixation step of heating the current collector on which the raw material has been applied to fix it as an electrode mixture layer; It includes: [Effects of the Invention]

[0010] The present disclosure can further reduce the resistance of the electrodes of an electricity storage device. The reason for this effect is presumed to be as follows. For example, it is presumed that the carbon fibers function as a conductive material, and that by orienting the long axis direction of the carbon fibers in the thickness direction of the electrode, a path for efficient electron conduction can be formed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing an example of the configuration of an electricity storage device 10. FIG. [Figure 2] 3A to 3C are explanatory diagrams showing an example of a method for manufacturing an electrode used in the electricity storage device 10. [Figure 3] Measurement results of fiber diameter D and fiber length L of carbon fiber. [Figure 4] 1 shows optical microscope images of electrode surfaces and cross-sectional schematic diagrams of electrode surfaces in Experimental Examples 1, 3, and 5. [Figure 5] 3D image of carbon fiber obtained by X-ray laminography measurement. [Figure 6] Histogram of the deviation angle θ relative to the film thickness direction in X-ray laminography measurements. [Figure 7] FIG. 1 is a graph showing the relationship between sheet resistance and the electric field during electrostatic printing in Experimental Examples 1 to 5. [Figure 8] FIG. 1 is a graph showing the relationship between the degree of orientation and the sheet resistivity in Experimental Examples 1 to 5. DETAILED DESCRIPTION OF THE INVENTION

[0012] (Electrodes for electricity storage devices) The electrode of this embodiment is used in an electricity storage device and includes a current collector and an electrode mixture layer. The electricity storage device may be any of a lithium secondary battery, a lithium ion secondary battery, a capacitor, a hybrid capacitor, an air battery, etc. The electrode becomes either a positive electrode or a negative electrode based on the potential of the counter electrode relative to the potential of the active material. The active material absorbs and releases carrier ions in the electricity storage device. The carrier ions are not particularly limited as long as they are used in electricity storage devices, and examples thereof include alkali metal ions and Group 2 element ions. Examples of alkali metal ions include lithium ions, sodium ions, and potassium ions. Examples of Group 2 element ions include magnesium ions and calcium ions. Here, a lithium ion secondary battery using lithium ions as a carrier will be mainly described as an example.

[0013] The current collector is a conductive member adjacent to the electrode mixture layer and collects current during charging and discharging. The current collector may be appropriately selected depending on the potential of the active material, and may be made of, for example, aluminum, titanium, stainless steel, nickel, iron, copper, calcined carbon, conductive polymers, conductive glass, or aluminum or copper whose surface has been treated with carbon, nickel, titanium, silver, or the like to improve adhesion, conductivity, and oxidation resistance. These surfaces may also be subjected to oxidation treatment. Examples of the current collector shape include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and fiber-group-formed. The thickness of the current collector is, for example, 1 to 500 μm. The amount of the active material composite formed may be appropriately determined depending on the desired performance required for the electricity storage device.

[0014] The electrode composite layer is formed on a current collector and contains an active material, a binder, and carbon fibers. This electrode composite layer may also contain a conductive material in addition to the carbon fibers. The active material may be a positive electrode active material. As the positive electrode active material, sulfides containing a transition metal element, oxides containing lithium and a transition metal element, etc. can be used. Specifically, transition metal sulfides such as TiS2, TiS3, MoS3, FeS2, and the basic composition formula is Li (1-x) MnO2 (0 < x < 1, etc., the same below), Li (1-x) Mn2O4, etc., lithium manganese composite oxides, the basic composition formula is Li (1-x) CoO2, etc., lithium cobalt composite oxides, the basic composition formula is Li (1-x) NiO2, etc., lithium nickel composite oxides, the basic composition formula is Li (1-x) Ni a Co b Mn c O2 (a + b + c = 1), Li (1-x) Ni a Co b Mn c O4 (a + b + c = 2), etc., lithium nickel cobalt manganese composite oxides, the basic composition formula is LiV2O3, etc., lithium vanadium composite oxides, the basic composition formula is V2O5, etc., transition metal oxides, etc. can be used. Also, the positive electrode active material may be lithium iron phosphate, etc. Among these, lithium transition metal composite oxides, for example, LiCoO2, LiNiO2, LiMnO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, etc. are preferred. Note that the "basic composition formula" means that other elements may be included.

[0015] Also, as the positive electrode active material, for example, it may be used for a capacitor that adsorbs and desorbs carrier ions. This positive electrode active material may, for example, contain a carbon material. The carbon material is not particularly limited, and examples include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, polyacenes, etc. Among these, activated carbons showing a high specific surface area are preferred. The carbon material has a specific surface area of 1000m2 / g or more, and 1500m 2 / g or more is more preferable. 2 / g or more, the discharge capacity can be further increased. The specific surface area of ​​this activated carbon is 3000 m 2 / g or less, and 2 It is considered that the positive electrode stores electricity by adsorbing and desorbing at least one of anions and cations contained in the ionically conductive medium, but it may also store electricity by inserting and desorbing at least one of anions and cations contained in the ionically conductive medium.

[0016] The active material may be a negative electrode active material. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys, and tin compounds; carbon materials capable of absorbing and releasing lithium ions; composite oxides containing multiple elements; and conductive polymers. Examples of carbon materials include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Among these, graphites such as artificial graphite and natural graphite are preferred because they have an operating potential close to that of metallic lithium, allow for charging and discharging at high operating voltages, suppress self-discharge when the carrier ion is lithium ion, and reduce irreversible capacity during charging. Examples of composite oxides include lithium titanium composite oxide and lithium vanadium composite oxide. Of these, carbon materials are preferred as negative electrode active materials from the standpoint of capacity, and composite oxides are preferred from the standpoint of safety.

[0017] The binder is a material that fixes the active material and carbon fibers. The binder may be, for example, a resin that bonds by heating. The binder preferably has ion conductivity in the electrolyte. Examples of such resins include one or more of polyvinylidene fluoride (PVdF), a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVdF-HFP), polymethyl methacrylate (PMMA), and a copolymer of polymethyl methacrylate and an acrylic polymer. Since the resin is used in electrodes, it is more preferable that the resin be a fluorine-containing resin in terms of potential stability. PVdF is preferred as the resin.

[0018] The carbon fibers have a fiber diameter D in the range of 1 μm or more and 20 μm or less. The carbon fibers have a fiber length L in the range of 5 μm or more and 500 μm or less. These carbon fibers do not include submicron-sized carbon fibers such as carbon nanofibers and carbon nanotubes. The electrode mixture layer may contain carbon nanofibers and carbon nanotubes, but these materials do not affect the degree of orientation of the carbon fibers. The fiber length L of the carbon fibers is preferably shorter than the thickness of the electrode mixture layer. The size of the carbon fibers is a value measured using an image particle size distribution analyzer.

[0019] The conductive material added other than carbon fiber may have, for example, a particulate, flake, fibrous, or tubular shape. Examples of the conductive material include one or more of acetylene black, carbon black, ketjen black, flake graphite, and carbon nanotubes. Of these, acetylene black is preferred. When the conductive material is carbon particles, the primary particle diameter d may be in the range of 10 nm to 100 nm. A particle diameter d of 10 nm or more is preferred because it facilitates the formation of conductive paths. The particle diameter d is preferably 20 nm or more and 50 nm or less. Furthermore, when the carbon material is flake carbon, the length a and width b may be the same or different, and the thickness c may be different from the length a and width b. The length a and width b may be in the range of 10 nm to 100 nm. It is preferable that the thickness c is shorter than the length a and width b, and may be in the range of 5 nm to 50 nm. Furthermore, when the conductive material is in the form of fibers or tubes, the size may be 1 / 10 or less of the carbon fiber.

[0020] The electrode mixture layer has a degree of orientation of carbon fibers oriented in the thickness direction of the electrode, determined based on the carbon fibers exposed on the electrode surface, of 50% or more. This degree of orientation is preferably higher, preferably 60% or more, and more preferably 70% or more. This degree of orientation is 100% or less, and may be 90% or less. A higher degree of orientation is preferable because the carbon fibers are more oriented in the thickness direction of the electrode, thereby further reducing resistance. Here, this degree of orientation is calculated by the formula: orientation (%) = n / N × 100, where N is the number of all carbon fibers exposed on the electrode surface, Lm is the median length of the carbon fibers contained in the electrode mixture layer, and n is the number of carbon fibers exposed on the electrode surface whose length is 3 / 7 × Lm or less.

[0021] This electrode has, for example, an electrode composite layer with a sheet resistance of 32 Ωcm 2 The sheet resistance is preferably lower than 20 Ωcm. 2 Less than 10 Ωcm is more preferable. 2Less than 5Ωcm is more preferable. 2 In addition, the electrode may have a volume resistivity of 3.0×10 or less. 3 The volume resistivity is preferably 1.0×10 Ωcm or less. 3 Ωcm or less is more preferable, and 5×10 2 Ωcm or less is more preferable, and 1.0×10 2 The electrode may have a resistance of Ωcm or less. The electrode mixture layer of this electrode may have a thickness in the range of 10 μm to 500 μm. As the thickness of the electrode mixture layer becomes thicker, the influence of the decrease in resistance due to the degree of orientation of the carbon fibers becomes greater. The thickness of this electrode may be 20 μm or more, 50 μm or more, 100 μm or more, 150 μm or more, or 200 μm or more. The electrode mixture layer of this electrode may have an electrode density of 2.0 (g / cm 3 ) or more, and 2.2 (g / cm 3 ) or more, and more preferably 2.4 (g / cm 3 ) or more. In addition, the electrode has an electrode density of the electrode mixture layer of 3.5 (g / cm 3 ) or less, and 3.0 (g / cm 3 ) or less, and more preferably 2.6 (g / cm 3 ) It is more preferable that the range is as follows.

[0022] The electrode mixture layer preferably contains 70% by mass or more and 95% by mass or less of the active material, binder, and carbon fiber as a whole. An active material content of 70% by mass or more can ensure sufficient capacity, while a content of 95% by mass or less is preferable because the relative amounts of binder and carbon fiber can be ensured, thereby ensuring strength and conductivity. The electrode mixture layer preferably contains 0.5% by mass or more and 5% by mass or less of the binder as a whole, relative to the active material, binder, and carbon fiber. A binder content of 0.5% by mass or more can ensure sufficient strength, while a content of 5% by mass or less can ensure the relative amount of active material, thereby ensuring capacity. The electrode mixture layer preferably contains 0.5% by mass or more and 5% by mass or less of the carbon fiber as a whole, relative to the active material, binder, and carbon fiber. A carbon fiber content of 0.5% by mass or more can sufficiently reduce resistance, while a carbon fiber content of 5% by mass or less can ensure the relative amount of active material, thereby ensuring capacity. Furthermore, when the electrode mixture layer contains a conductive material other than carbon fiber, the conductive material is preferably contained in a range of 0.5% by mass to 5% by mass based on the total of the active material, binder, carbon fiber, and conductive material. A conductive material content of 0.5% by mass or more can further improve conductivity, while a conductive material content of 5% by mass or less can further suppress a relative decrease in the amount of active material, thereby further suppressing a decrease in capacity, which is preferable.

[0023] (Electricity storage device) The electricity storage device of the present disclosure includes the above-described electrodes. This electricity storage device may include a positive electrode having a positive electrode active material and a negative electrode having a negative electrode active material, and at least one of the positive electrode and the negative electrode may be the above-described electrode. This electricity storage device may also include a positive electrode, a negative electrode, and an ion-conductive medium interposed between the positive electrode and the negative electrode and conducting carrier ions.

[0024] The ion-conducting medium may be a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, etc. Examples of the solvent for the non-aqueous electrolyte include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which may be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of a cyclic carbonate and a chain carbonate is preferred, as this combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charge and discharge, but also allows for a well-balanced viscosity of the electrolyte, the electric capacity of the resulting battery, and the battery output.

[0025] Examples of supporting salts include LiPF, LiBF, LiAsF, LiCF, SO, LiN(CF, SO), LiC(CF, SO), LiSbF, LiSiF, LiAlF, LiSCN, LiClO, LiCl, LiF, LiBr, LiI, and LiAlCl. From the viewpoint of electrical properties, it is preferable to use one or more salts selected from the group consisting of inorganic salts such as LiPF, LiBF, LiAsF, and LiClO, and organic salts such as LiCF, SO, LiN(CF, SO), and LiC(CF, SO). The concentration of this supporting salt in the nonaqueous electrolyte is preferably 0.1 mol / L to 5 mol / L, and more preferably 0.5 mol / L to 2 mol / L. A sufficient current density can be obtained when the supporting electrolyte is dissolved at a concentration of 0.1 mol / L or higher, and the electrolyte can be made more stable at a concentration of 5 mol / L or lower. In addition, a phosphorus-based, halogen-based, or other flame retardant may be added to this non-aqueous electrolyte.

[0026] Instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. Examples of the ion-conducting polymer include polymer gels composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, or vinylidene fluoride and a supporting salt. Furthermore, a combination of an ion-conducting polymer and a nonaqueous electrolyte can also be used. In addition to ion-conducting polymers, inorganic solid electrolytes, mixtures of organic polymer electrolytes and inorganic solid electrolytes, or inorganic solid powders bound by an organic binder can also be used as the ion-conducting medium.

[0027] The power storage device of the present disclosure may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of a lithium secondary battery, but examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0028] The shape of the electricity storage device is not particularly limited, and examples thereof include coin, button, sheet, laminate, cylindrical, flat, and rectangular shapes. The device may also be applied to large devices used in electric vehicles and the like. FIG. 1 is a schematic diagram showing an example of an electricity storage device 10. The electricity storage device 10 includes a positive electrode 12, a negative electrode 15, and an ion conductive medium 19. The positive electrode 12 includes a positive electrode active material layer 13 and a current collector 14. The negative electrode 15 includes a negative electrode active material layer 16 and a current collector 17. The ion conductive medium 19 conducts carrier ions and may be an electrolyte solution or a solid electrolyte. The electrode described above may be the positive electrode 12 or the negative electrode 15. When the positive electrode 12 is the above-described electrode, the positive electrode mixture layer 13 includes an active material 21, carbon fibers 22, and a binder 23. Carbon fibers 22 have a fiber diameter D in the range of 1 μm to 20 μm, and a fiber length L in the range of 5 μm to 500 μm. In addition, positive electrode mixture layer 13 has a degree of orientation of carbon fibers 22 oriented in the thickness direction of the electrode of 50% or more, determined based on carbon fibers 22 exposed on the electrode surface.

[0029] (Electrode manufacturing method) The method for manufacturing an electrode used in the electricity storage device of the present disclosure may be the method for manufacturing the electrode described above. In this manufacturing method, the contents described for the electrode described above, such as the materials and blending ratio ranges of each component, are applied as appropriate, and detailed description thereof will be omitted. This manufacturing method includes an application step and a fixation step. FIG. 2 is an explanatory diagram showing an example of a method for manufacturing an electrode used in the electricity storage device 10, where FIG. 2A is an explanatory diagram showing an example of a mixing process, FIG. 2B is an explanatory diagram showing an example of a coating process, and FIG. 2C is an explanatory diagram showing an example of a fixation process.

[0030] In the coating process, an electrostatic field is generated between the surface of the current collector and the active material, and a raw material containing a binder and carbon fiber is coated onto the surface of the current collector. The carbon fiber has a fiber diameter D of 1 μm to 20 μm and a fiber length L of 5 μm to 500 μm. A carbon material other than carbon fiber may be added to the raw material as a conductive material. It is preferable not to use a solvent in the coating process. That is, the coating process is preferably performed as a dry process in which no solvent is added to the raw material, and may be an electrostatic screen printing process (Figure 2B). In this coating process, a mixing process in which the raw material powder is dry-mixed before coating the raw material may be performed (Figure 2A). A mixer can be used for the mixing process. In the mixing process, the granular active material, binder, and optionally, granular conductive material may be mixed, and then carbon fiber is added and further mixed. In the coating process, the raw material is preferably coated in an electrostatic field strength of 1 kV / cm to 10 kV / cm. An electric field of 1 kV / cm or more is preferable because it allows the carbon fibers to be more oriented in the thickness direction. Furthermore, an electric field of 10 kV / cm or less provides good processing efficiency. In this coating process, it is more preferable to coat the raw material with an electrostatic field strength in the range of 2 kV / cm to 4 kV / cm. The raw material may be placed on a screen mesh and applied to the current collector through the screen mesh. The mesh size of the screen mesh is appropriately selected depending on the particle size of the raw material, etc. During coating of the raw material, the raw material placed on the screen mesh may be slid with a member such as a squeegee to press the raw material toward the current collector. The coating time may be, for example, such that the coating process is continued until the electrode mixture layer reaches the desired thickness.

[0031] In the fixation step, the current collector coated with the raw material is heated to fix the material into an electrode composite layer. For example, the fixation step may involve hot-pressing the current collector on which the raw material is formed ( FIG. 2C ). The heating temperature may be set appropriately depending on the type of binder, and may be, for example, 100°C or higher, 120°C or higher, or 150°C or higher, or 200°C or lower, 180°C or lower, or 160°C or lower. It is preferable to use a press pressure that is appropriately adjusted so that the orientation of the carbon fibers does not change after the coating process.

[0032] The electrode, electricity storage device, and electrode manufacturing method of the present embodiment described above in detail can further reduce the resistance of the electrode of the electricity storage device. The reason for this effect is presumed to be as follows. For example, this is because the carbon fibers function as a conductive material, and by orienting the long axis direction of the carbon fibers in the thickness direction of the electrode, a path for efficient electron conduction can be formed.

[0033] In general, in the electrodes of energy storage devices such as lithium-ion batteries and nickel-metal hydride batteries, attempts have been made to increase the thickness of the electrode composite layer containing active material, thereby reducing the relative consumption of current collectors and separators and increasing the energy density (the amount of energy per unit volume of the battery). However, increasing the thickness of the electrode increases the distance electrons must travel, thereby increasing internal resistance and significantly reducing battery capacity. In the aforementioned prior art (JP 2013-122883 A), an electrostatic field is applied in the direction of the electrode film thickness when applying an electrode slurry containing carbon nanofibers to the current collector, or before drying the solvent after application, thereby orienting the carbon nanofibers in the direction of the film thickness. However, carbon nanofibers generally have a high tendency to aggregate, making it difficult to uniformly disperse and maintain that state, making slurry management difficult. Another issue is the high energy consumption required for the slurry film formation process in which electrode slurry is applied to obtain an electrode film, due to the need to volatilize the solvent at high temperatures using a drying oven. In particular, a system for recovering and disposing of N-methyl-2-pyrrolidone (NMP), which is used as a solvent for the positive electrode slurry, is required. Furthermore, there is also the issue that, during the solvent drying process, carbon nanofibers that have been oriented in the film thickness direction may become oriented in-plane again. The electrode and its manufacturing method disclosed herein use carbon fibers with low cohesion, and the electrode mixture layer is applied dry without using a solvent. This allows for the carbon fibers to be easily oriented in the electrode thickness direction while further reducing energy consumption.

[0034] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure.

[0035] For example, in the above-described embodiment, the raw material powder is applied to the current collector without a solvent in the electrode manufacturing method, but this is not particularly limited, and the raw material powder may be applied to the current collector after adding a solvent. As long as the carbon fibers can be oriented in the thickness direction of the electrode, the electrode can be manufactured regardless of whether it is a dry method or a wet method.

[0036] The present disclosure may be any of the following [1] to

[10] . [1] An electrode used in an electricity storage device, A current collector; an electrode mixture layer formed on a current collector and including an active material, a binder, and carbon fibers having a fiber diameter D of 1 μm or more and 20 μm or less and a fiber length L of 5 μm or more and 500 μm or less; An electrode in which the degree of orientation of the carbon fibers in the electrode mixture layer, which is determined based on the carbon fibers exposed on the electrode surface, is 50% or more. [2] The electrode according to [1], wherein the degree of orientation is 60% or more. [3] The electrode according to [1] or [2], wherein the degree of orientation is calculated by the formula: orientation (%) = n / N × 100, where N is the number of all the carbon fibers exposed on the electrode surface, Lm is the median length of the carbon fibers contained in the electrode mixture layer, and n is the number of carbon fibers exposed on the electrode surface whose length is 3 / 7 × Lm or less. [4] The electrode according to any one of [1] to [3], wherein the electrode satisfies at least one of (1) to (4). (1) Sheet resistance is 32 Ωcm 2 The following is the result. (2) Volume resistivity is 3.0 × 10 3 It is less than Ωcm. (3) The thickness is in the range of 10 μm to 500 μm. (4) Electrode density is 2.0 (g / cm 3 ) or more 3.5 (g / cm 3 ) is in the following range. [5] The electrode according to any one of [1] to [4], wherein the active material is a composite oxide containing lithium and a transition metal. [6] An electricity storage device comprising the electrode according to any one of [1] to [5]. [7] A method for manufacturing an electrode used in an electricity storage device, comprising: an application step of generating an electrostatic field between the surface of a current collector and the active material, a binder, and carbon fibers having a fiber diameter D of 1 μm or more and 20 μm or less and a fiber length L of 5 μm or more and 500 μm or less, and applying the raw material to the surface of the current collector; a fixation step of heating the current collector on which the raw material has been applied to fix it as an electrode mixture layer; A method for manufacturing an electrode comprising the steps of: [8] The method for producing an electrode according to [7], wherein the coating step does not use a solvent. [9] The method for manufacturing an electrode according to [7] or [8], wherein in the coating step, the raw material is coated at an electrostatic field strength in the range of 1 kV / cm to 10 kV / cm.

[10] The method for manufacturing an electrode according to [7] or [8], wherein in the coating step, the raw material is coated at an electrostatic field strength in the range of 2 kV / cm to 4 kV / cm. [Example]

[0037] Specific examples of fabricating electrodes and electricity storage devices according to the present disclosure will be described below as experimental examples, with Experimental Example 1 corresponding to a comparative example and Experimental Examples 2 to 5 corresponding to working examples.

[0038] (Electrode fabrication) (1) Electrode powder mixing process 91 g of lithium nickel-cobalt manganese oxide was used as the active material, 3 g of acetylene black (HS100 manufactured by Denka Kogyo) as the conductive material, 3 g of carbon fiber (K223HM (50 μ) manufactured by Mitsubishi Chemical), and 3 g of polyvinylidene fluoride (HSV900 manufactured by Arkema) as the binder. A lab mill (OML-1 manufactured by Osaka Chemical) was used to mix the electrode powder. After mixing the active material, conductive material, and binder for 5 minutes, the carbon fiber was added and mixed for another 1 minute.

[0039] (Physical properties of carbon fiber) The fiber diameter and fiber length distributions of the carbon fibers used were measured. The length distribution of the carbon fibers was measured using an image particle size distribution analyzer (Microtrac-Bell, CamseizerX2) to measure the fiber diameter (D) and fiber length (L) distributions of the carbon fibers. The fiber diameter (D) distribution was calculated by taking the minimum diameter (Xmin) of the measured particles as the fiber diameter (D). The fiber length (Xlength) distribution was calculated using the minimum diameter (Xmin) of the measured particles and the maximum Feret diameter (XFemax) according to the following equation (1). Figure 3 shows the measurement results of the fiber diameter (D) and fiber length (L) of the carbon fibers. Figure 3A is a distribution map of the fiber diameter (D), Figure 3B is a distribution map of the fiber length (L), and Figure 3C is a photograph of the carbon fiber. As shown in Figure 3, the carbon fibers used had a median diameter (D50) of 12 μm and a median length (Lm) of 70 μm.

[0040]

number

[0041] (2) Electrode Film Formation (Experimental Examples 1 to 5) Using an electrostatic screen printing device (TS-1 manufactured by Berg Industries), the electrode powder was applied onto an aluminum foil (thickness 15 μm) current collector. A 150-wire, 60 μm (109 μm mesh) screen mesh was used. An electric field of 0, 0.6, 1.3, 2.5, or 3.8 kV / cm was applied between the screen plate and the coating surface. The resulting electrodes were designated Experimental Examples 1 to 5. The electrode powder was then fixed onto the current collector using a heated roll press (TAKUMI Giken SA6202) to a coating weight of 30 mg / cm. 2 The pressing temperature was 200°C, the roll speed was 0.2 m / min, and the linear pressure was 286 kg / cm.

[0042] (electronic resistance measurement) The electronic resistance of the electrode was measured using the four-terminal method. Two electrodes were sandwiched between an indenter and a load cell, with copper foil between them. A positive voltage and current wire was connected to one electrode, and a negative voltage and current wire was connected to the other electrode. The copper foil was placed to reduce the effect of contact resistance between the electrodes and measure the electronic resistance of the electrode. Area: 2 cm2 50 kg (25 kg / cm) 2 The value of the DC resistance when a load of 1000 kJ / cm was applied was taken as the electronic resistance. The volume resistivity Rv [Ωcm], the electronic resistance of the electrode Re [Ω], and the measurement area S [cm 2 ] and the thickness of the electrode t [cm], the volume resistivity of the electrode was calculated using equation (2). Here, the measurement area is the area of ​​the indenter 2 cm 2 It was decided. Rv [Ωcm] = (Re [Ω] × S [cm 2 ]) / t[cm] …Formula (2)

[0043] (Observation and analysis of electrode surfaces using an optical microscope) Optical microscope images of the electrode surface were taken using a digital microscope (Keyence VHX-7000). The pixel size was 1 μm / pixel. Using the image analysis software Image J, the carbon fibers exposed on the electrode surface were extracted by binarizing the brightness values ​​of the obtained observation images. Furthermore, a histogram of the longitudinal lengths (major diameters) of the extracted carbon fibers was obtained using Image J's particle analysis function. Of the carbon fibers exposed on the surface, those with a major diameter of 30 μm or less were considered to be oriented in the film thickness direction, and the ratio of the number of carbon fibers with a major diameter of 30 μm or less to the total number of carbon fibers exposed on the surface was calculated as the proportion of oriented carbon fibers. That is, when the number of all carbon fibers exposed on the electrode surface is N [pieces], the median length of the carbon fibers contained in the electrode composite layer is Lm [μm], and the number of carbon fibers exposed on the electrode surface with a length of 3 / 7 × Lm [μm] or less is n [pieces], the orientation degree (%) was calculated using the following formula (3). Orientation degree [%]=n[pieces] / N[pieces]×100...Formula (3)

[0044] (X-ray laminography measurement and orientation analysis) The validity of the orientation was examined from 3D transmission images of the electrode. 3D images of the electrode were obtained by X-ray laminography. X-ray laminography measurements were performed at the SPring-8 BL33XU (Toyota Beamline) large-scale synchrotron radiation facility. The X-ray energy was 16 keV, and the tilt angle of the rotation axis was 30°. An X-ray CMOS camera (Hamamatsu Photonics ORCA-flush4.0) was used as the detector, and images were taken at a pixel size of 0.325 μm / pixel. Images were taken every 0.1° of rotation with an exposure time of 100 msec, obtaining 360° (3601 images). The transmission images were reconstructed into 3D images using reconstruction software provided by JASRI. The carbon fiber region was extracted from the 3D image of the electrode obtained using X-ray tomography analysis software (Volume Graphics VGSTUDIO MAX), and orientation analysis was performed. A histogram of the deviation angle θ relative to the electrode thickness direction was obtained. Experimental Examples 1 and 5 were measured.

[0045] (Carbon fiber orientation evaluation) FIG. 4 shows optical microscope images and cross-sectional schematic diagrams of the electrode surfaces of Experimental Examples 1, 3, and 5, with FIG. 4A for Experimental Example 1, FIG. 4B for Experimental Example 3, and FIG. 4C for Experimental Example 5. FIG. 5 shows a three-dimensional image of carbon fibers obtained by X-ray laminography. FIG. 6 shows histograms of the deviation angle θ relative to the film thickness direction of Experimental Examples 1 and 5 obtained by X-ray laminography. As shown in FIG. 4, in Experimental Example 1, where no electric field was applied during electrostatic screen printing, many carbon fibers with their longitudinal direction exposed were observed. However, as the applied electric field increased, as in Experimental Example 3, where the applied electric field was 1.3 kV / cm, and Experimental Example 5, where the applied electric field was 3.8 kV / cm, the number of carbon fibers with their longitudinal direction exposed decreased, and many dot-like carbon fibers were observed. Furthermore, as shown in Figure 6, compared to Experimental Example 1, in which no electric field was applied, the electrode of Experimental Example 5, in which a film was formed by applying an electric field of 3.8 kV / cm, showed fewer carbon fibers oriented in the plane direction when the deviation angle θ was 80° to 90°, and more carbon fibers oriented in the thickness direction when the deviation angle θ was 80° or less. This indicates that, in the optical microscope images, the number of carbon fibers exposed in the longitudinal direction decreased and the number of carbon fibers appearing as dots increased as the applied electric field increased. This is because the electric field used when applying the raw material powder to the current collector caused the carbon fibers to be oriented in the thickness direction, reducing the area exposed on the electrode surface. Carbon fibers with a major axis of 3 / 7Lm, i.e., 30 μm or less, were defined as being oriented in the film thickness direction, and the ratio of the number (n) of carbon fibers with a length (L) of 30 μm or less to the total number (N) of carbon fibers exposed on the surface was calculated as the percentage of oriented carbon fibers.

[0046] (Results and Discussion) Fig. 7 is a graph showing the relationship between the sheet resistance and the electric field during electrostatic printing in Experimental Examples 1 to 5. Fig. 8 is a graph showing the relationship between the sheet resistance and the degree of orientation in Experimental Examples 1 to 5. The graph also shows the relationship between the electric field [kV / cm] during electrostatic printing, the electrode thickness [μm], and the basis weight of the electrode composite [mg / cm 2 ], electrode density [g / cm 3 ], percentage of oriented fibers [%], area resistivity [Ωcm 2The surface roughness [kJ / cm] and volume resistivity [Ωcm] are shown in Table 1. As shown in Figures 7 and 8 and Table 1, in Experimental Examples 1 to 5, it was found that as the electric field during electrostatic printing increased, the degree of orientation improved, i.e., the carbon fibers aligned in the thickness direction of the electrode. It was also found that a higher degree of orientation resulted in a lower sheet resistance and volume resistivity. The reason for this is presumed to be that, for example, orienting the carbon fibers as a conductive material in the thickness direction of the electrode forms a path for efficient electron conduction in the thickness direction of the electrode, thereby further reducing the electronic resistance of the electrode for an electricity storage device. It was found that the electric field during electrostatic printing is preferably 1.0 kV / cm or more, more preferably 2.0 kV / cm or more. It was also presumed that the electric field is preferably in the range of 10 kV / cm or less. It was also found that the degree of orientation of the carbon fibers oriented in the thickness direction of the electrode, determined based on the carbon fibers exposed on the electrode surface, is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more. It was also speculated that the degree of orientation may be 100% or less, or even 90% or less.

[0047] [Table 1]

[0048] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure. [Industrial Applicability]

[0049] The electrode, electricity storage device, and electrode manufacturing method disclosed in this specification can be used in the technical field of electricity storage devices such as secondary batteries. [Explanation of symbols]

[0050] 10 Electric storage device, 12 Positive electrode, 13 Positive electrode active material layer, 14 Current collector, 15 Negative electrode, 16 Negative electrode active material layer, 17 Current collector, 18 Separator, 19 Ion conductive medium, 21 Active material, 22 Carbon fiber, 23 Binder.

Claims

1. An electrode used in an electricity storage device, A current collector; an electrode mixture layer formed on a current collector and including an active material, a binder, and carbon fibers having a fiber diameter D of 1 μm or more and 20 μm or less and a fiber length L of 5 μm or more and 500 μm or less; the electrode mixture layer has a degree of orientation of carbon fibers oriented in the thickness direction of the electrode, determined based on the carbon fibers exposed on the electrode surface, of 50% or more; The degree of orientation can be calculated by the formula: orientation degree (%) = n / N × 100, where N is the number of all the carbon fibers exposed on the electrode surface, Lm is the median length of the carbon fibers contained in the electrode mixture layer, and n is the number of carbon fibers exposed on the electrode surface and having a length of 3 / 7 × Lm or less.

2. The electrode according to claim 1 , wherein the degree of orientation is 60% or more.

3. The electrode according to claim 1 or 2, wherein the electrode satisfies any one or more of (1) to (4). (1) Sheet resistance is 32 Ω cm 2 The following is the result. (2) Volume resistivity is 3.0 × 10 3 It is Ωcm or less. (3) The thickness is in the range of 10 μm or more and 500 μm or less. (4) Electrode density is 2.0 (g / cm 3 ) or more 3.5 (g / cm 3 ) is in the range below.

4. 3. The electrode according to claim 1, wherein the active material is a composite oxide containing lithium and a transition metal.

5. An electricity storage device comprising the electrode according to claim 1 or 2.

6. A method for manufacturing an electrode used in the electricity storage device according to claim 1 or 2, comprising: An electrostatic field is generated between the surface of the current collector, and the active material, the binder, and the fiber diameter D are 1 μm or more and 20 a coating step of coating a surface of the current collector with a raw material including carbon fibers having a fiber length L in the range of 5 μm to 500 μm; a fixation step of heating the current collector on which the raw material has been applied to fix it as an electrode mixture layer; A method for manufacturing an electrode comprising the steps of:

7. The method for manufacturing an electrode according to claim 6 , wherein in the coating step, no solvent is added to the raw material.

8. The method for manufacturing an electrode according to claim 6 , wherein in the coating step, the raw material is coated at an electrostatic field strength in the range of 1 kV / cm to 10 kV / cm.

9. The method for manufacturing an electrode according to claim 6 , wherein in the coating step, the raw material is coated at an electrostatic field strength in the range of 2 kV / cm to 4 kV / cm.

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