Method for manufacturing electrode, electrode, and power storage device

Abstract

To reduce conductive materials in an electrode of a power storage device and also suppress increase of the resistance of the electrode.SOLUTION: The method for manufacturing an electrode according to the present disclosure is a method for manufacturing an electrode used for a power storage device, and the method includes an application step of forming a fiber-containing layer so that a raw material including an active material, a binding material, and carbon fibers is aligned in the thickness direction of a mixture layer by generating an electrostatic field in the surface of a collector, and forming a protective layer adjacent to at least one surface of the fiber-containing layer by using a material which contains less carbon fibers than the fiber-containing layer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This specification discloses a method for manufacturing an electrode, an electrode, and an electrical storage device.

Background Art

[0002] Conventionally, as an electrode of an electrical storage device, for example, an electrode includes a current collector, an active material formed on the surface of the current collector, a conductive assistant including carbon nanofibers, and a binder, and an active material layer. The surface of the electrode is measured by X-ray diffraction method, and the orientation ratio obtained from the sum of the intensities of diffraction peaks derived from each crystal plane of the carbon nanofibers and the intensity of the diffraction peak derived from the (002) crystal plane of the carbon nanofibers is within a predetermined range (see, for example, Patent Document 1). This electrode is said to be able to provide an electrode or an electrical storage device with low electrical resistance.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the electrode of Patent Document 1 described above, although it is manufactured using a slurry containing carbon nanofibers, carbon nanofibers have high cohesiveness, making it difficult to manage the slurry and difficult to uniformly disperse and maintain that state. In addition, it has been desired to further reduce the usage amount of conductive materials such as carbon nanofibers and further suppress the increase in electrical resistance at that time.

[0005] The present disclosure has been made in view of such problems, and the main object is to provide a method for manufacturing an electrode, an electrode, and an electrical storage device that can further suppress an increase in the resistance of the electrode while reducing the conductive material.

Means for Solving the Problems

[0006] As a result of intensive research to achieve the above object, the present inventors have found that by using carbon fiber as a conductive material, providing a protective layer on the current collector side and / or the electrode surface side, and orienting this carbon fiber by an electrostatic field, it is possible to further suppress an increase in the resistance of the electrode while reducing the content of the conductive material, and have completed the invention disclosed in this specification.

[0007] That is, the method for manufacturing an electrode disclosed in this specification is A method for manufacturing an electrode used in a power storage device, An electrostatic field is generated between the surface of the current collector, and a fiber-containing layer is formed so that the carbon fibers are aligned in the thickness direction of the composite layer using a raw material containing an active material, a binder, and carbon fibers. A coating step of producing an electrode composite layer by forming a protective layer adjacent to at least one surface of the fiber-containing layer using a raw material having a lower carbon fiber content than the fiber-containing layer, which includes.

[0008] The electrode disclosed in this specification is An electrode used in a power storage device, A current collector, An electrode composite layer formed on the current collector and containing an active material, a binder, and carbon fibers, and includes The electrode composite layer has a protective layer including at least one of a current collector side protective layer on the current collector side and a surface side protective layer on the surface side of the electrode, and a fiber-containing layer adjacent to the protective layer and containing carbon fibers. The protective layer has an average orientation angle of the carbon fibers obtained by passing through the inside of the electrode composite layer when the carbon fibers along the plane direction are 90° of 75° or less.

[0009] The power storage device disclosed in this specification includes the above-described electrode.

Advantages of the Invention

[0010] In the present disclosure, it is possible to further suppress an increase in the resistance of an electrode while reducing the amount of a conductive material. The reason for obtaining such an effect is presumed as follows. For example, carbon fibers function as a conductive material, and it is presumed that by orienting the long axis direction of the carbon fibers in the thickness direction of the electrode, a path for efficiently conducting electrons can be formed. Further, in such an electrode, on the current collector side and the surface of the electrode, for example, carbon fibers may be oriented along the plane direction of the current collector and may not contribute to the formation of an effective electron conduction path in the film thickness direction. The reason for this is, for example, on the current collector side, in the initial stage of film formation, when the oriented carbon fibers collide with the current collector, there is no support, so they may fall in the in-plane direction. Also, on the electrode surface side, when pressed during immobilization, the carbon fibers near the surface may fall in the in-plane direction. Here, by providing a protective layer that does not contain carbon fibers or has a lower carbon fiber content on the current collector side and / or the surface side of the electrode, it is possible to further suppress the in-plane orientation in which carbon fibers are oriented along the plane direction. Therefore, in the present disclosure, due to the protective layer, carbon fibers are oriented along the film thickness direction, and the ratio of carbon fibers contributing to electron conduction can be increased, so that the electron resistance can be reduced with the minimum necessary amount of carbon fiber.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0012] (Method for manufacturing an electrode) The method for manufacturing an electrode used in the power storage device of the present disclosure is a method for manufacturing an electrode used in a power storage device. The power storage device may be any one of a lithium secondary battery, a lithium ion secondary battery, a capacitor, a hybrid capacitor, an air battery, and the like. The electrode becomes either a positive electrode or a negative electrode based on the potential of the counter electrode with respect to the potential of the active material. The active material stores and releases carrier ions of the power storage device. The carrier ions are not particularly limited as long as they are used in the power storage device, and examples thereof include alkali metal ions and group 2 element ions. Examples of the alkali metal ions include lithium ions, sodium ions, potassium ions, and the like. Examples of the group 2 element ions include magnesium ions, calcium ions, and the like. Here, a lithium ion secondary battery using lithium ions as carriers will be mainly described as an example below. This manufacturing method may include a coating step and further include an immobilization step. FIG. 1 is an explanatory diagram showing an example of a method for manufacturing an electrode used in the power storage device 10, and FIG. 1A shows an example of a mixing process, FIG. 1B shows an example of a coating process, and FIG. 1C shows an example of an immobilization process.

[0013] In the coating process, an electrostatic field is generated between the surface of the current collector and the discharge part that discharges the composite material, and a raw material containing an active material, a binder, and carbon fibers is applied to the surface of the current collector to form an electrode composite layer (FIG. 1B). In the coating process, it is preferable not to use a solvent. That is, the coating process is preferably performed as a dry process without adding a solvent to the raw material, and may also be an electrostatic screen printing process (FIG. 1B). In this coating process, a mixing process of dry-mixing the raw material powder before the coating process of the raw material may be performed (FIG. 1A). A mixer can be used for the mixing process. In the mixing process, after mixing the granular active material, the binder, and, if necessary, the granular conductive material, carbon fibers may be added and further mixed.

[0014] In the coating process, a fiber-containing layer including a central region in the thickness direction of the electrode composite layer and a protective layer adjacent to at least one surface of the fiber-containing layer using a raw material with a lower carbon fiber content than the fiber-containing layer are formed. The fiber-containing layer is formed using a raw material containing an active material, a binder, and carbon fibers, and is formed such that the carbon fibers are aligned in the thickness direction of the composite layer by an electrostatic field. The protective layer may include any one or more of a collector-side protective layer formed adjacent to the current collector side and a surface-side protective layer formed on the electrode surface side. The protective layer has a low carbon fiber content and is a layer that protects the carbon fibers in the adjacent fiber-containing layer, which are inclined in the film thickness direction, from becoming aligned in the plane direction. In this coating process, after forming the collector-side protective layer as the protective layer on the current collector, the fiber-containing layer may be formed, or after forming the fiber-containing layer, the surface-side protective layer as the protective layer may be formed on this fiber-containing layer, or after forming the surface-side protective layer as the protective layer on the substrate on the electrode surface side, the fiber-containing layer may be formed, or any one or more of these may be performed. Note that the substrate may be removed later. From the viewpoint of further suppressing the change in the orientation direction of the carbon fibers, it is preferable to perform the electrostatic screen printing process for both the formation of this protective layer and the formation of the fiber-containing layer.

[0015] FIG. 2 is an explanatory diagram regarding the orientation of carbon fibers in the electrode. FIG. 2A shows a diagram of applying carbon fibers onto the current collector by an electrostatic field, FIG. 2B shows a diagram of pressing and fixing the applied electrode composite layer, FIG. 2C shows a diagram of forming a surface-side protective layer, and FIG. 2D shows a diagram of forming a current-collecting-side protective layer and a surface-side protective layer. As shown in FIG. 2A, in electrostatic screen printing, at the initial stage of film formation on the current collector side, when the oriented carbon fibers collide with the current collector, there is no support, so they may fall along the plane direction. Also, as shown in FIG. 2B, on the electrode surface side, after applying the electrode composite layer, when pressing during immobilization, the carbon fibers near the surface may fall along the plane direction. In this manufacturing method, as shown in FIGS. 2C and 2D, by providing a surface-side protective layer 27 with few carbon fibers and a current-collecting-side protective layer 26, the orientation (in-plane orientation) of the carbon fibers along the plane direction is further suppressed.

[0016] In the coating process, it is preferable to apply the raw material in the range where the intensity of the electrostatic field is 1 kV / cm or more and 10 kV / cm or less. When the electric field is 1 kV / cm or more, the carbon fibers can be oriented more along the thickness direction, which is preferable. Also, when the electric field is 10 kV / cm or less, the processing efficiency is good. In this coating step, it is more preferable to apply the raw material in the range where the intensity of the electrostatic field is 2 kV / cm or more and 4 kV / cm or less. The raw material may be accommodated on a screen mesh and applied from this screen mesh to the current collector. The mesh size of the screen mesh is appropriately selected according to the particle size of the raw material and the like. When applying the raw material, the raw material accommodated on the screen mesh may be pressed against the current collector side by sliding a member such as a squeegee. The coating time may be, for example, to perform this coating process until the thickness of the electrode composite layer reaches the desired thickness.

[0017] In the coating process, it is preferable to form the protective layer in a range where the ratio B / L of the thickness B of the protective layer to the average fiber length L of the carbon fiber is 0.05 or more, more preferably 0.10 or more, and still more preferably 0.15 or more. When B / L is 0.05 or more, the electrode resistance can be further reduced, and thus the amount of carbon fiber used can be further reduced. In particular, when B / L is 0.05 or more, the surface-side protective layer is effective when it is within this range. Also, when B / L is 0.10 or more, the current-collecting-side protective layer is effective when it is within this range. Also, B / L preferably ranges from 0.5 or less, more preferably 0.3 or less, and still more preferably 0.25 or less. When B / L is 0.5 or less, an increase in electrode resistance can be further reduced when reducing the amount of carbon fiber used.

[0018] In the coating process, it is preferable to form the protective layer in a range where the ratio t / T of the thickness t of the protective layer to the thickness T of the electrode composite layer is from 0.1 to 0.5. From the viewpoint of the orientation of the carbon fibers in the fiber-containing layer, it is desirable that the protective layer be thicker, and from the viewpoint of resistance reduction, it is desirable that the protective layer be thinner because the carbon fiber content is small. This ratio t / T is more preferably 0.15 or more, and still more preferably 0.2 or more. Also, this ratio t / T is more preferably 0.4 or less, and still more preferably 0.3 or less.

[0019] The electrode composite layer formed in the coating process 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 carbon fibers. The blending amount of each material may be appropriately set according to the required performance of the electrode. For example, the electrode composite layer preferably contains more active material. As the blending amount of the active material, for example, 60% by mass or more is preferable, 70% by mass or more is more preferable, 80% by mass or more is further preferable, and 90% by mass or more is most preferable. Considering the blending of the binder and carbon fibers, the blending amount of the active material is preferably 98% by mass or less. As the blending amount of the binder, for example, 1% by mass or more is preferable, 1.5% by mass or more is more preferable, 2% by mass or more is further preferable, and 3% by mass or more may also be used. Also, the blending amount of the binder is preferably 5% by mass or less, preferably 3.5% by mass or less, and may also be 3% by mass or less. From the viewpoint of conductivity, a larger blending amount of carbon fibers is preferable, but from the viewpoint of discharge capacity, a smaller blending amount is preferable. As the blending amount of carbon fibers, for example, as a raw material of the fiber-containing layer, 5% by mass or less is preferable, 3.5% by mass or less is preferable, and 3% by mass or less may also be used. Also, this blending amount of carbon fibers is preferably 1% by mass or more, more preferably 1.5% by mass or more, further preferably 2% by mass or more, and may also be 3% by mass or more. As a raw material of the protective layer, the blending amount of carbon fibers is preferably less, preferably 1% by mass or less, preferably 0.5% by mass or less, and may also be 0% by mass. Also, as the blending amount of the conductive material other than carbon fibers, 1% by mass or more is preferable, 1.5% by mass or more is more preferable, 2% by mass or more is further preferable, and 3% by mass or more may also be used. Also, the blending amount of the conductive material is preferably 5% by mass or less, preferably 3.5% by mass or less, and may also be 3% by mass or less.

[0020] The active material may be a positive electrode active material. As the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, etc. can be used. Specifically, TiS 2 , TiS 3 , MoS 3 , FeS 2 and other transition metal sulfides, with a basic composition formula of Li (1-x) MnO 2 (0 < x < 1, etc., the same below) or Li(1-x) Mn 2 O 4 such as lithium manganese composite oxides with a basic composition formula of Li (1-x) CoO 2 such as lithium cobalt composite oxides with a basic composition formula of Li (1-x) NiO 2 such as lithium nickel composite oxides with a basic composition formula of Li (1-x) Ni a Co b Mn c O 2 (a + b + c = 1) or Li (1-x) Ni a Co b Mn c O 4 (a + b + c = 2), etc., such as lithium nickel cobalt manganese composite oxides with a basic composition formula of LiV 2 O 3 such as lithium vanadium composite oxides with a basic composition formula of V 2 O 5 Transition metal oxides such as these can be used. Also, the positive electrode active material may be, for example, lithium iron phosphate. Among these, lithium transition metal composite oxides, for example, LiCoO 2 , LiNiO 2 , LiMnO 2 , LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 etc. are preferable. Note that the "basic composition formula" means that other elements may be included.

[0021] 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 preferable. The carbon material preferably has a specific surface area of 1000 m 2 / g or more, and 1500 m 2It is more preferably 1000 m 2 / g or more. When the specific surface area is 1000 m 2 / g or more, the discharge capacity can be further increased. The specific surface area of this activated carbon is preferably 3000 m 2 / g or less, and may be 2000 m

[0022] The active material may be used as a negative electrode active material. Examples of the negative electrode active material include inorganic compounds such as lithium, lithium alloys, and tin compounds, carbon materials capable of occluding and releasing lithium ions, composite oxides containing a plurality of elements, and conductive polymers. Examples of the 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 have an operating potential close to that of metallic lithium, can be charged and discharged at a high operating voltage, suppress self-discharge when the carrier ion is lithium ion, and can reduce the irreversible capacity during charging, so they are preferable. Examples of the composite oxides include lithium titanium composite oxides and lithium vanadium composite oxides. Among these, the carbon material is preferable from the viewpoint of capacity, and the composite oxide is preferable from the viewpoint of safety as the negative electrode active material.

[0023] The binder is a material for fixing the active material and carbon fiber. As this binder, for example, a resin that is fixed by heating may be used. The binder preferably has ionic conductivity in an electrolytic solution, for example. Examples of such resins include polyvinylidene fluoride (PVdF), a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVdF-HFP), polymethyl methacrylate (PMMA), and one or more of a copolymer of polymethyl methacrylate and an acrylic polymer. In particular, since it is used for an electrode, this resin is more preferably a fluorine-containing resin from the viewpoint of potential stability. PVdF is preferable as this resin.

[0024] The carbon fiber may have a fiber diameter D in the range of 1 μm or more and 20 μm or less. Also, the carbon fiber may have a fiber length L in the range of 5 μm or more and 500 μm or less. This carbon fiber does not include submicron-sized ones such as carbon nanofibers and carbon nanotubes. The electrode mixture layer may contain carbon nanofibers and carbon nanotubes, but these materials are not involved in the degree of orientation of the carbon fiber. The fiber length L of the carbon fiber is preferably shorter than the thickness of the electrode mixture layer. The size of the carbon fiber is the value measured using an image type particle size distribution measuring device.

[0025] The conductive material added in addition to carbon fiber may have, for example, a particulate, flaky, fibrous or tubular shape. Examples of this conductive material include one or more of acetylene black, carbon black, ketjen black, flaky graphite and carbon nanotubes. Among these, acetylene black is preferred. When this conductive material is carbon particles, the particle size d of the primary particles may be in the range of 10 nm or more and 100 nm or less. When the particle size d is 10 nm or more, it is easy to form a conductive path, which is preferable. This particle size d is preferably 20 nm or more and preferably 50 nm or less. Further, when the carbon material is flaky carbon, the length a and the width b may be the same, may be different, or the thickness c may be different from the length a and the width b. The length a and the width b may be in the range of 10 nm or more and 100 nm or less. The thickness c is preferably shorter than the length a and the width b and may be in the range of 5 nm or more and 50 nm or less. Further, when the conductive material is fibrous or tubular, it may have a size of 1 / 10 or less than that of carbon fiber.

[0026] The current collector is a conductive member that is adjacent to the electrode composite layer and conducts current during charge and discharge. The current collector may be appropriately selected according to the potential of the active material, etc. For example, in addition to aluminum, titanium, stainless steel, nickel, iron, copper, fired carbon, conductive polymer, conductive glass, etc., for the purpose of improving adhesion, conductivity and oxidation resistance, those obtained by treating the surface of aluminum or copper with carbon, nickel, titanium, silver, etc. can be used. For these, it is also possible to perform surface oxidation treatment. Examples of the shape of the current collector include a foil shape, a film shape, a sheet shape, a net shape, a punched or expanded shape, a lath body, a porous body, a foam body, a formed body of a fiber group, etc. The thickness of the current collector is, for example, 1 to 500 μm. The formation amount of the active material composite may be appropriately set according to the desired performance required for the power storage device.

[0027] In the immobilization process, the current collector coated with the raw material is heated and subjected to a process of immobilization as the electrode mixture layer. In the immobilization process, for example, the current collector formed with the raw material may be heated and pressed for immobilization (FIG. 1C). The heating temperature may be appropriately set according to the type of the binder. For example, it may be 100 °C or higher, 120 °C or higher, 150 °C or higher, or may be 200 °C or lower, 180 °C or lower, 160 °C or lower, etc. Further, it is preferable to use a value obtained by appropriately adjusting the pressing pressure so that the orientation direction of the carbon fiber does not change after the coating process. In this way, the electrode can be manufactured.

[0028] (Electrode for energy storage device) The electrode of this embodiment is used in an energy storage device and includes a current collector and an electrode composite layer. Further, the electrode composite layer may include a protective layer and a fiber-containing layer. In this electrode, the contents described in the above-described method for manufacturing the electrode, such as the configuration, material, and range of the mixing ratio of each member, are appropriately applied, and detailed description thereof is omitted. The electrode composite layer has a protective layer including at least one of a current collecting side protective layer on the current collector side and a surface side protective layer on the surface side of the electrode, and a fiber-containing layer adjacent to the protective layer and containing carbon fibers. Further, in the protective layer, the average orientation angle of the carbon fibers obtained by passing through the inside of the electrode composite layer when the carbon fibers along the plane direction are 90° is 75° or less. This average orientation angle is obtained by analyzing the transmission image obtained by X-ray CT scanning. It is preferable that this protective layer has a current collecting side protective layer and a surface side protective layer having an average orientation angle of 75° or less. From the viewpoint of reducing resistance, it is preferable that the protective layer has a smaller average orientation angle, more preferably 72.5° or less, still more preferably 70° or less, and most preferably 67.5° or less or 65° or less. Further, in the entire electrode composite layer, it is preferable that the average orientation angle is 75° or less, more preferably 72.5° or less, still more preferably 70° or less, and most preferably 67.5° or less or 65° or less. Here, in the protective layer of the electrode, carbon fibers may enter. In the current collecting side protective layer, when the thickness of the electrode composite layer is 100, the region may be 30% or less, 20% or less, or 10% or less of the region on the current collector side. Further, in the surface side protective layer, when the thickness of the electrode composite layer is 100, the region may be 30% or less, 20% or less, or 10% or less of the region on the current collector side.

[0029] The electrode composite layer may have an orientation degree of carbon fibers oriented in the thickness direction of the electrode, which is determined based on the carbon fibers exposed on the electrode surface, of 50% or more. This orientation degree is preferably higher, preferably 60% or more, and more preferably 70% or more. This orientation degree is 100% or less, and may be 90% or less. If the orientation degree is higher, the carbon fibers are more oriented in the thickness direction of the electrode, so that the resistance can be further reduced, which is preferable. Here, this orientation degree refers to the ratio (%) of the oriented fibers. When the total number of all carbon fibers exposed on the electrode surface is N, the median length of the carbon fibers contained in the electrode composite layer is Lm, and the number of carbon fibers with a length of 3 / 7×Lm or less exposed on the electrode surface is n, the orientation degree (%) = n / N×100 can be obtained (Definition 1). This orientation degree can be obtained by simple processing through observation of the electrode surface.

[0030] Since the orientation degree in Definition 1 does not accurately reflect the orientation state of the carbon fibers inside the electrode, the orientation degree shall be the orientation degree in Definition 2 below. This orientation degree is obtained by analyzing the direction of each carbon fiber included in the image obtained by X-ray CT scan through image analysis. When the total number of all extracted carbon fibers is N (pieces) and the number of carbon fibers with an angle θ of 85° or less is n (pieces), the orientation degree (%) = n (pieces) / N (pieces)×100 (Definition 2). The orientation degree of the electrode composite layer may be, for example, 75% or more, preferably 85% or more, more preferably 87.5% or more, and still more preferably 90% or more.

[0031] This electrode preferably has, for example, a sheet resistance of the electrode composite layer of 32 Ωcm 2 or less. This sheet resistance is preferably lower, preferably 20 Ωcm 2 or less is more preferable, 10 Ωcm 2 or less is still more preferable, and 5 Ωcm 2 or less is also possible. Further, this electrode preferably has a volume resistivity of the electrode composite layer of 3.0×10 3 Ωcm or less. This volume resistivity is preferably lower, preferably 1.0×10 3 Ωcm or less is more preferable, 5×102 More preferably, it is below Ωcm, and it may be 1.0×10 2 Ωcm or less. Also, this electrode may have a thickness of the electrode composite layer in the range of 10 μm or more and 500 μm or less. When the thickness of the electrode composite layer becomes thicker, the influence of the decrease in resistance due to the orientation degree of carbon fibers becomes greater. The thickness of this electrode may be 20 μm or more, 50 μm or more, or may be 100 μm or more, 150 μm or more, 200 μm or more. Also, this electrode has an electrode density of the electrode composite layer of 2.0 (g / cm 3 ) or more, preferably in the range of 2.2 (g / cm 3 ) or more, and more preferably in the range of 2.4 (g / cm 3 ) or more. Also, this electrode has an electrode density of the electrode composite layer of 3.5 (g / cm 3 ) or less, preferably in the range of 3.0 (g / cm 3 ) or less, and more preferably in the range of 2.6 (g / cm 3 ) or less.

[0032] (Power storage device) The power storage device of the present disclosure includes the above-described electrode. This power storage device includes a positive electrode having a positive electrode active material and a negative electrode having a negative electrode active material, and one or more of the positive electrode and the negative electrode may be the above-described electrode. This power storage device may include a positive electrode, a negative electrode, and an ion conduction medium that is interposed between the positive electrode and the negative electrode and conducts carrier ions.

[0033] As the ion conductive medium, a non-aqueous electrolyte containing a supporting salt, a non-aqueous gel electrolyte, or the like can be used. Examples of the solvent for the non-aqueous electrolyte include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, and these can be used alone or in combination. Specifically, examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate, and 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-butyrolactone 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 cyclic carbonates and chain carbonates is preferred. According to this combination, not only are the cycle characteristics representing the battery characteristics during repeated charge and discharge excellent, but also the viscosity of the electrolyte, the electric capacity of the resulting battery, the battery output, etc. can be made well-balanced.

[0034] The supporting salt is, for example, LiPF 6 , LiBF 4 , LiAsF 6 , LiCF 3 SO 3 , LiN(CF 3 SO 2 ) 2 , LiC(CF 3 SO 2 ) 3 , LiSbF 6 , LiSiF 6 , LiAlF 4 , LiSCN, LiClO4 , LiCl, LiF, LiBr, LiI, LiAlCl 4 and the like. Among these, LiPF 6 , LiBF 4 , LiAsF 6 , LiClO 4 and other inorganic salts, and LiCF 3 SO 3 , LiN(CF 3 SO 2 ), 2 , LiC(CF 3 SO 2 ), 3 and other organic salts. It is preferable to use in combination one or more salts selected from the group consisting of these salts from the viewpoint of electrical characteristics. The concentration of this supporting salt in the non-aqueous electrolyte is preferably 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration at which the supporting salt is dissolved is 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolyte can be made more stable. Further, a flame retardant such as a phosphorus-based or halogen-based one may be added to this non-aqueous electrolyte.

[0035] In addition, instead of the liquid ion conduction medium, a solid ion conductive polymer can also be used as the ion conduction medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, vinylidene fluoride and a supporting salt can be used. Further, an ion conductive polymer and a non-aqueous electrolyte can also be used in combination. In addition, as the ion conduction medium, in addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder can be used.

[0036] The energy 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 use range of lithium secondary batteries. Examples include polymer non-woven fabrics such as polypropylene non-woven fabrics and polyphenylene sulfide non-woven fabrics, and thin microporous membranes of olefin-based resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0037] The shape of this energy storage device is not particularly limited, and examples include coin type, button type, sheet type, laminated type, cylindrical type, flat type, square type, etc. It may also be applied to large-scale ones used in electric vehicles and the like. FIG. 3 is a schematic diagram showing an example of the energy storage device 10. This energy storage device 10 has a positive electrode 12, a negative electrode 15, and an ion conduction medium 19. The positive electrode 12 has a positive electrode composite layer 13 and a current collector 14. The negative electrode 15 has a negative electrode composite layer 16 and a current collector 17. The ion conduction medium 19 conducts carrier ions and may be an electrolyte solution or a solid electrolyte. The above-described electrodes may be used as the positive electrode 12 or the negative electrode 15. When the positive electrode 12 is the above-described electrode, the positive electrode composite layer 13 contains an active material 21, carbon fibers 22, and a binder 23. The positive electrode composite layer 13 also contains a protective layer 25 and a fiber-containing layer 28. The protective layer 25 includes a current collection side protective layer 26 on the current collection body 14 side and a surface side protective layer 27 on the electrode surface side. The average orientation angle of the carbon fibers 22 obtained by passing through the inside of the positive electrode composite layer 13 when the carbon fibers 22 along the plane direction are 90° is 75° or less.

[0038] In the method for manufacturing an electrode, the electrode, and the power storage device of the present embodiment described in detail above, it is possible to further suppress an increase in the resistance of the electrode while reducing the carbon fiber as a conductive material. The reason for obtaining such an effect is presumed as follows. For example, carbon fiber functions as a conductive material, and by orienting the long axis direction of the carbon fiber in the thickness direction of the electrode, a path for efficiently conducting electrons can be formed. Further, in such an electrode, on the current collector side and the surface of the electrode, for example, carbon fibers may be oriented along the plane direction of the current collector and may not contribute to the formation of an effective electron conduction path in the film thickness direction. For example, on the current collector side, in the initial stage of film formation, when the oriented carbon fibers collide with the current collector, there is no support, so they may fall in the in-plane direction. Also, on the electrode surface side, when pressed during immobilization, the carbon fibers near the surface may fall in the in-plane direction. Here, by providing a protective layer that does not contain carbon fibers or has a lower carbon fiber content on the current collector side and / or the surface side of the electrode, it is possible to further suppress the in-plane orientation in which carbon fibers are oriented along the plane direction. Therefore, in this electrode, the protective layer causes the carbon fibers to be oriented along the film thickness direction, and the ratio of carbon fibers contributing to electron conduction can be increased. Thus, the electron resistance can be reduced with the minimum necessary amount of carbon fiber.

[0039] Generally, in the electrodes of power storage devices such as lithium-ion batteries and nickel-metal hydride batteries, attempts have been made to increase the energy density, which is the amount of energy per unit volume of the battery, by thickening the electrode composite layer containing the active material, thereby reducing the relative usage amounts of the current collector and the separator. However, when the electrode is thickened, the internal resistance increases due to the longer electron movement distance, and a significant decrease in battery capacity becomes a problem. In the above-described conventional technology (Japanese Patent Application Laid-Open No. 2013-122883), when applying an electrode slurry containing carbon nanofibers onto a current collector or before drying the solvent after application, a static electric field is applied in the film thickness direction of the electrode to orient the carbon nanofibers in the film thickness direction. However, carbon nanofibers generally have high cohesiveness, and it is not easy to uniformly disperse them and maintain that state, and it is difficult to manage the slurry. Also, in the slurry film-forming process of applying the electrode slurry to obtain an electrode film, since it is necessary to volatilize the solvent at a high temperature using a drying furnace, a large amount of energy is consumed, which is also a problem. In particular, N-methyl-2-pyrrolidone (NMP), which is used as a solvent for the positive electrode slurry, requires a system for recovery and treatment. Furthermore, in the solvent drying process, there is also a problem that the carbon nanofibers once oriented in the film thickness direction reorient in the plane. In the electrode and its manufacturing method of the present disclosure, since carbon fibers with low cohesiveness are used and the electrode composite layer is applied dry without using a solvent, it is possible to simply orient the carbon fibers in the electrode thickness direction while further reducing the energy consumption.

[0040] Note that the present disclosure is not limited to the above-described embodiments at all, and it goes without saying that various aspects can be implemented as long as they belong to the technical scope of the present disclosure.

[0041] For example, in the above-described embodiment, in the method for manufacturing an electrode, the raw material powder is applied onto the current collector without a solvent, but it is not particularly limited thereto, and a solvent may be added to the raw material powder and applied onto the current collector. As long as the carbon fibers can be oriented along the thickness direction of the electrode, the electrode can be manufactured regardless of whether it is dry or wet.

[0042] This disclosure may be as shown in any one of [1] to

[10] below. [1] A method for manufacturing an electrode used in a power storage device, generating an electrostatic field between the surface of a current collector and using a raw material containing an active material, a binder, and carbon fibers to form a fiber-containing layer such that the carbon fibers are aligned in the thickness direction of the composite layer, and forming a protective layer adjacent to at least one surface of the fiber-containing layer using a raw material having a lower carbon fiber content than the fiber-containing layer to produce an electrode composite layer, a coating step; A method for manufacturing an electrode including the above. [2] In the coating step, after forming a current collector-side protective layer as the protective layer on the current collector, forming the fiber-containing layer, or after forming the fiber-containing layer, forming a surface-side protective layer as the protective layer on the fiber-containing layer, or after forming a surface-side protective layer as the protective layer on a base material on the electrode surface side, forming the fiber-containing layer, any one or more of these are performed. The method for manufacturing an electrode according to [1]. [3] In the coating step, the protective layer is formed in a range where the ratio of the thickness of the protective layer to the average fiber length of the carbon fibers is 0.05 or more and 0.3 or less. The method for manufacturing an electrode according to [1] or [2]. [4] In the coating step, the protective layer is formed in a range where the ratio of the thickness of the protective layer to the average fiber length of the carbon fibers is 0.10 or more and 0.25 or less. The method for manufacturing an electrode according to any one of [1] to [3]. [5] In the coating step, the protective layer is formed in a range where the ratio of the thickness of the protective layer to the thickness of the electrode composite layer is 0.1 or more and 0.5 or less. The method for manufacturing an electrode according to any one of [1] to [4]. [6] A method for manufacturing an electrode according to any one of [1] to [5], a fixing step of heating the current collector coated with the raw material to fix it as an electrode composite layer; A method for manufacturing an electrode including the above. [7] An electrode used in a power storage device, a current collector, an electrode composite layer formed on the current collector and containing an active material, a binder, and carbon fibers, and comprising: The electrode composite layer has a protective layer including at least one of a current collecting side protective layer on the current collecting body side and a surface side protective layer on the surface side of the electrode, and a fiber-containing layer adjacent to the protective layer and containing carbon fibers. The protective layer has an average orientation angle of the carbon fibers obtained by passing through the inside of the electrode composite layer when the carbon fibers along the plane direction are 90° of 75° or less. [8] The electrode according to [7], wherein the protective layer has a current collecting side protective layer and a surface side protective layer with an average orientation angle of 75° or less. [9] The electrode according to [7] or [8], wherein the protective layer has an average orientation angle of 70° or less.

[10] A power storage device including the electrode according to any one of [7] to [9].

Examples

[0043] Hereinafter, examples of specifically manufacturing the electrode and the power storage device of the present disclosure will be described as experimental examples. Note that Experimental Example 1 corresponds to a reference example, and Experimental Examples 2 to 12 correspond to examples.

[0044] (Electrode manufacturing) (1) Mixing treatment of electrode powder 91 g of lithium nickel cobalt manganate as an active material, 3 g of acetylene black (HS100 manufactured by Denka Co., Ltd.) as a conductive material, 3 g of carbon fiber (K223HM (50 μm) manufactured by Mitsubishi Chemical Corporation), and 3 g of polyvinylidene fluoride (HSV900 manufactured by Arkema) as a binder were dry-mixed to obtain an electrode powder for a positive electrode. A lab mill (OML-1 manufactured by Osaka Chemical Co., Ltd.) was used for mixing the electrode powder. The active material, the conductive material, and the binder were mixed for 5 minutes, then the carbon fiber was added, and the mixture was further mixed for 1 minute.

[0045] (Physical properties of carbon fiber) The fiber diameter and fiber length distribution of the carbon fibers used were measured. Regarding the distribution of the carbon fiber lengths, the distribution of the fiber diameter D and fiber length L of the carbon fibers was measured using an image-based particle size distribution measuring device (manufactured by Microtrac Bell, Camseizer X2). The distribution of the fiber diameter D was calculated with the minimum diameter Xmin of the measured particles taken as the fiber diameter D. The distribution of the fiber length Xlength was calculated from the following formula (1) using the minimum diameter Xmin and the maximum Feret diameter XFe max of the measured particles. Figure 4 shows the measurement results of the fiber diameter D and fiber length L of the carbon fibers. Figure 4A is the distribution diagram of the fiber diameter D, Figure 4B is the distribution diagram of the fiber length L, and Figure 4C is the imaging photograph of the carbon fibers. As shown in Figure 4, for the carbon fibers used, the median diameter D50 of the fibers was 12 μm and the median length Lm was 70 μm.

[0046] [Number]

[0047] (2) Film formation of the electrode (Reference Examples 1 to 5) Using an electrostatic screen printing device (TS-1 manufactured by Berg Industries), electrode powder was applied onto an aluminum foil (thickness 15 μm) serving as a current collector. A 150-line - 60 μm (aperture 109 μm) screen mesh was used. An electric field of 0, 0.6, 1.3, 2.5, 3.8 kV / cm was applied between the screen plate and the coating surface. Each of the obtained electrodes was designated as Reference Examples 1 to 5. Further, the electrode powder was fixed onto the current collector using a heating roll press machine (Takumi Giken SA6202), and a positive electrode composite layer with a basis weight of 30 mg / cm 2 and a thickness of approximately 130 μm was laminated on the current collector to obtain a positive electrode. The pressing temperature was 200 °C, the feeding speed of the roll was 0.2 m / min, and the linear pressure was 286 kg / cm.

[0048] (Measurement of electrical resistance) The electrical resistance of the electrode was measured by the four-terminal method. Two electrodes were opposed via a copper foil and clamped with a pressure head and a load cell, and a plus voltage line and current line were connected to one electrode, and a minus voltage line and current line were connected to the other electrode. The copper foil was arranged to reduce the influence of the contact resistance between the electrodes and measure the electrical resistance of the electrode. With an area of 2 cm2 When a load of 50 kg (25 kg / cm 2 ) was applied to the piezoelectric element, the value of the DC resistance was taken as the electronic resistance. Also, when the volume resistivity Rv [Ωcm], the electronic resistance Re [Ω] of the electrode, the measurement area S [cm 2 , and the thickness t [cm] of the electrode were used, the volume resistivity of the electrode was calculated from Equation (2). Here, the measurement area was the area of the piezoelectric element, 2 cm 2 . Rv [Ωcm] = (Re [Ω] × S [cm 2 ) / t [cm] … Equation (2)

[0049] (Observation and Analysis of the Electrode Surface by Optical Microscope: Orientation Degree [Definition 1]) An optical microscope image of the surface of the formed electrode was obtained using a digital microscope (VHX-7000 manufactured by Keyence). The pixel size was 1 μm / pixel. Using the image analysis software Image J, carbon fibers exposed on the electrode surface were extracted by binarization processing from the luminance values of the obtained observation images. Furthermore, using the particle analysis function of Image J, a histogram of the length (major axis) in the longitudinal direction of the extracted carbon fibers was obtained. Among the carbon fibers exposed on the surface, those with a major axis 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 axis of 30 μm or less to the total number of carbon fibers exposed on the electrode surface was calculated as the ratio of the oriented carbon fibers. That is, when the total number of carbon fibers N [pieces] exposed on the electrode surface, the median length Lm [μm] of the carbon fibers contained in the electrode composite layer, and the number n [pieces] of carbon fibers with a length of 3 / 7 × Lm [μm] or less exposed on the electrode surface were used, the orientation degree (%) was taken as the value obtained by the following Equation (3). Orientation degree [%] = n [pieces] / N [pieces] × 100 … Equation (3)

[0050] (Evaluation of Carbon Fiber Orientation: Orientation Degree [Definition 1]) FIG. 5 is a conceptual diagram of the optical microscope images and cross-sections of the electrode surfaces in Experimental Examples 1, 3, and 5, where FIG. 5A is Reference Example 1, FIG. 5B is Reference Example 3, and FIG. 5C is Reference Example 5. As shown in FIG. 5, in electrostatic screen printing, in Experimental Example 1 where no electric field was applied, many carbon fibers with their longitudinal directions exposed were observed. In contrast, as in Reference Example 3 with an applied electric field of 1.3 kV / cm and Reference Example 5 with an applied electric field of 3.8 kV / cm, as the applied electric field increased, the number of carbon fibers with their longitudinal directions exposed decreased, and many carbon fibers that appeared punctate were observed. From this, in the optical microscope image, as the applied electric field increased, the number of carbon fibers with their longitudinal directions exposed decreased, and the number of carbon fibers that appeared punctate increased because the carbon fibers were oriented in the thickness direction by the electric field when the raw material powder was applied to the current collector, and the area exposed on the electrode surface decreased. Carbon fibers with a major axis of 3 / 7 Lm, that is, 30 μm or less, exposed on the surface were defined as those 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 ratio of the oriented carbon fibers (orientation degree: Definition 1).

[0051] (X-ray laminography measurement and orientation analysis) The validity of the orientation degree was examined from the three-dimensional transmission image of the electrode. A three-dimensional image of the electrode was obtained by X-ray laminography measurement. The X-ray laminography measurement was carried out at the large synchrotron radiation facility SPring-8 BL33XU (Toyota Beamline). The X-ray energy was set to 16 keV, and the tilt angle of the rotation axis was set to 30°. As the detector, an X-ray CMOS camera (ORCA-flush4.0 manufactured by Hamamatsu Photonics) was used, and imaging was performed with a pixel size of 0.325 μm / pixel. Imaging was performed at an exposure time of 100 msec for every 0.1° of the rotation angle, and 360° (3601 sheets) of transmission images were obtained. Using the reconstruction software provided by JASRI, the transmission images were reconstructed into three-dimensional images. The carbon fiber part was extracted from the three-dimensional image of the electrode obtained by the analysis software for X-ray tomography (VGSTUDIO MAX manufactured by Volume Graphics), orientation analysis was performed, and a histogram of the deviation angle θ with respect to the film thickness direction of the electrode was obtained.

[0052] (X-ray CT measurement: Orientation degree [Definition 2]) A three-dimensional image of the electrode was obtained by X-ray CT measurement. The X-ray CT measurement was performed using a high-resolution 3D X-ray microscope (Rigaku, nano3DX). Molybdenum was used as the X-ray source. Imaging was performed with a pixel size of 2.66 μm / pixel. Transmission images of 180° (3601 sheets) were obtained. Using reconstruction software, the transmission images were reconstructed into three-dimensional images. The carbon fiber portion was extracted from the three-dimensional image of the electrode obtained by analysis software for X-ray tomography (manufactured by Thermo fisher scientific, Amira), and orientation analysis was performed. A histogram of the angle θ with respect to the film thickness direction of the electrode was obtained. Among all the extracted carbon fibers, the orientation degree was defined as follows for the carbon fibers with an angle θ of 85° or less as the oriented carbon fibers. That is, when the number of all the extracted carbon fibers was N [pieces] and the number of carbon fibers with an angle θ of 85° or less was n [pieces], the orientation degree (%) was the value obtained by the following formula (4). Also, the average value of the orientation angles of all the carbon fibers was calculated as the average orientation angle. Furthermore, in order to clearly show the effects of the current collector side protective layer (B1 layer) and the surface side protective layer (B2 layer), the average orientation angle of the carbon fibers existing within 20 μm from the current collector or within 20 μm from the surface was also calculated. Orientation degree [%] = n [pieces] / N [pieces] × 100 … Formula (4)

[0053] (Results and discussion) Electric field [kV / cm] during electrostatic printing, total basis weight of the electrode composite material [mg / cm 2 , electrode thickness [μm], electrode density [g / cm 3 , ratio of oriented fibers [%-Definition 1,2], surface resistance [Ωcm 2The volume resistivity [Ωcm] is summarized in Table 1. As shown in Table 1, in Reference Examples 1 to 5, it was found that as the electric field during electrostatic printing increased, the degree of orientation improved, that is, the carbon fibers were aligned along the electrode thickness direction. Also, it was found that as this degree of orientation became higher, the surface resistance and volume resistivity decreased further. The reason for this is presumably that, for example, when carbon fibers as a conductive material are oriented in the electrode thickness direction, a path for efficiently conducting electrons in the electrode thickness direction can be formed, thus enabling further reduction of the electron resistance of the electrode for the power storage device. It was found that the electric field during electrostatic printing is preferably 1.0 [kV / cm] or more, and more preferably 2.0 [kV / cm] or more. Also, it was inferred that this electric field preferably ranges from 10 [kV / cm] or less. Also, the degree of orientation of the carbon fibers oriented in the electrode thickness direction determined based on the carbon fibers exposed on the electrode surface was found to be preferably 50% or more, more preferably 60% or more, and still more preferably 70% or more. Also, it was inferred that this degree of orientation may be 100% or less, 90% or less. Also, regarding the degree of orientation, it was confirmed that there is a good correlation between Definition 1 using the surface image of the electrode composite layer by an optical microscope and Definition 2 using the transmission image of the electrode by X-ray CT measurement.

[0054]

Table 1

[0055] Next, the protective layer was examined. As described above, it was found that in an electrode with a higher degree of orientation in which carbon fibers were tilted with respect to the electrode surface direction, its resistance was lower. On the other hand, on the current collector side and the surface of the electrode, for example, when the orientation angle is 80° to 90°, the carbon fibers are oriented in the in-plane direction and may not contribute to the formation of an effective electron conduction path in the film thickness direction. As a result, it becomes necessary to add more carbon fibers than necessary, which may inhibit the ion conduction path (voids) or become a factor in reducing the energy density (active material density). The reason for this is, for example, on the current collector side, at the initial stage of film formation, when the oriented carbon fibers collide with the current collector, there is no support, so it is considered that they will fall in the in-plane direction. Also, on the electrode surface side, when pressed during immobilization, it is considered that the carbon fibers near the surface will fall in the in-plane direction. For these reasons, it was considered to provide a protective layer that does not contain carbon fibers or has a lower carbon fiber content on the current collector side and the surface side of the electrode.

[0056] (Preparation of Electrode Powder A) The members of the above reference example were used. 90.4 g of lithium nickel cobalt manganate was weighed as the active material, 3 g of acetylene black as the conductive material, 3.6 g of carbon fiber, and 3 g of polyvinylidene fluoride as the binder were weighed and dry-mixed to obtain electrode powder A for the positive electrode. A labomill was used for mixing the electrode powder. The active material, acetylene black, and binder were mixed for 5 minutes, then carbon fiber was added and mixed for another 1 minute.

[0057] (Preparation of Electrode Powder B) 91 g of lithium nickel cobalt manganate was weighed as the active material, 3 g of acetylene black as the conductive material, and 3 g of polyvinylidene fluoride as the binder were weighed and dry-mixed to obtain electrode powder B for the positive electrode. A labomill was used for mixing the electrode powder. The composition of the electrode powder is summarized in Table 2.

[0058]

Table 2

[0059] (Film Formation of Electrode) Using an electrostatic screen printing apparatus, electrode powder was applied onto an aluminum foil (thickness 15 μm) serving as a current collector. A 150-line - 60 μm (mesh opening 109 μm) screen mesh was used. An electric field of 6 kV / cm was applied between the screen plate and the coating surface. Further, the electrode powder was fixed onto the current collector by a heating roll press machine, and a positive electrode composite layer with a basis weight of about 20 mg / cm 2 and a thickness of about 70 μm was laminated on the current collector to obtain a positive electrode. The pressing temperature was 200 °C, the feed rate of the roll was 0.2 m / min, and the linear pressure was 286 kg / cm.

[0060] (Without protective layer: Experimental Example 1) The electrode prepared without providing a protective layer under the conditions shown in Table 3 was taken as Experimental Example 1. Since there is no protective layer with few carbon fibers, the carbon fibers contained in the electrode have the largest value of 3.6 mass%.

[0061] (Current collector side protective layer: Experimental Examples 2 - 6) Under the conditions shown in Table 3, electrode powder B without carbon fibers was applied by electrostatic screen printing onto the current collector as a base to form a current collector side protective layer (B1 layer) as the protective layer, and then electrode powder A with carbon fibers was applied by electrostatic screen printing thereon to form a fiber-containing layer (A layer). The obtained electrodes were taken as Experimental Examples 2 - 6.

[0062] (Surface side protective layer: Experimental Examples 7 - 11) Under the conditions shown in Table 3, electrode powder A with carbon fibers was applied by electrostatic screen printing onto the current collector to form a fiber-containing layer (A layer), and then electrode powder B without carbon fibers was applied by electrostatic screen printing as a protective layer to form a surface side protective layer (B2 layer). The obtained electrodes were taken as Experimental Examples 7 - 11.

[0063] (Current collector side protective layer and surface side protective layer: Experimental Example 12) Under the conditions shown in Table 3, electrode powder B without carbon fiber was applied as a base on the current collector by electrostatic screen printing to form a current collector-side protective layer (B1 layer) as a protective layer, and electrode powder A with carbon fiber was applied thereon by electrostatic screen printing to form a fiber-containing layer (A layer). Further, electrode powder B without carbon fiber was applied as a protective layer thereon by electrostatic screen printing to form a surface-side protective layer (B2 layer). The obtained electrode was designated as Experimental Example 12.

[0064] (Results and Discussion) Figure 6 is a three-dimensional image of carbon fibers obtained from X-ray laminography measurement. Figure 7 is a histogram of the angle θ with respect to the film pressure direction of the electrodes in Experimental Examples 1 and 12. Figure 8 is a relationship diagram between the carbon fiber length and the degree of orientation in Experimental Examples 2 to 6, where Figure 8A is a cross-sectional view of the electrode model, Figure 8B is a relationship diagram between the degree of orientation (Definition 1) and the ratio of the thickness of the current collector side protective layer to the carbon fiber length, Figure 8C is a relationship diagram between the degree of orientation (Definition 2) and the ratio of the thickness of the current collector side protective layer to the carbon fiber length, Figure 8D is a relationship diagram between the average degree of orientation of the current collector side protective layer and the ratio of the thickness of the current collector side protective layer to the carbon fiber length, and Figure 8E is a relationship diagram between the sheet resistance and the ratio of the thickness of the current collector side protective layer to the carbon fiber length. Figure 9 is a relationship diagram between the carbon fiber length and the degree of orientation in Experimental Examples 7 to 11, where Figure 9A is a cross-sectional view of the electrode model, Figure 9B is a relationship diagram between the degree of orientation (Definition 1) and the ratio of the thickness of the surface side protective layer to the carbon fiber length, Figure 9C is a relationship diagram between the degree of orientation (Definition 2) and the ratio of the thickness of the surface side protective layer to the carbon fiber length, Figure 9D is a relationship diagram between the average degree of orientation of the surface side protective layer and the ratio of the thickness of the surface side protective layer to the carbon fiber length, and Figure 9E is a relationship diagram between the sheet resistance and the ratio of the thickness of the surface side protective layer to the carbon fiber length. Figure 10 is a relationship diagram between the sheet resistance and the degree of orientation in Experimental Examples 1 to 12 with respect to the amount of carbon fibers. Also, Table 3 summarizes the electrostatic field, total basis weight, basis weight of the fiber-containing layer (Layer A), basis weight of the current collector side protective layer (Layer B1), basis weight of the surface side protective layer (Layer B2), electrode thickness, thicknesses of Layers A, B1, and B2, ratio of Layer B1 thickness to average carbon fiber length, ratio of Layer B2 thickness to average carbon fiber length, electrode density, and amount of carbon fibers in the electrode for Experimental Examples 1 to 12. Also, Table 4 summarizes the ratio of oriented fibers (degree of orientation) according to Definitions 1 and 2, average orientation angle, average orientation angles near the current collector (current collector side protective layer) and near the surface (surface side protective layer), sheet resistance, and volume resistivity for Experimental Examples 1 to 12.

[0065]

Table 3

[0066]

Table 4

[0067] As shown in FIGS. 6 and Tables 3 and 4, in Experimental Examples 1 to 12, when an electrostatic field was generated between the surface of the current collector and the discharge part and the electrode binder powder was discharged, it was found that an electrode binder layer was obtained in which the carbon fibers were oriented along the thickness direction of the electrode binder layer with an angle rather than along the surface direction. Further, as shown in FIG. 7 and Tables 3 and 4, in Experimental Example 1, there were many carbon fibers with an orientation angle of 80 to 90° along the surface direction, but in Experimental Example 12, the carbon fibers with an orientation angle of 80 to 90° were reduced, and it was found that the angle of the carbon fibers was smaller, that is, they had a larger angle with respect to the surface direction.

[0068] Regarding Experimental Examples 2 to 6 provided with a current collecting side protective layer shown in Fig. 8A, including Tables 3 and 4, we will conduct an investigation. As shown in Fig. 8B, in Experimental Examples 2 to 6, in terms of Definition 1 calculated from the carbon fibers exposed on the electrode surface, the ratio (orientation degree) of the oriented fibers hardly changed regardless of the thickness of the current collecting side protective layer. On the other hand, as shown in Fig. 8C, it was found that the orientation degree of Definition 2 evaluated by X-ray CT measurement improved when the ratio (B1 / L) of the thickness of the current collecting side protective layer to the carbon fiber length L was 0.10 or more. It is presumed that this is because in the formation of the electrode composite layer that discharges the powder in the electrostatic field, the presence of the current collecting side protective layer that contains less carbon fiber suppresses the carbon fiber from aligning along the plane direction more effectively. This was also confirmed from the fact that as shown in Fig. 8D, when B1 / L was 0.10 or more, the average orientation angle in the vicinity of the current collector (within 20 μm) showed a lower value such as 5 to 10°. In this regard, it is presumed that the current collecting side protective layer formed by electrostatic screen printing is more preferable for improving the orientation angle of the carbon fiber because it has more space and the carbon fiber is more likely to penetrate compared to the one formed by applying and drying the paste-like raw material. Also, as shown in Fig. 8E, in Experimental Examples 2 to 6, the sheet resistance decreased in the range where B1 / L was 0.1 or more and 0.3 or less. The sheet resistance was slightly higher in Experimental Examples 2 to 6 compared to Experimental Example 1, and especially increased more when this ratio B1 / L was 0.3 or more. This is presumed to be because the absolute amount of carbon fiber decreased, and it is not that the orientation effect of the current collecting side protective layer decreased. In particular, it was found that when a current collecting side protective layer was provided and the carbon fiber was oriented so as not to be along the plane direction, a sufficiently low resistance value was shown even when the carbon fiber content was reduced.

[0069] Regarding Experimental Examples 7 to 11 provided with a surface-side protective layer shown in Fig. 9A, discussions will be made including Tables 3 and 4. As shown in Fig. 9B, in Experimental Examples 7 to 11, in Definition 1 calculated from carbon fibers exposed on the electrode surface, when the ratio (orientation degree) of the oriented fibers, i.e., the ratio (B2 / L) of the thickness of the surface-side protective layer to the carbon fiber length L, was 0.05 or more, the orientation degree of the carbon fibers exposed on the electrode surface was improved. On the other hand, when B2 / L was 0.2 or more, the carbon fibers were not exposed on the surface, and the orientation degree analysis according to Definition 1 could not be performed. As shown in Fig. 9C, in terms of the orientation degree according to Definition 2, no significant change in the orientation degree was observed depending on the thickness of the B2 layer. This is presumably because the influence of the in-plane orientation of the carbon fibers near the current collector is relatively large, so that in the orientation degree of Definition 2 for evaluating the entire electrode composite layer, the orientation effect of the surface layer is buried. This is also supported by the fact that as shown in Fig. 9D, when B2 / L is 0.05 or more, the average orientation angle in the vicinity of the surface (within 20 μm) shows a lower value such as 3 to 15°, indicating that the formation effect of the surface-side protective layer is clearly obtained. In this regard, also for the surface-side protective layer, compared with the one formed by applying and drying a paste-like raw material, the one formed by electrostatic screen printing is more preferable for improving the orientation angle of the carbon fibers because the electrode composite material is more likely to enter the space of the carbon fibers not along the in-plane direction. Further, as shown in Fig. 9E, in Experimental Examples 7 to 11, when B2 / L was in the range of 0.05 or more and 0.3 or less, the increase in the sheet resistance was suppressed despite the decrease in the carbon fibers. The sheet resistance was slightly higher in Experimental Examples 7 to 11 than in Experimental Example 1, and particularly increased more when this ratio was 0.3 or more. This is presumably because the absolute amount of the carbon fibers decreased, and the orientation effect of the surface-side protective layer did not decrease. In particular, it was found that when the carbon fibers were oriented so as not to be along the in-plane direction, even if the surface-side protective layer was provided and the carbon fiber content was reduced, a sufficiently low resistance value was shown.

[0070] As shown in FIG. 10, in Experimental Example 12 in which a current collecting side protective layer and a surface side protective layer were formed, since carbon fibers can be effectively utilized for the electron electromotion in the film pressing direction, it was clarified that the area resistance can be reduced while suppressing the amount of carbon fibers used to the minimum necessary. In Experimental Examples 5 and 6 and Experimental Examples 10 and 11, etc., although it can be said that the area resistance is larger, considering the state where the amount of carbon fibers is further reduced, it was presumed that the area resistance was sufficiently reduced.

[0071] Note that the present disclosure is not limited to the above-described embodiments at all, and it goes without saying that the present disclosure can be implemented in various modes as long as it belongs to the technical scope of the present disclosure.

Industrial Applicability

[0072] The method for manufacturing an electrode, the electrode, and the power storage device disclosed in this specification can be used in the technical field of power storage devices such as secondary batteries.

Explanation of Signs

[0073] 10 Power storage device, 12 Positive electrode, 13 Positive electrode composite layer, 14 Current collector, 15 Negative electrode, 16 Negative electrode composite layer, 17 Current collector, 18 Separator, 19 Ion conductive medium, 21 Active material, 22 Carbon fiber, 23 Binder, 25 Protective layer, 26 Current collecting side protective layer, 27 Surface side protective layer, 28 Fiber-containing layer.

Claims

1. A method for manufacturing an electrode used in an energy storage device, comprising: generating an electrostatic field between a surface of a current collector and forming a fiber-containing layer using a raw material containing an active material, a binder, and carbon fibers such that the carbon fibers are aligned in the thickness direction of a composite layer, and forming a protective layer adjacent to at least one surface of the fiber-containing layer using a raw material having a lower carbon fiber content than the fiber-containing layer to produce an electrode composite layer (coating step); A method for manufacturing an electrode comprising the above.

2. In the coating step, any one or more of the following operations are performed: forming a current collector-side protective layer as the protective layer on the current collector and then forming the fiber-containing layer; forming a surface-side protective layer as the protective layer on the fiber-containing layer after forming the fiber-containing layer; forming a surface-side protective layer as the protective layer on a base material on the electrode surface side and then forming the fiber-containing layer. The method for manufacturing an electrode according to claim 1.

3. In the coating step, the protective layer is formed in a range where the ratio of the thickness of the protective layer to the average fiber length of the carbon fibers is 0.05 or more and 0.3 or less. The method for manufacturing an electrode according to claim 1 or 2.

4. In the coating step, the protective layer is formed in a range where the ratio of the thickness of the protective layer to the average fiber length of the carbon fibers is 0.10 or more and 0.25 or less. The method for manufacturing an electrode according to claim 1 or 2.

5. In the coating step, the protective layer is formed in a range where the ratio of the thickness of the protective layer to the thickness of the electrode composite layer is 0.1 or more and 0.5 or less. The method for manufacturing an electrode according to claim 1 or 2.

6. A method for manufacturing an electrode according to claim 1 or 2, further comprising: heating the current collector coated with the raw material to immobilize it as an electrode composite layer (immobilization step). A method for manufacturing an electrode comprising the above.

7. An electrode used in an energy storage device, comprising: a current collector; an electrode composite layer formed on the current collector and containing an active material, a binder, and carbon fibers, wherein the electrode composite layer has a protective layer including at least one of a current collector-side protective layer on the current collector side and a surface-side protective layer on the surface side of the electrode, and a fiber-containing layer adjacent to the protective layer and containing carbon fibers. The average orientation angle of the carbon fibers obtained by passing through the inside of the electrode composite layer when the carbon fibers along the plane direction are set to 90° is 75° or less.

8. The electrode according to claim 7, wherein the protective layer has a current collector-side protective layer and a surface-side protective layer with an average orientation angle of 75° or less.

9. The electrode according to claim 7 or 8, wherein the protective layer has an average alignment angle of 70° or less. **Claim 10** A power storage device including the electrode according to claim 7 or 8.

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