JP7750226B2 - Electrode, power storage device, and method for manufacturing electrode - Google Patents

Description

This specification discloses an electrode, an electricity storage device, and a method for manufacturing the electrode.

Conventionally, proposed electrodes for electricity storage devices include, 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. When the electrode surface is measured using X-ray diffraction, the orientation ratio calculated from the sum of the intensities of the diffraction peaks resulting from each crystal plane of the carbon nanofiber and the intensity of the diffraction peak resulting from the (002) crystal plane of the carbon nanofiber falls within a predetermined range (see, for example, Patent Document 1). It is claimed that this electrode can provide an electrode and electricity storage device with low electrical resistance.

JP 2013-69928 A

However, the electrodes in Patent Document 1 mentioned above are manufactured using a slurry containing carbon nanofibers. However, carbon nanofibers have a high tendency to aggregate, making the slurry difficult to manage and making it difficult to uniformly disperse the fibers or maintain that state. As such, reducing the electrical resistance of electrodes used in electricity storage devices has not yet been sufficient, and there has been a demand for producing electrodes with low electrical resistance.

The present disclosure has been made in consideration of these issues, and its primary objective is to provide an electrode, an electricity storage device, and a method for manufacturing an electrode that can further reduce electrode resistance.

After extensive research to achieve the above-mentioned objective, the inventors discovered that electrode resistance 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, leading to the completion of the invention disclosed in this specification.

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.

The electricity storage device disclosed in this specification is equipped with the electrodes described above.

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:

This disclosure makes it possible to 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 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.

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

(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.

The current collector is a conductive member adjacent to the electrode composite layer that collects current during charging and discharging. The current collector can be selected appropriately depending on the potential of the active material, etc. Examples of materials that can be used include aluminum, titanium, stainless steel, nickel, iron, copper, calcined carbon, conductive polymers, and conductive glass. In addition, materials such as aluminum and copper whose surfaces have been treated with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and oxidation resistance can also be used. These surfaces can also be subjected to oxidation treatment. Current collector shapes include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and fiber-formed. The thickness of the current collector is, for example, 1 to 500 μm. The amount of active material composite formed can be appropriately determined depending on the desired performance of the electricity storage device.

The electrode mixture layer is formed on the current collector and contains an active material, a binder, and carbon fibers. This electrode mixture layer may contain a conductive material in addition to the carbon fibers. The active material may be a positive electrode active material. Examples of the positive electrode active material that can be used include sulfides containing transition metal elements and oxides containing lithium and transition metal elements. Specifically, transition metal sulfides such as _TiS2 , _TiS3 , _MoS3 , and _FeS2 ; lithium manganese composite oxides having a basic composition formula such as Li _{(1-x) MnO2} (0 _< x<1, etc., the same applies below) or Li _{(1-x) Mn2O4} ; lithium cobalt composite oxides having a basic composition formula such as Li _{(1-x) CoO2} ; lithium nickel composite oxides having a basic composition formula such as Li( _{1-x ) NiO2} ; lithium _{nickel cobalt} manganese composite oxides having a basic composition formula such as Li _{( 1-x)NiaCobMncO2 (a+b+c=1) or Li(1-x ) NiaCobMncO4 ( a +} b+c= ₂ ); Lithium vanadium composite oxides having a basic composition formula such as _V2O5 , _etc. , and transition metal oxides having a basic composition formula such as _V2O5 , etc., can be used. The positive electrode active material may also be lithium iron phosphate, etc. Among these, lithium transition metal composite oxides, such as _LiCoO2 , _LiNiO2 , _LiMnO2 , and LiNi1 _{/3Co1/ 3Mn1 / 3O2} , are preferred. The term "basic composition formula" means that other elements may be included.

The positive electrode active material may be used, for example, in a capacitor that adsorbs and desorbs carrier ions. This positive electrode active material may include, for example, a carbon material. Examples of carbon materials include, but are not limited to, activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. The carbon material preferably has a specific surface area of 1000 m ² /g or more, more preferably 1500 m ² /g or more. A specific surface area of 1000 m ² /g or more can further increase the discharge capacity. The specific surface area of the activated carbon is preferably 3000 ^{m 2} /g or less, and may be 2000 m ² /g or less, for ease of preparation. It is considered that the positive electrode stores electricity by adsorbing and desorbing at least one of anions and cations contained in the ion-conducting medium, but it may also store electricity by inserting and desorbing at least one of anions and cations contained in the ion-conducting medium.

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. Of these, graphites such as artificial graphite and natural graphite are preferred because they have an operating potential close to that of metallic lithium, enable 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 perspective of capacity, while composite oxides are preferred from the perspective of safety.

The binder is a material that fixes the active material and carbon fiber. This binder may be, for example, a resin that hardens when heated. It is preferable that the binder be ionically conductive 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 this resin is used in electrodes, it is particularly preferable that the resin be a fluorine-containing resin in terms of potential stability. PVdF is a preferable resin.

The carbon fibers have a fiber diameter D in the range of 1 μm or more and 20 μm or less. The carbon fibers also 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 carbon fiber size is measured using an image particle size distribution analyzer.

The conductive material added in addition to carbon fiber may be, for example, particulate, flake, fibrous, or tubular. Examples of such conductive materials 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 greater is preferred, as it facilitates the formation of conductive paths. This particle diameter d is preferably 20 nm or greater 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, if the conductive material is fibrous or tubular, it may be 1/10 or less the size of carbon fiber.

The electrode composite layer has a degree of orientation of carbon fibers oriented in the electrode thickness direction, 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 electrode thickness direction, thereby further reducing resistance. Here, this degree of orientation is calculated as follows: orientation (%) = n/N x 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 composite layer, and n is the number of carbon fibers exposed on the electrode surface with a length of 3/7 x Lm or less.

In this electrode, for example, the sheet resistance of the electrode mixture layer is preferably 32 ^Ωcm2 or less. This sheet resistance is preferably lower, more preferably 20 ^Ωcm2 or less, even more preferably 10 ^Ωcm2 or less, and may be 5 Ωcm2 ^or less. In addition, in this electrode, the volume resistivity of the electrode mixture layer is preferably 3.0 × ¹⁰³ Ωcm or less. This volume resistivity is preferably lower, more preferably 1.0 × ¹⁰³ Ωcm or less, even more preferably 5 × ¹⁰² Ωcm or less, and may be 1.0 × ¹⁰² Ωcm or less. In addition, in this electrode, the thickness of the electrode mixture layer may be in the range of 10 μm or more and 500 μm or less. 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 preferably has an electrode density of 2.0 (g/ ^cm3 ) or more, more preferably 2.2 (g/ ^cm3 ) or more, and even more preferably 2.4 (g/ ^cm3 ) or more. The electrode mixture layer of this electrode preferably has an electrode density of 3.5 (g/ ^cm3 ) or less, more preferably 3.0 (g/ ^cm3 ) or less, and even more preferably 2.6 (g/ ^cm3 ) or less.

The electrode mixture layer preferably contains 70% to 95% by mass of active material relative to the total active material, binder, and carbon fiber. An active material content of 70% by mass or more ensures sufficient capacity, while a content of 95% by mass or less is preferable because the relative amounts of binder and carbon fiber can be maintained, ensuring strength and conductivity. The electrode mixture layer preferably contains 0.5% to 5% by mass of binder relative to the total active material, binder, and carbon fiber. A binder content of 0.5% by mass or more ensures sufficient strength, while a content of 5% by mass or less ensures the relative amount of active material, ensuring capacity. The electrode mixture layer preferably contains 0.5% to 5% by mass of carbon fiber relative to the total 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 ensures the relative amount of active material, ensuring capacity. Furthermore, when the electrode mixture layer contains a conductive material other than carbon fiber, it is preferable that the conductive material be contained in a range of 0.5% by mass or more and 5% by mass or less relative to 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.

(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.

The ion-conducting medium can be a non-aqueous electrolyte solution containing a supporting salt or a non-aqueous gel electrolyte solution. Examples of solvents for non-aqueous electrolyte solutions include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which can 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. Of these, a combination of cyclic carbonates and chain carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery's performance over repeated charge and discharge, but also allows for a well-balanced electrolyte viscosity, the resulting battery's electrical capacity, and battery output.

Examples of supporting salts _include LiPF6, _LiBF4 , _LiAsF6 , _LiCF3SO3 , LiN( _{CF3SO2 ) 2} , LiC( _{CF3SO2 ) 3} , _LiSbF6 , _LiSiF6 , _LiAlF4 , LiSCN, _LiClO4 , LiCl, _LiF , LiBr, LiI, and _LiAlCl4 . Among these, _it is preferable to use a combination of one or more salts selected from _the group consisting of inorganic salts such as _LiPF6 , _LiBF4 , _LiAsF6 , and _LiClO4 , and organic salts such as _LiCF3SO3 , LiN( _{CF3SO2 ) 2} , and LiC( _CF3SO2 ) ₃ , from the viewpoint of electrical properties. 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, more preferably 0.5 mol/L or more and 2 mol/L or less. When the concentration of the dissolving supporting salt 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 more stable. In addition, a phosphorus-based, halogen-based, or other flame retardant may be added to the non-aqueous electrolyte.

In place of a liquid ion-conducting medium, a solid ion-conducting polymer can also be used as the ion-conducting medium. Examples of ion-conducting polymers that can be used 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, ion-conducting polymers can also be used in combination with nonaqueous electrolyte solutions. Furthermore, in addition to ion-conducting polymers, inorganic solid electrolytes, mixtures of organic polymer electrolytes and inorganic solid electrolytes, or inorganic solid powders bound with an organic binder can also be used as ion-conducting media.

The power storage device of the present disclosure may include a separator between the negative electrode and positive electrode. There are no particular limitations on the separator, as long as its composition can withstand the range of use of lithium secondary batteries. Examples include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous membranes made of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

The shape of this electricity storage device is not particularly limited, but examples include coin, button, sheet, laminate, cylindrical, flat, and rectangular shapes. It may also be used in large devices such as electric vehicles. Figure 1 is a schematic diagram showing an example of an electricity storage device 10. This electricity storage device 10 has a positive electrode 12, a negative electrode 15, and an ion-conducting medium 19. The positive electrode 12 has a positive electrode active material layer 13 and a current collector 14. The negative electrode 15 has a negative electrode active material layer 16 and a current collector 17. The ion-conducting medium 19 conducts carrier ions and may be an electrolyte solution or a solid electrolyte. The above-mentioned electrode may be the positive electrode 12 or the negative electrode 15. When the positive electrode 12 is the above-mentioned electrode, the positive electrode composite layer 13 includes an active material 21, carbon fibers 22, and a binder 23. The 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. Furthermore, the positive electrode composite layer 13 has a degree of orientation of the carbon fibers 22 oriented in the thickness direction of the electrode of 50% or more, determined based on the carbon fibers 22 exposed on the electrode surface.

(Electrode manufacturing method)
The manufacturing method of an electrode used in the energy storage device of the present disclosure may be the method for manufacturing the electrode described above. In this manufacturing method, the contents described in the above-described electrode, 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. Figure 2 is an explanatory diagram showing an example of a manufacturing method of an electrode used in the energy storage device 10, where Figure 2A shows an example of a mixing process, Figure 2B shows an example of a coating process, and Figure 2C shows an example of a fixation process.

In the coating process, an electrostatic field is generated between the collector and the surface, and raw materials containing an active material, a binder, and carbon fiber are applied to the collector surface. 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 materials 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 materials, and may be an electrostatic screen printing process (Figure 2B). This coating process may also involve a dry mixing process in which the raw material powder is mixed before coating (Figure 2A). A mixer can be used for the mixing process. The mixing process may involve mixing the granular active material, binder, and, if necessary, granular conductive material, followed by adding the carbon fiber and further mixing. In the coating process, the raw materials are preferably applied in an electrostatic field strength of 1 kV/cm to 10 kV/cm. An electric field of 1 kV/cm or higher 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 lower 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 mesh screen and applied to the current collector through the mesh screen. The mesh size of the screen is selected appropriately depending on the particle size of the raw material. During coating of the raw material, a member such as a squeegee may be slid across the raw material placed on the mesh screen to press the raw material toward the current collector. The coating time may be determined, for example, so that the coating process is continued until the electrode mixture layer reaches the desired thickness.

In the fixation process, the current collector coated with the raw material is heated to fix the material into an electrode composite layer. For example, the fixation process may involve hot-pressing the current collector on which the raw material has been formed (Figure 2C). The heating temperature may be set appropriately depending on the type of binder, but 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 also 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.

The electrode, electricity storage device, and electrode manufacturing method of this embodiment described in detail above 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, 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.

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 amounts of current collectors and separators used and increasing the energy density (the amount of energy per unit volume of the battery). However, thickening the electrode increases the distance electrons must travel, resulting in increased internal resistance and a significant decrease in battery capacity. In the aforementioned prior art (JP 2013-122883), an electrostatic field is applied in the direction of the electrode film thickness when applying electrode slurry containing carbon nanofibers to the current collector, or before drying the solvent after application, thereby aligning the carbon nanofibers in the direction of the film thickness. However, carbon nanofibers generally have a high tendency to agglomerate, making it difficult to uniformly disperse and maintain that state, making slurry management difficult. Another issue is the slurry film formation process in which electrode slurry is applied to obtain an electrode film, which requires high-temperature evaporation of the solvent using a drying oven, resulting in high energy consumption. In particular, N-methyl-2-pyrrolidone (NMP), which is used as a solvent for the positive electrode slurry, requires a system for recovering and disposing of it. Furthermore, there is a problem that carbon nanofibers that have been oriented in the film thickness direction may become oriented in-plane again during the solvent drying process. The electrode and manufacturing method disclosed herein use carbon fibers with low cohesion, and the electrode mixture layer is applied dry without the use of a solvent. This allows for easy alignment of the carbon fibers in the electrode thickness direction while further reducing energy consumption.

It goes without saying that the present disclosure is in no way limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure.

For example, in the above-described embodiment, the electrode manufacturing method involves applying the raw material powder to the current collector without a solvent, but this is not particularly limited; 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 or wet method.

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, wherein 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.
[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 and having a length of 3/7 × Lm or less.
[4] The electrode according to any one of [1] to [3], wherein the electrode satisfies any one or more of (1) to (4).
(1) The sheet resistance is 32 Ωcm ² or less.
(2) The volume resistivity is 3.0×10 ³ Ωcm or less.
(3) The thickness is in the range of 10 μm or more and 500 μm or less.
(4) The electrode density is in the range of 2.0 (g/cm ³ ) to 3.5 (g/cm ³ ).
[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 manufacturing 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.

Below, specific examples of the fabrication of electrodes and electricity storage devices according to the present disclosure are described as experimental examples. Note that Experimental Example 1 corresponds to a comparative example, and Experimental Examples 2 to 5 correspond to working examples.

(Electrode Fabrication)
(1) Electrode Powder Mixing Treatment 91 g of lithium nickel cobalt manganese oxide as the active material, 3 g of acetylene black (HS100 manufactured by Denka Kogyo Kogyo) as the conductive material, 3 g of carbon fiber (K223HM (50 μ) manufactured by Mitsubishi Chemical Corporation), and 3 g of polyvinylidene fluoride (HSV900 manufactured by Arkema) as the 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 to mix the active material, conductive material, and binder for 5 minutes, and then the carbon fiber was added and mixed for another 1 minute.

(Physical properties of carbon fiber)
The fiber diameter and fiber length distribution of the carbon fiber used were measured. The distribution of the length of the carbon fiber was measured using an image particle size distribution measuring device (Microtrac Bell, Camseizer X2) to measure the fiber diameter D and fiber length L of the carbon fiber. The distribution of the fiber diameter D was calculated using the minimum diameter Xmin of the measured particles 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 maximum Feret diameter XFemax of the measured particles. Figure 3 shows the measurement results of the fiber diameter D and fiber length L of the carbon fiber, where Figure 3A is a distribution diagram of the fiber diameter D, Figure 3B is a distribution diagram of the fiber length L, and Figure 3C is a photograph of the carbon fiber. As shown in Figure 3, the carbon fiber used had a median diameter D50 of 12 μm and a median length Lm of 70 μm.

(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), resulting in a positive electrode having a basis weight of 30 mg/cm ² and a thickness of approximately 130 μm laminated on the current collector. The press temperature was 200°C, the roll feed rate was 0.2 m/min, and the linear pressure was 286 kg/cm.

(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, facing each other via copper foil. 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 influence of electrode-to-electrode contact resistance and measure the electrode's electronic resistance. The value of DC resistance when a load of 50 kg (25 kg/cm ² ) was applied to an indenter with an area of 2 cm ² was taken as the electronic resistance. The volume resistivity of the electrode was calculated using equation (2) where Rv [Ωcm], Re [Ω], the measurement area S [cm ² ], and the electrode thickness t [cm] are the indenter area. Here, the measurement area was taken as the indenter area of 2 cm ² .
Rv[Ωcm]=(Re[Ω]×S[cm ² ])/t[cm] …Formula (2)

(Observation and analysis of electrode surface using an optical microscope)
Optical microscope images of the surface of the deposited electrode were obtained 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 binarization processing from the brightness values of the obtained observation image. Furthermore, a histogram of the longitudinal length (major axis) of the extracted carbon fibers was obtained using the particle analysis function of Image J. Of 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 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 mixture 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 determined by the following formula (3).
Orientation degree [%] = n [pieces] / N [pieces] × 100 ... Formula (3)

(X-ray laminography measurement and orientation analysis)
The appropriateness of the degree of orientation was examined from the 3D transmission image of the electrode. 3D images of the electrode were obtained by X-ray laminography measurement. X-ray laminography measurement was performed at the SPring-8BL33XU (Toyota Beamline) large 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 (ORCA-flush 4.0 manufactured by Hamamatsu Photonics) was used as the detector, and images were taken with a pixel size of 0.325 μm/pixel. Images were taken every 0.1° of rotation angle with an exposure time of 100 msec, obtaining 360° (3601 images) of transmission images. Using reconstruction software provided by JASRI, the transmission images were reconstructed into 3D images. The carbon fiber portion was extracted from a three-dimensional image of the electrode obtained using analysis software for X-ray tomography (VGSTUDIO MAX manufactured by Volume Graphics), and orientation analysis was performed to obtain a histogram of the deviation angle θ relative to the film thickness direction of the electrode.

(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 being Experimental Example 1, FIG. 4B being Experimental Example 3, and FIG. 4C being Experimental Example 5. FIG. 5 shows three-dimensional images of carbon fibers obtained by X-ray laminography measurement. 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 measurement. As shown in FIG. 4 , in Experimental Example 1, in which 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, in which the applied electric field was 1.3 kV/cm, and Experimental Example 5, in which 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 a decrease in carbon fibers oriented in the plane direction where the deviation angle θ was 80° to 90°, and an increase in carbon fibers oriented in the thickness direction where the deviation angle θ was 80° or less. From this, it was found that, in the optical microscope image, as the applied electric field increased, the number of carbon fibers exposed in the longitudinal direction decreased and the number of carbon fibers appearing as dots increased. This was because the electric field used when applying the raw material powder to the current collector caused the carbon fibers to orient 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.

(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 degree of orientation and the sheet resistance in Experimental Examples 1 to 5. Table 1 also shows the electric field [kV/cm], electrode thickness [μm], electrode composite weight [mg/cm ² ], electrode density [g/cm ³ ], oriented fiber ratio [%], sheet resistance [Ωcm ² ], and volume resistivity [Ωcm] during electrostatic printing in Experimental Examples 1 to 5. As shown in FIGS. 7 and 8 and Table 1, in Experimental Examples 1 to 5, the degree of orientation was found to be improved as the electric field during electrostatic printing increased, i.e., the carbon fibers aligned along the electrode thickness direction. It was also found that a higher degree of orientation resulted in a further reduction in the 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 electrode thickness direction can form a path for efficient electron conduction in the electrode thickness direction, 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, and more preferably 2.0 kV/cm or more. It was also inferred that this electric field is preferably in the range of 10 kV/cm or less. It was also found that the 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, is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more. It was also inferred that this degree of orientation may be 100% or less, or even 90% or less.

It goes without saying that the present disclosure is in no way limited to the above-described examples, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure.

The electrodes, electricity storage devices, and electrode manufacturing methods disclosed in this specification can be used in the technical field of electricity storage devices such as secondary batteries.

10 Electricity 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 (9)

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. The electrode described in claim 1, wherein the degree of orientation is 60% or more. The electrode according to claim 1 or 2, wherein the electrode satisfies any one or more of (1) to (4).
(1) The sheet resistance is 32 Ωcm ² or less.
(2) The volume resistivity is 3.0×10 ³ Ωcm or less.
(3) The thickness is in the range of 10 μm or more and 500 μm or less.
(4) The electrode density is in the range of 2.0 (g/cm ³ ) to 3.5 (g/cm ³ ). The electrode described in claim 1 or 2, wherein the active material is a composite oxide containing lithium and a transition metal. An electricity storage device comprising the electrode described in claim 1 or 2. 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: The electrode manufacturing method described in claim 6, wherein no solvent is added to the raw material during the coating process. 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. 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.