Negative electrode for power storage device, and power storage device

Abstract

This negative electrode (22) for a power storage device comprises a negative electrode current collector (22a) having a first surface (22a1), and a negative electrode active material layer (22b) formed on a first surface (22a1) of the negative electrode current collector (22a). The basis weight of the negative electrode active material layer (22b) is 20 mg / cm2 or greater. The negative electrode active material layer (22b) contains graphite, an aqueous binder having a glass transition temperature of less than 7°C, and single-walled carbon nanotubes.

Description

Negative electrode for power storage device, and power storage device

[0001] The present invention relates to a negative electrode for an electricity storage device and to an electricity storage device.

[0002] Patent Document 1 discloses a flat-type energy storage device configured by stacking multiple energy storage cells in series. The energy storage cell includes a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes. The positive electrode has a positive electrode active material layer formed on one side of a foil-shaped positive electrode current collector. The negative electrode has a negative electrode active material layer formed on one side of a foil-shaped negative electrode current collector, and is disposed so that the negative electrode active material layer faces the positive electrode active material layer of the positive electrode. The energy storage device disclosed in Patent Document 1 is formed by stacking multiple such energy storage cells such that the positive electrode current collector and the negative electrode current collector are in contact with each other.

[0003] Patent Document 2 discloses a manufacturing method for continuously manufacturing electrodes using a roll. The electrode manufacturing method disclosed in Patent Document 2 includes a coating step and a drying step. The coating step is a step of coating an electrode mixture containing an active material, a solvent, etc. onto a current collector fed from a feed roll. The drying step is a step of drying the coating layer of the electrode mixture coated on the current collector after the coating step. The current collector after the drying step is wound up on a winding roll.

[0004] JP2017-16825A JP11-102696A

[0005] One method for increasing the energy density of a power storage cell is to increase the basis weight of the active material layer. However, when this configuration is applied to a negative electrode having an active material layer provided on the surface of a current collector, a problem occurs in that the active material layer is easily detached from the current collector during the manufacturing of a power storage device using the negative electrode.

[0006] As a result of intensive research by the present inventors, it was found that the cause of the detachment of the active material layer is the bending load applied to the active material layer during or after the production of the negative electrode. In other words, as the coating weight of the active material layer increases, the flexibility of the active material layer decreases. As a result, when a bending load is applied to the active material layer, the active material layer cannot withstand the bending load and cracks occur in the active material layer. Due to this cracking in the active material layer, the active material layer is likely to detach from the current collector during the production of the electricity storage device.

[0007] During or after the production of the negative electrode, a bending load is applied to the active material layer when, for example, the current collector passes through a guide roll for changing the transport direction, or when the current collector is wound around a winding roll, resulting in the current collector being bent along the outer periphery of the roll.

[0008] The negative electrode for an electricity storage device that solves the above-described problems includes a current collector having a first surface and a negative electrode active material layer formed on the first surface of the current collector, wherein the negative electrode active material layer has a basis weight of 20 mg / cm. ２ In the negative electrode for a power storage device described above, the negative electrode active material layer contains graphite, which is a negative electrode active material capable of absorbing and releasing charge carriers, a water-based binder having a glass transition temperature of less than 7°C, and single-walled carbon nanotubes.

[0009] The aqueous binder preferably contains a compound having an aromatic ring, and the compound having an aromatic ring is preferably a polymer containing styrene as a structural unit.

[0010] The compound having an aromatic ring is preferably styrene-butadiene rubber. The G / D ratio of the graphite is preferably 3 or more. The content of the aqueous binder in the negative electrode active material layer is preferably 2.0% by mass or more and 4.0% by mass or less, and the content of the single-walled carbon nanotubes in the negative electrode active material layer is preferably 0.001% by mass or more and 0.02% by mass or less.

[0011] The current collector preferably has a second surface facing the opposite side to the first surface, and is a bipolar electrode in which the negative electrode active material layer is formed on the first surface of the current collector and a positive electrode active material layer is formed on the second surface.

[0012] The positive electrode active material layer contains a positive electrode active material capable of absorbing and releasing charge carriers, and the positive electrode active material layer has a basis weight of 50 mg / cm ２ and the content of the positive electrode active material in the positive electrode active material layer is 97 mass % or more, and the positive electrode active material layer preferably contains a water-based binder having a glass transition temperature of less than 7°C and single-walled carbon nanotubes.

[0013] The energy storage device that solves the above problem is an energy storage device comprising: a positive electrode having a positive electrode active material layer provided on a first surface of a positive electrode current collector; a negative electrode having a negative electrode active material layer provided on a first surface of a negative electrode current collector, the negative electrode active material layer being disposed so as to face the positive electrode active material layer of the positive electrode; a separator disposed between the positive electrode active material layer and the negative electrode active material layer; and a spacer disposed between first surfaces of the positive electrode current collector and the negative electrode current collector and adhered to the first surfaces of the positive electrode current collector and the negative electrode current collector, wherein the negative electrode is a negative electrode for the above energy storage device.

[0014] According to the present invention, cracking of the negative electrode active material layer due to bending load can be suppressed.

[0015] 1 is a cross-sectional view of a negative electrode according to a first embodiment; FIG. 2 is a cross-sectional view of a power storage device; FIG. 3 is a cross-sectional view of a bipolar electrode according to a second embodiment;

[0016] An embodiment of the present invention will be described below with reference to the drawings. [First embodiment] <Negative electrode> The negative electrode of the first embodiment is used as an electrode of a power storage device. The power storage device is, for example, a secondary battery such as a nickel-metal hydride secondary battery or a lithium-ion secondary battery. The power storage device may also be an electric double layer capacitor. The following describes the case where the negative electrode is an electrode of a lithium-ion secondary battery.

[0017] 1 , the negative electrode 22 includes a negative electrode current collector 22 a having a first surface 22 a 1 and a negative electrode active material layer 22 b formed on the first surface 22 a 1 of the negative electrode current collector 22 a. In this embodiment, the negative electrode current collector 22 a corresponds to the current collector described in the claims.

[0018] [Negative Electrode Current Collector] The negative electrode current collector 22a is a chemically inactive electrical conductor that continues to pass current through the negative electrode active material layer 22b during discharge or charge of the lithium ion secondary battery. The thickness of the negative electrode current collector 22a is, for example, 4 μm or more, preferably 6 μm or more. The thickness of the negative electrode current collector 22a is, for example, 20 μm or less, preferably 10 μm or less.

[0019] An example of the negative electrode current collector 22a is a copper current collector whose surface serving as the first surface 22a1 is made of copper. The copper current collector may be a single substance entirely made of copper, or a composite having a portion made of copper and a portion made of a material other than copper. An example of the single substance is copper foil. An example of the composite is a multilayer structure in which the layer making up the first surface 22a1 is a copper layer, or a substrate whose surface including the first surface 22a1 is coated with a copper film.

[0020] The negative electrode current collector 22a may be formed of a material other than copper. Examples of materials other than copper include metal materials, conductive resin materials, and conductive inorganic materials. Examples of the metal materials include aluminum, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc., as defined in JIS G 4305:2015). Examples of the conductive resin materials include resins obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as needed.

[0021] [Negative Electrode Active Material Layer] The negative electrode active material layer 22b is provided on the first surface 22a1 of the negative electrode current collector 22a. The coating weight of the negative electrode active material layer 22b is 20 mg / cm ２ or more, preferably 30 mg / cm ２ The weight per unit area is 20 mg / cm ２By doing so, it is possible to effectively increase the energy density of a lithium ion secondary battery using the negative electrode 22. Furthermore, the increase in the energy density of the lithium ion secondary battery improves battery characteristics such as battery capacity and long-term output.

[0022] The weight of the negative electrode active material layer 22b is, for example, 50 mg / cm ２ or less, preferably 40 mg / cm ２ The basis weight is 50 mg / cm or less. ２ When the negative electrode active material layer 22b has a thickness of 20 mg / cm or less, the thickness of the negative electrode active material layer 22b is reduced, thereby enabling a reduction in the height of a lithium ion secondary battery using the negative electrode 22. ２ That's all.

[0023] The thickness of the negative electrode active material layer 22b is, for example, 125 μm or more and 556 μm or less. The density of the negative electrode active material layer 22b is 0.9 g / cm ３ 1.6g / cm or more ３ The negative electrode active material layer 22b may be one that has been subjected to a press treatment. The thickness of the pressed negative electrode active material layer 22b is, for example, 125 μm or more, and preferably 187 μm or more. The thickness of the pressed negative electrode active material layer 22b is, for example, 500 μm or less, and preferably 400 μm or less. The density of the pressed negative electrode active material layer 22b is, for example, 1.0 g / cm ３ 1.6g / cm or more ３ The following is the result.

[0024] The negative electrode active material layer 22b contains a negative electrode active material, a specific negative electrode binder, and single-walled carbon nanotubes. (Negative Electrode Active Material) The negative electrode active material is capable of absorbing and releasing charge carriers such as lithium ions. The negative electrode active material is graphite. Examples of graphite include natural graphite, artificial graphite, hard carbon (non-graphitizable carbon), and soft carbon (easily graphitizable carbon). Examples of artificial graphite include highly oriented graphite and mesocarbon microbeads. One type of graphite may be used alone, or multiple types may be used in combination.

[0025] The graphite preferably contains a large amount of graphite structure. For example, the G / D ratio of the graphite determined by Raman spectroscopy is 2 or more, preferably 3 or more. The G / D ratio of the graphite is, for example, 5 or less, preferably 4 or less. The G / D ratio is determined by Raman spectroscopy using an argon ion laser beam at a G / D ratio of 1570 to 1620 cm －１ The intensity of the peaks present in the range of IA, 1350 to 1370 cm －１ When the intensity of the peak present in the range is defined as IB, the ratio IB / IA is expressed. When graphite containing a large amount of graphite structure is used, π-π interactions occur between the single-walled carbon nanotubes in the negative electrode active material layer 103. This allows the specific negative electrode binder and the single-walled carbon nanotubes to work together when a bending load is applied to the negative electrode active material layer 103.

[0026] The average particle diameter (D50) of the graphite is, for example, 10 μm or more and 20 μm or less. The average particle diameter (D50) of the graphite can be measured using a laser diffraction particle size analyzer. The content of the negative electrode active material in the negative electrode active material layer 22b is, for example, 90 mass% or more, preferably 95 mass% or more, and more preferably 96.5 mass% or more. The content of the negative electrode active material in the negative electrode active material layer 103 is, for example, 98 mass% or less, preferably 97.5 mass% or less. By making the content of the negative electrode active material 90 mass% or more, the energy density of a lithium ion secondary battery using the negative electrode 22 can be effectively increased. Furthermore, the increase in the energy density of the lithium ion secondary battery improves battery characteristics such as battery capacity and long-term output.

[0027] (Specific Negative Electrode Binder) The specific negative electrode binder is an aqueous binder having a glass transition temperature of less than 7° C. The aqueous binder is a binder that is dispersed or dissolved in an aqueous solvent and mixed with the negative electrode active material before use.

[0028] The glass transition temperature of the specific negative electrode binder is lower than 7° C., preferably equal to or lower than 0° C., and more preferably equal to or lower than −5° C. When the specific negative electrode binder has a glass transition temperature of lower than 7° C., the flexibility of the negative electrode active material layer 103 is improved.

[0029] The glass transition temperature is the glass transition onset temperature in a DSC curve measured using a differential scanning calorimeter (DSC) at a heating rate of 10°C / min. The DSC curve is measured using a method in accordance with JIS K 7121 (1987), except that the heating rate is 10°C / min. The glass transition onset temperature is the temperature at which the temperature begins to change from the low-temperature baseline in the DSC curve.

[0030] Examples of the specific negative electrode binder include various resin materials, for example, fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, acrylic resins, polymers containing styrene as a constituent unit, carboxymethyl cellulose, alginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester crosslinked bodies, and starch-acrylic acid graft polymers.

[0031] Examples of the acrylic resin include polyacrylic acid, polymethacrylic acid, polyacrylic acid ester, polymethacrylic acid ester, copolymers containing acrylic acid as a constituent unit, and polyacrylamide. The acrylic resin is preferably an acrylic resin other than polyacrylic acid.

[0032] Examples of polymers containing styrene as a constituent unit include styrene-butadiene rubber and styrene-acrylic rubber. The specific negative electrode binder contained in the negative electrode active material layer 103 may be one type or two or more types.

[0033] The glass transition temperature of a specific negative electrode binder can be adjusted by changing the degree of polymerization of the resin material and the ratio of the constituent units that make up the resin material. For example, when the negative electrode binder is styrene-butadiene rubber, the glass transition temperature of the negative electrode binder can be adjusted by changing the ratio of styrene and butadiene, which are the constituent units of styrene-butadiene rubber. Specifically, the glass transition temperature of the negative electrode binder can be lowered by increasing the ratio of butadiene to styrene. In other words, the glass transition temperature of the negative electrode binder can be lowered by decreasing the ratio of styrene to butadiene.

[0034] The specific negative electrode binder is preferably a compound having an aromatic ring. In this case, within the negative electrode active material layer 22b, π-π interactions occur between the aromatic ring of the specific negative electrode binder and the single-walled carbon nanotubes, and between the aromatic ring of the specific negative electrode binder and the graphite. This allows the specific negative electrode binder and the single-walled carbon nanotubes to work together, and the specific negative electrode binder and the graphite to work together, when a bending load is applied to the negative electrode active material layer 103. Examples of compounds having an aromatic ring include polymers containing styrene as a structural unit, such as styrene-butadiene rubber.

[0035] The content of the specific anode binder in the anode active material layer 22b is, for example, 2.0% by mass or more, preferably 2.3% by mass or more. Increasing the content of the specific anode binder can improve the binding property within the anode active material layer 22b. The content of the specific anode binder in the anode active material layer 22b is, for example, 4.0% by mass or less, preferably 3.0% by mass or less. The specific anode binder is a component within the anode active material layer 22b that does not contribute to charge and discharge. Reducing the content of the component that does not contribute to charge and discharge can increase the energy density of the lithium-ion secondary battery. Furthermore, increasing the energy density of the lithium-ion secondary battery improves battery characteristics such as battery capacity and long-term output.

[0036] (Single-walled carbon nanotubes) Carbon nanotubes can be broadly divided into two types: single-walled carbon nanotubes and multi-walled carbon nanotubes. Single-walled carbon nanotubes are cylindrical bodies formed by a single graphene sheet rolled up seamlessly. In contrast, multi-walled carbon nanotubes are composites in which multiple single-walled carbon nanotubes of different diameters are contained within a single single-walled carbon nanotube. Therefore, single-walled carbon nanotubes are more flexible than multi-walled carbon nanotubes.

[0037] The negative electrode active material layer 22b contains single-walled carbon nanotubes. In the negative electrode active material layer 22b, the single-walled carbon nanotubes may be present in a bundle of, for example, several tens of single-walled carbon nanotubes.

[0038] The fiber length and fiber diameter of the single-walled carbon nanotubes contained in the negative electrode active material layer 22b are not particularly limited. The fiber length of the single-walled carbon nanotubes is, for example, 5 μm or more and 1000 μm or less. The fiber diameter of the carbon nanotubes is, for example, 1 nm or more and 20 nm or less. The fiber length and fiber diameter of the single-walled carbon nanotubes can be measured using an electron microscope such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0039] The content of single-walled carbon nanotubes in the negative electrode active material layer 103 is, for example, 0.001% by mass or more, and preferably 0.005% by mass or more. The content of single-walled carbon nanotubes in the negative electrode active material layer 103 is, for example, 0.02% by mass or less, and preferably 0.015% by mass or less. Furthermore, the mass ratio of the specific negative electrode binder to the single-walled carbon nanotubes in the negative electrode active material layer 103 (specific negative electrode binder / single-walled carbon nanotubes) is, for example, 100 or more and 4000 or less.

[0040] (Other Components) The negative electrode active material layer 22b may contain other components other than the negative electrode active material, the specific negative electrode binder, and the single-walled carbon nanotubes as needed. Examples of other components include other binders, dispersants, conductive additives, electrolytes (polymer matrices, ion-conductive polymers, electrolyte solutions, etc.), and electrolyte supporting salts (lithium salts) for increasing ion conductivity. The types and contents of the other components are not particularly limited, and conventionally known knowledge about lithium-ion secondary batteries may be referenced as appropriate.

[0041] Examples of the dispersant include sodium salt of carboxymethylcellulose and ammonium salt of carboxymethylcellulose. The content of the dispersant in the negative electrode active material layer 22b is, for example, 1.0% by mass or less, preferably 0.5% by mass or less. The content of the dispersant in the negative electrode active material layer 22b is, for example, 0.3% by mass or more.

[0042] Examples of the conductive additive include acetylene black, carbon black, and graphite. The content of the conductive additive in the negative electrode active material layer 22b is, for example, an amount such that the total amount of the conductive additive and the single-walled carbon nanotubes is 0.5% by mass or less, and preferably 0.1% by mass or less.

[0043] <Method for Manufacturing Negative Electrode> Next, a description will be given of a method for manufacturing the negative electrode 22. The method for manufacturing the negative electrode 22 includes a coating step of coating the negative electrode current collector 22 a with a negative electrode composite material, and a drying step of drying the negative electrode composite material coated on the negative electrode current collector 22 a.

[0044] The negative electrode mixture is a mixture that becomes the negative electrode active material layer 22b upon solidification. The negative electrode mixture is, for example, a slurry. The negative electrode mixture contains a negative electrode active material, a specific negative electrode binder, single-walled carbon nanotubes, and an aqueous solvent, and may contain other components as necessary. The negative electrode active material, the specific negative electrode binder, the single-walled carbon nanotubes, and other components contained in the negative electrode mixture are the same as those described in the negative electrode active material layer section above.

[0045] The aqueous solvent is water or a mixed solvent of water and an organic solvent. The aqueous solvent is preferably a solvent in which the mass proportion of water is 50 to 100 mass%. The aqueous solvent is blended into the negative electrode composite so that the solids proportion of the negative electrode composite is 50 mass% or more and 65 mass% or less.

[0046] The coating step is a step of forming a coating layer of the negative electrode composite by applying the negative electrode composite to the first surface 22a1 of the negative electrode current collector 22a. Examples of methods for applying the negative electrode composite include a roll method, a die coating method, a reverse roll method, a doctor blade method, a knife method, a gravure method, a dipping method, and a squeeze method. The negative electrode composite may be applied continuously or intermittently to the first surface 22a1 of the negative electrode current collector 22a. The thickness of the coating layer of the negative electrode composite is such that the weight of the finally obtained negative electrode active material layer 22b is 20 mg / cm. ２ The thickness is adjusted to the above value. The length and width of the coating layer of the negative electrode composite are set appropriately depending on the size of the lithium ion secondary battery.

[0047] The drying step is a step of removing the aqueous solvent and solidifying the coating layer by drying the coating layer formed on the negative electrode current collector 22a. The coating layer solidified through the drying step becomes the negative electrode active material layer 22b. Examples of methods for drying the coating layer include natural drying, low-temperature air, hot air, vacuum, infrared rays, far-infrared rays, electron beams, and microwaves. Two or more of these drying methods may be combined. The drying temperature is, for example, 20°C or higher and 120°C or lower, and preferably 40°C or higher and 100°C or lower.

[0048] Furthermore, in order to increase the electrode density, a compression step of compressing the negative electrode active material layer 22b may be performed after the drying step. This compression step corresponds to the above-mentioned pressing process. Examples of methods for compressing the negative electrode active material layer 22b include a die pressing method and a calendar pressing method. The pressing pressure is, for example, 0.1 t / cm. ２ More than 10t / cm ２ or less, preferably 0.5 t / cm ２ 5.0t / cm or more ２ The following is the result.

[0049] Furthermore, when a compression step is performed, the thickness of the negative electrode active material layer 22b subjected to the compression step, i.e., the thickness of the negative electrode active material layer 22b before the pressing process, is, for example, 167 μm or more, and preferably 250 μm or more. The thickness of the negative electrode active material layer 22b before the pressing process is, for example, 556 μm or less, and preferably 444 μm or less. The density of the negative electrode active material layer 22b before the pressing process is, for example, 0.9 g / cm ３ 1.2g / cm or more ３ The following is the result.

[0050] A winding step of winding the negative electrode current collector 22 a into a roll may be performed between each of the above steps or at least once after all of the steps have been performed. In addition, a drying step may be performed again after the compression step.

[0051] Here, the method for producing the negative electrode includes a bending step of bending the negative electrode current collector 22 a on which the negative electrode active material layer 22 b is formed after the drying step. The curved state means that at least a portion of the negative electrode active material layer 22 b is curved to a specific radius of curvature or less, for example, 26 mm or less, or 25.5 mm or less.

[0052] Examples of the bending process include a winding process in which the negative electrode current collector 22a on which the negative electrode active material layer 22b is formed is wound around a core material, and a bending process in which the negative electrode current collector 22a is curved along the circumferential surface of a cylindrical body. An example of the winding process is the winding process described above. An example of the bending process is a process in which the negative electrode current collector 22a on which the negative electrode active material layer 22b is formed is conveyed by rollers, and the negative electrode current collector 22a is curved along the circumferential surface of a cylindrical body such as a guide roller or a tension roller. The bending process may be performed once or multiple times.

[0053] <Electricity Storage Device> Next, a description will be given of an example of an electricity storage device to which the negative electrode 22 is applied. The electricity storage device to which the negative electrode 22 is applied is, for example, a lithium ion secondary battery used in batteries for various vehicles such as forklifts, hybrid vehicles, and electric vehicles.

[0054] 2 , the energy storage device 10 includes a cell stack 30 (laminate) in which a plurality of energy storage cells 20 are stacked in a stacking direction. Hereinafter, the stacking direction of the energy storage cells 20 will be simply referred to as the stacking direction. Each energy storage cell 20 includes a positive electrode 21, a negative electrode 22, a separator 23, and a spacer 24.

[0055] The positive electrode 21 includes a positive electrode current collector 21a and a positive electrode active material layer 21b provided on a first surface 21a1 of the positive electrode current collector 21a. In a plan view, the positive electrode active material layer 21b is formed in the center of the first surface 21a1 of the positive electrode current collector 21a. In a plan view, the peripheral portion of the first surface 21a1 of the positive electrode current collector 21a is a positive electrode uncoated portion 21c where the positive electrode active material layer 21b is not provided. In a plan view, the positive electrode uncoated portion 21c is arranged to surround the periphery of the positive electrode active material layer 21b.

[0056] The negative electrode 22 includes a negative electrode current collector 22a and a negative electrode active material layer 22b provided on a first surface 22a1 of the negative electrode current collector 22a. In a plan view, the negative electrode active material layer 22b is formed in the center of the first surface 22a1 of the negative electrode current collector 22a. In a plan view, the peripheral portion of the first surface 22a1 of the negative electrode current collector 22a is a negative electrode uncoated portion 22c where the negative electrode active material layer 22b is not provided. In a plan view, the negative electrode uncoated portion 22c is arranged to surround the periphery of the negative electrode active material layer 22b.

[0057] The positive electrode 21 and the negative electrode 22 are arranged such that the positive electrode active material layer 21b and the negative electrode active material layer 22b face each other in the stacking direction. That is, the opposing direction of the positive electrode 21 and the negative electrode 22 coincides with the stacking direction. The negative electrode active material layer 22b is formed to be the same size as the positive electrode active material layer 21b or is formed to be slightly larger than the positive electrode active material layer 21b. When the negative electrode active material layer 22b is formed to be slightly larger than the positive electrode active material layer 21b, the entire formation region of the positive electrode active material layer 21b is located within the formation region of the negative electrode active material layer 22b in a planar view.

[0058] The positive electrode current collector 21a has a second surface 21a2 opposite to the first surface 21a1. The positive electrode 21 is a monopolar electrode in which neither the positive electrode active material layer 21b nor the negative electrode active material layer 22b is formed on the second surface 21a2 of the positive electrode current collector 21a. The negative electrode current collector 22a has a second surface 22a2 opposite to the first surface 22a1. The negative electrode 22 is a monopolar electrode in which neither the positive electrode active material layer 21b nor the negative electrode active material layer 22b is formed on the second surface 22a2 of the negative electrode current collector 22a.

[0059] The separator 23 is disposed between the positive electrode 21 and the negative electrode 22, and is a member that separates the positive electrode 21 and the negative electrode 22 to prevent short circuits due to contact between the two electrodes, while allowing lithium ions to pass through.

[0060] The separator 23 is, for example, a porous sheet or nonwoven fabric containing a polymer that absorbs and retains electrolyte. Examples of materials that make up the separator 23 include polyolefins such as polypropylene and polyethylene, and polyesters. The separator 23 may have a single-layer structure or a multi-layer structure. The multi-layer structure may include, for example, an adhesive layer, a ceramic layer as a heat-resistant layer, or the like.

[0061] The spacer 24 is disposed between the first surface 21a1 of the positive electrode current collector 21a of the positive electrode 21 and the first surface 22a1 of the negative electrode current collector 22a of the negative electrode 22, and on the outer circumferential side of the positive electrode active material layer 21b and the negative electrode active material layer 22b, and is adhered to both the positive electrode current collector 21a and the negative electrode current collector 22a. The spacer 24 maintains a gap between the positive electrode current collector 21a and the negative electrode current collector 22a to prevent short circuits between the current collectors and to provide a liquid-tight seal between the current collectors.

[0062] The spacer 24 extends along the peripheral edges of the positive electrode current collector 21 a and the negative electrode current collector 22 a in a plan view and is formed in a frame shape surrounding the positive electrode current collector 21 a and the negative electrode current collector 22 a. The spacer 24 is disposed between the positive electrode uncoated portion 21 c on the first surface 21 a 1 of the positive electrode current collector 21 a and the negative electrode uncoated portion 22 c on the first surface 22 a 1 of the negative electrode current collector 22 a.

[0063] Examples of materials that can be used to form the spacer 24 include various resin materials such as polyethylene (PE), modified polyethylene (modified PE), polystyrene (PS), polypropylene (PP), modified polypropylene (modified PP), ABS resin, and AS resin.

[0064] An enclosed space S is formed inside the energy storage cell 20 and is surrounded by a frame-shaped spacer 24, the positive electrode 21, and the negative electrode 22. A separator 23 and an electrolyte are housed in the enclosed space S. The peripheral portion of the separator 23 is embedded in the spacer 24.

[0065] The electrolyte is, for example, a liquid electrolyte. Examples of the liquid electrolyte include a liquid electrolyte containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the electrolyte salt include LiClO ４ , LiAsF ６ , LiPF ６ , LiBF ４ , LiCF ３ SO ３ , LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ Known lithium salts such as those listed above can be used. Furthermore, known solvents such as cyclic carbonates, cyclic esters, chain carbonates, chain esters, and ethers can be used as the non-aqueous solvent. These known solvent materials may be used in combination of two or more.

[0066] The spacer 24 can prevent the electrolyte contained in the sealed space S from leaking to the outside by sealing the sealed space S between the positive electrode 21 and the negative electrode 22. The spacer 24 can also prevent moisture from entering the sealed space S from the outside of the energy storage device 10. Furthermore, the spacer 24 can prevent gas generated from the positive electrode 21 or the negative electrode 22 due to, for example, a charge / discharge reaction from leaking to the outside of the energy storage device 10.

[0067] The cell stack 30 has a structure in which a plurality of storage cells 20 are stacked so that the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a are in contact with each other, thereby connecting the plurality of storage cells 20 that make up the cell stack 30 in series.

[0068] In the cell stack 30, two adjacent energy storage cells 20 in the stacking direction form a pseudo bipolar electrode 25 in which the mutually contacting positive electrode current collector 21 a and negative electrode current collector 22 a are regarded as a single current collector. The pseudo bipolar electrode 25 includes a current collector having a structure in which the positive electrode current collector 21 a and the negative electrode current collector 22 a are stacked, a positive electrode active material layer 21 b formed on one surface of the current collector, and a negative electrode active material layer 22 b formed on the other surface.

[0069] The energy storage device 10 includes a pair of current-carrying bodies, consisting of a positive electrode current-carrying plate 40 and a negative electrode current-carrying plate 50, which are arranged to sandwich the cell stack 30 in the stacking direction of the cell stack 30. The positive electrode current-carrying plate 40 and the negative electrode current-carrying plate 50 are each made of a material with excellent conductivity.

[0070] The positive electrode current-carrying plate 40 is electrically connected to the second surface 21a2 of the positive electrode current collector 21a of the positive electrode 21 arranged outermost at one end in the stacking direction. The negative electrode current-carrying plate 50 is electrically connected to the second surface 22a2 of the negative electrode current collector 22a of the negative electrode 22 arranged outermost at the other end in the stacking direction. Charging and discharging of the energy storage device 10 are performed through terminals provided on the positive electrode current-carrying plate 40 and the negative electrode current-carrying plate 50, respectively.

[0071] Next, the operation and effect of the first embodiment will be described. (1-1) The negative electrode 22 for the power storage device includes a negative electrode current collector 22a having a first surface 22a1, and a negative electrode active material layer 22b formed on the first surface 22a1 of the negative electrode current collector 22a. The coating weight of the negative electrode active material layer 22b is 20 mg / cm. ２ The negative electrode active material layer 22b contains graphite, a water-based binder (specific negative electrode binder) having a glass transition temperature of less than 7° C., and single-walled carbon nanotubes.

[0072] According to the above configuration, the negative electrode active material layer 22b has high flexibility due to the cooperation of the aqueous binder contained in the negative electrode active material layer 22b, which has a glass transition temperature of less than 7°C, and the single-walled carbon nanotubes. This makes it possible to suppress cracking of the negative electrode active material layer 22b that occurs when a bending load is applied to the negative electrode active material layer 22b during the bending step or the like in the manufacturing stage of the negative electrode 22. As a result, it is possible to suppress detachment of the negative electrode active material layer 22b from the negative electrode current collector 22a when a process that applies a load to the negative electrode 22 is performed during the manufacturing of an electricity storage device using the negative electrode 22.

[0073] (1-2) The aqueous binder (specific positive electrode binder) contains a compound having an aromatic ring. In this case, π-π interactions occur between the aromatic ring of the aqueous binder and the single-walled carbon nanotubes, making the aqueous binder and the single-walled carbon nanotubes more likely to move together. This results in a negative electrode active material layer 22b with higher flexibility.

[0074] (1-3) The energy storage device 10 includes a positive electrode 21, a negative electrode 22, a separator 23, and a spacer 24. The positive electrode 21 has a positive electrode active material layer 21b provided on a first surface 21a1 of a positive electrode current collector 21a. The negative electrode 22 has a negative electrode active material layer 22b provided on a first surface 22a1 of a negative electrode current collector 22a, and is disposed such that the negative electrode active material layer 22b faces the positive electrode active material layer 21b of the positive electrode 21. The separator 23 is disposed between the positive electrode active material layer 21b and the negative electrode active material layer 22b. The spacer 24 is disposed between the first surfaces 21a1, 22a1 of the positive electrode current collector 21a and the negative electrode current collector 22a, and is bonded to the first surfaces 21a1, 22a1 of the positive electrode current collector 21a and the negative electrode current collector 22a.

[0075] When manufacturing an electricity storage device having the above configuration, it is necessary to perform a process of stacking the positive electrode 21 and the negative electrode 22 via the separator 23 and the spacer 24, and a process of bonding the positive electrode 21 and the spacer 24. Therefore, compared to the case of manufacturing a general prismatic battery, a laminated battery, or a cylindrical battery, a load is applied to the negative electrode active material layer 22b of the negative electrode 22 multiple times, which makes it more likely that the negative electrode active material layer 22b will detach from the negative electrode current collector 22a during manufacturing. Therefore, when the negative electrode 22 of this embodiment is applied to the electricity storage device 10 having the above configuration, the effect of suppressing detachment of the negative electrode active material layer 22b from the negative electrode current collector 22a can be more significantly obtained.

[0076] Second Embodiment Next, a second embodiment embodied in a bipolar electrode will be described. The bipolar electrode can be used, for example, in place of the pseudo bipolar electrode 25 in the power storage device 10 described above.

[0077] 3, the bipolar electrode 100 includes a bipolar current collector 101, a positive electrode active material layer 102, and a negative electrode active material layer 103. In this embodiment, the bipolar current collector 101 corresponds to the current collector described in the claims.

[0078] The bipolar current collector 101 is a chemically inactive electrical conductor that continues to pass current through the positive electrode active material layer 102 and the negative electrode active material layer 103 during discharging or charging of the power storage device. The bipolar current collector 101 has a shape that can be bent and deformed into a curved state, such as a foil shape. The bipolar current collector 101 has a first surface 101b and a second surface 101a facing the opposite side to the first surface 101b. The first surface 101b and the second surface 101a are surfaces that are perpendicular to the thickness direction of the bipolar current collector 101.

[0079] The bipolar current collector 101 is a laminate formed by integrally joining a positive electrode current collector 104 and a negative electrode current collector 105 in the thickness direction. The second surface 101a of the bipolar current collector 101 is formed by the positive electrode current collector 104. The first surface 101b of the bipolar current collector 101 is formed by the negative electrode current collector 105. The positive electrode active material layer 102 is provided on the second surface 101a of the bipolar current collector 101. The negative electrode active material layer 103 is provided on the first surface 101b of the bipolar current collector 101.

[0080] [Negative Electrode Current Collector and Negative Electrode Active Material Layer] The negative electrode current collector 105 is a chemically inactive electrical conductor for continuously flowing current to the negative electrode active material layer 103 during discharging or charging of the lithium ion secondary battery. Each component of the negative electrode current collector 105 is similar to that of the negative electrode current collector 22a of the first embodiment. Each component of the negative electrode active material layer 103 is similar to that of the negative electrode active material layer 22b of the first embodiment.

[0081] [Positive Electrode Current Collector] The positive electrode current collector 104 is a chemically inactive electrical conductor that continues to pass current through the positive electrode active material layer 102 during discharge or charge of the lithium ion secondary battery. The thickness of the positive electrode current collector 104 is, for example, 5 μm or more, and preferably 10 μm or more. The thickness of the positive electrode current collector 21 a is, for example, 100 μm or less, and preferably 60 μm or less.

[0082] An example of the positive electrode current collector 104 is an aluminum current collector whose surface, which becomes the second surface 101a, is made of aluminum. The aluminum current collector may be a simple substance made entirely of aluminum, or a composite having a portion made of aluminum and a portion made of a material other than aluminum. An example of the simple substance is aluminum foil. An example of the composite is a multilayer structure in which the layer making up the second surface 101a is an aluminum layer, or a substrate whose surface, including the second surface 101a, is coated with an aluminum film.

[0083] The positive electrode current collector 104 may be formed of a material other than aluminum. Examples of materials other than aluminum include metal materials, conductive resin materials, and conductive inorganic materials. Examples of the metal materials include copper, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc., as defined in JIS G 4305:2015). Examples of the conductive resin materials include resins obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as needed.

[0084] The second surface 101 a of the positive electrode current collector 104 may be coated with a known protective layer such as a carbon coating layer. The second surface 101 a of the positive electrode current collector 104 may be treated by a known method such as plating.

[0085] An example of a preferred combination of the positive electrode current collector 104 and the negative electrode current collector 105 that constitute the bipolar current collector 101 is a case in which the positive electrode current collector 104 is made of aluminum foil and the negative electrode current collector 105 is made of copper foil. Examples of the bipolar current collector 101 include a current collector in which aluminum foils are bonded together, a current collector in which aluminum foil and copper foil are bonded together, and a current collector in which the surface of aluminum foil is plated with copper.

[0086] [Positive Electrode Active Material Layer] The weight of the positive electrode active material layer 102 is, for example, 50 mg / cm ２ and preferably greater than 60 mg / cm ２ More preferably, 70 mg / cm ２ The above basis weight is 50 mg / cm ２ By making the bipolar electrode 100 exceeding 100 kJ / cm 2 , it is possible to effectively increase the energy density of a lithium ion secondary battery using the bipolar electrode 100. Furthermore, the increase in the energy density of the lithium ion secondary battery improves battery characteristics such as battery capacity and long-term output.

[0087] The weight of the positive electrode active material layer 102 is, for example, 90 mg / cm ２ or less, preferably 80 mg / cm ２ The basis weight is 90 mg / cm or less.２ When the thickness of the positive electrode active material layer 102 is set to 50 mg / cm or less, the thickness of the positive electrode active material layer 102 is reduced, thereby enabling a reduction in the height of the lithium ion secondary battery using the bipolar electrode 100. ２ That's all.

[0088] The thickness of the positive electrode active material layer 102 is, for example, 200 μm or more and 600 μm or less. The density of the positive electrode active material layer 102 is, for example, 1.5 g / cm ３ 2.5g / cm or more ３ The positive electrode active material layer 102 may be one that has been subjected to a press treatment. The thickness of the pressed positive electrode active material layer 102 is, for example, 200 μm or more, and preferably 240 μm or more. The thickness of the pressed positive electrode active material layer 102 is, for example, 530 μm or less, and preferably 470 μm or less. The density of the pressed positive electrode active material layer 102 is, for example, 1.7 g / cm ３ 2.5g / cm or more ３ The following is the result.

[0089] The positive electrode active material layer 102 contains a positive electrode active material, a specific positive electrode binder, and single-walled carbon nanotubes. (Positive Electrode Active Material) The positive electrode active material is capable of absorbing and releasing charge carriers such as lithium ions. The positive electrode active material is not particularly limited, and any material that can be used as a positive electrode active material for lithium ion secondary batteries is employed. Examples of positive electrode active materials include polyanionic compounds having an olivine structure, lithium composite metal oxides having a layered rock salt structure, and metal oxides having a spinel structure. One type of positive electrode active material may be used alone, or multiple types may be used in combination.

[0090] The positive electrode active material is preferably a polyanionic compound having an olivine structure (hereinafter referred to as an olivine active material). Examples of the olivine active material include a compound represented by the general formula LiM ｈ P.O. ４ Examples of compounds represented by the general formula: LiM ｈ P.O. ４In the formula, M is at least one element selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, and Mo, and h is a value that satisfies 0<h<2. Specific examples of olivine-type active materials include olivine-type lithium iron phosphate (LiFePO ４ ) and olivine-type lithium manganese iron phosphate (LiMnFePO ４ ) are listed.

[0091] The particle shape of the positive electrode active material is not particularly limited, but is preferably a granule formed by agglomeration of primary particles of the positive electrode active material. The use of granules increases the flexibility of the positive electrode active material layer 21b. The particle diameter of the primary particles constituting the granules is, for example, 50 nm to 500 nm. The particle diameter of the primary particles is determined by small angle X-ray scattering (SAXS). Specifically, the particle diameter of the primary particles is a value defined as the average particle diameter (D50) obtained from the particle diameter distribution of the primary particles, assuming that the primary particles are spherical. The average particle diameter (D50) of the granules is, for example, 5 μm to 15 μm. The average particle diameter (D50) of the granules can be measured using a laser diffraction particle size analyzer.

[0092] The positive electrode active material is preferably a particle having a carbon coating on its surface. In this case, hydrophobic interaction occurs between the particle surface of the positive electrode active material and the single-walled carbon nanotubes in the positive electrode active material layer 102. This allows the positive electrode active material and the single-walled carbon nanotubes to work together when a bending load is applied to the positive electrode active material layer 102. A preferred example of a particle of positive electrode active material having a carbon coating is a particle having a core composed of a granulated body of an olivine-type active material and a carbon coating formed on the surface of the core.

[0093] The content of the positive electrode active material in the positive electrode active material layer 102 is, for example, 97% by mass or more, preferably 98% by mass or more, and more preferably 98.5% by mass or more. The content of the positive electrode active material in the positive electrode active material layer 102 is, for example, 99% by mass or less, and preferably 98.8% by mass or less. By making the content of the positive electrode active material 97% by mass or more, it is possible to effectively increase the energy density of a lithium ion secondary battery using the bipolar electrode 100. Furthermore, the increase in the energy density of the lithium ion secondary battery improves battery characteristics such as battery capacity and long-term output.

[0094] (Specific Positive Electrode Binder) The specific positive electrode binder is an aqueous binder having a glass transition temperature of less than 7° C. The aqueous binder is a binder that is dispersed or dissolved in an aqueous solvent and mixed with a positive electrode active material before use.

[0095] The glass transition temperature of the specific positive electrode binder is less than 7° C., preferably 0° C. or less, and more preferably −5° C. or less. When the glass transition temperature of the specific positive electrode binder is less than 7° C., the flexibility of the positive electrode active material layer 102 is improved. The glass transition temperature of the specific positive electrode binder is, for example, −50° C. or more.

[0096] Examples of specific positive electrode binders include various resin materials, for example, fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, acrylic resins, polymers containing styrene as a constituent unit, carboxymethyl cellulose, alginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester crosslinks, and starch-acrylic acid graft polymers.

[0097] Examples of the acrylic resin include polyacrylic acid, polymethacrylic acid, polyacrylic acid ester, polymethacrylic acid ester, copolymers containing acrylic acid as a constituent unit, and polyacrylamide. The acrylic resin is preferably an acrylic resin other than polyacrylic acid.

[0098] Examples of polymers containing styrene as a constituent unit include styrene-butadiene rubber and styrene-acrylic rubber. The specific positive electrode binder contained in the positive electrode active material layer 21b may be one type or two or more types.

[0099] The glass transition temperature of a specific positive electrode binder can be adjusted by changing the degree of polymerization of the resin material and the ratio of the constituent units that make up the resin material. For example, if the positive electrode binder is styrene-butadiene rubber, the glass transition temperature of the positive electrode binder can be adjusted by changing the ratio of styrene and butadiene, which are the constituent units of styrene-butadiene rubber. Specifically, the glass transition temperature of the positive electrode binder can be lowered by increasing the ratio of butadiene to styrene. In other words, the glass transition temperature of the positive electrode binder can be lowered by decreasing the ratio of styrene to butadiene.

[0100] The specific positive electrode binder is preferably a compound having an aromatic ring. In this case, π-π interactions occur between the aromatic ring of the specific positive electrode binder and the single-walled carbon nanotubes in the positive electrode active material layer 102. This allows the specific positive electrode binder and the single-walled carbon nanotubes to work together when a bending load is applied to the positive electrode active material layer 102. Examples of compounds having an aromatic ring include polymers containing styrene as a structural unit, such as styrene-butadiene rubber.

[0101] The content of the specific positive electrode binder in the positive electrode active material layer 102 is, for example, 0.8 mass % or more, preferably 0.9 mass % or more, and more preferably 1.0 mass % or more. By increasing the content of the specific positive electrode binder, the binding strength within the positive electrode active material layer 102 can be improved.

[0102] The content of the specific positive electrode binder in the positive electrode active material layer 102 is, for example, 2.0 mass % or less, preferably 1.5 mass % or less, and more preferably 1.3 mass % or less. The specific positive electrode binder is a component in the positive electrode active material layer 102 that does not contribute to charge and discharge. By reducing the content of the component that does not contribute to charge and discharge, the energy density of the lithium ion secondary battery can be increased. Furthermore, by increasing the energy density of the lithium ion secondary battery, battery characteristics such as battery capacity and long-term output can be improved.

[0103] (Single-Walled Carbon Nanotubes) In the positive electrode active material layer 102, the single-walled carbon nanotubes may be present in a state of being bundled together, for example, in a number of tens of pieces.

[0104] The fiber length and fiber diameter of the single-walled carbon nanotubes contained in the positive electrode active material layer 102 are not particularly limited. The fiber length of the single-walled carbon nanotubes is, for example, 5 μm or more and 1000 μm or less. The fiber diameter of the carbon nanotubes is, for example, 1 nm or more and 10 nm or less. The fiber length and fiber diameter of the single-walled carbon nanotubes can be measured using an electron microscope such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0105] The content of single-walled carbon nanotubes in the positive electrode active material layer 102 is, for example, 0.01% by mass or more, and preferably 0.03% by mass or more. The content of single-walled carbon nanotubes in the positive electrode active material layer 102 is, for example, 0.1% by mass or less, and preferably 0.07% by mass or less. Furthermore, the mass ratio of the specific positive electrode binder to the single-walled carbon nanotubes in the positive electrode active material layer 102 (specific positive electrode binder / single-walled carbon nanotubes) is, for example, 8 or more and 200 or less.

[0106] (Other Components) The positive electrode active material layer 102 can contain other components other than the positive electrode active material, the specific positive electrode binder, and the single-walled carbon nanotubes as needed. Examples of other components include other binders, dispersants, conductive additives, electrolytes (polymer matrices, ion-conductive polymers, electrolyte solutions, etc.), and electrolyte supporting salts (lithium salts) for increasing ion conductivity. The types and contents of other components are not particularly limited, and conventionally known knowledge about lithium-ion secondary batteries can be referenced as appropriate.

[0107] Examples of dispersants include sodium salts of carboxymethylcellulose and ammonium salts of carboxymethylcellulose. The content of the dispersant in the positive electrode active material layer 102 is, for example, 2.0% by mass or less, preferably 1.0% by mass or less, and more preferably 0.5% by mass or less. The content of the dispersant in the positive electrode active material layer 102 is, for example, 0.3% by mass or more.

[0108] Examples of the conductive additive include acetylene black, carbon black, and graphite. The content of the conductive additive in the positive electrode active material layer 102 is, for example, an amount such that the total amount of the conductive additive and the single-walled carbon nanotubes is 1.0% by mass or less, preferably 0.5% by mass or less, and more preferably 0.1% by mass or less.

[0109] <Method for Manufacturing Bipolar Electrode> Next, a description will be given of a method for manufacturing the bipolar electrode 100. The method for manufacturing the bipolar electrode 100 includes a negative electrode forming step and a positive electrode forming step.

[0110] The negative electrode forming step includes a negative electrode applying step of applying a negative electrode composite to first surface 101b of bipolar current collector 101, and a negative electrode drying step of drying the negative electrode composite applied to bipolar current collector 101. The negative electrode applying step and the negative electrode drying step are the same as the applying step and the drying step described in the above-mentioned method for manufacturing a negative electrode, except that bipolar current collector 101 for bipolar electrode 100 is used instead of negative electrode current collector 22a.

[0111] The positive electrode forming step includes a positive electrode applying step of applying a positive electrode composite to the second surface 101a of the bipolar current collector 101, and a positive electrode drying step of drying the positive electrode composite applied to the bipolar current collector 101.

[0112] The positive electrode mixture is a mixture that becomes the positive electrode active material layer 102 upon solidification. The positive electrode mixture is, for example, a slurry. The positive electrode mixture contains a positive electrode active material, a specific positive electrode binder, single-walled carbon nanotubes, and an aqueous solvent, and may contain other components as necessary. The positive electrode active material, the specific positive electrode binder, the single-walled carbon nanotubes, and other components contained in the positive electrode mixture are the same as those described in the section on the positive electrode active material layer above.

[0113] The aqueous solvent is water or a mixed solvent of water and an organic solvent. The aqueous solvent is preferably a solvent in which the mass proportion of water is 50 to 100 mass%. The aqueous solvent is blended into the positive electrode mixture so that the solids proportion of the positive electrode mixture is 60 mass% or more and 80 mass% or less.

[0114] The order in which the negative electrode forming step and the positive electrode forming step are performed is arbitrary. For example, the negative electrode forming step may be performed after the positive electrode forming step, or the positive electrode forming step may be performed after the negative electrode forming step. Alternatively, some or all of the negative electrode forming step and the positive electrode forming step may be performed simultaneously. For example, after the negative electrode composite is applied to the second surface 101 a of the bipolar current collector 101 and the positive electrode composite is applied to the second surface 101 a of the bipolar current collector 101 simultaneously, the coated layer of the negative electrode composite and the coated layer of the positive electrode composite may be dried simultaneously.

[0115] Furthermore, in order to increase the electrode density, after either or both of the negative electrode drying step and the positive electrode drying step have been performed, a compression step may be performed to compress either or both of the negative electrode active material layer 103 and the positive electrode active material layer 102. The compression step is the same as the compression step described in the above-mentioned method for producing a positive electrode.

[0116] Furthermore, when the compression step is performed on the positive electrode active material layer 102, the thickness of the positive electrode active material layer 102 subjected to the compression step, i.e., the thickness of the positive electrode active material layer 102 before the pressing step, is, for example, 227 μm or more, and preferably 273 μm or more. The thickness of the positive electrode active material layer 102 before the pressing step is, for example, 600 μm or less, and preferably 533 μm or less. The density of the positive electrode active material layer 102 before the pressing step is, for example, 1.5 g / cm ３ 2.2g / cm or more ３ The following is the result.

[0117] Between each of the above steps, or at least partially after all steps have been performed, a winding step of winding the bipolar current collector 101 into a roll may be performed. Furthermore, after the compression step, one or both of the negative electrode drying step and the positive electrode drying step may be performed again.

[0118] Here, the method for manufacturing the bipolar electrode 100 includes a bending step of bending the bipolar current collector 101 on which the anode active material layer 103 is formed after the anode drying step. The bending step is the same as the bending step described in the above method for manufacturing the anode. Note that when the anode forming step is performed after the anode forming step, the bending step may be performed during the positive electrode forming step.

[0119] The bipolar electrode 100 of the second embodiment also provides the same effects as those (1-1) to (1-3) described for the negative electrode 22 of the first embodiment. Furthermore, according to the second embodiment, the following effects can be further obtained.

[0120] (2-1) The bipolar electrode 100 includes a bipolar current collector 101 having a first surface 101b and a second surface 101a facing the opposite side to the first surface 101b. A positive electrode active material layer 102 is formed on the second surface 101a of the bipolar current collector 101, and a negative electrode active material layer 103 is formed on the first surface 101b. The configuration of the negative electrode active material layer 103 is similar to that of the negative electrode active material layer 22b of the first embodiment.

[0121] The bipolar electrode 100 requires that active material layers be formed on both the first surface 101b and the second surface 101a of the bipolar current collector 101. Therefore, compared to a monopolar electrode in which an electrode is formed on only one surface of the current collector, the bipolar electrode 100 requires more bending steps during manufacture, in which a bending load is applied to the negative electrode active material layer 103, making the negative electrode active material layer 103 more likely to crack. Therefore, when the negative electrode active material layer 103 of the bipolar electrode 100 is configured to include a water-based binder with a glass transition temperature of less than 7°C and single-walled carbon nanotubes, the effect of suppressing cracking of the negative electrode active material layer 103 can be more significantly achieved.

[0122] (2-2) The positive electrode active material layer 102 contains a positive electrode active material. The basis weight of the positive electrode active material layer 102 is 50 mg / cm ２ The content of the positive electrode active material in the positive electrode active material layer 102 is 97 mass % or more. The positive electrode active material layer 102 contains a water-based binder having a glass transition temperature of less than 7° C. and single-walled carbon nanotubes.

[0123] According to the above configuration, the positive electrode active material layer 102 has high flexibility due to the cooperation of the aqueous binder contained therein, which has a glass transition temperature of less than 7°C, and the single-walled carbon nanotubes. This also makes it possible to suppress cracking of the positive electrode active material layer 102 that occurs when a bending load is applied to the positive electrode active material layer 102. As a result, it is also possible to suppress detachment of the positive electrode active material layer 102 from the bipolar current collector 101 when a process that applies a load to the bipolar electrode 100 is performed during the manufacture of an electricity storage device.

[0124] This embodiment can be modified as follows: This embodiment and the following modifications can be combined with each other to the extent that no technical contradiction occurs. The configuration of the positive electrode active material layer 102 of the bipolar electrode 100 is not limited to the above embodiment, and any known configuration of a positive electrode active material layer used in the negative electrode of a power storage device can be applied.

[0125] The planar shapes of the negative electrode current collector 22a and the negative electrode active material layer 22b are not particularly limited. For example, they may be polygonal, such as rectangular, or may be circular or elliptical. The planar shapes of the positive electrode current collector 21a and the positive electrode active material layer 21b are not particularly limited. For example, they may be polygonal, such as rectangular, or may be circular or elliptical.

[0126] The negative electrode active material layer 22b may be provided on the second surface 22a2 of the negative electrode current collector 22a. The number of the power storage cells 20 constituting the power storage device 10 is not particularly limited. For example, the number of the power storage cells 20 constituting the power storage device 10 may be one.

[0127] Examples that further specify the above-described embodiment are described below. <Preparation of Negative Electrode> Graphite having a G / D ratio of 3.4, styrene-butadiene rubber (SBR), single-walled carbon nanotubes (CNT), and carboxymethyl cellulose sodium salt (CMC) were mixed in the proportions shown in Tables 1 and 2, and water was added to this mixture to prepare a negative electrode mixture with a solids ratio of 56 mass %.

[0128] The "Type of CNT" column in Tables 1 and 2 shows the type of carbon nanotube used. The amount of carbon nanotube blended in each example was adjusted so that the electrode resistance was approximately the same. The "Tg (°C) of SBR" column in Tables 1 and 2 shows the glass transition temperature of the styrene-butadiene rubber used. The glass transition temperature is the glass transition onset temperature in a DSC curve measured using DSC in accordance with JIS K 7121 (1987), except that the heating rate was 10°C / min.

[0129] A copper foil with a thickness of 8 μm was prepared as a current collector. The negative electrode composite was applied to one surface of the current collector in the form of a film using a doctor blade method. The applied negative electrode composite was heated at 50°C to dry and solidify the negative electrode composite, resulting in a negative electrode composite having a coating weight of 17 mg / cm on the current collector. ２ or 38 mg / cm ２ The negative electrodes of Examples 1 and 2, Comparative Examples 1 to 3, and Reference Examples 1 and 2 were fabricated, each having a negative electrode active material layer of the above formula.

[0130] <Bending Test> The negative electrode of each example was cut into a piece of 5 cm x 10 cm and used as a test sample. The test sample was wrapped around half of the outer periphery of a cylinder with a diameter of 50 mm, with the side on which the negative electrode active material layer was formed facing outward, and the surface condition of the negative electrode active material layer was then visually evaluated. The results are shown in Tables 1 and 2. The evaluation criteria were as follows:

[0131] "◯": No cracks on the surface of the negative electrode active material layer. "X": Cracks are present on the surface of the negative electrode active material layer.

[0132]

[0133]

[0134] Examples 1 and 2 are examples in which the negative electrode active material layer contains an aqueous binder having a glass transition temperature of less than 7°C and single-walled carbon nanotubes. No cracks were observed in the negative electrode active material layers of Examples 1 and 2. On the other hand, cracks were observed in the negative electrode active material layer of Comparative Example 1, in which an aqueous binder having a glass transition temperature of 7°C was used instead of the aqueous binder having a glass transition temperature of less than 7°C. Furthermore, cracks were observed in the negative electrode active material layer of Comparative Example 2, in which multi-walled carbon nanotubes were used instead of single-walled carbon nanotubes, and in the negative electrode active material layer of Comparative Example 3, which did not contain carbon nanotubes. These results demonstrate that the effect of suppressing cracking in the negative electrode active material layer in Examples 1 and 2 is not the sole effect of the aqueous binder having a glass transition temperature of less than 7°C or the sole effect of the single-walled carbon nanotubes, but rather the effect achieved by the cooperation of both components.

[0135] Although detailed experimental data is omitted, a similar bending test was carried out using a 25 mm diameter cylinder instead of the 50 mm diameter cylinder for Examples 1 and 2. As a result, no cracks were found in the negative electrode active material layers of Examples 1 and 2.

[0136] Reference Examples 1 and 2 are examples in which an aqueous binder with a glass transition temperature of 7°C is used and no carbon nanotubes are contained, and the negative electrode active material layer has a different basis weight. ２In Reference Example 1, no cracks were found in the negative electrode active material layer, whereas the coating weight was 38 mg / cm ２ In Reference Example 2, cracks were observed in the negative electrode active material layer. Thus, even when an aqueous binder with a glass transition temperature of 7°C or higher is used and single-walled carbon nanotubes are not contained, cracks do not occur in the negative electrode active material layer when the basis weight of the negative electrode active material layer is small. Therefore, it can be seen that cracks in the negative electrode active material layer caused by the application of bending stress are a phenomenon that occurs when the basis weight of the negative electrode active material layer is increased. Therefore, the effect of suppressing cracks in the negative electrode active material layer by using an aqueous binder with a glass transition temperature of less than 7°C and single-walled carbon nanotubes in combination is an effect that cannot be predicted from a configuration in which the basis weight of the negative electrode active material layer is small.

[0137] <Preparation of bipolar electrodes and bending test> LiFePO with carbon coating layer ４ Granules of (LFP), styrene-butadiene rubber (SBR), single-walled carbon nanotubes (CNT), and carboxymethyl cellulose sodium salt (CMC) were mixed in the blending ratios shown in Table 3, and water was added to this mixture to prepare a positive electrode mixture with a solids ratio of 75 mass%.

[0138] Graphite having a G / D ratio of 3.4, styrene-butadiene rubber (SBR), single-walled carbon nanotubes (CNT), and carboxymethyl cellulose sodium salt (CMC) were mixed in the blending ratios shown in Table 3, and water was added to this mixture to prepare a negative electrode mixture having a solids ratio of 56 mass%.

[0139] A 30 μm thick aluminum foil with an 8 μm thick copper foil laminated on one surface of the current collector was prepared. A film of the positive electrode composite was applied to the aluminum foil side of the current collector using a doctor blade method. The applied positive electrode composite was heated at 50°C to dry and solidify the positive electrode composite, resulting in a coating weight of 75 mg / cm on the current collector. ２Then, the negative electrode composite was applied to the copper foil side of the current collector in the form of a film using a doctor blade method. The applied negative electrode composite was heated at 50°C to dry and solidify the negative electrode composite, thereby forming a positive electrode active material layer with a coating weight of 37.8 mg / cm on the current collector. ２ Thus, a bipolar electrode having a positive electrode active material layer and a negative electrode active material layer was fabricated.

[0140]

[0141] Two test samples were used: one cut from the fabricated bipolar electrode to a size of 5 cm x 10 cm, and the other cut from the fabricated bipolar electrode to a size of 120 cm x 150 cm. The test sample was wrapped around half of the outer periphery of a 50 mm diameter cylinder, with the side on which the negative electrode active material layer was formed facing outward. The surface conditions of the negative electrode active material layer and the positive electrode active material layer were then visually evaluated. As a result, no cracks were observed in either the negative electrode active material layer or the positive electrode active material layer for either test sample.

[0142] 22... Negative electrode 22a... Negative electrode current collector 22a1... First surface 22b... Negative electrode active material layer 100... Bipolar electrode 101... Bipolar current collector 101b... First surface 101a... Second surface 102... Positive electrode active material layer 103... Negative electrode active material layer 104... Positive electrode current collector 105... Negative electrode current collector

Claims

1. The current collector comprises a current collector having a first surface and a negative electrode active material layer formed on the first surface of the current collector, wherein the basis weight of the negative electrode active material layer is 20 mg / cm². ２ The negative electrode for the energy storage device is as described above, The aforementioned negative electrode active material layer is Graphite, a negative electrode active material capable of intercepting and releasing charge carriers, A water-based binder having a glass transition temperature of less than 7°C, A negative electrode for an energy storage device containing single-walled carbon nanotubes.

2. The anode for an energy storage device according to claim 1, wherein the aqueous binder comprises a compound having an aromatic ring.

3. The negative electrode for an energy storage device according to claim 2, wherein the compound having the aromatic ring is a polymer containing styrene as a constituent unit.

4. The compound having the aromatic ring is styrene-butadiene rubber, the negative electrode for an energy storage device according to claim 2.

5. The negative electrode for an energy storage device according to any one of claims 1 to 4, wherein the G / D ratio of the graphite is 3 or more.

6. The content of the aqueous binder in the negative electrode active material layer is 2.0% by mass or more and 4.0% by mass or less. The negative electrode for an energy storage device according to any one of claims 1 to 4, wherein the content of the single-walled carbon nanotubes in the negative electrode active material layer is 0.001% by mass or more and 0.02% by mass or less.

7. The current collector has a second surface facing the opposite side of the first surface, A negative electrode for an energy storage device according to any one of claims 1 to 4, wherein the negative electrode active material layer is formed on the first surface of the current collector and the positive electrode active material layer is formed on the second surface of the current collector.

8. The positive electrode active material layer contains a positive electrode active material capable of intercepting and releasing charge carriers. The basis weight of the positive electrode active material layer is 50 mg / cm². ２ It is greater than and the above The content of the positive electrode active material in the positive electrode active material layer is 97% by mass or more. The positive electrode active material layer is A water-based binder having a glass transition temperature of less than 7°C, A negative electrode for an energy storage device according to claim 7, comprising single-walled carbon nanotubes.

9. A positive electrode having a positive electrode active material layer provided on the first surface of the positive electrode current collector, A negative electrode is provided with a negative electrode active material layer on the first surface of the negative electrode current collector, and the negative electrode is arranged such that the negative electrode active material layer faces the positive electrode active material layer of the positive electrode. A separator is disposed between the positive electrode active material layer and the negative electrode active material layer, A power storage device comprising a spacer disposed between the first surfaces of the positive electrode current collector and the negative electrode current collector, and adhered to the first surfaces of the positive electrode current collector and the negative electrode current collector, The negative electrode is the negative electrode for the energy storage device according to any one of claims 1 to 4, in the energy storage device.