Electrode for lithium ion secondary battery and lithium ion secondary battery

Abstract

This electrode comprises a current collector, and a positive electrode active material layer (102) formed on the surface of the current collector. The surface area of the positive electrode active material layer (102) is at least 1 m2. The positive electrode active material layer (102) comprises a main surface positioned on the side opposite from the surface facing the current collector, and groove sections (103) opened to the main surface. In a plan view of the main surface, the maximum distance D, which is the maximum value of a specific distance, is 60 mm or less. The specific distance is the shorter distance among a distance from an outer peripheral edge of the positive electrode active material layer (102) and a distance from the groove sections (103) at any point in an island part (104), which is a section of the main surface where the groove sections (103) are not provided.

Description

[Technical Field]

[0001] The present invention relates to an electrode for a lithium ion secondary battery and a lithium ion secondary battery. [Background technology]

[0002] Patent Document 1 discloses a bipolar structure electricity storage device constructed by stacking a plurality of individually manufactured electricity storage cells in series. The electricity storage cell includes a positive electrode having a positive electrode active material layer formed on one side of a foil-shaped positive electrode current collector, a negative electrode having a negative electrode active material layer formed on one side of a foil-shaped 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, and a separator disposed between the positive electrode and the negative electrode.

[0003] In the above-described energy storage device, a plurality of the energy storage cells are electrically connected in series by being stacked such that the positive electrode current collectors and the negative electrode current collectors are in contact with each other. In this case, current flows in the stacking direction of the energy storage cells. Therefore, the above-described energy storage device can secure a larger area for the current path and can obtain a higher output, compared to an energy storage device having a structure in which the energy storage cells are electrically connected in series through tabs drawn out from each energy storage cell. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-16825 A Summary of the Invention [Problem to be solved by the invention]

[0005] One possible method for increasing the capacity of a storage cell is to form a large electrode active material layer to increase the amount of active material held in the electrode active material layer. Here, in the case of an energy storage device having a structure in which current flows in the stacking direction of the energy storage cells, such as the above-mentioned energy storage device, forming a thick electrode active material layer to make the electrode active material layer large can cause a problem of increased electrical resistance. Furthermore, depending on the application of the energy storage device, an increase in the height dimension of the energy storage device due to thickening of the electrode active material layer may be avoided. For example, in the case of an energy storage device used as a battery placed under the floor of the passenger compartment of a vehicle such as an electric vehicle or hybrid vehicle, it is preferable that the energy storage device be as thin and flat as possible. The height of the battery placed under the passenger compartment floor is approximately 20 cm at most.

[0006] Therefore, the inventors have investigated the capacity increase of a lithium ion secondary battery having a structure in which current flows in the stacking direction of the storage cells by increasing the area of ​​the electrode active material layer, i.e., by increasing the planar size of the electrode active material layer. Specifically, the capacity required for a lithium ion secondary battery for an electric vehicle is approximately 50 kWh to 100 kWh.

[0007] Even when considering the constituent components and density of the electrode active material layer, in order to obtain a capacity of 50 kWh or more while keeping the height of the lithium-ion secondary battery to 20 cm or less, the area of ​​the electrode active material layer must be 1 m 2 The area of ​​the electrode active material layer must be 1 m or more. 2 When the lithium-ion secondary battery described above was fabricated and its properties were evaluated, a new problem was discovered: the electrical resistance of the electrodes increased. This increase in electrical resistance does not occur in conventional planar lithium-ion secondary batteries, and is therefore thought to be a phenomenon specific to when the area of ​​the electrode active material layer is increased beyond a certain level. [Means for solving the problem]

[0008] The electrode for a lithium ion secondary battery is an electrode for a lithium ion secondary battery having a bipolar structure, and comprises a current collector and a positive electrode active material layer formed on the surface of the current collector, and the area of ​​the positive electrode active material layer is 1 m 2The positive electrode active material layer has a main surface located on the opposite side to a surface facing the current collector, and a groove portion opening into the main surface, and when, in a plan view of the main surface, a specific distance is defined as the shorter of the distance from the outer peripheral edge of the positive electrode active material layer to an arbitrary point within an island portion, which is a portion of the main surface where the groove portion is not provided, and the distance from the groove portion to the arbitrary point, the maximum value of the specific distance is 60 mm or less.

[0009] In the electrode for a lithium ion secondary battery, it is preferable that the positive electrode active material layer has a shape having a longitudinal direction and a lateral direction, and the groove portion has a linear shape extending in the longitudinal direction of the positive electrode active material layer.

[0010] In the electrode for a lithium ion secondary battery, the positive electrode active material layer has a rectangular shape having a longitudinal direction and a lateral direction, the groove portion has a linear shape extending in the longitudinal direction of the positive electrode active material layer, and the aspect ratio of the island portion is 12 or more.

[0011] In the electrode for a lithium ion secondary battery, the positive electrode active material layer has a thickness of 250 μm or more. The lithium ion secondary battery is a bipolar structure lithium ion secondary battery and includes electrodes for the lithium ion secondary battery.

[0012] The lithium ion secondary battery has a capacity of 50 kWh or more. [Effects of the Invention]

[0013] According to the present invention, in an electrode for a bipolar lithium ion secondary battery, an increase in electrical resistance due to an increase in the area of ​​an active material layer can be suppressed. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a cross-sectional view of an electrode for a power storage device. [Figure 2] FIG. 2 is a plan view of an electrode for a power storage device. [Figure 3] FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2. [Figure 4] FIG. 3 is an enlarged view of part A in FIG. 2. [Figure 5] FIG. [Figure 6] FIG. 10 is a cross-sectional view of a modified bipolar electrode. DETAILED DESCRIPTION OF THE INVENTION

[0015] An embodiment of the present invention will now be described with reference to the drawings. (electrode) The electrode of this embodiment is used as a positive electrode or a negative electrode of a bipolar structure electricity storage device in which a plurality of electricity storage cells are stacked in series. The electricity storage device is a lithium ion secondary battery.

[0016] As shown in FIG. 1, electrode 100 is an electrode for a lithium ion secondary battery, and includes current collector 101 and active material layer 102 provided on first surface 101a of current collector 101. [Current collector] The current collector 101 is a chemically inactive electrical conductor that allows current to continue to flow through the active material layer 102 during charging or discharging of the lithium ion secondary battery. The current collector 101 is, for example, in the form of a foil. The foil-shaped current collector 101 has a thickness of, for example, 1 μm or more and 100 μm or less, and preferably 10 μm or more and 60 μm or less. The current collector 101 can be made of, for example, a metal material, a conductive resin material, a conductive inorganic material, or the like.

[0017] Examples of the metal material include copper, aluminum, nickel, titanium, and stainless steel. Examples of the conductive resin material include resins obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as needed.

[0018] When the electrode 100 is used as a positive electrode of a power storage device, the current collector 101 is preferably an aluminum current collector made of aluminum. The aluminum current collector may be made of aluminum alone or an aluminum alloy. Examples of aluminum alloys include Al-Mn alloys, Al-Mg alloys, and Al-Mg-Si alloys. The aluminum content in the aluminum layer is, for example, 50% by mass or more, and preferably 70% by mass or more.

[0019] The current collector 101 may have multiple layers including one or more layers containing the above-mentioned metal material or conductive resin material. The surface of the current collector 101 may be covered with a known protective layer such as a carbon coating layer. The surface of the current collector 101 may be treated by a known method such as plating.

[0020] [Active material layer] The active material layer 102 is formed on the first surface 101 a of the current collector 101 . The active material layer 102 contains an active material that can absorb and release lithium ions.

[0021] When the electrode 100 is used as a positive electrode of a power storage device, the active material contained in the active material layer 102 is a positive electrode active material. As the positive electrode active material, any material that can be used as a positive electrode active material for a lithium ion secondary battery, such as a lithium composite metal oxide having a layered rock salt structure, a metal oxide having a spinel structure, or a polyanionic compound, may be used. Two or more positive electrode active materials may also be used in combination. A specific example of the positive electrode active material is olivine-type lithium iron phosphate (LiFePO4), which is a polyanionic compound.

[0022] When the electrode 100 is used as the negative electrode of a power storage device, the active material contained in the active material layer 102 is a negative electrode active material. The negative electrode active material may be Li, carbon, a metal compound, or an element or compound thereof that can be alloyed with lithium, and can be used as a negative electrode active material for a lithium-ion secondary battery. Examples of carbon 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. Examples of elements that can be alloyed with lithium include silicon and tin.

[0023] There are no particular limitations on the content of the active material in the active material layer 102. The content of the active material in the active material layer 102 is, for example, 96% by mass or more and less than 100% by mass. The active material layer 102 may further contain, as necessary, a conductive aid for improving electrical conductivity, a binder, an electrolyte (a polymer matrix, an ion-conductive polymer, a liquid electrolyte, etc.), an electrolyte supporting salt (lithium salt) for improving ion conductivity, etc. The components contained in the active material layer, the blending ratio of these components, and the thickness of the active material layer are not particularly limited, and conventionally known knowledge about lithium ion secondary batteries may be referred to as appropriate.

[0024] The conductive additive is added to increase the conductivity of the electrode 100. Examples of the conductive additive include acetylene black, carbon black, graphite, and carbon nanotubes (CNT).

[0025] Examples of binders include 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 such as polyacrylic acid and polymethacrylic acid; styrene-butadiene rubber; carboxymethyl cellulose; alginates such as sodium alginate and ammonium alginate; water-soluble cellulose ester crosslinkers; and starch-acrylic acid graft polymers. These binders can be used alone or in combination. Examples of solvents or dispersion media include water and N-methyl-2-pyrrolidone.

[0026] [Dimensions and shape of active material layer] 2, in a plan view seen from the stacking direction of the current collector 101 and the active material layer 102 (hereinafter simply referred to as a plan view), the active material layer 102 is formed in the center of the first surface 101a of the current collector 101. In the plan view, the peripheral portion of the first surface 101a of the current collector 101 is an uncoated portion where the active material layer 102 is not provided. The uncoated portion is arranged so as to surround the periphery of the active material layer 102 in the plan view.

[0027] The thickness, density, basis weight, and porosity of the active material layer 102 are not particularly limited, and conventionally known knowledge about lithium ion secondary batteries may be referred to as appropriate. Specific examples of the thickness, density, basis weight, and porosity of the active material layer 102 when the active material layer 102 is a positive electrode active material layer and when the active material layer 102 is a negative electrode active material layer are described below.

[0028] <Thickness, Density, Weight per Unit Area, and Porosity in the Case of Positive Electrode Active Material Layer> The thickness of the active material layer 102 is, for example, 250 μm or more, and preferably 300 μm or more. The thickness of the active material layer 102 is, for example, 600 μm or less, and preferably 500 μm or less. By increasing the thickness of the active material layer 102, the capacity of the power storage cell can be increased.

[0029] The density of the active material layer 102 is, for example, 1.6 g / cm 3 or more, preferably 1.8 g / cm 3 The density of the active material layer 102 is, for example, 2.5 g / cm 3 or less, preferably 2.3 g / cm 3 The reason is as follows: By increasing the density of the active material layer 102, the capacity of the power storage cell can be increased.

[0030] The weight of the active material layer 102 is, for example, 50 mg / cm 2 or more, preferably 60 mg / cm 2 More preferably, it is 70 mg / cm or more. 2 The weight of the active material layer 102 is, for example, 90 mg / cm 2 or less, preferably 80 mg / cm 2 The reason is as follows: By increasing the basis weight of the active material layer 102, the capacity of the electricity storage cell can be increased.

[0031] The porosity of the active material layer 102 is, for example, 30% or more, and preferably 35% or more. The porosity of the active material layer 102 is, for example, 55% or less, and preferably 45% or less.

[0032] <Thickness, Density, Weight per Unit Area, and Porosity in the Case of Negative Electrode Active Material Layer> The thickness of the active material layer 102 is, for example, 200 μm or more, and preferably 250 μm or more. The thickness of the active material layer 102 is, for example, 600 μm or less, and preferably 500 μm or less. By increasing the thickness of the active material layer 102, the capacity of the power storage cell can be increased.

[0033] The density of the active material layer 102 is, for example, 1.1 g / cm 3 or more, preferably 1.2 g / cm 3 The density of the active material layer 102 is, for example, 1.7 g / cm 3 and preferably 1.5 g / cm 3 The reason is as follows: By increasing the density of the active material layer 102, the capacity of the power storage cell can be increased.

[0034] The weight of the active material layer 102 is, for example, 30 mg / cm 2 or more, preferably 33 mg / cm 2 More preferably, 35 mg / cm 2 The weight of the active material layer 102 is, for example, 50 mg / cm 2 or less, preferably 45 mg / cm 2 The reason is as follows: By increasing the basis weight of the active material layer 102, the capacity of the electricity storage cell can be increased.

[0035] The porosity of the active material layer 102 is, for example, 30% or more, and preferably 35% or more. The porosity of the active material layer 102 is, for example, 55% or less, and preferably 45% or less.

[0036] The area of ​​the active material layer 102, that is, the area of ​​the range where the active material layer 102 is formed on the first surface 101a of the current collector 101, is 1 m 2 The area of ​​the active material layer 102 is preferably 1.2 m 2 More preferably, 1.4 m 2 The area of ​​the active material layer 102 is, for example, 3 m 2 In this specification, the area of ​​the active material layer 102 is an area including the grooves 103 described below.

[0037] The planar shape of the active material layer 102 is not particularly limited. Examples of the planar shape of the active material layer 102 include polygonal, circular, and elliptical shapes. When the active material layer 102 is rectangular, the aspect ratio of the active material layer 102 in planar view is, for example, 1 to 2.5, and preferably 1 to 2. The vertical length L1 of the active material layer 102 is, for example, 500 mm to 1500 mm, and the horizontal length L2 is, for example, 800 mm to 3000 mm. The above-described planar shape of the active material layer 102, as well as the aspect ratio, vertical length L1, and horizontal length L2 of the active material layer 102, are applicable to both positive electrode active material layers and negative electrode active material layers.

[0038] [Groove part of active material layer] When the electrode 100 is used as a positive electrode of a power storage device, the active material layer 102 is provided with a groove 103. Below, the groove 103 will be described using an example in which the active material layer 102 has a horizontally long rectangular shape in a planar view. Hereinafter, the active material layer 102 will be referred to as a positive electrode active material layer 102.

[0039] 2 and 3, the positive electrode active material layer 102 has a groove 103 with a rectangular cross section that opens to a main surface 102a. The main surface 102a is the surface of the positive electrode active material layer 102 that is located opposite to the surface facing the current collector 101.

[0040] In a plan view, the grooves 103 extend in the horizontal direction, which is the longitudinal direction of the positive electrode active material layer 102. The grooves 103 have a constant width from one end to the other in the horizontal direction and are formed linearly. A plurality of grooves 103 are formed in parallel in the vertical direction at a constant pitch in the positive electrode active material layer 102. The bottom surfaces of the grooves 103 are formed by the current collector 101. The grooves 103 are slit-shaped. The cross-sectional shape of the grooves 103 is rectangular.

[0041] Here, in a plan view, the portions of the main surface 102a of the positive electrode active material layer 102 where the grooves 103 are not formed are defined as island portions 104 of the positive electrode active material layer 102. As described above, the grooves 103 are linear and extend in the horizontal direction of the positive electrode active material layer 102, and a plurality of the grooves 103 are arranged parallel to each other in the vertical direction. Therefore, each island portion 104 of the positive electrode active material layer 102 is formed in a horizontally elongated rectangular shape.

[0042] 4, the island portion 104 has an outer edge 104a, which is an edge that forms the outer periphery of the positive electrode active material layer 102, and a groove edge 104b, which is an edge that forms the groove portion 103. In this specification, the outer periphery of the active material layer 102 means the outer periphery of the range that includes the groove portion 103 and the island portion 104.

[0043] The island portion 104 is formed in a shape such that the distance from the outer edge 104a and the groove edge 104b satisfies the following conditions: In other words, the groove portion 103 is formed in the positive electrode active material layer 102 so that the island portion 104 is formed in a shape such that the distance from the outer edge 104a and the groove edge 104b satisfies the following conditions:

[0044] The above condition is that, when the specific distance is the shorter of the distance L3 from the outer edge 104a or the distance L4 from the groove edge 104b at any point P within the island portion 104, the maximum specific distance is 60 mm or less. This condition means that the distance to the nearest point on the periphery of the island portion 104 at all points within the island portion 104 is 60 mm or less. Hereinafter, the maximum specific distance will be referred to as the maximum distance D. If the island portion 104 has a horizontally elongated rectangular shape, the point at which the maximum distance D is taken is on a line S1 that extends horizontally and bisects the island portion 104, and where the distance L3 to the outer edge 104a is longer than the distance L4 to the groove edge 104b. In this case, the maximum distance D is half the width H of the island portion 104.

[0045] The maximum distance D is 60 mm or less, preferably 40 mm or less, and more preferably 20 mm or less. By shortening the maximum distance D, the effect of suppressing an increase in electrical resistance is improved.

[0046] The width H of the island portion 104, which is the length in the short direction, is, for example, 120 mm or less, preferably 80 mm or less, and more preferably 40 mm or less. The aspect ratio of the island portion 104 is, for example, 12 or more, preferably 15 or more, and more preferably 17 or more. The aspect ratio of the island portion 104 is, for example, 40 or less.

[0047] The ratio of the total area of ​​the island portions 104 to the area of ​​the positive electrode active material layer 102 is, for example, 90% or more and 99% or less. By increasing this ratio, the capacity of the power storage cell can be increased.

[0048] The width of the grooves 103 is, for example, 0.5 mm or more. By making the width of the grooves 103 0.5 mm or more, the effect of suppressing an increase in electrical resistance can be significantly achieved. Furthermore, by increasing the width of the grooves 103, the process of forming the positive electrode active material layer 102 having the grooves 103 becomes easier and the time required to inject the electrolyte through the grooves 103 can be shortened. Furthermore, the width of the grooves 103 is, for example, 3 mm or more. In this case, in addition to the above effects, the electrolyte retention function, i.e., the ability to retain the electrolyte within the grooves 103, can be improved. Furthermore, the width of the grooves 103 is, for example, 8 mm or less or 3 mm or less. By reducing the width of the grooves 103, the proportion of the total area of ​​the island portions 104 in the area of ​​the positive electrode active material layer 102 can be increased. The formation pitch of the grooves 103 is the sum of the width of the grooves 103 and the width H of the island portions 104.

[0049] The grooves 103 are formed so as to reach the current collector 101. That is, the bottom of the grooves 103 is the first surface 101a of the current collector 101, and the groove depth of the grooves 103 is the same as the thickness of the positive electrode active material layer 102. If a protective layer such as a carbon coating layer is applied to the first surface 101a of the current collector 101, the protective layer becomes the bottom of the grooves 103. By forming the grooves 103 so as to reach the current collector 101, the flow path cross-sectional area of ​​the grooves 103 as a flow path for the electrolyte can be maximized.

[0050] The method for forming the grooves 103 is not particularly limited. For example, the grooves 103 may be formed by applying a composite material, which will become the positive electrode active material layer 102 upon solidification, to the current collector 101 and then applying the composite material by slit coating to form the positive electrode active material layer 102. Specifically, a die coater is prepared in which an obstacle such as a shim that partially prevents the composite material from being discharged is provided at the discharge port of the slit die. The composite material is then applied using the die coater, thereby allowing the composite material to be applied in a shape having grooves. Alternatively, the grooves 103 may be formed by partially scraping the main surface 102a of the positive electrode active material layer 102 that has been formed in a shape without the grooves 103.

[0051] (Electricity storage device) Next, an example of an electricity storage device to which the electrode 100 is applied will be described. The power storage device to which the electrode 100 is applied is, for example, a lithium ion secondary battery used in batteries for various vehicles such as forklifts, hybrid cars, and electric cars.

[0052] As shown in FIG. 5, the energy storage device 10 includes a cell stack 30 (laminated body) in which a plurality of energy storage cells 20 are stacked in a stacking direction. Hereinafter, the stacking direction of the plurality of 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. In each energy storage cell 20, both the positive electrode 21 and the negative electrode 22 are the above-described electrodes 100. That is, the positive electrode 21 is the electrode 100 including the groove portion 103, and the negative electrode 22 is the electrode 100 without the groove portion 103. Note that the groove portion 103 is not shown in FIG. 5.

[0053] 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. When the positive electrode 21 is an electrode 100, the positive electrode current collector 21a is a current collector 101, and the positive electrode active material layer 21b is an active material layer 102.

[0054] 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 so as to surround the periphery of the positive electrode active material layer 21b.

[0055] 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. When the negative electrode 22 is an electrode 100, the negative electrode current collector 22a is a current collector 101, and the negative electrode active material layer 22b is an active material layer 102.

[0056] 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. The negative electrode uncoated portion 22c is arranged to surround the periphery of the positive electrode active material layer 21b in a plan view. 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. In other words, the facing 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 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 plan view.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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 bonded 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.

[0061] The spacer 24 extends along the peripheral edges of the positive electrode current collector 21a and the negative electrode current collector 22a in a plan view and is formed in a frame shape surrounding the peripheries of the positive electrode current collector 21a and the negative electrode current collector 22a. The spacer 24 is disposed between the positive electrode uncoated portion 21c on the first surface 21a1 of the positive electrode current collector 21a and the negative electrode uncoated portion 22c on the first surface 22a1 of the negative electrode current collector 22a.

[0062] 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.

[0063] An enclosed space S is formed inside the energy storage cell 20 and is surrounded by a frame-shaped spacer 24, a positive electrode 21, and a 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.

[0064] The electrolyte is a liquid electrolyte. Examples of liquid electrolytes include a liquid electrolyte containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Known lithium salts such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, LiN(FSO2)2, and LiN(CF3SO2)2 can be used as the electrolyte salt. Known solvents such as cyclic carbonates, cyclic esters, chain carbonates, chain esters, and ethers can be used as the non-aqueous solvent. Two or more of these known solvent materials may be used in combination. A preferred example of a combination of solvent materials is a solvent that combines a cyclic ester and a chain ester. A solvent that combines a cyclic ester and a chain ester has low viscosity, improving the fluidity of the liquid electrolyte. By improving the fluidity of the liquid electrolyte, it becomes easier to achieve the effect of suppressing an increase in electrical resistance.

[0065] 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.

[0066] The cell stack 30 has a structure in which a plurality of power 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, whereby the plurality of power storage cells 20 constituting the cell stack 30 are connected in series.

[0067] Here, 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 21a and negative electrode current collector 22a 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 21a and negative electrode current collector 22a are stacked, a positive electrode active material layer 21b formed on one surface of the current collector, and a negative electrode active material layer 22b formed on the other surface.

[0068] 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.

[0069] 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.

[0070] The energy storage device 10 is charged and discharged through terminals provided on the positive electrode current-carrying plate 40 and the negative electrode current-carrying plate 50. The material for the positive electrode current-carrying plate 40 may be the same as the material for the positive electrode current collector 21a. The positive electrode current-carrying plate 40 may be made of a metal plate that is thicker than the positive electrode current collector 21a used in the cell stack 30. The material for the negative electrode current-carrying plate 50 may be the same as the material for the negative electrode current collector 22a. The negative electrode current-carrying plate 50 may be made of a metal plate that is thicker than the negative electrode current collector 22a used in the cell stack 30.

[0071] As described above, the positive electrode 21 constituting each storage cell 20 of the energy storage device 10 is an electrode 100 having a groove 103, and the negative electrode 22 is an electrode 100 not having a groove 103. The area of ​​each of the positive electrode active material layer 21b of the positive electrode 21 and the negative electrode active material layer 22b of the negative electrode 22 is 1 m 2 As a result, the planar size of the power storage device 10 is 1 m 2 That's all.

[0072] The height of the energy storage device 10 is, for example, 20 cm or less. The lower limit of the height of the energy storage device 10 may be set to a value that allows the energy storage device 10 to obtain a required capacity depending on the areas of the positive electrode active material layer 21b and the negative electrode active material layer 22b of the negative electrode 22. For example, assuming a lithium ion secondary battery used in batteries for various vehicles such as electric vehicles, the energy storage device 10 as a whole, including the energy storage cells 20, has a capacity per area of, for example, 20 kWh / m 2 The lower limit of the height of the power storage device 10 is set so that the capacity per area is equal to or greater than 25 kWh / m. It is preferable that the capacity per area is higher. 2 More than 30kWh / m 2 More than 33kWh / m 2 or more than 35kWh / m 2 The capacity of the power storage device 10 is preferably 50 kWh or more. The capacity of the power storage device 10 is, for example, 100 kWh or less.

[0073] Next, the operation of this embodiment will be described. According to this embodiment, in a bipolar structure electricity storage device, an increase in electrical resistance caused by an increase in the area of ​​the active material layer of the electrode can be suppressed. The mechanism of the increase in electrical resistance caused by an increase in the area of ​​the active material layer is as follows.

[0074] The negative electrode active material contained in the negative electrode active material layer 22b expands and contracts by absorbing and releasing lithium ions during charging and discharging. The expansion of the negative electrode active material causes the liquid electrolyte impregnated in the negative electrode active material layer 22b to be pushed out and discharged from the negative electrode active material layer 22b, while the contraction of the negative electrode active material causes the liquid electrolyte to be absorbed into the negative electrode active material layer 22b. Thus, the negative electrode 22 repeatedly absorbs and discharges the liquid electrolyte into the negative electrode active material layer 22b during charging and discharging.

[0075] During charging, the negative electrode active material expands, which is a reaction in which lithium is taken up into the negative electrode active material, so the liquid electrolyte discharged by the expansion of the negative electrode active material has a low lithium ion concentration. In other words, the liquid electrolyte with a low lithium ion concentration is discharged from the negative electrode active material layer 22b.

[0076] Here, a sufficient amount of liquid electrolyte with a high lithium ion concentration is present near the main surface of the negative electrode active material layer 22b, so even if the liquid electrolyte with a low lithium ion concentration discharged from the negative electrode active material layer 22b is mixed, the lithium ion concentration of the liquid electrolyte remains almost unchanged.

[0077] Meanwhile, near the outer periphery of the negative electrode active material layer 22b, the liquid electrolyte with a low lithium ion concentration discharged from the negative electrode active material layer 22b is mixed, causing a phenomenon in which the lithium ion concentration of the liquid electrolyte decreases. During discharge, when the negative electrode active material contracts, the liquid electrolyte with a reduced lithium ion concentration is taken into the negative electrode active material layer 22b. Therefore, with repeated charge and discharge, the lithium ion concentration of the liquid electrolyte near the outer periphery of the negative electrode active material layer 22b gradually decreases, and accordingly, the lithium ion concentration in the outer periphery of the negative electrode active material layer 22b also gradually decreases.

[0078] The larger the area of ​​the negative electrode active material layer 22b, the less likely it is that the liquid electrolyte present at the outer periphery of the negative electrode active material layer 22b will be replaced with the liquid electrolyte present near the central portion of the negative electrode active material layer 22b, i.e., near the main surface close to the center in the planar direction. Therefore, when the negative electrode active material layer 22b has a large area, the decrease in the lithium ion concentration of the liquid electrolyte near the outer periphery of the negative electrode active material layer 22b and the decrease in the lithium ion concentration in the outer periphery of the negative electrode active material layer 22b become more significant.

[0079] As a result, the lithium ion concentration in the negative electrode active material layer 22b is unevenly distributed, with the lithium ion concentration being low in the peripheral portion and high in the central portion. This uneven distribution of lithium ion concentration results in portions in the negative electrode active material layer 22b that have high electrical resistance and are less reactive, and portions that have low electrical resistance and are more reactive. The difference in resistance between portions of the negative electrode active material layer 22b increases the electrical resistance of the electrode.

[0080] In particular, when the density of the negative electrode active material layer 22b is high or the porosity of the negative electrode active material layer 22b is low, the amount of active material per unit area in the negative electrode active material layer 22b increases, and therefore, more lithium is incorporated into the negative electrode active material layer 22b in the reaction during one charge. As a result, the deviation in the lithium ion concentration in the negative electrode active material layer 22b reaches a level sufficient to increase the electrical resistance of the electrode even with fewer charge-discharge cycles. Therefore, when the density of the negative electrode active material layer 22b is high or the porosity of the negative electrode active material layer 22b is low, the above-mentioned problem of increased electrical resistance of the electrode occurs even with fewer charge-discharge cycles.

[0081] Similarly, when the density of the positive electrode active material layer 21b is high or the porosity of the positive electrode active material layer 21b is low, the reaction on the negative electrode active material layer 22b side is more likely to proceed, and more lithium is taken up into the negative electrode active material layer 22b in the reaction during one charge. Therefore, when the density of the positive electrode active material layer 21b is high or the porosity of the positive electrode active material layer 21b is low, the above-mentioned problem of an increase in the electrical resistance of the electrode also occurs with a small number of charge-discharge cycles.

[0082] In this embodiment, the grooves 103 are provided in the positive electrode active material layer 21b of the positive electrode 21 so that the maximum distance D in the island portions 104 is 60 mm or less. The island portions 104 are portions of the main surface of the positive electrode active material layer 21b where the grooves 103 are not formed. The maximum distance D is the maximum value of the shorter of the distance L3 from the outer edge 104a and the distance L4 from the groove edge 104b at each point on the island portion 104.

[0083] In this case, a flow of liquid electrolyte occurs through the grooves 103 of the positive electrode active material layer 21b, which attempts to uniform the lithium ion concentration. For example, during charging, the liquid electrolyte present near the main surface of the negative electrode active material layer 22b flows into the grooves 103 of the positive electrode active material layer 21b that face the negative electrode active material layer 22b across the separator 23. At this time, the liquid electrolyte flows from the entire periphery of the grooves 103 toward the nearest groove 103. The liquid electrolyte that has flowed into the grooves 103 then flows through the grooves 103 and is discharged from the ends of the grooves 103 to near the outer periphery of the positive electrode active material layer 21b. During discharging, the liquid electrolyte present near the outer periphery of the negative electrode active material layer 22b flows toward the center of the main surface of the negative electrode active material layer 22b to make up for the liquid electrolyte that flowed into the grooves 103 during charging. Furthermore, part of the liquid electrolyte present near the outer periphery of the negative electrode active material layer 22b also flows into the grooves 103 located near the outer periphery.

[0084] Due to this large flow of the liquid electrolyte, the liquid electrolyte with a high lithium ion concentration present near the main surface of the negative electrode active material layer 22b is supplied to the vicinity of the outer periphery of the negative electrode active material layer 22b through the grooves 103 and the vicinity of the outer periphery of the positive electrode active material layer 21b, while the liquid electrolyte with a low lithium ion concentration present near the outer periphery of the negative electrode active material layer 22b is supplied to the center of the main surface of the negative electrode active material layer 22b.

[0085] This causes the liquid electrolyte with a high lithium ion concentration present near the main surface of the negative electrode active material layer 22b to be replaced by the liquid electrolyte with a low lithium ion concentration present near the outer periphery of the negative electrode active material layer 22b. Then, the liquid electrolyte with a high lithium ion concentration and the liquid electrolyte with a low lithium ion concentration mix together in many locations within the grooves 103 and around the negative electrode active material layer 22b. As a result, a local decrease in the lithium ion concentration of the liquid electrolyte near the outer periphery of the negative electrode active material layer 22b is suppressed. This suppresses a local decrease in the lithium ion concentration in the outer periphery of the negative electrode active material layer 22b, and also suppresses an increase in the electrical resistance of the electrode due to a bias in the lithium ion concentration.

[0086] Furthermore, when the grooves 103 are provided in the positive electrode active material layer 21b, a normal electrode reaction occurs in the portions of the negative electrode active material layer 22b of the negative electrode 22 that overlap with the island portions 104 of the positive electrode active material layer 21b. On the other hand, the electrode reaction is weaker in the portions of the negative electrode active material layer 22b of the negative electrode 22 that overlap with the grooves 103 of the positive electrode active material layer 21b compared to the portions that overlap with the island portions 104. Therefore, in the negative electrode active material layer 22b, portions where the electrode reaction is weak are formed in a regular arrangement that coincides with the grooves 103 in the planar direction. In other words, by providing the grooves 103 in the positive electrode active material layer 21b, portions where the electrode reaction is weak are indirectly formed in the negative electrode active material layer 22b. The portions where the electrode reaction is weak are finely distributed in a shape that coincides with the grooves 103 across the entire planar direction of the negative electrode active material layer 22b.

[0087] In the portions of the negative electrode active material layer 22b where the electrode reaction is weak, the discharge of the liquid electrolyte with a low lithium ion concentration is reduced. Therefore, the portions where the discharge of the liquid electrolyte with a low lithium ion concentration is reduced are finely distributed across the entire surface of the negative electrode active material layer 22b. This causes a small flow that attempts to equalize the small differences in lithium ion concentration throughout the liquid electrolyte present around the negative electrode active material layer 22b, including the vicinity of the central portion of the negative electrode active material layer 22b.

[0088] Even this small flow causes the liquid electrolyte present at the outer periphery of the negative electrode active material layer 22 b to be replaced by the liquid electrolyte present near the center of the negative electrode active material layer 22 b, thereby suppressing a local decrease in the lithium ion concentration in the outer periphery of the negative electrode active material layer 22 b and suppressing an increase in the electrical resistance of the electrode due to the uneven distribution of lithium ion concentration.

[0089] Next, the effects of this embodiment will be described. (1) The electrode 100 for a power storage device is a positive electrode for a power storage device 10 having a bipolar structure in which a plurality of power storage cells 20 are stacked in series. The electrode 100 includes a current collector 101 and a positive electrode active material layer 102 formed on the surface of the current collector 101. The area of ​​the positive electrode active material layer 102 is 1 m2 That is all. The positive electrode active material layer 102 has a main surface 102a located on the side opposite to the surface facing the current collector 101, and grooves 103 opening into the main surface 102a. In a plan view of the main surface 102a, a maximum distance D, which is the maximum value of the specific distance, is 60 mm or less. The specific distance is the shorter of the distance from the outer periphery of the positive electrode active material layer 102 to an arbitrary point within an island portion 104, which is a portion of the main surface 102a where the grooves 103 are not provided, and the distance from the grooves 103.

[0090] According to the above configuration, an increase in electrical resistance due to an increase in the area of ​​the negative electrode active material layer 22b can be suppressed. (2) The positive electrode active material layer 102 has a shape having a longitudinal direction and a lateral direction, and the grooves 103 are linear and extend in the longitudinal direction of the positive electrode active material layer 102 .

[0091] An electrode 100 including a positive electrode active material layer 102 having a shape with a longitudinal direction and a lateral direction is prone to deformation due to its own weight, with the longitudinal ends sagging. Furthermore, when grooves 103 are formed in the positive electrode active material layer 102, the positive electrode active material layer 102 is prone to bending at the portions where the grooves 103 are formed.

[0092] Therefore, when grooves 103 extending in the lateral direction of the positive electrode active material layer 102 are formed, the direction of bending when the longitudinal end deforms to droop overlaps with the direction in which the grooves 103 are formed. This makes it even more likely that the positive electrode will deform in the direction in which the longitudinal end droops. Specifically, during the transport process in manufacturing the electrode, for example, when the positive electrode is transported by suction, the electrode is likely to droop due to its own weight, resulting in deformation of the positive electrode. This problem also occurs, although to a lesser extent, when grooves 103 extending in a direction intersecting the longitudinal direction are formed.

[0093] On the other hand, when grooves 103 extending in the longitudinal direction of the positive electrode active material layer 102 are formed, the direction of bending when the longitudinal end deforms to droop and the direction in which the grooves 103 make the layer more likely to bend are perpendicular to each other. This makes it possible to prevent the electrode from being more likely to deform in the direction in which the longitudinal end droops. Furthermore, the positive electrode active material layer 102 having linear grooves 103 extending in the longitudinal direction can be easily formed by slit coating using a die coater with an obstacle such as a shim provided in the slit die.

[0094] (3) The grooves 103 are linear and extend in the longitudinal direction of the positive electrode active material layer 102. The aspect ratio of the island portions 104 is 12 or greater. By forming the island portion 104 in a shape with a large aspect ratio, that is, in a long and narrow shape, it is possible to increase the proportion of the area of ​​the island portion 104 to the entire area of ​​the positive electrode active material layer 102 while shortening the maximum distance D. Increasing the proportion of the area of ​​the island portion 104 to the entire area of ​​the positive electrode active material layer 102 allows the capacity of the storage cell 20 to be increased.

[0095] (4) The thickness of the positive electrode active material layer 102 is 250 μm or more. When the thickness of the positive electrode active material layer 102 is large, the grooves 103 can be formed deeper. By forming the grooves 103 deeper, the difference in electrode reaction between the portions of the negative electrode active material layer 22b that overlap with the island portions 104 of the positive electrode active material layer 21b and the portions that overlap with the grooves 103 becomes larger. As a result, the liquid electrolyte present around the negative electrode active material layer 22b is more likely to flow.

[0096] (5) In the electricity storage device 10, the thickness of the negative electrode active material layer 22b is 200 μm or more. When the thickness of the negative electrode active material layer 22b is large, the amount of active material per unit area in the negative electrode active material layer 22b increases. In this case, the amount of lithium ions absorbed and released per unit area of ​​the negative electrode active material layer 22b also increases, which makes the lithium ion concentration in the negative electrode active material layer 22b more likely to become uneven. As a result, the electrical resistance of the electrode also tends to increase. Therefore, when the thickness of the negative electrode active material layer 22b is large, it is particularly effective to employ the configuration (1) above, and the effect of (1) above can be more significantly achieved.

[0097] (6) In the electricity storage device 10, the porosity of one or both of the positive electrode active material layer 21b and the negative electrode active material layer 22b is 30% or more. According to the above configuration, the rate at which the bias in the lithium ion concentration in the negative electrode active material layer 22b increases with increasing charge / discharge cycles is reduced. Therefore, by combining this with the above configuration (1), it is possible to more effectively suppress the increase in electrical resistance caused by increasing the area of ​​the negative electrode active material layer 22b.

[0098] (7) Of the positive electrode 21 and negative electrode 22 that constitute the bipolar electrode 25, only the positive electrode 21 is an electrode 100 that has a groove portion 103. According to the above configuration, the decrease in capacity caused by providing the grooves 103 can be suppressed compared to when both the positive electrode 21 and the negative electrode 22 are electrodes 100 having the grooves 103 .

[0099] This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility. The planar shape of the grooves 103 is not limited to the above embodiment. For example, instead of the linear grooves 103 extending in the longitudinal direction of the positive electrode active material layer 102, linear grooves 103 extending in the lateral direction or linear grooves 103 extending in a direction intersecting the longitudinal and lateral directions may be used. Furthermore, grooves 103 extending in different directions may be combined, such as in a lattice pattern. Furthermore, instead of the linear grooves 103, wave-shaped, concentric, or irregularly shaped grooves may be used.

[0100] The width of the grooves 103 may be the same or different. Also, instead of the grooves 103 having a constant width, the grooves 103 may have varying widths. The cross-sectional shape of the groove 103 is not limited to a rectangular shape. Other cross-sectional shapes of the groove 103 include, for example, a V-shaped cross-section, a U-shaped cross-section, and a trapezoidal cross-section.

[0101] The grooves 103 may have a shape that does not reach the current collector 101, that is, a shape in which the bottom is formed by the positive electrode active material layer 102. In this case, the groove depth of the grooves 103 is, for example, 50% or more of the thickness of the positive electrode active material layer 102.

[0102] In the above embodiment, of the positive electrode 21 and the negative electrode 22 constituting the pseudo bipolar electrode 25, only the positive electrode 21 is the electrode 100 having the grooves 103. However, both the positive electrode 21 and the negative electrode 22 may be electrodes 100 having the grooves 103. When the grooves 103 are provided in the negative electrode active material layer 22b, lithium may be deposited in the grooves 103 of the negative electrode active material layer 22b if the following condition is met: the grooves 103 provided in the negative electrode active material layer 22b have a shape that reaches the current collector 101, and the portions of the positive electrode active material layer 21b that overlap with the grooves 103 of the negative electrode active material layer 22b in the stacking direction are island portions 104. Therefore, when the grooves 103 are provided in the negative electrode active material layer 22b, it is preferable to provide the grooves 103 in a portion that overlaps with the portion of the positive electrode active material layer 21b where the grooves 103 are provided in the stacking direction.

[0103] The electrode 100 may be an electrode having a bipolar structure in which the positive electrode 21 and the negative electrode 22 are integrated together. An example of the electrode 100 embodied as an electrode having a bipolar structure in which the positive electrode 21 and the negative electrode 22 are integrated together will be described with reference to Fig. 6 .

[0104] The bipolar electrode 100 shown in Fig. 6 includes a bipolar current collector 105. The bipolar current collector 105 is a laminate formed by integrally joining a foil-shaped positive electrode current collector 106 and a foil-shaped negative electrode current collector 107 in the thickness direction. Examples of the bipolar current collector 105 include a current collector formed by bonding two pieces of aluminum foil together, and a current collector formed by bonding an aluminum foil and a copper foil together.

[0105] An active material layer 108 configured as a positive electrode active material layer is provided on a first surface 105a of the bipolar current collector 105 formed by the positive electrode current collector 106. An active material layer 109 configured as a negative electrode active material layer is provided on a second surface 106a of the bipolar current collector 105 formed by the negative electrode current collector 107.

[0106] Of the active material layers 108 and 109, at least the active material layer 108 configured as the positive electrode active material layer is an active material layer that satisfies the requirements of the active material layer 102 described in the above embodiment. In other words, the bipolar electrode 100 has a structure in which only the positive electrode active material layer is an active material layer having the grooves 103, or a structure in which both the positive electrode active material layer and the negative electrode active material layer are active material layers having the grooves 103.

[0107] The specific configuration of the energy storage device 10 to which the electrode 100 is applied is not particularly limited as long as at least one positive electrode corresponds to the electrode 100. For example, the number of energy storage cells 20 constituting the energy storage device 10 may be one. The energy storage device 10 may also include a restraining member that applies a restraining load to the cell stack 30 in the stacking direction. The energy storage device 10 may also include an electrode 100 configured as a bipolar electrode. [Example]

[0108] Hereinafter, a more specific example of the above embodiment will be described. <Preparation of positive electrode> A positive electrode composite was prepared by mixing LiFePO4 as a positive electrode active material with styrene-butadiene rubber and carboxymethyl cellulose as binders, and adding water to this mixture. The positive electrode composite was applied to the surface of aluminum foil as a positive electrode current collector in the form of a film with slits using a slit die. The applied positive electrode composite was then dried and solidified by heat treatment to prepare the positive electrodes of Test Examples 1 to 3, in which a positive electrode active material layer with slit-shaped grooves was formed on the positive electrode current collector.

[0109] The positive electrode active material layer of the positive electrodes of Test Examples 1 to 3 has a rectangular shape in plan view. The positive electrodes of Test Examples 1 to 3 have the same overall dimensions of 1155 mm long x 1476.5 mm wide x 250 μm thick, but the shape of the grooves in plan view is different. The grooves of the positive electrode of Test Example 1 are linear, 3 mm wide, and extend laterally in plan view, with multiple grooves arranged vertically at a 122 mm pitch. The grooves of the positive electrode of Test Example 2 are linear, 2 mm wide, and extend laterally in plan view, with multiple grooves arranged vertically at an 82 mm pitch. The grooves of the positive electrode of Test Example 3 are linear, 1 mm wide, and extend laterally in plan view, with multiple grooves arranged vertically at a 42 mm pitch. The island width H, maximum distance D between the island portions, and aspect ratios of the island portions in the positive electrode active material layers of Test Examples 1 to 3 are as shown in Table 1.

[0110] The positive electrode composite was applied to the surface of an aluminum foil serving as a positive electrode current collector using a doctor blade method in the form of a film without slits. The applied positive electrode composite was then dried and solidified by heat treatment to produce a positive electrode of Test Example 4, in which a positive electrode active material layer without grooves was formed on the positive electrode current collector. The overall dimensions of the positive electrode active material layer of Test Example 4 were the same as those of the positive electrodes of Test Examples 1 to 3.

[0111] <Creating an energy storage device> A bipolar electrode battery having a structure in which 30 storage cells were stacked was fabricated by combining the positive electrodes, negative electrodes, and separators of Test Examples 1 to 4. The electrode battery was housed in a battery case, and an electrolyte was poured into the battery case, which was then sealed to obtain a lithium ion secondary battery.

[0112] The negative electrode used had a copper negative electrode current collector and a negative electrode active material layer made of graphite as the negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a dispersant. No grooves were formed in the negative electrode active material layer. The negative electrode active material layer of the negative electrode had a rectangular shape in plan view. The overall dimensions of the negative electrode active material layer were 1155 mm long, 1476.5 mm wide, and 250 μm thick. A polyethylene separator was used. The electrolyte solution used was a mixed solvent of ethylene carbonate and methyl propionate in a volume ratio of 15:85, with lithium hexafluorophosphate dissolved to a concentration of 1.2 M.

[0113] <Evaluation of the resistance of the power storage device> Each lithium-ion secondary battery was subjected to 15 cycles of constant current charging at 1.3 C for 30 minutes followed by constant current discharging at 0.2 C for 195 minutes. During this cycle, the discharge resistance at a 50% SOC (State of Charge) was measured after 0 cycle (i.e., the state before the first cycle), 5 cycles, 10 cycles, and 15 cycles. The rate of change in discharge resistance after 5 cycles, 10 cycles, and 15 cycles relative to the discharge resistance after 0 cycle (hereinafter referred to as the resistance change rate) was calculated. The results are shown in Table 1.

[0114] The discharge resistance was measured as follows. After adjusting the SOC of the lithium-ion secondary battery to 50%, a constant current discharge of 1C was performed for 10 seconds. The voltage before discharge (V ocv ) to the voltage reached when discharging for 10 seconds (V ccv ) divided by the current value to determine the discharge resistance.

[0115] <Evaluation of the bias in salt concentration in the active material layer> Furthermore, for the lithium ion secondary batteries using the positive electrodes of Test Examples 1 to 3, the salt concentration was measured at each location in the negative electrode active material layer. Specifically, the negative electrode was removed from the lithium ion secondary battery after 15 cycles, and the salt concentration (lithium hexafluorophosphate concentration) was measured at three specific points on the main surface of the negative electrode active material layer. Then, the maximum salt concentration difference, which was the maximum value of the salt concentration differences at the three specific points, was calculated. The results are shown in Table 1.

[0116] As shown in FIG. 4 , the three specific points are located on the main surface of the negative electrode active material layer and are opposite to the three points on the main surface of the positive electrode active material layer: the outer peripheral end point P1, the groove side end point P2, and the center point P3; that is, points that overlap in the stacking direction. The outer peripheral end point P1 is located on the outer peripheral edge of the positive electrode active material layer in the vertical direction in a planar view. The groove side end point P2 is located at the edge of the island portion of the positive electrode active material layer, and is 738.25 mm away from the outer peripheral edge of the positive electrode active material layer in the vertical direction in a planar view. The center point P3 is located at the edge of the island portion of the positive electrode active material layer, and is 738.25 mm away from the outer peripheral edge of the positive electrode active material layer in the vertical direction in a planar view. The center point P3 is located on a line S1 (specific line) in the island portion of the positive electrode active material layer, and is 738.25 mm away from the outer peripheral edge of the positive electrode active material layer in the planar view. The line S1 extends in the horizontal direction and bisects the island portion 104.

[0117] [Table 1]

[0118] As shown in Table 1, in Test Example 4, the rate of resistance change increases significantly with an increase in the number of cycles. This result indicates that the electrical resistance increases as the lithium-ion secondary battery is repeatedly charged and discharged. Although detailed data is omitted, a similar test was conducted using an electrode with the same configuration as Test Example 4, except that the area of ​​the negative electrode active material layer was reduced, and no increase in resistance due to repeated charging and discharging was confirmed. In this test, a negative electrode with an overall dimension of a negative electrode active material layer of 250 mm length x 350 mm width x 250 mm thickness was used. From these results, it can be seen that the increase in resistance due to repeated charging and discharging occurs when the area of ​​the negative electrode active material layer is large, for example, when the area of ​​the negative electrode active material layer is large, for example, when the area of ​​the negative electrode active material layer is small ... 2 It can be seen that this is a phenomenon specific to the above cases.

[0119] On the other hand, Test Examples 1 to 3, in which grooves were provided in the positive electrode active material layer, showed a smaller rate of resistance change after each cycle than Test Example 4, in which grooves were not provided in the positive electrode active material layer, and the increase in resistance due to repeated charge and discharge was suppressed. In particular, Test Example 3, in which the maximum distance D of the positive electrode active material layer was 30 mm or less, maintained the rate of resistance change after each cycle at nearly 100%, and the increase in resistance due to repeated charge and discharge was significantly suppressed. These results demonstrate that by providing grooves in the positive electrode active material layer so as to shorten the maximum distance D, the increase in resistance due to repeated charge and discharge can be suppressed.

[0120] Next, the salt concentration measurement results showed that the negative electrode active material layer after 15 cycles had a salt concentration bias depending on the region, with the salt concentration higher in the center and lower in the outer periphery in a plan view. The greater this salt concentration bias, i.e., the greater the maximum salt concentration difference, the greater the resistance change rate. It is also known that the salt concentration bias in the negative electrode active material layer creates areas with high and low electrical resistance in the negative electrode active material layer, and that the greater the difference in resistance between regions in the negative electrode active material layer, the greater the electrical resistance of the secondary battery.

[0121] From these points, the following knowledge can be obtained regarding the increase in resistance due to repeated charge and discharge when the area of ​​the negative electrode active material layer is large. Repeated charge and discharge cycles result in a difference in salt concentration between the central and peripheral portions of the negative electrode active material layer in plan view. When the area of ​​the negative electrode active material layer is large, the distance between the central and peripheral portions of the negative electrode active material layer in plan view increases, making the salt concentration imbalance due to repeated charge and discharge more pronounced. When the difference in resistance between different portions of the negative electrode active material layer due to the salt concentration imbalance becomes large enough to affect the electrical resistance of the secondary battery, the resistance increases due to repeated charge and discharge. Furthermore, by providing a groove in the positive electrode active material layer that shortens the maximum distance D, as in Test Examples 1 to 3, the salt concentration imbalance in the negative electrode active material layer can be reduced and the increase in resistance due to repeated charge and discharge can be suppressed. [Explanation of symbols]

[0122] 100...electrode 101...current collector 102...Active material layer 102a…Main surface 103...Groove 104...Island 10...Electricity storage device (lithium ion secondary battery) 21...Positive electrode 22...Negative electrode

Claims

1. An electrode for a bipolar lithium ion secondary battery, a current collector and a positive electrode active material layer formed on the surface of the current collector; The area of ​​the positive electrode active material layer is 1 m ２ That's all, the positive electrode active material layer has a main surface located on the opposite side to a surface facing the current collector, and a groove portion opening into the main surface, an electrode for a lithium ion secondary battery, wherein, in a plan view of the main surface, when a specific distance is defined as the shorter of a distance from an outer peripheral edge of the positive electrode active material layer to an arbitrary point within an island portion, which is a portion of the main surface where no groove portion is provided, or a distance from the groove portion to the arbitrary point, the maximum specific distance is 60 mm or less.

2. the positive electrode active material layer has a shape having a longitudinal direction and a lateral direction, 2. The electrode for a lithium ion secondary battery according to claim 1, wherein the groove portion is linear and extends in the longitudinal direction of the positive electrode active material layer.

3. the positive electrode active material layer has a rectangular shape having a longitudinal direction and a lateral direction, the groove portion is linear and extends in the longitudinal direction of the positive electrode active material layer, 2. The electrode for a lithium ion secondary battery according to claim 1, wherein the aspect ratio of the island portion is 12 or more.

4. 2. The electrode for a lithium ion secondary battery according to claim 1, wherein the thickness of the positive electrode active material layer is 250 μm or more.

5. A bipolar lithium ion secondary battery, A lithium ion secondary battery comprising the electrode according to any one of claims 1 to 4.

6. 6. The lithium ion secondary battery according to claim 5, having a capacity of 50 kWh or more.

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