Electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

This electrode is provided with: a current collector; and a positive electrode active material layer (102) formed on the surface of the current collector. The area of the positive electrode active material layer (102) is 1 m2 or more. The positive electrode active material layer (102) is provided with: a main surface positioned on the opposite side from the surface facing the current collector; and a groove section (103) that opens in the main surface. In a plan view of the main surface, a maximum distance D, which is the maximum value of the specific distance, is 60 mm or less. The specific distance is the shorter distance between the distance from the outer peripheral edge of the positive electrode active material layer (102) and the distance from the groove portion (103) at an arbitrary point in an island portion (104), which is a portion of the main surface where the groove portion (103) is 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 Art

[0002] Patent Document 1 discloses a bipolar structure power storage device formed by stacking a plurality of separately manufactured power storage units in series. The power storage unit comprises: a positive electrode formed by forming a positive electrode active material layer on one surface of a foil-shaped positive electrode current collector; a negative electrode formed by forming a negative electrode active material layer on one surface of a foil-shaped negative electrode current collector, the negative electrode active material layer being arranged in a manner that the negative electrode active material layer faces the positive electrode active material layer of the positive electrode; and a separator arranged between the positive electrode and the negative electrode.

[0003] In the above-mentioned power storage device, the plurality of the above-mentioned power storage cells are electrically connected in series by being stacked in such a manner that the positive electrode collector and the negative electrode collector are in contact. In this case, the current flows in the stacking direction of the power storage cells. Therefore, compared with a power storage device having a structure in which each power storage cell is electrically connected in series through a tab led out from each power storage cell, the above-mentioned power storage device can ensure a larger area of ​​the current path, and as a result, a higher output can be obtained.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2017-16825 Summary of the invention

[0007] Problem that the invention aims to solve

[0008] As a method of increasing the capacity of the power storage cell, a method of increasing the amount of active material held in the electrode active material layer by forming the electrode active material layer larger can be considered.

[0009] Here, in the case of a storage device having a structure in which current flows in the stacking direction of the storage unit, such as the above-mentioned storage device, if the electrode active material layer is formed thicker to make the electrode active material layer larger, the problem of increased resistance will arise. In addition, depending on the application of the above-mentioned storage device, there is also a situation where the increase in the height dimension of the above-mentioned storage device caused by thickening the electrode active material layer should be avoided. For example, in the case of a storage device used as a battery configured under the floor of a vehicle such as an electric vehicle or a hybrid vehicle, it is preferred that the storage device is as thin and flat as possible. The maximum height of the battery configured under the floor of the vehicle is about 20 cm.

[0010] Therefore, the inventors of the present invention have studied the increase in capacity caused by increasing the area of ​​the electrode active material layer, that is, increasing the planar size of the electrode active material layer, with respect to a lithium-ion secondary battery having a structure in which current flows in the stacking direction of the storage unit. Specifically, the capacity required for a lithium-ion secondary battery for electric vehicles is about 50 kWh to 100 kWh.

[0011] Even if the composition and density of the electrode active material layer are taken into consideration, in order to make the height of the lithium ion secondary battery less than 20 cm and obtain a capacity of 50 kWh or more, the area of ​​the electrode active material layer needs to be 1 m 2 The electrode active material layer is made in an area of ​​1m 2 When the above lithium-ion secondary batteries were used and their properties were evaluated, a new problem of increased electrode resistance was found. Since this increase in resistance does not occur in conventional lithium-ion secondary batteries with a planar size, it can be considered that this is a phenomenon unique to the case where the area of ​​the electrode active material layer is increased to a certain area or more.

[0012] Solutions for solving problems

[0013] An electrode for a lithium ion secondary battery is an electrode for a lithium ion secondary battery with a bipolar structure, comprising: a current collector; and a positive electrode active material layer formed on the surface of the current collector, wherein the area of ​​the positive electrode active material layer is 1 m 2 As described above, the positive electrode active material layer comprises: a main surface, which is located on the opposite side of the surface opposite to the collector; and a groove portion, which is open on the main surface. When the main surface is viewed from above, the shorter distance between the distance from the outer peripheral edge of the positive electrode active material layer and the distance from the groove portion at any point in the island portion of the main surface where the groove portion is not provided is set as a specific distance, and the maximum value of the specific distance is less than 60 mm.

[0014] In the above-mentioned electrode for lithium ion secondary battery, preferably, the positive electrode active material layer has a shape having a long side direction and a short side direction, and the groove portion is a straight line extending in the long side direction of the positive electrode active material layer.

[0015] In the above-mentioned electrode for lithium ion secondary battery, the positive electrode active material layer is rectangular with a long side direction and a short side direction, the groove portion is a straight line extending in the long side direction of the positive electrode active material layer, and the aspect ratio of the island portion is greater than 12.

[0016] In the above-mentioned electrode for lithium ion secondary battery, the thickness of the positive electrode active material layer is 250 μm or more.

[0017] A lithium ion secondary battery is a bipolar lithium ion secondary battery having the above-mentioned electrodes for the lithium ion secondary battery.

[0018] In the above-mentioned lithium ion secondary battery, the capacity is 50 kWh or more.

[0019] Effects of the Invention

[0020] According to the present invention, in an electrode for a lithium ion secondary battery having a bipolar structure, an increase in resistance due to an increase in the area of ​​an active material layer can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a cross-sectional view of an electrode for an electricity storage device.

[0022] Figure 2 It is a top view of an electrode for an electricity storage device.

[0023] Figure 3 yes Figure 2 3-3 line cross-section diagram.

[0024] Figure 4 yes Figure 2 Magnified view of part A.

[0025] Figure 5 It is a cross-sectional view of the power storage device.

[0026] Figure 6 It is a cross-sectional view of a bipolar electrode according to a modified example. DETAILED DESCRIPTION

[0027] Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

[0028] (electrode)

[0029] The electrode of this embodiment is used as a positive electrode or a negative electrode of a bipolar structure power storage device in which a plurality of power storage cells are stacked in series. The power storage device is a lithium ion secondary battery.

[0030] like Figure 1 As shown, the electrode 100 is an electrode for a lithium ion secondary battery, and includes a current collector 101 and an active material layer 102 provided on a first surface 101 a of the current collector 101 .

[0031] [Current Collector]

[0032] The current collector 101 is a chemically inert electrical conductor for allowing current to continue to flow through the active material layer 102 during the discharge or charge period of the lithium ion secondary battery. The current collector 101 is, for example, in the form of a foil. The thickness of the foil-shaped current collector 101 is, for example, 1 μm or more and 100 μm or less, preferably 10 μm or more and 60 μm or less. As a material constituting the current collector 101, for example, a metal material, a conductive resin material, a conductive inorganic material, etc. can be used.

[0033] Examples of the metal material include copper, aluminum, nickel, titanium, and stainless steel. Examples of the conductive resin material include conductive polymer materials or resins obtained by adding conductive fillers to non-conductive polymer materials as needed.

[0034] In the case of the electrode 100 used as a positive electrode of an electric storage device, the current collector 101 is preferably an aluminum current collector composed of aluminum. The aluminum current collector may be composed of a single substance of aluminum 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, preferably 70% by mass or more.

[0035] The current collector 101 may also include a plurality of layers including one or more layers containing the aforementioned metal material or conductive resin material. The surface of the current collector 101 may also be coated with a known protective layer such as a carbon coating. The surface of the current collector 101 may also be treated by a known method such as plating.

[0036] [Active material layer]

[0037] The active material layer 102 is formed on the first surface 101 a of the current collector 101 .

[0038] The active material layer 102 contains an active material that can absorb and release lithium ions.

[0039] In the case of the electrode 100 used as the positive electrode of the 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, it is sufficient to use a material that can be used as a positive electrode active material for a lithium ion secondary battery, such as a lithium composite metal oxide with a layered rock salt structure, a metal oxide with a spinel structure, a polyanion compound, etc. In addition, two or more positive electrode active materials can also be used at the same time. As a specific example of the positive electrode active material, olivine-type lithium iron phosphate (LiFePO4) as a polyanion compound can be cited.

[0040] In the case of the electrode 100 used as the negative electrode of the power storage device, the active material contained in the active material layer 102 is a negative electrode active material. As the negative electrode active material, any material that can be used as a negative electrode active material for a lithium ion secondary battery, such as Li, carbon, a metal compound, or an element or its compound that can be alloyed with lithium, can be used. As carbon, for example, natural graphite, artificial graphite, or hard carbon (difficult to graphitize carbon) or soft carbon (easy to graphitize carbon) can be listed. As artificial graphite, for example, highly oriented graphite (graphite), mesophase carbon microbeads, etc. can be listed. As elements that can be alloyed with lithium, for example, silicon (silicon) and tin can be listed.

[0041] The content of the active material in the active material layer 102 is not particularly limited. The content of the active material in the active material layer 102 is, for example, 96 mass % or more and less than 100 mass %.

[0042] The active material layer 102 may further include a conductive additive, a binder, an electrolyte (polymer matrix, ion-conductive polymer, liquid electrolyte, etc.) for improving electrical conductivity, and an electrolyte supporting salt (lithium salt) for improving ion conductivity, etc., as required. The components included in the active material layer or the mixing ratio of the components and the thickness of the active material layer are not particularly limited, and can be appropriately referred to the previously known views on lithium-ion secondary batteries.

[0043] The conductive additive is added to improve the conductivity of the electrode 100. Examples of the conductive additive include acetylene black, carbon black, graphite, and carbon nanotubes (CNT).

[0044] As the binder, 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 polyamide-imide, resins containing alkoxysilyl groups, acrylic resins such as polyacrylic acid or methacrylic acid, styrene-butadiene rubber, carboxymethyl cellulose, alginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester crosslinked products, starch-acrylic acid graft polymers, etc. can be exemplified. These binders can be used alone or in combination. Solvents or dispersion media, for example, water, N-methyl-2-pyrrolidone, etc. are used.

[0045] [Size and shape of active material layer]

[0046] like Figure 2As shown, in a plan view (hereinafter referred to as a plan view) viewed from the stacking direction of the current collector 101 and the active material layer 102, the active material layer 102 is formed in the central portion of the first surface 101a of the current collector 101. The peripheral portion of the first surface 101a of the current collector 101 in a plan view becomes 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 a plan view.

[0047] The thickness, density, weight per unit area, and porosity of the active material layer 102 are not particularly limited, and can be appropriately referred to the conventionally known viewpoints on lithium-ion secondary batteries. For each of the case where the active material layer 102 is a positive electrode active material layer and the case where the active material layer 102 is a negative electrode active material layer, specific examples of the thickness, density, weight per unit area, and porosity of the active material layer 102 are described below.

[0048] <Thickness, density, weight per unit area, and porosity in the case of a positive electrode active material layer>

[0049] The thickness of the active material layer 102 is, for example, 250 μm or more, preferably 300 μm or more. The thickness of the active material layer 102 is, for example, 600 μm or less, 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.

[0050] The density of the active material layer 102 is, for example, 1.6 g / cm 3 Above, preferably 1.8 g / cm 3 The density of the active material layer 102 is, for example, 2.5 g / cm 3 Below, preferably 2.3 g / cm 3 By increasing the density of the active material layer 102 , the capacity of the power storage cell can be increased.

[0051] The weight per unit area of ​​the active material layer 102 is, for example, 50 mg / cm 2 Above, preferably 60 mg / cm 2 More preferably, 70 mg / cm 2 The weight per unit area of ​​the active material layer 102 is, for example, 90 mg / cm 2 Below, preferably 80 mg / cm 2 By increasing the weight per unit area of ​​the active material layer 102 , the capacity of the power storage cell can be increased.

[0052] The porosity of the active material layer 102 is, for example, 30% or more, or preferably 35% or more. The porosity of the active material layer 102 is, for example, 55% or less, or preferably 45% or less.

[0053] <Thickness, density, basis weight and porosity in the case of negative electrode active material layer>

[0054] The thickness of the active material layer 102 is, for example, 200 μm or more, preferably 250 μm or more. The thickness of the active material layer 102 is, for example, 600 μm or less, 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.

[0055] The density of the active material layer 102 is, for example, 1.1 g / cm 3 Above, preferably 1.2 g / cm 3 The density of the active material layer 102 is, for example, 1.7 g / cm 3 Below, preferably 1.5 g / cm 3 By increasing the density of the active material layer 102 , the capacity of the power storage cell can be increased.

[0056] The weight per unit area of ​​the active material layer 102 is, for example, 30 mg / cm 2 Above, preferably 33 mg / cm 2 More preferably, 35 mg / cm 2 The weight per unit area of ​​the active material layer 102 is, for example, 50 mg / cm 2 Below, preferably 45 mg / cm 2 By increasing the weight per unit area of ​​the active material layer 102 , the capacity of the power storage cell can be increased.

[0057] The porosity of the active material layer 102 is, for example, 30% or more, or preferably 35% or more. The porosity of the active material layer 102 is, for example, 55% or less, or preferably 45% or less.

[0058] The area of ​​the active material layer 102, that is, the area of ​​the first surface 101a of the current collector 101 where the active material layer 102 is formed, is 1 m 2 The area of ​​the active material layer 102 is preferably 1.2 m 2 More than 1.4m is more preferred 2 In addition, the area of ​​the active material layer 102 is, for example, 3 m 2 In the present specification, the area of ​​the active material layer 102 includes the area of ​​the groove portion 103 described later.

[0059] The top view shape of the active material layer 102 is not particularly limited. The top view shape of the active material layer 102 is, for example, a polygonal shape, a circular shape, or an elliptical shape. When the active material layer 102 is rectangular, when viewed from above, the aspect ratio of the active material layer 102 is, for example, greater than 1 and less than 2.5, preferably greater than 1 and less than 2. The longitudinal length L1 of the active material layer 102 is, for example, greater than 500 mm and less than 1500 mm, and the lateral length L2 is, for example, greater than 800 mm and less than 3000 mm. In addition, the above-mentioned top view shape of the active material layer 102, the aspect ratio of the active material layer 102, the longitudinal length L1, and the lateral length L2 values ​​can be applied when the active material layer 102 is any one of the positive electrode active material layer and the negative electrode active material layer.

[0060] [Groove portion of active material layer]

[0061] In the case of the electrode 100 used as a positive electrode of an electric storage device, a groove portion 103 is provided in the active material layer 102. Hereinafter, the groove portion 103 will be described by taking as an example a case where the plan view shape of the active material layer 102 is a horizontally long rectangular shape. Hereinafter, the active material layer 102 is described as a positive electrode active material layer 102.

[0062] like Figure 2 and Figure 3 As shown, the positive electrode active material layer 102 is provided with grooves 103 having a rectangular cross section and opening on a main surface 102a. The main surface 102a is a surface of the positive electrode active material layer 102 located on the opposite side to the surface facing the current collector 101.

[0063] In a plan view, the groove 103 extends in a lateral direction which is the long side direction of the positive electrode active material layer 102. The groove 103 has a fixed width from one end to the other end in the lateral direction and is formed in a straight line. In the positive electrode active material layer 102, a plurality of grooves 103 are formed in parallel in the longitudinal direction at a fixed interval. The bottom surface of the groove 103 is formed by the collector 101. The groove 103 is slit-shaped. The cross-sectional shape of the groove 103 is rectangular.

[0064] Here, in a plan view, the portion of the main surface 102a of the positive electrode active material layer 102 where the groove portion 103 is not formed is set as the island portion 104 of the positive electrode active material layer 102. As described above, the groove portion 103 is a straight line extending in the lateral direction of the positive electrode active material layer 102, and a plurality of groove portions 103 are arranged in parallel in the longitudinal direction. Therefore, each island portion 104 of the positive electrode active material layer 102 is formed in a horizontally long rectangular shape.

[0065] like Figure 4As shown, the island 104 includes an outer edge 104a, which is an edge constituting the outer periphery of the positive electrode active material layer 102, and a groove edge 104b, which is an edge constituting the groove 103. In this specification, the outer periphery of the active material layer 102 refers to the outer periphery of the range of the groove 103 and the island 104 combined.

[0066] The island 104 is formed in a shape in which the distance from the outer edge 104a and the groove edge 104b satisfies the following conditions. In other words, the groove 103 is formed in the positive electrode active material layer 102 so as to form the island 104 in a shape satisfying the following conditions.

[0067] The above condition is that when the shorter distance between the distance L3 from the outer edge 104a and the distance L4 from the groove edge 104b at any point P in the island 104 is set as the specific distance, the maximum value of the specific distance is 60 mm or less. This condition means that at all points in the island 104, the distance to the nearest point on the periphery of the island 104 is 60 mm or less. Hereinafter, the maximum value of the specific distance is recorded as the maximum distance D. In the case where the island 104 is a horizontally long rectangular shape, the point in the island 104 where the maximum distance D is taken is a point on the straight line S1 extending in the horizontal direction and bisecting the island 104, where the distance L3 to the outer edge 104a is longer than the distance L4 to the groove edge 104b. And, the maximum distance D in this case becomes a value of 1 / 2 of the width H of the island 104.

[0068] 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 the increase in resistance is improved.

[0069] The width H, which is the length in the short-side direction of the island portion 104 , is, for example, 120 mm or less, preferably 80 mm or less, and more preferably 40 mm or less.

[0070] The aspect ratio of the island portion 104 is, for example, not less than 12, preferably not less than 15, and more preferably not less than 17. The aspect ratio of the island portion 104 is, for example, not more than 40.

[0071] The ratio of the total area of ​​the islands 104 to the area of ​​the positive electrode active material layer 102 is, for example, 90% or more and 99% or less. By increasing the above ratio, the capacity of the power storage cell can be increased.

[0072] The width of the groove portion 103 is, for example, 0.5 mm or more. By setting the width of the groove portion 103 to be 0.5 mm or more, the effect of suppressing the increase in resistance can be significantly obtained. In addition, by increasing the width of the groove portion 103, the operation of forming the positive electrode active material layer 102 having the groove portion 103 becomes easy, and the operation time of injecting the electrolyte through the groove portion 103 can be shortened. In addition, the width of the groove portion 103 is, for example, 3 mm or more. In this case, in addition to the above-mentioned effects, the electrolyte retention function, that is, the ability to retain the electrolyte in the groove portion 103, can also be improved. In addition, the width of the groove portion 103 is, for example, 8 mm or less or 3 mm or less. By reducing the width of the groove portion 103, the proportion of the total area of ​​the island portion 104 in the area of ​​the positive electrode active material layer 102 can be increased. In addition, the formation spacing of the groove portion 103 is the sum of the width of the groove portion 103 and the width H of the island portion 104.

[0073] The groove 103 is formed to reach the current collector 101. That is, the bottom of the groove 103 is the first surface 101a of the current collector 101, and the groove depth of the groove 103 is the same as the thickness of the positive electrode active material layer 102. In addition, when a protective layer such as a carbon coating is applied to the first surface 101a of the current collector 101, the protective layer becomes the bottom of the groove 103. By forming the groove 103 in a manner to reach the current collector 101, the flow path cross-sectional area of ​​the groove 103 as the flow path of the electrolyte can be maximized.

[0074] The method for forming the groove portion 103 is not particularly limited. As a method for forming the groove portion 103, for example, when the composite material that becomes the positive electrode active material layer 102 by curing is applied to the collector 101 to form the positive electrode active material layer 102, a method of coating the composite material by slit coating can be cited. Specifically, a die coater (die coater) is prepared in which obstacles such as shims that partially hinder the discharge of the composite material are set at the discharge port of the slit die. Then, by using the die coater to coat the composite material, the composite material can be coated into a shape having a groove portion. In addition, as another method, the groove portion 103 can also be formed by partially cutting the main surface 102a of the positive electrode active material layer 102 formed into a shape without the groove portion 103.

[0075] (Electricity storage device)

[0076] Next, an example of a power storage device to which the electrode 100 is applied will be described.

[0077] The power storage device to which the electrode 100 is applied is, for example, a lithium ion secondary battery used in batteries of various vehicles such as a forklift, a hybrid car, and an electric car.

[0078] like Figure 5The structure shown is that, in the power storage device 10, there is a unit stack body 30 (stacked body) in which a plurality of power storage cells 20 are stacked (stacked) in a stacking direction. Hereinafter, the stacking direction of the plurality of power storage cells 20 is referred to as the stacking direction. Each power storage cell 20 includes a positive electrode 21, a negative electrode 22, a separator 23, and a spacer 24. Both the positive electrode 21 and the negative electrode 22 of each power storage cell 20 are the above-mentioned electrodes 100. That is, the positive electrode 21 is an electrode 100 having a groove portion 103, and the negative electrode 22 is an electrode 100 not having a groove portion 103. In addition, in Figure 5 In the figure, the groove portion 103 is omitted.

[0079] The positive electrode 21 includes a positive electrode current collector 21 a and a positive electrode active material layer 21 b provided on a first surface 21 a 1 of the positive electrode current collector 21 a. When the positive electrode 21 is the electrode 100 , the positive electrode current collector 21 a is the current collector 101 , and the positive electrode active material layer 21 b is the active material layer 102 .

[0080] The positive electrode active material layer 21b is formed in the central portion of the first surface 21a1 of the positive electrode collector 21a in a plan view. The peripheral portion of the first surface 21a1 of the positive electrode collector 21a in a plan view is a positive electrode uncoated portion 21c where the positive electrode active material layer 21b is not provided. The positive electrode uncoated portion 21c is arranged so as to surround the positive electrode active material layer 21b in a plan view.

[0081] 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 the electrode 100, the negative electrode current collector 22a is the current collector 101, and the negative electrode active material layer 22b is the active material layer 102.

[0082] When viewed from above, the negative electrode active material layer 22b is formed in the central part of the first surface 22a1 of the negative electrode collector 22a. The peripheral edge of the first surface 22a1 of the negative electrode collector 22a when viewed from above becomes 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 in a manner surrounding the positive electrode active material layer 21b when viewed from above. The positive electrode 21 and the negative electrode 22 are arranged in a manner such that the positive electrode active material layer 21b and the negative electrode active material layer 22b are opposite to each other in the stacking direction. That is, the relative direction of the positive electrode 21 and the negative electrode 22 is consistent with the stacking direction. The negative electrode active material layer 22b is formed to be equal in size to the positive electrode active material layer 21b or is formed to be one circle larger than the positive electrode active material layer 21b. When the negative electrode active material layer 22 b is formed slightly larger than the positive electrode active material layer 21 b , the entire formation region of the positive electrode active material layer 21 b is located within the formation region of the negative electrode active material layer 22 b in plan view.

[0083] The positive electrode current collector 21a has a second surface 21a2 as the surface opposite to the first surface 21a1. The positive electrode 21 is an electrode of a single-pole structure 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 as the surface opposite to the first surface 22a1. The negative electrode 22 is an electrode of a single-pole structure 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.

[0084] The separator 23 is a member disposed between the positive electrode 21 and the negative electrode 22 , and is a member that prevents a short circuit caused by contact between the positive electrode 21 and the negative electrode 22 and allows lithium ions to pass therethrough.

[0085] The separator 23 is, for example, a porous sheet or nonwoven fabric containing a polymer that absorbs and retains electrolytes. As materials constituting the separator 23, for example, polyolefins such as polypropylene and polyethylene, polyesters, etc. can be cited. The separator 23 can have a single-layer structure or a multi-layer structure. The multi-layer structure can have, for example, an adhesive layer, a ceramic layer as a heat-resistant layer, etc.

[0086] The separator 24 is disposed between the first surface 21a1 of the positive electrode collector 21a of the positive electrode 21 and the first surface 22a1 of the negative electrode collector 22a of the negative electrode 22 and is located on the outer peripheral 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 collector 21a and the negative electrode collector 22a. The separator 24 maintains the interval between the positive electrode collector 21a and the negative electrode collector 22a to prevent a short circuit between the collectors, and seals the collectors liquid-tightly.

[0087] The spacer 24 is formed in a frame shape extending along the peripheral edge of the positive electrode collector 21a and the negative electrode collector 22a and surrounding the positive electrode collector 21a and the negative electrode collector 22a in a plan view. The spacer 24 is arranged between the positive electrode uncoated portion 21c of the first surface 21a1 of the positive electrode collector 21a and the negative electrode uncoated portion 22c of the first surface 22a1 of the negative electrode collector 22a.

[0088] Examples of the material constituting 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.

[0089] A sealed space S surrounded by a frame-shaped separator 24 , the positive electrode 21 , and the negative electrode 22 is formed inside the power storage cell 20 . A separator 23 and an electrolyte are accommodated in the sealed space S. The peripheral edge of the separator 23 is buried in the separator 24 .

[0090] The electrolyte is a liquid electrolyte. As the liquid electrolyte, for example, a liquid electrolyte comprising a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent can be cited. As the electrolyte salt, known lithium salts such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, LiN(FSO2)2, and LiN(CF3SO2)2 can be used. In addition, as the non-aqueous solvent, known solvents such as cyclic carbonates, cyclic esters, chain carbonates, chain esters, and ethers can be used. In addition, two or more of these known solvent materials can also be used in combination. A preferred example of a combination of solvent materials is a solvent composed of a cyclic ester and a chain lipid. Since the solvent composed of a cyclic ester and a chain lipid is low in viscosity, the fluidity of the liquid electrolyte is improved. By improving the fluidity of the liquid electrolyte, it is easy to obtain the effect of suppressing the increase in resistance.

[0091] The separator 24 can suppress leakage of the electrolyte contained in the sealed space S to the outside by sealing the sealed space S between the positive electrode 21 and the negative electrode 22. In addition, the separator 24 can suppress the intrusion of moisture from the outside of the power storage device 10 into the sealed space S. Moreover, the separator 24 can suppress leakage of gas generated from the positive electrode 21 or the negative electrode 22 by, for example, charge and discharge reactions to the outside of the power storage device 10.

[0092] 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 contacts the second surface 22a2 of the negative electrode current collector 22a. Thus, the plurality of storage cells 20 constituting the cell stack 30 are connected in series.

[0093] Here, in the unit stack 30, two storage cells 20 adjacent in the stacking direction form a pseudo bipolar electrode 25 in which the positive current collector 21a and the negative current collector 22a connected to each other are regarded as one current collector. The pseudo bipolar electrode 25 includes: a current collector having a structure in which the positive current collector 21a and the negative current collector 22a are stacked; a positive active material layer 21b formed on one side of the current collector; and a negative active material layer 22b formed on the other side of the current collector.

[0094] The power storage device 10 includes a pair of current-carrying bodies arranged to sandwich the unit stack 30 in the stacking direction thereof, and including a positive current-carrying plate 40 and a negative current-carrying plate 50. The positive current-carrying plate 40 and the negative current-carrying plate 50 are each made of a material having excellent electrical conductivity.

[0095] The positive electrode current conducting plate 40 is electrically connected to the second surface 21a2 of the positive electrode collector 21a of the outermost positive electrode 21 at one end in the stacking direction. The negative electrode current conducting plate 50 is electrically connected to the second surface 22a2 of the negative electrode collector 22a of the outermost negative electrode 22 at the other end in the stacking direction.

[0096] The charging and discharging of the power storage device 10 is performed by terminals respectively provided on the positive electrode current supply plate 40 and the negative electrode current supply plate 50. As a material constituting the positive electrode current supply plate 40, for example, the same material as the material constituting the positive electrode current collector 21a can be used. The positive electrode current supply plate 40 can also be composed of a metal plate thicker than the positive electrode current collector 21a used in the unit stack 30. As a material constituting the negative electrode current supply plate 50, for example, the same material as the material constituting the negative electrode current collector 22a can be used. The negative electrode current supply plate 50 can also be composed of a metal plate thicker than the negative electrode current collector 22a used in the unit stack 30.

[0097] As described above, the positive electrode 21 of each storage cell 20 constituting the 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. Since 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 are each 1 m 2 Therefore, the planar size of the power storage device 10 is 1 m 2 above.

[0098] The height dimension of the power storage device 10 is, for example, less than 20 cm. The lower limit of the height dimension of the power storage device 10 can be set to a value that can obtain the capacity required by the power storage device 10 according to the area of ​​the positive electrode active material layer 21 b and the negative electrode active material layer 22 b of the negative electrode 22. For example, in the case of a lithium ion secondary battery used as a battery for various vehicles such as electric vehicles, the power storage device 10 as a whole, which is composed of the power storage units 20, can be set to have a capacity of, for example, 20 kWh / m 2 The above method sets the lower limit of the height dimension of the power storage device 10. The capacity per area is preferably a higher value. The capacity per area is preferably 25 kWh / m 2 Above, 30kWh / m 2 Above, 33kWh / m 2 Above, or 35kWh / m 2In addition, 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.

[0099] Next, the operation of this embodiment will be described.

[0100] According to this embodiment, in a bipolar structure power storage device, it is possible to suppress an increase in resistance due to an increase in the area of ​​the active material layer of the electrode. The mechanism of the increase in resistance due to an increase in the area of ​​the active material layer is as follows.

[0101] The negative electrode active material contained in the negative electrode active material layer 22b expands and contracts by absorbing and releasing lithium ions during charge and discharge. At this time, due to the expansion of the negative electrode active material, the liquid electrolyte originally impregnated in the negative electrode active material layer 22b is squeezed out of the negative electrode active material layer 22b and discharged, and due to the contraction of the negative electrode active material, the liquid electrolyte is absorbed into the negative electrode active material layer 22b. In this way, in the negative electrode 22, the discharge and absorption of the liquid electrolyte relative to the negative electrode active material layer 22b are repeated as the charge and discharge are carried out.

[0102] During charging, the negative electrode active material expands, and lithium is taken into the negative electrode active material. Therefore, the liquid electrolyte discharged by the expansion of the negative electrode active material is in a state of low lithium ion concentration. That is, the liquid electrolyte with low lithium ion concentration is discharged from the negative electrode active material layer 22b.

[0103] Here, the liquid electrolyte with a high lithium ion concentration is abundantly present near the main surface of the negative electrode active material layer 22b, so that the lithium ion concentration of the liquid electrolyte hardly changes even if the liquid electrolyte with a low lithium ion concentration discharged from the negative electrode active material layer 22b is mixed.

[0104] On the other hand, near the outer periphery of the negative electrode active material layer 22b, the lithium ion concentration of the liquid electrolyte is reduced by mixing with the liquid electrolyte with a low lithium ion concentration discharged from the negative electrode active material layer 22b. In addition, when the negative electrode active material is discharged while shrinking, the liquid electrolyte with a reduced lithium ion concentration is taken into the negative electrode active material layer 22b. Therefore, by repeatedly charging and discharging, the lithium ion concentration of the liquid electrolyte near the outer periphery of the negative electrode active material layer 22b gradually decreases, and with this, the lithium ion concentration of the peripheral part of the negative electrode active material layer 22b also gradually decreases.

[0105] The larger the area of ​​the negative electrode active material layer 22b, the more difficult it is for the liquid electrolyte present at the outer periphery of the negative electrode active material layer 22b to be exchanged with the liquid electrolyte present near the central portion of the negative electrode active material layer 22b, that is, near the main surface close to the center in the plane direction. Therefore, when the negative electrode active material layer 22b has a large area, the reduction in the lithium ion concentration of the liquid electrolyte near the outer periphery of the negative electrode active material layer 22b and the reduction in the lithium ion concentration of the outer peripheral portion of the negative electrode active material layer 22b become more significant.

[0106] As a result, in the negative electrode active material layer 22b, the lithium ion concentration is low in the peripheral part and high in the central part. Due to the lithium ion concentration deviation, a part with high resistance and difficult to react and a part with low resistance and easy to react are generated in the negative electrode active material layer 22b. In addition, due to the resistance difference generated in each part of the negative electrode active material layer 22b, the resistance of the electrode increases.

[0107] 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, since the amount of active material per unit area in the negative electrode active material layer 22b increases, more lithium is taken into the negative electrode active material layer 22b in the reaction during one charge. As a result, the deviation of the lithium ion concentration in the negative electrode active material layer 22b will reach a level that increases the resistance of the electrode in a smaller number of charge and 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 resistance of the electrode will occur in a smaller number of charge and discharge cycles.

[0108] 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 becomes easy to proceed, so more lithium is taken 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 increased electrode resistance will also occur in a small number of charge and discharge cycles.

[0109] In the present embodiment, the groove portion 103 is provided in the positive electrode active material layer 21b of the positive electrode 21 so that the maximum distance D in the island portion 104 is 60 mm or less. The island portion 104 is a portion of the main surface of the positive electrode active material layer 21b where the groove portion 103 is not formed. The maximum distance D is the maximum value of the shorter distance between the distance L3 from the outer edge 104a and the distance L4 from the groove edge 104b at each point of the island portion 104.

[0110] In this case, a flow of liquid electrolyte intended to make the lithium ion concentration uniform occurs through the groove portion 103 of the positive electrode active material layer 21b. For example, during charging, the liquid electrolyte present near the main surface of the negative electrode active material layer 22b flows into the groove portion 103 of the positive electrode active material layer 21b opposite to the negative electrode active material layer 22b via the separator 23. At this time, the liquid electrolyte flows into the groove portion 103 from the entire periphery of the groove portion 103 toward the nearest groove portion 103. Then, the liquid electrolyte flowing into the groove portion 103 flows in the groove portion 103 and is discharged from the end of the groove portion 103 to the vicinity of the outer peripheral edge of the positive electrode active material layer 21b. During discharge, in order to supplement the liquid electrolyte flowing into the groove portion 103 during charging, the liquid electrolyte present near the outer peripheral edge of the negative electrode active material layer 22b flows toward the central side of the main surface of the negative electrode active material layer 22b. In addition, a portion of the liquid electrolyte present near the outer peripheral edge of the negative electrode active material layer 22b also flows into the groove portion 103 located near the outer peripheral edge.

[0111] By such a 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 groove portion 103 and the vicinity of the outer periphery of the positive electrode active material layer 21b. On the other hand, 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 central side of the main surface of the negative electrode active material layer 22b.

[0112] As a result, the liquid electrolyte with a high lithium ion concentration originally present near the main surface of the negative electrode active material layer 22b is exchanged with the liquid electrolyte with a low lithium ion concentration originally present near the outer periphery of the negative electrode active material layer 22b. In addition, in many places inside the groove 103 and around the negative electrode active material layer 22b, the liquid electrolyte with a high lithium ion concentration and the liquid electrolyte with a low lithium ion concentration are mixed. As a result, the reduction in the local lithium ion concentration of the liquid electrolyte near the outer periphery of the negative electrode active material layer 22b is suppressed. In addition, the reduction in the local lithium ion concentration of the peripheral part of the negative electrode active material layer 22b is suppressed, and the increase in electrode resistance caused by the deviation of the lithium ion concentration is suppressed.

[0113] In addition, when the groove portion 103 is provided in the positive electrode active material layer 21b, a normal electrode reaction is performed in the portion of the negative electrode active material layer 22b of the negative electrode 22 that overlaps with the island portion 104 of the positive electrode active material layer 21b. On the other hand, the electrode reaction is weaker in the portion of the negative electrode active material layer 22b of the negative electrode 22 that overlaps with the groove portion 103 of the positive electrode active material layer 21b than in the portion that overlaps with the island portion 104. Therefore, in the negative electrode active material layer 22b, the portion where the electrode reaction is weak is formed in a regular arrangement consistent with the groove portion 103 in the surface direction thereof. That is, by providing the groove portion 103 in the positive electrode active material layer 21b, the portion where the electrode reaction is weak is indirectly formed in the negative electrode active material layer 22b. And, the portion where the electrode reaction is weak is finely distributed in a shape consistent with the groove portion 103 relative to the entire surface direction of the negative electrode active material layer 22b.

[0114] In the portion 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 decreases. Therefore, relative to the entire surface direction of the negative electrode active material layer 22b, the portion where the discharge of the liquid electrolyte with a low lithium ion concentration decreases is finely distributed. As a result, a small flow that evens out the slight lithium ion concentration difference is also generated in the entire liquid electrolyte around the negative electrode active material layer 22b, including the vicinity of the central portion of the negative electrode active material layer 22b.

[0115] This small flow also causes the exchange of the liquid electrolyte present at the outer periphery of the negative electrode active material layer 22 b with the liquid electrolyte present near the central portion of the negative electrode active material layer 22 b. As a result, the decrease in the local lithium ion concentration at the outer periphery of the negative electrode active material layer 22 b is suppressed, and the increase in the resistance of the electrode caused by the deviation of the lithium ion concentration is suppressed.

[0116] Next, the effects of this embodiment will be described.

[0117] (1) The electrode 100 for the power storage device is a positive electrode for the power storage device 10 of 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 m 2 The positive electrode active material layer 102 has a main surface 102a located on the opposite side of the surface opposite to the current collector 101 and a groove portion 103 opened in the main surface 102a. When the main surface 102a is viewed from above, the maximum value of the specific distance, i.e., the maximum distance D, is less than 60 mm. The specific distance is the shorter distance between the distance from the outer peripheral edge of the positive electrode active material layer 102 and the distance from the groove portion 103 at any point in the island portion 104, i.e., the portion of the main surface 102a where the groove portion 103 is not provided.

[0118] According to the above configuration, it is possible to suppress an increase in resistance due to an increase in the area of ​​the negative electrode active material layer 22 b.

[0119] (2) The positive electrode active material layer 102 has a shape having a long side direction and a short side direction, and the groove portion 103 is a straight line extending in the long side direction of the positive electrode active material layer 102 .

[0120] The electrode 100 having the positive electrode active material layer 102 having a shape in the long side direction and the short side direction is easily deformed by its own weight so that the end in the long side direction droops. In addition, when the positive electrode active material layer 102 is formed with the groove portion 103, the positive electrode active material layer 102 is easily bent in the portion where the groove portion 103 is formed.

[0121] Therefore, in the case of forming a groove portion 103 extending in the short side direction of the positive electrode active material layer 102, the direction of bending when deforming in a manner that the end portion in the long side direction droops overlaps with the direction that becomes easy to bend due to the formation of the groove portion 103. As a result, it is easier to produce deformation of the positive electrode in the direction in which the end portion in the long side direction droops. Specifically, in the case of a handling process when manufacturing an electrode, for example, when the positive electrode is transported by adsorption, the electrode is easy to droop due to its own weight, so the positive electrode will be deformed. This problem also occurs in the case of forming a groove portion 103 extending in a direction intersecting the long side direction, although there are differences in degree.

[0122] On the other hand, when the groove 103 extending in the long-side direction of the positive electrode active material layer 102 is formed, the bending direction when deformed in a manner that the end in the long-side direction droops and the direction that becomes easy to bend due to the formation of the groove 103 become mutually orthogonal. Thus, it is possible to avoid a situation in which the electrode is more likely to be deformed in the direction in which the end in the long-side direction droops. In addition, the positive electrode active material layer 102 having a linear groove 103 extending in the long-side direction can be easily formed by slit coating using a die coater with obstacles such as gaskets provided in the slit die.

[0123] (3) The groove portion 103 is in a linear shape extending in the longitudinal direction of the positive electrode active material layer 102. The aspect ratio of the island portion 104 is 12 or more.

[0124] By forming the island 104 in a shape with a large aspect ratio, that is, an elongated shape, the ratio of the area of ​​the island 104 to the total area of ​​the positive electrode active material layer 102 can be increased while shortening the maximum distance D. By increasing the ratio of the area of ​​the island 104 to the total area of ​​the positive electrode active material layer 102, the capacity of the power storage cell 20 can be increased.

[0125] (4) The thickness of the positive electrode active material layer 102 is 250 μm or more.

[0126] When the thickness of the positive electrode active material layer 102 is thick, the groove portion 103 can be formed deeper. By forming the groove portion 103 deeper, the difference in electrode reaction between the portion of the negative electrode active material layer 22b overlapping with the island portion 104 of the positive electrode active material layer 21b and the portion overlapping with the groove portion 103 becomes larger. As a result, the liquid electrolyte present around the negative electrode active material layer 22b is easy to flow.

[0127] (5) In the power storage device 10 , the thickness of the negative electrode active material layer 22 b is 200 μm or more.

[0128] When the thickness of the negative electrode active material layer 22b is thick, the amount of active material per unit area in the negative electrode active material layer 22b increases. In this case, since the amount of lithium ions absorbed and released per unit area of ​​the negative electrode active material layer 22b also increases, it is also easy for the lithium ion concentration in the negative electrode active material layer 22b to deviate. As a result, the resistance of the electrode is also easy to increase. Therefore, when the thickness of the negative electrode active material layer 22b is thick, it is particularly effective to adopt the above-mentioned (1) structure, and the effect of the above-mentioned (1) can be obtained more significantly.

[0129] (6) In the power storage device 10 , the porosity of one or both of the positive electrode active material layer 21 b and the negative electrode active material layer 22 b is 30% or more.

[0130] According to the above configuration, the rate at which the variation in lithium ion concentration in the negative electrode active material layer 22b increases with respect to the charge and discharge cycle is reduced. Therefore, by combining with the configuration (1) above, the increase in resistance caused by the increase in the area of ​​the negative electrode active material layer 22b can be more effectively suppressed.

[0131] (7) Of the positive electrode 21 and the negative electrode 22 constituting the bipolar electrode 25 , only the positive electrode 21 is formed as the electrode 100 having the groove portion 103 .

[0132] According to the above configuration, compared with a case where both the positive electrode 21 and the negative electrode 22 are electrodes 100 having the grooves 103 , a decrease in capacity due to the provision of the grooves 103 can be suppressed.

[0133] In addition, the present embodiment can be implemented by the following modifications. The present embodiment and the following modifications can be implemented in combination with each other within the scope of no technical contradiction.

[0134] ○ The top view shape of the groove portion 103 is not limited to the above-mentioned embodiment. For example, instead of the linear groove portion 103 extending in the long side direction of the positive electrode active material layer 102, a linear groove portion 103 extending in the short side direction or a linear groove portion 103 extending in a direction intersecting the long side direction and the short side direction may be provided. In addition, groove portions 103 extending in different directions may be combined like a grid. In addition, instead of the linear groove portion 103, a wavy shape, a concentric circle shape, or an irregular shape may be provided.

[0135] The widths of the grooves 103 may be all the same or different from each other. In addition, instead of the grooves 103 having a fixed width, the grooves 103 may have a variable width.

[0136] The cross-sectional shape of the groove portion 103 is not limited to a rectangular shape. Other cross-sectional shapes of the groove portion 103 include, for example, a V-shaped cross section, a U-shaped cross section, and a trapezoidal cross section.

[0137] The groove 103 may not reach the current collector 101 , that is, the bottom may be formed by the positive electrode active material layer 102 . In this case, the groove depth of the groove 103 is, for example, 50% or more of the thickness of the positive electrode active material layer 102 .

[0138] In the above embodiment, only the positive electrode 21 of the positive electrode 21 and the negative electrode 22 constituting the pseudo-bipolar electrode 25 is provided as the electrode 100 having the groove portion 103, but both the positive electrode 21 and the negative electrode 22 may be provided as the electrode 100 having the groove portion 103. In addition, when the groove portion 103 is provided in the negative electrode active material layer 22b, lithium may be deposited in the groove portion 103 of the negative electrode active material layer 22b when the following conditions are satisfied. The conditions are that the groove portion 103 provided in the negative electrode active material layer 22b is in a shape that reaches the current collector 101, and the portion of the positive electrode active material layer 21b that overlaps with the groove portion 103 of the negative electrode active material layer 22b in the stacking direction is the island portion 104. Therefore, when the groove portion 103 is provided in the negative electrode active material layer 22b, it is preferable to provide the groove portion 103 in the portion that overlaps with the portion of the positive electrode active material layer 21b where the groove portion 103 is provided in the stacking direction.

[0139] The electrode 100 may also be a bipolar structure electrode in which the positive electrode 21 and the negative electrode 22 are integrated. Figure 6 , an example of the electrode 100 embodied as an electrode of a bipolar structure in which the positive electrode 21 and the negative electrode 22 are integrated will be described.

[0140] Figure 6The electrode 100 of the bipolar structure shown in the figure has a bipolar current collector 105. The bipolar current collector 105 is a laminated body formed by bonding a foil-shaped positive electrode current collector 106 and a foil-shaped negative electrode current collector 107 together in the thickness direction. Examples of the bipolar current collector 105 include a current collector formed by bonding aluminum foils to each other and a current collector formed by bonding aluminum foil and copper foil.

[0141] An active material layer 108 is provided as a positive electrode active material layer on a first surface 105a formed of a positive electrode current collector 106 of the bipolar current collector 105. An active material layer 109 is provided as a negative electrode active material layer on a second surface 106a formed of a negative electrode current collector 107 of the bipolar current collector 105.

[0142] At least the active material layer 108 configured as the positive electrode active material layer among the active material layer 108 and the active material layer 109 is an active material layer that satisfies the requirements of the active material layer 102 described in the above embodiment. That is, the bipolar structure electrode 100 has a structure in which only the positive electrode active material layer is an active material layer having the groove portion 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 groove portion 103.

[0143] ○ The specific structure of the power storage device 10 to which the electrode 100 is applied is not particularly limited as long as at least one positive electrode is a structure equivalent to the electrode 100. For example, the number of power storage cells 20 constituting the power storage device 10 may be one. In addition, the power storage device 10 may be provided with a restraining member that applies a restraining load to the unit stack 30 in the stacking direction. In addition, the power storage device 10 may be provided with an electrode 100 configured as a bipolar electrode.

[0144] Example

[0145] Hereinafter, examples in which the above-described embodiment is further concretized will be described.

[0146] <Production of positive electrode>

[0147] LiFEPO4 as a positive electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose were mixed, and water was added to the mixture to prepare a positive electrode composite material. For the surface of the aluminum foil as the positive electrode collector, the positive electrode composite material was coated into a film with slits using a slit die. The coated positive electrode composite material was dried and cured by heating the coated positive electrode composite material to produce positive electrodes of Test Examples 1 to 3 in which a positive electrode active material layer with a slit-shaped groove portion was formed on the positive electrode collector.

[0148] The top view shape of the positive electrode active material layer of the positive electrode of Test Examples 1 to 3 is rectangular. In the positive electrodes of Test Examples 1 to 3, the overall size of the positive electrode active material layer is the same as 1155mm in length × 1476.5mm in width × 250μm in thickness, and the top view shape of the groove portion is different. The groove portion of the positive electrode of Test Example 1 is a straight line with a width of 3mm extending in the horizontal direction when viewed from above, and multiple grooves are arranged in the vertical direction at a spacing of 122mm. The groove portion of the positive electrode of Test Example 2 is a straight line with a width of 2mm extending in the horizontal direction when viewed from above, and multiple grooves are arranged in the vertical direction at a spacing of 82mm. The groove portion of the positive electrode of Test Example 3 is a straight line with a width of 1mm extending in the horizontal direction when viewed from above, and multiple grooves are arranged in the vertical direction at a spacing of 42mm. The values ​​of the width H of the island portion, the maximum distance D in the island portion, and the aspect ratio of the island portion in the positive electrode active material layer of Test Examples 1 to 3 are shown in Table 1.

[0149] In addition, the positive electrode composite material was applied to the surface of the aluminum foil as the positive electrode collector in a film-like shape without slits using a doctor blade method. The applied positive electrode composite material was dried and solidified by heat treatment to prepare a positive electrode of Test Example 4 in which a positive electrode active material layer without grooves was formed on the positive electrode collector. The overall size of the positive electrode active material layer of Test Example 4 was the same as that of the positive electrodes of Test Examples 1 to 3.

[0150] <Production of power storage device>

[0151] A bipolar electrode battery having a structure in which 30 storage cells were stacked was prepared by combining the positive electrodes, negative electrodes, and separators of Test Examples 1 to 4. The electrode battery was housed in a battery case, an electrolyte was injected, and the battery case was sealed to obtain a lithium ion secondary battery.

[0152] As the negative electrode, a negative electrode having a negative electrode collector composed of copper, and a negative electrode active material layer composed of graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a dispersant was used. No grooves were formed in the negative electrode active material layer. The top view shape of the negative electrode active material layer of the negative electrode was rectangular. The overall size of the negative electrode active material layer was 1155 mm in length × 1476.5 mm in width × 250 μm in thickness. As a separator, a separator composed of polyethylene was used. As an electrolyte, an electrolyte in which lithium hexafluorophosphate was dissolved in a mixed solvent in which ethylene carbonate and methyl propionate were mixed in a volume ratio of 15:85 in a manner to give a concentration of 1.2 M was used.

[0153] <Evaluation of resistance of power storage device>

[0154] For each lithium-ion secondary battery obtained, after a constant current charge of 1.3C for 30 minutes, a constant current discharge of 0.2C for 195 minutes was set as one cycle, and the operation was repeated for 15 cycles. When the above cycle was repeated, the discharge resistance at SOC (State of Charge) 50% was measured after 0 cycles, 5 cycles, 10 cycles, and 15 cycles, which were the states before the first cycle. Then, the rate of change of the discharge resistance after 5 cycles, 10 cycles, and 15 cycles relative to the discharge resistance after 0 cycles (hereinafter referred to as the resistance change rate) was calculated. The results are shown in Table 1.

[0155] The discharge resistance was measured as follows. After the SOC of the lithium-ion secondary battery was adjusted to 50%, a constant current discharge of 1C for 10 seconds was performed. The voltage before discharge (V ocv ) to the voltage reached after 10 seconds of discharge (V ccv ) is divided by the current value to obtain the discharge resistance.

[0156] <Evaluation of variation in salt concentration in active material layer>

[0157] In addition, for the lithium ion secondary battery using the positive electrode of Test Examples 1 to 3, the salt concentration of each part of the negative electrode active material layer was measured. If described in detail, the negative electrode was taken out from the lithium ion secondary battery after 15 cycles, and the salt concentration (lithium hexafluorophosphate concentration) at three specific points on the main surface of the negative electrode active material layer was measured. Then, the maximum salt concentration difference was calculated as the maximum value of the salt concentration difference at the three specific points. The results are shown in Table 1.

[0158] like Figure 4 As shown, the above-mentioned three specific points are points on the back side of the three points of the peripheral side endpoint P1, the groove side endpoint P2 and the center point P3 on the main surface of the negative electrode active material layer, that is, points overlapping with these three points in the stacking direction. The peripheral side endpoint P1 is a point located on the longitudinal peripheral edge of the positive electrode active material layer when viewed from above. The groove side endpoint P2 is a point at a distance L3 of 738.25 mm from the longitudinal peripheral edge of the positive electrode active material layer when viewed from above, and is a point located on the edge of the island portion of the positive electrode active material layer. The center point P3 is a point at a distance L3 of 738.25 mm from the longitudinal peripheral edge of the positive electrode active material layer when viewed from above, and is a point on the straight line S1 (specific line) in the island portion of the positive electrode active material layer. The above-mentioned straight line S1 is a straight line extending in the horizontal direction and dividing the island portion 104 into two equal parts.

[0159] [Table 1]

[0160]

[0161] As shown in Table 1, in Test Example 4, as the number of cycles increases, the resistance change rate increases significantly. This result shows that the resistance increases by repeatedly charging and discharging the lithium-ion secondary battery. In addition, although detailed data are omitted, except for the fact that the area of ​​the negative electrode active material layer is reduced, when the same test was carried out using an electrode with the same structure as Test Example 4, no increase in resistance caused by repeated charging and discharging could be confirmed. In this test, a negative electrode with an overall size of a negative electrode active material layer of 250 mm in length × 350 mm in width × 250 mm in thickness was used. From these results, it can be seen that the resistance increase caused by repeated charging and discharging is when the area of ​​the negative electrode active material layer is large, for example, 1 mm 2 The above situation is unique.

[0162] On the other hand, in Test Examples 1 to 3 in which grooves are provided in the positive electrode active material layer, the resistance change rate after each cycle becomes smaller than in Test Example 4 in which grooves are not provided in the positive electrode active material layer, and the resistance increase caused by repeated charge and discharge is suppressed. In particular, in Test Example 3 in which the maximum distance D of the positive electrode active material layer is less than 30 mm, the resistance change rate after each cycle is maintained at almost 100%, and the resistance increase caused by repeated charge and discharge is significantly suppressed. From these results, it can be seen that by providing grooves in the positive electrode active material layer in a manner that shortens the maximum distance D, the resistance increase caused by repeated charge and discharge can be suppressed.

[0163] Secondly, from the results of the salt concentration measurement, it can be seen that in the negative electrode active material layer after 15 cycles, there is a deviation in salt concentration according to the position, such as a high salt concentration on the central side when viewed from above and a low salt concentration on the peripheral side. Moreover, the greater the deviation of the salt concentration, that is, the greater the maximum salt concentration difference, the greater the resistance change rate. In addition, it is known that the deviation of the salt concentration in the negative electrode active material layer causes the negative electrode active material layer to produce a part with high resistance and a part with low resistance. The greater the resistance difference at each part in the negative electrode active material layer, the greater the resistance of the secondary battery.

[0164] From these points of view, the following findings were obtained regarding the increase in resistance due to repeated charge and discharge when the area of ​​the negative electrode active material layer is large.

[0165] When charging and discharging are repeated, a difference in salt concentration will occur between the center side and the periphery side of the negative electrode active material layer when viewed from above. When the area of ​​the negative electrode active material layer is large, the deviation in salt concentration caused by repeated charging and discharging becomes more significant because the distance between the center side and the periphery side of the negative electrode active material layer when viewed from above is far. If the resistance difference at each location in the negative electrode active material layer caused by the deviation in salt concentration becomes large to the extent that it affects the resistance of the secondary battery, an increase in resistance caused by repeated charging and discharging will occur. In addition, by providing a groove portion that shortens the maximum distance D in the positive electrode active material layer as in Test Examples 1 to 3, the deviation in salt concentration in the negative electrode active material layer can be reduced, and the increase in resistance caused by repeated charging and discharging can be suppressed.

[0166] Description of Reference Numerals

[0167] 100…Electrode

[0168] 101…Current Collector

[0169] 102…Active material layer

[0170] 102a…main surface

[0171] 103…groove

[0172] 104…Island

[0173] 10…Electrical storage device (lithium-ion secondary battery)

[0174] 21…Positive electrode

[0175] 22…negative pole.

Claims

1. An electrode for a lithium ion secondary battery, which is an electrode for a lithium ion secondary battery with a bipolar structure, characterized in that: The invention comprises: a current collector; and a positive electrode active material layer formed on a surface of the current collector. The area of ​​the positive electrode active material layer is 1 m 2 above, The positive electrode active material layer includes: a main surface located on the opposite side to the surface facing the current collector; and a groove portion opened on the main surface. When looking down at the main surface, when the shorter distance between the distance from the outer peripheral edge of the positive electrode active material layer and the distance from the groove portion at any point in the island portion of the main surface where the groove portion is not provided is set as a specific distance, the maximum value of the specific distance is less than 60 mm.

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

3. The electrode for a lithium ion secondary battery according to claim 1, wherein The positive electrode active material layer is rectangular with a long side direction and a short side direction. The groove is in a straight line extending in the longitudinal direction of the positive electrode active material layer. The aspect ratio of the island portion is 12 or more.

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

5. A lithium ion secondary battery, which is a bipolar lithium ion secondary battery, characterized in that: A method of manufacturing the present invention comprising the electrode according to any one of claims 1 to 4.

6. The lithium ion secondary battery according to claim 5, wherein: The capacity of the lithium ion secondary battery is 50 kWh or more.

Images