Electrode and power storage device

Abstract

To provide an electrode and a power storage device capable of improving durability.SOLUTION: The electrode includes a current collector and a composite layer attached to the current collector. The composite layer includes an active material and carbon nanotubes. The active material includes activated carbon, and the activated carbon has an oxygen content of 3.0 mass% or less as measured by carbon-nitrogen analysis on the basis of the inert-gas fusion method. The proportion of the number of end portions of the carbon nanotubes per unit volume of the composite layer is 4.5×109 cm3 to 4.5×1011 per cm3. The power storage device includes a positive electrode and a negative electrode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode and an electricity storage device that include activated carbon and carbon nanotubes. [Background technology]

[0002] An electronic device (electricity storage device) having a positive electrode (electrode) containing activated carbon and carbon nanotubes is known. The prior art disclosed in Patent Document 1 reduces the resistance of the electrode by using carbon nanotubes. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-19738 Summary of the Invention [Problem to be solved by the invention]

[0004] In prior art, the high activity of activated carbon and carbon nanotubes can easily accelerate the decomposition of materials contained in electrodes and electricity storage devices, which can reduce the durability of the electrodes and electricity storage devices.

[0005] The present invention has been made to solve this problem, and has an object to provide an electrode and an electricity storage device that can improve durability. [Means for solving the problem]

[0006] To achieve this object, a first aspect of the present invention is an electrode comprising a current collector and a composite layer adhered to the current collector, the composite layer comprising an active material and carbon nanotubes, the active material comprising activated carbon, and the activated carbon having an oxygen content of 3.0 mass% or less as measured by carbon-nitrogen analysis using an inert gas fusion method.

[0007] A second aspect is an electrode comprising a current collector and a composite layer attached to the current collector, the composite layer comprising an active material and carbon nanotubes, the active material comprising activated carbon, and a ratio of the number of ends of the carbon nanotubes per unit volume of the composite layer being 4.5×10 9 pieces / cm 3 Over 4.5 x 10 11 pieces / cm 3 The following is the result.

[0008] In the third aspect, the ratio of the number of ends of the carbon nanotubes per unit volume of the composite layer in the first aspect is 4.5 × 10 9 pieces / cm 3 Over 4.5 x 10 11 pieces / cm 3 The following is the result.

[0009] A fourth aspect is an electricity storage device, which includes a positive electrode and a negative electrode, and the positive electrode is the electrode according to any one of the first to third aspects. [Effects of the Invention]

[0010] According to the present invention, the activated carbon contained in the mixture layer has an oxygen content of 3.0 mass% or less as measured by carbon-nitrogen analysis using an inert gas fusion method, or the ratio of the number of carbon nanotube ends per unit volume of the mixture layer is 4.5 × 10 9 pieces / cm 3 Over 4.5 x 10 11 pieces / cm 3 Since the temperature is less than 100°C, the activity of activated carbon and carbon nanotubes can be reduced, thereby improving the durability of electrodes and electricity storage devices. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view of an electricity storage device according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic cross-sectional view of an electricity storage device 10 according to one embodiment. The electricity storage device 10 includes, in order, a positive electrode 11, a separator 14, and a negative electrode 15. The electricity storage device 10 is an element that converts chemical energy into electrical energy and vice versa.

[0013] 1 illustrates an electricity storage device 10 having one set of a positive electrode 11, a separator 14, and a negative electrode 15, but the invention is not limited to this and may include a plurality of sets of these. The electricity storage device 10 is not limited to one having a stacked structure, but may also have a wound structure in which the positive electrode 11 and the negative electrode 15 are stacked with the separator 14 interposed therebetween and wound.

[0014] Examples of the electricity storage device 10 include a lithium ion battery and a lithium ion capacitor. Examples of lithium ion capacitors include an electric double layer capacitor, a redox capacitor that utilizes a redox reaction of an electrode or a redox reaction of ions in a non-aqueous electrolyte, and a hybrid capacitor that combines an electric double layer and a redox reaction, or that combines them with a secondary battery material.

[0015] The positive electrode 11 includes a current collector 12 and a composite layer 13 attached to the current collector 12. There are no particular limitations on the material of the current collector 12, and examples include Cu, Al, a Cu alloy, an Al alloy, and stainless steel. There are also no particular limitations on the shape of the current collector 12. The current collector 12 may be a porous foil having a plurality of holes penetrating the current collector 12.

[0016] The composite layer 13 includes an active material and carbon nanotubes. The active material includes activated carbon. Activated carbon obtained by carbonizing and activating a mineral-based, plant-based, or resin-based carbonaceous material is preferably used. Examples of mineral-based carbonaceous materials include coal (lignite, brown coal, bituminous coal, anthracite, etc.), cokes, infusibilized pitch, and oil carbon. Examples of plant-based carbonaceous materials include charcoal, coconut shells, sawdust, wood chips, and grass peat. Examples of resin-based carbonaceous materials include phenolic resin.

[0017] The properties of activated carbon are determined by its pore structure and surface chemical structure. Examples of chemical species that govern the surface chemical structure include oxygen, hydrogen, nitrogen, and sulfur. Oxygen exists as a polar group and plays an important role as a reaction site for interfacial phenomena and surface modification. The activated carbon contained in the composite layer 13 preferably has an oxygen content of 3.0 mass% or less as measured by carbon-nitrogen analysis using an inert gas fusion method. This is to reduce the reaction at the reaction sites of the activated carbon. One example of a method for reducing the oxygen content of activated carbon to 3.0 mass% or less is hydrogen reduction treatment in a high-temperature atmosphere of approximately 900-1200°C. The oxygen content of activated carbon is measured using an inert gas fusion-infrared absorption method.

[0018] Mixture layer 13 may contain an active material other than activated carbon. Examples of the active material other than activated carbon include graphite, low-crystalline carbon, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), and carbon black.

[0019] The carbon nanotubes electrically connect the active materials so that the active materials are not isolated in the composite layer 13. Furthermore, the number Ne of carbon nanotube ends per unit volume of the composite layer 13 calculated by the following formula (1) is 4.5×10 9 pieces / cm 3 Over 4.5 x 10 11 pieces / cm 3 It is preferable that the number of carbon nanotube ends is less than 100. This is because the reaction at the ends of the carbon nanotubes can be reduced by controlling the number of highly active carbon nanotube ends while reducing the electrical resistance of the mixture layer 13.

[0020] Ne=2·D·P / (ρ·A·L·100) …Equation (1) D: Density of the composite layer 13 (g / cm 3 ) P: Ratio (wt%) of carbon nanotubes in the composite layer 13 ρ: density of carbon nanotubes (= 1.3 g / cm 3 ) A: Cross-sectional area of ​​the carbon nanotube (cm 2 ) L: Length of carbon nanotube (cm)

[0021] D in formula (1) is calculated by taking an image of the cross section of the composite layer 13 using a scanning electron microscope (SEM), calculating the area of ​​each material constituting the composite layer 13 relative to the area of ​​the image, and then considering the ratio of the area of ​​each material as the volume ratio (volume ratio). The volume ratio is multiplied by the known density of each material to calculate the density (g / cm 3 )

[0022] In formula (1), P is the ratio of the mass of the carbon nanotubes contained in the mixture layer 13 to the total mass of the materials constituting the mixture layer 13. ρ in formula (1) is the known density of the carbon nanotubes (=1.3 g / cm 3 )

[0023] A in equation (1) is the value obtained by multiplying the square of the radius, calculated using the outer diameter of the carbon nanotube as the diameter, by pi (cross-sectional area (cm 2 )). In equation (1), L is the length (cm) of one carbon nanotube. A and L can be obtained by taking an SEM image of the carbon nanotube, extracting several samples from the SEM image, measuring the diameter and length of the samples, and then taking the arithmetic average. The cross-sectional area and length listed in the carbon nanotube catalog or specifications can also be used as A and L.

[0024] In formula (1), the value of D·P / (ρ·A·L·100) is multiplied by 2 because one carbon nanotube has two ends. Therefore, formula (1) can be used to calculate the number of carbon nanotube ends per unit volume of composite layer 13.

[0025] The composite layer 13 may contain a conductive agent other than carbon nanotubes. Examples of the conductive agent other than carbon nanotubes include graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, carbon black, carbon nanotubes, carbon fibers, and fibrous or coiled metals.

[0026] The composite layer 13 may contain a binder and a thickener. The binder binds the active material and the conductive agent. Examples of the binder include fluororesin, acrylic resin, polyolefin, and rubber. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. Examples of the acrylic resin include polyacrylic acid, polyacrylonitrile, and polyacrylamide. Examples of the polyacrylic acid include polymethacrylic acid ester, polyacrylic acid ester, and sodium polyacrylate. Examples of the polyolefin include polyethylene and polypropylene. Examples of the rubber include styrene-butadiene rubber. The binder is preferably polyacrylic acid or polyacrylamide, which have high tensile strength. An example of the thickener is carboxymethyl cellulose.

[0027] The binder has a tensile strength of 60 N / mm at a pulling speed of 5 mm / min. 2 This is preferable because it improves the mechanical strength of the network structure in which the binder is bound to the carbon nanotubes. The binder film is a test piece obtained by creating a film of approximately uniform thickness using a solution in which the binder is dissolved in a solvent, and then evaporating the solvent by natural drying. The test piece has gripping portions at both ends of the gauge section that can be attached to the chuck of the tensile tester. The gauge section is a square plate with a length of 10 mm, a width of 10 mm, and a thickness of 3 mm or less. The test piece can be formed by pouring the solution into a test piece mold, or the film can be formed and then cut out to the size of the test piece.

[0028] To determine the tensile strength, the test piece is attached to the chuck of the tensile testing machine, and then the chuck is pulled away at a rate of 5 mm / min to apply a tensile force in the longitudinal direction of the gauge section, and the maximum tensile force (N) is measured. 2 ) is the value obtained by dividing the maximum tensile force by the cross-sectional area of ​​the gauge section before the tensile force is applied.

[0029] The ratio of carbon nanotubes to binder contained in composite layer 13 is preferably 0.0018 or more and 0.9 or less, calculated by dividing the ratio (mass %) of carbon nanotubes to composite layer 13 by the ratio (mass %) of binder to composite layer 13. This is to create a network structure in composite layer 13 in which the binder is bound to the carbon nanotubes, thereby increasing the mechanical strength of composite layer 13.

[0030] Negative electrode 15 is formed by stacking current collector 16 and composite layer 17. Examples of the material for current collector 16 include a metal selected from Ni, Ti, Fe, and Al, an alloy containing two or more of these elements, and stainless steel.

[0031] The composite layer 17 contains an active material and a conductive agent. Examples of the active material include carbon-based materials such as graphite, low-crystalline carbon, graphitizable carbon, and non-graphitizable carbon, and silicon-containing materials. Examples of the silicon-containing materials include Si, Si-Li alloys, and compounds containing Si and O as constituent elements (hereinafter referred to as "SiO x " where 0.5≦X≦1.5 is an example.

[0032] SiO x Silicon oxide (SiO) is an oxide of silicon, and is exemplified by those having a structure in which microcrystalline or amorphous silicon is dispersed in an amorphous SiO matrix. x Since SiO has poor conductivity, x A conductive layer is provided to cover the surface of SiO x It is preferable to combine SiO with a conductive material. x Granulation is an example of a means for combining the active material with the conductive material. Examples of materials for the conductive layer include metals such as Pt and Os, and carbon. Examples of conductive materials include graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, carbon black, carbon nanotubes, carbon fiber, and fibrous or coiled metals. The proportion of silicon in the active material is preferably 10% by mass or more and 50% by mass or less.

[0033] The separator 14 is made of a porous material that is durable against the active materials and electrolyte solution contained in the positive electrode 11 and the negative electrode 15, and that is non-electron-conductive but allows lithium ions to pass through. Examples of the separator 14 include nonwoven fabrics and porous films made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0034] The electrolyte solution contains a lithium salt dissolved in a solvent. The lithium salt is a compound used to transfer lithium ions between the positive electrode 11 and the negative electrode 15. The anion of the lithium salt is a halide ion (I - , Cl - , Br - etc.), SCN - , BF4 - , BF3(CF3) - , BF3(C2F5) - , PF6 - , ClO4 - , SbF6 - , N(SO2F)2 - , N(SO2CF3)2 - , N(SO2C2F5)2 - , B(C6H5)4 - , B(O2C2H4)2 - , C(SO2F)3 - , C(SO2CF3)3 - , CF3COO - , CF3SO2O - , C6F5SO2O - , B(O2C2O2)2 - is exemplified.

[0035] The solvent for the electrolyte solution is not particularly limited as long as it is liquid in the temperature range in which the electricity storage device 10 is used. Examples of the solvent include carbonate esters, aliphatic carboxylic acid esters, phosphate esters, γ-lactones, ethers, nitriles, sulfolane, dimethyl sulfoxide, fluorous solvents, and ionic liquids. A mixture of these solvents is also acceptable.

[0036] The electricity storage device 10 is manufactured, for example, as follows. A slurry is prepared by dispersing an active material and carbon nanotubes in a solution in which a binder is dissolved. A dispersion medium such as an organic solvent such as N-methyl-2-pyrrolidone or water, or a thickener may be added to the slurry. The prepared slurry is applied to the current collector 12, and then dried to obtain the positive electrode 11 in which the composite layer 13 is formed on the current collector 12. The dried composite layer 13 may be rolled using a roller or the like.

[0037] Next, a slurry is prepared by dispersing an active material and a conductive agent in a solution containing a dissolved binder. The slurry is applied onto current collector 16 and then dried to obtain negative electrode 15 in which composite layer 17 is formed on current collector 16.

[0038] Next, Li is supplied to negative electrode 15, and lithium ions are pre-doped in an amount corresponding to at least a portion of the irreversible capacity. Pre-doping can be exemplified by a method in which current collector 16 and metallic lithium are short-circuited with a separator disposed between composite layer 17 and metallic lithium, and negative electrode 15, the separator, and metallic lithium are immersed in an electrolyte solution. Electrons flow from metallic lithium to current collector 16 due to the potential difference between current collector 16 and metallic lithium, and at the same time, the metallic lithium is ionized and released into the electrolyte solution, and the lithium ions in the electrolyte solution are supported by composite layer 17.

[0039] Pre-doping with lithium ions results in obtaining negative electrode 15 in which composite layer 17 is lithiated. During pre-doping, a current may be passed between current collector 16 and metallic lithium. The lithiation of composite layer 17 means that some of the atoms constituting composite layer 17 have been replaced with lithium atoms, that is, composite layer 17 contains lithium.

[0040] After cutting the positive electrode 11, separator 14, and negative electrode 15 into predetermined shapes, the positive electrode 11, separator 14, and negative electrode 15 are stacked in this order, terminals (not shown) are connected to the current collectors 12 and 16, respectively, and the resulting product is sealed in a case (not shown) together with the electrolyte to obtain the electricity storage device 10. [Example]

[0041] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0042] (Measurement of oxygen content in activated carbon) Using an oxygen, nitrogen, and hydrogen analyzer (LECO Japan, LLC, ONH836), activated carbon was heated and melted in an inert gas stream using the impulse heating method, and the oxygen content of the activated carbon was measured using the non-dispersive infrared absorption method.

[0043] (Preparation of positive electrode) The activated carbon, conductive agent, and polyacrylic acid (binder) whose oxygen content had been measured were placed in pure water and mixed in a mixer to prepare a slurry. Various slurries were prepared by changing the mixing amounts so that the ratio was 80-95 parts by weight of activated carbon, 0.01-5 parts by weight of conductive agent, and 3-10 parts by weight of binder, assuming a total of 100 parts by weight of activated carbon, conductive agent, and binder.

[0044] Carbon nanotubes or acetylene black were used as the conductive agent. Carbon nanotubes with a diameter of 8 nm and a length of 35 μm and those with a diameter of 8 nm and a length of 2000 μm were used. The slurry was applied to a current collector made of 30 μm thick aluminum foil, vacuum dried at 70°C for 1 hour, and then punched into squares with sides of 20 mm to obtain various positive electrodes. The amount of slurry applied was 5.0 mg / cm, which was the combined mass of activated carbon, conductive agent, and binder. 2 It was adjusted so that

[0045] (Preparation of negative electrode) Graphite, acetylene black (conductive agent), styrene butadiene rubber (binder), and carboxymethyl cellulose (thickener) were weighed out in a mass ratio of 93:1:5:1, added to pure water, and mixed in a mixer to prepare a slurry. The slurry was applied to a current collector made of 8 μm thick copper foil, vacuum dried at 70 °C for 1 hour, and then punched out into a square with a side length of 25 mm to obtain a positive electrode. The coating amount of the slurry was 6.5 mg / cm, with the combined mass of graphite, conductive agent, binder, and thickener. 2 It was adjusted so that

[0046] (Pre-doping of negative electrode) A separator and a metallic lithium foil (counter electrode) pressed onto copper foil were attached in that order on top of the negative electrode composite layer to prepare a half-cell. The half-cell was immersed in the electrolyte, then evacuated and sealed. A current was passed between the copper foil current collector of the half-cell and the copper foil counter electrode, and pre-doping was performed by charging at a constant current of 0.1 C to 0 V at 25 °C, followed by constant voltage charging to 0.01 C. The electrolyte was a solution of lithium bis(fluorosulfonyl)imide (LiFSI) as a lithium salt (1 mol / dm 3 ) was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate.

[0047] (Fabrication of electricity storage devices) After removing the lithiated negative electrode from the half cell, a separator and a positive electrode were attached in that order on the negative electrode composite layer to prepare various cells. The electrolyte was poured into the cell, and then the cell was degassed under vacuum. A potential of 3.8 V was applied between the positive and negative electrodes to age the cell, and the cell was then sealed under vacuum to obtain the power storage devices (lithium ion capacitors) of Samples 1-14 shown in Table 1. In the power storage device of Samples 1-3, the conductive agent in the positive electrode was acetylene black (the conductive agent did not contain carbon nanotubes).

[0048] [Table 1]

[0049] (Measurement and determination of initial resistance) The energy storage device was charged at a constant current of 50 C to 3.8 V at room temperature. After 30 minutes of constant voltage charging at 3.8 V, the voltage drop E0 and current I0 were measured at the moment the device was switched to constant voltage discharge at 50 C, and the initial resistance R0 = E0 / I0 was calculated.

[0050] Based on the initial resistance of the power storage device in Sample No. 1 where the conductive agent of the positive electrode was acetylene black, samples with an initial resistance of 90% or less of the initial resistance of Sample No. 1 were determined as A, samples with an initial resistance of more than 90% and less than 99% of the initial resistance of Sample No. 1 were determined as B, and samples with an initial resistance of 99% or more of the initial resistance of Sample No. 1 were determined as C.

[0051] The increase rate U (%) of the resistance R1 after the high-temperature test with respect to the initial resistance R0 was calculated by the formula U = (R1 - R0) / R0·100. Samples with U≦40% were determined as A, samples with 40% < U≦60% were determined as B, and samples with U>60% were determined as C.

[0052] The oxygen content (wt%) of the activated carbon contained in the positive electrode of each sample, the number of ends of the carbon nanotubes (CNT) of the positive electrode calculated based on formula (1) (10 8 pieces / cm 3 )、the determination of the initial resistance and the determination of the resistance increase rate were shown in Table 1.

[0053] As shown in Table 1, for the power storage devices in Sample No. 2 - 13 where the oxygen content of the activated carbon was 3.0 mass% or less, the determination of the resistance increase rate was A or B. However, for the power storage devices in Sample No. 1 and 14 where the oxygen content of the activated carbon exceeded 3.0 mass%, the determination of the resistance increase rate was C. It is presumed that in Sample No. 1 and 14 where the oxygen content of the activated carbon exceeded 3.0 mass%, the decomposition of the electrolyte was accelerated at high temperature due to the activity of the activated carbon, resulting in a high resistance after the high-temperature test.

[0054] [[ID=X]] When the initial resistance of sample No. 1-3, which does not contain carbon nanotubes in the positive electrode, is compared with the initial resistance of sample No. 4-14, which does contain carbon nanotubes in the positive electrode, sample No. 4-14 was evaluated as A or B, while sample No. 1-3 was evaluated as C. Therefore, it was revealed that when the positive electrode contains activated carbon and carbon nanotubes and the oxygen content of the activated carbon measured by carbon-nitrogen analysis using an inert gas fusion method is 3.0 mass% or less, the resistance of the positive electrode can be reduced, and deterioration of the positive electrode can be reduced, thereby improving the durability of the electricity storage device.

[0055] In addition, the ratio of the number of carbon nanotube ends per unit volume of the composite layer is 4.5 × 10 9 pieces / cm 3 Over 4.5 x 10 11 pieces / cm 3 The resistance increase rate of sample No. 6-11 is below the ratio of the number of carbon nanotube ends per unit volume of the composite layer of 4.5 × 10 11 pieces / cm 3 Comparing the resistance increase rate of Sample Nos. 12-14, which exceeds , with Sample Nos. 6-11, the evaluation was A, while Sample Nos. 12-14 were evaluated as B or C. It is presumed that Sample Nos. 12-14 had a high resistance after the high-temperature test because the decomposition of the electrolyte was accelerated at high temperatures due to the activity of the ends of the carbon nanotubes.

[0056] Furthermore, the initial resistance of sample No. 6-11 and the ratio of the number of carbon nanotube ends per unit volume of the composite layer were 4.5 × 10 9 pieces / cm 3 When compared with the initial resistance of Sample Nos. 1-5, which was less than 1.0, Sample Nos. 6-11 were evaluated as A, while Sample Nos. 1-5 were evaluated as B or C. Therefore, it is clear that the positive electrode contains activated carbon and carbon nanotubes, and the ratio of the number of carbon nanotube ends per unit volume of the composite layer is 4.5 × 10 9 pieces / cm 3 Over 4.5 x 10 11 pieces / cm 3It has been revealed that when the positive electrode resistance is less than or equal to 100%, the positive electrode deterioration can be reduced and the durability of the electricity storage device can be improved.

[0057] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0058] In the embodiment, the electricity storage device 10 has been described as including a negative electrode 15 in which a composite layer 17 is provided on one surface of a current collector 16, and a positive electrode 11 in which a composite layer 13 is provided on one surface of a current collector 12, but this is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to an electricity storage device including electrode layers (so-called bipolar electrodes) in which a composite layer 13 and a composite layer 17 are provided on both surfaces of a current collector 12. If bipolar electrodes and separators 14 are alternately stacked and housed in a case (not shown), an electricity storage device with a so-called bipolar structure can be obtained.

[0059] In the embodiment, the positive electrode 11 is described in which the composite layer 13 is provided on one surface of the current collector 12, but this is not necessarily limited to this. It is of course possible to provide the composite layer 13 on both surfaces of the current collector 12.

[0060] In the embodiment, a lithium ion capacitor has been described as an example of the electricity storage device 10 including the positive electrode 11, but the present invention is not necessarily limited to this. Another electricity storage device including the positive electrode 11 is a lithium ion battery. Examples of lithium ion batteries include those in which an electrolytic solution passes through the separator 14, those in which the separator 14 is made of a solid electrolyte, those in which the separator 14 contains a polymer or gel electrolyte, and those in which the separator 14 contains clay composite layers 13, 17. [Explanation of symbols]

[0061] 10. Energy storage devices 11 Positive electrode 12 Current collector 13 Mixed material layer 15 Negative electrode

Claims

1. a current collector; and a composite layer attached to the current collector, the composite layer includes an active material and carbon nanotubes, The active material is an electrode containing activated carbon, The activated carbon has an oxygen content of 3.0 mass % or less as measured by carbon-nitrogen analysis using an inert gas fusion method.

2. a current collector; and a composite layer attached to the current collector, the composite layer includes an active material and carbon nanotubes, The active material is an electrode containing activated carbon, The ratio of the number of ends of the carbon nanotubes per unit volume of the composite layer is 4.5 × 10 ９ pieces / cm ３ 4.5 x 10 １１ pieces / cm ３ The electrodes are as follows:

3. The ratio of the number of ends of the carbon nanotubes per unit volume of the composite layer is 4.5 × 10 ９ pieces / cm ３ 4.5 x 10 １１ pieces / cm ３ 2. The electrode of claim 1, wherein:

4. An electricity storage device including a positive electrode and a negative electrode, The electricity storage device, wherein the positive electrode is the electrode according to claim 1 .

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