Electrode for a power storage device and power storage device

Abstract

This electrode (100) for a power storage device comprises: a current collector (101) having a first surface (101a); and an active material layer (102) formed on the first surface (101a) of the current collector (101). The thickness of the active material layer (102) is 250 μm or more. A carbon coat layer (C) is provided to the first surface (101a) of the current collector (101). The active material layer (102) includes an active material capable of absorbing and releasing a charge carrier, an aqueous binder, and single-walled carbon nanotubes. The content of the single-walled carbon nanotubes in the active material layer (102) is 0.035-0.08 mass%.

Description

Technical Field

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

Background Art

[0002] The power storage device disclosed in Patent Document 1 includes a positive electrode and a negative electrode in which an active material layer is formed on one side of a current collector, and a separator disposed between the positive electrode and the negative electrode. The positive electrode and the negative electrode are arranged such that the active material layers face each other with the separator interposed therebetween. The active material layers of the positive electrode and the negative electrode contain conductive fibers as a conductive aid for enhancing the electron conductivity in the active material layer.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As one method of increasing the energy density of the above power storage device, it is conceivable to thicken the active material layer of the electrode to increase the ratio of the active material layer in the power storage device. The present inventors newly found that in an electrode in which the active material layer is thickly formed, the long-time output of the power storage device may decrease by including carbon nanotubes in the active material layer. Further, the present inventors found that by including carbon nanotubes in the active material layer of the electrode, the peeling strength of the active material layer with respect to the current collector provided with the carbon coat layer is improved.

Means for Solving the Problems

[0005] ​The electrode for a power storage device that achieves the above object includes a current collector having a first surface and an active material layer formed on the first surface of the current collector, and is an electrode for a power storage device in which the thickness of the active material layer is 250 μm or more. A carbon coating layer is provided on the first surface of the current collector. The active material layer contains an active material capable of occluding and releasing charge carriers, an aqueous binder, and single-walled carbon nanotubes. The content of the single-walled carbon nanotubes in the active material layer is 0.035% by mass or more and 0.08% by mass or less.

[0006] In the electrode for the power storage device described above, the density of the active material layer is 1.8 g / cm 3 or more, which is preferable. In the electrode for the power storage device described above, the aqueous binder preferably contains styrene-butadiene rubber, and the content of the styrene-butadiene rubber is preferably 1.2% by mass or more and 1.8% by mass or less.

[0007] In the electrode for the power storage device described above, the current collector is an aluminum current collector made of aluminum and is preferably applied as a positive electrode. In the electrode for the power storage device described above, the content of the active material in the active material layer is preferably 96% by mass or more.

[0008] In the electrode for the power storage device described above, the thickness of the carbon coating layer is preferably 0.1 μm or more and 5.0 μm or less.

Advantages of the Invention

[0009] According to the present invention, the long-time output of the power storage device and the peeling strength of the active material layer with respect to the current collector are improved.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0011] Hereinafter, an embodiment embodying the present invention will be described with reference to the drawings. (Electrode) The electrode of this embodiment is used as the positive electrode or the negative electrode of an energy storage device. The energy storage device is, for example, a secondary battery such as a nickel-metal hydride secondary battery or a lithium-ion secondary battery. Further, the energy storage device may be an electric double layer capacitor. Hereinafter, the case where it is an electrode of a lithium-ion secondary battery will be described.

[0012] As shown in FIG. 1, the electrode 100 includes a current collector 101 and an active material layer 102 provided on the first surface 101a of the current collector 101. [Current Collector] The current collector 101 is a chemically inert electrical conductor for continuously passing an electric current through the active material layer 102 during discharge or charging 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 the material constituting the current collector 101, for example, a metal material, a conductive resin material, a conductive inorganic material, etc. can be used.

[0013] Examples of the above metal materials include copper, aluminum, nickel, titanium, and stainless steel. Examples of the above conductive resin materials include conductive polymer materials or resins obtained by adding conductive fillers to non-conductive polymer materials as necessary.

[0014] When it is the electrode 100 applied as the positive electrode of the power storage device, the current collector 101 is preferably an aluminum current collector made of aluminum. The aluminum current collector may be made of a single aluminum or an aluminum alloy. Examples of the aluminum alloy include an Al-Mn alloy, an Al-Mg alloy, and an Al-Mg-Si alloy. The content ratio of aluminum in the aluminum layer 11a is, for example, 50% by mass or more, preferably 70% by mass or more.

[0015] A carbon coating layer C is provided on the entire first surface 101a of the current collector 101. The thickness of the carbon coating layer C is, for example, 0.1 μm or more, preferably 0.2 μm or more. By setting the thickness of the carbon coating layer C to 0.1 μm or more, the peel strength of the active material layer 102 with respect to the current collector 101 is improved. The thickness of the carbon coating layer C is, for example, 5.0 μm or less, preferably 2.0 μm or less. By setting the thickness of the carbon coating layer C to 5.0 μm or less, the volume ratio of the active material layer 102 in the power storage device to which the electrode 100 is applied can be increased. As a result, the energy density of the power storage device to which the electrode 100 is applied can be increased.

[0016] The carbon coating layer C is not particularly limited, and a known carbon coating layer used for the current collector of the electrode can be applied. An example of the carbon coating layer C includes carbon particles and a coating layer binder. The carbon coating layer C may contain other components other than the carbon particles and the coating layer binder, if necessary.

[0017] Examples of the carbon particles include known carbon materials applied to the carbon coating layer such as graphite, acetylene black, and carbon black. The carbon particles contained in the carbon coating layer C may be of one kind or a combination of two or more kinds. The content ratio of the carbon particles in the carbon coating layer C is, for example, 30% by mass or more and 90% by mass or less.

[0018] Examples of the materials constituting the coating layer binder include acrylic resins, cellulose derivatives, and carboxy-modified styrene-butadiene rubber. Examples of the acrylic resin include homopolymers of acrylic monomers such as acrylic acid, methacrylic acid, and (meth)acrylic acid esters, or (meth)acrylic copolymers containing the above acrylic monomers. In the present embodiment, (meth)acrylic acid means acrylic acid or methacrylic acid.

[0019] Examples of the (meth)acrylic acid ester include methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, and isobutyl (meth)acrylate.

[0020] In the above (meth)acrylic copolymer, examples of the other comonomers copolymerized with the acrylic monomer include α-olefins, styrene, α-methylstyrene, vinyltoluene, acrylonitrile, methacrylonitrile, and vinyl acetate. These comonomers can exist in the acrylic resin in the form of a random copolymer, a graft copolymer, or a block copolymer. Examples of the (meth)acrylic copolymer include silicon-modified acrylic styrene resin, carboxy-modified acrylic styrene resin, and hydroxyl-modified acrylic resin.

[0021] Examples of the cellulose derivative include carboxymethyl cellulose and salts of carboxymethyl cellulose. Examples of the salt of carboxymethyl cellulose include the sodium salt or ammonium salt of carboxymethyl cellulose. The material constituting the coating layer binder may be one kind or a combination of two or more kinds. The content ratio of the coating layer binder in the carbon coating layer C is, for example, 10% by mass or more and 70% by mass or less.

[0022] The carbon coating layer C can be formed, for example, by applying a carbon paste containing carbon particles and a coating layer binder to the first surface 101a of the current collector 101 and then solidifying the film of the applied carbon paste.

[0023] [Active material layer] The active material layer 102 is formed on the carbon coating layer C on the first surface 101a of the current collector 101. The active material layer 102 contains an active material capable of occluding and releasing charge carriers such as lithium ions, an aqueous binder, and single-walled carbon nanotubes (SWCNT). Hereinafter, single-walled carbon nanotubes will be referred to as carbon nanotubes.

[0024] When it is the electrode 100 applied 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, those that can be used as the positive electrode active material of a lithium ion secondary battery, such as a lithium composite metal oxide having a layered rock salt structure, a metal oxide having a spinel structure, and a polyanion-based compound, may be adopted. Also, two or more positive electrode active materials may be used in combination. A specific example of the positive electrode active material is olivine-type lithium iron phosphate (LiFePO4) which is a polyanion-based compound.

[0025] When it is the electrode 100 applied 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, those that can be used as the negative electrode active material of a lithium ion secondary battery, such as Li, or carbon, a metal compound, an element capable of alloying with lithium or a compound thereof, may be adopted. Examples of carbon include natural graphite, artificial graphite, or hard carbon (carbon with difficulty in graphitization) or soft carbon (carbon with ease in graphitization). Examples of artificial graphite include highly oriented graphite, mesocarbon microbeads, etc. Examples of elements capable of alloying with lithium include silicon and tin.

[0026] 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% by mass or more, preferably 97% by mass or more. By setting the content of the active material in the active material layer 102 to 96% by mass or more, the volume ratio of the active material in the active material layer 102 can be increased. As a result, the energy density of the power storage device to which the electrode 100 is applied can be increased. The content of the active material in the active material layer 102 is, for example, 98.965% by mass or less, preferably 98% by mass or less.

[0027] The aqueous binder is a binder that can be dissolved or dispersed in an aqueous solvent, and is a binder used by mixing with the positive electrode active material in a state of being dispersed or dissolved in the aqueous solvent. The aqueous binder is not particularly limited, and a conventionally known material can be used as the aqueous binder contained in the active material layer of the lithium-ion secondary battery. Examples of the aqueous binder include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, acrylic resins such as poly(meth)acrylic acid, styrene-butadiene rubber, carboxymethyl cellulose, alginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester cross-linked products, and starch-acrylic acid graft polymers. The aqueous binder contained in the active material layer 102 may be one type or two or more types.

[0028] The content of the aqueous binder in the active material layer 102 is not particularly limited. The content of the aqueous binder in the active material layer 102 is, for example, 1.0% by mass or more, preferably 1.3% by mass or more. The content of the aqueous binder in the active material layer 102 is, for example, 2.5% by mass or less, preferably 2.1% by mass or less.

[0029] Note that the active material layer 102 preferably contains styrene-butadiene rubber as an aqueous binder. The content of styrene-butadiene rubber in the active material layer 102 is, for example, 1.2% by mass or more and 2.5% by mass or less. Further, when the content of styrene-butadiene rubber is 1.2% by mass or more, the adhesion of the active material layer 102 to the current collector 101 is improved. Also, the content of styrene-butadiene rubber is preferably 1.8% by mass or less. In this case, the degree of bending of the active material layer 102 is reduced, thereby improving the ionic conductivity and the output performance of the power storage device.

[0030] The fiber length and fiber diameter of the carbon nanotubes are not particularly limited. The fiber length of the carbon nanotubes is, for example, 5 μm or more and 1000 μm or less. The fiber diameter of the carbon nanotubes is, for example, 1.6 nm or more and 100 μm or less. Further, the fiber length distribution of the carbon nanotubes is preferably a fiber length distribution in which there is only one main peak in the graph showing the fiber length distribution.

[0031] The content of carbon nanotubes in the active material layer 102 is 0.035% by mass or more and 0.08% by mass or less. By setting the content of carbon nanotubes to 0.035% by mass or more, the peel strength of the active material layer 102 with respect to the current collector 101 is improved. Also, by setting the content of carbon nanotubes to 0.08% by mass or less, the long-term output of the power storage device is improved.

[0032] The active material layer 102 can contain other components other than the above-described three components of the active material, the aqueous binder, and the carbon nanotubes, if necessary. Examples of the other components include a conductive aid, an electrolyte (polymer matrix, ion-conductive polymer, electrolyte solution, etc.), and an electrolyte supporting salt (lithium salt) for enhancing ionic conductivity. Examples of the conductive aid include acetylene black, carbon black, and graphite. The types and contents of the other components are not particularly limited, and conventionally known knowledge about lithium-ion secondary batteries can be appropriately referred to.

[0033] The active material layer 102 is formed thicker than normal from the viewpoint of increasing the energy density of the power storage device. The thickness t of the active material layer 102 is 250 μm or more. Also, the thickness t of the active material layer 102 is, for example, 750 μm or less.

[0034] The density of the active material layer 102 is not particularly limited. The density of the active material layer 102 is, for example, 1.8 g / cm 3 or more. When the density of the active material layer 102 is high, a decrease in the long-term output of the power storage device due to the inclusion of carbon nanotubes is likely to occur. Also, the density of the active material layer 102 is, for example, 2.4 g / cm 3 or less.

[0035] (Power storage device) Next, an example of a power storage device to which the electrode 100 is applied will be described. The power storage device to which the electrode 100 is applied is, for example, a power storage module used in batteries of various vehicles such as forklifts, hybrid vehicles, and electric vehicles. In this embodiment, the case where the power storage device 10 is a lithium-ion secondary battery will be exemplified.

[0036] As shown in FIG. 3, the power storage device 10 includes a cell stack 30 (laminated body) in which a plurality of power storage cells 20 are stacked (laminated) in the stacking direction. Hereinafter, the stacking direction of the plurality of power storage cells 20 will simply be 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. Either one or both of the positive electrode 21 and the negative electrode 22 of the power storage cell 20 are the above-described electrode 100. In FIG. 3, the illustration of the carbon coating layer C is omitted.

[0037] The positive electrode 21 includes a positive electrode current collector 21a and a positive electrode active material layer 21b provided on the first surface 21a1 of the positive electrode current collector 21a. When the positive electrode 21 is the electrode 100, the positive electrode current collector 21a is the current collector 101, and the positive electrode active material layer 21b is the active material layer 102.

[0038] In a plan view seen from the stacking direction (hereinafter simply referred to as the plan view), the positive electrode active material layer 21b is formed at the center of the first surface 21a1 of the positive electrode current collector 21a. The peripheral portion of the first surface 21a1 of the positive electrode current collector 21a in the plan view is a positive electrode non-coated portion 21c where the positive electrode active material layer 21b is not provided. The positive electrode non-coated portion 21c is arranged so as to surround the periphery of the positive electrode active material layer 21b in the plan view.

[0039] The negative electrode 22 includes a negative electrode current collector 22a and a negative electrode active material layer 22b provided on the 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.

[0040] In the plan view, the negative electrode active material layer 22b is formed at the center of the first surface 22a1 of the negative electrode current collector 22a. The peripheral portion of the first surface 22a1 of the negative electrode current collector 22a in the plan view is a negative electrode non-coated portion 22c where the negative electrode active material layer 22b is not provided. The negative electrode non-coated portion 22c is arranged so as to surround the periphery of the positive electrode active material layer 21b in the plan view. The positive electrode 21 and the negative electrode 22 are arranged such that the positive electrode active material layer 21b and the negative electrode active material layer 22b face each other in the stacking direction. That is, the facing direction of the positive electrode 21 and the negative electrode 22 coincides with the stacking direction. The negative electrode active material layer 22b is formed to have the same size as the positive electrode active material layer 21b or to be slightly larger than the positive electrode active material layer 21b. When the negative electrode active material layer 22b is formed to be slightly larger than the positive electrode active material layer 21b, the entire formation region of the positive electrode active material layer 21b is located within the formation region of the negative electrode active material layer 22b in the plan view.

[0041] The positive electrode current collector 21a has a second surface 21a2 on the side opposite to the first surface 21a1. The positive electrode 21 is an electrode with a monopolar 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 on the side opposite to the first surface 22a1. The negative electrode 22 is an electrode with a monopolar 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.

[0042] The separator 23 is disposed between the positive electrode 21 and the negative electrode 22, and is a member that prevents a short circuit due to contact between the two electrodes by isolating the positive electrode 21 and the negative electrode 22, while allowing charge carriers such as lithium ions to pass through.

[0043] The separator 23 is, for example, a porous sheet or nonwoven fabric containing a polymer that absorbs and retains an electrolyte. Examples of the material constituting the separator 23 include polyolefins such as polypropylene and polyethylene, and polyester. The separator 23 may have a single-layer structure or a multilayer structure. The multilayer structure may have, for example, an adhesive layer, a ceramic layer as a heat-resistant layer, and the like.

[0044] The spacer 24 is disposed between the first surface 21a1 of the positive electrode current collector 21a of the positive electrode 21 and the first surface 22a1 of the negative electrode current collector of the negative electrode 22, and on the outer peripheral side of the positive electrode active material layer 21b and the negative electrode active material layer 22b, and is adhered to both the positive electrode current collector 21a and the negative electrode current collector 22a. The spacer 24 maintains the distance between the positive electrode current collector 21a and the negative electrode current collector, prevents a short circuit between the current collectors, and seals the space between the current collectors in a liquid-tight manner.

[0045] In plan view, the spacer 24 extends along the peripheral portions of the positive electrode current collector 21a and the negative electrode current collector 22a, and is formed in a frame shape surrounding the positive electrode current collector 21a and the negative electrode current collector 22a. The spacer 24 is disposed between the non-coated portion 21c of the first surface 21a1 of the positive electrode current collector 21a and the non-coated portion 22c of the first surface 22a1 of the negative electrode current collector 22a.

[0046] 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.

[0047] Inside the power storage cell 20, a sealed space S surrounded by a frame-shaped spacer 24, a positive electrode 21, and a negative electrode 22 is formed. The separator 23 and the electrolyte are accommodated in the sealed space S. Note that the peripheral portion of the separator 23 is in a state of being buried in the spacer 24.

[0048] Examples of the electrolyte include a liquid electrolyte and a polymer gel electrolyte containing an electrolyte held in a polymer matrix. Examples of the liquid electrolyte include a liquid electrolyte containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the electrolyte salt, known lithium salts such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, LiN(FSO2)2, and LiN(CF3SO2)2 can be used. Also, as the non-aqueous solvent, known solvents such as cyclic carbonates, cyclic esters, chain carbonates, chain esters, and ethers can be used. Note that two or more of these known solvent materials may be used in combination.

[0049] By sealing the sealed space S between the positive electrode 21 and the negative electrode 22, the spacer 24 can suppress the leakage of the electrolyte accommodated in the sealed space S to the outside. Also, the spacer 24 can suppress the intrusion of moisture from the outside of the power storage device 10 into the sealed space S. Furthermore, the spacer 24 can suppress, for example, the leakage of gas generated from the positive electrode 21 or the negative electrode 22 due to charge and discharge reactions or the like to the outside of the power storage device 10.

[0050] The cell stack 30 has a structure in which a plurality of power storage cells 20 are stacked such that the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a are in contact with each other. As a result, the plurality of power storage cells 20 constituting the cell stack 30 are connected in series.

[0051] Here, in the cell stack 30, a pseudo bipolar electrode 25 is formed by two adjacent power storage cells 20 in the stacking direction, regarding the positive electrode current collector 21a and the negative electrode current collector 22a in contact with each other as a single current collector. The pseudo bipolar electrode 25 includes a current collector having a structure in which the positive electrode current collector 21a and the negative electrode current collector 22a are stacked, a positive electrode active material layer 21b formed on one surface of the current collector, and a negative electrode active material layer 22b formed on the other surface of the current collector.

[0052] The power storage device 10 includes a pair of current conductors, namely a positive electrode current conducting plate 40 and a negative electrode current conducting plate 50, which are arranged so as to sandwich the cell stack 30 in the stacking direction of the cell stack 30. The positive electrode current conducting plate 40 and the negative electrode current conducting plate 50 are each made of a material with excellent conductivity.

[0053] The positive electrode current conducting plate 40 is electrically connected to the second surface 21a2 of the positive electrode current 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 current collector 22a of the outermost negative electrode 22 at the other end in the stacking direction.

[0054] Charging and discharging of the power storage device 10 are performed through terminals provided on each of the positive electrode current conducting plate 40 and the negative electrode current conducting plate 50. As the material constituting the positive electrode current conducting plate 40, for example, the same material as the material constituting the positive electrode current collector 21a can be used. The positive electrode current conducting plate 40 may be made of a metal plate thicker than the positive electrode current collector 21a used in the cell stack 30. As the material constituting the negative electrode current conducting plate 50, for example, the same material as the material constituting the negative electrode current collector 22a can be used. The negative electrode current conducting plate 50 may be made of a metal plate thicker than the negative electrode current collector 22a used in the cell stack 30.

[0055] Next, the operation of this embodiment will be described. The image shown in FIG. 2 is an electron micrograph of the first surface 101a of the current collector 101 after peeling the active material layer 102 from the current collector 101 of the electrode 100. In the image, the spherical objects are the active material, and the fibrous objects are carbon nanotubes. The carbon nanotubes are in a state of being entangled with the surface of the active material and are also adhered to the carbon coating layer C of the current collector 101 so as to take root. By setting the content of carbon nanotubes in the active material layer 102 to 0.035% by mass or more, a state in which the carbon nanotubes adhere to the active material and the carbon coating layer C can be preferably formed. As a result, the peeling strength of the active material layer 102 with respect to the current collector 101 is improved.

[0056] As described above, when carbon nanotubes are contained in the active material layer 102, in addition to the effect of improving the electron conductivity of the active material layer 102 as a conductive assistant, an effect of improving the peeling strength of the active material layer 102 with respect to the current collector 101 can be obtained. On the other hand, in an electrode in which the thickness of the active material layer 102 is 250 μm or more, when the active material layer 102 contains carbon nanotubes, a problem occurs in that the long-term output of the power storage device decreases.

[0057] This problem is caused by the carbon nanotubes in the active material layer 102 decreasing the ionic conductivity of the active material layer 102. More specifically, when the active material layer 102 contains carbon nanotubes, the degree of bending of the active material layer 102 increases, and as a result, the ionic conductivity of the active material layer 102 decreases. And when the thickness of the active material layer 102 increases, the ease of movement of charge carriers such as lithium ions in the active material layer 102 becomes a strong factor that determines the rate-determining step of the long-term output of the power storage device, that is, the upper limit of the long-term output. As a result, the decrease in the ionic conductivity of the active material layer 102 appears as a decrease in the long-term output of the power storage device.

[0058] The inventors have found that by setting the content of carbon nanotubes in the active material layer 102 to 0.08% by mass or less, it is possible to suppress the decrease in the long-term output of the above-described power storage device. Therefore, by using the electrode 100 of the present embodiment in which the content of carbon nanotubes in the active material layer 102 is 0.08% by mass or less, the long-term output of the power storage device can be improved.

[0059] Next, the operation and effects of the present embodiment will be described. (1) The electrode 100 for a power storage device includes a current collector 101 having a first surface 101a and an active material layer 102 formed on the first surface 101a of the current collector 101. The thickness of the active material layer 102 is 250 μm or more. A carbon coating layer C is provided on the first surface 101a of the current collector 101. The active material layer 102 contains an active material capable of occluding and releasing charge carriers, an aqueous binder, and carbon nanotubes. The content of carbon nanotubes in the active material layer 102 is 0.035% by mass or more and 0.08% by mass or less.

[0060] According to the above configuration, the long-term output of the power storage device is improved. Further, according to the above configuration, the peel strength of the active material layer 102 with respect to the current collector 101 is improved. (2) The density of the active material layer 102 is 1.8 g / cm 3 or more.

[0061] The higher the density of the active material layer 102, the more likely the degree of bending of the active material layer 102 becomes when carbon nanotubes are contained, and as a result, the decrease in the long-term output of the power storage device also increases. Therefore, when the density of the active material layer 102 is high, the effect of improving the long-term output of the power storage device described in (1) above can be obtained more remarkably.

[0062] (3) The aqueous binder contains styrene-butadiene rubber. The content of styrene-butadiene rubber in the active material layer 102 is 1.2% by mass or more and 1.8% by mass or less. By setting the content of styrene-butadiene rubber to 1.2% by mass or more, the peel strength of the active material layer 102 with respect to the current collector 101 is improved. Further, by setting the content of styrene-butadiene rubber to 1.8% by mass or less, the degree of bending of the active material layer 102 is reduced. As a result, the ionic conductivity of the active material layer 102 is improved and the output performance of the power storage device is improved.

[0063] (4) The content of the active material in the active material layer 102 is 96% by mass or more. According to the above configuration, the energy density of the power storage device to which the electrode 100 is applied can be increased.

[0064] (5) The thickness of the carbon coating layer C is 0.1 μm or more and 5.0 μm or less. According to the above configuration, the peel strength of the active material layer 102 with respect to the current collector 101 is improved. Further, according to the above configuration, the energy density of the power storage device to which the electrode 100 is applied can be increased.

[0065] Note that the present embodiment can be implemented with the following modifications. The present embodiment and the following modification examples can be implemented in combination with each other within a technically non-conflicting range. ○ The formation range of the carbon coating layer C on the first surface 101a of the current collector 101 may be changed. For example, the carbon coating layer C may be formed only in the range where the active material layer 102 is formed on the first surface 101a, or the carbon coating layer C may be partially formed in a part of the range.

[0066] ○ The electrode 100 may be an electrode having a bipolar structure. An example of the electrode 100 embodied as an electrode having a bipolar structure will be described with reference to FIG. 4. The bipolar structure electrode 100 shown in FIG. 4 includes a bipolar current collector 103. The bipolar current collector 103 is a laminate in which a foil-shaped positive electrode current collector 104 and a foil-shaped negative electrode current collector 105 are integrally joined in the thickness direction. Examples of the bipolar current collector 103 include a current collector in which aluminum foils are bonded together and a current collector in which an aluminum foil and a copper foil are bonded together.

[0067] On the first surface 103a of the bipolar current collector 103 formed by the positive electrode current collector 104, a carbon coating layer C is provided. And on the first surface 103a, an active material layer 102a configured as a positive electrode active material layer is provided via the carbon coating layer C. Also, on the second surface 103b of the bipolar current collector 103 formed by the negative electrode current collector 105, a carbon coating layer C is provided. And on the second surface 103b, an active material layer 102b configured as a negative electrode active material layer is provided via the carbon coating layer C.

[0068] Either one or both of the active material layer 102a and the active material layer 102b are active material layers that satisfy the requirements of the active material layer 102 described in the above embodiment. In the case where the active material layer 102a is an active material layer that does not satisfy the requirements of the active material layer 102 described in the above embodiment, the carbon coating layer C on the first surface 103a can be omitted. Similarly, in the case where the active material layer 102b is an active material layer that does not satisfy the requirements of the active material layer 102 described in the above embodiment, the carbon coating layer C on the second surface 103b can be omitted.

[0069] ○ The power storage device 10 to which the electrode 100 is applied is not particularly limited as long as at least one positive electrode or at least one negative electrode corresponds to the electrode 100. For example, the number of power storage cells 20 constituting the power storage device 10 may be 1. Also, the power storage device 10 may be provided with a restraint member that applies a restraint load in the stacking direction to the cell stack 30. Also, the power storage device 10 may be provided with the electrode 100 configured as a bipolar electrode.

Example

[0070] Hereinafter, examples in which the above embodiment is further embodied will be described. <Fabrication of Positive Electrode> LiFePO4, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and single-walled carbon nanotubes (CNT) were mixed at the compounding ratios shown in Table 1 and Table 2, and water was added to this mixture to prepare a positive electrode composite material with a solid content ratio of 64% by mass. LiFePO4 is the positive electrode active material. Styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are aqueous binders.

[0071] As the positive electrode current collector, a carbon-coated aluminum foil with a thickness of 30 μm was prepared. A carbon coating with a thickness of 1 μm was provided on one surface of this carbon-coated aluminum foil. The positive electrode composite material was applied in a film form to the surface of the positive electrode current collector provided with the carbon coating using the doctor blade method. The applied positive electrode composite material was heat-treated at 50 °C to dry and solidify the positive electrode composite material, thereby producing positive electrode sheets for Test Examples 1 to 10 in which a positive electrode active material layer with a thickness of 250 μm was formed on the positive electrode current collector. The density of the positive electrode active material layer of each obtained positive electrode sheet was measured. As a result, the density of the positive electrode active material layer of each positive electrode sheet was all 1.8 g / cm 3 There was.

[0072] <Measurement of peel strength> Using the peel test device (manufactured by Minebea Co., Ltd., LTS-50N-S300), the positive electrode sheets of Test Examples 1 to 10 cut into 2.5 cm × 4 cm were used as measurement samples, and a 90-degree peel test was performed in accordance with JIS K 6854-1. By dividing the strength measured in the 90-degree peel test by the line width (2.5 cm), the peel strength of the positive electrode active material layer with respect to the positive electrode current collector in the measurement sample was calculated. The results are shown in Table 1 and Table 2.

[0073]

Table 1

[0074]

Table 2

[0075] <Fabrication of the energy storage device> A positive electrode obtained by cutting the positive electrode sheets of Test Examples 1 to 10 into a rectangular shape of 30 mm in length and 25 mm in width, a negative electrode, and a separator were combined to form an electrode body battery. The electrode body battery was housed in a battery case, an electrolytic solution was injected, and the battery case was sealed to obtain a lithium ion secondary battery.

[0076] As the negative electrode, a negative electrode having a negative electrode current collector made of copper, graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a dispersant was used. As the separator, a separator made of polyethylene was used. As the electrolytic solution, an electrolytic solution in which lithium hexafluorophosphate was dissolved at a concentration of 1.2 M in a mixed solvent obtained by mixing methyl propionate and ethylene carbonate at a volume ratio of 15:85 was used.

[0077] <Evaluation of the output performance of the energy storage device> (Evaluation of long-term output) In this evaluation, lithium ion secondary batteries obtained using the positive electrode sheets of Test Examples 6 to 10 showing high peel strength were used. For each lithium ion secondary battery, constant current (CC) charging was performed until the voltage reached 3.75 V. Then, constant power discharge was performed at 200 to 300 mW at 40°C, and the power outputtable for 2500 seconds (2500 s output) was calculated. A graph plotting the values of the 2500 s output against the CNT content in each test example is shown in FIG. 5.

[0078] (Evaluation of short-term output) In this evaluation, lithium-ion secondary batteries obtained using the positive electrode sheets of Test Examples 2 to 4 and 6 to 10 were used. For each lithium-ion secondary battery, constant current (CC) charging was performed at a direct current of 10 mA until the voltage of the negative electrode with respect to the positive electrode reached 3.75 V, and then discharged until the state of charge (SOC) reached 80%. Thereafter, it was discharged at a current of 25 °C and a 10C rate for 10 seconds and the output at 10 seconds (10s output) was calculated. A graph plotting the values of the 10s output against the CNT content in each test example is shown in Fig. 6.

[0079] As shown in the graph of Fig. 5, the 2500s output, which is a parameter indicating the long-term output, is almost constant in the range where the CNT content is 0.04 to 0.08 mass%, and significantly decreases as the CNT content increases up to 0.10 mass%. On the other hand, as shown in the graph of Fig. 6, the 10s output, which is a parameter indicating the short-term output, increases as the CNT content increases in the range where the CNT content is 0.04 mass% or less, and is almost constant in the range where the CNT content is 0.04 mass% or more. From these results, it can be seen that adding CNTs of 0.10 mass% or more does not affect the short-term output but decreases the long-term output.

[0080] Also, although detailed test data is omitted, similar tests were conducted using positive electrode sheets in which the thickness of the positive electrode active material layer of the positive electrode sheets of Test Examples 1 to 10 was changed from 250 μm to 120 μm. As a result, no decrease was confirmed in either the short-term output or the long-term output when CNTs of 0.10 mass% or more were added. Therefore, it can be seen that the decrease in the long-term output caused by adding CNTs of 0.10 mass% or more is a phenomenon specific to the case where the active material layer is formed thick. From these results, it can be seen that when the thickness of the active material layer is 250 μm or more, by setting the CNT content in the active material layer to 0.08 mass% or less, the long-term output is improved compared to the case where the CNT content exceeds 0.08 mass%.

[0081] <Evaluation of Flexure Degree> A symmetric model cell for measuring the degree of bending was fabricated using a positive electrode obtained by cutting the positive electrode sheets of Test Example 7 and Test Example 9 into a predetermined shape, a separator, and an electrolytic solution. As the separator, a separator made of polyethylene was used. As the electrolytic solution, an electrolytic solution in which lithium hexafluorophosphate was dissolved at a concentration of 1.2 M in a mixed solvent obtained by mixing methyl propionate and ethylene carbonate at a volume ratio of 15:85 was used.

[0082] Based on the following formula (1), the degree of bending τ of the positive electrode active material layer of the positive electrode sheet of each test example was calculated. The results are shown in Table 3. Degree of bending τ = (R ion ·A·K·ε) / 2d …(1) R ion : Ion resistance A: Electrode area (8.06 cm 2 ) K: Ionic conductivity of the electrolytic solution ε: Porosity of the positive electrode active material layer d: Thickness of the positive electrode active material layer (250 μm) Ion resistance R ion was derived from the real component of the extremely low frequency of the measured symmetric cell impedance of the symmetric model cell (= ion resistance R ion / 3).

[0083] The ionic conductivity K of the electrolytic solution was calculated from the measured value of the resistance at 25 °C and 10 kHz of a sample in which the electrolytic solution having the above composition was enclosed in a cell equipped with a platinum electrode. The porosity ε of the positive electrode active material layer was measured using the mercury intrusion method.

[0084]

Table 3

[0085] Also, when the active material layer is formed thick, the influence of the decrease in ionic conductivity becomes large. Therefore, according to the idea that the long-term output decreases based on the decrease in the ionic conductivity of the active material layer due to CNT, it can also be explained that the decrease in long-term output is a phenomenon peculiar to the case where the active material layer is formed thick.

[0086] <Test on the content of styrene-butadiene rubber (SBR)> Positive electrode sheets of Test Examples 11 to 13 were produced in the same manner as Test Examples 1 to 10, except that the mixing ratio was changed to the mixing ratio shown in Table 4. The density of the positive electrode active material layer of each obtained positive electrode sheet was measured. As a result, the density of the positive electrode active material layer of each positive electrode sheet was all 2.0 g / cm 3 Thereafter. Also, a positive electrode sheet of Test Example 14 was produced in the same manner as Test Examples 1 to 10, except that the thickness of the positive electrode active material layer was changed from 250 μm to 370 μm and the mixing ratio was changed to the mixing ratio shown in Table 4. As a result of measuring the density of the positive electrode active material layer of the positive electrode sheet of Test Example 14, the density was 2.0 g / cm 3 Thereafter.

[0087] The peeling strength between the positive electrode active material layer and the positive electrode current collector in the positive electrode sheets of Test Examples 11 to 13 and the positive electrode sheet of Test Example 14 was calculated by the same method as described above. Also, the degree of bending of the positive electrode active material layer of Test Examples 11 to 13 was calculated by the same method as described above. The results are shown in Table 4.

[0088] Furthermore, lithium-ion secondary batteries were fabricated using the positive electrode sheets of Test Examples 11 to 13 by the same method as described above. For each of the fabricated lithium-ion secondary batteries, after constant current (CC) charging was performed until the voltage with respect to the positive electrode reached 3.75 V, the capacity C 0.01 and the capacity C1 when discharging at 1 C were determined. Then, based on the following formula (2), ΔSOC (%) was calculated from the capacity C 0.01 and the capacity C1. The results are shown in Table 4.

[0089] ΔSOC (%) = (C1 / C 0.01 ) × 100…(2)

[0090]

Table 4

[0091] As shown in Table 4, the peel strength of Test Example 14 in which the thickness of the positive electrode active material layer was changed from 250 μm to 370 μm was 0.35 N / cm. From the results, it can be seen that the thickness of the active material layer does not strongly affect the peel strength.

[0092] Next, the technical idea that can be grasped from the above-described embodiments and modification examples will be described below. A power storage device comprising an electrode having an active material layer formed on a first surface of a current collector, wherein a carbon coating layer is provided on the first surface of the current collector, the thickness of the active material layer is 250 μm or more, the active material layer contains an active material capable of occluding and releasing charge carriers, an aqueous binder, and carbon nanotubes, and the content of the carbon nanotubes in the active material layer is 0.035% by mass or more and 0.08% by mass or less.

Description of reference numerals

[0093] C... Carbon coating layer 100... Electrode 101... Current collector 101a... First surface 102... Active material layer

Claims

1. An electrode for a power storage device, comprising a current collector having a first surface and an active material layer formed on the first surface of the current collector, wherein the thickness of the active material layer is 250 μm or more, a carbon coating layer is provided on the first surface of the current collector, the active material layer contains an active material capable of occluding and releasing charge carriers, an aqueous binder, and single-walled carbon nanotubes, and the content of the single-walled carbon nanotubes in the active material layer is 0.035% by mass or more and 0.08% by mass or less.

2. The density of the active material layer is 1.8 g / cm ３ The electrode for a power storage device according to claim 1, wherein the density is 1.8 g / cm or more.

3. The aqueous binder contains styrene-butadiene rubber, and the content of the styrene-butadiene rubber is 1.2% by mass or more and 1.8% by mass or less. The electrode for a power storage device according to Claim 1 or Claim 2.

4. The current collector is an aluminum current collector made of aluminum and is applied as a positive electrode. The electrode for a power storage device according to Claim 1 or Claim 2.

5. The content of the active material in the active material layer is 96% by mass or more. The electrode for a power storage device according to Claim 1 or Claim 2.

6. The thickness of the carbon coating layer is 0.1 μm or more and 5.0 μm or less. The electrode for a power storage device according to Claim 1 or Claim 2.

7. A power storage device including an electrode in which an active material layer is formed on a first surface of a current collector, a carbon coating layer is provided on the first surface of the current collector, the thickness of the active material layer is 250 μm or more, the active material layer contains an active material capable of occluding and releasing charge carriers, an aqueous binder, and carbon nanotubes, and the content of the carbon nanotubes in the active material layer is 0.035% by mass or more and 0.08% by mass or less.

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