JP2022027413A - Power storage device - Google Patents

Abstract

To suppress the formation of a wrinkle or crease by shrinkage of an active material layer in a current collector.SOLUTION: A power storage device 10 comprises a structure arranged by repeatedly laminating a negative electrode 22 having a negative electrode active material layer 22b provided on a first face 22a1 of a foil-like negative electrode current collector 22a, a positive electrode 21 having a positive electrode active material layer 21b provided on a first face 21a1 of a positive electrode current collector 21a, provided that the positive electrode active material layer 21b is opposed to the negative electrode active material layer 22b of the negative electrode 22, and a separator 23 disposed between the positive electrode active material layer 21b and the negative electrode active material layer 22b, provided that the negative electrode 22 and the positive electrode 21 are laminated so that a second face 22a2 of the negative electrode current collector 22a, and a second face 21a2 of the positive electrode current collector 21a are put in contact with each other. The negative electrode active material layer 22b contains a negative electrode active material, CMC, and SBR; the content of CMC is 0.5 mass% or more and 1.3 mass% or less, and the content of SBR is 2.2 mass% or more and 5.0 mass% or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a power storage device.

Patent Document 1 and Patent Document 2 disclose a flat type power storage device configured by stacking a plurality of individually manufactured power storage cells in series. The storage cell has a positive electrode having a positive electrode active material layer formed on one side of a foil-shaped positive electrode current collector and a negative electrode active material layer formed on one side of a foil-shaped negative electrode current collector. It includes a negative electrode arranged so that the layer faces the positive electrode active material layer of the positive electrode, and a separator arranged between the positive electrode and the negative electrode. In the power storage device, a plurality of the power storage cells are laminated so that the positive electrode current collector and the negative electrode current collector are in contact with each other, specifically, the positive electrode active material layer in the positive electrode current collector is not formed. It is formed by laminating the surface and the surface of the negative electrode current collector on which the negative electrode active material layer is not formed so as to be in contact with each other.

Japanese Unexamined Patent Publication No. 2017-16825 Japanese Patent No. 66366607

As one of the methods for increasing the energy density of the storage cell, a method for increasing the basis weight of the active material layer can be considered. However, when the basis weight of the active material layer of the power storage device having the above configuration is increased, the problems described below occur. That is, in an electrode having a structure in which an active material layer is provided on the surface of a foil-shaped current collector, when the amount of the active material layer is increased, wrinkles are generated in the current collector due to shrinkage of the active material layer during electrode manufacturing. It will be easier to do. The power storage device having the above configuration has a conductive portion formed by bringing the surfaces of the current collector, which are not provided with the active material layer, into contact with each other. Therefore, when the current collector is wrinkled, the adhesion between the current collectors in the conductive portion is lowered and the contact resistance is increased.

The present invention has been made in view of such circumstances, and an object thereof is to suppress wrinkles generated in a current collector due to shrinkage of an active material layer in a power storage device using a foil-shaped current collector. ..

The power storage device that achieves the above object is a negative electrode having a negative electrode active material layer provided on the first surface of a foil-shaped negative electrode current collector having a thickness of 1 to 100 μm, and a positive electrode activity on the first surface of the positive electrode current collector. A material layer is provided, and the positive electrode active material layer is arranged between the positive electrode arranged so as to face the negative electrode active material layer of the negative electrode, and between the positive electrode active material layer and the negative electrode active material layer. The separators are repeatedly laminated, and the second surface of the negative electrode current collector on the opposite side of the first surface and the second surface of the positive electrode current collector on the opposite side of the first surface come into contact with each other. A power storage device having a structure in which the negative electrode and the positive electrode are laminated as described above, and having a conductive portion superimposed on a portion located on the opposite side of the portion provided with the negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material, carboxymethyl cellulose, and styrene-butadiene rubber, and the content of the carboxymethyl cellulose is 0.5% by mass or more and 1.3% by mass or less. The content of the styrene-butadiene rubber is 2.2% by mass or more and 5.0% by mass or less.

In the power storage device, the degree of etherification of the carboxymethyl cellulose is preferably 0.85 or more and 1.1 or less.
In the power storage device, the basis weight of the negative electrode active material layer is preferably 17 mg / cm ² or more.

In the power storage device, the positive electrode active material layer is formed on the negative electrode and the first surface of the positive electrode current collector, and the positive electrode active material layer is arranged so as to face the negative electrode active material layer of the negative electrode. It has a structure in which a positive electrode and a separator arranged between the positive electrode active material layer and the negative electrode active material layer are repeatedly laminated, and the conductive portion is the positive electrode current collector of the positive electrode. It is preferable that the second surface of the negative electrode current collector and the second surface of the positive electrode current collector on the opposite side of the first surface are in contact with each other.

In the power storage device, the mass ratio of the styrene-butadiene rubber to the carboxymethyl cellulose is preferably 1.69 or more and 3 or less.
According to each of the above configurations, carboxymethyl cellulose and styrene-butadiene rubber are used as the binder component contained in the negative electrode active material layer, and the content thereof is set within the above-mentioned specific range, whereby the negative electrode active material is used. Shrinkage of the negative electrode active material layer during layer production is suppressed. As a result, it is possible to suppress the occurrence of wrinkles in the foil-shaped negative electrode current collector due to the shrinkage of the negative electrode active material layer. As a result, it is possible to suppress a decrease in the adhesion of the contact portion between the second surfaces of the negative electrode current collector and the positive electrode current collector, and an increase in contact resistance due to the decrease in the adhesion. By adopting a negative electrode active material layer that does not easily cause wrinkles in the negative electrode current collector, it becomes easy to increase the amount of the negative electrode active material layer with respect to the negative electrode current collector and increase the energy density of the storage cell. ..

According to the present invention, with respect to a power storage device using a foil-shaped current collector, wrinkles generated in the current collector due to shrinkage of the active material layer can be suppressed.

Sectional drawing of the power storage device. An explanatory diagram of a manufacturing method of a power storage device. (A) is a photograph of the second surface of the negative electrode current collector of Test Example 1, and (b) is a photograph of the second surface of the negative electrode current collector of Test Example 2. The graph which shows the relationship between the mass ratio (SBR / CMC) and the capacity retention rate.

Hereinafter, an embodiment embodying the present invention will be described with reference to the drawings.
The power storage device 10 shown in FIG. 1 is a power storage module used for batteries of various vehicles such as forklifts, hybrid vehicles, and electric vehicles. The power storage device 10 is, for example, a secondary battery such as a nickel hydrogen secondary battery or a lithium ion secondary battery. The power storage device 10 may be an electric double layer capacitor or an all-solid-state battery. In this embodiment, a case where the power storage device 10 is a lithium ion secondary battery is illustrated.

As shown in FIG. 1, the power storage device 10 includes a cell stack 30 (laminated body) in which a plurality of power storage cells 20 are stacked (stacked) in the stacking direction. Hereinafter, the stacking direction of the plurality of storage cells 20 is simply referred to as a stacking direction. Each storage cell 20 includes a positive electrode 21, a negative electrode 22, a separator 23, and a spacer 24.

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. The positive electrode active material layer 21b is formed in the central portion of the first surface 21a1 of the positive electrode current collector 21a in a plan view (hereinafter, simply referred to as a plan view) viewed from the stacking direction. The peripheral edge of the first surface 21a1 of the positive electrode current collector 21a in a plan view is a positive electrode uncoated portion 21c in which the positive electrode active material layer 21b is not provided. The positive electrode uncoated portion 21c is arranged so as to surround the periphery of the positive electrode active material layer 21b in a plan view.

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. In a plan view, the negative electrode active material layer 22b is formed in the central portion of the first surface 22a1 of the negative electrode current collector 22a. The peripheral edge of the first surface 22a1 of the negative electrode current collector 22a in a plan view is a negative electrode uncoated portion 22c in which the negative electrode active material layer 22b is not provided. The negative electrode uncoated portion 22c is arranged so as to surround the negative electrode active material layer 22b in a plan view.

The positive electrode 21 and the negative electrode 22 are arranged so 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 opposite directions of the positive electrode 21 and the negative electrode 22 coincide with the stacking direction. The negative electrode active material layer 22b is formed to be one size larger than the positive electrode active material layer 21b, and the entire formation region of the positive electrode active material layer 21b is located within the formation region of the negative electrode active material layer 22b in a plan view. There is.

The positive electrode current collector 21a has a second surface 21a2 which is a surface opposite to the first surface 21a1. The positive electrode 21 is an electrode having 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 which is a surface opposite to the first surface 22a1. The negative electrode 22 is an electrode having 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 negative electrode current collector 22a. The specific configurations of the materials constituting the positive electrode 21 and the negative electrode 22 will be described later.

The separator 23 is a member arranged between the positive electrode 21 and the negative electrode 22 and allowing a charge carrier such as lithium ion to pass through while separating the positive electrode 21 and the negative electrode 22 to prevent a short circuit due to contact between the two electrodes. ..

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

The spacer 24 is between the first surface 22a1 of the positive electrode current collector 21a of the positive electrode 21 and the first surface 22a1 of the negative electrode current collector 22a of the negative electrode 22 and more than the positive electrode active material layer 21b and the negative electrode active material layer 22b. It is arranged on the outer peripheral side and is bonded to both the positive electrode current collector 21a and the negative electrode current collector 22a. The spacer 24 contains an insulating material and insulates between the positive electrode current collector 21a and the negative electrode current collector 22a to prevent a short circuit between the current collectors.

Examples of the material constituting the spacer 24 include various resin materials such as polyethylene (PE), polystyrene, ABS resin, modified polypropylene (modified PP), and acrylonitrile styrene (AS) resin.

The spacer 24 extends along the peripheral edges of the positive electrode current collector 21a and the negative electrode current collector 22a in a plan view, and is formed in a frame shape surrounding the positive electrode current collector 21a and the negative electrode current collector 22a. ing. The spacer 24 is arranged between the positive electrode uncoated portion 21c on the first surface 21a1 of the positive electrode current collector 21a and the negative electrode uncoated portion 22c on the first surface 22a1 of the negative electrode current collector 22a in the stacking direction. ing.

Inside the storage cell 20, a space S surrounded by a frame-shaped spacer 24, a positive electrode 21, and a negative electrode 22 is formed. The space S houses the separator 23 and the electrolyte. The peripheral portion of the separator 23 is in a state of being buried in the spacer 24.

Therefore, the spacer 24 also functions as a sealing portion for sealing the space S between the positive electrode 21 and the negative electrode 22, and can suppress the permeation of the electrolyte contained in the space S to the outside. Further, the spacer 24 can suppress the intrusion of moisture from the outside of the power storage device 10 into the space S. Further, the spacer 24 can suppress the gas generated from the positive electrode 21 or the negative electrode 22 from leaking to the outside of the power storage device 10 due to, for example, a charge / discharge reaction.

The electrolyte housed in the space S is a liquid electrolyte (electrolyte solution). 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 LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (FSO ₂ ) ₂ , and LiN (CF ₃ SO ₂ ) ₂ can be used. Further, as the non-aqueous solvent, known solvents such as cyclic carbonates, cyclic esters, chain carbonates, chain esters, ethers and the like can be used. In addition, you may use two or more kinds of these known solvent materials in combination.

The cell stack 30 has a structure in which a plurality of storage cells 20 are superposed so that the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a are in direct contact with each other. As a result, a plurality of storage cells 20 constituting the cell stack 30 are connected in series. The contact portion between the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a is to have high adhesion, for example, in surface contact from the viewpoint of suppressing an increase in contact resistance. It is preferable to provide a wider range.

Here, in the cell stack 30, the two storage cells 20 adjacent to each other in the stacking direction provide a pseudo bipolar electrode 25 in which the positive electrode current collector 21a and the negative electrode current collector 22a in contact with each other are regarded as one current collector. It is formed. The pseudo bipolar electrode 25 includes a current collector having a structure in which a positive electrode current collector 21a and a negative electrode current collector 22a are superposed, and a positive electrode active material layer 21b formed on one surface of the current collector. It includes a negative electrode active material layer 22b formed on the other side surface.

The spacer 24 of each storage cell 20 has an outer peripheral portion 24a extending outward from each edge portion of the positive electrode current collector 21a and the negative electrode current collector 22a. The outer peripheral portion 24a projects in a direction orthogonal to the stacking direction from each edge portion of the positive electrode current collector 21a and the negative electrode current collector 22a when viewed from the stacking direction. The storage cells 20 adjacent to each other in the stacking direction are integrated by joining the outer peripheral portions 24a of the respective spacers 24. Examples of the method for joining the adjacent spacers 24 to each other include known welding methods such as heat welding, ultrasonic welding, and infrared welding.

The power storage device 10 includes a pair of energizing bodies composed of a positive electrode energizing plate 40 and a negative electrode energizing plate 50 arranged so as to sandwich the cell stack 30 in the stacking direction of the cell stack 30. The positive electrode energizing plate 40 and the negative electrode energizing plate 50 are each made of a material having excellent conductivity.

The positive electrode energizing plate 40 is electrically connected to the second surface 21a2 of the positive electrode current collector 21a of the positive electrode 21 arranged on the outermost side at one end in the stacking direction. The negative electrode energizing plate 50 is electrically connected to the second surface 22a2 of the negative electrode current collector 22a of the negative electrode 22 arranged on the outermost side at the other end in the stacking direction. The negative electrode current-carrying plate 50 is in a state of being overlapped so as to be in contact with the second surface 22a2 of the negative electrode current collector 22a. Similar to the contact portion between the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a, the contact portion between the negative electrode current collector plate 50 and the second surface 22a2 of the negative electrode current collector 22a From the viewpoint of suppressing an increase in contact resistance, it is preferable to increase the adhesion.

The power storage device 10 is charged and discharged through terminals provided on each of the positive electrode energizing plate 40 and the negative electrode energizing plate 50. As the material constituting the positive electrode current-carrying plate 40, for example, the same material as the material constituting the positive electrode current collector 21a can be used. The positive electrode current-carrying plate 40 may be made of a metal plate thicker than the positive electrode current collector 21a used for the cell stack 30.

As the material constituting the negative electrode current-carrying plate 50, for example, the same material as the material constituting the negative electrode current collector 22a can be used. The negative electrode current-carrying plate 50 may be made of a metal plate thicker than the negative electrode current collector 22a used for the cell stack 30.

In the present embodiment, the positive electrode current collector 21a and the negative electrode current collecting plate 50 electrically connected to the negative electrode current collector 22a correspond to the conductive portions superimposed on the second surface 22a2 of the negative electrode current collector 22a. The positive electrode current collector 21a and the negative electrode current collector plate 50 are in contact with a portion of the second surface 22a2 of the negative electrode current collector 22a located on the opposite side of the portion where the negative electrode active material layer 22b on the first surface 22a1 side is provided. It is superposed on the negative electrode current collector 22a so as to have a portion.

Next, the details of the positive electrode current collector 21a and the positive electrode active material layer 21b constituting the positive electrode 21, and the negative electrode current collector 22a and the negative electrode active material layer 22b constituting the negative electrode 22 will be described.
<Positive current collector>
The positive electrode current collector 21a is a chemically inert electric conductor for continuing to flow a current through the positive electrode active material layer 21b during discharging or charging of the lithium ion secondary battery. In the present embodiment, the positive electrode current collector 21a is an aluminum foil. As the material constituting the positive electrode current collector 21a, for example, a metal material, a conductive resin material, a conductive inorganic material, or the like can be used.

Examples of the conductive resin material include a conductive polymer material and a resin obtained by adding a conductive filler to a non-conductive polymer material as needed. The positive electrode current collector 21a may include a plurality of layers including one or more layers including the above-mentioned metal material or conductive resin material. The surface of the positive electrode current collector 21a may be covered with a known protective layer. The surface of the positive electrode current collector 21a may be treated by a known method such as plating treatment.

The positive electrode current collector 21a may have a form such as a foil, a sheet, a film, a wire, a rod, a mesh, or a clad material. The positive electrode current collector 21a may be, for example, a metal foil such as a copper foil, a nickel foil, a titanium foil, or a stainless steel foil, in addition to the aluminum foil. From the viewpoint of ensuring mechanical strength, the current collector may be a stainless steel foil (for example, SUS304, SUS316, SUS301, SUS304, etc. specified in JIS G 4305: 2015).

The positive electrode current collector 21a may be an alloy foil of the above metal. The positive electrode current collector 21a may be a foil containing a base material coated with an aluminum film. In the case of the foil-shaped positive electrode current collector 21a, the thickness of the positive electrode current collector 21a is, for example, 1 to 100 μm.

<Positive electrode active material layer>
The positive electrode active material layer 21b contains a positive electrode active material that can occlude and release charge carriers such as lithium ions. As the positive electrode active material, 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 having a layered rock salt structure, a metal oxide having a spinel structure, and a polyanionic compound, may be adopted. Further, two or more kinds of positive electrode active materials may be used in combination. In the present embodiment, the positive electrode active material layer 21b contains olivine-type lithium iron phosphate (LiFePO ₄ ) as a polyanionic compound.

The positive electrode active material layer 21b is a conductive auxiliary agent, a binder, an electrolyte (polymer matrix, an ionic conductive polymer, an electrolytic solution, etc.) for enhancing electrical conductivity, and an electrolyte support for enhancing ionic conductivity, if necessary. It may further contain salts (lithium salts) and the like. The components contained in the positive electrode active material layer 21b, the compounding ratio of the components, and the thickness of the positive electrode active material layer 21b are not particularly limited, and conventionally known knowledge about a lithium ion secondary battery can be appropriately referred to. The thickness of the positive electrode active material layer 21b is, for example, 2 to 150 μm.

The conductive auxiliary agent is added to increase the conductivity of the positive electrode 21. The conductive auxiliary agent is, for example, acetylene black, carbon black, graphite or the like.
Examples of the binder include fluororesins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide-based resins such as polyimide and polyamideimide, resins containing an alkoxysilyl group, and poly (poly). Examples thereof include acrylic resins such as meta) acrylic acid, styrene-butadiene rubbers, carboxymethyl cellulose, arginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester cross-linking products, and starch-acrylic acid graft polymers. These binders can be used alone or in combination. As the solvent or dispersion medium, for example, water, N-methyl-2-pyrrolidone and the like are used.

<Negative electrode current collector>
The negative electrode current collector 22a is in the form of a foil having a thickness of 1 to 100 μm. In the present embodiment, the negative electrode current collector 22a is a copper foil. As the material constituting the negative electrode current collector 22a, for example, a metal material, a conductive resin material, a conductive inorganic material, or the like can be used.

Examples of the conductive resin material include a conductive polymer material and a resin obtained by adding a conductive filler to a non-conductive polymer material as needed. The negative electrode current collector 22a may include a plurality of layers including one or more layers including the above-mentioned metal material or conductive resin material. The surface of the negative electrode current collector 22a may be covered with a known protective layer. The surface of the negative electrode current collector 22a may be treated by a known method such as plating.

The negative electrode current collector 22a may be, for example, a metal foil such as an aluminum foil, a nickel foil, a titanium foil, or a stainless steel foil, in addition to the copper foil. From the viewpoint of ensuring mechanical strength, the negative electrode current collector 22a may be a stainless steel foil (for example, SUS304, SUS316, SUS301, SUS304, etc. specified in JIS G 4305: 2015). The negative electrode current collector 22a may be an alloy foil of the above metal. The negative electrode current collector 22a may be a foil containing a base material coated with a copper film.

<Negative electrode active material layer>
The negative electrode active material layer 22b contains, as essential components, a negative electrode active material capable of occluding and releasing charge carriers such as lithium ions, carboxymethyl cellulose (hereinafter referred to as CMC), and styrene-butadiene rubber (hereinafter referred to as SBR). It is described.) And.

Examples of the negative electrode active material include Li, carbon, a metal compound, an element that can be alloyed with lithium, or a compound thereof. Examples of carbon include natural graphite, artificial graphite, hard carbon (non-graphitizable carbon) or soft carbon (easy graphitizable carbon). Examples of artificial graphite include highly oriented graphite and mesocarbon microbeads. Examples of elements that can be alloyed with lithium include silicon and tin.

The content of the negative electrode active material in the negative electrode active material layer 22b is, for example, 94.0% by mass or more and 97.3% by mass or less.
The degree of etherification of CMC is preferably 0.85 or more and 1.1 or less, and more preferably 0.90 or more and 1.05 or less. In the case of the negative electrode active material layer 22b containing the negative electrode active material, CMC and SBR, the degree of etherification is first specified between the degree of etherification of CMC and the degree of bending τ of the negative electrode active material layer 22b. In the range below the value, the degree of bending τ decreases as the degree of etherification increases, and in the range exceeding the second specific value, the degree of bending τ increases as the degree of etherification increases.

Therefore, by setting the degree of etherification of CMC to 0.85 or more and 1.1 or less, the tortuosity τ of the negative electrode active material layer 22b can be reduced. By reducing the tortuosity τ of the negative electrode active material layer 22b, the ion resistance, which is the resistance when ions pass through the electrode pores, is reduced, and the battery characteristics of the power storage device 10 are improved.

The content of CMC in the negative electrode active material layer 22b is 0.5% by mass or more and 1.3% by mass or less. By setting the CMC content in the above range, the occurrence of wrinkles in the negative electrode current collector 22a can be suppressed.

The content of CMC is preferably 0.6% by mass or more, and more preferably 0.7% by mass or more. The CMC content is preferably 1.2% by mass or less, more preferably 1.1% by mass or less.

The content of SBR in the negative electrode active material layer 22b is 2.2% by mass or more and 5.0% by mass or less. By setting the SBR content to 2.2% by mass or more, the occurrence of wrinkles in the negative electrode current collector 22a can be suppressed, and the peel strength between the negative electrode current collector 22a and the negative electrode active material layer 22b can be reduced. Can be suppressed. By setting the SBR content to 5.0% by mass or less, the resistance of the negative electrode can be reduced.

When the peel strength between the negative electrode current collector 22a and the negative electrode active material layer 22b is further improved, the SBR content is preferably 4.4% by mass or more, preferably 5.5% by mass or more. It is more preferable to have. The SBR content is preferably 7.0% by mass or less, and more preferably 6.0% by mass or less.

The total content of CMC and SBR in the negative electrode active material layer 22b is preferably 6.0% by mass or less, and more preferably 5.5% by mass or less. By reducing the total of the CMC content and the SBR content, the tortuosity τ of the negative electrode active material layer 22b can be reduced. As a result, the ion resistance, which is the resistance when ions pass through the electrode pores, is reduced, and the battery characteristics of the power storage device 10 are improved.

The negative electrode active material layer 22b may contain other components other than the negative electrode active material, CMC, and SBR, if necessary. Other components include, for example, a conductive auxiliary agent for enhancing electrical conductivity, a binder other than CMC and SBR, an electrolyte (polymer matrix, an ionic conductive polymer, an electrolytic solution, etc.), and an electrolyte for enhancing ionic conductivity. Supporting salt (lithium salt) can be mentioned.

The conductive auxiliary agent is added to increase the conductivity of the negative electrode 22. The conductive auxiliary agent is, for example, acetylene black, carbon black, graphite or the like.
Examples of the binder include fluororesins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide-based resins such as polyimide and polyamideimide, resins containing an alkoxysilyl group, and poly (poly). Meta) Acrylic resins such as acrylic acid, alginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester cross-linking products, and starch-acrylic acid graft polymers can be exemplified. These binders can be used alone or in combination. As the solvent, for example, water, N-methyl-2-pyrrolidone and the like are used.

The thickness and basis weight of the negative electrode active material layer 22b are not particularly limited, and conventionally known knowledge about a lithium ion secondary battery can be appropriately referred to. However, from the viewpoint of increasing the energy density of the storage cell 20, it is preferable to increase the basis weight of the negative electrode active material layer 22b.

The thickness of the negative electrode active material layer 22b is, for example, preferably 150 μm or more, and more preferably 200 μm or more. The thickness of the negative electrode active material layer 22b is, for example, 500 μm or less. The basis weight of the negative electrode active material layer 22b is, for example, preferably 17 mg / cm ² or more, and more preferably 25 mg / cm ² or more. The basis weight of the negative electrode active material layer 22b is, for example, 60 mg / cm ² or less.

When the grain size of the negative electrode active material layer 22b is 17 mg / cm ² or more, the mass ratio of SBR to CMC (SBR / CMC) in the negative electrode active material layer 22b is preferably 1.69 or more and 3 or less. It is more preferably 0 or more and 2.6 or less. The content of CMC in the negative electrode active material layer 22b is 0.5% by mass or more and 1.3% by mass or less, and the content of SBR is 2.2% by mass or more and 5.0% by mass or less. Therefore, the lower limit of the value that the mass ratio (SBR / CMC) can take is 1.69 (= 2.2 / 1.3).

When the basis weight of the negative electrode active material layer 22b is large, various irregularities such as thickness unevenness, press unevenness, and resistance unevenness are likely to occur in the negative electrode active material layer 22b. Various irregularities generated in the negative electrode active material layer 22b cause a capacity deterioration of the negative electrode active material layer 22b. By setting the mass ratio (SBR / CMC) to 1.69 or more and 3 or less, it is possible to suppress the capacity deterioration of the negative electrode active material layer 22b.

Next, a method of manufacturing the power storage device 10 of the present embodiment will be described.
As shown in FIG. 2, the power storage device 10 is manufactured by going through an electrode forming step S1, a storage cell forming step S2, and a cell stack forming step S3 in this order.

<Electrode forming process>
The electrode forming step S1 includes a positive electrode forming step of forming the positive electrode 21 and a negative electrode forming step of forming the negative electrode 22.

The positive electrode forming step is not particularly limited, and a known method applied to the formation of the positive electrode 21 including the positive electrode current collector 21a and the positive electrode active material layer 21b can be used. For example, a positive electrode mixture that becomes a positive electrode active material layer 21b by solidifying is attached to the first surface 21a1 of the positive electrode current collector 21a so as to have a predetermined thickness, and then a solidification process according to the positive electrode mixture is applied. 21 can be formed by performing the above.

In the negative electrode forming step, a negative electrode mixture that becomes a negative electrode active material layer 22b by solidifying is attached to the first surface 22a1 of the negative electrode current collector 22a, and then a predetermined solidification treatment is performed to form the negative electrode 22. This is the process of forming.

The negative electrode mixture contains a negative electrode active material, CMC, SBR, and a solvent as essential components. Further, the negative electrode mixture may contain other components described in the above-mentioned negative electrode active material layer column, if necessary.

The negative electrode active material contained in the negative electrode mixture is the same as that described in the negative electrode active material layer column above. When the total mass of the solid components contained in the negative electrode mixture is 100 parts by mass, the content of the negative electrode active material is, for example, 94.0 parts by mass or more and 97.3 parts by mass or less.

The CMC contained in the negative electrode mixture is the same as that described in the negative electrode active material layer column above.
When the total mass of the solid components contained in the negative electrode mixture is 100 parts by mass, the content of CMC is 0.5 parts by mass or more and 1.3 parts by mass or less. The content of the CMC is preferably 0.6 parts by mass or more, and more preferably 0.7 parts by mass or more. The CMC content is preferably 1.2 parts by mass or less, and more preferably 1.1 parts by mass or less. By setting the CMC content to 0.5 parts by mass or more, it becomes easy to prepare a fluid negative electrode mixture.

The SBR contained in the negative electrode mixture is the same as that described in the negative electrode active material layer column above. When the total mass of the solid components contained in the negative electrode mixture is 100 parts by mass, the SBR content is 2.2 parts by mass or more and 5.0 parts by mass or less. The content of the SBR is preferably 4.0 parts by mass or less, and more preferably 3.0 parts by mass or less.

The solvent is, for example, water, N-methylpyrrolidone (NMP). Two or more kinds of solvents may be used in combination. The solvent is preferably a solvent containing water as a main component, for example, a solvent in which the mass ratio of water in the solvent is 50 to 100% by mass.

Here, when a solvent containing water as a main component is used, it is preferable to use a CMC having a degree of etherification of 1.1 or less from the viewpoint of improving the stability of the viscosity of the negative electrode mixture over time. When the degree of etherification of CMC decreases, the hydrophobicity of CMC increases and the solubility in water decreases. As a result, the hydrophobic part of CMC is easily adsorbed on the surface of the negative electrode active material such as carbon having high hydrophobicity in the negative electrode mixture, and the dispersion stability of the negative electrode active material in the solvent containing water as a main component is improved. It can be improved and the increase in the viscosity of the negative electrode mixture over time can be suppressed.

The solvent is added to the negative electrode mixture so that the solid ratio of the negative electrode mixture is, for example, 60 to 65% by mass. The viscosity of the negative electrode mixture is, for example, 5000 to 50,000 mPa · s at 25 ° C.

In the negative electrode forming step, the method of adhering the negative electrode mixture to the negative electrode current collector 22a is not particularly limited, and a known method applied to the formation of an electrode such as a roll coating method can be used. The thickness of the negative electrode mixture attached to the negative electrode current collector 22a is set so that the basis weight of the solid component contained in the negative electrode mixture is within the range of the basis weight of the negative electrode active material layer 22b described above. Further, the solidification treatment in the negative electrode forming step is a treatment for removing the solvent by drying the negative electrode mixture adhering to the negative electrode current collector 22a by heating or the like.

<Storage cell formation process>
In the storage cell forming step S2, first, the positive electrode 21 and the negative electrode 22 are arranged so that the positive electrode active material layer 21b and the negative electrode active material layer 22b face each other in the stacking direction with the separator 23 sandwiched between them, and the positive electrode 21 and the negative electrode are arranged. The spacer 24 is arranged between 22 and on the outer peripheral side of the positive electrode current collector 21a and the negative electrode current collector 22a.

After that, the positive electrode 21, the negative electrode 22, and the separator 23 and the spacer 24 are joined by welding to form an assembly in which each member is integrated. Examples of the welding method for the spacer 24 include known welding methods such as heat welding, ultrasonic welding, and infrared welding.

Next, the electrolyte is injected into the space S inside the assembly through the injection port provided in a part of the spacer 24, and then the injection port is sealed. As a result, the storage cell 20 is formed.
<Cell stack formation process>
In the cell stack forming step S3, first, a plurality of storage cells 20 are stacked so as to face the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a. At this time, the second surface 21a2 of the positive electrode current collector 21a of one storage cell 20 and the second surface 22a2 of the negative electrode current collector 22a of the other storage cell 20 are brought into contact with each other. After that, the plurality of storage cells 20 are integrated by joining the outer peripheral portions 24a of the spacers 24 in the storage cells 20 adjacent to each other in the stacking direction.

Next, the positive electrode current-carrying plate 40 is superposed and electrically connected to the second surface 21a2 of the positive electrode current collector 21a of the positive electrode 21 arranged at one end in the stacking direction. Similarly, the negative electrode current-carrying plate 50 is superposed and electrically connected to the second surface 22a2 of the negative electrode current collector 22a of the negative electrode 22 arranged at the other end in the stacking direction. At this time, the second surface 22a2 of the negative electrode current collector 22a and the negative electrode current collecting plate 50 are brought into contact with each other.

In the present embodiment, the positive electrode current collector 21a and the negative electrode current-carrying plate 50 correspond to the conductive portion in which the negative electrode current collector 22a is in surface contact with the second surface 22a2. Further, in the cell stack forming step S3, the step of stacking the plurality of storage cells 20 and the step of connecting the negative electrode energizing plate 50 are the overlapping steps of bringing the conductive portion into contact with the second surface 22a2 of the negative electrode current collector 22a of the negative electrode 22. Corresponds to.

Next, the operation of this embodiment will be described.
The power storage device 10 uses CMC and SBR as the binder component contained in the negative electrode active material layer 22b, and sets their contents within a specific range. As a result, the shrinkage of the negative electrode active material layer 22b due to the removal of the solvent from the CMC during the production of the negative electrode active material layer 22b can be suppressed, and the occurrence of wrinkles in the foil-shaped negative electrode current collector 22a can be suppressed. .. As a result, it is possible to suppress a decrease in adhesion at the contact portion between the second surface 21a2 of the positive electrode current collector 21a and the second surface 22a2 of the negative electrode current collector 22a, and an increase in contact resistance due to the decrease in adhesion.

Next, the effect of this embodiment will be described.
(1) The power storage device 10 includes a negative electrode 22 in which a negative electrode active material layer 22b is provided on a first surface 22a1 of a foil-shaped negative electrode current collector 22a having a thickness of 1 to 100 μm, and a first surface of the positive electrode current collector 21a. The positive electrode active material layer 21b is provided on 21a1, and the positive electrode 21 is arranged so that the positive electrode active material layer 21b faces the negative electrode active material layer 22b of the negative electrode 22, and the positive electrode active material layer 21b and the negative electrode active material layer 22b. The separator 23 arranged between the two is repeatedly laminated, and the negative electrode 22 and the positive electrode 21 are in contact with the second surface 22a2 of the negative electrode current collector 22a and the second surface 21a2 of the positive electrode current collector 21a. Has a laminated structure. The negative electrode active material layer 22b contains a negative electrode active material, CMC, and SBR, the content of CMC is 0.5% by mass or more and 1.3% by mass or less, and the content of SBR is 2. It is 2% by mass or more and 5.0% by mass or less.

According to the above configuration, the negative electrode active material layer 22b that does not easily wrinkle the foil-shaped negative electrode current collector can be obtained. As a result, it becomes easy to increase the basis weight of the negative electrode active material layer 22b with respect to the negative electrode current collector 22a to increase the energy density of the storage cell 20.

(2) The degree of etherification of CMC is preferably 0.85 or more and 1.1 or less.
In this case, the tortuosity τ of the negative electrode active material layer 22b can be reduced. As a result, the ion resistance, which is the resistance when ions pass through the electrode pores of the negative electrode active material layer 22b, is reduced, and the battery characteristics of the power storage device 10 are improved.

(3) The basis weight of the negative electrode active material layer 22b is preferably 17 mg / cm ² or more.
The larger the basis weight of the negative electrode active material layer 22b, the more likely it is that wrinkles will occur in the negative electrode current collector 22a. Therefore, when the basis weight of the negative electrode active material layer 22b is large, the effect of (1) above can be obtained more remarkably.

(4) The method for manufacturing the power storage device 10 includes a negative electrode forming step of forming a negative electrode 22 by adhering and drying a negative electrode mixture on the first surface 22a1 of the negative electrode current collector 22a, and a negative electrode current collector 22a of the negative electrode 22. It has a stacking step of bringing the conductive portion into contact with the second surface 22a2 of the above. The negative electrode mixture contains a negative electrode active material, CMC, SBR, and a solvent. When the total mass of the solid components contained in the negative electrode mixture is 100 parts by mass, the CMC content is 0.5 parts by mass or more and 1.3 parts by mass or less, and the SBR content is 2.2. It is 50 parts by mass or more and 5.0 parts by mass or less. According to the above configuration, the effect of the above (1) can be obtained.

(5) The solvent contained in the negative electrode mixture is a solvent containing water as a main component, and the degree of etherification of CMC contained in the negative electrode mixture is preferably 1.1 or less.
In this case, the dispersion stability of the negative electrode active material in the solvent is improved, and the stability of the viscosity of the negative electrode mixture over time is improved. As a result, when the process of adhering the negative electrode mixture to the negative electrode current collector 22a in the electrode forming step is continuously performed for a long period of time, uneven coating of the negative electrode mixture is less likely to occur, and the continuous process becomes easier. ..

(6) The basis weight of the negative electrode active material layer 22b is preferably 17 mg / cm ² or more, and the mass ratio of SBR to CMC (SBR / CMC) is preferably 1.69 or more and 3 or less.
According to the above configuration, it is possible to suppress the capacity deterioration of the negative electrode active material layer 22b due to the large basis weight of the negative electrode active material layer 22b. It should be noted that this effect is more remarkable as the basis weight of the negative electrode active material layer 22b increases.

In addition, this embodiment can be changed and carried out as follows. The present embodiment and the following modified examples can be implemented in combination with each other within a technically consistent range.
○ The electrolyte is not limited to the liquid electrolyte, and may be a solid electrolyte such as a polymer gel electrolyte containing an electrolyte held in the polymer matrix. Further, the separator 23 itself may be composed of an electrolyte such as a polymer electrolyte or an inorganic electrolyte.

○ The spacer 24 may be joined or fixed to only one of the positive electrode current collector 21a and the negative electrode current collector 22a.
○ The spacer 24 may have a function of maintaining a distance between the positive electrode 21 and the negative electrode 22 to prevent a short circuit between the current collectors, and seals the space S between the positive electrode 21 and the negative electrode 22. It may not function as a sealing portion. For example, when the electrolyte is a solid electrolyte, a spacer 24 that does not function as a sealing portion can be adopted. Further, the shape of the spacer 24 is not limited to the frame shape surrounding the positive electrode current collector 21a and the negative electrode current collector 22a, and can be appropriately changed according to the function required for the spacer 24.

○ In order to improve the thermal stability of the positive electrode 21, a heat resistant layer may be provided on one or both of the first surface 21a1 and the second surface 21a2 of the positive electrode current collector 21a. Examples of the heat-resistant layer include a layer containing inorganic particles and a binder, and may also contain an additive such as a thickener. The same applies to the negative electrode 22.

○ In order to improve the conductive contact between both members between the positive electrode current collecting plate 40 and the positive electrode current collector 21a, a conductive layer in close contact with the positive electrode current collector 21a may be arranged. Examples of the conductive layer include a layer containing carbon such as acetylene black or graphite, and a layer having a hardness lower than that of the positive electrode current collector 21a such as a plating layer containing Au or the like. Further, a similar conductive layer may be arranged between the negative electrode current-carrying plate 50 and the negative electrode current collector 22a. In this case, the conductive layer may be the conductive portion instead of the negative electrode current-carrying plate 50.

○ Regarding the method of manufacturing the power storage device, the spacer 24 may be joined to other members in the power storage cell forming step, and the outer peripheral portions 24a of the spacers 24 in the power storage cells 20 adjacent to each other in the stacking direction may be joined at the same time.

Next, the technical ideas that can be grasped from the above-described embodiment and modified examples are described below.
(A) A negative electrode having a negative electrode active material layer provided on the first surface of a foil-shaped negative electrode current collector having a thickness of 1 to 100 μm, and a second electrode on the opposite side of the first surface of the negative electrode current collector. A power storage device having two surfaces, a conductive portion superimposed on a portion located on the opposite side of the portion provided with the negative electrode active material layer, wherein the negative electrode active material layer is a negative electrode active material and a negative electrode active material. It contains carboxymethyl cellulose and styrene-butadiene rubber, and the content of the carboxymethyl cellulose is 0.5% by mass or more and 1.3% by mass or less, and the content of the styrene-butadiene rubber is 2.2% by mass. A power storage device characterized by being% or more and 5.0% by mass or less.

(B) A negative electrode having a negative electrode active material layer provided on the first surface of a foil-shaped negative electrode current collector having a thickness of 1 to 100 μm, and a second electrode on the opposite side of the first surface of the negative electrode current collector. A method for manufacturing a power storage device having two surfaces, a conductive portion superimposed on a portion located on the opposite side of a portion provided with the negative electrode active material layer, and the first one of the negative electrode current collector. It has a negative electrode forming step of forming the negative electrode by adhering and drying the negative electrode mixture on the surface, and a stacking step of bringing the conductive portion into contact with the second surface of the negative electrode current collector of the negative electrode. The negative electrode mixture contains a negative electrode active material, carboxymethyl cellulose, styrene-butadiene rubber, and a solvent, and when the total mass of the solid components contained in the negative electrode mixture is 100 parts by mass, the carboxymethyl cellulose is said. Is 0.5 parts by mass or more and 1.3 parts by mass or less, and the content of the styrene-butadiene rubber is 2.2 parts by mass or more and 5.0 parts by mass or less. How to manufacture the device.

Hereinafter, examples in which the above embodiment is further embodied will be described.
<Test 1>
The negative electrode active material, CMC, and SBR were mixed at the ratios shown in Table 1, and water was added to the mixture to prepare a negative electrode mixture having a solid content ratio of 65% by mass. The negative electrode mixture was applied in the form of a film on one surface of the negative electrode current collector using the doctor blade method. The applied negative electrode mixture is heat-treated at a dew point of -40 ° C for 6 hours at 100 ° C to remove water in the negative electrode mixture, whereby a negative electrode having a grain size of 17 mg / cm ² is placed on the negative electrode current collector. The negative electrode sheets of Test Example 1 and Test Example 2 in which the active material layer was formed were prepared.

The negative electrode active material, CMC, SBR, and negative electrode current collector used in Test 1 are as follows.
Negative electrode active material: Graphite with an average particle size (D50) of 3.3 μm CMC: MAC350HC manufactured by Nippon Paper Industries, Ltd. (etherification degree 0.83)
SBR: TRD2001 manufactured by JSR Corporation
Negative electrode current collector: Copper foil with a thickness of 15 μm Both sides of the negative electrode sheets of Test Example 1 and Test Example 2 were visually observed to confirm the presence or absence of wrinkles in the negative electrode current collector. The results are shown in Table 1. FIG. 3 (a) is a photograph showing the second surface of the negative electrode current collector of the negative electrode of Test Example 1, and FIG. 3 (b) shows the second surface of the negative electrode current collector of the negative electrode of Test Example 2. It is a photograph.

表１に示すように、ＣＭＣの含有量が１．３質量％を超える試験例１では、負極集電体に皺が確認された。一方、ＣＭＣの含有量が１．３質量％以下である試験例２では、負極集電体に皺は確認されなかった。

As shown in Table 1, in Test Example 1 in which the CMC content exceeded 1.3% by mass, wrinkles were confirmed in the negative electrode current collector. On the other hand, in Test Example 2 in which the CMC content was 1.3% by mass or less, no wrinkles were confirmed in the negative electrode current collector.

<Test 2>
Negative electrode sheets of Test Examples 3 to 7 were prepared by the same method as in Test 1 except that the ratios of the negative electrode active material, CMC, and SBR were changed to the ratios shown in Table 2.

Both sides of the negative electrode sheets of Test Examples 3 to 7 were visually observed to confirm the presence or absence of wrinkles in the negative electrode current collector. The results are shown in Table 2.
Using a peeling test device (LTS-50N-S300 manufactured by MINEBEA) as a measurement sample obtained by cutting the negative electrode sheets of Test Examples 3 to 7 into 2.5 cm × 4 cm, 90 conforming to JIS K 6854-1. A peeling test was performed. The peel strength between the negative electrode active material layer and the negative electrode current collector in the measurement sample was calculated by dividing the strength measured in the 90 degree peeling test by the line width (2.5 cm). The results are shown in Table 2.

表２に示すように、ＣＭＣの含有量が０．５質量％未満である試験例５及び試験例７、並びにＳＢＲの含有量が２．２質量％未満である試験例６及び試験例７では、負極集電体に皺が確認された。また、表２の結果から、負極活物質層と負極集電体との間の剥離強度は、ＣＭＣの含有量に対する依存度は低く、ＳＢＲの含有量に対する依存度が大きい傾向が確認できる。そして、ＳＢＲの含有量を２．２質量％以上とした場合には、負極活物質層と負極集電体との間の剥離強度の低下が抑制されている。

As shown in Table 2, in Test Examples 5 and 7 in which the CMC content is less than 0.5% by mass, and in Test Examples 6 and 7 in which the SBR content is less than 2.2% by mass. , Wrinkles were confirmed on the negative electrode current collector. Further, from the results in Table 2, it can be confirmed that the peel strength between the negative electrode active material layer and the negative electrode current collector tends to have a low dependence on the CMC content and a large dependence on the SBR content. When the SBR content is 2.2% by mass or more, the decrease in peel strength between the negative electrode active material layer and the negative electrode current collector is suppressed.

<Test 3>
Negative electrode sheets of Test Examples 8 to 12 were prepared by the same method as in Test 1 except that the ratios of the negative electrode active material, CMC, and SBR were changed to the ratios shown in Table 3.

A symmetric model cell for measuring tortuosity was prepared using a negative electrode, a separator, and an electrolytic solution obtained by cutting the prepared negative electrode sheet into a predetermined shape. As the separator, a separator made of polyethylene was used. The electrolytic solution is a mixed solvent in which dimethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, and ethylene carbonate are mixed at a volume ratio of 40:35:5:20, and lithium hexafluorophosphate is added to a concentration of 1.2 M. An electrolytic solution dissolved in is used.

The tortuosity τ of the negative electrode active material layer of the negative electrode sheet of each test example was calculated based on the following formula (1). The results are shown in Table 3.
_Tortuosity τ = (Rion ・ A ・ K ・ ε) / 2d… (1)
Rion: _Ion resistance A: Electrode area (8.06 cm ² )
K: Ion conductivity of the electrolytic solution ε: Void ratio of the negative electrode active material layer d: Thickness of the negative electrode active material layer (0.1 mm)
The ion resistance _Rion was derived from the extremely low frequency real number component (= _ion resistance Rion / 3) of the measured symmetric cell impedance by measuring the symmetric cell impedance of the symmetric model cell.

The ion conductivity K of the electrolytic solution was calculated from the measured resistance at 25 ° C. and 10 kHz of the sample in which the electrolytic solution having the above composition was sealed in a cell provided with a platinum electrode.
The porosity ε of the negative electrode active material layer was measured by using the mercury intrusion method.

表３に示すように、ＣＭＣとＳＢＲの含有量の合計が増加するにしたがって、屈曲度τが増加する傾向が確認できる。

As shown in Table 3, it can be confirmed that the tortuosity τ tends to increase as the total content of CMC and SBR increases.

<Test 4>
Negative electrode sheets of Test Examples 13 to 16 were prepared by the same method as in Test 1 except that the ratios of the negative electrode active material, CMC, and SBR, and the type of CMC were changed. The ratios of the negative electrode active material, CMC, and SBR are as shown in Table 4. In Test 4, a CMC having an etherification degree of 0.83, 0.90, 0.93, or 1.36 was used as the CMC.

The tortuosity τ of the negative electrode active material layer of the negative electrode sheet of each test example was calculated by the same method as in Test 3. The results are shown in Table 4.

表４に示すように、ＣＭＣのエーテル化度が０．８５以上１．１以下である試験例１４及び試験例１５は、ＣＭＣのエーテル化度が上記範囲外である試験例１３及び試験例１６と比較して屈曲度τが低くなった。

As shown in Table 4, in Test Example 14 and Test Example 15 in which the degree of etherification of CMC is 0.85 or more and 1.1 or less, Test Example 13 and Test Example 16 in which the degree of etherification of CMC is out of the above range. The bending degree τ was lower than that of the above.

<Test 5>
Negative electrode sheets of Test Examples 17 to 18 were prepared by the same method as in Test 1 except that the ratios of the negative electrode active material, CMC, and SBR were changed to the ratios shown in Table 5. The tortuosity τ of the negative electrode active material layer of the negative electrode sheet of each test example was calculated by the same method as in Test 3. The results are shown in Table 5.

Further, the prepared negative electrode sheet was cut into a rectangular shape having a length of 3.1 mm and a width of 2.6 mm, and a negative electrode, a positive electrode, and a separator were combined to form an electrode body battery. A lithium ion secondary battery was obtained by accommodating the electrode body battery in the battery case and injecting an electrolytic solution to seal the battery case.

As the positive electrode, a positive electrode having a positive electrode current collector made of aluminum and a positive electrode active material layer made of LiFePO ₄ , polyvinylidene fluoride, and acetylene black was used. As the separator, a separator made of polyethylene was used. The same electrolytic solution as in Test 3 was used.

The obtained lithium ion secondary battery is charged with a constant current (CC) at a direct current of 1.9 mA until the voltage to the positive electrode at the negative electrode reaches 4.0 V, and then is charged with a constant voltage (CV) to 4.0 V. I went for 2 hours. Thirty minutes after charging was completed, discharge was performed until the voltage with respect to the positive electrode at the negative electrode became 2.5 V. The current value of 1C was set to 38.5 mA, and the 1C discharge characteristic was calculated based on the following formula. The results are shown in Table 5.

1C discharge characteristic (%) = (1C CC discharge capacity / 0.33C CC + CV (22.5V, 2 hours) capacity) × 100

表５に示すように、ＣＭＣとＳＢＲの含有量の合計を低くすること、及びＣＭＣのエーテル化度を上記の特定範囲とすることにより、屈曲度τが大きく低下する。そして、屈曲度τを低下させた試験例１８は、屈曲度τが高い試験例１７と比較して、１Ｃ放電特性が大きく向上した。

As shown in Table 5, by lowering the total content of CMC and SBR and setting the degree of etherification of CMC within the above-mentioned specific range, the tortuosity τ is greatly reduced. Then, in Test Example 18 in which the bending degree τ was reduced, the 1C discharge characteristic was greatly improved as compared with Test Example 17 in which the bending degree τ was high.

<Test 6>
The negative electrode active material, CMC, and SBR were mixed at the ratios shown in Table 6, and water was added to this mixture to prepare a negative electrode mixture of Test Examples 19 to 21 having a solid content ratio of 58% by mass. The negative electrode active material, CMC, and SBR used in Test 6 are the same as in Test 1. The prepared negative electrode mixture was stored in an environment of 25 ° C. and 60% humidity while stirring in a stirring container, and the presence or absence of fluidity after 3 days was visually confirmed. The results are shown in Table 6.

表６に示すように、ＣＭＣの含有量が０．５質量％以上である試験例１９及び試験例２０では、所定時間経過後も流動性が維持された。

As shown in Table 6, in Test Example 19 and Test Example 20 in which the CMC content was 0.5% by mass or more, the fluidity was maintained even after the lapse of a predetermined time.

<Test 7>
The negative electrode mixture of Test Examples 22 to 27 was prepared by the same method as in Test 6 except that the ratios of the negative electrode active material, CMC, and SBR, and the types of CMC and SBR were changed. In Test 7, a CMC having an etherification degree of either 0.90 or 1.36 was used as the CMC. As the SBR, any one of AL1002 manufactured by Nippon A & L Inc., AL2001 manufactured by Nippon A & L Inc., and TRD2001 manufactured by JSR Corporation was used.

The viscosity of the prepared negative electrode mixture immediately after adjustment and the viscosity of the negative mixture were stored in an environment of 25 ° C. while stirring in a stirring container, and the viscosity after 7 days was measured, and the viscosity increase rate after 7 days was measured. Calculated. The results are shown in Table 7.

表７に示す結果から、負極合材の粘度の経時的な上昇率に関して、ＳＢＲの種類に対する依存度は低く、ＣＭＣのエーテル化度に対する依存度が大きい傾向が確認できる。そして、ＣＭＣのエーテル化度が１．１以下である試験例２２，２４，２６では、時間経過による負極合材の粘度の上昇が抑制されている。

From the results shown in Table 7, it can be confirmed that the dependence on the type of SBR is low and the dependence on the degree of etherification of CMC is large with respect to the rate of increase in the viscosity of the negative electrode mixture over time. In Test Examples 22, 24, and 26 in which the degree of etherification of CMC is 1.1 or less, the increase in the viscosity of the negative electrode mixture with the passage of time is suppressed.

<Test 8>
The negative electrode active material, CMC, and SBR were mixed at the ratios shown in Table 8, and water was added to this mixture to prepare a negative electrode mixture having a solid content ratio of 63.7% by mass.
The negative electrode mixture was applied in the form of a film on one surface of the negative electrode current collector using the doctor blade method. The applied negative electrode mixture is heat-treated at a dew point of -40 ° C for 6 hours at 100 ° C to remove water in the negative electrode mixture, whereby a negative electrode having a grain size of 30 mg / cm ² is placed on the negative electrode current collector. The negative electrode sheets of Test Examples 28 to 31 on which the active material layer was formed were prepared.

The negative electrode active material, CMC, and negative electrode current collector used in Test 8 are as follows.
Negative electrode active material: Graphite with an average particle size (D50) of 15.3 μm CMC: CMC with an etherification degree of 0.89
Negative electrode current collector: Copper foil with a thickness of 15 μm A negative electrode formed by cutting a prepared negative electrode sheet into a rectangular shape of 3.1 mm in length × 2.6 mm in width, a positive electrode, and an electrode body battery by combining a separator. did. A lithium ion secondary battery was obtained by accommodating the electrode body battery in the battery case and injecting an electrolytic solution to seal the battery case.

As the positive electrode, a positive electrode having a positive electrode current collector made of aluminum and a positive electrode active material layer made of LiFePO ₄ , polyvinylidene fluoride, and acetylene black was used. As the separator, a separator made of polyethylene was used. As the electrolytic solution, LiPF ₆ was dissolved in a mixed solvent in which ethylene carbonate and methyl propionate were mixed at a volume ratio of 15:85 at a concentration of 1.2 mol / L to prepare a mother liquor, which was equivalent to 1% by mass with respect to the mother liquor. The amount of lithium difluoro (oxalate) borate (LiDFOB) and the amount of vinylene carbonate corresponding to 1% by mass were added and dissolved.

The obtained lithium ion secondary battery is charged with a DC current of 7.58 mA until the voltage to the positive electrode of the negative electrode reaches 3.75 V, and 10 minutes after the charging is completed, the DC current is 25.3 mA. The current was discharged until the voltage of the negative electrode with respect to the positive electrode became 3.0 V.

Five cycles of charging and discharging were performed with the above discharging and charging as one cycle, and the capacity retention rate was calculated based on the following formula. The above test for calculating the capacity retention rate was carried out twice, and the average value of the capacity retention rate was calculated. The results are shown in Table 8. Further, FIG. 4 shows a graph showing the relationship between the mass ratio of SBR to CMC (SBR / CMC) in the negative electrode active material layer of the negative electrode sheet and the capacity retention rate of the lithium ion secondary battery.

Capacity retention rate (%) = (Discharge capacity after 5 cycles / Discharge capacity in the first cycle) x 100

表８及び図４のグラフに示すように、容量維持率は、上記質量比に応じて変化する。具体的には、容量維持率は、上側に凸となる放物線状に変化するとともに、上記質量比が２．３付近で最大値を取る。したがって、上記質量比を１．６９以上３以下の範囲とすることにより容量維持率を高めることができるとともに、負極活物質層の目付量が大きいことに起因する負極活物質層の容量劣化の抑制できる。

As shown in the graphs of Table 8 and FIG. 4, the capacity retention rate changes according to the mass ratio. Specifically, the capacity retention rate changes in a parabolic shape that is convex upward, and takes a maximum value when the mass ratio is around 2.3. Therefore, the capacity retention rate can be increased by setting the mass ratio to the range of 1.69 or more and 3 or less, and the capacity deterioration of the negative electrode active material layer due to the large basis weight of the negative electrode active material layer is suppressed. can.

10 ... power storage device, 20 ... power storage cell, 21 ... positive electrode, 21a ... positive electrode current collector, 21b ... positive electrode active material layer, 22 ... negative electrode, 22a ... negative electrode current collector, 22b ... negative electrode active material layer, 23 ... separator, 24 ... spacer, 30 ... cell stack, 40 ... positive electrode energizing plate, 50 ... negative electrode energizing plate.

Claims (4)

A negative electrode having a negative electrode active material layer provided on the first surface of a foil-shaped negative electrode current collector having a thickness of 1 to 100 μm and a positive electrode active material layer provided on the first surface of the positive electrode current collector are provided. A positive electrode in which the active material layer is arranged so as to face the negative electrode active material layer of the negative electrode and a separator arranged between the positive electrode active material layer and the negative electrode active material layer are repeatedly laminated and laminated. The negative electrode and the positive electrode are laminated so that the second surface on the opposite side of the first surface of the negative electrode current collector and the second surface on the opposite side of the first surface of the positive electrode current collector are in contact with each other. It is a power storage device with a structure
The negative electrode active material layer contains a negative electrode active material, carboxymethyl cellulose, and styrene-butadiene rubber.
The content of the carboxymethyl cellulose is 0.5% by mass or more and 1.3% by mass or less.
A power storage device characterized in that the content of the styrene-butadiene rubber is 2.2% by mass or more and 5.0% by mass or less. The power storage device according to claim 1, wherein the degree of etherification of the carboxymethyl cellulose is 0.85 or more and 1.1 or less. The power storage device according to claim 1 or 2, wherein the negative electrode active material layer has a basis weight of 17 mg / cm ² or more. The power storage device according to claim 3, wherein the mass ratio of the styrene-butadiene rubber to the carboxymethyl cellulose is 1.69 or more and 3 or less.

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