WO2025142877A1 - Electrode and battery - Google Patents

Abstract

An electrode mixture layer (12) of an electrode (10) according to the present disclosure contains an electrode active material, a rubber-based binder, and a water-soluble binder. The electrode mixture layer (12) satisfies relational expressions (1), (2), and (3) when: division layers obtained by dividing the electrode mixture layer (12) into four layers in the thickness direction are a first division layer (12a), a second division layer (12b), a third division layer (12c), and a fourth division layer (12d) in this order from the front surface side of the electrode mixture layer (12) toward the electrode current collector (11) side; the mass ratios (%) of rubber-based binders contained in the first division layer, the second division layer, the third division layer, and the fourth division layer to the total mass of the rubber-based binders contained in the electrode mixture layer (12) are A1, B1, C1, and D1, respectively; and the mass ratios (%) of the water-soluble binders contained in the first division layer, the second division layer, the third division layer, and the fourth division layer to the total mass of the water-soluble binders contained in the electrode mixture layer (12) are A2, B2, C2, and D2, respectively.　(1) A1+D1<B1+C1, (2) A2+D2<B2+C2, (3) A1<A2

Description

Electrodes and batteries

This disclosure relates to electrodes and batteries.

In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications that require high capacity, such as in-vehicle applications and power storage applications. The electrodes that make up such batteries have a significant impact on the performance of the battery. For this reason, various studies have been conducted on electrodes.

An electrode is usually composed of a current collector and an electrode mixture layer containing an electrode active material and a binder. For example, Patent Document 1 discloses a technique for improving the adhesion between layers in an electrode having a multi-layer electrode mixture layer while suppressing the amount of binder. Specifically, Patent Document 1 discloses a technique for making more binder present in specific regions in a laminate for a secondary battery having a first active material layer and a second active material layer that function as the electrode mixture layer, and a current collector, in order to improve the adhesion between the active material layers and between the current collector and the active material layers.

JP 2020-161299 A

The technology disclosed in Patent Document 1 makes it possible to improve the adhesion between layers and avoid the problem of delamination when the electrode mixture layer is composed of multiple layers. However, electrodes are required not only to avoid such delamination problems, but also to improve the characteristics of the battery.

The present disclosure provides an electrode that can reduce the internal resistance of a battery.

The electrode of the present disclosure comprises:
An electrode current collector;
an electrode mixture layer disposed on the electrode current collector;
Equipped with
The electrode mixture layer contains an electrode active material and a binder,
The binder contains a rubber-based binder and a water-soluble binder,
The four divided layers resulting from dividing the electrode mixture layer into four equal parts in the thickness direction are designated, in order from the surface side of the electrode mixture layer toward the electrode current collector side, as a first divided layer, a second divided layer, a third divided layer, and a fourth divided layer, and the mass ratio (%) of the rubber-based binder contained in the first divided layer to the total mass of the rubber-based binder contained in the electrode mixture layer is designated as A1, the mass ratio (%) of the rubber-based binder contained in the second divided layer is designated as B1, the mass ratio (%) of the rubber-based binder contained in the third divided layer is designated as C1, and the mass ratio (%) of the rubber-based binder contained in the fourth divided layer is designated as B2, B3, C4, and C5. When the mass ratio (%) of the rubber-based binder contained is D1, the mass ratio (%) of the water-soluble binder contained in the first division layer relative to the total mass of the water-soluble binder contained in the electrode mixture layer is A2, the mass ratio (%) of the water-soluble binder contained in the second division layer is B2, the mass ratio (%) of the water-soluble binder contained in the third division layer is C2, and the mass ratio (%) of the water-soluble binder contained in the fourth division layer is D2, the electrode mixture layer satisfies the following relational expressions (1), (2), and (3).
A1+D1<B1+C1...(1)
A2+D2<B2+C2...(2)
A1<A2...(3)

The technology disclosed herein can provide an electrode that can reduce the internal resistance of a battery.

FIG. 1 is a cross-sectional view showing a schematic configuration of an electrode according to a first embodiment. FIG. 2 is a vertical cross-sectional view illustrating an example of a battery according to the second embodiment.

[Foundation of the present disclosure]
The present inventors have conducted extensive research and found that in order to improve the characteristics of a battery, specifically to reduce the internal resistance, it is necessary to appropriately control the distribution of the binder in the electrode, taking into account the type of binder, which has led the present inventors to conceive of the electrode of the present disclosure described below.

Below, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments.

[Embodiments of the present disclosure]
(Embodiment 1)
1 is a cross-sectional view showing a schematic configuration of an electrode according to embodiment 1. An electrode 10 according to embodiment 1 includes an electrode current collector 11 and an electrode mixture layer 12 disposed on the electrode current collector 11. The electrode mixture layer 12 contains an electrode active material and a binder. The binder contains a rubber-based binder and a water-soluble binder. That is, the electrode mixture layer 12 contains both a rubber-based binder and a water-soluble binder.

Here, the four divided layers resulting from dividing the electrode mixture layer 12 into four equal parts in the thickness direction are designated as a first divided layer 12a, a second divided layer 12b, a third divided layer 12c, and a fourth divided layer 12d, in that order from the surface 121 side of the electrode mixture layer 12 toward the electrode current collector 11 side. Furthermore, the mass ratio (%) of the rubber-based binder contained in the first divided layer 12a to the total mass of the rubber-based binder contained in the electrode mixture layer 12 is designated as A1, the mass ratio (%) of the rubber-based binder contained in the second divided layer 12b is designated as B1, the mass ratio (%) of the rubber-based binder contained in the third divided layer 12c is designated as C1, and the mass ratio (%) of the rubber-based binder contained in the fourth divided layer 12d is designated as D1. Furthermore, the mass ratio (%) of the water-soluble binder contained in the first division layer 12a to the total mass of the water-soluble binder contained in the electrode mixture layer 12 is A2, the mass ratio (%) of the water-soluble binder contained in the second division layer 12b is B2, the mass ratio (%) of the water-soluble binder contained in the third division layer 12c is C2, and the mass ratio (%) of the water-soluble binder contained in the fourth division layer 12d is D2. In this case, the electrode mixture layer 12 satisfies the following relational expressions (1), (2), and (3).
A1+D1<B1+C1...(1)
A2+D2<B2+C2...(2)
A1<A2...(3)

In this disclosure, the surface 121 of the electrode mixture layer 12 refers to the surface that comes into contact with the electrolyte when the electrode 10 is configured into a battery, and is located on the opposite side to the surface that faces the electrode current collector 11.

The first divided layer 12a, the second divided layer 12b, the third divided layer 12c, and the fourth divided layer 12d are virtual layers obtained by assuming that the electrode mixture layer 12 is divided into four equal parts in the thickness direction. Therefore, the electrode mixture layer 12 is not limited to a configuration consisting of multiple layers. The electrode mixture layer 12 may be formed of a single layer, or may be formed of multiple layers. Furthermore, when the electrode mixture layer 12 is formed of multiple layers, the number of layers is not particularly limited. The electrode mixture layer 12 may have a two-layer structure, a three-layer structure, a four-layer structure, or a five-layer structure or more.

The distribution of the rubber-based binder and the water-soluble binder in the electrode mixture layer 12 is controlled so that the electrode mixture layer 12 satisfies the above-mentioned relational expressions (1), (2), and (3), so that the electrode 10 according to the first embodiment can reduce the internal resistance of the battery, specifically the direct current internal resistance (DCR).

When the above relational expressions (1) and (2) are satisfied, the rubber-based binder and the water-soluble binder are both present in a larger mass ratio in the middle region in the thickness direction of the electrode mixture layer 12. That is, in the region including the surface 121 of the electrode mixture layer 12 and the region close to the electrode current collector 11, the rubber-based binder and the water-soluble binder are present in a smaller mass ratio than in the middle region. By distributing both the rubber-based binder and the water-soluble binder in this manner, it is considered that the resistance at the contact interface between the electrode mixture layer 12 and the electrode current collector 11, and at the contact interface between the electrode 10 and the electrolyte when the electrode 10 constitutes a battery, for example, can be effectively reduced. Furthermore, when the above relational expression (3) is satisfied, in the region including the surface 121 of the electrode mixture layer 12, the mass ratio A2 of the water-soluble binder, which is generally low resistance, is greater than the mass ratio A1 of the rubber-based binder, which is generally high resistance. It is believed that by controlling the distribution of the rubber-based binder and the water-soluble binder in this manner in the region including the surface 121 of the electrode mixture layer 12, it is possible to effectively reduce the resistance at the contact interface between the electrode 10 and the electrolyte when the electrode 10 constitutes a battery. For these reasons, it is believed that an electrode 10 having an electrode mixture layer 12 that satisfies the above relational expressions (1), (2), and (3) can reduce the internal resistance of the battery.

An electrode 10 having an electrode mixture layer 12 that satisfies the above relational expression (3) can also improve the cycle characteristics of the battery.

In addition, the electrode 10 according to embodiment 1, in which the electrode mixture layer 12 satisfies both of the above relational expressions (1) and (2), can achieve excellent adhesion in the electrode mixture layer 12, as well as excellent adhesion between the electrode mixture layer 12 and the current collector 11, similar to the electrode disclosed in Patent Document 1.

Here, in this specification, a rubber-based binder means a binder containing a rubber-based polymer compound. The rubber-based polymer compound is not particularly limited as long as it is a polymer compound having rubber elasticity. Examples of rubber-based polymer compounds include styrene butadiene rubber, high styrene rubber, ethylene propylene rubber, butyl rubber, chloroprene rubber, butadiene rubber, isoprene rubber, acrylonitrile butadiene rubber, acrylonitrile rubber, fluororubber, acrylic rubber, and silicone rubber. The rubber-based binder may be at least one selected from the group consisting of, for example, styrene butadiene rubber, high styrene rubber, ethylene propylene rubber, butyl rubber, chloroprene rubber, butadiene rubber, isoprene rubber, acrylonitrile butadiene rubber, acrylonitrile rubber, fluororubber, acrylic rubber, and silicone rubber.

In addition, in this specification, the water-soluble binder means a binder containing a water-soluble polymer compound. The water-soluble polymer compound is not particularly limited, but examples thereof include synthetic polymer-based water-soluble polymer compounds (hereinafter referred to as "synthetic polymer compounds") and polysaccharide-based water-soluble polymer compounds (hereinafter referred to as "polysaccharide polymer compounds"). The water-soluble binder may contain one type selected from synthetic polymer compounds and polysaccharide polymer compounds alone, or two or more types mixed together. Examples of synthetic polymer compounds include polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, polymethacrylic acid, polyacrylic acid, and derivatives thereof. Examples of polysaccharide polymer compounds include cellulose or a salt thereof, and carboxymethylcellulose or a salt thereof (CMC-Na, etc.). The water-soluble binder may contain at least one selected from the group consisting of, for example, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, derivatives of polyvinyl alcohol, derivatives of polyvinylpyrrolidone, derivatives of polyethylene oxide, polymethacrylic acid (MAA), salts of polymethacrylic acid (MAA-Na, MAA-K, etc., which may also be partially neutralized salts), polyacrylic acid (PAA), salts of polyacrylic acid (PAA-Na, PAA-K, etc., which may also be partially neutralized salts), cellulose, salts of cellulose, carboxymethylcellulose, and salts of carboxymethylcellulose.

The mass ratio of the rubber-based binder in the first divided layer 12a, the second divided layer 12b, the third divided layer 12c, and the fourth divided layer 12d can be determined by elemental mapping of the cross section along the thickness direction of the electrode mixture layer 12. Quantitative analysis of the rubber-based binder by elemental mapping can be performed by an electron probe microanalyzer (EPMA). When the rubber-based binder to be measured contained in the electrode mixture layer 12 is, for example, a rubber-based polymer compound having a C=C bond, bromine, for example, is added to the C=C bond, and the bromine is analyzed by elemental mapping using EPMA on the cross section of the electrode mixture layer 12. This allows the content of the rubber-based binder in each of the divided layers to be determined. Bromine selectively undergoes an addition reaction with the C=C bond, so it is suitable as an element for staining the rubber-based polymer compound having a C=C bond in the electrode mixture layer 12. The method of adding bromine to the rubber-based polymer compound is not particularly limited, and may involve a gas-phase reaction in a bromine atmosphere, or may involve immersing the electrode 10 in a bromine solution. When the rubber-based binder to be measured contained in the electrode mixture layer 12 is a rubber-based polymer compound that does not have a C=C bond, the content of the rubber-based binder may be measured by elemental mapping using an EPMA using a cross section of the electrode mixture layer 12 along the thickness direction that has been stained with osmium tetroxide.

The EPMA conditions are set, for example, as follows:
Electron beam diameter: Approximately 1 μm Acceleration voltage: Approximately 15 kV Beam current: Approximately 50 nA Number of pixels: Approximately 255 x 255 pixels Measurement time: Approximately 30 msec/pixel

The mass ratio of the water-soluble binder in the first divided layer 12a, the second divided layer 12b, the third divided layer 12c, and the fourth divided layer 12d can be determined by elemental mapping of a cross section along the thickness direction of the electrode mixture layer 12. Quantitative analysis of the water-soluble binder by elemental mapping can be performed by EPMA. If the water-soluble binder to be measured contained in the electrode mixture layer 12 is, for example, a water-soluble polymer compound having a carboxyl group, the carboxyl group can be modified with, for example, ruthenium, and the ruthenium can be analyzed by elemental mapping by EPMA to determine the content of the water-soluble binder in each of the divided layers. Even if the water-soluble binder to be measured contained in the electrode mixture layer 12 is a water-soluble polymer compound without a carboxyl group, the content of the water-soluble binder can be measured by modifying the water-soluble polymer compound with a metal element that can modify the specific functional group constituting the water-soluble polymer compound, and analyzing the element used for the modification by elemental mapping.

The electrode mixture layer 12 may satisfy the following relational expressions (4) and (5).
A1<B1...(4)
A2<B2...(5)

The electrode mixture layer 12 satisfies the above relational expressions (4) and (5), i.e., the mass ratio A1 of the rubber-based binder in the first divided layer 12a is smaller than the mass ratio B1 of the rubber-based binder in the second divided layer 12b, and the mass ratio A2 of the water-soluble binder in the first divided layer 12a is smaller than the mass ratio B2 of the water-soluble binder in the second divided layer 12b, so that the electrode 10 according to embodiment 1 can further reduce the internal resistance of the battery.

The electrode mixture layer 12 may satisfy the following relational formula (6).
B2+C2<B1+C1...(6)

Since the electrode mixture layer 12 satisfies the above relational expression (6), that is, since the mass ratio of the rubber-based binder (i.e., B1+C1) is greater than the mass ratio of the water-soluble binder (i.e., B2+C2) in the intermediate region in the thickness direction of the electrode mixture layer 12 composed of the second divided layer 12b and the third divided layer 12c, the electrode 10 according to embodiment 1 can further reduce the internal resistance of the battery.

The electrode mixture layer 12 may satisfy the following relational expression (7).
57%<B1+C1<65%...(7)

By the electrode mixture layer 12 satisfying the above relational expression (7), i.e., by the mass ratio of the rubber binder (i.e., B1+C1) being greater than 57% and less than 65% in the intermediate region in the thickness direction of the electrode mixture layer 12 constituted by the second divided layer 12b and the third divided layer 12c, the electrode 10 according to embodiment 1 can further reduce the internal resistance of the battery. B1+C1 may be 58% or more, or 59% or more. Also, B1+C1 may be 64% or less, or 63% or less.

The electrode mixture layer 12 may satisfy the following relational formula (8).
55%<B2+C2<60%...(8)

By the electrode mixture layer 12 satisfying the above relational expression (8), i.e., by the mass ratio of the water-soluble binder (i.e., B2+C2) being greater than 55% and less than 60% in the intermediate region in the thickness direction of the electrode mixture layer 12 constituted by the second divided layer 12b and the third divided layer 12c, the electrode 10 according to embodiment 1 can further reduce the internal resistance of the battery. B2+C2 may be 56% or more. Also, B2+C2 may be 59% or less, or 58% or less.

The content of the rubber-based binder in the electrode mixture layer 12 is, for example, 0.5% by mass or more and 2.0% by mass or less.

The content of the water-soluble binder in the electrode mixture layer 12 is, for example, 1.0% by mass or more and 3.0% by mass or less.

The rubber-based binder content in the binder contained in the electrode mixture layer 12 is, for example, 14.0% by mass or more and 66.0% by mass or less.

The content of the water-soluble binder in the binder contained in the electrode mixture layer 12 is, for example, 33.0% by mass or more and 86.0% by mass or less.

The mass ratio of the rubber-based binder to the water-soluble binder contained in the electrode mixture layer 12 is not particularly limited, but for example, the content of the rubber-based binder is lower than the content of the water-soluble binder.

When the tortuosity of the first segment layer 12a in the thickness direction is τA and the tortuosity of the fourth segment layer 12d in the thickness direction is τD, the electrode mixture layer 12 may satisfy the following relational formula (9).
τA<τD...(9)

In the electrode mixture layer, the tortuosity is an index showing the degree of curvature of the voids (pores) formed in the electrode mixture layer through which the electrolyte passes. The smaller the tortuosity, the less curvature there is in the path of the voids. The tortuosity of the electrode mixture layer is the value obtained by dividing the path (path length) from the start point to the end point of the void in the electrode mixture layer by the straight-line distance from the start point to the end point of the void in the electrode mixture layer. When the path length is the same as the straight-line distance from the start point to the end point of the void in the electrode mixture layer, the tortuosity is 1.

The tortuosity τA of the first divided layer 12a is the value obtained by dividing the path length from the start point to the end point of the gap in the first divided layer 12a by the linear distance from the start point to the end point of the gap in the first divided layer 12a. The tortuosity τD of the fourth divided layer 12d is the value obtained by dividing the path length from the start point to the end point of the gap in the fourth divided layer 12d by the linear distance from the start point to the end point of the gap in the fourth divided layer 12d. The electrode mixture layer 12 satisfies the above relational expression (9), that is, the tortuosity τD of the fourth divided layer 12d located on the electrode current collector 11 side is increased to increase the packing density of the electrode active material particles, and the tortuosity τA of the first divided layer 12a located on the surface side is decreased to improve the rate characteristics of the battery. As a result, the electrode 10 according to embodiment 1 can realize a battery with improved cycle characteristics during high-rate charge and discharge.

The electrode mixture layer 12 may satisfy the following relational formula (10).
0.3≦τA/τD<1 (10)

When the electrode mixture layer 12 satisfies the above relational expression (10), that is, when the ratio (τA/τD) of the tortuosity τA of the first divided layer 12a to the tortuosity τD of the fourth divided layer 12d is 0.3 or more and less than 1, a battery with improved cycle characteristics during high-rate charging and discharging can be realized. Hereinafter, the ratio of the tortuosity τA of the first divided layer 12a to the tortuosity τD of the fourth divided layer 12d is referred to as the tortuosity ratio τA/τD.

The electrode mixture layer 12 may satisfy the following relational formula (11).
0.4≦τA/τD<0.8 (11)

When the electrode mixture layer 12 satisfies the above relational expression (11), i.e., when the tortuosity ratio τA/τD is 0.4 or more and less than 0.8, a battery with further improved cycle characteristics during high-rate charging and discharging can be realized.

The curvature ratio τA of the first dividing layer 12a is, for example, 1.2 or more and 4.5 or less, and may be 1.5 or more and 3.0 or less.

The curvature ratio τD of the fourth divided layer 12d is, for example, 1.5 or more and 5.0 or less, and may be 2.0 or more and 4.0 or less.

In this specification, the tortuosity τA of the first divided layer 12a and the tortuosity τD of the fourth divided layer 12d are calculated by the following formulas (I) and (II), respectively. In formula (I), fA is the path length (abbreviated as path length) of the medial axis penetrating the surfaces facing each other in the thickness direction in the first divided layer 12a, and sA is the length of the straight line connecting the start point and end point of the path of fA (abbreviated as straight line distance of the end point). In formula (II), fD is the path length (abbreviated as path length) of the medial axis penetrating the surfaces facing each other in the thickness direction in the fourth divided layer 12d, and sD is the length of the straight line connecting the start point and end point of the path of fD (abbreviated as straight line distance of the end point). The sample for evaluating the tortuosity is evaluated in a fully discharged state.
τA=fA/sA...(I)
τD=fD/sD...(II)

The above path length and linear distance are determined by cross-sectional observation and image analysis of the electrode mixture layer 12 using a 3D scanning electron microscope (3D SEM, for example, Ethos NX-5000 manufactured by Hitachi High-Tech Corporation).

The specific method for calculating the curvature ratio is as follows:

(1) Construction of a three-dimensional structure The electrode mixture layer 12 for constructing a three-dimensional structure by 3DSEM is placed on the sample stage of the 3DSEM, and continuous cross-sectional slicing and cross-sectional observation are performed alternately. The observation is performed at an acceleration voltage of 5 kV. The obtained two-dimensional continuous images are binarized using three-dimensional image analysis software (e.g., EXFACT VR manufactured by Japan Visual Science Co., Ltd.), and the images are connected to construct a three-dimensional structure. The three-dimensional structure is preferably 100 μm × 100 μm × 100 μm or more.

(2) Identification of the first divided layer 12a and the fourth divided layer 12d In the image of the three-dimensional structure obtained in (1) above, when the thickness is divided into four equal parts, the region located closest to the surface is identified as the first divided layer 12a, and the region located closest to the electrode collector 11 is identified as the fourth divided layer 12d.

(3) Determination of fA, fD, sA, and sD in the first divided layer 12a and the fourth divided layer 12d In the image of the three-dimensional structure obtained in (1) above, the first divided layer 12a and the fourth divided layer 12d were identified by (2) above, and the gaps extracted by binarization were thinned to determine the axis (medial axis) passing through the center of the gap. Media axes present in the cube that penetrate in a direction perpendicular to the surface of the electrode current collector 11 were extracted, and the shortest path for those with branches in one path was determined as the path length (fA and fD) of the first divided layer 12a and the fourth divided layer 12d, respectively.

(4) Calculation of the tortuosity of the first divided layer 12a and the fourth divided layer 12d The tortuosity of the first divided layer 12a and the second divided layer 12d is calculated according to the above formulas (I) and (II) using the average values of the path lengths (fA and fD) of the first divided layer 12a and the fourth divided layer 12d obtained in (3) above and the average values of the straight-line distances (sA and sD) connecting the paths.

The electrode 10 of embodiment 1 may be a negative electrode or a positive electrode. The electrode 10 of embodiment 1 is, for example, a negative electrode. Both the negative electrode and the positive electrode may have the configuration of the electrode 10 described above.

The electrode 10 of the first embodiment can be produced, for example, by applying an electrode mixture slurry, in which an electrode mixture containing an electrode active material and a binder is dispersed in a dispersion medium, to the surface of an electrode current collector and drying the coating to form an electrode mixture layer 12. The coating film after drying may be rolled as necessary. The electrode mixture layer 12 may be formed on one surface or both surfaces of the electrode current collector 11.

In forming the electrode mixture layer 12, the number of times the electrode mixture slurry is applied is not particularly limited. For example, the electrode mixture layer 12 may be formed by a single application of the electrode mixture slurry. In this case, by appropriately adjusting the drying conditions of the coating film, it is possible to prepare an electrode mixture layer 12 in which the arrangement of the rubber-based binder and the water-soluble binder is controlled so as to satisfy the above relational expressions (1), (2), and (3). Furthermore, by appropriately adjusting the drying conditions of the coating film, it is possible to prepare an electrode mixture layer 12 in which the arrangement of the rubber-based binder and the water-soluble binder is controlled so as to further satisfy one or more of the above relational expressions (4) to (8).

The electrode mixture layer 12 may be formed by multiple coatings of the electrode mixture slurry. In this case, for example, coating may be performed using multiple electrode mixture slurries with different compositions. In this way, when multiple electrode mixture slurries with different compositions are used, for example, multiple electrode mixture slurries in which at least one selected from the group consisting of the content ratio of the rubber-based binder and the water-soluble binder and the compounding ratio of the materials constituting the electrode active material (for example, the compounding ratio of artificial graphite and natural graphite) are appropriately adjusted are prepared, and the electrode mixture layer 12 can be prepared by selecting appropriate electrode mixture slurries for the electrode collector side region, the intermediate region, and the surface region. This makes it easier to prepare the electrode mixture layer 12 in which the arrangement of the rubber-based binder and the water-soluble binder is controlled so as to satisfy the above relational expressions (1), (2), and (3). The above artificial graphite and natural graphite are examples of carbon materials, which are examples of electrode active materials. Furthermore, by selecting an appropriate electrode mixture slurry in this manner, it is possible to produce an electrode mixture layer 12 in which the arrangement of the rubber-based binder and the water-soluble binder is controlled so as to further satisfy one or more of the above relational expressions (4) to (8).

In order to prepare the electrode mixture layer 12 so that the tortuosity τA of the first divided layer 12a and the tortuosity τD of the fourth divided layer 12d satisfy the above relational expression (9), for example, multiple types of electrode active materials may be used. For example, active materials A and B made of different materials are prepared as electrode active materials, and multiple electrode mixture slurries having different blending ratios of active materials A and B are prepared. For example, by changing the blending ratio of active materials A and B between the electrode mixture slurry for preparing the region corresponding to the first divided layer 12a and the electrode mixture slurry for preparing the region corresponding to the fourth divided layer 12d, the tortuosity τA of the first divided layer 12a and the tortuosity τD of the fourth divided layer 12d can be controlled to satisfy the above relational expression (9).

Below, each component of the electrode 10 of embodiment 1 is described in detail.

[Electrode current collector]
When the electrode 10 is a negative electrode, the electrode current collector 11 is a negative electrode current collector. As the negative electrode current collector, a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof can be used. The sheet or film may be porous or non-porous. As the sheet or film, a metal foil, a metal mesh, or the like is used. A carbon material such as carbon may be applied to the surface of the negative electrode current collector as a conductive auxiliary material.

When the electrode 10 is a positive electrode, the electrode current collector 11 is a positive current collector. As the positive current collector, a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof can be used. Aluminum and its alloys are suitable as materials for the positive current collector because they are inexpensive and easy to form into a thin film. The sheet or film may be porous or non-porous. As the sheet or film, a metal foil, a metal mesh, or the like is used. A carbon material such as carbon may be applied to the surface of the positive current collector as a conductive auxiliary material.

[Electrode mixture layer]
The electrode mixture layer 12 contains an electrode active material and a binder.

The thickness of the electrode mixture layer 12 is, for example, 50 μm or more and 150 μm or less.

When the electrode 10 is a negative electrode, the electrode mixture layer 12 is a negative electrode mixture layer, and the negative electrode mixture layer includes a negative electrode active material. The negative electrode active material can be a material capable of absorbing and releasing metal ions (e.g., lithium ions). The negative electrode active material includes, for example, at least one selected from the group consisting of carbon materials and materials capable of forming an alloy with lithium. An example of the carbon material is graphite. In addition, it is preferable to include a Si-based material in order to increase the capacity of the secondary battery. Here, the Si-based material means a material containing Si. Examples of the Si-based material include Si, Si alloys, and Si compounds. In addition, the Si-based material may be, for example, a composite particle including an ion-conducting phase and a silicon phase (from one point of view, silicon particles) dispersed in the ion-conducting phase. The ion-conducting phase is a phase that conducts ions, and includes, for example, at least one selected from the group consisting of a silicate phase, an aluminate phase, a carbon phase, and a silicon oxide phase. One of these negative electrode active materials may be used, or two or more of them may be used in combination.

The negative electrode mixture layer may contain at least one negative electrode active material selected from the group consisting of graphite and Si-based materials. Graphite is recommended because it is less likely to deteriorate even when repeatedly charged and discharged at a deep depth. Carbon materials other than graphite may be used as the negative electrode active material. Si-based materials have a larger capacity than graphite, which is advantageous for increasing the capacity of the battery. In order to achieve high capacity while maintaining good cycle characteristics, graphite and Si-based materials may be used in combination as the negative electrode active material.

When the Si-based material is the above composite particle and the ion-conducting phase is a carbon phase, the carbon phase may be composed of, for example, amorphous carbon. Examples of the amorphous carbon that composes the carbon phase include hard carbon, soft carbon, and other amorphous carbon. Amorphous carbon is a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.34 nm.

When the Si-based material is the composite particle and the ion-conducting phase is a silicon oxide phase, the main component of the silicon oxide phase may be silicon dioxide. Here, the main component of the silicon oxide phase is, for example, a component that is 95% by mass or more and 100% by mass or less of the silicon oxide phase. The composition of the composite particle including the silicon oxide phase and the silicon phase dispersed therein can be expressed as SiO _x as a whole. SiO _x has a structure in which silicon particles are dispersed in amorphous SiO ₂ , for example. The content ratio x of oxygen to silicon is, for example, preferably 0.5≦x<2.0, more preferably 0.8≦x≦1.5.

When the Si-based material is the above-mentioned composite particle and the ion-conducting phase is a silicate phase, the silicate phase may satisfy the following conditions (A) and/or (B).
(A) The silicate phase contains at least one element selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements of the long form periodic table).
(B) The silicate phase contains an element L. The element L is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanoids, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Lanthanoids is a general term for 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

Regarding the above condition (A), examples of alkali metal elements include lithium (Li), potassium (K), and sodium (Na). Examples of Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). By including an alkali metal element and/or a Group 2 element, the irreversible capacity of the silicate phase may be reduced. A silicate phase containing lithium (hereinafter, sometimes referred to as "lithium silicate phase") is preferable in that it has, for example, a small irreversible capacity and a high initial charge/discharge efficiency.

The lithium silicate phase may be an oxide phase containing Li, Si, and O, and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O/Si is, for example, greater than 2 and less than 4. Preferably, O/Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li/Si is, for example, greater than 0 and less than 4.

The lithium silicate phase may contain a lithium silicate phase represented by the formula _{Li2zSiO (2+z) (} 0<z<2), or may be composed of the lithium silicate phase. It is preferable that z satisfies the relationship 0<z<1, and more preferably z=1/2 (i.e., _{Li2Si2O5 )} .

The Si-based material may also include composite particles containing an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase, and a coating layer covering at least a portion of the surface of the composite particles.

The coating layer present on the surface of the composite particle includes, for example, a conductive layer. By forming a conductive layer on the surface of the composite particle, the conductivity of the Si-based material may be increased. The conductive material constituting the conductive layer is preferably a conductive material containing carbon. Examples of conductive materials containing carbon include conductive carbon materials. Examples of conductive carbon materials include carbon black, graphite, amorphous carbon (amorphous carbon) with low crystallinity, etc. Amorphous carbon is preferable because it has a large buffering effect on the silicon phase that changes in volume during charging and discharging. The amorphous carbon may be easily graphitized carbon (soft carbon) or difficult to graphitize carbon (hard carbon). Examples of carbon black include acetylene black and ketjen black. The thickness of the conductive layer may be, for example, in the range of 1 nm or more and 200 nm or less. The thickness of the conductive layer can be measured by observing the cross section of the Si-containing material using a SEM (scanning electron microscope) or a TEM (transmission electron microscope).

The content of the Si-based material is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more, based on the total amount of the negative electrode active material, in terms of increasing the capacity of the secondary battery, etc. Also, in terms of increasing the capacity of the secondary battery and suppressing swelling of the negative electrode, the content of the Si-based material is preferably 3% by mass or more and 30% by mass or less, more preferably 5% by mass or more and 25% by mass or less, and even more preferably 8% by mass or more and 20% by mass or less, based on the total amount of the negative electrode active material.

When the electrode 10 is a negative electrode, the explanation of the binder in the negative electrode mixture layer is the same as the explanation of the binder in the electrode mixture layer 12 described above. That is, the negative electrode mixture layer contains a rubber-based binder and a water-soluble binder, and satisfies the above relational expressions (1), (2), and (3). The negative electrode mixture layer may further satisfy one or more of the above relational expressions (4) to (8).

When the electrode 10 is a negative electrode, the negative electrode mixture layer may satisfy the tortuosity characteristics of the electrode mixture layer 12 described above. That is, the negative electrode mixture layer may satisfy the above relational expression (9), and may further satisfy the above relational expression (10) or (11).

The negative electrode mixture layer may further contain other materials such as a conductive additive.

The conductive additive is used to reduce the resistance of the negative electrode. Examples of the conductive additive include carbon materials and conductive polymer compounds. Examples of the carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compounds include polyaniline, polypyrrole, and polythiophene.

When the electrode 10 is a positive electrode, the electrode mixture layer 12 is a positive electrode mixture layer, and the positive electrode mixture layer includes a positive electrode active material. The positive electrode active material can be a material capable of absorbing and releasing metal ions (e.g., lithium ions). As the positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanion materials, fluorinated polyanion materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc. can be used. In particular, when a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate is used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

When the electrode 10 is a positive electrode, the explanation of the binder in the positive electrode mixture layer is the same as the explanation of the binder in the electrode mixture layer 12 described above. That is, the positive electrode mixture layer contains a rubber-based binder and a water-soluble binder, and satisfies the above relational expressions (1), (2), and (3). The positive electrode mixture layer may further satisfy one or more of the above relational expressions (4) to (8).

When the electrode 10 is a positive electrode, the positive electrode mixture layer may satisfy the tortuosity characteristics of the electrode mixture layer 12 described above. That is, the positive electrode mixture layer may satisfy the above relational expression (9), and may further satisfy the above relational expression (10) or (11).

The positive electrode mixture layer may further contain other materials such as a conductive additive. Materials that can be used in the negative electrode mixture layer as a conductive additive can also be used in the positive electrode mixture layer.

(Embodiment 2)
The battery according to the second embodiment includes a positive electrode, a negative electrode, and an electrolyte. At least one selected from the group consisting of the positive electrode and the negative electrode is the electrode according to the first embodiment. With this configuration, the battery according to the second embodiment can reduce the internal resistance. The negative electrode may be the electrode according to the first embodiment.

FIG. 2 is a longitudinal cross-sectional view showing a schematic example of a battery according to embodiment 2. Battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed within the battery case and is in contact with the electrolyte.

The battery case is composed of a case body 25, which is a cylindrical metal container with a bottom, and a sealing body 26 that seals the opening of the case body 25. A gasket 37 is disposed between the case body 25 and the sealing body 26. The gasket 37 ensures that the battery case is airtight. Inside the case body 25, insulating plates 27 and 28 are disposed on both ends of the electrode group 24 in the direction of the winding axis of the electrode group 24.

The case body 25 has, for example, a step 31. The step 31 can be formed by partially pressing the side wall of the case body 25 from the outside. The step 31 may be formed in an annular shape on the side wall of the case body 25 along the circumferential direction of an imaginary circle defined by the case body 25. At this time, the sealing body 26 is supported, for example, by the surface of the step 31 on the opening side.

The sealing body 26 includes a filter 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cap 36. In the sealing body 26, these components are layered in this order. The sealing body 26 is attached to the opening of the case body 25 so that the cap 36 is located on the outside of the case body 25 and the filter 32 is located on the inside of the case body 25.

Each of the above-mentioned components constituting the sealing body 26 is, for example, disk-shaped or ring-shaped. The above-mentioned components are electrically connected to each other, except for the insulating member 34.

The electrode group 24 has a positive electrode 21, a separator 22, and a negative electrode 23. The positive electrode 21, the separator 22, and the negative electrode 23 are all strip-shaped. The width direction of the strip-shaped positive electrode 21 and negative electrode 23 is, for example, parallel to the winding axis of the electrode group 24. The separator 22 is disposed between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and the negative electrode 23 are wound in a spiral shape with the separator 22 interposed between these electrodes.

When observing the cross section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 24, the positive electrodes 21 and negative electrodes 23 are stacked alternately in the radial direction of an imaginary circle defined by the case body 25, with a separator 22 interposed between these electrodes.

The positive electrode 21 is electrically connected to the cap 36, which also serves as a positive electrode terminal, via the positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, near the center of the positive electrode 21 in the longitudinal direction of the positive electrode 21. The positive electrode lead 29 extends from the positive electrode 21 to the filter 32 through a through hole formed in the insulating plate 27. The other end of the positive electrode lead 29 is welded, for example, to the surface of the filter 32 facing the electrode group 24.

The negative electrode 23 is electrically connected to the case body 25, which also serves as a negative electrode terminal, via the negative electrode lead 30. One end of the negative electrode lead 30 is connected, for example, to an end of the negative electrode 23 in the longitudinal direction of the negative electrode 23. The other end of the negative electrode lead 30 is welded, for example, to the inner bottom surface of the case body 25.

The components of the battery 100 are described in detail below.

The positive electrode 21 includes a material that has the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 21 includes, for example, a positive electrode active material. The positive electrode 21 may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode current collector and the positive electrode mixture layer are as described in embodiment 1. The positive electrode 21 may be the electrode 10 according to embodiment 1.

The negative electrode 23 includes a material that has the property of absorbing and releasing metal ions (e.g., lithium ions). The negative electrode 23 includes, for example, a negative electrode active material. The negative electrode 23 may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode current collector and the negative electrode mixture layer are as described in embodiment 1. The negative electrode 23 may be the electrode 10 according to embodiment 1.

The electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte may be, for example, 0.5 mol/L or more and 2 mol/L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

Cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, nitriles, amides, etc. may be used as non-aqueous solvents. One selected from these solvents may be used, or two or more may be used in combination.

Examples of the lithium salt that can be used include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO ₂ C ₂ F ₅ ) ₂ ), LiAsF ₆ , LiCF ₃ SO ₃ , and lithium difluoro(oxalato)borate. One selected from these electrolyte salts may be used, or two or more may be used in combination.

Usually, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator 22 has high ion permeability and has appropriate mechanical strength and insulating properties. The separator 22 may be a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 22 may be made of a material such as a polymer. The polymer may be a polyolefin such as polypropylene or polyethylene.

In the battery according to the second embodiment, the electrolyte may be impregnated into a polymer provided as a separator, for example. In other words, the battery according to the second embodiment may have a structure in which the electrolyte and the polymer are used in combination.

The battery according to the second embodiment may further include a solid electrolyte as an electrolyte. That is, the battery according to the present disclosure may have a hybrid structure in which an electrolytic solution and a solid electrolyte are used in combination. Examples of solid electrolyte materials are halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In the present disclosure, a "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main component of the anions. A "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main component of the anions. An "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main component of the anions. The main component of the anions refers to the anion with the largest substance amount among all the anions constituting the solid electrolyte.

As an example of the structure of the battery according to the second embodiment, the configuration example shown in FIG. 2 is described, that is, a cylindrical nonaqueous electrolyte secondary battery in which a wound electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and an electrolyte solution are housed in an exterior body. However, the battery according to the present disclosure is not limited to this configuration example. The battery according to the second embodiment may be in any form, such as a square type, a coin type, a button type, a laminate type, etc. Also, as the electrode group in the battery according to the second embodiment, instead of the wound type electrode group, an electrode group of another form, such as an electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, may be used.

Other Embodiments
(Additional Note)
The above description of the embodiments discloses the following techniques.

(Technology 1)
An electrode current collector;
an electrode mixture layer disposed on the electrode current collector;
Equipped with
The electrode mixture layer contains an electrode active material and a binder,
The binder contains a rubber-based binder and a water-soluble binder,
The four divided layers resulting from dividing the electrode mixture layer into four equal parts in the thickness direction are designated, in order from the surface side of the electrode mixture layer toward the electrode current collector side, as a first divided layer, a second divided layer, a third divided layer, and a fourth divided layer, and the mass ratio (%) of the rubber-based binder contained in the first divided layer to the total mass of the rubber-based binder contained in the electrode mixture layer is designated as A1, the mass ratio (%) of the rubber-based binder contained in the second divided layer is designated as B1, the mass ratio (%) of the rubber-based binder contained in the third divided layer is designated as C1, and the mass ratio (%) of the rubber-based binder contained in the fourth divided layer is designated as B2, B3, C4, and C5. When the mass ratio (%) of the rubber-based binder contained in the electrode mixture layer is D1, the mass ratio (%) of the water-soluble binder contained in the first division layer relative to the total mass of the water-soluble binder contained in the electrode mixture layer is A2, the mass ratio (%) of the water-soluble binder contained in the second division layer is B2, the mass ratio (%) of the water-soluble binder contained in the third division layer is C2, and the mass ratio (%) of the water-soluble binder contained in the fourth division layer is D2, the electrode mixture layer satisfies the following relational expressions (1), (2), and (3):
electrode.
A1+D1<B1+C1...(1)
A2+D2<B2+C2...(2)
A1<A2...(3)

With this configuration, the electrode according to Technology 1 can reduce the internal resistance of the battery.

(Technology 2)
The electrode mixture layer satisfies the following relations (4) and (5):
The electrode according to technique 1.
A1<B1...(4)
A2<B2...(5)

With this configuration, the electrode according to Technology 2 can further reduce the internal resistance of the battery.

(Technology 3)
The electrode mixture layer satisfies the following relational formula (6):
3. The electrode according to claim 1 or 2.
B2+C2<B1+C1...(6)

With this configuration, the electrode according to Technology 3 can further reduce the internal resistance of the battery.

(Technology 4)
The electrode mixture layer satisfies the following relational formula (7):
The electrode according to any one of claims 1 to 3.
57%<B1+C1<65%...(7)

With this configuration, the electrode according to Technology 4 can further reduce the internal resistance of the battery.

(Technology 5)
The electrode mixture layer satisfies the following relational formula (8):
5. An electrode according to any one of claims 1 to 4.
55%<B2+C2<60%...(8)

With this configuration, the electrode according to Technology 5 can further reduce the internal resistance of the battery.

(Technology 6)
When the tortuosity in the thickness direction of the first segment layer is τA and the tortuosity in the thickness direction of the fourth segment layer is τD, the electrode mixture layer satisfies the following relational expression (9):
6. An electrode according to any one of claims 1 to 5.
τA<τD...(9)

With this configuration, the electrode according to Technology 6 can realize a battery with improved cycle characteristics during high-rate charging and discharging.

(Technology 7)
The electrode mixture layer satisfies the following relational formula (10):
Electrode according to technique 6.
0.3≦τA/τD<1 (10)

With this configuration, the electrode according to Technology 7 can realize a battery with improved cycle characteristics during high-rate charging and discharging.

(Technology 8)
The electrode mixture layer satisfies the following relational formula (11):
Electrodes according to technique 7.
0.4≦τA/τD<0.8 (11)

With this configuration, the electrode according to Technology 8 can realize a battery with improved cycle characteristics during high-rate charging and discharging.

(Technology 9)
The electrode is a negative electrode.
9. An electrode according to any one of claims 1 to 8.

With this configuration, the electrode according to Technology 9 can more effectively reduce the internal resistance of the battery.

(Technology 10)
A positive electrode and
A negative electrode;
An electrolyte;
Equipped with
At least one selected from the group consisting of the positive electrode and the negative electrode is an electrode according to any one of techniques 1 to 8.
battery.

This configuration allows the battery according to Technology 10 to have a reduced internal resistance.

(Technology 11)
The negative electrode is an electrode according to any one of techniques 1 to 8.
The battery according to claim 10.

This configuration allows the battery according to Technology 11 to reduce internal resistance more effectively.

The present disclosure will now be described in more detail using examples. The following examples are merely examples of one embodiment and are not intended to be limiting.

[Example 1]
(Preparation of negative electrode mixture slurry)
As the negative electrode active materials, three active materials, a first active material, a second active material, and a third active material, were prepared. The first active material was graphite, which was artificial graphite. The second active material was graphite, which was natural graphite. The third active material was a Si-based material. The Si-based material used as the third active material was a composite material with a silicon oxide phase as an ion-conducting phase. Styrene butadiene rubber (SBR) was prepared as a rubber-based binder. Carboxymethyl cellulose (CMC) was prepared as a water-soluble binder.

In Example 1, two negative electrode mixture slurries, a first slurry and a second slurry, were prepared.

For the first slurry, a mixed active material was used in which the first active material, the second active material, and the third active material were mixed in a mass ratio of first active material:second active material:third active material = 72:18:10. Water was added as a dispersion medium to a negative electrode mixture in which the mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2, and the mixture was stirred using a mixer to prepare the first slurry.

For the second slurry, a mixed active material was used in which the second active material and the third active material were mixed in a mass ratio of 90:10 (mass ratio). The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of 100:1:2 (mass ratio of mixed active material:rubber-based binder:water-soluble binder), and water was added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the second slurry.

(Preparation of negative electrode)
Copper foil was used as the negative electrode current collector. The second slurry was applied to both sides of this negative electrode current collector, and the coating was dried to form a film with a thickness of 100 μm. The first slurry was applied to the surface of each of the films prepared, and the coating was dried to form a film with a thickness of 30 μm, which was then rolled to form a negative electrode mixture layer. That is, the negative electrode mixture layer was formed by the film formed with the first slurry and the film formed with the second slurry. The region approximately corresponding to the first divided layer of the negative electrode mixture layer was prepared using the first slurry, and the region approximately corresponding to the second divided layer, the third divided layer, and the fourth divided layer was prepared using the second slurry. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a part of the negative electrode.

(Preparation of Positive Electrode Active Material)
The composite hydroxide represented by _{[ Ni0.90Al0.05Mn0.05} ](OH) ₂ obtained by _the coprecipitation method was calcined at 500°C for 8 hours to obtain a composite oxide ( _{Ni0.90Al0.05Mn0.05O2} ). Next, LiOH and the composite oxide were mixed so that the molar ratio _of Li to the total amount of Ni, Al, and Mn was 1.03 _{: 1} to obtain a mixture. This mixture was calcined from room temperature to 650°C at a heating rate of 2.0°C/min under an oxygen flow with an oxygen concentration of 95% (flow rate of 2mL/min per 10 ^cm3 and 5L/min per kg of the mixture), and then calcined from 650°C to 780°C at a heating rate of 0.5°C/min to obtain a lithium-containing composite oxide.

(Preparation of Positive Electrode)
The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 98:1:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied onto a positive electrode collector made of aluminum foil, the coating was dried and compressed, and then the positive electrode collector was cut into a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was disposed on both sides of the positive electrode core. An exposed portion in which the surface of the positive electrode collector was exposed was provided in a part of the positive electrode.

(Preparation of non-aqueous electrolyte (electrolyte solution))
A non-aqueous electrolyte (electrolytic solution) was prepared by dissolving _LiPF6 at a concentration of 1.2 mol/L in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (25 °C).

(Preparation of test cells (secondary batteries))
An aluminum lead was attached to the exposed part of the positive electrode, and a nickel lead was attached to the exposed part of the negative electrode, and the positive and negative electrodes were spirally wound through a polyolefin separator to prepare a wound electrode body. Insulating plates were placed on the top and bottom of the electrode body, and the electrode body was housed in an outer can. The negative electrode lead was welded to the bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing body. An electrolyte was poured into the outer can, and the opening of the outer can was sealed with a sealing body via a gasket to prepare a secondary battery as a test cell.

[Example 2]
(Preparation of negative electrode mixture slurry)
The negative electrode active materials used were the same first active material, second active material, and third active material as in Example 1. In addition, the same materials as in Example 1 were used for the rubber binder and water-soluble binder.

In Example 2, two negative electrode mixture slurries, a first slurry and a second slurry, were prepared.

For the first slurry, a mixed active material was used in which the first active material and the third active material were mixed in a mass ratio of first active material:third active material = 90:10. The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the first slurry.

In the second slurry, the second active material and the third active material were used as the negative electrode active material. A mixed active material was used in which the second active material:third active material = 90:10 (mass ratio). The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the second slurry.

(Preparation of negative electrode)
Copper foil was used as the negative electrode current collector. The second slurry was applied to both sides of this negative electrode current collector, and the coating was dried to form a film with a thickness of 100 μm. The first slurry was applied to the surface of each of the films prepared, and the coating was dried to form a film with a thickness of 30 μm, which was then rolled to form a negative electrode mixture layer. That is, the negative electrode mixture layer was formed by the film formed with the first slurry and the film formed with the second slurry. The region approximately corresponding to the first divided layer of the negative electrode mixture layer was prepared using the first slurry, and the regions approximately corresponding to the second divided layer, the third divided layer, and the fourth divided layer were prepared using the second slurry.

(Preparation of Positive Electrode Active Material and Positive Electrode)
The positive electrode active material and the positive electrode were prepared in the same manner as in Example 1.

(Preparation of non-aqueous electrolyte (electrolyte solution))
The non-aqueous electrolyte was prepared in the same manner as in Example 1.

(Preparation of test cells (secondary batteries))
A secondary battery was produced as a test cell in the same manner as in Example 1, except that the negative electrode produced in Example 2 was used.

[Example 3]
(Preparation of negative electrode mixture slurry)
The negative electrode active materials used were the same first active material, second active material, and third active material as in Example 1. In addition, the same materials as in Example 1 were used for the rubber binder and water-soluble binder.

In Example 3, two negative electrode mixture slurries, a first slurry and a second slurry, were prepared.

For the first slurry, a mixed active material was used in which the first active material, the second active material, and the third active material were mixed in a mass ratio of first active material:second active material:third active material = 72:18:10. The mixed active material, a rubber-based binder, and a water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:1.5 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the first slurry.

In the second slurry, the second active material and the third active material were used as the negative electrode active material. A mixed active material was used in which the second active material:third active material = 90:10 (mass ratio). The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the second slurry.

(Preparation of negative electrode)
Copper foil was used as the negative electrode current collector. The second slurry was applied to both sides of this negative electrode current collector, and the coating was dried to form a film with a thickness of 100 μm. The first slurry was applied to the surface of each of the films prepared, and the coating was dried to form a film with a thickness of 30 μm, which was then rolled to form a negative electrode mixture layer. That is, the negative electrode mixture layer was formed by the film formed with the first slurry and the film formed with the second slurry. The region approximately corresponding to the first divided layer of the negative electrode mixture layer was prepared using the first slurry, and the regions approximately corresponding to the second divided layer, the third divided layer, and the fourth divided layer were prepared using the second slurry.

(Preparation of Positive Electrode Active Material and Positive Electrode)
The positive electrode active material and the positive electrode were prepared in the same manner as in Example 1.

(Preparation of non-aqueous electrolyte (electrolyte solution))
The non-aqueous electrolyte was prepared in the same manner as in Example 1.

(Preparation of test cells (secondary batteries))
A secondary battery was produced as a test cell in the same manner as in Example 1, except that the negative electrode produced in Example 3 was used.

[Example 4]
(Preparation of negative electrode mixture slurry)
The negative electrode active materials used were the same first active material, second active material, and third active material as in Example 1. In addition, the same materials as in Example 1 were used for the rubber binder and water-soluble binder.

In Example 4, two negative electrode mixture slurries, a first slurry and a second slurry, were prepared.

For the first slurry, a mixed active material was used in which the first active material, the second active material, and the third active material were mixed in a mass ratio of first active material:second active material:third active material = 90:0:10. Water was added as a dispersion medium to a negative electrode mixture in which the mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:1.5, and the mixture was stirred using a mixer to prepare the first slurry.

In the second slurry, the second active material and the third active material were used as the negative electrode active material. A mixed active material was used in which the second active material:third active material = 90:10 (mass ratio). The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the second slurry.

(Preparation of negative electrode)
Copper foil was used as the negative electrode current collector. The second slurry was applied to both sides of this negative electrode current collector, and the coating was dried to form a film with a thickness of 100 μm. The first slurry was applied to the surface of each of the films prepared, and the coating was dried to form a film with a thickness of 30 μm, which was then rolled to form a negative electrode mixture layer. That is, the negative electrode mixture layer was formed by the film formed with the first slurry and the film formed with the second slurry. The region approximately corresponding to the first divided layer of the negative electrode mixture layer was prepared using the first slurry, and the regions approximately corresponding to the second divided layer, the third divided layer, and the fourth divided layer were prepared using the second slurry.

(Preparation of Positive Electrode Active Material and Positive Electrode)
The positive electrode active material and the positive electrode were prepared in the same manner as in Example 1.

(Preparation of non-aqueous electrolyte (electrolyte solution))
The non-aqueous electrolyte was prepared in the same manner as in Example 1.

(Preparation of test cells (secondary batteries))
A secondary battery was produced as a test cell in the same manner as in Example 1, except that the negative electrode produced in Example 4 was used.

[Comparative Example 1]
(Preparation of negative electrode mixture slurry)
The negative electrode active materials used were the same first active material, second active material, and third active material as in Example 1. In addition, the same materials as in Example 1 were used for the rubber binder and water-soluble binder.

For the first slurry, a mixed active material was used in which the first active material, the second active material, and the third active material were mixed in a mass ratio of first active material:second active material:third active material = 54:36:10. The mixed active material, a rubber-based binder, and a water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the first slurry.

For the second slurry, a mixed active material was used in which the first active material, the second active material, and the third active material were mixed in a mass ratio of first active material:second active material:third active material = 45:45:10. Water was added as a dispersion medium to a negative electrode mixture in which the mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2, and the mixture was stirred using a mixer to prepare the second slurry.

(Preparation of negative electrode)
Copper foil was used as the negative electrode current collector. The second slurry was applied to both sides of this negative electrode current collector, and the coating was dried to form a film with a thickness of 65 μm. The first slurry was applied to the surface of each of the films prepared, and the coating was dried to form a film with a thickness of 65 μm, which was then rolled to form a negative electrode mixture layer. That is, the negative electrode mixture layer was formed by the film formed with the first slurry and the film formed with the second slurry. The regions approximately corresponding to the first and second divided layers of the negative electrode mixture layer were prepared using the first slurry, and the regions approximately corresponding to the third and fourth divided layers were prepared using the second slurry.

(Preparation of Positive Electrode Active Material and Positive Electrode)
The positive electrode active material and the positive electrode were prepared in the same manner as in Example 1.

(Preparation of non-aqueous electrolyte (electrolyte solution))
The non-aqueous electrolyte was prepared in the same manner as in Example 1.

(Preparation of test cells (secondary batteries))
A secondary battery was produced as a test cell in the same manner as in Example 1, except that the negative electrode produced in Comparative Example 1 was used.

[Comparative Example 2]
(Preparation of negative electrode mixture slurry)
The negative electrode active materials used were the same first active material, second active material, and third active material as in Example 1. In addition, the same materials as in Example 1 were used for the rubber binder and water-soluble binder.

In Comparative Example 2, two negative electrode mixture slurries, a first slurry and a second slurry, were prepared.

For the first slurry, a mixed active material was used in which the first active material and the third active material were mixed in a mass ratio of first active material:third active material = 90:10. The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:2:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the first slurry.

In the second slurry, the second active material and the third active material were used as the negative electrode active material. A mixed active material was used in which the second active material:third active material = 90:10 (mass ratio). The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the second slurry.

(Preparation of negative electrode)
Copper foil was used as the negative electrode current collector. The second slurry was applied to both sides of this negative electrode current collector, and the coating was dried to form a film with a thickness of 100 μm. The first slurry was applied to the surface of each of the films prepared, and the coating was dried to form a film with a thickness of 30 μm, which was then rolled to form a negative electrode mixture layer. That is, the negative electrode mixture layer was formed by the film formed with the first slurry and the film formed with the second slurry. The region approximately corresponding to the first divided layer of the negative electrode mixture layer was prepared using the first slurry, and the regions approximately corresponding to the second divided layer, the third divided layer, and the fourth divided layer were prepared using the second slurry.

(Preparation of Positive Electrode Active Material and Positive Electrode)
The positive electrode active material and the positive electrode were prepared in the same manner as in Example 1.

(Preparation of non-aqueous electrolyte (electrolyte solution))
The non-aqueous electrolyte was prepared in the same manner as in Example 1.

(Preparation of test cells (secondary batteries))
A secondary battery was produced as a test cell in the same manner as in Example 1, except that the negative electrode produced in Comparative Example 2 was used.

[Comparative Example 3]
(Preparation of negative electrode mixture slurry)
The negative electrode active materials used were the same first active material, second active material, and third active material as in Example 1. In addition, the same materials as in Example 1 were used for the rubber binder and water-soluble binder.

In Comparative Example 3, a mixed active material was used in which the first active material, the second active material, and the third active material were mixed in a mass ratio of 45:45:10. This mixed active material, a rubber-based binder, and a water-soluble binder were mixed in a mass ratio of 100:1:2, and water was added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare a negative electrode slurry.

(Preparation of negative electrode)
A copper foil was used as a negative electrode current collector. The negative electrode slurry was applied to both sides of the negative electrode current collector, and the coating was dried to form a film having a thickness of 130 μm. The coating was then rolled to form a negative electrode mixture layer.

(Preparation of Positive Electrode Active Material and Positive Electrode)
The positive electrode active material and the positive electrode were prepared in the same manner as in Example 1.

(Preparation of non-aqueous electrolyte (electrolyte solution))
The non-aqueous electrolyte was prepared in the same manner as in Example 1.

(Preparation of test cells (secondary batteries))
A secondary battery was produced as a test cell in the same manner as in Example 1, except that the negative electrode produced in Comparative Example 3 was used.

[Comparative Example 4]
(Preparation of negative electrode mixture slurry)
The negative electrode active materials used were the same first active material, second active material, and third active material as in Example 1. In addition, the same materials as in Example 1 were used for the rubber binder and water-soluble binder.

In Comparative Example 4, two negative electrode mixture slurries, a first slurry and a second slurry, were prepared.

For the first slurry, a mixed active material was used in which the first active material and the third active material were mixed in a mass ratio of 90:10 (first active material:third active material). The mixed active material, a rubber-based binder, and a water-soluble binder were mixed in a mass ratio of 100:0.5:1 (mixed active material:rubber-based binder:water-soluble binder), and water was added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the first slurry.

In the second slurry, the second active material and the third active material were used as the negative electrode active material. A mixed active material was used in which the second active material:third active material = 90:10 (mass ratio). The mixed active material, rubber-based binder, and water-soluble binder were mixed in a mass ratio of mixed active material:rubber-based binder:water-soluble binder = 100:1:2 to prepare a negative electrode mixture. Water was then added as a dispersion medium to the negative electrode mixture, which was then stirred using a mixer to prepare the second slurry.

(Preparation of negative electrode)
Copper foil was used as the negative electrode current collector. The second slurry was applied to both sides of this negative electrode current collector, and the coating was dried to form a film with a thickness of 100 μm. The first slurry was applied to the surface of each of the films prepared, and the coating was dried to form a film with a thickness of 30 μm, which was then rolled to form a negative electrode mixture layer. That is, the negative electrode mixture layer was formed by the film formed with the first slurry and the film formed with the second slurry. The region approximately corresponding to the first divided layer of the negative electrode mixture layer was prepared using the first slurry, and the regions approximately corresponding to the second divided layer, the third divided layer, and the fourth divided layer were prepared using the second slurry.

(Preparation of Positive Electrode Active Material and Positive Electrode)
The positive electrode active material and the positive electrode were prepared in the same manner as in Example 1.

(Preparation of non-aqueous electrolyte (electrolyte solution))
The non-aqueous electrolyte was prepared in the same manner as in Example 1.

(Preparation of test cells (secondary batteries))
A secondary battery was produced as a test cell in the same manner as in Example 1, except that the negative electrode produced in Comparative Example 4 was used.

[Measurement of the mass ratio of rubber binder in negative electrode mixture layer]
For the negative electrodes of the respective Examples and Comparative Examples, the mass ratios A1, B1, C1, and D1 of the rubber binder in the negative electrode mixture layer were measured by element mapping using the following method. The results are shown in Table 1.

Quantitative analysis of the rubber binder by elemental mapping was performed by EPMA. The rubber binder used in the examples and comparative examples was SBR, i.e., a rubber polymer compound having a C=C bond. Bromine was added to the C=C bond of SBR, and the cross section of the negative electrode mixture layer was analyzed for bromine by elemental mapping using EPMA. This allowed the content of the rubber binder in each divided layer to be determined. The method of adding bromine to the rubber polymer compound involved immersing the negative electrode in a bromine solution. The EPMA conditions in the examples and comparative examples were as follows.
Electron beam diameter: 1 μm
Acceleration voltage: 15 kV
Beam current: 50 nA
Number of pixels: 255 x 255 pixels
Measurement time: 30msec/pixel

[Measurement of mass ratio of water-soluble binder in negative electrode mixture layer]
For the negative electrodes of the respective Examples and Comparative Examples, the mass ratios A2, B2, C2, and D2 of the water-soluble binder in the negative electrode mixture layer were measured by element mapping in the following manner. The results are shown in Table 1.

Quantitative analysis of the water-soluble binder by elemental mapping was carried out by EPMA. The water-soluble binder used in the examples and comparative examples was CMC, i.e., a water-soluble polymer compound having a carboxyl group. Ruthenium was added to the carboxyl group, and the ruthenium bromine was analyzed by elemental mapping by EPMA for the cross section of the negative electrode mixture layer. This resulted in the content of the water-soluble binder in each divided layer. The method of adding ruthenium to the water-soluble polymer compound involved immersing the negative electrode in a ruthenium-containing solution. The conditions of EPMA were as follows.
Electron beam diameter: 1 μm
Acceleration voltage: 15 kV
Beam current: 50 nA
Number of pixels: 255 x 255 pixels
Measurement time: 30msec/pixel

[Measurement of Tortuosity in Negative Electrode Mixture Layer]
For the negative electrodes of each Example and Comparative Example, the tortuosity τA of the first divided layer and the tortuosity τD of the fourth divided layer were obtained using the method described in embodiment 1. The tortuosity ratio τA/τD was calculated from the values of the tortuosity τA and the tortuosity τD. The results are shown in Table 1.

[Measurement of DCR]
Using the test cells of each of the Examples and Comparative Examples, the DCR of the cells was measured by the following method.

For each of the test cells of the examples and comparative examples, initial charge and discharge were carried out under the following conditions.
(Charge/discharge conditions)
Initial charge/discharge conditions: Constant current charging was performed at a current of 0.5 It (625 mA) until the battery voltage reached 4.2 V. Furthermore, constant voltage charging was performed at a voltage of 4.2 V until the current value reached 0.02 It (25 mA). Then, constant current discharging was performed at a current of 0.5 It (625 mA) until the battery voltage reached 2.5 V.

Then, charging and discharging were performed under the following conditions, and the initial direct current internal resistance (DCR) value was examined using the following formula (a). The results are shown in Table 1.

(Charge/discharge conditions)
At a temperature of 25° C., the battery was charged at a constant current of 0.3 It (375 mA) until the battery voltage reached 3.79 V. The battery was then charged at a constant voltage of 3.79 V until the current reached 0.02 It (25 mA). After a 2-hour pause, the battery was discharged at a current of 0.2 It (250 mA) for 10 seconds.

(DCR calculation formula)
DCR (mΩ) = (voltage immediately before discharge start - voltage 10 seconds after discharge start) / (discharge current density x electrode area) ... (a)

The results are shown in Table 1. Note that Table 1 shows relative values when the DCR value of Comparative Example 1 is taken as the reference (100%).

(Consideration)
As shown in Table 1, the batteries of Examples 1 to 4 in which a negative electrode having a negative electrode mixture layer satisfying the above relational expressions (1), (2), and (3) was used had a lower DCR than the batteries of Comparative Examples 1 to 4 in which an electrode having an electrode mixture layer satisfying the above relational expressions (1), (2), and (3) was not used. That is, the electrode having an electrode mixture layer satisfying the above relational expressions (1), (2), and (3) was able to improve the internal resistance of the battery.

The technology disclosed herein is useful for batteries such as lithium-ion secondary batteries.

Claims (11)

An electrode current collector;
an electrode mixture layer disposed on the electrode current collector;
Equipped with
The electrode mixture layer contains an electrode active material and a binder,
The binder contains a rubber-based binder and a water-soluble binder,
The four divided layers resulting from dividing the electrode mixture layer into four equal parts in the thickness direction are designated, in order from the surface side of the electrode mixture layer toward the electrode current collector side, as a first divided layer, a second divided layer, a third divided layer, and a fourth divided layer, and the mass ratio (%) of the rubber-based binder contained in the first divided layer to the total mass of the rubber-based binder contained in the electrode mixture layer is designated as A1, the mass ratio (%) of the rubber-based binder contained in the second divided layer is designated as B1, the mass ratio (%) of the rubber-based binder contained in the third divided layer is designated as C1, and the mass ratio (%) of the rubber-based binder contained in the fourth divided layer is designated as B2, B3, C4, and C5. When the mass ratio (%) of the rubber-based binder contained in the electrode mixture layer is D1, the mass ratio (%) of the water-soluble binder contained in the first division layer relative to the total mass of the water-soluble binder contained in the electrode mixture layer is A2, the mass ratio (%) of the water-soluble binder contained in the second division layer is B2, the mass ratio (%) of the water-soluble binder contained in the third division layer is C2, and the mass ratio (%) of the water-soluble binder contained in the fourth division layer is D2, the electrode mixture layer satisfies the following relational expressions (1), (2), and (3):
electrode.
A1+D1<B1+C1...(1)
A2+D2<B2+C2...(2)
A1<A2...(3) The electrode mixture layer satisfies the following relations (4) and (5):
2. The electrode of claim 1.
A1<B1...(4)
A2<B2...(5) The electrode mixture layer satisfies the following relational formula (6):
2. The electrode of claim 1.
B2+C2<B1+C1...(6) The electrode mixture layer satisfies the following relational formula (7):
2. The electrode of claim 1.
57%<B1+C1<65%...(7) The electrode mixture layer satisfies the following relational formula (8):
2. The electrode of claim 1.
55%<B2+C2<60%...(8) When the tortuosity in the thickness direction of the first segment layer is τA and the tortuosity in the thickness direction of the fourth segment layer is τD, the electrode mixture layer satisfies the following relational expression (9):
2. The electrode of claim 1.
τA<τD...(9) The electrode mixture layer satisfies the following relational formula (10):
7. The electrode of claim 6.
0.3≦τA/τD<1 (10) The electrode mixture layer satisfies the following relational formula (11):
8. The electrode of claim 7.
0.4≦τA/τD<0.8 (11) The electrode is a negative electrode.
2. The electrode of claim 1. A positive electrode and
A negative electrode;
An electrolyte;
Equipped with
At least one selected from the group consisting of the positive electrode and the negative electrode is the electrode according to any one of claims 1 to 8.
battery. The negative electrode is an electrode according to any one of claims 1 to 8.
The battery of claim 10.

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