Battery electrode

Abstract

To improve rate characteristics.SOLUTION: A battery electrode includes a substrate and a negative electrode active material layer. The negative electrode active material layer is disposed on a surface of the substrate. The negative electrode active material layer contains graphite and binder. The negative electrode active material layer has a basis weight of 25 mg / cm2 or more and density of 1.2 to 1.6 g / cm3. In the entire cross section parallel to a thickness direction of the negative electrode active material layer, an area fraction of the binder is 1.9% or more. The cross section includes a first region and a second region. The first region is disposed between the second region and the substrate. Relations of "1.0<(τ1 / τ2)<4.4" and "τ2<2.1" are satisfied. "τ1" indicates a degree of flexion in the first region. "τ2" indicates the degree of flexion in the second region.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to electrodes for batteries. [Background technology]

[0002] JP-A-2023-537139 discloses a negative electrode plate in which the degree of bending of a first negative electrode active material layer (deep portion) is smaller than the degree of bending of a second negative electrode active material layer (shallow portion). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2023-537139 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present disclosure is to improve rate characteristics. [Means for solving the problem]

[0005] 1. A battery electrode includes a substrate and a negative electrode active material layer. The negative electrode active material layer is disposed on the surface of the substrate. The negative electrode active material layer includes graphite and a binder. The negative electrode active material layer has a thickness of 25 mg / cm. 2 and 1.2 to 1.6 g / cm 3 The negative electrode active material layer has a density of 1.9% or more. In the entire cross section parallel to the thickness direction of the negative electrode active material layer, the area fraction of the binder is 1.9% or more. The cross section includes a first region and a second region. The first region is disposed between the second region and the substrate. The relationships "1.0<(τ1 / τ2)<4.4" and "τ2<2.1" are satisfied. "τ1" indicates the degree of bending in the first region. "τ2" indicates the degree of bending in the second region.

[0006] During long-term discharge, lithium (Li) ions move from shallow to deep within the negative electrode active material layer. The tortuosity indicates the complexity of the Li ion migration path (voids) in the thickness direction of the negative electrode active material layer. The closer the tortuosity is to 1, the simpler (more linear) the migration path is thought to be. The further the tortuosity is from 1, the more complex the migration path is thought to be. The closer the tortuosity is to 1, the more the supply of Li ions in the thickness direction is promoted, which is thought to improve rate characteristics.

[0007] To increase energy density, a high-density and high-weight negative electrode active material layer is required. However, high-density and high-weight increase the degree of bending. In other words, high-density and high-weight increase the rate characteristics. The negative electrode active material layer contains a binder in addition to the negative electrode active material (graphite). To mitigate the deterioration of rate characteristics due to high-density and high-weight, it is possible to reduce the amount of binder. Reducing the binder increases voids (pathways for Li-ion migration). In other words, it is expected that the rate characteristics will improve. However, on the other hand, a lack of binder may result in insufficient adhesive strength between the negative electrode active material layer and the substrate. For these reasons, a binder of 25 mg / cm has been used up to now. 2 High weight of 1.2g / cm 3 Under the conditions of the above-mentioned high density and binder amount of 1.9% or more, it was difficult to achieve a flexion degree of less than 2.1.

[0008] The present disclosure provides a negative electrode active material layer with two regions having different degrees of curvature, thereby improving rate characteristics while maintaining sufficient adhesive strength in a high-basis-weight, high-density negative electrode active material layer. The first region may be referred to as a "deep portion" or "deep layer," etc. The first region is located closer to the substrate than the second region. The second region may be referred to as a "shallow portion" or "surface layer," etc. The second region is located closer to the surface of the negative electrode active material layer than the first region. The ratio "τ1 / τ2" of the degree of curvature "τ1" of the first region to the degree of curvature "τ2" of the second region is greater than 1.0 and less than 4.4. When the degree of curvature ratio "τ1 / τ2" is 1.0 or less, the migration distance of Li ions in the shallow portion becomes relatively long, which may reduce the utilization rate of the negative electrode active material in the deep portion. When the tortuosity ratio "τ1 / τ2" exceeds 1, the migration distance of Li ions in the shallow portion becomes relatively short, which can improve the utilization of the negative electrode active material in the deep portion. By improving the utilization of the negative electrode active material in the deep portion, improvement in rate characteristics is expected. However, when the tortuosity ratio "τ1 / τ2" is 4.4 or more, it can become difficult for Li ions to migrate in the deep portion. As a result, the utilization of the negative electrode active material in the deep portion may actually decrease. Therefore, the present disclosure specifies the tortuosity ratio "τ1 / τ2" to be greater than 1.0 and less than 4.4.

[0009] 2. The battery electrode described in the above item "1" may include, for example, the following configuration: The area fraction of the binder in the first region is 2.9% or more, and the area fraction of the binder in the second region is 1.0% or less.

[0010] The relatively large amount of binder in the first region (deep part) is expected to improve adhesive strength. The relatively small amount of binder in the second region (shallow part) tends to make the ratio of the curvatures in the second region, "τ1 / τ2," more than 1.

[0011] 3. The battery electrode according to the above item "1" or "2" may have, for example, the following configuration: The graphite includes artificial graphite, and 80% or more of the graphite by mass fraction relative to the total amount of graphite has an aspect ratio of 1.6 or less.

[0012] By ensuring that the proportion of artificial graphite with an aspect ratio of 1.6 or less is 80% or more, improvement in rate characteristics is expected.

[0013] 4. The battery electrode according to any one of the above items "1" to "3" may include, for example, the following configuration: The negative electrode active material layer has a thickness of "0.033≦I (110) / I (002) " relationship is satisfied. "I (110) " indicates the diffraction intensity of the (110) plane in the X-ray diffraction profile of the negative electrode active material layer. (002) " indicates the diffraction intensity of the (002) plane in the X-ray diffraction profile of the negative electrode active material layer.

[0014] Ratio of diffraction intensities "I (110) / I (002) " is an index of the orientation state. Hereinafter, the ratio of the diffraction intensities "I (110) / I (002) " is also referred to as "degree of orientation." It is believed that the higher the degree of orientation, the more the long axis of the graphite particles is aligned in the thickness direction of the negative electrode active material layer. When the degree of orientation is 0.033 or more, improved rate characteristics are expected. For example, when a magnetic field is applied during drying of the slurry, the graphite can be oriented in the thickness direction of the negative electrode active material layer.

[0015] 5. The battery electrode according to any one of the above items "1" to "4" may have the following configuration: In a cross section, the area ratio of the second region to the entire negative electrode active material layer is 50 to 70%.

[0016] When the area fraction of the second region is between 50 and 70%, an improvement in the rate characteristic is expected.

[0017] Hereinafter, one embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment") and one example of the present disclosure (hereinafter may be abbreviated as "this example") will be described. However, this embodiment and this example do not limit the technical scope of the present disclosure. This embodiment and this example are illustrative in all respects. This embodiment and this example are non-restrictive. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the claims. For example, it is originally intended that any configurations may be extracted from this embodiment and arbitrarily combined. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic cross-sectional view showing an example of a battery electrode according to an embodiment of the present invention. [Figure 2] 10 is a table showing experimental results. DETAILED DESCRIPTION OF THE INVENTION

[0019] -Terms and phrases- Geometric terms should not be interpreted in a strict sense. Examples of geometric terms include "parallel," "perpendicular," and the like. For example, directions, angles, distances, and the like may be displaced relative to one another as long as substantially the same or similar functions are obtained. Geometric terms may include, for example, tolerances, errors, and the like in design, work, manufacturing, and the like. The dimensional relationships in each figure may not match the actual dimensional relationships. The dimensional relationships in each figure may be changed to aid the reader's understanding. For example, length, width, thickness, and the like may be changed. Some components may be omitted.

[0020] Numerical ranges such as "m to n%" include the upper and lower limits unless otherwise specified. That is, "m to n%" indicates a numerical range of "m% or more and n% or less." Furthermore, "m% or more and n% or less" includes "more than m% and less than n%." "Equal to or more" and "equal to or less" are represented by inequality signs with an equal sign "≦, ≧." "More than" and "less than" are represented by inequality signs without an equal sign "<, >." A numerical value arbitrarily selected from within the numerical range may be used as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described elsewhere in this specification, in a table, a figure, or the like.

[0021] All numerical values ​​are modified by the term "about." The term "about" may mean, for example, ±5%, ±3%, ±1%, etc. All numerical values ​​may be approximate values ​​that may vary depending on the application of the disclosed technology. All numerical values ​​may be expressed with significant figures. Unless otherwise specified, measured values ​​may be average values ​​of multiple measurements. The number of measurements may be three or more, five or more, or ten or more. In general, the more measurements are made, the more reliable the average value is expected to be. Measured values ​​may be rounded to the nearest significant figure. Measured values ​​may include errors, such as those associated with the detection limits of the measuring device.

[0022] The "weight" of the negative electrode active material layer indicates the mass per unit area of ​​the negative electrode active material layer. 2 " is used as a unit of basis weight. The "density" of the negative electrode active material layer indicates the apparent density of the negative electrode active material layer. The apparent density is determined by dividing the basis weight of the negative electrode active material layer by the thickness of the negative electrode active material layer. "g / cm 3 " is used as a unit of density. The basis weight and density are measured at five or more points in the negative electrode active material layer. The arithmetic average of the measurements at five or more points is used.

[0023] The "degree of curvature" of each region is measured as follows: A focused ion beam scanning electron microscope (FIB-SEM) is prepared. The sample (negative electrode active material layer) is sliced ​​once every 50 nm using an FIB, and cross-sectional SEM images (tomographic images) are obtained by performing SEM scanning. This operation is repeated multiple times. A three-dimensional structure is reconstructed from all the acquired tomographic images, thereby obtaining a 3D image of the negative electrode active material layer. The 3D image is analyzed using simulation software "GeoDict" (manufactured by Math2Market GmbH), and the degrees of curvature "τ1, τ2" of each region are determined.

[0024] The "area fraction" of the binder is measured by the following method. For example, a binder dyeing process may be performed on a cross-sectional sample of the negative electrode active material layer. For example, styrene butadiene rubber (SBR) may be dyed with osmium oxide. A binder mapping analysis is performed on the cross-sectional sample using SEM-EDX (Energy Dispersive X-ray Spectrometry). Pixels corresponding to the binder in the negative electrode active material layer are counted. The number of pixels corresponding to the binder is divided by the number of pixels in the entire negative electrode active material layer to determine the area fraction of the binder in the negative electrode active material layer. The area fraction is expressed as a percentage (%). The area fraction of the binder in each region can also be measured in a similar manner.

[0025] The "aspect ratio" of graphite is measured by the following method. A cross-sectional sample is prepared by cutting the negative electrode active material layer. The cross-sectional sample includes a cross section parallel to the thickness direction of the negative electrode active material layer. For example, the observation target area may be cleaned using a Cross Section Polisher (registered trademark) or the like. A cross-sectional SEM image is obtained by observing the cross-sectional sample using an SEM. Ten or more graphite particles are randomly extracted from the cross-sectional SEM image. The major axis diameter and minor axis diameter of the extracted particles are measured. The major axis diameter "φ1" indicates the diameter connecting the two most distant points on the outline of the particle. The minor axis diameter "φ2" indicates the largest diameter among the diameters perpendicular to the major axis diameter. The aspect ratio is the ratio of the major axis diameter to the minor axis diameter "φ1 / φ2." The arithmetic average of the aspect ratios of ten or more particles is considered to be the "aspect ratio."

[0026] The "degree of orientation" of the negative electrode active material layer is measured by the following method. The XRD profile of the negative electrode active material layer is measured by XRD (X-ray diffraction). The X-ray source is CuKα radiation. The measurement range is "10°≦2θ≦90°". In the XRD profile, the diffraction peak of the (002) plane can be detected in the range of "25°≦2θ≦30°". The area (integral intensity) of the diffraction peak of the (002) plane is the diffraction intensity "I (002) The diffraction peak of the (110) plane can be detected in the range of "75°≦2θ≦80°". The area of ​​the diffraction peak of the (110) plane is the diffraction intensity "I (110) " The diffraction intensity "I (110) " is the diffraction intensity "I (002) " is divided by ", the degree of orientation "I (110) / I (002) " is required.

[0027] -Battery electrode- One aspect of the present disclosure is a battery electrode. Another aspect of the present disclosure is a battery including the battery electrode. The present disclosure can be applied to any battery. The battery may be, for example, a monopolar battery, a bipolar battery, a non-aqueous battery, or a lithium ion battery.

[0028] FIG. 1 is a schematic cross-sectional view showing an example of a battery electrode according to this embodiment. Hereinafter, the battery electrode may be abbreviated as "electrode." The electrode 200 may be, for example, in a sheet form. The electrode 200 may be, for example, the negative electrode of a monopolar lithium-ion battery. The cross section of FIG. 1 is parallel to the thickness direction (Z direction) of the electrode 200. The electrode 200 includes a substrate 210 and a negative electrode active material layer 220.

[0029] The substrate 210 supports the negative electrode active material layer 220. The substrate 210 may be, for example, sheet-shaped. The thickness of the substrate 210 may be, for example, 1 to 50 μm, 3 to 30 μm, or 5 to 15 μm. The substrate 210 is conductive. The substrate 210 may include, for example, a metal foil. The substrate 210 may include, for example, at least one selected from the group consisting of Cu, Ni, Zn, Pb, Al, Ti, Fe, Ag, Au, and a conductive resin. The substrate 210 may include, for example, a Cu foil or a Cu alloy foil. The substrate 210 may have, for example, a multilayer structure. For example, the substrate 210 may be formed by bonding a Cu foil and an Al foil together.

[0030] The negative electrode active material layer 220 is disposed on the surface of the substrate 210. The negative electrode active material layer 220 may be disposed on only one surface of the substrate 210. The negative electrode active material layer 220 may be disposed on both surfaces of the substrate 210. When the electrode 200 is for a bipolar battery, the negative electrode active material layer 220 may be disposed on one surface (front surface) of the substrate 210, and a positive electrode active material layer (not shown) may be disposed on the other surface (back surface).

[0031] The thickness of the negative electrode active material layer 220 may be, for example, 10 μm or more, 50 μm or more, 100 μm or more, 150 μm or more, 200 μm or more, 300 μm or more, 400 μm or more, or 500 μm or more. The thickness of the negative electrode active material layer 220 may be, for example, 1000 μm or less, 500 μm or less, 400 μm or less, 300 μm or less, or 200 μm or less.

[0032] The weight of the negative electrode active material layer 220 is 25 mg / cm2 The weight per unit area of ​​the negative electrode active material layer 220 is, for example, 30 mg / cm 2 More than 35mg / cm 2 More than 40mg / cm 2 More than 45mg / cm 2 or more than 50 mg / cm 2 The weight per unit area of ​​the negative electrode active material layer 220 may be, for example, 100 mg / cm 2 Below, 75mg / cm 2 Below, 50mg / cm 2 Below 40mg / cm 2 or less than 30 mg / cm 2 It may be the following:

[0033] The density of the negative electrode active material layer 220 is 1.2 to 1.6 g / cm 3 The density is 1.6 g / cm 3 When the density of the negative electrode active material layer 220 exceeds 1.3 g / cm, it may be difficult to achieve a desired degree of bending. 3 More than 1.4g / cm 3 or more, or 1.5g / cm 3 The density of the negative electrode active material layer 220 may be, for example, 1.5 g / cm 3 Below, 1.4g / cm 3 or less than 1.3g / cm 3 It may be the following:

[0034] The negative electrode active material layer 220 includes a first region 221 and a second region 222. The negative electrode active material layer 220 may be composed of the first region 221 and the second region 222. Each region may form a layer. The first region 221 is disposed between the second region 222 and the substrate 210. The first region 221 may be in direct contact with the substrate 210, for example. The first region 221 may include the interface between the substrate 210 and the negative electrode active material layer 220, for example. The second region 222 may include the surface of the negative electrode active material layer 220, for example. That is, the second region 222 may be exposed on the surface of the negative electrode active material layer 220.

[0035] In a cross section of the anode active material layer 220, the area fraction of the second region 222 with respect to the entire anode active material layer 220 may be, for example, 50 to 70%. The area fraction of the second region 222 may be, for example, 60% or more, or 60% or less. In the same cross section, the area fraction of the first region 221 with respect to the entire anode active material layer 220 may be, for example, 30 to 50%. The area fraction of the first region 221 with respect to the entire anode active material layer 220 may be, for example, 40% or more, or 40% or less. Note that, in a cross section of the anode active material layer 220, the area fraction of each region with respect to the entire anode active material layer 220 is considered to be equal to the ratio of the thickness of each region with respect to the thickness of the anode active material layer 220 in the same cross section.

[0036] The negative electrode active material layer 220 may further include additional regions (a third region, a fourth region, etc.) in addition to the first region 221 and the second region 222. The additional regions may be distinguished from the first region 221 and the second region 222, for example, by at least one of their composition and structure. For example, the additional region may be disposed between the substrate 210 and the first region 221. For example, the additional region may be disposed between the first region 221 and the second region 222. For example, the additional region may be disposed between the surface of the negative electrode active material layer 220 and the second region 222.

[0037] The second region 222 has a smaller degree of curvature than the first region 221. The ratio "τ1 / τ2" of the degree of curvature "τ1" of the first region 221 to the degree of curvature "τ2" of the second region 222 is greater than 1.0 and less than 4.4. The ratio of curvature "τ1 / τ2" may be, for example, 1.2 or more, 1.6 or more, 2.0 or more, 2.4 or more, 2.8 or more, 3.2 or more, 3.6 or more, or 4.0 or more. The ratio of curvature "τ1 / τ2" may be, for example, 4.0 or less, 3.6 or less, 3.2 or less, 3.0 or less, less than 3.0, 2.8 or less, 2.4 or less, 2.0 or less, 1.6 or less, or 1.2 or less.

[0038] However, the degree of curvature "τ2" of the second region 222 is less than 2.1. The degree of curvature "τ2" of the second region 222 may be, for example, 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, 1.6 or less, 1.5 or less, 1.4 or less, 1.3 or less, 1.2 or less, or 1.1 or less. The degree of curvature "τ2" of the second region 222 may be, for example, 1 or more, 1.1 or more, 1.2 or more, 1.3 or more, 1.4 or more, 1.5 or more, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more.

[0039] The degree of curvature "τ1" of the first region 221 may be, for example, 2.1 or more, 2.5 or more, 3.0 or more, 3.5 or more, 4.0 or more, 4.5 or more, 5.5 or more, 6.0 or more, 6.5 or more, 7.0 or more, 7.5 or more, or 8.0 or more. The degree of curvature "τ1" of the first region 221 may be, for example, less than 8.5, 8.0 or less, 7.5 or less, 7.0 or less, 6.5 or less, 6.0 or less, 5.5 or less, 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, 3.0 or less, or 2.5 or less.

[0040] The degree of curvature of each region can be adjusted by any method. For example, "separate coating" may be performed. That is, multi-layer coating may be performed using two types of slurries with different compositions. The smaller the binder amount, the smaller the degree of curvature tends to be. For example, a magnetic field may be applied to the coating film (slurry). The magnetic field orients the graphite, which tends to reduce the degree of curvature. For example, the drying temperature of the coating film may be adjusted. The lower the drying temperature (the slower the drying rate), the smaller the degree of curvature tends to be. For example, by appropriately combining these methods, a negative electrode active material layer 220 that satisfies the relationships "1.0<(τ1 / τ2)<4.4" and "τ2<2.1" can be formed.

[0041] The negative electrode active material layer 220 includes graphite 2 and binder 4. The negative electrode active material layer 220 may be composed of, for example, 0.1 to 10% by mass of binder 4, with the remainder being graphite 2. The cross section of the negative electrode active material layer 220 may be composed of, for example, 1.9% or more by area of ​​binder 4, with the remainder being graphite 2. In addition to the graphite 2 and the binder 4, the negative electrode active material layer 220 may further include, for example, a conductive material, a thickener, an inorganic filler, etc.

[0042] The first region 221 and the second region 222 each independently contain graphite 2 and binder 4. The graphite 2 contained in the first region 221 may be the same as or different from the graphite 2 contained in the second region 222. The binder 4 contained in the first region 221 may be the same as or different from the binder 4 contained in the second region 222. Graphite is a negative electrode active material. Each of the first region 221 and the second region 222 may further contain an additional negative electrode active material in addition to graphite. Each of the first region 221 and the second region 222 may contain, for example, at least one selected from the group consisting of silicon (Si), silicon oxide (SiO), a silicon-carbon composite material (Si-C), a silicon-based alloy, tin, tin oxide, and lithium titanate. The mass fraction of the other negative electrode active materials relative to the total negative electrode active materials may be, for example, 50% or less, 40% or less, 30% or less, 20% or less, 10% or less, 5% or less, or 1% or less.

[0043] The graphite may be natural graphite or artificial graphite. The surface of the graphite may be coated with a carbon material. The carbon material may include, for example, soft carbon, hard carbon, amorphous carbon, low-crystalline carbon, etc. The D50 of the graphite may be, for example, 1 μm or more, 5 μm or more, or 10 μm or more. The D50 of the graphite may be, for example, 30 μm or less, 20 μm or less, 15 μm or less, or 10 μm or less. "D50" refers to the particle size at which the cumulative total is 50% in a volume-based particle size distribution (cumulative distribution). The particle size distribution can be measured by a laser diffraction method.

[0044] The aspect ratio of the graphite may be, for example, 1 to 4. The aspect ratio of the graphite may be, for example, 1.2 or more, 1.4 or more, 1.6 or more, 2.0 or more, 2.3 or more, 2.8 or more, 3.2 or more, or 3.6 or more. The aspect ratio of the graphite may be, for example, 3.6 or less, 3.2 or less, 2.8 or less, 2.3 or less, 2.0 or less, 1.6 or less, 1.4 or less, or 1.2 or less. For example, 80% or more of the graphite, by mass fraction, relative to the total amount of graphite, may have an aspect ratio of 1.6 or less. The proportion of the graphite having an aspect ratio of 1.6 or less may be, for example, 85% or more, 90% or more, or 95% or more. The proportion of the graphite having an aspect ratio of 1.6 or less may be, for example, 100% or less, 95% or less, 90% or less, or 85% or less.

[0045] Graphite orientation "I (110) / I (002) " may be, for example, 0.033 or more. The degree of orientation may be, for example, 0.050 or more, or 0.075 or more. The degree of orientation may be, for example, 0.100 or less, 0.075 or less, or 0.050 or less.

[0046] The binder 4 may contain, for example, at least one selected from the group consisting of SBR, acrylate butadiene rubber (ABR), polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), acrylic resin (acrylic acid ester copolymer), methacrylic resin (methacrylic acid ester copolymer), polyvinyl alcohol (PVA), and derivatives thereof.

[0047] In the cross section of the negative electrode active material layer 220, the area fraction of the binder 4 relative to the entire negative electrode active material layer 220 is 1.9% or more. This area fraction may be, for example, 2.5% or more, 3.0% or more, 3.5% or more, 4.0% or more, 4.5% or more, or 5.0% or more. This area fraction may be, for example, 10% or less, 7.5% or less, 7.0% or less, 6.5% or less, 6.0% or less, 5.5% or less, 5.0% or less, 4.5% or less, 4.0% or less, 3.5% or less, 3.0% or less, or 2.5% or less.

[0048] In the negative electrode active material layer 220, the binder 4 may be uniformly distributed, for example. In the negative electrode active material layer 220, the binder 4 may be distributed in a gradational manner, for example. For example, the area fraction of the binder 4 in the first region 221 may be higher than the area fraction of the binder 4 in the second region 222. The area fraction of the binder 4 in the first region 221 may be 2.9% or more, for example. The area fraction of the binder 4 in the first region 221 may be 3.0% or more, 3.5% or more, 4.0% or more, 4.5% or more, or 5.0% or more, for example. The area fraction of the binder 4 in the first region 221 may be, for example, 10% or less, 7.5% or less, 7.0% or less, 6.5% or less, 6.0% or less, 5.5% or less, 5.0% or less, 4.5% or less, 4.0% or less, 3.5% or less, 3.0% or less, or 2.5% or less. The area fraction of the binder 4 in the second region 222 may be, for example, 1.0% or less. The area fraction of the binder 4 in the second region 222 may be, for example, 0.9% or less, 0.8% or less, 0.7% or less, 0.6% or less, 0.5% or less, 0.4% or less, 0.3% or less, or 0.2% or less. The area fraction of the binder 4 in the second region 222 may be, for example, 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, 0.5% or more, 0.6% or more, 0.7% or more, 0.8% or more, or 0.9% or more. [Example]

[0049] - Electrode (negative electrode) manufacturing - No.1 The following materials were prepared: Negative electrode active material: artificial graphite (SG), aspect ratio: 1.6 Binder: SBR Thickener: Carboxymethyl cellulose (CMC) Dispersion medium: water Substrate: Cu foil (thickness: 15 μm)

[0050] In No. 1, "separate coating" was carried out. SG, SBR, CMC, and water were mixed to prepare a first slurry. The solid content ratio was "SG / CMC / SBR = 97 / 0.6 / 2.4 (mass ratio)." The first slurry was applied to the substrate to form a first region (deep part). The first region had a basis weight of 14 mg / cm after drying. 2 It was formed so that

[0051] The second slurry was prepared by mixing SG, SBR, CMC, and water. The solids ratio was SG / CMC / SBR = 98.6 / 0.6 / 0.8 (mass ratio). The second slurry was applied on top of the first slurry to form a second region (shallow part). The second region had a basis weight of 14 mg / cm2 after drying. 2 That is, the total basis weight was 28 mg / cm 2 is.

[0052] A magnetic field was applied to the coating film (first region and second region). After the application of the magnetic field, the coating film was dried to form a negative electrode active material layer. The drying temperature (hot air temperature) was 50°C. The negative electrode active material layer was compressed to produce an electrode (negative electrode). After compression, the density of the negative electrode active material layer was 1.2 g / cm 3 It was.

[0053] No.2 An electrode was prepared similar to No. 1, except that no magnetic field was applied to the coating.

[0054] No.3 In No. 3, a negative electrode active material layer was formed without separate coating. SG, SBR, CMC, and water were mixed to prepare a slurry. The solid content was SG / CMC / SBR=98.8 / 0.6 / 1.6 (mass ratio). The slurry was applied to the substrate to form a coating film. A magnetic field was applied to the coating film. After the magnetic field was applied, the coating film was dried to form a negative electrode active material layer. The weight per unit area after drying was 28 mg / cm. 2 After drying, the negative electrode active material layer was compressed to produce an electrode. After compression, the density of the negative electrode active material layer was 1.2 g / cm 3 It was.

[0055] No.4 An electrode was fabricated similarly to No. 1, except that an SG with an aspect ratio of 2.3 was used.

[0056] No.5 An electrode was prepared similar to No. 3, except that no magnetic field was applied to the coating.

[0057] No.6 An electrode was prepared in the same manner as No. 5, except that the drying temperature was changed to 25°C.

[0058] No.7 An electrode was fabricated in the same manner as No. 1, except that the amount of binder in the first slurry was increased.

[0059] -Manufacturing of evaluation cells- The following materials were prepared: Cathode active material: LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM) Conductive material: Acetylene black (AB) Binder: PVdF Dispersion medium: N-methyl-pyrrolidone (NMP) Base material: Al foil (thickness: 30 μm) Separator: PE porous sheet Electrolyte: LiPF6 (concentration: 1.0mol / L), EC+DMC+EMC Exterior: Pouch made of aluminum laminated film

[0060] NCM, AB, PVdF, and NMP were mixed to prepare a slurry. The solids were mixed in a ratio of NCM / AB / PVdF = 97.8 / 0.8 / 1.4 (mass ratio). The slurry was applied to a substrate to form a positive electrode active material layer. The positive electrode active material layer was dried. The positive electrode active material layer was compressed to produce a positive electrode.

[0061] The positive electrode, separator, and negative electrode were stacked in this order to form a power generating element, and the power generating element and electrolyte were sealed in an outer casing to produce an evaluation cell (rated capacity: 155 mA).

[0062] -evaluation- Figure 2 is a table showing the experimental results. The discharge capacity of the evaluation cell was measured at a rate of 0.1C and a rate of 1C. "C" is the symbol representing the rate. At a rate of 1C, the rated capacity is passed over one hour. The discharge capacity at a rate of 1C (1C discharge capacity) was divided by the discharge capacity at a rate of 0.1C (0.1C discharge capacity) to calculate "1C discharge capacity / 0.1C discharge capacity." The larger the "1C discharge capacity / 0.1C discharge capacity," the better the rate characteristics.

[0063] -result- No.3 The application of a magnetic field (orientation of graphite) tends to reduce the degree of curvature, but a degree of curvature of less than 2.1 has not been achieved.

[0064] No.6 Lowering the drying temperature tends to improve binder migration and therefore reduce the degree of bending, but a degree of bending of less than 2.1 has not been achieved.

[0065] No.2 The degree of tortuosity can be locally reduced by coating different layers, but a degree of tortuosity of less than 2.1 has not been achieved.

[0066] No.1 When the conditions "1.0<(τ1 / τ2)<4.4" and "τ2<2.1" are met, there is a tendency for the rate characteristics to improve. [Explanation of symbols]

[0067] 2 graphite, 4 binder, 200 electrode, 210 substrate, 220 negative electrode active material layer, 221 first region, 222 second region.

Claims

1. a substrate and a negative electrode active material layer, the negative electrode active material layer is disposed on a surface of the substrate, the negative electrode active material layer contains graphite and a binder, The negative electrode active material layer has a thickness of 25 mg / cm 2 or more, and 1.2 to 1.6 g / cm 3 and has a density of an area fraction of the binder in an entire cross section parallel to a thickness direction of the negative electrode active material layer is 1.9% or more; the cross-section includes a first region and a second region; the first region is disposed between the second region and the substrate; The relationships 1.0<(τ1 / τ2)<4.4 and τ2<2.1 are satisfied, The τ1 indicates the degree of bending in the first region, and The τ2 indicates the degree of bending in the second region, Electrodes for batteries.

2. The area fraction of the binder in the first region is 2.9% or more, and an area fraction of the binder in the second region is 1.0% or less; The battery electrode according to claim 1 .

3. The graphite includes artificial graphite, and 80% or more of the graphite by mass fraction relative to the total amount of the graphite has an aspect ratio of 1.6 or less. The battery electrode according to claim 1 or 2.

4. The negative electrode active material layer has a value of 0.033≦I (110) / I (002) Fulfilling the relationship, I (110) indicates a diffraction intensity of the (110) plane in the X-ray diffraction profile of the negative electrode active material layer, and I (002) indicates the diffraction intensity of the (002) plane in the X-ray diffraction profile of the negative electrode active material layer, The battery electrode according to claim 1 or 2.

5. In the cross section, an area fraction of the second region relative to the entire negative electrode active material layer is 50 to 70%; The battery electrode according to claim 1 or 2.

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