Electrode

Abstract

The present disclosure is an electrode. The 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 contains first graphite particles, second graphite particles, and Si particles. The first graphite particles have an aspect ratio of 6-20. The second graphite particles have an aspect ratio of 2.7 or less. In a cross section parallel to the thickness direction of the negative electrode active material layer, the orientation angle is 58 DEG or more.

Description

Technical Field

[0001] The present disclosure relates to an electrode. Background Art

[0002] Japanese Patent Application Laid-Open No. 2016-103347 discloses a negative electrode including carbon particles and non-carbon particles. Summary of the Invention

[0003] There is room for improvement in the cycle characteristics of a mixed system of graphite particles and silicon-containing particles (hereinafter also referred to as "Si particles"). During charging, each particle may expand. During discharging, each particle may contract. The expansion behavior and the contraction behavior may differ depending on the type of particle. Since the expansion and contraction of each particle are repeated, the arrangement of each particle may change. Due to the change in the arrangement, for example, there is a possibility that the conductive path is interrupted and particles isolated from the conductive network are generated. As a result, it may not be possible to obtain the desired cycle characteristics.

[0004] An object of the present disclosure is to improve the cycle characteristics.

[0005] 1. An electrode including a substrate and a negative electrode active material layer.

[0006] The negative electrode active material layer is disposed on the surface of the substrate.

[0007] The negative electrode active material layer includes first graphite particles, second graphite particles, and silicon-containing particles.

[0008] The first graphite particles have an aspect ratio of 6 to 20.

[0009] The second graphite particles have an aspect ratio of 2.7 or less.

[0010] In a cross section of the negative electrode active material layer parallel to the thickness direction, the orientation angle is 58° or more.

[0011] The orientation angle represents the average value of a first angle and a second angle.

[0012] The first angle represents the angle formed by the major axis of the first graphite particles and the surface of the substrate.

[0013] The second angle represents the angle formed by the major axis of the second graphite particles and the surface of the substrate.

[0014] The orientation angle is an index of the orientation state of the graphite particles in the negative electrode active material layer. The orientation angle can take a value of 0° to 90°. It can be considered that the larger the orientation angle, the more the major axis of the graphite particles is along the thickness direction of the negative electrode active material layer. For example, when forming the negative electrode active material layer, by applying a magnetic field to the graphite particles, the graphite particles can be oriented. By orienting the graphite particles so that the orientation angle becomes 58° or more, it is possible to expect to promote ion conduction in the thickness direction.

[0015] Moreover, the negative electrode active material layer contains two types of graphite particles. The first graphite particles relatively have a large particle size and a high aspect ratio. The second graphite particles relatively have a small particle size and a low aspect ratio. By the orientation of the first graphite particles, relatively large voids may be formed between the first graphite particles. When the first graphite particles exist alone, due to the volume change during charge and discharge, the position of the first graphite particles may move toward the adjacent void side. When the first graphite particles are oriented in the thickness direction, the voids are adjacent to the first graphite particles in the in-plane direction. Therefore, the first graphite particles will transfer in the in-plane direction. By the transfer of the first graphite particles in the in-plane direction, adverse conditions such as interruption of the conductive path may occur. By the presence of the second graphite particles in addition to the first graphite particles, it is expected that the second graphite particles enter the voids between the first graphite particles. By disposing the second graphite particles in the voids, it is expected to hinder the transfer of the first graphite particles.

[0016] Moreover, Si particles can also be disposed in the voids. During charging, the Si particles may expand rapidly. The voids formed by the combination of the first graphite particles and the second graphite particles can absorb the rapid expansion of the Si particles. Therefore, during charging, the expansion of the entire negative electrode active material layer can be alleviated. During discharging, the Si particles may contract rapidly. Since the Si particles and the second graphite particles are adjacent to each other in the voids, it is considered that it is difficult for the particles to be isolated and the structure to collapse. By the complementarity of the above actions, an improvement in the cycle characteristics can be expected.

[0017] 2. The electrode according to the above "1" may include, for example, the following configuration. This is because, according to this configuration, there is a possibility of improving the cycle characteristics.

[0018] The first graphite particles have a particle size of 27 to 66 μm.

[0019] 3. The electrode according to the above "1" or "2" may include, for example, the following configuration. This is because, according to this configuration, there is a possibility of improving the cycle characteristics.

[0020] The second graphite particles have a particle size of 5 to 22 μm.

[0021] 4. The electrode according to any one of the above "1" to "3" may include, for example, the following configuration. This is because, according to this configuration, there is a possibility of improving the cycle characteristics.

[0022] The ratio of the particle size of the first graphite particles to the particle size of the second graphite particles is 2.58 to 11.60.

[0023] 5. The electrode according to any one of the above “1” to “4” may include the following configuration, for example. This is because there is a possibility of improving the cycle characteristics according to this configuration.

[0024] The negative electrode active material layer contains silicon-containing particles with a mass fraction of 25% or less. The silicon-containing particles have a particle size of 11 μm or less.

[0025] Hereinafter, embodiments of the present disclosure (hereinafter abbreviated as “the present embodiment”) and examples of the present disclosure (hereinafter abbreviated as “the present example”) will be described. However, the present embodiment and the present example do not limit the technical scope of the present disclosure. The present embodiment and the present example are illustrative in all aspects. The present embodiment and the present example are non-restrictive. The technical scope of the present disclosure includes all changes within the meaning equivalent to the description in the claims and within the scope. For example, extracting any configurations from the present embodiment and combining them arbitrarily are also predetermined from the beginning. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The features, advantages, and technical and industrial significance of typical embodiments of the present invention will be described below with reference to the accompanying drawings, in which the same reference numerals represent the same elements, wherein:

[0027] Figure 1 is an explanatory diagram of the orientation angle;

[0028] Figure 2 is a conceptual diagram of the electrode in the present embodiment;

[0029] Figure 3 is a table showing the experimental results. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] MAIN TERMS

[0031] “Aspect ratio” represents the ratio of the major axis diameter to the minor axis diameter. The major axis diameter represents the maximum Feret diameter. The minor axis diameter represents the minimum Feret diameter. The maximum Feret diameter and the minimum Feret diameter of each particle are measured in a cross-sectional scanning electron microscope (SEM) image of the negative electrode active material layer. The maximum Feret diameter and the minimum Feret diameter can be determined by image analysis software. The cross-section is parallel to the thickness direction of the negative electrode active material layer. The observation magnification can be adjusted according to the size of the particles. The observation magnification is, for example, 700 times. Graphite particles having an aspect ratio of 6 to 20 are regarded as “first graphite particles”. Graphite particles having an aspect ratio of 2.7 or less are regarded as “second graphite particles”.

[0032] The "orientation angle" is measured through the following steps. Prepare cross-sectional SEM images of 5 negative electrode active material layers. The 5 cross-sectional SEM images are taken at separate positions. Figure 1 It is an explanatory diagram of the orientation angle. In the image, determine the major axis L of the particle. The major axis L is a straight line passing through the maximum Feret diameter F max of the particle. When the particle is the first graphite particle, the angle θ formed by the major axis L and the surface of the substrate 10 is the first angle. When the particle is the second graphite particle, the formed angle θ is the second angle. The formed angle θ can take values from 0° to 90°. The formed angle θ can be determined by image analysis software. In the 5 cross-sectional SEM images, the average value of the angle θ (the first angle) formed by the first graphite particles in the image and the angle θ (the second angle) formed by the second graphite particles is regarded as the orientation angle.

[0033] Unless otherwise specified, elements expressed in the singular form also include the plural form. For example, "particle" includes not only "1 particle" but also "multiple particles (particle group)" and "aggregate of particles (powder)".

[0034] "Particle size" represents the average value of the maximum Feret diameter. In the 5 cross-sectional SEM images, the average value of the maximum Feret diameter of the first graphite particles in the image is regarded as the "particle size of the first graphite particles". The same applies to the particle sizes of the second graphite particles and Si particles.

[0035] "Silicon-containing particle (Si particle)" means a particle containing Si. The Si particle can contain, for example, at least one selected from Si, SiO, Si-based alloy, and Si-C. "Si-C" represents a composite material containing Si and carbon (C). In Si-C, Si may or may not form a compound with C. C can be amorphous or crystalline.

[0036] The stoichiometric composition formula represents a representative example of a compound. The compound may also have a non-stoichiometric composition. For example, "SiO" is not limited to a compound having a molar ratio (mole ratio) of "Si:O = 1:1". Unless otherwise specified, "SiO" means a compound containing Si and O in an arbitrary molar ratio. For example, trace elements may be doped in the compound. A part of Si and O may also be replaced with other elements.

[0037] Geometric terms should not be construed in a strict sense. As geometric terms, for example, "parallel", "perpendicular", "orthogonal", etc. can be exemplified. For example, "parallel" can also deviate slightly from "parallel" in the strict sense. Geometric terms can include tolerances, errors, etc. in design, operation, manufacturing, etc. The dimensional relationships in each figure are sometimes inconsistent with the actual dimensional relationships. In order to help readers understand, the dimensional relationships in each figure are sometimes changed. For example, the length, width, thickness, etc. are sometimes changed. Sometimes a part of the composition is also omitted.

[0038] Unless otherwise specified, numerical ranges such as "m to n%" include the upper and lower limits. That is, "m to n%" represents a numerical range of "m% or more and n% or less". In addition, "m% or more and n% or less" includes "more than m% and less than n%". "Or more" and "or less" can be represented by the inequality sign "≤" with an equal sign. "More than" and "less than" can be represented by the inequality sign "<" without an equal sign.

[0039] Electrode

[0040] Figure 2 is a conceptual diagram showing the electrode in this embodiment. In Figure 2 it, the Z-axis direction is the thickness direction. The X-axis direction and the Y-axis direction are in-plane directions respectively. The electrode 100 is an electrode for a battery. The electrode 100 can be, for example, a negative electrode for a single-pole battery. The electrode 100 can also be, for example, for a bipolar battery. The electrode 100 can be, for example, an electrode for a liquid-based lithium-ion battery. The electrode 100 can be, for example, an electrode for an all-solid-state lithium-ion battery. The electrode 100 includes a substrate 10 and a negative electrode active material layer 20.

[0041] Substrate

[0042] The substrate 10 supports the negative electrode active material layer 20. The substrate 10 can be, for example, sheet-shaped. The thickness of the substrate 10 can be, for example, 1 to 50 μm, or 5 to 30 μm. The substrate 10 has conductivity. The substrate 10 can include, for example, a metal foil, etc. The substrate 10 can include at least one selected from Cu, Ni, Zn, Pb, Al, Ti, Fe, Ag, Au, and a conductive resin. The substrate 10 can include, for example, a Cu foil, a Cu alloy foil, etc. The substrate 10 can have a multilayer structure. For example, the substrate 10 can be formed by laminating a Cu foil and an Al foil.

[0043] Negative electrode active material layer

[0044] The negative electrode active material layer 20 is disposed on the surface of the substrate 10. The negative electrode active material layer 20 may be disposed only on one side of the substrate 10. The negative electrode active material layer 20 may also be disposed on both sides of the substrate 10. When the electrode 100 is an electrode for a bipolar battery, a negative electrode active material layer 20 may be disposed on one side (the front surface) of the substrate 10 and a positive electrode active material layer (not shown) may be disposed on the other side (the back surface). The thickness of the negative electrode active material layer 20 may be, for example, 10 to 1000 μm, or 100 to 500 μm.

[0045] The negative electrode active material layer 20 contains first graphite particles 21, second graphite particles 22, and Si particles 23. The first graphite particles 21, the second graphite particles 22, and the Si particles 23 are respectively negative electrode active materials. The first graphite particles 21 and the second graphite particles 22 contain graphite. The graphite may be artificial graphite or natural graphite. The first graphite particles 21 and the second graphite particles 22 only need to contain graphite, and for example, may further contain low-crystalline carbon, amorphous carbon, etc. Sometimes the first graphite particles 21 and the second graphite particles 22 contain components other than graphite. In this case, the mass fraction of graphite in the first graphite particles 21 and the second graphite particles 22 may be, for example, 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more.

[0046] Mixing ratio

[0047] The mixing ratio (mass ratio) of the first graphite particles 21 to the second graphite particles 22 may be, for example, "first graphite particles: second graphite particles = 9:1" to "first graphite particles: second graphite particles = 1:9". The mixing ratio may also be, for example, "first graphite particles: second graphite particles = 9:1" to "first graphite particles: second graphite particles = 5:5". The mixing ratio may also be, for example, "first graphite particles: second graphite particles = 8:2" to "first graphite particles: second graphite particles = 6:4".

[0048] Aspect ratio (AR1) of the first graphite particles

[0049] The first graphite particles 21 have an aspect ratio (AR1) of 6 to 20. The aspect ratio (AR1) may be, for example, 8 or more, 9 or more, 11 or more, or 19 or more. The aspect ratio (AR1) may be, for example, 19 or less, 11 or less, 9 or less, or 8 or less.

[0050] Particle size (d1) of the first graphite particles

[0051] The first graphite particle 21 may have a particle size (d1) of, for example, 27 to 66 μm. The particle size (d1) may be, for example, 33 μm or more, 34 μm or more, 46 μm or more, 57 μm or more, or 58 μm or more. The particle size (d1) may be, for example, 58 μm or less, 57 μm or less, 46 μm or less, 34 μm or less, or 33 μm or less. The particle size (d1) may be, for example, 30 to 50 μm.

[0052] Aspect ratio (AR2) of the second graphite particle

[0053] The second graphite particle 22 has an aspect ratio (AR2) of 2.7 or less. The aspect ratio (AR2) may be, for example, 2.2 or less, 2.1 or less, 1.9 or less, or 1.6 or less. The aspect ratio (AR2) may be, for example, 1 or more, 1.2 or more, 1.4 or more, or 1.6 or more.

[0054] Particle size (d2) of the second graphite particle

[0055] The second graphite particle 22 has a particle size (d2) of, for example, 5 to 22 μm. The particle size (d2) may be, for example, 18 μm or less, 16 μm or less, 14 μm or less, 12 μm or less, 10 μm or less, or 6 μm or less. The particle size (d2) may be, for example, 1 μm or more, 3 μm or more, 6 μm or more, 10 μm or more, or 12 μm or more. The particle size (d2) may be, for example, 5 to 12 μm.

[0056] Particle size ratio (d1 / d2)

[0057] The particle size ratio (d1 / d2) is the ratio of the particle size (d1) of the first graphite particle 21 to the particle size (d2) of the second graphite particle 22. The particle size ratio (d1 / d2) may be, for example, 2.58 to 11.60. The particle size ratio (d1 / d2) may be, for example, 2.83 or more, 3.40 or more, 4.83 or more, 5.50 or more, or 11.40 or more. The particle size ratio (d1 / d2) may be, for example, 11.40 or less, 5.50 or less, 4.83 or less, 3.40 or less, or 2.83 or less. The particle size ratio (d1 / d2) may be, for example, 3.30 to 7.25.

[0058] Orientation angle

[0059] In the negative electrode active material layer 20, the orientation angle is 58° or more. The orientation angle may be, for example, 59° or more, 61° or more, 62° or more, 64° or more, or 66° or more. The orientation angle may be, for example, 90° or less, 80° or less, 70° or less, 66° or less, 64° or less, or 62° or less.

[0060] Mass fraction of Si particles

[0061] The negative electrode active material layer 20 contains Si particles 23 with a mass fraction of, for example, 25% or less. The mass fraction of the Si particles 23 can be, for example, 13% or less, or 3.5% or less. The mass fraction of the Si particles 23 can be, for example, 1% or more, 2% or more, or 3.5% or more.

[0062] Particle size (d3) of the Si particles

[0063] The Si particles 23 can have a particle size (d3) of 11 μm or less, for example. The particle size (d3) can be, for example, 6 μm or less, or 1.5 μm or less. The particle size (d3) can be, for example, 0.5 μm or more, 1 μm or more, or 1.5 μm or more.

[0064] Other components

[0065] The negative electrode active material layer may contain, in addition to the negative electrode active material, a conductive material, a thickening material, an adhesive, etc. The conductive material can form an electron conduction path. The conductive material can contain, for example, at least one selected from acetylene black (AB), Ketjen black (registered trademark), vapor grown carbon fiber (VGCF), carbon nanotube (CNT), and graphene flake (GF). The compounding amount of the conductive material relative to 100 parts by mass of the negative electrode active material can be, for example, 0.1 to 10 parts by mass.

[0066] The thickening material can impart viscosity to the negative electrode paste. The thickening material can contain, for example, at least one selected from sodium alginate, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylpyrrolidone (PVP). The compounding amount of the thickening material relative to 100 parts by mass of the negative electrode active material can be, for example, 0.1 to 10 parts by mass.

[0067] The adhesive can bind solids to each other. The adhesive can contain, for example, at least one selected from styrene-butadiene rubber (SBR), acrylate-butadiene rubber (ABR), polyacrylonitrile (PAN), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), acrylic resin (acrylate copolymer), methacrylic resin (methacrylate copolymer), and polyvinyl alcohol (PVA). The compounding amount of the adhesive relative to 100 parts by mass of the negative electrode active material can be, for example, 0.1 to 10 parts by mass.

[0068] Fabrication of the specimen

[0069] Fabrication of the electrode

[0070] Figure 3This is a table showing the experimental results. Electrodes numbered 1 to 14 were fabricated through the following steps. A graphite mixture was prepared by mixing the first graphite particles and the second graphite particles at a "7:3 (mass ratio)". A negative electrode paste was prepared by mixing the graphite mixture, Si particles, SBR, CMC, a conductive material, and ion-exchanged water. The composition of the solid components was "graphite mixture: Si particles: SBR: CMC: conductive material = (98.3 - a - b): a: 0.5: 1.2: b (mass ratio)". As the substrate, a Cu foil (thickness: 10 μm, long strip) was prepared. A coating film was formed by coating the negative electrode paste on both sides of the substrate. A magnetic field was applied to the coating film by passing the coating film through the gap between a pair of neodymium magnets. The magnetic flux density of the magnets was 0.5 T. After applying the magnetic field, the coating film was dried, thereby fabricating the negative electrode active material layer. The electrode (negative electrode sheet) was fabricated by compressing the negative electrode active material layer. In the negative electrode sheet, the orientation angle and the like were measured through the aforementioned steps.

[0071] Fabrication of the evaluation unit

[0072] The evaluation unit (lithium-ion battery) was fabricated through the following steps. A positive electrode paste was prepared by mixing LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 (particle size: 5 μm), AB, PVdF, and N-methyl-2-pyrrolidone. The composition of the solid components was "LiNi 1 / 3Mn 1 / 3 Co 1 / 3 O2: AB: PVdF = 92: 5: 3 (mass ratio)". As the substrate, an Al foil (thickness: 15 μm, long strip) was prepared. A coating film was formed by coating the positive electrode paste on both sides of the substrate. The positive electrode active material layer was fabricated by drying the coating film. The counter electrode (positive electrode sheet) was fabricated by compressing the positive electrode active material layer.

[0073] A separator was prepared. The separator included a resin porous membrane and a heat-resistant layer. The resin porous membrane (thickness: 24 μm) had a three-layer structure (polyethylene layer / polypropylene layer / polyethylene layer). The heat-resistant layer (thickness: 4 μm) was formed on one side of the resin porous membrane.

[0074] A laminate was formed by laminating the positive electrode sheet, the separator, the negative electrode sheet, and the separator. A wound-type power generation element was formed by winding the laminate into a spiral shape. The power generation element was formed into a flat shape by flattening it in the radial direction. External terminals were connected to the power generation element. The power generation element was housed in a metal case. An electrolyte was injected into the metal case. After injecting the electrolyte, the metal case was sealed. The evaluation unit was fabricated according to the above. The composition of the electrolyte is as follows.

[0075] Electrolyte composition

[0076] Solvent: "Ethylene carbonate : Dimethyl carbonate : Ethyl methyl carbonate = 3 : 3 : 4 (volume ratio)"

[0077] Solute: LiPF6 (Concentration: 1 mol / L)

[0078] Activation treatment

[0079] The first charge and discharge were carried out under the following conditions.

[0080] Ambient temperature: 25°C

[0081] Charging: Constant current - constant voltage method, Current during constant current: 1 / 3C, Voltage during constant voltage: 4.2V, Cut-off current: 1 / 50C

[0082] Discharging: Constant current method, Current: 1 / 3C, Cut-off voltage: 3V

[0083] Evaluation

[0084] Measurement of initial capacity

[0085] The initial capacity (initial discharge capacity) was measured by charge and discharge under the following conditions.

[0086] Charging: Constant current - constant voltage method, Current during constant current: 1 / 3C, Voltage during constant voltage: 4.1V, Cut-off current: 1 / 50C

[0087] Discharging: Constant current method, Current: 1 / 3C, Cut-off voltage: 3V

[0088] Cycle characteristics

[0089] The charge and discharge under the following conditions were taken as 1 cycle, and the charge and discharge were carried out for 300 cycles.

[0090] Ambient temperature: 25°C

[0091] Current: 0.5C

[0092] Range of state of charge (SOC): 0% - 100%

[0093] After 300 cycles, the capacity after cycling was measured in the same way as the initial capacity. The capacity retention rate was obtained by dividing the capacity after cycling by the initial capacity. It can be considered that the higher the capacity retention rate, the better the cycle characteristics.

[0094] Results

[0095] At Figure 3In the table, compared with No.1 to No.7, the cycle characteristics of No.8 to No.14 are improved. No.8 to No.14 satisfy all of the following conditions (a) to (c). No.1 to No.7 do not satisfy one or more of the following conditions (a) to (c).

[0096] (a) The aspect ratio (AR1) of the first graphite particles is 6 to 20.

[0097] (b) The aspect ratio (AR2) of the second graphite particles is 2.7 or less.

[0098] (c) The orientation angle is 58° or more.

Claims

1. An electrode, comprising 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 first graphite particles, second graphite particles and silicon-containing particles. The first graphite particles have an aspect ratio of 6 to 20, The second graphite particles have an aspect ratio of 2.7 or less, In the cross section of the negative electrode active material layer parallel to the thickness direction, the orientation angle is 58° or more, The orientation angle represents the average value of the first angle and the second angle. The first angle represents an angle formed by a long axis of the first graphite particle and the surface of the substrate, and the second angle represents an angle formed by a long axis of the second graphite particle and the surface of the substrate.

2. The electrode according to claim 1, The first graphite particles have a particle size of 27 to 66 μm.

3. The electrode according to claim 1, The second graphite particles have a particle size of 5 to 22 μm.

4. The electrode according to any one of claims 1 to 3, The ratio of the particle size of the first graphite particles to the particle size of the second graphite particles is 2.58 to 11.

60.

5. The electrode according to any one of claims 1 to 3, The negative electrode active material layer contains the silicon-containing particles in an amount of 25% by mass or less, and The silicon-containing particles have a particle size of 11 μm or less.

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