Electrode

Abstract

To improve cycle characteristics.SOLUTION: An 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 includes first graphite particles, second graphite particles, and Si 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 a cross section parallel to the thickness direction of the negative electrode active material layer, the orientation angle is 58° or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to an electrode.

Background Art

[0002] Japanese Unexamined Patent Application Publication No. 2016-103347 discloses a negative electrode including carbon particles and non-carbon particles.

Prior Art Document

Patent Document

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a mixed system of graphite particles and silicon-containing particles (hereinafter also referred to as "Si particles"), there is room for improvement in cycle characteristics. During charging, each particle can expand. During discharging, each particle can contract. Depending on the type of particle, the expansion behavior and the contraction behavior can be different. By repeating the expansion and contraction of each particle, the arrangement of each particle can change. Due to the change in arrangement, for example, the conductive path may be interrupted, or particles isolated from the conductive network may be generated. As a result, it may not be possible to obtain the desired cycle characteristics.

[0005] An object of the present disclosure is to improve cycle characteristics.

Means for Solving the Problems

[0006] 1. 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 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 a cross-section parallel to the thickness direction of the negative electrode active material layer, the orientation angle is 58° or more. The orientation angle represents the average value of a first angle and a second angle. The first angle represents the angle formed by the major axis of the first graphite particles and the surface of the substrate. The second angle represents the angle formed by the major axis of the second graphite particles and the surface of the substrate.

[0007] 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 from 0° to 90°. The larger the orientation angle, the more likely the major axis of the graphite particles is considered to be along the thickness direction of the negative electrode active material layer. For example, when the negative electrode active material layer is formed, a magnetic field can be applied to the graphite particles so that the graphite particles can be oriented. By orienting the graphite particles so that the orientation angle is 58° or more, it is expected that ion conduction in the thickness direction is promoted.

[0008] Furthermore, the negative electrode active material layer includes two types of graphite particles. The first graphite particles have a relatively large particle size and a high aspect ratio. The second graphite particles have a relatively small particle size and a low aspect ratio. When the first graphite particles are oriented, relatively large voids can 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 can shift 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 shift in the in-plane direction. When the first graphite particles shift in the in-plane direction, inconveniences such as the conduction path being interrupted may occur. In addition to the first graphite particles, the presence of the second graphite particles is expected to allow the second graphite particles to enter the voids between the first graphite particles. By disposing the second graphite particles in the voids, it is expected that the shift of the first graphite particles is inhibited.

[0009] Furthermore, Si particles can also be disposed in the voids. During charging, the Si particles can 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 can contract rapidly. Since the Si particles and the second graphite particles are adjacent to each other within the voids, it is considered that particle isolation and structural collapse are less likely to occur. Through the synergistic effect of the above actions, an improvement in cycle characteristics is expected.

[0010] 2. The electrode described in the above "1" may include, for example, the following configuration. This is because the cycle characteristics may be improved by this configuration. The first graphite particles have a particle size of 27 to 66 μm.

[0011] 3. The electrode described in the above "1" or "2" may include, for example, the following configuration. This is because the cycle characteristics may be improved by this configuration. The second graphite particles have a particle size of 5 to 22 μm.

[0012] 4. The electrode described in any one of the above "1" to "3" may include, for example, the following configuration. This is because the cycle characteristics may be improved by this configuration. 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.

[0013] 5. The electrode described in any one of the above "1" to "4" may include, for example, the following configuration. This is because the cycle characteristics may be improved by this configuration. 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.

[0014] Hereinafter, embodiments of the present disclosure (hereinafter may be abbreviated as "the present embodiments"), and examples of the present disclosure (hereinafter may be abbreviated as "the present examples") will be described. However, the present embodiments and the present examples do not limit the technical scope of the present disclosure. The present embodiments and the present examples are illustrative in all respects. The present embodiments and the present examples are non-limiting. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configuration is extracted from the present embodiments and their arbitrary combinations are also initially planned.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0016] <Main Terms> "Aspect ratio" indicates the ratio of the major axis diameter to the minor axis diameter. The major axis diameter indicates the maximum Feret diameter. The minor axis diameter indicates the minimum Feret diameter. The maximum Feret diameter and the minimum Feret diameter of each particle are measured in a cross-sectional SEM (Scanning Electron Microscope) image of the negative electrode active material layer. The maximum Feret diameter and the minimum Feret diameter can be specified 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".

[0017] The "orientation angle" is measured by the following procedure. Five cross-sectional SEM images of the negative electrode active material layer are prepared. The five cross-sectional SEM images are taken at different positions. FIG. 1 is an explanatory diagram of the orientation angle. In the image, the major axis L of the particle is specified. The major axis L is a straight line passing through the maximum Feret diameter F max of the particle. When the particle is a 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 a second graphite particle, the formed angle θ is the second angle. The formed angle θ can take a value from 0° to 90°. The formed angle θ can be specified by image analysis software. In the five cross-sectional SEM images, the average value of the angle θ (first angle) formed by the first graphite particles in the image and the angle θ (second angle) formed by the second graphite particles is regarded as the orientation angle.

[0018] Elements expressed in the singular form include the plural form as well, unless otherwise specified. For example, "particle" includes not only "one particle" but also "a plurality of particles (particle group)" and "an aggregate of particles (powder)".

[0019] The "particle size" indicates the average value of the maximum Feret diameter. In the five 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.

[0020] "Silicon-containing particles (Si particles)" indicate particles containing Si. The Si particles may contain, for example, at least one selected from the group consisting of Si, SiO, Si-based alloys, and Si-C. "Si-C" indicates a composite material containing Si and carbon (C). In Si-C, Si may or may not form a compound with C. C may be amorphous or crystalline.

[0021] The stoichiometric composition formula shows representative examples of compounds. The compound may 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" indicates a compound containing Si and O in any molar ratio. For example, the compound may be doped with trace elements. A part of Si and O may be replaced by another element.

[0022] Geometric terms should not be construed in a strict sense. Examples of geometric terms include, for example, "parallel", "perpendicular", "orthogonal", etc. For example, "parallel" may deviate somewhat from "parallel" in the strict sense. Geometric terms may include, for example, tolerances, errors, etc. in design, operation, manufacturing, etc. The dimensional relationships in each figure may not match the actual dimensional relationships. The dimensional relationships in each figure may be changed to assist the reader's understanding. For example, the length, width, thickness, etc. may be changed. Some configurations may also be omitted.

[0023] A numerical range such as "from m to n%" includes the upper limit value and the lower limit value unless otherwise specified. That is, "from m to n%" indicates a numerical range of "m% or more and n% or less". Also, "m% or more and n% or less" includes "more than m% and less than n%". "Or more" and "or less" are represented by the inequality sign with an equal sign "≦". "More than" and "less than" are represented by the inequality sign without an equal sign "<".

[0024] <Electrode> Figure 2 is a conceptual diagram showing the electrode in the present embodiment. In Figure 2, 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 for a battery. The electrode 100 may be, for example, a negative electrode for a monopolar battery. The electrode 100 may be, for example, for a bipolar battery. The electrode 100 may be, for example, for a liquid-based lithium-ion battery. The electrode 100 may be, for example, for an all-solid-state lithium-ion battery. The electrode 100 includes a substrate 10 and a negative electrode active material layer 20.

[0025] · Substrate The substrate 10 supports the negative electrode active material layer 20. The substrate 10 may be, for example, sheet-shaped. The thickness of the substrate 10 may be, for example, 1 to 50 μm, or 5 to 30 μm. The substrate 10 has conductivity. The substrate 10 may contain, for example, a metal foil or the like. The substrate 10 may contain at least one selected from the group consisting of, for example, Cu, Ni, Zn, Pb, Al, Ti, Fe, Ag, Au, and a conductive resin. The substrate 10 may contain, for example, a Cu foil, a Cu alloy foil, or the like. The substrate 10 may have, for example, a multilayer structure. For example, the substrate 10 may be formed by laminating a Cu foil and an Al foil.

[0026] ·Negative electrode active material layer 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 on only one side of the substrate 10. The negative electrode active material layer 20 may be disposed on both sides of the substrate 10. When the electrode 100 is for a bipolar battery, the negative electrode active material layer 20 may be disposed on one surface (front surface) of the substrate 10, and a positive electrode active material layer (not shown) may be disposed on the other surface (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.

[0027] 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 each a negative electrode active material. The first graphite particles 21 and the second graphite particles 22 contain graphite. The graphite may be artificial graphite or natural graphite. As long as the first graphite particles 21 and the second graphite particles 22 contain graphite, they may further contain, for example, low-crystalline carbon, amorphous carbon, or the like. When the first graphite particles 21 and the second graphite particles 22 contain components other than graphite, 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.

[0028] ·Mixing ratio The mixing ratio (mass ratio) of the first graphite particles 21 and the second graphite particles 22 may be, for example, from "first graphite particles: second graphite particles = 9:1" to "first graphite particles: second graphite particles = 1:9". The mixing ratio may be, for example, from "first graphite particles: second graphite particles = 9:1" to "first graphite particles: second graphite particles = 5:5". The mixing ratio may be, for example, from "first graphite particles: second graphite particles = 8:2" to "first graphite particles: second graphite particles = 6:4".

[0029] ·Aspect ratio (AR1) of the first graphite particles 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.

[0030] ·Particle size (d1) of the first graphite particles The first graphite particles 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.

[0031] ·Aspect ratio (AR2) of the second graphite particles The second graphite particles 22 have 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.

[0032] ·Particle size (d2) of the second graphite particles 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.

[0033] ·Particle size ratio (d1 / d2) 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.

[0034] ·Orientation angle 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.

[0035] ·Mass fraction of Si particles The negative electrode active material layer 20 contains, by mass fraction, for example, 25% or less of Si particles 23. The mass fraction of the Si particles 23 may be, for example, 13% or less, or 3.5% or less. The mass fraction of the Si particles 23 may be, for example, 1% or more, 2% or more, or 3.5% or more.

[0036] ·Particle size (d3) of Si particles The Si particles 23 may have a particle size (d3) of, for example, 11 μm or less. The particle size (d3) may be, for example, 6 μm or less, or 1.5 μm or less. The particle size (d3) may be, for example, 0.5 μm or more, 1 μm or more, or 1.5 μm or more.

[0037] ·Other components In addition to the negative electrode active material, the negative electrode active material layer may further contain a conductive material, a thickening material, a binder, and the like. The conductive material can form an electron conduction path. The conductive material may contain, for example, at least one selected from the group consisting of acetylene black (AB), Ketjen black (registered trademark), vapor-grown carbon fiber (VGCF), carbon nanotube (CNT), and graphene flake (GF). The blending amount of the conductive material may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material.

[0038] The thickening material can impart viscosity to the negative electrode paste. The thickening material may contain, for example, at least one selected from the group consisting of sodium alginate, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylpyrrolidone (PVP). The blending amount of the thickening material may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material.

[0039] The binder can bond solids together. The binder may contain, for example, at least one selected from the group consisting of 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 blending amount of the binder may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material.

Examples

[0040] <Preparation of Samples> · Fabrication of Electrodes Figure 3 is a table showing experimental results. Electrodes according to Nos. 1 to 14 were fabricated by the following procedure. 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 solid content formulation was "(graphite mixture:Si particles:SBR:CMC:conductive material = (98.3 - a - b):a:0.5:1.2:b (mass ratio))". As a substrate, a Cu foil (thickness: 10 μm, long strip) was prepared. A coating film was formed by applying the negative electrode paste to 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 magnet was 0.5 T. After applying the magnetic field, the coating film was dried to fabricate a negative electrode active material layer. The negative electrode active material layer was compressed to fabricate an electrode (negative electrode sheet). In the negative electrode sheet, the orientation angle and the like were measured by the above-described procedure.

[0041] · Fabrication of Evaluation Cell An evaluation cell (lithium-ion battery) was fabricated by the following procedure. LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2 (particle size: 5 μm), AB, PVdF, and N-methyl-2-pyrrolidone were mixed to prepare a positive electrode paste. The solid content formulation was "(LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2:AB:PVdF = 92:5:3 (mass ratio))". As a substrate, an Al foil (thickness: 15 μm, long strip) was prepared. A coating film was formed by applying the positive electrode paste to both sides of the substrate. The coating film was dried to fabricate a positive electrode active material layer. The positive electrode active material layer was compressed to fabricate a counter electrode (positive electrode sheet).

[0042] 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.

[0043] A laminate was formed by laminating a positive electrode sheet, a separator, a negative electrode sheet, and a separator. A wound-type power generation element was formed by winding the laminate in a spiral shape. The power generation element was formed into a flat shape by being crushed in the radial direction. An external terminal was connected to the power generation element. The power generation element was housed in a metal case. An electrolytic solution was injected into the metal case. After the injection of the electrolytic solution, the metal case was sealed. Thus, an evaluation cell was fabricated. The composition of the electrolytic solution was as follows.

[0044] · Electrolytic solution composition Solvent: "Ethylene carbonate:Dimethyl carbonate:Ethyl methyl carbonate = 3:3:4 (volume ratio)" Solute: LiPF6 (concentration: 1 mol / L)

[0045] · Activation treatment The first charge and discharge were carried out under the following conditions. Ambient temperature: 25°C Charge: Constant current-constant voltage method, current during constant current: 1 / 3C, voltage during constant voltage: 4.2V, cut-off current: 1 / 50C Discharge: Constant current method, current: 1 / 3C, cut-off voltage: 3V

[0046] <Evaluation> · Measurement of initial capacity The initial capacity (initial discharge capacity) was measured by charge and discharge under the following conditions. Charge: Constant current-constant voltage method, current during constant current: 1 / 3C, voltage during constant voltage: 4.1V, cut-off current: 1 / 50C Discharge: Constant current method, current: 1 / 3C, cut-off voltage: 3V

[0047] · Cycle characteristics Taking the charge and discharge under the following conditions as one cycle, charge and discharge were carried out for 300 cycles. Ambient temperature: 25 °C Current: 0.5C Range of state of charge (SOC): 0% to 100%

[0048] After 300 cycles, similar to the initial capacity, the capacity after cycling was measured. By dividing the capacity after cycling by the initial capacity, the capacity retention rate was obtained. The higher the capacity retention rate, the better the cycle characteristics are considered to be.

[0049] <Results> In the table of Figure 3, Nos. 8 to 14 have improved cycle characteristics compared to Nos. 1 to 7. Nos. 8 to 14 satisfy all of the following conditions (a) to (c). Nos. 1 to 7 do not satisfy any one or more of the following conditions (a) to (c). (a) The aspect ratio (AR1) of the first graphite particles is from 6 to 20. (b) The aspect ratio (AR2) of the second graphite particles is 2.7 or less. (c) The orientation angle is 58° or more.

Explanation of symbols

[0050] 10 Substrate, 20 Negative electrode active material layer, 21 First graphite particles, 22 Second graphite particles, 23 Si particles, 100 Electrode, F max Maximum Feret diameter, L Major axis, θ Angle formed.

Claims

1. comprising a substrate and a negative electrode active material layer, wherein 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 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 a cross-section parallel to the thickness direction of the negative electrode active material layer, the orientation angle is 58° or more, the orientation angle represents the average value of a first angle and a second angle, the first angle represents the angle formed by the major axis of the first graphite particles and the surface of the substrate, and the second angle represents the angle formed by the major axis of the second graphite particles and the surface of the substrate, an electrode.

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

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

4. 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, the electrode according to any one of Claims 1 to 3.

5. the negative electrode active material layer contains the silicon-containing particles with a mass fraction of 25% or less, and the silicon-containing particles have a particle size of 11 μm or less, the electrode according to any one of Claims 1 to 3.

Images