Negative electrode of a secondary battery, method for manufacturing the negative electrode, and secondary battery using the negative electrode

Abstract

To provide a negative electrode containing a Si-containing particle and a graphite particle, capable of suppressing a capacity deterioration when repeating charging and discharging of a secondary battery.SOLUTION: A negative electrode disclosed herein comprises a negative electrode collector, and a negative electrode active material layer supported by the negative electrode collector. The negative electrode active material layer contains: a first graphite particle; a second graphite particle; a first Si-C composite particle; and a second Si-C composite particle. The Si content in the first Si-C composite particle is larger than the Si content in the second Si-C composite particle. A compression modulus of the first graphite particle is smaller than a compression modulus of the second graphite particle. The first Si-C composite particle is contacted to the first graphite particle more than the second graphite particle. The second Si-C composite particle is contacted to the second graphite particle more than the first graphite particle.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a negative electrode of a secondary battery and a method for manufacturing the same. The present invention also relates to a secondary battery using the negative electrode.

Background Art

[0002] In recent years, secondary batteries have been suitably used for portable power sources such as personal computers and mobile terminals, and power sources for driving vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In the application of power sources for driving vehicles, particularly in the application of power sources for driving BEVs, from the viewpoint of extending the cruising range of vehicles, further increase in the capacity of secondary batteries is desired. As a high-capacity negative electrode active material, Si-containing particles are known, and it is known that secondary batteries can be increased in capacity by Si-containing particles (see, for example, Patent Documents 1 and 2). Patent Documents 1 and 2 disclose a technique of using Si-containing particles and graphite particles in combination as a negative electrode active material.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, while Si-containing particles have a high capacity, they exhibit a large volume change due to expansion / contraction during charge and discharge of the secondary battery. Therefore, when Si-containing particles and graphite particles are used in combination, especially when the proportion of Si-containing particles is large, the fillability of these particles decreases when the secondary battery is repeatedly charged and discharged, resulting in a broken conductive path and generation of internal stress. Therefore, when Si-containing particles and graphite particles are used in combination, there is a problem that the cycle characteristics of the secondary battery deteriorate. Specifically, there is a problem that the capacity degradation is large when the secondary battery is repeatedly charged and discharged.

[0006] In view of the above circumstances, an object of the present invention is to provide a negative electrode containing Si-containing particles and graphite particles, which can suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

Means for Solving the Problems

[0007] The negative electrode disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer contains first graphite particles, second graphite particles, first Si-C composite particles, and second Si-C composite particles. The Si content in the first Si-C composite particles is greater than the Si content in the second Si-C composite particles. The compressive elastic modulus of the first graphite particles is smaller than the compressive elastic modulus of the second graphite particles. The first Si-C composite particles are in contact with the first graphite particles more than the second graphite particles. The second Si-C composite particles are in contact with the second graphite particles more than the first graphite particles.

[0008] According to such a configuration, it is possible to provide a negative electrode containing Si-containing particles and graphite particles, which can suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

[0009] From another aspect, the method for manufacturing a negative electrode of a secondary battery disclosed herein includes a step of preparing a first granulated body in which first Si-C composite particles and first graphite particles are aggregated, and a second granulated body in which second Si-C composite particles and second graphite particles are aggregated, wherein the Si content in the first Si-C composite particles is larger than the Si content in the second Si-C composite particles, and the compression elastic modulus of the first graphite particles is smaller than the compression elastic modulus of the second graphite particles; a step of mixing the first granulated body and the second granulated body in a dispersion medium to prepare a negative electrode paste; a step of coating the negative electrode paste on a negative electrode current collector; and a step of drying the coated negative electrode paste.

[0010] According to the negative electrode obtained by such a configuration, excellent capacity degradation resistance can be imparted when the secondary battery is repeatedly charged and discharged.

[0011] From another aspect, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the above-described negative electrode.

[0012] According to such a configuration, a secondary battery having excellent capacity degradation resistance when repeatedly charged and discharged can be provided.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for the implementation of the present invention can be understood as design matters of those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and common general knowledge in the art. Also, in the following drawings, members and parts having the same function are denoted by the same reference numerals for description. Further, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships. In this specification, the numerical range expressed as "A to B" includes A and B.

[0015] Note that in this specification, the "secondary battery" refers to a power storage device that can be repeatedly charged and discharged. Also, in this specification, the "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0016] The negative electrode disclosed herein is used in a secondary battery, preferably in a lithium-ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically showing an example of the negative electrode 60 according to this embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0017] As shown in the figure, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0018] As shown in the figure example, a negative electrode active material layer non-formation portion 62a where the negative electrode active material layer 64 is not provided may be provided at one end in the width direction of the negative electrode 60. In the negative electrode active material layer non-formation portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-formation portion 62a can function as a current collection portion. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0019] The shape of the negative electrode current collector 62 is, in the illustrated example, foil-shaped (or sheet-shaped), but is not limited thereto. The negative electrode current collector 62 may be in various forms such as rod-shaped, plate-shaped, mesh-shaped, etc. As the material of the negative electrode current collector 62, a metal with good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be used as in a conventional lithium-ion secondary battery, and among them, copper is preferable. As the negative electrode current collector 62, a copper foil is particularly preferable.

[0020] The dimensions of the negative electrode current collector 62 are not particularly limited and may be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 6 μm or more and 20 μm or less.

[0021] The negative electrode active material layer 64 contains a negative electrode active material. In the present embodiment, at least first graphite particles, second graphite particles, first Si-C composite particles, and second Si-C composite particles are used as the negative electrode active material. This will be described in detail with reference to FIG. 2. FIG. 2 is a schematic cross-sectional view showing the negative electrode active material particles contained in the negative electrode active material layer 64 shown in FIG. 1. As shown in FIG. 2, the negative electrode active material layer 64 includes first graphite particles 12, second graphite particles 14, first Si-C composite particles 16, and second Si-C composite particles 18.

[0022] The graphite constituting the first graphite particles 12 and the second graphite particles 14 may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in a form in which the graphite is coated with an amorphous carbon material.

[0023] The shapes of the first graphite particles 12 and the second graphite particles 14 are not particularly limited, and may be flaky, spherical, or the like.

[0024] The average particle diameters (D50) of the first graphite particles 12 and the second graphite particles 14 are not particularly limited. The average particle diameters (D50) of the graphite particles 12 and the second graphite particles 14 are, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, still more preferably 12 μm to 20 μm, and most preferably 15 μm to 20 μm.

[0025] In this specification, the "average particle diameter (D50)" refers to the median diameter (D50), and in the volume-based particle size distribution based on the laser diffraction / scattering method, it refers to the particle diameter corresponding to a cumulative frequency of 50% by volume from the side of the fine particles with a smaller particle diameter. The average particle diameter (D50) can be determined using a commercially available laser diffraction / scattering type particle size distribution measuring device or the like.

[0026] The first graphite particles 12 and the second graphite particles 14 have different compression elastic moduli, and the compression elastic modulus (E1) of the first graphite particles 12 is smaller than the compression elastic modulus (E2) of the second graphite particles 14.

[0027] The compression elastic modulus is also called Young's modulus, and the compression elastic modulus (E) is a value obtained from the formula compression elastic modulus (E) = compression stress (σ) / compression strain (ε). The higher the compression elastic modulus of the graphite particles, the harder the graphite particles and the more difficult they are to deform. The actions of the first graphite particles 12 and the second graphite particles 14 with different compression elastic moduli will be described later.

[0028] The compression elastic modulus (E1) of the first graphite particles 12 is not particularly limited as long as it is smaller than the compression elastic modulus (E2) of the second graphite particles 14. The compression elastic modulus (E1) of the first graphite particles 12 is preferably 130 MPa or less, more preferably 120 MPa or less, still more preferably 110 MPa or less, and particularly preferably 100 MPa or less. The compression elastic modulus (E1) of the first graphite particles 12 may be 10 MPa or more, 30 MPa or more, or 50 MPa or more.

[0029] The compression elastic modulus (E2) of the second graphite particle 14 is not particularly limited as long as it is larger than the compression elastic modulus (E1) of the first graphite particle 12. The compression elastic modulus (E2) of the second graphite particle 14 is, for example, more than 10 MPa, preferably more than 130 MPa, more preferably 140 MPa or more, still more preferably 150 MPa or more, and particularly preferably 160 MPa or more. The compression elastic modulus (E2) of the second graphite particle 14 may be 300 MPa or less, 250 MPa or less, or 200 MPa or less.

[0030] The compression elastic moduli of the first graphite particle 12 and the second graphite particle 14 can be measured using a micro compression tester. According to the micro compression tester, the compression elastic modulus can be measured for one graphite particle. Specifically, using a micro compression tester, one graphite particle is compressed in the vertical direction, and the displacement amount and stress during compression are measured. The compression elastic modulus (MPa) = compression stress σ (MPa) / compression strain ε (where compression strain ε = compression displacement amount / average particle diameter (D50) of graphite), and the compression elastic modulus is calculated. Here, the measurement is performed until the graphite particle is displaced by 10%. When the graphite particle breaks before being displaced by 10%, the compression elastic modulus is calculated using the value of the compression stress immediately before the break and the compression displacement amount. The compression elastic modulus is determined for five graphite particles, and the average value is calculated. This average value is adopted as the compression elastic modulus of the first graphite particle 12 and the second graphite particle 14.

[0031] Note that graphite particles with various compression elastic moduli are known and available. It is known that the physical properties of graphite particles change depending on shape, anisotropy, crystallite size, pore size, porosity, etc. Here, the porosity and shape tend to have a large influence on the compression elastic modulus of graphite particles, and graphite particles may be appropriately selected in consideration of this.

[0032] The mass ratio of the first graphite particle 12 to the second graphite particle 14 is not particularly limited as long as the effects of the present invention can be obtained. The mass ratio of the first graphite particle 12 to the total of the first graphite particle 12 and the second graphite particle 14 is, for example, 10% by mass to 90% by mass, preferably 30% by mass to 70% by mass.

[0033] The first Si-C composite particles 16 and the second Si-C composite particles 18 are composed of an Si-C composite material. The Si-C composite material typically includes a carbon domain and an Si-containing domain.

[0034] The carbon domain is, for example, a carbide of a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite or the like. The carbon domain preferably constitutes a carbon matrix. Thus, the Si-C composite material is preferably a material in which a plurality of Si-containing domains are dispersed in a carbon matrix. In this case, it is advantageous because the carbon matrix can relieve the volume change due to the expansion / contraction of the Si-containing domains.

[0035] The Si-containing domain contains Si and is composed of, for example, Si, silicon oxide (SiO x ), silicon nitride (SiNx), silicon carbide (SiCx), etc. The Si-containing domain is preferably composed of Si. The Si-containing domain may be fine particles. The average particle diameter of the Si-containing domain is, for example, 50 nm or less, and may be 5 nm to 50 nm. The "average particle diameter of the Si-containing domain" can be obtained as follows. First, the negative electrode active material layer 64 is processed by FIB (focused ion beam) to prepare a sample for scanning transmission electron microscope (STEM) observation. Then, after elemental analysis of the sample by EDX elemental mapping, a BF image (bright field image) and a HAADF image (high angle annular dark field image) are acquired. The diameter of the Si-containing domain can be obtained from the contrast and shape obtained by the BF image and the HAADF image. The diameters of 10 or more arbitrarily selected Si-containing domains are obtained, and the average value thereof is taken as the "average particle diameter of the Si-containing domain" here.

[0036] The Si-C composite material includes, for example, one in which Si-containing nanoparticles are dispersed inside a carbon material; one in which Si-containing nanoparticles are dispersed in the pores of a porous carbon material (e.g., granulated porous graphite); and the like. The Si-C composite material may be one in which Si-containing fine particles are attached to the surface of carbon particles; one in which carbon fine particles are attached to the surface of Si-containing particles; and the like. From the viewpoint of suppressing the volume change of Si, those in which Si-containing nanoparticles are dispersed inside a carbon material and those in which Si-containing nanoparticles are dispersed in the pores of a porous carbon material are preferable, and those in which Si-containing nanoparticles are dispersed in the pores of a porous carbon material are more preferable.

[0037] In this embodiment, the Si content (S1) in the first Si-C composite particle 16 is larger than the Si content (S2) in the second Si-C composite particle 18.

[0038] The Si content (S1) in the first Si-C composite particle 16 is not particularly limited as long as it is larger than the Si content (S2) in the second Si-C composite particle 18. Since the higher the Si content, the higher the capacity of the secondary battery can be, the Si content (S1) in the first Si-C composite particle 16 is preferably 45% by mass or more, more preferably 50% by mass or more, still more preferably 55% by mass or more, and particularly preferably 60% by mass or more. On the other hand, since the higher the Si content, the higher the expansion rate of the first Si-C composite particle 16, the Si content (S1) in the first Si-C composite particle 16 is preferably 80% by mass or less, more preferably 75% by mass or less, still more preferably 70% by mass or less.

[0039] The Si content (S2) in the second Si-C composite particle 18 is not particularly limited as long as it is larger than the Si content (S1) in the first Si-C composite particle 16. The larger the Si content, the higher the capacity of the secondary battery can be. Therefore, the Si content (S2) in the second Si-C composite particle 18 is preferably 20% by mass or more, more preferably 30% by mass or more, still more preferably 35% by mass or more, and particularly preferably 40% by mass or more. On the other hand, the larger the Si content, the higher the expansion rate of the second Si-C composite particle 18. Therefore, the Si content (S2) in the second Si-C composite particle 18 is preferably 55% by mass or less, more preferably 50% by mass or less, still more preferably 45% by mass or less.

[0040] The ratio (S1 / S2) of the Si content ratio (S1) in the first Si-C composite particle 16 to the Si content ratio (S2) in the second Si-C composite particle 18 is preferably 1.1 or more, more preferably 1.2 or more, and still more preferably 1.3 or more. The ratio (S1 / S2) may be 4 or less, 3.5 or less, 3 or less, or 2.5 or less. The Si content ratio (S1) in the first Si-C composite particle 16 and the Si content ratio (S2) in the second Si-C composite particle 18 can be measured according to a known method (e.g., high-frequency inductively coupled plasma (ICP) emission spectrometry).

[0041] The average particle diameters (D50) of the first Si-C composite particle 16 and the second Si-C composite particle 18 are not particularly limited. The average particle diameters (D50) of the first Si-C composite particle 16 and the second Si-C composite particle 18 are, for example, 1 μm to 15 μm, preferably 1 μm to 10 μm, and more preferably 4 μm to 10 μm, respectively.

[0042] The average particle diameter (D50) of the first Si-C composite particles 16 may be larger than, smaller than, or the same as the average particle diameter (D50) of the second Si-C composite particles 18. Since the first Si-C composite particles 16 have a higher Si content ratio than the second Si-C composite particles 18, the volume change during charge and discharge is large. To reduce the influence of this volume change, it is preferable that the average particle diameter (D50) of the first Si-C composite particles 16 is smaller than the average particle diameter (D50) of the second Si-C composite particles 18.

[0043] The first Si-C composite particles 16 and the second Si-C composite particles 18 can be produced according to known methods. For example, various production methods of Si-C composite material particles are known (see, for example, Japanese Patent Application Laid-Open No. 2015-38862, International Publication No. 2014 / 046144, and prior art documents cited in the said international publication). Therefore, the first Si-C composite particles 16 and the second Si-C composite particles 18 can be produced according to this known method.

[0044] The mass ratio between the first Si-C composite particles 16 and the second Si-C composite particles 18 is not particularly limited as long as the effects of the present invention can be obtained. The mass ratio of the first Si-C composite particles 16 to the total of the first Si-C composite particles 16 and the second Si-C composite particles 18 is, for example, 15% by mass to 85% by mass, preferably 25% by mass to 80% by mass, and more preferably 40% by mass to 60% by mass.

[0045] In the present embodiment, the first Si-C composite particles 16 are in contact with the first graphite particles 12 more than the second graphite particles 14. Also, the second Si-C composite particles 18 are in contact with the second graphite particles 14 more than the first graphite particles 12.

[0046] Therefore, in the present embodiment, two types of Si-C composite particles with different Si contents and two types of graphite particles with different compression elastic moduli are used, and a large number of the first graphite particles 12 with a small compression elastic modulus are arranged around the first Si-C composite particles 16 with a large Si content, and a large number of the second graphite particles 14 with a large compression elastic modulus are arranged around the second Si-C composite particles 18 with a small Si content.

[0047] With such a configuration, it is possible to suppress capacity degradation when the secondary battery is repeatedly charged and discharged. The reason is considered as follows.

[0048] Si-C composite particles have a large volume change due to expansion / contraction when the secondary battery is charged and discharged. Therefore, when Si-C composite particles and graphite particles are used in combination, the volume change of the Si-C composite particles during charge and discharge causes a change in the filling state of the Si-C composite particles and the graphite particles in the negative electrode active material layer, resulting in the occurrence of a broken conduction path or internal stress, which causes capacity degradation when the secondary battery is repeatedly charged and discharged.

[0049] Here, the larger the Si content in the Si-C composite particles, the larger the expansion rate when the secondary battery is charged. Also, the larger the compression elastic modulus of the graphite particles, the harder the graphite particles are. Therefore, in the present embodiment, the first Si-C composite particles 16 with a large expansion rate are in contact with a large number of the first graphite particles 12 with low hardness among the graphite particles, and the second Si-C composite particles 18 with a small expansion rate are in contact with a large number of the second graphite particles 14 with high hardness among the graphite particles.

[0050] When the first Si-C composite particles 16 with a large expansion rate are in contact with a large number of the first graphite particles 12 with low hardness, the first graphite particles 12 can follow the expansion / contraction of the first Si-C composite particles 16 during charge and discharge, thereby suppressing the broken conduction path associated with the expansion / contraction of the first Si-C composite particles 16.

[0051] On the other hand, when the second Si-C composite particles 18 with a small expansion rate are in contact with a large number of the second graphite particles 14 with high hardness, the second graphite particles 14 can suppress the expansion / contraction of the second Si-C composite particles 18 and relieve the internal stress. From the above results, it is possible to suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

[0052] In addition, in this specification, when it is said that X particles are in contact with more Z particles than Y particles, it means that the total contact area between one X particle and the Z particles around it is larger than the total contact area between one X particle and the Y particles around it. Therefore, regarding whether the first Si-C composite particles 16 and the second Si-C composite particles 18 are in contact with more of the first graphite particles 12 or the second graphite particles 14, for example, a cross-sectional scanning electron microscope (SEM) image of the negative electrode active material layer 64 can be obtained and confirmed. Specifically, in the SEM image, let the perimeter of one Si-C composite particle be A, the total perimeter of the portions where one Si-C composite particle is in contact with the first graphite particles 12 be B1, and the total perimeter of the portions where one Si-C composite particle is in contact with the second graphite particles 14 be B2. If B1 > B2, it can be determined that it is in contact with more of the first graphite particles 12, and if B2 > B1, it can be determined that it is in contact with more of the second graphite particles 14. This evaluation is performed and judged for 30 or more arbitrarily selected Si-C composite particles.

[0053] Therefore, for the first Si-C composite particles 16, preferably 0.5 < B1 / A ≤ 1 and 0 ≤ B2 / A < 0.5 are satisfied, and more preferably 0.7 < B1 / A ≤ 1 and 0 ≤ B2 / A < 0.3 are satisfied. For the second Si-C composite particles 18, preferably 0.5 < B2 / A ≤ 1 and 0 ≤ B1 / A < 0.5 are satisfied, and more preferably 0.7 < B2 / A ≤ 1 and 0 ≤ B1 / A < 0.3 are satisfied. The values of B1 / A and B2 / A adopt the average values of 30 or more arbitrarily selected Si-C composite particles.

[0054] In addition, the arrangement in which the first Si-C composite particles 16 are in contact with more of the first graphite particles 12 than the second graphite particles 14 and the second Si-C composite particles 18 are in contact with more of the second graphite particles 14 than the first graphite particles 12 is not limited to that shown in FIG. 2.

[0055] The mass ratio of the first Si-C composite particles 16 to the total of the first graphite particles 12 and the first Si-C composite particles 16 is preferably 10% by mass or more, more preferably 15% by mass or more, still more preferably 20% by mass or more, even more preferably 25% by mass or more, and most preferably 28% by mass or more. On the other hand, the mass ratio of the first Si-C composite particles 16 to the total of the first graphite particles 12 and the first Si-C composite particles 16 is preferably 60% by mass or less, more preferably 50% by mass or less, still more preferably 47% by mass or less, even more preferably 33% by mass or less, and most preferably 30% by mass or less.

[0056] The mass ratio of the second Si-C composite particles 18 to the total of the second graphite particles 14 and the second Si-C composite particles 18 is preferably 10% by mass or more, more preferably 15% by mass or more, still more preferably 20% by mass or more, even more preferably 25% by mass or more, and most preferably 28% by mass or more. On the other hand, the mass ratio of the second Si-C composite particles 18 to the total of the second graphite particles 14 and the second Si-C composite particles 18 is preferably 60% by mass or less, more preferably 50% by mass or less, still more preferably 47% by mass or less, even more preferably 33% by mass or less, and most preferably 30% by mass or less.

[0057] The negative electrode active material layer 64 may contain components other than the negative electrode active material, and examples thereof include a binder, a conductive material, and the like. As the binder, for example, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), etc. can be used. CMC also functions as a thickener. Examples of the conductive material include carbon black such as acetylene black, carbon fiber, carbon nanotube (CNT), etc. Among them, CNT is preferred. When CNT is used as the conductive material, the negative electrode active material layer 64 may contain a dispersant for CNT.

[0058] The content of the negative electrode active material in the negative electrode active material layer 64 (that is, relative to the total mass of the negative electrode active material layer 64) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less.

[0059] In the negative electrode active material layer 64, at least a part of the first Si-C composite particles 16 and at least a part of the first graphite particles 12 may form a first granulated body in which they are aggregated. Similarly, at least a part of the second Si-C composite particles 18 and at least a part of the second graphite particles 14 may form a second granulated body in which they are aggregated. The first granulated body is a particulate matter in which the first Si-C composite particles 16 and the first graphite particles 12 are agglomerated. The second granulated body is a particulate matter in which the second Si-C composite particles 18 and the second graphite particles 14 are agglomerated. According to the first granulated body and the second granulated body, in the negative electrode active material layer 64, it becomes easy to bring the first Si-C composite particles 16 into contact with the first graphite particles 12 more than the second graphite particles 14, and to bring the second Si-C composite particles 18 into contact with the second graphite particles 14 more than the first graphite particles 12.

[0060] In the first granulated body, the first Si-C composite particles 16 and the first graphite particles 12 may be bound by a first binder. In the second granulated body, the second Si-C composite particles 18 and the second graphite particles 14 may be bound by a second binder.

[0061] As the first binder and the second binder, CMC and / or PAA are suitable. The first granulated body and the second granulated body may further contain a conductive material. Also, the granulated bodies themselves and the granulated bodies and the negative electrode current collector 62 may be bound by a third binder (e.g., SBR, etc.).

[0062] The thickness of the negative electrode active material layer 64 is not particularly limited, but for example, it is 10 μm or more and 400 μm or less, preferably 20 μm or more and 300 μm or less.

[0063] The density of the negative electrode active material layer 64 is not particularly limited, but for example, it is 0.7 g / cm 3 or more, preferably 1.0 g / cm 3 or more, more preferably 1.2 g / cm 3 or more. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm 3 or less, and may be 2.0 g / cm 3 or less.

[0064] The negative electrode 60 may include members other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) adjacent to the negative electrode active material layer 64 may be provided on the negative electrode active material layer non-forming portion 62a. The insulating layer contains, for example, an insulating inorganic filler or the like.

[0065] The manufacturing method of the negative electrode 60 is not particularly limited. For example, an aggregate in which the first Si-C composite particles 16 and the first graphite particles 12 are agglomerated in advance is prepared, and an aggregate in which the second Si-C composite particles 18 and the second graphite particles 14 are agglomerated is prepared, and the negative electrode 60 is manufactured using these. In the negative electrode active material layer 64, a state can be easily obtained in which the first Si-C composite particles 16 are in contact with the first graphite particles 12 more than the second graphite particles 14, and the second Si-C composite particles 18 are in contact with the second graphite particles 14 more than the first graphite particles 12.

[0066] Therefore, a preferred method for manufacturing the negative electrode 60 includes a step of preparing a first granulated body in which the first Si-C composite particles 16 and the first graphite particles 12 are aggregated, and a second granulated body in which the second Si-C composite particles 18 and the second graphite particles 14 are aggregated (hereinafter, also referred to as the "granulated body preparation step"). Here, the Si content (S1) in the first Si-C composite particles 16 is greater than the Si content (S2) in the second Si-C composite particles 18, and the compression elastic modulus of the first graphite particles 12 is smaller than the compression elastic modulus of the second graphite particles 14; a step of mixing the first granulated body and the second granulated body in a dispersion medium to prepare a negative electrode paste (hereinafter, also referred to as the "paste preparation step"); a step of coating the negative electrode paste on a negative electrode current collector (hereinafter, also referred to as the "coating step"); and a step of drying the coated negative electrode paste (hereinafter, also referred to as the "drying step").

[0067] In this specification, the term "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, and includes so-called "slurry", "ink", etc. Hereinafter, each step will be described.

[0068] The granulated body preparation step can be carried out by adopting a known granulation method using two or more kinds of particles.

[0069] For example, using a known stirring granulator, the first Si-C composite particles 16 and the first graphite particles 12 are granulated to produce the first granulated body. When the first binder is used during granulation, the first Si-C composite particles 16 and the first graphite particles 12 can be bound together, making it easier to produce the first granulated body. Similarly, using a known stirring granulator, the second Si-C composite particles 18 and the second graphite particles 14 are granulated to produce the second granulated body. When the second binder is used during granulation, the second Si-C composite particles 18 and the second graphite particles 14 can be bound together, making it easier to produce the second granulated body.

[0070] As the first binder and the second binder, CMC and / or PAA are preferred. When using the first binder and the second binder, a solvent capable of dissolving them may be used.

[0071] Therefore, specifically, for example, in a stirring granulator, while stirring the first Si-C composite particles 16 and the first graphite particles 12, a solution of the first binder is dropped to granulate into particles, whereby the first granulated body can be produced. Similarly, for example, in a stirring granulator, while stirring the second Si-C composite particles 18 and the second graphite particles 14, a solution of the second binder is dropped to granulate into particles, whereby the second granulated body can be produced.

[0072] Also, a conductive material may be added during granulation. At this time, the conductive material can be introduced into the first granulated body and the second granulated body, and a good conductive path can be formed in the first granulated body and the second granulated body.

[0073] The paste preparation step can be carried out according to a known method. Specifically, for example, the first granulated body, the second granulated body, and optional components (e.g., conductive material, third binder) can be mixed with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc.

[0074] At this time, it is preferable to use a third binder as the optional component. As the third binder, SBR is preferable. By using the third binder, the granulated bodies can be firmly bound together, and the granulated body and the negative electrode current collector 62 can be firmly bound.

[0075] The coating step can be carried out according to a known method. Specifically, for example, the obtained negative electrode paste can be coated on the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, die coater, etc., to perform the coating step.

[0076] The drying process can be carried out according to a known method. Specifically, for example, from the negative electrode current collector 62 coated with the negative electrode paste, the dispersion medium is removed using a drying device such as a drying furnace, thereby forming the negative electrode active material layer 64. Thus, the drying process can be performed. The drying temperature and drying time may be appropriately determined according to the solid content concentration of the negative electrode paste and are not particularly limited. The drying temperature is, for example, 60°C or higher and 200°C or lower, preferably 70°C or higher and 150°C or lower. The drying time is, for example, 10 seconds or longer and 30 minutes or shorter, preferably 30 seconds or longer and 10 minutes or shorter.

[0077] After the drying process, a step of pressing the negative electrode active material layer 64 may further be performed. The pressing process can be carried out according to a known method. Specifically, the pressing process can be performed by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. By the pressing process, the first graphite particles 12, the second graphite particles 14, the first Si-C composite particles 16, and the second Si-C composite particles 18 contained in the negative electrode active material layer 64 can be densely filled. In the above manner, the negative electrode 60 can be obtained.

[0078] According to the above method, in the paste preparation process or the like, some of the first graphite particles 12 and / or the first Si-C composite particles 16 may fall off from the first granulated body, and some of the second graphite particles 14 and / or the second Si-C composite particles 18 may fall off from the second granulated body. However, in the negative electrode active material layer 64, at least some of the first Si-C composite particles 16 and at least some of the first graphite particles 12 maintain the first granulated body in which they are aggregated, and at least some of the second Si-C composite particles 18 and at least some of the second graphite particles 14 maintain the second granulated body in which they are aggregated. Therefore, a state can be obtained in which the first Si-C composite particles 16 are in contact with the first graphite particles 12 more than with the second graphite particles 14, and the second Si-C composite particles 18 are in contact with the second graphite particles 14 more than with the first graphite particles 12.

[0079] According to the negative electrode 60 according to the present embodiment, excellent resistance to capacity deterioration can be imparted when the secondary battery is repeatedly charged and discharged. Further, since the negative electrode 60 according to the present embodiment uses a negative electrode active material containing Si, the secondary battery can be made to have a high capacity. Therefore, the secondary battery using the negative electrode 60 according to the present embodiment has a high capacity and excellent cycle characteristics.

[0080] Therefore, from another aspect, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, an embodiment of the secondary battery disclosed herein will be described with reference to FIGS. 3 and 4, taking a lithium-ion secondary battery as an example. The following configuration example is a flat rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0081] The lithium-ion secondary battery 100 shown in FIG. 3 is a sealed lithium-ion secondary battery 100 constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight and highly thermally conductive metal material such as aluminum is used.

[0082] As shown in FIGS. 3 and 4, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are overlapped via two long separator sheets 70 and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of a long negative electrode current collector 62. The non-formed portion 52a of the positive electrode active material layer (that is, the portion where the positive electrode current collector 52 is exposed without the formation of the positive electrode active material layer 54) and the non-formed portion 62a of the negative electrode active material layer (that is, the portion where the negative electrode current collector 62 is exposed without the formation of the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction of the wound electrode body 20 (that is, the sheet width direction orthogonal to the longitudinal direction). A positive electrode current collecting plate 42a and a negative electrode current collecting plate 44a are joined to the non-formed portion 52a of the positive electrode active material layer and the non-formed portion 62a of the negative electrode active material layer, respectively.

[0083] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in a lithium ion secondary battery may be used. Examples thereof include sheets or foils made of metals with good conductivity (for example, aluminum, nickel, titanium, stainless steel, etc.). As the positive electrode current collector 52, an aluminum foil is preferable.

[0084] The dimensions of the positive electrode current collector 52 are not particularly limited and may be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0085] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material having a known composition used in a lithium ion secondary battery may be used. Specifically, for example, as the positive electrode active material, a lithium composite oxide, a lithium transition metal phosphate compound, etc. can be used. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, a spinel structure, an olivine structure, etc.

[0086] As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable. Specific examples thereof include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel manganese-based composite oxides, lithium nickel cobalt manganese-based composite oxides, lithium nickel cobalt aluminum-based composite oxides, lithium iron nickel manganese-based composite oxides, and the like.

[0087] In addition, in this specification, the term "lithium nickel cobalt manganese-based composite oxide" includes oxides containing one or more additional elements other than those in addition to the oxides composed of Li, Ni, Co, Mn, and O as constituent elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn, etc. Further, the additional element may be a semi-metal element such as B, C, Si, P, etc., or a non-metal element such as S, F, Cl, Br, I, etc. This also applies to the above-mentioned lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel manganese-based composite oxides, lithium nickel cobalt aluminum-based composite oxides, lithium iron nickel manganese-based composite oxides, and the like.

[0088] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium manganese iron phosphate, and the like.

[0089] These cathode active materials may be used alone or in combination of two or more. As the cathode active material, a lithium nickel cobalt manganese-based composite oxide is particularly preferable because of its excellent various properties such as initial resistance characteristics.

[0090] The average particle diameter (D50) of the positive electrode active material is not particularly limited, but for example, it is 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0091] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, etc. As the conductive material, for example, carbon black such as acetylene black (AB); carbon fibers such as vapor grown carbon fiber (VGCF) and carbon nanotube (CNT); and other carbon materials (e.g., graphite, etc.) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVdF) etc. can be used.

[0092] The content of the positive electrode active material in the positive electrode active material layer 54 (that is, the content of the positive electrode active material with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, but preferably 70% by mass or more, more preferably 80% by mass or more, and still more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0093] The thickness per side of the positive electrode active material layer 54 is not particularly limited, but is usually 10 μm or more, preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, preferably 300 μm or less.

[0094] As the negative electrode sheet 60, the above-described negative electrode 60 is used.

[0095] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0096] The thickness of the separator 70 is not particularly limited, but for example, it is 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less. The air permeability of the separator 70 obtained by the Gurley test method is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0097] The non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones, which are used in the electrolytes of general lithium-ion secondary batteries, can be used without particular limitation. Among them, carbonates are preferable, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As an example, the non-aqueous solvent consists only of carbonates. As another example, the non-aqueous solvent contains carbonates and esters such as methyl acetate.

[0098] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0099] In addition, as long as the effects of the present invention are not significantly impaired, the non-aqueous electrolyte may contain components other than the above-described components, for example, film-forming agents such as vinylene carbonate (VC) and oxalato complex; gas generators such as biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and various other additives.

[0100] The lithium ion secondary battery 100 is suppressed in capacity deterioration when charge and discharge are repeated, and also has a high capacity. The lithium ion secondary battery 100 can be used for various applications. Suitable applications include drive power sources mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Further, the lithium ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium ion secondary battery 100 can typically also be used in the form of an assembled battery in which a plurality of them are connected in series and / or in parallel.

[0101] As described above, as an example, the rectangular lithium ion secondary battery 100 including the flat wound electrode body 20 has been described. However, the lithium ion secondary battery can also be configured as a lithium ion secondary battery including a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). Further, the lithium ion secondary battery can also be configured as a cylindrical lithium ion secondary battery, a laminated case type lithium ion secondary battery, or the like.

[0102] Also, according to a known method, the lithium ion secondary battery 100 can be configured as an all-solid-state lithium ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0103] In addition, the negative electrode 60 according to the present embodiment is suitable for the negative electrode of a lithium ion secondary battery, but can be constructed and used as the negative electrode of other secondary batteries, and the other secondary batteries can be configured according to known methods.

[0104] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not intended to be limited to those shown in such embodiments.

[0105] <Fabrication of Negative Electrode> [Example 1] As negative electrode active materials, the following were prepared. The Si content ratios of the first Si-C composite particles and the second Si-C composite particles were measured using a commercially available ICP analyzer. The average particle diameter (D50) of each particle was measured using a commercially available laser diffraction / scattering type particle size distribution measuring device. The compression elastic modulus of the first graphite particles and the second graphite particles was measured by the method described later. First Si-C composite particles: Si-C composite material, Si content ratio = 60% by mass, average particle diameter (D50) = 6 μm Second Si-C composite particles: Si-C composite material, Si content ratio = 40% by mass, average particle diameter (D50) = 8 μm First graphite particles: compression elastic modulus = 80 MPa, average particle diameter (D50) = 15 μm Second graphite particles: compression elastic modulus = 180 MPa, average particle diameter (D50) = 14 μm

[0106] As a conductive material, single-walled carbon nanotubes (SWCNTs) were prepared. The SWCNTs were prepared in the form of an aqueous dispersion with a solid content concentration of 2%. As binders, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared.

[0107] A first mixed material containing the first graphite particles, the first Si-C composite particles, SWCNTs, CMC, and PAA in a mass ratio of 70:30:0.1:1:1 was prepared by the following procedure.

[0108] First, the first Si-C composite particles, the SWCNT dispersion, and the dispersion medium (water) were mixed using a disper at 3000 rpm to prepare a preliminary mixed paste. On the other hand, the first graphite particles, CMC, and PAA were dry-blended using a stirring granulator. To the obtained dry mixture, the preliminary mixed paste and the dispersion medium (water) were added and kneaded using a stirring granulator. Thereby, a first mixed material containing a first granulated body in which the first Si-C composite particles and the first graphite particles were aggregated was obtained.

[0109] Next, except for using the second Si-C composite particles instead of the first Si-C composite particles, in the same manner as above, the second graphite particles, the second Si-C composite particles, single-walled carbon nanotubes (SWCNT), carboxymethyl cellulose (CMC), and polyacrylic acid (PAA) were used to prepare a second mixed material containing them in a mass ratio of 70:30:0.1:1:1. This second mixed material contained a second granulated body in which the second Si-C composite particles and the second graphite particles were aggregated.

[0110] The first mixed material and the second mixed material were put into a planetary mixer at a mass ratio of 50:50. Further, 1.5 parts by mass of SBR with respect to 100 parts by mass of the negative electrode active material (that is, a total of 100 parts by mass of the first Si-C composite particles, the second Si-C composite particles, the first graphite particles, and the second graphite particles) was put into the planetary mixer. Further, a dispersion medium was added and these were diluted and mixed using a planetary mixer to obtain a negative electrode paste.

[0111] The prepared negative electrode paste was applied to the surface of a copper foil with a thickness of 10 μm and dried to form a negative electrode active material layer. The obtained sheet was processed into a predetermined size after roll pressing to obtain a negative electrode sheet.

[0112] <Measurement of Compressive Elastic Modulus of Graphite Particles> For the measurement of the compression elastic modulus of the first graphite particles and the second graphite particles, a micro compression tester "MTC-211" (manufactured by Shimadzu Corporation) was used. Using this tester, one graphite particle was compressed in the vertical direction, and the displacement amount and stress during compression were measured. Based on the following formula, the compression elastic modulus was obtained. Here, the measurement was performed until the graphite particle was displaced by 10%. When fracture occurred in the graphite particle before 10% displacement, the compression elastic modulus was calculated using the value of the compression stress immediately before fracture and the compression displacement amount. For each of the first graphite particles and the second graphite particles, measurements were performed on 5 graphite particles, and the average value of the compression elastic moduli of the 5 graphite particles was adopted as the compression elastic modulus of the first graphite particles and the second graphite particles. Compression elastic modulus (MPa) = Compression stress σ (MPa) / Compression strain ε (Compression strain ε = Compression displacement amount / Average particle diameter of graphite (D50))

[0113] [Example 2] The negative electrode of Example 2 was obtained in the same manner as in Example 1, except that the mass ratio of the first Si-C composite particles to the first graphite particles in the first mixed material was changed to 28:72, and the mass ratio of the first mixed material to the second mixed material was changed to 60:40.

[0114] [Example 3] The negative electrode of Example 3 was obtained in the same manner as in Example 1, except that the mass ratio of the first Si-C composite particles to the first graphite particles in the first mixed material was changed to 32:68, and the mass ratio of the first mixed material to the second mixed material was changed to 40:60.

[0115] [Example 4] The negative electrode of Example 4 was obtained in the same manner as in Example 1, except that the mass ratio of the first Si-C composite particles to the first graphite particles in the first mixed material was changed to 26:74, and the mass ratio of the first mixed material to the second mixed material was changed to 80:20.

[0116] [Example 5] The negative electrode of Example 5 was obtained in the same manner as in Example 1, except that the mass ratio of the first Si-C composite particles to the first graphite particles in the first mixed material was changed to 45:55, and the mass ratio of the first mixed material to the second mixed material was changed to 20:80.

[0117] Comparative Example 1 A negative electrode paste containing first graphite particles, second graphite particles, first Si-C composite particles, second Si-C composite particles, SWCNT, CMC, PAA, and SBR in a mass ratio of 35:35:15:15:0.1:1:1:1.5 was prepared by the following procedure.

[0118] First, the first Si-C composite particles, the second Si-C composite particles, the SWCNT dispersion, and the dispersion medium (water) were mixed using a disper at 3000 rpm to prepare a preliminary mixed paste. On the other hand, the first graphite particles, the second graphite particles, CMC, and PAA were dry-blended using a stirring granulator. To the obtained dry mixture, the preliminary mixed paste and the dispersion medium (water) were added and kneaded using a stirring granulator. Thereby, a third mixed material was obtained.

[0119] The negative electrode paste was obtained by dilution mixing of the third mixed material, SBR, and the dispersion medium using a planetary mixer.

[0120] The prepared negative electrode paste was applied to the surface of a copper foil with a thickness of 10 μm and dried to form a negative electrode active material layer. The obtained sheet was processed into a predetermined size after roll pressing to obtain a negative electrode sheet of Comparative Example 1.

[0121] Comparative Example 2 A negative electrode of Comparative Example 2 was obtained in the same manner as in Example 1, except that the second Si-C composite particles contained in the second mixed material were changed to first Si-C composite particles.

[0122] Comparative Example 3 A negative electrode of Comparative Example 3 was obtained in the same manner as in Example 1, except that the second graphite particles contained in the second mixed material were changed to first graphite particles.

[0123] Comparative Example 4 The negative electrode of Comparative Example 4 was obtained in the same manner as in Example 1, except that the first graphite particles contained in the first composite material were changed to second graphite particles, and the second graphite particles contained in the second composite material were changed to first graphite particles.

[0124] <Fabrication of Lithium-Ion Secondary Battery for Evaluation> As the positive electrode active material powder, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste was applied to the surface of an aluminum foil with a thickness of 15 μm and dried to form a positive electrode active material layer. After roll-pressing the positive electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a positive electrode sheet.

[0125] A separator made of porous polyolefin was prepared. Leads were attached to each of the negative electrode sheet and the positive electrode sheet prepared above, and they were laminated via the separator to fabricate an electrode assembly. This was housed together with a non-aqueous electrolyte in a case made of an aluminum laminate film. As the non-aqueous electrolyte, a solution in which LiPF6 as a supporting salt was dissolved at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 15:5:40:40 was used. Then, the case was sealed to obtain a lithium-ion secondary battery for evaluation.

[0126] <Cycle Performance Evaluation> Each of the fabricated lithium-ion secondary batteries for evaluation was placed in an environment at 25°C. Each lithium-ion secondary battery for evaluation was subjected to constant current charging up to 4.2 V at a current value of 0.4C, and then constant voltage charging was performed until the current value reached 0.1C. Next, each lithium-ion secondary battery for evaluation was discharged at a constant current of 0.4C to 2.5 V. Then, the discharge capacity at this time was measured to obtain the initial capacity.

[0127] The charge and discharge described above was repeated 250 cycles with one cycle of the charge and discharge. The discharge capacity after 250 cycles was determined in the same manner as the initial capacity. As an index of the cycle characteristics, the capacity retention rate (%) was determined from (discharge capacity after 250 cycles of charge and discharge / initial capacity) × 100. The results are shown in Table 1.

[0128]

Table 1

[0129] From the results in Table 1, particularly from the comparison of Examples 1 to 5 and Comparative Examples 1 to 4, it can be seen that when a large number of the first Si-C composite particles with a high Si content are in contact with a large number of the first graphite particles with a low compression elastic modulus, and a large number of the second Si-C composite particles with a low Si content are in contact with a large number of the second graphite particles with a high compression elastic modulus, the capacity retention rate after 250 cycles of charge and discharge becomes high. Therefore, according to the negative electrode disclosed herein, it can be seen that capacity degradation during repeated charge and discharge of the secondary battery can be suppressed.

[0130] As described above, specific examples of the present invention have been described in detail, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples exemplified above.

[0131] That is, the negative electrode of the secondary battery, its manufacturing method, and the secondary battery disclosed herein are as follows in items [1] to [8]. [1] A negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, The negative electrode active material layer contains first graphite particles, second graphite particles, first Si-C composite particles, and second Si-C composite particles, The Si content in the first Si-C composite particles is greater than the Si content in the second Si-C composite particles, The compression elastic modulus of the first graphite particles is smaller than the compression elastic modulus of the second graphite particles, The first Si-C composite particles are in contact with the first graphite particles more than the second graphite particles, The second Si-C composite particles are in contact with the second graphite particles more than with the first graphite particles. Negative electrode. [2] The Si content in the first Si-C composite particles is 45% by mass or more and 80% by mass or less, the Si content in the second Si-C composite particles is 20% by mass or more and 55% by mass or less, The negative electrode according to item [1], wherein the Si content in the first Si-C composite particles relative to the Si content in the second Si-C composite particles is 1.1 or more. [3] The compression elastic modulus of the first graphite particles is 130 MPa or less, and the compression elastic modulus of the second graphite particles is 140 MPa or more. The negative electrode according to item [1] or [2]. [4] At least some of the first Si-C composite particles and at least some of the first graphite particles form a first granule in which they are aggregated, and at least some of the second Si-C composite particles and at least some of the second graphite particles form a second granule in which they are aggregated. The negative electrode according to any one of items [1] to [3]. [5] In the first granule, the first Si-C composite particles and the first graphite particles are bound by a first binder, and in the second granule, the second Si-C composite particles and the second graphite particles are bound by a second binder. The negative electrode according to item [4]. [6] The mass ratio of the first Si-C composite particles to the total of the first graphite particles and the first Si-C composite particles is 10% by mass to 60% by mass, The mass ratio of the second Si-C composite particles to the total of the second graphite particles and the second Si-C composite particles is 10% by mass to 60% by mass. The negative electrode according to any one of items [1] to [5]. [7] A step of preparing a first granule in which first Si-C composite particles and first graphite particles are aggregated, and a second granule in which second Si-C composite particles and second graphite particles are aggregated, wherein the Si content in the first Si-C composite particles is greater than the Si content in the second Si-C composite particles, and the compression elastic modulus of the first graphite particles is smaller than the compression elastic modulus of the second graphite particles; A step of mixing the first granule and the second granule in a dispersion medium to prepare a negative electrode paste; A step of coating the negative electrode paste on a negative electrode current collector; A step of drying the coated negative electrode paste; A method for manufacturing a negative electrode comprising the above steps. [8] In the first granule, the first Si-C composite particles and the first graphite particles are bound by a first binder, and in the second granule, the second Si-C composite particles and the second graphite particles are bound by a second binder. The manufacturing method according to item [7]. [9] The mass ratio of the first Si-C composite particles to the total of the first graphite particles and the first Si-C composite particles is 10% by mass to 60% by mass, The mass ratio of the second Si-C composite particles to the total of the second graphite particles and the second Si-C composite particles is 10% by mass to 60% by mass. The manufacturing method according to item [7] or [8].

[10] A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode according to any one of items [1] to [6]. A secondary battery.

Explanation of symbols

[0132] 12 First graphite particles 14 Second graphite particles 16 First Si-C composite particles 18 Second Si-C composite particles 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode sheet (positive electrode) 52 Positive electrode current collector 52a Non-formed part of positive electrode active material layer 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode) 62 Negative electrode current collector 62a Non-formed part of negative electrode active material layer 64 Negative electrode active material layer 70 Separator sheet (separator) 100 Lithium-ion secondary battery

Claims

1. A negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, wherein the negative electrode active material layer contains first graphite particles, second graphite particles, first Si—C composite particles, and second Si—C composite particles, the Si content in the first Si—C composite particles is greater than the Si content in the second Si—C composite particles, the compression elastic modulus of the first graphite particles is smaller than the compression elastic modulus of the second graphite particles, the first Si—C composite particles are in contact with the first graphite particles more than with the second graphite particles, and the second Si—C composite particles are in contact with the second graphite particles more than with the first graphite particles. Negative electrode.

2. the Si content in the first Si—C composite particles is 45% by mass or more and 80% by mass or less, the Si content in the second Si—C composite particles is 20% by mass or more and 55% by mass or less, and the ratio of the Si content in the first Si—C composite particles to the Si content in the second Si—C composite particles is 1.1 or more. The negative electrode according to claim 1.

3. The compression elastic modulus of the first graphite particles is 130 MPa or less, and the compression elastic modulus of the second graphite particles is 140 MPa or more. The negative electrode according to claim 1.

4. At least some of the first Si—C composite particles and at least some of the first graphite particles form a first granule in which they are aggregated, and at least some of the second Si—C composite particles and at least some of the second graphite particles form a second granule in which they are aggregated. The negative electrode according to claim 1.

5. In the first granule, the first Si—C composite particles and the first graphite particles are bound by a first binder, and in the second granule, the second Si—C composite particles and the second graphite particles are bound by a second binder. The negative electrode according to claim 4.

6. The mass ratio of the first Si—C composite particles to the total of the first graphite particles and the first Si—C composite particles is 10% by mass to 60% by mass, and the mass ratio of the second Si—C composite particles to the total of the second graphite particles and the second Si—C composite particles is 10% by mass to 60% by mass. The negative electrode according to claim 1.

7. A step of preparing a first granule in which first Si—C composite particles and first graphite particles are aggregated, and a second granule in which second Si—C composite particles and second graphite particles are aggregated, wherein the Si content in the first Si—C composite particles is larger than the Si content in the second Si—C composite particles, and the compression elastic modulus of the first graphite particles is smaller than the compression elastic modulus of the second graphite particles. A step of mixing the first granule and the second granule in a dispersion medium to prepare a negative electrode paste. A step of coating the negative electrode paste on a negative electrode current collector. A step of drying the coated negative electrode paste. A method for manufacturing a negative electrode comprising the above steps.

8. In the first granule, the first Si—C composite particles and the first graphite particles are bound by a first binder, and in the second granule, the second Si—C composite particles and the second graphite particles are bound by a second binder. The manufacturing method according to claim 7.

9. The mass ratio of the first Si—C composite particles to the total of the first graphite particles and the first Si—C composite particles is 10% by mass to 60% by mass, and the mass ratio of the second Si—C composite particles to the total of the second graphite particles and the second Si—C composite particles is 10% by mass to 60% by mass. The manufacturing method according to claim 7.

10. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte. The secondary battery, wherein the negative electrode is the negative electrode according to claim 1. ​

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