Negative electrode for secondary battery, method for manufacturing negative electrode, and secondary battery using negative electrode

Abstract

The present disclosure relates to a negative electrode for a secondary battery, a method for manufacturing the negative electrode, and a secondary battery using the negative electrode. Provided is a negative electrode which contains Si-containing particles and graphite particles, and which is capable of suppressing capacity deterioration during repeated charging and discharging of a secondary battery. A negative electrode of a secondary battery according to the present disclosure includes a negative electrode current collector, and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles. The content ratio of Si in the first Si-containing particles is smaller than the content ratio of Si in the second Si-containing particles. When the density of a molded article obtained by press-molding 1 g of the particles in the uniaxial direction at 25 DEG C and 60 MPa into a sheet shape having a diameter of 20 mm is defined as the compaction density, the compaction density of the first Si-containing particles is 0.9 g / cm3 or more, and the compaction density of the second Si-containing particles is less than 0.9 g / cm3.

Description

Technical Field

[0001] This disclosure relates to the negative electrode of a secondary battery and a method for manufacturing the same. Background Technology

[0002] In recent years, rechargeable batteries have become suitable for use as power sources for personal computers, portable terminals, electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In applications for power supplies used in vehicle propulsion, particularly for battery electric vehicles (BEVs), there is a desire for further increases in the capacity of secondary batteries, from the perspective of extending the vehicle's driving range. As a high-capacity negative electrode active material, silicon particles are known, and it is known that the inclusion of silicon particles can enable high-capacity secondary batteries (see, for example, Japanese Patent Application Publication No. 2015-38862). Japanese Patent Application Publication No. 2015-38862 discloses a technology that uses silicon particles and graphite particles such as natural graphite as a negative electrode active material. Summary of the Invention

[0004] However, while Si-containing particles have high capacity, they also result in significant volume changes due to expansion and contraction during charging and discharging. Therefore, when Si-containing particles and graphite particles are used together, the filling capacity of these particles decreases during repeated charging and discharging of the secondary battery, potentially leading to the disruption of conductive pathways. Consequently, the use of Si-containing particles and graphite particles results in reduced cycle characteristics of the secondary battery, specifically significant capacity degradation during repeated charging and discharging.

[0005] In view of the above-mentioned actual situation, the purpose of this disclosure is to provide a negative electrode containing Si particles and graphite particles, which can suppress capacity degradation of secondary batteries during repeated charging and discharging.

[0006] The negative electrode of the secondary battery disclosed herein includes a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer contains graphite particles, a first type of Si-containing particles, and a second type of Si-containing particles. The Si content in the first type of Si-containing particles is smaller than the Si content in the second type of Si-containing particles. When the density of the molded body obtained by pressing 1g of particles into a sheet with a diameter of 20mm in the uniaxial direction at 25°C and 60MPa is defined as the compaction density, the compaction density of the first type of Si-containing particles is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

[0007] Based on this configuration, a negative electrode containing Si particles and graphite particles can be provided, which can suppress capacity degradation during repeated charging and discharging of secondary batteries.

[0008] From another perspective, the method for manufacturing the negative electrode of the secondary battery disclosed herein includes: a step of preparing a negative electrode paste containing graphite particles, a first Si-containing particle, a second Si-containing particle, and a dispersion medium; a step of coating the negative electrode paste onto a negative electrode current collector; a step of drying the coated negative electrode paste to form a negative electrode active material layer; and a step of pressing the negative electrode active material layer. The Si content in the first Si-containing particle is smaller than the Si content in the second Si-containing particle. When the density of the molded body obtained by pressing 1g of particles into a sheet with a diameter of 20mm in the uniaxial direction at 25°C and 60MPa is defined as the particle compaction density, the compaction density of the first Si-containing particle is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

[0009] The negative electrode obtained by adopting this structure can impart excellent capacity degradation resistance to repeated charge and discharge cycles of the secondary battery.

[0010] From another perspective, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the aforementioned negative electrode.

[0011] Based on this configuration, a secondary battery with excellent capacity degradation resistance during repeated charge and discharge can be provided. Attached Figure Description

[0012] Figure 1 A cross-sectional view illustrating the configuration of the negative electrode of a secondary battery according to an embodiment of the present disclosure.

[0013] Figure 2 To illustrate Figure 1 A cross-sectional view of the particle composition contained in the negative electrode active material layer.

[0014] Figure 3 A cross-sectional view is shown to schematically illustrate the structure of a lithium-ion secondary battery constructed using the negative electrode of a secondary battery according to an embodiment of the present disclosure.

[0015] Figure 4 To show Figure 3 A schematic exploded view of the structure of the wound electrode body of a lithium-ion secondary battery. Detailed Implementation

[0016] The embodiments of this disclosure will be described below with reference to the accompanying drawings. It should be noted that any matters necessary for the implementation of this disclosure that are not mentioned in this specification can be understood by those skilled in the art based on prior art. This disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in the field. Furthermore, in the following drawings, components and parts that perform the same function are labeled with the same reference numerals. Additionally, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships. It should be noted that in this specification, the numerical range represented as "A to B" includes both A and B.

[0017] It should be noted that in this specification, "secondary battery" refers to an energy storage device capable of repeated charging and discharging. Additionally, in this specification, "lithium-ion secondary battery" refers to a secondary battery that utilizes lithium ions as charge carriers and achieves charging and discharging through the movement of charges accompanying lithium ions between the positive and negative electrodes.

[0018] The negative electrode disclosed herein is used in secondary batteries, preferably in lithium-ion secondary batteries. (Refer to...) Figure 1 An embodiment of the negative electrode disclosed herein will be described in detail. Figure 1 The cross-sectional view shown is schematically 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. Figure 1 The negative electrode 60 shown in this embodiment is the negative electrode of a lithium-ion secondary battery.

[0019] 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 disposed on the negative electrode current collector 62. The negative electrode active material layer 64 may be disposed on only one side of the negative electrode current collector 62, or it may be disposed on both sides of the negative electrode current collector 62 as shown in the example. Preferably, the negative electrode active material layer 64 is disposed on both sides of the negative electrode current collector 62.

[0020] As shown in the example, a non-formed portion 62a of the negative electrode active material layer 64, where no negative electrode active material layer 64 is formed, can be provided at one end of the negative electrode 60 in the width direction. In the non-formed portion 62a, the negative electrode current collector 62 is exposed, and the non-formed portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0021] The negative electrode current collector 62 is shown in the example as a foil (or sheet), but is not limited to this. The negative electrode current collector 62 can be in various forms, such as rod-shaped, plate-shaped, or mesh-shaped. As with conventional lithium-ion secondary batteries, a metal with good conductivity (such as copper, nickel, titanium, stainless steel, etc.) can be used as the material for the negative electrode current collector 62, with copper being preferred. Copper foil is particularly preferred as the negative electrode current collector 62.

[0022] There is no particular limitation on the size of the negative electrode current collector 62, which can be appropriately determined according to the battery design. When copper foil is used as the negative electrode current collector 62, there is no particular limitation on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 6 μm or more and 20 μm or less.

[0023] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, at least graphite particles, first Si-containing particles, and second Si-containing particles are used. Figure 2 A detailed explanation. Figure 2 To show Figure 1 A schematic cross-sectional view of the particles contained in the negative electrode active material layer 64 is shown. Figure 2 As shown, the negative electrode active material layer 64 comprises graphite particles 12, a first Si-containing particle 14, and a second Si-containing particle 16. It should be noted that... Figure 2 For illustrative purposes only; therefore, the number, distribution, etc., of particles are not limited to... Figure 2 The content shown.

[0024] The graphite constituting the graphite particles 12 can be natural graphite or artificial graphite, and the graphite can be amorphous carbon-coated graphite in the form of amorphous carbon material.

[0025] The shape of the graphite particles 12 is not particularly limited and can be flake-like, spherical, etc. The graphite particles 12 are preferably spherical graphite particles. When the graphite particles 12 are spherical, the sphericity of the graphite particles 12 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0026] It should be noted that in this specification, "circularity" refers to the ratio of the circumference of a circle with the same area as the particle's projected area to the circumference of the particle's projected image (i.e., circularity = circumference of a circle with the same area as the particle's projected area / circumference of the particle's projected image). Therefore, the closer the circularity is to 1, the closer the particle's projected image is to a perfect circle, and the closer the particle is to a perfect sphere. Circularity can be calculated, for example, by using a commercially available static automatic image analysis device to determine the circularity of more than 100 particles and calculate its average value.

[0027] There is no particular limitation on the average particle size (D50) of the graphite particles 12. The average particle size (D50) of the graphite particles 12 is, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

[0028] It should be noted that in this specification, the term "average particle size (D50)" refers to the median diameter (D50), which is the particle size in a volume-based particle size distribution based on laser diffraction-scattering methods that corresponds to a cumulative frequency of 50% by volume from the side of the smallest particle. The average particle size (D50) can be determined using commercially available laser diffraction-scattering particle size distribution measuring devices, etc.

[0029] When 1g of graphite particles 12 are pressurized in a uniaxial direction to form a sheet with a diameter of 20mm, the density of the formed body is preferably 1.7g / cm³. 3 The molding pressure is 20 MPa or less, more preferably 10 MPa to 18 MPa. This molding pressure can be easily measured using an automatic powder resistance measurement system (e.g., the "MCP-PD600" manufactured by Nitto Seiko Analytech) and a 20 mm diameter probe (equivalent to a mold).

[0030] The proportion of graphite particles 12, which is the sum of graphite particles 12, first Si-containing particles 14, and second Si-containing particles 16, is preferably 40% to 90% by mass, more preferably 45% to 85% by mass, and even more preferably 50% to 80% by mass.

[0031] The first Si-containing particle 14 and the second Si-containing particle 16 can be, for example, particles from a Si-C composite material. Si-C composite materials typically contain carbon domains and Si-containing domains. It should be noted that the first Si-containing particle 14 and the second Si-containing particle 16 may not be Si-C composite materials, but may be Si particles, Si oxide particles, etc.

[0032] Carbon domains can be, for example, carbides of carbon precursors (e.g., petroleum asphalt, coal tar pitch, phenolic resins, etc.); graphite, etc. The carbon domains preferably constitute a carbon matrix. Therefore, Si-C composite materials are preferably materials in which multiple Si-containing domains are dispersed in a carbon matrix. In this case, the carbon matrix can mitigate the volume changes caused by the expansion / contraction of the Si-containing domains, which is advantageous.

[0033] Domains containing Si include Si, for example, those composed of Si, Si oxide (SiO2). x ), Si nitride (SiN) x ), Si carbide (SiC) xIt is composed of, etc. The domains containing Si are preferably composed of Si, and Si oxide (SiO2). x It consists of at least one of the following: The domains containing Si can be microparticles. The oxygen content in the domains containing Si is preferably 10% by mass or less.

[0034] The average particle size of the Si-containing domains is, for example, less than 50 nm, and can be between 5 nm and 50 nm. It should be noted that the "average particle size of the Si-containing domains" can be determined as follows: First, the negative electrode active material layer 64 is processed using FIB (Focused Ion Beam) to prepare a sample for observation by a scanning transmission electron microscope (STEM). Then, elemental analysis of the sample is performed using EDX elemental mapping to obtain BF (bright field) and HAADF (high angle scattering annular dark field) images. The diameter of the Si-containing domains can be determined from the contrast and shape obtained using the BF and HAADF images. The diameters of arbitrarily selected 10 or more Si-containing domains are determined, and their average value is set as the "average particle size of the Si-containing domains" here.

[0035] Si-C composite materials can be, for example, materials in which Si-containing microparticles are dispersed within a carbon material; or materials in which Si-containing microparticles are incorporated into the pores of granulated porous graphite. Si-C composite materials can also be materials in which Si-containing microparticles are attached to the surface of carbon particles; or materials in which carbon microparticles are attached to the surface of Si-containing particles. From the viewpoint of suppressing volume changes of Si, materials in which Si nanoparticles are dispersed within a carbon material and materials in which Si nanoparticles are dispersed within the pores of a porous carbon material are preferred; materials in which Si nanoparticles are dispersed within the pores of a porous carbon material are even more preferred.

[0036] The proportion of Si in the first Si-containing particle 14 (S1) is smaller than the proportion of Si in the second Si-containing particle 16 (S2). There are no particular limitations as long as the proportions of Si in the first Si-containing particle 14 (S1) and the second Si-containing particle 16 (S2) satisfy this relationship. However, if these Si proportions are too low, the improvement in cycle characteristics will be smaller, and the capacity enhancement effect of the secondary battery may be excessively reduced. On the other hand, if these Si proportions are too high, the volume change caused by the expansion / contraction of the first Si-containing particle 14 and the second Si-containing particle 16 during repeated charge-discharge cycles of the secondary battery may be excessively increased.

[0037] Therefore, the Si content (S1) in the first Si-containing particle 14 is preferably 20% to 55% by mass, more preferably 25% to 45% by mass. The Si content (S2) in the second Si-containing particle 16 is preferably 45% to 80% by mass, more preferably 55% to 75% by mass.

[0038] Furthermore, the ratio (S1 / S2) of the Si content ratio (S1) in the first Si-containing particle 14 to the Si content ratio (S2) in the second Si-containing particle 16 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and even more preferably 0.40 to 0.75.

[0039] Here, the density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet under uniaxial pressure at 25℃ and 60MPa is defined as the compaction density. This compaction density serves as an indicator of the particle filling capacity; the higher the compaction density, the higher the particle filling capacity. The compaction density of the first Si-particle-containing 14 is 0.90 g / cm³. 3 That's all. On the other hand, the compaction density of the second Si-particle-containing 16 is less than 0.90 g / cm³. 3 .

[0040] Thus, in this disclosure, in addition to graphite particles 12, a first Si-containing particle 14 with a low Si content and high compaction density is used in combination as the negative electrode active material, along with a second Si-containing particle 16 with a high Si content and low compaction density. Specifically, the compaction density of the Si-containing particles conventionally used is typically less than 0.90 / cm³. 3 Therefore, the first Si-containing particle 14 has a higher compaction density compared to conventionally used Si-containing particles. Based on this particle combination, capacity degradation during repeated charge-discharge cycles of the secondary battery can be suppressed. The reasons for this are as follows.

[0041] One reason for capacity degradation during repeated charge-discharge cycles of a secondary battery is the disruption of conductive pathways. With respect to Si-containing particles containing a low Si content, the expansion and contraction of the secondary battery during charge-discharge cycles are minimal. Therefore, by increasing the compaction density of the first Si-containing particles 14 with a low Si content, as disclosed in this disclosure—that is, by increasing the filling capacity of the first Si-containing particles 14 with a low Si content—the filling state around the first Si-containing particles 14 can be maintained even after repeated charge-discharge cycles of the secondary battery. This suppresses the disruption of conductive pathways during repeated charge-discharge cycles of the secondary battery.

[0042] On the other hand, secondary batteries containing a high proportion of Si particles exhibit significant expansion and contraction during charging and discharging. Therefore, by setting a low compaction density of the second Si particle 16 with a high Si content, as disclosed in this disclosure—that is, by setting a low filling density of the second Si particle 16 with a high Si content—deformation around the second Si particle 16 caused by its expansion / contraction can be suppressed even after repeated charging and discharging of the secondary battery. This suppresses the disruption of the conductive path during repeated charging and discharging of the secondary battery.

[0043] Therefore, by combining the first Si-containing particles 14 with a low Si content and high pressure density with the second Si-containing particles 16 with a high Si content and low pressure density, it is possible to suppress capacity degradation caused by the disconnection of the conductive path during repeated charging and discharging of the secondary battery.

[0044] The preferred compaction density of the first Si-particle-containing 14 is 0.95 g / cm³. 3 The above, more preferably 1.00 g / cm 3 The above is further preferred to be 1.1 g / cm³. 3 The above, especially preferred, is 1.2 g / cm³. 3 That's all. On the other hand, the compaction density of the first Si-containing particle 14 can be 2.3 g / cm³. 3 Below, 2.0g / cm 3 Below, 1.8g / cm 3 Below, or 1.5g / cm 3 the following.

[0045] The preferred compaction density of the second Si-particle-containing 16 is 0.88 g / cm³. 3 The preferred value is 0.86 g / cm³. 3 The following is a further preferred value: 0.85 g / cm³ 3 Below. On the other hand, the compaction density of the second Si-containing particle 16 is preferably 0.50 g / cm³. 3 The above, more preferably 0.65 g / cm³ 3 The above is further preferred to be 0.75 g / cm³. 3 The above, especially preferred, is 0.83 g / cm³. 3 above.

[0046] It should be noted that compaction density can be easily measured, for example, by using an automated powder resistance measurement system (such as the “MCP-PD600” manufactured by Nitto Seiko Analytech) and a 20mm diameter detector (equivalent to a mold).

[0047] It should be noted that the compaction density is affected by the roundness of the Si-containing particles. Increasing the roundness of the Si-containing particles tends to increase the compaction density. Therefore, if the roundness of the Si-containing particles is made to be 0.85–1 (especially 0.90–1), the compaction density is likely to reach 0.90 g / cm³. 3 That's all. On the other hand, if the sphericity of the Si particles is less than 0.85, the compaction density easily becomes less than 0.90 g / cm³. 3 .

[0048] Furthermore, the particle size of the Si-containing particles also affects the compaction density. Therefore, since it is easy to adjust to 0.90 g / cm³, the compaction density is...3 Given the above compaction density, the average particle size (D50) of the first Si-containing particles 14 is preferably 2 μm to 10 μm, more preferably 5 μm to 10 μm. Furthermore, it is easy to adjust to less than 0.90 g / cm³. 3 The compaction density is such that the average particle size (D50) of the second Si-containing particles 16 is preferably 2 μm to 10 μm, more preferably 5 μm to 10 μm, and even more preferably 6 μm to 9 μm.

[0049] The true density of Si-containing particles also affects the compaction density. Therefore, by adjusting the composition (the proportion of constituent elements) of the first Si-containing particle 14 and the second Si-containing particle 16, the compaction density can be finely adjusted.

[0050] The ratio of the average particle size (D50) of the graphite particles 12 to the average particle size (D50) of the first Si-containing particles 14 (D50 of graphite particles 12 / D50 of the first Si-containing particles 14) is not particularly limited. From the viewpoint of particularly high filling performance, the ratio (D50 of graphite particles 12 / D50 of the first Si-containing particles 14) is preferably 1.0 to 8.0, more preferably 1.0 to 5.0, even more preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0051] The ratio of the average particle size (D50) of the graphite particles 12 to the average particle size (D50) of the second Si-containing particles 16 (D50 of graphite particles 12 / D50 of the second Si-containing particles 16) is not particularly limited. From the viewpoint of particularly high filling performance, the ratio (D50 of graphite particles 12 / D50 of the second Si-containing particles 16) is preferably 1.0 to 8.0, more preferably 1.0 to 5.0, even more preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0052] The mass ratio of the first Si-containing particle 14 to the second Si-containing particle 16 is not particularly limited as long as the effects of this disclosure are achieved, for example, it is 10:90 to 90:10. This mass ratio (first:second) is preferably 20:80 to 80:20, more preferably 30:70 to 70:30, even more preferably 35:65 to 70:30, and particularly preferably 45:55 to 60:40.

[0053] The proportion of the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16, and the total content of the first Si-containing particles 14 and the second Si-containing particles 16 is preferably 10% to 60% by mass, more preferably 15% to 55% by mass, and even more preferably 20% to 50% by mass.

[0054] It should be noted that the first Si-containing particle 14 and the second Si-containing particle 16 can be manufactured according to known methods. It should be noted that various methods for manufacturing particles of Si-C composite materials are known (for example, see Japanese Patent Application Publication No. 2015-38862, International Publication No. 2014 / 046144, and other prior art documents listed in that international publication).

[0055] The negative electrode active material layer 64 may contain components other than the negative electrode active material; examples include adhesives and conductive materials. Examples of adhesives include styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVdF). CMC also functions as a thickener. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are preferred. When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a CNT dispersant.

[0056] The content of negative electrode active material in the negative electrode active material layer 64 (i.e., relative to the total mass of the negative electrode active material layer 64) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less. The content of conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less.

[0057] There is no particular limitation on the thickness of the negative electrode active material layer 64, for example, it is 10 μm or more and 400 μm or less, preferably 20 μm or more and 300 μm or less.

[0058] There is no particular limitation on the density of the negative electrode active material layer 64, for example, it can be 0.7 g / cm³. 3 The above, preferably 1.0 g / cm³ 3 The above, more preferably 1.2 g / cm³ 3 That's all. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm³. 3 The following can be 2.0 g / cm³ 3 the following.

[0059] The negative electrode 60 may include components 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 non-forming portion 62a of the negative electrode active material layer. This insulating layer may contain, for example, an insulating inorganic filler.

[0060] The negative electrode 60 is manufactured using a method comprising the following steps: a step of preparing a negative electrode paste containing graphite particles 12, first Si-containing particles 14, second Si-containing particles 16, and a dispersion medium (hereinafter also referred to as the "paste preparation step"); a step of coating the prepared negative electrode paste onto the negative electrode current collector 62 (hereinafter also referred to as the "coating step"); a step of drying the coated negative electrode paste to form a negative electrode active material layer 64 (hereinafter also referred to as the "drying step"); and a step of pressing the negative electrode active material layer 64 (hereinafter also referred to as the pressing step). In this manufacturing method, the Si content ratio (S1) in the first Si-containing particles 14 is smaller than the Si content ratio (S2) in the second Si-containing particles 16. The aforementioned compaction density of the first Si-containing particles 14 is 0.9 g / cm³. 3 The above-mentioned compaction density of the second Si-particle-containing 16 is less than 0.9 g / cm³. 3 .

[0061] It should be noted that in this specification, the term "paste" refers to a mixture obtained by dispersing part or all of the solid components in a dispersion medium, including the so-called "slurry" or "ink".

[0062] The paste preparation process can be carried out by mixing graphite particles 12, first Si-containing particles 14, second Si-containing particles 16, and optional components (e.g., binders, conductive materials, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., in accordance with a known method.

[0063] The coating process can be performed using known methods. Specifically, for example, the coating process can be performed by applying the obtained negative electrode paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, a notched wheel coater, a slot coater, or a die coater.

[0064] The drying process can be carried out using known methods. Specifically, for example, the dispersion medium is removed from the negative electrode current collector 62 coated with the negative electrode paste using a drying apparatus such as a drying oven, thereby forming a negative electrode active material layer 64. This allows the drying process to be performed. The drying temperature and drying time can be appropriately determined based on the solid component 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 higher and 30 minutes or lower, preferably 30 seconds or higher and 10 minutes or lower.

[0065] The pressing process can be performed using known methods. Specifically, the pressing process can be performed by applying pressure to the negative electrode active material layer 64 formed above using a roller press or the like. By using the pressing process, the negative electrode active material layer 64 is compressed to a specified density, thereby tightly filling the graphite particles 12, the first Si-containing particles 14, and the second Si-containing particles 16. In this way, the negative electrode 60 is obtained.

[0066] According to the negative electrode 60 of this embodiment, the secondary battery can be endowed with excellent capacity degradation resistance during repeated charge and discharge. Furthermore, since the negative electrode 60 of this embodiment uses a negative electrode active material containing Si, the secondary battery can achieve high capacity. Therefore, the secondary battery using the negative electrode 60 of this embodiment has high capacity and excellent cycle characteristics.

[0067] Therefore, from another perspective, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 described in the above-described embodiment. The following uses a lithium-ion secondary battery as an example, referring to... Figure 3 and Figure 4 An embodiment of the secondary battery disclosed herein will be described. The following configuration example is a flat, square lithium-ion secondary battery having a flat-shaped wound electrode body and a flat-shaped battery casing.

[0068] Figure 3 The lithium-ion secondary battery 100 shown is a sealed lithium-ion secondary battery constructed by housing a flat, wound electrode body 20 and a non-aqueous electrolyte (not shown) within a flat, square battery casing (i.e., outer container) 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 designed to open the internal pressure if the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. The battery casing 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0069] like Figure 3 and Figure 4As shown, the wound electrode body 20 has a positive electrode sheet 50 and a negative electrode sheet 60 that are wound along the length direction by overlapping two elongated separator sheets 70. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed on one or both (in this case, both) sides of the elongated positive electrode current collector 52 along the length direction. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed on one or both (in this case, both) sides of the elongated negative electrode current collector 62 along the length direction. The non-formed portions 52a of the positive electrode active material layer (i.e., the portions of the positive electrode current collector 52 where the positive electrode active material layer 54 is not formed) and the non-formed portions 62a of the negative electrode active material layer (i.e., the portions of the negative electrode current collector 62 where the negative electrode active material layer 64 is not formed) are formed so that they extend outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned length direction). A positive electrode current collector 42a and a negative electrode current collector 44a are respectively bonded to the non-forming portion 52a of the positive electrode active material layer and the non-forming portion 62a of the negative electrode active material layer.

[0070] As the positive current collector 52 constituting the positive electrode sheet 50, a known positive current collector used in lithium-ion secondary batteries can be used. Examples of such current collectors include sheets or foils made of metals with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive current collector 52.

[0071] There is no particular limitation on the size of the positive current collector 52, which can be appropriately determined according to the battery design. When aluminum foil is used as the positive current collector 52, there is no particular limitation on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0072] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition of positive electrode active material used in lithium-ion secondary batteries can be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. There are no particular limitations on the crystal structure of the positive electrode active material; it can be a layered structure, a spinel structure, an olivine structure, etc.

[0073] As a lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferred. Specific examples include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides.

[0074] It should be noted that in this specification, the term "lithium-nickel-cobalt-manganese composite oxide" is a term that includes oxides containing one or more additional elements besides Li, Ni, Co, Mn, and O as constituent elements. Examples of such additional elements include transition metals and typical metallic elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. Additionally, the added elements may be half-metallic elements such as B, C, Si, and P, and non-metallic elements such as S, F, Cl, Br, and I. This also applies to the aforementioned lithium-nickel composite oxides, lithium-cobalt composite oxides, lithium-manganese composite oxides, lithium-nickel-manganese composite oxides, lithium-nickel-cobalt-aluminum composite oxides, and lithium-iron-nickel-manganese composite oxides.

[0075] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0076] These positive electrode active materials can be used alone or in combination of two or more. Lithium-nickel-cobalt-manganese composite oxides are particularly preferred as positive electrode active materials due to their superior initial resistance and other properties.

[0077] There is no particular limitation on the average particle size (D50) of the positive electrode active material, 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.

[0078] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as lithium triphosphate, conductive materials, and binders. As conductive materials, carbon black such as acetylene black (AB), carbon fibers such as fumed carbon fiber (VGCF) and carbon nanotubes (CNT), and other carbon materials (such as graphite) are preferred. As binders, polyvinylidene fluoride (PVdF) can be used, for example.

[0079] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative 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 even 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, more preferably 0.2% by mass or more and 10% by mass or less. The content of 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, more preferably 0.3% by mass or more and 15% by mass or less. The content of 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, more preferably 0.5% by mass or more and 10% by mass or less.

[0080] The thickness of each single side of the positive electrode active material layer 54 is not particularly limited, but is generally 10 μm or more, preferably 20 μm or more. On the other hand, the thickness is generally 400 μm or less, preferably 300 μm or less.

[0081] The negative electrode 60 described above is used as the negative electrode sheet 60.

[0082] As the separator 70, examples include porous sheets (membranes) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. This porous sheet can be a single-layer structure or a multi-layered structure with two or more layers (e.g., a three-layer structure with PP layers stacked on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0083] The thickness of the separator 70 is not particularly limited, 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 Grie test method is not particularly limited, but preferably 350 seconds / 100cc or less.

[0084] Non-aqueous electrolytes typically contain 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 used in general lithium-ion secondary battery electrolytes can be used without particular limitation. Among these, carbonates are preferred, and specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene monofluorocarbonate (FEC), ethylene difluorocarbonate (DFEC), methyl monofluorodifluoromethyl carbonate (F-DMC), and dimethyl trifluorocarbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As an example, the non-aqueous solvent may contain only carbonates. As another example, the non-aqueous solvent may contain carbonates and esters such as methyl acetate.

[0085] As the supporting salt, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) are preferably used (LiPF6 is preferred). The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0086] It should be noted that the aforementioned non-aqueous electrolyte may contain components other than those mentioned above, provided that it does not significantly impair the effectiveness of this disclosure, such as film-forming agents like vinylene carbonate (VC) and oxalic acid complexes; gas generators like biphenyl (BP) and cyclohexylbenzene (CHB); and various additives such as thickeners.

[0087] Regarding the lithium-ion secondary battery 100, capacity degradation during repeated charge and discharge is suppressed, and it possesses high capacity. The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power supplies for electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Additionally, the lithium-ion secondary battery 100 can also be used as a battery for small energy storage devices. The lithium-ion secondary battery 100 can also typically be used in the form of a battery pack consisting of multiple batteries connected in series and / or in parallel.

[0088] The above description, as an example, illustrates a square lithium-ion secondary battery 100 comprising a flat, wound electrode body 20. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries comprising stacked electrode bodies (i.e., electrode bodies in which multiple positive and multiple negative electrodes are alternately stacked). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, laminated shell type lithium-ion secondary batteries, etc.

[0089] In addition, according to known methods, the lithium-ion secondary battery 100 can also be configured as an all-solid-state lithium-ion secondary battery that uses a solid electrolyte instead of a non-aqueous electrolyte.

[0090] Furthermore, the negative electrode 60 involved in this embodiment is suitable for the negative electrode of lithium-ion secondary batteries, but it can also be constructed as the negative electrode of other secondary batteries, which can be constructed according to known methods.

[0091] The following describes in detail the embodiments related to this disclosure, but it is not intended to limit this disclosure to the contents shown in the embodiments.

[0092] <Making the Negative Electrode>

[0093] [Example 1]

[0094] Prepare the following substances as negative electrode active materials.

[0095] First type of Si-containing particles: Si-C composite material, Si content = 40% by mass, average particle size (D50) = 7 μm, compaction density = 1.2 g / cm³ 3

[0096] The second type of Si-containing particle: Si-C composite material, Si content = 62% by mass, average particle size (D50) = 6 μm, compaction density = 0.85 g / cm³ 3

[0097] Graphite particles (first and second graphite particles): average particle size (D50) = 15 μm; 1 g of graphite particles are pressed uniaxially to form a density of 1.7 g / cm³. 3 Furthermore, the molding pressure for a 20mm diameter tablet is 15MPa.

[0098] It should be noted that the Si content ratio of the first and second Si-containing particles was determined using a commercially available ICP-OEC apparatus. The average particle size (D50) of each particle was determined using a commercially available laser diffraction-scattering particle size distribution measuring device. The compaction density of the first and second Si-containing particles was determined using the following method.

[0099] One gram of either the first or second Si-containing particle sample was measured and placed on the detector (20 mm diameter) of the "MCP-PD600" (manufactured by Nitto Seiko Analytech) automated powder resistivity measuring system. At 25°C, the load and displacement under uniaxial pressure were measured using this system. Based on this, the bulk density of the molded body at a pressure of 60 MPa was determined and set as the compaction density. Additionally, one gram of graphite particles was measured and placed on the detector (20 mm diameter) of the "MCP-PD600" (manufactured by Nitto Seiko Analytech) automated powder resistivity measuring system. At 25°C, the load and displacement under uniaxial pressure were measured using this system. Based on this, the density of the molded body was determined to be 1.7 g / cm³. 3 The molding pressure at that time.

[0100] As binders, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) are prepared. Additionally, a dispersion of single-layer carbon nanotubes (SWCNTs) is prepared as a conductive material.

[0101] The following steps were used to prepare a negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, CMC, PAA, SBR and SWCNT in a mass ratio of 60:20:20:1:1:1:0.1.

[0102] First, graphite particles, first Si-containing particles, second Si-containing particles, CMC, and PAA are dry-mixed using a planetary mixer. The resulting mixture, SWCNT dispersion, and dispersion medium are then kneaded using a planetary mixer. SBR is added, followed by the dispersion medium, and the mixture is thoroughly mixed to prepare the negative electrode paste.

[0103] A negative electrode active material layer is formed by coating a prepared negative electrode paste onto the surface of a 10 μm thick copper foil and drying it. After rolling the negative electrode active material layer, the resulting sheet is processed to a specified size to obtain the negative electrode sheet.

[0104] [Example 2]

[0105] In addition to being the first Si-containing particle, it uses a compaction density of 1.0 g / cm³. 3 Except for the particles of Si-C composite material containing 51% by mass of Si, the negative electrode of Example 2 was obtained using the same method as in Example 1.

[0106] [Example 3]

[0107] Except for changing the composition of the negative electrode paste, i.e. using a negative electrode paste containing graphite particles, first Si particles, second Si particles, CMC, PAA, SBR, and SWCNT in a mass ratio of 60:16:24:1:1:1:0.1, the negative electrode sheet of Example 3 was obtained by the same method as in Example 1.

[0108] [Example 4]

[0109] Except for changing the composition of the negative electrode paste, i.e., using a negative electrode paste containing graphite particles, first Si particles, second Si particles, CMC, PAA, SBR, and SWCNT in a mass ratio of 60:12:28:1:1:1:0.1, the negative electrode sheet of Example 4 was obtained by the same method as in Example 1.

[0110] [Example 5]

[0111] In addition to being the second Si-containing particle, a compaction density of 0.8 g / cm³ was used. 3 Except for the particles of Si-C composite material containing 64% by mass of Si, the negative electrode of Example 5 was obtained using the same method as in Example 1.

[0112] [Example 6]

[0113] In addition to being the second Si-containing particle, it uses a compaction density of 0.72 g / cm³. 3 Except for the particles of Si-C composite material containing 58% by mass of Si, the negative electrode of Example 6 was obtained using the same method as in Example 1.

[0114] [Comparative Example 1]

[0115] In addition to being the first Si-containing particle, it uses a compaction density of 0.8 g / cm³. 3 The first type of Si-C composite material contains 43% by mass of Si particles, which are used as the second type of Si particles, and the compaction density is 1.1 g / cm³. 3 Except for the particles of Si-C composite material containing 60% by mass of Si, the negative electrode of Comparative Example 1 was obtained using the same method as in Example 1.

[0116] [Comparative Example 2]

[0117] In addition to being the second Si-containing particle, it uses a compaction density of 1.1 g / cm³. 3 Except for the particles of Si-C composite material containing 60% by mass of Si, the negative electrode of Comparative Example 2 was obtained using the same method as in Example 1.

[0118] [Comparative Example 3]

[0119] In addition to being the first Si-containing particle, it uses a compaction density of 0.8 g / cm³. 3 The first type of Si-C composite material contains 43% by mass of Si particles, which are used as the second type of Si particles, and the compaction density is 0.85 g / cm³. 3 Except for the particles of Si-C composite material containing 62% by mass of Si, the negative electrode of Comparative Example 3 was obtained using the same method as in Example 1.

[0120] [Comparative Example 4]

[0121] The negative electrode sheet of Comparative Example 4 was obtained by the same method as in Example 1, except that a negative electrode paste containing graphite particles, a first Si-containing particle, CMC, PAA, SBR, and SWCNT in a mass ratio of 60:40:1:1:1:0.1 was used instead of a second Si-containing particle.

[0122] [Comparative Example 5]

[0123] The negative electrode of Comparative Example 5 was obtained using the same method as in Example 1, except that a negative electrode paste containing graphite particles, a second Si-containing particle, CMC, PAA, SBR, and SWCNT in a mass ratio of 60:40:1:1:1:0.1 was used instead of the first Si-containing particle.

[0124] <Evaluation of the fabrication of lithium-ion secondary batteries>

[0125] LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are mixed with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste is coated onto the surface of a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. The positive electrode active material layer is then rolled to obtain a sheet of specified dimensions, yielding the positive electrode sheet.

[0126] Prepare a separator made of porous polyolefin. Attach wires to the negative and positive electrode sheets prepared above, and stack them using the separator to form the electrode body. Contain this electrode body together with a non-aqueous electrolyte in a housing made of aluminum laminate. The non-aqueous electrolyte is a liquid obtained by dissolving LiPF6 (as a supporting salt) 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) in a volume ratio of 15:5:35:45. Then, seal the housing to obtain an evaluation lithium-ion secondary battery.

[0127] <Cyclic Characteristic Evaluation>

[0128] The prepared evaluation lithium-ion secondary batteries were placed in an environment of 25°C. Each evaluation lithium-ion secondary battery was charged with a constant current of 0.4C until 4.2V, then charged with a constant voltage until the current reached 0.1C. Next, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.4C until 2.5V. The discharge capacity at this point was then measured to determine the initial capacity.

[0129] The above charge-discharge cycle was repeated 200 times and considered as one charge-discharge cycle. The discharge capacity after 200 cycles was calculated using the same method as the initial capacity. As an indicator of cycle performance, the capacity retention rate (%) was calculated by multiplying (discharge capacity after 200 charge-discharge cycles / initial capacity) by 100. The results are shown in Table 1.

[0130] Table 1

[0131] Table 1

[0132]

[0133] As shown in Table 1, the proportion of Si in the first Si-containing particle is smaller than that in the second Si-containing particle, and the compaction density of the first Si-containing particle is 0.9 g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 Under these conditions, the capacity retention rate after 200 charge-discharge cycles is significantly high. Therefore, it can be seen that the negative electrode according to this disclosure can suppress capacity degradation during repeated charge-discharge cycles of the secondary battery.

[0134] The specific examples of this disclosure have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations to the specific examples illustrated above.

[0135] That is, the negative electrode of the secondary battery disclosed herein, its manufacturing method and the secondary battery are the following items [1] to

[10] .

[0136] [1] Negative electrode, which is the negative electrode of a secondary battery including a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector.

[0137] The negative electrode active material layer contains graphite particles, a first type of Si-containing particles, and a second type of Si-containing particles.

[0138] The proportion of Si in the first Si-containing particle is smaller than the proportion of Si in the second Si-containing particle.

[0139] When the density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet under uniaxial pressure at 25℃ and 60MPa is defined as the compressive density, the compressive density of the first Si-containing particle is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

[0140] [2] According to the negative electrode described in item [1], wherein the compaction density of the first Si-containing particle is 0.95 g / cm³. 3 Above and 1.8g / cm 3 the following.

[0141] [3] The negative electrode according to item [1] or [2], wherein the compaction density of the second Si-containing particles is 0.65 g / cm³. 3 Above and 0.85g / cm 3 the following.

[0142] [4] The negative electrode according to any one of items [1] to [3], wherein the mass ratio of the first Si-containing particle to the second Si-containing particle is 20:80 to 80:20.

[0143] [5] The negative electrode according to any one of items [1] to [4], wherein the ratio of the Si content in the first Si-containing particle to the Si content in the second Si-containing particle is 0.10 to 0.90.

[0144] [6] The negative electrode according to any one of items [1] to [5], wherein the Si content in the first Si-containing particle is 20% to 55% by mass, and the Si content in the second Si-containing particle is 45% to 80% by mass.

[0145] [7] The negative electrode according to any one of items [1] to [6], wherein the proportion of the graphite particles relative to the total of the graphite particles, the first Si-containing particles and the second Si-containing particles is 40% to 90% by mass.

[0146] [8] The negative electrode according to any one of items [1] to [7], wherein the ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the first Si-containing particles is 1.0 to 8.0.

[0147] The ratio of the average particle size (D50) of the graphite particles to the average particle size (D50) of the second Si-containing particles is 1.0 to 8.0.

[0148] [9] A method for manufacturing the negative electrode of a secondary battery, comprising:

[0149] The process for preparing a negative electrode paste containing graphite particles, a first type of Si-containing particles, a second type of Si-containing particles, and a dispersion medium;

[0150] The process of coating the negative electrode current collector with the negative electrode paste;

[0151] The process of drying the coated negative electrode paste to form a negative electrode active material layer; and

[0152] The process of pressing the negative electrode active material layer.

[0153] The proportion of Si in the first Si-containing particle is smaller than the proportion of Si in the second Si-containing particle.

[0154] When the density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet under uniaxial pressure at 25°C and 60MPa is defined as the particle compaction density, the compaction density of the first Si-containing particle is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

[0155]

[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 [8].

Claims

1. A negative electrode, which is a negative electrode of a secondary battery including a negative electrode current collector, and a negative electrode active material layer supported by the negative electrode current collector, the negative electrode active material layer contains graphite particles, first Si-containing particles, and second Si-containing particles, a Si content ratio in the first Si-containing particles is smaller than a Si content ratio in the second Si-containing particles, The tap density of the first Si-containing particles is 0.9 g / cm3 or more 3 The tap density of the second Si-containing particles is less than 0.9 g / cm3 3 .

2. The negative electrode according to claim 1, wherein The first Si-containing particles have a tap density of 0.95 g / cm 3 Above and 1.8 g / cm 3 Below.

3. The negative electrode according to claim 1, wherein The tap density of the second Si-containing particles is 0.65 g / cm 3 above and 0.85 g / cm 3 below.

4. The negative electrode according to claim 1, wherein a mass ratio of the first Si-containing particles to the second Si-containing particles is 20:80 to 80:

20.

5. The negative electrode according to claim 1, wherein a ratio of the Si content ratio in the first Si-containing particles to the Si content ratio in the second Si-containing particles is 0.10 to 0.

90.

6. The negative electrode according to claim 1, wherein the Si content ratio in the first Si-containing particles is 20 mass% to 55 mass%, and the Si content ratio in the second Si-containing particles is 45 mass% to 80 mass%.

7. The negative electrode according to claim 1, wherein a Si content ratio in the first Si-containing particles is smaller than a Si content ratio in the second Si-containing particles, 8. The negative electrode according to claim 1, wherein a ratio of the Si content ratio in the first Si-containing particles to the Si content ratio in the second Si-containing particles is 0.10 to 0.

90. the Si content ratio in the first Si-containing particles is 20 mass% to 55 mass%, and the Si content ratio in the second Si-containing particles is 45 mass% to 80 mass%. a Si content ratio in the first Si-containing particles is smaller than a Si content ratio in the second Si-containing particles, 9. A method for manufacturing a negative electrode of a secondary battery, comprising: a step of preparing a negative electrode paste containing graphite particles, first Si-containing particles, second Si-containing particles, and a dispersion medium; a step of coating the negative electrode paste on a negative electrode current collector; a step of drying the coated negative electrode paste to form a negative electrode active material layer; and a step of pressing the negative electrode active material layer, In a case where the density of a molded body obtained by press-molding 1 g of the particles into a tablet shape of 20 mm in diameter at 25°C under 60 MPa in a uniaxial direction is defined as the tap density of the particles, the tap density of the first Si-containing particles is 0.9 g / cm3 3 The above, and the tap density of the second Si-containing particles is less than 0.9 g / cm3 3 . a Si content ratio in the first Si-containing particles is smaller than a Si content ratio in the second Si-containing particles, 10. A secondary battery, which is a secondary battery including a positive electrode, a negative electrode, and an electrolyte, the negative electrode being the negative electrode according to claim 1.

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