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

Abstract

The present invention 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 containing Si-containing particles and graphite particles, which is capable of suppressing an increase in resistance when a secondary battery is repeatedly charged and discharged. 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 includes a first layer located on the surface layer portion side and a second layer located on the negative electrode current collector side. The first layer contains first graphite particles and first Si-containing particles. The second layer contains second graphite particles and second Si-containing particles. The content ratio of Si in the first Si-containing particles is greater 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 less than 0.9 g / cm3, and the compaction density of the second Si-containing particles is 0.9 g / cm3 or more.

Description

Technical Field

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

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

[0003] In applications for power supplies to drive vehicles, particularly battery electric vehicles (BEVs), there is a desire to further increase the capacity of secondary batteries from the viewpoint of extending the vehicle's driving range. As a high-capacity negative electrode active material, silicon-containing particles are known, and it is known that using silicon-containing 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-containing particles in combination with graphite particles such as natural graphite as a negative electrode active material. Summary of the Invention

[0004] However, after in-depth research, the inventors discovered that in the previous technology that used both Si-containing particles and graphite particles as negative electrode active materials, there was a problem of a large increase in resistance when the secondary battery was repeatedly charged and discharged.

[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 the increase in resistance during repeated charging and discharging of secondary batteries.

[0006] The negative electrode of the secondary battery disclosed herein comprises: 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 comprises a first layer located on the surface side and a second layer located on the negative electrode current collector side. The first layer contains first graphite particles and first Si-containing particles. The second layer contains second graphite particles and second Si-containing particles. The Si content in the first Si-containing particles is greater than the Si content in the second 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 Si-containing particles is less than 0.9g / cm³. 3 Furthermore, the compressibility density of the second Si-containing particle is 0.9 g / cm³. 3 above.

[0007] Based on this configuration, a negative electrode containing Si particles and graphite particles can be provided, which can suppress the increase in resistance during repeated charging and discharging of the secondary battery.

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

[0009] Based on this configuration, a negative electrode capable of suppressing the increase in resistance during repeated charging and discharging of a secondary battery can be manufactured.

[0010] Furthermore, 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, it is possible to provide a secondary battery that uses a negative electrode containing Si particles and graphite particles while exhibiting a small increase in resistance during repeated charging and discharging of the secondary battery. 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 showing the composition of particles of the negative electrode active material contained in the negative electrode active material layer.

[0014] Figure 3 A cross-sectional view illustrating 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 relating to this disclosure will now be described with reference to the accompanying drawings. It should be noted that matters not mentioned in this specification but necessary for the implementation of this disclosure can be grasped as design matters 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 indicated by 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 denoted as "A to B" includes both A and B.

[0017] It should be noted that in this specification, the term "secondary battery" refers to an energy storage device capable of repeated charging and discharging. Furthermore, in this specification, the term "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 the accompanying charge 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, may 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] In the example shown, the negative electrode current collector 62 is in the shape of a foil (or sheet), but is not limited to this. The negative electrode current collector 62 can be in various shapes such as rod-shaped, plate-shaped, or sieve-shaped. As for the material of the negative electrode current collector 62, similar to conventional lithium-ion secondary batteries, a metal with good conductivity (such as copper, nickel, titanium, stainless steel, etc.) can be used, 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] like Figure 1 As shown, the negative electrode active material layer 64 has a multi-layer structure, specifically, it has a first layer 64a located on the surface side of the negative electrode active material layer 64 and a second layer 64b located on the side of the negative electrode current collector 62. Figure 1 As shown, the first layer 64a is the upper layer of the negative electrode active material layer 64, and the second layer 64b is the lower layer of the negative electrode active material layer 64. It should be noted that, without significantly impairing the effects of the present invention, the negative electrode active material layer 64 may further have layers other than the first layer 64a and the second layer 64b. For example, the negative electrode active material layer 64 may have an intermediate layer between the first layer 64a and the second layer 64b, in which the components of these layers are mixed (hybridized).

[0024] The negative electrode active material layer 64 contains negative electrode active material. For this purpose, [the following is used]... Figure 2 A detailed explanation. Figure 2 To show Figure 1 The diagram shows a schematic cross-sectional view of the particles of the negative electrode active material contained in the negative electrode active material layer 64. This is for illustrative purposes. Figure 2 For illustrative purposes only; therefore, the number, distribution, etc., of particles are not limited to... Figure 2 The content shown.

[0025] Regarding the negative electrode active material, the first layer 64a contains first graphite particles 12 and first Si-containing particles 14. The second layer 64b contains second graphite particles 16 and second Si-containing particles 18. Therefore, in the first layer 64a, at least the first graphite particles 12 and the first Si-containing particles 14 are used as the negative electrode active material, and in the second layer 64b, at least the second graphite particles 16 and the second Si-containing particles 18 are used as the negative electrode active material. The volume change caused by the expansion / contraction associated with charging and discharging of Si-containing particles is large, but by using them in conjunction with graphite particles, the disconnection of the conductive path caused by the volume change of Si-containing particles can be suppressed.

[0026] The graphite constituting the first graphite particle 12 and the second graphite particle 16 can be natural graphite or artificial graphite. Graphite can also be amorphous carbon-coated graphite in the form of amorphous carbon material.

[0027] The shape of the first graphite particle 12 and the second graphite particle 16 is not particularly limited, and can be flake-like, spherical, etc. The first graphite particle 12 and the second graphite particle 16 are preferably spherical graphite particles. When the first graphite particle 12 and the second graphite particle 16 are spherical, the roundness of the first graphite particle 12 and the second graphite particle 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0028] It should be noted that the term "circularity" in this specification refers to the ratio of the circumference of a circle with the same area as the projected area of ​​the particle to the circumference of the particle's projected image (i.e., circularity = circumference of a circle with the same area as the projected area of ​​the particle / 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 determined, for example, by using a commercially available static automatic image analysis device to calculate the average circularity of more than 100 particles.

[0029] There are no particular limitations on the average particle size (D50) of the first graphite particle 12 and the average particle size (D50) of the second graphite particle 16. The average particle size (D50) of the first graphite particle 12 and the average particle size (D50) of the second graphite particle 16 are each, 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.

[0030] It should be noted that the term "average particle size (D50)" in this specification refers to the median diameter (D50), which, in a volume-based particle size distribution based on laser diffraction and scattering, is the particle size corresponding to 50% by volume of the cumulative frequency from the side of the smallest particle. The average particle size (D50) can be determined using commercially available laser diffraction and scattering particle size distribution measuring devices.

[0031] When 1g of the first graphite particles 12 are pressed in a uniaxial direction to form a tablet with a diameter of 20mm, the density of the molded body is preferably 1.7g / cm³. 3 The molding pressure is 20 MPa or less, more preferably 10 MPa to 18 MPa. When 1 g of the second graphite particles 16 are pressed in a uniaxial direction to form a compressed sheet with a diameter of 20 mm, the density of the molded body is preferably 1.7 g / cm³. 3The 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 measuring system (e.g., the "MCP-PD600" manufactured by Nitto Seiko Analitek Co., Ltd.) and a 20 mm diameter probe (equivalent to a mold).

[0032] The first graphite particle 12 and the second graphite particle 16 can be the same graphite particle or different graphite particles. It is preferable to use the same graphite particle as the first graphite particle 12 and the second graphite particle 16.

[0033] The first Si-containing particle 14 and the second Si-containing particle 18 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 18 may not be a Si-C composite material, but may be Si particles, Si oxide particles, etc.

[0034] The carbon regions are, for example, carbides of carbon precursors (e.g., petroleum asphalt, coal tar pitch, phenolic resins, etc.); graphite, etc. The carbon regions preferably constitute a carbon matrix. Therefore, Si-C composite materials are preferably materials in which multiple Si-containing regions 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 regions, which is therefore advantageous.

[0035] The Si-containing region contains Si, for example, Si, Si oxide (SiO2). x ), Si nitride (SiN) x ), Si carbide (SiC) x It is composed of, etc. The Si-containing region is preferably composed of Si, and Si oxide (SiO₂). x It consists of at least one of the following: The Si-containing region may be particulate. The oxygen content in the Si-containing region is preferably less than 10% by mass.

[0036] The average particle size of the Si-containing region 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 region" 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, after elemental analysis of the sample using EDX elemental mapping, BF (bright-field) and HAADF (high-angle scattering annular dark-field) images are obtained. The diameter of the Si-containing region 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 regions are determined, and their average value is set as the "average particle size of the Si-containing region" here.

[0037] Si-C composite materials can be, for example, products in which Si-containing microparticles are dispersed inside a carbon material; or products in which Si-containing microparticles enter the pores of granulated porous graphite. Si-C composite materials can also be products in which Si-containing microparticles are attached to the surface of carbon particles; or products in which carbon particles are attached to the surface of Si-containing particles. From the viewpoint of suppressing volume changes of Si, products in which Si nanoparticles are dispersed inside a carbon material and products in which Si nanoparticles are dispersed within the pores of a porous carbon material are preferred, and products in which Si nanoparticles are dispersed within the pores of a porous carbon material are even more preferred.

[0038] In this embodiment, the Si content (S1) in the first Si-containing particles 14 is greater than the Si content (S2) in the second Si-containing particles 18. There are no particular limitations on the Si content (S1) in the first Si-containing particles 14 and the Si content (S2) in the second Si-containing particles 18, as long as this relationship is satisfied. However, if these Si content proportions are too low, the high-capacity effect of the secondary battery may be excessively reduced. On the other hand, if these Si content proportions are too high, the volume change caused by the expansion / contraction of the first Si-containing particles 14 and the second Si-containing particles 18 during repeated charging and discharging of the secondary battery may become excessively large.

[0039] Therefore, the Si content (S1) in the first Si-containing particle 14 is preferably 45% to 80% by mass, more preferably 55% to 75% by mass. The Si content (S2) in the second Si-containing particle 18 is preferably 20% to 55% by mass, more preferably 25% to 45% by mass.

[0040] Furthermore, the ratio (S2 / S1) of the Si content ratio (S2) in the second Si-containing particle 18 to the Si content ratio (S1) in the first Si-containing particle 14 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and even more preferably 0.40 to 0.75.

[0041] Here, 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 compaction density. This compaction density is 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-14-containing material is less than 0.90g / cm³. 3 On the other hand, the compaction density of the second Si-particle-18 is 0.90 g / cm³. 3 above.

[0042] Thus, in the upper layer 64a of the negative electrode active material layer 64, in addition to the first graphite particles 12, first Si-containing particles 14 with a high Si content and low compaction density are used; in the lower layer 64b of the negative electrode active material layer 64, in addition to the second graphite particles 16, second Si-containing particles 18 with a low Si content and high compaction density are used. Specifically, the compaction density of the Si-containing particles conventionally used is typically less than 0.90 / cm³. 3 Therefore, the second Si-containing particle 18 has a higher compaction density compared to conventionally used Si-containing particles. This significantly suppresses the increase in resistance during repeated charge-discharge cycles of the secondary battery. The reasons for this are as follows.

[0043] When a secondary battery is repeatedly charged and discharged, the expansion and contraction of the negative electrode active material particles cause non-aqueous electrolyte to flow out of the negative electrode active material layer 64 and into it. If this repeated outflow and inflow of non-aqueous electrolyte leads to uneven concentration of the supporting salt within the non-aqueous electrolyte layer 64, it results in increased resistance. The change in the flowability (in other words, the fluidity) of the non-aqueous electrolyte within the negative electrode active material layer 64 due to repeated charging and discharging is a primary cause of this uneven concentration of the supporting salt.

[0044] In this embodiment, by using first Si-containing particles 14 with a high Si content and low pressure density (i.e., Si-containing particles with large expansion / contraction and low filling density) in the upper layer (i.e., the first layer 64a) of the negative electrode active material layer 64, deformation of the upper layer of the negative electrode active material layer 64 is suppressed. On the other hand, in the lower layer of the negative electrode active material layer 64, second Si-containing particles 18 with a low Si content and high pressure density (i.e., Si-containing particles with small expansion / contraction and high filling density) are used. As a result, particles become less mobile in the lower layer, and deformation of the lower layer of the negative electrode active material layer 64 is suppressed. As a result, even with repeated charge and discharge, the liquid flowability of the non-aqueous electrolyte in the negative electrode active material layer 64 is well maintained, and uneven concentration of supporting salts is less likely to occur. Therefore, the increase in resistance during repeated charge and discharge of the secondary battery is suppressed.

[0045] The preferred compaction density of the first Si-particle-containing 14 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 first Si-particle-containing 14 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] The preferred compaction density of the second Si-particle-containing 18 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 second Si-particle-18 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.

[0047] 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 Analite Co., Ltd.) and a probe (equivalent to a mold) with a diameter of 20 mm.

[0048] 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 easily reaches 0.90 g / cm³. 3 That's all. On the other hand, if the sphericity of the Si-containing particles is made less than 0.85, the compaction density can easily become less than 0.90 g / cm³. 3 .

[0049] Furthermore, the particle size of the Si-containing particles also affects the compaction density. Therefore, 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 first Si-containing particles 14 is preferably 2 μm to 10 μm, more preferably 5 μm to 10 μm, and even more preferably 6 μm to 9 μm. Furthermore, it is easy to adjust to 0.90 g / cm³. 3 Given the above compaction density, the average particle size (D50) of the second Si-containing particles 18 is preferably 2 μm to 10 μm, more preferably 5 μm to 10 μm.

[0050] The true density of Si-containing particles also affects the compaction density. Therefore, the compaction density can be finely adjusted by adjusting the composition (the proportion of constituent elements) of the first Si-containing particle 14 and the second Si-containing particle 18.

[0051] The ratio of the average particle size (D50) of the first graphite particles 12 to the average particle size (D50) of the first Si-containing particles 14 (D50 of the first 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 the first 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.

[0052] The ratio of the average particle size (D50) of the second graphite particles 16 to the average particle size (D50) of the second Si-containing particles 18 (D50 of the second graphite particles 16 / D50 of the second Si-containing particles 18) is not particularly limited. From the viewpoint of particularly high filling performance, the ratio (D50 of the second graphite particles 16 / D50 of the second Si-containing particles 18) 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.

[0053] It should be noted that the first Si-containing particle 14 and the second Si-containing particle 18 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).

[0054] In the first layer 64a, the mass ratio of the first Si-containing particles 14 relative to the total mass of the first graphite particles 12 and the first Si-containing particles 14 is preferably 10% to 60% by mass, more preferably 15% to 50% by mass, and even more preferably 20% to 40% by mass.

[0055] In the second layer 64b, the mass ratio of the second Si-containing particles 18 relative to the total of the second graphite particles 16 and the second Si-containing particles 18 is preferably 10% to 60% by mass, more preferably 15% to 50% by mass, and even more preferably 20% to 40% by mass. It should be noted that this mass ratio of the first Si-containing particles 14 in the first layer 64a may be the same as or different from the mass ratio of the second Si-containing particles 18 in the second layer 64b.

[0056] The negative electrode active material contained in the first layer 64a may consist only of the first graphite particles 12 and the first Si-containing particles 14. However, without impairing the effects of the present invention (e.g., less than 10% by mass of the total amount of negative electrode active material contained in the first layer 64a), the first layer 64a may further contain negative electrode active materials other than the first graphite particles 12 and the first Si-containing particles 14.

[0057] The negative electrode active material contained in the second layer 64b may consist only of the second graphite particles 16 and the second Si-containing particles 18. However, without impairing the effects of the present invention (e.g., less than 10% by mass of the total amount of negative electrode active material contained in the second layer 64b), the second layer 64b may further contain negative electrode active materials other than the second graphite particles 16 and the second Si-containing particles 18.

[0058] In the negative electrode active material layer 64, the ratio (T2 / T1) of the thickness (T2) of the second layer 64b to the thickness (T1) of the first layer 64a is not particularly limited as long as the effects of the present invention are achieved, and is, for example, 5 / 95 to 95 / 5. From the viewpoint of further suppressing the increase in resistance during repeated charging and discharging of the secondary battery, the ratio (T2 / T1) is preferably 10 / 90 to 90 / 10, more preferably 10 / 90 to 80 / 20, and even more preferably 30 / 70 to 60 / 40.

[0059] The negative electrode active material layer 64 may contain components other than the negative electrode active material; examples include adhesives and conductive materials. As adhesives, examples include styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVdF). CMC also functions as a tackifier. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are preferred. When using CNTs as conductive materials, the negative electrode active material layer 64 may contain a CNT dispersant.

[0060] The content of the negative electrode active material in the first layer 64a (i.e., relative to the total mass of the first layer 64a) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of the binder in the first layer 64a 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 the conductive material in the first layer 64a 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.

[0061] The content of the negative electrode active material in the second layer 64b (i.e., relative to the total mass of the second layer 64b) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of the binder in the second layer 64b 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 the conductive material in the second layer 64b 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] The negative electrode 60 can be suitably manufactured, for example, by a manufacturing method including the following steps: a step of mixing the second graphite particles 16 and the second Si-containing particles 18 in a dispersion medium to prepare a second layer forming paste (hereinafter also referred to as the "lower layer forming paste preparation step"); a step of mixing the first graphite particles 12 and the first Si-containing particles 14 in a dispersion medium to prepare a first layer forming paste (hereinafter also referred to as the "upper layer forming paste preparation step"); a step of applying the second layer forming paste onto the negative electrode current collector 62 and drying it to form a second layer 64b (lower layer) (hereinafter also referred to as the "lower layer forming step"); a step of applying the first layer forming paste onto the second layer 64b and drying it to form a first layer 64a (upper layer) (hereinafter also referred to as the "upper layer forming step"); and a step of pressing the formed first layer 64a and second layer 64b (hereinafter also referred to as the "pressing step"). In this manufacturing method, the Si content (S1) in the first Si-containing particles 14 is greater than the Si content (S2) in the second Si-containing particles 18. The aforementioned compaction density of the first Si-containing particles 14 is less than 0.9 g / cm³. 3 Furthermore, the aforementioned compaction density of the second Si-particle-18-containing material is 0.9 g / cm³. 3 above.

[0066] It should be noted that 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, including terms such as "slurry" and "ink".

[0067] The preparation process of the paste for lower layer formation can be carried out by mixing the second graphite particles 16, the second Si-containing particles 18, 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.

[0068] The upper layer forming paste preparation step can be performed by mixing the first graphite particles 12, the first Si-containing particles 14, and optional components (e.g., binders, conductive materials, etc.) with a dispersion medium (e.g., water) using a known mixing apparatus, stirring apparatus, or similar method, according to a known method. It should be noted that the upper layer forming paste preparation step can be performed in parallel with the lower layer forming paste preparation step. Alternatively, the upper layer forming paste preparation step can be performed in parallel with or after the lower layer forming step.

[0069] The lower layer formation process can be performed using known methods. Specifically, for example, it can be performed by applying a lower layer forming paste onto the negative electrode current collector 62 using a known coating apparatus and then drying it. By drying, the lower layer (second layer 64b) is formed.

[0070] The upper layer formation process can be performed according to known methods. Specifically, for example, it can be performed by applying an upper layer forming paste onto the formed lower layer using a known coating apparatus and then drying it. By drying, the upper layer (first layer 64a) is formed, forming the negative electrode active material layer 64.

[0071] The pressing process can be performed using known methods. Specifically, the pressing process can be performed by applying pressure to the upper and lower layers (i.e., the negative electrode active material layer 64) formed above using a roller press or the like. By employing the pressing process, the negative electrode active material layer 64 is compressed to a predetermined density, thereby tightly filling the negative electrode active material particles.

[0072] According to the negative electrode 60 of this embodiment, the increase in resistance during repeated charging and discharging of the secondary battery can be suppressed. Furthermore, since the negative electrode 60 of this embodiment uses a negative electrode active material containing Si, the secondary battery can achieve high capacity.

[0073] 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. A lithium-ion secondary battery is listed below, and an embodiment of the secondary battery disclosed herein is referred to. Figure 3 and Figure 4 The following is an example of a flat, square lithium-ion secondary battery with a flat, wound electrode body and a flat, battery casing.

[0074] Figure 3The 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 packaging 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 release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, an injection port (not shown) for injecting the non-aqueous electrolyte is provided in the battery casing 30. 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.

[0075] like Figure 3 and Figure 4 As shown, the wound electrode body 20 has a positive electrode sheet 50 and a negative electrode sheet 60, which are formed by two elongated diaphragm sheets 70 overlapping and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive active material layer 54 is formed on one or both (in this case, both) sides of the elongated positive current collector 52 in the longitudinal direction. The negative electrode sheet 60 has a configuration in which a negative active material layer 64 is formed on one or both (in this case, both) sides of the elongated negative current collector 62 in the longitudinal direction. The non-formed portions 52a of the positive active material layer (i.e., the portions where the positive active material layer 54 is not formed and thus the positive current collector 52 is exposed) and the non-formed portions 62a of the negative active material layer (i.e., the portions where the negative active material layer 64 is not formed and thus the negative current collector 62 is exposed) are formed in a manner that overflows 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 longitudinal direction). Positive current collector 42a and negative current collector 44a are respectively bonded to the non-formed portion 52a of the positive active material layer and the non-formed portion 62a of the negative active material layer.

[0076] As the positive current collector 52 constituting the positive electrode sheet 50, a known positive current collector for 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.

[0077] 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.

[0078] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition for 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.

[0079] 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.

[0080] It should be noted that in this specification, the term "lithium-nickel-cobalt-manganese composite oxide" refers to oxides containing one or more additional elements besides Li, Ni, Co, Mn, and O. 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. The same 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.

[0081] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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 is 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 is 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 is 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 is 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.

[0086] 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.

[0087] The negative electrode 60 described above is used as the negative electrode plate 60.

[0088] As the diaphragm 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 laminated on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the diaphragm 70.

[0089] The thickness of the diaphragm 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 diaphragm 70 obtained by the Gray test is not particularly limited, but preferably 350 seconds / 100cc or less.

[0090] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As a 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), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), methyl fluorodifluoromethyl carbonate (F-DMC), and difluorotrifluorodimethyl 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 may contain only carbonates. As another example, the non-aqueous solvent may contain carbonates and esters such as methyl acetate.

[0091] 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.

[0092] It should be noted that, provided it does not significantly impair the effects of this disclosure, the aforementioned non-aqueous electrolyte may also contain components other than those listed above, 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 tackifiers.

[0093] The lithium-ion secondary battery 100 exhibits minimal resistance increase during repeated charge and discharge cycles. Furthermore, the lithium-ion secondary battery 100 possesses high capacity. The lithium-ion secondary battery 100 can be used for various applications. Preferred applications include its use as a power source in vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Additionally, the lithium-ion secondary battery 100 can be used as a battery in small energy storage devices. Typically, the lithium-ion secondary battery 100 can also be used in the form of multiple batteries connected in series and / or parallel.

[0094] The above description, as an example, describes a square lithium-ion secondary battery 100 including a flat, wound electrode body 20. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries including stacked electrode bodies (i.e., electrode bodies in which multiple positive electrodes 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.

[0095] In addition, according to known methods, the lithium-ion secondary battery 100 can also be constructed as an all-solid-state lithium-ion secondary battery that uses a solid electrolyte instead of a non-aqueous electrolyte.

[0096] Furthermore, the negative electrode 60 involved in this embodiment is suitable as the negative electrode of a lithium-ion secondary battery, but can also be constructed and used as the negative electrode of other secondary batteries, which can be constructed according to known methods.

[0097] The following describes in detail embodiments related to this disclosure, but it is not intended to limit this disclosure to the contents shown in these embodiments.

[0098] <Making the Negative Electrode>

[0099] [Example 1]

[0100] The following substances were prepared as negative electrode active materials.

[0101] First type of Si-containing particle: Si-C composite material, Si content = 65% by mass, average particle size (D50) = 7 μm, compaction density = 0.85 g / cm³ 3

[0102] Second type of Si-containing particle: Si-C composite material, Si content = 35% by mass, average particle size (D50) = 6 μm, compaction density = 1.3 g / cm³ 3

[0103] Graphite particles (first graphite particle and second graphite particle): average particle size (D50) = 15 μm; 1 g of graphite particles are pressurized in a uniaxial direction to achieve a density of 1.7 g / cm³. 3 Furthermore, the molding pressure for a 20mm diameter tablet is 15MPa.

[0104] 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.

[0105] One gram of either the first or second Si-containing particle sample was measured and placed on the probe (20 mm diameter) of the "MCP-PD600" (manufactured by Nitto Seiko Analitek Co., Ltd.) automated powder resistivity measuring system. Using this system at 25°C, the load and displacement under uniaxial pressure were measured. 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 probe (20 mm diameter) of the "MCP-PD600" (manufactured by Nitto Seiko Analitek Co., Ltd.) automated powder resistivity measuring system. Using this system at 25°C, the load and displacement under uniaxial pressure were measured. Based on this, the density of the molded body was determined to be 1.7 g / cm³. 3 The molding pressure at that time.

[0106] Single-layer carbon nanotubes (SWCNTs) were prepared as conductive materials. SWCNTs were prepared in the form of a dispersion. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared as binders.

[0107] A paste for forming an upper layer containing graphite particles, a first type of Si particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 60:40:0.1:1:1:1.5 was prepared using the following steps. A paste for forming a lower layer containing graphite particles, a second type of Si particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 60:40:0.1:1:1:1.5 was also prepared using the following steps.

[0108] Graphite particles, first Si-containing particles, CMC, and PAA were dry-mixed using a planetary mixer. The resulting dry mixture, SWCNT dispersion, and dispersion medium were then kneaded using a planetary mixer. Subsequently, SBR and additional dispersion medium were added to the planetary mixer for dilution and mixing, thereby obtaining a paste for upper layer formation.

[0109] Graphite particles, second Si-containing particles, CMC, and PAA were dry-mixed using a planetary mixer. The resulting dry mixture, SWCNT dispersion, and dispersion medium were then kneaded using a planetary mixer. Subsequently, SBR and additional dispersion medium were added to the planetary mixer for dilution and mixing, thereby obtaining a paste for lower layer formation.

[0110] The lower layer is formed by coating a 10 μm thick copper foil with a paste and drying it. Then, the upper layer is formed by coating the lower layer with a paste and drying it. This creates a multi-layered negative electrode active material layer. After rolling the negative electrode active material layer, the resulting sheet is processed into a specified size to obtain the negative electrode sheet.

[0111] [Example 2]

[0112] In addition to being used as the second Si-containing particle, a compaction density of 1.0 g / cm³ was employed. 3 Except for the particles containing 51% by mass of Si-C composite material, the negative electrode of Example 2 was obtained using the same method as in Example 1.

[0113] [Example 3]

[0114] In addition to being the first Si-containing particle, it used 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 3 was obtained using the same method as in Example 1.

[0115] [Example 4]

[0116] Except for changing the ratio (T2 / T1) of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) to 20 / 80, the negative electrode of Example 4 was obtained by the same method as in Example 1.

[0117] [Example 5]

[0118] Except for changing the ratio (T2 / T1) of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) to 80 / 20, the negative electrode of Example 5 was obtained by the same method as in Example 1.

[0119] [Comparative Example 1]

[0120] Except for using an upper layer forming paste to form the lower layer and a lower layer forming paste to form the upper layer, the negative electrode of Comparative Example 1 was obtained using the same method as in Example 1. Therefore, in Comparative Example 1, the first Si-containing particles and the second Si-containing particles were used interchangeably.

[0121] [Comparative Example 2]

[0122] In addition to being the first Si-containing particle, it used a compaction density of 1.0 g / cm³. 3 The first part contains Si particles in a Si-C composite material with a mass ratio of 51% by weight. These particles, used as the second Si-containing particles, have a compaction density of 0.85 g / cm³. 3Except for the Si-C composite material particles containing 65% by mass of Si, the negative electrode of Comparative Example 2 was obtained using the same method as in Example 1.

[0123] [Comparative Example 3]

[0124] Except that the lower layer was formed using an upper layer forming paste, the upper layer was formed using a lower layer forming paste, and the ratio (T2 / T1) of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) was changed to 10 / 90, the negative electrode of Comparative Example 3 was obtained by the same method as in Example 1.

[0125] [Comparative Example 4]

[0126] A paste for forming the lower layer and a paste for forming the upper layer were mixed in a 1:1 mass ratio of their solid components to prepare a paste for forming the negative electrode active material layer. This paste was coated onto the surface of a 10 μm thick copper foil and dried to form the negative electrode active material layer. After rolling the negative electrode active material layer, the resulting sheet was processed to a specified size to obtain the negative electrode sheet of Comparative Example 4. Note that the thickness of the negative electrode sheet of Comparative Example 4 is the same as that of Example 1.

[0127] [Comparative Example 5]

[0128] A paste was applied to the surface of a 10 μm thick copper foil and dried to form a negative electrode active material layer. The negative electrode active material layer was then rolled and the resulting sheet was processed to a specified size to obtain the negative electrode sheet of Comparative Example 5. It should be noted that the thickness of the negative electrode sheet of Comparative Example 5 is the same as that of Example 1.

[0129] <Evaluation of Resistance Increase Rate>

[0130] First, using the negative electrodes of each embodiment and each comparative example, an evaluation lithium-ion secondary battery was fabricated as described below.

[0131] LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode paste was prepared by mixing O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder with N-methylpyrrolidone (NMP) at a mass ratio of NCM:AB:PVdF = 100:1:1. This paste was 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 was then rolled to obtain a sheet of specified dimensions, yielding the positive electrode sheet.

[0132] A porous polyolefin separator was prepared. Leads were attached to the prepared negative and positive electrode sheets, and the electrodes were stacked via the separator to form an electrode body. This electrode body was then housed together with a non-aqueous electrolyte in an aluminum laminate casing. For the non-aqueous electrolyte, a product containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 15:5:40:40, with LiPF6 dissolved as a supporting salt at a concentration of 1.0 mol / L, was used. The casing was then sealed, resulting in an evaluation lithium-ion secondary battery.

[0133] Next, the prepared 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 0.2C until 4.2V, and then charged with a constant voltage until the current reached 0.1C. This constituted the initial charging. Next, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.2C until 2.5V.

[0134] Next, the lithium-ion secondary batteries used for evaluation were adjusted to 50% SOC using constant current-constant voltage charging and stored at 25°C for 1 hour. Then, they were discharged at a constant current of 1C for 10 seconds. The difference between the open-circuit voltage (OCV) and the closed-circuit voltage (CCV) after 10 seconds of discharge was divided by the discharge current after 10 seconds of discharge, and the DC resistance was calculated based on the following formula. This calculated value was used as the initial DC resistance.

[0135] DC resistance = [OCV - CCV (after 10 seconds of discharge)] / Discharge current (after 10 seconds of discharge)

[0136] At 25°C, the evaluation lithium-ion secondary batteries were adjusted to 50% SOC using constant current-constant voltage charging. Then, a charge-discharge cycle of 1.5C constant current for 400 seconds followed by 0.75C constant current for 800 seconds was repeated for 400 cycles. The DC resistance after 400 cycles was then calculated using the same method as above. The resistance increase rate (%) was then calculated based on the following formula.

[0137] Resistance increase rate = (DC resistance after 400 cycles / initial DC resistance) × 100

[0138] Table 1

[0139]

[0140] As shown in Table 1, in the upper layer of the negative electrode active material layer, except for the first graphite particles, a high Si content and low compaction density (specifically, a compaction density of less than 0.9 g / cm³) are used. 3The first Si-containing particles, located in the lower layer of the negative electrode active material layer, except for the second graphite particles, use a low Si content and high compaction density (specifically, a compaction density of 0.9 g / cm³). 3 In the case of the second Si-containing electrode (as described above), the rate of increase in resistance is very small. Therefore, it can be seen that the negative electrode according to this disclosure can suppress the increase in resistance during repeated charging and discharging of a secondary battery while using a negative electrode containing both Si-containing particles and graphite particles.

[0141] 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.

[0142] That is, the negative electrode of the secondary battery, its manufacturing method and the secondary battery disclosed herein are the following items [1] to

[10] .

[0143] [1] The negative electrode is the negative electrode of a secondary battery, comprising 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 comprises a first layer on the surface side and a second layer on the negative electrode current collector side. The first layer contains first graphite particles and first Si-containing particles, and the second layer contains second graphite particles and second Si-containing particles. The Si content in the first Si-containing particles is greater than the Si content in the second Si-containing particles. When the density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet shape in a uniaxial direction at 25°C and 60MPa is defined as the compaction density, the compaction density of the first Si-containing particles is less than 0.9g / cm³. 3 Furthermore, the compressibility density of the second Si-containing particle is 0.9 g / cm³. 3 above.

[0144] [2] According to the negative electrode described in item [1], wherein the first Si-containing particle compaction density is 0.65 g / cm³. 3 Above and 0.85g / cm 3 the following.

[0145] [3] The negative electrode according to item [1] or [2], wherein the compaction density of the second Si-containing particles is 0.95 g / cm³. 3 Above and 1.8g / cm 3 the following.

[0146] [4] The negative electrode according to any one of items [1] to [3], wherein the ratio of the thickness of the second layer to the thickness of the first layer is 10 / 90 to 90 / 10.

[0147] [5] The negative electrode according to any one of items [1] to [4], wherein the first Si-containing particle and the second Si-containing particle are particles of Si-C composite material.

[0148] [6] The negative electrode according to any one of items [1] to [5], wherein the ratio of the Si content in the second Si-containing particles to the Si content in the first Si-containing particles is 0.10 to 0.90.

[0149] [7] The negative electrode according to any one of items [1] to [6], wherein the Si content in the first Si-containing particles is 45% to 80% by mass, and the Si content in the second Si-containing particles is 20% to 55% by mass.

[0150] [8] The negative electrode according to any one of items [1] to [7], wherein, in the first layer, the mass ratio of the first Si-containing particles relative to the total of the first graphite particles and the first Si-containing particles is 10% to 60% by mass, and in the second layer, the mass ratio of the second Si-containing particles relative to the total of the second graphite particles and the second Si-containing particles is 10% to 60% by mass.

[0151] [9] A method for manufacturing the negative electrode of a secondary battery, comprising: a step of mixing second graphite particles and second Si-containing particles in a dispersion medium to prepare a paste for forming a second layer; a step of mixing first graphite particles and first Si-containing particles in a dispersion medium to prepare a paste for forming a first layer; a step of coating the paste for forming the second layer onto a negative electrode current collector and drying it to form a second layer; a step of coating the paste for forming the first layer onto the second layer and drying it to form a first layer; and a step of pressing the formed first layer and second layer, wherein the Si content in the first Si-containing particles is greater than the Si content in the second Si-containing particles, and 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, and the compaction density of the first Si-containing particles is less than 0.9g / cm³. 3 Furthermore, the compressibility density of the second Si-containing particle is 0.9 g / cm³. 3 above.

[0152]

[10] A secondary battery is 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. Negative electrode, which is the negative electrode of a secondary battery comprising a negative electrode current collector and a layer of negative electrode active material supported by the negative electrode current collector, wherein, The negative electrode active material layer includes a first layer located on the surface side and a second layer located on the negative electrode current collector side. The first layer contains first graphite particles and first Si-containing particles. The second layer contains second graphite particles and second Si-containing particles. The first Si-containing particle contains a higher proportion of Si than the second Si-containing particle. 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 less than 0.9g / cm³. 3 Furthermore, the compressibility density of the second Si-containing particle is 0.9 g / cm³. 3 above.

2. The negative electrode according to claim 1, wherein, The first Si-containing particle compaction density is 0.65 g / cm³. 3 Above and 0.85g / cm 3 the following.

3. The negative electrode according to claim 1, wherein, The second Si-containing particle compaction density is 0.95 g / cm³. 3 Above and 1.8g / cm 3 the following.

4. The negative electrode according to claim 1, wherein, The ratio of the thickness of the second layer to the thickness of the first layer is 10 / 90 to 90 / 10.

5. The negative electrode according to claim 1, wherein, The first Si-containing particle and the second Si-containing particle are particles of the Si-C composite material.

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

90.

7. The negative electrode according to claim 1, wherein, The first Si-containing particle contains 45% to 80% Si by mass, and the second Si-containing particle contains 20% to 55% Si by mass.

8. The negative electrode according to claim 1, wherein, In the first layer, the mass percentage of the first Si-containing particles relative to the total mass of the first graphite particles and the first Si-containing particles is 10% to 60% by mass. In the second layer, the mass ratio of the second Si-containing particles relative to the total of the second graphite particles and the second Si-containing particles is 10% to 60% by mass.

9. A method for manufacturing the negative electrode of a secondary battery, comprising: The process of mixing second graphite particles and second Si-containing particles in a dispersion medium to prepare a paste for forming a second layer; The process of mixing first graphite particles and first Si-containing particles in a dispersion medium to prepare a paste for forming a first layer; The process of forming the second layer by coating the negative electrode current collector with a paste and drying it. The process of forming the first layer by applying a paste onto the second layer and drying it to form the first layer; and The process of pressing the first and second layers together. The proportion of Si in the first Si-containing particles is greater than that in the second Si-containing particles. 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 less than 0.9g / cm³. 3 Furthermore, the compressibility density of the second Si-containing particle is 0.9 g / cm³. 3 above.

10. A secondary battery is a battery that includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode as described in claim 1.

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