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

Abstract

The present invention provides a negative electrode containing Si-containing particles and graphite particles, which can suppress the increase in resistance when a secondary battery is repeatedly charged and discharged. [Solution] The negative electrode of the secondary battery of this disclosure 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 a molded body obtained by pressing 1 g of particles uniaxially at 25°C and 60 MPa to form a tablet with 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. 3 The value is less than the specified value, and the compaction density of the aforementioned second Si-containing particles is 0.9 g / cm³. 3 That's all.

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 said negative electrode. [Background technology]

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

[0003] For vehicle power supply applications, particularly for BEVs, there is a demand for higher capacity secondary batteries from the perspective of extending the vehicle's driving range. Si-containing particles are known as high-capacity negative electrode active materials, and it is known that secondary batteries can be made higher capacity by using Si-containing particles (see, for example, Patent Document 1). Patent Document 1 discloses a technology that uses Si-containing particles and graphite particles such as natural graphite in combination as negative electrode active materials. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-38862 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, after diligent research by the inventors, it was found that the conventional technology using both Si-containing particles and graphite particles as the negative electrode active material has a problem in that the resistance increases significantly when the secondary battery is repeatedly charged and discharged.

[0006] In view of the above circumstances, the present disclosure aims to provide a negative electrode containing Si-containing particles and graphite particles that can suppress the increase in resistance when a secondary battery is repeatedly charged and discharged.

Means for Solving the Problem

[0007] The negative electrode of the secondary battery of the present disclosure includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer 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 Si content ratio in the first Si-containing particles is larger than the Si content ratio in the second Si-containing particles. When the density of the compact obtained by pressing 1 g of particles uniaxially at 25°C and 60 MPa into a tablet shape with a diameter of 20 mm is defined as the consolidation density, the consolidation density of the first Si-containing particles is less than 0.9 g / cm 3 and the consolidation density of the second Si-containing particles is 0.9 g / cm 3 or more.

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

[0009] From another aspect, the method for manufacturing a secondary battery of the present disclosure 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 on a negative electrode current collector and drying it to form a second layer; a step of coating the paste for forming the first layer on the second layer and drying it to form a first layer; and a step of pressing the formed first layer and the second layer. The Si content ratio in the first Si-containing particles is larger than the Si content ratio in the second Si-containing particles. When the density of the compact obtained by pressing 1 g of particles uniaxially at 25°C and 60 MPa into a tablet shape with a diameter of 20 mm is defined as the consolidation density, the consolidation density of the first Si-containing particles is less than 0.9 g / cm 3The value is less than the specified value, and the compaction density of the aforementioned second Si-containing particles is 0.9 g / cm³. 3 That's all.

[0010] This configuration makes it possible to manufacture a negative electrode that can suppress the increase in resistance when a secondary battery is repeatedly charged and discharged.

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

[0012] With this configuration, it is possible to provide a secondary battery that exhibits a small increase in resistance when repeatedly charged and discharged, even while using a negative electrode containing Si-containing particles and graphite particles. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic cross-sectional view showing the configuration of the negative electrode of a secondary battery according to one embodiment of the present disclosure. [Figure 2] Figure 1 is a schematic cross-sectional view showing the composition of the particles of the negative electrode active material contained in the negative electrode active material layer. [Figure 3] This is a schematic cross-sectional view showing the configuration of a lithium-ion secondary battery constructed using the negative electrode of a secondary battery according to one embodiment of the present disclosure. [Figure 4] Figure 3 is a schematic exploded view showing the configuration of the wound electrode body of a lithium-ion secondary battery. [Modes for carrying out the invention]

[0014] Embodiments relating to this disclosure will be described below with reference to the drawings. Matters not mentioned herein but necessary for the implementation of this disclosure can be understood as design matters for those skilled in the art based on prior art. This disclosure can be implemented based on the contents disclosed herein and common technical knowledge in the art. In the following drawings, members and parts that perform the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships. In this specification, the numerical range expressed as "A~B" includes A and B.

[0015] In this specification, "secondary battery" refers to an energy storage device that can be repeatedly charged and discharged. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging through the transfer of charge associated with lithium ions between the positive and negative electrodes.

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

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

[0018] As shown in the illustrated example, a portion 62a without a negative electrode active material layer 64 may be provided at one end of the negative electrode 60 in the width direction. In the portion 62a without a negative electrode active material layer, the negative electrode current collector 62 is exposed, and the portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

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

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

[0021] As shown in Figure 1, the negative electrode active material layer 64 has a multilayer structure, specifically comprising 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 negative electrode current collector 62 side. As shown in Figure 1, 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. The negative electrode active material layer 64 may further have layers other than the first layer 64a and the second layer 64b, as long as this does not significantly impede the effects of the present invention. 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.

[0022] The negative electrode active material layer 64 contains the negative electrode active material. This will be explained in detail using Figure 2. Figure 2 is a schematic cross-sectional view showing the particles of the negative electrode active material contained in the negative electrode active material layer 64 shown in Figure 1. Note that Figure 2 is a schematic diagram, and therefore the number and distribution of particles are not limited to those shown in Figure 2.

[0023] 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. Although Si-containing particles undergo large volume changes due to expansion / contraction during charging and discharging, using them in combination with graphite particles can suppress the interruption of the conductive path caused by the volume change of Si-containing particles.

[0024] The graphite constituting the first graphite particles 12 and the second graphite particles 16 may be natural graphite or artificial graphite, or it may be amorphous carbon-coated graphite in which the graphite is coated with an amorphous carbon material.

[0025] The shape of the first graphite particles 12 and the second graphite particles 16 is not particularly limited and may be flaky, spherical, or the like. The first graphite particles 12 and the second graphite particles 16 are preferably spheroidized graphite particles. When the first graphite particles 12 and the second graphite particles 16 are spherical, the circularity of the first graphite particles 12 and the second graphite particles 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0026] In this specification, "circularity" refers to the ratio of the circumference of a perfect circle having the same area as the projected area of ​​the particle to the circumference of the particle projection image (i.e., circularity = circumference of a perfect circle having the same area as the projected area of ​​the particle / circumference of the particle projection image). Therefore, the closer the circularity is to 1, the closer the particle projection 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 analyzer to determine the circularity of 100 or more particles and calculating the average value.

[0027] The average particle diameter (D50) of the first graphite particles 12 and the average particle diameter (D50) of the second graphite particles 16 are not particularly limited. The average particle diameter (D50) of the first graphite particles 12 and the average particle diameter (D50) of the second graphite particles 16 are, 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] In this specification, "average particle size (D50)" refers to the median diameter (D50), which is the particle size corresponding to the cumulative frequency of 50% by volume from the smaller particle size side in a volume-based particle size distribution based on laser diffraction and scattering methods. The average particle size (D50) can be determined using commercially available laser diffraction and scattering type particle size distribution analyzers.

[0029] When 1 g of primary graphite particles 12 are compressed uniaxially to form a tablet shape with a diameter of 20 mm, the density of the molded body is 1.7 g / cm³. 3 The molding pressure at this time is preferably 20 MPa or less, and more preferably 10 MPa to 18 MPa. When 1 g of secondary graphite particles 16 are pressed in a uniaxial direction to form a tablet shape with a diameter of 20 mm, the density of the molded body is 1.7 g / cm³. 3 The molding pressure at which this occurs is preferably 20 MPa or less, and more preferably 10 MPa to 18 MPa. This molding pressure can be easily measured using an automatic powder resistance measuring system (e.g., "MCP-PD600" manufactured by Nitto Seiko Analytech Co., Ltd.) and a 20 mm diameter probe (corresponding to a mold).

[0030] The first graphite particles 12 and the second graphite particles 16 may be the same graphite particles, or different graphite particles may be used. It is preferable to use the same graphite particles for the first graphite particles 12 and the second graphite particles 16.

[0031] For example, the first Si-containing particles 14 and the second Si-containing particles 18 can be particles of a Si-C composite material. Si-C composite materials typically contain carbon domains and Si-containing domains. However, the first Si-containing particles 14 and the second Si-containing particles 18 do not have to be Si-C composite materials; they may be Si particles, Si oxide particles, etc.

[0032] Carbon domains are, for example, carbonized forms of carbon precursors (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc. Preferably, carbon domains constitute a carbon matrix. Therefore, a Si-C composite material is preferably a material in which multiple Si-containing domains are dispersed within a carbon matrix. In this case, the carbon matrix is ​​advantageous because it can mitigate volume changes due to the expansion / contraction of the Si-containing domains.

[0033] The Si-containing domain contains Si, for example, Si, Si oxide (SiO x It is composed of Si, Si nitride (SiNx), Si carbide (SiCx), etc. The Si-containing domain is preferably Si and Si oxide (SiO x It consists of at least one of the following. The Si-containing domains may be fine particles. The oxygen content in the Si-containing domains is preferably 10% by mass or less.

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

[0035] Si-C composite materials include, for example, those in which fine particles containing Si are dispersed inside a carbon material; or those in which fine particles containing Si are embedded in the pores of granulated porous graphite; and so on. Si-C composite materials may also include those in which fine particles containing Si are attached to the surface of carbon particles; or those in which carbon fine particles 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 inside a carbon material and materials in which Si nanoparticles are dispersed in the pores of a porous carbon material are preferred, and materials in which Si nanoparticles are dispersed in the pores of a porous carbon material are more preferred.

[0036] In this embodiment, the Si content ratio (S1) in the first Si-containing particles 14 is greater than the Si content ratio (S2) in the second Si-containing particles 18. The Si content ratio (S1) in the first Si-containing particles 14 and the Si content ratio (S2) in the second Si-containing particles 18 are not particularly limited as long as this relationship is satisfied. However, if these Si content ratios are too low, the effect of increasing the capacity of the secondary battery may become too small. On the other hand, if these Si content ratios are too high, the volume change due to expansion / contraction of the first Si-containing particles 14 and the second Si-containing particles 18 may become too large when the secondary battery is repeatedly charged and discharged.

[0037] Therefore, the Si content ratio (S2) in the first Si-containing particles 14 is preferably 45% to 80% by mass, more preferably 55% to 75% by mass. The Si content ratio (S1) in the second Si-containing particles 18 is preferably 20% to 55% by mass, more preferably 25% to 45% by mass.

[0038] Also, the ratio (S2 / S1) of the Si content ratio (S2) in the second Si-containing particles 18 to the Si content ratio (S1) in the first Si-containing particles 14 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and still more preferably 0.40 to 0.75.

[0039] Here, the density of the molded body obtained by pressing 1 g of the particles uniaxially at 25°C and 60 MPa to form a tablet with a diameter of 20 mm is defined as the consolidation density. This consolidation density is an index of the packing property of the particles. The higher the consolidation density of the particles, the higher the packing property of the particles. The consolidation density of the first Si-containing particles 14 is less than 0.90 g / cm 3 On the other hand, the consolidation density of the second Si-containing particles 18 is 0.90 g / cm 3 or more.

[0040] Thus, in the first layer 64a, which is the upper layer of the negative electrode active material layer 64, in addition to the first graphite particles 12, the first Si-containing particles 14 with a high Si content ratio and a low consolidation density are used. In the second layer 64b, which is the lower layer of the negative electrode active material layer 64, in addition to the second graphite particles 16, the second Si-containing particles 18 with a low Si content ratio and a high consolidation density are used. In particular, the consolidation density of the conventionally used Si-containing particles is usually less than 0.90 / cm 3 Therefore, the second Si-containing particles 18 have a higher consolidation density than the conventionally used Si-containing particles. Thereby, an increase in resistance when the secondary battery is repeatedly charged and discharged can be significantly suppressed. The reason is considered as follows.

[0041] When a secondary battery is repeatedly charged and discharged, the expansion and contraction of the negative electrode active material particles cause the non-aqueous electrolyte to flow out of the negative electrode active material layer 64 and to flow into the negative electrode active material layer 64. At this time, if the concentration of the supporting salt contained in the non-aqueous electrolyte becomes uneven due to the repeated outflow and inflow of the non-aqueous electrolyte within the negative electrode active material layer 64, it will lead to an increase in resistance. One of the factors that causes this unevenness in the concentration of the supporting salt is the change in the flowability (in other words, liquid flowability) of the non-aqueous electrolyte within the negative electrode active material layer 64 due to repeated charging and discharging.

[0042] In this embodiment, the deformation of the upper layer of the negative electrode active material layer 64 is suppressed by using first Si-containing particles 14 with a high Si content and low compaction density, i.e., Si-containing particles that expand / contract greatly and have low packing properties, in the upper layer (i.e., the first layer 64a) of the negative electrode active material layer 64. On the other hand, second Si-containing particles 18 with a low Si content and high compaction density, i.e., Si-containing particles that expand / contract little and have high packing properties, are used in the lower layer of the negative electrode active material layer 64. As a result, particle movement in the lower layer is made difficult, and deformation of the lower layer of the negative electrode active material layer 64 is suppressed. Consequently, even after repeated charging and discharging, the liquid flow properties of the non-aqueous electrolyte in the negative electrode active material layer 64 are maintained well, and unevenness in the concentration of the supporting salt is less likely to occur. Therefore, the increase in resistance when the secondary battery is repeatedly charged and discharged is suppressed.

[0043] The compaction density of the first Si-containing particle 14 is preferably 0.88 g / cm³. 3 The following, and more preferably 0.86 g / cm³ 3 The following, and more preferably 0.85 g / cm³ 3 The following applies. On the other hand, the compaction density of the first Si-containing particle 14 is preferably 0.50 g / cm³. 3 The above is more accurate, and more preferably 0.65 g / cm³ 3 The above is preferable, and more preferably 0.75 g / cm³ 3 The above is the most preferred, and is particularly preferably 0.83 g / cm³. 3 That's all.

[0044] The compaction density of the second Si-containing particle 18 is preferably 0.95 g / cm³. 3The above, and more preferably 1.00 g / cm³ 3 The above is preferable, and more preferably 1.1 g / cm³. 3 The above is the most preferred, and is particularly preferably 1.2 g / cm³. 3 That concludes the explanation. On the other hand, the compaction density of the second Si-containing particle 18 is 2.3 g / cm³. 3 Below 2.0g / cm 3 Below 1.8g / cm 3 The following, or 1.5 g / cm³ 3 The following is acceptable:

[0045] Furthermore, the compaction density can be easily measured, for example, by using an automated powder resistance measurement system (e.g., "MCP-PD600" manufactured by Nitto Seiko Analytech Co., Ltd.) and a 20 mm diameter probe (equivalent to a mold).

[0046] Furthermore, the compaction density is affected by the circularity of the Si-containing particles. Increasing the circularity of the Si-containing particles tends to increase the compaction density. Therefore, if the circularity of the Si-containing particles is set to 0.85 to 1 (especially 0.90 to 1), the compaction density will be 0.90 g / cm³. 3 This tends to be the case. On the other hand, if the circularity of the Si-containing particles is less than 0.85, the compaction density becomes 0.90 g / cm³. 3 It tends to be less than that.

[0047] Furthermore, the particle size of the Si-containing particles also affects the compaction density. Therefore, 0.90 g / cm³ 3 Because it is easy to adjust the compaction density to less than 0.90 g / cm³, 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. 3 Because it is easy to adjust the compaction density to the above levels, the average particle size (D50) of the second Si-containing particles 18 is preferably 2 μm to 10 μm, and more preferably 5 μm to 10 μm.

[0048] The true density of the Si-containing particles also affects the compaction density. Therefore, the compaction density can be finely adjusted by adjusting the composition (proportion of constituent elements) of the first Si-containing particles 14 and the second Si-containing particles 18.

[0049] The ratio of the average particle diameter (D50) of the first graphite particles 12 to the average particle diameter (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. Particularly from the viewpoint of high packing 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.

[0050] The ratio of the average particle diameter (D50) of the second graphite particles 16 to the average particle diameter (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. Particularly from the viewpoint of high packing 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.

[0051] The first Si-containing particles 14 and the second Si-containing particles 18 can be manufactured according to known methods. Various methods for manufacturing particles of Si-C composite materials are known (see, for example, Japanese Patent Publication No. 2015-38862, International Publication No. 2014 / 046144, and the prior art documents cited in said International Publication).

[0052] In the first layer 64a, the mass ratio of the first Si-containing particles 14 to the total of the first graphite particles 12 and the first Si-containing particles 14 is preferably 10% to 60%, more preferably 15% to 50%, and even more preferably 20% to 40%.

[0053] In the second layer 64b, the mass ratio of the second Si-containing particles 18 to the total of the second graphite particles 16 and the second Si-containing particles 18 is preferably 10% to 60%, more preferably 15% to 50%, and even more preferably 20% to 40%. The mass ratio of the first Si-containing particles 14 in the first layer 64a and the mass ratio of the second Si-containing particles 18 in the second layer 64b may be the same or different.

[0054] 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, 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, within a range that does not impede the effects of the present invention (for example, 10% by mass or less of the total amount of negative electrode active material contained in the first layer 64a).

[0055] 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, 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, within a range that does not impede the effects of the present invention (for example, 10% by mass or less of the total amount of negative electrode active material contained in the second layer 64b).

[0056] In the negative electrode active material layer 64, the ratio (T2 / T1) of the thickness 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 obtained, but is for example 5 / 95 to 95 / 5. From the viewpoint of further suppressing the increase in resistance when the secondary battery is repeatedly charged and discharged, 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.

[0057] The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as binders and conductive materials. Examples of binders include styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickening agent. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are particularly preferred. When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a CNT dispersant.

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

[0059] 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, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less.

[0060] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 400 μm or less, and preferably 20 μm or more and 300 μm or less.

[0061] The density of the negative electrode active material layer 64 is not particularly limited, but for example, 0.7 g / cm³ 3 The above is preferable, preferably 1.0 g / cm³. 3 The above is more preferable, or 1.2 g / cm³. 3That concludes the explanation. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm³. 3 The following is the case: 2.0 g / cm³ 3 The following is acceptable:

[0062] 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 portion 62a where the negative electrode active material layer is not formed. This insulating layer may contain, for example, an insulating inorganic filler.

[0063] The negative electrode 60 can be suitably manufactured by a manufacturing method comprising, for example, a step of preparing a paste for forming the second layer by mixing second graphite particles 16 and second Si-containing particles 18 in a dispersion medium (hereinafter also referred to as the "lower layer forming paste preparation step"); a step of preparing a paste for forming the first layer by mixing first graphite particles 12 and first Si-containing particles 14 in a dispersion medium (hereinafter also referred to as the "upper layer forming paste preparation step"); a step of coating the second layer forming paste onto the negative electrode current collector 62 and drying it to form the second layer 64b (lower layer) (hereinafter also referred to as the "lower layer forming step"); a step of coating the first layer forming paste onto the second layer 64b and drying it to form the 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 referred to as the "pressing step"). In this manufacturing method, the Si content (S1) in the first Si-containing particle 14 is greater than the Si content (S2) in the second Si-containing particle 18. The compaction density of the first Si-containing particle 14 is 0.9 g / cm³. 3 The above-mentioned compaction density of the second Si-containing particle 18 is less than 0.9 g / cm³. 3 That's all.

[0064] In this specification, "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, and includes so-called "slurry," "ink," etc.

[0065] The paste preparation step for forming the lower layer can be carried out by mixing the second graphite particles 16, the second Si-containing particles 18, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0066] The upper layer paste preparation step can be carried out by mixing the first graphite particles 12, the first Si-containing particles 14, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method. The upper layer paste preparation step may be carried out in parallel with the lower layer paste preparation step. The upper layer paste preparation step may be carried out in parallel with or after the lower layer formation step.

[0067] The lower layer formation process can be carried out according to a known method. Specifically, for example, a paste for forming the lower layer can be applied onto the negative electrode current collector 62 using a known coating apparatus and then dried. The lower layer (second layer 64b) is formed by drying.

[0068] The upper layer formation process can be carried out according to a known method. Specifically, for example, an upper layer formation paste can be applied to the formed lower layer using a known coating apparatus and then dried. By drying, the upper layer (first layer 64a) is formed, and the negative electrode active material layer 64 is formed.

[0069] The pressing process can be carried out according to a known method. Specifically, the pressing process can be performed by applying pressure to the formed upper and lower layers (i.e., the negative electrode active material layer 64) using a roller press or the like. The pressing process compresses the negative electrode active material layer 64 to a predetermined density, thereby densely filling it with negative electrode active material particles.

[0070] The negative electrode 60 according to this embodiment can suppress the increase in resistance when the secondary battery is repeatedly charged and discharged. Furthermore, since the negative electrode 60 according to this embodiment uses a negative electrode active material containing Si, the capacity of the secondary battery can be increased.

[0071] Therefore, from another perspective, the secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, an embodiment of the secondary battery disclosed herein will be described with reference to Figures 3 and 4, using a lithium-ion secondary battery as an example. The following configuration example is a flat rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0072] The lithium-ion secondary battery 100 shown in Figure 3 is a sealed lithium-ion secondary battery 100 constructed by housing a flat-shaped wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight metal material with good thermal conductivity such as aluminum is used.

[0073] As shown in Figures 3 and 4, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are superimposed on each other via two elongated separator sheets 70 and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated negative electrode current collector 62. The portion 52a where the positive electrode active material layer is not formed (i.e., the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the portion 62a where the negative electrode active material layer is not formed (i.e., the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed) are formed to protrude outward from both ends of the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are joined to the positive electrode active material layer not formed portion 52a and the negative electrode active material layer not formed portion 62a, respectively.

[0074] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in lithium-ion secondary batteries may be used, and examples include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0075] The dimensions of the positive electrode current collector 52 are not particularly limited and can be determined as appropriate according to the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 7 μm or more and 20 μm or less.

[0076] 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 may be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, spinel structure, olivine structure, etc.

[0077] Preferably, lithium transition metal composite oxides are lithium transition metal composite oxides containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel-manganese-based composite oxides, lithium nickel-cobalt-manganese-based composite oxides, lithium nickel-cobalt-aluminum-based composite oxides, and lithium iron-nickel-manganese-based composite oxides.

[0078] In this specification, "lithium nickel cobalt manganese composite oxide" is a term that encompasses not only oxides whose constituent elements are Li, Ni, Co, Mn, and O, but also oxides that contain one or more additive elements other than these. Examples of such additive elements include transition metal elements and main group metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additive elements may also be metalloid elements such as B, C, Si, and P, or nonmetallic elements such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc.

[0079] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0080] These positive electrode active materials may be used individually or in combination of two or more. Lithium nickel cobalt manganese composite oxides are particularly preferred as positive electrode active materials due to their excellent properties, such as initial resistance characteristics.

[0081] The average particle size (D50) of the positive electrode active material is not particularly limited, but is, for example, 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.

[0082] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, conductive material, binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor-processed carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF), etc.

[0083] The content of positive electrode active material in the positive electrode active material layer 54 (i.e., the content of 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, and 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, and 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, and more preferably 0.5% by mass or more and 10% by mass or less.

[0084] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, and preferably 300 μm or less.

[0085] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0086] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such porous sheets may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0087] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability obtained by the Gurley test method of the separator 70 is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0088] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents commonly used in lithium-ion secondary battery electrolytes, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without particular limitation. Among these, carbonates are preferred, with specific examples including ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used individually or in appropriate combinations of two or more. For example, the non-aqueous solvent may consist solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0089] Suitable supporting salts include lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI), preferably LiPF6. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0090] Furthermore, the above-mentioned non-aqueous electrolyte may contain components other than those described above, such as film-forming agents like vinylene carbonate (VC) and oxalat complexes; gas-generating agents like biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and other additives, as long as they do not significantly impair the effects of the present disclosure.

[0091] The lithium-ion secondary battery 100 exhibits a small increase in resistance when repeatedly charged and discharged. Furthermore, the lithium-ion secondary battery 100 has a high capacity. The lithium-ion secondary battery 100 can be used in a variety of applications. Suitable applications include power supplies for vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium-ion secondary battery 100 can also be used as a storage battery for small-scale power storage devices. Typically, the lithium-ion secondary battery 100 can also be used in the form of a battery pack, where multiple batteries are connected in series and / or parallel.

[0092] The above describes a rectangular lithium-ion secondary battery 100 equipped with a flattened wound electrode body 20 as an example. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries equipped with a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are stacked alternately). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, laminated case type lithium-ion secondary batteries, and the like.

[0093] Furthermore, according to known methods, the lithium-ion secondary battery 100 can also be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0094] Furthermore, while the negative electrode 60 according to this embodiment is suitable as the negative electrode of a lithium-ion secondary battery, it can be constructed and used as the negative electrode of other secondary batteries, and other secondary batteries can be constructed according to known methods.

[0095] The following describes examples relating to this disclosure in detail, but this disclosure is not intended to be limited to those shown in such examples.

[0096] <Fabrication of the negative electrode> [Example 1] The following materials were prepared as the negative electrode active material. First Si-containing particles: Si-C composite material, Si content ratio = 65% by mass, average particle diameter (D50) = 7μm, consolidation density 0.85g / cm 3 Second Si-containing particles: Si-C composite material, Si content ratio = 35% by mass, average particle diameter (D50) = 6μm, consolidation density 1.3g / cm 3 Graphite particles (first and second graphite particles): Average particle size (D50) = 15 μm. 1 g of graphite particles is compressed uniaxially to achieve a density of 1.7 g / cm³. 3 Furthermore, the molding pressure when forming a tablet shape with a diameter of 20 mm was 15 MPa.

[0097] The Si content of the first Si-containing particles and the second Si-containing particles was measured using a commercially available ICP-OEC instrument. The average particle size (D50) of each particle was measured using a commercially available laser diffraction / scattering particle size distribution analyzer. The compaction density of the first Si-containing particles and the second Si-containing particles was measured by the following method.

[0098] One g of either primary or secondary Si-containing particles was weighed out as a powder measurement sample and placed on the probe (20 mm in diameter) of the automatic powder resistance measurement system "MCP-PD600" (manufactured by Nitto Seikou Analytech Co., Ltd.). At 25°C, the load and displacement when pressurized in a uniaxial direction were measured using this automatic powder resistance measurement system. Based on this, the bulk density of the molded body at a pressure of 60 MPa was determined and defined as the consolidation density. In addition, one g of graphite particles was weighed out as a powder measurement sample and placed on the probe (20 mm in diameter) of the automatic powder resistance measurement system "MCP-PD600" (manufactured by Nitto Seikou Analytech Co., Ltd.). At 25°C, the load and displacement when pressurized in a uniaxial direction were measured using this automatic powder resistance measurement system. Based on this, the density of the molded body was determined to be 1.7 g / cm³. 3 The molding pressure was then calculated.

[0099] Single-walled carbon nanotubes (SWCNTs) were prepared as the conductive material. The SWCNTs were prepared in the form of a dispersion. Carboxymethylcellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared as binders.

[0100] A paste for forming the upper layer, containing graphite particles, primary Si-containing particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 60:40:0.1:1:1:1.5, was prepared according to the following procedure. A paste for forming the lower layer, containing graphite particles, secondary Si-containing particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 60:40:0.1:1:1:1.5, was also prepared according to the following procedure.

[0101] Graphite particles, primary Si-containing particles, CMC, and PAA were dry-blended using a planetary mixer. The resulting dry mixture, SWCNT dispersion, and dispersion medium were kneaded using a planetary mixer. Furthermore, SBR and additional dispersion medium were added to the planetary mixer and diluted and mixed to obtain a paste for forming the upper layer.

[0102] Graphite particles, secondary Si-containing particles, CMC, and PAA were dry-blended using a planetary mixer. The resulting dry mixture, SWCNT dispersion, and dispersion medium were kneaded using a planetary mixer. Furthermore, SBR and additional dispersion medium were added to the planetary mixer and diluted and mixed to obtain a paste for forming the lower layer.

[0103] The prepared lower layer paste was applied to the surface of a 10 μm thick copper foil and dried to form the lower layer of the negative electrode active material layer. Furthermore, the prepared upper layer paste was applied on top of the lower layer and dried to form the upper layer. This created a multi-layered negative electrode active material layer. After roll pressing the negative electrode active material layer, the resulting sheet was processed to a predetermined size to obtain a negative electrode sheet.

[0104] [Example 2] As a particle containing secondary Si, the compressed density is 1.0 g / cm³. 3 The negative electrode sheet of Example 2 was obtained in the same manner as in Example 1, except that particles of a Si-C composite material with a Si content of 51% by mass were used.

[0105] [Example 3] As a first Si-containing particle, the compressed density is 0.72 g / cm³. 3 The negative electrode sheet of Example 3 was obtained in the same manner as in Example 1, except that particles of a Si-C composite material with a Si content of 58% by mass were used.

[0106] [Example 4] The negative electrode sheet of Example 4 was obtained in the same manner as in Example 1, except that the ratio of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) (T2 / T1) was changed to 20 / 80.

[0107] [Example 5] The negative electrode sheet of Example 5 was obtained in the same manner as in Example 1, except that the ratio of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) (T2 / T1) was changed to 80 / 20.

[0108] [Comparative Example 1] The negative electrode sheet of Comparative Example 1 was obtained in the same manner as in Example 1, except that the lower layer was formed using an upper layer forming paste and the upper layer was formed using a lower layer forming paste. Therefore, in Comparative Example 1, the first Si-containing particles and the second Si-containing particles were swapped.

[0109] [Comparative Example 2] As a first Si-containing particle, the compressed density is 1.0 g / cm³. 3 Using particles of a Si-C composite material with a Si content of 51% by mass, the second Si-containing particles had a compressed density of 0.85 g / cm³. 3 A negative electrode sheet for Comparative Example 2 was obtained in the same manner as in Example 1, except that particles of a Si-C composite material with a Si content of 65% by mass were used.

[0110] [Comparative Example 3] A negative electrode sheet for Comparative Example 3 was obtained in the same manner as in Example 1, 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 of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) (T2 / T1) was changed to 10 / 90.

[0111] [Comparative Example 4] A paste for forming the negative electrode active material layer was prepared by mixing a paste for forming the lower layer and a paste for forming the upper layer so that the mass ratio of their solid contents was 1:1. This paste was applied to the surface of a 10 μm thick copper foil and dried to form the negative electrode active material layer. After roll pressing the negative electrode active material layer, the resulting sheet was processed to a predetermined size to obtain the negative electrode sheet of Comparative Example 4. The thickness of the negative electrode sheet of Comparative Example 4 was the same as that of Example 1.

[0112] [Comparative Example 5] A negative electrode active material layer was formed by applying a base layer forming paste to the surface of a 10 μm thick copper foil and drying it. After roll pressing the negative electrode active material layer, the resulting sheet was processed to a predetermined size to obtain the negative electrode sheet of Comparative Example 5. The thickness of the negative electrode sheet of Comparative Example 5 was the same as that of Example 1.

[0113] <Evaluation of Resistance Increase Rate> First, evaluation lithium-ion secondary batteries were prepared using the negative electrodes of each example and each comparative example as follows.

[0114] LiNi as a 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) in a mass ratio of NCM:AB:PVdF = 100:1:1. This paste was applied to the surface of a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. After roll pressing the positive electrode active material layer, the resulting sheet was processed to a predetermined size to obtain a positive electrode sheet.

[0115] A separator made of porous polyolefin was prepared. Leads were attached to the negative electrode sheet and positive electrode sheet prepared above, and the electrodes were stacked via the separator to create an electrode body. This was then placed in an aluminum laminate film case along with a non-aqueous electrolyte. The non-aqueous electrolyte was a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40, with LiPF6 dissolved as a supporting salt at a concentration of 1.0 mol / L. The case was then sealed to obtain a lithium-ion secondary battery for evaluation.

[0116] Next, each of the prepared lithium-ion secondary batteries for evaluation was placed in an environment of 25°C. Each evaluation lithium-ion secondary battery was charged with a constant current of 0.2C up to 4.2V, and then charged with a constant voltage until the current value was 0.1C. Initial charging was performed in this manner. Subsequently, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.2C down to 2.5V.

[0117] Next, each evaluation lithium-ion secondary battery was adjusted to a state of charge (SOC) of 50% by constant current-constant voltage charging and stored for 1 hour at 25°C. Afterward, it was discharged at a constant current of 1C for 10 seconds. The DC resistance was determined by dividing the difference between the open-circuit voltage (OCV) and the closed-circuit voltage (CCV) 10 seconds after discharge by the discharge current 10 seconds after discharge, i.e., based on the following formula. This determined value was defined as the initial DC resistance. DC resistance = [OCV-CCV (after 10 seconds of discharge)] / discharge current (after 10 seconds of discharge)

[0118] Under conditions of 25°C, each evaluation lithium-ion secondary battery was adjusted to a state of charge (SOC) of 50% by constant current-constant voltage charging. Then, a charge-discharge cycle was repeated 400 times, consisting of constant current charging at 1.5C for 400 seconds followed by constant current discharge at 0.75C for 800 seconds. Afterward, the DC resistance after 400 cycles was determined using the same method as above. The resistance increase rate (%) was then calculated based on the following formula. Resistance increase rate = (DC resistance after 400 cycles / initial DC resistance) × 100

[0119] [Table 1]

[0120] From the results in Table 1, in addition to the first graphite particles, the upper layer of the negative electrode active material contains a high Si content and low compaction density (specifically, a compaction density of 0.9 g / cm³). 3 Using first Si-containing particles (less than 0.9 g / cm³), in the lower layer of the negative electrode active material layer, in addition to second graphite particles, a low Si content and high-pressure compaction density (specifically, a compaction density of 0.9 g / cm³) is used. 3 As shown above, when using the second Si-containing particles, the resistance increase rate is very small. Therefore, it can be seen that the negative electrode of this disclosure can suppress the resistance increase when the secondary battery is repeatedly charged and discharged, even when using a negative electrode containing both Si-containing particles and graphite particles.

[0121] 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 technologies described in the claims include various modifications and changes to the specific examples described above.

[0122] In other words, the negative electrode of the secondary battery disclosed herein, the method for manufacturing the same, and the secondary battery are the following items [1] to

[10] . [1] Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, The negative electrode of a secondary battery, 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 aforementioned second layer contains secondary graphite particles and secondary Si-containing particles, The Si content in the first Si-containing particle is greater than the Si content in the second Si-containing particle. When the density of a molded body obtained by pressurizing 1 g of particles uniaxially at 25°C and 60 MPa to form a tablet with 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. 3 The value is less than the specified value, and the compaction density of the aforementioned second Si-containing particles is 0.9 g / cm³. 3 That's all for the negative electrode. [2] The compaction density of the first Si-containing particles is 0.65 g / cm³. 3 More than 0.85g / cm 3 The negative electrode is as follows, as described in item [1]. [3] The compaction density of the second Si-containing particles is 0.95 g / cm³. 3 More than 1.8g / cm 3 The negative electrode as described in item [1] or [2] below. [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. [5] The negative electrode according to any one of items [1] to [4], wherein the first Si-containing particles and the second Si-containing particles are each particles of a Si-C composite material. [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. [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. [8] In the first layer, the mass ratio of the first Si-containing particles to the total of the first graphite particles and the first Si-containing particles is 10% by mass to 60% by mass. The negative electrode according to any one of items [1] to [7], wherein in the second layer, the mass ratio of the second Si-containing particles to the total of the second graphite particles and the second Si-containing particles is 10% by mass to 60% by mass. [9] A step of preparing a paste for forming the second layer by mixing the second graphite particles and the second Si-containing particles in a dispersion medium. A step of preparing a paste for forming the first layer by mixing first graphite particles and first Si-containing particles in a dispersion medium. The process involves applying the second layer-forming paste onto the negative electrode current collector and drying it to form the second layer. The steps include applying the paste for forming the first layer onto the second layer and drying it to form the first layer, and The process includes pressing the formed first and second layers, The Si content in the first Si-containing particle is greater than the Si content in the second Si-containing particle. When the density of a molded body obtained by pressurizing 1 g of particles uniaxially at 25°C and 60 MPa to form a tablet with 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. 3 The value is less than the specified value, and the compaction density of the aforementioned second Si-containing particles is 0.9 g / cm³. 3 That's all. A method for manufacturing the negative electrode of a secondary battery.

[10] Positive electrode, negative electrode, electrolyte, A secondary battery equipped with, A secondary battery wherein the negative electrode is the negative electrode described in any one of items [1] to [8]. [Explanation of Symbols]

[0123] 12. First Graphite Particles 14 1st Si-containing particles 16. Second-order graphite particles 18 2nd Si-containing particles 20 Wound electrode body 30 Battery Cases 36 Safety valve 42 Positive terminal 42a Positive electrode current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheets (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheets (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator Sheets (Separators) 100 Lithium-ion rechargeable batteries

Claims

1. Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, The negative electrode of a secondary battery, 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 aforementioned second layer contains second graphite particles and second Si-containing particles, The Si content in the first Si-containing particle is greater than the Si content in the second Si-containing particle. When the density of a molded body obtained by pressing 1 g of particles uniaxially at 25°C and 60 MPa to form a tablet with a diameter of 20 mm is defined as the compaction density, the compaction density of the first Si-containing particle is 0.9 g / cm³. ３ The value is less than the specified value, and the compaction density of the second Si-containing particle is 0.9 g / cm³. ３ That's all for the negative electrode.

2. The compaction density of the first Si-containing particle is 0.65 g / cm³. ３ 0.85g / cm or more ３ The negative electrode according to claim 1, which is as follows:

3. The compaction density of the second Si-containing particle is 0.95 g / cm³. ３ 1.8g / cm or more ３ The negative electrode according to claim 1, which is as follows:

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 particles and the second Si-containing particles are each particles of a Si-C composite material.

6. The negative electrode according to claim 1, 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.

7. The negative electrode according to claim 1, 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.

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

9. A step of preparing a paste for forming the second layer by mixing second graphite particles and second Si-containing particles in a dispersion medium. A step of preparing a paste for forming the first layer by mixing first graphite particles and first Si-containing particles in a dispersion medium. The process involves applying the second layer-forming paste onto the negative electrode current collector and drying it to form the second layer. The steps include applying the paste for forming the first layer onto the second layer and drying it to form the first layer, and The process includes pressing the formed first and second layers, The Si content in the first Si-containing particle is greater than the Si content in the second Si-containing particle. When the density of a molded body obtained by pressing 1 g of particles uniaxially at 25°C and 60 MPa to form a tablet with a diameter of 20 mm is defined as the compaction density, the compaction density of the first Si-containing particle is 0.9 g / cm³. ３ The value is less than the specified value, and the compaction density of the second Si-containing particle is 0.9 g / cm³. ３ That's all. A method for manufacturing the negative electrode of a secondary battery.

10. Positive electrode, negative electrode, electrolyte, A secondary battery equipped with, A secondary battery wherein the negative electrode is the negative electrode described in claim 1.

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