The negative electrode of a secondary battery and a secondary battery using the said negative electrode

Abstract

The present invention provides a negative electrode containing Si-containing particles and graphite particles, which exhibits minimal swelling when repeatedly charged and discharged in a secondary battery. [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 smaller than the Si content in the second Si-containing particles. The first Si-containing particles and the second Si-containing particles are each coated with a conductive material. The ratio of the amount of conductive material coating (mass%) for the second Si-containing particles to the amount of conductive material coating (mass%) for the first Si-containing particles is 2.5 or more.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode for a secondary battery. This disclosure also relates to a secondary battery using 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, while Si-containing particles have high capacity, they exhibit significant volume changes due to expansion and contraction during charging and discharging of secondary batteries. Furthermore, when Si-containing particles and graphite particles are used together as the negative electrode active material, repeated charging and discharging of the secondary battery leads to increased internal stress due to the expansion of the negative electrode. Therefore, there is a need to develop a negative electrode containing Si-containing particles and graphite particles that exhibits less expansion when repeatedly charging and discharging of a secondary battery. Note that this negative electrode expansion refers to the volume of the negative electrode becoming larger than its initial volume in the same charging state (e.g., discharging state).

[0006] In view of the above circumstances, the present disclosure aims to provide a negative electrode containing Si-containing particles and graphite particles that exhibits minimal swelling when repeatedly charged and discharged in a secondary battery. [Means for solving the problem]

[0007] 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 smaller than the Si content in the second Si-containing particles. The first Si-containing particles and the second Si-containing particles are each coated with a conductive material. The ratio of the amount of conductive material coating (mass%) for the second Si-containing particles to the amount of conductive material coating (mass%) for the first Si-containing particles is 2.5 or more.

[0008] This configuration provides a negative electrode containing Si-containing particles and graphite particles that exhibits minimal swelling when repeatedly charged and discharged in a secondary battery.

[0009] In another respect, the secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode described above.

[0010] With this configuration, it is possible to provide a secondary battery in which the negative electrode does not swell significantly when repeatedly charged and discharged, even when using a negative electrode containing Si-containing particles and graphite particles. [Brief explanation of the drawing]

[0011] [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]

[0012] 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.

[0013] 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.

[0014] The negative electrode disclosed herein is used in a secondary battery, and preferably in a lithium-ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described while referring to FIG. 1. FIG. 1 is a cross-sectional view schematically showing an example of the negative electrode 60 according to the present embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to the present embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0015] As shown in the drawing, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0016] As in the illustrated example, a negative electrode active material layer non-formation portion 62a where the negative electrode active material layer 64 is not provided may be provided at one end in the width direction of the negative electrode 60. In the negative electrode active material layer non-formation portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-formation portion 62a can function as a current collection portion. However, the configuration for current collection from the negative electrode 60 is not limited to this.

[0017] The shape of the negative electrode current collector 62 is, in the illustrated example, foil-like (or sheet-like), but is not limited thereto. The negative electrode current collector 62 may be in various forms such as rod-like, plate-like, mesh-like, etc. As the material of the negative electrode current collector 62, a metal having good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be used as in a conventional lithium-ion secondary battery, and among them, copper is preferable. As the negative electrode current collector 62, a copper foil is particularly preferable.

[0018] The dimensions of the negative electrode current collector 62 are not particularly limited and may be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but for example, it is 5 μm or more and 35 μm or less, and preferably 6 μm or more and 20 μm or less.

[0019] As shown in FIG. 1, the negative electrode active material layer 64 has a multilayer structure. Specifically, it has a first layer 64a located on the surface layer 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. As shown in FIG. 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. Note that 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 the effects of the present invention are not significantly inhibited. 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.

[0020] The negative electrode active material layer 64 contains a negative electrode active material. This will be described in detail using FIG. 2. FIG. 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 FIG. 1. Note that FIG. 2 is a schematic diagram, and thus the number, distribution, etc. of the particles are not limited to those shown in FIG. 2.

[0021] 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 the volume change of the Si-containing particles due to expansion / contraction accompanying charge / discharge is large, by using them in combination with the graphite particles, it is possible to suppress the disconnection of the conductive path caused by the volume change of the Si-containing particles.

[0022] The graphite constituting the first graphite particles 12 and the second graphite particles 16 may be natural graphite or artificial graphite, or may be amorphous carbon-coated graphite in a form in which the graphite is coated with an amorphous carbon material.

[0023] The 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.

[0024] 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.

[0025] The average particle size (D50) of the first graphite particles 12 and the second graphite particles 16 is not particularly limited. The average particle size (D50) of the first graphite particles 12 and the second graphite particles 16 is, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] In this embodiment, the Si content ratio (S1) in the first Si-containing particles 14 is smaller than the Si content ratio (S2) in the second Si-containing particles 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 suppressing the expansion of the negative electrode when the secondary battery is repeatedly charged and discharged may be reduced. On the other hand, if these Si content ratios are too high, the volume change due to the 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.

[0034] Therefore, the Si content (S1) in the first Si-containing particle 14 is preferably 20% to 55% by mass, and more preferably 25% to 45% by mass. The Si content (S2) in the second Si-containing particle 18 is preferably 45% to 80% by mass, and more preferably 55% to 75% by mass.

[0035] Furthermore, the ratio (S1 / S2) of the Si content (S1) in the first Si-containing particle 14 to the Si content (S2) in the second Si-containing particle 18 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and even more preferably 0.40 to 0.75.

[0036] The average particle diameter (D50) of the first Si-containing particles 14 and the second Si-containing particles 18 is not particularly limited. The average particle diameter (D50) of the first Si-containing particles 14 and the second Si-containing particles 18 is, for example, 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm.

[0037] 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).

[0038] As shown in Figure 2, the first layer 64a contains a first conductive material 15. In the first layer 64a, the first Si-containing particles 14 are coated with the first conductive material 15. In addition, the second layer 64b contains a second conductive material 19. In the second layer 64b, the second Si-containing particles 18 are coated with the second conductive material 19.

[0039] In the illustrated example, the entire first Si-containing particle 14 and the entire second Si-containing particle 18 are coated with the first conductive material 15 and the second conductive material 19, respectively. Thus, the first conductive material 15 and the second conductive material 19 each form a coating layer. However, the first Si-containing particle 14 and the second Si-containing particle may be partially coated with the first conductive material 15 and the second conductive material 19, respectively.

[0040] Examples of conductive materials (i.e., the first conductive material 15 and the second conductive material 19) include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are particularly preferred. It is preferable that the first conductive material 15 and the second conductive material 19 are the same.

[0041] When carbon nanotubes (CNTs) are used as conductive materials, the type is not particularly limited, and for example, single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs) can be used. These can be used individually or in combination of two or more types. SWCNTs are preferred as CNTs because they can form more conductive paths between the negative electrode active materials. The CNTs may be manufactured by methods such as arc discharge, laser ablation, or chemical vapor deposition.

[0042] The average length of the CNTs is not particularly limited. If the average length of the CNTs is too long, the CNTs tend to aggregate, reducing their dispersibility. Therefore, the average length of the CNTs is preferably 15 μm or less, more preferably 8.0 μm or less, and even more preferably 5.0 μm or less. On the other hand, if the average length of the CNTs is too short, it tends to become difficult to form conductive paths between the negative electrode active materials. Therefore, the average length of the CNTs is preferably 0.3 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more.

[0043] The average diameter of the CNTs is not particularly limited, and is, for example, 0.1 nm to 50 nm, preferably 0.3 nm to 30 nm.

[0044] The average length and diameter of carbon nanotubes (CNTs) can be determined by taking electron microscope images of CNTs and calculating the average length and diameter of 100 or more CNTs. Specifically, for example, a CNT dispersion is diluted and dried to prepare a sample for measurement. This sample is then observed using a scanning electron microscope (SEM) to determine the length and diameter of 100 or more CNTs, and the average value is calculated. If the CNTs have re-aggregated, the length and diameter are determined for the bundle of aggregated CNTs.

[0045] In this embodiment, the amount of coating (C1) (mass%) of the first Si-containing particle 14 by the first conductive material 15 is less than the amount of coating (C2) (mass%) of the second Si-containing particle 18 by the second conductive material 19. The amount of coating (C1) is the ratio (%) of the mass of the first conductive material 15 to the mass of the first Si-containing particle 14, and the amount of coating (C2) is the ratio (%) of the mass of the second conductive material 19 to the mass of the second Si-containing particle 18.

[0046] Furthermore, the ratio (C2 / C1) of the amount of conductive material covering the second Si-containing particle (C2) to the amount of conductive material covering the first Si-containing particle (C1) is 2.5 or greater.

[0047] 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, first Si-containing particles 14 coated with the first conductive material 15 are used, and 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, second Si-containing particles 18 coated with the second conductive material 19 to a coating amount of 2.5 times or more than that of the first Si-containing particles 14 are used. As a result, swelling of the negative electrode 60 when the secondary battery is repeatedly charged and discharged can be significantly suppressed. The reason for this is thought to be as follows.

[0048] In other words, in the negative electrode active material layer 64, the upper layer (i.e., the first layer 64a) is the one that expands the most when the secondary battery is repeatedly charged and discharged. Therefore, Si-containing particles with a low Si content, which result in less expansion and contraction, are used in the upper layer, and these particles are further coated with a relatively small amount of conductive material. This makes it possible to suppress the expansion and contraction of the Si-containing particles and side reactions caused by charging and discharging.

[0049] On the other hand, the lower layer (i.e., the second layer 64b) uses Si-containing particles with a high Si content, which results in large expansion and contraction, and these particles are further coated with a relatively large amount of conductive material. This makes it possible to suppress the interruption of the conductive path during charging and discharging of the secondary battery. Therefore, it is possible to suppress the swelling of the negative electrode associated with the interruption of the conductive path (i.e., swelling due to uneven battery reaction and localized concentration of reaction and stress). Consequently, the swelling of the negative electrode 60 when the secondary battery is repeatedly charged and discharged can be significantly suppressed across the entire negative electrode active material layer 64.

[0050] The above coating ratio (C2 / C1) is preferably 2.8 or more, more preferably 3.0 or more, even more preferably 5.0 or more, and particularly preferably 7.0 or more. The above coating ratio (C2 / C1) may be 50 or less, 30 or less, 20 or less, or 15 or less.

[0051] The content of the conductive material in the negative electrode active material layer 64 (i.e., the first layer 64a and the second layer 64b) is not particularly limited as long as the effects of the present invention are obtained. The amount of conductive material is, for example, 0.02% to 10% by mass relative to the negative electrode active material, and may be appropriately determined depending on the type of conductive material. When the conductive material is CNT, the content of the conductive material in the negative electrode active material layer 64 is preferably 0.02% to 3.0% by mass, and more preferably 0.05% to 1.0% by mass, relative to the negative electrode active material.

[0052] Therefore, the amount of coating (C1) of the first Si-containing particle 14 by the first conductive material 15, and the amount of coating (C2) of the second Si-containing particle 18 by the second conductive material 19, can be appropriately set by considering the content of the conductive material in the negative electrode active material layer 64, the ratio of the coating amounts (C2 / C1), and the mass ratio of the first Si-containing particle 14 and the second Si-containing particle 18.

[0053] When the first conductive material 15 is CNT, the amount of coating (C1) of the first Si-containing particles 14 by the first conductive material 15 is, for example, 0.05 mass% to 2.0 mass%, preferably 0.05 mass% to 1.0 mass%, more preferably 0.05 mass% to 0.25 mass%, and particularly preferably 0.07 mass% to 0.24 mass%. When the second conductive material 19 is CNT, the amount of coating (C2) of the second Si-containing particles 18 by the second conductive material 19 is, for example, 0.10 mass% to 3.0 mass%, preferably 0.20 mass% to 1.5 mass%, more preferably 0.25 mass% to 0.70 mass%, and particularly preferably 0.27 mass% to 0.55 mass%.

[0054] The ratio of the total mass of the second conductive material 19 covering the second Si-containing particles 18 to the total mass of the first conductive material 15 covering the first Si-containing particles 14 is preferably greater than 1, more preferably 1.5 or more, even more preferably 2 or more, and particularly preferably 4 or more.

[0055] Furthermore, the first conductive material 15 and the second conductive material 19 may have a CNT dispersant attached to them. A known CNT dispersant can be used as the CNT dispersant.

[0056] 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%.

[0057] 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.

[0058] 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).

[0059] 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).

[0060] 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 swelling of the negative electrode 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.

[0061] The negative electrode active material layer 64 may contain components other than the negative electrode active material and conductive material, such as a binder. Examples of binders include styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickening agent.

[0062] 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 first layer 64a 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.

[0063] 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 second layer 64b 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.

[0064] 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.

[0065] 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. 3 That 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:

[0066] 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.

[0067] The negative electrode 60 is prepared by, for example, a step of preparing first Si-containing particles 14 coated with a first conductive material 15 and second Si-containing particles 18 coated with a second conductive material 19 (hereinafter also referred to as the "coated particle preparation step"), where the Si content ratio (S1) in the first Si-containing particles 14 is smaller than the Si content ratio (S2) in the second Si-containing particles 18, and the ratio of the amount of conductive material coating (mass%) on the second Si-containing particles 18 to the amount of conductive material coating (mass%) on the first Si-containing particles 14 is 2.5 or more; and the second Si-containing particles 18 coated with the second conductive material 19 and the second graphite particles 16 are mixed in a dispersion medium. The product can be suitably manufactured by a manufacturing method comprising: a step of preparing a paste for forming the lower layer (hereinafter also referred to as the "lower layer forming paste preparation step"); a step of mixing the first Si-containing particles 14 and the first graphite particles 12 coated with the first conductive material 15 in a dispersion medium to prepare a paste for forming the upper layer (hereinafter also referred to as the "upper layer forming paste preparation step"); a step of applying the lower layer forming paste onto the negative electrode current collector 62 and drying it to form the lower layer (hereinafter also referred to as the "lower layer forming step"); and a step of applying the upper layer forming paste onto the lower layer and drying it to form the upper layer (hereinafter also referred to as the "upper layer forming step").

[0068] 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.

[0069] The preparation step for coated particles can be carried out according to known methods. Specifically, for example, first Si-containing particles 14 with a high Si content and second Si-containing particles 18 with a low Si content are prepared. Conductive materials (first conductive material and second conductive material) are also prepared. Known coating methods can be used to coat the first Si-containing particles 14 and second Si-containing particles 18 with the conductive material. For example, a method in which Si-containing particles and conductive material are dispersed in a dispersion medium and then dried by heating or reduced pressure as necessary; a method in which a slurry in which Si-containing particles and conductive material are dispersed in a dispersion medium is spray-dried; a method in which Si-containing particles and conductive material are subjected to mechanofusion; a method in which Si-containing particles and conductive material are subjected to mechanical milling; a method in which conductive material is deposited on Si-containing particles by chemical vapor deposition (CDV), and so on.

[0070] Furthermore, the coating of the first Si-containing particles 14 and the second Si-containing particles 18 with the conductive material shall be carried out such that the coating amount ratio (C2 / C1) is 2.5 or greater.

[0071] 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 coated with the second conductive material 19, and an optional component (e.g., a binder) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0072] The upper layer paste preparation step can be carried out by mixing the first graphite particles 12, the first Si-containing particles 14 coated with the first conductive material 15, and an optional component (e.g., a binder) 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.

[0073] 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.

[0074] 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.

[0075] After the drying process, a further step of pressing the negative electrode active material layer 64 may be performed. The pressing process can be carried out according to a known method.

[0076] According to the negative electrode 60 of this embodiment, swelling of the negative electrode 60 when the secondary battery is repeatedly charged and discharged can be suppressed. Furthermore, since the negative electrode 60 of this embodiment uses a negative electrode active material containing Si, the capacity of the secondary battery can be increased.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] The lithium-ion secondary battery 100 has suppressed negative electrode swelling during repeated charging and discharging, and therefore exhibits low reaction force. Furthermore, the lithium-ion secondary battery 100 has high capacity. The lithium-ion secondary battery 100 can be used for various 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] <Fabrication of the negative electrode> [Example 1] The following materials were prepared as the negative electrode active material. 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. First Si-containing particles: Si-C composite material, Si content ratio = 35% by mass, average particle diameter (D50) = 8μm Second Si-containing particles: Si-C composite material, Si content ratio = 65% by mass, average particle diameter (D50) = 7μm Graphite particles (first and second graphite particles): Average particle size (D50) = 15 μm

[0103] A paste for forming the upper layer, containing graphite particles, primary Si-containing particles, primary conductive material, CMC, PAA, and SBR in a mass ratio of 70:30:0.1:1:1:1, was prepared according to the following procedure. A paste for forming the lower layer, containing graphite particles, secondary Si-containing particles, secondary conductive material, CMC, PAA, and SBR in a mass ratio of 80:20:0.1:1:1:1, was also prepared according to the following procedure.

[0104] Single-walled carbon nanotubes (SWCNTs) were prepared as the conductive material. The SWCNTs were prepared in the form of a dispersion. The first Si-containing particles, the SWCNT dispersion, and the dispersion medium were mixed using a disperser at a rotation speed of 3000 rpm. In this way, particles were obtained in which the surface of the first Si-containing particles was coated with SWCNTs (first conductive material). Similarly, the second Si-containing particles, the SWCNT dispersion, and the dispersion medium were mixed using a disperser at a rotation speed of 3000 rpm. In this way, particles were obtained in which the surface of the second Si-containing particles was coated with SWCNTs (second conductive material).

[0105] We prepared carboxymethylcellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) as binders.

[0106] Graphite particles, CMC, and PAA were dry-blended using a planetary mixer. The resulting dry mixture, along with the first Si-containing particles coated with the first conductive material, were added to the planetary mixer along with a dispersion medium and kneaded. 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.

[0107] Graphite particles, CMC, and PAA were dry-blended using a planetary mixer. The resulting dry mixture, along with the second Si-containing particles coated with the second conductive material, were added to the planetary mixer along with a dispersion medium and mixed. Furthermore, SBR and additional dispersion medium were added to the planetary mixer and diluted to obtain a paste for forming the lower layer.

[0108] 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.

[0109] [Example 2] The negative electrode sheet of Example 2 was obtained in the same manner as in Example 1, except that the first Si-containing particles and the second Si-containing particles were changed to those described below, and the blending mass ratio of the first conductive material in the upper layer was changed from 0.1 to 0.2. First Si-containing particles: Si-C composite material, Si content ratio = 40% by mass, average particle diameter (D50) = 7μm Second Si-containing particles: Si-C composite material, Si content ratio = 60% by mass, average particle diameter (D50) = 6μm

[0110] [Example 3] The negative electrode sheet of Example 3 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 10 / 90.

[0111] [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 90 / 10.

[0112] [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 20 / 80.

[0113] [Example 6] The negative electrode sheet of Example 6 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.

[0114] [Example 7] The negative electrode sheet of Example 7 was obtained in the same manner as in Example 1, except that the blending mass ratio of the second conductive material in the lower layer was changed from 0.5 to 0.3.

[0115] [Example 8] The negative electrode sheet of Example 7 was obtained in the same manner as in Example 1, except that the blending mass ratio of the first conductive material in the upper layer was changed from 0.1 to 0.2, and the blending mass ratio of the second conductive material in the lower layer was changed from 0.5 to 0.4.

[0116] [Comparative Example 1] 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 1. The thickness of the negative electrode sheet of Comparative Example 1 was the same as that of Example 1.

[0117] [Comparative Example 2] A negative electrode sheet of Comparative Example 2 was obtained in the same manner as in Example 1, except that the lower layer was formed using the paste for upper layer formation and the upper layer was formed using the paste for lower layer formation. Therefore, in Comparative Example 2, the first Si-containing particles coated with the first conductive material and the second Si-containing particles coated with the second conductive material were used in reverse.

[0118] [Comparative Example 3] A negative electrode sheet of Comparative Example 3 was obtained in the same manner as in Example 1, except that the blending mass ratio of the first conductive material in the upper layer was changed from 0.1 to 0.5.

[0119] [Comparative Example 4] A negative electrode sheet of Comparative Example 4 was obtained in the same manner as in Example 1, except that the blending mass ratio of the second conductive material in the lower layer was changed from 0.5 to 0.1.

[0120] [Comparative Example 5] The paste for lower layer formation was applied to the surface of a copper foil with a thickness of 10 μm and dried to form a negative electrode active material layer. After roll pressing the negative electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a negative electrode sheet of Comparative Example 5. The thickness of the negative electrode sheet of Comparative Example 5 was made the same as that of Example 1.

[0121] <Evaluation of electrode plate expansion rate> The thickness of the negative electrodes of each Example and each Comparative Example was measured. This thickness was taken as the initial thickness (T0). Using this negative electrode, an evaluation lithium ion secondary battery was fabricated as follows.

[0122] LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste was applied to the surface of an aluminum foil with a thickness of 15 μm and dried to form a positive electrode active material layer. After roll pressing the positive electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a positive electrode sheet.

[0123] 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.

[0124] 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 0.4C up to 4.2V, and then charged with a constant voltage until the current value was 0.1C. Subsequently, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.4C up to 2.5V.

[0125] The above charge-discharge cycle was repeated 250 times, with one cycle being defined as the charge-discharge cycle described above. Each lithium-ion secondary battery used for evaluation was disassembled under an argon atmosphere, the negative electrode was cleaned by immersion in DMC, and then dried. The thickness of the negative electrode was then measured and defined as the thickness after the charge-discharge cycle (Tc). The percentage change in negative electrode thickness before and after the charge-discharge cycle was calculated using (Tc / T0-1) × 100. The results are shown in Table 1.

[0126] [Table 1]

[0127] The results in Table 1 show that when low Si-containing particles coated with a first conductive material are used in addition to first graphite particles in the upper layer of the negative electrode active material layer, and high Si-containing particles coated with a second conductive material to 2.5 times or more the amount of coating of the first Si-containing particles are used in addition to second graphite particles in the lower layer of the negative electrode active material layer, the electrode plate expansion rate is very small. Therefore, it can be seen that with the negative electrode of this disclosure, even when using a negative electrode containing both Si-containing particles and graphite particles, the expansion of the negative electrode when repeatedly charging and discharging the secondary battery is small.

[0128] 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.

[0129] In other words, the negative electrode of the secondary battery and the secondary battery disclosed herein are the following items [1] to [9]. [1] Negative electrode current collector and A negative electrode comprising 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 aforementioned second layer contains secondary graphite particles and secondary Si-containing particles, The Si content in the first Si-containing particle is smaller than the Si content in the second Si-containing particle. The first Si-containing particles and the second Si-containing particles are each coated with a conductive material. A negative electrode in which the ratio of the amount of conductive material covering the second Si-containing particles (mass%) to the amount of conductive material covering the first Si-containing particles (mass%) is 2.5 or more. [2] The negative electrode according to item [1], wherein the conductive material is a carbon nanotube. [3] The negative electrode according to item [2], wherein the amount of conductive material covering the first Si-containing particles is 0.05 mass% to 2.0 mass%, and the amount of conductive material covering the second Si-containing particles is 0.10 mass% to 3.0 mass%. [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 ratio of the Si content in the first Si-containing particles to the Si content in the second Si-containing particles is 0.10 to 0.90. [6] The negative electrode according to any one of items [1] to [5], wherein the Si content in the first Si-containing particles is 20% to 55% by mass, and the Si content in the second Si-containing particles is 45% to 80% by mass. [7] The negative electrode according to any one of items [1] to [6], wherein the first Si-containing particles and the second Si-containing particles are each particles of a Si-C composite material. [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] 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]

[0130] 12. First Graphite Particles 14 1st Si-containing particles 15. First conductive material 16. Second-order graphite particles 18 2nd Si-containing particles 19. Second conductive material 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 A negative electrode comprising 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 aforementioned second layer contains second graphite particles and second Si-containing particles, The Si content in the first Si-containing particle is smaller than the Si content in the second Si-containing particle. The first Si-containing particles and the second Si-containing particles are each coated with a conductive material. A negative electrode in which the ratio of the amount of conductive material covering the second Si-containing particles (mass%) to the amount of conductive material covering the first Si-containing particles (mass%) is 2.5 or more.

2. The negative electrode according to claim 1, wherein the conductive material is a carbon nanotube.

3. The negative electrode according to claim 2, wherein the amount of conductive material covering the first Si-containing particles is 0.05% to 2.0% by mass, and the amount of conductive material covering the second Si-containing particles is 0.10% to 3.0% by mass.

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 ratio of the Si content in the first Si-containing particles to the Si content in the second Si-containing particles is 0.10 to 0.

90.

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

7. 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.

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. 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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