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

Abstract

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

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a negative electrode of a secondary battery and a manufacturing method thereof. The present disclosure also relates to a secondary battery using the negative electrode. BACKGROUND

[0002] In recent years, secondary batteries are suitably used for portable power sources of personal computers, mobile terminals, and the like, vehicle drive power sources of electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like.

[0003] In the vehicle drive power source use, particularly the drive power source use of BEVs, from the viewpoint of extending the cruising distance of the vehicle, it is desired that the secondary battery be further high in capacity. As a negative electrode active material high in capacity, Si-containing particles are known, and it is known that the secondary battery can be made high in capacity by using the Si-containing particles (for example, refer to Japanese Patent Application Publication No. 2015-38862). In Japanese Patent Application Publication No. 2015-38862, a technique of using Si-containing particles and graphite particles such as natural graphite as negative electrode active materials is disclosed. SUMMARY

[0004] However, in the case of Si-containing particles, the capacity is high, on the other hand, the volume change due to expansion / contraction of the secondary battery at the time of charge / discharge is large. Also, in the case where Si-containing particles and graphite particles are used as negative electrode active materials, when the secondary battery is repeatedly charged and discharged, there is a problem that the internal stress increases due to expansion of the negative electrode. Therefore, for a negative electrode containing Si-containing particles and graphite particles, it is desired to develop a negative electrode that is small in expansion when the secondary battery is repeatedly charged and discharged. Note that the expansion of the negative electrode means that the volume of the negative electrode becomes larger than the initial volume at the same state of charge (for example, the discharged state).

[0005] In view of the above-described actual circumstances, an object of the present disclosure is to provide a negative electrode that is a negative electrode containing Si-containing particles and graphite particles and is small in expansion when a secondary battery is repeatedly charged and discharged.

[0006] 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 by the negative electrode current collector. The negative electrode active material layer includes a first layer on the side of a surface layer portion, and a second layer on the side of the negative electrode current collector. 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 smaller than the Si content ratio in the second Si-containing particles. In the case where the density of a molded body obtained by pressing 1 g of particles at 25°C under 60 MPa in the uniaxial direction and molding into a tablet having a diameter of 20 mm is defined as the tap density, the tap density of the first Si-containing particles is 0.9 g / cm 3The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

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

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

[0009] The negative electrode obtained by adopting this structure can suppress the swelling of the negative electrode during repeated charging and discharging of the secondary battery.

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

[0011] Based on this configuration, it is possible to provide a secondary battery that uses a negative electrode containing Si particles and graphite particles and exhibits minimal expansion of the negative electrode during repeated charging and discharging of the secondary battery. Attached Figure Description

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

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

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

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

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

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

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

[0019] As shown in the figure, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 disposed on the negative electrode current collector 62. The negative electrode active material layer 64 may be disposed on only one side of the negative electrode current collector 62, or it may be disposed on both sides of the negative electrode current collector 62 as shown in the example. Preferably, the negative electrode active material layer 64 is disposed on both sides of the negative electrode current collector 62.

[0020] As shown in the example, a non-formed portion 62a of the negative electrode active material layer 64, which is not provided at one end of the negative electrode 60 in the width direction, can be provided. In the non-formed portion 62a, the negative electrode current collector 62 is exposed, and the non-formed portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

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

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

[0023] like Figure 1 As shown, the negative electrode active material layer 64 has a multi-layer structure, specifically, it has a first layer 64a located on the surface side of the negative electrode active material layer 64 and a second layer 64b located on the side of the negative electrode current collector 62. Figure 1 As shown, the first layer 64a is the upper layer of the negative electrode active material layer 64, and the second layer 64b is the lower layer of the negative electrode active material layer 64. Furthermore, the negative electrode active material layer 64 may further include layers other than the first layer 64a and the second layer 64b, without significantly impairing 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, which mixes the components of these layers.

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

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

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

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

[0028] To clarify, in this specification, "circularity" refers to the ratio of the circumference of a true circle with the same area as the particle's projected area to the circumference of the particle's projected image (i.e., circularity = circumference of a true circle with the same area as the particle's projected area / circumference of the particle's projected image). Therefore, the closer the circularity is to 1, the closer the particle's projected image is to a true circle, and the closer the particle is to a true sphere. Regarding circularity, for example, it can be determined by using a commercially available static automatic image analysis device to calculate the circularity of more than 100 particles and then calculating its average value.

[0029] There are no particular limitations on the average particle size (D50) of the first graphite particle 12 and the average particle size (D50) of the second graphite particle 16. The average particle size (D50) of the first graphite particle 12 and the average particle size (D50) of the second graphite particle 16 are each, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

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

[0031] When 1g of the first graphite particles 12 are pressed in a uniaxial direction to form a tablet with a diameter of 20mm, the preferred density of the formed body is 1.7g / cm³. 3 The molding pressure is 20 MPa or less, more preferably 10 MPa to 18 MPa. When 1 g of the second graphite particles 16 are pressed in a uniaxial direction to form a tablet with a diameter of 20 mm, the density of the molded body is preferably 1.7 g / cm³. 3The molding pressure is 20 MPa or less, more preferably 10 MPa to 18 MPa. This molding pressure can be easily measured using an automatic powder resistance measurement system (e.g., Nitto Seiko Analytical Technology Co., Ltd.'s "MCP-PD600") and a 20 mm diameter probe (equivalent to a mold).

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

[0033] The first Si-containing particle 14 and the second Si-containing particle 18 can be, for example, particles from a Si-C composite material. Si-C composite materials typically contain carbon domains and Si-containing domains. It should be noted that the first Si-containing particle 14 and the second Si-containing particle 18 may not be Si-C composite materials, but may be Si particles, Si oxide particles, etc.

[0034] Carbon domains can be, for example, carbides of carbon precursors (such as petroleum asphalt, coal tar pitch, phenolic resin, etc.) or graphite. Preferably, the carbon domains constitute a carbon matrix. Therefore, Si-C composite materials are preferably materials in which multiple Si-containing domains are dispersed within a carbon matrix. In this case, the carbon matrix can mitigate volume changes caused by the expansion / contraction of the Si-containing domains, which is advantageous.

[0035] Si-containing domains contain Si, for example, Si, Si oxide (SiO2). x It is composed of Si nitrides (SiNx), Si carbides (SiCx), etc. The Si domains are preferably composed of Si, Si oxides, and Si oxides (SiO2). x It consists of at least one of the following: The Si domains may be microparticles. The oxygen content in the Si domains is preferably 10% by mass or less.

[0036] The average particle size of the Si domains is, for example, less than 50 nm, and can be between 5 nm and 50 nm. It should be explained that the "average particle size of the Si domains" can be determined as follows: First, the negative electrode active material layer 64 is processed using FIB (focused ion beam) to prepare a sample for observation by a scanning transmission electron microscope (STEM). Then, after elemental analysis of the sample using EDX elemental mapping, BF (bright field) and HAADF (high angle scattering annular dark field) images are obtained. The diameter of the Si domains can be determined from the contrast and shape obtained using the BF and HAADF images. The diameters of arbitrarily selected 10 or more Si domains are determined, and their average value is set as the "average particle size of the Si domains" here.

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

[0038] 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. There are no particular limitations regarding 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, as long as this relationship is satisfied. However, if these Si content ratios are too low, the effect of suppressing the swelling of the negative electrode during repeated charging and discharging of the secondary battery may be reduced. On the other hand, if these Si content ratios are too high, the volume change caused by the expansion / contraction of the first Si-containing particles 14 and the second Si-containing particles 18 during repeated charging and discharging of the secondary battery may become excessively large.

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

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

[0041] The density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet under uniaxial pressure at 25℃ and 60MPa is defined as the compaction density. This compaction density serves as an indicator of the particle filling capacity; the higher the compaction density, the higher the particle filling capacity. The compaction density of the first Si-particle-containing 14 is 0.90 g / cm³. 3 That's all. On the other hand, the compaction density of the second Si-particle-containing 18 is less than 0.90 g / cm³. 3 .

[0042] Thus, in the upper layer 64a of the negative electrode active material layer 64, in addition to the first graphite particles 12, first Si-containing particles 14 with a low Si content and high compaction density are used; in the lower layer 64b of the negative electrode active material layer 64, in addition to the second graphite particles 16, second Si-containing particles 18 with a high Si content and low compaction density are used. Specifically, the compaction density of the Si-containing particles conventionally used is typically less than 0.90 / cm³. 3 Therefore, the first Si-containing particle 14 has a higher compaction density compared to conventionally used Si-containing particles. This significantly suppresses the swelling of the negative electrode 60 during repeated charge-discharge cycles of the secondary battery. The reasons for this are as follows.

[0043] That is, in the negative electrode active material layer 64, the upper layer (i.e., the first layer 64a) is the one that expands the most during repeated charging and discharging of the secondary battery. Therefore, the upper layer uses first Si-containing particles 14 with a low Si content ratio and high pressure density, i.e., Si-containing particles with small expansion / contraction and high filling capacity. As a result, in the upper layer, the particles become easy to move, and the particles follow the expansion / contraction caused by charging and discharging, thus mitigating stress and suppressing the expansion of the negative electrode.

[0044] On the other hand, a second Si-containing particle 18 with a high Si content and low compressive density is used in the lower layer, i.e., Si-containing particles with large expansion / contraction and low filling density. As a result, the particles become less mobile in the lower layer, suppressing the disconnection of the conductive path during charging and discharging. Therefore, the swelling of the negative electrode associated with the disconnection of the conductive path (i.e., uneven battery reaction, localized reaction, swelling due to stress concentration, etc.) can be suppressed. As a result, the negative electrode active material layer 64 as a whole can significantly suppress the swelling of the negative electrode 60 during repeated charging and discharging of the secondary battery.

[0045] The preferred compaction density of the first Si-particle-containing 14 is 0.95 g / cm³. 3 The above, more preferably 1.00 g / cm 3 The above is further preferred to be 1.1 g / cm³. 3 The above, especially preferred, is 1.2 g / cm³. 3 That's all. On the other hand, the compaction density of the first Si-particle-containing 14 can be 2.3 g / cm³. 3 Below, 2.0g / cm 3 Below, 1.8g / cm 3 Below, or 1.5g / cm 3 the following.

[0046] The preferred compaction density of the second Si-particle-containing 18 is 0.88 g / cm³. 3 The preferred value is 0.86 g / cm³. 3 The following is a further preferred value: 0.85 g / cm³3 Below. On the other hand, the compaction density of the second Si-particle-containing 18 is preferably 0.50 g / cm³. 3 The above, more preferably 0.65 g / cm³ 3 The above is further preferred to be 0.75 g / cm³. 3 The above, especially preferred, is 0.83 g / cm³. 3 above.

[0047] To explain, compaction density can be easily measured, for example, by using an automated powder resistance measurement system (such as the "MCP-PD600" manufactured by Nitto Seiko Analytical Technology Co., Ltd.) and a probe (equivalent to a mold) with a diameter of 20 mm.

[0048] It should be noted that the compaction density is affected by the roundness of the Si-containing particles. Increasing the roundness of the Si-containing particles tends to increase the compaction density. Therefore, if the roundness of the Si-containing particles is made to be 0.85–1 (especially 0.90–1), the compaction density easily reaches 0.90 g / cm³. 3 That's all. On the other hand, if the sphericity of the Si-containing particles is made less than 0.85, the compaction density can easily become less than 0.90 g / cm³. 3 .

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

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

[0051] The ratio of the average particle size (D50) of the first graphite particles 12 to the average particle size (D50) of the first Si-containing particles 14 (D50 of the first graphite particles 12 / D50 of the first Si-containing particles 14) is not particularly limited. From the viewpoint of particularly high filling performance, the ratio (D50 of the first graphite particles 12 / D50 of the first Si-containing particles 14) is preferably 1.0 to 8.0, more preferably 1.0 to 5.0, even more preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0052] The ratio of the average particle size (D50) of the second graphite particles 16 to the average particle size (D50) of the second Si-containing particles 18 (D50 of the second graphite particles 16 / D50 of the second Si-containing particles 18) is not particularly limited. From the viewpoint of particularly high filling performance, the ratio (D50 of the second graphite particles 16 / D50 of the second Si-containing particles 18) is preferably 1.0 to 8.0, more preferably 1.0 to 5.0, even more preferably 1.2 to 3.0, and particularly preferably 1.4 to 2.5.

[0053] Furthermore, the first Si-containing particle 14 and the second Si-containing particle 18 can be manufactured according to known methods. Furthermore, various methods for manufacturing particles of the Si-C composite material are known (for example, see Japanese Patent Application Publication No. 2015-38862, International Publication No. 2014 / 046144, and other prior art documents listed in that international publication).

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

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

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

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

[0058] 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, and is for example 5 / 95 to 95 / 5. From the viewpoint of further suppressing the swelling of the negative electrode during repeated charging and discharging of the secondary battery, the ratio (T2 / T1) is preferably 10 / 90 to 90 / 10, more preferably 20 / 80 to 90 / 10, and even more preferably 40 / 60 to 70 / 30.

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

[0060] The content of the negative electrode active material in the first layer 64a (i.e., relative to the total mass of the first layer 64a) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of the binder in the first layer 64a is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the first layer 64a is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less.

[0061] The content of the negative electrode active material in the second layer 64b (i.e., relative to the total mass of the second layer 64b) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of the binder in the second layer 64b is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the second layer 64b is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less.

[0062] There is no particular limitation on the thickness of the negative electrode active material layer 64, for example, it is 10 μm or more and 400 μm or less, preferably 20 μm or more and 300 μm or less.

[0063] There is no particular limitation on the density of the negative electrode active material layer 64, for example, it can be 0.7 g / cm³. 3 The above, preferably 1.0 g / cm³ 3 The above, more preferably 1.2 g / cm³ 3That's all. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm³. 3 The following can be 2.0 g / cm³ 3 the following.

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

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

[0066] To clarify, in this specification, "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, including so-called "slurry," "ink," etc.

[0067] The preparation process of the paste for lower layer formation can be carried out by mixing the second graphite particles 16, the second Si-containing particles 18, and optional components (e.g., binders, conductive materials, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., in accordance with known methods.

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

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

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

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

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

[0073] Therefore, from another perspective, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 described in the above-described embodiment. A lithium-ion secondary battery is listed below; for one embodiment of the secondary battery disclosed herein, please refer to... Figure 3 and Figure 4 The following is an example of a flat, square lithium-ion secondary battery with a flat, wound electrode body and a flat, battery casing.

[0074] Figure 3The lithium-ion secondary battery 100 shown is a sealed lithium-ion secondary battery 100 constructed by housing a flat, wound electrode body 20 and a non-aqueous electrolyte (not shown) within a flat, square battery casing (i.e., outer packaging container) 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 designed to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. The battery casing 30 is made of a lightweight, thermally conductive metal material such as aluminum.

[0075] 20 winding electrode bodies Figure 3 and Figure 4 As shown, the positive electrode 50 and negative electrode 60 are formed by overlapping and winding two elongated separator sheets 70 along their length. The positive electrode 50 has a configuration in which a positive active material layer 54 is formed on one or both (in this case, both) sides of the elongated positive current collector 52 along its length. The negative electrode 60 has a configuration in which a negative active material layer 64 is formed on one or both (in this case, both) sides of the elongated negative current collector 62 along its length. The non-formed portions 52a of the positive active material layer (i.e., the portions where the positive current collector 52 is exposed without the formation of the positive active material layer 54) and the non-formed portions 62a of the negative active material layer (i.e., the portions where the negative current collector 62 is exposed without the formation of the negative active material layer 64) are formed extending outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned length direction). Positive current collector 42a and negative current collector 44a are respectively bonded to the non-formed portion 52a of the positive active material layer and the non-formed portion 62a of the negative active material layer.

[0076] As the positive current collector 52 constituting the positive electrode sheet 50, a known positive current collector for lithium-ion secondary batteries can be used. Examples of such current collectors include sheets or foils made of metals with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive current collector 52.

[0077] There is no particular limitation on the size of the positive current collector 52, which can be appropriately determined according to the battery design. When aluminum foil is used as the positive current collector 52, there is no particular limitation on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0078] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition for lithium-ion secondary batteries can be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. There are no particular limitations on the crystal structure of the positive electrode active material; it can be a layered structure, a spinel structure, an olivine structure, etc.

[0079] As a lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferred. Specific examples include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides.

[0080] It should be noted that in this specification, "lithium-nickel-cobalt-manganese composite oxide" refers to oxides containing one or more additional elements besides Li, Ni, Co, Mn, and O as constituent elements. Examples of such additional elements include transition metals and typical metals such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. Additionally, the added elements may be half-metals such as B, C, Si, and P, or non-metals such as S, F, Cl, Br, and I. This also applies to the aforementioned lithium-nickel composite oxides, lithium-cobalt composite oxides, lithium-manganese composite oxides, lithium-nickel-manganese composite oxides, lithium-nickel-cobalt-aluminum composite oxides, and lithium-iron-nickel-manganese composite oxides.

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

[0082] These positive electrode active materials can be used alone, or in combination of two or more. Lithium-nickel-cobalt-manganese composite oxides are particularly preferred as positive electrode active materials due to their superior properties, such as initial resistance.

[0083] There is no particular limitation on the average particle size (D50) of the positive electrode active material, for example, it is 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0084] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as lithium triphosphate, conductive materials, and binders. As conductive materials, carbon black such as acetylene black (AB), carbon fibers such as fumed carbon fiber (VGCF) and carbon nanotubes (CNT), and other carbon materials (such as graphite) are preferred. As binders, polyvinylidene fluoride (PVdF) can be used, for example.

[0085] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but preferably 0.1% by mass or more and 15% by mass or less, more preferably 0.2% by mass or more and 10% by mass or less. The content of conductive material in the positive electrode active material layer 54 is not particularly limited, but preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.3% by mass or more and 15% by mass or less. The content of binder in the positive electrode active material layer 54 is not particularly limited, but preferably 0.4% by mass or more and 15% by mass or less, more preferably 0.5% by mass or more and 10% by mass or less.

[0086] The thickness of each single side of the positive electrode active material layer 54 is not particularly limited, but is generally 10 μm or more, preferably 20 μm or more. On the other hand, the thickness is generally 400 μm or less, preferably 300 μm or less.

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

[0088] As the separator 70, examples include porous sheets (membranes) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. This porous sheet can be a single-layer structure or a multi-layered structure with two or more layers (e.g., a three-layer structure with PP layers stacked on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0089] The thickness of the separator 70 is not particularly limited, for example, it is 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less. The air permeability of the separator 70 obtained by the Grie test method is not particularly limited, but preferably 350 seconds / 100cc or less.

[0090] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As a non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones, which are commonly used in electrolytes of lithium-ion secondary batteries, can be used without particular limitation. Among these, carbonates are preferred, and specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene monofluorocarbonate (FEC), ethylene difluorocarbonate (DFEC), difluoromethyl difluoromethyl carbonate (F-DMC), and dimethyl trifluorocarbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As an example, the non-aqueous solvent may contain only carbonates. As another example, the non-aqueous solvent may contain carbonates and esters such as methyl acetate.

[0091] As the supporting salt, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) are preferably used (LiPF6 is preferred). The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0092] Furthermore, the non-aqueous electrolyte may contain components other than those mentioned above, provided that it does not significantly impair the effectiveness of this disclosure, such as film-forming agents like vinylene carbonate (VC) and oxalic acid complexes; gas generators like biphenyl (BP) and cyclohexylbenzene (CHB); and various additives like thickeners.

[0093] In the case of the lithium-ion secondary battery 100, the swelling of the negative electrode during repeated charging and discharging is suppressed, resulting in low reactivity. Furthermore, the lithium-ion secondary battery 100 has a high capacity. The lithium-ion secondary battery 100 can be used for various applications. Preferred applications include its use as a power source in vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Additionally, the lithium-ion secondary battery 100 can be used as a battery in small energy storage devices. The lithium-ion secondary battery 100 can also typically be used in the form of a battery pack consisting of multiple batteries connected in series and / or parallel.

[0094] The above description, as an example, illustrates a square lithium-ion secondary battery 100 comprising a flat, wound electrode body 20. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries comprising stacked electrode bodies (i.e., electrode bodies in which multiple positive electrodes and multiple negative electrodes are alternately stacked). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, stacked-casing lithium-ion secondary batteries, and the like.

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

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

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

[0098] <Making the Negative Electrode>

[0099] [Example 1]

[0100] Prepare the following substances as negative electrode active materials.

[0101] First type of Si-containing particle: Si-C composite material, Si content = 35% by mass, average particle size (D50) = 6 μm, compaction density = 1.3 g / cm³ 3

[0102] Second type of Si-containing particle: Si-C composite material, Si content = 65% by mass, average particle size (D50) = 7 μm, compaction density = 0.85 g / cm³ 3

[0103] Graphite particles (first and second graphite particles): average particle size (D50) = 15 μm; 1 g of graphite particles are pressed uniaxially to form a density of 1.7 g / cm³. 3 Furthermore, the molding pressure for tablets with a diameter of 20mm is 15MPa.

[0104] Furthermore, the Si content ratio of the first and second Si-containing particles was determined using a commercially available ICP-OEC apparatus. The average particle size (D50) of each particle was determined using a commercially available laser diffraction scattering particle size distribution measuring device. The compaction density of the first and second Si-containing particles was determined using the following method.

[0105] One gram of either the first or second Si-containing particle sample was measured and placed on the probe (20 mm diameter) of the "MCP-PD600" (manufactured by Nitto Seiko Analytical Technology Co., Ltd.) automated powder resistivity measuring system. At 25°C, the load and displacement under uniaxial pressure were measured using this automated powder resistivity measuring system. Based on this, the bulk density of the molded body at a pressure of 60 MPa was determined and set as the compaction density. Additionally, one gram of graphite particles was measured and placed on the probe (20 mm diameter) of the "MCP-PD600" (manufactured by Nitto Seiko Analytical Technology Co., Ltd.) automated powder resistivity measuring system. At 25°C, the load and displacement under uniaxial pressure were measured using this automated powder resistivity measuring system. Based on this, the density of the molded body was determined to be 1.7 g / cm³. 3 The molding pressure at that time.

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

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

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

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

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

[0111] [Example 2]

[0112] As the first Si-containing particle, a compaction density of 1.0 g / cm³ was used. 3 The Si-C composite material contains particles in a proportion of 51% by mass. Otherwise, the negative electrode of Example 2 is obtained by the same method as in Example 1.

[0113] [Example 3]

[0114] The ratio (T2 / T1) of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) is changed to 10 / 90. Otherwise, the negative electrode of Example 3 is obtained by the same method as in Example 1.

[0115] [Example 4]

[0116] The ratio (T2 / T1) of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) is changed to 90 / 10. Otherwise, the negative electrode of Example 4 is obtained by the same method as in Example 1.

[0117] [Example 5]

[0118] The ratio (T2 / T1) of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) is changed to 20 / 80. Otherwise, the negative electrode of Example 5 is obtained by the same method as in Example 1.

[0119] [Example 6]

[0120] The ratio (T2 / T1) of the thickness of the upper layer (T2) to the thickness of the lower layer (T1) is changed to 80 / 20. Otherwise, the negative electrode of Example 6 is obtained by the same method as in Example 1.

[0121] [Example 7]

[0122] As the first Si-containing particle, a compaction density of 1.2 g / cm³ was used. 3 The first part contains Si particles in a Si-C composite material with a Si content of 40% by mass, which are used as the second Si-containing particles, and the second part uses particles with a compaction density of 0.8 g / cm³. 3 The Si-C composite material contains particles in a proportion of 64% by mass. Otherwise, the negative electrode of Example 7 is obtained by the same method as in Example 1.

[0123] [Example 8]

[0124] As the second Si-containing particle, a compaction density of 0.72 g / cm³ was used. 3 The negative electrode of Example 8 was obtained by using the same method as in Example 1, except that Si contains 58% by mass of Si-C composite material particles.

[0125] [Comparative Example 1]

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

[0127] [Comparative Example 2]

[0128] The lower layer was formed using an upper layer forming paste, and the upper layer was formed using a lower layer forming paste. Otherwise, the negative electrode of Comparative Example 2 was obtained using the same method as in Example 1. Therefore, in Comparative Example 2, the first Si-containing particles and the second Si-containing particles were used interchangeably.

[0129] [Comparative Example 3]

[0130] A negative electrode active material layer was formed by coating a 10 μm thick copper foil with a paste and drying it. The negative electrode active material layer was then rolled and the resulting sheet was processed to a specified size to obtain the negative electrode sheet of Comparative Example 3. It should be noted that the thickness of the negative electrode sheet of Comparative Example 3 was the same as that of Example 1.

[0131] <Evaluation of Plate Expansion Rate>

[0132] The thickness of the negative electrode in each embodiment and each comparative example was measured. This thickness was set as the initial thickness (T0). Using this negative electrode, an evaluation lithium-ion secondary battery was fabricated as described below.

[0133] LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are mixed with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste is coated onto the surface of a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. The positive electrode active material layer is then rolled to obtain a sheet of specified dimensions, resulting in the positive electrode sheet.

[0134] Prepare a separator made of porous polyolefin. Attach leads to the prepared negative and positive electrode sheets respectively, and stack them via the separator to form the electrode body. Contain this electrode body together with a non-aqueous electrolyte in a housing made of aluminum laminated film. For the non-aqueous electrolyte, a non-aqueous electrolyte prepared by dissolving LiPF6 as a supporting salt at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40 is used. Then, seal the housing to obtain an evaluation lithium-ion secondary battery.

[0135] Next, the prepared lithium-ion secondary batteries were placed in an environment of 25°C. Each evaluation lithium-ion secondary battery was charged with a constant current of 0.4C until it reached 4.2V, and then charged with a constant voltage until the current reached 0.1C. Next, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.4C until it reached 2.5V.

[0136] The above charge-discharge cycle was considered as one cycle, and 250 charge-discharge cycles were repeated. Each evaluation lithium-ion secondary battery was disassembled under an argon atmosphere, and the negative electrode was immersed in DMC, cleaned, and dried. The thickness of the negative electrode was then measured and defined as the thickness after charge-discharge cycles (Tc). The rate of change (%) of the negative electrode thickness before and after charge-discharge cycles was calculated using (Tc / T0-1)×100. The results are shown in Table 1.

[0137] [Table 1]

[0138] Table 1

[0139]

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

[0141] The specific examples of this disclosure have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes technical solutions that are modified or altered from the specific examples described above.

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

[10] .

[0143] [1] A negative electrode, which is the negative electrode of a secondary battery including a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector, wherein the negative electrode active material layer includes a first layer located on the surface side and a second layer located on the negative electrode current collector side, the first layer contains first graphite particles and first Si-containing particles, the second layer contains second graphite particles and second Si-containing particles, the proportion of Si in the first Si-containing particles is smaller than the proportion of Si in the second Si-containing particles, and the density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet in a uniaxial direction at 25°C and 60MPa is defined as the compaction density, the compaction density of the first Si-containing particles is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

[0144] [2] According to the negative electrode described in item [1], wherein the first Si-containing particle compaction density is 0.95 g / cm³. 3 Above and 1.8g / cm 3 the following.

[0145] [3] The negative electrode according to item [1] or [2], wherein the compaction density of the second Si-containing particles is 0.65 g / cm³. 3 Above and 0.85g / cm 3 the following.

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

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

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

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

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

[0151] [9] A method for manufacturing the negative electrode of a secondary battery includes: a step of mixing second graphite particles and second Si-containing particles in a dispersion medium to prepare a paste for forming a second layer; a step of mixing first graphite particles and first Si-containing particles in a dispersion medium to prepare a paste for forming a first layer; a step of coating the paste for forming the second layer onto a negative electrode current collector and drying it to form a second layer; a step of coating the paste for forming the first layer onto the second layer and drying it to form a first layer; and a step of pressing the formed first layer and second layer together, wherein the Si content in the first Si-containing particles is smaller than the Si content in the second Si-containing particles, and the density of the molded body obtained by pressing 1g of particles into a sheet with a diameter of 20mm under uniaxial direction at 25°C and 60MPa is defined as the compaction density, and the compaction density of the first Si-containing particles is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

[0152]

[10] A secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is the negative electrode according to any one of items [1] to [8].

Claims

1. A negative electrode, comprising a negative electrode current collector and a layer of negative electrode active material supported by the negative electrode current collector, wherein, The negative electrode active material layer includes a first layer located on the surface side and a second layer located on the negative electrode current collector side. The first layer contains first graphite particles and first Si-containing particles. The second layer contains second graphite particles and second Si-containing particles. The proportion of Si in the first Si-containing particle is smaller than the proportion of Si in the second Si-containing particle. When the density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet under uniaxial pressure at 25℃ and 60MPa is defined as the compressive density, the compressive density of the first Si-containing particle is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

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

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

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

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

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

90.

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

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

9. A method for manufacturing the negative electrode of a secondary battery, comprising: The process of mixing second graphite particles and second Si-containing particles in a dispersion medium to prepare a paste for forming a second layer; The process of mixing first graphite particles and first Si-containing particles in a dispersion medium to prepare a paste for forming a first layer; The process of forming the second layer involves coating the negative electrode current collector with a paste and drying it to form the second layer. The steps of forming the first layer by applying a paste onto the second layer and drying it to form the first layer; and The process of pressing the first and second layers together. Wherein, the proportion of Si in the first Si-containing particles is smaller than the proportion of Si in the second Si-containing particles, and the density of the molded body obtained by pressing 1g of particles into a 20mm diameter sheet under uniaxial pressure at 25℃ and 60MPa is defined as the compaction density, the compaction density of the first Si-containing particles is 0.9g / cm³. 3 The above, and the compaction density of the second Si-containing particle is less than 0.9 g / cm³. 3 .

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

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