Negative electrode for secondary battery and secondary battery using same

Abstract

The invention provides a negative electrode for a secondary battery and a secondary battery using the negative electrode. Provided is a negative electrode containing silicon-containing particles and graphite particles, the negative electrode being small in swelling when a secondary battery is repeatedly charged and discharged. A negative electrode for a secondary battery according to the present disclosure is a negative electrode for a secondary battery including a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer includes a lower layer on the negative electrode current collector side and an upper layer on the surface side. The upper layer contains first graphite particles and first silicon-containing particles as negative electrode active materials, and the lower layer contains second graphite particles and second silicon-containing particles as negative electrode active materials. The expansion ratio P1 of the first silicon-containing particles is smaller than the expansion ratio P2 of the second silicon-containing particles, and the average particle diameter M1 of the first silicon-containing particles is smaller than the average particle diameter M2 of the second silicon-containing particles.

Description

TECHNICAL FIELD

[0001] The present application relates to a negative electrode for a secondary battery and a manufacturing method of a secondary battery using the same. BACKGROUND

[0002] In recent years, secondary batteries are used as portable power sources for personal computers, mobile terminals, and the like, as vehicle drive power sources for 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 for BEVs, from the viewpoint of extending the cruising distance of the vehicle, it is desirable to further increase the capacity of the secondary battery. As a negative electrode active material having a high capacity, silicon-containing particles are known, and it is known that the secondary battery can be made high-capacity by using silicon-containing particles (for example, see Patent Literature 1). In Patent Literature 1, a technique is disclosed in which a plurality of particles containing elemental Si, a Si compound, and carbon are used as a negative electrode active material.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2010-033830 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] However, the silicon-containing particles have a high capacity, but on the other hand, have a large volume change due to expansion / contraction during charging and discharging of the secondary battery. Furthermore, in the case where the silicon-containing particles and graphite particles are used together as a negative electrode active material, if the secondary battery is repeatedly charged and discharged, there is a problem that the internal stress increases due to swelling of the negative electrode. Therefore, for a negative electrode containing silicon-containing particles and graphite particles, it is desirable to develop a negative electrode that has little swelling during repeated charging and discharging of the secondary battery. Note that the swelling of the negative electrode means that the volume of the negative electrode becomes larger than the initial volume under the same state of charge (for example, a state close to full charge at around 80% of SOC).

[0009] In view of the above-described actual circumstances, an object of the present disclosure is to provide a negative electrode that contains silicon-containing particles and graphite particles and has little swelling during repeated charging and discharging of a secondary battery.

[0010] MEANS FOR SOLVING THE PROBLEMS

[0011] The negative electrode disclosed herein is a negative electrode for a secondary battery including: a negative electrode current collector, and a negative electrode active material layer supported by the negative electrode current collector, the negative electrode active material layer including a lower layer on the negative electrode current collector side and an upper layer on the surface side, the upper layer containing first graphite particles and first silicon-containing particles as negative electrode active materials, the lower layer containing second graphite particles and second silicon-containing particles as negative electrode active materials, the first silicon-containing particles having an expansion ratio P1 smaller than an expansion ratio P2 of the second silicon-containing particles, and the first silicon-containing particles having an average particle diameter M1 smaller than an average particle diameter M2 of the second silicon-containing particles.

[0012] According to such a configuration, when used as a negative electrode for a secondary battery and subjected to charge and discharge, the swelling of the negative electrode active material layer on the surface side can be further reduced in the negative electrode active material layer formed of the two-layer structure on the negative electrode current collector side and on the surface side. Furthermore, the negative electrode active material on the negative electrode current collector side is affected by the expansion of the negative electrode active material on the surface side, and thus the expansion can be hindered. As a result, the swelling of the entire negative electrode active material layer can be suppressed. Moreover, a negative electrode for a secondary battery containing silicon-containing particles and graphite particles, which is a negative electrode having small swelling during repeated charge and discharge of a secondary battery, can be provided.

[0013] In a preferred embodiment of the negative electrode disclosed herein, the ratio P1 / P2 of the above P1 to the above P2 is 0.5 to 0.8. Thus, by using a negative electrode active material that is more difficult to expand on the surface side, a configuration in which the negative electrode active material layer on the surface side is difficult to expand is achieved, and thus the suppression of the swelling of the negative electrode can be more significantly achieved.

[0014] In a preferred embodiment of the negative electrode disclosed herein, the ratio M1 / M2 of the above M1 to the above M2 is 0.4 to 0.6. Thus, by using a negative electrode active material that has a smaller particle diameter after expansion on the surface side, the suppression of the swelling of the negative electrode can be more significantly achieved.

[0015] In a preferred embodiment of the negative electrode disclosed herein, the ratio T1:T2 of the thickness T1 of the above upper layer to the thickness T2 of the above lower layer is 10:90 to 90:10. Thus, by using a negative electrode active material that is more difficult to expand on the surface side, a configuration in which the negative electrode active material layer on the surface side is difficult to expand is achieved, and thus the suppression of the swelling of the negative electrode can be more significantly achieved.

[0016] In a preferred embodiment of the negative electrode disclosed herein, the mass ratio N1 of the first silicon-containing particles relative to the total mass of the first graphite particles and the first silicon-containing particles in the upper layer is 10 to 60% by mass, and the mass ratio N2 of the second silicon-containing particles relative to the total mass of the second graphite particles and the second silicon-containing particles in the lower layer is 10 to 60% by mass. Thus, by making the surface-side negative electrode active material layer less prone to expansion, the suppression of negative electrode expansion can be achieved more significantly.

[0017] The secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode disclosed herein.

[0018] With this configuration, the swelling of the negative electrode that accompanies charging and discharging can be suppressed, and a high-capacity secondary battery can be achieved. Attached Figure Description

[0019] Figure 1 The cross-sectional view shown schematically is 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.

[0020] Figure 2 To indicate Figure 1 A schematic cross-sectional view of the particles of the negative electrode active material contained in the negative electrode active material layer 64 shown.

[0021] Figure 3 A diagram illustrating the configuration of a lithium-ion secondary battery constructed using a negative electrode according to one embodiment.

[0022] Figure 4 To indicate Figure 3 A schematic exploded view of the structure of the wound electrode body of a lithium-ion secondary battery. Detailed Implementation

[0023] The preferred embodiments of the technology disclosed herein are described below. It should be noted that matters necessary for implementing the technology disclosed herein, other than those specifically mentioned in this specification, can be grasped by those skilled in the art based on prior art in this field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in this field. Furthermore, in the accompanying drawings described in this specification, the same reference numerals are used to describe components or parts that perform the same function, and sometimes repeated descriptions are omitted or simplified. Additionally, the dimensional relationships (length, width, thickness, etc.) in the drawings may not necessarily reflect actual dimensional relationships.

[0024] In this specification, the term "secondary battery" refers to all energy storage devices capable of repeated charging and discharging through the movement of charge carriers between the positive and negative electrodes. It includes the concepts of so-called storage batteries (chemical batteries) such as lithium-ion secondary batteries and sodium-ion secondary batteries, and capacitors (physical batteries) such as lithium-ion capacitors (LIC). The main constituent materials of the secondary batteries involved in this disclosure are described below. It should be noted that for constituent materials of secondary batteries not described herein, conventionally known materials can be used.

[0025] 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 schematically is 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.

[0026] 1. Negative electrode

[0027] (1) Composition of the negative electrode

[0028] like Figure 1 As shown, the negative electrode 60 includes a negative current collector 62 and a negative active material layer 64 supported by the negative current collector 62. In other words, the negative electrode 60 includes a negative current collector 62 and a negative active material layer 64 disposed on the negative current collector 62. The negative active material layer 64 may be disposed only on one side of the negative current collector 62, or it may be disposed as follows: Figure 1 The negative electrode current collector 62 is disposed on both sides as shown. The negative electrode active material layer 64 is preferably disposed on both sides of the negative electrode current collector 62.

[0029] 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.

[0030] like Figure 1 As shown, a non-formed portion 62a of the negative electrode active material layer 64, where no negative electrode active material layer 64 is formed, can be provided at one end of the negative electrode 60 in the width direction. In the non-formed portion 62a, the negative electrode current collector 62 is exposed, and the non-formed portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0031] In the example shown, the negative electrode current collector 62 is in the shape of a foil (or sheet), but is not limited to this. The negative electrode current collector 62 can be in various shapes such as rod-shaped, plate-shaped, or sieve-shaped. As for the material of the negative electrode current collector 62, similar to existing 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.

[0032] 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.

[0033] 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.

[0034] The negative electrode active material layer 64 contains a negative electrode active material. For it, use... Figure 2 A detailed explanation. Figure 2 To indicate 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.

[0035] Regarding the negative electrode active material, the first layer 64a contains first graphite particles 12 and first silicon-containing particles 14. The second layer 64b contains second graphite particles 16 and second silicon-containing particles 18. Therefore, in the first layer 64a, at least the first graphite particles 12 and the first silicon-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 silicon-containing particles 18 are used as the negative electrode active material. The silicon-containing particles experience large volume changes due to expansion / contraction during charging and discharging; by using them in conjunction with graphite particles, the breakage of the conductive path caused by the volume change of the silicon-containing particles can be suppressed.

[0036] 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.

[0037] 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.

[0038] It should be noted that the term "circularity" in this specification refers to the ratio of the circumference of a true circle having the same area as the projected area of ​​the particle to the circumference of the particle's projected image (i.e., circularity = circumference of a true circle having the same area as the projected area of ​​the particle / circumference of the particle's projected image). Therefore, the closer the circularity is to 1, the closer the particle's projected image is to a true circle, and the closer the particle is to a true sphere. Circularity can be determined, for example, 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.

[0039] 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.

[0040] It should be noted that the term "average particle size (D50)" in this specification refers to the median diameter (D50), which, in a volume-based particle size distribution based on laser diffraction and scattering, is the particle size corresponding to 50% by volume of the cumulative frequency from the side of the smallest particle. The average particle size (D50) can be determined using commercially available laser diffraction and scattering particle size distribution measuring devices.

[0041] 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.

[0042] The first silicon-containing particle 14 and the second silicon-containing particle 18 can be, for example, a product containing Si particles dispersed inside a carbon material; or a product containing Si particles inserted into the pores of granulated porous graphite. The Si-C composite material can be a product containing Si particles attached to the surface of carbon particles; or a product containing carbon particles attached to the surface of Si-containing particles. From the viewpoint of suppressing Si volume changes, products containing Si nanoparticles dispersed inside a carbon material and products containing Si nanoparticles dispersed in the pores of a porous carbon material are preferred, and products containing Si nanoparticles dispersed in the pores of a porous carbon material are more preferred.

[0043] As an example of the first silicon-containing particle 14 and the second silicon-containing particle 18, particles of a Si-C composite material can be used, for example. Si-C composite materials typically contain carbon domains and Si-containing domains. It should be noted that the first silicon-containing particle 14 and the second silicon-containing particle 18 may not be a Si-C composite material, but may be Si particles, Si oxide particles, etc.

[0044] Carbon domains can be, for example, carbides of carbon precursors (e.g., petroleum asphalt, coal tar pitch, phenolic resins, etc.); graphite, etc. 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.

[0045] Si-containing domains contain Si, for example, Si, Si oxide (SiO2). x ), Si nitride (SiN) x ), Si carbide (SiC) x It is composed of, etc. The Si-containing domains are preferably composed of Si, and Si oxide (SiO2). x It consists of at least one of the following: Si domains. The Si domains may be microparticles. The oxygen content in the Si domains is preferably less than 10% by mass.

[0046] 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 noted 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, the sample is subjected to elemental analysis using EDX surface scanning, obtaining BF (bright field image) and HAADF (high angle scattering annular dark field image). 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.

[0047] This specification does not specifically limit the silicon content ratio (S1) in the first silicon-containing particle 14 and the silicon content ratio (S2) in the second silicon-containing particle 18. However, if these silicon content ratios are too low, it may be impossible to achieve high capacity in the secondary battery. On the other hand, if these silicon content ratios are too high, the volume change caused by the expansion / contraction of the first silicon-containing particle 14 and the second silicon-containing particle 18 during repeated charging and discharging of the secondary battery may become excessively large.

[0048] Therefore, in the case of the first silicon-containing particle 14, for example, which is composed of a Si-C composite material, the silicon content (S1) in the particle is preferably 20% to 55% by mass, more preferably 25% to 45% by mass. The silicon content (S2) in the second silicon-containing particle 18 is preferably 45% to 80% by mass, more preferably 55% to 75% by mass.

[0049] Furthermore, the first silicon-containing particle 14 and the second silicon-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).

[0050] In this specification, the expansion ratio P and the average particle size M are used to define the first silicon-containing particle 14 and the second silicon-containing particle 18. Furthermore, these values ​​are independent of each other and are not dependent on the silicon content ratio mentioned above.

[0051] • Expansion rate P

[0052] In this specification, the expansion rate P is an indicator representing the degree of expansion of Si-C-containing particles before and after charging, which can be derived, for example, by the following steps.

[0053] Before charging, the electrode plates were cross-sectionally processed, and an SEM image of the processed surface was obtained before the battery was fabricated. After CCCV charging (e.g., 0.01C charging - 4.2V - 0.005C cutoff) was performed at 25°C, the battery was disassembled, and an SEM image of the cross-sectional processed part was obtained. The area of ​​any Si-C particles before and after charging was calculated using ImageJ, and the expansion rate P can be derived from the following equation (1). In equation (1), the cross-sectional area of ​​the Si-C particles obtained from the SEM image is raised to the power of 1 / 2 to convert it into a length index, and then raised to the power of 3 to convert it into a volume index.

[0054] Expansion rate P = (Cross-sectional area of ​​silicon-containing particles after charging) 3 / 2 / (Cross-sectional area of ​​silicon-containing particles before charging) 3 / 2 Equation (1)

[0055] • Average particle size M

[0056] As described above, the average particle size M refers to the median diameter (D50), which, in a volume-based particle size distribution based on laser diffraction and scattering, is the particle size corresponding to 50% of the cumulative frequency from the side of the smallest particle. The average particle size (D50) can be determined using commercially available laser diffraction and scattering particle size distribution measuring devices.

[0057] Furthermore, the negative electrode active material layer 64 may contain components other than the negative electrode active material. Examples of such components include adhesives and conductive materials. Examples of adhesives include styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a 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.

[0058] (2) Suppression of negative electrode plate expansion

[0059] When a secondary battery is charged and discharged, the negative electrode active material layer near the negative electrode current collector is significantly affected by the expansion of the negative electrode active material located further to the surface, tending to hinder expansion. Conversely, the negative electrode active material located near the surface of the negative electrode active material layer has fewer factors hindering expansion. That is, in the negative electrode active material layer 64, which has a two-layer structure, the upper layer (i.e., the first layer 64a) expands the most during repeated charging and discharging of the secondary battery. The following explanation focuses on the expansion rate P, average particle size M, content, and thickness T of the Si-C particles (Si-C composite material is used here) in the first layer 64a and the second layer 64b, to illustrate the suppression of negative electrode expansion.

[0060] (A) Expansion rate P

[0061] In one embodiment of the negative electrode active material layer, a first silicon-containing particle 14 with a low expansion rate (i.e., an expansion rate of 180% or less) is used in the upper layer; that is, silicon-containing particles with small expansion / contraction. Therefore, the particles can move easily in the upper layer, following the expansion / contraction caused by charging and discharging, thus mitigating stress and suppressing the expansion of the negative electrode.

[0062] On the other hand, a second silicon-containing particle 18 with a high expansion rate (i.e., an expansion rate greater than 180%) is used in the lower layer, that is, silicon-containing particles with a large expansion / contraction ratio. 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 (such as uneven battery reaction, localized reaction, and swelling due to stress concentration) 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.

[0063] From the viewpoint of increasing the capacity of the secondary battery, the ratio (P1 / P2) of the expansion rate P2 of the second silicon-containing particle 18 to the expansion rate P1 of the first silicon-containing particle 14 is preferably 0.50 or more, more preferably 0.55 or more, and particularly preferably 0.60 or more. On the other hand, from the viewpoint of suppressing the expansion of the negative electrode plate and preventing the disconnection of the conductive path during charging and discharging, it is preferably 0.80 or less, more preferably 0.75 or less, and particularly preferably 0.70 or less.

[0064] (B) Average particle size M

[0065] In one embodiment of the negative electrode active material layer, a first silicon-containing particle 14 with a small particle size (i.e., a particle size of 4 μm or less) is used in the upper layer; that is, silicon-containing particles with a relatively small particle size after expansion. As a result, the particles in the upper layer become easier to move, and the particles follow the expansion / contraction caused by charging and discharging, thus easing stress and suppressing the expansion of the negative electrode.

[0066] On the other hand, a second silicon-containing particle 18 with a large particle size (greater than 4 μm) is used in the lower layer; that is, silicon-containing particles with a large particle size after expansion / large shrinkage and low filling capacity. 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 (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.

[0067] From the viewpoint of increasing the capacity of secondary batteries, the ratio (P1 / P2) of the average particle size M2 of the second silicon-containing particles 18 to the average particle size M1 of the first silicon-containing particles 14 is preferably 0.40 or more, more preferably 0.44 or more, and particularly preferably 0.48 or more. On the other hand, from the viewpoint of suppressing the swelling of the negative electrode plate and preventing the circuit from being interrupted during charging and discharging, it is preferably 0.60 or less, more preferably 0.56 or less, and particularly preferably 0.52 or less.

[0068] The ratio of the average particle size (D50) of the first graphite particles 12 to the average particle size (D50) of the first silicon-containing particles 14 (D50 of the first graphite particles 12 / D50 of the first silicon-containing particles 14) is not particularly limited. From the viewpoint of high filling performance, the ratio (D50 of the first graphite particles 12 / D50 of the first silicon-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.

[0069] The ratio of the average particle size (D50) of the second graphite particles 16 to the average particle size (D50) of the second silicon-containing particles 18 (D50 of the second graphite particles 16 / D50 of the second silicon-containing particles 18) is not particularly limited. From the viewpoint of high filling performance, the ratio (D50 of the second graphite particles 16 / D50 of the second silicon-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.

[0070] (C) content

[0071] In one embodiment of the negative electrode active material layer, the upper layer contains a relatively small amount of second silicon-containing particles 18 with low expansion rate and small particle size. As a result, the particles in the upper layer become more mobile, following the expansion / contraction caused by charging and discharging, thus mitigating stress and suppressing the expansion of the negative electrode.

[0072] On the other hand, the lower layer contains a relatively large amount of first silicon-containing particles 14 with high expansion rate and large particle size. As a result, the particles become less mobile in the lower layer, suppressing the disconnection of the conductive path during charging and discharging. Therefore, it is possible to suppress the swelling of the negative electrode associated with the disconnection of the conductive path (such as uneven battery reaction, localized reaction, and swelling caused by stress concentration). 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.

[0073] 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 negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less.

[0074] 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 negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less.

[0075] In the first layer 64a, from the viewpoint of maximizing the capacity of the secondary battery, the mass ratio N1 of the first silicon-containing particles 14 relative to the total mass of the first graphite particles 12 and the first silicon-containing particles 14 is preferably 10% by mass or more, more preferably 15% by mass or more. On the other hand, from the viewpoint of suppressing the swelling of the negative electrode plate and preventing the circuit from being interrupted during charging and discharging, it is preferably 60% by mass or less, more preferably 40% by mass or less, and particularly preferably 20% or less.

[0076] In the second layer 64b, from the viewpoint of maximizing the capacity of the secondary battery, the mass ratio N2 of the second silicon-containing particles 18 relative to the total mass of the second graphite particles 16 and the second silicon-containing particles 18 is preferably 10% by mass or more, more preferably 15% by mass or more. On the other hand, from the viewpoint of preventing the circuit from being interrupted during charging and discharging, it is preferably 60% by mass or less, more preferably 40% by mass or less, and particularly preferably 20% or less.

[0077] The negative electrode active material contained in the first layer 64a may consist only of the first graphite particles 12 and the first silicon-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 silicon-containing particles 14, within a range that does not impede 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).

[0078] The negative electrode active material contained in the second layer 64b may consist only of the second graphite particles 16 and the second silicon-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 silicon-containing particles 18, within a range that does not impede 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).

[0079] (D) Thickness T of the first and second layers

[0080] In one embodiment of the negative electrode, the upper layer of the two layers comprising the negative electrode active material layer is formed with a relatively thin thickness. This suppresses swelling in the upper layer, thereby suppressing overall swelling of the negative electrode.

[0081] On the other hand, the lower layer is formed with a relatively thick layer. This allows particles to move more easily within the lower layer, mitigating stress as they follow the expansion / contraction caused by charging and discharging, thus suppressing the swelling of the negative electrode. Therefore, it can suppress the swelling of the negative electrode associated with the disconnection of the conductive path (such as uneven battery reaction, localized reactions, or swelling due to stress concentration). 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.

[0082] From the viewpoint of suppressing the expansion of the negative electrode plate, the ratio of the thickness T2 of the lower layer to the thickness T1 of the upper layer (T1:T2) is preferably 10:90 to 90:10, more preferably 20:80 to 80:20, and particularly preferably 30:70 to 70:30.

[0083] (3) Manufacturing of the negative electrode

[0084] The negative electrode 60 can be suitably manufactured by a manufacturing method including, for example, mixing the second graphite particles 16 and the second silicon-containing particles 18 in a dispersion medium to prepare a second layer forming paste (hereinafter also referred to as "lower layer negative electrode composite paste"); mixing the first graphite particles 12 and the first silicon-containing particles 14 in a dispersion medium to prepare a first layer forming paste (hereinafter also referred to as "upper layer negative electrode composite paste"); coating and drying the second layer forming paste on the negative electrode current collector 62 to form a second layer 64b (lower layer) (hereinafter also referred to as "lower layer forming process"); coating and drying the first layer forming paste on the second layer 64b to form a first layer 64a (upper layer) (hereinafter also referred to as "upper layer forming process"); and pressing the formed first layer 64a and second layer 64b (hereinafter also referred to as "pressing process").

[0085] It should be noted that in this specification, the term "paste" refers to a mixture in which part or all of the solid components are dispersed in a dispersion medium, including terms such as "slurry" and "ink".

[0086] The preparation process of the paste for lower layer formation can be carried out by mixing the second graphite particles 16, the second silicon-containing particles 18, and optional components (e.g., adhesives, conductive materials, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., in accordance with a known method.

[0087] The upper layer forming paste preparation step can be performed by mixing the first graphite particles 12, the first silicon-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., according to a known method. 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.

[0088] 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.

[0089] 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 to 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.

[0090] There is no particular limitation on the density of the negative electrode active material layer 64, for example, it can be 0.7 g / cm³. 3 The above, preferably 1.0 g / cm³ 3 The above, more preferably 1.2 g / cm³ 3 That's all. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm³. 3 The following can be 2.0 g / cm³ 3 the following.

[0091] 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 or the like, 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.

[0092] 2. Secondary battery

[0093] 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.

[0094] Therefore, from another perspective, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 described in the above-described embodiment. The following describes an embodiment of the secondary battery disclosed herein, using a lithium-ion secondary battery as an example. Figure 3 and Figure 4The following is an example of a flat, square lithium-ion secondary battery with a flat, wound electrode body and a flat, battery casing.

[0095] Figure 3 The diagram schematically illustrates the configuration of a lithium-ion secondary battery constructed using a negative electrode according to one embodiment. Figure 3 The lithium-ion secondary battery 100 shown is a sealed lithium-ion secondary battery constructed by housing a flat, wound electrode body 20 and a non-aqueous electrolyte (not shown) within a flat, square battery casing (i.e., outer container) 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 designed to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, an injection port (not shown) for injecting the non-aqueous electrolyte is provided in the battery casing 30. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. The battery casing 30 is made of a lightweight, thermally conductive metal material such as aluminum.

[0096] Figure 4 To indicate Figure 3 An exploded view of the structure of the wound electrode body in a lithium-ion secondary battery. The wound electrode body 20 is shown below. Figure 3 and Figure 4 As shown, the positive electrode 50 and negative electrode 60 are formed by two elongated separator sheets 70 overlapping and wound in the longitudinal direction. 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 in the longitudinal direction. 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 in the longitudinal direction. 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 in a manner that overflows outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned longitudinal direction). Positive current collector plate 42a and negative current collector plate 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] It should be noted that in this specification, the term "lithium-nickel-cobalt-manganese composite oxide" refers to oxides containing one or more additional elements besides Li, Ni, Co, Mn, and O as constituent elements. Examples of such additional elements include transition metals and typical metallic elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. Additionally, the added elements may be half-metallic elements such as B, C, Si, and P, and non-metallic elements such as S, F, Cl, Br, and I. This also applies to the aforementioned lithium-nickel composite oxides, lithium-cobalt composite oxides, lithium-manganese composite oxides, lithium-nickel-manganese composite oxides, lithium-nickel-cobalt-aluminum composite oxides, and lithium-iron-nickel-manganese composite oxides.

[0102] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0103] These positive electrode active materials can be used alone, or in combination of two or more. Lithium-nickel-cobalt-manganese composite oxides are particularly preferred as positive electrode active materials due to their superior initial resistance and other properties.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] The negative electrode 60 described above is used as the negative electrode plate 60.

[0109] 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 laminated on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0110] 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 Gray test is not particularly limited, but preferably 350 seconds / 100cc or less.

[0111] 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 general 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), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), fluoromethyl difluoromethyl carbonate (F-DMC), and difluorotrifluoromethyl carbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As an example, the non-aqueous solvent may contain only carbonates. As another example, the non-aqueous solvent may contain carbonates and esters such as methyl acetate.

[0112] 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.

[0113] Furthermore, the aforementioned non-aqueous electrolyte may contain components other than those mentioned above, provided that it does not significantly impair the effectiveness of this disclosure, such as film-forming agents like vinylene carbonate (VC) and oxalic acid complexes; gas generators like biphenyl (BP) and cyclohexylbenzene (CHB); and various additives such as tackifiers.

[0114] The lithium-ion secondary battery 100 suppresses the swelling of the negative electrode during repeated charging and discharging, thus exhibiting low reaction force. 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 typically be used even in the form of a battery pack consisting of multiple batteries connected in series and / or parallel.

[0115] The above description, as an example, describes a square lithium-ion secondary battery 100 including a flat, wound electrode body 20. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries including stacked electrode bodies (i.e., electrode bodies in which multiple positive electrodes and multiple negative electrodes are alternately stacked). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, stacked-casing lithium-ion secondary batteries, etc.

[0116] In addition, according to known methods, the lithium-ion secondary battery 100 can also be constructed as an all-solid-state lithium-ion secondary battery that uses a solid electrolyte instead of a non-aqueous electrolyte.

[0117] Furthermore, the negative electrode 60 involved in this embodiment is suitable as the negative electrode of a lithium-ion secondary battery, but can also be constructed and used as the negative electrode of other secondary batteries, which can be constructed according to known methods.

[0118] "evaluate"

[0119] 1. Experimental Example

[0120] The following describes test examples related to the technology disclosed herein, but it is not intended to limit the technology disclosed herein to these test examples.

[0121] (1) Example 1

[0122] Negative electrode fabrication

[0123] The graphite particles C and the second silicon-containing particles (expansion rate P2: 200%, particle size M2: 7 μm), which are used as the negative electrode active material, SWCNTs, which are used as the conductive material, and CMC, PAA, and SBR, which are used as the binders, are weighed so that the mass ratio is C: second silicon-containing particles: SWCNT: CMC: PAA: SBR = 85:15:0.1:1:1:1.5. Among them, the raw materials other than SWCNT and SBR are dry mixed, SWCNTs and dispersion medium are mixed and kneaded, and then SBR and dispersion medium are added and diluted and mixed to prepare a negative electrode composite paste for the lower layer. Furthermore, in order to coat the binder (CMC / PAA) around the active material, the pressure load on the paste needs to be optimized. The ideal solid fraction B0 of the paste for optimizing the pressure load is derived by the following formula (1).

[0124] B0 = 100 - A0 = {100 / (100+A1)} × 100 Equation (1) B0: Ideal solid fraction [%)

[0125] A0: The torque required for mixing results in the maximum moisture content [%).

[0126] A1: The amount of water [mL] needed to make 100g of the mixture.

[0127] Secondly, except that the first silicon-containing particles (expansion rate P1: 130%, particle size M1: 3.5μm) were used instead of the second silicon-containing particles, the upper layer negative electrode paste was prepared using the same method.

[0128] Then, the aforementioned lower layer negative electrode composite paste is coated onto the negative electrode core (copper foil, 10 μm) and dried. The aforementioned upper layer negative electrode composite paste is then coated onto the dried lower layer negative electrode composite paste, such that the ratio of the upper layer thickness T1 to the lower layer thickness T2 is 50:50. After drying, a two-layer structure of negative electrode active material (expansion ratio P1 / P2 = 0.65, particle size ratio M1 / M2 = 0.5) is formed on the negative electrode core. Then, the negative electrode plate is obtained by pressing, calendering, and processing to the specified dimensions.

[0129] Positive electrode production

[0130] Lithium nickel cobalt manganese composite oxide (NCM) as the positive electrode active material, polyvinylidene fluoride (PVdF) as the binder, and acetylene black (AB) as the conductive material were weighed to a mass ratio of NCM:PVdF:AB = 100:1:1 and mixed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode composite paste. This positive electrode composite paste was coated onto a long strip of positive electrode core (aluminum foil, 15 μm thick) and dried. Then, it was pressed, calendered, and processed to the specified dimensions to obtain the positive electrode plate.

[0131] Leads were installed on the negative and positive electrodes, and the electrodes were stacked together via separators to create an electrode body. The electrode body was then inserted into an outer casing made of aluminum laminates, a non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to create a test battery (laminated battery).

[0132] The non-aqueous electrolyte used a product in which LiPF6 was dissolved in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC = 15:5:40:40 to become 1M.

[0133] (2) Example 2

[0134] Except that the expansion ratio P1 / P2 was 0.8, the test cells were made in the same manner as in Example 1.

[0135] (3) Example 3

[0136] Except that the thickness ratio of the upper layer to the lower layer, T1:T2, was 70:30, the test cell was made in the same manner as in Example 1.

[0137] (4) Example 4

[0138] Except that the thickness ratio of the upper layer to the lower layer, T1:T2, was 30:70, the test cell was made in the same manner as in Example 1.

[0139] (5) Comparison Example 1

[0140] Except that the negative electrode active material layer was made in the same manner as in Example 1, the test cell was fabricated. Furthermore, the composition of the composite paste forming the negative electrode active material layer was adjusted to a mass ratio of C: second silicon-containing particles: first silicon-containing particles: SWCNT: CMC: PAA: SBR = 85: 7.5: 7.5: 0.1: 1: 1: 1.5.

[0141] (6) Comparison Example 2

[0142] Except that the expansion ratio P1 / P2 was 1.5, the test cells were made in the same manner as in Example 1.

[0143] (7) Comparative Example 3

[0144] Except that the particle size ratio M1 / M2 was 1.0, the test cells were made in the same manner as in Example 1.

[0145] 2. Evaluation Test

[0146] (1) Determination of the expansion rate P of the first silicon-containing particle and the second silicon-containing particle

[0147] Before charging, the negative electrode plate was cross-sectionally processed, and SEM images of the processed surface were obtained. After obtaining the SEM images, an experimental battery was fabricated and CCCV charging (0.01C_4.2V_0.005C cutoff) was performed at 25℃. After charging, the battery was disassembled, and SEM images of the cross-sectional processed part were obtained. The area of ​​any Si-C particles before and after charging was calculated using ImageJ, and the expansion rate P was calculated using the following formula (2).

[0148] Expansion rate P = (Cross-sectional area of ​​silicon-containing particles after charging) 3 / 2 / (Cross-sectional area of ​​silicon-containing particles before charging) 3 / 2 Equation (2)

[0149] (2) Evaluation of negative electrode plate expansion rate

[0150] Experimental batteries were fabricated, and a cycle of CCCV charging (0.4C - 4.2V - 0.1C cutoff) - CC discharging (0.4C - 2.5V cutoff) was set at 25°C, and 250 cycles of repeated charging and discharging were performed. Furthermore, the expansion rate of the negative electrode plate was derived from the following equation (3).

[0151] Negative plate expansion rate = {(thickness of the test cell after 250 cycles) / (thickness of the test cell before 250 cycles) - 1} × 100 Equation (3)

[0152] 3. Evaluation Results

[0153] The test results for each sample are summarized in Tables 1 and 2.

[0154] Table 1

[0155]

[0156] Table 2

[0157] Table 2

[0158]

[0159] Based on the above results, it is confirmed that the negative electrode active material layer in Examples 1 to 4 has a two-layer structure and the upper layer has a more difficult-to-expand shape, thus the negative electrode plate has a lower expansion rate.

[0160] On the other hand, although Comparative Example 1 contains two types of Si-C particles in the negative electrode active material layer, since the structure of the negative electrode active material layer is a single layer, it does not effectively suppress the expansion rate of the negative electrode plate.

[0161] In addition, although the negative electrode active material layer in Comparative Examples 2 and 3 has a two-layer structure, the upper layer is in a form that is more prone to expansion, so it does not effectively reduce the expansion rate of the negative electrode plate.

[0162] The results above confirm that in order to reduce the expansion rate of the negative electrode plate, the negative electrode active material layer needs to be two layers and the upper layer needs to be in a shape that is more difficult to expand.

[0163] As described above, the disclosures set forth in the following items are included in this specification.

[0164] Item 1: Negative electrode, which is a negative electrode for 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 lower layer located on the side of the negative electrode current collector and an upper layer located on the surface side, the upper layer containing first graphite particles and first silicon-containing particles as negative electrode active materials, the lower layer containing second graphite particles and second silicon-containing particles as negative electrode active materials, the expansion rate P1 of the first silicon-containing particles is smaller than the expansion rate P2 of the second silicon-containing particles, and the average particle size M1 of the first silicon-containing particles is smaller than the average particle size M2 of the second silicon-containing particles.

[0165] Item 2: The negative electrode according to Item 1, wherein the ratio of P1 to P2, P1 / P2, is 0.5 to 0.8.

[0166] Item 3: The negative electrode according to Item 1 or 2, wherein the ratio of M1 to M2, M1 / M2, is 0.4 to 0.6.

[0167] Item 4: The negative electrode according to any one of items 1 to 3, wherein the ratio of the thickness T1 of the upper layer to the thickness T2 of the lower layer, T1 / T2, is 10:90 to 90:10.

[0168] Item 5: The negative electrode according to any one of items 1 to 4, wherein the mass ratio N1 of the first silicon-containing particles relative to the total mass of the first graphite particles and the first silicon-containing particles in the upper layer is 10 to 60% by mass, and the mass ratio N2 of the second silicon-containing particles relative to the total mass of the second graphite particles and the second silicon-containing particles in the lower layer is 10 to 60% by mass.

[0169] Item 6: 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 5.

[0170] Explanation of reference numerals in the attached figures

[0171] 12 First Graphite Particle

[0172] 14 First silicon-containing particles

[0173] 16 Second graphite particles

[0174] 18 Second silicon-containing particles

[0175] 20. Winded electrode body

[0176] 30 Battery casing

[0177] 36 Safety valve

[0178] 42 Positive extremes

[0179] 42a Positive Current Collector

[0180] 44 Negative extremes

[0181] 44a Negative Current Collector

[0182] 50 Positive Electrode Sheets (Positive Electrode)

[0183] 52 Positive current collector

[0184] 52a Non-forming portion of the positive electrode active material layer

[0185] 54 Positive electrode active material layer

[0186] 60 Negative electrode plate (negative electrode)

[0187] 62 Negative current collector

[0188] 62a Non-forming portion of the negative electrode active material layer

[0189] 64 Negative Electrode Active Material Layer

[0190] 70. Partition plate (partition)

[0191] 100 Lithium-ion Secondary Battery

Claims

1. A negative electrode is a secondary battery negative electrode comprising a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer comprises a lower layer located on the side of the negative electrode current collector and an upper layer located on the surface side. The upper layer contains first graphite particles and first silicon-containing particles as negative electrode active materials, and the lower layer contains second graphite particles and second silicon-containing particles as negative electrode active materials. The expansion rate P1 of the first silicon-containing particles is smaller than the expansion rate P2 of the second silicon-containing particles, and the average particle size M1 of the first silicon-containing particles is smaller than the average particle size M2 of the second silicon-containing particles.

2. The negative electrode according to claim 1, wherein, The ratio of P1 to P2, P1 / P2, is 0.5 to 0.

8.

3. The negative electrode according to claim 1, wherein, The ratio of M1 to M2, M1 / M2, is 0.4 to 0.

6.

4. The negative electrode according to claim 1, wherein, The ratio of the thickness T1 of the upper layer to the thickness T2 of the lower layer, T1:T2, is 10:90 to 90:

10.

5. The negative electrode according to claim 1, wherein, The mass ratio N1 of the first silicon-containing particles in the upper layer relative to the total mass of the first graphite particles and the first silicon-containing particles is 10-60% by mass, and the mass ratio N2 of the second silicon-containing particles in the lower layer relative to the total mass of the second graphite particles and the second silicon-containing particles is 10-60% by mass.

6. A secondary battery is a battery that consists of a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode according to any one of claims 1 to 5.

Images