Negative electrode for secondary battery and secondary battery using the negative electrode

Abstract

The present invention provides a negative electrode containing Si-containing particles and graphite particles that exhibits minimal swelling and rapid performance degradation when repeatedly charged and discharged in a secondary battery. [Solution] The negative electrode of the secondary battery according to this disclosure is a negative electrode for a secondary battery comprising a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer comprises a lower layer located on the negative electrode current collector side and an upper layer located on the surface side. The upper layer contains first graphite particles and first Si-containing particles as negative electrode active material, and the lower layer contains second graphite particles and second Si-containing particles as negative electrode active material. The average circularity C1 of the first Si-containing particles is greater than the average circularity C2 of the second Si-containing particles.

Description

[Technical Field]

[0001] This invention relates to a negative electrode for a secondary battery and a secondary battery using the negative electrode. [Background technology]

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

[0003] For vehicle power supply applications, particularly for BEVs, there is a demand for higher capacity secondary batteries from the perspective of extending the vehicle's driving range. Si-containing particles are known as high-capacity negative electrode active materials, and it is known that secondary batteries can be made higher capacity using Si-containing particles (see, for example, Patent Documents 1 and 2). Patent Document 1 discloses a lithium-ion secondary battery in which the battery capacity and charge-discharge cycle characteristics are improved by using composite particles of silicon oxide particles, graphite particles, and a low-crystallinity carbon material covering at least a portion of these as the negative electrode active material, wherein the particles have an average circularity of 0.90 to 0.99. Patent Document 2 describes a silicon compound (SiO₂) as a negative electrode active material. x A lithium-ion secondary battery is disclosed that contains silicon compound particles (0.5 ≤ x ≤ 1.6), with an average circularity of 0.93 or higher, and with a proportion of particles having an average circularity of 0.85 or lower being 5% or less, thereby improving battery capacity and cycle characteristics. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2019-175851 [Patent Document 2] Japanese Patent Publication No. 2017-092009 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, while Si-containing particles improve the capacity of secondary batteries, they also cause significant volume changes due to expansion and contraction during charging and discharging. Furthermore, when using Si-containing particles with relatively high average circularity and graphite particles together as the negative electrode active material, repeated charging and discharging of the secondary battery leads to a problem where the internal stress increases due to the expansion of the negative electrode due to the lack of excess void space. Therefore, there is a need to develop a negative electrode containing Si-containing particles and graphite particles that exhibits less expansion when repeatedly charging and discharging the secondary battery. Note that this negative electrode expansion refers to the volume of the negative electrode becoming larger than its initial volume in the same charge state (for example, a state close to full charge around 80% SOC). In addition, if there is little excess void space within the negative electrode active material layer, rapid degradation of the secondary battery may occur due to a decrease in liquid flowability.

[0006] In view of the above circumstances, the present disclosure aims to provide a negative electrode for a secondary battery containing Si-containing particles and graphite particles, which reduces the degree of swelling when the secondary battery is repeatedly charged and discharged, and suppresses rapid deterioration due to a decrease in liquid flowability. [Means for solving the problem]

[0007] The negative electrode disclosed herein is a negative electrode for a secondary battery comprising a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer comprises a lower layer located on the negative electrode current collector side and an upper layer located on the surface side. The upper layer contains first graphite particles and first Si-containing particles as negative electrode active material, and the lower layer contains second graphite particles and second Si-containing particles as negative electrode active material. The average circularity C1 of the first Si-containing particles is greater than the average circularity C2 of the second Si-containing particles.

[0008] In this configuration, the negative electrode active material layer has a two-layer structure consisting of a surface layer and a negative electrode current collector layer. In this layer, the negative electrode active material is densely packed in the surface layer and sparsely packed in the negative electrode active material layer on the negative electrode current collector side. When charging and discharging, the upper layer, which is prone to deformation, can easily follow the expansion and contraction of the negative electrode active material, thereby suppressing negative electrode swelling by mitigating internal stress and enabling higher capacity secondary batteries. On the other hand, in the lower layer, where deformation is less likely, the negative electrode active material is less likely to move, thus suppressing negative electrode swelling by preventing the breakdown of the conductive path. Furthermore, in the lower layer, the sparse packing of the negative electrode active material is maintained even after repeated charging and discharging of the secondary battery, thereby suppressing rapid deterioration of the secondary battery due to a decrease in liquid flow.

[0009] In one preferred embodiment of the negative electrode disclosed herein, C1 is 0.9 or more and 1.0 or less, and C2 is 0.5 or more and less than 0.9. This allows for more dense packing of the negative electrode active material in the upper layer and more sparse packing of the negative electrode active material in the lower layer, thereby suitably achieving higher capacity of the secondary battery, suppression of negative electrode swelling during charging and discharging, and rapid degradation of the secondary battery due to reduced liquid flow.

[0010] In one preferred embodiment of the negative electrode disclosed herein, the average circularity D1 of the first graphite particles is 0.85 or more and 1.0 or less. This allows the negative electrode active material to be more densely packed in the upper layer, thereby suitably achieving higher capacity of the secondary battery and suppression of negative electrode swelling associated with charging and discharging.

[0011] In one preferred embodiment of the negative electrode disclosed herein, the ratio T1:T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer is 10:90 to 90:10. This allows for a more dense packing of the negative electrode active material in the upper layer and a more sparse packing of the negative electrode active material in the lower layer, thereby suitably achieving higher capacity of the secondary battery, suppression of negative electrode swelling due to charging and discharging, and rapid degradation of the secondary battery due to reduced liquid flow.

[0012] In one preferred embodiment of the negative electrode disclosed herein, the mass ratio N1 of the first Si-containing particles to the total mass of the first graphite particles and the first Si-containing particles in the upper layer is 10 to 60% by mass, and the mass ratio N2 of the second Si-containing particles to the total mass of the second graphite particles and the second Si-containing particles in the lower layer is 10 to 60% by mass. This allows for a more densely packed negative electrode active material in the upper layer and a more sparsely packed negative electrode active material in the lower layer, thereby suitably achieving higher capacity of the secondary battery, suppression of negative electrode swelling during charging and discharging, and rapid degradation of the secondary battery due to reduced liquid flow.

[0013] The secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is one of the negative electrodes disclosed herein.

[0014] This configuration makes it possible to realize a secondary battery that achieves high capacity, suppression of negative electrode swelling during charging and discharging, and suppression of rapid degradation due to reduced liquid flow. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a schematic cross-sectional view showing an example of the negative electrode 60 according to this embodiment, and is a cross-sectional view along the thickness direction and the width direction. [Figure 2] Figure 2 is a schematic cross-sectional view showing the particles of the negative electrode active material contained in the negative electrode active material layer 64 shown in Figure 1. [Figure 3] Figure 3 is a schematic diagram showing the configuration of a lithium-ion secondary battery constructed using a negative electrode according to one embodiment. [Figure 4] Figure 4 is a schematic exploded view showing the configuration of the wound electrode body of the lithium-ion secondary battery shown in Figure 3. [Modes for carrying out the invention]

[0016] Hereinafter, preferred embodiments of the technology disclosed herein will be described. Matters other than those specifically mentioned in this specification and necessary for the implementation of the technology disclosed herein can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. In the drawings described in this specification, members and parts having the same function are denoted by the same reference numerals for description, and duplicate descriptions may be omitted or simplified. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not necessarily reflect the actual dimensional relationships. Also, A~B indicating a numerical range indicates A or more and B or less, and thus includes a numerical range exceeding A and less than B.

[0017] As used herein, the term "secondary battery" refers to all storage devices capable of repeated charge and discharge with the movement of charge carriers between the positive and negative electrodes, and includes 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). Hereinafter, the main constituent materials of the secondary battery according to the present disclosure will be described. For the constituent materials of the secondary battery not described herein, those known in the art can be used.

[0018] The negative electrode disclosed herein is used in a secondary battery, and preferably in a lithium-ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically showing an example of the negative electrode 60 according to the present embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to the present embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0019] 1. Negative Electrode (1) Configuration of Negative Electrode As shown in Figure 1, the negative electrode 60 comprises a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 comprises a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided on only one side of the negative electrode current collector 62, or it may be provided on both sides of the negative electrode current collector 62 as shown in Figure 1. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0020] The negative electrode 60 may include components other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) adjacent to the negative electrode active material layer 64 may be provided on the portion 62a where the negative electrode active material layer is not formed. This insulating layer may contain, for example, an insulating inorganic filler.

[0021] As shown in Figure 1, a portion 62a without a negative electrode active material layer 64 may be provided at one end of the negative electrode 60 in the width direction. In the portion 62a without a negative electrode active material layer, the negative electrode current collector 62 is exposed, and the portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0022] The shape of the negative electrode current collector 62 is foil-like (or sheet-like) in Figure 1, but is not limited to this. The negative electrode current collector 62 may take various forms such as rod-like, plate-like, or mesh-like. As with conventional lithium-ion secondary batteries, a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.) can be used as the material for the negative electrode current collector 62, and copper is particularly preferred. Copper foil is especially preferred as the negative electrode current collector 62.

[0023] The dimensions of the negative electrode current collector 62 are not particularly limited and can be determined as appropriate according to the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 6 μm or more and 20 μm or less.

[0024] As shown in Figure 1, the negative electrode active material layer 64 has a multilayer structure, specifically comprising a first layer 64a located on the surface side of the negative electrode active material layer 64 and a second layer 64b located on the negative electrode current collector 62 side. As shown in Figure 1, the first layer 64a is the upper layer of the negative electrode active material layer 64, and the second layer 64b is the lower layer of the negative electrode active material layer 64. The negative electrode active material layer 64 may further have layers other than the first layer 64a and the second layer 64b, as long as this does not significantly impede the effects of the present invention. For example, the negative electrode active material layer 64 may have an intermediate layer between the first layer 64a and the second layer 64b in which the components of these layers are mixed.

[0025] The negative electrode active material layer 64 contains the negative electrode active material. This will be explained in detail using Figure 2. Figure 2 is a schematic cross-sectional view showing the particles of the negative electrode active material contained in the negative electrode active material layer 64 shown in Figure 1. Note that Figure 2 is a schematic diagram, and therefore the number and distribution of particles are not limited to those shown in Figure 2.

[0026] Regarding the negative electrode active material, the first layer 64a contains first graphite particles 12 and first Si-containing particles 14 as the negative electrode active material. The second layer 64b contains second graphite particles 16 and second Si-containing particles 18 as the negative electrode active material. Therefore, in the first layer 64a, at least the first graphite particles 12 and 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 second Si-containing particles 18 are used as the negative electrode active material. Although Si-containing particles undergo large volume changes due to expansion / contraction during charging and discharging, using them in combination with graphite particles can suppress the interruption of the conductive path caused by the volume change of Si-containing particles.

[0027] The graphite constituting the first graphite particles 12 and the second graphite particles 16 may be natural graphite or artificial graphite, or it may be amorphous carbon-coated graphite in which the graphite is coated with an amorphous carbon material.

[0028] The shape of the first graphite particles 12 is preferably spheroidal graphite particles. The circularity of the first graphite particles 12 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1, from the viewpoint of packing the negative electrode active material.

[0029] In this specification, "circularity" refers to the ratio of the circumference of a perfect circle having the same area as the projected area of ​​the particle to the circumference of the particle projection image (i.e., circularity = circumference of a perfect circle having the same area as the projected area of ​​the particle / circumference of the particle projection image). Therefore, the closer the circularity is to 1, the closer the particle projection image is to a perfect circle, and the closer the particle is to a perfect sphere. Circularity can be determined, for example, by using a commercially available static automatic image analyzer to determine the circularity of 100 or more particles and calculating the average value.

[0030] The average particle diameter (D50) of the first graphite particles 12 and the average particle diameter (D50) of the second graphite particles 16 are not particularly limited. The average particle diameter (D50) of the first graphite particles 12 and the average particle diameter (D50) of the second graphite particles 16 are, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

[0031] In this specification, "average particle size (D50)" refers to the median diameter (D50), which is the particle size corresponding to the cumulative frequency of 50% by volume from the smaller particle size side in a volume-based particle size distribution based on laser diffraction and scattering methods. The average particle size (D50) can be determined using commercially available laser diffraction and scattering type particle size distribution analyzers.

[0032] The first graphite particles 12 and the second graphite particles 16 may be the same graphite particles, or different graphite particles may be used. It is preferable to use the same graphite particles for the first graphite particles 12 and the second graphite particles 16.

[0033] The first Si-containing particles 14 and the second Si-containing particles 18 are, for example, fine particles containing Si dispersed inside a carbon material; fine particles containing Si embedded in the pores of granulated porous graphite; and so on. The Si-C composite material may be one in which fine particles containing Si are attached to the surface of carbon particles; or one in which carbon fine particles are attached to the surface of Si-containing particles. From the viewpoint of suppressing volume changes of Si, it is preferable that Si nanoparticles are dispersed inside a carbon material, and that Si nanoparticles are dispersed in the pores of a porous carbon material, and it is more preferable that Si nanoparticles are dispersed in the pores of a porous carbon material.

[0034] As examples of the first Si-containing particles 14 and the second Si-containing particles 18, for example, particles of Si-C composite material can be used. Si-C composite material typically contains carbon domains and Si-containing domains. Note that the first Si-containing particles 14 and the second Si-containing particles 18 do not have to be Si-C composite material, but may be Si particles, Si oxide particles, etc.

[0035] Carbon domains are, for example, carbonized forms of carbon precursors (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc. Preferably, carbon domains constitute a carbon matrix. Therefore, a Si-C composite material is preferably a material in which multiple Si-containing domains are dispersed within a carbon matrix. In this case, the carbon matrix is ​​advantageous because it can mitigate volume changes due to the expansion / contraction of the Si-containing domains.

[0036] The Si-containing domain contains Si, for example, Si, Si oxide (SiO x ), Si nitride (SiN x ), Si carbide (SiC x It is composed of the following. The Si-containing domain is preferably Si and Si oxide (SiO x It consists of at least one of the following. The Si-containing domains may be fine particles. The oxygen content in the Si-containing domains is preferably 10% by mass or less.

[0037] The average particle diameter of the Si-containing domains is, for example, 50 nm or less, and may be between 5 nm and 50 nm. The "average particle diameter of the Si-containing domains" can be determined as follows: First, the negative electrode active material layer 64 is processed with FIB (Focused Ion Beam) to prepare a sample for scanning transmission electron microscopy (STEM) observation. Then, the sample is subjected to elemental analysis by EDX elemental mapping, and BF images (bright-field images) and HAADF images (high-angle scattering annular dark-field images) are obtained. The diameter of the Si-containing domains can be determined from the contrast and shape obtained from the BF and HAADF images. The diameters of 10 or more arbitrarily selected Si-containing domains are determined, and their average value is taken as the "average particle diameter of the Si-containing domains" here.

[0038] In this specification, the Si content ratio (S1) in the first Si-containing particle 14 and the Si content ratio (S2) in the second Si-containing particle 18 are not particularly limited. However, if these Si content ratios are too low, it may not be possible to achieve high capacity in the secondary battery. On the other hand, if these Si content ratios are too high, the volume change due to expansion / contraction of the first Si-containing particle 14 and the second Si-containing particle 18 may become too large when the secondary battery is repeatedly charged and discharged.

[0039] Therefore, for example, in the case of the first Si-containing particle 14 made of Si-C composite material, the Si content (S1) in the particle is preferably 20% to 55% by mass, and more preferably 25% to 45% by mass. The Si content (S2) in the second Si-containing particle 18 is preferably 45% to 80% by mass, and more preferably 55% to 75% by mass.

[0040] The first Si-containing particles 14 and the second Si-containing particles 18 can be manufactured according to known methods. Various methods for manufacturing particles of Si-C composite materials are known (see, for example, Japanese Patent Publication No. 2015-38862, International Publication No. 2014 / 046144, and the prior art documents cited in said International Publication).

[0041] In this specification, the first Si-containing particles 14 and the second Si-containing particles 18 are defined by their average circularity D. The same method as the average circularity measurement method described above can be used to measure the average circularity D of the first Si-containing particles 14 and the second Si-containing particles 18.

[0042] The particle diameters of the first Si-containing particles 14 and the second Si-containing particles 18 are not particularly limited, but are measured by the same method as the method for measuring the average particle diameter (D50) described above. The average particle diameter (D50) is, for example, 1 μm to 20 μm, preferably 2 μm to 15 μm, more preferably 3 μm to 10 μm, and even more preferably 4 μm to 7 μm.

[0043] The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as binders and conductive materials. Examples of binders include styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickening agent. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are particularly preferred. When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a CNT dispersant.

[0044] (2) Suppression of negative electrode swelling and rapid deterioration of secondary batteries When a secondary battery is charged and discharged, the negative electrode active material near the negative electrode current collector is significantly affected by the expansion of the negative electrode active material located closer to the surface, and its expansion tends to be inhibited. Conversely, the negative electrode active material located near the surface of the negative electrode active material layer is less affected by factors that inhibit its expansion. In other words, in a negative electrode active material layer 64 with a two-layer structure, the upper layer (i.e., the first layer 64a) is the one that expands the most when the secondary battery is repeatedly charged and discharged. Below, the average circularity and content of Si-C-containing particles (here, Si-C composite material is used) in the first layer 64a and the second layer 64b, as well as the thickness T of the first layer 64a and the second layer 64b, will be explained from the perspective of suppressing negative electrode expansion and rapid degradation of the secondary battery.

[0045] (A) Mean circularity C In the negative electrode active material layer according to this disclosure, the upper layer uses first Si-containing particles 14, which have a higher average circularity than the second Si-containing particles, i.e., Si-containing particles that are densely packed within the negative electrode active material layer. As a result, in the upper layer, where deformation of the active material layer is likely to occur, stress is relieved by the upper layer following the expansion / contraction of the Si-containing particles due to charging and discharging. This makes it possible to achieve high capacity of the secondary battery and suppression of negative electrode expansion.

[0046] On the other hand, the lower layer uses second Si-containing particles 18, which have a lower average circularity C than the first Si-containing particles, i.e., Si-containing particles that are sparsely packed within the negative electrode active material layer. This makes it difficult for particles to move in the lower layer where deformation is less likely to occur, and thus suppresses the interruption of the conductive path during charging and discharging. Therefore, it is possible to suppress the swelling of the negative electrode associated with the interruption of the conductive path (such as swelling due to uneven battery reaction and localized concentration of reactions and stress). As a result, the swelling of the negative electrode 60 when the secondary battery is repeatedly charged and discharged can be suppressed across the entire negative electrode active material layer 64. Furthermore, in the lower layer, even after charging and discharging, the sparse packing of the negative electrode active material can be maintained within the negative electrode active material layer, thereby suppressing the rapid deterioration of the secondary battery due to electrolyte depletion.

[0047] The average circularity C1 of the first Si-containing particles 14 is preferably 0.9 or higher, more preferably 0.93 or higher, and particularly preferably 0.95 or higher, from the viewpoint of increasing the capacity of the secondary battery by densely filling it in the negative electrode active material layer. The upper limit of the average circularity C1 of the first Si-containing particles 14 is not particularly limited and may be, for example, 1.0 or less, or 0.98 or less. The average circularity C2 of the second Si-containing particles 18 is preferably less than 0.9, more preferably 0.87 or less, and particularly preferably 0.83 or less, from the viewpoint of suppressing rapid deterioration due to liquid depletion by sparsely filling the negative electrode active material layer. However, if the average circularity C2 of the second Si-containing particles 18 is too small, there is a risk of insufficient filling, so it is preferably 0.5 or more, more preferably 0.6 or more, and particularly preferably 0.7 or more.

[0048] (B) Content The content of the negative electrode active material in the first layer 64a (i.e., relative to the total mass of the first layer 64a) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less.

[0049] The content of the negative electrode active material in the second layer 64b (i.e., relative to the total mass of the second layer 64b) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less.

[0050] In the first layer 64a, the mass ratio N1 of the first Si-containing particles 14 to the total mass of the first graphite particles 12 and the first Si-containing particles 14 is preferably 10% by mass or more, and more preferably 15% by mass or more, from the viewpoint of increasing the capacity of the secondary battery. On the other hand, from the viewpoint of suppressing swelling of the negative electrode plate, it is preferably 60% by mass or less, more preferably 40% by mass or less, and particularly preferably 20% by mass or less.

[0051] In the second layer 64b, the mass ratio N2 of the second Si-containing particles 18 to the total mass of the second graphite particles 16 and the second Si-containing particles 18 is preferably 10% by mass or more, and more preferably 15% by mass or more, from the viewpoint of increasing the capacity of the secondary battery. On the other hand, from the viewpoint of suppressing swelling of the negative electrode plate, it is preferably 60% by mass or less, more preferably 40% by mass or less, and particularly preferably 20% by mass or less.

[0052] The negative electrode active material contained in the first layer 64a may consist only of the first graphite particles 12 and the first Si-containing particles 14. However, the first layer 64a may further contain negative electrode active materials other than the first graphite particles 12 and the first Si-containing particles 14, within a range that does not impede the effects of the present invention (for example, 10% by mass or less of the total amount of negative electrode active material contained in the first layer 64a).

[0053] The negative electrode active material contained in the second layer 64b may consist only of the second graphite particles 16 and the second Si-containing particles 18. However, the second layer 64b may further contain negative electrode active materials other than the second graphite particles 16 and the second Si-containing particles 18, within a range that does not impede the effects of the present invention (for example, 10% by mass or less of the total amount of negative electrode active material contained in the second layer 64b).

[0054] (C) Thickness T of the first and second layers The ratio of the thickness of the lower layer T2 to the thickness of the upper layer T1 (T1:T2) is preferable because it balances the effects of suppressing the swelling of the negative electrode plate and suppressing the rapid deterioration of the secondary battery due to electrolyte depletion. A ratio of 10:90 to 90:10 is preferable, 20:80 to 80:20 is more preferable, 30:70 to 70:30 is even more preferable, and 40:60 to 60:40 is particularly preferable.

[0055] Furthermore, the smaller the difference between the thickness T1 of the upper layer and the thickness T2 of the lower layer, the more uniformly the effects of this disclosure can be realized in both the upper and lower layers, and thus the effects of this disclosure can be more significantly realized in the negative electrode active material layer as a whole.

[0056] (4) Manufacturing of the negative electrode The negative electrode 60 can be suitably manufactured by a manufacturing method comprising, for example, the steps of: preparing a paste for forming the second layer (hereinafter also referred to as the "lower layer negative electrode composite paste") by mixing the second graphite particles 16 and the second Si-containing particles 18 in a dispersion medium; preparing a paste for forming the first layer (hereinafter also referred to as the "upper layer negative electrode composite paste") by mixing the first graphite particles 12 and the first Si-containing particles 14 in a dispersion medium; coating the second layer forming paste onto the negative electrode current collector 62 and drying it to form the second layer 64b (lower layer) (hereinafter also referred to as the "lower layer forming step"); coating the first layer forming paste onto the second layer 64b and drying it to form the first layer 64a (upper layer) (hereinafter also referred to as the "upper layer forming step"); and pressing the formed first layer 64a and second layer 64b (hereinafter also referred to as the "pressing step").

[0057] In this specification, "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, and includes so-called "slurry," "ink," etc.

[0058] The paste preparation step for forming the lower layer can be carried out by mixing the second graphite particles 16, the second Si-containing particles 18, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0059] The upper layer forming paste preparation process can be carried out by mixing the first graphite particles 12, the first Si-containing particles 14, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc. according to a known method. The upper layer forming paste preparation process may be carried out in parallel with the lower layer forming paste preparation process. The upper layer forming paste preparation process may be carried out in parallel with or after the lower layer forming process.

[0060] The lower layer forming process can be carried out according to a known method. Specifically, for example, it can be carried out by coating a lower layer forming paste on the negative electrode current collector 62 using a known coating device and drying it. By drying, the lower layer (the second layer 64b) is formed.

[0061] The upper layer forming process can be carried out according to a known method. Specifically, for example, it can be carried out by coating an upper layer forming paste on the formed lower layer using a known coating device and drying it. By drying, the upper layer (the first layer 64a) is formed, and the negative electrode active material layer 64 is formed.

[0062] The density of the negative electrode active material layer 64 is not particularly limited. For example, it is 0.7 g / cm 3 or more, preferably 1.0 g / cm 3 or more, and more preferably 1.2 g / cm 3 or more. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm 3 or less, and may be 2.0 g / cm 3 or less.

[0063] The pressing process can be carried out according to a known method. Specifically, the pressing process can be carried out by applying pressure to the formed upper and lower layers (i.e., the negative electrode active material layer 64) using a roller press or the like. By the pressing process, the negative electrode active material layer 64 is compressed to a predetermined density, and thereby the negative electrode active material particles are densely filled.

[0064] 2. Secondary battery According to the negative electrode 60 of this embodiment, swelling of the negative electrode 60 and rapid deterioration of the secondary battery can be suppressed when the secondary battery is repeatedly charged and discharged. Furthermore, since the negative electrode 60 of this embodiment uses a negative electrode active material containing Si, the capacity of the secondary battery can be increased.

[0065] Therefore, from another perspective, the secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, an embodiment of the secondary battery disclosed herein will be described with reference to Figures 3 and 4, using a lithium-ion secondary battery as an example. The following configuration example is a flat rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0066] Figure 3 is a schematic diagram showing the configuration of a lithium-ion secondary battery constructed using a negative electrode according to one embodiment. The lithium-ion secondary battery 100 shown in Figure 3 is a sealed lithium-ion secondary battery 100 constructed by housing a flat-shaped wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight metal material with good thermal conductivity such as aluminum is used.

[0067] Figure 4 is a schematic exploded view showing the configuration of the wound electrode body of the lithium-ion secondary battery shown in Figure 3. As shown in Figures 3 and 4, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are superimposed on each other via two elongated separator sheets 70 and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated negative electrode current collector 62. The portion 52a where the positive electrode active material layer is not formed (i.e., the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the portion 62a where the negative electrode active material layer is not formed (i.e., the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed) are formed to protrude outward from both ends of the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are joined to the positive electrode active material layer not formed portion 52a and the negative electrode active material layer not formed portion 62a, respectively.

[0068] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in lithium-ion secondary batteries may be used, and examples include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0069] The dimensions of the positive electrode current collector 52 are not particularly limited and can be determined as appropriate according to the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 7 μm or more and 20 μm or less.

[0070] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition of positive electrode active material used in lithium-ion secondary batteries may be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, spinel structure, olivine structure, etc.

[0071] Preferably, lithium transition metal composite oxides are lithium transition metal composite oxides containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel-manganese-based composite oxides, lithium nickel-cobalt-manganese-based composite oxides, lithium nickel-cobalt-aluminum-based composite oxides, and lithium iron-nickel-manganese-based composite oxides.

[0072] In this specification, "lithium nickel cobalt manganese composite oxide" is a term that encompasses not only oxides whose constituent elements are Li, Ni, Co, Mn, and O, but also oxides that contain one or more additive elements other than these. Examples of such additive elements include transition metal elements and main group metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additive elements may also be metalloid elements such as B, C, Si, and P, or nonmetallic elements such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc.

[0073] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0074] These positive electrode active materials may be used individually or in combination of two or more. Lithium nickel cobalt manganese composite oxides are particularly preferred as positive electrode active materials due to their excellent properties, such as initial resistance characteristics.

[0075] The average particle size (D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0076] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, conductive material, binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor-processed carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF), etc.

[0077] The content of positive electrode active material in the positive electrode active material layer 54 (i.e., the content of positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0078] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, and preferably 300 μm or less.

[0079] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0080] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such porous sheets may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0081] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability obtained by the Gurley test method of the separator 70 is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0082] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents commonly used in lithium-ion secondary battery electrolytes, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without particular limitation. Among these, carbonates are preferred, with specific examples including ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used individually or in appropriate combinations of two or more. For example, the non-aqueous solvent may consist solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0083] Suitable supporting salts include lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI), preferably LiPF6. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0084] Furthermore, the above-mentioned non-aqueous electrolyte may contain components other than those described above, such as film-forming agents like vinylene carbonate (VC) and oxalat complexes; gas-generating agents like biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and other additives, as long as they do not significantly impair the effects of the present disclosure.

[0085] The lithium-ion secondary battery 100 has suppressed negative electrode swelling during repeated charging and discharging, and therefore exhibits low reaction force. Furthermore, the lithium-ion secondary battery 100 has high capacity. The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power supplies for vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium-ion secondary battery 100 can also be used as a storage battery for small-scale power storage devices. Typically, the lithium-ion secondary battery 100 can also be used in the form of a battery pack, where multiple batteries are connected in series and / or parallel.

[0086] The above describes a rectangular lithium-ion secondary battery 100 equipped with a flattened wound electrode body 20 as an example. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries equipped with a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are stacked alternately). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, laminated case type lithium-ion secondary batteries, and the like.

[0087] Furthermore, according to known methods, the lithium-ion secondary battery 100 can also be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0088] Furthermore, while the negative electrode 60 according to this embodiment is suitable as the negative electrode of a lithium-ion secondary battery, it can be constructed and used as the negative electrode of other secondary batteries, and other secondary batteries can be constructed according to known methods.

[0089] ≪Rating≫ 1. Example Test The following describes test examples relating to the technology disclosed herein, but it is not intended that the technology disclosed herein is limited to these test examples.

[0090] (1) Example 1 Creating the negative electrode Secondary graphite particles C (average circularity: 0.9) and secondary Si-containing particles (average circularity C2: 0.8, average particle diameter M2: 7 μm) were weighed as negative electrode active materials, SWCNT as a conductive material, and CMC, PAA, and SBR as binders, with a mass ratio of C:secondary Si-containing particles:SWCNT:CMC:PAA:SBR = 85:15:0.1:1:1:1.5. Of these, the raw materials other than SWCNT and SBR were dry-mixed, then SWCNT and dispersion medium were mixed and kneaded into a solid paste, and then SBR and dispersion medium were added and diluted and mixed to create a negative electrode composite paste for the lower layer. Note that the above kneading process requires optimizing the pressure load on the paste in order to coat the active material with binder (CMC / PAA). To optimize this pressure load, the ideal solid content B0 of the paste was derived using the following equation (1). The ideal solid content ratio B0 of the paste is a value that depends on the conditions of the mixing process (e.g., the shape of the stirring blade, the rotation speed, etc.). B0 = 100 - A0 = 100 / (100 + A1) × 100 Equation (1) B0: Ideal solid content rate [%] A0: Moisture content [%] at which the torque required for mixing is maximized. A1: Moisture content [mL] when the mixture is 100g

[0091] Next, a negative electrode composite paste for the upper layer was prepared using the same manufacturing method, except that the first Si-containing particles (average circularity C1: 0.95, particle size M1: 7 μm) were used instead of the second Si-containing particles.

[0092] Then, the lower layer negative electrode composite paste was applied onto the negative electrode core (copper foil, 10 μm) and dried. After drying, the upper layer negative electrode composite paste was applied onto the lower layer negative electrode composite paste so that the ratio of the thickness T1 of the upper layer to the thickness T2 of the lower layer, T1:T2, was 50:50, and the mixture was dried to create a two-layer negative electrode active material layer on the negative electrode core. Subsequently, the material was rolled by press working and processed to the predetermined dimensions to obtain a negative electrode plate.

[0093] Creating a positive electrode A cathode composite paste was prepared by weighing lithium nickel cobalt manganese composite oxide (NCM) as the cathode active material, polyvinylidene fluoride (PVdF) as the binder, and acetylene black (AB) as the conductive material, in a mass ratio of NCM:PVdF:AB = 100:1:1, and mixing them in N-methyl-2-pyrrolidone (NMP). This cathode composite paste was applied to a long, strip-shaped cathode core (aluminum foil, 15 μm thick) and dried. Subsequently, it was rolled by press processing and processed to the specified dimensions to obtain a cathode plate.

[0094] Leads were attached to the negative electrode and positive electrode, and the electrodes were stacked with separators in between to create an electrode body. The fabricated electrode body was inserted into an outer casing made of aluminum laminate sheet, a non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to create a test cell (laminate cell). The non-aqueous electrolyte was prepared by dissolving LiPF6 in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC = 15:5:40:40 to a concentration of 1 M.

[0095] (2) Example 2 A test cell was prepared in the same manner as in Example 1, except that the average circularity of the first Si-containing particle was set to 0.9 and the average circularity of the second Si-containing particle was set to 0.85.

[0096] (3) Example 3 A test cell was prepared in the same manner as in Example 1, except that the thickness ratio T1:T2 between the upper and lower layers was set to 90:10.

[0097] (4) Example 4 A test cell was prepared in the same manner as in Example 1, except that the thickness ratio T1:T2 between the upper and lower layers was set to 10:90.

[0098] (5) Example 5 A test cell was prepared in the same manner as in Example 1, except that the thickness ratio T1:T2 between the upper and lower layers was set to 70:30.

[0099] (6) Example 6 A test cell was prepared in the same manner as in Example 1, except that the thickness ratio T1:T2 between the upper and lower layers was set to 30:70.

[0100] (7) Example 7 A test cell was prepared in the same manner as in Example 1, except that the average circularity of the secondary Si-containing particles was set to 0.5.

[0101] (8) Comparative Example 1 A test cell was prepared in the same manner as in Example 1, except that the negative electrode active material layer consisted of a single layer containing first Si-containing particles and second Si-containing particles. The composition of the composite paste forming the negative electrode active material layer was adjusted so that the mass ratio was C:second Si-containing particles:first Si-containing particles:SWCNT:CMC:PAA:SBR = 85:7.5:7.5:0.1:1:1:1.5.

[0102] (9) Comparative Example 2 A test cell was prepared in the same manner as in Example 1, except that a lower layer was formed using a paste containing first Si-containing particles and an upper layer was formed using a paste containing second Si-containing particles.

[0103] (10) Comparative Example 3 A test cell was prepared in the same manner as in Example 1, except that only the paste containing the first Si-containing particles was used and the negative electrode active material layer was made into one layer.

[0104] (11) Comparative Example 4 A test cell was prepared in the same manner as in Example 1, except that only the paste containing the second Si-containing particles was used and the negative electrode active material layer was made into one layer.

[0105] (12) Comparative Example 5 A test cell was prepared in the same manner as in Example 1, except that the average circularity of the secondary Si-containing particles was set to 0.4.

[0106] 2. Evaluation Test (1) Measurement of the average circularity of the first Si-containing particles and the second Si-containing particles. In this specification, the average circularity is derived using an image-based particle size distribution analyzer. Using a powder of Si-containing particles as a sample, the circumference length (L0) of a circle having the same area as the projection image of any Si-containing particle and the outer circumference length (L) of the projection image were measured and derived using the following equation (2). Then, the average circularity of Si-containing particles was derived from 3000 Si-containing particles. Average circularity C=L0 / L Equation (2)

[0107] (2) Evaluation of negative electrode plate expansion rate A test cell was prepared, and 250 cycles of CCCV charging (0.4C_4.2V_0.1C cut) - CC discharge (0.4C_2.5V cut) were performed in a 25°C environment. The negative electrode plate expansion rate was then derived from the following equation (3). Negative electrode plate expansion coefficient = {(thickness of test cell after 250 cycles / thickness of test cell before 250 cycles) - 1} × 100 Equation (3)

[0108] (3) Evaluation of resistance increase rate The test cell was subjected to CCCV charging at a constant current of 1.5C in a 25°C environment until the SOC reached 50% (1.5C_4.2V_0.1C cutoff). Afterward, it was stored at 25°C for 1 hour, followed by CC discharge at a constant current of 1.0C for 10 seconds (0.4C_2.5V cutoff). The resistance value at this time was defined as the initial resistance value. After measuring the initial resistance value as described above, a charge-discharge cycle consisting of CCCV charging (1.5C_4.2V_0.1C cut) and CC discharge (0.4C_2.5V cut) was repeated 50 times in a 25°C environment. The resistance was evaluated in the test cell after 50 cycles in a 25°C environment, and the resistance increase rate was derived using the following equation (4). In this specification, the rapid degradation of the secondary battery due to charging and discharging was evaluated by evaluating the above resistance increase rate. Resistance increase rate = Resistance value after 50 cycles / Initial resistance value × 100 Equation (4)

[0109] 3. Evaluation Results The test results for each sample are summarized in Tables 1 and 2.

[0110] [Table 1]

[0111] [Table 2]

[0112] Examples 1 to 7 show that the negative electrode active material layer has a two-layer structure, with the negative electrode active material more densely packed in the upper layer and more loosely packed in the lower layer. As a result, it was confirmed that the negative electrode plate expansion rate and high-rate cycle characteristics (resistance increase rate) are relatively low. In all of Comparative Examples 1 to 5, the configuration of the negative electrode active material layer was outside the scope of the technical concept of this disclosure, and it was confirmed that it was not possible to suppress both negative electrode swelling and rapid degradation of the secondary battery.

[0113] Based on these results, it was confirmed that in order to suppress negative electrode swelling and rapid degradation of secondary batteries, it is necessary to have two negative electrode active material layers, with the negative electrode active material with high circularity placed in the upper layer and the negative electrode active material with low circularity placed in the lower layer.

[0114] As described above, this specification includes the disclosures set forth in the following sections. Section 1: Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, A negative electrode for a secondary battery, The negative electrode active material layer comprises a lower layer located on the negative electrode current collector side and an upper layer located on the surface side. The upper layer contains first graphite particles and first Si-containing particles as negative electrode active material. The lower layer contains second graphite particles and second Si-containing particles as negative electrode active material. The average circularity C1 of the first Si-containing particles is greater than the average circularity C2 of the second Si-containing particles. Negative electrode.

[0115] Section 2: The negative electrode according to item 1, wherein C1 is 0.9 or more and 1.0 or less, and C2 is 0.5 or more and less than 0.9.

[0116] Section 3: The negative electrode according to item 1 or 2, wherein the average circularity D1 of the first graphite particles is 0.85 or more and 1.0 or less.

[0117] Section 4: The negative electrode according to any one of items 1 to 3, wherein the ratio T1:T2 of the thickness T1 of the upper layer to the thickness T2 of the lower layer is 10:90 to 90:10.

[0118] Section 5: The mass ratio N1 of the first Si-containing particles to the total mass of the first graphite particles and the first Si-containing particles in the upper layer is 10 to 60% by mass. The negative electrode according to any one of claims 1 to 4, wherein the mass ratio N2 of the second Si-containing particles to the total mass of the second graphite particles and the second Si-containing particles in the lower layer is 10 to 60% by mass.

[0119] Item 6: Positive electrode, negative electrode, electrolyte, A secondary battery equipped with The negative electrode is the negative electrode described in any one of items 1 to 5. Secondary battery. [Explanation of Symbols]

[0120] 12. First Graphite Particles 14 1st Si-containing particles 16. Second-order graphite particles 18 2nd Si-containing particles 20 Wound electrode body 30 Battery Cases 36 Safety valve 42 Positive terminal 42a Positive electrode current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheets (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheets (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator Sheets (Separators) 100 Lithium-ion rechargeable batteries

Claims

1. Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, A negative electrode for a secondary battery, The negative electrode active material layer comprises a lower layer located on the negative electrode current collector side and an upper layer located on the surface side. The upper layer contains first graphite particles and first Si-containing particles as negative electrode active material. The lower layer contains second graphite particles and second Si-containing particles as negative electrode active material. The average circularity C1 of the first Si-containing particles is greater than the average circularity C2 of the second Si-containing particles. Negative electrode.

2. The negative electrode according to claim 1, wherein C1 is 0.9 or more and 1.0 or less, and C2 is 0.5 or more and less than 0.

9.

3. The negative electrode according to claim 1, wherein the average circularity D1 of the first graphite particles is 0.85 or more and 1.0 or less.

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

10.

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

6. Positive electrode, negative electrode, electrolyte, A secondary battery equipped with The negative electrode is the negative electrode according to any one of claims 1 to 5. Secondary battery.

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