JP2024128914A - Secondary battery and method for manufacturing the same - Google Patents

Abstract

To provide a secondary battery comprising a negative electrode including graphite particles and a Si-based material as a negative electrode active material, and having excellent initial charging and discharging efficiency.SOLUTION: A secondary battery disclosed herein comprises an electrode body having a positive electrode and a negative electrode 60. The negative electrode 60 comprises a negative electrode current collector 62, and a negative electrode active material layer 64 disposed on the negative electrode current collector. The negative electrode active material layer 64 includes graphite particles 66 and Si-containing particles 67 as a negative electrode active material, and carbon nanotubes 68 as a conductive material. The Si-containing particle 67 is a porous body containing Si nanoparticles 67a with a mesh structure. In some pores 67b of the porous body, the carbon nanotubes 68 are disposed. When a weight of the Si-containing particles 67 is 100 wt%, a weight ratio X of the carbon nanotubes 68 to the Si-containing particles 67 is 0.02 wt% or more and 4 wt% or less.SELECTED DRAWING: Figure 3

Description

The present invention relates to a secondary battery and a method for manufacturing a secondary battery.

Secondary batteries such as lithium-ion secondary batteries are suitable for use as portable power sources for personal computers, mobile terminals, and the like, and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid vehicles (HEVs), and plug-in hybrid vehicles (PHEVs). The negative electrodes used in such secondary batteries generally have a configuration in which a negative electrode active material layer containing a negative electrode active material is disposed on a negative electrode current collector.

In recent years, the use of Si-based materials as negative electrode active materials has been considered in order to increase the capacity of secondary batteries (for example, Patent Documents 1 to 3). Patent Document 1 discloses a composite of silicon and a porous carbon material having micropores, mesopores, and/or macropores. Patent Document 2 discloses a silicon material whose precursor is amorphous silica produced from a plant-based raw material. Patent Document 3 discloses a negative electrode containing a negative electrode active material containing a Si-based material, carbon nanotubes having an outermost diameter of 5 nm or less, and carboxymethylcellulose having a weight-average molecular weight of 150,000 to 450,000.

Special table 2018-534720 publication JP 2021-38114 A International Publication No. 2022/070895

Si-based materials have a larger specific capacity than carbon materials such as graphite particles, but they also expand and contract significantly during charging and discharging, and their conductive paths tend to be easily broken. For this reason, when Si-based materials are used, the initial characteristics of the secondary battery (e.g., initial charge and discharge efficiency) are likely to decrease. Therefore, when Si-based materials are used as negative electrode active materials, there is still room for improvement in terms of improving the initial characteristics of secondary batteries.

The present invention has been made in consideration of these points, and aims to provide a secondary battery having a negative electrode containing graphite particles and a Si-based material as the negative electrode active material, and having excellent initial charge/discharge efficiency.

The secondary battery disclosed herein is a secondary battery equipped with an electrode body having a positive electrode and a negative electrode, the negative electrode being equipped with a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer including graphite particles and Si-containing particles as negative electrode active materials, and carbon nanotubes as a conductive material. The Si-containing particles are porous bodies containing Si nanoparticles in a mesh structure, and the carbon nanotubes are disposed in at least some of the pores of the porous body. When the weight of the Si-containing particles is taken as 100 wt%, the weight ratio X of the carbon nanotubes to the Si-containing particles is 0.02 wt% or more and 4 wt% or less.

According to this configuration, the Si-containing particles are porous bodies containing Si nanoparticles with a mesh-like structure, so that conductive paths are suitably formed. In addition, the Si-containing particles have multiple pores, and the carbon nanotubes are suitably arranged in the pores, so that conductive paths are also formed between the graphite particles and the Si-containing particles. Therefore, a secondary battery with excellent initial characteristics can be realized.

FIG. 1 is a diagram illustrating a schematic internal structure of a secondary battery according to an embodiment. FIG. 2 is a diagram illustrating a schematic configuration of an electrode body according to one embodiment. FIG. 3 is a diagram illustrating a negative electrode according to one embodiment. FIG. 4 is a flow chart showing a method for manufacturing a secondary battery according to an embodiment.

The following describes embodiments of the technology disclosed herein with reference to the drawings. Note that matters other than those specifically mentioned in this specification and necessary for implementing the technology disclosed herein (for example, the general configuration and manufacturing process of secondary batteries that do not characterize the technology disclosed herein) can be understood as design matters of a person skilled in the art based on the conventional technology in the field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in the field. Note that each drawing is drawn diagrammatically, and the dimensional relationships (length, width, thickness, etc.) do not necessarily reflect the actual dimensional relationships. In addition, in the drawings described below, the same reference numerals are used for members and parts that perform the same function, and duplicated descriptions may be omitted or simplified. In addition, the notation "A to B" (A and B are arbitrary numbers) indicating a range in this specification means A or more and B or less.

In this specification, the term "secondary battery" refers to a battery that can be repeatedly charged and discharged by the movement of charge carriers between the positive and negative electrodes. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and achieves charging and discharging by the movement of charge associated with the lithium ions between the positive and negative electrodes.

Figure 1 is a diagram showing a schematic internal structure of a secondary battery 100 according to one embodiment. As shown in Figure 1, the secondary battery 100 includes an electrode body 20 having a positive electrode 50 and a negative electrode 60, an electrolyte (not shown), and a battery case 30 that contains the electrode body 20 and the electrolyte. The secondary battery 100 shown in Figure 1 is a lithium ion secondary battery. The negative electrode 60 disclosed herein is preferably used as a negative electrode for a lithium ion secondary battery.

The battery case 30 is provided with a positive terminal 42 and a negative 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 to a predetermined level or higher. The battery case 30 is also provided with an injection port (not shown) for injecting a non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive current collector 42a. The negative terminal 44 is electrically connected to the negative current collector 44a. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum.

FIG. 2 is a schematic diagram showing the configuration of the electrode body 20. Here, the electrode body 20 is a flat-shaped wound electrode body. As shown in FIG. 2, the electrode body 20 has a configuration in which a long sheet-shaped positive electrode 50 (hereinafter also referred to as "positive electrode sheet 50") and a long sheet-shaped negative electrode 60 (hereinafter also referred to as "negative electrode sheet 60") are stacked via two long separators 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 (both sides here) of a long positive electrode 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 (both sides here) of a long negative electrode collector 62. As shown in FIG. 1 and FIG. 2, the positive electrode collector exposed portion 52a (i.e., the portion where the positive electrode collector 52 is exposed without the positive electrode active material layer 54) and the negative electrode collector exposed portion 62a (i.e., the portion where the negative electrode collector 62 is exposed without the negative electrode active material layer 64) are formed so as to protrude outward from both ends of the electrode body 20 in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode collector exposed portion 52a and the negative electrode collector exposed portion 62a are respectively joined to the positive electrode collector 42a and the negative electrode collector 44a.

The positive electrode collector 52 constituting the positive electrode sheet 50 may be a known positive electrode collector used in lithium ion secondary batteries, and is not particularly limited. For example, a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.) can be used. Aluminum foil is preferable as the positive electrode collector 52. The dimensions of the positive electrode collector 52 are not particularly limited and may be appropriately determined according to the battery design. When aluminum foil is used as the positive electrode collector 52, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a positive electrode active material having a known composition used in lithium ion secondary batteries may be used. Specifically, for example, as the positive electrode active material, a lithium composite oxide, a lithium transition metal phosphate compound (e.g., lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ )), or the like may be used. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more. Among them, lithium nickel cobalt manganese composite oxide can be preferably used as the positive electrode active material.

In this specification, the term "lithium nickel cobalt manganese composite oxide" includes oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements other than those. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a semimetal element such as Y/Z, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the above-mentioned lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as a conductive material, a binder, etc. As the conductive material, for example, carbon black such as acetylene black (AB), carbon fibers such as vapor grown carbon fiber (VGCF) and carbon nanotubes (CNT), and other carbon materials (e.g., graphite, etc.) may be suitably used. As the binder, for example, polyvinylidene fluoride (PVdF) may be used.

Although not particularly limited, the content of the conductive material is preferably 0.1 wt% to 10 wt%, and more preferably 1 wt% to 5 wt%, when the positive electrode active material is 100 wt%. The content of the binder is preferably 0.1 wt% to 10 wt%, and more preferably 1 wt% to 5 wt%, when the positive electrode active material is 100 wt%.

The thickness of each side of the positive electrode active material layer 54 is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

As the separator 70, various types of microporous sheets similar to those used conventionally can be used, such as microporous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such microporous resin sheets may have a single-layer structure or a multi-layer 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). The separator 70 may also have a heat-resistant layer (HRL).

The electrolyte can be the same as that used in the past, and for example, a non-aqueous electrolyte containing a supporting salt in an organic solvent (nonaqueous solvent) can be used. As the nonaqueous solvent, aprotic solvents such as carbonates, esters, ethers, etc. can be used. Among them, carbonates, for example, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be preferably adopted. Alternatively, fluorine-based solvents such as fluorinated carbonates such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC) can be preferably used. Such nonaqueous solvents can be used alone or in appropriate combination of two or more. As the supporting salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ can be preferably used. The concentration of the supporting salt is not particularly limited, but is preferably about 0.7 mol/L or more and 1.3 mol/L or less.
In addition, the nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt described above, so long as the effects of the present technology are not significantly impaired. For example, the nonaqueous electrolyte may contain various additives such as a gas generating agent, a film forming agent, a dispersing agent, and a thickening agent.

The negative electrode 60 of the secondary battery disclosed herein will be described below. FIG. 3 is a schematic diagram showing the negative electrode 60 of the secondary battery 100 disclosed herein. As shown in FIG. 3, the negative electrode 60 includes a negative electrode collector 62 and a negative electrode active material layer 64 disposed on the negative electrode collector 62. The negative electrode collector 62 may be a conventionally known one, and is not particularly limited. For example, a sheet or foil made of a metal such as copper, nickel, titanium, or stainless steel may be used. When copper foil is used as the negative electrode collector 62, its average thickness is not particularly limited, but is, for example, 5 μm or more and 30 μm or less, preferably 5 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less.

The negative electrode active material layer 64 contains at least a negative electrode active material. The negative electrode active material layer 64 contains at least graphite particles 66 and Si-containing particles 67 as the negative electrode active material. The Si-containing particles 67 are porous bodies containing Si nanoparticles 67a in a mesh structure. The Si-containing particles 67 have a plurality of pores 67b. Carbon nanotubes 68 are arranged in at least some of the plurality of pores 67b. The weight ratio X of the carbon nanotubes 68 to the Si-containing particles 67 is adjusted to be 0.02 wt% or more and 4 wt% or less when the weight of the Si-containing particles 67 is 100 wt%. With this configuration, the initial charge/discharge efficiency of the secondary battery 100 can be improved.

Although there is no intention to limit the technology disclosed herein, the reason for obtaining such an effect is presumed to be as follows. The conductive path is suitably improved by the Si-containing particle 67 being a porous body containing Si nanoparticles 67a having a mesh-like structure. In addition, the Si-containing particle 67 has a plurality of pores 67b, so that the carbon nanotubes 68 as conductive materials are suitably arranged around the Si-containing particle 67, and a good charging path is formed between the graphite particle 66 and the Si-containing particle 67. Therefore, the initial charge/discharge efficiency of the secondary battery 100 is improved. Even if the expansion and contraction associated with charging and discharging are repeated, the anchor effect is exerted, so that the carbon nanotubes 68 are unlikely to come off the plurality of pores 67b. Furthermore, since the Si nanoparticles 67a have a mesh-like structure, the expansion of the Si nanoparticles 67a can be suppressed, and the Si nanoparticles 67a are unlikely to be damaged by the expansion. Therefore, even if the expansion and contraction associated with charging and discharging are repeated, the conductive path is unlikely to be broken, and the cycle characteristics of the secondary battery 100 are improved.

As the graphite particles 66, for example, artificial graphite, natural graphite, etc. are used. The graphite particles 66 may have a coating layer of amorphous carbon on their surfaces. Although not particularly limited, it is preferable that the graphite particles 66 are approximately spherical. In this specification, the term "approximately spherical" includes a spherical shape, a rugby ball shape, etc., and refers to a shape with an average aspect ratio (the ratio of the superaxial length to the short-axis length in the smallest rectangle circumscribing the particle) of, for example, 1 to 2 (preferably 1 to 1.5).

The _D50 particle size of the graphite particles 66 is not particularly limited, but is preferably 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 25 μm or less. In this specification, the " _D50 particle size of the graphite particles" refers to a particle size that corresponds to a cumulative 50% from the fine particle side in a volume-based particle size distribution measured by a particle size distribution measurement based on a laser diffraction/light scattering method.

The Si-containing particle 67 is a porous body containing Si nanoparticles 67a having a mesh-like structure. The Si-containing particle 67 may contain components other than Si as long as it contains Si. Examples of the Si-containing particle 67 include SiOx, Si-C composites, and porous Si particles in which Si nanoparticles are dispersed. The porous body portion of the Si-containing particle 67 may be composed of, for example, Si as the main component, or carbon (C) as the main component. For example, a Si-C composite containing Si nanoparticles having a mesh-like structure and porous carbon particles is preferably used as the Si-containing particle 67. Alternatively, a Si particle containing Si nanoparticles having a mesh-like structure and porous Si particles is preferably used as the Si-containing particle 67. In this specification, "A is composed of B as the main component" means that B is the largest component by weight among the components that constitute A.

The Si-containing particles 67 are porous bodies having a plurality of pores 67b. The Si-containing particles 67 may have, for example, micropores, mesopores, and macropores. Here, the micropores are pores having a diameter of 2 nm or less, the mesopores are pores having a diameter of more than 2 nm and less than 50 nm, and the macropores are pores having a diameter of 50 nm or more. If the pore size is too large, there is a risk that the cycle characteristics of the secondary battery 100 will deteriorate due to erosion of the electrolyte. From this perspective, the pores 67b of the Si-containing particles 67 are preferably, for example, 1 nm or more and 300 nm or less, and may be 1 nm or more and 250 nm or less. The Si-containing particles 67 may have, for example, a nanoporous structure having a nano-sized porous structure.

Although not particularly limited, it is preferable that the Si-containing particles 67 contain pores with a diameter of 100 nm or more and pores with a diameter of 10 nm or less. By the Si-containing particles 67 having pores of 100 nm or more, the anchor effect is easily exerted, and the carbon nanotubes are suitably arranged on the surface of the Si-containing particles 67. Therefore, the initial charge and discharge efficiency of the secondary battery 100 can be improved. Furthermore, even if the Si-containing particles 67 repeatedly expand and contract with charging and discharging, the conductive material is unlikely to come off due to the anchor effect, and the conductive path is unlikely to be broken. Therefore, the cycle characteristics of the secondary battery 100 can also be improved. By the Si-containing particles 67 having pores of 10 nm or less, the erosion of the electrolyte and the expansion and contraction caused by charging and discharging can be suitably suppressed. This can improve the cycle characteristics of the secondary battery 100.

More preferably, the Si-containing particles 67 are adjusted so that the ratio (V ₁₀ /V ₁₀₀ ) of the log differential pore volume V ₁₀ of a pore with a diameter of ₁₀ nm to the log differential pore volume V 100 of a pore with a diameter of 100 nm is 1 or more. That is, it is preferable that the Si-containing particles 67 have a nanoporous structure in which there are more pores with a relatively small diameter (e.g., pores with a diameter of 10 nm) than pores with a relatively large diameter (e.g., pores with a diameter of 100 nm). This allows the initial charge/discharge efficiency and cycle characteristics of the secondary battery 100 to be suitably compatible. The ratio (V ₁₀ /V ₁₀₀ ) of V ₁₀ to V ₁₀₀ described above is preferably greater than 1, more preferably 1.2 or more, and may be 1.5 or more. Moreover, the ratio of _V10 to _V100 ( _V10 / _V100 ) is preferably, for example, 20 or less, and may be 10 or less.

The log differential pore volume V ₁₀₀ of a pore having a diameter of 100 nm and the log differential pore volume V ₁₀ of a pore having a diameter of 10 nm can be calculated based on the BJH method using a specific surface area and pore distribution measuring device. First, the Si-containing particles are heated and dried under vacuum to obtain a measurement sample. Next, the adsorption isotherm of the measurement sample is obtained using liquid nitrogen as a refrigerant and nitrogen gas (N ₂ gas) as an adsorption gas, and the obtained adsorption isotherm is analyzed by the BJH method to obtain the log differential pore volume distribution. Then, the log differential pore volume V ₁₀₀ of a pore having a diameter of 100 nm and the log differential pore volume V ₁₀ of a pore having a diameter of 10 nm can be obtained from the log differential pore volume distribution.

The Si-containing particles 67 have Si nanoparticles 67a with a mesh-like structure. The Si nanoparticles 67a are nano-sized (i.e., less than 1 μm) Si particles. The Si nanoparticles 67a may be present on the surface of the porous body and/or inside the pores 67b of the porous body. The Si nanoparticles 67a are preferably 100 nm or less, and more preferably 50 nm or less. This makes it possible to reduce the amount of expansion and contraction per particle of the Si nanoparticles 67a during charging and discharging, and the particles are less likely to break even if the expansion and contraction are repeated. Although not particularly limited, the average particle diameter of the Si nanoparticles 67a may be, for example, 5 nm or more. In this specification, the "average particle diameter of the Si nanoparticles" can be determined as follows. First, the negative electrode active material layer is processed with FIB (focused ion beam) to prepare a sample for observation with a scanning transmission electron microscope (STEM). Then, the sample is subjected to elemental analysis by EDX element mapping, and then a BF image (bright field image) and a HAADF image (high angle scattering annular dark field image) are obtained. The diameter of the Si nanoparticles can be determined from the contrast and shape obtained from the BF image and the HAADF image. The arithmetic mean of the diameters of at least 10 Si nanoparticles is defined as the "average particle size of the Si nanoparticles" here.

The Si nanoparticles 67a have a mesh structure. In this mesh structure, multiple voids are formed randomly or regularly. Because the Si nanoparticles 67a have a mesh structure, the conductive path is suitably improved. In addition, because the Si nanoparticles 67a have a mesh structure, excessive expansion and contraction of the Si nanoparticles 67a due to charging and discharging is suppressed.

Although not particularly limited, it is preferable that the Si-containing particle 67 has a plurality of pores 67b around the above-mentioned Si nanoparticles 67a. In particular, it is preferable that there are many small pores (e.g., pores with a diameter of 10 nm or less) around the Si nanoparticles 67a. This makes it possible to appropriately suppress the erosion of the electrolyte while mitigating the expansion and contraction that accompanies charging and discharging.

Although not particularly limited, the oxygen content of the Si-containing particles is preferably, for example, 10 wt % or less when the entire Si-containing particles are taken as 100 wt %. This can reduce side reactions caused by an excessive oxygen content, and can favorably improve the capacity and cycle characteristics of the secondary battery. The oxygen content can be measured by heating and melting in an inert gas using an oxygen analyzer.

The _D50 particle size of the Si-containing particles 67 is not particularly limited, but is preferably 1 μm or more and 15 μm or less, and more preferably 2 μm or more and 10 μm or less. In this specification, the " _D50 particle size of the Si-containing particles" refers to a particle size that corresponds to a cumulative 50% from the fine particle side in a volume-based particle size distribution measured by a particle size distribution measurement based on a laser diffraction/light scattering method.

The above-mentioned Si-containing particles 67 can be obtained, for example, by firing a plant containing Si. That is, the Si-containing particles 67 are preferably derived from plants. Specifically, rice (rice), barley, wheat, rye, and other rice husks, coconut shells, tea leaves, sugar cane, corn, and other plants are preferably used as raw materials. Among them, it is preferable that the Si-containing particles 67 are made from rice husks. Plants accumulate silicic acid absorbed from the soil around the cell walls. By firing this, a porous body containing Si nanoparticles 67a having a network structure derived from plants can be obtained. However, the Si-containing particles 67 may also be prepared by preparing a porous body mainly composed of Si or C and Si nanoparticles having a network structure, and introducing the Si nanoparticles into the porous body.

The negative electrode active material layer 64 may contain components other than the graphite particles 66 and Si-containing particles described above as the negative electrode active material (e.g., SiOx or hard carbon that does not have the structure described above) as long as the effect of the present technology is not significantly impaired.

Although not particularly limited, when the total weight of the negative electrode active material (the total weight of the graphite particles 66, the Si-containing particles 67, and the weight of other components that may be included as the negative electrode active material) is 100 wt%, the content of the Si-containing particles 67 is preferably 10 wt% to 60 wt%, and more preferably 20 wt% to 40 wt%. When the total weight of the negative electrode active material is 100 wt%, the content of the graphite particles 66 is preferably 40 wt% to 90 wt%, and more preferably 60 wt% to 80 wt%. That is, the weight ratio of the graphite particles 66 to the Si-containing particles is preferably adjusted to 90:10 to 40:60, and may be adjusted to 80:20 to 60:40. By adjusting the weight of the Si-containing particles 67 to the above range, the initial charge/discharge efficiency and the improvement of the cycle characteristics can be favorably achieved at the same time.

In the secondary battery 100 disclosed herein, carbon nanotubes (CNTs) 68 are used as the conductive material. The carbon nanotubes 68 are fibrous carbon having a structure in which graphene, which forms a carbon hexagonal network, is rolled into a cylindrical shape. The carbon nanotubes 68 have a high aspect ratio and excellent conductivity. For this reason, the carbon nanotubes 68 are easily entangled with the graphite particles 66 and the Si-containing particles 67, and the conductive path is favorably maintained. In addition, as described above, the Si-containing particles 67 have multiple pores 67b, and the carbon nanotubes 68 are arranged in these pores 67b, so that the carbon nanotubes 68 are unlikely to come off the Si-containing particles 67 even if they are repeatedly expanded and contracted due to charging and discharging, and the conductive path is favorably maintained.

Examples of carbon nanotubes 68 include single-walled carbon nanotubes (SWCNT) composed of one layer of graphene, double-walled carbon nanotubes (DWCNT) composed of two different SWCNTs, and multi-walled carbon nanotubes (MWCNT) composed of three or more different SWCNTs. From the viewpoint of further improving the capacity of the secondary battery 100, single-walled carbon nanotubes (SWCNT) are preferred.

Although not particularly limited, the average length of the carbon nanotubes 68 is preferably 1 μm or more and 10 μm or less, and more preferably 1 μm or more and 5 μm or less. When the carbon nanotubes 68 have a length in this range, the carbon nanotubes 68 are properly dispersed and a suitable conductive path is formed. In addition, the average diameter of the carbon nanotubes 68 is not particularly limited, but is preferably 1 nm or more and 100 nm or less, and more preferably 10 nm or more and 50 nm or less. The average length and average diameter of the carbon nanotubes 68 can be calculated, for example, by taking an electron microscope photograph of the carbon nanotubes, measuring the lengths and diameters of 30 or more carbon nanotubes, and averaging the measured values.

The Si-containing particles 67 have pores 67b, and the weight ratio between the Si-containing particles 67 and the carbon nanotubes 68 is appropriately adjusted, so that the carbon nanotubes 68 are suitably arranged around the Si-containing particles 67 (specifically, at least a part of the pores 67b of the Si-containing particles 67). This allows a conductive path to be suitably formed in the negative electrode active material layer 64, improving the initial charge/discharge efficiency. When the weight of the Si-containing particles 67 is 100 wt%, the weight ratio X of the carbon nanotubes 68 to the Si-containing particles 67 is preferably 0.02 wt% or more and 4 wt% or less, more preferably 0.2 wt% or more and 1 wt% or less, and even more preferably 0.2 wt% or more and 0.6 wt% or less. In other words, in the secondary battery 100 disclosed herein, the ratio of the carbon nanotube 68 content (wt%) to the Si-containing particle 67 content (wt%) (CNT/Si-containing particle) is preferably 0.0002 or more and 0.04 or less, more preferably 0.002 or more and 0.01 or less, and even more preferably 0.002 or more and 0.006 or less.

Although not particularly limited, when the total weight of the negative electrode active material is taken as 100 wt%, the content of the carbon nanotubes is preferably 0.01 wt% to 1.4 wt%, more preferably 0.1 wt% to 1 wt%, and even more preferably 0.1 wt% to 0.2 wt%. That is, the ratio of the negative electrode active material and the carbon nanotubes, by weight, is preferably negative electrode active material:CNT=100:0.01-100:1.4, more preferably negative electrode active material:CNT=100:0.1-100:1, and even more preferably negative electrode active material:CNT=100:0.1-100:0.2.

The negative electrode active material layer 64 may contain components (e.g., binders, etc.) other than the above-mentioned negative electrode active material (graphite particles 66 and Si-containing particles 67) and conductive material (carbon nanotubes). Conventionally known binders can be used. Examples of binders include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), etc. Among them, CMC, PAA, and SBR are preferably used. Although not particularly limited, it is more preferable to use CMC, PAA, and SBR in combination.

The total binder content is, for example, 1 wt% or more, preferably 3 wt% or more, and more preferably 3.5 wt% or more, when the negative electrode active material is 100 wt%. The total binder content is, for example, 10 wt% or less, preferably 8 wt% or less, and more preferably 5 wt% or less, when the negative electrode active material is 100 wt%.

It is presumed that because the Si-containing particles 67 are porous bodies having multiple pores, not only the carbon nanotubes 68 but also the binder (not shown) are arranged around the Si-containing particles 67. This makes it more difficult for the carbon nanotubes 68 to come off the Si-containing particles 67, which can improve the initial charge/discharge efficiency and cycle characteristics.

The thickness of each side of the negative electrode active material layer 64 is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

Although not particularly limited, the proportion of the negative electrode active material in the entire negative electrode active material layer 64 is, for example, 80 mass% or more, preferably 90 mass% or more, and more preferably 95 mass% or more. In addition, although not particularly limited, the proportion of the negative electrode active material in the entire negative electrode active material layer 64 may be, for example, 98 mass% or less.

<Secondary Battery Manufacturing Method>
As described above, the secondary battery 100 disclosed herein includes graphite particles 66, Si-containing particles 67, and carbon nanotubes 68, and the carbon nanotubes 68 are disposed in at least some of the pores of the Si-containing particles 67. Such a secondary battery 100 can be manufactured, for example, as follows.

Figure 4 is a flow chart showing one preferred embodiment of the method for manufacturing the secondary battery 100 disclosed herein. As shown in Figure 4, the method for manufacturing the secondary battery 100 disclosed herein includes a preparation step S10 for preparing each material, and a mixing step S20 for mixing the prepared materials. The mixing step S20 preferably includes a first mixing step S21 for mixing Si-containing particles and carbon nanotubes to prepare a first mixture, and a second mixing step S22 for mixing the first mixture, graphite particles, a binder, and a solvent to prepare a second mixture. However, the manufacturing method disclosed herein may further include other steps at any stage.

In the preparation step S10, at least the graphite particles 66, the Si-containing particles 67, and the carbon nanotubes 68 are prepared. In addition, in the preparation step S10, other necessary components (for example, a binder, a solvent, etc.) may be prepared. As described above, natural graphite or artificial graphite can be preferably used as the graphite particles 66. As the Si-containing particles 67, a porous body containing Si nanoparticles 67a having a mesh-like structure is prepared. As such Si-containing particles 67, for example, Si-C composites and/or Si particles derived from plants may be prepared. More preferably, as the Si-containing particles 67, Si-C composites and/or Si particles derived from rice husks may be prepared. As the carbon nanotubes 68, a water-soluble paste having a solid content of about 1% to 10% may be prepared.

As the binder, those exemplified above can be used without any particular limitation. As the solvent, both aqueous and non-aqueous solvents can be used. Typically, water or a mixed solvent mainly composed of water is preferably used. As the solvent component other than water constituting such a mixed solvent, one or more organic solvents (lower alcohols, lower ketones, etc.) that can be mixed uniformly with water can be appropriately selected and used. For example, it is preferable to use an aqueous solvent in which 80% by mass or more (more preferably 90% by mass or more, and even more preferably 95% by mass or more) of the aqueous solvent is water. A particularly preferred example is an aqueous solvent that is essentially composed of water.

The mixing step S20 may include a first mixing step S21 and a second mixing step S22. In the first mixing step S21, the Si-containing particles 67 and the carbon nanotubes 68 are mixed to prepare a first mixture. By mixing the Si-containing particles 67 and the carbon nanotubes 68 in advance and then mixing them with other materials, the carbon nanotubes 68 can be suitably arranged in some of the pores 67b of the Si-containing particles 67. This makes it easier for a conductive path to be formed, and the initial charge/discharge efficiency of the secondary battery 100 can be improved.

In the first mixing step S21, for example, powdered Si-containing particles 67 and paste-like carbon nanotubes 68 are fed into a stirring device and mixed. The stirring device is not particularly limited as long as it has a rotating stirrer (a stirring blade such as a dispersion blade or a turbine blade, a stirring vane). The rotation speed of the stirrer is not particularly limited, but may be, for example, about 1000 rpm to 5000 rpm.

In the second mixing step S22, the first mixture produced in the first mixing step S21 is mixed with the graphite particles 66, a binder, and a solvent to produce a second mixture. There are no particular limitations on the mixing device, and a mixing device similar to that used in the first mixing step S21 may be used. Although not particularly limited, when a powdered binder is used in the second mixing step S22, it is advisable to dry-mix the powdered graphite particles 66 and the binder, and then add the first mixture and a solvent to perform a solid kneading process. This can improve dispersibility.

The second mixture prepared above is applied to the negative electrode current collector 62 and dried. If necessary, a process of drying and pressing the negative electrode active material layer 64 arranged on the negative electrode current collector 62 may be performed. This allows the thickness and density of the negative electrode active material layer 64 to be adjusted.

The positive electrode active material layer 54 can be formed by dispersing the positive electrode active material, conductive material, and binder in an appropriate solvent (e.g., NMP) to prepare a paste (or slurry) composition, applying the composition to the surface of the positive electrode current collector 52, and drying it. Thereafter, the thickness and density of the positive electrode active material layer 54 can be adjusted by pressing as necessary.

The negative electrode 60 and positive electrode 50 prepared as above are stacked so that they are insulated by two separators 70. If necessary, the prepared stack is wound in the longitudinal direction around the winding axis, and the wound stack is pressed to prepare a flat-shaped wound electrode body. The wound electrode body is housed in a battery case 30, and nonaqueous electrolyte is poured through the liquid injection hole. The liquid injection hole is then sealed to hermetically seal the secondary battery 100. In this manner, the secondary battery 100 can be manufactured.

The above describes the configuration of the secondary battery 100 according to one embodiment and the manufacturing method of the secondary battery 100. The secondary battery 100 includes graphite particles 66, Si-containing particles 67, and carbon nanotubes 68. The carbon nanotubes 68 are arranged in at least a part of the pores 67b of the Si-containing particles 67, thereby improving the initial charge/discharge efficiency. Furthermore, the carbon nanotubes 68 are arranged in the pores 67b, which suitably suppresses the conductive path from being broken due to expansion and contraction caused by repeated charging and discharging, thereby improving the cycle characteristics of the secondary battery 100. The secondary battery 100 can be used for various purposes, but can be suitably used, for example, as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or truck. In particular, it is suitable as a power source for an electric vehicle (BEV; Battery Electric Vehicle). The secondary battery 100 can also be suitably used in the construction of a battery pack.

In the above-described secondary battery 100, a wound electrode body is exemplified as the electrode body 20, but this is not limited thereto. For example, the electrode body may be a laminated electrode body in which multiple approximately rectangular positive electrodes and multiple approximately rectangular negative electrodes are alternately stacked with separators interposed therebetween.

Test examples relating to the present invention are described below, but the present invention is not intended to be limited to those shown in the following test examples.

1. First Test In the first test, the type of Si-containing particles, the weight ratio of the Si-containing particles to the CNTs, and the mixing method were each changed, and the initial charge/discharge efficiency of the secondary battery was evaluated.

<Example 1>
First, graphite particles and Si-containing particles were prepared as the negative electrode active material. The Si-containing particles in Example 1 were Si-C composite particles derived from plants using rice husks as a raw material. Single-walled carbon nanotubes (SWCNT) were prepared as the conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as the binder. These were kneaded with water as a solvent so that the graphite particles:Si-containing particles:SWCNT:CMC:PAA:SBR=65:35:0.1:1:1:1.5 by weight was obtained, and a slurry for forming the negative electrode active material layer was prepared.

Specifically, the mixing and kneading of the slurry for forming the negative electrode active material layer was carried out as follows. First, the Si-containing particles, the SWCNT paste (solid content rate 2%), and the solvent were put into a container. The mixture was mixed at a rotation speed of 3000 rpm using a disper to prepare a first mixture. Next, the graphite particles, the CMC, and the PAA were dry-mixed using an agitation granulator, and the first mixture prepared above and the solvent were added to this mixed powder and kneaded. The solid content rate during the kneading was 65%. SBR and a solvent were further added to the kneaded mixture and mixed. In this way, the slurry for forming the negative electrode active material layer was prepared.
The solid content C (%) during kneading was determined as follows. Water was added to the mixed powder of graphite particles and Si-containing particles in the above ratio, and the moisture content at which the torque required for mixing was maximized was defined as A (%). The amount of moisture per 100 g of mixed powder corresponding to the moisture content A (%) was defined as B (ml). In this case, the solid content C (%) at which the maximum torque could be obtained could be calculated from the formula: C (%) = 100 - A = (100/(100 + B)) × 100.

The slurry for forming the negative electrode active material layer prepared above was applied in strips to both sides of a copper foil (thickness 10 μm). The slurry on the copper foil was then dried, pressed to a specified thickness, and processed to the specified dimensions to produce a negative electrode sheet.

Next, lithium nickel cobalt manganese composite oxide (NCM) was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and PVDF as the binder. These were mixed with N-methylpyrrolidone (NMP) as the solvent so that the weight ratio of NCM:AB:PVDF was 100:1:1 to prepare a slurry for forming the positive electrode active material layer. This slurry was applied in strips to both sides of an aluminum foil (thickness 15 μm). The slurry on the aluminum foil was then dried, pressed to a specified thickness, and processed to the specified dimensions to produce a positive electrode sheet.

The negative electrode sheet and the positive electrode sheet prepared above were laminated with a separator interposed therebetween to prepare a laminated electrode body. A current collecting lead was attached to each of the positive and negative electrode plates, and the laminated electrode body was inserted into an exterior body made of an aluminum laminate sheet. A non-aqueous electrolyte was injected into the interior of the exterior body, and the opening of the exterior body was sealed to prepare an evaluation battery of Example 1. As the separator, a porous polyolefin sheet having a three-layer structure of PP/PE/PP was used. As the non-aqueous electrolyte, a mixed solvent in which ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed to a volume ratio of EC:FEC:EMC:DMC=15:5:40:40 was used, in which LiPF ₆ as a supporting salt was dissolved at a concentration of 1 mol/L.

Table 1 shows the content (wt%) of the Si-containing particles when the weight of the negative electrode active material is 100 wt%, the weight ratio X (wt%) of the SWCNTs to the Si-containing particles when the weight of the Si-containing particles is 100 wt%, and the content (wt%) of the SWCNTs when the weight of the negative electrode active material is 100 wt%. The weight ratio X of the SWCNTs can be calculated from the formula: weight ratio X of the SWCNTs = ((SWCNT content)/(Si-containing particle content)) x 100.

<Examples 2, 3, 5 and 6>
The evaluation batteries of Examples 2, 3, 5, and 6 were prepared in the same manner as in Example 1, except that the weight ratio X (wt%) of the SWCNTs to the Si-containing particles, when the weight of the Si-containing particles was taken as 100 wt%, was changed as shown in Table 1.

<Example 4>
In Example 4, the step of premixing the Si-containing particles and SWCNT was not performed. That is, the Si-containing particles, the graphite particles, the CMC, and the PAA were dry-mixed using an agitation granulator, and the mixed powder was added with the paste-like SWCNT (solid content rate of 2%) and a solvent, and kneaded. The solid content rate during the kneading was 65%. SBR and a solvent were further added to the kneaded mixture and mixed. In this way, a slurry for forming a negative electrode active material layer was prepared. The evaluation battery of Example 4 was produced in the same manner as in Example 1, except as described above.

<Example 7>
The evaluation battery of Example 7 was produced in the same manner as in Example 1, except that Si--C composite particles produced by a CVD method were prepared as the Si-containing particles.

<Evaluation of initial charge/discharge efficiency>
In the first cycle, the battery was charged in a 25° C. environment using CCCV charging (at a rate of 0.05 C up to 4.2 V, then cut off at 0.05 C), and then discharged in CC mode (at a rate of 0.05 C to 2.5 V). The first charge capacity and the first discharge capacity were measured. The initial charge/discharge efficiency was calculated using the following formula (1). The results are shown in Table 1.
Initial charge/discharge efficiency (%)=((initial discharge capacity)/(initial charge capacity))×100 Formula (1)

As shown in Table 1, the evaluation batteries of Examples 1 to 4 have an initial charge/discharge efficiency of 82% or more. These results show that a secondary battery with excellent initial charge/discharge efficiency is realized by using graphite particles as the negative electrode active material and Si-containing particles, which are porous bodies containing Si nanoparticles with a mesh-like structure, with the weight ratio X of carbon nanotubes to the Si-containing particles being 0.02 wt% or more and 4 wt% or less when the weight of the Si-containing particles is taken as 100 wt%, and CNTs are arranged in at least some of the pores of the porous body.

In addition, when comparing Examples 1 to 3 in Table 1 with Example 4, it can be seen that by performing the first mixing step in which the Si-containing particles and SWCNTs are premixed and then the second mixing step, the CNTs are more easily arranged in the pores of the porous body, further improving the initial charge/discharge efficiency.

2. Second Test In the second test, the weight ratio of the Si-containing particles to the CNTs and the pore distribution ratio of the porous body were changed, and the cycle characteristics of the secondary battery were evaluated.

<Example 11>
An evaluation battery of Example 11 was produced in the same manner as in Example 1, except that the pore distribution ratio (V ₁₀ /V ₁₀₀ ) of the porous body of the Si-containing particles was 1.5.

<Examples 12 to 15>
The Si-containing particles were prepared so that the pore distribution ratio (V ₁₀ /V ₁₀₀ ) of the porous body was the value shown in Table 1. In addition, the weight ratio X of the carbon nanotubes to the Si-containing particles, where the weight of the Si-containing particles was taken as 100 wt %, was changed as shown in Table 1. Except for these, the evaluation batteries of Examples 12 to 15 were produced in the same manner as in Example 1.

<Calculation of pore distribution ratio>
The pore distribution ratio was calculated based on the BJH method using a specific surface area/pore distribution measuring device (ASAP2020 manufactured by Micromeritics). First, the Si-containing particles were heated and dried under vacuum to obtain a measurement sample. Next, the adsorption isotherm of the measurement sample was obtained using liquid nitrogen as a refrigerant and nitrogen gas ( _N2 gas) as an adsorption gas. The obtained adsorption isotherm was analyzed by the BJH method to obtain the log differential pore volume distribution. Then, from the log differential pore volume distribution, the log differential pore volume _V100 of the pores having a diameter of 100 nm and the log differential pore volume _V10 of the pores having a diameter of 10 nm were obtained, and the ratio of _V10 to _V100 ( _V10 / _V100 ) was obtained. The results are shown in Table 2.

<Evaluation of cycle capacity retention rate>
A cycle test was performed in which 250 cycles of charge and discharge were repeated, with one cycle being CCCV charging (0.4C rate up to 4.2V, then 0.1C cut) and then CC discharging (0.4C rate, 2.5V cut). The discharge capacity (initial capacity) at the first cycle and the discharge capacity at the 250th cycle were measured, and the cycle capacity retention rate was calculated by the following formula (2). It can be said that the higher the cycle capacity retention rate, the higher the cycle characteristics of the secondary battery. The results are shown in Table 2.
Cycle capacity retention rate (%)=((discharge capacity at 250th cycle)/(discharge capacity at 1st cycle))×100 (2)

As shown in Table 2, the capacity retention rate of the evaluation batteries of Examples 11 to 13 is 79% or more. These results show that the negative electrode active material is graphite particles, and the Si-containing particles are porous bodies containing Si nanoparticles with a mesh-like structure, and when the weight of the Si-containing particles is taken as 100 wt%, the weight ratio X of the carbon nanotubes to the Si-containing particles is 0.02 wt% or more and 4 wt% or less, and the CNTs are arranged in at least some of the pores of the porous body, thereby improving the cycle characteristics of the secondary battery.

Comparing Example 13 with Examples 11 and 12 in Table 2, it can be seen that the capacity retention rate is 86% or more when the pore distribution ratio ( _V10 / _V100 ) of the porous body is 1 or more. In other words, when the pore distribution ratio ( _V10 / _V100 ) of the porous body is 1 or more, the cycle characteristics of the secondary battery can be further improved.

Although several embodiments of the present invention have been described above, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. The technology described in the claims includes various modifications and changes to the above-exemplified embodiments. For example, it is possible to replace part of the above-mentioned embodiments with other modified forms, and it is also possible to add other modified forms to the above-mentioned embodiments. Furthermore, if a technical feature is not described as essential, it can also be deleted as appropriate.

As described above, specific aspects of the technology disclosed herein include those described in the following sections.
Item 1: A secondary battery comprising an electrode assembly having a positive electrode and a negative electrode, the negative electrode comprising a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer including graphite particles and Si-containing particles as negative electrode active materials and carbon nanotubes as a conductive material, the Si-containing particles being a porous body containing Si nanoparticles in a network structure, the carbon nanotubes being disposed in at least some pores of the porous body, and a weight ratio X of the carbon nanotubes to the Si-containing particles being 0.02 wt % or more and 4 wt % or less when the weight of the Si-containing particles is taken as 100 wt %.
Item 2: The secondary battery according to item 1, wherein the Si-containing particles are derived from plants.
Item 3: The secondary battery according to item 1 or 2, wherein the Si-containing particles have pores having a diameter of 100 nm or more and pores having a diameter of 10 nm or less, and a ratio of V ₁₀ to V ₁₀₀ (V ₁₀ /V ₁₀₀ ) is 1 or more, where V ₁₀₀ is a log differential pore volume of a pore having a diameter of 100 nm and V ₁₀ is a log differential pore volume of a pore having a diameter of 10 nm.
Item 4: The secondary battery according to any one of items 1 to 3, wherein the Si nanoparticles have an average particle diameter of 50 nm or less.
Item 5: The secondary battery according to any one of items 1 to 4, wherein the Si-containing particles are Si-C composite compounds containing the network-structured Si nanoparticles and porous carbon particles, and/or Si particles containing the network-structured Si nanoparticles and porous Si particles.
Item 6: The secondary battery according to any one of items 1 to 5, wherein the Si-containing particles have an oxygen content of 10 wt % or less.
Item 7: The secondary battery according to any one of items 1 to 6, wherein the content of the Si-containing particles is 10 wt % or more and 60 wt % or less when the total weight of the negative electrode active material is 100 wt %.
Item 8: A preparation step of preparing at least graphite particles and Si-containing particles as a negative electrode active material, and carbon nanotubes as a conductive material;
A method for manufacturing a secondary battery, comprising: a first mixing step of mixing the prepared Si-containing particles with carbon nanotubes to prepare a first mixture; and a second mixing step of mixing the first mixture with the graphite particles, a binder, and a solvent to prepare a second mixture, wherein the Si-containing particles prepared in the preparation step are a porous body containing network-structured Si nanoparticles.
Item 9: The manufacturing method according to item 8, wherein in the first mixing step, when the weight of the Si-containing particles is 100 wt %, a weight ratio X of the carbon nanotubes to the Si-containing particles is 0.02 wt % or more and 4 wt % or less.
Item 10: The manufacturing method according to item 8 or 9, wherein in the second mixing step, the content of the graphite particles is 40 wt % or more and 90 wt % or less when the weight of the negative electrode active material is 100 wt %.

20 Electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode (positive electrode sheet)
52 Positive electrode current collector 52a Positive electrode current collector exposed portion 54 Positive electrode active material layer 60 Negative electrode (negative electrode sheet)
62 Negative electrode current collector 62a Negative electrode current collector exposed portion 64 Negative electrode active material layer 66 Graphite particles 67 Si-containing particles 67a Si nanoparticles 67b Pore 68 Carbon nanotube (CNT)
70 Separator 100 Secondary battery

Claims (10)

A secondary battery including an electrode assembly having a positive electrode and a negative electrode,
the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector,
The negative electrode active material layer contains graphite particles and Si-containing particles as a negative electrode active material, and carbon nanotubes as a conductive material,
The Si-containing particle is a porous body containing Si nanoparticles having a network structure,
the carbon nanotubes are disposed in at least some of the pores of the porous body;
A secondary battery, wherein a weight ratio X of the carbon nanotubes to the Si-containing particles is 0.02 wt % or more and 4 wt % or less when the weight of the Si-containing particles is taken as 100 wt %. The secondary battery according to claim 1, wherein the Si-containing particles are derived from plants. The Si-containing particle has pores having a diameter of 100 nm or more and pores having a diameter of 10 nm or less,
3. The secondary battery according to claim 1, wherein when _V100 is a log differential pore volume of a pore having a diameter of ₁₀₀ nm and V10 is a log differential pore volume of a pore having a diameter of ₁₀ nm, the ratio of V10 to _V100 ( _V10 / _V100 ) is 1 or more. The secondary battery according to claim 1 or 2, wherein the Si nanoparticles have an average particle size of 50 nm or less. The secondary battery according to claim 1 or 2, wherein the Si-containing particles are Si-C composite compounds containing the network-structured Si nanoparticles and porous carbon particles, and/or Si particles containing the network-structured Si nanoparticles and porous Si particles. The secondary battery according to claim 1 or 2, wherein the Si-containing particles have an oxygen content of 10 wt% or less. The secondary battery according to claim 1 or 2, wherein the content of the Si-containing particles is 10 wt% or more and 60 wt% or less when the total weight of the negative electrode active material is 100 wt%. A preparation step of preparing at least graphite particles and Si-containing particles as a negative electrode active material, and carbon nanotubes as a conductive material;
a first mixing step of mixing the prepared Si-containing particles and carbon nanotubes to prepare a first mixture;
a second mixing step of mixing the first mixture, the graphite particles, a binder, and a solvent to prepare a second mixture;
Including,
The method for producing a secondary battery, wherein the Si-containing particles prepared in the preparing step are porous bodies containing Si nanoparticles in a network structure. The manufacturing method according to claim 8, wherein in the first mixing step, when the weight of the Si-containing particles is taken as 100 wt%, the weight ratio X of the carbon nanotubes to the Si-containing particles is 0.02 wt% or more and 4 wt% or less. The manufacturing method according to claim 8 or 9, wherein in the second mixing step, the content of the graphite particles is 40 wt% or more and 90 wt% or less when the weight of the negative electrode active material is 100 wt%.

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