Secondary battery and method of manufacturing the same

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

[Technical Field]

[0001] The present invention relates to a secondary battery and a method for manufacturing the secondary battery. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are suitably used as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc. 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.

[0003] In recent years, the use of Si-based materials as negative electrode active materials has been investigated with the aim of increasing the capacity of secondary batteries (e.g., 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 plant-derived raw materials. Patent Document 3 discloses a negative electrode containing a negative electrode active material containing a Si-based material, carbon nanotubes with an outermost diameter of 5 nm or less, and carboxymethylcellulose with a weight-average molecular weight of 150,000 to 450,000. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2018-534720 [Patent Document 2] Patent Publication No. 2021-38114 [Patent Document 3] International Publication No. 2022 / 070895 Summary of the Invention [Problem to be solved by the invention]

[0005] While Si-based materials have a larger specific capacity than carbon materials such as graphite particles, they also tend to expand and contract significantly during charging and discharging, making the conductive paths more susceptible to breakage. Therefore, when Si-based materials are used, the initial characteristics of secondary batteries (e.g., initial charge / discharge efficiency) tend to decrease. Therefore, when Si-based materials are used as negative electrode active materials, there is still room for improvement in improving the initial characteristics of secondary batteries.

[0006] The present invention has been made in view of the above points, and aims to provide a secondary battery having a negative electrode containing graphite particles and a Si-based material as a negative electrode active material, and having excellent initial charge / discharge efficiency. [Means for solving the problem]

[0007] The secondary battery disclosed herein is 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 includes 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 network 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.

[0008] According to this configuration, the Si-containing particles are porous bodies containing Si nanoparticles with a network structure, which allows for the formation of conductive paths. Furthermore, the Si-containing particles have multiple pores, and the carbon nanotubes are suitably arranged in the pores, which allows for the formation of conductive paths between the graphite particles and the Si-containing particles. Therefore, a secondary battery with excellent initial characteristics can be realized. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery according to one embodiment. [Figure 2] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly according to one embodiment. [Figure 3] FIG. 3 is a diagram schematically illustrating a negative electrode according to one embodiment. [Figure 4] FIG. 4 is a flowchart showing a method for manufacturing a secondary battery according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. Matters necessary for implementing the technology disclosed herein, other than those specifically mentioned in this specification (e.g., the general configuration and manufacturing process of a secondary battery that do not characterize the technology disclosed herein), can be understood as design matters for a person skilled in the art based on conventional technology 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. Each drawing is a schematic representation, and dimensional relationships (e.g., length, width, thickness) do not necessarily reflect actual dimensional relationships. In the drawings described below, components and parts that perform the same function are designated by the same reference numerals, and redundant descriptions may be omitted or simplified. In this specification, the notation "A to B" (A and B are arbitrary numbers) indicating a range means A or greater and B or less.

[0011] 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 a positive electrode and a negative electrode. 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.

[0012] FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery 100 according to one embodiment. As shown in FIG. 1, the secondary battery 100 includes an electrode assembly 20 having a positive electrode 50 and a negative electrode 60, an electrolyte (not shown), and a battery case 30 that accommodates the electrode assembly 20 and the electrolyte. The secondary battery 100 shown in FIG. 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.

[0013] 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 configured to release 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 a 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. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0014] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly 20. Here, the electrode assembly 20 is a flat-shaped wound electrode assembly. As shown in FIG. 2, the electrode assembly 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 together with two long separators 70 interposed therebetween 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 in this case) of a long 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 (both sides in this case) of a long negative electrode current collector 62. 1 and 2, the positive electrode current collector exposed portion 52a (i.e., a portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54) and the negative electrode current collector exposed portion 62a (i.e., a portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the electrode body 20. A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined to the positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a, respectively.

[0015] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current 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 preferred as the positive electrode current collector 52. The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on 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 to 35 μm, and preferably 7 μm to 20 μm.

[0016] The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a positive electrode active material of a known composition used in lithium ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds (e.g., lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4)). 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.

[0017] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, 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 is preferably used as the positive electrode active material.

[0018] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. 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 metalloid 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 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.

[0019] 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. Suitable conductive materials include 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). Suitable binders include polyvinylidene fluoride (PVdF).

[0020] 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%, based on 100 wt% of the positive electrode active material, and the content of the binder is preferably 0.1 wt% to 10 wt%, and more preferably 1 wt% to 5 wt%, based on 100 wt% of the positive electrode active material.

[0021] The thickness of the positive electrode active material layer 54 per side 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.

[0022] As the separator 70, various conventional microporous sheets 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). Separator 70 may also have a heat-resistant layer (HRL).

[0023] Conventional electrolytes can be used, for example, nonaqueous electrolytes containing a supporting salt in an organic solvent (nonaqueous solvent). Nonaqueous solvents include aprotic solvents such as carbonates, esters, and ethers. Among these, carbonates, such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), are preferred. 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) are preferred. These nonaqueous solvents can be used alone or in appropriate combinations of two or more. Lithium salts, such as LiPF, LiBF, and LiClO, are preferred as supporting salts. 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. The nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt described above, as 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 dispersant, and a thickener.

[0024] The negative electrode 60 of the secondary battery disclosed herein will now be described. 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 current collector 62 and a negative electrode active material layer 64 disposed on the negative electrode current collector 62. The negative electrode current collector 62 may be a conventionally known one and is not particularly limited. Examples include a sheet or foil made of a metal such as copper, nickel, titanium, or stainless steel. When copper foil is used as the negative electrode current collector 62, its average thickness is not particularly limited, but is, for example, 5 μm to 30 μm, preferably 5 μm to 20 μm, and more preferably 5 μm to 15 μm.

[0025] 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 network structure. The Si-containing particles 67 have a plurality of pores 67b. Carbon nanotubes 68 are disposed in at least some of the pores 67b. When the weight of the Si-containing particles 67 is taken as 100 wt%, 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. This configuration can improve the initial charge / discharge efficiency of the secondary battery 100.

[0026] While not intending to limit the technology disclosed herein, the reason for such effects is presumed to be as follows. The Si-containing particles 67 are porous bodies containing Si nanoparticles 67a with a network structure, which favorably improves the conductive path. Furthermore, the Si-containing particles 67 have a plurality of pores 67b, which favorably arrange the carbon nanotubes 68 serving as conductive materials around the Si-containing particles 67, thereby forming a favorable charging path between the graphite particles 66 and the Si-containing particles 67. This improves the initial charge / discharge efficiency of the secondary battery 100. Furthermore, even with repeated expansion and contraction due to charging and discharging, the carbon nanotubes 68 are less likely to come off the plurality of pores 67b due to the anchoring effect. Furthermore, the network structure of the Si nanoparticles 67a can suppress the expansion of the Si nanoparticles 67a, making the Si nanoparticles 67a less likely to be damaged by expansion. Therefore, even with repeated expansion and contraction due to charging and discharging, the conductive path is less likely to be broken, improving the cycle characteristics of the secondary battery 100.

[0027] 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, the graphite particles 66 are preferably substantially spherical. In this specification, the term "substantially spherical" encompasses spherical shapes, rugby ball shapes, etc., and refers to particles having an average aspect ratio (the ratio of the length in the superaxial direction to the length in the minor axis direction in the smallest rectangle circumscribing the particle) of, for example, 1 to 2 (preferably 1 to 1.5).

[0028] Graphite particles 66D 50 The particle diameter is not particularly limited, but is preferably, for example, 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 25 μm or less. 50 "Particle size" refers to the particle size corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by particle size distribution measurement based on the laser diffraction / light scattering method.

[0029] The Si-containing particles 67 are porous bodies containing Si nanoparticles 67a with a network structure. The Si-containing particles 67 may contain components other than Si, as long as they contain Si. Examples of the Si-containing particles 67 include SiOx, Si-C composites, and porous Si particles with Si nanoparticles dispersed therein. The porous portion of the Si-containing particles 67 may be composed primarily of Si or carbon (C). For example, a Si-C composite containing Si nanoparticles with a network structure and porous carbon particles is preferably used as the Si-containing particles 67. Alternatively, a Si particle containing Si nanoparticles with a network structure and porous Si particles is preferably used as the Si-containing particles 67. In this specification, the phrase "A is composed primarily of B" means that, among the components constituting A, B is the largest component by weight.

[0030] 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, micropores are pores having a diameter of 2 nm or less, mesopores are pores having a diameter of more than 2 nm and less than 50 nm, and 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 be reduced due to corrosion by 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, for example, 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.

[0031] Although not particularly limited, the Si-containing particles 67 preferably contain pores with a diameter of 100 nm or more and pores with a diameter of 10 nm or less. The Si-containing particles 67 having pores of 100 nm or more facilitate the exertion of an anchoring effect, allowing carbon nanotubes to be suitably arranged on the surfaces of the Si-containing particles 67. This improves the initial charge / discharge efficiency of the secondary battery 100. Furthermore, even if the Si-containing particles 67 repeatedly expand and contract during charge and discharge, the anchoring effect makes it difficult for the conductive material to come off, and the conductive path is unlikely to be broken. This can also improve the cycle characteristics of the secondary battery 100. The Si-containing particles 67 having pores of 10 nm or less effectively suppresses electrolyte erosion and expansion and contraction during charge and discharge. This improves the cycle characteristics of the secondary battery 100.

[0032] More preferably, the Si-containing particles 67 have a log differential pore volume V 100 Log differential pore volume V of pores with a diameter of 10 nm 10 Ratio of (V 10 / V 100) is preferably adjusted to be 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 relatively small diameters (e.g., pores with a diameter of 10 nm) than pores with relatively large diameters (e.g., pores with a diameter of 100 nm). This allows the secondary battery 100 to favorably achieve both the initial charge / discharge efficiency and the cycle characteristics. The above-mentioned V 100 V against 10 Ratio of (V 10 / V 100 ) is preferably greater than 1, more preferably 1.2 or more, and may be 1.5 or more. 100 V against 10 Ratio of (V 10 / V 100 ) is preferably, for example, 20 or less, and may be 10 or less.

[0033] Log differential pore volume V of a pore with a diameter of 100 nm 100 and the log differential pore volume V of a pore with a diameter of 10 nm 10 can be calculated based on the BJH method using a specific surface area and pore size distribution measurement device. First, Si-containing particles are heated and dried under vacuum to prepare a measurement sample. Next, an adsorption isotherm of the measurement sample is obtained using liquid nitrogen as a refrigerant and nitrogen gas (N2 gas) as the adsorption gas. The obtained adsorption isotherm is analyzed by the BJH method to determine the log differential pore volume distribution. Then, from the log differential pore volume distribution, the log differential pore volume V of pores with a diameter of 100 nm is calculated. 100 and the log differential pore volume V of a pore with a diameter of 10 nm 10 It is possible to find:

[0034] 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, more preferably 50 nm or less. This reduces the amount of expansion and contraction per Si nanoparticle 67a during charge and discharge, making them less likely to break even with repeated expansion and contraction. Although not particularly limited, the average particle diameter of the Si nanoparticles 67a may be, for example, 5 nm or more. Note that, in this specification, the "average particle diameter of Si nanoparticles" can be determined as follows. First, a sample for observation with a scanning transmission electron microscope (STEM) is prepared by FIB (focused ion beam) processing of the negative electrode active material layer. Then, the sample is subjected to elemental analysis by EDX element mapping, and a BF image (bright-field image) and a HAADF image (high-angle annular dark-field image) are obtained. The diameter of the Si nanoparticles can be determined from the contrast and shape obtained from the BF and HAADF images. The arithmetic mean of the diameters of at least 10 Si nanoparticles is defined as the "average particle size of Si nanoparticles" here.

[0035] The Si nanoparticles 67a have a mesh-like structure. In this mesh-like structure, a plurality of voids are formed randomly or regularly. The mesh-like structure of the Si nanoparticles 67a favorably improves the conductive path. Furthermore, the mesh-like structure of the Si nanoparticles 67a prevents the Si nanoparticles 67a from excessively expanding and contracting during charging and discharging.

[0036] Although not particularly limited, it is preferable that the Si-containing particles 67 have a plurality of pores 67b around the above-described Si nanoparticles 67a. In particular, it is preferable that many small-diameter pores (e.g., pores with a diameter of 10 nm or less) exist around the Si nanoparticles 67a. This can mitigate expansion and contraction caused by charge and discharge while suitably suppressing erosion of the electrolyte.

[0037] 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 excessive oxygen content, and can suitably improve the capacity and cycle characteristics of secondary batteries. The oxygen content can be measured by heating and melting the particles in an inert gas using an oxygen analyzer.

[0038] D of Si-containing particles 67 50 The particle diameter is not particularly limited, but is preferably, for example, 1 μm or more and 15 μm or less, and more preferably 2 μm or more and 10 μm or less. 50 "Particle size" refers to the particle size corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by particle size distribution measurement based on the laser diffraction / light scattering method.

[0039] The Si-containing particles 67 can be obtained by, for example, firing a plant containing Si. That is, the Si-containing particles 67 are preferably derived from plants. Specifically, plants such as rice (rice plant), barley, wheat, and rye husks, coconut shells, tea leaves, sugarcane, and corn are suitable as raw materials. Among these, rice husks are preferred as the raw material for the Si-containing particles 67. Plants accumulate silicic acid absorbed from the soil around their cell walls. By firing this, a porous body containing Si nanoparticles 67a with a plant-derived network structure can be obtained. However, the Si-containing particles 67 may also be prepared by separately preparing a porous body mainly composed of Si or C and Si nanoparticles with a network structure, and then introducing the Si nanoparticles into the porous body.

[0040] The negative electrode active material layer 64 may contain components other than the graphite particles 66 and the Si-containing particles (for example, SiOx or hard carbon that does not have the above-described structure) as the negative electrode active material, as long as the effects of the present technology are not significantly impaired.

[0041] 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 other components that may be included as the negative electrode active material) is taken as 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 taken as 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, both initial charge / discharge efficiency and cycle characteristics can be favorably improved.

[0042] The secondary battery 100 disclosed herein uses carbon nanotubes (CNTs) 68 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. Therefore, the carbon nanotubes 68 easily become entangled with the graphite particles 66 and the Si-containing particles 67, and the conductive paths are suitably maintained. Furthermore, as described above, the Si-containing particles 67 have a plurality of pores 67b. By arranging the carbon nanotubes 68 in these pores 67b, the carbon nanotubes 68 are unlikely to come off the Si-containing particles 67 even when they repeatedly expand and contract due to charge and discharge, and the conductive paths are suitably maintained.

[0043] Examples of the carbon nanotubes 68 include single-walled carbon nanotubes (SWCNTs) made of one layer of graphene, double-walled carbon nanotubes (DWCNTs) made of two different SWCNTs, and multi-walled carbon nanotubes (MWCNTs) made of three or more different SWCNTs. From the viewpoint of further improving the capacity of the secondary battery 100, single-walled carbon nanotubes (SWCNTs) are preferred.

[0044] 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 within this range, the carbon nanotubes 68 are properly dispersed, and a suitable conductive path is formed. Furthermore, although not particularly limited, the average diameter of the carbon nanotubes 68 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 determined, 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.

[0045] 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 some of the pores 67b of the Si-containing particles 67). This allows a conductive path to be suitably formed within the negative electrode active material layer 64, improving the initial charge / discharge efficiency. When the weight of the Si-containing particles 67 is taken as 100 wt%, the weight ratio X of the carbon nanotubes 68 to the Si-containing particles 67 is preferably 0.02 wt% to 4 wt%, more preferably 0.2 wt% to 1 wt%, and even more preferably 0.2 wt% to 0.6 wt%. 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.

[0046] Although not particularly limited, when the total weight of the negative electrode active material is taken as 100 wt%, the content of 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 to the carbon nanotubes, by weight, is preferably negative electrode active material:CNT=100:0.01 to 100:1.4, more preferably negative electrode active material:CNT=100:0.1 to 100:1, and even more preferably negative electrode active material:CNT=100:0.1 to 100:0.2.

[0047] The negative electrode active material layer 64 may contain components (e.g., a binder) other than the above-described 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), and polyvinylidene fluoride (PVDF). Of these, CMC, PAA, and SBR are preferably used. Although not particularly limited, it is more preferable to use CMC, PAA, and SBR in combination.

[0048] The total binder content is, for example, 1 wt% or more, preferably 3 wt% or more, and more preferably 3.5 wt% or more, relative to 100 wt% of the negative electrode active material, and 10 wt% or less, preferably 8 wt% or less, and more preferably 5 wt% or less, relative to 100 wt% of the negative electrode active material.

[0049] It is presumed that, because the Si-containing particles 67 are porous bodies having a plurality of 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.

[0050] The thickness of the negative electrode active material layer 64 per side 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.

[0051] 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. Furthermore, 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.

[0052] <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.

[0053] FIG. 4 is a flowchart illustrating one preferred embodiment of the method for manufacturing the secondary battery 100 disclosed herein. As shown in FIG. 4, the method for manufacturing the secondary battery 100 disclosed herein includes a preparation step S10 of preparing each material and a mixing step S20 of mixing the prepared materials. The mixing step S20 preferably includes a first mixing step S21 of mixing Si-containing particles with carbon nanotubes to prepare a first mixture, and a second mixing step S22 of mixing the first mixture with 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.

[0054] In the preparation step S10, at least graphite particles 66, Si-containing particles 67, and carbon nanotubes 68 are prepared. Other necessary components (e.g., binder, solvent, etc.) may also be prepared in the preparation step S10. 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 structure is prepared. As the Si-containing particles 67, for example, a plant-derived Si-C composite and / or Si particles may be prepared. More preferably, as the Si-containing particles 67, a rice husk-derived Si-C composite and / or Si particles may be prepared. As the carbon nanotubes 68, a water-soluble paste having a solid content of approximately 1% to 10% may be prepared.

[0055] 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 uniformly mixed 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.

[0056] The mixing step S20 may include a first mixing step S21 and a second mixing step S22. In the first mixing step S21, Si-containing particles 67 and 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 to form conductive paths, and the initial charge / discharge efficiency of the secondary battery 100 can be improved.

[0057] In the first mixing step S21, for example, powdered Si-containing particles 67 and paste-like carbon nanotubes 68 are put into a stirrer and mixed. The stirrer is not particularly limited as long as it has a rotating stirrer (a stirring blade or stirring vane such as a dispersion blade or turbine blade). The rotation speed of the stirrer is not particularly limited, but may be, for example, about 1000 rpm to 5000 rpm.

[0058] In the second mixing step S22, the first mixture prepared in the first mixing step S21 is mixed with graphite particles 66, a binder, and a solvent to prepare a second mixture. The stirring device is not particularly limited, and the same stirring device as 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 preferable to dry-mix the powdered graphite particles 66 and the binder, and then add the first mixture and a solvent and knead them together. This can improve dispersibility.

[0059] The second mixture prepared above is applied to the negative electrode current collector 62 and dried. If necessary, the negative electrode active material layer 64 disposed on the negative electrode current collector 62 may be subjected to a drying and pressing process. This allows the thickness and density of the negative electrode active material layer 64 to be adjusted.

[0060] The positive electrode active material layer 54 can be formed by dispersing the positive electrode active material, a conductive material, and a 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.

[0061] The negative electrode 60 and positive electrode 50 prepared above are stacked so as to be insulated by two separators 70. If necessary, the prepared laminate is wound in the longitudinal direction around the winding axis, and the wound laminate is pressed to prepare a flat wound electrode body. The wound electrode body is housed in a battery case 30, and a nonaqueous electrolyte is poured through the liquid pouring hole. The liquid pouring hole is then sealed, and the secondary battery 100 is hermetically sealed. In this manner, the secondary battery 100 can be manufactured.

[0062] The configuration of a secondary battery 100 according to one embodiment and a method for manufacturing the secondary battery 100 have been described above. The secondary battery 100 includes graphite particles 66, Si-containing particles 67, and carbon nanotubes 68. The carbon nanotubes 68 are disposed in at least some of the pores 67b of the Si-containing particles 67, thereby improving the initial charge / discharge efficiency. Furthermore, the carbon nanotubes 68 are disposed in the pores 67b, which effectively prevents the conductive paths from being broken due to expansion and contraction caused by repeated charge / discharge cycles, thereby improving the cycle characteristics of the secondary battery 100. The secondary battery 100 can be used for various purposes, but is particularly suitable as a power source (driving power source) for motors mounted on vehicles such as passenger cars and trucks. It is particularly suitable as a power source for electric vehicles (BEVs; Battery Electric Vehicles). The secondary battery 100 can also be used effectively in the construction of assembled batteries.

[0063] In the above-described secondary battery 100, a wound electrode body is exemplified as the electrode body 20, but this is not limited thereto, and the electrode body 20 may be, for example, a laminated electrode body, which is an electrode body in which a plurality of approximately rectangular positive electrodes and a plurality of approximately rectangular negative electrodes are alternately stacked with separators interposed therebetween.

[0064] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in the following test examples.

[0065] 1. First Exam 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 changed, and the initial charge-discharge efficiency of the secondary battery was evaluated.

[0066] <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 plant-derived Si-C composite particles made from rice husks. Single-walled carbon nanotubes (SWCNTs) were prepared as the conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared as binders. These were kneaded with water as a solvent so that the weight ratio of graphite particles:Si-containing particles:SWCNT:CMC:PAA:SBR was 65:35:0.1:1:1:1.5 to prepare a slurry for forming a negative electrode active material layer.

[0067] Specifically, the mixing and kneading of the slurry for forming the negative electrode active material layer was carried out as follows. First, Si-containing particles, a paste-like SWCNT (solid content of 2%), and a solvent were placed in a container. A disper was used to mix at a rotation speed of 3000 rpm to prepare a first mixture. Next, a stirring granulator was used to dry-mix graphite particles, CMC, and PAA, and the first mixture prepared above and a solvent were added to this mixed powder and kneaded. The solid content 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 the negative electrode active material layer was prepared. The solid content C (%) during kneading was determined as follows: Water was added to a mixed powder of graphite particles and Si-containing particles in the above ratio, and the moisture content A (%) was used to maximize the torque required for mixing. The amount of moisture per 100 g of mixed powder corresponding to the moisture content A (%) was used as B (ml). The solid content C (%) that can maximize the torque can be calculated using the formula: C (%) = 100 - A = (100 / (100 + B)) × 100.

[0068] The prepared negative electrode active material layer forming slurry 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 predetermined thickness, and processed to a predetermined size to produce a negative electrode sheet.

[0069] 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 in a weight ratio of NCM:AB:PVDF = 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 predetermined thickness, and processed to the predetermined dimensions to produce a positive electrode sheet.

[0070] The negative electrode sheet and the positive electrode sheet prepared above were stacked with a separator interposed therebetween to produce a laminated electrode body. Current-collecting leads were attached to the positive and negative electrode plates, respectively, and the laminated electrode body was inserted into an exterior body made of an aluminum laminate sheet. A nonaqueous electrolyte was poured into the interior of the exterior body, and the opening of the exterior body was sealed to produce an evaluation battery for Example 1. A porous polyolefin sheet with a three-layer structure of PP / PE / PP was used as the separator. The nonaqueous electrolyte was prepared by dissolving LiPF6 as a supporting electrolyte at a concentration of 1 mol / L in a mixed solvent of 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.

[0071] Table 1 shows the content (wt%) of Si-containing particles when the weight of the negative electrode active material is taken as 100 wt%, the weight ratio X (wt%) of SWCNTs to the Si-containing particles when the weight of the Si-containing particles is taken as 100 wt%, and the content (wt%) of SWCNTs when the weight of the negative electrode active material is taken as 100 wt%. The weight ratio X of SWCNTs can be calculated using the formula: weight ratio X of SWCNTs = ((content of SWCNTs) / (content of Si-containing particles)) × 100.

[0072] <Example 2, Example 3, Example 5 and Example 6> The evaluation batteries of Examples 2, 3, 5, and 6 were prepared in the same manner as Example 1, except that the weight ratio X (wt%) of SWCNTs to the Si-containing particles, where the weight of the Si-containing particles is taken as 100 wt%, was changed as shown in Table 1.

[0073] <Example 4> In Example 4, the step of pre-mixing the Si-containing particles and SWCNTs was not performed. That is, using an agitation granulator, the Si-containing particles, graphite particles, CMC, and PAA were dry-mixed, and then paste-like SWCNTs (solid content 2%) and a solvent were added to this mixed powder and kneaded. The solid content at the time of 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. Except for the above, the evaluation battery of Example 4 was fabricated in the same manner as in Example 1.

[0074] <Example 7> The evaluation battery of Example 7 was fabricated in the same manner as in Example 1, except that Si-C composite particles fabricated by a CVD method were prepared as the Si-containing particles.

[0075] <Evaluation of initial charge / discharge efficiency> The initial charge capacity and discharge capacity were measured in the first cycle, which consisted of CCCV charging (0.05C rate up to 4.2V, then 0.05C cut) at 25°C, followed by CC discharging (0.05C rate to 2.5V cut). 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 Equation (1)

[0076] [Table 1]

[0077] As shown in Table 1, the initial charge-discharge efficiency of the test batteries of Examples 1 to 4 was 82% or more. These results show that a secondary battery with excellent initial charge-discharge efficiency is realized by including graphite particles as the negative electrode active material and Si-containing particles, which are porous bodies containing Si nanoparticles with a network 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 by arranging CNTs in at least some of the pores of the porous body.

[0078] Furthermore, 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 performing the second mixing step, the CNTs are more easily arranged in the pores of the porous body, further improving the initial charge-discharge efficiency.

[0079] 2. Second Exam 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.

[0080] <Example 11> Pore ​​size distribution ratio (V 10 / V 100 An evaluation battery for Example 11 was produced in the same manner as in Example 1, except that ) was 1.5.

[0081] <Examples 12 to 15> As Si-containing particles, the pore size distribution ratio (V 10 / V 100) were prepared as shown in Table 1. In addition, when the weight of the Si-containing particles was taken as 100 wt%, the weight ratio X of the carbon nanotubes to the Si-containing particles was changed as shown in Table 1. Except for these, the evaluation batteries of Examples 12 to 15 were prepared in the same manner as in Example 1.

[0082] <Calculation of pore distribution ratio> The pore size distribution ratio was calculated based on the BJH method using a specific surface area and pore size distribution measuring device (ASAP2020 manufactured by Micromeritics). First, the Si-containing particles were heated and dried under vacuum to prepare a measurement sample. Next, an adsorption isotherm of the measurement sample was obtained using liquid nitrogen as a refrigerant and nitrogen gas (N2 gas) as the adsorption gas. The obtained adsorption isotherm was analyzed by the BJH method to determine the log differential pore volume distribution. Then, from the log differential pore volume distribution, the log differential pore volume V of pores with a diameter of 100 nm was calculated. 100 and the log differential pore volume V of a pore with a diameter of 10 nm 10 V 100 V against 10 Ratio of (V 10 / V 100 The results are shown in Table 2.

[0083] <Evaluation of cycle capacity retention rate> A cycle test was conducted in which 250 charge / discharge cycles were repeated at 25°C, with CCCV charging (0.4C rate up to 4.2V, then 0.1C cut) followed by CC discharging (0.4C rate, 2.5V cut). The discharge capacity at the first cycle (initial capacity) and the discharge capacity at the 250th cycle were measured, and the cycle capacity retention rate was calculated using the following formula (2). The higher the cycle capacity retention rate, the better 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 Formula (2)

[0084] [Table 2]

[0085] As shown in Table 2, the capacity retention rates of the test batteries of Examples 11 to 13 were 79% or more. These results show that the cycle characteristics of a secondary battery can be improved by including graphite particles as the negative electrode active material and Si-containing particles, which are porous bodies containing Si nanoparticles with a network structure, and by having the weight ratio X of carbon nanotubes to the Si-containing particles be 0.02 wt% or more and 4 wt% or less when the weight of the Si-containing particles is taken as 100 wt%, and by having CNTs arranged in at least some of the pores of the porous body.

[0086] Comparing Example 13 with Example 11 and Example 12 in Table 2, the pore size distribution ratio (V 10 / V 100 ) is 1 or more, the capacity retention rate is 86% or more. 10 / V 100 ) is 1 or more, the cycle characteristics of the secondary battery can be further improved.

[0087] 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 common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0088] 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, wherein the negative electrode comprises 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 containing 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 Si-containing particles have pores with a diameter of 100 nm or more and pores with a diameter of 10 nm or less, and the log differential pore volume of the pores with a diameter of 100 nm is V 100 , log differential pore volume V of a pore with a diameter of 10 nm 10 When V 100 V against 10 Ratio of (V 10 / V 100 Item 3. The secondary battery according to item 1 or 2, wherein the number of ions in a molecule is 1 or more. Item 4: The secondary battery according to any one of Items 1 to 3, wherein the Si nanoparticles have an average particle size 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 taken as 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 Si nanoparticles in a network structure. 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 Item 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%. [Explanation of symbols]

[0089] 20 Electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode (positive electrode sheet) 52 Positive electrode current collector 52a Exposed part of positive electrode current collector 54 Cathode active material layer 60 Negative electrode (negative electrode sheet) 62 Negative electrode current collector 62a Exposed part of negative electrode current collector 64 Negative electrode active material layer 66 graphite particles 67 Si-containing particles 67a Si nanoparticles 67b pore 68 Carbon nanotubes (CNTs) 70 Separator 100 Secondary battery

Claims

1. 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 negative electrode active materials, and carbon nanotubes as a conductive material, The Si-containing particles are porous bodies containing Si nanoparticles in a network structure, the Si-containing particles have pores with a diameter of 100 nm or more and pores with a diameter of 10 nm or less, The log differential pore volume of pores having a diameter of 100 nm is V 100 , and the log differential pore volume of pores having a diameter of 10 nm is V 10 , and the ratio of V 10 to V 100 (V 10 / V 100 ) is 1 or more; the carbon nanotubes are disposed in at least some of the pores of the porous body; A secondary battery, wherein the weight ratio X of the carbon nanotubes to the Si-containing particles is 0.02 wt % or more and 4 wt % or less, where the weight of the Si-containing particles is 100 wt %.

2. The secondary battery according to claim 1 , wherein the Si nanoparticles have an average particle size of 50 nm or less.

3. 3. The secondary battery according to claim 1, 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.

4. 3. The secondary battery according to claim 1, wherein the Si-containing particles have an oxygen content of 10 wt % or less.

5. 3. The secondary battery according to claim 1, wherein the content of the Si-containing particles is 10 wt% or more and 60 wt% or less, with the total weight of the negative electrode active material being 100 wt%.

6. 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 Si-containing particles prepared in the preparing step are porous bodies containing Si nanoparticles in a network structure, the Si-containing particles have pores with a diameter of 100 nm or more and pores with a diameter of 10 nm or less, The log differential pore volume of pores having a diameter of 100 nm is V 100 , and the log differential pore volume of pores having a diameter of 10 nm is V 10 , and the ratio of V 10 to V 100 (V 10 / V 100 ) is 1 or more; performing the first mixing step so that the carbon nanotubes are disposed in at least some of the pores of the porous body; 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 100 wt % in the first mixing step.

7. 7. The method according to claim 6, wherein in the second mixing step, the weight of the negative electrode active material is 100 wt%, and the content of the graphite particles is 40 wt% or more and 90 wt% or less.

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