Negative electrode for secondary battery, and secondary battery

Abstract

Provided is a negative electrode for a secondary battery capable of suppressing deterioration in charge / discharge cycle characteristics of the battery. A negative electrode for a secondary battery according to one embodiment of the present disclosure is provided with a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, the negative electrode mixture layer containing carbon nanotubes and a compound of a Si-containing material and a silane coupling agent, and the silane coupling agent having a phenyl group and at least one of an amine group and a methyl group.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a secondary battery and a secondary battery. Background Art

[0002] For negative electrodes for secondary batteries such as non-aqueous electrolyte secondary batteries, carbon-based materials are usually used, but in order to increase the capacity of the battery, materials containing Si have been studied for use in negative electrodes.

[0003] For example, Patent Documents 1 to 3 disclose negative electrodes for secondary batteries (Patent Documents 2 to 4) including a negative electrode active material containing a material containing Si and carbon nanotubes having functional groups such as carboxyl groups.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-176140

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2007-242386

[0008] Patent Document 3: Japanese Patent Application Laid-Open No. 2009-507338 Summary of the Invention

[0009] However, since the volume change (expansion and contraction) of the Si-containing material during charge and discharge is large, if charge and discharge are repeated, the conduction path of the negative electrode mixture layer containing the Si-containing material is cut off due to the large volume change of the Si-containing material, and there is a problem that the charge and discharge cycle characteristics are likely to deteriorate.

[0010] Therefore, an object of the present disclosure is to provide a negative electrode for a secondary battery that can suppress deterioration of the charge and discharge cycle characteristics of the battery, and a secondary battery including the negative electrode for a secondary battery.

[0011] A negative electrode for a secondary battery according to one aspect of the present disclosure is characterized by including a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, the negative electrode mixture layer containing a compound of a Si-containing material and a silane coupling agent, and carbon nanotubes, and the silane coupling agent having at least one of an amino group and a methyl group, and a phenyl group.

[0012] In addition, a secondary battery according to one aspect of the present disclosure is characterized by including the above-described negative electrode for a secondary battery.

[0013] According to the present disclosure, it is possible to provide a negative electrode for a secondary battery that can suppress deterioration of the charge and discharge cycle characteristics of the battery, and a secondary battery including the negative electrode for a secondary battery. Brief Description of the Drawings

[0014] Figure 1It is a longitudinal sectional view of a secondary battery as an example of an embodiment.

[0015] Figure 2 It is a diagram schematically showing the composition of a compound of a Si-containing material and a silane coupling agent, and a carbon nanotube, which is an example of an embodiment. Detailed Embodiment

[0016] Hereinafter, an example of an embodiment will be described in detail. The drawings referred to in the description of the embodiment are schematically drawn, and the dimensional ratios of the constituent elements depicted in the drawings may sometimes differ from the actual ones.

[0017] Figure 1 It is a longitudinal sectional view of a secondary battery as an example of an embodiment. Figure 1 The secondary battery 10 shown includes: a wound electrode body 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a non-aqueous electrolyte, insulating plates 18 and 19 respectively disposed above and below the electrode body 14, and a battery case 15 for housing the above components. The battery case 15 is composed of a bottomed cylindrical case body 16 and a sealing body 17 that closes the opening of the case body 16. It should be noted that other forms of electrode bodies such as a laminated electrode body in which a positive electrode and a negative electrode are alternately laminated with a separator interposed therebetween may be used instead of the wound electrode body 14. In addition, as the battery case 15, examples include metal cases such as cylindrical, square, coin-shaped, and button-shaped cases, and resin cases (so-called laminated types) formed by laminating resin sheets.

[0018] The non-aqueous electrolyte has lithium ion conductivity. The non-aqueous electrolyte can be a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0019] The liquid electrolyte (electrolyte solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles, amides, and mixed solvents of two or more of them can be used. As an example of the non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and their mixed solvents can be cited. The non-aqueous solvent may contain a halogenated product (for example, fluoroethylene carbonate) in which at least a part of the hydrogen of these solvents is substituted with a halogen atom such as fluorine. As the electrolyte salt, a lithium salt such as LiPF6 is used.

[0020] As the solid electrolyte, for example, solid or gel polymer electrolytes, inorganic solid electrolytes, etc. can be used. As the inorganic solid electrolyte, publicly known materials (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) used in all-solid-state lithium-ion secondary batteries, etc. can be used. The polymer electrolyte contains, for example, a lithium salt and a matrix polymer, or contains a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of the polymer material include fluororesins, acrylic resins, polyether resins, etc.

[0021] The housing main body 16 is, for example, a metal container having a bottomed cylindrical shape. A gasket 28 is provided between the housing main body 16 and the sealing body 17 to ensure the airtightness inside the battery. The housing main body 16 has, for example, a protruding portion 22 that supports the sealing body 17 and in which a part of the side surface portion protrudes inward. The protruding portion 22 is preferably formed in a ring shape along the circumferential direction of the housing main body 16, and the sealing body 17 is supported on its upper surface.

[0022] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a lid 27 are laminated in order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and the members other than the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their central portions, and the insulating member 25 is sandwiched between their peripheral portions. If the internal pressure of the secondary battery 10 rises due to heat generated by an internal short circuit or the like, for example, the lower valve body 24 is deformed and broken in such a way as to push the upper valve body 26 toward the lid 27 side, and the current path between the lower valve body 24 and the upper valve body 26 is cut off. If the internal pressure further rises, the upper valve body 26 is broken, and gas is discharged from the opening portion of the lid 27.

[0023] In Figure 1 In the secondary battery 10 shown, the positive electrode lead 20 attached to the positive electrode 11 extends toward the sealing body 17 side through the through hole of the insulating plate 18, and the negative electrode lead 21 attached to the negative electrode 12 extends toward the bottom side of the housing main body 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, by welding or the like, and the lid 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the housing main body 16 by welding or the like, and the housing main body 16 becomes the negative electrode terminal.

[0024] Hereinafter, the positive electrode 11, the negative electrode 12, and the separator 13 will be described in detail.

[0025] [Positive Electrode]

[0026] The positive electrode 11 includes a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. As the positive electrode current collector, a foil of a metal such as aluminum or aluminum alloy that is stable within the potential range of the positive electrode, a film with such a metal disposed on the surface layer, etc. can be used. The positive electrode mixture layer is composed of, for example, a positive electrode active material, a binder material, a conductive material, etc. The positive electrode mixture layer is preferably formed on both sides of the positive electrode current collector. The positive electrode 11 can be manufactured, for example, by coating a positive electrode mixture slurry containing a positive electrode active material, a binder material, a conductive material, etc. on the positive electrode current collector, drying and calendering the coated film to form a positive electrode mixture layer on the surface of the positive electrode current collector.

[0027] The positive electrode active material is, for example, a lithium composite oxide capable of reversibly inserting / extracting lithium. As the metal elements contained in the lithium composite oxide, for example, Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, etc. can be cited. Among them, it is preferable to contain at least one of Ni, Co, and Mn. As an example of a suitable lithium composite oxide, a composite oxide represented by the general formula LiMO2 (M is Ni and X, X is a metal element other than Ni, and the proportion of Ni is 50 mol% or more and 95 mol% or less relative to the total molar amount of the metal elements other than Li) can be cited. X in the above formula can be, for example, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, etc.

[0028] As the conductive material contained in the positive electrode mixture layer, fibrous carbon such as carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, and carbon materials such as graphite can be exemplified. As the binder material contained in the positive electrode mixture layer, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc. can be exemplified. Styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (which can be PAA-Na, PAA-K, etc., and partially neutralized salts), polyethylene oxide (PEO), polyvinyl alcohol (PVA), etc. can be cited.

[0029] [Negative electrode]

[0030] The negative electrode 12 includes a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. As the negative electrode current collector, for example, a foil of a metal such as copper or copper alloy that is stable within the potential range of the negative electrode, a film with such a metal disposed on the surface layer, etc. can be used.

[0031] The negative electrode mixture layer contains a compound of a Si-containing material as a negative electrode active material and a silane coupling agent, and carbon nanotubes as a conductive material. As described later, by having a predetermined structure, the silane coupling agent ensures a conduction path and suppresses a decrease in charge-discharge cycle characteristics.

[0032] The negative electrode 12 can be manufactured, for example, by coating a negative electrode mixture slurry prepared by mixing a negative electrode active material, a silane coupling agent, a conductive material, etc. on a negative electrode current collector, drying and rolling the coating film, and forming a negative electrode mixture layer on the surface of the negative electrode current collector.

[0033] Examples of the Si-containing material as the negative electrode active material include Si particles, alloy particles containing Si, and composite particles containing Si. They can be used alone or in combination of two or more. It should be noted that a conductive layer may be formed on the surface of the Si-containing material. The conductive layer contains, for example, conductive carbon and covers 30% to 70% of the surface area of the Si-containing material. The coverage rate of the conductive layer can be calculated using, for example, XPS (X-ray photoelectron spectroscopy).

[0034] Si particles are usually obtained by a vapor phase method, micro-grinding silicon cut powder, etc., and can be manufactured by any method. Examples of the alloy particles containing Si include alloys containing Si and a metal selected from alkali metals, alkaline earth metals, transition metals, rare earth metals, or a combination thereof.

[0035] Examples of the composite particles containing Si include a lithium ion conductive phase and Si particles dispersed in the lithium ion conductive phase. The lithium ion conductive phase is, for example, at least one selected from a silicon oxide phase, a silicate phase, and a carbon phase.

[0036] For example, from the viewpoint of high lithium ion conductivity, etc., the silicate phase preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium. Among them, from the viewpoint of high lithium ion conductivity, etc., the silicate phase is preferably a silicate phase containing lithium (hereinafter, sometimes referred to as a lithium silicate phase).

[0037] The lithium silicate phase is represented, for example, by the formula: Li 2z SiO 2＋z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., z preferably satisfies the relationship of 0 < z < 1, and more preferably z = 1 / 2.

[0038] Composite particles in which Si particles are dispersed in the silicon oxide phase are represented, for example, by the general formula SiO x (preferably in the range of 0 < x < 2, more preferably in the range of 0.5 ≤ x ≤ 1.6). Composite particles in which Si particles are dispersed in the carbon phase are represented, for example, by the general formula Si x C yIt is represented by (preferably in the range of 0 < x ≤ 1 and 0 < y ≤ 1).

[0039] The content of Si particles constituting the composite particles is, for example, in the range of 30 mass% to 80 mass%, 35 mass% to 75 mass%, or 55 mass% to 70 mass%. The content of Si particles can be measured by Si-NMR.

[0040] (Measurement conditions of Si-NMR)

[0041] Measuring device: manufactured by Varian, solid nuclear magnetic resonance spectrometer (INOVA-400)

[0042] Probe: Varian 7mm CPMAS-2

[0043] MAS: 4.2kHz

[0044] MAS speed: 4kHz

[0045] Pulse: DD (45° pulse + signal acquisition time 1H decoupling)

[0046] Repetition time: 1200sec

[0047] Observation width: 100kHz

[0048] Observation center: around -100ppm

[0049] Signal acquisition time: 0.05sec

[0050] Number of accumulations: 560

[0051] Sample amount: 207.6mg

[0052] The crystallite size of Si particles constituting the composite particles is, for example, 10nm to 30nm, or 15nm to 25nm. The crystallite size of Si particles is calculated by the Scherrer formula based on the full width at half maximum of the analytical peak attributed to the Si (111) plane in the X-ray diffraction pattern of Si particles.

[0053] The average particle diameter of Si particles constituting the composite particles is, for example, 500nm or less before the first charge and 400nm or less after the first charge. The average particle diameter of Si particles is a value obtained by averaging the maximum diameters of any 100 Si particles observed by SEM (scanning electron microscope) of the cross section of the composite particles.

[0054] The negative electrode active material preferably contains a negative electrode material with a smaller degree of expansion and contraction during charge and discharge compared to the Si-containing material. As the negative electrode material, a carbon material capable of reversibly inserting / extracting lithium is preferably included. As the carbon material, for example, graphite, easily graphitizable carbon, hardly graphitizable carbon, etc. can be cited. Among them, graphite with excellent charge-discharge stability and less irreversible capacity is preferred. Graphite refers to a material having a graphite crystal, and for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. can be cited.

[0055] For example, from the aspects of increasing the battery capacity and suppressing the reduction of charge-discharge cycle characteristics, etc., the content of the Si-containing material is preferably in the range of 1% by mass to 15% by mass relative to the total mass of the negative electrode active material. The content of the carbon material used as the negative electrode active material is preferably 85% by mass to 99% by mass relative to the total mass of the negative electrode active material. The content of the negative electrode active material is, for example, 85% by mass or more, 90% by mass or more, or 95% by mass or more relative to the total mass of the negative electrode binder layer.

[0056] The compound of the Si-containing material and the silane coupling agent is formed by covalently bonding the silanol group of the silane coupling agent to the hydroxyl group on the surface of the Si-containing material. For example, the silanol group is formed in the silane coupling agent by reacting with water, and this silanol group undergoes a dehydration condensation reaction with the hydroxyl group on the surface of the Si-containing material, thereby forming the compound of the Si-containing material and the silane coupling agent. In addition, at the same time as the above reaction, the silanol groups can also form siloxane (Si-O-Si) bonds with each other.

[0057] The silane coupling agent has at least one of an amino group and a methyl group, and a phenyl group. By the π-π interaction between the benzene ring of the silane coupling agent and the benzene ring of the carbon nanotube, the isolation of the Si-containing material can be suppressed, and the conduction path can be ensured. In addition, since the silane coupling agent has at least one of an amino group and a methyl group, as will be described later, when the carbon nanotube has an acidic functional group, an attractive force acts between the amino group or methyl group and the acidic functional group, and the isolation of the Si-containing material can be more significantly suppressed.

[0058] The silane coupling agent is not particularly limited as long as it has the above characteristics, and it can be N-phenyl-3-aminopropyltrimethoxysilane represented by the general formula I. N-phenyl-3-aminopropyltrimethoxysilane has an amino group and a phenyl group.

[0059] [Chemical formula 1]

[0060]

[0061] As another example of the silane coupling agent, p-styryltrimethoxysilane represented by the general formula II can be cited. p-styryltrimethoxysilane has a methyl group and a phenyl group.

[0062] [Chemical Formula 2]

[0063]

[0064] The addition amount of the silane coupling agent is preferably 0.01% by mass to 0.8% by mass, more preferably 0.01% by mass to 0.7% by mass, still more preferably 0.05% by mass to 0.5% by mass, and particularly preferably 0.05% by mass to 0.3% by mass, relative to the total mass of the negative electrode active material. When the addition amount of the silane coupling agent exceeds 0.8% by mass, the viscosity of the negative electrode mixture paste becomes too high, and it may be impossible to form a negative electrode mixture layer.

[0065] The content analysis of the coupling agent in the negative electrode mixture layer can be carried out by Py-GC / MS.

[0066] (Measurement conditions of Py-GC / MS)

[0067] Measurement device: Py-GC / MS (GC part: GC-2010 manufactured by Shimadzu Corporation, MS part: GCMS-QP2010Plus)

[0068] Thermal analysis device: PY-2020iD manufactured by Frontier Laboratories

[0069] Heating furnace temperature: 550 °C

[0070] Column: UA-5

[0071] Mass range: m / z = 5 - 500

[0072] Carrier gas: helium

[0073] Sample amount: 25.0 mg

[0074] The carbon nanotubes as the conductive material preferably have acidic functional groups. Figure 2 It is a diagram schematically showing the composition of a Si-containing material, a compound of a silane coupling agent, and carbon nanotubes, which is an example of an embodiment. As Figure 2 shown, it is presumed that π-π interaction occurs between the benzene ring from the silane coupling agent and the benzene ring of the carbon nanotubes, and there is an attractive force between the amino group or methyl group and the acidic functional group, thereby more significantly suppressing the isolation of the Si-containing material. Thus, even if a large volume change occurs in the Si-containing material due to repeated charge and discharge, it is possible to suppress the disconnection of the conduction path in the negative electrode mixture layer and suppress the deterioration of the charge-discharge cycle characteristics.

[0075] The acidic functional group may include at least one functional group selected from carboxyl group, sulfo group, and hydroxyl group. The amount of the acidic functional group in the carbon nanotubes is, for example, 0.01 to 0.25 mmol / g.

[0076] As a method for imparting acidic functional groups to carbon nanotubes, there is no particular limitation. For example, carbon nanotubes can be added to a mixed acid of sulfuric acid and nitric acid and allowed to react for a specified time to impart acidic functional groups to the carbon nanotubes. During the reaction, it is desirable to stir the mixed acid. The reaction time is not particularly limited, and for example, it is preferably 1 hour or more. In addition, the reaction temperature is not particularly limited, and a range of 20°C to 45°C is preferred.

[0077] Analysis of the acidic functional groups imparted to the carbon nanotubes can be carried out by TPD-MS (thermal programmed desorption - mass spectrometry).

[0078] (Measurement conditions for TPD-MS)

[0079] Measuring device: Gas chromatograph - mass spectrometer (GC part: 7890 manufactured by Agilent Technologies, MS part: MS - 60030BU)

[0080] Temperature conditions: Heat from 100°C to 1000°C at 20°C / min and hold for 10 minutes

[0081] Carrier gas: Helium

[0082] Flow rate: 50 mL / min

[0083] Measured mass number: m / z = 10 - 600

[0084] That there is an attractive force between at least one of the amino group and the methyl group from the silane coupling agent and the acidic functional groups possessed by the carbon nanotubes can be confirmed, for example, by observation using SEM (scanning electron microscope). Specifically, by observing the negative electrode mixture layer scraped from a copper foil under the following conditions using SEM, it is possible to confirm that the carbon nanotubes are adsorbed on the surface of the Si - containing material.

[0085] SEM: JSM7001F manufactured by JEOL

[0086] Accelerating voltage: 15 kv

[0087] Magnification: 20,000 times

[0088] Carbon nanotubes include single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. A single-walled carbon nanotube (SWCNT) is a carbon nanotube structure in which a single layer of graphene sheets forms a cylindrical shape. A double-walled carbon nanotube is a carbon nanotube structure in which two layers of graphene sheets are concentrically stacked to form a cylindrical shape. A multi-walled carbon nanotube is a carbon nanotube structure in which three or more layers of graphene sheets are concentrically stacked to form a cylindrical shape. It should be noted that a graphene sheet refers to a layer in which carbon atoms in the sp2 hybrid orbitals that make up a graphite crystal are located at the vertices of a regular hexagon. The shape of the carbon nanotube is not limited, and examples include needle-like, cylindrical tubular, fishbone-like (fishbone or cup stack type), flake-like (flat disk-like), and coil-like shapes.

[0089] For example, from the aspect of further suppressing the degradation of charge-discharge cycle characteristics, etc., the fiber length of the carbon nanotube is preferably 0.5 μm to 500 μm, more preferably 1 μm to 100 μm, further preferably 1 μm to 10 μm, and particularly preferably 1 μm to 5 μm. The fiber length of the carbon nanotube can be obtained by measuring the lengths of any 50 carbon nanotubes by field emission scanning microscopy (FE-SEM) and calculating the arithmetic mean.

[0090] For example, from the aspect of further suppressing the degradation of charge-discharge cycle characteristics, etc., the outermost diameter of the carbon nanotube is preferably 0.5 nm to 20 nm, more preferably 1 nm to 10 nm, further preferably 1 nm to 5 nm. The outermost diameter of the carbon nanotube can be obtained by measuring the outer diameters of any 50 carbon nanotubes by field emission scanning microscopy (FE-SEM) or transmission electron microscopy (TEM) and calculating the arithmetic mean.

[0091] For example, from the aspect of further suppressing the degradation of charge-discharge cycle characteristics, etc., the content of the carbon nanotube is preferably 0.01% by mass to 1% by mass, more preferably 0.01% by mass to 0.1% by mass, relative to the total mass of the negative electrode active material.

[0092] In addition to carbon nanotubes, the conductive material may also contain a conductive material other than carbon nanotubes. Examples of the conductive material other than carbon nanotubes include carbon blacks (CB) such as acetylene black and Ketjen black.

[0093] The negative electrode mixture layer may further contain a binder material. Examples of the binder material contained in the negative electrode mixture layer include styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, etc. Among them, SBR and NBR are preferred, and SBR is particularly preferred. They can be used alone, or two or more of them can be used in combination. For example, relative to the total mass of the negative electrode active material, the content of the binder material in the negative electrode mixture layer is 0.5% by mass to 5% by mass.

[0094] The negative electrode mixture layer may further contain a thickener. Examples of the thickener include carboxymethyl cellulose (CMC) or its salts (such as CMC-Na), polyacrylic acid (PAA) or its salts (which can be PAA-Na, PAA-K, etc., and partially neutralized salts), polyethylene oxide (PEO), polyvinyl alcohol (PVA), etc. They can be used alone, or two or more of them can be used in combination. For example, relative to the total mass of the negative electrode active material, the content of the thickener in the negative electrode mixture layer is 0.5% by mass to 10% by mass.

[0095] [Separator]

[0096] The separator 13 uses, for example, a porous sheet having ion permeability and insulation. Specific examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, etc. As the material of the separator 13, olefin resins such as polyethylene, polypropylene, and copolymers containing at least one of ethylene and propylene, cellulose, etc. are suitable. The separator 13 can be either a single-layer structure or a laminated structure. A heat-resistant layer or the like can also be formed on the surface of the separator 13.

[0097] Examples

[0098] Hereinafter, the present invention will be further described by way of examples, but the present invention is not limited to these examples.

[0099] <Example 1>

[0100] [Production of Carbon Nanotubes with Acidic Functional Groups]

[0101] Carbon nanotubes with a fiber length of 5 μm and an outermost diameter of 2 nm were put into a mixed acid of sulfuric acid / nitric acid and stirred at 40 °C for 12 hours. After the treatment, they were filtered through a glass filter, and the carbon nanotubes remaining on the glass filter were washed with pure water and then naturally dried overnight. The dried sample was analyzed by TPD-MS, and as a result, sulfonic groups, carboxyl groups, and hydroxyl groups were all confirmed. In the examples, the above sample was used as carbon nanotubes with acidic functional groups. It should be noted that the carbon nanotubes without acidic functional groups used in Example 4 and Comparative Example 1 were the untreated carbon nanotubes before being put into the mixed acid among the above.

[0102] [Fabrication of negative electrode]

[0103] SiO as a Si-containing material was mixed with graphite at a mass ratio of 8:92, and this mixture was used as the negative electrode active material. Then, the negative electrode active material, N-phenyl-3-aminopropyltrimethoxysilane, carbon nanotubes with acidic functional groups, sodium carboxymethyl cellulose, and styrene-butadiene rubber were mixed at a mass ratio of 100:0.1:0.02:1.0:1.0. An appropriate amount of water was added to this mixture for kneading to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was coated on both sides of a negative electrode current collector formed of a copper foil with a thickness of 10 μm, and the coating film was dried. The dried coating film was calendered using a calender roll to fabricate a negative electrode having negative electrode mixture layers formed on both sides of the negative electrode current collector.

[0104] [Preparation of electrolyte]

[0105] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70. LiPF6 was dissolved in this mixed solvent to a concentration of 1.4 mol / L. In addition, 2 mass% of vinylene carbonate (VC) was dissolved relative to the total amount of the electrolyte. This was used as the electrolyte of the example.

[0106] [Fabrication of secondary battery]

[0107] A separator composed of a microporous polyethylene film was disposed between metallic Li and the negative electrode, and after winding them, they were formed into a flat shape to fabricate a wound electrode body. After accommodating this electrode body and the above electrolyte in an outer package composed of an aluminum laminate, the inside of the outer package was decompressed to allow the electrolyte to infiltrate into the separator, and then the opening of the outer package was sealed to fabricate a secondary battery.

[0108] <Example 2>

[0109] In the fabrication of the negative electrode, p-styryltrimethoxysilane was used instead of N-phenyl-3-aminopropyltrimethoxysilane, and a secondary battery was fabricated in the same manner as in Example 1 except for this.

[0110] <Example 3>

[0111] In the production of the negative electrode, the addition amount of N-phenyl-3-aminopropyltrimethoxysilane was changed to 0.4% by mass relative to the total mass of the negative electrode active material. Except for this, a secondary battery was produced in the same manner as in Example 1.

[0112] <Example 4>

[0113] In the production of the negative electrode, the addition amount of N-phenyl-3-aminopropyltrimethoxysilane was changed to 0.4% by mass relative to the total mass of the negative electrode active material, and carbon nanotubes without acidic functional groups were added instead of carbon nanotubes with acidic functional groups. Except for this, a secondary battery was produced in the same manner as in Example 1.

[0114] <Example 5>

[0115] In the production of the negative electrode, the addition amount of N-phenyl-3-aminopropyltrimethoxysilane was changed to 0.8% by mass relative to the total mass of the negative electrode active material. Except for this, a secondary battery was produced in the same manner as in Example 1.

[0116] <Comparative Example 1>

[0117] In the production of the negative electrode, N-phenyl-3-aminopropyltrimethoxysilane was not added, and carbon nanotubes without acidic functional groups were added instead of carbon nanotubes with acidic functional groups. Except for this, a secondary battery was produced in the same manner as in Example 1.

[0118] <Comparative Example 2>

[0119] In the production of the negative electrode, N-phenyl-3-aminopropyltrimethoxysilane was not added. Except for this, a secondary battery was produced in the same manner as in Example 1.

[0120] [Evaluation of capacity retention rate]

[0121] After the secondary batteries of each example and each comparative example were subjected to constant current charging (current 0.1It, termination voltage 0.005V) in a temperature environment of 25°C, constant voltage charging was performed (voltage 0.005V, termination current 0.01It). Then, constant current discharging was performed (current 0.1It, termination voltage 1.5V). This charge and discharge was taken as one cycle, and 10 cycles were performed. The capacity retention rate in the charge and discharge cycles of the secondary batteries of each example and each comparative example was calculated by the following formula. It should be noted that the higher the capacity retention rate, the more the reduction in charge and discharge cycle characteristics is suppressed.

[0122] Capacity retention rate (%) = (Discharge capacity in the 10th cycle / Discharge capacity in the 1st cycle) × 100

[0123] [Evaluation of Viscosity of Slurry]

[0124] The viscosity of the negative electrode mixture slurry prepared in each example and each comparative example was measured using a viscosity measuring device under the following conditions. Based on the measurement results of conditions (1) and (2), the influence on coatability was judged from the viscosity. The case where the coating film was okay and could be coated in a good state was marked as 〇, and the case where the coating film was okay but could not be coated in a good state when the viscosity became larger than that was marked as △.

[0125] Viscosity measuring device: TV-22 viscometer manufactured by Toki Sangyo Co., Ltd.

[0126] Rotation speed and measurement time: Condition (1) rotation speed 2 rpm, time 60 seconds; Condition (2) rotation speed 20 rpm, time 60 seconds

[0127] The evaluation results of each example and each comparative example are summarized in Table 1. Among them, the capacity retention rate is based on the result of Comparative Example 1 (100%), and the results of other examples and comparative examples are shown.

[0128] [Table 1]

[0129]

[0130] Examples 1 to 5 all showed a higher capacity retention rate than Comparative Examples 1 and 2. From this, it can be known that by making the negative electrode mixture layer contain a compound of a Si-containing material and a silane coupling agent having a specified structure, and carbon nanotubes, the reduction of charge-discharge cycle characteristics can be suppressed. It should be noted that in Example 5, the evaluation result of the viscosity of the slurry was △, and it can be considered that if the addition amount of the coupling agent is further increased, the formation of the positive electrode mixture layer becomes difficult.

[0131] The present disclosure will be further described by the following embodiments.

[0132] Constitution 1:

[0133] A negative electrode for a secondary battery, which includes a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector,

[0134] The negative electrode mixture layer contains a Si-containing material as a negative electrode active material, a compound of a silane coupling agent, and carbon nanotubes,

[0135] The silane coupling agent has at least one of an amino group and a methyl group, and a phenyl group.

[0136] Constitution 2:

[0137] According to the negative electrode for a secondary battery described in Constitution 1, wherein the carbon nanotubes have acidic functional groups.

[0138] Constitution 3:

[0139] The negative electrode for a secondary battery according to Configuration 2, wherein the acidic functional group includes at least one functional group selected from a carboxyl group, a sulfo group, and a hydroxyl group.

[0140] Configuration 4:

[0141] The negative electrode for a secondary battery according to Configuration 2 or 3, wherein the compound is formed by covalently bonding a silanol group of the silane coupling agent to a hydroxyl group on the surface of the Si-containing material.

[0142] A π-π interaction occurs between the benzene ring from the silane coupling agent in the compound and the benzene ring of the carbon nanotube.

[0143] An attractive force acts between at least one of an amino group and a methyl group from the silane coupling agent in the compound and the acidic functional group of the carbon nanotube.

[0144] Configuration 5:

[0145] The negative electrode for a secondary battery according to any one of Configurations 1 to 4, wherein the fiber length of the carbon nanotube is 0.5 μm to 500 μm.

[0146] Configuration 6:

[0147] The negative electrode for a secondary battery according to any one of Configurations 1 to 5, wherein the outermost diameter of the carbon nanotube is 0.5 nm to 20 nm.

[0148] Configuration 7:

[0149] The negative electrode for a secondary battery according to any one of Configurations 1 to 6, wherein the negative electrode active material includes a carbon material.

[0150] In the negative electrode binder layer, the content of the carbon nanotube is 0.01% by mass to 1.0% by mass based on the total mass of the negative electrode active material.

[0151] Configuration 8:

[0152] The negative electrode for a secondary battery according to any one of Configurations 1 to 7, wherein the addition amount of the silane coupling agent is 0.01% by mass to 0.8% by mass based on the total mass of the negative electrode active material.

[0153] Configuration 9:

[0154] A secondary battery including the negative electrode for a secondary battery according to any one of Configurations 1 to 8.

[0155] Explanation of reference numerals

[0156] 10: Secondary battery, 11: Positive electrode, 12: Negative electrode, 13: Separator, 14: Electrode body, 15: Battery case, 16: Case main body, 17: Sealing body, 18, 19: Insulating plate, 20: Positive electrode lead, 21: Negative electrode lead, 22: Protrusion, 23: Filter, 24: Lower valve body, 25: Insulating member, 26: Upper valve body, 27: Cover, 28: Gasket.

Claims

1. A negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, wherein the negative electrode mixture layer contains a compound of a Si-containing material as a negative electrode active material and a silane coupling agent, and carbon nanotubes, the silane coupling agent having at least one of an amino group and a methyl group, and a phenyl group.

2. The negative electrode for a secondary battery according to claim 1, wherein, The carbon nanotubes have acidic functional groups.

3. The negative electrode for a secondary battery according to claim 2, wherein, The acidic functional groups include at least one functional group selected from a carboxyl group, a sulfo group, and a hydroxyl group.

4. The negative electrode for a secondary battery according to claim 2, wherein, The compound is formed by covalently bonding a silanol group of the silane coupling agent to a hydroxyl group on the surface of the Si-containing material, a benzene ring from the silane coupling agent in the compound undergoes π-π interaction with a benzene ring of the carbon nanotubes, and an attractive force acts between at least one of the amino group and the methyl group from the silane coupling agent in the compound and the acidic functional group of the carbon nanotubes.

5. The negative electrode for a secondary battery according to claim 1, wherein, The fiber length of the carbon nanotubes is 0.5 μm to 500 μm.

6. The negative electrode for a secondary battery according to claim 1, wherein, The outermost diameter of the carbon nanotubes is 0.5 nm to 20 nm.

7. The negative electrode for a secondary battery according to claim 1, wherein The negative electrode active material contains a carbon material, in the negative electrode mixture layer, the content of the carbon nanotubes is 0.01% by mass to 1.0% by mass relative to the total mass of the negative electrode active material.

8. The negative electrode for a secondary battery according to claim 1, wherein, The addition amount of the silane coupling agent is 0.01% by mass to 0.8% by mass relative to the total mass of the negative electrode active material.

9. A secondary battery, comprising the negative electrode for a secondary battery according to any one of claims 1 to 8.

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