Negative electrode for secondary batteries, and secondary battery

Abstract

Disclosed is a negative electrode (12) for secondary batteries, which includes a negative electrode current collector (40) and a negative electrode mixture layer (41) that is formed on the surface of the negative electrode current collector (40). This negative electrode (12) for secondary batteries is characterized in that: the negative electrode mixture layer (41) includes, as a negative electrode active material, a carbon material and an Si-containing material; the negative electrode mixture layer (41) includes, as a conductive agent, a first conductive agent that has a G / D ratio of not less than 20 but less than 65 as determined by Raman spectroscopic measurement, and a second conductive agent that has a G / D ratio of not less than 0.7 but less than 20 as determined by Raman spectroscopic measurement; and the ratio (W2 / W1) of the mass (W2) of the second conductive agent to the mass (W1) of the first conductive agent is less than 2.0.

Description

Negative electrode for secondary battery and secondary battery

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

[0002] In recent years, from the viewpoint of increasing the capacity of secondary batteries, the use of a silicon (Si)-containing material, which can occlude more lithium ions per unit mass than carbon-based active materials such as graphite, as the negative electrode active material contained in the negative electrode mixture layer has been considered. Patent Document 1 discloses a secondary battery in which the negative electrode mixture layer contains carbon nanotubes as a conductive agent.

[0003] International Publication No. 2017 / 007013

[0004] Incidentally, the Si-containing material undergoes a larger change in volume (expansion / contraction) due to the absorption of lithium ions than a carbon-based active material. Therefore, when charge and discharge are repeated, the large volume change in the Si-containing material causes the conductive path of the negative electrode mixture layer containing the Si-containing material to be cut, resulting in a problem that the charge and discharge cycle characteristics are likely to deteriorate.

[0005] A negative electrode for a secondary battery according to one aspect of the present disclosure is a negative electrode for a secondary battery including 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 includes, as negative electrode active materials, a carbon material and a Si-containing material, and, as conductive agents, a first conductive agent having a G / D ratio obtained by Raman spectroscopy of 20 or more and less than 65, and a second conductive agent having a G / D ratio obtained by Raman spectroscopy of 0.7 or more and less than 20, and wherein the ratio (W2 / W1) of the mass (W1) of the second conductive agent to the mass (W2) of the first conductive agent is less than 2.0.

[0006] A secondary battery according to one aspect of the present disclosure includes the above-described negative electrode, a positive electrode, and an electrolyte.

[0007] According to the negative electrode for a secondary battery according to one aspect of the present disclosure, the charge-discharge cycle characteristics of the secondary battery can be improved.

[0008] 1 is an axial cross-sectional view of a secondary battery according to an embodiment of the present invention;

[0009] Hereinafter, an example of an embodiment of a secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0010] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified as a secondary battery, but the battery outer can is not limited to a cylindrical outer can. The secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic outer can, a coin battery with a coin-shaped outer can, or a pouch-type battery with an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, and may be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0011] FIG. 1 is a cross-sectional view of a secondary battery 10 according to an embodiment. As shown in FIG. 1 , the secondary battery 10 includes a wound electrode assembly 14, an electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom and an open axial end, and the opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery will be referred to as the "top" and the bottom side of the outer can 16 will be referred to as the "bottom."

[0012] The electrolyte has ion conductivity (for example, lithium ion conductivity). The electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0013] The liquid electrolyte (electrolytic solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0014] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. Examples of the matrix polymer include a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.).

[0015] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length and width directions. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0016] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0017] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0018] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14 will be described in detail below, with particular reference to the negative electrode 12. [Positive Electrode] The positive electrode 11 includes a positive electrode current collector 30 and a positive electrode mixture layer 31 disposed on the positive electrode current collector 30. The positive electrode current collector 30 can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface. The positive electrode mixture layer 31 includes a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be fabricated, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and the like onto the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode current collector 30.

[0020] The positive electrode mixture layer 31 contains particulate lithium metal composite oxide as a positive electrode active material. The lithium metal composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal element constituting the lithium metal composite oxide is, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, Al, and Mn. Examples of suitable composite oxides include lithium metal composite oxides containing Ni, Co, and Mn, and lithium metal composite oxides containing Ni, Co, and Al.

[0021] The lithium-containing composite oxide is, for example, a secondary particle formed by the aggregation of a plurality of primary particles. The volume-based median diameter (D50) of the composite oxide is not particularly limited, but is, for example, 3 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less. When the composite oxide is a secondary particle formed by the aggregation of primary particles, the D50 of the composite oxide refers to the D50 of the secondary particles. D50 refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the composite oxide (as well as that of the negative electrode active material) can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as a dispersion medium.

[0022] The average particle size of the primary particles constituting the lithium-containing composite oxide is, for example, 0.05 μm or more and 1 μm or less, and is calculated by averaging the diameters of the circumscribed circles of the primary particles extracted by analyzing a scanning electron microscope (SEM) image of the cross section of the secondary particles.

[0023] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, and other carbon materials. Examples of the binder contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and the like. Furthermore, these resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like.

[0024] [Negative Electrode] The negative electrode 12 includes a negative electrode current collector 40 and a negative electrode mixture layer 41 formed on the surface of the negative electrode current collector 40. The negative electrode current collector 40 may be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film having such a metal disposed on its surface. The thickness of the negative electrode current collector 40 is not particularly limited, but is preferably 1 μm or more and 50 μm or less, and more preferably 5 μm or more and 20 μm or less, from the viewpoint of balancing the strength and weight reduction of the negative electrode 12.

[0025] The negative electrode mixture layer 41 includes, as negative electrode active materials, a carbon material and a Si-containing material. As will be described in detail later, the negative electrode mixture layer 41 also includes, as conductive agents, a first conductive agent having a G / D ratio obtained by Raman spectroscopy of 20 or more and less than 65, and a second conductive agent having a G / D ratio obtained by Raman spectroscopy of 0.7 or more and less than 20.

[0026] The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry prepared by mixing a negative electrode active material including a carbon material and a Si-containing material, a conductive agent including a first conductive agent and a second conductive agent, etc., onto the negative electrode current collector 40, drying and rolling the coating, and forming the negative electrode mixture layer 41 on the surface of the negative electrode current collector 40.

[0027] The carbon material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, it is preferable to use at least artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, or a mixture thereof. The volume-based median diameter (D50) of the carbon material is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less.

[0028] The carbon material preferably includes a first carbon material and a second carbon material having different BET specific surface areas. The BET specific surface area of ​​the first carbon material is, for example, 0.9 m ２ / g or more, 6.5m ２ / g or less, and the BET specific surface area of ​​the second carbon material is, for example, 2.5 m ２ / g over 8.0m ２ / g or less. In general, the smaller the BET specific surface area of ​​a carbon material, the more suppressed the reaction with the electrolyte during charge and discharge, and therefore the charge and discharge cycle characteristics of the battery tend to be improved. On the other hand, the smaller the BET specific surface area of ​​a carbon material, the lower the lithium ion acceptability, which may result in a decrease in the input characteristics of the battery. Therefore, by using a first carbon material and a second carbon material having different BET specific surface areas as the carbon material, it is possible to improve the charge and discharge cycle characteristics while maintaining the input characteristics of the battery. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0029] The Si-containing material may be any material containing Si, and examples thereof include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. The volume-based median diameter (D50) of the composite material is generally smaller than the volume-based median diameter (D50) of graphite. The volume-based median diameter (D50) of the composite material is, for example, 1 μm or more and 15 μm or less. Note that one type of Si-containing material may be used alone, or two or more types may be used in combination.

[0030] The content of the Si-containing material is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, relative to the total mass of the negative electrode mixture layer 41. By setting the content of the Si-containing material to 20% by mass or more, relative to the total mass of the negative electrode mixture layer 41, a high capacity battery can be achieved. Furthermore, the content of the Si-containing material is preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 40% by mass or more, relative to the total mass of the negative electrode mixture layer 41. The Si-containing material undergoes a larger volume change (expansion / contraction) during charge / discharge than carbon materials. Therefore, if the content of the Si-containing material exceeds 60% by mass, the conductive path of the negative electrode mixture layer 41 containing the Si-containing material is more likely to be disconnected, which may lead to isolation of the Si-containing material and a decrease in the charge / discharge cycle characteristics of the battery. Therefore, the content of the Si-containing material is preferably 10 mass% or more and 60 mass% or less, more preferably 15 mass% or more and 55 mass% or less, and even more preferably 20 mass% or more and 50 mass% or less, relative to the total mass of the negative electrode mixture layer 41.

[0031] A suitable Si-containing material is a composite particle including an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The Si phase is formed by dispersing Si in the form of fine particles. The composite particle may also have a conductive layer that covers a portion of the surface of the ion-conducting phase. The conductive layer is made of a material with higher conductivity than the ion-conducting phase and forms a good conductive path in the negative electrode mixture layer 41. The conductive layer contains, for example, conductive carbon and covers 30% to 70% of the surface area of ​​the ion-conducting phase. The coverage of the conductive layer can be calculated, for example, using X-ray photoelectron spectroscopy (XPS).

[0032] The ion-conducting phase is a continuous phase composed of an aggregate of particles finer than the Si phase, and is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

[0033] The silicate phase preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium, for example, from the viewpoint of high lithium ion conductivity, etc. Among these, a silicate phase containing lithium (hereinafter sometimes referred to as a lithium silicate phase) is preferred, for example, from the viewpoint of high lithium ion conductivity, etc.

[0034] The lithium silicate phase can be, for example, a compound of the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, and the like, z preferably satisfies the relationship 0<z<1, and more preferably z=1 / 2.

[0035] An example of a suitable composite material containing Si has a sea-island structure in which fine Si is dispersed uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The oxygen to silicon content (x) is, for example, 0.5≦x<2.0, preferably 0.8≦x≦1.5.

[0036] The content of the Si phase constituting the composite particles is, for example, 30% by mass or more and 80% by mass or less. The content of the Si phase can be measured by Si-NMR. The measurement conditions are, for example, as follows: Measurement device: Solid-state nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1 H decoupled) Repetition time: 1200 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0037] The crystallite size of the Si phase constituting the composite particles is, for example, 10 nm or more and 30 nm or less. The crystallite size of the Si phase is calculated by the Scherrer equation from the half-width of the analytical peak assigned to the Si (111) plane in the X-ray diffraction pattern of the Si phase.

[0038] The ratio (V1 / V2) of the volume (V1) in the charged state to the volume (V2) in the discharged state of the Si-containing material is preferably 2.1 or less. However, if (V1 / V2) is too small, the battery capacity tends to decrease. Therefore, (V1 / V2) of the Si-containing material is preferably 1.5 or more, more preferably 1.6 or more. An example of a suitable range of (V1 / V2) of the Si-containing material is 1.5 or more and 2.1 or less, more preferably 1.6 or more and 2.0 or less. If (V1 / V2) of the Si-containing material is within this range, it becomes easy to achieve both high capacity and excellent cycle characteristics.

[0039] In this specification, the ratio (V1 / V2) of the volume (V1) of the silicon-containing material in a charged state to the volume (V2) of the silicon-containing material in a discharged state is measured by the following method. (1) A battery to be evaluated is disassembled, and the negative electrode plate is cut out. A single-electrode cell is prepared using metallic Li as the counter electrode and an ionic liquid as the electrolyte, with the particle cross-section of the silicon-containing material exposed. (2) The single-electrode cell is charged at 0.002 C in a temperature environment of 25°C until the cell voltage reaches 5 mV, and then discharged at 0.05 C until the cell voltage reaches 1.0 V, and the particle cross-section of the silicon-containing material is observed in situ with an SEM. (3) From the change in the particle cross-sectional area of ​​the silicon-containing material, the particle volume (V1) of the silicon-containing material in a charged state and the particle volume (V2) of the silicon-containing material in a discharged state are determined, and (V1 / V2) is calculated.

[0040] As described above, the negative electrode mixture layer 41 contains, as conductive agents, a first conductive agent having a G / D ratio obtained by Raman spectroscopy of 20 or more and less than 65, and a second conductive agent having a G / D ratio obtained by Raman spectroscopy of 0.7 or more and less than 20. The G / D ratio is determined by the D-Band (1300 cm) in the Raman spectroscopy spectrum. －１ Above, 1350cm －１ G-Band (1550 cm or less) peak intensity－１ Above, 1600cm －１ A conductive material with a high G / D ratio has a small number of defects and is highly crystalline.

[0041] The Raman spectroscopy spectrum of the conductive agent can be measured using a Raman spectroscopy device (for example, NRS-5500 manufactured by JASCO Corporation). The Raman spectroscopy spectrum of the conductive agent is measured by dispensing a dispersion liquid in which the conductive agent is dispersed onto a slide and flattening it with a spatula to prepare a sample. The measurement conditions are, for example, as follows: Measurement time: 5 seconds Number of accumulations: 2 Neutral density filter OD: 0.3 Objective lens magnification: 100x Measurement range: 950 cm －１ Above, 1900cm －１ below

[0042] As described above, Si-containing materials undergo larger volume changes (expansion and contraction) during charge and discharge than carbon materials. Therefore, when the negative electrode mixture layer 41 contains a Si-containing material as the negative electrode active material, the Si-containing material becomes increasingly isolated as charge and discharge are repeated, which may result in a deterioration in the charge and discharge cycle characteristics of the battery. As a result of studies by the present inventors, it has been found that the charge and discharge cycle characteristics of a battery are significantly improved when the negative electrode mixture layer 41 contains, as conductive agents, a first conductive agent having a G / D ratio of 20 or more but less than 65 and a second conductive agent having a G / D ratio of 0.7 or more but less than 20, and the ratio (W2 / W1) of the mass (W2) of the second conductive agent to the mass (W1) of the first conductive agent is less than 2.0. Although the detailed mechanism is unclear, it is speculated that by including two types of conductive agents with different G / D ratios and by setting the mixing ratio of the first conductive agent to the second conductive agent within a predetermined range, a strong conductive path is formed between the carbon material as the negative electrode active material and the Si-containing material, and the conductive path is unlikely to be broken even if a volume change occurs in the Si-containing material.

[0043] When the negative electrode mixture layer 41 contains only a first conductive agent having a G / D ratio of 20 or more and less than 65 as the conductive agent, the battery charge / discharge cycle characteristics are sufficiently improved, but the negative electrode slurry viscosity increases and processability deteriorates. The first conductive agent can form a relatively long conductive path between the negative electrode active materials. However, the first conductive agent tends to have a small contact area with the negative electrode active material, and the conductive path formed by the first conductive agent is easily broken by volume changes of the Si-containing material. Furthermore, when the negative electrode mixture layer 41 contains only a second conductive agent having a G / D ratio of 0.7 or more and less than 20 as the conductive agent, the battery charge / discharge cycle characteristics are not sufficiently improved. The second conductive agent is likely to be arranged so as to cover a portion of the surface of the negative electrode active material, and the contact area with the negative electrode active material is large. However, the conductive path formed by the second conductive agent is relatively short and easily broken by volume changes of the Si-containing material. That is, when the negative electrode mixture layer 41 contains, as conductive agents, a first conductive agent having a G / D ratio of 20 or more and less than 65, and a second conductive agent having a G / D ratio of 0.7 or more and less than 20, a relatively long conductive path can be formed while ensuring the contact area between the negative electrode active material and the conductive agent, and an increase in viscosity of the negative electrode slurry can also be suppressed. Note that the G / D ratio is used as an index indicating the characteristics of the conductive agent because the present inventors have found that there is an excellent correlation between the G / D ratio of the conductive agent and the charge / discharge cycle characteristics of the battery.

[0044] The first conductive agent and the second conductive agent are preferably carbon nanotubes (CNTs). Examples of carbon nanotubes include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), including double-walled carbon nanotubes. Single-walled carbon nanotubes (SWCNTs) are carbon nanostructures in which a single layer of graphene sheets forms a cylindrical shape, while multi-walled carbon nanotubes are carbon nanostructures in which two or more layers of graphene sheets are concentrically stacked to form a cylindrical shape. A graphene sheet refers to a layer in which carbon atoms in the sp2 hybrid orbitals that form graphite crystals are located at the vertices of a regular hexagon. The shape of the carbon nanotubes is not limited, but examples include needles, cylindrical tubes, fishbone-shaped (fishbone or cup-stacked), trump-shaped (platelet), and coil-shaped. From the viewpoint of satisfying the above-mentioned G / D ratio range, the first conductive agent is preferably a single-walled carbon nanotube, and the second conductive agent is preferably a multi-walled carbon nanotube. Hereinafter, an example will be described in which the first conductive agent and the second conductive agent are carbon nanotubes.

[0045] The ratio (W2 / W1) of the mass (W2) of the second conductive agent in the negative electrode mixture layer 41 to the mass (W1) of the first conductive agent in the negative electrode mixture layer 41 is preferably 0.01 or more, more preferably 0.1 or more, and even more preferably 0.2 or more. When (W2 / W1) is 0.01 or more, it is easy to ensure the contact area between the negative electrode active material and the conductive agent, thereby further improving the charge / discharge cycle characteristics of the battery. When (W2 / W1) is less than 0.01, the viscosity of the negative electrode mixture slurry may become too high, which may prevent the formation of the negative electrode mixture layer 41. When (W2 / W1) is less than 2.0, it is sufficient, but is preferably 1.5 or less, and more preferably 1.0 or less. When (W2 / W1) is 2.0 or more, the length of the conductive path formed by the conductive agent is likely to be short, making it difficult to sufficiently improve the charge / discharge cycle characteristics of the battery. Therefore, (W2 / W1) is preferably 0.01 or more and less than 2.0, more preferably 0.1 or more and 1.5 or less, and even more preferably 0.2 or more and 1.0 or less.

[0046] The sum (W1 + W2) of the mass of the first conductive agent (W1) and the mass of the second conductive agent (W2) is preferably 0.01 mass% or more, more preferably 0.02 mass% or more, and even more preferably 0.03 mass% or more, relative to the total mass of the negative electrode mixture layer 41. When (W1 + W2) is 0.01 mass% or more, a good conductive path is formed in the negative electrode mixture layer 41, and the charge / discharge cycle characteristics of the battery can be further improved. Furthermore, (W1 + W2) is preferably 0.8 mass% or less, more preferably 0.7 mass% or less, and even more preferably 0.6 mass% or less, relative to the total mass of the negative electrode mixture layer 41. When (W1 + W2) exceeds 0.8 mass%, the amount of negative electrode active material in the negative electrode mixture layer 41 decreases, which may result in a decrease in battery capacity. Therefore, (W1+W2) is preferably 0.01 mass% or more and 0.8 mass% or less, more preferably 0.02 mass% or more and 0.7 mass% or less, and even more preferably 0.03 mass% or more and 0.6 mass% or less, relative to the total mass of the negative electrode mixture layer 41.

[0047] The negative electrode mixture layer 41 may contain a conductive agent other than the first conductive agent and the second conductive agent. Examples of conductive agents other than the first conductive agent and the second conductive agent include conductive agents having a G / D ratio of 65 or more. From the viewpoint of more significantly exhibiting the effects of the present disclosure, the sum (W1 + W2) of the mass (W1) of the first conductive agent and the mass (W2) of the second conductive agent is preferably 90 mass% or more, and more preferably 95 mass% or more, of the total mass of the conductive agents contained in the negative electrode mixture layer 41.

[0048] The first conductive agent and the second conductive agent preferably have one or more functional groups (hereinafter referred to as "acidic functional groups") selected from the group consisting of a carboxy group, a sulfo group, and a hydroxy group. In this case, an attractive force acts between the acidic functional groups and the negative electrode active material, thereby suppressing isolation of the Si-containing material during charge and discharge, and further improving the charge and discharge cycle characteristics of the battery. The amount of the acidic functional groups in the first conductive agent and the second conductive agent is, for example, 0.01 mmol / g or more and 0.25 mmol / g or less.

[0049] The method for imparting acidic functional groups to the conductive agent is not particularly limited, but for example, the conductive agent can be added to a mixed acid of sulfuric acid and nitric acid and reacted for a predetermined time to impart acidic functional groups to the conductive agent. It is desirable to stir the mixed acid during the reaction. The reaction time is not particularly limited, but is preferably, for example, 1 hour or more. The reaction temperature is also not particularly limited, but is preferably in the range of 20°C or higher and 45°C or lower.

[0050] The acidic functional groups imparted to the conductive agent can be analyzed by TPD-MS (thermal evolved gas analysis). The measurement conditions are, for example, as follows: Measurement device: gas chromatograph mass spectrometer (GC section: 7890 manufactured by Agile Technologies, MS section: MS-60030BU) Temperature conditions: temperature increased from 100°C to 1000°C at 20°C / min and held for 10 minutes Carrier gas: helium Flow rate: 50 mL / min Measurement mass number: m / z = 10 or more and 600 or less

[0051] The fiber length of the first conductive agent is preferably 0.5 μm or more, more preferably 1.0 μm or more, and even more preferably 2.0 μm or more. When the fiber length of the first conductive agent is 0.5 μm or more, a relatively long conductive path can be formed between the negative electrode active materials. Furthermore, from the viewpoint of productivity, the fiber length of the first conductive agent is preferably 500 μm or less, more preferably 450 μm or less, and even more preferably 400 μm or less. Therefore, the fiber length of the first conductive agent is preferably 0.5 μm or more and 500 μm or less, more preferably 1.0 μm or more and 450 μm or less, and even more preferably 2.0 μm or more and 400 μm or less. The fiber length of the first conductive agent can be determined by measuring the lengths of 50 arbitrary carbon nanotubes using a field emission scanning electron microscope (FESEM) and calculating the arithmetic average. The same applies to the method for measuring the fiber length of the second conductive agent.

[0052] The fiber length of the second conductive agent is preferably 0.2 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more. When the fiber length of the first conductive agent is 0.2 μm or more, a relatively long conductive path can be formed between the negative electrode active materials. Furthermore, the fiber length of the second conductive agent is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. When the fiber length of the second conductive agent exceeds 30 μm, the contact area between the second conductive agent and the negative electrode active material tends to be small, which may make it difficult to form a strong conductive path between the negative electrode active materials. Therefore, the fiber length of the second conductive agent is preferably 0.2 μm or more and 30 μm or less, more preferably 0.5 μm or more and 25 μm or less, and even more preferably 1.0 μm or more and 20 μm or less.

[0053] The outermost diameter of the first conductive agent is preferably 0.5 nm or more, more preferably 0.7 nm or more, and even more preferably 1.0 nm or more. If the outermost diameter of the first conductive agent is less than 0.5 nm, the conductive path formed by the first conductive agent may be easily broken due to volume changes of the Si-containing material. Furthermore, the outermost diameter of the first conductive agent is preferably less than 5.0 nm, more preferably 4.5 nm or less, and even more preferably 4.0 nm or less. If the outermost diameter of the first conductive agent is less than 5.0 nm, the fiber length of the first conductive agent tends to be long, and a relatively long conductive path can be formed between the negative electrode active materials. Therefore, the outermost diameter of the first conductive agent is preferably 0.5 nm or more but less than 5.0 nm, more preferably 0.7 nm or more but 4.5 nm or less, and even more preferably 1.0 nm or more but 4.0 nm or less. The outer diameter of the first conductive agent can be determined by measuring the outer diameters of 50 arbitrary carbon nanotubes using a field emission scanning electron microscope (FE-SEM) or a transmission electron microscope (TEM) and calculating the arithmetic average. The outer diameter of the second conductive agent can be determined in the same manner.

[0054] The outermost diameter of the second conductive agent is preferably 5.0 nm or more, more preferably 6.0 nm or more, and even more preferably 7.0 nm or more. When the outermost diameter of the second conductive agent is 5.0 nm or more, the contact area between the second conductive agent and the negative electrode active material is increased, making it easier to form a strong conductive path between the negative electrode active materials. Furthermore, the outermost diameter of the second conductive agent is preferably 20.0 nm or less, more preferably 17.5 nm or less, and even more preferably 15.0 nm or less. When the outermost diameter of the second conductive agent exceeds 20.0 nm, the fiber length of the second conductive agent tends to be excessively short, which may make it difficult to form a conductive path between the negative electrode active materials. Therefore, the outermost diameter of the second conductive agent is preferably 5.0 nm or more and 20.0 nm or less, more preferably 6.0 nm or more and 17.5 nm or less, and even more preferably 7.0 nm or more and 15.0 nm or less.

[0055] The negative electrode mixture layer 41 may further contain a binder. Examples of binders contained in the negative electrode mixture layer 41 include fluorine-containing resins such as styrene butadiene rubber (SBR), nitrile-butadiene rubber (NBR), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF), as well as polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. Among these, SBR and NBR are preferred, and SBR is particularly preferred. These may be used alone or in combination of two or more. The content of the binder in the negative electrode mixture layer 41 is, for example, 0.5% by mass or more and 5% by mass or less with respect to the total mass of the mixture layer 41.

[0056] The negative electrode mixture layer 41 may further contain a thickener. Examples of thickeners include carboxymethyl cellulose (CMC) or a salt thereof (CMC-Na, etc.), polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyethylene oxide (PEO), and polyvinyl alcohol (PVA). These may be used alone or in combination of two or more. The content of the thickener in the negative electrode mixture layer 41 is, for example, 0.5% by mass or more and 10% by mass or less with respect to the total mass of the negative electrode mixture layer 41.

[0057] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0058] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metals such as Ti, Al, Si, and Mg, and phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0059] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0060] Example 1 Preparation of First Conductive Agent Single-walled carbon nanotubes with a fiber length of 5 μm and an outermost diameter of 1.6 nm were placed in a mixed acid of sulfuric acid and nitric acid, and stirred at 40°C for 12 hours. After the treatment, the mixture was filtered through a glass filter, and the carbon nanotubes remaining on the glass filter were washed with pure water and then air-dried overnight. When the dried sample was analyzed by TPD-MS, the presence of sulfo groups, carboxyl groups, and hydroxyl groups was confirmed. Furthermore, the G / D ratio of the first conductive agent was measured by Raman spectroscopy, and was found to be 40.

[0061] [Preparation of Second Conductive Agent] Multi-walled carbon nanotubes with a fiber length of 1 μm and an outermost diameter of 10 nm were placed in a mixed acid of sulfuric acid and nitric acid and stirred at 40°C for 12 hours. After the treatment, the mixture was filtered through a glass filter, and the carbon nanotubes remaining on the glass filter were washed with pure water and then air-dried overnight. When the dried sample was analyzed by TPD-MS, the presence of sulfo groups, carboxyl groups, and hydroxyl groups was confirmed. Furthermore, the G / D ratio of the second conductive agent was measured by Raman spectroscopy and found to be 10.

[0062] [Preparation of negative electrode] A carbon material having a BET specific surface area of ​​4 m ２ / g of first graphite particles, and a BET specific surface area of ​​6 m ２Graphite particles containing 0.1g of second graphite particles and SiO as a Si-containing material were mixed in a mass ratio of 70:30, and this mixture was used as a negative electrode active material. The first conductive agent and the second conductive agent were mixed in a mass ratio of 100:1, and this mixture was used as a conductive agent. That is, the ratio (W2 / W1) of the mass of the second conductive agent (W2) to the mass of the first conductive agent (W1) was 0.01. The negative electrode active material, the conductive agent, sodium carboxymethyl cellulose, and styrene butadiene rubber were then mixed in a mass ratio of 100:0.02:1.0:1.0. An appropriate amount of water was added to this mixture and kneaded to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil with a thickness of 10 μm, and the coating was dried. The dried coating film was rolled using a pressure roller to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector.

[0063] [Preparation of Electrolyte Solution] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:70. LiPF ６ was dissolved in the electrolyte to a concentration of 1.4 mol / L. Furthermore, 2 mass % of vinylene carbonate (VC) was dissolved in the electrolyte to the total amount of the electrolyte. This was used as the electrolyte of the example.

[0064] [Fabrication of Secondary Battery] A separator made of a polyethylene microporous membrane was placed between the metallic Li and the negative electrode, and then rolled up and formed into a flat shape to prepare a wound electrode assembly. This electrode assembly and the above-mentioned electrolyte solution were housed in an exterior body made of aluminum laminate, and the interior of the exterior body was depressurized to impregnate the separator with the electrolyte solution. The opening of the exterior body was then sealed to prepare a secondary battery.

[0065] [Evaluation of Capacity Retention Rate] The secondary battery thus fabricated was subjected to constant current charging (current 0.1 It, end voltage 0.005 V) in a temperature environment of 25°C, followed by constant voltage charging (voltage 0.005 V, end current 0.01 It). Thereafter, the secondary battery was subjected to constant current discharging (current 0.1 It, end voltage 1.5 V). This cycle of charging and discharging constitutes one cycle, and 10 cycles were performed. The capacity retention rate of the secondary battery in the charge-discharge cycles was calculated using the following formula. Note that a higher capacity retention rate indicates improved charge-discharge cycle characteristics. Capacity retention rate = (discharge capacity at 10th cycle / discharge capacity at 1st cycle)

[0066] [Evaluation of Slurry Viscosity] The viscosity of the negative electrode mixture slurry prepared in the preparation of the negative electrode was measured under the following conditions using a viscosity measuring device, and the influence on the coatability was judged from the measurement results under conditions (1) and (2) based on the viscosity. ◯ indicates that there was no problem with the coating film and it could be coated in a good state; △ indicates that there was no problem with the coating film but if the viscosity increased any further it would not be possible to coat in a good state; and × indicates that there was a problem with the coating film and it would not be possible to coat in a good state. Viscosity measuring device: TV-22 viscometer manufactured by Toki Sangyo Co., Ltd. Rotation speed and measurement time: Condition (1) rotation speed 2 rpm, time 60 seconds, Condition (2) rotation speed 20 rpm, time 60 seconds

[0067] Example 2 A test cell was fabricated and evaluated in the same manner as in Example 1, except that the first conductive agent and the second conductive agent were mixed in a mass ratio of 90:10 and this mixture was used as the conductive agent in the fabrication of the negative electrode. That is, the ratio (W2 / W1) of the mass of the second conductive agent (W2) to the mass of the first conductive agent (W1) was 0.11.

[0068] Example 3 A test cell was fabricated and evaluated in the same manner as in Example 1, except that the first conductive agent and the second conductive agent were mixed in a mass ratio of 80:20 and this mixture was used as the conductive agent in the fabrication of the negative electrode. That is, the ratio (W2 / W1) of the mass of the second conductive agent (W2) to the mass of the first conductive agent (W1) was 0.25.

[0069] Example 4 A test cell was fabricated and evaluated in the same manner as in Example 1, except that the first conductive agent and the second conductive agent were mixed in a mass ratio of 70:30 and this mixture was used as the conductive agent in the fabrication of the negative electrode. That is, the ratio (W2 / W1) of the mass of the second conductive agent (W2) to the mass of the first conductive agent (W1) was 0.43.

[0070] Example 5 A test cell was prepared and evaluated in the same manner as in Example 1, except that the first conductive agent and the second conductive agent were mixed in a mass ratio of 50:50 and this mixture was used as the conductive agent in the preparation of the negative electrode. In other words, the ratio (W2 / W1) of the mass of the second conductive agent (W2) to the mass of the first conductive agent (W1) was 1.0.

[0071] Example 6 A test cell was fabricated and evaluated in the same manner as in Example 1, except that the first conductive agent and the second conductive agent were mixed at a mass ratio of 120:1 and this mixture was used as the conductive agent in the fabrication of the negative electrode. That is, the ratio (W2 / W1) of the mass of the second conductive agent (W2) to the mass of the first conductive agent (W1) was 0.008.

[0072] Comparative Example 1 A test cell was prepared and evaluated in the same manner as in Example 1, except that in preparing the negative electrode, only the first conductive agent was used as the conductive agent, and the second conductive agent was not used.

[0073] Comparative Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that the first conductive agent and the second conductive agent were mixed in a mass ratio of 50:100 and this mixture was used as the conductive agent in the preparation of the negative electrode. In other words, the ratio (W2 / W1) of the mass of the second conductive agent (W2) to the mass of the first conductive agent (W1) was 2.0.

[0074] Comparative Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that in preparing the negative electrode, only the second conductive agent was used as the conductive agent, and the first conductive agent was not used.

[0075] Comparative Example 4 A test cell was fabricated and evaluated in the same manner as in Example 1, except that in fabricating the negative electrode, only the second conductive agent was used as the conductive agent and the content of the conductive agent in the negative electrode mixture layer was increased. More specifically, the negative electrode active material, the second conductive agent, sodium carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 100:0.02:1.0:1.0.

[0076] The evaluation results of the capacity retention rates and the viscosity of the slurries of the secondary batteries of the Examples and Comparative Examples are shown in Table 1. The capacity retention rates of the secondary batteries of the Examples and Comparative Examples shown in Table 1 are expressed relative to the capacity retention rate of the secondary battery of Comparative Example 1, which is set to 100.

[0077]

[0078] As shown in Table 1, the secondary batteries of the examples have improved capacity retention rates compared to the secondary batteries of the comparative examples. In other words, by including a carbon material and a Si-containing material as the negative electrode active material and using a first conductive agent having a G / D ratio of 20 or more and less than 65 and a second conductive agent having a G / D ratio of 0.7 or more and less than 20 as the conductive agents, the charge-discharge cycle characteristics can be improved. On the other hand, the results of comparative examples 3 and 4 show that when only the second conductive agent is used, even if the content of the second conductive agent is increased, the capacity retention rate is hardly improved. This is presumably because the conductive path formed by the second conductive agent is relatively short and easily broken by a volume change of the Si-containing material.

[0079] Furthermore, in Example 6, in which the content ratio of the first conductive agent was increased, the slurry viscosity became excessively high and it was not possible to apply the slurry in a good condition. From this result, it can be said that if the content ratio of the first conductive agent is excessively increased, the capacity retention rate is improved, but productivity may decrease.

[0080] The present disclosure is further described by the following embodiments. Aspect 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, as a negative electrode active material, a carbon material and a Si-containing material, and, as conductive agents, a first conductive agent having a G / D ratio obtained by Raman spectroscopy of 20 or more and less than 65, and a second conductive agent having a G / D ratio obtained by Raman spectroscopy of 0.7 or more and less than 20. Aspect 2: A negative electrode for a secondary battery according to Aspect 1, wherein the first conductive agent and the second conductive agent are carbon nanotubes. Aspect 3: A negative electrode for a secondary battery according to Aspect 1 or 2, wherein a ratio (W2 / W1) of the mass (W2) of the second conductive agent to the mass (W1) of the first conductive agent is 0.01 or more and less than 2.0. Configuration 4: The negative electrode for a secondary battery according to any one of Configurations 1 to 3, wherein the sum (W1+W2) of the mass of the first conductive agent (W1) and the mass of the second conductive agent (W2) is 0.01 mass% or more and 0.8 mass% or less with respect to the total mass of the negative electrode mixture layer. Configuration 5: The negative electrode for a secondary battery according to any one of Configurations 1 to 4, wherein the first conductive agent and the second conductive agent have one or more functional groups selected from the group consisting of a carboxy group, a sulfo group, and a hydroxy group. Configuration 6: The negative electrode for a secondary battery according to Configuration 2, wherein the fiber length of the first conductive agent is 0.5 μm or more and 500 μm or less. Configuration 7: The negative electrode for a secondary battery according to Configuration 2, wherein the fiber length of the second conductive agent is 0.2 μm or more and 30 μm or less. Configuration 8: The negative electrode for a secondary battery according to Configuration 2, wherein the outermost peripheral diameter of the first conductive agent is 0.5 nm or more and less than 5.0 nm. Configuration 9: The negative electrode for a secondary battery according to Configuration 2, wherein the outermost peripheral diameter of the second conductive agent is 5.0 nm or more and less than 20.0 nm. Configuration 10: The negative electrode for a secondary battery according to any one of Configurations 1 to 9, wherein the content of the Si-containing material is 10 mass % or more and 60 mass % or less, relative to the total mass of the negative electrode mixture layer. Configuration 11: The negative electrode for a secondary battery according to any one of Configurations 1 to 10, wherein the ratio (V1 / V2) of the volume (V1) of the Si-containing material in a charged state to the volume (V2) of the Si-containing material in a discharged state is 1.5 or more.Aspect 12: The negative electrode for a secondary battery according to any one of Aspects 1 to 11, wherein the Si-containing material comprises an ion-conducting phase and an Si phase dispersed in the ion-conducting phase, the ion-conducting phase being at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. Aspect 13: The carbon material comprises a first carbon material and a second carbon material having different BET specific surface areas, the BET specific surface area of ​​the first carbon material being 0.9 m. ２ / g or more, 6.5m ２ / g or less, and the BET specific surface area of ​​the second carbon material is 2.5 m ２ / g over 8.0m ２ / g or less. 14. A secondary battery comprising the negative electrode for a secondary battery according to any one of configurations 1 to 13, a positive electrode, and an electrolyte.

[0081] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Positive electrode current collector, 31 Positive electrode mixture layer, 40 Negative electrode current collector, 41 Negative electrode mixture layer

Claims

1. A negative electrode for a secondary battery comprising a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector, wherein the negative electrode mixture layer contains, as a negative electrode active material, a carbon material and a Si-containing material, and as conductive agents, a first conductive agent having a G / D ratio obtained by Raman spectroscopy of 20 or more and less than 65, and a second conductive agent having a G / D ratio obtained by Raman spectroscopy of 0.7 or more and less than 20, wherein a ratio (W2 / W1) of a mass (W1) of the second conductive agent to a mass (W2) of the first conductive agent is less than 2.

0.

2. The negative electrode for a secondary battery according to claim 1, wherein the first conductive material and the second conductive material are carbon nanotubes.

3. The negative electrode for a secondary battery as described in claim 1, wherein the ratio (W2 / W1) of the mass (W2) of the second conductive material to the mass (W1) of the first conductive material is 0.01 or more and less than 2.

0.

4. The negative electrode for a secondary battery as described in claim 1, wherein the sum (W1+W2) of the mass (W1) of the first conductive material and the mass (W2) of the second conductive material is 0.01 mass% or more and 0.8 mass% or less with respect to the total mass of the negative electrode mixture layer.

5. The negative electrode for a secondary battery according to claim 1, wherein the first conductive agent and the second conductive agent have one or more functional groups selected from the group consisting of a carboxy group, a sulfo group, and a hydroxy group.

6. The negative electrode for a secondary battery according to claim 2, wherein the fiber length of the first conductive agent is 0.5 μm or more and 500 μm or less.

7. The negative electrode for a secondary battery according to claim 2, wherein the fiber length of the second conductive agent is 0.2 μm or more and 30 μm or less.

8. The negative electrode for a secondary battery according to claim 2, wherein the outermost peripheral diameter of the first conductive material is 0.5 nm or more and less than 5.0 nm.

9. The negative electrode for a secondary battery according to claim 2, wherein the outermost peripheral diameter of the second conductive material is 5.0 nm or more and less than 20.0 nm.

10. The negative electrode for a secondary battery according to claim 1, wherein the content of the Si-containing material is 10 mass % or more and 60 mass % or less with respect to the total mass of the negative electrode mixture layer.

11. The negative electrode for a secondary battery according to claim 1, wherein a ratio (V1 / V2) of a volume (V1) of the Si-containing material in a charged state to a volume (V2) of the Si-containing material in a discharged state is 1.5 or more.

12. The negative electrode for a secondary battery according to claim 1, wherein the Si-containing material comprises an ion-conducting phase and a Si phase dispersed in the ion-conducting phase, and the ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

13. The carbon material includes a first carbon material and a second carbon material having different BET specific surface areas, and the BET specific surface area of ​​the first carbon material is 0.9 m ２ / g or more, 6.5m ２ / g or less, and the BET specific surface area of ​​the second carbon material is 2.5 m ２ / g over 8.0m ２ The negative electrode for a secondary battery according to claim 1 , wherein the molecular weight of the negative electrode is 1 / g or less.

14. A secondary battery comprising: the negative electrode for secondary batteries according to any one of claims 1 to 13; a positive electrode; and an electrolyte.

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