Negative electrode for secondary battery, and secondary battery

Abstract

A negative electrode for a secondary battery, said negative electrode comprising a negative electrode mixture layer that includes a negative electrode active material, single-walled carbon nanotubes, and an aromatic sulfonate, wherein the negative electrode active material includes a Si-containing material, the Si-containing material content in the negative electrode active material is 15 mass% to 75 mass%, the ratio X1 of the mass of the single-walled carbon nanotubes included in the negative electrode mixture layer to the mass of the negative electrode active material is 0.015% to 3.0%, and the ratio Y1 of the mass of the aromatic sulfonate included in the negative electrode mixture layer to the mass of the negative electrode active material is 0.02% to 4.0%.

Description

Negative electrode for secondary battery and secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-052949, filed on March 28, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a negative electrode for a secondary battery and a secondary battery.

[0003] As secondary batteries are required to have higher energy densities, there is hope for the use of silicon (Si)-containing materials that alloy with lithium as negative electrode active materials with high theoretical capacity densities.

[0004] Patent Document 1 proposes "a negative electrode for a lithium ion secondary battery, comprising a negative electrode active material layer containing a silicon-based active material and carbon nanotubes having an average diameter of 10 nm or more and 120 nm or less and an average length of 0.5 μm or more and 20 μm or less, wherein the content of the carbon nanotubes is 0.1 mass % or more and 2 mass % or less with respect to the total mass of the negative electrode active material layer."

[0005] JP 2016-110876 A

[0006] A negative electrode for a secondary battery has a negative electrode mixture layer formed by applying a slurry (negative electrode slurry) containing a negative electrode active material to the surface of a negative electrode current collector and drying the coating. When the negative electrode active material contains a Si-containing material, the Si-containing material undergoes a large volume change during charge and discharge, which gradually reduces the conductive paths in the negative electrode mixture layer and tends to reduce the conductivity of the negative electrode mixture layer. The greater the content of the Si-containing material in the negative electrode active material, the more significant the reduction in the conductive paths, and the more likely it is that the capacity retention rate will decrease.

[0007] When carbon nanotubes are used as in Patent Document 1, the reduction of conductive paths in the negative electrode mixture layer is suppressed. However, carbon nanotubes tend to aggregate in the negative electrode slurry and tend to form structures due to intermolecular forces with the polymer component, resulting in uneven distribution. This tendency becomes more pronounced as the content of Si-containing material in the negative electrode active material increases and the amount of carbon nanotubes used increases. Therefore, it is important to suppress the aggregation and structure formation of carbon nanotubes and increase the dispersibility of carbon nanotubes in the negative electrode slurry.

[0008] One aspect of the present disclosure relates to a negative electrode for a secondary battery, including a negative electrode mixture layer including a negative electrode active material, single-walled carbon nanotubes, and an aromatic sulfonate, wherein the negative electrode active material includes a Si-containing material, and a content of the Si-containing material in the negative electrode active material is 15 mass% or more and 75 mass% or less, a ratio X1 of a mass of the single-walled carbon nanotubes contained in the negative electrode mixture layer to a mass of the negative electrode active material is 0.015% or more and 3.0% or less, and a ratio Y1 of a mass of the aromatic sulfonate contained in the negative electrode mixture layer to a mass of the negative electrode active material is 0.02% or more and 4.0% or less.

[0009] According to the present disclosure, the decrease in the capacity retention rate of a secondary battery is suppressed. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of its configuration and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] 1 is a longitudinal sectional view schematically illustrating a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

[0011] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0012] The present disclosure encompasses any combination of two or more claims arbitrarily selected from the appended claims, i.e., any combination of two or more claims arbitrarily selected from the appended claims may be combined unless a technical contradiction arises.

[0013] [Negative Electrode] The present disclosure relates to a negative electrode for a secondary battery. Hereinafter, a negative electrode according to the present disclosure will also be referred to as a "negative electrode (N)." Secondary batteries to which the present disclosure is applied include non-aqueous electrolyte secondary batteries. Non-aqueous electrolyte secondary batteries include lithium ion secondary batteries that use a liquid non-aqueous electrolyte (electrolytic solution), solid-state batteries that include a gel electrolyte, and all-solid-state batteries. A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. A separator is usually disposed between the positive electrode and the negative electrode.

[0014] The negative electrode (N) has a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector. The negative electrode mixture layer is composed of a negative electrode mixture and may be in the form of a layer or a film. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector. The dispersion medium is not particularly limited, but examples thereof include water, alcohol, and N-methyl-2-pyrrolidone (NMP).

[0015] The negative electrode mixture layer (i.e., negative electrode mixture) of the negative electrode (N) contains a negative electrode active material, single-walled carbon nanotubes, and an aromatic sulfonate. The single-walled carbon nanotubes function as a conductive agent. The aromatic sulfonate functions as a dispersant for the single-walled carbon nanotubes. The negative electrode mixture layer may further contain polymer components such as a negative electrode binder and a thickener.

[0016] The negative electrode active material includes a Si-containing material. The content of the Si-containing material in the negative electrode active material (hereinafter also referred to as "content (Si)") is 15% by mass or more and 75% by mass or less, and may be greater than 30% by mass and less than 75% by mass, 31% by mass or more and 75% by mass or less, or 35% by mass or more and 75% by mass or less. By increasing the proportion of the Si-containing material relative to the total amount of the negative electrode active material in this manner, a high-capacity negative electrode (N) can be obtained. The negative electrode active material accounts for most of the mass of the negative electrode mixture layer. Therefore, the content of the Si-containing material in the negative electrode mixture layer is slightly smaller than the above range. The content of the Si-containing material in the negative electrode mixture layer may be, for example, 13% by mass or more and 74% by mass or less, 26% by mass or more and 74% by mass or less, 27% by mass or more and 74% by mass or less, or 30% by mass or more and 74% by mass or less.

[0017] The ratio X1 of the mass of the single-walled carbon nanotubes (hereinafter also referred to as "SWCNTs") contained in the negative electrode mixture layer to the mass of the negative electrode active material is 0.015% or more and 3.0% or less, or may be 0.03% or more and 2.0% or less, or may be 0.05% or more and 1.0% or less.

[0018] SWCNTs have a particularly significant effect of increasing the conductivity of the negative electrode mixture layer when the ratio of the Si-containing material to the total amount of the negative electrode active material is high, as in the above range. In other words, when the content (Si) is less than 15% by mass, an improvement in the conductivity of the negative electrode mixture layer is observed, but the effect is not significant. On the other hand, when the content (Si) is 15% by mass or more, the improvement in the conductivity of the negative electrode mixture layer is significant, and when the content (Si) exceeds 30% by mass, the improvement in the conductivity of the negative electrode mixture layer is very significant. Furthermore, the capacity retention rate of the secondary battery improves in accordance with the improvement in conductivity.

[0019] On the other hand, if the content (Si) exceeds 75 mass%, the dispersibility of SWCNTs decreases, and the effect of increasing the conductivity of the negative electrode mixture layer is not sufficiently achieved. This tendency is particularly pronounced when the negative electrode active material contains a carbon material such as graphite. This is presumably due to the increased affinity between SWCNTs and carbon materials.

[0020] The remarkable improvement in the conductivity of the negative electrode mixture layer when the content (Si) is 15% by mass or more is a phenomenon not observed with multi-walled carbon nanotubes (hereinafter also referred to as "MWCNT"). In order to improve the conductivity of the negative electrode mixture layer using MWCNT, a larger amount of MWCNT is required than in the case of SWCNT. Therefore, when the content (Si) is 15% by mass or more, the dispersibility of the required amount of MWCNT decreases, and disadvantages such as a decrease in the voids in the negative electrode mixture layer become significant.

[0021] The ratio Y1 of the mass of the aromatic sulfonate contained in the negative electrode mixture layer to the mass of the negative electrode active material is 0.02% or more and 4.0% or less, or may be 0.05% or more and 2.0% or less, or 0.08% or more and 1.5% or less, or may be 0.1% or more and 1.0% or less.

[0022] Aromatic sulfonates have a high affinity with SWCNT. This is thought to be due to the π-π interaction between the aromatic ring of aromatic sulfonic acid and SWCNT. It is thought that the adsorption of aromatic sulfonates onto SWCNT suppresses the interaction between SWCNTs, thereby suppressing the aggregation and uneven distribution of SWCNTs. The interaction between SWCNTs and the interaction between SWCNTs and polymer components is suppressed by the anionic sulfonate group (SO ３ － This is thought to be due to the repulsion of the negative charges of the

[0023] The ratios X1 and Y1 may satisfy 0.15X1≦Y1≦4X1+0.6. The ratio X1 is the ratio of the mass of SWCNT to the mass of the negative electrode active material, and the ratio Y1 is the ratio of the mass of the aromatic sulfonate necessary to disperse the SWCNT to the mass of the negative electrode active material. Therefore, when both the SWCNT and the aromatic sulfonate exhibit high effects, a linear correlation is observed between them. By blending the SWCNT and the aromatic sulfonate in the negative electrode mixture so as to satisfy 0.15X1≦Y1≦4X1+0.6, a negative electrode with well-balanced physical properties can be more easily manufactured.

[0024] Similarly, from the viewpoint of exerting the high effects of both SWCNT and aromatic sulfonate, the ratio Y2 of the mass of the aromatic sulfonate contained in the negative electrode mixture layer to the mass of the single-walled carbon nanotubes may be 5% or more and 3000% or less, or 10% or more and 200% or less.

[0025] The ratio X2 of the mass of SWCNTs to the mass of the Si-containing material contained in the negative electrode mixture layer is 0.02% or more and 4% or less, and may be 0.04% or more and 3% or less. The required amount of SWCNTs may be affected by the Si-containing material among the negative electrode active materials. By controlling the ratio X2, it is possible to further enhance the effect of improving the conductivity of the negative electrode mixture layer.

[0026] Similarly, the required amount of aromatic sulfonate may be affected by the Si-containing material, among other negative electrode active materials. The ratio Y3 of the mass of the aromatic sulfonate contained in the negative electrode mixture layer to the mass of the Si-containing material may be 0.0025% (or 0.025%) or more and 6% or less, or may be 0.005% (or 0.05%) or more and 3% or less.

[0027] (SWCNT) Single-walled carbon nanotubes (SWCNTs) will be described. Carbon nanotubes (CNTs) are carbon materials with nanometer diameters that have a structure in which a sheet (graphene) of a six-membered ring network formed by carbon atoms is rolled into a cylindrical shape, and have excellent electrical conductivity. A CNT with one graphene layer constituting the cylindrical structure is called a single-walled CNT (SWCNT). A CNT with multiple graphene layers constituting the cylindrical structure is called a multi-walled CNT (MWCNT).

[0028] SWCNTs behave differently from MWCNTs because they are thin, flexible, and have a strong interaction with the aromatic sulfonate used as a dispersant. SWCNTs are effective in ensuring contact between the Si-containing material and its surroundings when voids form around the Si-containing material when it expands during charging and contracts during discharging. SWCNTs have a strong ability to form and maintain conductive paths.

[0029] From the viewpoint of improving the conductivity of the negative electrode mixture layer containing a large amount of Si-containing material with as little material as possible, the average diameter of the SWCNTs is, for example, preferably 3 nm or less, more preferably 2 nm or less, and may be less than 1 nm. The average diameter of the SWCNTs is preferably 0.8 nm or more. In this case, the strength of the SWCNTs is sufficiently ensured, and the conductive path through the SWCNTs is easily maintained during charge and discharge.

[0030] The negative electrode mixture layer may contain a small amount of MWCNT. However, the proportion of SWCNTs to the total CNTs is preferably 90% or more, and more preferably 95% or more. The proportion of SWCNTs to the total CNTs is determined by the following method.

[0031] A scanning electron microscope (SEM) is used to obtain an image of the cross section of the negative electrode mixture layer or the CNTs. Using the SEM image, a number of CNTs (e.g., approximately 50 to 200) are randomly selected and observed, the number of SWCNTs is determined, and the ratio of the number of SWCNTs to the total number of selected CNTs is calculated.

[0032] From the viewpoint of ensuring many conductive paths between the Si-containing material and its surroundings, the average length of the SWCNTs is preferably 1 μm or more and 100 μm or less, and more preferably 5 μm or more and 20 μm or less.

[0033] The average diameter and average length of the SWCNTs are determined by taking an image of the cross section of the negative electrode mixture layer or the CNTs using a SEM and / or a transmission electron microscope (TEM), measuring the lengths and diameters of a number of randomly selected SWCNTs (e.g., about 50 to 200) using the image, and averaging the measured values. The length of the SWCNTs means the length when they are assumed to be stretched linearly.

[0034] The SWCNTs can be confirmed, for example, by an SEM image of a cross section of the negative electrode mixture layer. Examples of methods for analyzing the SWCNTs include Raman spectroscopy and thermogravimetric analysis.

[0035] Note that CNT is one type of negative electrode conductive additive, and usable conductive additives are not limited to CNT. For example, carbon black such as acetylene black may be used in combination with CNT.

[0036] (Aromatic Sulfonate) As described above, the aromatic ring of the aromatic sulfonate has a high affinity with SWCNT due to π-π interactions, and also has the effect of suppressing interactions between SWCNTs and between SWCNTs and polymer components.

[0037] Aromatic sulfonates are salts of sulfonic acid with an aromatic ring, and the sulfonic acid dissociates to form an anionic sulfonate group (SO ３ － The sulfonate group may be directly bonded to the aromatic ring. ３ － When R is represented by the formula (I), R may be a ring-constituting element of an aromatic ring.

[0038] Specific examples of aromatic rings constituting the aromatic sulfonate include aromatic rings having a skeleton such as furan, pyrrole, imidazole, thiophene, phosphole, pyrazole, oxazole, isoxazole, thiazole, benzene, pyridine, pyrazine, pyrimidine, pyridazine, triazine, benzofuran, isobenzofuran, indole, isoindole, benzothiophene, benzophosphole, benzimidazole, purine, indazole, benzoxazole, benzisoxazole, benzothiazole, naphthalene, quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline, anthracene, and pyrene. Among these, aromatic rings having a skeleton such as benzene or naphthalene are preferred in view of ease of availability.

[0039] The aromatic sulfonate may be an aromatic hydroxysulfonate having a hydroxyl group bonded to the aromatic ring. The hydroxyl group bonded to the aromatic ring can form a hydrogen bond with a hydroxyl group (e.g., a silanol group) present on the surface of the Si-containing material. Therefore, the aromatic hydroxysulfonate has a high affinity with the Si-containing material and can bond strongly to both the SWCNT and the Si-containing material. The number of hydroxyl groups bonded to one aromatic ring is not limited, but one or two is preferred.

[0040] Specific examples of aromatic hydroxysulfonates include phenolsulfonates, sulfosalicylates, sulfophthalates, cresolsulfonates, naphtholsulfonates, catecholsulfonates, benzyl alcohol sulfonates, aminonaphtholsulfonates, and ligninsulfonates.

[0041] The aromatic sulfonate may be a metal salt. Examples of the metal salt include sodium salt, lithium salt, potassium salt, ammonium salt, calcium salt, and magnesium salt. Among these, at least one selected from the group consisting of sodium aromatic sulfonate, lithium aromatic sulfonate, and potassium aromatic sulfonate is preferred because it is easily ionically dissociated and inexpensive.

[0042] The weight average molecular weight (Mw) of the aromatic sulfonate may be 5,000 or more, 5,000 to 500,000, 5,000 to 300,000, or 5,000 to 100,000. In other words, the aromatic sulfonate may be a polymer (aromatic sulfonic acid polymer salt).

[0043] The weight average molecular weight is a weight average molecular weight calculated in terms of standard polystyrene measured by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as a solvent, or a weight average molecular weight calculated in terms of standard polyethylene glycol measured by GPC using water, dimethylformamide (DMF), or dimethyl sulfoxide (DMSO) as a solvent.

[0044] The aromatic sulfonic acid polymer salt does not need to be composed solely of aromatic sulfonate units, but may be a polymer containing aromatic sulfonate units. For example, 30 mol % or more, or even 50 mol % or more, or 80 mol % or more of the aromatic rings constituting the aromatic sulfonic acid polymer salt may be aromatic rings of an aromatic sulfonate (e.g., an aromatic hydroxysulfonate). For example, the aromatic sulfonic acid polymer salt may contain units of an aromatic compound other than an aromatic sulfonate. That is, the aromatic sulfonic acid polymer salt may be a copolymer of an aromatic sulfonate and another aromatic compound.

[0045] The aromatic sulfonic acid polymer salt may contain a unit other than an aromatic ring. The unit other than an aromatic ring may be a linking group that links the aromatic rings together. The linking group is typically an alkylene group such as a methylene group or an ethylene group, but is not particularly limited.

[0046] An example of an aromatic sulfonic acid polymer salt is a formaldehyde condensate of an aromatic sulfonate. That is, the aromatic sulfonic acid polymer salt may be a novolac resin containing an aromatic ring having a sulfonate group. Such a formaldehyde condensate has a structure in which the aromatic rings of the aromatic sulfonate are linked to each other by methylene groups. Because the formaldehyde condensate is flexible and has a network structure, it has a high affinity with both SWCNTs and negative electrode active materials and can play a role in reinforcing the formation of conductive paths by the SWCNTs.

[0047] Among these, formaldehyde condensates of phenolsulfonates are preferred because they are easily available and inexpensive. That is, the aromatic sulfonate may be a polyphenolsulfonate. The polyphenolsulfonate may contain phenol units that do not have sulfonate groups. For example, 30 mol% or more, further 50 mol% or more, or 80 mol% or more of the aromatic rings constituting the polyphenolsulfonate may be phenolsulfonates.

[0048] The aromatic sulfonates may be used alone or in combination of two or more.

[0049] The structural formula of polyphenolsulfonic acid sodium salt, which is a suitable example of a formaldehyde condensate, is shown below.

[0050]

[0051] (Negative electrode active material) The negative electrode active material refers to a material contained in a negative electrode and capable of electrochemically absorbing and desorbing lithium ions. Here, the negative electrode active material includes a Si-containing material, and the content of the Si-containing material in the negative electrode active material (Si content) is 15 mass% or more and 75 mass% or less.

[0052] A carbon material is preferred as the negative electrode active material to be used in combination with the Si-containing material. Compared to the Si-containing material, the carbon material expands and contracts less during charging and discharging, and therefore, when used in combination with the Si-containing material, the capacity retention rate of the battery is easily improved. The content of the carbon material (excluding the carbon phase serving as the lithium ion conductive phase described below) in the negative electrode active material may be, for example, 25% by mass or more and 85% by mass or less. That is, the remainder of the negative electrode active material other than the Si-containing material may be a carbon material.

[0053] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among these, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles. One type of carbon material may be used alone, or two or more types may be used in combination.

[0054] (Si-containing material) A Si-containing material is a type of alloy material and is a general term for materials containing silicon (Si). Silicon alone undergoes a large volume change during charging and discharging, causing it to become pulverized. Therefore, Si-containing materials in which silicon is combined with other materials are used.

[0055] The content of silicon (Si) contained in the Si-containing material is, for example, 20% or more and 80% or less, and may be 30% or more and 70% or less.

[0056] The Si-containing material may be, for example, a composite material including a lithium ion conductive phase and a silicon phase dispersed within the lithium ion conductive phase. The average particle size of such a Si-containing material may be, for example, 1 μm to 20 μm or 5 μm to 12 μm. Within this particle size range, stress caused by volumetric changes in the Si-containing material during charge and discharge can be easily alleviated, making it easier to achieve a better capacity retention rate.

[0057] The average particle size of the Si-containing material refers to the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by Horiba Ltd. can be used as the measuring device.

[0058] The lithium ion conductive phase is preferably at least one selected from the group consisting of a silicate phase, a silicon oxide phase, and a carbon phase. Specifically, the Si-containing material may be at least one selected from the group consisting of the following first composite material, second composite material, and third composite material.

[0059] <First Composite Material> The first composite material includes a silicon oxide phase and a first silicon phase dispersed in the silicon oxide phase. The first composite material may include a conductive material that coats at least a portion of its surface. The first composite material is superior among Si-containing materials in that it has high stability and small volume change. The high stability is thought to be due to the small particle size of the first silicon phase dispersed in the silicon oxide phase, which makes deep charging difficult. On the other hand, the silicon oxide phase has a relatively large number of sites that irreversibly trap lithium ions, and therefore tends to have a large irreversible capacity among Si-containing materials. The trapping of lithium ions by the silicon oxide phase is thought to increase the structural stability of the first composite material and contribute to suppressing volume change.

[0060] The first composite material can be obtained, for example, by heating silicon oxide in a non-oxidizing atmosphere containing an inert gas such as argon to carry out a disproportionation reaction. In the disproportionation reaction, Si crystallites can be uniformly formed in the silicon oxide phase. The size of the silicon phase formed by the disproportionation reaction is small, for example, with an average particle size of less than 100 nm, and can also be in the range of 5 nm to 50 nm. The main component of the silicon oxide (for example, 95 to 100 mass%) can be silicon dioxide. That is, the first composite material can be a silicon dioxide containing SiO ２ phase and SiO ２ The first composite material may have a first silicon phase dispersed within the first silicon phase. ｘ The range of the x value may be 0<x<2, but is preferably 0.9≦x≦1.1, and x=1 may also be acceptable.

[0061] <Second Composite Material> The second composite material includes a silicate phase and a second silicon phase dispersed within the silicate phase. The second composite material is synthesized by combining silicate and silicon in any combination, which makes it easy to increase the silicon phase content and is suitable for achieving high capacity. The silicate phase has an advantage in that it has few sites that irreversibly trap lithium ions and has a small irreversible capacity. The second composite material may also include a conductive material that coats at least a portion of its surface.

[0062] The content of the second silicon phase in the second composite material may be, for example, 40% by mass or more and 80% by mass or less, which makes it easier to achieve both high capacity and improved cycle characteristics of the battery.

[0063] The silicate phase may contain, for example, at least one element selected from the group consisting of Group 1 and Group 2 elements of the long periodic table. Examples of Group 1 and Group 2 elements of the long periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements may include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), and titanium (Ti). Among these, a silicate phase containing lithium (lithium silicate phase) is preferred due to its small irreversible capacity and high initial charge / discharge efficiency. That is, the second composite material may include a lithium silicate phase and a second silicon phase dispersed within the lithium silicate phase.

[0064] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. This is advantageous in terms of stability and lithium ion conductivity. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase (Li / Si) is, for example, greater than 0 and less than 4. Examples of elements other than Li, Si, and O that may be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0065] The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚ It may have a composition expressed as (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z preferably satisfies the relationship 0<z<1, and more preferably z=½.

[0066] The average particle size of the second silicon phase is, for example, 500 nm or less, or may be 400 nm or less, or 200 nm or less. When the second silicon phase has such an average particle size, the capacity of the second composite material can be easily increased. On the other hand, if the average particle size is 500 nm or less, the volume change of the second silicon phase during charge and discharge becomes relatively small, thereby improving the structural stability of the second composite material. Here, the average particle size of the second silicon phase is a value after at least the first charge.

[0067] The composition of the second composite material can be analyzed, for example, by the following method. The battery is disassembled, the negative electrode is removed, washed with a non-aqueous solvent such as ethylene carbonate, and dried. After that, a cross-section of the negative electrode mixture layer is processed using a cross-section polisher (CP) to obtain a sample. A backscattered electron image of the sample cross-section is obtained using a field emission scanning electron microscope (FE-SEM), and the cross-section of the second composite material is observed. Qualitative and quantitative analysis of the elements of the observed second composite material can be performed using an Auger electron spectroscopy (AES) analyzer (accelerating voltage 10 kV, beam current 10 nA).

[0068] For example, Li ２ｚ SiO ２＋ｚ In the case of the second composite particles represented by the formula (1), the ratio of 2z to (2+z) can be calculated from the obtained lithium (Li) content and oxygen (O) content.

[0069] <Third Composite Material> The third composite material includes a carbon phase and a third silicon phase dispersed within the carbon phase. The third composite material is synthesized by combining the raw materials for the carbon phase and silicon in an arbitrary combination, which makes it easy to increase the content of the silicon phase and is suitable for achieving high capacity. The carbon phase itself can electrochemically absorb and release lithium ions, thereby achieving capacity.

[0070] The content of the third silicon phase in the third composite material may be, for example, 40% by mass or more and 80% by mass or less, which makes it easier to achieve both high capacity and improved cycle characteristics of the battery.

[0071] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other. Amorphous carbon can be obtained, for example, by sintering a carbon source in an inert atmosphere and pulverizing the resulting sintered body. The third composite material can be obtained, for example, by mixing a carbon source with silicon particles, stirring the mixture while crushing it with a mixer such as a ball mill, and then firing the mixture in an inert atmosphere. Examples of carbon sources that may be used include sugars such as carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, and sucrose, as well as water-soluble resins.

[0072] The average particle size of the third silicon phase is, for example, 500 nm or less, or may be 400 nm or less, or 200 nm or less. When the third silicon phase has such an average particle size, the capacity of the second composite material can be easily increased. On the other hand, if the average particle size is 500 nm or less, the volume change of the third silicon phase during charge and discharge becomes relatively small, thereby improving the structural stability of the third composite material. Here, the average particle size of the third silicon phase is a value after at least the first charge.

[0073] The content of the silicon phase contained in the Si-containing material (first to third composite materials) can be measured by Si-NMR. Desirable measurement conditions for Si-NMR are shown below.

[0074] Measurement equipment: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) 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: around -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0075] The average grain size of the silicon phase contained in the Si-containing material (first to third composite materials) can be measured from a cross-sectional SEM (scanning electron microscope) photograph of the Si-containing material. Specifically, the average grain size of the silicon phase can be determined by averaging the maximum grain sizes of 100 randomly selected silicon phases.

[0076] (Negative electrode binder) The negative electrode mixture usually contains a negative electrode binder. As the negative electrode binder, an acrylic resin is preferred. The acrylic resin is a polymer component having a carboxyl group, and has a strong binding force, firmly solidifying the negative electrode mixture layer and enhancing the adhesion between the negative electrode mixture layer and the negative electrode current collector.

[0077] The acrylic resin preferably contains a polymer containing at least one selected from the group consisting of (meth)acrylic acid units and (meth)acrylate units. The acrylic resin may be a homopolymer or a copolymer. In the copolymer, the total content of the (meth)acrylic acid units and (meth)acrylate units is preferably, for example, 50 mol % or more, and more preferably 80 mol % or more. Note that "(meth)acrylic acid" means at least one selected from the group consisting of "acrylic acid" and "methacrylic acid".

[0078] The acrylic resin preferably contains at least a (meth)acrylate unit, which is advantageous for facilitating preparation of a negative electrode slurry and improving battery characteristics. Examples of the (meth)acrylate include alkali metal salts such as lithium salts and sodium salts, and ammonium salts.

[0079] Specific examples of acrylic resins include polyacrylic acid, polymethacrylic acid, copolymers containing repeating units of acrylic acid and / or methacrylic acid (acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, etc.), salts thereof, etc. One type of acrylic resin may be used alone, or two or more types may be used in combination.

[0080] The negative electrode binder may contain a polymer component other than acrylic resin. Examples include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; and polyimide resins such as polyimide and polyamideimide. A rubber-like material such as styrene-butadiene copolymer rubber (SBR) may also be used as the binder.

[0081] (Thickener) The negative electrode mixture may contain a thickener. As the thickener, carboxymethyl cellulose (CMC) resin is preferable. The CMC resin may be CMC or a CMC salt. Examples of the CMC salt include alkali metal salts such as lithium salt and sodium salt, and ammonium salt. One type of thickener may be used alone, or two or more types may be used in combination.

[0082] The ratio Z of the mass of the polymer component (e.g., the sum of the negative electrode binder and the thickener) contained in the negative electrode mixture layer to the mass of the negative electrode active material is, for example, 1% or more and 15% or less, or may be 1% or more and 10% or less, or 1% or more and 5% or less.

[0083] (Negative electrode current collector) As the negative electrode current collector, a non-porous conductive substrate (metal foil or the like) or a porous conductive substrate (mesh, net, punched sheet or the like) is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy and the like. The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 μm to 50 μm, or 5 to 20 μm.

[0084] Next, a secondary battery according to an embodiment of the present invention will be described in detail. The secondary battery includes the above-described negative electrode, for example, a positive electrode as described below, and a non-aqueous electrolyte.

[0085] [Positive Electrode] The positive electrode comprises, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture contains a positive electrode active material as an essential component and may contain, as optional components, a positive electrode binder, a positive electrode conductive additive, and the like. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The dispersion medium is not particularly limited, but examples include water, alcohol, and N-methyl-2-pyrrolidone (NMP).

[0086] The positive electrode active material includes a material that electrochemically absorbs and releases lithium ions. A lithium transition metal composite oxide is preferred as the material that electrochemically absorbs and releases lithium ions. Examples of the lithium transition metal composite oxide include layered compounds with a rock salt crystal structure, spinel compounds, and polyanion compounds. Among these, a layered compound containing Ni, in which the proportion of Ni relative to all metal elements other than Li is 90 mol % or more, is preferred in terms of achieving high capacity.

[0087] An example of a preferred layered compound is a compound having the composition of formula (C): Li α Ni(1-x1-x2-x3-y)Co ｘ１ Mn ｘ２ Al ｘ３ M ｙ O ２＋βand a lithium transition metal composite oxide represented by the formula (C) where 0.95≦α≦1.05, 0.8≦1-x1-x2-x3-y≦0.99, 0≦x1≦0.1, 0≦x2≦0.1, 0≦x3≦0.1, 0≦y≦0.1, and −0.05≦β≦0.05. M is at least one element selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y.

[0088] The positive electrode current collector is, for example, a metal foil, and examples of the material include stainless steel, aluminum, aluminum alloy, and titanium.

[0089] Examples of the positive electrode binder include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF), polyolefin resins such as polyethylene and polypropylene, polyamide resins such as aramid resin, polyimide resins such as polyimide and polyamideimide, vinyl resins such as polyacrylonitrile and polyvinyl acetate, polyvinylpyrrolidone, polyethersulfone, etc. These may be used alone or in combination of two or more.

[0090] Examples of the positive electrode conductive assistant include carbon black such as acetylene black, conductive fibers such as CNT, etc. These may be used alone or in combination of two or more.

[0091] [Separator] A separator is usually interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0092] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a solvent (non-aqueous solvent) and a solute dissolved in the solvent. Examples of the solute include lithium salts. Various additives may be added to the non-aqueous electrolyte.

[0093] Known materials can be used as the solvent. Examples of solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0094] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ etc.), lithium salts of fluorine-containing acids (LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ etc.), lithium salt of fluorine-containing acid imide (LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２and lithium halides (LiCl, LiBr, LiI, etc.). The lithium salts may be used alone or in combination of two or more.

[0095] The concentration of the lithium salt in the nonaqueous electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By setting the lithium salt concentration within this range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained.

[0096] [Secondary Battery] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any form, such as a cylindrical type, a prismatic type, a coin type, a button type, or a laminate type.

[0097] The structure of the secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery 10 that is an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0098] The secondary battery 10 includes an electrode group 18, an electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode plate 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode plate 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode plate 15 and a negative electrode plate 16 with a separator 17 interposed therebetween.

[0099] (Additional Notes) The above description discloses the following technologies. (Technology 1) A negative electrode for a secondary battery, comprising a negative electrode mixture layer including a negative electrode active material, single-walled carbon nanotubes, and an aromatic sulfonate, wherein the negative electrode active material includes a Si-containing material, and the content of the Si-containing material in the negative electrode active material is 15% by mass or more and 75% by mass or less, a ratio X1 of the mass of the single-walled carbon nanotubes contained in the negative electrode mixture layer to the mass of the negative electrode active material is 0.015% or more and 3.0% or less, and a ratio Y1 of the mass of the aromatic sulfonate contained in the negative electrode mixture layer to the mass of the negative electrode active material is 0.02% or more and 4.0% or less. (Technology 2) The negative electrode for a secondary battery according to Technology 1, wherein the average diameter of the single-walled carbon nanotubes is 3 nm or less. (Technology 3) The negative electrode for a secondary battery according to Technology 1 or 2, wherein the weight-average molecular weight of the aromatic sulfonate is 5,000 or more. (Technology 4) The negative electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the aromatic sulfonate comprises at least one selected from the group consisting of a sodium salt of aromatic sulfonate, a lithium salt of aromatic sulfonate, and a potassium salt of aromatic sulfonate. (Technology 5) The negative electrode for a secondary battery according to any one of Technologies 1 to 4, wherein the aromatic sulfonate is a polyphenol sulfonate. (Technology 6) The negative electrode for a secondary battery according to any one of Technologies 1 to 5, wherein the content of the Si-containing material in the negative electrode mixture layer is greater than 30 mass% and not more than 75 mass%. (Technology 7) The negative electrode for a secondary battery according to any one of Technologies 1 to 6, wherein the ratio Y1 is 0.05% or more and 2.0% or less. (Technology 8) The negative electrode for a secondary battery according to any one of Technologies 1 to 7, wherein the ratio X1 and the ratio Y1 satisfy 0.15X1≦Y1≦4X1+0.6. (Technology 9) The negative electrode for a secondary battery according to any one of Technologies 1 to 8, wherein the Si-containing material comprises a lithium ion conductive phase and a silicon phase dispersed in the lithium ion conductive phase. (Technology 10) The negative electrode for a secondary battery according to any one of Technologies 1 to 9, wherein the lithium ion conductive phase includes at least one phase selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase.(Technology 11) The negative electrode for a secondary battery according to any one of Techniques 1 to 10, wherein the negative electrode mixture layer contains a polymer component, and a ratio Z of the mass of the polymer component contained in the negative electrode mixture layer to the mass of the negative electrode active material is 1% or more and 15% or less. (Technology 12) The negative electrode for a secondary battery according to Technique 11, wherein the polymer component contains at least one resin selected from the group consisting of an acrylic resin and a carboxymethyl cellulose resin. (Technology 13) A secondary battery comprising the negative electrode for a secondary battery according to any one of Techniques 1 to 12, a positive electrode, and a non-aqueous electrolyte.

[0100] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0101] Comparative Examples AC1 to AC32 [Fabrication of Negative Electrodes] A third composite material having an average particle size of 5 μm was prepared as a Si-containing material. The third composite material contained an amorphous carbon phase and a silicon phase dispersed within the carbon phase. The content of the silicon phase in the third composite material was 50 mass %. The average particle size of the silicon phase was 20 nm.

[0102] A negative electrode slurry was prepared containing a negative electrode active material (a mixture of 10% by mass of Si-containing material and 90% by mass of graphite), CMC, lithium polyacrylate (PAAL), and styrene-butadiene copolymer rubber (SBR) in a mass ratio of 100:1:1:1, CNT in a ratio X1 shown in Table 1, and aromatic sulfonate in a ratio Y1 shown in Table 1. The negative electrode slurry was applied to both sides of copper foil serving as a negative electrode current collector, the coating was dried, and then rolled to a density of 1.2 g / cm. 3 A negative electrode mixture layer of the above formula was formed to obtain a negative electrode.

[0103] X1: Ratio of the mass of SWCNT to the mass of the negative electrode active material: in the range of 0.01% to 1%. X1': Ratio of the mass of MWCNT to the mass of the negative electrode active material: in the range of 0.05% to 3%. Y1: Ratio of the mass of aromatic sulfonate to the mass of the negative electrode active material: in the range of 0% to 1.0%.

[0104] SWCNT: average diameter 1.5 nm, average length 5 μm MWCNT: average diameter 6 nm, average length 0.5 μm

[0105] Aromatic sulfonate: a compound represented by the following general formula:

[0106]

[0107] Polyphenol sulfonic acid sodium salt (weight average molecular weight 8000) represented by the formula

[0108] [Preparation of Positive Electrode] Lithium nickel composite oxide (LiNi ０．８ Co ０．１８ Al ０．０２ O ２ ), acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5, and N-methyl-2-pyrrolidone (NMP) was added, followed by stirring using a mixer to prepare a positive electrode slurry. The positive electrode slurry was applied to both sides of an aluminum foil serving as a positive electrode current collector, the coating was dried, and then rolled to a density of 3.6 g / cm. 3 A positive electrode mixture layer of the above was formed to obtain a positive electrode.

[0109] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving a lithium salt in a non-aqueous solvent. The non-aqueous solvent used was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC) in a volume ratio of 20:70:5:5. The lithium salt used was LiPF ６ LiPF in a non-aqueous electrolyte was used. ６ The concentration was set to 1.35 mol / L.

[0110] [Fabrication of Non-Aqueous Electrolyte Secondary Batteries] A tab was attached to each electrode, and the positive and negative electrodes were spirally wound with a separator interposed therebetween so that the tabs were positioned at the outermost periphery to fabricate an electrode group. The electrode group was inserted into an exterior body made of aluminum laminate film, vacuum dried at 105°C for 2 hours, and then a non-aqueous electrolyte was poured into the exterior body. The opening of the exterior body was sealed to obtain batteries AC1 to AC32 of Comparative Examples AC1 to AC32.

[0111]

[0112] Comparative Examples BC1 to BC23, Examples BE1 to BE9 Batteries BC1 to BC23 of Comparative Examples BC1 to BC23 and batteries BE1 to BE9 of Examples BE1 to BE9 were fabricated using the proportions X1, X1′, and Y1 shown in Table 2 in the same manner as in Table 1, except that a mixture of 15 mass % of a Si-containing material and 85 mass % of graphite was used as the negative electrode active material.

[0113]

[0114] Comparative Examples CC1 to CC23, Examples CE1 to CE9 Batteries CC1 to CC23 of Comparative Examples CC1 to CC23 and batteries CE1 to CE9 of Examples CE1 to CE9 were produced using the proportions X1, X1′, and Y1 shown in Table 3 in the same manner as in Table 1, except that a mixture of 35 mass % of a Si-containing material and 65 mass % of graphite was used as the negative electrode active material.

[0115]

[0116] Comparative Examples DC1 to DC7, Examples DE1 to DE9 Batteries DC1 to DC7 of Comparative Examples DC1 to DC7 and batteries DE1 to DE9 of Examples DE1 to DE9 were produced in the same manner as in Table 1, except that a mixture of 50 mass % Si-containing material and 50 mass % graphite was used as the negative electrode active material, with the proportions X1 and Y1 shown in Table 4. No examples using MWCNT were carried out.

[0117]

[0118] Comparative Examples EC1 to EC7, Examples EE1 to EE9 Batteries EC1 to EC7 of Comparative Examples EC1 to EC7 and batteries EE1 to EE9 of Examples EE1 to EE9 were produced in the same manner as in Table 1, except that a mixture of 75 mass % of Si-containing material and 25 mass % of graphite was used as the negative electrode active material, with the proportions X1 and Y1 shown in Table 5. No examples using MWCNT were produced.

[0119]

[0120] Comparative Examples FC1 to FC16 Batteries FC1 to FC16 of Comparative Examples FC1 to FC16 were produced in the same manner as in Table 1, except that a mixture of 80 mass % of Si-containing material and 20 mass % of graphite was used as the negative electrode active material, with the proportions X1 and Y1 shown in Table 6. No examples using MWCNT were carried out.

[0121]

[0122] Each battery prepared above was evaluated using the following method. The battery was charged at a constant current of 0.2 It in an environment of 25°C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.015 It. Subsequently, constant current discharge was performed at a current of 0.2 It until the voltage reached 2.75 V. The rest period between charge and discharge was 10 minutes. Charging and discharging were performed in an environment of 25°C. Charge and discharge were repeated, and the ratio (percentage) of the discharge capacity at the 200th cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate. In each table, the results of a battery in which the aromatic sulfonate content Y1 was 0 mass% and the SWCNT content X1 was 1 mass% (i.e., battery AC4 in Table 1, battery BC4 in Table 2, battery CC4 in Table 3, battery DC4 in Table 4, battery EC4 in Table 5, and battery FC4 in Table 6) are set as a reference value of 100, and the capacity retention rates of the other batteries are shown in Tables 1 to 6. The larger the value, the better the capacity retention rate.

[0123] From Tables 1 to 3, it can be seen that the trends in the results of the capacity retention rate are completely different between a battery using SWCNT and aromatic sulfonate in a predetermined ratio and a battery using MWCNT and aromatic sulfonate in a predetermined ratio; that the effects of SWCNT and aromatic sulfonate are particularly high when the content of the Si-containing material in the negative electrode active material is in the range of 15 to 75 mass% (preferably in the range of 30 to 75 mass%); and that the effect of improving the capacity retention rate is particularly large when the ratios X1 and Y1 satisfy the relationship 0.15X1≦Y1≦4X1+0.6.

[0124] A non-aqueous electrolyte secondary battery comprising the negative electrode for a secondary battery according to the present invention is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0125] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0126] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode plate, 15a: Positive electrode lead, 16: Negative electrode plate, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove portion, 23: First insulating plate, 24: Second insulating plate

Claims

1. A negative electrode for a secondary battery, comprising a negative electrode mixture layer including a negative electrode active material, single-walled carbon nanotubes, and an aromatic sulfonate, wherein the negative electrode active material includes a Si-containing material, the content of the Si-containing material in the negative electrode active material being 15% by mass or more and 75% by mass or less, a ratio X1 of the mass of the single-walled carbon nanotubes contained in the negative electrode mixture layer to the mass of the negative electrode active material being 0.015% or more and 3.0% or less, and a ratio Y1 of the mass of the aromatic sulfonate contained in the negative electrode mixture layer to the mass of the negative electrode active material being 0.02% or more and 4.0% or less.

2. The negative electrode for a secondary battery according to claim 1, wherein the single-walled carbon nanotubes have an average diameter of 3 nm or less.

3. The negative electrode for a secondary battery according to claim 1, wherein the aromatic sulfonate has a weight average molecular weight of 5,000 or more.

4. The negative electrode for a secondary battery according to claim 1, wherein the aromatic sulfonate comprises at least one selected from the group consisting of sodium aromatic sulfonate, lithium aromatic sulfonate, and potassium aromatic sulfonate.

5. The negative electrode for a secondary battery according to claim 1, wherein the aromatic sulfonate is a polyphenol sulfonate.

6. The negative electrode for a secondary battery according to claim 1, wherein the content of the Si-containing material in the negative electrode mixture layer is greater than 30 mass % and not more than 75 mass %.

7. The negative electrode for a secondary battery according to claim 1, wherein the ratio Y1 is 0.05% or more and 2.0% or less.

8. The negative electrode for a secondary battery according to claim 1, wherein the ratio X1 and the ratio Y1 satisfy the relationship 0.15X1≦Y1≦4X1+0.

6.

9. The negative electrode for a secondary battery according to claim 1, wherein the Si-containing material comprises a lithium ion conductive phase and a silicon phase dispersed within the lithium ion conductive phase.

10. The negative electrode for a secondary battery according to claim 1, wherein the lithium ion conductive phase comprises at least one phase selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase.

11. The negative electrode for a secondary battery according to claim 1, wherein the negative electrode mixture layer contains a polymer component, and a ratio Z of the mass of the polymer component contained in the negative electrode mixture layer to the mass of the negative electrode active material is 1% or more and 15% or less.

12. The negative electrode for a secondary battery according to claim 11, wherein the polymer component comprises at least one selected from the group consisting of acrylic resins and carboxymethyl cellulose resins.

13. A secondary battery comprising the negative electrode for secondary batteries according to any one of claims 1 to 12, a positive electrode, and a non-aqueous electrolyte.

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