Negative electrode for secondary battery and secondary battery

Abstract

This negative electrode for a secondary battery is provided with a negative electrode mixture layer comprising an Si compound-containing negative electrode active material, a conductive material and a binding material; the conductive material includes single-layer carbon nanotubes with a outermost diameter of at most 5 nm, and the binding material contains carboxymethyl cellulose salt with a weight average molecular weight of 150,000-450,000.

Description

[Technical Field]

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

[0002] Si compounds are alloying materials that can be alloyed with lithium, and are known to be able to store more lithium ions per unit volume than carbon-based active materials such as graphite, making them promising for use as negative electrode active materials in secondary batteries.

[0003] However, because silicon compounds undergo large volume changes (expansion and contraction) during charge and discharge, repeated charge and discharge can break the conductive network between the negative electrode active materials, resulting in a problem of reduced charge and discharge cycle performance.

[0004] To address these issues, Patent Document 1 discloses a technology in which carbon nanotubes are added to a negative electrode composite layer containing a Si compound to prevent disconnection of the conductive network between negative electrode active materials due to expansion and contraction of the Si compound, thereby suppressing deterioration of charge-discharge cycle characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-110876 Summary of the Invention

[0006] According to the technology of Patent Document 1, although the deterioration of charge-discharge cycle characteristics at a normal rate is suppressed, the effect of suppressing the deterioration of charge-discharge cycle characteristics at a high rate is not sufficiently obtained.

[0007] A negative electrode for a secondary battery according to one embodiment of the present disclosure includes a negative electrode composite layer having a negative electrode active material containing a Si compound, a conductive material, and a binder, wherein the conductive material contains single-walled carbon nanotubes having an outermost diameter of 5 nm or less, and the binder contains a carboxymethyl cellulose salt having a weight-average molecular weight of 150,000 or more and 450,000 or less.

[0008] A secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode is the above-described negative electrode for a secondary battery. [Effects of the Invention]

[0009] According to one embodiment of the present disclosure, the use of a negative electrode active material containing a Si compound can suppress deterioration in charge-discharge cycle characteristics at high rates. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] A negative electrode for a secondary battery according to one embodiment of the present disclosure includes a negative electrode composite layer having a negative electrode active material containing a Si compound, a conductive material, and a binder, wherein the conductive material contains single-walled carbon nanotubes having an outermost diameter of 5 nm or less, and the binder contains a carboxymethyl cellulose salt having a weight-average molecular weight of 150,000 or more and 450,000 or less. The negative electrode for a secondary battery according to one embodiment of the present disclosure can suppress deterioration of charge-discharge cycle characteristics at high rates. The mechanism by which this effect is achieved is unclear, but the following is thought to be the case.

[0012] When a carboxymethyl cellulose salt is added as a binder to a slurry containing a negative electrode active material and carbon nanotubes during negative electrode production, the hydrophilicity of the carboxymethyl cellulose salt generally repels the hydrophobic carbon nanotubes, resulting in aggregation of the carbon nanotubes. While single-walled carbon nanotubes are more likely to form a conductive network with a negative electrode active material containing a Si compound than multi-walled carbon nanotubes, the single-walled carbon nanotubes are more hydrophobic than multi-walled carbon nanotubes, and therefore aggregate in the presence of the carboxymethyl cellulose salt, preventing the formation of a sufficient conductive network between the negative electrode active material containing a Si compound and the single-walled carbon nanotubes. However, after extensive research, the inventors have found that by adjusting the weight-average molecular weight of the carboxymethyl cellulose salt to between 150,000 and 450,000, the hydrophilicity can be moderated while maintaining binding performance. Therefore, by using a carboxymethyl cellulose salt with a weight-average molecular weight of 150,000 or more and 450,000 or less, as in this embodiment, aggregation of single-walled carbon nanotubes is suppressed, and a conductive network between the negative electrode active material containing a Si compound and the single-walled carbon nanotubes is sufficiently formed, which is thought to suppress deterioration of charge-discharge cycle characteristics at high rates.

[0013] Hereinafter, an example of an embodiment will be described in detail with reference to the drawings. Note that the nonaqueous electrolyte secondary battery of the present disclosure is not limited to the embodiment described below. Furthermore, the drawings referred to in the description of the embodiment are schematic.

[0014] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing member 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, other types of electrode bodies may be used, such as a laminated electrode body formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include cylindrical, prismatic, coin-shaped, button-shaped, or other metal exterior cans, and pouch exteriors formed by laminating a resin sheet and a metal sheet.

[0015] Case body 16 is, for example, a cylindrical metal outer can with a bottom. A gasket 28 is provided between case body 16 and sealing body 17 to ensure airtightness inside the battery. Case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports sealing body 17. Protruding portion 22 is preferably formed in an annular shape along the circumferential direction of case body 16, and supports sealing body 17 on its upper surface.

[0016] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a filter 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 to one another at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, 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 ruptures, and gas is discharged from the opening of the cap 27.

[0017] In the secondary battery 10 shown in Fig. 1, a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in an insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside an insulating plate 19 and extends toward the bottom side of the case body 16. The positive electrode lead 20 is connected to the underside of a filter 23, which is the bottom plate 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 filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

[0018] Each component of the secondary battery 10 will be described in detail below.

[0019] [Negative electrode] The negative electrode 12 includes a negative electrode current collector made of, for example, a metal foil and a negative electrode composite layer formed on the current collector. The negative electrode current collector may be, for example, a foil of a metal such as copper that is stable within the potential range of the negative electrode, or a film with such a metal disposed on its surface. The negative electrode composite layer includes a negative electrode active material containing graphite particles and a Si compound, a binder, and a conductive material.

[0020] The negative electrode 12 can be produced, for example, by preparing a negative electrode composite slurry containing a negative electrode active material, a binder, a conductive material, and the like, applying this negative electrode composite slurry onto a negative electrode current collector, drying it to form a negative electrode composite layer, and then performing a compression step in which the negative electrode composite layer is compressed using a rolling roller or the like.

[0021] The negative electrode active material contains a Si compound. The Si compound may be any material capable of absorbing and releasing lithium ions, but from the viewpoint of increasing the capacity of the secondary battery, the Si compound preferably contains a lithium ion conductive phase and Si particles dispersed in the lithium ion conductive phase, and the lithium ion conductive phase is preferably at least one selected from a silicate phase, a silicon oxide phase, and a carbon phase.

[0022] Preferably, a conductive coating made of a highly conductive material is formed on the surface of the Si compound particles. Examples of materials constituting the conductive coating include at least one selected from carbon materials, metals, and metal compounds. Among these, carbon materials such as amorphous carbon are preferred. The carbon coating can be formed, for example, by a CVD method using acetylene, methane, or the like, or by a method in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with a silicon-based active material and heat-treated. Alternatively, the conductive coating may be formed by adhering a conductive filler such as carbon black to the surface of the Si compound particles using a binder.

[0023] Specific examples of the Si compound include Si compound A containing a silicate phase and Si particles dispersed in the silicate phase, Si compound B containing a silicon oxide phase and Si particles dispersed in the silicon oxide phase, and Si compound C containing a carbon phase and Si particles dispersed in the carbon phase. These may be used alone or in combination of two or more.

[0024] From the viewpoint of high lithium ion conductivity, the silicate phase of the Si compound A preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, etc. Among these, a silicate phase containing lithium (hereinafter sometimes referred to as a lithium silicate phase) is preferred.

[0025] The content of silicon particles in the Si compound A is preferably 30% by mass or more and 80% by mass or less, more preferably 35% by mass or more and 75% by mass or less, and more preferably 55% by mass or more and 70% by mass or less, from the viewpoints of increasing capacity and improving charge-discharge cycle characteristics.

[0026] The average particle size of the Si particles is, for example, preferably 500 nm or less, more preferably 200 nm or less, and even more preferably 50 nm or less before the first charge, from the viewpoint of suppressing cracks in the Si particles themselves. After the first charge, the average particle size of the Si particles is preferably 400 nm or less, and more preferably 100 nm or less. The average particle size of the Si particles is measured by observing a cross-sectional SEM (scanning electron microscope) photograph of the Si compound. Specifically, the average particle size of the Si particles is obtained by averaging the maximum diameters of any 100 Si particles.

[0027] The lithium silicate phase is represented, for example, by the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2.

[0028] The Si compound B in which Si particles are dispersed in the silicon oxide phase is represented, for example, by the general formula SiO x (preferably in the range of 0 < x < 2, more preferably in the range of 0.5 ≤ x ≤ 1.6). The Si compound C in which Si particles are dispersed in the carbon phase is represented, for example, by the general formula Si x C y (preferably in the ranges of 0 < x ≤ 1 and 0 < y ≤ 1, more preferably in the ranges of 0.3 ≤ x ≤ 0.45 and 0.7 ≤ y ≤ 0.55). The content and average particle size of the Si particles in the Si compounds B and C may be the same as those in the Si compound A.

[0029] The content of the Si compound in the negative electrode active material is preferably 1% by mass or more and 10% by mass or less with respect to the mass of the negative electrode active material, for example, in terms of suppressing an increase in the capacity of the secondary battery and a decrease in the charge-discharge cycle characteristics.

[0030] The negative electrode active material preferably contains graphite particles, for example, in terms of suppressing a decrease in the charge-discharge cycle characteristics of the secondary battery. The graphite particles are not particularly limited, such as natural graphite and artificial graphite. The interplanar spacing (d of the (002) plane of the graphite particles by wide-angle X-ray diffraction 002) is, for example, preferably 0.3354 nm or more, more preferably 0.3357 nm or more, and preferably less than 0.340 nm, more preferably 0.338 nm or less. The crystallite size (Lc(002)) of the graphite particles determined by X-ray diffraction is, for example, preferably 5 nm or more, more preferably 10 nm or more, and preferably 300 nm or less, more preferably 200 nm or less. The interplanar spacing (d 002 When the surface roughness (Tc) and the crystallite size (Lc(002)) satisfy the above ranges, the battery capacity of the secondary battery tends to be larger than when the surface roughness (Tc) and the crystallite size (Lc(002)) do not satisfy the above ranges.

[0031] The content of graphite particles in the negative electrode active material is preferably 80% by mass or more and 90% by mass or less relative to the mass of the negative electrode active material, for example, from the viewpoint of increasing the capacity of the secondary battery and suppressing deterioration of the charge-discharge cycle characteristics.

[0032] The content of the negative electrode active material in the negative electrode mixture layer is, for example, preferably 85 mass % or more, more preferably 90 mass % or more, and even more preferably 95 mass % or more, relative to the mass of the negative electrode mixture layer.

[0033] The conductive material contained in the negative electrode composite layer includes single-walled carbon nanotubes. Single-walled carbon nanotubes (SWCNTs) are carbon nanostructures in which a single layer of graphene sheets forms a cylindrical shape. Note that a graphene sheet refers to a layer in which carbon atoms in sp2 hybrid orbitals that form graphite crystals are located at the vertices of a regular hexagon. The shape of the single-walled carbon nanotubes is not limited, but examples of such shapes include needles, cylindrical tubes, fishbone shapes (fishbone or cup stacked shapes), playing card shapes (platelets), and coil shapes.

[0034] The outermost diameter (i.e., fiber diameter) of the single-walled carbon nanotubes may be 5 nm or less, for example, in order to facilitate the formation of a conductive network with the negative electrode active material, but is preferably 1 nm or more and 3 nm or less. The outermost diameter of the single-walled carbon nanotubes 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 then calculating the arithmetic average.

[0035] The fiber length of the single-walled carbon nanotubes is preferably 500 nm or more and 200 μm or less, and more preferably 1 μm or more and 100 μm or less, from the viewpoint of efficiently forming a conductive network with the negative electrode active material, etc. The fiber length of the single-walled carbon nanotubes can be determined by measuring the lengths of 50 random single-walled carbon nanotubes using a field emission scanning electron microscope (FE-SEM) and taking the arithmetic average.

[0036] The content of the single-walled carbon nanotubes is preferably 0.001% by mass or more and 0.1% by mass or less, and more preferably 0.001% by mass or more and 0.01% by mass or less, relative to the mass of the negative electrode active material, for example, in terms of efficiently forming a conductive network together with the negative electrode active material.

[0037] In addition to single-walled carbon nanotubes, the conductive material may also contain multi-walled carbon nanotubes, which are carbon nanostructures formed by concentrically stacking two or more graphene sheets to form a cylindrical shape, as long as the effects of the present disclosure are not impaired.

[0038] The conductive material may contain a particulate conductive material as needed. Examples of particulate conductive materials include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. When a particulate conductive material is used, it is preferable that the primary particle diameter is 5 nm or more and 100 nm or less, and that the aspect ratio is less than 10.

[0039] The binder includes a carboxymethyl cellulose salt, such as carboxymethyl cellulose sodium salt (CMC-Na) or carboxymethyl cellulose potassium salt (CMC-K).

[0040] The weight-average molecular weight of the carboxymethyl cellulose salt may be 150,000 or more and 450,000 or less to suppress deterioration of charge-discharge cycle performance at high rates, but is preferably 200,000 or more and 400,000 or less. By ensuring that the weight-average molecular weight of the carboxymethyl cellulose salt satisfies the above range, as described above, the hydrophilicity can be moderately reduced while maintaining binding performance, thereby suppressing aggregation of single-walled carbon nanotubes in the presence of the carboxymethyl cellulose salt. As a result, a sufficient conductive network is formed between the negative electrode active material containing a Si compound and the single-walled carbon nanotubes, suppressing deterioration of charge-discharge cycle performance at high rates. The weight-average molecular weight is measured by gel permeation chromatography (GPC).

[0041] The degree of etherification of the carboxymethyl cellulose salt is preferably 0.5 to 1.5, more preferably 0.6 to 1.1, in order to suppress deterioration of charge-discharge cycle performance at high rates. By setting the degree of etherification of the carboxymethyl cellulose salt within the above range, hydrophilicity can be further reduced while maintaining binding performance, thereby further suppressing aggregation of single-walled carbon nanotubes in the presence of the carboxymethyl cellulose salt. As a result, a conductive network between the negative electrode active material containing a Si compound and the single-walled carbon nanotubes is more fully formed, thereby further suppressing deterioration of charge-discharge cycle performance at high rates. The degree of etherification is measured by ashing titration. The weight-average molecular weight of the carboxymethyl cellulose salt can be controlled, for example, by adjusting the raw cellulose and the conditions for crushing the raw cellulose, and the degree of etherification can be controlled, for example, by adjusting the conditions for reaction with chloroacetic acid and its salts, and / or an alkaline agent.

[0042] The content of the carboxymethyl cellulose salt is preferably 1% by mass or more and 3% by mass or less, and more preferably 1.5% by mass or more and 2.5% by mass or less, relative to the mass of the negative electrode active material, in order to suppress a decrease in charge-discharge cycle characteristics at high rates, for example.

[0043] In addition to carboxymethyl cellulose salt, the binder may contain, for example, fluorine-based resin, PAN, polyimide-based resin, acrylic-based resin, polyolefin-based resin, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0044] [Positive electrode] The positive electrode 11 is composed of a positive electrode current collector, such as a metal foil, and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum, that is stable within the potential range of the positive electrode, or a film with such a metal disposed on its surface. The positive electrode composite layer contains, for example, a positive electrode active material, a binder, a conductive material, etc.

[0045] The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, etc. onto a positive electrode current collector, drying the slurry to form a positive electrode composite layer, and then performing a compression step in which the positive electrode composite layer is compressed using a rolling roller or the like.

[0046] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li xMn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of multiple kinds. In terms of achieving high capacity of the secondary battery, the positive electrode active material is Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), etc., and it is preferable to contain lithium nickel composite oxides.

[0047] Examples of the conductive material include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more kinds.

[0048] Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, and polyolefin-based resins. These may be used alone or in combination of two or more kinds.

[0049] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and a separator whose surface is coated with a material such as an aramid-based resin or ceramic may be used.

[0050] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (electrolytic solution) and may be a solid electrolyte using a gel polymer or the like. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0051] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, and γ-butyrolactone.

[0052] Examples of the above-mentioned ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, etc., and chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl, etc.

[0053] As the above-mentioned halogenated derivatives, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP), etc.

[0054] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O)F4), LiPF 6-x (C n [[ID=K12]]F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10, LiCl, LiBr, LiI, lithium chloroborane, lower aliphatic lithium carboxylates, borates such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 Examples of the lithium salt include imide salts such as SiO2) (where l and m are integers of 1 or more). The lithium salt may be used alone or in combination. Among these, LiPF6 is preferred from the viewpoints of ionic conductivity, electrochemical stability, etc. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of solvent.

[0055] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0056] Example 1 [Preparation of negative electrode] Graphite particles and a Si compound were mixed in a mass ratio of 90:10. This mixture was used as the negative electrode active material. Single-walled carbon nanotubes (SWCNTs) with an outermost diameter of 1 to 3 nm and a weight-average molecular weight of 20 × 10 6 Carboxymethylcellulose sodium salt (CMC-Na) with a degree of etherification of 1.1 was prepared. These materials were then mixed so that the mass ratio of negative electrode active material: SWCNT: CMC-Na: styrene-butadiene copolymer rubber (SBR) was 100:0.01:1.5:2 to prepare a negative electrode composite slurry. This slurry was applied to both sides of a copper foil current collector by a doctor blade method, and after drying the coating, the coating was compressed with a rolling roller to produce a negative electrode in which a negative electrode composite layer was formed on both sides of the negative electrode current collector.

[0057] [Preparation of positive electrode] The positive electrode active material is aluminum-containing lithium nickel cobalt oxide (LiNi 0.88 Co 0.09 Al 0.03O2) was used. 100 parts by mass of the above positive electrode active material, 1 part by mass of acetylene black, and 0.9 parts by mass of polyvinylidene fluoride were mixed in a solvent of N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode composite slurry. This slurry was applied to both sides of a 15 μm-thick aluminum foil, the coating was dried, and then the coating was rolled using a rolling roller to produce a positive electrode in which a positive electrode composite layer was formed on both sides of the positive electrode current collector.

[0058] [Preparation of non-aqueous electrolyte] LiPF6 was dissolved at a concentration of 1.4 mol / L in a non-aqueous solvent made by mixing ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75, and then 3 mass% of vinylene carbonate and 0.5 mass% of 1,6-diisocyanate hexane were added to prepare a non-aqueous electrolyte.

[0059] [Secondary battery production] (1) An aluminum positive electrode lead was attached to the positive electrode current collector, and a nickel-copper-nickel negative electrode lead was attached to the negative electrode current collector. After that, a polyethylene separator was placed between the positive and negative electrodes and wound to prepare a wound electrode body. (2) Insulating plates were placed above and below the electrode body, the negative electrode lead was welded to the case body, and the positive electrode lead was welded to the sealing member, and the electrode body was housed within the case body. (3) After the non-aqueous electrolyte was injected into the case body under reduced pressure, the open end of the case body was crimped to a sealing member via a gasket, completing the secondary battery.

[0060] <Example 2> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na having a degree of etherification of 1.1 was used.

[0061] Example 3 Weight average molecular weight 40×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na having a degree of etherification of 1.1 was used.

[0062] Example 4 A secondary battery was fabricated in the same manner as in Example 1, except that the CMC-Na of Example 2 was used and the negative electrode active material:SWCNT:CMC-Na:SBR were mixed so that the mass ratio thereof was 100:0.001:1.5:2.

[0063] <Example 5> A secondary battery was fabricated in the same manner as in Example 1, except that the CMC-Na of Example 2 was used and the negative electrode active material:SWCNT:CMC-Na:SBR were mixed so that the mass ratio thereof was 100:0.05:1.5:2.

[0064] Example 6 A secondary battery was fabricated in the same manner as in Example 1, except that the CMC-Na of Example 2 was used and the negative electrode active material:SWCNT:CMC-Na:SBR were mixed so that the mass ratio thereof was 100:0.1:1.5:2.

[0065] Example 7 Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with a degree of etherification of 0.5 was used.

[0066] Example 8 Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with a degree of etherification of 0.6 was used.

[0067] Example 9 Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with a degree of etherification of 1.5 was used.

[0068] Example 10 Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with a degree of etherification of 1.6 was used.

[0069] Example 11 Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with a degree of etherification of 0.3 was used.

[0070] Example 12 Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with a degree of etherification of 1.8 was used.

[0071] <Comparative Example 1> Weight average molecular weight 10×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 1.1 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0072] <Comparative Example 2> Weight average molecular weight 20×10 6 A secondary battery was fabricated in the same manner as in Comparative Example 1, except that CMC-Na with an etherification degree of 1.1 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0073] <Comparative Example 3> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with a degree of etherification of 1.1 was used and mixed so that the mass ratio of the negative electrode active material:CMC-Na:SBR was 100:1.5:2.

[0074] <Comparative Example 4> Weight average molecular weight 40×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 1.1 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0075] <Comparative Example 5> Weight average molecular weight 50×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 1.1 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0076] <Comparative Example 6> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 0.3 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0077] <Comparative Example 7> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 0.5 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0078] <Comparative Example 8> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 0.6 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0079] <Comparative Example 9> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 1.5 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0080] <Comparative Example 10> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 1.6 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0081] <Comparative Example 11> Weight average molecular weight 30×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na with an etherification degree of 1.8 was used and that the mass ratio of negative electrode active material:CMC-Na:SBR was mixed to be 100:1.5:2.

[0082] <Comparative Example 12> Weight average molecular weight 10×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na having a degree of etherification of 1.1 was used.

[0083] <Comparative Example 13> Weight average molecular weight 50×10 6 A secondary battery was fabricated in the same manner as in Example 1, except that CMC-Na having a degree of etherification of 1.1 was used.

[0084] <Comparative Example 14> A secondary battery was fabricated in the same manner as in Example 1, except that multi-walled carbon nanotubes (MWCNTs) having an outermost diameter of 11 nm were used and that CMC-Na of Example 2 was used.

[0085] [High-rate charge / discharge cycle test] Each secondary battery of the Examples and Comparative Examples was charged at a constant current of 1 C in a temperature environment of 25° C. until the battery voltage reached 4.2 V, and then discharged at a constant current of 1 C until the battery voltage reached 2.5 V. This high-rate charge / discharge cycle was repeated 50 times, and the capacity retention rate was calculated using the following formula.

[0086] Capacity retention rate (%) = (50th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 The results of the capacity retention rate in the high-rate charge-discharge cycle test for the examples and comparative examples are summarized in Table 1. Note that a higher capacity retention rate indicates that the deterioration of the charge-discharge cycle characteristics at high rates is more suppressed.

[0087] [Table 1]

[0088] As can be seen from Table 1, all of Examples 1 to 12 exhibited higher capacity retention rates than Comparative Examples 1 to 14. Therefore, by using single-walled carbon nanotubes having an outermost diameter of 5 nm or less as the conductive material and a carboxymethyl cellulose salt having a weight-average molecular weight of 150,000 or more and 450,000 or less as the binder, it is possible to suppress the deterioration of charge-discharge cycle characteristics at high rates. [Explanation of symbols]

[0089] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 Case body 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Overhang 23 Filters 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket

Claims

1. a negative electrode mixture layer including a negative electrode active material containing a Si compound, a conductive material, and a binder; the conductive material includes single-walled carbon nanotubes having an outermost diameter of 5 nm or less, the binder contains a carboxymethyl cellulose salt having a weight average molecular weight of 150,000 or more and 450,000 or less, The negative electrode for a secondary battery, wherein the content of the single-walled carbon nanotubes is 0.001% by mass or more and 0.05% by mass or less relative to the mass of the negative electrode active material.

2. 2. The negative electrode for a secondary battery according to claim 1, wherein the outermost diameter of the single-walled carbon nanotubes is 1 nm or more and 3 nm or less.

3. 3. The negative electrode for secondary batteries according to claim 1, wherein the content of the single-walled carbon nanotubes is 0.001% by mass or more and 0.01% by mass or less with respect to the mass of the negative electrode active material.

4. 4. The negative electrode for a secondary battery according to claim 1, wherein the degree of etherification of the carboxymethyl cellulose salt is 0.5 or more and 1.6 or less.

5. 5. The negative electrode for a secondary battery according to claim 4, wherein the degree of etherification of the carboxymethyl cellulose salt is 0.6 or more and 1.1 or less.

6. 6. The negative electrode for a secondary battery according to claim 1, wherein the weight average molecular weight of the carboxymethyl cellulose salt is 200,000 or more and 400,000 or less.

7. 7. The negative electrode for a secondary battery according to claim 1, wherein the Si compound comprises a lithium ion conductive phase and silicon particles dispersed in the lithium ion conductive phase.

8. 8. The negative electrode for a secondary battery according to claim 7, wherein the lithium ion conductive phase contains at least one of silicate and silicon oxide.

9. A positive electrode, a negative electrode, and a non-aqueous electrolyte are provided. A secondary battery, wherein the negative electrode is the negative electrode for a secondary battery according to any one of claims 1 to 8.

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