Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

A negative electrode for a nonaqueous-electrolyte secondary battery comprises a negative-electrode mixture that includes: a negative-electrode active material that can electrochemically store and discharge lithium ions; carbon nanotubes; and an acrylic resin. The negative-electrode active material includes a silicon-containing material, and the average diameter of the carbon nanotubes is 5 nm or less.

Description

[Technical Field]

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

[0002] A non-aqueous electrolyte secondary battery, such as a lithium ion secondary battery, includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode mixture containing a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions. For example, a material containing silicon, which has a large capacity for absorbing lithium ions, is used as the negative electrode active material.

[0003] Patent Document 1 proposes a negative electrode for a lithium ion secondary battery, which has a negative electrode active material layer containing a silicon-based active material and carbon nanotubes (CNTs) having an average diameter of 10 nm or more and 120 nm or less and an average length within a specific range, and in which the CNT content is within a specific range. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-110876 Summary of the Invention

[0005] A conductive path is formed between the silicon-based active material and the surrounding negative electrode active material via the CNT. However, the CNTs described in Patent Document 1 have a large average diameter of 10 nm or more and high rigidity, which prevents the CNTs from being sufficiently interposed between the silicon-based active material and the surrounding negative electrode active material, and therefore may result in insufficient contact between the CNTs and the silicon-based active material and the surrounding negative electrode active material. Furthermore, because CNTs are hydrophobic, insufficient contact between the CNTs and the silicon-based active material may be formed. If the above contacts are not sufficiently formed, a conductive path may not be sufficiently formed, resulting in insufficient improvement in cycle characteristics.

[0006] One aspect of the present disclosure relates to a negative electrode for a non-aqueous electrolyte secondary battery, comprising a negative electrode mixture including a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes, and an acrylic resin, wherein the negative electrode active material includes a silicon-containing material, and the carbon nanotubes have an average diameter of 5 nm or less.

[0007] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is the above-described negative electrode.

[0008] According to the present disclosure, the cycle characteristics of a non-aqueous electrolyte secondary battery can be improved. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Non-aqueous electrolyte secondary battery negative electrode] A negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode mixture containing a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes (hereinafter also referred to as CNTs), and an acrylic resin. The negative electrode active material contains a silicon-containing material (hereinafter also referred to as Si-containing material). The CNTs have an average diameter of 5 nm or less.

[0011] When the average diameter of the CNTs is 5 nm or less, the rigidity of the CNTs is low, allowing the CNTs to be sufficiently interposed between the Si-containing material and the surrounding negative electrode active material. Therefore, the CNTs easily form contact points with the Si-containing material and the surrounding negative electrode active material, and a sufficient conductive path (conductive network) is formed between the Si-containing material and the surrounding negative electrode active material via the CNTs. This improves cycle performance.

[0012] The acrylic resin has excellent adhesive strength and can firmly bond the Si-containing material to the surrounding CNTs. The acrylic resin firmly maintains the contact between the CNTs and the Si-containing material, ensuring a sufficient conductive path. When the Si-containing material expands and contracts due to charging and discharging, the acrylic resin firmly maintains the adhesion between the CNTs and the Si-containing material, preventing the isolation of the Si-containing material (interruption of the conductive network) that accompanies this expansion and contraction. This improves cycle characteristics.

[0013] When the negative electrode mixture contains CNTs with an average diameter of 5 nm or less and an acrylic resin, the CNTs easily form contact points with the Si-containing material and the surrounding negative electrode active material, and even a small amount of CNTs (e.g., a CNT content of 0.1 mass% or less) can efficiently form a conductive path. Therefore, the amount of CNTs can be reduced and the amount of negative electrode active material can be increased, enabling a high capacity. Furthermore, the amount of CNTs contained in the negative electrode slurry during negative electrode preparation can be reduced. When the amount of CNTs in the negative electrode slurry is small, CNT aggregation is suppressed, improving the reliability of the resulting battery (negative electrode) and making it easier to achieve stable cycle characteristics.

[0014] From the viewpoint of improving the reliability and cycle characteristics of the battery, the CNT content in the negative electrode mixture may be 0.1% by mass or less, 0.08% by mass or less, or 0.05% by mass or less, based on the total amount of the negative electrode active material. The lower limit of the CNT content may be 0.001% by mass or 0.003% by mass. For example, the CNT content in the negative electrode mixture may be 0.001% by mass or more and 0.05% by mass or less, based on the total amount of the negative electrode active material. Furthermore, from the viewpoint of further improving the reliability and cycle characteristics of the battery, the CNT content in the negative electrode mixture is preferably 0.005% by mass or more and 0.05% by mass or less, and more preferably 0.005% by mass or more and 0.02% by mass or less, based on the total amount of the negative electrode active material.

[0015] The content of the acrylic resin in the negative electrode mixture is preferably 0.1% by mass or more and 1% by mass or less, based on the total amount of the negative electrode active material. When the content of the acrylic resin in the negative electrode mixture is 0.1% by mass or more, based on the total amount of the negative electrode active material, the adhesion between the CNTs and the Si-containing material is likely to be improved. When the content of the acrylic resin in the negative electrode mixture is 1% by mass or less, based on the total amount of the negative electrode active material, the resistance of the negative electrode is likely to be reduced, a sufficient amount of negative electrode active material is ensured, and high capacity is likely to be achieved.

[0016] From the viewpoint of improving cycle characteristics, the mass ratio of acrylic resin to CNT in the negative electrode mixture (hereinafter also referred to as (acrylic resin / CNT)) may be 0.2 or more and 500 or less. (acrylic resin / CNT) is preferably 2 or more and 100 or less. In this case, the effect of improving the adhesion between the Si-containing material and CNT by the acrylic resin and the effect of forming a conductive path by the CNT are easily achieved in a well-balanced manner, further improving cycle characteristics. (acrylic resin / CNT) is more preferably 20 or more and 100 or less.

[0017] (carbon nanotubes) CNTs are nanometer-sized carbon materials with a cylindrical structure consisting of a sheet of graphene, a six-membered ring network formed by carbon atoms. They have excellent electrical conductivity. When the number of graphene layers making up the cylindrical structure is one, they are called single-walled carbon nanotubes (SWCNTs). When the number of layers is multiple, they are called multi-walled carbon nanotubes (MWCNTs).

[0018] From the viewpoint of improving cycle characteristics, the average diameter of the CNTs is preferably 1 nm or more and 5 nm or less, more preferably 1 nm or more and 3 nm or less. When the average diameter of the CNTs is 1 nm or more, the strength of the CNTs is sufficiently ensured, and the conductive path through the CNTs is easily maintained during charge and discharge.

[0019] CNTs with an average diameter of 5 nm or less may contain a large amount of SWCNTs and a small amount of MWCNTs. When the average diameter of CNTs is 5 nm or less, for example, 50% or more of the CNTs are SWCNTs. 90% or more of the CNTs may also be SWCNTs. The proportion of SWCNTs in the CNTs refers to the ratio of the number of SWCNTs to the total number of CNTs.

[0020] The proportion of SWCNTs in the CNTs contained in the negative electrode mixture is determined by the following method.

[0021] A scanning electron microscope (SEM) is used to obtain an image of the cross section of the negative electrode mixture layer or CNTs. Using the SEM image, several 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.

[0022] From the viewpoint of ensuring a conductive path between the Si-containing material and the surrounding negative electrode active material, the average length of the CNTs is preferably 1 μm or more and 100 μm or less, and more preferably 5 μm or more and 20 μm or less.

[0023] The average length and diameter of the CNTs are determined by obtaining an image of the cross section of the negative electrode mixture layer or the CNTs using an SEM and / or a transmission electron microscope (TEM), measuring the lengths and diameters of a number of randomly selected CNTs (e.g., about 50 to 200) using the image, and averaging the lengths and diameters. The length of the CNTs refers to the length when they are linear.

[0024] The CNTs can be confirmed, for example, by an SEM image of a cross section of the negative electrode mixture layer. Methods for analyzing the CNTs include, for example, Raman spectroscopy and thermogravimetric analysis.

[0025] (acrylic resin) The acrylic resin is an acrylic binder containing a carboxyl group, and has a strong binding force, which serves to firmly solidify the negative electrode mixture (layer) and increase the adhesion between the negative electrode mixture layer and the negative electrode current collector.

[0026] 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 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. In this specification, "(meth)acrylic acid" means at least one selected from the group consisting of "acrylic acid" and "methacrylic acid".

[0027] The acrylic resin preferably contains at least a (meth)acrylate unit, which facilitates the preparation of a negative electrode slurry and is advantageous for improving battery characteristics. In this case, examples of the (meth)acrylate include alkali metal salts such as lithium salts and sodium salts, and ammonium salts.

[0028] 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.

[0029] The weight-average molecular weight of the acrylic resin is preferably not less than 3,000 and not more than 10,000,000. When the weight-average molecular weight of the acrylic resin is within the above range, the acrylic resin is able to sufficiently improve cycle characteristics and reduce internal resistance, while also suppressing gelation (increase in viscosity) of the negative electrode slurry, facilitating the production of the negative electrode.

[0030] (Si-containing material) The Si-containing material preferably includes a composite material in which silicon particles (fine Si phase) are dispersed in a lithium ion conductive phase (matrix). The lithium ion conductive phase preferably contains at least one selected from the group consisting of a SiO2 phase, a silicate phase, and a carbon phase. The lithium ion conductive phase can form an amorphous phase. Since the stress generated due to the expansion and contraction of the silicon particles during charge and discharge is relaxed by the lithium ion conductive phase, the composite material is advantageous for improving cycle characteristics. The Si-containing material may include a composite material in which silicon particles are dispersed in a SiO2 phase, a composite material in which silicon particles are dispersed in a silicate phase, a composite material in which silicon particles are dispersed in a carbon phase, and the like.

[0031] The SiO2 phase is an amorphous phase containing 95% by mass or more of silicon dioxide. The composite material in which silicon particles are dispersed in the SiO2 phase is SiO x represented by, where x is, for example, 0.5 ≦ x < 2, and preferably 0.8 ≦ x ≦ 1.6. SiO x is obtained, for example, by heat-treating silicon monoxide and separating it into a SiO2 phase and a fine Si phase by disproportionation reaction. When observing the particle cross-section of SiO x using a transmission electron microscope (TEM), Si particles dispersed in the SiO2 phase can be confirmed.

[0032] The silicate phase preferably contains at least one of an alkali metal element (Group 1 element other than hydrogen in the long-period type periodic table) and a Group 2 element in the long-period type periodic table. The alkali metal elements include lithium (Li), potassium (K), sodium (Na), and the like. The Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and the like. The lithium silicate phase can have a composition represented by the formula: Li 2y SiO 2+y (0 < y < 2). y may be 1 / 2 or 1. The composite material in which silicon particles are dispersed in the silicate phase can be obtained, for example, by pulverizing a mixture of a silicate and raw material silicon while stirring with a ball mill or the like to make it into fine particles, and then heat-treating the mixture in an inert atmosphere.

[0033] The average particle size of the silicon particles (before the first charge) dispersed within the silicate phase may be 50 nm or more and 500 nm or less, or 100 nm or more and 400 nm or less. The average particle size of the silicon particles is obtained by calculating the average of the maximum diameters of 100 randomly selected silicon particles using an SEM image of the cross section of the composite material. The content of the silicon particles dispersed within the silicate phase may be 30 mass % or more and 95 mass % or less, or 35 mass % or more and 75 mass % or less, based on the total mass of the composite material.

[0034] The carbon phase contains, for example, amorphous carbon with low crystallinity. The amorphous carbon may be, for example, easily graphitized carbon (hard carbon) or hardly graphitized carbon (soft carbon). A composite material in which silicon particles are dispersed within the carbon phase can be obtained, for example, by pulverizing a mixture of a carbon source and raw silicon while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere. For example, sugars such as carboxymethyl cellulose (CMC) or water-soluble resins such as polyvinylpyrrolidone are used as the carbon source.

[0035] The composition of the Si-containing material can be determined, for example, by obtaining a backscattered electron image of the cross section of the negative electrode mixture layer using a field emission scanning electron microscope (FE-SEM), observing the Si-containing material particles, and performing elemental analysis on the observed Si-containing material particles. Elemental analysis can be performed using, for example, an electron probe microanalyzer (EPMA). The composition of the lithium ion conductive phase can also be determined by the above analysis.

[0036] The Si-containing material is, for example, a particulate material. The average particle size (D50) of the Si-containing material is, for example, 1 μm or more and 25 μm or less, and preferably 4 μm or more and 15 μm or less. Within the above range, good battery performance is likely to be obtained. In this specification, the average particle size (D50) refers to the particle size (volume-average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by the laser diffraction scattering method. For example, the "LA-750" manufactured by HORIBA Ltd. can be used as a measuring device.

[0037] When the lithium ion conductive phase is an SiO2 phase or a silicate phase, at least a portion of the particle surface of the Si-containing material may be coated with a conductive layer to improve conductivity. The conductive layer contains a conductive material such as conductive carbon. The amount of the conductive layer is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the total of the Si-containing material particles and the conductive layer. The Si-containing material particles having a conductive layer on their surface can be obtained, for example, by mixing coal pitch or the like with the Si-containing material particles and heat-treating the mixture in an inert atmosphere.

[0038] (carbon materials) The negative electrode active material may further include a carbon material that electrochemically absorbs and releases lithium ions. The carbon material expands and contracts less during charge and discharge than the Si-containing material. The combined use of the Si-containing material and the carbon material can maintain better contact between the negative electrode active material particles and between the negative electrode mixture layer and the negative electrode current collector during repeated charge and discharge. This means that the high capacity of the Si-containing material can be imparted to the negative electrode while improving cycle performance. From the viewpoint of increasing capacity and improving cycle performance, the proportion of the carbon material in the total of the Si-containing material and the carbon material is preferably 98% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 75% by mass or more and 95% by mass or less.

[0039] Examples of carbon materials used for the negative electrode active material include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. One type of carbon material may be used alone, or two or more types may be used in combination.

[0040] Among these, graphite is preferred as the carbon material because of its excellent charge / discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, graphitizable carbon, or non-graphitizable carbon.

[0041] Graphite is a carbon material with a developed graphite-type crystal structure. The interplanar spacing d002 of the (002) plane of graphite measured by X-ray diffraction may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. The crystallite size Lc(002) of graphite may be, for example, 5 nm or more, or 5 nm or more and 200 nm or less. The crystallite size Lc(002) is measured, for example, by the Scherrer method. When the interplanar spacing d002 of the (002) plane of graphite and the crystallite size Lc(002) are within the above ranges, high capacity is easily obtained.

[0042] [Nonaqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode includes the above-described negative electrode mixture.

[0043] The nonaqueous electrolyte secondary battery will be described in detail below.

[0044] (Negative electrode) The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. 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.

[0045] The negative electrode mixture contains, as essential components, a negative electrode active material, CNTs, and an acrylic resin, and may contain, as optional components, a conductive agent other than CNTs, a binder other than the acrylic resin, and the like.

[0046] Examples of conductive agents other than CNT include carbons such as acetylene black, metals such as aluminum, etc. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0047] Examples of binders other than acrylic resins include resin materials other than acrylic resins. Examples of such resin materials 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. Rubber-like materials such as styrene-butadiene copolymer rubber (SBR) may also be used as binders.

[0048] Examples of binders include carboxymethyl cellulose (CMC) and its modified forms (including salts such as Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), etc. One type of binder may be used alone, or two or more types may be used in combination.

[0049] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0050] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but may be 1 to 50 μm or 5 to 20 μm.

[0051] (positive electrode) The positive electrode may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium such as NMP onto the surface of the positive electrode current collector and drying it. The dried coating film may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces. The positive electrode mixture contains, as an essential component, a positive electrode active material, and may contain, as optional components, a binder, a conductive agent, and the like.

[0052] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal is used. Examples of the transition metal include Ni, Co, Mn, etc. Examples of the composite oxide containing lithium and a transition metal include, for example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c 、Li a Ni 1-b M b O c 、Li a Mn2O4, Li a Mn[[ID=D37]] 2-b M b O4. Here, a = 0 to 1.2, b = 0 to 0.9, c = 2.0 to 2.3. M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Note that the a value indicating the molar ratio of lithium increases or decreases during charge and discharge.

[0053] Among them, Li a Ni b M 1-b O2 (where 0 < a ≤ 1.2, 0.3 ≤ b ≤ 1, and M is at least one selected from the group consisting of Mn, Co, and Al.) is preferable. From the perspective of increasing the capacity, it is more preferable to satisfy 0.85 ≤ b ≤ 1. From the perspective of the stability of the crystal structure, Lia Ni b Co c Al d O2 (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c ≤ 0.15, 0 < d ≤ 0.1, b + c + d = 1) is more preferable.

[0054] As the binder, the resin materials exemplified for the negative electrode can be used. As the conductive agent, the same ones as those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite and artificial graphite may be used.

[0055] The shape and thickness of the positive electrode current collector can be selected respectively from the shapes and ranges according to the negative electrode current collector. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, etc. can be exemplified.

[0056] (Non-aqueous electrolyte) [[ID=二十一]]The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte excellent in ionic conductivity and having appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above. [[ID=二十二]] [[ID=二十三]]

[0057] [[ID=二十四]] [[ID=二十五]]As the non-aqueous solvent, for example, cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, etc. are used. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), etc. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, etc. The non-aqueous solvent may be used alone or in combination of two or more. [[ID=二十六]]

[0058] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 Examples of the lithium salt include lithium phosphates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of the borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonyl imide (LiN(CF3SO2)(C4F9SO2)), lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2), etc. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0059] (separator) It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may 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.

[0060] An example of the structure of a nonaqueous electrolyte 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 a nonaqueous 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 nonaqueous electrolyte secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0061] Hereinafter, the structure of a prismatic nonaqueous electrolyte secondary battery as an example of the nonaqueous electrolyte secondary battery according to the present disclosure will be described with reference to Fig. 1. Fig. 1 is a schematic perspective view of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away.

[0062] The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a non-aqueous electrolyte housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0063] One end of a negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. In other words, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. The electrolyte injection hole provided in the sealing plate 5 is closed by a sealing plug 8.

[0064] Examples of the present disclosure will be specifically described below, but the present invention is not limited to the following examples.

[0065] Examples 1 to 9 [Preparation of negative electrode] An appropriate amount of water was added to the negative electrode mixture and mixed to obtain a negative electrode slurry. The negative electrode mixture was a mixture of a negative electrode active material, a binder, and a conductive agent.

[0066] The negative electrode active material was a mixture of Si-containing material and graphite (average particle size (D50) 25 μm). The mass ratio of the Si-containing material to the graphite was 10:90. The Si-containing material contained SiO 2 whose surface was coated with a conductive layer containing conductive carbon. x The conductive layer was coated with SiO particles (x=1, average particle size (D50) 5 μm). x The amount was 5 parts by mass per 100 parts by mass of the total of the particles and the conductive layer.

[0067] The binders used were acrylic resin, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR). The acrylic resin used was lithium polyacrylate (PAA-Li). The conductive material used was CNTs (diameter 1.2-2.0 nm, average diameter approximately 1.6 nm, average length approximately 5 μm) containing 90% or more SWCNTs.

[0068] The contents of CNT and acrylic resin in the negative electrode mixture (mass ratio to the total negative electrode active material) were set to the values ​​shown in Table 1. The content of CMC-Na in the negative electrode mixture was 1 mass% relative to the total negative electrode active material. The content of SBR in the negative electrode mixture was 1 mass% relative to the total negative electrode active material.

[0069] Next, the negative electrode slurry was applied to the surface of the copper foil, the coating was dried, and then the copper foil was rolled to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm ) on both sides of the copper foil. 3 ) was formed to obtain a negative electrode.

[0070] [Preparation of positive electrode] Lithium-containing composite oxide (LiNi 0.8 Co0.18 Al 0.02 2.5 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were added to 95 parts by mass of ethanol (O2) and mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm) on both sides of the aluminum foil. 3 ) was formed to obtain a positive electrode.

[0071] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was obtained by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 3:7).

[0072] [Fabrication of non-aqueous electrolyte secondary battery] An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween in an inert gas atmosphere to produce a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet having an Al layer, and the nonaqueous electrolyte was injected thereinto, followed by sealing the exterior body to produce a nonaqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, a portion of the positive electrode lead and the negative electrode lead were each exposed to the outside from the exterior body. In Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 to 9 are designated A1 to A9, respectively.

[0073] Example 10 The conductive agent used was CNT (average diameter approximately 5 nm) containing 50% SWCNT. The contents of CNT and acrylic resin in the negative electrode mixture (mass ratio to the total negative electrode active material) were set to the values ​​shown in Table 1. Battery A10 of Example 10 was fabricated in the same manner as Battery A1 of Example 1, except for the above.

[0074] Comparative Example 1 In the preparation of the negative electrode, the negative electrode mixture did not contain acrylic resin. The content of CNT in the negative electrode mixture was 0.12 mass% with respect to the total negative electrode active material. Battery B1 of Comparative Example 1 was prepared in the same manner as Battery A1 of Example 1 except for the above.

[0075] Comparative Example 2 In the preparation of the negative electrode, the negative electrode mixture did not contain CNT. The content of the acrylic resin in the negative electrode mixture was 1 mass% with respect to the total negative electrode active material. Battery B2 of Comparative Example 2 was prepared in the same manner as Battery A1 of Example 1 except for the above.

[0076] Comparative Example 3 Battery B3 of Comparative Example 3 was produced in the same manner as battery A1 of Example 1, except that in the production of the negative electrode, CNT and acrylic resin were not included in the negative electrode mixture.

[0077] Comparative Example 4 The conductive agent used was CNT (average diameter approximately 9 nm) containing 100% MWCNT. The contents of CNT and acrylic resin in the negative electrode mixture (mass ratio to the total negative electrode active material) were set to the values ​​shown in Table 1. Battery B4 of Comparative Example 4 was fabricated in the same manner as Battery A1 of Example 1, except for the above.

[0078] Comparative Example 5 Battery B5 of Comparative Example 5 was produced in the same manner as Battery A5 of Example 5, except that in the production of the negative electrode, no acrylic resin was added to the negative electrode mixture.

[0079] The batteries obtained above were evaluated as follows.

[0080] [Charge / discharge cycle test] (charging) The battery was charged at a constant current of 0.5 C (180 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C (18 mA).

[0081] (discharge) A constant current discharge was carried out at a current of 0.7 C (252 mA) until the voltage reached 2.5 V.

[0082] The rest time between charging and discharging was 10 minutes, and charging and discharging were carried out in an environment of 25°C.

[0083] Charge and discharge were repeated under the above conditions. The ratio (percentage) of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate. The evaluation results are shown in Table 1.

[0084] [Table 1]

[0085] Batteries A1 to A10 exhibited higher capacity retention rates than batteries B1 to B5. Batteries B1 and B5 exhibited lower capacity retention rates because the negative electrode mixture did not contain acrylic resin. Battery B2 exhibited lower capacity retention rates because the negative electrode mixture did not contain CNT. Battery B3 exhibited lower capacity retention rates because the negative electrode mixture did not contain CNT or acrylic resin. Battery B4 exhibited lower capacity retention rates because the negative electrode mixture contained large CNTs with an average diameter of 9 nm. Battery A5, which used acrylic resin with a small amount of CNT, exhibited a higher capacity retention rate than battery B5, which used acrylic resin with a small amount of CNT and did not use acrylic resin.

[0086] Furthermore, the initial capacity of the negative electrodes obtained in Example 5 and Comparative Examples 1 and 2 was determined by the following procedure.

[0087] An evaluation cell was constructed using the negative electrode obtained above, a Li electrode as a counter electrode, and the non-aqueous electrolyte described above. + ) was charged at a constant current of 0.01C until the voltage reached 1V (vs. Li / Li + The negative electrode was discharged at a constant current of 0.1 C until the discharge capacity reached 100. The discharge capacity of the negative electrode at this time was determined as the initial capacity. The initial capacity was expressed as an index, with the initial capacity of the negative electrode of Example 5 being set to 100.

[0088] The negative electrode of Example 5 had a higher initial capacity than the negative electrodes of Comparative Examples 1 and 2. In the negative electrode of Comparative Example 1, the addition of CNTs improved the conductivity, but the amount of CNTs was large, which reduced the amount of active material, resulting in a lower initial capacity than the negative electrode of Example 5. In the negative electrode of Comparative Example 2, the amount of acrylic resin added was large and no CNTs were included, resulting in a high resistance and a lower initial capacity than the negative electrode of Example 5. [Industrial Applicability]

[0089] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0090] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal

Claims

1. The battery includes a negative electrode mixture including a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes, and an acrylic resin; the negative electrode active material includes a silicon-containing material, The carbon nanotubes have an average diameter of 5 nm or less, In the negative electrode mixture, a mass ratio of the acrylic resin to the carbon nanotubes is 20 or more and 100 or less, The negative electrode for a non-aqueous electrolyte secondary battery, wherein the content of the carbon nanotubes in the negative electrode mixture is 0.003 mass % or more and 0.3 mass % or less with respect to the total amount of the negative electrode active material.

2. The battery includes a negative electrode mixture including a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes, and an acrylic resin; the negative electrode active material includes a silicon-containing material, The carbon nanotubes have an average diameter of 5 nm or less, In the negative electrode mixture, a mass ratio of the acrylic resin to the carbon nanotubes is 20 or more and 100 or less, The negative electrode for a non-aqueous electrolyte secondary battery, wherein the content of the acrylic resin in the negative electrode mixture is 0.02 mass % or more and 1.5 mass % or less with respect to the total amount of the negative electrode active material.

3. The battery includes a negative electrode mixture including a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes, and an acrylic resin; the negative electrode active material includes a silicon-containing material, The carbon nanotubes have an average diameter of 5 nm or less, In the negative electrode mixture, a mass ratio of the acrylic resin to the carbon nanotubes is 20 or more and 100 or less, the content of the carbon nanotubes in the negative electrode mixture is 0.003 mass% or more and 0.3 mass% or less with respect to the total amount of the negative electrode active material, The negative electrode for a non-aqueous electrolyte secondary battery, wherein the content of the acrylic resin in the negative electrode mixture is 0.02 mass % or more and 1.5 mass % or less with respect to the total amount of the negative electrode active material.

4. 4. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 1, wherein the carbon nanotubes have an average diameter of 1 nm or more and 3 nm or less.

5. 5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon nanotubes include single-walled carbon nanotubes.

6. 6. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing material includes a composite material having a lithium ion conductive phase and silicon particles dispersed in the lithium ion conductive phase.

7. The lithium ion conductive phase is SiO ２ phase, silicate phase, and carbon phase; 7. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 6, wherein the silicate phase contains at least one element selected from the group consisting of alkali metal elements and Group 2 elements.

8. A positive electrode, a negative electrode, and a non-aqueous electrolyte, A non-aqueous electrolyte secondary battery, wherein the negative electrode is the negative electrode according to any one of claims 1 to 7.

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