Negative electrode and secondary battery

Abstract

To provide an electrode, particularly a negative electrode, with the excellent rate characteristic in use for a battery, and also provide a battery using the negative electrode.SOLUTION: A negative electrode includes a current collector, and an active material layer provided on a surface of the current collector and including an active material, a conductive auxiliary agent, and a binder. The active material layer includes a continuous path continuing from a surface in contact with the current collector to a surface opposite to the surface in contact with the current collector. When a ratio of a length L of the path to an average thickness x of the active material layer in the active material layer is a tortuosity factor τ and the number of paths, n, expressing the tortuosity factor τ are measured, the mode τmax of the tortuosity factor is 1.55 or less. When the number of paths expressing the mode τmax of the tortuosity factor is nmax and the tortuosity factor smaller than the mode τmax of the tortuosity factor among the tortuosity factors expressing the number of paths corresponding to 50% of the number of paths nmax expressing the mode τmax of the tortuosity factor is τ50, τ50 is 1.48 or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] In an electrode used in a lithium ion battery or the like, an active material layer is formed on a current collector. There are many methods for forming an active material layer on a current collector when producing an electrode, but they can be roughly divided into a wet method and a dry method from the viewpoint of whether or not the electrode mixture containing the active material is made into a slurry. In the wet method, an active material, a polymer, a solvent, etc. are mixed to obtain a slurry, and the obtained slurry is applied to a current collector and dried to form an electrode. On the other hand, in the dry method, an electrode is formed without going through a process of applying a solvent and an active material in a slurried state. For example, an electrode is formed by forming a granule containing an active material in advance and transferring it to a current collector.

[0003] Patent Document 1 discloses a method for producing an electrode by a wet method for producing a lithium ion secondary battery using an electrode slurry in which an active material, a conductive agent, and a solvent are mixed. In Patent Document 1, the adhesion between the electrode and the current collector is improved by adjusting the moisture content. In addition, Patent Document 2 discloses a technique for forming a granulated body containing an active material in advance and transferring it onto a current collector. Patent Document 2 discloses a method for producing an electrode sheet, including a granulation step for obtaining a wet granulated body containing active material particles, a binder, and a solvent, a molding step for obtaining a compact by molding an aggregate of the granulated body obtained in the granulation step into a planar or block shape, and a step for forming the compact obtained in the molding step into a film and transferring it to a current collector. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2017 / 188043 [Patent Document 2] JP 2015-201318 A Summary of the Invention [Problem to be solved by the invention]

[0005] In lithium-ion secondary batteries, in order to meet the demand for rapid charging and discharging, it is desirable to use electrodes that have excellent rate characteristics, i.e., that can withstand charging and discharging at a large current. However, even when the techniques disclosed in Patent Document 1 and Patent Document 2 are used, the rate characteristics of the electrodes are sometimes insufficient.

[0006] As a result of investigations, the present inventors have found that an electrode with good rate characteristics can be obtained by appropriately forming paths (ion paths between active materials) in the active material layer through which lithium ions can pass.

[0007] The present invention has been made in view of the above circumstances, and provides an electrode, particularly a negative electrode, which has good rate characteristics when applied to a battery, and a secondary battery including the same. [Means for solving the problem]

[0008] That is, according to the present invention, there are provided the following negative electrode and secondary battery. 1. A current collector; an active material layer provided on a surface of the current collector, the active material layer including an active material, a conductive additive, and a binder; A negative electrode comprising: the active material layer has a continuous path extending from a surface in contact with the current collector to a surface opposite to the surface in contact with the current collector, In the active material layer, the ratio of the length L of the path to the average thickness x of the active material layer is defined as a tortuosity τ, and when the number n of paths exhibiting the tortuosity τ is measured, the most frequent tortuosity τ is max is less than or equal to 1.55, The most frequent value τ of the curvature ratio max Let n be the number of paths that show max The most frequent value of the curvature ratio is τ max The number of paths that indicate max Among the curve rates τ that indicate 50% of the number of routes, the most frequent value τ of the curve rate maxThe curvature ratio is smaller than τ 50 Then, τ 50 is 1.48 or less.

[0009] 2. A secondary battery in which the negative electrode described in 1 above and a positive electrode are stacked in multiple layers with a separator between them, optionally rolled up, and housed in an outer container, and a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode are extended from the outer container. Effect of the Invention

[0010] According to the present invention, it is possible to provide a secondary battery having good rate characteristics and a negative electrode used therein. [Brief description of the drawings]

[0011] [Figure 1] 1 shows distribution curves of the tortuosity τ of a path penetrating an active material layer in the thickness direction. The solid line is the distribution curve of the active material layer of the present invention (Example 1), the dashed line is the distribution curve of the active material layer of Comparative Example 1, and the dotted line is the distribution curve of the active material layer of Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described.

[0013] One embodiment of the present invention is a negative electrode including a current collector and an active material layer provided on a surface of the current collector, the active material layer including an active material, a conductive additive, and a binder, wherein the active material layer has a continuous path extending from a surface in contact with the current collector to a surface opposite to the surface in contact with the current collector, In the active material layer, the ratio of the length L of the path to the average thickness x of the active material layer is defined as a tortuosity τ, and when the number n of paths exhibiting the tortuosity τ is measured, the most frequent tortuosity τ is max is less than or equal to 1.55, The most frequent value τ of the curvature ratio max Let n be the number of paths that show max The most frequent value of the curvature ratio is τ max The number of paths that indicatemax Among the curve rates τ that indicate 50% of the number of routes, the most frequent value τ of the curve rate max The curvature ratio is smaller than τ 50 Then, τ 50 is 1.48 or less.

[0014] In one embodiment, the negative electrode is not particularly limited, but refers to a negative electrode used in a non-aqueous secondary battery material. The negative electrode is composed of a current collector and an active material layer provided on the surface of the current collector, and the active material layer includes an active material, a conductive assistant, and a binder. As the current collector, for example, a metal such as copper, nickel, titanium, or stainless steel can be used. As the current collector, it is preferable to use a flat metal, particularly a metal foil. The current collector will be described in detail later.

[0015] As the active material, a carbon-based active material can be used. As the carbon-based active material, natural graphite, artificial graphite, hard carbon, soft carbon, carbon black, or any mixture thereof can be selected. Natural graphite includes natural graphite with amorphous carbon coated on the particle surface, and similarly, artificial graphite includes artificial graphite with amorphous carbon coated on the particle surface. These natural graphite and artificial graphite can be used in the form of primary particles, particles in which primary particles are aggregated to form secondary particles, or mixtures thereof. In addition, the negative electrode active material may be a mixture of a carbon-based active material and a silicon-based active material. The negative electrode active material may include a metal material such as aluminum, lithium, silver, bismuth, calcium, cerium, indium, magnesium, tin, zinc, or nickel. The negative electrode active material will be described in detail later.

[0016] The conductive assistant is a material for reducing the resistance of the electrode. Examples of the conductive assistant include carbon black such as acetylene black and ketjen black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon nanotubes, carbon nanofibers, carbon nanobrushes, and other carbon fibers. The conductive assistant will be described in detail later.

[0017] Examples of the binder include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF), conductive polymers such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles, synthetic rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), and acrylonitrile butadiene rubber (NBR), and polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. Also, examples of the binder that can be used include polyacrylic acid, polymethacrylic acid, lithium polyacrylate, sodium polyacrylate, potassium polyacrylate, sodium polymethacrylate, and potassium polymethacrylate; ethyl polyacrylate, ethyl polyacrylate, butyl polyacrylate, methyl polymethacrylate, ethyl polymethacrylate, butyl polymethacrylate, polyacrylamide, and polyacrylonitrile, and any mixtures thereof. Furthermore, as the binder, carboxymethylcellulose (hereinafter referred to as "CMC"), which is a derivative of cellulose, or a metal salt of carboxymethylcellulose (for example, sodium carboxymethylcellulose, potassium carboxymethylcellulose) can be used. The binder will be described in detail later.

[0018] In one embodiment, the active material layer has a continuous path that connects the surface in contact with the current collector to the surface opposite the surface in contact with the current collector. Here, the path refers to a gap or hole in the active material layer, and in one embodiment, it is preferable that the path penetrates from the surface in contact with the current collector to the surface opposite the surface in contact with the current collector. In one embodiment, the presence of such a penetrating path in the active material layer allows ions to pass from the surface in contact with the current collector to the surface opposite the surface in contact with the current collector, or from the surface opposite the surface in contact with the current collector to the surface in contact with the current collector.

[0019] The active material layer has a certain thickness, and the average thickness of the active material layer is x. On the other hand, when the length of the path is L, the tortuosity τ can be calculated as the ratio of the length L of the path to the average thickness x of the active material layer. The tortuosity τ is a value that indicates how many times the length of the path penetrating in the thickness direction of the active material layer is the length in the thickness direction, that is, the degree of detour of the path, and is also called the degree of bending. The minimum value of the tortuosity is 1. A tortuosity value of 1 indicates that the path penetrates straight in the thickness direction of the active material layer, and a tortuosity value greater than 1 indicates that the path is bent or penetrates with one or more detour points.

[0020] The tortuosity τ of the active material layer can be determined by a method that combines continuous cross-section preparation using a focused ion beam device and observation using a scanning electron microscope, or by an X-ray CT (computed tomography) method. For a certain active material layer, the number n of paths exhibiting a tortuosity τ is measured and the number of paths is plotted against the tortuosity, allowing the distribution of paths present in the active material layer to be evaluated. For example, FIG. 1 shows a plot of the number n of paths exhibiting a tortuosity τ measured and the number of paths against the tortuosity. For example, when the number n of paths with a tortuosity τ value of 2.0 is large, it can be evaluated that the active material layer has a considerable number of bends and detours in the paths penetrating in the thickness direction of the active material layer. On the other hand, when the number n of paths with a tortuosity τ value of 1.0 is large, it can be evaluated that the active material layer has a large number of paths penetrating straight in the thickness direction of the active material layer. In this way, the peak of the number n of paths with a tortuosity τ, i.e., the mode τ of the tortuosity, max The shape of the distribution curve of the tortuosity τ of the path is useful for evaluating the state of the path in the active material layer. The present inventors have found that by making the mode of the tortuosity τ of the active material layer as small as possible, it is possible to prepare an electrode for a battery that requires charging and discharging at a large current without impeding the passage of ions in the active material layer (i.e., reducing the resistance of the active material layer).

[0021] In one embodiment, the most frequent value τ of the curvature ratio max It is preferable that the distribution of the tortuosity ratio is 1.55 or less. Here, referring to FIG. 1, the distribution of the tortuosity ratio is explained. The most frequent value τ of the tortuosity ratio ismax Let n be the number of paths that show max The most frequent value of the curvature ratio is τ max The number of paths that indicate max Among the curve rates τ that indicate 50% of the number of routes, the most frequent curve rate τ max The curvature ratio is smaller than τ 50 In this case, τ 50 is preferably 1.48 or less. Of the three distribution curves depicted in FIG. 1, the solid line is the distribution curve of the active material layer in the negative electrode of one embodiment. Meanwhile, the distribution curves represented by the dotted and dashed lines are those of the active material layer in the negative electrode of the prior art. Comparing the distribution curves of the tortuosity of the paths penetrating in the thickness direction of the active material layer of one embodiment with those of the prior art, it can be seen that there are many paths with a small tortuosity in the active material layer of the present invention. In one embodiment, the most frequent value τ of the tortuosity max The number of paths that indicate max It is particularly preferable that the surface area is equal to or larger than 1790. This makes it possible to sufficiently secure paths for ions to pass through the active material layer, which is believed to reduce the resistance of the active material layer.

[0022] In one embodiment, the active material layer preferably includes granules in which the active material, the conductive assistant, and the binder are aggregated. Here, the granules refer to granules (granules) in which the above-mentioned active material, the conductive assistant, and the binder, and optionally a solvent, are used as raw materials and aggregated. The active material layer used in the negative electrode of one embodiment is formed by applying the granules in which the active material, the conductive assistant, and the binder are aggregated to the surface of the current collector by a dry coating method. First, the granules are formed so as to have a desired surface and internal structure (specifically, a desired path structure), and the granules are applied to the surface of the current collector by a dry coating method, and pressed to obtain a desired density while adjusting the pressing conditions, thereby forming an active material layer having the above-mentioned desired tortuosity distribution. The reason why an active material layer having a desired tortuosity distribution can be formed by forming an active material layer using granules in which the active material, the conductive assistant, and the binder are aggregated is not clear, but it is thought that this is because the granules gradually collapse during pressing, so that the active material layer is formed without eliminating the voids between the active material particles. Compared with an active material layer formed by a wet coating method using a slurry containing a mixture of an active material, a conductive additive, and a binder, the active material layer in one embodiment is presumed to have a greater or equal number of paths penetrating the thickness direction of the active material layer, and to have a smaller tortuosity of the paths overall.

[0023] The tortuosity can be adjusted by the pressing conditions such as the pressing pressure and the number of presses even if the particle size of the active material and the amount of binder are the same, and whether the wet method or the dry method is used, but the adjustment method is not limited to this. It was found that when the active material layer is formed by the dry method, the through-path width can be made larger than that of the wet method even if the amount of voids per volume is the same as that of the wet method, and the tortuosity can be easily controlled. However, even in the dry method, if the density of the active material layer is increased without adjusting the pressing conditions, the granules are crushed to a state close to the original active material particles, and as a result, the tortuosity of the paths present in the formed active material layer becomes larger overall.

[0024] Here, the average particle size of the granules is preferably 15 μm or more, more preferably 20 μm or more, and even more preferably 40 μm or more, and is preferably 90 μm or less, more preferably 80 μm or less, and even more preferably 70 μm or less. In one embodiment, the average particle size of the granules means the particle size at an integrated value of 50% (median size: D50) in the particle size distribution (volume basis) measured by a laser diffraction scattering method.

[0025] From the viewpoint of adhesion of the conductive assistant to the active material surface, the average particle size of the active material particles is preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. And, it is preferably 20.0 μm or less, more preferably 10.0 μm or less, and even more preferably 5.0 μm or less. In one embodiment, the average particle size of the active material particles means the particle size at an integrated value of 50% (median size: D50) in the particle size distribution (volume basis) measured by a laser diffraction scattering method.

[0026] The negative electrode active material includes a carbon-based active material. The carbon-based active material is preferably natural graphite, artificial graphite, hard carbon, soft carbon, or any mixture thereof. Here, graphite is a carbon material having hexagonal plate-like crystals in a hexagonal system, and may be called graphite or the like. The natural graphite and artificial graphite include natural graphite coated with amorphous carbon, and artificial graphite coated with amorphous carbon.

[0027] Here, amorphous carbon refers to a carbon material that is amorphous as a whole and has a random network structure of microcrystals, which may have a structure similar to that of graphite in part. Examples of amorphous carbon include carbon black, coke, activated carbon, carbon fiber, carbon nanotubes, hard carbon, soft carbon, and mesoporous carbon. When using artificial graphite, it is preferable that the interlayer distance d value (d002) is 0.33 nm or more.

[0028] The crystal structure of artificial graphite is generally thinner than that of natural graphite. When artificial graphite is used as a negative electrode active material for a non-aqueous electrolyte secondary battery, particularly a lithium ion secondary battery, it is necessary that the artificial graphite has an interlayer distance that allows lithium ions to be inserted. The interlayer distance that allows lithium ions to be inserted and removed can be estimated by the d value (d002), and if the d value is 0.33 nm or more, lithium ions can be inserted and removed without any problems. In addition, the negative electrode active material may include a silicon-based active material (A) containing SiOx (wherein x is a number that satisfies 0.5≦x≦1.6) and a carbon-based active material.

[0029] In one embodiment, when the entire granules are taken as 100 parts by mass, the content of the active material is preferably 95.0 parts by mass or more, more preferably 96 parts by mass or more, even more preferably 97 parts by mass or more, and preferably 99.5 parts by mass or less, more preferably 99.0 parts by mass or less, even more preferably 98.5 parts by mass or less. The proportion of the active material involved in the formation of the granules is at least 95% or more, preferably 97% or more, and even more preferably 99% or more.

[0030] The conductive assistant preferably contains at least one selected from the group consisting of carbon black, carbon nanofibers, and carbon nanotubes. The carbon black used as the conductive assistant preferably has an aggregate structure in which carbon nanoparticles are connected in a beaded shape. As the carbon black, acetylene black, ketjen black, etc. can be used. Carbon nanotubes formed of a substance composed of carbon atoms such as graphene into a cylindrical shape with a diameter of several nm and a length of several mm can also be used. In particular, single-walled carbon nanotubes formed in a substantially single layer around the cylinder, or multi-walled carbon nanotubes formed in a multiple layer around the cylinder can be used. Among these, it is preferable to use single-walled or multi-walled carbon nanotubes.

[0031] In one embodiment, when the entire granule is taken as 100 parts by mass, the content of the conductive assistant is preferably 0.01 parts by mass or more, preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably 0.3 parts by mass or more, and preferably 2.0 parts by mass or less, more preferably 1.5 parts by mass or less, and even more preferably 0.5 parts by mass or less.

[0032] In one embodiment, the granules further contain a binder. It is preferable that the conductive assistant and the binder are present on the surface of the active material particles, and the binder and the conductive assistant are present on the surface and inside of the granules. In this way, when the granules are pressed, the desired density can be obtained without excessively compressing the pore size between the active material particles.

[0033] The binder may contain, for example, one or more of polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyimide, polyamideimide, polyamide, polyvinyl alcohol, polyvinyl butyral, etc., but is not limited thereto, and can be appropriately changed depending on the solvent. It is preferable to use at least two or more of a first binder having strong adhesive power and a second binder that provides flexibility after drying in combination, and it is preferable to include, for example, polyacrylic acid and styrene butadiene rubber, etc.

[0034] In one embodiment, when the entire granules are taken as 100 parts by mass, the content of the binder is preferably 0.3 parts by mass or more, more preferably 0.5 parts by mass or more, even more preferably 1.0 parts by mass or more, and preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, and even more preferably 2.0 parts by mass or less.

[0035] A method for manufacturing a negative electrode according to one embodiment preferably includes a step of forming granules by spraying and adding a dispersion liquid in which a conductive assistant is dispersed to the active material particles while mixing and stirring the active material particles, and a step of placing the granules on a current collector and pressing the granules. The method for forming the granules is not particularly limited, and examples of the method include a spray granulation method, an agitation granulation method, a fluidized bed granulation method, a rolling granulation method, an extrusion granulation method, and a compression granulation method. These granulation methods may be selected or combined as desired. An example of the spray granulation method is a spray drying granulation method. In the spray drying granulation method, for example, a dispersion liquid in which a negative electrode active material and a conductive assistant are dispersed is sprayed into a high-temperature gas in a spray dryer to perform granulation. In the agitation granulation method, for example, a dispersion liquid in which a conductive assistant is dispersed is sprayed and added to the negative electrode active material particles while mixing and stirring the negative electrode active material to perform granulation. The average particle size of the granulated particles is preferably 15 μm or more, more preferably 20 μm or more, and even more preferably 40 μm or more, and is preferably 90 μm or less, more preferably 80 μm or less, and even more preferably 70 μm or less. The average particle size of the granulated particles means the particle size (median size: D50) at an integrated value of 50% in the particle size distribution (volume basis) measured by a laser diffraction scattering method. By adjusting the number of presses and the press pressure for the active material layer containing the granules thus formed, the adhesion between the current collector and the active material layer is high, and the desired pore paths between the active material particles are maintained to obtain the desired electrode density.

[0036] By attaching a dispersion containing a conductive assistant and a binder that have been highly dispersed in advance to active material particles without containing any other organic solvent, it is possible to prepare an active material having a conductive assistant uniformly formed on the surface. The diameter of the granules formed varies depending on the equipment used to mix and stir these materials, but it is possible to control the diameter by the amount and timing of spraying the dispersion of the conductive assistant and the binder, the mixing and stirring times, and other conditions. In addition, by classifying the formed granules through a sieve, it is possible to obtain granules with any particle size distribution.

[0037] In one embodiment, the material of the current collector is not particularly limited, but when the current collector is a positive electrode current collector, it is preferably aluminum foil. When the current collector is a negative electrode current collector, it is selected from the group consisting of copper, stainless steel, nickel, titanium, or an alloy thereof, and copper is particularly preferred. The shape of the current collector is not particularly limited, and may be foil, flat plate, or mesh. For example, the thickness is 0.001 mm or more and 0.5 mm or less.

[0038] When the granules are placed on the surface of the current collector or the surface of the current collector and pressed, the pressing method is not particularly limited as long as the electrode granules containing the electrode active material can be deposited on the current collector. In one embodiment, it is preferable to use a roll-type pressure molding method. In the roll-type pressure molding method, a long current collector is arranged so that the current collector passes between a pair of rolls while the current collector is unwound from a roll body wound in a roll shape and wound up. The granules are supplied onto the current collector before passing through the rolls, and are compressed when passing between the rolls to form an electrode sheet. In order to adjust the supply amount of the granules, it is preferable to supply the thickness while smoothing the surface with a squeegee or the like before supplying them between the rolls. It is preferable to previously form a current collector surface layer containing a binder to enhance the binding property with the granules on the current collector. By adjusting the number of times of pressing the electrode sheet, the pressing pressure, the temperature, etc., it is possible to adjust the adhesion between the active material layer and the current collector and the length of the path of the voids between the active material particles.

[0039] A second embodiment of the present invention is a secondary battery in which the above-mentioned negative electrode and positive electrode are stacked in multiple layers with a separator interposed therebetween, optionally rolled up, and housed in an outer container, and a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode are drawn out from the outer container. The secondary battery of the second embodiment can be produced using the above-mentioned negative electrode in accordance with a known method. In the second embodiment, the positive electrode is not particularly limited, but refers to a positive electrode used in a non-aqueous secondary battery material. For example, the positive electrode is composed of a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector, and the active material layer includes a positive electrode active material, a conductive additive, and a binder. The positive electrode active material layer including the positive electrode active material, the conductive additive, and the binder can be formed on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of stainless steel, aluminum, nickel, titanium, or a positive electrode current collector in which the surface of aluminum or stainless steel is surface-treated with carbon, nickel, titanium, or silver.

[0040] Positive electrode active materials include composite oxides of lithium and transition metals such as lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-manganese-cobalt composite oxide, lithium-nickel-manganese-aluminum composite oxide, and lithium-nickel-cobalt-manganese-aluminum composite oxide; transition metal sulfides such as TiS2, FeS, and MoS2; MnO, V2O5, and V6O 13 , TiO2, or other transition metal oxides; and olivine-type lithium phosphate. The olivine-type lithium phosphate contains, for example, one or more elements selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, lithium, phosphorus, and oxygen. These compounds may have some elements partially substituted with other elements in order to improve their properties. Among them, the positive electrode active material is Li a Ni b Mn c Co d M xIt is preferable to use lithium nickel manganese cobaltate represented by O2 (where a, b, c, d, and x satisfy 0.9 ≦ a ≦ 1.2, 0 < b < 1, 0 < c ≦ 0.5, 0 < d ≦ 0.5, 0 ≦ x ≦ 0.3, and b + c + d = 1, and M is at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr). Examples of the binder that forms the positive electrode active material layer together with the positive electrode active material include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF); conductive polymers such as polyanilines, polythiophenes, polyacetylenes, and polypyrroles; synthetic rubbers such as styrene-butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), and acrylonitrile-butadiene rubber (NBR); or polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. Examples of the conductive assistant include carbon blacks such as acetylene black and ketjen black; activated carbon; graphite; mesoporous carbon; fullerenes; carbon fibers such as carbon nanofibers, carbon nanotubes, and carbon nanobrushes. In addition, electrode additives generally used for electrode formation, such as thickeners, dispersants, and stabilizers, can be appropriately used in the positive electrode active material layer.

[0041] In the second embodiment, examples of the separator include porous separators. Examples of the form of the separator include membranes, films, non-woven fabrics, etc. Examples of the porous separator include polyolefin-based porous separators such as polypropylene-based and polyethylene-based; porous separators formed of polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride hexafluoropropylene copolymer, etc.

[0042] The separator may contain inorganic particles such as ceramic particles or may further contain a layer containing inorganic particles.

[0043] In the secondary battery of the second embodiment, a negative electrode and a positive electrode are laminated with a separator therebetween, and may be wound and housed in an outer container. The secondary battery is formed by drawing out a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode from the outer container. For the outer container, for example, a flexible film having a resin layer on the front and back sides of a metal layer such as aluminum as a base material, or a can made of a metal such as aluminum, iron, or stainless steel can be used. The positive electrode terminal can be made of aluminum or an aluminum alloy, and the negative electrode terminal can be made of copper or a copper alloy or a copper alloy plated with nickel. The shape of the secondary battery according to the second embodiment may be, for example, a square, cylindrical, coin or button shape housed in an outer container using an aluminum can, an iron can, a stainless steel can, etc. Also preferred is a pouch-shaped secondary battery housed in an outer container mainly made of an aluminum film.

[0044] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various configurations other than those described above may also be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of the present invention are included in the present invention. EXAMPLES

[0045] EXAMPLES Hereinafter, the present invention will be described with reference to examples and comparative examples, but the present invention is not limited to these.

[0046] <Production of negative electrodes> Example 1 Separately from the formation of the conductive layer and the adhesive layer, granulated particles for the negative electrode active material layer were prepared. A negative electrode dispersion liquid was prepared by highly dispersing the negative electrode active material particles, which were artificial graphite with an average particle size of 12 μm and silicon monoxide with an average particle size of 5 μm, single-wall carbon nanotubes (SWCNT) as a conductive assistant, and polyacrylic acid (PAA) and styrene butadiene rubber (SBR) as binders together with a solvent. Next, the negative electrode dispersion liquid prepared by the above procedure was sprayed by rotating the atomizer at a predetermined rotation speed in a spray dryer to obtain granules. The mass ratio of artificial graphite / silicon monoxide / SWCNT / PAA / SBR was 93.45:4.0:0.05:0.5:2.0. Artificial graphite from Nippon Graphite Industries Co., Ltd., PAA from Fujifilm Wako Pure Chemical Industries, Ltd., and SWCNT from OCSiALl can be used.

[0047] Next, the obtained granules were supplied at a constant supply rate onto a current collector traveling from the unwinding side to the winding side of the electrode coater. An adhesive auxiliary layer containing carbon black and binders (CMC and SBR) had been formed in advance on the surface of the current collector. After the granules were supplied onto the current collector, the amount of granules supplied to the press roll by the squeegee roll was adjusted before the granules were compressed by the press roll, and then the granules were passed between the press rolls to form an active material layer of the desired density. In the embodiment, the density was increased to 1.65 g / cm by passing through the press once at a linear pressure of 0.9 t / cm. 3 A negative electrode of 1000 nm was obtained.

[0048] Comparative Example 1 The same granules as in Example 1 were used, and the roll press conditions were a press roll of 0.3 t / cm linear pressure, and the density was 1.65 g / cm. 3 A negative electrode of 1000 nm was obtained.

[0049] Comparative Example 2 A slurry containing graphite as the negative electrode active material, acetylene black as a conductive additive, and styrene-butadiene copolymer rubber (SBR) as a binder was prepared, and applied to the copper foil as the negative electrode current collector using a die coater, dried, and then pressed with a roll press machine to produce a density of 1.65 g / cm. 3 The graphite used was from Nippon Graphite Industries Co., Ltd.

[0050] <Battery manufacturing> The positive electrode active material is LiNi with an average particle size of 4.5 μm. 0.92 Mn 0.03 Co 0.05 A slurry containing lithium nickel manganese cobalt oxide represented by O2, carbon black as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder was applied to an aluminum foil as a positive electrode current collector, and then pressed to produce a positive electrode with a density of 3.5 g / cm. 3 A positive electrode of 1000 nm was obtained.

[0051] The eight positive electrode layers and the nine negative electrode layers of Example 1 obtained previously were laminated via a polyolefin-based porous separator, and a negative electrode terminal electrically connected to each negative electrode and a positive electrode terminal electrically connected to each positive electrode were provided to obtain a laminate. A laminate was also prepared for the negative electrode of Comparative Example 1 or 2 in the same manner. Next, an electrolyte solution in which LiPF6 was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (ethylene carbonate:diethyl carbonate=3:7 (volume ratio)) to a concentration of 1.0 mol / L was placed in a container with a flexible film case together with the obtained laminate, and the container was sealed with the positive electrode terminal and the negative electrode terminal pulled out to the outside to obtain a secondary battery.

[0052] <Rate characteristics> The fabricated battery was charged at a constant current of 0.2C up to 4.2V, then switched to a constant voltage after reaching 4.2V and charged down to 0.015C, and discharged at constant currents of 0.2C and 3C down to 2.5V, and the rate characteristics were determined as 3C / 0.2C, which is the ratio of the capacity of the 3C discharge to the 0.2C discharge at 2.5V. Generally, the higher the current value, the more likely the capacity is to decrease, so the closer to 100%, the better the rate characteristics are.

[0053] <curvature rate> The curvature ratio was measured by X-ray CT analysis using Rigaku Nano-3DX. 3 )μm 3 And the field of view (67.6 3 )μm 3 One side of this field of view was parallel to the electrode thickness direction from the current collector toward the electrode surface in the active material layer, that is, one side of 67.6 μm corresponds to the average thickness x of the active material layer. The results are shown in Table 1. The distribution curves of the tortuosity of Example 1 and Comparative Examples 1 and 2 are shown in FIG.

[0054] [Table 1]

[0055] As shown in Table 1 and FIG. 1, good rate characteristics were obtained in this example. This is presumably because the most frequent value of the tortuosity, which is an index of the length of the lithium ion path, is short at 1.55, and the number of paths is large. In addition, the number of paths on the side with an even smaller tortuosity is also small at 1.48, which is half the number of paths at the most frequent tortuosity. In other words, it can be seen that the active material layer of Example 1 has many shorter paths. On the other hand, in Comparative Examples 1 and 2, in which negative electrodes having active material layers in which there were more paths with a large tortuosity were used, the rate characteristics were inferior to those of the Examples.

[0056] Although the negative electrode and secondary battery of the present invention have been described in detail above, the present invention is not limited to the above-mentioned embodiments and examples, and various improvements and modifications may be made without departing from the spirit and scope of the present invention.

Claims

1. Current collector and, A negative electrode comprising an active material layer on the surface of the current collector, which includes an active material, a conductive additive, and a binder, The active material layer has a continuous path connecting from the surface in contact with the current collector to the surface opposite the surface in contact with the current collector, The active material layer comprises a granular body formed by the aggregation of the active material, the conductive additive, and the binder. The average particle size of the granulated material is 15 μm or more and 90 μm or less. In the active material layer, the ratio of the length L of the path to the average thickness x of the active material layer is defined as the curvature ratio τ, and when the number n of paths exhibiting the curvature ratio τ is measured, the mode of the curvature ratio τ ｍａｘ The value is 1.55 or less. The mode τ of the aforementioned curvature ｍａｘ n is the number of paths that show this. ｍａｘ The mode of the curve ratio is τ ｍａｘ The number of paths that show n ｍａｘ Among the curvature ratios τ representing 50% of the number of paths, the mode of the curvature ratio τ ｍａｘ A curvature smaller than τ ５０ When that happens, τ ５０ A negative electrode characterized by having a coefficient of 1.48 or less.

2. A current collector and A negative electrode comprising an active material layer on the surface of the current collector, which includes an active material, a conductive additive, and a binder, The active material layer has a continuous path connecting from the surface in contact with the current collector to the surface opposite the surface in contact with the current collector, In the active material layer, the ratio of the length L of the path to the average thickness x of the active material layer is defined as the curvature ratio τ, and when the number n of paths exhibiting the curvature ratio τ is measured, the mode of the curvature ratio τ max is 1.55 or less. Let n max be the number of paths that exhibit the mode of the curvature ratio τ max, and let τ 50 be the curvature ratio among the number of paths that represent 50% of the number of paths that exhibit the mode of the curvature ratio τ max n max, which is smaller than the mode of the curvature ratio τ max. Then, if τ 50 is 1.48 or less, The mode τ of the curvature factor ｍａｘ the number n of paths indicating ｍａｘ is 1790 or more, wherein the negative electrode.

3. The mode τ of the aforementioned curvature ｍａｘ The number of paths that show n ｍａｘ The negative electrode according to claim 1, wherein the value is 1790 or more.

4. The negative electrode according to any one of claims 1 to 3, wherein the active material is the negative electrode active material.

5. A secondary battery comprising a plurality of negative electrodes and positive electrodes as described in claim 4, stacked via a separator, optionally wound together, and housed in an outer container, with a positive electrode terminal electrically connected to the positive electrode and a negative electrode terminal electrically connected to the negative electrode being drawn out from the outer container.

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