Positive electrode and secondary battery

Abstract

The purpose of the present invention is to provide an electrode that can achieve favorable rate characteristics when applied to a battery.　The present invention relates to a positive electrode comprising a current collecting foil and, on the main surface of the current collecting foil, an active material layer including an active material, a conductive assistant, and a binder. The active material layer includes routes along which ions can pass, said routes stretching from the main surface side of the current collecting foil to the surface side of the current collecting foil that is opposite from the main surface side. The positive electrode is such that the most frequent value of curvatures τ is 1.59 or less, and among curvatures lower than the maximum value of the count number of conductive paths expressing the most frequent value of curvatures, the most frequent value of the curvatures at 50% of the count number of conductive paths is the curvature that is lower than the most frequent value of curvatures and is 1.52 or less.

Description

Positive electrodes and secondary batteries

[0001] The present invention relates to a positive electrode and a secondary battery.

[0002] Electrodes used in lithium-ion batteries and the like have an active material layer formed on a current collector. There are many methods for forming an active material layer on a current collector when producing an electrode. These methods can be broadly divided into wet methods and dry methods, depending on whether or not the electrode mixture containing the active material is slurried. In the wet method, an active material, a polymer, a solvent, and the like are mixed to obtain a slurry, and the resulting 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 undergoing a step of applying a slurry of a solvent and an active material. For example, an electrode is formed by forming granules containing the active material in advance and pressing them onto a current collector.

[0003] Patent Document 1 discloses a wet electrode manufacturing method for producing a lithium-ion secondary battery using an electrode slurry containing an active material, a conductive agent, and a solvent. Patent Document 1 improves adhesion between the electrode and the current collector by adjusting the water content. Patent Document 2 also discloses a technology in which granules containing an active material are formed in advance and then transferred onto a current collector. Patent Document 2 discloses a method for manufacturing an electrode sheet, including a granulation step for obtaining wet granules containing active material particles, a binder, and a solvent; a molding step for forming an aggregate of the granules obtained in the granulation step into a planar or block-shaped compact; and a process for forming the compact obtained in the molding step into a film and transferring the film to a current collector.

[0004] International Publication No. 2017 / 188043 Japanese Patent Application Laid-Open No. 2015-201318

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

[0006] As a result of investigations, the inventors have found that by appropriately forming paths in the active material layer through which lithium ions can pass (ion paths between active materials), it is possible to obtain an electrode, particularly a positive electrode, with good rate characteristics, and a secondary battery including the same.

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

[0008] That is, the present invention provides the following positive electrode and secondary battery: 1. A positive electrode including a current collector and an active material layer provided on the 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 that connects from a surface that contacts the current collector to a surface opposite to the surface that contacts the current collector, and wherein 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 showing the tortuosity τ is measured, the mode τ of the tortuosity ｍａｘ is 1.59 or less, and the most frequent value τ of the curvature ratio ｍａｘ The number of paths that show ｍａｘ and the most frequent value τ of the curvature ratio 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 ｍａｘ The curvature ratio is smaller than τ ５０ Then, τ ５０ A positive electrode characterized in that:

[0009] 2. A secondary battery in which a plurality of positive electrodes and negative electrodes described in 1 above are stacked with separators interposed therebetween, optionally wound together, 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.

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

[0011] 1 shows distribution curves of the tortuosity τ of the path penetrating the 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.

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

[0013] One embodiment of the present invention is a positive 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. Here, the active material layer has a continuous path connecting 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, a tortuosity τ is defined as a ratio of a length L of the path to an average thickness x of the active material layer, and when the number n of paths showing the tortuosity τ is measured, the mode τ of the tortuosity ｍａｘ is 1.59 or less, and the most frequent value τ of the curvature ratio ｍａｘ The number of paths that show ｍａｘ and the most frequent value τ of the curvature ratio 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 ｍａｘ The curvature ratio is smaller than τ ５０ Then, τ ５０ is 1.52 or less.

[0014] In one embodiment, the positive electrode refers to, but is not limited to, a positive electrode used in a non-aqueous secondary battery material. The positive 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 additive, and a binder. Examples of the current collector that can be used include metals such as stainless steel, aluminum, nickel, and titanium, and current collectors in which the surface of aluminum or stainless steel is surface-treated with carbon, nickel, titanium, or silver. It is preferable to use a planar metal, particularly a metal foil, as the current collector. The current collector will be described in detail below. Examples of 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; TiS ２ , FeS, MoS ２ transition metal sulfides such as MnO, V ２ O ５ , V ６ O １３ , TiO ２ 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, as well as lithium, phosphorus, and oxygen. These compounds may have some elements partially substituted with other elements in order to improve their properties.

[0015] The conductive additive is a material for reducing the resistance of the electrode. Examples of the conductive additive include carbon black 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 commonly used for electrode formation, such as thickeners, dispersants, and stabilizers, can be appropriately used in the positive electrode active material layer.

[0016] Examples of binders 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. Examples of binders that can be used include polyacrylic acid, polymethacrylic acid, sodium polyacrylate, potassium polyacrylate, sodium polymethacrylate, potassium polymethacrylate; ethyl polyacrylate, ethyl polyacrylate, butyl polyacrylate, methyl polymethacrylate, ethyl polymethacrylate, butyl polymethacrylate, and any mixtures thereof. Furthermore, as the binder, carboxymethyl cellulose (hereinafter referred to as "CMC"), which is a cellulose derivative, or a metal salt of carboxymethyl cellulose (for example, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose) can also be used. The binder will be described in detail later.

[0017] 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 void or hole in the active material layer, and in one embodiment, it is preferably a path that penetrates the active material layer 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.

[0018] The active material layer has a certain predetermined thickness, and the average thickness of the active material layer is denoted by x. Meanwhile, when the length of the path is denoted by L, the tortuosity τ can be calculated as the ratio of the path length L 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 the thickness direction of the active material layer is compared to the length in the thickness direction, i.e., the degree of detour of the path, and is also called the bending degree. The minimum value of the tortuosity is 1. A tortuosity value of 1 indicates that the path penetrates straight through the active material layer in the thickness direction, and a tortuosity value greater than 1 indicates that the path is curved or penetrates with one or more detour points.

[0019] The tortuosity τ of an active material layer can be determined by a method combining continuous cross-section preparation using a focused ion beam device with scanning electron microscope observation, or by X-ray CT (computed tomography). For a given 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. The number n of paths exhibiting a tortuosity τ is measured, and the number of paths is plotted against the tortuosity, as shown in FIG. 1, for example. For example, if 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, if 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. Thus, the peak of the number n of paths exhibiting a tortuosity τ, i.e., the mode τ of the tortuosity, can be determined.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 produce an electrode for a battery that requires charging and discharging at a large current without impeding the passage of ions through the active material layer (i.e., reducing the resistance of the active material layer).

[0020] In one embodiment, the most frequent value τ of the curvature ratio ｍａｘ It is preferable that the curve ratio is 1.59 or less. Here, the distribution of the curve ratio will be described with reference to FIG. 1. The most frequent curve ratio τ ｍａｘ 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 curvature rates τ that represent 50% of the number of routes, the most frequent value τ ｍａｘ The curvature ratio is smaller than τ ５０ In this case, τ ５０ is preferably 1.52 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 positive electrode of one embodiment. On the other hand, the distribution curves represented by the dotted and dashed lines are those of the active material layer in the positive 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 the active material layer of the present invention has many paths with a small tortuosity. In one embodiment, the mode value τ of the tortuosity ｍａｘ The number of paths that indicate ｍａｘ It is particularly preferable that the ionic conductivity is not less than 570. This is thought to ensure sufficient passage of ions in the active material layer, thereby reducing the resistance of the active material layer.

[0021] In one embodiment, the active material layer preferably includes granules in which the active material, conductive additive, and binder are aggregated. Here, the granules refer to granules (granules) obtained by aggregating a mixture of the active material, conductive additive, binder, and, optionally, a solvent. In one embodiment, the active material layer used in the positive electrode is formed by applying granules in which the active material, conductive additive, and binder are aggregated to the surface of a current collector using a dry coating method. First, granules are formed to have the desired surface and internal structure (specifically, the desired path structure), and then the granules are applied to the current collector surface using a dry coating method. The active material layer can be formed with the desired tortuosity distribution by adjusting the pressing conditions to obtain the desired density. The reason why an active material layer with the desired tortuosity distribution can be formed by using granules in which the active material, conductive additive, and binder are aggregated is unclear, but it is thought to be because the granules gradually break down during pressing, allowing the active material layer to be formed without eliminating voids between the active material particles. Compared to 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 larger or similar number of paths penetrating the thickness direction of the active material layer, and the tortuosity of the paths is smaller overall.

[0022] The tortuosity can be adjusted by changing 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 has been found that when an active material layer is formed by a dry method, the thickness of the through-holes can be made larger than that of the wet method, even if the amount of voids per volume is similar to that of the wet method, making it easier to control the tortuosity. However, even when using a dry method, if the density of the active material layer is increased without adjusting the pressing conditions, the granules will be 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 will increase overall.

[0023] 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 50% of the integrated value (median size: D50) in the particle size distribution (volume basis) measured by a laser diffraction scattering method.

[0024] From the viewpoint of adhesion of the conductive additive 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. It is also 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 refers to the particle size at 50% of the integrated value (median diameter: D50) in the particle size distribution (volume basis) measured by a laser diffraction scattering method.

[0025] 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; TiS ２ , FeS, MoS ２ transition metal sulfides such as MnO, V ２ O ５ , V ６ O １３ , TiO ２and one or more transition metal oxides selected from the group consisting of olivine-type lithium phosphates. The olivine-type lithium phosphates contain, 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, as well as lithium, phosphorus, and oxygen. These compounds may have some elements partially substituted with other elements to improve their properties. Among these, the positive electrode active material is preferably a Li-based phosphate having an average particle size of 3 to 15 μm. ａ Ni ｂ Mn ｃ Co ｄ M ｘ O ２ (wherein 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 element selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr) is preferably used.

[0026] In one embodiment, when the total amount of the granules is 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, and even more preferably 98.5 parts by mass or less. The proportion of the active material involved in forming the granules is at least 95% or more, preferably 97% or more, and even more preferably 99% or more.

[0027] The conductive additive 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 additive preferably has an aggregate structure in which nanoparticle-sized carbons are linked together like beads. Examples of carbon black that can be used include acetylene black and ketjen black. Carbon nanotubes, which are formed from carbon-atom-based materials such as graphene into cylindrical shapes with a diameter of several nanometers and a length of several millimeters, can also be used. In particular, bundles of single-walled carbon nanotubes, each of which has a substantially single layer around the cylinder, or multi-walled carbon nanotubes, each of which has multiple layers around the cylinder, can be used. Among these, single-walled or multi-walled carbon nanotubes are preferred.

[0028] In one embodiment, when the total amount of the granules is 100 parts by mass, the content of the conductive additive 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, even more preferably 0.5 parts by mass or less.

[0029] In one embodiment, the granules further contain a binder. Preferably, the conductive additive and binder are present on the surface of the active material particles, and the binder and conductive additive 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 reducing the pore size between the active material particles.

[0030] 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 to these and can be changed appropriately depending on the solvent. Preferably, a combination of at least two or more of a first binder having strong adhesive strength and a second binder that provides flexibility after drying is used, and for example, a binder containing polyacrylic acid and styrene butadiene rubber, etc. is preferred.

[0031] In one embodiment, when the total amount of the granules is 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 part by mass or more, and preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, even more preferably 2.0 parts by mass or less.

[0032] A method for producing a positive electrode according to one embodiment preferably includes the steps of spraying a dispersion of a conductive additive onto the active material particles while mixing and stirring the active material particles to form granules, and placing the granules on a current collector and pressing them. The method for forming the granules is not particularly limited, and examples include spray granulation, stirring granulation, fluidized bed granulation, rolling granulation, extrusion granulation, and compression granulation. These granulation methods may be selected or combined as desired. Examples of spray granulation methods include spray-drying granulation. In the spray-drying granulation method, for example, a dispersion of a negative electrode active material and a conductive additive is sprayed into high-temperature gas in a spray dryer to form granules. In the stirring granulation method, for example, a dispersion of a conductive additive is sprayed onto the negative electrode active material particles while mixing and stirring the negative electrode active material to form granules. By adjusting the number of presses and the pressure applied to the active material layer containing the granules thus formed, it is possible to obtain a desired electrode density while maintaining high adhesion between the current collector and the active material layer and maintaining the desired pore paths between the active material particles.

[0033] By applying a dispersion containing a highly dispersed conductive additive and a binder to active material particles without the addition of other organic solvents, an active material having a uniformly dispersed conductive additive on its surface can be prepared. The diameter of the granules formed varies depending on the equipment used to mix and stir these materials, but can be controlled by adjusting the amount and timing of spraying the dispersion of conductive additive and binder, as well as the mixing and stirring times. Furthermore, by classifying the formed granules through a sieve, granules with any particle size distribution can be obtained.

[0034] In one embodiment, the material of the current collector is not particularly limited, but when the current collector is a positive electrode current collector, aluminum foil is preferred. When the current collector is a negative electrode current collector, a material selected from the group consisting of copper, stainless steel, nickel, titanium, or an alloy thereof is preferred, with copper being particularly preferred. The shape of the current collector is not particularly limited, and may be foil, flat, or mesh-like. For example, the thickness is 0.001 mm or more and 0.5 mm or less.

[0035] When granules are placed on the surface of a current collector or current collector and pressed, the pressing method is not particularly limited as long as it can deposit electrode granules containing an electrode active material on the current collector. In one embodiment, a roll-type pressure molding method is preferably used. 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 rolled-up roll and then wound up. The granules are supplied onto the current collector before passing through the rolls and compressed as they pass between the rolls to form an electrode sheet. To adjust the amount of granules supplied, it is preferable to smooth the surface of the granules with a squeegee or the like before supplying them between the rolls to increase their thickness. It is preferable to previously form a current collector surface layer containing a binder to enhance adhesion to the granules on the current collector. By adjusting the number of times, pressure, temperature, etc., of pressing the electrode sheet, it is possible to adjust the adhesion between the active material layer and the current collector and the length of the voids between the active material particles.

[0036] A second embodiment of the present invention is a secondary battery in which a plurality of the above-described positive electrodes and negative electrodes are stacked with a separator interposed therebetween, optionally wound, 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 fabricated using the above-described negative electrode in accordance with a known method. In the second embodiment, the negative electrode refers to, but is not particularly limited to, a negative electrode used in a non-aqueous secondary battery material. For example, the negative electrode is composed of a negative electrode current collector and a negative electrode active material layer provided on the surface of the negative electrode current collector, and the active material layer includes a negative electrode active material, a conductive additive, and a binder. The negative electrode active material layer including the negative electrode active material, the conductive additive, and the binder can be formed on at least one surface of the negative electrode current collector.

[0037] 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 with hexagonal plate-like crystals and is sometimes called graphite. Natural graphite and artificial graphite include natural graphite coated with amorphous carbon and artificial graphite coated with amorphous carbon.

[0038] Here, amorphous carbon refers to a carbon material that is amorphous as a whole and has a random network structure of microcrystals, which may partially have a structure similar to graphite. 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.

[0039] The crystalline structure of artificial graphite is generally thinner than that of natural graphite. When artificial graphite is used as a negative electrode active material for nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, it must have 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 (where x is a number that satisfies 0.5≦x≦1.6) and a carbon-based active material. Examples of binders that form the negative electrode active material layer together with the negative 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); and polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. Examples of conductive additives include carbon black 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 commonly used for electrode formation, such as thickeners, dispersants, and stabilizers, can be used appropriately in the negative electrode active material layer.

[0040] In the second embodiment, the separator may be, for example, a porous separator. The separator may be in the form of a membrane, a film, a nonwoven fabric, or the like. Examples of the porous separator include polyolefin-based porous separators such as polypropylene-based and polyethylene-based separators; and porous separators formed from polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, or the like.

[0041] The separator may contain inorganic particles such as ceramic particles, or may further include a layer containing inorganic particles.

[0042] In a second embodiment of the secondary battery, a plurality of positive electrodes and negative electrodes are stacked with a separator interposed therebetween, optionally wound, and housed in an outer container. The secondary battery is formed by extending 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. The outer container can be, for example, a flexible film having a resin layer on both sides of a metal layer such as aluminum as a base material, or a metal can made of aluminum, iron, stainless steel, or the like. The positive electrode terminal can be made of aluminum or an aluminum alloy, and the negative electrode terminal can be made of copper, a copper alloy, or a nickel-plated copper or copper alloy. The shape of the secondary battery according to the second embodiment can be, for example, a rectangular, cylindrical, coin, or button shape housed in an outer container such as an aluminum can, iron can, or stainless steel can. Pouch-shaped secondary batteries housed in an outer container primarily made of aluminum film are also preferred.

[0043] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention.

[0044] EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these.

[0045] <Production of Positive Electrode> (Example 1) A dispersion solution of the conductive additive was prepared by highly dispersing single-walled carbon nanotubes (SWCNT) as a conductive additive and polyvinylidene fluoride (PVDF) as a binder together with N-methyl-2-pyrrolidone (NMP) as a solvent. Next, LiNi particles with an average particle diameter of 4.5 μm, which are positive electrode active material particles, were added. ０．９２ Mn ０．０３ Co ０．０５ O ２While stirring, the conductive additive dispersion solution prepared by the above procedure was sprayed into the mixer, thereby adhering the conductive additive to the surface of the positive electrode active material particles. Next, the active material particles with the conductive additive attached were placed in a stirring granulator, and the stirring blades and chopper were rotated at a predetermined rotation speed. A predetermined amount of the prepared conductive additive dispersion solution was sprayed at a time, while the conductive additive, active material particles, and binder were mixed and stirred to obtain granules. The mass ratio of the solid content in the granules was 98.0:0.5:1.5 (positive electrode active material / SWCNT / PVDF). The artificial graphite manufactured by Nippon Graphite Industries Co., Ltd., the PVDF manufactured by Kureha Corporation, and the SWCNT manufactured by OCSiAl Corporation could be used.

[0046] 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 a binder (PVDF) had been previously formed on the surface of the current collector. After supplying the granules onto the current collector, the amount of granules supplied to the press roll was adjusted by a squeegee roll before the granules were compressed by the press roll, and the granules were then passed between the press rolls to form an active material layer of the desired density. In the examples, a density of 3.50 g / cm was achieved by passing the granules through the press once at a linear pressure of 1.0 t / cm. ３ A negative electrode of 1000 .mu.m was obtained.

[0047] Comparative Example 1: The same granules as in Example 1 were used, and the press conditions were a press roll of 0.4 t / cm linear pressure, and the density was 3.50 g / cm. ３ A positive electrode of 1000 .mu.m was obtained.

[0048] <Production of Battery> A slurry of a mixture of artificial graphite and silicon monoxide as an active material, single-walled carbon nanotubes (SWCNT) as a conductive additive, and styrene butadiene rubber (SBR) as a binder dispersed in a solvent was applied to a copper foil as a negative electrode current collector, and then pressed to a density of 1.6 g / cm. ３ A negative electrode of 1000 .mu.m was obtained.

[0049] The nine negative electrode layers and eight positive electrode layers of the previously obtained example or comparative example were stacked 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. Next, a mixed solvent of ethylene carbonate and diethyl carbonate (ethylene carbonate:diethyl carbonate=3:7 (volume ratio)) was added with LiPF as an electrolyte. ６ The electrolyte solution in which the above was dissolved to a concentration of 1.0 mol / L was placed together with the obtained laminate in a container with a flexible film as a case, and the container was sealed with the positive electrode terminal and the negative electrode terminal extended to the outside, thereby obtaining a secondary battery.

[0050] <Rate characteristics> The prepared battery was charged at a constant current of 0.2 C up to 4.2 V, and after reaching 4.2 V, the charge was switched to a constant voltage and charged down to 0.015 C, and discharged at a constant current of 0.2 C and 3 C down to 2.5 V, and the rate characteristics were determined as 3 C / 0.2 C, which is the ratio of the capacity at 2.5 V between the 3 C discharge and the 0.2 C discharge. Generally, the capacity tends to decrease more easily as the current value increases, so the closer the current value is to 100%, the better the rate characteristics are.

[0051] <Tortuosity> The tortuosity was measured by X-ray CT analysis using a nano-3DX manufactured by Rigaku Corporation. ３ ) μm ３ and the field of view range (67.6 ３ ) μm ３ One side of this field of view was parallel to the electrode thickness direction in the active material layer from the current collector toward the electrode surface, i.e., 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 for Example 1 and Comparative Example 1 are also shown in FIG.

[0052]

[0053] As shown in Table 1 and Figure 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 paths, is short at 1.58, and the number of paths is large. Furthermore, 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 Example 1, which uses a positive electrode having an active material layer with a larger number of paths with a larger tortuosity, the rate characteristics are inferior to those of the examples.

[0054] The positive electrode for a secondary battery and the secondary battery of the present invention have been described in detail above, but the present invention is not limited to the above-mentioned embodiments and examples, and various improvements and modifications may be made within the scope of the present invention.

Claims

1. A positive electrode comprising: 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 that connects from a surface that contacts the current collector to a surface opposite to the surface that contacts the current collector, and wherein 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 in the active material layer, and when the number n of paths showing the tortuosity τ is measured, the most frequent tortuosity τ is ｍａｘ is 1.59 or less, and the most frequent value τ of the curvature ratio is ｍａｘ 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 ratios τ that indicate 50% of the number of routes, the most frequent value τ of the curve ratio ｍａｘ The curvature ratio is smaller than τ ５０ Then, τ ５０ is 1.52 or less.

2. The positive electrode according to claim 1, wherein the active material layer includes a granule in which the active material, the conductive additive, and the binder are bound together.

3. The positive electrode according to claim 1 or 2, wherein the average particle size of the granules is from 20 μm to 100 μm.

4. The mode τ ｍａｘ The number of paths that indicate ｍａｘ The positive electrode according to any one of claims 1 to 3, wherein the number of is 570 or more.

5. The positive electrode according to any one of claims 1 to 4, wherein the active material is a positive electrode active material.

6. A secondary battery in which a positive electrode according to any one of claims 1 to 5 and a negative electrode are stacked in multiple layers with a separator between them, optionally rolled up, 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 pulled out from the outer container.

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