Conductive composite for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

A conductive composite for lithium ion secondary batteries, which is a composite of a polymer and a conductive material, and which has a particle diameter of 140-320 nm (inclusive) at a cumulative percentage of 50% when the particle diameter of an aqueous dispersion of the composite is measured by a dynamic light scattering method. And the particle diameter of 90% in cumulative percentage is 700 nm or more and 3400 nm or less.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrically conductive composite for a lithium-ion secondary battery, a negative electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery.

[0002] This application claims priority based on Japanese Patent Application No. 2023-039922 filed on March 14, 2023, and the contents thereof are hereby incorporated by reference. BACKGROUND

[0003] Lithium-ion secondary batteries are widely used as power sources for mobile devices such as mobile phones and notebook computers, and for hybrid electric vehicles and the like.

[0004] The capacity of a lithium-ion secondary battery mainly depends on the active material of the electrode. Graphite is generally used as the negative electrode active material, but a negative electrode active material with higher capacity is required. Therefore, silicon (Si) having a theoretical capacity much larger than that of graphite (372 mAh / g) has attracted attention.

[0005] A negative electrode active material containing Si undergoes a large volume expansion during charging. The volume expansion of the negative electrode active material is a cause of a decrease in the cycle characteristics of the battery. If the negative electrode active material volume expands, for example, the conductive path between the negative electrode active materials is cut off, or peeling occurs at the interface between the negative electrode active material layer and the current collector, or cracks occur in the SEI (Solid Electrolyte Interphase) coating film, resulting in decomposition of the electrolyte, and the like. These can decrease the cycle characteristics of the battery.

[0006] For example, Patent Literature 1 discloses an electrode for a lithium-ion secondary battery having a composite of carbon nanotubes and a binder. The composite of carbon nanotubes and a binder improves the adhesion between active material particles and the conductivity of the electrode.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT LITERATURE

[0009] Patent Literature 1: Japanese Patent Application Publication No. 2015-153714 (A) SUMMARY

[0010] PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] It is required to improve the adhesion and conductivity between negative electrode active materials and between the negative electrode active material layer and the current collector, and to improve the cycle characteristics of a lithium-ion secondary battery.

[0012] The present disclosure was made in view of the above problems, and aims to provide a structure capable of improving the cycle characteristics of a lithium-ion secondary battery.

[0013] Means for solving the technical problem

[0014] In order to solve the above technical problem, the following means are provided.

[0015] (1) The electrically conductive composite for a lithium ion secondary battery according to a first aspect is a composite of a polymer and an electrically conductive material. When a water dispersion of the composite is subjected to particle size measurement by dynamic light scattering, the particle size at which the cumulative percentage reaches 50% is 140 nm or more and 320 nm or less, and the particle size at which the cumulative percentage reaches 90% is 700 nm or more and 3400 nm or less.

[0016] (2) In the electrically conductive composite for a lithium ion secondary battery according to the above aspect, the polymer can include a carboxyl group or a carboxylate.

[0017] (3) In the electrically conductive composite for a lithium ion secondary battery according to the above aspect, the electrically conductive material can include any one or more selected from the group consisting of activated carbon, graphite, carbon black, and carbon nanotubes.

[0018] (4) The negative electrode for a lithium ion secondary battery according to a second aspect includes the electrically conductive composite for a lithium ion secondary battery according to the above aspect and a Si-containing negative electrode active material.

[0019] (5) The lithium ion secondary battery according to a third aspect has a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is the negative electrode for a lithium ion secondary battery according to the above aspect.

[0020] Effects of the Invention

[0021] The lithium ion secondary battery according to the above aspect has excellent cycle characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a schematic view of a lithium ion secondary battery according to a first embodiment.

[0023] Figure 2 is a scanning electron microscope photograph of a negative electrode active material according to the first embodiment.

[0024] Figure 3 is a schematic view of a particle size distribution of an electrically conductive composite according to the first embodiment.

[0025] Explanation of Symbols

[0026] 10: Separator

[0027] 20: Positive electrode

[0028] 22: Positive electrode current collector

[0029] 24: positive electrode active material layer

[0030] 30: negative electrode

[0031] 32: negative electrode current collector

[0032] 34: negative electrode active material layer

[0033] 40: power generating element

[0034] 50: exterior body

[0035] 52: metal foil

[0036] 54: resin layer

[0037] 60, 62: terminal

[0038] 100: lithium ion secondary battery DETAILED DESCRIPTION

[0039] Hereinafter, an embodiment will be described in detail with appropriate reference to the drawings. In the following description, the drawings used in the description are sometimes shown with portions that are to be features enlarged for the sake of easy understanding of the features, and sometimes the size ratio and the like of each constituent element are different from the actual ones. The materials, sizes, and the like exemplified in the following description are one example, and the present application is not limited to these, and can be appropriately changed and implemented within the scope of the gist thereof.

[0040] < Lithium ion secondary battery >

[0041] Figure 1 is a schematic view of a lithium ion secondary battery according to the first embodiment. Figure 1 The lithium ion secondary battery 100 shown has a power generating element 40 and an exterior body 50, and a nonaqueous electrolyte solution (not shown). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of terminals 60, 62 connected to the power generating element 40. The nonaqueous electrolyte solution is contained in the exterior body 50. In the lithium ion secondary battery 100, the power generating element 40 is connected to the outside via the terminals 60, 62. Figure 1 In the lithium ion secondary battery 100, one power generating element 40 is exemplified in the exterior body 50, but a plurality of power generating elements 40 can be stacked.

[0042] (Power generating element)

[0043] The power generating element 40 has a separator 10, a positive electrode 20, and a negative electrode 30.

[0044] < Positive electrode >

[0045] The positive electrode 20 has, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one face of the positive electrode current collector 22.

[0046] [Positive electrode current collector]

[0047] The positive electrode current collector 22 is, for example, an electrically conductive sheet. The positive electrode current collector 22 is, for example, a thin sheet of metal such as aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is suitable for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0048] [Positive electrode active material layer]

[0049] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 can also contain, as necessary, an electrically conductive aid, a binder.

[0050] The positive electrode active material contains an electrode active material capable of reversibly performing adsorption and release of lithium ions, detachment and insertion (intercalation) of lithium ions, or doping and de-doping of lithium ions and counter anions.

[0051] The positive electrode active material contains, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMnO2), lithium manganite (LiMn2O4), and a compound of the general formula: LiNi x Co y Mn z M a O2 (in the general formula, x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithium vanadium compound (LiV2O5), an olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr or VO), lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material can also be an organic substance. For example, the positive electrode active material can also be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacenic molecule.

[0052] The positive electrode active material can also be a material not containing lithium. The material not containing lithium is, for example, FeF3, a conjugated polymer containing an organic conductive substance, a Scheffer phase compound, a transition metal chalcogen compound, a vanadium oxide, a niobium oxide, or the like. The material not containing lithium can be used by using only one kind of material or by using a plurality of kinds in combination. In the case where the positive electrode active material is a material not containing lithium, for example, discharge is initially performed. Lithium is inserted in the positive electrode active material by the discharge. Furthermore, the material not containing lithium can also be pre-doped with lithium in a chemical or electrochemical manner.

[0053] The conductive aid improves the electron conductivity between the positive electrode active materials. The conductive aid is, for example, carbon powder, carbon nanotube, carbon material, metal fine powder, a mixture of carbon material and metal fine powder, and conductive oxide. The carbon powder is, for example, carbon black, acetylene black, Ketjen black, or the like. The metal fine powder is, for example, copper, nickel, stainless steel, iron, or the like.

[0054] The content of the conductive aid in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive aid is 0.5 mass% or more and 20 mass% or less, preferably 1 mass% or more and 5 mass% or less, with respect to the total mass of the positive electrode active material, the conductive aid, and the binder.

[0055] The binder in the positive electrode active material layer 24 binds the positive electrode active materials to each other. The binder can use various known materials. In addition, the binder can be the conductive composite for the negative electrode active material layer 34 described later. The binder is preferably a material that is not dissolved in the electrolyte, has oxidation resistance, and has adhesiveness. The binder is, for example, fluorine resin. The binder is, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamide-imide (PAI), polybenzimidazole (PBI), polyether sulfone (PES), polyacrylic acid and a copolymer thereof, a metal ion crosslinked body of polyacrylic acid and a copolymer thereof, polypropylene (PP) or polyethylene (PE) grafted with maleic anhydride, or a mixture thereof. The binder for the positive electrode active material layer is particularly preferably PVDF.

[0056] The content of the binder in the positive electrode active material layer 24 is not particularly limited. For example, the content of the binder is 1 mass% or more and 15 mass% or less, preferably 1.5 mass% or more and 5 mass% or less, with respect to the total mass of the positive electrode active material, the conductive aid, and the binder. If the content of the binder is small, the adhesion strength of the positive electrode 20 becomes weak. If the content of the binder is high, the binder is electrochemically inert and does not contribute to the discharge capacity, and thus the energy density of the lithium ion secondary battery 100 becomes low.

[0057] [Negative electrode]

[0058] The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[0059] [Negative electrode current collector]

[0060] The negative electrode current collector 32 is, for example, a conductive sheet material. The negative electrode current collector 32 can use the same material as the positive electrode current collector 22.

[0061] [Negative electrode active material layer]

[0062] The negative electrode active material layer 34 contains a negative electrode active material and a conductive composite.

[0063] The negative electrode active material contains, for example, silicon or a silicon compound. The silicon compound is, for example, a silicon alloy, silicon oxide, or the like. The silicon or silicon compound can be crystalline or amorphous. The amorphous silicon or silicon compound can be produced by a melt spun method, a gas atomization method, or the like. The negative electrode active material can also be an active material other than silicon.

[0064] Although not particularly limited, the content of silicon in the negative electrode active material layer can be 40% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, or 95% by mass or more and 100% by mass or less, with respect to the mass of the negative electrode active material layer.

[0065] The silicon alloy is represented by SiXn n (Si). X is a cation. X is, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K, or the like. n satisfies 0 ≤ n ≤ 0.5. The silicon oxide is represented by SiOx x . x satisfies, for example, 0.8 ≤ x ≤ 2. The silicon oxide can consist only of SiO2, only of SiO, or a mixture of SiO and SiO2. In addition, the silicon oxide can lack a portion of oxygen.

[0066] The negative electrode active material can be a composite of silicon or a silicon compound. The composite is a composite in which at least a portion of the surface of a particle of silicon or a silicon compound is coated with a conductive material. The conductive material is, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or the like. For example, a silicon-carbon composite material (Si-C) is one example of the composite. The amount of the conductive material coated with respect to the particle of silicon or a silicon compound is, for example, 0.01% by mass or more and 30% by mass or less, preferably 0.1% by mass or more and 20% by mass or less, with respect to the total mass of the composite. The composite can be produced, for example, by a mechanical alloying method, a chemical vapor deposition method, a wet method, a method in which a polymer is coated and then thermally decomposed and carbonized, or the like.

[0067] The specific surface area of the negative electrode active material, which is obtained by the BET method, is, for example, 0.5 m 2 / g or more and 100 m 2 / g or less, preferably 1.0 m 2 / g or more and 20 m 2The specific surface area is preferably 0.5 m2 / g or more, more preferably 1.0 m2 / g or more, and even more preferably 2.0 m2 / g or more. When the specific surface area is less than 0.5 m2 / g, Li ions are difficult to intercalate and deintercalate among the negative electrode active materials. When the specific surface area is large, a large amount of binder is required for electrode formation, and the capacity per unit volume becomes small.

[0068] Figure 2 FIG. 1 is a scanning electron microscope (SEM) image of the negative electrode active material 1 according to the present embodiment. The negative electrode active material 1 is, for example, amorphous and has ridges on the surface. The surface of the negative electrode active material 1 is composed of a plurality of faces, and the boundaries of these faces are the ridges.

[0069] The average length of the ridges 2 is, for example, 0.5 μm or more, and preferably 1.0 μm or more and 5.0 μm or less. The average length of the ridges 2 is, for example, calculated by adding the lengths of the ridges 2 present in 10 scanning electron microscope images taken at a magnification of 10,000 times, and dividing the sum by the number of the negative electrode active materials 1 containing the ridges 2.

[0070] The angle formed by the two faces sandwiching the ridge 2 is, for example, 150° or less, and preferably 60° or more and 120° or less.

[0071] In addition, the average number of the ridges 2 contained in one negative electrode active material 1 is, for example, 20 or less, and preferably 8 or more and 15 or less.

[0072] The particle diameter of the negative electrode active material is, for example, 0.5 μm or more, and preferably 1.0 μm or more and 5.0 μm or less. The particle diameter of the negative electrode active material can be measured by a scanning electron microscope by measuring the cross section of the negative electrode, and taking the average of the major axes of 50 particles measured at random. In addition, the particle diameter of the negative electrode active material cannot be measured by a dynamic light scattering method. This is because the negative electrode active material has a large bulk density and precipitates in water. When the negative electrode active material and the conductive composite are extracted from the negative electrode, the particle diameter of the conductive composite can be evaluated by allowing the negative electrode active material to settle and evaluating the supernatant. The average length of the ridges 2 is, for example, calculated by adding the lengths of the ridges 2 present in 10 scanning electron microscope images taken at a magnification of 10,000 times, and dividing the sum by the number of the negative electrode active materials 1 containing the ridges 2.

[0073] The conductive composite is a composite of a polymer and a conductive material. The composite is formed by the polymer and the conductive material being mutually adhered and integrated. The polymer constituting the conductive composite is not limited to one kind, and can be two or more kinds. The conductive material constituting the conductive composite is not limited to one kind, and can be two or more kinds. The conductive composite is a plurality of particles having a particle size distribution described later.

[0074] The polymer constituting the electrically conductive composite is, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid, a copolymer of acrylic acid and ethylene vinyl alcohol, a copolymer of sodium acrylate and ethylene vinyl alcohol, sodium carboxymethyl cellulose, poly norbornene dicarboxylic acid, polymethacrylic acid, polyvinyl alcohol, polyimide, polyamide.

[0075] The polymer constituting the electrically conductive composite may, for example, also contain a carboxyl group or a carboxylate. The carboxylate is a salt obtained by substituting a part of the carboxyl group with a metal ion such as Na. The polymer may also contain both a carboxyl group and a carboxylate. A polymer containing both a carboxyl group and a carboxylate can be obtained, for example, by ion exchange of a part of the carboxyl group of a polymer having a carboxyl group. The carboxyl group is responsible for the binding of the negative electrode active material to the electrically conductive composite, and the adhesion of the negative electrode active material to the electrically conductive composite is improved by the electrically conductive composite having a carboxyl group. In addition, when the polymer contains a carboxylate, it is possible to prevent intramolecular crosslinking of the polymer, thereby suppressing the gelation of the electrically conductive composite.

[0076] For example, polyacrylic acid, carboxymethyl cellulose (CMC), a copolymer of acrylic acid and ethylene vinyl alcohol, a copolymer of sodium acrylate and ethylene vinyl alcohol, sodium carboxymethyl cellulose, poly norbornene dicarboxylic acid, polymethacrylic acid contain a carboxyl group or a carboxylate.

[0077] The electrically conductive material may, for example, use the same material as the electrically conductive aid contained in the positive electrode. The electrically conductive material may, for example, also contain any one or more selected from the group consisting of activated carbon, graphite, carbon black, and carbon nanotubes. These electrically conductive materials become good conductors when they are compounded with the polymer. Although not particularly limited, in the case where the electrically conductive material contains any one or more selected from the group consisting of activated carbon, graphite, carbon black, and carbon nanotubes, the content of the one or more selected from the group consisting of activated carbon, graphite, carbon black, and carbon nanotubes can be 1 mass% or more and 100 mass% or less, can be 10 mass% or more and 100 mass% or less, or can be 50 mass% or more and 100 mass% or less, with respect to the total mass of the electrically conductive material.

[0078] The electrically conductive composite has a particle size D50 of 140 nm or more and 320 nm or less at which the cumulative percentage reaches 50% and a particle size D90 of 700 nm or more and 3400 nm or less at which the cumulative percentage reaches 90% when the particle size of the water dispersion is measured by the dynamic light scattering method. Figure 3 is a schematic view of the particle size distribution of the electrically conductive composite according to the first embodiment. Figure 3The particle size distribution is expressed in terms of frequency distribution. The particle size distribution is determined based on scattered high intensity. The particle diameter D50 at which the cumulative percentage is 50% is the particle diameter at which the cumulative distribution is 50%, and the particle diameter D90 at which the cumulative percentage is 90% is the particle diameter at which the cumulative distribution is 90%.

[0079] If the conductive composite satisfies the above particle size relationship, the particle size distribution of the conductive composite has a moderate deviation. Spaces of various sizes exist between the negative electrode active materials. This is because the sizes of the negative electrode active materials have a deviation, and the shapes of the negative electrode active materials are not fixed. The conductive composite having a moderate deviation in the particle size distribution can exist in the spaces of various sizes between the negative electrode active materials, and can improve the adhesion between the negative electrode active materials.

[0080] The content rate of the conductive composite in the negative electrode active material layer 34 is not particularly limited. For example, the content of the conductive composite is 1% by mass or more and 20% by mass or less, and preferably 3% by mass or more and 15% by mass or less, with respect to the total mass of the negative electrode active material and the conductive composite. If the content rate of the conductive composite is low, the adhesion strength between the negative electrode active materials in the negative electrode 30 and the adhesion strength between the negative electrode active material layer 34 and the negative electrode current collector 32 are weakened. If the content rate of the conductive composite is high, the conductive composite does not contribute to the discharge capacity, and thus the energy density of the lithium ion secondary battery 100 is low.

[0081] The negative electrode active material layer 34 can also include components other than the negative electrode active material and the conductive composite. For example, a crosslinking agent can also be included. If a crosslinking agent is added to the negative electrode active material layer 34, the conductive composites easily coagulate with each other, and the deviation in the distribution of the conductive composites can be controlled. The crosslinking agent can be, for example, a metal compound composed of an organic compound such as pyromellitic acid, 5,5-diethylbarbituric acid, 1,2,4-trihydroxybenzene, 1,3,5-trihydroxybenzene, 1,2,3-trihydroxybenzene, phthalic acid, isophthalic acid, terephthalic acid, cyanuric acid, and an organic ligand such as acetylacetone or bipyridine dicarboxylic acid, and a metal such as titanium, zirconium oxide, nickel, cobalt, and manganese. Among these, a titanium compound is preferred, and examples include titanium lactate, titanium triethanol amine, titanium lactate ammonium salt, titanium diethanol amine, titanium aminoethylaminoethanolate, and the like.

[0082] <Separator>

[0083] The separator 10 is sandwiched by the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30, and prevents short-circuiting of the positive electrode 20 and the negative electrode 30. The separator 10 extends in the in-plane direction of the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0084] The separator 10 has, for example, a porous structure that is electrically insulating. The separator 10 is, for example, a single layer of a polyolefin film, a laminate. The separator 10 can also be an extended film of a mixture of polyethylene or polypropylene, or the like. The separator 10 can be a fibrous nonwoven fabric made of a material selected from at least one of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 can also be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, a sulfide-based solid electrolyte. The separator 10 can also be, for example, an inorganic-coated separator. The inorganic-coated separator is formed by coating a mixture of a resin such as PVDF or CMC and an inorganic substance such as alumina or silica on the surface of the above-described film. The inorganic-coated separator has excellent heat resistance, and inhibits the deposition of transition metals eluted from the positive electrode onto the surface of the negative electrode.

[0085] <electrolyte>

[0086] An electrolyte is enclosed in the exterior body 50, and is impregnated in the power generating element 40. The nonaqueous electrolyte has, for example, a nonaqueous solvent and an electrolyte salt. The electrolyte salt is dissolved in the nonaqueous solvent.

[0087] The electrolyte can use a publicly known electrolyte. The electrolyte contains, for example, a nonaqueous solvent and an electrolyte salt.

[0088] The electrolyte salt is, for example, a lithium salt. The electrolyte can use, for example, LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, and LiN(FSO2)2, or the like. The lithium salt can be used alone as one kind, or two or more kinds can be used in combination. From the viewpoint of ionization degree, the electrolyte preferably contains LiPF6.

[0089] The nonaqueous solvent is, for example, an aprotic organic solvent. The organic solvent is, for example, a cyclic carbonate, a chain carbonate, an ether, a mixture thereof, or the like.

[0090] The cyclic carbonate solvates the electrolyte. The cyclic carbonate is, for example, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate. The cyclic carbonate preferably contains at least fluoroethylene carbonate. Fluoroethylene carbonate (FEC) has a high redox potential, and is easily reduced and decomposed. By the reduction and decomposition of a part of the fluoroethylene carbonate (FEC), the electrolyte or the remaining solvent in the electrolyte is difficult to be decomposed. In addition, the fluoroethylene carbonate (FEC) forms a thin and stable coating film (SEI coating film) on the surface of the negative electrode active material as a whole at the initial stage of use of the lithium ion secondary battery. The SEI coating film prevents the direct contact of the negative electrode active material with the electrolyte, and prevents the decomposition of the electrolyte.

[0091] The chain carbonate reduces the viscosity of the cyclic carbonate. Examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. In addition, the non-aqueous solvent may also include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, and 1,2-diethoxyethane.

[0092] <Exterior body>

[0093] The outer casing 50 seals the power generation element 40 and the non-aqueous electrolyte therein, thereby preventing the non-aqueous electrolyte from leaking out and preventing moisture from entering the lithium ion secondary battery 100 from outside.

[0094] For example, Figure 1 As shown, the exterior body 50 includes a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated with a polymer film (resin layer 54) from both sides.

[0095] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the resin layer 54 may differ between the inner and outer sides. For example, the outer side material may be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), while the inner polymer film material may be polyethylene (PE) or polypropylene (PP).

[0096] <Terminal>

[0097] Terminals 60 and 62 are connected to the negative electrode 30 and the positive electrode 20, respectively. Terminal 60 connected to the negative electrode 30 is the negative terminal, and terminal 62 connected to the positive electrode 20 is the positive terminal. Terminals 60 and 62 are responsible for external electrical connection. Terminals 60 and 62 are formed from conductive materials such as aluminum, nickel, and copper. The connection method can be welding or screw fastening. To prevent short circuits, terminals 60 and 62 are preferably protected with insulating tape.

[0098] [Method for manufacturing lithium-ion secondary battery]

[0099] The lithium ion secondary battery 100 is manufactured by separately preparing the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte solution, and the outer casing 50 and assembling them. An example of a method for manufacturing the lithium ion secondary battery 100 will be described below.

[0100] The negative electrode 30 is produced, for example, by sequentially performing a slurry preparation step, an electrode coating step, a drying step, and a rolling step.

[0101] The slurry preparation step is a step of preparing a slurry by mixing a negative electrode active material (silicon or a silicon compound), a conductive composite, and a solvent.

[0102] The negative electrode active material is, for example, an amorphous material. For example, a material including prescribed ridges can be selected from commercially available negative electrode active materials. If the negative electrode active material is amorphous, the negative electrode active materials are difficult to most closely pack with each other. If a gap for volume expansion at the time of charge and discharge is ensured within the negative electrode active material layer 34, electrode collapse is difficult to occur.

[0103] The negative electrode active material can be a material in which active material particles and a conductive material are mixed and compounded while a shearing force is applied. If the active material particles are mixed while a shearing force is applied to the extent that the active material particles are not deteriorated, the surfaces of the active material particles are coated with the conductive material. In addition, the particle diameter of the negative electrode active material can be adjusted by the extent of the mixing. In addition, the negative electrode active material after preparation can be sieved to make the particle diameters uniform.

[0104] The conductive composite is obtained by compounding a polymer and a conductive material. For example, when the polymer and the conductive material are added to a solvent such as water and stirred, the polymer and the conductive material are compounded. At this time, the polymer and the conductive material are compounded by increasing the interaction therebetween. For example, the interaction between the polymer and the conductive material is increased by adding the conductive material charged with a corona charging gun to the solvent or by activating the surface of the conductive material by irradiating ultraviolet rays during stirring.

[0105] In addition, the particle size distribution of the conductive composite can be changed by, for example, changing the kind of solvent, the stirring speed, the temperature, the stirring time, and the like, and adjusted by controlling them. For example, by selecting a good solvent of the polymer as the solvent and appropriately adding a poor solvent thereto, the aggregation of the polymer can be changed. If the aggregation of the polymer changes, the particle size distribution of the conductive composite changes. Furthermore, by adjusting the stirring speed, the temperature of the solvent, the stirring time, and the like, the probability of collision between the conductive composites changes, and thus the particle size distribution of the conductive composite can also be controlled.

[0106] In addition, a cross-linking agent can be added to the polymer and the conductive material when the conductive composite is prepared. If the cross-linking agent is added, the conductive composites easily aggregate with each other, and the particle size distribution of the conductive composite can be easily controlled within a prescribed range.

[0107] The solvent is, for example, water. The composition ratio of the negative electrode active material and the conductive composite is preferably 70 wt% to 99 wt% : 1 wt% to 30 wt% in terms of mass ratio. The mass ratio thereof is adjusted so that the entirety becomes 100 wt%.

[0108] The electrode coating step is a step of coating the slurry on the surface of the negative electrode current collector 32. The method of coating the slurry is not particularly limited. For example, a slit die coating method, a doctor blade method can be used as the method of coating the slurry.

[0109] The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 on which the slurry is coated is dried in an atmosphere of 60°C or higher and 200°C or lower. By such a step, the negative electrode active material layer 34 is formed on the negative electrode current collector 32.

[0110] The calendering step can be performed as necessary. The calendering step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The calendering step is performed using a roll press device or the like, for example.

[0111] The positive electrode 20 can be produced by the same steps as the negative electrode 30. The separator 10 and the exterior body 50 can use commercially available substances.

[0112] Next, the positive electrode 20 and the negative electrode 30 are stacked with the separator 10 interposed therebetween, and the power generating element 40 is produced. In the case where the power generating element 40 is a jelly-roll, the positive electrode 20, the negative electrode 30, and the separator 10 are wound with one end side thereof as the axis.

[0113] Finally, the power generating element 40 is enclosed in the exterior body 50. A non-aqueous electrolyte is injected into the exterior body 50. By performing pressure reduction, heating, or the like after the non-aqueous electrolyte is injected, the non-aqueous electrolyte is impregnated in the power generating element 40. By sealing the exterior body 50 by heating or the like, a lithium ion secondary battery 100 can be obtained. Further, instead of injecting the electrolyte into the exterior body 50, the power generating element 40 can be impregnated in the electrolyte.

[0114] The lithium ion secondary battery 100 according to the first embodiment is excellent in cycle characteristics by having the electrically conductive composite having a particle size distribution satisfying a prescribed condition. This is considered to be because the electrically conductive composite having a moderate deviation in the particle size distribution exists in spaces of various sizes among the negative electrode active materials, whereby the adhesion between the negative electrode active materials and between the negative electrode active material layer 34 and the negative electrode current collector 32 is improved. Further, it is considered to be because the electrically conductive composite exists between the negative electrode active materials and between the negative electrode active material layer 34 and the negative electrode current collector 32, whereby the electrically conductive path therebetween can be ensured.

[0115] The embodiments of the present application have been described in detail with reference to the drawings, but the structures in each of the embodiments and combinations thereof are one example, and addition, omission, substitution, and other changes of the structures can be made within the scope of the gist of the present application.

[0116] (Example)

[0117] [Example 1]

[0118] The positive electrode slurry is applied to one side of a 15 μm thick aluminum foil. The positive electrode slurry is prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0119] The positive electrode active material uses Li x CoO2. Acetylene black was used as a conductive additive. Polyvinylidene fluoride (PVDF) was used as a binder. N-methyl-2-pyrrolidone was used as a solvent. A positive electrode slurry was prepared by mixing 97 parts by mass of a positive electrode active material, 1 part by mass of a conductive additive, 2 parts by mass of a binder, and 70 parts by mass of a solvent. The positive electrode active material loading in the dried positive electrode active material layer was 25 mg / cm 2 The solvent was removed from the positive electrode slurry in a drying oven to produce a positive electrode active material layer. The positive electrode active material layer was pressed using a roller press to produce a positive electrode.

[0120] Next, negative electrode slurry was applied to one side of a copper foil having a thickness of 10 μm. The negative electrode slurry was prepared by mixing a negative electrode active material, a conductive composite, and a solvent.

[0121] The negative electrode active material is silicon with a particle size (D50) of 3μm. The conductive composite uses a composite of PVDF, carbon black, and carbon nanotubes. PVDF, carbon black, and carbon nanotubes were irradiated in dimethylformamide at 10mW / cm 2 The mixture was composited under ultraviolet light of 365 nm at 60° C. and 1000 rpm for 1 hour. After composited, dimethylformamide was removed to obtain a conductive composite.

[0122] The solvent used was N-methylpyrrolidone. 90 parts by mass of the negative electrode active material and 10 parts by mass of the conductive composite were mixed in water to prepare a negative electrode slurry. The negative electrode active material loading in the dried negative electrode active material layer was 2.5 mg / cm 2 The solvent was removed from the negative electrode slurry in a drying oven to produce a negative electrode active material layer. The negative electrode active material layer was pressed by roller pressing and then dried at 150° C. or higher for 5 hours in a nitrogen atmosphere.

[0123] Next, an electrolyte solution was prepared. The solvent used was a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) at a mass ratio of 11:89. LiPF6 was used as the electrolytic salt, with a LiPF6 concentration of 1 mol / L.

[0124] (Fabrication of Lithium-ion Secondary Batteries for Evaluation)

[0125] The prepared negative electrode and positive electrode were stacked via a separator (a porous polyethylene sheet) in such a manner that the positive electrode active material layer and the negative electrode active material layer faced each other, to obtain a laminate. A negative electrode lead made of nickel was attached to the negative electrode of the laminate. A positive electrode lead made of aluminum was attached to the positive electrode of the laminate. The positive electrode lead and the negative electrode lead were welded by an ultrasonic welder. The laminate was inserted into an outer packaging body of an aluminum laminate film, and heat-sealed except for one portion around the periphery to form a closed portion. Then, after the electrolyte solution was finally injected into the outer packaging body, the remaining one portion was sealed by heat-sealing while being reduced in pressure by a vacuum sealer, to produce a lithium-ion secondary battery.

[0126] (Measurement of capacity retention rate after 500 cycles)

[0127] The cycle characteristics of the lithium-ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge-discharge tester (manufactured by Hokuto Denko Corporation).

[0128] Constant-current charging was performed at a charge rate of 0.5 C (a current value at which charging is completed in 1 hour when constant-current charging is performed at 25°C), constant-current constant-voltage charging (CC-CV charging) was performed until the battery voltage became 4.2 V, and constant-current discharging was performed at a discharge rate of 1.0 C until the battery voltage became 2.5 V (CC discharging). The discharge capacity after the end of the charge-discharge was measured, and the battery capacity Q1 before the cycle test was calculated.

[0129] The battery in which the battery capacity Q1 was calculated was charged again by constant-current constant-voltage charging at a charge rate of 0.5 C using the secondary battery charge-discharge tester until the battery voltage became 4.2 V, and discharged by constant-current discharging at a discharge rate of 1.0 C until the battery voltage became 2.5 V. The above charge-discharge was counted as one cycle, and 500 cycles of charge-discharge were performed. Then, the discharge capacity after 500 cycles of charge-discharge was measured, and the battery capacity Q2 after 500 cycles was calculated.

[0130] The capacity retention rate E after 500 cycles was calculated from the capacities Q1 and Q2 calculated above. The capacity retention rate E was calculated from E = Q2 / Q1 x 100. The capacity retention rate of Example 1 was 69%.

[0131] (Example 2)

[0132] Example 2 differs from Example 1 in that the particle size distribution of the conductive composite was changed, and a cross-linking agent was added when the conductive composite was compounded. The particle size distribution of the conductive composite was changed by adding the cross-linking agent. The cross-linking agent in Example 2 was pyromellitic acid. The capacity retention rate after 500 cycles was calculated in the same manner as in Example 1.

[0133] (Example 3)

[0134] Example 3 differs from Example 1 in that the solvent at the time of preparing the conductive composite is changed from dimethylformamide to water, and the material of the polymer constituting the conductive composite and the particle size distribution of the conductive composite are changed. In Example 3, the polymer constituting the conductive composite is set to polyacrylic acid. Polyacrylic acid has a carboxyl group. The carboxyl group and the carboxylate in the conductive composite can be found by extracting the conductive composite from the electrode prepared and analyzing it. In this conductive composite, -COOLi and -COOH are highly water-soluble, and can also be easily extracted from the electrode. The capacity retention rate after 500 cycles is found in the same manner as in Example 1.

[0135] (Example 4)

[0136] Example 4 differs from Example 3 in that the particle size distribution of the conductive composite is changed, and a cross-linking agent is added at the time of preparing the conductive composite. The particle size distribution of the conductive composite is changed by adding the cross-linking agent. The cross-linking agent in Example 4 is set to pyromellitic acid. The capacity retention rate after 500 cycles is found in the same manner as in Example 3.

[0137] (Examples 5, 6)

[0138] Examples 5 and 6 differ from Example 4 in that the kind of the cross-linking agent added at the time of preparing the conductive composite is changed. By changing the kind of the cross-linking agent, the particle size distribution of the conductive composite changes. The cross-linking agent in Example 5 is terephthalic acid. The cross-linking agent in Example 6 is cyanuric acid. The capacity retention rate after 500 cycles is found in the same manner as in Example 4.

[0139] (Example 7)

[0140] Example 7 differs from Example 3 in that the material of the polymer constituting the conductive composite, the particle size distribution of the conductive composite, and the cross-linking agent added at the time of preparing the conductive composite are changed. In Example 7, the polymer constituting the conductive composite is set to a polymer in which polyvinyl alcohol and polyacrylic acid are mixed. The cross-linking agent in Example 7 is 1,2,4-trihydroxybenzene. The capacity retention rate after 500 cycles is found in the same manner as in Example 3.

[0141] (Examples 8, 9)

[0142] Example 8, 9 differ from Example 3 in that the material of the polymer constituting the conductive composite, the material of the conductive material, and the particle size distribution of the conductive composite, and a crosslinking agent is added at the time of compounding the conductive composite. In Example 8, 9, the polymer constituting the conductive composite is set to a polymer in which carboxymethyl cellulose and polyacrylic acid are mixed. In addition, Example 8 sets the electric conductor constituting the conductive composite to graphite and carbon nanotube. Example 9 sets the electric conductor constituting the conductive composite to carbon black and activated carbon. The crosslinking agent in Example 8, 9 is set to a titanium complex. The capacity retention rate after 500 cycles is found in the same manner as Example 3.

[0143] (Comparative Example 1)

[0144] Comparative Example 1 differs from Example 1 in that the particle size distribution of the conductive composite is changed. The particle size distribution of the conductive composite is changed by stirring in dimethylformamide at 25°C, 500 rpm for 0.5 hours while irradiating 10 mW / cm2of 365 nm ultraviolet light. The capacity retention rate after 500 cycles is found in the same manner as Example 1. 2

[0145] (Comparative Example 2)

[0146] Comparative Example 2 differs from Example 4 in that the particle size distribution of the conductive composite is changed. The particle size distribution of the conductive composite is changed by stirring in water at 80°C, 1500 rpm for 10 hours while irradiating 10 mW / cm2of 365 nm ultraviolet light. The capacity retention rate after 500 cycles is found in the same manner as Example 4. 2

[0147] (Comparative Example 3)

[0148] Comparative Example 3 differs from Example 4 in that the particle size distribution of the conductive composite is changed. The particle size distribution of the conductive composite is changed by stirring in water at 80°C, 1500 rpm for 15 hours while irradiating 10 mW / cm2of 365 nm ultraviolet light. The capacity retention rate after 500 cycles is found in the same manner as Example 4. 2

[0149] The conditions of the conductive composite and the measurement results of Example 1 to 9 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0150]

[0151] ​​​The capacity maintenance rates of Examples 1 to 9 are higher than those of Comparative Examples 1 to 3. It is considered that this is because the electrically conductive composite contained in the negative electrode active material layer 34 enters various sizes of spaces between the active materials and between the active materials and the current collector foil. The electric conductivity and adhesion between them are improved by the electrically conductive composite, and thus the lithium ion secondary batteries according to Examples 1 to 9 have excellent cycle characteristics.

[0152] Industrial Applicability

[0153] According to the present application, a lithium ion secondary battery having excellent cycle characteristics can be provided.

Claims

1. An electrically conductive composite for a lithium ion secondary battery, wherein the electrically conductive composite for a lithium ion secondary battery is a composite of a polymer and an electrically conductive material, when a water dispersion of the composite is subjected to particle size measurement by a dynamic light scattering method, a particle size at which a cumulative percentage reaches 50% is 140 nm or more and 320 nm or less, a particle size at which a cumulative percentage reaches 90% is 700 nm or more and 3400 nm or less.

2. The electrically conductive composite for a lithium ion secondary battery according to claim 1, wherein the polymer contains a carboxyl group or a carboxylate.

3. The electrically conductive composite for a lithium ion secondary battery according to claim 1, wherein the electrically conductive material contains any one or more selected from the group consisting of activated carbon, graphite, carbon black, and carbon nanotube.

4. A negative electrode for a lithium ion secondary battery, wherein comprises: the electrically conductive composite for a lithium ion secondary battery according to claim 1, and a Si-containing negative electrode active material.

5. A lithium ion secondary battery, wherein has: a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, the negative electrode is the negative electrode for a lithium ion secondary battery according to claim 4.

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