Positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

A positive electrode (20) for a lithium-ion secondary battery has a positive electrode current collector (22) and a positive electrode active material layer (24), wherein the positive electrode active material layer (24) contains a positive electrode active material (1) and a fluorine-free binder composition, the binder composition is a mixture of a first synthetic resin (3) and cellulose nanofibers (2), the content of the binder composition is 1.0-4.0 mass% inclusive, the content of the first synthetic resin (3) is 0.2-1.0 mass% inclusive, and the content of the cellulose nanofibers (2) is 1.0-3.5 mass% inclusive.

Description

Positive electrode for lithium-ion secondary batteries and lithium-ion secondary batteries

[0001] This disclosure relates to a positive electrode for a lithium-ion secondary battery and a lithium-ion secondary battery. This application claims priority under Japanese Patent Application No. 2025-051612, filed in Japan on March 26, 2025, the contents of which are incorporated herein by reference.

[0002] Lithium-ion rechargeable batteries are widely used as power sources for mobile devices such as cell phones and laptops, as well as hybrid cars.

[0003] In conventional lithium-ion secondary batteries, polyvinylidene fluoride (PVDF) and other materials have been used as binders for the positive electrode active material layer to improve electrode characteristics. For example, Patent Document 1 discloses a positive electrode active material on which carbon derived from cellulose nanofibers is supported in order to improve conductivity, but PVD is used as the binder when manufacturing the positive electrode active material layer.

[0004] However, from the perspective of reducing environmental impact, regulations on per- and polyfluorinated compounds (PFAS: Per- and Polyfluoroalkyl Substances) have been considered in recent years. For this reason, PFAS-free electrodes are attracting attention.

[0005] For example, Patent Document 2 discloses the use of a layer obtained using a slurry composition containing an electrode active material, a conductive additive, and a cellulose fiber aqueous dispersion as a binder as an electrode. Also, for example, Patent Document 3 discloses improving the durability of a negative electrode by using a negative electrode active material and a binder containing an acrylic polymer and cellulose nanofibers in the negative electrode active material layer.

[0006] Japanese Patent No. 6193505, International Publication No. 2013 / 042720, JP 2025-500323

[0007] As described in Patent Document 2, when a cellulose fiber aqueous dispersion is used as a binder, the adhesion to the electrode is low, and the addition of resin to the binder as an auxiliary agent is considered. However, as described in Patent Document 3, if the amount of resin in the binder is large, the surface of the active material is covered with the binder, and the rate characteristics deteriorate.

[0008] This disclosure has been made in view of the above-mentioned issues and aims to provide a positive electrode for lithium-ion secondary batteries and a lithium-ion secondary battery with excellent rate characteristics.

[0009] To solve the above problems, the following means are provided.

[0010] (1) The positive electrode for a lithium-ion secondary battery according to the first embodiment comprises a positive electrode current collector and a positive electrode active material layer, wherein the positive electrode active material layer comprises a positive electrode active material and a fluorine-free binder composition, the binder composition being a mixture of a first synthetic resin and cellulose nanofibers, the content of the binder composition being 1.0% by mass or more and 4.0% by mass or less, the content of the first synthetic resin being 0.2% by mass or more and 1.0% by mass or less, and the content of the cellulose nanofibers being 1.0% by mass or more and 3.5% by mass or less.

[0011] (2) In the positive electrode for a lithium-ion secondary battery according to the above embodiment (1), the first synthetic resin may include one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene.

[0012] (3) In the positive electrode for a lithium-ion secondary battery according to the above embodiment (1) or (2), the average diameter of the cellulose nanofiber may be 5 nm or more and 50 nm or less.

[0013] (4) In a positive electrode for a lithium-ion secondary battery according to any of the above embodiments (1) to (3), a carbon coating layer is disposed between the positive electrode current collector and the positive electrode active material layer, wherein the carbon coating layer comprises a conductive carbon material and a second synthetic resin, and the second synthetic resin may include one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene.

[0014] (5) The lithium-ion secondary battery according to the second embodiment comprises a positive electrode for a lithium-ion secondary battery according to any of embodiments (1) to (4) above, a negative electrode, and a separator disposed between the positive electrode for the lithium-ion secondary battery and the negative electrode.

[0015] The positive electrode for a lithium-ion secondary battery according to the above embodiment improves the rate characteristics of the lithium-ion secondary battery.

[0016] This is a schematic diagram showing the stacked structure of the positive electrode for a lithium-ion secondary battery according to the first embodiment. This is a schematic diagram showing a modified example of the stacked structure of the positive electrode for a lithium-ion secondary battery according to the first embodiment. This is a schematic diagram of a lithium-ion secondary battery according to the first embodiment.

[0017] The embodiments will be described in detail below, with reference to the figures as appropriate. The drawings used in the following description may be enlarged for convenience to clearly illustrate the features, and the dimensional ratios of each component may differ from those in reality. The materials, dimensions, etc., exemplified in the following description are examples only, and this disclosure is not limited to them. It is possible to modify and implement them as appropriate without changing the technical requirements.

[0018] "Lithium-ion secondary battery" Fig. 3 is a schematic diagram of the lithium-ion secondary battery according to the first embodiment. A lithium-ion secondary battery 100 shown in Fig. 3 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (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 and 62 connected to the power generating element 40. The non-aqueous electrolyte is accommodated in the exterior body 50. Although Fig. 3 illustrates a case where one power generating element 40 is provided in the exterior body 50, a plurality of power generating elements 40 may be stacked. The lithium-ion secondary battery 100 may be of any type such as cylindrical, prismatic, laminated, or button-type.

[0019] (Power Generating Element) The power generating element 40 includes a positive electrode 20, a negative electrode 30, and a separator 10 disposed between the positive electrode 20 and the negative electrode 30.

[0020] <Positive Electrode> Fig. 1 is a schematic diagram of the laminated structure of the positive electrode 20 for a lithium-ion secondary battery according to the first embodiment. The positive electrode 20 includes, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is preferably in contact with at least one surface of the positive electrode current collector 22.

[0021] Fig. 2 is a schematic diagram of a modified example of the laminated structure of the positive electrode 20 for a lithium-ion secondary battery according to the first embodiment. In the positive electrode 20, for example, a carbon coating layer 23 may be disposed between the positive electrode current collector 22 and the positive electrode active material layer 24. In this case, it is preferable that one surface of the carbon coating layer 23 is in contact with at least one surface of the positive electrode current collector 22, and the other surface of the carbon coating layer 23 is in contact with at least one surface of the positive electrode active material layer 24.

[0022] [Positive Electrode Current Collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate of aluminum, copper, nickel, titanium, stainless steel, or the like. Lightweight aluminum is suitably used 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.

[0023] [Positive Electrode Active Material Layer] The positive electrode active material layer 24 comprises the positive electrode active material 1 and a fluorine-free binder composition. The positive electrode active material layer 24 may optionally contain a conductive auxiliary agent 4.

[0024] The positive electrode active material 1 includes an electrode active material capable of reversibly progressing occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions.

[0025] The positive electrode active material 1 is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobaltate (LiCoO ２ ), lithium nickelate (LiNiO ２ ), lithium manganate (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and a compound represented by the general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ (in the general formula, x+y+z+a=1, 0≤x<1, 0≤y<1, 0≤z<1, 0≤a<1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithium vanadium compound (LiV ２ O ５ ), olivine-type LiMPO ４ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), lithium titanate (Li ４ Ti ５ O １２ ), LiNi ｘ Co ｙ Al ｚ O ２ where 0.9 < x+y+z < 1.1. The positive electrode active material 1 may be an organic material. For example, the positive electrode active material 1 may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0026] The positive electrode active material 1 may be a lithium-free material. The lithium-free material is, for example, FeF ３These include conjugated polymers containing organic conductive materials, Schevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. Lithium-free materials may be used individually or in combination. If the positive electrode active material 1 is a lithium-free material, for example, a discharge is performed first. Lithium is inserted into the positive electrode active material 1 by the discharge. Alternatively, lithium may be pre-doped chemically or electrochemically into the lithium-free material of the positive electrode active material 1.

[0027] The content of the positive electrode active material 1 in the positive electrode active material layer 24 is not particularly limited. For example, the content of the positive electrode active material 1 in the total mass (100% by mass) of the positive electrode active material 1, conductive additive 4, and binder composition constituting the positive electrode active material layer 24 is preferably 90% by mass or more and 98% by mass or less.

[0028] The binder composition binds the positive electrode active materials 1 together, the positive electrode active materials 1 together with the positive electrode current collector 22 or carbon coating layer 23, the positive electrode active materials 1 together with the conductive additive 4, and the conductive additive 4 together with the positive electrode current collector 22 or carbon coating layer 23. The binder composition is a mixture of the first synthetic resin 3 and cellulose nanofiber 2.

[0029] In the positive electrode active material layer 24, the cellulose nanofibers 2 contained in the binder composition are mixed so as to be between the surface of the positive electrode active material 1 and the first synthetic resin 3, thereby suppressing excessive coating of the surface of the positive electrode active material 1 by the first synthetic resin 3.

[0030] The content of the binder composition in the total mass (100% mass) of the positive electrode active material 1, conductive additive 4, and binder composition constituting the positive electrode active material layer 24 is 1.0% by mass or more and 4.0% by mass or less, preferably 1.5% by mass or more and 2.5% by mass or less. The content of the binder composition is the sum of the content of the first synthetic resin 3 and the content of the cellulose nanofiber 2. If the content of the binder composition is 4.0% by mass or less, it is possible to form the film of the positive electrode active material layer 24 while maintaining the capacity of the lithium-ion secondary battery. On the other hand, if the content of the binder composition is less than 1.0% by mass, it is difficult to form the film of the positive electrode active material layer 24.

[0031] The first synthetic resin 3 may contain one or more synthetic resins selected from the group consisting of acrylic resin, styrene-butadiene rubber, carboxymethylcellulose, polyacrylonitrile, polyimide, polyamideimide, and polyethylene. Furthermore, from the viewpoint of having excellent oxidation resistance and suppressing oxidative decomposition of the synthetic resin itself even at high potentials, it is more preferable that the first synthetic resin 3 contains one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene.

[0032] The content of the first synthetic resin 3 in the total mass (100% by mass) of the positive electrode active material 1, conductive additive 4, and binder composition constituting the positive electrode active material layer 24 is 0.2% by mass or more and 1.0% by mass or less, preferably 0.3% by mass or more and 0.6% by mass or less.

[0033] By setting the content of the first synthetic resin 3 to a range of 0.2% by mass or more and 1.0% by mass or less, it is possible to suppress the first synthetic resin 3 from covering the positive electrode active material 1, thereby increasing the area on which lithium ions enter and exit the positive electrode active material 1 and improving the rate characteristics. If the content of the first synthetic resin 3 is less than 0.2% by mass, it becomes difficult to obtain good adhesion between the positive electrode active materials 1 themselves, between the positive electrode active material 1 and the positive electrode current collector 22 or carbon coating layer 23, between the positive electrode active material 1 and the conductive additive 4, and between the conductive additive 4 and the positive electrode current collector 22 or carbon coating layer 23. On the other hand, if the content of the first synthetic resin 3 exceeds 1.0% by mass, the surface of the positive electrode active material 1 is easily covered with the first synthetic resin 3, increasing the reaction resistance on the surface of the positive electrode active material 1 and reducing the rate characteristics.

[0034] The cellulose nanofibers 2 suppress excessive coating of the surface of the positive electrode active material 1 by the first synthetic resin 3, thereby reducing the reaction resistance on the surface of the positive electrode active material 1 and improving the rate characteristics. Examples of cellulose nanofibers 2 include plant-derived cellulose fibers, bacterial-derived cellulose fibers, and chemically synthesized cellulose fibers. These cellulose nanofibers 2 may be used alone or in combination of two or more types.

[0035] The cellulose nanofibers 2 preferably have an average diameter of 5 nm or more and 50 nm or less, and more preferably have an average diameter of 10 nm or more and 30 nm or less. When the average diameter of the cellulose nanofibers 2 is within this range, the cellulose nanofibers 2 are uniformly dispersed in the positive electrode active material layer 24, thereby suppressing excessive coating of the positive electrode active material 1 surface by the first synthetic resin 3 throughout the positive electrode active material layer 24. Consequently, the charge-discharge reaction of the positive electrode active material 1 in the positive electrode 20 is made uniform, and the resistance during the charge-discharge reaction is reduced, thus improving the rate characteristics.

[0036] The average diameter of cellulose nanofiber 2 was determined by observing the cross-section of cellulose nanofiber 2 using a scanning electron microscope (SEM), observing 50 points, and calculating the average value.

[0037] The content of cellulose nanofibers 2 in the total mass (100% by mass) of the positive electrode active material 1, conductive additive 4, and binder composition constituting the positive electrode active material layer 24 is 1.0% by mass or more and 3.5% by mass or less, preferably 1.2% by mass or more and 2% by mass or less. When the content of cellulose nanofibers 2 satisfies this range, the rate characteristics are improved.

[0038] In particular, when the content of the first synthetic resin 3 and the content of the cellulose nanofiber 2 satisfy the ranges of this embodiment 1, the network structure of the first synthetic resin 3 and the cellulose nanofiber 2 is optimized, and excellent rate characteristics can be obtained.

[0039] The conductive additive 4 enhances the electronic conductivity between the positive electrode active material 1. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include copper, nickel, stainless steel, and iron powders.

[0040] The content of the conductive additive in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive additive in the total mass (100% by mass) of the positive electrode active material 1, conductive additive 4, and binder composition constituting the positive electrode active material layer 24 is 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 5% by mass or less.

[0041] [Carbon Coating Layer] In the positive electrode 20, a carbon coating layer 23 may be placed between the positive electrode current collector 22 and the positive electrode active material layer 24. The carbon coating layer 23 is placed to improve the adhesion between the positive electrode current collector 22 and the positive electrode active material layer 24. The carbon coating layer 23 comprises a conductive carbon material and a second synthetic resin.

[0042] The conductive carbon material used in the carbon coating layer 23 is a conductive carbon material, and one or more can be selected from the group consisting of graphene, multilayer graphene, carbon nanotubes, carbon black, graphite, easily graphitizable carbon materials (soft carbon), and difficult-to-graphitize carbon materials (hard carbon).

[0043] The content of conductive carbon material in the carbon coating layer 23 is not particularly limited. For example, the content of conductive carbon material in the total mass (100% by mass) of the conductive carbon material constituting the carbon coating layer 23 and the second synthetic resin is preferably 50% by mass or more and 90% by mass or less.

[0044] The second synthetic resin used in the carbon coating layer 23 may be the same as or different from the first synthetic resin 3 used in the positive electrode active material layer 24. The second synthetic resin may contain one or more synthetic resins selected from the group consisting of acrylic resin, styrene-butadiene rubber, carboxymethylcellulose, polyacrylonitrile, polyimide, polyamideimide, and polyethylene. Furthermore, from the viewpoint of having excellent oxidation resistance and suppressing oxidative decomposition of the synthetic resin itself even at high potentials, it is more preferable that the second synthetic resin contains one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene.

[0045] The content of the second synthetic resin in the carbon coating layer 23 is not particularly limited. For example, the content of the second synthetic resin in the total mass (100% by mass) of the conductive carbon material and the second synthetic resin constituting the carbon coating layer 23 is preferably 10% by mass or more and 50% by mass or less.

[0046] In this embodiment, when the carbon coating layer 23 is placed between the positive electrode current collector 22 and the positive electrode active material layer 24, the adhesion between the positive electrode active material layer 24 containing cellulose nanofibers 2 and the positive electrode current collector 22 is improved. Furthermore, if one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene are used as the second synthetic resin for the carbon coating layer 23, oxidative decomposition of the synthetic resin itself is suppressed even at high potentials, thereby improving the current collection efficiency in the charge-discharge reaction and obtaining excellent rate characteristics.

[0047] <Negative Electrode> The negative electrode 30 includes, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is in contact with at least one surface of the negative electrode current collector 32.

[0048] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 is, for example, a thin metal plate such as aluminum, copper, nickel, titanium, or stainless steel. The negative electrode current collector 32 preferably contains copper. The negative electrode current collector 32 may also be, for example, rolled copper foil or electrolytic copper foil. The average thickness of the negative electrode current collector 32 is, for example, 10 μm or more and 30 μm or less.

[0049] [Negative electrode active material layer] The negative electrode active material layer 34 contains negative electrode active material. The negative electrode active material layer 34 may also contain a binder, conductive additive, dispersion stabilizer, etc., as needed.

[0050] The negative electrode active material can be any compound capable of intercalating and releasing ions, and known negative electrode active materials used in lithium-ion secondary batteries can be used. Examples of negative electrode active materials include metallic lithium, lithium alloys, carbon materials, and materials that can be alloyed with lithium. Examples of carbon materials include graphite (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature calcined carbon, which are capable of intercalating and releasing ions. Examples of materials that can be alloyed with lithium include silicon, tin, zinc, lead, and antimony. Materials that can be alloyed with lithium may be these elemental metals, or alloys or oxides containing these elements. Furthermore, materials that can be alloyed with lithium may be composites in which at least a portion of their surface is coated with a conductive material (e.g., a carbon material).

[0051] The amount of negative electrode active material contained in the negative electrode active material layer 34 is not particularly limited, similar to the amount of positive electrode active material contained in the positive electrode active material layer 24. For example, the amount of negative electrode active material in the total mass (100% by mass) of the negative electrode active material, conductive additive, and binder constituting the negative electrode active material layer 34 is preferably 90% by mass or more and 98% by mass or less.

[0052] The binder can be the same as the binder composition used in the positive electrode active material layer 24 of the positive electrode 20. In addition to the binder composition used in the positive electrode active material layer 24, one or more of the following may be used as the binder: for example, polyvinyl alcohol (PVA), polyamide (PA), polybenzimidazole (PBI), polyethersulfone (PES), styrene-butadiene rubber, ethylene-propylene rubber, carboxymethylcellulose (CMC), etc.

[0053] The binder content in the negative electrode active material layer 34 is not particularly limited. For example, the binder content in the total mass (100% by mass) of the negative electrode active material, conductive additive, and binder is preferably 0.5% by mass or more and 20% by mass or less. If the binder content is low, the adhesive strength of the negative electrode 30 will be weakened. If the binder content is high, the binder is electrochemically inert and does not contribute to the discharge capacity, so the energy density of the lithium-ion secondary battery 100 will be low.

[0054] The conductive additive in the negative electrode active material layer 34 enhances the electronic conductivity between the negative electrode active materials. The conductive additive can be the same as the one used in the positive electrode active material layer 24 of the positive electrode 20.

[0055] The content of the conductive additive in the negative electrode active material layer 34 is not particularly limited. For example, the content of the conductive additive in the total mass (100% by mass) of the negative electrode active material, conductive additive, and binder is preferably 5% by mass or more and 20% by mass or less.

[0056] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30 and prevents a short circuit between them. The separator 10 spreads in plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0057] The separator 10 may have, for example, an electrically insulating porous structure. The separator 10 may be, for example, a single layer or laminate of a polyolefin film. The separator 10 may also be a stretched film of a mixture of polyethylene or polypropylene. The separator 10 may also be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may also be, for example, a solid electrolyte. The solid electrolyte may be, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 may also be an inorganic coated separator. An inorganic coated separator is obtained by coating the surface of the above film with a mixture of resin such as PVDF or CMC and inorganic substances such as alumina or silica. Inorganic coated separators have excellent heat resistance and suppress the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0058] (Non-aqueous electrolyte) The non-aqueous electrolyte is sealed inside the outer casing 50 and impregnated into the power generation element 40. If the separator 10 is a solid electrolyte, it does not need to contain the non-aqueous electrolyte. A known electrolyte can be used as the non-aqueous electrolyte. The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in the non-aqueous solvent.

[0059] The electrolyte is, for example, a lithium salt. The electrolyte is, for example, LiPF ６ LiClO ４ LiBF ４ LiCF ３ SO ３ LiCF ３ CF ２ SO ３ LiC (CF ３ SO ２ ) ３ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ CF ２ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN (CF ３ CF ２ CO) ２ , LiBOB, LiN(FSO ２ ) ２ These are examples. A single lithium salt may be used alone, or two or more may be used in combination. From the viewpoint of degree of ionization, the electrolyte is LiPF ６ It is preferable that it contains [the specified element]. The concentration of the electrolyte is, for example, 0.8 mol / L or more and 5.0 mol / L or less.

[0060] The non-aqueous solvent is not particularly limited as long as it is a solvent commonly used in lithium-ion secondary batteries. The solvent may include, for example, a cyclic carbonate compound, a linear carbonate compound, a cyclic ester compound, or a linear ester compound. The solvent may also contain a mixture of these in any proportion. Examples of cyclic carbonate compounds include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, vinylene carbonate, etc. Examples of linear carbonate compounds include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of cyclic ester compounds include γ-butyrolactone, etc. Examples of linear ester compounds include propyl propionate, ethyl propionate, ethyl acetate, etc.

[0061] (Outer casing) The outer casing 50 seals the power generation element 40 and the non-aqueous electrolyte inside. The outer casing 50 prevents the non-aqueous electrolyte from leaking out and prevents moisture and other substances from entering the lithium-ion secondary battery 100 from the outside.

[0062] The outer casing 50, as shown in Figure 3 for example, has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The outer casing 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0063] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used for the resin layer 54. The materials constituting the resin layer 54 may differ between the inside and outside. For example, a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), can be used as the outer material, while polyethylene (PE) or polypropylene (PP) can be used as the material for the inner polymer film.

[0064] (Terminals) Terminals 60 and 62 are connected to the negative electrode 30 and the positive electrode 20, respectively. Terminal 62, connected to the positive electrode 20, is the positive terminal, and terminal 60, connected to the negative electrode 30, is the negative terminal. Terminals 60 and 62 are responsible for electrical connections to the outside. Terminals 60 and 62 are made of conductive materials such as aluminum, nickel, and copper. The connection method may be welding or screw fastening. It is preferable to protect terminals 60 and 62 with insulating tape to prevent short circuits.

[0065] "Method for Manufacturing a Lithium-Ion Secondary Battery" A lithium-ion secondary battery 100 is manufactured by preparing a negative electrode 30, a positive electrode 20, a separator 10, an electrolyte, and an outer casing 50, and assembling them. An example of the manufacturing method for a lithium-ion secondary battery 100 is described below.

[0066] The positive electrode 20 is obtained by applying a paste-like positive electrode slurry (coating) to at least one surface of the positive electrode current collector 22 and drying it to form a positive electrode active material layer 24. A commercially available product can be used for the positive electrode current collector 22.

[0067] There are no particular restrictions on the method of applying the positive electrode slurry. For example, the slit die coating method and the doctor blade method can be used as methods for applying the positive electrode slurry.

[0068] When preparing the cathode slurry, first, a dispersion is obtained by dispersing cellulose nanofibers in a solvent. The solvent can be, for example, water or N-methyl-2-pyrrolidone. Next, the first synthetic resin is dissolved in the dispersion. The first synthetic resin may be dissolved in the solvent beforehand.

[0069] Next, a positive electrode slurry is prepared by mixing a positive electrode active material and a conductive additive with a dispersion containing cellulose nanofibers and a synthetic resin. The composition ratio of the positive electrode active material, cellulose nanofibers, the first synthetic resin, and the conductive additive in the positive electrode slurry is preferably 90% to 97% by mass: 1.0% to 3.5% by mass: 0.2% to 1.0% by mass: 0.5% to 2.0% by mass, in mass ratio. These mass ratios are adjusted so that the total mass ratio is 100% by mass. A metal container such as SUS is preferred when preparing the positive electrode slurry.

[0070] Next, the solvent is removed from the positive electrode slurry. For example, the positive electrode current collector 22 coated with the positive electrode slurry can be dried in an atmosphere of 80°C to 150°C. By this procedure, a positive electrode 20 is obtained in which a positive electrode active material layer 24 is formed on the positive electrode current collector 22.

[0071] The positive electrode on which the positive electrode active material layer 24 is formed may be pressed using a roll press or the like if necessary. The linear pressure of the roll press will vary depending on the material used, but it will be adjusted so that the density of the positive electrode active material layer 24 reaches a predetermined value. The relationship between the density of the positive electrode active material layer 24 and the linear pressure is determined by prior studies that take into account the relationship with the material ratio constituting the positive electrode active material layer 24.

[0072] In the positive electrode 20, if a carbon coating layer 23 is placed between the positive electrode current collector 22 and the positive electrode active material layer 24, it is preferable to apply the carbon slurry to at least one surface of the positive electrode current collector 22 before applying the paste-like positive electrode slurry to form the carbon coating layer 23.

[0073] There are no particular restrictions on the method of applying the carbon slurry. For example, the slit die coating method and the doctor blade method can be used as methods for applying the cathode slurry.

[0074] When preparing a carbon slurry, a conductive carbon material, a second synthetic resin, and a solvent are mixed to create the slurry. The solvent can be, for example, N-methyl-2-pyrrolidone or pure water. The preferred mass ratio of the conductive carbon material to the second synthetic resin in the carbon slurry is 50% to 90% by mass: 10% to 50% by mass. These mass ratios are adjusted so that the total mass ratio is 100% by mass. A metal container, such as stainless steel (SUS), is preferred for slurry preparation.

[0075] Next, the solvent is removed from the carbon slurry. For example, the positive electrode current collector 22 coated with the carbon slurry can be dried in an atmosphere of 80°C to 150°C. In this procedure, a carbon coating layer 23 is formed on the positive electrode current collector 22. Then, the paste-like positive electrode slurry is applied to at least one surface of the carbon coating layer 23 and dried to form a positive electrode active material layer 24, thereby obtaining the positive electrode 20.

[0076] Next, the negative electrode 30 is prepared. The negative electrode 30 can be prepared in the same way as the positive electrode 20. A paste-like negative electrode slurry is applied to at least one surface of the negative electrode current collector 32. The negative electrode slurry is made by mixing a negative electrode active material, a binder, a conductive additive, and a solvent and forming it into a paste. The negative electrode slurry may be obtained in the same way as the positive electrode slurry. The negative electrode 30 is obtained by applying the negative electrode slurry to the negative electrode current collector 32 and drying it.

[0077] Next, the positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to create a power generation element 40. If the power generation element 40 is a wound body, the positive electrode 20, negative electrode 30, and separator 10 are wound around one end of each as an axis.

[0078] Next, the power generation element 40 is sealed in the casing 50. The non-aqueous electrolyte is injected into the casing 50. By injecting the non-aqueous electrolyte and then applying reduced pressure, heating, etc., the non-aqueous electrolyte impregnates the power generation element 40. By sealing the casing 50 with heat, etc., a lithium-ion secondary battery 100 is obtained.

[0079] In the lithium-ion secondary battery 100 according to this embodiment, the positive electrode active material layer 24 in the positive electrode 20 comprises a positive electrode active material 1 and a fluorine-free binder composition, the binder composition being a mixture of a first synthetic resin 3 and cellulose nanofibers 2 in a predetermined ratio. Therefore, the positive electrode active material 1 is suppressed from being covered by the first synthetic resin 3, increasing the area on which lithium ions enter and exit the positive electrode active material 1, thereby improving the rate characteristics. Furthermore, when the first synthetic resin 3 used in this embodiment is one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene, these synthetic resins have excellent oxidation resistance, and oxidative decomposition of the synthetic resin itself is suppressed even at high potentials. Therefore, the utilization efficiency of lithium ions is improved, and the rate characteristics are enhanced. In addition, when cellulose nanofibers 2 having a predetermined average diameter are used as the cellulose nanofibers 2, the cellulose nanofibers 2 are uniformly dispersed in the positive electrode active material layer 24. Therefore, the first synthetic resin 3 can suppress excessive coating of the surface of the positive electrode active material 1 across the entire positive electrode active material layer 24, resulting in a more uniform charge-discharge reaction of the positive electrode active material 1 in the positive electrode 20 and reduced resistance during the charge-discharge reaction. As a result, the rate characteristics are improved. Furthermore, by arranging a carbon coating layer 23 between the positive electrode current collector 22 and the positive electrode active material layer 24 in the positive electrode 20, the adhesion between the positive electrode active material layer 24 containing cellulose nanofibers 2 and the positive electrode current collector 22 is improved. In addition, one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene, which have excellent oxidation resistance, are used as the second synthetic resin in the carbon coating layer 23. As a result, oxidative decomposition of the synthetic resin itself is suppressed even at high potentials, improving the current collection efficiency in the charge-discharge reaction and improving the rate characteristics.

[0080] Although an example of this embodiment has been described in detail above with reference to the drawings, the configurations and their combinations in this embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the technical requirements of this disclosure.

[0081] "Example 1" <Fabrication of the positive electrode> Lithium cobalt oxide was used as the positive electrode active material, carbon black was used as the conductive additive, cellulose nanofibers with an average diameter of 4 nm were used, and the synthetic resin (acrylic resin) shown in Table 1 was used as the first synthetic resin. N-methyl-2-pyrrolidone was used as the solvent.

[0082] Cellulose nanofibers were uniformly dispersed in a solvent, and then the first synthetic resin was added and dissolved. This prepared a dispersion containing cellulose nanofibers and the first synthetic resin. Next, a positive electrode active material and a conductive additive were added to the dispersion and mixed to prepare a positive electrode slurry. In the positive electrode slurry, in a total of 100% by mass of the mixture of positive electrode active material, conductive additive, cellulose nanofibers, and the first synthetic resin, the proportions were 95.3% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, 2.5% by mass of the cellulose nanofibers, and 0.2% by mass of the first synthetic resin.

[0083] A positive electrode current collector made of 15 μm thick aluminum has a coating of positive electrode active material at a rate of 12 mg / cm² on one surface. ２ The positive electrode slurry was applied in such a manner. After application, the solvent was removed by drying at 100°C, and the resulting coating was rolled to obtain a positive electrode having a positive electrode active material layer.

[0084] <Fabrication of the negative electrode> A negative electrode active material mixture was prepared by mixing the negative electrode active material, a conductive additive, and a binder. Artificial graphite was used as the negative electrode active material, carbon black as the conductive additive, and SBR / CMC (styrene-butadiene rubber / carboxymethylcellulose) as the binder. The mass ratio of the negative electrode active material, conductive additive, and binder was 90% by mass: 5.0% by mass: 5.0% by mass. This negative electrode active material mixture was dispersed in distilled water to prepare a negative electrode slurry. The negative electrode slurry was then applied to one surface of a 10 μm thick copper negative electrode current collector. After application, it was dried at 100°C to remove the solvent and prepare a negative electrode having a negative electrode active material layer.

[0085] Next, the electrolyte was prepared. EC / DEC (ethylene carbonate / diethyl carbonate: mixing ratio 30% / 70% by mass) was used as the solvent for the electrolyte.

[0086] (Fabrication of Lithium-ion Secondary Battery for Evaluation) The fabricated negative electrode and positive electrode were stacked with a separator (porous polyethylene sheet) in between, so that the positive electrode active material layer and the negative electrode active material layer faced each other, to obtain a laminate. A nickel negative electrode lead was attached to the negative electrode of the laminate. An aluminum positive electrode lead was attached to the positive electrode of the laminate. The positive electrode lead and negative electrode lead were welded together using an ultrasonic welding machine. This laminate was inserted into an aluminum laminate film enclosure and a closed section was formed by heat sealing all but one corner around the perimeter. Finally, the electrolyte was injected into the enclosure, and then the remaining corner was sealed by heat sealing under reduced pressure using a vacuum sealing machine to fabricate a lithium-ion secondary battery.

[0087] (Rate Characteristics) The rate characteristics of the lithium-ion secondary battery were determined. The rate characteristics were measured using a secondary battery charge / discharge test apparatus. The rate characteristics (%) were evaluated with a voltage range from 4.4V to 3.0V, and with 1C = 1000mAh per full cell design capacity. The rate characteristics were measured as follows: CCCV charging (constant current constant voltage charging, termination current value 0.05C) was performed with a current value of 0.2C, and then discharged with a current value of 5.0C, and the 5.0C discharge capacity at this time was measured. Next, CCCV charging (constant current constant voltage charging, termination current value 0.05C) was performed with a current value of 0.2C, and then discharged with a current value of 0.2C, and the 0.2C discharge capacity at this time was measured. The rate characteristics (5C capacity retention rate (%)) were calculated using the following formula (1). (5C capacity retention rate (%)) = (5.0C discharge capacity) / (0.2C discharge capacity) × 100 ... (1) The rate characteristic of the lithium-ion secondary battery of Example 1 was 65%.

[0088] (First synthetic resin, second synthetic resin) In each example and comparative example, the types of resins used as the first synthetic resin and the second synthetic resin are shown in Tables 1 to 12, respectively. The SBR / CMC shown in Table 6 is styrene-butadiene rubber / carboxymethyl cellulose.

[0089] (Cellulose Nanofibers) In each example and comparative example, cellulose nanofibers having the average diameters shown in Tables 1 to 12 were used. The average diameter of the cellulose nanofibers was calculated by observing the cross-section of the cellulose nanofibers using a scanning electron microscope (SEM), observing 50 points, and determining the average value.

[0090] "Examples 2 to 7" In Examples 2 to 7, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 1, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Example 2, when obtaining the positive electrode slurry, 94.3% by mass of positive electrode active material, 2.0% by mass of conductive additive, 3.5% by mass of cellulose nanofiber, and 0.2% by mass of the first synthetic resin were mixed (total 100% by mass). In Example 3, when obtaining the positive electrode slurry, 96.5% by mass of positive electrode active material, 2.0% by mass of conductive additive, 1.0% by mass of cellulose nanofiber, and 0.5% by mass of the first synthetic resin were mixed (total 100% by mass). In Example 4, when obtaining the positive electrode slurry, 95% by mass of positive electrode active material, 2.0% by mass of conductive additive, 2.5% by mass of cellulose nanofiber, and 0.5% by mass of the first synthetic resin were mixed (total 100% by mass). In Example 5, when obtaining the positive electrode slurry, 94% by mass of positive electrode active material, 2.0% by mass of conductive additive, 3.5% by mass of cellulose nanofiber, and 0.5% by mass of the first synthetic resin were mixed (total 100% by mass). In Example 6, when obtaining the positive electrode slurry, 96% by mass of positive electrode active material, 2.0% by mass of conductive additive, 1.0% by mass of cellulose nanofiber, and 1.0% by mass of the first synthetic resin were mixed (total 100% by mass). In Example 7, when obtaining the positive electrode slurry, 94.5% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, 2.5% by mass of cellulose nanofiber, and 1.0% by mass of the first synthetic resin were mixed (total 100% by mass). In Examples 2 to 7, the rate characteristics of the lithium-ion secondary battery were determined in the same manner as in Example 1. The results are shown in Table 2.

[0091] "Examples 8 to 14" In Example 8, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 1, except that the first synthetic resin was changed to polyacrylonitrile when manufacturing the positive electrode. In Example 9, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 2, except that the first synthetic resin was changed to polyacrylonitrile when manufacturing the positive electrode. In Example 10, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 3, except that the first synthetic resin was changed to polyacrylonitrile when manufacturing the positive electrode. In Example 11, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 4, except that the first synthetic resin was changed to polyacrylonitrile when manufacturing the positive electrode. In Example 12, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 5, except that the first synthetic resin was changed to polyacrylonitrile when manufacturing the positive electrode. In Example 13, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 6, except that the first synthetic resin was changed to polyacrylonitrile when fabricating the positive electrode. In Example 14, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 7, except that the first synthetic resin was changed to polyacrylonitrile when fabricating the positive electrode. In Examples 8 to 14, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 2.

[0092] "Examples 15 to 17" In Example 15, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 3, except that the first synthetic resin was changed to polyimide when manufacturing the positive electrode. In Example 16, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 4, except that the first synthetic resin was changed to polyimide when manufacturing the positive electrode. In Example 17, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 5, except that the first synthetic resin was changed to polyimide when manufacturing the positive electrode. In Examples 15 to 17, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 4.

[0093] "Examples 18 to 20" In Example 18, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 3, except that the first synthetic resin was changed to polyamide-imide when fabricating the positive electrode. In Example 19, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 4, except that the first synthetic resin was changed to polyamide-imide when fabricating the positive electrode. In Example 20, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 5, except that the first synthetic resin was changed to polyamide-imide when fabricating the positive electrode. In Examples 18 to 20, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 4.

[0094] "Examples 21 to 23" In Example 21, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 3, except that the first synthetic resin was changed to polyethylene when manufacturing the positive electrode. In Example 22, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 4, except that the first synthetic resin was changed to polyethylene when manufacturing the positive electrode. In Example 23, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 5, except that the first synthetic resin was changed to polyethylene when manufacturing the positive electrode. In Examples 21 to 23, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 4.

[0095] "Examples 24 to 26" In Example 24, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 10, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 5 nm. In Example 25, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 11, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 5 nm. In Example 26, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 12, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 5 nm. In Examples 24 to 26, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 6.

[0096] "Examples 27 to 29" In Example 27, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 10, except that the cellulose nanofibers used to fabricate the positive electrode were changed to cellulose nanofibers with an average diameter of 22 nm. In Example 28, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 11, except that the cellulose nanofibers used to fabricate the positive electrode were changed to cellulose nanofibers with an average diameter of 22 nm. In Example 29, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 12, except that the cellulose nanofibers used to fabricate the positive electrode were changed to cellulose nanofibers with an average diameter of 22 nm. In Examples 27 to 29, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 6.

[0097] Examples 30 to 32 In Example 30, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 10, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 35 nm. In Example 31, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 11, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 35 nm. In Example 32, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 12, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 35 nm. In Examples 30 to 32, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 6.

[0098] "Examples 33 to 35" In Example 33, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 10, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 50 nm. In Example 34, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 11, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 50 nm. In Example 35, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 12, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 50 nm. In Examples 33 to 35, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 6.

[0099] Examples 36 to 38 In Example 36, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 10, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 52 nm. In Example 37, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 11, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 52 nm. In Example 38, an evaluation lithium-ion secondary battery was fabricated in the same manner as in Example 12, except that the cellulose nanofibers used to fabricate the positive electrode were replaced with cellulose nanofibers having an average diameter of 52 nm. In Examples 36 to 38, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 6.

[0100] "Example 39" <Preparation of Positive Electrode> Carbon black was used as the conductive carbon material, and the synthetic resin shown in Table 6 (polyacrylonitrile) was used as the second synthetic resin. N-methyl-2-pyrrolidone was used as the solvent. A carbon slurry was prepared by mixing the conductive carbon material and the second synthetic resin in a ratio of 50 parts by mass:50 parts by mass. The conductive carbon material was applied to one surface of a positive electrode current collector made of aluminum with a thickness of 15 μm at a rate of 2.0 mg / cm². ２ A carbon slurry was applied in this manner. After application, the solvent was removed by drying at 100°C to obtain a carbon coating layer.

[0101] Next, lithium cobalt oxide was used as the positive electrode active material, carbon black as the conductive additive, cellulose nanofibers with an average diameter of 35 nm were used, and the synthetic resin shown in Table 5 (polyacrylonitrile) was used as the first synthetic resin. N-methyl-2-pyrrolidone was used as the solvent.

[0102] Cellulose nanofibers were uniformly dispersed in a solvent, and then the first synthetic resin was added and dissolved. This prepared a dispersion containing cellulose nanofibers and the first synthetic resin. Next, a positive electrode active material and a conductive additive were added to the dispersion and mixed to prepare a positive electrode slurry. In the positive electrode slurry, in a total of 100% by mass of the mixture of positive electrode active material, conductive additive, cellulose nanofibers, and the first synthetic resin, the composition was 95% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, 2.5% by mass of the cellulose nanofibers, and 0.5% by mass of the first synthetic resin.

[0103] The amount of positive electrode active material applied to one surface of the carbon coating layer is 12 mg / cm². ２ The positive electrode slurry was applied in such a manner. After application, the solvent was removed by drying at 100°C, and the resulting coating was rolled to obtain a positive electrode having a positive electrode active material layer.

[0104] <Fabrication of the negative electrode> A negative electrode having a negative electrode active material layer was fabricated in the same manner as in Example 1.

[0105] Next, the electrolyte was prepared in the same manner as in Example 1.

[0106] (Preparation of lithium-ion secondary battery for evaluation) A lithium-ion secondary battery was prepared in the same manner as in Example 1, using a positive electrode active material layer, a negative electrode active material layer, and an electrolyte.

[0107] (Rate Characteristics) The rate characteristics of the lithium-ion secondary battery of Example 39 were determined in the same manner as in Example 1, and the rate characteristics were 86%.

[0108] "Examples 40 to 44" In Example 40, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 39, except that the second synthetic resin was changed to polyimide when manufacturing the positive electrode. In Example 41, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 39, except that the second synthetic resin was changed to polyamide-imide when manufacturing the positive electrode. In Example 42, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 39, except that the second synthetic resin was changed to polyethylene when manufacturing the positive electrode. In Example 43, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 39, except that the second synthetic resin was changed to acrylic resin when manufacturing the positive electrode. In Example 44, an evaluation lithium-ion secondary battery was manufactured in the same manner as in Example 39, except that the second synthetic resin was changed to SBR / CMC when manufacturing the positive electrode. In Examples 40 to 44, the rate characteristics of the lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 6.

[0109] "Comparative Examples 1 to 5" In Comparative Examples 1 to 5, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 8, except that cellulose nanofibers were not used when preparing the positive electrode, and the mixing ratio of the positive electrode active material, conductive additive, and first synthetic resin was changed. In Comparative Example 1, when obtaining the positive electrode slurry, 97.9% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, and 0.1% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 2, when obtaining the positive electrode slurry, 97.8% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, and 0.2% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 3, when obtaining the positive electrode slurry, 97.5% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, and 0.5% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 4, when obtaining the positive electrode slurry, 97% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, and 1.0% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 5, when obtaining the positive electrode slurry, 96.8% by mass of the positive electrode active material, 2.0% by mass of the conductive additive, and 1.2% by mass of the first synthetic resin were mixed (total 100% by mass).

[0110] "Comparative Examples 6 to 8" In Comparative Examples 6 to 8, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 1, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 6, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 97.4% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.1% by mass (total 100% by mass). In Comparative Example 7, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 93.9% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 4.0% by mass, and first synthetic resin: 0.1% by mass (total 100% by mass). In Comparative Example 8, when obtaining the positive electrode slurry, the following were mixed (total 100% by mass): positive electrode active material: 92.9% by mass, conductive additive: 1.9% by mass, cellulose nanofiber: 4.0% by mass, and the first synthetic resin: 1.2% by mass.

[0111] In Comparative Examples 1 to 8, the rate characteristics of lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 8.

[0112] "Comparative Examples 9 to 13" In Comparative Examples 9 to 13, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 8, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 9, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 97.4% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.1% by mass (total 100% by mass). In Comparative Example 10, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 96.9% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 1.0% by mass, and first synthetic resin: 0.1% by mass (total 100% by mass). In Comparative Example 11, when obtaining the positive electrode slurry, 95.4% by mass of positive electrode active material, 2.0% by mass of conductive additive, 2.5% by mass of cellulose nanofiber, and 0.1% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 12, when obtaining the positive electrode slurry, 94.4% by mass of positive electrode active material, 2.0% by mass of conductive additive, 3.5% by mass of cellulose nanofiber, and 0.1% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 13, when obtaining the positive electrode slurry, 93.9% by mass of positive electrode active material, 2.0% by mass of conductive additive, 4.0% by mass of cellulose nanofiber, and 0.1% by mass of the first synthetic resin were mixed (total 100% by mass).

[0113] "Comparative Examples 14 and 16" In Comparative Examples 14 and 16, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 8, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 14, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 97.3% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.2% by mass (total 100% by mass). In Comparative Example 16, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 93.8% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 4.0% by mass, and first synthetic resin: 0.2% by mass (total 100% by mass).

[0114] "Comparative Examples 17 and 18" In Comparative Examples 17 and 18, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 8, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 17, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 97% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.5% by mass. In Comparative Example 18, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 93.6% by mass, conductive additive: 1.9% by mass, cellulose nanofiber: 4.0% by mass, and first synthetic resin: 0.5% by mass.

[0115] "Comparative Examples 19 to 21" In Comparative Examples 19 to 21, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 8, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 19, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 96.5% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 1.0% by mass (total 100% by mass). In Comparative Example 20, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 93.6% by mass, conductive additive: 1.9% by mass, cellulose nanofiber: 3.5% by mass, and first synthetic resin: 1.0% by mass (total 100% by mass). In Comparative Example 21, when obtaining the positive electrode slurry, the following were mixed (total 100% by mass): positive electrode active material: 93.1% by mass, conductive additive: 1.9% by mass, cellulose nanofiber: 4.0% by mass, and the first synthetic resin: 1.0% by mass.

[0116] "Comparative Examples 22 to 26" In Comparative Examples 22 to 26, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 8, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 22, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 96.3% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 1.2% by mass (total 100% by mass). In Comparative Example 23, when obtaining the positive electrode slurry, the following was mixed: positive electrode active material: 95.8% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 1.0% by mass, and first synthetic resin: 1.2% by mass (total 100% by mass). In Comparative Example 24, when obtaining the positive electrode slurry, 94.3% by mass of positive electrode active material, 2.0% by mass of conductive additive, 2.5% by mass of cellulose nanofiber, and 1.2% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 25, when obtaining the positive electrode slurry, 93.4% by mass of positive electrode active material, 1.9% by mass of conductive additive, 3.5% by mass of cellulose nanofiber, and 1.2% by mass of the first synthetic resin were mixed (total 100% by mass). In Comparative Example 26, when obtaining the positive electrode slurry, 92.9% by mass of positive electrode active material, 1.9% by mass of conductive additive, 4.0% by mass of cellulose nanofiber, and 1.2% by mass of the first synthetic resin were mixed (total 100% by mass).

[0117] In Comparative Examples 9 to 26, the rate characteristics of lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 10.

[0118] "Comparative Examples 27 and 28" In Comparative Examples 27 and 28, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 15, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 27, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 97% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.5% by mass. In Comparative Example 28, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 93.6% by mass, conductive additive: 1.9% by mass, cellulose nanofiber: 4.0% by mass, and first synthetic resin: 0.5% by mass.

[0119] "Comparative Examples 29 and 30" In Comparative Examples 29 and 30, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 18, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 29, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 97% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.5% by mass. In Comparative Example 30, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 93.6% by mass, conductive additive: 1.9% by mass, cellulose nanofiber: 4.0% by mass, and first synthetic resin: 0.5% by mass.

[0120] "Comparative Examples 31 and 32" In Comparative Examples 31 and 32, evaluation lithium-ion secondary batteries were prepared in the same manner as in Example 21, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 31, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 97% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.5% by mass. In Comparative Example 32, when obtaining the positive electrode slurry, the following was mixed (total 100% by mass): positive electrode active material: 93.6% by mass, conductive additive: 1.9% by mass, cellulose nanofiber: 4.0% by mass, and first synthetic resin: 0.5% by mass.

[0121] "Comparative Example 33" In Comparative Example 33, a lithium-ion secondary battery for evaluation was prepared in the same manner as in Example 24, except that the mixing ratio of the positive electrode active material, conductive additive, cellulose nanofiber, and first synthetic resin was changed when preparing the positive electrode. In Comparative Example 33, when obtaining the positive electrode slurry, the following were mixed (total 100% by mass): positive electrode active material: 97% by mass, conductive additive: 2.0% by mass, cellulose nanofiber: 0.5% by mass, and first synthetic resin: 0.5% by mass.

[0122] In Comparative Examples 27 to 33, the rate characteristics of lithium-ion secondary batteries were determined in the same manner as in Example 1. The results are shown in Table 12.

[0123] The conditions and measurement results for Examples 1 to 44 and Comparative Examples 1 to 33 are summarized in Tables 1 to 12.

[0124]

[0125]

[0126]

[0127]

[0128]

[0129]

[0130]

[0131]

[0132]

[0133]

[0134]

[0135]

[0136] Examples 1 to 44 all exhibited superior rate characteristics compared to Comparative Examples 1 to 33. In particular, Examples 8 to 44, which used one of the preferred first synthetic resins—polyacrylonitrile, polyimide, polyamideimide, or polyethylene—showed even better rate characteristics than Examples 1 to 7. This is thought to be because these first synthetic resins have excellent oxidation resistance, and oxidative decomposition of the synthetic resin itself is suppressed even at high potentials, thus improving the rate characteristics even at a high potential state of 4.2V.

[0137] Furthermore, in Examples 24 to 38, the rate characteristics were superior in Examples 24 to 35, where the average diameter of the cellulose nanofibers was within a more preferable range, compared to Examples 36 to 38. This is thought to be because the cellulose nanofibers were sufficiently dispersed, which suppressed excessive coating of the positive electrode active material surface by the first synthetic resin, thus improving the rate characteristics.

[0138] Furthermore, in Examples 39 to 44, where a carbon coating layer was provided between the positive electrode current collector and the positive electrode active material layer, the rate characteristics were even better than those of Examples 1 to 38. This is thought to be because the carbon coating layer improved the adhesion between the positive electrode current collector and the positive electrode active material layer, and because the second synthetic resin used in the carbon coating layer has excellent oxidation resistance, oxidative decomposition of the synthetic resin itself was suppressed even at high potentials, resulting in improved rate characteristics even at a high potential state of 4.2V.

[0139] In Comparative Examples 1 to 5, the positive electrode active material layer did not contain cellulose nanofibers, making it easy for the positive electrode active material to be covered by the first synthetic resin. This increased the reaction resistance on the surface of the positive electrode active material, which is thought to have resulted in a decrease in rate characteristics. In Comparative Examples 6 to 8, the content of either cellulose nanofibers or the first synthetic resin was inappropriate, which is thought to have resulted in a decrease in rate characteristics. In Comparative Examples 9 to 33, the content of either cellulose nanofibers, the first synthetic resin, or the binder composition was inappropriate, which is thought to have resulted in a decrease in rate characteristics.

[0140] The positive electrode of this embodiment is suitably applied to lithium-ion secondary batteries.

[0141] 1. Positive electrode active material 2. Cellulose nanofiber 3. First synthetic resin 4. Conductive additive 10. Separator 20. Positive electrode 22. Positive electrode current collector 23. Carbon coating layer 24. Positive electrode active material layer 30. Negative electrode 32. Negative electrode current collector 34. Negative electrode active material layer 40. Power generation element 50. Outer casing 52. Metal foil 54. Resin layer 60, 62. Terminals 100. Lithium-ion secondary battery

Claims

1. A positive electrode for a lithium-ion secondary battery, comprising a positive electrode current collector and a positive electrode active material layer, wherein the positive electrode active material layer comprises a positive electrode active material and a fluorine-free binder composition, the binder composition being a mixture of a first synthetic resin and cellulose nanofibers, the content of the binder composition being 1.0% by mass or more and 4.0% by mass or less, the content of the first synthetic resin being 0.2% by mass or more and 1.0% by mass or less, and the content of the cellulose nanofibers being 1.0% by mass or more and 3.5% by mass or less.

2. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the first synthetic resin comprises one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene.

3. The positive electrode for a lithium-ion secondary battery according to claim 1, wherein the average diameter of the cellulose nanofibers is 5 nm or more and 50 nm or less.

4. A positive electrode for a lithium-ion secondary battery according to claim 1, wherein a carbon coating layer is disposed between the positive electrode current collector and the positive electrode active material layer, the carbon coating layer comprises a conductive carbon material and a second synthetic resin, and the second synthetic resin comprises one or more synthetic resins selected from the group consisting of polyacrylonitrile, polyimide, polyamideimide, and polyethylene.

5. A lithium-ion secondary battery comprising a positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 4, a negative electrode, and a separator disposed between the positive electrode for a lithium-ion secondary battery and the negative electrode.

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