Positive electrode active material layer for lithium-ion secondary battery, positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

A positive electrode active material layer (24) for a lithium-ion secondary battery contains a positive electrode active material (1) that occludes and releases lithium ions, a conductive auxiliary agent (2), and a binder (3). The binder (3) contains a carboxymethyl cellulose salt and at least one polymer. The breaking strength y (MPa) of the binder (3) and the breaking elongation x (%) of the binder (3) satisfy 3x ≥ y ≥ x and x > 6.

Description

Positive electrode active material layer for lithium-ion secondary batteries, positive electrode for lithium-ion secondary batteries, and lithium-ion secondary batteries

[0001] This disclosure relates to a positive electrode active material layer for lithium-ion secondary batteries, a positive electrode for lithium-ion secondary batteries, and lithium-ion secondary batteries. This application claims priority under Japanese Patent Application No. 2025-055418, filed in Japan on March 28, 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 for hybrid vehicles.

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

[0004] For example, Patent Document 1 discloses a positive electrode active material layer containing an olivine-type phosphoric acid compound, a binder containing an acrylic acid ester polymer, a conductive additive, and a carboxymethylcellulose salt. In Patent Document 1, by configuring the positive electrode active material layer in this way, the volumetric efficiency is kept within a predetermined range, thereby suppressing the decrease in battery voltage.

[0005] Furthermore, Patent Document 2 discloses that a binder composition containing an acrylic polymer is used in a slurry for electrodes of a lithium-ion secondary battery, and that the dispersion stability of the electrode slurry is improved by controlling the content of monomers and oligomers in the acrylic polymer.

[0006] International Publication No. 2024 / 204452, Japanese Patent No. 6090213

[0007] Lithium-ion secondary batteries require excellent cycle characteristics. However, conventional binders that replace PVDF have presented challenges in improving cycle characteristics.

[0008] The present disclosure has been made in view of the above problems, and an object thereof is to provide a positive electrode active material layer for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery that are excellent in cycle characteristics.

[0009] In order to solve the above problems, the following means are provided.

[0010] (1) A positive electrode active material layer for a lithium ion secondary battery according to a first aspect includes a positive electrode active material that occludes and releases lithium ions, a conductive auxiliary agent, and a binder, wherein the binder contains a carboxymethyl cellulose salt and at least one polymer, the breaking strength y (MPa) of the binder and the breaking elongation x (%) of the binder satisfy 3x≧y≧x and x>6.

[0011] (2) In the positive electrode active material layer for a lithium ion secondary battery according to the above aspect (1), the polymer may contain a dried product of at least any one of an acrylic emulsion polymer and an acrylonitrile-based emulsion polymer.

[0012] (3) In the positive electrode active material layer for a lithium ion secondary battery according to the above aspect (1) or (2), the degree of etherification of the carboxymethyl cellulose salt may be 0.85 or more.

[0013] (4) In the positive electrode active material layer for a lithium ion secondary battery according to any one of the above aspects (1) to (3), the positive electrode active material is LiCoO ２ , LiNiO ２ , LiMnO ２ , LiMn ２ O ４ , LiFePO ４ , a compound represented by the general formula: Li(Ni ｘ Co ｙ Mn ｚ )O ２ , and a compound selected from the group consisting of a compound represented by the general formula: Li(Ni ｘ Co ｙ Al ｗ )O ２ and containing one or more composite metal oxides, wherein the general formula: Li(Ni ｘ Coｙ Mn ｚ ) O ２ In this case, x + y + z = 1, and the general formula is Li(Ni ｘ Co ｙ Al ｗ ) O ２ In this case, x + y + w = ​​1 is also acceptable.

[0014] (5) In a positive electrode active material layer for a lithium-ion secondary battery according to any of the above embodiments (1) to (4), the positive electrode active material is Li(Ni １／３ Co １／３ Mn １／３ ) O ２ , Li(Ni ０．５ Co ０．２ Mn ０．３ ) O ２ , and Li(Ni ０．８ Co ０．１ Mn ０．１ ) O ２ It may contain one or more composite metal oxides selected from the group consisting of the following.

[0015] (6) In a positive electrode active material layer for a lithium-ion secondary battery according to any of the above embodiments (1) to (5), the positive electrode active material is Li(Ni １／３ Co １／３ Al １／３ ) O ２ , Li(Ni ０．５ Co ０．２ Al ０．３ ) O ２ , and Li(Ni ０．８ Co ０．１ Al ０．１ ) O ２ It may contain one or more composite metal oxides selected from the group consisting of the following.

[0016] (7) In a positive electrode active material layer for a lithium-ion secondary battery according to any of the above embodiments (1) to (6), the positive electrode active material may include positive electrode active material particles and a layer provided on the surface of the positive electrode active material particles and containing organic groups.

[0017] (8) The positive electrode for a lithium-ion secondary battery according to the second embodiment comprises a positive electrode active material layer for a lithium-ion secondary battery according to any of embodiments (1) to (7) above, and a positive electrode current collector.

[0018] (9) A lithium-ion secondary battery according to the third embodiment comprises a positive electrode for a lithium-ion secondary battery according to embodiment (8), a negative electrode, and a separator disposed between the positive electrode for a lithium-ion secondary battery and the negative electrode.

[0019] The lithium-ion secondary battery using the positive electrode active material layer for a lithium-ion secondary battery according to the above embodiment and the positive electrode for a lithium-ion secondary battery according to the above embodiment exhibits excellent cycle characteristics.

[0020] This is a schematic cross-sectional view of the positive electrode active material layer for a lithium-ion secondary battery according to the first embodiment. This is a schematic view of the lithium-ion secondary battery according to the first embodiment.

[0021] 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.

[0022] "Lithium-ion secondary battery" Figure 2 is a schematic diagram of a lithium-ion secondary battery according to the first embodiment. The lithium-ion secondary battery 100 shown in Figure 2 comprises a power generation element 40, an outer casing 50, and a non-aqueous electrolyte (not shown). The outer casing 50 covers the periphery of the power generation element 40. The power generation element 40 is connected to the outside by a pair of terminals 60, 62 connected to the power generation element 40. The non-aqueous electrolyte is contained within the outer casing 50. In Figure 2, a case in which there is one power generation element 40 inside the outer casing 50 is illustrated, but multiple power generation elements 40 may be stacked. The lithium-ion secondary battery 100 may also be cylindrical, prismatic, laminated, button-shaped, etc.

[0023] (Power generation element) The power generation element 40 comprises a positive electrode 20, a negative electrode 30, and a separator 10 positioned between the positive electrode 20 and the negative electrode 30.

[0024] <Positive Electrode> 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.

[0025] [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 such as aluminum, copper, nickel, titanium, or stainless steel. 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.

[0026] [Positive electrode active material layer] The positive electrode active material layer 24, as shown in the schematic cross-sectional diagram of the positive electrode active material layer for lithium-ion secondary batteries in Figure 1, includes a positive electrode active material 1 that intercepts and deintercepts lithium ions, a conductive additive 2, and a binder 3.

[0027] The positive electrode active material 1 includes an electrode active material capable of reversibly carrying out intercalation (intercalation) of lithium ions, or doping and dedoping of lithium ions and counteranions.

[0028] The positive electrode active material 1 is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO2). ２ ), lithium nickelate (LiNiO ２ ), lithium manganese (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), lithium iron phosphate (LiFePO) ４ ), general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ Compounds of (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, Cr), lithium vanadium compounds (LiV ２ O ５ ), olivine-type LiMPO ４(However, M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li ４ Ti ５ O １２ ), LiNi ｘ Co ｙ Al ｗ O ２ (0.9 < x + y + w < 1.1). The positive electrode active material 1 may be an organic substance. For example, the positive electrode active material 1 may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0029] The positive electrode active material 1 may be a lithium-free material. Examples of lithium-free materials include FeF ３ These include conjugated polymers containing organic conductive materials, Schevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. The 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.

[0030] The positive electrode active material 1 is more preferably LiCoO ２ LiNiO ２ LiMnO ２ LiMn ２ O ４ LiFePO ４ , general formula: Li(Ni ｘ Co ｙ Mn ｚ ) O ２ Compounds of the same name, and general formula: Li(Ni ｘ Co ｙ Al ｗ ) O ２ It is preferable that it contains one or more composite metal oxides selected from the group consisting of the following compounds. In this case, the general formula is Li(Ni ｘ Co ｙ Mn ｚ ) O ２ In this case, it is preferable that x + y + z = 1, and the general formula is: Li(Niｘ Co ｙ Al ｗ )O ２ It is preferred that x+y+w=1. These composite metal oxides are composite oxides containing one or more selected from the group consisting of Ni, Co, and Mn, have properties of high redox potential and high energy density, and are preferable as materials for lithium ion secondary batteries for portable devices such as mobile phones and power tools.

[0031] General formula: Li(Ni ｘ Co ｙ Mn ｚ )O ２ As the compound, Li(Ni １／３ Co １／３ Mn １／３ )O ２ , Li(Ni ０．５ Co ０．２ Mn ０．３ )O ２ , and Li(Ni ０．８ Co ０．１ Mn ０．１ )O ２ One or more selected from the group consisting of can be selected. General formula: Li(Ni ｘ Co ｙ Al ｗ )O ２ As the compound, Li(Ni １／３ Co １／３ Al １／３ )O ２ , Li(Ni ０．５ Co ０．２ Al ０．３ )O ２ , and Li(Ni ０．８ Co ０．１ Al ０．１ )O ２ One or more selected from the group consisting of can be selected.

[0032] The positive electrode active material 1 preferably comprises positive electrode active material particles and a layer provided on the surface of the positive electrode active material particles that contains organic groups. The positive electrode active material particles contain the electrode active material described above. The surface layer more preferably contains organopolysiloxane (silane condensate). For example, a mixture is obtained by mixing positive electrode active material particles, a silane coupling agent (e.g., trimethoxyphenylsilane), water, and methyl ethyl ketone, and this mixture is heated. As a result, the silane coupling agent hydrolyzes to produce silanol groups. The silanol groups are R-Si(OH) ３This is represented as , where R is an organic group in the silane coupling agent, for example, an alkyl group. The generated silanol group then forms a hydrogen bond with the hydroxyl groups on the surface of the positive electrode active material particles, followed by dehydration condensation. As a result, a layer containing organopolysiloxane (silane condensate) can be provided on the surface of the positive electrode active material particles. The organopolysiloxane is bonded to the surface of the positive electrode active material particles via covalent bonds (Si-O-M, where M is the metal element of the positive electrode active material particles). If the amount of positive electrode active material 1 is 100% by mass, the content of the surface layer is preferably 0.01% by mass or more and 0.5% by mass or less, and more preferably 0.02% by mass or more and 0.1% by mass or less. The inclusion of a surface layer containing organic groups in the positive electrode active material 1 improves the cycle characteristics. Typically, the positive electrode active material 1 is a metal oxide, and many hydroxyl groups are present on its surface. These hydroxyl groups readily form hydrogen bonds with the hydroxyl groups of carboxymethylcellulose salt, making it easy for carboxymethylcellulose salt to adhere to the surface of positive electrode active material 1. Because carboxymethylcellulose salt is hard, if positive electrode active materials 1 are bonded together by carboxymethylcellulose salt, it can become a starting point for cracks and fractures, potentially degrading the cycle characteristics. Therefore, coating the surface of positive electrode active material 1 with organic groups such as methyl groups can suppress the bonding between carboxymethylcellulose salt and positive electrode active material 1. This improves the cycle characteristics. Methods for analyzing the surface layer include time-of-flight secondary ion mass spectrometry (TOF-SIMS), transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS). If the surface layer contains organopolysiloxane (silane condensate), in TOF-SIMS, Si ＋ , SiO ＋ Fragment ions from liberated organic groups are observed. TEM-EDS can detect the amount of Si on the surface. If organic groups containing hydrocarbons are present on the surface, XPS can observe bond energies such as C-C / C=C / C-H. If the surface layer contains organopolysiloxanes (silane condensates), Si is present, and ICP-MS can measure the amount of organic groups and silanol groups in the surface layer from the amount of Si contained in the sample.

[0033] 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 2, and binder 3 constituting the positive electrode active material layer 24 is preferably 90% by mass or more and 99% by mass or less, and more preferably 96% by mass or more and 98% by mass or less.

[0034] The conductive additive 2 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.

[0035] The content of the conductive additive 2 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 2, and binder 3 constituting the positive electrode active material layer 24 is preferably 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 5% by mass or less.

[0036] The binder 3 bonds the positive electrode active materials 1 together, and the interface between the positive electrode active materials 1 and the positive electrode current collector 22. The binder 3 is a mixture of a carboxymethylcellulose salt and at least one polymer.

[0037] The positive electrode active material 1 contained in the positive electrode active material layer 24 undergoes significant volume expansion and contraction during charging and discharging. This volume expansion and contraction of the positive electrode active material 1 may cause cracks to form in the positive electrode active material layer 24 or delamination to occur at the interface between the positive electrode active material layer 24 and the positive electrode current collector 22, raising concerns that the cycle characteristics of the lithium-ion secondary battery may deteriorate.

[0038] In this embodiment, the binder 3 is composed of a mixture of a carboxymethylcellulose salt and at least one polymer, and the binder 3 satisfies the relationship between its breaking strength y (MPa) and breaking elongation x (%) being 3x ≥ y ≥ x and x > 6. With a binder 3 whose breaking strength y (MPa) and breaking elongation x (%) satisfy this relationship, the binder 3 follows the volume expansion and volume contraction of the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, improving the bonding between the positive electrode active materials 1 and at the interface between the positive electrode active material layer 24 and the positive electrode current collector 22, thereby obtaining a lithium-ion secondary battery 100 with excellent cycle characteristics.

[0039] The elongation at break x (%) is preferably x > 6, in the range of 100 ≥ x > 6, and more preferably in the range of 30 ≥ x ≥ 7. When the elongation at break x (%) satisfies x > 6, the binder 3 follows the volume expansion and contraction of the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, improving the cycle characteristics of the lithium-ion secondary battery 100. On the other hand, if the elongation at break x (%) is 6% or less, the elasticity of the binder 3 is low, making it difficult for the binder 3 to follow the volume expansion of the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, and strain is likely to occur in the positive electrode active material layer 24.

[0040] The breaking strength y (MPa) is in the range of 3x ≥ y ≥ x, preferably in the range of 200 ≥ y ≥ 18, and more preferably in the range of 70 ≥ y ≥ 20. When the breaking strength y (MPa) satisfies the range of 3x ≥ y ≥ x, the binder 3 follows the volume expansion and contraction of the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, improving the cycle characteristics of the lithium-ion secondary battery 100. On the other hand, if the breaking strength y (MPa) is less than x (MPa), the binder has difficulty following the volume contraction of the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, causing the binder 3 to break and reducing the bonding between the positive electrode active material 1 and at the interface between the positive electrode active material layer 24 and the positive electrode current collector 22, resulting in reduced conductivity. Furthermore, if the breaking strength y (MPa) exceeds 3x (MPa), the binder has difficulty following the volume contraction of the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, making it easy for strain to occur in the positive electrode active material layer 24.

[0041] The elongation at break x (%) and the breaking strength y (MPa) of the binder 3 are measured by the following method. A substrate made of polyethylene terephthalate resin is coated with a mixture of carboxymethylcellulose salt and at least one polymer using a film applicator, and dried at 100°C to obtain a binder film with a thickness of 100 μm. Next, the binder film is peeled from the substrate and 10 strip-shaped measurement samples with a width of 10 mm and a length of 40 mm are cut out. Using a tensile testing machine (manufactured by Imada Co., Ltd., FSA-1KE-50N), the measurement samples are pulled at a speed of 100 mm / min at room temperature (25°C to 30°C), and the average value of the strength at which the measurement sample breaks (the value obtained by dividing the tensile load value by the cross-sectional area of ​​the measurement sample) is calculated and taken as the breaking strength y (MPa). The elongation at break x (%) is calculated by the following formula (1). Elongation at break x (%) = 100 × (L - Lo) / Lo (1) In equation (1), Lo is the length of the sample before the test, and L is the length of the sample at the time of break.

[0042] The type of carboxymethylcellulose salt used in binder 3 is not particularly limited, but examples include alkali metal salts of carboxymethylcellulose and alkaline earth metal salts of carboxymethylcellulose. Specific examples include lithium carboxymethylcellulose, sodium carboxymethylcellulose, potassium carboxymethylcellulose, magnesium carboxymethylcellulose, and calcium carboxymethylcellulose, and one or more of these carboxymethylcellulose salts can be used. In particular, sodium carboxymethylcellulose is preferred because it has a high effect in improving cycle characteristics.

[0043] The degree of etherification of the carboxymethylcellulose salt is preferably 0.85 or higher. Furthermore, the degree of etherification of the carboxymethylcellulose salt is preferably 0.95 or lower. A degree of etherification of 0.85 or higher further improves the cycle characteristics. In the carboxymethylcellulose salt, there are three hydroxyl groups per glucose unit. The degree of etherification is determined by the carboxymethyl group (-CH) among these three hydroxyl groups. ２This indicates the average number of hydroxyl groups substituted with COOM (where M is a metal ion). As mentioned above, the degree of etherification indicates the substitution ratio of hydroxyl groups in the carboxymethylcellulose salt. When the degree of etherification (substitution ratio) is small and there are many hydroxyl groups, the carboxymethylcellulose salts tend to connect with each other through hydrogen bonds, resulting in a rigid structure. In this case, the electrode becomes hard and brittle, which may worsen the cycle characteristics. On the other hand, when the degree of etherification is 0.85 or higher, the degree of etherification (substitution ratio) is large, and there are few hydroxyl groups, the carboxymethylcellulose salts do not easily form hydrogen bonds with each other and repel each other to some extent, resulting in a softer structure and improved cycle characteristics. The degree of etherification of carboxymethylcellulose salt is measured by acid-base titration (neutralization titration). For example, it is measured by the following procedure: Dry the carboxymethylcellulose salt sample and weigh it. Then dissolve the sample in pure water. Add hydrochloric acid of a known concentration to this aqueous solution. This sufficiently converts the carboxymethyl groups into the H-type (acid type, -CH₂) ２ Convert to COOH. Then, titrate with a sodium hydroxide aqueous solution of known concentration. As a result, the equivalent amount of carboxyl groups (meq / g, milliequivalent of carboxyl groups per gram of sample) contained in the sample can be determined from the amount of sodium hydroxide aqueous solution required for titration. Carboxymethyl group (-CH ２ Using the molecular weight of COO (58 g / mol) and the molecular weight of glucose monomer (cellulose alone) (162 g / mol), the degree of etherification (substitution ratio, DS) is calculated using the following formula: Degree of etherification (DS) = (162 × carboxyl equivalent) / {1000 - (58 × carboxyl equivalent)}

[0044] The carboxymethylcellulose salt content in the positive electrode active material layer 24 is preferably 0.5% by mass or more and 4% by mass or less, and more preferably 0.5% by mass or more and 0.7% by mass or less, based on the total mass (100% by mass) of the positive electrode active material 1, conductive additive 2, and binder 3 constituting the positive electrode active material layer 24.

[0045] The polymer used in binder 3 preferably contains at least one dried product of an acrylic emulsion polymer and an acrylonitrile emulsion polymer, from the viewpoint of having excellent oxidation resistance and suppressing oxidative decomposition of the synthetic resin itself even at high potentials. Examples of acrylic emulsion polymers include polystyrene-block-polyacrylic, styrene-acrylic copolymer, alkyl acrylate copolymer, styrene acrylate copolymer, acrylamide acrylate copolymer, urethane acrylate copolymer, or silicone acrylate copolymer. Examples of acrylonitrile emulsion polymers include polystyrene-block-polyacrylonitrile, styrene acrylonitrile copolymer, acrylonitrile acrylate copolymer, acrylonitrile butadiene rubber, or styrene acrylonitrile butadiene rubber. Among these polymers, it is particularly preferable to use one or more selected from polystyrene-block-polyacrylic and polystyrene-block-polyacrylonitrile, as they have a high effect in improving cycle characteristics.

[0046] The acrylic emulsion polymer and acrylonitrile emulsion polymer used in the binder 3 preferably have a glass transition temperature (Tg) of -60°C or higher and 60°C or lower, and more preferably -50°C or higher and 20°C or lower. When the binder 3 contains a dried acrylic emulsion polymer or acrylonitrile emulsion polymer with a glass transition temperature in this range, the binder 3 follows the volume expansion and contraction of the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, making it difficult for strain to occur in the positive electrode active material layer 24.

[0047] The polymer content in the positive electrode active material layer 24 is preferably 0.5% by mass or more and 5% by mass or less, more preferably 1% by mass or more and 3% by mass or less, and more preferably more than 1.2% by mass, based on the total mass (100% by mass) of the positive electrode active material 1, conductive additive 2, and binder 3 constituting the positive electrode active material layer 24.

[0048] The content of the binder 3 in the positive electrode active material layer 24 is not particularly limited. For example, the content of the binder 3 in the total mass (100% by mass) of the positive electrode active material 1, conductive additive 2, and binder 3 constituting the positive electrode active material layer 24 is 1% by mass or more and 6% by mass or less, preferably 1% by mass or more and 3.5% by mass or less. When the content of the binder 3 satisfies this range, the cycle characteristics are improved.

[0049] <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.

[0050] [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.

[0051] [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 binders, conductive additives, dispersion stabilizers, etc., as needed.

[0052] 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).

[0053] 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 contained in the negative electrode active material layer 34 is the same as the amount of positive electrode active material contained in the positive electrode active material layer 24.

[0054] The binder used can be the same as the binder used in the positive electrode active material layer 24 of the positive electrode 20. In addition to the binder used in the positive electrode active material layer 24, one or more binders selected from styrene-butadiene rubber, polyacrylic acid, polyurethane, polyimide, and polyamide may also be used.

[0055] 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.

[0056] 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.

[0057] 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 0.1% by mass or more and 3% by mass or less.

[0058] <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.

[0059] 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.

[0060] (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.

[0061] 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.

[0062] 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, and vinylene carbonate. Examples of linear carbonate compounds include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of cyclic ester compounds include γ-butyrolactone. Examples of linear ester compounds include propyl propionate, ethyl propionate, and ethyl acetate.

[0063] (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.

[0064] The outer casing 50, as shown in Figure 2 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).

[0065] 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.

[0066] (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.

[0067] "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.

[0068] 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.

[0069] 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.

[0070] When preparing the cathode slurry, first, a mixture of carboxymethylcellulose salt and polymer is prepared. As the polymer, an emulsion polymer, in which the polymer is dispersed in a solvent beforehand, may be used. Either an acrylic emulsion polymer or an acrylonitrile emulsion polymer can be selected as the emulsion polymer. The solvent can be, for example, water or N-methyl-2-pyrrolidone.

[0071] The mixed solution satisfies the following conditions: the elongation at break x (%) and the breaking strength y (MPa) of the binder 3 obtained using the mixed solution are 3x ≥ y ≥ x, and x > 6.

[0072] Next, a positive electrode slurry is prepared by adding the positive electrode active material and a conductive additive to a mixture containing carboxymethylcellulose salt and polymer, and mixing it using a blender, hybrid mixer, or the like. The composition ratio of the positive electrode active material, carboxymethylcellulose salt, polymer, and conductive additive in the positive electrode slurry is preferably 96% to 98% by mass: 0.5% to 0.7% by mass: more than 1.2% by mass: 1% to 5% by mass, in terms of 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] Next, the power generation element 40 is sealed in the casing 50. The non-aqueous electrolyte is injected into the casing 50. After injecting the non-aqueous electrolyte, the non-aqueous electrolyte is impregnated into the power generation element 40 by applying reduced pressure, heating, etc. By sealing the casing 50 with heat, etc., a lithium-ion secondary battery 100 is obtained.

[0078] In the lithium-ion secondary battery 100 according to this embodiment, the binder 3 in the positive electrode active material layer 24 of the positive electrode 20 is composed of a mixture of carboxymethylcellulose salt and at least one polymer. In this binder 3, the breaking strength y (MPa) and the breaking elongation x (%) satisfy 3x ≥ y ≥ x and x > 6, resulting in excellent cycle characteristics. This is thought to be because the binder 3 used in the positive electrode active material layer 24 has excellent strength and elasticity. Specifically, even if volume expansion occurs in the positive electrode active material 1 during charging and discharging of the lithium-ion secondary battery 100, strain is less likely to occur at the interface between the positive electrode active materials and at the interface between the positive electrode active material layer 24 and the positive electrode current collector 22 because the binder 3 has excellent elasticity. Furthermore, even if the volume of the positive electrode active material 1 shrinks during charging and discharging of the lithium-ion secondary battery 100, the interface between the positive electrode active materials and the interface between the positive electrode active material layer 24 and the positive electrode current collector 22 is less likely to break because the binder 3 has excellent strength. In other words, the lithium-ion secondary battery 100 according to this embodiment has excellent cycle characteristics because the binder 3, which has excellent elasticity and can strongly bond the interfaces between the positive electrode active materials and the interface between the positive electrode active material layer 24 and the positive electrode current collector 22, is used in the positive electrode active material layer 24.

[0079] 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.

[0080] "Example 1" <Preparation of the positive electrode> Lithium cobalt oxide (W01T47LILBPw01, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the positive electrode active material, carbon black (Super-P, manufactured by Imerys Japan Co., Ltd.) was used as the conductive additive, sodium carboxymethylcellulose (Sunrose MAC200HC, manufactured by Nippon Paper Industries Co., Ltd.) was used as the carboxymethylcellulose salt, and polystyrene-block-polyacrylic (746991, manufactured by Sigma-Aldrich Japan LLC) was used as the polymer. Pure water was used as the solvent.

[0081] An acrylic emulsion polymer was prepared by first dispersing the polymer in pure water, which is the solvent. A carboxymethylcellulose salt was then added to create a mixture containing the carboxymethylcellulose salt and the polymer.

[0082] To measure the elongation at break x (%) and tensile strength y (MPa) of the binder, a portion of the mixture of carboxymethylcellulose salt and polymer was taken as a measurement sample. The mixture of carboxymethylcellulose salt and polymer was coated onto a substrate made of polyethylene terephthalate resin using a film applicator and dried at 100°C to obtain a binder film with a thickness of 100 μm. Next, the binder film was peeled from the substrate and 10 strip-shaped measurement samples, each 10 mm wide and 40 mm long, were cut out. Using a tensile testing machine (FSA-1KE-50N, manufactured by IMADA Co., Ltd.), the measurement samples were pulled at a speed of 100 m / min at room temperature (25°C), and the average value of the strength at which the measurement sample broke (the tensile load value divided by the cross-sectional area of ​​the measurement sample) was calculated and defined as the tensile strength y (MPa). The elongation at break x (%) was calculated using the following formula (1). Elongation at break x (%) = 100 × (L - Lo) / Lo (1) In equation (1), Lo is the length of the sample before testing, and L is the length of the sample at break. The elongation at break x (%) of the binder used in Example 1 was 9%, and the breaking strength y (MPa) was 22 MPa. The results are shown in Table 2.

[0083] Next, a positive electrode slurry was prepared by adding the positive electrode active material and a conductive additive to a mixture of carboxymethylcellulose salt and polymer and mixing them together. In the positive electrode slurry, the following proportions were used in the total 100% by mass of the mixture of positive electrode active material, conductive additive, carboxymethylcellulose salt, and polymer: positive electrode active material: 96.0% by mass, conductive additive: 2.00% by mass, carboxymethylcellulose salt: 0.7% by mass, and polymer: 1.3% by mass.

[0084] A positive electrode current collector made of 15 μm thick aluminum has a coating of positive electrode active material at a rate of 15 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.

[0085] <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. Graphite (CGB-20, manufactured by Nippon Graphite Industries Co., Ltd.) was used as the negative electrode active material, carbon black as the conductive additive, and a mixture of styrene-butadiene rubber (TRD104A, manufactured by ENEOS Material Co., Ltd.) and carboxymethylcellulose sodium (Sunrose MAC200HC, manufactured by Nippon Paper Industries Co., Ltd.) as the binder. The amounts of the negative electrode active material, conductive additive, and binder in the negative electrode active material mixture were set to a mass ratio of 96% by mass: 0.5% by mass: 3.5% by mass (total 100% 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.

[0086] Next, the electrolyte was prepared. As the solvent for the electrolyte, a mixture of 30 vol% ethyl carbonate and 70 vol% diethyl carbonate was used, in which 1 mol / L of lithium hexafluoride salt was dissolved.

[0087] (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. Finally, the electrolyte was injected into the enclosure, and then the remaining corner was sealed by heat sealing while depressurizing the inside of the enclosure using a vacuum sealing machine, thereby fabricating a lithium-ion secondary battery.

[0088] (Cycle Characteristics) (Measurement of Capacity Retention Rate After 100 Cycles) The cycle characteristics of the lithium-ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.).

[0089] The battery was charged at a constant current of 0.5C until the voltage reached 4.2V, and then discharged at a constant current of 1.0C until the voltage reached 2.5V. A charge rate of 0.5C is the current value at which charging is completed in 2 hours at 25°C. A discharge rate of 1.0C is the current value at which discharge is completed in 1 hour at 25°C. The discharge capacity after the end of charging and discharging was detected, and the battery capacity Q before the cycle test was determined. １ They sought it.

[0090] The above battery capacity Q １ The battery whose capacity was determined was then again charged using a secondary battery charge / discharge test device with a constant current charge rate of 0.5C until the battery voltage reached 4.2V, and then discharged with a constant current discharge rate of 1.0C until the battery voltage reached 2.5V. The above charge / discharge was counted as one cycle, and 100 charge / discharge cycles were performed. After that, the discharge capacity after 100 charge / discharge cycles was detected, and the battery capacity Q after 100 cycles was determined. ２ They sought it.

[0091] The capacity Q calculated above １ Q ２From this, the capacity retention rate E after 100 cycles was calculated. The capacity retention rate E is given by E = Q ２ / Q １ It can be calculated by multiplying by 100. The volume retention rate in Example 1 was 99.3%. The results are shown in Table 2.

[0092] (Carboxymethylcellulose salts) The carboxymethylcellulose salts shown in Tables 1 and 3 are described below. In Tables 1 and 3, carboxymethylcellulose salts are referred to as CMC salts. ・MAC200HC: Carboxymethylcellulose sodium (Sunrose MAC200HC, Nippon Paper Industries Co., Ltd.) ・MAC350HC: Carboxymethylcellulose sodium (Sunrose MAC350HC, Nippon Paper Industries Co., Ltd.) Even with the same model number, the degree of etherification of carboxymethylcellulose salts differed depending on the lot. The degree of etherification of the carboxymethylcellulose salts used in each example was measured using the method described in the embodiment. The measured degrees of etherification are shown in Tables 1 and 3.

[0093] (Polymers) The polymers shown in Tables 1 and 3 are described below. • Polystyrene-block-polyacrylic: (746991, manufactured by Sigma-Aldrich Japan LLC) • Polystyrene-block-polyacrylonitrile: (182850, manufactured by Sigma-Aldrich Japan LLC)

[0094] "Examples 2, 3, Comparative Example 1, and 2" Examples 2, 3, Comparative Example 1, and Comparative Example 2 differ from Example 1 in that the degree of etherification of the carboxymethylcellulose salt, the content of the carboxymethylcellulose salt, and the content of the polymer were changed as shown in Table 1. Other conditions in Examples 2, 3, Comparative Example 1, and Comparative Example 2 were basically the same as in Example 1, and the elongation at break x (%), breaking strength y (MPa), and volume retention rate after 100 cycles of the binder were measured. The results are shown in Tables 1 and 2.

[0095] "Examples 4-6" Examples 4-6 differ from Example 2 in that the type of carboxymethylcellulose salt and the degree of etherification of the carboxymethylcellulose salt were changed as shown in Table 1. Other conditions in Examples 4-6 were basically the same as in Example 2, and the elongation at break x (%), breaking strength y (MPa), and volume retention rate after 100 cycles of the binder were measured. The results are shown in Tables 1 and 2.

[0096] "Example 7" Example 7 differs from Example 1 in that polystyrene-block-polyacrylonitrile was used as the polymer instead of polystyrene-block-polyacrylic. Other conditions in Example 7 were basically the same as in Example 1, and the elongation at break x (%), breaking strength y (MPa), and volume retention rate after 100 cycles of the binder were measured. The results are shown in Tables 1 and 2. "Examples 8, 9, Comparative Example 3, Comparative Example 4" Examples 8, 9, Comparative Example 3, and Comparative Example 4 differ from Example 7 in that the degree of etherification of the carboxymethylcellulose salt, the content of the carboxymethylcellulose salt, and the content of the polymer were changed as shown in Table 1. Other conditions in Examples 8, 9, Comparative Example 3, and Comparative Example 4 were basically the same as in Example 7, and the elongation at break x (%), breaking strength y (MPa), and volume retention rate after 100 cycles of the binder were measured. The results are shown in Tables 1 and 2.

[0097] "Examples 10-12" Examples 10-12 differ from Example 8 in that the type of carboxymethylcellulose salt and the degree of etherification of the carboxymethylcellulose salt were changed as shown in Table 1. Other conditions in Examples 10-12 were basically the same as in Example 8, and the elongation at break x (%), breaking strength y (MPa), and volume retention rate after 100 cycles of the binder were measured. The results are shown in Tables 1 and 2.

[0098] Examples 13-18: 150 parts by mass of lithium cobalt oxide (W01T47LILBPw01, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the positive electrode active material, 0.5 parts by mass of a silane coupling agent (trimethoxyphenylsilane, manufactured by Shin-Etsu Chemical Co., Ltd.), 0.2 parts by mass of pure water, and 100 parts by mass of methyl ethyl ketone were mixed. The resulting mixture was stirred for 24 hours, and then heated at 100°C to volatilize the methyl ethyl ketone. This surface-treated the positive electrode active material, providing a layer containing organic groups (a layer containing organopolysiloxane (silane condensate)) on the surface of the positive electrode active material particles.

[0099] In Example 13, a lithium-ion secondary battery was fabricated in the same manner as in Example 1, except that the surface-treated positive electrode active material described above was used, and the capacity retention rate after 100 cycles was measured. The elongation at break x (%) and tensile strength y (MPa) of the binder in Example 13 were the same as the values ​​in Example 1. In Example 14, a lithium-ion secondary battery was fabricated in the same manner as in Example 2, except that the surface-treated positive electrode active material described above was used, and the capacity retention rate after 100 cycles was measured. The elongation at break x (%) and tensile strength y (MPa) of the binder in Example 14 were the same as the values ​​in Example 2. In Example 15, a lithium-ion secondary battery was fabricated in the same manner as in Example 3, except that the surface-treated positive electrode active material described above was used, and the capacity retention rate after 100 cycles was measured. The elongation at break x (%) and tensile strength y (MPa) of the binder in Example 15 were the same as the values ​​in Example 3. In Example 16, a lithium-ion secondary battery was fabricated in the same manner as in Example 4, except that the surface-treated positive electrode active material described above was used, and the capacity retention rate after 100 cycles was measured. The elongation at break x (%) and tensile strength y (MPa) of the binder in Example 16 were the same as the values ​​in Example 4. In Example 17, a lithium-ion secondary battery was fabricated in the same manner as in Example 5, except that the surface-treated positive electrode active material described above was used, and the capacity retention rate after 100 cycles was measured. The elongation at break x (%) and tensile strength y (MPa) of the binder in Example 17 were the same as the values ​​in Example 5. In Example 18, a lithium-ion secondary battery was fabricated in the same manner as in Example 6, except that the surface-treated positive electrode active material described above was used, and the capacity retention rate after 100 cycles was measured. The elongation at break x (%) and tensile strength y (MPa) of the binder in Example 18 were the same as the values ​​in Example 6.

[0100] The conditions and measurement results for Examples 1-12 and Comparative Examples 1-4 are summarized in Tables 1 and 2. The conditions and measurement results for Examples 13-18 are summarized in Tables 3 and 4.

[0101]

[0102]

[0103]

[0104]

[0105] In Examples 1 to 12, the cycle characteristics of the lithium-ion secondary battery were superior to those of Comparative Examples 1 to 4. In Examples 1 to 12, the tensile strength y (MPa) of the binder and the tensile elongation x (%) of the binder satisfied 3x ≥ y ≥ x and x > 6. Therefore, it is considered that the cycle characteristics of the lithium-ion secondary battery were superior. In addition, in Examples 1 to 12, the amount of carboxymethylcellulose salt in the positive electrode active material layer was 0.7% by mass or less, and the amount of polymer was more than 1.2% by mass. Therefore, it is presumed that the binder had sufficient elongation and strength, resulting in good cycle characteristics of the lithium-ion secondary battery. In particular, when the degree of etherification of the carboxymethylcellulose salt was 0.85 or higher (Examples 1 to 5, 7 to 11), the cycle characteristics of the lithium-ion secondary battery were even better. Furthermore, compared to cases where the positive electrode active material did not include a layer containing organic groups (Examples 1-6), the cycle characteristics of the lithium-ion secondary battery were even better when the positive electrode active material included positive electrode active material particles and a layer containing organic groups provided on the surface of the positive electrode active material particles (Examples 13-18).

[0106] In Comparative Examples 1 and 3, the binder used in the positive electrode active material layer did not satisfy either the fracture elongation x (%) or the fracture strength y (MPa) of 3x ≥ y ≥ x, nor x > 6. Therefore, it is thought that the binder had inferior strength and elasticity, resulting in reduced cycle characteristics. In Comparative Examples 2 and 4, the binder used in the positive electrode active material layer satisfied x > 6 for fracture elongation x (%), but did not satisfy the fracture strength y (MPa) of 3x ≥ y ≥ x. Therefore, it is thought that the binder had inferior strength, resulting in reduced cycle characteristics.

[0107] The positive electrode active material layer of this embodiment is suitably applied to lithium-ion secondary batteries.

[0108] 1 Positive electrode active material 2 Conductive additive 3 Binding agent 10 Separator 20 Positive electrode 22 Positive electrode current collector 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 active material layer for a lithium-ion secondary battery, comprising a positive electrode active material that intercepts and deintercepts lithium ions, a conductive additive, and a binder, wherein the binder comprises a carboxymethylcellulose salt and at least one polymer, and the tensile strength y (MPa) of the binder and the tensile elongation x (%) of the binder satisfy 3x ≥ y ≥ x and x > 6.

2. The positive electrode active material layer for a lithium-ion secondary battery according to claim 1, wherein the polymer comprises a dried product of at least one of an acrylic emulsion polymer and an acrylonitrile emulsion polymer.

3. The positive electrode active material layer for a lithium-ion secondary battery according to claim 1, wherein the degree of etherification of the carboxymethylcellulose salt is 0.85 or higher.

4. The positive electrode active material is LiCoO ２ , LiNiO ２ , LiMnO ２ , LiMn ２ O ４ , LiFePO ４ , a compound of the general formula: Li(Ni ｘ Co ｙ Mn ｚ )O ２ , and a compound of the general formula: Li(Ni ｘ Co ｙ Al ｗ )O ２ , comprising one or more composite metal oxides selected from the group consisting of the above compounds, wherein in said general formula Li(Ni ｘ Co ｙ Mn ｚ )O ２ , x + y + z = 1, and in said general formula Li(Ni ｘ Co ｙ Al ｗ )O ２ , x + y + w = 1. The positive electrode active material layer for a lithium ion secondary battery according to claim 1.

5. The positive electrode active material is Li(Ni １／３ Co １／３ Mn １／３ ) O ２ , Li(Ni ０．５ Co ０．２ Mn ０．３ ) O ２ , and Li(Ni ０．８ Co ０．１ Mn ０．１ ) O ２ A positive electrode active material layer for a lithium-ion secondary battery according to claim 1, comprising one or more composite metal oxides selected from the group consisting of the following.

6. The positive electrode active material is Li(Ni １／３ Co １／３ Al １／３ ) O ２ , Li(Ni ０．５ Co ０．２ Al ０．３ ) O ２ , and Li(Ni ０．８ Co ０．１ Al ０．１ ) O ２ A positive electrode active material layer for a lithium-ion secondary battery according to claim 1, comprising one or more composite metal oxides selected from the group consisting of the following.

7. The positive electrode active material layer for a lithium-ion secondary battery according to claim 1, wherein the positive electrode active material comprises positive electrode active material particles and a layer provided on the surface of the positive electrode active material particles and containing organic groups.

8. A positive electrode for a lithium-ion secondary battery comprising a positive electrode active material layer for a lithium-ion secondary battery according to any one of claims 1 to 7, and a positive electrode current collector.

9. A lithium-ion secondary battery comprising a positive electrode for a lithium-ion secondary battery as described in claim 8, 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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