Composition for lithium ion secondary battery, electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

In a composition according to the present disclosure, with respect to the total amount of an electrically conductive carbon material (A), an olefin-based resin (B), a binder resin (C), a synthetic rubber (D), and an additive (E), (A) is contained at 10-60 mass%, (B) at 30-70 mass%, (C) at 1-30 mass%, (D) at 3-20 mass%, and (E) at 0-50 mass%, and the ratio of the content of (A) to the content of (B) is 0.25-2.00.

Description

[Technical Field]

[0001] The present disclosure relates to a composition, an electrode, a lithium ion secondary battery, and a method for producing the composition. [Background technology]

[0002] In recent years, lithium-ion secondary batteries have been widely used as power sources for electronic devices, electric vehicles, and electrical storage. In particular, there has been a recent demand for batteries with high capacity, high output, and high energy density that can be installed in hybrid vehicles and the like. Such lithium-ion secondary batteries have the advantage of high energy density. However, the use of lithium metal and lithium ions requires adequate safety measures.

[0003] Patent Document 1 discloses a positive electrode for a lithium ion secondary battery that improves the safety of the lithium ion secondary battery. The positive electrode disclosed in Patent Document 1 includes a positive electrode current collector and a positive electrode composite layer. The positive electrode composite layer is laminated on at least one side of the positive electrode current collector. The positive electrode composite layer includes a positive electrode composite layer and an undercoat layer. The undercoat layer is formed between the positive electrode current collector and the positive electrode composite layer. The undercoat layer includes a conductive additive, a binder, and thermally expandable microcapsules. The thermally expandable microcapsules have a maximum volume expansion temperature of 70°C or higher and 180°C or lower.

[0004] Patent Document 1: International Publication No. 2019 / 189866 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a demand for a lithium-ion secondary battery that is not only excellent in safety but also excellent in battery performance, in which DC resistance at the initial stage of charge / discharge and an increase in DC resistance after long-term storage of the lithium-ion secondary battery in a high-temperature environment are suppressed. The output of a lithium-ion secondary battery in which DC resistance at the initial stage of charge / discharge is suppressed is superior to the output of a lithium-ion secondary battery in which DC resistance at the initial stage of charge / discharge is not suppressed. The life of a lithium-ion secondary battery in which an increase in DC resistance after long-term storage in a high-temperature environment is suppressed is superior to the life of a lithium-ion secondary battery in which an increase in DC resistance after long-term storage in a high-temperature environment is not suppressed.

[0006] In view of the above circumstances, an object of the present disclosure is to provide a composition, an electrode, a lithium ion secondary battery, and a method for producing the composition, which can provide a lithium ion secondary battery that is excellent in safety and in which an increase in DC resistance during initial charging and discharging and an increase in DC resistance after long-term storage in a high-temperature environment are suppressed. [Means for solving the problem]

[0007] The means for solving the above problems include the following embodiments.

[0008] <1> Relative to the total amount of the conductive carbon material (A), the olefin-based resin (B), the binder resin (C) which is a resin other than the olefin-based resin (B), the synthetic rubber (D), and the additive (E), 10% by mass to 60% by mass of the conductive carbon material (A), 30% by mass to 70% by mass of the olefin-based resin (B); 1% by mass to 30% by mass of the binder resin (C); 1% by mass to 20% by mass of the synthetic rubber (D); and 0% by mass to 50% by mass of the additive (E), A composition, wherein the ratio of the content of the conductive carbon material (A) to the content of the olefin resin (B) is 0.25 to 2.00. <2> The olefin resin (B) includes a polyethylene resin or a polypropylene resin. <1> The composition described in <3> The olefin-based resin (B) includes a water-dispersible olefin-based resin. <1> or <2> The composition described in <4> The particle size of the olefin-based resin (B) is 0.1 μm to 9.0 μm, and the softening point of the olefin-based resin (B) is 70° C. or higher. <1> ~ <3> The composition according to any one of the preceding claims. <5> the ratio of the content of the synthetic rubber (D) to the content of the olefin resin (B) is 0.07 to 0.25; <1> ~ <4> The composition according to any one of the preceding claims. <6> The binder resin (C) contains carboxymethyl cellulose or polyvinylidene fluoride. <1> ~ <5> The composition according to any one of the preceding claims. <7> The synthetic rubber (D) contains styrene-butadiene rubber. <1> ~ <6> The composition according to any one of the preceding claims. <8> The additive (E) contains at least one of a thermally expandable microcapsule having a maximum volume expansion temperature of 70°C or more and 180°C or less and an inorganic oxide filler. <1> ~ <7> The composition according to any one of the preceding claims. <9> The conductive carbon material (A), the olefin-based resin (B), the binder resin (C) which is a resin other than the olefin-based resin (B), and the synthetic rubber (D), A composition having a ratio (D99 / D10) of particle size distribution D99 to particle size distribution D10 measured by a laser light diffraction scattering method of 35 or less. <10> The particle size distribution D99 measured by a laser light diffraction scattering method is 20 μm or less. <9> The composition described in <11> the ratio (D99 / D50) of the particle size distribution D99 to the particle size distribution D50 measured by a laser light diffraction scattering method is 20 or less; <9> or <10> The composition described in <12> a current collector; <1> ~ <11> An electrode comprising an undercoat layer containing the composition according to any one of the above items and a composite layer. <13> The current collector, the undercoat layer, and the composite layer are laminated in this order. <12> The electrode according to claim 1. <14> The electrode is a positive electrode. <12> or <13> The electrode according to claim 1. <15> The aforementioned <12> ~ <14> A lithium ion secondary battery comprising the electrode according to any one of the above. <16> mixing a conductive carbon material (A), an olefin-based resin (B), a binder resin (C) that is a resin other than the olefin-based resin (B), and a synthetic rubber (D) to prepare a mixture; Dispersing the mixture using a dispersing roll; A method for producing a composition comprising: [Effects of the Invention]

[0009] The present disclosure provides a composition, an electrode, a lithium ion secondary battery, and a method for producing the composition, which can provide a lithium ion secondary battery that is excellent in safety and in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a laminated battery, which is an example of the lithium ion secondary battery of the present disclosure. [Figure 2] FIG. 2 is a cross-sectional view of a positive electrode in a lithium ion secondary battery according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is a cross-sectional view of a negative electrode in a lithium ion secondary battery according to an embodiment of the present disclosure. [Figure 4] FIG. 4 is a side view of a three-roll mill according to an embodiment used in the manufacturing method of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved. In this specification, (meth)acrylate means acrylate or methacrylate.

[0012] Hereinafter, embodiments of the composition, electrode, lithium ion secondary battery, and method for producing the composition of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference characters, and description thereof will not be repeated.

[0013] (1) First embodiment The composition of the first embodiment of the present disclosure contains, relative to the total amount of the conductive carbon material (A), the olefin-based resin (B), the binder resin (C) which is a resin other than the olefin-based resin (B), the synthetic rubber (D), and the additive (E) (hereinafter, may be simply referred to as the "total amount of (A) to (E)"). 10% by mass to 60% by mass of the conductive carbon material (A), 30% by mass to 70% by mass of the olefin-based resin (B); 1% by mass to 30% by mass of the binder resin (C); 1% by mass to 20% by mass of the synthetic rubber (D); and 0% by mass to 50% by mass of the additive (E), The ratio of the content of the conductive carbon material (A) to the content of the olefin resin (B) (hereinafter, sometimes referred to as "mass ratio (A / B)") is 0.25 to 2.00.

[0014] In the present disclosure, the term "conductive carbon material" refers to a carbon material having a volume resistivity at 20°C of less than 40 Ω·cm, preferably less than 3 Ω·cm. In the present disclosure, the term "olefin-based resin" refers to a resin containing structural units derived from an olefin. Specifically, the term "olefin-based resin" refers to a homopolymer of an olefin, a copolymer of two or more olefins, or a copolymer of an olefin and another monomer. In the present disclosure, the term "additives" refers to solid components other than the conductive carbon material (A), the olefin resin (B), the binder resin (C), and the synthetic rubber (D). In the present disclosure, the terms "content" and "addition amount" are considered to be substantially the same.

[0015] The composition of the first embodiment, having the above-described configuration, can provide a lithium ion secondary battery that is excellent in safety and in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed. In other words, the composition of the first embodiment can provide a lithium ion secondary battery that has a good balance between safety and battery performance (i.e., output and life).

[0016] (1.1) Use of the composition The composition of the first embodiment is not particularly limited and is suitably used for electrode components of lithium-ion secondary batteries, etc. Specifically, the composition of the first embodiment is preferably used to form an undercoat layer included in an electrode of a lithium-ion secondary battery including an electrode in which an undercoat layer and a composite layer are laminated in this order on at least one main surface of a current collector. In particular, the composition of the first embodiment is more preferably used to form an undercoat layer included in a positive electrode of a lithium-ion secondary battery including a positive electrode in which an undercoat layer and a composite layer are laminated in this order on at least one main surface of a current collector.

[0017] In this disclosure, the term "current collector" refers to a sheet-like material that collects electrons generated from an active material in a lithium-ion secondary battery and supplies the electrons to the active material. The term "main surfaces of the current collector" refers to a pair of opposing surfaces with the largest area among multiple pairs of opposing surfaces.

[0018] (1.2) Conductive carbon material (A) The composition of the first embodiment contains a conductive carbon material (A), which reduces the electrical resistance of the composition of the first embodiment.

[0019] Examples of the conductive carbon material (A) include graphite, carbon black, conductive carbon fiber, and fullerene. Examples of graphite include artificial graphite and natural graphite (e.g., flake graphite, lump graphite, and amorphous graphite). Examples of conductive carbon fibers include carbon nanotubes, carbon nanofibers, and carbon fibers. The conductive carbon material (A) may be used alone or in combination of two or more.

[0020] The shape of the conductive carbon material (A) is not particularly limited, and may be particulate. When the conductive carbon material (A) is particulate, the particle size of the conductive carbon material (A) is not particularly limited. From the viewpoint of dispersing the conductive carbon material (A) among the particles contained in the undercoat layer and functioning as a conductive assistant, the particle size of the conductive carbon material (A) is preferably 5 μm or less, more preferably 1 μm to 4 μm. In this case, the primary particle size of the particles of the conductive carbon material (A) is preferably 0.5 μm or less, more preferably 0.1 μm to 0.4 μm. The particle size of the conductive carbon material (A) is the particle size (particle size distribution D50, median diameter) corresponding to the cumulative 50% by volume from the fine particle side in the volume-based particle size distribution measured by a particle size distribution measuring device based on the laser light diffraction scattering method.

[0021] The conductive carbon material (A) may be a commercially available product. Examples of commercially available carbon black include "Super P" (manufactured by TIMCAL). Examples of commercially available flake graphite include "KS-6" (manufactured by TIMREX).

[0022] The content of the conductive carbon material (A) is 10% by mass to 60% by mass with respect to the total amount of (A) to (E). When the content of the conductive carbon material (A) is within the above range, the number of contact points between the conductive carbon materials (A) in the composition of the first embodiment is large, and the electrical resistance of the composition of the first embodiment at room temperature can be reduced by the percolation effect. Furthermore, when the temperature of the lithium ion secondary battery rises suddenly, contact between the conductive carbon materials (A) is difficult to maintain, and the electrical resistance of the undercoat layer increases, allowing the resulting lithium ion secondary battery to exhibit a shutdown function. In other words, the safety of the lithium ion secondary battery is excellent. The shutdown function includes preventing the progress of the battery reaction in the lithium ion secondary battery. The "battery reaction" refers to the insertion and desorption reactions of lithium ions that occur between the positive electrode and the negative electrode. The higher the content of the conductive carbon material (A), the better the battery performance of the lithium ion secondary battery tends to be. From the viewpoint of ensuring the shutdown function, the content of the conductive carbon material (A) is preferably 45 mass % or less, and more preferably 35 mass % or less, based on the total amount of (A) to (E). From the viewpoint of ensuring battery performance, the content of the conductive carbon material (A) is preferably 15% by mass or more, and more preferably 25% by mass or more, based on the total amount of (A) to (E).

[0023] The mass ratio (A / B) is 0.25 to 2.00. When the mass ratio (A / B) is within the above range, it is possible to achieve both the shutdown function and a reduced electrical resistance of the undercoat layer to ensure battery performance (hereinafter referred to as "low resistance of the undercoat layer"). From the viewpoint of ensuring the shutdown function, the mass ratio (A / B) is preferably 1.50 or less, more preferably 0.90 or less, even more preferably 0.75 or less, particularly preferably 0.60 or less, and even more preferably 0.55 or less. From the viewpoint of ensuring the battery performance, the mass ratio (A / B) is preferably 0.27 or more, more preferably 0.30 or more, even more preferably 0.35 or more, and particularly preferably 0.50 or more.

[0024] (1.3) Olefin resin (B) The composition of the first embodiment contains an olefin resin (B). As a result, when the composition of the first embodiment is used as a raw material for the undercoat layer, the lithium ion secondary battery is less likely to experience thermal runaway, and the safety of the lithium ion secondary battery is ensured.

[0025] The material of the olefin resin (B) is not particularly limited. The softening point of the olefin resin (B) is preferably 150°C or lower, more preferably 140°C or lower, and even more preferably 135°C or lower, from the viewpoint of more effectively exhibiting the shutdown function by melting the olefin resin (B) in a lower temperature range during a sudden temperature rise of the lithium ion secondary battery (hereinafter referred to as "effective exhibiting the shutdown function"). The softening point of the olefin resin (B) is preferably 70°C or higher, more preferably 90°C or higher, even more preferably 110°C or higher, and particularly preferably 120°C or higher, from the viewpoint of maintaining the shape of the olefin resin (B) before and after the drying treatment performed in the manufacturing process of the positive electrode (hereinafter referred to as "shape maintenance of the olefin resin (B) during the positive electrode drying step"). The softening point of the olefin resin (B) is preferably 70°C to 150°C. The softening point of the olefin resin (B) is a value measured according to JIS K2207 (ring and ball method).

[0026] Specific examples of the olefin resin (B) include polyethylene, polypropylene, ethylene-vinyl acetate copolymer (EVA), polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, polyvinylidene fluoride, polyamide, polystyrene, polyacrylonitrile, polyethylene oxide, and polymethyl (meth)acrylate. Among these, the olefin resin (B) preferably contains a polyethylene resin or a polypropylene resin, from the viewpoints of both effectively exhibiting the shutdown function and maintaining the shape of the olefin resin (B) during the positive electrode drying process. The olefin resin (B) may be used alone or in combination of two or more.

[0027] In the present disclosure, the term "polyethylene resin" refers to a resin containing ethylene as a main component. Examples of polyethylene resins include ethylene homopolymers and copolymers. Among these, the polyethylene resin is preferably a copolymer of ethylene and at least one α-olefin.

[0028] The olefin resin (B) preferably contains a water-dispersible olefin resin, which makes the lithium ion secondary battery less susceptible to thermal runaway when the composition of the first embodiment is used as a raw material for the undercoat layer, thereby improving the safety of the lithium ion secondary battery.

[0029] In the present disclosure, the term "water-dispersible olefin-based resin" refers to an olefin-based resin that can be dispersed in water without the addition of at least one of a surfactant and an organic solvent.

[0030] Examples of the water-dispersible olefin resin material include polyethylene, polyethylene elastomer, polyolefin ionomer, and EVA.

[0031] The content of the water-dispersible olefin resin is not particularly limited, and is preferably 20% by mass or more, more preferably 50% by mass or more, and may be 100% by mass, based on the total amount of the olefin resin (B).

[0032] The shape of the olefin resin (B) is not particularly limited, and may be particulate. When the olefin resin (B) is particulate, the particle size of the olefin resin (B) is not particularly limited. From the viewpoint of adjusting the particle size of the olefin resin (B) so that it falls within the optimum film thickness range of the undercoat layer, the particle size of the olefin resin (B) is preferably 9.0 μm or less, more preferably 4.0 μm or less, and even more preferably 2.0 μm or less. From the viewpoint of processability of the composition (e.g., slurry for the undercoat layer), the particle size of the olefin resin (B) is preferably 0.1 μm or more, more preferably 0.5 μm or more. The particle size of the olefin resin (B) is preferably 0.1 μm to 9.0 μm. The smaller the particle size of the olefin resin (B), the more likely the olefin resin (B) to aggregate. The particle size of the olefin resin (B) is a value measured by the Cole counter method.

[0033] The particle size of the olefin resin (B) is preferably 0.1 μm to 9.0 μm, and the softening point of the olefin resin (B) is preferably 70° C. or higher. As a result, when the composition of the first embodiment is used as a raw material for the undercoat layer, the lithium ion secondary battery is less susceptible to thermal runaway, and the safety of the lithium ion secondary battery is improved.

[0034] The content of the olefin resin (B) is 30% by mass to 70% by mass relative to the total amount of (A) to (E). If the content of the olefin resin (B) is within the above range, the shutdown function can be ensured. The higher the content of the olefin resin (B), the better the safety of the lithium ion secondary battery, but the lower the battery performance of the lithium ion secondary battery tends to be. In other words, there is a trade-off between the safety and the battery performance of the lithium ion secondary battery. From the viewpoint of low resistance of the undercoat layer, the content of the olefin resin (B) is preferably 65% ​​by mass or less, more preferably 60% by mass or less, and even more preferably 55% by mass or less, based on the total amount of (A) to (E). From the viewpoint of effectively exerting the shutdown function, the content of the olefin resin (B) is preferably 35% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more, based on the total amount of (A) to (E).

[0035] The olefin resin (B) may be a commercially available product. Examples of commercially available water-dispersible olefin resins include the Chemipearl (registered trademark) series (aqueous polyolefin dispersions) manufactured by Mitsui Chemicals, Inc. Examples of finely particulate aqueous dispersions of low-molecular-weight polyethylene include WP100, W100, W200, W300, W308, W310, W400, W401, W410, W4005, W500, WF640, W700, W800, W900, W950, P301W, and WH201.

[0036] (1.4) Binder resin (C) The composition of the first embodiment contains a binder resin (C). When the composition of the first embodiment is used as a raw material for an undercoat layer, the binder resin (C) improves the physical properties of the undercoat layer (e.g., electrolyte permeability and peel strength) and can also improve the battery performance of a lithium ion secondary battery.

[0037] The binder resin (C) is a resin other than the olefin-based resin (B). The material of the binder resin (C) is not particularly limited, and examples thereof include carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF copolymers, hydroxypropyl methyl cellulose, polyvinyl alcohol, hydroxypropyl cellulose, and diacetyl cellulose. PVDF copolymers are copolymers of vinylidene fluoride and other monomers, such as PVDF-HFP (hexafluoropropylene) and PVDF-PEO (polyoxyethylene). The binder resin (C) may be used alone or in combination of two or more. Among these, it is preferable that the binder resin (C) contains CMC or PVDF, from the viewpoint of low resistance of the undercoat layer.

[0038] The binder resin (C) may be a commercially available product. Commercially available CMC products include "1130", "1140", "1240", "1250", "1260", "1330", "2200", and "DL100L" manufactured by Daicel Miraize Co., Ltd. Commercially available PVDF products include the Kureha (registered trademark) KF Polymer series manufactured by Kureha Corporation, such as "W#1100," "W#1300," "W#1700," "W#7200," and "W#7300."

[0039] The content of the binder resin (C) is 1% by mass to 30% by mass relative to the total amount of (A) to (E). The shutdown function depends on the balance between the content of the binder resin (C) and the content of the olefin-based resin (B). When the content ratio of the binder resin (C) to the total amount of (A) to (E) is high, the content ratio of the olefin-based resin (B) to the total amount of (A) to (E) becomes low. In this case, the shutdown function may not function. When the content of the binder resin (C) is within the above range, it is possible to achieve both processability of the composition (e.g., slurry for the undercoat layer) and ensuring the shutdown function. The content of the binder resin (C) is preferably 15% by mass or less, and more preferably 10% by mass or less, based on the total amount of (A) to (E), from the viewpoint of ensuring the shutdown function through the content of the olefin resin (B). From the viewpoint of processability of the composition (for example, slurry for undercoat layer), the content of the binder resin (C) is preferably 2% by mass or more, more preferably 5% by mass or more, based on the total amount of (A) to (E).

[0040] (1.5) Synthetic rubber (D) The composition of the first embodiment contains a synthetic rubber (D). When the composition of the first embodiment is used as a raw material for the undercoat layer, the adhesion between the undercoat layer and the current collector is improved, and the battery resistance can be reduced.

[0041] Examples of the synthetic rubber (D) include styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, hydrogenated styrene butadiene rubber (HSBR), butylene rubber, polybutadiene, and polyisoprene rubber. The synthetic rubber (D) may be used alone or in combination of two or more. Among these, from the viewpoint of low resistance of the undercoat layer, the synthetic rubber (D) is preferably a rubber appropriately mixed with a water-dispersible binder such as an SBR emulsion, and more preferably contains SBR.

[0042] The shape of the synthetic rubber (D) is not particularly limited, and it may be in the form of particles.

[0043] The content of the synthetic rubber (D) is 1% by mass to 20% by mass based on the total amount of (A) to (E). When the content of the synthetic rubber (D) is within the above range, it is possible to achieve both adhesion to the current collector and a reduction in the electrical resistance of the composition itself. From the viewpoint of reducing the electrical resistance of the composition itself, the content of the synthetic rubber (D) is preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less, based on the total amount of (A) to (E). From the viewpoint of adhesion to the current collector, the content of the synthetic rubber (D) is preferably 3% by mass or more, and more preferably 5% by mass or more, based on the total amount of (A) to (E).

[0044] The ratio of the content of the synthetic rubber (D) to the content of the olefin resin (B) (hereinafter referred to as the "mass ratio (D / B)") is not particularly limited, but is preferably 0.01 to 0.7. When the mass ratio (D / B) is within the above range, when the composition of the first embodiment is used as a raw material for the undercoat layer, it is possible to achieve both good adhesion between the undercoat layer and the current collector and effective demonstration of the shutdown function. From the viewpoint of effectively exerting the shutdown function, the mass ratio (D / B) is more preferably 0.50 or less, even more preferably 0.25 or less, and particularly preferably 0.17 or less. When the composition of the first embodiment is used as a raw material for the undercoat layer, the mass ratio (D / B) is more preferably 0.07 or more, and even more preferably 0.08 or more, from the viewpoint of maintaining the adhesion between the undercoat layer and the current collector.

[0045] The synthetic rubber (D) may be a commercially available product, such as "TRD2001" (aqueous dispersion of styrene-butadiene rubber particles) manufactured by JSR Corporation.

[0046] (1.6) Additives (E) The composition of the first embodiment may contain an additive (E) as needed, which makes it possible to impart various functions to the composition of the first embodiment depending on the type of additive (E).

[0047] Examples of additive (E) include thermally expandable microcapsules having a maximum volume expansion temperature of 70°C or higher and 180°C or lower (hereinafter, simply referred to as "thermally expandable microcapsules"), inorganic oxide fillers, curable resin fillers, scaly fillers, etc. Among these, additive (E) preferably contains at least one of thermally expandable microcapsules and inorganic oxide fillers.

[0048] In this disclosure, the term "thermally expandable microcapsules" refers to those having an outer shell made of a thermoplastic resin and a volatile expanding agent encapsulated in the outer shell. Specifically, when the thermally expandable microcapsules are in contact with an electrolyte solution (described later) and exposed to a temperature (e.g., 70°C to 160°C) just before the onset of thermal runaway due to abnormal heat generation in a lithium-ion secondary battery, they rapidly soften and foam, causing a volume expansion. In the present disclosure, the "volume expansion starting temperature" refers to the temperature at which the volatile expanding agent contained in the thermally expandable microcapsules gasifies and the thermally expandable microcapsules begin to expand in volume. In the present disclosure, "maximum volume expansion temperature" refers to the temperature at which the expansion volume of the thermally expandable microcapsule is maximum when the volatile expansion agent contained in the thermally expandable microcapsule is gasified and the thermally expandable microcapsule expands in volume.

[0049] The content of additive (E) is 0% by mass to 50% by mass with respect to the total amount of (A) to (E). If the content of additive (E) is within the above range, the shutdown function can be more effectively exhibited due to the properties of additive (E). The content of the additive (E) has little effect on the content ratio of the conductive carbon material (A) and the olefin resin (B), and from the viewpoints of effectively exhibiting the shutdown function and maintaining the battery performance, the content is preferably 40 mass % or less, and more preferably 30 mass % or less, based on the total amount of (A) to (E). From the viewpoint of effectively exhibiting the shutdown function, the content of the additive (E) is preferably 5% by mass or more, and more preferably 10% by mass or more, based on the total amount of (A) to (E).

[0050] (1.6.1) Thermally expandable microcapsules The composition of the first embodiment may contain thermally expandable microcapsules as needed. The composition of the first embodiment does not necessarily have to contain thermally expandable microcapsules. When the composition of the first embodiment contains thermally expandable microcapsules and is used as a raw material for the undercoat layer, the thermally expandable microcapsules undergo volumetric expansion due to abnormal heat generation in the lithium ion secondary battery, thereby efficiently increasing the DC resistance of the electrode and improving the safety of the lithium ion secondary battery.

[0051] The volume expansion starting temperature of the thermally expandable microcapsules is preferably 120°C to 130°C. The maximum volume expansion temperature of the thermally expandable microcapsules is preferably 145°C to 155°C.

[0052] The thermoplastic resin constituting the outer shell preferably contains a (co)polymer containing vinylidene chloride and a (co)polymer containing (meth)acrylonitrile. This allows the thermoplastic resin constituting the outer shell to have excellent electrolyte resistance, thermoplasticity, and gas barrier properties. In particular, from the viewpoint of electrolyte resistance, the thermoplastic resin constituting the outer shell is preferably a (co)polymer containing (meth)acrylonitrile as the main component (51% by mass or more). The raw material of the thermoplastic resin constituting the outer shell may contain a crosslinkable monomer in addition to a polymerizable monomer in order to improve the foaming properties and heat resistance of the resulting thermally expandable microcapsules.

[0053] The boiling point of the volatile expanding agent is preferably selected so that the maximum volume expansion temperature of the thermally expandable microcapsules is higher than the softening point of the olefin-based resin (B). The boiling point is preferably selected so that the volume expansion starting temperature of the thermally expandable microcapsules is equal to the softening point of the olefin-based resin (B). This effectively increases the DC resistance of the electrode when the temperature of the undercoat layer rises above the softening point of the olefin-based resin (B) due to heat generation in the lithium-ion secondary battery. As a result, the safety of the lithium-ion secondary battery is further improved. Examples of volatile blowing agents include low-molecular-weight hydrocarbons with a boiling point of 100°C or less, non-flammable or flame-retardant compounds, etc. Low-molecular-weight hydrocarbons are preferably used as low-molecular-weight hydrocarbons with a boiling point of 100°C or less. Examples of low-molecular-weight hydrocarbons include propane, propylene, n-butane, isobutane, butene, isobutene, isopentane, neopentane, n-pentane, n-hexane, isohexane, heptane, and petroleum ether. Examples of non-flammable or flame-retardant compounds include halogenated hydrocarbons (e.g., methyl chloride, methylene chloride, fluorotrichloromethane, difluorodichloromethane, chlorotrifluoromethane, etc.) and chlorofluorocarbons. Volatile blowing agents may be used alone or in combination of two or more.

[0054] The particle size of the thermally expandable microcapsules is not particularly limited, but is preferably 1 μm to 40 μm, more preferably 3 μm to 30 μm, and even more preferably 5 μm to 25 μm. The particle size of the thermally expandable microcapsules is the particle size (particle size distribution D50, median diameter) corresponding to the cumulative 50% by volume from the fine particle side in the volume-based particle size distribution measured using a particle size distribution measuring device based on the laser light diffraction scattering method.

[0055] The content of the thermally expandable microcapsules is 0% by mass to 50% by mass relative to the total amount of (A) to (E). The content of the thermally expandable microcapsules has little effect on the content ratio of the conductive carbon material (A) and the olefin resin (B), and from the viewpoint of maintaining the shutdown function and battery performance, the content is preferably 50 mass% or less, more preferably 40 mass% or less, of the total amount of (A) to (E). The content of the thermally expandable microcapsules is preferably 5% by mass or more, and more preferably 10% by mass or more, of the total amount of (A) to (E), from the viewpoint of enhancing the shutdown function due to the thermal expansion of the thermally expandable microcapsules in the event of abnormal heat generation.

[0056] The thermally expandable microcapsules may be commercially available products, such as the Matsumoto Microsphere (registered trademark) series manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd., the EXPANCEL (registered trademark) series manufactured by Akzo Nobel, and the ADVANCELL (registered trademark) series manufactured by Sekisui Chemical Co., Ltd.

[0057] (1.6.2) Inorganic oxide fillers The inorganic oxide filler may contain an inorganic oxide filler as needed. The composition of the first embodiment may not contain an inorganic oxide filler. When the composition of the first embodiment contains an inorganic oxide filler, the inorganic oxide filler functions as a filling material. Increasing the content of inorganic oxide filler contributes to the heat resistance of the positive electrode, while minimizing the content of inorganic oxide filler forms an undercoat layer that adheres closely to the positive electrode composite layer, melting the undercoat layer in the event of an internal short circuit and contributing to improved safety. Furthermore, by selecting the type and physical properties of the inorganic oxide filler, it is possible to decompose the electrolyte and generate gas when the battery is overcharged.

[0058] Examples of inorganic oxide fillers include aluminum oxide (α-Al2O3, γ-Al2O3), aluminum hydroxide (Al(OH)3), boehmite (AlOOH)), magnesia (magnesium oxide: MgO), magnesium hydroxide (Mg(OH)2), zirconia (ZrO2), titania (TiO2), silica (SiO2), silicon dioxide (SiO2), silicon carbide (SiC), aluminum nitride (AlN), boron nitride (BN), mica, and graphite oxide (e.g., expanded graphite). These inorganic oxide fillers may be used alone or in combination of two or more. Among these, it is preferable that the inorganic oxide filler contains aluminum oxide.

[0059] The shape of the inorganic oxide filler is not particularly limited, and examples thereof include spherical, needle-like, ellipsoidal, plate-like, scale-like, etc. The particle size of the inorganic oxide filler is not particularly limited, and is preferably 0.01 μm to 5 μm. The particle size of the inorganic oxide filler indicates the particle size (particle size distribution D50, median diameter) corresponding to the cumulative 50% by volume from the fine particle side in the volume-based particle size distribution measured by a particle size distribution measuring device based on the laser light diffraction scattering method.

[0060] The content of the inorganic oxide filler is 0% by mass to 50% by mass relative to the total amount of (A) to (E). The content of the inorganic oxide filler has little effect on the content ratio of the conductive carbon material (A) and the olefin resin (B), and from the viewpoint of maintaining the shutdown function and battery performance, the content is preferably 40 mass % or less, more preferably 30 mass % or less, based on the total amount of (A) to (E). The content of the inorganic oxide filler is preferably 5% by mass or more, and more preferably 10% by mass or more, based on the total amount of (A) to (E), from the viewpoint of suppressing the fluidity of the olefin resin (B) dissolved in the undercoat layer at high temperatures and maintaining the shutdown function for a long period of time.

[0061] (1.7) Non-solid content The composition of the first embodiment may contain non-solid components. For example, when the undercoat layer is formed from a slurry for the undercoat layer, the undercoat layer may contain various compounding components derived from the slurry for the undercoat layer. Examples of non-solid components include various compounding components derived from the slurry for the undercoat layer (e.g., thickeners, surfactants, dispersants, wetting agents, antifoaming agents, etc.), water, etc.

[0062] (2) Second embodiment The composition of the second embodiment of the present disclosure contains a conductive carbon material (A), an olefin-based resin (B), a binder resin (C) that is a resin other than the olefin-based resin (B), and a synthetic rubber (D), and has a ratio (D99 / D10) of particle size distribution D99 (hereinafter also simply referred to as "D99") to particle size distribution D10 (hereinafter also simply referred to as "D10") measured by a laser light diffraction scattering method of 35 or less.

[0063] In this disclosure, "particle size distribution D99 measured by laser light diffraction scattering" refers to the particle size corresponding to the cumulative 99% by volume from the fine particle side in the volume-based particle size distribution measured by a particle size distribution measuring device based on laser light diffraction scattering. The method for measuring D99 is the same as that described in the Examples. In the present disclosure, the "particle size distribution D10 measured by laser light diffraction scattering" refers to the particle size corresponding to the cumulative 10% by volume from the fine particle side in the volume-based particle size distribution measured by a particle size distribution measuring device based on laser light diffraction scattering. The method for measuring D10 is the same as that described in the Examples.

[0064] The composition of the second embodiment is similar to the composition of the first embodiment, except that the ratio (D99 / D10) is 35 or less, and the contents of the conductive carbon material (A), the olefin resin (B), the binder resin (C), and the synthetic rubber (D) and the mass ratio (A / B) do not necessarily have to be within a specific range. The description of the composition of the first embodiment can be applied to the description of the composition of the second embodiment. Hereinafter, descriptions that overlap with the description of the composition of the first embodiment will be omitted.

[0065] The composition of the second embodiment, having the above-described configuration, can provide a lithium ion secondary battery that is excellent in safety and in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed. In other words, the composition of the second embodiment can provide a lithium ion secondary battery that has a good balance between safety and battery performance (i.e., output and life). This effect is presumably due to, but not limited to, the following reasons. A ratio (D99 / D10) of 35 or less indicates that the number of aggregates of components in the composition is small (in other words, that the number of components dispersed in the composition is large). The lower the ratio (D99 / D10), the fewer the number of aggregates of components in the composition. Uniform dispersion of the conductive carbon material (A) in the composition facilitates the formation of a uniform conductive path within the undercoat layer. This suppresses the initial DC resistance during battery charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment. Furthermore, uniform dispersion of the olefin resin (B) in the composition facilitates efficient performance of the shutdown function. As a result, it is presumed that the composition of the second embodiment, having the above-described configuration, can provide a lithium ion secondary battery that is excellent in safety and in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed.

[0066] The ratio (D99 / D10) is 35 or less, and from the viewpoint of improving the safety and battery performance of the lithium ion secondary battery, it is more preferably 3.0 to 30.0, even more preferably 3.0 to 22.0, and particularly preferably 3.0 to 15.0. Methods for adjusting the ratio (D99 / D10) within the above range include a kneader, a two-roll mill, a three-roll mill, a planetary mixer, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, an attritor, a high-shear mixer, a Disper, a Filmix, an ultrasonic wave, and a high-pressure homogenizer.

[0067] The D99 of the composition of the second embodiment is not particularly limited, but is preferably 20 μm or less. The smaller the aggregates of fine particles, the better the dispersibility of the materials contained in the undercoat composition. When D99 is 20 μm or less, the safety and battery performance of the lithium ion secondary battery are more likely to be improved than when D99 is greater than 20 μm. From the viewpoint of improving the safety and battery performance of lithium ion secondary batteries, D99 of the composition is more preferably 2.0 μm to 20.0 μm, even more preferably 2.0 μm to 15.0 μm, and particularly preferably 2.0 μm to 10.0 μm. Methods for adjusting the D99 of the composition to fall within the above range include kneaders, two-roll mills, three-roll mills, planetary mixers, ball mills, horizontal sand mills, vertical sand mills, annular bead mills, attritors, high-shear mixers, Disper mixers, Filmix mixers, ultrasonic and high-pressure homogenizers.

[0068] The D10 of the composition of the second embodiment is not particularly limited, and is preferably 0.1 μm to 1.5 μm, more preferably 0.1 μm to 1.0 μm, and even more preferably 0.1 μm to 0.7 μm. If D10 is within the above range, the dispersibility of the materials contained in the undercoat composition is excellent, and the safety and battery performance of the lithium ion secondary battery are more likely to be improved than when D10 is outside the above range. Methods for adjusting the D10 of the composition of the second embodiment to fall within the above range include a kneader, a two-roll mill, a three-roll mill, a planetary mixer, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, an attritor, a high-shear mixer, a Disper, a Filmix, ultrasonic waves, and a high-pressure homogenizer.

[0069] The particle size distribution D90 (hereinafter simply referred to as "D90") of the composition of the second embodiment measured by a laser light diffraction scattering method is not particularly limited, but is preferably 1.0 μm to 10.0 μm, more preferably 1.0 μm to 7.0 μm, and even more preferably 1.0 μm to 4.0 μm. If D90 is within the above range, the dispersibility of the undercoat composition is excellent, and the safety and battery performance of the lithium ion secondary battery are likely to be improved. In the present disclosure, "particle size distribution D90 measured by a laser light diffraction scattering method" refers to the particle size corresponding to the cumulative 90% by volume from the fine particle side in the volume-based particle size distribution measured using a particle size distribution measuring device based on the laser light diffraction scattering method. The method for measuring D90 is the same as that described in the Examples. Methods for adjusting the D90 of the composition to fall within the above range include kneaders, two-roll mills, three-roll mills, planetary mixers, ball mills, horizontal sand mills, vertical sand mills, annular bead mills, attritors, high-shear mixers, Disper mixers, Filmix mixers, ultrasonic and high-pressure homogenizers.

[0070] The particle size distribution D50 (hereinafter simply referred to as "D50") of the composition of the second embodiment measured by a laser light diffraction scattering method is not particularly limited, but is preferably 0.5 μm to 5.0 μm, more preferably 0.5 μm to 3.0 μm, and even more preferably 0.5 μm to 2.0 μm. If D50 is within the above range, the dispersibility of the undercoat composition is excellent, and the safety and battery performance of the lithium ion secondary battery are likely to be improved. In the present disclosure, "particle size distribution D50 measured by a laser light diffraction scattering method" refers to the particle size corresponding to a cumulative 50% by volume from the fine particle side in a volume-based particle size distribution measured using a particle size distribution measuring device based on the laser light diffraction scattering method. The method for measuring D50 is the same as that described in the Examples. Methods for adjusting the D50 of the composition to fall within the above range include a kneader, a two-roll mill, a three-roll mill, a planetary mixer, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, an attritor, a high-shear mixer, a Disper, a Filmix, ultrasonic waves, and a high-pressure homogenizer.

[0071] The ratio of D99 to D50 (D99 / D50) is preferably 20 or less. A ratio (D99 / D50) of 20 or less indicates that the size of aggregates is suppressed relative to the median diameter of fine particles in the composition. A ratio (D99 / D50) of 20 or less provides excellent dispersibility of the undercoat composition. This makes it easier to improve the safety and battery performance of lithium-ion secondary batteries than when the ratio (D99 / D50) exceeds 20. From the viewpoint of improving the safety and battery performance of the lithium ion secondary battery, the ratio (D99 / D50) is more preferably 2.0 to 16.0, even more preferably 2.0 to 10.0, and particularly preferably 2.0 to 6.0. Methods for adjusting the ratio (D99 / D50) within the above range include a kneader, a two-roll mill, a three-roll mill, a planetary mixer, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, an attritor, a high-shear mixer, a Disper, a Filmix, an ultrasonic wave, and a high-pressure homogenizer.

[0072] (2.1) Use of the composition The use of the composition of the second embodiment is not particularly limited, and examples thereof include the same uses as those exemplified as the use of the composition of the first embodiment.

[0073] (2.2) Conductive carbon material (A) The composition of the second embodiment contains a conductive carbon material (A). Examples of the conductive carbon material (A) include the same materials as those exemplified as the conductive carbon material (A) of the first embodiment.

[0074] The content of the conductive carbon material (A) is not particularly limited, and is preferably 10% by mass to 60% by mass relative to the total amount of (A) to (E). The content of the conductive carbon material (A) is more preferably 45% by mass or less, even more preferably 35% by mass or less, particularly preferably 25% by mass or less, and even more preferably 19% by mass or less, relative to the total amount of (A) to (E). The content of the conductive carbon material (A) is more preferably 15% by mass or more, relative to the total amount of (A) to (E). The technical significance of setting the content of the conductive carbon material (A) within the above range is the same as the technical significance described in the first embodiment.

[0075] The mass ratio (A / B) is preferably 0.25 to 2.00. The mass ratio (A / B) is more preferably 1.50 or less, even more preferably 0.90 or less, particularly preferably 0.75 or less, even more preferably 0.60 or less, and most preferably 0.55 or less. The mass ratio (A / B) is more preferably 0.27 or more, even more preferably 0.30 or more, particularly preferably 0.35 or more, and even more preferably 0.50 or more. The technical significance of keeping the mass ratio (A / B) within the above range is the same as the technical significance described in the first embodiment.

[0076] (2.3) Olefin resin (B) The composition of the second embodiment contains an olefin-based resin (B). Examples of the olefin-based resin (B) include the same resins as those exemplified as the olefin-based resin (B) of the first embodiment.

[0077] The content of the olefin resin (B) is not particularly limited, and is preferably 30% to 80% by mass relative to the total amount of (A) to (E). The content of the olefin resin (B) is more preferably 70% by mass or less, even more preferably 65% ​​by mass or less, and particularly preferably 60% by mass or less, relative to the total amount of (A) to (E). The content of the olefin resin (B) is more preferably 35% by mass or more, even more preferably 40% by mass or more, particularly preferably 50% by mass or more, and even more preferably 55% by mass or more, relative to the total amount of (A) to (E). The technical significance of setting the content of the olefin resin (B) within the above range is the same as the technical significance described in the first embodiment.

[0078] (2.4) Binder resin (C) The composition of the second embodiment contains a binder resin (C). Examples of the binder resin (C) include the same resins as those exemplified as the binder resin (C) of the first embodiment.

[0079] The content of the binder resin (C) is not particularly limited, and is preferably 1% by mass to 30% by mass relative to the total amount of (A) to (E). The content of the binder resin (C) is more preferably 15% by mass or less, and even more preferably 10% by mass or less, relative to the total amount of (A) to (E). The content of the binder resin (C) is more preferably 2% by mass or more, and even more preferably 5% by mass or more, relative to the total amount of (A) to (E). The technical significance of setting the content of the binder resin (C) within the above range is the same as the technical significance described in the first embodiment.

[0080] (2.5) Synthetic rubber (D) The composition of the second embodiment contains a synthetic rubber (D). Examples of the synthetic rubber (D) include the same rubbers as those exemplified as the synthetic rubber (D) of the first embodiment.

[0081] The content of synthetic rubber (D) is not particularly limited, and is preferably 1% by mass to 20% by mass relative to the total amount of (A) to (E). The content of synthetic rubber (D) is more preferably 15% by mass or less, even more preferably 12% by mass or less, and particularly preferably 10% by mass or less, relative to the total amount of (A) to (E). The content of synthetic rubber (D) is more preferably 3% by mass or more, and particularly preferably 5% by mass or more, relative to the total amount of (A) to (E). The technical significance of setting the content of synthetic rubber (D) within the above range is the same as the technical significance described in the first embodiment.

[0082] (2.6) Additives (E) The composition of the second embodiment may contain an additive (E) as needed. Examples of the additive (E) include the same additives as those exemplified as the additive (E) of the first embodiment.

[0083] The content of additive (E) is not particularly limited, and is preferably 0% by mass to 50% by mass relative to the total amount of (A) to (E). The content of additive (E) is more preferably 40% by mass or less, and particularly preferably 30% by mass or less, relative to the total amount of (A) to (E). The content of additive (E) is more preferably 5% by mass or more, and even more preferably 10% by mass or more, relative to the total amount of (A) to (E). The technical significance of setting the content of additive (E) within the above range is the same as the technical significance described in the first embodiment.

[0084] (2.7) Non-solids The composition of the second embodiment may contain non-solid components, such as the same non-solid components as those exemplified in the first embodiment.

[0085] (3) Electrode The electrode of the present disclosure comprises a current collector, an undercoat layer containing the composition of the first or second embodiment of the present disclosure, and a composite layer.

[0086] In the present disclosure, the term "electrode" refers to at least one of the positive electrode and the negative electrode of a lithium ion secondary battery.

[0087] The electrode of the present disclosure has the above-described configuration, and therefore can provide a lithium ion secondary battery that is excellent in safety and in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed.

[0088] In the electrode of the present disclosure, the current collector, the undercoat layer, and the composite layer are preferably laminated in this order. As a result, the electrode of the present disclosure increases the electrical resistance between the current collector and the composite layer when the temperature of the lithium-ion secondary battery rises suddenly. This prevents the lithium-ion secondary battery from overheating. Therefore, the electrode of the present disclosure can improve the safety of the lithium-ion secondary battery. Furthermore, the electrode of the present disclosure can prevent an increase in the DC resistance of the lithium-ion secondary battery even when the lithium-ion secondary battery is stored for a long period of time in a high-temperature environment.

[0089] The electrode of the present disclosure may be a positive electrode or a negative electrode. In particular, the electrode of the present disclosure is preferably a positive electrode. When the electrode of the present disclosure is a positive electrode, the electrical resistance of the positive electrode increases, and the resulting lithium ion secondary battery can exhibit a shutdown function.

[0090] (3.1) Positive electrode The positive electrode of the present disclosure may include a current collector (hereinafter also referred to as a "positive electrode current collector"), an undercoat layer, and a composite layer (hereinafter also referred to as a "positive electrode composite layer").

[0091] The positive electrodes of the present disclosure include a first positive electrode configuration, a second positive electrode configuration, a third positive electrode configuration, and a fourth positive electrode configuration. The first positive electrode configuration indicates a configuration in which an undercoat layer and a positive electrode mixture layer are laminated in this order on both main surfaces of a current collector. The second positive electrode configuration indicates a configuration in which an undercoat layer and a positive electrode composite layer are laminated in this order on one main surface of a current collector, and an undercoat layer is laminated on the other main surface of the current collector. The third positive electrode configuration shows a configuration in which an undercoat layer and a positive electrode composite layer are laminated in this order on one main surface of a current collector, and a positive electrode composite layer is laminated on the other main surface of the current collector. The fourth positive electrode configuration indicates a configuration in which an undercoat layer and a positive electrode composite layer are laminated in this order on one main surface of a current collector, and neither the undercoat layer nor the positive electrode composite layer is laminated on the other main surface of the current collector.

[0092] (3.1.1) Positive electrode current collector Examples of materials for the positive electrode current collector include aluminum, nickel, stainless steel (SUS), copper, etc. "Aluminum" includes pure aluminum and aluminum alloys.

[0093] (3.1.2) Undercoat layer The undercoat layer is formed using the composition of the first or second embodiment of the present disclosure.

[0094] The position where the undercoat layer is formed may be selected appropriately depending on the coating pattern of the positive electrode composite layer (e.g., intermittent coating, stripe coating, etc.) as long as it is formed on at least a part of at least one main surface of the positive electrode current collector.

[0095] The thickness of the undercoat layer is not particularly limited. The thickness of the undercoat layer is preferably 50 μm or less, more preferably 20 μm or less, from the viewpoint of further suppressing the DC resistance of the lithium ion secondary battery in normal conditions. The thickness of the undercoat layer is preferably 0.1 μm or more, more preferably 0.2 μm or more, from the viewpoint of increasing the DC resistance when the lithium ion secondary battery abnormally heats up and more reliably exhibiting the shutdown function.

[0096] (3.1.3) Positive electrode composite layer The positive electrode mixture layer contains a positive electrode active material and a binder.

[0097] (3.1.3.1) Positive electrode active material The positive electrode active material is not particularly limited as long as it is a material that can absorb and release lithium ions, and can be adjusted appropriately depending on the application of the lithium ion secondary battery.

[0098] Examples of the positive electrode active material include a first oxide and a second oxide. The first oxide contains lithium (Li) and nickel (Ni) as constituent metal elements. The second oxide contains Li, Ni, and at least one metal element other than Li and Ni as its constituent metal elements. Examples of metal elements other than Li and Ni include transition metal elements and typical metal elements. The second oxide preferably contains the metal elements other than Li and Ni in an amount equivalent to or less than Ni in atomic number terms. The metal element other than Li and Ni may be, for example, at least one selected from the group consisting of Co, Mn, Al, Cr, Fe, V, Mg, Ca, Na, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. The positive electrode active material may be used alone or in combination of two or more.

[0099] The positive electrode active material preferably contains a lithium-containing composite oxide (hereinafter sometimes referred to as "NCM") represented by the following formula (X): The lithium-containing composite oxide (X) has the advantages of high energy density per unit volume and excellent thermal stability.

[0100] LiNi a Co b Mn c O2… Formula (X)

[0101] In formula (X), a, b, and c each independently represent a number greater than 0 and less than 1, and the sum of a, b, and c is 0.99 or greater and 1.00 or less.

[0102] A specific example of NCM is LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Co 0.3 Mn 0.2O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 Examples include O2.

[0103] The positive electrode active material may contain a lithium-containing composite oxide (hereinafter, sometimes referred to as "NCA") represented by the following formula (Y).

[0104] Li t Ni 1-x-y Co x Al y O2... Formula (Y)

[0105] In formula (Y), t is 0.95 or more and 1.15 or less, x is 0 or more and 0.3 or less, y is 0.1 or more and 0.2 or less, and the sum of x and y is less than 0.5.

[0106] A specific example of NCA is LiNi 0.8 Co 0.15 Al 0.05 Examples include O2.

[0107] The content of the positive electrode active material is preferably 10% by mass or more, more preferably 30% by mass or more, even more preferably 50% by mass or more, and particularly preferably 70% by mass or more, based on the total amount of the positive electrode mixture layer. The content of the positive electrode active material is preferably 99.9 mass % or less, and more preferably 99 mass % or less, relative to the total amount of the positive electrode mixture layer.

[0108] (3.1.3.2) Binder Examples of binders include polyvinyl acetate, polymethyl methacrylate, nitrocellulose, fluororesin, and rubber particles. Examples of fluororesins include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. Examples of rubber particles include styrene-butadiene rubber particles and acrylonitrile rubber particles. Among these, from the viewpoint of improving the oxidation resistance of the positive electrode mixture layer, it is preferable that the binder contains a fluororesin. The binder may be used alone or in combination of two or more kinds. The binder content is preferably 0.1% by mass or more and 4% by mass or less relative to the total amount of the positive electrode mixture layer, from the viewpoint of achieving both the physical properties of the positive electrode mixture layer (for example, electrolyte permeability, peel strength, etc.) and battery performance.

[0109] (3.1.3.3) Conductive additives The positive electrode mixture layer may further contain a conductive additive. As the conductive aid, known conductive aids can be used. As known conductive aids, conductive carbon materials are preferred. Examples of conductive carbon materials include graphite, carbon black, conductive carbon fiber, and fullerene. Examples of conductive carbon fibers include carbon nanotubes, carbon nanofibers, and carbon fiber. Examples of graphite include artificial graphite and natural graphite. Examples of natural graphite include flake graphite, lump graphite, and amorphous graphite. The conductive aids may be used alone or in combination of two or more. The material of the conductive aid may be a commercially available product.

[0110] (3.1.3.4) Other ingredients The positive electrode mixture layer may contain other components such as a thickener, a surfactant, a dispersant, a wetting agent, and an antifoaming agent.

[0111] (3.2) Negative electrode The negative electrode of the present disclosure includes a current collector (hereinafter also referred to as "negative electrode current collector") and a mixture layer (hereinafter also referred to as "negative electrode mixture layer"), and may also include an undercoat layer.

[0112] Negative electrodes of the present disclosure may include a first negative electrode configuration, a second negative electrode configuration, a third negative electrode configuration, and a fourth negative electrode configuration. The first negative electrode configuration indicates a configuration in which an undercoat layer and a negative electrode mixture layer are laminated in this order on both main surfaces of a current collector. The second negative electrode configuration indicates a configuration in which an undercoat layer and a negative electrode composite layer are laminated in this order on one main surface of a current collector, and an undercoat layer is laminated on the other main surface of the current collector. The third negative electrode configuration shows a configuration in which an undercoat layer and a negative electrode composite material layer are laminated in this order on one main surface of a current collector, and a negative electrode composite material layer is laminated on the other main surface of the current collector. The fourth negative electrode configuration indicates a configuration in which an undercoat layer and a negative electrode composite layer are laminated in this order on one main surface of a current collector, and neither the undercoat layer nor the negative electrode composite layer is laminated on the other main surface of the current collector.

[0113] (3.2.1) Negative electrode current collector Examples of materials for the negative electrode current collector include copper, aluminum, nickel, stainless steel (SUS), and nickel-plated steel.

[0114] (3.2.2) Undercoat layer The undercoat layer is the same as that exemplified as the undercoat layer of the positive electrode.

[0115] (3.2.3) Negative electrode composite layer The negative electrode mixture layer contains a negative electrode active material and a binder.

[0116] (3.2.3.1) Negative electrode active material The negative electrode active material is not particularly limited as long as it is a material capable of absorbing and releasing lithium ions. The negative electrode active material is preferably at least one material selected from the group consisting of metallic lithium, lithium-containing alloys, metals or alloys capable of alloying with lithium, oxides capable of doping and dedoping lithium ions, transition metal nitrides capable of doping and dedoping lithium ions, and carbon materials capable of doping and dedoping lithium ions. Among these, the negative electrode active material is preferably a carbon material capable of doping and dedoping lithium ions (hereinafter referred to as "carbon material").

[0117] Examples of carbon materials include carbon black, activated carbon, graphite materials, and amorphous carbon materials. These carbon materials may be used alone or in combination of two or more. The form of the carbon material is not particularly limited, and examples include fibrous, spherical, and flake forms. The particle size of the carbon material is not particularly limited, and is preferably 5 μm or more and 50 μm or less, more preferably 20 μm or more and 30 μm or less. Examples of amorphous carbon materials include hard carbon, coke, mesocarbon microbeads (MCMB) fired at 1500° C. or less, and mesophase pitch carbon fiber (MCF). Examples of graphite materials include natural graphite and artificial graphite. Examples of artificial graphite include graphitized MCMB and graphitized MCF. The graphite material may contain boron. The graphite material may be coated with a metal or amorphous carbon. Examples of metal materials that coat the graphite material include gold, platinum, silver, copper, and tin. The graphite material may be a mixture of amorphous carbon and graphite.

[0118] (3.2.3.2) Binder Examples of the binder contained in the negative electrode mixture layer include the same binders as those exemplified as the binder contained in the positive electrode mixture layer. The binder contained in the negative electrode mixture layer may be the same as or different from the binder contained in the positive electrode mixture layer. The content of the binder contained in the negative electrode mixture layer is not particularly limited, and may be the same as the content of the binder contained in the positive electrode mixture layer exemplified above.

[0119] (3.2.3.3) Conductive additives The negative electrode mixture layer preferably contains a conductive additive. Examples of the conductive additive include the same conductive additives as those exemplified as the conductive additives that can be contained in the positive electrode mixture layer.

[0120] (3.2.3.4) Other ingredients In addition to the above components, the negative electrode mixture layer may contain other components such as a thickener, a surfactant, a dispersant, a wetting agent, and an antifoaming agent.

[0121] (4) Lithium-ion secondary battery The lithium ion secondary battery of the present disclosure includes the electrode of the present disclosure.

[0122] A lithium ion secondary battery generally comprises an outer casing, a positive electrode, a negative electrode, a separator, and an electrolyte. The outer casing accommodates the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte. The positive electrode is capable of absorbing and releasing lithium ions. The negative electrode is capable of absorbing and releasing lithium ions. The separator separates the positive electrode and the negative electrode.

[0123] In the lithium ion secondary battery of the present disclosure, at least one of the positive electrode and the negative electrode is the electrode of the present disclosure. When one of the positive electrode and the negative electrode of the lithium ion secondary battery of the present disclosure is the electrode of the present disclosure, the other of the positive electrode and the negative electrode may be a known electrode used in lithium ion secondary batteries. Hereinafter, a case where the positive electrode and the negative electrode are the electrodes of the present disclosure will be described.

[0124] (4.1) Exterior body The shape of the exterior body is not particularly limited and may be appropriately selected depending on the application of the lithium ion secondary battery, etc. Examples of the exterior body include an exterior body including a laminate film and an exterior body consisting of a battery can and a battery can lid.

[0125] (4.2) Positive and negative electrodes The positive electrode is the positive electrode of the present disclosure. The negative electrode is the negative electrode of the present disclosure.

[0126] (4.3) Separator The separator may be, for example, a porous resin flat plate. Materials for the porous resin flat plate include resins and nonwoven fabrics containing such resins. Examples of resins include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polyester, cellulose, and polyamide. Among these, the separator is preferably a porous resin sheet having a single layer or multilayer structure. The porous resin sheet is mainly made of one or more polyolefin resins. The thickness of the separator is preferably 5 μm or more and 30 μm or less. The separator is preferably disposed between the positive electrode and the negative electrode.

[0127] (4.4) Non-aqueous electrolyte The non-aqueous electrolyte contains an electrolyte and a non-aqueous solvent.

[0128] (4.4.1) Electrolyte The electrolyte preferably contains at least one of a fluorine-containing lithium salt (hereinafter sometimes referred to as a "fluorine-containing lithium salt") and a fluorine-free lithium salt.

[0129] Examples of the fluorine-containing lithium salt include inorganic acid anion salts and organic acid anion salts. Examples of inorganic acid anion salts include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), and lithium hexafluorotantalate (LiTaF6). Examples of organic acid anion salts include lithium trifluoromethanesulfonate (LiCF3SO3), etc. Among these, LiPF6 is particularly preferred as the fluorine-containing lithium salt. Fluorine-free lithium salts include lithium perchlorate (LiClO4), lithium tetrachloroaluminate (LiAlCl4), and lithium decachlorodecaborate (Li2B 10 Cl 10 ) etc.

[0130] When the electrolyte contains a fluorine-containing lithium salt, the content of the fluorine-containing lithium salt is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, based on the total amount of the electrolyte.

[0131] When the fluorine-containing lithium salt contains lithium hexafluorophosphate (LiPF6), the content of lithium hexafluorophosphate (LiPF6) is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, based on the total amount of the electrolyte.

[0132] When the non-aqueous electrolyte contains an electrolyte, the concentration of the electrolyte in the non-aqueous electrolyte is preferably 0.1 mol / L or more and 3 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less.

[0133] When the non-aqueous electrolyte contains lithium hexafluorophosphate (LiPF6), the concentration of lithium hexafluorophosphate (LiPF6) in the non-aqueous electrolyte is preferably 0.1 mol / L or more and 3 mol / L or less, more preferably 0.5 mol / L or more and 2 mol / L or less.

[0134] (4.4.2) Non-aqueous solvents The non-aqueous electrolyte generally contains a non-aqueous solvent.

[0135] Examples of non-aqueous solvents include cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, fluorine-containing chain carbonates, aliphatic carboxylic acid esters, fluorine-containing aliphatic carboxylic acid esters, γ-lactones, fluorine-containing γ-lactones, cyclic ethers, fluorine-containing cyclic ethers, chain ethers, fluorine-containing chain ethers, nitriles, amides, lactams, nitromethane, nitroethane, sulfolane, trimethyl phosphate, dimethyl sulfoxide, dimethyl sulfoxide phosphate, etc. The non-aqueous solvents may be used alone or in combination of two or more.

[0136] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and trifluoropropylene carbonate. Examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and dipropyl carbonate (DPC). Examples of fluorine-containing chain carbonates include methyl 2,2,2-trifluoroethyl carbonate. Examples of aliphatic carboxylic acid esters include methyl formate, methyl acetate, methyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylbutyrate, ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, ethyl isobutyrate, and ethyl trimethylbutyrate. Examples of the fluorine-containing aliphatic carboxylic acid esters include methyl difluoroacetate, methyl 3,3,3-trifluoropropionate, ethyl difluoroacetate, and 2,2,2-trifluoroethyl acetate. Examples of γ-lactones include γ-butyrolactone and γ-valerolactone. Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,3-dioxane, and 1,4-dioxane. Examples of chain ethers include 1,2-ethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, 1,2-dimethoxyethane, and 1,2-dibutoxyethane. Examples of fluorine-containing chain ethers include HCF2CF2CH2OCF2CF2H, CF3CF2CH2OCF2CF2H, HCF2CF2CH2OCF2CFHCF3, CF3CF2CH2OCF2CFHCF3, CF 13 OCH3, C6F 13 OC2H5, C8F 17 OCH3, C8F 17 OC2H5, CF3CFHCF2CH(CH3)OCF2CFHCF3, HCF2CF2OCH(C2H5)2, HCF2CF2OC4H9, HCF2CF2OCH2CH(C2H5)2, HCF2CF2OCH2CH(CH3)2, etc. Examples of nitriles include acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, and 3-methoxypropionitrile. Examples of amides include N,N-dimethylformamide. Examples of lactams include N-methylpyrrolidinone, N-methyloxazolidinone, and N,N'-dimethylimidazolidinone.

[0137] The non-aqueous solvent preferably contains at least one selected from the group consisting of cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, and fluorine-containing chain carbonates. In this case, the total proportion of the cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, and fluorine-containing chain carbonates is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, based on the total amount of the non-aqueous solvent.

[0138] The non-aqueous solvent preferably contains at least one selected from the group consisting of cyclic carbonates and chain carbonates. In this case, the total proportion of the cyclic carbonates and chain carbonates in the non-aqueous solvent is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, based on the total amount of the non-aqueous solvent.

[0139] The content of the non-aqueous solvent is preferably 99% by mass or less, more preferably 97% by mass or less, and further preferably 90% by mass or less, based on the total amount of the non-aqueous electrolyte. The content of the non-aqueous solvent is preferably 60% by mass or more, and more preferably 70% by mass or more, based on the total amount of the non-aqueous electrolyte.

[0140] The intrinsic viscosity of the non-aqueous solvent is preferably 10.0 mPa·s or less at 25° C., from the viewpoint of further improving the dissociation property of the electrolyte and the mobility of ions.

[0141] (4.4.3) Electrolyte Additives The non-aqueous solvent may contain an electrolyte additive. This can make it difficult for side reactions, which are not the original battery reactions, to proceed during the charge / discharge cycle of the lithium-ion secondary battery. The battery reaction is a reaction in which lithium ions enter and exit (intercalate) the positive electrode and the negative electrode. Side reactions include a reductive decomposition reaction of the non-aqueous electrolyte by the negative electrode, an oxidative decomposition reaction of the non-aqueous electrolyte by the positive electrode, and elution of metal elements from the positive electrode active material. The electrolyte additive is not particularly limited, and any known additive can be used. For example, the additives described in JP-A-2019-153443 can be used.

[0142] (5) An example of a lithium-ion secondary battery An example of a lithium ion secondary battery according to an embodiment of the present disclosure will be specifically described with reference to Figures 1 and 2. Figure 1 is a cross-sectional view of a lithium ion secondary battery 1 according to an embodiment of the present disclosure. Figure 2 is a cross-sectional view of a positive electrode 11 in the lithium ion secondary battery 1 according to an embodiment of the present disclosure. Figure 3 is a cross-sectional view of a negative electrode 12 in the lithium ion secondary battery 1 according to an embodiment of the present disclosure.

[0143] A lithium ion secondary battery 1 according to an embodiment of the present disclosure is a laminated type. As shown in FIG. 1 , the lithium ion secondary battery 1 includes a battery element 10, a positive electrode lead 21, a negative electrode lead 22, and an exterior body 30. The battery element 10 is enclosed inside the exterior body 30. The exterior body 30 is formed of a laminate film. The positive electrode lead 21 and the negative electrode lead 22 are each attached to the battery element 10. The positive electrode lead 21 and the negative electrode lead 22 are each led out in opposite directions from the inside to the outside of the exterior body 30.

[0144] As shown in FIG. 1, the battery element 10 is formed by laminating a positive electrode 11, a separator 13, and a negative electrode 12. 2, positive electrode 11 is formed by forming positive electrode composite layer 11B on both main surfaces of undercoat layer-attached positive electrode current collector 11A. Undercoat layer-attached positive electrode current collector 11A is formed by forming undercoat layer 111 on both main surfaces of positive electrode current collector 110. 3, negative electrode 12 is formed by forming negative electrode composite layer 12B on both main surfaces of undercoat layer-equipped negative electrode current collector 12A. Undercoat layer-equipped negative electrode current collector 12A is formed by forming undercoat layer 121 on both main surfaces of negative electrode current collector 120. As shown in FIG. 1, a positive electrode composite layer 11B formed on one main surface of a positive electrode current collector 11A with an undercoat layer of a positive electrode 11 and a negative electrode composite layer 12B formed on one main surface of a negative electrode current collector 12A with an undercoat layer of a negative electrode 12 adjacent to the positive electrode 11 face each other via a separator 13.

[0145] A non-aqueous electrolyte solution is injected inside the exterior body 30. The non-aqueous electrolyte solution permeates the positive electrode mixture layer 11B, the separator 13, and the negative electrode mixture layer 12B. In the lithium ion secondary battery 1, one cell layer 14 is formed by adjacent positive electrode mixture layer 11B, separator 13, and negative electrode mixture layer 12B.

[0146] In this embodiment, the lithium ion secondary battery 1 is a stacked type, but the present disclosure is not limited thereto, and the lithium ion secondary battery 1 may be, for example, a wound type. A wound type battery is formed by stacking a positive electrode, a separator, a negative electrode, and a separator in this order and winding them into a layered configuration. The wound type battery may be a cylindrical type or a rectangular type. 1 , the directions in which the positive electrode lead 21 and the negative electrode lead 22 protrude from the inside to the outside of the exterior body 30 are opposite directions relative to the exterior body 30, but the present disclosure is not limited thereto. For example, the directions in which the positive electrode lead and the negative electrode lead protrude from the inside to the outside of the exterior body 30 may be the same direction relative to the exterior body 30.

[0147] (6) Method for producing the composition The method for producing a composition according to the present disclosure includes mixing a conductive carbon material (A), an olefin-based resin (B), a binder resin (C) other than the olefin-based resin (B), and a synthetic rubber (D) to prepare a mixture (hereinafter also referred to as a "mixing step"), and dispersing the mixture using a dispersing roll (hereinafter also referred to as a "dispersing step"). The mixing step and the dispersing step are carried out in this order. As long as they are carried out in this order, there may be other steps (for example, a step of kneading the mixture) between the mixing step and the dispersing step.

[0148] The method for producing a composition according to the present disclosure has the above-described configuration, and therefore provides a composition that can be used to produce a lithium ion secondary battery that is excellent in safety and in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed.

[0149] (6.1) Mixing process In the mixing step, the conductive carbon material (A), the olefin-based resin (B), the binder resin (C), and the synthetic rubber (D) are mixed to prepare a mixture. The mixing method is not particularly limited and may be any known method. The amounts of the conductive carbon material (A), the olefin-based resin (B), the binder resin (C), the synthetic rubber (D), and the additive (E) are not particularly limited and may be the same as the amounts exemplified for the contents of the conductive carbon material (A), the olefin-based resin (B), the binder resin (C), the synthetic rubber (D), and the additive (E) in the composition of the second embodiment.

[0150] (6.2) Dispersion process In the dispersion step, the mixture is dispersed using a dispersion roll. This results in a composition in which the raw materials are more dispersed than in the mixture before the dispersion step. As a result, the resulting composition can be used to form a lithium ion secondary battery in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed compared to when the dispersion step is not performed. The process of dispersing the mixture using a dispersing roll is also called a “dispersion process.” The mixture may be kneaded in advance before the dispersion process.

[0151] The dispersion method is not particularly limited, and examples thereof include a method using a kneading device. Examples of kneading devices include a kneader, a two-roll mill, a three-roll mill, a planetary mixer, a ball mill, a horizontal sand mill, a vertical sand mill, an annular bead mill, an attritor, a high-shear mixer, a Disper, a Filmix, an ultrasonic wave, and a high-pressure homogenizer. Of these, a three-roll mill is preferred as the kneading device from the viewpoint of dispersing the fine particles more uniformly in the mixture. When a kneading device is used, methods for dispersing a mixture include batch dispersion, pass dispersion, and circulation dispersion. Two or more methods for dispersing a mixture may be combined. "Batch dispersion" refers to a method for dispersing a mixture using only a kneading device without using piping or the like. "Pass dispersion" refers to a method for dispersing a mixture by passing the mixture through the kneading device using a kneading system. The kneading system comprises a kneading device, a first tank that supplies the mixture to the kneading device via piping, and a second tank that receives the mixture after the kneading process. "Circulation dispersion" refers to a method for dispersing the mixture by returning the mixture in the second tank after the kneading process to the first tank and circulating it.

[0152] (6.2.1) Three-roll mill A three-roll mill 90 according to an embodiment will be described with reference to Fig. 4. In Fig. 4, the reference symbol D1 indicates the rotation direction of the first roll 91 (hereinafter also referred to as the "feed roll 91"). The reference symbol D2 indicates the rotation direction of the second roll 92 (hereinafter also referred to as the "intermediate roll 92"). The reference symbol D3 indicates the rotation direction of the third roll 93 (hereinafter also referred to as the "finishing roll 93").

[0153] As shown in Fig. 4, the three-roll mill 90 includes a feed roll 91, an intermediate roll 92, a finishing roll 93, and a blade 94. The feed roll 91 and the intermediate roll 92 are arranged with a first distance L1 (see Fig. 4) between them. The intermediate roll 92 and the finishing roll 93 are arranged with a second distance L2 (see Fig. 4) between them. The blade 94 is arranged so as to come into contact with the surface of the finishing roll 93.

[0154] In the three-roll mill 90, as shown in Fig. 4, the mixture 1110 is stored in the upper space formed by the feed roll 91 and the intermediate roll 92. The mixture 1110 adhering to the surface of the feed roll 91 is transferred to the surface of the intermediate roll 92. The mixture 1110 adhering to the surface of the intermediate roll 92 is transferred to the surface of the finishing roll 93. The mixture 1110 adhering to the surface of the finishing roll 93 is scraped off by the blade 94.

[0155] The feed roll 91, the intermediate roll 92, and the finishing roll 93 may each be a roll used in a known three-roll mill. The feed roll 91, the intermediate roll 92, and the finishing roll 93 each have approximately the same diameter. The blade 94 may be, for example, a doctor blade.

[0156] The rotation speeds of the feed roll 91, the intermediate roll 92, and the finishing roll 93 are not particularly limited, and may be faster in the order of the feed roll 91, the intermediate roll 92, and the finishing roll 93. The rotation speed of the feed roll 91 is not particularly limited but is preferably 5 rpm to 84 rpm, more preferably 16 rpm to 56 rpm. The rotation speed of the intermediate roll 92 is not particularly limited but is preferably 16 rpm to 240 rpm, more preferably 50 rpm to 160 rpm. The rotation speed of the finishing roll 93 is not particularly limited but is preferably 50 rpm to 600 rpm, more preferably 150 rpm to 400 rpm. The ratio of the rotation speed of the intermediate roll 92 to the rotation speed of the charge roll 91 (rotation speed of the intermediate roll 92 / rotation speed of the charge roll 91) is preferably 2.0 to 4.0, more preferably 2.5 to 3.5, from the viewpoint of uniformly dispersing the fine particles in the mixture. The ratio of the rotation speed of the finishing roll 93 to the rotation speed of the charge roll 91 (rotation speed of the finishing roll 93 / rotation speed of the charge roll 91) is preferably 5.0 to 12.0, more preferably 6.5 to 10.0, from the viewpoint of uniformly dispersing the fine particles in the mixture.

[0157] The first distance L1 is not particularly limited, and is preferably 0 mm to 0.05 mm, more preferably 0 mm to 0.02 mm, from the viewpoint of uniformly dispersing the fine particles in the mixture. The second distance L2 is not particularly limited, and is preferably 0 mm to 0.05 mm, more preferably 0 mm to 0.02 mm, from the viewpoint of uniformly dispersing the fine particles in the mixture.

[0158] The number of dispersion treatments (hereinafter also referred to as "number of treatments") may be one or more. In particular, the number of treatments is preferably three. When the number of treatments is three, the conductive carbon material (A) and the olefin resin (B) in the mixture are more likely to be uniformly dispersed than when the number of treatments is two or less or four or more. As a result, a composition is obtained that can be used to form a lithium ion secondary battery in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are further suppressed. [Example]

[0159] Hereinafter, embodiments of the present disclosure will be described in detail with reference to examples, but the present disclosure is not limited to the description of these examples.

[0160] The products used in the examples and comparative examples are as follows. The physical properties of each product are catalog values. <Conductive carbon material (A)> Super-P: "Super-P" manufactured by TIMCAL (conductive carbon black, solid content: 100% by mass) KS-6: TIMREX's "KS-6" (flake graphite, solid content: 100% by mass) <Water-dispersible olefin resin (B)> W300: "Chemipearl (registered trademark) W300" manufactured by Mitsui Chemicals, Inc. (aqueous dispersion of low molecular weight polyethylene, solid content: 40% by mass, particle size: 3.0 μm, softening point (ring and ball method): 132°C) W401: "Chemipearl (registered trademark) W401" manufactured by Mitsui Chemicals, Inc. (aqueous dispersion of low molecular weight polyethylene, solid content: 40% by mass, particle size: 1.0 μm, softening point (ring and ball method): 110°C) W700: "Chemipearl (registered trademark) W700" manufactured by Mitsui Chemicals, Inc. (aqueous dispersion of low molecular weight polyethylene, solid content: 40% by mass, particle size: 1.0 μm, softening point (ring and ball method): 132°C) W900: "Chemipearl (registered trademark) W900" manufactured by Mitsui Chemicals, Inc. (aqueous dispersion of low molecular weight polyethylene, solid content: 40% by mass, particle size: 0.6 μm, softening point (ring and ball method): 132°C) W950: "Chemipearl (registered trademark) W950" manufactured by Mitsui Chemicals, Inc. (aqueous dispersion of low molecular weight polyethylene, solid content: 40% by mass, particle size: 0.6 μm, softening point (ring and ball method): 113°C) WP100: "Chemipearl (registered trademark) WP100" manufactured by Mitsui Chemicals, Inc. (aqueous dispersion of low molecular weight polypropylene, solid content concentration: 40% by mass, particle size: 1.0 μm, softening point (ring and ball method): 148°C) P301W: Mitsui Chemicals' P301W (low molecular weight polyethylene, solid content: 100% by mass, particle size: 3.0 μm, softening point (ring and ball method): 132°C) <Binder resin (C)> CMC: "2200" manufactured by Daicel Miraize Co., Ltd. (sodium carboxymethyl cellulose, solid content: 100% by mass) PVDF: Kureha Corporation's "W#7200" (vinylidene fluoride resin, solid content: 100% by mass) <Synthetic rubber (D)> SBR: JSR Corporation's "TRD2001" (aqueous dispersion of styrene butadiene rubber particles, solid content: 50% by mass) <Additive (E)> MC: Matsumoto Microsphere (registered trademark) FN-100SS (microcapsules, solid content: 100% by mass) manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd. Al2O3: "Aluminum oxide" manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (average particle size: 2 μm, solid content: 100% by mass)

[0161] [1] Preparation of a positive electrode current collector with an undercoat layer [1.1] Preparation of slurry for undercoat layer (Example 1-1) A 5 L planetary dispenser was used to prepare the undercoat layer slurry. 29 parts by mass of "Super-P" (A) and 116.0 parts by mass of "MC" (E) were mixed for 10 minutes to obtain a mixture (preparation step A). To the mixture, 564.0 parts by mass of "CMC aqueous solution" (C) was added and mixed for a further 20 minutes to obtain a first mixture (preparation step B). The "CMC aqueous solution" (C) was prepared by adding "CMC" (C) to distilled water so that the content of "CMC" (C) was 1.2% by mass relative to the total amount of "CMC aqueous solution" (C). The first mixed solution was added with 215.0 parts by mass of "CMC aqueous solution" (C) and mixed for 20 minutes. This addition operation was repeated three times to obtain a second mixed solution (preparation step C). The total amount of "CMC aqueous solution" (C) added to the first mixed solution was 645.0 parts by mass. To the second mixture, 29.0 parts by mass of "SBR" (D) and 290.0 parts by mass of "W401" (B) were added, and the mixture was kneaded for 20 minutes and then vacuum degassed for 30 minutes (preparation step D). In this way, a slurry (composition) for the undercoat layer with a solid content concentration of 17.3% by mass was prepared.

[0162] (Examples 1-2 to 1-14, 1-18, and 1-19) A slurry (composition) for the undercoat layer was prepared in the same manner as in Example 1-1, except that the materials and amounts added in each of the preparation steps A to D were changed as shown in Tables 1 and 2.

[0163] (Examples 1-15 and 1-16) A slurry (composition) for the undercoat layer was prepared in the same manner as in Example 1-1, except that the additive materials and amounts added in each of preparation steps A to D were changed as shown in Table 2, and the addition operation was performed twice in preparation step C.

[0164] [Table 1]

[0165] [Table 2]

[0166] In Table 1, the "total amount added" in the preparation step C indicates the total amount of "aqueous CMC solution (C)" added to the first mixed liquid.

[0167] (Examples 1-17) A 5 L planetary dispenser was used to prepare the undercoat layer slurry. 116.0 parts by mass of "Super-P" (A), 87 parts by mass of "P301W" (B), and 8.7 parts by mass of a vacuum-dried product of "SBR" (D) were mixed for 10 minutes to obtain a third mixed liquid. (Preparation step A). To the third mixed liquid, 50 parts by mass of "N-methylpyrrolidone" (hereinafter referred to as "NMP") was added, and the mixture was further mixed for 10 minutes to obtain a fourth mixed liquid (preparation step B). To the fourth mixed solution, 250 parts by mass of "PVDF solution" (C) was added and kneaded for 25 minutes, after which 400 parts by mass of "PVDF solution" (C) was added and kneaded for 25 minutes, and then 329 parts by mass of "PVDF solution" (C) was added and kneaded for 25 minutes to obtain a fifth mixed solution (preparation step C). The "PVDF solution" (C) was prepared by adding "PVDF" (C) to "NMP" so that the content of "PVDF" (C) was 8% by mass relative to the total amount of "PVDF solution" (C). To adjust the viscosity, 75 parts by mass of NMP was added to the fifth mixed solution and mixed for 15 minutes, and then 75 parts by mass of NMP was added and mixed for 15 minutes, followed by vacuum degassing for 30 minutes (Preparation step D). In this way, a slurry (composition) for the undercoat layer having a solid content concentration of 20.9% by mass was prepared.

[0168] Example 2-1 A 5 L planetary dispenser was used to prepare the undercoat layer slurry. A three-roll mill (BR-300HCVIII, manufactured by Imex Co., Ltd.) was used for dispersing the slurry for the undercoat layer. 58 parts by mass of "Super-P" (A) was pre-mixed for 5 minutes (preparation step A). 229.0 parts by mass of "CMC aqueous solution" (C) was added to "Super-P" (A) powder and mixed for another 20 minutes to obtain a sixth mixture (preparation step B). "CMC aqueous solution" (C) was prepared by adding "CMC" (C) to distilled water so that the content of "CMC" (C) was 1.2% by mass relative to the total amount of "CMC aqueous solution" (C). The sixth mixed solution was added with 327.0 parts by mass of "CMC aqueous solution" (C) and mixed for 20 minutes. This addition operation was repeated three times to obtain a seventh mixed solution (preparation step C). The total amount of "CMC aqueous solution" (C) added to the sixth mixed solution was 981.0 parts by mass. To the seventh mixed liquid, 29.0 parts by mass of "SBR" (D) and 507.5 parts by mass of "W300" (B) were added, and the mixture was kneaded for 20 minutes and vacuum degassed for 30 minutes to obtain an eighth mixed liquid (preparation step D). The eighth mixed solution was dispersed twice using the three-roll mill shown in Figure 4 under conditions of a roll gap of 10 μm and a roll rotation speed of 300 rpm (gear ratio 1:3:9) (Preparation step E). That is, the rotation speed of the charge roll 91 was 33.3 rpm, the rotation speed of the intermediate roll 92 was 100 rpm, the rotation speed of the finishing roll 93 was 300 rpm, the first gap L1 was 10 μm, and the second gap L2 was 10 μm. In this way, a slurry (composition) for an undercoat layer with a solid content concentration of 16.1 mass% was prepared.

[0169] (Examples 2-2 to 2-9) A slurry (composition) for the undercoat layer was prepared in the same manner as in Example 2-1, except that the additive materials, additive amounts, and three-roll mill conditions of the three-roll mill in preparation steps A to E were changed as shown in Table 7.

[0170] [Table 3]

[0171] (Comparative Example 1) A 5 L planetary dispenser was used to prepare the undercoat layer slurry. Add 50 parts by mass of "NMP" to 116.0 parts by mass of "Super-P" (A) and mix for 10 minutes. After mixing for 1 minute, a mixed solution was obtained. To the mixture, 250 parts by mass of "PVDF solution" (C) was added and kneaded for 25 minutes, after which 800 parts by mass of "PVDF solution" (C) was added and kneaded for 25 minutes, and then 1,125 parts by mass of "PVDF solution" (C) was added and kneaded for 25 minutes, followed by vacuum degassing for 30 minutes. "PVDF solution" (C) was prepared by adding "PVDF" (C) to "NMP" so that the content of "PVDF" (C) was 8% by mass of the total amount of "PVDF solution" (C). In this way, a slurry (composition) for the undercoat layer having a solid content concentration of 12.4 mass % was prepared.

[0172] (Comparative Example 2) A 5 L planetary dispenser was used to prepare the undercoat layer slurry. 232 parts by mass of "Super-P" (A) was added to 902.0 parts by mass of "CMC aqueous solution" (C) and mixed for an additional 20 minutes to obtain a mixed solution. "CMC aqueous solution" (C) was prepared by adding "CMC" (C) to distilled water so that the content of "CMC" (C) was 1.2% by mass based on the total amount of "CMC aqueous solution" (C). The mixed solution was added with 505.0 parts by mass of "CMC aqueous solution" (C) and mixed for 20 minutes. This addition operation was repeated three times to obtain a mixed solution. The total amount of "CMC aqueous solution" (C) added to the mixed solution was 1,516 parts by mass. To the mixed liquid, 58.0 parts by mass of "SBR" (D) was added, and the mixture was kneaded for 20 minutes and then vacuum degassed for 30 minutes to prepare a slurry (composition) for the undercoat layer with a solid content concentration of 10.7% by mass.

[0173] (Comparative Example 3) A slurry (composition) for the undercoat layer was prepared in the same manner as in Example 1-17, except that the additive materials and amounts added in each of preparation steps A to D were changed as shown in Table 3, and the addition operation was performed once in preparation step D.

[0174] [Table 4]

[0175] (Comparative Examples 4 to 10) Except for changing the materials and amounts added in each of the preparation steps A to D as shown in Table 4, a slurry (composition) for the undercoat layer was prepared in the same manner as in Example 1-1.

[0176] [Table 5]

[0177] [1.2] Coating and drying of undercoat layer A die coater was used to apply the undercoat layer slurry. The coating thickness after drying is 3 μm (coating weight is approximately 0.2 mg / cm 2 The slurry for the undercoat layer was applied to one main surface of an aluminum foil (thickness 20 μm, width 200 mm, positive electrode current collector) and dried so that the thickness after drying was 3 μm. Next, the slurry for the undercoat layer was similarly applied to the other main surface (uncoated surface) of the aluminum foil and dried so that the thickness after drying was 3 μm. In this way, an aluminum foil roll (positive electrode current collector with undercoat layer) was obtained, with undercoat layers coated on both sides.

[0178] [2] Positive electrode preparation [2.1] Preparation of cathode composite slurry A 5 L planetary dispenser was used to prepare the positive electrode mixture slurry. "NCM523" (manufactured by Umicore, composition formula: LiNi) as the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2) 1520 mass parts and "Super-P" as a conductive additive 30 parts by mass of "KS-6" (flake graphite manufactured by TIMREX) as a conductive additive were mixed for 10 minutes to obtain a mixture for a positive electrode. 50 parts by mass of NMP was added to the positive electrode mixture and mixed for 20 minutes to obtain a first positive electrode mixture. To the first positive electrode mixture, 350 parts by mass of the "PVDF solution" was added and kneaded for 30 minutes, after which 260 parts by mass of the "PVDF solution" was added and kneaded for 15 minutes, and then 220 parts by mass of the "PVDF solution" was added and kneaded for 15 minutes to obtain a second positive electrode mixture. The "PVDF solution" was prepared by adding "PVDF" to "NMP" so that the "PVDF" content was 8 mass% of the total amount of the "PVDF solution." To adjust the viscosity, 80 parts by mass of "NMP" was added to the second positive electrode mixture and mixed for 30 minutes, followed by vacuum degassing for 30 minutes. In this way, a positive electrode mixture slurry with a solid content concentration of 65 mass % was prepared.

[0179] [2.2] Coating and drying A die coater was used to apply the positive electrode mixture slurry. The mass of the positive electrode mixture layer (coating film after drying) is 19.0 mg / cm 2 The positive electrode composite slurry was applied to one main surface (i.e., the undercoat layer) of a positive electrode current collector with an undercoat layer (aluminum foil thickness: 20 μm, thickness of one undercoat layer: 3 μm, width: 200 mm) and dried so that the mass of the positive electrode composite layer (coated film after drying) became 19.0 mg / cm. 2 The positive electrode mixture slurry was similarly applied to the other main surface (that is, the undercoat layer) of the positive electrode current collector with the undercoat layer so that the thickness became as shown in the figure, and then dried. The positive electrode roll thus obtained (the total amount of coating on both sides was 38.0 mg / cm 2 ) was dried in a vacuum drying oven at 130°C for 12 hours.

[0180] [2.3] Press A 35-ton press was used to press the positive electrode roll. The gap between the upper and lower rolls was adjusted to press the positive electrode to a density of 2.9±0.05 g / cm. 3 The mixture was pressed using a 35-ton press machine to obtain a sheet of 1000g.

[0181] [2.4] Slit The positive electrode roll was slit to obtain the area of ​​the positive electrode composite layer (front surface: 56 mm × 334 mm, back surface: 56 mm × 408 mm) and an area for tab welding margins, thereby obtaining a positive electrode in which an undercoat layer was laminated on aluminum foil.

[0182] [3] Negative electrode production [3.1] Preparation of negative electrode mixture slurry A 5 L planetary dispenser was used to prepare the negative electrode mixture slurry. 1050 parts by mass of "natural graphite" as the negative electrode active material and "Super-P" (conductive carbon, BET specific surface area 62 m) as the conductive additive. 2 / g) and 11 parts by mass were mixed for 10 minutes. A mixture for a negative electrode was obtained. To the negative electrode mixture, 450 parts by mass of the "CMC aqueous solution" was added, and the mixture was further mixed for 20 minutes to obtain a first negative electrode mixture. To the first negative electrode mixed solution, 150 parts by mass of the "CMC aqueous solution" was added and mixed for an additional 30 minutes, then 293.5 parts by mass of the "CMC aqueous solution" was added and mixed for an additional 30 minutes, and then 450 parts by mass of water as a solvent was added and mixed for 15 minutes to obtain a second negative electrode mixed solution. To the second negative electrode mixture, 45 parts by mass of "SBR aqueous solution" (manufactured by JSR Corporation, solid content: 50% by mass) was added, and the mixture was kneaded for 15 minutes, followed by vacuum degassing for 10 minutes. In this way, a negative electrode mixture slurry with a solid content concentration of 45 mass % was prepared.

[0183] [3.2] Coating and drying The negative electrode mixture slurry was applied using a die coater. The mass of the negative electrode mixture layer (coating film after drying) is 11.0 mg / cm 2 The negative electrode composite slurry was applied to one main surface of a copper foil (thickness 10 μm, negative electrode current collector) and dried so that the mass of the negative electrode composite layer (coated film after drying) became 11.0 mg / cm. 2 The negative electrode composite slurry was applied to the other main surface of the copper foil and dried. 2) was dried in a vacuum drying oven at 120°C for 12 hours.

[0184] [3.3] Press A small press was used to press the negative electrode roll. The gap between the upper and lower rolls was adjusted to press the negative electrode roll to a density of 1.45±0.05 g / cm. 3 The mixture was pressed using a small press to obtain a final product.

[0185] [3.4] Slit The negative electrode roll was slit to obtain a negative electrode having an area for the negative electrode composite layer (front surface: 58 mm×372 mm, back surface: 58 mm×431 mm) and an area for a tab welding margin.

[0186] [4] Fabrication of a wound battery (design capacity 1Ah) [4.1] Winding The separator used was a porous polyethylene film (60.5 mm x 450 mm) with a porosity of 45% by volume and a thickness of 25 μm. The negative electrode, separator, positive electrode with an undercoat layer, and separator obtained above were stacked and wound together, and then press-molded. Next, an aluminum tab was bonded to the margin of the positive electrode with the undercoat layer using an ultrasonic bonding machine, and a nickel tab was bonded to the margin of the negative electrode using an ultrasonic bonding machine. This was sandwiched between laminate films, and three sides were heat-sealed. This resulted in an exterior body with an opening (hereinafter referred to as the "first exterior body").

[0187] [4.2] Injection of non-aqueous electrolyte Ethylene carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:EMC:DMC = 3:3:4 to obtain a mixed solvent, and LiPF6 was dissolved in the mixed solvent to a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte. Before injecting the nonaqueous electrolyte, the first exterior body was dried under reduced pressure at 70°C for 12 hours in a vacuum dryer. After injecting 4.7±0.1 g of the electrolyte into the first exterior body, the opening of the first exterior body was heat-sealed while drawing a vacuum. This produced a lithium-ion secondary battery precursor.

[0188] [4.3] Activation process The lithium ion secondary battery precursor was kept at room temperature (25°C) for 24 hours. Next, the lithium ion secondary battery precursor was charged at a constant current of 0.05C for 4 hours (0.05C-CC) and then rested for 12 hours. It was then charged at a constant current and constant voltage of 0.1C to 4.2V (0.1C-CCCV), rested for 30 minutes, and then discharged at a constant current of 0.1C to 2.8V (0.1C-CC). Furthermore, a charge-discharge cycle (charging to 4.2V at 0.1C-CCCV and discharging to 2.8V at 0.1C-CC) was repeated five times. The lithium ion secondary battery precursor was then stored at 25°C for five days in a fully charged state of 4.2V (SOC 100%). In this way, wound-type batteries (lithium ion secondary batteries) of Examples 1-1 to 1-19, Comparative Examples 1 to 10, and Reference Example were obtained.

[0189] [5] Evaluation method The following particle size distribution measurements were carried out on the slurries for the undercoat layer of Examples 1-1 to 1-19, Examples 2-1 to 2-9, Comparative Examples 1 to 10, and Reference Example. The following forced internal short circuit test, initial DCIR measurement, and DCIR evaluation after high-temperature storage were performed on the wound-type batteries of Examples 1-1 to 1-19, Examples 2-1 to 2-9, Comparative Examples 1 to 10, and Reference Example. The evaluation results are shown in Table 3.

[0190] [5.1] Particle size distribution measurement 5] Evaluation method 1.0 g of the undercoat layer slurry (hereinafter also referred to as "sample") of Examples 1-1 to 1-19, 2-1 to 2-9, Comparative Examples 1 to 10, and the Reference Example was placed in a 100 ml beaker with 50 ml of water. The sample was crushed with a spatula and gently mixed with a dropper. The sample was then dispersed in water using a tabletop ultrasonic cleaner (Honda Electronics Co., Ltd., Model W-113). The output of the tabletop ultrasonic cleaner was set to 100 W and 28 kHz, and the dispersion was carried out for 60 seconds. After that, bubbles formed on the surface of the aqueous solution in the beaker were removed, and the volumetric particle size distribution of the sample was measured using a laser light diffraction scattering method with a particle size distribution analyzer (Microtrac MT3300EXII, Microtrac-Bell Corporation). From this measurement, the D10, D50, D90, and D99 in the volumetric particle size distribution were determined. The measurement results for D10, D50, D90 and D99 are shown in Table 7.

[0191] [5.2] Forced internal short circuit test (nail penetration test) A wound battery (design capacity 1 Ah) was charged at a constant current and constant voltage (0.1 C-CCCV) at 0.1 C up to 4.2 V in a temperature environment of 25°C, and a nail penetration test was conducted (nail diameter 3 mm, nail penetration speed 1.0 mm / sec). A nail with a diameter of 3 mm was inserted into the center of the wound battery (cell) at a speed of 1.0 mm / sec, short-circuiting the positive and negative electrodes inside the battery container. The short-circuit behavior of the battery was then observed. The nail penetration test was carried out on the same batteries multiple times (3 to 6 times), and the total number of test batteries was evaluated according to the following criteria: Acceptable ratings for the nail penetration test are "A," "B," and "C."

[0192] A: The percentage of batteries whose internal temperature exceeded 300°C was less than 20% of the total number of tested batteries. B: The percentage of batteries whose internal temperature exceeded 300°C was 20% or more and less than 50% of the total number of test batteries. C: The percentage of batteries whose internal temperature exceeded 300°C was 50% or more and less than 80% of the total number of test batteries. D: The percentage of batteries whose internal temperature exceeded 300°C was 80% or more of the total number of test batteries.

[0193] [5.3] Initial DCIR Assessment [5.3.1] Initial DCIR measurement A wound battery (design capacity 1 Ah) was charged at a constant current and constant voltage (0.1 C-CCCV) up to 4.2 V in a temperature environment of 25°C, and the initial DCIR was measured. The battery was discharged at a constant current of 0.1 C for 10 seconds (0.1 C-CC-10 s), and then charged at a constant current of 0.1 C for 10 seconds (0.1 C-CC-10 s). Next, the battery was subjected to constant current discharge at 0.2 C for 10 seconds (0.2 C-CC-10 s), and constant current charge at 0.2 C for 10 seconds (0.2 C-CC-10 s). Next, the battery was discharged at a constant current of 0.5 C for 10 seconds (0.5 C-CC-10 s), and then charged at a constant current of 0.5 C for 10 seconds (0.5 C-CC-10 s). Next, the battery was discharged at a constant current of 1.0 C for 10 seconds (1.0 C-CC-10 s), and then charged at a constant current of 1.0 C for 10 seconds (1.0 C-CC-10 s). Next, the battery was discharged at a constant current of 2.0 C for 10 seconds (2.0 C-CC-10s), and then charged at a constant current of 2.0 C for 10 seconds (2.0-CC-10s). The first DC resistance (DCIR) was calculated based on each voltage drop (= voltage before discharge started - voltage 10 seconds after discharge started) due to "CC10s discharge" at each discharge rate of 0.1C to 2.0C and each current value (i.e., each current value corresponding to a discharge rate of 0.1C to 2.0C).

[0194] [5.3.2] Evaluation method Based on the above measured values, a first relative value was calculated with the measured value of the first direct current resistance (DCIR) of the reference example (without undercoat layer) set at 100, and the initial DCIR was evaluated according to the following criteria. The first DC resistance measurements and first relative values ​​are shown in Table 3. Acceptable ratings for the initial DCIR rating are "A," "B," and "C."

[0195] A: The first relative value is 100 or less. B: The first relative value is greater than 100 and less than or equal to 110. C: The first relative value is greater than 110 and less than or equal to 130. D: The first relative value exceeds 130.

[0196] [5.4] DCIR evaluation after high temperature storage [5.4.1] DCIR measurement after high temperature storage The wound battery (design capacity 1 Ah) was charged at 0.1 C to 4.2 V at a constant current and constant voltage (0.1 C-CCCV) in a 25°C environment, and then left in a charged state for 28 days at 60°C to obtain a post-high-temperature storage battery. A DCIR evaluation process similar to the above-described initial DCIR evaluation process was performed to determine the second DC resistance (DCIR).

[0197] [5.4.2] Evaluation method Based on the above measured values, a second relative value was calculated with the measured value of the second direct current resistance (DCIR) of the reference example (without undercoat layer) set at 100, and the DCIR after high-temperature storage was evaluated according to the following criteria. The second measured DC resistance value and the second relative value are shown in Table 3. Acceptable ratings for DCIR rating after high temperature storage are "A", "B" and "C".

[0198] A: The second relative value is 100 or less. B: The second relative value is greater than 100 and less than or equal to 130. C: The second relative value is greater than 130 and less than or equal to 150. D: The second relative value exceeds 150.

[0199] [Table 6]

[0200] [Table 7]

[0201] In Table 6, the amount (mass%) of water-dispersible polyolefin resin (B) added indicates the mass ratio of the solid content of the water-dispersible polyolefin resin (B) to the total of 100 mass% of the conductive carbon material (A), water-dispersible olefin resin (B), binder resin (C), synthetic rubber (D), and additive (E). In Table 5, the amount (mass%) of synthetic rubber (D) added indicates the mass ratio of the solid content of synthetic rubber (D) to 100% by mass, which is the total of the conductive carbon material (A), water-dispersible olefin resin (B), binder resin (C), synthetic rubber (D), and additive (E). In Table 7, in the dispersion step, "-" in the interval (μm) and number of treatments (times) indicates that the dispersion step was not performed. In Table 7, "distance (μm)" indicates each of the first distance L1 (see FIG. 4) and the second distance L2 (see FIG. 4). The first distance L1 and the second distance L2 are the same.

[0202] [6] Evaluation [6.1] Comparative Examples 1 to 10 The composition of Comparative Example 1 did not contain the olefin resin (B) and the synthetic rubber (D). The content of the binder resin (C) was not within the range of 1% by mass to 30% by mass. The composition of Comparative Example 2 did not contain the olefin resin (B), and the content of the conductive carbon material (A) was not within the range of 10% by mass to 60% by mass. Therefore, the evaluation results of the nail penetration test for Comparative Examples 1 and 2 were "D." This result revealed that the compositions of Comparative Examples 1 and 2 could not be used to form lithium ion secondary batteries with excellent safety.

[0203] The composition of Comparative Example 3 did not contain synthetic rubber (D). The content of conductive carbon material (A) was not within the range of 10% by mass to 60% by mass. The content of binder resin (C) was not within the range of 1% by mass to 30% by mass. The mass ratio (A / B) was not within the range of 0.25 to 2.00. Therefore, the evaluation results of the initial DCIR and the DCIR after high-temperature storage for Comparative Example 3 were both "D." This result revealed that the composition of Comparative Example 3 could not be used to form a lithium ion secondary battery in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment were suppressed.

[0204] In the compositions of Comparative Examples 4 to 7, the content of the olefin resin (B) was not within the range of 30% to 70% by mass, and the mass ratio (A / B) was not within the range of 0.25 to 2.00. Therefore, the DCIR evaluation results after high-temperature storage for Comparative Examples 4 and 5 were "D." This result shows that the compositions of Comparative Examples 4 and 5 cannot be used to form lithium ion secondary batteries in which an increase in DC resistance after long-term storage in a high-temperature environment is suppressed. The results of the initial DCIR evaluation and the DCIR evaluation after high-temperature storage of Comparative Example 6 were both "D." This result revealed that the composition of Comparative Example 6 could not be used to form a lithium ion secondary battery in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment were suppressed. The evaluation result of the nail penetration test for Comparative Example 7 was "D." This result revealed that the composition of Comparative Example 7 could not be used to form a lithium ion secondary battery with excellent safety.

[0205] In the compositions of Comparative Examples 8 to 10, the content of the conductive carbon material (A) was not within the range of 10% by mass to 60% by mass, the content of the olefin resin (B) was not within the range of 30% by mass to 70% by mass, and the mass ratio (A / B) was not within the range of 0.25 to 2.00. Therefore, the evaluation results of the nail penetration test for Comparative Examples 8 to 10 were "D." This result showed that the compositions of Comparative Examples 8 to 10 could not be used to form lithium ion secondary batteries with excellent safety.

[0206] From the above, it was found that the compositions of Comparative Examples 1 to 10 could not be used to form lithium ion secondary batteries that are excellent in safety and have suppressed DC resistance during initial charging and discharging and an increase in DC resistance after long-term storage in a high-temperature environment.

[0207] [6.2] Examples 1-1 to 1-19 The compositions of Examples 1-1 to 1-19 contained 10% by mass to 60% by mass of a conductive carbon material (A), 30% by mass to 70% by mass of an olefin resin (B), 1% by mass to 30% by mass of a binder resin (C), 1% by mass to 20% by mass of a synthetic rubber (D), and 0% by mass to 50% by mass of an additive (E), with a mass ratio (A / B) of 0.25 to 2.00. Therefore, the evaluation results of the nail penetration test for Examples 1-1 to 1-19 were either "A" to "C." Furthermore, the evaluation results of the initial DCIR evaluation and the DCIR evaluation after high-temperature storage for Examples 1-1 to 1-19 were either "A" to "C." These results demonstrate that the compositions of Examples 1-1 to 1-19 are excellent in safety and can be used to form lithium ion secondary batteries in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed.

[0208] Comparing Examples 1-1 to 1-19, the evaluation results of Examples 1-1 to 1-7 in the nail penetration test were "A," and the evaluation results of the initial DCIR evaluation and the DCIR evaluation after high-temperature storage were "B" or "C." Therefore, it was found that the compositions of Examples 1-1 to 1-7 could be used to form lithium ion secondary batteries that have a good balance between safety and battery performance. In particular, the results of the nail penetration test and the relative values ​​of the initial DCIR evaluation and the DCIR evaluation after high-temperature storage revealed that the compositions of Examples 1-4 to 1-7 could be used to form lithium ion secondary batteries with a better balance between safety and battery performance. In the compositions of Examples 1-4 to 1-7, the content of the olefin resin (B) was 50% by mass to 60% by mass, and the mass ratio (A / B) was 0.40 to 0.60. In particular, it was found that the composition of Example 1-4 can be used to produce a lithium ion secondary battery with an excellent balance between safety and battery performance. This is presumably because, among Examples 1-4 to 1-7, the particle size of the olefin resin (B) is relatively small (e.g., 0.5 μm or more and 2.0 μm or less) and the softening point of the olefin resin (B) is relatively high (e.g., 120°C or more and 135°C or less). When the particle size of the olefin resin (B) is relatively small, the film thickness of the undercoat layer can be kept within an optimum range. When the softening point of the olefin resin (B) is relatively high, the shape of the olefin resin (B) can be maintained during the positive electrode drying process.

[0209] Comparing Examples 1-12 to 1-19, it was found that as the content of the conductive carbon material (A) increased, the evaluation results of the initial DCIR evaluation and the DCIR evaluation after high-temperature storage approached "A," and the battery performance of the lithium ion secondary battery tended to become better.

[0210] [6.3] Examples 2-1 to 2-9 The compositions of Examples 2-1 to 2-9 contained a conductive carbon material (A), an olefin resin (B), a binder resin (C), and a synthetic rubber (D), and had a ratio (D99 / D10) of 35 or less. Therefore, the evaluation results of the nail penetration test for Examples 2-1 to 2-9 were "A" or "B." Furthermore, the evaluation results of the initial DCIR evaluation and the DCIR evaluation after high-temperature storage for Examples 2-1 to 2-9 were "A" or "B." These results demonstrate that the compositions of Examples 2-1 to 2-9 are excellent in safety and can be used to form lithium ion secondary batteries in which the DC resistance at the initial stage of charge / discharge and the increase in DC resistance after long-term storage in a high-temperature environment are suppressed. In addition, it was found that the compositions of Examples 2-1 to 2-9 can be used to produce lithium ion secondary batteries with superior safety and battery performance. This is presumably because, by carrying out the dispersion step, the conductive carbon material (A) and the olefin-based resin (B) are less likely to aggregate and are uniformly dispersed in the composition, which makes it easier for the conductive carbon material (A) to function as a conductive assistant and for the olefin-based resin (B) to exhibit its shutdown function.

[0211] Comparing Examples 2-1 to 2-9, Examples 2-1, 2-2, 2-8, and 2-9 showed that, despite the fact that the amount of conductive carbon material (A) functioning as a conductive additive was small (i.e., the content of conductive carbon material (A) was 25% by mass or less), the initial DCIR evaluation result was "A" and the DCIR evaluation result after high-temperature storage was "B." In other words, it was found that the battery performance of the lithium ion secondary battery was excellent. This is presumably because, by performing the dispersion step, the conductive carbon material (A) was uniformly dispersed in the composition even when the amount of conductive carbon material (A) added was small, and the conductive carbon material (A) functioned efficiently as a conductive additive.

[0212] The disclosure of Japanese Patent Application No. 2021-205441, filed on December 17, 2021, is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. relative to the total amount of the conductive carbon material (A), the olefin-based resin (B), the binder resin (C) which is a resin other than the olefin-based resin (B), the synthetic rubber (D), and the additive (E), 10% by mass to 60% by mass of the conductive carbon material (A); 35% by mass to 70% by mass of the olefin-based resin (B); 1% by mass to 30% by mass of the binder resin (C); 1% by mass to 20% by mass of the synthetic rubber (D); The additive (E) is contained in an amount of 0% by mass to 50% by mass, the olefin-based resin (B) is a water-dispersible olefin-based resin, the water-dispersible olefin-based resin is a polyethylene-based resin or a polypropylene resin, the additive (E) is at least one of a thermally expandable microcapsule having a maximum volume expansion temperature of 70°C or more and 180°C or less, an inorganic oxide filler, a curable resin filler, and a scaly filler; a ratio of the content of the conductive carbon material (A) to the content of the olefin resin (B) being 0.25 to 2.00;

2. The composition for a lithium ion secondary battery according to claim 1 , wherein the olefin-based resin (B) comprises a water-dispersible olefin-based resin.

3. 3. The composition for lithium ion secondary batteries according to claim 1, wherein the particle size of the olefin-based resin (B) is 0.1 μm to 9.0 μm, and the softening point of the olefin-based resin (B) is 70° C. or higher.

4. The composition for lithium ion secondary batteries according to any one of claims 1 to 3, wherein the ratio of the content of the synthetic rubber (D) to the content of the olefin-based resin (B) is 0.07 to 0.

25.

5. The composition for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the binder resin (C) contains carboxymethyl cellulose or polyvinylidene fluoride.

6. The composition for lithium ion secondary batteries according to any one of claims 1 to 5, wherein the synthetic rubber (D) contains styrene-butadiene rubber.

7. The composition for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the additive (E) comprises at least one of a thermally expandable microcapsule having a maximum volume expansion temperature of 70°C or higher and 180°C or lower and an inorganic oxide filler.

8. An electrode for a lithium ion secondary battery, comprising: a current collector; an undercoat layer containing the composition according to any one of claims 1 to 7; and a composite layer.

9. The electrode for a lithium ion secondary battery according to claim 8 , wherein the current collector, the undercoat layer, and the composite layer are laminated in this order.

10. The electrode for a lithium ion secondary battery according to claim 8 or 9, wherein the electrode is a positive electrode.

11. A lithium ion secondary battery comprising the electrode according to any one of claims 8 to 10.

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