Conductive composition for undercoat layer, current collector with undercoat layer, electrode, and lithium-ion secondary battery

Abstract

To provide a composition which becomes a lithium ion secondary battery with excellent safety and battery performance.SOLUTION: A composition of the present disclosure, contains: a conductive carbon material (A) containing a conductive carbon fiber (A1); an olefin resin (B); a binder resin (C) other than the olefin resin (B); larger than 5 mass% and 30 mass% or less of the conductive carbon fiber (A1); 60 mass% to 90 mass% of the olefin resin (B); one mass% to 30 mass% of the binder resin (C) to a total amount of a synthetic rubber (D); and one mass% to 20 mass% of the synthetic rubber (D). A ratio of a content amount of the conductive carbon material (A) against a content amount of the olefin resin (B) is 0.08 to 0.32.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to compositions, current collectors with undercoat layers, electrodes, and lithium-ion secondary batteries. [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 is a growing demand for high-capacity, high-output, and high-energy-density batteries that can be installed in hybrid vehicles and other applications. Such lithium-ion secondary batteries offer the advantage of high energy density. However, because they utilize lithium metal and lithium ions, sufficient safety measures are necessary.

[0003] Patent Document 1 discloses a conductive composition for forming an underlayer of an electrode for a non-aqueous electrolyte secondary battery, which has the function of increasing the internal resistance when the internal temperature of the battery rises. The conductive composition disclosed in Patent Document 1 contains a conductive carbon material (A), a water-soluble resin (B), water-dispersible resin fine particles (C), and an aqueous liquid medium (D). The water-dispersible resin fine particles (C) include at least olefin-based resin fine particles. Of the total solid content of the conductive composition, the content of the conductive carbon material (A) is 10 to 50% by mass, the content of the water-soluble resin (B) is 10 to 50% by mass, the content of the water-dispersible resin fine particles (C) is 30 to 70% by mass, and the proportion of olefin-based resin fine particles contained in the water-dispersible resin fine particles (C) is 50 to 100% by mass. Specifically in Patent Document 1, carbon black (i.e., "Denka Black HS-100" or "Ketjen Black EC-300J") is used as the conductive carbon material (A). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Specification of Patent No. 5939346 [Overview of the Initiative]

Problems to be Solved by the Invention

[0005] However, in a lithium secondary ion battery formed using the conductive composition disclosed in Patent Document 1, there is a possibility that excellent safety and excellent battery performance cannot be achieved simultaneously.

[0006] In view of the above circumstances, an object of the present disclosure is to provide a composition, a current collector with an undercoat layer, an electrode, and a lithium ion secondary battery that can be used to form a lithium ion secondary battery excellent in safety and battery performance.

Means for Solving the Problems

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

[0008] <1> A conductive carbon material (A) containing conductive carbon fibers (A1), an olefin resin (B), a binder resin (C) which is a resin other than the olefin resin (B), and a synthetic rubber (D), wherein, based on the total amount of the conductive carbon fibers (A1), the olefin resin (B), the binder resin (C), and the synthetic rubber (D), the content of the conductive carbon fibers (A1) is more than 5% by mass and 30% by mass or less, the content of the olefin resin (B) is 60% by mass to 90% by mass, the content of the binder resin (C) is 1% by mass to 30% by mass, the content of the synthetic rubber (D) is 1% by mass to 20% by mass, and a composition in which the ratio of the content of the conductive carbon material (A) to the content of the olefin resin (B) is 0.08 to 0.32. <2> The composition according to <1>, wherein the conductive carbon material (A) further contains conductive carbon particles (A2). <3> The composition according to <1> or <2>, wherein the conductive carbon fibers (A1) are at least one selected from carbon nanotubes and carbon fibers. <4> The content of the conductive carbon material (A) is 7% by mass to 30% by mass with respect to the total amount of the conductive carbon material (A), the olefin resin (B), the binder resin (C), and the synthetic rubber (D), and the composition according to any one of <1> to <3> above. <5> The composition according to any one of <1> to <4> above, wherein the olefin resin (B) contains a polyethylene-based resin or a polypropylene-based resin. <6> The composition according to any one of <1> to <5> above, wherein the olefin resin (B) contains a water-dispersible olefin resin. <7> The composition according to any one of <1> to <6> above, wherein the particle size of the olefin resin (B) is 0.1 μm to 9.0 μm and the softening point of the olefin resin (B) is 70°C or higher. <8> The composition according to any one of <1> to <7> above, wherein the binder resin (C) contains carboxymethyl cellulose or polyvinylidene fluoride. <9> The composition according to any one of <1> to <8> above, wherein the synthetic rubber (D) contains styrene-butadiene rubber. <10> The composition according to any one of <1> to <9> above, which is used for an undercoat layer of an electrode of a lithium-ion secondary battery. <11> The composition according to <10> above, wherein the electrode is a positive electrode. <12> A current collector, An undercoat layer laminated on at least one main surface of the current collector, The undercoat layer is a current collector with an undercoat layer containing the solid content of the composition according to any one of <1> to <11> above. <13> The current collector with an undercoat layer according to <12>, A composite layer laminated on the undercoat layer, An electrode comprising <14> A lithium-ion secondary battery comprising the electrode according to <13>.

Advantages of the Invention

[0009] According to this disclosure, a composition that can be used to make a lithium-ion secondary battery with excellent safety and battery performance, a current collector with an undercoat layer, electrodes, and a lithium-ion secondary battery are provided. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view showing a laminated battery, which is an example of a lithium-ion secondary battery of the present disclosure. [Figure 2] This is a cross-sectional view of the positive electrode in a lithium-ion secondary battery according to an embodiment of the present disclosure. [Figure 3] This is a cross-sectional view of the negative electrode in a lithium-ion secondary battery according to an embodiment of the present disclosure. [Modes for carrying out the invention]

[0011] In this specification, a numerical range represented by "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively. In this specification, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, provided that their intended purpose is achieved. In this specification, (meth)acrylate means acrylate or methacrylate.

[0012] Embodiments of the composition, current collector with undercoat layer, electrodes, and lithium-ion secondary battery of this disclosure will be described below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and will not be repeated in the description.

[0013] (1) Composition The composition of this disclosure is a conductive carbon material (A) containing conductive carbon fibers (A1), an olefin resin (B), a binder resin (C) which is a resin other than the olefin resin (B), and synthetic rubber (D) in total amount (hereinafter also simply referred to as "the total amount of (A) to (D)"). The conductive carbon fiber (A1) comprises more than 5% by mass and 30% by mass or less, The aforementioned olefin resin (B) is 60% to 90% by mass, The aforementioned binder resin (C) is 1% to 30% by mass, The synthetic rubber (D) contains 1% to 20% by mass, A composition in which 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 0.08 to 0.32.

[0014] In this disclosure, "conductive carbon material" refers to a carbon material having a volume resistivity of less than 40 Ω·cm, preferably less than 3 Ω·cm, at 20°C. In this disclosure, "conductive carbon fiber" refers to a fibrous conductive carbon material among conductive carbon materials. "Fibrous" refers to an elongated shape, specifically a shape in which the ratio of the major axis (fiber length) to the minor axis (fiber diameter) of the conductive carbon material (major axis / minor axis) exceeds 100. In this disclosure, "olefin-based resin" refers to a resin containing structural units derived from olefins. More specifically, "olefin-based resin" refers to an olefin homopolymer, a copolymer of two or more olefins, or a copolymer of an olefin and another monomer. In this disclosure, "content" and "amount added" shall be considered to be substantially the same.

[0015] Because the composition of this disclosure has the above-described structure, it can be used to make a lithium-ion secondary battery that is excellent in terms of safety and battery performance.

[0016] (1.1) Uses of the composition The compositions of this disclosure are not particularly limited and are suitably used in components of electrodes for lithium-ion secondary batteries, more suitably used in undercoat layers for electrodes of lithium-ion secondary batteries, and even more suitably used in undercoat layers for positive electrodes of lithium-ion secondary batteries. More specifically, the compositions of this disclosure are preferably used to form the undercoat layer included in a lithium-ion secondary battery having an electrode in which an undercoat layer and an composite layer are laminated in that order on at least one main surface of a current collector. In particular, the compositions of this disclosure are more preferably used to form the undercoat layer included in the positive electrode of a lithium-ion secondary battery having a positive electrode in which an undercoat layer and an composite layer are laminated in that order on at least one main surface of a current collector.

[0017] In this disclosure, "current collector" refers to a sheet-like material used in a lithium-ion secondary battery to collect electrons generated from the active material and to supply electrons to the active material. "Main surface of the current collector" refers to the pair of opposing surfaces with the largest area among a plurality of opposing pairs of surfaces.

[0018] (1.2) Conductive carbon material (A) The compositions of this disclosure contain a conductive carbon material (A). The conductive carbon material (A) contains conductive carbon fibers (A1). This reduces the electrical resistance of the compositions of this disclosure.

[0019] (1.2.1) Conductive carbon fiber (A1) The conductive carbon material (A) includes conductive carbon fibers (A1). This makes it easier to reduce the electrical resistance of the composition of this disclosure.

[0020] The fiber diameter of the conductive carbon fiber (A1) is not particularly limited, but is preferably 1 nm to 300 nm, more preferably 3 nm to 150 nm, even more preferably 5 nm to 100 nm, and most preferably 10 nm to 70 nm. The fiber length of the conductive carbon fiber (A1) is not particularly limited, but is preferably 1 μm to 100 μm, more preferably 3 μm to 70 μm, even more preferably 5 μm to 50 μm, and most preferably 5 μm to 30 μm. One method for measuring fiber diameter and fiber length is to randomly select 50 fibers from scanning electron microscope images and calculate them by averaging the results.

[0021] Examples of conductive carbon fibers (A1) include carbon fiber (CF, fiber diameter: 1000nm~10000nm), carbon nanotube (CNT, fiber diameter: 1nm~1000nm), graphene nanoribbon (GNR, fiber diameter: 1nm~10nm), and fullerene nanowhiskers (fiber diameter: 1nm~10nm).

[0022] "Carbon fiber" refers to a fibrous (i.e., solid) carbon material that consists almost entirely of carbon elements. "Carbon nanotubes" refer to needle-shaped carbon fibers created by seamlessly sealing graphene, which is formed from a network of six-membered carbon rings, into a cylindrical shape. A "graphene nanoribbon" refers to a strip-like material obtained by cutting graphene, which is formed from a network of six-membered carbon rings, into nanometer-sized widths. "Fullerene nanowhiskers" refer to carbon fibers in which fullerenes are linked together in a whisker-like structure, forming linear single crystals, with the fiber diameter of these linear single crystals being on the nanometer scale.

[0023] Examples of carbon nanotubes include single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), multi-walled carbon nanotubes (MWNTs), and vapor-grown carbon fibers (VGCFs). Conductive carbon fiber (A1) may be used alone or in combination of two or more types.

[0024] From the viewpoint of balancing conductive performance and cost, the conductive carbon fiber (A1) preferably contains at least one selected from carbon nanotubes and carbon fibers, more preferably contains carbon nanotubes or carbon fibers, and even more preferably contains carbon nanotubes. In particular, the carbon nanotube preferably contains at least one of multi-walled carbon nanotubes (MWNTs) and vapor-processed carbon fibers (VGCFs), more preferably contains multi-walled carbon nanotubes (MWNTs), and even more preferably contains multi-walled carbon nanotubes (MWNTs).

[0025] The conductive carbon fiber (A1) may be a commercially available product. Examples of commercially available multi-walled carbon nanotubes (MWNTs) include "HX-N1" and "HX-N3" from QINGDAO HAOXIN. Examples of commercially available vapor-phase carbon fibers (VGCFs) include "VGCF(registered trademark)-H" from Showa Denko K.K. As a commercially available dispersion containing conductive carbon fiber (A1), "HX-WS-1", "HX-WS-3", and "HX-NS-1" from QINGDAO HAOXIN may be used.

[0026] (1.2.2) Conductive carbon particles (A2) The conductive carbon material (A) may or may not contain conductive carbon particles (A2). By including conductive carbon particles (A2) in addition to conductive carbon fibers (A1), the conductive performance of the conductive carbon material (A) is improved by forming hybrid conductive paths that connect lines and points, and the cost competitiveness can be improved by reducing the content of conductive carbon fibers (A1).

[0027] In this disclosure, "conductive carbon particles" refers to particulate conductive carbon materials. "Particulate" refers to granular materials, specifically those with a ratio of the major axis (minimum diameter of the particle) to the minor axis (minimum diameter of the particle) (major axis / minor axis) of 1.0 to 5.0.

[0028] The particle size of the conductive carbon particles (A2) is not particularly limited. From the viewpoint of dispersing the conductive carbon particles (A2) between each particle in the undercoat layer and functioning as a conductive additive, the particle size of the conductive carbon particles (A2) is preferably 5 μm or less, more preferably 1 μm to 4 μm. In this case, the primary particle diameter of the conductive carbon particles (A2) is preferably 0.5 μm or less, more preferably 0.001 μm to 0.4 μm. The particle size of conductive carbon particles (A2) is the particle size (D50 particle size, 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 analyzer based on laser diffraction and light scattering.

[0029] Examples of conductive carbon particles (A2) include graphite, carbon black, and fullerene. Examples of graphite include artificial graphite and natural graphite (e.g., flake graphite, lump graphite, earthy graphite, etc.). Conductive carbon particles (A2) may be used individually or in combination of two or more types.

[0030] The conductive carbon particles (A2) may be commercially available. Examples of commercially available carbon black include "Super P" (manufactured by Timcal). Examples of commercially available flake graphite include "KS-6" (manufactured by Timrex).

[0031] (1.2.3) Content The content of conductive carbon fiber (A1) is greater than 5% by mass and less than or equal to 30% by mass of the total amount of (A) to (D). If the content of conductive carbon fiber (A1) is within the above range, the number of contact points between conductive carbon fibers (A1) in the composition of this disclosure is large, and the electrical resistance of the composition of this disclosure at room temperature can be reduced by the percolation effect. Furthermore, when the temperature of the lithium-ion secondary battery rises rapidly, contact between conductive carbon fibers (A1) is difficult to maintain, and the electrical resistance of the undercoat layer increases, so the resulting lithium-ion secondary battery can 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 of the lithium-ion secondary battery. The battery reaction refers to the insertion and removal reactions of lithium ions that take place between the positive electrode and the negative electrode. The higher the content of conductive carbon material (A), including conductive carbon fiber (A1), the better the battery performance of lithium-ion secondary batteries tends to be.

[0032] The content of conductive carbon material (A) is not particularly limited as long as the content of conductive carbon fiber (A1) is within the above range. From the viewpoint of ensuring the shutdown function and battery performance, it is preferably more than 5% by mass and 30% by mass, more preferably 7% by mass to 25% by mass, even more preferably 10% by mass to 16% by mass, and particularly preferably 10% by mass to 13% by mass, relative to the total amount of (A) to (D).

[0033] The content of conductive carbon fiber (A1) is more than 5% by mass and 30% by mass or less relative to the total amount of (A) to (D). From the viewpoint of ensuring the shutdown function and battery performance, it is preferably more than 5% by mass and 20% by mass or less, more preferably 7% by mass to 15% by mass, and even more preferably 7% by mass to 10% by mass.

[0034] When the conductive carbon material (A) contains conductive carbon particles (A2), the content of conductive carbon particles (A2) is preferably within the following range. The content of conductive carbon particles (A2) is preferably 0.1% to 10% by mass, more preferably 0.5% to 5% by mass, and even more preferably 1% to 3% by mass, relative to the total amount of (A) to (D), from the viewpoint of ensuring battery performance by improving conductivity as an auxiliary to conductive carbon fibers (A1) and ensuring shutdown function. The mass ratio (A2 / A1) of the content of conductive carbon particles (A2) to the content of conductive carbon fibers (A1) is not particularly limited, but is preferably 0 to 0.7, more preferably 0 to 0.5, and even more preferably 0 to 0.3, from the viewpoint of ensuring the shutdown function and battery performance.

[0035] The mass ratio (A / B) is between 0.08 and 0.32. Within this range, it is possible to achieve both the shutdown function and the reduction of the electrical resistance of the undercoat layer to ensure battery performance (hereinafter referred to as "low resistance of the undercoat layer"). The mass ratio (A / B) is preferably 0.08 to 0.25, more preferably 0.10 to 0.20, and even more preferably 0.10 to 0.14, from the viewpoint of ensuring the shutdown function and battery performance.

[0036] (1.3) Olefin resins (B) The composition of this disclosure comprises an olefin resin (B). As a result, when the composition of this disclosure is used as a raw material for the undercoat layer, the lithium-ion secondary battery is less prone to thermal runaway, and the safety of the lithium-ion secondary battery is ensured.

[0037] The material of the olefin resin (B) is not particularly limited. The softening point of the olefin resin (B) is preferably 70°C to 150°C. The upper limit of the softening point is preferably 140°C or lower, more preferably 135°C or lower, from the viewpoint of enabling the shutdown function to be more effectively performed by melting the olefin resin (B) in a lower temperature range when the temperature of the lithium-ion secondary battery rises rapidly (hereinafter referred to as "effective execution of the shutdown function"). The lower limit of the softening point of the olefin resin (B) is preferably 90°C or higher, more preferably 110°C or higher, and even more preferably 120°C or higher, from the viewpoint of maintaining the shape of the olefin resin (B) before and after the drying process performed in the manufacturing process of the positive electrode (hereinafter referred to as "shape retention of olefin resin (B) during the positive electrode drying process"). The softening point of the olefin resin (B) is shown as the value measured by JIS K2207 (ring-ball method).

[0038] Specifically, examples of olefin resin materials (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. In particular, the material of the olefin resin (B) is preferably polyethylene resin or polypropylene resin, from the viewpoint of achieving both effective shutdown function and shape retention 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 types.

[0039] In this disclosure, "polyethylene resin" refers to a resin whose main component is ethylene. Examples of polyethylene resins include homopolymers or copolymers of ethylene. In particular, the polyethylene resin is preferably a copolymer of ethylene and at least one α-olefin. In this disclosure, "polypropylene resin" refers to a resin whose main component is propylene. Examples of polypropylene resins include homopolymers or copolymers of propylene. In particular, the polypropylene resin is preferably a copolymer of propylene and at least one α-olefin (excluding propylene). In this disclosure, the principal component refers to the component that has the highest content ratio (mol%) among the constituent units contained in the resin or polymer.

[0040] The olefin resin (B) preferably includes a water-dispersible olefin resin. This makes the lithium-ion secondary battery less susceptible to thermal runaway when the composition of this disclosure is used as a raw material for the undercoat layer, thereby improving the safety of the lithium-ion secondary battery.

[0041] In this disclosure, "water-dispersible olefin resin" refers to fine particles of an olefin resin that can be dispersed in water without the addition of any surfactant or organic solvent.

[0042] Examples of water-dispersible olefin resins include polyethylene, polyethylene elastomers, polyolefin ionomers, and EVA.

[0043] The content of the water-dispersible olefin resin is not particularly limited, but is preferably 20% to 100% by mass, more preferably 50% to 100% by mass, and may be 100% by mass, relative to the total amount of the olefin resin (B).

[0044] The shape of the olefin resin (B) is not particularly limited and may be particulate. If the shape of the olefin resin (B) is particulate, the particle size of the olefin resin (B) is not particularly limited. The particle size of the olefin resin (B) is preferably 0.1 μm to 9.0 μm, more preferably 0.5 μm to 4.0 μm, and even more preferably 0.5 μm to 2.0 μm, from the viewpoint of adjusting the particle size of the olefin resin (B) to fall within the optimal undercoat layer thickness range and the processability of the composition (e.g., undercoat layer slurry). The smaller the particle size of the olefin resin (B), the more likely it is to aggregate. The particle size of olefin resin (B) is shown as the value measured by the Coulter counter method.

[0045] 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 this disclosure is used as a raw material for the undercoat layer, the lithium-ion secondary battery is less prone to thermal runaway, and the safety of the lithium-ion secondary battery is improved.

[0046] The content of olefin resin (B) is 60% to 90% by mass relative to the total amount of (A) to (D). If the content of olefin resin (B) is within the above range, the shutdown function can be ensured. The higher the olefin resin (B) content, the safer the lithium-ion secondary battery becomes, but the battery performance tends to decrease. In other words, there is a trade-off relationship between the safety and battery performance of lithium-ion secondary batteries. The content of olefin resin (B) is preferably 65% ​​to 83% by mass, more preferably 72% to 83% by mass, and even more preferably 77% to 83% by mass, relative to the total amount of (A) to (D), from the viewpoint of low resistance and effective shutdown function of the undercoat layer. A lower content of olefin resin (B) tends to reduce the shutdown function.

[0047] The olefin resin (B) may be a commercially available product. Examples of commercially available water-dispersible olefin resins include the ChemiPearl® series (polyolefin aqueous dispersion) manufactured by Mitsui Chemicals, Inc. Examples of low molecular weight polyethylene dispersed into fine aqueous particles include W300, W400, W410, W700, W4005, W401, W500, WF640, W900, W950, WH201, WP100, and P301W.

[0048] (1.4) Binder resin (C) The composition of the present disclosure comprises a binder resin (C). When the composition of the present disclosure is used as a raw material for an undercoat layer, the binder resin (C) can improve the physical properties of the undercoat layer (e.g., electrolyte penetration and peel strength) and improve the battery performance of a lithium-ion secondary battery.

[0049] The material of the binder resin (C) is not particularly limited, and examples include carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF copolymer, polyvinylpyrrolidone (PVP), hydroxypropyl methylcellulose, polyvinyl alcohol, hydroxypropyl cellulose, and diacetylcellulose. PVDF copolymer is a copolymer of vinylidene fluoride and another monomer, such as PVDF-HFP (hexafluoropropylene) and PVDF-PEO (polyoxyethylene). The binder resin (C) may be used alone or in combination of two or more types. In particular, the binder resin (C) preferably contains CMC or PVDF from the viewpoint of low resistance of the undercoat layer.

[0050] The binder resin (C) may be a commercially available product. Examples of commercially available CMC products include "1130," "1140," "1240," "1250," "1260," "1330," "2200," and "DL100L," all manufactured by Daicel Mirise Co., Ltd. Commercially available PVDF products include the Kureha® KF Polymer series manufactured by Kureha Corporation, such as "W#1100", "W#1300", "W#1700", "W#7200", "W#7300", and "L#7208".

[0051] The content of binder resin (C) is 1% to 30% by mass relative to the total amount of (A) to (D). The shutdown function depends on the balance between the content of binder resin (C) and the content of olefin resin (B). If the ratio of binder resin (C) to the total amount of (A) to (D) is high, the ratio of olefin resin (B) to the total amount of (A) to (D) will be low. In this case, the shutdown function may not work. If the content of binder resin (C) is within the above range, it is possible to achieve both processability of the composition (e.g., slurry for undercoat layer) and the assurance of the shutdown function. The content of the binder resin (C) is preferably 2% to 15% by mass, more preferably 5% to 10% by mass, relative to the total amount of (A) to (D), from the viewpoint of ensuring the shutdown function by the content of the olefin resin (B) and the processability of the composition (e.g., slurry for the undercoat layer).

[0052] (1.5) Synthetic rubber (D) The composition of the present disclosure includes synthetic rubber (D). When the composition of the present disclosure is used as a raw material for an undercoat layer, the adhesion between the undercoat layer and the positive electrode current collector is improved, thereby reducing battery resistance.

[0053] Examples of synthetic rubber (D) materials include styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, hydrogenated styrene-butadiene rubber (HSBR), butylene rubber, polybutadiene, and polyisoprene rubber. Synthetic rubber (D) may be used alone or in combination of two or more types. In particular, synthetic rubber (D) is preferably a mixture of a water-dispersible binder such as an SBR emulsion, more preferably containing SBR, and even more preferably being SBR, from the viewpoint of low resistance of the undercoat layer.

[0054] The shape of the synthetic rubber (D) is not particularly limited and may be granular.

[0055] The content of synthetic rubber (D) is 1% to 20% by mass relative to the total amount of (A) to (D). If the content of synthetic rubber (D) is within the above range, it is possible to achieve both adhesion to the positive electrode current collector and a reduction in the electrical resistance of the composition itself. The content of synthetic rubber (D) is preferably 3% to 15% by mass, more preferably 5% to 12% by mass, and even more preferably 5% to 10% by mass, relative to the total amount of (A) to (D), from the viewpoint of reducing the electrical resistance of the composition itself and adhesion to the positive electrode current collector.

[0056] The mass ratio (D / B) of the synthetic rubber (D) content to the olefin resin (B) content is not particularly limited, but is preferably 0.01 to 0.7. If the mass ratio (D / B) is within the above range, when the composition of this disclosure is used as a raw material for the undercoat layer, it is possible to achieve both good adhesion between the undercoat layer and the positive electrode current collector and effective shutdown function. The mass ratio (D / B) is more preferably 0.05 to 0.50, even more preferably 0.05 to 0.25, and particularly preferably 0.06 to 0.15, from the viewpoint of effectively exhibiting the shutdown function and maintaining adhesion between the undercoat layer and the positive electrode current collector when the composition of this disclosure is used as a raw material for the undercoat layer.

[0057] The synthetic rubber (D) may be a commercially available product. Examples of commercially available synthetic rubber (D) include "TRD2001" (aqueous dispersion of styrene-butadiene rubber particles) manufactured by JSR Corporation.

[0058] (1.6) Additives (E) The compositions of the present disclosure may or may not contain additive (E) as needed. By including additive (E) in the compositions of the present disclosure, various functions can be imparted to the compositions of the present disclosure depending on the type of additive (E).

[0059] In this disclosure, "additive" refers to solid components other than conductive carbon material (A), olefin resin (B), binder resin (C), and synthetic rubber (D).

[0060] Examples of additive (E) include thermally expandable microcapsules having a maximum volume expansion temperature of 70°C to 180°C (hereinafter sometimes simply referred to as "thermally expandable microcapsules"), inorganic oxide fillers, curable resin fillers, flake-shaped fillers, and the like. In particular, it is preferable that additive (E) contains at least one of thermally expandable microcapsules and inorganic oxide fillers.

[0061] In this disclosure, "thermally expandable microcapsule" refers to a microcapsule comprising an outer shell made of a thermoplastic resin and a volatile expanding agent enclosed within the outer shell. Specifically, when a thermally expandable microcapsule is in contact with an electrolyte solution (described later) and is exposed to a temperature just before thermal runaway due to abnormal heat generation in a lithium-ion secondary battery begins (for example, 70°C to 160°C), it rapidly softens, foams, and undergoes volume expansion. In this disclosure, "volume expansion initiation temperature" refers to the temperature at which the volatile expanding agent encapsulated in the thermally expandable microcapsules gasifies and the thermally expandable microcapsules begin to expand in volume. In this disclosure, "maximum volume expansion temperature" refers to the temperature at which the expansion volume of the thermally expandable microcapsule is maximized when the volatile expanding agent encapsulated in the thermally expandable microcapsule gasifies and the thermally expandable microcapsule expands in volume.

[0062] If the composition of this disclosure contains additive (E), the content of additive (E) is preferably within the following range. The content of additive (E) is preferably 0% to 50% by mass relative to the total amount of (A) to (E). If the content of additive (E) is within the above range, the properties of additive (E) can make the shutdown function more effective. The amount of additive (E) has little effect on the ratio of conductive carbon material (A) and olefin resin (B), and from the viewpoint of effective performance of the shutdown function, maintenance of battery performance, and effective performance of the shutdown function, it is preferably 5% to 40% by mass, more preferably 10% to 30% by mass or less, relative to the total amount of (A) to (E).

[0063] (1.6.1) Thermally expandable microcapsules The compositions of this disclosure may optionally contain thermally expandable microcapsules. The compositions of this disclosure may not contain thermally expandable microcapsules. When the composition of the present disclosure contains thermally expandable microcapsules and is used as a raw material for an undercoat layer, the thermally expandable microcapsules can expand in volume due to abnormal heat generation in a lithium-ion secondary battery, thereby efficiently increasing the DC resistance of the electrodes and improving the safety of the lithium-ion secondary battery.

[0064] The temperature at which the volume expansion of the thermally expandable microcapsules begins is preferably 120°C to 130°C. The temperature at which the volume expansion of the thermally expandable microcapsules is preferably 145°C to 155°C.

[0065] The thermoplastic resin constituting the outer shell preferably contains a (co)polymer containing vinylidene chloride and a (co)polymer containing (meth)acrylonitrile. This results in a thermoplastic resin constituting the outer shell having 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 with (meth)acrylonitrile as the main component (51% by mass or more). The raw materials for the thermoplastic resin constituting the outer shell may include a crosslinkable monomer in addition to a polymerizable monomer in order to improve the foaming properties and heat resistance of the resulting heat-expandable microcapsules.

[0066] It is preferable to select the boiling point of the volatile expansion agent such that the maximum volume expansion temperature of the thermally expandable microcapsules is higher than the softening point of the olefin resin (B). It is also preferable to select the boiling point of the volatile expansion agent such that the volume expansion initiation temperature of the thermally expandable microcapsules is equivalent to the softening point of the olefin resin (B). As a result, when the temperature of the undercoat layer rises above the softening point of the olefin resin (B) due to the heat generated by the lithium-ion secondary battery, the DC resistance of the electrodes increases efficiently. Consequently, the safety of the lithium-ion secondary battery is further improved. Examples of volatile swelling agents include low molecular weight hydrocarbons with a boiling point of 100°C or less, and non-flammable or flame-retardant compounds. Examples of low molecular weight hydrocarbons with a boiling point of 100°C or less 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 swelling agents may be used individually or in combination of two or more.

[0067] 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 (D50 particle size, median diameter) corresponding to the cumulative 50% volume from the fine particle side in the volume-based particle size distribution measured by a particle size distribution analyzer based on laser diffraction and light scattering.

[0068] If the composition contains thermally expandable microcapsules, the content of thermally expandable microcapsules is preferably 5% to 50% by mass, more preferably 10% to 40% by mass, relative to the total amount of (A) to (D). Within this range, the influence on the content ratio of conductive carbon material (A) and olefin resin (B) is minimal, which is preferable from the viewpoint of maintaining the shutdown function, battery performance, and enhancing the shutdown function by thermal expansion of the thermally expandable microcapsules in the event of abnormal heat generation.

[0069] The thermally expandable microcapsules may be commercially available. Examples of commercially available thermally expandable microcapsules include the "Matsumoto Microspheres®" series from Matsumoto Oil & Fat Pharmaceutical Co., Ltd., the "EXPANCEL®" series from AkzoNobel, and the "ADVANCELL®" series from Sekisui Chemical Co., Ltd.

[0070] (1.6.2) Inorganic oxide fillers The inorganic oxide filler may optionally contain an inorganic oxide filler. The compositions of this disclosure do not necessarily have to contain an inorganic oxide filler. If the composition of this disclosure contains an inorganic oxide filler, the inorganic oxide filler functions as a filler. Increasing the inorganic oxide filler content contributes to the heat resistance of the positive electrode, while minimizing the inorganic oxide filler content results in an undercoat layer that adheres closely to the positive electrode composite layer. This undercoat layer melts during an internal short circuit, contributing to improved safety. Furthermore, by selecting the type and properties of the inorganic oxide filler, it is possible to decompose the electrolyte and generate gas during battery overcharging.

[0071] 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) and boron nitride (BN), mica, and graphite oxide such as expanded graphite. Inorganic oxide fillers may be used individually or in combination of two or more. Among these, it is preferable that the inorganic oxide filler contains aluminum oxide.

[0072] The shape of the inorganic oxide filler is not particularly limited and can be spherical, needle-shaped, ellipsoidal, plate-shaped, or flaky. The particle size of the inorganic oxide filler is not particularly limited, but is preferably 0.01 μm to 5 μm. The particle size of the inorganic oxide filler is the particle size (D50 particle size, 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 analyzer based on laser diffraction and light scattering.

[0073] When the composition of this disclosure contains an inorganic oxide filler, the amount of inorganic oxide filler has little influence on the ratio of conductive carbon material (A) to olefin resin (B). From the viewpoint of maintaining the shutdown function, battery performance, and suppressing the fluidity of the olefin resin (B) that has melted in the undercoat layer at high temperatures, thereby maintaining the shutdown function for a long period of time, the amount of inorganic oxide filler is preferably 5% to 40% by mass, more preferably 10% to 30% by mass, relative to the total amount of (A) to (E).

[0074] (1.7) Non-solids The compositions of this disclosure may contain non-solid components. For example, when an undercoat layer is formed from an undercoat slurry, the undercoat layer may contain various components derived from that undercoat slurry. Non-solid components include various components derived from the undercoat slurry (e.g., thickeners, as well as surfactants, dispersants, wetting agents, defoaming agents, etc.) and water.

[0075] (2) Current collector with undercoat layer The current collector with an undercoat layer of the present disclosure comprises a current collector and an undercoat layer. The undercoat layer is laminated on at least one main surface of the current collector. The undercoat layer contains the solids of the composition of the present disclosure.

[0076] In this disclosure, "current collector with undercoat layer" refers to an electrode component of a lithium-ion secondary battery. In this disclosure, “solids of the composition” includes a conductive carbon material (A), an olefin resin (B), a binder resin (C), and synthetic rubber (D), and may optionally include an additive (E).

[0077] Because the current collector with an undercoat layer of this disclosure has the above configuration, the current collector with an undercoat layer of this disclosure increases the electrical resistance between the current collector and the composite material layer described later when the temperature of the lithium-ion secondary battery rises rapidly. This suppresses overheating of the lithium-ion secondary battery. Therefore, the current collector with an undercoat layer of this disclosure can improve the safety of the lithium-ion secondary battery. Furthermore, the current collector with an undercoat layer of this disclosure can suppress the 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.

[0078] When undercoat layers are laminated on both main surfaces of the current collector, the configuration of one undercoat layer (e.g., material, shape, film thickness, etc.) may be the same as or different from that of the other undercoat layer.

[0079] Hereinafter, a current collector with an undercoat layer used in the positive electrode of a lithium-ion secondary battery will be referred to as an "undercoated positive electrode current collector," and a current collector with an undercoat layer used in the negative electrode of a lithium-ion secondary battery will be referred to as an "undercoated negative electrode current collector."

[0080] (2.1) Current collector The material of the current collector is not particularly limited. When a current collector is used as a component of the positive electrode (hereinafter sometimes referred to as "positive electrode current collector"), examples of materials for the positive electrode current collector include aluminum, nickel, stainless steel (SUS), and copper. "Aluminum" includes pure aluminum or aluminum alloys. When a current collector is used as a component of the negative electrode (hereinafter sometimes referred to as "negative electrode current collector"), examples of materials for the negative electrode current collector include copper, aluminum, nickel, stainless steel (SUS), and nickel-plated steel.

[0081] (2.2) Undercoat layer The undercoat layer is formed using the composition of the present disclosure.

[0082] The undercoat layer only needs to be formed on at least a portion of the main surface of at least one of the current collectors, and can be appropriately selected according to the coating pattern of the composite layer (e.g., intermittent coating, stripe coating, etc.).

[0083] The thickness of the undercoat layer is not particularly limited, and is, for example, 0.1 μm to 50 μm or less. From the viewpoint of further suppressing the DC resistance of the lithium-ion secondary battery under normal conditions, the upper limit of the undercoat layer thickness is preferably 20 μm or less. From the viewpoint of increasing the DC resistance and ensuring more reliable shutdown function during abnormal heat generation of the lithium-ion secondary battery, the lower limit of the undercoat layer thickness is preferably 0.2 μm or more.

[0084] (3) Electrode The electrode of the present disclosure comprises a current collector with an undercoat layer and an asphalt mixture layer. The asphalt mixture layer is laminated on the undercoat layer of the current collector with an undercoat layer of the present disclosure.

[0085] In this disclosure, "electrode" refers to at least one of the positive electrode and the negative electrode of a lithium-ion secondary battery.

[0086] When the positive electrode and negative electrode of a lithium-ion secondary battery are each equipped with a current collector with an undercoat layer, the current collector with an undercoat layer included in the positive electrode and the current collector with an undercoat layer included in the negative electrode may be the same or different. The material of the composite layer contained in the positive electrode (hereinafter sometimes referred to as the "positive electrode composite layer") and the material of the composite layer contained in the negative electrode (hereinafter sometimes referred to as the "negative electrode composite layer") are different.

[0087] (3.1) Positive electrode The positive electrode of the present disclosure comprises a current collector with an undercoat layer and a positive electrode composite layer.

[0088] The positive electrode of this disclosure includes 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 shows an undercoat layer and a positive electrode composite layer laminated in that order on both main surfaces of the current collector. The second positive electrode configuration shows an undercoat layer and a positive electrode composite layer laminated in that order on the main surface of one of the current collectors, and an undercoat layer laminated on the main surface of the other of the current collectors. The third positive electrode configuration shows an undercoat layer and a positive electrode composite layer laminated in that order on the main surface of one of the current collectors, and a positive electrode composite layer laminated on the main surface of the other of the current collectors. The fourth positive electrode configuration shows an undercoat layer and a positive electrode composite layer laminated in that order on the main surface of one of the current collectors, while neither the undercoat layer nor the positive electrode composite layer is laminated on the main surface of the other of the current collectors.

[0089] (3.1.1) Cathode composite layer The positive electrode composite layer contains a positive electrode active material and a binder.

[0090] (3.1.1.1) Positive electrode active material The positive electrode active material is not particularly limited as long as it is a material capable of intercalating and releasing lithium ions, and can be appropriately adjusted depending on the application of the lithium-ion secondary battery.

[0091] Examples of positive electrode active materials include primary oxides and secondary oxides. The first oxide consists of lithium (Li) and nickel (Ni) as its constituent metallic elements. The second oxide contains Li, Ni, and at least one metal element other than Li and Ni as constituent metal elements. Examples of metal elements other than Li and Ni include transition metals and typical metals. Preferably, the second oxide contains the metal element other than Li and Ni in an amount equivalent to or less than Ni in terms of atomic number. The metal element other than Li and Ni may be 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 types.

[0092] 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 advantage of having a high energy density per unit volume and excellent thermal stability.

[0093] LiRing a Co b Mn c O2… Formula (X)

[0094] In formula (X), a, b, and c are each independently greater than 0 and less than 1, and the sum of a, b, and c is 0.99 or more and 1.00 or less.

[0095] Specific examples of NCM include LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2, 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 O2 and the like.

[0096] The positive electrode active material may contain a lithium-containing composite oxide represented by the following formula (Y) (hereinafter sometimes referred to as "NCA").

[0097] Li t Ni 1-x-y Co x Al y O2... Formula (Y)

[0098] 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.

[0099] Specific examples of NCA include LiNi 0.8 Co 0.15 Al 0.05 O2 and the like.

[0100] The content of the positive electrode active material is preferably 10% by mass to 99.9% by mass, more preferably 30% by mass to 99% by mass, still more preferably 50% by mass to 99% by mass, and particularly preferably 70% by mass to 99% by mass with respect to the total amount of the positive electrode composite material layer.

[0101] (3.1.1.2) 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 composite layer, the binder preferably contains a fluororesin. The binder may be used alone or in combination of two or more types. From the viewpoint of balancing the physical properties of the positive electrode composite layer (e.g., electrolyte permeability, peel strength, etc.) with battery performance, the binder content is preferably 0.1% by mass or more and 4% by mass or less of the total amount of the positive electrode composite layer.

[0102] (3.1.1.3) Conductive additives The positive electrode composite layer may further contain a conductive additive. As the material of the conductive additive, known conductive additives can be used. Among the known conductive additives, conductive carbon materials are preferred. Examples of conductive carbon materials include graphite, carbon black, conductive carbon fibers, and fullerenes. Examples of conductive carbon fibers include carbon nanotubes and carbon fibers. Examples of graphite include artificial graphite and natural graphite. Examples of natural graphite include flake graphite, lump graphite, and earthy graphite. The conductive additive may be used alone or in combination of two or more types. The material of the conductive additive may be a commercially available product.

[0103] (3.1.1.4) Other ingredients The positive electrode composite layer may contain other components. These other components include thickeners, surfactants, dispersants, wetting agents, and defoaming agents.

[0104] (3.2) Negative electrode The negative electrode of the present disclosure comprises a current collector with an undercoat layer and a negative electrode composite layer.

[0105] The negative electrode of this disclosure includes 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 shows a structure in which an undercoat layer and a negative electrode composite layer are laminated in that order on the main surfaces of both current collectors. The second negative electrode configuration shows an undercoat layer and a negative electrode composite layer laminated in that order on the main surface of one of the current collectors, and an undercoat layer 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 layer are laminated in that order on the main surface of one of the current collectors, and a negative electrode composite layer is laminated on the main surface of the other of the current collectors. The fourth negative electrode configuration shows an undercoat layer and a negative electrode composite layer laminated in that order on the main surface of one of the current collectors, while neither the undercoat layer nor the negative electrode composite layer is laminated on the main surface of the other of the current collectors.

[0106] (3.2.1) Negative electrode composite layer The negative electrode composite layer contains a negative electrode active material and a binder.

[0107] (3.2.1.1) Negative electrode active material The negative electrode active material is not particularly limited as long as it is a material capable of intercalating and releasing lithium ions. Preferably, the negative electrode active material is at least one selected from the group consisting of, for example, metallic lithium, lithium-containing alloys, metals or alloys that can be alloyed with lithium, oxides that can be doped and dedoped with lithium ions, transition metal nitrides that can be doped and dedoped with lithium ions, and carbon materials that can be doped and dedoped with lithium ions. Among these, the negative electrode active material is preferably a carbon material capable of doping and dedoping with lithium ions (hereinafter referred to as "carbon material").

[0108] Examples of carbon materials include carbon black, activated carbon, graphite materials, and amorphous carbon materials. These carbon materials may be used individually or in mixtures of two or more. The form of the carbon material is not particularly limited and can be fibrous, spherical, or flake-shaped, for example. The particle size of the carbon material is not particularly limited, but is preferably 5 μm to 50 μm, and more preferably 20 μm to 30 μm. Examples of amorphous carbon materials include hard carbon, coke, mesocarbon microbeads (MCMB) fired at temperatures below 1500°C, 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. Graphite materials may contain boron. Graphite materials may be coated with metal or amorphous carbon. Examples of metals used to coat the graphite material include gold, platinum, silver, copper, and tin. Graphite materials may also be mixtures of amorphous carbon and graphite.

[0109] (3.2.1.2) Binder The binders included in the negative electrode composite layer are the same as those exemplified for the binders included in the positive electrode composite layer. The binder contained in the negative electrode composite layer may be the same as or different from the binder contained in the positive electrode composite layer. The binder content in the negative electrode composite layer is not particularly limited and may be the same as that exemplified for the binder content in the positive electrode composite layer.

[0110] (3.2.1.3) Conductive additives The negative electrode composite layer preferably contains a conductive additive. Examples of conductive additives include those similar to those exemplified as conductive additives that may be included in the positive electrode composite layer.

[0111] (3.2.1.4) Other ingredients The negative electrode composite layer may contain other components in addition to the above-mentioned components. Examples of other components include thickeners, surfactants, dispersants, wetting agents, and defoaming agents.

[0112] (4) Lithium-ion secondary batteries The lithium-ion secondary battery of this disclosure comprises the electrodes of this disclosure.

[0113] A lithium-ion secondary battery generally comprises an outer casing, a positive electrode, a negative electrode, a separator, and an electrolyte. The outer casing houses the positive electrode, negative electrode, separator, and non-aqueous electrolyte. The positive electrode is capable of intercalating and releasing lithium ions. The negative electrode is also capable of intercalating and releasing lithium ions. The separator separates the positive electrode and the negative electrode.

[0114] In the lithium-ion secondary battery of this disclosure, at least one of the positive electrode and the negative electrode is the electrode of this disclosure. If one of the positive electrode and the negative electrode of the lithium-ion secondary battery of this disclosure is the electrode of this disclosure, the other of the positive electrode and the negative electrode may be any known electrode used in lithium-ion secondary batteries. The following describes the case where the positive electrode and negative electrode are the electrodes of this disclosure.

[0115] (4.1) Exterior There are no particular limitations on the shape of the outer casing, and it is selected appropriately depending on the application of the lithium-ion secondary battery. Examples of outer casings include those containing a laminate film, and those consisting of a battery can and a battery can lid.

[0116] (4.2) Positive and negative electrodes The positive electrode is the positive electrode of this disclosure. The negative electrode is the negative electrode of this disclosure.

[0117] (4.3) Separator Examples of separators include porous resin plates. Materials for the porous resin plates include resins and nonwoven fabrics containing these resins. Examples of resins include polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polyester, cellulose, and polyamide. Among these, the separator is preferably a single-layer or multi-layer porous resin sheet. The porous resin sheet is mainly composed of one or more types of polyolefin resins. The thickness of the separator is preferably 5 μm to 30 μm. The separator is preferably placed between the positive electrode and the negative electrode.

[0118] (4.4) Non-aqueous electrolyte Non-aqueous electrolytes contain an electrolyte and a non-aqueous solvent.

[0119] (4.4.1) Electrolyte The electrolyte preferably contains at least one of a lithium salt containing fluorine (hereinafter sometimes referred to as "fluorinated lithium salt") and a lithium salt that does not contain fluorine.

[0120] Examples of fluorinated lithium salts include inorganic acid anionic salts and organic acid anionic salts. Examples of inorganic acid anionic salts include lithium hexafluoride phosphate (LiPF6), lithium borate tetrafluoride (LiBF4), lithium arsenate hexafluoride (LiAsF6), and lithium tantalate hexafluoride (LiTaF6). Examples of organic acid anionic salts include lithium trifluoromethanesulfonate (LiCF3SO3). Among these, LiPF6 is particularly preferred as a fluorine-containing lithium salt. Lithium salts that do not contain fluorine include lithium perchlorate (LiClO4), lithium aluminate tetrachloride (LiAlCl4), and lithium decachlorodecaborate (Li2B 10 Cl 10 ) are some examples.

[0121] When the electrolyte contains a fluorinated lithium salt, the content of the fluorinated lithium salt is preferably 50% to 100% by mass, more preferably 60% to 100% by mass, and even more preferably 80% to 100% by mass, relative to the total amount of the electrolyte.

[0122] When the fluorinated lithium salt contains lithium hexafluoride phosphate (LiPF6), the content of lithium hexafluoride phosphate (LiPF6) is preferably 50% to 100% by mass, more preferably 60% to 100% by mass, and even more preferably 80% to 100% by mass, relative to the total amount of the electrolyte.

[0123] 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, more preferably 0.5 mol / L or more and 2 mol / L or less.

[0124] When the non-aqueous electrolyte contains lithium hexafluoride phosphate (LiPF6), the concentration of lithium hexafluoride phosphate (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.

[0125] (4.4.2) Non-aqueous solvents Non-aqueous electrolytes generally contain a non-aqueous solvent.

[0126] Examples of non-aqueous solvents include cyclic carbonates, fluorinated cyclic carbonates, linear carbonates, fluorinated linear carbonates, aliphatic carboxylic acid esters, fluorinated aliphatic carboxylic acid esters, γ-lactones, fluorinated γ-lactones, cyclic ethers, fluorinated cyclic ethers, linear ethers, fluorinated linear ethers, nitriles, amides, lactams, nitromethane, nitroethane, sulfolane, trimethyl phosphate, dimethyl sulfoxide, and dimethyl sulfoxide phosphate. Non-aqueous solvents may be used individually or in combination of two or more.

[0127] 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 linear 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 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 linear 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, and C6F 13 OCH3, C6F 13 OC2H5, C8F 17 O CH3, C8F 17 Examples include OC2H5, CF3CFHCF2CH(CH3)OCF2CFHCF3, HCF2CF2OCH(C2H5)2, HCF2CF2OC4H9, HCF2CF2OCH2CH(C2H5)2, and HCF2CF2OCH2CH(CH3)2. Examples of nitriles include acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, and 3-methoxypropionitrile. Examples of amides include N,N-dimethylformamide. Examples of lactam compounds include N-methylpyrrolidinone, N-methyloxazolidinone, and N,N'-dimethylimidazolidinone.

[0128] The non-aqueous solvent preferably contains at least one selected from the group consisting of cyclic carbonates, fluorine-containing cyclic carbonates, linear carbonates, and fluorine-containing linear carbonates. In this case, the total proportion of cyclic carbonates, fluorine-containing cyclic carbonates, linear carbonates, and fluorine-containing linear 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.

[0129] The non-aqueous solvent preferably contains at least one selected from the group consisting of cyclic carbonates and linear carbonates. In this case, the total proportion of cyclic carbonates and linear 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, relative to the total amount of the non-aqueous solvent.

[0130] The content of the non-aqueous solvent is preferably 60% to 99% by mass, more preferably 70% to 97% by mass, and even more preferably 70% to 90% by mass, relative to the total amount of the non-aqueous electrolyte.

[0131] 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 of the electrolyte and the mobility of ions.

[0132] (4.4.3) Electrolyte additives The non-aqueous solvent may contain electrolyte additives. This makes it less likely for side reactions, which are not part of the primary battery reaction, to occur during the charge-discharge cycle of a lithium-ion secondary battery. The battery reaction involves the intercalation of lithium ions into and out of the positive and negative electrodes. Side reactions include the reductive decomposition of the non-aqueous electrolyte by the negative electrode, the oxidative decomposition of the non-aqueous electrolyte by the positive electrode, and the elution of metal elements from the positive electrode active material. There are no particular restrictions on the electrolyte additive; any known additive can be used as is. For example, the additive described in Japanese Patent Publication No. 2019-153443 can be used.

[0133] (5) An example of a lithium-ion secondary battery An example of a lithium-ion secondary battery according to an embodiment of this 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 this disclosure. Figure 2 is a cross-sectional view of the positive electrode 11 in the lithium-ion secondary battery 1 according to an embodiment of this disclosure. Figure 3 is a cross-sectional view of the negative electrode 12 in the lithium-ion secondary battery 1 according to an embodiment of this disclosure.

[0134] The lithium-ion secondary battery 1 according to the embodiment of this disclosure is of the stacked type. As shown in Figure 1, the lithium-ion secondary battery 1 comprises a battery element 10, a positive electrode lead 21, a negative electrode lead 22, and an outer casing 30. The battery element 10 is enclosed inside the outer casing 30. The outer casing 30 is made of laminate film. The positive electrode lead 21 and the negative electrode lead 22 are attached to the battery element 10. The positive electrode lead 21 and the negative electrode lead 22 are led out in opposite directions from the inside to the outside of the outer casing 30.

[0135] As shown in Figure 1, the battery element 10 is made up of a stack of a positive electrode 11, a separator 13, and a negative electrode 12. As shown in Figure 2, the positive electrode 11 has a positive electrode composite layer 11B formed on both main surfaces of the positive electrode current collector 11A with an undercoat layer. The positive electrode current collector 11A has an undercoat layer 111 formed on both main surfaces of the positive electrode current collector 110. As shown in Figure 3, the negative electrode 12 is formed by having a negative electrode composite material layer 12B formed on both main surfaces of the negative electrode current collector 12A with an undercoat layer. The negative electrode current collector 12A with an undercoat layer is formed by having an undercoat layer 121 formed on both main surfaces of the negative electrode current collector 120. As shown in Figure 1, the positive electrode composite material layer 11B formed on one main surface of the positive electrode current collector 11A with an undercoat layer of the positive electrode 11 and the negative electrode composite material layer 12B formed on one main surface of the negative electrode current collector 12A with an undercoat layer of the negative electrode 12 adjacent to the positive electrode 11 are facing each other via a separator 13.

[0136] A non-aqueous electrolyte is injected into the interior of the outer casing 30. The non-aqueous electrolyte permeates the positive electrode composite layer 11B, the separator 13, and the negative electrode composite layer 12B. In the lithium-ion secondary battery 1, a single cell layer 14 is formed by the adjacent positive electrode composite layer 11B, the separator 13, and the negative electrode composite layer 12B.

[0137] In this embodiment, the lithium-ion secondary battery 1 is of the stacked type, but the disclosure is not limited thereto, and the lithium-ion secondary battery 1 may be of, for example, the wound type. The wound type is formed by stacking a positive electrode, a separator, a negative electrode, and a separator in this order and winding them in layers. The wound type includes cylindrical or rectangular shapes. In this embodiment, as shown in Figure 1, the directions in which the positive lead 21 and the negative lead 22 protrude from the inside to the outside of the casing 30 are opposite to the casing 30, but the disclosure is not limited thereto. For example, the directions in which the positive lead and the negative lead protrude from the inside to the outside of the casing 30 are the same with respect to the casing 30. [Examples]

[0138] The embodiments relating to this disclosure will be described in detail below with reference to the examples. However, this disclosure is not limited in any way to the descriptions of these examples.

[0139] The products used in the examples and comparative examples are as follows. The physical properties of each product are catalog values.

[0140] <Conductive carbon material (A)> <Products containing carbon fiber (A1) and binder resin (C1) (A1, C1)> • HX-WS-1: Aqueous dispersion of "HX-WS-1" (multilayer carbon nanotube "HX-N1" (A1) (manufactured by QINGDAO HAOXIN), fiber diameter: 30nm~60nm, fiber length: 10μm~20μm, fiber solid content concentration: 10% by mass, binder resin (C1) (PVP (polyvinylpyrrolidone)) solid content concentration: 2% by mass) • HX-NS-1: HX-NS-1 is a dispersion of NMP (N-methylpyrrolidone) of multi-wall carbon nanotubes "HX-N1" (A1) (manufactured by QINGDAO HAOXIN). Fiber diameter: 30nm~60nm, fiber length: 10μm~20μm, solid content concentration of fibers: 10% by mass, solid content concentration of binder resin (C1) (PVP): 2% by mass. • HX-WS-3: Aqueous dispersion of "HX-WS-3" (multilayer carbon nanotube "HX-N3" (A1) (manufactured by QINGDAO HAOXIN), fiber diameter: 5nm~10nm, fiber length: over 50μm, fiber solid content concentration: 10% by mass, binder resin (C1) (PVP) solid content concentration: 2% by mass) <Carbon fiber (A1)> • VGCF: "VGCF(registered trademark)-H" manufactured by Showa Denko Corporation (vapor-phase carbon fiber, fiber diameter: 150 nm, fiber length: 80 μm, fiber solid content concentration: 100% by mass) <Carbon particles (A2)> • Super-P: "Super-P" manufactured by TIMCAL (conductive carbon black, solid content concentration: 100% by mass) • KS-6: TIMREX's "KS-6" (flaky graphite, solid content concentration: 100% by mass)

[0141] <Olefin-based resin (B)> • W300: "Chemipearl (registered trademark) W300" manufactured by Mitsui Chemicals, Inc. (Aqueous dispersion of low molecular weight polyethylene, solid content concentration: 40% by mass, particle size: 3.0 μm, softening point (ring-ball method): 132°C) • W900: "Chemipearl (registered trademark) W900" manufactured by Mitsui Chemicals, Inc. (Aqueous dispersion of low molecular weight polyethylene, solid content concentration: 40% by mass, particle size: 0.6 μm, softening point (ring-ball method): 132°C) • W950: "Chemipearl (registered trademark) W950" manufactured by Mitsui Chemicals, Inc. (Aqueous dispersion of low molecular weight polyethylene, solid content concentration: 40% by mass, particle size: 0.6 μm, softening point (ring-ball method): 113°C) • WP100: "Chemipearl (registered trademark) W100" manufactured by Mitsui Chemicals, Inc. (Aqueous dispersion of low molecular weight polyethylene, solid content concentration: 40% by mass, particle size: 1.0 μm, softening point (ring-ball method): 148°C) • P301W: A product developed by Mitsui Chemicals, Inc., "P301W" (low molecular weight polyethylene, solid content concentration: 100% by mass, particle size: 3.0 μm, softening point (ring-ball method): 132°C)

[0142] <Binder resin (C2)> • CMC: "2200" manufactured by Daicel Mirise Co., Ltd. (carboxymethylcellulose sodium, solid content concentration: 100% by mass) • PVDF: "L#7208" manufactured by Kureha Corporation (NMP solution of vinylidene fluoride resin, solid content concentration: 8% by mass) • PVP (Polyvinylpyrrolidone): Polyvinylpyrrolidone contained in the carbon fibers (A1) "HX-WS-1", "HX-NS-1", and "HX-WS-3" mentioned above.

[0143] <Synthetic rubber (D)> • SBR: "TRD2001" manufactured by JSR Corporation (aqueous dispersion of styrene-butadiene rubber particles, solids content: 50% by mass)

[0144] [1] Fabrication of a positive electrode current collector with an undercoat layer [1.1] Preparation of slurry for undercoat layer (Example 1) The slurry for the undercoat layer was prepared using a 5L planetary dispensing device. 600.0 parts by mass of "W900" (B) and 1250.0 parts by mass of "CMC aqueous solution" (C2) were added and mixed for 10 minutes to obtain the first mixture (Preparation step A). ​​"CMC aqueous solution" (C2) was prepared by adding "CMC" (C2) to distilled water so that the "CMC" (C2) content was 1.2% by mass relative to the total amount of "CMC aqueous solution" (C2). To the first mixture, 100.0 parts by mass of "HX-WS-3" (A1,C1) was added and kneaded for 15 minutes. Then, another 100.0 parts by mass of "HX-WS-3" (A1,C1) was added and kneaded for another 15 minutes. Finally, another 100.0 parts by mass of "HX-WS-3" (A1,C1) was added and kneaded for another 15 minutes to obtain the second mixture (Preparation step B). 30.0 parts by mass of "SBR" (D) were added to the second mixture, kneaded for 20 minutes, and vacuum degassed for 30 minutes. In this way, a slurry (composition) for the undercoat layer with a solid content of 14.0% by mass was prepared (Preparation step C).

[0145] (Examples 2-5, 7-9, Comparative Example 1, Comparative Example 6) The undercoat slurry (composition) was prepared in the same manner as in Example 1, except that the additive materials and amounts added in each of the preparation steps A to C were changed as shown in Table 1.

[0146] [Table 1]

[0147] In Table 1, "Total Addition Amount" in Preparation Step B refers to the total amount of "HX-WS-3" (A1,C1) or "HX-WS-1" (A1,C1) added to the first mixture.

[0148] (Example 6) The slurry for the undercoat layer was prepared using a 5L planetary dispensing device. 40.0 parts by mass of "VGCF" (A1) was mixed with 300.0 parts by mass of "CMC aqueous solution" (C2), and after mixing for 15 minutes, another 150.0 parts by mass of "CMC aqueous solution" (C2) was added and kneaded for 15 minutes, and then another 150.0 parts by mass of "CMC aqueous solution" (C2) was added and kneaded for 15 minutes to obtain the first mixed solution (Preparation step A). ​​"CMC aqueous solution" (C2) was prepared by adding "CMC" (C2) to distilled water so that the "CMC" (C2) content was 1.2% by mass relative to the total amount of "CMC aqueous solution" (C2). To the first mixture, 400.0 parts by mass of "W900" (B) and 533.3 parts by mass of "CMC aqueous solution" (C2) were added and kneaded for 15 minutes. Then, another 400.0 parts by mass of "W900" (B) and 533.3 parts by mass of "CMC aqueous solution" (C2) were added and kneaded for another 15 minutes to obtain the second mixture (Preparation step B). 40.0 parts by mass of "SBR" (D) were added to the second mixture, kneaded for 20 minutes, and degassed under vacuum for 30 minutes. In this way, a slurry (composition) for the undercoat layer with a solid content of 15.7% by mass was prepared (Preparation step C).

[0149] (Example 10) The slurry for the undercoat layer was prepared using a 5L planetary dispensing device. 9.0 parts by mass of "Super-P" (A2) and 250.0 parts by mass of "CMC aqueous solution" (C2) were added and mixed for 15 minutes to obtain the first mixture (Preparation step A). ​​"CMC aqueous solution" (C2) was prepared by adding "CMC" (C2) to distilled water so that the "CMC" (C2) content was 1.2% by mass relative to the total amount of "CMC aqueous solution" (C2). To the first mixture, 210.0 parts by mass of "W900" (B), 70.0 parts by mass of "HX-WS-1" (A1,C1), and 300.0 parts by mass of water were added and kneaded for 15 minutes. Then, another 210.0 parts by mass of "W900" (B), 70.0 parts by mass of "HX-WS-1" (A1,C1), and 300.0 parts by mass of water were added and kneaded for another 15 minutes. Finally, another 210.0 parts by mass of "W900" (B), 70.0 parts by mass of "HX-WS-1" (A1,C1), and 300.0 parts by mass of water were added and kneaded for another 15 minutes to obtain the second mixture (Preparation step B). 30.0 parts by mass of "SBR" (D) were added to the second mixture, kneaded for 20 minutes, and then vacuum degassed for 30 minutes. In this way, an undercoat layer slurry (composition) with a solid content of 15.0% by mass was prepared (Preparation step C).

[0150] (Examples 11-15, Comparative Examples 2-5) The undercoat layer slurry (composition) was prepared in the same manner as in Example 10, except that the additive materials and amounts added in each of the preparation steps A to C were changed as shown in Table 2.

[0151] [Table 2]

[0152] In Table 2, "Total Addition Amount" in Preparation Step A and Preparation Step B refers to the total amount of "CMC aqueous solution" (C2), "W900" (B), "HX-WS-1" (A1, C1), and water added to the first or second mixture.

[0153] (Example 16) A 5L planetary dispersant was used to prepare the slurry for the undercoat layer. 15.0 parts by mass of "Super-P" (A2), 300.0 parts by mass of "HX-NS-1" (A1, C1), 180.0 parts by mass of "P301W" (B), and 9.0 parts by mass of vacuum-dried "SBR" (D) were mixed for 10 minutes to obtain the first mixed solution (Preparation step A). To the first mixture, 50 parts by mass of "N-methylpyrrolidone" (hereinafter referred to as "NMP") was added and mixed for a further 10 minutes to obtain the second mixture (preparation step B). To the second mixture, 250.0 parts by mass of "PVDF solution" (C2) was added and kneaded for 25 minutes. Then, another 250.0 parts by mass of "PVDF solution" (C2) was added and kneaded for another 25 minutes. Finally, another 250.0 parts by mass of "PVDF solution" (C2) was added and kneaded for another 25 minutes to obtain the third mixture (preparation step C). "PVDF solution" (C2) was prepared by adding "PVDF" (C2) to "NMP" such that the "PVDF" (C2) content was 8% by mass relative to the total amount of "PVDF solution" (C2). To adjust the viscosity, 75 parts by mass of "NMP" were added to the third mixture and mixed for 15 minutes, then another 75 parts by mass of "NMP" were added and mixed for another 15 minutes, followed by vacuum degassing for 30 minutes (preparation step D). In this way, a slurry (composition) for the undercoat layer with a solid content of 20.6% by mass was prepared.

[0154] [1.2] Undercoat layer coating and drying A die coater was used to apply the slurry for the undercoat layer. The coating thickness after drying is 3 μm (coating weight is approximately 0.2 mg / cm²). 2 The undercoat slurry was applied to one main surface of an aluminum foil (20 μm thick, 200 mm wide, positive electrode current collector) and dried. Then, the undercoat slurry was similarly applied to the other main surface (uncoated surface) of the aluminum foil so that the coating thickness after drying was 3 μm, and dried. In this way, an aluminum foil roll (positive electrode current collector with undercoat layer) was obtained with undercoat layers coated on both sides.

[0155] [2] Cathode fabrication [2.1] Preparation of cathode composite slurry A 5L planetary discharger was used to prepare the cathode composite slurry. "NCM811" (manufactured by Umicore, composition formula: LiNi) as a positive electrode active material. 0.8 Co 0.1 Mn 0.1 A mixture for the positive electrode was obtained by mixing 1520 parts by mass of O2, 30 parts by mass of "Super-P" (manufactured by Timcal, conductive carbon) as a conductive additive, and 30 parts by mass of "KS-6" (manufactured by Timrex, flaky graphite) as a conductive additive for 10 minutes. 50 parts by mass of "NMP" were added to the positive electrode mixture and mixed for 20 minutes to obtain the first positive electrode mixture. To the mixture for the first positive electrode, 350 parts by mass of "PVDF solution" was added and kneaded for 30 minutes. Then, another 260 parts by mass of "PVDF solution" was added and kneaded for 15 minutes. Finally, another 220 parts by mass of "PVDF solution" was added and kneaded for 15 minutes to obtain the mixture for the second positive electrode. The "PVDF solution" was prepared by adding "PVDF" to "NMP" such that the "PVDF" content was 8% by mass relative to the total amount of "PVDF solution". To adjust the viscosity, 80 parts by mass of "NMP" were added to the mixture for the second positive electrode and mixed for 30 minutes, followed by vacuum degassing for 30 minutes. In this way, a cathode composite slurry with a solid content concentration of 65% by mass was prepared.

[0156] [2.2] Coating and drying A die coater was used to coat the cathode composite slurry. The mass of the positive electrode composite layer (coated film after drying) is 19.0 mg / cm². 2 To achieve this, the positive electrode mixture 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, undercoat layer thickness: 3 μm, width: 200 mm) and dried. Then, the mass of the positive electrode mixture layer (coated film after drying) was 19.0 mg / cm³. 2To achieve this, the positive electrode mixture slurry was similarly applied to the other main surface (i.e., the undercoat layer) of the positive electrode current collector with an undercoat layer and dried. The resulting double-sided coated cathode roll (coating amount: 38.0 mg / cm² total on both sides) 2 The product was dried in a vacuum-drying oven at 130°C for 12 hours.

[0157] [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, and the positive electrode was pressed to a density of 2.9 ± 0.05 g / cm³. 3 It was pressed using a 35-ton press to achieve this result.

[0158] [2.4] Slit The positive electrode roll was slit to obtain an area for the positive electrode composite layer (front: 56 mm x 334 mm, back: 56 mm x 408 mm) and an area for tab welding margin, thereby obtaining a positive electrode with an undercoat layer laminated on aluminum foil (hereinafter also referred to as "positive electrode with undercoat layer").

[0159] [3] Negative electrode preparation [3.1] Preparation of negative electrode slurry A 5L planetary dispensing system 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 m²) as a conductive additive. 2 A negative electrode mixture was obtained by mixing 11 parts by mass of ( / g) with the mixture for 10 minutes. 450 parts by mass of "CMC aqueous solution" were added to the negative electrode mixture and mixed for a further 20 minutes to obtain the first negative electrode mixture. To the first negative electrode mixture, 150 parts by mass of "CMC aqueous solution" was added and mixed for a further 30 minutes. Then, 293.5 parts by mass of "CMC aqueous solution" was added and mixed for another 30 minutes. Finally, 450 parts by mass of water, the solvent, was added and mixed for 15 minutes to obtain the second negative electrode mixture. 45 parts by mass of "SBR aqueous solution" (manufactured by JSR, solid content concentration: 50% by mass) was added to the mixture for the second negative electrode and 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% by mass was prepared.

[0160] [3.2] Coating and drying A die coater was used to coat the negative electrode mixture slurry. The mass of the negative electrode composite layer (coated film after drying) is 11.0 mg / cm³. 2 To achieve this, the negative electrode mixture slurry was applied to one main surface of a copper foil (10 μm thick, negative electrode current collector) and dried. Then, the other main surface (uncoated surface) of the copper foil was similarly coated with the negative electrode mixture slurry, so that the mass of the negative electrode mixture layer (coated film after drying) was 11.0 mg / cm³. 2 The negative electrode slurry was applied and dried to achieve the desired result. The resulting double-sided coated negative electrode roll (coating amount: 22.0 mg / cm² total on both sides) 2 The product was dried in a vacuum-drying oven at 120°C for 12 hours.

[0161] [3.3] Press A small press was used to press the negative electrode roll. The gap between the upper and lower rolls was adjusted, and the negative electrode roll was pressed to a density of 1.45 ± 0.05 g / cm³. 3 It was pressed using a small press machine to achieve this result.

[0162] [3.4] Slit The negative electrode roll was slit to obtain the negative electrode by obtaining the area of ​​the negative electrode composite layer (front: 58 mm x 372 mm, back: 58 mm x 431 mm) and the area for tab welding margin.

[0163] [4] Fabrication of wound-type battery (designed capacity 1Ah) [4.1] Winding A porous polyethylene membrane (60.5 mm × 450 mm) with a void ratio of 45% by volume and a thickness of 25 μm was used as the separator. The negative electrode, separator, positive electrode with undercoat layer, and separator obtained above were stacked and wound together, then press-molded. Next, an aluminum tab was joined to the blank portion of the positive electrode where the undercoat layer was placed using an ultrasonic bonding machine, and a nickel tab was joined to the blank portion of the negative electrode using an ultrasonic bonding machine. This was then sandwiched between laminate films, and three sides were heat-sealed. This resulted in an outer casing with an opening (hereinafter simply referred to as "outer casing").

[0164] [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. LiPF6 was dissolved in this mixed solvent to a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte. Before injecting the non-aqueous electrolyte, the above-mentioned casing was dried under reduced pressure in a vacuum dryer at 70°C for 12 hours. After injecting 4.7 ± 0.1 g of electrolyte into the casing, the opening of the casing was heat-sealed while under vacuum. This yielded a lithium-ion secondary battery precursor. Hereafter, the casing with the heat-sealed opening will also be simply referred to as the "battery container."

[0165] [4.3] Activation treatment 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 with a constant current at 0.05C for 4 hours (0.05C-CC), followed by a rest period of 12 hours. After that, it was charged with a constant current and constant voltage at 0.1C to 4.2V (0.1C-CCCV), rested for 30 minutes, and then discharged with a constant current at 0.1C to 2.8V (0.1C-CC). Furthermore, the charge-discharge cycle (charging to 4.2V with 0.1C-CCCV and discharging to 2.8V with 0.1C-CC) was repeated 5 times. After that, the lithium-ion secondary battery precursor was stored at 25°C for 5 days with a full charge of 4.2V (SOC 100%). Thus, wound-type batteries (lithium-ion secondary batteries) for Examples 1 to 16, Comparative Examples 1 to 6, and the Reference Example were obtained.

[0166] [5] Evaluation method The wound-type batteries of Examples 1 to 16, Comparative Examples 1 to 6, and the Reference Example underwent the following forced internal short-circuit test, initial DCIR measurement, and DCIR evaluation after high-temperature storage. The evaluation results are shown in Table 3.

[0167] [5.1] Safety evaluation (nail penetration test) A wound-type battery (designed capacity 1Ah) was charged to 4.2V at 0.1C using constant current constant voltage (0.1C-CCCV) in a 25°C temperature environment, and then subjected to a nail penetration test (nail diameter 3mm, nail penetration speed 1.0mm / sec). A 3mm diameter nail was inserted into the center of a wound-type battery (cell) used as a test battery at a speed of 1.0 mm / second, short-circuiting the positive and negative electrodes inside the battery container. The short-circuit behavior of the test battery was observed. Specifically, the surface temperature of the battery container (hereinafter also referred to as the "internal temperature of the test battery") was measured over time using a thermocouple attached to the surface of the battery container. Multiple nail-piercing tests (3 to 6 times) were performed on the test batteries at the same level, and the safety of the total number of test batteries was evaluated according to the following criteria. The safety evaluation results are shown in Table 3. Acceptable safety ratings are "A", "B", and "C".

[0168] A: The percentage of test batteries whose internal temperature exceeded 300°C was less than 30% of the total number of test batteries. B: The percentage of test batteries whose internal temperature exceeded 300°C was between 30% and 50% of the total number of test batteries. C: The percentage of test batteries whose internal temperature exceeded 300°C was between 50% and 75% of the total number of test batteries. D: The percentage of test batteries whose internal temperature exceeded 300°C was 75% or more of the total number of test batteries.

[0169] [5.2] Initial DCIR evaluation [5.2.1] Initial DCIR measurement A wound-type battery (designed capacity 1Ah) was charged to 4.2V at 0.1C using constant current constant voltage (0.1C-CCCV) in a 25°C environment, and initial DCIR measurements were performed. The battery was discharged at 0.1C for 10 seconds with a constant current (0.1C-CC-10s) and then charged at 0.1C for 10 seconds with a constant current (0.1C-CC-10s). Next, the battery was discharged at 0.2C for 10 seconds with a constant current (0.2C-CC-10s) and then charged at 0.2C for 10 seconds with a constant current (0.2C-CC-10s). Next, the battery was discharged at 0.5C for 10 seconds with a constant current (0.5C-CC-10s), and then charged at 0.5C for 10 seconds with a constant current (0.5C-CC-10s). Next, the battery was discharged at 1.0C for 10 seconds with a constant current (1.0C-CC-10s), and then charged at 1.0C for 10 seconds with a constant current (1.0C-CC-10s). Next, the battery was discharged at 2.0C for 10 seconds with a constant current (2.0C-CC-10s) and then charged at 2.0C for 10 seconds with a constant current (2.0-CC-10s). The first DC resistance (DCIR) was determined based on the voltage drop (= voltage before discharge - voltage 10 seconds after discharge) and current value (i.e., the current value corresponding to the discharge rates of 0.1C to 2.0C) for each "CC10s discharge" at discharge rates from 0.1C to 2.0C.

[0170] [5.2.2] Evaluation Method Based on the above measurements, the first relative value was calculated by setting the measured value of the first DC resistance (DCIR) of the reference example (without undercoat layer) to 100, and an initial DCIR evaluation was performed according to the following criteria. The first relative values ​​and judgment results are shown in Table 3. The acceptable initial DCIR evaluations are "A", "B", and "C".

[0171] A: The first relative value is 105 or less. B: The first relative value is greater than 105 and less than or equal to 115. C: The first relative value is greater than 115 and less than or equal to 130. D: The first relative value exceeds 130.

[0172] [5.3] DCIR evaluation after storage at 60°C [5.3.1] DCIR measurement after storage at 60℃ The fabricated wound-type battery (designed capacity 1Ah) was charged to 4.2V at 0.1C using constant current and constant voltage (0.1C-CCCV) in a 25°C environment, and then left to stand in a 60°C atmosphere for 28 days while charged. A battery that had been stored at high temperature was thus obtained. After high-temperature storage, the battery underwent the same DCIR evaluation process as described above for the initial DCIR evaluation, and the second DC resistance (DCIR) was determined.

[0173] [5.3.2] Evaluation Method Based on the measured values ​​of the second DC resistance (DCIR) of the battery after high-temperature storage, a second relative value was calculated by setting the measured value of the second DC resistance (DCIR) of the reference example (without undercoat layer) to 100, and the DCIR evaluation after storage at 60°C was performed according to the following criteria. The second relative value and judgment results are shown in Table 3. The acceptable DCIR evaluations after storage at 60°C are "A", "B", and "C".

[0174] A: The second relative value is 120 or less. B: The second relative value is greater than 120 and less than or equal to 140. C: The second relative value is greater than 140 and less than or equal to 160. D: The second relative value exceeds 160.

[0175] [5.4] Overall rating Based on the safety evaluation results, the initial DCIR evaluation results, and the DCIR evaluation results after storage at 60°C, a comprehensive evaluation was conducted according to the following criteria. The results are shown in Table 3. The acceptable overall rating is "A".

[0176] A: None of the safety evaluation results, initial DCIR evaluation results, or DCIR evaluation results after storage at 60°C resulted in a "D" rating. B: The safety evaluation results, initial DCIR evaluation results, and DCIR evaluation results after storage at 60°C all contain a "D" rating.

[0177] [Table 3]

[0178] In Table 3, "(A1)" indicates conductive carbon fiber (A1), and "(A2)" indicates conductive carbon particles (A2). In Table 3, "HX-N3" in Example 1 refers to the multi-walled carbon nanotube "HX-N3" (A1) contained in "HX-WS-3" (A1, C1). In Table 3, "HX-N1" in Comparative Examples 1 to 6, Examples 2 to 5, and Examples 7 to 16 refers to the multi-walled carbon nanotube "HX-N1" (A1) contained in "HX-WS-1" (A1,C1) or "HX-NS-1" (A1,C1). In Table 3, the content (mass%) of water-dispersible polyolefin resin (B) indicates the ratio of the mass of the solids of water-dispersible polyolefin resin (B) to the total mass of (A) to (D) (100%). In Table 3, "(C1)" in Comparative Examples 1 to 6, Examples 1 to 5, and Examples 7 to 16 refers to the binder resin (C1) (PVP) contained in "HX-WS-1" (A1,C1), "HX-WS-3" (A1,C1), or "HX-NS-1" (A1,C1). In Table 3, the content (mass%) of synthetic rubber (D) indicates the ratio of the mass of the solid content of synthetic rubber (D) to the total mass of (A) to (D) (100%).

[0179] The composition of Comparative Example 1 did not contain a conductive carbon material (A), an olefin resin (B), a binder resin (C), or synthetic rubber (D). Therefore, the safety evaluation result for Comparative Example 1 was "D". The compositions of Comparative Examples 2 to 4 contained a conductive carbon material (A), an olefin resin (B), a binder resin (C), and synthetic rubber (D). However, the content of conductive carbon fiber (A1) was not within the range of more than 5% by mass and less than or equal to 30% by mass relative to the total amount of (A) to (D). Therefore, the evaluation results of the initial DCIR and DCIR after storage at 60°C for Comparative Examples 2 to 4 were "D". The composition of Comparative Example 5 contains a conductive carbon material (A), an olefin resin (B), a binder resin (C), and synthetic rubber (D). However, the content of the olefin resin (B) relative to the total amount of (A) to (D) was not within the range of 60% to 90% by mass. The mass ratio (A / B) was not within the range of 0.08 to 0.32. Therefore, the safety evaluation result for Comparative Example 5 was "D". The composition of Comparative Example 6 contains a conductive carbon material (A), an olefin resin (B), a binder resin (C), and synthetic rubber (D). However, the content of the olefin resin (B) relative to the total amount of (A) to (D) was not within the range of 60% to 90% by mass. The content of the binder resin (C) was not within the range of 1% to 30% by mass. The content of the synthetic rubber (D) was not within the range of 1% to 20% by mass. The mass ratio (A / B) was not within the range of 0.08 to 0.32. Therefore, the safety evaluation result for Comparative Example 6 was "D". These results indicate that the compositions of Comparative Examples 1 to 6 cannot be used to create lithium-ion secondary batteries that are superior in terms of safety and battery performance.

[0180] On the other hand, in the compositions of Examples 1 to 16, the content of conductive carbon fiber (A1) relative to the total amount of (A) to (D) was in the range of more than 5% by mass and 30% by mass or less. The content of olefin resin (B) was in the range of 60% to 90% by mass. The content of binder resin (C) was in the range of 1% to 30% by mass. The content of synthetic rubber (D) was in the range of 1% to 20% by mass. The mass ratio (A / B) was in the range of 0.08 to 0.32. Therefore, in Examples 1 to 16, the safety evaluation result was "A", "B", or "C", the initial DCIR evaluation result was "A", "B", or "C", and the DCIR evaluation result after storage at 60°C was "A", "B", or "C". These results show that the compositions of Examples 1 to 16 can be used to create lithium-ion secondary batteries with excellent safety and battery performance.

[0181] Comparing Examples 1 to 16, the safety evaluation results of Examples 1 to 3 and Example 5 were "A" or "B", the evaluation result of the initial DCIR was "A", and the evaluation result of the DCIR after storage at 60°C was "B". Therefore, among Examples 1 to 16, it was found that the compositions of Examples 1 to 3 and Example 5 can be lithium-ion secondary batteries with excellent balance between safety (i.e., shutdown function) and battery performance. The first composition includes Examples 1 to 3 and Example 5 among Examples 1 to 16. The first composition is Based on the total amount of the conductive carbon material (A) composed of conductive carbon fibers (A1), olefin resin (B), binder resin (C), and synthetic rubber (D), more than 5% by mass and 30% by mass or less of conductive carbon fibers (A1), 60% to 90% by mass of olefin resin (B), 1% to 16% by mass of binder resin (C), 1% to 14% by mass of synthetic rubber (D), and contains the mass ratio (A / B) is 0.10 to 0.32, the fiber diameter of the conductive carbon fibers (A1) is 1 nm to 100 nm, the softening point of the olefin resin (B) is 120°C to 150°C.

[0182] Comparing Examples 1 to 3 and Example 5, in Example 2, the safety evaluation result was "A", the evaluation result of the initial DCIR was "A", and the evaluation result of the DCIR after storage at 60°C was "B". Therefore, among Examples 1 to 3 and Example 5, it was found that the composition of Example 2 can be a lithium-ion secondary battery with better balance between safety (i.e., shutdown function) and battery performance. The second composition includes Example 2 among Examples 1 to 16. The second composition is Based on the total amount of the conductive carbon material (A) composed of conductive carbon fibers (A1), olefin resin (B), binder resin (C), and synthetic rubber (D), Conductive carbon fiber (A1) greater than 5% by mass and less than 30% by mass, Olefin resin (B) 60% to 90% by mass, Binder resin (C) 1% to 16% by mass, It contains synthetic rubber (D) at a concentration of 1% to 14% by mass, The mass ratio (A / B) is between 0.10 and 0.32. The conductive carbon fiber (A1) has a fiber diameter of 20 nm to 100 nm. The softening point of olefin resin (B) is 120°C to 140°C. The particle size of the olefin resin (B) is 0.1 μm to 2.0 μm. [Explanation of symbols]

[0183] 1. Lithium-ion rechargeable battery 10 Battery elements 11 Positive electrode 11A Positive electrode current collector with undercoat layer 11B Positive electrode composite layer 110 Positive electrode current collector 111 Undercoat Layer 12 Negative electrode 12A Negative electrode current collector with undercoat layer 12B Negative electrode composite layer 120 Negative electrode current collector 121 Undercoat Layer 13 Separator 14 single cell layers 21 Positive lead 22 Negative lead 30 Exterior

Claims

1. With respect to the total amount of conductive carbon material (A) containing conductive carbon fiber (A1), olefin resin (B), binder resin (C) which is a resin other than the olefin resin (B), and synthetic rubber (D), The content of the conductive carbon material (A) is 7% by mass to 30% by mass, The conductive carbon fiber (A1) is present in an amount of 7% to 30% by mass, The aforementioned olefin resin (B) is present in an amount of 60% to 90% by mass, The aforementioned binder resin (C) is 1% to 30% by mass, The synthetic rubber (D) contains 1% to 20% by mass, A conductive composition for an undercoat layer, wherein the ratio of the content of the conductive carbon material (A) to the content of the olefin resin (B) is 0.08 to 0.

32.

2. The conductive composition for an undercoat layer according to claim 1, wherein the conductive carbon material (A) further comprises conductive carbon particles (A2).

3. The conductive composition for an undercoat layer according to claim 1, wherein the conductive carbon fiber (A1) is at least one selected from carbon nanotubes and carbon fibers.

4. The conductive composition for an undercoat layer according to claim 1, wherein the olefin resin (B) comprises a polyethylene resin or a polypropylene resin.

5. The conductive composition for an undercoat layer according to claim 1, wherein the olefin resin (B) comprises a water-dispersible olefin resin.

6. The conductive composition for an undercoat layer according to claim 1, wherein the particle size of the olefin resin (B) is 0.1 μm to 9.0 μm, and the softening point of the olefin resin (B) is 70°C or higher.

7. The conductive composition for an undercoat layer according to claim 1, wherein the binder resin (C) comprises carboxymethylcellulose or polyvinylidene fluoride.

8. The conductive composition for an undercoat layer according to claim 1, wherein the synthetic rubber (D) comprises styrene-butadiene rubber.

9. A conductive composition for an undercoat layer, as described in claim 1, used in the undercoat layer of an electrode of a lithium-ion secondary battery.

10. The conductive composition for an undercoat layer according to claim 9, wherein the electrode is a positive electrode.

11. Current collector and, The current collector comprises an undercoat layer laminated on at least one main surface, The undercoat layer comprises a solid component of the conductive composition for undercoat layer described in any one of claims 1 to 10, wherein the undercoat layer is a current collector with an undercoat layer.

12. A current collector with an undercoat layer as described in Claim 11, The composite layer laminated on the undercoat layer, An electrode equipped with

13. A lithium-ion secondary battery comprising the electrode described in Claim 12.

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