Negative electrode for lithium-ion secondary battery, lithium-ion secondary battery, and lithium-ion secondary battery module

Abstract

Provided is a negative electrode for a lithium-ion secondary battery, the electrode containing a negative electrode current collector layer and a negative electrode active material layer (2). A negative electrode active material contained in the negative electrode active material layer (2) contains a silicon-based active material. A binder contained in the negative electrode active material layer (2) contains a polycarboxylic acid-based polymer. The tensile strength of the negative electrode current collector layer is 250 MPa or more, as measured under conditions of a tensile speed of 200 mm / min and a temperature of 25°C in accordance with JIS Z 2241: 2011.

Description

Negative electrode for lithium-ion secondary batteries, lithium-ion secondary batteries, and lithium-ion secondary battery modules

[0001] This invention relates to a negative electrode for a lithium-ion secondary battery, a lithium-ion secondary battery, and a lithium-ion secondary battery module.

[0002] Carbon-based and silicon-based active materials are known as negative electrode active materials for lithium-ion secondary batteries. Silicon-based active materials are attracting attention because they can produce lithium-ion secondary batteries with higher capacity compared to those using carbon-based active materials. An example of a lithium-ion secondary battery technology using silicon-based active materials as the negative electrode active material is the one described in Patent Document 1.

[0003] Patent Document 1 describes a surface with a wetting tension of 32 to 44 mN / m and an conductivity of 2.7 × 10⁻⁶ ７ ~5.7 x 10 ７ The negative electrode described comprises a negative electrode current collector with an S / m ratio and an active material-containing layer provided on the surface of the negative electrode current collector, wherein the active material-containing layer contains a negative electrode active material and a binder. Patent Document 1 describes that it is possible to provide a negative electrode with excellent output characteristics in a battery.

[0004] Japanese Patent Publication No. 2023-127717

[0005] However, lithium-ion secondary batteries using silicon-based active materials as the negative electrode active material exhibited significant expansion and contraction of the silicon-based active material during charging and discharging, resulting in room for improvement in terms of cycle characteristics compared to lithium-ion secondary batteries using carbon-based active materials as the negative electrode active material.

[0006] The present invention provides a negative electrode for a lithium-ion secondary battery that can produce a lithium-ion secondary battery with an improved balance of capacity and cycle characteristics, as well as a lithium-ion secondary battery and a lithium-ion secondary battery module with an improved balance of capacity and cycle characteristics.

[0007] The inventors diligently conducted research to achieve the above objectives. As a result, they discovered that a lithium-ion secondary battery with an improved balance of capacity and cycle characteristics can be obtained by including a silicon-based active material in the negative electrode active material layer, a polycarboxylic acid-based polymer in the binder in the negative electrode active material layer, and a predetermined tensile strength in the negative electrode current collector layer, thus completing the present invention.

[0008] According to the present invention, the following lithium-ion secondary battery is provided.

[0009] [1] A negative electrode for a lithium-ion secondary battery comprising a negative electrode current collector layer and a negative electrode active material layer, wherein the negative electrode active material contained in the negative electrode active material layer comprises a silicon-based active material, the binder contained in the negative electrode active material layer comprises a polycarboxylic acid polymer, and the tensile strength of the negative electrode current collector layer, measured in accordance with JIS Z 2241:2011 under conditions of a tensile speed of 200 mm / min and 25°C, is 250 MPa or more. [2] The negative electrode for a lithium-ion secondary battery according to [1], wherein when the entire negative electrode active material layer is 100 parts by mass, the content of the polycarboxylic acid polymer in the negative electrode active material layer is 1 part by mass or more and 10 parts by mass or less. [3] The silicon-based active material comprises SiO ｘ[1] or [2] A negative electrode for a lithium-ion secondary battery according to [1] or [2], comprising one or more selected from the group consisting of (0 < x ≤ 2), Si / C comprising Si-C composite particles comprising silicon and carbon material, and Si. [4] A negative electrode for a lithium-ion secondary battery according to any one of [1] to [3], wherein when the entire negative electrode active material layer is 100 parts by mass, the content of the silicon-based active material in the negative electrode active material layer is 5 parts by mass or more. [5] A negative electrode for a lithium-ion secondary battery according to any one of [1] to [4], wherein the silicon-based active material comprises Si / C comprising Si-C composite particles comprising silicon and carbon material. [6] A negative electrode for a lithium-ion secondary battery according to any one of [1] to [5], wherein the negative electrode active material further comprises a carbon-based active material. [7] A negative electrode for a lithium-ion secondary battery according to [6], wherein when the entire negative electrode active material layer is 100 parts by mass, the content of the carbon-based active material in the negative electrode active material layer is 50 parts by mass or more and 95 parts by mass or less. [8] The negative electrode for a lithium-ion secondary battery according to any one of [1] to [7], wherein the negative electrode active material layer further comprises a conductive additive. [9] The negative electrode for a lithium-ion secondary battery according to [8], wherein when the total amount of the negative electrode active material layer is 100 parts by mass, the content of the conductive additive in the negative electrode active material layer is 0.01 parts by mass or more and 2 parts by mass or less.

[10] The negative electrode for a lithium-ion secondary battery according to [8] or [9], wherein the conductive additive comprises carbon nanotubes.

[11] The specific surface area of ​​the carbon nanotubes, as measured by the nitrogen adsorption BET method, is 300 m². ２

[10] A negative electrode for a lithium-ion secondary battery, wherein the negative electrode is greater than or equal to 1 / g.

[12] A negative electrode for a lithium-ion secondary battery, wherein the aspect ratio of the carbon nanotube is 1000 or greater, according to

[10] or

[11] .

[13] A negative electrode for a lithium-ion secondary battery, wherein the thickness of the negative electrode current collector layer is 1 μm or more and 20 μm or less, according to any one of [1] to

[12] .

[14] A negative electrode for a lithium-ion secondary battery, wherein the tensile strength of the binder, measured in accordance with JIS K 7127:1999 at a tensile speed of 200 mm / min and 25°C, is 110 MPa or more and 340 MPa or less, according to any one of [1] to

[13] .

[15] A negative electrode for a lithium-ion secondary battery, wherein the negative electrode current collector layer contains copper foil, according to any one of [1] to

[14] .

[16] A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery, according to any one of [1] to

[15] .

[17] A lithium-ion secondary battery according to

[16] , wherein the energy density is 700 Wh / L or more on a cell basis.

[18] A lithium-ion secondary battery module comprising the lithium-ion secondary battery according to

[16] or

[17] .

[0010] According to the present invention, it is possible to provide a negative electrode for a lithium-ion secondary battery that can obtain a lithium-ion secondary battery with an improved balance of capacity and cycle characteristics, as well as a lithium-ion secondary battery and a lithium-ion secondary battery module with an improved balance of capacity and cycle characteristics.

[0011] This is a schematic cross-sectional view illustrating an example of a lithium-ion secondary battery according to this embodiment.

[0012] Embodiments of the present invention will be described below with reference to the drawings. In all drawings, similar components are denoted by the same reference numerals, and their descriptions are omitted as appropriate. Also, the drawings are schematic diagrams and do not necessarily correspond to the actual dimensional ratios. In the specification, unless otherwise specified, the notation "A to B" regarding numerical ranges means A or more and B or less. For example, 1 to 5% means 1% or more and 5% or less.

[0013] 1. Negative electrode for lithium-ion secondary battery The negative electrode for lithium-ion secondary battery of this embodiment is a negative electrode for lithium-ion secondary battery comprising a negative electrode current collector layer and a negative electrode active material layer, wherein the negative electrode active material contained in the negative electrode active material layer contains a silicon-based active material, and the binder contained in the negative electrode active material layer contains a polycarboxylic acid-based polymer, and the tensile strength of the negative electrode current collector layer, measured in accordance with JIS Z 2241:2011 method under conditions of a tensile speed of 200 mm / min and 25°C, is 250 MPa or more.

[0014] Although the mechanism by which the negative electrode for lithium-ion secondary batteries of this embodiment solves the above-mentioned problems is not clear, it is thought that the tensile strength of the negative electrode current collector layer being within the above numerical range suppresses the expansion of the entire negative electrode due to the silicon-based active material contained in the negative electrode active material layer. This is thought to improve the performance balance of the capacity and cycle characteristics of the resulting lithium-ion secondary battery.

[0015] In the negative electrode for the lithium-ion secondary battery of this embodiment, the tensile strength of the negative electrode current collector layer, measured in accordance with JIS C 6515:1998, is 250 MPa or more. From the viewpoint of further improving the performance balance of the capacity and cycle characteristics of the resulting lithium-ion secondary battery, it is preferably 300 MPa or more, more preferably 350 MPa or more, even more preferably 400 MPa or more, even more preferably 450 MPa or more, even more preferably 500 MPa or more, and even more preferably 550 MPa or more. There is no particular upper limit to the tensile strength of the negative electrode current collector layer, but it may be, for example, 2000 MPa or less, 1500 MPa or less, or 1000 MPa or less. Therefore, the tensile strength of the negative electrode current collector layer is preferably 300 MPa to 2000 MPa, more preferably 350 MPa to 2000 MPa, even more preferably 400 MPa to 1500 MPa, even more preferably 450 MPa to 1500 MPa, even more preferably 500 MPa to 1000 MPa, and even more preferably 550 MPa to 1000 MPa, from the viewpoint of further improving the performance balance of the capacity and cycle characteristics of the resulting lithium-ion secondary battery. The tensile strength of the negative electrode current collector layer can be measured by the method described in the examples. <Negative electrode active material layer> The negative electrode for the lithium-ion secondary battery of this embodiment includes a negative electrode current collector layer and a negative electrode active material layer. (Negative electrode active material) The negative electrode active material included in the negative electrode active material layer of this embodiment includes a silicon-based active material from the viewpoint of improving the battery capacity of the resulting lithium-ion secondary battery.

[0016] The silicon-based active material in this embodiment is preferably SiO ｘ (0 < x ≤ 2), comprising one or more selected from the group consisting of Si / C including silicon and carbon material Si-C composite particles, and Si, more preferably SiO ｘ(0 < x ≤ 2), and includes one or more selected from the group consisting of Si / C containing Si-C composite particles containing silicon and a carbon material. From the viewpoint of further improving the cycle characteristics of the obtained lithium-ion secondary battery, preferably, it includes Si / C containing Si-C composite particles containing silicon and a carbon material. Further, the silicon-based active material is preferably in powder form.

[0017] From the viewpoint of further improving the battery performance of the obtained lithium-ion secondary battery, the Si / C of the present embodiment preferably contains a porous carbon material in the Si-C composite particles, and silicon is present in at least a part of the pores of the porous carbon material.

[0018] In the present embodiment, as a method for confirming that the Si-C composite particles in the Si / C contain silicon and a carbon material, and silicon is present in at least a part of the pores of the porous carbon material in the Si-C composite particles, for example, regarding the cross-section of the Si-C composite particles in the Si / C, using a scanning electron microscope, an energy dispersive X-ray spectroscopy detector, and image analysis software, secondary electrons are selected as the detection target, and under the conditions of an acceleration voltage of 3 kV, a mapping integration number of 20 times, and a magnification of 3000 times, element mapping of silicon and carbon is performed and observed.

[0019] Examples of the porous carbon material constituting the Si-C composite particles include activated carbon, aggregates of carbon fibers or aggregates of carbon nanotubes, carbon obtained by heat-treating resins or organic substances, hard carbon, and the like. The porous carbon material can be produced by a known production method such as a method for producing activated carbon or heat treatment of a polymer, but commercially available ones may also be purchased, as long as silicon can be generated or incorporated into the pores of the porous carbon, and it is not limited thereto.

[0020] Median diameter D in the volume frequency particle size distribution of the Si / C of the present embodiment by the laser diffraction scattering method ５０From the perspective of further improving the performance balance between the energy density and rapid charging performance of the resulting lithium-ion secondary battery, it is preferably 1.0 µm or more, more preferably 2.0 µm or more, still more preferably 3.0 µm or more, and further preferably 20.0 µm or less, more preferably 15.0 µm or less, still more preferably 10.0 µm or less. Also, for the same reason, it is preferably 1.0 µm or more and 20.0 µm or less, more preferably 2.0 µm or more and 15.0 µm or less, still more preferably 3.0 µm or more and 10.0 µm or less.

[0021] When the total amount of the negative electrode active material layer of this embodiment is 100 parts by mass, the content of the silicon-based active material in the negative electrode active material layer is preferably 5 parts by mass or more, more preferably 8 parts by mass or more, still more preferably 10 parts by mass or more. There is no particular limitation on the upper limit value of the content of the silicon-based active material in the negative electrode active material layer, but for example, it may be 40 parts by mass or less, may be 30 parts by mass or less, or may be 25 parts by mass or less. Therefore, the content of the silicon-based active material in the negative electrode active material layer is preferably 5 parts by mass or more and 40 parts by mass or less, more preferably 8 parts by mass or more and 30 parts by mass or less, still more preferably 10 parts by mass or more and 25 parts by mass or less.

[0022] The negative electrode active material of this embodiment preferably further contains a carbon-based active material. Examples of the carbon-based active material include graphite, amorphous carbon, diamond-like carbon, fullerene, carbon nanotube, carbon nanohorn, etc., and among these, it is preferable to contain graphite. Also, there is no particular limitation on the type of graphite, but from the perspective of further improving the battery performance of the resulting lithium-ion secondary battery, it is preferably graphite containing amorphous carbon on the surface. Also, it is preferably powdered graphite.

[0023] When the total amount of the negative electrode active material layer in this embodiment is 100 parts by mass, the content of carbon-based active material in the negative electrode active material layer is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, even more preferably 75 parts by mass or more, and preferably 95 parts by mass or less, more preferably 90 parts by mass or less, even more preferably 88 parts by mass or less, and even more preferably 85 parts by mass or less. Also, when the total amount of the negative electrode active material layer in this embodiment is 100 parts by mass, the content of carbon-based active material in the negative electrode active material layer is preferably 50 parts by mass or more and 95 parts by mass or less, more preferably 60 parts by mass or more and 90 parts by mass or less, even more preferably 70 parts by mass or more and 88 parts by mass or less, and even more preferably 75 parts by mass or more and 85 parts by mass or less.

[0024] (Binder) The binder contained in the negative electrode active material layer of this embodiment contains a polycarboxylic acid polymer. In this specification, a polycarboxylic acid polymer means a polymer containing an unsaturated carboxylic acid or a salt thereof as a monomer unit. Examples of unsaturated carboxylic acids or salts thereof include acrylic acid or a salt thereof, methacrylic acid or a salt thereof, maleic acid or a salt thereof, fumaric acid or a salt thereof, and among these, preferably one or more selected from the group consisting of acrylic acid or a salt thereof and methacrylic acid or a salt thereof, more preferably acrylic acid or a salt thereof, and even more preferably acrylic acid. In addition, alkali metal salts (e.g., Li, Na, K, etc.) are preferred for the salt. The polycarboxylic acid polymer of this embodiment preferably contains one or more selected from the group consisting of polyacrylic acid or a salt thereof and polymethacrylic acid or a salt thereof, more preferably polyacrylic acid or a salt thereof, and even more preferably polyacrylic acid.

[0025] When the total amount of the negative electrode active material layer in this embodiment is 100 parts by mass, the content of the polycarboxylic acid polymer in the negative electrode active material layer is preferably 1 part by mass or more, more preferably 1.3 parts by mass or more, even more preferably 1.5 parts by mass or more, even more preferably 2.0 parts by mass or more, and preferably 10 parts by mass or less, more preferably 9.0 parts by mass or less, even more preferably 8.0 parts by mass or less, and even more preferably 6.0 parts by mass or less. Also, when the total amount of the negative electrode active material layer in this embodiment is 100 parts by mass or more, more preferably 1.3 parts by mass or more and 9.0 parts by mass or less, even more preferably 1.5 parts by mass or more and 8.0 parts by mass or less, and even more preferably 2.0 parts by mass or more and 6.0 parts by mass or less.

[0026] The tensile strength of the binder, measured in accordance with JIS K 7127:1999 at a tensile speed of 200 mm / min and 25°C, is preferably 110 MPa or higher, more preferably 130 MPa or higher, even more preferably 150 MPa or higher, even more preferably 170 MPa or higher, and preferably 340 MPa or lower, more preferably 320 MPa or lower, even more preferably 300 MPa or lower, and even more preferably 280 MPa or lower, measured in accordance with JIS K 7127:1999 at a tensile speed of 200 mm / min and 25°C, for similar reasons, is preferably 110 MPa or higher and 340 MPa or lower, more preferably 130 MPa or higher and 320 MPa or lower, even more preferably 150 MPa or higher and 300 MPa or lower, and even more preferably 170 MPa or higher and 280 MPa or lower. Test specimens for measuring the tensile strength of the binder can be prepared, for example, by the following method. (Method) The binder is dissolved in distilled water, the resulting solution is applied to a PET (polyethylene terephthalate) film, heat-treated at 130°C for 3 hours, and dried to produce a binder film with a thickness of 30 μm. Furthermore, the obtained binder film is cut into 80 mm x 20 mm test specimens. The tensile strength of the binder can be measured more specifically by the method described in the examples.

[0027] (Conductive aid) From the viewpoint of further improving the performance balance between the capacity and cycle characteristics of the resulting lithium ion secondary battery, the negative electrode active material layer of the present embodiment preferably further contains a conductive aid.

[0028] Examples of the conductive aid in the negative electrode active material layer of the present embodiment include carbon fibers such as carbon nanofibers; carbon blacks such as acetylene black and ketjen black; carbon materials such as activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes. Among these, one of them may be used alone, or two or more of them may be used in combination. Among these, from the viewpoint of further improving the performance balance between the capacity and cycle characteristics of the resulting lithium ion secondary battery, the conductive aid in the negative electrode active material layer of the present embodiment preferably contains a carbon material, more preferably contains carbon nanotubes, and still more preferably contains single-walled carbon nanotubes.

[0029] When the total amount of the negative electrode active material layer of the present embodiment is 100 parts by mass, the content of the conductive aid in the negative electrode active material layer is preferably 0.01 part by mass or more, more preferably 0.02 part by mass or more, still more preferably 0.03 part by mass or more, and preferably 2 parts by mass or less, more preferably 1.5 parts by mass or less, still more preferably 1.0 part by mass or less, still more preferably 0.5 part by mass or less, and still more preferably 0.3 part by mass or less. Also, when the total amount of the negative electrode active material layer of the present embodiment is 100 parts by mass, the content of the conductive aid in the negative electrode active material layer is preferably 0.01 part by mass or more and 2 parts by mass or less, more preferably 0.02 part by mass or more and 1.5 parts by mass or less, still more preferably 0.03 part by mass or more and 1.0 part by mass or less, still more preferably 0.03 part by mass or more and 0.5 part by mass or less, and still more preferably 0.03 part by mass or more and 0.3 part by mass or less.

[0030] The specific surface area of the carbon nanotubes in the negative electrode active material layer of the present embodiment measured by the nitrogen adsorption BET method is preferably 300 m ２ / g or more, more preferably 350 m ２ / g or more, more preferably 400m ２ / g or more, more preferably 450m ２ / g or more, more preferably 500m ２ It is 1500 m or more. There is no particular limit to the upper limit of the specific surface area of ​​carbon nanotubes in the negative electrode active material layer of this embodiment, but for example, 1500 m ２ It may be less than / g, and 1200m ２ It may be less than / g, and 1000m ２ It may be less than / g. Therefore, the specific surface area of ​​carbon nanotubes in the negative electrode active material layer is preferably 300 m from the viewpoint of further improving the performance balance of the capacity and cycle characteristics of the resulting lithium-ion secondary battery. ２ / g or more 1500m ２ / g or less, more preferably 350m ２ / g or more 1200m ２ / g or less, more preferably 400m ２ / g or more 1000m ２ / g or less, more preferably 450m ２ / g or more 1000m ２ / g or less, more preferably 500m ２ / g or more 1000m ２ It is less than or equal to / g. The specific surface area of ​​carbon nanotubes can be measured by the method described in the examples.

[0031] The aspect ratio of the carbon nanotubes in this embodiment is preferably 1000 or more, more preferably 10000 or more, even more preferably 20000 or more, even more preferably 30000 or more, and even more preferably 40000 or more, from the viewpoint of further improving the performance balance of the capacity and cycle characteristics of the resulting lithium-ion secondary battery. There is no particular upper limit to the aspect ratio of the carbon nanotubes in this embodiment, but it may be, for example, 10000000 or less, 5000000 or less, 2000000 or less, 1000000 or less, or 750000 or less. Therefore, from the viewpoint of further improving the performance balance of the capacity and cycle characteristics of the resulting lithium-ion secondary battery, the aspect ratio of the carbon nanotubes is preferably 1,000 to 1,000,000, more preferably 10,000 to 5,000,000, even more preferably 20,000 to 2,000,000, even more preferably 30,000 to 1,000,000, and even more preferably 40,000 to 750,000.

[0032] <Negative electrode current collector layer> The negative electrode for the lithium-ion secondary battery of this embodiment includes a negative electrode current collector layer.

[0033] The negative electrode current collector layer of this embodiment includes a negative electrode current collector formed of, for example, aluminum, stainless steel, nickel, titanium, or an alloy thereof, and preferably a negative electrode current collector formed of copper. The shape of the negative electrode current collector may be, for example, foil, a plate, or a mesh, and preferably foil. Therefore, the negative electrode current collector layer of this embodiment preferably includes copper foil.

[0034] In this embodiment, the thickness of the negative electrode current collector layer is preferably 1 μm or more, more preferably 2 μm or more, even more preferably 4 μm or more, even more preferably 5 μm or more, even more preferably 6 μm or more, and preferably 20 μm or less, more preferably 18 μm or less, even more preferably 16 μm or less, even more preferably 14 μm or less, even more preferably 12 μm or less, and even more preferably 10 μm or less, for similar reasons.

[0035] 2. Lithium-ion secondary battery: The lithium-ion secondary battery of this embodiment is equipped with a negative electrode for lithium-ion secondary batteries of this embodiment, and therefore the balance of capacity and cycle characteristics is improved.

[0036] The lithium-ion secondary battery of this embodiment will be described with reference to the figures. Figure 1 is a schematic cross-sectional view showing an example of the lithium-ion secondary battery of this embodiment. As shown in Figure 1, the lithium-ion secondary battery 10 comprises a negative electrode for the lithium-ion secondary battery of this embodiment, an electrolyte, and a positive electrode. A separator 5 can also be provided between the positive electrode and the negative electrode. Multiple pairs of positive and negative electrodes can be provided.

[0037] The lithium-ion secondary battery 10 has a positive electrode comprising a positive electrode current collector 3 made of a metal such as aluminum foil and a positive electrode active material layer 1 containing a positive electrode active material provided thereon, and a negative electrode comprising a negative electrode current collector 4 made of a metal such as copper foil and a negative electrode active material layer 2 containing a negative electrode active material provided thereon. The positive electrode and the negative electrode are laminated via a separator 5 made of a nonwoven fabric or a polypropylene microporous film, for example, so that the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other. This electrode pair is housed in a container formed of an outer casing 6, 7 made of an aluminum laminate film, for example. A positive electrode tab 9 is connected to the positive electrode current collector 3 and a negative electrode tab 8 is connected to the negative electrode current collector 4, and these tabs are pulled out of the container. An electrolyte is injected into the container and sealed. A structure in which a group of electrodes, each consisting of multiple laminated electrodes, is housed in the container is also possible.

[0038] The lithium-ion secondary battery 10 can be manufactured according to known methods. For electrodes, for example, laminates or wound bodies can be used. For the outer casing, metal casings or aluminum laminate casings can be used as appropriate. The shape of the battery may be any shape, such as coin type, button type, sheet type, cylindrical type, prismatic type, or flat type.

[0039] In the lithium-ion secondary battery of this embodiment, the positive electrode preferably comprises a positive electrode active material layer containing a positive electrode active material and a positive electrode current collector. The positive electrode active material layer of this embodiment preferably comprises a positive electrode active material and a binder, and more preferably comprises a positive electrode active material, a binder and a conductive additive.

[0040] The positive electrode active material in the positive electrode active material layer of this embodiment may be, for example, a composite oxide of lithium and a transition metal such as lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-manganese-cobalt composite oxide, lithium-nickel-manganese-aluminum composite oxide, lithium-nickel-cobalt-manganese-aluminum composite oxide; TiS２ FeS, MoS ２ Transition metal sulfides such as MnO, V ２ O ５ , V ６ O １３ , TiO ２ Examples include transition metal oxides such as olivine-type lithium phosphate oxide, and these may be used individually or in combination of two or more.

[0041] Examples of conductive additives in the positive electrode active material layer of this embodiment include carbon fibers such as carbon nanofibers; carbon blacks such as acetylene black and Ketjenblack; and carbon materials such as activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes. One of these may be used alone, or two or more may be used in combination.

[0042] Examples of binders in the positive electrode active material layer of this embodiment include fluorine-based binders such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); and aqueous binders such as styrene-butadiene rubber. One of these may be used alone, or two or more may be used in combination.

[0043] The positive electrode current collector of this embodiment may be made of, for example, aluminum, stainless steel, nickel, titanium, or an alloy thereof. The shape of the positive electrode current collector may be, for example, foil, a flat plate, or a mesh. The thickness of the positive electrode current collector is not particularly limited, but for example, it is 1 μm or more and 50 μm or less.

[0044] The electrolyte in this embodiment may include, for example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), and butylene carbonate (BC); linear carbonates such as ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and dipropyl carbonate (DPC); aliphatic carboxylic acid esters; γ-lactones such as γ-butyrolactone; linear ethers; and organic solvents such as cyclic ethers, to which lithium hexafluoride phosphate (LiPF) is added. ６), lithium borofluoride (LiBF ４ ), LiFSI, lithium perchlorate (LiClO ４ Examples include solutions of lithium salts such as ), and the organic solvent may be used alone or in combination of two or more.

[0045] The separator of this embodiment is, for example, mainly made of a porous membrane, woven fabric, nonwoven fabric, etc., and as the resin component, for example, polyolefin resins such as polypropylene and polyethylene, polyester resins, acrylic resins, styrene resins, or nylon resins can be used. In addition, if necessary, a layer containing inorganic particles may be formed in the separator, and examples of inorganic particles include insulating oxides, nitrides, sulfides, carbides, etc.

[0046] As the outer casing of this embodiment, for example, a case or can made of a flexible film can be used, and from the viewpoint of reducing the weight of the battery, it is preferable to use a flexible film. The flexible film can be one in which a resin layer is provided on both the front and back surfaces of a metal base layer. The metal layer can be selected to have barrier properties such as preventing leakage of electrolyte and intrusion of moisture from the outside, and can be made of aluminum, stainless steel, etc. A heat-sealable resin layer, such as a modified polyolefin, is provided on at least one surface of the metal layer. The outer casing is formed by facing the heat-sealable resin layers of the flexible film toward each other and heat-sealing the area around the part that houses the electrode laminate. A resin layer such as a nylon film or polyester film can be provided on the surface of the outer casing opposite to the surface on which the heat-sealable resin layer is formed.

[0047] The energy density of the lithium-ion secondary battery of this embodiment is preferably 700 Wh / L or more, more preferably 720 Wh / L or more, even more preferably 740 Wh / L or more, even more preferably 750 Wh / L or more, even more preferably 760 Wh / L or more, even more preferably 780 Wh / L or more, and even more preferably 800 Wh / L or more, on a cell basis. There is no particular upper limit to the energy density of the lithium-ion secondary battery of this embodiment, but for example it may be 1000 Wh / L or less, or 950 Wh / L or less. Furthermore, the energy density of the lithium-ion secondary battery of this embodiment is preferably 700 Wh / L or more and 1000 Wh / L or less, more preferably 720 Wh / L or more and 1000 Wh / L or less, even more preferably 740 Wh / L or more and 1000 Wh / L or less, even more preferably 750 Wh / L or more and 1000 Wh / L or less, even more preferably 760 Wh / L or more and 1000 Wh / L or less, even more preferably 780 Wh / L or more and 1000 Wh / L or less, and even more preferably 800 Wh / L or more and 950 Wh / L or less.

[0048] The energy density of the lithium-ion secondary battery in this embodiment can be measured, for example, by the following method 1.

[0049] (Method 1) The lithium-ion secondary battery of this embodiment is charged at 7.2 mA, and after the upper limit voltage reaches 4.2 V, it is charged at a constant voltage until the total charging time is 12 hours. Next, it is discharged at a constant current of 7.2 mA until the lower limit voltage reaches 2.5 V. Then, it is charged again under the same conditions, left in a constant temperature bath at 45°C for 3 days, discharged again under the same conditions, and then charged and discharged one more time. At this time, the capacity and average voltage at the time of the last discharge are measured. Next, after the last discharge, the thickness of the lithium-ion secondary battery is measured. Then, based on the above measured values, the electrode area of ​​the electrode stack, the thickness of the laminate casing, the thickness of the negative electrode current collector, and the thickness of the positive electrode current collector in the positive electrode, the cell-equivalent energy density E (Wh / L) is calculated from the following formula. (Energy density E) = (Capacity at the last discharge) × (Average voltage at the last discharge) / (Electrode area of ​​the electrode stack) / ((Thickness of lithium-ion secondary battery) - (Thickness of laminate casing) - (Thickness of positive electrode current collector) / 2 - (Thickness of negative electrode current collector) / 2)

[0050] 3. Lithium-ion secondary battery module The lithium-ion secondary battery module of this embodiment comprises the lithium-ion secondary battery of this embodiment. Since the lithium-ion secondary battery of this embodiment has an improved balance of capacity and cycle characteristics, the lithium-ion secondary battery module of this embodiment also has an improved balance of capacity and cycle characteristics.

[0051] The lithium-ion secondary battery module of this embodiment preferably includes two or more lithium-ion secondary batteries of this embodiment connected in series or in parallel. The lithium-ion secondary battery module of this embodiment more preferably includes a housing capable of accommodating two or more lithium-ion secondary batteries of this embodiment connected in series or in parallel. The lithium-ion secondary battery module of this embodiment may also be a battery pack that includes one or more selected from the group consisting of a protection circuit to protect the lithium-ion secondary batteries from overcurrent, a balance circuit to equalize the voltage between the electrodes of the lithium-ion secondary batteries, a controller to control the lithium-ion secondary batteries, a cooler capable of cooling the lithium-ion secondary batteries, and a heater capable of heating the lithium-ion secondary batteries.

[0052] The lithium-ion secondary battery pack of this embodiment can be used in a battery system comprising a plurality of electrically connected lithium-ion secondary battery modules and a battery control system. Examples of battery systems include battery packs, stationary battery systems, battery systems for the power of automobiles, battery systems for auxiliary equipment in automobiles, and battery systems for emergency power supplies.

[0053] The embodiments of the present invention have been described above, but these are merely examples, and various other configurations can also be adopted.

[0054] It should be noted that the present invention is not limited to the embodiments described above, and any modifications, improvements, etc., that can achieve the objectives of the present invention are included in the present invention.

[0055] The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto.

[0056] <Tensile Strength of Negative Electrode Current Collector Layer> For each negative electrode current collector layer, the tensile strength was measured using a tensile testing machine (LOP-1000N, manufactured by Imada Co., Ltd.) in accordance with JIS Z 2241:2011, with test specimens (current collector layers cut to 80 mm x 20 mm) measured at a tensile speed of 200 mm / min, 25°C, 50% humidity, and a chuck distance of 60 mm. The average of five measured values ​​was used for each tensile strength. The results are shown in Table 2. The tensile direction is the MD direction.

[0057] <Tensile Strength of Binder Films> Each binder was dissolved in the solvent according to the formulations listed in Table 1. The resulting solutions were applied to PET (polyethylene terephthalate) films, heat-treated at 130°C for 3 hours, and dried to produce binder films with a thickness of 30 μm. After the temperature of each binder film had cooled to room temperature, they were immediately cut into 80 mm x 20 mm test pieces. The thickness was measured using a micrometer (Mitutoyo, model: CLM2-10QMB, minimum display: 1 μm). The tensile strength of each test piece was then measured using a tensile testing machine (LOP-1000N, IMADA Corporation) in accordance with JIS K 7127:1999, under the conditions of a tensile speed of 200 mm / min, 25°C, 50% humidity, and a chuck distance of 60 mm. The average of five measured values ​​was used for each tensile strength. The results are shown in Table 1.

[0058]

[0059] <Measurement of Specific Surface Area of ​​Negative Electrode Active Material and Conductive Additive> The specific surface area of ​​the negative electrode active material and conductive additive was measured using a fully automatic specific surface area measuring device (Macsorb HM-1208, manufactured by Mountec Co., Ltd.) in accordance with JIS Z 8830:2013, using the BET flow method and the single-point method. In the above measurements, nitrogen was used as the adsorbent gas. Degassing treatment was performed by adding nitrogen and heating at 300°C for 1 hour to remove the gas adsorbed on the surface.

[0060] <Measurement of Aspect Ratio (Fiber Length / Fiber Diameter) of Single-Walled Carbon Nanotubes> Using a scanning electron microscope, single-walled carbon nanotubes were observed under conditions of 5000x magnification and a field of view of 20 μm × 20 μm. The fiber diameter and fiber length of the single-walled carbon nanotubes were measured, and the aspect ratio was calculated. A total of 100 nanotubes were measured, and their average values ​​were used. More specifically, single-walled carbon nanotubes were dispersed in ethanol, spread on aluminum foil, dried, and then measured from 100 nanotubes where both ends of the fiber could be observed.

[0061] <Median diameter D of the negative electrode active material> ５０ Measurement > The median diameter D at which the cumulative value reaches 50% is determined from the volume frequency particle size distribution measured by laser diffraction scattering using a laser diffraction particle size distribution analyzer (SALD-2300, manufactured by Shimadzu Corporation). ５０ The following was determined. Here, the negative electrode active material was suspended in a dispersion medium and measured after sufficient ultrasonic dispersion. Five measurements were taken, and the average value was used for each measurement.

[0062] <Preparation of Negative Electrode> For each example and comparative example, the negative electrode active material, conductive additive, and binder were mixed in the proportions shown in Table 2, and an appropriate amount of water was added to prepare a negative electrode active material slurry. Next, the negative electrode active material slurry was applied to each negative electrode current collector layer, with an initial charge capacity of 4.3 mAh / cm² per unit area. ２ After applying the material in the desired amount, it was dried to obtain a negative electrode laminate. Next, using a roll press, the negative electrode laminate was processed to a density of 1.65 g / cm³. ３ The negative electrodes for each embodiment and each comparative example were obtained by pressing with a pressure such that the electrodes were pressed.

[0063] <Fabrication of the positive electrode> Lithium nickel-cobalt manganese oxide (LiNi ０．９ Co ０．０５ Mn ０．０５ O ２A cathode active material slurry was prepared by adding an appropriate amount of N-methyl-2-pyrrolidone to a solid component consisting of 1.5 parts by mass of polyvinylidene fluoride and 1.0 part by mass of single-walled carbon nanotubes to 97.5 parts by mass of (the material in question). Next, the cathode active material slurry was applied to a 12 μm thick aluminum foil, which serves as the cathode current collector, with an initial charge capacity of 4.0 mAh / cm² per unit area. ２ After applying the coating in an amount that would result in the desired coating, the material was dried to obtain a positive electrode laminate. Next, using a roll press, the positive electrode laminate was processed to a density of 3.5 g / cm³. ３ The positive electrodes for each embodiment and comparative example were obtained by pressing with a pressure such that the electrodes were pressed.

[0064] <Preparation of Non-Aqueous Electrolyte> The non-aqueous electrolyte was prepared by mixing an organic solvent with a supporting salt. The cyclic carbonate ethylene carbonate and the chain carbonate ethylmethyl carbonate were adjusted to a volume ratio of 3 / 7, and lithium hexafluoride phosphate (LiPF) was added as the supporting salt. ６ A solution (concentration: 1.2 mol / L) and fluoroethylene carbonate (concentration relative to the organic solvent: 6% by mass) were used as additives.

[0065] <Fabrication of Lithium-Ion Secondary Batteries> The positive electrode and the negative electrode of each example and comparative example were cut to 3 cm x 3 cm and placed opposite each other with a separator in between to fabricate an electrode laminate. A 10 μm thick microporous polyethylene film with ceramic coating on both sides was used as the separator. Next, the electrode laminate and the non-aqueous electrolyte were placed inside a laminate casing molded from a film mainly composed of aluminum, and positive electrode tabs and negative electrode tabs were connected to the negative electrode and positive electrode, respectively. The lithium-ion secondary batteries of each example and comparative example were then fabricated by sealing the periphery of the laminate casing. Here, one end of the positive electrode tab is connected to the positive electrode and the other end is led out of the casing, and one end of the negative electrode tab is connected to the negative electrode and the other end is led out of the casing.

[0066] <Energy Density> For each example and comparative example of lithium-ion secondary battery, charging was performed at 7.2 mA, and after the upper voltage limit reached 4.2 V, charging was performed at a constant voltage until the total charging time was 12 hours. Next, the battery was discharged at a constant current of 7.2 mA until the lower voltage limit reached 2.5 V. Then, it was charged again under the same conditions, left in a constant temperature bath at 45°C for 3 days, discharged again under the same conditions, and then charged and discharged one more time. At this time, the capacity and average voltage at the time of the final discharge were measured. Next, the thickness of the lithium-ion secondary battery was measured after the final discharge. Then, based on the above measurements, the electrode area of ​​the electrode stack, the thickness of the laminate casing, the thickness of the negative electrode current collector, and the thickness of the positive electrode current collector, the cell-equivalent energy density E (Wh / L) was calculated using the following formula. The results are shown in Table 2. (Energy density E) = (Capacity at the last discharge) × (Average voltage at the last discharge) / (Electrode area of ​​the electrode stack) / ((Thickness of lithium-ion secondary battery) - (Thickness of positive electrode current collector) / 2 - (Thickness of negative electrode current collector) / 2)

[0067] <Cycle Test> The lithium-ion secondary batteries of each example and comparative example were placed in a constant temperature bath at 45°C and charged at 30mA. After the upper voltage limit reached 4.2V, charging was continued at a constant voltage until the total charging time was 2.5 hours. Next, the batteries were discharged at a constant current of 30mA until the lower voltage limit reached 2.5V. This charge-discharge cycle was then repeated 500 times. The ratio of the discharge capacity after the first discharge to the initial charge capacity was calculated as the initial efficiency (%), and the ratio of the discharge capacity after the 500th discharge to the discharge capacity after the first discharge was calculated as the capacity retention rate (%). The results are shown in Table 2.

[0068]

[0069] Details of each component in Tables 1 and 2 are as follows: <Negative electrode active material> ・Si / C (Si / C powder, D ５０ : 4.8 μm) ・Graphite (artificial graphite containing amorphous carbon on the surface, D ５０ : 10.5μm, specific surface area: 1.6m ２ / g)

[0070] <Preparation of Si / C powder> Porous carbon material (D ５０ : 4.8 μm, specific surface area: 1678 m ２The material ( / g) was placed in a tubular furnace, the furnace was purged with argon gas, and then a mixed gas of 2 mol% silane gas and 98 mol% nitrogen gas was flowed into the furnace at a flow rate of 300 sccm. The process was carried out under conditions of 500°C, 760 Torr, and 120 minutes. The product was then cooled to room temperature to obtain Si / C powder.

[0071] Cross-sections of Si-C composite particles contained in the obtained Si / C powder were analyzed using a scanning electron microscope (Hitachi High-Tech Corporation, SU3500), an energy-dispersive X-ray spectrometer (Oxford Instruments, Ultim Max 40), and image analysis software (Oxford Instruments, Aztec). Secondary electrons were selected as the target of detection, and elemental mapping of silicon and carbon was performed under the conditions of an acceleration voltage of 3 kV, 20 mapping integrations, and a magnification of 3000x. This confirmed that the Si-C composite particles contained silicon, and that silicon was present in at least a portion of the pores of the porous carbon material within the Si-C composite particles.

[0072] <Negative electrode current collector layer> ・Copper foil 1 (SEED_6, manufactured by Nippon Electrolytic Co., Ltd., tensile strength: 600 MPa, thickness: 6 μm) ・Copper foil 2 (S battery foil, manufactured by SK nexilis Company, tensile strength: 600 MPa, thickness: 8 μm) ・Copper foil 3 (W battery foil, manufactured by SK nexilis Company, tensile strength: 400 MPa, thickness: 8 μm) ・Copper foil 4 (U battery foil, manufactured by SK nexilis Company, tensile strength: 800 MPa, thickness: 8 μm) ・Copper foil 5 (SEED_6, manufactured by Nippon Electrolytic Co., Ltd., tensile strength: 560 MPa, thickness: 6 μm) ・Copper foil 6 (SEED_6, manufactured by Nippon Electrolytic Co., Ltd., tensile strength: 580 MPa, thickness: 6 μm) - Copper foil 7 (B battery foil, SK nexilis Company, tensile strength: 300 MPa, thickness: 6 μm) - Copper foil 8 (YB_6, manufactured by Nippon Electrolytic Co., Ltd., tensile strength: 150 MPa, thickness: 6 μm) - Copper foil 9 (V battery foil, manufactured by SK nexilis Company, tensile strength: 400 MPa, thickness: 6 μm) Although copper foils 1, 5, and 6 have the same product name (SEED_6), they differ in tensile strength because they are from different manufacturing lots. The tensile strength can be adjusted by controlling the crystal grain size of the copper foil.

[0073] <Binders> ・PAA (Polyacrylic acid, manufactured by Sumitomo Seika Co., Ltd., AquaCharge) ・SBR (Styrene-butadiene rubber, BM-1100H, manufactured by Nippon Zeon Co., Ltd.)

[0074] <Conductive additive> ・SWCNT1 (Single-walled carbon nanotube, aspect ratio: 100,000, specific surface area: 800 m²) ２ / g) ・SWCNT2 (Single-walled carbon nanotube, aspect ratio: 500,000, specific surface area: 600 m²) ２ / g) ・SWCNT3 (Single-walled carbon nanotube, aspect ratio: 50,000, specific surface area: 600 m²) ２ / g)

[0075] This application claims priority based on Japanese Patent Application No. 2025-019812, filed on 10 February 2025, and incorporates all of its disclosures herein.

[0076] 1. Positive electrode active material layer 2. Negative electrode active material layer 3. Positive electrode current collector 4. Negative electrode current collector 5. Separator 6. Outer casing 7. Outer casing 8. Negative electrode tab 9. Positive electrode tab 10. Lithium-ion secondary battery

Claims

1. A negative electrode for a lithium-ion secondary battery comprising a negative electrode current collector layer and a negative electrode active material layer, wherein the negative electrode active material contained in the negative electrode active material layer contains a silicon-based active material, the binder contained in the negative electrode active material layer contains a polycarboxylic acid-based polymer, and the tensile strength of the negative electrode current collector layer, measured in accordance with JIS Z 2241:2011 under conditions of a tensile speed of 200 mm / min and 25°C, is 250 MPa or more.

2. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein when the total amount of the negative electrode active material layer is 100 parts by mass, the content of the polycarboxylic acid polymer in the negative electrode active material layer is 1 part by mass or more and 10 parts by mass or less.

3. The silicon-based active material is SiO ｘ The negative electrode for a lithium-ion secondary battery according to claim 1 or 2, comprising one or more selected from the group consisting of (0 < x ≤ 2), Si / C comprising Si-C composite particles containing silicon and carbon material, and Si.

4. The negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 3, wherein when the total amount of the negative electrode active material layer is 100 parts by mass, the content of the silicon-based active material in the negative electrode active material layer is 5 parts by mass or more.

5. The negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 4, wherein the silicon-based active material includes Si / C, which includes Si-C composite particles containing silicon and carbon material.

6. The negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 5, wherein the negative electrode active material further comprises a carbon-based active material.

7. The negative electrode for a lithium-ion secondary battery according to claim 6, wherein when the total amount of the negative electrode active material layer is 100 parts by mass, the content of the carbon-based active material in the negative electrode active material layer is 50 parts by mass or more and 95 parts by mass or less.

8. The negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 7, wherein the negative electrode active material layer further comprises a conductive additive.

9. The negative electrode for a lithium-ion secondary battery according to claim 8, wherein when the total amount of the negative electrode active material layer is 100 parts by mass, the content of the conductive additive in the negative electrode active material layer is 0.01 parts by mass or more and 2 parts by mass or less.

10. The negative electrode for a lithium-ion secondary battery according to claim 8 or 9, wherein the conductive additive comprises carbon nanotubes.

11. The specific surface area of ​​the carbon nanotube, as measured by the nitrogen adsorption BET method, is 300 m². ２ The negative electrode for a lithium-ion secondary battery according to claim 10, wherein the value is 1 / g or more.

12. The negative electrode for a lithium-ion secondary battery according to claim 10 or 11, wherein the aspect ratio of the carbon nanotube is 1000 or more.

13. The negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 12, wherein the thickness of the negative electrode current collector layer is 1 μm or more and 20 μm or less.

14. The negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 13, wherein the tensile strength of the binder, measured in accordance with JIS K 7127:1999 under conditions of a tensile speed of 200 mm / min and 25°C, is 110 MPa or more and 340 MPa or less.

15. The negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 14, wherein the negative electrode current collector layer includes copper foil.

16. A lithium-ion secondary battery comprising a negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 15.

17. The lithium-ion secondary battery according to claim 16, wherein the energy density is 700 Wh / L or more on a cell basis.

18. A lithium-ion secondary battery module comprising the lithium-ion secondary battery according to claim 16 or 17.

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