Negative electrode active material, negative electrode, lithium-ion secondary battery, and lithium-ion secondary battery module

Abstract

This negative electrode active material comprises a silicon-based active material and graphite, wherein the modal value of the peak with the largest area in a histogram of the frequency distribution of the D / G ratio of the graphite, determined by a predetermined method, is 1.10 to 2.50 inclusive.

Description

Negative electrode active material, negative electrode, lithium-ion secondary battery and lithium-ion secondary battery module

[0001] The present invention relates to a negative electrode active material, a negative electrode, 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.

[0003] Patent Document 1 describes a negative electrode active material layer containing a carbonaceous negative electrode active material, wherein the degree of orientation I of the carbonaceous negative electrode active material layer is measured by X-ray diffraction. ００４ / I １１０ However, a negative electrode active material layer is described in which the D / G frequency distribution of the carbonaceous negative electrode active material, which is the ratio of the peak intensity of the D band to the peak intensity of the G band obtained by Raman mapping measurement, has a mode of 0.50 or more and 0.80 or less, and the full width at half maximum of the peak having the mode is 0.3 or more and 0.6 or less. Furthermore, Patent Document 1 describes that a negative electrode active material layer having improved resistance can be provided.

[0004] Japanese Patent Publication No. 2024-154149

[0005] Lithium-ion secondary batteries using both silicon-based active materials and graphite as negative electrode active materials sometimes suffered from reduced cycle performance due to gas generation during charging and discharging.

[0006] The present invention provides a negative electrode active material that can reduce the amount of gas generated in the resulting lithium-ion secondary battery.

[0007] The inventors diligently conducted research to achieve the above objectives. As a result, they discovered for the first time that by including graphite in a silicon-based active material and a negative electrode active material containing graphite, the amount of gas generated in the resulting lithium-ion secondary battery can be reduced by having the mode of the largest peak in the histogram of the frequency distribution of the D / G ratio fall within a predetermined range, thus completing the present invention.

[0008] According to the present invention, the following negative electrode active material, negative electrode, lithium-ion secondary battery, and lithium-ion secondary battery module are provided.

[0009] [1] A negative electrode active material comprising a silicon-based active material and graphite, wherein the mode of the peak with the largest area in the histogram of the frequency distribution of the D / G ratio of the graphite according to Method 1 below is 1.10 or more and 2.50 or less. (Method 1) The Raman mapping of the graphite is measured using a laser Raman microscope. Then, the peak area of ​​the G band and the peak area of ​​the D band are determined in the Raman spectrum of each pixel. Then, the D / G ratio is calculated from the peak area of ​​the G band and the peak area of ​​the D band for each pixel. Then, a histogram of the frequency distribution of the D / G ratio is created by plotting the frequency of the D / G ratio. [2] The negative electrode active material according to [1] above, wherein the full width at half maximum of the peak with the largest area is 0.40 or more and 1.10 or less. [3] The negative electrode active material according to [1] or [2] above, wherein the minimum value of the D / G ratio at the peak with the largest area is 0.01 or more. [4] A negative electrode active material according to any of [1] to [3] above, wherein the gas generation rate by Method 2 below is 10.0% or less. (Method 2) The gas generation rate of a lithium-ion secondary battery prepared by the <Method for Manufacturing a Lithium-ion Secondary Battery> below is measured by the <Method for Measuring Gas Generation Rate> below. <Method for Manufacturing a Lithium-ion Secondary Battery> A negative electrode active material slurry is prepared by adding water to a solid component consisting of 96.9 parts by mass of the negative electrode active material, 3.0 parts by mass of polyacrylic acid, and 0.1 parts by mass of single-walled carbon nanotubes. Next, the negative electrode active material slurry is applied to copper foil, and the initial charge capacity per unit area is 4.3 mAh / cm². ２ After coating in this manner, the material is dried to obtain a negative electrode laminate consisting of a negative electrode active material layer and the copper foil. Next, the negative electrode laminate is subjected to a process where the density of the negative electrode active material layer is 1.50 g / cm³. ３ Press it in this manner to obtain the negative electrode. Next, lithium nickel cobalt manganese oxide (LiNi ０．９ Co ０．０５ Mn ０．０５ O ２) To a solid content consisting of 97.5 parts by mass, 1.5 parts by mass of polyvinylidene fluoride and 1.0 part by mass of multi-walled carbon nanotubes, N-methyl-2-pyrrolidone is added to prepare a positive electrode active material slurry. Next, the positive electrode active material slurry is applied onto an aluminum foil such that the initial charge capacity per unit area is 4.0 mAh / cm ２ , followed by drying to obtain a positive electrode laminate composed of a positive electrode active material layer and the aluminum foil. Next, the positive electrode laminate is pressed such that the density of the positive electrode active material layer is 3.50 g / cm ３ to obtain a positive electrode. Next, an electrode laminate in which the negative electrode and the positive electrode are disposed opposite to each other with a separator interposed therebetween, and a non-aqueous electrolyte are placed in a laminate outer package. Then, a positive electrode tab and a negative electrode tab are connected to the positive electrode and the negative electrode, respectively, and the periphery of the laminate outer package is sealed to produce a lithium ion secondary battery. <Method for Measuring Gas Generation Rate> The lithium ion secondary battery is placed in a constant temperature bath at 45°C. Next, the battery is charged at a charge rate of 1.0 C, and after the upper limit voltage reaches 4.25 V, charging is performed at a constant voltage until the charge rate reaches 0.05 C. Next, discharging is performed at a constant current at a discharge rate of 1.0 C until the lower limit voltage reaches 2.8 V. Next, this charge-discharge cycle is repeated 750 times. Then, the gas generation rate is calculated from the following formula (1). Formula (1): (Gas generation rate) = {(Volume of the lithium ion secondary battery after 750 charge-discharge cycles) - (Volume of the lithium ion secondary battery before the first charge-discharge cycle)} / (Volume of the lithium ion secondary battery before the first charge-discharge cycle) × 100 [5] The negative electrode active material according to any one of [1] to [4] above, wherein the silicon-based active material contains Si / C including Si-C composite particles. [6] The negative electrode active material according to any one of [1] to [5] above, wherein the volume-based median diameter D of the graphite measured by a laser diffraction scattering method ５０ is 1.0 µm or more and 30.0 µm or less. [7] The graphite has a specific surface area of 0.1 m measured by the BET method in accordance with JIS Z8830:2013 ２ / g or more and 5.0 m ２[1] to [6] above, wherein the negative electrode active material is less than or equal to / g. [8] The negative electrode active material according to any one of [1] to [7] above, wherein the graphite content is 50% by mass or more and 99% by mass or less when the total negative electrode active material is 100% by mass. [9] A negative electrode comprising a negative electrode active material layer containing the negative electrode active material according to any one of [1] to [8] above, and a negative electrode current collector layer.

[10] The negative electrode according to [9] above, wherein the binder further contained in the negative electrode active material layer contains a polycarboxylic acid polymer.

[11] The negative electrode according to [9] or

[10] above, wherein the negative electrode active material layer further contains a conductive additive.

[12] The negative electrode according to

[11] above, wherein the conductive additive contains a carbon material.

[13] The negative electrode according to

[12] above, wherein the carbon material contains carbon nanotubes.

[14] The negative electrode according to any one of [9] to

[13] above, wherein the negative electrode current collector layer contains copper foil.

[15] A lithium-ion secondary battery comprising a negative electrode as described in any of [9] to

[14] above, a positive electrode, and a separator.

[16] A lithium-ion secondary battery module comprising the lithium-ion secondary battery as described in

[15] above.

[0010] According to the present invention, it is possible to provide a negative electrode active material that can reduce the amount of gas generated in the resulting lithium-ion secondary battery.

[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 active material The negative electrode active material of this embodiment comprises a silicon-based active material and graphite, wherein the mode of the histogram of the frequency distribution of the D / G ratio of the graphite, calculated by Method 1 below, is 1.10 or more and 2.50 or less.

[0014] (Method 1) The Raman mapping of graphite is measured using a laser Raman microscope. Then, the peak area of ​​the G band and the peak area of ​​the D band are determined in the Raman spectrum of each pixel. Next, the D / G ratio is calculated from the peak areas of the G band and the D band for each pixel. Then, a histogram of the frequency distribution of the D / G ratio is created by plotting the frequency of the D / G ratio. Here, the measurement conditions for the graphite Raman mapping are, for example, objective lens magnification: 50x, objective lens numerical aperture: 0.80, excitation wavelength: 532 nm, excitation light intensity: 14.5 mW, excitation light intensity density: 1.08 × 10⁻¹⁶. ４ W / cm ２ The conditions were as follows: exposure time: 5 seconds, number of integrations: 1, diffraction grating: 300 gr / mm, slit width: 50 μm, pixel size: 0.828 μm × 0.828 μm, number of pixels: 200 × 57 pixels, measurement area: 166 μm × 47.2 μm, and measurement mode: XY-imaging.

[0015] Although the mechanism by which the negative electrode active material of this embodiment solves the above-mentioned problems is not clear, it is thought that the reaction between the negative electrode active material and the electrolyte can be reduced because the mode value is within the above numerical range. This is thought to reduce the amount of gas generated in the resulting lithium-ion secondary battery.

[0016] [Silicon-based active material] The silicon-based active material of this embodiment is preferably SiO ｘ (0 < x ≤ 2), comprising one or more selected from the group consisting of Si / C including Si-C composite particles and Si, more preferably SiO ｘ The silicon-based active material in this embodiment is preferably in powder form.

[0017] The Si-C composite particles of this embodiment preferably include silicon and carbon material. In the Si-C composite particles of this embodiment, from the viewpoint of further improving the balance of cycle characteristics and energy density performance of the resulting lithium-ion secondary battery, it is preferable that the carbon material in the Si-C composite particles includes a porous carbon material, and that silicon is present in at least a portion of the pores of the porous carbon material.

[0018] The porous carbon material constituting the Si-C composite particles includes, for example, one or more selected from the group consisting of activated carbon, aggregates of carbon fibers, aggregates of carbon nanotubes, carbon obtained by heat treatment of resins or organic materials, and hard carbon. The porous carbon material can be produced by known manufacturing methods such as those for producing activated carbon or by heat treatment of polymers, but it may also be purchased commercially. The porous carbon material is not limited to those that can generate or incorporate silicon into the pores of the porous carbon.

[0019] In this embodiment, the method for producing Si-C composite particles is not particularly limited, but for example, median diameter D ５０ The surface area is 4.0–10.0 μm, and the specific surface area is 1600–1800 m². ２ This can be obtained by a manufacturing method in which porous carbon material in a quantity of / g is placed in a tubular furnace, the inside of the tubular furnace is replaced with argon gas, and then a mixed gas of silane gas and nitrogen gas, with silane gas content of 1 to 3 mol%, is flowed into the tubular furnace at a flow rate of 250 to 350 sccm, and the process is carried out under conditions of 450 to 550°C, 700 to 800 Torr, and 90 to 150 minutes.

[0020] In this embodiment, a method for confirming that the Si-C composite particles contain silicon and carbon material, and that the silicon in the Si-C composite particles is present in at least a portion of the pores of the porous carbon material, is to perform elemental mapping of silicon and carbon on a cross-section of the Si-C composite particles using a scanning electron microscope, an energy-dispersive X-ray spectroscopic detector, and image analysis software, selecting secondary electrons as the detection target, and observing the results under conditions of an acceleration voltage of 3 kV, 20 mapping integrations, and a magnification of 3000x.

[0021] The volume-based median diameter D of the silicon-based active material in this embodiment, as determined by laser diffraction scattering. ５０ From the viewpoint of further improving the balance between the energy density and rapid charging performance of the resulting lithium-ion secondary battery, the particle size is preferably 1.0 μm to 20.0 μm, more preferably 2.0 μm to 15.0 μm, and even more preferably 3.0 μm to 10.0 μm.

[0022] When the total amount of the negative electrode active material in this embodiment is considered to be 100% by mass, the content of silicon-based active material in the negative electrode active material of this embodiment is preferably 1% by mass or more and 50% by mass or less, more preferably 5% by mass or more and 40% by mass or less, even more preferably 10% by mass or more and 30% by mass or less, and even more preferably 12% by mass or more and 20% by mass or less.

[0023] [Graphite] The graphite in this embodiment preferably includes graphite containing amorphous carbon on its surface, and more preferably includes both graphite containing amorphous carbon on its surface and graphite not containing amorphous carbon on its surface, from the viewpoint of further improving the battery performance of the resulting lithium-ion secondary battery. Furthermore, the graphite in this embodiment is preferably in powder form.

[0024] In the histogram of the frequency distribution of the D / G ratio of the graphite of this embodiment, the mode of the peak with the largest area (hereinafter also referred to as the mode of the peak with the largest area) is 1.10 or more and 2.50 or less, from the viewpoint of reducing the amount of gas generated in the resulting lithium-ion secondary battery. From the viewpoint of further reducing the amount of gas generated in the resulting lithium-ion secondary battery, it is preferably 1.15 or more and 2.30 or less, more preferably 1.20 or more and 2.20 or less, even more preferably 1.25 or more and 2.10 or less, even more preferably 1.30 or more and 2.00 or less, even more preferably 1.40 or more and 1.85 or less, even more preferably 1.45 or more and 1.80 or less, and even more preferably 1.50 or more and 1.75 or less. More specifically, the mode of the peak with the largest area of ​​the graphite of this embodiment can be measured by the method described in the example.

[0025] In the histogram of the frequency distribution of the D / G ratio of graphite according to Method 1 above, the half-width of the peak with the largest area is preferably 0.40 to 1.10, more preferably 0.45 to 1.00, even more preferably 0.50 to 0.95, and even more preferably 0.55 to 0.90, from the viewpoint of further reducing the amount of gas generated in the resulting lithium-ion secondary battery.

[0026] In the graphite of this embodiment, the minimum value of the D / G ratio at the peak with the largest area in the histogram of the frequency distribution of the D / G ratio according to Method 1 above (hereinafter also referred to as the minimum value of the D / G ratio) is preferably 0.01 or higher, more preferably 0.05 or higher, even more preferably 0.10 or higher, and still more preferably 0.20 or higher, from the viewpoint of further reducing the amount of gas generated in the resulting lithium-ion secondary battery. The upper limit of the minimum value of the D / G ratio of the graphite of this embodiment is not particularly limited, but for example it may be 1.00 or lower, 0.90 or lower, 0.80 or lower, or 0.70 or lower. In the graphite of this embodiment, the minimum value of the D / G ratio is preferably 0.01 or higher and 1.00 or lower, more preferably 0.05 or higher and 0.90 or lower, even more preferably 0.10 or higher and 0.80 or lower, and still more preferably 0.20 or higher and 0.70 or lower, from the viewpoint of further reducing the amount of gas generated in the resulting lithium-ion secondary battery.

[0027] The graphite in this embodiment has, for example, a median diameter D ５０ The surface area is 1 to 20 μm and the specific surface area is 0.1 to 20 m². ２ This method involves placing 1 / g of graphite in a reaction vessel, reducing the pressure inside the vessel to 1-500 Pa, then flowing a mixed gas of 0.1-3 mol% acetylene gas, 0.1-3 mol% methane gas, and nitrogen gas into the reaction vessel at a flow rate of 0.1-1000 sccm, and forming amorphous carbon on the surface of the graphite by chemical vapor deposition under conditions of 600-1300°C, 1-500 Pa, and 0.5-3 hours.

[0028] The median diameter D of the graphite in this embodiment, based on volume, as determined by laser diffraction scattering. ５０From the viewpoint of further improving the balance between the energy density and rapid charging performance of the resulting lithium-ion secondary battery, the particle size is preferably 1.0 μm to 30.0 μm, more preferably 3.0 μm to 25.0 μm, and even more preferably 5.0 μm to 20.0 μm.

[0029] The specific surface area of ​​the graphite in this embodiment, measured by the BET method in accordance with JIS Z8830:2013, is preferably 0.1 m², from the viewpoint of further improving the balance between the energy density and rapid charging performance of the resulting lithium-ion secondary battery. ２ / g or more 5.0m ２ / g or less, more preferably 0.5m ２ / g or more 3.0m ２ / g or less, more preferably 1.0m ２ / g or more 2.5m ２ It is less than or equal to / g.

[0030] When the total amount of the negative electrode active material in this embodiment is considered to be 100% by mass, the graphite content in the negative electrode active material of this embodiment is preferably 50% by mass or more and 99% by mass or less, more preferably 60% by mass or more and 95% by mass or less, even more preferably 70% by mass or more and 90% by mass or less, and even more preferably 80% by mass or more and 88% by mass or less.

[0031] [Characteristics of the negative electrode active material] The gas generation rate (hereinafter also referred to as the gas generation rate) of the negative electrode active material of this embodiment by the method 2 below is preferably 10.0% or less, more preferably 9.0% or less, even more preferably 7.0% or less, even more preferably 5.0% or less, even more preferably 4.5% or less, even more preferably 4.0% or less, and even more preferably 3.5% or less, from the viewpoint of further reducing the amount of gas generated in the resulting lithium-ion secondary battery. The lower limit of the gas generation rate is not particularly limited, but for example it may be 0.1% or more, 0.5% or more, or 1.0% or more. The gas generation rate may be, for example, 0.1% or more and 10.0% or less, 0.5% or more and 9.0% or less, 1.0% or more and 7.0% or less, 1.0% or more and 5.0% or less, 1.0% or more and 4.5% or less, 1.0% or more and 4.0% or less, or 1.0% or more and 3.5% or less.

[0032] (Method 2) The gas generation rate of the lithium-ion secondary battery prepared according to the <Method for Manufacturing a Lithium-ion Secondary Battery> described below is measured according to the <Method for Measuring Gas Generation Rate> described below. More specifically, the gas generation rate can be measured by the method described in the Examples.

[0033] <Method for manufacturing lithium-ion secondary batteries> A negative electrode active material slurry is prepared by adding water to a solid component consisting of 96.9 parts by mass of negative electrode active material, 3.0 parts by mass of polyacrylic acid, and 0.1 parts by mass of single-walled carbon nanotubes. Next, the negative electrode active material slurry is applied to copper foil, with an initial charge capacity of 4.3 mAh / cm² per unit area. ２ After coating in this manner, the material is dried to obtain a negative electrode laminate consisting of a negative electrode active material layer and the copper foil. Next, the negative electrode laminate is subjected to a process where the density of the negative electrode active material layer is 1.50 g / cm³. ３ Press it in this manner to obtain the negative electrode. Next, lithium nickel cobalt manganese oxide (LiNi ０．９ Co ０．０５ Mn ０．０５ O ２A cathode active material slurry is prepared by adding N-methyl-2-pyrrolidone to a solid component consisting of 97.5 parts by mass of ) 1.5 parts by mass of polyvinylidene fluoride and 1.0 part by mass of multi-walled carbon nanotubes. Next, the cathode active material slurry is placed on aluminum foil, with an initial charge capacity of 4.0 mAh / cm² per unit area. ２ After coating in this manner, it is dried to obtain a positive electrode laminate consisting of a positive electrode active material layer and the aluminum foil. Next, the positive electrode laminate is subjected to a process where the density of the positive electrode active material layer is 3.50 g / cm³. ３ The material is pressed to obtain the positive electrode. Next, the electrode stack, in which the negative electrode and positive electrode are positioned opposite each other with a separator in between, and the non-aqueous electrolyte are placed inside a laminate casing. Then, the positive electrode tab and negative electrode tab are connected to the negative electrode and positive electrode, respectively, and the perimeter of the laminate casing is sealed to fabricate a lithium-ion secondary battery.

[0034] <Method for Measuring Gas Generation Rate> A lithium-ion secondary battery is placed in a constant temperature bath at 45°C. Next, it is charged at a charge rate of 1.0C, and after the upper limit voltage reaches 4.25V, it is charged at a constant voltage until the charge rate reaches 0.05C. Next, it is discharged at a discharge rate of 1.0C at a constant current until the lower limit voltage reaches 2.8V. This charge-discharge cycle is then repeated 750 times. Next, the gas generation rate is calculated from the following formula (1). Formula (1): (Gas generation rate) = {(Volume of lithium-ion secondary battery after 750 charge-discharge cycles) - (Volume of lithium-ion secondary battery before the first charge-discharge cycle)} / (Volume of lithium-ion secondary battery before the first charge-discharge cycle) × 100

[0035] 2. Negative Electrode The negative electrode of this embodiment comprises a negative electrode active material layer containing the negative electrode active material of this embodiment and a negative electrode current collector layer. The negative electrode active material layer of this embodiment is provided, for example, on the negative electrode current collector layer of this embodiment. Because the negative electrode of this embodiment comprises a negative electrode active material layer containing the negative electrode active material of this embodiment, the amount of gas generated in the resulting lithium-ion secondary battery can be reduced. The negative electrode of this embodiment is preferably a negative electrode for a lithium-ion secondary battery.

[0036] [Negative Electrode Active Material Layer] The negative electrode active material layer of this embodiment contains the negative electrode active material of this embodiment, from the viewpoint of reducing the amount of gas generated in the resulting lithium-ion secondary battery. When the total amount of the negative electrode active material layer of this embodiment is considered to be 100% by mass, the content of the negative electrode active material of this embodiment in the negative electrode active material layer is preferably 50% by mass or more and 99% by mass or less, more preferably 70% by mass or more and 98% by mass or less, even more preferably 90% by mass or more and 98% by mass or less, and even more preferably 95% by mass or more and 97.5% by mass or less.

[0037] In this embodiment, the density of the negative electrode active material layer is preferably 0.7 g / cm³, from the viewpoint of improving the performance balance between the energy density and cycle characteristics of the resulting lithium-ion secondary battery. ３ 2.0g / cm or more ３ More preferably, 0.9 g / cm³ ３ 1.8g / cm or more ３ More preferably, 1.0 g / cm³ ３ 1.7g / cm or more ３ More preferably, 1.3 g / cm³ ３ 1.6g / cm or more ３ The following applies:

[0038] In this embodiment, the thickness of the negative electrode active material layer is preferably 10 μm to 200 μm, more preferably 15 μm to 150 μm, and even more preferably 20 μm to 100 μm, from the viewpoint of improving the balance between the energy density and rapid charging performance of the resulting lithium-ion secondary battery.

[0039] (Binder) The negative electrode active material layer of this embodiment preferably further comprises a binder. The binder in the negative electrode active material layer of this embodiment comprises, for example, one or more selected from the group consisting of fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF); polycarboxylic acid polymers such as poly(meth)acrylic acid; conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole; synthetic rubbers such as styrene-butadiene rubber, butadiene rubber, chloroprene rubber, isoprene rubber, and acrylonitrile-butadiene rubber; and polysaccharides such as carboxymethylcellulose (CMC), xanthan gum, guar gum, and pectin. Among these, the binder in the negative electrode active material layer of this embodiment preferably comprises one or more selected from the group consisting of fluororesin, polycarboxylic acid polymer, and synthetic rubber, from the viewpoint of further improving the battery performance of the resulting lithium-ion secondary battery, more preferably comprises one or more selected from the group consisting of polyvinylidene fluoride, polycarboxylic acid polymer, and styrene-butadiene rubber, and even more preferably comprises a polycarboxylic acid polymer.

[0040] The binder further included in the negative electrode active material layer of this embodiment preferably includes a polycarboxylic acid polymer, from the viewpoint of improving the cycle characteristics of the resulting lithium-ion secondary battery. In this embodiment, "polycarboxylic acid polymer" refers to a polymer containing an unsaturated carboxylic acid or a salt thereof as monomer units. The unsaturated carboxylic acid or a salt thereof includes, for example, one or more selected from the group consisting of acrylic acid or a salt thereof; methacrylic acid or a salt thereof; maleic acid or a salt thereof; and fumaric acid or a salt thereof. The unsaturated carboxylic acid or a salt thereof preferably includes one or more selected from the group consisting of acrylic acid or a salt thereof and methacrylic acid or a salt thereof, more preferably includes acrylic acid or a salt thereof, and even more preferably includes acrylic acid. The salt of the unsaturated carboxylic acid preferably includes an alkali metal salt of the unsaturated carboxylic acid. The alkali metal salt of the unsaturated carboxylic acid includes, for example, one or more selected from the group consisting of lithium salt of the unsaturated carboxylic acid, sodium salt of the unsaturated carboxylic acid, and potassium salt of the unsaturated carboxylic acid. The polycarboxylic acid polymer of this embodiment preferably comprises one or more selected from the group consisting of polyacrylic acid or a salt thereof and polymethacrylic acid or a salt thereof, and more preferably comprises polyacrylic acid or a salt thereof.

[0041] When the entire negative electrode active material layer of this embodiment is considered to be 100% by mass, the binder content in the negative electrode active material layer is preferably 1.0% by mass or more and 10% by mass or less, more preferably 1.5% by mass or more and 8.0% by mass or less, and even more preferably 2.0% by mass or more and 5.0% by mass or less, from the viewpoint of further improving the battery performance of the resulting lithium-ion secondary battery.

[0042] (Conductive additive) The negative electrode active material layer of this embodiment preferably further contains a conductive additive from the viewpoint of further improving the performance balance between the energy density and cycle characteristics of the resulting lithium-ion secondary battery.

[0043] The conductive additive in the negative electrode active material layer of this embodiment includes, for example, one or more selected from the group consisting of carbon fibers such as carbon nanofibers; carbon black such as acetylene black and Ketjen black; and carbon materials such as activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes. The conductive additive in the negative electrode active material layer of this embodiment preferably includes a carbon material from the viewpoint of further improving the performance balance between the energy density and cycle characteristics of the resulting lithium-ion secondary battery. The carbon material of this embodiment preferably includes carbon nanotubes, and more preferably includes single-walled carbon nanotubes, from the viewpoint of further improving the performance balance between the energy density and cycle characteristics of the resulting lithium-ion secondary battery.

[0044] The specific surface area of ​​the carbon nanotubes in this embodiment, as measured by the nitrogen adsorption BET method, is preferably 300 m², from the viewpoint of further improving the performance balance between the energy density and cycle characteristics of the resulting lithium-ion secondary battery. ２ / g or more, more preferably 400m ２ / g or more, more preferably 500m ２ / g or more, more preferably 600m ２ / g or more, more preferably 700m ２ It is 1 / g or more. There is no particular upper limit on the specific surface area of ​​the carbon nanotubes in this embodiment, but for example, 3000 m ２ It may be less than / g, and 2500m ２ It may be less than / g, and 2000m ２ It may be less than / g. The specific surface area of ​​the carbon nanotube in this embodiment is preferably 300 m² from the viewpoint of further improving the performance balance of energy density and cycle characteristics of the resulting lithium-ion secondary battery. ２ / g or more 3000m ２ / g or less, more preferably 400m ２ / g or more 2500m ２ / g or less, more preferably 500m ２ / g or more 2000m ２ / g or less, more preferably 600m ２ / g or more 2000m２ / g or less, more preferably 700m ２ / g or more 2000m 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.

[0045] The aspect ratio of the carbon nanotube in this embodiment is preferably 10 or more, more preferably 100 or more, even more preferably 500 or more, even more preferably 1000 or more, and even more preferably 1500 or more, from the viewpoint of further improving the performance balance between energy density and cycle characteristics of the lithium-ion secondary battery. There is no particular upper limit to the aspect ratio of the carbon nanotube in this embodiment, but it may be, for example, 10000 or less, 8000 or less, 5000 or less, 4000 or less, or 3000 or less. The aspect ratio of the carbon nanotube in this embodiment is preferably 10 to 10000, more preferably 100 to 8000, even more preferably 500 to 5000, even more preferably 1000 to 4000, and even more preferably 1500 to 3000, from the viewpoint of further improving the performance balance between energy density and cycle characteristics of the lithium-ion secondary battery. The aspect ratio of the carbon nanotube can be measured by the method described in the examples.

[0046] When the entire negative electrode active material layer of this embodiment is considered to be 100% by mass, the content of the conductive additive in the negative electrode active material layer is preferably 0.01% by mass or more and 2.0% by mass or less, more preferably 0.02% by mass or more and 1.0% by mass or less, even more preferably 0.04% by mass or more and 0.5% by mass or less, and even more preferably 0.05% by mass or more and 0.3% by mass or less, from the viewpoint of further improving the performance balance of the energy density and cycle characteristics of the resulting lithium-ion secondary battery.

[0047] [Negative Electrode Current Collector Layer] The negative electrode current collector layer of this embodiment includes a negative electrode current collector formed from one or more materials selected from the group consisting of copper, aluminum, stainless steel, nickel, titanium, and alloys thereof, and preferably includes a negative electrode current collector formed from copper. The shape of the negative electrode current collector is, for example, foil, flat plate, mesh, etc., and is preferably foil. The negative electrode current collector layer of this embodiment preferably includes copper foil.

[0048] In this embodiment, the thickness of the negative electrode current collector layer is preferably 1 μm to 20 μm, more preferably 2 μm to 18 μm, even more preferably 3 μm to 16 μm, even more preferably 4 μm to 14 μm, even more preferably 5 μm to 12 μm, and even more preferably 6 μm to 10 μm, from the viewpoint of further improving the performance balance of energy density and cycle characteristics of the resulting lithium-ion secondary battery.

[0049] 3. Lithium-ion secondary battery The lithium-ion secondary battery of this embodiment comprises the negative electrode, positive electrode, and separator of this embodiment. Because the lithium-ion secondary battery of this embodiment is equipped with the negative electrode of this embodiment, the amount of gas generated is reduced.

[0050] 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 the negative electrode, positive electrode, and separator 5 of this embodiment.

[0051] The lithium-ion secondary battery 10 comprises a positive electrode having a positive electrode active material layer 1 and a positive electrode current collector layer 3, and a negative electrode having a negative electrode active material layer 2 and a negative electrode current collector layer 4. The positive electrode and the negative electrode are stacked via a separator 5 so that, for example, the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other, thereby forming an electrode pair. This electrode pair is housed in a container formed by, for example, an outer casing 6, 7. A positive electrode tab 9 is connected to the positive electrode current collector layer 3, and a negative electrode tab 8 is connected to the negative electrode current collector layer 4, with the positive electrode tab 9 and the negative electrode tab 8 extending out of the container. An electrolyte is sealed inside the container. The lithium-ion secondary battery 10 can also have a structure in which an electrode group, each consisting of multiple stacked electrode pairs, is housed in the container.

[0052] The lithium-ion secondary battery 10 can be manufactured according to known methods. The electrodes are, for example, laminates, wound bodies, etc. The casing is, for example, a metal casing, an aluminum laminate casing, etc. The shape of the battery is, for example, coin-type, button-type, sheet-type, cylindrical, prismatic, flat-type, etc.

[0053] [Positive electrode] 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 layer. The positive electrode active material layer of this embodiment is provided, for example, on the positive electrode current collector layer of this embodiment.

[0054] The positive electrode active material in the positive electrode active material layer of this embodiment is, 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 composite oxide, lithium-nickel-cobalt-manganese composite oxide; TiS ２ FeS, MoS ２ Transition metal sulfides such as MnO, V ２ O ５, V ６ O １３ , TiO ２ The positive electrode active material in the positive electrode active material layer of this embodiment preferably contains a composite oxide of lithium and a transition metal, more preferably one or more composite oxides selected from the group consisting of 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, and lithium-nickel-cobalt-manganese composite oxide, and even more preferably contains a lithium-nickel-manganese-cobalt composite oxide.

[0055] From the viewpoint of further improving the energy density, the content of the positive electrode active material in the positive electrode active material layer of this embodiment is preferably 50.0 parts by mass or more and 99.9 parts by mass or less, more preferably 75.0 parts by mass or more and 99.5 parts by mass or less, even more preferably 85.0 parts by mass or more and 99.0 parts by mass or less, even more preferably 90.0 parts by mass or more and 98.5 parts by mass or less, and even more preferably 95.0 parts by mass or more and 98.0 parts by mass or less, when the total volume of the positive electrode active material layer is 100.0 parts by mass.

[0056] The positive electrode active material layer of this embodiment preferably further includes a binder. The binder in the positive electrode active material layer of this embodiment includes one or more selected from the group consisting of, for example, fluorine-based binders such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), and aqueous binders such as styrene-butadiene rubber.

[0057] The positive electrode active material layer of this embodiment preferably further comprises a conductive additive. The conductive additive in the positive electrode active material layer of this embodiment includes, for example, one or more selected from the group consisting of carbon fibers such as carbon nanofibers; carbon black such as acetylene black and Ketjen black; and carbon materials such as activated carbon, mesoporous carbon, fullerenes, and carbon nanotubes.

[0058] The positive electrode current collector layer of this embodiment includes a positive electrode current collector formed from one or more materials selected from the group consisting of, for example, aluminum, stainless steel, nickel, titanium, and alloys thereof. The shape of the positive electrode current collector is, for example, foil, flat plate, mesh, etc. The thickness of the positive electrode current collector is, for example, 1 μm or more and 50 μm or less.

[0059] [Separator] The separator in this embodiment is, for example, a porous membrane, woven fabric, or nonwoven fabric made of resin. The resin component constituting the separator includes, for example, one or more selected from the group consisting of polyolefin resins such as polypropylene and polyethylene; polyester resins; acrylic resins; styrene resins; and nylon resins. In addition, if necessary, the separator may include a layer containing inorganic particles. The inorganic particles include, for example, one or more selected from the group consisting of insulating oxides, nitrides, sulfides, and carbides.

[0060] [Electrolyte] The electrolyte of this embodiment preferably includes a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent. The organic solvent includes, for example, one or more selected from the group consisting of 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 cyclic ethers. The lithium salt is, for example, lithium hexafluoride phosphate (LiPF) ６ ), lithium borofluoride (LiBF ４), lithium bis(fluorosulfonyl)imide (LiFSI) and lithium perchlorate (LiClO ４ Includes one or more selected from the group consisting of ) etc.

[0061] [Outer Body] The outer body of this embodiment is, for example, a flexible film, a can case, a case made of a flexible film, etc., and a flexible film is preferably used from the viewpoint of reducing the weight of the lithium-ion secondary battery. The flexible film is, for example, a laminate in which resin layers are provided on the front and back surfaces of a metal layer that serves as a base material. A heat-sealable resin layer made of, for example, modified polyolefin is provided on at least one surface of the metal layer. On the surface opposite to the surface on which the heat-sealable resin layer is provided, a resin layer made of, for example, nylon resin, polyester resin, etc. The metal layer is made of a material that has barrier properties such as preventing leakage of electrolyte and intrusion of moisture from the outside, and is, for example, a layer made of aluminum, stainless steel, etc. The outer body of this embodiment can be 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.

[0062] 4. Lithium-ion secondary battery module The lithium-ion secondary battery module of this embodiment is equipped with the lithium-ion secondary battery of this embodiment. Since the lithium-ion secondary battery of this embodiment has reduced gas generation, the lithium-ion secondary battery module of this embodiment has reduced gas generation.

[0063] The lithium-ion secondary battery module of this embodiment preferably comprises 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 preferably further comprises a housing capable of housing the lithium-ion secondary batteries of this embodiment. The lithium-ion secondary battery module of this embodiment preferably further comprises 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.

[0064] The lithium-ion secondary battery module 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. The battery system includes, for example, one or more types selected from the group consisting of battery packs, stationary battery systems, automotive power battery systems, automotive auxiliary battery systems, and emergency power battery systems.

[0065] The embodiments of the present invention have been described above, but these are merely examples, and various other configurations can also be adopted.

[0066] 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.

[0067] The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto.

[0068] For graphites 1 to 6, the mode, the full width at half maximum, and the minimum value of the peak with the largest area in the histogram of the frequency distribution of the D / G ratio were calculated using the method described below. The results are shown in Table 2.

[0069] <Creation of a histogram of the frequency distribution of the D / G ratio of graphite> 50 mg of graphite was placed on a glass slide (white plate) and leveled with the back of a spatula to prepare a measurement sample. Next, Raman mapping was measured for each measurement sample using a laser Raman microscope (RAMANtouch, manufactured by Nanophoton Inc.). The measurement settings were: objective lens magnification: 50x, objective lens numerical aperture: 0.80, excitation wavelength: 532 nm, excitation light intensity: 14.5 mW, excitation light intensity density: 1.08 × 10⁻¹⁶ ４ W / cm ２ The measurement was performed under the following conditions: exposure time: 5 seconds, number of integrations: 1, diffraction grating: 300 gr / mm, slit width: 50 μm, pixel size: 0.828 μm × 0.828 μm, number of pixels: 200 × 57 pixels, measurement area: 166 μm × 47.2 μm, and measurement mode: XY-imaging. Next, Raman spectra were created for each pixel. Then, the peak area of ​​the G band and the peak area of ​​the D band were determined for each pixel's Raman spectrum. Next, the D / G ratio was calculated for each pixel from the peak area of ​​the G band and the peak area of ​​the D band. Finally, a histogram of the frequency distribution of the D / G ratio was created by plotting the frequency of the D / G ratio.

[0070] Furthermore, graphite samples 2-3 and 5-6 showed one peak in the histogram. Graphite samples 1 and 4 showed two peaks in the histogram. Hereafter, when two peaks are present in the histogram, the peak with the smaller D / G ratio value will be referred to as the first peak M. １ The peak with the larger D / G ratio is the second peak M. ２ These are called respectively. In graphite 1 and 4, the second peak M ２ However, it was the largest mountain in terms of area.

[0071] <Calculation of the Mode of the Largest Area Peak> First, in order to determine the peak with the largest area, the area of ​​the peaks in the histogram was calculated using the following procedure. The mode of the peak determined by the following method was taken as the mode of the peak with the largest area.

[0072] (Method for calculating mountain area: when there is one mountain) The area of mountain M was obtained by the following procedure. First, for mountain M, the mode m of mountain M, which is the D / G ratio value with the maximum frequency, was obtained. Next, when the frequency at the mode m is defined as A, among the D / G ratio values at which the frequency in mountain M is A / 2, the value xl that is the minimum D / G ratio value was obtained. In this case, among the D / G ratio values at which the frequency in mountain M is A / 2, the value xu that is the maximum D / G ratio value was also obtained. Then, the area of the histogram in the interval from value xl to value xu was defined as the area S of mountain M.

[0073] (Method for calculating mountain area: when there are two mountains) According to the following procedure, the first mountain M １ area S １ and the second mountain M ２ area S ２ were obtained respectively. First, the first mountain M １ mode m １ and the second mountain M ２ mode m ２ were obtained respectively. Next, between mode m １ and mode m ２ among the D / G ratio values in the interval, the value xv with the minimum frequency was obtained.

[0074] Next, for mode m １ when the frequency therein is defined as A １ , for the first mountain M １ among the D / G ratio values at which the frequency is A １ / 2, the value xl that is the minimum D / G ratio value １ was obtained. Note that if there is no D / G ratio value at which the frequency in the first mountain M is A / 2 between the D / G ratio value 0 and the mode m １ , xl １ is set to 0 when no D / G ratio value at which the frequency is A １ / 2 exists. Next, between the mode m １ =0 was set. Next, between the mode m １ and the value xv, if there exists a D / G ratio value at which the frequency in the first mountain M is A １ , the frequency is A １ / 2, this value is defined as value xu １ . On the other hand, between the mode m １ and the value xv, for the first mountain M １ the frequency is A １If there is no D / G ratio value that is equal to / 2, then xu １ Let's assume it equals xv.

[0075] Next, the mode m ２ Frequency in A ２ In that case, the second mountain M ２ In this case, the frequency is A ２ Among the D / G ratio values ​​that result in / 2, xu is the value with the maximum D / G ratio. ２ Next, we calculated the value xv and the mode m. ２ Between them, the second mountain M ２ In this case, the frequency is A ２ If there is a D / G ratio value that is 2, then that value is xl ２ On the other hand, the value xv and the mode m ２ Between them, the second mountain M ２ In this case, the frequency is A ２ If there is no D / G ratio value that is equal to / 2, then xl ２ Let's assume it equals xv.

[0076] Next, value xl １ ~value xu １ The area of ​​the histogram for the interval is the first peak M １ Area S １ And the value xl ２ ~value xu ２ The area of ​​the histogram for the interval is the second peak M ２ Area S ２ And then, S １ > S ２ In that case, the first mountain M １ We defined the mountain with the largest area as S. １ ≤S ２ In that case, the second mountain M ２ This was designated as the mountain with the largest area.

[0077] <Calculation of the full width at half maximum for each peak> The full width at half maximum (HM) for each peak in the histogram was calculated using the following procedure.

[0078] (Method for calculating the half-width of a peak; in the case of one peak) For a peak M, the difference between the value xl and the value xu (xu - xl) was defined as the half-width HM of peak M.

[0079] (Method for calculating the half-width of a peak; in the case of two peaks) Follow the procedure below for the first peak M１ Half-width HM １ And the second mountain M ２ Half-width HM ２ We calculated xu. １ If < xv, then the value xu １ and value xl １ The difference (xu １ ―xl １ ) to half-width HM １ That was the case. On the other hand, xu １ If the value is xv, the mode is m １ and value xl １ Twice the difference { (m １ ―xl １ ) × 2}, half-width HM １ xv < xl ２ If so, the value xu ２ and value xl ２ The difference (xu ２ ―xl ２ ) to half-width HM ２ That was the case. On the other hand, xl ２ If the value is xv, then the value xu ２ and the mode m ２ Twice the difference between (xu ２ ―> ２ ) × 2}, half-width HM ２ That's what I decided.

[0080] (Calculation of the minimum D / G ratio at the peak with the largest area) For the peak with the largest area determined by the method described above, the minimum D / G ratio at the peak with the largest area was found from the histogram of that peak. Note that for graphite 1 and 4, where there were two peaks in the histogram, the second peak M was the peak with the largest area. ２ Then, the first mountain M １ Because they overlapped, the largest mountain (the second mountain M) ２ The minimum value of the D / G ratio in ) was not determined.

[0081] <Measurement of specific surface area of ​​negative electrode active material and conductive additive> The specific surface area 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.

[0082] <Median diameter D of negative electrode active material and positive 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 median diameter D of the positive electrode active material was determined. Here, the negative electrode active material was suspended in a dispersion medium (0.1% by mass sodium hexametaphosphate aqueous solution), ultrasonically dispersed, and then measured. Five measurements were taken, and the arithmetic mean of the obtained values ​​was adopted. The median diameter D of the positive electrode active material was determined using the same method. ５０ They sought it.

[0083] <Measurement of Aspect Ratio (Fiber Length / Fiber Diameter) of Carbon Nanotubes> Using a scanning electron microscope, 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 carbon nanotubes were measured, and the aspect ratio was calculated. A total of 100 samples were measured, and the average values ​​were used for each.

[0084] Next, lithium-ion secondary batteries were fabricated for each example using the following procedure, and the gas generation rate was calculated. The results are shown in Table 1.

[0085] <Fabrication of the negative electrode> 96.9 parts by mass of negative electrode active material, 3.0 parts by mass of polyacrylic acid (manufactured by Sumitomo Seika Co., Ltd.), and single-walled carbon nanotubes (aspect ratio: 2000, specific surface area: 1200 m²) in the proportions shown in Table 1. ２ A negative electrode active material slurry was prepared by adding an appropriate amount of water to a solid content of 0.1 parts by mass ( / g). Next, the negative electrode active material slurry was applied to a copper foil with a thickness of 8 μm, which served as the negative electrode current collector, with an initial charge capacity of 4.3 mAh / cm² per unit area. ２ After coating in this manner, the material was dried to obtain a negative electrode laminate consisting of a negative electrode active material layer and copper foil. Next, using a roll press, the negative electrode laminate was processed so that the density of the negative electrode active material layer was 1.50 g / cm³. ３ The negative electrodes for each example were obtained by pressing them in this manner. By adjusting the amount of negative electrode active material slurry applied, the initial charge capacity per unit area was 4.3 mAh / cm². ２ The negative electrode active material slurry was coated onto the copper foil in such a manner.

[0086] <Fabrication of the positive electrode> Lithium nickel-cobalt manganese oxide (LiNi ０．９ Co ０．０５ Mn ０．０５ O ２ Median diameter D ５０ 97.5 parts by mass of (4.0 μm), 1.5 parts by mass of polyvinylidene fluoride (Solvey), and multi-walled carbon nanotubes (aspect ratio: 150, specific surface area: 275 m²). ２ A positive electrode active material slurry was prepared by adding an appropriate amount of N-methyl-2-pyrrolidone to a solid content of 1.0 part by mass (g). Next, the positive electrode active material slurry was applied to a 12 μm thick aluminum foil, which served as the positive electrode current collector, with an initial charge capacity of 4.0 mAh / cm² per unit area. ２ After coating in this manner, the material was dried to obtain a positive electrode laminate consisting of a positive electrode active material layer and aluminum foil. Next, using a roll press, the positive electrode laminate was processed so that the density of the positive electrode active material layer was 3.50 g / cm³. ３ The positive electrode was obtained by pressing it in this manner.

[0087] <Preparation of Non-Aqueous Electrolyte> The organic solvent was prepared by adjusting the volume ratio of ethylene carbonate to ethyl methyl carbonate to 3:7. Next, lithium hexafluoride phosphate was added to the organic solvent as a supporting salt, and fluoroethylene carbonate was added as an additive. Lithium hexafluoride phosphate was added to the organic solvent to a concentration of 1.2 mol / L. Fluoroethylene carbonate was added to the organic solvent to a concentration of 6% by mass relative to the organic solvent.

[0088] <Fabrication of Lithium-ion Secondary Batteries> The positive electrode and the negative electrode of each example were cut to 3 cm x 3 cm and placed opposite each other with a separator in between to create 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. Then, a negative electrode tab was connected to the negative electrode and a positive electrode tab was connected to the positive electrode, and the periphery of the laminate casing was sealed to create each example of a lithium-ion secondary battery. Here, one end of the positive electrode tab was connected to the positive electrode and the other end was led out of the casing. Similarly, one end of the negative electrode tab was connected to the negative electrode and the other end was led out of the casing.

[0089] <Measurement of Gas Generation Rate> Each lithium-ion secondary battery was placed in a constant temperature bath at 45°C. Next, it was charged at a charge rate of 1.0C, and after the upper limit voltage reached 4.25V, it was charged at a constant voltage until the charge rate became 0.05C. Next, it was discharged at a discharge rate of 1.0C at a constant current until the lower limit voltage became 2.8V. This charge-discharge cycle was then repeated 750 times. Next, the gas generation rate was calculated from the above formula (1).

[0090]

[0091]

[0092] Details of each component in Tables 1 and 2 are as follows: <Negative electrode active material> ・Silicon-based active material Si / C (Si-C composite particles, median diameter D) ５０ : 4.8 μm) Graphite 1 (a mixture of artificial graphite coated with amorphous carbon on the surface and artificial graphite without amorphous carbon on the surface (mass ratio 7:3), median diameter D ５０ : 12.3μm, specific surface area: 1.5m ２ / g) Graphite 2 (artificial graphite with amorphous carbon coating on the surface, median diameter D) ５０ : 13.5μm, specific surface area: 1.4m ２ / g) Graphite 3 (artificial graphite with amorphous carbon coating on the surface, median diameter D) ５０ : 17.0μm, specific surface area: 1.1m ２ / g) Graphite 4 (a mixture of artificial graphite coated with amorphous carbon on the surface and graphite 6 (mass ratio 7:3), median diameter D ５０ : 12.7μm, specific surface area: 1.2m ２ / g) Graphite 5 (artificial graphite that does not contain amorphous carbon on the surface, median diameter D) ５０ : 16.0μm, specific surface area: 2.2m ２ / g) Graphite 6 (artificial graphite that does not contain amorphous carbon on the surface, median diameter D) ５０ :9.9μm, specific surface area: 1.8m ２ / g)

[0093] <Fabrication of Si-C composite particles> Porous carbon material (median diameter D) ５０ : 4.8μm, specific surface area: 1700m ２ The 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 composite particles.

[0094] Cross-sections of the obtained Si-C composite particles 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 contain silicon, and that silicon is present in at least a portion of the pores of the porous carbon material within the Si-C composite particles.

[0095] <Preparation of Graphite 1> Artificial graphite without amorphous carbon on its surface was placed in the reaction vessel. Next, after reducing the pressure inside the reaction vessel, a mixed gas of 1 mol% acetylene gas, 1 mol% methane gas, and 98 mol% nitrogen gas was flowed into the reaction vessel at a flow rate of 50 sccm, and amorphous carbon was formed on the surface of the artificial graphite by chemical vapor deposition under the conditions of 900°C, 100 Pa, and 1 hour. Next, the product was cooled to room temperature, and artificial graphite with amorphous carbon coated on its surface (median diameter D) was formed. ５０ : 13.5μm, specific surface area: 1.4m ２ We obtained artificial graphite (median diameter D) (hereinafter also referred to as graphite A) which does not contain amorphous carbon on its surface. ５０ :9.5μm, specific surface area: 1.8m ２ Graphite A (hereinafter also referred to as graphite B) was prepared. Graphite 1 was obtained by mixing graphite A and graphite B in a mass ratio of graphite A:graphite B = 7:3.

[0096] <Preparation of Graphite 2> Artificial graphite without amorphous carbon on its surface was placed in a reaction vessel. Next, the pressure inside the reaction vessel was reduced, and a mixed gas of 1 mol% acetylene gas, 1 mol% methane gas, and 98 mol% nitrogen gas was flowed into the reaction vessel at a flow rate of 50 sccm. Amorphous carbon was formed on the surface of the artificial graphite by chemical vapor deposition under the conditions of 900°C, 100 Pa, and 1 hour. Next, the product was cooled to room temperature to obtain Graphite 2, which is artificial graphite coated with amorphous carbon on its surface.

[0097] <Preparation of Graphite 3> Artificial graphite without amorphous carbon on its surface was placed in a reaction vessel. Next, the pressure inside the reaction vessel was reduced, and a mixed gas of 1 mol% acetylene gas, 1 mol% methane gas, and 98 mol% nitrogen gas was flowed into the reaction vessel at a flow rate of 50 sccm. Amorphous carbon was formed on the surface of the artificial graphite by chemical vapor deposition under the conditions of 900°C, 200 Pa, and 1 hour. Next, the product was cooled to room temperature to obtain graphite 3, which is artificial graphite coated with amorphous carbon on its surface.

[0098] <Preparation of Graphite 4> Artificial graphite without amorphous carbon on its surface was placed in the reaction vessel. Next, after reducing the pressure inside the reaction vessel, a mixed gas of 1 mol% acetylene gas, 1 mol% methane gas, and 98 mol% nitrogen gas was flowed into the reaction vessel at a flow rate of 50 sccm, and amorphous carbon was formed on the surface of the artificial graphite by chemical vapor deposition under the conditions of 900°C, 100 Pa, and 1 hour. Next, the product was cooled to room temperature, and artificial graphite with amorphous carbon coated on its surface (median diameter D) was formed. ５０ : 14.0μm, specific surface area: 0.9m ２ Graphite C (hereinafter also referred to as graphite C) was obtained. Graphite C and graphite 6 were mixed in a mass ratio of graphite C:graphite 6 = 7:3 to obtain graphite 4.

[0099] <Graphite 5> Artificial graphite that does not contain amorphous carbon on the surface (median diameter D) ５０ : 16.0μm, specific surface area: 2.2m ２ ( / g) was used.

[0100] <Graphite 6> Artificial graphite that does not contain amorphous carbon on the surface (median diameter D) ５０ :9.9μm, specific surface area: 1.8m ２ ( / g) was used.

[0101] This application claims priority based on Japanese Patent Application No. 2025-051940, filed on 26 March 2025, and incorporates all of its disclosures herein.

[0102] 1. Positive electrode active material layer 2. Negative electrode active material layer 3. Positive electrode current collector layer 4. Negative electrode current collector layer 5. Separator 6. Outer casing 7. Outer casing 8. Negative electrode tab 9. Positive electrode tab 10. Lithium-ion secondary battery

Claims

It contains silicon-based active material and graphite, A negative electrode active material wherein, in the histogram of the frequency distribution of the D / G ratio of the aforementioned graphite according to Method 1 below, the mode of the peak with the largest area is 1.10 or more and 2.50 or less. (Method 1) The Raman mapping of the graphite is measured using a laser Raman microscope. Next, the peak area of ​​the G band and the peak area of ​​the D band are determined in the Raman spectrum of each pixel. Then, the D / G ratio is calculated from the peak areas of the G band and the D band for each pixel. Finally, a histogram of the frequency distribution of the D / G ratio is created by plotting the frequency of the D / G ratio. 　 The negative electrode active material according to claim 1, wherein the half-width of the peak with the largest area is 0.40 or more and 1.10 or less. 　 The negative electrode active material according to claim 1 or 2, wherein the minimum value of the D / G ratio in the mountain with the largest area is 0.01 or more. 　 The negative electrode active material according to any one of claims 1 to 3, wherein the gas generation rate by the method 2 described below is 10.0% or less. (Method 2) For lithium-ion secondary batteries manufactured according to the method described below, the gas generation rate is measured according to the method described below. <Method for manufacturing lithium-ion secondary batteries> A negative electrode active material slurry is prepared by adding water to a solid component consisting of 96.9 parts by mass of the negative electrode active material, 3.0 parts by mass of polyacrylic acid, and 0.1 parts by mass of single-walled carbon nanotubes. Next, the negative electrode active material slurry is applied to copper foil, with an initial charge capacity of 4.3 mAh / cm² per unit area. ２ After coating in this manner, the material is dried to obtain a negative electrode laminate consisting of a negative electrode active material layer and the copper foil. Next, the negative electrode laminate is subjected to a process where the density of the negative electrode active material layer is 1.50 g / cm³. ３ Press it in this manner to obtain the negative electrode. 　 Next, lithium nickel cobalt manganese (LiNi ０．９ Co ０．０５ Mn ０．０５ O ２ A cathode active material slurry is prepared by adding N-methyl-2-pyrrolidone to a solid component consisting of 97.5 parts by mass of ) 1.5 parts by mass of polyvinylidene fluoride and 1.0 part by mass of multi-walled carbon nanotubes. Next, the cathode active material slurry is placed on aluminum foil, with an initial charge capacity of 4.0 mAh / cm² per unit area. ２ After coating in this manner, it is dried to obtain a positive electrode laminate consisting of a positive electrode active material layer and the aluminum foil. Next, the positive electrode laminate is subjected to a process in which the density of the positive electrode active material layer is 3.50 g / cm³. ３ Press it in this manner to obtain the positive electrode. 　 Next, an electrode stack in which the negative electrode and the positive electrode are arranged opposite each other with a separator in between, and a non-aqueous electrolyte are placed inside a laminated casing. Then, a positive electrode tab and a negative electrode tab are connected to the negative electrode and the positive electrode, respectively, and the periphery of the laminated casing is sealed to produce a lithium-ion secondary battery. <Method for measuring gas generation rate> The lithium-ion secondary battery is placed in a constant temperature bath at 45°C. Next, it is charged at a charge rate of 1.0C, and after the upper limit voltage reaches 4.25V, it is charged at a constant voltage until the charge rate becomes 0.05C. Next, it is discharged at a discharge rate of 1.0C with a constant current until the lower limit voltage reaches 2.8V. Then, this charge and discharge cycle is repeated 750 times. Next, the gas generation rate is calculated from the following formula (1). Equation (1): (Gas generation rate) = {(Volume of the lithium-ion secondary battery after the 750th charge / discharge cycle) - (Volume of the lithium-ion secondary battery before the first charge / discharge cycle)} / (Volume of the lithium-ion secondary battery before the first charge / discharge cycle) × 100. 　 The negative electrode active material according to any one of claims 1 to 4, wherein the silicon-based active material contains Si / C including Si-C composite particles. 　 The volume-based median diameter D of the graphite measured by laser diffraction scattering method ５０ is 1.0 μm or more and 30.0 μm or less, the negative electrode active material according to any one of claims 1 to 5. 　 The specific surface area of ​​the aforementioned graphite, measured according to JIS Z8830:2013 using the BET method, is 0.1 m². ２ / g or more 5.0m ２ The negative electrode active material according to any one of claims 1 to 6, wherein the amount is less than or equal to / g. 　 The negative electrode active material according to any one of claims 1 to 7, wherein, when the total amount of the negative electrode active material is 100% by mass, the graphite content is 50% by mass or more and 99% by mass or less. 　 A negative electrode active material layer comprising the negative electrode active material described in any one of claims 1 to 8, A negative electrode comprising a negative electrode current collector layer. 　 The negative electrode according to claim 9, wherein the binder further contained in the negative electrode active material layer comprises a polycarboxylic acid polymer. 　 The negative electrode according to claim 9 or 10, wherein the negative electrode active material layer further comprises a conductive additive. 　 The negative electrode according to claim 11, wherein the conductive additive comprises a carbon material. 　 The negative electrode according to claim 12, wherein the carbon material includes carbon nanotubes. 　 The negative electrode according to any one of claims 9 to 13, wherein the negative electrode current collector layer includes copper foil. 　 A negative electrode according to any one of claims 9 to 14, Positive electrode and, A lithium-ion secondary battery equipped with a separator. 　 A lithium-ion secondary battery module comprising the lithium-ion secondary battery described in claim 15.

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