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

Abstract

To provide a negative electrode that enables the production of a lithium-ion secondary battery with improved cycle characteristics.SOLUTION: A negative electrode includes a negative electrode active material layer including a negative electrode active material, a conductive additive, and a binder, and the negative electrode active material includes graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, and the proportion of fibers (P) in the conductive additive having a fiber length of 1.0 μm or more, whose fiber length is three times or more the median diameter D50 in a volume frequency particle size distribution of the Si / C powder measured by a laser diffraction scattering method, is 50% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a negative electrode, a lithium ion secondary battery, and a lithium ion secondary battery module. [Background technology]

[0002] As the negative electrode active material, Si / C powder is sometimes used. Patent Document 1 describes a composite particle made of a Si-C composite material, which has an object to provide a composite particle that achieves a high silicon utilization rate in a lithium ion secondary battery and is resistant to oxidation when dispersed in water. The composite particle contains a carbon material and silicon, and has a silicon content of 30% by mass or more and 80% by mass or less, and a true density of 1.80 g / cm as determined by dry density measurement using helium gas. 3 More than 1.99g / cm 3 or less, and in the Raman spectrum of the composite particle, a peak is 450 to 495 cm -1 and the intensity of the peak is I Si and the G band intensity (1580 cm -1 (peak intensity near I G Then, I Si / I G is 1.3 or less, and the atomic number ratios of Si, O and C in the narrow spectrum of X-ray photoelectron spectroscopy of the composite particle are respectively Si , A O , and A C The ratio of SiO2 and SiO in the Si species ratio determined by the Si2p spectrum state analysis is B SiO2 , B SiO Then, A Si is 0.05 or more, and at least one of the following formulas (1) and (2) is satisfied. Y ≥ 0.75 … (1) Y≧-0.32X+0.81 …(2) [In formulas (1) and (2), X = I Si / I G and Y=A C / (A C +ASi ×(B SiO2 +B SiO )) is. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-059283 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the investigations of the present inventors, it has become clear that the cycle characteristics of a lithium ion secondary battery having a negative electrode using a negative electrode active material containing graphite powder and Si / C powder may be reduced.

[0005] The present invention provides a negative electrode that enables a lithium ion secondary battery with improved cycle characteristics to be obtained, as well as a lithium ion secondary battery and a lithium ion secondary battery module with improved cycle characteristics. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have discovered a negative electrode comprising a negative electrode active material layer including a negative electrode active material, a conductive additive, and a binder, wherein the negative electrode active material includes graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, and wherein the conductive additive contains fibers having a fiber length of 1.0 μm or more, and the fiber length is 1.0 μm or more, and the median diameter D 50 The inventors have found that a negative electrode in which the proportion of the number of fibers (P) that is three times or more of the total number of fibers (P) is 50% or more can improve the cycle characteristics of the resulting lithium ion secondary battery, and have completed the present invention.

[0007] According to the present invention, there are provided the following negative electrode, lithium ion secondary battery, and lithium ion secondary battery module.

[0008] [1] A negative electrode including a negative electrode active material layer containing a negative electrode active material, a conductive additive, and a binder, the negative electrode active material includes graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material; Among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is determined by the median diameter D 50 A negative electrode in which the proportion of the number of fibers (P) that is three times or more of the above is 50% or more. [2] Among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is a particle diameter D at which a cumulative value in a volume frequency particle size distribution of the graphite powder by a laser diffraction scattering method is 10%. 10 The negative electrode according to [1], wherein the proportion of the number of fibers (Q) that is equal to or greater than this is 12% or more. [3] The negative electrode according to [1] or [2], wherein the conductive additive comprises one or more selected from the group consisting of carbon nanotubes, carbon nanohorns, graphene, carbon nanobrushes, and carbon black. [4] The negative electrode according to [3], wherein the conductive additive contains carbon nanotubes. [5] The negative electrode according to any one of [1] to [4], wherein the content of the conductive additive in the negative electrode active material layer is 0.01 parts by mass or more and 5.00 parts by mass or less, when the total amount of the negative electrode active material layer is 100.00 parts by mass. [6] The median diameter D in the volume frequency particle size distribution of the Si / C powder measured by the laser diffraction scattering method 50 The negative electrode according to any one of [1] to [5], wherein the average particle size is 1.0 μm or more and 20.0 μm or less. [7] The negative electrode according to any one of [1] to [6], wherein the carbon material in the Si-C composite particles comprises a porous carbon material, and the silicon is present in at least part of the pores of the porous carbon material. [8] The particle diameter D at which the cumulative value reaches 10% in the volume frequency particle size distribution by the laser diffraction scattering method of the graphite powder 10 The negative electrode according to any one of [1] to [7], wherein the average particle size is 1.0 μm or more and 10.0 μm or less. [9] The median diameter D in the volume frequency particle size distribution of the graphite powder measured by the laser diffraction scattering method 50 The negative electrode according to any one of [1] to [8], wherein the average particle size is 3.0 μm or more and 30.0 μm or less.

[10] The negative electrode according to any one of [1] to [9], wherein the graphite powder contains graphite particles having amorphous carbon on the surface thereof.

[11] The negative electrode according to any one of [1] to

[10] , wherein the graphite powder contains artificial graphite particles.

[12] The content of the graphite powder in the negative electrode active material layer and the content of the Si / C powder in the negative electrode active material layer are respectively set to W C and W SiC When W C / W SiC The negative electrode according to any one of [1] to

[11] , wherein the value of is 1.0 or more and 20.0 or less.

[13] The graphite powder has a median diameter D 50 The graphite powder (A) and the graphite powder (B) are two different graphite powders, The median diameter D of the graphite powder (A) 50 is the median diameter D of the graphite powder (B). 50 The negative electrode according to any one of [1] to

[12] , wherein the negative electrode has a diameter of 100 nm or more.

[14] The median diameter D of the graphite powder (A) 50 D A , the median diameter D of the graphite powder (B) 50 D B When D B / D A The negative electrode according to

[13] , wherein the value of is 0.40 or more and less than 1.00.

[15] The negative electrode according to

[13] or

[14] , wherein the graphite powder (A) contains graphite particles containing amorphous carbon on their surfaces, and the graphite powder (B) contains graphite particles not containing amorphous carbon on their surfaces.

[16] The content of the graphite powder (A) in the graphite powder and the content of the graphite powder (B) in the graphite powder are respectively W A and W B When W B / W A The negative electrode according to any one of

[13] to

[15] , wherein the value of is 0.1 or more and 10.0 or less.

[17] The negative electrode according to any one of [1] to

[16] , wherein the binder contains one or more selected from the group consisting of fluororesins, polycarboxylic acid polymers, and synthetic rubbers.

[18] The negative electrode according to any one of [1] to

[17] , which has a capacity retention rate C of 80% or more as measured by the following method: (method) The negative electrode and lithium nickel cobalt manganese oxide (Li(Ni 0.9 Co 0.05 Mn 0.05 A lithium ion secondary battery is fabricated by placing an electrode laminate, in which a positive electrode containing a cellulose acylate (C1) and a positive electrode containing a cellulose acylate (C2) facing each other with a polyethylene separator interposed therebetween, and a nonaqueous electrolyte solution in a laminate outer casing formed by processing an aluminum-based film. A positive electrode tab and a negative electrode tab are connected to the negative electrode and the positive electrode, respectively, and sealing the periphery of the laminate outer casing. The lithium ion secondary battery is then placed in a thermostatic chamber at 45°C and charged at 30 mA. After the upper voltage reaches 4.2 V, the battery is charged at a constant voltage until the total charge time is 2.5 hours. The battery is then discharged at a constant current of 30 mA until the lower voltage reaches 2.5 V. This charge / discharge cycle is then repeated 300 times, and the ratio of the 300th discharge capacity to the first discharge capacity is calculated, which is defined as the capacity retention rate C (%).

[19] A lithium ion secondary battery comprising the negative electrode according to any one of [1] to

[18] .

[20]

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

[19] . [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a negative electrode that enables a lithium ion secondary battery with improved cycle characteristics to be obtained, as well as a lithium ion secondary battery and a lithium ion secondary battery module with improved cycle characteristics. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic cross-sectional view showing an example of a lithium ion secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. To avoid complexity, when there are multiple identical components in the same drawing, only one of them may be labeled with a symbol, and not all of them. The drawings are for illustrative purposes only, and the shapes and dimensional ratios of the components in the drawings do not necessarily correspond to the actual products.

[0012] In this embodiment, "A to B" indicating a numerical range means A or more and B or less unless otherwise specified. In this embodiment, the fiber refers to one having an aspect ratio (fiber length / fiber diameter) of 3 or more.

[0013] <Negative electrode> The negative electrode of this embodiment is a negative electrode having a negative electrode active material layer including a negative electrode active material, a conductive additive, and a binder, wherein the negative electrode active material includes graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, and among fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is equal to or less than the median diameter D 50 The proportion of the number of fibers (P) that is three times or more of the above is 50% or more.

[0014] According to the studies of the present inventors, in a negative electrode comprising a negative electrode active material layer including a negative electrode active material, a conductive additive, and a binder, the negative electrode active material including graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, the fiber length of the fibers in the conductive additive is greater than or equal to the median diameter D in the volume frequency particle size distribution of the Si / C powder measured by a laser diffraction scattering method. 50 They found that there is a correlation between the ratio of the number of fibers that is three times or more of the total number of fibers and the cycle characteristics of lithium-ion secondary batteries.

[0015] As a result of further investigations by the present inventors based on the above findings, it has been found that in a negative electrode comprising a negative electrode active material layer including a negative electrode active material, a conductive additive, and a binder, the negative electrode active material including graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, among fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is less than or equal to the median diameter D 50 The inventors have found that the cycle characteristics of the resulting lithium ion secondary battery can be improved by setting the proportion of the number of fibers (P), which is three times or more of the total number of fibers (P), to 50% or more, and have completed the present invention.

[0016] In the negative electrode of this embodiment, among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is a median diameter D 50 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the number ratio of the fibers (P) that is three times or more is 50% or more, preferably 52% or more, more preferably 54% or more, even more preferably 56% or more, even more preferably 58% or more, even more preferably 60% or more, even more preferably 65% ​​or more, and even more preferably 70% or more. In the negative electrode of this embodiment, the upper limit of the number ratio of the fibers (P) is not particularly limited, but may be, for example, 100% or less, 99% or less, or 95% or less.

[0017] In the negative electrode of this embodiment, among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is a median diameter D 50 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the number ratio of fibers (P) that is three times or more is preferably 50% or more and 100% or less, more preferably 52% or more and 100% or less, even more preferably 54% or more and 100% or less, even more preferably 56% or more and 99% or less, even more preferably 58% or more and 99% or less, even more preferably 60% or more and 99% or less, even more preferably 65% ​​or more and 95% or less, and even more preferably 70% or more and 95% or less.

[0018] In the negative electrode of this embodiment, among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is a particle diameter D at which the cumulative value in the volume frequency particle size distribution of graphite powder by laser diffraction scattering method is 10%. 10 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the number ratio of the fibers (Q) is preferably 12% or more, more preferably 13% or more, even more preferably 14% or more, even more preferably 15% or more, even more preferably 17% or more, even more preferably 18% or more, and even more preferably 21% or more. In the negative electrode of this embodiment, the upper limit of the number ratio of the fibers (Q) is not particularly limited, and may be, for example, 70% or less, 50% or less, 40% or less, 35% or less, or 30% or less.

[0019] In the negative electrode of this embodiment, among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is a particle diameter D at which the cumulative value in the volume frequency particle size distribution of graphite powder by laser diffraction scattering method is 10%. 10From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the number ratio of the fibers (Q) is preferably 12% or more and 70% or less, more preferably 13% or more and 70% or less, even more preferably 14% or more and 50% or less, even more preferably 15% or more and 50% or less, even more preferably 17% or more and 40% or less, even more preferably 18% or more and 35% or less, and even more preferably 21% or more and 30% or less.

[0020] In this embodiment, the volume frequency particle size distribution of the Si / C powder can be measured by the following method, for example. First, the Si / C powder is suspended in a dispersion medium and ultrasonically dispersed. Next, the volume frequency particle size distribution of the Si / C powder is measured using a laser diffraction scattering method using a laser diffraction particle size distribution analyzer. The measurement is performed five times, and the average value can be used.

[0021] In this embodiment, the volume frequency particle size distribution of the graphite powder can be measured by the following method, for example. First, graphite powder is suspended in a dispersion medium and ultrasonically dispersed. Next, a laser diffraction particle size distribution analyzer is used to measure the volume frequency particle size distribution of the graphite powder by the laser diffraction scattering method. Measurements are performed five times, and the average value can be used.

[0022] In this embodiment, the method for measuring the number ratio of fibers (P) and the number ratio of fibers (Q) among the fibers having a fiber length of 1.0 μm or more in the conductive additive may be, for example, the following method. First, the negative electrode current collector is separated from the negative electrode to obtain a negative electrode active material layer. Next, the negative electrode active material layer is added to the organic solvent N-methyl-2-pyrrolidone and heated at 100°C for 9 hours while stirring to obtain a slurry. Next, the slurry is left to stand for 9 hours to separate into a supernatant and a sediment, and the supernatant is collected. Next, the supernatant is dropped onto a new aqueous substrate and dried to obtain powder (X). The powder (X) thus obtained contains a conductive additive as the main component and may also contain a negative electrode active material and binder. However, the conductive additive, negative electrode active material, and binder can be separately observed using a scanning electron microscope at a magnification of 5000x, for example. Next, using a scanning electron microscope, the powder (X) is observed at any position under conditions of 5000x magnification and a field of view of 20 μm × 20 μm, and the fiber length is measured for all fibers in the conductive additive whose both ends can be seen and whose fiber diameter is 0.1 nm or more and whose fiber length is 1.0 μm or more. The observation is continued while changing the position until a total of 200 or more fibers can be observed, and the fiber length of each fiber is measured. Next, the fiber length (μm) of each measured fiber and the median diameter D of the Si / C powder used as the raw material for the negative electrode active material are calculated. 50 (μm) indicates that the fiber length of the conductive additive fibers with a fiber diameter of 0.1 nm or more and a fiber length of 1.0 μm or more is the median diameter D 50 The percentage (%) of fibers (P) that are three times or more of the fiber length (μm) of each fiber measured and the cumulative 10% diameter D of the graphite powder used as the raw material for the negative electrode active material were calculated. 10 (μm) is the particle diameter D of the conductive additive, which is the particle diameter of the fibers having a fiber diameter of 0.1 nm or more and a fiber length of 1.0 μm or more, at which the cumulative value in the volume frequency particle size distribution of the graphite powder by the laser diffraction scattering method is 10%. 10 The percentage (%) of fibers (Q) that are equal to or greater than this is calculated.

[0023] In this embodiment, as a method for adjusting the fiber length of the conductive aid, for example, a method of using commercially available conductive aids having different fiber lengths, or a method of adjusting the production conditions of the conductive aid such as heat treatment to produce conductive aids having different fiber lengths can be mentioned. Further, two or more types of conductive aids having different fiber lengths may be mixed to adjust the fiber length of the conductive aid.

[0024] <Negative electrode active material> The negative electrode active material of this embodiment contains graphite powder and Si / C powder.

[0025] <Si / C powder> The Si / C powder of this embodiment contains Si-C composite particles containing silicon and a carbon material. From the viewpoint of further improving the battery performance of the obtained lithium ion secondary battery, the Si / C powder of this embodiment preferably contains a porous carbon material as the 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.

[0026] In this embodiment, as a method for confirming that the Si-C composite particles in the Si / C powder 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 powder, 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, elemental mapping of silicon and carbon is performed and observed.

[0027] 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 to these.

[0028] The median diameter D in the volume frequency particle size distribution of the Si / C powder of this embodiment measured by the laser diffraction scattering method 50 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the average particle size is preferably 1.0 μm or more and 20.0 μm or less, more preferably 2.0 μm or more and 17.0 μm or less, even more preferably 3.0 μm or more and 14.0 μm or less, even more preferably 3.5 μm or more and 12.0 μm or less, and even more preferably 4.0 μm or more and 10.0 μm or less.

[0029] The particle diameter D at which the cumulative value reaches 10% in the volume frequency particle size distribution of the Si / C powder according to this embodiment, as determined by the laser diffraction scattering method, is 10 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the average particle size is preferably 0.1 μm or more and 10.0 μm or less, more preferably 0.5 μm or more and 8.0 μm or less, even more preferably 1.0 μm or more and 6.0 μm or less, and even more preferably 1.5 μm or more and 5.0 μm or less.

[0030] The particle diameter D at which the cumulative value reaches 90% in the volume frequency particle size distribution of the Si / C powder according to this embodiment, as determined by the laser diffraction scattering method, is 90 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the average particle size is preferably 3.0 μm or more and 30.0 μm or less, more preferably 5.0 μm or more and 25.0 μm or less, even more preferably 6.0 μm or more and 20.0 μm or less, even more preferably 7.0 μm or more and 17.0 μm or less, and even more preferably 8.0 μm or more and 15.0 μm or less.

[0031] The particle diameter D at which the cumulative value reaches 10% in the volume frequency particle size distribution of the Si / C powder according to this embodiment, as determined by the laser diffraction scattering method, is 10 , 90% particle diameter D 90 and median diameter D 50 It is calculated from (D 90 -D 10 ) / D 50From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the value of is preferably 0.50 or more and 3.00 or less, more preferably 0.70 or more and 2.50 or less, even more preferably 0.90 or more and 2.00 or less, even more preferably 1.00 or more and 1.80 or less, and even more preferably 1.05 or more and 1.60 or less.

[0032] From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the content of the Si / C powder in the negative electrode active material of this embodiment, when the total amount of the negative electrode active material is taken as 100.00 parts by mass, is preferably 1.00 parts by mass or more and 50.00 parts by mass or less, more preferably 5.00 parts by mass or more and 40.00 parts by mass or less, even more preferably 10.00 parts by mass or more and 30.00 parts by mass or less, even more preferably 15.00 parts by mass or more and 25.00 parts by mass or less, and even more preferably 18.00 parts by mass or more and 22.00 parts by mass or less.

[0033] In this embodiment, the method for producing the Si / C powder is not particularly limited. For example, the Si / C powder may have a median diameter of 4.0 to 10.0 μm and a specific surface area of ​​1000 to 1800 m. 2 The porous carbon material having a molecular weight of 1 / g is placed in a tubular furnace, the atmosphere inside the furnace is replaced with argon gas, and then a mixed gas of silane gas and nitrogen gas, in which the silane gas is 1 to 3 mol %, is flowed into the tubular furnace at a flow rate of 250 to 350 sccm, and the material is treated under conditions of 450 to 550°C, 700 to 800 Torr, and for 90 to 150 minutes.

[0034] <Graphite powder> The median diameter D in the volume frequency particle size distribution of the graphite powder of this embodiment measured by the laser diffraction scattering method 50 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the average particle size is preferably 3.0 μm or more and 30.0 μm or less, more preferably 5.0 μm or more and 25.0 μm or less, even more preferably 7.0 μm or more and 20.0 μm or less, even more preferably 9.0 μm or more and 16.0 μm or less, and even more preferably 9.5 μm or more and 13.0 μm or less.

[0035] The particle diameter D at which the cumulative value reaches 10% in the volume frequency particle size distribution of the graphite powder of this embodiment measured by the laser diffraction scattering method 10 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the average particle size is preferably 1.0 μm or more and 10.0 μm or less, more preferably 3.0 μm or more and 9.0 μm or less, even more preferably 4.0 μm or more and 8.0 μm or less, and even more preferably 5.0 μm or more and 7.0 μm or less.

[0036] The particle diameter D at which the cumulative value reaches 90% in the volume frequency particle size distribution of the graphite powder of this embodiment measured by the laser diffraction scattering method 90 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the average particle size is preferably 5.0 μm or more and 40.0 μm or less, more preferably 10.0 μm or more and 35.0 μm or less, even more preferably 12.0 μm or more and 30.0 μm or less, even more preferably 14.0 μm or more and 27.0 μm or less, and even more preferably 16.0 μm or more and 24.0 μm or less.

[0037] The particle diameter D at which the cumulative value reaches 10% in the volume frequency particle size distribution of the graphite powder of this embodiment measured by the laser diffraction scattering method 10 , 90% particle diameter D 90 and median diameter D 50 It is calculated from (D 90 -D 10 ) / D 50 From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the value of is preferably 0.50 or more and 3.00 or less, more preferably 0.70 or more and 2.50 or less, even more preferably 0.90 or more and 2.00 or less, even more preferably 1.00 or more and 1.70 or less, and even more preferably 1.10 or more and 1.50 or less.

[0038] The graphite powder of this embodiment preferably contains graphite particles containing amorphous carbon on the surface thereof, from the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery.

[0039] The graphite powder of this embodiment preferably contains artificial graphite particles from the viewpoint of further improving the battery performance of the resulting lithium ion secondary battery.

[0040] These graphite powders can be obtained by, for example, classifying commercially available graphite powders using a sieve with an appropriate opening ratio and wire diameter, thereby obtaining a median diameter D 50 , cumulative 10% diameter D 10 and cumulative 90% diameter D 90 The graphite powder containing graphite particles with amorphous carbon on the surface can be obtained, for example, by coating 2 to 5 parts by weight of amorphous carbon with 100 parts by weight of commercially available graphite powder by a method such as arc ion plating, sputtering, or plasma CVD. Examples of commercially available graphite powder include graphite powder manufactured by Nippon Graphite Industries Co., Ltd. and graphite powder manufactured by JFE Chemical Corporation.

[0041] From the viewpoint of further improving the cycle characteristics of the resulting lithium-ion secondary battery, the content of the graphite powder in the negative electrode active material of this embodiment is preferably 50.00 parts by mass or more and 99.00 parts by mass or less, more preferably 60.00 parts by mass or more and 95.00 parts by mass or less, even more preferably 70.00 parts by mass or more and 90.00 parts by mass or less, and even more preferably 75.00 parts by mass or more and 85.00 parts by mass or less, when the total amount of the negative electrode active material is taken as 100.00 parts by mass.

[0042] The median diameter D in the volume frequency particle size distribution of the graphite powder and Si / C powder of this embodiment measured by the laser diffraction scattering method 50 D respectively C and D SiC When D C / D SiC From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the value of is preferably 0.1 or more and 10.0 or less, more preferably 0.5 or more and 7.0 or less, even more preferably 0.8 or more and 5.0 or less, even more preferably 1.0 or more and 4.0 or less, and even more preferably 1.1 or more and 3.0 or less.

[0043] In the negative electrode active material of this embodiment, the content of graphite powder in the negative electrode active material layer and the content of Si / C powder in the negative electrode active material layer are respectively set to W C and W SiC When W C / W SiC From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the value of is preferably 1.0 or more and 20.0 or less, more preferably 2.0 or more and 15.0 or less, even more preferably 2.5 or more and 10.0 or less, even more preferably 3.0 or more and 7.0 or less, and even more preferably 3.5 or more and 5.0 or less.

[0044] From the viewpoint of further improving the cycle characteristics of the resulting lithium-ion secondary battery, the total content of the graphite powder and Si / C powder in the negative electrode active material of this embodiment is preferably 80.00 parts by mass or more and 100.00 parts by mass or less, more preferably 90.00 parts by mass or more and 100.00 parts by mass or less, even more preferably 95.00 parts by mass or more and 100.00 parts by mass or less, and even more preferably 99.00 parts by mass or more and 100.00 parts by mass or less, when the total amount of the negative electrode active material is taken as 100.00 parts by mass.

[0045] <Graphite powder (A), graphite powder (B)> From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the graphite powder of this embodiment preferably has a median diameter D 50 The graphite powder (A) and the graphite powder (B) are two types of graphite powder with different median diameters, D 50 is the median diameter D of graphite powder (B) 50 Greater than.

[0046] When the graphite powder of this embodiment contains graphite powder (A) and graphite powder (B), the median diameter D in the volume frequency particle size distribution of the graphite powder (A) measured by a laser diffraction scattering method is 50 D A , the median diameter D in the volume frequency particle size distribution of graphite powder (B) by laser diffraction scattering method 50 D B When D B / DA From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the value of is preferably 0.40 or more and less than 1.00, more preferably 0.50 or more and 0.90 or less, even more preferably 0.55 or more and 0.80 or less, and still more preferably 0.60 or more and 0.75 or less.

[0047] When the graphite powder of the present embodiment contains graphite powder (A) and graphite powder (B), from the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, preferably, the graphite powder (A) contains graphite particles containing amorphous carbon on the surface, and the graphite powder (B) contains graphite particles not containing amorphous carbon on the surface.

[0048] When the graphite powder of this embodiment contains graphite powder (A) and graphite powder (B), the content of graphite powder (A) in the graphite powder and the content of graphite powder (B) in the graphite powder are respectively defined as W A and W B When W A / W B From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the value of is preferably 0.1 or more and 10.0 or less, more preferably 0.3 or more and 5.0 or less, even more preferably 0.5 or more and 3.0 or less, and still more preferably 0.7 or more and 1.5 or less.

[0049] <Method of manufacturing negative electrode active material> The method for producing the negative electrode active material of this embodiment can include, for example, dry-mixing the raw materials, graphite powder and Si / C powder, using a mixer such as a small mill mixer, a V-type mixer, a rocking mixer, a ball mill, or a vibration mill.

[0050] <Negative electrode active material layer> The negative electrode active material layer of this embodiment contains a negative electrode active material, a conductive additive, and a binder.

[0051] From the viewpoint of further improving the cycle characteristics of the resulting lithium-ion secondary battery, the content of the negative electrode active material in the negative electrode active material layer of this embodiment, when the total amount of the negative electrode active material layer is taken as 100.00 parts by mass, is preferably 50.00 parts by mass or more and 100.00 parts by mass or less, more preferably 75.00 parts by mass or more and 99.90 parts by mass or less, even more preferably 85.00 parts by mass or more and 99.50 parts by mass or less, even more preferably 90.00 parts by mass or more and 99.00 parts by mass or less, even more preferably 95.00 parts by mass or more and 98.50 parts by mass or less, even more preferably 96.00 parts by mass or more and 98.00 parts by mass or less, and even more preferably 96.30 parts by mass or more and 97.50 parts by mass or less.

[0052] <Conductive additive> From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the conductive additive of this embodiment preferably contains one or more selected from the group consisting of carbon nanotubes, carbon nanohorns, graphene, carbon nanobrushes and carbon black, more preferably contains carbon nanotubes, and even more preferably contains single-walled carbon nanotubes.

[0053] The carbon nanotubes of this embodiment are materials in which a six-membered ring network (graphene) of carbon atoms has a single- or multi-layer coaxial tube structure. They can be broadly classified as single-walled carbon nanotubes or multi-walled carbon nanotubes. While either type of carbon nanotube may be used, particularly when used in a negative electrode active material containing silicon, which exhibits large expansion and contraction, it is preferable to use aggregated carbon nanotubes that are longer in diameter and length than individual carbon nanotubes by dispersing and aggregating these carbon nanotubes in a solution in advance to prevent rupture of the conductive network between the negative electrode active material. Examples of solutions include, but are not limited to, water and N-methyl-2-pyrrolidone. The fiber length can be adjusted using known dispersion methods and dispersion conditions. The average fiber length of the aggregated carbon nanotubes used in the negative electrode active material layer or positive electrode active material layer is preferably 0.3 μm to 30 μm, more preferably 1 μm to 20 μm, and even more preferably 1 μm to 15 μm, from the viewpoint of further improving the cycle characteristics of the resulting lithium-ion secondary battery.

[0054] From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the average fiber diameter of the carbon nanotubes of this embodiment before aggregation is preferably 0.5 nm or more and 15 nm or less, more preferably 1 nm or more and 8 nm or less, and even more preferably 1 nm or more and 5 nm or less.

[0055] From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the content of the conductive additive in the negative electrode active material layer of this embodiment is preferably 0.01 parts by mass or more and 5.00 parts by mass or less, more preferably 0.02 parts by mass or more and 3.00 parts by mass or less, even more preferably 0.03 parts by mass or more and 1.00 parts by mass or less, even more preferably 0.04 parts by mass or more and 0.50 parts by mass or less, and even more preferably 0.07 parts by mass or more and 0.30 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.00 parts by mass.

[0056] From the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the content of carbon nanotubes in the conductive additive of this embodiment is preferably 50 parts by mass or more and 100 parts by mass or less, more preferably 70 parts by mass or more and 100 parts by mass or less, even more preferably 80 parts by mass or more and 100 parts by mass or less, even more preferably 90 parts by mass or more and 100 parts by mass or less, and even more preferably 95 parts by mass or more and 100 parts by mass or less, when the total amount of the conductive additive in the negative electrode active material layer is taken as 100 parts by mass.

[0057] <Binder> Examples of binders in this embodiment include 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 (SBR), butadiene rubber (BR), chloroprene rubber (CR), isoprene rubber (IR), and acrylonitrile butadiene rubber (NBR); and polysaccharides such as carboxymethyl cellulose (CMC), xanthan gum, guar gum, and pectin. These may be used alone or in combination of two or more. Among these, from the viewpoint of further improving the cycle characteristics of the resulting lithium ion secondary battery, the binder of the present embodiment preferably contains one or more selected from the group consisting of fluororesin, polycarboxylic acid polymer, and synthetic rubber, more preferably contains one or more selected from the group consisting of polyvinylidene fluoride, polycarboxylic acid polymer, and styrene butadiene rubber, even more preferably contains a polycarboxylic acid polymer, and even more preferably contains poly(meth)acrylic acid.

[0058] From the viewpoint of further improving the cycle characteristics of the resulting lithium-ion secondary battery, the content of the binder in the negative electrode active material layer of this embodiment is preferably 0.10 parts by mass or more and 10.00 parts by mass or less, more preferably 1.00 parts by mass or more and 7.00 parts by mass or less, and even more preferably 2.00 parts by mass or more and 5.00 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.00 parts by mass.

[0059] From the viewpoint of further improving battery performance, the thickness of the negative electrode active material layer of this embodiment is preferably 10 μm or more and 250 μm or less, more preferably 20 μm or more and 200 μm or less, and even more preferably 50 μm or more and 150 μm or less.

[0060] The density of the negative electrode active material layer of this embodiment is preferably 0.50 g / cm from the viewpoint of further improving the battery performance. 3 More than 3.00g / cm 3 or less, more preferably 1.00 g / cm 3 More than 2.50g / cm 3 or less, more preferably 1.30 g / cm 3 More than 2.00g / cm 3 The following is the result.

[0061] <Negative electrode current collector> Preferably, the negative electrode of this embodiment further includes a negative electrode current collector, from the viewpoint of further improving the battery performance of the resulting lithium ion secondary battery.

[0062] The negative electrode current collector of this embodiment may be formed of, for example, copper, stainless steel, nickel, titanium, or an alloy thereof. The negative electrode current collector may be in the form of, for example, a foil, a flat plate, or a mesh. The thickness of the negative electrode current collector is not particularly limited, but is, for example, 1 μm or more and 50 μm or less.

[0063] <Capacity maintenance rate C> From the viewpoint of further improving the cycle characteristics of the resulting lithium-ion secondary battery, the capacity retention rate C of the negative electrode of this embodiment, as measured by the method described below, is preferably 80% or more, more preferably 82% or more, even more preferably 84% or more, even more preferably 87% or more, even more preferably 90% or more, and even more preferably 93% or more. The upper limit of the capacity retention rate C of the negative electrode of this embodiment, as measured by the method described below, is not particularly limited, but may be, for example, 100% or less, or 99% or less. (method) The negative electrode and lithium nickel cobalt manganese oxide (Li(Ni 0.9 Co 0.05 Mn 0.05 A lithium-ion secondary battery is fabricated by placing an electrode laminate, in which a positive electrode containing a cellulose acylate (C1) and a positive electrode (C2) are arranged opposite each other via a polyethylene separator, and a nonaqueous electrolyte solution in a laminate outer casing formed by processing an aluminum-based film. A positive electrode tab and a negative electrode tab are connected to the negative electrode and the positive electrode, respectively, and sealing the periphery of the laminate outer casing. The lithium-ion secondary battery is then placed in a thermostatic chamber at 45°C and charged at 30 mA. After the upper voltage reaches 4.2 V, it is charged at a constant voltage until the total charge time is 2.5 hours. It is then discharged at a constant current of 30 mA until the lower voltage reaches 2.5 V. This charge / discharge cycle is then repeated 300 times, and the ratio of the 300th discharge capacity to the first discharge capacity is calculated, which is defined as the capacity retention rate C (%).

[0064] From the viewpoint of further improving the cycle characteristics of the resulting lithium-ion secondary battery, the capacity retention rate C of the negative electrode of this embodiment, as determined by the above method, is preferably 80% or more and 100% or less, more preferably 82% or more and 100% or less, even more preferably 84% or more and 100% or less, even more preferably 87% or more and 99% or less, even more preferably 90% or more and 99% or less, and even more preferably 93% or more and 99% or less.

[0065] <Lithium-ion secondary battery> The lithium ion secondary battery of this embodiment includes the negative electrode of this embodiment. The lithium ion secondary battery of this embodiment includes the negative electrode of this embodiment, and therefore has improved cycle characteristics.

[0066] The lithium ion secondary battery of this embodiment will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing an example of the lithium ion secondary battery of this embodiment. As shown in FIG. 1, the lithium ion secondary battery 10 includes the negative electrode of this embodiment, an electrolyte, and a positive electrode. A separator 5 can be provided between the positive electrode and the negative electrode. A plurality of electrode pairs of a positive electrode and a negative electrode can be provided.

[0067] The lithium-ion secondary battery 10 includes a positive electrode including 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 disposed thereon, and a negative electrode including 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 disposed thereon. The positive electrode and negative electrode are stacked, for example, with a separator 5 made of a nonwoven fabric, a polypropylene microporous film, or the like, interposed between them, 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 exterior bodies 6 and 7 made of, for example, aluminum laminate film. 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, with these tabs extending outside the container. An electrolyte solution is poured into the container and sealed. Alternatively, a container may contain an electrode group in which multiple electrode pairs are stacked.

[0068] The lithium ion secondary battery 10 can be fabricated according to a known method. The electrodes can be, for example, laminates or wound bodies. The exterior can be a metal exterior or an aluminum laminate exterior. The battery can be in any shape, such as a coin, button, sheet, cylindrical, rectangular, or flat shape.

[0069] Examples of the positive electrode active material in the positive electrode active material layer of this embodiment include composite oxides of lithium and transition metals 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, and lithium-nickel-cobalt-manganese-aluminum composite oxide; transition metal sulfides such as TiS, FeS, and MoS; MnO, VO, and VO. 13 transition metal oxides such as TiO2; olivine-type lithium phosphate; and the like. One of these may be used alone, or two or more may be used in combination.

[0070] Examples of the conductive additive in the positive electrode active material layer of this embodiment include carbon fibers such as carbon nanofibers; carbon blacks such as acetylene black and ketjen black; 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.

[0071] Examples of the binder 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.

[0072] The positive electrode current collector of this embodiment may be formed of, for example, aluminum, stainless steel, nickel, titanium, or an alloy thereof. The positive electrode current collector may be in the form of, for example, a foil, a flat plate, or a mesh. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 1 μm or more and 50 μm or less.

[0073] Examples of the electrolyte solution of this embodiment include organic solvents such as cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), and butylene carbonate (BC); chain 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; chain ethers; and cyclic ethers, in which lithium salts such as lithium hexafluorophosphate (LiPF), lithium fluoroborate (LiBF), LiFSI, and lithium perchlorate (LiClO) are dissolved. These organic solvents may be used singly or in combination of two or more.

[0074] The separator of this embodiment is made of, for example, a porous film, woven fabric, nonwoven fabric, etc., mainly made of resin, and the resin component can be, for example, a polyolefin resin such as polypropylene or polyethylene, a polyester resin, an acrylic resin, a styrene resin, a nylon resin, etc. If necessary, the separator may be formed with a layer containing inorganic particles, and examples of the inorganic particles include insulating oxides, nitrides, sulfides, carbides, etc.

[0075] The exterior body of this embodiment can be, for example, a case or a can case made of a flexible film, and from the viewpoint of reducing the weight of the battery, it is preferable to use a flexible film. The flexible film can be a metal layer serving as a base material, with resin layers provided on both sides. The metal layer can be selected from those having barrier properties, such as preventing leakage of the electrolyte solution and infiltration of moisture from the outside, and aluminum, stainless steel, etc. can be used. A heat-sealable resin layer, such as a modified polyolefin, is provided on at least one side of the metal layer. The heat-sealable resin layers of the flexible films are placed opposite each other, and the periphery of the portion housing the electrode stack is heat-sealed to form the exterior body. A resin layer, such as a nylon film or a polyester film, can be provided on the surface of the exterior body opposite the side on which the heat-sealable resin layer is formed.

[0076] <Lithium-ion secondary battery module> The lithium ion secondary battery module of this embodiment includes the lithium ion secondary battery of this embodiment. Since the lithium ion secondary battery of this embodiment has improved cycle characteristics, the lithium ion secondary battery module of this embodiment also has improved cycle characteristics.

[0077] 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 parallel. More preferably, the lithium-ion secondary battery module of this embodiment includes a housing capable of accommodating two or more lithium-ion secondary batteries of this embodiment connected in series or parallel. The lithium-ion secondary battery module of this embodiment further preferably includes one or more components selected from the group consisting of a protection circuit that protects the lithium-ion secondary batteries from overcurrent, a balancing circuit that equalizes the voltage between the electrodes of the lithium-ion secondary batteries, a controller that controls the lithium-ion secondary batteries, a cooler that can cool the lithium-ion secondary batteries, and a heater that can heat the lithium-ion secondary batteries.

[0078] The lithium-ion secondary battery module of this embodiment can be used in a battery system including a plurality of electrically connected lithium-ion secondary battery modules and a battery control system. Examples of battery systems include battery packs, stationary storage battery systems, automotive power storage battery systems, automotive auxiliary storage battery systems, and emergency power storage battery systems.

[0079] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Example]

[0080] Embodiments of the present invention will be described in detail based on examples and comparative examples. Note that the present invention is not limited only to the examples.

[0081] In the production of the negative electrode active material, the following graphite powders were used.

[0082] <Graphite powder (A)> · Graphite powder 1 (artificial graphite containing amorphous carbon on the surface, D 10 : 8.4 μm, D 50 : 13.6 μm, D 90 : 21.1 μm, (D 90 -D 10 ) / D 50 : 0.93) · Graphite powder 2 (artificial graphite containing amorphous carbon on the surface, D 10 : 9.0 μm, D 50 : 14.5 μm, D 90 : 36.0 μm, (D 90 -D 10 ) / D 50 : 1.86) · Graphite powder 3 (artificial graphite containing amorphous carbon on the surface, D 10 : 5.5 μm, D 50 : 10.1 μm, D 90 : 17.0 μm, (D 90 -D 10 ) / D 50 : 1.14)

[0083] <Graphite powder (B)> · Graphite powder 4 (artificial graphite not containing amorphous carbon on the surface, D 10 : 4.9 μm, D 50 : 9.6 μm, D 90 : 17.6 μm, (D 90 -D 10 ) / D 50 : 1.32)

[0084] <Production of Si / C powder 1> Porous carbon material 1 (D 50 : 4.8 μm, specific surface area: 1678 m 2 / g) was placed in a tubular furnace, and after replacing the inside of the tubular furnace with argon gas, a mixed gas of 2 mol% silane gas and 98 mol% nitrogen gas was flowed into the tubular furnace at a flow rate of 300 sccm, and the treatment was carried out while maintaining the conditions of 500 °C, 760 Torr, and 120 minutes. Subsequently, the product was cooled to room temperature to obtain Si / C powder 1.

[0085] Regarding the cross-section of the Si-C composite particles contained in the obtained Si / C powder 1, using a scanning electron microscope (SU3500 manufactured by Hitachi High-Technologies Corporation), an energy-dispersive X-ray spectroscopy detector (Ultim Max 40 manufactured by Oxford Instruments), and image analysis software (Aztec manufactured by Oxford Instruments), secondary electrons were selected for 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, elemental mapping of silicon and carbon was performed, and it was confirmed that the Si-C composite particles contain silicon and that silicon exists in at least a part of the pores of the porous carbon material.

[0086] Regarding the obtained Si / C powder 1, from the volume frequency particle size distribution measured by the laser diffraction scattering method using a laser diffraction type particle size distribution measuring device (SALD-2300 manufactured by Shimadzu Corporation), the particle size D at which the cumulative value becomes 10% 10 , the particle size D at which the cumulative value becomes 90% 90 , the median diameter D 50 and (D 90 - D 10 ) / D 50 values were determined. Here, Si / C powder 1 was suspended in a dispersion medium and ultrasonically dispersed before measurement. The measurement was carried out 5 times, and the average values were adopted respectively. The results are shown in Table 2.

[0087] <Preparation of Si / C Powder 2> Si / C powder 2 was obtained in the same manner as Si / C powder 1, except that porous carbon material 2 (D 50 : 9.0 μm, specific surface area: 1635 m 2 / g) was used instead of porous carbon material 1.

[0088] Elemental mapping was performed on the cross section of the Si-C composite particles contained in the obtained Si / C powder 2 using the same method as for the above-mentioned Si / C powder 1, and it was confirmed that the Si-C composite particles contained silicon, and that the silicon in the Si-C composite particles was present in at least some of the pores of the porous carbon material.

[0089] The obtained Si / C powder 2 was subjected to the same method as the Si / C powder 1 to measure the cumulative 10% diameter D 10 , cumulative 90% diameter D 90 , median diameter D 50 and (D 90 -D 10 ) / D 50 The results are shown in Table 2.

[0090] (Examples 1 to 6, Comparative Examples 1 and 2) <Preparation of graphite powder> Graphite powder (A) and graphite powder (B) were mixed in the compounding ratio shown in Table 1 to obtain graphite powders of Examples 1 to 6 and Comparative Examples 1 and 2.

[0091] The graphite powders of each example and each comparative example were measured for the cumulative 10% diameter D 10 , cumulative 90% diameter D 90 , median diameter D 50 , (D 90 -D 10 ) / D 50 The cumulative 90% diameter D 90 and median diameter D 50 The graphite powder was suspended in a dispersion medium and ultrasonically dispersed, and then the measurement was performed. The measurement was performed five times, and the average value was used for each. The results are shown in Table 2.

[0092] <Preparation of negative electrode active material> Graphite powder and Si / C powder were mixed in the compounding ratios shown in Table 1 to obtain negative electrode active materials of Examples 1 to 6 and Comparative Examples 1 and 2.

[0093] The following conductive additives and binders were used in the preparation of the negative electrode: The average fiber length of the conductive additive is the average fiber length after dispersion in a solvent and aggregation.

[0094] <Conductive additive> Single-walled carbon nanotubes 1 (average fiber length: 4.4 μm) Single-walled carbon nanotubes 2 (average fiber length: 5.7 μm) Single-walled carbon nanotubes 3 (average fiber length: 3.6 μm) Single-walled carbon nanotubes 4 (average fiber length: 2.4 μm) Multi-walled carbon nanotubes 1 (average fiber length: 4.3 μm) Multi-walled carbon nanotubes 2 (average fiber length: 3.8 μm) Carbon black 1 (Timcal, C65) <Binder> Polyacrylic acid 1 (Sumitomo Seika Chemicals, Aquacharge)

[0095] <Preparation of negative electrode> For the negative electrode active materials of each Example and Comparative Example, the negative electrode active material, conductive additive, and binder were mixed in the blending ratio shown in Table 1, 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 a copper foil having a thickness of 8 μm, which was a negative electrode current collector, to obtain a negative electrode current collector having an initial charge capacity per unit area of ​​4.3 mAh / cm. 2 The negative electrode laminate was then pressed using a roll press machine to a density of 1.65 g / cm. 3 The negative electrodes of the examples and comparative examples were obtained by pressing the negative electrodes with a pressure of 0.1 to 1.0.

[0096] <Number ratio of fiber (P), number ratio of fiber (Q)> For the negative electrodes of each example and comparative example, the negative electrode current collector was separated from the negative electrode to obtain a negative electrode active material layer. The negative electrode active material layer was then added to N-methyl-2-pyrrolidone, an organic solvent, and heated at 100°C for 9 hours while stirring to obtain a slurry. The slurry was then allowed to stand for 9 hours to separate into a supernatant and a sediment, and the supernatant was collected. The supernatant was then dropped onto a new aqueous substrate and dried to obtain powder (X). Powder (X) thus obtained contained a conductive additive as the main component, with a portion containing a negative electrode active material and binder. The conductive additive, negative electrode active material, and binder could be separately observed using a scanning electron microscope (SU3500, manufactured by Hitachi High-Tech Corporation) at a magnification of 5000x. Next, using a scanning electron microscope, the powder (X) was observed at an arbitrary position under conditions of 5000x magnification and a field of view of 20 μm × 20 μm. Of the fibers in the conductive additive in which both ends of the fibers could be seen, the fiber length was measured for all fibers with a fiber diameter of 0.1 nm or more and a fiber length of 1.0 μm or more. The observation position was changed and the fiber length of each fiber was measured until a total of 200 or more fibers could be observed. Next, the fiber length (μm) of each measured fiber and the median diameter D of the Si / C powder used as the raw material for the negative electrode active material were measured. 50 From (μm), for the negative electrodes of each Example and each Comparative Example, among the fibers in the conductive additive having a fiber diameter of 0.1 nm or more and a fiber length of 1.0 μm or more, the fiber length is determined to be the median diameter D in the volume frequency particle size distribution of the Si / C powder by the laser diffraction scattering method. 50 The percentage (%) of fibers (P) having a diameter of at least three times the diameter of the fiber (P) was calculated. 10 From (μm), for the negative electrodes of each Example and each Comparative Example, among the fibers in the conductive additive having a fiber diameter of 0.1 nm or more and a fiber length of 1.0 μm or more, the particle diameter D at which the cumulative value in the volume frequency particle size distribution of graphite powder by laser diffraction scattering method is 10% is calculated. 10 The percentage (%) of fibers (Q) that met the above criteria was calculated, and the results are shown in Table 2.

[0097] <Preparation of positive electrode> Lithium nickel cobalt manganese oxide (Li(Ni 0.9 Co 0.05 Mn 0.05 A positive electrode active material slurry was prepared by adding an appropriate amount of N-methyl-2-pyrrolidone to a solid content consisting of 97.50 parts by mass of )O2), 1.50 parts by mass of polyvinylidene fluoride, and 1.00 parts by mass of single-walled carbon nanotubes. Next, the positive electrode active material slurry was applied to a 12 μm thick aluminum foil positive electrode current collector so that the initial charge capacity per unit area was 4.0 mAh / cm. 2 The positive electrode laminate was then coated in an amount such that the density was 3.5 g / cm using a roll press. 3 The positive electrode was obtained by pressing the positive electrode with a pressure of 0.015.

[0098] <Preparation of non-aqueous electrolyte> The non-aqueous electrolyte was prepared by mixing an organic solvent and a supporting salt. The volume ratio of cyclic carbonate (ethylene carbonate) to chain carbonate (ethyl methyl carbonate) was adjusted to 3:7, and lithium hexafluorophosphate (LiPF6) (concentration: 1.2 mol / L) was used as the supporting salt, and fluoroethylene carbonate (concentration relative to the organic solvent: 6% by mass) was used as the additive.

[0099] <Fabrication of lithium-ion secondary batteries> The positive electrode and the negative electrode of each Example and Comparative Example were cut into 3 cm x 3 cm pieces and placed opposite each other with a separator interposed therebetween to produce an electrode laminate. The separator used was a 10 μm-thick microporous polyethylene film with a ceramic coating on both sides. The electrode laminate and nonaqueous electrolyte were then placed in a laminate outer casing formed by processing an aluminum-based film. A positive electrode tab and a negative electrode tab were connected to the negative electrode and the positive electrode, respectively, and the periphery of the laminate outer casing was sealed to produce a lithium-ion secondary battery of each Example and Comparative Example. One end of the positive electrode tab was connected to the positive electrode and the other end was extended outside the outer casing, and one end of the negative electrode tab was connected to the negative electrode and the other end was extended outside the outer casing.

[0100] <Capacity maintenance rate C> The lithium-ion secondary batteries of each Example and Comparative Example were placed in a thermostatic chamber at 45°C and charged at 30 mA. After the upper voltage limit reached 4.2 V, they were charged at a constant voltage until the total charge time reached 2.5 hours. They were then discharged at a constant current of 30 mA until the lower voltage limit reached 2.5 V. This charge / discharge cycle was then repeated 300 times, and the ratio of the 300th discharge capacity to the first discharge capacity was calculated, giving the capacity retention rate C (%). The results are shown in Table 2.

[0101] [Table 1]

[0102] [Table 2] [Explanation of symbols]

[0103] 1 Cathode active material layer 2 Negative electrode active material layer 3 Positive electrode current collector 4 Negative electrode current collector 5 Separator 6. Exterior body 7. Exterior body 8 Negative electrode tab 9 Positive tab 10 Lithium-ion secondary battery

Claims

1. A negative electrode including a negative electrode active material layer containing a negative electrode active material, a conductive additive, and a binder, the negative electrode active material includes graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material; Among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is a median diameter D ５０ a ratio of the number of fibers (P) that is three times or more of the total number of fibers (P) is 50% or more;

2. Among the fibers in the conductive additive having a fiber length of 1.0 μm or more, the fiber length is a particle diameter D at which a cumulative value in a volume frequency particle size distribution of the graphite powder by a laser diffraction scattering method is 10%. １０ The negative electrode according to claim 1 , wherein the proportion of the number of fibers (Q) that is equal to or greater than 12% is 12% or greater.

3. 3. The negative electrode according to claim 1, wherein the conductive additive comprises one or more selected from the group consisting of carbon nanotubes, carbon nanohorns, graphene, carbon nanobrushes, and carbon black.

4. The negative electrode according to claim 3 , wherein the conductive additive comprises carbon nanotubes.

5. The negative electrode according to any one of claims 1 to 4, wherein the content of the conductive additive in the negative electrode active material layer is 0.01 parts by mass or more and 5.00 parts by mass or less when the total amount of the negative electrode active material layer is 100.00 parts by mass.

6. The median diameter D in the volume frequency particle size distribution of the Si / C powder measured by the laser diffraction scattering method ５０ The negative electrode according to any one of claims 1 to 5, wherein the average particle size is 1.0 µm or more and 20.0 µm or less.

7. 7. The negative electrode according to claim 1, wherein the carbon material in the Si—C composite particles comprises a porous carbon material, and the silicon is present in at least part of the pores of the porous carbon material.

8. The particle diameter D at which the cumulative value reaches 10% in the volume frequency particle size distribution by the laser diffraction scattering method of the graphite powder １０ The negative electrode according to any one of claims 1 to 7, wherein the thickness is 1.0 µm or more and 10.0 µm or less.

9. The median diameter D in the volume frequency particle size distribution of the graphite powder measured by the laser diffraction scattering method ５０ The negative electrode according to any one of claims 1 to 8, wherein the thickness is 3.0 µm or more and 30.0 µm or less.

10. 10. The negative electrode according to claim 1, wherein the graphite powder comprises graphite particles having amorphous carbon on the surface thereof.

11. The negative electrode according to any one of claims 1 to 10, wherein the graphite powder contains artificial graphite particles.

12. The content of the graphite powder in the negative electrode active material layer and the content of the Si / C powder in the negative electrode active material layer are respectively set to W Ｃ and W ＳｉＣ When W Ｃ / W ＳｉＣ The negative electrode according to any one of claims 1 to 11, wherein the value of is 1.0 or more and 20.0 or less.

13. The graphite powder has a median diameter D ５０ The graphite powder (A) and the graphite powder (B) are two different graphite powders, The median diameter D of the graphite powder (A) ５０ is the median diameter D of the graphite powder (B). ５０ The negative electrode according to any one of claims 1 to 12, wherein the negative electrode has a mass ratio of 1:1 or more.

14. The median diameter D of the graphite powder (A) ５０ D Ａ , the median diameter D of the graphite powder (B) ５０ D Ｂ When this is done, D Ｂ / D Ａ The negative electrode according to claim 13 , wherein the value of is equal to or greater than 0.40 and less than 1.

00.

15. 15. The negative electrode according to claim 13, wherein the graphite powder (A) comprises graphite particles having amorphous carbon on their surfaces, and the graphite powder (B) comprises graphite particles having no amorphous carbon on their surfaces.

16. The content of the graphite powder (A) in the graphite powder and the content of the graphite powder (B) in the graphite powder are respectively W Ａ and W Ｂ When W Ｂ / W Ａ The negative electrode according to any one of claims 13 to 15, wherein the value of is 0.1 or more and 10.0 or less.

17. 17. The negative electrode according to claim 1, wherein the binder comprises one or more selected from the group consisting of fluororesins, polycarboxylic acid polymers, and synthetic rubbers.

18. The negative electrode according to any one of claims 1 to 17, wherein the capacity retention rate C measured by the following method is 80% or more. (method) The negative electrode and lithium nickel cobalt manganese oxide (Li(Ni ０．９ Co ０．０５ Mn ０．０５ A lithium ion secondary battery is fabricated by placing an electrode laminate, in which a positive electrode containing a cation exchange resin (C2) and a positive electrode (C3) are arranged opposite each other via a polyethylene separator, and a nonaqueous electrolyte solution in a laminate outer casing formed by processing an aluminum-based film. A positive electrode tab and a negative electrode tab are connected to the negative electrode and the positive electrode, respectively, and sealing the periphery of the laminate outer casing. The lithium ion secondary battery is then placed in a thermostatic chamber at 45°C and charged at 30 mA. After the upper voltage reaches 4.2 V, the battery is charged at a constant voltage until the total charge time is 2.5 hours. The battery is then discharged at a constant current of 30 mA until the lower voltage reaches 2.5 V. This charge / discharge cycle is then repeated 300 times, and the ratio of the 300th discharge capacity to the first discharge capacity is calculated, which is defined as the capacity retention rate C (%).

19. A lithium ion secondary battery comprising the negative electrode according to any one of claims 1 to 18.

20. A lithium ion secondary battery module comprising the lithium ion secondary battery according to claim 19.

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