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

Abstract

To provide a negative electrode active material that enables the production of a lithium ion secondary battery with improved energy density.SOLUTION: A negative electrode active material includes graphite powder, and Si / C powder containing Si-C composite particles containing silicon and a carbon material, and when the breaking strengths of the graphite powder and the Si / C powder measured in accordance with JIS R 1639-5:2007 are CS1 and CS2, respectively, the value of CS1 / CS2 is 0.32 or less.SELECTED DRAWING: None

Description

[Technical Field]

[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. [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+A Si ×(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 energy density of a lithium ion secondary battery using a negative electrode active material containing graphite powder and Si / C powder may be low.

[0005] The present invention provides a negative electrode active material and a negative electrode that can provide a lithium ion secondary battery with improved energy density, as well as a lithium ion secondary battery and a lithium ion secondary battery module with improved energy density. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above problems. As a result, they have found that a negative electrode active material containing graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, where CS1 and CS2 are the breaking strengths of the graphite powder and the Si / C powder measured in accordance with JIS R 1639-5:2007, respectively, and having a CS1 / CS2 value of 0.32 or less, can improve the energy density 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 active material, negative electrode, lithium ion secondary battery, and lithium ion secondary battery module.

[0008] [1] graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material; The negative electrode active material has a CS1 / CS2 value of 0.32 or less, where CS1 and CS2 are the breaking strengths of the graphite powder and the Si / C powder measured in accordance with JIS R 1639-5:2007, respectively. [2] The negative electrode active material according to [1], wherein the CS1 is 10 MPa or more and 90 MPa or less. [3] 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 active material according to [1] or [2], wherein the average particle size is 3.0 μm or more and 30.0 μm or less. [4] 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 , 90% particle diameter D 90 and median diameter D 50 It is calculated from (D 90 -D 10 ) / D 50 The negative electrode active material according to any one of [1] to [3], wherein the value of is 0.50 or more and 3.00 or less. [5] 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 active material according to any one of [1] to [4], wherein the average particle size is 1.0 μm or more and 20.0 μm or less. [6] The negative electrode active material according to any one of [1] to [5], wherein the carbon material in the Si-C composite particles comprises a porous carbon material, and the silicon is present in at least a portion of the pores of the porous carbon material. [7] The negative electrode active material according to any one of [1] to [6], wherein the graphite powder contains graphite particles having amorphous carbon on the surface thereof. [8] The negative electrode active material according to any one of [1] to [7], wherein the graphite powder contains artificial graphite particles. [9] The negative electrode active material according to any one of [1] to [8], wherein the content of the graphite powder in the negative electrode active material is 50.0 parts by mass or more and 99.0 parts by mass or less, when the total amount of the negative electrode active material is 100.0 parts by mass.

[10] The negative electrode active material according to any one of [1] to [9], wherein the content of the Si / C powder in the negative electrode active material is 1.0 part by mass or more and 50.0 parts by mass or less, when the total amount of the negative electrode active material is 100.0 parts by mass.

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

[10] , wherein, when the content of the graphite powder in the negative electrode active material is W1 and the content of the Si / C powder in the negative electrode active material is W2, the value of W1 / W2 is 1.0 or more and 20.0 or less.

[12] 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 active material according to any one of [1] to

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

[13] 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 active material according to

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

[14] The negative electrode active material according to

[12] or

[13] , wherein the graphite powder (A) contains graphite particles containing amorphous carbon on the surface thereof, and the graphite powder (B) contains graphite particles not containing amorphous carbon on the surface thereof.

[15] The content of the graphite powder (A) in the graphite powder is W A The content of the graphite powder (B) in the graphite powder is W B When W A / W BThe negative electrode active material according to any one of

[12] to

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

[16] The negative electrode active material according to any one of [1] to

[15] , which has an energy density E of 800 Wh / L or more in terms of a cell, as measured by the following method: (method) A suitable amount of water is added to a solid content of 96.9 parts by mass of the negative electrode active material, 0.1 parts by mass of single-walled carbon nanotubes, and 3.0 parts by mass of polyacrylic acid to prepare a negative electrode active material slurry. Next, the negative electrode active material slurry is applied to a copper foil negative electrode current collector to form a negative electrode current collector having an initial charge capacity per unit area of ​​4.3 mAh / cm. 2 After applying the coating amount so that the density becomes 1.65 g / cm, the coating is dried to obtain a negative electrode laminate. 3 The negative electrode is then pressed with a pressure so that 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 crystalline silicon dioxide (C2) and a positive electrode (C1) 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 charged at 7.2 mA, and after the upper voltage reaches 4.2 V, it is charged at a constant voltage until the total charge time is 12 hours. It is then discharged at a constant current of 7.2 mA until the lower voltage reaches 2.5 V. It is then charged again under the same conditions, left in a constant temperature bath at 45°C for 3 days, and discharged again under the same conditions. The capacity and average voltage at the final discharge are measured. After the final discharge, the thickness of the lithium-ion secondary battery is measured. Next, the energy density E (Wh / L) converted into a cell is calculated using the following formula based on the above measured values, the electrode area of ​​the electrode laminate, the thickness of the laminate exterior body, the thickness of the negative electrode current collector, and the thickness of the positive electrode current collector in the positive electrode. (Energy density E) = (Capacity at final discharge) × (Average voltage at final discharge) / (Electrode area of ​​electrode laminate) / ((Thickness of lithium-ion secondary battery) - (Thickness of laminate exterior) - (Thickness of positive electrode current collector) / 2 - (Thickness of negative electrode current collector) / 2)

[17] A negative electrode comprising the negative electrode active material according to any one of [1] to

[16] .

[18]

[17] A lithium ion secondary battery comprising the negative electrode according to

[17] .

[19]

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

[18] . [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a negative electrode active material and a negative electrode that enable a lithium ion secondary battery with improved energy density to be obtained, as well as a lithium ion secondary battery and a lithium ion secondary battery module with improved energy density. [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.

[0013] <Negative electrode active material> The negative electrode active material of this embodiment includes graphite powder and Si / C powder containing Si-C composite particles containing silicon and a carbon material, and when the breaking strengths of the graphite powder and Si / C powder measured in accordance with JIS R 1639-5:2007 are CS1 and CS2, respectively, the value of CS1 / CS2 is 0.32 or less.

[0014] According to the investigations of the present inventors, it has been found that in a negative electrode active material containing graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, there is a correlation between the ratio of the fracture strengths of the graphite powder and the Si / C powder and the energy density of the resulting lithium ion secondary battery.

[0015] Based on the above findings, the inventors have conducted further studies and have found that in a negative electrode active material comprising graphite powder and Si / C powder containing Si-C composite particles containing silicon and a carbon material, when the breaking strengths of the graphite powder and the Si / C powder measured in accordance with JIS R 1639-5:2007 are CS1 and CS2, respectively, by setting the CS1 / CS2 value to 0.32 or less, the energy density of the resulting lithium ion secondary battery can be improved, thereby completing the present invention.

[0016] In the negative electrode active material of this embodiment, when the fracture strengths of the graphite powder and the Si / C powder measured in accordance with JIS R 1639-5:2007 are CS1 and CS2, respectively, the value of CS1 / CS2 is 0.32 or less, preferably 0.31 or less, more preferably 0.30 or less, even more preferably 0.29 or less, even more preferably 0.28 or less, even more preferably 0.27 or less, even more preferably 0.26 or less, even more preferably 0.25 or less, and even more preferably 0.24 or less, from the viewpoint of further improving the energy density of the resulting lithium ion secondary battery. In the negative electrode active material of this embodiment, the lower limit of the value of CS1 / CS2 is not particularly limited, and may be, for example, 0.01 or more, 0.05 or more, 0.10 or more, 0.13 or more, 0.15 or more, or 0.18 or more.

[0017] In the negative electrode active material of this embodiment, when the breaking strengths of the graphite powder and the Si / C powder measured in accordance with JIS R 1639-5:2007 are CS1 and CS2, respectively, the value of CS1 / CS2 is, from the viewpoint of further improving the energy density of the resulting lithium ion secondary battery, preferably 0.01 or more and 0.32 or less, more preferably 0.01 or more and 0.31 or less, even more preferably 0.01 or more and 0.30 or less, even more preferably 0.01 or more and 0.29 or less, even more preferably 0.05 or more and 0.28 or less, even more preferably 0.10 or more and 0.27 or less, even more preferably 0.13 or more and 0.26 or less, even more preferably 0.15 or more and 0.25 or less, and even more preferably 0.18 or more and 0.24 or less.

[0018] In the negative electrode active material of this embodiment, the breaking strength CS1 of the graphite powder measured in accordance with JIS R 1639-5:2007 is preferably 10 MPa or more and 90 MPa or less, more preferably 30 MPa or more and 85 MPa or less, even more preferably 40 MPa or more and 80 MPa or less, and still more preferably 50 MPa or more and 78 MPa or less, from the viewpoint of further improving the energy density of the resulting lithium ion secondary battery.

[0019] In the negative electrode active material of this embodiment, the breaking strength CS2 of the Si / C powder measured in accordance with JIS R 1639-5:2007 is preferably 50 MPa or more and 500 MPa or less, more preferably 100 MPa or more and 450 MPa or less, even more preferably 130 MPa or more and 420 MPa or less, even more preferably 150 MPa or more and 370 MPa or less, even more preferably 180 MPa or more and 330 MPa or less, and even more preferably 200 MPa or more and 300 MPa or less, from the viewpoint of further improving the energy density of the resulting lithium ion secondary battery.

[0020] In this embodiment, the breaking strength of the graphite powder or Si / C powder can be measured by the following method, for example. First, for the graphite powder or Si / C powder, using a micro-compression tester, the test force P (N) when the powder particles are broken is measured by a indenter. That is, after spraying a very small amount of the powder on the sample stage, for any 100 particles, each particle is compressed one by one with the indenter to obtain the test force P (N) when the powder particles are broken. Also, the particle diameter d (mm) of each particle is measured in two orthogonal directions (the X direction and the Y direction in the observation image of the micro-compression tester), and the average value thereof is used. Next, the fracture strength CS of each particle is obtained from the test force P (N), the particle diameter d (mm), and the circumference ratio π by the following formula, and the average value of the fracture strength CS for 100 particles is taken as the fracture strength of the powder. CS = 2.48×P / (π×d 2 )

[0021] In this embodiment, as a method for adjusting the fracture strength of the graphite powder and the Si / C powder, for example, a method of using commercially available graphite powder and Si / C powder with different fracture strengths, or a method of adjusting the manufacturing conditions of the graphite powder and the Si / C powder such as surface treatment and heat treatment to produce graphite powder and Si / C powder with different fracture strengths can be mentioned. Also, two or more kinds of graphite powders with different fracture strengths may be mixed to adjust the fracture strength of the graphite powder, and two or more kinds of Si / C powders with different fracture strengths may be mixed to adjust the fracture strength of the Si / C powder.

[0022] <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.

[0023] In this embodiment, a method for confirming that the Si-C composite particles in the Si / C powder contain silicon and carbon material and that the silicon in the Si-C composite particles is present in at least some of the pores of the porous carbon material can be, for example, by observing a cross section of the Si-C composite particle in the Si / C powder using a scanning electron microscope, an energy dispersive X-ray spectroscopic detector, and image analysis software, selecting secondary electrons as the detection target, and performing elemental mapping of silicon and carbon under conditions of an acceleration voltage of 3 kV, a mapping accumulation number of 20, and a magnification of 3000x.

[0024] Examples of porous carbon materials that make up the Si-C composite particles include activated carbon, aggregates of carbon fibers, aggregates of carbon nanotubes, carbon obtained by heat treating resins or organic materials, hard carbon, etc. Porous carbon materials can be produced by methods for producing activated carbon or known production methods that involve heat treating polymers, but commercially available products may also be purchased, and are not limited to these, as long as silicon can be produced or incorporated into the pores of the porous carbon.

[0025] 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 energy density 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.

[0026] 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 energy density 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.

[0027] 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 energy density 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.

[0028] 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 50 From the viewpoint of further improving the energy density 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.

[0029] 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.

[0030] From the viewpoint of further improving the energy density of the resulting lithium ion secondary battery, the content of the Si / C powder in the negative electrode active material of this embodiment is preferably 1.0 parts by mass or more and 50.0 parts by mass or less, more preferably 5.0 parts by mass or more and 45.0 parts by mass or less, even more preferably 8.0 parts by mass or more and 42.0 parts by mass or less, even more preferably 15.0 parts by mass or more and 40.0 parts by mass or less, and even more preferably 18.0 parts by mass or more and 37.0 parts by mass or less, when the total amount of the negative electrode active material is taken as 100.0 parts by mass.

[0031] 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 ​​1600 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.

[0032] <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 energy density 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 8.0 μm or more and 16.0 μm or less, and even more preferably 9.0 μm or more and 14.0 μm or less.

[0033] 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 energy density 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 2.0 μm or more and 9.0 μm or less, even more preferably 2.5 μm or more and 8.5 μm or less, and even more preferably 3.0 μm or more and 8.0 μm or less.

[0034] 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 energy density 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.

[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 , 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 energy density 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 0.80 or more and 2.00 or less, even more preferably 0.90 or more and 1.70 or less, and even more preferably 1.00 or more and 1.50 or less.

[0036] 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.

[0037] The graphite powder of this embodiment preferably contains graphite particles containing amorphous carbon on the surface thereof, from the viewpoint of further improving the energy density of the resulting lithium ion secondary battery.

[0038] 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.

[0039] 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 can be obtained by adjusting the thickness of the graphite powder. 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. In this case, the breaking strength of the graphite powder can be adjusted by adjusting the thickness of the amorphous carbon coating layer. Examples of commercially available graphite powder include graphite powder manufactured by Nippon Graphite Industries Co., Ltd. and graphite powder manufactured by JFE Chemical Corporation.

[0040] From the viewpoint of further improving the energy density of the resulting lithium-ion secondary battery, the content of the graphite powder in the negative electrode active material of this embodiment, when the total amount of the negative electrode active material is taken as 100.0 parts by mass, is preferably 50.0 parts by mass or more and 99.0 parts by mass or less, more preferably 55.0 parts by mass or more and 95.0 parts by mass or less, even more preferably 58.0 parts by mass or more and 92.0 parts by mass or less, even more preferably 60.0 parts by mass or more and 85.0 parts by mass or less, and even more preferably 63.0 parts by mass or more and 82.0 parts by mass or less.

[0041] 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 where D1 and D2 are, respectively, the value of D1 / D2 is, from the viewpoint of further improving the energy density of the obtained lithium ion secondary battery, preferably 0.5 or more and 10.0 or less, more preferably 0.8 or more and 7.0 or less, even more preferably 1.0 or more and 5.0 or less, even more preferably 1.4 or more and 4.0 or less, and even more preferably 1.8 or more and 3.0 or less.

[0042] In the negative electrode active material of this embodiment, when the content of graphite powder in the negative electrode active material is W1 and the content of Si / C powder in the negative electrode active material is W2, the value of W1 / W2 is preferably 1.0 or more and 20.0 or less, more preferably 1.2 or more and 15.0 or less, even more preferably 1.4 or more and 10.0 or less, even more preferably 1.6 or more and 7.0 or less, and even more preferably 1.8 or more and 5.0 or less, from the viewpoint of further improving the energy density of the obtained lithium ion secondary battery.

[0043] From the viewpoint of further improving the energy density 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 80.0 parts by mass or more and 100.0 parts by mass or less, more preferably 90.0 parts by mass or more and 100.0 parts by mass or less, even more preferably 95.0 parts by mass or more and 100.0 parts by mass or less, and even more preferably 99.0 parts by mass or more and 100.0 parts by mass or less, when the total amount of the negative electrode active material is taken as 100.0 parts by mass.

[0044] <Graphite powder (A), graphite powder (B)> From the viewpoint of further improving the energy density 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.

[0045] 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) and the graphite powder (B) by the laser diffraction scattering method is 50 D respectively A and D B When D B / D AFrom the viewpoint of further improving the energy density 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 0.95 or less, even more preferably 0.55 or more and 0.90 or less, and still more preferably 0.60 or more and 0.85 or less.

[0046] When the graphite powder of the present embodiment contains graphite powder (A) and graphite powder (B), from the viewpoint of further improving the energy density 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.

[0047] 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 is W A , the content of graphite powder (B) in the graphite powder is W B When W A / W B From the viewpoint of further improving the energy density of the resulting lithium ion secondary battery, the value of is preferably 0.1 or more and 10.0 or less, more preferably 0.1 or more and 8.0 or less, even more preferably 0.2 or more and 6.0 or less, and even more preferably 0.2 or more and 5.0 or less.

[0048] <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.

[0049] <Energy density E> From the viewpoint of further improving the energy density of the resulting lithium-ion secondary battery, the energy density E of the negative electrode active material of this embodiment, measured by the method described below, is preferably 800 Wh / L or more, more preferably 810 Wh / L or more, even more preferably 820 Wh / L or more, even more preferably 830 Wh / L or more, even more preferably 840 Wh / L or more, even more preferably 855 Wh / L or more, even more preferably 865 Wh / L or more, and even more preferably 875 Wh / L or more, in terms of cell conversion. The upper limit of the energy density E of the negative electrode active material of this embodiment, measured by the method described below, is not particularly limited, but may be, for example, 1500 Wh / L or less, 1200 Wh / L or less, or 1000 Wh / L or less. (method) A negative electrode active material slurry was prepared by adding an appropriate amount of water to a solid content consisting of 96.9 parts by mass of negative electrode active material, 0.1 parts by mass of single-walled carbon nanotubes, and 3.0 parts by mass of polyacrylic acid. The negative electrode active material slurry was then applied to a copper foil negative electrode current collector so that the initial charge capacity per unit area was 4.3 mAh / cm. 2 After applying the coating amount so that the density becomes 1.65 g / cm, the coating is dried to obtain a negative electrode laminate. 3 The negative electrode is then pressed with a pressure so that the negative electrode and lithium nickel cobalt manganese oxide (Li(Ni 0.9 Co 0.05 Mn 0.05A 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 charged at 7.2 mA, and after the upper voltage reaches 4.2 V, it is charged at a constant voltage until the total charge time is 12 hours. It is then discharged at a constant current of 7.2 mA until the lower voltage reaches 2.5 V. It is then charged again under the same conditions, left in a constant temperature bath at 45°C for 3 days, and discharged again under the same conditions. The capacity and average voltage at the final discharge are measured. After the final discharge, the thickness of the lithium-ion secondary battery is measured. Next, based on the above measured values, the electrode area of ​​the electrode laminate, the thickness of the laminate exterior body, the thickness of the negative electrode current collector, and the thickness of the positive electrode current collector in the positive electrode, the cell-equivalent energy density E (Wh / L) is calculated using the following formula. (Energy density E) = (Capacity at final discharge) × (Average voltage at final discharge) / (Electrode area of ​​electrode laminate) / ((Thickness of lithium-ion secondary battery) - (Thickness of laminate exterior) - (Thickness of positive electrode current collector) / 2 - (Thickness of negative electrode current collector) / 2)

[0050] From the viewpoint of further improving the energy density of the resulting lithium-ion secondary battery, the energy density E of the negative electrode active material of this embodiment obtained by the above method is, in cell conversion, preferably 800 Wh / L to 1500 Wh / L, more preferably 810 Wh / L to 1500 Wh / L, even more preferably 820 Wh / L to 1500 Wh / L, still more preferably 830 Wh / L to 1500 Wh / L, even more preferably 840 Wh / L to 1200 Wh / L, still more preferably 855 Wh / L to 1200 Wh / L, still more preferably 865 Wh / L to 1000 Wh / L, and still more preferably 875 Wh / L to 1000 Wh / L.

[0051] <Negative electrode> The negative electrode of this embodiment includes the negative electrode active material of this embodiment. The negative electrode of this embodiment preferably includes a negative electrode active material layer containing the negative electrode active material of this embodiment, and a negative electrode current collector, from the viewpoint of further improving the battery performance of the resulting lithium-ion secondary battery. The negative electrode active material layer of this embodiment preferably includes the negative electrode active material of this embodiment and a binder, and more preferably includes the negative electrode active material of this embodiment, a binder, and a conductive additive, from the viewpoint of further improving the battery performance of the resulting lithium-ion secondary battery.

[0052] From the viewpoint of further improving the energy density of the resulting lithium ion secondary battery, the content of the negative electrode active material of this embodiment in the negative electrode active material layer of this embodiment is preferably 50.0 parts by mass or more and 100.0 parts by mass or less, more preferably 75.0 parts by mass or more and 99.9 parts by mass or less, even more preferably 85.0 parts by mass or more and 99.5 parts by mass or less, even more preferably 90.0 parts by mass or more and 99.0 parts by mass or less, even more preferably 95.0 parts by mass or more and 98.5 parts by mass or less, and even more preferably 96.0 parts by mass or more and 98.0 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.0 parts by mass.

[0053] Examples of the conductive additive in the negative 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. Among these, the conductive additive in the negative electrode active material layer of this embodiment preferably contains a carbon material, more preferably contains carbon nanotubes, and even more preferably contains single-walled carbon nanotubes, from the viewpoint of further improving the battery performance of the resulting lithium-ion secondary battery.

[0054] From the viewpoint of further improving the battery performance 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 5.0 parts by mass or less, more preferably 0.03 parts by mass or more and 1.0 parts by mass or less, even more preferably 0.05 parts by mass or more and 0.5 parts by mass or less, and even more preferably 0.07 parts by mass or more and 0.3 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.0 parts by mass.

[0055] Examples of the binder in the negative electrode active material layer of this embodiment include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyvinyl fluoride (PVF); polycarboxylic acid-based 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 battery performance of the resulting lithium ion secondary battery, the binder in the negative electrode active material layer 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.

[0056] From the viewpoint of further improving the battery performance of the resulting lithium ion secondary battery, the content of the binder in the negative electrode active material layer of this embodiment is preferably 0.1 parts by mass or more and 10.0 parts by mass or less, more preferably 1.0 parts by mass or more and 7.0 parts by mass or less, and even more preferably 2.0 parts by mass or more and 5.0 parts by mass or less, when the total amount of the negative electrode active material layer is taken as 100.0 parts by mass.

[0057] From the viewpoint of further improving the battery performance of the resulting lithium ion secondary battery, 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.

[0058] 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 of the resulting lithium ion secondary battery. 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.

[0059] 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.

[0060] <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 containing the negative electrode active material of this embodiment, and therefore has improved energy density.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] In the lithium-ion secondary battery of this embodiment, the positive electrode preferably includes a positive electrode active material layer containing a positive electrode active material and a positive electrode current collector. The positive electrode active material layer of this embodiment preferably includes a positive electrode active material and a binder, and more preferably includes a positive electrode active material, a binder, and a conductive additive.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] <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 an improved energy density, the lithium ion secondary battery module of this embodiment also has an improved energy density.

[0073] 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.

[0074] 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.

[0075] 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]

[0076] The embodiments of the present invention will be described in detail based on examples and comparative examples, but the present invention is not limited to the examples.

[0077] In preparing the negative electrode active material, the following graphite powder was used.

[0078] <Graphite powder (A)> Graphite powder 1 (artificial graphite containing amorphous carbon on the surface, D 10 :8.4μm, D 50 :13.6μm, D 90 :21.1μm, (D 90 -D 10 ) / D 50 :0.93) Graphite powder 2 (artificial graphite containing amorphous carbon on the surface, D 10 :9.0μm, D 50 :14.5μm, D 90 :36.0μm, (D 90 -D 10 ) / D 50 :1.86) Graphite powder 3 (artificial graphite containing amorphous carbon on the surface, D 10 :6.0μm, D 50 :12.0μm, D 90 : 19.0 μm, (D 90 -D 10 ) / D 50 :1.08) Graphite powder 4 (artificial graphite containing amorphous carbon on the surface, D 10 :6.0μm, D 50 :12.0μm, D 90 : 20.0 μm, (D 90 -D 10 ) / D 50 :1.17) Graphite powder 5 (artificial graphite containing amorphous carbon on the surface, D 10 :10.0μm, D 50 :30.0μm, D 90 :36.0μm, (D 90 -D 10 ) / D 50 :0.87)

[0079] <Graphite powder (B)> · Graphite powder 6 (synthetic graphite without 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)

[0080] <Preparation 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. 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 under the conditions of 500 °C, 760 Torr, and 120 minutes. Then, the product was cooled to room temperature to obtain Si / C powder 1.

[0081] Regarding the cross-section of the Si-C composite particles contained in the obtained Si / C powder 1, using a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, SU3500), an energy-dispersive X-ray spectroscopic analyzer (manufactured by Oxford Instruments, Ultim Max 40), and image analysis software (manufactured by Oxford Instruments, Aztec), secondary electrons were selected as the detection target, and silicon and carbon elemental mapping was performed under the conditions of an acceleration voltage of 3 kV, a mapping integration number of 20 times, and a magnification of 3000 times. 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.

[0082] Regarding the obtained Si / C powder 1, using a micro-compression tester (manufactured by Shimadzu Corporation, MCT-510), the test force P (N) when the particles of Si / C powder 1 are broken was measured by a plunger. That is, after spraying a very small amount of Si / C powder 1 on the sample stage, for any 100 grains, each grain was compressed one by one with a plunger, and the test force P (N) when the particles of Si / C powder 1 were broken was obtained. Also, the particle diameter d (mm) of each grain was measured in two orthogonal directions (the X direction and the Y direction in the observation image of the micro-compression tester), and the average value was used. Next, the fracture strength CS of each grain was obtained from the test force P (N), the particle diameter d (mm), and the pi π by the following formula, and the average value of the fracture strength CS for 100 grains was taken as the fracture strength CS2 of Si / C powder 1. The results are shown in Table 1. CS = 2.48×P / (π×d 2 )

[0083] 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 particle size distribution measuring device (manufactured by Shimadzu Corporation, SALD-2300), the particle diameter D at which the cumulative value becomes 10% 10 , the particle diameter D at which the cumulative value becomes 90% 90 , the median diameter D 50 and (D 90 -D 10 ) / D 50 values were obtained. Here, Si / C powder 1 was suspended in a dispersion medium and ultrasonicated before measurement. The measurement was performed 5 times, and the average values were adopted respectively. The results are shown in Table 1.

[0084] <Preparation of Si / C Powder 2 and Si / C Powder 3> Except that porous carbon material 2 (D 50 : 4.4 μm, specific surface area: 1750 m 2 / g) and porous carbon material 3 (D 50 : 9.0 μm, specific surface area: 1635 m 2 / g) were used respectively instead of porous carbon material 1, Si / C powder 2 and Si / C powder 3 were obtained in the same manner as Si / C powder 1.

[0085] Elemental mapping was performed on the cross sections of the Si-C composite particles contained in the obtained Si / C powder 2 and Si / C powder 3 using the same method as for Si / C powder 1 above.It was confirmed that both 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.

[0086] The resulting Si / C powder 2 and Si / C powder 3 were measured for fracture strength CS2 and cumulative 10% diameter D, respectively, in the same manner as for the Si / C powder 1. 10 , cumulative 90% diameter D 90 , median diameter D 50 and (D 90 -D 10 ) / D 50 The results are shown in Table 1.

[0087] (Examples 1 to 13, 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 13 and Comparative Examples 1 and 2.

[0088] The graphite powders of each example and each comparative example were measured for fracture strength CS1 and cumulative 10% diameter D 10 , cumulative 90% diameter D 90 , median diameter D 50 , (D 90 -D 10 ) / D 50 The graphite powder was suspended in a dispersion medium and ultrasonically dispersed before measurement. Measurements were carried out five times, and the average values ​​were used for each. The results are shown in Table 1.

[0089] <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 13 and Comparative Examples 1 and 2.

[0090] <Preparation of negative electrode> For each of the negative electrode active materials in each of the Examples and Comparative Examples, a negative electrode active material slurry was prepared by adding an appropriate amount of water to a solid content consisting of 96.9 parts by mass of the negative electrode active material, 0.1 parts by mass of single-walled carbon nanotubes, and 3.0 parts by mass of polyacrylic acid. The negative electrode active material slurry was then applied to an 8 μm-thick copper foil negative electrode current collector to form 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 1000 kJ / cm.sup.2 or less.

[0091] <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 97.5 parts by mass of )O2), 1.5 parts by mass of polyvinylidene fluoride, and 1.0 part 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.

[0092] <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.

[0093] <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.

[0094] <Energy density E> The lithium-ion secondary batteries of each example and comparative example were charged at 7.2 mA, and after the upper limit voltage reached 4.2 V, they were charged at a constant voltage until the total charge time reached 12 hours. They were then discharged at a constant current of 7.2 mA until the lower limit voltage reached 2.5 V. They were then charged again under the same conditions, left in a thermostatic chamber at 45°C for 3 days, and discharged again under the same conditions. The capacity and average voltage at the final discharge were measured. After the final discharge, the thickness of the lithium-ion secondary battery was measured. The cell-equivalent energy density E (Wh / L) was calculated using the following formula based on the above measurements, the electrode area of ​​the electrode laminate, the thickness of the laminate outer casing, the thickness of the negative electrode current collector, and the thickness of the positive electrode current collector. The results are shown in Table 1. (Energy density E) = (Capacity at final discharge) × (Average voltage at final discharge) / (Electrode area of ​​electrode laminate) / ((Thickness of lithium-ion secondary battery) - (Thickness of positive electrode current collector) / 2 - (Thickness of negative electrode current collector) / 2)

[0095] [Table 1] [Explanation of symbols]

[0096] 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. graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material; The fracture strength of the graphite powder and the Si / C powder measured in accordance with JIS R 1639-5:2007 was determined as CS. １ and C.S. ２ When this is done, CS １ / CS ２ The negative electrode active material has a value of 0.32 or less.

2. Said CS １ The negative electrode active material according to claim 1 , wherein the stress is 10 MPa or more and 90 MPa or less.

3. 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 active material according to claim 1 or 2, wherein the average particle diameter is 3.0 μm or more and 30.0 μm or less.

4. 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 , 90% particle diameter D ９０ and median diameter D ５０ (D ９０ -D １０ ) / D ５０ 4. The negative electrode active material according to claim 1, wherein the value of is 0.50 or more and 3.00 or less.

5. 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 active material according to any one of claims 1 to 4, wherein the average particle size is 1.0 µm or more and 20.0 µm or less.

6. 6. The negative electrode active material 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 a portion of the pores of the porous carbon material.

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

8. The negative electrode active material according to any one of claims 1 to 7, wherein the graphite powder contains artificial graphite particles.

9. The content of the graphite powder in the negative electrode active material is 50.0 parts by mass or more and 99.0 parts by mass or less, when the total amount of the negative electrode active material is 100.0 parts by mass. The negative electrode active material according to any one of claims 1 to 8.

10. The content of the Si / C powder in the negative electrode active material is 1.0 parts by mass or more and 50.0 parts by mass or less when the total amount of the negative electrode active material is 100.0 parts by mass. The negative electrode active material according to any one of claims 1 to 9.

11. The content of the graphite powder in the negative electrode active material is W １ , the content of the Si / C powder in the negative electrode active material is W ２ When W １ / W ２ The negative electrode active material according to any one of claims 1 to 10, wherein the value of is 1.0 or more and 20.0 or less.

12. 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 active material according to any one of claims 1 to 11, wherein the negative electrode active material has a molecular weight of 1.0 or more.

13. 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 active material according to claim 12 , wherein the value of

14. 14. The negative electrode active material according to claim 12 or 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.

15. The content of the graphite powder (A) in the graphite powder is W Ａ The content of the graphite powder (B) in the graphite powder is W Ｂ When W Ａ / W Ｂ The negative electrode active material according to any one of claims 12 to 14, wherein the value of is 0.1 or more and 10.0 or less.

16. The negative electrode active material according to any one of claims 1 to 15, wherein the energy density E measured by the following method is 800 Wh / L or more in terms of a cell. (method) A suitable amount of water is added to a solid content of 96.9 parts by mass of the negative electrode active material, 0.1 parts by mass of single-walled carbon nanotubes, and 3.0 parts by mass of polyacrylic acid to prepare a negative electrode active material slurry. Next, the negative electrode active material slurry is applied to a copper foil negative electrode current collector to form a negative electrode current collector having an initial charge capacity per unit area of ​​4.3 mAh / cm. ２ Then, the negative electrode laminate is pressed into a roll press machine so that the density of the negative electrode laminate is 1.65 g / cm. ３ Then, the negative electrode and lithium nickel cobalt manganese oxide (Li(Ni ０．９ Co ０．０５ Mn ０．０５ A lithium-ion secondary battery was 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 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 were connected to the negative electrode and the positive electrode, respectively, and sealing the periphery of the laminate outer casing. The lithium-ion secondary battery was then charged at 7.2 mA, and after the upper voltage reached 4.2 V, it was charged at a constant voltage until the total charge time was 12 hours. It was then discharged at a constant current of 7.2 mA until the lower voltage reached 2.5 V. It was then charged again under the same conditions, left in a thermostatic chamber at 45°C for 3 days, and discharged again under the same conditions. The capacity and average voltage at the final discharge were measured. After the final discharge, the thickness of the lithium-ion secondary battery was measured. Next, the energy density E (Wh / L) converted into a cell is calculated using the following formula based on the above measured values, the electrode area of ​​the electrode laminate, the thickness of the laminate exterior body, the thickness of the negative electrode current collector, and the thickness of the positive electrode current collector in the positive electrode. (Energy density E) = (Capacity at final discharge) × (Average voltage at final discharge) / (Electrode area of ​​electrode laminate) / ((Thickness of lithium ion secondary battery) - (Thickness of laminate exterior body) - (Thickness of positive electrode current collector) / 2 - (Thickness of negative electrode current collector) / 2)

17. A negative electrode comprising the negative electrode active material according to any one of claims 1 to 16.

18. A lithium ion secondary battery comprising the negative electrode according to claim 17.

19. A lithium ion secondary battery module comprising the lithium ion secondary battery according to claim 18.

Images