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 has improved adhesion and a balance of performance in the energy density and cycle characteristics of the resulting lithium-ion secondary battery.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 the specific surface areas (m2 / g) of the graphite powder and the Si / C powder, determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013, are S1 and S2, respectively, and when the total amount of the negative electrode active material is taken as 1.00, the mass ratio of the graphite powder to the negative electrode active material is W1, and the mass ratio of the Si / C powder to the negative electrode active material is W2, the value of S1×W1+S2×W2 is 1.10 or more and 3.40 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] The inventors' studies have revealed that a negative electrode active material containing graphite powder and Si / C powder may have reduced adhesion, and that a lithium ion secondary battery using a negative electrode active material containing graphite powder and Si / C powder may have reduced energy density or cycle characteristics.

[0005] The present invention provides a negative electrode active material with improved adhesion and improved performance balance between the energy density and cycle characteristics of the resulting lithium ion secondary battery, a negative electrode with improved performance balance between the energy density and cycle characteristics of the resulting lithium ion secondary battery, and a lithium ion secondary battery and lithium ion secondary battery module with improved performance balance between the energy density and cycle characteristics. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have discovered a negative electrode active material comprising graphite powder and Si / C powder containing Si-C composite particles containing silicon and a carbon material, wherein the specific surface areas (m2) of the graphite powder and the Si / C powder are determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013. 2 / g) are S1 and S2, respectively, and when the total amount of the negative electrode active material is taken as 1.00, the mass ratio of the graphite powder to the negative electrode active material is W1, and the mass ratio of the Si / C powder to the negative electrode active material is W2. It has been found that a negative electrode active material having a value of S1×W1+S2×W2 of 1.10 or more and 3.40 or less can improve the performance balance of the energy density and cycle characteristics of the resulting lithium ion secondary battery, and the present invention has been completed based on this finding.

[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] A negative electrode active material comprising: graphite powder; and Si / C powder containing Si-C composite particles containing silicon and a carbon material, The specific surface area (m) of the graphite powder and the Si / C powder was determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013. 2 / g) are S1 and S2, respectively, When the total amount of the negative electrode active material is taken as 1.00, the mass ratio of the graphite powder to the negative electrode active material is taken as W1, and the mass ratio of the Si / C powder to the negative electrode active material is taken as W2, A negative electrode active material in which the value of S1×W1+S2×W2 is 1.10 or more and 3.40 or less. [2] The above S1 is 0.8m 2 / g or more 3.0m 2 / g or less of the negative electrode active material according to [1]. [3] The S2 is 2.0m 2 / g or more 15.0m 2 / g or less of the negative electrode active material according to [1] or [2]. [4] The negative electrode active material according to any one of [1] to [3], wherein W1 is 0.50 or more and 0.99 or less. [5] The negative electrode active material according to any one of [1] to [4], wherein W2 is 0.01 or more and 0.50 or less. [6] The negative electrode active material according to any one of [1] to [5], wherein the value of W1 / W2 is 1.0 or more and 20.0 or less. [7] 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 any one of [1] to [6], wherein the average particle size is 3.0 μm or more and 30.0 μm or less. [8] 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 [7], wherein the average particle size is 1.0 μm or more and 20.0 μm or less. [9] The negative electrode active material according to any one of [1] to [8], 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.

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

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

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

[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) and the graphite powder (B) 50 D respectively A and 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] When the total amount of the graphite powder is taken as 100 parts by mass, 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 represented by W A and W B When W B / W A The 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 a peel strength S of 10 mN / mm or more according to the following method 1: (Method 1) 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 The negative electrode active material layer is then dried to obtain a negative electrode laminate including the negative electrode current collector and the negative electrode active material layer. Next, the negative electrode laminate is pressed using a roll press machine to a density of 1.65 g / cm. 3 The negative electrode is then cut into a size of 10 mm x 100 mm to prepare a test piece. The test piece is then attached to a tensile tester to determine whether the tensile strength per electrode cross-sectional area is approximately 15 N / mm. 2 The test piece is then attached to a peel tester, and the negative electrode active material layer is peeled off in a 90° direction at a pulling rate of 50 mm / min, and the peel strength S (mN / mm) is measured.

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

[16] , which has an energy density E of 800 Wh / L or more in terms of a cell, as measured by the following method 2: (Method 2) 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 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 thermostatic chamber at 45°C for 3 days, and discharged again under the same conditions, and then charged and discharged once more. 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)

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

[17] , which has a capacity retention rate C of 85% or more according to the following method 3: (Method 3) 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 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 negative electrode comprising the negative electrode active material according to any one of [1] to

[18] .

[20]

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

[19] . [twenty one]

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

[20] . [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a negative electrode active material with improved adhesion and improved performance balance between the energy density and cycle characteristics of the resulting lithium ion secondary battery, a negative electrode with improved performance balance between the energy density and cycle characteristics of the resulting lithium ion secondary battery, and a lithium ion secondary battery and lithium ion secondary battery module with improved performance balance between the energy density and 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.

[0013] <Negative electrode active material> The negative electrode active material of this embodiment is a negative electrode active material containing graphite powder and Si / C powder containing Si-C composite particles containing silicon and a carbon material, and the specific surface area (m ) of the graphite powder and the Si / C powder is determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013. 2 / g) are S1 and S2, respectively, and when the total amount of the negative electrode active material is 1.00, the mass ratio of the graphite powder to the negative electrode active material is W1, and the mass ratio of the Si / C powder to the negative electrode active material is W2. The value of S1×W1+S2×W2 is 1.10 or more and 3.40 or less.

[0014] According to the investigations of the present inventors, in a negative electrode active material containing graphite powder and Si / C powder including Si-C composite particles containing silicon and a carbon material, it has been found that there is a correlation between the specific surface area of ​​the graphite powder and the Si / C powder, adhesion, and the performance balance of the energy density and cycle characteristics of the resulting lithium ion secondary battery.

[0015] As a result of further investigations by the present inventors based on the above findings, it was found that in a negative electrode active material containing graphite powder and Si / C powder containing Si-C composite particles containing silicon and a carbon material, the specific surface area (m 2 / g) are S1 and S2, respectively, and when the total amount of the negative electrode active material is 1.00, the mass ratio of the graphite powder to the negative electrode active material is W1, and the mass ratio of the Si / C powder to the negative electrode active material is W2. It has been found that by setting the value of S1×W1+S2×W2 to 1.10 or more and 3.40 or less, the adhesion and the performance balance of the energy density and cycle characteristics of the resulting lithium ion secondary battery can be improved, and the present invention has been completed based on this finding.

[0016] In the negative electrode active material of this embodiment, the specific surface area (m) of the graphite powder and the Si / C powder determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013 2 / g) are S1 and S2, respectively, and when the total amount of the negative electrode active material is taken as 1.00, the mass ratio of the graphite powder to the negative electrode active material is W1, and the mass ratio of the Si / C powder to the negative electrode active material is W2. From the viewpoint of further improving the performance balance of adhesion, the energy density, and the cycle characteristics of the resulting lithium ion secondary battery, the value of S1×W1+S2×W2 is 1.10 or more and 3.40 or less, preferably 1.30 or more and 3.35 or less, more preferably 1.50 or more and 3.30 or less, even more preferably 1.70 or more and 3.26 or less, even more preferably 1.90 or more and 3.23 or less, even more preferably 2.00 or more and 3.20 or less, even more preferably 2.10 or more and 3.17 or less, even more preferably 2.20 or more and 3.15 or less, and even more preferably 2.50 or more and 3.13 or less.

[0017] In this embodiment, the value of S1×W1+S2×W2 can be understood as a weighted average value of the specific surface areas of the graphite powder and the Si / C powder, weighted by the mass ratio of the graphite powder and the Si / C powder to the negative electrode active material, respectively.

[0018] The graphite powder of this embodiment has a specific surface area S1 determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013, and is preferably 0.8 m from the viewpoint of further improving the performance balance between adhesion, the energy density, and the cycle characteristics of the resulting lithium ion secondary battery. 2 / g or more 3.0m 2 / g or less, more preferably 1.0m 2 / g or more 2.5m 2 / g or less, more preferably 1.2m 2 / g or more 2.2m 2 / g or less, more preferably 1.4m 2 / g or more 2.0m 2 / g or less.

[0019] The specific surface area S2 of the Si / C powder of this embodiment, determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013, is preferably 2.0 m from the viewpoint of further improving the performance balance between adhesion, the energy density, and the cycle characteristics of the resulting lithium ion secondary battery. 2 / g or more 15.0m 2 / g or less, more preferably 3.0m 2 / g or more 12.0m 2 / g or less, more preferably 4.0m 2 / g or more 10.0m 2 / g or less, more preferably 5.0m 2 / g or more 9.0m 2 / g or less, more preferably 6.0m 2 / g or more 8.0m 2 / g or less.

[0020] In this embodiment, as a method for adjusting the specific surface areas of the graphite powder and the Si / C powder, for example, a method using commercially available graphite powders and Si / C powders having different specific surface areas, or a method of adjusting the production conditions of the graphite powder and the Si / C powder such as surface treatment and heat treatment to produce graphite powders and Si / C powders having different specific surface areas can be mentioned. Further, two or more types of graphite powders having different specific surface areas may be mixed to adjust the specific surface area of the graphite powder, and two or more types of Si / C powders having different specific surface areas may be mixed to adjust the specific surface area of the Si / C powder.

[0021] From the viewpoint of further improving the adhesion, the energy density of the obtained lithium-ion secondary battery, and the performance balance of the cycle characteristics, when the total amount of the negative electrode active material is 1.00, the mass ratio W1 of the graphite powder to the negative electrode active material of this embodiment is preferably 0.50 or more and 0.99 or less, more preferably 0.60 or more and 0.95 or less, still more preferably 0.65 or more and 0.92 or less, and still more preferably 0.70 or more and 0.85 or less.

[0022] From the viewpoint of further improving the adhesion, the energy density of the obtained lithium-ion secondary battery, and the performance balance of the cycle characteristics, when the total amount of the negative electrode active material is 1.00, the mass ratio W2 of the Si / C powder to the negative electrode active material of this embodiment is preferably 0.01 or more and 0.50 or less, more preferably 0.05 or more and 0.40 or less, still more preferably 0.08 or more and 0.35 or less, and still more preferably 0.15 or more and 0.30 or less.

[0023] When the mass ratios of the graphite powder and the Si / C powder to the negative electrode active material of this embodiment are W1 and W2 respectively, from the viewpoint of further improving the adhesion, the energy density of the obtained lithium-ion secondary battery, and the performance balance of the cycle characteristics, the value of W1 / W2 is preferably 1.0 or more and 20.0 or less, more preferably 1.5 or more and 15.0 or less, still more preferably 2.0 or more and 12.0 or less, still more preferably 2.4 or more and 10.0 or less, and still more preferably 2.7 or more and 5.0 or less.

[0024] <Si / C powder> The Si / C powder of this embodiment includes Si-C composite particles containing silicon and a carbon material. From the viewpoint of further improving the battery performance of the resulting lithium-ion secondary battery, the Si / C powder of this embodiment preferably has the carbon material in the Si-C composite particles containing a porous carbon material, and silicon present in at least part of the pores of the porous carbon material.

[0025] 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.

[0026] 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.

[0027] 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 adhesion and the performance balance of the energy density and cycle characteristics of the resulting lithium ion secondary battery, the thickness is preferably 1.0 μm or more and 20.0 μm or less, more preferably 2.0 μm or more and 16.0 μm or less, even more preferably 3.0 μm or more and 13.0 μm or less, even more preferably 3.5 μm or more and 10.0 μm or less, and even more preferably 4.0 μm or more and 6.0 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 From the viewpoint of further improving the performance balance of adhesion and the energy density and cycle characteristics of the resulting lithium ion secondary battery, the thickness is preferably 0.1 μm or more and 10.0 μm or less, more preferably 0.5 μm or more and 7.0 μm or less, even more preferably 1.0 μm or more and 5.0 μm or less, and even more preferably 1.5 μm or more and 3.0 μm or less.

[0029] 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 performance balance of adhesion and the energy density and cycle characteristics of the resulting lithium ion secondary battery, the thickness 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 15.0 μm or less, and even more preferably 8.0 μm or more and 10.0 μm or less.

[0030] 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 performance balance among the adhesion and the energy density and 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 1.30 or more and 1.60 or less.

[0031] 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.

[0032] From the viewpoint of further improving the performance balance of adhesion and the energy density and 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 is preferably 1.0 part by mass or more and 50.0 parts by mass or less, more preferably 5.0 parts by mass or more and 40.0 parts by mass or less, even more preferably 8.0 parts by mass or more and 35.0 parts by mass or less, and even more preferably 15.0 parts by mass or more and 30.0 parts by mass or less, when the total amount of the negative electrode active material is taken as 100.0 parts by mass.

[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 2.0 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.

[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 performance balance of adhesion and the energy density and cycle characteristics of the resulting lithium ion secondary battery, the thickness 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 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 method10 From the viewpoint of further improving the adhesion and the balance of the energy density and cycle characteristics of the resulting lithium ion secondary battery, the thickness 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 8.0 μm or less, and even more preferably 3.0 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 performance balance of adhesion and the energy density and cycle characteristics of the resulting lithium ion secondary battery, the thickness 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 performance balance among the adhesion, the energy density and 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] 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.

[0039] The graphite powder of this embodiment preferably contains graphite particles containing amorphous carbon on the surface, from the viewpoint of further improving the adhesiveness and the performance balance of the energy density and cycle characteristics of the resulting lithium ion secondary battery.

[0040] 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.

[0041] 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.

[0042] From the viewpoint of further improving the performance balance between adhesion and the energy density and 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 50.0 parts by mass or more and 99.0 parts by mass or less, more preferably 60.0 parts by mass or more and 95.0 parts by mass or less, even more preferably 65.0 parts by mass or more and 92.0 parts by mass or less, and still more preferably 70.0 parts by mass or more and 85.0 parts by mass or less, when the total amount of the negative electrode active material is taken as 100.0 parts by mass.

[0043] 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 are D1 and D2, respectively, the value of D1 / D2 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 1.1 or more and 4.0 or less, and even more preferably 1.5 or more and 3.0 or less, from the viewpoint of further improving the performance balance of the adhesion and the energy density and cycle characteristics of the obtained lithium ion secondary battery.

[0044] From the viewpoint of further improving the performance balance of adhesion and the energy density and 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 80.0 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.

[0045] <Graphite powder (A), graphite powder (B)> From the viewpoint of further improving the performance balance between adhesion and the energy density and 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) 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 performance balance between adhesion and the energy density and 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 0.95 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.

[0047] When the graphite powder of the present embodiment contains graphite powder (A) and graphite powder (B), from the viewpoint of further improving the performance balance of adhesion and the energy density and cycle characteristics of the resulting lithium ion secondary battery, preferably, graphite powder (A) contains graphite particles containing amorphous carbon on the surface, and 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 the graphite powder (A) in the graphite powder and the content of the graphite powder (B) in the graphite powder are respectively expressed as W A and W B When W B / W A From the viewpoint of further improving the performance balance between adhesion and the energy density and 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 7.0 or less, even more preferably 0.5 or more and 4.0 or less, even more preferably 0.7 or more and 2.0 or less, and still more preferably 0.8 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] <Peel strength S> From the viewpoint of further improving adhesion, the negative electrode active material of this embodiment has a peel strength S of preferably 10 mN / mm or more, more preferably 11 mN / mm or more, even more preferably 12 mN / mm or more, and even more preferably 13 mN / mm or more, measured by the following method 1. The upper limit of the peel strength S of the negative electrode active material of this embodiment measured by the following method 1 is not particularly limited, but may be, for example, 30 mN / mm or less, or 20 mN / mm or less. (Method 1) 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 The negative electrode active material layer is then dried to obtain a negative electrode laminate including the negative electrode current collector and the negative electrode active material layer. Next, the negative electrode laminate is pressed using a roll press machine to a density of 1.65 g / cm. 3 The negative electrode is then pressed with a pressure so that the negative electrode is 10 mm x 100 mm in size to prepare a test piece. The test piece is then attached to a tensile tester to determine whether the tensile strength per electrode cross-sectional area is approximately 15 N / mm 2 The test piece is then attached to a peel tester, and the negative electrode active material layer is peeled off in a 90° direction at a pulling rate of 50 mm / min, and the peel strength S (mN / mm) is measured.

[0051] From the viewpoint of further improving adhesion, the peel strength S of the negative electrode active material of this embodiment by the above method 1 is preferably 10 mN / mm or more and 30 mN / mm or less, more preferably 11 mN / mm or more and 30 mN / mm or less, even more preferably 12 mN / mm or more and 20 mN / mm or less, and still more preferably 13 mN / mm or more and 20 mN / mm or less.

[0052] <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 Method 2 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 Method 2 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 2) 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)

[0053] 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, when measured by the above method 2, is 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 1200 Wh / L, even more preferably 840 Wh / L to 1200 Wh / L, still more preferably 855 Wh / L to 1200 Wh / L, even more preferably 865 Wh / L to 1000 Wh / L, and still more preferably 875 Wh / L to 1000 Wh / L.

[0054] <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 active material of this embodiment, measured by Method 3 below, is preferably 85% or more, more preferably 88% or more, even more preferably 90% or more, even more preferably 92% or more, even more preferably 93% or more, even more preferably 94% or more, even more preferably 95% or more, and even more preferably 96% or more. The upper limit of the capacity retention rate C of the negative electrode active material of this embodiment, measured by Method 3 below, is not particularly limited, but may be, for example, 100% or less. (Method 3) 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 0.05 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 (%).

[0055] 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 active material of this embodiment, as determined by the above method 3, is preferably 85% or more and 100% or less, more preferably 88% or more and 100% or less, even more preferably 90% or more and 100% or less, even more preferably 92% or more and 100% or less, even more preferably 93% or more and 100% or less, even more preferably 94% or more and 100% or less, even more preferably 95% or more and 100% or less, and even more preferably 96% or more and 100% or less.

[0056] <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.

[0057] From the viewpoint of further improving the performance balance between the energy density and cycle characteristics 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] <Lithium-ion secondary battery> The lithium ion secondary battery of this embodiment includes the negative electrode of this embodiment. Because the lithium ion secondary battery of this embodiment includes the negative electrode containing the negative electrode active material of this embodiment, the performance balance between energy density and cycle characteristics is improved.

[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] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] Examples of the electrolyte solution of this embodiment include those obtained by dissolving lithium salts such as lithium hexafluorophosphate (LiPF), lithium fluoroborate (LiBF), LiFSI, and lithium perchlorate (LiClO) in 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. These organic solvents may be used singly or in combination of two or more.

[0075] 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.

[0076] 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.

[0077] <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 performance balance between energy density and cycle characteristics, the lithium-ion secondary battery module of this embodiment also has an improved performance balance between energy density and cycle characteristics.

[0078] 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.

[0079] 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.

[0080] 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]

[0081] 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.

[0082] In preparing the negative electrode active material, the following graphite powder was used.

[0083] <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)

[0084] <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)

[0085] <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.

[0086] 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-Technologies Corporation, SU3500), an energy-dispersive X-ray spectroscopy detector (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 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 silicon exists in at least a part of the pores of the porous carbon material in the Si-C composite particles.

[0087] 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 (manufactured by Shimadzu Corporation, SALD-2300), the particle diameter D 10 at which the cumulative value becomes 10%, the particle diameter D 90 at which the cumulative value becomes 90%, the median diameter D 50 and (D 90 -D 10 ) / D 50The value was determined. Here, Si / C powder 1 was suspended in a dispersion medium and ultrasonicated before measurement. The measurement was performed five times, and the average value was adopted respectively. The results are shown in Table 1.

[0088] For the obtained Si / C powder 1, using a fully automatic specific surface area measuring device (manufactured by Mountech Co., Ltd., Macsorb HM-1208), in accordance with JIS Z 8830:2013, the specific surface area S2 was determined by the BET flow method and the one-point method. The results are shown in Table 1.

[0089] <Preparation of Si / C Powder 2, Si / C Powder 3, and Si / C Powder 4> Instead of porous carbon material 1, porous carbon material 2 (D 50 : 9.0 μm, specific surface area: 1635 m 2 / g), porous carbon material 3 (D 50 : 10.0 μm, specific surface area: 1613 m 2 / g), and porous carbon material 4 (D 50 : 3.0 μm, specific surface area: 1775 m 2 / g) were used respectively, and Si / C powder 2, Si / C powder 3, and Si / C powder 4 were obtained in the same manner as Si / C powder 1.

[0090] Regarding the cross-sections of the Si-C composite particles contained in the obtained Si / C powder 2, Si / C powder 3, and Si / C powder 4, elemental mapping was performed in the same manner as the above Si / C powder 1. As a result, it was confirmed that all Si-C composite particles contain silicon, and in the Si-C composite particles, silicon exists in at least a part of the pores of the porous carbon material.

[0091] For the obtained Si / C powder 2, Si / C powder 3, and Si / C powder 4, in the same manner as the above Si / C powder 1, the specific surface area S2, cumulative 10% diameter D 10 , cumulative 90% diameter D 90 , median diameter D 50 and the value of (D 90 -D 10 ) / D 50 were measured respectively. The results are shown in Table 1.

[0092] (Examples 1 to 7, 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 7 and Comparative Examples 1 and 2.

[0093] The graphite powders of each example and each comparative example were measured for specific surface area S1 and cumulative 10% diameter D by the same method as for the Si / C powder 1. 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.

[0094] <Preparation of negative electrode active material> Graphite powder and Si / C powder were mixed in the ratios shown in Table 1 to obtain the negative electrode active materials of Examples 1 to 7 and Comparative Examples 1 and 2. For the negative electrode active material of each Example and Comparative Example, the value of S1×W1+S2×W2 was calculated from the specific surface area S1 of the graphite powder, the specific surface area S2 of the Si / C powder, the mass ratio W1 of the graphite powder to the negative electrode active material, and the mass ratio W2 of the Si / C powder to the negative electrode active material. The results are shown in Table 1.

[0095] <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 active material layer was then dried to obtain a negative electrode laminate including the negative electrode current collector and the negative electrode active material layer. Next, the negative electrode laminate was pressed using a roll press 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.

[0096] <Peel strength S> For the negative electrodes of each example and comparative example, the negative electrodes were cut into a size of 10 mm x 100 mm to prepare test pieces. Then, the test pieces were once attached to a tensile tester (Imada Co., Ltd., ZP-200N) to measure the tensile strength per electrode cross-sectional area of ​​about 15 N / mm 2 The test piece was then removed from the tensile tester. The test piece was then attached to a peel tester (Imada Co., Ltd., ZP-5N) and the negative electrode active material layer was peeled off in a 90° direction at a pulling rate of 50 mm / min to measure the peel strength S (mN / mm). The results are shown in Table 1.

[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 97.5 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.

[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] <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)

[0101] <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 1.

[0102] [Table 1] [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 active material comprising: graphite powder; and Si / C powder containing Si-C composite particles containing silicon and a carbon material, The specific surface area (m ) of the graphite powder and the Si / C powder was determined by the BET flow method and the single-point method in accordance with JIS Z 8830:2013. ２ / g) are S １ and S ２ year, When the total amount of the negative electrode active material is taken as 1.00, the mass ratio of the graphite powder to the negative electrode active material is W １ , the mass ratio of the Si / C powder to the negative electrode active material is W ２ When S １ ×W １ +S ２ ×W ２ The negative electrode active material has a value of 1.10 or more and 3.40 or less.

2. The S １ is 0.8m ２ / g or more 3.0m ２ The negative electrode active material according to claim 1 , wherein the SiO 2 content is 1 / g or less.

3. The S ２ is 2.0 m ２ / g or more 15.0m ２ The negative electrode active material according to claim 1 or 2, wherein the Cr content is 0.15 / g or less.

4. The W １ The negative electrode active material according to any one of claims 1 to 3, wherein is 0.50 or more and 0.99 or less.

5. The W ２ 5. The negative electrode active material according to claim 1, wherein the value of ρ is 0.01 or more and 0.50 or less.

6. W １ / W ２ The negative electrode active material according to any one of claims 1 to 5, wherein the value of is 1.0 or more and 20.0 or less.

7. 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 any one of claims 1 to 6, wherein the average particle size is 3.0 µm or more and 30.0 µm or less.

8. 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 7, wherein the average particle size is 1.0 µm or more and 20.0 µm or less.

9. 9. 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.

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

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

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) and the graphite powder (B) ５０ D respectively Ａ and 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. When the total amount of the graphite powder is taken as 100 parts by mass, 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 represented by W Ａ and W Ｂ When W Ｂ / W Ａ 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 a peel strength S measured by the following method 1 is 10 mN / mm or more. (Method 1) 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 The negative electrode active material layer is then dried to obtain a negative electrode laminate including the negative electrode current collector and the negative electrode active material layer. Next, the negative electrode laminate is pressed using a roll press machine to a density of 1.65 g / cm ３ The negative electrode is then cut into a size of 10 mm x 100 mm to prepare a test piece. The test piece is then attached to a tensile tester to determine whether the tensile strength per electrode cross-sectional area is about 15 N / mm. ２ The test piece is then attached to a peel tester, and the negative electrode active material layer is peeled off in a 90° direction at a pulling rate of 50 mm / min, and the peel strength S (mN / mm) is measured.

17. The negative electrode active material according to any one of claims 1 to 16, wherein the energy density E measured by the following Method 2 is 800 Wh / L or more in terms of a cell. (Method 2) 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)

18. The negative electrode active material according to any one of claims 1 to 17, wherein a capacity retention rate C measured by the following method 3 is 85% or more. (Method 3) 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 is fabricated by placing an electrode laminate, in which a positive electrode containing a cation exchanger (C) and a nonaqueous electrolyte solution are arranged opposite each other via a polyethylene separator, 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 negative electrode comprising the negative electrode active material according to any one of claims 1 to 18.

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

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

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