JP2025154046A - Lithium-ion secondary battery and lithium-ion secondary battery module - Google Patents

Abstract

To provide a lithium-ion secondary battery with improved cycle characteristics and rapid charging capabilities.SOLUTION: A lithium ion secondary battery includes a negative electrode including a negative electrode active material layer, a positive electrode including a positive electrode active material layer, a separator, and an electrolyte. The negative electrode active material layer includes a mixed negative electrode active material including graphite powder and Si/C powder including Si-C composite particles containing silicon and a carbon material. In an X-ray diffraction spectrum of the mixed negative electrode active material measured in accordance with JIS K 0131:1996 using CuKα radiation having a wavelength of 1.5406Å as a radiation source under a condition of a tube voltage of 40 kV and a tube current of 40 kV by means of an X-ray diffractometer, when the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° or more and less than 27.5° is defined as IC, and the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5° or more and 29.5° or less is defined as ISi, the value of (ISi/IC)×100(%) is 0.20% or more and 2.00% or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a lithium-ion secondary battery and a lithium-ion secondary battery module.

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 that achieves a high silicon utilization rate in a lithium ion secondary battery and is resistant to oxidation when dispersed in water, and describes a composite particle containing a carbon material and silicon, in which the silicon content is 30% by mass or more and 80% by mass or less, and the true density measured by dry density measurement using helium gas is 1.80 g/cm ³ or more and 1.99 g/cm ³ or less, and in the Raman spectrum of the composite particle, a peak is present at 450 to 495 cm ⁻¹ , and when the intensity of the peak is I _Si and the intensity of the G band (peak intensity around 1580 cm ⁻¹ ) is I _G , I _Si /I _G is 1.3 or less, and the atomic number ratios of Si, O and C measured by narrow spectrum of X-ray photoelectron spectroscopy of the composite particle are A _Si , A _O , and A _C , respectively, and the ratio of SiO ₂ to SiO in the Si species ratio measured by Si2p spectrum state analysis is B Disclosed is a composite particle in which, assuming _SiO2 and B _SiO , 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 _{SiO 2} + B _SiO )).]

JP 2023-059283 A

The inventors' research has revealed that lithium-ion secondary batteries using a negative electrode active material layer containing graphite powder and Si/C powder may exhibit reduced cycle characteristics and rapid chargeability.

The present invention provides a lithium-ion secondary battery and a lithium-ion secondary battery module with improved cycle characteristics and rapid charging capabilities.

The present inventors have conducted extensive research to solve the above problems. As a result, they have found a lithium ion secondary battery comprising a negative electrode including a negative electrode active material layer, a positive electrode including a positive electrode active material layer, a separator, and an electrolyte, wherein the negative electrode active material layer comprises a negative electrode active material including graphite powder and Si/C powder including Si—C composite particles containing silicon and a carbon material, and wherein an X-ray diffraction spectrum of the negative electrode active material is measured in accordance with JIS K 0131:1996 using CuKα radiation having a wavelength of 1.5406 Å as a radiation source with an X-ray diffractometer under conditions of a tube voltage of 40 kV and a tube current of 40 mA, where I _C is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° to less than 27.5°, and I _Si is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5 _° to 29.5 _° , and The present inventors have found that a lithium ion secondary battery in which the value of (%) × 100(%) is 0.20% or more and 2.00% or less can improve cycle characteristics and rapid chargeability, and have completed the present invention.

The present invention provides the following lithium-ion secondary battery and lithium-ion secondary battery module.

[1]
A lithium ion secondary battery comprising: a negative electrode including a negative electrode active material layer; a positive electrode including a positive electrode active material layer; a separator; and an electrolyte solution,
the negative electrode active material layer includes a negative electrode active material including graphite powder and Si/C powder including Si-C composite particles containing silicon and a carbon material;
In an X-ray diffraction spectrum of the negative electrode active material measured in accordance with JIS K 0131:1996 using CuKα radiation of a wavelength of 1.5406 Å as a radiation source under conditions of a tube voltage of 40 kV and a tube current of 40 mA using an X-ray diffractometer, where _IC is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° or more and less than 27.5°, and IS is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5 _° or more and 29.5° or less, the value of ( _ISi / _IC ) x 100(%) is 0.20% or more and 2.00% or less.
[2]
The lithium ion secondary battery according to [1], wherein the graphite powder has an I _D /I _G value of 0.080 or more and 0.300 or less, as measured by the following Method 1:
(Method 1)
In accordance with JIS K 0137:2010, the graphite powder is irradiated with an argon laser using a laser Raman spectrometer under the conditions of an excitation wavelength of 532 nm, an entrance slit width of 200 μm, an exposure time of 15 seconds, an accumulation count of 2, and a diffraction grating of 600 lines/mm, and a Raman spectrum is measured. Next, from the Raman spectrum, peak intensities _I and _I at ₁₃₆₀ ^cm and 1580 ^cm are determined, respectively. Next, the value of _I / _I is determined from I and _I.
[3]
The lithium ion secondary battery according to [1] or [2], wherein the Si/C powder has a median diameter D ₅₀ of 6.0 μm or more and less than 10.0 μm in a volume frequency particle size distribution measured by a laser diffraction scattering method.
[4]
The lithium ion secondary battery according to any one of [1] to [3], 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.
[5]
The lithium ion secondary battery according to any one of _[ 1] to [4], wherein a value of W _C /W SiC is 1.0 or more and 20.0 or less, where W _C is the content of the graphite powder in the negative electrode active material layer and W _SiC is the content of the Si/C powder in the negative electrode active material layer.
[6]
The lithium ion secondary battery according to any one of [1] to [5], wherein the electrolyte solution contains a lithium salt/halogen-containing EC-based electrolyte solution.
[7]
The lithium ion secondary battery according to [6], wherein the lithium salt/halogen-containing EC-based electrolyte solution contains one or more halogen-containing ethylene carbonates selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, and trichloroethylene carbonate.
[8]
The lithium ion secondary battery according to any one of [1] to [7], wherein the graphite powder contains graphite particles having amorphous carbon on the surface thereof.
[9]
The lithium ion secondary battery according to any one of [1] to [8], wherein the graphite powder contains artificial graphite particles.
[10]
the graphite powder comprises graphite powder (A) and graphite powder (B), which are two types of graphite powder having different median diameters _D50 in a volume frequency particle size distribution measured by a laser diffraction scattering method;
The lithium ion secondary battery according to any one of [1] to [9], wherein the median diameter D ₅₀ of the graphite powder (A) is larger than the median diameter D ₅₀ of the graphite powder (B).
[11]
[11] The lithium ion secondary battery according to [10], 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.
[12]
[12] The lithium ion secondary battery according to any one of [1] to [11], wherein the negative electrode active material layer further contains one or more conductive additives selected from the group consisting of carbon nanotubes, carbon nanohorns, graphene, carbon nanobrushes, and carbon black.
[13]
The lithium ion secondary battery according to any one of [1] to [12], wherein the capacity retention rate C according to the following method 2 is 85% or more.
(Method 2)
The lithium ion secondary battery is placed in a thermostatic chamber at 45° C. and charged at 30 mA. After the upper limit voltage reaches 4.2 V, the battery is charged at a constant voltage until the total charging time reaches 2.5 hours. The battery is then discharged at a constant current of 30 mA until the lower limit 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 (%).
[14]
The lithium ion secondary battery according to any one of [1] to [13], wherein the 2C/1C cycle capacity retention rate C _QC according to the following Method 3 is 88% or more.
(Method 3)
A charge/discharge cycle test (charge rate: 2.0 C, discharge rate: 1.0 C, temperature: 25° C., upper limit voltage: 4.25 V, lower limit voltage: 2.5 V, number of cycles: 300) is performed on the lithium ion secondary battery. Next, the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle of the lithium ion secondary battery is calculated, and this ratio is defined as the 2C/1C cycle capacity retention rate C _QC (%).
[15]
A lithium ion secondary battery module comprising the lithium ion secondary battery according to any one of [1] to [14].

The present invention provides a lithium-ion secondary battery and a lithium-ion secondary battery module with improved cycle characteristics and rapid charging capabilities.

1 is a schematic cross-sectional view showing an example of a lithium ion secondary battery according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
To avoid complexity, when there are a plurality of identical components in the same drawing, only one of them may be labeled with a reference symbol, and not all of them may be labeled with a reference symbol.
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.

In this embodiment, "A to B" indicating a numerical range means A or more and B or less unless otherwise specified.

<Lithium-ion secondary battery>
The lithium ion secondary battery of this embodiment is a lithium ion secondary battery including a negative electrode including a negative electrode active material layer, a positive electrode including a positive electrode active material layer, a separator, and an electrolyte solution, and the negative electrode active material layer includes a negative electrode active material including graphite powder and Si/C powder including Si-C composite particles containing silicon and a carbon material, and the X-ray diffraction spectrum of the negative electrode active material is measured in accordance with JIS K 0131:1996 using CuKα radiation having a wavelength of 1.5406 Å as a radiation source with an X-ray diffractometer under conditions of a tube voltage of 40 kV and a tube current of 40 mA, where I _C is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° or more and less than 27.5°, and I _Si is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5° or more and 29.5° or less, and (I _Si /I _C ) × 100(%) is 0.20% or more and 2.00% or less.

The inventors' studies have found that in a lithium-ion secondary battery comprising a negative electrode including a negative electrode active material layer, a positive electrode including a positive electrode active material layer, a separator, and an electrolyte, where the negative electrode active material layer contains a negative electrode active material including graphite powder and Si/C powder containing Si-C composite particles containing silicon and a carbon material, there is a correlation between the ratio of the peak intensity of the diffraction peak derived from silicon to the peak intensity of the diffraction peak derived from graphite in the X-ray diffraction spectrum of the negative electrode active material and the cycle characteristics and rapid chargeability of the lithium-ion secondary battery.

As a result of further investigations by the present inventors based on the above findings, they have found that a lithium ion secondary battery includes a negative electrode including a negative electrode active material layer, a positive electrode including a positive electrode active material layer, a separator, and an electrolyte, wherein the negative electrode active material layer includes a negative electrode active material including Si/C powder containing graphite powder and Si— _C composite particles containing silicon and a carbon material, and in the lithium ion secondary battery, the negative electrode active material has a ratio _{(I Si /} I _C The present inventors have found that the cycle characteristics and rapid chargeability can be improved by adjusting the value of (%)×100(%) to 0.20% or more and 2.00% or less, and have completed the present invention.

Here, in the X-ray diffraction spectrum of a negative electrode active material containing graphite powder and Si/C powder containing Si-C composite particles containing silicon and a carbon material, the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° or more and less than 27.5° corresponds to the peak derived from the 002 plane of graphite, and the maximum diffraction peak present in the diffraction angle 2θ range of 27.5° or more and 29.5° or less corresponds to the peak derived from the 111 plane of silicon.

In the X-ray diffraction spectrum of the negative electrode active material of this embodiment, measured in accordance with JIS K 0131:1996 using CuKα radiation of 1.5406 Å wavelength as a radiation source under conditions of a tube voltage of 40 kV and a tube current of 40 mA by an X-ray diffractometer, the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° or more and less than 27.5° is defined as I _C , and the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5° or more and 29.5° or less is defined as I Si , where I _{Si is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5° or more and 29.5° or less, and I Si} is the following: (I _Si /I _C From the viewpoint of further improving the cycle characteristics and the rapid chargeability, the value of (%) is 0.20% or more and 2.00% or less, preferably 0.20% or more and 1.50% or less, more preferably 0.20% or more and 1.20% or less, even more preferably 0.20% or more and 1.00% or less, even more preferably 0.30% or more and 0.90% or less, even more preferably 0.40% or more and 0.80% or less, and even more preferably 0.50% or more and 0.70% or less.

In this embodiment, examples of methods for adjusting the value of ( _ISi / _IC ) x 100 in the X-ray diffraction spectrum of the negative electrode active material include a method using commercially available graphite powder and Si/C powder with different crystallinity, and a method for adjusting the manufacturing conditions of the graphite powder and Si/C powder, such as raw materials and heat treatment conditions, to manufacture graphite powder and Si/C powder with different crystallinity. In addition, the value of ( _ISi / _IC ) x 100 may be adjusted by mixing two or more graphite powders with different crystallinity, or the value of ( _ISi / _IC ) x 100 may be adjusted by mixing two or more Si/C powders with different crystallinity.

<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 cycle characteristics and rapid chargeability, the Si/C powder of this embodiment preferably includes a carbon material in the Si-C composite particles that contains a porous carbon material, and silicon is present in at least some of the pores of the porous carbon material.

In this embodiment, a method for confirming that the Si-C composite particles in the Si/C powder contain silicon and a 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 performed by, for example, observing a cross section of a 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, 20 mapping accumulations, and 3000x magnification.

Examples of porous carbon materials that make up Si-C composite particles include activated carbon, carbon fiber aggregates, carbon nanotube aggregates, carbon obtained by heat treating resins or organic materials, hard carbon, etc. Porous carbon materials can be produced using methods for producing activated carbon or known methods for heat treating polymers, but commercially available products may also be purchased. They are not limited to these, and any material can be used as long as it is possible to produce or incorporate silicon into the pores of the porous carbon.

The median diameter _D50 in the volume frequency particle size distribution of the Si/C powder of this embodiment measured by a laser diffraction scattering method is preferably 6.0 μm or more and less than 10.0 μm, more preferably 6.5 μm or more and 9.5 μm or less, even more preferably 7.0 μm or more and 9.0 μm or less, and still more preferably 8.0 μm or more and 9.0 μm or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

The particle diameter D ₉₀ at which the cumulative value reaches 90% in the volume frequency particle size distribution of the Si/C powder of this embodiment measured by the laser diffraction scattering method is preferably 10.5 μm or more and 16.0 μm or less, more preferably 11.0 μm or more and 15.5 μm or less, even more preferably 11.5 μm or more and 15.0 μm or less, even more preferably 13.0 μm or more and 15.0 μm or less, and even more preferably 14.0 μm or more and 15.0 μm or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

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. Then, the volume frequency particle size distribution of the Si/C powder is measured by 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.

The specific surface area of the Si/C powder of this embodiment, determined by the BET flow method and single-point method in accordance with JIS Z 8830:2013, is preferably 5.0 m 2 /g or less, more preferably 4.5 m ² /g or less, even more preferably 3.5 m ² /g or less, and even more preferably 2.5 m ² /g or less, from the viewpoint of further improving cycle characteristics and rapid ^{chargeability} .
The lower limit of the specific surface area 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 not particularly limited, but may be, for example, 0.1 m ² /g or more, 0.5 m ² /g or more, or 1.0 m ² /g or more.

The specific surface area of the Si/C powder of this embodiment, determined by the BET flow method and single-point method in accordance with JIS Z 8830:2013, is preferably 0.1 m ² /g or more and 5.0 m ² /g or less, more preferably 0.1 m 2 /g or more and 4.5 m ² /g or less, even more preferably 0.5 m ² /g or more and 3.5 m ² /g or less, and even more preferably 1.0 m ² /g or more and 2.5 ^{m 2 /} g or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

From the viewpoint of further improving cycle characteristics and rapid chargeability, the content of Si/C powder in the negative electrode active material of this embodiment, when the total amount of the negative electrode active material is taken as 100.0 parts by mass, 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 40.0 parts by mass or less, even more preferably 10.0 parts by mass or more and 30.0 parts by mass or less, even more preferably 13.0 parts by mass or more and 25.0 parts by mass or less, and even more preferably 18.0 parts by mass or more and 22.0 parts by mass or less.

In this embodiment, the method for producing the Si/C powder is not particularly limited. For example, the Si/C powder can be obtained by a production method in which a porous carbon material having a median diameter of 4.0 to 12.0 μm and a specific surface area of 1000 to 1800 m ² /g is placed in a tubular furnace, the atmosphere in the tubular furnace is replaced with argon gas, and then a mixed gas of silane gas and nitrogen gas, with silane gas at 1 to 3 mol %, is flowed into the tubular furnace at a flow rate of 250 to 350 sccm, and the powder is processed under conditions of 400 to 550°C, 700 to 800 Torr, and 60 to 150 minutes.

Here, in order to produce a Si/C powder from which a negative electrode active material having a small _ISi can be obtained, that is, a Si/C powder having low silicon crystallinity and a small peak intensity of the diffraction peak derived from the silicon 111 plane in the X-ray diffraction spectrum, it is preferable to produce the powder under conditions of a treatment temperature of 480°C or higher and a treatment time of 100 minutes or longer.

<Graphite powder>
From the viewpoint of further improving cycle characteristics and rapid chargeability, the graphite powder of this embodiment has an _I / _I ratio of preferably 0.080 to 0.300, more preferably 0.085 to 0.250, even more preferably 0.090 to 0.200, and still more preferably 0.120 to 0.180. By setting the _I / _I ratio of the graphite powder within the above ranges, the cycle characteristics and rapid chargeability can be further improved.

In the present embodiment, the value of I _D /I _G of the graphite powder can be determined, for example, by the following method.
First, in accordance with JIS K 0137:2010, a laser Raman spectrometer is used to irradiate graphite powder with an argon laser under the conditions of an excitation wavelength of 532 nm, an entrance slit width of 200 μm, an exposure time of 15 seconds, two integration times, and a diffraction grating of 600 lines/mm, to measure the Raman spectrum. Next, the peak intensities _I and _I at 1360 ^cm and 1580 ^cm are determined from the Raman spectrum. Next, the value _I / _I is calculated from _I and _I.

The graphite powder of this embodiment has a median diameter _D50 in a volume frequency particle size distribution measured by a laser diffraction scattering method, which is preferably 3.0 μm or more and 16.0 μm or less, more preferably 5.0 μm or more and 15.0 μm or less, even more preferably 7.0 μm or more and 14.0 μm or less, still more preferably 8.0 μm or more and 13.0 μm or less, still more preferably 9.0 μm or more and 12.0 μm or less, and still more preferably 10.0 μm or more and 11.0 μm or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

The graphite powder of this embodiment has a particle diameter _D90 , at which the cumulative value reaches 90% in a volume frequency particle size distribution measured by a laser diffraction scattering method, of preferably 5.0 μm or more and 30.0 μm or less, more preferably 8.0 μm or more and 27.0 μm or less, even more preferably 10.0 μm or more and 24.0 μm or less, still more preferably 13.0 μm or more and 20.0 μm or less, and still more preferably 16.0 μm or more and 18.0 μm or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

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, the volume frequency particle size distribution of the graphite powder is measured by 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.

The graphite powder of this embodiment preferably contains graphite particles with amorphous carbon on the surface, from the perspective of further improving cycle characteristics and rapid charging performance.

The graphite powder of this embodiment preferably contains artificial graphite particles, from the viewpoint of further improving cycle characteristics and rapid charging performance.

These graphite powders can be obtained, for example, by adjusting the median diameter _D50 , cumulative 10% diameter _D10 , and cumulative 90% diameter _D90 of commercially available graphite powder by, for example, classifying the powder using a sieve with an appropriate opening ratio and wire diameter. 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 arc ion plating, sputtering, plasma CVD, or other methods. Examples of commercially available graphite powders include graphite powder manufactured by Nippon Graphite Industries Co., Ltd. and graphite powder manufactured by JFE Chemical Corporation.

From the viewpoint of further improving cycle characteristics and rapid chargeability, the content of 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 60.0 parts by mass or more and 95.0 parts by mass or less, even more preferably 70.0 parts by mass or more and 90.0 parts by mass or less, even more preferably 75.0 parts by mass or more and 87.0 parts by mass or less, and even more preferably 77.0 parts by mass or more and 83.0 parts by mass or less.

In the negative electrode active material of this embodiment, when the content of graphite powder in the negative electrode active material is W _C and the content of Si/C powder in the negative electrode active material is W _SiC , the value of W _C /W _SiC is preferably 1.0 or more and 20.0 or less, more preferably 2.0 or more and 15.0 or less, even more preferably 2.5 or more and 10.0 or less, even more preferably 3.0 or more and 7.0 or less, and still more preferably 3.5 or more and 5.0 or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

From the viewpoint of further improving cycle characteristics and rapid chargeability, the total content of 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.

<Graphite Powder (A), Graphite Powder (B)>
From the viewpoint of further improving cycle characteristics and rapid chargeability, the graphite powder of the present embodiment preferably contains graphite powder (A) and graphite powder (B), which are two types of graphite powders having different median diameters _D50 in volume frequency particle size distributions measured by a laser diffraction scattering method, and the median diameter _D50 of the graphite powder (A) is larger than the median diameter _D50 of the graphite powder (B).

When the graphite powder of the present embodiment contains graphite powder (A) and graphite powder (B), when the median diameters _D50 in the volume frequency particle size distributions of graphite powder (A) and graphite powder (B) measured by a laser diffraction scattering method are defined as D _A and D _B , respectively, the value of D _B /D _A 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 0.60 or more and 0.90 or less, and still more preferably 0.70 or more and 0.85 or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

When the graphite powder of this embodiment contains graphite powder (A) and graphite powder (B), from the viewpoint of further improving cycle characteristics and rapid chargeability, 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.

When the graphite powder of the present embodiment contains graphite powder (A) and graphite powder (B), when the content of graphite powder (A) in the graphite powder is W _A and the content of graphite powder (B) in the graphite powder is W _B , the value of W _A /W _B is preferably 0.1 or more and 10.0 or less, more preferably 0.3 or more and 5.0 or less, even more preferably 0.5 or more and 3.0 or less, 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, from the viewpoint of further improving cycle characteristics and rapid chargeability.

<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.

<Negative electrode>
The negative electrode of the present embodiment includes a negative electrode active material layer containing the negative electrode active material of the present embodiment, and from the viewpoint of further improving cycle characteristics and rapid chargeability, preferably includes a negative electrode active material layer containing the negative electrode active material of the present embodiment and a negative electrode current collector.
From the viewpoint of further improving cycle characteristics and rapid chargeability, the negative electrode active material layer of this embodiment preferably contains the negative electrode active material of this embodiment and a binder, and more preferably contains the negative electrode active material of this embodiment, a binder, and a conductive additive.

From the viewpoint of further improving cycle characteristics and rapid chargeability, the content of the negative electrode active material of this embodiment in the negative electrode active material layer of this embodiment, when the total amount of the negative electrode active material layer is taken as 100.0 parts by mass, 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.

From the viewpoint of further improving cycle characteristics and rapid chargeability, the negative electrode active material layer of this embodiment preferably contains one or more conductive additives selected from the group consisting of carbon nanotubes, carbon nanohorns, graphene, carbon nanobrushes, and carbon black, more preferably contains carbon nanotubes, and even more preferably contains single-walled carbon nanotubes.

When the negative electrode active material layer of this embodiment contains carbon nanotubes, the average fiber length of the carbon nanotubes is preferably 1.0 μm or more and 5.0 μm or less, more preferably 2.0 μm or more and 4.0 μm or less, and even more preferably 2.5 μm or more and 3.5 μm or less, from the viewpoint of further improving cycle characteristics and rapid chargeability.

From the viewpoint of further improving cycle characteristics and rapid chargeability, 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.

Examples of binders 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. Among these, from the viewpoint of further improving battery performance, the binder in the negative electrode active material layer of this embodiment preferably contains one or more materials selected from the group consisting of fluororesin, polycarboxylic acid polymer, and synthetic rubber, more preferably contains one or more materials 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.

From the viewpoint of further improving battery performance, 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.

From the viewpoint of further improving battery performance, the thickness of the negative electrode active material layer in 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.

From the viewpoint of further improving battery performance, the density of the negative electrode active material layer of this embodiment is preferably 0.50 g/cm or ^more and 3.00 g/cm ^{or less} , more preferably 1.00 g/cm ^{or more} and 2.50 g/cm ^{or less} , and even more preferably 1.30 g/cm or ^more and 2.00 g/cm ^or less.

The negative electrode current collector of this embodiment may be formed of, for example, copper, stainless steel, nickel, titanium, or an alloy thereof. The shape of the negative electrode current collector may be, 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.

<Positive electrode>
The positive electrode of this embodiment includes a positive electrode active material layer, and preferably includes a positive electrode active material layer and a positive electrode current collector from the viewpoint of further improving battery performance.
From the viewpoint of further improving battery performance, the positive electrode active material layer of the present embodiment preferably contains a positive electrode active material, more preferably contains a positive electrode active material and a binder, and even more preferably contains a positive electrode active material, a binder, and a conductive additive.

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 ₂ , FeS, and MoS ₂ ; transition metal oxides such as MnO, V ₂ O ₅ , V ₆ O ₁₃ , and TiO ₂ ; and olivine-type lithium phosphate. These may be used alone or in combination of two or more. Among these, from the viewpoint of further improving battery performance, the positive electrode active material in the positive electrode active material layer of the present embodiment preferably contains a composite oxide of lithium and a transition metal, more preferably contains a lithium-nickel-manganese-cobalt composite oxide, and even more preferably contains lithium nickel cobalt manganese oxide.

From the viewpoint of further improving battery performance, the content of the positive electrode active material in the positive electrode active material layer of this embodiment is preferably 50.0 parts by mass or more and 100.0 parts by mass or less, more preferably 75.0 parts by mass or more and 100.0 parts by mass or less, even more preferably 85.0 parts by mass or more and 100.0 parts by mass or less, even more preferably 90.0 parts by mass or more and 100.0 parts by mass or less, and even more preferably 95.0 parts by mass or more and 100.0 parts by mass or less, when the total amount of the positive electrode active material layer is taken as 100.0 parts by mass.

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. Among these, from the viewpoint of further improving battery performance, the conductive additive in the positive 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 battery performance, the content of the conductive additive in the positive 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 0.3 parts by mass or more and 5.0 parts by mass or less, even more preferably 0.5 parts by mass or more and 3.0 parts by mass or less, and even more preferably 0.7 parts by mass or more and 1.5 parts by mass or less, when the total amount of the positive electrode active material layer is taken as 100.0 parts by mass.

Examples of binders in the positive electrode active material layer of this embodiment include fluorine-based binders such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); and aqueous binders such as styrene-butadiene rubber. One of these may be used alone, or two or more may be used in combination. Of these, from the perspective of further improving battery performance, the binder in the positive electrode active material layer of this embodiment preferably contains a fluorine-based binder, and more preferably contains polyvinylidene fluoride.

From the perspective of further improving battery performance, the content of the binder in the positive 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 0.5 parts by mass or more and 5.0 parts by mass or less, even more preferably 1.0 parts by mass or more and 3.0 parts by mass or less, and even more preferably 1.2 parts by mass or more and 2.0 parts by mass or less, when the total amount of the positive electrode active material layer is taken as 100.0 parts by mass.

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.

<Electrolyte>
The electrolyte solution of the present embodiment preferably contains a lithium salt and an organic solvent, from the viewpoint of further improving battery performance.

Examples of the lithium salt in the electrolyte solution of this embodiment include lithium salts such as lithium hexafluorophosphate, lithium tetrafluorophosphate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium bis(fluoroethylsulfonyl)imide. One of these may be used alone, or two or more may be used in combination. Among these, the lithium salt of this embodiment preferably includes lithium hexafluorophosphate, from the viewpoint of further improving battery performance.

From the perspective of further improving battery performance, the content of lithium salt in the electrolyte solution of this embodiment is preferably 1.0 parts by mass or more and 30.0 parts by mass or less, more preferably 5.0 parts by mass or more and 20.0 parts by mass or less, and even more preferably 10.0 parts by mass or more and 15.0 parts by mass or less, when the total amount of the electrolyte solution is 100.0 parts by mass.

Examples of organic solvents in the electrolyte solution of this embodiment include 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; gamma-lactones such as gamma-butyrolactone; chain ethers; and cyclic ethers. One of these may be used alone, or two or more may be used in combination. Among these, from the viewpoint of further improving battery performance, the organic solvent in the electrolyte solution of this embodiment preferably includes one or more selected from the group consisting of cyclic carbonates and chain carbonates, and more preferably includes one or more selected from the group consisting of ethylene carbonate, vinylene carbonate, and ethyl methyl carbonate. Furthermore, the electrolyte solution of this embodiment may contain the above compound as an organic solvent or as an additive.

From the perspective of further improving battery performance, the content of the organic solvent in the electrolyte solution of this embodiment is preferably 70.0 parts by mass or more and 99.0 parts by mass or less, more preferably 80.0 parts by mass or more and 95.0 parts by mass or less, and even more preferably 83.0 parts by mass or more and 90.0 parts by mass or less, when the total amount of the electrolyte solution is 100.0 parts by mass.

From the viewpoint of further improving battery performance, the electrolyte solution of this embodiment preferably contains a lithium salt/halogen-containing EC-based electrolyte solution. The lithium salt/halogen-containing EC-based electrolyte solution of this embodiment is a nonaqueous electrolyte solution containing a lithium salt, an organic solvent, and halogen-containing ethylene carbonate.

From the perspective of further improving battery performance, the lithium salt/halogen-containing EC-based electrolyte solution of this embodiment preferably contains one or more halogen-containing ethylene carbonates selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, and trichloroethylene carbonate, and more preferably contains fluoroethylene carbonate.

From the viewpoint of further improving battery performance, the content of halogen-containing ethylene carbonate in the electrolyte solution of this embodiment is preferably 0.1 parts by mass or more and 10.0 parts by mass or less, more preferably 0.5 parts by mass or more and 5.0 parts by mass or less, and even more preferably 1.0 parts by mass or more and 3.0 parts by mass or less, when the total amount of the electrolyte solution is 100.0 parts by mass.

<Separator>
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.

<Exterior body>
The lithium ion secondary battery of this embodiment may further include an exterior body.
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.

The lithium-ion secondary battery of this embodiment will be described using the drawings. FIG. 1 is a schematic cross-sectional view showing an example of a 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 also be provided between the positive electrode and the negative electrode. Multiple electrode pairs of positive and negative electrodes can be provided.

The lithium-ion secondary battery 10 has 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 and negative electrodes, including the positive electrode active material layer 1 and the negative electrode active material layer 2, are stacked facing each other via a separator 5 made of a nonwoven fabric, a polypropylene microporous film, or the like. This electrode pair is housed in a container formed by 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. A container may also be used to house an electrode group consisting of multiple stacked electrode pairs.

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

<Capacity maintenance rate C>
From the viewpoint of further improving cycle characteristics, the capacity retention rate C of the lithium ion secondary battery of this embodiment 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 94% or more, and even more preferably 96% or more. The upper limit of the capacity retention rate C of the lithium ion secondary battery of this embodiment is not particularly limited, but may be, for example, 100% or less.
In this embodiment, the cycle characteristics of the lithium ion secondary battery can be evaluated using the capacity retention rate C as an index.

From the viewpoint of further improving cycle characteristics, the capacity retention rate C of the lithium ion secondary battery of this embodiment 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 94% or more and 100% or less, and even more preferably 96% or more and 100% or less.

In this embodiment, the capacity retention rate C of the lithium ion secondary battery can be measured by, for example, the following method.
First, the lithium ion secondary battery is placed in a thermostatic chamber at 45°C and charged at 30 mA. After the upper limit voltage reaches 4.2 V, it is charged at a constant voltage until the total charging time is 2.5 hours. Next, it is discharged at a constant current of 30 mA until the lower limit voltage reaches 2.5 V. Next, this charge/discharge cycle is 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 (%).

<2C/1C cycle capacity retention rate C _QC >
From the viewpoint of further improving the rapid chargeability, the 2C/1C cycle capacity retention rate C _QC of the lithium ion secondary battery of this embodiment is preferably 88% or more, more preferably 89% or more, even more preferably 90% or more, and still more preferably 93% or more. The upper limit of the 2C/1C cycle capacity retention rate C _QC of the lithium ion secondary battery of this embodiment is not particularly limited, but may be, for example, 100% or less.
In this embodiment, the rapid chargeability of a lithium ion secondary battery can be evaluated using the 2C/1C cycle capacity retention rate C _QC as an index.

From the viewpoint of further improving the rapid chargeability, the 2C/1C cycle capacity retention rate C _QC of the lithium ion secondary battery of this embodiment is preferably 88% or more and 100% or less, more preferably 89% or more and 100% or less, even more preferably 90% or more and 100% or less, and still more preferably 93% or more and 100% or less.

In this embodiment, the 2C/1C cycle capacity retention rate C _QC of a lithium ion secondary battery can be measured by, for example, the following method.
First, a charge/discharge cycle test (charge rate: 2.0 C, discharge rate: 1.0 C, temperature: 25°C, upper limit voltage: 4.25 V, lower limit voltage: 2.5 V, number of cycles: 300) is performed on the lithium ion secondary battery. Next, the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle of the lithium ion secondary battery is calculated, and this is defined as the 2C/1C cycle capacity retention rate C _QC (%).

<Lithium-ion secondary battery module>
The lithium ion secondary battery module of this embodiment includes the lithium ion secondary battery of this embodiment. The lithium ion secondary battery of this embodiment has improved cycle characteristics and rapid chargeability, and therefore the lithium ion secondary battery module of this embodiment has improved cycle characteristics and rapid chargeability.

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 housing 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.

The lithium-ion secondary battery module of this embodiment can be used in a battery system comprising multiple 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.

The above describes embodiments of the present invention, but 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 objectives of the present invention are included in the present invention.

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

The following graphite powder was used to prepare the negative electrode active material:

<Graphite powder (A)>
Graphite powder 1 (artificial graphite containing amorphous carbon on the surface, D ₅₀ : 12.0 μm)
Graphite powder 2 (artificial graphite containing amorphous carbon on the surface, D ₅₀ : 14.5 μm)
Graphite powder 3 (artificial graphite containing amorphous carbon on the surface, D ₅₀ : 30.0 μm)

<Graphite powder (B)>
Graphite powder 4 (artificial graphite not containing amorphous carbon on the surface, D ₅₀ : 9.6 μm)

<Preparation of Si/C Powder 1>
Porous carbon material 1 ( _D50 : 8.5 μm, specific surface area: 1566 ^m2 /g) was placed in a tubular furnace, and the inside of the tubular furnace was replaced with argon gas. Then, 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 material was treated under the conditions of 500°C, 760 Torr, and 120 minutes. The product was then cooled to room temperature to obtain Si/C powder 1.

A cross-section of the Si-C composite particles contained in the obtained Si/C powder 1 was examined using a scanning electron microscope (SU3500, manufactured by Hitachi High-Technologies Corporation), an energy dispersive X-ray spectrometer (Ultim Max 40, manufactured by Oxford Instruments), and image analysis software (Aztec, manufactured by Oxford Instruments). Secondary electrons were selected as the detection target, and elemental mapping of silicon and carbon was performed at an acceleration voltage of 3 kV, 20 mapping accumulations, and 3000x magnification. This confirmed that the Si-C composite particles contained silicon, and that the silicon in the Si-C composite particles was present in at least some of the pores of the porous carbon material.

The specific surface area of the obtained Si/C powder 1 was measured by the BET flow method and the single-point method in accordance with JIS Z 8830:2013 using a fully automatic specific surface area measuring device (Macsorb HM-1208, manufactured by Mountec Co., Ltd.). Furthermore, the particle diameter D 90 and median diameter D ₅₀ at which the cumulative value reached 90% were determined from the volume frequency particle size distribution measured by a laser diffraction scattering method using a laser diffraction particle size distribution measuring device (SALD- ₂₃₀₀ , manufactured by Shimadzu Corporation). Here, the Si/C powder 1 was suspended in a dispersion medium and ultrasonically dispersed, followed by measurement. Five measurements were performed, and the average values were used. The results are shown in Table 1.

<Si/C powder 2>
Si/C powder 2 was obtained in the same manner as Si/C powder 1, except that porous carbon material 2 (D ₅₀ : 7.5 μm, specific surface area: 1675 m ² /g) was used instead of porous carbon material 1.

<Preparation of Si/C Powder 3 and Si/C Powder 4>
Si/C powders 3 and 4 were obtained in the same manner as Si/C powder 1, except that porous carbon material 3 (D ₅₀ : 5.5 μm, specific surface area: 1756 m ² /g) and porous carbon material 4 (D ₅₀ : 10.0 μm, specific surface area: 1655 m ² /g) were used instead of porous carbon material 1, the treatment temperature of the porous carbon material in the tubular furnace was changed to 450°C, and the treatment time of the porous carbon material in the tubular furnace was changed to 90 minutes.

Elemental mapping was performed on the cross sections of the Si-C composite particles contained in the obtained Si/C powders 2, 3, and 4 using the same method as for Si/C powder 1 above. It was confirmed that the Si-C composite particles in all cases contained silicon, and that the silicon in the Si-C composite particles was present at least partially within the pores of the porous carbon material.

The specific surface area, cumulative 90% diameter _D90 , and median diameter _D50 of the obtained Si/C powders 2, 3, and 4 were measured in the same manner as for the Si/C powder 1. The results are shown in Table 1.

(Examples 1 to 3, 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 3 and Comparative Examples 1 and 2.

For the graphite powders of each Example and Comparative Example, Raman spectra were measured in accordance with JIS K 0137:2010 using a triple laser Raman spectrometer (RAMANOR T64000, manufactured by HORIBA Jobin Yvon) under the conditions of an excitation wavelength of 532 nm, an entrance slit width of 200 μm, an exposure time of 15 seconds, two integration times, and a diffraction grating of 600 lines/mm. The graphite powder was irradiated with an argon laser and the Raman spectra were then measured. The peak intensities _I and _I at 1360 ^cm and 1580 ^cm were then determined from the Raman spectra. The _I / _I ratio for the graphite powders of each Example and Comparative Example was then calculated from _{I and I.} The results are shown in Table 1.

The cumulative 90% diameter _D90 and median diameter _D50 of the graphite powders of each Example and Comparative Example were measured using the same method as for the Si/C powder 1. Here, the graphite powder was suspended in a dispersion medium and ultrasonically dispersed before measurement. The measurement was performed five times, and the average value was used for each. The results are shown in Table 1.

<Preparation of negative electrode active material>
The graphite powder and the Si/C powder were mixed in the compounding ratios shown in Table 1 to obtain the negative electrode active materials of Examples 1 to 3 and Comparative Examples 1 and 2.

For the negative electrode active materials of each Example and Comparative Example, in accordance with JIS K 0131:1996, an X-ray diffraction spectrum was measured using CuKα radiation of 1.5406 Å wavelength as a radiation source with a fully automatic multipurpose X-ray diffractometer (Rigaku Corporation, SmartLab 3kW) under conditions of a tube voltage of 40 kV and a tube current of 40 mA. The peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° to less than 27.5° was defined as I _C , and the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5° to 29.5° was defined as I _Si , and the value of ( _ISi / _IC ) × 100(%) was calculated. The results are shown in Table 1.

<Preparation of negative electrode>
For the negative electrode active material of each Example and each Comparative Example, 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 (average fiber length: 3.0 μm), and 3.0 parts by mass of polyacrylic acid. Next, the negative electrode active material slurry was applied to an 8 μm thick copper foil negative electrode current collector in an amount such that the initial charge capacity per unit area was 4.3 mAh / cm ² , and then dried to obtain a negative electrode laminate. Next, using a roll press, the negative electrode laminate was pressed at a pressure such that the density was 1.65 g / cm ³ , and a negative electrode of each Example and Comparative Example was obtained.

<Preparation of positive electrode>
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 _lithium nickel cobalt manganese oxide (Li( _{Ni0.9Co0.05Mn0.05} ) _O2 ), 1.5 parts by mass _of polyvinylidene fluoride, and ^1.0 part by mass of single-walled carbon nanotubes. The positive electrode active material slurry was then applied to a 12 μm thick aluminum foil positive electrode current collector in an amount such that the initial charge capacity per unit area was 4.0 mAh/cm2, and then dried to obtain a positive electrode laminate. The positive electrode laminate was then pressed using a roll press at a pressure such that the density was 3.5 g/ ^cm3 , to obtain a positive electrode.

<Preparation of non-aqueous electrolyte>
The nonaqueous electrolyte solution was prepared by mixing an organic solvent, a lithium salt, and an additive. The organic solvent was prepared by mixing ethylene carbonate, a cyclic carbonate, and ethyl methyl carbonate, a chain carbonate, in a volume ratio of 3/7. Next, 12 parts by mass of lithium hexafluorophosphate as a lithium salt and 2 parts by mass of fluoroethylene carbonate and 2 parts by mass of vinylene carbonate as additives were added to 84 parts by mass of the prepared organic solvent and mixed to obtain a nonaqueous electrolyte solution.

<Fabrication of Lithium-ion Secondary Battery>
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. Here, 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.

<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.

<2C/1C cycle capacity retention rate C _QC >
A charge-discharge cycle test (charge rate: 2.0 C, discharge rate: 1.0 C, temperature: 25°C, upper limit voltage: 4.25 V, lower limit voltage: 2.5 V, number of cycles: 300) was conducted on the lithium ion secondary batteries of each Example and Comparative Example. Next, the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle of each lithium ion secondary battery of each Example and Comparative Example was determined, and this was taken as the 2C/1C cycle capacity retention rate C _QC (%). The results are shown in Table 1.

REFERENCE SIGNS LIST 1 Positive electrode active material layer 2 Negative electrode active material layer 3 Positive electrode current collector 4 Negative electrode current collector 5 Separator 6 Exterior body 7 Exterior body 8 Negative electrode tab 9 Positive electrode tab 10 Lithium ion secondary battery

Claims (15)

A lithium ion secondary battery comprising: a negative electrode including a negative electrode active material layer; a positive electrode including a positive electrode active material layer; a separator; and an electrolyte solution,
the negative electrode active material layer includes a negative electrode active material including graphite powder and Si/C powder including Si-C composite particles containing silicon and a carbon material;
In an X-ray diffraction spectrum of the negative electrode active material measured in accordance with JIS K 0131:1996 using CuKα radiation of a wavelength of 1.5406 Å as a radiation source under conditions of a tube voltage of 40 kV and a tube current of 40 mA using an X-ray diffractometer, where _IC is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 25.5° or more and less than 27.5°, and IS is the peak intensity of the maximum diffraction peak present in the diffraction angle 2θ range of 27.5 _° or more and 29.5° or less, the value of ( _ISi / _IC ) x 100(%) is 0.20% or more and 2.00% or less. 2. The lithium ion secondary battery according to claim 1, wherein the graphite powder has an I _D /I _G value measured by the following Method 1 of 0.080 or more and 0.300 or less.
(Method 1)
In accordance with JIS K 0137:2010, the graphite powder is irradiated with an argon laser using a laser Raman spectrometer under the conditions of an excitation wavelength of 532 nm, an entrance slit width of 200 μm, an exposure time of 15 seconds, an accumulation count of 2, and a diffraction grating of 600 lines/mm, and a Raman spectrum is measured. Next, from the Raman spectrum, peak intensities _I and _I at ₁₃₆₀ ^cm and 1580 ^cm are determined, respectively. Next, the value of _I / _I is determined from I and _I. 3. The lithium ion secondary battery according to claim 1, wherein the Si/C powder has a median diameter _D50 of 6.0 μm or more and less than 10.0 μm in a volume frequency particle size distribution determined by a laser diffraction scattering method. The lithium-ion secondary battery described in any one of claims 1 to 3, wherein the carbon material in the Si-C composite particles includes a porous carbon material, and the silicon is present in at least some of the pores of the porous carbon material. 5. The lithium ion secondary battery according to claim 1, wherein the value of W _C /W _SiC is 1.0 or more and 20.0 or less, where W _C is the content of the graphite powder in the negative electrode active material and W SiC is the content of the _Si /C powder in the negative electrode active material. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein the electrolyte solution comprises a lithium salt/halogen-containing EC-based electrolyte solution. The lithium ion secondary battery according to claim 6, wherein the lithium salt/halogen-containing EC-based electrolyte solution contains one or more halogen-containing ethylene carbonates selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, and trichloroethylene carbonate. The lithium-ion secondary battery described in any one of claims 1 to 7, wherein the graphite powder contains graphite particles with amorphous carbon on the surface. The lithium-ion secondary battery according to any one of claims 1 to 8, wherein the graphite powder contains artificial graphite particles. the graphite powder comprises graphite powder (A) and graphite powder (B), which are two types of graphite powder having different median diameters _D50 in a volume frequency particle size distribution measured by a laser diffraction scattering method;
The lithium ion secondary battery according to any one of claims 1 to 9, wherein the median diameter _D50 of the graphite powder (A) is larger than the median diameter _D50 of the graphite powder (B). The lithium-ion secondary battery described in claim 10, wherein the graphite powder (A) contains graphite particles having amorphous carbon on their surfaces, and the graphite powder (B) contains graphite particles having no amorphous carbon on their surfaces. The lithium-ion secondary battery described in any one of claims 1 to 11, wherein the negative electrode active material layer further contains one or more conductive additives selected from the group consisting of carbon nanotubes, carbon nanohorns, graphene, carbon nanobrushes, and carbon black. The lithium ion secondary battery according to any one of claims 1 to 12, wherein the capacity retention rate C according to the following method 2 is 85% or more.
(Method 2)
The lithium ion secondary battery is placed in a thermostatic chamber at 45° C. and charged at 30 mA. After the upper limit voltage reaches 4.2 V, the battery is charged at a constant voltage until the total charging time reaches 2.5 hours. The battery is then discharged at a constant current of 30 mA until the lower limit 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 (%). The lithium ion secondary battery according to any one of claims 1 to 13, wherein the 2C/1C cycle capacity retention rate C _QC according to the following Method 3 is 88% or more.
(Method 3)
A charge/discharge cycle test (charge rate: 2.0 C, discharge rate: 1.0 C, temperature: 25° C., upper limit voltage: 4.25 V, lower limit voltage: 2.5 V, number of cycles: 300) is performed on the lithium ion secondary battery. Next, the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle of the lithium ion secondary battery is calculated, and this ratio is defined as the 2C/1C cycle capacity retention rate C _QC (%). A lithium ion secondary battery module comprising the lithium ion secondary battery according to any one of claims 1 to 14.