JP2021109803A - Ultrathin graphite sheet-silicon powder complex having interposition, inclusion, crosslinked structure, method for producing the same, lithium ion battery negative electrode, and lithium ion battery - Google Patents

Abstract

To provide a negative electrode active material capable of more easily being produced while keeping cycle characteristics.SOLUTION: A graphite silicon powder complex is produced by mixing expanded graphite and silicon powder in an aprotic solvent; the aprotic solvent is preferably at least one selected from aromatic solvents, halogen-based solvents and amide-based solvents; ultrasonic irradiation is preferably conducted in the process of mixing; the expanded graphite is preferably delaminated in the aprotic solvent to be turned into a sheet having a thickness of 15 nm or less.SELECTED DRAWING: Figure 9

Description

The present invention relates to a negative electrode active material.

As a negative electrode active material in a non-aqueous secondary battery such as a lithium ion battery, a composite of a carbon material such as graphite and silicon is known (for example, Patent Documents 1 and 2). However, the charge / discharge characteristics are not always sufficient, and as the number of cycles increases, the discharge capacity decreases.
On the other hand, Patent Document 3 discloses that a silicon powder coated with a carbon film can be produced by heating the silicon powder in an organic gas, and the obtained carbon film-coated silicon powder is used as a negative electrode active material. It has been described that its use improves charge / discharge cycle characteristics.

JP-A-2015-38852 Japanese Unexamined Patent Publication No. 2016-149340 Japanese Unexamined Patent Publication No. 2018-85316

Although the carbon film-coated silicon powder of Patent Document 3 has excellent charge / discharge cycle characteristics, it cannot be said that it is convenient because its production requires heating in an organic gas.
The present invention has been made by paying attention to the above circumstances, and an object of the present invention is that it can be manufactured more easily while exhibiting cycle characteristics equal to or higher than those of the carbon film-coated silicon powder of Patent Document 3. The purpose is to provide a negative electrode active material.

The negative electrode active material according to the present invention is a graphite silicon powder complex, and the constituent requirements of the present invention are specifically as follows.
[1] A method for producing a graphite-silicon powder composite, which comprises mixing expanded graphite and silicon powder in an aprotic solvent.
[2] The method for producing a graphite silicon powder composite according to the above [1], wherein the aprotic solvent is at least one selected from an aromatic solvent, a halogen solvent, and an amide solvent.
[3] The method for producing a graphite silicon powder complex according to the above [1] or [2], which irradiates ultrasonic waves at the time of mixing.
[4] Production of the graphite silicon powder complex according to any one of the above [1] to [3], wherein the expanded graphite is delaminated in an aprotic solvent to form a sheet having a thickness of 15 nm or less. Method.
[5] A method for producing a graphite-silicon powder composite, which comprises mixing a graphite sheet having a thickness of 15 nm or less and silicon powder in a solvent.
[6] The method for producing a graphite silicon powder complex according to the above [4] or [5], wherein the major axis in the plane direction of the graphite sheet having a thickness of 15 nm or less is 0.1 μm or more.
[7] The method for producing a graphite silicon powder complex according to the above [1] to [6], wherein the silicon powder has at least one property selected from the following (a) to (d).
(A) Cumulative 50% particle size of the silicon powder on a volume basis is 300 nm or less (b) The silicon powder is in the form of flakes (c) The silicon powder is cut from a crystalline silicon ingot. The powder or a pulverized product thereof (d) The cumulative 50% particle size based on the number of the silicon powder calculated based on the crystallite size distribution from the X-ray diffraction pattern is 3 to 100 nm [8]. The method for producing a graphite silicon powder composite according to any one of the above [1] to [7], wherein the hard carbon or the hard carbon precursor does not coexist when the graphite is mixed with the solvent.
[9] A graphite-silicon powder complex in which silicon powder is sandwiched between graphite sheets having a thickness of 15 nm or less.
[10] A graphite-silicon powder complex in which silicon powder is wrapped in a graphite sheet having a thickness of 15 nm or less.
[11] The graphite silicon powder complex according to the above [9] or [10], which does not contain at least one selected from hard carbon, soft carbon, and precursors thereof.
[12] A negative electrode in which a negative electrode material layer containing a plurality of graphite silicon powder complexes according to any one of [9] to [11] is laminated on the surface of a current collector.
[13] The negative electrode according to the above [12], wherein the negative electrode material layer contains a plurality of aggregated graphite sheets having a thickness of 50 nm or more, and a plurality of the graphite silicon powder complexes are sandwiched between the aggregated graphite sheets.
[14] The graphite sheet having a thickness of 1 to 200 nm containing a graphite sheet having a thickness of 15 nm or less and an agglutinated graphite sheet having a thickness of 50 nm or more is inserted at intervals of 0.2 to 5 μm in the cross section of the negative electrode material layer. The negative electrode according to [12].
[15] The negative electrode according to any one of [12] to [14] above, wherein the cracks existing on the surface of the negative electrode material layer are crosslinked with a graphite sheet having a thickness of 15 nm or less.

According to the present invention, it is possible to provide a negative electrode active material exhibiting cycle characteristics equal to or higher than that of carbon film-coated silicon powder, and the negative electrode active material can be easily produced.

FIG. 1-1 is a graph showing the particle size distribution of the silicon powder used in Examples and Comparative Examples by a dynamic light scattering type particle size measuring device. FIG. 1-2 (a) is a diagram showing an X-ray diffraction pattern of the silicon powder used in Examples and Comparative Examples, and FIG. 1-2 (b) is a distribution and mode of crystallite size obtained from the X-ray diffraction pattern. It is a figure which shows the diameter and the cumulative 50% particle diameter. FIG. 2 is a transmission electron micrograph of the silicon powder used in Examples and Comparative Examples. FIG. 3 is a transmission electron micrograph of expanded graphite delaminated in an aprotic solvent (N-methyl-2-pyrrolidone). FIG. 4 is a transmission electron micrograph of the end face of expanded graphite delaminated in an aprotic solvent (N-methyl-2-pyrrolidone). FIG. 5 is a scanning electron micrograph of a graphite sheet dispersed in ethanol. FIG. 6 shows an atomic force microscope image and a cross-sectional thickness distribution of a graphite sheet dispersed with ethanol. FIG. 7 is a transmission electron micrograph of a graphite sheet dispersed with ethanol. FIG. 8 is a scanning electron micrograph of a graphite sheet / silicon powder complex of a comparative example. FIG. 9 is a diagram in which a scanning transmission electron micrograph and a scanning transmission electron micrograph of the ultrathin graphite sheet / silicon powder composite of the example are elementally mapped by an energy dispersive X-ray analyzer. FIG. 10 is a scanning electron micrograph of the spheroidized graphite silicon powder composite in the negative electrode cross section of the example. FIG. 11 is a graph showing the relationship between the number of cycles of a battery (half cell) using the negative electrode active material obtained in Examples and Comparative Examples and the discharge capacity. FIG. 12 is a graph showing the relationship between the number of cycles of a battery (half cell) using the negative electrode active material obtained in Examples and Comparative Examples and the charge capacity. FIG. 13 is a graph showing the relationship between the number of cycles of a battery (half cell) using the negative electrode active material obtained in Examples and Comparative Examples and the discharge capacity. FIG. 14 is a scanning electron micrograph of the surface and cross section of the negative electrode material layer existing in the negative electrode of the example. FIG. 15 is a diagram of element mapping of a cross section of an electrode using a graphite silicon powder composite with an energy dispersive X-ray analyzer. FIG. 16 is a transmission electron micrograph of the negative electrode active material present in the negative electrode of the example. FIG. 17 is a graph showing the relationship between the number of cycles of a battery (full cell) using the negative electrode active material obtained in the examples and the discharge capacity. FIG. 18A is a scanning electron micrograph of the electrode surface using the negative electrode active material (ultra-thin graphite sheet: silicon = 1: 5 (mass ratio)) after charging and discharging for 300 cycles in the example. Is. FIG. 18B is a partially enlarged photograph of FIG. 18A. FIG. 18 (c) is a partially enlarged photograph of FIG. 18 (b). FIG. 19 is a diagram in which the electrodes observed in FIG. 18 are ultrasonically dispersed in ethanol, and the scanning transmission electron micrograph and the scanning transmission electron micrograph are elementally mapped by an energy dispersive X-ray analyzer. FIG. 20 is a scanning electron micrograph of the electrode surface using the comparative negative electrode active material (carbon-coated silicon powder) after charging and discharging for 300 cycles in the example.

1. 1. Negative electrode active material In the present invention, a graphite silicon powder composite is produced as a negative electrode active material. The graphite silicon powder composite can be produced by mixing expanded graphite and silicon powder in an aprotic solvent.

The expanded graphite is obtained by immersing graphite such as scaly graphite in an acid (for example, a mixed acid composed of sulfuric acid and nitric acid) to allow the acid to penetrate between the graphite layers, washing with water and drying as necessary, and then heating. This refers to graphite that can be produced by means of graphite in which the layers are partially expanded by gasifying the acid between the layers by heating. The expanded graphite may be a commercially available product or may be produced from graphite.

In the present invention, by bringing the expanded graphite into contact with an aprotic solvent, the expanded graphite is appropriately peeled between the layers, and as a result, the silicon powder can be appropriately sandwiched or wrapped (including inclusion, wrapping, etc.). Therefore, when used as a negative electrode active material for a secondary battery, the charge / discharge cycle characteristics can be sufficiently enhanced.

Examples of the aprotic solvent include aromatic hydrocarbon solvents such as toluene, xylene and mesitylene, halogenated hydrocarbon solvents such as chlorobenzene, dichlorobenzene, trichlorobenzene, chlorotoluene, chloroethane and chloromethane, dimethylformamide and N. Examples include amide solvents such as −methyl-2-pyrrolidone. These aprotic solvents may be used alone or in combination of two or more. As the aprotic solvent, an amide solvent is preferable.

The expanded graphite after contacting with an aprotic solvent and delaminating is an ultrathin graphite sheet. The major axis (mode diameter) in the plane direction of the ultrathin graphite sheet in the aprotic solvent is, for example, 0.1 μm or more, preferably 0.2 μm or more. The major axis is, for example, 100 μm or less, preferably 50 μm or less, and more preferably 30 μm or less.
From the viewpoint of improving the cycle characteristics, the major axis (mode diameter) may be, for example, 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 6 μm or more. On the contrary, from the viewpoint of improving the initial discharge capacity, the major axis (mode diameter) may be, for example, 20 μm or less, preferably 15 μm or less, more preferably 10 μm or less, and even more preferably 7 μm or less.

The thickness (mode) of the ultrathin graphite sheet in the aprotic solvent is, for example, 0.1 nm or more, preferably 1 nm or more, and more preferably 3 nm or more. The thickness may be, for example, 15 nm or less, 12 nm or less, or 8 nm or less. By making the graphite sheet thinner, it becomes easier to sandwich or enclose silicon. Furthermore, the amount of graphite relative to silicon can be reduced, and the theoretical capacity can be increased. On the other hand, by not making the graphite sheet excessively thin, it is possible to prevent the graphite sheet from wrinkling without sandwiching or wrapping the silicon.

As the silicon powder, for example, silicon chips machined from a crystalline silicon ingot or a pulverized product thereof can be used. By using crystalline silicon, the electrical properties are improved. Further, as the pulverization, for example, it is preferable to perform pulverization using a mortar or pestle, and a crusher can be used for the pulverization.

The cumulative 50% particle size of the silicon powder determined by the dynamic light scattering type particle size measuring device on a volume basis is, for example, 10 nm or more, preferably 30 nm or more, and more preferably 60 nm or more. The cumulative 50% particle size is, for example, 300 nm or less, preferably 200 nm or less, and more preferably 150 nm or less. The mode diameter of the silicon powder determined by the dynamic light scattering type particle size measuring device is, for example, 10 nm or more, preferably 30 nm or more, more preferably 60 nm or more, and for example, 300 nm or less, preferably 200 nm or less, more preferably. It is 150 nm or less.
The cumulative 50% particle size based on the number of silicon powders calculated based on the crystallite size distribution from the X-ray diffraction pattern is, for example, 3 nm or more, preferably 7 nm or more, more preferably 10 nm or more, for example. It is 100 nm or less, preferably 70 nm or less, and more preferably 50 nm or less. The mode diameter of the silicon powder calculated based on the crystallite size distribution from the X-ray diffraction pattern is, for example, 3 nm or more, preferably 7 nm or more, more preferably 10 nm or more, and for example, 100 nm or less, preferably 70 nm or less. , More preferably 50 nm or less.
When the silicon powder has an appropriate size, the electrical characteristics and the ionic conduction characteristics of the silicon electrode are improved, and the silicon powder is easily sandwiched or wrapped in the ultrathin graphite sheet. Further, as the particle size of the silicon powder becomes larger, the aggregation and peeling of the silicon due to the insertion of lithium ions into the silicon are likely to occur. However, in the present invention, the silicon is sandwiched or wrapped in the ultrathin sheet. Therefore, even if the silicon powder is enlarged within the range of the particle size, the peeling can be suppressed.

The shape of the silicon powder is not particularly limited, and may be powdery, granular, spherical, plate-like (flake-like), lump-like, fibrous, or the like, but flake-like is preferable. When it is in the form of flakes, it is likely to be appropriately sandwiched between ultrathin graphite sheets and encapsulated in an aprotic solvent, and the cycle characteristics are likely to be improved.

The thickness (mode) of the flake-shaped silicon powder is, for example, 0.5 nm or more, preferably 1.0 nm or more, and more preferably 2.0 nm or more. The thickness is, for example, 100 nm or less, preferably 50 nm or less, and more preferably 30 nm or less. By adjusting the thickness of the flake-shaped silicon powder, the silicon powder is easily sandwiched between the ultrathin graphite sheets and easily encapsulated, and the cycle characteristics are likely to be improved.

The amount of silicon powder is, for example, about 0.01 to 100 parts by mass, preferably about 0.1 to 50 parts by mass, and more preferably about 0.8 to 10 parts by mass with respect to 1 part by mass of expanded graphite. ..

The temperature at which the expanded graphite and the silicon powder are mixed in the aprotic solvent is not particularly limited and may be equivalent to that of the environment. The temperature is, for example, 0 ° C. or higher, preferably 10 ° C. or higher, and is, for example, the boiling point or lower of the solvent, preferably 80 ° C. or lower, and more preferably 50 ° C. or lower.

When the expanded graphite and the silicon powder are mixed in an aprotic solvent, ultrasonic waves may be applied if necessary. Ultrasonic irradiation facilitates delamination of expanded graphite and sandwiching or encapsulation of silicon powder by the graphite sheeted by the delamination. Further, the expanded graphite may be irradiated with ultrasonic waves in advance in an aprotic solvent before being mixed with the silicon powder. Sonication before mixing facilitates sheet formation of expanded graphite. Both sonication before mixing and sonication during mixing may be performed.

The graphite silicon powder composite obtained by mixing expanded graphite and silicon powder in an aprotic solvent can be isolated by separating from the solvent by an appropriate method. For separation from the solvent, for example, filtration, centrifugation, concentration and the like can be used, and suction filtration is preferable.

The graphite silicon powder complex isolated from the solvent is dried, if necessary. The drying temperature is, for example, 10 ° C. or higher, preferably 20 ° C. or higher, more preferably 30 ° C. or higher, and for example, 200 ° C. or lower, preferably 150 ° C. or lower, more preferably 100 ° C. or lower. The pressure at the time of drying may be atmospheric pressure or reduced pressure.

Further, the graphite silicon powder complex isolated from the solvent may be sphericalized if necessary. By making it spherical, it is possible to further suppress the diffusion of a part of the detached electrode into the electrolytic solution. Sphericalization is possible, for example, by colliding particles (graphite silicon powder composites) with each other in a high-speed airflow and compounding the other particles on the surface of one particle, and spheroidization by such an airflow. A "hybridization system" (trade name) manufactured by Nara Machinery Co., Ltd. can be used for this.

The graphite silicon powder composite produced as described above has a structure in which silicon powder is sandwiched between ultrathin graphite sheets having a thickness of 15 nm or less (more preferably, only the front surface and the back surface of the silicon powder). It has a structure in which the side surface is covered with an ultra-thin graphite sheet (also referred to as an encapsulated structure or a wrapped structure), and may be hereinafter referred to as an ultra-thin graphite sheet / silicon powder composite. When the silicon powder is sandwiched or wrapped in a thin graphite sheet, it is prevented that the silicon powder separates from the ultrathin graphite sheet and diffuses into the electrolytic solution even if the volume of silicon changes due to charging and discharging, and the machine also works. Since the graphite is also improved, the cycle characteristics when used as the negative electrode active material of the secondary battery are improved. Further, even if the weight of graphite in the negative electrode active material is reduced in order to improve the theoretical capacity, the conductivity is improved.
Further, the spherical graphite silicon powder complex is a complex in which silicon powder is sandwiched or encapsulated in a layer by a graphite sheet and headed. The mode diameter of the spherical graphite silicon powder complex is, for example, 10 nm or more, preferably 20 nm or more, more preferably 30 nm or more, and for example, 2 μm or less, preferably 1 μm or less, more preferably 500 nm or less.

The negative electrode active material of the present invention may also contain soft carbon, hard carbon and the like, but preferably does not contain at least one (preferably all) of these or precursors thereof. In order to contain soft carbon and hard carbon, it is necessary to calcin a mixture of their precursors (particularly hard carbon precursors), silicon and graphite, which complicates the production of the negative electrode active material.

Note that soft carbon is a carbide of an easily graphitized material, and examples of the easily graphitized material (soft carbon precursor) include coal-based pitch, petroleum-based pitch, mesophase pitch, coke, and low-molecular-weight heavy oil. Can be mentioned. Hard carbon is a carbide of a refractory material, and examples of the refractory material (hard carbon precursor) include phenol resin, epoxy resin, melamine resin, urea resin, aniline resin, cyanate resin, furan resin, and the like. Thermocurable resins such as ketone resins, unsaturated polyester resins and urethane resins; thermoplastic resins such as polyolefins, styrene resins, AS resins, ABS resins, polyesters, polycarbonates, polyacetals and polyethers are included.

2. Negative electrode The negative electrode active material (graphite silicon powder composite) is used as a negative electrode material composition together with a binder and the like, and by laminating the negative electrode material composition on the current collector surface, a non-aqueous secondary battery such as a lithium ion battery is used. Can be used as the negative electrode of. When the negative electrode active material (graphite silicon powder composite) of the present invention is used, graphite that does not sandwich or encapsulate silicon aggregates during drying to form a thick graphite sheet, which functions as a wall or pillar and is a negative electrode. Increase the mechanical strength of the material layer.

The thickness of the aggregated graphite sheet that functions as a wall or pillar material is, for example, 50 nm or more, preferably 60 nm or more, more preferably 70 nm or more, and for example, 3 μm or less, preferably 1 μm or less, more preferably 500 nm or less. More preferably, it is 200 nm or less.

A graphite sheet having a thickness of 1 to 200 nm (preferably a graphite sheet having a thickness of 3 to 150 nm, more preferably a graphite sheet having a thickness of 5 to 100 nm) containing the ultrathin graphite sheet and an agglomerated graphite sheet that functions as a wall or pillar material. , It is preferable that the negative electrode material layer is inserted every 0.2 to 5 μm (preferably every 0.5 to 4 μm, more preferably every 0.8 to 3 μm) in the cross section.

Cracks may be present on the surface of the negative electrode material layer. Even if there are cracks on the surface of the negative electrode material layer, it is possible to exhibit excellent electrical characteristics by cross-linking the cracks with the ultrathin graphite sheet.

The binder constituting the negative electrode material composition may be, for example, a water-insoluble polymer or a water-soluble polymer (thickener). Examples of the water-insoluble polymer include synthetic rubbers such as styrene-butadiene rubber, isoprene rubber, and ethylene-propylene rubber; styrene-butadiene-styrene block copolymer or its hydrogen additive, styrene-ethylene-butadiene, and styrene copolymer weight. Thermoplastic elastomers such as coalesced, styrene / isoprene, styrene block copolymers or hydrogenated products thereof; syndiotactic-1,2-polybutadiene, ethylene / vinyl acetate copolymers, ethylene and α- with 3 to 12 carbon atoms. Soft resins such as copolymers with olefins; thermoplastic resins such as polyethylene, polypropylene, polyethylene terephthalate and aromatic polyamide; fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride can be mentioned. Examples of the water-soluble polymer include polyacrylic acid, polyvinyl alcohol, and carboxymethyl cellulose. These binders may be used alone or in combination of two or more.
The amount of the binder is, for example, 5 parts by mass or more, preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and for example, 100 parts by mass or less, preferably 100 parts by mass, based on 100 parts by mass of the negative electrode active material. It is 70 parts by mass or less, more preferably 50 parts by mass or less.

The negative electrode material composition may contain a conductive auxiliary agent in addition to the negative electrode active material and the binder. Conductive auxiliaries are useful in mediating electrical bonding between negative electrode active materials and can reduce the internal resistance of the electrodes. Examples of the conductive auxiliary agent include carbon materials such as carbon black, acetylene black, ketjen black, carbon nanofibers, and carbon nanotubes. These conductive auxiliaries may be used alone or in combination of two or more.
The amount of the conductive auxiliary agent is, for example, 0 parts by mass or more, preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and for example, 100 parts by mass or less, preferably 100 parts by mass, based on 100 parts by mass of the negative electrode active material. It is 70 parts by mass or less, more preferably 50 parts by mass or less.

The negative electrode material composition can be dispersed in water, an organic solvent, or the like to form a paste. The electrode material layer can be formed by applying this paste to the surface of the current collector and drying it. Examples of the organic solvent include isopropyl alcohol, N-methylpyrrolidone, dimethylformamide and the like, and these may be one kind or a combination of two or more kinds.

As the current collector, copper, a copper alloy, a metal such as stainless steel, nickel, titanium, carbon, or the like can be used. These current collectors may be in the form of a film (such as a metal foil) or may have a three-dimensional structure. As a current collector having a three-dimensional structure, plain woven wire mesh, lath net, expanded metal, punching metal, metal foam, metal woven fabric, metal non-woven fabric, carbon fiber woven fabric, carbon fiber non-woven fabric, and carbon-coated porous material. Examples include a material (sponge, etc.) and porous graphite.

3. 3. Lithium-ion secondary battery The negative electrode can be combined with a positive electrode, an electrolyte, a separator, or the like to form a non-aqueous secondary battery (preferably a lithium-ion secondary battery).

The positive electrode is formed by forming a positive electrode material layer containing a positive electrode active material and a binder on the surface of the current collector, and the binder, the current collector and the like can be the same as those of the negative electrode. In the case of a lithium ion secondary battery, the positive electrode active material is, for example, a metal oxide such as chromium oxide, titanium oxide, cobalt oxide, vanadium pentoxide; LiCoO ₂ , LiNiO ₂ , LiNi _1-x Co _x O ₂ , LiNi 1 _{-x-yCo x Al y O 2} , LiMnO 2, LiMn 2 O 4, LiFeO 2, LiFePO 4 lithium-containing composite metal oxides such as titanium sulfide, chalcogen compounds of transition metals such as molybdenum sulfide, polyacetylene, polyparaphenylene , Conductive polymers such as polypyrrole, and the like.

In the case of a lithium ion secondary battery, the electrolyte may be an electrolyte solution composed of a lithium salt and a solvent, a solid electrolyte, a gel electrolyte, and a hybrid of the electrolyte solution and the solid electrolyte. Electrolyte is preferred. Lithium salts of the electrolytic solution include LiCl, LiAlCl ₄ , LiClO ₄ , LiBr, LiSiF ₆ , LiPF _{6 , LiAsF 6} , LiBF ₄ , LiB (C ₆ H ₅ ), LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , LiN ( CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ CH ₂ OSO ₂ ) ₂ , LiN (CF ₃ CF ₃ OSO ₂ ) ₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN (CF ₃ ) ₂ CHOSO ₂₂ , LiBC ₆ H ₃ (CF ₃ ) ₂₄ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) _{3, and the} like.

Solvents for the electrolytic solution include carbonate esters such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and fluoroethylene carbonate, carboxylic acid esters such as γ-butyrolactone, ethyl acetate and trimethyl orthoformate, and 1,1-dimethoxyethane. , 1,2-Dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, ethers such as diethyl ether, sulfolane, methyl Thioethers such as sulfolane and tetrahydrothiophene, sulfoxide compounds such as dimethylsulfoxide, nitrile compounds such as acetonitrile, chloronitrile and propionitrile, nitro compounds such as nitromethane and nitrobenzene, amide compounds such as dimethylformamide and N-methylpyrrolidone, benzoyl chloride. , Haroxides such as benzoyl bromide, 3-methyl-2-oxazoline, trimethylborate, tetramethyl silicate and the like. One type of these solvents may be used, or two or more types may be combined.

As the separator of the lithium ion secondary battery, a known separator can be appropriately used, and a polyolefin-based porous sheet manufactured from polyethylene, polypropylene or the like is preferable.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples as well as the present invention, and appropriate modifications are made to the extent that it can be adapted to the gist of the above and the following. Of course, it is possible to carry out, and all of them are included in the technical scope of the present invention.

Production Example 1 <Silicon powder>
Silicon chips washed with water were crushed with a grinder (manufactured by Ishikawa Factory Co., Ltd., model 20D), and then further crushed with a mortar to obtain silicon powder. The particle size of the obtained silicon powder was measured using a dynamic light scattering type particle size measuring device (ELSZ-1000S manufactured by Otsuka Electronics Co., Ltd.). The measurement result of the particle size is shown in FIG. 1-1. X-ray diffraction of silicon powder Using an X-ray diffractometer (manufactured by Rigaku Co., Ltd., fully automatic multipurpose X-ray diffractometer SmartLab), the X-ray diffraction pattern is measured, and the obtained peak shape is determined by software (manufactured by Rigaku Co., Ltd., The particle size distribution of the silicon powder was evaluated by analysis using CSDA). These results are shown in Figure 1-2. Further, the silicon powder was photographed with a transmission electron microscope (JEM-ARM200F manufactured by JEOL Ltd.), and the transmission electron microscope photograph thereof is shown in FIG.
As shown in FIG. 1-1, D50 showing a cumulative 50% of the silicon powder on a volume basis was 72.9 nm in the volume particle size distribution. In the X-ray diffraction pattern (FIG. 1-2 (a)), a peak attributed to the silicon crystal was observed, and when the crystallite size was calculated from the full width at half maximum of the peak using the Scherrer equation, Si (111) and Si ( In 220) and Si (311), it was 19.3 nm, 15.0 nm, and 13.3 nm. Since the crystallite size differs depending on the crystal orientation, it can be seen that the shape of the Si powder is anisotropic. Further, in the particle size distribution from the peak shape attributed to Si (111) (FIG. 1-2 (b)), D50, which has a mode diameter of 15 nm, which is the mode, and shows a cumulative 50% on a number basis, is It was 18.7 nm. Further, as shown in FIG. 2, the silicon powder is in the form of flakes and has a major axis in the plane direction of about 5 to 150 nm ((a) in FIG. 2) and a major axis in the plane direction of about 0.6 to. There are two types, one with a thickness of about 1 μm ((b) in FIG. 2), and the thickness of the silicon powder having a major axis in the plane direction of about 5 to 150 nm is 3.5 to 10 nm ((c) in FIG. 2). ))Met.

Production Example 2 <Ultra-thin graphite sheet dispersion>
0.2 g of expanded graphite (manufactured by Ito Graphite Industry Co., Ltd., EC-10) in 300 ml of N-methyl-2-pyrrolidone solvent using an ultrasonic cleaner (manufactured by SND Co., Ltd., US-2, 120 W) 20 The expanded graphite was delaminated by time treatment to obtain an ultrathin graphite sheet dispersion. The ultrathin graphite sheet in the dispersion was photographed with a transmission electron microscope (JEM-ARM200F, manufactured by JEOL Ltd.), and the transmission electron microscope photographs thereof are shown in FIGS. 3 and 4. As shown in FIG. 3, the major axis of the ultrathin graphite sheet in the plane direction was 0.4 to 13 μm, and as shown in FIG. 4, the thickness was 1 to 14 nm.

Comparative Production Example 1 <Graphite Sheet Dispersion Solution>
Expanded by treating 0.2 g of expanded graphite (manufactured by Ito Graphite Industry Co., Ltd., EC-10) in 300 ml of ethanol solvent using an ultrasonic cleaner (manufactured by SND Co., Ltd., US-2, 120 W) for 20 hours. Graphite was delaminated to obtain a graphite sheet dispersion. A scanning electron micrograph (JSM-6335F, manufactured by JEOL Ltd.) of the dispersed expanded graphite sheet is scanned in FIG. 5, and an atomic force microscope image (VN-8010, manufactured by Keyence Ltd.) is scanned in FIG. A scanning electron micrograph (manufactured by JEOL Ltd., by JSM-6335F) is shown in FIG. From FIGS. 5 and 6, the thickness of the graphite sheet was 16 to 28 nm. From FIG. 7, the major axis of the graphite sheet was 17 μm. The roundness of the edge of the graphite sheet as seen in the thin graphite sheet was not observed, and it was in the shape of a thin plate.

Example 1 <Negative electrode active material 1: Ultrathin graphite sheet / silicon powder complex>
0.2 g of the silicon powder obtained in Production Example 1 was dispersed in an N-methyl-2-pyrrolidone solvent using an ultrasonic cleaner (manufactured by SND Co., Ltd., US-2, 120 W) for 1 hour or more. A silicon powder dispersion was obtained. To this silicon powder dispersion, the entire amount of the ultrathin graphite sheet dispersion obtained in the same manner as in Production Example 2 was added, and ultrasonic waves were further irradiated for 1 hour or more to obtain an ultrathin graphite sheet / silicon powder dispersion. This graphite sheet / silicon powder dispersion is filtered through a suction filter equipped with a PTFE membrane filter (F-3030-013 manufactured by Flon Industries, Ltd.), and the ultrathin graphite sheet collected on the PTFE membrane filter. The silicon powder composite was dried in the air at 80 ° C. for 6 hours to obtain a disk-shaped graphite sheet-silicon powder composite (negative electrode active material 1) (ultra-thin graphite sheet: silicon = 1: 1 (mass ratio)). ..

Comparative Example 1 <Comparative Negative Electrode Active Material 1: Graphite Sheet / Silicon Powder Complex>
0.2 g of the silicon powder obtained in Production Example 1 was dispersed in ethanol using an ultrasonic cleaner (manufactured by SND Co., Ltd., US-2, 120 W) for 1 hour or more to obtain a silicon powder dispersion. .. To this silicon powder dispersion, the entire amount of the graphite sheet dispersion obtained in the same manner as in Comparative Production Example 1 was added, and ultrasonic waves were further irradiated for 1 hour or more to obtain an ultrathin graphite sheet / silicon powder dispersion. This graphite sheet / silicon powder dispersion is filtered by a suction filter equipped with a PTFE membrane filter (F-3030-013 manufactured by Flon Industries, Ltd.), and the graphite sheet / silicon powder collected on the PTFE membrane filter. The composite was dried in the air at 80 ° C. for 6 hours to obtain a disk-shaped graphite sheet-silicon powder composite (comparative negative electrode active material 1) (ultra-thin graphite sheet: silicon = 1: 1 (mass ratio)).
A scanning electron micrograph (according to JSM-6335F, manufactured by JEOL Ltd.) of this graphite sheet-silicon powder composite (comparative negative electrode active material 1) is shown in FIG. In FIG. 8, the white or light gray in the frame (V) is a silicon powder whose outline is clearly observed and which is present on the graphite sheet. The white or light gray color in the frame (W) is a silicon powder whose outline is unclearly observed and exists between the graphite sheets. As can be seen from this scanning electron micrograph, the silicon powder was inserted between the graphite sheets in a dispersed state.

Example 2 <Negative electrode active material 2: Ultrathin graphite sheet / silicon powder complex>
The 1 g of silicon powder obtained in Production Example 1 was dispersed in 300 ml of N-methyl-2-pyrrolidone solvent using an ultrasonic cleaner (manufactured by SND Co., Ltd., US-2, 120 W) for 1 hour or more. A silicon powder dispersion was obtained. To this silicon powder dispersion, the entire amount of the ultrathin graphite sheet dispersion obtained in the same manner as in Production Example 2 was added, and ultrasonic waves were further irradiated for 1 hour or more to obtain an ultrathin graphite sheet / silicon powder dispersion. This ultra-thin graphite sheet / silicon powder dispersion is filtered by a suction filter equipped with a PTFE membrane filter (F-3030-013 manufactured by Flon Industries, Ltd.), and the ultra-thin graphite collected on the PTFE membrane filter. The sheet / silicon powder composite is dried in the air at 80 ° C. for 6 hours to form a disk-shaped ultrathin graphite sheet / silicon powder composite (negative electrode active material 2) (ultrathin graphite sheet: silicon = 1: 5 (mass ratio)). ) Was obtained.
This ultrathin graphite sheet-silicon powder composite (negative electrode active material 2) was photographed with a scanning transmission electron microscope (JEM-ARM200F, manufactured by JEOL Ltd.), and further, an element mapping image was taken with an energy dispersive X-ray analyzer. Also got. FIG. 9 (a) is a scanning transmission electron microscope photograph taken, FIG. 9 (b) is an elemental mapping of silicon by an energy dispersive X-ray analyzer in this region, and FIG. 9 (c) is carbon. Elemental mapping of. Flake-shaped silicon particles were inserted between the ultra-thin graphite sheets in a dispersed state.

Example 3 <Negative electrode active material 3: Spheroidized graphite silicon powder complex>
The disk-shaped ultrathin graphite sheet / silicon powder composite obtained in Example 2 is subjected to a high-speed airflow impact method powder surface modifier (Nara Machinery Co., Ltd., hybridization system (model number: NHS-0)). And treated at a rotor rotation speed of 6800 rpm for 3 minutes to make an ultrathin graphite sheet / silicon powder composite spherical. The obtained spherical graphite silicon powder composite (negative electrode active material 2) was photographed with a scanning electron microscope (JSM-6335F, manufactured by JEOL Ltd.) (FIG. 10). The particle size of the spherical graphite silicon powder complex was mainly 50 to 150 nm as shown in FIG.

Comparative Example 2 <Comparative Negative Electrode Active Material 2: Carbon Coated Silicon Powder>
The silicon powder obtained in Production Example 1 was heated to 1000 ° C. in a rotating kiln in a 100% hydrogen (purity 99.95%) atmosphere under atmospheric pressure, and then 100% ethylene (purity 99.95%) under atmospheric pressure. The atmosphere was changed to a purity of 99.5%), and the mixture was cooled to room temperature in a 100% hydrogen atmosphere to prepare an amorphous carbon-coated silicon powder having a weight ratio of silicon powder to carbon film of 10: 1. When the obtained carbon-coated silicon powder (comparative negative electrode active material 2) was observed with a transmission electron microscope (JEM-ARM200F, manufactured by JEOL Ltd.), the thickness of the carbon film was uniform at 3 to 4 nm.

<First evaluation>
The states and characteristics of the negative electrode active materials obtained in Examples 1 to 3 and Comparative Examples 1 and 2 were evaluated as follows. Further, the silicon powder of Production Example 1 (comparative negative electrode active material 3) was used as the negative electrode active material and evaluated in the same manner.

1. 1. Coating slurry To 50 mg of the negative electrode active material, Ketjen black as a conductive auxiliary agent, a 2 wt% aqueous solution of polyacrylic acid as a binder, and a 4 wt% aqueous solution of polyvinyl alcohol as a binder were added to a dairy pot. To prepare a coating slurry in which the negative electrode active material, Ketjen black, polyacrylic acid and polyvinyl alcohol were blended so as to have a weight ratio of 6: 2: 1: 1.

2. The slurry for negative electrode coating was applied onto a rolled copper foil with an applicator, temporarily dried at room temperature, and punched into a circle having a diameter of 11.3 mm using an electrode punch to obtain a negative electrode. After vacuum drying this negative electrode, the weight was measured with an electronic balance, and it was confirmed that the electrode weight after subtracting the copper foil weight was 1.08 mg. Next, the negative electrode was heated and dried at 150 ° C. in vacuum for 6 hours.

3. 3. A CR2032 type coin battery was assembled using the vacuum-heated and dried negative electrode, a lithium foil having a diameter of 13 mm as a counter electrode, a vacuum-heated and dried polyethylene separator, and an electrolytic solution in a glove box having a half-cell argon atmosphere. As the electrolytic solution, 1M ethylene carbonate of lithium hexafluorophosphate to which 10% by mass of fluoroethylene carbonate was added: diethyl carbonate = 1: 1 electrolytic solution was used.

4. Cell characteristics After leaving the half cell obtained as described above for 12 hours, charge / discharge measurement was performed. In the 1st to 5th cycles, the crystalline silicon powder is fully amorphized at 0.05C (charge / discharge rate of full charge or full discharge in 20 hours), and in the 6th and subsequent cycles, 0.5C (full charge in 2 hours). Alternatively, charging / discharging was performed at a charging / discharging rate of (charge / discharging rate for full discharge). The cell voltage range was 0.01 to 1.5V.
FIG. 11 shows the relationship between the number of cycles and the discharge capacity in the half cell using the negative electrode active material of Example 1 and Comparative Example 1. The relationship between (a) to (c) in the figure and the negative electrode active material is as follows.
(A) Comparative negative electrode active material 1 (graphite sheet / silicon powder composite) (ethanol dispersion) (graphite sheet: silicon = 1: 1 (mass ratio))
(B) Negative electrode active material 1 (ultra-thin graphite sheet / silicon powder composite) (N-methyl-2-pyrrolidone solvent dispersion) (ultra-thin graphite sheet: silicon = 1: 1 (mass ratio))
(C) Comparative negative electrode active material 3 (silicon powder)
As shown in FIG. 11, the discharge capacities at the 100th cycle were 594 mAh / g for (a) (comparative negative electrode active material 1), 801 mAh / g for (b) (negative electrode active material 1), and (c) ( The comparative negative electrode active material 3) was 53 mAh / g. The discharge capacity of (a) (comparative negative electrode active material 1) is 1.6 times the theoretical capacity (372 mAh / g) of the graphite electrode, and the discharge capacity of (b) (negative electrode active material 1) is the theoretical capacity of the graphite electrode. It was 2.15 times that of. It is considered that when a thinner graphite sheet is used, the conductivity is improved by increasing the number of graphite sheets per unit weight, and the graphite sheet further encloses the silicon powder by increasing the flexibility.

FIG. 12 shows the relationship between the number of cycles and the charge capacity in the half cell using the negative electrode active materials of Examples 2 and 3 and Comparative Examples 2 and 3, and FIG. 13 shows the relationship between the number of cycles and the discharge capacity. The relationship between (a), (b), (c), and (d) in the figure and the negative electrode active material is as follows.
(A) Negative electrode active material 2 (Example 2. Ultra-thin graphite sheet / silicon powder composite) (Ultra-thin graphite sheet: silicon = 1: 5 (mass ratio))
(B) Negative electrode active material 3 (Example 3. Spherical ultrathin graphite sheet / silicon powder complex)
(C) Comparative negative electrode active material 2 (Comparative example 2. Carbon-coated silicon powder)
(D) Comparative negative electrode active material 3 (silicon powder)
The theoretical capacity per unit weight of the active material is 3040 mAh / g for the negative electrode active material 2 and the negative electrode active material 3, 3257 mAh / g for the comparative negative electrode active material 2, and 3578 mAh / g for the comparative negative electrode active material 3.
As shown in FIGS. 12 and 13, the charge / discharge capacities at the 300th cycle were 1273 mAh / g for (a) (negative electrode active material 2), 1141 mAh / g for (b) (negative electrode active material 3), and (c). ) (Comparative negative electrode active material 2) was 1114 mAh / g, and (d) (Comparative negative electrode active material 3) was 50 mAh / g. The discharge capacity of (a) (negative electrode active material 2) was 3.4 times the theoretical capacity (372 mAh / g) of the graphite electrode, and 1.14 times that of (c) (comparative negative electrode active material 2). The negative electrode active material 1 had the highest capacity in most cycles, and the comparative negative electrode active material 2 had a larger capacity decrease from the 230th cycle than the negative electrode active material 2 and the negative electrode active material 3.

5. State of Electrode Active Material in Electrode A scanning electron microscope (Nippon Denshi Co., Ltd.) shows the surface and cross section of the electrode obtained in the above "2. Negative electrode" when the negative electrode active material 2 of Example 2 is used as the negative electrode active material. A scanning electron micrograph obtained by observing with JSM-6335F manufactured by JSM-6335F is shown in FIG. 14 (FIGS. 14 (a) and 14 (b) are surface photographs, and FIG. 14 (c) is a cross-sectional photograph. FIG. 14 (d) is a conceptual diagram showing the relationship between FIGS. 14 (a), (b), and (c)), an energy-distributed X-ray analyzer (manufactured by Nippon Denshi Co., Ltd., JSM-6335F). The element mapping photograph measured by the above is shown in FIG. 15, and the transmission electron micrograph obtained by observing with a transmission electron microscope (manufactured by Nippon Denshi Co., Ltd., JEM-ARM200F) is shown in FIG.
In the scanning electron micrograph of FIG. 14, thin films, dark thin lines and flat surfaces (indicated by X in the figure) are on graphite sheets, bright angular particles (indicated by Y in the figure) are on silicon powder, and dark agglomerated particles (indicated by Y in the figure). (Indicated by Z) corresponds to the conductive auxiliary agent and the binder. In FIG. 14 (a), the electrode surface was covered with the graphite sheet (X), and the silicon powder (Y) was sandwiched between the graphite sheets (X). In FIG. 14 (b), as shown by the arrows, A graphite sheet (X) thinner than 10 nm was crosslinked at the cracks on the electrode surface. In FIG. 14C, by observing the cross section of the electrode, a graphite sheet (X) having a thickness of 8 to 80 nm was inserted every 1 to 2 μm. As shown in FIG. 14, the negative electrode active material 2 is sandwiched between graphite sheets or crosslinked with a graphite sheet in the electrode, and it is considered that the conductivity and mechanical strength of the electrode are improved.
As shown in FIG. 16, the silicon powder (Y) was encapsulated in the ultrathin graphite sheet (X), and the conductive auxiliary agent (Z) entered the gap.

<Second evaluation>
The characteristics of the negative electrode active material 2 obtained in Example 2 were evaluated as follows.
1. 1. Charging / Discharging Characteristics Using the negative electrode active material 2 (ultra-thin graphite sheet / silicon powder complex) of Example 2 as the negative electrode active material, a full cell was prepared by the following procedure.
First, a negative electrode was produced in the same manner as in the first evaluation.
On the other hand, a conductive additive (acetylene black and carbon nanofiber) and a binder (2% by weight aqueous solution of carboxymethyl cellulose and 40% by weight aqueous solution of styrene-butadiene rubber) were added to 2.5 g of the _{positive electrode active material LiFePO 4 to rotate.} -Mix with a revolving mixer (Sinky Co., Ltd., AR-100) to prepare a coating slurry in which the weight ratio of the positive electrode active material, the conductive auxiliary agent and the binder is 85:10: 5. did. The coating slurry was applied onto rolled aluminum foil with an applicator, temporarily dried at room temperature, and punched into a circle having a diameter of 11.3 mm using an electrode punch to obtain a positive electrode. After vacuum drying this positive electrode, the weight was measured with an electronic balance, and it was confirmed that the electrode weight after subtracting the aluminum foil weight was 2 mg. Next, the positive electrode was heated and dried in vacuum at 150 ° C. for 6 hours.
A CR2032 type coin battery was assembled using the vacuum-heat-dried negative electrode and positive electrode, the vacuum-heat-dried polyethylene separator, and the electrolytic solution in a glove box having an argon atmosphere. As the electrolytic solution, 1M ethylene carbonate of lithium hexafluorophosphate to which 10% by mass of fluoroethylene carbonate was added: diethyl carbonate = 1: 1 electrolytic solution was used.
It was fully charged until the cell voltage reached 3.46 V, the discharge capacity was limited to 1300 mAh / g, and charging and discharging were repeated. In charging / discharging, the cell voltage upper limit and the discharge capacity were set with the intention that the volume change of silicon would be sufficiently small. The relationship between the number of cycles and the discharge capacity is shown in FIG. By limiting the discharge capacity, as is clear from FIG. 17, the discharge capacity of 1300 mAh / g can be maintained for 700 cycles or more, and the cell life can be remarkably improved.

<Third evaluation>
The negative electrode active material 2 (ultra-thin graphite sheet / silicon powder composite) (ultra-thin graphite sheet: silicon = 1: 5 (mass ratio)) of Example 2 after charging / discharging for 300 cycles in the first evaluation. FIG. 18A shows a scanning electron micrograph obtained by observing the surface of the electrode used with a scanning electron microscope (JSM-6335F, manufactured by Nippon Denshi Co., Ltd.). In FIG. 18 (a), cracks due to the volume change of silicon were observed, but no isolated islands were observed. In FIG. 18 (b), which is an enlarged view of this crack region, a graphite sheet (X) covering the electrode surface and a region (Y + Z) in which silicon powder and a conductive auxiliary agent coexist were observed even after 300 cycles. Furthermore, although the electrodes were cracked due to the change in the volume of silicon, a structure was observed in which the cracks were crosslinked at multiple points indicated by the arrows. As shown in FIG. 18 (c), which is a further enlarged view of the crosslinked portion indicated by the thick arrow, an ultrathin graphite sheet (X) to which silicon powder (Y), a conductive auxiliary agent, or the like was attached was observed. It is considered that this crosslinked structure improves the conductivity and mechanical strength of the electrode. Further, on the surface of the crack, a structure in which the silicon powder (Y) was sandwiched or encapsulated between the ultrathin graphite sheets (X) was observed. It is considered that this structure improves the conductivity of the electrode and suppresses the peeling of silicon. Further, a scanning transmission electron microscope photograph obtained by ultrasonically dispersing the electrodes in ethanol and observing with a scanning transmission electron microscope (JEM-ARM200F manufactured by JEOL Ltd.) is shown in FIG. 19 (a). An elemental mapping image of silicon by an energy dispersive X-ray analyzer in the region is shown in FIG. 19 (b), and a carbon mapping image is shown in FIG. 19 (c). No significant agglomeration of the silicon and graphite sheets was observed, and the silicon was sandwiched or encapsulated in the ultrathin graphite sheet while maintaining the major axis of 10 nm to 1 μm and the thickness of 30 nm or less through which the electron beam passes. (Structure of graphite silicon powder composite) was observed. According to the structure of the graphite silicon powder complex sandwiched or encapsulated in the ultrathin graphite sheet, it is considered that the peeling of silicon is suppressed and contributes to the improvement of long-term cycle characteristics.
Comparison of Comparative Example 2 after charging and discharging for 300 cycles in the first evaluation Regarding the electrode surface using the negative electrode active material 2 (carbon-coated silicon powder), a scanning electron microscope (manufactured by JEOL Ltd., JSM-6335F) ) Is shown in FIG. 20 as a scanning electron micrograph obtained by observing. Similar to FIG. 19 (a), in FIG. 20 (a), cracks due to the volume change of silicon were observed. Further magnifying and observing the cracked portion, as seen in FIG. 20 (b), unlike the case of the negative electrode active material 2, no crosslinked structure was observed between the cracks, and the cracked portion became an isolated island.

The graphite silicon powder composite of the present invention can be used as a negative electrode active material for a secondary battery.

Claims (15)

A method for producing a graphite-silicon powder composite, which comprises mixing expanded graphite and silicon powder in an aprotic solvent. The method for producing a graphite silicon powder composite according to claim 1, wherein the aprotic solvent is at least one selected from an aromatic solvent, a halogen solvent, and an amide solvent. The method for producing a graphite silicon powder complex according to claim 1 or 2, wherein ultrasonic waves are applied at the time of mixing. The method for producing a graphite silicon powder composite according to any one of claims 1 to 3, wherein the expanded graphite is delaminated in an aprotic solvent to form a sheet having a thickness of 15 nm or less. A method for producing a graphite-silicon powder composite, which comprises mixing a graphite sheet having a thickness of 15 nm or less and silicon powder in a solvent. The method for producing a graphite silicon powder complex according to claim 4 or 5, wherein the major axis in the plane direction of the graphite sheet having a thickness of 15 nm or less is 0.1 μm or more. The method for producing a graphite silicon powder composite according to claim 1 to 6, wherein the silicon powder has at least one property selected from the following (a) to (d).
(A) The cumulative 50% particle size of the silicon powder based on the volume is 300 nm or less. (B) The silicon powder is in the form of flakes. (C) The silicon powder is cut from a crystalline silicon ingot. The powder or a pulverized product thereof (d) The cumulative 50% particle size based on the number of the silicon powders calculated based on the crystallite size distribution from the X-ray diffraction pattern is 3 to 100 nm. The method for producing a graphite silicon powder composite according to any one of claims 1 to 7, wherein the hard carbon or the hard carbon precursor does not coexist when the graphite and the solvent are mixed. A graphite silicon powder complex in which silicon powder is sandwiched between graphite sheets having a thickness of 15 nm or less. A graphite silicon powder complex in which silicon powder is wrapped in a graphite sheet having a thickness of 15 nm or less. The graphite silicon powder complex according to claim 9 or 10, which does not contain at least one selected from hard carbon, soft carbon, and a precursor thereof. A negative electrode in which a negative electrode material layer containing a plurality of graphite silicon powder composites according to any one of claims 9 to 11 is laminated on the surface of a current collector. The negative electrode according to claim 12, wherein the negative electrode material layer contains a plurality of aggregated graphite sheets having a thickness of 50 nm or more, and a plurality of the graphite silicon powder composites are sandwiched between the aggregated graphite sheets. 15. The negative electrode described. The negative electrode according to any one of claims 12 to 14, wherein cracks existing on the surface of the negative electrode material layer are crosslinked with a graphite sheet having a thickness of 15 nm or less.

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