JP6772435B2 - Negative electrode active material for lithium ion secondary batteries and its manufacturing method - Google Patents

Description

The present invention relates to a negative electrode active material for a lithium ion secondary battery and a method for producing the same.

With the increasing performance of mobile devices such as smartphones and tablet terminals and the widespread use of vehicles equipped with lithium-ion secondary batteries such as EVs and PHEVs, there is an increasing demand for higher capacity lithium-ion secondary batteries. Currently, graphite is mainly used as the negative electrode material for lithium-ion secondary batteries, but in order to further increase the capacity, the negative electrode material using silicon particles that have a high theoretical capacity and can occlude and release lithium ions. Development is active.

On the other hand, when lithium is occluded by charging, these silicon particles remarkably expand in volume, and the structure of the negative electrode using these is also destroyed to cut the conductivity. Therefore, there is a problem that the capacity of the negative electrode using silicon particles is remarkably reduced as the cycle progresses.

To solve this problem, a method has been proposed in which these silicon particles are finely pulverized and composited with graphite or the like. In such composite particles, even if the silicon fine particles are alloyed with lithium and expanded, the expansion per particle is small and the conductivity is ensured by graphite, so that these materials are not used alone as the negative electrode material. It is known that the cycle characteristics are significantly improved. For example, Patent Document 1 describes a mixing step of mixing expanded graphite or flaky graphite having a specific surface area of 30 m ² / g or more with a battery active material capable of combining lithium ions to obtain a mixture, and spheroidizing the mixture. A composite negative electrode active material for a lithium secondary battery, which comprises a spheroidizing step of producing a substantially spherical composite negative electrode active material for a lithium secondary battery, which is treated and contains a battery active material capable of combining with graphite and lithium ions. The production method is disclosed, and it is described that the negative electrode active material that can be combined with lithium ions contains at least one element selected from Si, Sn, Al, Sb, and In, and the average particle size is preferably 1 μm or less. ing.

By using such silicon fine particles, the expansion per particle due to lithium insertion during charging is reduced, and the cycle characteristics are good at the start of use, but the silicon fine particles that occlude lithium soften with expansion during charging. , Particles are attached to each other. As a result, there is a problem that the fine particles become coarse, the expansion per particle becomes large, the structure of the composite particle is destroyed, and the cycle characteristics deteriorate.

Japanese Patent No. 5227483

The present invention relates to a negative electrode active material for a lithium ion secondary battery, which is a composite containing silicon-based fine particles, carbon having an amorphous residue and / or fine crystal carbon, and graphite, and has a large discharge capacity. The present invention is to provide a negative electrode active material that provides a lithium ion secondary battery having a long cycle life, and a method for producing the same.

As a result of diligent studies to solve the above problems, the present inventors have an average particle size (D50) of 0.5 μm or less, the surface is doped with one or more metals, and silicon is measured by XRD. Using 10 to 50% by weight of silicon-based fine particles having a single-phase peak pattern, amorphous carbon and / or microcrystalline carbon, and graphite as the balance, a negative electrode active material for a lithium ion secondary battery is used. We have found that a lithium ion secondary battery capable of suppressing mutual adhesion between silicon-based fine particles during charging and having good cycle characteristics can be obtained, and have completed the present invention.

Hereinafter, the negative electrode active material for a lithium ion secondary battery of the present invention will be described in detail.

The silicon-based fine particles referred to in the present invention have one or more metals doped on the surface, have a single-phase silicon peak pattern as measured by XRD, and are a metal that can be combined with lithium contained in the negative electrode active material ( Unlike Si, Sn, Al, etc.) and alloys and metal oxides containing these, it is possible to suppress mutual adhesion between fine particles during charging, and the effect of improving cycle characteristics is great. The metals used for doping include Ag, Al, Bi, Cd, Co, Cr, Cu, Ga, Ge, In, Li, Mg, Mn, Ni, Pb, Sb, Sn, Ti, Ta, V, W, Y, One or more combinations selected from Zn are desirable.

In the negative electrode active material for a lithium ion secondary battery of the present invention, the average particle size of the silicon-based fine particles is 0.5 μm or less, preferably 0.3 μm or less and 0.1 μm or more. If it is larger than 0.5 μm, cracks due to expansion stress occur between the silicon-based fine particles and the carbon precursor, the conductive path is cut off, and cycle deterioration tends to be severe. If it is smaller than 0.1 μm, the initial Coulomb efficiency of charge / discharge may decrease. D50 is a volume average particle diameter measured by a laser diffraction method or a dynamic light scattering method.

The content of the silicon-based fine particles is 10 to 50% by weight, preferably 15 to 40% by weight. When the content of the silicon-based fine particles is less than 10% by weight, a sufficiently large capacity cannot be obtained as compared with the conventional graphite, and when it is larger than 50% by weight, cycle deterioration tends to be severe.

The amorphous carbon and / or microcrystalline carbon referred to in the present invention includes easily graphitized carbon (soft carbon) that is graphitized by heat treatment exceeding 2000 ° C. and non-graphitized carbon (hard carbon) that is difficult to graphitize. ..

As the graphite referred to in the present invention, crystals having a graphene layer parallel to the c-axis, graphene delaminated by ultrasonic waves or the like can be used. Crystals in which the graphene layer is parallel to the c-axis include natural graphite obtained by refining ore, artificial graphite obtained by graphitizing the pitch of petroleum or coal, or graphite layers that are expanded by heat treatment after acid treatment and oxidation treatment. It is desirable that expanded graphite or a crushed product of expanded graphite, which has been partially peeled off to form an accordion, is compressed. The shape is preferably scaly, oval or spherical, columnar or fibrous. Alternatively, the particle size of graphite is not particularly limited as long as it is smaller than the particle size of the negative electrode active material for a lithium ion secondary battery. The composite with graphite enhances the conductivity and strength of the negative electrode active material for the lithium ion secondary battery of the present invention, and improves the charge / discharge rate characteristics and cycle characteristics. The interplanar spacing d002 of the (002) plane measured by X-ray diffraction of the graphite particles is preferably 0.338 nm or less, which means highly graphitized graphite. When d002 exceeds this value, the effect of improving conductivity by graphite becomes small.

The negative electrode active material for a lithium ion secondary battery of the present invention is preferably a composite particle having a rounded shape and an average particle size (D50) of 1 to 40 μm, and particularly preferably 2 to 30 μm. If D50 is less than 1 μm, it becomes bulky and it becomes difficult to produce a high-density electrode, and if it exceeds 40 μm, the unevenness of the applied electrode becomes severe and it becomes difficult to produce a uniform electrode. Further, the average particle size of the silicon-based fine particles is 1/5 or less of the average particle size of the negative electrode active material, and amorphous carbon and / or microcrystalline carbon covers the surface of the negative electrode active material. It is preferable to have.

Composite particles with a rounded shape are those with rounded corners, spherical or spheroidal, disc or oval, thick and rounded corners, or those. Is a deformed one with rounded corners. The rounded shape increases the bulk density of the composite particles and increases the packing density when used as a negative electrode. Further, since the carbon precursor covers the particle surface of the negative electrode active material for the lithium ion secondary battery, the lithium ions solvated in the electrolytic solution during the charging / discharging process are separated from the solvent on the surface of the carbon precursor. Since only lithium ions react with silicon-based fine particles and / or graphite, decomposition products of the solvent are less likely to be produced, and charging / discharging efficiency is increased.

The negative electrode active material for a lithium ion secondary battery of the present invention has a structure in which the silicon-based fine particles are sandwiched between amorphous carbon and / or fine crystalline carbon and a thin graphite layer having a thickness of 0.2 μm or less. It is preferable that the structure is laminated and / or spreads in a mesh pattern, and the graphite thin layer is curved and covered near the surface of the negative electrode active material particles for a lithium ion secondary battery.

The graphite thin layer referred to in the present invention is expanded graphite or expanded graphite which is expanded by heat treatment after acid treatment and oxidation treatment of the above-mentioned graphite, and a part of the graphite layers is peeled off to form an accordion. It is composed of one layer of graphene (thickness 0.0003 μm) to several hundred layers (thickness ~ 0.2 μm) produced by receiving a compressive force on the pulverized product of graphite, graphene delaminated by ultrasonic waves, or the like. The thinner the graphite thin layer, the thinner the silicon-based fine particles sandwiched between the graphite thin layers and the carbon precursor layer, and the better the transfer of electrons to the silicon-based fine particles, resulting in a thickness of 0.2 μm. If it exceeds, the electron transfer effect of the graphite thin layer diminishes. When the graphite thin layer is linear when viewed in cross section, it is preferable for electron transfer that the length is at least half the particle size of the negative electrode active material for a lithium ion secondary battery, and that of the negative electrode active material for a lithium ion secondary battery. It is more preferable that the size is about the same as the particle size. When the graphite thin layer is mesh-like, it is preferable for electron transfer that the graphite thin layer network is connected over half or more of the particle size of the negative electrode active material for the lithium ion secondary battery, and the negative electrode for the lithium ion secondary battery. It is more preferable that the size is about the same as the particle size of the active material.

In the present invention, it is preferable that the graphite thin layer is curved and covered near the particle surface of the negative electrode active material for a lithium ion secondary battery. By making such a shape, the electrolytic solution invades from the graphite thin layer end face, and the silicon-based fine particles and the graphite thin layer end face are in direct contact with the electrolytic solution, and a reactant is formed during charging and discharging, which reduces efficiency. Risk is reduced.

The negative electrode active material for a lithium ion secondary battery of the present invention preferably has a specific surface area of 0.5 to 120 m ² / g by the BET method, and more preferably 0.5 to 50 m ² / g.

Next, a method for producing the negative electrode active material for a lithium ion secondary battery of the present invention will be described.

The method for producing a polar active material for a lithium ion secondary battery of the present invention includes a step of mixing silicon-based fine particles, a carbonaceous substance, and the remaining graphite, a step of compacting, and a step of pulverizing and spheroidizing the composite particles. It includes a step of forming and a step of firing the composite particles in an inert atmosphere.

In the production of silicon-based fine particles, first, silicon raw materials (states such as ingots, wafers, and powders) are crushed with a crusher, and in some cases, a classifier is used to make silicon fine particles having an average particle size (D50) of 0.3 μm or less. Adjust to. The silicon fine particles are used as an alcohol slurry, and the metal salt or metal oxide is adhered to the surface of the silicon fine particles by a method such as dissolving a metal salt in the slurry or dispersing the metal oxide and drying it, and further, nitrogen, argon, etc. are not added. It is produced by reducing it by heating it in a mixed gas such as hydrogen or carbon monoxide in an active gas.

The metals, metal salts and metal oxides used are Ag, Al, Bi, Cd, Co, Cr, Cu, Ga, Ge, In, Li, Mg, Mn, Ni, Pb, Sb, Sn, Ti, Ta and V. , W, Y, Zn, preferably one or more simple substances, salts, oxides, and one or more simple substances, salts selected from Al, Cu, Mg, Ni, Pb, Zn. , Oxides are more preferred.

The carbonaceous material is not particularly limited as long as it is a polymer mainly composed of carbon and becomes amorphous carbon and / or microcrystalline carbon by heat treatment in an inert gas atmosphere. Coal-based pitch, synthetic pitch, tars, cellulose, sculose, polyvinyl chloride, polyvinyl alcohol, phenol resin, furan resin, furfuryl alcohol, polystyrene, epoxy resin, polyacrylonitrile, melamine resin, acrylic resin, polyamideimide resin, polyamide Resin, polystyrene resin, etc. can be used.

The remaining graphite is natural graphite, artificial graphite obtained by graphitizing the pitch of petroleum or coal, or they are acid-treated and oxidized, and then expanded by heat treatment to peel off a part of the graphite layers and accordion. Expanded graphite in the form, a crushed product of expanded graphite, graphene delaminated by ultrasonic waves or the like can be used. The remaining graphite is adjusted to a size that can be used in the mixing process in advance, and the particle size before mixing is 1 to 100 μm for natural graphite or artificial graphite, crushed expanded graphite or expanded graphite, and 5 μm to 5 mm for graphene. Degree.

The mixing of the silicon-based fine particles, the carbonaceous material, and the remaining graphite can be performed by adding the silicon-based fine particles, the carbonaceous material, and the remaining graphite to the solvent, dispersing and mixing them, and then removing the solvent. The solvent used is not particularly limited as long as it can dissolve carbonaceous substances. For example, when pitch and tars are used as carbonaceous substances, quinoline, pyridine, toluene, benzene, tetrahydrofuran, creosote oil and the like can be used, and when polyvinyl chloride is used, tetrahydrofuran, cyclohexanone, nitrobenzene and the like can be used. When a phenol resin or a furan resin is used, ethanol, methanol or the like can be used.

As the mixing device, a combination of a stirring tank and a homomixer, a kneader, a Nauter mixer, a Ladyge mixer, a Henschel mixer, a high speed mixer and the like can be used. After mixing in the solvent with these, the solvent can be removed continuously with one of these devices equipped with a heat drying function, or separately by using a vibration dryer, paddle dryer, thin film evaporator, etc. This is done using a dryer. It is preferable to remove the solvent until the residual ratio of the solvent in the mixture becomes 5% by weight or less.

Consolidation of the mixture with silicon-based fine particles, carbonaceous material, and the rest of graphite is compressed by a compressor such as a roll press or roller compactor until the light bulk density is 400 g / L or more, and then 0.1 to 5 mm by a crusher. It is preferable to crush it into flakes.

As a method for crushing the consolidated product, a dry crusher such as a jet mill, a hammer mill, a pin mill, or a disc mill is used for crushing. Further, in order to adjust the particle size distribution after pulverization, dry classification such as wind power classification and sieving is used. In the type in which the crusher and the classifier are integrated, it is possible to crush and classify at the same time. The target average particle size (D50) after pulverization / classification is 1 to 40 μm.

As a method for spheroidizing the pulverized material, there are a method of passing through a dedicated spheroidizing device and a method of spheroidizing by adjusting the above-mentioned processing time. Dedicated spherical devices include Hosokawa Micron's Faculti (registered trademark), Nobilta (registered trademark), Mechanofusion (registered trademark), Nippon Coke Industries' COMPOSI, Nara Kikai Seisakusho's hybridization system, and EarthTechnica Examples include Cryptron Orb and Cryptron Eddie.

The composite particles obtained by pulverization are fired in an argon gas or nitrogen gas stream, or in an inert atmosphere such as vacuum. The firing temperature is preferably 600 to 1000 ° C. If the firing temperature is less than 600 ° C., the irreversible capacity becomes large due to insufficient sintering of the carbonaceous material, and the cycle characteristics deteriorate, so that the characteristics of the battery tend to deteriorate.

According to the present invention, the expansion volume per particle is reduced by atomizing the average particle size (D50) of the silicon-based fine particles to 0.5 μm or less, and the mutual adhesion between the silicon-based fine particles is suppressed by metal doping on the surface. Lithium ion secondary with excellent cycle characteristics by securing a conductive path by combining silicon-based fine particles with amorphous carbon and / or microcrystalline carbon or amorphous carbon and / or microcrystalline carbon and graphite. A negative electrode active material for a battery can be obtained.

It is a secondary electron image by FE-SEM of the negative electrode active material particle cross section for a lithium ion secondary battery obtained in Example 1. It is the XRD measurement result of the silicon-based fine particles and silicon fine particles described in Example 1, Comparative Examples 1 and 2, and Reference Example 1. It is an SEM image of the negative electrode surface after the charge / discharge test of Reference Example 1 and Comparative Example 2.

Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

Example 1
"Preparation of silicon-based fine particles whose surface is doped with metal"
20% by weight of metallic silicon with a purity of 3.5N is mixed with ethanol, wet pulverized using a zirconia media bead mill, and a silicon fine particle slurry having an average particle size (D50) of 0.16 μm on a laser diffraction type particle size distribution meter. Got A copper nitrate ethanol solution was added to this silicon fine particle slurry so that the amount of Cu added to silicon was 4% by weight, dried at 120 ° C. to remove ethanol, and then in a hydrogen / nitrogen mixed atmosphere for 1 hour at 600 ° C. It was calcined to obtain silicon-based fine particles. When these fine particles were XRD-measured, a diffraction pattern of only crystalline silicon was obtained.

"Preparation of negative electrode active material for lithium ion secondary batteries"
Natural graphite having a particle diameter of about 0.5 mm (width in the (200) plane direction) and a thickness of about 0.02 mm was immersed in concentrated sulfuric acid containing 1% by weight of sodium nitrate and 7% by weight of potassium permanganate for 24 hours. Then, it was washed with water and dried to obtain acid-treated graphite. This acid-treated graphite was passed through a mullite tube at 850 ° C. through which nitrogen gas was passed, and the expanded graphite was collected. The width of expanded graphite in the (200) plane direction was about 0.5 mm, which was the same as the original graphite value, but the thickness was about 4 mm, which expanded about 200 times, and the appearance was coiled. Graphite was observed by SEM. The layer was peeled off and it was confirmed that it was accordion-like.

60 g of silicon-based fine particles as an ethanol slurry at a slurry concentration of 20%, 24 g of the expanded graphite, 10 g of a resole-type phenol resin as a carbonaceous substance, and 1 L of ethanol are placed in a stirring container and mixed and stirred with a homomixer for 1 hour. did. Then, the solvent was removed from the mixed solution with a rotary evaporator at 60 ° C. under reduced pressure to obtain about 40 g of a mixed dried product (light bulk density 80 g / L).

This mixed dried product was compacted to a light bulk density of 440 g / L with a 3-roll mill.

Next, this compacted product was pulverized and sphericalized with a new power mill to obtain a spherical powder having a light bulk density of 650 g / L. The obtained powder was calcined in a tube furnace in a nitrogen gas atmosphere at 900 ° C. for 1 hour. Then, it was passed through a sieve having an opening of 45 μm to obtain a negative electrode active material for a lithium ion secondary battery of the present invention having an average particle diameter (D50) of 20 μm and a light bulk density of 810 g / L.

FIG. 1 shows a secondary electron image of a cross section of the negative electrode active material for a lithium ion secondary battery of the present invention cut with an Ar ion beam by FE-SEM. Inside the particles, a structure in which silicon-based fine particles were sandwiched between carbon precursors and graphite thin layers (11) having a thickness of 0.2 μm or less (12) spread like a mesh and were laminated. Amorphous carbon and / or microcrystalline carbon adhered to and covered the silicon-based fine particles, and the graphite thin layer (11) was curved to cover the particles in the vicinity of the particle surface of the negative electrode active material. Further, the specific surface area of the negative electrode active material by the BET method was 18 m ² / g. In addition, the XRD measurement showed that the interplanetary spacing d002 of the (002) plane of graphite was 0.336 nm, and very broad diffraction lines derived from amorphous carbon and / or microcrystalline carbon and amorphous carbon were also observed. Was done. A lithium ion secondary battery using the negative electrode active material for the lithium ion secondary battery of the present invention was produced as follows.

"Manufacturing negative electrodes for lithium-ion secondary batteries"
The obtained negative electrode active material for a lithium ion secondary battery was 95.5% by weight (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive auxiliary agent, and 0.5% by weight as a binder. A slurry containing a negative electrode mixture was prepared by mixing 1.5% by weight of carboxymethyl cellulose (CMC), 2.5% by weight of styrene butadiene rubber (SBR), and water.

The obtained slurry was applied to a copper foil with a solid content coating amount of 3 mg / cm ² and dried in the air at 110 ° C. for 0.5 hours. After drying, it was punched into a circle of 14 mmφ, uniaxially pressed at a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to obtain a negative electrode for a lithium ion secondary battery.

"Preparation of evaluation cell"
The evaluation cell is produced by dipping the negative electrode, the polypropylene separator, the glass filter, the counter electrode, metallic lithium, and the stainless foil of the base material into the screw cell in the glove box, and then laminating them in this order. did. As the electrolytic solution, ethylene carbonate and diethyl carbonate were mixed in a volume ratio of 1: 1, LiPF6 was added to a concentration of 1.2 mol / L, and fluoroethylene carbonate was further added in an amount of 2% by volume.

"Evaluation conditions"
The evaluation cell was cycle tested in a constant temperature room at 25 ° C. Charging was carried out at a constant current of 2 mA to 0.01 V, and then at a constant voltage of 0.01 V until the current value reached 0.2 mA. Further, the discharge was performed up to a voltage value of 1.5 V with a constant current of 2 mA. The cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after the charge / discharge test 50 times under the charge / discharge conditions with the initial discharge capacity.

The results of the cell evaluation were an initial discharge capacity of 615 mAh / g and a 50th discharge capacity of 567 mAh / g, and the capacity retention rate was as high as 91%, which was good.

Comparative Example 1
A negative electrode active material for a lithium ion secondary battery, a negative electrode, and an evaluation cell were prepared in this order by the same method as in Example 1 except that the surface of the silicon fine particles was not doped with metal, and the cells were evaluated.

The results of the cell evaluation were an initial discharge capacity of 850 mAh / g and a 50th discharge capacity of 574 mAh / g, and the capacity retention rate was 67%, which tended to deteriorate faster than in Example 1.

Reference example 1
20% by weight of metallic silicon having a purity of 3.5N was mixed with ethanol, and a silicon fine particle slurry having an average particle size (D50) of 0.21 μm was obtained with a laser diffraction type particle size distribution meter using a zirconia media bead mill. A copper nitrate ethanol solution was added to this silicon fine particle slurry so that the amount of Cu added to silicon was 0.5% by weight, dried at 120 ° C. to remove ethanol, and then at 600 ° C. in a hydrogen / nitrogen mixed atmosphere. It was calcined for 1 hour to obtain silicon-based fine particles. When these fine particles were XRD-measured, a diffraction pattern of only crystalline silicon was obtained.

70% by weight (content in total solid content) of the obtained silicon-based fine particles as a negative electrode active material for a lithium ion secondary battery without being combined with amorphous carbon and / or microcrystalline carbon and graphite. The same applies hereinafter), 10% by weight of carbon nanotubes as a conductive auxiliary agent and 20% by weight of a polyimide binder as a binder were mixed to prepare a slurry containing a negative electrode mixture.

The obtained slurry was applied to a copper foil with a solid content coating amount of 3 mg / cm ² , and dried at 110 ° C. in the atmosphere for 0.5 hours. After drying, it was punched into a circle of 14 mmφ, uniaxially pressed under the condition of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to obtain a negative electrode for a lithium ion secondary battery. The cell preparation method and evaluation conditions for evaluation were the same as in Example 1.

The results of the cell evaluation were an initial discharge capacity of 2673 mAh / g and a 50th discharge capacity of 568 mAh / g, and the capacity retention rate was 49%, which tended to be larger than that of Example 1 and Comparative Example 1 in which composite particles were formed. ..

When the shape of the silicon-based fine particles after charging and discharging was observed by SEM, the effect of suppressing mutual adhesion between the particles was confirmed (Fig. 3).

Comparative Example 2
The silicon fine particles obtained in Reference Example 1 were not metal-doped, and then a negative electrode and an evaluation cell were prepared in the same manner as in Reference Example 1 to evaluate the cells.

The results of the cell evaluation were an initial discharge capacity of 3248 mAh / g and a 50th discharge capacity of 1521 mAh / g, and the 50-cycle maintenance rate was 47%, which was slightly lower than that of Comparative Example 2. When the shape of the silicon-based fine particles after charging and discharging was observed by SEM, mutual adhesion between the particles was confirmed (Fig. 3).

Table 1 shows the battery evaluation results of Examples, Reference Examples and Comparative Examples.

11 Graphite thin layer inside the negative electrode active material 12 Si fine particles inside the negative electrode active material

Claims (4)

Silicon having an average particle size (D50) of 0.5 μm or less by a particle size distribution meter of a laser diffraction method or a dynamic light scattering method, Cu doped on the surface, and a silicon single-phase peak pattern measured by XRD. It contains 10 to 50% by weight of system fine particles, the balance is amorphous carbon and / or fine crystal carbon, and graphite, the average particle size (D50) is 1 to 40 μm, and the specific surface area by the BET method is 0.5. ~ 50 m ² / g , petroleum pitch, carbon pitch, synthetic pitch, tars, cellulose, sucrose, polyvinyl chloride, polyvinyl alcohol, phenol resin, furan resin, amorphous alcohol, polystyrene, epoxy resin, poly Amorphous carbon and / or microcrystalline carbon, which are carbons derived from acrylonitrile, melamine resin, acrylic resin, polyamideimide resin, polyamide resin or polyimide resin, cover the surface of the negative electrode active material, and silicon-based fine particles are formed. A negative electrode active material for a lithium ion secondary battery, characterized in that it has a structure sandwiched between thin graphite layers having a thickness of 0.2 μm or less together with amorphous carbon and / or fine crystal carbon. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the amorphous carbon and / or the microcrystalline carbon is carbon derived from a phenol resin. A step of mixing silicon-based fine particles, a carbonaceous substance that becomes amorphous carbon and / or microcrystalline carbon by heat treatment, and graphite, a step of compacting, and a step of crushing and spheroidizing to obtain substantially spherical composite particles. The method for producing a negative electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the step of forming and the step of firing the composite particles in an inert gas atmosphere are included. 3. Claim 3 characterized by using silicon-based particles produced from a step of adhering one or more kinds of Cu salts or Cu oxides other than silicon to a silicon surface and a step of reducing Cu salts or Cu oxides. The method for producing a negative electrode active material for a lithium ion secondary battery according to the above method.

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