JP6961980B2 - Composite active material for lithium secondary battery and its manufacturing method - Google Patents

Description

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

With the miniaturization and weight reduction of electronic materials and the progress of development of HEVs or EVs, there is an increasing demand for the development of batteries having a large capacity, high-speed charge / discharge characteristics, good cycle characteristics, and excellent safety. Among them, lithium ion secondary batteries (lithium secondary batteries) are attracting attention as the most promising batteries.

However, as a prerequisite for the development of a lithium secondary battery exhibiting excellent performance, a negative electrode material, a positive electrode material, an electrolytic solution, a separator, a current collector, etc. having excellent various performances have been developed, and their characteristics are sufficiently exhibited. The battery design must be made.

Among them, since the negative electrode material determines the basic battery characteristics, the development of materials having more excellent characteristics such as charge / discharge capacity is being actively carried out. For example, Patent Document 1 discloses a composite active material for a lithium secondary battery capable of producing a lithium secondary battery having a large charge / discharge capacity, high-speed charge / discharge characteristics, and good cycle characteristics, and a method for producing the same. Has been done. Similarly, by adding a metal element, a composite active material for a lithium secondary battery containing a carbon substance derived from tar pitch while having a high charge / discharge capacity is disclosed (see, for example, Patent Document 2).

However, although the composite active material for a lithium secondary battery to which a metal element is added has excellent characteristics such as charge / discharge capacity, there is a problem that the charge / discharge cycle life tends to be shortened. Therefore, a composite active material containing soft carbon to improve the charge / discharge cycle life is also disclosed (see, for example, Patent Document 3).

However, even with these methods, the suppression of irreversible expansion was insufficient when a long cycle was carried out.

Japanese Patent No. 5227483 Japanese Patent No. 3289231 Japanese Patent No. 4281099

On the other hand, in recent years, from the viewpoint of battery use safety, it has been required that the volume of the electrode material does not expand even after repeated charging and discharging. If the volume expansion of the electrode material is large, the electrolytic solution leaks and the battery life is shortened. Further, in recent years, the required characteristics for battery materials have been greatly increased, and the required levels for cycle characteristics have been further increased. Further, from the viewpoint of battery design, there is an increasing demand for reducing the volume expansion of the electrode material during charging.

The present inventors produced a composite active material for a lithium secondary battery containing silicon as a battery active material capable of combining with lithium ions according to the production method described in Patent Document 1 described above, and obtained the lithium secondary. When the expansion characteristics of an electrode material containing a composite active material for a battery (for example, a negative electrode material) were evaluated, after several cycles, an irreversible large volume expansion beyond the theoretical expansion associated with the alloying of lithium and silicon was performed. Was observed, and it was found that further improvement was necessary. Further, regarding the cycle characteristics, although the conventional required level is satisfied, the higher required level of recent years is not satisfied, and further improvement is required. Further, during charging, the volume expansion of the electrode was observed due to the theoretical expansion due to the alloying of lithium and silicon, and further improvement was required from the viewpoint of battery design.

In view of the above circumstances, the present invention is a lithium secondary battery capable of producing an electrode material in which volume expansion is suppressed during charging and volume expansion is suppressed even after repeated charging and discharging, and exhibits excellent cycle characteristics. It is an object of the present invention to provide a composite active material for a lithium secondary battery capable of producing the same, and a method for producing the same.

Another object of the present invention is to provide a lithium secondary battery containing the above-mentioned composite active material for a lithium secondary battery.

As a result of diligent studies on the prior art, the present inventors have found that the above problems can be solved by the following configurations.
(1) There are voids between the Si or Si alloy and the graphite thin layer and the Si or Si alloy, between the graphite thin layer and the graphite thin layer, and between the Si or Si alloy and the Si or Si alloy, and the void ratio is 2. ~ 50% composite active material for lithium secondary batteries.
(2) The average particle size (D50) of the active material is 1 to 40 μm, the specific surface area is 0.5 to 45 m ² / g, the average pore diameter is 10 to 40 nm, and the open pore volume is 0.06 cm ³ / g or less. The composite active material for a lithium secondary battery according to (1).
(3) Lithium according to (1) or (2), wherein the average particle size (D50) of the Si or Si alloy is 0.01 to 5 μm, and carbon at least covers the surface of the active material. Composite active material for secondary batteries.
(4) The Si or Si alloy has a structure sandwiched between carbon thin layers of graphite having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or mesh pattern, and the graphite thin layers are spread. The composite active material for a lithium secondary battery according to (1) or (2), wherein the active material particles are curved near the surface of the active material particles to cover the active material particles, and the surface of the outermost layer is covered with carbon.
(5) The composite for a lithium secondary battery according to any one of (1) to (4), wherein the content of the Si or Si alloy is 10 to 80 parts by mass, and the content of the carbon is 90 to 10 parts by mass. Active material.
(6) A step of mixing Si or Si alloy, a carbon precursor, a void forming material, and graphite as necessary, a step of granulating and compacting, and a substantially spherical composite by pulverizing and spheroidizing the mixture. A step of forming particles, a step of calcining the composite particles in an inert atmosphere, and / or a step of removing the void forming material by dissolution in a solvent, and mixing the carbon precursor with the composite particles or calcined powder. The composite for a lithium secondary battery according to any one of (1) to (5), which comprises a step of obtaining a composite particle having a carbon film coated with a calcined powder or carbon by heating the step and a mixture thereof in an inert atmosphere. Manufacturing method of active material.
(7) The carbon-coated composite particles, spherical composite particles or calcined powder obtained in (6) and the carbon precursor are calcined in an inert atmosphere to coat the inside and outside of the composite particles or calcined powder with a carbon film. The method for producing a composite active material for a lithium secondary battery according to (6), wherein the step is performed.
(8) The lithium secondary battery according to (6) or (7), wherein after the step of coating with a vapor phase, a step of wind classifying the crushed and spherically treated powder or calcined powder or carbon-coated powder is performed. A method for producing a composite active material for use.
(9) A step of calcining the composite particles and the calcined powder together with the carbon precursor in an inert atmosphere and heating the carbon precursor in the inert atmosphere to coat the carbon film with the calcined powder or the carbon-coated composite particles or carbon. The method for producing a composite active material for a lithium secondary battery according to any one of (7) to (9), wherein the temperature of the step of coating the inside and outside of the calcined powder with the gas phase is 300 to 1200 ° C., respectively.
(10) A lithium secondary battery containing the composite active material for the lithium secondary battery according to any one of (1) to (5).

According to the present invention, it is possible to manufacture an electrode material in which volume expansion during charging and volume expansion is suppressed even after repeated charging and discharging, and it is possible to manufacture a lithium secondary battery exhibiting excellent cycle characteristics. A composite active material for a lithium secondary battery and a method for producing the same can be provided.

Further, according to the present invention, it is also possible to provide a lithium secondary battery containing the composite active material for the lithium secondary battery.

Hereinafter, the composite active material for a lithium secondary battery of the present invention and a method for producing the same will be described in detail.

The composite active material for a lithium secondary battery of the present invention has voids between a Si or Si alloy and a graphite thin layer and a Si or Si alloy, or between a graphite thin layer and a graphite thin layer, and has a porosity of 2 to 2. It is 50%, preferably 4 to 50%, particularly preferably 10 to 50%, and having the voids suppresses volume expansion during charging and has excellent cycle characteristics.

The particle size (D50: 50% volume particle size) of the composite active material for a lithium secondary battery is not particularly limited, but 2 to 40 μm is preferable, and 5 to 35 μm is more preferable, because the effect of the present invention is more excellent. ~ 30 μm is more preferable.

The particle size (D90: 90% volume particle size) is not particularly limited, but 10 to 75 μm is preferable, 10 to 60 μm is more preferable, and 20 to 45 μm is further preferable, in that the effect of the present invention is more excellent.

Further, the particle size (D10: 10% volume particle size) is not particularly limited, but 1 to 20 μm is preferable, and 2 to 10 μm is more preferable, because the effect of the present invention is more excellent.

D10, D50 and D90 correspond to the cumulative 10%, cumulative 50% and cumulative 90% particle sizes from the fine particle side of the cumulative particle size distribution measured by the laser diffraction scattering method, respectively.

At the time of measurement, the composite active material for a lithium secondary battery is added to the liquid and vigorously mixed using ultrasonic waves or the like, and the prepared dispersion liquid is introduced into the apparatus as a sample to perform the measurement. As the liquid, it is preferable to use water, alcohol, or a low volatility organic solvent for work. At this time, it is preferable that the obtained particle size distribution map shows a normal distribution.

The composite active material for a lithium secondary battery of the present invention ^{preferably has a specific surface area of 0.5 to 45 m 2} / g, more preferably 0.5 to 30 m ² / g, and particularly preferably 0.5 to 10 m ² / g. Is. Within this range, the solid electrolyte layer (SEI) formed on the surface of the active material due to contact with the electrolytic solution and charge / discharge can be suppressed, and the initial Coulomb efficiency and the capacity retention rate can be improved.

The average pore diameter is preferably 10 to 40 nm, more preferably 10 to 30 nm, and particularly preferably 10 to 20 nm. Further preferably open pore volume is less than 0.06 cm ³ / g, more preferably 0.04 cm ³ / g, particularly preferably at most 0.02 cm ³ / g. By setting the average pore diameter and the volume of the open pores in this range, it is possible to suppress the invasion of the electrolytic solution into the active material and improve the capacity retention rate and the hyperexpansion rate.

The specific surface area (BET specific surface area), average pore diameter, and open pore volume of the composite active material for a lithium secondary battery are measured by vacuum drying the sample at 300 ° C. for 30 minutes and then measuring it by the nitrogen adsorption multipoint method.

The composite active material for a lithium secondary battery of the present invention has a structure in which the battery active material is sandwiched between carbon and a thin graphite layer having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or mesh pattern. It is preferable that carbon curves near the surface of the active material particles to cover the active material particles on the surface of the outermost layer.

When the thickness exceeds 0.2 μm, the electron transfer effect of the graphite thin layer diminishes. When the graphite thin layer is linear in cross section, its length is preferably at least half the size of the composite active material particles for a lithium secondary battery for electron transfer, and the size of the composite material particles for a lithium secondary battery It is more preferable that the degree is equivalent. When the graphite thin layer is in the form of a mesh, it is preferable for electron transfer that the network of the graphite thin layer is connected over half or more of the size of the active material particles, and it is more preferable that the size of the graphite thin layer is about the same as the size of the active material particles. ..

In the composite active material for a lithium secondary battery of the present invention, it is preferable that carbon curves near the surface of the active material particles on the surface of the outermost layer to cover the active material particles. With such a shape, the electrolytic solution invades the surface of the outermost layer from the carbon end face, and the carbon end face and the electrolytic solution come into direct contact with the surface of the battery active material and the outermost layer, and a reactant is formed during charging and discharging. And the risk of reduced efficiency is reduced.

Si in the present invention refers to general-purpose grade metallic silicon having a purity of about 98% by weight, chemical grade metallic silicon having a purity of 2 to 4N, polysilicon having a higher purity than 4N obtained by chlorination and distillation purification, and single crystal growth. Ultra-high-purity single-crystal silicon that has undergone the precipitation process by the method, or those obtained by doping them with Group 13 or Group 15 elements of the periodic table to form p-type or n-type, and polishing and cutting of wafers generated in the semiconductor manufacturing process. It is not particularly limited as long as it has a purity higher than that of general-purpose grade metallic silicon, such as waste wafers and waste wafers that have become defective in the process.

The Si alloy referred to in the present invention is an alloy containing Si as a main component. In the Si alloy, as the element contained in addition to Si, one or more of the elements of Groups 2 to 15 of the periodic table is preferable, and the selection and / or the amount of the element added at which the melting point of the phase contained in the alloy is 900 ° C. or more. Is preferable.

In the composite active material for a lithium secondary battery of the present invention, the average particle size (D50) of the Si compound is preferably 0.01 to 5 μm, more preferably 0.01 to 1 μm, and particularly preferably 0.05 to 0. It is 0.6 μm. If it is smaller than 0.01 μm, the capacity and initial efficiency are severely lowered due to surface oxidation, and if it is larger than 5 μm, cracks are severely caused by expansion due to lithium insertion, and cycle deterioration is likely to be severe. The average particle size (D50) is the volume average particle size measured by a laser particle size distribution meter.

The content of the Si compound is preferably 10 to 80 parts by mass, and particularly preferably 15 to 50 parts by mass. When the content of the Si compound is less than 10 parts by mass, a sufficiently large capacity cannot be obtained as compared with conventional graphite, and when it is larger than 80 parts by mass, cycle deterioration tends to be severe.

Examples of graphite include natural graphite materials and artificial graphite, and among them, flaky graphite obtained by flaking natural graphite, which is usually called graphite, is preferable.

In the present specification, the flaky graphite is intended to be graphite having 400 or less layers of graphene sheets. The graphene sheets are mainly bonded to each other by van der Waals force.

As for the number of graphene sheets laminated in the flaky graphite, the battery active material that can be combined with lithium ions and the flaky graphite are more evenly dispersed, and the expansion of the battery material using the composite active material for the lithium secondary battery is further suppressed. And / or, in that the cycle characteristics of the lithium secondary battery are more excellent (hereinafter, simply "the point where the effect of the present invention is more excellent"), 300 layers or less is preferable, 200 layers or less is more preferable, and 150 layers or less. Layers and below are more preferred. From the viewpoint of handleability, 5 or more layers are preferable.

The number of graphene sheets laminated in the flaky graphite can be measured using a transmission electron microscope (TEM).

The average thickness of the flaky graphite is preferably 40 nm or less, more preferably 22 nm or less, in that the effect of the present invention is more excellent. The lower limit is not particularly limited, but it is usually 4 nm or more in many cases because the manufacturing procedure becomes complicated.

As a method for measuring the average thickness, flaky graphite is observed by electron microscope observation (TEM), the thickness of 10 or more layers of laminated graphene sheets in the flaky graphite is measured, and the value is calculated. By averaging, the average thickness is obtained.

The flaky graphite is obtained by exfoliating the graphite compound between its layer surfaces and flaking it.

Examples of flaky graphite include so-called expanded graphite.

Expanded graphite contains graphite. For example, scaly graphite is treated with concentrated sulfuric acid, nitric acid, hydrogen peroxide solution, etc., and these chemicals are intercalated in the gaps between graphene sheets and further heated. It is obtained by widening the gaps in the graphene sheet as the intercalated drug solution evaporates. As will be described later, a predetermined composite active material for a lithium secondary battery can be produced using expanded graphite as a starting material. That is, expanded graphite can also be used as the graphite in the composite active material for the lithium secondary battery.

Further, as the graphite, expanded graphite which has been subjected to a spheroidizing treatment can also be mentioned. The procedure for spheroidization will be described in detail later. As will be described later, when the expanded graphite is spheroidized, it is spheroidized together with other components (for example, precursors of hard carbon and soft carbon, battery active material capable of combining with lithium ions, etc.). The process may be carried out.

The specific surface area of graphite is not particularly limited, but 10 m ² / g or more is preferable, and 20 m ² / g or more is more preferable, in that the effect of the present invention is more excellent. The upper limit is not particularly limited, but the specific surface area is ^{preferably 200 m 2} / g or less because the manufacturing procedure becomes complicated and synthesis is difficult.

The specific surface area of graphite was measured by using the BET method (JIS Z 8830, one-point method) by nitrogen adsorption.

Graphite preferably has a purity of 99.9% by weight or more, an impurity amount of 1000 ppm or less, an S amount of 0.3% by weight or less, and / or a BET specific surface area of 200 m ² / g or less. If the purity is less than 99.9% by weight or the amount of impurities is more than 1000 ppm, the irreversible capacity due to the formation of SEI derived from impurities increases, so that the initial charge / discharge efficiency, which is the discharge capacity with respect to the initial charge capacity, becomes low. Tend. Further, when the amount of S is higher than 0.3% by weight, the irreversible capacity is similarly increased, so that the initial charge / discharge efficiency is lowered. More preferably, the amount of S is 0.1% by weight or less. When the BET specific surface area of graphite ^{is higher than 40 m 2} / g, the area that reacts with the electrolytic solution becomes large, so that the initial charge / discharge efficiency becomes low.

Impurities were measured by ICP emission spectroscopic analysis of the following 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd. , Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U) by semi-quantitative value of impurities. The amount of S is determined by combustion absorption treatment by an oxygen flask combustion method, filtering by a filter, and ion chromatography (IC) measurement.

The carbon other than the graphite thin layer in the present invention is not particularly limited as long as it becomes carbon when sintered or heat-treated, and a carbonaceous material other than the graphite thin layer is preferable, and the carbonaceous material is amorphous or fine. Examples of crystalline carbon substances include easily graphitized carbon (soft carbon) that is graphitized by heat treatment exceeding 2000 ° C. and non-graphitizable carbon (hard carbon) that is difficult to graphitize.

Hard carbon is preferably obtained by carbonizing a precursor such as a resin or a resin composition. By carbonizing, the resin or resin composition is carbonized and can be used as a carbon material for a lithium ion secondary battery. Examples of the resin or resin composition used as a raw material (precursor) of hard carbon include polymer compounds (for example, thermosetting resin and thermoplastic resin). The thermosetting resin is not particularly limited, and for example, a phenol resin such as a novolak type phenol resin or a resol type phenol resin; an epoxy resin such as a bisphenol type epoxy resin or a novolak type epoxy resin; a melamine resin; a urea resin; an aniline resin; Examples thereof include cyanate resin; furan resin; ketone resin; unsaturated polyester resin; and urethane resin. Further, modified products obtained by modifying these with various components can also be used.

The thermoplastic resin is not particularly limited, and for example, polyethylene, polystyrene, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, polyethylene terephthalate, polycarbonate, polyacetal, polyphenylene ether, and poly. Examples thereof include butylene terephthalate, polyphenylene sulfide, polysulfone, polyether sulfone, and polyether ether ketone.

One or a combination of two or more of these can be used.

Among these, particularly preferable hard carbon raw materials (precursors) include phenol resins such as novolak type phenol resins and resol type phenol resins.

The shape of the hard carbon precursor is not particularly limited, and any shape such as powder, plate, granular, fibrous, lump, and spherical can be used. These precursors are preferably dissolved in the solvent used when mixing the various components.

The weight average molecular weight of the precursor of the hard carbon used is preferably 1000 or more, and more preferably 1,000,000 or less in that the effect of the present invention is more excellent.

Soft carbon is preferably obtained by carbonizing a precursor such as a resin or a resin composition. By carbonizing, the resin or resin composition is carbonized and can be used as a carbon material for a lithium ion secondary battery. The resin or resin composition used as a raw material (precursor) for soft carbon is not particularly limited, and is limited to coal-based pitch (for example, coal tar pitch), petroleum-based pitch, mesophase pitch, coke, low-molecular-weight heavy oil, and the like. Alternatively, examples thereof include coal-based pitches (for example, coal tar pitches), petroleum-based pitches, mesophase pitches, coke, low-molecular-weight heavy oils, and derivatives thereof. Among them, soft carbon obtained from a precursor such as a coal-based pitch is preferable because the effect of the present invention is more excellent.

The shape of the soft carbon precursor is not particularly limited, and any shape such as powder, plate, granular, fibrous, lump, and spherical can be used. These precursors are preferably dissolved in the solvent used when mixing the various components.

The weight average molecular weight of the precursor of the soft carbon used is preferably 1000 or more, and more preferably 1,000,000 or less because the effect of the present invention is more excellent.

When the composite active material for a lithium secondary battery of the present invention contains carbon other than the graphite thin layer, the content of carbon other than the graphite thin layer is preferably 90 to 10 parts by mass, particularly preferably 60 to 10 parts by mass. When the content of carbon other than the graphite thin layer is less than 10 parts by mass, the carbon other than the graphite thin layer cannot cover the Si compound, the conductive path becomes insufficient, and capacity deterioration is likely to occur, and 90 mass. If it is larger than the part, sufficient capacity cannot be obtained.

When the composite active material for a lithium secondary battery of the present invention contains carbon and graphite other than the graphite thin layer, the respective contents are preferably 5 to 60 parts by mass and 20 to 80 parts by mass, and 10 to 55 parts by mass. The ratio of parts by mass to 30 to 70 parts by mass is particularly preferable. When the content of carbon other than the graphite thin layer is less than 5 parts by mass, the carbon other than the graphite thin layer cannot cover the Si compound and graphite, the adhesion between the Si compound and graphite becomes insufficient, and the active material particles. Is likely to be difficult to form. Further, when it is larger than 60 parts by mass, the effect of graphite having higher conductivity than carbon other than the graphite thin layer is not sufficiently brought out. On the other hand, when the content of graphite is less than 20 parts by mass, the effect of graphite having higher conductivity than the carbonaceous material is not sufficient, and when it is more than 80 parts by mass, a sufficiently large capacity can be obtained as compared with the conventional graphite. No.

The method for producing a composite active material for a lithium secondary battery of the present invention includes a step of mixing Si or Si alloy, a carbon precursor, a void forming material, and graphite as necessary, a step of granulating and compacting, and a mixture. To form substantially spherical composite particles and / or to remove the void forming material by dissolution in a solvent, to calcin the composite particles in an inert atmosphere, and a carbon precursor. This includes a step of mixing the body with the composite particles or the calcined powder, and a step of heating the mixture in an inert atmosphere to obtain a calcined powder or a carbon-coated composite particle of the carbon film.

As the raw material Si compound, it is preferable to use a powder having an average particle size (D50) of 0.01 to 5 μm. In order to obtain a Si compound having a predetermined particle size, the raw material (state of ingot, wafer, powder, etc.) of the above-mentioned Si compound is pulverized by a pulverizer, and in some cases, a classifier is used. In the case of lumps such as ingots and wafers, they can be first pulverized using a coarse crusher such as a jaw crusher. After that, for example, a ball mill, a medium stirring mill, or a roller that uses a crushing medium such as a ball or a bead to crush the crushed material by using the impact force, the frictional force, and the compressive force generated by the kinetic energy. Rotation of a roller mill that crushes objects at high speed, a jet mill that collides with lining materials at high speed or collides with particles, and crushes by the impact force of the impact, and a rotor with hammers, blades, pins, etc. fixed. It can be finely crushed using a hammer mill, pin mill, disc mill that crushes the material to be crushed by using the impact force of the above, a colloid mill that uses the shearing force, and a high-pressure wet facing collision type disperser "Ultimizer". ..

Both wet and dry pulverization can be used. For further fine pulverization, for example, a wet bead mill is used, and the diameter of the beads is gradually reduced to obtain very fine particles. Further, in order to adjust the particle size distribution after pulverization, dry classification, wet classification or sieving classification can be used. Dry classification mainly uses airflow, and the processes of dispersion, separation (separation of fine particles and coarse particles), collection (separation of solid and gas), and discharge are performed sequentially or simultaneously, and interference between particles and particles Pretreatment (adjustment of moisture, dispersibility, humidity, etc.) is performed before classification so that the classification efficiency is not reduced due to the shape, turbulence of airflow, velocity distribution, influence of static electricity, etc., or the moisture content of the airflow used. And adjust the oxygen concentration. In the dry type with an integrated classifier, crushing and classification are performed at once, and it is possible to obtain a desired particle size distribution.

As another method for obtaining a Si compound having a predetermined particle size, a method of heating the Si compound with a plasma, a laser, or the like to evaporate it and coagulating it in an inert atmosphere, CVD, plasma CVD, etc. using a gas raw material, etc. These methods are suitable for obtaining ultrafine particles of 0.1 μm or less.

The carbon precursor of the raw material is not particularly limited as long as it is a carbon-based compound mainly composed of carbon and becomes a carbonaceous substance by heat treatment in an inert atmosphere, and the graphitized carbon (soft carbon) and graphite are not particularly limited. Examples thereof include non-graphitized carbon (hard carbon) which is difficult to convert.

The void forming material referred to in the present invention is a liquid or solid organic substance or an inorganic substance, which is vaporized and evaporated by heat treatment at 150 ° C. or higher, a substance soluble in an aqueous solvent or a non-aqueous solvent, and most of which is vaporized by thermal decomposition. It is a substance that evaporates, but part of it remains as a component different from that at the time of addition. Phenolic resins such as glucose, fructose, saccharin, sorbitol, novolak type phenol resin, resole type phenol resin, natural rubber, styrene butadiene rubber, butadiene rubber, chloroprene rubber, butyl rubber, rubbers such as nitrile rubber, polystyrene, polyethylene, polychloride Resins such as vinylidene, polyvinylidene fluoride, polypropylene, polycarbonate, polyurethane, acrylic, etc. are mentioned, and among them, glycerin, diglycerin, polyglycerin, menthol, PEG, polyvinylpyrrolidone, sucrose, glucose, fructose, novolak type phenol resin, resole. Type phenol resin, styrene butadiene rubber, polypropylene are preferable, and glycerin, diglycerin, polyglycerin, menthol, PEG, polyvinylpyrrolidone, sucrose, and resole type phenol resin are particularly preferable.

As the raw material graphite, natural graphite, artificial graphite obtained by graphitizing the pitch of petroleum or coal, or the like can be used, and scaly, oval or spherical, columnar or fibrous or the like is used. Further, after the graphite is acid-treated and oxidized, the graphite is expanded by heat treatment, and a part of the graphite layers is peeled off to form an accordion-like expanded graphite or a crushed product of expanded graphite, or the layers are subjected to ultrasonic waves or the like. Peeled graphene or the like can also be used. Expanded graphite or a crushed product of expanded graphite is superior in flexibility to other graphites, and in the step of forming composite particles described later, the crushed particles are re-bonded to facilitate substantially spherical composite particles. Can be formed into. In the above points, it is preferable to use expanded graphite or a pulverized product of expanded graphite. The raw material graphite is prepared in advance to a size that can be used in the mixing process, 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.

Mixing with these Si compounds, carbon precursors, void forming materials and, if necessary, graphite, if the carbon precursors are softened and liquefied by heating, the Si compounds and carbon precursors are heated. This can be done by kneading the void forming material and, if necessary, graphite. When the carbon precursor is soluble in a solvent, a Si compound, a carbon precursor, a void forming material, and if necessary graphite are added to the solvent, and Si is added to the solution in which the carbon precursor is dissolved. This can be done by dispersing and mixing the compound, carbon precursor, void forming material and, if necessary, graphite, and then removing the solvent. The solvent used is not particularly limited as long as it can dissolve the carbon precursor. For example, when pitch and tars are used as carbon precursors, quinoline, pyridine, toluene, benzene, tetrahydrofuran, cleosort oil and the like can be used, and when polyvinyl chloride is used, tetrahydrofuran, cyclohexanone, nitrobenzene and the like can be used. It can be used, and when a phenol resin or a furan resin is used, ethanol, methanol or the like can be used.

As a mixing method, a kneader can be used when the carbon precursor is heated and softened. When a solvent is used, in addition to the above-mentioned kneader, a Nauter mixer, a Reedige mixer, a Henschel mixer, a high-speed mixer, a homomixer and the like can be used. In addition, the jacket is heated by these devices, and then the solvent is removed by a vibration dryer, a paddle dryer, or the like.

By solidifying the carbon precursor or continuing stirring in the process of removing the solvent for a certain period of time with these devices, the Si compound, the carbon precursor, the void forming material, and, if necessary, the mixture with graphite are granulated.・ It is compacted. Further, the carbon precursor is solidified or the mixture after removing the solvent is compressed by a compressor such as a roller compactor and roughly pulverized by a crusher, whereby granulation and consolidation can be performed. The size of these granulated / consolidated products is preferably 0.1 to 5 mm from the viewpoint of ease of handling in the subsequent pulverization step.

Granulation and compaction methods include ball mills and medium stirring mills that use compressive force to crush crushed materials, roller mills that crush crushed materials using the compressive force of rollers, and lining materials at high speed. Jet mills that collide with each other or collide with each other and crush by the impact force of the impact, hammer mills and pin mills that crush the crushed material by using the impact force of the rotation of the rotor with fixed hammers, blades, pins, etc. , A dry crushing method such as a disc mill is preferable. 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, crushing and classification are performed at once, and a desired particle size distribution can be obtained.

As a method of crushing and spheroidizing the granulated / consolidated mixture, a method of crushing by the above-mentioned crushing method to adjust the particle size and then passing through a dedicated spheroidizing device, and the above-mentioned jet mill or rotor There is a method of repeating the method of crushing the material to be crushed by utilizing the impact force due to rotation, or a method of making it spherical by extending the processing time. Dedicated spherical devices include Hosokawa Micron's Faculty (registered trademark), Nobilta (registered trademark), Mechanofusion (registered trademark), Nippon Coke Industries, Inc. COMPOSI, Nara Machinery Co., Ltd.'s hybridization system, and EarthTechnica Co., Ltd. Examples include Cryptron Orb and Cryptron Eddie.

By performing the above pulverization and spheroidizing treatment, substantially spherical composite particles can be obtained.

The obtained composite particles are calcined in an argon gas or nitrogen gas stream, in a vacuum, or the like in order to carbonize the carbon precursor component and remove the void forming material. The firing temperature is preferably 300 to 1200 ° C, particularly preferably 600 to 1200 ° C. When the firing temperature is less than 300 ° C., the electric resistance between the graphite layer and Si inside the composite particles and the composite particles increases due to the residual unthermally decomposed components of the carbon precursor and the void forming material, so that the discharge capacity Tends to decrease. On the other hand, when the calcination temperature exceeds 1200 ° C., there is a strong possibility that the reaction between the Si compound and the amorphous carbon or graphite derived from the carbon precursor occurs, and the discharge capacity tends to decrease.

In addition to the removal by firing, the void forming material may be dissolved in a solvent such as water or alcohol and extracted by a method such as filtration or centrifugation. In the case of dissolution extraction, it is preferable to use a solvent that does not dissolve the carbon precursor, and examples of the solvent include water, methanol, ethanol, isopropanol, acetone, toluene, tetrahydrofuran, etc., among which water, Methanol, ethanol, acetone, tetrahydrofuran and the like are preferable.

Further, in the composite active material for a lithium secondary battery of the present invention, carbon-coated composite particles, spherical composite particles or calcined powder obtained in the previous step and a carbon precursor are fired in an inert atmosphere to form a carbon film. Is preferably produced by performing a step of coating the inside and outside of the composite particles or the calcined powder.

Examples of the carbon precursor used include coal-based pitch (for example, coal tar pitch), petroleum-based pitch, mesophase pitch, coke, low-molecular-weight heavy oil, and the like.

When carbon-coated composite particles, spherical composite particles or calcined powder and a carbon precursor are fired in an inert atmosphere to coat the carbon film inside and outside the composite particles or calcined powder, the carbon precursor is squeezed or the like. By putting it in a container and heating it in an inert atmosphere so that it does not come into direct contact with the composite particles, or by adding a hydrocarbon gas such as methane, ethane, ethylene, acetylene, or propylene to the inert atmosphere and heating it. It is preferable to coat the inside and outside of the carbon film with the calcined powder, the carbon-coated composite particles, or the carbon-coated calcined powder with a gas phase.

Further, the composite active material for a lithium secondary battery of the present invention is manufactured by performing a step of coating a carbon film with a gas phase and then a step of wind-classifying a spherically treated powder, a calcined powder or a carbon-coated powder. It is preferable to do so.

As a method of wind power classification, powder is put into a wind power classification device such as ATP-50 manufactured by Hosokawa Micron, and the particle size of the powder to be classified is adjusted by adjusting the operating conditions such as the rotor rotation speed and the differential pressure. It is possible to control.

The composite active material for a lithium secondary battery of the present invention is useful as an active material used as a battery material (electrode material) used in a lithium secondary battery.

The method for producing the negative electrode for a lithium secondary battery using the composite active material is not particularly limited, and a known method can be used.

For example, a composite active material for a lithium secondary battery and a binder can be mixed, made into a paste using a solvent, and coated on a copper foil to obtain a negative electrode for a lithium secondary battery.

As the current collector, in addition to the copper foil, a current collector having a three-dimensional structure is preferable in that the battery cycle is more excellent. Examples of the material of the current collector having the three-dimensional structure include carbon fiber, sponge-like carbon (a sponge-like resin coated with carbon), and metal.

As a current collector having a three-dimensional structure (porous current collector), as a porous body of a metal or carbon conductor, a plain woven wire net, an expanded metal, a lath net, a metal foam, a metal woven cloth, a metal non-woven fabric, etc. Examples thereof include carbon fiber woven fabrics and carbon fiber non-woven fabrics.

As the binder used, known materials can be used, and for example, fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene, SBR, polyethylene, polyvinyl alcohol, carboxymethyl cellulose, polyacrylic acid, glue and the like are used. Be done.

Examples of the solvent include water, isopropyl alcohol, N-methylpyrrolidone, dimethylformamide and the like.

When making a paste, as described above, a known stirrer, mixer, kneader, kneader or the like may be used for stirring and mixing, if necessary.

When preparing a coating slurry using a composite active material for a lithium secondary battery, it is preferable to add conductive carbon black, carbon nanotubes or a mixture thereof as a conductive material. The shape of the composite active material for a lithium secondary battery obtained by the above step is often relatively granular (particularly substantially spherical), and the contact between the particles tends to be point contact. In order to avoid this adverse effect, a method of blending carbon black, carbon nanotubes or a mixture thereof in the slurry can be mentioned. Since carbon black, carbon nanotubes, or a mixture thereof can be concentratedly aggregated in the capillary portion formed by contact with the composite active material when the slurry solvent is dried, it is necessary to prevent contact breakage (increased resistance) due to the cycle. Can be done.

The blending amount of carbon black, carbon nanotubes or a mixture thereof is not particularly limited, but is preferably 0.2 to 4 parts by mass and 0.5 to 2 parts by mass with respect to 100 parts by mass of the composite active material for a lithium secondary battery. More preferably, it is by mass. Examples of carbon nanotubes include single-wall carbon nanotubes and multi-wall carbon nanotubes.
(Positive electrode)
As the positive electrode used in the lithium secondary battery having the negative electrode obtained by using the composite active material, a positive electrode using a known positive electrode material can be used.

As a method for producing a positive electrode, a known method can be mentioned, and a method of applying a positive electrode mixture composed of a positive electrode material, a binder and a conductive agent to the surface of a current collector can be mentioned. As the cathode material (cathode active material), chromium oxide, titanium oxide, cobalt oxide, or metal oxides such as vanadium _{pentoxide, LiCoO 2, LiNiO 2, LiNi} 1-y Co y O 2, LiNi 1-x-y _{Co x Al y O 2, LiMnO} 2, LiMn 2 O 4, LiFeO 2 lithium metal oxides such as titanium sulfide, chalcogen compounds of transition metals such as molybdenum sulfide, or polyacetylene, polyparaphenylene, conductive polypyrrole Examples thereof include a conjugated polymer substance having.
(Electrolytic solution)
As the electrolytic solution used in the lithium secondary battery having a negative electrode obtained by using the composite active material, a known electrolytic solution can be used.

For example, as an electrolyte salt contained in the _{electrolyte, LiPF 6, LiBF 4, LiAsF} 6, LiClO 4, LiB (C 6 H 5), LiCl, LiBr, 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 ₂ } ₂ , LiB {C ₆ H ₃ (CF ₃ ) ₂ } ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF _6, etc. Lithium salts can be used. In particular, LiPF ₆ and LiBF ₄ are preferable from the viewpoint of oxidation stability.

The electrolyte salt concentration in the electrolyte solution is preferably 0.1 to 5 mol / liter, more preferably 0.5 to 3 mol / liter.

Examples of the solvent used in the electrolytic solution include carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate and diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran and 2 -Methyl tetrahydrofuran, γ-butyrolactone, 1,3-dioxofuran, 4-methyl-1,3-dioxolane, anisole, ethers such as diethyl ether, sulfolanes, thioethers such as methyl sulfolane, acetonitrile, chloronitrile, propionitrile, etc. Nitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethylolthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethylsulfoxide, 3-methyl-2- Aprotonic organic solvents such as oxazoline, ethylene glycol and dimethylsulfite can be used.

Instead of the electrolytic solution, a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte may be used. Examples of the polymer compound constituting the matrix of the polymer solid electrolyte or the polymer gel electrolyte include ether-based polymer compounds such as polyethylene oxide and its crosslinked product, methacrylate-based polymer compounds such as polymethacrylate, and acrylate-based compounds such as polyacrylate. Polymer compounds, fluoropolymer compounds such as polyvinylidene fluoride (PVDF) and vinylidene fluoride-hexafluoropropylene copolymer are preferred. These can also be mixed and used. From the viewpoint of redox stability and the like, fluoropolymer compounds such as PVDF and vinylidene fluoride-hexafluoropropylene copolymer are particularly preferable.
(Separator)
A known material can be used as the separator used in the lithium secondary battery having a negative electrode obtained by using the composite active material. Examples thereof include woven fabrics, non-woven fabrics, and microporous membranes made of synthetic resins. A microporous membrane made of synthetic resin is preferable, and among them, a polyolefin-based microporous membrane is preferable in terms of film thickness, membrane strength, membrane resistance, and the like. Specifically, it is a microporous membrane made of polyethylene and polypropylene, or a microporous membrane made of a composite thereof.

The lithium secondary battery uses the above-mentioned negative electrode, positive electrode, separator, electrolyte, and other battery components (for example, current collector, gasket, sealing plate, case, etc.) and is cylindrical, square, or according to a conventional method. It can have a form such as a button type.

The lithium secondary battery of the present invention is used in various portable electronic devices, and in particular, a notebook computer, a notebook word processor, a palm top (pocket) computer, a mobile phone, a mobile fax, a mobile printer, a headphone stereo, a video camera, and a portable television. , Portable CD, Portable MD, Electric Shaving Machine, Electronic Notebook, Transceiver, Electric Tool, Radio, Tape Recorder, Digital Camera, Portable Copy Machine, Portable Game Machine, etc. In addition, there are two types of vehicles such as electric vehicles, hybrid vehicles, vending machines, electric carts, power storage systems for road leveling, household power storage, distributed power storage systems (built into stationary electrical appliances), and emergency power supply systems. It can also be used as a secondary battery.

It is a cross-sectional SEM image of the composite active material produced in Example 1 of the present invention. It is a cross-sectional SEM image of the composite active material produced in Example 2 of the present invention. It is a cross-sectional SEM image of the composite active material produced in Example 3 of the present invention. It is a cross-sectional SEM image of the composite active material produced in Example 3 of the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.
<Example 1>
(Preparation of expanded graphite)
A scaly natural graphite having an average particle diameter of 1 mm was immersed in a mixed acid of 9 parts by mass of sulfuric acid and 1 part by mass of nitric acid at room temperature for 1 hour, and then the mixed acid was removed with a No. 3 glass filter to obtain acid-treated graphite. Further, the acid-treated graphite was washed with water and then dried. When 5 g of the dried acid-treated graphite was stirred in 100 g of distilled water and the pH was measured 1 hour later, the pH was 6.7. The dried acid-treated graphite was put into a vertical electric furnace under a nitrogen atmosphere set at 850 ° C. to obtain expanded graphite. The bulk density of expanded graphite was 0.002 g / cm ³ , and the specific surface area was 45 m ² / g.

(Mixing process)
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol in an amount of 21% by weight, and a finely pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was carried out for 6 hours to obtain an average particle size (D50). D50) An ultrafine Si slurry having a BET specific surface area of 0.3 μm and a dry BET specific surface area of 100 m ² / g was obtained.

Acid-treated natural graphite having a particle size of 0.3 mm (width in the direction of the (200) plane) and a thickness of 10 μm was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 12 L / min, and heated to 850 ° C. with an electric heater. It was passed through a Murite tube having a length of 1 m and an inner diameter of 20 mm, and was released into the atmosphere from the end face, gas such as sulfurous acid was exhausted to the upper part, and expanded graphite was collected in the lower part in a stainless steel container. The width of the expanded graphite in the (200) plane direction was 0.3 mm, which was the same as that of the original graphite, but the thickness was 2.4 mm, which expanded 240 times, and the appearance was coiled. Was peeled off, and it was confirmed that it was accordion-like.

286 g of the ultrafine Si slurry, 91 g of the expanded graphite, 18 g of a resole-type phenol resin (weight average molecular weight (Mw) = 1500), 49 g of glycerin, and 2 L of ethanol are placed in a stirring container and mixed with an in-line mixer for 22 minutes. Stirred. Then, the mixed solution was transferred to a rotary evaporator, heated to 40 ° C. in a warm bath while rotating, and depressurized by a vacuum pump to remove the solvent. Then, it was spread on a vat in a draft and dried for 2 hours while being exhausted, passed through a mesh having a mesh size of 2 mm, and dried for another day to obtain 199 g of a mixed dry matter (light bulk density 190 g / L).

(Press process)
This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and compacted to a light bulk density of 293 g / L.

(Spheroidization process)
Next, this granulated / consolidated product is put into a hybridization system of a crushing / spheroidizing device, crushed at a rotor peripheral speed of 100 m / s for 30 minutes, and at the same time sphericalized, and has a substantially spherical shape with a light bulk density of 603 g / L. A composite powder was obtained.

(Baking process)
The obtained powder was placed in a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to carbonize the phenol resin and vaporize glycerin, which is a void forming material, at the same time. As a result, a substantially spherical fired powder consisting of 65 parts by mass of graphite, 30 parts by mass of Si content, and 5 parts by mass of carbonaceous material (hard carbon content derived from phenol resin) was obtained.

After that, a mesh with an opening of 45 μm was passed through, and the bulk density was 788 g / L, and the average particle size (D).
A substantially spherical calcined powder having a size of 50) of 40.4 μm was obtained.

(Carbon coating with coal tar pitch)
After mixing 23 g of the obtained substantially spherical calcined powder and 18 g of coal tar pitch, 18 g of quinoline was added, and the mixture was stirred for 10 minutes, and then calcined and coated by the following method.

(Baking)
The coal tar pitch was denatured to soft carbon by heating the mixture at 600 ° C. for 2 hours with flowing nitrogen (4 L / min). As a result, the graphite content is 50 parts by mass, the Si content is 23 parts by mass, and the carbon material is 27 parts by mass (hard carbon content derived from phenol resin is 4 parts by mass, and soft carbon content derived from coal tar pitch is 23 parts by mass). ) Was obtained.

(Crushing / sieving)
The obtained composite active material was crushed by a stamp mill and then pulverized by a ball mill, and passed through a mesh having a mesh size of 45 μm to obtain a pulverized powder having a light bulk density of 685 g / L.

(Wind power classification)
The obtained pulverized powder was put into a wind classifying device (50 ATP) manufactured by Hosokawa Micron, and the classifying rotor was speed-classified at 15,000 rpm to obtain a powder having a light bulk density of 187 g / L.

(Carbon coating with vapor phase coat)
The powder obtained by wind classification is set in a quartz pipe, the inside of the pipe is evacuated by a rotary pump, nitrogen gas having a flow rate of 200 SCCM and ethylene gas having a flow rate of 100 SCCM are flowed into the pipe, and heated to 1000 ° C. with an electric heater. Then, carbon coating was performed by holding the state for 2 hours. The weight increase due to the carbon coating was 7.8%, which resulted in a graphite content of 46 parts by mass, a Si content of 21 parts by mass, and a carbonaceous material of 33 parts by mass (4% by mass of hard carbon derived from phenol resin). A composite active material for a lithium secondary battery was obtained, which consisted of parts, coal tar pitch, and a soft carbon content of 29 parts by mass derived from the vapor phase coat).

Its physical characteristics are as follows. Particle size distribution D50: 11.2 μm, D90: 19.8 μm, BET specific surface area: 5.2 m ² / g, average pore diameter: 13.5 nm, open pore volume: 0.021 cm ³ / g, shape: substantially spherical.

A secondary electron image of the particle cross section of the composite active material by SEM (scanning electron microscope) is shown in FIG.

The cross-sectional SEM observation of the obtained composite active material for a lithium secondary battery was performed, and the void ratio ((cross-sectional area of intraparticle voids / cross-sectional area of the entire particle) X100) was determined from the SEM image and found to be 4%. ..

(Manufacturing of negative electrode for lithium secondary battery)
With respect to 95.5% by weight of the obtained composite active material (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive auxiliary agent and 4% by weight of gelled polyacrylic acid as a binder. % And water were mixed to prepare a slurry containing a negative electrode mixture.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the amount of solid content applied was 2.8 mg / cm ^2, and dried at 110 ° C. for 2 hours in a vacuum dryer. After drying, it is punched into a circular shape 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 form a negative electrode mixture layer having a thickness of 29 μm. A negative electrode for the next battery was obtained.

"Preparation of evaluation cell"
In the glove box, the negative electrode, the polypropylene separator of 24 mmφ, the glass filter of 21 mmφ, the metallic lithium of 18 mmφ and the thickness of 0.2 mm, and the stainless steel foil of the base material are dipped into the electrolytic solution in the screw cell in the glove box. Then, they were laminated in this order, and finally the lid was screwed in to prepare. As the electrolytic solution, ethylene carbonate and diethyl carbonate were used as a mixed solvent having a volume ratio of 1: 1 and FEC (fluoroethylene carbonate) was used, and LiPF ₆ was dissolved at a concentration of 1.2 vol / L. The evaluation cell was further placed in a closed glass container containing silica gel, and an electrode passed through a silicon rubber lid was connected to the charging / discharging device.

The evaluation cell is charged at a constant current of 0.44 mA at 0.1 C to 0.005 V in a constant temperature room at 25 ° C., and then charged at a constant voltage of 0.005 V at 0.05 C until the current value reaches 0.02 mA. As a result, the initial charge capacity was 887 mAh / g. After that, the evaluation cell was disassembled in the argon atmosphere in the glove box, the electrode film thickness was measured with a micrometer, and the initial charge expansion rate ((post-charge electrode film thickness / pre-charge electrode film thickness x100)) was 135%. there were.
<Example 2>
The steps of preparing and mixing the expanded graphite are the same as in <Example 1>.

The prepared expanded graphite (21 g), sucrose (29 g) and pure water (500 mL) were placed in a stirring container, and the mixture was stirred and mixed with an in-line mixer for 10 minutes. Then, the mixed solution was transferred to a container of a rotary evaporator, heated to 80 ° C. in a warm bath while rotating, and depressurized by a vacuum pump to remove most of the water to obtain a mixed cake of expanded graphite and sucrose. Then, the mixed cake was dried in a drying oven at 130 ° C. for 5 hours to obtain an expanded graphite-sucrose mixture.

120 g of the above-mentioned ultrafine Si slurry, 70 g of the above expanded graphite-sucrose mixture, 6.3 g of glycerin and 1 L of ethanol were placed in a stirring container, and the mixture was mixed and stirred with an in-line mixer for 22 minutes. Then, the mixed solution was transferred to a rotary evaporator, heated to 40 ° C. in a warm bath while rotating, and depressurized by a vacuum pump to remove the solvent. Then, it was spread on a vat in a draft and dried for 2 hours while being exhausted, passed through a mesh having a mesh size of 2 mm, and dried for another day to obtain 96 g of a mixed dry matter (light bulk density 178 g / L).

(Press process)
This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and compacted to a light bulk density of 362 g / L.

(Spheroidization process)
Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 360 seconds while being water-cooled, and at the same time sphericalized to obtain a substantially spherical composite powder having a light bulk density of 425 g / L.

(Baking process)
The obtained powder was placed in a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to vaporize glycerin, which is a void forming material, and carbonize sucrose at the same time. As a result, a substantially spherical calcined powder consisting of 61 parts by mass of graphite, 30 parts by mass of Si content, and 9 parts by mass of carbonaceous material (hard carbon content derived from sucrose) was obtained.

Then, a mesh having a mesh size of 45 μm was passed through the powder to obtain a substantially spherical calcined powder having a light bulk density of 444 g / L.
(Carbon coating with coal tar pitch)
After mixing 47 g of the obtained substantially spherical calcined powder and 37 g of coal tar pitch, 37 g of quinoline was added, and the mixture was stirred for 10 minutes, and then calcined and coated by the following method.

(Baking)
The coal tar pitch was denatured to soft carbon by heating the mixture at 600 ° C. for 2 hours with flowing nitrogen (13.4 L / min). As a result, the graphite content is 47 parts by mass, the Si content is 23 parts by mass, and the carbon material is 30 parts by mass (7 parts by mass of hard carbon derived from sucrose, 23 parts by mass of soft carbon derived from coal tar pitch). A composite active material consisting of

(Crushing / sieving)
The obtained composite active material was crushed by a stamp mill and then pulverized by a ball mill, and passed through a mesh having a mesh size of 45 μm to obtain a pulverized powder having a light bulk density of 385 g / L.
(Wind power classification)
The obtained pulverized powder was put into a wind classifying device (50 ATP) manufactured by Hosokawa Micron, and the classifying rotor was speed-classified at 15,000 rpm to obtain a powder having a light bulk density of 154 g / L.

(Carbon coating with vapor phase coat)
The powder obtained by wind classification is set in a quartz pipe, the inside of the pipe is evacuated by a rotary pump, nitrogen gas having a flow rate of 200 SCCM and ethylene gas having a flow rate of 100 SCCM are flowed into the pipe, and heated to 1000 ° C. with an electric heater. Then, carbon coating was performed by maintaining the state for a long time. The weight increase due to the carbon coating was 15.2%, which resulted in a graphite content of 41 parts by mass, a Si content of 20 parts by mass, and a carbonaceous material of 39 parts by mass (6 mass of hard carbon derived from sucrose). A composite active material for a lithium secondary battery was obtained, which consisted of parts, coal tar pitch, and a soft carbon content of 33 parts by mass derived from the vapor phase coat).

Its physical characteristics are as follows. Particle size distribution D50: 12.1 μm, D90: 24.4 μm, BET specific surface area: 5.7 m ² / g, average pore diameter: 13.6 nm, open pore volume: 0.023
A composite active material for a lithium secondary battery having a cm of ³ / g, a light bulk density of 203 g / L, and a shape: substantially spherical shape was obtained.

A secondary electron image of the particle cross section of the composite active material by SEM (scanning electron microscope) is shown in FIG.

The cross-sectional SEM observation of the obtained composite active material for a lithium secondary battery was performed, and the void ratio ((cross-sectional area of intraparticle voids / cross-sectional area of the entire particle) x 100) was determined from the SEM image and found to be 8%. ..

(Manufacturing of negative electrode for lithium ion secondary battery)
With respect to 95.5% by weight of the obtained composite active material (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive auxiliary agent and 4% by weight of gelled polyacrylic acid as a binder. % And water were mixed to prepare a slurry containing a negative electrode mixture.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the amount of solid content applied was 2.7 mg / cm ^2, and dried at 110 ° C. for 2 hours in a vacuum dryer. After drying, it is punched into a circular shape 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 2 hours under vacuum to form a negative electrode mixture layer having a thickness of 30 μm. A negative electrode for the next battery was obtained.

"Preparation of evaluation cell"
The evaluation cell is charged at a constant current of 0.44 mA at 0.1 C to 0.005 V in a constant temperature room at 25 ° C., and then charged at a constant voltage of 0.005 V at 0.05 C until the current value reaches 0.02 mA. As a result, the initial charge capacity was 890 mAh / g. After that, the evaluation cell was disassembled in the argon atmosphere in the glove box, the electrode film thickness was measured with a micrometer, and the initial charge expansion rate ((post-charge electrode film thickness / pre-charge electrode film thickness x100)) was 137%. there were.
<Example 3>
The steps of preparing and mixing the expanded graphite are the same as in <Example 1>.
80 g of ultrafine Si slurry, 23 g of expanded graphite, 24 g of sucrose, 12 g of menthol and 1 L of ethanol were placed in a stirring container, and the mixture was mixed and stirred with an in-line mixer for 10 minutes. Then, the mixed solution was transferred to a rotary evaporator, heated to 40 ° C. in a warm bath while rotating, and depressurized by a vacuum pump to remove the solvent. Then, it was spread on a vat in a draft and dried for 2 hours while being exhausted, passed through a mesh having a mesh size of 2 mm, and dried for another day to obtain 80 g of a mixed dry matter (light bulk density 209 g / L).

(Press process)
This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and compacted to a light bulk density of 215 g / L.

(Spheroidization process)
Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 360 seconds while being water-cooled, and at the same time sphericalized to obtain a substantially spherical composite powder having a light bulk density of 278 g / L.

(Baking process)
The obtained powder was placed in a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to vaporize menthol, which is a void forming material, and carbonize sucrose. As a result, a substantially spherical fired powder consisting of 58 parts by mass of graphite, 30 parts by mass of Si content, and 12 parts by mass of carbonaceous material (hard carbon content derived from sucrose resin) was obtained.

Then, a mesh having a mesh size of 45 μm was passed through to obtain a substantially spherical calcined powder having a light bulk density of 157 g / L and an average particle size (D50) of 10.1 μm.

(Carbon coating with coal tar pitch)
After mixing 12 g of the obtained substantially spherical calcined powder and 9.4 g of coal tar pitch, 25 g of quinoline was added, and the mixture was stirred for 10 minutes, and then calcined and coated by the following method.

(Baking)
The coal tar pitch was denatured to soft carbon by heating the mixture at 600 ° C. for 2 hours with flowing nitrogen (13.4 L / min). As a result, the graphite content is 45 parts by mass, the Si content is 23 parts by mass, and the carbon material is 32 parts by mass (9 parts by mass of hard carbon derived from sucrose, 23 parts by mass of soft carbon derived from coal tar pitch). A composite active material consisting of

(Crushing / sieving)
The obtained composite active material was pulverized in a mortar and passed through a mesh having an opening of 45 μm to obtain a pulverized powder having a light bulk density of 279 g / L and an average particle size (D50) of 9.2 μm.

(Carbon coating with vapor phase coat)
The obtained powder is set in a quartz tube, the inside of the tube is evacuated by a rotary pump, nitrogen gas having a flow rate of 200 SCCM and ethylene gas having a flow rate of 100 SCCM are flowed into the tube, and the mixture is heated to 1000 ° C. with an electric heater. Carbon coating was performed by maintaining the state for a long time. The weight increase due to the carbon coating was 14.9%, which resulted in a graphite content of 39 parts by mass, a Si content of 20 parts by mass, and a carbonaceous material of 41 parts by mass (8 parts by mass of hard carbon derived from sucrose). , Coultal pitch, and a composite active material for a lithium secondary battery, which comprises 33 parts by mass of soft carbon derived from a vapor phase coat).

A secondary electron image of the composite active material by SEM (scanning electron microscope) is shown in FIG.

The void ratio ((cross-sectional area of intraparticle voids / cross-sectional area of the entire particle) x 100) was determined from the cross-sectional SEM image of the obtained composite active material for a lithium secondary battery and found to be 24%.
"Manufacturing a negative electrode for a lithium ion secondary battery"
With respect to the obtained composite active material of 89.5% 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 10% by weight of gelled polyacrylic acid as a binder. % And water were mixed to prepare a slurry containing a negative electrode mixture.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the amount of solid content applied was 2.1 mg / cm ^2, and dried at 110 ° C. in a vacuum dryer for 0.5 hours. .. After drying, it is punched into a circular shape 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 2 hours under vacuum to form a negative electrode mixture layer having a thickness of 23 μm. A negative electrode for the next battery was obtained.

"Preparation of evaluation cell"
The evaluation cell is charged at a constant current of 0.44 mA at 0.1 C to 0.005 V in a constant temperature room at 25 ° C., and then charged at a constant voltage of 0.005 V at 0.05 C until the current value reaches 0.02 mA. As a result, the initial charge capacity was 1075 mAh / g. After that, the evaluation cell was disassembled in the argon atmosphere in the glove box, the electrode film thickness was measured with a micrometer, and the initial charge expansion rate ((post-charge electrode film thickness / pre-charge electrode film thickness x100)) was 141%. there were.
<Comparative example 1>
(Preparation of expanded graphite)
The steps of preparing and mixing the expanded graphite are the same as in <Example 1>.

945 g of the ultrafine Si slurry, 240 g of the expanded graphite, 100 g of a resole-type phenol resin (weight average molecular weight (Mw) = 230), and 4 L of ethanol were placed in a stirring container, and the mixture was mixed and stirred with an in-line mixer for 22 minutes. Then, the mixed solution was transferred to a rotary evaporator, heated to 40 ° C. in a warm bath while rotating, and depressurized by a vacuum pump to remove the solvent. Then, it was spread on a vat in a draft and dried for 2 hours while being exhausted, passed through a mesh having a mesh size of 2 mm, and dried for another day to obtain 471 g of a mixed dry matter (light bulk density 201 g / L).

(Press process)
This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and compacted to a light bulk density of 419 g / L.

(Spheroidization process)
Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 360 seconds while being water-cooled, and at the same time sphericalized to obtain a substantially spherical composite powder having a light bulk density of 439 g / L.

(Baking process)
The obtained powder was placed in a quartz boat and calcined at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to simultaneously carbonize the phenol resin. As a result, a substantially spherical fired powder consisting of 60 parts by mass of graphite, 30 parts by mass of Si content, and 10 parts by mass of carbonaceous material (hard carbon content derived from phenol resin) was obtained.

Then, a mesh having a mesh size of 45 μm was passed through the powder to obtain a substantially spherical calcined powder having a light bulk density of 505 g /.

(Carbon coating with coal tar pitch)
After mixing 50 g of the obtained substantially spherical calcined powder and 40 g of coal tar pitch, 40 g of quinoline was added, and the mixture was stirred for 10 minutes, and then calcined and coated by the following method.

(Baking)
The coal tar pitch was denatured to soft carbon by heating the mixture at 600 ° C. for 2 hours with flowing nitrogen (13.4 L / min). As a result, the graphite content is 46 parts by mass, the Si content is 23 parts by mass, and the carbon material is 31 parts by mass (hard carbon content derived from phenol resin is 8 parts by mass, and soft carbon content derived from coal tar pitch is 23 parts by mass). ) Was obtained.

(Crushing / sieving)
The obtained composite active material is crushed by a stamp mill and then crushed by a ball mill, and passed through a mesh having a mesh size of 45 μm to obtain a pulverized powder having a light bulk density of 453 g / L and an average particle size (D50) of 12.5 μm. rice field.

(Carbon coating with vapor phase coat)
The obtained powder is set in a quartz tube, the inside of the tube is evacuated by a rotary pump, nitrogen gas having a flow rate of 200 SCCM and ethylene gas having a flow rate of 100 SCCM are flowed into the tube, and the mixture is heated to 1000 ° C. with an electric heater. Carbon coating was performed by maintaining the state for a long time. The weight increase due to the carbon coating was 8.2%, which resulted in a graphite content of 43 parts by mass, a Si content of 21 parts by mass, and a carbonaceous material of 36 parts by mass (7 mass of hard carbon derived from phenol). A composite active material for a lithium secondary battery was obtained, which consisted of parts, coal tar pitch, and a soft carbon content of 29 parts by mass derived from the vapor phase coat).

Its physical characteristics are as follows. Particle size distribution D50: 24.2 μm, D90: 44.9 μm, BET specific surface area: 6.1 m ² / g, average pore diameter: 15.6 nm, open pore volume: 0.028 cm ³ / g, shape: substantially spherical.

A secondary electron image of the particle cross section of the composite active material by SEM (scanning electron microscope) is shown in FIG.

The cross-sectional SEM observation of the obtained composite active material for a lithium secondary battery was performed, and the void ratio ((cross-sectional area of intraparticle voids / cross-sectional area of the entire particle) x 100) was determined from the SEM image and found to be 1%. ..
"Manufacturing a negative electrode for a lithium ion secondary battery"
With respect to 95.5% by weight of the obtained composite active material (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive auxiliary agent and 4% by weight of gelled polyacrylic acid as a binder. % And water were mixed to prepare a slurry containing a negative electrode mixture.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the amount of solid content applied was 2.6 mg / cm ^2, and dried at 110 ° C. in a vacuum dryer for 0.5 hours. .. After drying, it is punched into a circular shape 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 2 hours under vacuum to form a negative electrode mixture layer having a thickness of 20 μm. A negative electrode for the next battery was obtained.

"Preparation of evaluation cell"
The evaluation cell is charged at a constant current of 0.44 mA at 0.1 C to 0.005 V in a constant temperature room at 25 ° C., and then charged at a constant voltage of 0.005 V at 0.05 C until the current value reaches 0.02 mA. As a result, the initial charge capacity was 1075 mAh / g. After that, the evaluation cell was disassembled in the argon atmosphere in the glove box, the electrode film thickness was measured with a micrometer, and the initial charge expansion rate ((post-charge electrode film thickness / pre-charge electrode film thickness x100)) was 200%. there were.

Claims (9)

It contains Si or Si alloy , graphite and carbonaceous material, and has voids in one or more places between the graphite thin layer and Si or Si alloy, between the graphite thin layer and the graphite thin layer, and between Si or Si alloy and Si or Si alloy. It has a porosity of 2 to 50%, an average particle size (D50) of the active material of 1 to 40 μm, a specific surface area of 0.5 to 45 m ² / g, an average pore diameter of 10 to 40 nm, and an open pore volume. A composite active material for a lithium secondary battery having a specific surface area of 0.06 cm ³ / g or less. The composite active material for a lithium secondary battery according to claim 1, wherein the Si or Si alloy has an average particle size (D50) of 0.01 to 5 μm, and carbon covers at least the surface of the active material. .. The Si or Si alloy has a structure sandwiched between thin graphite layers having a thickness of 0.2 μm or less together with carbon, and the structure spreads in a laminated and / or mesh pattern, and the thin graphite layers are active material particles. The composite active material for a lithium secondary battery according to claim 1, wherein the compound active material is curved near the surface of the graphite and covers the active material particles, and the surface of the outermost layer is covered with carbon. 10 to 80 parts by weight the content of the Si or Si alloy, 5-60 parts by weight the content of graphite, according to any one of claims 1 to 3, characterized in that 20 to 80 parts by weight carbonaceous material Lithium secondary battery composite active material. A step of mixing Si or Si alloy, a carbon precursor, a void forming material, and graphite, a step of granulating and compacting, and a step of crushing and spheroidizing the mixture to form substantially spherical composite particles, the composite. Includes a step of calcining the particles in an inert atmosphere, a step of mixing the carbon precursor and the calcined powder, and a step of heating the mixture in the inert atmosphere to obtain composite particles having a carbon film coated with carbon. The method for producing a composite active material for a lithium secondary battery according to any one of claims 1 to 4. Composite particles of carbon coated obtained in claim 5, and according to claim 5, characterized in that a step of coating the calcined carbon film in and out of the composite particles and a carbon precursor in an inert atmosphere A method for producing a composite active material for a lithium secondary battery. The lithium secondary battery according to claim 5 or 6 , wherein after the step of coating with a gas phase, a step of wind-classifying the pulverized and spherically treated powder or calcined powder or carbon-coated powder is performed. Method for producing composite active material. The temperature of the step of firing the composite particles together with the carbon precursor in an inert atmosphere and the step of heating the carbon precursor in an inert atmosphere to coat the inside and outside of the calcined powder in which the carbon film is carbon-coated with a vapor phase is set. The method for producing a composite active material for a lithium secondary battery according to any one of claims 5 to 7, wherein the temperature is 300 to 1200 ° C., respectively. A lithium secondary battery containing the composite active material for a lithium secondary battery according to any one of claims 1 to 4.

Images