JP6961981B2 - 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 initial charge / discharge capacity, there is a problem that the cycle life tends to be shortened by repeating charging / discharging.

In recent years, the required characteristics for battery materials have become extremely high, and the required levels for cycle characteristics have also increased. 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 cycle characteristics of electrode materials (for example, negative electrode materials) containing composite active materials for batteries were evaluated, they met the conventional required level, but did not meet the higher required level these days, and further improvement is required. Met.

Therefore, a composite active material containing soft carbon to improve the charge / discharge cycle life is also disclosed (see, for example, Patent Document 3). One of the battery characteristics is the Coulomb efficiency defined as "nth discharge capacity (Li desorption amount) / nth charge capacity (Li insertion amount)". Since it is considered that a high level of cycle life can be maintained by improving the coulomb efficiency, it is required to improve the coulomb efficiency, but the above-mentioned Patent Document 3 does not describe the coulomb efficiency.

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

In view of the above circumstances, the present invention is a composite active material for a lithium secondary battery capable of producing an electrode material having high Coulomb efficiency and capable of producing a lithium secondary battery exhibiting excellent cycle characteristics, and a composite active material thereof. The subject is to provide a manufacturing method.

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) In the secondary battery composite active material, the thermal decomposition start temperature when the active material is heated at a temperature rising rate of 5 ° C./min in an air atmosphere is 530 ° C. or higher, and the average pore diameter is 10 to 40 nm. A composite active material for a lithium secondary battery, characterized in that the volume of open pores is 0.06 cm ^{3 / g or less.}
(2) The active material is characterized in that the thermal decomposition reaction of the active material when the temperature rises to 1000 ° C. in an air atmosphere is an exothermic reaction, and the maximum weight loss at that time is 50% or more. The composite active material for a lithium secondary battery according to (1).
(3) The composite active material for a lithium secondary battery according to (1) or (2), wherein the temperature difference of the exothermic peak in the thermal decomposition reaction of the active material is 150 ° C. or less.
(4) The lithium ion according to any one of (1) to (3), wherein the active material has a D50 of 1 to 40 μm and a BET specific surface area of 0.5 to ^{20 m 2 / g according to the BET method.} Composite active material for next battery.
(5) The D / G, which is the ratio of the G band derived from crystalline carbon to the D band derived from amorphous carbon in Raman spectroscopy of the active material, is 1.0 or more (1) to (4). ), The composite active material for a lithium secondary battery.
(6) A step of mixing Si or Si alloy, a carbon precursor, and a graphite component if necessary, a step of granulating and compacting, and a step of pulverizing and spheroidizing the mixture to form substantially spherical composite particles. A step, a step of calcining the composite particles in an inert atmosphere, a step of mixing the carbon precursor and the composite particles or calcined particles, and a calcined powder coated with carbon by heating the mixture in an inert gas atmosphere. Alternatively, the method for producing a composite active material for a lithium secondary battery according to any one of (1) to (5), which comprises a step of obtaining carbon-coated composite particles.
(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 particles 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 any one of (1) to (5). A lithium secondary battery containing the composite active material.

According to the present invention, for a lithium secondary battery capable of producing an electrode material having high coulombic efficiency even after repeated initial charging and discharging, and capable of producing a lithium secondary battery exhibiting excellent cycle characteristics. A composite active material and a method for producing the same can be provided.

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 secondary battery composite active material of the present invention has a thermal decomposition start temperature of 530 ° C. or higher when the active material is heated at a temperature rising rate of 5 ° C./min in an air atmosphere.

In the present invention, by satisfying the above requirements, the electrolytic solution is a secondary battery composite active material having a structure in which the composite grains or the calcined powder are coated with carbon species around the composite grains or the calcined powder. Since it does not come into direct contact with the composite particles or the calcined powder, the reaction with the electrolytic solution can be suppressed, and as a result, the cycle characteristics are improved.

The secondary battery composite active material of the present invention has a composition containing carbon and Si as main components, and in air, carbon reacts with oxygen in the air to cause an exothermic reaction due to oxidative decomposition. This decomposition temperature varies depending on the type and structure of the carbon species, but since graphite, which is a carbon species that is the main component of the composite, retains a crystal structure, the decomposition start temperature exceeds 700 ° C. On the other hand, as the carbon species for coating the composite grains or the calcined powder, a carbon species having a decomposition temperature of more than 530 ° C. is preferable, and the present invention has been made. A carbon seed having a decomposition temperature of 530 ° C. or lower cannot suppress the reaction with the electrolytic solution because the interaction with the composite granules or the calcined powder is weak.

The carbon type to be coated is not particularly limited as long as it is an amorphous carbon that interacts with the composite grains or the calcined powder, and examples thereof include hard carbon described later and soft carbon described later.

Further, in the secondary battery composite active material of the present invention, the thermal decomposition reaction of the active material when the temperature is raised to 1000 ° C. in the atmospheric atmosphere is an exothermic reaction, and the maximum weight loss at that time is 50% or more. Is preferable. If the maximum weight loss is less than 50%, it is difficult to maintain the structure of the complex because the Si atoms cannot be covered with graphite.

In the thermal decomposition reaction of the active material of the present invention, the temperature difference of the exothermic peak is preferably 150 ° C. or less. The exothermic reaction temperature changes the decomposition start temperature depending on the type and structure of the carbon species to be coated, in addition to graphite, which is the main component of the composite, but the carbon species coated on the graphite surface, which is the main component of the composite, is Along with interacting with graphite, the exothermic energy generated during the oxidative decomposition reaction of the coated carbon species accelerates the decomposition of the composite, and the temperature difference between the exothermic peaks becomes 150 ° C. or less. When the exothermic peak temperature exceeds 150 ° C., the interaction between the amorphous carbon used for coating and graphite is weak, or the composite grains or calcined powder are not uniformly coated, so that the cycle characteristics are deteriorated.

Since the secondary battery composite active material of the present invention has a structure in which the surface of the graphite structure is coated with carbon, it is the ratio of the G band derived from crystalline carbon and the D band derived from amorphous carbon in Raman spectroscopy. The D / G is preferably 1.0 or more.

The secondary battery composite active material of the present invention preferably has a D50 of 1 to 40 μm, more preferably 2 to 20 μm, and particularly preferably 3 to 10 μm in order to bring out the performance of the active material. If the particle size is too small, it is preferably 1 μm or more from the viewpoint of powder handling and agglutination. If the particle size becomes too large, current collection and dropout during expansion are likely to occur, so the particle size is preferably 40 μm or less.

D90 is preferably 2 to 60 μm, more preferably 3 to 40 μm, and particularly preferably 5 to 30 μm. When D90 is 2 μm or less, powder agglutination is likely to occur, which makes it difficult not only to handle but also to produce an electrode having good dispersibility. In particular, if coarse particles exceeding 60 μm are present in D90, the non-uniformity of the electrode slurry and uniform coating on the electrodes cannot be performed, so that the battery performance such as cycle characteristics deteriorates.

D50 and D90 correspond to a cumulative 50% and a cumulative 90% particle size 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.

Since the secondary battery composite active material of the present invention is an index showing the reactivity with the electrolytic solution, the BET specific surface area by the BET method ^{is preferably 0.5 to 20} m 2 / g, and more preferably 0. It is 5 to 15 m2 / g, particularly preferably 1 to 10 m2 / g.

The average pore diameter is 10 to 40 nm, preferably 10 to 30 nm, and particularly preferably 10 to 20 nm. Open pore volume is less than 0.06 cm ³ / g, preferably 0.04 cm ³ / g, particularly preferably at most 0.01 cm ³ / g.

By setting the average pore diameter, the volume of the open pores, and the BET specific surface area within this range, it is possible to suppress the intrusion of the electrolytic solution into the composite active material for the lithium secondary battery, 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 has a structure in which a Si compound is sandwiched between thin graphite layers having a thickness of 0.2 μm or less, and the structure is laminated and / or spreads like a mesh, and the thin graphite layer. Is curved near the surface of the composite active material particles for the lithium secondary battery to cover the composite active material particles for the lithium secondary battery.

The composite active material for a lithium secondary battery of the present invention may be a composite active material for a lithium secondary battery containing a Si or Si alloy (together, a Si compound) and a carbonaceous substance or a carbonaceous substance and a graphite component. preferable.

Si is general-purpose grade metallic silicon with a purity of about 98% by weight, chemical grade metallic silicon with a purity of 2 to 4N, polysilicon with a higher purity than 4N obtained by chlorination and distillation purification, and a precipitation step by the single crystal growth method. Ultra-high-purity single-crystal silicon that has undergone It is not particularly limited as long as it has a purity higher than that of general-purpose grade metallic silicon, such as a waste wafer that has become defective in the above.

The Si alloy 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, the D50 of the Si compound is preferably 0.01 to 0.6 μm, more preferably 0.05 to 0.5 μm, and particularly preferably 0.1 to 0.4 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 0.6 μm, cracks are severely caused by expansion due to lithium insertion, and cycle deterioration is likely to be severe.

The D90 of the Si compound is preferably 0.01 to 1.0 μm, more preferably 0.05 to 0.6 μm, and particularly preferably 0.1 to 0.5 μm.

Note that D50 and D90 are volume average particle diameters measured by a laser particle size distribution meter.

The BET specific surface area of the Si compound by the BET method ^{is preferably 40 to 300 m 2} / g, more preferably 60 to 300 m ² / g, and particularly preferably 60 to ²⁰⁰ m 2 / g.

The carbonaceous material is an amorphous or microcrystalline carbon substance, and examples thereof include easily graphitized carbon (soft carbon) which is graphitized by heat treatment exceeding 2000 ° C. and non-graphitizable carbon (hard carbon) which 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 that serves as a 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 precursors of hard carbon include novolak-type phenol resins, phenol resins such as resol-type phenol resins, and the like.

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, more preferably 1,000,000 or more, in that the effect of the composite active material for a lithium secondary battery 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 as a precursor of 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, or theirs. Derivatives and the like can be mentioned, with solid or liquids such as coal-based pitches (eg, coal tar pitches), petroleum-based pitches, mesophase pitches, coke, low-molecular-weight heavy oils, or derivatives thereof. Among them, soft carbon obtained from a precursor such as a coal-based pitch is preferable because the effect of the composite active material for a lithium secondary battery 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 can also be dissolved in the solvent used to mix the various components.

The weight average molecular weight of the precursor of the soft carbon used is preferably 1000 or more, more preferably 1,000,000 or more, in that the effect of the composite active material for a lithium secondary battery is more excellent.

Examples of the graphite component 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.

The number of laminated graphene sheets in the flake graphite is such that the Si compound and the flake graphite are more evenly dispersed, the expansion of the battery material using the composite active material for the lithium secondary battery is further suppressed, and / or In terms of more excellent cycle characteristics of the lithium secondary battery, 300 layers or less are preferable, 200 layers or less are more preferable, and 150 layers or less are further preferable. 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 composite active material for a lithium secondary battery 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, the thinned graphite is observed with an electron microscope (TEM), the thickness of 10 or more layers of laminated graphene sheets in the thinned graphite is measured, and the values are arithmetically averaged. By doing so, the average thickness can be 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 component in the composite active material for a lithium secondary battery.

Further, as a graphite component, 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 subjected to the spheroidizing treatment, the spheroidizing treatment is carried out together with other components (for example, a precursor of hard carbon, a precursor of soft carbon, a Si compound, etc.). You may.

The specific surface area of the graphite component 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 composite active material for a lithium secondary battery 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 the graphite component was measured by using the BET method (JIS Z 8830, one-point method) by nitrogen adsorption.

The graphite component 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 by the BET method of 40 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 the graphite component ^{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.

In the composite active material for a lithium secondary battery of the present invention, the content of the Si compound when the Si compound and the carbonaceous material are preferably contained 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. The content of the carbonaceous material is preferably 90 to 10 parts by mass, and particularly preferably 60 to 10 parts by mass. If the content of the carbonaceous material is less than 10 parts by mass, the carbonaceous material cannot cover the Si compound, the conductive path is insufficient, and capacity deterioration is likely to occur. If the content is larger than 90 parts by mass, the capacity is sufficient. I can't get it.

The content of the Si compound when the Si compound, the carbonaceous substance, and the graphite component are preferably contained is preferably 10 to 60 parts by mass, the content of the carbonaceous substance is preferably 5 to 60 parts by mass, and particularly 10 to 55 parts by mass. preferable. When the content of the carbonaceous material is less than 5 parts by mass, the carbonaceous material cannot cover the Si compound and graphite, the adhesion between the Si compound and graphite becomes insufficient, and the formation of composite active material particles for a lithium secondary battery is formed. Is likely to be difficult. On the other hand, if it is larger than 60 parts by mass, the effect of graphite having higher conductivity than that of carbonaceous material cannot be sufficiently brought out. The content of the graphite component is preferably 20 to 80 parts by mass, and particularly preferably 30 to 70 parts by mass. When the content of the graphite component 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 larger than 80 parts by mass, a sufficiently large capacity cannot be obtained as compared with the conventional graphite. ..

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

The method for producing a composite active material for a lithium secondary battery includes a step of mixing Si or Si alloy, a carbon precursor, and a graphite component as necessary, a step of granulating and compacting, and a crushing and spheroidizing treatment of the mixture. A step of forming substantially spherical composite particles, a step of calcining the composite particles in an inert atmosphere, a step of mixing the carbon precursor and the composite particles or the calcined powder, and a mixture thereof in an inert atmosphere. It includes a step of obtaining carbon-coated calcined powder or carbon-coated composite particles by heating.

As the raw material Si compound, it is preferable to use a powder having a 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 the non-graphitized carbon (hard carbon) which is difficult to be formed.

As the raw material graphite component, 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, these graphite components are acid-treated and oxidized, and then 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 by ultrasonic waves or the like. Graphene or the like delaminated 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 graphite component of the raw material 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 for graphene. It is about 5 mm.

Mixing with these Si compounds, carbon precursors and, if necessary, graphite components, if the carbon precursors are to be softened and liquefied by heating, the Si compounds, carbon precursors and further are required under heating. This can be done by kneading the graphite component according to the above. When the carbon precursor is soluble in a solvent, a Si compound, a carbon precursor, and a graphite component as needed are added to the solvent, and the Si compound and carbon are added to the solution in which the carbon precursor is dissolved. This can be done by dispersing and mixing the precursor and, if necessary, the graphite component 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, and, if necessary, the mixture with the graphite component are granulated and consolidated. Will be done. 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, or in a vacuum. 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 residual unpyrolyzed component of the carbon precursor increases the electrical resistance between the graphite layer and Si inside the composite particles and the composite particles, so that the discharge capacity tends to decrease. It is in. On the other hand, when the firing temperature exceeds 1200 ° C., there is a strong possibility that the reaction between the Si compound and the amorphous carbon or graphite component derived from the carbon precursor will occur, and the discharge capacity tends to decrease.

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. It is preferable to carry out a step of coating the inside and outside of the composite particles or the fired particles to produce the particles.

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 particles, methane is used in the inert atmosphere. , Ethane, ethylene, acetylene, propylene and other hydrocarbon gases are added and heated to coat the inside and outside of carbon-coated calcined powder or carbon-coated composite particles or carbon-coated calcined powder with a vapor phase in a vapor phase. Is preferable. As for the hydrocarbon gas, ethylene and propylene are particularly preferable from the viewpoint of performing a dense carbon coating. In particular, in order to uniformly coat the composite particles or the calcined powder with carbon, it is preferable to heat the composite particles or the calcined powder with a batch type or continuous type rotary kiln that can be stirred by rotation during firing. As a condition for carbon coating using a hydrocarbon gas by a rotary kiln, the temperature is preferably 300 to 1200 ° C. If the temperature is less than 300 ° C., the quality of the carbon coating film is poor and the BET specific surface area is high, so that the electrolytic solution is immersed and the cycle maintenance rate tends to decrease.

The flow rate of the hydrocarbon gas per minute is preferably 0.3 to 3 vol% with respect to the volume of the heating unit. If it is less than 0.3 vol%, the carbon film deposition efficiency is poor, and if it is higher than 3 vol%, the carbon film quality is poor.

Further, the composite active material for a lithium secondary battery of the present invention is a powder or calcined powder that has been pulverized and spherically treated, or a carbon-coated powder or a CVD carbon-coated powder after the step of coating the carbon film with the gas phase. It is preferable to carry out a step of classifying the body by wind force to manufacture the body.

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 a composite active material for a lithium secondary battery used as a battery material (electrode material) used in a lithium secondary battery.

The method for producing a negative electrode for a lithium secondary battery using a composite active material for a lithium secondary battery 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 for the lithium secondary battery when the slurry solvent is dried, the contact is broken (increased resistance) due to the cycle. ) Can be prevented.

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 for the lithium secondary battery, 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)
A known electrolytic solution can be used as the electrolytic solution used in the lithium secondary battery having a negative electrode obtained by using the composite active material for the lithium secondary battery.

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 for the lithium secondary battery. 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 containing the composite active material for a lithium secondary battery of the present invention has a Coulomb efficiency when charging and discharging are repeated 20 times, as compared with a lithium secondary battery using another composite active material for a lithium secondary battery. It's expensive. More specifically, it is 99% or more as compared with commercially available artificial graphite. The coulomb efficiency tends to gradually increase by repeating the charge / discharge cycle and become constant from about 20 times, and the coulomb efficiency at the 20th time is confirmed in order to reflect the long-term cycle maintenance rate.

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 TG-DTA curve of the composite active material for a silicon-based lithium secondary battery produced in Example 1 of this invention. It is a Raman spectroscopic spectrum of the composite active material for a silicon-based lithium secondary battery produced in Example 1 of this invention. It is a TG-DTA curve of the composite active material for a silicon-based lithium secondary battery manufactured in Reference Example 1 of this invention. It is a Raman spectroscopic spectrum of a composite active material for a silicon-based lithium secondary battery manufactured in Reference Example 1 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)
21% by weight of chemical grade metallic Si (purity 3N) having a D50 of 7 μm was mixed with ethanol, and fine pulverization using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. An ultrafine Si slurry having a 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.

1049 g of the ultrafine Si slurry, 300 g of the expanded graphite, 125 g of a resole-type phenol resin (weight average molecular weight (Mw) = 370), and 5 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 evacuated by an aspirator 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 a mixed dry product (light bulk density 212 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 391 g / L.

(Spheroidization process)
Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 300 seconds while being water-cooled, and at the same time sphericalized to obtain a substantially spherical composite powder having a light bulk density of 452 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 calcined powder having a graphite component content of 60 parts by mass, a Si content of 30 parts by mass, and a carbonaceous substance of 10 parts by mass (hard carbon content derived from phenol resin) was obtained.

Then, a substantially spherical calcined powder having a light bulk density of 466 g / L, a D50 of 22.0 μm, and a BET specific surface area of 65 m ^{2 / g was obtained through a mesh having an opening of 45 μm.}

(Carbon coating with coal tar pitch)
After mixing 150 g of the obtained substantially spherical calcined powder and 118 g of coal tar pitch with a ball mill, 150 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 into soft carbon by heating the mixture at 600 ° C. for 2 hours at a heating rate of 5 ° C./min while flowing nitrogen (4 L / min). As a result, the content of graphite component is 60 parts by mass, the content of Si is 30 parts by mass, and the content of carbonaceous material is 40 parts by mass (content of hard carbon derived from phenol resin is 10 parts by mass, content of soft carbon derived from coal tar pitch is 30 parts by mass). Part) was obtained as a composite active material for a silicon-based lithium secondary battery.

(Crushing / sieving)
The obtained composite active material for a silicon-based lithium secondary battery is crushed by a stamp mill, crushed by a ball mill, passed through a mesh having a mesh size of 45 μm, and crushed powder having a light bulk density of 405 g / L and a D50 of 15.9 μm. Got

(Carbon coating with vapor phase coat)
100 g of carbon-coated crushed powder produced by coal tar pitch was put into a batch rotary kiln TRB-0-1E manufactured by Takasago Industry Co., Ltd. CVD carbon coating was performed by heating for 1 hour. The amount of weight increase due to CVD carbon coating was 17% on average and 4.0 standard deviation measured at three locations in the same batch. As a result, the content of graphite component is 60 parts by mass, the content of Si is 30 parts by mass, and the content of carbonaceous material is 59 parts by mass (content of hard carbon derived from phenol resin is 10 parts by mass, content of soft carbon derived from coal tar pitch is 32 parts by mass). A composite active material for a silicon-based lithium secondary battery was obtained, which consisted of 17 parts by mass of a CVD carbon film).

Its physical characteristics are as follows. Particle size distribution D50: 8.0 μm, D90: 15.8 μm, BET specific surface area: 4.9 m ² / g, open pore volume: 0.02 cm ³ / g, shape: substantially spherical.

The results of thermal analysis in the atmosphere are shown in FIG. As a thermal analyzer, Bruker AX TG-DTA2020SA was used, about 5 mg of a sample _{was placed in an Al 2} O ₃ pan, and 50 ml / min of atmospheric gas Air was added. The temperature rise rate was 5 ° C./min. The change in thermal behavior with respect to _{Al 2} O ₃ which is a standard sample under the above conditions was investigated. The decomposition start temperature was determined by drawing a tangent line before and after the start of weight loss and defining the temperature at the intersection as the decomposition start temperature. As a result, the decomposition start temperature was 605 ° C and 657 ° C, and the difference temperature was 52 ° C. The maximum weight loss up to 1000 ° C. was 77.1%.

The Raman spectroscopic measurement spectrum is shown in FIG. The Raman spectroscope used an NRS-5100 manufactured by Nippon Spectroscopy, a laser wavelength of 532 nm, a laser intensity of 1.7 mW, and a grating of 1800 l / mm, an exposure time of 100 seconds, and a total of two measurements. The amount of defects (D band / G band ratio) determined from Raman spectroscopy was 1.15.

Further, the BET specific surface area of the substantially spherical mixture before carrying out the above (coating with coal tar pitch) was 63.3 m ² / g, and the obtained substantially spherical composite active material for a silicon-based lithium secondary battery was obtained. The BET specific surface area is 1.5 m ² / g, and the BET specific surface area is significantly reduced. Therefore, the structure is covered with soft carbon and a CVD carbon film in which the graphite component and Si are carbonaceous substances. You can see that.

(Manufacturing of negative electrode for lithium secondary battery)
To 95.5% by weight of the obtained composite active material for a silicon-based lithium secondary battery (content in the total solid content; the same applies hereinafter), 0.5% by weight of Super C65 manufactured by Timcal as a conductive auxiliary agent was added. , 4% by weight of CLP manufactured by Wako Pure Chemical Industries, Ltd. was mixed with water as a binder 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 4.9 mg / cm ^2, and dried at 110 ° C. for 2 hours in a vacuum dryer. After drying, the lithium ion secondary battery is punched into a circular shape of 14 mmφ ^{, uniaxially pressed under the condition of a pressure of 3 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 24 μm. Negative electrode for use 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. The electrolytic solution used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, the additive was FEC (fluoroethylene carbonate), and LiPF ₆ was dissolved to a concentration of 1.2 vol / L. bottom. 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 was cycle tested in a thermostatic chamber at 25 ° C. Charging was carried out at a constant current of 2.2 mA to 0.01 V, and then at a constant voltage of 0.01 V until the current value reached 0.2 mA. The discharge was performed with a constant current of 2.2 mA up to a voltage value of 1.5 V. The initial discharge capacity and the initial charge / discharge efficiency are the results of the initial charge / discharge test. In addition, the cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after the charge / discharge test 20 times under the charge / discharge conditions with the initial discharge capacity.
<Reference example 1>
(Preparation of expanded graphite)
Expanded graphite was prepared in the same manner as in Example 1.
(Mixing process)
21% by weight of chemical grade metallic Si (purity 3N) having a D50 of 7 μm was mixed with ethanol, and fine pulverization using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. An ultrafine Si slurry having a 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.

1049 g of the ultrafine Si slurry, 300 g of the expanded graphite, 125 g of a resole-type phenol resin (weight average molecular weight (Mw) = 370), and 5 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 evacuated by an aspirator 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 a mixed dry matter (light bulk density 235 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 467 g / L.

(Spheroidization process)
Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 300 seconds while being water-cooled, and at the same time sphericalized to obtain a substantially spherical composite powder having a light bulk density of 422 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 calcined powder having a graphite component content of 60 parts by mass, a Si content of 30 parts by mass, and a carbonaceous substance of 10 parts by mass (hard carbon content derived from phenol resin) was obtained.

Then, a substantially spherical calcined powder having a light bulk density of 599 g / L, a D50 of 20.0 μm, and a BET specific surface area of 64 m ^{2 / g was obtained through a mesh having an opening of 45 μm.}
(Carbon coating with coal tar pitch)
After mixing 150 g of the obtained substantially spherical calcined powder and 118 g of coal tar pitch with a ball mill, 150 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 into soft carbon by heating the mixture at 600 ° C. for 2 hours at a heating rate of 5 ° C./min while flowing nitrogen (4 L / min). As a result, the content of graphite component is 60 parts by mass, the content of Si is 30 parts by mass, and the content of carbonaceous material is 40 parts by mass (content of hard carbon derived from phenol resin is 10 parts by mass, content of soft carbon derived from coal tar pitch is 30 parts by mass). Part) was obtained as a composite active material for a silicon-based lithium secondary battery.

(Crushing / sieving)
The obtained composite active material for a silicon-based lithium secondary battery is crushed by a stamp mill, crushed by a ball mill, passed through a mesh having a mesh size of 45 μm, and crushed powder having a light bulk density of 462 g / L and a D50 of 15.9 μm. Got

Its physical characteristics are as follows. Particle size distribution D50: 5.7 μm, D90: 10.6 μm, BET specific surface area: 25.6 ^{m 2} / g, open pore volume: 0.04 cm ³ / g, shape: substantially spherical.

The results of thermal analysis in the atmosphere are shown in FIG. As a thermal analyzer, Bruker AX TG-DTA2020SA was used, about 5 mg of a sample _{was placed in an Al 2} O ₃ pan, and 50 ml / min of atmospheric gas Air was added. The temperature rise rate was 5 ° C./min. The change in thermal behavior with respect to _{Al 2} O ₃ which is a standard sample under the above conditions was investigated. The decomposition start temperature was determined by drawing a tangent line before and after the start of weight loss and defining the temperature at the intersection as the decomposition start temperature. As a result, the decomposition start temperature was 506 ° C and 666 ° C, and the difference temperature was 16052 ° C. The maximum weight loss up to 1000 ° C. was 79.0%.

The Raman spectroscopic measurement spectrum is shown in FIG. The Raman spectroscope used NRS-5100 manufactured by Nippon Spectroscopy, a laser wavelength of 532 nm, a laser intensity of 5.7 mW, and a grating of 1800 l / mm, an exposure time of 50 seconds, and a total of two measurements. The amount of defects (D band / G band ratio) determined from Raman spectroscopy was 0.4.

(Manufacturing of negative electrode for lithium secondary battery)
To 95.5% by weight of the obtained composite active material for a silicon-based lithium secondary battery (content in the total solid content; the same applies hereinafter), 0.5% by weight of Super C65 manufactured by Timcal as a conductive auxiliary agent was added. , 4% by weight of CLP manufactured by Wako Pure Chemical Industries, Ltd. was mixed with water as a binder 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 1.6 mg / cm ^2, and dried at 110 ° C. for 2 hours in a vacuum dryer. After drying, the lithium ion secondary battery is punched into a circular shape of 14 mmφ ^{, uniaxially pressed under the condition of a pressure of 3 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 15 μm. Negative electrode for use 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. The electrolytic solution used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, the additive was FEC (fluoroethylene carbonate), and LiPF ₆ was dissolved to a concentration of 1.2 vol / L. bottom. 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 was cycle tested in a thermostatic chamber at 25 ° C. Charging was carried out at a constant current of 0.34 mA to 0.01 V, and then at a constant voltage of 0.01 V until the current value reached 0.2 mA. The discharge was performed with a constant current of 0.34 mA up to a voltage value of 1.5 V. The initial discharge capacity and the initial charge / discharge efficiency are the results of the initial charge / discharge test. In addition, the cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after the charge / discharge test 20 times under the charge / discharge conditions with the initial discharge capacity.

Claims (7)

In the secondary battery composite active material, the thermal decomposition start temperature when the active material is heated at a temperature rising rate of 5 ° C./min in an air atmosphere is 530 ° C. or higher, the average pore diameter is 10 to 40 nm, and the volume of open pores is open. A composite active material for a lithium secondary battery, characterized in that it is 0.06 cm ^{3 / g or less.} The first aspect of the present invention is that the thermal decomposition reaction of the active material when the temperature of the active material is raised to 1000 ° C. in an air atmosphere is an exothermic reaction, and the maximum weight loss at that time is 50% or more. The above-mentioned composite active material for a lithium secondary battery. The composite active material for a lithium secondary battery according to claim 1 or 2, wherein the temperature difference of the exothermic peak is 150 ° C. or less in the thermal decomposition reaction of the active material. The composite activity for a lithium secondary battery according to any one of claims 1 to 3, wherein the active material has a D50 of 1 to 40 μm and a BET specific surface area according to the BET method is 0.5 to ^{20 m 2 / g.} material. The claim is characterized in that the G band derived from crystalline carbon in Raman spectroscopy of the active material is composed of two or more , and the D / G which is the ratio with the D band derived from amorphous carbon is 1.0 or more. The composite active material for a lithium secondary battery according to any one of 1 to 4. A step of mixing Si or Si alloy, a carbon precursor, and a graphite component as necessary, a step of granulating and compacting, and a step of pulverizing and spheroidizing the mixture to form substantially spherical composite particles. A step of calcining the composite particles in an inert atmosphere, a step of mixing the carbon precursor with the composite particles or calcined particles, and heating the mixture in an inert atmosphere to carbon-coated calcined powder or carbon-coated. Step of obtaining composite particles, obtained carbon-coated composite particles, spherical composite particles or calcined powder and carbon precursor are calcined in an inert atmosphere, and a carbon film is vaporized inside and outside the composite particles or calcined particles. method for producing a composite active material for lithium secondary battery according to claim 1, the step of CVD carbon coating, characterized in containing Mukoto. The lithium secondary battery according to any one of claims 1 to 5. A lithium secondary battery containing the composite active material.

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