JP6961948B2 - Composite active material for silicon-based lithium secondary batteries and its manufacturing method - Google Patents

Description

The present invention relates to a silicon-based 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 coulombic efficiency defined as "nth discharge capacity ÷ nth charge capacity". 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 silicon-based 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. An object of the present invention is to provide and a method for producing the same.

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 a composite active material for a silicon-based lithium secondary battery containing a Si or Si alloy, a carbonaceous material or a carbonaceous material, and a graphite component, the BET specific surface area by the BET method is 0.1 to 10 m ² / g. , Average particle size (D50) is 3-40 μm, open pore volume is 0.06 cm ³ / g or less, and composite activity for silicon-based lithium secondary battery that does not have a broad peak at 10 to 20 ° in diffraction by an X-ray diffractometer. material.
(2) The average particle size (D50) of Si or Si alloy is 0.01 to 0.6 μm, D90 is 0.01 to 1.0 μm, and the BET specific surface area by the BET method is 40 to 300 m ² / g. The composite active material for a silicon-based lithium secondary battery according to (1).
(3) The composite for a silicon-based lithium secondary battery according to (1) or (2), wherein the content of the Si or Si alloy is 10 to 80 parts by mass, and the content of the carbonaceous substance is 90 to 10 parts by mass. Active material.
(4) The content of the Si or Si alloy is 10 to 60 parts by mass, the content of the carbonaceous substance is 5 to 60 parts by mass, and the content of the graphite component is 20 to 80 parts by mass (1) or ( The composite active material for a silicon-based lithium secondary battery according to 2).
(5) The Si or Si alloy has a structure sandwiched between a graphite thin layer having a thickness of 0.2 μm or less together with a carbonaceous material, and the structure spreads in a laminated and / or mesh pattern, and the graphite thin layer Is curved near the surface of the composite active material particles for a silicon-based lithium secondary battery to cover the composite active material particles for a silicon-based lithium secondary battery, and the outermost layer is covered with a carbonaceous material (1) to The composite active material for a silicon-based lithium secondary battery according to any one of (4).
(6) The graphite component contains 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd) by ICP emission spectroscopy. , Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U) with a purity of 99.9% by weight or more (1000 ppm or less) determined from the semi-quantitative value of impurities, and ion chromatography (IC) by the oxygen flask combustion method. ) The composite active material for a silicon-based lithium secondary battery according to any one of (1) to (5), wherein the amount of S according to the measurement method is 0.3% by weight or less and / or the BET specific surface area is 40 m ^{2 / g or less.} ..
(7) 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 the calcined powder, and a carbon-coated calcined powder by heating the mixture in the inert atmosphere. Alternatively, the method for producing a composite active material for a silicon-based lithium secondary battery according to any one of (1) to (6), which comprises a step of obtaining carbon-coated composite particles.
(8) The carbon-coated composite particles, spherical composite particles or calcined powder obtained in (7) and the hydrocarbon gas are calcined in a calcining furnace capable of uniformly mixing in an inert atmosphere to form a CVD carbon film. The method for producing a composite active material for a silicon-based lithium secondary battery according to (7), wherein a step of coating the inside and outside of particles or calcined powder is performed.
(9) The method for producing a composite active material for a silicon-based lithium secondary battery according to (7) or (8), wherein the firing furnace capable of uniformly mixing according to (8) is a rotary kiln.
(10) Any of (7) to (9), after the step of coating with the vapor phase, a step of wind-classifying the pulverized and spherically treated powder or calcined powder, carbon-coated powder, or CVD carbon-coated powder. A method for producing a composite active material for a silicon-based lithium secondary battery described in the above.
(11) A step of calcining the composite particles and the calcined powder together with the carbon precursor in an inert atmosphere and carbon-coated calcined powder or carbon-coated composite particles or carbon-coated by heating the hydrocarbon gas in the inert atmosphere. The method for producing a composite active material for a silicon-based lithium secondary battery according to any one of (7) to (10), wherein the temperature of the step of coating the inside and outside of the calcined powder with CVD carbon is 300 to 1200 ° C.

According to the present invention, for a silicon-based lithium secondary battery capable of producing an electrode material having high Coulomb efficiency even after repeated 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 silicon-based 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 silicon-based lithium secondary battery of the present invention is a composite active material for a silicon-based lithium secondary battery containing a Si or Si alloy (hereinafter, Si compound), a carbonaceous substance, or a carbonaceous substance and a graphite component. The BET specific surface area of the active material according to the BET method is 0.1 to 10 m ² / g, the average particle size (D50) is ³ to 40 μm, the open pore volume is 0.06 cm 3 / g or less, and the X-ray diffractometer is used. It is a composite active material for a silicon-based lithium secondary battery that does not have a broad peak at 10 to 20 ° in diffraction.

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 silicon-based lithium secondary battery, the average particle size (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. It is 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 average particle size (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.

The average particle size (D50) and the average particle size (D90) are volume average particle sizes 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 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 hard carbon precursor used is preferably 1000 or more, more preferably 1,000,000 or more, in that the effect of the composite active material for a silicon-based 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 silicon-based 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 solids or liquids such as 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 silicon-based 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 silicon-based 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 graphene sheets laminated in the flaky graphite is such that the Si compound and the flaky graphite are more evenly dispersed, the expansion of the battery material using the composite active material for a silicon-based lithium secondary battery is further suppressed, and / Alternatively, 300 layers or less are preferable, 200 layers or less are more preferable, and 150 layers or less are further preferable, in that the cycle characteristics of the lithium secondary battery are more excellent. 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 silicon-based 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 silicon-based 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 silicon-based 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 may be carried out together with other components (for example, precursors of hard carbon and soft carbon, Si compounds, etc.). ..

^{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 silicon-based 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 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 silicon-based lithium secondary battery of the present invention, the content of the Si compound when containing the Si compound and the carbonaceous material 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 containing the Si compound, the carbonaceous material, and the graphite component is preferably 10 to 60 parts by mass, the content of the carbonaceous material is preferably 5 to 60 parts by mass, and particularly preferably 10 to 55 parts by mass. .. When the content of the carbonaceous material is less than 10 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 composite active material particles for a silicon-based lithium secondary battery Is likely to be difficult to form. 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. ..

The particle size (D50: 50% volumetric particle size) of the composite active material for a silicon-based lithium secondary battery of the present invention is 3 to 40 μm in that the effect of the composite active material for a silicon-based lithium secondary battery is more excellent. , 5 to 35 μm, more preferably 5 to 30 μm.

The particle size (D90: 90% volume particle size) is not particularly limited, but 10 to 75 μm is preferable, and 10 to 60 μm is more preferable, in that the effect of the composite active material for a silicon-based lithium secondary battery is more excellent. 20 to 45 μm is more preferable.

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, in that the effect of the composite active material for a silicon-based lithium secondary battery 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, a composite active material for a silicon-based lithium secondary battery is added to a liquid and vigorously mixed using ultrasonic waves or the like, and the prepared dispersion liquid is introduced into an apparatus as a sample to perform 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 silicon-based lithium secondary battery has a BET specific surface area of 0.1 to 10 m ² / g, preferably 0.5 to 9 m ² / g, according to the BET method. Within this range, the solid electrolyte layer (SEI) formed on the surface of the composite active material for silicon-based lithium secondary batteries due to contact with the electrolytic solution and charging / discharging can be suppressed, and the initial coulomb efficiency and 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. Open pore volume is at most 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 and the open pore volume within this range, it is possible to suppress the intrusion of the electrolytic solution into the composite active material for a silicon-based 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 silicon-based lithium secondary battery are measured by the nitrogen adsorption multipoint method after vacuum-drying the sample at 300 ° C. for 30 minutes. ..

The composite active material for a silicon-based lithium secondary battery does not have a broad peak at 10 ° to 20 ° in X-ray diffraction. The 10 ° to 20 ° Broad Peak is considered to exhibit a long-period structure derived from the pores, and by not having this peak, immersion of the electrolytic solution can be suppressed, and the capacity retention rate and the Coulomb efficiency can be improved.

The method for measuring X-ray diffraction is a powder X-ray diffraction method for a composite active material for a silicon-based lithium secondary battery. The measurement conditions are that copper Kα rays are used as the characteristic X-rays, 40 kV, 40 mA, and a scan speed of 2 ° / min, and that there is no Broad Peak at 10 ° to 20 ° under these conditions.

The composite active material for a silicon-based 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 spreads in a laminated and / or mesh pattern, and graphite. It is preferable that the thin layer is curved near the surface of the composite active material particles for the silicon-based lithium secondary battery to cover the composite active material particles for the silicon-based lithium secondary battery.

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, it is preferable that the length is at least half the size of the composite active material particles for a silicon-based lithium secondary battery for electron transfer, and the composite active for a silicon-based lithium secondary battery. It is more preferable that the size is about the same as the size of the substance particles. When the graphite thin layer is mesh-like, it is preferable for electron transfer that the graphite thin layer is connected over half the size of the composite active material particles for a silicon-based lithium secondary battery, and the silicon-based lithium secondary battery. It is more preferable that the size is about the same as the size of the composite active material particles for use.

In the composite active material for a silicon-based lithium secondary battery, the graphite thin layer may be curved near the surface of the composite active material particle for the silicon-based lithium secondary battery to cover the composite active material particle for the silicon-based lithium secondary battery. preferable. With such a shape, there is a risk that the electrolytic solution will invade from the graphite thin layer end face and the Si compound or the graphite thin layer end face will come into direct contact with the electrolytic solution to form a reactant during charging and discharging, resulting in reduced efficiency. Is reduced.

The composite active material for a silicon-based lithium secondary battery preferably has a variation in the amount of CVD carbon coating in the same lot of 5 or less as a standard deviation. When the standard deviation is 5 or less, there is little variation in the CVD carbon coating, and a uniform carbon coating is provided. The carbon coating amount can be obtained by, for example, calculating from the weight increase amount before and after the CVD carbon coating and the decrease amount of the Si compound concentration in the composite active material for a silicon-based lithium secondary battery. The chemical vapor deposition (CVD) carbon coating is preferably obtained by heating a hydrocarbon gas such as methane, ethylene, acetylene or propylene in an inert atmosphere such as nitrogen or argon.

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

The method for producing a composite active material for a silicon-based 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 step of crushing and spheroidizing the mixture. A step of forming a substantially spherical composite particle by a chemical treatment, a step of firing the composite particle in an inert atmosphere, a step of mixing the carbon precursor and the composite particle or the calcined powder, and an inert mixture thereof. It includes a step of obtaining carbon-coated calcined powder or carbon-coated composite particles by heating in an atmosphere.

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 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 silicon-based lithium secondary battery of the present invention, carbon-coated composite particles, spherical composite particles or calcined powder obtained in the previous step and hydrocarbon gas are uniformly mixed in an inert atmosphere. It is preferable to carry out a step of firing in a possible firing furnace to coat the inside and outside of the composite particles or the calcined powder with the CVD carbon film.

Carbon-coated composite particles, spherical composite particles or calcined powder, carbon precursor and hydrocarbon gas are calcined in a calcining furnace capable of uniform mixing in an inert atmosphere, and the CVD carbon film is formed inside and outside the composite particles or calcined powder. In the case of coating with carbon-coated calcined powder or carbon-coated composite particles or carbon-coated by adding a hydrocarbon gas such as methane, ethane, ethylene, acetylene, and propylene to an inert atmosphere and heating the gas. It is preferable to coat the inside and outside of the fired powder with CVD carbon in a gas phase. 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 silicon-based lithium secondary battery of the present invention is obtained by pulverizing and spherically treating powder or calcined powder or carbon-coated powder or CVD carbon coating after a step of coating a carbon film with a gas phase. It is preferable to carry out a step of wind classifying the powder to produce the powder.

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 silicon-based lithium secondary battery of the present invention is useful as a composite active material for a silicon-based lithium secondary battery 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 for a silicon-based lithium secondary battery is not particularly limited, and a known method can be used.

For example, a silicon-based 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 silicon-based 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 silicon-based 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 a silicon-based lithium secondary battery when the slurry solvent is dried, contact breakage due to the cycle (contact breakage due to the cycle) Increased resistance) 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, preferably 0.5 parts by mass, based on 100 parts by mass of the composite active material for a silicon-based lithium secondary battery. More preferably, it is ~ 2 parts 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 silicon-based 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 a silicon-based 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 a separator used in a lithium secondary battery having a negative electrode obtained by using the composite active material for a silicon-based 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 using the composite active material for a silicon-based lithium secondary battery of the present invention has a Coulomb efficiency when charging and discharging are repeated 20 times, and another composite active material for a silicon-based lithium secondary battery is used. It is more expensive than a lithium secondary battery. 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 an SEM image of the composite active material for a silicon-based lithium secondary battery manufactured in Example 1 of this invention. It is a cross-sectional SEM image of the composite active material for a silicon-based lithium secondary battery manufactured in Example 1 of the present invention. It is an X-ray diffraction pattern of the composite active material for a silicon-based lithium secondary battery manufactured in Example 1 of this 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)
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 finely pulverized using zirconia beads having a diameter of 0.3 mm for 6 hours to obtain an average particle size (D50). An ultrafine Si slurry having a BET specific surface area of 0.2 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.

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 266 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 457 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 509 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 635 g / L, an average particle size (D50) of 22.0 μm, and a BET specific surface area of 63.3 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 has a light bulk density of 405 g / L and an average particle size (D50) of 15. A 9.9 μm ground powder was obtained.

(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 (capacity 100 L) manufactured by Takasago Industry Co., Ltd., and the heating rate was 200 ° C./hr, ethylene 2 L / min, and nitrogen 8 L / min. CVD carbon coating was performed by heating at 900 ° C. for 1 hour while flowing. 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: 32 μm, D90: 52 μm, BET specific surface area: 1.5 m ² / g, open pore volume: 0.003 cm ³ / g, shape: substantially spherical.

The secondary electron image of the composite active material for a silicon-based lithium secondary battery by SEM (scanning electron microscope) is shown in FIG. 1, and the secondary electron image of the particle cross section of the composite active material for a silicon-based lithium secondary battery is shown in FIG. Shown in 2.

From this, it can be seen that the composite active material for a silicon-based lithium secondary battery has a structure in which the graphite component and the Si compound are covered with soft carbon.

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.

The obtained composite active material for a silicon-based lithium secondary battery does not have a peak at 10 to 20 ° in X-ray diffraction (FIG. 3).

(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.
<Example 2>
(Classification process)
30 g of the CVD carbon-coated powder prepared in Example 1 was prepared.

This composite active material for a silicon-based lithium secondary battery is put into a wind power classifier (ATP-50 manufactured by Hosokawa Micron) and classified at a classifier rotation speed of 15,000 rpm. The average particle size (D50) is 8 μm and D90 is 16 μm. , A composite active material for a silicon-based lithium secondary battery having a BET specific surface area of 4.9 m ² / g, an open pore volume of 0.02 cm ³ / g, and a light bulk density of 220 g / L was obtained.

Further, the BET specific surface area of the substantially spherical mixture before carrying out the above (coating with coal tar pitch, which is a carbonaceous material) was 63.3 m ² / g, and the obtained substantially spherical silicon-based lithium secondary battery was used. The BET specific surface area of the composite active material is 4.9 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 carbon substances. You can see that it is taking.

(Manufacture of negative electrode for lithium ion 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. , CLPA manufactured by Wako Pure Chemical Industries, Ltd. as a binder was mixed with water in an amount of 4% by weight 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 4.2 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 20 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 10 times under the charge / discharge conditions with the initial discharge capacity.

Further, in the obtained composite active material for a silicon-based lithium secondary battery, an electrolytic solution (ethylene carbonate and diethyl carbonate was used as a mixed solvent having a volume ratio of 1: 1 and LIPF ₆ was dissolved at a concentration of 1.2 mol / L. Then, 2% by volume of fluoroethylene carbonate was added to the mixture), soaked in an argon atmosphere for 12 hours, dried, and the cross section was measured with EDS to measure the element F, which is an electrolytic solution component. As a result of confirming the presence or absence, no immersion was observed.
<Example 3>
The expanded graphite was prepared, crushed and sieved in the same manner as in Example 1.
(Carbon coating with vapor phase coat)
100 g of carbon-coated pulverized powder with a coal tar pitch was put into a rotary kiln, and CVD carbon coating was performed by heating at 950 ° C. for 1 hour while flowing a heating rate of 200 ° C./hr, propylene 1 L / min, and nitrogen 9 L / min. rice field. The weight increase due to the CVD carbon coating was 19%. 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 61 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 19 parts by mass of a CVD carbon film). Its physical characteristics are as follows. Particle size distribution D50: 30 μm, D90: 50 μm, BET specific surface area: 0.9 m ² / g, open pore volume: 0.003 cm ³ / g, shape: substantially spherical.

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 0.9 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.

(Manufacture of negative electrode for lithium ion secondary battery)
With respect to the obtained composite active material for a silicon-based lithium secondary battery, a negative electrode for a lithium ion secondary battery was obtained in the same manner as in Example 2, and an evaluation cell was prepared and evaluated.
<Example 4>
30 g of the CVD carbon-coated powder prepared in Example 3 was prepared.

This composite active material for a silicon-based lithium secondary battery was classified by the same method as in Example 2, and the average particle size (D50) was 7 μm, D90 was 12 μm, the BET specific surface area was 4.2 m ² / g, and the volume of open pores. : A composite active material for a silicon-based lithium secondary battery having a light bulk density of 220 g / L and ^{0.02 cm 3 / g was obtained.}
(Manufacture of negative electrode for lithium ion secondary battery)
With respect to the obtained composite active material for a silicon-based lithium secondary battery, a negative electrode for a lithium ion secondary battery was obtained in the same manner as in Example 2, and an evaluation cell was prepared and evaluated.
<Example 5>
The expanded graphite was prepared, crushed and sieved in the same manner as in Example 1.
(Carbon coating with vapor phase coat)
100 g of carbon-coated crushed powder by coal tar pitch is put into a rotary kiln, and the temperature rise rate is 200 ° C./hr, ethylene 0.5 L / min, nitrogen 1 L / min, and CVD by heating at 950 ° C. for 1.7 hours. Carbon coating was applied. The weight increase due to the CVD carbon coating was 27%. 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 69 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 27 parts by mass of a CVD carbon film). This composite active material for a silicon-based lithium secondary battery was classified under the same equipment and conditions as in Example 2, and the average particle size (D50) was 9 μm, D90 was 19 μm, and the BET specific surface area was 7.1 m ² / g. A composite active material for a silicon-based lithium secondary battery having an open pore volume: 0.02 cm ^{3 / g was obtained.}

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 7.1 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.

(Manufacture of negative electrode for lithium ion secondary battery)
With respect to the obtained composite active material for a silicon-based lithium secondary battery, a negative electrode for a lithium ion secondary battery was obtained in the same manner as in Example 2, and an evaluation cell was prepared and evaluated.
<Reference example 1>
The substantially spherical calcined powder obtained in Example 1 was used as the composite active material for the silicon-based lithium secondary battery of Reference Example 1, and the obtained composite active material for the silicon-based lithium secondary battery was 95.5% by weight (total solid content). 0.5% by weight of Lithium Super C65 as a conductive auxiliary agent, 4% by weight of CLPA manufactured by Wako Pure Chemical Industries as a binder, and water are mixed with respect to the content in the negative electrode. An agent-containing slurry was prepared.

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 3.9 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 19 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.
<Comparative example 1>
Compared to 95.5% by weight of commercially available artificial graphite (content in total solid content; the same applies hereinafter), 0.5% by weight of Super C65 manufactured by Timcal as a conductive auxiliary agent and CLPA manufactured by Wako Pure Chemical Industries as a binder. Was mixed with 4% by weight and water 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 4.7 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 22 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. 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.

As shown in Table 1 above, the half cell using the composite active material for a silicon-based lithium secondary battery of the present invention shown in Example 1 shows a high capacity of 900 mAh / g or more, but 99.0. It was confirmed that the 20th Coulomb efficiency was as high as% or more. The half cell using the composite active material for a silicon-based lithium secondary battery of the present invention shown in Examples 3 to 5 shows a high capacity of 850 mAh / g or more, but a high 20th coulomb of 98.0% or more. It was confirmed to show efficiency. In Reference Example 1, the efficiency of the 20th coulomb was inferior, and in Comparative Example 1, the charge / discharge capacity was inferior.

Claims (9)

And Si or Si alloy, a carbonaceous material and graphite components, comprise Ri Na, graphite components and Si or Si alloy has a covered structure in soft carbon and CVD carbon coating is carbonaceous material, silicon-based lithium secondary In the composite active material for batteries, the BET specific surface area by the BET method is 0.1 to 10 m ² / g, the average particle size (D50) is ³ to 40 μm, the open pore volume is 0.06 cm 3 / g or less, and the X-ray diffractometer. A composite active material for a silicon-based lithium secondary battery that does not have a broad peak at 10 to 20 ° in the diffraction by. Claim 1 in which the average particle size (D50) of Si or a Si alloy is 0.01 to 0.6 μm, D90 is 0.01 to 1.0 μm, and the BET specific surface area by the BET method is 40 to 300 m ^{2 / g.} The composite active material for a silicon-based lithium secondary battery described in. The composite active material for a silicon-based lithium secondary battery according to claim 1 or 2, wherein the content of the Si or Si alloy is 10 to 80 parts by mass, and the content of the carbonaceous substance is 90 to 10 parts by mass. The first or second claim, wherein the content of the Si or Si alloy is 10 to 60 parts by mass, the content of the carbonaceous substance is 5 to 60 parts by mass, and the content of the graphite component is 20 to 80 parts by mass. Composite active material for silicon-based lithium secondary batteries. The Si or Si alloy has a structure sandwiched between a graphite thin layer having a thickness of 0.2 μm or less together with a carbonaceous material, and the structure spreads in a laminated and / or mesh pattern, and the graphite thin layer is silicon-based. Any of claims 1 to 4, which are curved near the surface of the composite active material particles for a lithium secondary battery to cover the composite active material particles for a silicon-based lithium secondary battery, and the outermost layer surface is covered with a carbonaceous material. The composite active material for silicon-based lithium secondary batteries described in Crab. A step of mixing Si or Si alloy, a carbon precursor, and a graphite component, a step of granulating and compacting, a step of crushing and spheroidizing the mixture to form substantially spherical composite particles, and the step of forming the composite particles. A step of calcining in an inert atmosphere, a step of mixing the carbon precursor and the composite particles or the calcined powder, and heating the mixture in the inert atmosphere to produce carbon-coated calcined powder or carbon-coated composite particles. may include a step, further resulting carbon-coated composite particles, the sphericity composite particles or fired powder and firing to CVD carbon target film and a hydrocarbon gas at a uniform mixture can be fired furnace in an inert atmosphere The method for producing a composite active material for a silicon-based lithium secondary battery according to any one of claims 1 to 5, wherein the step of coating the inside and outside of the composite particles or the calcined powder is performed. The method for producing a composite active material for a silicon-based lithium secondary battery according to claim 6, wherein the uniformly-mixable firing furnace is a rotary kiln. The method for producing a composite active material for a silicon-based lithium secondary battery according to claim 6 or 7, wherein the step of wind-classifying the powder coated with the CVD carbon film is performed. The step of calcining the composite particles and the calcined powder together with the carbon precursor in an inert atmosphere and the carbon-coated calcined powder or the carbon-coated composite particles or the carbon-coated calcined powder by heating the hydrocarbon gas in the inert atmosphere. The method for producing a composite active material for a silicon-based lithium secondary battery according to any one of claims 6 to 8, wherein the temperature of the step of coating the inside and outside with CVD carbon is 300 to 1200 ° C., respectively.

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