JP2022190387A - Composite active material for lithium secondary battery and manufacturing method for the same - Google Patents

Abstract

To provide a composite active material for a lithium secondary battery with high capacity retention and coulombic efficiency.SOLUTION: The composite active material for a lithium secondary battery includes Si or a Si compound, amorphous carbon, and a void. Si or a Si compound is present around the void, and Si or a Si compound is adhered to amorphous carbon.SELECTED DRAWING: Figure 1

Description

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

Lithium secondary batteries are widely used in consumer electronics due to their characteristics of relatively high energy density, light weight and long life. However, with the development of electric vehicles, the demand for development of batteries with large capacity, high-speed charging/discharging characteristics, good cycle characteristics, and excellent safety is increasing. Such high power applications require electrodes with higher specific capacities than those used in existing lithium secondary batteries.

Carbon-based materials (e.g., graphite) are currently used as the main negative electrode material in commercial lithium secondary batteries, and their charge capacity in the form of graphite is about 372 milliampere hours per gram (mAh/g). is. In recent years, silicon has been actively studied as a high-capacity negative electrode material to replace carbon. The theoretical capacity of silicon is about 4200 mAh/g, which is more than ten times that of graphite. However, when using silicon as a negative electrode material, it is necessary to solve problems such as the low electronic conductivity of silicon, the collapse of particles due to large volume changes of silicon during charging and discharging, and the continuous decomposition of the electrolyte.

In order to solve these problems, efforts have been made to combine electronically conductive materials and silicon, or to introduce voids to mitigate the volume change. Non-Patent Document 1 partially achieves improvement in electron conductivity and capacity by combining silicon and carbon. However, continuous decomposition of the electrolytic solution due to expansion and contraction cannot be suppressed, and there are problems such as lower charge-discharge efficiency than graphite. Moreover, in Patent Document 1, it is reported that by encapsulating a composite active material in a conductive matrix, conductivity and reduction in volume change can be achieved. Although there is no specific description of the material synthesis method and battery evaluation results in the patent, since there are no voids around the active material, the stress due to volume change cannot be directly relieved, and the continuous electrolyte solution It can be easily assumed that the deterioration of the charge-discharge efficiency is large because the decomposition of the battery cannot be suppressed.

As previously mentioned, there remains a need for improved negative electrodes for use in lithium secondary batteries. Above all, achieving stable negative electrode performance is a challenge for many high-capacity materials.

Japanese Patent No. 6449154

Kong Lijuan et al. , Electrochimica Acta, 2016, 198, 144-155.

On the other hand, in recent years, from the viewpoint of safety in use of batteries, not only high capacity but also electrode materials that do not expand in volume even after repeated charging and discharging are required. If the volume expansion of the electrode material is large, leakage of the electrolyte occurs and the service life of the battery is shortened. Moreover, in recent years, the required properties of electrode materials have increased significantly, and the required level of cycle properties has also increased.

In view of the above circumstances, an object of the present invention is to provide a composite active material for a lithium secondary battery with high capacity retention rate and coulombic efficiency.

As a result of intensive studies on conventional techniques, the inventors of the present invention changed the structure of silicon by performing aging before evaluating cycle characteristics to form a structure (hollow structure) along the inner wall surface of the carbon matrix. As a result, the contact area between the silicon and the carbon matrix increases, the lithium conductivity improves, the lithium supply efficiency during charging and discharging improves, and the long-term cycle stability also improves.

The present invention has the following embodiments.

A composite active material for a lithium secondary battery containing Si or a Si compound, amorphous carbon, and voids, wherein Si or the Si compound is present around the voids, and the Si or the Si compound adheres to the amorphous carbon A composite active material for a lithium secondary battery characterized by

According to the composite active material for a lithium secondary battery of the present invention, a lithium secondary battery having a high capacity retention rate, a high coulombic efficiency, and excellent cycle characteristics can be realized.

In the composite active material for a lithium secondary battery, the ratio of the pore volume to the volume of the Si or Si compound is preferably 0.5-50.

In the above composite active material for a lithium secondary battery, it is preferable that the average number of Si or Si compounds accommodated in one of the pores is 4 or less.

In the above composite active material for a lithium secondary battery, the shortest distance between the Si or Si compound and the inner wall surface of the void accommodating the Si or Si compound is preferably 10 nm or less.

In the above composite active material for a lithium secondary battery, it is preferable that the shortest distance between each of the plurality of voids and the voids arranged around the voids is 1.0 μm or less.

Preferably, the composite active material for a lithium secondary battery further has an outer layer, and the outer layer contains crystalline carbon or amorphous carbon having a pore diameter of 10 nm or more.

In the composite active material for lithium secondary batteries, the crystalline carbon preferably satisfies at least one of the following conditions (1) to (3).
(1) 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) purity obtained from semi-quantitative values of impurities is 99% by weight or more.
(2) The amount of S determined by an ion chromatography (IC) measurement method using an oxygen flask combustion method is 1% by weight or less.
(3) A BET specific surface area of 100 m ² /g or less.

The particle size (D50) of the composite active material for lithium secondary batteries is preferably 0.3 to 50 μm.

The BET specific surface area of the composite active material for lithium secondary batteries is preferably 100 m ² /g or less.

The present invention also provides a method for producing the aforementioned composite active material for a lithium secondary battery, comprising: a first step of coating the Si or Si compound with a polymer film to obtain first particles; A second step of mixing or coating one particle with an amorphous carbon precursor to obtain a second particle, a third step of aggregating and firing the second particles to form a fired body, and charging and discharging. a method for producing a composite active material for a lithium secondary battery, including a fourth step of

According to the method for producing a composite active material for a lithium secondary battery of the present invention, it is possible to produce an electrode material whose volume change is suppressed during the initial charge, and a lithium secondary battery having a high capacity and excellent cycle characteristics is produced. A realizable composite active material for a lithium secondary battery can be produced.

In the manufacturing method described above, the polymer film is preferably formed using a monomer, an initiator and a dispersant.

The above production method preferably includes a step of carbon-coating the fired body after the third step. In the above production method, the amorphous carbon precursor is preferably polyacrylonitrile.

The present invention also provides an electrode composition for a lithium secondary battery containing the composite active material for a lithium secondary battery.

According to the electrode composition for a lithium secondary battery of the present invention, it is possible to prepare an electrode material in which the volume change is suppressed at the time of the first charge, and lithium can realize a lithium secondary battery having a high capacity and excellent cycle characteristics. Electrodes for secondary batteries can be produced.

Further, the present invention is an electrode containing the composite active material for a lithium secondary battery.

ADVANTAGE OF THE INVENTION According to the electrode of this invention, the volume change is suppressed at the time of initial charge, and it can implement|achieve the lithium secondary battery which has high capacity|capacitance and was excellent in cycling characteristics.

In the present invention, “amorphous carbon” refers to carbon having an X-ray diffraction peak half width of 3° or more on the (002) plane.

"Crystalline carbon" refers to carbon whose X-ray diffraction peak half width of the (002) plane is less than 3°.

INDUSTRIAL APPLICABILITY According to the present invention, a composite active material for a lithium secondary battery is capable of producing an electrode material with suppressed volume expansion after the first charge, and realizing a lithium secondary battery with high capacity and excellent cycle characteristics. , an electrode composition for a lithium secondary battery, an electrode for a lithium secondary battery, and a method for producing a composite active material for a lithium secondary battery.

<Composite Active Material for Lithium Secondary Batteries>
The composite active material (composite material) for a lithium secondary battery of the present invention will be described in detail below with reference to FIGS. 1 and 6 while showing an example of the present invention. FIG. 1 is a schematic diagram showing an example of the composite active material for a lithium secondary battery of the present invention, and FIG. 6 is a cross-sectional SEM image (×30,000) of the composite active material produced in Example 1 of the present invention. .

The composite active material for lithium secondary batteries of the present invention is a composite active material for lithium secondary batteries containing a Si-based material and amorphous carbon, wherein the Si-based material is included in the amorphous carbon. and a composite active material for a lithium secondary battery in which a plurality of structures in which the Si-based material is enclosed in the amorphous carbon are present. In the composite active material for a lithium secondary battery of the present invention, voids are included in the amorphous carbon, and the voids are present in the Si-based material.

Specifically, as shown in FIG. 1, the composite active material 100 for lithium secondary batteries of the present invention includes a matrix 1 having a plurality of voids 3 and a Si-based material 2 accommodated in the voids 3. , the matrix 1 is a composite active material for a lithium secondary battery containing amorphous carbon. Si-based materials are Si or Si alloys.

According to the composite active material for a lithium secondary battery of the present invention, a lithium secondary battery with high capacity retention rate and coulombic efficiency can be realized.

The composite active material for a lithium secondary battery of the present invention has a structure in which a plurality of voids 3 are included in amorphous carbon and voids 3 are included in Si-based material 2, as shown in FIG. That is, the matrix 1 contains a plurality of voids 3 , and the Si-based material 2 contains the voids 3 .

Furthermore, since the composite active material 100 has a plurality of voids 3 in the matrix 1 and contains the voids 3 in the Si-based material 2, Si-based cores are formed in the voids in the shell made of amorphous carbon. It has the following advantages compared to a collection of core-shell particles containing material. That is, the matrix 1 sufficiently secures conduction paths for electrons and lithium ions, and resistance to movement of electrons and lithium ions is reduced. Therefore, deterioration in rate characteristics and deterioration in capacity retention rate due to charging and discharging are suppressed.

As described above, the composite active material for lithium secondary batteries of the present invention is useful as a composite active material used for electrode materials (especially negative electrode materials) used in lithium secondary batteries.

The composite active material for lithium secondary batteries of the present invention contains amorphous carbon. The amorphous carbon content is preferably 20% by mass or more, more preferably 40% by mass or more, and particularly preferably 100% by mass. When the amorphous carbon content is 20% by mass or more, the denseness is improved, and the infiltration of the electrolytic solution into the composite active material can be suppressed, thereby improving the cycle characteristics.

The voids in the composite active material for a lithium secondary battery of the present invention are introduced to relax the expansion stress of Si or Si compounds. Therefore, the ratio of void volume to Si or Si compound volume is preferably 0.5 to 50, more preferably 1 to 30, even more preferably 2 to 10, still more preferably 3 to 10, 3 to 7 are particularly preferred. When the volume ratio is within this range, expansion of the composite active material is moderated, and the volumetric capacity of the composite active material is less likely to decrease.

Methods for calculating the ratio of void volume to Si or Si compound volume include the following methods.

First, the negative electrode for a lithium secondary battery is cut in the vertical direction (thickness direction) of the electrode using a cross section processing device. Alternatively, the composite active material is cut. A cross-section polisher is preferably used as an apparatus for cross-section processing in order to obtain a clearer image. After that, the obtained cross-sectional portion is observed using a microscope. As the microscope used here, it is preferable to use a field emission scanning electron microscope (FE-SEM) because it is necessary to obtain sufficient resolution and observation range. The microscopic image obtained here is hereinafter referred to as a "cross-sectional SEM image". After that, two transparent sheets were placed on the printed image of the secondary electron image obtained. Fill in the corresponding part with a pen. As the transparent sheet, it is preferable to use an OHP sheet (overhead projector sheet) because of its good workability. Next, each image is converted to JPEG or TIFF data, binarized using Nano Hunter NS2K-Pro (Nano System Co., Ltd.), and the area S1 of the portion corresponding to Si or Si compound and the portion corresponding to the void , the area S2 of is calculated. After that, the areas S1 and S2 thus calculated are converted into the volume V1 of the portion corresponding to Si or Si compound and the volume V2 of the portion corresponding to the void. At this time, assuming that the portion corresponding to Si or the Si compound and the portion corresponding to the void are circles, the radius r of the circle is calculated using the following formula (A), and based on the calculated radius r, The volume V is calculated using the following formula (B). Thus, the ratio of void volume to Si or Si compound volume can be calculated.
r=(area/π) ^1/2 (A)
V=4πr ³ /3 (B)
The half width of the X-ray diffraction peak of the (002) plane of amorphous carbon in the present invention is preferably 15° or less, more preferably 3° to 12°. When the half width of the X-ray diffraction peak of the (002) plane of amorphous carbon is 15° or less, the initial charge/discharge efficiency is improved.

In the composite active material for a lithium secondary battery of the present invention, the average number of Si or Si compounds contained in each void contained in the amorphous carbon is preferably 4 or less, more preferably 2 or less. is. When the average number is 4 or less, the electron conductivity is further ensured, and the expansion of Si or Si compound is efficiently alleviated.

As a method for calculating the average number of Si or Si compounds contained in each void contained in amorphous carbon, the following method can be used.

For the cross-sectional SEM image of the composite active material obtained by the above method, a plurality of voids containing Si or Si compounds are selected, and the total number of Si or Si compounds contained in those voids is measured. . The number of voids to be selected should be as large as possible, but from the viewpoint of workability, it is 10 or more, preferably 20 or more. By calculating the average value by dividing the total number of measured Si or Si compounds by the number of measured voids, the average number of Si or Si compounds accommodated per void in amorphous carbon can be calculated.

The shortest distance between Si or the Si compound in the composite active material for a lithium secondary battery of the present invention and the inner wall surface of the void containing Si or the Si compound is preferably 10 nm or less. When the shortest distance is 10 nm or less, the conductivity of electrons and lithium ions is further improved. As a method of calculating the shortest distance between Si or a Si compound in the composite active material and the inner wall surface of the void containing Si or the Si compound, there is a method of measuring the distance on a cross-sectional SEM image. The shortest distance is more preferably 10 nm or less, and even more preferably 5 nm or less. In particular, the shortest distance is preferably 0 nm. That is, it is preferable that Si or the Si compound and the inner wall surface of the void are in contact with each other. In this case, when the Si or Si compound expands during charging, the stress applied to the inner wall surfaces of the voids by Si or the Si compound is more effectively reduced.

In the composite active material for a lithium secondary battery of the present invention, in each of the plurality of voids, the shortest distance to the voids arranged around the voids is 1.0 μm or less, more preferably 0.7 μm or less, more preferably 0.7 μm or less. is preferably 0.5 μm or less. In this case, contact between Si or the Si compound and the electrolyte is further suppressed, and the life of the battery is further improved. Further, when the shortest distance is 1.0 μm or less, the electron transfer effect of the matrix 1 is enhanced. In terms of electron transfer, the matrix 1 preferably has a mesh shape when viewed in cross section.

The particle size (D50: 50% volume particle size) of the composite active material for lithium secondary batteries of the present invention is preferably 0.3 to 50 μm, more preferably 0.3 to 40 μm. When the particle size (D50) of the composite active material for a lithium secondary battery is 0.3 to 50 μm, the smoothness of the electrode surface and the density of the composite active material in the electrode can be improved. That is, when the particle size is 0.3 μm or more, aggregates composed of a plurality of composite active materials are less likely to be formed when an electrode-forming composition containing a composite active material for a lithium secondary battery is applied. Since the coatability of the electrode-forming composition is improved, the smoothness of the electrode surface can be further improved. Further, when the particle size is 50 μm or less, the filling property of the composite active material in the electrode can be further improved, and the density of the composite active material in the electrode can be further improved.

The particle size (D90: 90% volume particle size) is preferably 1 to 75 μm, more preferably 2 to 60 μm. When the particle size (D90) of the composite active material for a lithium secondary battery is 1 to 75 μm, the smoothness of the electrode surface and the density of the composite active material in the electrode can be improved. That is, when the particle size is 1 μm or more, aggregates composed of a plurality of composite active materials are less likely to be formed when an electrode-forming composition containing a composite active material for a lithium secondary battery is applied. Since the coatability of the forming composition is improved, the smoothness of the electrode surface can be further improved. Further, when the particle size is 75 μm or less, the filling property of the composite active material in the electrode can be further improved, and the density of the composite active material in the electrode can be further improved.

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

In the measurement, the composite active material for lithium secondary batteries was added to the liquid and vigorously mixed using ultrasonic waves, etc., and the prepared dispersion liquid was introduced as a sample into a device (laser particle size distribution analyzer), and the particles were measured. A diameter (D50 or D90) measurement is taken. If the composite active material and the liquid do not mix well, a surfactant or the like may be added as necessary. As the liquid, it is preferable to use water, alcohol, or a low-volatility organic solvent for workability.

The composite active material for lithium secondary batteries of the present invention preferably has a BET specific surface area of 100 m ² /g or less, more preferably 0.5 to 70 m ² /g, particularly preferably 0.5 to 30 m ² /g. is g. With a BET specific surface area of 100 m ² /g or less, a solid electrolyte layer (SEI) formed on the surface of the active material by contact with the electrolytic solution and charging and discharging while introducing voids inside the composite active material. can be suppressed, and the initial charge/discharge efficiency and capacity retention rate can be improved.

The BET specific surface area is a value measured using the BET method (JIS Z 8830, one-point method) by nitrogen adsorption.

The Si referred to in the present invention is not particularly limited as long as it has a purity equal to or higher than that of general-purpose grade metallic silicon. Specifically, such Si includes general-purpose grade metallic silicon with a purity of about 98% by weight, chemical grade metallic silicon with a purity of 2 to 4N, and polysilicon with a purity higher than 4N that has been chlorinated and purified by distillation. , ultra-high-purity single-crystal silicon that has undergone a deposition process by a single-crystal growth method, or doped with an element of group 13 or 15 of the periodic table to make it p-type or n-type, wafers generated in the semiconductor manufacturing process scraps from polishing and cutting, and waste wafers that have become defective in the process.

The Si compound referred to in the present invention refers to a Si alloy and a non-metallic compound containing Si.

A Si alloy is an alloy containing Si as a main component. In the Si alloy, the element other than Si is preferably one or more elements of Groups 2 to 15 of the periodic table, and preferably an element in which the melting point of the phase contained in the alloy is 900° C. or higher. Among the elements of groups 2 to 15 of the periodic table, groups 2 to 4, 7, 8, and 11 to 14 are preferred.

Nonmetallic compounds containing Si are oxides, carbides, nitrides, fluorides, sulfides, phosphides, and hydrides containing Si as a main component.

In the composite active material for lithium secondary batteries of the present invention, the particle size (D50) of Si or Si compound is preferably 0.01 to 5 μm, more preferably 0.01 to 1 μm, and particularly preferably 0 0.05 to 0.6 μm. When D50 is 0.01 μm or more, the capacity and initial efficiency are less likely to decrease due to surface oxidation. When D50 is 5 μm or less, cracking due to expansion due to lithium insertion is less likely to occur, and cycle deterioration is less likely to occur. The particle diameter (D50) is the volume average diameter measured with a laser particle size distribution meter.

The content of Si or Si compound is preferably 5 to 80 parts by mass, particularly preferably 15 to 50 parts by mass, based on the total of 100 parts by mass of Si or Si compound and amorphous carbon. When the content of Si or Si compound is 5 parts by mass or more with respect to the total of 100 parts by mass of Si or Si compound and amorphous carbon, a sufficiently large capacity can be obtained. When the content of Si or Si compound is 80 parts by mass or less with respect to a total of 100 parts by mass of Si or Si compound and amorphous carbon, cycle deterioration is less likely to occur.

The amorphous carbon of the present invention is not particularly limited as long as it is amorphous carbon, but amorphous or microcrystalline carbonaceous materials other than graphite are preferable as the amorphous carbon.

Amorphous or microcrystalline carbonaceous materials other than graphite are obtained by firing precursors of amorphous carbon. Examples of such amorphous carbon precursors include polyaniline, polypyrrole, polyacrylonitrile, polyvinyl alcohol, polyglycerin, polyparaphenylene vinylene, polyimide resin, resorcinol-formaldehyde resin, phenol resin, epoxy resin, melamine resin, Urea resin, cyanate resin, furan resin, ketone resin, unsaturated polyester resin, urethane resin, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, pyrrole, dopamine, ammonium alginate, cellulose, glucose, saccharin , sugars such as fructose, coal-based pitch (eg, coal tar pitch), petroleum-based pitch, mesophase pitch, coke, low-molecular-weight heavy oil, or derivatives thereof. Among them, polyaniline, polypyrrole, polyacrylonitrile, polyvinyl alcohol, polyimide resin, resorcinol-formaldehyde resin, phenolic resin, sugars such as dopamine, glucose, saccharin, fructose, coal-based pitch (eg, coal tar pitch), petroleum-based pitch, or Derivatives thereof are preferred, and particularly preferred are polyaniline, polyacrylonitrile, polyvinyl alcohol, phenol resin, coal-based pitch (eg, coal tar pitch), and derivatives thereof.

The temperature (firing temperature) for firing the amorphous carbon precursor may be any temperature at which carbonization occurs, preferably 300 to 1500°C, particularly preferably 500 to 1300°C, and more preferably 600°C. ~1100°C. When the firing temperature is 300° C. or higher, carbonization tends to proceed. On the other hand, when the firing temperature is 1500° C. or lower, the reaction between Si or the Si compound and the inert gas described later tends to be less likely to occur, and the decrease in discharge capacity tends to be less likely to occur.

Firing is preferably performed in an inert gas atmosphere, and examples of the inert gas used include nitrogen, argon, and helium. Among them, nitrogen is preferred.

In the composite active material for a lithium secondary battery of the present invention, the content of amorphous carbon is preferably 20 to 95 parts by mass, preferably 30 to 85 parts by mass, based on a total of 100 parts by mass of Si or Si compound and amorphous carbon. Parts by mass are particularly preferred. When the content of amorphous carbon is 20 parts by mass or more in a total of 100 parts by mass of Si or Si compound and amorphous carbon, amorphous carbon can cover Si or Si compound, and the conductive path is sufficient. As a result, capacity deterioration is less likely to occur. When the content of amorphous carbon is 95 parts by mass or less based on the total of 100 parts by mass of Si or Si compound and amorphous carbon, a sufficient capacity is likely to be obtained.

The composite active material for a lithium secondary battery of the present invention may further have an outer layer, and the outer layer may contain crystalline carbon or amorphous carbon having a pore size of 10 nm or more. The presence of crystalline carbon in the outer layer improves adhesion between the composite active materials when an electrode is produced by press molding, and improves the smoothness of the surface of the composite active material. As a result, the electrode density is further improved. In addition, when the composite active material further has an outer layer containing amorphous carbon on the outside, the pore diameter of the amorphous carbon is preferably 10 nm or more and 1000 nm or less, more preferably 10 nm or more and 500 nm or less, and particularly preferably. is 10 nm or more and 200 nm or less. When the pore diameter is within this range, the compatibility between the composite active material and the electrolytic solution is improved, and the charge capacity and rate characteristics of the lithium secondary battery are improved. Specifically, when the pore diameter of the amorphous carbon is 10 nm or more, the compatibility between the composite active material and the electrolytic solution is improved, and when the pore diameter is 1000 nm or less, the electrode density is less likely to decrease, and lithium In the secondary battery, the reaction between Si and the electrolytic solution is less likely to occur, and the capacity retention rate is less likely to decrease.

The pore size in the outer layer can be measured by the BJH method. As a measuring device, an automatic specific surface area/pore size distribution measuring device Tristar 3000 manufactured by Shimadzu Corporation can be used.

The amorphous carbon may be the same as or different from the amorphous carbon contained in the composite active material for lithium secondary batteries. When the amorphous carbon contained in the outer layer is different from the amorphous carbon contained in the composite active material for lithium secondary batteries, the charge/discharge efficiency can be further improved.

The crystalline carbon of the present invention is not particularly limited as long as it has crystallinity, but carbon derived from graphite is preferable as the crystalline carbon.

Graphite-derived carbon is obtained by firing graphite. Examples of such graphite include natural graphite materials and artificial graphite. Among them, exfoliated graphite obtained by exfoliating natural graphite, which is usually called graphite, is preferable.

In the present specification, exfoliated graphite means graphite having 400 or less layers of graphene sheets. Note that the graphene sheets are bonded together mainly by van der Waals forces.

The number of layers of graphene sheets in the exfoliated graphite is such that the battery active material capable of combining with lithium ions and the exfoliated graphite are more uniformly dispersed, and the expansion of the battery material using the composite active material for lithium secondary batteries is more efficient. It is preferably 300 or less, more preferably 200 or less, and even more preferably 150 or less in terms of suppression and/or better cycle characteristics of the lithium secondary battery. The number of graphene sheet layers is preferably 5 or more from the viewpoint of handleability.

The number of graphene sheet layers in exfoliated graphite can be measured using a transmission electron microscope (TEM).

The average thickness of the exfoliated graphite is preferably 40 nm or less, more preferably 22 nm or less, from the viewpoint of better rate characteristics of the lithium secondary battery. The average thickness of exfoliated graphite is preferably 4 nm or more because the production procedure is simplified.

In addition, as a method for measuring the average thickness, exfoliated graphite is observed with a TEM, the thickness of the layer of graphene sheets laminated in 10 or more exfoliated graphite is measured, and the values are arithmetically averaged. A method of obtaining an average thickness is used.

Exfoliated graphite is obtained by exfoliating a graphite compound between its layers and exfoliating it into flakes.

Examples of exfoliated graphite include so-called expanded graphite.

Expanded graphite contains graphite. Expanded graphite is produced, for example, by treating flake graphite with concentrated sulfuric acid, nitric acid, hydrogen peroxide solution, etc., and intercalating these chemicals in the gaps between graphene sheets. Further, it is obtained by widening the gaps between the graphene sheets when the intercalated chemical solution is vaporized by heating. 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. In other words, expanded graphite can also be used as the graphite in the composite active material for lithium secondary batteries.

In addition, expanded graphite that has been subjected to a spheroidizing treatment can also be used as graphite. The procedure of the sphering process will be detailed later. As will be described later, when the expanded graphite is subjected to a spheronization treatment, other components (for example, hard carbon and soft carbon precursors, battery active materials that can be combined with lithium ions, etc.) are used together with spheroidization. Processing may be performed.

Crystalline carbon or graphite preferably satisfies at least one of the following (1) to (3).
(1) The purity is 99% by weight or more, or the amount of impurities is 10000 ppm or less.
(2) S content is 1% by weight or less (3) BET specific surface area is 100 m ² /g or less.

When the purity is 99% by weight or more, or the amount of impurities is 10000 ppm or less, the irreversible capacity due to the formation of SEI derived from impurities decreases, so the initial charge-discharge efficiency, which is the discharge capacity relative to the initial charge capacity, tends to be difficult to decrease. .

When the amount of S is 1% by weight or less, the irreversible capacity is decreased as in (1), and the initial charge/discharge efficiency is less likely to decrease. More preferably, the S content is 0.5% by weight or less.

The BET specific surface area of crystalline carbon or graphite is more preferably 5 to 100 m ² /g, particularly preferably 20 to 50 m ² /g. When the BET specific surface area of the crystalline carbon or graphite is 100 m ² /g or less, the area where the crystalline carbon or graphite reacts with the electrolytic solution can be reduced, so the initial charge/discharge efficiency is less likely to decrease.

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

The amount of impurities is measured by ICP emission spectrometry, 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) are measured by semi-quantitative impurity values. In addition, the amount of S is measured by ion chromatography (IC) after subjecting graphite to combustion absorption treatment by the oxygen flask combustion method, filtering it with a filter.
<Method for producing composite active material for lithium secondary battery>
Next, a method for producing a composite active material for lithium secondary batteries according to the present invention will be described with reference to FIGS. FIG. 2 is a schematic cross-sectional view showing an example of the first particles in the method for producing a composite active material for lithium secondary batteries of the present invention, and FIG. FIG. 4 is a schematic cross-sectional view showing an example of two particles, and FIG. 4 is a schematic cross-sectional view showing an example of an aggregate of the second particles in FIG.

As shown in FIGS. 2 to 4, the method for producing a composite active material for a lithium secondary battery of the present invention is a method for producing the above-described composite active material for a lithium secondary battery, comprising Si or a Si compound, A first step of coating a polymer film to obtain first particles, a second step of mixing or coating the first particles with an amorphous carbon precursor to obtain second particles, and a plurality of second particles. It includes a third step of assembling and sintering the aggregate to form a sintered body, and a fourth step of charging and discharging.

According to the method for producing a composite active material for a lithium secondary battery of the present invention, it is possible to produce an electrode material whose volume change is suppressed during the initial charge, and a lithium secondary battery having a high capacity and excellent cycle characteristics is produced. A realizable composite active material for a lithium secondary battery can be produced.

In the production method described above, the polymer film is formed using, for example, a monomer, an initiator and, if necessary, a dispersant. This solution preferably contains a dispersing agent.

In the manufacturing method described above, firing is performed, for example, in an inert atmosphere.

The third step includes a step of mixing the second particles with crystalline carbon as necessary to obtain a first mixture, and a step of granulating and compacting the first mixture to obtain a second mixture. and pulverizing and spheronizing the second mixture to form substantially spherical composite particles, and firing the composite particles as a fired body in an inert atmosphere.

The above production method may or may not further include a step of carbon-coating the fired body after the third step, but it is preferable to include it.

As Si or Si compound, it is preferable to use a powder having a particle size (D50) of 0.01 to 5 μm. In order to obtain Si or a Si compound having a predetermined particle size, the Si or Si compound raw material (ingot, wafer, powder, etc.) is pulverized by a pulverizer, and a classifier is used in some cases. When the raw material of Si or Si compound is a lump such as an ingot or a wafer, it can be pulverized using a coarse pulverizer such as a jaw crusher and then pulverized using a fine pulverizer. Examples of such fine pulverization devices include ball mills, medium agitating mills, and the like, in which pulverizing media such as balls and beads are moved, and impact, friction, and compression forces generated by the kinetic energy of the pulverization media are used to pulverize the material. Roller mills that use the compressive force of rollers to pulverize, jet mills that pulverize materials by colliding them against the lining material at high speed or colliding particles with each other, and pulverize them by the impact force of the impact, hammers, blades, pins, etc. Hammer mills, blade mills, pin mills, and disk mills that use the impact force generated by the rotation of a fixed rotor to crush crushed materials, colloid mills that use shear force, and the high-pressure wet opposed-collision disperser "Ultimizer." etc. can be used.

Either wet pulverization or dry pulverization can be used for the pulverization. For further fine pulverization, very fine particles can be obtained by, for example, using a wet bead mill to gradually reduce the diameter of the beads. In addition, dry classification, wet classification, or sieving classification can be used in order to adjust the particle size distribution after pulverization. Dry classification mainly uses air flow, and the processes of dispersion, separation (separation of fine and coarse particles), collection (separation of solid and gas), and discharge are performed sequentially or simultaneously, and interference between particles, particle In order not to reduce the classification efficiency due to the shape, airflow turbulence, velocity distribution, static electricity, etc., pretreatment (adjustment of moisture, dispersibility, humidity, etc.) is performed before classification, or the moisture content of the airflow used is and adjust the oxygen concentration. In the dry type with an integrated classifier, pulverization and classification are performed at once, and a desired particle size distribution can be obtained.

Another method of obtaining Si or a Si compound with a predetermined particle size is a method of heating Si or a Si compound by plasma or laser to evaporate it and then solidifying it in an inert atmosphere, or CVD using a gas raw material. and plasma CVD, and these methods are suitable for obtaining ultrafine particles of 0.1 μm or less.

The surface of the Si or Si compound may or may not be modified, but it is preferable to modify the surface of the Si or Si compound particles in advance in order to promote the reaction between the Si or Si compound and the monomer. The term "modification" as used herein refers to a process of changing the surface state of Si or a Si compound through a chemical reaction using a surface modifier to facilitate coating with a polymer (polymer film). As the surface modifier, it is preferable to use one or more compounds selected from the group consisting of molecules containing an alkoxide group, a carboxyl group or a hydroxy group in the molecule, bases and oxidizing agents. Specific surface modifiers include, for example, vinyl-based surface modifiers such as vinyltrimethoxysilane and vinyltriethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, and 3-glycidoxypropylmethyldiethoxysilane. , 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane and other epoxy-based surface modifiers, p-styryltrimethoxysilane and other styryl-based surface modifiers, 3-methacryloxypropylmethyl Methacrylic surface modifiers such as dimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, acrylics such as 3-acryloxypropyltrimethoxysilane surface modifiers, isocyanurate surface modifiers such as tris-(trimethoxysilylpropyl) isocyanurate or isocyanate surface modifiers such as 3-isocyanatopropyltriethoxysilane, tetraethoxysilane, hydrochloric acid, peroxide Oxidizing agents such as hydrogen, nitric acid, sulfuric acid, potassium permanganate, potassium dichromate, sodium hypochlorite, chromium trioxide, ammonium persulfate, potassium persulfate, ammonia, sodium hydroxide, potassium hydroxide, sodium bicarbonate , potassium hydrogen carbonate, sodium carbonate, potassium carbonate and the like. Surface modifiers are preferably 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, tetraethoxysilane, peroxide One or more selected from the group consisting of hydrogen oxide, nitric acid, hydrochloric acid, ammonia and sodium hydroxide, particularly preferably one or more selected from the group consisting of 3-methacryloxypropyltrimethoxysilane, tetraethoxysilane, hydrochloric acid and ammonia. .

When using a surface modifier, it is preferable to add 0.1 to 800 parts by mass of the surface modifier to 100 parts by mass of Si or Si compound. In order to prevent aggregation of particles during the modification reaction, a polycarboxylic acid-based stabilizer may be added as necessary. In order to promote the reforming reaction, if necessary, a compound that dissolves in water such as ammonia, sodium hydroxide, potassium hydroxide, or sodium hydrogen carbonate and exhibits alkalinity, or a compound that dissolves in water such as hydrochloric acid, nitric acid, acetic acid, or sulfuric acid. Residual reaction accelerators such as compounds exhibiting acidity may be added. Ammonia, hydrochloric acid, or nitric acid is preferred because it has high reactivity and leaves no metal compound. When a residual reaction accelerator is used, it is preferable to add 0.005 to 54 parts by mass of the residual reaction accelerator to 100 parts by mass of Si or Si compound. The solvent used for the reaction may be any solvent capable of dissolving the surface modifier. Such solvents include water, ethanol, methanol, acetone, dimethylformamide, tetrahydrofuran, toluene, hexane, dichloromethane, chloroform, and the like. As the solvent, a mixed solvent of two or more of these may be used as necessary. When modifying the Si particle surface using 3-methacryloxypropyltrimethoxysilane or tetraethoxysilane as the surface modifier, it is preferable to use a mixed solvent of water and ethanol. The ratio of each solvent in the mixed solvent is preferably 10 to 100 parts by mass of water per 100 parts by mass of ethanol. When the ratio of water to ethanol in the mixed solvent is within this range, the Si or Si compound in the solvent can be easily dispersed, and the reforming reaction can proceed sufficiently.

After modifying the surface of Si or a Si compound, if necessary, the surface-modified Si or Si compound may be pulverized using a ball mill or a bead mill to form fine particles. Zirconia or alumina is preferable for the balls used for crushing. The crushing time is preferably 1 to 24 hours, more preferably 1 to 12 hours.

Moreover, after the surface-modified Si or Si compound is pulverized and atomized, the atomized surface-modified Si or Si compound may be separated by centrifugation if necessary. At this time, in the centrifugal separation, the solvent used for modifying the Si or Si compound surface may be replaced with water.

During the reaction of Si or a Si compound and a monomer, each raw material is uniformly mixed using a general mixer or stirrer such as a magnetic stirrer, a three-one motor, a homomixer, an in-line mixer, a bead mill, or a ball mill. The reaction temperature is preferably 40-100°C. Also, the reaction time is preferably 0.5 to 72 hours, more preferably 0.5 to 24 hours. When the reaction time is within this range, the reaction between Si or the Si compound and the monomer proceeds sufficiently, and the productivity is less likely to decrease.

Examples of monomers to be reacted with Si or Si compounds include styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, methacrylic acid. methacrylic acids such as isobutyl, tert-butyl methacrylate, 2-ethylhexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, triethylene glycol methacrylate; system, itaconic anhydride, itaconic acid, acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, sec-butyl acrylate, isobutyl acrylate, tert acrylate acrylic acid-based monomers such as -butyl, 2-ethylhexyl acrylate, isobornyl acrylate, benzyl acrylate, phenyl acrylate, glycidyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, and hydroxybutyl acrylate , methacrylamide, N-methylacrylamide, N,N'-dimethylacrylamide, N-tert-butylmethacrylamide, Nn-butylmethacrylamide, N-methylolmethacrylamide, N-ethylolmethacrylamide, etc. acrylamide-based monomers such as N,N'-methylenebisacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide, Nn-butylacrylamide, N-methylol acrylamide, N-ethylolacrylamide, Vinyl benzoate, diethylaminostyrene, diethylamino alpha-methylstyrene, p-vinylbenzenesulfonic acid, p-vinylbenzenesulfonic acid sodium salt, p-vinylbenzenesulfonic acid lithium salt, divinylbenzene, vinyl acetate, butyl acetate, vinyl chloride, fluorine Vinyl chloride, vinyl bromide, maleic anhydride, N-phenylmaleimide, N-butylmaleimide, N-vinylpyrrolidone, N-vinylcarbazole, acrylonitrile, aniline, pyrrole, polyol-based monomers used in urethane polymerization or isocyanate-based monomers monomers. Polymeric monomers are preferably styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, isobutyl methacrylate, tert- Methacrylic acid monomers such as butyl, 2-ethylhexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, triethylene glycol methacrylate, itaconic acid anhydride, itaconic acid, acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, sec-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, acrylic acrylic monomers such as 2-ethylhexyl acid, isobornyl acrylate, benzyl acrylate, phenyl acrylate, glycidyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, and hydroxybutyl acrylate; divinylbenzene; Acrylonitrile, more preferably styrene, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, Acrylonitrile, particularly preferably styrene, methyl methacrylate, methyl acrylate or acrylonitrile.

Examples of initiators to be used include azo compounds such as azobisisobutyronitrile, potassium persulfate, ammonium persulfate, benzoyl peroxide, diisobutyryl peroxide, di-n-propylperoxydicarbonate, diisopropylperoxydicarbonate. Carbonate, dilauroyl peroxide, dibenzoyl peroxide, 1,1-di(tert-hexylperoxy)cyclohexane, 1,1-di(tert-butylperoxy)cyclohexane, tert-butyl hydroperoxide and diisobutyryl peroxide , tert-hexylperoxyisopropyl monocarbonate, tert-butylperoxyisopropyl monocarbonate, 2,5-di-methyl-2,5-di(benzoylperoxy)hexane, tert-butylperoxyacetate, di-tert-hexyl peroxide , di-tert-butyl peroxide, diisopropylbenzene hydroperoxide, tert-butyl hydroperoxide and the like.

Solvents used in obtaining the monomer slurry include, for example, water, ethanol, methanol, isopropyl alcohol, propanol, and toluene, preferably water, ethanol, or methanol, and particularly preferably water or ethanol. These can be used alone or in combination of two or more.

The monomer content in the monomer slurry is preferably 0.5 to 20% by weight, particularly preferably 0.5 to 10% by weight. When the monomer content is within this range, the coating around Si or Si compound can have a sufficient thickness.

The initiator content in the monomer slurry is preferably 0.01 to 3% by weight, particularly preferably 0.01 to 1% by weight.

The monomer slurry preferably contains a dispersant to improve the dispersibility of Si or Si compound or to promote polymerization. Examples of the dispersant include polyvinyl alcohol, polyvinylpyrrolidone, sodium styrenesulfonate, lithium styrenesulfonate, ammonium styrenesulfonate, styrenesulfonic acid-based dispersants such as ethyl styrenesulfonate, carboxystyrene, polyacrylic acid, Polycarboxylic acid-based dispersants such as polymethacrylic acid, naphthalenesulfonic acid formalin condensation-based dispersants, polyethylene glycol, polycarboxylic acid partial alkyl ester-based dispersants, polyether-based dispersants, polyalkylenepolyamine-based dispersants, alkylsulfonic acids system dispersants, quaternary ammonium-based dispersants, higher alcohol alkylene oxide-based dispersants, polyhydric alcohol ester-based dispersants, alkylpolyamine-based dispersants or polyphosphate-based dispersants, preferably polyacrylic acid-based dispersants agents (dispersants), styrenesulfonic acid-based dispersants and polyvinylpyrrolidone, particularly preferably styrenesulfonic acid-based dispersants and polyvinylpyrrolidone.

The content of the dispersant in the monomer slurry is preferably 3% by weight or less, particularly preferably 0.001 to 2% by weight. When the amount of the dispersing agent is within this range, aggregation of Si or Si compounds is less likely to proceed. Alternatively, the film thickness of the polymer (polymer film) surrounding Si or the Si compound becomes less likely to become thin.

The monomer slurry may contain a polymerization accelerator to facilitate polymerization. Examples of the polymerization accelerator include pH adjusters such as sodium hydrogen carbonate and potassium hydroxide. The polymerization accelerator is preferably sodium bicarbonate.

The obtained polymer film coated with Si or Si compound is removed by firing, which will be described later, to form voids.

As a method of mixing or coating Si or a Si compound (first particles) coated with a polymer film with an amorphous carbon precursor, any one of the following production methods 1 and 2 can be used.

Production method 1: A method of mixing or coating a polymer, which is a precursor of amorphous carbon, around a polymer film by polymerization.

Production method 2: A method of mixing or coating Si or a Si compound (first particles) coated with a polymer film and an amorphous carbon precursor by wet or dry mixing.

The polymer that is the precursor of amorphous carbon used in production method 1 is not particularly limited as long as it is carbonized by baking to become amorphous carbon. Examples include polyimide resin, polyaniline, polypyrrole, polyvinyl alcohol, polyacrylonitrile, polyparaphenylenevinylene, and the like. Among them, polyaniline, polypyrrole and polyacrylonitrile are preferred. Polyaniline and polyacrylonitrile are particularly preferred as the polymer that serves as a precursor of amorphous carbon. Among these, polyacrylonitrile is more preferable. In this case, a more dense matrix is obtained, and penetration of the electrolytic solution into the composite active material is suppressed, thereby suppressing decomposition of the electrolytic solution inside the composite active material and improving cycle characteristics.

When mixing or coating the polymer by polymerization, the initiators, monomers, dispersants, polymerization accelerators and solvents described above can be used.

At this time, since the first particles act as seeds and promote the reaction, it is not necessary to add a surface modifier, but the above-described surface modifier may be added as necessary.

The precursor of amorphous carbon used in production method 2 is not particularly limited as long as it becomes amorphous carbon after firing. , polyimide resin, resorcinol-formaldehyde resin, phenol resin, epoxy resin, melamine resin, urea resin, cyanate resin, furan resin, ketone resin, unsaturated polyester resin, urethane resin, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene- Styrene (ABS) resin, pyrrole, dopamine, ammonium alginate, cellulose, glucose, saccharin, sugars such as fructose, coal-based pitch (e.g., coal tar pitch), petroleum-based pitch, mesophase pitch, coke, low-molecular heavy oil, Alternatively, derivatives thereof and the like are included. Among them, polyaniline, polypyrrole, polyacrylonitrile, polyvinyl alcohol, polyimide resin, resorcinol-formaldehyde resin, phenolic resin, sugars such as dopamine, glucose, saccharin, fructose, coal-based pitch (eg, coal tar pitch), petroleum-based pitch, or Derivatives thereof and the like are preferred. Precursors of amorphous carbon are particularly preferably polyaniline, polyacrylonitrile, polyvinyl alcohol, phenolic resins, coal-based pitch (eg coal tar pitch) or derivatives thereof.

The method of mixing the first particles and the amorphous carbon precursor is not particularly limited, and examples include a method of mixing the dried first particles and the amorphous carbon precursor in a solid state, and a method of drying the first particles. A method of impregnating a substance with a slurry containing an amorphous carbon precursor and mixing, a method of adding an amorphous carbon precursor to a slurry containing the first particles and mixing in a liquid phase, and the like can be used.

As a method for mixing the dried first particles and the amorphous carbon precursor in a solid state, for example, after drying the first particles, the dried first particles and the amorphous carbon precursor are mixed in a mortar. Mixing in a mortar, mixing using a ball mill, bead mill, pot mill, roller mill, jet mill or the like is preferred, and mixing in a mortar or ball mill is particularly preferred.

As a method of impregnating and mixing the dried first particles with the slurry containing the amorphous carbon precursor, for example, after drying the first particles, the amorphous carbon precursor is dissolved in a solvent. A method in which a solution or dispersion is prepared to form a slurry, and the dried first particles are added to the solution or slurry and mixed; A method of preparing a slurry, adding it to the dried product of the first particles, and mixing them, or the like can be used. The solvent is not particularly limited as long as it can dissolve or disperse the precursor of amorphous carbon. Aromatic compounds, pyridine, piperidine, cyclohexanone, cyclohexane, hexane, ethyl acetate, acetone, dichloromethane, chloroform, creosote oil, glycerin, water, etc. Among them, alcohols such as ethanol and methanol, ethers such as tetrahydrofuran aromatic compounds such as toluene and xylene, cyclohexanone and water are preferred, and ethanol, xylene and water are particularly preferred. The mixing method is not particularly limited, and general mixers and stirrers such as magnetic stirrers, three-one motors, homomixers, in-line mixers, bead mills and ball mills can be used.

The method of adding the amorphous carbon precursor to the slurry containing the first particles and mixing in the liquid phase is not particularly limited. A mixer or stirrer can be used. The solvent for the slurry is not particularly limited, and examples include alcohols such as ethanol, methanol and isopropyl alcohol, ethers such as tetrahydrofuran and diethyl ether, aromatic compounds such as benzene, nitrobenzene, toluene and xylene, pyridine and piperidine. , cyclohexanone, cyclohexane, hexane, ethyl acetate, acetone, dichloromethane, chloroform, creosote oil, glycerin, water, and the like. Among them, alcohols such as ethanol and methanol, ethers such as tetrahydrofuran, aromatic compounds such as toluene and xylene, cyclohexanone and water are preferred, and ethanol, xylene and water are particularly preferred.

The temperature at which a plurality of mixtures (second particles) in which the first particles are mixed or coated with the amorphous carbon precursor is aggregated and fired is preferably 300 to 1500 ° C., more preferably 500 to 1300 ° C. and particularly preferably 600 to 1100°C. When the firing temperature is 300° C. or higher, the polymer formed around Si or the Si compound is less likely to remain, resulting in a decrease in the initial volume discharge capacity, a decrease in the initial charge/discharge efficiency, and an increase in the initial electrode expansion coefficient. unlikely to occur. On the other hand, when the firing temperature is 1500° C. or lower, the reaction between Si or the Si compound and the inert gas described later tends to be less likely to occur, and the decrease in discharge capacity tends to be less likely to occur.

Firing is preferably performed in an inert gas atmosphere. As the inert gas, nitrogen, argon, helium, etc. are preferable, and nitrogen is particularly preferable.

By performing this baking, the polymer film around Si or the Si compound is volatilized, voids are generated around the Si or the Si compound, and the amorphous carbon precursor of the aggregate of the second particles is carbonized. , composite particles of amorphous carbon and Si or a Si compound are produced as a sintered body.

The fourth step of charging and discharging can be performed not only after the third step, but also as a negative electrode for a lithium secondary battery, which will be described later.

As a specific charging/discharging process, for example, a cyclic charging/discharging process can be performed in a constant temperature environment of 20 to 65° C. in a coin cell. Regarding the cycle charge/discharge conditions in the charge/discharge process, the charge/discharge current is preferably 0.01 to 10 C, particularly preferably 0.05 to 5 C, and the charge/discharge voltage is 0.010 to 1.5 V vs. It can be done within the Li/Li+ range. Charging can be performed by constant current-constant voltage charging (CCCV charging), and discharging can be performed by constant current discharging (CC discharging). Also, the number of cycles is preferably 1 to 100 cycles, particularly preferably 50 to 100 cycles.

As a method of carbon-coating the composite particles after the third step, there is a method of carbon-coating by a CVD (Chemical Vapor Deposition) method, a method of vaporizing a carbon precursor under heating to coat with carbon, or a method of applying a carbon precursor to the composite particles. A method of carbon-coating by mixing the body and then firing it may be mentioned.

In the method of carbon-coating the composite particles by the CVD method, the carbon-coating can be performed by heating the carbon compound.

Examples of carbon compounds used here include methane, ethylene, acetylene, propylene, benzene, toluene, xylene, naphthalene, anthracene, pyrene, acenaphthylene, dihydroanthracene, diphenylene sulfide, thioxanthene, thianthrene, carbazole, acridine, condensed poly( Cyclic phenazine compounds and the like can be mentioned, among which ethylene, acetylene, propylene, toluene, xylene, naphthalene, anthracene and the like are preferred, and ethylene, anthracene, toluene and the like are particularly preferred.

The temperature for heating the carbon compound is preferably 300 to 1500°C, particularly preferably 500 to 1100°C.

In the CVD method, the composite particles may be coated with the carbon compound as carbon, and the pressure may be normal pressure or reduced pressure.

The carbon precursor used in the method of vaporizing the carbon precursor under heating to coat with carbon is not particularly limited as long as it becomes carbon after firing. Examples of such carbon precursors include polyaniline, polypyrrole, polyacrylonitrile, polyvinyl alcohol, polyglycerin, polyparaphenylenevinylene, polyimide resin, resorcinol-formaldehyde resin, phenol resin, epoxy resin, melamine resin, urea resin, cyanate resin. , furan resin, ketone resin, unsaturated polyester resin, urethane resin, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, pyrrole, dopamine, ammonium alginate, cellulose, sugars such as glucose, saccharin, fructose , coal-based pitch (eg, coal tar pitch), petroleum-based pitch, mesophase pitch, coke, low-molecular-weight heavy oil, or derivatives thereof. Among them, polyaniline, polypyrrole, polyacrylonitrile, polyvinyl alcohol, polyimide resin, resorcinol-formaldehyde resin, phenolic resin, sugars such as dopamine, glucose, saccharin, fructose, coal-based pitch (eg, coal tar pitch), petroleum-based pitch, or Derivatives thereof are preferred, and particularly preferred are polyaniline, polyacrylonitrile, polyvinyl alcohol, phenol resin, coal-based pitch (eg, coal tar pitch), and derivatives thereof.

The heating temperature may be any temperature at which the carbon precursor is vaporized, preferably 300 to 1500.degree. C., particularly preferably 500 to 1300.degree. C., and more preferably 600 to 1100.degree. When the temperature is 300° C. or higher, the carbon precursor is less likely to remain, and the initial volumetric discharge capacity is less likely to decrease, the initial charge/discharge efficiency is less likely to decrease, and the initial electrode expansion coefficient is less likely to increase. On the other hand, when the temperature is 1500° C. or lower, the reaction between Si or Si compound and an inert gas described later tends to be less likely to occur, and the decrease in discharge capacity tends to be less likely to occur.

It is preferable to vaporize the carbon precursor under heating in an inert gas atmosphere. Nitrogen, argon, helium, etc. are mentioned as an inert gas to be used. Among them, nitrogen is preferred.

The carbon precursor coating method in the method of mixing the carbon precursor with the composite particles and then calcining the carbon precursor is the same as the method of mixing the first particles and the amorphous carbon precursor described above. can be used.

The carbon precursor in the method of mixing the composite particles with the carbon precursor and then firing the carbon precursor to coat with carbon is the same as the carbon precursor used in the method of vaporizing the carbon precursor under heating to coat with carbon. can be used.

The firing temperature may be any temperature at which the carbon precursor vaporizes, preferably 300 to 1500.degree. C., particularly preferably 500 to 1300.degree. C., and more preferably 600 to 1100.degree. When the temperature is 300° C. or higher, the carbon precursor is less likely to remain, and the initial volumetric discharge capacity is less likely to decrease, the initial charge/discharge efficiency is less likely to decrease, and the initial electrode expansion coefficient is less likely to increase. On the other hand, when the temperature is 1500° C. or lower, the reaction between Si or Si compound and an inert gas described later tends to be less likely to occur, and the decrease in discharge capacity tends to be less likely to occur.

When the carbon precursor is calcined, it is preferably carried out in an inert gas atmosphere. Examples of the inert gas to be used include nitrogen, argon, and helium, among which nitrogen is preferred.

In the present invention, natural graphite, artificial graphite obtained by graphitizing petroleum or coal pitch, and the like can be used as the crystalline carbon that is optionally mixed with the second particles. As the shape of the crystalline carbon, scale-like, oval-like or spherical, columnar or fiber-like is used. In addition, expanded graphite obtained by subjecting the crystalline carbon to an acid treatment or an oxidation treatment, followed by heat treatment to expand the graphite layers so that a portion of the graphite layers are exfoliated to form an accordion, or a pulverized product of the expanded graphite, or ultrasonic waves, etc. Graphene or the like, which is delaminated by a method, can also be used. Expanded graphite or pulverized expanded graphite is more flexible than other crystalline carbons, and in the step of forming composite particles described later, the pulverized particles are re-bonded to form substantially spherical composite particles. can be easily formed. In view of the above, it is preferable to use expanded graphite or pulverized expanded graphite. The particle size of the crystalline carbon before being mixed with the second particles is preferably 1 to 100 μm for natural graphite and artificial graphite, and 5 μm to 5 mm for expanded graphite, pulverized expanded graphite, and graphene.

When mixing crystalline carbon with the second particles, the carbon compound can be added because Si or the Si compound and the crystalline carbon can be bound more. As the carbon compound, it is preferable that Si or a Si compound and crystalline carbon can be combined and that there is no residual carbon component after firing. Glycerols such as triglycerol fatty acid esters, glycols such as menthol, pentaerythritol, dipentaerythritol, tripentaerythritol, ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, polyethylene oxide, trimethylolpropane, poly(diallyldimethylammonium chloride) ), polyvinylpyrrolidone, and the like, preferably glycerin, polyglycerin, poly(diallyldimethylammonium chloride), polyvinylpyrrolidone, and particularly preferably glycerin.

When the high secondary particles are mixed with crystalline carbon, it is preferable to use a solvent. Examples of the solvent include quinoline, pyridine, toluene, benzene, tetrahydrofuran, creosote oil, tetrahydrofuran, cyclohexanone, and nitrobenzene. , glycerin, menthol, polyvinyl alcohol, water, ethanol, methanol can be used.

As a method for mixing the second particles and crystalline carbon, when the total concentration of solids composed of the second particles and crystalline carbon in the slurry containing the second particles and crystalline carbon is high, a kneader ( kneader) or Loedige mixer can be used. When a solvent is used, a three-one motor, a stirrer, a Nauta mixer, a Loedige mixer, a Henschel mixer, a high-speed mixer, a homomixer, an in-line mixer, etc. can be used in addition to the kneader described above.

When using a solvent to remove the solvent, the jacket can be heated directly with these devices, or the solvent can be removed with a vibration dryer, paddle dryer, rotary evaporator, thin film evaporator, spray dryer, conical dryer, vacuum dryer, etc. can do. Solid-liquid separation can be carried out with an apparatus such as a centrifugal separator, a filter press, a suction filter, or a pressure filter before the drying operation. If excess carbon compounds remain, the composite active materials will be linked together after firing, and after that, the pulverization and crushing process will be required, and it will cause a decrease in the capacity of the negative electrode. preferably.

By continuing stirring in the process of solvent removal in these apparatuses for 1 to 100 hours, a mixture (first mixture) of Si or a Si compound, crystalline carbon, and optionally a carbon compound is granulated and compacted. be. Further, the mixture after removing the solvent can be compressed with a compressor such as a roller compactor, and coarsely pulverized with a crusher to be granulated and compacted. The size of these granulated/consolidated products is preferably 0.1 to 5 mm for ease of handling in the subsequent pulverization step.

The methods of granulation and compaction include ball mills that use compressive force to pulverize crushed materials, medium agitating mills, roller mills that crush crushed materials using compressive force from rollers, and crushed materials that are used as lining materials at high speed. Jet mills that use the impact force generated by colliding or colliding particles with each other to pulverize, hammer mills and blades that pulverize materials using the impact force generated by the rotation of a rotor with hammers, blades, pins, etc. Dry pulverization methods such as mills, pin mills and disk mills are preferred. In addition, dry classification such as wind classification and sieving is used in order to adjust the particle size distribution after pulverization. In a type in which a pulverizer and a classifier are integrated, pulverization and classification are performed at once, and a desired particle size distribution can be obtained.

As a method of pulverizing and spheronizing the mixture (second mixture) obtained by granulation and compaction, there is a method of pulverizing by the above pulverization method to adjust the particle size and passing it through a dedicated spheronization device; There is a method of repeating the above-mentioned method of pulverizing the crushed material using the impact force caused by the rotation of the jet mill or rotor, or a method of prolonging the treatment time to make the crushed material spherical. Dedicated sphering devices include Hosokawa Micron's Faculty (trade name), Nobilta (trade name), and Mechanofusion (trade name), Nippon Coke Industry's COMPOSI, Nara Machinery's Hybridization System, and Earth Technica's. Kryptron Orb, Kryptron Eddy, etc. can be mentioned.

Composite particles having a substantially spherical shape can be obtained by performing the pulverization and spheronization treatment.

The obtained composite particles are fired to obtain a fired body.

The firing temperature is preferably 300 to 1200°C, particularly preferably 600 to 1200°C. When the firing temperature is 300° C. or higher, unthermally decomposed components of the polymer film coated with Si or Si compound are less likely to remain, the initial charge/discharge efficiency is less likely to decrease, and the initial charge expansion coefficient is less likely to increase. On the other hand, when the firing temperature is 1200° C. or lower, the reaction between Si or Si compound and carbon is less likely to occur, and the discharge capacity is less likely to decrease.

Firing of the composite particles is preferably carried out in an inert gas atmosphere. Nitrogen, argon, helium, etc. are mentioned as an inert gas to be used. Among them, nitrogen is preferred.

As a method for carbon-coating the fired body, there is a method of carbon-coating by the above-described CVD method, a method of vaporizing a carbon precursor under heating and coating with carbon, or a method of mixing a carbon precursor with the fired powder and further firing. Therefore, a method of carbon coating can be used.
<Electrode composition for lithium secondary battery>
The electrode composition for lithium secondary batteries of the present invention comprises the composite active material for lithium secondary batteries of the present invention, a binder, and a solvent.

According to the electrode composition for a lithium secondary battery of the present invention, it is possible to prepare an electrode material in which the volume change is suppressed at the time of the first charge, and lithium can realize a lithium secondary battery having a high capacity and excellent cycle characteristics. Electrodes for secondary batteries can be produced.

As the binder to be used, known materials can be used. Glue is used.

Examples of solvents include water, isopropyl alcohol, N-methylrollidone, dimethylformamide, and the like.

The electrode composition for a lithium secondary battery of the present invention can be obtained by mixing the composite active material for a lithium secondary battery of the present invention with a binder and forming a paste using a solvent. In addition, when making a paste, if necessary, a known stirrer, mixer, kneader, kneader, etc. may be used to stir and mix the composite active material for lithium secondary batteries, the binder and the solvent. good.
<Electrodes for lithium secondary batteries>
Next, the lithium secondary battery electrode of the present invention will be described with reference to FIG. FIG. 5 is a schematic diagram showing an example of the electrode of the present invention. As shown in FIG. 5, the lithium secondary battery electrode 200 of the present invention includes the lithium secondary battery composite active material 100 described above.

According to the lithium secondary battery electrode of the present invention, it is possible to realize a lithium secondary battery that is suppressed in volume change at the time of initial charging, has a high capacity, and has excellent cycle characteristics.

The lithium secondary battery electrode of the present invention is useful as a negative electrode of a lithium secondary battery.

A known method can be used to manufacture a negative electrode for a lithium secondary battery using the composite active material for a lithium secondary battery of the present invention.

For example, a negative electrode mixture-containing slurry is prepared as the aforementioned electrode composition for a lithium secondary battery. A negative electrode for a lithium secondary battery can be obtained by applying the negative electrode mixture-containing slurry onto a current collector, such as a copper foil.

In addition to the copper foil, the current collector is preferably a current collector having a three-dimensional structure because the cycle of the battery is superior. Materials for the current collector having a three-dimensional structure include, for example, carbon fiber, sponge-like carbon (sponge-like resin coated with carbon), and metals other than copper.

Current collectors having a three-dimensional structure (porous current collectors) include porous conductors of metals and carbon. Examples of such conductive porous materials include plain-woven wire mesh, expanded metal, lath mesh, metal foam, metal woven fabric, metal non-woven fabric, carbon fiber woven fabric, and carbon fiber non-woven fabric.

When preparing a slurry containing a negative electrode mixture using a composite active material for a lithium secondary battery, it is preferable to add one or more selected from the group consisting of conductive carbon black, carbon nanofibers and carbon nanotubes as a conductive material. . Even if the shape of the composite active material for lithium secondary batteries obtained by the above steps is granular (particularly approximately spherical), the group consisting of carbon black, carbon nanofibers and carbon nanotubes is added to the slurry containing the negative electrode mixture. By blending more than one selected, the contact between particles of the composite active material is less likely to be point contact. At least one selected from the group consisting of carbon black, carbon nanofibers and carbon nanotubes can concentrate in the capillary portion formed by contacting the composite active material for a lithium secondary battery during drying of the slurry solvent. Therefore, disconnection of contacts (increase in resistance) due to cycles can be prevented.

The amount of one or more selected from the group consisting of carbon black, carbon nanofibers and carbon nanotubes is preferably 0.2 to 4 parts by mass with respect to 100 parts by mass of the composite active material for lithium secondary batteries. , more preferably 0.5 to 2 parts by mass. Carbon nanotubes include single-wall carbon nanotubes and multi-wall carbon nanotubes.
<Lithium secondary battery>
A lithium secondary battery includes the above-described negative electrode as an electrode, a positive electrode, an electrolytic solution, and a separator. The lithium secondary battery may further include other battery components (eg current collector, gasket, sealing plate, case, etc.). A lithium secondary battery can have a shape such as a cylindrical shape, a rectangular shape, or a button shape according to a conventional method.
(positive electrode)
As a positive electrode used in a lithium secondary battery having a negative electrode obtained by using the composite active material for lithium secondary batteries of the present invention, a positive electrode using a known positive electrode material can be used.

Examples of the method for manufacturing the positive electrode include known methods, such as a method of applying a positive electrode mixture comprising a positive electrode material, a binder and a conductive agent to the surface of a current collector. Examples of positive electrode materials (positive electrode active materials) include metal oxides such as chromium oxide, titanium oxide, cobalt oxide, vanadium pentoxide, LiCoO ₂ , LiNiO ₂ , LiNi _1-y Co _y O ₂ , and LiNi _1-xy . Lithium metal _oxides such as _CoxAlyO2 , _LiMnO2 , _LiMn2O4 , and _{LiFeO2 ;} transition metal _chalcogen compounds such as titanium sulfide and molybdenum sulfide; and conductive materials such as polyacetylene, polyparaphenylene, and polypyrrole. Examples include conjugated polymer substances.
(Electrolyte)
Known electrolytes can be used as electrolytes used in lithium secondary batteries having a negative electrode obtained by using the composite active material for lithium secondary batteries of the present invention.

For example, electrolyte salts contained in the electrolyte include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB(C ₆ H ₅ ) ₄ , LiCl, LiBr, LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , LiN(CF _3SO2 ) _{2 , LiC ( CF3SO2} ) ₃ , _{LiN ( CF3CH2OSO2} ) ₂ , _{LiN ( CF3CF3OSO2} ) ₂ , _{LiN ( HCF2CF2CH2OSO2} ) ₂ , LiN{( _CF3 ) _{2CHOSO2}2, LiB{C6H3(CF3)2 } 4 , LiN ( SO2CF3 ) 2 , LiC ( SO2CF3} ) ₃ , _LiAlCl4 or _LiSiF6 , etc. can be used. LiPF ₆ and LiBF ₄ are particularly preferred 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.

Solvents used in the electrolytic solution include, for example, carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate and diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2 - methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxofuran, 4-methyl-1,3-dioxolane, anisole, ethers such as diethyl ether, sulfolane, thioethers such as methylsulfolane, acetonitrile, chloronitrile, propionitrile, etc. nitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2- Aprotic organic solvents such as oxazoline, ethylene glycol or dimethylsulfite can be used.

A polymer electrolyte such as a solid polymer electrolyte or a polymer gel electrolyte may be used instead of the electrolytic solution. Polymer compounds that make up the matrix of solid polymer electrolytes or polymer gel electrolytes include ether-based polymer compounds such as polyethylene oxide and its crosslinked products, methacrylate-based polymer compounds such as polymethacrylate, and acrylate-based compounds such as polyacrylate. Polymer compounds, fluorine-based polymer compounds such as polyvinylidene fluoride (PVDF) and vinylidene fluoride-hexafluoropropylene copolymer are preferred. These can also be mixed and used. From the viewpoint of oxidation-reduction stability, fluorine-based polymer compounds such as PVDF and vinylidene fluoride-hexafluoropropylene copolymer are particularly preferable.
(separator)
A known material can be used as a separator for a lithium secondary battery having a negative electrode obtained by using the composite active material for a lithium secondary battery of the present invention. Examples of separators include woven fabrics, non-woven fabrics, synthetic resin microporous membranes, and the like. Synthetic resin microporous membranes are preferred, and polyolefin microporous membranes are particularly preferred 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 combining these.

INDUSTRIAL APPLICABILITY The lithium secondary battery of the present invention is used in various portable electronic devices, particularly notebook personal computers, notebook word processors, palmtop (pocket) personal computers, mobile phones, mobile facsimiles, mobile printers, headphone stereos, video cameras, and mobile televisions. , portable CDs, portable MDs, electric shavers, electronic notebooks, transceivers, power tools, radios, tape recorders, digital cameras, portable copiers, portable game machines, and the like. In addition, the lithium secondary battery of the present invention can be used in electric vehicles, hybrid vehicles, vending machines, electric carts, power storage systems for load leveling, household power storage devices, distributed power storage systems (built into stationary electrical appliances), emergency It can also be used as a secondary battery for a time power supply system.

1 is a schematic cross-sectional view showing an example of a composite active material for a lithium secondary battery of the present invention; FIG. FIG. 3 is a schematic cross-sectional view showing an example of first particles in the method for producing a composite active material for lithium secondary batteries of the present invention. FIG. 3 is a schematic cross-sectional view showing an example of second particles in the method for producing a composite active material for lithium secondary batteries of the present invention. 4 is a schematic cross-sectional view showing an example of an aggregate of second particles in FIG. 3. FIG. It is a schematic diagram which shows an example of the electrode of this invention. 1 is a cross-sectional SEM image (×30,000) of the composite active material produced in Example 1 of the present invention.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these.

<Example 1>
(Silicon surface modification step)
An ethanol slurry containing silicon particles with a D50 of 200 nm was put into a beaker so that the amount of silicon was 87.5 g, ultrasonic irradiation was performed for 15 minutes, and then ethanol was added so that the total amount of ethanol was 2212 g. A slurry was obtained. After that, 192.5 g of a polycarboxylic acid-based dispersant, 5.0 g of hydrochloric acid, and 700 g of water were added to the silicon slurry, and the mixture was stirred for 30 minutes at a rotation speed of 250 rpm. After that, 175 g of tetraethoxysilane (TEOS) was added to the above slurry and the temperature was raised to 70°C. After stirring at 70° C. for 12 hours, the resulting silicon slurry was centrifuged at 4800 rpm for 25 minutes and re-dispersed in ethanol. The obtained slurry was subjected to ball milling using zirconia balls with a diameter of 1.0 mm for 8 hours to obtain a silicon slurry. This was subjected to centrifugal separation under the conditions of a rotation speed of 4800 rpm and a rotation time of 60 minutes, and redispersed with water.

(Silicon coating process)
The above slurry was weighed to a silicon solid content of 22.2 g, transferred to a round-bottomed flask, and additional water was added to bring the total amount of water to 6116 g. After purging the inside of the flask system with nitrogen, the liquid temperature was raised to 35°C. After that, 0.84 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added into the flask and stirred for 30 minutes. 140.3 g of distilled styrene monomer and 0.65 g of lithium p-styrenesulfonate (LiSS) dissolved in 80 g of water were added and stirred for 2 hours. After that, the liquid temperature was raised to 62° C., and 1.8 g of ammonium persulfate (APS) dissolved in 80 g of water was added. Thereafter, heating and stirring were continued for 10 hours under reflux. The resulting reaction solution was subjected to centrifugal separation under conditions of a rotation speed of 4800 rpm and a rotation time of 5 minutes, and the resulting precipitate was redispersed in water. The resulting supernatant was further subjected to centrifugal separation under the conditions of 4,800 rpm and 15 minutes of rotation, and the resulting precipitate was redispersed in water. The resulting supernatant was further subjected to centrifugal separation under the conditions of 4800 rpm and 30 minutes of rotation, and the obtained precipitate was redispersed in water. Finally, the obtained supernatant was subjected to centrifugal separation under the conditions of 4800 rpm for 45 minutes of rotation, and the resulting precipitate was redispersed in water to obtain a polystyrene-coated silicon slurry.

An aqueous slurry of polystyrene-coated silicon was weighed into a beaker to a solids content of 5.0 g and additional water was added to bring the total amount of water to 1380 g. After purging the inside of the flask system with nitrogen, the liquid temperature was raised to 35°C. After that, 36.3 g of acrylonitrile monomer was added and stirred for 2 hours. After that, the liquid temperature was raised to 62° C., and 1.1 g of APS dissolved in 50 g of water was added. Thereafter, heating and stirring were continued for 10 hours under reflux. The resulting reaction solution was centrifuged under the conditions of 4800 rpm for 25 minutes, and re-dispersed with ethanol to obtain a polymer-coated silicon slurry.

After that, the polymer-coated silicon was heated to 65° C. while reducing the pressure with a diaphragm pump to remove the solvent and dried to obtain a mixed dried product.

(Crushing process)
The dried mixed product was placed in a blade mill and pulverized at 15,000 rpm for 150 seconds while cooling with water, and simultaneously spheroidized.

(Baking process)
The obtained powder was placed in a quartz boat and fired in a tubular furnace at a maximum temperature of 900° C. for 1 hour while flowing nitrogen gas. A sintered powder was thus obtained.

(Carbon coating process by vapor phase coating)
The fired powder thus obtained was placed in a rotary firing furnace, and a nitrogen gas flow rate of 267 SCCM and an ethylene gas flow rate of 133 SCCM were passed through the tube, and heated to 920° C. with an electric heater while rotating at 2 rpm. By maintaining this state for 57 minutes, carbon coating was performed to obtain a composite active material for a lithium secondary battery.

(Preparation of negative electrode for lithium secondary battery)
With respect to 92.5% by weight of the obtained composite active material for lithium secondary batteries (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive agent and polycarboxylic acid as a binder 7.0% by weight of the system binder and water were mixed to prepare a negative electrode mixture-containing slurry.

The resulting negative electrode mixture-containing slurry was applied to a copper foil having a thickness of 10 μm using an applicator so that the solid content coating amount was 2.0 mg/cm ² , and dried in a vacuum dryer at 90° C. for 12 hours. Dried. After drying, the negative electrode was punched into a circle of 14 mmφ, roll-pressed at room temperature at a feed rate of 1 m/min and a pressure of 4.0 t/cm ² , and further heat-treated at 110°C for 2 hours under vacuum to obtain a negative electrode having a thickness of 32 µm. A negative electrode for a lithium secondary battery having a mixture layer formed thereon was obtained.

(Preparation and Evaluation of Cycle Characteristic Evaluation Cell and Preparation of Composite Material Having Hollow Structure of Encapsulated Substance)
The coin cell for evaluation was prepared by dipping the negative electrode, a glass filter of 21 mmφ, a metal lithium of 16 mmφ and a thickness of 0.6 mm, and a stainless steel foil as a base material into the coin cell in a glove box, and then laminating them in this order. , and finally screwed on the lid. The electrolytic solution is a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1:1, to which 2% by volume of FEC (fluoroethylene carbonate) is added, and LiPF6 is dissolved to a concentration of 1.2 mol/liter. I used something else. The coin cell for evaluation was placed in a sealed glass container containing silica gel, and the electrodes were connected to a charging/discharging device through a silicon rubber lid.

The coin cell for evaluation was cycle-tested in a temperature-controlled room at 25°C. After charging at 0.3C, charging was performed at a constant voltage of 0.010V until the current value reached 0.03C.

Discharge was performed at a constant current of 0.3C to a voltage value of 1.5V. The initial discharge capacity and the initial charge/discharge efficiency were the results of the initial charge/discharge test.

In addition, the cycle characteristics were evaluated as the charge-discharge efficiency at the 50th and 100th cycles when the charge-discharge test was performed 100 times under the charge-discharge conditions. The charge/discharge efficiency of the eye was 99.9%.

From the cross-sectional SEM image of FIG. 6, the composite active material has Si or Si alloy included in amorphous carbon before charging and discharging, and there are multiple structures in which Si or Si alloy is included in the amorphous carbon. However, the crystalline carbon encloses voids, and cross-sectional SEM images after charging and discharging show that the encapsulating material adheres to the inner wall of the amorphous carbon.

<Comparative Example 1>
(Silicon coating process)
A slurry obtained in the same manner as in Example 1 was weighed so that the silicon solid content was 16.6 g, transferred to a round bottom flask, and additional water was added so that the total amount of water was 4590 g. After purging the inside of the flask system with nitrogen, the liquid temperature was raised to 35°C. After that, 0.63 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added to the flask and stirred for 30 minutes. 52.6 g of distilled styrene monomer and 0.48 g of lithium p-styrenesulfonate (LiSS) dissolved in 60 g of water were added and stirred for 2 hours. After that, the liquid temperature was raised to 62° C., and 1.3 g of ammonium persulfate (APS) dissolved in 60 g of water was added. Thereafter, heating and stirring were continued for 10 hours under reflux. The resulting reaction solution was centrifuged under the conditions of 4800 rpm and 45 minutes of rotation, and re-dispersed with ethanol to obtain a polystyrene-coated silicon slurry.

The above ethanol slurry of polystyrene-coated silicon was weighed into a beaker so that the solid content was 8.7 g, and ethanol was added so that the total amount of ethanol was 1273 g. 8.0 g of hexadecyltrimethylammonium bromide dispersed in 248 mL of water was added to the obtained slurry, and the mixture was stirred with a magnetic stirrer for 10 minutes. After that, 34.6 g of ammonium hydroxide and 268 mL of water were added to the slurry and stirred for 30 minutes. After that, 16.9 g of tetraethoxysilane was added and stirred for 3 hours. The resulting slurry was subjected to centrifugal separation under the conditions of 4800 rpm and 10 minutes of rotation, and the supernatant was discarded. The sediment was dried under reduced pressure to obtain Si@void@SiO ₂ particle powder.

(Baking process)
The obtained powder was placed in a quartz boat and fired in a tubular furnace at a maximum temperature of 900° C. for 1 hour while flowing nitrogen gas to obtain a fired powder.

(Carbon coating process by vapor phase coating)
The obtained fired powder was introduced into a firing furnace, and gas phase coating was performed using coal tar pitch as a carbon precursor while flowing nitrogen. The firing conditions were a heating rate of 5° C./min, heating at 300° C. for 1 hour, 600° C. for 5 hours, and 900° C. for 1 hour. A shell-structured particle into which carbon was introduced was obtained by vapor-phase coating.

(Preparation of negative electrode for lithium secondary battery)
57 parts by mass of graphite was added to 100 parts by mass of the obtained shell-structured particles to obtain a composite active material for a lithium secondary battery as a comparative example.

With respect to 92.5% by weight of the obtained composite active material for lithium secondary batteries (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive agent and polycarboxylic acid as a binder 7.0% by weight of the system binder and water were mixed to prepare a negative electrode mixture-containing slurry.

The resulting negative electrode mixture-containing slurry was applied to a copper foil having a thickness of 10 μm using an applicator so that the solid content coating amount was 2.0 mg/cm ² , and dried in a vacuum dryer at 90° C. for 12 hours. Dried. After drying, the lithium secondary was punched into a circle of 14 mmφ, uniaxially pressed under the condition of a pressure of 1.0 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 battery negative electrode was obtained.

(Preparation and Evaluation of Cycle Characteristic Evaluation Cell)
The coin cell for evaluation was prepared by dipping the negative electrode for a lithium secondary battery, a glass filter of 21 mmφ, a metal lithium of 16 mmφ and a thickness of 0.6 mm, and a stainless steel foil as a base material of the lithium secondary battery in a glove box in an electrolytic solution. , were laminated in this order, and finally a lid was screwed and manufactured. The electrolytic solution is a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1:1, to which 2% by volume of FEC (fluoroethylene carbonate) is added, and LiPF6 is dissolved to a concentration of 1.2 mol/liter. I used something else. The coin cell for evaluation was placed in a sealed glass container containing silica gel, and the electrodes were connected to a charging/discharging device through a silicon rubber lid.

The coin cell for evaluation was cycle-tested in a temperature-controlled room at 25°C. After charging at 0.3C, charging was performed at a constant voltage of 0.010V until the current value reached 0.03C.

Discharge was performed at a constant current of 0.3C to a voltage value of 1.5V. The initial discharge capacity and the initial charge/discharge efficiency were the results of the initial charge/discharge test.

In addition, the cycle characteristics were evaluated as the charge-discharge efficiency at the 50th and 100th cycles when the charge-discharge test was performed 100 times under the charge-discharge conditions. The charge/discharge efficiency of the eye was 99.0%.

In the examples, the encapsulating substance adheres to the inner wall of the amorphous carbon, thereby increasing the contact area and improving the charge-discharge efficiency during the cycle. That is, the charge/discharge cycle characteristics are good.

1 amorphous carbon 2 Si compound 3 void

Claims (15)

A composite active material for a lithium secondary battery containing Si or a Si compound, amorphous carbon, and voids, wherein Si or the Si compound is present around the voids, and the Si or the Si compound adheres to the amorphous carbon A composite active material for a lithium secondary battery, characterized by comprising: 2. The composite active material for a lithium secondary battery according to claim 1, wherein the ratio of the volume of said pores to the volume of said Si or Si compound is 0.5-50. 3. The composite active material for a lithium secondary battery according to claim 1, wherein the average number of said Si or Si compounds contained in each said void contained in said matrix is 4 or less. The composite active material for a lithium secondary battery according to any one of claims 1 to 3, wherein the shortest distance between said Si or Si compound and the inner wall surface of said void containing said Si or Si compound is 10 nm or less. . The composite active material for a lithium secondary battery according to any one of claims 1 to 4, wherein each of the plurality of pores has a shortest distance to the surrounding pores of 1.0 µm or less. further comprising an outer layer outside the composite active material for a lithium secondary battery;
The composite active material for a lithium secondary battery according to any one of claims 1 to 5, wherein the outer layer contains crystalline carbon or amorphous carbon having a pore size of 10 nm or more. 7. The composite active material for a lithium secondary battery according to claim 6, wherein said crystalline carbon satisfies at least one of the following conditions (1) to (3).
(1) 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) purity obtained from semi-quantitative values of impurities is 99% by weight or more.
(2) The amount of S determined by an ion chromatography (IC) measurement method using an oxygen flask combustion method is 1% by weight or less.
(3) A BET specific surface area of 100 m ² /g or less. The composite active material for a lithium secondary battery according to any one of claims 1 to 70, wherein the composite active material has a particle size (D50) of 0.3 to 50 µm. The composite active material for a lithium secondary battery according to any one of claims 1 to 8, wherein the composite active material has a BET specific surface area of 100 m ² /g or less. A method for producing the composite active material for a lithium secondary battery according to claim 1,
a first step of obtaining first particles by coating the Si or Si compound with a polymer film;
a second step of mixing or coating the first particles with an amorphous carbon precursor to obtain second particles;
A method for producing a composite active material for a lithium secondary battery, comprising: a third step of aggregating and firing a plurality of said second particles to form a fired body; and a fourth step of further charging and discharging. 11. The method of manufacturing a composite active material for a lithium secondary battery according to claim 10, wherein the polymer film is formed using a monomer, an initiator and a dispersant. 12. The method for producing a composite active material for a lithium secondary battery according to claim 10, further comprising a step of carbon-coating the fired body after the third step. The method for producing a composite active material for a lithium secondary battery according to any one of claims 10 to 12, wherein the amorphous carbon precursor is polyacrylonitrile. An electrode composition for lithium secondary batteries, comprising the composite active material for lithium secondary batteries according to any one of claims 1 to 9. An electrode for lithium secondary batteries comprising the composite active material for lithium secondary batteries according to any one of claims 1 to 9.

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