JP7293645B2 - Composite active material for lithium secondary battery and manufacturing method thereof - Google Patents

Description

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

As electronic materials become smaller and lighter, and development of HEVs or EVs progresses, there is an increasing demand for the development of batteries with large capacity, high-speed charge/discharge characteristics, good cycle characteristics, and excellent safety. Among them, the lithium secondary battery is attracting attention as the most promising battery. However, as a premise for the development of lithium secondary batteries exhibiting excellent performance, the development of materials such as negative electrode materials, positive electrode materials, electrolytic solutions, separators, or current collectors that are excellent in various performances, and those materials A battery design that fully produces these characteristics must be made.

Among these materials, the negative electrode material determines the basic battery characteristics. Therefore, development of materials with better characteristics such as charge/discharge capacity is being actively carried out. Patent Document 1 discloses a composite active material for a lithium secondary battery capable of producing a lithium secondary battery having a large charge/discharge capacity, high-speed charge/discharge characteristics, and good cycle characteristics, and a method for producing the same. there is Similarly, by adding a metal element, it is disclosed that a composite active material for a lithium secondary battery containing carbonaceous matter derived from tar pitch or ethylene gas has a high charge-discharge capacity (Patent Document 2). , 3).

These composite active materials for lithium secondary batteries are excellent in characteristics such as charge/discharge capacity. However, the composite active material for lithium secondary batteries expands greatly during the first charge, which poses a serious problem in actual use. In addition, although these composite active materials for lithium secondary batteries have higher capacity retention rates than those reported in the past, they do not reach the properties that are required these days.

In order to suppress the expansion of the composite active material for lithium secondary batteries during charging, an attempt has been made to suppress the expansion during charging by providing appropriate voids within each carbon/silicon particle. . (See Non-Patent Document 1).

However, the composite active material for lithium secondary batteries obtained by these methods has a problem that the initial charge/discharge efficiency is lowered because the amorphous carbon is distributed around the voids in a network-like manner. In addition, these composite active materials for lithium secondary batteries have a low initial volumetric discharge capacity, and do not meet the characteristics required in recent years.

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

N Liu et al. , Nano Lett. , 2012, 12, 3315-3321

On the other hand, in recent years, from the viewpoint of safety in use of batteries, it is required that the initial volumetric discharge capacity is high and that the volume of the electrode material does not expand even after repeated charging and discharging. If the discharge capacity of the electrode material is low, it will need to be charged frequently when used in smartphones or electric vehicles. In addition, when the volume expansion of the electrode material is large, leakage of the electrolyte occurs and battery life 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, which is an electrode material whose volume expansion is suppressed during the initial charge and which exhibits excellent cycle characteristics, and a method for producing the same. and

As a result of earnestly examining the prior art, the inventors have found that the above problems can be solved by the following configuration.
(1)
A composite active material for a lithium secondary battery containing Si or a Si alloy and crystalline carbon, having a structure having voids around the Si or Si alloy, and having a maximum discharge in a charge/discharge test of the lithium secondary battery. A composite active material for a lithium secondary battery, wherein the discharge capacity after 20 charge/discharge cycles is 70.0% or more of the capacity.
(2)
The composite active material for a lithium secondary battery according to (1), which has an initial volume discharge capacity of 640 mAh/cc or more in a charge/discharge test of a lithium secondary battery using the composite active material for a lithium secondary battery as a negative electrode active material.
(3)
The lithium secondary battery according to (1) or (2), which has an initial charge/discharge efficiency of 66.0% or more in a charge/discharge test of the lithium secondary battery using the composite active material for a lithium secondary battery as a negative electrode active material. for composite active materials.
(4)
The lithium secondary according to any one of (1) to (3), which has an initial charge expansion rate of 165% or less in a charge/discharge test of a lithium secondary battery using the composite active material for lithium secondary batteries as a negative electrode active material. Composite active material for batteries.
(5)
The Si or Si alloy exists between crystalline carbons, and the void volume in the composite active material for lithium secondary batteries measured by SEM image observation is 2 of the volume of the entire composite active material for lithium secondary batteries. % to 90%, the composite active material for a lithium secondary battery according to any one of (1) to (4).
(6)
The composite active material for lithium secondary batteries according to any one of (1) to (5), further comprising amorphous carbon.
(7)
The ratio of the pore volume in the composite active material for lithium secondary batteries measured by SEM image observation to the volume of Si or Si alloy in the composite active material for lithium secondary batteries is 0.5 to 200 (1) The composite active material for a lithium secondary battery according to any one of (6).
(8)
The composite active material for a lithium secondary battery according to any one of (1) to (7), wherein the Si or Si alloy has a particle size (D50) of 0.01 to 5 μm.
(9)
The Si or Si alloy has a structure sandwiched between crystalline carbon layers having a thickness of 0.5 μm or less, the structure spreads in a lamination and/or network shape, and the crystalline carbon layer is a lithium secondary The composite active material for lithium secondary batteries according to any one of (1) to (8), which is curved near the surface of the composite active material particles for batteries to cover the composite active material particles for lithium secondary batteries.
(10)
The crystalline carbon contains 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) has a purity of 99% by weight or more as determined by semi-quantitative values of impurities, and an ion chromatography (IC) measurement method using an oxygen flask combustion method. The composite active material for lithium secondary batteries according to any one of (1) to (9), which has a BET specific surface area of 1% by weight or less and/or a BET specific surface area of 100 m ² /g or less.
(11)
After modifying the Si or Si alloy with a surface modifier as necessary, a polymer monomer, an initiator and, if necessary, a dispersant are added, and after coating the Si or Si alloy with a polymer film, graphite and A step of mixing a carbon compound as necessary, a step of granulating and consolidating, a step of pulverizing and spheroidizing the mixture to form substantially spherical composite particles, and forming the composite particles in an inert atmosphere. The method according to any one of (1) to (10), which includes a step of firing, a step of mixing the carbon compound and the composite particles or the fired particles as necessary, and a step of heating the mixture in an inert atmosphere. and a method for producing a composite active material for a lithium secondary battery.
(12)
The method for producing a composite active material for lithium secondary batteries according to (11), wherein an oxidizing agent or a modifier containing a metal alkoxide group, a carboxyl group, or a hydroxyl group in the molecule is used as the surface modifier.
(13)
Polymeric monomers include styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, Methyl methacrylates such as 2-ethylhexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, triethylene glycol methacrylate, itaconic anhydride substance, 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 acid Acrylic acids such as 2-ethylhexyl, isobornyl acrylate, benzyl acrylate, phenyl acrylate, glycidyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, methacrylamide, N-methyl Methacrylamides such as acrylamide, N,N'-dimethylacrylamide, N-tert-butylmethacrylamide, Nn-butylmethacrylamide, N-methylolmethacrylamide, N-ethylolmethacrylamide, N,N Acrylamides such as '-methylenebisacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide, Nn-butylacrylamide, N-methylolacrylamide, N-ethylolacrylamide, vinyl benzoate, diethylaminostyrene, Diethylamino alpha-methylstyrene, p-vinylbenzenesulfonic acid, p-vinylbenzenesulfonic acid sodium salt, divinylbenzene, vinyl acetate, butyl acetate, vinyl chloride, vinyl fluoride, vinyl bromide, maleic anhydride, N-phenylmaleimide , N-butylmaleimide, N-vinylpyrrolidone, N-vinylcarbazole, acrylonitrile, aniline, pyrrole, polyol or isocyanate used in urethane polymerization (11) or (12) and a method for producing a composite active material for a lithium secondary battery.

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 of producing a lithium secondary battery exhibiting excellent cycle characteristics. and a method for producing the same.

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

Hereinafter, the composite active material for lithium secondary batteries and the method for producing the same of the present invention will be described in detail while showing an example of the present invention.

A composite active material for a lithium secondary battery of the present invention is a composite active material for a lithium secondary battery containing Si or a Si alloy and crystalline carbon, and has a structure having voids around the Si or Si alloy. and that the discharge capacity after 20 charge/discharge cycles is 70.0% or more compared to the maximum discharge capacity in a charge/discharge test of a lithium secondary battery using the composite active material for lithium secondary batteries as a negative electrode active material. It is a composite active material for lithium secondary batteries characterized by:

A composite active material for a lithium secondary battery of the present invention contains Si or a Si alloy and crystalline carbon. As shown in FIG. 1, Si or a Si alloy exists between crystalline carbon atoms, thereby having a structure with voids around the Si or Si alloy. By using the composite active material for a lithium secondary battery having such a structure as a negative electrode active material of a lithium secondary battery, lithium having a high initial volumetric discharge capacity, a high initial charge/discharge efficiency, and a high initial volumetric discharge capacity can be obtained. A secondary battery is obtained. In addition, by suppressing the expansion of the composite active material, the lithium secondary battery has excellent cycle characteristics.

The composite active material for lithium secondary batteries of the present invention is compared with the maximum discharge capacity in a charge-discharge test of a lithium secondary battery using the composite active material for lithium secondary batteries as a negative electrode active material. Discharge capacity (hereinafter also referred to as "capacity retention rate after 20 cycles") is 70.0% or more, preferably 93.2% or more, more preferably 97.3% or more, and particularly preferably 99.0%. % or more.

The capacity retention rate after 20 cycles can be obtained from the following formula.

Capacity retention rate after 20 cycles (%) = Discharge capacity after 20 cycles (mAh/g)
/ maximum discharge capacity (mAh / g) × 100
The maximum discharge capacity is the discharge per unit weight of the negative electrode active material during the 1st to 20th discharges in a charge/discharge test of a lithium secondary battery using the composite active material for lithium secondary batteries of the present invention as a negative electrode active material. Maximum capacity.

The initial volumetric discharge capacity can be obtained from the following formula.

Initial volume discharge capacity (mAh/cc) = Initial charge capacity (mAh/g)
× electrode density before charging (g / cc) ÷ initial charge expansion rate (%) × initial charge / discharge efficiency (%)
The initial charge capacity is the discharge capacity per unit weight of the negative electrode active material during the first charge in the charge/discharge test.

The electrode density before charging is the content of the negative electrode active material per unit volume of the negative electrode before charging, and can be obtained from the following formula.

Electrode density (g/cc) = (weight of lithium secondary battery negative electrode - weight of electrode substrate)
/ ((thickness of lithium secondary battery negative electrode - thickness of electrode base material) x electrode area)
The initial charge expansion rate is the initial charge expansion rate in a charge and discharge test of a lithium secondary battery using the composite active material for lithium secondary batteries of the present invention as a negative electrode active material, that is, the electrode film thickness of the lithium secondary battery negative electrode before charging. , is the electrode film thickness of the lithium secondary battery negative electrode after the first charge in the charge-discharge test, and can be obtained from the following formula.

Initial charging expansion rate (%) = electrode film thickness after charging/electrode film thickness before charging x 100
The electrode film thickness is the electrode thickness of the negative electrode for a lithium secondary battery, and can be measured using a micrometer. The electrode film thickness after charging and the electrode film thickness before charging are the electrode film thicknesses after the initial charging and at the time of electrode preparation, respectively.

The initial charge/discharge efficiency is the initial charge/discharge efficiency in a charge/discharge test of a lithium secondary battery using the composite active material for lithium secondary batteries of the present invention as a negative electrode active material, that is, the first charge/discharge in the charge/discharge test. , is the ratio of the discharge capacity per unit weight of the negative electrode during discharge to the charge capacity per unit weight of the negative electrode during charge, and can be obtained from the following equation.

Initial charge/discharge efficiency (%) = Initial discharge capacity (mAh/g)
/ Initial charge capacity (mAh / g) × 100
The initial volume discharge capacity is preferably 640 mAh/cc or more, more preferably 672 mAh/cc or more, still more preferably 750 mAh/cc or more, particularly preferably 800 mAh/cc or more, most preferably 863 mAh/cc or more. The upper limit of the initial volume discharge capacity is preferably 1500 mAh/cc or less.

The initial charge/discharge efficiency is preferably 66.0% or higher, more preferably 71.1% or higher, particularly preferably 81.0% or higher, and most preferably 82.4% or higher.

The initial charge expansion rate is preferably 165% or less, more preferably 150% or less, particularly preferably 140% or less, and most preferably 126% or less.

Here, as a charge/discharge test for evaluating the capacity retention rate after 20 cycles, the initial volumetric discharge capacity, the initial charge/discharge efficiency, and the expansion coefficient during the initial charge, a charge/discharge test using the following charge/discharge conditions can be mentioned.

Charging is performed at a measurement temperature of 25±2° C., constant current charging at 0.5 mA from the initial voltage to 0.005 V, and then constant voltage charging at a voltage of 0.005 V until the current value reaches 0.03 mA. .

Discharge is constant current discharge at 0.5 mA in a voltage range of 0.005V to 1.5V at a measurement temperature of 25±2°C.

A lithium secondary battery using the composite active material for a lithium secondary battery of the present invention as a negative electrode active material, which is subjected to the charge/discharge test, may be a lithium secondary battery having the following configuration.

The lithium secondary battery in the evaluation of the initial charge expansion rate and the initial charge / discharge efficiency is a screw-cell type lithium secondary battery equipped with a negative electrode, a counter electrode, a glass filter, a polypropylene separator and an electrolyte (hereinafter referred to as "evaluation It is also called “screw cell for use”). In the evaluation screw cell, the negative electrode, the polypropylene separator, the glass filter and the counter electrode were each moistened with the electrolytic solution, and then the evaluation screw cell was prepared in the order of the negative electrode, the polypropylene separator, the glass filter and the counter electrode. It is preferably prepared by filling the inside and fixing it with a screw-cell lid with a torque of 0.9 N/cm ² . A lithium foil having a thickness of 0.6 mm is preferably used as the counter electrode.

The lithium secondary battery in the evaluation of the capacity retention rate after 20 cycles was a CR2032 type coin cell type lithium secondary battery equipped with a negative electrode, a counter electrode, a separator and an electrolytic solution (hereinafter also referred to as "evaluation coin cell"). is. In the evaluation coin cell, the negative electrode, the glass filter, and the counter electrode were each moistened with an electrolytic solution, and then the negative electrode, the glass filter, and the counter electrode were filled in the evaluation coin cell in that order, and the coin cell top lid was fitted. It is preferable to prepare by

In the screw cell for evaluation and the coin cell for evaluation, the negative electrode is a negative electrode using the composite active material for a lithium secondary battery of the present invention as a negative electrode active material, and the counter electrode is metallic lithium crimped to a stainless steel foil. The electrolytic solution contains a solvent and an electrolyte salt, and the solvent is a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 2% by volume of fluoroethylene carbonate (EC/DEC= 1/1: volume ratio), and the electrolyte is lithium hexafluorophosphate (LiPF ₆ ). The LiPF ₆ content in the electrolyte is preferably 1.2 mol/liter.

In the screw cell for evaluation and the coin cell for evaluation, the negative electrode is preferably a negative electrode produced by the following method.

A negative electrode mixture slurry consisting of a negative electrode active material, a conductive aid, a binder and pure water is obtained, applied to an electrode base material, dried in a vacuum, and the negative electrode mixture is layered on the electrode base material. Further, the negative electrode mixture is pressure-bonded to the electrode base material, and then heat-treated in a vacuum atmosphere to form a negative electrode for a lithium secondary battery.

The negative electrode mixture slurry contains 92.5 to 95.5% by weight of the negative electrode active material for lithium secondary batteries, 0.5% by weight of the conductive aid, and 4.0 to 7.0% by weight of the binder, and the balance is is preferably pure water. Preferably, the conductive agent is acetylene black, the binder is polyacrylic acid, and the electrode substrate is copper foil.

The pressure bonding of the negative electrode mixture to the electrode substrate is preferably carried out by uniaxial pressing at a pressure of 0.6 t/cm ² or roll pressing at a feed rate of 1 m/min and a pressure of 4.0 t/cm ² . The vacuum drying conditions are preferably a pressure of 1000 Pa or less, 110° C. and 0.5 hours, or a pressure of 1000 Pa or less, 90° C. and 12 hours. °C and 3 hours are preferred.

A composite active material for a lithium secondary battery of the present invention is a composite active material for a lithium secondary battery containing Si or a Si alloy (hereinafter collectively referred to as "Si compound") and crystalline carbon, wherein the Si Alternatively, a Si alloy exists between crystalline carbons, and the void volume in the composite active material for lithium secondary batteries measured by SEM image observation is 2% to 90% of the total volume of the composite active material for lithium secondary batteries. %. In the composite active material for a lithium secondary battery of the present invention, the structure having voids around the Si compound particularly relaxes the expansion stress caused by the expansion of the Si compound during charging between the Si compound and the crystalline carbon. In particular, the void volume in the lithium secondary battery composite active material measured by SEM image observation exceeds 1% of the total volume of the lithium secondary battery composite active material. .

The voids in the composite active material for lithium secondary batteries of the present invention are introduced to relax the expansion stress of Si or Si alloys. The voids around Si or Si alloy measured by SEM image observation are preferably 2 to 90%, more preferably 10 to 65%, of the total volume of the composite active material for lithium secondary batteries. , particularly preferably 10 to 50%, more preferably 15 to 50%.

The ratio of the pore volume in the composite active material for lithium secondary batteries measured by SEM image observation to the Si or Si alloy volume in the composite active material for lithium secondary batteries of the present invention is 0.5 to 200. is preferred, particularly preferably 0.5-185, more preferably 0.5-10.

Methods for calculating the void include the following methods.

First, the negative electrode for a lithium secondary battery is cut in the vertical direction of the electrode using a cross section processing device. A cross-section polisher is preferably used as an apparatus for cross-section processing in order to obtain a clearer image. In addition, the composite active material powder for a lithium secondary battery, that is, the powdered composite active material for a lithium secondary battery of the present invention is embedded in an epoxy resin and then cut, and then a cross section obtained using a microscope. observe the part. The microscope used here is a field emission scanning electron microscope (FE-SEM) because it is necessary to obtain sufficient resolution and observation range. After that, two transparent sheets were superimposed on the printed image of the secondary electron image obtained. - Fill in the gaps with a pen. As the transparent sheet, an OHP sheet (sheet for overhead projector) is used 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 of the composite active material for lithium secondary batteries and the area of the voids. Calculate Finally, the porosity in the composite active material for lithium secondary batteries is calculated from the formula: porosity (%) = area of voids (μm ² )/area of composite active material for lithium secondary batteries (μm ² ) × 100. can be calculated.

The particle size (D50: 50% volume particle size) of the composite active material for a lithium secondary battery of the present invention is preferably 50 μm or less, more preferably 45 μm or less, and further preferably 40 μm or less, from the viewpoint that the effect of the present invention is more excellent. preferable.

The particle size (D90: 90% volume particle size) is preferably 75 μm or less, more preferably 65 μm or less, and even more preferably 55 μm or less, from the viewpoint that the effects of the present invention are more excellent.

Furthermore, the particle size (D10: 10% volume particle size) is preferably 30 µm or less, more preferably 20 µm or less, from the viewpoint that the effects of the present invention are more excellent.

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

In addition, when measuring the particle size, the composite active material for lithium secondary batteries is added to the liquid and mixed vigorously while using ultrasonic waves to prepare a dispersion liquid, and the prepared dispersion liquid is placed in the device as a measurement sample. It may be introduced and the particle size may be measured.

When the composite active material for lithium secondary batteries does not blend well with the liquid, that is, when the composite active material for lithium secondary batteries is difficult to disperse uniformly in the liquid, a surfactant or the like may be added as necessary. good. As the liquid, it is preferable to use water, alcohol, or a low-volatile organic solvent for operational reasons.

The composite active material for a lithium secondary battery of the present invention preferably exhibits a normal distribution in the particle size distribution diagram obtained.

The composite active material for lithium secondary batteries of the present invention preferably has a BET specific surface area of 0.5 to 200 m ² /g, more preferably 0.5 to 150 m ² /g, particularly preferably 0.5 to 130 m ² /g. g, more preferably 1 to 100 m ² /g, still more preferably 1 to 80 m ² /g. When the BET specific surface area is within this range, voids are introduced inside the composite active material for a lithium secondary battery. In other words, when the BET specific surface area is within this range, the particles of the composite active material for lithium secondary batteries of the present invention have sufficient voids. This suppresses the formation of a solid electrolyte layer (SEI) formed on the surface of the composite active material for lithium secondary batteries due to contact with the electrolytic solution and charge/discharge. As a result, the initial volumetric discharge capacity, the initial charge/discharge efficiency, and the capacity retention rate after 20 cycles are improved.

The BET specific surface area of the composite active material for a lithium secondary battery is a value measured by a nitrogen adsorption multipoint method after vacuum-drying a sample at 200° C. for 20 minutes.

The composite active material for a lithium secondary battery of the present invention preferably has a structure in which the Si compound is sandwiched between crystalline carbon layers having a thickness of 0.2 μm or less. A structure in which a Si compound is sandwiched between crystalline carbons spreads in a lamination and/or mesh pattern to form a crystalline carbon layer. The crystalline carbon layer is curved near the surface of the Si compound particles to cover the Si compound particles and the voids existing in the vicinity thereof, and the crystalline carbon constructs a three-dimensional network, that is, It is preferable to have a structure in which a structure in which the Si compound is partially contained in the voids formed by the crystalline carbon is repeated.

If the thickness of the crystalline carbon layer is 0.5 μm or less, the electron transfer effect of the crystalline carbon layer is reduced. When the crystalline carbon layer is linear when viewed in cross section, its length is preferably at least half the size of the composite active material particles for lithium secondary batteries for electron transfer, and the size of the composite material particles for lithium secondary batteries. It is more preferable that it is equivalent to . When the crystalline carbon layer has a network shape, it is preferable for electron transfer that the network of the crystalline carbon layer is connected over half or more of the particle size of the composite active material for lithium secondary batteries. More preferably, it is approximately the same size as the particle size of the composite active material.

In the composite active material for a lithium secondary battery of the present invention, the crystalline carbon layer may curve near the surface of the particles of the composite active material for a lithium secondary battery to cover the particles of the composite active material for a lithium secondary battery. preferable. With such a shape, a reactant is formed during charge/discharge due to direct contact between the Si compound or the end surface of the crystalline carbon layer and the electrolyte due to penetration of the electrolyte solution from the end surface of the crystalline carbon layer. Reduces the risk of inefficiency.

Si in the present invention is general-purpose grade metallic silicon with a purity of about 98% by weight, chemical grade metallic silicon with a purity of 2 to 4N, chlorinated and distilled polysilicon with a purity higher than 4N, Ultra-pure single crystal silicon that has undergone a deposition process by a single crystal growth method, or doped with an element of group 13 or group 15 of the periodic table to make it p-type or n-type, generated in the semiconductor manufacturing process There is no particular limitation as long as the purity is equal to or higher than that of general-purpose grade metal silicon, such as scraps of wafer polishing or cutting, and waste wafers that have become defective in the process. Si is preferably metallic silicon with a purity of 2 to 4N.

The Si alloy referred to in the present invention is an alloy containing Si as a main component. In the Si alloy, the element contained in addition to Si is preferably one or more elements of Groups 2 to 15 of the periodic table, and preferably an element whose phase contained in the alloy has a melting point of 900° C. or higher.

In the composite active material for a lithium secondary battery of the present invention, the Si compound preferably has a particle size (D50) of 0.01 to 5 μm, more preferably 0.01 to 1 μm, and particularly preferably 0.05 to 0.05 μm. 6 μm, more preferably 0.1 to 0.5 μm. If it is 0.01 μm or more, the reduction in charge/discharge capacity and the reduction in initial charge/discharge efficiency due to surface oxidation of the Si compound are less likely to occur. On the other hand, when the particle diameter (D50) of the Si compound is 5 μm or less, cracking due to expansion due to lithium insertion during charge/discharge is less likely to occur, and cycle deterioration, ie, deterioration of cycle characteristics, is likely to be suppressed. The particle size (D50) is determined in the same manner as described above.

In the composite active material for a lithium secondary battery of the present invention, the Si compound has a structure sandwiched between crystalline carbon layers having a thickness of 0.5 μm or less, and the structure spreads in a laminated and/or network fashion. Preferably, the crystalline carbon layer is curved near the surface of the composite active material particles for lithium secondary batteries to cover the composite active material particles for lithium secondary batteries.

The content of the Si compound is preferably 10 to 80 parts by mass, particularly preferably 15 to 50 parts by mass, and more preferably 20 to 50 parts by mass. When the content of the Si compound is less than 10 parts by mass, it is easy to obtain a sufficiently large capacity compared to a conventional negative electrode using graphite as a negative electrode active material. decrease tends to be small.

The crystalline carbon of the present invention is not particularly limited as long as it becomes crystalline carbon when fired, and graphite-derived carbon is particularly preferred.

The graphite that becomes crystalline carbon when fired in the present invention includes natural graphite material, artificial graphite, and the like, and among them, exfoliated graphite, which is usually called graphite, is preferable.

In the present specification, exfoliated graphite means graphite having 400 or less layers of graphene sheets. The graphene sheets are mainly bonded to each other by the van der Waals force, that is, the graphene sheets are stacked together by the van der Waals force.

The number of layers of graphene sheets in exfoliated graphite is such that the Si compound and exfoliated graphite are more uniformly dispersed, so that the expansion of the electrode material using the composite active material for lithium secondary batteries is further suppressed. , and/or in that the cycle characteristics of the lithium secondary battery are more excellent (hereinafter simply referred to as "the point at which the effects of the present invention are more excellent"), preferably 300 layers or less, more preferably 200 layers or less, and 150 layers or less. More preferred. From the standpoint of handleability, the number of laminated graphene sheets is preferably 5 or more.

The number of stacked graphene sheets 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 that the effects of the present invention are more excellent. The lower limit of the average thickness of exfoliated graphite is preferably 4 nm or more because the production procedure is simplified.

In addition, the method for measuring the average thickness of the exfoliated graphite is to observe the exfoliated graphite by electron microscope observation (TEM), and measure the thickness of 10 or more layers of the laminated graphene sheets in the exfoliated graphite. , the average thickness of the exfoliated graphite is obtained by arithmetically averaging the values.

Exfoliated graphite is graphite obtained by flaking by exfoliating between layers of graphene sheets contained in a graphite compound, specifically natural graphite.

Examples of exfoliated graphite include so-called expanded graphite.

Expanded graphite contains graphite. For example, flake graphite is treated with chemicals such as concentrated sulfuric acid, nitric acid, and hydrogen peroxide, and these chemicals are intercalated in the gaps between graphene sheets. After heating, the gaps between the graphene sheets are widened when the intercalated chemical is vaporized by further heating. As will be described later, a 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 the spheroidizing treatment, it is possible to perform the spheroidizing treatment together with other components (for example, precursors of hard carbon and soft carbon). good.

The crystalline carbon preferably has a purity of 99% by weight or more, an impurity amount of 10000 ppm or less, an S amount of 1% by weight or less, and/or a BET specific surface area of 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. be. Also, when the amount of S is 1% by weight or less, the irreversible capacity similarly decreases, so 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 graphite is preferably 5 to 100 m ² /g, particularly preferably 20 to 50 m ² /g. When the BET specific surface area is 100 m ² /g or less, the area that reacts with the electrolytic solution is small, so the initial charge/discharge efficiency is less likely to decrease.

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

Impurities are 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). The amount of S is measured by ion chromatography (IC) after combustion and absorption treatment by the oxygen flask combustion method followed by filter filtration.

In the composite active material for lithium secondary batteries of the present invention, the content of crystalline carbon is preferably 95 to 10 parts by mass, particularly preferably 70 to 10 parts by mass. When the content of crystalline carbon is less than 10 parts by mass, the crystalline carbon can cover the Si compound, providing a sufficient conductive path. As a result, deterioration of the initial volumetric discharge capacity is less likely to occur. On the other hand, when the content of crystalline carbon exceeds 95 parts by mass, the initial volumetric discharge capacity is difficult to increase.

More preferably, the composite active material for lithium secondary batteries of the present invention contains amorphous carbon in the inner and/or outer layer portion of the composite active material for lithium secondary batteries.

The amorphous carbon is not particularly limited as long as it becomes amorphous carbon when fired, and amorphous or microcrystalline carbonaceous materials other than graphite are particularly preferred.

The amorphous or microcrystalline carbonaceous materials other than graphite that becomes amorphous carbon when fired in the present invention include easily graphitizable carbon (soft carbon) that graphitizes by heat treatment exceeding 2000 ° C. and hard carbon that graphitizes. Examples include non-graphitizable carbon (hard carbon) and graphite-like aromatic compounds, preferably soft carbon or hard carbon, and particularly preferably soft carbon.

Hard carbon is preferably obtained by carbonizing a precursor such as a resin or resin composition. By carbonizing, the resin or resin composition is carbonized, and the hard carbon obtained thereby can be used as a carbon material for lithium secondary batteries. Examples of resins or resin compositions that serve as raw materials (precursors) for hard carbon include polymer compounds (eg, thermosetting resins and thermoplastic resins). Thermosetting resins include, for example, phenolic resins such as novolac type phenolic resins and resol type phenolic resins; epoxy resins such as bisphenolic epoxy resins and novolac type epoxy resins; melamine resins; urea resin; aniline resin; cyanate resin; furan resin; ketone resin; unsaturated polyester resin; Modified products obtained by modifying these with various components can also be used.

Examples of thermoplastic resins include polyethylene, polystyrene, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, polyethylene terephthalate, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyether ether ketone, and the like.

One or more of these can be used in combination.

Among these, particularly preferred raw materials (precursors) for hard carbon include phenolic resins such as novolac type phenolic resins and resol type phenolic resins.

The shape of the hard carbon precursor can be any shape such as powder, plate, granule, fibrous, massive and spherical. These precursors are preferably dissolved in the solvent used when mixing the various components.

The weight-average molecular weight of the hard carbon precursor to be used is preferably 100 or more, and more preferably 1,000,000 or less, from the viewpoint that the effect of the present invention is more excellent.

Soft carbon is preferably obtained by carbonizing a precursor such as a resin or resin composition. By carbonizing, the resin or resin composition is carbonized, and the soft carbon thus obtained can be used as a carbon material for lithium secondary batteries. Examples of resins or resin compositions that serve as raw materials (precursors) for soft carbon include coal-based pitch (eg, coal tar pitch), petroleum-based pitch, mesophase pitch, coke, low-molecular-weight heavy oil, and derivatives thereof, preferably coal-based pitch (eg, coal tar pitch), petroleum-based pitch, mesophase pitch, coke, low-molecular-weight heavy oil, or derivatives thereof. Soft carbon obtained from precursors such as coal-based pitch, and more preferably soft carbon derived from coal tar pitch, is preferable in that the effect of the present invention is more excellent.

Precursors of soft carbon can be of any shape such as powder, plate, granule, fibrous, massive and spherical. These precursors are preferably dissolved in the solvent used when mixing the various components.

The weight-average molecular weight of the soft carbon precursor to be used is preferably 1,000 or more, and more preferably 1,000,000 or less, from the viewpoint that the effects of the present invention are more excellent.

Examples of graphite-like aromatic compounds include dopamine hydrochloride, dihydroxyphenylalanine, dopamine derivatives such as 5,6-dihydroxyquindole, polyphenols such as tannic acid, catechins, anthocyanins, rutin and isoflavones. catecholamines, gallic acid, pyrogallol, galacetophenone and the like.

One or more of these can be used in combination.

Among these, particularly preferred graphite-like aromatic compounds include dopamine derivatives such as dopamine hydrochloride, dihydroxyphenylalanine, and 5,6-dihydroxykihindole.

As a method for producing a composite active material for a lithium secondary battery of the present invention, Si or a Si alloy is coated with a surface modifier if necessary, and then a polymer monomer, an initiator and, if necessary, a dispersant are added. , After coating a polymer film on Si or Si alloy, a step of mixing graphite and, if necessary, a carbon compound, a step of granulating and consolidating, and pulverizing and spheroidizing the mixture to form a substantially spherical composite A step of forming particles, a step of firing the composite particles in an inert atmosphere, a step of mixing the carbon compound and the composite particles or the fired powder as necessary, and a step of and a method including a step of mixing composite particles or sintered powder and a step of heating the mixture in an inert atmosphere.

As a preferred production method, after coating Si or Si alloy with a surface modifier, a polymer monomer, an initiator and a dispersant are added, and after coating Si or Si alloy with a polymer film, graphite and a carbon compound are mixed. granulating and consolidating; pulverizing and spheronizing the mixture to form substantially spherical composite particles; and firing the composite particles in an inert atmosphere. mentioned. Further, after the step of firing the composite particles in an inert atmosphere, a method further comprising the steps of mixing the carbon compound body with the composite particles or the fired powder and heating the mixture in an inert atmosphere. can also be used.

It is preferable to use a powder having a particle size (D50) of 0.01 to 5 μm as the Si compound. In order to obtain a Si compound having a predetermined particle size, the Si compound raw material (ingot, wafer, powder, etc.) is pulverized by a pulverizer, and a classifier is used in some cases. In the case of lumps such as ingots and wafers, they can be first pulverized using a coarse pulverizer such as a jaw crusher. After that, for example, by moving grinding media such as balls or beads, using the impact force, friction force, and compression force due to the kinetic energy, a ball mill, a medium agitating mill, or a , a roller mill that pulverizes using the compressive force of rollers, a jet mill that pulverizes the material by colliding it with the lining material at high speed or colliding with each other, and pulverizing by the impact force caused by the impact, and a hammer Hammer mill, pin mill, disk mill that crushes crushed materials using the impact force generated by the rotation of a rotor with fixed blades, pins, etc., colloid mill that uses shear force, and high-pressure wet type It can be finely pulverized using a counter-collision type disperser "Ultimizer" or the like.

Both wet and dry pulverization can be used. 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 airflow used Adjust the moisture and oxygen concentration of the water. 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 for obtaining a Si compound having a predetermined particle size is to evaporate the Si compound by heating it with plasma, laser, etc., and solidify it in an inert atmosphere, or to use a gas raw material. There are methods such as CVD and plasma CVD, and these methods are suitable for obtaining ultrafine particles of 0.1 μm or less.

In order to promote the reaction between the Si compound and the polymer monomer, it is preferable to previously modify the surface of the Si compound particles with a surface modifier such as a silane coupling agent, if necessary. The surface modifier preferably contains an oxidizing agent or a metal alkoxide group, a carboxyl group, or a hydroxyl group in the molecule. Epoxy systems such as 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, p-styryl styryl-based such as trimethoxysilane; methacrylic-based such as 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane; - acrylics such as acryloxypropyltrimethoxysilane, isocyanurates such as tris-(trimethoxysilylpropyl)isocyanurate or isocyanates such as 3-isocyanate-propyltriethoxysilane, tetraethoxysilanes, Oxidizing agents such as hydrogen peroxide, nitric acid, sulfuric acid, potassium permanganate, potassium dichromate, sodium hypochlorite, chromium trioxide, ammonium persulfate, potassium persulfate, etc., preferably 3-methacryloxypropyl. One or more selected from the group consisting of methyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane and tetraethoxysilane, particularly preferably 3-methacryloxysilane. One or two selected from roxypropyltrimethoxysilane and tetraethoxysilane.

When using a surface modifier, it is preferable to add 0.1 to 100 parts by mass of the surface modifier to 100 parts by mass of the 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 modification reaction, if necessary, a water-soluble alkaline compound such as ammonia, sodium hydroxide, potassium hydroxide or sodium hydrogen carbonate, or a water-soluble acidic compound such as hydrochloric acid, nitric acid, acetic acid or sulfuric acid is used. A residual reaction accelerator such as a compound having Ammonia or hydrochloric acid is preferred because it has high reactivity and leaves no metal compound. When using the residual reaction accelerator, it is preferable to add 0.005 to 54 parts by mass of the residual reaction accelerator to 100 parts by mass of the Si compound. The solvent used for the reaction may be any solvent as long as it dissolves the surface modifier, and examples thereof include water, ethanol, methanol, acetone, dimethylformamide, tetrahydrofuran, toluene, hexane, chloroform, and the like, if necessary. A mixed solvent may be used. When using 3-methacryloxypropyltrimethoxysilane or tetraethoxysilane as a surface modifier to modify the Si compound particle surface, 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 ethanol in the mixed solvent is within this range, the Si compound in the solvent is easily stabilized, and the modification reaction proceeds sufficiently.

After coating the Si compound with the surface modifier, if necessary, the Si compound particles may be pulverized and finely divided using a ball mill or a bead mill. Balls used for pulverization are preferably made of zirconia or alumina. The crushing time is preferably 1 to 24 hours, more preferably 1 to 12 hours.

Further, after pulverizing and atomizing the Si compound particles, it is preferable to replace the solvent used in modifying the surface of the Si compound particles by centrifugal separation with water, if necessary.

During the reaction of Si compound and polymer monomer, general mixers and stirrers such as magnetic stirrer, three-one motor, homomixer, in-line mixer, bead mill and ball mill are used. It is preferable to uniformly mix each raw material. The reaction temperature is preferably room temperature. 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 modification reaction proceeds sufficiently and the productivity is less likely to decrease.

By adding a polymer monomer and an initiator to a Si compound, the obtained polymer monomer slurry is polymerized to form a polymer, and a polymer film covering is obtained around the Si compound. be.

Polymeric monomers to be reacted with Si compounds include, for example, styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, and methacrylate. isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, triethylene glycol methacrylate, etc. methyl methacrylate, itaconic 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, 2-ethylhexyl acrylate, isobornyl acrylate, benzyl acrylate, phenyl acrylate, glycidyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, etc. Acid-based, methacrylamide, N-methylacrylamide, N,N'-dimethylacrylamide, N-tert-butylmethacrylamide, Nn-butylmethacrylamide, N-methylolmethacrylamide, N-ethylolmethacrylamide Acrylamides such as methacrylamides such as N,N'-methylenebisacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide, Nn-butylacrylamide, N-methylol-acrylamide, and N-ethylol-acrylamide vinyl benzoate, diethylaminostyrene, diethylamino alpha-methylstyrene, p-vinylbenzenesulfonic acid, sodium p-vinylbenzenesulfonate, divinylbenzene, vinyl acetate, butyl acetate, vinyl chloride, vinyl fluoride, vinyl bromide , maleic anhydride, N-phenylmaleimide, N-butylmaleimide, N-vinylpyrrolidone, N-vinylcarbazole, acrylonitrile, aniline, pyrrole, polyol or isocyanate used in urethane polymerization. , preferably styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, methacrylic acid 2-ethylhexyl, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, triethyleneglycol methacrylate, methyl methacrylates, itaconic 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, 2-acrylate acrylic acids such as ethylhexyl, isobornyl acrylate, benzyl acrylate, phenyl acrylate, glycidyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, and hydroxybutyl acrylate; acrylonitrile; 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, methacryl methyl acid or methyl acrylate.

Examples of initiators to be used include azo compounds such as azobisisobutyronitrile, potassium persulfate, ammonium persulfate, benzoyl peroxide, diisobutyryl peroxide, di-n-propyl peroxydicarbonate, diisopropyl per- Oxydicarbonate, dilauroyl peroxide, dibenzoyl peroxide, 1,1-di(tert-hexylperoxy)cyclohexane, 1,1-di(tert-butylperoxy)cyclohexane, tert-butylhydroperoxide and diisobutyrylperoxide , tert-hexylperoxyisopropyl monocarbonate, tert-butylperoxyisopropyl monocarbonate, 2,5-di-methyl-2,5-di(benzoylperoxy)hexane, tert-butylperoxyacetate, Organic peroxides such as di-tert-hexyl peroxide, di-tert-butyl peroxide, diisopropylbenzene hydroperoxide and tert-butyl hydroperoxide can be mentioned.

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

The content of the polymer monomer in the polymer monomer slurry is preferably 0.5 to 20% by weight, particularly preferably 1.5 to 10% by weight. When the content of the polymer monomer is within this range, the coating around the Si compound has a sufficient thickness, and as a result, the void volume around the Si compound becomes sufficient. As a result, the expansion of the Si compound during Li charging is sufficiently alleviated, and the aggregation of the Si compounds is less likely to proceed.

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

The polymer monomer slurry preferably contains a dispersant in order to improve the dispersibility of the Si compound or promote polymerization. Examples of the dispersant include polyvinylpyrrolidone and styrenesulfonic acid. Sodium, styrenesulfonic acids such as lithium styrenesulfonate, ammonium styrenesulfonate, styrenesulfonic acid ethyl ester, etc., polycarboxylic acids such as carboxystyrene, polyacrylic acid, polymethacrylic acid, naphthalenesulfonic acid formalin condensation, polyethylene glycol -, polycarboxylic acid partial alkyl ester system, polyether system, polyalkylene polyamine system, alkyl sulfonic acid system, quaternary ammonium system, higher alcohol alkylene oxide system, polyhydric alcohol ester system, alkyl polyamine system, or Examples include polyphosphate-based additives, preferably polyacrylic acid-based additives, styrenesulfonic acid-based additives and polyvinylpyrrolidone, particularly preferably styrenesulfonic acid-based additives and polyvinylpyrrolidone.

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

The polymer monomer slurry preferably contains a polymerization accelerator in order to accelerate polymerization. Examples of the polymerization accelerator include pH adjusters such as sodium hydrogen carbonate and potassium hydroxide. is preferably sodium bicarbonate.

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

Natural graphite, artificial graphite obtained by graphitizing petroleum or coal pitch, and the like can be used as graphite, and the graphite is used in the form of flakes, ovals, spheres, cylinders, or fibers. In addition, expanded graphite obtained by subjecting the graphite to an acid treatment, oxidation treatment, and then heat treatment to expand the graphite so that a portion of the graphite layers are exfoliated to form an accordion, pulverized expanded graphite, or ultrasonic waves, etc. Graphene or the like, which is delaminated by a method, can also be used. Expanded graphite or its pulverized product has superior flexibility compared to other graphites, and in the step of forming composite particles described later, the pulverized particles are re-bonded to easily form substantially spherical composite particles. can do. In view of the above, it is preferable to use expanded graphite or its pulverized material as the graphite. The raw material graphite is adjusted in advance to a size that can be used in the mixing process. degree.

After coating the Si compound with the polymer film, it is preferable to add a carbon compound because the Si compound and graphite can be further bound when mixed with graphite. As the carbon compound, it is preferable that the crystalline carbon obtained from the Si compound and graphite can be combined and that there is no residual carbon component after firing. Glycerols such as fatty acid esters, triglycerin fatty acid esters, menthol, pentaerythritol, dipentaerythritol, tripentaerythritol, ethylene glycol, propylene glycol, diethylene glycol, Glycols such as polyethylene glycol, polyethylene oxide and trimethylolpropane, polyvinylpyrrolidone and the like, preferably polyvinylpyrrolidone, menthol or glycerin, particularly preferably glycerin.

After the Si compound is coated with the polymer film, it is preferable to use a solvent when mixing graphite and, if necessary, the carbon compound. Examples of the solvent include quinoline, pyridine, toluene, benzene, tetrahydrofuran, Creosote oil, tetrahydrofuran, cyclohexanone, nitrobenzene, glycerin, menthol, polyvinyl alcohol, water, ethanol or methanol can be used.

As a mixing method, when the slurry concentration is high, a kneader (kneader) or a Loedige mixer can be used. When using a solvent, in addition to the above-mentioned kneader, three-one motor, stirrer, Nauta mixer, Redige mixer, Henschel mixer, high speed mixer, homomixer, in-line mixer, etc. can be used.

When a solvent is used for mixing and the solvent is to be removed, the solvent can be removed by jacket heating with these apparatuses, or by using a vibration dryer, a paddle dryer, or the like. Prior to the drying operation, solid-liquid separation can be carried out with devices such as centrifuges, filter presses, suction filters, pressure filters and the like. By removing the excess carbon compound, it becomes unnecessary to connect the composite active materials for a lithium secondary battery after firing, and to pulverize and pulverize them. Furthermore, it is preferable to perform a solid-liquid separation operation because the cause of the decrease in the capacity of the negative electrode is eliminated.

By continuing stirring in the process of removing the solvent in these apparatuses for a certain period of time, the mixture of the Si compound, graphite and, if necessary, the carbon compound is granulated and compacted. 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 granulate and consolidate. 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 consolidation include ball mills that use compressive force to pulverize the material, medium agitation mills, roller mills that use the compression force of rollers to pulverize the material, and high-speed crushing. A jet mill that crushes by the impact force caused by the collision with the lining material or by colliding the particles with each other, and the impact force by the rotation of the rotor with fixed hammers, blades, pins, etc. is used. A dry pulverization method such as a hammer mill, pin mill or disc mill is preferred, and a roller mill is particularly 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.

Further, by increasing the number of times of granulation and compaction, it is possible to improve the dispersibility of the Si compound in the graphite. The number of times of granulation/consolidation is preferably 1 to 10 times, more preferably 2 to 10 times, and particularly preferably 2 to 7 times. When the number of times of granulation and compaction is 10 or less, the crystallinity of graphite is less likely to deteriorate, and the initial charge/discharge efficiency tends to be less likely to decrease.

As a method of pulverizing and spheroidizing the granulated and compacted mixture, there is a method of pulverizing by the above-mentioned pulverization method to adjust the particle size and passing it through a dedicated spheronization device, and a method of passing it through a dedicated spheronization device. A method of repeating the method of pulverizing the crushed object using the impact force due to the rotation of the rotor, or a method of prolonging the treatment time to make the crushed object spherical. Dedicated sphering devices include Faculty (trade name), Nobilta (trade name), and Mechanofusion (trade name) from Hosokawa Micron Corporation, COMPOSI from Nippon Coke Kogyo Co., Ltd., Hybridization System from Nara Machinery Works, Ltd. Cryptoron Orb and Cryptoron Eddy manufactured by Earth Technica Co., Ltd., and the like.

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

The obtained composite particles are fired in an inert atmosphere such as argon gas or nitrogen gas.

The firing temperature is preferably 300 to 1200°C, particularly preferably 600 to 1200°C, more preferably 800 to 1000°C. When the firing temperature is 300° C. or higher, the polymer film coated with the Si compound is less likely to remain, resulting in a decrease in the initial volumetric discharge capacity, a decrease in the initial charge/discharge efficiency, and an increase in the initial electrode expansion coefficient. On the other hand, when the firing temperature is 1200° C. or lower, the reaction between the Si compound and graphite is less likely to occur, and the decrease in discharge capacity tends to be less likely to occur.

The composite active material for lithium secondary batteries of the present invention is useful as a negative electrode active material used as an electrode material for lithium secondary batteries.

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, the composite active material for a lithium secondary battery of the present invention and a binder are mixed and made into a paste using a solvent to obtain a slurry containing a negative electrode mixture. 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, as porous bodies of metal or carbon conductors, plain weave wire mesh, expanded metal, lath mesh, metal foam, metal woven fabric, metal non-woven fabric, Examples thereof include carbon fiber woven fabric and carbon fiber non-woven fabric.

As the binder to be used, known materials can be used, for example, fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene, styrene-butadiene rubber (SBR), polyethylene, polyvinyl alcohol, and carboxymethyl cellulose. , polyacrylic acid, glue, or the like is used.

Examples of solvents include water, isopropyl alcohol, N-methylpyrrolidone and dimethylformamide. In addition, when making a paste, if necessary, a known stirrer, mixer, kneader, kneader, etc. are used to stir the composite active material for lithium secondary batteries, the binder and the solvent. May be mixed.

When preparing a negative electrode mixture slurry using a composite active material for a lithium secondary battery, it is preferable to add conductive carbon black, carbon nanotubes, or a mixture thereof as a conductive material. The shape of the composite active material for a lithium secondary battery obtained by the above process is often relatively granular (particularly approximately spherical), and the particles of the composite active material for a lithium secondary battery are separated from each other. Contact tends to be point contact. In order to avoid this adverse effect, there is a method of adding carbon black, carbon nanotubes or a mixture thereof to the negative electrode mixture slurry. Carbon black, carbon nanotubes, or a mixture thereof can be intensively aggregated in the capillary portion formed by contact with the composite active material for a lithium secondary battery when the slurry solvent is dried. It is possible to prevent disconnection of contacts (increased resistance).

The amount of carbon black, carbon nanotubes, or a mixture thereof is preferably 0.2 to 4 parts by mass, preferably 0.5 to 2 parts by mass, with respect to 100 parts by mass of the composite active material for lithium secondary batteries. more preferred. Carbon nanotubes include single-wall carbon nanotubes and multi-wall carbon nanotubes.
(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 Co _x Al _y O ₂ , LiMnO ₂ , LiMn ₂ O ₄ and LiFeO ₂ ; transition metal chalcogen compounds such as titanium sulfide and molybdenum sulfide; A conductive conjugated polymer substance and the like can be mentioned.
(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 electrolytic solution include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB(C ₆ H ₅ ), LiCl, LiBr, LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , LiN(CF _{3 SO2} ) 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 Lithium salts 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, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and other carbonates, 1, 1- or 1 , 2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxofuran, 4-methyl-1,3-dioxolane, anisole, diethyl ether, etc. Ether, sulfolane, thioether such as methylsulfolane, nitrile such as acetonitrile, chloronitrile, propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl ortho Aprotic organic solvents such as formate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethylsulfoxide, 3-methyl-2-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 constituting the matrix of the polymer solid electrolyte or polymer gel electrolyte include ether-based polymer compounds such as polyethylene oxide and its crosslinked products, methacrylate-based polymer compounds such as polymethacrylate, and polyacrylate. Acrylate-based high molecular compounds such as hexafluoropropylene, polyvinylidene fluoride (PVDF), and fluorine-based high molecular compounds such as vinylidene fluoride-hexafluoropropylene copolymer are preferred. A mixture of these can also be used. From the viewpoint of oxidation-reduction stability, fluorine-based polymer compounds such as PVDF and vinylidene fluoride-hexafluoropropylene copolymer are particularly preferable.
(separator)
Known materials can be used as a separator for a lithium secondary battery having a negative electrode obtained by using the composite active material for lithium secondary batteries of the present invention. For example, woven fabrics, non-woven fabrics, synthetic resin microporous membranes, and the like are exemplified. 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.

The lithium secondary battery uses the above-described negative electrode, positive electrode, separator, electrolyte solution, and other battery components (for example, current collector, gasket, sealing plate, case, etc.), and is formed into a cylindrical shape according to a conventional method. , rectangular or button-shaped.

The lithium secondary battery of the present invention is used in various portable electronic devices, especially notebook computers, notebook word processors, palmtop (pocket) computers, mobile phones, mobile facsimiles, mobile printers, Headphone stereo, video camera, portable TV, portable CD, portable MD, electric shaver, electronic notebook, transceiver, electric tools, radio, tape recorder, digital camera, portable copier It can be used for machines, portable game machines, and the like. In addition, electric vehicles, hybrid vehicles, vending machines, electric carts, power storage systems for road leveling, household power storage devices, distributed power storage systems (built into stationary electrical appliances), emergency power supply It can also be used as a secondary battery for a system or the like.

3 is a cross-sectional SEM image of a composite active material for a lithium secondary battery produced in Example 3 of the present invention; 1 is an SEM image of a composite active material for a lithium secondary battery produced in Example 1 of the present invention; 1 is a cross-sectional SEM image (resin-embedded) of a composite active material for a lithium secondary battery produced in Example 1 of the present invention; 4 is a cross-sectional SEM image of the composite active material for lithium secondary batteries produced in Example 4 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>
(Preparation of expanded graphite)
After immersing scale-like natural graphite having an average particle size of 1 mm in a mixed acid of 9 parts by mass of sulfuric acid and 1 part by mass of nitric acid at room temperature for 1 hour, the mixed acid was removed with a No. 3 glass filter to obtain acid-treated graphite. Further, the acid-treated graphite was washed with water and then dried. 5 g of dried acid-treated graphite was stirred in 100 g of distilled water, and after 1 hour the pH was measured and found to be 6.7. The dried acid-treated graphite was placed in a horizontal electric furnace set at 850° C. under a nitrogen atmosphere to obtain expanded graphite. The expanded graphite had a bulk density of 0.015 g/cm ³ and a BET specific surface area of 24 m ² /g.

(Si pulverization step)
Chemical grade metal Si (purity 3N) with a particle size (D50) of 7 μm was mixed with ethanol at 21% by weight, and finely pulverized using zirconia beads with a diameter of 0.3 mm. An ultrafine Si slurry having a particle size (D50) of 0.2 μm and a BET specific surface area when dried of 100 m ² /g was obtained.

(Si surface modification step)
197 g of the pulverized Si slurry was placed in a beaker, subjected to ultrasonic irradiation for 15 minutes, and then 412 g of additional ethanol was added to obtain a Si slurry. Thereafter, 45 g of ammonium hydroxide and 200 g of water were mixed, added to the above Si slurry, and stirred for 1 hour at a rotation speed of 250 rpm using a stirring blade. After that, 25 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added to the above Si slurry. After stirring at room temperature for 24 hours, the obtained Si slurry was centrifuged at 6000 rpm for 30 minutes and re-dispersed with ethanol.

After repeating this centrifugal separation treatment and ethanol redispersion operation three times, it is again centrifuged under the same conditions, and after removing the supernatant, it is redispersed with ethanol, resulting in a particle size (D50) of 0.2 μm. 3-methacryloxypropyltrimethoxysilane-coated Si particles (hereinafter abbreviated as MPS-Si) were obtained.

(Si coating step)
The above MPS-Si slurry was weighed so that the solid content was 8 g, then ultrasonic irradiation was performed for 15 minutes, and ethanol was added so that the total amount of ethanol was 504 g. 10.67 g of polyvinylpyrrolidone (manufactured by Tokyo Chemical Industry Co., Ltd., grade name: polyvinylpyrrolidone K30) was sampled, added to 29.33 g of water, and dissolved while being irradiated with ultrasonic waves. After purging a 1 L round-bottomed flask with nitrogen, the above MPS-Si slurry and polyvinylpyrrolidone aqueous solution were poured, 32 g of distilled styrene monomer and 0.53 g of azoisobisbutyronitrile were added, and an oil bath was removed. The temperature was raised to 60°C. Thereafter, heating was continued for 6 hours under reflux to obtain a polystyrene-coated Si slurry.

66 g of the polystyrene-coated Si slurry (13.5 g as Si), 31.5 g of the expanded graphite, 4.5 g of glycerin, and 0.5 L of ethanol were placed in a stirring vessel and mixed with a homogenizer for 15 minutes. A liquid mixture was obtained by mixing and stirring. The mixture was then transferred to a rotary evaporator, heated to 50° C. with a hot bath while rotating, and vacuumed with an aspirator to remove the solvent. After that, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 110 g of a mixed dried product (light-packed bulk density of 164 g/L).

(Pressing process)
This dried mixed product was passed through a three-roll mill twice, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 299 g/L.

(Spheroidization process)
Next, the granulated and compacted product was placed in a new power mill, water-cooled, pulverized at 21000 rpm for 360 seconds, and simultaneously spheroidized to obtain a substantially spherical composite powder having a light bulk density of 371 g/L.

(Classification process)
The substantially spherical composite powder was placed in an air classifier (ATP-50 manufactured by Hosokawa Micron) and classified at a classifier rotation speed of 15,000 rpm to obtain a substantially spherical composite fine powder having a light bulk density of 197 g/L.

(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. As a result, a sintered powder composed of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a light bulk density of 115 g/L and a particle size (D50) of 10 μm (composite active material for lithium secondary battery).

SEM (Scanning Electron Microscope) secondary electron images of particle cross sections of the composite active material for lithium secondary batteries are shown in FIGS. From this, it can be seen that the composite active material for a lithium secondary battery of this example has a structure in which void structures exist around the silicon particles and Si or Si alloy exists between the crystalline carbons.

(Preparation of negative electrode for lithium secondary battery)
With respect to 95.5% 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 4.0% 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 18 μm using an applicator so that the solid content coating amount was 2.5 mg/cm ² , and dried at 110° C. in a vacuum dryer. Dried for 0.5 hours. After drying, the lithium secondary was punched into a circle of 14 mmφ, uniaxially pressed at a pressure of 0.6 t/cm ² , and heat-treated at 110° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 32 μm. A battery negative electrode was obtained.

(Preparation and evaluation of cells for evaluating expansion rate of initial charge)
The screw cell for evaluation was prepared by inserting the above negative electrode, a polypropylene separator of 24 mmφ, a glass filter of 21 mmφ, metallic lithium of 18 mmφ and a thickness of 0.2 mm into the screw cell in a glove box, and stainless steel foil as the base material. were dipped in an electrolytic solution, laminated in this order, and finally a lid was screwed. 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 1.2% of LiPF ₆ is added. Those dissolved so as to have a concentration of mol/liter were used.

The evaluation cell 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 evaluation cell was subjected to a charge/discharge test in a constant temperature room at 25°C. Charging is performed by constant current-constant voltage charging. After charging at 0.1 C to 0.005 V at a constant current of 0.5 mA, the current value is 0.03 mA (= 0.5/20) at a constant voltage of 0.005 V. It was performed at 0.05 C until the The initial charge capacity was 1307 mAh/g. After that, the screw cell for evaluation was dismantled in a glove box in an argon atmosphere, and the electrode film thickness was measured with a micrometer. As a result, the initial charging expansion rate ((electrode film thickness after charging/electrode film thickness before charging×100)) was 129%.

(Preparation and evaluation of capacity retention rate evaluation cell after 20 cycles)
The coin cell for evaluation was prepared by dipping the negative electrode, a glass filter of 21 mmφ, a metal lithium of 18 mmφ and a thickness of 0.2 mm, and a stainless steel foil as a base material in the electrolytic solution in a glove box. They were laminated in order, and finally a lid was screwed on. 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 1.2% of LiPF ₆ is added. Those dissolved so as to have a concentration of mol/liter were used. 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 cover.

The coin cell for evaluation was cycle-tested in a temperature-controlled room at 25°C. After charging at a constant current of 0.5 mA to 0.005 V at 0.1 C, the battery was charged at a constant voltage of 0.005 V at 0.05 C until the current value reached 0.03 mA (=0.5/20). rice field.

Discharge was performed to a voltage value of 1.5 V at a constant current of 0.5 mA. 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 capacity retention rate after 20 cycles by comparing the discharge capacity when the charge/discharge test was performed 20 times under the above charge/discharge conditions with the maximum discharge capacity, and the initial charge/discharge efficiency was 66. 3%, the initial volume discharge capacity was 644 mAh/cc, and the capacity retention rate after 20 cycles was 96.7%. Here, the initial volume discharge capacity is calculated by initial charge capacity (mAh/g)×electrode density before charge (g/cc)/initial charge expansion rate (%)×initial charge/discharge efficiency (%).

<Example 2>
After preparing a polystyrene-coated Si slurry in the same manner as the Si pulverization step and the Si coating step in Example 1, the obtained slurry was allowed to stand for about 1 hour, and after removing the supernatant solution with a dropper, ethanol was added. - solution was added. This operation was repeated twice to remove free polystyrene particles.

Then, 13.4 g of the polystyrene-coated Si (Si), 58.3 g of the expanded graphite, 8.3 g of glycerin, and 3.9 L of ethanol were placed in a stirring vessel and mixed and stirred with a homomixer for 15 minutes. The mixture was then transferred to a rotary evaporator, heated to 50° C. with a hot bath while rotating, and evacuated with an aspirator to remove the solvent. Then, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 119 g of a mixed dried product (light-packed bulk density of 137 g/L).

(Pressing process)
This dried mixed product was passed through a three-roll mill twice, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 229 g/L.

(Spheroidization process)
Next, this granulated/consolidated product is put into a particle design/surface modification device (Hybridization system NHS-1L manufactured by Nara Machinery Co., Ltd.), pulverized at 100 m/sec for 30 minutes, and simultaneously spheroidized to obtain a light bulk density. 316 g/L of approximately spherical composite powder was obtained.

(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. As a result, a sintered powder composed of 84 parts by mass of crystalline carbon (derived from graphite) and 16 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a particle size (D50) of 11 μm.

A composite active material for a lithium secondary battery, a negative electrode, a cell for evaluating the initial charge expansion rate, and a screw cell for evaluating the capacity retention rate after 20 cycles were prepared in the same manner as in Example 1, and a charge-discharge test was performed. The initial charge capacity was 1032 mAh/g, the initial charge expansion rate was 143%, the initial charge/discharge efficiency was 71.1%, the initial discharge volume capacity was 672 mAh/cc, and the capacity retention rate after 20 cycles was 99.7%. rice field.

From this, it can be seen that the composite active material for a lithium secondary battery has a structure in which void structures exist around silicon particles and Si or Si alloy exists between crystalline carbons.

<Example 3>
After obtaining pulverized Si by the same method as the Si pulverization step of Example 1, a polystyrene-coated Si slurry was obtained in the same manner as the Si surface coating step of Example 1 without performing the Si surface coating step. rice field.

Thereafter, 32 g of the polystyrene-coated Si (Si), 74.7 g of the expanded graphite, 10.7 g of glycerin and 1.2 L of ethanol were placed in a stirring vessel and mixed and stirred for 20 minutes with a homogenizer. The mixture was then transferred to a rotary evaporator, heated to 50° C. with a hot bath while rotating, and evacuated with an aspirator to remove the solvent. Then, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 210 g of a mixed dried product (light-packed bulk density of 198 g/L).

(Pressing process)
This dried mixed product was passed twice through a three-roll mill, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 442 g/L.

Next, 56.1 g of this granulated/consolidated product is put into a particle design/surface modification device (Hybridization system NHS-1L manufactured by Nara Machinery Co., Ltd.), pulverized at 100 m/sec for 15 minutes, and simultaneously spheroidized. A substantially spherical composite powder having a light bulk density of 323 g/L was obtained.

(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. As a result, a sintered powder composed of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a light bulk density of 217 g/L and a particle size (D50) of 12 μm.

FIG. 1 shows a secondary electron image of a particle cross section of the composite active material for a lithium secondary battery obtained by SEM (scanning electron microscope).

A composite active material for a lithium secondary battery, a negative electrode, a cell for evaluating the initial charge expansion rate, and a screw cell for evaluating the capacity retention rate after 20 cycles were prepared in the same manner as in Example 1, and a charge-discharge test was performed. The initial charge capacity was 1604 mAh/g, the initial charge expansion rate was 132%, the initial charge/discharge efficiency was 78.7%, the initial discharge volume capacity was 822 mAh/cc, and the capacity retention rate after 20 cycles was 93.2%. rice field.

<Example 4>
The pulverized Si obtained by the same method as the Si pulverizing step of Example 1 was weighed so that the solid content was 8 g, and then ultrasonic irradiation was performed for 15 minutes, so that the total amount of ethanol was 504 g. Additional ethanol was added. 10.67 g of polyvinylpyrrolidone K30 was sampled, added to 29.33 g of water, and dissolved while being irradiated with ultrasonic waves. After purging a 1 L round-bottomed flask with nitrogen, the pulverized Si slurry and polyvinylpyrrolidone aqueous solution were poured, 53.35 g of distilled styrene monomer and 0.53 g of azobisisobutyronitrile were added, and the mixture was placed in an oil bath. was raised to 60°C. Thereafter, heating was continued for 10 hours under reflux to obtain a polystyrene-coated Si slurry.

Thereafter, 32 g of the polystyrene-coated Si (Si), 74.7 g of the expanded graphite, 10.7 g of glycerin and 1.2 L of ethanol were placed in a stirring vessel and mixed and stirred for 20 minutes with a homogenizer. The mixture was then transferred to a rotary evaporator, heated to 50° C. with a hot bath while rotating, and evacuated with an aspirator to remove the solvent. After that, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 287 g of a mixed dried product (light-packed bulk density: 175 g/L).

(Pressing process)
This dried mixed product was passed twice through a three-roll mill, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 377 g/L.

(Spheroidization process)
Next, 40.9 g of this granulated/consolidated product is put into a particle design/surface modification device (Hybridization system NHS-1L manufactured by Nara Machinery Co., Ltd.), pulverized at 100 m/sec for 15 minutes, and simultaneously spheroidized. A substantially spherical composite powder having a light bulk density of 307 g/L was obtained.

(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. As a result, a sintered powder composed of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a sintered powder having a light bulk density of 178 g/L and a particle size (D50) of 13 μm. A composite active material for a lithium secondary battery, a negative electrode, a cell for evaluating the initial charge expansion rate, and a screw cell for evaluating the capacity retention rate after 20 cycles were prepared in the same manner as in Example 1, and a charge-discharge test was performed. The initial charge capacity was 1453 mAh/g, the initial charge expansion rate was 126%, the initial charge/discharge efficiency was 78.9%, the initial discharge volume capacity was 827 mAh/cc, and the capacity retention rate after 20 cycles was 95.0%. rice field.

<Example 5>
The pulverized Si obtained by the same method as the Si pulverizing step of Example 1 was weighed so that the solid content was 4.9 g, followed by ultrasonic irradiation for 15 minutes, and 700 g of water was added. After purging the 1 L round-bottomed flask with nitrogen, the pulverized Si slurry was poured thereinto and the oil bath was heated to 80.degree. Thereafter, 11.3 g of methyl methacrylate monomer distilled under reflux and 0.07 g of potassium persulfate dissolved in 20 g of water were added dropwise over 30 minutes, and heating was continued for 3 hours to form a polymethyl methacrylate Si slurry. - got.

After that, 19.6 g of polymethyl methacrylate Si, 45.7 g of expanded graphite and 6.5 g of glycerin were placed in a stirring vessel and mixed and stirred for 40 minutes with a homogenizer. The mixture was then transferred to a rotary evaporator, heated to 80° C. with a hot bath while rotating, and evacuated with an aspirator to remove the solvent. Then, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 93 g of a mixed dried product (light-packed bulk density: 131 g/L).

(Pressing process)
This dried mixed product was passed through a three-roll mill twice, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 160 g/L.

(Spheroidization process)
Next, 68 g of this granulated/consolidated product is put into a particle design/surface modification device (Hybridization system NHS-1L manufactured by Nara Machinery Co., Ltd.), pulverized at 40 m / sec for 15 minutes, and simultaneously spheroidized. A substantially spherical composite powder with a density of 314 g/L was obtained.

(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. As a result, a sintered powder composed of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a sintered powder having a light bulk density of 256 g/L and a particle size (D50) of 15 μm. A composite active material for a lithium secondary battery, a negative electrode, a cell for evaluating the initial charge expansion rate, and a screw cell for evaluating the capacity retention rate after 20 cycles were prepared in the same manner as in Example 1, and a charge-discharge test was performed. The initial charge capacity was 1659 mAh/g, the initial charge expansion rate was 163%, the initial charge/discharge efficiency was 82.5%, the initial discharge volume capacity was 832 mAh/cc, and the capacity retention rate after 20 cycles was 75.0%. rice field.

<Example 6>
The pulverized Si obtained by the same method as the Si pulverizing step of Example 1 was weighed so that the solid content was 8 g, and then ultrasonic irradiation was performed for 15 minutes, so that the total amount of ethanol was 504 g. Additional ethanol was added. 10.67 g of polyvinylpyrrolidone K30 was sampled, added to 29.33 g of water, and dissolved while being irradiated with ultrasonic waves. After purging a 1 L round-bottomed flask with nitrogen, the pulverized Si slurry and polyvinylpyrrolidone aqueous solution were poured, 53.35 g of distilled styrene monomer and 0.53 g of azoisobutyronitrile were added, and an oil bath was added. The temperature was raised to 60°C. Thereafter, heating was continued for 10 hours under reflux to obtain a polystyrene-coated Si slurry.

Thereafter, 32 g of the polystyrene-coated Si (Si), 74.7 g of the expanded graphite, 10.7 g of glycerin and 1.2 L of ethanol were placed in a stirring vessel and mixed and stirred for 20 minutes with a homogenizer. The mixture was then transferred to a rotary evaporator, heated to 50° C. with a hot bath while rotating, and evacuated with an aspirator to remove the solvent. After that, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 287 g of a mixed dried product (light-packed bulk density: 175 g/L).

(Pressing process)
This dried mixed product was passed twice through a three-roll mill, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 377 g/L.

(Spheroidization process)
Next, 80 g of this granulated/consolidated product is put into a particle design/surface modification device (Hybridization system NHS-1L manufactured by Nara Machinery Co., Ltd.), pulverized at 40 m / sec for 15 minutes, and simultaneously spheroidized. A substantially spherical composite powder with a density of 382 g/L was obtained.
(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. As a result, a sintered powder composed of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a light bulk density of 195 g/L and a particle size (D50) of 31 μm.

The obtained calcined powder (100 parts by mass) was stirred with coal tar pitch (carbonization degree 60%, 8.35 parts by mass) for 30 minutes, and then calcined and coated by the following method.

(Baking process)
Using a rotary firing furnace, while flowing nitrogen (0.3 L/min), the temperature increase rate is 30 ° C./min, and the mixture is heated at 600 ° C. for 2 hours while rotating at 2 rpm. - Modification of lupit into soft carbon. As a result, a fired powder containing 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 5 parts by mass of amorphous carbon (soft carbon derived from coal tar pitch) was obtained.

(Carbon coating process by vapor phase coating)
The obtained powder was set in a rotary sintering furnace, and after the inside of the tube was evacuated by a rotary pump, nitrogen gas at a flow rate of 200 SCCM and ethylene gas at a flow rate of 100 SCCM were flowed into the tube, and the tube was rotated at 2 rpm. Carbon coating was performed by heating to 920° C. with an electric heater and maintaining that state for 3 minutes. The weight increase due to the carbon coating is 5% by weight, resulting in 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, 10 parts by mass of amorphous carbon (5 parts by mass of soft carbon derived from coal tar pitch parts, and 5 parts by mass of soft carbon derived from the gas phase coat).

After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a particle size (D50) of 33 μm. A composite active material for a lithium secondary battery, a negative electrode, a cell for evaluating the initial charge expansion rate, and a screw cell for evaluating the capacity retention rate after 20 cycles were prepared in the same manner as in Example 1, and a charge-discharge test was performed. The initial charge capacity was 1264 mAh/g, the initial charge expansion rate was 140%, the initial charge/discharge efficiency was 82.4%, the initial discharge volume capacity was 863 mAh/cc, and the capacity retention rate after 20 cycles was 97.3%. rice field.

<Example 7>
(Si surface modification step)
The pulverized Si obtained by the same method as the Si pulverization step of Example 1 was weighed so that the solid content was 40 g, and then ultrasonic irradiation was performed for 15 minutes, so that the total amount of ethanol was 1018 g. Additional ethanol was added to obtain a Si slurry. Then, 88 g of a polycarboxylic acid-based dispersant, 36 g of ammonium hydroxide and 320 g of water were added to the Si slurry, and the mixture was stirred for 1 hour with a stirring blade at a rotation speed of 400 rpm. After that, 80 g of tetraethoxysilane (TEOS) was added to the above Si slurry. After stirring at room temperature for 1.5 hours, the obtained Si slurry was centrifuged at 4800 rpm for 25 minutes and re-dispersed with ethanol. The obtained slurry was subjected to ball milling using zirconia balls with a diameter of 1.0 mm for 8 hours to obtain Si slurry with a particle size (D50) of 0.25 μm. 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.

(Si coating step)
The above slurry was weighed so that the Si solid content was 13.85 g, transferred to a round-bottomed flask, and additional water was added to bring the total amount of water to 3386 g. After purging the inside of the flask system with nitrogen, the liquid temperature was raised to 35°C. After that, 0.465 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added into the flask and stirred for 30 minutes. 77.7 g of distilled styrene monomer and 0.39 g of sodium p-styrenesulfonate dissolved in 40 g of water were added and stirred for 2 hours. After that, the liquid temperature was raised to 62° C., and 1.7 g of ammonium persulfate dissolved in 40 g of water was added. Thereafter, heating and stirring were continued for 10 hours under reflux. The resulting reaction solution was centrifuged at a rotation speed of 4800 rpm for a rotation time of 45 minutes to remove free polystyrene, and the precipitate was redispersed in ethanol to obtain a polystyrene-coated Si slurry. rice field.

Thereafter, 9.95 g of the polystyrene-coated Si as Si, 23.2 g of the expanded graphite, 3.32 g of glycerin, and 450 mL of ethanol were placed in a stirring vessel and mixed and stirred for 20 minutes with a homogenizer. The mixture was then transferred to a rotary evaporator, heated to 50° C. with a hot bath while rotating, and evacuated with an aspirator to remove the solvent. Then, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 85 g of a mixed dried product (light-packed bulk density of 183 g/L).

(Pressing process)
This dried mixture was passed through a three-roll mill six times, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 205 g/L.

(Spheroidization process)
Next, 27 g of this granulated/consolidated product is put into a particle design/surface modification device (Hybridization system NHS-1L manufactured by Nara Machinery Co., Ltd.), pulverized at 40 m / sec for 15 minutes, simultaneously spheroidized, and lightly packed. A substantially spherical composite powder with a density of 264 g/L was obtained.

(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. As a result, a sintered powder composed of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a sintered powder having a light bulk density of 207 g/L and a particle size (D50) of 13 μm.

The obtained calcined powder (100 parts by mass) was stirred with coal tar pitch (carbonization degree 60%, 50 parts by mass) for 30 minutes, and then calcined and coated by the following method.

Using a rotary firing furnace, while flowing nitrogen (0.3 L/min), the temperature increase rate is 30 ° C./min, and the mixture is heated at 600 ° C. for 2 hours while rotating at 2 rpm. - Modification of lupit into soft carbon. As a result, a fired powder composed of 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 30 parts by mass of amorphous carbon (soft carbon derived from coal tar pitch) was obtained.

(Carbon coating process by vapor phase coating)
The obtained powder was set in a rotary sintering furnace, and after the inside of the tube was evacuated by a rotary pump, nitrogen gas at a flow rate of 200 SCCM and ethylene gas at a flow rate of 100 SCCM were flowed into the tube, and the tube was rotated at 2 rpm. Carbon coating was performed by heating to 920° C. with an electric heater and maintaining this state for 25 minutes. The weight increase due to the carbon coating is 17% by weight, resulting in 70 parts by weight of crystalline carbon (derived from graphite), 30 parts by weight of Si, 47 parts by weight of amorphous carbon (30 parts by weight of soft carbon derived from coal tar pitch parts, and 17 parts by mass of soft carbon derived from the gas phase coat). After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a particle size (D50) of 27 μm.

(Preparation of negative electrode for lithium secondary battery and battery evaluation)
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 18 μm using an applicator so that the solid content coating amount was 2.5 mg/cm ² , and dried at 90° C. in a vacuum dryer. Dried for 12 hours. After drying, it is punched into a circle of 14 mmφ, roll-pressed at 100° C., a feed rate of 1 m/min, and a pressure of 4.0 t/cm ² , and further heat-treated at 110° C. for 3 hours under vacuum to obtain a thickness of 32 μm. Thus, a negative electrode for a lithium secondary battery was obtained in which a negative electrode mixture layer of No.

Thereafter, a cell for evaluating the expansion rate of the initial charge and a screw cell for evaluating the capacity retention rate after 20 cycles were prepared in the same manner as in Example 1, and a charge-discharge test was performed. The initial charge expansion rate was 115%, the initial charge/discharge efficiency was 73.1%, the initial discharge volume capacity was 718 mAh/cc, and the capacity retention rate after 20 cycles was 99.7%.

<Example 8>
(Si surface modification step)
The pulverized Si obtained by the same method as the Si pulverizing step of Example 1 was weighed so that the solid content was 52.5 g, and then ultrasonic irradiation was performed for 15 minutes, and the total amount of ethanol was 1327 g. Ethanol was additionally added to obtain a Si slurry. After that, 116 g of a polycarboxylic acid-based dispersant, 3.5 g of 10 mol/L hydrochloric acid and 420 g of water were added to the Si slurry, and the mixture was stirred for 30 minutes with a stirring blade at a rotation speed of 250 rpm. After that, 105 g of tetraethoxysilane (TEOS) was added to the Si slurry, and the liquid temperature was raised to 70.degree. After stirring at 70° C. for 12 hours, the obtained Si slurry was centrifuged at 4800 rpm for 25 minutes and re-dispersed with 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 Si slurry with a particle size (D50) of 0.24 μm. 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.

(Si coating step)
The above slurry was weighed so that the Si solid content was 13.85 g, transferred to a round-bottomed flask, and additional water was added to bring the total amount of water to 3823 g. After purging the inside of the flask system with nitrogen, the liquid temperature was raised to 35°C. After that, 0.53 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added into the flask and stirred for 30 minutes. 87.8 g of distilled styrene monomer and 0.41 g of lithium p-styrenesulfonate dissolved in 40 g of water were added and stirred for 2 hours. After that, the liquid temperature was raised to 62° C., and 0.56 g of ammonium persulfate dissolved in 40 g of water was added. Thereafter, heating and stirring were continued for 10 hours under reflux. The resulting reaction solution was centrifuged at a rotation speed of 4800 rpm for a rotation time of 45 minutes to remove free polystyrene, and the precipitate was redispersed in ethanol to obtain a polystyrene-coated Si slurry. rice field.

Thereafter, 10.37 g of the polystyrene-coated Si (Si), 24.20 g of the expanded graphite, 3.46 g of glycerin and 500 mL of ethanol were placed in a stirring vessel and mixed and stirred for 20 minutes with a homogenizer. The mixture was then transferred to a rotary evaporator, heated to 50° C. with a hot bath while rotating, and evacuated with an aspirator to remove the solvent. Then, it was spread on a vat in a fume hood, dried for 1 hour while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 78 g of a mixed dried product (light-packed bulk density: 162 g/L).

(Pressing process)
This dried mixture was passed through a three-roll mill six times, passed through a sieve with an opening of 1 mm, and granulated and compacted to a light bulk density of 185 g/L.

(Spheroidization process)
Next, 59 g of this granulated/consolidated product was put into a particle design/surface modification device (Hybridization system NHS-1L manufactured by Nara Machinery Co., Ltd.), pulverized at 40 m / sec for 15 minutes, and simultaneously spheroidized and lightly packed. A substantially spherical composite powder with a density of 247 g/L was obtained.

(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. As a result, a sintered powder composed of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a light bulk density of 177 g/L and a particle size (D50) of 14 μm.

The obtained calcined powder (100 parts by mass) was stirred with coal tar pitch (carbonization degree 60%, 50 parts by mass) for 30 minutes, and then calcined and coated by the following method.

Using a rotary kiln, the mixture is heated at 300° C. for 2 hours and at 600° C. for 1 hour while flowing nitrogen (0.4 L/min) at a temperature increase rate of 30° C./min and rotating at 1 rpm. Thus, the coal tar pitch was modified into soft carbon. As a result, a fired powder composed of 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 30 parts by mass of amorphous carbon (soft carbon derived from coal tar pitch) was obtained.

(Carbon coating process by vapor phase coating)
The obtained powder was set in a rotary sintering furnace, and after the inside of the tube was evacuated by a rotary pump, nitrogen gas at a flow rate of 266 SCCM and ethylene gas at a flow rate of 133 SCCM were flowed into the tube, and the tube was rotated at 1 rpm. Carbon coating was performed by heating to 1000° C. with an electric heater and maintaining the state for 1 hour. The weight increase due to the carbon coating is 5% by weight, resulting in 70 parts by weight of crystalline carbon (derived from graphite), 30 parts by weight of Si, 35 parts by weight of amorphous carbon (30 parts by weight of soft carbon derived from coal tar pitch parts, and 5 parts by mass of soft carbon derived from the gas phase coat). After that, it was passed through a mesh with an opening of 45 μm to obtain a fired powder having a particle size (D50) of 25 μm.

(Preparation of negative electrode for lithium secondary battery and battery evaluation)
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 11 μm using an applicator so that the solid content coating amount was 2.5 mg/cm ² , and dried at 90° C. in a vacuum dryer. Dried for 12 hours. 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 under vacuum at 110°C for 2 hours to form a negative electrode mixture layer having a thickness of 24 µm. A battery negative electrode was obtained.

After that, a cell for evaluating the expansion rate of the initial charge and a screw cell for evaluating the capacity retention rate after 20 cycles were prepared in the same manner as in Example 1, and a charge-discharge test was performed. The initial charge expansion rate was 142%, the initial charge/discharge efficiency was 83.7%, the initial discharge volume capacity was 651 mAh/cc, and the capacity retention rate after 20 cycles was 98.4%.

<Comparative Example 1>
(Preparation of expanded graphite)
The steps of preparing and mixing expanded graphite were the same as <Example 1>.

945 g of the ultrafine Si slurry, 240 g of the expanded graphite, 100 g of resol-type phenolic resin (weight average molecular weight (Mw) = 230), and 4 L of ethanol were placed in a stirring vessel, and an in-line mixer was used. was mixed and stirred for 22 minutes. The mixture was then transferred to a rotary evaporator, heated to 40° C. with a hot bath while rotating, and depressurized with a vacuum pump to remove the solvent. Then, it was spread on a vat in a fume hood, dried for 2 hours while exhausting air, passed through a mesh with an opening of 2 mm, and further dried for 1 day to obtain 471 g of a mixed dried product (light-packed bulk density: 201 g/L).

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

(Spheroidization process)
Next, the granulated and compacted product was placed in a new power mill and ground at 21000 rpm for 360 seconds while cooling with water, and at the same time spheroidized to obtain a substantially spherical composite powder having a light bulk density of 439 g/L.

(Baking process)
The obtained powder was placed in a quartz boat and fired in a tubular furnace at a maximum temperature of 900° C. for 1 hour while flowing nitrogen gas to carbonize the phenolic resin at the same time. As a result, a substantially spherical sintered powder composed of 60 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 10 parts by mass of amorphous carbon (hard carbon derived from phenolic resin) was obtained.

After that, it was passed through a mesh with an opening of 45 μm to obtain a substantially spherical sintered powder having a light bulk density of 505 g/.

(Carbon coating process with coal tar pitch)
After 50 g of the obtained substantially spherical sintered powder and 40 g of coal tar pitch were mixed, 40 g of quinoline was added, and after stirring for 10 minutes, the mixture was sintered and coated by the following method.

(Baking process)
The coal tar pitch was transformed into soft carbon by heating the mixture at 600° C. for 2 hours under nitrogen flow (13.4 L/min). As a result, 60 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, 40 parts by mass of amorphous carbon (10 parts by mass of hard carbon derived from phenolic resin, soft carbon derived from coal tar pitch) A composite active material for a lithium secondary battery consisting of 30 parts by mass of carbon) was obtained.

(Crushing/sieving process)
The obtained composite active material for lithium secondary batteries was pulverized in a stamp mill, pulverized in a ball mill, passed through a mesh with an opening of 45 μm, and passed through a mesh having a light bulk density of 453 g/L and a particle diameter (D50) of 12.5 g/L. A ground powder of 5 μm was obtained.

(Carbon coating process by vapor phase coating)
The obtained powder was set in a quartz tube, and after the inside of the tube was evacuated by a rotary pump, nitrogen gas at a flow rate of 200 SCCM and ethylene gas at a flow rate of 100 SCCM were flowed into the tube and heated to 1000 by an electric heater. It was heated up to °C and maintained in that state for a period of time to form a carbon coating. The weight increase due to the carbon coating was 8.2% by weight, resulting in 60 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 48 parts by mass of amorphous carbon (hard carbon derived from phenol A composite active material for a lithium secondary battery consisting of 10 parts by mass and 38 parts by mass of soft carbon derived from the gas phase coat) was obtained.

Its physical properties are as follows. Particle diameter D50: 24 μm, D90: 45 μm, BET specific surface area: 6 m ² /g, average pore diameter: 15.6 nm, open pore volume: 0.028 cm ³ /g, shape: substantially spherical.

A cross-sectional SEM observation of the obtained composite active material for a lithium secondary battery was performed, and the porosity was found from the SEM image to be 1%, with no voids around Si.

(Preparation of negative electrode for lithium secondary battery and battery evaluation)
With respect to 95.5% 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 aid and gelling as a binder A negative electrode mixture-containing slurry was prepared by mixing 4% by weight of polyacrylic acid and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content coating amount was 2.6 mg/cm ² , and dried in a vacuum dryer at 110° C. for 0.5 hours. Dried. After drying, the lithium secondary was punched into a circle of 14 mmφ, uniaxially pressed under the condition of a pressure of 0.6 t/cm ² , and further heat-treated at 110°C for 2 hours under vacuum to form a negative electrode mixture layer having a thickness of 20 µm. A battery negative electrode was obtained.

The cell for evaluation was charged in a constant temperature room at 25° C. at a constant current of 0.44 mA to 0.005 V at 0.1 C, then at a constant voltage of 0.005 V at 0.05 C until the current value reached 0.02 mA. As a result, the initial charge capacity was 1075 mAh/g. After that, the evaluation cell was dismantled in an argon atmosphere in the glove box, and the electrode film thickness was measured with a micrometer. was 200%.

In Comparative Example 1, since the Si compound was not coated with a polymer film, the void volume in the composite active material for a lithium secondary battery was small and the expansion rate was high.

1 crystalline carbon 2 Si compound 3 void

Claims (3)

After modifying Si or Si alloy with a surface modifier as necessary, a polymer monomer, an initiator and a dispersant are added, Si or Si alloy is coated with a polymer film, graphite and optionally carbon a step of mixing the compound, a step of granulating and compacting, a step of pulverizing and spheronizing the mixture to form substantially spherical composite particles, and a step of firing the composite particles in an inert atmosphere; A composite active material for a lithium secondary battery containing Si or a Si alloy and crystalline carbon, comprising the step of mixing the carbon compound with the composite particles or the sintered particles, and heating the mixture in an inert atmosphere, if necessary. A method for producing a material, which has a structure having voids around the Si or Si alloy, the composite active material for a lithium secondary battery is used as a negative electrode active material, and the negative electrode active material is 95.5% by weight. On the other hand, a negative electrode containing 0.5% by weight of acetylene black as a conductive aid and 4.0% by weight of a polycarboxylic acid binder as a binder, a counter electrode made of metallic lithium crimped to a stainless steel foil, a separator, and fluoroethylene It is a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 2% by volume of carbonate (EC/DEC = 1/1: volume ratio), and the electrolyte is hexafluoride. Using a CR2032 type coin cell type lithium secondary battery with an electrolyte that is lithium phosphate (LiPF6), at a measurement temperature of 25 ± 2 ° C, a constant current of 0.5 mA from the initial voltage to 0.005 V After charging, constant voltage charging at a voltage of 0.005 V until the current value reaches 0.03 mA, and a constant voltage of 0.5 mA in a voltage range of 0.005 V to 1.5 V at a measurement temperature of 25 ± 2 ° C. A method for producing a composite active material for a lithium secondary battery, wherein the discharge capacity after 20 charge/discharge cycles is 70.0% or more of the maximum discharge capacity in a charge/discharge test by current discharge. 2. The method for producing a composite active material for a lithium secondary battery according to claim 1, wherein an oxidizing agent or a modifier containing a metal alkoxide group, a carboxyl group or a hydroxyl group in the molecule is used as the surface modifier. Polymer monomers are styrene, methyl methacrylic acid, acrylic acid, methacrylamide, acrylamide, vinyl benzoate, diethylaminostyrene, diethylamino alpha-methylstyrene, p-vinylbenzenesulfonic acid, sodium p-vinylbenzenesulfonate. Salt, divinylbenzene, vinyl acetate, butyl acetate, vinyl chloride, vinyl fluoride, vinyl bromide, maleic anhydride, N-phenylmaleimide, N-butylmaleimide, N-vinylpyrrolidone, N-vinylcarbazole, acrylonitrile, aniline , pyrrole, polyol used in urethane polymerization, or isocyanate.

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