JP7516848B2 - Silicon or silicon alloy, composite active material for lithium secondary battery containing same, and method for producing same - Google Patents

Description

The present invention relates to silicon or a silicon alloy, a composite active material for a lithium secondary battery containing the same, and a method for producing the same.

Demand for higher capacity lithium secondary batteries is growing with the increasing performance of mobile devices such as smartphones and tablet terminals, and the widespread use of vehicles equipped with lithium secondary batteries such as EVs and PHEVs. Currently, graphite is mainly used as the negative electrode material for lithium secondary batteries, but in order to further increase capacity, there has been active development of negative electrode materials using metals such as silicon and tin, which have high theoretical capacity and are capable of absorbing and releasing lithium ions, or alloys with other elements.

On the other hand, it is known that composite active materials for lithium secondary batteries, which are made of metal materials capable of absorbing and releasing lithium ions, undergo a significant volume expansion when alloyed with lithium by charging. This causes the active materials to crack and become finer, and furthermore, the structure of negative electrodes using these materials is destroyed, cutting off their conductivity. Therefore, a problem with negative electrodes using these metal materials is that their capacity decreases significantly over cycles.

To address this issue, a method has been proposed in which the metal materials are coated with a polymer, and then the polymer is removed to form voids after compounding with a carbonaceous material or graphite. It is known that such composite particles significantly improve cycle characteristics because the active material as a whole does not expand due to the voids around the metal material when the metal material is alloyed with lithium and expands in volume. For example, Patent Document 1 discloses that by producing a lithium secondary battery using a composite active material for lithium secondary batteries having a structure with voids around Si or a Si alloy as the active material, the discharge capacity after 20 charge/discharge cycles is 70.0% or more compared to the maximum discharge capacity in a charge/discharge test. However, there is no description of the oxygen content or the amount of surface hydroxyl groups in the Si or Si alloy.

In addition, Patent Document 2 discloses that silicon-based particles, characterized in that the oxygen content (wt%) divided by the BET specific surface area ( ^m2 /g) is 0.1 or less, and a composite active material for lithium secondary batteries containing the silicon-based particles exhibit high initial charge/discharge efficiency. However, since the oxygen content is low, it is difficult to form a void precursor on the surface of the silicon or silicon alloy. If the void precursor cannot be formed, it is difficult to form voids on the surface of the silicon or silicon alloy, and it is easy to predict that the expansion rate during lithium ion charging will be high and the capacity retention rate will be low.

In response to the ever-increasing required characteristics of lithium secondary batteries, there is a demand for the development of a negative electrode material that can achieve both an initial coulombic efficiency and an initial expansion rate that is better than the negative electrode materials using the composite active materials for lithium secondary batteries obtained in Patent Documents 1 and 2, as well as the development of Si or Si alloys to be used therein.

WO2019-131519 JP 2017-112057 A

In recent years, from the perspective of battery safety, it has become necessary for the electrode material to not expand in volume even after repeated charging and discharging. If the electrode material expands in volume significantly, electrolyte leakage occurs and the battery life is shortened. On the other hand, if the initial charge/discharge efficiency is low, the lithium ions contained in the positive electrode cannot be used effectively, resulting in problems such as reduced capacity per weight and high manufacturing costs. Therefore, there is a demand for materials with high initial charge/discharge efficiency, and it is necessary to achieve both of these properties.

In view of the above circumstances, the present invention aims to provide a composite active material for lithium secondary batteries that is an electrode material in which volume expansion during initial charging is suppressed and that exhibits excellent initial charge/discharge efficiency, as well as Si or Si alloys used therein, and a method for producing the same.

As a result of thorough investigation of the prior art, the present inventors have found that the above-mentioned problems can be solved by the following configuration.
(1) Si or a Si alloy having an average particle size (D50) of 0.01 to 0.6 μm, an average particle size (D90) of 0.01 to 1.0 μm, a BET specific surface area of 40 to 300 m ² /g as measured by the BET method, and an oxygen content of 10% by weight or more and less than 20% by weight.
(2) Si or a Si alloy according to (1), wherein the number of surface hydroxyl groups is 1.0/nm ² to 3.0/nm ² .
(3) Si or a Si alloy according to (1) or (2), in which the value obtained by dividing the oxygen content (wt %) by the BET specific surface area (m ² /g) is 0.1 to 0.4.
(4) A composite active material for a lithium secondary battery comprising the Si or Si alloy according to any one of (1) to (3) above and crystalline carbon, the composite active material for a lithium secondary battery being characterized in having a void structure around the Si or Si alloy.
(5) The composite active material for lithium secondary batteries according to (4), wherein the composite active material for lithium secondary batteries has a structure in which Si or a Si alloy is surrounded by crystalline carbon, 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.
(6) The composite active material for lithium secondary batteries according to (4) or (5), characterized in that the Si or Si alloy is sandwiched between crystalline carbon layers having a thickness of 0.5 μm or less, the structure spreads in a laminated and/or mesh-like manner, and the crystalline carbon layers are curved near the surfaces of the composite active material for lithium secondary batteries to cover the composite active material for lithium secondary batteries.
(7) The composite active material for lithium secondary batteries according to any one of (4) to (6), characterized in that the crystalline carbon has a purity of 99% by weight or more as determined from semi-quantitative impurity values of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U) by ICP atomic emission spectroscopy, an S amount of 1% by weight or less as determined by an ion chromatography (IC) measurement method using an oxygen flask combustion method, and/or a BET specific surface area of 100 ^m2 /g or less.
(8) A method for producing a composite active material for a lithium secondary battery according to any one of (4) to (7), comprising the steps of: adding a polymer monomer, an initiator, and, if necessary, a dispersant to the Si or Si alloy according to any one of (1) to (3); coating the Si or Si alloy with a polymer film; mixing the resulting mixture with graphite and, if necessary, a carbon compound; granulating and compacting the mixture; pulverizing and spheronizing the mixture to form substantially spherical composite particles; calcining the composite particles in an inert atmosphere; mixing, if necessary, a carbon compound with the composite particles or the calcined particles; and heating the mixture in an inert atmosphere.
(9) The polymer monomer is a methyl methacrylate such as styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, or triethylene glycol methacrylate. acrylates, 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, and other acrylic acid-based compounds; methacrylic acid Methacrylamides such as N,N'-methylenebisacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide, N-n-butylacrylamide, N-methylolmethacrylamide, and N-ethylolmethacrylamide; acrylamides such as N,N'-methylenebisacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide, N-n-butylacrylamide, N-methylolacrylamide, and N-ethylolacrylamide; vinyl benzoate, diethyl acrylamide, and the like. The method for producing a composite active material for a lithium secondary battery according to (8), wherein the active material is selected from the group consisting of aminostyrene, 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, a polyol-based material or an isocyanate-based material used in urethane polymerization.

The present invention provides a composite active material for a secondary battery that has high initial charge/discharge efficiency and suppresses volume expansion after initial charging, as well as a specific Si or Si alloy used in the composite active material and a method for producing the same.

The Si or Si alloy of the present invention, as well as the composite active material for lithium secondary batteries and the method for producing the same, will be described in detail below, showing an example of the present invention.

The Si or Si alloy (collectively, Si compound) of the present invention has an average particle size (D50) of 0.01 to 0.6 μm, preferably 0.03 to 0.5 μm, more preferably 0.05 to 0.5 μm, an average particle size (D90) of 0.01 to 1.0 μm, preferably 0.03 to 0.8 μm, more preferably 0.05 to 0.06 μm, a BET specific surface area by the BET method of 40 to 300 m ² /g, preferably 40 to 250 m ² /g, more preferably 40 to 200 m ² /g, particularly preferably 60 to 200 m ² /g, and an oxygen content of 10% by weight or more and less than 20% by weight, more preferably 11% by weight or more and less than 19% by weight. If D50 or D90 is smaller than this range, the productivity of Si or Si alloy decreases, while if it is larger than this range, there is a problem that the aggregation of Si or Si alloy is large and uniform polymer coating cannot be performed. Furthermore, if the oxygen content is greater than this range, lithium ions are more likely to be trapped in the Si or Si alloy, resulting in a decrease in the initial charge/discharge efficiency. If the oxygen content is less than this range, the Si or Si alloy surface cannot be coated with the polymer, resulting in a high initial expansion coefficient.

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

When measuring the average particle size, a dispersion liquid is prepared by using ultrasonic waves or the like after diluting the Si or Si alloy slurry as is or, if necessary, diluting it with a liquid, and the prepared dispersion liquid is introduced into the device as a measurement sample to measure the average particle size. The liquid to be used for dilution should be one in which Si or Si alloy can be dispersed, and it is preferable to use water, alcohol, or a low-volatility organic solvent.

When the Si or Si alloy does not mix well with the liquid, i.e., when it is difficult to disperse the Si or Si alloy uniformly in the liquid, a surfactant or the like may be added as necessary.

The BET specific surface area is a value measured by the nitrogen adsorption method after the sample is vacuum dried at 200°C for 20 minutes. It may be measured by either the single-point method or the multi-point method.

The oxygen content can be measured using an oxygen analyzer manufactured by LECO using the impulse furnace melting-infrared absorption method.

It is preferable that the particle size distribution diagram obtained for Si or Si alloys shows a normal distribution.

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

The term "Si alloy" as used in this invention refers to an alloy whose main component is Si. In the Si alloy, the elements contained other than Si are preferably one or more elements from groups 2 to 15 of the periodic table, and are preferably elements whose phase contained in the alloy has a melting point of 900°C or higher.

The Si or Si alloy of the present invention has a surface hydroxyl group amount of 1.0/nm ² to 3.0/nm ² , preferably 1.2/nm ² to 2.7/nm ² , and more preferably 1.3/nm ² to 2.5/nm ² . If the number of surface hydroxyl groups is less than this range, it is difficult to coat the Si or Si alloy surface with a polymer, and there is a problem that the initial expansion rate of the composite active material for lithium secondary batteries is high. If the number of surface hydroxyl groups is greater than this range, the problem of lithium ions being trapped on the Si or Si alloy surface during charging becomes significant, and there is a problem that the initial charge/discharge efficiency is reduced.

A conventional titration method can be used to measure the amount of surface hydroxyl groups. First, the Si or Si alloy slurry is dried at a temperature of 80°C to 110°C, and if necessary, lightly crushed in a mortar to obtain Si or Si alloy powder. This powder is added to a mixed solvent of 30 cc of 20% by weight sodium chloride aqueous solution and 10 cc of ethanol, and lightly stirred. Then, while stirring the slurry with a stirrer tip, 0.1 mol/L NaOH aqueous solution is dropped to adjust the pH to 4. Then, the NaOH aqueous solution is further dropped until the pH of the solution stabilizes at 9. The amount of surface hydroxyl groups of Si or Si alloy is given by the following formula.

Amount of surface hydroxyl groups (number/nm ² )=c×V×NA/C×S
c (mol/L): concentration of NaOH aqueous solution used in titration V (L): volume of sodium hydroxide solution required to increase pH from 4 to 9 NA (pieces/mol) = Avogadro's number C (g): weight of Si or Si alloy S (nm ² /g): BET specific surface area of Si or Si alloy Titration may be performed manually or with an automatic titrator. In addition, when stabilizing the pH of the solution at pH 4 or pH 9, 0.1 mol/L of HCl may be used as necessary. The BET specific surface area is a value measured by a nitrogen adsorption single-point method or a multipoint method after vacuum drying the sample at 200°C for 20 minutes.

In the Si or Si alloy of the present invention, the value obtained by dividing the oxygen content (wt%) by the BET specific surface area ( ^m2 /g) is 0.1 to 0.4, preferably 0.1 to 0.3, and more preferably 0.1 to 0.2. If the value obtained by dividing the oxygen content (wt%) by the BET specific surface area ( ^m2 /g) is smaller than this range, it is difficult to coat the Si or Si alloy surface with a polymer film, and there is a problem that the initial expansion rate of the composite active material for lithium secondary batteries is high, and if it is larger than this range, Li ions are easily trapped on the Si or Si alloy surface during charging, and there is a problem that the initial charge/discharge efficiency is reduced.

The method for producing Si or Si alloy of the present invention will be explained.

As the raw material for Si or Si alloy, it is preferable to use powder with an average particle size (D50) of 0.01 to 10 μm. To obtain a Si compound with a specified average particle size, the raw material for the above-mentioned Si compound (ingot, wafer, powder, etc.) is pulverized in a pulverizer, and in some cases a classifier is used. In the case of lumps such as ingots and wafers, they can first be pulverized using a coarse pulverizer such as a jaw crusher. The material can then be finely pulverized using a ball mill, a media agitator mill, or a roller mill, which uses the compression force of rollers to pulverize the material, by moving a pulverizing medium such as balls or beads, and using the impact, friction, and compression forces generated by the kinetic energy of the pulverization medium; a jet mill, which pulverizes the material by colliding it at high speed with the lining material or with each other, and using the impact force generated by the impact; a hammer mill, pin mill, or disk mill, which pulverize the material by using the impact force generated by the rotation of a rotor to which hammers, blades, pins, etc. are fixed; a colloid mill that uses shear force; or a high-pressure wet opposed collision type disperser called an "ultimizer."

Both wet and dry grinding can be used. For further fine grinding, for example, a wet bead mill can be used to gradually reduce the bead diameter, thereby obtaining very fine particles. In addition, to adjust the particle size distribution after grinding, dry classification, wet classification, or sieving classification can be used. Dry classification mainly uses airflow, and the processes of dispersion, separation (separation of fine particles and coarse particles), collection (separation of solids and gas), and discharge are performed sequentially or simultaneously. In order to prevent a decrease in classification efficiency due to interference between particles, particle shape, airflow turbulence, velocity distribution, static electricity, etc., pretreatment is performed before classification (adjustment of moisture, dispersibility, humidity, etc.), or the moisture and oxygen concentration of the airflow used is adjusted. In dry types with an integrated classifier, grinding and classification are performed at the same time, making it possible to achieve the desired particle size distribution.

Other methods for obtaining Si or Si alloys with a specified average particle size include heating and evaporating a Si compound using plasma or laser, etc., and solidifying it in an inert atmosphere, or using CVD or plasma CVD using a gas source. These methods are suitable for obtaining ultrafine particles of 0.1 μm or less.

In order to adjust the oxygen content or the amount of surface hydroxyl groups, the surface of the Si or Si alloy is chemically treated. For example, the Si or Si alloy can be treated with an oxidizing agent such as hydrogen peroxide, nitric acid, sulfuric acid, potassium permanganate, potassium dichromate, sodium hypochlorite, chromium trioxide, ammonium persulfate, potassium persulfate, or an alkaline solution such as ammonia, sodium hydroxide, potassium hydroxide, calcium hydroxide, or magnesium hydroxide. If necessary, the surface of the Si or Si alloy can be modified with a metal alkoxide or a compound containing a carboxyl group or a hydroxyl group. Specific examples of surface modification compounds include tetraethoxysilane and tetramethoxysilane, and are not particularly limited as long as the oxygen content and the amount of surface hydroxyl groups can be adjusted to the above ranges.

When using a surface modification compound, it is preferable to add 0.1 to 300 parts by mass of the surface modification compound to 100 parts by mass of Si or Si alloy. 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, a reaction accelerator such as ammonia, sodium hydroxide, potassium hydroxide, or sodium bicarbonate, which dissolves in water and shows alkalinity, or hydrochloric acid, nitric acid, acetic acid, or sulfuric acid, which dissolves in water and shows acidity, may be added as necessary. The reaction accelerator is preferably ammonia, hydrochloric acid, nitric acid, or acetic acid because it is highly reactive and does not leave any metal compound. When using a reaction accelerator, it is preferable to add 0.005 to 54 parts by mass of the reaction accelerator to 100 parts by mass of the Si compound. The solvent used in the reaction may be any solvent in which the surface modification compound dissolves, such as water, ethanol, methanol, acetone, dimethylformamide, tetrahydrofuran, toluene, hexane, or chloroform, and a mixed solvent may be used as necessary. When modifying the surface of a Si compound using tetraethoxysilane as a surface modification compound, it is preferable to use a mixed solvent of water and ethanol. The ratio of each solvent in the mixed solvent is preferably 10 to 100 parts by mass of water per 100 parts by mass of ethanol. When the ratio of water in the mixed solvent is within this range, the Si or Si alloy in the solvent is likely to be stable, and the modification reaction is likely to proceed sufficiently.

After the Si or Si alloy is coated with the surface-modifying compound, the Si or Si alloy may be pulverized and atomized using a ball mill or a bead mill, if necessary. The balls used for crushing are preferably zirconia or alumina. The crushing time is preferably 1 to 24 hours, more preferably 1 to 12 hours.

In addition, after the Si or Si alloy is pulverized and atomized, it is preferable to replace the solvent used to modify the Si or Si alloy surface with water by centrifugation, if necessary.

The composite active material for lithium secondary batteries of the present invention is a composite active material for lithium secondary batteries containing the above-mentioned Si or Si alloy and crystalline carbon, and is characterized in that it has a void structure around the Si or Si alloy, and the voids are introduced to relieve the expansion stress of the Si or Si alloy.

In the composite active material for lithium secondary batteries of the present invention, the structure having voids around the Si compound is particularly a structure having spaces between the Si compound and the crystalline carbon to relieve the expansion stress caused by the expansion of the Si compound during charging.

Among the structures in which the Si compound is surrounded by voids in the composite active material for lithium secondary batteries, it is particularly preferred that the composite active material for lithium secondary batteries has a structure in which Si or a Si alloy is surrounded by crystalline carbon, 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, more preferably 10 to 65%, particularly preferably 10 to 50%, and even more preferably 15 to 50%. By using a composite active material for lithium secondary batteries having such a structure as the active material for a lithium secondary battery, the expansion of the composite active material for lithium secondary batteries is suppressed, and the lithium secondary battery has excellent cycle characteristics.

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

The following methods can be used to calculate voids:

First, the negative electrode for lithium secondary batteries is cut in the vertical direction of the electrode using a cross-section processing device. As a device used for cross-section processing, a cross-section polisher is preferably used in order to obtain a clearer image. In addition, the composite active material powder for lithium secondary batteries, that is, the powdered composite active material for lithium secondary batteries of the present invention is embedded in an epoxy resin, and then cut, and the obtained cross-section portion is observed using a microscope. The microscope used here is a field emission scanning electron microscope (FE-SEM) because it is necessary to obtain sufficient resolution and observation range. Then, two transparent sheets are placed on top of the printed image of the obtained secondary electron image, and the outline of the composite active material for lithium secondary batteries is drawn on one sheet, and the voids on the other sheet are filled in with a pen. As the transparent sheet, an OHP sheet (overhead projector sheet) is used because of its good workability. Next, each image is converted to JPEG or TIFF data and binarized using Nano Hunter NS2K-Pro (Nano Systems Co., Ltd.), and the area of the composite active material for lithium secondary batteries and the area of the voids are calculated. 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.

The content of the Si compound in the composite active material for lithium secondary batteries is preferably 5 to 80 parts by mass, more preferably 10 to 50 parts by mass, and particularly preferably 15 to 50 parts by mass. If the content of the Si compound is less than 5 parts by mass, a sufficiently large capacity is likely to be obtained compared to conventional negative electrodes that use graphite as a composite active material for lithium secondary batteries. On the other hand, if the content of the Si compound is more than 80 parts by mass, cycle deterioration, i.e., the decrease in cycle characteristics, is likely to be small.

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, and 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, and the conductive path becomes sufficient. This makes it difficult for the initial volumetric discharge capacity to deteriorate. On the other hand, when the content of crystalline carbon exceeds 95 parts by mass, the charging capacity is difficult to increase.

There are no particular limitations on the crystalline carbon, so long as it becomes crystalline carbon when fired, and carbon derived from graphite is particularly preferred.

The graphite that becomes crystalline carbon when fired in the present invention includes natural graphite and artificial graphite, among which exfoliated graphite, commonly called graphite, is preferred.

In this specification, exfoliated graphite refers to graphite with 400 or fewer graphene sheet layers. The graphene sheets are bonded together primarily by van der Waals forces, that is, the graphene sheets are stacked together by van der Waals forces.

The number of layers of the graphene sheets in the exfoliated graphite is preferably 300 layers or less, more preferably 200 layers or less, and even more preferably 150 layers or less, in order to more uniformly disperse the Si compound and the exfoliated graphite, thereby further suppressing the expansion of the electrode material using the composite active material for lithium secondary batteries, and/or to improve the cycle characteristics of the lithium secondary battery (hereinafter simply referred to as "the advantages of the present invention"). From the viewpoint of ease of handling, the number of layers of the graphene sheets is preferably 5 layers or more.

The number of graphene sheets stacked 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, and more preferably 22 nm or less, in order to obtain a better effect of the present invention. The lower limit of the average thickness of the exfoliated graphite is preferably 4 nm or more, in order to simplify the manufacturing procedure.

The average thickness of the exfoliated graphite is measured by observing the exfoliated graphite using a transmission electron microscope (TEM), measuring the thickness of 10 or more layers of stacked graphene sheets in the exfoliated graphite, and calculating the arithmetic average of the values to obtain the average thickness of the exfoliated graphite.

Exfoliated graphite is graphite obtained by exfoliating the graphene sheets contained in graphite compounds, specifically natural graphite, between the layers.

An example of exfoliated graphite is so-called expanded graphite.

Expanded graphite contains graphite, and can be obtained, for example, by treating flake graphite with chemicals such as concentrated sulfuric acid, nitric acid, or hydrogen peroxide, intercalating these chemicals into the gaps in the graphene sheet, and then heating the sheet to expand the gaps in the graphene sheet as the intercalated chemicals evaporate. As described below, a composite active material for lithium secondary batteries can be manufactured 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.

Graphite also includes expanded graphite that has been subjected to a spheroidizing process. The procedure for the spheroidizing process will be described in detail later. As will be described later, when performing the spheroidizing process on expanded graphite, the spheroidizing process may be performed together with other components (e.g., precursors of hard carbon and soft carbon, etc.).

The crystalline carbon preferably has a purity of 99% by weight or more, an impurity content of 10,000 ppm or less, an S content 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 impurity content is 10,000 ppm or less, the irreversible capacity due to the formation of SEI derived from impurities is small, so the initial charge/discharge efficiency, which is the discharge capacity relative to the initial charge capacity, tends to be less likely to be low. Similarly, when the S content is 1% by weight or less, the irreversible capacity is small, so the initial charge/discharge efficiency is less likely to be low. 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, and 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 electrolyte is small, so the initial charge/discharge efficiency is less likely to be low.

The composite active material for lithium secondary batteries of the present invention has a structure in which Si or a Si alloy is sandwiched between crystalline carbon layers having a thickness of 0.5 μm or less, and the structure is spread in a laminated and/or mesh-like manner, and it is preferable that the crystalline carbon layers are curved near the surface of the composite active material for lithium secondary batteries to cover the composite active material for lithium secondary batteries.

In the composite active material for lithium secondary batteries of the present invention, the crystalline carbon layer is curved near the surface of the Si compound to cover the Si compound and the voids present in its vicinity, and the crystalline carbon forms a three-dimensional network, i.e., it is preferable that the structure is a repeated structure in which the Si compound is contained in part of the voids formed by the crystalline carbon.

If the thickness of the crystalline carbon layer exceeds 0.5 μm, the electron transfer effect of the crystalline carbon layer is weakened. If the crystalline carbon layer is linear when viewed in cross section, it is preferable for electron transfer that its length is at least half the size of the composite active material for lithium secondary batteries, and more preferably is approximately the same as the size of the composite active material for lithium secondary batteries. If the crystalline carbon layer is mesh-like, it is preferable for electron transfer that the mesh of the crystalline carbon layer is connected over at least half the size of the composite active material for lithium secondary batteries, and more preferably is approximately the same as the size of the composite active material for lithium secondary batteries.

In the composite active material for lithium secondary batteries of the present invention, it is preferable that the crystalline carbon layer bends near the surface of the composite active material for lithium secondary batteries to cover the composite active material for lithium secondary batteries. Such a shape reduces the risk of a reaction product being formed during charging and discharging due to direct contact between the electrolyte and the Si compound or the end face of the crystalline carbon layer caused by the electrolyte penetrating from the end face of the crystalline carbon layer, resulting in a decrease in charge and discharge efficiency.

The crystalline carbon preferably has a purity of 99% by weight or more as determined from the semi-quantitative impurity values of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U) by ICP atomic emission spectroscopy, an S amount of 1% by weight or less as determined by an ion chromatography (IC) measurement method using an oxygen flask combustion method, and/or a BET specific surface area of 100 ^m2 /g or less.

Impurities are measured using ICP emission spectrometry, measuring the semi-quantitative impurity values of the following 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U). The amount of S is measured using ion chromatography (IC) after filtering after combustion and absorption treatment using the oxygen flask combustion method.

It is further preferable that the composite active material for lithium secondary batteries of the present invention contains amorphous carbon in the inner and/or outer layer portions of the composite active material for lithium secondary batteries.

There are no particular limitations on the amorphous carbon, so long as it becomes amorphous carbon when fired, and amorphous or microcrystalline carbonaceous materials other than graphite are particularly preferred.

Amorphous or microcrystalline carbonaceous materials other than graphite that become amorphous carbon when fired include easily graphitized carbon (soft carbon), which is graphitized by heat treatment at temperatures above 2000°C, non-graphitized carbon (hard carbon), which is difficult to graphitize, and graphite-like aromatic compounds. At least one of soft carbon and hard carbon is preferable, and soft carbon is particularly preferable.

Hard carbon is preferably obtained by carbonizing a precursor such as a resin or a resin composition. By carbonizing the resin or resin composition, the hard carbon 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 hard carbon include polymer compounds (e.g., thermosetting resins, thermoplastic resins). Examples of thermosetting resins include phenolic resins such as novolac-type phenolic resins and resole-type phenolic resins, epoxy resins such as bisphenol-type epoxy resins and novolac-type epoxy resins, melamine resins, urea resins, aniline resins, isocyanate resins, furan resins, ketone resins, unsaturated polyester resins, and urethane resins. Modified products of these resins modified 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, and polyetheretherketone. One or a combination of two or more of these may be used.

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

Hard carbon precursors can be in any shape, including powder, plates, granules, fibers, chunks, and spheres. It is preferable that these precursors dissolve in the solvent used when mixing the various components.

The weight average molecular weight of the hard carbon precursor used is preferably 100 or more, and more preferably 1,000,000 or less, in order to obtain a better effect of the present invention.

The soft carbon is preferably obtained by carbonizing a precursor such as a resin or a resin composition. The resin or resin composition is carbonized by carbonization, and the soft carbon obtained by carbonization can be used as a carbon material for lithium secondary batteries. Examples of the resin or resin composition that is the raw material (precursor) of the soft carbon include coal-based pitch (e.g., coal thal pitch), petroleum-based pitch, mesophase pitch, coke, low molecular weight heavy oil, and derivatives thereof, and coal-based pitch (e.g., coal thal pitch), petroleum-based pitch, mesophase pitch, coke, low molecular weight heavy oil, and derivatives thereof are preferred. In terms of superior effects of the present invention, the soft carbon is preferably soft carbon obtained from a precursor such as coal-based pitch, and more preferably soft carbon derived from coal thal pitch.

Soft carbon precursors can be in any shape, including powder, plates, granules, fibers, chunks, and spheres. These precursors are preferably soluble in the solvent used to mix the various components.

The weight average molecular weight of the soft carbon precursor used is preferably 1,000 or more, and more preferably 1,000,000 or less, in order to obtain a better effect of the present invention.

Examples of graphite-like aromatic compounds include dopamine derivatives such as dopamine hydrochloride, dihydroxyphenylalanine, and 5,6-dihydroxykihindole, polyphenols such as tannic acid, catechin, anthocyanin, rutin, and isoflavone, catecholamine, gallic acid, pyrogallol, and galacetophenone, as well as materials obtained by carbonizing and calcining these compounds.

One or more of these can be used in combination.

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

The content of amorphous carbon is preferably 1 to 80 parts by mass, more preferably 3 to 60 parts by mass, and particularly preferably 5 to 50 parts by mass. If the content of amorphous carbon is less than 1 part by mass, the reaction between the composite active material for lithium secondary batteries and the electrolyte cannot be suppressed, and there is a problem that the charge/discharge efficiency does not improve even if the battery is repeatedly charged and discharged. If the content of amorphous carbon is more than 80 parts by mass, the irreversible capacity of the amorphous carbon itself is large, and there is a problem that the initial charge/discharge efficiency of the composite active material for lithium secondary batteries also decreases.

The average particle size (D50) of the composite active material for lithium secondary batteries of the present invention is preferably 0.1 μm to 50 μm, more preferably 0.3 μm to 40 μm, and particularly preferably 0.5 μm to 30 μm. If the average particle size is outside this range, coating unevenness is likely to occur during electrode coating, which is not preferable. The BET specific surface area of the composite active material for lithium secondary batteries is preferably 0.5 to 200 m ² /g, more preferably 1 m ² /g to 150 m ² /g, and particularly preferably 1 m ² /g to 100 m ² /g. If the BET specific surface area is large, the reaction with the electrolyte increases, causing a problem of a decrease in the initial charge/discharge efficiency.

The method for producing a composite active material for lithium secondary batteries of the present invention is characterized by including the steps of adding a polymer monomer, an initiator, and, if necessary, a dispersant to Si or a Si alloy, coating the Si or Si alloy with a polymer film, mixing the resulting mixture with graphite and, if necessary, a carbon compound, granulating and compacting the mixture, pulverizing and spheronizing the mixture to form substantially spherical composite particles, firing the composite particles in an inert atmosphere, and, if necessary, mixing the composite particles or the fired particles with a carbon compound, and heating the mixture in an inert atmosphere.

In order to promote the reaction of the Si or Si alloy that has undergone the above process with the polymer monomer, it is preferable to modify the surface of the Si or Si alloy in advance as necessary with a surface modifier such as a silane coupling agent. The surface modifier preferably contains a metal alkoxide group, a carboxyl group, or a hydroxyl group in the molecule. Specific surface modifiers include, for example, vinyl-based such as vinyltrimethoxysilane and vinyltriethoxysilane, epoxy-based such as 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltrimethoxysilane, and 3-glycidoxypropyltriethoxysilane, styryl-based such as p-styryltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, and 3-methacryloxypropylmethyldiethoxysilane, and 3-methacryloxypropylmethyldiethoxysilane. Examples of the silane include methacryl-based compounds such as 3-methacryloxypropyltriethoxysilane, acrylic compounds such as 3-acryloxypropyltrimethoxysilane, isocyanurate compounds such as tris-(trimethoxysilylpropyl)isocyanurate, and isocyanate compounds such as 3-isocyanatepropyltriethoxysilane. Of these, at least one selected from the group consisting of 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, and 3-methacryloxypropyltriethoxysilane is preferred, and 3-methacryloxypropyltrimethoxysilane is particularly preferred.

During the reaction between the Si compound and the polymer monomer, it is preferable to mix the raw materials uniformly using a general mixer or stirrer such as a magnetic stirrer, three-one motor, homomixer, in-line mixer, bead mill, or ball mill. The reaction temperature is preferably 25°C. The reaction time is preferably 0.5 to 72 hours, and more preferably 0.5 to 24 hours. By keeping the reaction time within this range, the modification reaction proceeds sufficiently and productivity is less likely to decrease.

By adding a polymer monomer and an initiator to a Si compound, the resulting polymer monomer slurry is polymerized to form a polymer, resulting in a coating of a polymer film around the Si compound.

Examples of polymer monomers to be reacted with the Si compound include methyl methacrylates such as styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, and triethylene glycol 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, and acrylic acid. Acrylic acid-based compounds such as isobornyl acrylate, benzyl acrylate, phenyl acrylate, glycidyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, and hydroxybutyl acrylate; methacrylamide-based compounds such as methacrylamide, N-methylacrylamide, N,N'-dimethylacrylamide, N-tert-butylmethacrylamide, N-n-butylmethacrylamide, N-methylolmethacrylamide, and N-ethylolmethacrylamide; acrylamide-based compounds such as N,N'-methylenebisacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide, N-n-butylacrylamide, N-methylolacrylamide, and N-ethylolacrylamide; vinyl benzoate, diethylaminostyrene, diethylaminoalpha-methylstyrene, p-vinylbenzenesulfonic acid ... Examples of the methacrylic acid include sodium sulfonate, divinylbenzene, vinyl acetate, butyl acetate, vinyl chloride, vinyl fluoride, vinyl bromide, maleic anhydride, N-phenylmaleimide, N-butylmaleimide, N-vinylpyrrolidone, N-vinylcarbazole, acrylonitrile, aniline, pyrrole, and polyols or isocyanates used in urethane polymerization, and preferably methyl methacrylates such as styrene, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, sec-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, and triethylene glycol methacrylate. Examples of the acrylic acid-based compounds include acrylic acid-based compounds such as acrylic acid-based compounds, 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, and hydroxybutyl acrylate, and acrylonitrile. More preferred are styrene, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, and acrylonitrile. Particularly preferred are styrene, methyl methacrylate, and methyl acrylate.

Examples of initiators used include azo compounds such as azobisisobutyronitrile, potassium persulfate, ammonium persulfate, benzoyl peroxide, diisobutyryl peroxide, di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, dilauroyl peroxide, dibenzoyl peroxide, 1,1-di(tert-hexylperoxy)cyclohexane, 1,1-di(tert-butylperoxy)cyclohexane, and tert-butyl hydroperoxy. Examples of organic peroxides include diisobutyryl peroxide, tert-hexylperoxyisopropyl monocarbonate, tert-butylperoxyisopropyl monocarbonate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, tert-butylperoxyacetate, di-tert-hexylperoxide, di-tert-butylperoxide, diisopropylbenzenehydroperoxide, and tert-butylhydroperoxide.

Examples of solvents used to prepare a slurry of polymer monomers include water, ethanol, methanol, isopropyl alcohol, propanol, and toluene, and preferably water, ethanol, or methanol, and more preferably water or ethanol. These can be used alone or in combination of two or more.

The polymer monomer content in the polymer monomer slurry is preferably 0.5 to 30% by weight, and particularly preferably 1.5 to 20% by weight. With the polymer monomer content in this range, the coating around the Si compound is sufficiently thick, and as a result, the amount of voids around the Si compound is sufficient. This sufficiently alleviates the expansion of the Si compound during Li charging, and also makes it difficult for aggregation between the Si compounds to progress.

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

In the polymer monomer slurry, it is preferable to contain a dispersant in order to improve the dispersibility of the Si compound or to promote polymerization. Examples of the dispersant include polyvinylpyrrolidone, styrene sulfonate-based agents such as sodium styrene sulfonate, lithium styrene sulfonate, ammonium styrene sulfonate, and styrene sulfonate ethyl ester, polycarboxylic acid-based agents such as carboxystyrene, polyacrylic acid, and polymethacrylic acid, naphthalene sulfonate-formaldehyde condensation agents, polyethylene glycol, polycarboxylic acid partial alkyl ester-based agents, polyether-based agents, polyalkylene polyamine-based agents, alkyl sulfonic acid-based agents, quaternary ammonium-based agents, higher alcohol alkylene oxide-based agents, polyhydric alcohol ester-based agents, alkyl polyamine-based agents, and polyphosphate-based agents. Polyacrylic acid-based additives, styrene sulfonate-based agents, and polyvinylpyrrolidone are preferred, and styrene sulfonate-based agents and polyvinylpyrrolidone are particularly preferred.

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

The polymer monomer slurry preferably contains a polymerization promoter to promote polymerization. Examples of the polymerization promoter include pH adjusters such as sodium bicarbonate or potassium hydroxide, and preferably sodium bicarbonate.

The resulting polymer film coated with the Si compound is removed by firing, as described below, to form voids.

Graphite can be natural graphite, artificial graphite made by graphitizing petroleum or coal pitch, etc., and can be in the form of flakes, ovals, spheres, cylinders, or fibers. In addition, expanded graphite or crushed expanded graphite, which is expanded by heat treatment after acid treatment or oxidation treatment and has some of the graphite layers peeled off to form an accordion shape, or graphene whose layers are peeled off by ultrasonic waves or the like, can also be used. Expanded graphite or crushed expanded graphite has better flexibility than other graphites, and in the process of forming composite particles described below, the crushed particles can easily recombine to form approximately spherical composite particles. From the above points, it is preferable to use expanded graphite or crushed expanded graphite as graphite. The graphite raw material is used after being adjusted to a size that can be used in the mixing process in advance, and the particle size before mixing is 1 to 100 μm for natural graphite and artificial graphite, and about 5 μm to 5 mm for expanded graphite or crushed expanded graphite and graphene.

It is preferable to add a carbon compound because it can better bind the Si compound and graphite when the Si compound is mixed with the graphite after coating the Si compound with a polymer film. The carbon compound is preferably one that can bind the crystalline carbon obtained from the Si compound and graphite and does not leave any carbon residue after firing. Examples of the carbon compound include glycerin-based compounds such as glycerin, diglycerin, triglycerin, polyglycerin, diglycerin fatty acid esters, and triglycerin fatty acid esters; glycol-based compounds such as menthol, pentaerythritol, dipentaerythritol, tripentaerythritol, ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, polyethylene oxide, and trimethylolpropane; and polyvinylpyrrolidone. Polyvinylpyrrolidone, menthol, or glycerin are preferred, and glycerin is particularly preferred.

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

When the slurry concentration is high, a kneader or Loedige mixer can be used as a mixing method. When a solvent is used, in addition to the kneaders mentioned above, a three-one motor, stirrer, Nauta mixer, Loedige 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 mixture can be jacket heated in these devices as is, or the solvent can be removed using a vibration dryer or paddle dryer. Before the drying process, solid-liquid separation can be performed using a device such as a centrifuge, filter press, suction filter, or pressure filter. By removing excess carbon compounds, the process of bonding the composite active materials for lithium secondary batteries together after firing and the process of crushing and disintegrating them are unnecessary. Furthermore, it is preferable to perform these solid-liquid separation processes, as this eliminates the cause of the capacity decrease of the negative electrode.

In these devices, the mixture of the Si compound, graphite, and optionally the carbon compound is granulated and compacted by continuing the stirring during the solvent removal process for a certain period of time. In addition, the mixture after the solvent removal can be granulated and compacted by compressing it with a compressor such as a roller compactor and coarsely crushing it with a crusher. The size of these granulated and compacted products is preferably 0.1 to 5 mm for ease of handling in the subsequent crushing process.

The methods of granulation and compaction are preferably dry grinding methods such as ball mills, which use compression force to grind the material to be crushed, media agitation mills, roller mills, which use compression force from rollers to grind the material, jet mills, which collide the material to be crushed against a lining material or against each other at high speed to grind the material by the impact force from the impact, hammer mills, pin mills, and disk mills, which use the impact force from the rotation of a rotor to which hammers, blades, pins, etc. are fixed, and among these, roller mills are particularly preferred. In addition, to adjust the particle size distribution after grinding, dry classification such as air classification and sieving is used. In types where the grinder and classifier are integrated, grinding and classification are performed at the same time, making it possible to achieve the desired particle size distribution.

In addition, by increasing the number of granulation and compaction processes, the dispersibility of the Si compound in the graphite can be improved. The number of granulation and compaction processes is preferably 1 to 10, more preferably 2 to 10, and particularly preferably 2 to 7. If the number of granulation and compaction processes is 10 or less, the crystallinity of the graphite tends not to deteriorate, and the initial charge/discharge efficiency tends not to decrease.

Methods for crushing and spheronizing the granulated and compacted mixture include crushing the material by the above-mentioned crushing method to adjust the particle size, then passing the material through a dedicated spheronizing device, or crushing the material using the impact force of the rotation of a jet mill or rotor as described above, and repeating this process or extending the processing time to make the material spheroidal. Examples of dedicated spheronizing devices include Faculty (product name), Nobilta (product name), and Mechanofusion (product name) from Hosokawa Micron Corporation, COMPOSI from Nippon Coke & Co., Ltd., Hybridization System from Nara Machinery Works, and Cryptron Orb and Cryptron Eddy from Ars Technica.

By carrying out the above-mentioned grinding and spheronization processes, it is possible to obtain roughly spherical composite particles.

The resulting 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, and more preferably 800 to 1100°C. If the firing temperature is 300°C or higher, the polymer film covering the Si compound is less likely to remain, and the initial volumetric discharge capacity is less likely to decrease, and furthermore, the initial charge/discharge efficiency is less likely to decrease, and the initial electrode expansion rate is less likely to increase. On the other hand, if the firing temperature is 1200°C or lower, the reaction between the Si compound and graphite is less likely to occur, and a 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 an active material used in electrode materials 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 lithium secondary batteries of the present invention is mixed with a binder, and the mixture is made into a paste using a solvent to form a negative electrode mixture-containing slurry. The negative electrode mixture-containing slurry can be applied to a current collector, for example, copper foil, to form a negative electrode for a lithium secondary battery.

In addition to copper foil, a collector having a three-dimensional structure is preferable as the current collector because it has a better battery cycle. Examples of materials for a current collector having a three-dimensional structure include carbon fiber, sponge-like carbon (carbon coated on sponge-like resin), and metals other than copper.

Current collectors with a three-dimensional structure (porous current collectors) include porous bodies made of metal or carbon conductors, such as plain woven wire mesh, expanded metal, lath mesh, metal foam, woven metal fabric, nonwoven metal fabric, woven carbon fiber fabric, and nonwoven carbon fiber fabric.

The binder used may be a known material, such as fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene, styrene butadiene rubber (SBR), polyethylene, polyvinyl alcohol, carboxymethyl cellulose, polyacrylic acid, or glue.

Examples of the solvent include water, isopropyl alcohol, N-methylpyrrolidone, and dimethylformamide. When forming the paste, the composite active material for lithium secondary batteries, the binder, and the solvent may be stirred and mixed using a known stirrer, mixer, kneader, or the like, as necessary.

When preparing the negative electrode mixture slurry using the composite active material for lithium secondary batteries, 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 lithium secondary batteries obtained by the above process is often relatively granular (particularly, approximately spherical), and the particles of the composite active material for lithium secondary batteries tend to contact each other at points. In order to avoid this problem, a method can be used in which carbon black, carbon nanotubes, or a mixture thereof is added to the negative electrode mixture slurry. Carbon black, carbon nanotubes, or a mixture thereof can be concentrated and aggregated in the capillary portion formed by the contact of the composite active material for lithium secondary batteries when the slurry solvent dries, so that it is possible to prevent contact breakage (increased resistance) that occurs with cycles.

The amount of carbon black, carbon nanotubes or a mixture thereof is preferably 0.2 to 4 parts by mass, more preferably 0.5 to 2 parts by mass, based on 100 parts by mass of the composite active material for lithium secondary batteries. Examples of carbon nanotubes include single-wall carbon nanotubes and multi-wall carbon nanotubes.
(Positive electrode)
As the 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.

The method for producing the positive electrode includes a known method, such as a method of applying a positive electrode mixture consisting of a positive electrode material, a binder, and a conductive agent to the surface of a current collector. The positive electrode material (positive electrode active material) includes metal oxides such as chromium oxide, titanium oxide, cobalt oxide, and vanadium pentoxide, lithium metal oxides such as _{LiCoO 2} , LiNiO ₂ , LiNi _1-y Co y O ₂ , LiNi 1- _{x -y} Co _x Al _y O 2 , LiMnO ₂ , LiMn ₂ O ₄ , and LiFeO ₂ , transition metal chalcogen compounds such as titanium sulfide and molybdenum sulfide, and conductive conjugated polymeric substances such as polyacetylene, polyparaphenylene, and polypyrrole.
(Electrolyte)
As the electrolyte 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 known electrolyte can be used.

For example, electrolyte salts contained in the electrolyte solution _{include LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5 ) , LiCl , LiBr , LiCF3SO3 , LiCH3SO3, LiN(CF3SO2 ) 2 , LiC} (CF3SO2) ₃ , _{LiN ( CF3CH2OSO2 ) 2} , LiN _{( CF3CF3OSO2} ) ₂ , _{LiN ( HCF2CF2CH2OSO2 ) 2} , LiN{( _CF3 ) _{2CHOSO2 } 2 , LiB {C6H3 ( CF3} ) ₂ } ₄ , LiN(SO ₂ CF ₃ ) ₂ , LiC(SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ , or other lithium salts can be used. In particular, LiPF ₆ and LiBF ₄ are preferred from the viewpoint of oxidation stability.

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

Examples of solvents used in the electrolyte include carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxofuran, 4-methyl-1,3-dioxolane, anisole, ethers such as diethyl ether, and thiol such as sulfolane and methylsulfolane. Nitriles such as ether, acetonitrile, chloronitrile, and propionitrile, and aprotic organic solvents such as trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazoline, ethylene glycol, and dimethyl sulfite can be used.

Instead of the electrolytic solution, a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte may be used. As the polymer compound constituting the matrix of the polymer solid electrolyte or polymer gel electrolyte, an ether-based polymer compound such as polyethylene oxide or its crosslinked product, a methacrylate-based polymer compound such as polymethacrylate, an acrylate-based polymer compound such as polyacrylate, or a fluorine-based polymer compound such as polyvinylidene fluoride (PVDF) or a vinylidene fluoride-hexafluoropropylene copolymer is preferred. These may also be used in combination. From the viewpoint of oxidation-reduction stability, a fluorine-based polymer compound such as PVDF or a vinylidene fluoride-hexafluoropropylene copolymer is particularly preferred.
(Separator)
A separator 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 can be made of a known material. Examples include woven fabric, nonwoven fabric, and synthetic resin microporous membranes. Synthetic resin microporous membranes are preferred, and polyolefin microporous membranes are particularly preferred in terms of membrane thickness, membrane strength, membrane resistance, and the like. Specific examples include polyethylene and polypropylene microporous membranes, and microporous membranes made by combining these.

Lithium secondary batteries can be made in the usual manner into cylindrical, square, or button shapes using the above-mentioned negative electrodes, positive electrodes, separators, electrolytes, and other battery components (e.g., current collectors, gaskets, sealing plates, cases, etc.).

Lithium secondary batteries are used in various portable electronic devices, particularly in notebook personal computers, notebook word processors, palmtop (pocket) personal computers, mobile phones, portable fax machines, portable printers, headphone stereos, video cameras, portable televisions, portable CDs, portable MDs, electric shavers, electronic organizers, walkie-talkies, power tools, radios, tape recorders, digital cameras, portable copiers, and portable game machines. They can also be used as secondary batteries in electric vehicles, hybrid vehicles, vending machines, electric carts, load leveling storage systems, home storage batteries, distributed power storage systems (built into stationary electrical appliances), and emergency power supply systems.

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)
Scaly natural graphite with an average particle size of 1 mm was immersed in a mixed acid of 9 parts by mass of sulfuric acid and 1 part by mass of nitric acid at 25°C for 1 hour, and then the mixed acid was removed using a No. 3 glass filter to obtain acid-treated graphite. The acid-treated graphite was further washed with water and then dried. 5 g of the dried acid-treated graphite was stirred in 100 g of distilled water, and the pH was measured after 1 hour, and found to be 6.7. The dried acid-treated graphite was placed in a horizontal electric furnace in a nitrogen atmosphere set at 850°C 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 process)
Chemical grade metallic silicon (purity 3N) with an average particle size (D50) of 7 μm was mixed with ethanol at 21% by weight, and finely ground in a wet bead mill using zirconia beads with a diameter of 0.3 mm for 6 hours to obtain an ultrafine particle silicon slurry with an average particle size (D50) of 0.2 μm and a BET specific surface area when dried of 100 ^m2 /g.

(Si surface modification step)
The crushed silicon was weighed out so that the solid content was 40 g, and then ultrasonic irradiation was performed for 15 minutes. Additional ethanol was added so that the total amount of ethanol became 1011 g to obtain a silicon slurry. After that, 88 g of a polycarboxylic acid dispersant, 7.2 g of ammonium hydroxide, and 320 g of water (aqueous ammonia solution, NH3aq.) were added to the above-mentioned Si slurry, and the mixture was stirred at 25° C. at a rotation speed of 250 rpm using a stirring blade. The mixture was stirred for 6 hours. The resulting silicon slurry was then centrifuged at a rotation speed of 4800 rpm for 25 minutes, and redispersed in ethanol. The mixture was milled for 8 hours using a ball mill with 1.0 mm zirconia balls to obtain a silicon slurry with an average particle size (D50) of 0.24 μm. The mixture was then rotated at 4,800 rpm for 60 minutes. It was centrifuged and redispersed in water. The slurry was dried to obtain a Si powder having an oxygen content of 17.9%, a surface hydroxyl group content of 1.3/nm ² , and a ratio of oxygen content (wt %)/BET specific surface area (m ² /g). The value was 0.10. The physical properties of Si are shown in Table 1.

(Si coating process)
The above slurry was weighed so that the Si solid content was 13.85 g and transferred to a round-bottom flask, and additional water was added so that the total water amount was 3823 g. After purging the flask with nitrogen, the liquid temperature was raised to 35 ° C. Then, 0.525 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added to the flask and stirred for 30 minutes. 87.7 g of distilled styrene monomer and 0.44 g of sodium p-styrenesulfonate dissolved in 50 g of water were added and stirred for 2 hours. Then, the liquid temperature was raised to 62 ° C., and 1.93 g of ammonium persulfate dissolved in 50 g of water was added. Then, heating and stirring were continued for 10 hours under reflux. The obtained reaction liquid was centrifuged at a rotation speed of 4800 rpm and a rotation time of 45 minutes to remove free polystyrene, and the precipitate was redispersed with ethanol to obtain polystyrene-coated Si.

Then, 9.01 g of the polystyrene-coated Si, 21.0 g of the expanded graphite, 3.0 g of glycerin, and 705 mL of ethanol were placed in a stirring vessel and mixed and stirred for 20 minutes with a homomixer. The mixture was then transferred to a rotary evaporator, heated to 50°C in a warm bath while rotating, and evacuated with an aspirator to remove the solvent. The mixture was then spread out on a tray in a draft and dried for 2 hours while evacuating, passed through a mesh with 2 mm openings, and dried for another day to obtain 76 g of a mixed dry product (light bulk density 173 g/L).

(Pressing process)
This mixed and dried product was passed through a triple roll mill six times, then passed through a sieve with 1 mm openings, and granulated and compacted to a light bulk density of 200 g/L.

(Spheronization process)
Next, 63 g of this granulated and compacted material was placed in a particle design/surface modification device (Nara Machinery Manufacturing Co., Ltd. Hybridization System NHS-1L), pulverized at 40 m/sec for 15 minutes, and simultaneously spheronized to obtain an approximately spherical composite powder with a loose bulk density of 231 g/L.

(Firing process)
The obtained powder was placed in a quartz boat and fired in a tubular furnace with nitrogen gas flowing at a maximum temperature of 900° C. for 1 hour, thereby obtaining a fired powder consisting of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si.

The powder was then passed through a mesh with 45 μm openings to obtain a fired powder with a light bulk density of 168 g/L.

The resulting fired powder (100 parts by mass) was stirred with coal tar pitch (carbonization degree 60%, 50 parts by mass) for 15 minutes, and then fired and coated using the following method.

The coal tar pitch was modified into soft carbon by heating the mixture at 600°C for 2 hours in a rotary sintering furnace while rotating at 2 rpm and flowing nitrogen (0.4 L/min) at a temperature increase rate of 30°C/min. This resulted in a sintered powder consisting 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).

(Carbon coating process by vapor phase coating)
The obtained powder was set in a rotary sintering furnace, and the inside of the tube was evacuated with a rotary pump, followed by flowing nitrogen gas at a flow rate of 0.27 L/m and ethylene gas at a flow rate of 0.13 L/m into the tube, and heating to 920 ° C. with an electric heater while rotating at 2 rpm, and carbon coating was performed by maintaining that state for 25 minutes. The weight increase due to carbon coating was 15 wt%, and this resulted in a composite active material for lithium secondary batteries consisting of 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 45 parts by mass of amorphous carbon (30 parts by mass of soft carbon derived from coal tar pitch, 15 parts by mass of soft carbon derived from gas phase coat). Thereafter, the powder was passed through a mesh with an opening of 45 μm to obtain a sintered powder with an average particle size (D50) of 22 μm.

(Preparation of negative electrodes for lithium secondary batteries and battery evaluation)
92.5% by weight (content in the total amount of solids; the same applies below) of the obtained composite active material for lithium secondary batteries was mixed with 0.5% by weight of acetylene black as a conductive assistant, 7.0% by weight of a polycarboxylic acid-based binder as a binder, and water to prepare a negative electrode mixture-containing slurry.

The obtained 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/ ^cm2 , and dried for 12 hours in a vacuum dryer at 90° C. After drying, it was punched out into a circle of 14 mmφ, roll pressed under conditions of 100° C., a feed rate of 1 m/min, and a pressure of 4.0 t/ ^cm2 , and further heat-treated in vacuum at 110° C. for 3 hours to obtain a negative electrode for a lithium secondary battery having a negative electrode mixture layer formed thereon.

After that, 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 conducted. The initial charge capacity was 961 mAh/g, the initial charge expansion rate was 137%, the initial charge/discharge efficiency was 85%, and the capacity retention rate after 20 cycles was 98%.

Example 2
(Si surface modification step)
The crushed Si obtained by the same method as the Si crushing process of Example 1 was weighed so that the solid content was 52.5 g, and then ultrasonic irradiation was performed for 15 minutes, and additional ethanol was added so that the total amount of ethanol was 1327 g to obtain a Si slurry. Then, 116 g of a polycarboxylic acid dispersant, 3.5 g of 10 mol/L hydrochloric acid, and 420 g of water were added to the above-mentioned Si slurry, and stirring was performed for 30 minutes under the condition of a rotation speed of 250 rpm using a stirring blade. Then, 105 g of tetraethoxysilane (TEOS) was added to the above-mentioned Si slurry, and the liquid temperature was raised to 70 ° C. Stirring was performed for 12 hours at 70 ° C., and then the obtained Si slurry was centrifuged under the condition of a rotation speed of 4800 rpm and a rotation time of 25 minutes, and redispersed with ethanol. The obtained slurry was pulverized in a ball mill using zirconia balls with a diameter of 1.0 mm for 8 hours to obtain a Si slurry with an average particle size (D50) of 0.24 μm. This was centrifuged at a rotation speed of 4800 rpm for 60 minutes, and redispersed in water. The Si powder obtained by drying this slurry had an oxygen content of 14.3%, a surface hydroxyl group amount of 1.6/nm ² , and an oxygen content (wt %)/BET specific surface area (m ² /g) value of 0.11. The physical properties of Si are shown in Table 1.

(Si coating process)
The above slurry was weighed so that the Si solid content was 13.85 g and transferred to a round-bottom flask, and additional water was added so that the total water amount was 3823 g. After purging the flask with nitrogen, the liquid temperature was raised to 35 ° C. Then, 0.53 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added to 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. Then, the liquid temperature was raised to 62 ° C., and 0.56 g of ammonium persulfate dissolved in 40 g of water was added. Then, heating and stirring were continued for 10 hours under reflux. The obtained reaction liquid was centrifuged under conditions of a rotation speed of 4800 rpm and a rotation time of 45 minutes to remove free polystyrene, and the precipitate was redispersed in ethanol to obtain polystyrene-coated Si.

Then, 10.37 g of the polystyrene-coated 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 homomixer. The mixture was then transferred to a rotary evaporator, heated to 50°C in a warm bath while rotating, and evacuated with an aspirator to remove the solvent. The mixture was then spread out on a tray in a draft and dried for 1 hour while evacuating, passed through a mesh with 2 mm openings, and dried for another day to obtain 78 g of a mixed dry product (light bulk density 162 g/L).

(Pressing process)
This mixed and dried product was passed through a triple roll mill six times, then passed through a sieve with 1 mm openings, and granulated and compacted to a light bulk density of 185 g/L.

(Spheronization process)
Next, 59 g of this granulated and compacted material was placed in a particle design/surface modification device (Hybridization System NHS-1L manufactured by Nara Machinery Manufacturing Co., Ltd.), pulverized at 40 m/sec for 15 minutes, and simultaneously spheronized to obtain an approximately spherical composite powder with a loose bulk density of 247 g/L.

(Firing process)
The obtained powder was placed in a quartz boat and fired in a tubular furnace with nitrogen gas flowing at a maximum temperature of 900° C. for 1 hour, thereby obtaining a fired powder consisting of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si.

The powder was then passed through a mesh with 45 μm openings to obtain a fired powder with a light bulk density of 177 g/L and an average particle size (D50) of 14 μm.

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

Using a rotary kiln, the mixture was heated at 300°C for 2 hours and 600°C for 1 hour while rotating at 1 rpm with nitrogen flowing (0.4 L/min) at a temperature increase rate of 30°C/min, thereby modifying the coal tar pitch to soft carbon. This resulted in a fired powder consisting 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).

(Carbon coating process by vapor phase coating)
The obtained powder was set in a rotary sintering furnace, and the inside of the tube was evacuated with a rotary pump, followed by flowing nitrogen gas at a flow rate of 266 SCCM and ethylene gas at a flow rate of 133 SCCM into the tube, and heated to 1000 ° C. with an electric heater while rotating at 1 rpm, and carbon coating was performed by maintaining that state for 1 hour. The weight increase due to carbon coating was 5 wt%, and this resulted in a composite active material for lithium secondary batteries consisting of 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 35 parts by mass of amorphous carbon (30 parts by mass of soft carbon derived from coal tar pitch, and 5 parts by mass of soft carbon derived from gas phase coat). Thereafter, the powder was passed through a mesh with an opening of 45 μm to obtain a sintered powder with an average particle size (D50) of 25 μm.

(Preparation of negative electrodes for lithium secondary batteries and battery evaluation)
92.5% by weight (content in the total amount of solids; the same applies below) of the obtained composite active material for lithium secondary batteries was mixed with 0.5% by weight of acetylene black as a conductive assistant, 7.0% by weight of a polycarboxylic acid-based binder as a binder, and water to prepare a negative electrode mixture-containing slurry.

The obtained negative electrode mixture-containing slurry was applied to a copper foil of 11 μm using an applicator, and dried for 12 hours in a vacuum dryer at 90° C. After drying, it was punched out into a circle of 14 mmφ, roll pressed under conditions of 100° C., a feed rate of 1 m/min, and a pressure of 4.0 t/ ^cm2 , and further heat-treated in vacuum at 110° C. for 2 hours to obtain a negative electrode for a lithium secondary battery having a negative electrode mixture layer formed thereon.

Thereafter, 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 expansion rate was 142%, the initial charge/discharge efficiency was 84%, and the capacity retention rate after 20 cycles was 98%.
Example 3
(Silicon crushing process)
Chemical grade metal silicon (purity 3N) with an average particle size (D50) of 7 μm was mixed with water at 16% by weight, and finely pulverized in a wet bead mill using zirconia beads with a diameter of 0.3 mm to obtain a silicon slurry with an average particle size (D50) of 255 nm and a BET specific surface area when dry of 68 m ² /g.
(Silicon surface modification process)
The pulverized silicon slurry was weighed so that the solid content was 300 g, and then ultrasonic irradiation was performed for 15 minutes, and additional water was added so that the total water amount was 5000 g to obtain a silicon slurry. Then, 0.54 g of ammonium hydroxide was added to the silicon slurry, and stirring was performed for 1 hour using a magnetic stirrer at a rotation speed of 500 rpm. Then, 600 g of tetraethoxysilane (TEOS) was added to the slurry. Stirring was performed for 5 hours at 25°C, and then tetraethoxysilane was removed using a separatory funnel to obtain a silicon slurry with an average particle size (D50) of 0.27 μm. The oxygen content of the Si powder obtained by drying this slurry was 18.7%, the surface hydroxyl group amount was 2.2 pieces/nm ² , and the value of oxygen content (wt%)/BET specific surface area (m ² /g) was 0.18. The physical properties of Si are shown in Table 1.

(Polymer coating process)
The slurry was weighed so that the silicon solid content was 80 g and transferred to a round-bottom flask, and additional water was added so that the total water amount was 6965 g. After purging the flask with nitrogen, the liquid temperature was raised to 35 ° C. Then, 12.1 g of MPS was added to the flask and stirred for 30 minutes. 507 g of distilled styrene monomer and 2.54 g of NaSS dissolved in 50 g of water were added and stirred for 2 hours. Then, the liquid temperature was raised to 62 ° C., and 11.1 g of APS dissolved in 375 g of water was added at a rate of 8 cc / h using a syringe pump. Then, heating and stirring were continued under reflux for 10 hours to obtain a polymer-coated silicon slurry.

Then, 64 g of the polystyrene-coated Si, 16,540 g of total water, 32 g of a 20 wt % aqueous solution of polydiallyldimethylammonium chloride, and 149.3 g of the expanded graphite were placed in a stirring vessel and mixed and stirred in a homomixer. The mixture was then filtered under reduced pressure in vacuum to remove the solvent. The mixture was then spread out on a tray in a draft and dried while ventilating, and further dried for one day to obtain 651 g of a mixed dried product.

(Pressing process)
This mixed and dried product was passed through a triple roll mill six times, then passed through a sieve with 1 mm openings, and granulated and compacted to a light bulk density of 211 g/L.

(Spheronization process)
Next, 1,223 g of this granulated and compacted material was placed in a particle design/surface modification device (Hybridization System NHS-1L manufactured by Nara Machinery Manufacturing Co., Ltd.), pulverized at 40 m/sec for 15 minutes, and simultaneously spheronized to obtain an approximately spherical composite powder with a loose bulk density of 271 g/L.

The resulting composite powder (100 parts by mass) was mixed with coal tar pitch (carbonization degree 40%, 75 parts by mass) for 20 minutes, and then fired and coated using the following method.

The coal thal pitch was modified into soft carbon by heating the mixture to 920°C for 1 hour while rotating in a rotary kiln at a temperature increase rate of 5°C/min and flowing nitrogen. This resulted in a fired powder consisting 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 thal pitch). The mixture was then passed through a mesh with 45μm openings to obtain fired powder with a loose bulk density of 252g/L.

(Carbon coating process by vapor phase coating)
The obtained powder was set in a rotary sintering furnace, and the inside of the tube was evacuated with a rotary pump, followed by flowing nitrogen gas at a flow rate of 3.6 L/min and ethylene gas at a flow rate of 1.8 L/min into the tube, and heating to 920 ° C. with an electric heater while rotating, and carbon coating was performed by maintaining that state for 7 minutes. The weight increase due to carbon coating was 26 wt%, and this resulted in a composite active material for lithium secondary batteries consisting of 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 56 parts by mass of amorphous carbon (30 parts by mass of soft carbon derived from coal tar pitch, and 26 parts by mass of soft carbon derived from gas phase coat). Thereafter, the powder was passed through a mesh with an opening of 45 μm to obtain a sintered powder with an average particle size (D50) of 25 μm.

(Preparation of negative electrodes for lithium secondary batteries and battery evaluation)
92.5% by weight (content in the total amount of solids; the same applies below) of the obtained composite active material for lithium secondary batteries was mixed with 0.5% by weight of acetylene black as a conductive assistant, 7.0% by weight of a polycarboxylic acid-based binder as a binder, and water to prepare a negative electrode mixture-containing slurry.

The obtained negative electrode mixture-containing slurry was applied to a copper foil having a thickness of 11 μm using an applicator, and dried for 12 hours in a vacuum dryer at 90° C. After drying, the foil was punched out into a circle having a diameter of 14 mm, roll pressed under conditions of 100° C., a feed rate of 1 m/min, and ^a pressure of 4.0 t/cm2, and further heat-treated in a vacuum at 110° C. for 2 hours to obtain a negative electrode for a lithium secondary battery having a negative electrode mixture layer formed thereon.

After that, 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 expansion rate was 137%, the initial charge/discharge efficiency was 83%, and the capacity retention rate after 20 cycles was 97%. The physical properties of Si are shown in Table 1.

<Comparative Example 1>
(Si surface modification step)
The crushed Si was weighed so that the solid content was 40 g, and then ultrasonic irradiation was performed for 15 minutes, and additional ethanol was added so that the total amount of ethanol was 1018 g to obtain a Si slurry. Then, 88 g of a polycarboxylic acid dispersant, 36 g of ammonium hydroxide, and 320 g of water were added to the above Si slurry, and stirring was performed for 1 hour using a stirring blade at a rotation speed of 400 rpm. Then, 80 g of tetraethoxysilane (TEOS) was added to the above Si slurry. Stirring was performed for 1.5 hours at 25°C, and then the obtained Si slurry was centrifuged at a rotation speed of 4800 rpm and a rotation time of 25 minutes, and redispersed with ethanol. The obtained slurry was subjected to a ball mill using a zirconia ball with a diameter of 1.0 mm for 8 hours to obtain a Si slurry with an average particle size (D50) of 0.25 μm. This was centrifuged at 4,800 rpm for 60 minutes and redispersed in water. The oxygen content of the Si powder obtained by drying this slurry was 43.0%, the number of surface hydroxyl groups was 3.5/ ^nm2 , and the oxygen content (wt%)/BET specific surface area ( ^m2 /g) was 0.50. The physical properties of Si are shown in Table 1.

(Si coating process)
The above slurry was weighed so that the Si solid content was 13.85 g and transferred to a round-bottom flask, and additional water was added so that the total water amount was 3386 g. After purging the flask with nitrogen, the liquid temperature was raised to 35 ° C. Then, 0.465 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added to 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. Then, the liquid temperature was raised to 62 ° C., and 1.7 g of ammonium persulfate dissolved in 40 g of water was added. Then, heating and stirring were continued for 10 hours under reflux. The obtained reaction liquid was centrifuged under conditions of a rotation speed of 4800 rpm and a rotation time of 45 minutes to remove free polystyrene, and the precipitate was redispersed with ethanol to obtain polystyrene-coated Si.

Then, 9.95 g of the polystyrene-coated 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 homomixer. The mixture was then transferred to a rotary evaporator, heated to 50°C in a warm bath while rotating, and evacuated with an aspirator to remove the solvent. The mixture was then spread out on a tray in a draft and dried for 2 hours while evacuating, passed through a mesh with 2 mm openings, and dried for another day to obtain 85 g of a mixed dry product (light bulk density 183 g/L).

(Pressing process)
This mixed and dried product was passed through a triple roll mill six times, then passed through a sieve with 1 mm openings, and granulated and compacted to a light bulk density of 205 g/L.

(Spheronization process)
Next, 27 g of this granulated and compacted material was placed in a particle design/surface modification device (Hybridization System NHS-1L manufactured by Nara Machinery Manufacturing Co., Ltd.), pulverized at 40 m/sec for 15 minutes, and simultaneously spheronized to obtain an approximately spherical composite powder with a loose bulk density of 264 g/L.

(Firing process)
The obtained powder was placed in a quartz boat and fired in a tubular furnace with nitrogen gas flowing at a maximum temperature of 900° C. for 1 hour, thereby obtaining a fired powder consisting of 70 parts by mass of crystalline carbon (derived from graphite) and 30 parts by mass of Si.

The powder was then passed through a mesh with 45 μm openings to obtain a fired powder with a light bulk density of 207 g/L and an average particle size (D50) of 13 μm.

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

The coal thal pitch was modified into soft carbon by heating the mixture at 600°C for 2 hours in a rotary sintering furnace while rotating at 2 rpm and flowing nitrogen (0.3 L/min) at a temperature increase rate of 30°C/min. This resulted in a sintered powder consisting 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 thal pitch).

(Carbon coating process by vapor phase coating)
The obtained powder was set in a rotary sintering furnace, and the inside of the tube was evacuated with a rotary pump, followed by flowing nitrogen gas at a flow rate of 200 SCCM and ethylene gas at a flow rate of 100 SCCM into the tube, and heating to 920 ° C. with an electric heater while rotating at 2 rpm, and maintaining that state for 25 minutes to perform carbon coating. The weight increase due to carbon coating was 17 wt%, and this resulted in a composite active material for lithium secondary batteries consisting of 70 parts by mass of crystalline carbon (derived from graphite), 30 parts by mass of Si, and 47 parts by mass of amorphous carbon (30 parts by mass of soft carbon derived from coal tar pitch, 17 parts by mass of soft carbon derived from gas phase coat). Thereafter, the powder was passed through a mesh with an opening of 45 μm to obtain a sintered powder with an average particle size (D50) of 27 μm.

(Preparation of negative electrodes for lithium secondary batteries and battery evaluation)
92.5% by weight (content in the total amount of solids; the same applies below) of the obtained composite active material for lithium secondary batteries was mixed with 0.5% by weight of acetylene black as a conductive assistant, 7.0% by weight of a polycarboxylic acid-based binder as a binder, and water to prepare a negative electrode mixture-containing slurry.

The obtained 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/ ^cm2 , and dried for 12 hours in a vacuum dryer at 90° C. After drying, it was punched out into a circle of 14 mmφ, roll pressed under conditions of 100° C., a feed rate of 1 m/min, and a pressure of 4.0 t/ ^cm2 , and further heat-treated in vacuum at 110° C. for 3 hours to obtain a negative electrode for a lithium secondary battery having a negative electrode mixture layer having a thickness of 32 μm.

Thereafter, 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 expansion rate was 115%, the initial charge/discharge efficiency was 73%, and the capacity retention rate after 20 cycles was 100%.
<Comparative Example 2>
A Si slurry was prepared in the same manner as in the Si pulverization step of Example 1. The oxygen content of the Si powder obtained by drying this slurry was 8.8%, the amount of surface hydroxyl groups was 1.5/nm ² , and the value of oxygen content (wt%)/BET specific surface area (m ² /g) was 0.08. This Si slurry was weighed out so that the Si solid content was 0.36 g and transferred to a round-bottom flask, and a solvent was added so that the total amount of water was 24.6 g and the total amount of ethanol was 60.2 g. After purging the flask with nitrogen, the liquid temperature was raised to 35°C. Then, 0.596 g of 3-methacryloxypropyltrimethoxysilane (MPS) was added to the flask and stirred for 30 minutes. 2.5 g of distilled styrene monomer and 0.025 g of sodium p-styrenesulfonate dissolved in 4 g of water were added and stirred for 2 hours. The liquid temperature was then raised to 62°C, and 0.26 g of potassium persulfate dissolved in 4 g of water was added. Heating and stirring was then continued for 10 hours under reflux. The resulting reaction liquid was centrifuged at a rotation speed of 4800 rpm for 45 minutes to remove free polystyrene, and the precipitate was redispersed in ethanol, but it was not possible to cover the periphery of Si with polystyrene.

Comparative Example 1 had a high oxygen content and a high amount of surface hydroxyl groups, and thus had a low initial charge-discharge efficiency, while Comparative Example 2 had a low oxygen content and a low value of the acid content (wt %)/BET specific surface area ( ^m2 /g), and therefore the Si compound could not be coated with a polymer film.

Claims (9)

Si or Si alloy characterized in that the average particle size (D50) is 0.01 to 0.6 μm, the average particle size (D90) is 0.01 to 1.0 μm, the BET specific surface area measured by the BET method is 40 to 300 m ² /g, the oxygen content is 10% by weight or more and less than 20% by weight, and the number of surface hydroxyl groups is 1.0/nm ² to 3.0/nm ² . 2. The Si or Si alloy according to claim 1 , wherein the value obtained by dividing the oxygen content (wt %) by the BET specific surface area (m ² /g) is 0.1 to 0.4. 3. A composite active material for a lithium secondary battery comprising the Si or Si alloy according to claim 1 or 2 and crystalline carbon, the composite active material for a lithium secondary battery having a void structure around the Si or Si alloy. 4. The composite active material for lithium secondary batteries according to claim 3, wherein the composite active material for lithium secondary batteries has a structure in which Si or a Si alloy is surrounded by crystalline carbon, and a 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. 5. The composite active material for lithium secondary batteries according to claim 3 or 4, characterized in that the structure is such that Si or a Si alloy is sandwiched between crystalline carbon layers having a thickness of 0.5 μm or less, the structure is spread in a laminated and/or mesh -like manner, and the crystalline carbon layers are curved near the surfaces of the composite active material for lithium secondary batteries to cover the composite active material for lithium secondary batteries. 6. The composite active material for lithium secondary batteries according to any one of claims 3 to 5, characterized in that the crystalline carbon has a purity of 99% by weight or more as determined from semi-quantitative impurity values of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co, Cu, Mo, Pb, Sb, Se, Th, Tl, U) by ICP atomic emission spectroscopy, an S amount of 1% by weight or less as determined by an ion chromatography (IC) measurement method using an oxygen flask combustion method, and/or a BET specific surface area of 100 ^m2 / g or less. A method for producing a composite active material for a lithium secondary battery according to any one of claims 3 to 6, comprising the steps of: adding a polymer monomer, an initiator, and, if necessary, a dispersant to the Si or Si alloy according to claim 1 or 2; coating the Si or Si alloy with a polymer film; mixing the resulting mixture with graphite and, if necessary, a carbon compound; granulating and compacting the mixture; pulverizing and spheronizing the mixture to form substantially spherical composite particles; calcining the composite particles in an inert atmosphere; and, if necessary , mixing a carbon compound with the composite particles or the calcined particles and heating the mixture in an inert atmosphere. 8. The method for producing a composite active material for a lithium secondary battery according to claim 7 , wherein the polymer monomer is styrene, methacrylic acid, methyl methacrylate, acrylic acid, methyl acrylate, 2-hydroxyethyl acrylate, divinylbenzene, acrylonitrile, aniline, or pyrrole . 8. The method for producing a composite active material for a lithium secondary battery according to claim 7, wherein the initiator is azobisisobutyronitrile, potassium persulfate, ammonium persulfate or benzoyl peroxide.