Lithium-ion secondary battery negative electrode active material

Abstract

Provided are: carbon layer-containing β-silicon carbide particles each having a β-silicon carbide particle with a carbon layer disposed on the surface thereof, and characterized in that the carbon layer and the β-silicon carbide of the β-silicon carbide particle are in direct contact with each other; a lithium ion secondary battery negative electrode active material containing the carbon layer-containing β-silicon carbide particles; a lithium ion secondary battery negative electrode containing the lithium ion secondary battery negative electrode active material; and a lithium ion secondary battery having the lithium ion secondary battery negative electrode.

Description

Negative electrode active material for lithium-ion secondary batteries

[0001] The present invention relates to a negative electrode active material for a lithium ion secondary battery. More specifically, the present invention relates to a negative electrode active material for a lithium ion secondary battery, a method for producing the same, a negative electrode including the negative electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery having the negative electrode.

[0002] Due to the rapid development of portable electronic devices, the automotive industry, and other areas, lithium-ion secondary batteries, which are small and highly durable, have attracted attention. Conventional lithium-ion secondary batteries primarily use graphite as the active material for their negative electrodes. However, the discharge capacity of lithium-ion secondary batteries using graphite as the negative electrode active material is at most approximately 300 mAh / g. Therefore, in recent years, the use of silicon particles (theoretical discharge capacity: 4200 mAh / g), which has a theoretical discharge capacity much higher than that of graphite, as the negative electrode active material has been investigated.

[0003] However, when silicon particles are used as the negative electrode active material, the volume of the silicon particles expands by about three times when lithium ions are loaded into the silicon particles. As a result, the silicon particles are broken down as lithium ions are repeatedly loaded into and released from the silicon particles, resulting in fine silicon particles that are electrically isolated. In addition, a coating layer is formed on the broken surfaces of the silicon particles, which is thought to result in a deterioration in the charge-discharge cycle characteristics of the secondary battery (see, for example, paragraph

[0003] of Patent Document 1).

[0004] Therefore, as a negative electrode active material that prevents electrical isolation of silicon particles due to expansion and contraction of the silicon particles, a negative electrode active material for lithium ion secondary batteries has been proposed in which scaly silicon particles are attached to the surface of a carbon substrate, with parts of the scaly silicon particles sticking into the carbon substrate (see, for example, claim 1 of Patent Document 1).

[0005] The negative electrode active material is said to prevent electrical isolation of silicon particles due to their expansion and contraction. However, the initial discharge capacity of a lithium-ion secondary battery using the negative electrode active material is approximately 800 mAh / g (see, for example, Figure 12 of Patent Document 1). Furthermore, the discharge capacity of a lithium-ion secondary battery using a negative electrode active material in which graphite contains 10 to 20 mass % silicon is approximately 400 to 500 mAh / g.

[0006] Under these circumstances, research is being conducted into negative electrode active materials for lithium ion secondary batteries that have excellent durability against charge and discharge and a high discharge capacity (see, for example, Patent Document 2 and Non-Patent Documents 1 and 2).

[0007] In recent years, there has been a demand for the development of a negative electrode active material that can replace the relatively inexpensive and easily available silicon carbide used in lithium-ion secondary batteries, and that has a high discharge capacity and excellent durability against charge and discharge.

[0008] JP 2016-143462 A JP 2022-040471 A

[0009] Nandan, R.; Takamori, N.; Higashimine, K.; Badam, R.; Matsumi, NJ Mater. Chem. A. 2022, 10, 5230. “Zinc Blende Inspired Rational Designing of beta-SiC Based Resilient Anode Material for Lithium-Ion Batteries”Review article: DOI:10.1039 / c7cs00858a

[0010] The present invention has been made in view of the above-mentioned conventional technology in order to develop a next-generation lithium ion secondary battery that can replace lithium ion secondary batteries that use silicon carbide as a negative electrode active material, and its objective is to provide carbon layer-containing β-silicon carbide particles that have a high discharge capacity and excellent durability against charging and discharging and that can be suitably used as a negative electrode active material for lithium ion secondary batteries, a method for producing the same, a negative electrode active material for lithium ion secondary batteries that contains the carbon layer-containing β-silicon carbide particles, a negative electrode for lithium ion secondary batteries that contains the negative electrode active material for lithium ion secondary batteries, and a lithium ion secondary battery that has the negative electrode for lithium ion secondary batteries.

[0011] The present invention provides: (1) carbon-layer-containing β-silicon carbide particles having a carbon layer on the surface of the β-silicon carbide particles, characterized in that the β-silicon carbide of the β-silicon carbide particles and the carbon layer are in direct contact with each other; (2) carbon-layer-containing β-silicon carbide particles according to (1), in which the entire surface of the β-silicon carbide particles is substantially covered with a carbon layer; (3) carbon-layer-containing β-silicon carbide particles according to (1), in which the average particle size of the β-silicon carbide particles is 0.1 to 30 μm; (4) carbon-layer-containing β-silicon carbide particles according to (1), in which the thickness of the carbon layer is 5 to 800 nm; and (5) (6) A method for producing carbon layer-containing β-silicon carbide particles having a carbon layer on the surface of the β-silicon carbide particles, the method comprising: acid-washing the β-silicon carbide particles to expose the β-silicon carbide on the surface of the β-silicon carbide particles; adhering a carbon layer-forming material to the surface of the β-silicon carbide particles where the β-silicon carbide is exposed; and then firing the β-silicon carbide particles to which the carbon layer-forming material is adhered. (7) A method for producing carbon layer-containing β-silicon carbide particles according to (6), in which the β-silicon carbide particles are acid-washed with an acid solution. (8) A method for producing carbon layer-containing β-silicon carbide particles according to (5), in which the acid used in the acid solution is an inorganic acid or an organic acid. (9) The method for producing carbon layer-containing β-silicon carbide particles according to (7) above, wherein the inorganic acid is hydrofluoric acid, hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid, or phosphoric acid; (10) The method for producing carbon layer-containing β-silicon carbide particles according to (6) above, wherein the temperature of the acid solution used for pickling the β-silicon carbide particles is 0 to 90°C;(11) The method for producing carbon layer-containing β-silicon carbide particles according to (6), wherein the temperature of the acid solution when pickling the β-silicon carbide particles is 5 to 40°C; (12) The method for producing carbon layer-containing β-silicon carbide particles according to (5), wherein the material for forming the carbon layer is a phenylamine-based polymer, a polyimide-based polymer, a polybenzimidazole-based polymer, a polyphenylene oxide-based polymer, an acrylonitrile-based polymer, or a (meth)acrylic polymer; (13) The method for producing carbon layer-containing β-silicon carbide particles according to (5), wherein the material for forming the carbon layer is a phenylamine-based polymer, a polyimide-based polymer, or a polybenzimidazole-based polymer; (14) (15) A method for producing carbon layer-containing β-silicon carbide particles according to (5), comprising immersing β-silicon carbide particles in a solution of a carbon layer-forming material, or spraying the solution of the carbon layer-forming material onto the surfaces of the β-silicon carbide particles, or adhering a heated and melted carbon layer-forming material to the surfaces of the β-silicon carbide particles, thereby adhering the carbon layer-forming material to the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed; (16) A negative electrode active material for a lithium ion secondary battery, comprising the carbon layer-containing β-silicon carbide particles according to (1); (17) A negative electrode for a lithium ion secondary battery, comprising the negative electrode active material for a lithium ion secondary battery according to (16); and (18) The present invention relates to a lithium ion secondary battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the negative electrode is the negative electrode for a lithium ion secondary battery according to (17).

[0012] According to the present invention, there are provided carbon layer-containing β-silicon carbide particles which have a high discharge capacity and excellent durability against charge and discharge and can be suitably used as a negative electrode active material for a lithium ion secondary battery, a method for producing the same, a negative electrode active material for a lithium ion secondary battery containing the carbon layer-containing β-silicon carbide particles, a negative electrode for a lithium ion secondary battery containing the negative electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery having the negative electrode for a lithium ion secondary battery.

[0013]

[0023] FIG. 1 is a photograph, substituted for a drawing, showing a high-resolution transmission electron microscope image of conventional β-silicon carbide particles.

[0024] FIG. 2 is a photograph, substituted for a drawing, showing a transmission electron microscope image of β-silicon carbide particles from which the silicon oxide coating has been removed.

[0025] FIG. 3 is a photograph, substituted for a drawing, showing a transmission electron microscope image of carbon layer-containing β-silicon carbide particles.

[0026] FIG. 4 is a graph showing the change in battery capacity when a charge-discharge cycle test is performed using half cells in which various β-silicon carbide particles are used as the negative electrode active material.

[0027] FIG. 5 is a graph showing the results of cyclic voltammetry measurements of half cells in which various β-silicon carbide particles are used as the negative electrode active material.

[0028] FIG. 6 is a graph showing the results of a charge-discharge cycle test using half cells in which various β-silicon carbide particles are used as the negative electrode active material.

[0029] FIG. 7 is a graph showing the results of a charge-discharge cycle test using full cells in which carbon layer-containing β-silicon carbide particles are used as the negative electrode active material.

[0029] FIG. 8 is an optical photograph, substituted for a drawing, showing the results of a light-emitting diode lighting test using the full cells.

[0014] The theoretical discharge capacity of a lithium-ion secondary battery using graphite particles as the negative electrode active material is 372 mAh / g, so as long as graphite particles are used, it is difficult to create a lithium-ion secondary battery with a high discharge capacity.

[0015] In contrast, lithium-ion secondary batteries that use silicon particles as the negative electrode active material have the advantage of having a theoretical discharge capacity much higher than that of graphite, as mentioned above. However, silicon particles expand to approximately 400% by volume during repeated charge-discharge cycles of the lithium-ion secondary battery, which can cause damage to the silicon particles themselves or the solid electrolyte interface (coating) formed on the surface of the silicon particles. If the coating on the surface of the silicon particles is damaged or cracks occur in the silicon particles, the electrolyte may decompose at the damaged or cracked site, or the internal resistance of the silicon particles may increase.

[0016] When silicon monoxide particles are used as the negative electrode active material, charge and discharge can be more easily controlled than with silicon particles, but the same phenomenon as when silicon particles are used as described above can occur, and the internal resistance of the silicon monoxide particles can increase.

[0017] Another option is to use commercially available silicon carbide as the negative electrode active material. The use of conventional commercially available silicon carbide as the negative electrode active material does not have the drawbacks of silicon particles and silicon monoxide particles. However, lithium ion secondary batteries using silicon carbide as the negative electrode active material have a drawback of poor battery performance, as the initial discharge capacity is approximately 50 mAh / g.

[0018] Therefore, the present inventors have conducted extensive research focusing on commercially available silicon carbide particles. As a result, they have conducted acid washing of the β-silicon carbide particles to expose the β-silicon carbide on the surfaces of the β-silicon carbide particles, directly attaching a carbon layer-forming material to the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed, and then firing the β-silicon carbide particles with the carbon layer-forming material attached to them to prepare a fired body. It has been found that in the fired body, the β-silicon carbide of the β-silicon carbide particles is in direct contact with the carbon layer.

[0019] Furthermore, it was found that when the sintered body was used as a negative electrode active material, the discharge capacity of a lithium ion secondary battery using the negative electrode active material surprisingly increased remarkably to 930 mAh / g. The present invention was completed based on this finding.

[0020] The carbon layer-containing β-silicon carbide particles of the present invention are β-silicon carbide particles having a carbon layer on the surface thereof, and are characterized in that the β-silicon carbide of the β-silicon carbide particles and the carbon layer are in direct contact with each other.

[0021] The carbon layer-containing β-silicon carbide particles of the present invention can be prepared, for example, by acid-washing β-silicon carbide particles to expose β-silicon carbide on the surfaces of the β-silicon carbide particles, adhering a carbon layer-forming material to the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed, and then firing the β-silicon carbide particles to which the carbon layer-forming material is adhered, thereby forming a carbon layer on the surfaces of the β-silicon carbide particles in which the β-silicon carbide of the β-silicon carbide particles is in direct contact with the carbon layer.

[0022] Next, the present invention will be described in detail, but the present invention is not limited to the embodiments described below, and various changes and modifications can be made by those skilled in the art within the scope of the technical ideas described in this specification.

[0023] [β-Silicon Carbide Particles] β-Silicon carbide (β-SiC) particles have the properties of having a low thermal expansion coefficient and density compared to silicon, and high mechanical strength. β-Silicon carbide particles are readily available commercially, but can also be prepared by conventional methods using the thermal decomposition of organic gases containing silicon and carbon. Silicon oxide is inevitably present on the surface of β-silicon carbide particles.

[0024] In the present invention, commercially available β-silicon carbide particles can be used as the β-silicon carbide particles. Alternatively, the β-silicon carbide particles can be carbon layer-containing β-silicon carbide particles described in JP 2022-40471 A, which contain β-silicon carbide and nitrogen-doped carbon, with β-silicon carbide domains dispersed in a nitrogen-doped carbon matrix. The carbon layer-containing β-silicon carbide particles are suitable for use in the present invention because a negative electrode active material using these carbon layer-containing β-silicon carbide particles has excellent charge / discharge durability and a high discharge capacity.

[0025] Examples of the shape of the β-silicon carbide particles include spherical, oval sphere, cube, rectangular parallelepiped, triangular pyramid, square pyramid, needle, rod, cylinder, scale, plate, and irregular shape, but the present invention is not limited to these examples.

[0026] The average particle size of the β-silicon carbide particles is not particularly limited, but is preferably 0.1 to 30 μm from the viewpoint of improving the dispersion stability of the carbon layer-containing β-silicon carbide particles in the negative electrode. The average particle size of the β-silicon carbide particles means a value obtained by randomly selecting 100 β-silicon carbide particles from an image taken with a scanning electron microscope, calculating the average values ​​of the vertical and horizontal axes for each β-silicon carbide particle, summing the average values ​​for the 100 β-silicon carbide particles, and dividing the sum by 100.

[0027] [Carbon Layer-Containing β-Silicon Carbide Particles] In the carbon layer-containing β-silicon carbide particles of the present invention, the β-silicon carbide of the β-silicon carbide particles is in direct contact with the carbon layer.

[0028] The carbon layer-containing β-silicon carbide particles of the present invention can be produced, for example, by acid-washing β-silicon carbide particles to expose β-silicon carbide on the surfaces of the β-silicon carbide particles, adhering a carbon layer-forming material to the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed, and then firing the β-silicon carbide particles with the carbon layer-forming material attached. Because this method is employed in the present invention, carbon layer-containing β-silicon carbide particles in which the β-silicon carbide and the carbon layer of the β-silicon carbide particles are in direct contact with each other can be obtained.

[0029] An acid solution is used when pickling the β-silicon carbide particles. An aqueous medium can be used as the solvent for the acid solution. The aqueous medium is usually water, but the aqueous medium may contain a hydrophilic organic solvent within a range that does not impair the object of the present invention.

[0030] Examples of the hydrophilic organic solvent include monohydric alcohols such as methanol, ethanol, isopropyl alcohol, n-propyl alcohol, and allyl alcohol; polyhydric alcohols such as ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, heptanediol, and dipropylene glycol; ketones such as acetone, methyl ethyl ketone, and methyl propyl ketone; aliphatic organic acid alkyl esters such as methyl formate, ethyl formate, methyl acetate, and methyl acetoacetate; and alkyl ethers of polyhydric alcohols such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and dipropylene glycol monomethyl ether. However, the present invention is not limited to these examples. These hydrophilic organic solvents may be used alone or in combination of two or more.

[0031] The acid used in the acid solution includes inorganic acids and organic acids. The inorganic acids and organic acids may be used alone or in combination. From the viewpoint of easily pickling the β-silicon carbide particles, it is preferable that the inorganic acids and organic acids are soluble in an aqueous medium.

[0032] Examples of inorganic acids include hydrofluoric acid, hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid, phosphoric acid, etc., but the present invention is not limited to these examples. These inorganic acids may be used alone or in combination of two or more.

[0033] Examples of organic acids include water-soluble organic acids such as formic acid, acetic acid, chloroacetic acid, dichloroacetic acid, trichloroacetic acid, and citric acid, but the present invention is not limited to these examples. These organic acids may be used alone or in combination of two or more.

[0034] Of the acids used in the acid solution, inorganic acids are preferred from the viewpoint of improving operability when pickling the β-silicon carbide particles, and hydrofluoric acid, hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid, and phosphoric acid are more preferred, with hydrofluoric acid being even more preferred from the viewpoint of efficiently pickling the β-silicon carbide particles.

[0035] The acid concentration in the acid solution varies depending on the type of acid and cannot be determined in general. Therefore, it is preferable to adjust the acid concentration in the acid solution appropriately depending on the type of acid. For example, when hydrofluoric acid is used as the acid solution, the concentration of hydrogen fluoride in the hydrofluoric acid is about 3 to 40 mass%.

[0036] The temperature of the acid solution used to pickle the β-silicon carbide particles is not particularly limited. The temperature of the acid solution is preferably 0 to 90°C from the viewpoint of efficiently pickling the β-silicon carbide particles, more preferably 5 to 40°C in consideration of the efficiency and safety of pickling the β-silicon carbide particles, and even more preferably room temperature from the viewpoint of improving energy efficiency of pickling the β-silicon carbide particles.

[0037] Examples of methods for pickling β-silicon carbide particles include immersing the β-silicon carbide particles in an acid solution and spraying the β-silicon carbide particles with an acid solution, but the present invention is not limited to these methods. Of these methods, the method of immersing the β-silicon carbide particles in an acid solution is preferred from the viewpoint of efficiently and safely pickling the β-silicon carbide particles.

[0038] The acid washing of the β-silicon carbide particles is carried out until β-silicon carbide is exposed on the surface of the β-silicon carbide particles, from the viewpoint of preparing a negative electrode active material for a lithium ion secondary battery having a high discharge capacity and excellent durability against charge and discharge. The larger the region where β-silicon carbide is exposed on the surface of the β-silicon carbide particles, the more preferable, from the viewpoint of preparing a negative electrode active material for a lithium ion secondary battery having a high discharge capacity and excellent durability against charge and discharge. It is more preferable that the surface of the β-silicon carbide particles is exposed. Whether β-silicon carbide is exposed on the surface of the β-silicon carbide particles can be confirmed, for example, by observing the β-silicon carbide particles using a high-resolution transmission electron microscope (HRTEM) or the like.

[0039] The pickled β-silicon carbide particles can be separated from the acid solution by, for example, centrifugation, filtration, or the like.

[0040] In order to remove the acid solution adhering to the β-silicon carbide particles from the β-silicon carbide particles, it is preferable to wash the β-silicon carbide particles after pickling with water. There are no particular limitations on the type of water used for washing, and examples of water include pure water and distilled water. There are also no particular limitations on the ambient temperature and water temperature when washing the β-silicon carbide particles with water, and room temperature is usually preferred. Methods for washing the β-silicon carbide particles with water include immersing the β-silicon carbide particles in water and spraying water on the surfaces of the β-silicon carbide particles, but the present invention is not limited to these methods. Among these methods, immersing the β-silicon carbide particles in water is preferred from the viewpoint of efficiently washing the β-silicon carbide particles with water.

[0041] In this manner, silicon oxide that is inevitably present on the surfaces of the β-silicon carbide particles can be removed from the surfaces of the β-silicon carbide particles, thereby exposing β-silicon carbide on the surfaces of the β-silicon carbide particles.

[0042] Next, a carbon layer-forming material is adhered to the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed. From the viewpoint of preventing the formation of a silicon oxide film on the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed due to oxygen and moisture contained in the air, it is preferable to quickly adhere the carbon layer-forming material to the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed.

[0043] The carbon layer-forming material is a precursor of the carbon that constitutes the carbon layer of the carbon layer-containing β-silicon carbide particles of the present invention.

[0044] The carbon layer forming material is not particularly limited as long as it forms a carbon layer on the surface of the β-silicon carbide particles.

[0045] Examples of carbon layer-forming materials include phenylamine-based polymers such as polydopamine, polyimide-based polymers, polybenzimidazole-based polymers, polyphenylene oxide-based polymers, acrylonitrile-based polymers, and (meth)acrylic polymers, but the present invention is not limited to these examples. All of these polymers form a graphite carbon layer upon firing, and are therefore suitable for use in the present invention. These polymers may be used alone or in combination of two or more. Among the carbon layer-forming materials, from the viewpoint of efficiently adhering the carbon layer-forming material to the surface of the β-silicon carbide particles, phenylamine-based polymers, polyimide-based polymers, polybenzimidazole-based polymers, polyphenylene oxide-based polymers, acrylonitrile-based polymers, and (meth)acrylic polymers are preferred, with phenylamine-based polymers, polyimide-based polymers, and polybenzimidazole-based polymers being more preferred.

[0046] Examples of methods for adhering a carbon layer-forming material to the surfaces of β-silicon carbide particles where the β-silicon carbide is exposed include a method of immersing β-silicon carbide particles in a solution of the carbon layer-forming material, a method of applying a solution of the carbon layer-forming material to the surfaces of the β-silicon carbide particles by spraying or the like, and a method of adhering a heated and melted carbon layer-forming material to the surfaces of the β-silicon carbide particles, but the present invention is not limited to these methods. Among these methods, from the viewpoint of uniformly adhering the carbon layer-forming material to the surfaces of the β-silicon carbide particles, the method of immersing β-silicon carbide particles in a solution of the carbon layer-forming material and the method of applying a solution of the carbon layer-forming material to the surfaces of the β-silicon carbide particles by spraying or the like are preferred, and the method of immersing β-silicon carbide particles in a solution of the carbon layer-forming material is more preferred.

[0047] The solution of the carbon layer-forming material can be a solution in which the carbon layer-forming material is dissolved in an organic solvent. The type of organic solvent varies depending on the type of carbon layer-forming material and cannot be determined in general. Therefore, it is preferable to select the organic solvent appropriately depending on the type of carbon layer-forming material.

[0048] Examples of the organic solvent include halogen-containing solvents such as dichloromethane, dichloroethane, and chloroform; alcohol-based organic solvents such as methanol, ethanol, isopropanol, n-butanol, and tert-butyl alcohol; ether-based organic solvents such as propylene glycol methyl ether, dipropylene glycol methyl ether, ethyl cellosolve, butyl cellosolve, and diglyme; ester-based organic solvents such as ethyl acetate, butyl acetate, and cellosolve acetate; ketone-based organic solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and diacetone alcohol; amide-based organic solvents such as dimethylformamide; and aromatic organic solvents such as benzene, toluene, and xylene. However, the present invention is not limited to these examples. These solvents may be used alone, or two or more types may be used in combination as necessary. It is preferable to select and use from these solvents a solvent suitable for the carbon layer-forming material.

[0049] The concentration of the carbon layer-forming material in the solution of the carbon layer-forming material cannot be determined in general because it differs depending on the type of carbon layer-forming material. From the viewpoint of improving coatability, the concentration of the carbon layer-forming material in the solution of the carbon layer-forming material is usually preferably about 5 to 30 mass %.

[0050] The thickness of the carbon layer-forming material attached to the surface of the β-silicon carbide particles is adjusted so that the carbon layer formed by firing the carbon layer-forming material has a desired thickness, which will be described later.

[0051] Next, the β-silicon carbide particles having the carbon layer forming material attached to the surface thereof are fired to obtain carbon layer-containing β-silicon carbide particles.

[0052] The atmosphere in which the β-silicon carbide particles having the carbon layer-forming material adhered to their surfaces are fired is not particularly limited, and may be air or an inert gas such as argon gas or nitrogen gas. However, an inert gas is preferred from the viewpoint of avoiding the influence of oxygen contained in the air.

[0053] The firing temperature of the β-silicon carbide particles having a carbon layer-forming material adhered to their surfaces varies depending on the type of carbon layer-forming material and cannot be determined in general, so it is preferable to adjust it appropriately depending on the type of carbon layer-forming material. The firing temperature is equal to or higher than the temperature required to carbonize the carbon layer-forming material but lower than the temperature at which the carbide of the carbon layer-forming material decomposes, and is usually around 500 to 1200°C.

[0054] The firing time for β-silicon carbide particles having a carbon layer-forming material attached to their surfaces cannot be determined in general because it varies depending on the type of carbon layer-forming material, the firing temperature, etc. The firing time is usually the time required from the start of firing the carbon layer-forming material until the carbon layer-forming material is sufficiently carbonized.

[0055] By firing the β-silicon carbide particles having the carbon layer-forming material adhered to their surfaces in the manner described above, it is possible to obtain carbon layer-containing β-silicon carbide particles in which the β-silicon carbide of the β-silicon carbide particles is in direct contact with the carbon layer.

[0056] The thickness of the carbon layer in the carbon layer-containing β-silicon carbide particles is preferably 5 nm or more, more preferably 10 nm or more, even more preferably 20 nm or more, and even more preferably 30 nm or more, from the viewpoint of obtaining a negative electrode active material for a lithium ion secondary battery that has a high discharge capacity and excellent durability against charge and discharge. There is no particular limitation on the upper limit of the thickness of the carbon layer in the carbon layer-containing β-silicon carbide particles, and the upper limit is usually 800 nm. Therefore, from the viewpoint of obtaining a negative electrode active material for a lithium ion secondary battery that has a high discharge capacity and excellent durability against charge and discharge, the thickness of the carbon layer in the carbon layer-containing β-silicon carbide particles is preferably 5 to 800 nm, more preferably 10 to 800 nm, even more preferably 20 to 800 nm, and even more preferably 30 to 800 nm.

[0057] In the carbon layer-containing β-silicon carbide particles, the entire surface of the β-silicon carbide particles is preferably substantially covered with a carbon layer, from the viewpoint of obtaining a negative electrode active material for a lithium ion secondary battery having a high discharge capacity and excellent durability against charge and discharge. Here, "substantially covering the entire surface of the β-silicon carbide particles with a carbon layer" means that the surface of the β-silicon carbide particles is completely covered with a carbon layer, and that there are areas on the surface of the β-silicon carbide particles that are not completely covered with a carbon layer, to the extent that the object of the present invention is not impaired. In the carbon layer-containing β-silicon carbide particles, from the viewpoint of obtaining a negative electrode active material for a lithium ion secondary battery having a high discharge capacity and excellent durability against charge and discharge, it is preferable that 90% or more of the surface of the β-silicon carbide particles is covered with a carbon layer, and more preferably 95% or more of the surface of the β-silicon carbide particles is covered with a carbon layer.

[0058] The carbon layer-containing β-silicon carbide particles of the present invention have excellent durability against charge and discharge because the β-silicon carbide of the β-silicon carbide particles is in direct contact with the carbon layer. Furthermore, a negative electrode active material containing the carbon layer-containing β-silicon carbide particles has the advantage of having a high discharge capacity.

[0059] Therefore, the carbon layer-containing β-silicon carbide particles of the present invention can be suitably used as a negative electrode active material for lithium ion secondary batteries.

[0060] [Negative Electrode Active Material for Lithium Ion Secondary Battery] The negative electrode active material for a lithium ion secondary battery of the present invention contains the carbon layer-containing β-silicon carbide particles of the present invention.

[0061] The negative electrode active material for a lithium ion secondary battery of the present invention may be composed solely of the carbon layer-containing β-silicon carbide particles of the present invention, or may contain other negative electrode active materials within the scope of the present invention. Examples of other negative electrode active materials include powders of carbon substances such as graphite, coke, etc., but the present invention is not limited to these examples.

[0062] [Lithium Ion Secondary Battery] The lithium ion secondary battery of the present invention has the same structure as commonly used lithium secondary ion batteries.

[0063] The lithium ion secondary battery of the present invention has a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The shape of the lithium ion secondary battery may be, for example, a cylindrical type or a laminated type, but the present invention is not limited to these examples.

[0064] When the lithium ion secondary battery of the present invention is, for example, a CR2025 coin battery, the negative electrode, separator, and non-aqueous electrolyte are housed in a case. In the following, an embodiment in which the lithium ion secondary battery is a CR2025 coin battery will be described, but the present invention is not limited to this embodiment.

[0065] The case used in the CR2025 coin battery is hollow, has an opening, and doubles as a positive electrode container. A lid is provided at the opening of the case, and the lid also doubles as a negative electrode lid. A gasket is provided between the case and the lid to maintain an insulating and sealed state between the case and the lid. The space between the case and the lid contains an electrode and a nonaqueous electrolyte.

[0066] (1) Electrodes The electrodes consist of a positive electrode, a negative electrode, and a separator, arranged in this order: positive electrode, separator, and negative electrode. The positive electrode is in contact with the inner surface of the case, and the negative electrode is in contact with the inner surface of the lid of the case.

[0067] [Positive Electrode] The positive electrode may be the same as the positive electrode used in commonly used lithium secondary ion batteries, and the present invention is not limited by the composition and structure of the positive electrode.

[0068] The positive electrode can be produced, for example, by mixing a positive electrode active material, a conductive material, and a binder in a predetermined ratio, adding a solvent, and if necessary, activated carbon, a viscosity-adjusting additive, and the like, in appropriate amounts to the resulting mixture, kneading the resulting mixture to prepare a positive electrode composite paste, and then applying the positive electrode composite paste to the surface of a current collector, press-molding the paste as necessary, and drying the paste.

[0069] Representative examples of the positive electrode active material include lithium and lithium-manganese composite oxides, but the present invention is not limited to these examples. Examples of the conductive material include carbon black such as acetylene black, natural graphite, artificial graphite, and expanded graphite, but the present invention is not limited to these examples. Examples of the binder include polyvinylidene fluoride (PVDF), 1-methyl-2-pyrrolidone (NMP), polytetrafluoroethylene (PTFE), fluororubber, and ethylene-propylene-diene rubber, but the present invention is not limited to these examples. Examples of the solvent include N-methyl-2-pyrrolidone, but the present invention is not limited to these examples.

[0070] The current collector may be the same as a current collector used in a commonly used lithium secondary ion battery, and the present invention is not limited by the type of the current collector. Materials used for the current collector include, for example, metals such as copper, zinc, aluminum, stainless steel, titanium, nickel, tungsten, and zirconium, but the present invention is not limited to these examples. These metals may be used alone or in combination.

[0071] [Negative electrode (negative electrode for lithium ion secondary battery)] The negative electrode contains the negative electrode active material for lithium ion secondary battery of the present invention. Therefore, the negative electrode uses the negative electrode active material for lithium ion secondary battery of the present invention. The negative electrode can be produced, for example, by mixing the negative electrode active material and a binder and adding an appropriate amount of solvent to the resulting mixture to prepare a paste-like negative electrode mixture, applying the negative electrode mixture to the surface of a metal foil current collector such as copper, drying it, and optionally press-molding and drying it.

[0072] The negative electrode active material for a lithium ion secondary battery of the present invention may be composed solely of the negative electrode active material for a lithium ion secondary battery of the present invention, or may contain other negative electrode active materials other than the negative electrode active material for a lithium ion secondary battery to the extent that the object of the present invention is not impaired. Examples of the other negative electrode active materials include powders such as graphite and carbon black, but the present invention is not limited to these examples. Examples of the binder include polyvinylidene fluoride (PVDF), 1-methyl-2-pyrrolidone (NMP), polytetrafluoroethylene (PTFE), fluororubber, and ethylene-propylene-diene rubber, but the present invention is not limited to these examples. Examples of the solvent include N-methyl-2-pyrrolidone, but the present invention is not limited to these examples.

[0073] [Separator] A separator is used between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode, retains the electrolyte, and forms a path for lithium ions to move between the positive electrode and the negative electrode. The separator can be a foil membrane (thin film) having a plurality of micropores formed therein. Materials used for the separator include, for example, polyolefin resins such as polyethylene and polypropylene, polyamide, polyimide, cellulose, porous glass, and glass fiber, but the present invention is not limited to these examples. Separators are readily available commercially. In the present invention, commercially available separators may be used.

[0074] (2) Nonaqueous Electrolyte The lithium ion secondary battery of the present invention uses a nonaqueous electrolyte as the electrolyte. Examples of nonaqueous electrolytes include cyclic carbonates such as ethylene carbonate, diethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; and ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane, but the present invention is not limited to these examples. These nonaqueous electrolytes may be used alone, or two or more types may be used in combination as necessary.

[0075] The lithium ion secondary battery of the present invention uses the negative electrode active material for lithium ion secondary batteries of the present invention as the negative electrode active material, and therefore has a high discharge capacity and excellent durability against charge and discharge.

[0076] When the lithium ion secondary battery of the present invention is, for example, a coin-type battery, the initial discharge capacity is 1.2 mAh or more, and even after 175 or more charge / discharge cycles, the decrease in discharge capacity is small and a discharge capacity of about 1.0 mAh or more is maintained. As a result, the capacity retention rate is about 80% or more, and the battery is excellent in terms of the maintenance of discharge capacity and durability against charge / discharge.

[0077] Therefore, the lithium ion secondary battery of the present invention is expected to be used in a variety of applications, such as portable electronic devices such as smartphones and tablet personal computers, hybrid vehicles, and electric vehicles.

[0078] Next, the present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0079] [Raw Materials] β-silicon carbide particles, hydrofluoric acid (hydrogen fluoride concentration: 48%), polyacrylic acid, sodium carboxymethylcellulose (number average molecular weight: approximately 250,000), and a solution of 1.0 M LiPF as an electrolyte in a mixed solvent consisting of 50% by volume of ethylene carbonate and 50% by volume of diethylene carbonate were obtained from Sigma-Aldrich.

[0080] 1.0 M Tris-HCl buffer (pH: 8.5) was obtained from Nippon Gene Co., Ltd., and 3-hydroxytyramine hydrochloride was obtained from Tokyo Chemical Industry Co., Ltd.

[0081] Acetylene black was obtained from Denka Corporation and used as a conductive material when preparing the slurry. Copper foil for batteries (manufactured by Nikola Co., Ltd., thickness: approximately 20 μm) was used as the current collector for the cathode. Lithium metal (manufactured by Honjo Metals Co., Ltd., thickness: approximately 1 mm) was used as the reference electrode and counter electrode for the half-cell. Polypropylene film (manufactured by Asahi Kasei Corporation, trade name: Celgard 2500) was used as the separator for a CR2025-type lithium-ion secondary battery. LiCoO2-coated aluminum foil (manufactured by Piotrec Co., Ltd., current density: 1.5 mAh / cm) was used as the current collector for the half-cell. 2 ] was used as the anode current collector of the full cell.

[0082] Reference Example Commercially available conventional β-silicon carbide particles were observed using a transmission electron microscope (TEM) [manufactured by Hitachi Industrial Products, Ltd., product number: HF-2000] and a high-resolution transmission electron microscope (HRTEM) [manufactured by JEOL Ltd., product number: JEM-2010]. The high-resolution transmission electron microscope (HRTEM) images are shown in Figures 1(a) to 1(d).

[0083] The HRTEM images shown in Figures 1(a) to (d) confirmed that commercially available β-silicon carbide (SiC) particles have a silicon oxide (SiOx) coating on their surfaces, as indicated by the arrows in Figure 1.

[0084] The reason why a silicon oxide coating exists on the surface of β-silicon carbide particles is partly because the β-silicon carbide particles are produced by the Antison process, which involves mixing silica sand with carbon and firing the resulting mixture at temperatures of 1600 to 2500°C. It is believed that when β-silicon carbide particles are produced by this process, a silicon oxide layer forms on the surface of the β-silicon carbide particles due to silicon's inherent high chemical affinity for oxygen. In addition, when β-silicon carbide particles are left in the air, they are susceptible to oxidation upon contact with oxygen and moisture contained in the air, which is why a silicon oxide layer is formed on the surface of the β-silicon carbide particles.

[0085] Production Example 1: As described above, a silicon oxide coating was present on the surface of β-silicon carbide particles. This silicon oxide coating was then removed. More specifically, the β-silicon carbide particles were immersed in 6 M hydrofluoric acid at 50°C for approximately 12 hours, and the hydrofluoric acid was then centrifuged to separate the hydrofluoric acid-treated β-silicon carbide particles from the hydrofluoric acid. The hydrofluoric acid-treated β-silicon carbide particles were then washed several times with deionized water and methanol. The β-silicon carbide particles were observed using a transmission electron microscope (TEM) (product number: HF-2000, manufactured by Hitachi Industrial Products, Ltd.). The results are shown in Figures 2(a) to 2(d). Figure 2 is a photographic representation of β-silicon carbide particles from which the silicon oxide coating had been removed, taken using a transmission electron microscope. In Figure 2, SiCA represents β-silicon carbide.

[0086] The results shown in Figures 2(a) to (d) show that β-silicon carbide is present in the acid-treated β-silicon carbide particles, but the silicon oxide coating that was present on the surface of the β-silicon carbide particles has been removed.

[0087] Production Example 2 Dopamine hydrochloride (3-hydroxytyramine hydrochloride) was used as a precursor of the carbon layer-forming material. A dopamine hydrochloride solution was prepared by dissolving dopamine hydrochloride in a 50% by volume aqueous methanol solution at room temperature to a concentration of 1 mg / mL, and the solution was continuously stirred for 2 hours.

[0088] 5 g of the β-silicon carbide particles from which the silicon oxide coating had been removed, obtained in Production Example 1, was added to 10 mL of the dopamine hydrochloride solution obtained above. To the resulting mixture was added 1.0 M Tris-HCl buffer solution while stirring to adjust the pH of the mixture to 8.5, and ultrasonic waves were continuously applied to the mixture while stirring for 48 hours, thereby obtaining a mixture containing β-silicon carbide particles coated with polydopamine (hereinafter referred to as polydopamine-coated β-silicon carbide particles) as β-silicon carbide particles having a carbon layer-forming material attached thereto.

[0089] The mixture containing the polydopamine-coated β-silicon carbide particles obtained above was centrifuged to separate and extract the polydopamine-coated β-silicon carbide particles from the solvent contained in the mixture, and in order to completely remove moisture from the polydopamine-coated β-silicon carbide particles, the polydopamine-coated β-silicon carbide particles were further freeze-dried to prepare dried polydopamine-coated β-silicon carbide particles, which were used in the following examples.

[0090] Example 1 The dried polydopamine-coated β-silicon carbide particles obtained in Production Example 2 were placed in an electric furnace, and while argon gas was being introduced into the electric furnace, the polydopamine-coated β-silicon carbide particles were heated to 800°C at a heating rate of 5°C / min, baked at the same temperature for 2 hours, and then allowed to cool to room temperature, thereby obtaining carbon layer-containing β-silicon carbide particles.

[0091] The carbon layer-containing β-silicon carbide particles obtained above were observed with a transmission electron microscope (TEM) [product number HF-2000, manufactured by Hitachi Industrial Products, Ltd.]. The results are shown in Figure 3. Figure 3 is a photograph, used as a substitute for a drawing, showing an image of the carbon layer-containing β-silicon carbide particles taken with a transmission electron microscope. In Figure 3, SiCA represents β-silicon carbide, and Carbon represents the carbon layer.

[0092] The results shown in Figure 3 indicate that the carbon layer-containing β-silicon carbide particles have a carbon layer formed on the surface of the β-silicon carbide particle, and that the thickness of the carbon layer is approximately 10 to 20 nm. Furthermore, the results shown in Figure 3 indicate that the entire surface of the β-silicon carbide particle in the carbon layer-containing β-silicon carbide particle is covered with the carbon layer.

[0093] Comparative Example 1 Dried polydopamine-coated β-silicon carbide particles were prepared in the same manner as in Production Example 2, except that the β-silicon carbide particles treated with hydrofluoric acid in Production Example 2 were replaced with β-silicon carbide particles of the Reference Example that had not been treated with hydrofluoric acid and had a silicon oxide coating.

[0094] Next, carbon layer-containing β-silicon carbide particles (hereinafter referred to as carbon layer-silicon oxide coating-containing β-silicon carbide particles) were obtained in the same manner as in Example 1, except that the polydopamine-coated β-silicon carbide particles obtained above were used instead of the dried polydopamine-coated β-silicon carbide particles obtained in Production Example 2.

[0095] [Preparation of Slurry] As the negative electrode active material, β-silicon carbide particles, the acid-treated β-silicon carbide particles described above, the polydopamine-coated β-silicon carbide particles obtained above, the carbon layer-containing β-silicon carbide particles obtained above, or the carbon layer-silicon oxide coating-containing β-silicon carbide particles obtained above were used.

[0096] 70 g of the negative electrode active material, 15 g of acetylene black, 7.5 g of polyacrylic acid as a binder, and 7.5 g of carboxymethyl cellulose were mixed in a ball mill to obtain a uniform composition, and a suitable amount of water was used as a solvent when preparing the slurry.

[0097] [Fabrication of Negative Electrode] The slurry obtained above was applied to a copper foil (length: 20 mm, width: 100 mm, thickness: 20 μm) using a doctor blade, and dried at a temperature of 80° C. under reduced pressure for about 12 hours. The copper foil was then pressed with a roll to fabricate a negative electrode plate with a coating thickness of 0.06 mm after drying.

[0098] [Half-cell assembly] To incorporate the negative electrode plate obtained above into a CR2025-type coin battery, a 15 mm diameter disk was punched out. The resulting disk was used as the negative electrode, lithium foil as the counter electrode, a polypropylene separator (manufactured by Asahi Kasei Corporation, product name: Celgard 2500, thickness: 25 μm) as the separator between the negative electrode and the counter electrode, and a nonaqueous electrolyte prepared by dissolving LiPF6 to a concentration of 1 M in a solvent mixture of ethylene carbonate and diethylene carbonate in a 1:1 mass ratio. A half-cell was assembled in an argon gas atmosphere. The obtained half-cell was left at room temperature for 12 hours for stabilization.

[0099] [Assembly of a Full Cell] A full cell was fabricated by using LiCoO2 as the positive electrode instead of lithium foil in the half cell described above. Before fabricating the full cell, the negative and positive electrodes were charged and discharged at potentials of 1.5 to 4.2 V using half cells. This charging and discharging is useful not only for minimizing the initial irreversible loss of lithium ions but also for activating the electrodes.

[0100] [Electrochemical Properties 1] The electrochemical properties were examined using a glove box (manufactured by MBRAUN, product name: Labstar) and a charge / discharge device (manufactured by Biological Scientific Instruments, product number: BCS-805).

[0101] The battery capacity of the half-cells obtained above was investigated by repeatedly charging and discharging the cells at different current densities (mA / g). As a result, the coulombic efficiency (charge-discharge efficiency) of the half-cell using β-silicon carbide particles as the anode active material was 96.2% in the first five cycles, while the coulombic efficiency of the half-cells using other anode active materials reached 100% in the first five cycles.

[0102] Furthermore, the coulombic efficiency after 128 charge / discharge cycles was approximately 93.7% for the half-cell using β-silicon carbide particles as the negative electrode active material, whereas it was approximately 100% for the half-cell using acid-treated β-silicon carbide particles, the half-cell using polydopamine-coated β-silicon carbide particles as the negative electrode active material, and the half-cell using carbon-layer-containing β-silicon carbide particles as the negative electrode active material.

[0103] FIG. 4(a) shows the battery capacity of half cells using β-silicon carbide particles and acid-treated β-silicon carbide particles as the negative electrode active material, and FIG. 4(b) shows the battery capacity of half cells using polydopamine-coated β-silicon carbide particles and carbon layer-containing β-silicon carbide particles as the negative electrode active material.

[0104] In FIG. 4( a), symbol A shows the change in battery capacity when a charge-discharge cycle test was performed using a half cell that used β-silicon carbide particles as the negative electrode active material, and symbol B shows the change in battery capacity when a cycle test was performed using a half cell that used acid-treated β-silicon carbide particles as the negative electrode active material.

[0105] In FIG. 4( b), symbol C shows the change in battery capacity when a charge-discharge cycle test was conducted using a half cell in which polydopamine-coated β-silicon carbide particles were used as the negative electrode active material, and symbol D shows the change in battery capacity when a charge-discharge cycle test was conducted using a half cell in which carbon layer-containing β-silicon carbide particles were used as the negative electrode active material.

[0106] 4, at a current density of 50 mA / g, the initial battery capacity of the half cell using β-silicon carbide particles as the anode active material was approximately 51 mAh / g (symbol A), the initial battery capacity of the half cell using acid-treated β-silicon carbide particles as the anode active material was approximately 73 mAh / g (symbol B), and the initial battery capacity of the half cell using polydopamine-coated β-silicon carbide particles as the anode active material was approximately 367 mAh / g (symbol C). In contrast, at a current density of 50 mA / g, the initial battery capacity of the half cell using carbon-layered β-silicon carbide particles as the anode active material was approximately 678 mAh / g (symbol D).

[0107] From the above results, it is clear that the carbon layer-containing β-silicon carbide particles significantly increase the battery capacity compared to conventional β-silicon carbide particles.

[0108] [Electrochemical Properties 2] Cyclic voltammetry of the half-cell was measured at room temperature using a potentiostat (Biological, product number: VSM). The potential range during cyclic voltammetry measurement was 0 to 3 V, and the scan rate was adjusted to 0.5 mV / s. Cyclic voltammetry (scan rate: 0.5 mV / s) was measured for a half-cell using β-silicon carbide particles as the negative electrode active material and a half-cell using β-silicon carbide particles containing a carbon layer as the negative electrode active material. The results are shown in Figure 5.

[0109] In FIG. 5, symbol E indicates the cyclic voltammetry of the half cell in which the carbon layer-containing β-silicon carbide particles are used as the negative electrode active material, and symbol F indicates the cyclic voltammetry of the half cell in which the β-silicon carbide particles are used as the negative electrode active material.

[0110] The results shown in FIG. 5 indicate that the half cell (E) using the carbon layer-containing β-silicon carbide particles as the negative electrode active material has a broader cyclic voltammetry profile than the half cell (F) using β-silicon carbide particles as the negative electrode active material, thereby significantly improving the lithiation and delithiation currents.

[0111] [Electrochemical Characteristics 3] A charge-discharge cycle test was conducted at a current density of 100 mA / g using a half-cell in which the carbon layer-containing β-silicon carbide particles were used as the negative electrode active material and a half-cell in which the carbon layer-silicon oxide coating-containing β-silicon carbide particles were used as the negative electrode active material. The results are shown in Figure 6.

[0112] In FIG. 6, symbol G is a graph showing the results of a charge-discharge cycle test on a half cell using carbon layer-containing β-silicon carbide particles as the negative electrode active material, and symbol H is a graph showing the results of a charge-discharge cycle test on a half cell using carbon layer-silicon oxide coating-containing β-silicon carbide particles as the negative electrode active material.

[0113] The results shown in FIG. 6 show that the half cell (initial battery capacity: approximately 900 mAh / g) (symbol G) using the carbon layer-containing β-silicon carbide particles as the negative electrode active material has a significantly higher initial battery capacity than the half cell (initial battery capacity: approximately 600 mAh / g) (symbol H) using the carbon layer-silicon oxide coating-containing β-silicon carbide particles as the negative electrode active material.

[0114] It is also apparent that the half cell (symbol G) in which the carbon layer-containing β-silicon carbide particles are used as the negative electrode active material maintains a high battery capacity even after 300 charge / discharge cycles.

[0115] Therefore, it is highly likely that negative electrode active materials using carbon layer-containing β-silicon carbide particles will be put to practical use as new negative electrodes in the near future, replacing negative electrode active materials using conventional β-silicon carbide particles.

[0116] [Electrochemical Properties 4] A full cell using carbon layer-containing β-silicon carbide particles as the negative electrode active material was subjected to a charge-discharge cycle test at a current density of 60 mA / g. The results are shown in FIG.

[0117] From the results shown in FIG. 7, it can be seen that the initial discharge capacity of the full cell using the carbon layer-containing β-silicon carbide particles as the negative electrode active material was 1.1 mAh, and the discharge capacity after 175 cycles was maintained at about 80% of the initial discharge capacity.

[0118] Therefore, since the negative electrode active material using the carbon layer-containing β-silicon carbide particles maintains the initial discharge capacity without a significant decrease, it is expected that in the near future it will be put to practical use as a full-cell negative electrode active material, replacing negative electrode active materials using conventional β-silicon carbide particles.

[0119] Next, a lighting test of a light-emitting diode was carried out using the full cell obtained above, and the results are shown in FIG.

[0120] In Figure 8, the upper row is an optical photograph of the light-emitting diode before lighting, and the lower row is an optical photograph of the light-emitting diode after lighting. The black circle in the text in the upper row of Figure 8 is the light-emitting diode before lighting, and the white circle in the text in the lower row of Figure 8 is the light-emitting diode after lighting.

[0121] As shown in FIG. 8, the full cell using the carbon layer-containing β-silicon carbide particles as the negative electrode active material was able to brightly light up the light-emitting diode, which indicates that the full cell can be used as a power source for devices using light-emitting diodes, such as portable electronic devices.

[0122] Therefore, lithium ion secondary batteries using the negative electrode active material for lithium ion secondary batteries of the present invention are expected to replace lithium ion secondary batteries using conventional negative electrode active materials.

[0123] The negative electrode active material of the present invention is expected to be a next-generation negative electrode active material for lithium ion secondary batteries that can replace conventional negative electrode active materials that use silicon carbide particles. Furthermore, lithium ion secondary batteries that use negative electrodes containing the negative electrode active material for lithium ion secondary batteries of the present invention have high discharge capacity and excellent durability against charge and discharge, and are therefore expected to be used in a wide range of applications, such as portable electronic devices such as smartphones and tablet personal computers, hybrid vehicles, and electric vehicles.

Claims

1. Carbon layer-containing β-silicon carbide particles having a carbon layer on the surface of the β-silicon carbide particles, characterized in that the β-silicon carbide of the β-silicon carbide particles and the carbon layer are in direct contact with each other.

2. The carbon layer-containing β-silicon carbide particles according to claim 1, wherein the entire surface of the β-silicon carbide particles is substantially covered with the carbon layer.

3. The carbon layer-containing β-silicon carbide particles according to claim 1, wherein the average particle size of the β-silicon carbide particles is 0.1 to 30 μm.

4. The carbon layer-containing β-silicon carbide particles according to claim 1, wherein the carbon layer has a thickness of 5 to 800 nm.

5. A method for producing carbon layer-containing β-silicon carbide particles having a carbon layer on the surface of the β-silicon carbide particles, the method comprising the steps of: pickling the β-silicon carbide particles to expose β-silicon carbide on the surface of the β-silicon carbide particles; adhering a carbon layer-forming material to the surface of the β-silicon carbide particles where the β-silicon carbide is exposed; and then firing the β-silicon carbide particles to which the carbon layer-forming material is adhered.

6. The method for producing carbon layer-containing β-silicon carbide particles according to claim 5, wherein the β-silicon carbide particles are pickled with an acid solution.

7. The method for producing carbon layer-containing β-silicon carbide particles according to claim 6, wherein the acid used in the acid solution is an inorganic acid or an organic acid.

8. The method for producing carbon layer-containing β-silicon carbide particles according to claim 7, wherein the inorganic acid is hydrofluoric acid, hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid or phosphoric acid.

9. The method for producing carbon layer-containing β-silicon carbide particles according to claim 7, wherein the inorganic acid is hydrofluoric acid.

10. The method for producing carbon layer-containing β-silicon carbide particles according to claim 6, wherein the temperature of the acid solution used in pickling the β-silicon carbide particles is 0 to 90° C.

11. The method for producing carbon layer-containing β-silicon carbide particles according to claim 6, wherein the temperature of the acid solution used in pickling the β-silicon carbide particles is 5 to 40° C.

12. The method for producing carbon layer-containing β-silicon carbide particles according to claim 5, wherein the material for forming the carbon layer is a phenylamine-based polymer, a polyimide-based polymer, a polybenzimidazole-based polymer, a polyphenylene oxide-based polymer, an acrylonitrile-based polymer or a (meth)acrylic-based polymer.

13. The method for producing carbon layer-containing β-silicon carbide particles according to claim 5, wherein the material for forming the carbon layer is a phenylamine-based polymer, a polyimide-based polymer or a polybenzimidazole-based polymer.

14. A method for producing carbon layer-containing β-silicon carbide particles according to claim 5, comprising the steps of immersing β-silicon carbide particles in a solution of the carbon layer-forming material, spraying the solution of the carbon layer-forming material onto the surfaces of the β-silicon carbide particles, or attaching the carbon layer-forming material that has been heated and melted onto the surfaces of the β-silicon carbide particles, thereby attaching the carbon layer-forming material to the surfaces of the β-silicon carbide particles on which the β-silicon carbide is exposed.

15. A method for producing carbon layer-containing β-silicon carbide particles according to claim 5, comprising the steps of immersing β-silicon carbide particles in a solution of a carbon layer-forming material, and adhering the carbon layer-forming material to the surfaces of the β-silicon carbide particles where the β-silicon carbide is exposed.

16. A negative electrode active material for a lithium ion secondary battery comprising the carbon layer-containing β-silicon carbide particles according to claim 1.

17. A negative electrode for a lithium ion secondary battery comprising the negative electrode active material for a lithium ion secondary battery according to claim 16.

18. A lithium ion secondary battery having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the negative electrode is the negative electrode for a lithium ion secondary battery according to claim 17.

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