JP6451071B2 - Carbon silicon negative electrode active material for lithium ion secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a negative electrode active material for a lithium ion secondary battery and a method for producing the same.

  As mobile devices such as smartphones and tablet terminals have higher performance and vehicles equipped with lithium ion secondary batteries such as EV, HEV, and PHEV have spread, there is an increasing demand for higher capacity of lithium ion secondary batteries. At present, graphite is mainly used as the negative electrode material of lithium ion secondary batteries. However, for further increase in capacity, the theoretical capacity is high, and elements such as silicon and tin that can absorb and release lithium ions are used. Development of negative electrode materials using metals or alloys with other elements has been activated.

  On the other hand, it is known that an active material made of a metal material capable of inserting and extracting lithium ions significantly expands when alloyed with lithium by charging. Therefore, the active material is cracked and refined, and the structure of the negative electrode using these is also broken and the conductivity is cut. Therefore, the negative electrode using these metal materials has a problem that the capacity is remarkably lowered with the passage of cycles.

In response to this problem, a technique has been proposed in which these metal materials are made into fine particles and combined with carbonaceous material or graphite. Such composite particles have significantly higher cycle characteristics than the use of these materials alone as a negative electrode material, because these metal materials are alloyed with lithium and conductivity is ensured by carbonaceous materials and graphite even when they are miniaturized. It is known to improve. For example, in Patent Document 1, the active material of the negative electrode includes fine particles having a carbonaceous material layer formed on the surface, and the fine particles are composed of at least one element selected from Mg, Al, Si, Ca, Sn, and Pb. In addition, it is disclosed that the average particle diameter is 1 to 500 nm, and the atomic ratio of the fine particles in the active material is 15% or more. Patent Document 2 discloses a mixing step of mixing a battery active material capable of combining with lithium ions and graphite to obtain a mixture, and subjecting the mixture to a spheroidization treatment so that the battery active capable of combining with graphite and lithium ions is obtained. A method for producing a composite active material for a lithium secondary battery comprising a spheroidizing step for producing a substantially spherical composite active material for a lithium secondary battery containing the material, and a battery active material capable of combining with the lithium ion As for, at least one element selected from Si, Sn, Al, Sb, and In, graphite is preferably expanded graphite or flaky graphite having a specific surface area of 30 m ² / g or more.

  In this way, the cycle characteristics can be improved by encapsulating the battery active material that can be combined with lithium ions by expanded graphite or flake graphite, but the required characteristics have not yet been reached, and the cycle life has been further improved. It was sought after.

Japanese Patent Laid-Open No. 10-3920 Japanese Patent No. 5227483

  The present invention relates to a negative electrode active material for a lithium ion secondary battery comprising a carbonaceous material, graphite and Si, and a lithium ion secondary battery having a large discharge capacity and a long cycle life, and a method for producing the same Is to provide.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have a bulk density of 130% or less during heat treatment at 1100 to 1200 ° C. when heat-treating acid-treated graphite to obtain expanded graphite. It is composed of expanded graphite, containing 5 to 90% by weight of carbonaceous material and graphite, and sandwiched between thin graphite layers with a thickness of 0.2 μm or less together with the composite carbonaceous material. The negative electrode active material for a lithium ion secondary battery is characterized in that the negative electrode active material for a lithium ion secondary battery is spread in a network shape and the graphite thin layer is curved near the surface of the active material particles to cover the active material particles. Has been found that a lithium ion secondary battery having a large cycle life and a long cycle life can be obtained, and the present invention has been completed.

  That is, the present invention is a negative electrode active material for a lithium ion secondary battery comprising graphite, a carbonaceous material, and a Si compound, and is composed of expanded graphite having a bulk density of 130% or less during heat treatment at 1100 to 1200 ° C. Is a carbon-silicon-based negative electrode active material for a lithium ion secondary battery.

  Hereinafter, the carbon silicon-based negative electrode active material for a lithium ion secondary battery of the present invention will be described in detail.

  Graphite as used in the present invention is a crystal in which the graphene layer is parallel to the c-axis, and the graphite is expanded by heat treatment after acid treatment and oxidation treatment, and part of the graphite layer is peeled off to form an accordion shape. Expanded graphite or a pulverized product of expanded graphite. Examples of the acid used for the acid treatment include sulfuric acid, nitric acid, permanganic acid and the like. The bulk density of expanded graphite is preferably low. Normally, acid-treated graphite tends to expand when heat-treated at a high temperature. Therefore, when heat-treated at a high temperature of 1100 to 1200 ° C., the maximum expansion rate is obtained, and low bulk density expanded graphite is obtained. In addition, when heat treatment is performed at a temperature lower than this, the expansion coefficient tends to be low, and the bulk density tends to be high. The graphite of the present invention preferably has a bulk density of 130% or less during heat treatment at 1100 to 1200 ° C. A more preferable range is 120% or less. If it exceeds 130%, since the expansion coefficient is low, the function of covering the Si surface by the graphite thin layer to be described later is reduced, Si and the electrolytic solution are in direct contact, and a reactant is formed during charge and discharge, and during charge and discharge. The efficiency drop is severe. In parallel with this, when the bulk density increases, the expansion of the graphite is not sufficient, and the specific surface area value tends to decrease. In this case, it is preferable to have a specific surface area value of 50% or more during heat treatment at 1100 to 1200 ° C.

  The negative electrode active material for a lithium ion secondary battery of the present invention preferably contains 5 to 90% by weight, more preferably 20 to 60% by weight, as the content of carbonaceous material and graphite. When the content of carbonaceous material is less than 5% by weight, the carbonaceous material and graphite cannot cover the Si compound, the conductive path becomes insufficient, and capacity deterioration easily occurs. Is not enough.

  The carbonaceous material referred to in the present invention is an amorphous or microcrystalline carbon material, and easily graphitized carbon (soft carbon) that is graphitized by a heat treatment exceeding 2000 ° C. and non-graphitizable carbon (hard carbon) that is difficult to graphitize. )

  Moreover, it is preferable that content of the carbonaceous material said by this invention is 5 to 40 weight%. When the content of the carbonaceous material is less than 5% by weight, the carbonaceous material cannot cover the Si compound and graphite, adhesion between the Si compound and graphite becomes insufficient, and formation of active material particles tends to be difficult. Moreover, when larger than 40 weight%, the effect of the graphite whose electroconductivity is higher than a carbonaceous material is not fully drawn out.

  In the present invention, Si is a general grade metal silicon having a purity of about 98%, a chemical grade metal silicon having a purity of 2 to 4N, a chlorinated and purified by distillation using 4N, a single crystal growth method High-purity single crystal silicon that has undergone a deposition step by the above, or those doped with elements of Group 13 or 15 of the periodic table to be p-type or n-type, and polishing or cutting of wafers generated in the semiconductor manufacturing process There is no particular limitation as long as the purity is higher than that of general-purpose grade metal silicon, such as scraps and waste wafers that have become defective in the process.

In the carbon silicon-based negative electrode active material for a lithium ion secondary battery of the present invention, the average particle diameter D50 of Si is 0.01 to 0.5 μm. If it is smaller than 0.01 μm, the capacity and initial efficiency due to surface oxidation are drastically reduced, and if it is larger than 0.5 μm, cracks are severely caused by expansion due to lithium insertion, resulting in severe cycle deterioration. D50 is a volume average particle diameter measured by a laser diffraction method or a dynamic light scattering method. Moreover, the specific surface area measured by BET method is 23-300 m < ² > / g. When the specific surface area is less than 23 m ² / g, the particles are large, and cracks are severely caused by expansion due to lithium insertion, resulting in severe cycle deterioration.

  The content of Si is preferably 10 to 80% by weight, and more preferably 15 to 50% by weight. When the Si content is less than 10% by weight, a sufficiently large capacity cannot be obtained as compared with conventional graphite. When the Si content is more than 80% by weight, cycle deterioration becomes severe.

  The particle size of the graphite contained in the negative electrode active material of the present invention is not particularly limited as long as it is smaller than the size of the negative electrode active material particles, but the thickness of the graphite particles may be 1/5 or less of the average particle diameter D50 of the active material. preferable. Addition of graphite increases the conductivity and strength of the active material particles, and improves charge / discharge rate characteristics and cycle characteristics. The (002) plane spacing d002 measured by X-ray diffraction of graphite particles is preferably 0.338 nm or less, which means highly graphitized graphite. When d002 exceeds this value, the effect of improving conductivity by graphite becomes small.

  In the carbon silicon-based negative electrode active material for a lithium ion secondary battery of the present invention, the graphite thin layer is preferably 0.5 μm or less. When the thickness is 0.5 μm or less, the Si compound sandwiched between the graphite thin layers and the carbonaceous material layer are thinned, and the electron transfer to the Si compound is improved. The electron transfer effect of the thin layer is diminished. When the graphite thin layer is linear when viewed in cross section, its length is preferably at least half the size of the negative electrode active material particles for electron transfer, and more preferably about the same as the size of the negative electrode active material particles. When the graphite thin layer is network-like, it is preferable for electron transfer that the graphite thin layer network is connected to more than half of the size of the negative electrode active material particles, and it may be about the same size as the negative electrode active material particles. Further preferred.

  In the carbon silicon negative electrode active material for a lithium ion secondary battery of the present invention, the structure spreads in a layered and / or network form, and the thin graphite layer is curved near the surface of the active material particles. It is preferable to cover. With such a shape, there is a risk that the electrolyte enters from the end face of the graphite thin layer, the Si compound or the end face of the graphite thin layer is in direct contact with the electrolyte, and a reactant is formed during charge and discharge, resulting in reduced efficiency. To reduce.

  In the present invention, the graphite thin layer refers to expanded graphite or expanded graphite in which the above-mentioned graphite is subjected to acid treatment and oxidation treatment and then expanded by heat treatment, and a part of the graphite layer is peeled off to form an accordion shape. It is a graphite thin layer made of a pulverized product.

  The carbon silicon-based negative electrode active material for a lithium ion secondary battery of the present invention is a composite particle having a rounded average particle diameter D50 of 1 to 40 μm, preferably 2 to 30 μm. When D50 is less than 1 μm, it becomes bulky and it becomes difficult to produce a high-density electrode, and when it exceeds 40 μm, the unevenness of the applied electrode becomes intense and it becomes difficult to produce a uniform electrode. Moreover, it is preferable that the average particle diameter of Si is 1/5 or less of the average particle diameter of the negative electrode active material.

  Composite particles with rounded shapes are those in which the corners of particles produced by pulverization, etc. are rounded, spherical or spheroid shapes, discs or oval shapes with thickness and rounded corners, or those Is a deformed one with rounded corners. When the shape is rounded, the bulk density of the composite particles is increased, and the packing density when the negative electrode is formed is increased.

  Moreover, in the silicon-containing negative electrode active material for a lithium ion secondary battery of the present invention, the content of the Si compound is 10 to 60% by weight, the content of the carbonaceous material is 5 to 40% by weight, and the content of the graphite Is preferably 20 to 80% by weight.

In the carbon silicon-based negative electrode active material for a lithium ion secondary battery of the present invention, the specific surface area is more preferably 5 to 120 m ² / g. When the specific surface area is less than 5 μm, the particle diameter becomes large, and the unevenness of the applied electrode becomes intense, making it difficult to produce a uniform electrode. When the specific surface area exceeds 120 m ² / g, the surface area is large and The amount of reaction products increases, and the battery characteristics are adversely affected such as deterioration of cycle characteristics.

  Next, the manufacturing method of the carbon silicon type negative electrode active material for lithium ion secondary batteries of this invention is demonstrated.

  The method for producing a carbon silicon negative electrode active material for a lithium ion secondary battery of the present invention comprises a step of mixing expanded graphite, a carbon precursor and Si, a step of granulating and densifying, and a composite particle by pulverizing A step of forming, and a step of firing the composite particles in an inert atmosphere.

  Expanded graphite, which is a raw material, is natural graphite, artificial graphite obtained by graphitizing the pitch of petroleum or coal, etc., and graphite having a scaly, oval or spherical, cylindrical or fiber shape is treated with an acid, oxidized After the treatment, expanded graphite or expanded graphite pulverized product that is expanded by heat treatment and peels off part of the graphite layer to form an accordion shape is used. The particle size before mixing is about 5 μm to 5 mm. In expanded graphite, it can be expanded greatly by sufficiently performing acid treatment and increasing the temperature gradient of heat treatment, and the thickness of the graphite thin layer of the negative electrode active material obtained by sufficiently mixing and dispersing can be increased. Since it can be made thin, good electrical conductivity and cycle characteristics can be obtained.

  The carbon precursor as a raw material is not particularly limited as long as it is a polymer mainly composed of carbon and becomes a carbonaceous material by heat treatment in an inert gas atmosphere, but is not limited to petroleum pitch, coal pitch, synthetic pitch, Tar, cellulose, sucrose, polyvinyl chloride, polyvinyl alcohol, phenol resin, furan resin, furfuryl alcohol, polystyrene, epoxy resin, polyacrylonitrile, melamine resin, acrylic resin, polyamideimide resin, polyamide resin, polyimide resin, etc. it can.

As the raw material Si, a powder having an average particle diameter D50 of 0.01 to 0.5 μm and a specific surface area of 23 to 300 m ² / g measured by the BET method is used. In order to obtain Si having a predetermined particle diameter, the above-mentioned Si raw materials (ingot, wafer, powder, etc.) are pulverized by a pulverizer, and a classifier is used in some cases. In the case of a lump such as an ingot or a wafer, it can be first pulverized using a coarse pulverizer such as a jaw crusher. After that, for example, a ball or bead is used to move the grinding medium, and the impact force, frictional force, or compression force of the kinetic energy is used to grind the material to be crushed, the media agitation mill, or the compression force of the roller. Rotation of a roller mill that pulverizes, a jet mill that collides crushed objects with the lining material or collides with each other at high speed, and pulverizes by the impact force of the impact, and a rotor with a fixed hammer, blade, pin, etc. After pulverizing using a hammer mill, pin mill, disk mill that pulverizes the material to be crushed using the impact force of, colloid mill using shear force or high-pressure wet opposed collision disperser `` Ultimizer '', etc., Micronize.

  As a pulverization method, for example, a very fine particle can be obtained by using a wet bead mill and gradually reducing the diameter of the beads. Zirconia, which has high strength, is preferable as the medium used for the bead mill. The diameter of the beads is changed depending on the particle diameter of Si to be crushed. For example, if the average particle diameter D50 of Si is 10 to 40 μm, beads of 0.5 to 1 mm are used, and the average particle diameter D50 of Si is 0.00. When the average particle diameter D50 of Si is 0.1 to 0.5 μm, it is preferable to use 0.03 to 0.1 mm beads. When using beads smaller than 0.1 mm, it is preferable to use a centrifugal separation system for separating the beads and the slurry.

  As the dispersing agent during pulverization, alcohols such as methanol, ethanol and isopropyl alcohol, and hydrocarbon solvents such as hexane and toluene are preferable. Water is not suitable because of the intense oxidation of Si. Moreover, you may add the anionic, cationic, and nonionic dispersing agent for lowering | hanging a slurry viscosity as needed. The concentration of the slurry is not particularly specified, but includes 5 to 25% as a viscosity for performing efficient pulverization, preventing aggregation during pulverization and lowering the slurry viscosity, and more preferably 5 to 20%. %. If the concentration is lower than 5%, the pulverization efficiency is lowered, and if it is higher than 25%, the slurry viscosity increases, and pulverization may not be possible due to a decrease in pulverization efficiency or clogging.

  In order to adjust the particle size distribution after pulverization, dry classification, wet classification or sieving classification can be used. In the dry classification, the process of dispersion, separation (separation of fine particles and coarse particles), collection (separation of solid and gas), and discharge are performed sequentially or simultaneously, mainly using air flow. Pre-classification (adjustment of moisture, dispersibility, humidity, etc.) before classification, or the moisture in the airflow used so that the classification efficiency is not lowered due to the influence of shape, air flow disturbance, velocity distribution, static electricity, etc. It is done by adjusting the oxygen concentration. In a dry type in which a classifier is integrated, pulverization and classification are performed at a time, and a desired particle size distribution can be obtained.

  As another method for obtaining Si having a predetermined particle size, a method of obtaining Si by heating and evaporating a Si compound with plasma or laser and solidifying it in an inert gas, CVD or plasma CVD using a gas raw material, etc. These methods are suitable for obtaining ultrafine particles of 0.1 μm or less.

  When the carbon precursor is softened or liquefied by heating, mixing with these expanded graphite, carbon precursor, and Si can be performed by kneading graphite, carbon precursor, and Si under heating. it can. If the carbon precursor is dissolved in a solvent, expanded graphite, carbon precursor, and Si are added to the solvent, and the expanded graphite, carbon precursor, and Si are dispersed in the solution in which the carbon precursor is dissolved. , Mixing and then removing the solvent. The solvent to be used can be used without particular limitation as long as it can dissolve the carbon precursor. For example, when pitch or tar is used as the carbon precursor, quinoline, pyridine, toluene, benzene, tetrahydrofuran, creosote oil or the like can be used. When polyvinyl chloride is used, tetrahydrofuran, cyclohexanone, nitrobenzene or the like can be used. When phenol resin or furan resin is used, ethanol, methanol or the like can be used.

  As a mixing method, when the carbon precursor is heat-softened, a kneader (kneader) can be used. In the case of using a solvent, in addition to the above-described kneader, a Nauter mixer, a Roedige mixer, a Henschel mixer, a high speed mixer, a homomixer, or the like can be used. Further, the jacket is heated with these apparatuses, and then the solvent is removed with a vibration dryer, a paddle dryer or the like.

  With these apparatuses, the mixture of expanded graphite, Si, and carbon precursor is granulated and consolidated by solidifying the carbon precursor or continuing stirring in the process of removing the solvent for a certain period of time. Further, the carbon precursor is solidified or the mixture after removing the solvent is compressed by a compressor such as a roller compactor and coarsely pulverized by a crusher, whereby granulation and consolidation can be achieved. The size of the granulated / consolidated product is preferably 0.1 to 5 mm in view of ease of handling in the subsequent pulverization step.

  The granulated / consolidated material is pulverized by ball mill, medium agitation mill, roller mill for pulverizing using the compressive force of the roller, or lining material to be crushed at high speed. A jet mill that collides with each other or collides with each other and crushes by the impact force of the impact, a hammer mill that crushes the material to be crushed using the impact force of the rotation of a rotor with a fixed hammer, blade, pin, etc. A dry pulverization method such as a pin mill or a disk mill is preferred. In order to adjust the particle size distribution after pulverization, dry classification such as air classification and sieving is used. In the type in which the pulverizer and the classifier are integrated, pulverization and classification are performed at a time, and a desired particle size distribution can be obtained.

  The composite particles obtained by pulverization are fired in an argon gas or nitrogen gas stream or in an inert atmosphere such as a vacuum. The firing temperature is preferably 800 to 1000 ° C. When the firing temperature is less than 800 ° C., the irreversible capacity of the amorphous carbon derived from the carbon precursor is large, and the cycle characteristics are poor, so that the battery characteristics tend to deteriorate.

  The method for producing a carbon silicon-based negative electrode active material for a lithium ion secondary battery according to the present invention comprises a step of mixing and dispersing expanded graphite, Si, and a carbon precursor in a solvent in which the carbon precursor is dissolved, and granulation / thickness It is preferable to include a step of forming a composite particle having a round shape by pulverization and spheronization, and a step of firing the composite particle in an inert atmosphere.

  As a method of pulverizing the granulated / consolidated product and subjecting it to spheronization, it is pulverized by the above-mentioned pulverization method to adjust the particle size, and then passed through a dedicated spheronization device, and the above-mentioned jet mill or rotor rotation. There is a method of spheroidizing by repeating the method of pulverizing the material to be crushed by using the impact force of or by extending the processing time. Specialized spheroidizing devices include Hosokawa Micron's Faculty (registered trademark), Nobilta (registered trademark), Mechano-Fusion (registered trademark), Nippon Coke Industries' COMPOSI, Nara Machinery Co., Ltd. hybridization system, Earth Technica's Examples include kryptron orb and kryptron eddy.

  The carbon silicon-based negative electrode active material for a lithium ion secondary battery of the present invention thus obtained can be used as a negative electrode material for a lithium secondary battery.

  The negative electrode active material of the present invention is, for example, kneaded with an organic binder, a conductive additive and a solvent, and formed into a sheet shape, a pellet shape or the like, or applied to a current collector, and the current collector A negative electrode for a lithium secondary battery is formed by integrating with the body.

  As the organic binder, for example, polyethylene, polypropylene, ethylene propylene polymer, butadiene rubber, styrene butadiene rubber, butyl rubber, and a polymer compound having a large ion conductivity can be used. Polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide and the like can be used as the polymer compound having a high ionic conductivity. The content of the organic binder is preferably 3 to 20% by weight based on the whole negative electrode material. In addition to the organic binder, carboxymethyl cellulose, polysodium acrylate, other acrylic polymers, or fatty acid esters may be added as a viscosity modifier.

  The type of the conductive agent is not particularly limited, and may be any electron-conductive material that does not cause decomposition or alteration in the configured battery. Specifically, Al, Ti, Fe, Ni, Cu, Zn, Ag, Metal powder and metal fiber such as Sn, Si, or graphite such as natural graphite, artificial graphite, various coke powders, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, various resin fired bodies, etc. Etc. can be used. The addition amount of the conductive agent is 0 to 20% by weight, more preferably 1 to 10% by weight, based on the whole negative electrode material. When the amount of the conductive agent is small, the conductivity of the negative electrode material may be poor and the initial resistance tends to be high. On the other hand, an increase in the amount of conductive agent may lead to a decrease in battery capacity.

  There is no restriction | limiting in particular as said solvent, N-methyl- 2-pyrrolidone, a dimethylformamide, isopropanol, a pure water etc. are mentioned, There is no restriction | limiting in particular in the quantity. As the current collector, for example, a foil such as nickel or copper, a mesh, or the like can be used. The integration can be performed by a molding method such as a roll or a press.

  The negative electrode thus obtained has a cycle characteristic compared to a lithium secondary battery using conventional silicon as a negative electrode material by placing the positive electrode opposite to each other through a separator and injecting an electrolytic solution. In addition, a lithium secondary battery having excellent characteristics such as high capacity and high initial efficiency can be manufactured.

The material used for the positive electrode, for example _{LiNiO 2, LiCoO 2, LiMn 2} O 4, LiNi x Mn y Co 1-x-y O 2, LiFePO 4, Li 0.5 Ni 0.5 Mn 1.5 O 4 Li ₂ MnO ₃ —LiMO ₂ (M═Co, Ni, Mn), Li foil and the like can be used alone or in combination.

As the electrolytic solution, lithium salts such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ are used as non-aqueous solvents such as ethylene carbonate, diethyl carbonate, dimethoxyethane, dimethyl carbonate, tetrahydrofuran, and propylene carbonate. A so-called dissolved organic electrolyte solution can be used. Furthermore, ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used. The counter anion is not particularly limited, and examples thereof include BF ₄ ⁻ , PF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ^{− and the} like. The ionic liquid can be used by mixing with the organic electrolyte solvent described above. An SEI (solid electrolyte interface layer) forming agent such as vinylene carbonate or fluoroethylene carbonate can also be added to the electrolytic solution.

  In addition, a solid electrolyte obtained by mixing the above salts with polyethylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, or the like, or a derivative, mixture, or complex thereof can also be used. In this case, the solid electrolyte can also serve as a separator, and the separator becomes unnecessary. As the separator, for example, a nonwoven fabric mainly composed of polyolefin such as polyethylene or polypropylene, cloth, microporous film, or a combination thereof is used. can do.

  According to the present invention, the acid-treated graphite is electrolyzed by ensuring conductivity by enclosing Si by using graphite obtained by compressing expanded graphite having a bulk density of 130% or less during heat treatment at 1100 to 1200 ° C. Excellent cycle characteristics by suppressing the reaction between the liquid and silicon, and high initial efficiency can be obtained. In addition, a high bulk density negative electrode active material suitable for forming a high density negative electrode can be obtained by the production method of the present invention.

2 is a secondary electron image obtained by FE-SEM of a cross section of a negative electrode active material particle obtained in Example 1. FIG.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited to these Examples.

Example 1
Immerse natural graphite with a particle diameter of about 0.5 mm (width in the (200) plane direction) and a thickness of about 0.02 mm in a solution of concentrated sulfuric acid with 1 wt% sodium nitrate and 7 wt% potassium permanganate added for 24 hours. Then, it was washed with water and dried to obtain acid-treated graphite. This acid-treated graphite was put into a vibrating powder feeder, put on nitrogen gas at a flow rate of 10 L / min, passed through a quartz tube having a length of 1 m and an inner diameter of 11 mm heated to 1000 ° C. with an electric heater, and released from the end face to the atmosphere. A gas such as sulfurous acid was exhausted at the top, and expanded graphite was collected at the bottom in a stainless steel container. The bulk density of this expanded graphite was 2.2 g / L, and the specific surface area was 60 m ² / g. The bulk density of expanded graphite collected through acid-treated graphite in a mullite tube of the same size heated to 1150 ° C. with an electric heater is 2.0 g / L and the specific surface area is 65 m ² / g. The expansion coefficient was 110%. The appearance was coiled, and the graphite layer was peeled off by SEM observation, confirming the accordion shape.

20% by weight of chemical grade metal Si (purity 3.5N) having an average particle diameter D50 of 7 μm was mixed with methanol, and pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was performed for 5 hours. Was 0.30 μm, and a Si slurry having a BET surface area of 60 m ² / g during drying was obtained.

  24 g of the expanded graphite, 60 g of the Si slurry, 10 g of a resol type phenol resin, and 1 L of ethanol were placed in a stirring vessel, and mixed and stirred for 1 hour with a homomixer. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 60 ° C. with a warm bath while rotating, and evacuated with an aspirator to remove the solvent. Thereafter, it was spread on a bat in a draft and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 12 hours to obtain about 40 g of a dry mixture (light bulk density 80 g / L).

  This mixed dried product was passed through a three-roll mill twice, and granulated and consolidated to a particle size of about 2 mm and a light bulk density of 385 g / L.

Next, the granulated / consolidated product was placed in a new power mill and pulverized at 22000 rpm for 900 seconds while being cooled with water, and spheroidized at the same time to obtain a spheroidized powder having a light bulk density of 650 g / L. The obtained powder was put into an alumina boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace. Thereafter, a negative electrode active material having an average particle diameter D50 of 19 μm and a light bulk density of 761 g / L was obtained through a mesh having an opening of 45 μm. The specific surface area by the BET method using nitrogen gas was 50 m ² / g.

  In FIG. 1, the secondary electron image by the FE-SEM of the cross section which cut | disconnected the obtained negative electrode active material particle with Ar ion beam is shown. Inside the negative electrode active material particles, 0.05 to 0.2 μm long Si fine particles and a carbonaceous material (0.02) between the thin graphite layers (11) having a thickness of 0.02 to 0.2 μm (the gap is 0. 0). The structure sandwiched between (05-1 μm) spreads in a mesh pattern and is laminated. The carbonaceous material was in close contact with and covered the Si fine particles. Further, in the vicinity of the surface of the active material particles, the graphite thin layer (11) was curved to cover the active material particles.

  A lithium ion secondary battery using the obtained negative electrode active material was produced as follows.

"Preparation of negative electrode for lithium ion secondary battery"
The obtained negative electrode active material was 95.5% by weight (content in the total solid content, the same applies hereinafter), acetylene black 0.5% by weight as a conductive additive, and CMC 1.5% by weight as a binder. A negative electrode mixture-containing slurry was prepared by mixing 2.5% by weight of SBR and water.

The obtained slurry was applied to a copper foil having a thickness of 15 μm using an applicator so that the solid content was 3 mg / cm ² and dried at 110 ° C. in a stationary operation dryer for 0.5 hour. After drying, the lithium ion 2 was punched into a circle of 14 mmφ, uniaxially pressed under conditions of a pressure of 0.6 t / cm ² , and further heat-treated at 110 ° C. for 3 hours under vacuum to form a negative electrode mixture layer having a thickness of 30 μm. A negative electrode for a secondary battery was obtained.

"Production of evaluation cells"
In the glove box, the evaluation cell was prepared by dipping the negative electrode, a 24 mmφ polypropylene separator, a 21 mmφ glass filter, a 18 mmφ 0.2 mm thick metal lithium and a stainless steel foil of the base material into the electrolyte solution in the glove box. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolytic solution used was a mixture of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1 and LiPF ₆ dissolved to a concentration of 1.2 mol / L. The evaluation cell was further placed in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to the charge / discharge device.

"Evaluation conditions"
The evaluation cell was cycle tested in a constant temperature room at 25 ° C. Charging was performed after charging to 0.01 V at a constant current of 2 mA until the current value reached 0.2 mA at a constant voltage of 0.01 V. The discharge was performed at a constant current of 2 mA up to a voltage value of 1.5 V. The discharge capacity and the initial charge / discharge efficiency were the results of the initial charge / discharge test.

  In addition, the cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after 30 charge / discharge tests under the charge / discharge conditions with the initial discharge capacity.

Comparative Example 1
The same acid-treated graphite as in Example 1 was heated to 850 ° C. with an electric heater and passed through a quartz tube having a length of 1 m and an inner diameter of 11 mm, a bulk density of 2.9 g / L, a specific surface area of 34 m ² / g, and a 1150 ° C. product. A negative electrode active material, a negative electrode, and a cell for evaluation were prepared in the same manner as in Example 1 except that the expansion rate was 145%, and cell evaluation was performed.

  The results of Example 1 and Comparative Example 1 are shown in Table 1.

  As is clear from Table 1, the lithium ion secondary battery of Example 1 having a bulk density of 110% with respect to 1150 ° C. expanded graphite has a high capacity, high initial charge / discharge efficiency, and good charge / discharge cycle characteristics. It is.

  On the other hand, the lithium ion secondary battery of Comparative Example 1 having a high bulk density of 145% with respect to 1150 ° C. expanded graphite was inferior in initial capacity and cycle capacity maintenance rate.

  The negative electrode active material for lithium ion secondary battery and the method for producing the same of the present invention can be used for a lithium ion secondary battery that requires a high capacity and a long life.

11 Graphite thin layer inside negative electrode active material 12 Si fine particles inside negative electrode active material

Claims (4)

In a negative electrode active material for a lithium ion secondary battery comprising carbonaceous material and graphite and Si, natural graphite is immersed in concentrated sulfuric acid to which 1% by weight of sodium nitrate and 7% by weight of potassium permanganate are added for 24 hours. Thereafter, the acid-treated graphite obtained by washing and drying with water was heat-treated at 1100 to 1200 ° C. under nitrogen gas at a flow rate of 10 L / min. It is composed of expanded graphite having a carbonaceous material and graphite content of 5 to 90% by weight, and sandwiched between thin graphite layers having a thickness of 0.5 μm or less together with carbonaceous material in which Si fine particles are complexed. A negative electrode for a lithium ion secondary battery, characterized in that the structure extends in a laminated and / or network form, and the graphite thin layer is curved near the surface of the active material particles to cover the active material particles. Active material. The negative electrode active material is a composite particle having a round shape, an average particle diameter D50 of 1 to 40 μm, and a specific surface area by a BET method of 5 to 120 m ² / g, and a carbonaceous material covers at least the active material surface. The silicon-containing negative electrode active material for a lithium ion secondary battery according to claim 1, wherein: A process of mixing expanded graphite having a bulk density of 130% or less, a carbon precursor, and Si as raw materials based on the bulk density when heat-treating acid-treated graphite at 1100 to 1200 ° C. (100%). And a step of granulating and compacting, a step of forming a composite particle having a rounded shape by pulverization and spheronization, and a step of firing the composite particle in an inert gas atmosphere. The manufacturing method of the negative electrode active material for lithium ion secondary batteries of Claim 1 characterized by the above-mentioned. 4. The method for producing a silicon-containing negative electrode active material for a lithium ion secondary battery according to claim 3, wherein the temperature of the step of firing the composite particles in an inert atmosphere is 800 to 1000 ° C. 5.

Images