JP2016110969A - Negative electrode active material for lithium ion secondary battery, and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a negative electrode active material for a lithium ion secondary battery superior in cycle characteristics while keeping a high initial efficiency and a high battery capacity; and a method for manufacturing such a negative electrode active material.SOLUTION: A negative electrode active material for a lithium ion secondary battery comprises: a Si compound; and a carbonaceous material or a combination of a carbonaceous material and graphite. The negative electrode active material is obtained by the steps of: mixing a Si compound, a carbon precursor and as required, graphite powder; granulating and compacting the mixture; pulverizing the mixture to form composite particles; sintering the composite particles in an inert gas atmosphere; and performing a wind power classification of composite powder subjected to the pulverization and conglobation, or sintered powder.SELECTED DRAWING: None

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 EVs and PHEVs have been demanded to increase the 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% by weight or more.

  Patent Document 2 discloses metal-carbon composite particles in which metal particles are embedded in a plurality of phases of carbon, and the carbon contains graphite and amorphous carbon. The metal particles include Mg, Al, It is described that any one of Si, Zn, Ge, Bi, In, Pd, and Pt is used, and the average particle diameter is preferably 0.1 to 20 μm. Patent Document 3 discloses a so-called core shell in which the negative electrode active material includes graphite core particles, a carbon coating (shell) that covers the graphite core particles, and metal particles that are dispersed and positioned inside the carbon coating. The graphite core particles have an average particle size of 1 to 20 μm, the coating thickness of the carbon coating is 1 to 4 μm, and the metals alloyed with lithium include Cr, Sn, Si, Al, Mn, Ni , Zn, Co, In, Cd, Bi, Pb, and V, and at least one substance selected from the group consisting of V and an average particle size of 0.01 to 1.0 μm is preferable.

Furthermore, Patent Document 4 discloses a mixing step of obtaining a mixture by mixing expanded graphite or flaky graphite having a BET specific surface area of 30 m ² / g or more and a battery active material that can be combined with lithium ions, and adding a spherical shape to the mixture. A composite active material for a lithium secondary battery having a spheroidizing step of manufacturing a substantially spherical composite active material for a lithium secondary battery containing a battery active material that can be combined with graphite and lithium ions A method is disclosed, and the battery active material that can be combined with lithium ions contains at least one element selected from Si, Sn, Al, Sb, and In, and the average particle diameter is preferably 1 μm or less. Yes.

  In the method using the composite particles, the energy density of the negative electrode increases as the composite particles are densely packed in the negative electrode thin film, and the performance as a battery is improved. In addition, by uniformly and as isotropically filling the composite particles, lithium can be desorbed and inserted uniformly, local deterioration of the negative electrode can be avoided, and cycle life is improved. For example, Patent Document 5 discloses a negative electrode material for a lithium secondary battery including spherical graphite particles derived from scaly natural graphite particles, and describes that the circularity is preferably 0.85 or more. .

Japanese Patent Laid-Open No. 10-3920 JP 2000-272911 A JP 2010-129545 A Japanese Patent No. 5227483 JP2012-221951A

  The present invention relates to a negative electrode active material for a lithium ion secondary battery comprising Si or Si alloy (hereinafter collectively referred to as “Si compound”) and a carbonaceous material or a carbonaceous material and graphite in combination. Another object of the present invention is to provide a negative electrode active material that can provide a lithium ion secondary battery having high energy density and cycle life, and a method for producing the same.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the composite material is a negative electrode active material for a lithium ion secondary battery comprising a Si compound and a carbonaceous material or a carbonaceous material and graphite. By controlling the particle size and shape, it was found that a negative electrode active material providing a lithium ion secondary battery having high energy density and cycle life can be obtained, and the present invention has been completed.

  That is, the present invention relates to a negative electrode active material for a lithium ion secondary battery comprising Si or a Si alloy and a carbonaceous material or a carbonaceous material and graphite, wherein the average particle size (D50) of the negative electrode active material is 1 to 40 μm. And a negative active material for a lithium ion secondary battery characterized by being substantially spherical composite particles having an average circularity of 0.7 to 1.0.

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

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

  The Si alloy referred to in the present invention is an alloy containing Si as a main component. In the Si alloy, the element contained other than Si is preferably one or more elements of Groups 2 to 15 of the periodic table, and the selection and / or addition amount of the element having a melting point of the phase contained in the alloy of 900 ° C. or more. Is preferred.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, the average particle size (D50) of the Si compound is preferably 0.01 to 5 μm, more preferably 0.01 to 1 μm, particularly preferably 0.05 to. 0.6 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 5 μm, cracking is severely caused by expansion due to lithium insertion, and cycle deterioration tends to be severe. The average particle size (D50) is a volume average particle size measured with a laser particle size distribution meter.

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

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. )

  In the negative electrode active material for a lithium ion secondary battery of the present invention, when a carbonaceous material is contained, the content of the carbonaceous material is preferably 90 to 20% by weight, particularly preferably 40 to 20% by weight. When the content of carbonaceous material is less than 20% by weight, the carbonaceous material cannot cover the Si compound, and the conductive path becomes insufficient, and capacity deterioration is likely to occur severely. It is not obtained.

  The graphite referred to in the present invention is a crystal whose graphene layer is parallel to the c-axis, natural graphite obtained by refining ore, artificial graphite obtained by graphitizing the pitch of oil or coal, etc. There are oval or spherical, cylindrical or fiber shapes. In addition, these graphites are subjected to acid treatment, oxidation treatment, and then expanded by heat treatment. Part of the graphite layer is exfoliated to form an accordion shape, or a pulverized product of expanded graphite, or an ultrasonic layer or the like. Exfoliated graphene or the like can also be used. 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, and the thickness of the graphite particles is 1/5 or less of the average particle diameter (D50) of the active material. It is 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 present invention, graphite has a purity of 99.9% by weight or more, or an impurity amount of 1000 ppm or less, an S amount of 0.3% by weight or less, and / or a BET specific surface area of 40 m ² / g or less. Is preferred. If the purity is less than 99.9% by weight or the amount of impurities is more than 1000 ppm, the irreversible capacity due to the formation of SEI derived from impurities increases, so the initial charge / discharge efficiency, which is the discharge capacity with respect to the initial charge capacity, decreases. Tend. Moreover, since the irreversible capacity | capacitance similarly becomes high when S amount becomes higher than 0.3 weight%, initial charge / discharge efficiency becomes low. More preferably, the amount of S is preferably 0.1% by weight or less. If the BET specific surface area of graphite is higher than 40 m ² / g, the area that reacts with the electrolytic solution increases, so the initial charge / discharge efficiency is likely to be low.

  Impurities were analyzed by ICP emission spectroscopic analysis with the following 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co). , Cu, Mo, Pb, Sb, Se, Th, Tl, U). In addition, the amount of S is measured by ion chromatography (IC) measurement after filtering and filtering with an oxygen flask combustion method.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, when a carbonaceous material and graphite are contained, the respective contents are preferably 5 to 40% by weight and 20 to 80% by weight, and 8 to 30% by weight. A proportion of 40 to 70% by weight is particularly preferred. 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. On the other hand, when the graphite content is less than 20% by weight, the effect of graphite having higher conductivity than the carbonaceous material is not sufficient, and when it is more than 80% by weight, a sufficiently large capacity can be obtained compared to conventional graphite. Absent.

  The negative electrode active material for a lithium ion secondary battery of the present invention is a substantially spherical composite particle having an average particle diameter (D50) of 1 to 40 μm, preferably 2 to 30 μm, particularly preferably 2 to 20 μm. It is. When the average particle diameter (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 the Si compound is 1/5 or less of the average particle diameter of the negative electrode active material, and the carbonaceous material covers at least the active material surface.

The substantially spherical composite particles are those in which the corners of particles generated by pulverization, etc. are removed, spherical or spheroid shapes, discs or oval shapes with rounded corners, or deformed Including those having rounded corners, the circularity is 0.7 to 1.0. The circularity was measured by image analysis of a particle image taken with a scanning electron microscope. That is, when the projected area (A) and the perimeter (PM) of a particle are measured from a photograph and the area of a perfect circle having the same perimeter (PM) is (B), the circularity is defined as A / B. The When the radius of the true circle is r, PM = 2πr and B = πr ² are established, and from this, the circularity A / B = A × 4π / (PM) ² is calculated. Thereby, the average value of the substantially spherical particles excluding the flat fine particles having a minor axis length of less than 1 μm among the arbitrary 100 or more composite particles was defined as the average circularity of the composite particles. In addition, flat fine particles are those in which the corners of particles generated by pulverization, etc. are rounded, those having a round shape with a disc or oval shape, or those in which they are deformed and rounded in corners, etc. The short axis length of the particle image taken with a scanning electron microscope was less than 1 μm. The content of the flat fine particles was defined as the total projected area of the flat fine particles divided by the total projected area of all the particles. 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. Further, since the carbonaceous material covers at least the active material surface, lithium ions solvated in the electrolytic solution during the charge / discharge process are separated from the solvent on the surface of the carbonaceous material, and only lithium ions are Si compounds. Since it reacts with graphite and / or graphite, it becomes difficult to produce a decomposition product of the solvent, and the efficiency of charging and discharging is increased.

  When the average circularity of the substantially spherical composite particles decreases, the bulk density decreases, the packing density when the negative electrode is made, and the contact points and regions between the composite particles decrease. Due to the volume expansion / contraction, the probability that the electrical continuity is interrupted increases, and the cycle capacity retention rate tends to decrease. When the composite particles are composed of substantially spherical particles and flat composite fine particles, the flat fine particles fill the gaps between the substantially spherical particles as the content of the flat fine particles increases. Electrical conduction is maintained even in volume expansion and contraction. When the average particle size (D50) is 1 to 10 μm and the flat particles having a minor axis length of less than 1 μm measured by SEM image observation are contained in an amount of 1 wt% to 80 wt%, the negative electrode active material has an excellent cycle capacity. Indicates the maintenance rate. When the content of the flat fine particles is less than 1% by weight and / or when the circularity of the substantially spherical particles is less than 0.7, the effect of improving the cycle capacity retention rate is not recognized.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, the Si compound is sandwiched between graphite thin layers having a thickness of 0.2 μm or less together with the carbonaceous material, and the structure is laminated and / or It is preferable that the graphite thin layer is spread in a network shape, the graphite thin layer is curved near the surface of the active material particles to cover the active material particles, and the surface of the outermost layer is covered with the carbonaceous material.

  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. , Pulverized material, graphene delaminated by ultrasonic waves, or the like, or graphite made of a graphene layer (thickness 0.0003 μm) to several hundred layers (thickness 0.2 μm) produced by receiving a compressive force. It is a thin layer. When the thickness of the graphite thin layer is thinner, the Si compound sandwiched between the graphite thin layers and the carbonaceous material layer become thinner, and the transmission of electrons to the Si compound is improved, and the thickness exceeds 0.2 μm. The electron transfer effect of the graphite 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 present invention, the graphite thin layer is preferably curved near the surface of the active material particles to cover the active material particles. By adopting 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 reaction product is formed during charge and discharge, resulting in reduced efficiency. Is reduced.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, the Si compound content is preferably 10 to 80% by weight, and the carbonaceous material content is preferably 90 to 20% by weight.

  In the 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 20%. It is preferably ˜80% by weight.

In the lithium ion secondary battery negative electrode active material of the present invention, the BET specific surface area is preferably 0.5 to 80 m ² / g.

In the negative electrode active material for a lithium ion secondary battery of the present invention, the carbonaceous material is a carbon material formed by carbonizing a carbon precursor described later inside the negative electrode active material. Therefore, the lithium ions solvated in the electrolyte during charging and discharging have a structure that is difficult to directly contact the Si compound and / or graphite, and the BET specific surface area is 0.5 to 60 m ² / g. Moreover, since the reaction between the carbonaceous material and the electrolytic solution on the surface is kept small, the charge / discharge efficiency is further increased.

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

  The method for producing a negative electrode active material for a lithium ion secondary battery of the present invention comprises a step of mixing an Si compound, a carbon precursor, and, if necessary, a graphite, a step of granulating and densifying, and a pulverized composite It includes a step of forming particles and a step of firing the composite particles in an inert atmosphere.

  As the Si compound as a raw material, it is preferable to use a powder having an average particle diameter (D50) of 0.01 to 5 μm. In order to obtain a Si compound having a predetermined particle diameter, the above-described Si compound raw material (ingot, wafer, powder, etc.) is pulverized by a pulverizer, and in some cases, a classifier is used. 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. It can be finely pulverized by using a hammer mill, pin mill, disk mill that pulverizes the material to be crushed using the impact force of the colloid, a colloid mill that uses shear force, or a high-pressure wet-on-front collision disperser "Ultimizer". .

  The pulverization can be used for both wet and dry processes. For further fine pulverization, very fine particles can be obtained, for example, by using a wet bead mill and gradually reducing the diameter of the beads. 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. Adjust 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 a Si compound having a predetermined particle size, a method in which the Si compound is heated and evaporated by plasma or laser and solidified in an inert gas, or a CVD or plasma CVD using a gas raw material is used. These methods are suitable for obtaining ultrafine particles of 0.1 μm or less.

  The carbon precursor of the raw material is not particularly limited as long as it is a carbon-based compound mainly composed of carbon and becomes a carbonaceous material by heat treatment in an inert gas atmosphere. Petroleum pitch, coal pitch, synthetic pitch , Tars, 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. Can be used. Furthermore, in the step of forming composite particles described later, it is preferable to use a carbon precursor having a strong binding force in that the pulverized particles can be rebound to form substantially spherical composite particles. In particular, when the carbon precursor is a carbon-based compound and the weight average molecular weight (Mw) is 1000 or less, a strong binding force is expressed, which is preferable.

  As the raw material graphite, natural graphite, artificial graphite obtained by graphitizing the pitch of petroleum or coal, and the like can be used, and scaly, oval or spherical, cylindrical or fiber-like are used. In addition, these graphites are subjected to acid treatment, oxidation treatment, and then expanded by heat treatment, and a portion of the graphite layer is exfoliated to form an accordion shape, or a pulverized product of expanded graphite, or an ultrasonic wave, etc. Exfoliated graphene or the like can also be used. Expanded graphite or a pulverized product of expanded graphite is superior in flexibility to other graphites, and in the process of forming composite particles, which will be described later, the pulverized particles can be rebound to easily form substantially spherical composite particles. Can be formed. In view of the above, it is preferable to use expanded graphite or a pulverized product of expanded graphite. The raw material graphite is preliminarily adjusted to a size that can be used in the mixing process, and the particle size before mixing is 1 to 100 μm for natural graphite or artificial graphite, or 5 μm to 5 mm for expanded graphite or expanded graphite pulverized product, graphene Degree.

  Mixing with these Si compounds, carbon precursors, and, if necessary, graphite, when the carbon precursors are softened or liquefied by heating, the Si compounds, carbon precursors, and further necessary under heating Accordingly, it can be performed by kneading graphite. When the carbon precursor is dissolved in a solvent, the Si compound, the carbon precursor, and, if necessary, graphite are added to the solvent, and the Si compound and the carbon precursor are dissolved in the solution in which the carbon precursor is dissolved. Body, and if necessary, graphite can be dispersed and mixed, and then the solvent can be removed. 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, and 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 devices, the carbon precursor is solidified, or stirring in the process of solvent removal is continued for a certain amount of time, so that the Si compound, the carbon precursor, and, if necessary, the mixture with graphite are granulated and consolidated. The 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 600 to 1200 ° C. When the firing temperature is less than 600 ° 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. On the other hand, when the firing temperature exceeds 1200 ° C., there is a strong possibility that a reaction between the Si compound and the amorphous carbon or graphite derived from the carbon precursor occurs, and the discharge capacity tends to decrease.

  The method for producing a negative electrode active material for a lithium ion secondary battery of the present invention comprises a step of mixing and dispersing a Si compound, a carbon precursor, and optionally graphite in a solvent in which the carbon precursor is dissolved, It is preferable to include a step of densification, a step of forming composite particles having a round shape by pulverization and spheronization, and a step of firing the composite particles 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 method for producing a negative electrode active material for a lithium ion secondary battery according to the present invention comprises a step of mixing and dispersing a Si compound, a carbon precursor, expanded graphite or flaky graphite in a solvent in which the carbon precursor is dissolved, It is preferable to include a step of grain / thickening, a step of pulverizing and spheronizing to form substantially spherical composite particles, and a step of firing the composite particles in an inert atmosphere.

  Expanded graphite and flaky graphite are made from acid-treated graphite obtained by acid-treating and oxidizing natural graphite and artificial graphite. Expanded graphite is an acid-treated graphite that is expanded by heat treatment, and part of the graphite layer is peeled off to form an accordion. In addition, exfoliated graphite is pulverized, or graphene delaminated with ultrasonic waves or the like is flaky graphite. 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 negative electrode active material for lithium ion secondary batteries of the present invention thus obtained can be used as a negative electrode material for lithium secondary batteries.

  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 The negative electrode for a lithium secondary battery is integrated 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 auxiliary agent is not particularly limited, and may be any electron conductive material that does not cause decomposition or alteration in the constructed battery. Specifically, Al, Ti, Fe, Ni, Cu, Zn, Ag , Sn, Si and other metal powders and metal fibers, 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. Graphite or the like can be used. The addition amount of the conductive assistant 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 auxiliary agent is small, the negative electrode material may have poor conductivity, and the initial resistance tends to be high. On the other hand, an increase in the amount of conductive aid 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 with a separator interposed therebetween 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) or the like can be used alone or in combination.

As an electrolytic solution, a lithium salt such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF _{3 is used as} a non-aqueous solvent 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, by making the composite particles into substantially spherical particles having a high bulk density, a negative electrode active material suitable for forming a negative electrode having a high energy density and excellent cycle characteristics can be obtained. Moreover, excellent cycle characteristics and high initial efficiency can be obtained by suppressing the reaction between the electrolytic solution and silicon by reducing the expansion volume per particle by the fine silicon particles and by combining the carbonaceous material.

2 is a secondary electron image of the negative electrode active material particles obtained in Example 1 by SEM. 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. 2 is a secondary electron image of the negative electrode active material particles obtained in Example 2 by SEM. 3 is a secondary electron image by SEM of negative electrode active material particles obtained in Example 3. FIG. 4 is a secondary electron image obtained by FE-SEM of a cross section of a negative electrode active material particle obtained in Example 3. FIG. 4 is a secondary electron image of the negative electrode active material particles obtained in Example 4 by SEM. 7 is a secondary electron image of the negative electrode active material particles obtained in Example 5 by SEM. 4 is a secondary electron image of the negative electrode active material particles obtained in Example 6 by SEM. 2 is a secondary electron image of the negative electrode active material particles obtained in Comparative Example 1 by SEM. 3 is a secondary electron image of the negative electrode active material particles obtained in Comparative Example 2 by SEM. It is a secondary electron image by SEM of the negative electrode active material particles obtained in Comparative Example 3.

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

Example 1
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol at 25% by weight, and a finely pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. An ultrafine Si slurry having a D50) of 0.3 μm and a dry BET specific surface area of 60 m ² / g was obtained.

  Natural graphite having a particle diameter of 0.5 mm (width in the (200) plane direction) and a thickness of 0.02 mm is immersed in a solution obtained by adding 1 wt% sodium nitrate and 7 wt% potassium permanganate to concentrated sulfuric acid for 24 hours. Thereafter, it was washed with water and dried to obtain acid-treated graphite. This acid-treated graphite was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 10 L / min, passed through a mullite tube having a length of 1 m and an inner diameter of 11 mm heated to 850 ° 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 expanded graphite had a (200) plane width of 0.5 mm and maintained the original graphite value, but the thickness expanded to 4 mm and 200 times, the appearance was coiled, and the graphite layer was peeled off by SEM observation. The accordion was confirmed.

  24 g of the ultrafine Si slurry, 12 g of the expanded graphite, 5 g of a resol type phenolic resin (weight average molecular weight (Mw) = 460), 1.6 L of ethanol were placed in a stirring vessel, and after ultrasonic treatment for 15 minutes The mixture was stirred with a homomixer for 30 minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 65 ° 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 fume hood and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 2 days to obtain 20 g of a mixed dried product (light bulk density 67 g / L).

  This mixed dried product was passed through a three-roll mill twice, and granulated and consolidated to a particle size of 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 21000 rpm for 900 seconds while being cooled with water. At the same time, it was spheroidized 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 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.

  FIG. 1 shows an SEM image of the obtained negative electrode active material particles. The negative electrode active material particle graphite thin layer (12) is curved and has a substantially spherical shape covering the active material particle, the average circularity is 0.74, and the content of flat fine particles is 0% by weight. there were.

  In FIG. 2, the secondary electron image by the FE-SEM of the cross section which cut | disconnected the obtained negative electrode active material particle with the ion beam is shown. The negative electrode active material particles have a substantially spherical shape, and the inside of the negative electrode active material particles is a fine graphite layer having a thickness of 0.02 to 0.2 μm along with carbonaceous matter with Si fine particles having a length of 0.05 to 0.2 μm ( 11) (13) (gap is 0.05 to 1 μm), the structure sandwiched between the layers spreads and is laminated. The carbonaceous material was in close contact with and covered the Si fine particles. Further, near the surface of the active material particles, the graphite thin layer (12) was curved to cover the active material particles.

The BET specific surface area by the BET method using nitrogen gas was 50 m ² / g. In powder X-ray diffraction, a diffraction line corresponding to the (002) plane of graphite was observed, and d002 was 0.336 nm. In addition, a very broad diffraction line derived from amorphous carbonization of the carbonaceous material was observed in the vicinity thereof. A diffraction line corresponding to the (100) plane of Si was observed, and d002 was 0.314 nm.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.2% by weight (content in the total solid content, the same applies hereinafter), acetylene black 0.6% by weight as a conductive auxiliary, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.6 wt%, styrene butadiene rubber (SBR) 2.6 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.5 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 29 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 mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1. The cell for evaluation was further put in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

"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 2.2 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 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and 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.

Example 2
36 g of ultrafine Si slurry produced in the same manner as in Example 1, 18 g of expanded graphite, 7.5 g of resol type phenol resin (weight average molecular weight (Mw) = 490), and 2.4 L of ethanol were placed in a stirring vessel. And sonication for 15 minutes. Thereafter, the mixed solution was transferred to a rotary evaporator, heated to 50 ° 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 fume hood and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 2 days to obtain 32 g of a mixed dried product (light bulk density 66 g / L).

  This mixed dried product was passed through a three-roll mill twice, and granulated and consolidated to a particle size of 2 mm and a light bulk density of 340 g / L.

  Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 900 seconds while cooling with water, and at the same time, spheroidized to obtain a spheroidized powder having a light bulk density of 490 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 9 μm and a light bulk density of 567 g / L was obtained through a mesh having an opening of 45 μm.

FIG. 3 shows an SEM image of the obtained negative electrode active material. The negative electrode active material particle graphite thin layer (12) is curved and has a substantially spherical shape covering the active material particle, the average circularity is 0.77, and the content of flat fine particles is 0% by weight. The BET specific surface area by the BET method using nitrogen gas was 47 m ² / g. In powder X-ray diffraction, a diffraction line corresponding to the (002) plane of graphite was observed, and d002 was 0.336 nm. In addition, a very broad diffraction line derived from amorphous carbonization of the carbonaceous material was observed in the vicinity thereof. A diffraction line corresponding to the (100) plane of Si was observed, and d002 was 0.314 nm.

  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”
With respect to 90.9 wt% of the obtained negative electrode active material (content in the total solid content, the same applies hereinafter), acetylene black 0.4 wt% as a conductive auxiliary and polyvinylidene fluoride (PVDF) as a binder A negative electrode mixture-containing slurry was prepared by mixing 8.7% by weight of NMP.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 1.8 mg / cm ^2, and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 17 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, 16 mmφ and 0.2 mm thick metal lithium, and a stainless steel foil of the base material into the electrolyte solution. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolytic solution used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1. 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 with a constant current of 1.4 mA until the current value reached 0.2 mA with a constant voltage of 0.01 V. The discharge was performed at a constant current of 1.4 mA up to a voltage value of 1.5V. The initial discharge capacity and 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.

Example 3
Chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol in an amount of 21% by weight and subjected to fine grinding wet bead mill using zirconia beads having a diameter of 0.3 mm for 6 hours. D50) An ultrafine Si slurry having a diameter of 0.3 μm and a dry BET specific surface area of 100 m ² / g was obtained.

  Acid-treated natural graphite having a particle diameter of 0.3 mm (width in the (200) plane direction) and a thickness of 10 μm was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 12 L / min, and heated to 850 ° C. with an electric heater. It was passed through a mullite tube having a length of 1 m and an inner diameter of 20 mm, discharged from the end face to the atmosphere, and 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 expanded graphite had a (200) plane width of 0.3 mm and maintained the original graphite value, but the thickness expanded 2.4 times to 2.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

  95.7 g of the ultrafine Si slurry, 37.5 g of the expanded graphite, 23.5 g of a resol type phenolic resin (weight average molecular weight (Mw) = 370), and 5 L of ethanol were placed in a stirring vessel, and 60 minutes with a homomixer. Mix and stir for minutes. 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 fume hood and dried for 2 hours while being evacuated, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 80 g of a mixed dried product (light bulk density 87 g / L).

  This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a light bulk density of 528 g / L.

Next, this granulated / consolidated product is placed in a new power mill, water-cooled and pulverized at 21000 rpm for 900 seconds. At the same time, it is spheroidized, and the resulting bulk density is 633 g / L. While flowing nitrogen gas in a furnace, firing was performed at a maximum temperature of 900 ° C. for 1 hour. Thereafter, a mesh having an opening of 45 μm was passed through to obtain composite particles having an average particle diameter (D50) of 17.5 μm and a light bulk density of 807 g / L.

The composite particles are put into an air classifier (ATP-20 manufactured by Hosokawa Micron), classified at a classifier rotation speed of 60,000 rpm and an air volume of 8 m ³ / m, and fine particles are captured by a dust collecting bag filter, and the average particle diameter A negative electrode active material having a (D50) of 4.8 μm and a light bulk density of 204 g / L was obtained. FIG. 4 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.75, and the inclusion of flat fine particles The rate was 77.9% by weight. The BET specific surface area by the BET method using nitrogen gas was 56 m ² / g.

  In FIG. 5, the secondary electron image by the FE-SEM of the cross section which cut | disconnected the obtained negative electrode active material particle with the ion beam is shown. The negative electrode active material particles are composed of substantially spherical particles and flat fine particles, and inside the substantially spherical particles, a structure in which Si fine particles are sandwiched between carbonaceous materials and a graphite thin layer spreads in a network shape and is laminated. The carbonaceous material was in close contact with and covered the Si fine particles. Further, near the surface of the active material particles, the graphite thin layer was curved to cover the active material particles. Although the flat fine particles have a small number of layers, they have a structure similar to that of the substantially spherical particles, and the surface thereof is covered with a thin graphite layer or a carbon material.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.4% by weight (content in the total solid content; the same applies hereinafter), acetylene black 0.5% by weight as a conductive additive, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.6 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 1.5 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 16 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 electrolyte used was a mixture of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1, FEC (fluoroethylene carbonate), and LiPF ₆ dissolved to a concentration of 1.2 vol / L. The cell for evaluation was further put in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

  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 2.2 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 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and 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.

Example 4
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol in an amount of 24% by weight and subjected to a fine pulverization wet bead mill using zirconia beads having a diameter of 0.3 mm for 6 hours. D50) An ultrafine Si slurry having a diameter of 0.3 μm and a dry BET specific surface area of 100 m ² / g was obtained.

  Acid-treated natural graphite having a particle diameter of 0.3 mm (width in the (200) plane direction) and a thickness of 10 μm was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 12 L / min, and heated to 850 ° C. with an electric heater. It was passed through a mullite tube having a length of 1 m and an inner diameter of 20 mm, discharged from the end face to the atmosphere, and 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 expanded graphite had a (200) plane width of 0.3 mm and maintained the original graphite value, but the thickness expanded 2.4 times to 2.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

  98.8 g of the ultrafine Si slurry, 48.0 g of the expanded graphite, 20.0 g of a resol type phenol resin (weight average molecular weight (Mw) = 370), and 5.9 L of ethanol were placed in a stirring vessel. And stirred for 90 minutes. 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 fume hood and dried for 2 hours while evacuating, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 86 g of a mixed dried product (light bulk density 77 g / L).

  This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a light bulk density of 303 g / L.

  Next, the granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 900 seconds while being cooled with water, and simultaneously spheronized to obtain a spheroidized powder having a light bulk density of 478 g / L. The obtained powder was put into a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace. Thereafter, a mesh having an opening of 45 μm was passed through to obtain composite particles having an average particle diameter (D50) of 16.5 μm and a light bulk density of 573 g / L.

This composite particle is put into an air classifier (ATP-50 manufactured by Hosokawa Micron), fine particle is captured by a classifier rotation speed of 18,000 rpm, an air volume of 1.6 m ³ / min, and a cyclone collector. A negative electrode active material having a diameter (D50) of 5.9 μm and a light bulk density of 293 g / L was obtained. FIG. 6 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.74, and the inclusion of flat fine particles The rate was 1.8% by weight.
The BET specific surface area by the BET method using nitrogen gas was 30 m ² / g.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.6% by weight (content in the total solid content; the same applies hereinafter), acetylene black 0.5% by weight as a conductive auxiliary, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.4 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 2.5 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 21 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 electrolyte used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, FEC (fluoroethylene carbonate) as the additive, and LiPF ₆ dissolved to a concentration of 1.2 vol / L. did. The cell for evaluation was further put in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

  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 2.2 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 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and 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.

Example 5
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol at 25% by weight, and a finely pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. D50) An ultrafine Si slurry with 0.4 μm and a dry BET specific surface area of 60 m ² / g was obtained.

  Vibrating powder feeder using acid-treated high-purity natural graphite having a particle diameter of 0.15 mm (width in the (200) plane direction), a thickness of 10 μm, a purity of 99.9% by weight or more, and an S content of 0.3% by weight or less Put into nitrogen gas at a flow rate of 12 L / min and pass through a mullite tube with a length of 1 m and an inner diameter of 11 mm heated to 850 ° C. with an electric heater, discharged from the end face to the atmosphere, and a gas such as sulfurous acid exhausted to the top, Expanded graphite was collected in a stainless steel container at the bottom. The width of the expanded graphite in the (200) plane direction was 0.15 mm and the original graphite value was maintained, but the thickness expanded to 40 mm, 0.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

  466.4 g of the ultrafine Si slurry, 426.2 g of the expanded graphite, 86.5 g of a resol type phenolic resin (weight average molecular weight (Mw) = 460), and 6.4 L of ethanol are placed in a stirring vessel. And stirred for 26 minutes. 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 fume hood and dried for 2 hours while being evacuated, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 588 g of a mixed dried product (light bulk density 170 g / L).

  This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a lightly packed compact 308 g / L.

  Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 900 seconds while cooling with water, and at the same time, spheronized to obtain a spheroidized powder having a light bulk density of 437 g / L.

The obtained powder was put in a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to obtain composite particles having a light bulk density of 549 g / L. The composite particles are put into an air classifier (ATP-50 manufactured by Hosokawa Micron), and the fine particle is captured by a cyclone collector with a classifier rotating speed of 5000 rpm and an air volume of 1.6 m ³ / min, and the average particle diameter (D50) Was 10.0 μm, and a lightly loaded bulk density of 558 g / L was obtained. FIG. 7 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved to cover the active material particles, flat fine particles are contained, the average circularity is 0.70, and the inclusion of flat fine particles The rate was 1.2% by weight. The BET specific surface area by the BET method using nitrogen gas was 29.0 m ² / g.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.4% by weight (content in the total solid content; the same applies hereinafter), acetylene black 0.5% by weight as a conductive additive, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.6 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.0 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 22 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 electrolyte used is a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, FEC (fluoroethylene carbonate) as the additive, and LiPF ₆ dissolved to a concentration of 1.2 mol / L. did. The cell for evaluation was further put in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

  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 2.2 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 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and 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.

Example 6
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol at 25% by weight, and a finely pulverized wet bead mill using zirconia beads having a diameter of 0.3 mm was performed for 6 hours. D50) An ultrafine Si slurry with 0.4 μm and a dry BET specific surface area of 60 m ² / g was obtained.

  Vibrating powder feeder using acid-treated high-purity natural graphite having a particle diameter of 0.15 mm (width in the (200) plane direction), a thickness of 10 μm, a purity of 99.9% by weight or more, and an S content of 0.3% by weight or less Put into nitrogen gas at a flow rate of 12 L / min and pass through a mullite tube with a length of 1 m and an inner diameter of 11 mm heated to 850 ° C. with an electric heater, discharged from the end face to the atmosphere, and a gas such as sulfurous acid exhausted to the top, Expanded graphite was collected in a stainless steel container at the bottom. The width of the expanded graphite in the (200) plane direction was 0.15 mm and the original graphite value was maintained, but the thickness expanded to 40 mm, 0.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

  145.7 g of the ultrafine Si slurry, 133.2 g of the expanded graphite, 27 g of resol type phenolic resin (weight average molecular weight (Mw) = 460), 2 L of ethanol were placed in a stirring vessel, and 8.25 using an in-line mixer. Mix and stir for minutes. 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 fume hood and dried for 2 hours while being evacuated, passed through a mesh with a mesh opening of 2 mm, and further dried for 1 day to obtain 188 g of a mixed dried product (light bulk density 132 g / L).

  This mixed dried product was passed through a three-roll mill twice, passed through a sieve having an opening of 1 mm, and granulated and consolidated to a lightly packed compact 235 g / L.

  Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 900 seconds while being cooled with water. At the same time, it was spheroidized to obtain a spheroidized powder having a light bulk density of 476 g / L.

The obtained powder was put into a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace to obtain composite particles having a light bulk density of 641 g / L. Thereafter, a negative active material having an average particle diameter (D50) of 17.6 μm and a light bulk density of 629 g / L was obtained through a mesh having an opening of 45 μm. FIG. 8 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.72, and the inclusion of flat fine particles The rate was 1.1% by weight. The BET specific surface area by the BET method using nitrogen gas was 37 m ² / g.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.4% by weight (content in the total solid content; the same applies hereinafter), acetylene black 0.5% by weight as a conductive additive, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.6 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.6 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 36 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 electrolyte used is a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, FEC (fluoroethylene carbonate) as the additive, and LiPF ₆ dissolved to a concentration of 1.2 mol / L. did. The cell for evaluation was further put in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

  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 2.2 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 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and 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
In the mixing step, 36 g of ultrafine Si slurry, 18 g of expanded graphite, 7.5 g of resol type phenol resin (weight average molecular weight (Mw) = 3.6 × 10 ³ ), and 2.4 L of ethanol are put in a stirring vessel. A spherical powder having an average particle size (D50) of 4.2 μm and a light bulk density of 250 g / L was obtained in the same manner as in Example 2 except that the steps were performed.

  A negative electrode active material, a negative electrode, and an evaluation cell were prepared in this order from the spheroidized powder in the same manner as in Example 2, and the cells were evaluated.

FIG. 9 shows an SEM image of the obtained negative electrode active material. The particles were not substantially spherical, but were fine powder and flat particles, the average circularity was 0.65, and the content of flat particles was 0.3% by weight. The BET specific surface area by the BET method using nitrogen gas was 33 m ² / g.

  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”
With respect to 90.8% by weight of the obtained negative electrode active material (content in the total solid content, the same shall apply hereinafter), acetylene black 0.5% by weight as a conductive assistant, PVDF 8.7% by weight as a binder, NMP was mixed to prepare a negative electrode mixture-containing slurry.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 2.2 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 17 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, 16 mmφ and 0.2 mm thick metal lithium, and a stainless steel foil of the base material into the electrolyte solution. After that, the layers were laminated in this order, and finally the lid was screwed in. The electrolytic solution used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1. 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 with a constant current of 1.4 mA until the current value reached 0.2 mA with a constant voltage of 0.01 V. The discharge was performed at a constant current of 1.4 mA up to a voltage value of 1.5V. The initial discharge capacity and 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 2
A chemical grade metal Si (purity 3N) having an average particle size (D50) of 7 μm was mixed with ethanol in an amount of 23% by weight and subjected to a fine pulverization wet bead mill using zirconia beads having a diameter of 0.3 mm for 6 hours. D50) An ultrafine Si slurry having a diameter of 0.3 μm and a dry BET specific surface area of 100 m ² / g was obtained.

  Acid-treated natural graphite having a particle diameter of 0.3 mm (width in the (200) plane direction) and a thickness of 10 μm was placed in a vibrating powder feeder, placed on nitrogen gas at a flow rate of 12 L / min, and heated to 850 ° C. with an electric heater. It was passed through a mullite tube having a length of 1 m and an inner diameter of 20 mm, discharged from the end face to the atmosphere, and 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 expanded graphite had a (200) plane width of 0.3 mm and maintained the original graphite value, but the thickness expanded 2.4 times to 2.4 mm, the appearance was coiled, and the graphite layer was observed by SEM observation. Was peeled off and confirmed to be in the form of an accordion.

  102.6 g of the ultrafine Si slurry, 48.0 g of the expanded graphite, 20.0 g of a resol-type phenol resin (weight average molecular weight (Mw) = 370), and 5.9 L of ethanol were placed in a stirring vessel. And stirred for 90 minutes. 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 fume hood and dried for 2 hours while being evacuated, passed through a 2 mm mesh, and further dried for 1 day to obtain 86 g of a mixed dried product (light bulk density 66 g / L).

  This mixed dried product is passed through a three-roll mill twice, passed through a 1 mm sieve, granulated and compacted to a light bulk density of 287 g / L, and then heat-treated at 150 ° C. for 2 hours in the atmosphere. It was.

  Next, the granulated / consolidated product subjected to the heat treatment was placed in a new power mill and pulverized at 21000 rpm for 300 seconds while being cooled with water, and spheroidized at the same time to obtain a spheroidized powder having a light bulk density of 225 g / L. The obtained powder was put into a quartz boat and fired at a maximum temperature of 900 ° C. for 1 hour while flowing nitrogen gas in a tubular furnace, and then composite particles were obtained through a mesh having an opening of 45 μm.

This composite particle is put into an air classifier (ATP-50 manufactured by Hosokawa Micron), fine particle is captured by a classifier rotation speed of 18,000 rpm, an air volume of 1.6 m ³ / min, and a cyclone collector. A negative electrode active material having a diameter (D50) of 4.3 μm and a light bulk density of 270 g / L was obtained. FIG. 10 shows an SEM image of the obtained negative electrode active material particles. In addition to the substantially spherical particles in which the negative electrode active material particle graphite thin layer (12) is curved and covers the active material particles, flat fine particles are contained, the average circularity is 0.56, and the inclusion of flat fine particles The rate was 30.9% by weight. The BET specific surface area by the BET method using nitrogen gas was 47 m ² / g.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained negative electrode active material is 95.5% by weight (content in the total solid content; the same shall apply hereinafter), acetylene black 0.5% by weight as a conductive auxiliary, and carboxymethyl cellulose (CMC) 1 as a binder. A negative electrode mixture-containing slurry was prepared by mixing 0.5 wt%, styrene butadiene rubber (SBR) 2.5 wt%, and water.

The obtained slurry was applied to a copper foil having a thickness of 18 μm using an applicator so that the solid content was 3.1 mg / cm ² and dried at 110 ° C. in a vacuum dryer for 0.5 hours. . 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 28 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 electrolyte used was a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1, FEC (fluoroethylene carbonate) as the additive, and LiPF ₆ dissolved to a concentration of 1.2 vol / L. did. The cell for evaluation was further put in a sealed glass container containing silica gel, and an electrode through a silicon rubber lid was connected to a charge / discharge device (SM-8 manufactured by Hokuto Denko).

  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 2.2 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 2.2 mA up to a voltage value of 1.5V. The initial discharge capacity and 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 3
When the baked powder obtained in Example 3 was passed through a sieve having an opening of 45 μm, the obtained particles of 45 μm or more were passed through a mesh having an opening of 53 μm, and the average particle diameter (D50) was 54.8 μm. Composite particles having a light bulk density of 935 g / L were obtained. FIG. 11 shows an SEM image of the obtained negative electrode active material particles. The negative electrode active material particle graphite thin layer (12) is curved and has a substantially spherical shape covering the active material particle, the average circularity is 0.73, and the content of flat fine particles is 0% by weight. there were. The BET specific surface area by the BET method using nitrogen gas was 92 m ² / g.

  Table 1 shows the results of Examples 1 to 6 and Comparative Examples 1 to 3.

表１から明らかなように、実施例１〜２のリチウムイオン２次電池は、高容量で、初期充放電効率が高く、充放電サイクル特性が良好である。

As is clear from Table 1, the lithium ion secondary batteries of Examples 1 and 2 have a high capacity, high initial charge / discharge efficiency, and good charge / discharge cycle characteristics.

  The lithium ion secondary battery using the negative electrode active material containing the flat fine particles of Examples 3 to 6 in an amount of 1% to 80% has better charge / discharge cycle characteristics than Examples 1-2. In Examples 5 and 6, high-purity graphite is used as a raw material, so that the initial charge / discharge efficiency is a higher value.

  On the other hand, since the average circularity of the lithium ion secondary battery of Comparative Example 1 is low, the cycle retention rate is inferior to that of Examples 1-6. Although the lithium ion secondary battery of Comparative Example 2 contains an appropriate amount of flat particles, the average circularity is low, so that the charge / discharge cycle characteristics are inferior to those of Examples 3 to 5 containing a specific amount of flat particles. In Comparative Example 3, the composite particles were too large in size, so that the electrode could not be formed and evaluation was impossible.

  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 Thin graphite layer inside negative electrode active material 12 Thin graphite layer near negative electrode active material surface 13 Si fine particle and carbonaceous material layer

Claims (16)

In the negative electrode active material for a lithium ion secondary battery comprising Si or Si alloy, carbonaceous material or carbonaceous material and graphite, the average particle diameter (D50) of the negative electrode active material is 1 to 40 μm, and the average A negative electrode active material for a lithium ion secondary battery, wherein the negative electrode active material is a substantially spherical composite particle having a circularity of 0.7 to 1.0. The average particle diameter (D50) of the negative electrode active material is 1 to 10 μm, and the composite particles include 1 to 80% by weight of flat fine particles having a minor axis length of less than 1 μm measured by SEM image observation. The negative electrode active material for a lithium ion secondary battery according to claim 1. 3. The lithium ion secondary battery according to claim 1, wherein an average particle diameter (D50) of the Si or Si alloy is 0.01 to 5 μm, and a carbonaceous material covers at least an active material surface. Negative electrode active material. 3. The lithium ion secondary battery according to claim 1, wherein an average particle diameter (D50) of the Si or Si alloy is 0.01 to 1 μm, and a carbonaceous material covers at least an active material surface. Negative electrode active material. The Si or Si alloy has a structure sandwiched between carbonaceous materials and a thin graphite layer having a thickness of 0.2 μm or less, and the structure spreads in a laminated and / or network shape, and the thin graphite layer is an active material. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the active material particles are curved in the vicinity of the surface of the particles, and the surface of the outermost layer is covered with a carbonaceous material. Negative electrode active material. The graphite is composed of 26 elements (Al, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, V, Zn, Zr, Ag, As, Ba, Be, Cd, Co, Cu, by ICP emission spectroscopy. , Mo, Pb, Sb, Se, Th, Tl, U) Ion chromatography (IC) measurement by oxygen flask combustion method when the purity determined from the semi-quantitative value of impurities is 99.9% by weight or more, or the impurity amount is 1000 ppm or less amount S 0.3 wt% or less by law, and / or a negative electrode for a lithium ion secondary battery according to any one of claims 1 to 5, equal to or less than the BET specific surface area of 40 m 2 / ^g Active material. The lithium ion 2 according to any one of claims 1 to 6, wherein a content of the Si or Si alloy is 10 to 80% by weight, and a content of the carbonaceous material is 90 to 20% by weight. Negative electrode active material for secondary battery. The content of the Si or Si alloy 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 20 to 80% by weight. 6. The negative electrode active material for a lithium ion secondary battery according to any one of 6 above. BET specific surface area is 0.5-80 m < ² > / g, The negative electrode active material for lithium ion secondary batteries of any one of Claims 1-8 characterized by the above-mentioned. A step of mixing Si or an Si alloy, a carbon precursor, and graphite, a step of granulating and compacting, a step of grinding and spheroidizing the mixture to form substantially spherical composite particles, and the composite particles The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 9, further comprising a step of firing in an inert gas atmosphere. The method for producing a negative electrode active material for a lithium ion secondary battery according to claim 10, wherein in the spheronization treatment, the pulverized particles are rebound to form substantially spherical composite particles. A step of mixing Si or an Si alloy, a carbon precursor, and graphite, a step of granulating and compacting, a step of grinding and spheroidizing the mixture to form substantially spherical composite particles, and the composite particles The lithium ion 2 according to any one of claims 1 to 9, comprising a step of firing in an inert gas atmosphere and a step of air classification of the pulverized and spherically processed powder or the fired powder. The manufacturing method of the negative electrode active material for secondary batteries. In the spheronization treatment, the pulverized particles are rebound to form substantially spherical composite particles and flat fine particles, or the substantially spherical composite particles and flat fine particles are mixed, stirred, and classified. Item 13. A method for producing a negative electrode active material for a lithium ion secondary battery according to Item 12. The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 10 to 13, wherein the carbon precursor is a carbon-based compound having a weight average molecular weight (Mw) of 1000 or less. The method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 10 to 14, wherein the graphite is expanded graphite or flaky graphite. The temperature of the process which bakes a composite particle in inert atmosphere is 600-1200 degreeC, The manufacture of the negative electrode active material for lithium ion secondary batteries of any one of Claims 10-15 characterized by the above-mentioned. Method.

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