JP2015219989A - Negative electrode active material for lithium ion secondary battery and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for a lithium ion secondary battery, maintaining high battery capacity and excellent in cycle characteristics and coulomb efficiency characteristics.SOLUTION: A negative electrode active material for a lithium ion secondary battery is obtained by adding a carbonized material to a complex compound which comprises Si or a Si alloy having an average particle size of 0.01-5 μm and carbonaceous matter or carbonaceous matter and graphite, and has an average particle size of 1-40 μm.

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 become more sophisticated and energy-efficient vehicles such as HEVs, PHEVs, and EVs become more popular, the demand for lithium ion secondary batteries has become more complex. For example, in mobile devices, a material having a high durability and a high capacity is demanded at a deep charging depth (DOD) for the purpose of use. On the other hand, in-vehicle applications such as EV, since the charge depth can be controlled, a battery material having a high capacity and a high cycle characteristic is desired at a place where the charge depth is relatively shallow.

  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 actively conducted.

  On the other hand, it is known that an active material made of a metal material capable of inserting and extracting lithium ions causes volume expansion / contraction due to insertion / extraction of lithium into / from the metal material during charge / discharge. Yes. Therefore, it is said that the active material is made finer due to stress accompanying Li insertion / desorption, and the conductivity is cut due to structural breakdown of the negative electrode material. Therefore, the negative electrode using these metal materials has a problem that the battery life is remarkably reduced by repeated use.

  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 improved cycle characteristics compared to the case where these materials are used alone as a negative electrode material because these metal materials are alloyed with lithium and the conductivity is ensured by carbonaceous materials and graphite even if they are refined. It is known to do. For example, Patent Document 1 discloses a carbonaceous material and at least one metal selected from the group consisting of Ag, Zn, Al, Ga, In, Si, Ge, Sn, and Pb having an average particle diameter of 10 nm to 200 nm. It is disclosed that the nanometal fine particles comprising an element are contained, and the nanometal fine particles are contained in an amount of 3 wt% or more and 20 wt% or less with respect to the total weight of the carbonaceous material, the graphite material, and the nanometal fine particles. ing. In Patent Document 2, carbonaceous particles containing at least one or more selected from graphite or carbon black are arranged on the surface of the composite particles containing graphite particles, Si fine particles, and amorphous carbon (A). It is disclosed that the carbonaceous material negative electrode active material is coated with amorphous carbon (B). Further, in Patent Document 3, composite particles containing at least silicon and carbon around graphite particles and having a particle diameter smaller than that of the graphite particles are dispersed and arranged, and the graphite particles and the composite particles are amorphous. It is described that it was covered with a carbon film. Further, in Patent Document 4, a diffraction peak attributed to Si (111) is observed in X-ray diffraction, and the silicon crystal size determined by the sealer method based on the width of the diffraction line is 1 to It is disclosed that the surface of particles having a structure in which silicon microcrystals of 500 nm are dispersed in a silicon compound is coated with carbon.

In Patent Documents 1 and 2, fine particles of metal Si are uniformly mixed in graphite or a carbonaceous material, or the surface of Si fine particles is coated with amorphous carbon, but sufficient cycle characteristics cannot be ensured. In order to improve the cycle characteristics, Patent Documents 3 and 4 employ a method of depositing Si microcrystals in the SiOx phase and coating the particle surface with carbon. As a result, although the cycle characteristics are improved, as shown in Non-Patent Document 1, the SiOx phase reacts with Li and changes to electrochemically inactive Li ₄ SiO ₄ , so that the initial Coulomb efficiency is significantly reduced. To do.

JP 2004-213927 A JP 2008-277232 A Japanese Patent No. 4308446 Japanese Patent No. 3952180

M.M. Yamada Battery Technology 22, 72 (2010)

  The present invention relates to lithium ion 2 in which Si or Si alloy (hereinafter also referred to as “Si compound”) and a carbonaceous material or a composite material containing carbonaceous material and graphite are mixed and mixed with carbonized material in a highly dispersed state. An object is to provide a negative electrode active material for a secondary battery, a negative electrode active material having a large discharge capacity, a long cycle life even when an Si compound is used, and a lithium ion secondary battery having high Coulomb efficiency, and a method for producing the same.

  As a result of intensive studies to solve the above problems, the present inventors have surprisingly found that in 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, After compounding Si compound, carbonaceous material, and graphite, lithium ion with high capacity, long cycle life, and high coulomb efficiency even when Si compound is used by adding carbonized material and mixing with high dispersion. The inventors have found that a negative electrode active material that gives a secondary battery can be obtained, and have completed the present invention.

  That is, the present invention adds a carbonized product to Si or an Si alloy having an average particle size of 0.01 to 5 μm and a composite material having an average particle size of 1 to 40 μm made of carbonaceous material or carbonaceous material and graphite. A negative electrode active material for a lithium ion secondary battery.

  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%, a chemical grade metal silicon having a purity of 2 to 4N, a chlorinated and purified by distillation using 4N, a single crystal growth method Such as ultra-high-purity single-crystal silicon that has undergone a deposition process, wafer polishing and cutting waste generated in the semiconductor manufacturing process, waste wafers that have become defective in the process, etc. There is no particular limitation.

  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 0.01 to 5 μm, and more preferably 0.05 to 0.5 μm. If it is smaller than 0.01 μm, the capacity and initial efficiency due to surface oxidation are drastically reduced, and if it is larger than 5 μm, cracking is severely caused by expansion due to lithium insertion, and cycle deterioration tends to be severe. The average particle diameter (D50) is a volume average particle diameter measured with a laser particle size distribution meter.

  The average particle diameter of the composite particles composed of the Si compound of the present invention and a carbonaceous material or a carbonaceous material and graphite is 1 to 40 μm. When the average particle diameter of the composite particles is less than 1 μm, it is bulky and it becomes difficult to produce a high-density electrode, and there is a difficulty in handling because it is a fine powder with a small particle diameter. If the particle diameter exceeds 40 μm, the sheet cannot be produced unless the coating thickness of the negative electrode is increased, so that the electrode sheet resistance increases and the discharge capacity and cycle characteristics decrease.

  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 heat treatment exceeding 2000 ° C. and hard-graphitizable carbon (hard carbon) that is difficult to graphitize. )

  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.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, the amount of the Si compound present in the composite is preferably 10% by weight or more and 80% by weight or less, and more 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.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, the addition amount of the carbonized product added to and mixed with the composite product is 0.5 wt% or more and 99.5 wt% with respect to the total amount of the composite product and the added carbonized product. The following is preferable, More preferably, it is 10 to 90 weight%, More preferably, it is 20 to 80 weight%.

The carbonized material to be added in the present invention is not particularly limited, but has a conductivity, a charge / discharge capacity as a negative electrode active material due to insertion / extraction of Li, and a negative electrode excellent in cycle characteristics and coulomb efficiency. Material is preferred. Specific examples include the carbonaceous material and graphite. When such a material is used alone, the electrical resistivity is 1 × 10 ⁻⁵ Ωm or more, the initial discharge capacity is about 350 mAh / g, the cycle capacity maintenance rate after 50 cycles is 95% or more, and the initial Coulomb efficiency Is preferably 85% or more, and the 10th Coulomb efficiency is 98% or more, and the 50th Coulomb efficiency is 99% or more.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, the carbonized material to be added preferably has a particle size of 0.1 to 40 μm. When the particle diameter is less than 0.1 μm, it is very fine and difficult to mix with the composite particles. If the particle diameter exceeds 40 μm, the sheet cannot be produced unless the coating thickness of the negative electrode is increased, so that the electrode sheet resistance increases and the discharge capacity and cycle characteristics decrease.

  In the negative electrode active material for a lithium ion secondary battery according to the present invention, the ratio of the average particle size of the carbonized product added to the particle size of the composite (average particle size ratio) is 2.5 to 300%, and 10 to 200%. Is preferable, and 30 to 100% is more preferable. If the average particle size ratio is less than 2.5%, the particle size of the carbonized product to be added becomes extremely small as compared with the composite particles, and there is a possibility that homogeneous mixing with the composite particles becomes difficult. Further, if the average particle size ratio exceeds 300%, it may be difficult to ensure the conductivity of the composite particles.

  In the negative electrode active material for a lithium ion secondary battery of the present invention, it is preferable that the Si compound and the carbonaceous material are both sandwiched between thin graphite layers having a thickness of 0.5 μm or less. The active material particles are formed by spreading in a network, and the graphite thin layer is curved near the surface of the active material particles to cover the composite particles, and graphite or carbonaceous matter is arranged around the composite particles. It is preferable. If the thickness of the graphite thin layer exceeds 0.5 μm, the electronic conductivity of the graphite thin layer may be lowered.

  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 according to the present invention comprises a step of mixing an Si compound, a carbon precursor, and, if necessary, a graphite, a step of granulating and compacting, and a pulverizing mixture. The method includes a step of forming composite particles, a step of firing the composite particles in an inert gas atmosphere, and a step of mixing the composite and the carbonized product.

  As the Si compound as a raw material, a powder having an average particle size (D50) of 0.01 to 5 μm is used. 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. It is done by adjusting the oxygen concentration. In a dry type in which a classifier is integrated, pulverization and classification are performed at a time, and a desired particle size distribution can be obtained.

  As another method for obtaining 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, but is not limited to petroleum pitch, coal pitch, or 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.

  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. 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.

  A method of mixing the composite and the carbonized product is not particularly limited, but it is preferable to slurry the composite and the carbonized product using a dispersion solvent such as an organic solvent such as water or alcohol. Other methods include dry methods such as V-type mixers and screw mixers.

  In the step of mixing the composite and the carbonized product, it is possible to add a binder and a conductive aid necessary for producing the electrode. Thereby, not only the compounded material, the carbonized material, the binder, and the conductive auxiliary agent can be added and prepared at a time, but also the members can be homogeneously prepared.

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

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

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

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

  Then, battery performance is evaluated using a charging / discharging device. The battery evaluation conditions are not particularly limited, and examples thereof include a constant current method, a constant current constant voltage method, a constant capacity method, a constant power method, and a pulse method. In particular, the constant current method and the constant current constant voltage method are often used for evaluating battery characteristics when the charge / discharge depth (DOD) is close to 100%, and the constant capacity method and the constant power method are the charge / discharge depth (DOD). However, it can also be used for battery evaluation in a relatively shallow region.

  According to the present invention, the network of electrical conduction paths spreads in a network by reducing the expansion volume per particle by silicon of fine particles and by highly dispersed mixing of a composite of a dense carbonaceous material and graphite. It is possible to suppress a decrease in electrical conductivity during repeated charging and discharging, and high capacity and excellent cycle characteristics and stable Coulomb efficiency can be obtained.

2 is a low-magnification backscattered electron image obtained by FE-SEM of the cross-section of the negative electrode composite particles obtained in Example 1. 3 is a high-magnification backscattered electron image obtained by FE-SEM of a cross section of the negative electrode composite particles obtained in Example 1. FIG. 2 is a SEM photograph after the production of an electrode sheet of the negative electrode active material obtained in Example 1. 4 is a SEM photograph after production of an electrode sheet of a negative electrode active material obtained in Example 2.

  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 in an amount of 20% by weight. An ultrafine Si slurry having a D50) of 0.3 μm and a dry BET surface area of 100 m ² / g was obtained.

  A natural graphite having a flat shape with a particle diameter of about 0.5 mm and a thickness of about 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, and then washed with water. And dried to obtain acid-treated graphite. The acid-treated graphite was passed through a mullite tube having a length of 1 m and an inner diameter of 11 mm, which was heated to 1150 ° C. with an electric heater by flowing nitrogen gas at a flow rate of 14 L / min so as to obtain a supply rate of 5 g / min. As the sulfuric acid in the acid-treated graphite was decomposed and discharged into a gas such as sulfurous acid by the heat treatment, the acid-treated graphite expanded and was collected in a stainless steel container. The expansion coefficient calculated from the ratio of light bulk density before and after heat treatment was 350%. By SEM observation, it was confirmed that the graphite layer exfoliated and expanded in the thickness direction, and was an accordion-shaped powder.

  86 g of the ultrafine Si slurry, 20.6 g of the expanded graphite, 12.9 g of a resol type phenolic resin (ASBERY grade 3772), and 3.2 L of ethanol were placed in a stirring vessel, and 1 at 8000 rpm with a homomixer. Stir for hours. 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 12 hours to obtain about 50 g of a mixed dried product (light bulk density 80 g / L).

  This mixed dried product was passed through a three-roll mill twice, and granulated and consolidated to a particle size of about 2 mm and a lightly packed bulk density of 467 g / L.

  Next, this granulated / consolidated product was placed in a new power mill and pulverized at 21000 rpm for 15 minutes while being cooled with water. 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 mesh having an opening of 45 μm was passed through to obtain a composite having an average particle diameter (D50) of 18.6 μm and a lightly packed bulk density of 753 g / L.

  FIG. 1 shows a reflected electron image obtained by FE-SEM of a cross section obtained by cutting the obtained composite particles with an ion beam. The inside of the composite particles had a structure in which 0.05 to 1.0 μm long Si fine particles and a carbonaceous material were sandwiched between 0.02 to 0.5 μm thick graphite thin layers and spread in a network. . As the carbonized product to be added to the composite, commercial natural graphite (CGB10 made by Nippon Graphite) having an average particle size (D50) of 12.1 μm was used. The average particle size ratio of the composite and added graphite was 65%. The discharge capacity of the natural graphite used was 365 mAh / g, and the discharge capacity after repeated charging and discharging 50 times was 365 mAh / g, and the discharge capacity hardly changed. In addition, the initial Coulomb efficiency was 86.6%, the Coulomb efficiency after repeating 10 times charging / discharging was 99.0%, and the Coulomb efficiency after repeating 50 times charging / discharging was 99.7%.

“Preparation of negative electrode for lithium ion secondary battery”
The obtained composite and natural graphite (50:50 wt%) were weighed as a negative electrode active material, and the negative electrode active material was 95.5 wt% (content in the total amount of solid content, the same applies hereinafter). , After mixing acetylene black 0.5% by weight as a conductive aid, carboxymethyl cellulose (CMC) 1.5% by weight and styrene butadiene rubber (SBR) 2.5% by weight, water, and a rotating / revolving mixer ( The negative electrode active material was dispersed and mixed using a thin key foaming refining taro) 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 3 mg / cm ² and dried at 110 ° C. in a stationary operation dryer for 0.5 hour. An SEM photograph of the surface shape of the sheet electrode at this time is shown in FIG.

After drying, it was punched into a 13.8 mmφ circle, uniaxially pressed under a pressure of 0.6 t / cm ² , and further heat treated at 110 ° C. for 2 hours under vacuum to form a negative electrode mixture layer having a thickness of 30 μm. A negative electrode for an ion 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 in a volume ratio of 1: 1, dissolved LiPF ₆ to a concentration of 1.2 mol / L, and added with 2% by volume of fluoroethylene carbonate. 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).

"Evaluation conditions"
The evaluation cell was cycle tested in a constant temperature room at 25 ° C. Charging was performed until the current value reached 0.2 mA at a constant voltage of 0.01 V after charging to 0.01 V at a constant current of 3 mA. The discharge was performed at a constant current of 2 mA up to a voltage value of 1.5 V. 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 cycle capacity maintenance ratio by comparing the discharge capacity after the 50th charge / discharge test under the charge / discharge conditions with the initial discharge capacity.

Example 2
A negative electrode active material, a negative electrode, and an evaluation cell were prepared in the same manner as in Example 1 except that the obtained composite and the negative active material composed of 30: 70% by weight of natural graphite were used. did.

Comparative Example 1
Except for using a commercially available silicon (manufactured by China (Hanwa Kogyo Co., Ltd.)) having an average particle size of 7 μm as a composite, and using a negative active material consisting of 30% by weight of the composite and natural graphite 30% by weight. A negative electrode active material, a negative electrode, and an evaluation cell were prepared in the same manner as in Example 1, and the cells were evaluated.

Comparative Example 2
A negative electrode active material, a negative electrode, and an evaluation cell were prepared in the same manner as in Comparative Example 1 except that the composite material of Comparative Example 1 and the negative electrode active material composed of 40: 60% by weight of natural graphite were used. did.

Comparative Example 3
A negative electrode active material, a negative electrode, and an evaluation cell were prepared in the same manner as in Example 1, except that the natural graphite was not added as the negative electrode active material and only the composite of Example 1 was used. .

Comparative Example 4
A negative electrode active material, a negative electrode, and an evaluation cell were prepared in the same manner as in Example 1 except that the composite was not added as the negative electrode active material and only the natural graphite was used.

  The negative electrode active material preparation conditions of Examples 1 and 2 and the negative electrode active material preparation conditions of Comparative Examples 1 to 4 are shown in Table 1. Table 2 shows the results of Examples 1 and 2 and the results of Comparative Examples 1 to 4.

  As is clear from Table 2, the lithium ion secondary batteries of Examples 1 and 2 using a negative electrode active material prepared by adding Si after adding Si, a carbonaceous material, and graphite and then adding graphite have a high capacity, It can be seen that the discharge cycle characteristics are good and the coulomb efficiency is stable quickly.

  In contrast, the lithium ion secondary batteries of Comparative Examples 1 and 2 using a negative electrode active material prepared by simply adding graphite without compounding Si with a carbonaceous material or graphite have initial Coulomb efficiency and cycle capacity. It can be seen that the maintenance rate is greatly reduced. Furthermore, in Comparative Example 3 using a negative electrode active material prepared by combining Si, a carbonaceous material and graphite, it can be seen that the charge / discharge cycle characteristics and the 10th Coulomb efficiency are inferior. Furthermore, the negative electrode active material produced simply by adding graphite has a low discharge capacity. The calculation results of the battery characteristics of Examples 1 and 2 using the weighted average values of Comparative Examples 3 and 4 are shown in Calculation Examples 1 and 2. Examples 1 and 2 have higher cycle capacity retention ratio and 10th Coulomb efficiency than Calculation Examples 1 and 2, and the battery performance far exceeds the weighted average prediction by adding the composite of the present invention and graphite. It was found that was secured.

  The lithium ion secondary battery negative electrode active material and the manufacturing method thereof according to the present invention can be used for a lithium ion secondary battery that has a high capacity and requires a long life.

11 Graphite or carbonized layer (gray) in cross-sectional backscattered electron image of composite
12 Fine Si particles (white) in cross-sectional backscattered electron image of composite
13 Composite particles 14 Additive particles

Claims (7)

It is a mixture obtained by adding carbonized material to Si or Si alloy having an average particle size of 0.01 to 5 μm and a carbonaceous material or a composite material having an average particle size of 1 to 40 μm made of carbonaceous material and graphite. A negative electrode active material for a lithium ion secondary battery. The amount of Si or Si alloy is not less than 10% by weight and not more than 80% by weight in the composite, and the amount of carbonized product added to the composite is 0.5% by weight based on the total amount of composite and added carbonized product. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the negative electrode active material is 99.5% by weight or less. The average particle size of the carbonized product to be added is 0.1 to 40 µm, and the average particle size of the carbonized product is 2.5 to 300% with respect to the particle size of the composite product. The negative electrode active material for lithium ion secondary batteries as described in 2. Si or Si alloy and a carbonaceous material are both sandwiched between thin graphite layers having a thickness of 0.5 μm or less, and the structure spreads in a laminated and / or network form, and the thin graphite layer is active material particles. The lithium according to any one of claims 1 to 3, wherein the composite particle is curved in the vicinity of the surface of the metal, and graphite or carbonaceous matter is disposed around the composite particle. Negative electrode active material for ion secondary battery. A step of mixing Si or an Si alloy, a carbon precursor, and optionally graphite, a step of granulating and compacting, a step of pulverizing the mixture to form composite particles, and 5. The negative electrode active material for a lithium ion secondary battery according to claim 1, comprising a step of firing in an active gas atmosphere, and a step of mixing the composite and the carbonized product. Production method. 6. The production of a negative electrode active material for a lithium ion secondary battery according to claim 5, wherein in the step of mixing the composite and the carbonized product, the composite and the carbonized product are slurried using a solvent. Method. The step of mixing the composite and the carbonized product includes a step of adding a binder and a conductive additive necessary for producing the electrode. The negative electrode active material for a lithium ion secondary battery according to claim 6, Production method.

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