JP2012178344A - Compound material and method for manufacturing the same, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compound material which has, in use for a lithium ion secondary battery, high capacity, excellent cycle characteristics in repeated charging and discharging, and excellent charging and discharging efficiency at an early stage of a cycle.SOLUTION: The compound material is formed by combining particles including a solid phase A with Si as a main body and a fixed phase B comprised of an intermetallic compound of Si and at least one type selected from Cr and Ti, with carbon material.

Description

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

  In recent years, lithium ion secondary batteries used as a main power source for communication / electric / electronic devices such as mobile phones and personal computers have a high electromotive force and a high energy density.

  In the lithium ion secondary battery, when metallic lithium is used as the negative electrode material, lithium is a base metal, so that a high voltage can be taken out and the battery has a high energy density. However, when charging / discharging is repeated, lithium precipitates in a dendritic form from the negative electrode, penetrates through the separator as an insulator, and short-circuits with the positive electrode, so that there is a problem in that low reliability and charge / discharge cycle characteristics deteriorate. is there.

  In order to solve such a problem, at present, a battery using a carbon material capable of occluding and releasing lithium ions as a negative electrode material replacing metallic lithium has been put into practical use. Usually, when a carbon material is used as the negative electrode material, the charging reaction is a reaction in which lithium ions are occluded in the carbon material, so that metallic lithium does not precipitate and the problem of internal short circuit due to dendrites is solved.

  However, among the carbon materials that have been used so far, it is known that graphite-based carbon materials have a theoretical capacity limit of 372 mAh / g in discharge capacity. On the other hand, although non-graphitic carbon materials have a large discharge capacity, they have a large irreversible capacity, and there is a problem that a very large loss is generated at the battery design stage.

  Against this background, with increasing performance of electronic / electrical devices every year, there is an urgent need to increase the capacity of lithium ion secondary batteries.

  Therefore, in order to further increase the capacity of the lithium ion secondary battery, there is no possibility of causing an internal short circuit due to dendrites, and active research is being conducted on negative electrode materials having a higher theoretical capacity than carbon materials.

At present, alloy-based negative electrode materials containing elements such as Si and Sn are attracting attention. Metal elements such as Si and Sn can occlude and release lithium ions electrochemically, and can charge and discharge with a very large capacity compared to carbon materials.
For example, in Patent Document 1, a nonferrous metal silicide material composed of a transition element, and in Patent Document 2, particles including both a phase composed of Si, Sn, etc. and a phase composed of an intermetallic compound containing Si, Sn, etc. as one of constituent elements. It has been reported that a material composed of

JP-A-7-240201 JP 11-86853 A

However, the negative electrode materials described above have the following problems.
A lithium ion secondary battery using a non-ferrous metal silicide negative electrode material composed of a transition element disclosed in Patent Document 1 exhibits cycle characteristics equivalent to those of a natural graphite negative electrode material, but its battery capacity is 12 at maximum. % Is only increased, and a significant increase in capacity is desired.

The lithium ion secondary battery using the negative electrode material shown in Patent Document 2 has a phase of Si, Sn, etc. with a large volume change accompanying charging / discharging, and a volume change with charge / discharge of Mg ₂ Si, NiSi ₂ etc. By covering with a small phase, each other's volume change is relaxed and cycle characteristics are improved. However, there is a problem that the initial irreversible capacity is large, and there is a problem that a large loss occurs in the battery design stage.

  Accordingly, an object of the present invention is to provide a composite material having a high capacity, excellent cycle characteristics during repeated charge / discharge, and excellent charge / discharge efficiency at the initial cycle when used in a lithium ion secondary battery.

  As a result of intensive studies by the present inventors, the present inventors have found that the following composite materials are effective for solving the above problems, and have completed the present invention.

  In the present invention, particles including a solid phase A mainly composed of Si and a solid phase B composed of an intermetallic compound of Si and at least one selected from Cr and Ti are combined with a carbon material. Relates to a composite material.

  Moreover, it is preferable that the content rate of the said carbon material is 5 mass%-40 mass%.

The median particle diameter on a volume basis as measured by a laser diffraction particle size distribution meter is 0.1Myuemu～20myuemu, specific surface area determined from nitrogen adsorption measurements is 0.5m ² / g~15m ² / g, it is preferable true density of ^{2.2g / cm 3 ~4.0g / cm 3} .

  The crystallite size of Si in the solid phase A is preferably 40 nm or less.

  The solid phase A and the solid phase B are each composed of at least one of a crystalline region and an amorphous region, and 2θ = 40 derived from the solid phase B when a CuKα ray having a wavelength of 1.5406 nm is used as a radiation source. The X-ray diffraction peak intensity ratio of 2θ = 27.5 degrees to 30.5 degrees derived from the solid phase A with respect to the X-ray diffraction peak intensity of .5 degrees to 44.5 degrees is preferably 0.35 or less. .

  The composite material is preferably used for an electrode for a lithium ion secondary battery.

  In the present invention, an alloy containing at least one selected from Si, Cr, and Ti and a carbon material are pulverized and granulated in a ball mill in which a spherical medium made of iron, chrome steel, or zirconia is present. The manufacturing method of the said composite material including this.

  Furthermore, this invention relates to the negative electrode for lithium ion secondary batteries which has a collector and the negative electrode material layer which contains the said composite material provided on the said collector as an active material.

  The present invention also relates to a lithium ion secondary battery comprising a positive electrode, the negative electrode for a lithium ion secondary battery, and an electrolyte.

  ADVANTAGE OF THE INVENTION According to this invention, when it uses for a lithium ion secondary battery, it is excellent in the cycle characteristic at the time of repeated capacity | capacitance and charge / discharge, and the composite material excellent in the charge / discharge efficiency of the cycle initial stage is provided.

6 is a pattern diagram of the composite material produced in Example 2 by powder X-ray diffraction. FIG.

In the present specification, a numerical range indicated using “to” indicates a range including the numerical values described before and after “to” as the minimum value and the maximum value, respectively.
Further, in this specification, the amount of each component in the composition is the sum of the plurality of substances present in the composition unless there is a specific indication when there are a plurality of substances corresponding to each component in the composition. Means quantity.

<Composite material>
The composite material of the present invention is suitably used as a negative electrode material for lithium ion battery electrodes. The composite material is a particle including a solid phase A mainly composed of Si, and a solid phase B including an intermetallic compound of Si and at least one selected from Cr and Ti. Composite with carbon material.

  In this composite material, the solid phase A is a phase responsible for occlusion / release of lithium, and is an electrochemically reversible phase for reaction with lithium. The solid phase A is a phase mainly composed of Si. When the solid phase A is mainly composed of Si, the discharge capacity per unit mass or unit volume is increased.

  Furthermore, the solid phase A preferably contains additional elements such as boron, phosphorus, and transition metal elements. When the solid phase A contains the additive element, the low electrical conductivity of Si as a semiconductor is compensated. When the solid phase A contains an element other than Si, the Si content in the solid phase A is preferably 95 atm% to 99.99 atm%, more preferably 97 atm% to 99.99 atm%. More preferably, it is 98 atm% to 99.99 atm%.

  The solid phase B includes an intermetallic compound of Si and at least one selected from Cr and Ti. The intermetallic compound is a phase that plays a role of assisting the low electric conductivity of the solid phase A and a role of maintaining the particle shape of the particles even in the volume expansion at the time of charging of the solid phase A. Therefore, the solid phase B is preferably composed of a material having high electrical conductivity and high strength, and more preferably composed of a material having high affinity with the solid phase A.

Specifically, the solid phase B in the present invention preferably includes a silicide compound represented by at least one selected from CrSi ₂ and TiSi ₂ . The solid phase B may be composed of one of CrSi ₂ or TiSi ₂ , or may be composed of _two of CrSi ₂ and TiSi ₂ , from the viewpoint of improving charge / discharge efficiency and cycle characteristics. It is preferable to include two kinds of CrSi ₂ and TiSi ₂ .
Hereinafter, the solid phase B including two types of silicide compounds of CrSi ₂ and TiSi ₂ may be referred to as a (Cr, Ti) Si ₂ phase.

Further, the solid phase B may contain intermetallic compounds with Si other than CrSi ₂ and TiSi ₂ . For example, M ¹ Si ₂ and M ² Si ₂ (M ¹ and M ² represent transition metal elements other than Cr and Ti) may be present in the particles. Examples of transition metal elements other than Cr and Ti include Fe, Ni, Cu, Zr, Hf, V, Nb, Mo, Mn, Co, Mg, and W. Si other than CrSi ₂ and TiSi ₂ Examples of the intermetallic compound include FeSi ₂ and NiSi ₂ .

  As described above, the discharge capacity per unit mass or unit volume increases as the proportion of the solid phase A increases. On the other hand, as the proportion of the solid phase B increases, the electrical conductivity increases, and the particle shape is maintained even in volume expansion, so that the cycle characteristics are improved. From the balance of these two advantages, the ratio (at%) between the solid phase A and the solid phase B is preferably 90:10 to 70:30, and preferably 85:15 to 75:25. More preferably, it is 85: 15-80: 20.

  The particles containing the solid phase A and the solid phase B are combined with a carbon material to form a composite material. As used herein, the term “composite” refers to a state in which Si and a carbon material in a particle including solid phase A and solid phase B form a Si—C bond, or a single or plural particles and a carbon material, respectively. Are integrated to form an aggregate of fine particles that exhibit the same behavior. These states can be analyzed, for example, by analyzing an X-ray absorption fine structure (XAFS) or a TEM-EELS method (transmission electron microscope-electron energy loss spectroscopy).

  Examples of the carbon material include natural graphites such as flaky graphite and spherical graphite, and carbon blacks such as artificial graphite, acetylene black, ketjen black, and furnace black. These may be used alone or in combination.

  The carbon material is preferably added at 5% by mass to 40% by mass in the composite material, more preferably at 5% by mass to 35% by mass, and at 10% by mass to 30% by mass. More preferably. When the blending amount of the carbon material is 5% by mass or more, cycle characteristics tend to be improved. On the other hand, when the blending amount of the carbon material is 40% by mass or less, the capacity is increased.

  Furthermore, the composite material of the present invention may be coated with carbon or a polymer. By coating the composite material with carbon or polymer, the charge / discharge efficiency is improved. It is preferable to appropriately determine the coating amount so that the initial charge / discharge efficiency is not lowered by the coating and the characteristics of the negative electrode material are not degraded.

  Examples of the carbon or polymer coating method include a wet mixing method, a chemical vapor deposition method, and a mechanochemical method. The chemical vapor deposition method and the wet mixing method are preferable from the viewpoint that the reaction system can be uniformly controlled and the shape of the composite particles can be maintained. In the wet mixing method and the mechanochemical method, a polymer compound such as a phenol resin or a styrene resin, a carbonizable solid material such as pitch, or the like can be processed as a solid or a dissolved material.

  The carbon source for forming the coating layer is not particularly limited, and aliphatic hydrocarbons, aromatic hydrocarbons, alicyclic hydrocarbons, and the like can be used in the chemical vapor deposition method. Specific examples include methane, ethane, propane, toluene, benzene, xylene, styrene, naphthalene, cresol, anthracene, and derivatives thereof. These carbon sources may be used alone or in combination of two or more.

  Examples of the polymer to be coated include starch derivatives, polysaccharides, water-soluble cellulose derivatives, water-soluble synthetic resins, and the like. Specifically, starch, starch derivatives, dextrin, dextrin derivatives, cyclodextrin, cellulose derivatives. , Alginic acid, alginic acid derivatives, pullulan, glycogen, agarose, carrageenan, pectin, lignin derivatives, xyloglucan, gelatin, casein, arabinose, glucose, mannose, galactose, sucrose, maltose, lactose, cellobiose, trehalose, glycine, alanine, Valine, leucine, isoleucine, serine, threonine, cysteine, cystine, methionine, aspartic acid, glutamic acid, lysine, arginine, phenylalanine, tyrosine, hiss Jin, tryptophan, proline, oxyproline, glycylglycine, gallic acid, tea catechins, persimmon catechin, saccharin, saccharin salts, butynediol and the like. These polymers may be used alone or in combination of two or more.

  The volume-based median particle diameter (D50%) of the composite material is preferably 0.1 μm to 20 μm, and more preferably 0.5 μm to 10 μm. When the average particle size is 20 μm or less, the distribution of the composite material in the negative electrode becomes uniform when used for the negative electrode for a lithium ion secondary battery, and further, the expansion and contraction at the time of charge / discharge are made uniform. Degradation of characteristics can be suppressed. On the other hand, when the median particle diameter is 0.1 μm or more, the negative electrode density tends to increase and the capacity tends to increase. For measuring the median particle diameter, a known method such as a laser diffraction particle size distribution analyzer can be employed.

The specific surface area of the composite material is preferably 0.5m ² / g~15m ² / g, more preferably ^{1.0m 2 / g~14m 2 / g,} 3.0m 2 / g~13m More preferably, it is ² / g. When the specific surface area is 15 m ² / g or less, the irreversible capacity at the first cycle of the obtained lithium ion secondary battery decreases. Furthermore, the amount of the binder used can be suppressed when producing the negative electrode. When the specific surface area is 0.5 m ² / g or more, the contact area with the electrolytic solution increases, and the charge / discharge efficiency increases.
For the measurement of the specific surface area, a known method such as the BET method (nitrogen gas adsorption method) can be employed.

Further, the true density of the composite material has a correlation with the specific surface area, and the true density tends to increase as the specific surface area decreases. Preferably has a true density of ^{2.2g / cm 3 ~4.0g / cm 3} , more preferably ^{2.4g / cm 3 ~4.0g / cm 3} . For the measurement of the true density, a known method such as a gas phase substitution method can be employed.

Next, the crystal states of the solid phase A and the solid phase B in the composite material will be described.
The solid phase A and the solid phase B are preferably composed of at least one of a crystalline region and an amorphous region. When the composite material consisting of at least one of the crystalline and amorphous regions of the solid phase A and the solid phase B is used as an electrode material for a lithium ion secondary battery, volume change associated with charge / discharge is suppressed, Particle cracking is suppressed, and as a result, deterioration of cycle characteristics due to separation of the composite material from the current collector is suppressed.

  Here, the crystal states of the solid phase A and the solid phase B can be confirmed by using a powder X-ray diffractometer. A CuKα ray having a wavelength of 1.5406 nm is used as an X-ray source, and measurement is performed in the range of 2θ = 10 degrees to 80 degrees. Judgment is made based on the presence or absence of peaks attributed to the crystal planes of solid phase A and solid phase B in the obtained diffraction spectrum. When these phases are composed of fine crystalline regions, relatively broad diffraction peaks are observed. In addition, when the phase is composed of an amorphous region, the diffraction spectrum of the composite material obtained from the powder X-ray diffraction measurement shows a halo pattern in which the half width cannot be recognized, and the diffraction peak intensity decreases. .

The solid phase A may be crystalline, amorphous, or both crystalline and amorphous. From the viewpoint of improving the cycle characteristics, it is preferable that the proportion of the amorphous phase in the solid phase A is large. Specifically, the solid phase A contains both crystals and amorphous or amorphous. The quality case is preferred. More preferably, the X-ray diffraction peak intensity of 2θ = 27.5 ° to 30.5 ° derived from the solid phase A is not detected. Note that the fact that the diffraction peak is not observed means that the displacement from the baseline in the vicinity of the X-ray diffraction peak derived from the solid phase A is equal to or less than the noise level. It means 95% or less.
The solid phase B may be crystalline, amorphous, or both crystalline and amorphous. Preferably, the solid phase B includes both crystalline and amorphous. is there.

It is preferable that the crystal region of the solid phase A is further in a microcrystalline state. Specifically, the crystallite size of Si constituting the solid phase A is preferably 40 nm or less, more preferably 25 nm or less, and still more preferably 10 nm or less. When the crystallite size is 40 nm or less, volume change during charge and discharge of Si is suppressed, particle cracking is suppressed, and deterioration in cycle characteristics due to peeling from the current collector is suppressed.
The crystallite size of Si constituting the solid phase A can be directly observed with a transmission electron microscope (TEM), and can be determined from the half width of the peak obtained by powder X-ray diffraction analysis using the Scherrer equation.

  In the obtained diffraction spectrum, when there are a plurality of peaks attributed to the crystal plane of Si, the crystallite size can be calculated using the Scherrer equation for the main peak having the highest diffraction intensity. Good.

  The crystallite region of the solid phase B is also preferably in a microcrystalline state. The length of the crystallite region of the solid phase B can also be directly observed with a transmission electron microscope (TEM), and can be obtained from the half width of the peak obtained by powder X-ray diffraction analysis. The length of the crystallite region of the solid phase B is preferably 100 nm or less, preferably 80 nm or less, and preferably 70 nm or less.

  Area of X-ray diffraction peak intensity of 2θ = 27.5 degrees to 30.5 degrees derived from solid phase A with respect to X-ray diffraction peak intensity of 2θ = 40.5 degrees to 44.5 degrees derived from solid phase B The ratio (XRD intensity ratio) is preferably 0.35 or less, more preferably 0.30 or less, and still more preferably 0.15 or less. When the XRD intensity ratio is 0.35 or less, the length of the crystallite region of the solid phase B is suppressed, so that a specific amount of grain boundaries exist between the crystallites, and the volume of the particles during charge and discharge changes. There is a tendency for the cycle characteristics to improve following the dynamic strength.

As an example, a composite material in which an alloy particle in which the solid phase A is made of a Si phase and the solid phase B is made of a (Cr, Ti) Si ₂ phase and natural graphite is processed by a mechanical alloying method for 10 hours. The diffraction pattern is shown in FIG. FIG. 1 is a powder X-ray diffraction pattern of the composite material produced in Example 2 described later. The mechanical alloying method will be described later.

  In FIG. 1, the horizontal axis is the diffraction angle 2θ (degrees), and the vertical axis is the diffraction intensity. In the figure, ○ indicates a peak attributed to the crystal plane of the solid phase A, and ● indicates a peak attributed to the crystal plane of the solid phase B. When the XRD intensity ratio is calculated using the diffraction intensities of the solid phase A and the solid phase B (dotted line frame) in the range of the diffraction angle 2θ, it is 0.31.

<Production method of composite material>
The manufacturing method of the composite material of this invention is demonstrated.
The method for producing a composite material according to the present invention includes a ball mill in which a spherical medium made of iron, chromium steel, or zirconia is present in an alloy containing Si and at least one selected from Cr and Ti, and a carbon material. Including pulverization and granulation.

First, particles (alloy particles) including a solid phase A mainly composed of Si and a solid phase B composed of an intermetallic compound of Si and at least one selected from Cr and Ti are prepared. The alloy particles may be commercially available or may be prepared.
The alloy particles are manufactured by heating and melting each constituent element at a predetermined ratio in a melting tank to prepare a molten alloy, and rapidly solidifying the molten alloy. In addition, you may throw in each alloying element, such as a single-piece | unit state or solid solution, for each constituent element fuse | melted when producing a molten alloy. As a melting method, a conventionally known method such as a high-frequency melting method or an arc melting method can be used. As a method of rapid solidification, a conventionally known method such as a roll spinning method, a gas atomizing method, a melt drag method, a direct casting rolling method, or the like can be used.

Next, using the alloy particles and the carbon material, a composite material in which the particles composed of the solid phase A and the solid phase B and the carbon material are combined is obtained.
The composite material is produced by repeating pulverization and granulation in a ball mill in which a spherical medium made of iron, chrome steel, or zirconia is present. The method for producing the composite material is not particularly limited as long as the above-described desired physical property value of the composite material can be realized. Above all, if the mechanical alloying method is used in the production method, it is possible to simultaneously process the granulation process and the pulverization process of the alloy particles and the carbon material, and further, the particle diameter, specific surface area, and true density of the composite material to be produced. In addition, the crystal state can be easily controlled.

  In the mechanical alloying method, the alloy particles and the carbon material are put into an apparatus together with a spherical medium and vibrated at high speed. According to the mechanical alloying method, it is easy to combine with mechanical impact energy, and since heating and melting can be suppressed, crystal growth hardly occurs and fine particle formation is easy.

Examples of the apparatus in which spherical media used for the mechanical alloying method are present include a ball mill, a bead mill, a planetary ball mill, an attritor, and a vibration mill.
In the processing by the mechanical alloying method, the median particle diameter and solidity of a desired composite material can be determined by appropriately setting the amount of spherical media charged, the diameter of the spherical media, the processing time, etc. according to the size of the processing apparatus. The crystallite size of Si in phase A, the X-ray diffraction peak intensity ratio, and the like can be obtained.

  Specifically, for example, when the processing time by the mechanical alloying method is lengthened, the alloy particles become finer and the crystallite size of Si in the solid phase A becomes smaller. Further, as the composite of the alloy particles and the carbon material proceeds, the alloy particles are more uniformly distributed in the composite material. Further, the solid phase A becomes amorphous, and the X-ray diffraction peak intensity ratio (XRD intensity ratio, solid phase A / solid phase B) decreases.

  The composite material of the present invention can be applied to a negative electrode material and a negative electrode for lithium ion secondary batteries containing the composite material as an active material, and to all electrochemical devices having a charge / discharge mechanism that inserts and desorbs lithium ions, such as hybrid capacitors. Can be used. When used for a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery having a high capacity, excellent cycle characteristics during repeated charge / discharge, and excellent charge / discharge efficiency at the beginning of the cycle can be obtained.

<Anode for lithium ion secondary battery>
A negative electrode for a lithium ion secondary battery of the present invention (hereinafter sometimes abbreviated as “negative electrode”) includes a current collector, and a negative electrode material layer containing the composite material provided on the current collector as an active material, Have. For example, the negative electrode for a lithium ion secondary battery of the present invention is a coating obtained by mixing the composite material, an organic binder, a solvent such as a solvent or water, a thickener if necessary, and a conventionally known carbon-based negative electrode material. It is obtained by preparing a liquid and applying the coating liquid to a current collector, then drying the solvent or water and press-molding it. Generally, it is kneaded with an organic binder, a solvent, and the like, and formed into a sheet shape, a pellet shape, or the like.

  Although it does not specifically limit as said organic binder, For example, a styrene-butadiene copolymer; Ethylenically unsaturated carboxylic acid ester (For example, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (Meth) acrylonitrile and hydroxyethyl (meth) acrylate) and ethylenically unsaturated carboxylic acid (for example, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid) (meth) acrylic copolymer; poly Examples thereof include polymer compounds such as vinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, and polyamideimide.

  As the polymer compound having a high ionic conductivity, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile and the like can be used.

  These organic binders include those dispersed or dissolved in water and those dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP) depending on the respective physical properties. Among these, an organic binder whose main skeleton is polyacrylonitrile, polyimide, or polyamideimide is preferable because of its excellent adhesion, and the heat treatment temperature is as described below for an organic binder whose main skeleton is polyacrylonitrile. It is more preferable because it is low and the flexibility of the electrode is excellent. Examples of the organic binder having polyacrylonitrile as a main skeleton include, for example, a product obtained by adding acrylic acid for imparting adhesiveness and a linear ether group for imparting flexibility to a polyacrylonitrile skeleton (manufactured by Hitachi Chemical Co., Ltd., LSR7) can be used.

  The blending amount of the organic binder is preferably 0.1% by mass to 20% by mass with respect to 100% by mass of the total mass of the negative electrode material layer.

The negative electrode for lithium ion secondary batteries of this invention may use together the conventionally known carbon-type negative electrode material with the composite material of this invention, etc. as an active material contained in a negative electrode material layer. Depending on the type of carbon-based negative electrode material used in combination, an improvement in charge / discharge efficiency, an improvement in cycle characteristics, an effect of suppressing the expansion of the electrode, and the like can be obtained.
Conventionally known carbon-based negative electrode materials include natural graphites such as flaky graphite and spherical graphite, artificial graphite, and amorphous carbon. These carbon-based negative electrode materials can be mixed alone or in combination with the composite material.

Further, as the thickener for adjusting the viscosity, carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein, and the like are the organic binders described above. It is also preferred to use with.
Although there is no restriction | limiting in particular as a solvent used for mixing of an organic binder, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, (gamma) -butyrolactone, etc. are used.

  In addition, you may add a conductive support agent to the said coating liquid. Examples of the conductive assistant include graphite particles, carbon black, graphite, acetylene black, carbon fibers, carbon nanotubes, and conductive oxides and nitrides. These conductive assistants may be used alone or in combination of two or more. It is preferable that the content rate of a conductive support agent is 0.1 mass%-20 mass% with respect to 100 mass% of total mass of a negative electrode material layer.

  The negative electrode for a lithium ion secondary battery is prepared by preparing the coating solution and then applying the coating solution to, for example, a current collector, drying, and press-molding to integrate the current collector.

  Further, the material and shape of the current collector are not particularly limited. For example, if a belt-like material made of aluminum, copper, nickel, titanium, stainless steel or the like in a foil shape, a punched foil shape, a mesh shape, or the like is used. Good. A porous material such as porous metal (foamed metal) or carbon paper can also be used.

  The method for applying the coating liquid to the current collector is not particularly limited. For example, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method. And known methods such as screen printing. After the application, it is preferable to perform a rolling process using a flat plate press, a calender roll or the like, if necessary.

  Further, the integration of the coating liquid formed in a sheet shape, pellet shape, or the like with the current collector can be performed by a known method such as a roll, a press, or a combination thereof.

The negative electrode layer formed on the current collector and the negative electrode layer integrated with the current collector are preferably heat-treated according to the organic binder used. For example, when an organic binder having polyacrylonitrile as the main skeleton is used, the temperature is 100 ° C. to 180 ° C., and when using an organic binder having polyimide, polyamideimide as the main skeleton, 150 ° C. to 450 ° C. It is preferable to heat-treat with.
This heat treatment increases the strength by removing the solvent and curing the organic binder, thereby improving the adhesion between the particles and between the particles and the current collector. These heat treatments are preferably performed in an inert atmosphere such as helium, argon, nitrogen, or a vacuum atmosphere in order to prevent oxidation of the current collector during the treatment.

In addition, the negative electrode is preferably pressed (pressurized) before the heat treatment. The electrode density can be adjusted by applying pressure treatment. The negative electrode material for a lithium ion secondary battery of the present invention, it is ^{preferably, 1.5g / cm 3 ~1.85g / cm} 3 electrode density of 1.4g ^/ cm ³ ~1.9g ^/ cm 3 more ^preferably, it is more preferably from ^{1.6g / cm 3 ~1.8g / cm 3} . As for the electrode density, the higher the volume capacity, the better the adhesion and the cycle characteristics.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention includes a positive electrode, the negative electrode, and an electrolyte.
For example, the negative electrode can be a lithium ion secondary battery by disposing a positive electrode opposite to each other with a separator interposed therebetween and injecting an electrolytic solution containing an electrolyte.

  The positive electrode can be obtained by forming a positive electrode layer on the current collector surface in the same manner as the negative electrode. As the current collector in the positive electrode, the same current collector as described in the negative electrode can be used.

The material used for the positive electrode of the lithium ion secondary battery of the present invention is not particularly limited. For example, a metal compound, metal oxide, metal sulfide, or conductive polymer capable of doping or intercalating lithium ions. The material may be used and is not particularly limited. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and double oxides thereof (LiCo _x Ni _y Mn _z O ₂ , x + y + z = 1, 0 <x, 0 <y; LiNi _2-x Mn _x O ₄ , 0 <x ≦ 2)), lithium manganese spinel (LiMn ₂ O ₄ ), lithium vanadium compound, V ₂ O ₅ , V ₆ O ₁₃ , VO ₂ , MnO ₂ , TiO ₂ , MoV ₂ O ₈ , TiS ₂ , V ₂ S ₅ , VS ₂ , MoS ₂ , MoS ₃ , Cr ₃ O ₈ , Cr ₂ O ₅ , olivine type LiMPO ₄ (M: Co, Ni, Mn, Fe ), Conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc. alone or mixed Can be used. Among them, lithium nickelate (LiNiO ₂ ) and its double oxide (LiCo _x Ni _y Mn _z O ₂ , x + y + z = 1) are suitable as the positive electrode material used in the present invention because of their high capacity. In addition, you may use a positive electrode active material combining a conductive support agent.

Examples of the conductive assistant include graphite particles and carbon black. These conductive assistants may be used alone or in combination of two or more.
The positive electrode includes the above positive electrode active material, the same organic binder as the organic binder used in the negative electrode, such as polyvinylidene fluoride, and N-methyl-2-pyrrolidone and γ-butyl which are the same solvents used in the negative electrode. A positive electrode coating solution is prepared by mixing with a solvent such as lactone, this positive electrode coating solution is applied to at least one surface of a current collector such as an aluminum foil, the solvent is then removed by drying, and rolling is performed as necessary. Can be produced.

  The electrolyte solution used for the lithium ion secondary battery of the present invention is not particularly limited, and a known one can be used. For example, a non-aqueous lithium ion secondary battery can be manufactured by using a solution obtained by dissolving an electrolyte in an organic solvent as the electrolytic solution.

As the electrolyte, for _{example, LiPF 6, LiClO 4, LiBF} 4, LiClF 4, LiAsF 6, LiSbF 6, LiAlO 4, LiAlCl 4, LiN (CF 3 SO 2) 2, LiN (C 2 F 5 SO 2) 2 Examples thereof include lithium salts that generate anions that are difficult to solvate, such as LiC (CF ₃ SO ₂ ) ₃ , LiCl, and LiI.

Examples of the organic solvent include carbonates (propylene carbonate, ethylene carbonate, diethyl carbonate, etc.), lactones (γ-butyrolactone, etc.), chain ethers (1,2-dimethoxyethane, dimethyl ether, diethyl ether, etc.), Cyclic ethers (tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyldioxolane, etc.), sulfolanes (sulfolane, etc.), sulfoxides (dimethylsulfoxide, etc.), nitriles (acetonitrile, propionitrile, benzonitrile, etc.), amides Examples thereof include aprotic solvents such as polyamines (N, N-dimethylformamide, N, N-dimethylacetamide and the like) and polyoxyalkylene glycols (diethylene glycol and the like).
An organic solvent may be used independently and may be used as 2 or more types of mixed solvents.

  Various known separators can be used as the separator. Specific examples of the separator include paper, polypropylene, polyethylene, and glass fiber separators.

  The method for producing the lithium ion secondary battery of the present invention is not particularly limited except that the composite material of the present invention or the negative electrode for a lithium ion secondary battery of the present invention is used. Known positive electrode and lithium ion secondary battery use It can be produced by using a material such as an electrolytic solution and a separator and utilizing a known method for producing a lithium ion secondary battery.

  For example, first, two electrodes of a positive electrode and a negative electrode are wound through a separator made of a polyethylene microporous film. The obtained spiral wound group is inserted into a battery can, and a tab terminal previously welded to a negative electrode current collector is welded to the bottom of the battery can. Inject the electrolyte into the obtained battery can, weld the tab terminal that was previously welded to the positive electrode current collector to the battery lid, and place the lid on the top of the battery can via the insulating gasket A battery is obtained by caulking and sealing the part where the lid and the battery can are in contact.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated more concretely, this invention is not restrict | limited to the following Example. Unless otherwise specified, “part” and “%” are based on mass.

[Example 1]
(Production method of composite material)
In order to obtain the composite materials shown in Table 1, first, Si, Cr, and Ti are charged at a predetermined ratio, melted in an Ar atmosphere using high-frequency induction melting, and alloy particles are obtained from the obtained melt by a gas atomization method. It was. About this alloy particle, what was produced in Sanyo Special Steel Co., Ltd. was used.

Next, alloy alloy / natural graphite = 80/20 (mass ratio) was charged, and mechanical alloying treatment was performed using a vibration mill apparatus (spherical media made of spherical media chrome steel, manufactured by Chuo Kako Co., Ltd.). The composite material with the target carbon was obtained by carrying out for 30 hours.
The powder characteristics of the composite material were analyzed using the following method. Table 1 shows the results of the powder characteristics.

<Measurement of median particle diameter (D50%)>
A solution in which a composite material is dispersed in purified water together with a surfactant is placed in a sample water tank of a laser diffraction particle size distribution analyzer (SALD-2100) manufactured by Shimadzu Corporation, and circulated with a pump while applying ultrasonic waves. Measured by diffraction method. The volume cumulative 50% particle size (D50%) of the obtained particle size distribution was defined as the average particle size.

<Measurement of BET specific surface area>
The measurement was performed using a nitrogen adsorption / desorption measuring device (Macsorb HM model 1201) manufactured by Mountec Co., Ltd. The specific surface area was calculated by the BET method.

<Measurement of true density>
The true density was measured using a true density meter device (Micromeritics Accupic 1330) manufactured by Shimadzu Corporation.

<Measurement of Si crystallite size>
The composite material was analyzed using a powder X-ray diffractometer (XRD-6100) manufactured by Shimadzu Corporation using CuKα rays having a wavelength of 1.5406 nm as a radiation source. In the diffraction spectrum derived from the Si phase obtained by the analysis, from the half-value width of the diffraction peak with the strongest X-ray intensity observed in the range of 2θ = 10 degrees to 80 degrees, Scherrer's equation is used. The crystallite size was calculated.

<Measurement of XRD intensity ratio>
The composite material was analyzed using a powder X-ray diffractometer (XRD-6100) manufactured by Shimadzu Corporation using CuKα rays having a wavelength of 1.5406 nm as a radiation source. In the composite material, 2θ = 27.5 degrees to 30 derived from the Si phase with respect to the X-ray diffraction peak intensity of 2θ = 40.5 degrees to 44.5 degrees derived from the Si phase and the (Cr, Ti) Si ₂ phase. An X-ray diffraction peak intensity ratio (XRD intensity ratio) of .5 degrees was calculated.

(Production method of negative electrode)
75% by mass of the composite material powder prepared by the above method, 15% by mass of acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent, and LSR-7 (manufactured by Hitachi Chemical Co., Ltd.) as a binder powder and binder 10 mass% was added with respect to the total mass of, and it knead | mixed after that, and produced the uniform slurry. The slurry was applied to the glossy surface of the electrolytic copper foil, preliminarily dried at 90 ° C. for 2 hours, and further subjected to curing treatment by drying at 120 ° C. for 4 hours in a vacuum atmosphere. The negative electrode which has this was obtained.

(Production of lithium ion secondary battery)
Using the electrode obtained above as a negative electrode, lithium metal as a counter electrode, ethylene carbonate / ethyl methyl carbonate (1: 1 volume ratio) and vinyl carbonate (VC) (1.0% by mass) containing 1M LiPF ₆ as an electrolyte, A 2016 coin cell was prepared using a mixed liquid of fluoroethylene carbonate (FEC) (20% by volume), a polyethylene microporous film having a thickness of 25 μm as a separator, and a copper plate having a thickness suitable as a spacer.

(Battery evaluation)
<First discharge capacity, charge / discharge efficiency>
Each battery obtained above is put in a thermostat kept at 25 ° C., charged with constant current until 0 V at 0.43 mA (0.32 mA / cm ² ), and then the current flows at a constant voltage of 0 V. The battery was further charged until it decayed to a value corresponding to 0.043 mA, and the initial charge capacity was measured. After charging, the battery was discharged after a 30-minute pause. Discharge was performed at 0.43 mA (0.32 mA / cm ² ) until it reached 1.5 V, and the initial discharge capacity was measured. At this time, the capacity was converted to the mass of the composite material used. A value obtained by dividing the initial discharge capacity by the initial charge capacity was calculated as charge / discharge efficiency (%).

<Cycle characteristics>
Each battery obtained above is placed in a thermostat kept at 25 ° C., and is charged with constant current until 0 V at 0.65 mA (0.42 mA / cm ² ). The battery was further charged until it decayed to a value corresponding to 0.043 mA. After charging, the battery was discharged after a 30-minute pause. The discharge was performed at 0.65 mA (0.42 mA / cm ² ) until 1.5 V was reached. The cycle characteristics were evaluated by performing the cycle test 100 times with this charge-discharge cycle.
Cycle characteristics = 100th cycle discharge capacity / 1st cycle discharge capacity

[Example 2]
A composite material was produced and evaluated in the same manner as in Example 1 except that the mechanical alloying process in Example 1 was changed to 10 hours.

[Example 3]
In Example 1, a composite material was prepared and evaluated in the same manner as in Example 1 except that the mass ratio of charged alloy particles / natural graphite before mechanical alloying was mixed at 95/5. went.

[Example 4]
In Example 1, a composite material was prepared and evaluated in the same manner as in Example 1 except that the mass ratio of charged alloy particles / natural graphite before mechanical alloying was changed to 60/40. It was.

[Comparative Example 1]
In the method for producing the negative electrode of Example 1, the same evaluation as in Example 1 was performed using the alloy particles used in Example 1 instead of the composite material.

[Comparative Example 2]
In Example 2, a composite material was prepared and evaluated in the same manner as in Example 2 except that Si metal powder was used instead of the alloy particles.

  The evaluation results of the above examples and comparative examples are shown in Table 1 below.

  From Table 1, it can be seen that the composite materials for lithium ion secondary batteries of Examples are high capacity, negative electrode materials excellent in cycle characteristics and cycle charge and discharge efficiency. In addition, the discharge capacity of Example 4 is 540 mAh / g, and the discharge capacity is increased by 45% compared to the discharge capacity 372 mAh / g when the graphite-based carbon material is used.

Claims (9)

  A composite material in which particles including a solid phase A mainly composed of Si and a solid phase B composed of an intermetallic compound of Si and at least one selected from Cr and Ti are combined with a carbon material. The composite material according to claim 1, wherein a content ratio of the carbon material is 5 mass% to 40 mass%. The median particle diameter on a volume basis as measured by a laser diffraction particle size distribution meter is 0.1Myuemu～20myuemu, specific surface area determined from nitrogen adsorption measurements is 0.5m ² / g~15m ² / g, a true density the composite material according to claim 1 or claim 2 but is ^{2.2g / cm 3 ~4.0g / cm 3} .   The composite material according to any one of claims 1 to 3, wherein a crystallite size of Si in the solid phase A is 40 nm or less.   The solid phase A and the solid phase B are each composed of at least one of a crystalline region and an amorphous region, and 2θ = 40 derived from the solid phase B when a CuKα ray having a wavelength of 1.5406 nm is used as a radiation source. 2. The ratio of the X-ray diffraction peak intensity of 2θ = 27.5 degrees to 30.5 degrees derived from the solid phase A to the X-ray diffraction peak intensity of .5 degrees to 44.5 degrees is 0.35 or less. The composite material according to claim 4.   The composite material according to any one of claims 1 to 5, which is used for an electrode for a lithium ion secondary battery.   Grinding and granulating an alloy containing Si and at least one selected from Cr and Ti and a carbon material in a ball mill in which a spherical medium made of iron, chromium steel or zirconia is present. The manufacturing method of the composite material of any one of Claims 1-6. A negative electrode material layer comprising the current collector and the composite material according to any one of claims 1 to 6 as an active material, provided on the current collector,
A negative electrode for a lithium ion secondary battery.   A lithium ion secondary battery comprising a positive electrode, a negative electrode according to claim 8, and an electrolyte.