JP2005123175A - Composite particle, manufacturing method of the same, negative electrode material and negative electrode for lithium-ion secondary battery, and the lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode material, such that when used as a negative electrode for a lithium-ion secondary battery, ensures a high discharge capacity and an appreciable cycle characteristic and initial charge/discharge efficiency, and a lithium-ion secondary battery using the same; and to provide a composite particle that is suitable as a material of such a negative electrode material, and a manufacturing method of the same. <P>SOLUTION: The composite particle, which comprises a metal that can be alloyed with lithium, a graphitic material, and a carbonaceous material, has voids and has a 20% or higher ratio for voids around the metal to all the voids of the composite particles. The composite particle manufacturing method includes mixing of a metal, a graphitic material, and a precursor of a carbonaceous material A of a relatively low actual carbon rate after heating, and then mixing the mixture with a carbonaceous material B of a relatively higher actual carbon rate, after heating. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to composite particles containing a graphite material, a production method thereof, a negative electrode material and a negative electrode for a lithium ion secondary battery using the same, and a lithium ion secondary battery using the same.

Lithium ion secondary batteries having excellent characteristics of high voltage and high energy density compared to other secondary batteries are widely used as power sources for electronic devices. In recent years, miniaturization or performance enhancement of electronic devices has rapidly progressed, and there is an increasing demand for higher energy density of lithium ion secondary batteries.
At present, lithium ion secondary batteries generally use LiCoO _{2 for} the positive electrode and graphite for the negative electrode. However, although the graphite negative electrode is excellent in reversibility of charge and discharge, its discharge capacity has already reached a value close to the theoretical value 372 mAh / g corresponding to the intercalation compound LiC ₆ , in order to achieve further higher energy density. Needs to develop a negative electrode material having a larger discharge capacity than graphite.

Although metallic lithium has the highest discharge capacity as a negative electrode material, there is a problem that lithium is deposited in a dendritic state during charging, the negative electrode is deteriorated, and the charge / discharge cycle is shortened. In addition, lithium deposited in a dendrite shape may penetrate the separator and reach the positive electrode, causing a short circuit.
Therefore, a metal or a metal compound that forms an alloy with lithium has been studied as a negative electrode material that replaces metallic lithium. These alloy negative electrodes have discharge capacities far surpassing that of graphite, though not as much as metallic lithium. However, active materials are pulverized and peeled off due to volume expansion accompanying alloying, and a practical level of cycle characteristics has not yet been obtained.

In order to solve the drawbacks of the alloy negative electrode as described above, a composite of a metal or a metal compound and one or both of a graphite material and a carbonaceous material has been studied.
Patent Document 1 discloses that a metal or a metal material and a composite material made of a graphite material and a carbon material are used as an electrode material. In this composite material, the carbon material plays a role of bonding or covering a metal substance and a graphite material. This is also the ratio ID / IG (= R value) of the D band 1360 cm ⁻¹ peak intensity ID of the surface of the carbon material measured by Raman spectroscopy using an argon laser and the G band 1580 cm ⁻¹ peak intensity IG. Is disclosed to be 0.4 or more, which indicates that the carbon material is not graphitized. However, when the carbon material penetrates into the inside of the composite material, it is not possible to secure voids for buffering the expansion around the metal or metal compound, resulting in deterioration of cycle characteristics due to destruction of the composite particle structure There is.

  Patent Document 2 discloses a negative electrode material in which porous particles composed of silicon-containing particles and carbon-containing particles are coated with carbon. The carbon-containing particles correspond to a kind of graphite material. In this example of technology, the anode material is destroyed due to volume expansion when silicon and lithium are alloyed even though the anode material is positively made porous, and satisfactory cycle characteristics cannot be obtained. . Furthermore, since the carbon-containing particles (graphite material) are as small as 1 μm or less and easily cause a decomposition reaction of the electrolytic solution, a decrease in the initial charge / discharge efficiency resulting from the decomposition reaction of the electrolytic solution may become apparent.

Japanese Patent No. 3369589 Japanese Patent No. 3466576

  The present inventor has said that the composite particles of the prior art cannot absorb the expansion of a metal capable of forming an alloy with lithium while maintaining conductivity, so that the cycle characteristics deteriorate when used as a negative electrode material. As a result of estimation and diligent investigation, if a void larger than the average particle diameter of the metal is formed around the metal, the expansion of the metal at the time of alloy formation can be absorbed, and powdering and peeling of the composite particles can be prevented. It has been found that the conductivity can be maintained, and the present invention has been completed.

  The present invention has been made in view of the above-described knowledge, and when used as a negative electrode for a lithium ion secondary battery, a negative electrode material having a high discharge capacity and excellent cycle characteristics and initial charge / discharge efficiency. And a lithium ion secondary battery using the same. It is another object of the present invention to provide composite particles containing a graphite material suitable as a material for such a negative electrode material and a method for producing the same.

  The present invention provides a composite particle comprising a metal that can be alloyed with lithium, a graphite material, and a carbonaceous material, wherein the composite particle has voids, and the ratio of voids around the metal to the total voids of the composite particles is It is a composite particle characterized by being 20% or more.

  In the composite particles of the present invention, the metal is preferably silicon.

  In the composite particle of the present invention, it is preferable that the metal is silicon and a part of the silicon is an oxide.

  In the present invention, a metal that can be alloyed with lithium, a graphite material, and a precursor of carbonaceous material A are mixed, and the resultant composite particles have a higher residual carbon ratio than the precursor of carbonaceous material A. It is a method for producing composite particles, wherein the precursor of the carbonaceous material B is mixed and then heated.

  The method for producing composite particles of the present invention comprises mixing a metal that can be alloyed with lithium, a graphite material, and a precursor of the carbonaceous material A having a relatively low residual carbon ratio into composite particles, A method of mixing the precursor of the carbonaceous material B having a relatively high residual carbon ratio with the composite particles and heating to form voids around the metal is preferable.

  In the method for producing composite particles of the present invention, it is preferable that the carbon residue of the carbonaceous material A is 10% or more lower than the carbon residue of the carbonaceous material B.

  Moreover, this invention is a negative electrode material for lithium ion secondary batteries characterized by including one of the said composite particles.

  Moreover, this invention is a negative electrode for lithium ion secondary batteries using the said negative electrode material for lithium ion secondary batteries.

  The present invention also provides a lithium ion secondary battery using the negative electrode for a lithium ion secondary battery.

The lithium ion secondary battery produced using the negative electrode material containing the composite particles of the present invention has a high discharge capacity and is excellent in initial charge / discharge capacity and cycle characteristics.
Therefore, the lithium ion secondary battery using the negative electrode material of the present invention satisfies the recent demand for higher energy density, and is effective for downsizing and higher performance of equipment to be mounted.
Moreover, since the composite particle of the present invention can be produced using a material conventionally used as a material of the composite particle, there is an advantage that the material is easily obtained and the material cost is low. In addition, the method for producing composite particles of the present invention has an advantage that composite particles having a desired void can be stably produced by a relatively simple method.

Hereinafter, the present invention will be described more specifically.
(Composite particles)
The composite particle of the present invention is a composite particle mainly composed of a metal that can be alloyed with lithium, a graphite material, and a carbonaceous material. The composite particles contain a plurality of metal particles dispersedly and contain a plurality of large and small voids in a dispersed manner, and at least a part of each void is present around each metal. And it is the structure where the circumference | surroundings of a composite particle are surrounded by a carbonaceous material. In addition, the shape of the composite particles is not specified, and those having a size of about 3 to 50 μm can be manufactured, and are not particularly limited.

、好ましくは

の距離にある領域内に存在することが好ましい。金属がスズの場合には、半径の

、好ましくは

の距離にある領域内に存在することが好ましい。 In the composite particle of the present invention, the void around the metal is a void existing in direct contact with at least a part of the surface of the metal. Unless the voids are in direct contact with at least a part of the surface of the metal particles, the expansion of the metal particles cannot be absorbed, and the cycle characteristics are not sufficiently improved when used as a negative electrode material. The gap varies depending on the type of metal, so it cannot be said uniformly. However, when the metal is silicon, the center is from the center of the silicon, and when the silicon is spherical, the radius is not spherical. Considering the sphere corresponding to the volume, the radius

,Preferably

It is preferable that it exists in the area | region which exists in the distance. If the metal is tin, the radius

,Preferably

It is preferable that it exists in the area | region which exists in the distance.

The ratio of the voids around the metal to the total voids of the composite particles of the present invention must be 20% or more. If it is less than 20%, the metal cannot absorb the expansion when it forms an alloy with lithium. More preferably, it is 20 to 100%, More preferably, it is 40 to 100%, Most preferably, it is 50 to 100%.
Moreover, it is preferable that the ratio of the space | gap of the whole composite particle is 3 to 50%. If it is less than 3%, the metal may not be able to absorb expansion when it forms an alloy with lithium, and if it exceeds 50%, the strength of the composite particles may be insufficient.

The suitable composition (mass ratio) of the main component of the composite particle of the present invention is in the range of metal: graphite material: carbonaceous material = 1-20: 30-95: 4-50 when the total composite particle is 100. Yes, preferably in the range of 2-10: 60-93: 5-30.
If the metal composition is less than this range, when the negative electrode material containing the composite particles is used in a lithium ion secondary battery, the effect of improving the discharge capacity of the battery may be small, and conversely if it exceeds the range The effect of improving the cycle characteristics of the battery may be reduced.
If the composition of the graphite material deviates from the above range, it becomes difficult to form voids around the metal.
In addition, if the composition of the carbonaceous material deviates from this range, the effect of improving charge / discharge efficiency and cycle characteristics may not be sufficient.

The total void volume of the composite particles of the present invention can be obtained, for example, by measuring the composite particles that have been crushed to expose the cross section with a mercury porosimeter. Then, the porosity (volume ratio) of the entire composite particle is calculated.
The ratio of the voids around the metal to the total voids of the entire composite particles of the present invention is the total void area measured on a scanning electron micrograph (magnification 400 times) of the cross section of 50 composite particles, and the voids around all metals The average value of the ratio of the voids around the metal (area ratio) to the total voids of the entire composite particle obtained from the area of 50.

  In the composite particles of the present invention, since the voids are present around the metal, the cycle characteristics of the lithium ion secondary battery are improved. This is presumably because expansion and contraction of the metal during charge and discharge are buffered by the voids, and structural destruction of the negative electrode material containing the composite particles is suppressed. That is, for example, even when the metal itself is pulverized, since the form of the composite particles as the whole of the negative electrode material is maintained, the contact between the composite particles is maintained, the current collection is not impaired, It is estimated that it is possible to suppress the deterioration of cycle characteristics.

(Manufacture of composite particles)
The present invention can produce composite particles in which at least a part of voids exist around the metal using a mixture containing a metal that can be alloyed with lithium, a graphite material, and a precursor of a carbonaceous material. Any method can be used. The precursor of the carbonaceous material can be used after being melted, dispersed or dissolved.

The preferred composition (mass ratio) of the precursors A and B of the metal, the graphite material, and the carbonaceous material is as follows: metal: graphitic material: carbonaceous material = 1-15: 35-95 : 4 to 50, preferably 2 to 10: 60 to 93: 5 to 30. Specifically, metal: graphitic material: carbonaceous material A: carbonaceous material B = 1 to 15:35 to 95: 2 to 50: 2 to 40, preferably 2 to 10:60 to 93: It is the range of 3-30: 2-30.
When the metal is less than the range, the negative electrode material containing the composite particles may have a small effect of improving the discharge capacity of the battery when used in a lithium ion secondary battery. The effect of improving the cycle characteristics may be reduced.
If the carbonaceous material A is less than this range, the effect of improving the cycle characteristics may not be sufficient, and conversely if it is more than the range, the effect of improving the charge / discharge efficiency may not be sufficient.
On the other hand, if the carbonaceous material B is less than the range, the effect of improving the charge / discharge efficiency may not be sufficient, and conversely if it is more than the range, the effect of improving the cycle characteristics may not be sufficient.

  The method for producing composite particles of the present invention comprises mixing a metal that can be alloyed with lithium, a graphite material, and a precursor of carbonaceous material A having a relatively low residual carbon ratio, and further leaving the remaining composite particles in the resulting composite particles. In this method, precursors of carbonaceous material B having a relatively high carbon ratio are mixed and heated. In this production method, the heat treatment is preferably performed at a temperature at which the carbonaceous materials A and B of the composite particles can be substantially free of volatile substances.

The precursor is carbonized by heat treatment at a temperature of 600 ° C. or higher, preferably 800 ° C. or higher, and conductivity is imparted to the carbonaceous material. The heat treatment may be performed several times stepwise, or may be performed in the presence of a catalyst. Moreover, you may carry out in any atmosphere of oxidizing gas and non-oxidizing gas.
However, since the carbon and silicon react to generate SiC at 1500 ° C. or higher, the heating temperature needs to be lower than 1500 ° C. It is preferable that it is 1000-1200 degreeC. Moreover, it is preferable to mix using a dispersion medium suitably. It is preferable to remove the dispersion medium at a temperature not higher than the temperature at which the precursors of the carbonaceous materials A and B are not softened or decomposed.

  Further, it is preferable to adjust the particle size such as pulverization, sieving, and fine powder removal by classification at any stage before and after the heat treatment. In addition, when the heat treatment is performed at a relatively low temperature and the composite is flexible, an operation of rolling the composite or an operation of applying a high shearing force is applied, so that the composite becomes a nearly spherical shape, particularly a graphite material. When scaly graphite is used as one of the above, it is preferable because the scaly graphite is easily arranged concentrically. As a device capable of such operations, granulators such as GRANUREX [manufactured by Freud Sangyo Co., Ltd.], Newgra Machine [manufactured by Seishin Corporation], Agromaster [manufactured by Hosokawa Micron Co., Ltd.], roll mill, high These are compression shear type processing devices such as hybridization systems [made by Nara Machinery Co., Ltd.], mechano microsystems [made by Nara Machinery Co., Ltd.], mechano fusion systems [made by Hosokawa Micron Co., Ltd.], etc. be able to.

(Carbonaceous material)
The carbonaceous material has conductivity, and binds the metal and the graphite material, is an indispensable component as a binder, and can be obtained by heat-treating the precursor. Although the kind of precursor of carbonaceous material is not ask | required, in this invention, it is necessary to use 2 or more types from which the carbon residue of the carbonaceous material after carbonization differs. The difference in the remaining charcoal rate means that it is relatively preferably several percent or more, more preferably 10% or more. Here, the residual carbon ratio is based on the fixed carbon method of JIS K2425, and refers to the residual when the entire amount is carbonized by heating to 800 ° C. The percentage is expressed as a percentage.

  The precursor of the carbonaceous material is preferably tar pitches and / or resins. Tar pitches are preferably used as a precursor of the carbonaceous material B (that is, a precursor of a carbonaceous material having a relatively high residual carbon ratio) because the amount of voids after carbonization is relatively small. On the other hand, since the amount of voids after carbonization is relatively large, resins are preferable as a precursor of the carbonaceous material A (that is, a precursor of a carbonaceous material having a relatively low residual carbon ratio). Specifically, as petroleum-based or coal-based tar pitches, coal tar, tar light oil, tar medium oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, heavy Examples include oil. Examples of the resins include thermoplastic resins such as polyvinyl alcohol, phenol resins, and furan resins. For example, a phenol resin having a residual carbon ratio of 10 to 50% is used as the precursor of the carbonaceous material A, and a coal tar pitch having a residual carbon ratio of 50 to 90% is used as the precursor of the carbonaceous material B. Is preferred.

  Since the precursor of the carbonaceous material A having a relatively low residual carbon ratio generates many voids in the heated carbonaceous material, it mainly plays a role of void formation around the metal. On the other hand, the precursor of the carbonaceous material B having a relatively high residual carbon ratio has few voids generated in the carbonaceous material after heating and can form a dense carbonaceous material. It forms the outermost layer and plays the role of surrounding the composite particles. As a result, it is possible to reduce the irreversible capacity (improvement of initial charge / discharge efficiency) of a lithium ion secondary battery using a negative electrode material containing the same. Therefore, in the manufacturing process of the composite particles of the present invention, the precursor of the carbonaceous material A is first mixed with a metal or a graphite material, and then combined, and then the precursor of the carbonaceous material B is mixed and combined. It is necessary to make it.

  When the remaining carbon ratio of the precursors of the carbonaceous materials A and B is reversed, that is, when the mixing order of the precursors of the carbonaceous materials A and B is opposite to the above, the formation of voids is also the outermost layer. As a result, the cycle characteristics and the initial charge / discharge capacity of the lithium ion secondary battery using this as a negative electrode material cannot be improved. Further, when the carbonaceous material is derived from only one type of carbonaceous material precursor, there is no substantial difference in the residual carbon ratio, so either the formation of the voids or the reduction of the irreversible capacity Only one effect can be obtained.

(Metal that can be alloyed with lithium)
Metals that can be alloyed with lithium are Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, Sb, Ge, Ni, and the like. Two or more kinds of alloys may be used. The alloy may further contain elements other than those described above. Preferred metals are silicon Si and tin Sn, and particularly preferred is readily available silicon. In addition, since these metal compounds have a volume expansion due to alloying with lithium smaller than that of the metal, a part of the metal may be a compound such as an oxide, a nitride, or a carbide. Preferred are silicon oxide and tin oxide, and particularly preferred is readily available silicon oxide. When using a metal and a metal compound together, it is preferable that a metal compound shall be 10-95 mass% with respect to the total amount. Within this range, the effect of reducing expansion is exhibited. More preferred is 50 to 95% by mass.
The average particle diameter of the metal is preferably 10 μm or less, and more preferably 5 μm or less. If it exceeds 10 μm, the effect of improving the cycle characteristics may be reduced.
The shape of the metal is not particularly limited. Any of a granular shape, a spherical shape, a plate shape, a scale shape, a needle shape, a thread shape and the like may be used. Here, the average particle diameter means a particle diameter at which the cumulative frequency measured with a laser diffraction particle size meter is 50% in volume fraction.

(Graphite material)
The graphite material is not particularly limited as long as it can occlude and release lithium ions. Artificial graphite or natural graphite obtained by heat treatment (graphitization) of tar or pitch at a temperature of 1500 ° C. or higher is finally used, for example, part or all of which is made of graphite. Specifically, mesophase fired bodies, mesophase spherules, and cokes obtained by heat-treating and polycondensing easily graphitizable carbon materials such as petroleum-based or coal-based tar pitches are 1500 ° C. or higher, preferably 2800- It can be obtained by graphitization at 3300 ° C.
The shape of the graphite material may be any of a spherical shape, a block shape, a plate shape, a flaky shape, a fibrous shape, and the like, but a flaky shape or a shape close to a flaky shape is particularly preferable. Moreover, the above-mentioned various mixtures, granulated products, coatings, and laminates may be used. Further, it may be subjected to various chemical treatments in the liquid phase, gas phase, and solid phase, heat treatment, oxidation treatment, physical treatment, and the like.
The average particle diameter of the graphite material is preferably 1 to 30 μm, particularly preferably 3 to 15 μm.

The present invention is a negative electrode material for a lithium ion secondary battery containing the composite particles, or a negative electrode for a lithium ion secondary battery using the negative electrode material, and further a lithium ion secondary battery using the negative electrode. .
(Negative electrode)
The negative electrode for a lithium ion secondary battery of the present invention is produced according to a normal negative electrode molding method, but is not limited as long as it is a method capable of obtaining a chemically and electrochemically stable negative electrode. When preparing the negative electrode, it is preferable to use a negative electrode mixture prepared in advance by adding a binder to the composite particles of the present invention. As the binder, those showing chemical and electrochemical stability with respect to the electrolyte are preferable. For example, fluorine-based resin powders such as polytetrafluoroethylene and polyvinylidene fluoride, and resin powders such as polyethylene and polyvinyl alcohol Carboxymethyl cellulose and the like are used. These can also be used together. A binder is normally used in the ratio of about 1-20 mass% in the whole quantity of a negative electrode mixture.

  More specifically, first, the composite particles of the present invention are adjusted to a desired particle size by classification or the like, and a mixture obtained by mixing with a binder is dispersed in a solvent to prepare a negative electrode mixture in the form of a paste. That is, a slurry obtained by mixing the composite particles of the present invention and a binder with a solvent such as water, isopropyl alcohol, N-methylpyrrolidone, dimethylformamide, and the like, using a known stirrer, mixer, kneader, kneader, etc. Use to stir and mix to prepare paste. When the paste is applied to one or both sides of the current collector and dried, a negative electrode in which the negative electrode mixture layer is uniformly and firmly bonded is obtained. The film thickness of the negative electrode mixture layer is 10 to 200 μm, preferably 20 to 100 μm.

The negative electrode of the present invention can also be produced by dry-mixing the composite particles of the present invention and resin powders such as polyethylene and polyvinyl alcohol and hot pressing in a mold.
When the negative electrode mixture layer is formed and then subjected to pressure bonding such as pressing, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased.
The shape of the current collector used for producing the negative electrode is not particularly limited, but may be a foil shape, a mesh shape, a net shape such as expanded metal, or the like. The material for the current collector is preferably copper, stainless steel, nickel or the like. The thickness of the current collector is preferably about 5 to 20 μm in the case of a foil.
The negative electrode of the present invention includes a metal that can be alloyed with lithium, a composite particle containing a graphite material and a carbonaceous material, a graphite material such as natural graphite, and a carbonaceous material such as amorphous hard carbon, You may mix | blend organic substances, such as a phenol resin, metals, such as a silicon | silicone, metal compounds, such as a tin oxide.

(Lithium ion secondary battery)
A lithium ion secondary battery usually has a negative electrode, a positive electrode, and a non-aqueous electrolyte as main battery components. The positive electrode and the negative electrode are each composed of a lithium ion carrier, and lithium ions are occluded in the negative electrode during charging, and the negative electrode during discharging. By battery mechanism to detach from.
The lithium ion secondary battery of the present invention is not particularly limited except that the negative electrode material of the present invention is used as the negative electrode material, and other battery components such as a positive electrode, an electrolyte, and a separator are general lithium ion secondary batteries. According to the elements of

(Positive electrode)
The positive electrode is formed, for example, by applying a positive electrode mixture comprising a positive electrode material, a binder and a conductive agent to the surface of the current collector. The positive electrode material (positive electrode active material) is preferably selected from materials that can occlude / release a sufficient amount of lithium, and lithium such as lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides, and lithium compounds thereof. Containing compound, general formula M _X Mo ₆ S _8-y (wherein M is at least one transition metal element, X is a numerical value in the range of 0 ≦ X ≦ 4, Y is 0 ≦ Y ≦ 1) Chevrel phase compounds, activated carbon, carbon fiber and the like. The vanadium oxide is represented by V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , or V ₃ O ₈ .

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals. The composite oxide may be used alone or in combination of two or more. Specifically, the lithium-containing transition metal oxide is LiM ¹ _1-X M ² _X O ₂ (wherein M ¹ and M ² are at least one transition metal element, and X is in the range of 0 ≦ X ≦ 1. LiM ¹ _1-Y M ² _Y O ₄ (wherein M ¹ and M ² are at least one transition metal element, and Y is a value in the range of 0 ≦ Y ≦ 1). Indicated.
The transition metal elements represented by M ¹ and M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Ti, Cr , V, Al, etc. Preferred examples are LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _0.9 Co _0.1 O ₂ , LiNi _0.5 Mn _0.5 O ₂ and the like.
Examples of the lithium-containing transition metal oxide include lithium, transition metal oxides, hydroxides, salts, and the like as starting materials, and these starting materials are mixed in accordance with the composition of the desired metal oxide, and are mixed under an oxygen atmosphere. It can be obtained by firing at a temperature of ˜1000 ° C.

The positive electrode active material may be used alone or in combination of two or more. For example, a carbonate such as lithium carbonate can be added to the positive electrode. Moreover, when forming a positive electrode, conventionally well-known various additives, such as a electrically conductive agent and a binder, can be used suitably.
The positive electrode is produced by applying a positive electrode mixture comprising the positive electrode material, a binder, and a conductive agent for imparting conductivity to the positive electrode on both sides of the current collector to form a positive electrode mixture layer. As the binder, the same one as that used for producing the negative electrode can be used. As the conductive agent, known materials such as graphitized materials and carbon black are used.
The shape of the current collector is not particularly limited, but a foil shape or a mesh shape such as a mesh or expanded metal is used. The material of the current collector is aluminum, stainless steel, nickel or the like. The thickness is preferably 10 to 40 μm.

  Similarly to the negative electrode, the positive electrode mixture may be formed in a paste by dispersing the positive electrode mixture in a solvent, and the paste-like positive electrode mixture may be applied to a current collector and dried to form a positive electrode mixture layer. After forming the agent layer, pressure bonding such as pressing may be further performed. As a result, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.

(Nonaqueous electrolyte)
The non-aqueous electrolyte used in the lithium ion secondary battery of the present invention, an electrolyte salt used in the conventional non-aqueous electrolyte, for _{example, LiPF 6, LiBF 4, LiAsF} 6, LiClO 4, LiB (C 6 _{H 5), LiCl, LiBr,} LiCF 3 SO 3, LiCH 3 SO 3, LiN (CF 3 SO 2) 2, LiC (CF 3 SO 2) 3, LiN (CF 3 CH 2 OSO 2) 2, LiN (CF ₃ CF ₂ OSO ₂ ) ₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN ((CF ₃ ) ₂ CHOSO ₂ ) ₂ , LiB [(C ₆ H ₃ ((CF ₃ ) ₂ ] ₄ , LiAlCl ₄ Lithium salts such as LiSiF ₆ can be used, and LiPF ₆ and LiBF ₄ are particularly preferably used from the viewpoint of oxidation stability.
The electrolyte salt concentration in the electrolyte is preferably 0.1 to 5 mol / l, more preferably 0.5 to 3.0 mol / l.

  Examples of the solvent for preparing the non-aqueous electrolyte include carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2 -Methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, ethers such as anisole and diethyl ether, thioethers such as sulfolane and methylsulfolane, acetonitrile, chloronitrile, propionitrile, etc. Nitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethylorthoformate, nitrobenzene, benzoyl chloride, benzobromide An aprotic organic solvent such as yl, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, dimethyl sulfite can be used.

When the nonaqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or polymer gel electrolyte, a polymer compound gelled with a plasticizer (nonaqueous electrolyte) is used as a matrix. Examples of the matrix polymer compound include ether-based resins such as polyethylene oxide and cross-linked products thereof, polymethacrylate-based resins, polyacrylate-based resins, polyvinylidene fluoride (PVDF), and vinylidene fluoride-hexafluoropropylene copolymers. Fluorine resins can be used alone or in combination.
Among these, from the viewpoint of oxidation-reduction stability, it is preferable to use a fluorine-based resin such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer.
As the plasticizer to be used, the above electrolyte salt or non-aqueous solvent can be used. In the case of a polymer gel electrolyte, the electrolyte salt concentration in the non-aqueous electrolyte as a plasticizer is preferably from 0.1 to 5 mol / l, more preferably from 0.5 to 2.0 mol / l.

The production of the polymer electrolyte is not particularly limited. For example, a method of mixing a polymer compound constituting a matrix, a lithium salt and a non-aqueous solvent (plasticizer) and heating to melt and dissolve the polymer compound, for mixing A method in which a polymer compound, a lithium salt, and a non-aqueous solvent are dissolved in an organic solvent, and then the organic solvent for mixing is evaporated. A polymerizable monomer, a lithium salt, and a non-aqueous solvent are mixed. Alternatively, a method of polymerizing a polymerizable monomer by irradiating a molecular beam or the like to obtain a polymer compound can be exemplified.
The ratio of the nonaqueous solvent in the polymer electrolyte is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass. If it is less than 10% by mass, the electrical conductivity will be low, and if it exceeds 90% by mass, the mechanical strength will be weak and it will be difficult to form a film.

(Separator)
In the lithium ion secondary battery of the present invention, a separator can also be used. Although a separator is not specifically limited, For example, a woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. are mentioned. A synthetic resin microporous membrane is preferred, and among them, a polyolefin microporous membrane is preferred in terms of thickness, membrane strength, and membrane resistance. Specifically, it is a microporous membrane made of polyethylene and polypropylene, or a microporous membrane that combines these.
In the lithium ion secondary battery of the present invention, a polymer electrolyte can also be used.

  A lithium ion secondary battery using a polymer electrolyte is generally called a polymer battery, and is composed of a negative electrode using the composite particles of the present invention, a positive electrode, and a polymer electrolyte. For example, the negative electrode, the polymer electrolyte, and the positive electrode are laminated in this order, and are housed in a battery outer packaging material. In addition to this, a polymer electrolyte may be further arranged outside the negative electrode and the positive electrode.

  Furthermore, the structure of the lithium ion secondary battery of the present invention is arbitrary, and is not particularly limited with respect to its shape and form, and may be cylindrical, rectangular, depending on the application, mounted equipment, required charge / discharge capacity, etc. It can be arbitrarily selected from a mold, a coin mold, a button mold, and the like. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to include a means for detecting an increase in the internal pressure of the battery and shutting off the current when there is an abnormality such as overcharging. In the case of a polymer solid electrolyte battery or a polymer battery, a structure enclosed in a laminate film can also be used.

  EXAMPLES Next, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited to these examples. In Examples and Comparative Examples, evaluation button type secondary batteries having the configuration as shown in FIG. 1 were produced and evaluated. A real battery can be manufactured according to a well-known method based on the objective of this invention.

In Examples and Comparative Examples, the residual carbon ratio of the precursor of the carbonaceous material was measured as follows in accordance with the fixed carbon method of JIS K2425.
1 g of carbonaceous material was weighed into a crucible and heated in an electric furnace at 430 ° C. for 30 minutes without a lid. Then, it was made into a double crucible and heated in an electric furnace at 800 ° C. for 30 minutes to remove volatile components, and the remaining percentage was defined as the residual coal rate.
The average particle size of the composite particles was measured using a laser diffraction particle size distribution meter (manufactured by Seishin Co., Ltd., LS-5000), and the particle size was such that the cumulative frequency was 50% in volume fraction.
The porosity of the entire composite particles was determined by measuring the volume of all the voids using a mercury porosimeter and determining the ratio of the total volume of the composite particles.
The ratio of the voids around the metal to the total voids of the composite particles is obtained by calculating the area ratio of the void region two-dimensionally from the scanning electron microscope observation of the particle cross section. The average value was adopted. Here, if the void exists in direct contact with at least a part of the surface of the metal, the void around the metal is defined.

(Example 1)
(Manufacture of composite particles)
A slurry in which a metal silicon powder [manufactured by Kojundo Chemical Laboratory Co., Ltd., average particle size 2 μm] is dispersed in an ethanol solution of a phenol resin [manufactured by Sumitomo Bakelite Co., Ltd., residual carbon ratio 50%], and natural graphite [ (Made by Chuetsu Graphite Industries Co., Ltd., average particle size 10 μm) was kneaded at 150 ° C. for 1 hour using a biaxial heating kneader to obtain a kneaded product. At that time, the solid content ratio was adjusted to be phenol resin: silicon powder: natural graphite = 18: 6: 76.
Subsequently, a coal tar pitch oil [manufactured by JFE Chemical Co., Ltd., residual carbon ratio 60%] was mixed with a tar oil in a tar to prepare a coal tar pitch solution. The solution and the kneaded product were kneaded at 200 ° C. for 1 hour using a biaxial heating kneader. At that time, the solid content ratio was adjusted to be coal tar pitch: the kneaded material = 30: 70. After kneading, a vacuum was applied to remove the solvent in the kneaded product.

The obtained kneaded material was coarsely pulverized and then heated at 1000 ° C. for 10 hours to make the kneaded material substantially free of volatiles. That is, the phenol resin and coal tar pitch were carbonized. The average particle diameter of the obtained composite particles was 15 μm. Table 1 shows the composition of the kneaded material before heating, the composition of the obtained composite particles of silicon / natural graphite / carbonaceous material, the measured porosity of the entire composite particles, and the ratio of the voids around the metal to the total voids of the composite particles. It was shown to.
As shown in FIG. 2, the composite particle is a composite particle in which silicon 11 and natural graphite 12 are integrated with a carbonaceous material A derived from coal tar pitch and a carbonaceous material B derived from a phenol resin. There were voids around.

(Preparation of negative electrode mixture paste)
90% by mass of the composite particles and 10% by mass of polyvinylidene fluoride were placed in N-methylpyrrolidone, and stirred and mixed at 2000 rpm for 30 minutes using a homomixer to prepare an organic solvent-based negative electrode mixture.

(Production of working electrode)
The negative electrode mixture paste was applied to a copper foil with a uniform thickness, the solvent was volatilized in a vacuum at 90 ° C., dried, and the negative electrode mixture layer was pressed by a hand press. A copper foil and a negative electrode mixture layer were punched into a cylindrical shape having a diameter of 15.5 mm to produce a working electrode composed of a current collector and a negative electrode mixture adhered to the current collector.

(Production of counter electrode)
The lithium metal foil was pressed onto a nickel net and punched into a cylindrical shape with a diameter of 15.5 mm to produce a current collector made of nickel net and a counter electrode made of lithium metal foil in close contact with the current collector.

(Electrolyte / Separator)
LiPF ₆ was dissolved at a concentration of 1 mol / dm ^{3 in a} mixed solvent obtained by mixing 33 vol% ethylene carbonate and 67 vol% methyl ethyl carbonate to prepare a non-aqueous electrolyte. The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body to produce a separator impregnated with the electrolytic solution.

(Evaluation battery)
A button-type secondary battery shown in FIG. 1 was produced as an evaluation battery.
A separator 5 impregnated with an electrolytic solution was sandwiched between the negative electrode 2 in close contact with the current collector 7b and the positive electrode 4 in close contact with the current collector 7a. Thereafter, the outer cup 1 and the outer can 3 were combined so that the negative electrode current collector 7 b side was accommodated in the outer cup 1 and the positive electrode current collector 7 a side was accommodated in the outer can 3. At that time, an insulating gasket 6 was interposed between the peripheral edges of the outer cup 1 and the outer can 3, and both peripheral edges were caulked and sealed.
The evaluation battery was subjected to the following charge / discharge experiments at a temperature of 25 ° C., and the discharge capacity, initial charge / discharge efficiency, and cycle characteristics were calculated. The evaluation results are shown in Table 2.

(Discharge capacity and initial charge / discharge efficiency)
The constant current charging was performed until the circuit voltage reached 0 mV at a current value of 0.9 mA. When the circuit voltage reached 0 mV, switching was made to constant voltage charging, and the charging was continued until the current value reached 20 μA. The charging capacity was determined from the amount of electricity applied during that time. Then, it rested for 120 minutes. Next, constant current discharge was performed until the circuit voltage reached 1.5 V at a current value of 0.9 mA, and the discharge capacity was determined from the amount of electricity supplied during this period. The initial charge / discharge efficiency was calculated from the following equation. In this test, the process of occluding lithium into the graphite particles was charged and the process of detaching was defined as discharge.
Initial charge / discharge efficiency (%) = (first cycle discharge capacity / first cycle charge capacity) × 100

(Cycle characteristics)
An evaluation battery different from the evaluation battery that evaluated the discharge capacity and the initial charge / discharge efficiency was similarly produced, and the following evaluation was performed.
After constant current charging at a current value of 4.0 mA until the circuit voltage reached 0 mV, switching to constant voltage charging was continued until the current value reached 20 μA, and then rested for 120 minutes. Next, constant current discharge was performed at a current value of 4.0 mA until the circuit voltage reached 1.5 V, and the discharge capacity was obtained from the energization amount during this period. Charging / discharging was repeated 20 times. The cycle characteristics were calculated using the following formula.
Cycle characteristics = (discharge capacity in 20th cycle / discharge capacity in 1st cycle) × 100
Table 2 shows the evaluation results for battery characteristics (discharge capacity, initial charge / discharge efficiency, and cycle characteristics).
As shown in Table 2, the evaluation battery obtained using the composite particles of Example 1 as the working electrode exhibits a high discharge capacity and has a high initial charge / discharge efficiency. Furthermore, it exhibits excellent cycle characteristics.

(Example 2)
Composite particles were produced in the same manner as in Example 1, except that the solid content ratio of the composite particles in Example 1 was changed to phenol resin: silicon powder: natural graphite = 27: 6: 67. The average particle diameter of the obtained composite particles was 14 μm. Table 1 shows the composition of the kneaded product before heating, the composition of silicon: natural graphite: carbonaceous material of the composite particles, the measured porosity of the composite particles, and the ratio of the voids around the metal to the total voids of the composite particles. . In addition, a lithium ion secondary battery was produced using the composite particles, the negative electrode, and the nonaqueous electrolyte under the same method and conditions as in Example 1. The discharge capacity, initial charge / discharge efficiency and cycle characteristics of the battery were measured in the same manner as in Example 1, and the evaluation results are shown in Table 2.
As shown in Table 2, the evaluation battery obtained using the composite particles of Example 2 as the working electrode shows a high discharge capacity and has a high initial charge / discharge efficiency. Furthermore, it exhibits excellent cycle characteristics.

(Example 3)
The phenol resin and coal tar pitch used in Example 1 are different from each other in phenol resin [manufactured by JFE Chemical Co., Ltd .: residual carbon ratio 40%] and coal tar pitch [manufactured by JFE Chemical Co., Ltd .: residual carbon ratio 65%]. ] Composite particles were produced in the same manner as in Example 1, except that The average particle diameter of the obtained composite particles was 15 μm. Table 1 shows the composition of the kneaded product before heating, the composition of silicon: natural graphite: carbonaceous material of the composite particles, the measured porosity of the composite particles, and the ratio of the voids around the metal to the total voids of the composite particles. . In addition, a lithium ion secondary battery was produced using the composite particles, the negative electrode, and the nonaqueous electrolyte under the same method and conditions as in Example 1. The discharge capacity, initial charge / discharge efficiency and cycle characteristics of the battery were measured in the same manner as in Example 1, and the evaluation results are shown in Table 2.
As shown in Table 2, the evaluation battery obtained using the composite particles of Example 3 as the working electrode exhibits a high discharge capacity and has a high initial charge / discharge efficiency. Furthermore, it exhibits excellent cycle characteristics.

Example 4
In place of the silicon powder used in Example 1, tin powder (Aldrich, average particle size 1 μm) was used, and when mixed with natural graphite in an ethanol solution of phenol resin, the solid content ratio was phenol resin: tin. Composite particles were produced in the same manner and under the same conditions as in Example 1 except that powder: natural graphite = 18: 27: 55. The average particle diameter of the obtained composite particles was 12 μm. Table 1 shows the kneaded material before heating, the composition of tin powder of composite particles: natural graphite: carbonaceous material, the measured porosity of the composite particles, and the ratio of the voids around the metal to the total voids of the composite particles. It was. In addition, a lithium ion secondary battery was produced using the composite particles, the negative electrode, and the nonaqueous electrolyte under the same method and conditions as in Example 1. The discharge capacity, initial charge / discharge efficiency and cycle characteristics of the battery were measured in the same manner as in Example 1, and the evaluation results are shown in Table 2.
As shown in Table 2, the evaluation battery obtained using the composite particles of Example 4 as the working electrode exhibits a high discharge capacity and has a high initial charge / discharge efficiency. Furthermore, it exhibits excellent cycle characteristics.

(Example 5)
In Example 1, in addition to silicon powder, silicon monoxide powder [manufactured by Kojundo Chemical Laboratory Co., Ltd., average particle size 10 μm] was used, and 50 parts by mass of silicon powder and 50 parts by mass of silicon monoxide powder were blended. When mixing with natural graphite in an ethanol solution of phenol resin, the solid content ratio is phenol resin: (silicon powder + silicon monoxide powder): natural graphite = 17.6: 8.7: 73.8 The same method as in Example 1, except that the solid content ratio is equal to coal tar pitch: the kneaded material = 27.5: 72.5 when kneaded with the coal tar pitch solution. Composite particles were produced under the conditions described above. The average particle diameter of the obtained composite particles was 20 μm. The composition of the kneaded product before heating, the composite particle silicon: silicon monoxide: natural graphite: carbonaceous material, the measured porosity of the entire composite particle, and the ratio of the void around the metal to the total void of the composite particle are shown. It was shown in 1. In addition, a lithium ion secondary battery was produced using the composite particles, the negative electrode, and the nonaqueous electrolyte under the same method and conditions as in Example 1. The discharge capacity, initial charge / discharge efficiency and cycle characteristics of the battery were measured in the same manner as in Example 1, and the evaluation results are shown in Table 2.
As shown in Table 2, the evaluation battery obtained using the composite particles of Example 5 as the working electrode exhibits a high discharge capacity and has a high initial charge / discharge efficiency. Furthermore, it exhibits excellent cycle characteristics.

(Comparative Example 1)
The same metal, graphite material and carbonaceous material used in Example 1 were used, and these were kneaded together, that is, phenol resin, metal silicon powder, natural graphite and coal tar pitch were mixed with oil in tar. Then, after kneading simultaneously with a biaxial heating kneader, the kneaded product was heated to remove the solvent and dried. Composite particles were produced in the same manner as in Example 1 except that the obtained kneaded material was pulverized and fired at 1000 ° C. for 10 hours. The average particle diameter of the obtained composite particles was 15 μm. Table 1 shows the composition of the kneaded product before heating, the composition of silicon: natural graphite: carbonaceous material of the composite particles, the measured porosity of the composite particles, and the ratio of the voids around the metal to the total voids of the composite particles. . In addition, a lithium ion secondary battery was produced using the composite particles, the negative electrode, and the nonaqueous electrolyte under the same method and conditions as in Example 1. The discharge capacity, initial charge / discharge efficiency and cycle characteristics of the battery were measured in the same manner as in Example 1, and the evaluation results are shown in Table 2.
As shown in Table 2, in Comparative Example 1 in which no voids exist around the silicon particles, high initial charge / discharge efficiency and cycle characteristics cannot be obtained. This is presumably because the structure of the composite particles was destroyed by the expansion / contraction of the silicon particles during charge / discharge, resulting in a decrease in conductivity and separation of the active material from the current collector.

It is a schematic cross section which shows the structure of the button type evaluation battery for using for a charging / discharging test. It is a schematic diagram of the cross section of the composite particle of Example 1 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exterior cup 2 Working electrode 3 Exterior can 4 Counter electrode 5 Separator 6 Insulating gasket 7a, 7b Current collector 11 Metal silicon 12 Natural graphite 13 Carbon material derived from coal tar pitch 14 Carbonaceous material derived from phenol resin 15 Void

Claims (8)

  In composite particles made of a metal that can be alloyed with lithium, a graphite material, and a carbonaceous material, the composite particles have voids, and the ratio of voids around the metal to the total voids of the composite particles is 20% or more Composite particles characterized by being.   The composite particle according to claim 1, wherein the metal is silicon.   The composite particle according to claim 1, wherein the metal is silicon, and a part of the silicon is an oxide.   A metal that can be alloyed with lithium, a graphite material, and a precursor of carbonaceous material A are mixed, and the obtained composite particles are made of carbonaceous material B having a higher residual carbon ratio than the precursor of carbonaceous material A. A method for producing composite particles, wherein the precursor is mixed and then heated.   The method for producing composite particles according to claim 4, wherein a carbon residue rate of the carbonaceous material A is 10% or more lower than a carbon residue rate of the carbonaceous material B.   A negative electrode material for a lithium ion secondary battery, comprising the composite particles according to claim 1.   A negative electrode for a lithium ion secondary battery, wherein the negative electrode material for a lithium ion secondary battery according to claim 6 is used. A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 7.

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