JP2007031233A - Method for producing graphite material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a graphite material with high crystallinity by graphitizing a meso carbon spherule and/or a fired product thereof as a raw material. <P>SOLUTION: This method for producing the graphite material comprises graphitizing the meso carbon spherule and/or the fired product thereof in the presence of an iron element and a silicon element, where the rate of the total amount of the iron element and the silicon element to the total amount of the meso carbon spherule, the fired product of the meso carbon spherule, the iron element and the silicon element is 0.1-25 mass% and the rate of the iron element to the total amount of the iron element and the silicon element is 30-90 mass%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a graphite material which provides a negative electrode material for a lithium ion secondary battery having a high discharge capacity by graphitizing mesocarbon spherules and / or a fired product of mesocarbon spherules.

  Lithium-ion secondary batteries have excellent features such as high operating voltage, large battery capacity, long cycle life, and low environmental pollution. Therefore, conventional nickel-cadmium batteries and nickel It is widely used in place of hydrogen batteries. Lithium ion secondary batteries have become practical because it was discovered that carbon materials with intercalated lithium ions could be a stable active material instead of lithium metal, which had a safety problem as a negative electrode material. This is due to the recognition of the role of carbon materials in the practical application and performance improvement of lithium ion secondary batteries.

  With the recent increase in performance and functionality of portable electronic devices such as mobile phones and notebook computers, power consumption has increased, and further increase in capacity of lithium ion secondary batteries has been demanded. The capacity of the lithium ion secondary battery is particularly determined by the discharge capacity per mass of the carbon material for the negative electrode. The discharge capacity per mass is the theoretical capacity of 372 mAh / g is the limit, and the problem is to make the discharge capacity of the carbon material for the negative electrode as close as possible to the theoretical capacity of natural graphite. On the other hand, in order to improve the discharge capacity per lithium ion secondary battery, it is also important to improve the discharge capacity per volume. That is, it is important to improve the electrode density of the negative electrode plate and fill the negative electrode active material as much as possible.

However, natural graphite, which is said to have the highest discharge capacity per mass, originates from its scaly structure, and when trying to improve the electrode density, it is oriented parallel to the current collector, so the inside of the lithium ion active material Tended to be difficult to insert into.
Therefore, in order to prevent the orientation of the carbon material, a technique using a spherical carbon material, for example, a graphitized material of mesocarbon microspheres (mesophase microspheres) is known (Patent Document 1). However, even if mesocarbon microsphere graphitized materials are used, the discharge capacity does not reach that of natural graphite.

Therefore, it is conceivable to improve the discharge capacity by increasing the crystallinity of the carbon material made from mesocarbon microspheres. As one of the techniques for increasing the crystallinity of carbon materials, metal oxides (iron oxide, chromium oxide, aluminum oxide, silicon oxide, etc.) and / or metals (copper, platinum or palladium) are used as carbonization catalysts for mesophase microspheres. A technique for heating and carbonizing the small spheres is known (Patent Document 2). Also, graphitizable aggregates (coke powder, etc.) or graphite and graphitizable binders (pitch, tar, etc.), and graphitization catalyst (metals such as titanium, silicon, iron or oxides or carbides thereof) A technique of using and firing at a temperature of 2500 ° C. or higher and graphitizing is known (Patent Document 3).
However, the crystallinity of the graphite material obtained by these improved techniques is not sufficiently high, and the discharge capacity of a lithium ion secondary battery comprising a negative electrode material and a negative electrode using the graphite material is not satisfactory.

Japanese Patent Application Laid-Open No. 4-115457, page 3, upper left column Japanese Patent Application Laid-Open No. 2001-107057, paragraph 0053 Japanese Patent Application Laid-Open No. 2004-6358, paragraphs 0020-0023

  Therefore, the present invention provides a discharge capacity of a lithium ion secondary battery when a graphite material obtained by graphitization using mesocarbon microspheres and / or a fired product thereof as a raw material is used for a negative electrode material and a negative electrode. It is an object to provide a method for producing a graphite material that can be made high.

  The present invention relates to a method for producing a graphite material for graphitizing mesocarbon spherules and / or calcined mesocarbon spherules in the presence of iron element and silicon element. The ratio of the total amount of the iron element and the silicon element to the total amount of the fired small sphere, the iron element and the silicon element is 0.1 to 25% by mass, and the ratio of the iron element to the total amount of the iron element and the silicon element described above The ratio is 30 to 90% by mass, and the method for producing a graphite material.

  In the method for producing a graphite material according to the present invention, the iron element is preferably in the form of iron and / or an iron compound, and the silicon element is preferably in the form of silicon and / or a silicon compound.

  In the method for producing a graphite material of the present invention, the iron element and the silicon element are preferably in the form of a compound of iron and silicon.

  In the method for producing a graphite material of the present invention, the graphite material is preferably a negative electrode material for a lithium ion secondary battery.

  The graphite material obtained by graphitizing the mesocarbon spherules and / or the fired product of the present invention has high crystallinity, and when used as a negative electrode material for a lithium ion secondary battery, a lithium ion secondary having a high discharge capacity. A battery can be provided.

  As a result of detailed examination of the graphitization catalyst, the present inventor has graphitized the mesocarbon microspheres of the raw material by a synergistic effect when iron element (including iron compound) and silicon element (including silicon compound) are used in combination. Thus, the crystallinity was improved, and when the graphite material was used as the negative electrode material of a lithium ion secondary battery, it was found that the discharge capacity could be improved, and the method for producing the graphite material of the present invention was completed.

  The method for producing a graphite material according to the present invention uses mesocarbon spherules and / or calcined mesocarbon spherules (hereinafter, also simply referred to as mesocarbon spherules or spherules) using iron element and silicon element as a catalyst. This is a method of graphitizing by adjusting the abundance of iron element and silicon element within a specific range when graphitizing. As a result, the crystallinity of the graphite material can be increased, thereby increasing the discharge capacity of the lithium ion secondary battery. That is, the ratio of the total amount of iron element and silicon element to the total amount of mesocarbon microspheres, mesocarbon microspheres, iron element and silicon element is 0.1 to 25% by mass, and iron element and silicon element This is a method for producing the graphite material by adjusting the ratio of the iron element to the total amount of 30 to 90% by mass and firing and graphitizing.

(Mesocarbon microsphere)
Mesocarbon spherules, which are starting materials for the graphite material of the present invention, include coal tar, tar light oil, tar medium oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, oxygen-crosslinked petroleum pitch, heavy oil, etc. Preferably, it is prepared from coal tar pitch or petroleum pitch, for example, by the following method.
350 to 1000 ° C., preferably 400 to 600 ° C., more preferably 400 to petroleum or coal-based tar pitches containing 0.01 to 2% by mass, preferably 0.3 to 0.9% by mass of free carbon. When heat-treated at ˜450 ° C., spherical bodies having optical anisotropy, that is, mesocarbon microspheres are formed. The small spheres are present in the pitch after heat treatment in a proportion of 1 to 50% by mass, preferably 15 to 40% by mass. From this, it is possible to obtain mesocarbon spherules by extraction with tar oil or the like and filtration.

The mesocarbon spherule fired product used in the present invention is the mesocarbon spherule 300.
It is a carbon material obtained by heat treatment (firing) at ˜1200 ° C., and the volatile content is adjusted to 2.0 to 30% by mass, preferably 3 to 10% by mass. The calcination improves the crystallinity of the small spheres.
As a raw material for producing the graphite material of the present invention, the composition ratio in the case of using mesocarbon spherules and mesocarbon spherules in combination is not limited at all. The aggregation of the microspheres is small.
The average particle size of the mesocarbon microspheres and the fired product of the mesocarbon microspheres is several μm to several tens μm, preferably 10 to 70 μm, and more preferably 15 to 40 μm. The spherical shape is preferably as close as possible to a spherical shape, but may be an elliptical spherical shape, for example.

(Graphitization catalyst)
The graphitization catalyst used in the present invention is an iron element and a silicon element. Of course, an iron element and a silicon element mean that these metal elements themselves, an iron compound, and a silicon compound are included. Examples of the iron compound and silicon compound include inorganic compounds such as oxides, double oxides, chlorides, and nitrides, and organic compounds such as alkoxysilanes, chlorosilanes, and acetic acid oxides. Preferred are inorganic compounds, more preferred are oxides such as Fe ₂ O ₃ , Fe ₃ O ₄ , FeO, SiO ₂ , and SiO, double oxides, and ferrosilicon, and tetramethoxysilane and dimethyldimethoxy. Such as silane. Particularly preferred are Fe ₂ O ₃ , tetramethoxysilane and ferrosilicon.
The average particle size of the iron element and silicon element is preferably as small as possible in order to bring out the maximum effect with a small amount of use, and is preferably 1 nm to 100 μm, preferably 1 nm to 10 μm, more preferably 1 nm. ˜1 μm.

The total amount of iron element and silicon element is 0.1 to 25% by mass, preferably 1 to 20% by mass, based on the total amount of mesocarbon microspheres, calcined mesocarbon microspheres, iron element and silicon element. %, More preferably 5 to 15% by mass. When the total amount is less than 0.1% by mass, the catalytic effect is insufficient and the crystallinity of the graphite material cannot be sufficiently increased. On the other hand, if the total amount exceeds 25% by mass, there are iron elements and silicon elements that are not used, which is economically useless. Thus, deviations from this range must be avoided.
For example, when the iron element is iron oxide, the amount of the iron element is an amount that does not include oxygen.
Moreover, an iron element is used in the ratio of 30-90 mass% with respect to the total amount of an iron element and a silicon element, Preferably it is 40-80 mass%, More preferably, it is 50-70 mass%. When it deviates from this range, the yield of the graphite material is lowered, and the crystallinity of the obtained graphite material cannot be sufficiently increased. When the total amount is less than 30% by mass, the ability of the catalyst is lowered, and the crystallinity of the obtained graphite material cannot be sufficiently increased. On the other hand, when the total amount exceeds 90% by mass, iron and carbon react during graphitization to produce cementite (Fe ₃ C), and when cementite decomposes at high temperature, the surface of the mesocarbon microspheres There is a harmful effect of wearing out. Thus, deviations from this range must be avoided.
By limiting the total amount of iron element and silicon element and the amount of iron element to the above range, the crystallinity of the graphite material obtained by graphitization is increased, and the discharge capacity of the lithium ion secondary battery is synergistic. (See [FIG. 2]). From this, it is estimated that the iron element and the silicon element are exhibiting the effect more than a mere catalyst.

  The mixing of the mesocarbon spherules, the fired product thereof, the iron element, and the silicon element is not particularly limited, but the dispersion obtained by previously mixing the iron element and silicon element in a solvent, mixing and dispersing the mesocarbon spherule, It is preferable to graphitize after mixing with the fired product, filtering and separating, and drying. This is because mixing and dispersion of mesocarbon spherules and the like become uniform, and the graphitization efficiency is improved. As a dispersion solvent, an organic solvent such as acetone, toluene, or a tar oil is preferably used. The drying may be performed under conditions in which iron element and silicon element are not oxidized, for example, vacuum drying at 50 to 120 ° C. or hot air drying in a nitrogen atmosphere.

(Graphitization method)
The graphitization method of the present invention is performed, for example, by the following method.
(1) Heating the coal tar pitch to add the iron element and silicon element or their solvent dispersion to the heat-treated pitch that has produced mesocarbon spherules, and after mixing, add tar oil or the like, A method of heating, calcining and graphitizing the mesocarbon spherule mixture obtained by filtration, drying and washing if necessary. (2) Heating coal tar pitch to produce mesocarbon spherules. It was obtained by adding tar oil in the heat-treated pitch, filtering, if necessary, drying and washing, adding iron element and silicon element or their solvent dispersion, and mixing. Method of heating and graphitizing mesocarbon spherule mixture by heating (3) Mesocarbon spherules and / or calcined mesocarbon spherules containing iron element and silicon element or their solvent dispersion How pressure, and obtained by mixing, by heating a mixture of mesocarbon spherules and / or calcined product of the small spheres, calcined graphitized

The graphitization method is, for example, 2500 ° C. or more, preferably 2800 ° C. or more, more preferably in a non-oxidizing atmosphere such as vacuum, nitrogen atmosphere, argon atmosphere using a graphitization furnace such as a Tamman furnace or an Atchison furnace. Is graphitized by high-temperature heat treatment at a temperature around 3000 ° C. The time required for graphitization is 0.5 to 80 hours, preferably 2 to 20 hours.
In the case of agglomeration during graphitization, it is preferable to adjust the particle size by removing coarse particles having a particle size (maximum particle size) larger than the thickness of the negative electrode by classification.

  The graphite material obtained by the production method of the present invention can be used for various applications by taking advantage of its characteristics. In particular, the graphite material contains substantially no graphitization catalyst or volatile matter, and has a crystal structure. Since there is no distortion, lithium ions can enter the graphite structure. Therefore, when used for a negative electrode material or a negative electrode of a lithium ion secondary battery, it is possible to give a good result in increasing the discharge capacity of the secondary battery.

The lithium ion secondary battery is essentially a battery mechanism in which lithium ions are occluded in the negative electrode during charging and desorbed from the negative electrode during discharging. A lithium ion secondary battery usually has a negative electrode, a positive electrode, and a nonaqueous electrolyte as main battery components, and the positive and negative electrodes are each composed of a lithium ion carrier, and the nonaqueous solvent enters and exits between layers in the charge / discharge process. .
The lithium ion secondary battery preferably has a large discharge capacity per unit volume of the negative electrode. For this reason, it is desired that the negative electrode material constituting the negative electrode has high crystallinity.

In the present invention, the crystallinity of the graphite material can be determined from the interplanar spacing (d ₀₀₂ ) of the carbon network layer and the size of the crystallite in the C-axis direction (Lc) in the X-ray wide angle diffraction method. That is, using a CuKα ray as an X-ray source and high-purity silicon as a standard substance, a diffraction peak on the (002) plane was measured for a graphite material, and d ₀₀₂ and Lc were determined from the peak position and its half-value width, respectively. Is calculated. The calculation method is in accordance with the Gakushin Law, and a specific method is described in “Carbon Fiber” (Modern Editorial Company, published in March 1986), pages 733-742.
From the viewpoint of developing a high discharge capacity, d ₀₀₂ and Lc, which are indicators of the degree of development of the graphite structure of the graphite material of the present invention, are preferably d ₀₀₂ ≦ 0.3365 nm and Lc ≧ 40 nm. More preferably, d ₀₀₂ ≦ 0.3362 nm and Lc ≧ 50 nm.

The negative electrode material of the lithium ion secondary battery of the present invention is not particularly limited except that the mesocarbon spheroid graphite material is used, and other battery components are the elements of a general lithium ion secondary battery. Follow.
That is, the negative electrode material for a lithium ion secondary battery of the present invention is a composition of a graphite material of mesocarbon spherules with a binder such as polyvinylidene fluoride resin or a mixture of the binder. There is a negative electrode for a lithium ion secondary battery in which the negative electrode material is fixed to a current collector such as a copper foil.

(Negative electrode material, negative electrode)
The negative electrode material for a lithium-on secondary battery of the present invention is prepared, for example, as a negative electrode mixture paste, and is further produced into a negative electrode and a lithium-on secondary battery. Specifically, the graphitized material is adjusted to an appropriate particle size by classification or the like, and the negative electrode mixture paste is prepared by mixing the binder with a solution or dispersion using an organic solvent such as isopropyl alcohol or an aqueous solvent, The paste is applied to one or both sides of a current collector and dried to form a negative electrode mixture layer, whereby a negative electrode in which the negative electrode mixture layer is uniformly and firmly adhered to the current collector can be produced. Of course, the method is not limited to the method described above, and the graphite material and, for example, polyvinylidene fluoride resin are mixed with a solvent such as N-methylpyrrolidone, dimethylformamide, water, alcohol, etc. to form a slurry, and then collected. The negative electrode mixture layer can also be formed by applying to an electric body.
The binder preferably has chemical stability and electrochemical stability with respect to the electrolyte, such as polyvinylidene fluoride resin, polytetrafluoroethylene resin and other fluororesins, polyethylene, polyvinyl alcohol, carboxymethylcellulose, SBR and the like. Illustrated. These can also be used together.
The usage-amount of a binder is 1-20 mass% of the negative electrode mixture whole quantity, Preferably it is 3-10 mass%. The negative electrode mixture is applied to one or both sides of the current collector so as to have a layer thickness of 10 to 300 μm, preferably 40 to 100 μm.
The negative electrode mixture layer can contain a conductive agent such as carbon black and an additive such as carbon fiber.

The shape of the current collector used for the negative electrode is not particularly limited, and examples thereof include a foil shape or a net shape such as a mesh or an expanded metal. Examples of the current collector include copper, stainless steel, and nickel. In the case of a foil, the current collector preferably has a thickness of about 5 to 20 μm.
The negative electrode can be formed according to a normal molding method. For example, the graphitized material and the binder powder can be dry mixed and hot press molded in a mold. Furthermore, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased by performing pressure bonding such as pressurization after forming the negative electrode mixture layer.

(Positive electrode)
As the positive electrode active material, it is preferable to select a positive electrode active material that can absorb and desorb a sufficient amount of lithium ions. Examples of such positive electrode active materials include lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides (V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , V ₃ O _8, etc.) and lithium compounds. Lithium-containing compound of general formula M _x MO ₆ S _8-Y (where X is a numerical value in the range of 0 ≦ X ≦ 4, Y is 0 ≦ Y ≦ 1, and M represents a metal such as a transition metal) The chevrel phase compound represented by these, activated carbon, activated carbon fiber, etc. can be mentioned.
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. Specifically, the lithium-containing transition metal oxide is LiM (1) _1-X M (2) _X O ₂ (where X is a numerical value in the range of 0 ≦ X ≦ 1, M (1), M (2) represents at least one kind of transition metal) or LiM (1) _2-Y M (2) _Y O ₄ (where Y is a numerical value in the range of 0 ≦ Y ≦ 1, M (1), M (2) represents at least one transition metal).

Examples of the transition metal element represented by M include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, and Sn. Preferably, Co, Fe, Mn, Ti, Cr, V and Al are mentioned.
More specifically, lithium-containing transition metal oxides include LiCoO ₂ , Li _X Ni _Y M _1-Y O ₂ (M is a transition metal element excluding Ni, preferably Co, Fe, Mn, Ti, Cr, V Lithium composite oxide, LiNiO ₂ , LiMnO ₂ , LiMn ₂ O _4, etc., at least one selected from Al, 0.05 ≦ X ≦ 1.10, 0.5 ≦ Y ≦ 1.0 Can be mentioned.

The lithium-containing transition metal oxide as described above, for example, uses Li, transition metal oxides or salts as starting materials, and mixes these starting materials according to the composition, and in a temperature range of 600 to 1000 ° C. in an oxygen atmosphere. It can be obtained by firing. Note that the starting materials are not limited to oxides or salts, and can be synthesized from hydroxides or the like.
In the present invention, the positive electrode active material may be used alone or in combination of two or more. For example, a carbon salt such as lithium carbonate can be added to the positive electrode.

In order to form a positive electrode using such a positive electrode material, for example, a positive electrode mixture comprising a positive electrode active material, a binder, and a conductive agent for imparting conductivity to the electrode is applied to both sides of the current collector and dried. To form a positive electrode mixture layer. After the positive electrode mixture layer is formed, pressure bonding such as press pressing may be further performed. Thereby, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.
As the binder, a fluorine-based resin such as polyvinylidene fluoride resin or polytetrafluoroethylene resin used in the negative electrode, an elastomer such as SBR, polyethylene, or the like can be used. In forming the positive electrode mixture layer, conventionally known various additives such as a conductive agent and a binder can be appropriately used. As the conductive agent, for example, graphite particles such as carbon black are used.

  The shape of the current collector is not particularly limited, and a foil shape or a mesh shape such as a mesh or an expanded metal is used. For example, examples of the current collector include aluminum, stainless steel, and nickel. The thickness is preferably 10 to 40 μm.

(Electrolytes)
As the electrolyte used in the lithium ion secondary battery of the present invention, it can be used 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), LiC (CF 3 SO 2) 3, LiN (CF 3 CH 2 OSO 2) 2, LiN ( It is possible to use lithium salts such as 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 , LiSiF _6. it can. In particular, LiPF ₆ and LiBF ₄ are preferably used from the viewpoint of oxidation stability.
The electrolyte concentration in the electrolytic solution is preferably 0.1 to 5 mol / l, and more preferably 0.5 to 3.0 mol / l.

The non-aqueous electrolyte may be a liquid non-aqueous electrolyte or a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the non-aqueous electrolyte battery is configured as a so-called lithium ion battery, and in the latter case, the non-aqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte battery or a polymer gel electrolyte battery.
In the case of a liquid non-aqueous electrolyte, the solvent is ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltatrahydrofuran, γ-butyllactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, trimethylborate , Tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, dimethyl sulfite, etc. It can be used protic organic solvent.

When the nonaqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer electrolyte, the matrix polymer compound gelled with a plasticizer (nonaqueous electrolyte) is included. Fluorine polymers such as ether polymer compounds such as polyethylene oxide and cross-linked polymers thereof, polymethacrylate polymer compounds, polyacrylate polymer compounds, polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymers. Molecular compounds can be used alone or in combination.
Among these, it is preferable to use a fluorine-based polymer compound such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer from the viewpoint of redox stability.

  As the electrolyte salt and non-aqueous solvent constituting the plasticizer contained in these polymer solid electrolyte and polymer gel electrolyte, any of the above-mentioned ones can be used. In the case of a gel electrolyte, the electrolyte salt concentration in the non-aqueous electrolyte that is a plasticizer is preferably 0.1 to 5 mol / l, and more preferably 0.5 to 2.0 mol / l.

The method for producing such a solid electrolyte is not particularly limited. For example, a polymer compound that forms a matrix, a lithium salt, and a solvent are mixed, heated and melted, and an appropriate organic solvent for mixing is used. Method of evaporating the organic solvent for mixing after dissolving the molecular compound, lithium salt and solvent, and mixing the monomer, lithium salt and solvent, and irradiating them with ultraviolet rays, electron beams or molecular beams to form a polymer And the like.
Moreover, the addition ratio of the solvent in the solid electrolyte is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass. When the content is 10 to 90% by mass, the electrical conductivity is high, the mechanical strength is high, and the film is easily formed.

A separator can also be used in the lithium ion secondary battery of the present invention.
Although it does not specifically limit as a separator, For example, a woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. are mentioned. In particular, a synthetic resin porous membrane is preferably used. Among these, a polyolefin-based microporous membrane is preferable in terms of thickness, membrane strength, and membrane resistance. Specifically, it is a microporous film made of polyethylene and polypropylene, or a microporous film in which these are combined.

In the lithium ion secondary battery of the present invention, a gel electrolyte can be used because of the high initial charge / discharge efficiency.
In the gel electrolyte secondary battery, the negative electrode containing the mesocarbon microsphere graphite material of the present invention, the positive electrode, and the gel electrolyte are laminated in the order of the negative electrode, the gel electrolyte, and the positive electrode, for example, and housed in the battery outer packaging material. Consists of. In addition to this, a gel electrolyte may be further disposed outside the negative electrode and the positive electrode. In the gel electrolyte secondary battery using such a graphite material for the negative electrode, even when propylene carbonate is contained in the gel electrolyte and the graphite material powder has a small particle size that can sufficiently reduce impedance, Irreversible capacity can be kept small. Therefore, a large discharge capacity is obtained and a high initial charge / discharge efficiency is obtained.

  Furthermore, the structure of the lithium ion secondary battery of the present invention is arbitrary, and the shape and form thereof are not particularly limited, and can be arbitrarily selected from a cylindrical shape, a square shape, a coin shape, a button shape, and the like. Can do. 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 gel electrolyte battery, a structure enclosed in a laminate film can also be used.

Next, the present invention will be specifically described with reference to examples. The present invention is not limited to these examples.
In the following examples and comparative examples, an evaluation button type secondary battery having a configuration as shown in FIG. 1 was fabricated using a negative electrode containing a graphite material of a mesocarbon microsphere and a binder, and the battery characteristics were evaluated. evaluated. An actual battery can be produced according to a known method based on the concept of the present invention.
The average particle size of the small spheres, iron elements, and silicon elements was measured using a laser diffraction particle size distribution analyzer (manufactured by Seishin Co., Ltd., LS-5000), and the particle size was such that the cumulative frequency was 50% by volume fraction. .
As described above, the lattice spacing (d ₀₀₂ ) of the graphite material is an X-ray diffraction method using CuKα rays as X-rays and high-purity silicon as a standard substance [Sugirou Otani, carbon fiber, pages 733-742]. (1986), Modern Editing Company].
The true density of the graphite material was measured in accordance with JIS R7222, using a pycnometer and a liquid phase substitution method using butanol as a dispersion medium.

Example 1
The coal tar pitch was heated at 400 to 460 ° C. to produce mesocarbon microspheres in the coal tar pitch. Mesocarbon microspheres were extracted and filtered and separated using 600 parts by mass of tar middle oil (boiling range: 140 to 270 ° C.) with respect to 100 parts by mass of the heat treatment pitch. The small spheres were fired at 340 ° C. in a nitrogen gas atmosphere. The obtained mesocarbon microspheres contained 98.0% by mass of benzene insoluble components and 89.5% by mass of quinoline insoluble components. Volatiles were 8.5% by weight.

4.0% by mass of pure iron (average particle size of 50 nm) and 1.0% by mass of pure silicon (average particle size of 10 nm) were added to the fired microspheres (average particle size of 32 μm). To 100 g of the obtained mixture, 200 ml of tar oil was dropped, and the mixture was stirred and dispersed for 20 minutes using an ultrasonic cleaner. From the resulting dispersion, oil in tar was separated using a suction filter equipped with a membrane filter. The separated residue was dried at 60 ° C. for 2 hours using a vacuum dryer.
The dried mixture was filled in a graphite crucible, heated at 2800 ° C. for 5 hours using a Tamman furnace, fired and graphitized to obtain a graphite material. The physical properties of the graphite material are shown in Table 1.

90% by mass of the graphite material of the mesocarbon spherule and 10% by mass of polyvinylidene fluoride resin were mixed in an N-methylpyrrolidone solvent, and stirred at 500 rpm for 5 minutes using a homomixer. A paste was prepared.
The paste was applied to one side of a copper foil as a current collector to a uniform thickness using a doctor blade applicator having a clearance of 200 μm, and N-methylpyrrolidone was volatilized and dried at 90 ° C. in a vacuum. Next, after pressing with a roller press, a disk having a diameter of 15.5 mm was punched out to produce a working electrode.
The counter electrode was prepared by pressing a lithium metal foil (thickness 10 μm) against a nickel net (200 mesh) and punching out a disk having a diameter of 15.5 mm.

The non-aqueous electrolyte, a volume ratio of ethylene carbonate and ethyl methyl carbonate 1: 2 and the mixed solvent was prepared at a concentration comprised a LiPF ₆ and 1 mol / l. The obtained non-aqueous electrolyte was impregnated into a polypropylene porous body to produce a separator impregnated with the electrolyte.

The button type secondary battery shown in FIG. 1 was used as the evaluation battery.
In the evaluation battery, the outer cup 1 and the outer can 3 have a sealed structure in which the outer cup 1 and the outer can 3 are caulked with an insulating gasket 6 at the periphery.
In this evaluation battery, the separator 5 impregnated with the electrolytic solution was laminated between the working electrode 2 in close contact with the current collector 7b and the counter electrode 4 in close contact with the current collector 7a, and then the working electrode 2 was attached. The counter electrode 4 is accommodated in the outer can 3 in the outer cup 1, the outer cup 1 and the outer can 3 are combined, and the periphery of the outer cup 1 and the outer can 3 is caulked and sealed with an insulating gasket 6. Made.
The evaluation battery is a battery composed of a working electrode 2 containing graphitized mesocarbon spherules that can be used as a negative electrode active material in a real battery, and a counter electrode 4 made of a lithium metal foil.

The evaluation battery was subjected to the following charge / discharge test at a temperature of 25 ° C.
Constant current charging was performed until the circuit voltage reached 0 mV at a current value of 0.9 mA, and switching to constant voltage charging was performed when the circuit voltage reached 0 mV, and 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.
In this test, the process of occluding lithium ions in the negative electrode mixture was charged, and the process of desorbing from the negative electrode mixture was discharge.
Next, constant current charging is performed until the circuit voltage reaches 1.5 V at a current value of 0.9 mA. The charging capacity and discharging capacity are obtained from the energization amount during that time, and the irreversible capacity and initial charging and discharging efficiency are calculated from the following equations. did. Table 2 shows the discharge capacity (mAh / g), irreversible capacity (mAh / g), and initial charge / discharge efficiency (%) per gram of the graphite material.
Irreversible capacity (mAh / g) = charge capacity of first cycle (mAh / g)-discharge capacity of first cycle
Amount (mAh / g)
Initial charge / discharge efficiency (%) = (First cycle discharge capacity / First cycle charge capacity)
× 100

(Examples 2-6, 9-10, Comparative Examples 1-5)
A graphite material was produced by the same method and conditions as in Example 1 except that the raw materials shown in Table 1 were used. Further, a negative electrode material, a negative electrode, and an evaluation battery were produced. And the physical property measurement of the graphite material and the characteristic evaluation of the evaluation battery were performed under the same method and conditions as in Example 1. The results are shown in Table 2.
Moreover, the relationship between the ratio of the iron element to (iron element + silicon element) and the discharge capacity in Examples 1 to 3 and Comparative Examples 1 to 2 and 5 is shown in FIG.

(Example 7)
In the mesocarbon spherule of Example 1, iron (III) oxide and tetramethoxysilane [Si (OCH ₃ ) ₄ ] were dispersed in tar light oil so that iron was 3 mass% and silicon was 1 mass%. Added and stirred. From the resulting dispersion, oil in tar was separated using a suction filter equipped with a membrane filter. The separated residue was dried at 60 ° C. for 2 hours using a vacuum dryer. The residue after drying was graphitized under the same method and conditions as in Example 1 to produce a graphite material, and further, a negative electrode material, a negative electrode, and an evaluation battery were produced. And the physical property measurement of the graphite material and the characteristic evaluation of the evaluation battery were performed under the same method and conditions as in Example 1. The results are shown in Table 2.

(Example 8)
Except for using a fired product obtained by firing the mesocarbon spherules of Example 1 at 500 to 1000 ° C., graphitization was performed in the same manner and conditions as in Example 7 to produce a graphite material. A negative electrode and an evaluation battery were produced. And the physical property measurement of the graphite material and the characteristic evaluation of the evaluation battery were performed under the same method and conditions as in Example 1. The results are shown in Table 2.

  From FIG. 2, the discharge capacity of the lithium ion secondary battery has an effect greater than the sum, that is, a synergistic effect by using a specific amount of iron element and silicon element as a graphitization catalyst for mesocarbon microspheres. Is clear.

  The negative electrode material for a lithium ion secondary battery containing the mesocarbon microsphere graphite material of the present invention has a high discharge capacity when used as a negative electrode of a lithium ion secondary battery. Therefore, it is expected to be used as a battery for portable electronic devices such as mobile phones and notebook computers that require higher performance and higher functions.

It is sectional drawing of the evaluation battery for evaluating a charging / discharging characteristic. It is a graph which shows the relationship between the quantity of the iron element and silicon element which are graphitization catalysts, and the discharge capacity of a lithium ion secondary battery.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exterior cup 2 Working electrode 3 Exterior can 4 Counter electrode 5 Electrolyte solution impregnation separator 6 Insulation gasket 7a, 7b Current collector

Claims (4)

  A method for producing a graphite material, wherein a mesocarbon microsphere and / or a mesocarbon microsphere fired product is graphitized in the presence of an iron element and a silicon element, the mesocarbon microsphere and the mesocarbon microsphere being fired. The ratio of the total amount of the iron element and the silicon element to the total amount of the iron element and the silicon element is 0.1 to 25% by mass, and the ratio of the iron element to the total amount of the iron element and the silicon element is 30 to 30%. 90% by mass and a method for producing a graphite material.   2. The method for producing a graphite material according to claim 1, wherein the iron element is in the form of iron and / or an iron compound, and the silicon element is in the form of silicon and / or a silicon compound.   The method for producing a graphite material according to claim 1, wherein the iron element and the silicon element are in the form of a compound of iron and silicon.   The method for producing a graphite material according to any one of claims 1 to 3, wherein the graphite material is a negative electrode material for a lithium ion secondary battery.

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