JP2009173547A - Polycrystalline mesocarbon microsphere graphitized product, negative electrode active material, and lithium ion secondary battery - Google Patents

Abstract

When used as a negative electrode material, a lithium ion battery having a high charge / discharge capacity per volume, good discharge load characteristics and charge characteristics (cycle characteristics), small charge / discharge loss, and good charge / discharge efficiency. A polycrystalline mesocarbon microsphere graphitized product from which a secondary battery is obtained, a negative electrode active material using the same, and a lithium ion secondary battery using the same.
A polycrystal mesocarbon microsphere graphitized product having a mean particle size of 1 to 120 μm, comprising a plurality of crystals, the C-axis direction of each crystal being a random direction; using the graphitized product Negative electrode active material; a lithium ion secondary battery using the negative electrode active material.
[Selection] Figure 1

Description

  The present invention relates to a polycrystalline mesocarbon microsphere graphitized product suitable for a negative electrode material for a lithium ion secondary battery, a negative electrode active material using the same, and a lithium ion secondary battery using the same.

  Lithium ion secondary batteries have excellent characteristics such as high operating voltage, large battery capacity, long cycle life, and little impact on environmental pollution. Instead of nickel and hydrogen batteries, they are used in a wide range of fields and applications. Lithium ion secondary batteries are now available for practical use 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. It is. For this reason, the role of carbon materials for the practical application and performance improvement of lithium ion secondary batteries is significant.

  By the way, in recent years, in portable electronic devices such as mobile phones and notebook personal computers, power consumption has increased along with higher performance and higher functionality, and further increase in capacity of lithium ion secondary batteries to be mounted has been demanded. Yes. Until now, it has been considered that the capacity of a lithium ion secondary battery is largely governed by the discharge capacity per mass of the carbon material for the negative electrode. The discharge capacity per mass in the carbon material for the negative electrode is limited to 372 mAh / g, which is the theoretical capacity of high-purity natural graphite. Therefore, in order to increase the capacity of the lithium ion secondary battery, it is important to increase the active material density of the negative electrode carbon material and improve the discharge capacity per volume. If the density is increased, it is considered that the contact between the particles of the carbon material for the negative electrode increases, thereby increasing the electronic conductivity, improving the chargeability, and improving the discharge capacity per unit volume.

However, when the active material density of the negative electrode is increased for the purpose of improving the discharge capacity per volume using a carbon material such as scale-like natural graphite, there is a practical problem that the discharge load characteristics and the charge characteristics deteriorate. . The reason is that when the lithium ion secondary battery is charged, lithium ions in the electrolyte are inserted from the edge surface of the graphite structure of the negative electrode carbon material to form LiC ₈ , which corresponds to the hexagonal mesh surface of the graphite structure. It is difficult to insert lithium ions from the basal surface. For example, a carbon material having a regular graphite structure such as natural graphite, at a low density, the edge surface, which is a surface perpendicular to the basal surface, is easily exposed to the electrode surface, and lithium ions can be easily inserted. . However, when such a carbon material is press-molded at a high pressure in order to improve the active material density, the scale-like natural graphite is easily oriented, and the basal surface is easily exposed because the particles are in close contact with each other. Exposure is reduced. Therefore, it becomes difficult for lithium ions to be inserted between the layers of natural graphite crystals, lithium ion diffusibility deteriorates, and discharge load characteristics and charge characteristics deteriorate. Therefore, carbon materials such as natural graphite have a problem that the active material density cannot be increased and the discharge capacity per volume cannot be improved.

  On the other hand, control of the shape of the carbon material is effective in order to make it difficult to align even when press-molding at high pressure. For example, Patent Document 1 describes an example in which a substantially spherical mesocarbon microsphere is used as an active material of a negative electrode material of a lithium ion secondary battery. However, the mesocarbon microspheres described in Patent Document 1 are composed of Brooks Taylor type single crystals arranged in layers in a direction perpendicular to the diameter direction. Since this is a sphere, it is difficult to orientate compared to natural graphite, etc., but if the active material density in the negative electrode is increased for the purpose of increasing the discharge capacity per volume, it is oriented in the layer surface direction, and the lithium ion diffusibility is still poor. There was a problem that the discharge load characteristics and the charge characteristics deteriorated.

JP 2000-323127 A

  The object of the present invention is that when used as a negative electrode material for a lithium ion secondary battery, the discharge capacity per volume is high, the discharge load characteristics and the charge characteristics are good, the charge / discharge loss is small, and the charge / discharge is small. An object of the present invention is to provide a polycrystalline mesocarbon microsphere graphitized product from which a lithium ion secondary battery having good efficiency can be obtained, a negative electrode active material using the same, and a lithium ion secondary battery using the same.

  The present invention is a polycrystallized mesocarbon microsphere graphitized product having a mean particle diameter of 1 to 120 μm, which is composed of a plurality of crystals and the C-axis direction of each crystal is random.

  The polycrystalline mesocarbon microsphere graphitized product is preferably spherical as a negative electrode active material for a lithium ion secondary battery.

  Moreover, this invention is a negative electrode active material for lithium ion secondary batteries containing the said polycrystalline mesocarbon microsphere graphitized product.

  The present invention is also a lithium ion secondary battery comprising a negative electrode active material, a positive electrode active material, and a non-aqueous electrolyte, and using the polycrystalline mesocarbon microsphere graphitized product as the negative electrode active material.

  When the graphitized product of the present invention is used as a negative electrode material for a lithium ion secondary battery, the discharge capacity per volume is high, the discharge load characteristics and the charge characteristics (cycle characteristics) are good, and the charge / discharge loss is further reduced. A small lithium ion secondary battery with good charge / discharge efficiency can be obtained.

  Since the lithium ion secondary battery of the present invention uses the graphitized product of the present invention as the negative electrode active material, the discharge capacity per volume is high, and the discharge load characteristics and the charge characteristics (cycle characteristics) are good. Furthermore, it is possible to exhibit high battery performance with low charge / discharge loss and good charge / discharge efficiency.

1 is a schematic view of a cross section of a polycrystalline mesocarbon microsphere graphitized product obtained in Example 1 observed with a scanning electron microscope. FIG. 6 is a schematic view of a cross section of a single crystal mesocarbon microsphere graphitized product obtained in Comparative Example 1 observed by a scanning electron microscope. FIG. It is a graph which shows the relationship between the negative electrode active material density of the test battery produced in Example 1 and Comparative Examples 1 and 2, and the constant current charge capacity in the charge rate of 0.5C. It is a graph which shows the relationship between the discharge rate and discharge load characteristic of the test battery produced in Example 1 and Comparative Examples 1 and 2.

  Hereinafter, the polycrystalline mesocarbon microsphere graphitized product of the present invention (hereinafter also referred to as the graphitized product of the present invention), a production method thereof, and a lithium ion secondary battery using the same as a negative electrode active material will be described in detail. To do.

  The graphitized product of the present invention is composed of a plurality of crystals, the C-axis direction of each crystal being a random direction, and an average particle size of 1 to 120 μm of polycrystalline mesocarbon microspheres (hereinafter simply referred to as mesocarbon small particles). (Also called a sphere). That is, the graphitized product of the present invention is as shown in FIG. 1 schematically showing the observation result of the cross section of the polycrystalline mesocarbon microsphere graphitized product obtained in Example 1 described later by a scanning electron microscope. It is a polycrystal in which a plurality of crystals are aggregated via grain boundaries. Each crystal has a different crystal orientation. Therefore, the direction perpendicular to the lattice plane (002) of each crystal, that is, the C-axis direction of each crystal forms a random direction. For this reason, in the graphitized product of the present invention, the edge surface of the graphite crystal structure is exposed in a random direction on the surface.

  Therefore, even if the graphitized product of the present invention is press-molded and densified to bring the graphitized product into close contact with each other, the edge surface into which lithium ions are inserted can be exposed. Therefore, the diffusibility by the insertion of lithium ions from the edge surface is maintained. When such a graphitized product is used as a negative electrode active material for a lithium ion secondary battery, lithium ion having a high discharge capacity per volume, good discharge load characteristics and charge characteristics, and excellent other battery characteristics. A secondary battery can be manufactured. Here, the fact that the C-axis direction is a random direction can be evaluated by observation with a scanning electron microscope (500 to 2000 times) of the cross section of the graphitized product. If there is at least one other crystal whose C-axis direction is different by 45 ° or more with respect to one crystal observed in the cross section of one graphitized product, this graphitized product has a random C-axis direction of each crystal. Means that In the present invention, at least one such graphitized product is sufficient.

  The average particle size of the graphitized product of the present invention is determined according to the application, battery design conditions, etc., but is preferably 1 to 120 μm, more preferably 5 to 100 μm. When used as a negative electrode material for a lithium ion secondary battery, the average particle size is preferably 5 to 70 μm, and more preferably 12 to 40 μm.

Furthermore, the fact that the C-axis direction of the graphitized product of the present invention is a random direction is that an electrode plate is produced using a carbon material containing the graphitized product of the present invention, and this electrode plate is subjected to X-ray diffraction. It can also be confirmed by measuring the peak intensities I ₁₀₁ and I ₀₀₄ of the (101) and (004) planes of graphite and calculating the intensity ratio I ₁₀₁ / I ₀₀₄ . The larger this value, the more random the C-axis direction is. The electrode when the graphitized product of the present invention is used preferably has an active material density of 1.6 g / cm ³ and an intensity ratio I ₁₀₁ / I ₀₀₄ of 0.35 or more. By setting it to 0.35 or more, the diffusivity of lithium ions is improved, and the discharge load characteristics can be further improved.

  The graphitized product of the present invention is prepared by adding an organic solvent to a graphitizable carbon material containing mesocarbon microspheres formed by heat treatment, adjusting the viscosity so that the viscosity becomes 1 to 100 mPa · s, and then filtering. Can be produced by separating the mesocarbon spherules and graphitizing the obtained mesocarbon spherules. The said viscosity is a viscosity at the time of filtration, and is measured in 100-140 degreeC.

  The graphitizable carbon material is produced from a carbon material whose graphitization is promoted when subjected to high-temperature heat treatment, as described in, for example, JP-A-59-56486. Examples of the raw material for the graphitizable carbon material include petroleum-based or coal-based tars and pitches, specifically, coal tar, coal tar pitch, acenaphthylene, petroleum heavy oil, and the like.

  In the method for producing a graphitized product of the present invention, first, mesocarbon microspheres are produced by heat-treating the graphitizable carbon material. The heat treatment temperature is appropriately determined according to the kind of graphitizable carbon material, the heating method, and the like. For example, when coal tar pitch is used as the graphitizable carbon material, heat treatment is performed at 350 to 500 ° C., preferably 380 to 480 ° C. for 10 to 120 minutes, preferably 20 to 60 minutes. In particular, by adjusting the amount of free carbon contained in the coal tar pitch, the content of mesocarbon microspheres produced in the graphitizable carbon material can be controlled. The content of the mesocarbon microspheres to be generated is preferably 15 to 50% by mass. More preferably, it is 18-40 mass. When the high content mesocarbon spherules are generated in this way, the fine mesocarbon spherules can be coalesced and combined in the heat treatment process to effectively produce polycrystalline mesocarbon spherules. .

Next, the produced mesocarbon microspheres are separated from the graphitizable carbon material. For this purpose, an organic solvent is added to the graphitizable carbon material and the viscosity is adjusted to 1 to 100 mPa · s, preferably 1.0 to 50 mPa · s. In that case, you may stir as needed. The viscosity of the graphitizable carbon material containing conventional mesocarbon spherules is about 0.01 mPa · s, which is taken into consideration that filtration can be easily performed.
Examples of the organic solvent include hydrocarbons such as toluene and n-hexane, ethers such as tetrahydrofuran, ketones such as acetone, pyridines such as quinoline, tar light oil, tar heavy oil, and the like. is not.

  The mesocarbon spherules are separated from the graphitizable carbon material containing mesocarbon spherules with adjusted viscosity by filtration. At this time, if the viscosity is higher than 100 mPa · s, filtration becomes difficult, and if the viscosity is lower than 1 mPa · s, the pitch component which is a graphitizable carbon material other than the mesocarbon microspheres on the surface of the mesocarbon microspheres. When used as a negative electrode material, the discharge capacity is low and the charge / discharge loss is further increased. By filtering in the viscosity range, the graphitizable carbon material other than the mesocarbon spherules and the added organic solvent are simultaneously removed, and the mesocarbon spherules can be separated.

  Filtration is preferably performed under pressure from the viewpoint of productivity. The method of pressure filtration is not particularly limited, and for example, a pressure filter such as a candle type, a funder type, or a Nutsche type can be used. When a candle type is used, it is preferable to pressurize to a pressure of 100 to 500 kPa.

Next, the separated mesocarbon microspheres are graphitized at a temperature of 2000 ° C. or higher, preferably 2600 to 3200 ° C., to graphitize the polycrystalline mesocarbon microspheres to produce a graphitized product. The obtained graphitized product is adjusted to a predetermined average particle size by classification or the like.
Prior to graphitization of the mesocarbon spherules, if the spherules are baked at a temperature of 300 to 500 ° C. to form a carbide, fusion during graphitization can be avoided, so the average particle size of the graphitized product It is preferable for adjusting.

  The graphitized product of the present invention is particularly suitable as a negative electrode material for a lithium ion secondary battery, but is also used as a conductive material or a refractory for applications other than the negative electrode by taking advantage of its characteristics. You can also.

  The lithium ion secondary battery of the present invention includes a negative electrode active material, a positive electrode active material, and a non-aqueous electrolyte, and uses the graphitized product of the present invention as the negative electrode active material. A lithium ion secondary battery usually has a negative electrode, a positive electrode, and a non-aqueous electrolyte as main battery components, and the positive and negative electrodes are each composed of a lithium ion carrier, and the non-aqueous solvent enters and exits between the layers in the charge / discharge process. . In the battery, lithium ions are doped into the negative electrode during charging, and are dedoped from the negative electrode during discharging.

  The lithium ion secondary battery of the present invention is not particularly limited except that the graphitized product of the present invention is used as the negative electrode material, and other battery components conform to the elements of a general lithium ion secondary battery.

  The production of the negative electrode from the graphitized product according to the present invention can be performed by sufficiently extracting the performance of the graphitized product and having a high moldability for the powder and obtaining a chemically and electrochemically stable negative electrode. If it is a method, it will not restrict | limit at all, It can carry out according to a normal shaping method.

  At the time of producing the negative electrode, a negative electrode mixture obtained by adding a binder to the graphitized product of the present invention can be used. As the binder, it is preferable to use those having chemical stability and electrochemical stability with respect to the electrolyte. For example, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, For example, carboxymethylcellulose is used. These can also be used together. In general, the binder is preferably used at a ratio of about 1 to 20% by mass in the total amount of the negative electrode mixture.

Specifically, for example, the graphitized product of the present invention is adjusted to an appropriate particle size by classification or the like, and mixed with a binder to prepare a negative electrode mixture. This negative electrode mixture is usually used as a current collector. The negative electrode mixture layer is formed by coating on one or both sides.
At this time, if the negative electrode mixture is dispersed in a solvent and made into a paste, and then applied to the current collector and dried, a negative electrode mixture layer uniformly and firmly adhered to the current collector is formed. The paste can be prepared by stirring at about 300 to 3000 rpm with a wing homomixer. As the solvent, a normal solvent used for preparing a negative electrode mixture can be used.

For example, the graphitized product of the present invention and a fluorine resin powder such as polytetrafluoroethylene may be mixed and kneaded in a solvent such as isopropyl alcohol to obtain a paste, which may be applied. Also, the graphitized product of the present invention and a fluorine resin powder such as polyvinylidene fluoride or a water-soluble binder such as carboxymethyl cellulose are mixed with a solvent such as N-methylpyrrolidone, dimethylformamide, water, alcohol, etc. After that, this may be applied.
The coating thickness when the mixture of the graphitized product and the binder of the present invention is applied to the current collector is preferably 10 to 500 μm, more preferably 20 to 200 μm. After the negative electrode mixture layer is formed, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased by pressure bonding such as pressurization.
The negative electrode can also be produced by dry-mixing the graphitized product of the present invention and resin powders such as polyethylene and polyvinyl alcohol and hot pressing in a mold.

  The shape of the current collector used for the negative electrode is not particularly limited, but a foil or a net-like material such as a mesh or expanded metal is used. Examples of the current collector include copper, stainless steel, and nickel. The thickness is preferably about 5 to 20 μm.

The material of the positive electrode (positive electrode active material) of the lithium ion secondary battery of the present invention is not particularly limited as long as it is a material that can occlude and release a sufficient amount of lithium. For example, 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-containing compounds such as lithium compounds thereof, Chevrel phase represented by the formula M _X Mo ₆ S _8-y (where X is a numerical value in the range of 0 ≦ X ≦ 4, Y is a range of 0 ≦ Y ≦ 1, and M represents a metal such as a transition metal) A compound, activated carbon, activated carbon fiber, etc. can be used.

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-p M (2) _p O ₂ (wherein P is a numerical value in the range of 0 ≦ P ≦ 1, M (1), M (2) is composed of at least one transition metal element) or LiM (1) _2-Q M (2) _Q O ₄ (wherein Q is a numerical value in the range of 0 ≦ Q ≦ 1, M (1), M (2) is composed of at least one transition metal element).
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, Al is mentioned.

More specifically, lithium-containing transition metal oxides include LiCoO ₂ , Lip Ni _Q M _{1 -Q} O ₂ (M is the transition metal element excluding Ni, preferably Co, Fe, Mn, Ti, Cr, V , At least one selected from Al, 0.05 ≦ p ≦ 1.10, 0.5 ≦ q ≦ 1.0.) Lithium composite oxide, LiNiO ₂ , LiMnO ₂ , LiMn ₂ O _4, etc. Is mentioned.

The lithium-containing transition metal oxide as described above includes, for example, lithium, a transition metal oxide or a salt as a starting material, these starting materials are mixed according to the composition, and a temperature range of 600 to 1000 ° C. in an oxygen atmosphere. It can obtain by baking with. The starting material is not limited to oxides or salts but may be hydroxides.
In the present invention, the positive electrode active material may be used alone or in combination of two or more. For example, an alkali carbonate such as lithium carbonate can be added to the positive electrode.

  In order to form a positive electrode with such a positive electrode material, for example, a positive electrode mixture comprising a positive electrode material, a binder, and a conductive agent for imparting conductivity to the electrode is applied to both surfaces of the current collector. Form a layer. As the binder, any of those exemplified for the negative electrode can be used. As the conductive agent, for example, a carbon material, graphite or carbon black is used.

The shape of the positive electrode current collector is not particularly limited, and may be a box shape or a mesh shape such as a mesh or expanded metal. Examples of the current collector include aluminum, stainless steel, and nickel. The thickness is preferably 10 to 40 μm.
Also in the case of the positive electrode, like the negative electrode, the positive electrode mixture is dispersed in a solvent to form a paste, and the paste-like positive electrode mixture is applied to a current collector and dried to form a positive electrode mixture layer. Alternatively, 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.

  In forming the negative electrode and the positive electrode as described above, conventionally known various additives such as a conductive agent and a binder can be appropriately used.

The electrolyte used in the lithium ion secondary battery of the present invention is obtained by dissolving an electrolyte salt in a non-aqueous solvent. Examples of the electrolyte salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , 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 ) Lithium salts such as ₂ CHOSO ₂ ] ₂ , LiB [C ₆ H ₃ (CF ₃ ) ₂ ] ₄ , LiAlCl ₄ , LiSiF ₆ can be used. In particular, LiPF ₆ and LiBF ₄ are preferably used from the viewpoint of oxidation stability.
The electrolyte salt concentration in the non-aqueous electrolyte 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, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxy. Ethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, boric acid Trimethyl, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, Aprotic organic solvents such as dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, and dimethyl sulfite can be used.

When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or polymer gel electrolyte, it contains a matrix polymer compound gelled with a plasticizer (non-aqueous electrolyte). As the molecular compound, an ether-based polymer compound such as polyethylene oxide or a crosslinked product thereof, a fluorine-based resin such as polymethacrylate-based, polyacrylate-based, polyvinylidene fluoride, or vinylidene fluoride-hexafluoropropylene copolymer is used alone. Or it can be used as a mixture.
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 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 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 method for producing such a polymer 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, a polymer compound and lithium in an appropriate organic solvent. Method of evaporating organic solvent after dissolving salt and solvent, and polymerizing monomer, lithium salt and solvent as raw material for polymer electrolyte are mixed and irradiated with ultraviolet rays, electron beam or molecular beam for polymerization And a method for producing a polymer electrolyte.
Further, the addition ratio of the solvent in the polymer electrolyte is preferably 10 to 90% by mass because the electrical conductivity is high and the mechanical strength is high and the film is easily formed. More preferably, it is 30-80 mass%.

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. In particular, a synthetic resin microporous membrane is preferably used. Among these, a polyolefin microporous membrane is preferable 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.

  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 non-aqueous electrolyte battery with higher safety, it is desirable to have a means for detecting an increase in the internal pressure of the battery and shutting off the current when an abnormality such as overcharge occurs. 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.

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

Example 1
A coal tar pitch containing 1% by mass of free carbon was heated at 450 ° C. for 30 minutes to produce 35% by mass of mesocarbon microspheres (average particle size 32 μm) in the coal tar pitch. To this, oil in tar (boiling point: 180 to 300 ° C.) was added, the viscosity of coal tar pitch was adjusted to 1.5 mPa · s (125 ° C.), and the mixture was stirred at 125 ° C. for 1 hour. A graphitizable carbon material containing was obtained. This was poured into a pressure filter while being maintained at 125 ° C., pressurized with nitrogen gas and filtered to separate mesocarbon microspheres. The content of mesocarbon microspheres was calculated from the mass ratio of the mesocarbon remaining on the filter cloth to the raw material coal tar pitch. The average particle size of the mesocarbon spherules is a 50% volume equivalent cumulative particle size measured with a laser diffraction particle size distribution meter.

  The separated mesocarbon spherules were placed in a nitrogen atmosphere at 125 ° C. to remove the oil and dried. The spherules were fired at 450 ° C. in a nitrogen atmosphere for 5 hours, and further placed in a graphite crucible, heated in an argon atmosphere at a heating rate of 1000 ° C./hour, and graphitized at 3000 ° C. for 3 hours. Graphitized. This was classified to obtain mesocarbon microsphere graphitized products having an average particle size of 30 μm.

The obtained mesocarbon microsphere graphitized product was embedded in a resin, polished and subjected to argon ion sputtering, and the cross section of the mesocarbon microsphere graphitized product was observed with a scanning electron microscope. The observation result is shown as a schematic diagram in FIG.
From FIG. 1, it was found that the obtained mesocarbon microsphere graphitized product was composed of a plurality of crystals. In addition, crystals whose C-axis directions differ from each other by 45 ° or more were confirmed, and it was found that the C-axis directions are random directions.

  Further, polyvinylidene fluoride (PVDF) as a binder was mixed with the above-mentioned polycrystalline mesocarbon microsphere graphitized product so that the mass ratio of the polycrystallized mesocarbon microsphere graphitized product / PVDF was 90/10. Then, PVDF was dissolved in N-methylpyrrolidone and kneaded to prepare a pasty active material.

This pasty active material was applied to one side of a copper foil as a current collector using a doctor blade applicator having a clearance of 200 μm, dried at 100 ° C. for 12 minutes, and the active material density in the electrode was 1. A test electrode was manufactured by press-molding to 8 g / cm ³ and vacuum-drying at 130 ° C. overnight. This electrode was subjected to X-ray diffraction measurement using Cu—Kα rays as an X-ray source, and the peak intensity of the (101) plane and the peak intensity of the (004) plane were determined. The intensity ratio I ₁₀₁ / I ₀₀₄ was 0.36. The active material density was 1.6 g / cm ^3, and the strength ratio I ₁₀₁ / I ₀₀₄ of a test electrode produced in the same manner was 0.37.

A non-aqueous electrolyte solution prepared by dissolving LiPF ₆ at a concentration of 1 mol / kg in a mixed solvent (volume ratio of EC / EMC = 1/2) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). A test battery was prepared using a counter electrode made of lithium foil and a porous separator placed between the counter electrode and the test electrode. The discharge capacity per mass of the test battery was 350 mAh / g and the discharge capacity per volume was 630 mAh / cm ³ , and a very high discharge capacity per volume was obtained.

Moreover, the test battery was evaluated as follows. After performing constant current charging until the circuit voltage reaches 0 mV at a current value of 0.9 mA, when the circuit voltage reaches 0 mV, switching to constant voltage charging is continued until the current value reaches 20 μA. It was. 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 at a current value of 0.9 mA until the circuit voltage reached 1.5 V, and the discharge capacity was obtained from the energization amount during this period. This was the first cycle.
The charge / discharge efficiency of the test battery was calculated from the following formula (I).
Charging / discharging efficiency = (discharge capacity in the first cycle / charge capacity in the first cycle)
× 100 (I)
Further, the charge / discharge loss was calculated by the following formula (II).
Charge / discharge loss = Charge capacity in the first cycle−Discharge capacity in the first cycle
(II)

Here, the charge / discharge rate at which 100% of the discharge capacity is charged or discharged in one hour is referred to as 1C. That is, 0.1 C if it takes 10 hours to charge or discharge 100% of the discharge capacity, and 2 C if it takes 30 minutes.
As the discharge load characteristics, the ratio of the discharge amount when discharged at a rate of 2.5 C to the discharge amount when discharged at a rate of 0.1 C was determined. In the test battery using the active material of Example 1, it was 84%.
Table 1 summarizes the active material density and X-ray diffraction intensity ratio of the test electrode, and the electrolyte solution component, discharge capacity, charge / discharge loss, charge / discharge efficiency, and discharge load characteristics of the test battery.

(Comparative Example 1)
A coal tar pitch containing 0.5% by mass of free carbon was heated at 420 ° C. for 15 minutes to produce 12% by mass of mesocarbon microspheres (average particle size 19 μm) in the coal tar pitch. To this, tar light oil (boiling point: 80 to 120 ° C.) is added, the viscosity of coal tar pitch is adjusted to 0.01 mPa · s (125 ° C.), and the mixture is stirred at 125 ° C. for 1 hour. A graphitizable carbon material containing was obtained. This was poured into a pressure filter while being maintained at 125 ° C., pressurized with nitrogen gas and filtered to separate mesocarbon microspheres.

  The separated mesocarbon spherules were placed in a 125 ° C. nitrogen atmosphere to remove oil and dried. Then, after calcining at 450 ° C. for 5 hours in a nitrogen atmosphere and adjusting the particle size, it is further placed in a graphite crucible, heated in an argon atmosphere at a heating rate of 1000 ° C./hour, and graphitized at 3000 ° C. for 3 hours. The treatment was performed and graphitized. This was classified to obtain mesocarbon microsphere graphitized products having an average particle diameter of 17 μm.

The obtained mesocarbon microsphere graphitized product was embedded in a resin, polished and subjected to argon ion sputtering, and the cross section of the mesocarbon microsphere graphitized product was observed with a scanning electron microscope. The observation result is shown as a schematic diagram in FIG.
From FIG. 2, it was found that this mesocarbon microsphere graphitized product was not a completely parallel layered structure, but a single crystal structure in which the layers continued for a long time.

  Further, in the same manner as in Example 1, the obtained single crystal mesocarbon microsphere graphitized product was PVDF as a binder, and the mass ratio of single crystal mesocarbon microsphere graphitized product / PVDF was 90/10. Then, PVDF was dissolved with N-methylpyrrolidone and kneaded to prepare a pasty active material.

This pasty active material was applied to one side of a copper foil as a current collector using a doctor blade applicator having a clearance of 200 μm, dried at 100 ° C. for 12 minutes, and the active material density in the electrode was 1. A test electrode was manufactured by press-molding to 6 g / cm ³ and vacuum-drying at 130 ° C. overnight. This electrode was subjected to X-ray diffraction measurement using Cu—Kα rays as an X-ray source, and the peak intensity of the (101) plane and the peak intensity of the (004) plane were determined. The intensity ratio I ₁₀₁ / I ₀₀₄ was 0.33.

Using this test electrode, a test battery was produced in the same manner as in Example 1. The discharge capacity per mass of the test battery was 335 mAh / g, and the discharge capacity per volume was 536 mAh / cm ³ .
Moreover, the discharge load characteristic of the test battery using this active material was 75%.
Table 1 summarizes the active material density and X-ray diffraction intensity ratio of the test electrode, and the electrolyte solution component, discharge capacity, charge / discharge loss, charge / discharge efficiency, and discharge load characteristics of the test battery.

  Comparing the results of Comparative Example 1 with the results of Example 1, Example 1 using an active material composed of polycrystalline mesocarbon microsphere graphitized products in which the C-axis direction of each crystal forms a random direction. It can be seen that a lithium ion secondary battery having excellent discharge capacity per volume and discharge load characteristics, small charge / discharge loss, and good charge / discharge efficiency can be obtained.

(Comparative Example 2)
A test electrode was prepared by using scaly natural graphite having an average particle size of 20 μm as an active material and the active material density of 1.6 g / cm ³ in the same manner as in Comparative Example 1. This electrode was subjected to X-ray diffraction measurement using Cu—Kα rays as an X-ray source, and the peak intensity of the (101) plane and the peak intensity of the (004) plane were determined. The intensity ratio I ₁₀₁ / I ₀₀₄ was 0.05.

Using this test electrode, a test battery was produced in the same manner as in Example 1. Table 1 summarizes the evaluation results of the test electrode and the test battery. The discharge capacity per mass was 365 mAh / g, but the discharge capacity per volume was as low as 584 mAh / cm ³ because the active material density could not be increased. Moreover, it turned out that charging / discharging loss is as high as 45 mAh / g, and charging / discharging efficiency is also low. Further, the discharge load characteristics were inferior at 56%.

Further, using the active materials obtained in Example 1 and Comparative Examples 1 and 2, the active material density was changed to 1.5 g / cm ³ , 1.6 g / cm ³ and 1.7 g / cm ³ . For the test battery, the constant current charge capacity was measured at a constant current of 0.9 mA at a charge rate of 0.5 C until the circuit voltage reached 0 mV. The result is shown in FIG.
From FIG. 3, it can be seen that the test battery using the active material of Example 1 has better charging characteristics than the test battery using the active material of Comparative Examples 1 and 2.
In Example 1, the constant current charge capacity did not change to 352 mAh / g even when the active material density was 1.8 g / cm ³ . However, Comparative Example 1 can not increase the active material density to 1.8 g / cm ^3, the Comparative Example 2 increases the active material density to 1.8 g / cm ^3, in the step of drying the electrode, peeled I have. In both Comparative Examples 1 and 2, a test electrode having an active material density of 1.8 g / cm ³ or more could not be produced.

  Further, for the test batteries using the negative electrode materials obtained in Example 1 and Comparative Examples 1 and 2, the discharge rates were 0.1 C, 0.2 C, 0.5 C, 1.0 C, 1.5 C and 2.5 C. FIG. 4 shows the result of measuring the discharge load characteristics (ratio to the discharge capacity at 0.1 C) instead of. FIG. 4 shows that the test battery using the active material of Example 1 is superior in discharge load characteristics as compared with the test battery using the active materials of Comparative Examples 1 and 2.

  From FIG. 3 and FIG. 4, the graphitized product of polycrystalline mesocarbon spherules of Example 1 of the present invention can increase the active material density, has a high discharge capacity per volume, and has charging characteristics. In addition, a lithium ion secondary battery excellent in discharge load characteristics can be obtained. Moreover, since it is excellent in charging characteristics, the lithium ion secondary battery using the graphitized product of the present invention as an active material also has excellent cycle characteristics.

Claims (4)

  A polycrystalline mesocarbon microsphere graphitized product having a mean particle diameter of 1 to 120 μm, comprising a plurality of crystals, and the C-axis direction of each crystal is random.   The polycrystalline mesocarbon microsphere graphitized product according to claim 1, wherein the graphitized product is spherical.   A negative electrode active material for a lithium ion secondary battery comprising the polycrystalline mesocarbon microsphere graphitized product according to claim 1 or 2.   A lithium ion secondary battery comprising a negative electrode active material, a positive electrode active material, and a non-aqueous electrolyte, and using the polycrystalline mesocarbon microsphere graphitized product according to claim 1 or 2 as a negative electrode active material.

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