JP4554795B2 - Carbon material for negative electrode of lithium ion secondary battery and method for producing the same - Google Patents

Description

【００５９】
【発明の効果】
本発明によれば、リチウムイオン二次電池において、高放電容量で、高い初期充放電効率を得ることができる。
【図面の簡単な説明】
【図１】 本発明の実施例１で得られた炭素材料を電子顕微鏡撮影した図面代用写真である。
【図２】 従来の解砕または粉砕処理が施された炭素材料を電子顕微鏡撮影した図面代用写真である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carbon material for a negative electrode of a lithium ion secondary battery that exhibits a high discharge capacity and high initial charge / discharge efficiency, and a method for producing the same.
[0002]
[Prior art]
In recent years, with the miniaturization or high performance of electronic devices, there is an increasing demand for higher energy density of batteries. Under such circumstances, lithium secondary batteries using lithium for the negative electrode theoretically have not only high energy density, high voltage and long cycle life, but also from the viewpoint of environmental pollution. It is attracting attention as a next-generation secondary battery to replace the mainstream nickel-cadmium battery.
Especially when used as a negative electrode as it is, it has the ability to insert (occlude) and release (release) lithium ions instead of lithium metal, which has the problems of cycle life reduction and safety reduction due to dendrite precipitation. Since the discovery that a carbon material that can be prevented can be a stable negative electrode material, it has become possible to put lithium ion secondary batteries into practical use.
[0003]
Lithium ion secondary batteries are usually composed of two types of intercalation compounds with different oxidation-reduction potentials that function as lithium ion carriers as positive and negative electrode materials. Done between layers. Although the lithium ion insertion / removal mechanism itself in the negative electrode in a lithium ion secondary battery using a carbon material has not been fully elucidated at present, many proposals regarding the carbon material have been made.
Among these, graphite having excellent charge / discharge characteristics and high discharge capacity and potential flatness is considered promising (Japanese Patent Publication No. 62-23433, etc.). Particularly in recent years, it has been proposed that a graphite-based carbon material obtained by high-temperature treatment of mesophase carbon microspheres (mesocarbon microbeads) at 2000 ° C. or higher is suitable for a negative electrode of a lithium ion secondary battery (Japanese Patent Laid-Open No. 4-115458). No. 4, JP-A-4-188559, JP-A-4-190557, JP-A-4-332484, etc.). This is a microsphere of mesophase carbon microspheres of several to several tens of μm, has many lithium ion insertion / extraction sites, can take a large electrode filling rate, and has an appropriate graphite structure or similar to graphite by high-temperature heating. It is thought to be due to having a structure.
[0004]
The battery characteristics of graphite are said to be related to the graphite structure because of its similarity to the graphite intercalation compound, but the optimum graphite structure differs depending on the battery element such as electrolyte or solvent, and the negative electrode of the lithium ion secondary battery Various graphite structures have been proposed as carbon materials that give good performance. For example, the thickness (Lc) of the crystal lattice in the C-axis direction in carbon X-ray diffraction is 200 mm (20 nm), and the ratio Lc / La to the thickness (La) of the crystal lattice in the A-axis direction is 1.3 or more. Carbon material (Japanese Patent Laid-Open No. 4-190556), lattice spacing (d ₀₀₂ ) Is 3.45 mm (0.345 nm) and Lc is 300 mm (30 nm) or more (Japanese Patent Laid-Open No. 4-188559), d ₀₀₂ 3.35-3.40 mm (0.335-0.340 nm), Lc and La are 200 mm (20 nm) or more, and the true density is 2.00-2.25 g / cm. ^Three A carbon material (Japanese Patent Laid-Open No. 4-188559) is proposed.
[0005]
The graphitized mesophase carbon microspheres described above separate the optically anisotropic intermediate phase (spherulites) of macroaromatic polymers that precipitate around 350-450 ° C when the pitches are heat-treated from the pitch matrix. The spherulites separated from the pitch matrix can be obtained by heating them to high temperatures as they are, and they can be obtained as a strongly fused lump. In order to use this as a negative electrode material for batteries, it is necessary to pulverize or pulverize the massive heat-fused product into a powder form, but it is not easy to pulverize or grind this fused material.
[0006]
For this reason, for example, prior to heating the spherulites separated from the pitch matrix to a high temperature, it is proposed to prevent fusion during graphitization by adding primary firing (calcination) at a low temperature of about 350 ° C. . In order to prevent fusion during heating, when separating the spherulites from the pitch matrix, the pitch component to be fused carbonized is removed to 5% by mass or less using an extraction solvent such as benzene or quinoline. It is also proposed that this is preferable (Japanese Patent Laid-Open No. 7-169458).
[0007]
According to the method of the above publication, even if the mesophase carbon spherules are graphitized, they can be obtained with a particle size that can be used as a negative electrode of a lithium ion secondary battery with almost no fusion. Although crushing or crushing is not necessarily required to obtain, crushing or crushing can be easily performed, and the yield can be increased by adding crushing or crushing treatment after primary firing and / or graphitization. .
Further, the increase in the capacity of the lithium ion secondary battery greatly contributes to the filling rate of the negative electrode carbon material. For this reason, in commercial practice, the particle size is usually crushed or crushed.
[0008]
[Problems to be solved by the invention]
On the other hand, in the lithium ion secondary battery using graphite as a negative electrode material, the irreversible capacity (hereinafter also referred to as “irreversible capacity”) in the first cycle is remarkably increased, and the level is several tens to several hundred mAh / g at the first discharge. Indicates discharge capacity loss. That is, there is a problem that the initial charge / discharge efficiency is low. Not all of the causes have been clarified, but one of them is that graphite is active against the electrolyte. Specifically, the decomposition of the solvent or supporting electrolyte on the graphite surface has been reported. Has been. This decomposition reaction continues until the decomposition product is deposited and grows on the graphite (carbon) surface, and the electron has a thickness that cannot move directly from the graphite surface to a solvent or the like. Also, solvent molecules and lithium ions co-intercalate and the graphite surface layer peels off, and the newly exposed graphite surface may react with the electrolyte to increase the irreversible capacity (low initial charge / discharge efficiency) (Journal of Electrochemical Society, vol.137, 2009 (1990)).
[0009]
In view of the circumstances as described above, the present invention effectively reduces the irreversible capacity in the first cycle, obtains high initial charge / discharge efficiency, and provides a high-discharge capacity lithium ion secondary battery. It is an object to provide a carbon material for use, a method for producing such a carbon material, and a lithium ion secondary battery.
[0010]
[Means for Solving the Problems]
The inventors of the present invention have intensively studied to solve the above-described problems. As a result, the pulverization or pulverization treatment is performed after the primary firing or after the graphitization treatment in order to obtain graphite particles having a uniform particle size with a high yield. If only the particles having a molten surface layer formed during primary firing and graphitization are used as the carbon material of the negative electrode of the lithium ion secondary battery, the initial charge / discharge efficiency is reduced. Markedly It was found that it can be improved.
That is, the carbon material for a lithium ion secondary battery negative electrode of the present invention is 400-600 under inert atmosphere High-temperature heating at 2000 ° C. or higher is added after primary firing at No crushing or crushing It is a substantially single layer powdery carbon material consisting of mesophase carbon spherules alone and / or aggregates thereof, and has the molten surface layer formed in the primary firing and high-temperature heating process as it is.
[0011]
The maximum particle diameter of the lithium ion secondary battery negative electrode carbon material is preferably smaller than 150 μm.
Moreover, it is desirable not to contain fine powder having a particle size of 3 μm or less.
[0012]
In the method for producing a carbon material for a lithium ion secondary battery negative electrode according to the present invention, after separating mesophase carbon microspheres generated by heat treatment of pitches from the pitch matrix, 400-600 under inert atmosphere Primary firing at ℃, the resulting fired product Without crushing or crushing After classifying and removing coarse particles and lumps, heat at 2000 ° C or higher And leave the molten surface layer formed in the high-temperature heating process as it is .
[0013]
Classification of the fired product can be performed with a 200 mesh sieve.
Furthermore, it is desirable to remove fine powder having a particle size of 3 μm or less from air classification.
[0014]
In the above, it is desirable to separate and wash from the pitch matrix using an organic solvent so that the quinoline soluble content of the mesophase carbon microspheres is 20% by mass or less. The mesophase carbon microspheres thus separated preferably have a toluene soluble content of 10% by mass or less.
[0015]
Thereby, the above-described carbon material for a negative electrode of a lithium ion secondary battery of the present invention can be obtained.
In the present invention, a lithium ion secondary battery using a negative electrode formed from the carbon material is also provided. A lithium ion secondary battery usually includes a positive electrode and a nonaqueous electrolyte together with the negative electrode.
[0016]
In addition, it has been proposed to form a coating on the surface of graphite particles exposed by pulverization of a material in which graphite particles are fused and agglomerated with each other, and use them for the negative electrode of a lithium ion secondary battery, for example, in JP-A-11-310405. Is a carbon having a two-layer structure in which a surface layer made of a carbon material for coating formation is formed on the surface of the core carbon material (graphite), and part or all of the edge portion of the core crystal is coated with the carbon material for coating formation. Materials have been proposed. Thus, the coating layer formed on the graphite surface may peel off during use.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described more specifically.
As described above, 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 during the charge / discharge process. Done between layers. In essence, this is a battery mechanism in which lithium ions are doped into the negative electrode during charging and are dedoped from the negative electrode during discharging.
In the present invention, the following specific carbon materials are provided as negative electrode materials for such lithium ion secondary batteries. That is, the carbon material for a negative electrode of a lithium ion secondary battery of the present invention is composed of mesophase carbon microspheres alone and / or aggregates thereof subjected to high-temperature heating at 2000 ° C. or higher following primary firing at 350 to 900 ° C. It is a substantially single-layer powdery carbon material, and has the molten surface layer formed in the primary firing and high-temperature heating process as it is.
[0018]
Since mesophase carbon microspheres have low melting point components such as a pitch matrix component remaining on the surface, the components are melted and graphitized by primary firing. The carbon material of the present invention has a graphitized surface after being melted in this way. In the present invention, the surface graphitized after the low melting point component is melted is referred to as a molten surface layer.
The carbon material of the present invention essentially has no exposed edge and has a molten surface layer as it is.
In the present invention, having a substantially single layer means that a surface layer integrally formed with the inside of the particle as a fusion layer finally subjected to high-temperature heating at 2000 ° C. or higher is graphitized, but the carbon material is graphitized. This means that there is no coating layer formed after pulverizing or pulverizing or pulverizing graphite particles. Such a molten surface layer has no fear of peeling off.
[0019]
As described above, the graphitized mesophase carbon microspheres used as the negative electrode material of the lithium ion secondary battery are usually crushed or ground in any of the manufacturing processes. The This is an added product, and at least a part of the inside is exposed, but particles made of only a graphitized carbon material having a melted surface layer formed during primary firing and graphitization are used as negative electrodes for lithium ion secondary batteries. It is not known that the initial charge / discharge efficiency is remarkably improved as compared with products containing ordinary pulverized or pulverized products.
[0020]
The carbon material has a lattice spacing d in X-ray diffraction. ₀₀₂ Is preferably a graphite material having a true specific gravity of 2.2 or more. Here, the lattice spacing d ₀₀₂ Means a value measured by an X-ray diffraction method (Otani Sugirou, carbon fiber, P733-742 (1986) Modern Editing Co., Ltd.) using CuKα rays as X-rays and high-purity silicon as a standard substance.
[0021]
The carbon material for a negative electrode of the lithium ion secondary battery of the present invention preferably has a maximum particle size smaller than 150 μm, more preferably 108 μm or less, from the viewpoint of filling rate.
Moreover, it is desirable not to contain fine powder having a particle size of 3 μm or less.
Usually, those having an average particle size of more than 3 μm and not more than 108 μm are preferably used.
[0022]
The carbon material for a lithium ion secondary battery negative electrode as described above can be produced by the following method.
In the method for producing a carbon material for a negative electrode of a lithium ion secondary battery according to the present invention, the mesophase carbon microspheres produced by heat treatment of pitches were separated from the pitch matrix, and then subjected to primary firing at 350 to 900 ° C. After classifying the baked product to remove coarse particles and agglomerates, it is heated at a high temperature of 2000 ° C. or higher.
[0023]
In the mesophase carbon microspheres, when the petroleum pitch, coal tar pitch and other petroleum and coal-based pitches are heat-treated, the giant aromatic polymer produced in the pitch around 350 to 450 ° C. is precipitated in the pitch matrix. It is a microsphere having a particle size of several to several tens of μm obtained by separating the intermediate phase (spherulite) from the pitch matrix.
[0024]
In the above, it is desirable to separate and wash from the pitch matrix using an organic solvent so that the quinoline soluble part (QS) of the mesophase carbon microspheres is 20 mass% or less, preferably 10 mass% or less.
The mesophase carbon microspheres thus separated preferably have a toluene soluble content (TS) of 10% by mass or less, more preferably 5% by mass or less.
In this case, benzene, toluene, quinoline, tar middle oil, tar heavy oil, or the like can be used as the organic solvent. Two or more of these may be used in combination.
[0025]
The mesophase carbon spheres separated from the pitch matrix are , It heats to 400-600 degreeC and performs primary baking (calcination). In order to classify the fired product and remove coarse particles and agglomerates, a general classification method such as sieving or air classification can be employed. Classification is performed under conditions such that crushing and crushing are not performed. Specifically, it can be performed with a 200 mesh sieve.
Further, if necessary, fine powder is also removed and the minimum particle size is adjusted. It is preferable to remove fine powder having a particle size of 3 μm or less from air classification.
Primary firing temperature is 400 ° C Less than, it is easy to fuse by high temperature heating in the next process, 600 ° C If it is super, it is easy to fuse by primary firing.
[0026]
The calcined product from which coarse particles and agglomerates have been removed is then heated (graphitized) at a high temperature of 2000 ° C. or higher, preferably 2500 ° C. or higher, more preferably 2500 to 3200 ° C. After this graphitization treatment, the removal treatment of coarse particles and aggregates by classification as described above may be performed. If it is less than 2000 ° C., graphitization does not proceed sufficiently, and the discharge capacity becomes small.
[0027]
In the present invention, the particle size is adjusted by classification in any of the above processes, and is not pulverized or crushed. For this reason, the yield as the final product is not high compared with the method including crushing or crushing, but there is no exposure of the edge portion of the crystal, and it is suitable as a material for a negative electrode of a lithium ion secondary battery. Thus, a carbon material having particularly excellent initial charge / discharge efficiency can be obtained.
In the primary firing, mesophase carbon microspheres are simple spheres of several to several tens of μm. However, when the primary firing, a plurality of single particles are aggregated into aggregates of several tens of μm, and further aggregation proceeds. For example, it becomes a coarse particle of 300 μm or more or a lump of about several centimeters.
When coarse particles or lumps are used as the carbon material for the negative electrode, the unevenness is severe and it is preferable to remove them.
[0028]
<Lithium ion secondary battery>
In this invention, the lithium ion secondary battery using the negative electrode formed from the above carbon materials is also provided. The lithium ion secondary battery of the present invention is not particularly limited except that the above carbon material is used as the negative electrode material, and other battery components conform to the elements of a general lithium ion secondary battery. A lithium ion secondary battery usually includes a negative electrode, a positive electrode, and a nonaqueous electrolyte as main battery components.
[0029]
<Negative electrode>
Formation of the negative electrode from the carbon material can be carried out in accordance with a normal molding method, but the negative electrode is capable of sufficiently drawing out the performance of the carbon material, having high formability to the powder, and being chemically and electrochemically stable. There is no limitation as long as it can be obtained.
At the time of producing the negative electrode, a binder mixture obtained by adding a binder to a carbon material can be used.
As the binder, it is desirable to use one having chemical stability and electrochemical stability with respect to the electrolyte. For example, fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, and Carboxymethylcellulose and the like are used. These can also be used together.
In general, the binder is preferably used in an amount of about 1 to 20% by mass in the total amount of the negative electrode mixture.
[0030]
Specifically, for example, a carbon material 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 applied to one or both sides of a current collector. By coating, a negative electrode mixture layer can be formed.
In this case, a normal solvent can be used. The negative electrode mixture is dispersed in the solvent, made into a paste, and then applied to the current collector and dried to make the negative electrode mixture layer uniform and strong. Glued to the body.
More specifically, for example, a carbon material and a fluorine resin powder such as polytetrafluoroethylene may be mixed and kneaded in a solvent such as isopropyl alcohol and then applied. Further, after mixing the carbon material and a water-based binder such as fluorinated resin powder such as polyvinylidene fluoride or carboxymethylcellulose with a solvent such as N-methylpyrrolidone, dimethylformamide, water, alcohol or the like, Can be applied.
[0031]
The coating thickness when the mixture of the carbon material powder and the binder is applied to the current collector is suitably 10 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.
Also, the carbon material and resin powder such as polyethylene and polyvinyl alcohol can be dry-mixed and hot-press molded in a mold.
[0032]
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 foil, stainless steel foil, and nickel foil. In the case of a foil shape, the thickness of the current collector is preferably about 5 to 20 μm.
[0033]
<Positive electrode>
As a material for the positive electrode (positive electrode active material), it is preferable to select a material capable of doping / dedoping a sufficient amount of lithium. Examples of such positive electrode active materials include lithium-containing transition metal oxides, transition metal chalcogenides, and vanadium oxides (V ₂ O _Five , V ₆ O ₁₃ , V ₂ O _Four , V _Three O ₈ And lithium-containing compounds such as Li compounds thereof, general formula M _X Mo ₆ S _8-y (Wherein X is a numerical value in a range of 0 ≦ X ≦ 4, Y is a value in a range of 0 ≦ Y ≦ 1, M represents a metal such as a transition metal), a chevrel phase 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-X M (2) _X O ₂ (Wherein X is a numerical value in the range of 0 ≦ X ≦ 1, and M (1) and M (2) are composed of at least one transition metal element) or LiM (1) _2-y M (2) _y O _Four (Where Y is a numerical value in the range of 0 ≦ Y ≦ 1, and M (1) and M (2) are composed of at least one transition metal element).
Examples of the transition metal element represented by M in the formula include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., preferably Co, Fe, Mn, Ti, Cr, V and Al are mentioned.
[0034]
More specifically, as the lithium-containing transition metal oxide, LiCoO ₂ LixNi _y M _1-y O ₂ (M is the transition metal element excluding Ni, preferably at least one selected from Co, Fe, Mn, Ti, Cr, V, and Al, 0.05 ≦ x ≦ 1.10, 0.5 ≦ y ≦ 1. A lithium composite oxide represented by LiNiO ₂ LiMnO ₂ , LiMn ₂ O _Four Etc.
[0035]
The lithium-containing transition metal oxide as described above includes, for example, Li, transition metal oxides or salts as starting materials, these starting materials are mixed according to the composition, and a temperature of 600 ° C. to 2000 ° C. in an oxygen-existing atmosphere. It can be obtained by firing in a range. The starting material is 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.
[0036]
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. For example, a carbon material is used as the conductive agent.
[0037]
The shape of the current collector is not particularly limited, and may be a box shape or a mesh shape such as a mesh or expanded metal. For example, examples of the current collector include aluminum foil, stainless steel foil, and nickel foil. The thickness is preferably 10 to 40 μm.
In the case of the positive electrode, as in the case of the negative electrode, the positive electrode mixture is dispersed in a solvent to make 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 forming the positive electrode mixture layer, pressure bonding such as press pressing may be further performed. As a result, the positive electrode mixture layer is uniformly and firmly bonded to the current collector.
[0038]
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.
[0039]
<Electrolyte>
As an electrolyte used in the present invention, an electrolyte salt used in a normal nonaqueous electrolytic solution can be used, for example, LiPF. ₆ , LiBF _Four , LiAsF ₆ LiClO _Four , LiB (C ₆ H _Five ), LiCl, LiBr, LiCF _Three SO _Three , LiCH _Three SO _Three , LiN (CF _Three SO ₂ ) ₂ , LiC (CF _Three SO ₂ ) _Three , LiN (CF _Three CH ₂ OSO ₂ ) ₂ , LiN (CF _Three CF ₂ OSO ₂ ) ₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN ((CF _Three ) ₂ CHOSO ₂ ) ₂ , LiB [C ₆ H _Three (CF _Three ) ₂ ] _Four LiAlCl _Four , LiSiF ₆ Lithium salt etc. can be used. In particular, LiPF ₆ , LiBF _Four Is preferably used from the viewpoint of oxidation stability.
The electrolyte salt concentration in the electrolytic solution is preferably 0.1 to 5 mol / liter, and more preferably 0.5 to 3.0 mol / liter.
[0040]
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.
[0041]
In the case of a liquid non-aqueous electrolyte solution, the solvent is ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, trimethyl borate, silicic acid Tetramethyl, nitromethane, dimethylformamide, N -Aprotic organics such as methyl pyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, sulfite, dimethyl sulfite A solvent can be used.
[0042]
When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, it contains a matrix polymer gelled with a plasticizer (non-aqueous electrolyte). , Fluorinated polymers such as polyethylene oxide and its crosslinked polymers, polymethacrylates, polyacrylates, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, etc., alone or in combination. Can be used.
Among these, it is desirable to use a fluorine-based polymer such as polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer from the viewpoint of oxidation-reduction stability.
[0043]
As the electrolyte salt and non-aqueous solvent constituting the plasticizer contained in these polymer solid electrolyte and polymer gel electrolyte, any of those described above 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 / liter, and more preferably 0.5 to 2.0 mol / liter.
The method for producing such a solid electrolyte is not particularly limited. For example, a method in which a polymer compound, a lithium salt and a solvent (plasticizer) are mixed and heated to melt, or a suitable organic solvent for mixing is highly suitable. Method of evaporating the organic solvent for mixing after dissolving the molecular compound, lithium salt and solvent (plasticizer), and mixing the monomer, lithium salt and solvent (plasticizer), and then ultraviolet, electron beam or molecular beam And a method of forming a polymer by irradiation.
Moreover, 10-90 mass% is preferable, and, as for the addition ratio of the solvent (plasticizer) in the said solid electrolyte, More preferably, it is 30-80 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 filming will be difficult.
[0044]
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 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.
[0045]
In the lithium ion secondary battery of the present invention, the gel electrolyte can be used because the initial charge / discharge efficiency has been improved.
A gel electrolyte secondary battery is configured by laminating a negative electrode containing a carbon material, a positive electrode, and a gel electrolyte in the order of, for example, a negative electrode, a gel electrolyte, and a positive electrode, and housing the resultant in a battery exterior material. 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 carbon material for the negative electrode, even when propylene carbonate is contained in the gel electrolyte and the carbon material powder has a particle size small enough to reduce the impedance sufficiently, it is irreversible. Capacity is kept small. Therefore, a large discharge capacity is obtained and a high initial charge / discharge efficiency is obtained.
[0046]
Furthermore, the structure of the lithium ion secondary battery according to 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. be able to. 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.
[0047]
【Example】
EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited to these Examples.
The powder characteristics and charge / discharge characteristics of the graphite powders obtained in Examples and Comparative Examples were measured as follows. In the following, a circuit for evaluating a lithium ion secondary battery was produced and the charge / discharge characteristics were measured. However, an actual battery can be produced according to a known method based on the concept of the present invention.
[0048]
<Powder characteristic evaluation>
(1) Particle size distribution
The graphite powder was dispersed in distilled water to which a surfactant was added, and the particle size distribution was measured with a laser diffraction particle size distribution analyzer LS-5000 manufactured by Seishin Co.
(2) BET specific surface area
It was measured by the BET 1 point method by nitrogen gas adsorption method.
(3) X-ray diffraction
About 15% by mass of X-ray standard high-purity silicon powder is added to the graphite powder, mixed, packed in a data cell, and monochromatic with a graphite monochromator. The X-ray diffraction curve is measured, and the interlayer distance (lattice spacing d ₀₀₂ )
(4) Toluene soluble matter
The measurement was performed according to JIS K2425.
(5) Quinoline soluble content
The measurement was performed according to JIS K2425.
The graphite powder is dissolved in quinoline and heated at 75 ° C. for 30 minutes, and then suction filtered while hot using a crucible type filter 1G4 specified in JIS R3503. The filtrate is dried and the mass is measured. Diatomaceous earth is used as a filter aid.
[0049]
<Charge / discharge characteristics evaluation>
The charge / discharge capacity was measured under the following conditions under argon flow in the dry box.
Cell format: 3-pole beaker cell
Counter electrode and reference electrode: metallic lithium
Working electrode: 10 parts by weight of polyvinylidene fluoride (PVDF) was mixed with 100 parts by weight of graphite powder, and further N-methylpyrrolidone was added to dissolve PVDF sufficiently to form a paste, which was applied onto a copper foil. This was pre-dried at 100 ° C. and then pressed with a roll press, and further N-methylpyrrolidone was removed under vacuum at 100 ° C. to obtain a working electrode.
Electrolyte: 1M LiClO _Four / PC + EC + DMC (molar ratio 3: 5: 2)
PC: Propylene carbonate
EC: ethylene carbonate
DMC: Dimethyl carbonate
[0050]
Current density 0.2mA / cm ² When the circuit voltage reaches 0.4 mV, switch to constant potential charging at 0.1 V and continue charging until the current value reaches 20 μA, then 0.4 mA / cm ² The constant current discharge was performed. The capacity until the circuit voltage reached 2.5 V was defined as the discharge capacity.
The discharge capacity (mAh / g) per gram of graphite powder is shown in the table.
[0051]
<Initial charge / discharge efficiency>
The charge capacity and discharge capacity were determined from the energization amount in the first charge / discharge cycle, and the initial discharge efficiency was calculated by the following equation.
Initial charge / discharge efficiency = (discharge capacity / charge capacity) x 100 (%)
[0052]
Example 1
<Manufacture of mesophase carbon spheres>
Coal tar containing 0.5% by mass of free carbon (QI) was heat-treated at 350 ° C. for 0.5 hr, and further reheated at 450 ° C. for 0.2 hr to produce mesophase microspheres. Such heat-treated pitch was extracted using oil in tar (bp: 130 to 250 ° C.), and mesophase carbon microspheres were separated and filtered from the pitch matrix. The obtained mesophase carbon microspheres were subjected to primary firing (calcination) at 500 ° C. in a rotary kiln.
The calcined product was removed from a large coarse diameter (referred to as coarse powder) using a 200 mesh (sieve 75 μm) vibrating sieve. The obtained particle size-adjusted product was put into a graphite crucible, and was graphitized by raising the temperature to 3000 ° C. at a rate of 1000 ° C./hr in an argon atmosphere.
Powder characteristics of the graphite powder thus obtained (particle size distribution (average particle size, maximum particle size), specific surface area, crystalline lattice spacing d) ₀₀₂ ) And charge / discharge characteristics are shown in Table 1. An electron micrograph of the graphite powder is shown in FIG.
[0053]
(Example 2)
A graphite powder was produced in the same manner as in Example 1 except that tar heavy oil (bp: 200 to 300 ° C.) was used instead of tar middle oil as an extractant for the heat treatment pitch.
Table 1 shows the powder characteristics and charge / discharge characteristics of the obtained graphite powder.
[0054]
Example 3
A graphite powder was produced in the same manner as in Example 1 except that the calcined product was not subjected to sieving and subjected to graphitization treatment as it was.
Table 1 shows the powder characteristics and charge / discharge characteristics of the obtained graphite powder.
[0055]
(Comparative Example 1)
A graphite powder was produced in the same manner as in Example 1 except that the calcined product was pulverized with a hammer mill before passing through a 200-mesh (sieve 75 μm) vibrating sieve. An electron micrograph of the powder is shown in FIG.
Table 1 shows the powder characteristics and charge / discharge characteristics of the obtained graphite powder.
[0056]
(Comparative Example 2)
When primary baking was performed at 340 ° C. and graphitization treatment was performed in the same manner as in Example 1, it was in the form of a lump and charge / discharge characteristics could not be evaluated.
[0057]
(Comparative Example 3)
Table 1 shows the powder characteristics of commercially available artificial graphite powder and the charge / discharge characteristics when this powder is used as a carbon material for the working electrode.
[0058]
[Table 1]

[0059]
【The invention's effect】
According to the present invention, in a lithium ion secondary battery, high initial charge / discharge efficiency can be obtained with a high discharge capacity.
[Brief description of the drawings]
FIG. 1 is a drawing-substituting photograph of an electron microscope photograph of the carbon material obtained in Example 1 of the present invention.
FIG. 2 is a drawing-substituting photograph in which a conventional carbon material that has been crushed or crushed is taken with an electron microscope.

Claims (5)

Substantially composed of mesophase carbon microspheres alone and / or aggregates thereof that are subjected to primary baking at 400 to 600 ° C. in an inert atmosphere, followed by high-temperature heating at 2000 ° C. or higher, and are not pulverized or crushed. powdery carbonaceous material in a, the primary firing and the lithium ion secondary battery negative electrode carbon material as it has a melting surface layer formed by high-temperature heating process of a single layer.   The carbon material for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the maximum particle size is smaller than 150 µm. The mesophase carbon microspheres produced by heat treatment of pitches are separated from the pitch matrix, and then primary fired at 400 to 600 ° C. in an inert atmosphere, and the obtained fired products are classified without being crushed or disintegrated. 3. The method for producing a carbon material for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the coarse particles and the lump are removed and then heated at a high temperature of 2000 ° C. or higher.   The manufacturing method of the carbon material for lithium ion secondary battery negative electrodes of Claim 3 which classifies the said baked goods with a 200 mesh sieve.   A lithium ion secondary battery using a negative electrode formed from the carbon material according to claim 1.

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