JP2004063411A - Complex graphite material, its manufacturing method, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a complex graphite material and its manufacturing method, suitably used in a negative electrode of, in particular, a lithium ion secondary battery and a lithium ion polymer battery having high discharging capacity, charging and discharging efficiency and tap density, and to provide the lithium ion secondary battery and the lithium ion polymer battery using the same. <P>SOLUTION: This complex graphite material is prepared by mixing a graphite material (graphitized material of mesophase carbon small spherical bodies) of two-layered structure provided with a layer of crystalline property lower than that of inside, on a surface, metallic powder of silicon, tin and aluminum forming alloy with lithium, and carbon precursor, then heating and granulating the obtained mixture, and then reheating the mixture for carbonization. The graphite material is properly used as the negative electrode material of, in particular, the lithium ion secondary battery or the lithium ion polymer battery. Further it can be used as a conductive material for a fuel cell separator and the graphite for refractory, besides the negative electrode material, by utilizing its characteristic. <P>COPYRIGHT: (C)2004,JPO

Description

【００６５】
【表２】

【図面の簡単な説明】
【図１】本発明の複合黒鉛質材料の断面模式図。
【図２】従来の複合黒鉛質材料の断面模式図。
【符号の説明】
１：　複合黒鉛質材料
２：　黒鉛質材料
３：　高結晶性黒鉛質材料
４：　低結晶性黒鉛質材料
５：　複合層
６：　リチウムと合金形成可能な金属粉末
７：　炭素質材料[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a composite graphite material suitable for a negative electrode of a lithium ion secondary battery and a lithium ion polymer battery, a method for producing the same, and a negative electrode for a lithium ion secondary battery and a lithium ion polymer battery using the composite graphite material. And a lithium ion secondary battery and a lithium ion polymer battery using the negative electrode.
[0002]
[Prior art]
2. Description of the Related Art In recent years, with the miniaturization or high performance of electronic devices, demands for higher energy density of batteries have been increasing. Under such circumstances, lithium-ion secondary batteries using lithium for the negative electrode have attracted attention because of their advantages of high energy density and high voltage. In this lithium ion secondary battery, it is known that when lithium metal is used as it is as a negative electrode, lithium is deposited in a dendritic state at the time of charging, so that the negative electrode is deteriorated and the life of a charge / discharge cycle is short. In addition, there is also a danger that lithium precipitated in the form of dentite penetrates through the separator, reaches the positive electrode, and causes a short circuit.
[0003]
Therefore, as a negative electrode material, a carbon material capable of inserting and extracting lithium ions by intercalating lithium instead of lithium and capable of preventing lithium metal from being deposited, specifically, graphite or carbon having a turbostratic structure Materials are used. In particular, graphite has features of high discharge capacity, high initial efficiency, high discharge voltage, and flatness, and is most often used as a negative electrode active material in commercially available lithium ion secondary batteries. However, since graphite has a theoretical capacity of 372 mAh / g, an anode material having a higher capacity is desired, and a composite with a metal capable of forming an alloy with lithium such as silicon, tin, aluminum, silver, and zinc has been studied. ing. It has been reported that these metal powders form an alloy with lithium and exhibit a large charge / discharge capacity (J. of applied electrochemistry 23 (1993) P1-10), silicon (LiSi4.4) 2010 mAh / g, A value of 790 mAh / g tin (LiSn4.4) is shown. However, there is a problem in cycle characteristics and charge / discharge efficiency, and it has not been put to practical use.
[0004]
In order to solve the above problem, a method of compounding the metal and the carbonaceous material has been proposed.
For example, in an electrode material in which a carbon material A and a metal B are bonded or coated with another carbon material C, a proposal for regulating the ratio of the amorphous structure portion / crystal structure portion on the surface of the carbon material C (Japanese Unexamined Patent Application Publication No. No. 286763, JP-A-6-279112), a carbonaceous material, tar pitch and a metal such as aluminum or silicon are mixed, and the mixture is heat-treated and carbonized to produce a composite carbide. Carbon layer distance d ₀₀₂ (JP-A-8-231273).
[0005]
Further, in a composite carbon material for an electrode in which an organic material and a metal compound are coated on a graphite surface, there is a proposal for specifying the crystal size in the C-axis and A-axis directions of particulate graphite (Japanese Patent Application Laid-Open No. H11-279785). . Further, among composite carbon particles containing graphite, amorphous carbon and silicon, a composite carbon particle in which a plurality of graphite particles coated with amorphous carbon containing silicon are aggregated is known (JP-A-2000-203818). Publication). In addition, metal-carbon composite particles in which metal particles are embedded in both phases of amorphous carbon such as graphite and petroleum-based pitch have been proposed (Japanese Patent Application Laid-Open No. 2000-272911).
[0006]
All of the proposed composite graphite materials (including graphite particles and the like) have a large charge / discharge loss, low charge / discharge efficiency, and have not been put to practical use.
Some of the composite graphitic materials regulate the ratio of the amorphous structure portion to the crystal structure portion on the surface. However, the ratio between the surface and the internal crystallinity of the graphite portion itself (graphite core material) is controlled. It's not about the difference. Therefore, conventionally, there is no known composite graphite material that refers to the difference between the crystallinity of the surface of the graphite portion itself (graphite core material) and the crystallinity inside the graphite portion itself. There is no known composite graphitic material that mentions that it affects the discharge capacity and charge / discharge efficiency.
[0007]
The inventor of the present invention has found that, in the case of the prior art, the reactivity of a graphite material before compounding (hereinafter, sometimes referred to as a raw graphite material) is high, and this causes a decrease in the charge / discharge efficiency of the graphite material. The present inventors focused on forming a low-crystalline layer on the surface of the raw graphite material to form a two-layer structure. The present inventors have found that, on the low crystalline layer, a metal powder capable of forming an alloy with lithium is composited with a carbonaceous material to produce a composite graphitic material having a high discharge capacity and charge / discharge efficiency. Reached.
[0008]
[Problems to be solved by the invention]
The present invention relates to a composite graphite material having a high discharge capacity and charge / discharge efficiency, particularly a composite graphite material suitable for use in a negative electrode of a lithium ion secondary battery or a lithium ion polymer battery, a method for producing the same, and a composite graphite material thereof An object of the present invention is to provide a negative electrode of a lithium ion secondary battery or a lithium ion polymer battery using a material and a lithium ion secondary battery or a lithium ion polymer battery using the same.
[0009]
[Means for Solving the Problems]
The object of the present invention was obtained after mixing a raw graphite material of a two-layer structure having a layer with lower crystallinity than the inside, a metal powder capable of forming an alloy with lithium, and a carbonaceous precursor on the surface. This is achieved by heating and crushing the mixture followed by reheating to carbonize to obtain a composite graphitic material.
[0010]
That is, the present invention is a graphite material obtained by compounding a metal powder capable of forming an alloy with lithium with a carbon material on the surface of the graphite material, and the crystallinity of the surface of the graphite material is graphite. A composite graphitic material characterized by being lower than the crystallinity inside the material.
[0011]
The preferred graphite material is the case of graphitized mesophase carbon spherules.
[0012]
The preferred graphite material has a Raman spectrum of 1360 cm. ^-1 Peak intensity (I ₁₃₆₀ ) And 1580cm ^-1 Peak intensity (I ₁₅₈₀ ) Intensity ratio (R = I ₁₃₆₀ / I ₁₅₈₀ ) Is 0.05 or more, and the plane spacing d of the carbon netting layer by X-ray diffraction ₀₀₂ Is 0.3365 nm or less.
[0013]
The preferred metal capable of forming an alloy with lithium is at least one metal selected from the group consisting of silicon, tin and aluminum.
[0014]
The present invention provides a method for heating a mixture containing a graphite material, a metal powder capable of forming an alloy with lithium, and a carbonaceous precursor, the surface of which has lower crystallinity than the internal crystallinity, at 350 to 800 ° C. Crushing the material, heating the crushed material at 800 to 2000 ° C. to carbonize the carbonaceous precursor, and compounding the metal powder on the surface of the graphite material with the obtained carbonaceous material. This is a method for producing a composite graphite material.
The graphitic material is preferably a graphitized mesophase carbon spheroid.
[0015]
The present invention is a negative electrode for a lithium ion secondary battery, characterized by using any one of the above graphite materials.
[0016]
The present invention provides a lithium ion secondary battery using the negative electrode for a lithium ion secondary battery.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail.
The graphitic material used as a raw material of the composite graphitic material of the present invention is a graphite material having a two-layer structure whose surface crystallinity is lower than the internal crystallinity. The graphite material is a graphite material obtained by coating a graphite material with a carbonaceous precursor and then carbonizing the coating, or a graphite obtained by carbonizing mesophase carbon microspheres generated during pitch heat treatment. Quality material.
[0018]
Hereinafter, a method of carbonizing the mesophase carbon microspheres to obtain a raw graphite material will be described.
<Mesophase carbon sphere>
Mesophase carbon microspheres have an optical anisotropy of several μm to several tens of μm in particle size generated in a pitch matrix when a petroleum or coal pitch is heated at a temperature of about 350 to 450 ° C. It is a small sphere. The small spheres are extracted and separated from the pitch matrix together with a low molecular weight component using a solvent such as benzene, toluene, quinoline, medium tar oil, and heavy tar oil. After drying the separated mesophase carbon microspheres, primary firing is performed at a temperature of 350 ° C. or more, preferably 350 to 900 ° C., and the fired product is classified without crushing / crushing to remove coarse particles and aggregates. . At the time of the primary firing, a small amount of pitch or the like adhering to the surface of the small spheres is carbonized, and these form a thin layer on the surface of the small spheres. Classification can be carried out by a method generally used industrially, such as a sieve or an air classifier. For example, the target maximum particle size is adjusted using a 200-mesh sieve.
[0019]
The average particle size of the small spheres after the adjustment of the particle size is preferably from 2 to 100 μm, and particularly preferably from 5 to 50 μm. If the average particle size of the small spheres after the adjustment of the particle size is less than 2 μm, the specific surface area of the composite graphite material becomes large, and it becomes difficult to apply it to the electrode. If the average particle diameter of the small spheres exceeds 100 μm, the electrode surface becomes rough when the composite graphite material is applied to the electrode, which is not preferable. The specific surface area of the small spheres after the particle size adjustment was 5 m ² / G or less, preferably 1 m ² / G or less is particularly preferred.
Small spheres having the above characteristics are preferable because they can form a composite graphite material having a large tap density (filling density) as compared with a case where flaky natural graphite is used as a raw material. The removal of coarse particles and fine powder by classification can also be performed after graphitization.
[0020]
<Raw graphite material>
The mesophase microspheres are further subjected to a heat treatment at a temperature of 2,000 ° C. or higher, preferably 2500 ° C. or higher, to form a two-layer structure having a low crystallinity layer on the surface of highly crystalline mesophase microspheres (graphitic structure). Raw graphite material is manufactured.
[0021]
The crystallinity of the raw graphite material is evaluated by Raman spectrum using an argon laser. That is, of the nine types of lattice vibrations based on the graphite structure, 1580 cm corresponding to the E2g type vibration corresponding to the in-plane lattice vibration ^-1 1360 cm reflecting the Raman spectrum in the vicinity and disorder of the crystal structure mainly due to crystal defects in the surface layer, stacking irregularities, etc. ^-1 The nearby Raman spectrum is measured by a Raman spectrometer (NR1100, manufactured by JASCO Corporation) using an argon laser having a wavelength of 514.5 nm.
[0022]
From the peak intensity of each Raman spectrum, its intensity ratio (R = I ₁₃₆₀ / I ₁₅₈₀ ) Is calculated, and the larger the intensity ratio, the lower the crystallinity of the surface. The intensity ratio R is preferably R ≧ 0.05 from the viewpoint of reducing the irreversible capacity. If the crystallinity of the surface is high, R <0.05, and in this case, the irreversible capacity is large, and sufficient battery performance cannot be obtained. It is considered that this is because the degree of crystallinity of the surface layer is too large and the decomposition reaction of the electrolytic solution on the surface of the graphitic material tends to proceed.
[0023]
On the other hand, the average crystallinity of the graphitic material is determined by the plane spacing (d ₀₀₂ ) And the crystallite size in the C-axis direction (Lc). That is, using a CuKα ray as an X-ray source and high-purity silicon as a standard substance, a (002) diffraction peak was measured for the graphitic material, and d was determined from the peak position and its half-value width, respectively. ₀₀₂ , Lc. The calculation method is based on the Gakushin method, and a specific method is described in "Carbon Fiber" (published by Kindaisha Publishing Co., Ltd., March 1986), pages 733 to 742.
[0024]
D by X-ray diffraction as an index of the degree of development of the graphite structure of the raw graphite material of the present invention ₀₀₂ And Lc are d from the viewpoint of developing a high discharge capacity. ₀₀₂ ≦ 0.3365 nm, Lc ≧ 40 nm, preferably d ₀₀₂ It is particularly preferred that ≦ 0.3362 nm and Lc ≧ 50 nm. D of raw graphite material ₀₀₂ And Lc is d ₀₀₂ > 0.3365 nm and Lc <40 nm, the degree of development of the graphite structure is low, so that when used as a negative electrode of a lithium ion secondary battery, the doping amount of lithium is small and a high discharge capacity can be obtained. There are things you can't do.
[0025]
Further, the raw graphite material of the present invention may cause problems such as a decrease in initial charge / discharge efficiency and safety if the specific surface area is too large, so that the specific surface area by nitrogen gas adsorption BET method is 20 m. ² / G or less, preferably 5 m ² / G or less.
[0026]
Next, the raw graphite material is mixed with a metal powder capable of forming an alloy with lithium and a carbonaceous precursor, and the mixture is heated at 350 to 800 ° C, preferably 400 to 600 ° C. Is crushed, and the crushed material is heated to 800 to 2000 ° C., preferably 900 to 1500 ° C. to carbonize the carbonaceous precursor. The heating for carbonization melts the carbonaceous precursor, and the melt binds and coats the metal powder on the surface of the raw graphite material to contribute to the composite. If the carbonization temperature exceeds 2000 ° C., graphitization of the carbonaceous precursor starts, which is not preferable. The time required for carbonization is 0.5 to 50 hours, preferably 2 to 20 hours.
[0027]
When the mixture is heated in two stages, the former stage heating is performed at 350 to 800 ° C., and then crushing is performed, strong energy is not required for crushing aggregates, and crushing is performed from the interface between adjacent aggregates. Therefore, it is not possible to obtain a lump in which a substantial portion of the composite layer is peeled or collapsed, or a defective lump in which a substantial portion of the composite layer is excessively attached or fused. When the crushed material is heated to 800 to 2000 ° C. and carbonized, further aggregation does not substantially occur, and the composite graphite material having a high discharge capacity and charge / discharge efficiency of the present invention can be efficiently produced. The heating time in the former stage is 0.5 to 50 hours, preferably 2 to 20 hours.
[0028]
On the other hand, when the mixture is heated to 800 ° C. or more in one step and carbonized, a large amount of carbide coagulates and the cohesive force is large. In addition to the crushing from the interface (agglomerated surface) of the adjacent lumps, the composite layer of one lumps is crushed in a state of being fused with the other lumps. In other words, it is not preferable because a lump in which a substantial portion of the composite layer is peeled and collapsed and a lump in which a substantial portion of the composite layer is excessively adhered and fused are obtained.
[0029]
<Metal powder>
Metal powder used in the present invention, in a lithium ion secondary battery, is capable of forming an alloy with lithium, silicon, tin, aluminum, at least one selected from silver and zinc, preferably silicon, At least one selected from tin and aluminum. These metal powders form an alloy with lithium and have a discharge capacity of about 700 mAh / g, which is much larger than the graphite material. Therefore, these metal powders are mixed with the graphite material to form a composite material. This has the effect of increasing the discharge capacity of the material, but on the other hand may cause a problem of lowering the charge / discharge efficiency and cycle characteristics. Therefore, it is preferable to set the mixing ratio described later.
[0030]
The particle size of the metal powder is preferably as small as possible, but is 0.01 to 10 μm, preferably 0.05 to 1 μm. If it is less than 0.01 μm, the price of the metal powder will rise. As the metal, at least one selected from silicon, tin, aluminum, silver and zinc is used, and preferably, at least one selected from silicon, tin and aluminum is used.
The mixing ratio of the metal powder to 100 parts by mass of the raw graphite material is 0.1 to 50 parts by mass, preferably 1 to 20 parts by mass.
[0031]
<Carbon precursor>
The carbonaceous precursor is formed by bonding a metal powder to the surface of the graphite material when the raw graphite material or the like is heated to 800 ° C. or higher, such as a binder, a coating agent, etc. It functions as an agent. As the carbonaceous precursor, common carbonaceous precursors such as coal tar pitch, petroleum pitch, and various resins can be used, but use of coal tar pitch is preferred.
The mixing ratio of the carbonaceous precursor to 100 parts by mass of the raw graphite material is not particularly limited, but is 1 to 50 parts by mass, preferably 5 to 30 parts by mass.
[0032]
<Composite graphite material>
As shown in the schematic diagram of FIG. 1, the composite graphite material 1 of the present invention includes a substantially spherical graphite material 2 and a composite layer 5 formed in a layer so as to cover the entire surface of the material 2. It has a layer structure. The graphitic material 2 includes an inner (core) 3 having high crystallinity and a thin surface layer 4 having low crystallinity covering the surface. The composite layer 5 is composed of a metal powder 6 and a carbonaceous material 7.
On the other hand, as shown in the schematic diagram of FIG. 2, the conventional composite graphite material 1 has a substantially spherical graphite material 2 and a composite layer 5 formed in a layer so as to cover the entire surface of the material 2. This is the same as the present invention in that the composite layer 5 is composed of the metal powder 6 and the carbonaceous material 7. However, since natural graphite is used as the graphite material 2, it does not have the surface layer 4 as in the present invention.
[0033]
The average particle size of the composite graphite material of the present invention is 2 to 100 µm, preferably 5 to 50 µm. The thickness of the composite layer containing the metal powder formed on the surface of the graphitic material is preferably at the same level as the particle size of the metal powder. If the thickness is too small, the metal powder tends to be desorbed. If the thickness is too large, the ratio of the carbonaceous material (carbonized carbonaceous precursor) increases, which undesirably lowers the discharge capacity and charge / discharge efficiency.
[0034]
The composite graphite material of the present invention is produced by a conventional method as a negative electrode of a lithium ion secondary battery or a lithium polymer battery, and is applied as a negative electrode of a lithium ion secondary battery or a lithium polymer battery.
[0035]
<Lithium ion secondary battery>
Lithium ion secondary batteries usually have a negative electrode, a positive electrode, and a non-aqueous electrolyte as main battery components, and the positive and negative electrodes are each made of a lithium ion carrier, and the flow of a non-aqueous solvent in the charge / discharge process is performed between layers. .
In essence, it is a battery mechanism in which lithium ions are doped into the negative electrode during charging and de-doped from the negative electrode during discharging.
The lithium ion secondary battery of the present invention is not particularly limited except that the composite graphite material is used as the negative electrode, and the other battery components conform to those of a general lithium ion secondary battery.
[0036]
<Negative electrode>
Manufacture of the negative electrode from the composite graphite material is a molding method capable of sufficiently extracting the performance of the graphite material, and having high moldability to powder, and a chemically and electrochemically stable negative electrode. If there is no limitation, it can be carried out according to a usual molding method.
When manufacturing the negative electrode, a negative electrode mixture obtained by adding a binder to a composite graphite material can be used. As the binder, those having chemical stability to the electrolyte and electrochemical stability are preferably used.For example, polyvinylidene fluoride, a fluororesin such as polytetrafluoroethylene, polyethylene, polyvinyl alcohol, and furthermore Carboxymethyl cellulose is used. These can be used in combination. The binder is usually used at a ratio of about 1 to 20% by mass based on the whole amount of the negative electrode mixture.
[0037]
Specifically, for example, a composite graphite material is adjusted to an appropriate particle size by classification or the like, and mixed with a binder to prepare a negative electrode mixture. Alternatively, a negative electrode mixture layer is formed by coating on both surfaces.
At this time, if the negative electrode mixture is dispersed in a solvent to form 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 with a blade-type homomixer at about 300 to 3000 rpm. The solvent may be a usual solvent used for preparing the negative electrode mixture.
[0038]
For example, a paste may be obtained by mixing and kneading a composite graphite material and a fluorine-based resin powder such as polytetrafluoroethylene in a solvent such as isopropyl alcohol, and applying the paste. Further, after mixing a composite graphite material, a fluorine-based resin powder such as polyvinylidene fluoride or a water-soluble binder such as carboxymethyl cellulose with a solvent such as N-methylpyrrolidone, dimethylformamide or water or alcohol to form a slurry. It may be applied. When the mixture of the composite graphite material and the binder is applied to the current collector, the applied thickness is 10 to 500 μm, preferably 20 to 200 μm. After the formation of the negative electrode mixture layer, by performing pressure bonding such as press pressure, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased.
Alternatively, the composite graphite material and a resin powder such as polyethylene or polyvinyl alcohol are dry-mixed, and hot-pressed in a mold to produce a negative electrode.
[0039]
The shape of the current collector used for the negative electrode is not particularly limited, but a foil shape, a mesh shape such as a mesh or expanded metal, or the like is used. Examples of the current collector include copper, stainless steel, and nickel. In the case of a foil, the thickness of the current collector is preferably about 5 to 20 μm.
[0040]
<Positive electrode>
As the material of the positive electrode (positive electrode active material), it is preferable to select a material capable of doping / dedoping a sufficient amount of lithium. 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-containing compounds thereof, such as a lithium compound of the general formula M _X Mo ₆ S _8-y (Where X is a value in the range of 0 ≦ X ≦ 4, Y is a value in the range of 0 ≦ Y ≦ 1, and M represents a metal such as a transition metal), an activated carbon, an activated carbon fiber or the like. Can be used.
[0041]
The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals. The lithium-containing transition metal oxide is specifically LiM (1) _1-p M (2) _p O ₂ (Where P is a numerical value in the range of 0 ≦ P ≦ 1 and M (1) and M (2) are made of at least one transition metal element) or LiM (1) _2-Q M (2) _Q O ₄ (Where Q is a numerical value in the range of 0 ≦ Q ≦ 1 and M (1) and M (2) are made of at least one transition metal element).
In the above, examples of the transition metal element represented by M include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, and Sn, and preferably Co, Fe, Mn, Ti, and Cr. , V, and Al.
[0042]
As the lithium-containing transition metal oxide, more specifically, LiCoO ₂ , Lip Ni _Q M _1-Q O ₂ (M is the transition metal element other than Ni, preferably at least one selected from the group consisting of Co, Fe, Mn, Ti, Cr, V and Al, 0.05 ≦ p ≦ 1.10, 0.5 ≦ q ≦ 1. 0) LiNiO, LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ And the like.
[0043]
The above-mentioned lithium-containing transition metal oxides are, for example, lithium, an oxide or a salt of a transition metal as a starting material, and mixing these starting materials according to the composition. By sintering. The starting material is not limited to oxides or salts, and may be a hydroxide or the like.
In the present invention, as the positive electrode active substance, the above compounds 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.
[0044]
To form a positive electrode with such a positive electrode material, for example, a positive electrode mixture composed of 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.
[0045]
The shape of the current collector is not particularly limited, and a current collector having a box shape or a mesh shape such as a mesh or expanded metal is used. Examples of the current collector substrate include an aluminum foil, a stainless steel foil, and a nickel foil. The thickness is preferably from 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 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. After forming the positive electrode mixture layer, pressure bonding such as pressurization may be further performed. Thereby, the positive electrode mixture layer is uniformly and firmly adhered to the current collector.
[0046]
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.
[0047]
<Electrolyte>
As the electrolyte used in the present invention, an electrolyte salt used in a normal non-aqueous electrolyte can be used, for example, 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 ) ₂ CHOSO ₂ ) ₂ , LiB [C ₆ H ₃ (CF ₃ ) ₂ ] ₄ , LiAlCl ₄ , LiSiF ₆ And the like can be used. Especially LiPF ₆ , LiBF ₄ Is preferably used from the viewpoint of oxidation stability.
The electrolyte salt concentration in the electrolyte is preferably from 0.1 to 5 mol / L, more preferably from 0.5 to 3.0 mol / L.
[0048]
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 and a polymer gel electrolyte battery.
[0049]
When a liquid nonaqueous electrolyte solution is used, as a solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolan, 4-methyl-1,3-dioxolan, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, trimethyl borate, tetramethyl silicate, Nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl 2-oxazolidone, ethylene glycol, aprotic organic solvents such as dimethyl sulfite may be used.
[0050]
When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, it includes a matrix polymer gelled with a plasticizer (non-aqueous electrolyte). Is a polyethylene oxide or a crosslinked product thereof, such as an ether-based resin, a polymethacrylate-based, a polyacrylate-based, a polyvinylidene fluoride or a vinylidene fluoride-hexafluoropropylene copolymer-based fluorine-based resin, or the like alone or as a mixture. Can be used.
Among them, it is desirable to use a fluorine-based resin such as polyvinylidene fluoride or a vinylidene fluoride-hexafluoropropylene copolymer from the viewpoint of oxidation-reduction stability and the like.
[0051]
As the electrolyte salt and the non-aqueous solvent constituting the plasticizer contained in the polymer solid electrolyte and the polymer gel electrolyte, any of those described above can be used. In the case of the polymer gel electrolyte, the concentration of the electrolyte salt in the non-aqueous electrolyte as a plasticizer is preferably 0.1 to 5 mol / L, more preferably 0.5 to 2.0 mol / L.
Although the method for producing such a polymer electrolyte is not particularly limited, for example, a method of mixing a polymer forming a matrix, a lithium salt and a solvent, and melting by heating, a method of polymerizing a suitable organic solvent, a lithium salt and After dissolving the solvent, a method of evaporating the organic solvent, and mixing a polymerizable monomer, a lithium salt and a solvent as a raw material of the polymer electrolyte, and irradiating the mixture with an ultraviolet ray, an electron beam or a molecular beam to polymerize the mixture. Examples of the method include a method for producing a molecular electrolyte.
The proportion of the solvent in the solid electrolyte is preferably from 10 to 90% by mass, since the conductivity is high, the mechanical strength is high, and the film is easily formed, more preferably from 30 to 80% by mass. is there.
[0052]
In the lithium ion secondary battery of the present invention, a separator can be used.
The separator is not particularly limited, and examples thereof include a woven fabric, a nonwoven fabric, and a microporous membrane made of a synthetic resin. In particular, a synthetic resin microporous membrane is preferably used, and among them, a polyolefin-based microporous membrane is preferable in terms of thickness, film strength, and film resistance. Specifically, it is a microporous film made of polyethylene and polypropylene, or a microporous film obtained by combining these.
[0053]
Furthermore, the structure of the lithium ion secondary battery according to the present invention is arbitrary, and its shape and form are not particularly limited, and are arbitrarily selected from a cylindrical type, a square type, a coin type, a button type, and the like. be able to. In order to obtain a sealed non-aqueous electrolyte battery with higher safety, it is desirable to provide a means for interrupting the current by detecting an increase in battery internal pressure when an abnormality such as overcharging occurs. In the case of a polymer solid electrolyte battery or a polymer gel electrolyte battery, the structure may be such that the battery is sealed in a laminate film.
[0054]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
(Example 1)
Coal tar containing 0.5% by mass of free carbon (QI) was heat-treated at 350 ° C. for 0.5 hour, and further heat-treated at 450 ° C. for 0.2 hour to produce mesophase carbon microspheres. . Pitch was extracted from the coal tar after the heat treatment using tar heavy oil (boiling point: 200 to 300 ° C.), and mesophase carbon microspheres were obtained by filtration and separation from the pitch matrix. The mesophase carbon small sphere calcined product was calcined at 500 ° C. in a rotary kiln, and coarse particles (aggregates) were removed using a 200-mesh (75 μm mesh) vibrating sieve. The obtained particle-size-adjusted product was placed in a graphite crucible, heated at a rate of 1000 ° C./hour in an argon atmosphere, and carbonized at 3000 ° C. for 3 hours to obtain a graphitic material.
[0055]
D by X-ray diffraction of the graphite material ₀₀₂ Is 0.3362 nm, I by Raman spectroscopy ₁₃₆₀ / I ₁₅₈₀ Was 0.17. 3 parts by mass of silicon powder (average particle size: 1.9 μm) and 20 parts by mass of coal tar pitch (softening point: 90 ° C.) were mixed with 87 parts by mass of the graphite material at 150 ° C. for 1 hour using a kneader, and then mixed with 500 parts. Heated at 0 ° C for 3 hours. Thereafter, the heat-treated product was crushed, and the crushed material was heated at 1000 ° C. for 1 hour to carbonize the coal tar pitch to obtain a graphite material in which silicon powder was bonded to and coated on the surface of the graphite material. The carbonization rate of the coal tar pitch was 50%. The charge / discharge capacity, powder characteristics (particle size distribution, specific surface area), and tap density of the graphite material were evaluated by the following methods. The results are shown in Table 2.
[0056]
(Charging and discharging capacity)
The charge / discharge capacity was measured in a dry box under argon flow under the following conditions.
Cell format: 3-pole beaker cell
Counter electrode and reference electrode: lithium metal
Working electrode: Pressed product of copper foil coated with graphite powder
Manufacturing method of working electrode: A slurry obtained by mixing 10% by mass of PVDF with graphite powder and further dissolving PVDF with added N-methylpyrrolidone was applied on a copper foil. The coated copper foil was pre-dried at 100 ° C., and then pressed using a roll press. The processed product was dried at 100 ° C. under vacuum to remove N-methylpyrrolidone to obtain a working electrode.
[0057]
Electrolyte: 1 mol LiClO / (propylene carbonate / ethylene carbonate / dimethyl carbonate = 3/5/2)
Charge / discharge: Current density 1.0 mA / cm ² After charging at a constant current of 0.1 V and a constant potential at 0.1 V, 1.0 mA / cm ² Was discharged at a constant current. At this time, the charge capacity and the discharge capacity were measured. The difference between the charge capacity and the discharge capacity is the charge / discharge loss.
When the graphitic material of the present invention was used, as shown in Table 2, high discharge capacity and charge / discharge efficiency were exhibited.
[0058]
(Powder characteristics)
(1) Particle size distribution
The graphitic material of the present invention is dispersed in distilled water to which a surfactant has been added, and the particle size distribution is measured with a laser diffraction type particle size distribution analyzer LS-5000 (manufactured by Seishin). I asked.
(2) BET specific surface area
The specific surface area was measured by the BET one-point method by nitrogen gas adsorption.
(3) Tap density
Using a powder tester (manufactured by Hosokawa Micron Corp.), drop the powder into a 100 ml tapping cell in accordance with JIS K5101, fill the cell completely, tap 20 times with a stroke length of 10 mm, and perform 100 ml volume tapping. The weight of the powder inside was measured, and the tap density was determined.
[0059]
(Examples 2 to 4)
A graphitic material was prepared in the same manner as in Example 1, except that the amounts of silicon powder and coal tar pitch were changed as shown in Table 1. Table 2 shows the results of evaluating the charge / discharge capacity, powder characteristics (particle size distribution, specific surface area), and tap density of the graphite material.
[0060]
(Comparative Example 1)
A graphite material was prepared in the same manner as in Example 1 except that silicon powder and coal tar pitch were not blended. Table 2 shows the results of evaluating the charge / discharge capacity, powder characteristics (particle size distribution, specific surface area), and tap density of the graphite material.
[0061]
(Comparative Examples 2-3)
In Example 1, commercial natural graphite (particle size: 10 μm, 30 μm, d) was used instead of the prepared graphite particles. ₀₀₂ 0.3356 nm, I ₁₃₆₀ / I ₁₅₆₀ Was prepared in the same manner as in Example 1 except that 0.10) was used. Table 2 shows the results of evaluating the charge / discharge capacity, powder characteristics (particle size distribution, specific surface area), and tap density of the graphite material.
[0062]
(Examples 5 to 6)
A graphite material was prepared in the same manner as in Example 1 except that tin powder or aluminum powder having the compounding amount shown in Table 1 was used instead of silicon powder. Table 2 shows the results of evaluating the charge / discharge capacity, powder characteristics (particle size distribution, specific surface area), and tap density of the graphite material.
[0063]
【The invention's effect】
According to the present invention, a graphitic material having high discharge capacity, charge / discharge efficiency and tap density can be manufactured. The graphite material is particularly suitable as a negative electrode material of a lithium ion secondary battery or a lithium ion polymer battery. Further, by utilizing its features, it can also be used for applications other than the negative electrode material, for example, conductive materials for fuel cell separators and graphite for refractories.
[0064]
[Table 1]

[0065]
[Table 2]

[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a composite graphite material of the present invention.
FIG. 2 is a schematic cross-sectional view of a conventional composite graphite material.
[Explanation of symbols]
1: Composite graphite material
2: Graphite material
3: Highly crystalline graphite material
4: Low crystalline graphite material
5: Composite layer
6: Metal powder capable of forming an alloy with lithium
7: Carbonaceous material

Claims (8)

A graphite material obtained by compounding a metal powder capable of forming an alloy with lithium with a carbonaceous material on the surface of the graphite material, wherein the crystallinity of the surface of the graphite material is higher than the crystallinity inside the graphite material. Composite graphite material characterized by a low carbon content. The composite graphite material according to claim 1, wherein the graphite material is a graphitized mesophase carbon spheroid. In the peak intensity of 1360 cm ^-1 in the Raman spectrum of the graphite material _{(I 1360)} and the intensity ratio of the peak intensity of _{^{1580cm -1 (I 1580) (R}} = I 1360 / I 1580) is 0.05 or more, X-rays 3. The composite graphitic material according to claim 1, wherein a plane distance d ₀₀₂ of the carbon netting layer by a diffraction method is 0.3365 nm or less. The composite graphite material according to any one of claims 1 to 3, wherein the metal capable of forming an alloy with lithium is at least one metal selected from the group consisting of silicon, tin, and aluminum. After heating a mixture containing a graphitic material whose surface crystallinity is lower than the internal crystallinity, a metal powder capable of forming an alloy with lithium, and a carbonaceous precursor at 350 to 800 ° C., the heat-treated product is crushed. And heating the crushed material at 800 to 2000 ° C. to carbonize the carbonaceous precursor, and compounding the metal powder on the surface of the graphitic material with the obtained carbonaceous material. Method of manufacturing quality materials. The method according to claim 5, wherein the graphite material is a graphitized mesophase carbon spheroid. A negative electrode for a lithium ion secondary battery, comprising using the composite graphite material according to claim 1. A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 7.

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