JP4855696B2 - Negative electrode material for lithium ion secondary battery, method for producing the same, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a negative electrode material for a lithium ion secondary battery containing graphite and a metal, a production method thereof, a negative electrode for a lithium ion secondary battery using the same, and a lithium ion secondary battery using the same and having excellent discharge capacity and cycle characteristics.

  Lithium ion secondary batteries have a higher voltage and higher energy density than other secondary batteries, and are therefore widely used as power sources for electronic devices. In recent years, electronic devices have been rapidly reduced in size and performance, and there is an increasing demand for further improving the energy density of lithium ion secondary batteries.

Currently, lithium ion secondary batteries generally use LiCoO _{2 for} the positive electrode and graphite for the negative electrode. However, although the graphite negative electrode is excellent in charge / discharge reversibility, the discharge capacity has already reached a value close to the theoretical value (372 mAh / g) of the intercalation compound (LiC ₆ ). Therefore, in order to further increase the energy density of the battery, it is necessary to develop a negative electrode material having a discharge capacity larger than that of graphite.

  Metallic lithium has the maximum discharge capacity as a negative electrode material. However, since lithium is deposited in a dendritic state during charging and the negative electrode is deteriorated, there is a problem that the charge / discharge cycle of the battery is shortened. In addition, lithium deposited in a dendrite shape may penetrate the separator and reach the positive electrode, and the battery may be short-circuited.

  Therefore, a metal or a metal compound that forms an alloy with lithium has been studied as a negative electrode material replacing lithium metal. The discharge capacity of these alloy negative electrodes has a discharge capacity far exceeding that of graphite, although it does not reach that of metallic lithium. However, powder expansion and separation of the active material occur due to volume expansion accompanying alloying, and the cycle characteristics of the lithium ion secondary battery have not yet reached a practical level.

  In order to solve the drawbacks of the above-described alloy negative electrode, development of a negative electrode by combining a metal or metal compound that forms an alloy with lithium and a graphite material and / or a carbon material has been studied.

  In order to suppress metal pulverization / peeling accompanying volume expansion, it is effective to improve the adhesion between the metal and the graphite material and / or carbon material interface. However, there is a problem that sufficient adhesion cannot be obtained by simply mixing the metal and the graphite material, and the metal is liberated from the graphite and the cycle characteristics are deteriorated when charging and discharging are repeated.

  In order to solve such problems, Patent Document 1 (Japanese Patent Laid-Open No. 11-279785) uses a composite carbon material in which a coating layer derived from an organic material and a metal compound is formed on the surface of particulate graphite as an electrode. Technology is disclosed. In this composite carbon material, the coating layer derived from the organic material plays a role as a binder of graphite and metal.

Patent Document 2 (Japanese Patent Application Laid-Open No. 2004-185975) discloses a three-layer structure in which a metal that can be alloyed with lithium is fixed to a graphite particle surface by mechanochemical treatment, and a carbon layer is formed on the surface. A technique using a composite carbon material having a structure as an electrode is disclosed. In this composite carbon material, mechanochemical treatment is performed for the purpose of improving the adhesion between graphite and metal.
JP-A-11-279785 JP 2004-185975 A

  However, even in the composite carbon material described in Patent Document 1, when the coating layer is broken due to the expansion / contraction of the metal accompanying charge / discharge, no adhesion is secured at the interface between the graphite and the metal. Therefore, the liberation of both cannot be avoided, and the charge / discharge efficiency and cycle characteristics deteriorate.

  In the composite carbon material described in Patent Document 2, the mechanochemical treatment mechanically applies a shearing force to integrate the graphite and the metal, and releases the metal and the graphite due to the expansion and contraction of the metal. It does not have interfacial adhesion enough to completely suppress. Therefore, even with this composite carbon material, if the coating layer is destroyed due to metal expansion / contraction caused by charge / discharge, the charge / discharge efficiency and cycle characteristics are also lowered.

  As described above, the prior art has a problem that it is difficult to enhance the adhesion between a metal that can be alloyed with graphite and lithium, and to improve the charge / discharge efficiency and cycle characteristics.

  The present invention has been made in view of the above situation, and is used as a negative electrode material for a lithium ion secondary battery. A negative electrode material having a high discharge capacity and excellent cycle characteristics and initial charge / discharge efficiency is obtained. The purpose is to provide. Also, a negative electrode for a lithium ion secondary battery having a high discharge capacity, having excellent cycle characteristics and initial charge / discharge efficiency, and a lithium ion secondary battery using the negative electrode for the secondary battery, comprising the obtained negative electrode material The purpose is to provide.

In order to achieve the above object, the present invention has the following features.
[1] SiO ₂ adheres to at least a part of the surface of the massive graphite material having an aspect ratio of 5 or less, and Si adheres to at least a part of the surfaces of the massive graphite material and the SiO ₂ A negative electrode material for a lithium ion secondary battery.
[ 2 ] After attaching SiO ₂ by mechanochemical treatment to at least a part of the surface of the massive graphite material having an aspect ratio of 5 or less , Si is applied to at least a part of the surfaces of the massive graphite material and the SiO ₂ A method for producing a negative electrode material for a lithium ion secondary battery, characterized by being deposited by a vapor phase method.
[ 3 ] A negative electrode for a lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery as described in [1] above.
[ 4 ] A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to [ 3 ] above.

  By using the negative electrode material for a lithium ion secondary battery of the present invention, an excellent discharge capacity exceeding the theoretical capacity of graphite can be obtained, and at the same time, a lithium ion secondary battery exhibiting excellent initial charge / discharge efficiency and cycle characteristics can be obtained. it can.

  Therefore, the lithium ion secondary battery using the negative electrode material of the present invention satisfies the recent demand for higher energy density of the battery, and is effective in reducing the size and performance of the mounted device.

  Hereinafter, the present invention will be described more specifically.

[Negative electrode material]
In the negative electrode material of the present invention, a metal compound that does not alloy with lithium adheres to at least part of the surface of the massive graphite material, and further, lithium and an alloy adhere to at least part of the surfaces of the massive graphite material and the metal compound. It is a composite negative electrode material to which a metal that can be converted is attached. Here, the metal compound preferably has an average particle size smaller than the average particle size of the massive graphite material. The metal compound that is not alloyed with lithium is preferably deposited by mechanochemical treatment, and the metal that can be alloyed with lithium is preferably deposited by a vapor phase method.

  In the negative electrode material, the metal compound that is not alloyed with lithium is in close contact with both the massive graphite material and the metal that can be alloyed with lithium, and as a result, the massive graphite material and the metal that can be alloyed with lithium are used. The interface is strengthened, that is, the adhesiveness is improved, and the metal can be prevented from peeling and falling off due to charge / discharge, and the charge / discharge efficiency and cycle characteristics are improved. Moreover, since the electrical conductivity in the negative electrode material is improved with the strengthening of the interface, the charge / discharge efficiency and the cycle characteristics are improved. Further, since the graphite material is in a lump shape, voids are generated between the negative electrode materials, and the charge expansion of the metal that can be alloyed with lithium can be absorbed, thereby improving the cycle characteristics. Furthermore, if a metal compound that does not alloy with lithium is attached to the outer surface of the graphite by mechanochemical treatment, the surface area of the graphite increases due to the polishing effect, so that a metal that can be alloyed with lithium in particular is in the form of a film. In the case where it exists, the film thickness can be reduced.

  Any method may be used to attach the metal compound not alloyed with lithium to the massive graphite material, but it is preferable to use a mechanical method such as mechanochemical treatment. The adhesion form may be any form, but it is preferable that the metal compound not alloyed with lithium is dispersed in a state close to primary particles and uniformly distributed on the outer surface of the graphite. Further, the metal compound that does not alloy with lithium may exist in a state of being embedded in the massive graphite material.

  Attachment of the metal that can be alloyed with lithium to the massive graphite and the metal compound that is not alloyed with lithium may be a chemical bond, a physical bond, or a composite thereof, but is in a chemically bonded state. It is preferable. Moreover, although the said attachment form may be what form, it is preferable that it is a film | membrane form.

  The mass proportion of the metal compound not alloyed with lithium in the whole negative electrode material of the present invention is preferably 0.05 to 10%, particularly preferably 0.1 to 5%. When the mass ratio is less than 0.05%, the interfacial adhesion between the graphite material and the metal that can be alloyed with lithium may be insufficient, and cycle characteristics may be deteriorated. On the other hand, when the mass ratio exceeds 10%, the discharge capacity may be reduced.

  Moreover, it is preferable that the mass ratio of the metal which can be alloyed with the said lithium to the whole negative electrode material of this invention is 3-70%, especially 5-50%. When the mass ratio is less than 3%, the effect of improving the discharge capacity may be reduced. On the other hand, when the mass ratio exceeds 70%, the cycle characteristics may deteriorate.

  The mass ratio can be obtained by conversion from a qualitative analysis of a metal species by a known elemental quantitative analysis or X-ray diffraction method (XRD).

  When the adhesion state of the metal that can be alloyed with lithium is a film, the average thickness of the metal is approximately 1 nm to 2 μm, preferably 10 nm to 1 μm.

  The average thickness can be measured by observing the cross section of the negative electrode material with a scanning electron microscope or a transmission electron microscope, or analyzing the element concentration in the depth direction with a glow discharge emission spectrometer (GDS) or the like.

  The negative electrode material preferably has an average particle size of 1 to 50 μm. When the average particle diameter is less than 1 μm, it is difficult to adjust the negative electrode mixture paste when forming the negative electrode, and active edges of the graphite are likely to be exposed, and the initial charge / discharge efficiency may be reduced. When the average particle diameter exceeds 50 μm, it is difficult to adjust the thickness of the active material layer of the negative electrode. A more preferable average particle diameter is 3 to 40 μm. The average particle diameter can be measured by a laser diffraction particle size distribution meter.

  The negative electrode material includes a massive graphite material, a metal compound that is not alloyed with lithium, and a metal that can be alloyed with lithium as essential constituent components, but in addition to this, includes different types of carbonaceous materials, graphite materials, and inorganic materials. You may go out. Specifically, it may be a mixture of a different carbonaceous material or a graphite material, a granulated material, a coating material, or a laminate. Further, it may be subjected to various chemical treatments in the liquid phase, gas phase, and solid phase, heat treatment, oxidation treatment, physical treatment and the like. Also in this case, the average particle diameter of the negative electrode material as a whole is preferably 1 to 50 μm.

  The reason why the negative electrode material of the present invention exhibits excellent cycle characteristics is not clear, but because the interface between the metal that can be alloyed with lithium and the graphite is reinforced by a metal compound that does not alloy with lithium, When the metal can be alloyed with lithium is prevented from exfoliating and dropping due to expansion and contraction of the metal, the conductivity in the negative electrode material is improved, and when the metal that can be alloyed with lithium is expanded by charging, It is considered that the sufficient space is ensured between the negative electrode materials, the contact between the metal that can be alloyed with lithium and the electrolytic solution is maintained, and the like.

[Metal compounds that do not alloy with lithium]
The metal compound that does not alloy with lithium constituting the negative electrode material may be any compound such as a metal oxide, nitride, carbide, or hydroxide, but an oxide is particularly preferable. Examples include such as SiO ₂ and SnO _2, small particle size, is also easy SiO ₂ particularly preferably obtained.

[Metal that can be alloyed with lithium]
Examples of metals that can be alloyed with lithium constituting the negative electrode material include Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, Sb, Ge, Ni etc. can be mentioned, Preferably it is Si and Sn, Most preferably, it is Si. The metal that can be alloyed with lithium may be an alloy of two or more of the metals. In the alloy, an element other than the metal may be contained, and an oxide or a nitride may be formed. Moreover, it is preferable that the said metal contains an amorphous thing or an amorphous thing. When the metal is amorphous, expansion during charging is reduced. Therefore, a metal containing amorphous silicon is most preferable.

[Graphite]
The graphite material constituting the negative electrode material is not particularly limited as long as it is a graphite material that can occlude / release lithium ions as a negative electrode active material and is in a lump shape.

  Examples of the graphite material include those formed partly or entirely of graphite, for example, artificial graphite obtained by finally heat treating tar and pitch at 1500 ° C. or higher. Specifically, a mesophase calcined product, mesophase spherule, mesophase carbon fiber or coke obtained by polycondensation using petroleum-based and coal-based tars and pitches, which are called graphitizable carbon materials, is preferably 1500 ° C. As mentioned above, More preferably, what was obtained by graphitizing at 2800-3300 degreeC can be used. Further, even a scaly graphite material such as natural graphite can be used by making it into a lump by mechanical treatment.

  Here, the lump means that the ratio of the major axis length to the length in the minor axis direction (minor axis length) orthogonal thereto, that is, the aspect ratio is 5 or less, and particularly preferably 3 or less. . As the aspect ratio, a value obtained by observing the negative electrode material with a scanning electron microscope, measuring the major axis length and minor axis length of about 50 particles, and arithmetically averaging them can be used.

  From the viewpoint of obtaining a high discharge capacity, the graphite material preferably has high crystallinity. The crystallinity index is preferably 0.34 nm or less, and particularly preferably 0.337 nm or less in terms of the average lattice spacing d002 of the (002) plane by X-ray wide angle diffraction.

  The lattice spacing is measured by measuring the diffraction peak on the (002) plane of the graphite using CuKα ray as the X-ray source and high-purity silicon as the standard substance, and calculating d002 from the peak position. The method can be used. The calculation method follows the Japan Science and Technology Act (measurement method defined by the 117th Committee of the Japan Society for the Promotion of Science). Specifically, “Suguro Otani,“ Carbon Fiber ”, Modern Editorial Company, 1986. , Pp. 733-742, etc., can be used.

The specific surface area of the graphite is preferably in the range of 0.1 to 50 m ² / g, particularly 0.3 to ⁵ m ² / g. When the specific surface area is less than 0.1 m ² / g, the adhesion amount per unit area of the metal that can be alloyed with the lithium inevitably increases, and the metal is cracked or powdered during charging. There is. When the specific surface area is more than 50 m ² / g, the amount of adhesion per unit area of the metal is reduced, but the exposure rate of the active edge surface of the graphite is increased, the initial charge / discharge efficiency is reduced, or the negative electrode It becomes difficult to adjust the negative electrode mixture paste when forming the film. In addition, the value measured by the BET method by adsorption of nitrogen gas was used for the specific surface area.

  The graphite material may include different types of carbonaceous materials and graphite materials. In this case, the average value of crystallinity of the entire graphite material is preferably 0.34 nm or less in terms of the average lattice spacing d002 of the (002) plane by X-ray wide angle diffraction. Specifically, it may be a mixture of a different carbonaceous material or a graphite material, a granulated material, a coated material, or a laminate, and a carbonaceous material-coated material is particularly preferable. Further, it may be subjected to various chemical treatments in the liquid phase, gas phase, and solid phase, heat treatment, oxidation treatment, physical treatment and the like.

[Method for producing negative electrode material]
As a method for producing a negative electrode material of the present invention, a metal compound that does not alloy with lithium adheres to at least a part of the surface of the massive graphite material, and further, at least a part of the surfaces of the massive graphite material and the metal compound. Any method may be used as long as it can obtain a structure in which a metal capable of alloying with lithium is adhered, and a method for maximizing the effects of the present invention is illustrated below. Here, the metal compound preferably has an average particle size smaller than the average particle size of the massive graphite material.

  The adhesion of the metal compound not alloyed with lithium may be performed by any of mechanical, chemical, and physical methods, but a mechanical method is particularly preferable. Examples of the mechanical method include mechanochemical treatment that imparts mechanical energy such as compression, shearing, collision, and friction. Examples of such devices that can be operated include granulators such as GRANUREX (manufactured by Freund Sangyo Co., Ltd.), Newgra Machine (manufactured by Seishin Enterprise Co., Ltd.), Agromaster (manufactured by Hosokawa Micron Co., Ltd.), and roll mills. And compression shearing processing devices such as a hybridization system (manufactured by Nara Machinery Co., Ltd.), a mechanomicro system (manufactured by Nara Machinery Co., Ltd.), and a mechano-fusion system (Hosokawa Micron Co., Ltd.).

  The attachment of the metal that can be alloyed with lithium is not particularly limited as long as it is a method capable of attaching a metal or a metal organic compound to a graphite material in a gas phase or a liquid phase, but it is preferable to use a gas phase method. . Gas phase methods include vacuum vapor deposition, sputtering, ion plating, molecular beam epitaxy, and other PVD (Physical Vapor Deposition) methods, atmospheric pressure CVD (Chemical Vapor Deposition) methods, low pressure CVD methods, and plasma CVD methods. , MO (Magneto-optic) CVD method, CVD method such as optical CVD. Among these, the sputtering method is most preferable. As the sputtering method, a direct current sputtering method, a magnetron sputtering method, a high frequency sputtering method, a reactive sputtering method, a bias sputtering method, an ion beam sputtering method, or the like can be used.

In the sputtering method, a metal target is installed on the cathode side, and generally a glow discharge is generated between electrodes in an inert gas atmosphere of about 1 to 10 ⁻² Pa to ionize the inert gas and strike the target metal. This is a method of coating the target metal on the graphite material placed on the anode side. In this case, it is effective to mechanically stir the graphite material or apply vibration such as ultrasonic waves to move the graphite material so that the surface of the graphite material is uniformly coated with metal. A plurality of metals may be used as the metal. That is, a plurality of metals may be simultaneously sputtered to synthesize an alloy, or a plurality of metals may be laminated in order.

  The negative electrode material obtained in this way can be further mixed, coated and adhered with different carbonaceous materials, graphite materials, inorganic materials and the like according to the purpose. For example, the negative electrode material of the present invention can be further baked by coating a carbonaceous material with a thin film by CVD or attaching a carbonaceous material precursor in a liquid phase.

[Negative electrode]
As a negative electrode material constituting the negative electrode of the lithium ion secondary battery, known negative electrode materials and conductive materials can be mixed and used in addition to the negative electrode material of the present invention described above. For example, various negative electrode materials such as natural graphite, artificial graphite, mesophase globular graphite, mesophase carbon fiber graphite, mesophase calcined graphite, carbon black, acetylene black, ketjen black, vapor grown carbon fiber A conductive material such as can be mixed and used.

  The production of the negative electrode of the lithium ion secondary battery can be performed according to a normal method of forming a negative electrode, but is not limited as long as it is a molding method capable of obtaining a chemically and electrochemically stable negative electrode.

  Moreover, the negative electrode mixture which added the binder to this negative electrode material can be used at the time of preparation of a negative electrode. As the binder, those having chemical stability and electrochemical stability with respect to the electrolyte are preferably used. For example, fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, and styrene Butadiene rubber, carboxymethyl cellulose and the like are used. Moreover, 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.

  As a specific example of the preparation of the negative electrode, a negative electrode mixture is prepared by mixing the particles of the negative electrode material with a binder, and this negative electrode mixture is usually applied to one or both sides of a current collector to form a negative electrode mixture. The method of forming an agent layer is mentioned.

  A normal solvent for preparing a negative electrode can be used for preparing the negative electrode. When the negative electrode mixture is dispersed in a solvent and made into a paste, and then applied to the current collector and dried, the negative electrode mixture layer is uniformly and firmly adhered to the current collector. More specifically, for example, the negative electrode material particles and a fluorine-based resin powder such as polyvinylidene fluoride or a water-dispersible binder such as styrene butadiene rubber, or a water-soluble binder such as carboxymethyl cellulose are used. A paste is prepared by mixing with pyrrolidone, dimethylformaldehyde or a solvent such as water or alcohol to form a slurry, and then kneading with a kneader. When the paste is applied to one or both sides of the current collector and dried, a negative electrode in which the negative electrode mixture layer is uniformly and firmly bonded can be obtained. The film thickness of the negative electrode mixture layer is 10 to 200 μm, preferably 30 to 100 μm.

  Alternatively, the negative electrode material particles can be dry-mixed with resin powders such as polyethylene and polyvinyl alcohol as a binder, and hot-press molded in a mold to produce a negative electrode. However, dry mixing requires a large amount of binder to obtain sufficient strength of the negative electrode, and if the binder is excessive, the discharge capacity and rapid charge / discharge efficiency of the lithium ion secondary battery may be reduced. .

  When the negative electrode mixture layer is formed and then pressure bonding such as pressurization is performed, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased.

  The shape of the current collector used for the negative electrode is not particularly limited, and examples thereof include a foil shape or a net-like shape such as a mesh or an expandable metal. Examples of the material for 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.

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

  The lithium ion secondary battery of the present invention is not particularly limited except that the negative electrode is used, and other battery components are in accordance with elements of a general lithium ion secondary battery.

[Positive electrode]
As the positive electrode material (positive electrode active material) used in the lithium ion secondary battery of the present invention, a lithium compound is used, but it is preferable to select a material that can occlude / desorb a sufficient amount of lithium. For example, lithium-containing transition metal oxide, transition metal chalcogenide, vanadium oxide, other lithium-containing compounds, general formula M _x Mo ₆ S _8-Y (where X is 0 ≦ X ≦ 4, Y is 0 ≦ Y) A numerical value in the range of ≦ 1, and M represents at least one kind of transition metal) can be used. Examples of the vanadium _oxide, or the like can be used those represented by _{V 2 O 5, V 6 O} 13, V 2 O 4, V 3 O 8.

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals. The composite oxide may be used alone or in combination of two or more. Specifically, the lithium-containing transition metal oxide is LiM ¹ _1-X M ² _x O ₂ (where X is a numerical value in the range of 0 ≦ X ≦ 1, and M ¹ and M ² are at least one kind of transition. A metal element) or LiM ¹ _1-Y M ² _Y O ₄ (where Y is a numerical value in the range of 0 ≦ Y ≦ 1, and M ¹ and M ² are at least one transition metal element) It is. In the formula, transition metals represented by M ¹ and M ² are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, and the like. Preferably, Co, Mn, Cr, Ti, V, Fe, Al and the like are used. Specific examples include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _0.9 Co _0.1 O ₂ , LiNi _0.5 Co _0.5 O _{2 and the} like.

  Further, the lithium-containing transition metal oxide is, for example, lithium, transition metal oxide, salts and the like as a starting material, these starting materials are mixed according to the composition of the desired metal oxide, and 600 ~ It can be obtained by firing at a temperature of 1000 ° C. The starting materials are not limited to oxides and salts, and may be hydroxides.

  In the lithium ion secondary battery of the present invention, the positive electrode active material may be used alone or in combination of two or more of the above lithium compounds. Further, an alkali carbonate such as lithium carbonate can be added to the positive electrode.

  The positive electrode is formed by, for example, applying a positive electrode mixture composed of the lithium compound, a binder, and a conductive agent for imparting conductivity to the positive electrode on one or both sides of the current collector to form a positive electrode mixture layer. Produced. As the binder, the same one as that used for producing the negative electrode can be used. As the conductive agent, a carbon material such as graphite or carbon black is used.

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

  The shape of the current collector is not particularly limited, and a foil shape or a mesh shape such as a mesh or expanded metal is used. The current collector is made of aluminum, stainless steel, nickel, or the like. In the case of a foil, the thickness of the current collector is preferably about 10 to 40 μm.

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

  The electrolyte salt concentration in the electrolytic solution is preferably 0.1 to 5 mol / l, and more preferably 0.5 to 3.0 mol / l.

  The non-aqueous electrolyte may be a liquid non-aqueous electrolyte or a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the non-aqueous electrolyte battery is configured as a so-called lithium ion secondary battery, and in the latter case, the non-aqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte or a polymer gel electrolyte battery. .

  As a solvent for preparing the nonaqueous electrolyte solution, carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, ethers such as anisole and diethyl ether, thioethers such as sulfolane and methylsulfolane, acetonitrile, chloronitrile, propionitrile, etc. Nitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, Benzoyl, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, aprotic organic solvents such as dimethyl sulfite may be used.

  When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte or a polymer gel electrolyte, it is preferable to use a polymer gelled with a plasticizer (non-aqueous electrolyte) as a matrix. Examples of the polymer constituting the matrix include ether-based polymer compounds such as polyethylene oxide and cross-linked products thereof, polymethacrylate-based polymer compounds, polyacrylate-based polymer compounds, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene. It is particularly preferable to use a fluorine-based polymer compound such as a copolymer.

  The polymer solid electrolyte or polymer gel electrolyte is mixed with a plasticizer, and as the plasticizer, the electrolyte salt and the non-aqueous solvent can be used. In the case of a polymer gel electrolyte, the electrolyte salt concentration in the non-aqueous electrolyte solution that is a plasticizer is preferably 0.1 to 5 mol / l, and more preferably 0.5 to 2.0 mol / l.

  The method for producing the polymer solid electrolyte is not particularly limited. For example, a method of mixing a polymer compound constituting a matrix, a lithium salt, and a nonaqueous solvent (plasticizer) and heating to melt the polymer compound, an organic solvent A method in which a polymer compound, a lithium salt, and a non-aqueous solvent (plasticizer) are dissolved in, and an organic solvent for mixing is evaporated, a polymerizable monomer, a lithium salt, and a non-aqueous solvent (plasticizer) are mixed, and the mixture is mixed Examples thereof include a method of polymerizing a polymerizable monomer by irradiating an ultraviolet ray, an electron beam, a molecular beam or the like to obtain a polymer.

  Here, the ratio of the non-aqueous solvent (plasticizer) in the solid electrolyte is preferably 10 to 90% by mass, and more preferably 30 to 80% by mass. If it is less than 10% by mass, the electrical conductivity will be low, and if it exceeds 90% by mass, the mechanical strength will be weak and film formation will be difficult.

[Separator]
In the lithium ion secondary battery of the present invention, a separator can also be used.

  Although the material of a separator is not specifically limited, For example, a woven fabric, a nonwoven fabric, a synthetic resin microporous film, etc. can be used. As a material for the separator, a microporous membrane made of synthetic resin is suitable. Among them, a polyolefin microporous membrane is suitable in terms of thickness, membrane strength, and membrane resistance. Specifically, polyethylene and polypropylene microporous membranes, or microporous membranes composed of these are suitable.

[Lithium ion secondary battery]
The lithium ion secondary battery of the present invention comprises a negative electrode, a positive electrode, and a non-aqueous electrolyte containing a graphite material, which are configured as described above, in the order of, for example, a negative electrode, a non-aqueous electrolyte, and a positive electrode. Consists of housing. Further, a non-aqueous electrolyte may be disposed outside the negative electrode and the positive electrode.

  In addition, the structure of the lithium ion secondary battery of the present invention is not particularly limited, and the shape and form thereof are not particularly limited, and are cylindrical, depending on the application, mounted equipment, required charge / discharge capacity, and the like. , Square shape, coin shape, button shape, and the like. In order to obtain a sealed nonaqueous electrolyte battery with higher safety, it is preferable to use a battery equipped with means for detecting an increase in the internal pressure of the battery and shutting off the current when an abnormality such as overcharging occurs.

  In the case where the lithium ion secondary battery is a polymer solid electrolyte battery or a polymer gel electrolyte battery, a structure in which the lithium ion secondary battery is enclosed in a laminate film may be used.

  EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited to these Examples. Further, in the following examples and comparative examples, as shown in FIG. 1, a button type two for single electrode evaluation composed of a working electrode (negative electrode) 2 containing a graphite material and a counter electrode (positive electrode) 4 made of lithium foil. Next batteries were fabricated and evaluated. An actual battery can be produced according to a known method based on the concept of the present invention.

  The average particle size of the negative electrode material was measured with a laser diffraction particle size distribution meter, and the particle size was such that the cumulative frequency of the particle size distribution was 50% by volume.

  The specific surface area of the graphite particles was measured by the BET method.

  The aspect ratio of the graphite particles is an average value of the major axis length and minor axis length at arbitrary 100 positions, which was obtained by observation with a scanning electron microscope of 8000 times. The aspect ratio is the ratio of the major axis length (average value) to the minor axis length (average value).

  The mass ratio of the metal that can be alloyed with lithium in the negative electrode material was obtained by ashing the negative electrode material, performing elemental analysis by emission spectroscopy, and converting it to a concentration as a metal.

[Example 1]
[Production of negative electrode material]
Gas phase silica equivalent to 0.5% by mass (“AEROSIL50”, Nippon Aerosil Co., Ltd.) graphite powder obtained by processing scaly natural graphite into a spherical shape (manufactured by Chuetsu Graphite Industries, Ltd., WF-025, average particle size 25 μm) (Made by Co., Ltd., average particle size of 30 nm)), and the resulting mixture was processed at a peripheral speed of a rotating drum of 20 m / sec using a dry powder compounding device (Mechano-Fusion System, Hosokawa Micron Co., Ltd.). A composite material in which gas phase silica is dispersed and adhered to the surface of the graphite powder by applying compressive force and shear force repeatedly under conditions of a distance of 5 mm between the rotating drum and the internal member for 60 minutes, and performing mechanochemical treatment. Manufactured.

  Next, the composite material is disposed on the anode side stage of the DC bipolar sputtering apparatus, the 99.999% single crystal silicon target is disposed on the cathode side, and the pressure is 0.5 Pa, the voltage is 600 V, and the current is 0.5 A. After sputtering for 2 hours, the graphite material was stirred. Again, sputtering was performed for 2 hours under the same conditions as described above, and stirring was repeated. Thereafter, similar sputtering was further performed for 2 hours.

  As for the metal silicon adhering to the surface of the graphite material, it was confirmed from the analysis by the X-ray diffraction method that almost all the amount was amorphous silicon. The amount of amorphous silicon deposited by emission spectroscopic analysis was 10% by mass.

  When the obtained negative electrode material was visually observed with a scanning electron microscope, it was observed that amorphous silicon was film-like and adhered to the outer surface of the graphite material. The characteristics of the negative electrode material (average particle diameter, specific surface area, aspect ratio, amount of attached amorphous silicon, etc.) are shown in Table 1 below.

[Production of working electrode (negative electrode)]
4% by mass of the binder polyvinylidene fluoride was mixed with the negative electrode material prepared by the above method, and a solvent N-methylpyrrolidone was further added to prepare an organic solvent-based negative electrode mixture paste. This was applied to the copper foil to a uniform thickness, and further the solvent was volatilized at 90 ° C. in a vacuum and dried. Next, the negative electrode mixture applied on the copper foil was pressurized by a hand press. Furthermore, the working electrode (negative electrode) which consists of a negative mix layer (thickness 50 micrometers) closely_contact | adhered to collector copper foil (thickness 16 micrometers) was produced by punching in circular shape of diameter 15.5mm.

[Production of counter electrode (positive electrode)]
A lithium metal foil is pressed onto a nickel net and punched into a circular shape with a diameter of 15.5 mm, and a current collector made of nickel net and a counter electrode made of a lithium metal foil (thickness 0.5 mm) in close contact with the current collector ( Positive electrode) was prepared.

[Electrolyte, separator]
LiPF ₆ was dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate 33 vol% -methyl ethyl carbonate 67 vol% to prepare a non-aqueous electrolyte. The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body (thickness 20 μm) to produce a separator impregnated with the electrolytic solution.

[Production of evaluation battery]
A button-type secondary battery shown in FIG. 1 was prepared as an evaluation battery.

  The exterior cup 1 and the exterior can 3 were sealed by interposing an insulating gasket 6 at the peripheral portion thereof and caulking both peripheral portions. Inside, in order from the inner surface of the outer can 3, a current collector 7 a made of nickel net, a cylindrical counter electrode (positive electrode) 4 made of lithium foil, a separator 5 impregnated with an electrolyte, and a disk-like made of a negative electrode mixture A battery system in which a working electrode (negative electrode) 2 and a current collector 7b made of copper foil are laminated.

  In the evaluation battery, the separator 5 impregnated with the electrolytic solution was stacked between the working electrode 2 in close contact with the current collector 7b and the counter electrode 4 in close contact with the current collector 7a, and then the working electrode 2 was attached to the exterior cup. 1, the counter electrode 4 is accommodated in the outer can 3, the outer cup 1 and the outer can 3 are combined, and an insulating gasket 6 is interposed between the outer peripheral portion of the outer cup 1 and the outer can 3. The part was crimped and sealed.

  The evaluation battery is a battery composed of a working electrode 2 containing graphite particles that can be used as a negative electrode active material in a real battery and a counter electrode 4 made of a lithium metal foil.

  The evaluation battery produced as described above was subjected to the following charge / discharge test at a temperature of 25 ° C., and the initial charge / discharge efficiency and cycle characteristics were calculated. The evaluation results (discharge capacity, initial charge / discharge efficiency and cycle characteristics) are shown in Table 1 below.

[Initial charge / discharge efficiency]
After constant current charging of 0.9 mA until the circuit voltage reaches 0 mV, switching to constant voltage charging when the circuit voltage reaches 0 mV, and the charge capacity is obtained from the amount of current during which the current value reaches 20 μA. It was. Then, it rested for 120 minutes. Next, constant current discharge was performed until the circuit voltage reached 1.5 V at a current value of 0.9 mA, and the discharge capacity was determined from the amount of electricity supplied during this period. This was the first cycle. The initial charge / discharge efficiency was calculated from the following equation (1). In this test, the process of occluding lithium ions in the negative electrode material was charged, and the process of detaching lithium ions from the negative electrode material was discharge.

Initial charge / discharge efficiency (%) = (first cycle discharge capacity / first cycle charge capacity)
× 100 (1)
[Cycle characteristics]
Subsequently, after performing constant current charging of 4.0 mA until the circuit voltage reaches 0 mV, switching to constant voltage charging when the circuit voltage reaches 0 mV, and further continuing charging until the current value reaches 20 μA, Paused for 120 minutes. Next, constant current discharge was performed until the circuit voltage reached 1.5 V at a current value of 4.0 mA. This charge / discharge was repeated 100 times, and the cycle characteristics were calculated from the obtained discharge capacity using the following equation (2).

Cycle characteristics (%) = (discharge capacity in the 100th cycle / discharge capacity in the first cycle) × 100 (2)
[Example 2]
In Example 1 described above, mesophase microspheres (KMFC, manufactured by JFE Chemical Co., Ltd.) were graphitized at 3000 ° C. for 6 hours (average particle diameter 20 μm) instead of scaly graphite processed into a spherical shape. The negative electrode mixture was prepared, the negative electrode was produced, the lithium ion secondary battery was produced, and the battery was evaluated in the same manner as in Example 1 except that. The characteristics and evaluation results of the negative electrode material are also shown in Table 1 below.

  From Examples 1-2, it can be seen that the lithium ion secondary battery using the negative electrode material of the present invention has excellent discharge capacity, initial charge / discharge efficiency, and cycle characteristics.

[Comparative Example 1]
In Example 1, except that the adhesion of the vapor phase silica to the graphite material by mechanochemical treatment was not carried out, the preparation of the negative electrode mixture, the production of the negative electrode, the production of the lithium ion secondary battery, and the battery Evaluation was performed. The characteristics and evaluation results of the negative electrode material are also shown in Table 1 below.

  From the comparison between Example 2 and Comparative Example 1, it can be seen that the presence / absence of a metal compound that does not alloy with lithium improves the initial charge / discharge efficiency and the cycle characteristics.

[Comparative Example 2]
In Example 1, it replaced with what processed the scaly graphite into the spherical shape, and except using scaly natural graphite (Chuetsu Graphite Co., Ltd. make, BF15A, average particle diameter of 15 micrometers) similarly to Example 1. Preparation of the negative electrode mixture, preparation of the negative electrode, preparation of a lithium ion secondary battery, and evaluation of the battery were performed. The characteristics and evaluation results of the negative electrode material are also shown in Table 1 below.

  From the comparison between Example 1 and Comparative Example 2, it can be seen that the initial charge / discharge efficiency and the cycle characteristics are improved when the graphite is agglomerated and the aspect ratio is small.

  The negative electrode material for lithium ion secondary batteries of the present invention can be used for high performance lithium ion secondary batteries ranging from small to large, taking advantage of the characteristics.

It is sectional drawing of the evaluation battery for evaluating the battery characteristic of the negative electrode material of this invention.

Explanation of symbols

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

Claims (4)

SiO ₂ is attached to at least a part of the surface of the massive graphite material having an aspect ratio of 5 or less, and Si is attached to at least a part of the surfaces of the massive graphite material and the SiO _2. A negative electrode material for a lithium ion secondary battery. After attaching SiO ₂ to at least part of the surface of the massive graphite material having an aspect ratio of 5 or less by mechanochemical treatment, Si is applied to at least part of the surface of the massive graphite material and the SiO ₂ by a vapor phase method. A method for producing a negative electrode material for a lithium ion secondary battery.   A negative electrode for a lithium ion secondary battery, wherein the negative electrode material for a lithium ion secondary battery according to claim 1 is used.   A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 3.

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