JP2012138372A - Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material having high discharge capacity and achieving excellent cycle characteristics and initial charge/discharge efficiency when used as a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery made of the obtained negative electrode material, having high discharge capacity and achieving excellent cycle characteristics and initial charge/discharge efficiency, and a lithium ion secondary battery manufactured by using the negative electrode for a secondary battery.SOLUTION: In a negative electrode material for a lithium ion secondary battery, a layer of metal and/or a metal compound that can be alloyed with lithium is formed on at least a part of a surface of a graphitic material via a compound layer having metal atoms and carbon atoms that can be alloyed with lithium, and the graphitic material forms recesses and projections by allowing a fine graphite particle with an average particle size of 10 nm to 1 μm to attach on a surface of a mother particle using a graphite particle having a particle size larger than that of the fine graphite particle as the mother particle.

Description

  The present invention relates to a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

  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. These alloy negative electrodes have discharge capacities far surpassing that of graphite, though not as much as 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 a metal compound, which is alloyed 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.

  In Patent Documents 3 and 4 and the like, a metal that can be alloyed with lithium is placed on a current collector, a PVD (Physical Vapor Deposition) method such as a vacuum deposition method, a sputtering method, or an ion plating method, or a CVD (Chemical Vapor). A technique for forming a thin film by a deposition method is disclosed.

JP-A-11-279785 JP 2004-185975 A WO01 / 031720 Publication JP 2004-127561 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. In addition, since the outermost layer has a carbon layer, there is a problem that the charge / discharge capacity is low.

  Further, even in the methods described in Patent Documents 3 and 4, since no adhesion is secured at the interface between graphite and metal, when the metal repeatedly expands and contracts with charge and discharge, a crack occurs at the interface. The liberation is unavoidable and the charge / discharge efficiency and cycle characteristics deteriorate.

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

  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 by solving the above problems, and has a high discharge capacity, excellent cycle characteristics and initial charge / discharge. It aims at providing the negative electrode material from which efficiency is obtained. Further, a negative electrode for a lithium ion secondary battery having a high discharge capacity, excellent cycle characteristics and initial charge / discharge efficiency, and a lithium ion secondary battery using the negative electrode for the secondary battery, using 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] A layer of a metal and / or metal compound that can be alloyed with lithium is provided on at least a part of the surface of the graphite material via a compound layer having a metal atom and a carbon atom that can be alloyed with lithium, The graphite material is formed by forming irregularities by adhering fine graphite particles having an average particle diameter of 10 nm to 1 μm to the surface of the mother particles using graphite particles having a larger particle diameter as mother particles. A negative electrode material for a lithium ion secondary battery.

  [2] The lithium ion secondary as described in [1] above, wherein in the compound layer having a metal atom and a carbon atom that can be alloyed with lithium, the carbon atom is unevenly distributed on the graphite material side. Negative electrode material for batteries.

  [3] A negative electrode for a lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to [1] or [2].

  [4] A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to [3] above as the negative electrode.

  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.

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

  Hereinafter, the present invention will be described more specifically.

[Anode material for lithium ion secondary batteries]
The negative electrode material for a lithium ion secondary battery of the present invention (hereinafter also simply referred to as “negative electrode material”) is a compound layer having metal atoms and carbon atoms that can be alloyed with lithium on at least a part of the surface of the graphite material. This is a negative electrode material provided with a layer (outermost layer) of a metal and / or metal compound that can be alloyed with lithium via (interface layer). Here, in the compound layer having metal atoms and carbon atoms that can be alloyed with lithium, it is preferable that the carbon atoms contained in the compound layer are unevenly distributed on the graphite material side.

  In the negative electrode material, a metal and / or metal compound that can be alloyed with lithium is chemically bonded to the graphite through an interface layer, so that the adhesion between the two is excellent. Omission can be suppressed, and the initial charge / discharge efficiency and cycle characteristics are improved. Moreover, since the electrical conductivity in the negative electrode material is improved as the adhesive strength at the interface is improved, the initial charge / discharge efficiency and cycle characteristics are improved.

  The mass proportion of the metal that can be alloyed with lithium in the whole negative electrode material of the present invention is preferably 3 to 70%, particularly preferably 5 to 50%. The metal is present alone or as a compound in the outermost layer itself and coexists with carbon atoms in the interface layer. When the mass ratio of the metal 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).

  The average thickness of the metal and / or metal compound layer is generally in the range of 1 nm to 2 μm, preferably 10 nm to 1 μm. The average thickness can be measured by observing a cross section of the negative electrode material with a scanning electron microscope or a transmission electron microscope.

  The mass ratio of the metal in the interface layer (compound layer having a metal atom and a carbon atom that can be alloyed with lithium) in the entire negative electrode material of the present invention is 0% of the total metal amount in the negative electrode material. It is preferably 5 to 30%, particularly 1 to 10%. When the mass ratio is less than 0.5%, the adhesive strength at the interface is insufficient, and the cycle characteristics may be deteriorated. On the other hand, when the mass ratio exceeds 30%, the effect of improving the discharge capacity may be reduced.

  The carbon atom concentration in the interface layer (the compound layer having metal atoms and carbon atoms that can be alloyed with lithium) is preferably unevenly distributed on the graphite material side. As a result, the adhesion between the interface layer and the graphite material is improved, so that the metal can be prevented from falling off during charging and discharging, and the cycle characteristics are improved.

  The carbon concentration distribution can be confirmed by element mapping the particle cross section of the negative electrode material with an electron probe microanalyzer (EPMA). It can also be confirmed by measuring the element distribution in the depth direction by glow discharge emission analysis (GDS).

  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 contains a metal that can be alloyed with lithium and a graphite material as essential constituent components, but may further contain different types of carbonaceous material, graphite material, inorganic material, and metal. Specifically, it may be a carbonaceous material, a graphite material, an inorganic material, a mixture with a metal, a granulated material, a coating, 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 the adhesion strength between the metal and / or metal compound layer that can be alloyed with lithium and the graphite is alloyed with lithium. Since it is improved by a compound layer having possible metal atoms and carbon atoms, the metal and / or metal compound that can be alloyed with lithium is prevented from peeling and falling off due to expansion and contraction of the metal and / or metal compound. Is considered to contribute.

[Graphite]
The graphite material constituting the negative electrode material is not particularly limited as long as it can absorb and release lithium ions as the negative electrode active material.

  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.

  The shape of the graphite material is not particularly limited, and may be any of a scale shape, a lump shape, a spherical shape, a fibrous shape, and the like, but a lump shape or a spherical shape is preferable. The graphite material preferably has fine irregularities on the surface. The unevenness can be formed by attaching fine graphite particles such as carbon black to the surface of the mother particles by mechanical treatment such as mechanochemical treatment using graphite particles having a larger particle size as mother particles. . In this case, the average particle diameter of the fine graphite particles forming the irregularities is preferably 10 nm to 1 μm, and the average particle diameter of the mother particles is preferably 1 to 50 μm. When unevenness is present on the surface, the stress at the time of metal expansion is relaxed, so that the initial charge / discharge efficiency and cycle characteristics are improved. Moreover, since the particle indirect point is secured at the convex portion, the conductivity is improved, and the initial charge / discharge efficiency and the cycle characteristics are also improved.

  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 amount of the metal and / or metal compound that can be alloyed with lithium inevitably increases per unit area, and the metal and / or metal compound May cause cracking or powdering during charging. When the specific surface area is more than 50 m ² / g, the amount of the metal and / or metal compound deposited per unit area decreases, but the exposure rate of the active edge surface of the graphite increases and the initial charge / discharge efficiency is improved. It becomes difficult to adjust the negative electrode mixture paste when the negative electrode is formed or the negative electrode is formed. 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.

[Metals and metal compounds 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 these metals.

  The alloy may contain an element other than the metal, and may form a metal compound such as an oxide or a nitride. The metal and the metal compound are preferably amorphous or amorphous. This is because if it is amorphous, expansion during charging is reduced.

[Compound layer having metal atoms and carbon atoms that can be alloyed with lithium]
Examples of the metal species present in the compound layer (interface layer) having metal atoms and carbon atoms that can be alloyed with lithium include Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, and B. , Au, Pt, Pd, Sb, Ge, Ni, and the like. Si, Sn are preferable, and Si is particularly preferable. The metal that can be alloyed with lithium may be an alloy of two or more of these metals.

  In order to further improve the adhesion, the metal atoms in the interface layer are preferably the same as the metal atoms that can be alloyed with lithium and the metal atoms present in the metal compound layer (outermost layer).

  The mass ratio of the metal in the compound layer (interface layer) having metal atoms and carbon atoms that can be alloyed with lithium is not particularly limited, but the metal that can be alloyed with lithium and the metal present in the metal compound layer (outermost layer) 1-30% of is preferable. When the mass ratio is less than 1%, a sufficient adhesive effect cannot be obtained, and the cycle characteristics may be deteriorated. On the other hand, if it exceeds 30%, the effect of improving the discharge capacity may be small.

[Method for producing negative electrode material]
As a method for producing a negative electrode material of the present invention, a metal that can be alloyed with lithium and / or a compound layer having a metal atom and a carbon atom that can be alloyed with lithium on at least a part of the surface of the graphite material and / or Any method may be used as long as a structure having a metal compound layer can be obtained, and a method for maximizing the effects of the present invention will be exemplified below.

  The method of forming the metal and / or metal compound layer that can be alloyed with lithium on the surface of the graphite is preferably a vapor phase method. As the vapor phase method, a PVD (Physical Vapor Deposition) method such as a vacuum deposition method, a sputtering method, an ion plating method, a molecular beam epitaxy method, an atmospheric pressure CVD (Chemical Vapor Deposition) method, a low pressure CVD method, a plasma CVD method, etc. , MO (Magneto-optical) CVD methods, and CVD methods such as photo-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 move the graphite material mechanically by stirring the graphite material or applying vibration such as ultrasonic waves to uniformly coat the surface of the graphite material 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 compound layer having metal atoms and carbon atoms that can be alloyed with lithium is formed by introducing a gas containing hydrocarbons such as methane, ethane, propane, and butane into the apparatus during the sputtering. be able to. The mixing amount of the hydrocarbon is not particularly limited, but is preferably 50% or less by pressure with respect to the inert gas introduced as the ion source. A preferred lower limit of the pressure is 1%.

  At this time, the concentration distribution of carbon atoms in the compound layer can be controlled by introducing the hydrocarbon into the apparatus while adjusting the flow rate of the hydrocarbon. For example, carbon atoms can be unevenly distributed on the graphite particle side by decreasing the flow rate of the hydrocarbon continuously and / or stepwise.

  Here, continuously reducing the flow rate of hydrocarbon means reducing the flow rate of hydrocarbon at a predetermined reduction rate, and the reduction rate may be changed in the middle. Further, decreasing the flow rate of hydrocarbon stepwise means that after flowing hydrocarbon at a constant flow rate, the flow rate of hydrocarbon is decreased by a predetermined amount and this flow rate is allowed to flow for a predetermined time. The decrease in the flow rate of hydrocarbons may be performed continuously, stepwise, or a combination thereof.

  Finally, a negative electrode material for a lithium ion secondary battery of the present invention can be obtained by depositing a metal that can be alloyed with lithium by sputtering in the absence of a hydrocarbon gas. Here, the absence of hydrocarbon gas means that the hydrocarbon gas concentration is 0.1 vol% or less.

  Further, the negative electrode material for a lithium ion secondary battery of the present invention is obtained by subjecting a graphite material and a metal and / or metal compound that can be alloyed with lithium to mechanochemical treatment so that at least a part of the surface of the graphite material is obtained. After the metal and / or metal compound that can be alloyed with lithium are attached, the heat treatment can be performed in a temperature range of 900 to 1300 ° C.

  Here, the mechanochemical treatment is a method of imparting mechanical energy such as compression, shearing, collision, friction, etc. to an object to be treated. 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.).

  After the mechanochemical treatment, heat treatment is performed in a temperature range of 900 to 1300 ° C. By this heat treatment, a compound layer having metal atoms and carbon atoms that can be alloyed with lithium can be interposed between the graphite material and the metal and / or metal compound that can be alloyed with lithium. By adjusting the temperature of the heat treatment, carbon atoms in the compound layer can be unevenly distributed on the graphite material side. If the heat treatment temperature is less than 900 ° C, the reaction between the graphite and the metal and / or metal compound that can be alloyed with lithium may be insufficient. The abundance may increase and the capacity may decrease.

  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 the said 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. If this 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, but is preferably a foil shape or a net shape such as a mesh or expanded metal. Moreover, as a material of the said electrical power collector, copper, stainless steel, nickel, etc. are preferable. Moreover, it is preferable that the thickness of an electrical power collector shall be about 5-20 micrometers in the case of foil shape.

  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, but is preferably a foil shape or a mesh shape such as a mesh or expanded metal. Moreover, as a material of the said electrical power collector, aluminum, stainless steel, nickel, etc. are preferable. The thickness of the current collector is preferably about 10 to 40 μm in the case of a foil shape.

[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.

  A plasticizer is blended in the polymer solid electrolyte or polymer gel electrolyte, and the electrolyte salt or non-aqueous solvent can be used as the plasticizer. 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 in which a polymer compound constituting a matrix, a lithium salt, and a nonaqueous solvent (plasticizer) are mixed and heated to melt the polymer compound, organic After dissolving a polymer compound, a lithium salt, and a non-aqueous solvent (plasticizer) in a solvent, a method of evaporating an organic solvent for mixing, a polymerizable monomer, a lithium salt, and a non-aqueous solvent (plasticizer) are mixed, Examples include a method of obtaining a polymer by irradiating the mixture with ultraviolet rays, an electron beam, a molecular beam or the like to polymerize a polymerizable monomer.

  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, reference examples and comparative examples, as shown in FIG. 1, a single electrode evaluation comprising a working electrode (negative electrode) 2 containing a graphite material and a counter electrode (positive electrode) 4 made of lithium foil is used. A button-type secondary battery was fabricated and evaluated. An actual battery can be produced according to a known method based on the concept of the present invention.

  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.

  The carbon concentration distribution in the interface layer between the metal and graphite was confirmed by element mapping the particle cross section of the negative electrode material with an electron probe microanalyzer (EPMA). Furthermore, Si present in the metal and / or metal compound layer (uppermost layer) that can be alloyed with lithium and the compound layer (interface layer) that has metal atoms and carbon atoms that can be alloyed with lithium is quantified from elemental mapping. Then, the abundance ratio of Si in each layer was obtained, and the amount of Si present in each layer was calculated by multiplying the value by the total Si amount measured by emission spectroscopy. The abundance ratio was calculated from 20 fields of view of the cross section, and an addition average value was adopted.

[Reference Example 1]
[Production of negative electrode material]
As the graphite material, mesophase microspheres (KMFC manufactured by JFE Chemical Co., Ltd.) graphitized at 3000 ° C. for 6 hours (average particle size 20 μm) were used.

  The graphite material is arranged on the anode side stage of the DC bipolar sputtering apparatus, the single crystal silicon target having a purity of 99.999 mass% is arranged 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. During sputtering, methane having a pressure of 10% with respect to the pressure of argon was simultaneously introduced. The amount of methane introduced was gradually reduced as the sputtering progressed, and was adjusted to zero at the end of sputtering. Thereafter, sputtering was again performed for 4 hours under the condition of only argon gas, and stirring was repeated.

As a result of visual observation of the obtained negative electrode material with a scanning electron microscope, it was found that the silicon was entirely covered in a film shape.
From the cross-sectional observation, it was observed that an interface layer made of a compound of silicon and carbon was present between the graphite material and the silicon film.

  Furthermore, from elemental mapping by EPMA of the particle cross section, it was confirmed that the carbon concentration of the compound layer of silicon and carbon gradually decreased from the graphite side toward the silicon film side.

  The amounts of Si present in the outermost layer and the interface layer were 9% by mass and 1% by mass, respectively.

[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.

[Discharge capacity, 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 50th cycle / discharge capacity in the first cycle) × 100 (2)
[Example 1]
In the above Reference Example 1, except that the mesophase small sphere graphitized material is pretreated as follows, the preparation of the negative electrode mixture, the preparation of the negative electrode, the preparation of the lithium ion secondary battery, and the evaluation of the battery are the same as in the above Reference Example 1. Went. Table 1 shows the characteristics and evaluation results of the negative electrode material in the previous period.
(Preprocessing)
Graphitized mesophase spherules: 98 parts by mass, carbonaceous fine particles (Ketjen black, average particle size 30 nm) graphitized at 3000 ° C .: 2 parts by mass are added, and a dry powder compounding device (mechano-fusion system) , Manufactured by Hosokawa Micron Co., Ltd.), applying a compressive force and a shearing force repeatedly under the conditions of a peripheral speed of the rotating drum of 20 m / second, a processing time of 60 minutes, and a distance of 5 mm between the rotating drum and the internal member. Treated. When the obtained composite material was observed by SEM (scanning electron microscope), it was confirmed that fine graphite particles were dispersed and adhered to the surface of the graphite powder, and irregularities were formed on the surface.

[Reference Example 2]
In Reference Example 1, except that the amount of methane introduced was constant, preparation of the negative electrode mixture, preparation of the negative electrode, preparation of the lithium ion secondary battery, and evaluation of the battery were performed in the same manner as in Reference Example 1. The characteristics and evaluation results of the negative electrode material are also shown in Table 1.

[Reference Example 3]
Natural graphite (manufactured by Chuetsu Graphite Industries Co., Ltd., BF15A): 90 parts by mass, Si powder (manufactured by High Purity Chemical Laboratory, average particle size 2 μm): 10 parts by mass are added, and a dry powder compounding apparatus (mechanofusion system, Hosokawa Micron Co., Ltd.), mechanochemical treatment by repeatedly applying compressive force and shearing force under the conditions of a peripheral speed of the rotating drum of 20 m / s, a processing time of 60 minutes, and a distance of 5 mm between the rotating drum and the internal member. Was given. When the obtained composite material was observed by SEM, it was confirmed that Si particles were dispersed and adhered to the surface of the graphite powder.

  Thereafter, the obtained composite material was further heat-treated at 1100 ° C. for 10 hours. Elemental mapping by EPMA of the particle cross section of the finally obtained composite material confirmed that a SiC layer was formed at the interface between Si and graphite. The amounts of Si present in the outermost layer and the interface layer were 9% by mass and 1% by mass, respectively.

  From Example 1 and Reference Examples 1 to 3, 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 Reference Example 1, except that methane was not introduced during sputtering, the preparation of the negative electrode mixture, the production of the negative electrode, the production of the lithium ion secondary battery, and the evaluation of the battery were performed in the same manner as in Example 1. The characteristics and evaluation results of the negative electrode material are also shown in Table 1.

  From the comparison between Reference Example 1, Example 1 and Comparative Example 1, when an interface layer in which the carbon concentration gradually decreases from the graphite side to the metal side at the interface between the metal and graphite, the initial charge and discharge efficiency It can be seen that the cycle characteristics are improved.

  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.

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

Claims (4)

  A layer of a metal and / or metal compound that can be alloyed with lithium is provided on at least a part of the surface of the graphite material via a compound layer having a metal atom and a carbon atom that can be alloyed with lithium. Lithium ions characterized by forming irregularities by adhering fine graphite particles having an average particle diameter of 10 nm to 1 μm to the surface of the mother particles using graphite particles having a larger particle diameter as mother particles Negative electrode material for secondary batteries.   2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein in the compound layer having a metal atom and a carbon atom that can be alloyed with lithium, the carbon atom is unevenly distributed on the graphite side. .   A negative electrode for a lithium ion secondary battery, wherein the negative electrode material for a lithium ion secondary battery according to claim 1 or 2 is used.   A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 3 as the negative electrode.

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