JP2004210634A - COMPOSITE GRAPHITE PARTICLE, ITS PRODUCTION METHOD, Li ION SECONDARY BATTERY CATHODE MATERIAL, Li ION SECONDARY BATTERY CATHODE AND Li ION SECONDARY BATTERY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite graphite particle capable of obtaining a Li ion secondary battery having a high discharge capacity and a high initial charge-discharge efficiency, and also excellent in rapid discharge characteristics and cycle characteristics, when it is used as a cathode material for the Li ion secondary battery. <P>SOLUTION: The composite graphite particle has a carbon material obtained by heating and carbonizing a carbonizable material composed of only a resin or a resin material wherein a graphite prepared by pelletizing a scale-like graphite has the composite graphite particle of 0.50-20 mass% after being carbonized. A production method of the composite graphite particle, a Li ion secondary battery cathode material, a Li ion secondary battery cathode and a Li ion secondary battery, are provided. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

An object of the present invention is to provide composite graphite particles having high load characteristics and high cycle characteristics, high discharge capacity, and low irreversible capacity as a negative electrode material for a lithium ion secondary battery.
Further, the present invention relates to a lithium ion secondary battery and a constituent material thereof. Specifically, the present invention relates to a novel method for producing composite graphite particles composed of at least two kinds of materials having different physical properties. Further, a composite graphite particle having a novel structure obtained by this method, a negative electrode material of a lithium ion secondary battery containing the same, a negative electrode of a lithium ion secondary battery using the negative electrode material, and a lithium ion secondary battery using the negative electrode Next battery.

  In recent years, the miniaturization and weight reduction of electronic devices and communication devices are rapidly progressing, and there is a strong demand for miniaturization and weight reduction of secondary batteries used as power sources for driving these devices, with high energy density, A lithium ion secondary battery having a high voltage has been proposed. Lithium-ion secondary batteries use, for example, lithium cobaltate for the positive electrode, use a carbonaceous material such as graphite for the negative electrode, occlude lithium ions in the negative electrode during charging, and transfer these lithium ions to the negative electrode during discharging. Is to be released from

  Examples of the negative electrode material include MCMB (mesocarbon microbeads) derived from natural graphite which is a natural mineral resource and petroleum or coal-based heavy oil, or fine particles of mesophase pitch as described in Patent Document 1. Graphite is used as artificial graphite. In general, there is a correlation between the graphitization property of a carbonaceous material and the discharge capacity of a negative electrode material using the same. The better the graphitization property of a carbonaceous material, the higher the discharge capacity when used as an electrode. Tend to be. Therefore, scaly natural graphite having high crystallinity exhibits a discharge capacity comparable to the theoretical discharge capacity of graphite, 372 mAh / g. However, since the shape is scale-like, the coating property on the current collector is inferior, and the particles are oriented with respect to the current collector. Due to such physical problems, the electric resistance in the electrode surface increases, and as a result, when used as a negative electrode material, the irreversible capacity of the battery increases, or the high load characteristics deteriorate, There have been problems such as deterioration of cycle characteristics as a battery.

  2. Description of the Related Art In recent years, with the miniaturization or high performance of electronic devices, demands for higher energy density of batteries have been increasing. Lithium-ion secondary batteries are attracting attention because they can increase the voltage and can increase the energy density as compared with other secondary batteries. A lithium ion secondary battery has a negative electrode, a positive electrode, and a non-aqueous electrolyte as main components. Lithium ions generated from the non-aqueous electrolyte move between the negative electrode and the positive electrode during the discharging and charging processes, and become a secondary battery. Usually, a carbon material is used as the negative electrode material of the above-mentioned lithium ion secondary battery. As such a carbon material, graphite which has particularly excellent charge / discharge characteristics and exhibits a large discharge capacity and potential flatness is considered promising (Patent Document 2 and the like).

The more the three-dimensional crystal regularity (also referred to as crystallinity in the present invention) consisting of fused polycyclic hexagonal network planes (also referred to as carbon network planes in the present invention) develops, the more stable the intercalation compound with lithium becomes. Easy to form. Therefore, it is reported that the higher the crystallinity of graphite, the greater the amount of lithium inserted between the layers of the carbon mesh surface, and the greater the discharge capacity (Non-Patent Document 1, etc.). It has also been reported that various layer structures are formed depending on the amount of lithium inserted into the carbon netting layer, and exhibit a flat and high potential close to lithium metal in a region where they coexist (Non-Patent Document 2 and the like). Therefore, when a lithium ion secondary battery is assembled using graphite as a negative electrode material, high output can be obtained. Generally, the theoretical capacity when graphite is used as a negative electrode material is defined as the discharge capacity when graphite and lithium finally form LiC ₆ which is an ideal graphite intercalation compound. The discharge capacity is 372 mAh / g.

However, in a lithium ion secondary battery using graphite as a negative electrode material, as the crystallinity of graphite increases, side reactions such as decomposition of an electrolytic solution at the first charging tend to occur on the graphite surface. This side reaction is continued until the decomposition product accumulates and grows on the graphite surface, and the thickness of the graphite is such that electrons of the graphite cannot directly move to a solvent or the like. Since the side reaction at the time of the first charge does not participate in the battery reaction, the so-called irreversible capacity which cannot be taken out as an electric quantity in the first discharge process significantly increases. That is, there is a problem that the ratio of the initial discharge capacity to the initial charge capacity (also referred to as initial charge / discharge efficiency in the present invention) is reduced (Non-Patent Document 3 and the like). The irreversible capacity is represented by the following equation.
Irreversible capacity = initial charge capacity-initial discharge capacity

  Also, the initial charge / discharge efficiency may decrease due to co-intercalation of the solvent molecules and lithium ions, causing the graphite surface layer to peel off and the newly exposed graphite surface to react with the electrolyte. It has been reported (Non-Patent Document 4). As a means for compensating for such a decrease in the initial charge / discharge efficiency, a method of adding a positive electrode material of a secondary battery is known. However, the addition of the extra cathode material causes a new problem of reducing the energy density.

  As described above, in a lithium ion secondary battery using graphite as a negative electrode material, achieving both a large discharge capacity and a high initial charge / discharge efficiency is a conflicting demand depending on the crystallinity of graphite. As a method to solve this, a high-density graphite, which is advantageous for increasing the discharge capacity, is used as a nucleus, and its surface is coated with low-crystalline graphite or carbon, which is advantageous for improving the initial charge and discharge efficiency, to form a composite structure. A method has been proposed. This is because low-crystalline carbon has a low discharge capacity but a low decomposition reactivity to an electrolytic solution.

The prior art using such a composite material having a different crystallinity can be roughly classified as follows.
(1) Coating the surface of highly crystalline graphite as a nucleus with low crystalline carbon derived from a pyrolysis gas of an organic compound such as propane or benzene (for example, Patent Documents 3 and 4).
(2) A high crystalline graphite as a nucleus is coated or impregnated with a pitch or a curable resin in a liquid phase, and then fired at a temperature of about 1,000 ° C. to form a low crystalline carbon on a surface layer. (For example, Patent Documents 5 to 12).

  However, none of these methods is still sufficient for the recent demand level for increasing the discharge capacity. The method (1) has a complicated production process, high cost, and has a problem in productivity from the viewpoint of industrial production. Further, since the low-crystalline carbon on the surface is coated in an extremely thin film, there is a problem that the specific surface area is high and the initial charge / discharge efficiency is low. In the above method (2), the low-crystalline carbon in the surface layer is fused together when fired at about 1,000 ° C., and when this is crushed, the low-crystalline carbon in the surface layer is the core. There is a problem in that it is exfoliated from graphite, and powder characteristics such as specific surface area and bulk density, and battery characteristics such as initial charge / discharge efficiency are reduced.

  In the above methods (1) and (2), graphite as the nucleus and low-crystalline carbon of the surface layer have different expansion and contraction behaviors due to charge and discharge, and therefore, rapid charge and discharge and charge and discharge are repeated. As a result, the low-crystalline carbon of the surface layer may peel off, causing the same problem as described above.

  The discharge capacity of a battery largely depends on the discharge capacity per volume of graphite constituting the negative electrode. Therefore, in order to increase the discharge capacity of the battery, it is more advantageous to fill graphite having a large discharge capacity per unit weight (mAh / g) at a high density. However, when a negative electrode is formed by filling graphite at a high density, the adhesion between graphite and the low-crystalline carbon of the surface layer tends to be insufficient in the above methods (1) and (2). Then, the low-crystalline carbon film is peeled off from the graphite, the graphite surface having high reactivity with the electrolytic solution is exposed, and the initial charge / discharge efficiency may decrease.

  In addition, in Patent Document 12, it is shown in Examples that a graphite is coated with a pitch and then heat-treated at 2,800 ° C., but the formed film has low crystallinity (Ra value of Raman spectroscopy is 0). .32, and the method of measuring the R value will be described later), which causes the same problem as described above. In addition, although flaky graphite flakes are used as the core material, the graphite has a large aspect ratio and the orientation of the graphite when a negative electrode is manufactured also causes a decrease in rapid discharge characteristics and cycle characteristics.

  Apart from the above-mentioned prior art, in the case of Patent Document 13 in which the particle shape of the composite graphite is made closer to a sphere, a certain effect is recognized in the rapid charge / discharge characteristics and cycle characteristics. However, Patent Document 13 does not mention the difference in crystallinity between the outermost layer and the inside. According to the method of mixing and firing a plurality of flat cokes and pitches, pulverizing them so that the aspect ratio becomes 5 or less, and then graphitizing, only high-crystalline composite graphite particles can be obtained. And the initial charge / discharge efficiency remains low.

JP-A-9-251855 JP-B-62-23433 JP-A-4-368778 JP-A-5-275076 JP-A-4-368778 JP-A-5-94838 JP-A-5-217604 JP-A-6-84516 JP-A-7-302595 JP-A-11-54123 JP 2000-229924 A JP 2000-3708 A JP 2001-89118 A Electrochemistry and industrial physical chemistry, 61 (2), 1383 (1993) J. Electrochem. Soc., Vol. 140, 9, 2490 (1993) J. Electrochem. Soc., Vol. 117 222 (1970) J. Electrochem. Soc., Vol. 137, 2009 (1990)

Therefore, recently, even if the carbonaceous material is highly crystalline, a method of pulverizing the carbonaceous material, or a method such as mechanical fusion or the like, has a relatively bulky or sphere-like highly crystalline graphite or an aspect ratio of 3 or less. Natural graphite has begun to be produced. However, since these natural graphites have a relatively large specific surface area, when used as a negative electrode material of a lithium ion secondary battery, they have disadvantages such as high reactivity with an electrolytic solution and large irreversible capacity. . However, these natural graphites exhibit a discharge capacity close to the theoretical value, and there is a strong demand for their use.
Accordingly, a first object of the present invention is to provide composite graphite particles having high load characteristics and high cycle characteristics as a negative electrode material for a lithium ion secondary battery, and further having a high discharge capacity and a low irreversible capacity. is there.

  A second object of the present invention is to produce a carbon material having a composite structure with good productivity and at low cost. Furthermore, the present invention provides a lithium-ion secondary battery which uses lithium as a negative electrode material, which has a trade-off between performance, namely, high discharge capacity and high initial charge / discharge efficiency, as well as excellent rapid discharge characteristics and cycle characteristics. A third object is to obtain an ion secondary battery. Specifically, a novel composite graphite particle having both a large discharge capacity and a high initial charge / discharge efficiency, and capable of obtaining a lithium ion secondary battery having excellent rapid discharge characteristics and cycle characteristics, a method for producing the same, and a composite Another object is to provide a negative electrode material and a lithium ion secondary battery using graphite particles.

  The above object is achieved by the following invention. That is, the present invention provides a resin alone or a mixture of a resin and a pitch, in which spherical graphite particles obtained by granulating and spheroidizing flaky graphite by mechanical external force become 0.50 to 20% by mass of the composite graphite particles after carbonization. Provided is a composite graphite particle characterized by being coated with a carbon material obtained by heating and carbonizing.

  In the present invention, it is preferable that the resin is a phenol resin, a precursor of the phenol resin, or a monomer mixture of the phenol resin. Preferably, the pitch is a petroleum-based or coal-based pitch. Further, the mixing ratio of the resin and the pitch is preferably resin / pitch = 5/95 to 100/0 (mass ratio).

The granulated spheroidal graphite particles have an average particle size of 5 to 60 μm, an aspect ratio of 3 or less, a specific surface area of 0.5 to 10 m ² / g, and an Lc value measured by X-ray diffraction of 40 nm. above, La is 40nm or more, d ₀₀₂ is 0.337nm less, 1360 cm ^-1 peak intensity measured by Raman spectroscopy using an argon laser (I ₁₃₆₀₎ and 1580 cm ^-1 peak intensity (I ₁₅₈₀₎ of the ratio I ₁₃₆₀ / It is preferably made of highly crystalline artificial or natural graphite having an I ₁₅₈₀ (R value) of 0.06 to 0.30 and a half width of a ₁₅₈₀ cm ⁻¹ band of 10 to 60.

The carbon material has a ratio of ₁₃₆₀ / I ₁₅₈₀ (R value) of ₁₃₆₀ cm ^-1 peak intensity (I ₁₃₆₀ ) to ₁₅₈₀ cm ^-1 peak intensity (I ₁₅₈₀ ) measured by Raman spectroscopy using an argon laser. It is preferably from 0.05 to 0.40.

  Further, the present invention is to cover the spherical graphite particles obtained by granulating and spheroidal graphite by mechanical external force with a resin alone or a mixture of a resin and pitch, and heat-treat the coated particles to carbonize the coating layer. The present invention also provides a method for producing the composite graphite particles according to any one of the above aspects of the present invention. In the production method, the spherical graphite particles are coated with a resin alone or a mixture of a resin and a pitch, and then heat-treated at 200 to 300 ° C. in the air or under an inert atmosphere. Firing is preferred.

  Further, the present invention provides a negative electrode material for a lithium ion secondary battery, comprising any of the composite graphite particles of the present invention, and any of the composite graphite particles of the present invention is used as a negative electrode material. A lithium ion secondary battery is provided.

  Further, the present invention provides a method for producing composite graphite particles in which a carbonizable material containing a resin material is mixed with graphite and the resulting mixture is carbonized at 2,000 to 3,200 ° C. A method for producing composite graphite particles, characterized in that a carbonizable material is mixed so that 0.50 to 20% by mass of the particles becomes a carbon material obtained by heating and carbonizing a carbonizable material.

  In the above production method, it is preferable that the carbonizable material is a mixture of a resin material and tars, and the ratio of resin material / tars is 5/95 to 100/0 (mass ratio). It is preferable that the resin material is at least one selected from the group consisting of resin raw materials and resin precursors. Further, it is preferable that the resin material is at least one selected from the group consisting of a thermosetting resin raw material and a thermosetting resin precursor. Further, the thermosetting resin is preferably a phenol resin. Preferably, the graphite is obtained by granulating flaky graphite. Further, it is preferable that a carbonizable material containing a resin material is mixed with the graphite, and the mixture is heated at 200 to 300 ° C. and carbonized before carbonizing the obtained mixture.

Further, the present invention provides a composite graphite particle having a carbon material obtained by heating and carbonizing a carbonizable material containing a resin material that accounts for 0.50 to 20% by mass of the composite graphite particle after carbonization on at least a surface portion of the graphite. I do. The composite graphite particles are composite graphite particles having a carbon material on at least a surface portion of graphite having a plane spacing d ₀₀₂ of less than 0.337 nm in X-ray diffraction, and the aspect ratio of the composite graphite particles is 3 or less, peak intensity of 1580 cm ^-1 in the Raman spectrum of the composite graphite particle (I ₁₅₈₀₎ for the ratio (I ₁₅₈₀₎ of the peak intensity of _{^{1360cm -1 (I 1360) / (}} I 1360) of less than 0.3 from 0.1 or more Preferably, there is.

The carbon material preferably has an X-ray diffraction plane distance d ₀₀₂ of less than 0.343 nm and a ratio of graphite to the plane distance d ₀₀₂ of 1.001 or more to less than 1.02. Further, it is preferable that the carbonizable material is a mixture of a resin material and tars, and the ratio of resin material / tars is 5/95 to 100/0 (mass ratio).

  It is preferable that the resin material is at least one selected from the group consisting of resin raw materials and resin precursors. It is preferable that the resin material is at least one selected from the group consisting of a thermosetting resin raw material and a thermosetting resin precursor. Further, it is preferable that the thermosetting resin is a phenol resin. Preferably, the graphite is obtained by granulating flaky graphite.

  Further, the present invention provides a negative electrode material for a lithium ion secondary battery, comprising any one of the composite graphite particles of the present invention described above, a negative electrode using the negative electrode material for a lithium ion secondary battery, and Provided is a lithium ion secondary battery using a negative electrode.

  The present inventors have conducted intensive studies in order to solve the above-mentioned problems of the prior art, and as a result of these studies, particles obtained by granulating highly crystalline graphite such as natural graphite into a shape close to a sphere have been reduced in graphitization property. By mixing with a carbonizable material such as resin or pitch shown, coating the particle surface, and performing a high-temperature heat treatment, the carbonizable material is carbonized, the specific surface area of the graphite particles is easily reduced, and the load characteristics are reduced. I found a way to raise it. Since the composite graphite particles produced by this method have high graphitization properties and a low specific surface area, they are high discharge capacity, low irreversible capacity and high load characteristics materials as negative electrode materials for lithium ion secondary batteries. .

  According to the present invention, graphite particles having high load characteristics and high cycle characteristics as a negative electrode material for a lithium ion secondary battery, and having a high discharge capacity and a low irreversible capacity can be provided. Further, according to the present invention, by using a carbonizable material such as a mixture of resin and pitch as a coating material, it is possible to obtain an essentially high-density electrode while maintaining a high discharge capacity and rate characteristics. In addition, it is possible to easily produce an electrode having a uniform density with little dependency on the pressing pressure at the time of producing the electrode.

  Further, according to the method of the present invention, a carbon material having a composite structure can be produced with good productivity and at low cost. Moreover, according to this method, composite graphite particles suitable as a negative electrode material of a lithium ion secondary battery, which could not be produced conventionally, can be produced. A lithium ion secondary battery using this as a negative electrode material can highly achieve both high initial charge / discharge efficiency and large discharge capacity, which have been difficult to achieve conventionally. Furthermore, it has excellent rapid discharge characteristics and cycle characteristics. Therefore, the composite graphite particles of the present invention can satisfy the recent demand for higher density of battery energy. Further, the device on which the negative electrode material and the lithium secondary battery of the present invention are mounted can be reduced in size and improved in performance, and can widely contribute to society.

  Next, the present invention will be described in more detail with reference to preferred embodiments. The spheroidal graphite particles used in the present invention are spheroidal graphite particles obtained by granulating and spheroidizing artificial or natural highly graphitized flaky graphite with an external mechanical force. The mechanical external force refers to mechanical pulverization and granulation, and can make flake graphite into granulated spheroids. Examples of a flake graphite crushing device include a crushing device such as a counter jet mill (manufactured by Hosokawa Micron Corp.) and a current jet (manufactured by Nisshin Engineering Co., Ltd.). Milling to 60 μm, preferably 15-25 μm average particle size.

Although the pulverized product has an acute-angled surface, the pulverized product is granulated and used in the present invention. Examples of the granulating and spheroidizing apparatus for the pulverized product include granulating machines such as GRANUREX (manufactured by Freud Sangyo Co., Ltd.), NEW GRAM MACHINE (manufactured by Seisin Corporation), and Agromaster (manufactured by Hosokawa Micron Corporation), and hybridization. (Nara Machinery Co., Ltd.), Mechano Micros (Nara Machinery Co., Ltd.), Mechano Fusion System (Hosokawa Micron Co., Ltd.) and other shear compression processing devices can be used. "" Refers to flake graphite particles having an average particle size of 5 to 60 m, an aspect ratio of 3 or less, and a specific surface area of 0.5 to ¹⁰ m2 / g. Lc is the crystallite size Lc (002) of the graphite structure in the c-axis direction, and La is the crystallite size La (110) of the graphite structure in the a-axis direction.

The graphite particles used in the present invention have a crystallinity as measured by X-ray diffraction, Lc is 40 nm or more, La is 40 nm or more, d ₀₀₂ is 0.337 nm or less, Raman spectroscopy using an argon laser. half-width ratio I ₁₃₆₀ / I ₁₅₈₀ (R value) of 0.06 to 0.30, and 1580 cm ^-1 band of the measured 1360 cm ^-1 peak intensity (I ₁₃₆₀₎ and 1580 cm ^-1 peak intensity (I ₁₅₈₀₎ by Is preferably a high crystalline artificial or natural graphite particle having a particle size of 10 to 60. In the case of graphite having no such properties, the object of the present invention may not be sufficiently achieved. Lc is the crystallite size Lc (002) of the graphite structure in the c-axis direction, and La is the crystallite size La (110) of the graphite structure in the a-axis direction.

  The present invention is characterized in that the graphite particles are coated with a carbon material. As a material forming the carbon material, a carbonizable material capable of being carbonized is used. In particular, the resin is not particularly limited as long as it is various resins that can be easily carbonized, but a preferable material is a phenol resin alone or a mixture of a phenol resin and a pitch. The above-mentioned “phenol resin” includes a phenol resin itself (a highly condensed product of a phenol which may have a substituent and formaldehyde), an initial condensate of a phenol and formaldehyde (a precursor of a phenol resin), and Any mixture of phenols and formaldehyde (monomer mixture) can be used. In order to uniformly coat the graphite particles with these phenolic resins, it is preferable to use a phenolic resin precursor that can be melted or turned into solution by heat, or the above-mentioned monomer mixture that can be used as an aqueous solution. When a resin, particularly a phenolic resin alone, is used, the carbon material in the composite graphite particles of the present invention does not graphitize, and a coating layer of an appropriate carbon material is formed on the surface of the graphite particles, and a high discharge rate and The composite graphite particles of the present invention having a low specific surface area are obtained.

  The above resin can be used alone or as a mixture with pitch. As the pitch to be used, for example, a tar pitch obtained by concentrating and thermally polymerizing petroleum or coal tar is preferable. When a resin (especially a phenol resin) and a pitch are mixed and used as a coating material for graphite particles, the mixing ratio of the resin and the pitch is such that resin / pitch = 5/95 to 100/0 (mass ratio). It is preferable that the ratio be as follows (in the case of resin / pitch = 100/0, it is the same as the above-mentioned resin alone).

  If the use ratio of the resin is less than the above range, graphitization of the formed carbon material proceeds excessively, which is not preferable in terms of rate characteristics. When a resin and a pitch are used in combination, a preferable ratio of both of them is preferably 5/95 to 100/0, more preferably 30/70 to 70/30 in mass ratio of the resin and the pitch. By using the resin and the pitch together, the formed carbon material has a graphitization progress suppressed to a desired degree, a coating layer of an appropriate carbon material is formed on the graphite particle surface, and a high discharge rate and a low discharge rate are obtained. The composite graphite particles of the present invention having a specific surface area are obtained.

  In the step of coating the surface of the graphite particles with the carbon material, the graphite particles and the carbonizable material are charged into a mixer capable of applying a strong shearing stress, and kneaded in a temperature range equal to or higher than the liquefaction temperature of the carbonizable material. Thereby, the coating treatment of the graphite particles can be performed. Alternatively, the method can be carried out by dissolving the resin alone or a mixture of the resin and the pitch in a solvent capable of dissolving, mixing and stirring the solution of the carbonizable material and the graphite particles, and then removing the solvent by distillation. Alternatively, graphite particles are added to a resin monomer mixture or a low molecular weight resin (precursor of resin) or a pitch solution and stirred to adhere the resin or the resin / pitch composite on the particle surface, or while polymerizing. It can also be applied and coated.

  Furthermore, it is possible to coat the carbon material in a homogeneous or layered structure. Homogeneous means that the carbon material is a single component or a mixture of multiple components homogeneously.Coating with a layered structure means that after thin coating, another carbon material is coated again. Means For example, the surface of graphite particles is coated as a first layer with a phenolic resin composed of phenol and formaldehyde, and then the surface of graphite particles is coated with a xylenol resin composed of dimethylphenol (xylenol) and formaldehyde as a second layer. After coating the pitch as one layer, the phenolic resin can be coated as a second layer. By using such various techniques, the surface of the graphite particles can be coated with an arbitrary ratio of the carbon material.

  The content of the carbon material is a ratio (mass%) of carbon obtained by heating and carbonizing the carbonizable material to the composite graphite particles after carbonization.

  The content of the carbon material is 0.50 to 20% by mass of the entire composite graphite particles. Particularly, 8 to 12% by mass is preferable. If the amount is less than 0.5% by mass, a sufficient coating effect cannot be obtained, for example, a surface that is not substantially coated with the graphite particles is formed. On the other hand, if it exceeds 20% by mass, the coating layer is too thick, which not only has the adverse effect of reducing the discharge capacity of the obtained composite graphite particles, but also the residual carbon ratio of the carbonizable material (in most cases, 40 to 60%). In consideration of the above, the amount of the carbon material with respect to the graphite particles is large, and remarkable fusion of the graphite particles occurs in the coating (mixing), heating (curing), or carbonization process. When the fused particles are pulverized, the carbon material is cracked or peeled off, and the coating becomes non-uniform. In the above, when the content is relatively large, if preliminary firing is performed at a temperature of about 350 to 1,500 ° C. before carbonization, excessive gas generation during carbonization can be suppressed later, Operationally favorable. The content of the carbon material may be within the above range as an average of the entire composite graphite particles. Not all of the individual particles need to be within the above range, and some of the particles outside the above range may be included.

  In the present invention, carbonization is performed after the graphite particles are coated. In this step, it is desirable to provide a heating step at 200 to 300 ° C. In the heating step, since the carbonizable material, that is, light volatile components contained in the resin and the pitch are volatilized, it is necessary to raise the temperature over a sufficient time (for example, 4 to 6 hours). If this time is shortened, not only does the risk of sudden gas generation increase, but also the curing rate of the coating material does not reach the softening rate, and the adhesion of the coated graphite particles increases, which is not preferable in the production process.

Carbonization can be achieved by performing high-temperature baking at 2,000 to 3,200 ° C, but in the present invention, carbonization is preferably performed at 2,500 to 3,200 ° C, more preferably at 2,800 to 3,200 ° C. . As the carbonization method at this time, a general graphitization furnace typified by an Atisen furnace or the like can be used. The carbon material formed in such carbonization has a ratio I ₁₃₆₀ / I ₁₅₈₀ ( ₁₃₆₀ cm ⁻¹ peak intensity (I ₁₃₆₀ ) to ₁₅₈₀ cm ⁻¹ peak intensity (I ₁₅₈₀ ) measured by Raman spectroscopy using an argon laser. (R value) is preferably 0.05 to 0.40, and more preferably 0.05 to 0.20. The purpose of the present invention may not be sufficiently achieved with a carbon material having no such characteristics.

  The composite graphite particles of the present invention produced through the above steps are useful as a negative electrode material of a lithium ion secondary battery. For example, a slurry is prepared by using the composite graphite particles of the present invention as a binder, for example, a rubber-based fine particle binder (SBR) as an aqueous binder, water as a dispersion, and carboxymethyl cellulose (CMC) as a thickener. Alternatively, a negative electrode can be obtained by forming a slurry of polyvinylidene fluoride and an organic solvent solution of N-methylpyrrolidone as an organic solvent-based binder, applying the slurry on a copper foil, drying and pressing.

On the other hand, for example, a material obtained by dissolving LiPF _{6 in} a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) as an electrolytic solution, for example, lithium cobalt oxide as a positive electrode, a porous polypropylene film as a separator, and an electrolytic solution, for example, is used. Can be used to produce a battery. As described above, by coating the graphite particles with the carbon material using the resin alone or the mixture of the resin and the pitch, it is possible to obtain graphite particles for a negative electrode that have high load characteristics and are not excessively deformed during pressing. it can. Therefore, the composite graphite particles of the present invention are useful as a negative electrode material of a lithium ion secondary battery, and can provide a high performance lithium ion secondary battery.

Further, the best mode for carrying out the present invention with respect to the second and third objects will be described in detail.
The present invention relates to a method for producing composite graphite particles in which graphite is mixed with a carbonizable material containing a resin material, and the resulting mixture is carbonized at 2,000 ° C to 3,200 ° C. A method for producing composite graphite particles, characterized in that a carbonizable material is mixed so that 0.50 to 20% by mass of the carbonizable material is carbonized by heating and carbonizing the carbonizable material.

An object of the present invention relating to the above-mentioned production method is a composite graphite particle in which a carbonizable material containing a resin material is mixed with graphite, and the resulting mixture is carbonized at 2,000 to 3,200 ° C. The reason for paying attention to the composite graphite particles will be described below.
It is preferable to use granular graphite as the graphite. If flake-like or fibrous particles are not used, and preferably spherical particles are used, the shape of the composite graphite particles finally obtained is changed by the aspect ratio (the maximum diameter of the particles and the diameter orthogonal to the maximum diameter). Ratio) is small. As the aspect ratio becomes smaller, graphite crystals are arranged at random when a negative electrode is formed, the direction of expansion and contraction of graphite accompanying charge and discharge becomes uniform, and contact of graphite particles after charge and discharge can be maintained. Also, it is excellent in the liquid absorption and retention of the electrolyte. By these actions, rapid charge / discharge characteristics and cycle characteristics are excellent.

  As a material to be mixed with the granular graphite, a carbonizable material containing a resin material is used. By carbonizing at 2,000 to 3,200 ° C, a carbide having an excellent balance between discharge capacity and initial charge / discharge efficiency is obtained. Because it is obtained. When only the tar and the pitch in the prior art are used as the carbonizable material, the initial charge / discharge efficiency is reduced even if a high discharge capacity is obtained.

  Further, from the viewpoint of obtaining a high discharge capacity, the carbonization temperature is set to 2,000 to 3,200 ° C. This is because, when carbonized at about 1,000 ° C. as in the related art, although the initial charge / discharge efficiency is increased, the discharge capacity is reduced, and the rapid charge / discharge characteristics and cycle characteristics are reduced.

  In the production method of the present invention, it is preferable to use at least one selected from the group consisting of resin raw materials and resin precursors as the resin material. More preferably, at least one selected from the group consisting of a thermosetting resin raw material and a thermosetting resin precursor is used as the resin material. Compared with the case where the starting material is a resin having a high molecular weight, the use of a resin raw material or a resin precursor as a raw material makes it possible to suppress the destruction of the composite structure in the manufacturing process, and finally obtain composite graphite particles. The adhesion between the carbon material and graphite becomes very strong, which contributes to the improvement of the battery characteristics described later.

  It is more preferable to use a raw material or a precursor of a thermosetting resin because desired composite graphite particles can be obtained in a relatively small amount with a high carbonization rate. In addition, when a phenol resin is used as the thermosetting resin, that is, at least one selected from the group consisting of a phenol resin raw material and a phenol resin precursor is used as the resin material, the carbonization rate is particularly high and a relatively small amount of raw material is used. Is most preferable because it is possible to obtain composite graphite particles of That is, in the step of mixing the granular graphite and the carbonizable material, since the amount of the raw material of the resin material to be charged is small, the above-described problem of the fusion can be avoided.

  The most significant feature of the production method of the present invention is that the carbonizable material is mixed so that 0.50 to 20% by mass of the composite graphite particles after heating is carbonized by heating and carbonizing the carbonizable material. As in the prior art, when the content of the carbon material exceeds 20%, the content of the resin material relatively increases, and fusion between the particles occurs during the production process of the composite graphite particles. By limiting the content of the carbon material to 0.50 to 20% by mass, in the process of mixing and firing the resin raw material and the resin precursor, fusion between the materials is suppressed, and uneven distribution of the resin material and composite graphite are prevented. Aggregation of particles can be suppressed, and a suitable shape can be easily obtained. As a result, there is no need to add a crushing step by fusion, and the productivity is improved. The content of the carbon material may be within the above range as an average of the entire composite graphite particles. Not all of the individual particles need to be within the above range, and some of the particles outside the above range may be included.

  In the production method of the present invention, the carbonizable material is desirably a mixture of a resin material and tars. When the tars are used alone, the initial charge / discharge efficiency is reduced, but by mixing the resin material and the tars in an appropriate range, the crystallinity can be controlled so that the battery characteristics are maximized. . The ratio of resin material / tars is preferably in the range of 5/95 to 100/0.

  Further, it is preferable to use, as the graphite, one obtained by granulating flaky graphite, in particular, graphite formed into a spherical shape by a mechanical external force. By such an operation, the adhesion to the carbon material is further enhanced, and the penetration of the carbon material obtained by heating and carbonizing the carbonizable material into the gaps between the created granular graphites also contributes to the improvement of the adhesion. Since the adhesion between the carbon material and the granular graphite is high, they do not separate during the manufacturing process. Furthermore, it is possible to reduce the exposure of the edge portion of the graphite structure, which is highly reactive with the electrolytic solution, of the flaky graphite to the surface, and to achieve the desired battery characteristics even if the amount of the carbonizable material is reduced. Can be. Since the required amount of the carbonizable material is small, the shape of the graphite as a raw material can be directly reflected on the shape of the composite graphite particles, which is very efficient from the viewpoint of quality control and industrial production.

  Further, in the production method of the present invention, it is possible to mix the graphite with a carbonizable material containing a resin material, heat the mixture at 200 to 300 ° C. before carbonizing the obtained mixture, and then carbonize the mixture. preferable. By heating at 200 to 300 ° C. before carbonization, volatilization of volatiles contained in resin materials and tars, and curing of thermosetting resin when using raw materials or precursors of thermosetting resin material This increases the adhesion between the graphite and the carbonizable material, prevents the carbon material from peeling during charging and discharging of the composite graphite particles, and contributes to the improvement of battery characteristics. Note that this heating is preferably performed at a low heating rate.

  Next, among the composite graphite particles obtained by the method of the present invention, when the composite graphite particles are used for a negative electrode material of a lithium ion secondary battery, they have both a large discharge capacity and a high initial charge / discharge efficiency as well as ever before, and have a high speed. The invention of a novel composite graphite particle exhibiting discharge characteristics and cycle characteristics will be described more specifically.

The present inventors, at least on the surface portion of highly crystalline graphite, composite graphite particles having a carbon material having a lower crystallinity than graphite, the ratio of the carbon material is optimized from the viewpoint that battery characteristics are maximized, and In addition, the aspect ratio of the composite graphite particles and the crystallinity of the outermost layer were specified in appropriate ranges.
That is, the present invention is a composite graphite particle having a carbon material obtained by heating and carbonizing a carbonizable material containing a resin material that accounts for 0.50 to 20% by mass of the composite graphite particle after carbonization in at least a surface portion of the graphite. .

Further, the present invention provides a composite graphite particle having a carbon material on at least a surface portion of graphite having an interplanar spacing d ₀₀₂ of less than 0.337 nm in X-ray diffraction, wherein the composite graphite particle has an aspect ratio of 3 or less. the ratio of the peak intensity of _{^{1360cm -1 (I 1360) (I}} 1580) / (I 1360) is less than 0.3 0.1 or more to the peak intensity of 1580cm ^-1 (I ₁₅₈₀₎ in the Raman spectrum of the graphite particle It is a composite graphite particle.

[graphite]
Graphite constituting the core material of the composite graphite particles of the present invention, d ₀₀₂ is a measurement of X-ray diffraction is highly crystalline graphite shows less than 0.337 nm. As such graphite, commercially available flaky natural graphite is typical. As the graphite has higher crystallinity, the crystal grows regularly, and generally has a flaky shape. In addition, since the shape of the composite graphite particles finally obtained reflects the shape of the graphite, the shape of the graphite is preferably close to spherical, and the aspect ratio (the diameter of the particle orthogonal to the maximum diameter of the maximum diameter of the particle) is preferable. It is preferable to use granular graphite having a ratio of 3 or less. If the aspect ratio exceeds 3, graphite may be oriented when a negative electrode is manufactured using the same, and the battery characteristics may be deteriorated. However, this aspect ratio is preferably 3 or less as an average of the entire graphite, and the aspect ratio of all individual graphite particles does not need to be 3 or less, and some of the graphite particles include particles exceeding 3 in some cases. It may be. Such granular graphite can be produced, for example, by using flaky graphite as a raw material in the following manner.

  First, as the flaky graphite, a commercially available flake-like product may be used, or graphite of various shapes (coarse-grained natural graphite or artificial graphite) may be pulverized using a known pulverizing apparatus. it can. At this time, it is preferable to adjust the average particle size of the pulverized product to 5 to 60 μm. As a pulverizer, a counter jet mill (manufactured by Hosokawa Micron Corp.), a current jet (Nisshin Engineering Co., Ltd.), or the like can be used. Although flaky graphite obtained by pulverization or the like has an acute portion on its surface, in the present invention, it is granulated into granular graphite having a smooth surface by applying a mechanical external force thereto. Is preferred. In this granulation step, granulation is usually performed using a plurality of pulverized flaky graphite so that the aspect ratio becomes 3 or less. However, the present invention does not exclude the use of ground flake graphite alone. The method of granulation is not particularly limited, for example, a method of mixing a plurality of flaky graphite in the presence of a granulating auxiliary such as an adhesive or a resin, without using an adhesive for a plurality of flaky graphite. A method of applying a mechanical external force, a combination of both, and the like can be given. However, a method in which a mechanical external force is applied without using such a granulating aid to granulate into a spherical shape is most preferable.

  Examples of the above granulating apparatus include granulating machines such as GRANUREX (manufactured by Freund Corporation), New Glamachine (manufactured by Seishin Enterprise), Agromaster (manufactured by Hosokawa Micron Corporation), and a hybridization system (manufactured by ) Machines having shear compression processing capability such as Nara Machinery Co., Ltd., Mechano Micros (Nara Machinery Co., Ltd.), and Mechano Fusion System (Hosokawa Micron Co., Ltd.) can be used. Granulation can also be performed by operating the operating conditions using the above-mentioned pulverizer. The spherically shaped graphite may be any one obtained by rolling one flake graphite, or one obtained by aggregating and granulating a plurality of flake graphite. In particular, it is preferable that a plurality of flaky graphites are concentrically granulated.

[Carbon material]
The composite graphite particles of the present invention have a carbon material on at least a surface portion of graphite. The carbon material may be any as long as it gives the properties of the composite graphite particles described below. Usually, the carbon material is obtained by applying, impregnating, and / or mixing a carbonizable material including a resin material to the above-described graphite particles, and then performing a carbonization treatment by heating. The carbonizable material referred to in the present invention refers to a material that can be carbonized and / or graphitized by heating. Such heating is generally at least 700 ° C, preferably at 800 to 3,200 ° C. Therefore, the carbonization treatment referred to in the present invention includes the graphitization treatment. Particularly preferably, it is 2,000 to 3,200 ° C.

  Further, the term "at least the surface portion of graphite" as used in the present invention refers to the entire outer surface of graphite or a part thereof. Although it is a typical example of the present invention, for example, when the graphite particles obtained by the granulation treatment are secondary particles composed of a plurality of (scale-like) graphite, the outer surface of the secondary particles or a part thereof is Point. In the case of such secondary particles, the carbonizable material may penetrate into the interior of the secondary particles and be carbonized. Of course, the carbon material may be formed inside the graphite alone. However, in the composite graphite particles of the present invention, the outer surface of graphite is optimally coated with a carbon material. A preferred coverage is 50-100%.

  In the present invention, the carbonizable material described above is a mixture of a resin material and tars, and the mass ratio of the resin material to the tars is such that the ratio of resin material / tars is 5/95 to 100/0. Is preferred. More preferably, it is 30/70 to 70/30. When the ratio of the resin material is 5% by mass or more, the graphitization (crystallization) of the formed carbon material sufficiently proceeds, and at the same time, the effect of improving the initial charge / discharge efficiency becomes large. It is desirable to use a mixture of a resin material and tars because the degree of graphitization (crystallinity) of the carbon material can be adjusted so that the effect of the present invention is maximized.

  Tars referred to in the present invention refer to carbon materials precursors such as tar produced during wood carbonization, coal tar obtained from coal, and heavy oil produced from petroleum. Including. Specifically, coal pitch, bulk mesophase pitch, petroleum pitch and the like are exemplified. Each of these forms a graphite structure when heat-treated individually at about 3,000 ° C. Optically, it may be isotropic or anisotropic.

  The resin material in the present invention is preferably at least one selected from the group consisting of resin raw materials and resin precursors. The precursor of the resin includes a reaction intermediate, an oligomer, a polymerization intermediate, and the like. As an example of a resin raw material, a resin is obtained by heating, stirring and leaving a mixture of these raw materials, including a monomer and a polymerization initiator.

  In the present invention, it is preferable to use, as the resin material, at least one selected from the group consisting of a thermosetting resin raw material and a thermosetting resin precursor. When the thermosetting resin is carbonized at a high temperature, the resulting carbide has, on average, high crystallinity equivalent to graphite and may also include a graphite portion, but also a portion having a carbon turbulent layer structure. Therefore, in the present invention, it is referred to as a carbon material and is distinguished from core material graphite.

  As the thermosetting resin, a resin having a large amount of carbon remaining after heat treatment is desirable, and examples thereof include a urea resin, a maleic acid resin, a coumarone resin, a xylene resin, and a phenol resin. In the present invention, as the resin material, it is more preferable to use at least one selected from the group consisting of a phenol resin, a mixture of phenol resin monomers, and a phenol resin precursor. More specifically, a phenol resin itself (a highly condensed product of a phenol optionally having a substituent and an aldehyde represented by formaldehyde), an initial condensate of a phenol and an aldehyde ( Both phenolic resin precursors) and mixtures of phenols and aldehydes (monomer mixtures) can be used.

The crystallinity of the carbon material constituting the composite graphite particles of the present invention is preferably less than 0.343 nm, while the plane distance d ₀₀₂ of X-ray diffraction of graphite is less than 0.337 nm. If it is less than d ₀₀₂ of the carbon material is 0.343 nm, the discharge capacity is improved, thereby improving adhesion to the carbon material and graphite. The difference in crystallinity between graphite and the carbon material is more preferably such that the ratio of d ₀₀₂ of the carbon material to d ₀₀₂ of the graphite is in the range of 1.001 or more to less than 1.020. If it is 1.001 or more, the initial charge / discharge efficiency is further improved, and if it is less than 1.020, the adhesion of the carbon material is further improved.

[Composite graphite particles]
The composite graphite particle of the present invention is a composite graphite particle having a carbon material on at least a surface portion of graphite having a plane spacing d ₀₀₂ of less than 0.337 nm in X-ray diffraction, and an aspect ratio of the composite graphite particle is 3 or less. in, from 0.50 to 20% by weight of the composite graphite particles are the carbon material, the ratio of the peak intensity of 1360 cm ^-1 to the peak intensity of 1580cm ^-1 (I ₁₅₈₀₎ in the Raman spectrum of the composite graphite particle (I ₁₃₆₀₎ The fact that (I ₁₅₈₀ / I ₁₃₆₀ ) is 0.1 or more and less than 0.3 is shown again, and will be described in more detail.

It is preferable that the composite graphite particles also have a nearly spherical shape with an aspect ratio of 3 or less, similarly to the graphite used. If the aspect ratio exceeds 3, graphite may be oriented when a negative electrode is manufactured using the same, and the battery characteristics may be deteriorated. However, this aspect ratio is preferably 3 or less as an average of the entire composite graphite particles, and the aspect ratio of all the individual composite graphite particles does not need to be 3 or less, and some of the particles exceeding 3 are partially used. May be included. The composite graphite particles of the present invention have the above-mentioned graphite as a core material, and a carbon material having lower crystallinity than graphite exists at least on the surface thereof. Composite the surface of the crystalline graphite particles can be defined by the R value of Raman spectroscopy, 1360 cm ^-1 peak intensity measured by Raman spectroscopy using an argon laser (I ₁₃₆₀₎ and 1580 cm ^-1 peak intensity (I the ratio I ₁₃₆₀ / I ₁₅₈₀ (R value of ₁₅₈₀₎₎ is required to be 0.1 or more and less than 0.3. When the R value is less than 0.1 or 0.3 or more, the initial charge / discharge efficiency may decrease in any case. Particularly preferred R value is 0.1 to 0.2.

  In the present invention, the ratio of the carbon material is limited to the range of 0.50 to 20% by mass of the carbon material in the composite graphite particles. When the proportion of the carbon material is less than 0.50% by mass, it is difficult to completely cover the active graphite edge surface, and the initial charge / discharge efficiency may decrease. On the other hand, when it exceeds 20% by mass, the proportion of the carbon material having a relatively low discharge capacity is too large, and the discharge capacity of the composite graphite particles is reduced. In addition, the proportion of the raw materials (thermosetting resins and tar pitches) for forming the carbon material is large, and the particles are easily fused in the coating step and the subsequent heat treatment step, and the composite graphite particles finally obtained are obtained. In some cases, cracking or peeling may occur in a portion of the carbon material layer, and the initial charge / discharge efficiency may decrease. In particular, it is preferably 3 to 15% by mass, more preferably 8 to 12% by mass. The content of the carbon material may be within the above range as an average of the entire composite graphite particles. Not all of the individual particles need to be within the above range, and some of the particles outside the above range may be included.

Further, preferred physical property values of the composite graphite particles of the present invention include an average particle size of 5 to 60 μm, a specific surface area of 0.5 to 10 m ² / g, and a measured Lc value of X-ray diffraction of 40 nm or more, d ₀₀₂ is preferably 0.3370 nm or less. If the average particle diameter and the aspect ratio are in the ranges described above, the discharge capacity and the initial charge / discharge efficiency are high, and other battery characteristics such as rapid charge / discharge characteristics and cycle characteristics are further improved. When the specific surface area is 10 m ² / g or less, the viscosity of the negative electrode mixture paste (a mixture of the negative electrode material and the binder dispersion) when forming the negative electrode is easily adjusted, and the binding force of the binder is also improved. If Lc and d _{002 in} X-ray diffraction are within the above-mentioned values, a sufficient discharge capacity can be obtained. Lc is the crystallite size Lc (002) of the graphite structure in the c-axis direction, and La is the crystallite size La (110) of the graphite structure in the a-axis direction.

[Production method of composite graphite particles]
An example of a method for producing the composite graphite particles of the present invention described above will be described. As described above, it is desirable to use flaky graphite that has been previously shaped into a sphere by a granulation operation or the like. When the outer surface of the graphite particles is coated with a carbonizable material comprising a mixture of a thermosetting resin and a tar, the carbonizable material and the graphite particles are charged into a mixer, and the softening point of the carbonizable material is reduced. Kneading by applying a strong shearing force in the above temperature range, or a method of mixing a carbonizable material as a solution or a dispersion with graphite particles and then drying and removing a solvent or a dispersion medium is used. In particular, it is desirable to use a thermosetting resin as a low molecular weight substance (precursor of the resin) or a monomer mixture and coat the graphite particles and increase the molecular weight by heating at the same time. Similarly, when the carbonizable material contains tars, it is effective to promote polycondensation of the tars simultaneously with coating.

  In the present invention, a phenol resin is preferable as a thermosetting resin essential as a carbonizable material. When graphite particles are coated with a phenol resin, it is preferable to use a phenol resin precursor or a phenol resin monomer-containing material. The phenolic resin precursor or the monomer-containing phenolic resin can be easily melted or turned into solution by heating, and can be uniformly coated on graphite particles. Further, it has a feature that a phenol resin layer formed by heating at the same time as coating is firmly adhered to graphite particles.

  The carbonizable material can be coated with a plurality of compositions in a homogeneous or dispersed state. The carbonizable material can be coated multiple times, varying its composition. For example, a graphite layer is coated with a phenol resin composed of phenol and formaldehyde as a first layer, and then a second layer is coated with a xylenol resin composed of dimethylphenol (xylenol) and formaldehyde. And then a phenolic resin can be coated as the second layer.

  The coating amount of the carbonizable material may be set so that the ratio of the carbon material to the composite graphite particles is finally 0.50 to 20% by mass. It is preferable to heat the resin material at 200 to 300 ° C after mixing the carbonizable material with the graphite particles or simultaneously with the mixing process. In this heating step, the resin material is hardened and the light volatile components contained in the resin material and tars are volatilized. Therefore, it is preferable that the temperature is raised over a sufficient time, usually 4 hours or more. Maintaining such a heating time allows for complete mixing and smooth curing, so that the adhesion between the carbonizable material and the graphite particles is increased, and the carbon material is separated during the charging and discharging of the composite graphite particles. Can be prevented, which contributes to improvement of battery characteristics.

  After the curing step, if necessary, the particle size is preferably adjusted by crushing, sieving, or the like, followed by firing. The firing is preferably performed at 2,000 to 3,200 ° C. More preferably, 2,500 to 3,200 ° C, further preferably 2,800 to 3,200 ° C. For the firing treatment, a general graphitization furnace represented by an Acheson furnace can be used. It is desirable to carry out in a non-oxidizing atmosphere.

  The present invention also provides a negative electrode material containing any of the composite graphite particles described above. The composite graphite particles of the present invention can be diverted to applications other than the negative electrode, such as conductive materials for fuel cell separators and graphite for refractories, taking advantage of their characteristics. It is suitable as a negative electrode material. That is, the negative electrode material of the present invention is required to contain at least the above composite graphite particles. Therefore, the composite graphite particles of the present invention themselves are also the negative electrode material of the present invention.

Hereinafter, a negative electrode material of a lithium ion secondary battery, a negative electrode of a lithium ion secondary battery, and a lithium ion secondary battery using the composite graphite particles of the present invention will be described. [Negative electrode material and negative electrode for lithium ion secondary battery]
The present invention also provides a negative electrode material for a lithium ion secondary battery containing any of the composite graphite particles of the present invention described above. The negative electrode material of the present invention may include a negative electrode material other than the composite graphite particles of the present invention as long as the object of the present invention is not impaired. The negative electrode of the present invention is obtained by solidifying and / or shaping the above-described negative electrode material of the present invention. The formation of the negative electrode can be performed according to a normal molding method, but it is necessary to obtain a chemically and electrochemically stable negative electrode that sufficiently draws out the performance of the composite graphite particles and has a high shapeability for powder. There is no limitation as long as the method can be performed.

  When manufacturing the negative electrode, a negative electrode mixture obtained by adding a binder to composite graphite particles can be used. As the binder, it is desirable to use a binder having chemical stability and electrochemical stability with respect to the electrolyte and the electrolyte solution solvent. For example, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyethylene, polyvinyl alcohol, styrene butadiene rubber, carboxymethyl cellulose and the like are used. These can be used in combination. Usually, the binder is preferably used in an amount of about 1 to 20% by mass based on the whole amount of the negative electrode mixture.

  Specifically, the negative electrode mixture layer is prepared by mixing composite graphite particles, which have been adjusted to an appropriate particle size by classification or the like, with a binder to prepare a negative electrode mixture. It can be formed by applying to one or both sides of the body. At this time, a normal solvent can be used, and after dispersing the negative electrode mixture in the solvent to form a paste, and then applying and drying the current collector, the negative electrode mixture layer is uniformly and firmly formed. Can be obtained. The paste can be prepared by stirring with various mixers.

  For example, the composite graphite particles of the present invention and a fluorine-based resin powder such as polytetrafluoroethylene may be mixed and kneaded in a solvent such as isopropyl alcohol, and then applied to form a negative electrode mixture layer. Further, the composite graphite particles of the present invention, a water-soluble binder such as fluorinated resin powder such as polyvinylidene fluoride or carboxymethyl cellulose, mixed with a solvent such as N-methylpyrrolidone, dimethylformamide or water, alcohol and slurry. After that, coating may be performed to form a negative electrode mixture layer.

  The thickness of the negative electrode mixture comprising the mixture of the composite graphite particles and the binder of the present invention when applied to the current collector is preferably 10 to 300 μm. After the formation of the negative electrode mixture layer, by performing pressure bonding such as press pressure, the adhesive strength between the negative electrode mixture layer and the current collector can be further increased. In the lithium ion secondary battery of the present invention, the shape of the current collector used for the negative electrode is not particularly limited, but a foil shape or a mesh shape such as a mesh or expanded metal is used. Examples of the current collector include copper, stainless steel, and nickel. In the case of a foil, the thickness of the current collector is preferably about 5 to 20 μm.

The present invention further provides a lithium ion secondary battery using the above-described negative electrode.
[Lithium ion secondary battery]
A lithium-ion secondary battery generally includes a negative electrode material, a positive electrode material, and a non-aqueous electrolyte as main battery components. Each of the positive electrode material and the negative electrode material is a lithium ion carrier. This is a battery mechanism in which lithium ions are doped into the negative electrode during charging, and dedoped from the negative electrode during discharging. The lithium ion secondary battery of the present invention is not particularly limited except that a negative electrode material containing the composite graphite particles of the present invention is used. The other components are the same as those of a general lithium ion secondary battery.

(Positive electrode material)
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, and it is preferable to select a material capable of doping / dedoping a sufficient amount of lithium. For example, lithium-containing compounds such as lithium-containing transition metal oxides, transition metal chalcogenides, vanadium oxides and Li compounds thereof, and a general formula M _x Mo ₆ S _8-y (where X is 0 ≦ X ≦ 4 and Y is A numerical value in the range of 0 ≦ Y ≦ 1 and M represents a metal such as a transition metal), activated carbon, activated carbon fiber and the like. Vanadium oxides include those represented by V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , and 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. The lithium-containing transition metal oxide is specifically LiM (1) _1-x M (2) _x O ₂ (where X is a numerical value in the range of 0 ≦ X ≦ 1, M (1), M (1) (2) consists of at least one transition metal element) or LiM (1) _1-y M (2) _y O ₄ (where X is a numerical value in the range of 0 ≦ Y ≦ 1, and M (1) , M (2) comprises at least one transition metal element.)

  In the above formula, the transition metal elements represented by M (1) and M (2) are Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., and preferred is Co. , Fe, Mn, Ti, Cr, V, Al and the like. The lithium-containing transition metal oxide is, for example, Li or an oxide or a salt of a transition metal as a starting material, and these starting materials are mixed according to a desired composition of the metal oxide. It can be obtained by firing in a temperature range of 1,000 ° C. The starting materials are not limited to oxides and salts, but may be hydroxides and the like.

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

(Positive electrode)
Positive electrode, for example, by forming a positive electrode mixture layer by applying a positive electrode mixture of the lithium compound and a binder and a conductive agent for imparting conductivity to the electrode on one or both surfaces of the current collector can get. As the binder, any of those exemplified for 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 may be formed into a paste by dispersing the positive electrode mixture in a solvent, and the paste-shaped positive electrode mixture may be applied to a current collector and dried to form a positive electrode mixture layer. After forming the positive electrode mixture layer, pressure bonding such as pressing may be further performed. Thereby, the positive electrode mixture layer is uniformly and firmly adhered to the current collector. The shape of the current collector is not particularly limited, and may be a box, a mesh, or a mesh such as expanded metal. For example, examples of the current collector include an aluminum foil, a stainless steel foil, and a nickel foil. The thickness is preferably from 10 to 40 μm.

(Non-aqueous 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 ₃ OSO ₂ ) ₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN ((CF ₃ ) ₂ CHOSO ₂ ) ₂ , LiB ｛C ₆ H ₃ (CF ₃ ) ₂ ｝ ₄ , LiAlCl ₄ , LiSiF ₆ And the like. Particularly, LiPF ₆ and LiBF ₄ are preferable from the viewpoint of oxidation stability.

  The electrolyte salt concentration in the electrolytic solution is preferably from 0.1 to 5 mol / l, more preferably from 0.5 to 3.0 mol / l. The non-aqueous electrolyte may be a liquid non-aqueous electrolyte or a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the non-aqueous electrolyte battery is configured as a so-called lithium ion battery, and in the latter case, the non-aqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte or a polymer gel electrolyte battery.

  When a liquid non-aqueous electrolyte solution, as a solvent, ethylene carbonate, propylene carbonate, dimethyl carbonate, carbonates such as diethyl carbonate, 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, Ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxofuran, 4-methyl-1,3-dioxolan, anisole and diethyl ether; thioethers such as sulfolane and methylsulfolane; acetonitrile, chloronitrile and propio Nitriles such as nitriles, trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzochloride Le, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, may be used an aprotic organic solvent such as sulfite, dimethyl sulfite.

  When the non-aqueous electrolyte is a polymer electrolyte such as a polymer solid electrolyte and a polymer gel electrolyte, a polymer gelled with a plasticizer (non-aqueous electrolyte) is used as a matrix. Examples of the polymer constituting the matrix include ether polymers such as polyethylene oxide and crosslinked products thereof, acrylate polymer compounds such as polymethacrylate polymer compounds and polyacrylates, polyvinylidene fluoride (PVDF), and vinylidene fluoride. -A fluorine-based polymer compound such as a hexafluoropropylene copolymer is particularly preferred.

  The polymer solid electrolyte or polymer gel electrolyte contains a plasticizer. As the plasticizer, the above-mentioned electrolyte salts and non-aqueous solvents can be used. In the case of the polymer gel electrolyte, the concentration of the electrolyte salt in the non-aqueous electrolyte as a plasticizer is preferably from 0.1 to 5 mol / l, more preferably from 0.5 to 2.0 mol / l.

  The method for producing the solid electrolyte is not particularly limited. For example, a method of mixing a polymer forming a matrix, a lithium salt and a non-aqueous solvent (plasticizer) and heating to melt the polymer, A method of dissolving a molecular compound, a lithium salt and a non-aqueous solvent (plasticizer) and then evaporating an organic solvent, and mixing a polymerizable monomer, a lithium salt and a non-aqueous solvent (plasticizer) as a raw material of a polymer electrolyte Then, a method of irradiating the mixture with ultraviolet rays, an electron beam, a molecular beam, or the like to form a polymer can be used. Further, the addition rate of the non-aqueous solvent (plasticizer) in the solid electrolyte is preferably from 10 to 90% by mass, and more preferably from 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 the film will not be easily formed.

(separator)
In the lithium ion secondary battery of the present invention, a separator can be used. The material of the separator is not particularly limited, and examples thereof include a woven fabric, a nonwoven fabric, and a synthetic resin microporous membrane. In particular, a synthetic resin microporous membrane is preferable, and among them, a polyolefin-based microporous membrane is preferable in terms of thickness, film strength, film resistance, and the like. Specifically, it is a microporous membrane made of polyethylene or polypropylene, or a microporous membrane obtained by combining these.

  Also, a gel electrolyte can be used without using a separator. A secondary battery using a gel electrolyte, a negative electrode material containing the composite graphite particles, a positive electrode material and a gel electrolyte, for example, a negative electrode material, a gel electrolyte, laminated in the order of the positive electrode material, in the exterior material of the battery It is composed by containing. Further, a gel electrolyte may be provided outside the negative electrode material and the positive electrode material.

(Lithium ion secondary battery)
The gel electrolyte secondary battery is configured by stacking a negative electrode containing the graphite particles, a positive electrode, and a gel electrolyte in the order of, for example, a negative electrode, a gel electrolyte, and a positive electrode, and housing the stacked negative electrode in a battery exterior material. . Further, a gel electrolyte may be provided outside the negative electrode and the positive electrode. In the gel electrolyte secondary battery using the negative electrode material of the present invention for the negative electrode, even when the gel electrolyte contains propylene carbonate, the irreversible capacity in the first cycle can be suppressed to a small value.

  Further, the structure of the lithium ion secondary battery of the present invention is not particularly limited, its shape and form are not particularly limited, and depending on the application, mounted equipment, required charge / discharge capacity, etc., a cylindrical type, It can be arbitrarily selected from a square type, a coin type, a button type and the like. In order to obtain a sealed non-aqueous electrolyte battery with higher safety, it is preferable to provide a means for interrupting the current by detecting an increase in battery internal pressure when an abnormality such as overcharging occurs. In the case of a polymer solid electrolyte battery or a polymer gel electrolyte battery, the structure may be such that the battery is sealed in a laminate film.

  A lithium ion secondary battery using a polymer gel electrolyte is composed of a negative electrode using the negative electrode material of the present invention, a positive electrode, and a gel electrolyte. For example, it is manufactured by stacking a negative electrode, a gel electrolyte, and a positive electrode in this order, and housing the battery in a battery exterior material. In addition to this, a gel electrolyte may be further provided outside the negative electrode and the positive electrode. In the polymer gel electrolyte battery using the negative electrode material of the present invention, propylene carbonate can be contained in the gel electrolyte.

  Further, the structure of the lithium ion secondary battery of the present invention is arbitrary, and its shape and form are not particularly limited, and may be arbitrarily selected from a cylindrical type, a square type, a coin type, a button type and the like. Can be. In order to obtain a sealed non-aqueous electrolyte battery with higher safety, it is preferable to provide a means for interrupting the current by detecting an increase in battery internal pressure when an abnormality such as overcharging occurs. In the case of a battery using a gel electrolyte, a structure in which the battery is sealed in a laminate film may be used.

  Next, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following description, “%” is based on mass unless otherwise specified.

<Graphite>
As the graphite constituting the core material of the composite graphite particles of the present invention, granulated graphite A having the physical properties shown in Table 1 below were used in Examples 1 to 5 and Comparative Examples below, and granulated graphite B was used in Example 6 Used for The granulated graphite A is obtained by granulating flaky natural graphite having an average particle diameter of 30 μm by using a counter jet mill 200AFG manufactured by Hosokawa Micron Corporation at an air pressure of 300 kPa for 1 hour in the machine. Of the granulated graphite thus obtained, fine powder having a particle diameter of 5 μm or less and insufficient granulation was removed. Further, coarse powder was removed so as to be below a 75 μm sieve. As shown in Table 1 below, the granulated graphite used in each of the examples and comparative examples has a lower specific surface area than general flaky graphite while maintaining high crystallinity by granulation.

Various physical properties in Table 1 above were measured as follows.
Specific surface area (m ² / g): The BET specific surface area by nitrogen gas adsorption was determined.
Average particle size (μm): The particle size at which the cumulative frequency of the particle size distribution measured by a laser diffraction type particle size distribution meter becomes 50% by volume percentage.
-Aspect ratio: Image processing of a scanning electron microscope 300 times larger than that of graphite particles using an image analyzer (manufactured by Toyobo Co., Ltd.), and the aspect ratio of any 50 graphite particles (length in the major axis direction and (The ratio of the lengths in the orthogonal short axis direction).
Lc (nm), La (nm), and d ₀₀₂ (nm): X-ray diffraction method using CuKα ray as X-ray and high-purity silicon as standard material (JSPS 117th Committee) Specifically, it was measured by the method｝ described in “Carbon Fiber” (Sugio Otani, pp. 733-742 (1986), Kindaikisha). Value.
· R values: the wavelength of the peaks present in the region of 1570～1630Cm ^-1 and I _G in the Raman spectrum using argon laser beam having a wavelength of 514.5nm by JASCO Corp. NR-1800, 1350~1370cm ^{- When} the intensity of the peak existing in the region ¹ was defined as _ID , the ratio was defined as _ID / _IG ratio.
-Width at half maximum: Calculated from the Raman band width at half the height of the peak existing in the region of 1570 to 1630 cm ^{-1 in} the Raman spectrum.

Example 1
A solution obtained by adding and dissolving 25 g of a phenol resin (residual carbon ratio: 40%) to a mixture of 500 g of ethylene glycol and 2.5 g of hexamethylenetetramine was added to granulated graphite (graphite particles) (average particle size: 20 μm, aspect ratio). 2.0) 90 g was added and stirred in a dispersed state. Next, the solvent was distilled off at 150 ° C. under reduced pressure to obtain graphite particles mixed with the resin. The resin-mixed graphite particles were heated in air to 270 ° C. over 5 hours, and were further kept at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. Next, a pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and further carbonization was performed at 3,000 ° C. to obtain composite graphite particles of the present invention having a carbon material content of 10%.

In order to estimate the crystallinity of the carbon material of Example 1, a carbon material was prepared by adding the same heat history as in Example 1 without adding graphite particles to Example 1. The d ₀₀₂ measured by X-ray diffraction was 0.3366 nm and Lc 38 nm, and it was confirmed that the graphite particles used as the core material had slightly lower crystallinity than d ₀₀₂ and Lc (Table 1). In addition, the residual carbon ratio of the carbonizable material refers to the residue when heated to 800 ° C. and substantially the entire amount is carbonized in accordance with the fixed carbon method of JIS K 2425, expressed as a percentage. is there.

Example 2
110 g of graphite particles (average particle diameter: 20 μm, aspect ratio: 2.0) were added to a solution composed of 39 g of phenol monomer, 66 g of 37% formalin, and 4 g of hexamethylenetetramine, and stirred in a dispersed state. The mixture was heated to 90 ° C. to polymerize the monomer, and the graphite particles were mixed with the phenol resin. The mixture was filtered to obtain resin-mixed graphite particles. The mixed graphite particles had a resin content of 20% (residual carbon ratio: 10%). The temperature of the mixed graphite particles was raised to 270 ° C. in air over 5 hours, and the temperature was further maintained at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. The pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and the carbonization was further performed at 3,000 ° C. to obtain composite graphite particles of the present invention having a carbon material content of 10%.

Example 3
To a solution obtained by adding 6.7 g of coal-based pitch (softening point 105 ° C., residual carbon ratio 60%) and 15 g of a phenol resin (residual carbon ratio 40%) to a mixture of tar gas oil 500 g and hexamethylenetetramine 1.5 g. Then, 90 g of graphite particles (average particle size: 20 μm, aspect ratio: 2.0) were added and stirred in a dispersed state. Next, the tar gas oil as a solvent was distilled off at 150 ° C. under reduced pressure to obtain pitch / resin mixed graphite particles. The temperature of the mixed graphite particles was raised to 270 ° C. in air over 5 hours, and the temperature was further maintained at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. Next, a pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and further carbonization was performed at 3,000 ° C. to obtain composite graphite particles of the present invention having a carbon material content of 10%.

Example 4
Graphite particles in a solution obtained by mixing 20 g of phenol monomer, 33 g of 37% formalin, 2 g of hexamethylenetetramine, and 7.5 g of coal-based mesophase pitch fine powder (average particle size: 4 μm, softening point: 350 ° C., remaining carbon ratio: 80%) 110 g (average particle size: 20 μm, aspect ratio: 2.0) was added, and the mixture was stirred in a dispersed state. The above components were polymerized at 90 ° C. and mixed with graphite particles. Then, the mixed graphite particles were taken out by filtration. The graphite particles were mixed with the pitch composite resin (18% as a pitch composite resin component and 10% as a residual carbon ratio). The resin-mixed graphite particles were heated in air to 270 ° C. over 5 hours, and were further kept at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. Next, a pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and further carbonization was performed at 3,000 ° C. to obtain composite graphite particles of the present invention having a carbon material content of 10%.

Example 5
A phenol monomer was blended in Example 2 so that the carbon material content of the composite graphite particles was 5%, and carbonization was performed at 1,300 ° C instead of carbonization at 3,000 ° C. In the same manner as in Example 2, composite graphite particles were produced, and composite graphite particles having a carbon material content of 5% were obtained.

Example 6
In Example 1, composite graphite particles were produced in the same manner as in Example 1 except that granulated graphite B obtained by applying a mechanical external force to granulated graphite B was used as the graphite particles. The composite graphite particles having the content of the carbon material were obtained.

Comparative Example 1
1 g of a phenol resin (residual carbon ratio: 40%) is added to a mixture of 500 g of ethylene glycol and 0.1 g of hexamethylenetetramine, and 100 g of graphite particles (average particle size: 20 μm, aspect ratio: 2.0) are added to the solution, and dispersed. It was stirred in the state. The solvent was distilled off at 150 ° C. under reduced pressure to obtain resin-mixed graphite particles. The temperature of the mixed graphite particles was raised to 270 ° C. in air over 5 hours, and the temperature was further maintained at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. Pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and further carbonization was performed at 3,000 ° C. to obtain composite graphite particles of a comparative example having a carbon material content of 0.40%.

Comparative Example 2
100 g of graphite particles (average particle size: 20 μm, aspect ratio: 2.0) were added to a solution composed of a mixture of 60 g of a phenol resin (residual carbon ratio: 40%), 500 g of tar gas oil and 6 g of hexamethylenetetramine, and stirred in a dispersed state. did. The solvent was distilled off at 150 ° C. under reduced pressure to obtain resin-mixed graphite particles. The temperature of the mixed graphite particles was raised to 270 ° C. in air over 5 hours, and the temperature was further maintained at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. Pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and further carbonization was performed at 3,000 ° C., to obtain composite graphite particles of a comparative example having a carbon material content of 24%.

Comparative Example 3
A solution obtained by mixing 0.6 g of phenol resin (residual carbon ratio: 40%), 0.4 g of coal-based pitch (softening point: 105 ° C., residual carbon ratio: 60%), 500 g of tar gas oil, and 0.1 g of hexamethylenetetramine Then, 100 g of graphite particles (average particle size: 20 μm, aspect ratio: 2.0) were added and stirred in a dispersed state. Next, the tar gas oil as a solvent was distilled off at 150 ° C. under reduced pressure to obtain mixed graphite particles. The resin-mixed graphite particles were heated in air to 270 ° C. over 5 hours, and were further kept at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. Next, a pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and carbonization was further performed at 3,000 ° C. to obtain a composite graphite particle of a comparative example having a carbon material content of 0.48%. .

Comparative Example 4
Graphite particles (average particle diameter: 20 μm, aspect ratio: 30 g of phenolic resin (residual carbon ratio: 40%), 20 g of coal-based pitch (softening point: 105 ° C., residual carbon ratio: 60%), 500 g of tar gas oil, and 6 g of hexamethylenetetramine The mixture was stirred in a dispersed state. Next, tar gas oil as a solvent was distilled off at 150 ° C. under reduced pressure to obtain resin-mixed graphite particles. The resin-mixed graphite particles were heated in air to 270 ° C. over 5 hours, and were further kept at 270 ° C. for 2 hours and heated. This was crushed to be below a 75 μm sieve. Next, a pre-carbonization treatment was performed at 1,000 ° C. in a nitrogen atmosphere, and further a carbonization was performed at 3,000 ° C., thereby obtaining composite graphite particles of a comparative example having a carbon material content of 24%.

Comparative Example 5
In Example 1, graphite particles of a comparative example were obtained by performing only the same pre-carbonization treatment and carbonization as in Example 1 without using a carbonizable material.

Comparative Example 6
16.7 g of coal-based pitch (softening point: 105 ° C., residual carbon ratio: 60%) is dissolved in 500 g of tar gas oil, and 90 g of graphite particles (average particle size: 20 μm, aspect ratio: 2.0) are added to the solution and stirred in a dispersed state. did. Next, the tar gas oil as a solvent was distilled off at 150 ° C. under reduced pressure to obtain pitch-mixed graphite. The mixed graphite particles were subjected to a pre-carbonization treatment at 1,000 ° C. in a nitrogen atmosphere and pulverized so as to be under a 75 μm sieve. Further, by performing carbonization at 3,000 ° C., composite graphite particles of a comparative example corresponding to the prior art having a carbon material content of 10% were obtained.

Evaluation example 1
Using the composite graphite particles or the graphite particles of the above Examples and Comparative Examples, button-type secondary batteries for evaluation having the configuration shown in FIG. 1 were prepared and evaluated as described below. However, an actual battery can be manufactured according to a known method based on the concept of the present invention. In the battery for evaluation, the working electrode was expressed as a negative electrode, and the counter electrode was expressed as a positive electrode.

<Preparation of negative electrode mixture paste>
For 98% of the composite graphite particles, 1% of styrene-butadiene rubber and 1% of carboxymethylcellulose were added to water at a ratio of 1% to water as a binder to prepare a negative electrode mixture paste. This paste was used using the composite graphite particles of each of the examples and comparative examples (note that the paste of Comparative Example 5 was not a composite graphite particle).

<Production of negative electrode>
Each of the above negative electrode mixture pastes was applied on a copper foil (current collector) in a uniform thickness, and further heated to 90 ° C. in a vacuum to evaporate the solvent and dried. Next, the negative electrode mixture applied on the copper foil was pressed by a roller press, and further punched out together with the copper foil into a circular shape having a diameter of 15.5 mm, whereby a current collector 7b made of copper foil (having a thickness of 16 μm ) To produce a negative electrode 2 composed of a negative electrode mixture layer (50 μm thick) adhered to the negative electrode mixture layer.

<Manufacture of positive electrode>
The lithium metal foil is pressed against a nickel net and punched out into a cylindrical shape having a diameter of 15.5 mm, and a current collector 7a made of a nickel net and a positive electrode 4 made of a lithium metal foil (thickness 0.5 μm) adhered to the current collector. Was manufactured.

<Electrolyte>
LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate 33 mol% and ethyl methyl carbonate 67 mol% at a concentration of 1 mol / dm ³ to prepare a non-aqueous electrolyte. The obtained non-aqueous electrolyte was impregnated into a porous polypropylene body (thickness: 20 μm) to produce a separator 5 impregnated with the electrolyte.

<Production of battery for evaluation>
A button secondary battery having the structure shown in FIG. 1 was produced as an evaluation battery. First, a separator 5 impregnated with an electrolyte solution is laminated between a negative electrode (working electrode) 2 closely attached to the current collector 7b and a positive electrode (counter electrode) 4 closely adhered to the current collector 7a. Thereafter, the outer cup 1 and the outer can 3 are combined so that the negative electrode current collector 7b is accommodated in the outer cup 1 and the positive electrode current collector 7a is accommodated in the outer can 3. At that time, an insulating gasket 6 was interposed between the outer edges of the outer cup 1 and the outer can 3, and both outer edges were caulked to seal. The following charge / discharge test was performed on the evaluation battery manufactured as described above at a temperature of 25 ° C.

<Charging / discharging test>
Constant current charging is performed at a current value of 0.9 mA until the circuit voltage reaches 0 mV. Next, when the circuit voltage reached 0 mV, the mode was switched to constant voltage charging, charging was continued until the current value reached 20 μA, and the charging capacity was determined from the amount of current during that time. Thereafter, the operation was stopped for 120 minutes. Next, constant current discharge was performed at a current value of 0.9 mA until the circuit voltage reached 2.5 V, and the discharge capacity was determined from the amount of current supplied during this time. This was the first cycle. The initial charge / discharge efficiency was calculated from the following equation.

なお、この試験では、リチウムイオンを複合黒鉛粒子中にドープする過程を充電、複合黒鉛粒子から脱ドープする過程を放電とした。

In this test, the process of doping lithium ions into the composite graphite particles was defined as charging, and the process of undoping lithium ions from the composite graphite particles was defined as discharging.

Next, as a second cycle, after charging in the same manner as in the first cycle, constant current discharge is performed at a current value of 20 mA until the circuit voltage reaches 2.5 V, and the discharge capacity in the second cycle is determined based on the amount of current during this time. I asked. At this time, the rapid discharge efficiency was evaluated from the discharge capacity in the first cycle and the discharge capacity in the second cycle according to the following equation.

In addition to these evaluation tests, constant current charging is performed at a current value of 6 mA until the circuit voltage reaches 0 mV. Next, when the circuit voltage reaches 0 mV, switching to constant voltage charging is performed, and charging is continued until the current value reaches 20 μA. Thereafter, the operation was stopped for 120 minutes. Next, constant current discharge was performed at a current value of 6 mA until the circuit voltage reached 2.5 V. This charge / discharge was repeated for 20 cycles, and the discharge capacity in the 20th cycle was determined. The discharge capacity at the 1st cycle and the 20th cycle was obtained, and the cycle characteristics were calculated from the following equation.

Table 2 shows the characteristics of the composite graphite particles or the graphite particles of the examples and the comparative examples, and the battery characteristics of the battery for evaluation using these as a negative electrode material.
In Examples 1 to 6 of the present invention, the graphite particles have a carbon material set to an appropriate R value. It can be seen that although the discharge capacity is slightly reduced as compared with Comparative Example 5 having no carbon material, a high discharge capacity is maintained, and the initial charge / discharge efficiency, rapid discharge efficiency, and cycle characteristics are excellent. In particular, in Examples 2 and 4 in which a phenolic resin monomer was used as a resin material, rapid discharge efficiency and cycle characteristics were excellent.

  On the other hand, in Comparative Example 5 having no carbon material and Comparative Examples 1 and 3 in which the content of the carbon material is less than the range of the present invention, the initial charge / discharge efficiency, rapid discharge efficiency, and cycle characteristics are remarkably low. Conversely, in Comparative Examples 2 and 4 in which the content of the carbon material was larger than the range of the present invention, the carbon material was peeled off due to the crushing of the composite graphite particles fused during mixing, and the initial charge / discharge efficiency and the like were improved. Poor effect. Further, the discharge capacity is significantly reduced. Peeling of the carbon material can also be confirmed from an increase in the specific surface area.

  In the case of Comparative Example 6, which corresponds to the prior art in which no resin material is used for the carbon material, the crystallinity of the carbon material becomes too high, the R value decreases, and the initial charge / discharge efficiency decreases.

The measurements of the various characteristics in Tables 2-1 to 3 were performed as follows.
-Ratio of carbon material (%): The same thermal history as that of the composite graphite particles was imparted to the raw material of the carbon material to prepare a carbide of the carbon material alone, and the residual carbon ratio of the raw material was determined. The ratio of the carbon material in the composite graphite particles was calculated by conversion from the residual carbon ratio.
-Coverage: A composite graphite particle was polished to obtain a cross section, and the particle surface was observed at 1,000 times using a polarizing microscope. The exposed portion of a foreign carbon material different from the inner graphite present in the outermost layer is visually measured for 10 of the arbitrary particles, and the average of the ratio of the length of the carbon material coating portion to the contour length of the composite graphite particle is measured. The value was determined.
Specific surface area (m ² / g): same as in Table 1 above.
Average particle diameter: the same as in Table 1 above.
R value: the same as in Table 1 above.
_D002 : Same as in Table 1 above.
-Aspect ratio: Same as Table 1 above.

Evaluation example 2 (press pressure dependency)
A graphite electrode was prepared as described above using the composite graphite particles or graphite particles of Examples 1, 3, and 4 and Comparative Examples 3, 4, and 5, and the press pressure dependence of the electrode density was examined. The result shown in FIG. 2 was obtained. As shown in FIG. 2, the composite graphite particles or the graphite particles shown in Comparative Example 3 in which the content of the carbon material is low and Comparative Example 5 in which no carbonizable material is used are not completely spherical. The flake graphite is mechanically granulated into a round shape. However, the electrode density increased rapidly with respect to the pressing pressure in the process of manufacturing the negative electrode. This indicates that when the electrode is press-molded over several meters, the density tends to be non-uniform due to slight pressure fluctuation. On the other hand, the composite graphite particles using the carbonizable material composed of an appropriate amount of resin / pitch obtained in Examples 3 and 4 have reduced press pressure dependence of the density in any case, and have a constant electrode. It is understood that the material is easy to control the density. Further, Comparative Example 4, which has a large carbon material content, has low press pressure dependency, but cannot satisfy the battery characteristics as shown in Evaluation Example 1 described above.

  According to the present invention, graphite particles having high load characteristics and high cycle characteristics as a negative electrode material for a lithium ion secondary battery, and having a high discharge capacity and a low irreversible capacity can be provided. Further, according to the present invention, by using a mixture of a resin and a pitch as a covering material, an essentially high-density electrode can be obtained while maintaining high discharge capacity and rate characteristics (rapid discharge efficiency). In addition, it is possible to easily produce an electrode having a uniform density with little dependency on the pressing pressure at the time of producing the electrode.

  Further, according to the method of the present invention, a carbon material having a composite structure can be produced with good productivity and at low cost. Moreover, according to this method, composite graphite particles suitable as a negative electrode material of a lithium ion secondary battery, which could not be produced conventionally, can be produced. A lithium ion secondary battery using this as a negative electrode material can highly achieve both high initial charge / discharge efficiency and large discharge capacity, which have been difficult to achieve conventionally. Furthermore, it has excellent rapid discharge characteristics and cycle characteristics. Therefore, the composite graphite particles of the present invention can satisfy the recent demand for higher density of battery energy. Further, the device on which the negative electrode material and the lithium secondary battery of the present invention are mounted can be reduced in size and improved in performance, and can widely contribute to society.

FIG. 4 illustrates a cross section of a button secondary battery for evaluation. The electrode density-press pressure correlation diagram.

Explanation of reference numerals

1: External cup 2: Negative electrode (working electrode)
3: Outer can 4: Positive electrode (counter electrode)
5: separator 6: insulating gasket 7a: current collector 7b: current collector

Claims (28)

  Spheroidal graphite particles obtained by granulating flaky graphite by mechanical external force become 0.50 to 20% by mass of the composite graphite particles after carbonization, and are obtained by heating and carbonizing a resin alone or a mixture of a resin and a pitch. Composite graphite particles characterized by being coated with a material.   The composite graphite particles according to claim 1, wherein the resin is a phenol resin, a precursor of the phenol resin, or a monomer mixture of the phenol resin.   The composite graphite particles according to claim 1, wherein the pitch is a petroleum-based or coal-based pitch.   The composite graphite particles according to claim 1, wherein the mixing ratio of the resin and the pitch is such that the resin / pitch is 5/95 to 100/0 (mass ratio). The granulated spheroidal graphite particles have an average particle diameter of 5 to 60 μm, an aspect ratio of 3 or less, a specific surface area of 0.5 to 10 m ² / g, and a measured Lc value of X-ray diffraction of 40 nm or more, La Is 40 nm or more and d ₀₀₂ is 0.337 nm or less, and the ratio I ₁₃₆₀ / I ₁₅₈₀ (I ₁₃₆₀ ) of ₁₃₆₀ cm ⁻¹ peak intensity (I ₁₃₆₀ ) and ₁₅₈₀ cm ⁻¹ peak intensity (I ₁₅₈₀ ) measured by Raman spectroscopy using an argon laser. The composite graphite particle according to claim 1, comprising a highly crystalline artificial or natural graphite having an R value of 0.06 to 0.30 and a half width of 1580 cm- ¹ band of 10 to 60. When the carbon material has a ratio I ₁₃₆₀ / I ₁₅₈₀ (R value) of ₁₃₆₀ cm ⁻¹ peak intensity (I ₁₃₆₀ ) and ₁₅₈₀ cm ⁻¹ peak intensity (I ₁₅₈₀ ) measured by Raman spectroscopy using an argon laser of 0.05 to 50%. The composite graphite particle according to claim 1, wherein the particle diameter is 0.40.   The spheroidal graphite is granulated and spheroidized by mechanical external force, and the spherical graphite particles are coated with a resin alone or a mixture of a resin and a pitch, and the coated particles are heat-treated to carbonize the coating layer. The method for producing composite graphite particles according to any one of 1 to 6.   The method according to claim 7, wherein the spherical graphite particles are coated with a resin alone or a mixture of a resin and a pitch, then heat-treated at 200 to 300 ° C in the air or in an inert atmosphere, and then calcined at 2,000 to 3,200 ° C. A method for producing the composite graphite particles according to the above.   A negative electrode material for a lithium ion secondary battery, comprising the composite graphite particles according to claim 1.   A lithium ion secondary battery, wherein the composite graphite particles according to any one of claims 1 to 6 are used as a negative electrode material.   In a method for producing composite graphite particles, in which a carbonizable material containing a resin material is mixed with graphite and the resulting mixture is carbonized at 2,000 to 3,200 ° C., after carbonization, 0.50 of the composite graphite particles is produced. A method for producing composite graphite particles, wherein a carbonizable material is mixed so that about 20% by mass of the carbonizable material is carbonized by heating and carbonizing the carbonizable material.   The method for producing composite graphite particles according to claim 11, wherein the carbonizable material is a mixture of a resin material and tars, and the ratio of resin material / tars is 5/95 to 100/0 (mass ratio).   The method for producing composite graphite particles according to claim 11 or 12, wherein the resin material is at least one selected from the group consisting of resin raw materials and resin precursors.   The method for producing composite graphite particles according to claim 11 or 12, wherein the resin material is at least one selected from the group consisting of a thermosetting resin raw material and a thermosetting resin precursor.   The method for producing composite graphite particles according to claim 14, wherein the thermosetting resin is a phenol resin.   The method for producing composite graphite particles according to any one of claims 11 to 15, wherein the graphite is obtained by granulating flaky graphite.   17. The graphite according to claim 11, wherein a carbonizable material containing a resin material is mixed with the graphite, and the mixture is heated at 200 to 300 ° C. and carbonized before carbonizing the obtained mixture. Of producing composite graphite particles.   Composite graphite particles comprising a carbon material obtained by heating and carbonizing a carbonizable material containing a resin material in an amount of 0.50 to 20% by mass of the composite graphite particles after carbonization on at least a surface portion of the graphite. A Raman spectrum of a composite graphite particle having a carbon material on at least a surface portion of graphite having a plane spacing d ₀₀₂ of less than 0.337 nm in X-ray diffraction, wherein the composite graphite particle has an aspect ratio of 3 or less. The ratio (I ₁₅₈₀ ) / (I ₁₃₆₀ ) of the peak intensity (I ₁₃₆₀ ) at ₁₃₆₀ cm ^{−1 to} the peak intensity (I ₁₅₈₀ ) at ₁₅₈₀ cm ⁻¹ is 0.1 or more and less than 0.3 at 20. Composite graphite particles. 20. The composite graphite particles according to claim 19, wherein the plane distance d _{002 of the} carbon material in X-ray diffraction is less than 0.343 nm, and the ratio of the plane distance d ₀₀₂ of the graphite to the plane distance d ₀₀₂ is from 1.001 or more to less than 1.02.   The composite according to any one of claims 18 to 20, wherein the carbonizable material is a mixture of a resin material and tars, and the ratio of resin material / tars is 5/95 to 100/0 (mass ratio). Graphite particles.   The composite graphite particles according to any one of claims 18 to 21, wherein the resin material is at least one selected from the group consisting of a resin raw material and a resin precursor.   The composite graphite particles according to any one of claims 18 to 21, wherein the resin material is at least one selected from the group consisting of a raw material of a thermosetting resin and a precursor of the thermosetting resin.   The composite graphite particles according to claim 23, wherein the thermosetting resin is a phenol resin.   The composite graphite particles according to any one of claims 18 to 24, wherein the graphite is obtained by granulating flaky graphite.   A negative electrode material for a lithium ion secondary battery, comprising the composite graphite particles according to any one of claims 18 to 25.   A negative electrode comprising the negative electrode material for a lithium ion secondary battery according to claim 26.   A lithium ion secondary battery comprising the negative electrode according to claim 27.

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