Graphite material and method of producing the same, anode for lithium-ion secondary battery, and lithium-ion secondary battery

Abstract

To provide a graphite material exhibiting high initial charge / discharge efficiency and excellent input / output characteristics at a low temperature, when used as an anode of a lithium-ion secondary battery.SOLUTION: The graphite material has a volume-based mean particle diameter of 15.0 μm or smaller, a pore volume in a pore size range of 0.002 to 2 μm determined by a mercury intrusion method of greater than 0.100 ml / g, and a pore volume in a pore size range of 2 to 10 nm determined by a nitrogen adsorption method of 0.0030 ml / g or below.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a graphite material and a method for producing the same, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery. [Background technology]

[0002] Among various secondary batteries, lithium-ion batteries are known to have high volumetric and weight energy density and operating voltage, as well as good cycle characteristics and large current charge / discharge performance, and are used in a wide range of fields, including portable devices and electric tools. Graphite-based materials are the most widely used negative electrode materials for lithium-ion secondary batteries. Graphite-based materials are broadly divided into natural graphite and artificial graphite. Petroleum-based or coal-based coke is the main source of artificial graphite. Coke-based graphite is produced by crushing coke raw materials and graphitizing them as is, or by combining granulation and surface modification. On the other hand, natural graphite is produced by crushing flakes and combining granulation, spheroidization, and coating and sintering. In recent years, the demand for lithium-ion batteries with higher capacity and higher initial charge / discharge efficiency has been increasing due to the extension of the driving range of electric vehicles. On the other hand, if the negative electrode is made denser to increase the capacity, the voids in the negative electrode mixture layer disappear, making it difficult for the electrolyte to penetrate. Especially in cold regions, the electrolyte does not penetrate sufficiently into the negative electrode mixture layer, and the input / output characteristics tend to deteriorate. To solve this problem, for example, Patent Document 1 discloses natural graphite that is spheroidized by mechanically treating flake natural graphite. The spheroidization treatment of the flake natural graphite increases the pore volume in the pore diameter range of 2 to 4 nm and promotes the permeation of the electrolyte, thereby improving the low-temperature input / output characteristics. Patent Document 2, like Patent Document 1, shows spheroidized natural graphite obtained by mechanically treating flake natural graphite. By appropriately controlling the volume-based average particle size, and the pore volume and mode pore size in the pore size range of 0.01 to 1 μm or less, a moderately dense intra-particle void structure is formed, allowing the electrolyte to spread smoothly and efficiently inside the particles, improving low-temperature input / output characteristics. Patent Document 3 shows natural graphite obtained by subjecting spherical natural graphite to isotropic pressure treatment. The compression treatment reduces the volume of pores with diameters in the range of 2 to 2000 nm, preventing the organic binder used in the production of the negative electrode from entering the pores, and ensures electrical conductivity, thereby improving rapid charging characteristics. Patent Documents 4 and 5 show artificial graphite produced by crushing and classifying coke, and graphitizing it while blowing air into it. By oxidizing the surface of the coke in the graphitization process, the pore volume of pores with diameters of 0.4 μm or less is controlled within a certain range, improving high-speed charge / discharge performance. Patent Document 6 shows a negative electrode active material in which graphite powder A is graphitized by kneading and molding flake natural graphite and / or artificial graphite with a carbonaceous precursor, and graphite powder B is graphitized by coating spherical natural graphite with pitch and firing the mixture in an arbitrary ratio. By achieving both high density and electrolyte permeability, irreversible reactions are suppressed and cycle characteristics are improved. Patent Documents 7 and 8 show artificial graphite that is graphitized by kneading and molding coke powder, a carbonaceous precursor, and a graphitization catalyst. The graphitization catalyst produces large voids within the particles, and the pore volume of pores with diameters in the range of 0.10 to 8.0 μm is increased, improving high-load characteristics while achieving both high density and electrolyte permeability. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-186916 A [Patent Document 2] JP 2016-178074 A [Patent Document 3] JP 2015-181116 A [Patent Document 4] International Publication No. 2015 / 182560 [Patent Document 5] International Publication No. 2016 / 129557 [Patent Document 6] JP 2010-92649 A [Patent Document 7] International Publication No. 2021 / 044482 [Patent Document 8] International Publication No. 2015 / 147012 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in Patent Documents 1 and 2, while the pore volume is large and the electrolyte can easily penetrate, there is also an increase in a side reaction in which the electrolyte is decomposed on the edge surface of highly reactive graphite, which raises concerns about a decrease in the initial charge / discharge efficiency and gas generation. In Patent Document 3, the pressure treatment removes the voids within the particles, making it difficult for the electrolyte to penetrate into the particles, and the Li-ion insertion and desorption sites within the particles cannot be efficiently utilized, which may result in insufficient low-temperature input / output characteristics. In Patent Documents 4 and 5, the pore volume is increased by the oxidation reaction of the coke, but the pore volume is smaller than that of the natural graphite shown in Patent Documents 1 and 2, and therefore the pore volume is equivalent to that of natural graphite that has been subjected to pressure treatment as shown in Patent Document 3, and there is a possibility that the low-temperature input / output characteristics are insufficient. In Patent Document 6, the composite particles of coke powder and scaly natural graphite have a large average particle size and a large BET specific surface area. Because of this, the nanopore volume (pore volume with a pore diameter of 2 to 10 nm) in the particles is also large, which may cause side reactions in the electrolyte to proceed and reduce the initial charge / discharge efficiency. In Patent Documents 7 and 8, the graphitization catalyst attached to the inside of the particles volatilizes due to graphitization, generating a large number of nanopores (pores with a pore diameter of 2 to 10 nm) inside the particles. Therefore, the initial charge / discharge efficiency may decrease due to the progression of side reactions of the electrolyte in these pores.

[0005] The present invention has been made in view of the above background art, and an object of the present invention is to provide a graphite material that exhibits high initial charge-discharge efficiency and excellent low-temperature input-output characteristics when used in a negative electrode of a lithium-ion secondary battery. [Means for solving the problem]

[0006] As a result of extensive investigations, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0007] (1) The volume-based average particle size is 15.0 μm or less, The pore volume in the range of pore diameters of 0.002 to 2 μm as determined by mercury intrusion porosimetry is more than 0.100 ml / g; A graphite material having a pore volume of pores in the range of pore diameters of 2 to 10 nm, as determined by a nitrogen gas adsorption method, of 0.0030 ml / g or less. (2) The graphite material according to (1) above, having a pore volume in the pore diameter range of 2 to 4 nm, as determined by a nitrogen gas adsorption method, of 0.0022 ml / g or less. (3) The graphite material according to (1) or (2) above, in which the volume of pores within a pore diameter range of 0.01 to 1 μm, as determined by mercury intrusion porosimetry, is 0.07 ml / g or less. (4) the Raman R value is 0.010 to 0.060; The average spacing (d 002 The graphite material according to any one of the above (1) to (3), wherein the average particle diameter (nm) of the graphite particles is 0.3360 nm or less. (5) Specific surface area is 1.0 to 4.0 m 2 The graphite material according to any one of the above (1) to (4), wherein: (6) Tap density is 0.80 to 1.20 g / cm 3 The graphite material according to any one of the above (1) to (5), (7) The graphite material according to any one of (1) to (6) above, which has a graphite coating on at least a part of the surface of a granule of coke graphitized material and natural graphite. (8) A method for producing the graphite material according to any one of (1) to (7) above, comprising the steps of: A granulation step of mixing and granulating the coke, natural graphite and graphitic precursor; a graphitization step of heating the granules obtained in the granulation step to convert the coke into a coke graphitized material and convert the graphitic precursor into graphite. (9) A method for producing the graphite material according to any one of (1) to (7) above, comprising the steps of: A granulation step of mixing coke, natural graphite and a graphite precursor and granulating the mixture to obtain a granulated product having a coating of the graphite precursor on at least a part of a surface of the granulated product of the coke and the natural graphite; and a graphitization step of heating the granules obtained in the granulation step to convert the coke into a coke graphitized product and convert the graphitic precursor into graphite, thereby obtaining a graphite material having a graphitic coating on at least a portion of a surface of the granules of the coke graphitized product and natural graphite. (10) The method for producing a graphite material according to (8) or (9) above, wherein the coke is coal coke. (11) A negative electrode for a lithium ion secondary battery, comprising the graphite material according to any one of (1) to (7) above. (12) A lithium ion secondary battery having the negative electrode according to (11) above. Effect of the Invention

[0008] According to the present invention, it is possible to provide a graphite material that exhibits high initial charge / discharge efficiency and excellent low-temperature input / output characteristics when used in the negative electrode of a lithium ion secondary battery. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view showing a button-type secondary battery. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The graphite material and its manufacturing method, the negative electrode for a lithium ion secondary battery, and the lithium ion secondary battery of the present invention will be described below. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In addition, each component may be used alone or in combination of two or more. When two or more components are used in combination, the content of the components refers to the total content unless otherwise specified. In addition, when the material is used in the negative electrode of a lithium ion secondary battery, it is simply said that the present invention has "excellent effects" when it exhibits high initial charge / discharge efficiency and excellent low-temperature input / output characteristics.

[0011] [1] Graphite material The graphite material of the present invention (hereinafter also referred to as "the present graphite material") is a graphite material having a volume-based average particle size of 15.0 μm or less, a pore volume of pores with diameters of 0.002 to 2 μm determined by mercury intrusion porosimetry exceeding 0.100 ml / g, and a pore volume of pores with diameters of 2 to 10 nm determined by nitrogen gas adsorption being 0.0030 ml / g or less. By using the present graphite material in a negative electrode (negative electrode) for a lithium ion secondary battery, a lithium ion secondary battery having high initial charge / discharge efficiency and excellent low-temperature input / output characteristics can be obtained.

[0012] [Physical properties of graphite materials]

[0013] [Volume-based average particle size] The volume-based average particle size of the graphite material is 15.0 μm or less, preferably 14.5 μm or less, more preferably 5.0 to 14.5 μm, even more preferably 8.0 to 14.5 μm, and most preferably 9.0 to 14.5 μm. If the volume-based average particle size exceeds 15.0 μm, the diffusion resistance of lithium ions within the graphite particles increases, tending to deteriorate the input / output characteristics. In this specification, the volume-based average particle size is measured using a laser particle size distribution analyzer (Seishin Enterprise Co., Ltd., LMS-2000e) under conditions where ion-exchanged water is used as the dispersion medium and the amount of sample liquid is 40 mL.

[0014] [Pore volume in the pore diameter range of 0.002μm to 2μm determined by mercury intrusion porosimetry] The pore volume of the graphite material having a pore diameter in the range of 0.002 μm to 2 μm, as determined by mercury intrusion porosimetry, exceeds 0.100 ml / g. A preferred lower limit is 0.110 ml / g, more preferably 0.120 ml, and even more preferably 0.130 ml / g. Within the above range, the electrolyte can easily penetrate into the particles, which tends to be effective in improving low-temperature input / output characteristics. The upper limit of the pore volume in the range of pore diameters of 0.002 μm to 2 μm determined by mercury intrusion porosimetry is not particularly limited. However, if the pore volume in the above range is too large, the pore volume in the range of pore diameters of 2 nm to 10 nm determined by nitrogen gas adsorption method exceeds 0.0030 ml / g, and the initial charge / discharge efficiency may decrease. Therefore, the upper limit of the pore volume in the range of pore diameters of 0.002 μm to 2 μm of the graphite material is preferably 1.000 ml / g, more preferably 0.800 ml / g, even more preferably 0.600 ml / g, and particularly preferably 0.500 ml / g. The pore volume can be determined by pore size distribution measurement using mercury intrusion porosimetry (for example, AutoPoreIV 9500 manufactured by Shimadzu Corporation).

[0015] [Pore volume in the pore diameter range of 2 nm to 10 nm determined by nitrogen gas adsorption method] The pore volume of the graphite material in the pore diameter range of 2 nm to 10 nm, determined by nitrogen gas adsorption, is 0.0030 ml / g or less, preferably 0.0005 to 0.0030 ml / g, more preferably 0.0010 to 0.0029 ml / g, and even more preferably 0.0015 to 0.0028 ml / g. Within the above range, side reactions with the electrolyte are suppressed, and this tends to be effective in improving storage characteristics and initial charge / discharge efficiency. The pore volume can be measured by nitrogen gas adsorption. An Autosorb IQ (Anton-Paar) can be used as the measuring device. The sample is sealed in a powder cell and pretreated at 300°C under vacuum (1.3 Pa or less) for 3 hours, after which the adsorption isotherm (adsorbed gas: nitrogen) is measured at liquid nitrogen temperature. The resulting adsorption isotherm is used to determine the micropore distribution by BJH analysis, from which the pore volume in the pore diameter range of 2 nm to 10 nm is calculated.

[0016] [Pore volume in the pore diameter range of 2 nm to 4 nm determined by nitrogen gas adsorption method] The pore volume of the graphite material having a pore diameter in the range of 2 to 4 nm, as determined by a nitrogen gas adsorption method, is preferably 0.0022 ml / g or less, more preferably 0.0005 to 0.0022 ml / g, and more preferably 0.0010 to 0.0020 cm 3 / g is more preferable, and 0.0007 to 0.0015 ml / g is particularly preferable. Within the above range, side reactions with the electrolyte are further suppressed, which tends to be effective in improving storage characteristics and initial charge / discharge efficiency. The pore volume of pores having a diameter in the range of 2 to 4 nm determined by nitrogen gas adsorption is calculated by the same measurement method as the pore volume of pores having a diameter in the range of 2 to 10 nm described above.

[0017] [Pore volume in the pore diameter range of 0.01 to 1 μm determined by mercury intrusion porosimetry] The pore volume of the graphite material having a pore diameter in the range of 0.01 μm to 1 μm, determined by mercury intrusion porosimetry, is preferably 0.07 ml / g or less, more preferably 0.01 to 0.07 ml / g, further preferably 0.02 to 0.07 ml / g, and particularly preferably 0.03 to 0.07 ml / g. The pore volume can be determined by pore size distribution measurement using mercury intrusion porosimetry (for example, AutoPoreIV 9500 manufactured by Shimadzu Corporation). Within the above range, side reactions with the electrolyte are further suppressed, which tends to be effective in improving storage characteristics and initial charge / discharge efficiency.

[0018] [Raman R value] The Raman R value of the graphite material is preferably 0.010 to 0.060, more preferably 0.010 to 0.050, and most preferably 0.020 to 0.040. If the Raman R value of the graphite material is too small, the initial charge / discharge efficiency will be high, and if the Raman R value is too large, the low-temperature input / output resistance will be high. The Raman R value of the graphite material is determined, for example, as follows. Using a Raman spectrometer, perform microscopic Raman analysis 100 times at a wavelength of 532 nm to obtain a Raman spectrum. -1 Intensity I of the peak in the region D and G band (1570-1630cm -1 Intensity I of the peak in the region G The ratio of the Raman R value (I D / I G ) is calculated as follows.

[0019] [d 002 〕 This graphite material has an average interplanar spacing (d 002 It is preferable that the value of the optical fiber is 0.3360 nm or less. The method for measuring the lattice constant and crystallite size of carbonaceous materials by X-ray diffraction is the so-called Gakushin method, which was established by the 117th Committee of the Japan Society for the Promotion of Science, and is also specified in JISR 7651:2007. Based on this measurement method, the average interplanar spacing of the (002) plane, d 002 Find the value. d 002 The d value is one of the indicators of the crystallinity of carbonaceous materials. 002 A large value indicates low crystallinity, and a small value indicates high crystallinity.

[0020] [Specific surface area] The specific surface area of ​​the graphite material is preferably 1.0 to 4.0 m 2 / g, more preferably 1.5 to 3.5 m 2 / g, most preferably 1.8 to 3.0 m 2 / g. If the specific surface area is too small, the initial discharge capacity will be small, and if the specific surface area is too large, the initial charge / discharge efficiency will be small. The specific surface area was measured by the BET single point method.

[0021] [Tap density] The tap density of the graphite material is preferably 0.80 to 1.20 g / cm 3, more preferably 0.85 to 1.10 g / cm 3 , and most preferably 0.88 to 1.05 g / cm 3 It is. If the tap density is too small, the initial discharge capacity per volume will be small, whereas if the tap density is too large, the low-temperature input / output resistance will be large. The tap density is measured in accordance with JIS Z 2512:2012 using a tap density measuring device (tapping device) under conditions of a tap stroke of 3 mm and 300 taps.

[0022] [Graphite material structure] The graphite material of the present invention preferably has a graphite coating on at least a part of the surface of a granule of the coke graphitized material and natural graphite. Examples of a method for producing the graphite material of the present invention in this embodiment include the production method described below. The following are particularly preferred: (Size) Volume-based average particle size of coke graphitized material: 5.0 to 15.0 μm, more preferably 6.0 to 14.0 μm, and even more preferably 7.0 to 13.0 μm Volume-based average particle size of natural graphite: 2.0 to 9.0 μm, more preferably 3.0 to 8.0 μm, and even more preferably 4.0 to 7.0 μm (if the volume-based average particle size is too large, the low-temperature input / output resistance increases) (Content) Coke graphitized material: 55.0 to 80.0% by mass, more preferably 57.0 to 75.0% by mass, and even more preferably 58.0 to 73.0% by mass. Natural graphite: 45.0 to 20.0% by mass, more preferably 43.0 to 20.0% by mass, and even more preferably 42.0 to 25.0% by mass, If the coke graphitized material content is too high (if the natural graphite content is too low), the initial discharge capacity will be small and the low-temperature input / output resistance will be large. Conversely, if the coke graphitized material content is too low (if the natural graphite content is too high), the initial charge / discharge efficiency will be small. Graphite coating: 0.5 to 15.0 parts by mass, more preferably 1.0 to 15.0 parts by mass, and even more preferably 2.0 to 14.5 parts by mass, per 100 parts by mass of the total amount of the coke graphitized material and the natural graphite. When the graphite material is produced by the production method described later, the content of the graphitic coating is determined from the residual carbon ratio when the graphitic precursor is heated under the same conditions as those for graphitization.

[0023] [2] Manufacturing method of graphite material The method for producing a graphite material according to the present embodiment (hereinafter also referred to as "the present production method") is a method for producing the present graphite material described above. In the present production method, coke, natural graphite, and a graphitic precursor are granulated to obtain a granulated material, and then the granulated material is graphitized to obtain a graphitized material, thereby producing the present graphite material.

[0024] [Coke] Examples of coke include coal coke and petroleum coke. Coal coke is a metallic glossy, gray-black porous solid obtained by carbonizing coal at high temperatures (about 1000 to 1100°C). Petroleum coke is a coke obtained by pyrolyzing heavy fractions of petroleum at high temperatures. The coke may be coke before calcination (green coke) or may be calcined coke (calcine coke). Calcination of the coke is carried out at a temperature of about 900 to 1500° C. using, for example, a rotary kiln. It is preferable to use coal coke (coal-based coke) because the negative electrode has high density and excellent battery characteristics, and it is more preferable to use coal coke before calcination. When using uncalcined coke (green coke) as a raw material, it may be dried in advance at, for example, 100 to 200°C. The raw coke (raw coke) may be pulverized before use. The volume-based average particle size of the pulverized product is preferably 5.0 to 15.0 μm, more preferably 6.0 to 14.0 μm, and even more preferably 7.0 to 13.0 μm. The pulverization method may be performed using a conventional pulverizer.

[0025] [Natural graphite] As the natural graphite, for example, flake natural graphite, flat natural graphite such as flake natural graphite, and spheroidized natural graphite obtained by subjecting flake or flat natural graphite to spheroidization treatment can be used. Since flake particles are easier to granulate than spherical particles, it is preferable to use flake or flat natural graphite. In addition, in terms of ease of granulation, it is preferable that the natural graphite is a fine powder. Specifically, the volume-based average particle size of the natural graphite is preferably 9.0 μm or less, more preferably 8.0 μm or less, and even more preferably 7.0 μm or less. In addition, the lower limit of the volume-based average particle size of the natural graphite is preferably 1.0 μm, more preferably 2.0 μm, and most preferably 3.0 μm. The natural graphite may be natural graphite before purification or may be natural graphite that has been highly purified. In the present invention, purification generally means an operation of dissolving and removing ash, metals, etc. contained in low-purity natural graphite by treating in an acid such as hydrochloric acid, sulfuric acid, nitric acid, or hydrofluoric acid, or by combining a plurality of acid treatment steps. Usually, after the acid treatment step, a water washing treatment or the like is performed to remove the acid used in the purification treatment step. Also, instead of the acid treatment step, ash, metals, etc. may be evaporated and removed by treatment at a high temperature of 2000°C or more. Also, ash, metals, etc. may be removed by treatment in a halogen gas atmosphere such as chlorine gas during high-temperature heat treatment. Furthermore, these methods may be used in any combination.

[0026] [Graphitic precursor] The graphitic precursor binds the coke and / or natural graphite. Examples of graphite precursors include tar pitches and / or resins.Specific examples of heavy oils, particularly tar pitches, include coal tar, light tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, heavy oil, etc.Examples of resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid; thermosetting resins such as phenol resins and furan resins; and the like. The amount of the graphite precursor added is preferably 1 to 20 parts by mass, more preferably 2 to 15 parts by mass, and even more preferably 3 to 12 parts by mass, per 100 parts by mass of the total amount of the coke and natural graphite.

[0027] [Granulation] As a granulation method, a method of applying a mechanical external force to the coke and natural graphite to which the graphite precursor has been added is preferable. As a granulation device, for example, a kneader such as a pressure kneader or a two-roll mill, a pulverizer such as a rotary ball mill, a counter jet mill (manufactured by Hosokawa Micron Corporation), or a current jet (manufactured by Nisshin Engineering Co., Ltd.), etc. can be used. The volume-based average particle size of the granulated product is preferably from 1.00 to 15.0 μm, more preferably from 3.00 to 14.5 μm, and even more preferably from 5.00 to 14.3 μm.

[0028] [Graphitization] Next, the granulated material is graphitized by heating it in a non-oxidizing atmosphere to obtain a graphitized material. The heating temperature during graphitization (graphitization temperature) is preferably 2500° C. or higher, and more preferably 2800° C. or higher. On the other hand, the graphitization temperature is preferably 4000° C. or lower, and more preferably 3500° C. or lower. If the graphitization temperature is within this range, the crystallinity of the graphitized material will be good. Through the graphitization, the coke becomes graphitized coke and the graphitic precursor becomes graphite.

[0029] [3] Negative electrode for lithium-ion secondary batteries (negative electrode) The negative electrode for a lithium ion secondary battery (negative electrode) of this embodiment contains the present graphite material described above. The negative electrode of this embodiment is produced in a manner similar to that of a normal negative electrode. When preparing the negative electrode, it is preferable to use a negative electrode mixture prepared in advance by adding a binder to the above-mentioned present graphite material. The negative electrode mixture may contain other active materials and conductive agents. As the binder, one that is chemically and electrochemically stable with respect to the electrolyte is preferred. For example, fluorine-based resins such as polytetrafluoroethylene and polyvinylidene fluoride; resins such as polyethylene, polyvinyl alcohol and styrene-butadiene rubber; carboxymethyl cellulose; and the like are used, and two or more of these can also be used in combination. The binder is usually used in an amount of about 1 to 20 mass % based on the total amount of the negative electrode mixture. More specifically, first, the above-mentioned graphite material is optionally adjusted to a desired particle size by classification or the like. Then, a binder is added and mixed, and the resulting mixture is dispersed in a solvent to prepare a paste-like negative electrode mixture. Examples of the solvent include water, isopropyl alcohol, N-methylpyrrolidone, and dimethylformamide. For mixing and dispersion, a known stirrer, mixer, kneader, kneader, etc. are used. The prepared paste is applied to one or both sides of a current collector and dried. In this way, a negative electrode mixture layer (negative electrode) that is uniformly and firmly adhered to the current collector is obtained. The thickness of the negative electrode mixture layer is preferably 10 to 200 μm, more preferably 20 to 100 μm. After the negative electrode mixture layer is formed, pressure bonding such as pressing can be performed to further increase the adhesive strength between the negative electrode mixture layer (negative electrode) and the current collector. The shape of the current collector is not particularly limited, but may be, for example, a foil, a mesh, or a net such as an expanded metal. The material of the current collector is preferably copper, stainless steel, nickel, etc. The thickness of the current collector is preferably about 5 to 20 μm in the case of a foil.

[0030] [4] Lithium-ion secondary battery The lithium ion secondary battery of this embodiment has the negative electrode of this embodiment. In addition to the negative electrode, the lithium ion secondary battery further includes a positive electrode, a non-aqueous electrolyte, etc. The lithium ion secondary battery is constructed, for example, by stacking the negative electrode, the non-aqueous electrolyte, and the positive electrode in this order and housing them in an exterior material of the battery. Lithium ion secondary batteries can be selected from various types, such as cylindrical, square, coin, button, and laminate types, depending on the application, the device in which they are installed, and the required charge and discharge capacity.

[0031] [Positive electrode] It is preferable to select a material for the positive electrode (positive electrode active material) that can absorb / desorb a sufficient amount of lithium. In addition to lithium, the positive electrode active material may be, for example, a lithium-containing compound such as a lithium-containing transition metal oxide, a transition metal chalcogenide, vanadium oxide, and a lithium compound thereof; X Mo6S 8-Y (wherein M is at least one transition metal element, X is a number in the range of 0≦X≦4, and Y is a number in the range of 0≦Y≦1), activated carbon, activated carbon fiber, etc. Examples of vanadium oxide include V2O5, V6O 13 , V2O4, and V3O8. 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 kinds. The lithium-containing transition metal oxide is specifically LiM 1 1-X M 2 X O2 (M in the formula 1 , M 2 is at least one transition metal element, and X is a number in the range of 0≦X≦1), or LiM 1 1-Y M 2 Y O4 (M in the formula 1 , M 2 is at least one transition metal element, and Y is a number in the range of 0≦Y≦1. M 1 , M 2 The transition metal element represented by is Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, etc., and preferably is Co, Fe, Mn, Ti, Cr, V, Al, etc. Preferred specific examples are LiCoO2 (LCO), LiNiO2, LiMnO2, LiNi0.9 Co 0.1 O2, LiNi 0.5 Co 0.5 O2, etc. The lithium-containing transition metal oxide can be obtained, for example, by using lithium, transition metal oxides, hydroxides, salts, etc. as starting materials, mixing these starting materials according to the composition of the desired metal oxide, and firing the mixture at a temperature of 600 to 1000°C in an oxygen atmosphere. The positive electrode active material may be one of the above compounds or two or more of them. For example, a carbonate such as lithium carbonate may be added to the positive electrode. When forming the positive electrode, various additives such as a conventional conductive agent and a binder may be appropriately used. The positive electrode is produced, for example, by applying a positive electrode mixture consisting of a positive electrode active material, a binder, and a conductive agent for imparting electrical conductivity to the positive electrode to both sides of a current collector to form a positive electrode mixture layer. As the binder, the binder used in the preparation of the negative electrode can be used. As the conductive agent, known conductive agents such as graphitized materials and carbon black are used. The shape of the current collector is not particularly limited, but examples include a foil shape, a mesh shape, etc. The material of the current collector is aluminum, stainless steel, nickel, etc. The thickness of the current collector is preferably 10 to 40 μm. In the preparation of the positive electrode, similarly to the negative electrode, a paste-like positive electrode mixture may be applied to a current collector, dried, and then pressure-bonded by pressing or the like.

[0032] [Non-aqueous electrolyte] The non-aqueous electrolyte may be a liquid non-aqueous electrolyte (non-aqueous electrolyte liquid), or may be a polymer electrolyte such as a solid electrolyte or a gel electrolyte. In the former case, the nonaqueous electrolyte battery is configured as a so-called lithium ion secondary battery, while in the latter case, the nonaqueous electrolyte battery is configured as a polymer electrolyte battery such as a polymer solid electrolyte battery or a polymer gel electrolyte battery. As the non-aqueous electrolyte, lithium salts such as LiPF6, LiBF4, LiAsF6, LiClO4, LiB(CH5), LiCl, LiBr, LiCF3SO3, LiCH3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiN(CF3CH2OSO2)2, LiN(CF3CF2OSO2)2, LiN(HCF2CF2CH2OSO2)2, LiN((CF3)2CHOSO2)2, LiB[{CH3(CF3)2}]4, LiAlCl4, LiSiF6, etc., which are electrolyte salts used in ordinary non-aqueous electrolyte solutions, are used. From the viewpoint of oxidation stability, LiPF6 and LiBF4 are preferred. The concentration of the electrolyte salt in the nonaqueous electrolyte solution is preferably from 0.1 to 5.0 mol / L, and more preferably from 0.5 to 3.0 mol / L. Examples of solvents for preparing the non-aqueous electrolyte solution include carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate; ethers such as 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, and diethyl ether; thioethers such as sulfolane and methylsulfolane; nitriles such as acetonitrile, chloronitrile, and propionitrile; and aprotic organic solvents such as trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, and dimethyl sulfite. When the non-aqueous electrolyte is a polymer electrolyte such as a solid electrolyte or a gel electrolyte, it is preferable to use a polymer gelled with a plasticizer (nonaqueous electrolyte liquid) as the matrix. Suitable polymers for forming the matrix include ether-based polymer compounds such as polyethylene oxide and its crosslinked products; poly(meth)acrylate-based polymer compounds; and fluorine-based polymer compounds such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer. The concentration of the electrolyte salt in the nonaqueous electrolyte solution, which is a plasticizer, is preferably 0.1 to 5.0 mol / L, and more preferably 0.5 to 2.0 mol / L. In the polymer electrolyte, the proportion of the plasticizer is preferably from 10 to 90% by mass, more preferably from 30 to 80% by mass.

[0033] [Separator] In lithium ion secondary batteries, a separator may also be used. The separator is not particularly limited in material, but may be, for example, a woven fabric, a nonwoven fabric, or a synthetic resin microporous film. Of these, a synthetic resin microporous film is preferred, and a polyolefin microporous film is more preferred in terms of thickness, film strength, and film resistance. Suitable examples of the polyolefin microporous film include a polyethylene microporous film, a polypropylene microporous film, and a composite microporous film of these. EXAMPLES

[0034] The present invention will be specifically described below with reference to examples, although the present invention is not limited to the examples described below.

[0035] [Example 1]

[0036] [Preparation of graphite material] The raw coal-based coke before calcination was coarsely crushed, and then dried using a kneader to remove moisture. The obtained dried coke was crushed to a volume-based average particle size of 12.0 μm. Then, the dried coke (coke), crushed flake natural graphite (volume-based average particle size: 6 μm), and petroleum pitch (pitch) were mixed to a ratio of coke: flake natural graphite: pitch = 70: 30: 14 (mass ratio), and the mixture was stirred at 630 ° C. under a nitrogen atmosphere to perform granulation, and a granulated product (a granulated product having a petroleum pitch coating on at least a part of the surface of the granulated product of coke and flake natural graphite) was obtained. The obtained granules were crushed, then sealed in a graphite crucible and graphitized at 3000°C. The graphitized product (graphite material) obtained was sieved. The volume-based average particle size of the graphitized product obtained (a graphite material having a graphitic coating on at least a part of the surface of a granulated product of the coke graphitized product and the scaly natural graphite) was 11.3 μm. The volume-based average particle size of the coke and flake natural graphite used as the raw materials was the same even after graphitization. The same was true for the other examples and comparative examples.

[0037] [Physical properties of graphite materials] For the obtained graphite material, the pore volume of pores having a diameter of 2 nm to 4 nm, the pore volume of pores having a diameter of 0.002 μm to 2 μm, etc. were determined by the above-mentioned methods. All the results are shown in Table 1 below.

[0038] [Preparation of negative electrode mixture paste] A negative electrode mixture paste was prepared by adding graphite material (96 parts by mass), carboxymethyl cellulose (1.2 parts by mass) as a binder, styrene butadiene rubber (1.8 parts by mass) as a binder, and carbon black (1.0 part by mass) as a conductive agent to water and stirring.

[0039] [Preparation of negative electrode for button-type secondary battery] The prepared negative electrode mixture paste was applied to the copper foil in a uniform thickness, and the negative electrode mixture was 10.00±0.25mg / cm 2The electrode sheet was prepared by evaporating the solvent at 110° C. in a vacuum and drying to form a negative electrode mixture layer. The negative electrode mixture layer was then pressed by a roll press to obtain a negative electrode having a density of 1.50 g / cm. 3 The copper foil and the negative electrode mixture layer were then punched out into a cylindrical shape with a diameter of 15.5 mm, thus producing a negative electrode in close contact with a current collector made of copper foil.

[0040] [Preparation of Positive Electrode for Button-Type Secondary Battery] The lithium metal foil was pressed against the nickel net and punched out into a circular shape with a diameter of 15.5 mm, thereby producing a positive electrode made of lithium metal foil (thickness: 0.5 mm) adhered to a current collector made of nickel net.

[0041] [Preparation of button-type secondary battery] In order to evaluate the battery performance of the negative electrode material alone, a button-type secondary battery as shown in Figure 1 was fabricated. Fig. 1 is a cross-sectional view of a button-type secondary battery. In the button-type secondary battery shown in Fig. 1, the peripheral portions of an exterior cup 1 and an exterior can 3 are crimped via an insulating gasket 6 to form a sealed structure. Inside the sealed structure, a current collector 7a, a positive electrode 4, a separator 5, a negative electrode 2, and a current collector 7b are layered in this order from the inner surface of the exterior can 3 to the inner surface of the exterior cup 1. The button-type secondary battery shown in FIG. 1 was fabricated as follows. First, a non-aqueous electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (33% by volume) and methyl ethyl carbonate (67% by volume). The obtained non-aqueous electrolyte solution was impregnated into a polypropylene porous body (thickness: 20 μm) to prepare a separator 5 impregnated with the non-aqueous electrolyte solution. Next, the prepared separator 5 was sandwiched and laminated between the negative electrode 2 adhered to the current collector 7b made of copper foil and the positive electrode 4 adhered to the current collector 7a made of nickel net. Thereafter, the current collector 7b and the negative electrode 2 were housed inside the exterior cup 1, the current collector 7a and the positive electrode 4 were housed inside the exterior can 3, and the exterior cup 1 and the exterior can 3 were joined together. Furthermore, the peripheral parts of the exterior cup 1 and the exterior can 3 were crimped and sealed with an insulating gasket 6 interposed therebetween. In this manner, a button-type secondary battery was produced.

[0042] [Charge and discharge test of button-type secondary batteries] A charge-discharge test was conducted on the button-type secondary battery thus fabricated. In this test, the process of absorbing lithium ions into the negative electrode was defined as charging, and the process of desorbing lithium ions from the negative electrode was defined as discharging. First, the battery was charged at a constant current of 0.9 mA until the circuit voltage reached 1 mV. When the circuit voltage reached 1 mV, the battery was switched to constant voltage charging, and charging was continued until the current reached 20 μA. The charge capacity (unit: mAh / g) was calculated from the amount of current flowing during this period. After that, a 10-minute break was allowed. Next, the battery was discharged at a constant current of 0.9 mA until the circuit voltage reached 1.5 V. The initial discharge capacity (unit: mAh / g) was calculated from the amount of current flowing during this period. This was designated the first cycle. Furthermore, the initial charge / discharge efficiency was calculated from the following formula (1). The results are shown in Table 1 below. The higher the initial charge / discharge efficiency, the better the initial charge / discharge efficiency can be evaluated. In practice, the initial charge / discharge efficiency is preferably 90% or more. Initial charge / discharge efficiency [%] = 100 × {discharge capacity of the first cycle (initial discharge capacity) / charge capacity of the first cycle (initial charge capacity)} (1)

[0043] [Preparation of Laminated Secondary Battery Negative Electrode] The prepared negative electrode mixture paste was applied to the copper foil in a uniform thickness, and the negative electrode mixture was 10.00±0.25mg / cm 2The electrode sheet was prepared by evaporating the solvent at 110°C in a vacuum and drying to form a negative electrode mixture layer. The solvent was evaporating at 110°C in a vacuum and drying to form a negative electrode mixture layer. Next, an electrode sheet was cut to 2.2 cm x 3.2 cm. The negative electrode mixture layer was then pressed by a roll press to obtain a negative electrode having a density of 1.50 g / cm. 3 In this manner, a negative electrode was produced in close contact with the current collector made of copper foil.

[0044] [Preparation of positive electrode mixture paste for laminate type secondary battery] A positive electrode mixture paste was prepared by stirring LCO (93 parts by mass), PVdF (polyvinylidene fluoride) (4 parts by mass) as a binder, and carbon black (3 parts by mass) as a conductive agent.

[0045] [Preparation of Positive Electrode for Laminated Secondary Battery] The prepared negative electrode mixture paste was applied to an aluminum foil in a uniform thickness, and the positive electrode mixture was applied to the aluminum foil in a thickness of 23.0±0.5 mg / cm. 2 The electrode sheet was prepared by evaporating the solvent at 120°C in a vacuum and drying to form a positive electrode mixture layer. The electrode sheet was then cut to a size of 2 cm x 3 cm. The positive electrode mixture layer was then pressed by a roll press to obtain a positive electrode having a density of 3.20 g / cm. 3 In this manner, a positive electrode was produced in close contact with the current collector made of aluminum foil.

[0046] [Preparation of Laminated Secondary Battery] A separator (polypropylene porous body, thickness: 20 μm) was placed between the negative electrode and positive electrode for the laminated secondary battery prepared by the above method, and then combined. Vinylene carbonate was added as an additive to a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio = 3:7) so that the mixed solvent:vinylene carbonate = 99:1 (mass ratio), and 250 μl of an electrolyte in which LiPF6 was dissolved to 1.0 mol / L was injected to prepare a laminated secondary battery.

[0047] [Charge and discharge test of laminated secondary battery] In this test, the process of absorbing lithium ions into the negative electrode was called charging, and the process of desorbing lithium ions from the negative electrode was called discharging. First, the battery was charged at a constant current of 0.2C (the current value at which the rated capacity based on the discharge capacity at one hour rate is discharged in one hour is 1C, hereinafter the same) at 25°C to a SOC (charge rate) of 50% (fully charged state is SOC 100%, hereinafter the same) and then aged at 45°C for 24 hours. After that, the battery was charged at a constant current of 0.5C, and when the circuit voltage reached 4.1V, it was switched to constant voltage charging and charging was continued until the current value reached 0.05C. The battery was then discharged at a constant current of 0.1C until the circuit voltage reached 2.5V. Next, the battery was charged at a constant current of 1.0C up to SOC 50%, and then charged at a constant current of 0.5C, 1.0C, 1.5C, and 2.0C for 10 seconds in a low-temperature environment of 0°C, and the drop in battery voltage after 10 seconds of charging under each condition was measured. Each current value and the battery voltage drop after 10 seconds at each current value were plotted on a graph as the x-axis and y-axis values, respectively. Next, the low-temperature input / output resistance was calculated from the slope of the line obtained by linearly regressing the plot of the graph based on the following formula (2). The results are shown in Table 1 below. The lower the low-temperature input / output resistance value, the better the low-temperature input / output characteristics can be evaluated. In practice, the low-temperature input / output resistance is preferably 15Ω or less. Low-temperature input / output resistance [Ω] = voltage drop [V] / current value [A] (2)

[0048] [Example 2] An electrode and a battery were produced and evaluated in the same manner as in Example 1, except that the mixture ratio in the preparation of the graphite material was coke: flake natural graphite: pitch = 70: 30: 6 (mass ratio). The volume-based average particle size of the obtained graphitized material (a graphite material having a graphitic coating on at least a part of the surface of a granulated material of coke graphitized material and flake natural graphite) was 9.6 μm. The results are shown in Table 1 below.

[0049] [Example 3] An electrode and a battery were produced and evaluated in the same manner as in Example 1, except that the mixture ratio in the preparation of the graphite material was coke: flake natural graphite: pitch = 70: 30: 20 (mass ratio). The volume-based average particle size of the obtained graphitized material (a graphite material having a graphitic coating on at least a part of the surface of a granulated material of coke graphitized material and flake natural graphite) was 14.2 μm. The results are shown in Table 1 below.

[0050] [Example 4] Except for the volumetric average particle size of the dried coke, which was 8.0 μm, electrodes and batteries were produced and evaluated in the same manner as in Example 1. The volumetric average particle size of the obtained graphitized material (a graphite material having a graphitic coating on at least a part of the surface of a granulated material of the coke graphitized material and the scaly natural graphite) was 8.2 μm. The results are shown in Table 1 below.

[0051] [Example 5] Except for changing the volume-based average particle size of the flake natural graphite to 4.0 μm, electrodes and batteries were produced and evaluated in the same manner as in Example 1. The volume-based average particle size of the obtained graphitized product (a graphite material having a graphitic coating on at least a part of the surface of a granulated product of coke graphitized product and flake natural graphite) was 10.6 μm. The results are shown in Table 1 below.

[0052] [Example 6] Except for mixing the coke, flake natural graphite, and pitch in a ratio of 60:40:14 (mass ratio), electrodes and batteries were produced and evaluated in the same manner as in Example 1. The volume-based average particle size of the obtained graphitized material (a graphite material having a graphitic coating on at least a portion of the surface of a granulated material of the coke graphitized material and the flake natural graphite) was 10.6 μm. The results are shown in Table 1 below.

[0053] [Comparative Example 1] In the preparation of the graphite material, the volume-based average particle size of the flake natural graphite was set to 10 μm, and the mixture ratio of the mixture was set to coke: flake natural graphite: pitch = 70: 30: 20 (mass ratio). Except for this, electrodes and batteries were prepared and evaluated in the same manner as in Example 1. The volume-based average particle size of the obtained graphitized material was 15.5 μm. The results are shown in Table 1 below. In this example, the initial charge / discharge efficiency is high because the pore volume in the pore diameter range of 2 to 10 nm is small, but the low-temperature input / output characteristics are poor because the pore volume in the pore diameter range of 0.002 to 2 μm is small.

[0054] [Comparative Example 2] In the preparation of the graphite material, the mixture ratio of the mixture was coke: flake natural graphite: pitch = 100:0:14 (mass ratio), but the same procedure as in Example 1 was used to prepare and evaluate the electrode and battery. The volume-based average particle size of the obtained graphitized material was 14.9 μm. The results are shown in Table 1 below.

[0055] [Comparative Example 3] In the preparation of the graphite material, the volume-based average particle size of the flake natural graphite was set to about 10 μm, and the mixture ratio of the mixture was set to coke: flake natural graphite: pitch = 0: 100: 10 (mass ratio). Except for this, electrodes and batteries were prepared and evaluated in the same manner as in Example 1. The volume-based average particle size of the obtained graphitized material was 11.4 μm. The results are shown in Table 1 below.

[0056] [Table 1]

[0057] In Table 1, the parts by mass of the graphite coating represent the parts by mass relative to 100 parts by mass of the total of the coke graphitized material and the scaly natural graphite.

[0058] In Examples 1 to 6, the pore volume of the pores in the pore diameter range of 2 to 10 nm is 0.0030 ml / g or less, and the pore volume of the pores in the pore diameter range of 0.002 to 2 μm is more than 0.100 ml / g. Therefore, all of them have excellent initial charge / discharge efficiency and low temperature This is thought to be because the pore volume in the pore diameter range of 2 to 10 nm is small, which suppresses side reactions of the electrolyte, and because the pore volume in the pore diameter range of 0.002 to 2 μm is large, which allows the electrolyte to easily penetrate into the inside of the particles. [Explanation of symbols]

[0059] 1: Exterior cup 2: Negative electrode 3: Outer can 4: Positive electrode 5: Separator 6: Insulation gasket 7a: Current collector 7b: Current collector

Claims

1. The volume-based average particle size is 15.0 μm or less, The pore volume in the range of pore diameters of 0.002 to 2 μm as determined by mercury intrusion porosimetry is more than 0.100 ml / g; A graphite material having a pore volume of pores having diameters in the range of 2 to 10 nm, as determined by a nitrogen gas adsorption method, of 0.0030 ml / g or less.

2. 2. The graphite material according to claim 1, wherein the pore volume in the pore diameter range of 2 to 4 nm, as determined by a nitrogen gas adsorption method, is 0.0022 ml / g or less.

3. 2. The graphite material according to claim 1, wherein the pore volume in the pore diameter range of 0.01 to 1 μm, as determined by mercury intrusion porosimetry, is 0.07 ml / g or less.

4. The Raman R value is 0.010 to 0.060; The average spacing (d ００２ 2. The graphite material of claim 1, wherein the average particle diameter (nm) of the graphite particles is 0.3360 nm or less.

5. Specific surface area is 1.0 to 4.0 m ２ 2. The graphite material of claim 1, wherein the tensile strength is 1 / g.

6. Tap density is 0.80 to 1.20 g / cm ３ 2. The graphite material of claim 1 ,

7. The graphite material according to any one of claims 1 to 6, wherein the graphite material has a graphitic coating on at least a portion of a surface of a granule of the coke graphitized material and natural graphite.

8. A method for producing the graphite material according to any one of claims 1 to 6, comprising the steps of: A granulation step of mixing and granulating the coke, natural graphite and graphitic precursor; a graphitization step of heating the granules obtained in the granulation step to convert the coke into a coke graphitized material and convert the graphitic precursor into graphite.

9. A method for producing the graphite material according to any one of claims 1 to 6, comprising the steps of: A granulation step of mixing coke, natural graphite and a graphite precursor and granulating the mixture to obtain a granulated product having a coating of the graphite precursor on at least a part of a surface of the granulated product of the coke and the natural graphite; and a graphitization step of heating the granules obtained in the granulation step to convert the coke into a coke graphitized product and convert the graphitic precursor into graphite, thereby obtaining a graphite material having a graphitic coating on at least a portion of a surface of the granules of the coke graphitized product and natural graphite.

10. The method for producing a graphite material according to claim 9, wherein the coke is a coal coke.

11. A negative electrode for a lithium ion secondary battery, comprising the graphite material according to any one of claims 1 to 6.

12. A lithium ion secondary battery comprising the negative electrode according to claim 11.

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