JP2006228505A - Graphite particles for anode of lithium-ion secondary battery, its manufacturing method, as well as anode for lithium-ion secondary battery and lithium-ion secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery with higher capacity and charge/discharge efficiency, and also with better cycling characteristics and rapid charge/discharge properties than the conventional ones. <P>SOLUTION: Provided are the graphite particles for the anode of the lithium-ion secondary battery, having a mean particle diameter of 5 to 50 micrometers, a true specific gravity of 2.20 or larger, a specific surface area by nitrogen gas absorption of 8 m<SP>2</SP>/g or lower, a specific surface area by carbon dioxide absorption of 1 m<SP>2</SP>/g or lower, and an oxygen element richness measured by X-ray photoelectron spectroscopy (XPS) of 0.7 at% or higher, and its manufacturing method, as well as the anode for the lithium-ion secondary battery and the lithium-ion secondary battery using the graphite particles. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a graphite particle for a lithium ion secondary battery negative electrode, a method for producing the same, and a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the graphite particles. More specifically, a lithium-ion secondary battery having high capacity, high charge / discharge efficiency, excellent rapid charge / discharge characteristics and cycle characteristics, and suitable for use in portable devices, electric vehicles, power storage, etc., and to obtain the same The present invention relates to a graphite particle for a negative electrode of a lithium ion secondary battery, a production method thereof, and a negative electrode for a lithium ion secondary battery using the graphite particle.

  Conventionally, negative electrode materials for lithium ion secondary batteries include, for example, natural graphite particles, artificial graphite particles graphitized with coke, organic polymer materials, artificial graphite particles graphitized with pitch, etc., and graphite obtained by pulverizing these Particles, spherical graphite obtained by graphitizing mesophase carbon, and the like are used. In addition, a negative electrode for a lithium ion secondary battery using these graphite particles is generally mixed with graphite particles, an organic binder (binder) and a solvent to form a graphite paste, and this is applied to the surface of the copper foil. It is manufactured by applying and drying the solvent, and then adjusting the density by compressing with a roll or the like as necessary.

  Electrochemical characteristics such as charge / discharge potential, charge / discharge capacity, and cycle characteristics of the lithium ion secondary battery include crystallinity (crystal structure, crystallinity), surface morphology, internal structure, surface chemical composition, etc. of the graphite particles used in the negative electrode. Strongly depends on. Furthermore, the influence on the characteristic by SEI (Solid Electrolyte Interface) formed on the negative electrode surface at the time of the first charge is great. This SEI is formed by the reaction between the negative electrode and the electrolytic solution, and once formed, further reaction is suppressed, so that lithium can be inserted between the graphite layers. However, SEI is one of the causes that cause irreversible capacity. Further, the thermal stability related to the safety of the battery depends on the stability of SEI. Furthermore, because of the mechanism that SEI is formed by the reaction between the negative electrode and the electrolyte, the amount of oxygen functional groups such as carboxyl groups and carbonyl groups on the surface of the graphite particles, the surface crystallinity of the graphite particles, and the graphite such as the pore structure It is greatly affected by the surface structure of the particles.

  Heretofore, it has been proposed to perform surface modification of graphite particles in order to obtain good battery characteristics. For example, Patent Documents 1 to 3 disclose a method for surface-treating a carbon material by thermal plasma.

JP-A-10-92432 JP 2000-223121 A JP 2004-265733 A

  The present invention has a high capacity and charge / discharge efficiency compared to conventional lithium ion secondary batteries, and is excellent in cycle characteristics and rapid charge / discharge characteristics, and a lithium ion secondary battery for obtaining the same. It aims at providing the negative electrode for lithium ion secondary batteries which uses the graphite particle for secondary battery negative electrodes, its manufacturing method, and this graphite particle.

  That is, the present invention is characterized by the following items (1) to (7).

(1) Average particle diameter is 5 to 50 μm, true specific gravity is 2.20 or more, specific surface area by nitrogen gas adsorption is 8 m ² / g or less, specific surface area by carbon dioxide gas adsorption is 1 m ² / g or less, X-ray photoelectron spectroscopy spectrum A graphite particle for a negative electrode of a lithium ion secondary battery, wherein the oxygen element concentration measured by (XPS) is 0.7 at% or more.

  (2) The lithium ion secondary battery negative electrode according to (1), wherein the graphite particle material is subjected to thermal plasma treatment in a reducing atmosphere, an oxidizing atmosphere, or a reactive atmosphere. Graphite particles.

  (3) The graphite particles for a lithium ion secondary battery negative electrode according to (1) or (2) above, wherein an edge surface in the c-axis direction in which crystal faces are laminated on the surface is exposed.

(4) Reducing atmosphere, oxidizing atmosphere or reactivity against graphite particle raw materials having an average particle size of 5 to 50 μm, a true specific gravity of 2.20 or more, and a specific surface area by nitrogen gas adsorption of 10 m ² / g or less. A method for producing graphite particles for a negative electrode of a lithium ion secondary battery, characterized by performing a thermal plasma treatment in an atmosphere.

  (5) A thermal plasma treatment is performed on the surface of a graphite particle raw material having an average particle diameter of 5 to 50 μm and a true specific gravity of 2.20 or more in a reducing atmosphere, an oxidizing atmosphere or a reactive atmosphere. A method for producing graphite particles for a negative electrode of a lithium ion secondary battery, wherein an edge surface in the c-axis direction in which crystal faces are laminated is exposed.

  (6) Using the graphite particles for a lithium ion secondary battery negative electrode according to any one of claims 1 to 3 and / or the graphite particles for a lithium ion secondary battery negative electrode produced by the production method according to claims 4 to 5. A negative electrode for a lithium ion secondary battery.

  (7) A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 6.

  According to the present invention, compared to a conventional lithium ion secondary battery, the capacity and charge / discharge efficiency are high, the lithium ion secondary battery is excellent in cycle characteristics and rapid charge / discharge characteristics, and lithium for obtaining the same. It becomes possible to provide a graphite particle for an ion secondary battery negative electrode, a production method thereof, and a negative electrode for a lithium ion secondary battery using the graphite particle.

The graphite particles for the negative electrode of the lithium ion secondary battery of the present invention have an average particle diameter of 5 to 50 μm, a true specific gravity of 2.20 or more, a specific surface area of 8 m ² / g or less by nitrogen gas adsorption, and a specific surface area of carbon dioxide gas adsorption. 1 m ² / g or less and the oxygen element concentration measured by X-ray photoelectron spectroscopy (XPS) is 0.7 at% or more. Such graphite particles can improve the capacity, charge / discharge efficiency, and cycle characteristics of a lithium ion secondary battery including a negative electrode using the graphite particles. This is considered to be because it is possible to reduce both the decomposition of the electrolytic solution and the lithium ions that are fixed in the graphite particles and do not participate in charge / discharge during charging of the lithium ion secondary battery.

  The average particle size of the graphite particles for lithium ion secondary battery negative electrode of the present invention is required to be 5 to 50 μm, preferably 5 to 35 μm, more preferably 5 to 25 μm, and 8 to A thickness of 20 μm is particularly preferable. If the average particle size is less than 5 μm, the adhesion to the current collector tends to be insufficient, so that it is necessary to use more binder than usual when producing a negative electrode for a lithium ion secondary battery. This causes a problem that the resistance of the negative electrode increases. On the other hand, if the average particle size exceeds 50 μm, unevenness is likely to occur on the surface of the negative electrode of the lithium ion secondary battery to be produced, resulting in a large current density variation during charge / discharge, resulting in a decrease in charge / discharge cycle characteristics. Problem arises. In addition, the average particle diameter in this invention can be measured with a laser diffraction type particle size distribution meter, and when a particle is not spherical, let the equivalent diameter which converted the projection area into a circle be an average particle diameter.

  The true specific gravity of the graphite particles for lithium ion secondary battery negative electrode of the present invention is required to be 2.20 or more, preferably 2.21 or more, more preferably 2.22 or more, More preferably, it is 2.23 or more, and particularly preferably 2.24 or more. If the true specific gravity is less than 2.20, the discharge capacity of the lithium ion secondary battery to be produced tends to decrease. The true specific gravity can be measured by a known method such as a butanol substitution method.

The specific surface area of the lithium-ion secondary battery negative electrode graphite particles of the present invention, nitrogen gas (N ₂₎ measured specific surface area of 8m 2 ^{/ g} or less, a specific surface area as measured by carbon dioxide (CO ₂₎ is 1 m ² / It is necessary to satisfy simultaneously that it is below g. Thereby, the electrolyte solution decomposition at the time of charge and the lithium ion which is fixed in the graphite particles and does not participate in charge / discharge can be reduced at the same time, and the charge / discharge efficiency of the lithium ion secondary battery to be manufactured can be improved. Nitrogen gas _{(N 2)} specific surface area measured by is preferably in the range of 0.5~8m ² / g, more preferably in the range of ^{1~8m 2 / g, 2~5m 2 /} g It is particularly preferable that the range is When the specific surface area measured with nitrogen gas is less than 0.5 m ² / g, the input / output characteristics tend to deteriorate. When the specific surface area exceeds 8 m ² / g, the lithium ion secondary battery to be produced is As a result, the irreversible capacity of the first cycle increases, and as a result, the discharge capacity of the lithium ion secondary battery to be produced tends to decrease. As a result, there is a tendency that resistance increases and input / output characteristics deteriorate. The specific surface area measured by the carbon dioxide gas is preferably at 0.8 m ² / g or less, more preferably 0.5 m ² / g or less, particularly not less 0.1 m ² / g or less preferable. When the specific surface area measured with carbon dioxide exceeds 1.0 m ² / g, the irreversible capacity of the first cycle of the lithium ion secondary battery to be manufactured becomes large, and as a result, the discharge capacity of the lithium ion secondary battery to be manufactured Tends to be smaller. The specific surface area can be measured by a known method such as nitrogen gas adsorption or BET method using carbon dioxide gas adsorption.

  The oxygen element concentration measured by X-ray photoelectron spectroscopy (XPS) of the graphite particles for the negative electrode of the lithium ion secondary battery of the present invention is required to be 0.7 at% or more, and is 1.0 at% or more. It is preferably 1.5 at% or more, more preferably 2.0 at% or more. By making the oxygen element concentration measured by X-ray photoelectron spectroscopy (XPS) 0.7 at% or more, oxygen functional groups are introduced, and the inertness of the site that causes decomposition of the electrolyte during charging of the graphite particle surface A change occurs in the interaction between the electrolyte and the electrolytic solution, and the charge / discharge efficiency of the manufactured lithium ion secondary battery can be improved. When the oxygen element concentration measured by X-ray photoelectron spectroscopy (XPS) is less than 0.7 at%, the charge / discharge efficiency of the lithium ion secondary battery to be manufactured tends to decrease. The oxygen element concentration measured by the X-ray photoelectron spectrum is calculated from the intensity of the O1s peak from the element detected by the X-ray photoelectron spectrum having an irradiation X-ray intensity of 45 to 150 W.

  The interlayer distance d (002) between graphite crystals in the graphite particles for lithium ion secondary battery negative electrode of the present invention is preferably 3.38 angstroms or less, more preferably 3.37 angstroms or less. It is particularly preferable that the thickness is not more than angstrom. When the interlayer distance d (002) of the graphite crystal exceeds 3.38 angstroms, the discharge capacity of the lithium ion secondary battery to be produced tends to decrease. The crystallite size Lc (002) in the C-axis direction of the graphite crystal is preferably 300 angstroms or more and 600 angstroms or more from the viewpoint of improving the charge / discharge capacity of the lithium ion secondary battery to be produced. More preferably, it is 900 angstroms or more, more preferably 1000 angstroms or more. The interlayer distance d (002) of the graphite crystals and the crystallite size Lc (002) in the C-axis direction can be measured by a wide angle X-ray diffraction method using CuKα rays.

For lithium-ion secondary battery negative electrode graphite particles of the present invention, in the Raman spectrum measured by laser light having a wavelength of 532 nm, a peak appearing in the vicinity of 1350 cm ^-1 peak (G peak) appearing near 1580 cm ^-1 (D peak) The height intensity ratio (D / G) is preferably 0.05 or more, more preferably 0.08 or more, further preferably 0.10 or more, and 0.10 to 0.40. It is particularly preferred that When the intensity ratio (D / G) is less than 0.05 or exceeds 0.40, the charge / discharge efficiency of the lithium ion secondary battery to be produced tends to be reduced.

The graphite particles for lithium ion secondary battery negative electrode of the present invention have a pore volume in the range of 10 ² to 10 ⁶ angstroms measured by mercury porosimetry in the range of 0.2 to 2.5 cc / g. Is more preferable, is in the range of 0.4 to 2.0 cc / g, more preferably is in the range of 0.4 to 1.5 cc / g, and is in the range of 0.6 to 1.2 cc / g It is particularly preferred that If the pore volume of the pores in the range of 10 ² to 10 ⁶ angstroms is less than 0.2 cc / g, the cycle characteristics tend to deteriorate, and if it exceeds 2.5 cc / g, the anode material for lithium ion secondary batteries and There exists a tendency for the adhesive strength with an electric body to fall. The pore volume can be determined by measuring the pore size distribution by mercury porosimetry.

  The aspect ratio of the graphite particles for a lithium ion secondary battery negative electrode of the present invention is preferably 5 or less, more preferably 1.1 to 5, and further preferably 1.1 to 3. 2 to 2.5 is particularly preferable. When the aspect ratio exceeds 5, the negative electrode material for a lithium ion secondary battery tends to be oriented in the surface direction of the current collector, and as a result, the rapid charge / discharge characteristics, output characteristics and cycle characteristics of the resulting lithium ion secondary battery Tends to decrease. When the aspect ratio is less than 1.1, the contact area between the particles decreases, and the conductivity of the negative electrode to be produced tends to decrease. The aspect ratio is represented by A / B, where A is the length in the major axis direction of the graphite particles and B is the length in the minor axis direction. The aspect ratio in the present invention can be obtained by enlarging graphite particles with an electron microscope, selecting 10 particles arbitrarily, measuring A / B while changing the observation angle of the electron microscope, and taking the average value. Further, when the graphite particles have a thickness direction such as a flake shape, a plate shape, or a block shape, the thickness is set to a length B in the minor axis direction. Further, the graphite particles having an aspect ratio of 5 or less may be aggregated or bonded together, and the shape is changed so that the aspect ratio is 5 or less by applying mechanical force to one particle. It may be a thing, and what was produced combining these further may be sufficient.

  In the graphite particles for a negative electrode of the lithium ion secondary battery of the present invention, it is preferable that an edge surface in the C-axis direction in which crystal surfaces are laminated on the surface of the graphite particles is exposed. By exposing the edge surface in the C-axis direction in which crystal faces are laminated on the surface of the graphite particles, it becomes easy to charge and discharge lithium between graphite crystal layers, and rapid charge / discharge characteristics can be improved. The exposed state of the edge surface in the C-axis direction in which crystal faces are laminated on the surface of the graphite particles can be observed by enlarging the surface of the graphite particles with a scanning electron microscope to 3000 times or more.

  The graphite particles for lithium ion secondary battery negative electrode of the present invention having the above-mentioned characteristics are not particularly limited. For example, in a reducing atmosphere, an oxidizing atmosphere or a reactive atmosphere, the graphite particles are subjected to plasma treatment. It can manufacture by giving etc.

The graphite particle raw material is not particularly limited. For example, artificial graphite particles and / or natural graphite particles having an average particle diameter of 5 to 50 μm and a true specific gravity of 2.20 or more, or an average particle diameter of 5 to 50 μm. Thus, it is preferable to use artificial graphite particles and / or natural graphite particles having a true specific gravity of 2.20 or more and a specific surface area by nitrogen gas adsorption of 10 m ² / g or less. The shape of the graphite particle raw material is not particularly limited, but in order to make the thermal plasma treatment effective, for example, it is preferably spherical, massive, etc., and the aspect ratio is more preferably 5 or less, More preferably, the aspect ratio is 3 or less. In addition, the definition and measurement of each physical property of the above-described graphite particle raw material are the same as in the case of the above-described graphite particles for a negative electrode of a lithium ion secondary battery of the present invention.

Further, the bulk density of the graphite particle raw material is not particularly limited, but is preferably 0.2 g / cm ³ or more, and preferably 0.3 g / cm ³ or more from the viewpoint of the uniformity of the thermal plasma treatment described later. Is more preferably 0.4 g / cm ³ or more, and particularly preferably 0.5 g / cm ³ or more. The measurement of the bulk density is calculated from the volume and weight after dropping the graphite particle material from the top into a 100 ml graduated cylinder and placing 100 ml of the graduated cylinder 50 times from a height of 5 cm.

  Although there is no restriction | limiting in particular as said plasma processing, It is preferable from a viewpoint of improving the charging / discharging capacity | capacitance, charging / discharging efficiency, cycling characteristics, and rapid charging / discharging characteristic of the lithium ion secondary battery to produce. Thermal plasma is a plasma generated from medium pressure (about 10 to 70 kPa) to 1 atm. Unlike normal low-pressure plasma, a plasma close to thermal equilibrium is obtained. The material present in the system can be elevated to high temperatures. Thus, the thermal plasma allows both the generation of a high temperature phase and surface modification.

  The thermal plasma treatment can be performed according to, for example, “Masataka Ishigaki, Ceramics, 30 (1995) No. 11, 1013 to 1016”, Japanese Patent Laid-Open No. 7-31873, and Patent Documents 1 to 3. More specifically, for example, a high-frequency thermal plasma generator (thermal plasma torch) as shown in FIG. 1 can be used. In this method, a processing object is continuously introduced into the plasma torch, and the processing object is recovered at the lower part. The apparatus (torch) 10 in FIG. 1 has a structure in which a high-frequency coil 12 is wound outside a water-cooled double tube 11 and generates thermal plasma by high-frequency electromagnetic induction therein. A lid 13 is attached to the upper part of the water-cooled double tube 11, and a powder-feeding water-cooled probe 14 for supplying a graphite particle raw material powder and a carrier gas for thermal plasma processing is installed on the lid 13. The size of the plasma torch is not particularly limited, but in the case of the structure shown in FIG. 1, it is preferable that the tube diameter is 10 to 1000 mm, particularly about 50 to 100 mm, and the height is 50 to 3000 mm, particularly about 200 to 3000 mm. . Further, a central gas Gp for mainly forming a plasma flow and a sheath gas Gs for mainly wrapping the outside of the plasma flow are introduced into the apparatus (torch) 10. Hereinafter, the central gas, the sheath gas, and the carrier gas may be collectively referred to as plasma gas.

  As conditions for generating thermal plasma (processing conditions using thermal plasma), the frequency is usually 0.5 to 30 MHz, the input power is 3 to 200 kW, and the pressure inside the torch is 1 to 100 kPa. In particular, the frequency is 0.5 to 6 MHz. The input power is preferably 10 to 100 kW, and the pressure inside the torch is preferably 10 to 70 kPa.

In addition, the effect of thermal plasma treatment can be controlled by appropriately selecting the type of plasma gas. Plasma gas used, i.e., central gas, as the sheath gas and the carrier gas is not particularly limited, it is preferable to use at least Ar respectively, and _Ar, and at least one N _2, H 2, CO ₂ and CO It is more preferable to use together. In particular, it is preferable to use H ₂ or N ₂ in combination with Ar, and to add CO ₂ to these. In the plasma gas, the volume ratio of gases other than Ar is preferably 1 to 20%. By using at least Ar as the plasma gas, the effects of the present invention, particularly the first charge / discharge efficiency, can be improved. Further, since H ₂ is a high thermal conductivity compared to N _2, when using of H ₂ typically heating efficiency is higher. The sheath gas is preferably mixed with a diatomic gas such as H ₂ or N ₂ in order to protect the inner wall of the torch. The total flow rate of the central gas and the sheath gas is usually 2 to 200 liters / minute, preferably 30 to 130 liters / minute.

  The apparatus and conditions as described above enable thermal plasma treatment of the graphite particle raw material in a reducing, oxidizing or reactive atmosphere at 3,000 to 15,000 ° C. In the present invention, the residence time of the graphite particle raw material in the temperature range of 3,000 to 15,000 ° C. is preferably about 0.001 to 10 seconds, particularly about 0.02 to 0.5 seconds. Moreover, it is preferable that the quantity of the graphite particle raw material provided to a thermal plasma process shall be 0.001-0.5 kg by the introduction amount per minute. Therefore, the flow rate of the carrier gas may be 1 to 100 liters / minute.

The graphite particle raw material may be subjected to thermal plasma treatment alone, or may be mixed with a material other than the graphite particle raw material, such as an organic compound such as a metal, a metal compound, or a resin, and may be subjected to thermal plasma treatment. It is particularly preferable to perform a thermal plasma treatment together with the metal oxide. The metal oxide is not particularly limited, for example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), lithium manganate (LiMnO _2), and these mixed oxide _{(LiCoxNiyMnzO 2, X + Y +} X = 1 ), Lithium manganese spinel (LiMn ₂ O ₄ ), lithium vanadium compound, V ₂ O ₅ , olivine type LiMPO ₄ (M: Co, Ni, Mn, Fe) and the like are preferable. In addition, it is preferable that the mixture ratio of substances other than the graphite particle raw material in a mixture shall be 5 mass% or less.

  Performing the above thermal plasma treatment as a treatment for the graphite particle raw material is very effective because it greatly contributes to improving various characteristics of the lithium ion secondary battery of the present invention. You may manufacture the graphite particle for lithium ion secondary battery negative electrodes of this invention by processes other than a process.

  Examples of the treatment other than the thermal plasma treatment include a treatment for adding an active site to the surface of the graphite particle raw material (for example, a treatment with a functional group). If the graphite particles are exposed to air after such treatment, moisture adsorption and adsorption of functional groups that impair the electrode characteristics may occur, which may reduce the electrode characteristics to some extent, but this point will be described later. Thus, the graphite particles may be handled so as to avoid contact with air after the surface treatment. In addition, the process performed with respect to the graphite particle raw material in the present invention does not include the process of leaving in an oxidizing atmosphere such as air.

  Moreover, as another process, the grinding | pulverization or milling of a graphite particle raw material is contained, for example. By crushing or milling, the activity of the graphite particle raw material surface is improved, and the characteristics as an electrode material are improved. Means and specific conditions used for pulverization or milling may be set so as to improve the characteristics as an electrode material, and are not particularly limited, but the treatment is preferably performed in a rare gas. Moreover, as a grinding | pulverization means, it can carry out by rotating the pot in which the graphite particle and grinding media, such as an alumina ball | bowl, were put, for example. The processing time may be about 1 to 48 hours, for example.

  Still another treatment includes, for example, a treatment in which ultrasonic waves are applied to a specific organic solvent in which a graphite particle raw material is dispersed. In this treatment, the organic solvent is carbonized and deposited by applying traveling wave ultrasonic waves. Since precipitation occurs on the surface of the dispersed graphite particle raw material, a carbon coating free from adsorption of unnecessary functional groups or the like can be formed on the surface of the graphite particle raw material. Examples of the specific organic solvent include o-dichlorobenzene.

  The negative electrode for a lithium ion secondary battery of the present invention includes, for example, a graphite particle for a negative electrode of the lithium ion secondary battery of the present invention, a binder, various additives, etc. together with a solvent, a stirrer, a ball mill, a super sand mill, a pressure kneader, etc. Mix and disperse with a dispersing device to adjust the negative electrode layer coating material and apply it to the current collector to form the negative electrode layer, or form the paste negative electrode layer coating material into a sheet shape, pellet shape, etc. However, it can be obtained by integrating this with the current collector. In addition, it is preferable that the average particle diameter of the graphite particle for lithium ion secondary battery negative electrodes of this invention in the case of apply | coating the coating material for negative electrode layers is 15-40 micrometers.

  When manufacturing negative electrodes for lithium ion secondary batteries using graphite particles that have been subjected to surface treatment such as plasma treatment, it is best not to expose them to moisture and air as much as possible to improve the charge / discharge efficiency of lithium ion secondary batteries. It is preferable from the viewpoint. For example, in the thermal plasma processing apparatus shown in FIG. 1, the processed graphite particles deposited in the chamber 15 below the water-cooled double tube 11 are collected outside the chamber 15 without being exposed to air, and the air is left as it is. It is possible to obtain an electrode by bonding with a binder without touching. In order not to touch the air, for example, the take-out port of the chamber 15 may have an air lock structure. After taking out from the chamber 15, it is preferable to carry out electrode preparation in dry air having a dew point of −5 ° C. or lower or in an inert gas atmosphere such as argon until completion of electrode preparation. The dew point is more preferably −10 ° C., further preferably −30 ° C. or less, particularly preferably −50 ° C. or less, and most preferably −70 ° C.

  The binder is not particularly limited. For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether. Fluorine such as copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF) Resin, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP) TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluorine rubber (VDF-PFP-TFE fluorine rubber), Vinylidene fluorides such as vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluorine rubber (VDF-PFMVE-TFE fluorine rubber), vinylidene fluoride-chlorotrifluoroethylene fluorine rubber (VDF-CTFE fluorine rubber) -Based fluoro rubber, tetrafluoroethylene-propylene-based fluoro rubber (TFE-P-based fluoro rubber), tetrafluoroethylene-perfluoroalkyl vinyl ether-based fluoro rubber, and heat Sex fluororubber (e.g., manufactured by Daikin Industries DAI-EL Thermoplastic) or the like is preferable. Of course, binders other than fluorine-based ones such as styrene butadiene rubber (SBR) can also be used.

  The binder is usually used in the form of a powder dissolved or dispersed in a solvent (solvent), but may be used as a powder without using a solvent. The solvent to be used is a non-aqueous solvent. For example, various solvents such as methyl ethyl ketone, cyclohexanone, isophorone, N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide and toluene may be appropriately selected.

  Moreover, you may add an oxide to the said coating material for negative electrode layers. In this case, the oxide is preferably a metal oxide that can be mixed with the graphite particle raw material in the above-described thermal plasma treatment. The compounding amount of the oxide is preferably (mass of oxide / total mass of graphite particles and oxide) of 5% by mass or less.

  Moreover, it is preferable to mix a conductive support agent with the said coating material for negative electrode layers. Examples of the conductive assistant include carbon black, graphite, acetylene black, or an oxide or nitride that exhibits conductivity. The usage-amount of a conductive support agent should just be about 1-15 mass% of the graphite particle | grains of this invention.

  The binder is used in an amount of 80:20 to 99: 1, preferably in the range of 85:15 to 98: 2, with the weight ratio of the solid content and binder contained in the negative electrode layer coating. It is more preferable that By using the binder in such a range, the binding property is improved. In addition to the graphite particles of the present invention, the solid content can include additives such as oxides and conductive assistants.

  The material and shape of the current collector are not particularly limited in the case of a negative electrode, and a strip-shaped one made of aluminum, copper, nickel, titanium, stainless steel, etc. in a foil shape, a punched foil shape, a mesh shape, or the like. Use it. A porous material such as porous metal (foamed metal) or carbon paper can also be used.

  The method for applying the negative electrode layer coating to the current collector is not particularly limited. For example, a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure. Known methods such as a coating method and a screen printing method may be mentioned. After the application, a rolling process using a flat plate press, a calendar roll or the like is performed as necessary. Moreover, integration of the negative electrode layer coating material and the current collector formed into a sheet shape, pellet shape, or the like can be performed by a known method such as a roll, a press, or a combination thereof.

  The lithium ion secondary battery of the present invention can be obtained, for example, by placing the negative electrode and the positive electrode of the present invention facing each other with a separator interposed therebetween and injecting an electrolytic solution. In manufacturing the battery, it is desirable not to expose the negative electrode to air following the negative electrode manufacturing. Therefore, it is desirable to carry out at least the process until the negative electrode is sealed in the battery case in a rare gas using, for example, a glove box. Moreover, it is desirable to store the negative electrode in a dry rare gas until it is used in the battery manufacturing process.

  The positive electrode can be obtained by forming a positive electrode layer on the current collector surface in the same manner as the negative electrode. In this case, the current collector may be a band-shaped material made of a metal or an alloy such as aluminum, titanium, or stainless steel in a foil shape, a punched foil shape, a mesh shape, or the like.

The positive electrode material used for the positive electrode layer may be a metal compound, metal oxide, metal sulfide, or conductive polymer material that can be doped or intercalated with lithium ions, and is not particularly limited. Lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), and their double oxides (LiCoxNiyMnzO ₂ , X + Y + X = 1), lithium manganese spinel (LiMn ₂ O ₄ ), lithium Examples include vanadium compounds, V ₂ O ₅ , olivine-type LiMPO ₄ (M: Co, Ni, Mn, Fe), polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, and the like. Japanese Patent Publication No. 61-53828, Japanese Patent Publication No. 63- No. 59507 Those placing of, and the like. In addition, when using a metal oxide, a metal sulfide, etc. for positive electrode material, it is preferable to contain carbonaceous materials, such as a graphite, acetylene black, and ketjen black, as a electrically conductive agent.

The electrolytic solution is prepared by dissolving a lithium-containing electrolyte in a non-aqueous solvent. The lithium-containing electrolyte may be appropriately selected from, for example, LiClO ₄ , LiBF ₄ , LiPF _{6 and the} like, and Li (CF ₃ SO ₂ ) ₂ N, Li (C ₂ F ₅ SO ₂ ) ₂ N, etc. Lithium imide salt and LiB (C ₂ O ₄ ) ₂ can also be used. As the non-aqueous solvent, for example, ethers, ketones, carbonates, and the like can be appropriately selected from organic solvents exemplified in JP-A No. 63-121260, but carbonates are particularly used in the present invention. It is preferable. Among carbonates, it is preferable to use a mixed solvent in which ethylene carbonate is a main component and one or more other solvents are added. Usually, the mixing ratio is preferably ethylene carbonate: other solvent = 5 to 70:95 to 30 (volume ratio). Since ethylene carbonate has a high freezing point of 36.4 ° C and is solidified at room temperature, ethylene carbonate alone cannot be used as a battery electrolyte, but it can be mixed by adding one or more other solvents having a low freezing point. The freezing point of the solvent is lowered and it can be used. Other solvents are not particularly limited as long as the freezing point of ethylene carbonate can be lowered. For example, diethyl carbonate, dimethyl carbonate, propylene carbonate, 1,2-dimethoxyethane, methyl ethyl carbonate, γ-butyrolactone, γ- Parerolactone, γ-octanoic lactone, 1,2-diethoxyethane, 1,2-ethoxymethoxyethane, 1,2-dibutoxyethane, 1,3-dioxolanane, tetrahydrofuran, 2-methyltetrahydrofuran, 4,4- Examples include dimethyl-1,3-dioxane, butylene carbonate, and methyl formate. By using the above mixed solvent as the electrolytic solution and using the graphite particles of the present invention as the negative electrode active material, the battery capacity is remarkably improved, and the irreversible capacity ratio can be sufficiently reduced.

  Alternatively, a solid electrolyte obtained by gelling an electrolytic solution with an organic polymer or an electrolyte in which a lithium salt is dissolved in a polymer, for example, an electrolyte in which a lithium salt is dissolved in polyethylene oxide and does not contain an electrolytic solution at all may be used. it can. In addition, a composite material of a lithium ion conductive inorganic compound (for example, lithium iodide) and an organic polymer compound can be used.

  As the separator, for example, a nonwoven fabric mainly composed of polyolefin such as polyethylene and polypropylene, cloth, microporous film, or a combination thereof can be used. In addition, when it is set as the structure where the positive electrode and negative electrode of the lithium ion secondary battery to produce are not in direct contact, it is not necessary to use a separator.

  Although the structure of the lithium ion secondary battery of the present invention is not particularly limited, usually, a positive electrode and a negative electrode, and a separator provided as necessary, are wound into a flat spiral to form a wound electrode group, In general, these are laminated as a flat plate to form a laminated electrode plate group, and the electrode plate group is enclosed in an exterior body.

  FIG. 2 shows a configuration example of a lithium ion secondary battery. In this battery, a power generation element is produced by thermocompression bonding of a negative electrode 31, a separator 4 made of a solid electrolyte, and a positive electrode 51, and this is enclosed in a bag-shaped outer package 101. In FIG. 2, illustration of lead wires drawn out from the exterior body 101 is omitted. For the outer package 101, a laminate film in which a gas barrier layer made of an inorganic foil made of an inorganic material such as metal (Al or the like), glass, ceramics, a reinforcing layer made of a synthetic resin, and an adhesive layer is laminated is used. It is common. In order to form a bag-shaped exterior body using a film, the films may be thermally welded together or the films may be bonded together with an adhesive or a bonding member.

  The lithium ion secondary battery of the present invention is used as a paper-type battery, a button-type battery, a coin-type battery, a laminated battery, a cylindrical battery, or the like.

  In the above description, the graphite particles of the present invention have been described as being used for lithium ion secondary batteries, but the present invention can also be applied to electrochemical devices in general.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these.

Example 1
(Production and evaluation of graphite particles for negative electrodes of lithium ion secondary batteries)
Using the plasma torch shown in FIG. 1, massive artificial graphite particles having an average particle size of 21.4 μm, a true specific gravity of 2.24, and a specific surface area of 3.5 m ² / g by adsorption of nitrogen gas (manufactured graphite powder manufactured by Hitachi Chemical Co., Ltd.) Sample) was continuously sprayed and subjected to thermal plasma treatment to obtain graphite particles for a negative electrode of a lithium ion secondary battery. At the time of thermal plasma treatment, Ar + H ₂ mixed gas is used as a plasma gas at a gas flow ratio Ar: H ₂ = 93: 7, the pressure in the torch is 53 kPa, the frequency is 2 MHz, the input power is 40 kW, and the supply The speed was 4 g / min, and the time from the start of supply to the end of treatment was 5 minutes. According to the model calculation, the plasma temperature is 10,000 ° C. or higher.

  As a result of observing the graphite particles for a lithium ion secondary battery negative electrode obtained by thermal plasma treatment as described above with a scanning electron microscope (XL30 manufactured by Philip) at a magnification of 5000 times, a crystal plane was observed on the surface of the graphite particles. It was confirmed that the stacked edge surfaces in the c-axis direction were exposed. In addition, Table 1 shows various physical property values of the obtained graphite particles for a lithium ion secondary battery negative electrode. The measuring method of various physical property values is as follows.

<Average particle size>
A solution in which the obtained graphite particle sample for a lithium ion secondary battery negative electrode and a surfactant are dispersed in purified water is placed in a sample water tank of a laser diffraction particle size distribution analyzer (SALD-3000J, manufactured by Shimadzu Corporation). Measured by laser diffraction while circulating with a pump while applying ultrasonic waves. The 50% cumulative particle size of the obtained particle size distribution was defined as the average particle size.

<Specific surface area by nitrogen gas adsorption>
The obtained graphite particle sample for the negative electrode of a lithium ion secondary battery was vacuum-dried at 200 ° C. for 1 hour, and then the amount of nitrogen gas adsorbed was measured at liquid nitrogen temperature while cooling the sample with liquid nitrogen using Quantachrome AUTOSORB-1. Measured by the multipoint method and calculated according to the BET method.

<Specific surface area by carbon dioxide adsorption>
The obtained graphite particle sample for the negative electrode of a lithium ion secondary battery was vacuum-dried at 200 ° C. for 1 hour, and then the amount of carbon dioxide gas adsorbed was increased at a temperature of 273 K while cooling the sample with ice water using AUTASORB-1 manufactured by Quantachrome. Measured by the point method and calculated according to the BET method.

<Raman intensity ratio D / G>
Using a laser Raman spectrometer (NRS-1000 manufactured by JASCO Corporation), the obtained graphite particle sample for a negative electrode of a lithium ion secondary battery was magnified with a 20 × objective lens, and a laser beam having a wavelength of 532 nm and 3.9 mW. The sample was irradiated with Raman scattered light with a CCD detector, and the exposure time was 120 seconds and the number of integrations was 2 times. The wave number of the obtained spectrum was determined from the difference between the wave number of the peak obtained by measuring the range of 800 to 2000 cm ⁻¹ of indene (Wako primary reagent) under the same conditions and the theoretical wave number of each peak of indene. Correction was made using a calibration curve.

<True specific gravity>
It was measured by a butanol replacement method (JIS R 7212) using a specific gravity bottle.

<Oxygen element concentration by XPS>
The obtained graphite particle sample for a negative electrode of a lithium ion secondary battery was placed on an XPS sample table with a double-sided conductive tape in a glove box having a dew point of -70 ° C., sealed with a transfer vessel, and then XPS. After moving to a device (Shimadzu Corporation / Kratos AXIS-165), evacuating, the transfer vessel was opened and subjected to measurement. The measurement was calculated from the intensity of the O1s peak from an element detected in an X-ray photoelectron spectrum having a measurement area of 0.3 × 0.7 mm ² and an irradiation X-ray intensity of 45 to 150 W.

<Interlayer distance d002>
Using a wide-angle X-ray diffractometer (MultiFlex, manufactured by Rigaku Corporation), Cu-Kα rays were monochromatized with a monochromator and calculated from the angle of the (002) plane diffraction peak measured using high-purity silicon as a standard substance.

(Production and evaluation of lithium ion secondary battery)
Next, the obtained graphite particles for the negative electrode of the lithium ion secondary battery are sealed in a sealed container in a plasma processing apparatus, and taken out in a glove box in an argon gas atmosphere (dew point: −70 ° C. or lower, oxygen concentration: 10 ppm or lower). Thus, a sample electrode and a battery were produced.

  A sample electrode was prepared by adding 10% by weight of a solid content of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2pyrrolidone to 90% by weight of the obtained graphite particles for a negative electrode of a lithium ion secondary battery. The obtained graphite paste was applied to a rolled copper foil with a thickness of 10 μm in a circular shape with a diameter of 9.5 mmφ using a metal mask, and further dried under reduced pressure at 120 ° C. to remove N-methyl-2pyrrolidone. And manufactured.

In addition, the battery was prepared by laminating the sample electrode (negative electrode) prepared above together with a counter electrode (positive electrode) through a separator, and injecting an electrolyte solution to obtain a CR2016 type coin cell. For the counter electrode, metallic lithium whose surface was polished to remove the oxide film was used. In the electrolyte, LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC and DEC are in a volume ratio of 1: 2) to a concentration of 1.2 mol / liter. The solution was used. A polyethylene microporous film having a thickness of 25 μm was used as the separator.

Next, electrochemical measurement was performed on the battery obtained as described above. After charging to 0 V (Vvs. Li / Li ⁺ ) with a constant current of 0.25 mA / cm ² between the sample electrode and the counter electrode, until the current becomes 0.025 mA / cm ² with a constant voltage of 0 V Charged. Next, the battery was discharged at a constant current of 0.25 mA / cm ² until the voltage became 1.5 V (Vvs. Li / Li ⁺ ), and charge / discharge capacity and initial charge / discharge efficiency were measured. The results are shown in Table 1.

Further, the second cycle, after charging at a constant current of current density of 2 mA / cm ² until 0V (Vvs.Li/Li ^+), a voltage at a constant current of current density of 2 mA / cm ² is 1.5V (Vvs. The battery was discharged until Li / Li ⁺ ), and the rapid charge / discharge capacity was measured. The results are shown in Table 1.

  FIG. 3 shows the relationship (cycle characteristics) between the number of times charge / discharge is repeated under the same conditions as the charge / discharge capacity measurement and the discharge capacity retention rate.

Example 2
A negative electrode for a lithium ion secondary battery in the same manner as in Example 1 except that Ar + H ₂ + CO ₂ mixed gas was used as the gas for thermal plasma treatment at a gas flow ratio Ar: H ₂ : CO ₂ = 87: 7: 6. Graphite particles, a sample electrode, and a battery were prepared, and the same test as in Example 1 was performed. The results are shown in Table 1 and FIG. Further, when the graphite particles were observed in the same manner as in Example 1, it was confirmed that the edge surface in the c-axis direction in which crystal surfaces were laminated on the surface was exposed.

Comparative Example 1
A test electrode and a battery were prepared in the same manner as in Example 1 except that the massive artificial graphite particle powder used in Example 1 was used as it was as a graphite particle for a negative electrode of a lithium ion secondary battery without performing thermal plasma treatment. The same test as in Example 1 was performed. The results are shown in Table 1 and FIG. Further, when the graphite particles were observed in the same manner as in Example 1, it was not confirmed that the edge surface in the c-axis direction in which crystal faces were laminated on the surface was exposed.

Comparative Example 2
Instead of the bulk artificial graphite particles used in Example 1, coal pitch was used as a raw material and graphitized at 2800 ° C. to produce an average particle size of 10 μm, true specific gravity of 2.185, specific surface area of 5.2 m ² by nitrogen gas adsorption Except for using / g of graphite, graphite particles for a negative electrode of a lithium ion secondary battery, a sample electrode, and a battery were prepared in the same manner as in Example 1, and the same test as in Example 1 was performed. The results are shown in Table 1 and FIG. Further, when the graphite particles were observed in the same manner as in Example 1, it was not confirmed that the edge surface in the c-axis direction in which crystal faces were laminated on the surface was exposed.

  As shown in Table 1 and FIG. 3, it was shown that the lithium ion secondary batteries of the examples had high capacity, high charge / discharge efficiency, and excellent recycling characteristics and rapid charge / discharge characteristics.

Schematic of the high frequency thermal plasma generator. Sectional drawing which shows one form of a structure of a lithium ion secondary battery. The cycle characteristic of the lithium ion secondary battery produced in the Example and the comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Torch 11 Water-cooled double tube 12 High frequency coil 13 Lid 14 Water-cooled probe for powder supply 15 Chamber 31 Negative electrode 4 Separator 51 Positive electrode 101 Exterior body

Claims (7)

Average particle size is 5 to 50 μm, true specific gravity is 2.20 or more, specific surface area by nitrogen gas adsorption is 8 m ² / g or less, specific surface area by carbon dioxide gas adsorption is 1 m ² / g or less, X-ray photoelectron spectroscopy spectrum (XPS) A graphite particle for a negative electrode of a lithium ion secondary battery, characterized in that the oxygen element concentration measured by (1) is 0.7 at% or more.   The graphite particles for a lithium ion secondary battery negative electrode according to claim 1, wherein the graphite particle raw material is subjected to a thermal plasma treatment in a reducing atmosphere, an oxidizing atmosphere or a reactive atmosphere.   3. The graphite particles for a negative electrode of a lithium ion secondary battery according to claim 1, wherein an edge surface in the c-axis direction in which crystal faces are laminated on the surface is exposed. In a reducing atmosphere, an oxidizing atmosphere or a reactive atmosphere with respect to a graphite particle material having an average particle diameter of 5 to 50 μm, a true specific gravity of 2.20 or more, and a specific surface area by nitrogen gas adsorption of 10 m ² / g or less A method for producing graphite particles for a negative electrode of a lithium ion secondary battery, characterized by performing a thermal plasma treatment.   A graphite particle material having an average particle size of 5 to 50 μm and a true specific gravity of 2.20 or more is subjected to a thermal plasma treatment in a reducing atmosphere, an oxidizing atmosphere or a reactive atmosphere, and a crystal plane is formed on the surface. A method for producing graphite particles for a negative electrode of a lithium ion secondary battery, wherein the laminated c-axis edge surface is exposed.   It uses the graphite particle for lithium ion secondary battery negative electrodes in any one of Claims 1-3, and / or the graphite particle for lithium ion secondary battery negative electrodes produced with the manufacturing method of Claims 4-5. A negative electrode for a lithium ion secondary battery. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to claim 6.

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