WO2022210745A1 - 被覆炭素材、負極及び二次電池 - Google Patents
被覆炭素材、負極及び二次電池 Download PDFInfo
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- DENFJSAFJTVPJR-UHFFFAOYSA-N triethoxy(ethyl)silane Chemical compound CCO[Si](CC)(OCC)OCC DENFJSAFJTVPJR-UHFFFAOYSA-N 0.000 description 1
- CPUDPFPXCZDNGI-UHFFFAOYSA-N triethoxy(methyl)silane Chemical compound CCO[Si](C)(OCC)OCC CPUDPFPXCZDNGI-UHFFFAOYSA-N 0.000 description 1
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- UDUKMRHNZZLJRB-UHFFFAOYSA-N triethoxy-[2-(7-oxabicyclo[4.1.0]heptan-4-yl)ethyl]silane Chemical compound C1C(CC[Si](OCC)(OCC)OCC)CCC2OC21 UDUKMRHNZZLJRB-UHFFFAOYSA-N 0.000 description 1
- JXUKBNICSRJFAP-UHFFFAOYSA-N triethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane Chemical compound CCO[Si](OCC)(OCC)CCCOCC1CO1 JXUKBNICSRJFAP-UHFFFAOYSA-N 0.000 description 1
- RIUWBIIVUYSTCN-UHFFFAOYSA-N trilithium borate Chemical group [Li+].[Li+].[Li+].[O-]B([O-])[O-] RIUWBIIVUYSTCN-UHFFFAOYSA-N 0.000 description 1
- ZNOCGWVLWPVKAO-UHFFFAOYSA-N trimethoxy(phenyl)silane Chemical compound CO[Si](OC)(OC)C1=CC=CC=C1 ZNOCGWVLWPVKAO-UHFFFAOYSA-N 0.000 description 1
- DQZNLOXENNXVAD-UHFFFAOYSA-N trimethoxy-[2-(7-oxabicyclo[4.1.0]heptan-4-yl)ethyl]silane Chemical compound C1C(CC[Si](OC)(OC)OC)CCC2OC21 DQZNLOXENNXVAD-UHFFFAOYSA-N 0.000 description 1
- BPSIOYPQMFLKFR-UHFFFAOYSA-N trimethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane Chemical compound CO[Si](OC)(OC)CCCOCC1CO1 BPSIOYPQMFLKFR-UHFFFAOYSA-N 0.000 description 1
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Classifications
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B32/00—Carbon; Compounds thereof
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/362—Composites
- H01M4/366—Composites as layered products
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B32/00—Carbon; Compounds thereof
- C01B32/20—Graphite
- C01B32/21—After-treatment
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- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/133—Electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
- H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
- H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- the present invention relates to a coated carbon material, a negative electrode using the coated carbon material, a secondary battery including the negative electrode, and a method for producing the coated carbon material.
- lithium-ion secondary batteries which have a higher energy density than nickel-cadmium batteries and nickel-hydrogen batteries and are excellent in large-current charge-discharge characteristics.
- increasing the capacity of lithium-ion secondary batteries has been widely studied. , to achieve longer life.
- a carbon material such as graphite As a negative electrode active material.
- graphite with a high degree of graphitization has a capacity close to 372 mAh / g, which is the theoretical capacity for lithium absorption of graphite, when used as a negative electrode active material for a lithium ion secondary battery. It is known to be preferable as a negative electrode active material because of its excellent properties.
- the density of the active material layer containing the negative electrode material is increased in order to increase the capacity, the irreversible charge-discharge capacity during the initial cycle will increase, the large-current charge-discharge characteristics will decrease, and the cycle characteristics will deteriorate due to the destruction and deformation of the material. I had a problem with the drop.
- the surface of the carbon material is usually coated with a polymer compound used as a binder or the like or a non-aqueous electrolyte solution.
- the reaction forms a protective coating called SEI (Solid Electrolyte Interface). It is known that the SEI prevents contact between the carbon material and the electrolytic solution, suppresses the decomposition of the electrolytic solution by the active carbon material, and maintains the chemical stability of the negative electrode surface.
- Patent Document 1 natural graphite is subjected to spheroidization treatment (mechanical energy treatment) to produce spheroidized natural graphite, and the spheroidized natural graphite is used as nuclei.
- spheroidization treatment mechanical energy treatment
- a technique has been developed in which graphite is used and its surface is coated with amorphous carbon.
- spherical natural graphite disclosed in Patent Document 1 although high capacity and good rapid charge/discharge characteristics are obtained, excessive decomposition of the electrolytic solution occurs, resulting in poor initial irreversible capacity and charge/discharge cycle characteristics. It was inadequate and generated a large amount of side reaction product gas, and further improvement was required.
- Patent Document 3 discloses that surface oxygen functional groups are imparted to improve adhesiveness with water-soluble polymers.
- a method of impregnating spheroidized natural graphite with a water-soluble polymer is disclosed.
- Patent Document 4 an increase in internal resistance after charge-discharge cycles is suppressed, and for the purpose of improving cycle characteristics, the surface of the carbon material has a boron atom and a C—O—C bonding portion.
- a method of applying a coating comprising boron atoms and bridging sites interposed between the negative electrode active material is disclosed.
- the spherical natural graphite disclosed in Patent Document 1 has a high capacity and good rapid charge/discharge characteristics, but causes excessive decomposition of the non-aqueous electrolyte. Therefore, the initial charge/discharge efficiency and charge/discharge cycle characteristics were insufficient, and the amount of side reaction product gas generated was large, and further improvement was required.
- a carbon material is coated with an ion-conductive polymer or a water-soluble polymer disclosed in Patent Document 2
- the adhesiveness to the carbon material is insufficient and the electrolyte swells.
- the initial charge/discharge efficiency, charge/discharge cycle characteristics, and stability are still insufficient.
- the present invention has been made in view of such background art, and provides a coated carbon material capable of obtaining a secondary battery having excellent initial efficiency while maintaining capacity compared to the conventional technology, and as a result, , to provide a high-performance secondary battery.
- the present inventors have found that a coated carbon material having a specific coating formed on the surface of the carbon material, a negative electrode using the coated carbon material, and a secondary comprising the negative electrode The inventors have found that a battery can solve the above problems, and have completed the present invention.
- the present inventors consider the reason why the coated carbon material according to the present invention exhibits the above effects as follows. That is, the present inventors believe that it is important for the film coated on the carbon material to satisfy at least one condition selected from the following condition (1) and condition (2).
- Condition (1) The film contains an acetoacetyl group-containing resin. A resin having acetoacetyl groups as self-crosslinking groups crosslinks on the surface of the coated carbon material, suppressing swelling and elution of the resin, thereby improving slurry properties and suppressing side reactions with electrolyte due to efficient coating. It is considered that the effect of improving the initial efficiency brought about by this was able to be remarkably expressed.
- the film contains a polyvinyl alcohol-based resin and a crosslinked product of a silicon element-containing compound.
- a silicon element-containing compound as a cross-linking agent contained together with the polyvinyl alcohol-based resin cross-links on the surface of the coated carbon material, suppressing the swelling and elution of the polymer, thereby improving the slurry properties and the electrolyte solution through efficient coating. It is considered that the effect of improving the initial efficiency brought about by the suppression of side reactions was remarkably exhibited.
- the film further contains a boron element-containing compound (simply referred to as a boron compound)
- a boron element-containing compound simply referred to as a boron compound
- the crosslinked structure on the surface of the coated carbon material suppresses the elution of the boron compound, thereby further remarkably exhibiting the effect. It is thought that it was possible.
- the gist of the present invention is as follows.
- a coated carbon material obtained by coating a carbon material with a film A coated carbon material, wherein the coating comprises at least one compound selected from the following compound (X) and a crosslinked product of the following compound group (Y).
- Y acetoacetyl group-containing resin
- Y polyvinyl alcohol-based resin and silicon element-containing compound
- the boron element-containing compound is at least one compound selected from boron oxide, metaboric acid, tetraboric acid, borates, and alkoxides having 1 to 3 carbon atoms bonded to boron,
- the coated carbon material according to [9].
- a method for producing a coated carbon material in which a carbon material is coated with a coating comprising a step of mixing a carbon material with a compound (X) and/or a group of compounds (Y) below (X): Acetoacetyl group-containing resin (Y): Polyvinyl alcohol based resin and silicon element-containing compound [12] comprising a current collector and an active material layer formed on the current collector, A negative electrode, wherein the active material layer comprises the coated carbon material according to any one of [1] to [10].
- a secondary battery comprising a positive electrode, a negative electrode and an electrolyte, A secondary battery, wherein the negative electrode is the negative electrode according to [12].
- the coated carbon material of the present invention as a negative electrode active material for a secondary battery, it is possible to provide a secondary battery with excellent initial efficiency while maintaining capacity.
- a coated carbon material (which may be referred to as a negative electrode material), which is an embodiment of the present invention, is a coated carbon material capable of intercalating and deintercalating lithium ions, A coated carbon material obtained by coating a carbon material with a film, A coated carbon material, wherein the coating (also simply referred to as "film") contains at least one compound selected from the following compound (X) and a crosslinked compound of the following compound group (Y). is not particularly limited.
- Y Polyvinyl alcohol-based resin and silicon element-containing compound
- the coating is composed of a film containing the compound (X) and a film containing a crosslinked product of the compound group (Y).
- the film may be a coating comprising at least one selected membrane (which may be a coating consisting of said membrane).
- the film may or may not contain components other than the compound (X) and the crosslinked product of the compound group (Y), and is composed only of the compound (X). It may be composed only of the crosslinked product of the compound group (Y), or it may be a laminated film of a film containing the compound (X) and a film containing the crosslinked product of the compound group (Y).
- Carbon materials include graphite, amorphous carbon, and carbonaceous materials with a low degree of graphitization.
- graphite is commercially readily available, has a theoretically high charge/discharge capacity of 372 mAh/g, and is capable of performing at a high current density compared to other negative electrode active materials. It is preferable because the effect of improving the charge/discharge characteristics is large.
- Graphite with few impurities is preferable, and if necessary, it can be used after being subjected to various known purification treatments. Types of graphite include natural graphite, artificial graphite, and the like, and natural graphite is more preferable.
- those coated with a carbonaceous material such as amorphous carbon or graphitized material may be used.
- these can be used individually or in combination of 2 or more types.
- artificial graphite include coal tar pitch, coal-based heavy oil, atmospheric residual oil, petroleum-based heavy oil, aromatic hydrocarbons, nitrogen-containing cyclic compounds, sulfur-containing cyclic compounds, polyphenylene, polyvinyl chloride, polyvinyl Alcohol, polyacrylonitrile, polyvinyl butyral, natural polymer, polyphenylene sulfide, polyphenylene oxide, furfuryl alcohol resin, phenol-formaldehyde resin, imide resin, and other organic materials are baked and graphitized.
- the firing temperature can be in the range of 2500° C. or higher and 3200° C. or lower, and a silicon-containing compound, a boron-containing compound, or the like can be used as a graphitization catalyst during the firing.
- natural graphite include highly purified flake graphite and graphite that has been subjected to spheroidizing treatment. Among them, natural graphite that has been subjected to a spheroidizing treatment is more preferable from the viewpoint of the filling properties of particles and charge/discharge load characteristics.
- a device used for the spheroidizing treatment for example, a device that repeatedly applies mechanical action such as compression, friction, or shear force including interaction of graphite carbonaceous material particles mainly to impact force can be used. . Specifically, it has a rotor with many blades installed inside the casing, and when the rotor rotates at high speed, mechanical impact such as impact compression, friction, or shear force is applied to the carbon material introduced inside. Apparatuses that provide the action and surface treatment are preferred. Moreover, it is preferable to have a mechanism for repeatedly giving a mechanical action by circulating graphite.
- Examples of preferred devices for imparting mechanical action to carbon materials include Hybridization System (manufactured by Nara Machinery Co., Ltd.), Crypton (manufactured by Earthtechnica Corporation), CF Mill (manufactured by Ube Industries), Mechanofusion System (Hosokawa Micron Corporation). (manufactured by Tokuju Kosakusho Co., Ltd.), or Theta Composer (manufactured by Tokuju Kosakusho).
- the hybridization system manufactured by Nara Machinery Co., Ltd. is preferable.
- the peripheral speed of the rotating rotor is preferably 30 to 100 m/sec, more preferably 40 to 100 m/sec, and particularly preferably 50 to 100 m/sec.
- the treatment of imparting a mechanical action to the carbon material can be performed by simply passing graphite through, but it is preferable to circulate or retain the graphite in the apparatus for 30 seconds or more, and the treatment is preferably performed in the apparatus for 1 minute or more. It is more preferable to treat by circulation or retention.
- amorphous carbon include particles obtained by calcining bulk mesophase and particles obtained by subjecting a carbon precursor to infusibility treatment and calcining it.
- Examples of carbonaceous substances with a low degree of graphitization include those obtained by firing organic substances at a temperature of usually less than 2500°C.
- Organic substances include coal tar pitch, coal-based heavy oil such as dry distillation liquefied oil; straight-run heavy oil such as atmospheric residual oil and vacuum residual oil; Aromatic hydrocarbons such as acenaphthylene, decacyclene and anthracene; Nitrogen-containing cyclic compounds such as phenazine and acridine; Sulfur-containing cyclic compounds such as thiophene; Aliphatic cyclic compounds such as adamantane Compounds; polyphenylenes such as biphenyl and terphenyl; polyvinyl esters such as polyvinyl chloride, polyvinyl acetate and polyvinyl butyral; and thermoplastic polymers such as polyvinyl alcohol.
- the firing temperature can be 600° C. or higher, preferably 900° C. or higher, more preferably 950° C. or higher, and can be lower than 2500° C., The range is preferably 2000° C. or lower, more preferably 1400° C. or lower. Acids such as phosphoric acid, boric acid, and hydrochloric acid, and alkalis such as sodium hydroxide can be mixed with the organic matter during firing.
- carbon material particles obtained by coating the above-described natural graphite or artificial graphite with amorphous carbon and/or a graphite substance having a low degree of graphitization can also be used.
- the carbon material that constitutes the carbon material can also be used in combination with one or more of other carbon materials.
- the carbon material may contain a Si-containing compound.
- the Si-containing compound include a negative electrode active material containing a composite oxide phase containing lithium silicate and silicon particles dispersed in the composite oxide phase. be done.
- the volume-based average particle diameter (also referred to as “average particle diameter d50”) of the carbon material of the present embodiment is preferably 1 ⁇ m or more, more preferably 5 ⁇ m or more, still more preferably 10 ⁇ m or more, particularly preferably 15 ⁇ m or more, and most preferably , 16.5 ⁇ m or more.
- the average particle diameter d50 is preferably 50 ⁇ m or less, more preferably 40 ⁇ m or less, still more preferably 35 ⁇ m or less, particularly preferably 30 ⁇ m or less, and most preferably 25 ⁇ m or less.
- the average particle size d50 is within the above range, the increase in irreversible capacity of the secondary battery (particularly, non-aqueous secondary battery) obtained using the carbon material, the loss of the initial battery capacity can be suppressed, and the slurry can be applied. It is possible to suppress the occurrence of process problems such as streaking in the battery, the deterioration of high current density charge and discharge characteristics, and the deterioration of low temperature input/output characteristics.
- the average particle diameter d50 is defined by adding a carbon material to 10 mL of a 0.2% by mass aqueous solution of polyoxyethylene sorbitan monolaurate (an example of which is Tween 20 (registered trademark)), which is a surfactant. 0.01 g is suspended, and this is introduced as a measurement sample into a commercially available laser diffraction/scattering particle size distribution analyzer (eg LA-920 manufactured by HORIBA), and the measurement sample is irradiated with ultrasonic waves of 28 kHz at an output of 60 W for 1 minute. After that, it is defined as the volume-based median diameter measured by the measuring device.
- a commercially available laser diffraction/scattering particle size distribution analyzer eg LA-920 manufactured by HORIBA
- the circularity of the carbon material of the present embodiment is 0.88 or more, preferably 0.90 or more, and more preferably 0.91 or more. Also, the degree of circularity is preferably 1 or less, more preferably 0.98 or less, still more preferably 0.97 or less. When the circularity is within the above range, it tends to be possible to suppress the deterioration of the high current density charge/discharge characteristics of secondary batteries (especially non-aqueous secondary batteries).
- the circularity value for example, using a flow type particle image analyzer (for example, FPIA manufactured by Sysmex Industrial Co., Ltd.), about 0.2 g of a sample (carbon material) is added to polyoxyethylene (20) sorbitan, which is a surfactant.
- a flow type particle image analyzer for example, FPIA manufactured by Sysmex Industrial Co., Ltd.
- a sample carbon material
- polyoxyethylene (20) sorbitan which is a surfactant.
- the detection range is specified as 0.6 to 400 ⁇ m
- the particle size is Values measured for particles in the range 1.5-40 ⁇ m are used.
- the method for improving the degree of circularity is not particularly limited, but it is preferable to apply a spheroidizing treatment to make the particles spherical, since the shape of the inter-particle voids when used as a negative electrode will be in order.
- spheroidization include a method of mechanically approximating a sphere by applying shearing force and compressive force, and a mechanical/physical treatment of granulating a plurality of carbon material fine particles by the adhesive force possessed by the binder or the particles themselves. methods and the like.
- the tap density of the carbon material of the present embodiment is preferably 0.7 g/cm 3 or more, more preferably 0.8 g/cm 3 or more, still more preferably 0.85 g/cm 3 or more, and particularly preferably 0.8 g/cm 3 or more. 9 g/cm 3 or more, most preferably 0.95 g/cm 3 or more, preferably 1.3 g/cm 3 or less, more preferably 1.2 g/cm 3 or less, still more preferably 1.1 g/cm 3 3 or less.
- the tap density is within the above range, processability such as streaking during electrode plate production is improved, and high-speed charge/discharge characteristics are excellent.
- the intra-particle carbon density is less likely to increase, rollability is good, and there is a tendency to easily form a high-density negative electrode sheet.
- the tap density is measured by using a powder density measuring instrument, and dropping the carbon material of the present embodiment through a sieve with an opening of 300 ⁇ m into a cylindrical tap cell with a diameter of 1.6 cm and a volume capacity of 20 cm 3 to fill the cell to the full. After that, tapping with a stroke length of 10 mm is performed 1000 times, and the density is defined as the density obtained from the volume and mass of the sample at that time.
- the d value (interlayer distance) of the lattice plane (002 plane) obtained by X-ray diffraction according to the Gakushin method of the carbon material of the present embodiment is preferably 0.335 nm or more and less than 0.340 nm. .
- the d value is more preferably 0.339 nm or less, still more preferably 0.337 nm or less.
- the crystallinity of the graphite is high, so there is a tendency to suppress an increase in the initial irreversible capacity.
- 0.335 nm is the theoretical value of graphite.
- the crystallite size (Lc) of the carbon material determined by X-ray diffraction according to the Gakushin method is preferably 1.5 nm or more, more preferably 3.0 nm or more. Within the above range, the particles are not too low in crystallinity, and the reversible capacity is less likely to decrease when used as a secondary battery (particularly, a non-aqueous secondary battery).
- the lower limit of Lc is the theoretical value of graphite.
- Ash content is preferably 1% by mass or less, more preferably 0.5% by mass or less, and still more preferably 0.1% by mass, relative to the total mass of the carbon material. It is below. Moreover, it is preferable that the lower limit of the ash content is 1 ppm or more.
- the ash content is within the above range, in the case of a secondary battery (especially a non-aqueous secondary battery), the deterioration of battery performance due to the reaction between the carbon material and the electrolyte during charging and discharging can be suppressed to a negligible level. can.
- cost increases can be suppressed.
- the specific surface area (SA) of the carbon material of the present embodiment measured by the BET method is preferably 2 m 2 /g or more, more preferably 2.4 m 2 /g or more, still more preferably 2.6 m 2 /g or more, especially It is preferably 2.8 m 2 /g or more, most preferably 3.0 m 2 /g or more. Also, it is preferably 13 m 2 /g or less, more preferably 12 m 2 /g or less, still more preferably 11 m 2 /g or less, particularly preferably 10 m 2 /g or less, and most preferably 9 m 2 /g or less.
- the specific surface area is within the above range, it is possible to sufficiently secure the site where Li enters and exits, so the output characteristics of high-speed charge and discharge characteristics are excellent, and the activity of the active material with respect to the electrolyte solution can be moderately suppressed, so the initial irreversible capacity does not increase, and there is a tendency to manufacture high-capacity batteries.
- the negative electrode is formed using a carbon material, an increase in reactivity with the electrolyte can be suppressed, and gas generation can be suppressed, so it is preferable for secondary batteries (especially non-aqueous secondary batteries). can be provided.
- the BET specific surface area is measured by using a surface area meter (for example, Maxsorb HM Model-1210, manufactured by Mountec), and preliminarily drying the carbon material sample at 100°C under nitrogen flow for 30 minutes under reduced pressure, and then cooling to liquid nitrogen temperature. and is defined as a value measured by the nitrogen adsorption BET one-point method using nitrogen gas.
- a surface area meter for example, Maxsorb HM Model-1210, manufactured by Mountec
- the pore volume in the range of 10 nm to 1000 nm is a value measured using a mercury intrusion method (mercury porosimetry), and is preferably 0. 05 mL/g or more, more preferably 0.07 mL/g or more, still more preferably 0.1 mL/g or more, preferably 0.3 mL/g or less, more preferably 0.28 mL/g or less, More preferably, it is 0.25 mL/g or less.
- the pore volume in the range of 10 nm to 1000 nm is within the above range, it is difficult for the electrolyte solution (especially non-aqueous electrolyte solution) to enter the pores, and lithium ions are intercalated and desorbed during rapid charging and discharging. It is possible to further avoid the tendency that the cycle characteristics are deteriorated due to deposition of lithium metal accompanying the failure of the cycle time. Furthermore, it is possible to prevent the binder from being easily absorbed into the voids during the production of the electrode plate, thereby reducing the electrode plate strength and initial efficiency.
- the electrolyte solution especially non-aqueous electrolyte solution
- the total pore volume of the carbon material of the present embodiment is preferably 0.1 mL/g or more, more preferably 0.2 mL/g or more, still more preferably 0.25 mL/g or more, and particularly preferably 0.5 mL. / g or more.
- the total pore volume is preferably 10 mL/g or less, more preferably 5 mL/g or less, even more preferably 2 mL/g or less, and particularly preferably 1 mL/g or less.
- the average pore size of the carbon material of the present embodiment is preferably 0.03 ⁇ m or more, more preferably 0.05 ⁇ m or more, still more preferably 0.1 ⁇ m or more, and particularly preferably 0.5 ⁇ m or more.
- the average pore size is preferably 80 ⁇ m or less, more preferably 50 ⁇ m or less, and even more preferably 20 ⁇ m or less.
- a mercury porosimeter (Autopore 9520: manufactured by Micromeritex Co., Ltd.) can be used as a device for the mercury porosimetry.
- a sample carbon material
- a sample is weighed to a value of about 0.2 g, sealed in a powder cell, and pretreated by degassing at 25° C. under vacuum (50 ⁇ mHg or less) for 10 minutes.
- a vacuum of 4 psia (approximately 28 kPa) is applied to introduce mercury into the cell, the pressure is increased in steps from 4 psia (approximately 28 kPa) to 40,000 psia (approximately 280 MPa), and then reduced to 25 psia (approximately 170 kPa).
- the number of steps during pressure increase is 80 or more, and the amount of mercury intrusion is measured after an equilibrium time of 10 seconds at each step.
- the pore size distribution is calculated from the mercury intrusion curve thus obtained using the Washburn equation.
- the surface tension ( ⁇ ) of mercury is 485 dyne/cm, and the contact angle ( ⁇ ) is 140°.
- the average pore diameter is defined as the pore diameter when the cumulative pore volume is 50%.
- the true density of the carbon material of the present embodiment is preferably 1.9 g/cm 3 or more, more preferably 2 g/cm 3 or more, still more preferably 2.1 g/cm 3 or more, and particularly preferably 2.2 g. /cm 3 or more, and the upper limit is 2.26 g/cm 3 .
- the upper limit is the theoretical value of graphite.
- the aspect ratio of the powdered carbon material of the present embodiment is theoretically 1 or more, preferably 1.1 or more, and more preferably 1.2 or more. Also, the aspect ratio is preferably 10 or less, more preferably 8 or less, and still more preferably 5 or less. When the aspect ratio is within the above range, streaking of the slurry containing the carbon material (negative electrode forming material) is less likely to occur during electrode plate formation, and a uniform coated surface can be obtained. ) tends to avoid the deterioration of high current density charge/discharge characteristics.
- the aspect ratio is represented by A/B, where A is the longest diameter of the carbon material particles (carbon material) when observed three-dimensionally, and B is the shortest diameter among the diameters perpendicular to it.
- Observation of the carbon material particles is performed with a scanning electron microscope capable of magnified observation. Select any 50 carbon material particles fixed to the end surface of a metal with a thickness of 50 microns or less, rotate and tilt the stage on which the sample is fixed for each, measure A and B, and measure A/B Find the average value of
- the maximum particle size dmax of the carbon material of the present embodiment is preferably 200 ⁇ m or less, more preferably 150 ⁇ m or less, still more preferably 120 ⁇ m or less, particularly preferably 100 ⁇ m or less, and most preferably 80 ⁇ m or less.
- dmax is within the above range, it tends to be possible to suppress the occurrence of process problems such as streaking.
- the maximum particle size is defined as the value of the largest particle size at which particles are measured in the particle size distribution obtained when measuring the average particle size d50.
- the Raman R value of the carbon material of the present embodiment is preferably 0.1 or more, more preferably 0.15 or more, and still more preferably 0.2 or more. Also, the Raman R value is preferably 0.6 or less, more preferably 0.5 or less, and even more preferably 0.4 or less.
- the Raman R value is obtained by measuring the intensity I A of the peak P A near 1580 cm ⁇ 1 and the intensity I B of the peak P B near 1360 cm ⁇ 1 in the Raman spectrum obtained by Raman spectroscopy. Defined as calculated as the ratio (I B /I A ).
- “near 1580 cm- 1 " refers to the range of 1580 to 1620 cm- 1
- “near 1360 cm -1 " refers to the range of 1350 to 1370 cm- 1 .
- the Raman R value is within the above range, the crystallinity of the surface of the carbon material particles (carbon material) is difficult to increase, and when the density is increased, the crystals are difficult to orient in the direction parallel to the negative electrode plate, resulting in a decrease in load characteristics. tend to avoid. Furthermore, the crystals on the surface of the particles are less likely to be disturbed, suppressing an increase in reactivity with the electrolytic solution of the negative electrode, and avoiding a decrease in charge-discharge efficiency and an increase in gas generation in secondary batteries (especially non-aqueous secondary batteries). There is a tendency.
- the Raman spectrum can be measured with a Raman spectrometer.
- the particles to be measured are allowed to fall freely into the measurement cell to fill the sample, and while the measurement cell is irradiated with argon ion laser light, the measurement cell is rotated in a plane perpendicular to the laser light.
- the measurement conditions are as follows. Argon ion laser light wavelength: 514.5 nm Laser power on sample: 25mW Resolution: 4 cm -1 Measurement range: 1100 cm -1 to 1730 cm -1 Peak intensity measurement, peak half width measurement: background processing, smoothing processing (convolution 5 points by simple average)
- the DBP (dibutyl phthalate) oil absorption of the carbon material of the present embodiment is preferably 65 ml/100 g or less, more preferably 62 ml/100 g or less, still more preferably 60 ml/100 g or less, and particularly preferably 57 ml/100 g. It is below. Also, the DBP oil absorption is preferably 30 ml/100 g or more, more preferably 40 ml/100 g or more.
- the DBP oil absorption is within the above range, it means that the progress of spheroidization of the carbon material is sufficient, and there is a tendency that streaking is less likely to occur when the slurry containing the carbon material is applied. Since there is also a pore structure, there is a tendency to avoid lowering the reaction surface.
- the DBP oil absorption is defined as a measured value in accordance with ISO 4546, when 40 g of measurement material (carbon material) is charged, and the dropping rate is 4 ml/min, the rotation speed is 125 rpm, and the set torque is 500 N ⁇ m.
- Brabender's absorbometer E type can be used for the measurement.
- the particle diameter (d10) corresponding to cumulative 10% from the smaller particle side of the particle diameter measured on a volume basis of the carbon material of the present embodiment is preferably 30 ⁇ m or less, more preferably 20 ⁇ m or less, further preferably 17 ⁇ m or less. is 1 ⁇ m or more, more preferably 5 ⁇ m or more, still more preferably 10 ⁇ m or more, particularly preferably 11 ⁇ m or more, and most preferably 13 ⁇ m or more.
- d10 is defined as a value at which the particle frequency % becomes 10% by accumulation from the smaller particle size in the particle size distribution obtained when the average particle size d50 is measured.
- the particle size (d90) corresponding to cumulative 90% from the small particle side of the particle size measured on a volume basis of the carbon material of the present embodiment is preferably 100 ⁇ m or less, more preferably 70 ⁇ m or less, still more preferably 60 ⁇ m or less, and more preferably 60 ⁇ m or less. More preferably 50 ⁇ m or less, particularly preferably 45 ⁇ m or less, most preferably 42 ⁇ m or less, preferably 20 ⁇ m or more, more preferably 26 ⁇ m or more, still more preferably 30 ⁇ m or more, particularly preferably 34 ⁇ m or more.
- d90 is within the above range, it is possible to avoid a decrease in electrode strength and a decrease in initial charge-discharge efficiency in secondary batteries (especially non-aqueous secondary batteries), and the occurrence of process inconveniences such as streaking during slurry application. , deterioration of high-current-density charge-discharge characteristics, and deterioration of low-temperature input/output characteristics can also be avoided.
- the d90 is defined as a value at which the particle frequency % becomes 90% by accumulation from the small particle size in the particle size distribution obtained when the average particle size d50 is measured.
- the acetoacetyl group-containing resin in (X) When the film according to the present embodiment contains the acetoacetyl group-containing resin according to (X) above, the compound of the acetoacetyl group-containing resin may be a single compound or two compounds. More than one type of compound may be mixed. By using the acetoacetyl group-containing resin, it is possible to obtain a coated carbon material capable of obtaining a secondary battery whose capacity is maintained as compared with the conventional technology.
- the preferred structure of the acetoacetyl group-containing resin according to (X) of the present embodiment is at least one structure selected from the group consisting of a linear structure, a graft structure, a star structure, and a three-dimensional network structure. It is preferable to have The acetoacetyl group-containing resin may have a functional group other than the acetoacetyl group. hydroxyl group).
- the compound of (X) and the crosslinked product of the compound group of (Y) may overlap, the compound of (X) may be treated as excluding the crosslinked product of the compound group of (Y). Examples of the case where the above overlap occurs include the case where the polyvinyl alcohol-based resin (Y) is an acetoacetyl group-containing resin.
- the acetoacetyl group-containing resin improves the resistance of the secondary battery negative electrode active material (especially, non-aqueous secondary battery negative electrode active material) to the electrolyte solution, and makes it difficult for the coating of the coated carbon material to dissolve in the electrolyte solution. is more preferably sparingly soluble in the electrolytic solution (non-aqueous electrolytic solution).
- the acetoacetyl group-containing resin is soluble in the electrolytic solution (especially non-aqueous electrolytic solution), it can be rendered insoluble in the electrolytic solution by reacting and curing with the cross-linking agent. .
- poorly soluble in the electrolytic solution means that the acetoacetyl group-containing resin is mixed with ethylene carbonate and ethyl methyl carbonate (3:7 by volume) at a mass ratio of 1:50 (acetoacetyl group-containing resin: solvent). ratio) at 60° C. for 5 hours, the dry mass reduction rate before and after the immersion is 10% by mass or less.
- the resin of the resin portion excluding the acetoacetyl group of the acetoacetyl group-containing resin according to (X) of the present embodiment is not particularly limited, and specifically includes polyvinyl alcohol resin, acrylic polyol resin, polyester polyol resin, Alternatively, polyol-based resins such as polyether polyol resins, silicone resins, epoxy resins, acrylic resins having hydrolyzable silyl groups, or polyester resins are preferable, and polyvinyl alcohol-based resins, acrylic polyol resins, polyester polyol resins, hydrolyzable silyl An acrylic resin having a group or a polyester resin is more preferable, and a polyvinyl alcohol resin, an acrylic polyol resin, or a polyester polyol resin is particularly preferable, and a polyvinyl alcohol resin (e.g., polyvinyl alcohol resin ) is most preferred.
- polyvinyl alcohol resin e.g., polyvinyl alcohol resin
- Polyvinyl alcohol resin in (Y) When the film according to the present embodiment contains a crosslinked product of the polyvinyl alcohol resin according to (Y) and a silicon element-containing compound, the polyvinyl alcohol resin is a single compound. Alternatively, two or more types of compounds may be mixed. In addition, as a preferred structure of the polyvinyl alcohol-based resin according to (Y) of the present embodiment, at least one structure selected from the group consisting of a linear structure, a graft structure, a star structure, and a three-dimensional network structure. It is preferable to have The polyvinyl alcohol-based resin may have a substituent.
- This functional group is preferably a reactive substituent, and although the reactive substituent is not particularly limited, it has high reactivity and can form a covalent bond with a cross-linking agent having excellent water resistance and solvent resistance.
- an alcoholic hydroxyl group, a carboxyl group, a carbonyl group, a (meth)acrylic group, an epoxy group, a vinyl group, a hydrolyzable silyl group, a silanol group, a hydrosilyl group, or an acetoacetyl group are preferred, and an alcoholic hydroxyl group, a carboxyl group, a carbonyl A hydrolyzable silyl group, a silanol group, or an acetoacetyl group is more preferred, an alcoholic hydroxyl group, a carbonyl group, a silanol group, or an acetoacetyl group is more preferred, an alcoholic hydroxyl group, or an acetoacetyl group is particularly preferred, and an aceto Ace
- Polyvinyl alcohol-based resin is used because it improves the resistance of the secondary battery negative electrode active material (especially, the non-aqueous secondary battery negative electrode active material) to the electrolyte solution, and makes it difficult for the coating of the coated carbon material to elute into the electrolyte solution. More preferably, it is poorly soluble in an electrolytic solution (particularly, a non-aqueous electrolytic solution). However, in the present embodiment, even if the polyvinyl alcohol-based resin dissolves in the electrolytic solution (non-aqueous electrolytic solution), it can be rendered sparingly soluble in the electrolytic solution by reacting and curing with the cross-linking agent.
- poorly soluble in the electrolytic solution means that the cured product obtained by the reaction of the polyvinyl alcohol-based resin, or the above resin and the silicon element-containing compound as a cross-linking agent, ethyl carbonate and ethyl methyl carbonate (3: 7 volume ratio) at 60° C. for 5 hours, the dry mass reduction rate before and after the immersion is 10% by mass or less.
- the film containing the crosslinked product of the compound group (Y) of the present embodiment may or may not have a structure derived from a resin (another resin) other than the polyvinyl alcohol-based resin.
- a resin another resin
- specific examples of other resins are not particularly limited, they have a plurality of hydroxyl groups in the molecule or groups that become hydroxyl groups by hydrolysis, and react with a cross-linking agent to be sparingly soluble in water or an electrolytic solution.
- polyol resins such as acrylic polyol resins, polyester polyol resins, or polyether polyol resins, silanol, or silicone resins having hydrolyzable silyl groups, epoxy resins, hydrolyzable silyl
- acrylic resin or a polyester resin having a group is preferable, an acrylic polyol resin, a polyester polyol resin, an acrylic resin or a polyester resin having a hydrolyzable silyl group is more preferable, and an acrylic polyol resin or a polyester polyol resin is particularly preferable.
- PVOH-based resin Polyvinyl alcohol-based resin (hereinafter sometimes referred to as PVOH-based resin) is not particularly limited in its specific structure as long as it is a resin having a vinyl alcohol structural unit. Typically, it is obtained by saponifying polycarboxylic acid vinyl ester obtained by polymerizing a carboxylic acid vinyl ester monomer such as vinyl acetate, but it is not limited to this.
- PVOH-based resin examples include unmodified PVOH and modified PVOH-based resin.
- the modified PVOH-based resin may be a copolymerized modified PVOH-based resin synthesized by copolymerizing a monomer other than a vinyl ester-based monomer that provides a PVOH structural unit, or may be a copolymerized modified PVOH-based resin synthesized by synthesizing an unmodified PVOH. It may be a modified PVOH-based resin after modifying the chain or side chain with an appropriate compound.
- examples of copolymerized modified PVOH resins include PVOH resins having primary hydroxyl groups in side chains.
- PVOH-based resins include side-chain 1,2-diol-modified PVOH-based resins obtained by copolymerizing 3,4-diacetoxy-1-butene, vinylethylene carbonate, glycerin monoallyl ether, or the like; or 1, Hydroxymethylvinylidene diacetate such as 3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, or 1,3-dibutyronyloxy-2-methylenepropane;
- a PVOH-based resin having a hydroxymethyl group in a side chain obtained by saponification can be mentioned.
- unmodified PVOH or the modified PVOH resin is acetoacetic esterified, acetalized, urethanized, etherified, grafted, phosphorylated, or oxyalkylenated. methods and the like.
- both the above unmodified PVOH and modified PVOH can be used in this embodiment.
- PVOH which is easily soluble in cold water and could not be used as a film by itself in the past because it is easily eluted when it is made into a water slurry for electrode plate coating
- self-crosslinks in the above case (X) in the case of (Y) above, it can be made sparingly soluble by using a cross-linking agent, and can be suitably used as a carbon material coating.
- PVOH a PVOH containing an anion modifying group having a functional group excellent in lithium conductivity, such as a carboxylic acid group or a sulfonic acid group, or a nonionic modifying group having a hydroxyalkyl group or an oxyethylene group, etc., is contained in the side chain. PVOH and the like can also be used, and the resistance of the film can be lowered.
- the solubility of PVOH resin differs depending on the degree of saponification and polymerization.
- the degree of saponification of the PVOH-based resin is not particularly limited, but in the present embodiment, a cross-linking agent can be used in combination to make the resin insoluble, so a wide range of degrees of saponification can be selected.
- the degree of saponification is usually 70 mol % or more, preferably 78 to 100 mol %, particularly preferably 85 to 99.8 mol %.
- PVOH-based resins tend to have the highest elution rate in water when the degree of saponification is around 88 mol %, although there are some differences depending on the degree of polymerization, the type of modification, and the like. Therefore, in order to improve the water resistance of the cured product reacted with the cross-linking agent, it is preferable to make the degree of saponification higher or lower than around 88%.
- PVOH-based resin having a modifying group even if it has a high degree of saponification, it is difficult to crystallize and has high solubility in water.
- it is 98 mol % or more.
- the upper limit is usually 100 mol% or less, preferably 99.8 mol% or less.
- the degree of saponification is a value measured according to ISO 15023-2.
- low saponification PVOH having a degree of saponification of 38 to 55 mol % can be used in combination with a cross-linking agent.
- the average degree of polymerization of the PVOH-based resin is not particularly limited.
- the average degree of polymerization is usually 100 or more, preferably 200 or more, more preferably 250 or more. This range makes it easier to prevent the solubility from becoming too high. Moreover, it is usually 4,000 or less, preferably 3,500 or less, and more preferably 2,800 or less. This range makes it easier to prevent the solubility from becoming too low.
- Such average degree of polymerization is a value measured by an aqueous solution viscosity measurement method (ISO 15023-2).
- the PVOH-based resin only one resin may be used, or two or more resins may be blended and used.
- the structural units may differ, the degree of saponification may differ, and the average degree of polymerization may differ.
- the degree of saponification, the average degree of polymerization, etc. in the case of blending and using may be such that the average values of all the PVOH-based resins are within the above ranges.
- the PVOH-based resin may be partially modified. When modified, the modification rate of the PVOH resin is 90% within 60 minutes after dispersing 10 g of the resin particles in 100 g of water under stirring at 20° C., raising the temperature to 90° C. at 1° C./min under stirring. A range in which the mass % or more is dissolved is preferable.
- the film according to the present embodiment has a film derived from a silicon element-containing compound as the cross-linking agent according to (Y) above
- the silicon element-containing compound is not particularly limited, but an acetoacetyl group-containing PVOH resin and a cross-linking agent
- a crosslinked product (film) with is described as an example.
- a method for forming the crosslinked product (crosslinking method) for example, a heat treatment, a crosslinking agent treatment, an ultraviolet irradiation treatment, an electron beam irradiation treatment, or the like is used. Among them, a thermally crosslinked product crosslinked by heat treatment is preferred.
- the type of silicon element-containing compound as a cross-linking agent used in the cross-linking agent treatment is not particularly limited.
- a component having a three-dimensional siloxane crosslinked structure derived from a polycondensate and the like are included.
- the cross-linking agent in addition to the silicon element-containing compound described above, a cross-linking agent other than this (other cross-linking agent) may or may not be used.
- Specific examples of other cross-linking agents include: A known cross-linking agent for a PVOH-based resin having a carboxyl group, an acetoacetyl group, or the like can be used.
- monoaldehyde compounds such as formaldehyde or acetaldehyde
- aldehyde compounds such as polyaldehyde compounds such as glyoxal, glutaraldehyde, or dialdehyde starch
- metaxylenediamine, norbornanediamine 1,3-bisaminomethylcyclohexane, bisamino Propyl piperazine, 3,3′-dimethyl-4,4′-diaminodicyclohexylmethane, 4,4′-diaminodicyclohexylmethane, 4,4′-diaminodiphenylmethane, 3,3′,5,5′-tetramethyl-4 ,4′-diaminodiphenylmethane, 3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane, 3,3′-dimethyl-4,4′-diamino-5,
- the content of such a cross-linking agent is preferably 0.05 to 50 parts by mass, more preferably 0.5 to 35 parts by mass as a solid content with respect to 100 parts by mass of the PVOH resin. , particularly preferably 1 to 25 parts by mass. If the content of the cross-linking agent is too small, the effect of the cross-linking agent tends to be poor, and if it exceeds the upper limit, the unreacted cross-linking agent tends to elute or precipitate.
- Examples of the method for mixing the PVOH-based resin and the cross-linking agent include (i) a method of mixing an aqueous solution of the PVOH-based resin and an aqueous solution of the cross-linking agent, (ii) a method of spraying the aqueous solution of the cross-linking agent onto the solid PVOH-based resin, and (iii) ) A method of spraying an aqueous PVOH-based resin solution onto a solid cross-linking agent.
- the PVOH-based resin that is available on the market may be used, or it may be obtained by synthesis. When synthesized, it can be synthesized by a known method.
- the film according to the present embodiment has a film derived from a silicon element-containing compound as the cross-linking agent according to (Y) above, the surface and/or inner film of the coated carbon material contains a component having a three-dimensional crosslinked structure. It is more preferable to contain a component having a three-dimensional siloxane crosslinked structure derived from a hydrolysis polycondensate of an alkoxysilane and/or a low condensate thereof.
- the three-dimensional crosslinked structure is, for example, a structure in which an organic and/or inorganic crosslinker having two or more crosslinkable reactive groups is crosslinked
- the three-dimensional siloxane crosslinked structure refers to the alkoxysilane is formed by hydrolytic polycondensation of a trialkoxysilane having three alkoxy groups per molecule and/or a tetraalkoxysilane having four alkoxy groups per molecule with each other, a steric structure mainly composed of siloxane units Means mesh structure.
- the three-dimensional siloxane crosslinked structure is derived from a hydrolytic polycondensate of an alkoxysilane and/or a low condensate thereof, and has T units and/or Q units as constituent units.
- a T unit indicates a unit having three oxygen atoms bonded to a Si atom
- a Q unit indicates a unit having four oxygen atoms bonded to a Si atom.
- the alkoxysilane may contain units other than T units and Q units, such as M units having one oxygen atom bonded to a Si atom and D units having two oxygen atoms bonded to a Si atom.
- the T unit is usually 0 mol% or more, and usually 20 mol% or less, preferably 10 mol% or less, particularly preferably 5 mol% or less.
- the Q unit is usually 80 mol % or more, preferably 90 mol % or more, particularly preferably 95 mol % or more, and usually 100 mol % or less.
- the total amount of T units and Q units is usually 80 mol% or more, preferably 90 mol% or more, more preferably 95 mol% or more, and usually 100 mol% or less.
- Alkoxysilane is not particularly limited as long as it is a silane having an alkoxy group.
- alkoxy groups include aliphatic alkoxy groups having 1 to 10 carbon atoms such as methoxy, ethoxy, propoxy, and butoxy, phenoxy, and aryl.
- An aromatic alkoxy group having 6 to 15 carbon atoms such as an oxy group can be mentioned.
- Aliphatic alkoxy groups having 1 to 4 carbon atoms are preferred from the viewpoint of easy control of the hydrolysis reaction.
- Alkoxysilanes include monoalkoxysilanes, dialkoxysilanes, trialkoxysilanes, and tetraalkoxysilanes. More specifically, monoalkoxysilanes such as vinyldimethylethoxysilane; dialkyldialkoxysilanes such as dimethyldimethoxysilane; diaryldialkoxysilanes; 3-aminopropylmethyldimethoxysilane; or 3-[N-(2-aminoethyl ) amino group-containing dialkoxysilanes such as amino]propylmethyldimethoxysilane; mercapto group-containing dialkoxysilanes such as 3-mercaptopropylmethyldimethoxysilane; (meth)acryloyl groups such as 3-(meth)acryloxypropylmethyldimethoxysilane containing dialkoxysilane; alkenyl group-containing dialkoxysilane such as vinyldimethoxymethylsi
- trialkoxysilanes having a hydrosilyl group such as trimethoxysilane; alkyltrialkoxysilanes such as methyltriethoxysilane, methyltrimethoxysilane, ethyltrimethoxysilane, or ethyltriethoxysilane; phenyltrimethoxysilane, or aryltrialkoxysilanes such as phenyltriethoxysilane; mercapto group-containing trialkoxysilanes such as 3-mercaptopropyltrimethoxysilane; alkenyl group-containing trialkoxysilanes such as vinyltrimethoxysilane; 2-(meth)acryloxyethyl Trimethoxysilane, or (meth)acryloyl group-containing trialkoxysilanes such as 2-(meth)acryloxyethyltriethoxysilane; (glycidyloxyalkyl)trialkoxy
- tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, or tetrabutoxysilane are included.
- tetraalkoxysilanes or trialkoxysilanes are preferred, and tetraalkoxysilanes are more preferred, because when a composite film with a PVOH-based resin is formed, the three-dimensional siloxane crosslinked structure becomes highly crosslinked and the effect of suppressing swelling and dissolution of the film increases. preferable. Moreover, the proportion of trialkoxysilane may be increased when it is desired to impart flexibility to the resulting composite coating. These alkoxysilanes may be used alone or in combination of two or more.
- the purpose of the present embodiment is to introduce a three-dimensional crosslinked structure, more specifically, to suppress the solubility and swelling of the PVOH-based resin film by introducing a three-dimensional siloxane crosslinked structure, monoalkoxy, which is a low crosslinkable component, Silane or dialkoxysilane is used as an additive for imparting functionality such as flexibility and lithium ion conductivity, and the amount is kept to a minimum that does not promote dissolution or swelling of the PVOH-based resin component in the composite particles. is preferred.
- Alkoxysilane and/or its low condensate hydrolyzes in a solvent to form a three-dimensional siloxane crosslinked structure as a hydrolytic polycondensate.
- the term "low condensate” as used herein means an oligomer of about 2- to 10-mer of alkoxysilane, and may be an oligomer of about 2- to 8-mer, or may be an oligomer of about 2- to 5-mer.
- a lower alcohol having 1 to 4 carbon atoms such as methanol, ethanol or propanol, or a mixture thereof with water is usually used.
- the component having a three-dimensional crosslinked structure contained in the coated carbon material of the present embodiment more specifically, the component having a three-dimensional siloxane crosslinked structure has a Si content relative to the total mass of the coated carbon material, converted to SiO2 . is usually 0.01% by mass or more, preferably 0.05% by mass or more, and is usually 50% by mass or less, preferably 20% by mass or less.
- the Si content with respect to the total mass of the coated carbon material will be explained later in the description of the coating amount of the coated carbon material.
- the water content of the PVOH-based resin contained in the film of the coated carbon material It is possible to prevent thickening of the water slurry during coating of the electrode plate and clogging of the slurry filtration process. Further, when boric acid is used in combination to reduce film resistance, elution of boric acid from the film can be suppressed, and low resistance can be maintained over a long period of time.
- a component having a three-dimensional crosslinked structure such as a component having a three-dimensional siloxane crosslinked structure
- a component having a three-dimensional siloxane crosslinked structure is present, a trifunctional silicon unit ( T unit: RSiO 1.5 ) to which the carbon atoms of the organic group R are directly bonded and/or Alternatively, a group of broad peaks derived from a crosslinked product of tetrafunctional silicon units (Q units: SiO 2 ) not bonded to carbon atoms of organic groups is observed.
- the coated carbon material itself containing a component having a three-dimensional siloxane crosslinked structure may be used as a sample, and the coated carbon material particles are crushed to peel off the coating.
- the coating fine powder when dispersed in water may be collected by filtration or centrifugation, dried and used as a sample.
- the coated carbon material has a structure coated with a hybrid film of the PVOH-based resin and the SiO 2 layer, the hybrid film may be peeled off and recovered as a sample, and the PVOH-based resin in the coated carbon material can be collected by filtration or centrifugation, dried and used as a sample.
- the film according to the present embodiment has a film derived from a boron element-containing compound as the cross-linking agent according to (Y), from the viewpoint of reducing resistance, the film (Y) further includes a boron element-containing compound It is preferable to contain a boron element derived from.
- the content of the boron element-containing compound (including the portion corresponding to the structure derived from the boron element-containing compound) in the film of (Y) is not particularly limited, but the content with respect to the total mass of the coated carbon material is 0.5.
- boron element-containing compound 01% by mass or more, more preferably 0.03% by mass or more, even more preferably 0.05% by mass or more, and preferably 10% by mass or less, 5% by mass % or less, more preferably 1 mass % or less.
- the type of boron element-containing compound is not particularly limited, and examples thereof include boron oxide, metaboric acid, tetraboric acid, borate, alkoxide having 1 to 3 carbon atoms bonded to boron, lithium borate, and the like. At least one compound selected from boron oxide, metaboric acid, tetraboric acid, borates, and alkoxides having 1 to 3 carbon atoms bonded to boron, because it can be easily coated. is preferred.
- One of these boron element-containing compounds may be used alone, or two or more thereof may be used in combination.
- any additive that improves battery performance can be used in combination with the coating of the coated carbon material of the present embodiment within a range that does not affect the curability, water resistance, solvent resistance, and long-term characteristics of the coating.
- known surfactants and silane coupling agents that contribute to imparting wettability and adhesion to negative electrode active materials and binder resins, inorganic oxide particles that are effective in reducing the resistance of coatings, lithium compound particles, polyaniline sulfonic acid, etc.
- Preferred examples include conductive polymers, and organic compounds that form complex ions with lithium ions, such as polyethylene oxide and complex hydrides.
- the content of the compound (X) with respect to the total mass of the coated carbon material is particularly limited. However, it is usually 0.01% by mass or more, preferably 0.02% by mass or more, and more preferably 0.03% by mass or more, from the viewpoint of improving charge-discharge efficiency and reducing the specific surface area. , more preferably 0.04% by mass or more, particularly preferably 0.05% by mass or more, preferably 10% by mass or less, more preferably 5% by mass or less, It is more preferably 2% by mass or less, and particularly preferably 0.9% by mass or less.
- the content of the crosslinked product of the compound group (Y) with respect to the total mass of the coated carbon material is not particularly limited. It is usually 0.01% by mass or more, preferably 0.02% by mass or more, more preferably 0.03% by mass or more, further preferably 0.04% by mass or more, and 0.04% by mass or more. 05% by mass or more, preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 2% by mass or less, and 0.9% by mass. The following are particularly preferred.
- the volume-based average particle diameter (also referred to as “average particle diameter d50”) of the coated carbon material of the present embodiment is preferably 1 ⁇ m or more, more preferably 5 ⁇ m or more, still more preferably 10 ⁇ m or more, particularly preferably 15 ⁇ m or more, and most preferably. is greater than or equal to 16.5 ⁇ m.
- the average particle diameter d50 is preferably 50 ⁇ m or less, more preferably 40 ⁇ m or less, still more preferably 35 ⁇ m or less, particularly preferably 30 ⁇ m or less, and most preferably 25 ⁇ m or less.
- the average particle size d50 is within the above range, there is a tendency to suppress an increase in the irreversible capacity of a secondary battery (especially a non-aqueous secondary battery) obtained using the coated carbon material and a loss in the initial battery capacity, It is also possible to suppress the occurrence of process problems such as streaking in slurry application, deterioration of high current density charge/discharge characteristics, and deterioration of low temperature input/output characteristics.
- the average particle diameter d50 is measured by adding coated carbon to 10 mL of a 0.2% by mass aqueous solution of polyoxyethylene sorbitan monolaurate (an example of which is Tween 20 (registered trademark)), which is a surfactant. 0.01 g of the material is suspended, and this is introduced as a measurement sample into a commercially available laser diffraction/scattering particle size distribution analyzer (eg LA-920 manufactured by HORIBA), and the measurement sample is subjected to ultrasonic waves of 28 kHz at an output of 60 W for 1 minute. After irradiation, it is defined as the volume-based median diameter measured by the measuring device.
- a commercially available laser diffraction/scattering particle size distribution analyzer eg LA-920 manufactured by HORIBA
- the circularity of the coated carbon material of the present embodiment is 0.88 or more, preferably 0.90 or more, and more preferably 0.91 or more. Also, the degree of circularity is preferably 1 or less, more preferably 0.98 or less, and still more preferably 0.97 or less. When the degree of circularity is within the above range, there is a tendency that the deterioration of the high current density charge/discharge characteristics of secondary batteries (especially non-aqueous secondary batteries) can be suppressed.
- a flow-type particle image analyzer eg, FPIA manufactured by Sysmex Industrial Co., Ltd.
- FPIA flow-type particle image analyzer
- polyoxyethylene (20) which is a surfactant.
- the detection range is specified to 0.6 to 400 ⁇ m, and the particle size values measured for particles in the range of 1.5 to 40 ⁇ m are used.
- the method for improving the degree of circularity is not particularly limited, but it is preferable to apply a spheroidizing treatment to make the particles spherical, since the shape of the inter-particle voids when used as a negative electrode will be in order.
- spheroidization include a method of mechanically approximating a sphere by applying a shearing force or a compressive force, and a mechanical or physical method of granulating a plurality of coated carbon material fine particles by the adhesive force of the binder or the particles themselves. processing methods and the like.
- the tap density of the coated carbon material of the present embodiment is preferably 0.7 g/cm 3 or more, more preferably 0.8 g/cm 3 or more, still more preferably 0.85 g/cm 3 or more, and particularly preferably 0. .9 g/cm 3 or more, most preferably 0.95 g/cm 3 or more, preferably 1.3 g/cm 3 or less, more preferably 1.2 g/cm 3 or less, still more preferably 1.1 g/cm 3 cm 3 or less.
- the tap density is within the above range, processability such as streaking during electrode plate production is improved, and high-speed charge/discharge characteristics are excellent.
- the intra-particle carbon density is less likely to increase, rollability is good, and there is a tendency to easily form a high-density negative electrode sheet.
- the tap density is measured by using a powder density measuring instrument, and dropping the coated carbon material of the present embodiment through a sieve with an opening of 300 ⁇ m into a cylindrical tap cell having a diameter of 1.6 cm and a volume capacity of 20 cm 3 to fill the cell. After filling, the sample is tapped 1000 times with a stroke length of 10 mm.
- the d value (interlayer distance) of the lattice plane (002 plane) obtained by X-ray diffraction according to the Gakushin method of the coated carbon material of the present embodiment is preferably 0.335 nm or more and less than 0.340 nm. It is preferably 0.339 nm or less, still more preferably 0.337 nm or less.
- the d002 value is within the above range, the crystallinity of the graphite is high, so there is a tendency to suppress an increase in the initial irreversible capacity.
- 0.335 nm is the theoretical value of graphite.
- the crystallite size (Lc) of the coated carbon material determined by X-ray diffraction according to the Gakushin method is preferably 1.5 nm or more, more preferably 3.0 nm or more. Within this range, the particles are not too low in crystallinity, and a decrease in reversible capacity can be suppressed when used as a secondary battery (particularly, a non-aqueous secondary battery).
- the lower limit of Lc is the theoretical value of graphite.
- Ash content in the coated carbon material of the present embodiment is preferably 2% by mass or less, more preferably 1.5% by mass or less, and still more preferably 1.0% by mass, relative to the total mass of the coated carbon material. % by mass or less. Moreover, it is preferable that the lower limit of the ash content is 1 ppm or more.
- the ash content is within the above range, in the case of a secondary battery (especially a non-aqueous secondary battery), the deterioration of the battery performance due to the reaction between the coated carbon material and the electrolyte during charging and discharging can be suppressed to a negligible level. can be done.
- the production of the coated carbon material does not require a large amount of time, energy, and facilities for pollution control, the increase in cost can be suppressed.
- the specific surface area (SA) of the coated carbon material of the present embodiment measured by the BET method is preferably 1 m 2 /g or more, more preferably 2 m 2 /g or more, still more preferably 2.5 m 2 /g or more, and particularly preferably is greater than or equal to 2.8 m 2 /g, most preferably greater than or equal to 3 m 2 /g. Also, it is preferably 11 m 2 /g or less, more preferably 9 m 2 /g or less, even more preferably 8 m 2 /g or less, particularly preferably 7 m 2 /g or less, and most preferably 6 m 2 /g or less.
- the specific surface area is within the above range, it is possible to sufficiently secure the site where Li enters and exits, so the output characteristics of high-speed charge and discharge characteristics are excellent, and the activity of the active material with respect to the electrolyte solution can be moderately suppressed, so the initial irreversible capacity does not increase, and there is a tendency to manufacture high-capacity batteries.
- the coated carbon material is used to form the negative electrode, an increase in reactivity with the electrolyte can be suppressed, and gas generation can be suppressed. ) can be provided.
- the BET specific surface area is measured by using a surface area meter (for example, a specific surface area measuring device "Gemini 2360" manufactured by Shimadzu Corporation), and preliminarily drying the coated carbon material sample at 100°C under nitrogen flow for 3 hours under reduced pressure. Defined as the value measured by the nitrogen adsorption BET single point method after cooling to nitrogen temperature.
- the pore volume in the range of 10 nm to 1000 nm is a value measured using a mercury intrusion method (mercury porosimetry), and is preferably 0. 0.01 mL/g or more, more preferably 0.03 mL/g or more, still more preferably 0.05 mL/g or more, and preferably 0.3 mL/g or less, more preferably 0.25 mL/g or less , and more preferably 0.2 mL/g or less.
- the pore volume in the range of 10 nm to 1000 nm is within the above range, it is difficult for the electrolyte solution (especially non-aqueous electrolyte solution) to enter the pores, and lithium ions are intercalated and desorbed during rapid charging and discharging. It is possible to further avoid the tendency that the cycle characteristics are deteriorated due to deposition of lithium metal accompanying the failure of the cycle time. Furthermore, the binder is more likely to be absorbed into the voids during the production of the electrode plate, thereby avoiding the tendency of causing a decrease in the strength of the electrode plate and a decrease in initial efficiency.
- the electrolyte solution especially non-aqueous electrolyte solution
- the total pore volume of the coated carbon material of the present embodiment is preferably 0.1 mL/g or more, more preferably 0.2 mL/g or more, still more preferably 0.25 mL/g or more, and particularly preferably 0.25 mL/g or more. 5 mL/g or more.
- the total pore volume is preferably 10 mL/g or less, more preferably 5 mL/g or less, even more preferably 2 mL/g or less, and particularly preferably 1 mL/g or less.
- the average pore size of the coated carbon material of the present embodiment is preferably 0.03 ⁇ m or more, more preferably 0.05 ⁇ m or more, even more preferably 0.1 ⁇ m or more, and particularly preferably 0.5 ⁇ m or more.
- the average pore diameter is preferably 80 ⁇ m or less, more preferably 50 ⁇ m or less, and even more preferably 20 ⁇ m or less.
- a mercury porosimeter (Autopore 9520: manufactured by Micromeritex Co., Ltd.) can be used as a device for the mercury porosimetry.
- a sample (coated carbon material) is weighed to a value of about 0.2 g, sealed in a powder cell, and pretreated by degassing at 25° C. under vacuum (50 ⁇ mHg or less) for 10 minutes.
- a vacuum of 4 psia (approximately 28 kPa) is applied to introduce mercury into the cell, the pressure is increased in steps from 4 psia (approximately 28 kPa) to 40,000 psia (approximately 280 MPa), and then reduced to 25 psia (approximately 170 kPa).
- the number of steps during pressure increase is set to 80 or more, and the amount of mercury intrusion is measured after an equilibrium time of 10 seconds at each step.
- the pore size distribution is calculated from the mercury intrusion curve thus obtained using the Washburn equation.
- the surface tension ( ⁇ ) of mercury is 485 dyne/cm, and the contact angle ( ⁇ ) is 140°.
- the average pore diameter is defined as the pore diameter when the cumulative pore volume is 50%.
- the true density of the coated carbon material of the present embodiment is preferably 1.9 g/cm 3 or more, more preferably 2 g/cm 3 or more, still more preferably 2.1 g/cm 3 or more, and particularly preferably 2.1 g/cm 3 or more. It is 2 g/cm 3 or more, and the upper limit is 2.26 g/cm 3 . The upper limit is the theoretical value of graphite. Within this range, the crystallinity of the carbon is not too low, and an increase in the initial irreversible capacity of a secondary battery (especially a non-aqueous secondary battery) can be suppressed.
- the aspect ratio of the powdered coated carbon material of the present embodiment is 1 or more, preferably 1.1 or more, and more preferably 1.2 or more. Also, the aspect ratio is preferably 10 or less, more preferably 8 or less, even more preferably 5 or less, and particularly preferably 3 or less. When the aspect ratio is within the above range, streaking of the slurry containing the coated carbon material (negative electrode forming material) is less likely to occur during electrode plate formation, and a uniform coating surface can be obtained. There is a tendency to avoid deterioration of high current density charge/discharge characteristics of batteries).
- the aspect ratio is represented by A/B, where A is the longest diameter of the coated carbon material particles when observed three-dimensionally, and B is the shortest diameter among the diameters perpendicular to it.
- Observation of the coated carbon material particles is performed with a scanning electron microscope capable of magnified observation. Select any 50 coated carbon material particles fixed to the end face of a metal with a thickness of 50 microns or less, rotate and tilt the stage on which the sample is fixed for each, measure A and B, and measure A/ Calculate the average value of B.
- the maximum particle size dmax of the coated carbon material of the present embodiment is preferably 200 ⁇ m or less, more preferably 150 ⁇ m or less, even more preferably 120 ⁇ m or less, particularly preferably 100 ⁇ m or less, and most preferably 80 ⁇ m or less.
- dmax is within the above range, it tends to be possible to suppress the occurrence of process problems such as streaks.
- the maximum particle size is defined as the value of the largest particle size at which particles are measured in the particle size distribution obtained when measuring the average particle size d50.
- the Raman R value of the coated carbon material of the present embodiment is preferably 0.1 or more, more preferably 0.15 or more, and still more preferably 0.2 or more. Also, the Raman R value is preferably 0.6 or less, more preferably 0.5 or less, and even more preferably 0.4 or less.
- the Raman R value is obtained by measuring the intensity I A of the peak P A near 1580 cm ⁇ 1 and the intensity I B of the peak P B near 1360 cm ⁇ 1 in the Raman spectrum obtained by Raman spectroscopy. Defined as calculated as the ratio (I B /I A ). In this specification, “near 1580 cm -1 " refers to the range of 1580 to 1620 cm -1 , and “near 1360 cm -1 " refers to the range of 1350 to 1370 cm -1 .
- the crystallinity of the surface of the coated carbon material particles is difficult to increase, and when the density is increased, the crystals are difficult to orient in the direction parallel to the negative electrode plate, thereby avoiding deterioration in load characteristics. There is a tendency. Furthermore, the crystals on the surface of the particles are less likely to be disturbed, suppressing an increase in reactivity with the electrolytic solution of the negative electrode, thereby avoiding a decrease in charge-discharge efficiency and an increase in gas generation in secondary batteries (especially non-aqueous secondary batteries). There is a tendency.
- the Raman spectrum can be measured with a Raman spectrometer. Specifically, the particles to be measured are allowed to fall freely into the measurement cell to fill the sample, and while the measurement cell is irradiated with argon ion laser light, the measurement cell is rotated in a plane perpendicular to the laser light. Take measurements.
- the measurement conditions are as follows. Argon ion laser light wavelength: 514.5 nm Laser power on sample: 25mW Resolution: 4 cm -1 Measurement range: 1100 cm -1 to 1730 cm -1 Peak intensity measurement, peak half width measurement: background processing, smoothing processing (convolution 5 points by simple average)
- the DBP (dibutyl phthalate) oil absorption of the coated carbon material of the present embodiment is preferably 65 ml/100 g or less, more preferably 62 ml/100 g or less, still more preferably 60 ml/100 g or less, and particularly preferably 57 ml/100 g or less. 100 g or less. Also, the DBP oil absorption is preferably 30 ml/100 g or more, more preferably 40 ml/100 g or more.
- the DBP oil absorption is within the above range, it means that the degree of spheroidization of the coated carbon material is sufficiently advanced, and there is a tendency that streaking is less likely to occur when the slurry containing the coated carbon material is applied. Since there is also a pore structure inside, there is a tendency to avoid lowering the reaction surface.
- the DBP oil absorption is defined as a measured value in accordance with ISO 4546, when 40 g of the material to be measured (coated carbon material) is added, and the drip rate is 4 ml / min, the rotation speed is 125 rpm, and the set torque is 500 N m. .
- Brabender's absorbometer E type can be used.
- the particle diameter (d10) corresponding to cumulative 10% from the smaller particle side of the particle diameter measured on the volume basis of the coated carbon material of the present embodiment is preferably 30 ⁇ m or less, more preferably 20 ⁇ m or less, and still more preferably 17 ⁇ m or less. It is preferably 1 ⁇ m or more, more preferably 3 ⁇ m or more, still more preferably 5 ⁇ m or more, particularly preferably 8 ⁇ m or more, and most preferably 10 ⁇ m or more.
- d10 is defined as a value at which the particle frequency % becomes 10% by accumulation from the smaller particle size in the particle size distribution obtained when the average particle size d50 is measured.
- the particle size (d90) corresponding to cumulative 90% from the small particle side of the particle size measured on a volume basis of the coated carbon material of the present embodiment is preferably 100 ⁇ m or less, more preferably 70 ⁇ m or less, and even more preferably 60 ⁇ m or less. More preferably 50 ⁇ m or less, particularly preferably 45 ⁇ m or less, most preferably 42 ⁇ m or less, preferably 20 ⁇ m or more, more preferably 26 ⁇ m or more, even more preferably 30 ⁇ m or more, particularly preferably 34 ⁇ m or more.
- d90 is within the above range, it is possible to avoid a decrease in electrode strength and a decrease in initial charge-discharge efficiency in secondary batteries (especially non-aqueous secondary batteries), and the occurrence of process inconveniences such as streaking during slurry application. , deterioration of high-current-density charge-discharge characteristics, and deterioration of low-temperature input/output characteristics can also be avoided.
- the d90 is defined as a value at which the particle frequency % becomes 90% by accumulation from the small particle size in the particle size distribution obtained when the average particle size d50 is measured.
- the acetoacetyl group-containing resin related to (X) or the polyvinyl alcohol-based resin related to (Y) (hereinafter, these resins are collectively referred to as “resin”) ) can be evaluated by measuring the amount of resin eluted into the solution when the coated carbon material is immersed in a salt-free non-aqueous solvent at 25°C for 5 hours.
- the elution amount is preferably 20% by mass or less, more preferably 15% by mass or less, still more preferably 10% by mass or less, and particularly preferably 5% by mass or less of the total amount of resin contained in the coated carbon material. % by mass or less.
- the basal surface of the carbon material is coated with a film from the viewpoint of suppressing an increase in resistance.
- the coverage of the basal surface of the carbon material is not particularly limited, but from the viewpoint of efficiently improving the initial efficiency, it is usually 30% or more, preferably 40% or more, and preferably 50% or more. It is more preferably 60% or more, and is usually 100% or less, preferably 98% or less, more preferably 96% or less, and further preferably 95% or less. .
- the basal surface of the carbon material is covered with a film can be evaluated by simultaneously measuring the adsorption isotherm and the heat of adsorption using toluene gas.
- the surface of a carbon material having a heat of adsorption of 67 kJ/mol or more and having a high affinity for toluene is defined as a basal surface.
- the basal surface is coated with an organic compound.
- the coverage of the basal surface of the carbon material can be measured by the following procedure using a differential heat of adsorption measuring device. First, the adsorption isotherm and the heat of adsorption are simultaneously measured using toluene gas on the carbon material before being coated with an organic compound.
- a carbon material surface having a heat of adsorption of 67 kJ /mol or more and having a high affinity for toluene is defined as a basal surface.
- the adsorption isotherm and heat of adsorption were measured simultaneously using toluene gas in the same manner. Find the surface area.
- the basal surface specific surface area is lower than that of the raw carbon material.
- the basal surface coverage is calculated by the following formula (A).
- Basal surface coverage (%) [1-(basal surface specific surface area of coated carbon material)/(basal surface specific surface area of raw material carbon material)] x 100
- the coated carbon material (also referred to as a secondary battery negative electrode active material or simply a coated carbon material) according to the above embodiment is selected from, for example, a carbon material, a compound (X) below, and a compound group (Y) below.
- a method for producing a coated carbon material comprising the step of mixing at least one compound or group of compounds of (X): Acetoacetyl group-containing resin (Y): Polyvinyl alcohol-based resin and silicon element-containing compound Specifically, it can be produced by the following method. In the following description, for the sake of convenience, the carbon material (A), the coating material (B), the secondary battery negative electrode active material (C), and the solution (D) will be described.
- the acetoacetyl group-containing resin (B1) is added to an organic solvent, water, or a mixed solvent thereof in the mixing step of 1), After the solution (D) is mixed with the carbon material (A), the secondary battery negative electrode in which the carbon material (A) contains the coating material (B) is dried by heating and/or under reduced pressure in the drying step of 2). It is possible to obtain an active material for
- the solvent to be used is not particularly limited as long as the acetoacetyl group-containing resin (B1) is dissolved or dispersed, but water, ethyl methyl ketone, toluene, acetone, methyl isobutyl ketone, ethanol, methanol and the like are preferable. be done. Among them, water, ethyl methyl ketone, acetone, methyl isobutyl ketone, ethanol, or methanol are more preferable from the viewpoint of cost and ease of drying.
- the film according to the present embodiment contains a crosslinked product of the compound group (Y), the polyvinyl alcohol-based resin (B1) and a silicon element-containing compound (B2) as a crosslinker
- the boron compound (B3) is added to an organic solvent, water, or a mixed solvent thereof in the 1) mixing step, the solution (D) is mixed with the carbon material (A), and then heated in the 2) drying step. Or/and by drying under reduced pressure, a secondary battery negative electrode active material in which the carbon material (A) contains the coating material (B) can be obtained.
- the solution of the polyvinyl alcohol resin (B1), the solution of the silicon element-containing compound (B2) as a cross-linking agent, and the solution of the boron compound (B3) may be prepared separately, or the polyvinyl alcohol resin ( A solution of B1), a solution of the silicon element-containing compound (B2) and a solution of the boron compound (B3) may be added to the same solvent to prepare a solution.
- the polyvinyl alcohol-based resin (B1), a solution of the silicon element-containing compound (B2), and a solution of the boron compound (B3) from the viewpoint of the initial charge-discharge efficiency of the lithium ion secondary battery, it is preferable to separately prepare a solution of the polyvinyl alcohol-based resin (B1), a solution of the silicon element-containing compound (B2), and a solution of the boron compound (B3). .
- the solvent to be used is not particularly limited as long as the polyvinyl alcohol resin (B1), the silicon element-containing compound (B2) and the boron compound (B3) are dissolved or dispersed, but preferably water, ethyl methyl ketone, toluene, Acetone, methyl isobutyl ketone, ethanol, methanol, and the like. Among them, water, ethyl methyl ketone, acetone, methyl isobutyl ketone, ethanol, or methanol are more preferable from the viewpoint of cost and ease of drying.
- the method for mixing the carbon material (A) and the coating material (B) is not particularly limited. However, from the viewpoint of suppressing the initial amount of gas and the amount of stored gas, it is desirable that the surface of the carbon material (A) can be uniformly coated with the organic compound (B).
- a mixing method there are a method of stirring using a stirring blade in a fixed container, a method of mixing by rotating the container itself to roll the powder, and a method of fluidizing by air flow and mixing. mentioned. Among them, from the viewpoint of mixing uniformity, a method of stirring using a stirring blade in a fixed container is preferable. A type trough may be mentioned, but from the viewpoint of in-machine adhesion and uniform mixing, a horizontal cylindrical type is preferable.
- the shape of the stirring impeller is ribbon type, screw type, single paddle type, double paddle type, anchor type, or plow type for the horizontal axis type, and ribbon type or screw type for the vertical axis type.
- a planetary type, a conical screw type, or a lower high-speed rotating blade but from the viewpoint of uniform mixing, a plow type with a horizontal axis is preferable.
- the peripheral speed of the stirring blade is preferably 0.1 m/s or more, more preferably 1 m/s or more, still more preferably 2 m/s or more, and particularly preferably 3 m/s or more from the viewpoint of mixing uniformity. Yes, preferably 100 m/s or less, more preferably 80 m/s or less, still more preferably 50 m/s or less.
- the treatment time is preferably 0.5 min or longer, more preferably 1 min or longer, still more preferably 5 min or longer, preferably 5 hr or shorter, more preferably 1 hr or shorter, and still more preferably 20 min or shorter. When the treatment time is within the above range, more uniform mixing can be achieved while maintaining the treatment capacity.
- the mixing temperature is preferably 1°C or higher, more preferably 10°C or higher, preferably 100°C or lower, and more preferably 80°C or lower.
- the mixing temperature is within the above range, an increase in the viscosity of the coating material (B) can be suppressed, and more uniform mixing can be achieved. Also, the cost for temperature control can be reduced.
- the coating material (B) is preferably pre-diluted with a solvent.
- the ratio of the solution obtained by diluting the coating material (B) to the carbon material (A) is preferably 1% by mass or more, more preferably 5% by mass or more, preferably 200% by mass or less, and more preferably 150% by mass or less. It's good to By setting the ratio within the above range, the ratio can be uniformly mixed, and the drying time in the post-process can be shortened. Further, a solution of the acetoacetyl group-containing resin (B1) may be added during preparation of the slurry in which the carbon material (A) is dispersed.
- the slurry in which the carbon material (A) is dispersed is used in the step of applying the secondary battery negative electrode active material according to the present embodiment to the negative electrode surface in order to produce the secondary battery negative electrode. It is one of the aspects.
- the concentration of the acetoacetyl group-containing resin (B1) in the solvent when mixed with the carbon material (A) is preferably 0.01% by mass or more and 20% by mass or less. Within this range, it can be expected that the acetoacetyl group-containing resin (B1) is uniformly present on the surface of the carbon material (A) in the secondary battery negative electrode active material, and the effect can be obtained efficiently.
- the concentration of the acetoacetyl group-containing resin (B1) in the solution is preferably 0.03% by mass or more, more preferably 0.05% by mass or more, and preferably 15% by mass or less, More preferably, it is 10% by mass or less.
- the method for mixing the carbon material (A) and the coating material (B) is particularly limited. However, from the viewpoint of suppressing the initial amount of gas and the amount of stored gas, it is desirable that the surface of the carbon material (A) can be uniformly coated with the coating material (B).
- a mixing method there are a method of stirring using a stirring blade in a fixed container, a method of mixing by rotating the container itself to roll the powder, and a method of fluidizing by air flow and mixing. mentioned. Among them, a method of stirring using a stirring blade in a fixed container is preferable from the viewpoint of mixing uniformity.
- the fixed container at that time may be an inverted cone type, a vertical cylindrical type, a horizontal cylindrical type, or a U-shaped trough, but the horizontal cylindrical type is preferable from the viewpoint of in-machine adhesion and uniform mixing.
- the shape of the stirring impeller is ribbon type, screw type, single paddle type, double paddle type, anchor type, or plow type for the horizontal axis type, and ribbon type or screw type for the vertical axis type.
- the peripheral speed of the stirring blade is preferably 0.1 m/s or more, more preferably 1 m/s or more, still more preferably 2 m/s or more, and particularly preferably 3 m/s or more from the viewpoint of mixing uniformity. Yes, preferably 100 m/s or less, more preferably 80 m/s or less, still more preferably 50 m/s or less.
- the treatment time is preferably 0.5 min or longer, more preferably 1 min or longer, still more preferably 5 min or longer, preferably 5 hr or shorter, more preferably 1 hr or shorter, still more preferably 20 min or shorter. When the treatment time is within the above range, more uniform mixing can be achieved while maintaining the treatment capacity.
- the mixing temperature is preferably 1° C. or higher, more preferably 10° C. or higher, preferably 100° C. or lower, and more preferably 80° C. or lower.
- the mixing temperature is within the above range, an increase in the viscosity of the coating material (B) can be suppressed, and more uniform mixing can be achieved. Also, the cost for temperature control can be reduced.
- a solution of the polyvinyl alcohol resin (B1) and a solution of the silicon element-containing compound (B2) and the boron compound (B3) as cross-linking agents are prepared separately, these solutions and the carbon material (A) are mixed at the same time.
- the carbon material (A) may be mixed, or a solution of a polyvinyl alcohol-based resin (B1) or a solution of a silicon element-containing compound (B2), a boron compound (B3) Any two of the solutions and the carbon material (A) may be mixed, and then another solution may be added, or one kind may be mixed with the carbon material A in order.
- the order of mixing may be any of the solution of the polyvinyl alcohol resin (B1), the silicon element-containing compound (B2), and the boron compound (B3), and when mixing sequentially, a drying step may be interposed.
- the boron compound (B3) may or may not be included.
- the state after mixing any two or all of the solution of the polyvinyl alcohol resin (B1), the silicon element-containing compound (B2), and the boron compound (B3) solution is
- the coating material (B) is preferably diluted with a solvent in advance from the viewpoint of uniform mixing. More preferably, after diluting the polyvinyl alcohol resin (B1) with a solvent, it is mixed with the silicon element-containing compound (B2) and the boron compound (B3), and then mixed with the carbon material (A).
- the time until mixing with the carbon material (A) is preferably 1 hr. within 20 minutes, but preferably within 20 minutes from the viewpoint of tact time.
- the ratio of the solution obtained by diluting the coating material (B) to the carbon material (A) is preferably 1% by mass or more, more preferably 5% by mass or more, preferably 200% by mass or less, and more preferably 150% by mass or less. It's good to By setting it within the above range, the ratio can be uniformly mixed, and the drying time in the post-process can be shortened. Further, when preparing the slurry in which the carbon material (A) is dispersed, a solution of the polyvinyl alcohol resin (B1), a solution of the silicon element-containing compound (B2), and a solution of the boron compound (B3) may be added.
- the slurry in which the carbon material (A) is dispersed is used in the step of applying the secondary battery negative electrode active material according to the present embodiment to the negative electrode surface in order to produce the secondary battery negative electrode. It is one of the aspects.
- a solution of polyvinyl alcohol resin (B1), a solution of silicon element-containing compound (B2), and a solution of boron compound (B3) are separately prepared, and these solutions and carbon material (A) are mixed at the same time to prepare a slurry in which the active material (A) is dispersed. Further, since the surface of the carbon material (A) can be uniformly coated, the solution of the boron compound (B3) and the carbon material (A) are mixed at the same time, the mixed solution is filtered or dried, and then polyvinyl alcohol resin ( It is more preferred to mix the solution of B1) with the solution of silicon element-containing (B2).
- the concentration of the polyvinyl alcohol resin (B1), the silicon element-containing compound (B2), or the boron compound (B3) in the solvent when mixed with the carbon material (A) is preferably 0.01% by mass or more and 20% by mass, respectively. % by mass or less. Within this range, the polyvinyl alcohol-based resin (B1), the silicon element-containing compound (B2), and the boron compound (B3) are uniformly present on the surface of the carbon material (A) in the secondary battery negative electrode active material. can be expected, and the effect can be obtained efficiently.
- concentrations of the polyvinyl alcohol-based resin (B1), the silicon element-containing compound (B2), and the boron compound (B3) in the solution are each preferably 0.03% by mass or more, and more preferably 0.05% by mass or more. and preferably 15% by mass or less, more preferably 10% by mass or less.
- the above solution concentration is the concentration of the solution when it is brought into contact with the carbon material (A), and includes the solution of the polyvinyl alcohol resin (B1), the solution of the silicon element-containing compound (B2), and the boron compound (B3).
- the solution of is mixed with the carbon material (A) at the same time, or when these solutions are mixed with the carbon material (A) after mixing, the polyvinyl alcohol-based resin (B1) and the silicon element-containing compound (B2), Concentration for coating (B), which is the sum of boron compounds (B3).
- the polyvinyl alcohol resin when adding another solution after mixing the carbon material (A) with either the solution of the polyvinyl alcohol resin (B1), the silicon element-containing compound (B2), or the solution of the boron compound (B3), the polyvinyl alcohol resin These are the respective concentrations of the solution of (B1), the solution of the silicon element-containing compound (B2), and the solution of the boron compound (B3).
- the amounts of the polyvinyl alcohol-based resin (B1) and the silicon element-containing compound (B2) added can be adjusted as appropriate, and are preferably contained in the active material for the secondary battery negative electrode of the present embodiment described above. It is preferable to adjust the blending amount.
- Step (2) Drying Step
- the film according to the present embodiment contains the crosslinked product of (X) above, and when the solution of the acetoacetyl group-containing resin (B1) is dried by heating, the acetoacetyl group-containing resin (B1 ), and more preferably the boiling point of the solvent or higher. It is preferably 50°C or higher and 300°C or lower. Within this range, the drying efficiency is sufficient, the deterioration of battery performance due to residual solvent can be avoided, the decomposition of the acetoacetyl group-containing resin (B1) can be prevented, and the carbon material (A) and the acetoacetyl group-containing resin can be used together.
- the film according to the present embodiment contains a crosslinked product of the compound group (Y), the polyvinyl alcohol-based resin (B1) and / or the silicon element-containing compound (B2) as a crosslinking agent, the boron compound (B3)
- the temperature is preferably below the decomposition temperature of the polyvinyl alcohol resin (B1), the silicon element-containing compound (B2), and the boron compound (B3), and the temperature above the boiling point of the solvent. is more preferable. It is preferably 50°C or higher and 300°C or lower.
- the drying efficiency is sufficient, and deterioration of battery performance due to residual solvent can be avoided, and the polyvinyl alcohol resin (B1), the silicon element-containing compound (B2), and the boron compound (B3) are prevented from decomposing. Or, to easily prevent a decrease in the effect due to weakening of the interaction between the carbon material (A) and the polymer (B1) having a reactive substituent, the silicon element-containing compound (B2), and the boron compound (B3). can be done.
- Said temperature is preferably 250° C. or lower and preferably 100° C. or higher.
- the film according to this embodiment contains the compound (X) above
- the solution of the acetoacetyl group-containing resin (B1) is dried under reduced pressure
- the film according to this embodiment contains the compound (Y) above
- the pressure is expressed as gauge pressure (atmospheric pressure difference) is usually 0 MPa or less and -0.2 MPa or more. Within this range, drying can be performed relatively efficiently.
- the pressure is preferably -0.03 MPa or less, and preferably -0.15 MPa or more.
- the method for drying the mixture of the carbon material (A) and the coating material (B) is not particularly limited, but it is desirable to be able to coat uniformly from the viewpoint of suppressing the initial gas amount and the storage gas amount. .
- the coating material (B) can be efficiently adsorbed to the specific mesopore surfaces, and the effect of suppressing gas generation can be easily obtained.
- the heat transfer method includes a convection heat transfer method in which hot air is directly applied for drying, and a conductive heat transfer method in which heat is transferred from a heat medium through a conductive heating plate. From the viewpoint of yield, the conductive heat transfer method is preferable.
- the moving form of the material to be dried is static drying, in which the material is left standing to dry, hot air conveying drying, in which the material to be dried is dispersed in hot air or sprayed together with hot air, and agitation drying, in which the material is dried while being agitated.
- hot air conveying drying in which the material to be dried is dispersed in hot air or sprayed together with hot air
- agitation drying in which the material is dried while being agitated.
- the drying process may be performed in the same facility as the mixing process, or may be performed in separate facilities as long as the uniformity of mixing and the drying ability are maintained.
- a method of stirring drying there is a method in which the mixture is dried while stirring using a stirring blade in a fixed container, a method in which the container itself rotates and the powder is tumbled while drying, and a method in which hot air is blown from the bottom.
- the agitating vessel in this case may be of an inverted conical type, vertical cylindrical type, horizontal cylindrical type, U-shaped trough, or the like, but the horizontal cylindrical type is preferable from the viewpoint of yield, workability, and installation space.
- the shape of the stirring impeller is ribbon type, screw type, single-shaft paddle type, double-shaft paddle type, anchor type, plow type, or hollow wedge type for the horizontal shaft type, and ribbon type for the vertical shaft type. , screw type, conical screw type, or lower high-speed rotating blade, but preferably horizontal shaft type, single paddle type, or plow type.
- the peripheral speed of the stirring blade varies depending on the stirring/drying method, but from the viewpoint of uniformity, it is preferably 0.01 m/s or more, more preferably 0.2 m/s or more, still more preferably 1 m/s or more, and particularly preferably It is 2 m/s or more, preferably 40 m/s or less, more preferably 20 m/s or less, still more preferably 10 m/s or less.
- the types of heat medium include heat medium oil, steam, and electric heaters, but steam is preferable from the viewpoint of cost.
- the heat medium by flowing the heat medium through the agitating tank jacket, the agitating blade, or the agitating shaft, the heat is transferred to the material to be dried via the heat transfer surface. It is preferable to flow the heat medium through all of the .
- a horizontal cylindrical agitation vessel can be heated by flowing a heat medium through the agitation vessel jacket, and a dryer equipped with a horizontal shaft type plow-shaped agitating blade and two mutually meshing horizontal axis type hollow wedge-shaped agitating blades are installed.
- a CD dryer Karl, Ltd. that can be heated by flowing a heat medium through both the rotating shaft and the casing with a jacket that is placed horizontally.
- an inverted conical stirring tank Okawara Seisakusho
- Amixon Toyo Hitec
- a step of filtering a solution containing the carbon material (A) and the acetoacetyl group-containing resin (B1) may include a step of washing with As a result, the excess acetoacetyl group-containing resin (B1) that is not directly attached to the carbon material (A) can be removed, and low-temperature input/output can be achieved without reducing the effects of improving the initial efficiency and suppressing gas generation. It is preferable because it can improve the characteristics.
- the film according to the present embodiment contains a crosslinked product of the compound group (Y), prior to drying, the carbon material (A), the polyvinyl alcohol-based resin (B1) and the silicon element-containing compound (B2) as a crosslinking agent , filtering the solution containing the boron compound (B3), and washing the obtained residue with water.
- excess polyvinyl alcohol resin (B1), silicon element-containing compound (B2), and boron compound (B3) that are not directly attached to carbon material (A) can be removed, improving initial efficiency and generating gas. It is preferable because the low-temperature input/output characteristics can be improved without reducing the effect of suppressing the
- the film according to the present embodiment contains a crosslinked product of the compound group (Y)
- the active material for a secondary battery negative electrode of the present embodiment contains other components
- the acetoacetyl group-containing resin (B1 ) when the active material for a secondary battery negative electrode of the present embodiment contains other components, the acetoacetyl group-containing resin (B1 ), it is added to an organic solvent, water, or a mixed solvent thereof to form a solution, and after the solution is mixed with the carbon material (A), it is dried by heating and/or under reduced pressure.
- a solution of the other components may be prepared separately from the solution of the acetoacetyl group-containing resin (B1), or the same solution as the solution of the acetoacetyl group-containing resin (B1) may be prepared.
- a solution may be provided in addition to the solvent.
- the film according to the present embodiment contains a crosslinked product of the compound group (Y)
- the active material for a secondary battery negative electrode of the present embodiment contains other components
- the polyvinyl alcohol-based resin (B1) As with the silicon element-containing compound (B2) and the boron compound (B3), an organic solvent, water or a mixed solvent thereof is added to form a solution, and the solution is mixed with the carbon material (A) and then heated or / and dried by vacuum.
- a solution of the other components may be prepared separately from the solution of the polyvinyl alcohol resin (B1) and the silicon element-containing compound (B2).
- a solution may be prepared by adding to the same solvent as the solution of the polyvinyl alcohol-based resin (B1) or the silicon element-containing compound (B2).
- the negative electrode for a secondary battery of the present embodiment includes a current collector and a negative electrode active material layer formed on the current collector.
- a negative electrode for a non-aqueous secondary battery is preferably characterized by containing a coated carbon material. More preferably, it contains a binder. As the binder, one having an olefinically unsaturated bond in the molecule is used.
- styrene-butadiene rubber examples include styrene-butadiene rubber, styrene/isoprene/styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene/propylene/diene copolymer.
- styrene-butadiene rubber is preferable because of its easy availability.
- the strength of the negative electrode plate can be increased.
- the strength of the negative electrode is high, deterioration of the negative electrode due to charging and discharging can be suppressed, and the cycle life can be lengthened.
- the adhesive strength between the active material layer and the current collector is high, even if the content of the binder in the active material layer is reduced, when the negative electrode is wound to manufacture the battery, In addition, it is presumed that the problem of the active material layer peeling off from the current collector does not occur.
- the binder having olefinic unsaturated bonds in the molecule it is desirable to have a large molecular weight or a large proportion of unsaturated bonds.
- the weight average molecular weight is preferably in the range of 10,000 or more, more preferably 50,000 or more, and preferably 1,000,000 or less, more preferably 300,000 or less. is desirable.
- the number of moles of olefinic unsaturated bonds per 1 g of the total binder is preferably 2.5 ⁇ 10 ⁇ 7 mol or more, more preferably 8 ⁇ 10 ⁇ 7 mol or more, preferably 1 ⁇ 10 ⁇ 6 mol or less, more preferably 5 ⁇ 10 ⁇ 6 mol or less, is desirable.
- the binder may satisfy at least one of the regulations regarding the molecular weight and the regulation regarding the proportion of unsaturated bonds, but it is more preferable to satisfy both regulations at the same time. When the molecular weight of the binder having olefinic unsaturated bonds is within the above range, the mechanical strength and flexibility are excellent.
- the binder having olefinic unsaturated bonds has a degree of unsaturation of preferably 15% or more, more preferably 20% or more, still more preferably 40% or more, and preferably 90% or less, more preferably 80%. % or less.
- the degree of unsaturation represents the ratio (%) of double bonds to the repeating units of the polymer.
- a binder having no olefinically unsaturated bond can also be used in combination with the binder having an olefinically unsaturated bond as long as the effects of the present invention are not lost.
- the mixing ratio of the binder having no olefinic unsaturated bonds to the binder having olefinic unsaturated bonds is preferably 150% by mass or less, more preferably 120% by mass or less.
- binders having no olefinic unsaturated bonds include thickening polysaccharides such as methyl cellulose, carboxymethyl cellulose, starch, carrageenan, pullulan, guar gum and xanthan gum (xanthan gum), polyethers such as polyethylene oxide and polypropylene oxide, Vinyl alcohols such as polyvinyl alcohol and polyvinyl butyral; polyacids such as polyacrylic acid and polymethacrylic acid; metal salts of these polymers; fluorine-containing polymers such as polyvinylidene fluoride; alkane polymers such as polyethylene and polypropylene; A copolymer etc. are mentioned.
- the coated carbon material of the present embodiment is used in combination with the binder having the olefinic unsaturated bond described above, it is possible to reduce the ratio of the binder used in the active material layer compared to the conventional one.
- the coated carbon material of the present embodiment and a binder (this may optionally be a mixture of a binder having unsaturated bonds and a binder having no unsaturated bonds as described above). ) is preferably 90/10 or more, more preferably 95/5 or more, preferably 99.9/0.1 or less, more preferably 99.5/0 .5 or less.
- the ratio of the binder is within the above range, the decrease in capacity and increase in resistance can be suppressed, and the plate strength is also excellent.
- the coated carbon material of this embodiment is formed by dispersing the above-described coated carbon material of this embodiment and a binder in a dispersion medium to form a slurry, which is then applied to a current collector.
- a dispersion medium an organic solvent such as alcohol or water can be used.
- a conductive agent may be added to the slurry.
- the conductive agent include carbon black such as acetylene black, ketjen black, furnace black, artificial graphite powder, and fine powders of Cu, Ni, or alloys thereof having an average particle size of 1 ⁇ m or less.
- the amount of the conductive agent added is preferably about 10% by mass or less with respect to the coated carbon material of the present embodiment.
- a conventionally known current collector can be used as the current collector to which the slurry is applied.
- Specific examples include metal thin films such as rolled copper foil, electrolytic copper foil, and stainless steel foil.
- the thickness of the current collector is preferably 4 ⁇ m or more, more preferably 6 ⁇ m or more, and preferably 30 ⁇ m or less, more preferably 20 ⁇ m or less.
- This slurry was applied with a doctor blade to a width of 5 cm on a 20 ⁇ m thick copper foil as a current collector so that 10.0 ⁇ 0.3 mg/cm 2 of the coated carbon material adhered, and the temperature was 110° C. After drying for 30 minutes, the layer was roll-pressed using a roller with a diameter of 20 cm to adjust the density of the active material layer to 1.60 ⁇ 0.03 g/cm 3 to obtain an electrode sheet.
- the slurry on the current collector After coating the slurry on the current collector, it is preferably 60° C. or higher, more preferably 80° C. or higher, and preferably 200° C. or lower, more preferably 195° C. or lower, in dry air or an inert atmosphere. It dries to form an active layer.
- the thickness of the active material layer obtained by applying and drying the slurry is preferably 5 ⁇ m or more, more preferably 20 ⁇ m or more, still more preferably 30 ⁇ m or more, and preferably 200 ⁇ m or less, more preferably 100 ⁇ m or less, and even more preferably. It is 75 ⁇ m or less.
- the thickness of the active material layer is within the above range, it is excellent in practicality as a negative electrode due to the balance with the particle size of the coated carbon material, and it is possible to obtain a sufficient function of absorbing and releasing Li for a high-density current value. can.
- the density of the coated carbon material in the active material layer varies depending on the application, but in applications where capacity is important, it is preferably 1.55 g/cm 3 or more, more preferably 1.6 g/cm 3 or more, and even more preferably 1.65 g. /cm 3 or more, particularly preferably 1.7 g/cm 3 or more. Moreover, it is preferably 1.9 g/cm 3 or less. When the density is within the above range, the capacity of the battery per unit volume can be sufficiently ensured, and the rate characteristics are less likely to deteriorate.
- the coated carbon material of the present embodiment described above is used to produce a negative electrode for a secondary battery
- the method and selection of other materials are not particularly limited.
- a lithium ion secondary battery is produced using this negative electrode, there are no particular restrictions on the selection of members necessary for the battery configuration, such as the positive electrode and the electrolytic solution that constitute the lithium ion secondary battery. Details of the lithium ion secondary battery negative electrode and the lithium ion secondary battery using the coated carbon material of the present embodiment are exemplified below, but usable materials and manufacturing methods are limited to the following specific examples. not a thing
- the basic configuration of a secondary battery, particularly a lithium ion secondary battery, according to another embodiment of the present embodiment is the same as that of a conventionally known lithium ion secondary battery. and a negative electrode, and an electrolyte.
- This secondary battery is particularly preferably a non-aqueous secondary battery.
- the negative electrode the coated carbon material described above is used.
- the positive electrode is obtained by forming a positive electrode active material layer containing a positive electrode active material and a binder on a current collector.
- positive electrode active materials include metal chalcogen compounds capable of intercalating and deintercalating alkali metal cations such as lithium ions during charging and discharging.
- Metal chalcogen compounds include transition metal oxides such as vanadium oxides, molybdenum oxides, manganese oxides, chromium oxides, titanium oxides, and tungsten oxides, vanadium sulfides, and molybdenum sulfides.
- transition metal sulfides of titanium transition metal sulfides such as CuS, or transition metal phosphorus-sulfur compounds such as NiPS 3 and FePS 3 , transition metal selenium compounds such as VSe 2 or NbSe 3 , Fe 0.25 Transition metal composite oxides such as 0.75 S 2 or Na 0.1 CrS 2 , or transition metal composite sulfides such as LiCoS 2 or LiNiS 2 can be used.
- V2O5 , V5O13 , VO2 , Cr2O5 , MnO2 , TiO2 , MoV2O8 , LiCoO2 , LiNiO2 , LiMn2O4 , TiS2 , V2S 5 , Cr 0.25 V 0.75 S 2 or Cr 0.5 V 0.5 S 2 are preferred, and LiCoO 2 , LiNiO 2 or LiMn 2 O 4 or transition metals thereof are particularly preferred. It is a lithium-transition metal composite oxide partially substituted with another metal.
- These positive electrode active materials may be used singly or in combination.
- any known binder can be selected and used.
- examples thereof include inorganic compounds such as silicate and water glass, and resins having no unsaturated bonds such as Teflon (registered trademark) and polyvinylidene fluoride. Among these, resins having no unsaturated bonds are preferred. If a resin having an unsaturated bond is used as the resin that binds the positive electrode active material, it may be decomposed during the oxidation reaction.
- the weight average molecular weight of these resins is usually 10,000 or more, preferably 100,000 or more, and usually 3,000,000 or less, preferably 1,000,000 or less.
- a conductive material may be contained in the positive electrode active material layer in order to improve the conductivity of the electrode.
- the conductive material is not particularly limited as long as it can be mixed with the active material in an appropriate amount to impart conductivity, but it is usually acetylene black, carbon black, carbon powder such as graphite, or various metal fibers and powders. , or foil.
- the positive electrode plate is formed by making a slurry of the positive electrode active material and the binder with a solvent, applying it on the current collector, and drying it, in the same manner as the manufacturing of the negative electrode described above.
- Aluminum, nickel, stainless steel (SUS), or the like is used as the current collector of the positive electrode, but is not limited at all.
- an electrolytic solution (especially a non-aqueous electrolytic solution) obtained by dissolving a lithium salt in a solvent (especially a non-aqueous solvent), or a gel-like, rubber-like, or solid sheet of this electrolytic solution with an organic polymer compound or the like. Shaped ones are used.
- the solvent used in the electrolytic solution is not particularly limited, and can be appropriately selected and used from known solvents conventionally proposed as solvents for electrolytic solutions.
- chain carbonates such as diethyl carbonate, dimethyl carbonate, or ethyl methyl carbonate
- cyclic carbonates such as ethylene carbonate, propylene carbonate, or butylene carbonate
- chain ethers such as 1,2-dimethoxyethane
- - cyclic ethers such as methyltetrahydrofuran, sulfolane, or 1,3-dioxolane
- chain esters such as methyl formate, methyl acetate, or methyl propionate
- cyclic esters such as ⁇ -butyrolactone or ⁇ -valerolactone etc.
- any one of these solvents may be used alone, or two or more may be used in combination.
- a mixed solvent a combination of a mixed solvent containing a cyclic carbonate and a chain carbonate is preferable, and the cyclic carbonate is a mixed solvent of ethylene carbonate and propylene carbonate. It is particularly preferable in that the non-rejection property is improved.
- propylene carbonate is preferably in the range of 2% by mass to 80% by mass, more preferably in the range of 5% by mass to 70% by mass, and even more preferably in the range of 10% by mass to 60% by mass. If the proportion of propylene carbonate is lower than the above range, the ionic conductivity at low temperatures decreases. The co-insertion causes delamination and deterioration of the graphite-based negative electrode active material, resulting in a problem that sufficient capacity cannot be obtained.
- the lithium salt used in the electrolytic solution is also not particularly limited, and can be appropriately selected and used from known lithium salts known to be usable for this purpose.
- halides such as LiCl or LiBr
- perhalogenates such as LiClO4, LiBrO4 or LiClO4
- inorganic lithium salts such as inorganic fluoride salts such as LiPF6 , LiBF4 or LiAsF6
- Lithium salts may be used alone or in combination of two or more.
- concentration of the lithium salt in the electrolytic solution is usually in the range of 0.5 mol/L or more and 2.0 mol/L or less.
- specific examples of the organic polymer compound include poly(polyethylene) such as polyethylene oxide or polypropylene oxide.
- Ether-based polymer compound Crosslinked polymer of polyether-based polymer compound; Vinyl alcohol-based polymer compound such as polyvinyl alcohol or polyvinyl butyral; Insolubilized vinyl alcohol-based polymer compound; siloxane; vinyl-based polymer compounds such as polyvinylpyrrolidone, polyvinylidene carbonate, or polyacrylonitrile; or poly( ⁇ -methoxyoligooxyethylene methacrylate), poly( ⁇ -methoxyoligooxyethylene methacrylate-co-methyl methacrylate), or poly polymer copolymers such as (hexafluoropropylene-vinylidene fluoride);
- the electrolytic solution described above may further contain a film-forming agent.
- a film-forming agent include carbonate compounds such as vinylene carbonate, vinyl ethyl carbonate, or methylphenyl carbonate; alkene sulfides such as ethylene sulfide or propylene sulfide; 1,3-propanesultone or 1,4-butanesultone; or an acid anhydride such as maleic anhydride or succinic anhydride.
- an overcharge inhibitor such as diphenyl ether or cyclohexylbenzene may be added.
- the content of the additive is usually 10% by mass or less, preferably 8% by mass or less, more preferably 5% by mass or less, and particularly preferably 2% by mass or less. If the content of the additive is too large, other battery characteristics may be adversely affected, such as an increase in initial irreversible capacity, deterioration in low-temperature characteristics and rate characteristics.
- a polymer solid electrolyte that is a conductor of alkali metal cations such as lithium ions can also be used.
- polymer solid electrolytes include those obtained by dissolving a lithium salt in the aforementioned polyether-based polymer compound, polymers in which terminal hydroxyl groups of polyether are substituted with alkoxides, and the like.
- a porous separator such as a porous membrane or non-woven fabric is usually interposed between the positive electrode and the negative electrode to prevent a short circuit between the electrodes.
- the electrolytic solution is used by impregnating a porous separator.
- Polyolefins such as polyethylene and polypropylene, polyethersulfone, and the like are used as the material of the separator, and polyolefins are preferred.
- the form of the secondary battery of this embodiment is not particularly limited. Examples include a cylinder type in which a sheet electrode and a separator are formed in a spiral shape, a cylinder type in which a pellet electrode and a separator are combined to form an inside-out structure, and a coin type in which a pellet electrode and a separator are laminated.
- a coin shape in which a pellet electrode and a separator are laminated.
- housing the batteries of these forms in an arbitrary exterior case, they can be used in arbitrary shapes such as a coin shape, a cylindrical shape, or a rectangular shape.
- the procedure for assembling the secondary battery of the present embodiment is also not particularly limited, and it may be assembled according to an appropriate procedure according to the structure of the battery.
- a battery can be made by providing a separator, placing a positive electrode so as to face the negative electrode, and crimping it together with a gasket or a sealing plate.
- This slurry was applied to a copper foil having a thickness of 10 ⁇ m as a current collector in a width of 10 cm using a die coating machine so that 10.10 ⁇ 0.3 mg/cm 2 of the coated carbon material was attached.
- the density of the active material layer was adjusted to 1.60 ⁇ 0.03 g/cm 3 by roll pressing using a 20 cm roller to obtain an electrode sheet.
- the capacity (mAh/g) was obtained from the discharge capacity at the 3rd cycle.
- the initial efficiency shown in Table 2 is the initial efficiency of each example and comparative example when the initial efficiency of Comparative Example 1-1 is 100.0
- the initial efficiency shown in Table 3 is the initial efficiency of Comparative Example 1- 2 is the initial efficiency of each example and comparative example when the initial efficiency of No. 2 is set to 100.0.
- Example 1-1 100 g of spherical natural graphite (carbon material (A)) having SA of 6.4 m 2 /g and d50 of 17.3 ⁇ m, and 100 g of PVOH (B1-1) aqueous solution (PVOH (B1-1) solid content concentration of 0 .5% by weight) and were mixed in a glass vessel using a three-one motor. The obtained sample was dried and sieved to obtain a powdery coated carbon material (C). SA and initial efficiency of the obtained coated carbon material (C) were measured by the above-described measurement methods. Table 2 shows the results. In addition, as a result of evaluating whether or not the basal surface of the carbon material was covered with the film by the evaluation method described above, it was found that the film was covered.
- Example 1-2 100 g of spherical natural graphite (carbon material (A)) having SA of 6.4 m 2 /g and d50 of 17.3 ⁇ m, and 100 g of PVOH (B1-1) aqueous solution (PVOH (B1-1) solid content concentration of 1 .0% by mass) and were mixed in a glass container using a three-one motor.
- the obtained sample was dried and sieved to obtain a powdery coated carbon material (C).
- the properties of the obtained coated carbon material (C) were evaluated in the same manner as in Example 1-1. Table 2 shows the results.
- Table 2 shows the results.
- Table 2 shows the results.
- Table 2 shows the results.
- Table 2 shows the results.
- Table 2 shows the results.
- the basal surface of the carbon material was covered with the film by the evaluation method described above, it was found that the film was covered.
- Example 1-3 100 g of spherical natural graphite (carbon material (A)) having SA of 6.3 m 2 /g and d50 of 16.3 ⁇ m, and 100 g of PVOH (B1-2) aqueous solution (PVOH (B1-2) solid content concentration of 0 .5% by weight) and were mixed in a glass vessel using a three-one motor.
- the obtained sample was dried and sieved to obtain a powdery coated carbon material (C).
- SA, capacity, and initial efficiency of the obtained coated carbon material (C) were measured by the measurement methods described above. Table 3 shows the results.
- Table 3 shows the results.
- Table 3 shows the results.
- Example 1-3 coating with an acetoacetyl group-containing resin having a self-crosslinking group can reduce the SA of graphite, effectively suppressing the side reaction with the electrolyte, and the conventional technology.
- a good initial efficiency could be achieved while maintaining the capacity compared to the
- the use of the acetoacetyl group-containing resin made it possible to maintain the capacity as compared with the prior art. This is probably because the selective coating of the acetoacetyl group-containing resin on the basal surface does not inhibit deinsertion of Li ions at the edges.
- aqueous rubber dispersion 20.00 ⁇ 0.02 g of the coated carbon material, 0.7 mass% carboxymethylcellulose sodium salt (20.00 ⁇ 0.02 g of aqueous solution (0.14 g in terms of solid content), and styrene / butadiene 0.42 ⁇ 0.02 g (0.2 g in terms of solid content) of the aqueous rubber dispersion was stirred for 5 minutes with a foaming mixer manufactured by THINKY, and defoamed for 30 seconds to obtain a slurry.
- This slurry was spread on a copper foil with a thickness of 20 ⁇ m, which is a current collector, using an automatic coating machine manufactured by Tester Sangyo and a doctor blade so that 10.00 ⁇ 0.3 mg/cm 2 of the coated carbon material adhered.
- the electrode sheet was obtained by applying it to a thickness of 5 cm and roll pressing using a roller with a diameter of 20 cm to adjust the density of the active material layer to 1.60 ⁇ 0.03 g/cm 3 .
- Example 2-6 the electrode sheet prepared by the above method was punched into a disk shape with a diameter of 12.5 mm, and the lithium metal foil was punched into a disk shape with a diameter of 14 mm. It was the opposite.
- a separator (made of porous polyethylene film) was placed on each of the 2016 coin cells.
- the initial efficiency shown in Table 6 is the initial efficiency of each example and comparative example when the initial efficiency of Comparative Example 2-1 is 100.0
- the initial efficiency shown in Table 7 is the initial efficiency of Comparative Example 2- 5 is the initial efficiency of each example and comparative example when the initial efficiency of No. 5 is set to 100.0.
- the peel strength was measured by the following procedure using a light load type adhesion/film peeling analyzer (Kyowa Interface Science Co., Ltd. VPA-3S). That is, the electrode sheet prepared by the above method was dried at 110° C. for 24 hours, cut into pieces of 2.5 cm ⁇ 7 cm, and the negative electrode surface was faced to the test plate. Set it on the measuring device and attach the edge of the electrode plate sheet to the load cell. Peeling was performed at a peeling angle of 90° and a peeling speed of 50 mm/min, and the force required for peeling was measured.
- PVOH (B1-1), SiO 2 , and boron oxide are mixed so that the content of the film is 47.5% by mass, 47.5% by mass, and 5% by mass, respectively, and dried to form a composite membrane. Obtained.
- the elution rate of boron into water was measured by the above measurement method. Table 5 shows the results.
- Example 2-1 100 g of spherical natural graphite (carbon material (A)) having an SA of 6.4 m 2 /g and a d50 of 17.3 ⁇ m, an aqueous solution of PVOH (B1-1), and polytetramethoxysilane as a cross-linking agent (B2). 100 g of a solution mixed with a hydrolyzate (PVOH (B1-1) solid content concentration 0.5% by mass, polytetramethoxysilane hydrolyzate solid content concentration 0.5% by mass), and a three-one motor in a glass container was mixed using The obtained sample was dried and sieved to obtain a powdery coated carbon material (C).
- Example 2-2 As the carbon material (A), 100 g of spherical natural graphite having an SA of 6.4 m 2 /g and a d50 of 17.3 ⁇ m, and boron oxide as the boron compound (B3) were adjusted to a concentration of 0.5% by mass. The aqueous solution was mixed in a glass container using a three-one motor, and after drying, 100 g of a solution obtained by mixing the obtained powder, an aqueous solution of PVOH (B1-1), and a hydrolyzed solution of polytetramethoxysilane as a cross-linking agent (B2).
- Example 2-3 An aqueous solution prepared by adjusting 100 g of spherical natural graphite having an SA of 6.4 m 2 /g and a d50 of 17.3 ⁇ m as the carbon material (A) and boron oxide as the boron compound (B3) to a concentration of 0.5% by mass. was mixed in a glass container using a three-one motor, filtered, and the powder obtained by drying was mixed with an aqueous solution of PVOH (B1-1) and a hydrolyzed solution of polytetramethoxysilane as a cross-linking agent (B2).
- Example 2-4 100 g of a mixed solution (PVOH (B1-1) solid 0.5% by mass of polytetramethoxysilane, 0.5% by mass of polytetramethoxysilane hydrolyzate solids, and 0.5% by mass of boron oxide solids) in the same manner as in Example 2-1. Carried out. The obtained coated carbon material (C) was evaluated in the same manner as in Example 2-1. In addition, as a result of evaluating whether or not the basal surface of the carbon material was covered with the film by the evaluation method described above, it was found that the film was covered.
- PVOH PVOH (B1-1) solid 0.5% by mass of polytetramethoxysilane, 0.5% by mass of polytetramethoxysilane hydrolyzate solids, and 0.5% by mass of boron oxide solids
- Example 2-5 Granulated spherical natural graphite with SA of 11.9 m 2 /g and d50 of 15.7 ⁇ m was used as carbon material (A), PVOH (B1-1) aqueous solution and polytetramethoxy as cross-linking agent (B2)
- A carbon material
- B1-1 aqueous solution
- B2 polytetramethoxy as cross-linking agent
- 100 g of a mixed solution of silane hydrolyzate 100 g of a mixed solution of PVOH (B1-1) aqueous solution and polytetramethoxysilane hydrolyzate (PVOH (B1-1) solid content concentration 1.0% by mass, poly It was carried out in the same manner as in Example 2-1, except that the hydrolyzate of tetramethoxysilane (solid content concentration: 1.0% by mass) was used.
- the obtained coated carbon material (C) was evaluated in the same manner as in Example 2-1. Table 7 shows the results. In addition, as a result of evaluating whether or not the basal surface of the carbon material was covered with the film by the evaluation method described above, it was found that the film was covered.
- Example 2-6 100 g of spherical natural graphite (carbon material (A)) having SA of 6.3 m 2 /g and d50 of 16.3 ⁇ m, PVOH (B1-3) aqueous solution and polytetramethoxysilane (crosslinking agent (B2)) 100 g of a solution mixed with the hydrolyzate (PVOH (B1-3) solid content concentration 0.5% by mass, polytetramethoxysilane hydrolyzate solid content concentration 0.125% by mass), and three-one in a glass container Mixed using a motor. The obtained sample was dried and sieved to obtain a powdery coated carbon material (C).
- carbon material (A) having SA of 6.3 m 2 /g and d50 of 16.3 ⁇ m
- the coated carbon material of the present invention as an active material for the negative electrode of a secondary battery, it is possible to maintain the capacity compared to the conventional technology, have excellent high-temperature storage characteristics and input/output characteristics, and generate less lithium gas.
- An ion secondary battery can be provided.
- the manufacturing method of the said material since the number of processes is small, it can manufacture stably, efficiently, and at low cost.
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Abstract
Description
また、特許文献2に開示されているイオン伝導性高分子や水溶性高分子で炭素材料に被覆すると、炭素材料に対する接着性が不十分で、且つ電解液に対して膨潤性を有することから、このような炭素材料を負極用活物質として用いると初期充放電効率、充放電サイクル特性、及び安定性が未だ不十分であった。
すなわち、炭素材に被覆された被膜が、下記の条件(1)及び条件(2)から選択される少なくとも1つの条件を満たすことが重要であると本発明者らは考えている。
条件(1):被膜が、アセトアセチル基含有樹脂を含む。自己架橋性基としてアセトアセチル基を有する樹脂が被覆炭素材表面で架橋し、樹脂の膨潤や溶出を抑制することにより、スラリー性状の改善や、効率的な被覆による電解液との副反応の抑制がもたらす初期効率の向上という効果を顕著に発現することが出来たと考えられる。
条件(2):被膜が、ポリビニルアルコール系樹脂及びケイ素元素含有化合物の架橋物を含む。ポリビニルアルコール系樹脂と共に含まれる架橋剤としてのケイ素元素含有化合物が被覆炭素材表面で架橋し、ポリマーの膨潤や溶出を抑制することにより、スラリー性状の改善や、効率的な被覆による電解液との副反応の抑制がもたらす初期効率の向上という効果を顕著に発現することが出来たと考えられる。また、被膜中にホウ素元素含有化合物(単に、ホウ素化合物とも称する。)をさらに含む場合には、被覆炭素材表面の架橋構造物がホウ素化合物の溶出を抑制することでさらに効果を顕著に発現することが出来たと考えられる。
[1] 炭素材に被膜が被覆された被覆炭素材であって、
前記被膜が、下記(X)の化合物及び下記(Y)の化合物群の架橋物から選択される少なくとも1つを含む、被覆炭素材。
(X):アセトアセチル基含有樹脂
(Y):ポリビニルアルコール系樹脂及びケイ素元素含有化合物
[2] 前記炭素材が、黒鉛である、[1]に記載の被覆炭素材。
[3] 前記被膜が、前記炭素材のベーサル面に被覆される、[1]又は[2]に記載の被覆炭素材。
[4] 前記被膜が、前記(X)の化合物を含む、[1]~[3]のいずれかに記載の被覆炭素材。
[5] 前記アセトアセチル基含有樹脂が、水酸基を含む、[4]に記載の被覆炭素材。
[6] 前記アセトアセチル基含有樹脂が、アセトアセチル基を含むポリビニルアルコール系樹脂である、[4]又は[5]に記載の被覆炭素材。
[7] 前記被膜が、前記(Y)の化合物群の架橋物を含む、[1]~[3]のいずれかに記載の被覆炭素材。
[8] 前記ポリビニルアルコール系樹脂が、アセトアセチル基を含む、[7]に記載の被覆炭素材。
[9] 前記被膜が、更に、ホウ素元素含有化合物を含む、[7]又は[8]に記載の被覆炭素材。
[10] 前記ホウ素元素含有化合物が、酸化ホウ素、メタホウ酸、四ホウ酸、ホウ酸塩、及びホウ素に結合する炭素の数が1~3であるアルコキシドから選ばれる少なくとも1種の化合物である、[9]に記載の被覆炭素材。
[11] 炭素材に被膜が被覆された被覆炭素材の製造方法であって、
炭素材と、下記(X)の化合物及び/又は下記(Y)の化合物群と、を混合する工程を含む、被覆炭素材の製造方法
(X):アセトアセチル基含有樹脂
(Y):ポリビニルアルコール系樹脂及びケイ素元素含有化合物
[12] 集電体と、該集電体上に形成された活物質層と、を備え、
前記活物質層が、[1]~[10]のいずれかに記載の被覆炭素材を含む、負極。
[13] 正極、負極及び電解質を備える二次電池であって、
前記負極が、[12]に記載の負極である、二次電池。
本明細書において、「~」を用いて表される数値範囲は、「~」の前後に記載された数値を下限値及び上限値として含む範囲を意味し、「A~B」は、A以上B以下であることを意味する。
本発明の一実施形態である被覆炭素材(負極材と称してもよい。)は、リチウムイオンを吸蔵・放出可能な被覆炭素材であって、
炭素材に被膜が被覆された被覆炭素材であって、
前記被膜(単に「膜」とも称する。)が、下記(X)の化合物及び下記(Y)の化合物群の架橋物から選択される少なくとも1つを含む、被覆炭素材。であれば特に限定されない。
(X):アセトアセチル基含有樹脂
(Y):ポリビニルアルコール系樹脂及びケイ素元素含有化合物
上記被覆は、上記(X)の化合物を含む膜及び上記(Y)の化合物群の架橋物を含む膜から選択される少なくとも1つの膜を含む被覆(該膜からなる被膜であってもよい)であってよい。また、上記被膜は、上記(X)の化合物及び上記(Y)の化合物群の架橋物以外の成分を含んでいてもよく、含んでいなくともよく、(X)の化合物のみから構成されていてもよく、(Y)の化合物群の架橋物のみから構成されていてもよく、また、(X)の化合物を含む膜と(Y)の化合物群の架橋物を含む膜の積層膜であってもよい。
人造黒鉛としては、例えば、コールタールピッチ、石炭系重質油、常圧残油、石油系重質油、芳香族炭化水素、窒素含有環状化合物、硫黄含有環状化合物、ポリフェニレン、ポリ塩化ビニル、ポリビニルアルコール、ポリアクリロニトリル、ポリビニルブチラール、天然高分子、ポリフェニレンサルファイド、ポリフェニレンオキシド、フルフリルアルコール樹脂、フェノール-ホルムアルデヒド樹脂、又はイミド樹脂などの有機物を焼成し、黒鉛化したものが挙げられる。
天然黒鉛としては、例えば、高純度化した鱗片状黒鉛や球形化処理を施した黒鉛が挙げられる。中でも、粒子の充填性や充放電負荷特性の観点から、球形化処理を施した天然黒鉛が更に好ましい。
具体的には、ケーシング内部に多数のブレードを設置したローターを有し、そのローターが高速回転することによって、内部に導入された炭素材料に対して衝撃圧縮、摩擦、又はせん断力等の機械的作用を与え、表面処理を行なう装置が好ましい。また、黒鉛を循環させることによって機械的作用を繰り返し与える機構を有するものであるのが好ましい。
非晶質炭素としては、例えば、バルクメソフェーズを焼成した粒子や、炭素前駆体を不融化処理し、焼成した粒子が挙げられる。
焼成の際、有機物に燐酸、ホウ酸、塩酸などの酸類、水酸化ナトリウム等のアルカリ類などを混合することもできる。
炭素材を構成する炭素材料は、他の炭素材料の一種又は二種以上と組み合わせて使用することもできる。
以下は、原料となる炭素材の好ましい特徴を説明するものである。以下、適宜、本実施形態の炭素材、又は単に炭素材と称することがある。
本実施形態の炭素材の体積基準平均粒径(「平均粒径d50」とも記載する)は好ましくは1μm以上、より好ましくは5μm以上、更に好ましくは10μm以上、特に好ましくは15μm以上、最も好ましくは、16.5μm以上である。また平均粒径d50は、好ましくは50μm以下、より好ましくは40μm以下、更に好ましくは35μm以下、特に好ましくは30μm以下、最も好ましくは25μm以下である。平均粒径d50が、上記の範囲であれば、前記炭素材を用いて得られる二次電池(特に、非水系二次電池)の不可逆容量の増加、初期電池容量の損失を抑えられ、スラリー塗布における筋引きなどの工程不都合の発生、高電流密度充放電特性の低下、低温入出力特性の低下を抑えられる。
本実施形態の炭素材の円形度は、0.88以上、好ましくは0.90以上、より好ましくは0.91以上である。また、円形度は好ましくは1以下、より好ましくは0.98以下、更に好ましくは0.97以下である。円形度が上記範囲内であると、二次電池(特に、非水系二次電池)の高電流密度充放電特性の低下を抑制できる傾向にある。なお、円形度は以下の式で定義され、円形度が1のときに理論的真球となる。
円形度=(粒子投影形状と同じ面積を持つ相当円の周囲長)/(粒子投影形状の実際の周囲長)
本実施形態の炭素材のタップ密度は好ましくは0.7g/cm3以上、より好ましくは0.8g/cm3以上、更に好ましくは0.85g/cm3以上、特に好ましくは0.9g/cm3以上、最も好ましくは0.95g/cm3以上、好ましくは1.3g/cm3以下であり、より好ましくは1.2g/cm3以下であり、更に好ましくは1.1g/cm3以下である。
前記タップ密度は、粉体密度測定器を用い、直径1.6cm、体積容量20cm3の円筒状タップセルに、目開き300μmの篩を通して本実施形態の炭素材を落下させて、セルに満杯に充填した後、ストローク長10mmのタップを1000回行なって、その時の体積と試料の質量から求めた密度として定義する。
本実施形態の炭素材の、学振法によるX線回折で求めた格子面(002面)のd値(層間距離)は、好ましくは0.335nm以上、0.340nm未満である。ここで、d値はより好ましくは0.339nm以下、更に好ましくは0.337nm以下である。d002値が上記範囲内にあると、黒鉛の結晶性が高いため、初期不可逆容量が増加を抑制する傾向にある。ここで、0.335nmは黒鉛の理論値である。
また、学振法によるX線回折で求めた前記炭素材の結晶子サイズ(Lc)は、好ましくは1.5nm以上、より好ましくは3.0nm以上の範囲である。上記範囲内であると、結晶性が低過ぎない粒子となり、二次電池(特に、非水系二次電池)とした場合に可逆容量が減少し難くなる。なお、Lcの下限は黒鉛の理論値である。
本実施形態の炭素材に含まれる灰分は、炭素材の全質量に対して、好ましくは1質量%以下、より好ましくは0.5質量%以下であり、更に好ましくは0.1質量%以下である。また、灰分の下限は1ppm以上であることが好ましい。
灰分が上記範囲内であると二次電池(特に、非水系二次電池)とした場合に、充放電時の炭素材と電解液との反応による電池性能の劣化を無視できる程度に抑えることができる。また、炭素材の製造に多大な時間とエネルギーと汚染防止のための設備とを必要としないため、コストの上昇も抑えられる。
本実施形態の炭素材のBET法により測定した比表面積(SA)は、好ましくは2m2/g以上、より好ましくは2.4m2/g以上、更に好ましくは2.6m2/g以上、特に好ましくは2.8m2/g以上、最も好ましくは3.0m2/g以上である。また、好ましくは13m2/g以下、より好ましくは12m2/g以下、更に好ましくは11m2/g以下、特に好ましくは10m2/g以下、最も好ましくは9m2/g以下である。
また、炭素材を使用して負極を形成した場合の、その電解液との反応性の増加を抑制でき、ガス発生を抑えることができるため、好ましい二次電池(特に、非水系二次電池)を提供することができる。
本実施形態の炭素材において、10nm~1000nmの範囲の細孔容積は、水銀圧入法(水銀ポロシメトリー)を用いて測定した値であり、好ましくは0.05mL/g以上、より好ましくは0.07mL/g以上、更に好ましくは0.1mL/g以上であり、また、好ましくは0.3mL/g以下であり、より好ましくは0.28mL/g以下、更に好ましくは0.25mL/g以下である。
全細孔容積が上記範囲内であると、極板化時のバインダ量を過剰にする必要がなく、極板化時に増粘剤やバインダの分散効果も得られ易くなる。
平均細孔径が上記範囲内であると、極板化時のバインダ量を過剰にする必要がなく、電池の高電流密度充放電特性の低下を回避できる傾向にある。
引き続き、4psia(約28kPa)に減圧して前記セルに水銀を導入し、圧力を4psia(約28kPa)から40000psia(約280MPa)までステップ状に昇圧させた後、25psia(約170kPa)まで降圧させる。
なお、水銀の表面張力(γ)は485dyne/cm、接触角(ψ)は140°として算出する。平均細孔径は、累計細孔体積が50%となるときの細孔径として定義する。
本実施形態の炭素材の真密度は、好ましくは1.9g/cm3以上、より好ましくは2g/cm3以上、更に好ましくは2.1g/cm3以上、特に好ましくは2.2g/cm3以上であり、上限は2.26g/cm3である。上限は黒鉛の理論値である。真密度が上記範囲内であると、炭素の結晶性が低すぎず、二次電池(特に、非水系二次電池)とした場合、その初期不可逆容量の増大を抑制できる傾向にある。
本実施形態の炭素材の粉末状態でのアスペクト比は、理論上1以上であり、好ましくは1.1以上、より好ましくは1.2以上である。またアスペクト比は好ましくは10以下、より好ましくは8以下、更に好ましくは5以下である。
アスペクト比が上記範囲内であると、極板化時に炭素材を含むスラリー(負極形成材料)のスジ引きが起こり難く、均一な塗布面が得られ、二次電池(特に、非水系二次電池)の高電流密度充放電特性の低下を回避する傾向にある。
本実施形態の炭素材の最大粒径dmaxは、好ましくは200μm以下、より好ましくは150μm以下、更に好ましくは120μm以下、特に好ましくは100μm以下、最も好ましくは80μm以下である。dmaxが上記範囲内にあると、筋引きなどの工程不都合の発生を抑制できる傾向にある。
また、最大粒径は、平均粒径d50の測定の際に得られた粒度分布において、粒子が測定された最も大きい粒径の値として定義される。
本実施形態の炭素材のラマンR値は、その値は好ましくは0.1以上、より好ましくは0.15以上、更に好ましくは0.2以上である。また、ラマンR値は好ましくは0.6以下、より好ましくは0.5以下、更に好ましくは0.4以下である。
なお、前記ラマンR値は、ラマン分光法で求めたラマンスペクトルにおける1580cm-1付近のピークPAの強度IAと、1360cm-1付近のピークPBの強度IBとを測定し、その強度比(IB/IA)として算出されたものと定義する。
なお、本明細書において「1580cm-1付近」とは1580~1620cm-1の範囲を、「1360cm-1付近」とは1350~1370cm-1の範囲を指す。
前記ラマンスペクトルは、ラマン分光器で測定できる。具体的には、測定対象粒子を測定セル内へ自然落下させることで試料充填し、測定セル内にアルゴンイオンレーザー光を照射しながら、測定セルをこのレーザー光と垂直な面内で回転させながら測定を行なう。測定条件は以下の通りである。
アルゴンイオンレーザー光の波長 :514.5nm
試料上のレーザーパワー :25mW
分解能 :4cm-1
測定範囲 :1100cm-1~1730cm-1
ピーク強度測定、ピーク半値幅測定:バックグラウンド処理、スムージング処理(単純平均によるコンボリューション5ポイント)
本実施形態の炭素材のDBP(フタル酸ジブチル)吸油量は、好ましくは65ml/100g以下、より好ましくは62ml/1O0g以下、更に好ましくは60ml/100g以下、特に好ましくは57ml/100g以下である。また、DBP吸油量は好ましくは30ml/100g以上、より好ましくは40ml/100g以上である。
また、DBP吸油量は、ISO 4546に準拠し、測定材料(炭素材)を40g投入し、滴下速度4ml/min、回転数125rpm、設定トルク500N・mとしたときの測定値として定義される。測定には、例えばブラベンダー社製 アブソープトメーター E型を用いることができる。
本実施形態の炭素材の体積基準で測定した粒径の、小さい粒子側から累積10%に相当する粒径(d10)は好ましくは30μm以下、より好ましくは20μm以下、更に好ましくは17μm以下、好ましくは1μm以上、より好ましくは5μm以上、更に好ましくは10μm以上、特に好ましくは11μm以上、最も好ましくは13μm以上である。
d10は、平均粒径d50の測定の際に得られた粒度分布において、粒子の頻度%が小さい粒径から積算で10%となった値として定義される。
本実施形態の炭素材の体積基準で測定した粒径の、小さい粒子側から累積90%に相当する粒径(d90)は好ましくは100μm以下、より好ましくは70μm以下、更に好ましくは60μm以下、より更に好ましくは50μm以下、特に好ましくは45μm以下、最も好ましくは42μm以下、好ましくは20μm以上、より好ましくは26μm以上、更に好ましくは30μm以上、特に好ましくは34μm以上である。
d90は、平均粒径d50の測定の際に得られた粒度分布において、粒子の頻度%が小さい粒径から積算で90%となった値として定義される。
(X)におけるアセトアセチル基含有樹脂
本実施形態に係る被膜が上記(X)に係るアセトアセチル基含有樹脂を含む場合、このアセトアセチル基含有樹脂の化合物は、単一の化合物であっても二種類以上の化合物が混在していてもよい。アセトアセチル基含有樹脂を用いることにより、従来技術と比較して容量が維持された二次電池を得ることが可能な被覆炭素材を得ることができる。
また、本実施形態の上記(X)に係るアセトアセチル基含有樹脂の好ましい構造としては、直鎖構造、グラフト型構造、星型構造、及び三次元網目構造からなる群より選ばれる少なくとも一つの構造を有していることが好ましい。
アセトアセチル基含有樹脂は、アセトアセチル基以外の官能基を有していてよく、例えば、反応活性が高く耐水性および耐溶剤性に優れる共有結合を形成可能であることから、水酸基(特に、アルコール性水酸基)を有することが好ましい。
なお、(X)の化合物と、(Y)の化合物群の架橋物が重複する場合があるため、(X)の化合物は、(Y)の化合物群の架橋物を除くものとして扱ってよい。上記の重複が生じる場合とは、例えば、(Y)のポリビニルアルコール系樹脂がアセトアセチル基含有樹脂である場合等が挙げられる。
本実施形態に係る被膜が上記(Y)に係るポリビニルアルコール系樹脂とケイ素元素含有化合物の架橋物を含む場合、このポリビニルアルコール系樹脂は、単一の化合物であっても二種類以上の化合物が混在していてもよい。
また、本実施形態の上記(Y)に係るポリビニルアルコール系樹脂の好ましい構造としては、直鎖構造、グラフト型構造、星型構造、及び三次元網目構造からなる群より選ばれる少なくとも一つの構造を有していることが好ましい。
ポリビニルアルコール系樹脂は置換基を有していてよい。この官能基は反応性置換基であることが好ましく、反応性置換基としては、特に限定されないが、反応活性が高く架橋剤と耐水性および耐溶剤性に優れる共有結合を形成可能であることから、アルコール性水酸基、カルボキシル基、カルボニル基、(メタ)アクリル基、エポキシ基、ビニル基、加水分解性シリル基、シラノール基、ヒドロシリル基、又はアセトアセチル基が好ましく、アルコール性水酸基、カルボキシル基、カルボニル基、加水分解性シリル基、シラノール基、又はアセトアセチル基がより好ましく、アルコール性水酸基、カルボニル基、シラノール基、又はアセトアセチル基がさらに好ましく、アルコール性水酸基、又はアセトアセチル基が特に好ましく、アセトアセチル基が最も好ましい。置換される官能基は1種類であってもよく、2種類以上であってもよい。
ポリビニルアルコール系樹脂(以下、適宜PVOH系樹脂とする場合がある)は、ビニルアルコール構造単位を有する樹脂であれば、その具体的な構造は特に限定されず、典型的には酢酸ビニルなどのカルボン酸ビニルエステルモノマーを重合したポリカルボン酸ビニルエステルをケン化して得られるが、これに限られない。
変性PVOH系樹脂としては、PVOH構造単位を供与するビニルエステル系モノマー以外のモノマーを共重合することにより合成される共重合変性PVOH系樹脂であってもよいし、未変性PVOHを合成した後に主鎖または側鎖を適宜化合物で変性した後変性PVOH系樹脂であってもよい。
なお溶剤系で被覆する場合はケン化度38~55モル%の低ケン化のPVOHを架橋剤と組み合わせて使用することができる。
変性基を有するPVOH系樹脂の場合、平均重合度は通常100以上であり、200以上であることが好ましく、250以上であることがより好ましい。この範囲にすることで、溶解性が高くなりすぎることを防ぎやすくなる。また通常4000以下であり、3500以下であることが好ましく、2800以下であることがより好ましい。この範囲にすることにより、溶解性が低くなりすぎることを防ぎやすくなる。かかる平均重合度は水溶液粘度測定法(ISO 15023-2)で測定した値である。
また、PVOH系樹脂は部分的に変性されていてもよい。変性されている場合、PVOH系樹脂の変性率は、当該樹脂粒子10gを20℃の攪拌下の水100gに分散後、撹拌下1℃/分で90℃まで昇温し、60分以内に90質量%以上溶解する範囲が好ましい。
本実施形態に係る被膜が上記(Y)に係る架橋剤としてのケイ素元素含有化合物由来の膜を有する場合、このケイ素元素含有化合物は、特に限定されないが、アセトアセチル基含有PVOH系樹脂と架橋剤との架橋物(被膜)を例に説明する。
前記架橋物を形成する方法(架橋方法)としては、例えば、熱処理、架橋剤処理、紫外線照射処理、電子線照射処理等が用いられる。中でも好ましくは、熱処理により架橋された熱架橋物である。
架橋剤としては、上記のケイ素元素含有化合物に加え、これ以外の架橋剤(その他の架橋剤)が用いられていてもよく、又は用いられていなくともよく、その他の架橋剤の具体例は、カルボキシル基、アセトアセチル基などを有するPVOH系樹脂の架橋剤として公知のものを用いることができる。例えば、ホルムアルデヒド、もしくはアセトアルデヒド等のモノアルデヒド化合物;グリオキザール、グルタルアルデヒド、もしくはジアルデヒド澱粉等の多価アルデヒド化合物などのアルデヒド化合物;メタキシレンジアミン、ノルボルナンジアミン、1,3-ビスアミノメチルシクロヘキサン、ビスアミノプロピルピペラジン、3,3’-ジメチル-4,4’-ジアミノジシクロヘキシルメタン、4,4’-ジアミノジシクロヘキシルメタン、4,4’-ジアミノジフェニルメタン、3,3’,5,5’-テトラメチル-4,4’-ジアミノジフェニルメタン、3,3’,5,5’-テトラエチル-4,4’-ジアミノジフェニルメタン、3,3’-ジメチル-4,4’-ジアミノ-5,5’-ジエチルジフェニルメタン、4,4’-ジアミノジフェニルエーテル、ジアミノジフェニルスルフォン、1,2-フェニレンジアミン、1,3-フェニレンジアミン、1,4-フェニレンジアミン、3-メチル-1,2-フェニレンジアミン、4-メチル-1,2-フェニレンジアミン、2-メチル-1,3-フェニレンジアミン、4-メチル-1,3-フェニレンジアミン、2-メチル-4,6-ジエチル-1,3-フェニレンジアミン、2,4-ジエチル-6-メチル-1,3-フェニレンジアミン、2,4,6-トリメチル-1,3-フェニレンジアミン、もしくは2-クロロ-1,4-フェニレンジアミン等のアミン系化合物;メチロール化尿素、もしくはメチロール化メラミンなどのメチロール化合物;ヘキサメチレンテトラミン等のアンモニアとホルムアルデヒドとの反応物;ホウ酸、もしくはホウ砂などのホウ素化合物;塩基性塩化ジルコニル、硝酸ジルコニル、もしくは酢酸ジルコニウムアンモニウムなどのジルコニウム化合物;テトラメチルチタネートのようなチタンオルソエステル類;チタンエチルアセトアセトナートのようなチタンキレート類;ポリヒドロキシチタンステアレートのようなチタンアシレート類などのチタン化合物;アルミニウムアセチルアセトナートのようなアルミニウム有機酸キレート類などのアルミニウム化合物;シランカップリング剤などの有機反応性基を有するオルガノアルコキシシラン化合物;エチレングリコールジグリシジルエーテル、ポリエチレングリコールジグリシジルエーテル、グリセリンジグリシジルエーテル、グリセリントリグリシジルエーテル、ヘキサンジオールジグリシジルエーテル、もしくはトリメチロールプロパントリグリシジルエーテル等の多価エポキシ化合物;又は各種イソシアネート系化合物、もしくはポリアミドポリアミン-エピクロロヒドリン系樹脂などのポリアミドポリアミン-エピハロヒドリン系樹脂などが挙げられる。
本実施形態に係る被膜が、上記(Y)に係る架橋剤としてのケイ素元素含有化合物由来の膜を有する場合、被覆炭素材の表面及び/又は内部の被膜に三次元架橋構造を有する成分を含有することが好ましく、アルコキシシラン及び/又はその低縮合物の加水分解重縮合物に由来する三次元シロキサン架橋構造を有する成分を含有することがより好ましい。被膜が三次元シロキサン架橋構造を有する成分を含有することで、被膜に含まれるPVOH系樹脂の水スラリーや電解液への溶出を抑制し、且つ、被膜が膨潤することを抑制できる。
より具体的には、ビニルジメチルエトキシシランなどのモノアルコキシシラン;ジメチルジメトキシシランなどのジアルキルジアルコキシシラン;ジアリールジアルコキシシラン;3-アミノプロピルメチルジメトキシシラン、もしくは3-[N-(2-アミノエチル)アミノ]プロピルメチルジメトキシシラン等のアミノ基含有ジアルコキシシラン;3-メルカプトプロピルメチルジメトキシシラン等のメルカプト基含有ジアルコキシシラン;3-(メタ)アクリロキシプロピルメチルジメトキシシラン等の(メタ)アクリロイル基含有ジアルコキシシラン;ビニルジメトキシメチルシラン、もしくはビニルメチルジエトキシシラン等のアルケニル基含有ジアルコキシシラン;又は3-グリシジルオキシプロピルメチルジメトキシシラン、3-グリシジルオキシプロピルメチルジエトキシシラン、もしくは3-グリシジルオキシプロピルエチルジエトキシシランなどのエポキシ基含有ジアルコキシシラン類などのジアルコキシシランが挙げられる。
これらのアルコキシシランは1種のみ用いてもよく、2種以上を併用してもよい。
上記範囲で三次元架橋構造を有する成分、より具体的には、三次元シロキサン架橋構造を有する成分が被覆炭素材粒子に含有されることで、被覆炭素材の被膜に含まれるPVOH系樹脂の水への溶解が制限され、極板塗工時の水スラリーの増粘やスラリー濾過工程の目詰まりを防止することができる。また、被膜抵抗低減のためホウ酸を併用する場合には、被膜からのホウ酸の溶出を抑え、長期にわたり低い抵抗を維持することができる。
三次元シロキサン架橋構造を有する成分が存在していれば、固体29Si-NMRスペクトルの測定において、有機基Rの炭素原子が直接結合した3官能ケイ素単位(T単位:RSiO1.5)及び/又は有機基の炭素原子と結合していない4官能ケイ素単位(Q単位:SiO2)の架橋体に由来するブロードなピーク群が観測される。
本実施形態に係る被膜が、上記(Y)に係る架橋剤としてのホウ素元素含有化合物由来の膜を有する場合、抵抗を低減する観点から、この(Y)の膜は、更に、ホウ素元素含有化合物に由来するホウ素元素を含むことが好ましい。
(Y)の膜中のホウ素元素含有化合物(ホウ素元素含有化合物由来の構造に相当する部分も含む。)の含有量は、特段制限されないが、被覆炭素材の全質量に対する含有量として、0.01質量%以上であることが好ましく、0.03質量%以上であることがより好ましく、0.05質量%以上であることがさらに好ましく、また、10質量%以下であることが好ましく、5質量%以下であることがより好ましく、1質量%以下であることがさらに好ましい。
ホウ素元素含有化合物の種類は特段制限されず、例えば、酸化ホウ素、メタホウ酸、四ホウ酸、ホウ酸塩、ホウ素に結合する炭素の数が1~3であるアルコキシド、リチウムホウ酸塩、等が挙げられ、被覆が容易にできることから、酸化ホウ素、メタホウ酸、四ホウ酸、ホウ酸塩、及びホウ素に結合する炭素の数が1~3であるアルコキシドから選ばれる少なくとも1種の化合物であることが好ましい。これらのホウ素元素含有化合物は1種で単独で用いてもよいが、2種以上を併用してもよい。
・(X)の化合物、(Y)の化合物群の架橋物の含有量
被膜が、上記(X)の化合物を含む場合、被覆炭素材の全質量に対する(X)の化合物の含有量は特段制限されないが、充放電効率向上と比表面積低減効果の観点から、通常0.01質量%以上であり、0.02質量%以上であることが好ましく、0.03質量%以上であることがより好ましく、0.04質量%以上であることがさらに好ましく、0.05質量%以上であることが特に好ましく、また、10質量%以下であることが好ましく、5質量%以下であることがより好ましく、2質量%以下であることがさらに好ましく、0.9質量%以下であることが特に好ましい。
被膜が、上記(Y)の化合物群の架橋物を含む場合、被覆炭素材の全質量に対する(Y)の化合物群の架橋物の含有量は特段制限されないが、十分に架橋を起こす観点から、通常0.01質量%以上であり、0.02質量%以上であることが好ましく、0.03質量%以上であることがより好ましく、0.04質量%以上であることがさらに好ましく、0.05質量%以上であることが特に好ましく、また、10質量%以下であることが好ましく、5質量%以下であることがより好ましく、2質量%以下であることがさらに好ましく、0.9質量%以下であることが特に好ましい。
本実施形態の被覆炭素材の体積基準平均粒径(「平均粒径d50」とも記載する)は好ましくは1μm以上、より好ましくは5μm以上、更に好ましくは10μm以上、特に好ましくは15μm以上、最も好ましくは、16.5μm以上である。また平均粒径d50は、好ましくは50μm以下、より好ましくは40μm以下、更に好ましくは35μm以下、特に好ましくは30μm以下、最も好ましくは25μm以下である。平均粒径d50が上記範囲内の場合、前記被覆炭素材を用いて得られる二次電池(特に、非水系二次電池)の不可逆容量の増加、初期電池容量の損失を抑制する傾向にあり、またスラリー塗布における筋引きなどの工程不都合の発生、高電流密度充放電特性の低下、低温入出力特性の低下を抑制することもできる。
本実施形態の被覆炭素材の円形度は、0.88以上、好ましくは0.90以上、より好ましくは0.91以上である。また、円形度は好ましくは1以下、より好ましくは0.98以下、更に好ましくは0.97以下である。円形度が上記範囲内であると、二次電池(特に、非水系二次電池)の高電流密度充放電特性の低下を抑制できる傾向がある。なお、円形度は以下の式で定義され、円形度が1のときに理論的真球となる。
円形度=(粒子投影形状と同じ面積を持つ相当円の周囲長)/(粒子投影形状の実際の周囲長)
本実施形態の被覆炭素材のタップ密度は好ましくは0.7g/cm3以上、より好ましくは0.8g/cm3以上、更に好ましくは0.85g/cm3以上、特に好ましくは0.9g/cm3以上、最も好ましくは0.95g/cm3以上、好ましくは1.3g/cm3以下であり、より好ましくは1.2g/cm3以下であり、更に好ましくは1.1g/cm3以下である。
前記タップ密度は、粉体密度測定器を用い、直径1.6cm、体積容量20cm3の円筒状タップセルに、目開き300μmの篩を通して本実施形態の被覆炭素材を落下させて、セルに満杯に充填した後、ストローク長10mmのタップを1000回行なって、その時の体積と試料の質量から求めた密度として定義する。
本実施形態の被覆炭素材の、学振法によるX線回折で求めた格子面(002面)のd値(層間距離)は、好ましくは0.335nm以上、0.340nm未満であり、より好ましくは0.339nm以下、更に好ましくは0.337nm以下である。d002値が上記範囲内にあると、黒鉛の結晶性が高いため、初期不可逆容量が増加を抑制する傾向にある。ここで、0.335nmは黒鉛の理論値である。
また、学振法によるX線回折で求めた前記被覆炭素材の結晶子サイズ(Lc)は、好ましくは1.5nm以上、より好ましくは3.0nm以上の範囲である。この範囲内であると、結晶性が低過ぎない粒子となり、二次電池(特に、非水系二次電池)とした場合に可逆容量が減少することを抑制できる。なお、Lcの下限は黒鉛の理論値である。
本実施形態の被覆炭素材に含まれる灰分は、被覆炭素材の全質量に対して、好ましくは2質量%以下、より好ましくは1.5質量%以下であり、更に好ましくは1.0質量%以下である。また、灰分の下限は1ppm以上であることが好ましい。
灰分が上記範囲内であると二次電池(特に、非水系二次電池)とした場合に、充放電時の被覆炭素材と電解液との反応による電池性能の劣化を無視できる程度に抑えることができる。また、被覆炭素材の製造に多大な時間とエネルギーと汚染防止のための設備とを必要としないため、コストの上昇も抑えられる。
本実施形態の被覆炭素材のBET法により測定した比表面積(SA)は、好ましくは1m2/g以上、より好ましくは2m2/g以上、更に好ましくは2.5m2/g以上、特に好ましくは2.8m2/g以上、最も好ましくは3m2/g以上である。また、好ましくは11m2/g以下、より好ましくは9m2/g以下、更に好ましくは8m2/g以下、特に好ましくは7m2/g以下、最も好ましくは6m2/g以下である。
また、被覆炭素材を使用して負極を形成した場合の、その電解液との反応性の増加を抑制でき、ガス発生を抑えることができるため、好ましい二次電池(特に、非水系二次電池)を提供することができる。
本実施形態の被覆炭素材において、10nm~1000nmの範囲の細孔容積は、水銀圧入法(水銀ポロシメトリー)を用いて測定した値であり、好ましくは0.01mL/g以上、より好ましくは0.03mL/g以上、更に好ましくは0.05mL/g以上であり、また、好ましくは0.3mL/g以下であり、より好ましくは0.25mL/g以下、更に好ましくは0.2mL/g以下である。
全細孔容積が上記範囲内であると、極板化時のバインダ量を過剰にする必要がなく、極板化時に増粘剤やバインダの分散効果も得られ易くなる。
平均細孔径が上記範囲内であると、極板化時のバインダ量を過剰にする必要がなく、電池の高電流密度充放電特性の低下を回避できる傾向にある。
引き続き、4psia(約28kPa)に減圧して前記セルに水銀を導入し、圧力を4psia(約28kPa)から40000psia(約280MPa)までステップ状に昇圧させた後、25psia(約170kPa)まで降圧させる。
なお、水銀の表面張力(γ)は485dyne/cm、接触角(ψ)は140°として算出する。平均細孔径は、累計細孔体積が50%となるときの細孔径として定義する。
本実施形態の被覆炭素材の真密度は、好ましくは1.9g/cm3以上、より好ましくは2g/cm3以上、更に好ましくは2.1g/cm3以上、特に好ましくは2.2g/cm3以上であり、上限は2.26g/cm3である。上限は黒鉛の理論値である。この範囲内であると炭素の結晶性が低過ぎず、二次電池(特に、非水系二次電池)とした場合の、その初期不可逆容量が増大を抑制できる。
本実施形態の被覆炭素材の粉末状態でのアスペクト比は、理論上1以上であり、好ましくは1.1以上、より好ましくは1.2以上である。またアスペクト比は好ましくは10以下、より好ましくは8以下、更に好ましくは5以下、特に好ましくは3以下である。
アスペクト比が上記範囲内であると、極板化時に被覆炭素材を含むスラリー(負極形成材料)のスジ引きが起こり難く、均一な塗布面が得られ、二次電池(特に、非水系二次電池)の高電流密度充放電特性の低下を回避する傾向にある。
本実施形態の被覆炭素材の最大粒径dmaxは、好ましくは200μm以下、より好ましくは150μm以下、更に好ましくは120μm以下、特に好ましくは100μm以下、最も好ましくは80μm以下である。dmaxが上記範囲内にあると、筋引きなどの工程不都合の発生を抑制できる傾向にある。
また、最大粒径は、平均粒径d50の測定の際に得られた粒度分布において、粒子が測定された最も大きい粒径の値として定義される。
本実施形態の被覆炭素材のラマンR値は、その値は好ましくは0.1以上、より好ましくは0.15以上、更に好ましくは0.2以上である。また、ラマンR値は好ましくは0.6以下、より好ましくは0.5以下、更に好ましくは0.4以下である。
なお、前記ラマンR値は、ラマン分光法で求めたラマンスペクトルにおける1580cm-1付近のピークPAの強度IAと、1360cm-1付近のピークPBの強度IBとを測定し、その強度比(IB/IA)として算出されたものと定義する。
なお、本明細書において「1580cm-1付近」とは1580~1620cm-1の範囲を、「1360cm-1付近」とは1350~1370cm-1の範囲を指す。
アルゴンイオンレーザー光の波長 :514.5nm
試料上のレーザーパワー :25mW
分解能 :4cm-1
測定範囲 :1100cm-1~1730cm-1
ピーク強度測定、ピーク半値幅測定:バックグラウンド処理、スムージング処理(単純平均によるコンボリューション5ポイント)
本実施形態の被覆炭素材のDBP(フタル酸ジブチル)吸油量は、好ましくは65ml/100g以下、より好ましくは62ml/100g以下、更に好ましくは60ml/100g以下、特に好ましくは57ml/100g以下である。また、DBP吸油量は好ましくは30ml/100g以上、より好ましくは40ml/100g以上である。
また、DBP吸油量は、ISO 4546に準拠し、測定材料(被覆炭素材)を40g投入し、滴下速度4ml/min、回転数125rpm、設定トルク500N・mとしたときの測定値として定義される。測定には、例えばブラベンダー社製 アブソープトメーター E型を用いることができる。
本実施形態の被覆炭素材の体積基準で測定した粒径の、小さい粒子側から累積10%に相当する粒径(d10)は好ましくは30μm以下、より好ましくは20μm以下、更に好ましくは17μm以下、好ましくは1μm以上、より好ましくは3μm以上、更に好ましくは5μm以上、特に好ましくは8μm以上、最も好ましくは10μm以上である。
d10は、平均粒径d50の測定の際に得られた粒度分布において、粒子の頻度%が小さい粒径から積算で10%となった値として定義される。
本実施形態の被覆炭素材の体積基準で測定した粒径の、小さい粒子側から累積90%に相当する粒径(d90)は好ましくは100μm以下、より好ましくは70μm以下、更に好ましくは60μm以下、より更に好ましくは50μm以下、特に好ましくは45μm以下、最も好ましくは42μm以下、好ましくは20μm以上、より好ましくは26μm以上、更に好ましくは30μm以上、特に好ましくは34μm以上である。
d90は、平均粒径d50の測定の際に得られた粒度分布において、粒子の頻度%が小さい粒径から積算で90%となった値として定義される。
本実施形態の被覆炭素材における、(X)に係るアセトアセチル基含有樹脂又は(Y)に係るポリビニルアルコール系樹脂(以下、これらの樹脂を総称して、単に「樹脂」とも称する。)の溶出性は、塩を含まない非水系溶媒に、25℃で被覆炭素材を5時間浸漬した際に、溶液への樹脂の溶出量を測定することにより評価することができる。
溶出量は好ましくは、被覆炭素材に含有される樹脂全量の20質量%以下とすることができ、より好ましくは15質量%以下であり、更に好ましくは10質量%以下であり、特に好ましくは5質量%以下である。
上記溶出性の評価で使用される溶媒は、塩を含まないエチレンカーボネートとエチルメチルカーボネートの混合溶媒(容積比=3:7)とする。
本実施形態の被覆炭素材において、抵抗の増加を抑制する観点から、被膜は炭素材のベーサル面に被覆されていることが好ましい。さらに、炭素材のベーサル面の被覆率は特段制限されないが、効率的に初期効率を向上させる観点から、通常30%以上であり、40%以上であることが好ましく、50%以上であることがより好ましく、60%以上であることがさらに好ましく、また、通常100%以下であり、98%以下であることが好ましく、96%以下であることがより好ましく、95%以下であることがさらに好ましい。
炭素材のベーサル面に被膜が被覆されているか否かについては、トルエンガスを用いて吸着等温線と吸着熱とを同時測定することで評価することができる。具体的には、吸着熱が67kJ/mol以上のトルエンと親和性の高い炭素材表面をベーサル面と定義し、被覆前の炭素材と比較してこの領域のトルエンの吸着量が減少することで、べーサル面が有機化合物で被覆されていることを確認できる。
炭素材のベーサル面の被覆率の測定方法は、微分吸着熱測定装置によって次の手順により測定することができる。
先ず、有機化合物を被覆する前の炭素材について、トルエンガスを用いて吸着等温線と吸着熱を同時測定する。吸着熱が67kJ/mol以上のトルエンと親和性の高い炭素材表面をベーサル面と定義し、トルエンの分子断面積=5.5×10-19m2とベーサル面へのトルエン吸着量から、炭素材のベーサル面比表面積を求める。
次に、有機化合物を被覆した被覆炭素材についても同様にトルエンガスを用いて吸着等温線と吸着熱を同時測定し、吸着熱が67kJ/mol以上のベーサル面へのトルエン吸着量からベーサル面比表面積を求める。このとき、原料の炭素材のベーサル面の一部が有機化合物で被覆されると、トルエンとの親和性が低下して吸着熱が小さくなるため、有機化合物で被覆された被覆炭素材においては、原料の炭素材に比べてベーサル面比表面積が低下する。
ベーサル面被覆率は以下の式(A)により算出する。
式(A)
ベーサル面被覆率(%)=[1-(被覆炭素材のベーサル面比表面積)/(原料の炭素材のベーサル面比表面積)]×100
上記の実施形態に係る被覆炭素材(二次電池負極用活物質、又は単に被膜炭素材ともいう)は、例えば、炭素材と、下記(X)の化合物及び下記(Y)の化合物群から選択されるの少なくとも1つの化合物又は化合物群と、を混合する工程を含む、被覆炭素材の製造方法である。
(X):アセトアセチル基含有樹脂
(Y):ポリビニルアルコール系樹脂及びケイ素元素含有化合物
具体的には、以下の方法により製造することができる。
なお、以下の説明において、便宜上、炭素材(A)、被覆材(B)、二次電池負極用活物質(C)、溶液(D)として説明する。
本実施形態に係る被膜が上記(X)の化合物を含む場合、前記アセトアセチル基含有樹脂(B1)を、1)混合工程で有機溶媒、水又はこれらの混合溶媒に加え、その溶液(D)を炭素材(A)と混合した後、2)乾燥工程で加熱又は/及び減圧によって乾燥させることによって、炭素材(A)が被覆材(B)を含有した二次電池負極用活物質を得ることができる。
本実施形態に係る被膜が上記(Y)の化合物群の架橋物を含む場合、前記ポリビニルアルコール系樹脂(B1)及び架橋剤としてのケイ素元素含有化合物(B2)及び任意に前記ホウ素化合物(B3)を、1)混合工程で有機溶媒、水又はこれらの混合溶媒に加え、その溶液(D)を炭素材(A)と混合した後、2)乾燥工程で加熱又は/及び減圧によって乾燥させることによって、炭素材(A)が被覆材(B)を含有した二次電池負極用活物質を得ることができる。
(X)の化合物
本実施形態に係る被膜が上記(X)の化合物を含む場合、炭素材(A)と被覆材(B)を混合する方法としては、特に制約はないが、初期ガス量の抑制や保存ガス量の抑制の観点で炭素材(A)の表面に有機化合物(B)を均一にコートできることが望ましい。
混合方法としては、固定された容器内で撹拌翼を用いて撹拌をする方法や、容器自体が回転して粉体を転動させて混合する方法や、気流によって流動化させて混合する方法が挙げられる。中でも、混合均一性の観点から固定された容器内で撹拌翼を用いて撹拌をする方法が好ましい
その際の固定された容器は、逆円錐型、縦置き円筒型、横置き円筒型、又はU型トラフが挙げられるが、機内付着と均一混合性の観点では好ましくは横置き円筒型である。
そして、例えば、横置き円筒型の容器で水平軸方式の鋤型の撹拌翼を有するミキサーを用いることが好ましい。
処理時間は好ましくは0.5min以上、より好ましくは1min以上、更に好ましくは5min以上、好ましくは5hr以下、より好ましくは1hr以下、更に好ましくは20min以下である。処理時間が上記範囲内にあると、処理能力を維持しつつより均一に混合することができる。
また、炭素材(A)を分散させたスラリーの作製時に、アセトアセチル基含有樹脂(B1)の溶液を添加してもよい。これは、負極板に二次電池負極用活物質を塗布した後に、架橋性置換基を有するポリマー(B1)の溶媒を乾燥することでも、初期充放電効率改善、ガス発生抑制効果が得られ、製造プロセスを簡略化できるためである。
前記溶液中のアセトアセチル基含有樹脂(B1)の濃度は、好ましくは0.03質量%以上であり、より好ましくは0.05質量%以上であり、また、好ましくは15質量%以下であり、より好ましくは10質量%以下である。
本実施形態に係る被膜が上記(Y)の化合物群の架橋物を含む場合、炭素材(A)と被覆材(B)を混合する方法としては、特に制約はないが、初期ガス量の抑制や保存ガス量の抑制の観点で炭素材(A)の表面に被覆材(B)を均一にコートできることが望ましい。
混合方法としては、固定された容器内で撹拌翼を用いて撹拌をする方法や、容器自体が回転して粉体を転動させて混合する方法や、気流によって流動化させて混合する方法が挙げられる。中でも、混合均一性の観点から固定された容器内で撹拌翼を用いて撹拌をする方法が好ましい。
その際の固定された容器は、逆円錐型、縦置き円筒型、横置き円筒型、又はU型トラフが挙げられるが、機内付着と均一混合性の観点では好ましくは横置き円筒型である。
そして、例えば、横置き円筒型の容器で水平軸方式の鋤型の撹拌翼を有するミキサーを用いることが好ましい。
処理時間は好ましくは0.5min以上、より好ましくは1min以上、更に好ましくは5min以上、好ましくは5hr以下、より好ましくは1hr以下、更に好ましくは20min以下である。処理時間が上記範囲内にあると、処理能力を維持しつつより均一に混合することができる。
ポリビニルアルコール系樹脂(B1)の溶液と、架橋剤としてのケイ素元素含有化合物(B2)、ホウ素化合物(B3)の溶液を別途用意した場合、これらの溶液と炭素材(A)とを同時に混合してもよく、これらの溶液を混合した後に炭素材(A)を混合してもよく、あるいはポリビニルアルコール系樹脂(B1)の溶液又はケイ素元素含有化合物(B2)の溶液、ホウ素化合物(B3)の溶液のうちいずれか2種類と炭素材(A)を混合した後にもう1種類の溶液を加えてもよいし、1種類ずつ順番に炭素材Aと混合してもよい。混合順はポリビニルアルコール系樹脂(B1)の溶液と、ケイ素元素含有化合物(B2)、ホウ素化合物(B3)のどれからでもよく、順次混合する場合には間に乾燥工程を挟んでもよい。ホウ素化合物(B3)は含んでも含まなくてもよい。
また、炭素材(A)を分散させたスラリーの作製時に、ポリビニルアルコール系樹脂(B1)の溶液と、ケイ素元素含有化合物(B2)、ホウ素化合物(B3)の溶液を添加してもよい。これは、負極板に二次電池負極用活物質を塗布した後に、ポリビニルアルコール系樹脂(B1)と、ケイ素元素含有(B2)、ホウ素化合物(B3)の溶媒を乾燥することでも、初期充放電効率改善、ガス発生抑制効果が得られ、製造プロセスを簡略化できるためである。
前記溶液中のポリビニルアルコール系樹脂(B1)又はケイ素元素含有化合物(B2)、ホウ素化合物(B3)の濃度はそれぞれ、好ましくは0.03質量%以上であり、より好ましくは0.05質量%以上であり、また、好ましくは15質量%以下であり、より好ましくは10質量%以下である。
また、ポリビニルアルコール系樹脂(B1)及びケイ素元素含有化合物(B2)の添加量は適宜調整可能であり、上述した、本実施形態の二次電池負極用活物質中における好ましい含有量となるように配合量を調節することが好ましい。
本実施形態に係る被膜が上記(X)の架橋物を含む場合、アセトアセチル基含有樹脂(B1)の溶液について加熱により乾燥を行なう場合、アセトアセチル基含有樹脂(B1)の分解温度以下の温度であることが好ましく、また溶媒の沸点以上の温度とすることがより好ましい。好ましくは50℃以上、300℃以下である。この範囲であれば、乾燥効率が十分であり、かつ溶媒残存による電池性能の低下が避けられ、かつアセトアセチル基含有樹脂(B1)の分解防止や、炭素材(A)とアセトアセチル基含有樹脂(B1)との相互作用が弱くなることによる効果の低減防止を容易に図ることができる。
また、本実施形態に係る被膜が上記(Y)の化合物群の架橋物を含む場合、ポリビニルアルコール系樹脂(B1)及び/または架橋剤としてのケイ素元素含有化合物(B2)、ホウ素化合物(B3)の溶液について加熱により乾燥を行なう場合、ポリビニルアルコール系樹脂(B1)及びケイ素元素含有化合物(B2)、ホウ素化合物(B3)の分解温度以下の温度であることが好ましく、また溶媒の沸点以上の温度とすることがより好ましい。好ましくは50℃以上、300℃以下である。この範囲であれば、乾燥効率が十分であり、かつ溶媒残存による電池性能の低下が避けられ、かつポリビニルアルコール系樹脂(B1)及びケイ素元素含有化合物(B2)、ホウ素化合物(B3)の分解防止や、炭素材(A)と反応性置換基を有するポリマー(B1)及びケイ素元素含有化合物(B2)、ホウ素化合物(B3)との相互作用が弱くなることによる効果の低減防止を容易に図ることができる。
本実施形態に係る被膜が上記(X)の化合物を含む場合、アセトアセチル基含有樹脂(B1)の溶液について減圧により乾燥を行なうとき、又は、本実施形態に係る被膜が上記(Y)の化合物群の架橋物を含む場合、ポリビニルアルコール系樹脂(B1)及び/またはケイ素元素含有化合物(B2)、ホウ素化合物(B3)の溶液について減圧により乾燥を行なうとき、圧力は、ゲージ圧表記(大気圧との差)で通常0MPa以下、-0.2MPa以上である。この範囲であれば、比較的効率よく乾燥を行うことができる。圧力は、好ましくは-0.03MPa以下であり、また、好ましくは-0.15MPa以上である。
伝熱方式は熱風を直接あてて乾燥させる対流伝熱式、熱媒体から伝導加熱板を通して熱伝える伝導伝熱式が挙げられるが、歩留りの観点から伝導伝熱式が好ましい。
撹拌翼の形状は水平軸方式であれば、リボン型、スクリュー型、単軸パドル型、複軸パドル型、イカリ型、鋤型、又は中空くさび型であり、垂直軸方式であれば、リボン型、スクリュー型、円錐スクリュー型、又は下部高速回転羽、が挙げられるが、好ましくは水平軸方式の単軸パドル型、又は鋤型である。
伝導伝熱方式の場合、熱媒の種類は熱媒体油、蒸気、電気ヒーターが挙げられるが、コスト面から蒸気が好ましい。また、熱媒は撹拌槽ジャケット、撹拌翼、もしくは撹拌軸に流すことで、伝熱面を介して被乾燥物へ熱を伝えるが、伝熱効率の観点から、撹拌槽ジャケットと撹拌翼と撹拌軸のすべてに熱媒を流すことが好ましい。
本実施形態に係る被膜が上記(Y)の化合物群の架橋物を含む場合、乾燥に先立ち、炭素材(A)、ポリビニルアルコール系樹脂(B1)及び架橋剤としてのケイ素元素含有化合物(B2)、ホウ素化合物(B3)を含む溶液を濾過する工程、及び得られた残渣を水で洗浄する工程を含んでいてもよい。これにより炭素材(A)に直接付着していない余分なポリビニルアルコール系樹脂(B1)及びケイ素元素含有化合物(B2)、ホウ素化合物(B3)を除去することができ、初期効率の向上やガス発生の抑制といった効果を低減させることなく、低温入出力特性を向上させられるため好ましい。
その他の成分を添加する場合には、アセトアセチル基含有樹脂(B1)の溶液とは別に、その他の成分の溶液を用意してもよいし、アセトアセチル基含有樹脂(B1)の溶液と同一の溶媒に加えて溶液を用意してもよい。
また、本実施形態に係る被膜が上記(Y)の化合物群の架橋物を含む場合、本実施形態の二次電池負極用活物質にその他の成分を含有させるときには、ポリビニルアルコール系樹脂(B1)やケイ素元素含有化合物(B2)、ホウ素化合物(B3)と同様に、有機溶媒、水又はこれらの混合溶媒に加えて溶液とし、その溶液を、炭素材(A)と混合した後、加熱又は/及び減圧によって乾燥させる。
有機化合物(B3)等のその他の成分を添加する場合には、ポリビニルアルコール系樹脂(B1)やケイ素元素含有化合物(B2)の溶液とは別に、その他の成分の溶液を用意してもよいし、ポリビニルアルコール系樹脂(B1)やケイ素元素含有化合物(B2)の溶液と同一の溶媒に加えて溶液を用意してもよい。
本実施形態の二次電池用負極(以下適宜「電極シート」ともいう。)は、集電体と、集電体上に形成された負極活物質層とを備えると共に、活物質層は少なくとも上述した被覆炭素材を含有することを特徴とし、非水系二次電池用負極であることが好ましい。更に好ましくはバインダを含有する。
バインダとしては、分子内にオレフィン性不飽和結合を有するものを用いる。その種類は特に制限されないが、具体例としては、スチレン-ブタジエンゴム、スチレン・イソプレン・スチレンゴム、アクリロニトリル-ブタジエンゴム、ブタジエンゴム、エチレン・プロピレン・ジエン共重合体などが挙げられる。このようなオレフィン性不飽和結合を有するバインダを用いることにより、活物質層の電解液に対する膨潤性を低減することができる。中でも入手の容易性から、スチレン-ブタジエンゴムが好ましい。
本実施形態においては、オレフィン性不飽和結合を有さないバインダも、本発明の効果が失われない範囲において、上述のオレフィン性不飽和結合を有するバインダと併用することができる。オレフィン性不飽和結合を有するバインダに対する、オレフィン性不飽和結合を有さないバインダの混合比率は、好ましくは150質量%以下、より好ましくは120質量%以下の範囲である。
オレフィン性不飽和結合を有さないバインダの例としては、メチルセルロース、カルボキシメチルセルロース、澱粉、カラギナン、プルラン、グアーガム、ザンサンガム(キサンタンガム)等の増粘多糖類、ポリエチレンオキシド、ポリプロピレンオキシド等のポリエーテル類、ポリビニルアルコール、ポリビニルブチラール等のビニルアルコール類、ポリアクリル酸、ポリメタクリル酸等のポリ酸、或いはこれらポリマーの金属塩、ポリフッ化ビニリデン等の含フッ素ポリマー、ポリエチレン、ポリプロピレンなどのアルカン系ポリマー及びこれらの共重合体などが挙げられる。
このスラリーを、集電体である厚さ20μmの銅箔上に、被覆炭素材が10.0±0.3mg/cm2付着するように、ドクターブレードを用いて幅5cmに塗布し、110℃で30分乾燥後、直径20cmのローラを用いてロールプレスして、活物質層の密度が1.60±0.03g/cm3になるよう調整し電極シートを得た。
スラリーを塗布、乾燥して得られる活物質層の厚さは、好ましくは5μm以上、より好ましくは20μm以上、更に好ましくは30μm以上、また、好ましくは200μm以下、より好ましくは100μm以下、更に好ましくは75μm以下である。活物質層の厚みが上記範囲内であると、被覆炭素材の粒径との兼ね合いから負極としての実用性に優れ、高密度の電流値に対する十分なLiの吸蔵・放出の機能を得ることができる。
本実施形態の別の実施形態に係る二次電池、特にリチウムイオン二次電池の基本的構成は、従来公知のリチウムイオン二次電池と同様であり、通常、リチウムイオンを吸蔵・放出可能な正極及び負極、並びに電解質を備える。この二次電池は、特に、非水系二次電池であることが好ましい。負極としては、上述した被覆炭素材を用いる。
正極は、正極活物質及びバインダを含有する正極活物質層を、集電体上に形成したものである。
正極板は、前記したような負極の製造と同様の手法で、正極活物質やバインダを溶剤でスラリー化し、集電体上に塗布、乾燥することにより形成する。正極の集電体としては、アルミニウム、ニッケル、又はステンレススチール(SUS)などが用いられるが、何ら限定されない。
電解質としては、溶媒(特に、非水系溶媒)にリチウム塩を溶解させた電解液(特に、非水系電解液)や、この電解液を有機高分子化合物等によりゲル状、ゴム状、又は固体シート状にしたものなどが用いられる。
また、上述の電解液に有機高分子化合物を含ませ、ゲル状、ゴム状、或いは固体シート状にして使用する場合、有機高分子化合物の具体例としては、ポリエチレンオキシド、もしくはポリプロピレンオキシド等のポリエーテル系高分子化合物;ポリエーテル系高分子化合物の架橋体高分子;ポリビニルアルコール、もしくはポリビニルブチラールなどのビニルアルコール系高分子化合物;ビニルアルコール系高分子化合物の不溶化物;ポリエピクロルヒドリン;ポリフォスファゼン;ポリシロキサン;ポリビニルピロリドン、ポリビニリデンカーボネート、もしくはポリアクリロニトリルなどのビニル系高分子化合物;又はポリ(ω-メトキシオリゴオキシエチレンメタクリレート)、ポリ(ω-メトキシオリゴオキシエチレンメタクリレート-co-メチルメタクリレート)、もしくはポリ(ヘキサフルオロプロピレン-フッ化ビニリデン)等のポリマー共重合体などが挙げられる。
また、電解質として、リチウムイオン等のアルカリ金属カチオンの導電体である高分子固体電解質を用いることもできる。高分子固体電解質としては、前述のポリエーテル系高分子化合物にリチウムの塩を溶解させたものや、ポリエーテルの末端水酸基がアルコキシドに置換されているポリマーなどが挙げられる。
なお、実施例中の略号は、以下のとおりである。
・PVOH(B1-1):アセトアセチル基含有ポリビニルアルコール(平均重合度1500)
・PVOH(B1-2):アセトアセチル基含有ポリビニルアルコール(平均重合度500)
・PVOH(B1-3):アセトアセチル基不含有ポリビニルアルコール(平均重合度500)
・ラベリン:ナフタリンスルホン酸ナトリウムホルマリン縮合物
<電極シートの作製>
実験1に係る実施例又は比較例の被覆炭素材を用い、活物質層密度1.60±0.03g/cm3の活物質層を有する極板を作製した。具体的には、被覆炭素材20.00±0.02gに、1質量%カルボキシメチルセルロースナトリウム塩水溶液を20.00±0.02g(固形分換算で0.200g)、及びスチレン・ブタジエンゴム水性ディスパージョン0.42±0.02g(固形分換算で0.2g)を、THINKY製泡取り練太郎で5分間撹拌し、30秒脱泡してスラリーを得た。
上記方法で作製した電極シートを直径12.5mmの円盤状に打ち抜き、リチウム金属箔を直径14mmの円板状に打ち抜き対極とした。両極の間には、エチレンカーボネートとエチルメチルカーボネートの混合溶媒(容積比=3:7)に、LiPF6を1mol/Lになるように溶解させ、ビニレンカーボネートを2質量%添加した電解液を含浸させたセパレータ(多孔性ポリエチレンフィルム製)を置き、2032コイン型電池をそれぞれ作製した。
上述の方法で作製した二次電池(2032コイン型電池)を用いて、下記の測定方法で電池充放電時の容量を測定した。
電解液二次電池を、25℃において24時間静置後、25℃で0.04Cに相当する定電流でリチウム対極に対して5mVまで充電した後、5mVの定電圧で0.004Cになるまで充電を実施し、0.08Cの定電流で1.5Vまで放電した。これを3回繰り返しコイン電池評価を完了させた。(1サイクル目の放電容量)÷(1サイクル目の充電容量)×100から初期効率(%)を求めた。容量(mAh/g)は3サイクル目の放電容量から求めた。表2に示す初期効率は、比較例1-1の初期効率を100.0とした場合の各実施例および比較例の初期効率であり、また、表3に示す初期効率は、比較例1-2の初期効率を100.0とした場合の各実施例および比較例の初期効率である。
実施例又は比較例に記載のポリビニルアルコール膜を膜質量(W1)の50倍の質量の純水に25℃で1日間浸漬し、複合膜を取り出して表面に付着した純水を取り除き、質量(W2)を測定、その後、110℃で5時間減圧乾燥して質量(W3)を測定し、下記式を用いて膨潤率および溶解率を算出した。
膨潤率=(W2-W3)/W3×100
溶解率=(W1-W3)/W1×100
全自動比表面積測定装置(マウンテック製、マックソーブHM Model-1210)を用い、被覆炭素材試料に対して窒素流通下100℃、30分の予備乾燥を行なった後、液体窒素温度まで冷却し、窒素ガスを用い、BET1点法によって測定した。
トルエンガスを用いて吸着等温線と吸着熱とを同時測定することにより、被膜が炭素材のベーサル面に被覆されているか否かを評価した。
SAが6.4m2/g、d50が17.3μmである球形化天然黒鉛(炭素材(A))を100gと、PVOH(B1-1)水溶液100g(PVOH(B1-1)固形分濃度0.5質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について、前記測定法でSA、初期効率を測定した。結果を表2に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
SAが6.4m2/g、d50が17.3μmである球形化天然黒鉛(炭素材(A))を100gと、PVOH(B1-1)水溶液100g(PVOH(B1-1)固形分濃度1.0質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について実施例1-1と同様に特性を評価した。結果を表2に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
SAが6.4m2/g、d50が17.3μmである球形化天然黒鉛を用いて、前記方法により電極シートを作製、初期効率を測定した。結果を表2に示す。
SAが6.3m2/g、d50が16.3μmである球形化天然黒鉛(炭素材(A))を100gと、PVOH(B1-2)水溶液100g(PVOH(B1-2)固形分濃度0.5質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について、前記測定法でSA、容量、初期効率を測定した。結果を表3に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
SAが6.3m2/g、d50が16.3μmである球形化天然黒鉛を用いて、前記方法により電極シートを作製、初期効率を測定した。結果を表3に示す。
SAが6.3m2/g、d50が16.3μmである球形化天然黒鉛(炭素材(A))を100gと、PVOH(B1-3)水溶液100g(PVOH(B1-3)固形分濃度0.5質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について、前記測定法でSA、容量、初期効率を測定した。結果を表3に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
<電極シートの作製>
後述の実施例2-1~2-5又は比較例2-1、2-2の被覆炭素材を用い、活物質層密度1.60±0.03g/cm3の活物質層を有する極板を作製した。具体的には、被覆炭素材20.00±0.02gに、1質量%カルボキシメチルセルロースナトリウム塩(水溶液を20.00±0.02g(固形分換算で0.200g)、及びスチレン・ブタジエンゴム水性ディスパージョン0.42±0.02g(固形分換算で0.2g)を、THINKY製泡取り練太郎で5分間撹拌し、30秒脱泡してスラリーを得た。
また、後述の実施例2-6又は比較例2-3~2-5の被覆炭素材を用い、活物質層密度1.60±0.03g/cm3の活物質層を有する極板を作製した。具体的には、被覆炭素材20.00±0.02gに、0.7質量%カルボキシメチルセルロースナトリウム塩(水溶液を20.00±0.02g(固形分換算で0.14g)、及びスチレン・ブタジエンゴム水性ディスパージョン0.42±0.02g(固形分換算で0.2g)を、THINKY製泡取り練太郎で5分間撹拌し、30秒脱泡してスラリーを得た。
後述の実施例2-1~2-5又は比較例2-1、2-2について上記方法で作製した電極シートを直径12.5mmの円盤状に打ち抜き、リチウム金属箔を直径14mmの円板状に打ち抜き対極とした。両極の間には、エチレンカーボネートとエチルメチルカーボネートの混合溶媒(容積比=3:7)に、LiPF6を1mol/Lになるように溶解させ、ビニレンカーボネートを2質量%添加した電解液を含浸させたセパレータ(多孔性ポリエチレンフィルム製)を置き、2032コイン型電池をそれぞれ作製した。
また、後述の実施例2-6又は比較例2-3~2-5について上記方法で作製した電極シートを直径12.5mmの円盤状に打ち抜き、リチウム金属箔を直径14mmの円板状に打ち抜き対極とした。両極の間には、エチレンカーボネートとエチルメチルカーボネートの混合溶媒(容積比=3:7)に、LiPF6を1mol/Lになるように溶解させ、ビニレンカーボネートを2質量%添加した電解液を含浸させたセパレータ(多孔性ポリエチレンフィルム製)を置き、2016コイン型電池をそれぞれ作製した。
上述の方法で作製した二次電池(2032コイン型電池および2016コイン型電池)を用いて、下記の測定方法で電池充放電時の容量を測定した。
電解液二次電池を、25℃において24時間静置後、25℃で0.04Cに相当する定電流でリチウム対極に対して5mVまで充電した後、5mVの定電圧で0.004Cになるまで充電を実施し、0.08Cの定電流で1.5Vまで放電した。これを3回繰り返しコイン電池評価を完了させた。(1サイクル目の放電容量)÷(1サイクル目の充電容量)×100から初期効率(%)を求めた。表6に示す初期効率は、比較例2-1の初期効率を100.0とした場合の各実施例および比較例の初期効率であり、また、表7に示す初期効率は、比較例2-5の初期効率を100.0とした場合の各実施例および比較例の初期効率である。
実施例又は比較例に記載のポリビニルアルコールとシリカの複合膜を膜質量(W1)の50倍の質量の純水に25℃で1日間浸漬し、複合膜を取り出して表面に付着した純水を取り除き、質量(W2)を測定、その後、110℃で5時間減圧乾燥して質量(W3)を測定し、下記式を用いて膨潤率および溶解率を算出した。
膨潤率=(W2-W3)/W3×100
溶解率=(W1-W3)/W1×100
実施例または比較例に記載のポリビニルアルコールとシリカと酸化ホウ素の複合膜を膜質量(W1)の100倍の質量の純水中で25℃にて1時間撹拌し、上澄み液を濾別して、ICPによりホウ素の溶出量を定量した。
全自動比表面積測定装置(マウンテック製、マックソーブHM Model-1210)を用い、被覆炭素材試料に対して窒素流通下100℃、30分の予備乾燥を行なった後、液体窒素温度まで冷却し、窒素ガスを用い、BET1点法によって測定した。
ピール強度は軽荷重タイプ粘着・皮膜剥離解析装置(協和界面科学社製VPA-3S)によって次の手順により測定した。
即ち、上述の方法によって作製した電極シートを110℃で24時間乾燥させた後に2.5cm×7cmにカットし、負極面を試験板に向き合わせて幅2cmの両面テープで貼り付け、試験板を測定装置にセットし、極板シートの端部をロードセルに貼り付ける。剥離角度90°、剥離速度50mm/minで剥離を行い、剥離に必要な力を測定した。
トルエンガスを用いて吸着等温線と吸着熱とを同時測定することにより、被膜が炭素材のベーサル面に被覆されているか否かを評価した。
ポリテトラメトキシシラン(SiO2換算Si含有量52質量%)をメタノール溶媒中、アルミニウムアセチルアセトンを触媒として加水分解し、固形分がSiO2換算で16質量%となるように加水分解液を調液した。
PVOH(B1-1)水溶液(固形分濃度10質量%)と、前記SiO2換算で、固形分が16質量%であるポリテトラメトキシシランの加水分解液と、をPVOH(B1-1)とSiO2含有量が膜中でそれぞれ80質量%、20質量%となるように混合、乾燥して複合膜を得た。前記測定法で水への膨潤、溶解率を測定した。結果を表4に示す。
固形分5質量%のPVOH(B1-1)と、SiO2換算で、固形分が16質量%であるポリテトラメトキシシランの加水分解液と、酸化ホウ素換算で固形分2質量%の酸化ホウ素水溶液と、をPVOH(B1-1)とSiO2、酸化ホウ素の含有量が膜中でそれぞれ47.5質量%、47.5質量%、5質量%となるように混合、乾燥して複合膜を得た。前記測定法で水へのホウ素の溶出率を測定した。結果を表5に示す。
固形分5質量%のPVOH(B1-1)と、酸化ホウ素換算で固形分2質量%の酸化ホウ素水溶液と、をPVOH(B1-1)と酸化ホウ素の膜中含有量がそれぞれ95質量%、5質量%となるように混合、乾燥して複合膜を得た。前記測定法で水へのホウ素の溶出率を測定した。結果を表5に示す。
SiO2換算で、固形分が16質量%であるポリテトラメトキシシランの加水分解液と、酸化ホウ素換算で固形分2質量%の酸化ホウ素水溶液と、をSiO2、酸化ホウ素の含有量がそれぞれ95質量%、5質量%となるように混合、乾燥してSiO2/酸化ホウ素複合膜を得た。前記測定法で水へのホウ素の溶出率を測定した。結果を表5に示す。
SAが6.4m2/g、d50が17.3μmである球形化天然黒鉛(炭素材(A))を100gと、PVOH(B1-1)水溶液及び架橋剤(B2)としてポリテトラメトキシシランの加水分解液を混合した溶液100g(PVOH(B1-1)固形分濃度0.5質量%、ポリテトラメトキシシランの加水分解物固形分濃度0.5質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について、前記測定法でSA、初期効率を測定した。結果を表6に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
炭素材(A)として、SAが6.4m2/g、d50が17.3μmである球形化天然黒鉛を100gと、ホウ素化合物(B3)として酸化ホウ素を0.5質量%の濃度に調整した水溶液をガラス容器内でスリーワンモーターを用いて混合し、乾燥後、得られた粉末と、PVOH(B1-1)水溶液及び架橋剤(B2)としてポリテトラメトキシシランの加水分解液を混合した溶液100g(PVOH(B1-1)固形分濃度0.25質量%、ポリテトラメトキシシランの加水分解物固形分濃度0.25質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について実施例2-1と同様に評価した。結果を表6に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
炭素材(A)として、SAが6.4m2/g、d50が17.3μmである球形化天然黒鉛を100gとホウ素化合物(B3)として酸化ホウ素を0.5質量%の濃度に調整した水溶液をガラス容器内でスリーワンモーターを用いて混合、濾過した後、乾燥して得られた粉末を、PVOH(B1-1)水溶液及び架橋剤(B2)としてポリテトラメトキシシランの加水分解液を混合した溶液100g(PVOH(B1-1)固形分濃度0.8質量%、ポリテトラメトキシシランの加水分解物固形分濃度0.2質量%)と、をガラス容器内でスリーワンモーターを用いて混合、濾過後に乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について実施例2-1と同様に評価した。結果を表6に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
PVOH(B1-1)水溶液と、架橋剤(B2)としてのポリテトラメトキシシランの加水分解液と、ホウ素化合物(B3)としての酸化ホウ素と、を混合した溶液100g(PVOH(B1-1)固形分濃度0.5質量%、ポリテトラメトキシシランの加水分解物固形分濃度0.5質量%、酸化ホウ素固形分濃度0.5質量%)を用いた以外は実施例2-1と同様にして実施した。得られた被覆炭素材(C)について実施例2-1と同様に評価した。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
SAが6.4m2/g、d50が17.3μmである球形化天然黒鉛を用いて、前記方法により電極シートを作製、初期効率を測定した。結果を表6に示す。
炭素材(A)として、SAが11.9m2/g、d50が15.7μmである造粒球形化天然黒鉛を使用し、PVOH(B1-1)水溶液及び架橋剤(B2)としてポリテトラメトキシシランの加水分解液を混合した溶液100gとして、PVOH(B1-1)水溶液とポリテトラメトキシシランの加水分解液を混合した溶液100g(PVOH(B1-1)固形分濃度1.0質量%、ポリテトラメトキシシランの加水分解物固形分濃度1.0質量%)を用いた以外は実施例2-1と同様にして実施した。得られた被覆炭素材(C)について実施例2-1と同様に評価した。結果を表7に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
SAが11.9m2/g、d50が15.7μmである造粒球形化天然黒鉛を用いて、前記方法により電極シートを作製、初期効率を測定した。結果を表7に示す。
SAが6.3m2/g、d50が16.3μmである球形化天然黒鉛(炭素材(A))を100gと、PVOH(B1-3)水溶液及びポリテトラメトキシシラン(架橋剤(B2))の加水分解液を混合した溶液100g(PVOH(B1-3)固形分濃度0.5質量%、ポリテトラメトキシシランの加水分解物固形分濃度0.125質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について、前記測定法でSA、初期効率、ピール強度を測定した。結果を表8に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。また、容量維持も確認された。
SAが6.3m2/g、d50が16.3μmである球形化天然黒鉛(炭素材(A))を100gと、PVOH(B1-3)水溶液を混合した溶液100g(PVOH(B1-3)固形分濃度0.5質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について、前記測定法でSA、初期効率、ピール強度を測定した。結果を表8に示す。また、前記評価方法により被膜が炭素材のベーサル面に被覆されているか否かを評価した結果、被覆されていることが分かった。
SAが6.3m2/g、d50が16.3μmである球形化天然黒鉛(炭素材(A))を100gと、ラベリン(ナフタリンスルホン酸ナトリウムホルマリン縮合物)の水溶液と、架橋剤(B2)としてポリテトラメトキシシランの加水分解液と、を混合した溶液100g(ラベリン固形分濃度0.5質量%、ポリテトラメトキシシランの加水分解物固形分濃度0.125質量%)と、をガラス容器内でスリーワンモーターを用いて混合した。得られたサンプルを乾燥して、篩処理を行い、粉末状の被覆炭素材(C)を得た。得られた被覆炭素材(C)について、前記測定法でSA、初期効率、ピール強度を測定した。結果を表8に示す。
SAが6.3m2/g、d50が16.3μmである球形化天然黒鉛を用いて、前記方法により電極シートを作製、初期効率、ピール強度を測定した。結果を表8に示す。
Claims (13)
- 炭素材に被膜が被覆された被覆炭素材であって、
前記被膜が、下記(X)の化合物及び下記(Y)の化合物群の架橋物から選択される少なくとも1つを含む、被覆炭素材。
(X):アセトアセチル基含有樹脂
(Y):ポリビニルアルコール系樹脂及びケイ素元素含有化合物 - 前記炭素材が、黒鉛である、請求項1に記載の被覆炭素材。
- 前記被膜が、前記炭素材のベーサル面に被覆される、請求項1又は2に記載の被覆炭素材。
- 前記被膜が、前記(X)の化合物を含む、請求項1~3のいずれか1項に記載の被覆炭素材。
- 前記アセトアセチル基含有樹脂が、水酸基を含む、請求項4に記載の被覆炭素材。
- 前記アセトアセチル基含有樹脂が、アセトアセチル基を含むポリビニルアルコール系樹脂である、請求項4又は5に記載の被覆炭素材。
- 前記被膜が、前記(Y)の化合物群の架橋物を含む、請求項1~3のいずれか1項に記載の被覆炭素材。
- 前記ポリビニルアルコール系樹脂が、アセトアセチル基を含む、請求項7に記載の被覆炭素材。
- 前記被膜が、更に、ホウ素元素含有化合物を含む、請求項7又は8に記載の被覆炭素材。
- 前記ホウ素元素含有化合物が、酸化ホウ素、メタホウ酸、四ホウ酸、ホウ酸塩、及びホウ素に結合する炭素の数が1~3であるアルコキシドから選ばれる少なくとも1種の化合物である、請求項9に記載の被覆炭素材。
- 炭素材に被膜が被覆された被覆炭素材の製造方法であって、
炭素材と、下記(X)の化合物及び/又は下記(Y)の化合物群と、を混合する工程を含む、被覆炭素材の製造方法。
(X):アセトアセチル基含有樹脂
(Y):ポリビニルアルコール系樹脂及びケイ素元素含有化合物 - 集電体と、該集電体上に形成された活物質層と、を備え、
前記活物質層が、請求項1~10のいずれか1項に記載の被覆炭素材を含む、負極。 - 正極、負極及び電解質を備える二次電池であって、
前記負極が、請求項12に記載の負極である、二次電池。
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