WO2015029128A1 - Negative electrode active material, negative electrode mixture using same, negative electrode and lithium ion secondary battery - Google Patents

Negative electrode active material, negative electrode mixture using same, negative electrode and lithium ion secondary battery Download PDF

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WO2015029128A1
WO2015029128A1 PCT/JP2013/072806 JP2013072806W WO2015029128A1 WO 2015029128 A1 WO2015029128 A1 WO 2015029128A1 JP 2013072806 W JP2013072806 W JP 2013072806W WO 2015029128 A1 WO2015029128 A1 WO 2015029128A1
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negative electrode
lithium ion
electrode active
ion secondary
secondary battery
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PCT/JP2013/072806
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French (fr)
Japanese (ja)
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岡井　誠
京谷　隆
康人 干川
孝文 石井
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株式会社日立製作所
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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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • 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
    • H01M4/04—Processes of manufacture in general
    • H01M4/0402—Methods of deposition of the material
    • H01M4/0421—Methods of deposition of the material involving vapour deposition
    • 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
    • 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
    • H01M4/134—Electrodes based on metals, Si or alloys
    • 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
    • H01M4/139—Processes of manufacture
    • H01M4/1393—Processes of manufacture of electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • 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
    • H01M4/139—Processes of manufacture
    • H01M4/1395—Processes of manufacture of electrodes based on metals, Si or alloys
    • 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
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • 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
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/386—Silicon or alloys based on silicon
    • 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
    • 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
    • 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
    • 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 negative electrode active material, a negative electrode mixture using the same, a negative electrode, and a lithium ion secondary battery.
  • Graphite-based carbonaceous materials are widely used as negative electrode active materials for lithium ion secondary batteries.
  • Patent Document 1 describes that a C / Si / O composite material is produced by growing a Si phase from a liquid phase on the surface of graphite (expanded graphite) and performing a heat treatment.
  • Non-Patent Document 1 describes an example in which silicon nanoparticles and thermally expanded graphite oxide are mechanically mixed and the characteristics are measured as a negative electrode active material.
  • the present invention prevents irreversible battery capacity reduction due to expansion and destruction of silicon nanoparticles, and provides lithium ion secondary batteries having excellent charge / discharge cycle characteristics. It is an object to provide a secondary battery.
  • the negative electrode active material for a lithium ion secondary battery of the present invention includes a carbonaceous material having electrical conductivity and silicon nanoparticles, and the silicon nanoparticles are chemically bonded to the surface of the carbonaceous material. It is characterized by having.
  • the present invention it is possible to provide a lithium ion secondary battery having excellent charge / discharge cycle characteristics by preventing electrical isolation due to expansion and contraction of silicon nanoparticles accompanying lithium ion insertion and release.
  • FIG. 2 is a scanning electron micrograph in which a part of the thermally expanded graphite oxide of FIG. 1 is further enlarged. It is a scanning electron micrograph which shows the thermal expansion graphite oxide which attached the silicon nanoparticle which is a negative electrode active material of a 1st Example. It is the scanning electron micrograph which expanded further the surface of the thermal expansion graphite oxide of FIG. It is a transmission electron micrograph which shows the silicon nanoparticle which grew on the surface of the thermal expansion graphite oxide which comprises the negative electrode active material of a 1st Example. 6 is a transmission electron micrograph showing a bonding surface of the silicon nanoparticles of FIG. 5.
  • FIG. 12B is a side view of the silicon nanoparticles of FIG. 12A. It is a fragmentary sectional view which shows the internal structure of the lithium ion secondary battery of this invention.
  • the present invention relates to a lithium ion secondary battery using an organic solvent containing a lithium salt as an electrolyte, and is composed of silicon nanoparticles and a carbonaceous material having electrical conductivity, and the silicon nanoparticles are more than the average curvature thereof.
  • the present invention relates to a lithium ion secondary battery using a composite material bonded to the surface of a carbonaceous material as a negative electrode active material through a flat portion having a small curvature.
  • the silicon nanoparticles are nanoparticles substantially composed of silicon alone.
  • the diameter of silicon nanoparticles In order to prevent expansion and destruction of silicon nanoparticles due to insertion of lithium ions, it is effective to reduce the diameter of silicon nanoparticles.
  • the extent to which the diameter of the silicon nanoparticles should be reduced depends on the battery operating conditions such as the charge / discharge rate, but is preferably about 100 nm or less. More desirably, it is 30 nm or less.
  • the silicon nanoparticles and the thermally expanded graphite oxide are simply in contact with each other simply by mechanically mixing the silicon nanoparticles and the thermally expanded graphite oxide as in the configuration described in Non-Patent Document 1.
  • the silicon nanoparticles and the thermally expanded graphite oxide are physically separated, and as a result, the silicon nanoparticles are electrically isolated.
  • the C / Si / O composite material described in Patent Document 1 has room for improvement in that the electrical conductivity is inferior to that of silicon alone because carbon and oxygen are chemically bonded to silicon.
  • the above problem is solved by realizing a state in which silicon nanoparticles having high electrical conductivity are strongly bonded to the surface of thermally expanded graphite oxide. Specifically, a state in which the silicon nanoparticles are bonded to the surface of the thermally expanded graphite oxide through a flat portion having a curvature smaller than the average curvature is realized. Such a bonded state can be realized by producing silicon nanoparticles on the surface of thermally expanded graphite oxide by a vapor phase growth method.
  • the thermally expanded graphite oxide was obtained by the heat treatment of the so-called graphite oxide oxidized by the Hummer method, that is, a mixture of sulfuric acid, sodium nitrate and potassium permanganate at 1050 ° C. for 2 hours in an argon atmosphere.
  • FIG. 1 is a scanning electron micrograph (SEM image) of thermally expanded graphite oxide.
  • a very thin graphite layer can be made from a graphite structure having an initial thickness of several tens of ⁇ m.
  • FIG. 2 is a high-magnification scanning electron micrograph of thermally expanded graphite oxide.
  • this thermally expanded graphite oxide has a structure in which a single layer of a honeycomb-like crystal lattice made of carbon atoms is overlapped by several to several tens of layers.
  • FIG. 3 is a scanning electron micrograph of thermally expanded graphite oxide having silicon nanoparticles attached to the surface, which is the negative electrode active material of the first example. This is produced by the vapor phase growth method described in detail later.
  • the particles that appear whitish in the form of particles are silicon nanoparticles, which are uniformly present on the surface of the thermally expanded graphite oxide.
  • FIG. 4 is an observation of the thermally expanded graphite oxide with the silicon nanoparticles shown in FIG. 3 attached to the surface at a high magnification.
  • spherical silicon nanoparticles 402 are uniformly attached to the surface of the thermally expanded graphite oxide 401 with an interval (approximately 30 to 100 nm).
  • the average diameter of the silicon nanoparticles 402 is about 30 nm.
  • the average diameter was calculated by statistically processing the diameter in one direction (horizontal direction) of the silicon nanoparticles 402 in the figure.
  • FIG. 5 and FIG. 6 are photographs (TEM images) obtained by photographing silicon nanoparticles on the surface of thermally expanded graphite oxide with a transmission electron microscope.
  • the portion that looks black and spherical is the silicon nanoparticle 501, and the portion that appears behind it is thermally expanded graphite oxide.
  • the silicon nanoparticle 601 shown in FIG. 6 is an oblique view of the bonding surface 602 (surface indicated by the arrow) between the silicon nanoparticle 601 and the thermally expanded graphite oxide. It can be seen that the bonding surface 602 is a flat surface. That is, it can be seen that the silicon nanoparticles 601 are bonded to the surface of the carbonaceous material via a flat portion (bonding surface 602) having a curvature smaller than the average curvature.
  • the bonding surface 602 is also referred to as an adhesion surface.
  • FIG. 13 shows the internal structure of the lithium ion secondary battery of the present invention.
  • 1310 is a positive electrode
  • 1311 is a separator
  • 1312 is a negative electrode
  • 1313 is a battery can
  • 1314 is a positive current collector tab
  • 1315 is a negative current collector tab
  • 1316 is an inner lid
  • 1317 is an internal pressure release valve
  • 1318 is a gasket
  • 1319 is a positive temperature coefficient resistance element (PTC resistance element; PTC is an abbreviation for Positive Temperature Coefficient)
  • 1320 is a battery lid.
  • the positive electrode was produced by the following procedure.
  • LiMn 2 O 4 was used as the positive electrode active material.
  • graphite powder and acetylene black were added as conductive materials.
  • the mixing ratio of the positive electrode active material, graphite powder, and acetylene black is 85.0: 7.0: 2.0 on a mass basis.
  • PVDF polyvinylidene fluoride
  • NMP 1-methyl-2-pyrrolidone
  • the positive electrode mixture refers to a mixture of a positive electrode active material, a binder and the like.
  • This slurry was applied uniformly and evenly on both sides of an aluminum foil having a thickness of 20 ⁇ m using an applicator. After the application, it was compression molded by a roll press so that the electrode density was 2.55 g / cm 3 . This was cut with a cutting machine to produce a positive electrode having a thickness of 100 ⁇ m, a length of 900 mm, and a width of 54 mm.
  • the mixing ratio of the positive electrode active material, graphite powder, acetylene black, and polyvinylidene fluoride is 85.0: 7.0: 2.0: 6.0 on a mass basis.
  • the negative electrode was produced by the following procedure.
  • the silicon and carbon composite materials described in the first and second examples were used as the negative electrode active material.
  • a solution obtained by dissolving PVDF in NMP as a binder was added to the composite material.
  • the mixing ratio of the composite material and the binder is 95.0: 5.0 on a mass basis.
  • the negative electrode mixture refers to a mixture of a negative electrode active material, a binder and the like.
  • This slurry was applied uniformly and evenly on both sides of a rolled copper foil having a thickness of 10 ⁇ m with a coating machine. After application, the electrode was compression-molded with a roll press to make the electrode density 1.3 g / cm 3 . This was cut with a cutting machine to produce a negative electrode having a thickness of 110 ⁇ m, a length of 950 mm, and a width of 56 mm.
  • the positive electrode current collecting tab 1314 and the negative electrode current collecting tab 1315 were ultrasonically welded to the positive electrode 1310 produced as described above and the uncoated portion (current collector exposed surface) of the negative electrode 1312, respectively.
  • An aluminum lead piece was used for the positive electrode current collecting tab 1314, and a nickel lead piece was used for the negative electrode current collecting tab 1315.
  • a separator 1311 made of a porous polyethylene film having a thickness of 30 ⁇ m was sandwiched between the positive electrode 1310 and the negative electrode 1312, and the positive electrode 1310, the separator 1311, and the negative electrode 1312 were wound.
  • the wound body was housed in a battery can 1313, and the negative electrode current collecting tab 1315 was connected to the bottom of the battery can 1313 by a resistance welder.
  • the positive electrode current collecting tab 1314 was connected to the bottom surface of the inner lid 1316 by ultrasonic welding.
  • a non-aqueous electrolyte was injected.
  • the solvent of the electrolytic solution was composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), and the volume ratio was 1: 1: 1.
  • the electrolyte is LiPF 6 at a concentration of 1 mol / L (about 0.8 mol / kg). Such an electrolytic solution was dropped from above the electrode group, and the battery lid 1320 was caulked and sealed in the battery can 1313 to obtain a lithium ion secondary battery.
  • the fabricated battery was charged to 4.20 V at a current corresponding to 0.3 CA at 25 ° C., and then charged at a constant voltage until the current became 0.03 C at 4.20 V. After 30 minutes of rest, constant current discharge was performed to 2.7 V with a constant current corresponding to 0.3 CA. This was repeated for 3 cycles to initialize the battery capacity at the third cycle, and the measured battery capacity was defined as the initial battery capacity.
  • the initial battery capacity was 1.15 Ah.
  • Cycle capacity retention rate (%) (battery capacity after 500 cycles) / (initial battery capacity) Moreover, the storage test was done at 50 degreeC. The battery was charged to 4.20 V with a current corresponding to 0.3 CA, and then constant voltage charging was performed until the current became 0.03 C at 4.20 V. After a 30-minute rest, it was stored for 3 months in a thermostatic bath at 50 ° C. After storage, the sample was taken out from the thermostat and allowed to stand at 25 ° C. for 3 hours, and the capacity was measured. The battery capacity was measured as described above. The storage capacity maintenance rate after storage for 3 months was calculated by the following formula.
  • FIG. 7 is a schematic cross-sectional view showing a bonded state of the silicon nanoparticles of FIG. 6 which is the first embodiment.
  • the surface of a thermally expanded graphite oxide 701 has a structure in which silicon nanoparticles 702 are bonded via a flat portion 703 having a certain area.
  • the thermally expanded graphite oxide 701 has a thin film structure, the shape is close to an ellipse or a rectangle, and the length of the longest portion is about 100 ⁇ m.
  • the film thickness is 100 nm or less, and it is possible to make it 10 nm or less by devising the oxidation condition and the thermal expansion condition.
  • the specific surface area is 100 m 2 / g or more, and more silicon nanoparticles 702 can be bonded to the surface. Further preferred range of the specific surface area is 100 ⁇ 1000m 2 / g.
  • the silicon nanoparticles 702 are substantially spherical and are joined to the thermally expanded graphite oxide 701 through a flat portion 703 having a curvature smaller than the average curvature. Excluding the bonding surface of the silicon nanoparticle 702 with the thermally expanded graphite oxide 701, it is twice the average radius of curvature, that is, the average diameter is 100 nm or less, more preferably 30 nm or less. Since the silicon nanoparticles 702 are produced on the surface of the thermally expanded graphite oxide 701 by a vapor phase growth method, the joint surface between the two is inevitably in a form along the surface shape of the thermally expanded graphite oxide 701. The surface of the thermally expanded graphite oxide 701 is almost flat and has a gentle curvature depending on the location.
  • the silicon nanoparticle 702 and the carbon are joined by a covalent bond between silicon (silicon) and carbon. Therefore, the bonding force is very strong, and the electrical conduction characteristics between the two are also very good.
  • FIG. 8 schematically shows the structure of a composite material composed of silicon and carbon, which is the negative electrode active material of the first embodiment.
  • a plurality of silicon nanoparticles 802 are joined to the surface of the carbonaceous material through a certain area on the surface of the thermally expanded graphite oxide 801.
  • the thermally expanded graphite oxide 801 has a thin film structure, the shape is close to an ellipse or a rectangle, and the length of the longest portion is about 100 ⁇ m. Further, the film thickness is 100 nm or less, and it is possible to make it 10 nm or less by devising the oxidation condition and the thermal expansion condition. In particular, when the film thickness is 10 nm or less, the specific surface area is 100 m 2 / g or more, and more silicon nanoparticles 802 can be bonded to the surface.
  • the silicon nanoparticles 802 are substantially spherical and are joined to the thermally expanded graphite oxide 801 through a flat portion having a curvature smaller than the average curvature.
  • the average radius of curvature is twice, that is, the average diameter is 100 nm or less, and more preferably 30 nm or less. Since the silicon nanoparticles 802 are formed on the surface of the thermally expanded graphite oxide 801 by a vapor phase growth method, the joint surface between the two is inevitably in a form along the surface shape of the thermally expanded graphite oxide 801.
  • the surface of the thermally expanded graphite oxide 801 is almost flat and has a gentle curvature depending on the location.
  • the number of silicon nanoparticles 802 bonded to the surface of the same thermally expanded graphite oxide 801 can be controlled to some extent by changing the growth conditions in the vapor phase growth method. Basically, the number can be increased by increasing the growth time.
  • the joint between the silicon nanoparticle 802 and the thermally expanded graphite oxide 801 is joined by a covalent bond between silicon and carbon. Therefore, the bonding force is very strong, and the electrical conduction characteristics between the two are also very good.
  • the mass ratio of the thermally expanded graphite oxide 801 and the silicon nanoparticles 802 is 1: 1.
  • a lithium ion secondary battery using a composite material composed of silicon and carbon having the above structure as a negative electrode active material was produced.
  • a battery capacity per mass of the negative electrode active material was 1000 mAh / g, and a storage capacity retention rate after 5000 cycles was 95%, and a high capacity and long life lithium ion secondary battery could be realized.
  • FIG. 9 is a schematic cross-sectional view showing the negative electrode active material of the second embodiment.
  • This embodiment is different from the first embodiment in that the surface of the silicon nanoparticle 902 other than the bonding surface is covered with a coating 903 mainly composed of carbon.
  • the surface of the thermally expanded graphite oxide 901 has a structure in which silicon nanoparticles 902 are bonded via a flat portion 904 having a certain area. Further, the surface of the silicon nanoparticles 902 other than the bonding surface (flat portion 904) is covered with a coating 903 containing carbon as a main component.
  • the thermally expanded graphite oxide 901 has a thin film structure, the shape is close to an ellipse or a rectangle, and the length of the longest portion is about 100 ⁇ m. Further, the film thickness is 100 nm or less, and it is possible to make it 10 nm or less by devising the oxidation condition and the thermal expansion condition. In particular, when the film thickness is 10 nm or less, the specific surface area is 100 m 2 / g or more, and more silicon nanoparticles 902 can be bonded to the surface.
  • the silicon nanoparticles 902 are substantially spherical, and are joined to the thermally expanded graphite oxide 901 through a flat portion 904 having a curvature smaller than the average curvature. Excluding the bonding surface of the silicon nanoparticles 902 with the thermally expanded graphite oxide 901, it is twice the average radius of curvature, that is, the average diameter is 100 nm or less, and more preferably 30 nm or less. Since the silicon nanoparticles 902 are produced on the surface of the thermally expanded graphite oxide 901 by a vapor phase growth method, the joint surface between the two is necessarily in a form along the surface shape of the thermally expanded graphite oxide 901. The surface of the thermally expanded graphite oxide 901 is almost flat and has a gentle curvature depending on the location.
  • the joint between the silicon nanoparticle 902 and the thermally expanded graphite oxide 901 is joined by a covalent bond between silicon and carbon. Therefore, the bonding force is very strong, and the electrical conduction characteristics between the two are also very good.
  • the thermally expanded graphite oxide 901 of the silicon nanoparticles 902 is formed by thermal vapor deposition using propylene as a raw material gas.
  • a film 903 mainly composed of carbon and having a multilayer structure of nanographene (a structure in which a single layer of graphite is laminated) is formed on the surface other than the joint portion.
  • the film 903 mainly composed of carbon is 10 nm. In some cases, it is possible to use a coating 903 mainly composed of carbon having a thickness of 1 to 30 nm.
  • the film thickness of the coating 903 containing carbon as a main component can be adjusted by changing the growth time in the thermal vapor deposition method.
  • the surface of the silicon nanoparticle 902 By covering the surface of the silicon nanoparticle 902 with the coating 903 containing carbon as a main component, the surface of the silicon nanoparticle 902 can be prevented from being oxidized and stable electric conductivity can be secured. The life of the lithium ion secondary battery can be extended.
  • FIG. 9 shows a structure in which one silicon nanoparticle 902 is bonded to the surface of thermally expanded graphite oxide 901. However, at least a plurality of silicon nanoparticles 902 are the same through a bonding surface having a certain area. It is also possible to use a structure bonded to the surface of the thermally expanded graphite oxide 901.
  • a lithium ion secondary battery using a composite material composed of silicon and carbon having the above structure as a negative electrode active material was produced. As a result, it was possible to realize a high-capacity and long-life lithium ion secondary battery with a battery capacity per mass of the negative electrode active material of 1000 mAh / g and a storage capacity retention rate of 950 cycles after 95%.
  • FIG. 10 shows the structure of the negative electrode of the lithium ion secondary battery.
  • the negative electrode shown in this drawing has a structure in which a composite material in which silicon nanoparticles 1002 are bonded to the surface of thermally expanded graphite oxide 1001 is press-molded on a negative electrode current collector 1003.
  • a film 903 mainly composed of carbon in FIG. 9 may be formed on the surface of the silicon nanoparticle 1002.
  • the negative electrode active material is mixed with a binder and applied to the negative electrode current collector 1003.
  • FIG. 11 is a schematic view showing a thermal vapor phase growth apparatus for forming silicon nanoparticles on the surface of thermally expanded graphite oxide.
  • the thermal vapor phase growth apparatus includes a reaction furnace 1101 having a raw material installation unit 1102.
  • the reaction furnace 1101 is made of quartz and has a diameter of 5 cm and a length of 40 cm.
  • the reaction furnace 1101 is connected to a first pipe for supplying hydrogen, a second pipe for supplying silicon tetrachloride (SiCl 4 ) together with hydrogen, and a third pipe for discharging exhaust gas to the outside.
  • the second pipe is provided with a container 1103 in which liquid silicon tetrachloride (SiCl 4 ) serving as a silicon raw material is placed. By bubbling with hydrogen gas (H 2 ), silicon tetrachloride (SiCl 4 ) is provided. Is introduced into the reactor 1101.
  • the third pipe is provided with a container 1104 containing a sodium hydroxide aqueous solution so as to absorb and remove harmful acid gases and the like.
  • the hydrogen is used for reducing and removing oxygen present on the surface of the thermally expanded graphite by setting the inside of the reaction furnace 1101 as a reducing atmosphere. Therefore, any gas other than hydrogen can be used as long as it can be a reducing atmosphere.
  • the first pipe and the second pipe are provided with flow controllers 1105 and 1106 (Mass Flow Controller) and a plug 1107 so that the flow rates of hydrogen and silicon tetrachloride can be adjusted respectively. .
  • flow controllers 1105 and 1106 Mass Flow Controller
  • plug 1107 so that the flow rates of hydrogen and silicon tetrachloride can be adjusted respectively.
  • the vapor pressure of silicon tetrachloride at 20 ° C. is 30 kPa.
  • the amount of silicon tetrachloride introduced is 34%.
  • the amount of silicon tetrachloride introduced (mixing ratio of hydrogen and silicon tetrachloride) can be controlled by adjusting the flow rate using the flow rate control units 1105 and 1106 and the plug 1107.
  • An example of a procedure for producing a negative electrode active material (a procedure for growing nanosilicon on the surface of thermally expanded graphite oxide) is as follows.
  • the thermally expanded graphite oxide was placed in the raw material container and installed in the raw material installation unit 1102 inside the reaction furnace 1101. Hydrogen was allowed to flow through the first pipe at a flow rate of 200 mL / min, and the second pipe was closed. In this state, the temperature of the reactor 1101 was increased from room temperature to 1000 ° C. at a rate of 10 ° C./min.
  • the hydrogen flow rate was changed to 100 mL / min, and the hydrogen flow rate of the second pipe was set to 100 mL / min. Under this condition, 17% silicon tetrachloride can be introduced.
  • the hydrogen flow rate of the first pipe was changed to 200 mL / min, held at 1000 ° C. for 30 minutes, and then naturally cooled.
  • silicon nanoparticles having an average diameter of 30 nm could be formed on the surface of the thermally expanded graphite oxide.
  • the ratio of silicon nanoparticles to the total composite material is 20% by mass.
  • the ratio of silicon nanoparticles can be adjusted by changing the growth time.
  • the ratio of silicon nanoparticles to the whole negative electrode active material is desirably 20 to 80% by mass.
  • the temperature is lowered at a rate of 10 ° C./min before the natural cooling, and when the temperature reaches 800 ° C., the first pipe Then, argon gas containing 5% propylene gas is introduced at a flow rate of 200 mL / min, and after 1 hour, it is switched to 200 mL / min pure argon gas, held for 30 minutes, and then naturally cooled.
  • argon gas containing 5% propylene gas is introduced at a flow rate of 200 mL / min, and after 1 hour, it is switched to 200 mL / min pure argon gas, held for 30 minutes, and then naturally cooled.
  • the carbon which comprises the film formed in this way has a nano graphene structure.
  • Flake graphite having an average diameter of 10 to 100 ⁇ m is immersed in a solution containing concentrated sulfuric acid, sodium nitrate and potassium permanganate for several days and stirred. This is the so-called Hummers method. After oxidation by the Hummers method, heat-expanded graphite oxide was produced by further heat treatment in an argon atmosphere at 600 to 1100 ° C. for 30 minutes to 3 hours.
  • the film thickness in the C-axis direction of the thermally expanded graphite oxide is 1.0 to 100.0 nm, and can be adjusted to some extent by changing the oxidation conditions and the foaming treatment conditions.
  • Flaked graphite and benzoyl peroxide powder are mixed and oxidized by heat treatment at 110 ° C. for 10 minutes. This is further subjected to heat treatment at 600 to 1100 ° C. for 30 minutes to 3 hours in an argon atmosphere to obtain thermally expanded graphite oxide.
  • FIG. 12A is a schematic perspective view showing the shape of silicon nanoparticles constituting the negative electrode active material of the present invention.
  • FIG. 12B is a side view of the silicon nanoparticles of FIG. 12A. These figures show the case where the silicon nanoparticles are assumed to be spherical and the joint surface with the thermally expanded graphite oxide is assumed to be a flat surface.
  • the area of the joint surface is A 2 and the area of the other spherical portion is A 1 .
  • the radius of the sphere and r, and the distance from the center of the sphere to the joining surfaces and xr (provided that 0 ⁇ x ⁇ 1), the ratio of A 2 to the total area A 1 + A 2 is, (x-1) / (X-3).
  • thermally expanded graphite oxide was used as the carbonaceous material, but desirable results can be obtained even if the carbonaceous material is fine graphite, carbon nanotube or carbon nanohorn. Since these have a regular arrangement of carbon atoms like the thermally expanded graphite oxide, they have high electrical characteristics such as conductivity.

Abstract

Disclosed is a negative electrode active material for lithium ion secondary batteries, which contains a carbonaceous material having electrical conductivity and silicon nanoparticles having bonding surfaces that are chemically bonded to the surface of the carbonaceous material. Consequently, electrical isolation due to expansion and contraction of silicon nanoparticles accompanying intercalation and deintercalation of lithium ions is prevented, thereby enabling achievement of a lithium ion secondary battery that has excellent charge/discharge cycle characteristics.

Description

負極活物質並びにこれを用いた負極合剤、負極及びリチウムイオン二次電池Negative electrode active material and negative electrode mixture, negative electrode and lithium ion secondary battery using the same
　本発明は、負極活物質並びにこれを用いた負極合剤、負極及びリチウムイオン二次電池に関する。 The present invention relates to a negative electrode active material, a negative electrode mixture using the same, a negative electrode, and a lithium ion secondary battery.
　リチウムイオン二次電池の負極活物質として、黒鉛系の炭素質材料が広く用いられている。 Graphite-based carbonaceous materials are widely used as negative electrode active materials for lithium ion secondary batteries.
　黒鉛にリチウムイオンが挿入した際の化学量論的組成は、ＬｉＣ６であり、その理論容量は３７２ｍＡｈ／ｇと算出できる。これに対して、シリコンにリチウムイオンが挿入した際の化学量論的組成は、Ｌｉ１５Ｓｉ４であり、その理論容量は３５７１ｍＡｈ／ｇと算出できる。このように、シリコンは、黒鉛に比べて９．６倍のリチウムを貯蔵できる魅力的な材料である。 Stoichiometric composition when the lithium ions are inserted into the graphite is LiC 6, the theoretical capacity can be calculated to be 372 mAh / g. On the other hand, the stoichiometric composition when lithium ions are inserted into silicon is Li 15 Si 4 , and its theoretical capacity can be calculated as 3571 mAh / g. Thus, silicon is an attractive material that can store 9.6 times as much lithium as graphite.
　しかしながら、シリコン粒子にリチウムイオンを挿入すると体積が３倍程度に膨張するため、リチウムイオンの挿入と放出を繰り返す間にシリコン粒子が力学的に破壊する。シリコン粒子が破壊すると、破壊した微小粒子が電気的に孤立し、破壊面に新しい電気化学的被覆層ができて不可逆的な電池の容量が減少する。これらの影響により、充放電サイクル特性が著しく低下するという問題があった。 However, when lithium ions are inserted into the silicon particles, the volume expands to about three times, so that the silicon particles are dynamically destroyed during repeated insertion and release of lithium ions. When silicon particles break down, the broken microparticles are electrically isolated and a new electrochemical coating layer is created on the broken surface, reducing the irreversible battery capacity. Due to these effects, there is a problem that the charge / discharge cycle characteristics are remarkably deteriorated.
　特許文献１には、黒鉛（膨張黒鉛）の表面に液相からＳｉ相を成長させ、熱処理することにより、Ｃ／Ｓｉ／Ｏ複合材料を作製することが記載されている。 Patent Document 1 describes that a C / Si / O composite material is produced by growing a Si phase from a liquid phase on the surface of graphite (expanded graphite) and performing a heat treatment.
　非特許文献１には、シリコンナノ粒子と熱膨張酸化黒鉛とを機械的に混合し、負極活物質として特性を測定した例が記載されている。 Non-Patent Document 1 describes an example in which silicon nanoparticles and thermally expanded graphite oxide are mechanically mixed and the characteristics are measured as a negative electrode active material.
特開２００６－０５９５５８号公報JP 2006-059558 A
　本発明は、シリコンナノ粒子と炭素とを含む複合材料を用いる負極活物質において、シリコンナノ粒子の膨張破壊による不可逆的な電池の容量減少を防止し、優れた充放電サイクル特性を有するリチウムイオン二次電池を提供することを課題とする。 In the negative electrode active material using a composite material containing silicon nanoparticles and carbon, the present invention prevents irreversible battery capacity reduction due to expansion and destruction of silicon nanoparticles, and provides lithium ion secondary batteries having excellent charge / discharge cycle characteristics. It is an object to provide a secondary battery.
　本発明のリチウムイオン二次電池用負極活物質は、電気伝導性を有する炭素質材料と、シリコンナノ粒子と、を含み、シリコンナノ粒子は、炭素質材料の表面に化学的に結合した接合面を有することを特徴とする。 The negative electrode active material for a lithium ion secondary battery of the present invention includes a carbonaceous material having electrical conductivity and silicon nanoparticles, and the silicon nanoparticles are chemically bonded to the surface of the carbonaceous material. It is characterized by having.
　本発明によれば、リチウムイオン挿入および放出に伴う、シリコンナノ粒子の膨張収縮による電気的孤立を防止し、優れた充放電サイクル特性を有するリチウムイオン二次電池を提供することができる。 According to the present invention, it is possible to provide a lithium ion secondary battery having excellent charge / discharge cycle characteristics by preventing electrical isolation due to expansion and contraction of silicon nanoparticles accompanying lithium ion insertion and release.
熱膨張酸化黒鉛の走査型電子顕微鏡写真である。It is a scanning electron micrograph of thermally expanded graphite oxide. 図１の熱膨張酸化黒鉛の一部を更に拡大した走査型電子顕微鏡写真である。FIG. 2 is a scanning electron micrograph in which a part of the thermally expanded graphite oxide of FIG. 1 is further enlarged. 第一の実施例の負極活物質である、シリコンナノ粒子を付着させた熱膨張酸化黒鉛を示す走査型電子顕微鏡写真である。It is a scanning electron micrograph which shows the thermal expansion graphite oxide which attached the silicon nanoparticle which is a negative electrode active material of a 1st Example. 図３の熱膨張酸化黒鉛の表面を更に拡大した走査型電子顕微鏡写真である。It is the scanning electron micrograph which expanded further the surface of the thermal expansion graphite oxide of FIG. 第一の実施例の負極活物質を構成する、熱膨張酸化黒鉛の表面に成長したシリコンナノ粒子を示す透過型電子顕微鏡写真である。It is a transmission electron micrograph which shows the silicon nanoparticle which grew on the surface of the thermal expansion graphite oxide which comprises the negative electrode active material of a 1st Example. 図５のシリコンナノ粒子の接合面を示す透過型電子顕微鏡写真である。6 is a transmission electron micrograph showing a bonding surface of the silicon nanoparticles of FIG. 5. 図６のシリコンナノ粒子の接合状態を示す模式断面図である。It is a schematic cross section which shows the joining state of the silicon nanoparticle of FIG. 図６のシリコンナノ粒子の接合状態を示す模式斜視図である。It is a model perspective view which shows the joining state of the silicon nanoparticle of FIG. 第二の実施例の負極活物質を示す模式断面図である。It is a schematic cross section which shows the negative electrode active material of a 2nd Example. 本発明の負極活物質を含む負極の構造を示す模式断面図である。It is a schematic cross section which shows the structure of the negative electrode containing the negative electrode active material of this invention. 本発明の負極活物質を製造するための熱気相成長装置を示す概略構成図である。It is a schematic block diagram which shows the thermal vapor phase growth apparatus for manufacturing the negative electrode active material of this invention. 本発明の負極活物質を構成するシリコンナノ粒子の形状を示す模式斜視図である。It is a model perspective view which shows the shape of the silicon nanoparticle which comprises the negative electrode active material of this invention. 図１２Ａのシリコンナノ粒子の側面図である。FIG. 12B is a side view of the silicon nanoparticles of FIG. 12A. 本発明のリチウムイオン二次電池の内部構造を示す部分断面図である。It is a fragmentary sectional view which shows the internal structure of the lithium ion secondary battery of this invention.
　本発明は、リチウム塩を含有する有機溶媒を電解液とするリチウムイオン二次電池において、シリコンナノ粒子と、電気伝導性を有する炭素質材料とから成り、シリコンナノ粒子が、その平均曲率よりも小さい曲率を有する平坦部を介して、炭素質材料表面と接合している複合材料を、負極活物質として用いたことを特徴とするリチウムイオン二次電池に関する。ここで、シリコンナノ粒子は、実質的にケイ素単体で構成されたナノ粒子である。 The present invention relates to a lithium ion secondary battery using an organic solvent containing a lithium salt as an electrolyte, and is composed of silicon nanoparticles and a carbonaceous material having electrical conductivity, and the silicon nanoparticles are more than the average curvature thereof. The present invention relates to a lithium ion secondary battery using a composite material bonded to the surface of a carbonaceous material as a negative electrode active material through a flat portion having a small curvature. Here, the silicon nanoparticles are nanoparticles substantially composed of silicon alone.
　リチウムイオン挿入によるシリコンナノ粒子の膨張破壊を防止するためには、シリコンナノ粒子の直径を小さくすることが有効である。シリコンナノ粒子の直径をどの程度小さくすれば良いかは、充放電速度等の電池動作条件に依存するが、概ね１００ｎｍ以下が望ましい。更に望ましくは３０ｎｍ以下である。 In order to prevent expansion and destruction of silicon nanoparticles due to insertion of lithium ions, it is effective to reduce the diameter of silicon nanoparticles. The extent to which the diameter of the silicon nanoparticles should be reduced depends on the battery operating conditions such as the charge / discharge rate, but is preferably about 100 nm or less. More desirably, it is 30 nm or less.
　しかしながら、シリコンナノ粒子の直径を小さくするだけでは、良好な充放電サイクル特性を実現することが出来ない。直径の小さいシリコンナノ粒子は、リチウムイオン挿入および放出により、膨張破壊には至らないが、炭素の３倍以上の膨張および収縮を繰り返すことになる。そのため、この膨張収縮サイクルの過程で、電気的に孤立するシリコンナノ粒子が増加し、その結果、電池容量が低下する。 However, good charge / discharge cycle characteristics cannot be realized simply by reducing the diameter of the silicon nanoparticles. Silicon nanoparticles having a small diameter do not lead to expansion and destruction due to insertion and release of lithium ions, but they repeat expansion and contraction three times or more that of carbon. Therefore, in the process of the expansion / contraction cycle, electrically isolated silicon nanoparticles increase, and as a result, the battery capacity decreases.
　非特許文献１に記載されている構成のようにシリコンナノ粒子と熱膨張酸化黒鉛とを機械的に混合しただけでは、シリコンナノ粒子と熱膨張酸化黒鉛とが単に接触しているだけであるため、充放電サイクルの過程でシリコンナノ粒子と熱膨張酸化黒鉛とが物理的に離れ、結果としてシリコンナノ粒子が電気的に孤立する。 Since the silicon nanoparticles and the thermally expanded graphite oxide are simply in contact with each other simply by mechanically mixing the silicon nanoparticles and the thermally expanded graphite oxide as in the configuration described in Non-Patent Document 1. In the charge / discharge cycle, the silicon nanoparticles and the thermally expanded graphite oxide are physically separated, and as a result, the silicon nanoparticles are electrically isolated.
　また、特許文献１に記載されているＣ／Ｓｉ／Ｏ複合材料は、炭素及び酸素がケイ素に化学結合しているため、ケイ素単体に比べて電気伝導性が劣る点で改善の余地がある。 Also, the C / Si / O composite material described in Patent Document 1 has room for improvement in that the electrical conductivity is inferior to that of silicon alone because carbon and oxygen are chemically bonded to silicon.
　そこで、本発明では、電気伝導性の高いシリコンナノ粒子が、熱膨張酸化黒鉛の表面と強く接合した状態を実現することにより、上記課題を解決する。具体的には、シリコンナノ粒子が、その平均曲率よりも小さい曲率を有する平坦部を介して、熱膨張酸化黒鉛表面と接合した状態を実現する。このような接合状態は、熱膨張酸化黒鉛の表面にシリコンナノ粒子を気相成長法により作製することにより実現することができる。 Therefore, in the present invention, the above problem is solved by realizing a state in which silicon nanoparticles having high electrical conductivity are strongly bonded to the surface of thermally expanded graphite oxide. Specifically, a state in which the silicon nanoparticles are bonded to the surface of the thermally expanded graphite oxide through a flat portion having a curvature smaller than the average curvature is realized. Such a bonded state can be realized by producing silicon nanoparticles on the surface of thermally expanded graphite oxide by a vapor phase growth method.
　以下、本発明のシリコンナノ粒子と熱膨張酸化黒鉛との複合材料の形態について、図面を用いて説明する。 Hereinafter, the form of the composite material of the silicon nanoparticles of the present invention and thermally expanded graphite oxide will be described with reference to the drawings.
　熱膨張酸化黒鉛は、Ｈｕｍｍｅｒ法、すなわち、硫酸、硝酸ナトリウムおよび過マンガン酸カリウムの混合物により酸化した、いわゆる酸化黒鉛をアルゴン雰囲気１０５０℃で２時間、熱処理をすることにより得た。 The thermally expanded graphite oxide was obtained by the heat treatment of the so-called graphite oxide oxidized by the Hummer method, that is, a mixture of sulfuric acid, sodium nitrate and potassium permanganate at 1050 ° C. for 2 hours in an argon atmosphere.
　図１は、熱膨張酸化黒鉛の走査型電子顕微鏡写真（ＳＥＭ画像）である。 FIG. 1 is a scanning electron micrograph (SEM image) of thermally expanded graphite oxide.
　上記方法により、当初の厚さが数十μｍの黒鉛構造から非常に膜厚の薄い黒鉛層を作ることができる。 By the above method, a very thin graphite layer can be made from a graphite structure having an initial thickness of several tens of μm.
　図２は、熱膨張酸化黒鉛の高倍率の走査電子顕微鏡写真である。 FIG. 2 is a high-magnification scanning electron micrograph of thermally expanded graphite oxide.
　この写真から、超薄膜の黒鉛層が形成できていることがわかる。更に高倍率の観察により、熱膨張酸化黒鉛の膜厚は、平均で１０ｎｍ以下であることがわかった。また、比表面積は１００ｍ２／ｇであった。 From this photograph, it can be seen that an ultra-thin graphite layer was formed. Furthermore, it was found by observation at a high magnification that the film thickness of the thermally expanded graphite oxide was 10 nm or less on average. The specific surface area was 100 m 2 / g.
　この熱膨張酸化黒鉛は、炭素原子からなるハニカム状の結晶格子の単一層が数層乃至数十層程度重なった構造を有すると考える。 It is considered that this thermally expanded graphite oxide has a structure in which a single layer of a honeycomb-like crystal lattice made of carbon atoms is overlapped by several to several tens of layers.
　図３は、第一の実施例の負極活物質である、シリコンナノ粒子を表面に付着させた熱膨張酸化黒鉛の走査型電子顕微鏡写真である。これは、後に詳述する気相成長法により作製したものである。 FIG. 3 is a scanning electron micrograph of thermally expanded graphite oxide having silicon nanoparticles attached to the surface, which is the negative electrode active material of the first example. This is produced by the vapor phase growth method described in detail later.
　本図において、粒状に白っぽく見えるものがシリコンナノ粒子であり、熱膨張酸化黒鉛の表面に均一に存在することがわかる。 In this figure, it can be seen that the particles that appear whitish in the form of particles are silicon nanoparticles, which are uniformly present on the surface of the thermally expanded graphite oxide.
　図４は、図３に示すシリコンナノ粒子を表面に付着させた熱膨張酸化黒鉛を高倍率で観察したものである。 FIG. 4 is an observation of the thermally expanded graphite oxide with the silicon nanoparticles shown in FIG. 3 attached to the surface at a high magnification.
　本図において、熱膨張酸化黒鉛４０１の表面には、球状のシリコンナノ粒子４０２が間隔（およそ３０～１００ｎｍ）をあけてむらなく付着している。シリコンナノ粒子４０２の平均直径は約３０ｎｍである。ここで、平均直径は、本図におけるシリコンナノ粒子４０２の一方向（水平方向）の直径を統計処理することにより算出した。 In this figure, spherical silicon nanoparticles 402 are uniformly attached to the surface of the thermally expanded graphite oxide 401 with an interval (approximately 30 to 100 nm). The average diameter of the silicon nanoparticles 402 is about 30 nm. Here, the average diameter was calculated by statistically processing the diameter in one direction (horizontal direction) of the silicon nanoparticles 402 in the figure.
　図５及び図６は、熱膨張酸化黒鉛の表面のシリコンナノ粒子を透過型電子顕微鏡により撮影した写真（ＴＥＭ画像）である。 FIG. 5 and FIG. 6 are photographs (TEM images) obtained by photographing silicon nanoparticles on the surface of thermally expanded graphite oxide with a transmission electron microscope.
　図５において、黒く球状に見える部分がシリコンナノ粒子５０１であり、その背後に見えるものが熱膨張酸化黒鉛である。 In FIG. 5, the portion that looks black and spherical is the silicon nanoparticle 501, and the portion that appears behind it is thermally expanded graphite oxide.
　図６に示すシリコンナノ粒子６０１は、シリコンナノ粒子６０１と熱膨張酸化黒鉛との接合面６０２（矢印で示す面）を斜めから見たものである。接合面６０２は、平坦面であることがわかる。すなわち、シリコンナノ粒子６０１は、その平均曲率よりも小さい曲率を有する平坦部（接合面６０２）を介して炭素質材料の表面に接合されていることがわかる。ここで、接合面６０２は、付着面ともいう。 The silicon nanoparticle 601 shown in FIG. 6 is an oblique view of the bonding surface 602 (surface indicated by the arrow) between the silicon nanoparticle 601 and the thermally expanded graphite oxide. It can be seen that the bonding surface 602 is a flat surface. That is, it can be seen that the silicon nanoparticles 601 are bonded to the surface of the carbonaceous material via a flat portion (bonding surface 602) having a curvature smaller than the average curvature. Here, the bonding surface 602 is also referred to as an adhesion surface.
　以上のように、一定面積の平坦部を介してシリコンナノ粒子と熱膨張酸化黒鉛表面とが強く接合した状態を実現することができる。これにより、シリコンナノ粒子の膨張収縮による電気的孤立を防止し、優れた充放電サイクル特性を有するリチウムイオン二次電池を提供することが可能である。 As described above, it is possible to realize a state in which the silicon nanoparticles and the surface of the thermally expanded graphite oxide are strongly bonded via the flat portion having a certain area. Thereby, it is possible to provide a lithium ion secondary battery that prevents electrical isolation due to expansion and contraction of silicon nanoparticles and has excellent charge / discharge cycle characteristics.
　図１３は、本発明のリチウムイオン二次電池の内部構造を示したものである。 FIG. 13 shows the internal structure of the lithium ion secondary battery of the present invention.
　本図において、１３１０は正極、１３１１はセパレータ、１３１２は負極、１３１３は電池缶、１３１４は正極集電タブ、１３１５は負極集電タブ、１３１６は内蓋、１３１７は内圧開放弁、１３１８はガスケット、１３１９は正温度係数抵抗素子（ＰＴＣ抵抗素子。ＰＴＣは、Ｐｏｓｉｔｉｖｅ　Ｔｅｍｐｅｒａｔｕｒｅ　Ｃｏｅｆｆｉｃｉｅｎｔの略称である。）、１３２０は電池蓋である。これらがリチウムイオン二次電池１３００を構成している。電池蓋１３２０は、内蓋１３１６、内圧開放弁１３１７、ガスケット１３１８及び正温度係数抵抗素子１３１９からなる一体化部品である。 In this figure, 1310 is a positive electrode, 1311 is a separator, 1312 is a negative electrode, 1313 is a battery can, 1314 is a positive current collector tab, 1315 is a negative current collector tab, 1316 is an inner lid, 1317 is an internal pressure release valve, 1318 is a gasket, 1319 is a positive temperature coefficient resistance element (PTC resistance element; PTC is an abbreviation for Positive Temperature Coefficient), and 1320 is a battery lid. These constitute a lithium ion secondary battery 1300. The battery lid 1320 is an integrated part composed of an inner lid 1316, an internal pressure release valve 1317, a gasket 1318, and a positive temperature coefficient resistance element 1319.
　正極は、以下の手順により作製した。 The positive electrode was produced by the following procedure.
　正極活物質には、ＬｉＭｎ２Ｏ４を用いた。その正極活物質に、導電材として黒鉛粉末及びアセチレンブラックを添加した。正極活物質、黒鉛粉末及びアセチレンブラックの混合比は、質量基準で８５．０：７．０：２．０である。 LiMn 2 O 4 was used as the positive electrode active material. To the positive electrode active material, graphite powder and acetylene black were added as conductive materials. The mixing ratio of the positive electrode active material, graphite powder, and acetylene black is 85.0: 7.0: 2.0 on a mass basis.
　さらに、結着剤（バインダ）としてポリフッ化ビニリデン（以下、ＰＶＤＦと略記する。）を１－メチル－２－ピロリドン（以下、ＮＭＰと略記する。）に溶解した溶液を加え、プラネタリーミキサーで混合した。さらに、真空下でスラリー中の気泡を除去し、均質な正極合剤スラリーを調製した。ここで、正極合剤とは、正極活物質、結着剤等を混合したものをいう。 Further, a solution in which polyvinylidene fluoride (hereinafter abbreviated as PVDF) is dissolved in 1-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) is added as a binder, and mixed with a planetary mixer. did. Furthermore, bubbles in the slurry were removed under vacuum to prepare a homogeneous positive electrode mixture slurry. Here, the positive electrode mixture refers to a mixture of a positive electrode active material, a binder and the like.
　このスラリーを、塗布機を用いて厚さ２０μｍのアルミニウム箔の両面に均一かつ均等に塗布した。塗布後、ロールプレス機により電極密度が２．５５ｇ／ｃｍ３になるように圧縮成形した。これを切断機で裁断し、厚さ１００μｍ、長さ９００ｍｍ、幅５４ｍｍの正極を作製した。 This slurry was applied uniformly and evenly on both sides of an aluminum foil having a thickness of 20 μm using an applicator. After the application, it was compression molded by a roll press so that the electrode density was 2.55 g / cm 3 . This was cut with a cutting machine to produce a positive electrode having a thickness of 100 μm, a length of 900 mm, and a width of 54 mm.
　正極活物質、黒鉛粉末、アセチレンブラック及びポリフッ化ビニリデンの混合比は、質量基準で８５．０：７．０：２．０：６．０である。 The mixing ratio of the positive electrode active material, graphite powder, acetylene black, and polyvinylidene fluoride is 85.0: 7.0: 2.0: 6.0 on a mass basis.
　負極は、以下の手順により作製した。 The negative electrode was produced by the following procedure.
　負極活物質は、上述の第一の実施例及び第二の実施例に記載したシリコンと炭素の複合材料を用いた。その複合材料に、結着剤（バインダ）としてＰＶＤＦをＮＭＰに溶解した溶液を加えた。複合材料と結着剤との混合比は、質量基準で９５．０：５．０である。 As the negative electrode active material, the silicon and carbon composite materials described in the first and second examples were used. A solution obtained by dissolving PVDF in NMP as a binder was added to the composite material. The mixing ratio of the composite material and the binder is 95.0: 5.0 on a mass basis.
　それをプラネタリーミキサーで混合し、真空下でスラリー中の気泡を除去して、均質な負極合剤スラリーを調製した。ここで、負極合剤とは、負極活物質、結着剤等を混合したものをいう。 It was mixed with a planetary mixer, and bubbles in the slurry were removed under vacuum to prepare a homogeneous negative electrode mixture slurry. Here, the negative electrode mixture refers to a mixture of a negative electrode active material, a binder and the like.
　このスラリーを塗布機で厚さ１０μｍの圧延銅箔の両面に均一かつ均等に塗布した。塗布後、その電極をロールプレス機によって圧縮成形して、電極密度が１．３ｇ／ｃｍ３とした。これを切断機で裁断し、厚さ１１０μｍ、長さ９５０ｍｍ、幅５６ｍｍの負極を作製した。 This slurry was applied uniformly and evenly on both sides of a rolled copper foil having a thickness of 10 μm with a coating machine. After application, the electrode was compression-molded with a roll press to make the electrode density 1.3 g / cm 3 . This was cut with a cutting machine to produce a negative electrode having a thickness of 110 μm, a length of 950 mm, and a width of 56 mm.
　上述のように作製した正極１３１０と、負極１３１２の未塗布部（集電板露出面）とに、それぞれ正極集電タブ１３１４および負極集電タブ１３１５を超音波溶接した。正極集電タブ１３１４にはアルミニウム製リード片を、負極集電タブ１３１５にはニッケル製リード片を用いた。その後、厚さ３０μｍの多孔性ポリエチレンフィルムからなるセパレータ１３１１を正極１３１０と負極１３１２との間に挟み込み、正極１３１０、セパレータ１３１１、負極１３１２を捲回した。この捲回体を電池缶１３１３に収納し、負極集電タブ１３１５を電池缶１３１３の缶底に抵抗溶接機により接続した。正極集電タブ１３１４は、内蓋１３１６の底面に超音波溶接により接続した。 The positive electrode current collecting tab 1314 and the negative electrode current collecting tab 1315 were ultrasonically welded to the positive electrode 1310 produced as described above and the uncoated portion (current collector exposed surface) of the negative electrode 1312, respectively. An aluminum lead piece was used for the positive electrode current collecting tab 1314, and a nickel lead piece was used for the negative electrode current collecting tab 1315. Thereafter, a separator 1311 made of a porous polyethylene film having a thickness of 30 μm was sandwiched between the positive electrode 1310 and the negative electrode 1312, and the positive electrode 1310, the separator 1311, and the negative electrode 1312 were wound. The wound body was housed in a battery can 1313, and the negative electrode current collecting tab 1315 was connected to the bottom of the battery can 1313 by a resistance welder. The positive electrode current collecting tab 1314 was connected to the bottom surface of the inner lid 1316 by ultrasonic welding.
　上部の電池蓋１３２０を電池缶１３１３に取り付ける前に、非水電解液を注入した。電解液の溶媒は、エチレンカーボネート（ＥＣ）とジメチルカーボネート（ＤＭＣ）とジエチルカーボネート（ＤＥＣ）からなり、体積比は１：１：１とした。電解質は、濃度１ｍｏｌ／Ｌ（約０．８ｍｏｌ／ｋｇ）のＬｉＰＦ６である。このような電解液を電極群の上から滴下し、電池蓋１３２０を電池缶１３１３に、かしめて密封し、リチウムイオン二次電池を得た。 Before attaching the upper battery lid 1320 to the battery can 1313, a non-aqueous electrolyte was injected. The solvent of the electrolytic solution was composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), and the volume ratio was 1: 1: 1. The electrolyte is LiPF 6 at a concentration of 1 mol / L (about 0.8 mol / kg). Such an electrolytic solution was dropped from above the electrode group, and the battery lid 1320 was caulked and sealed in the battery can 1313 to obtain a lithium ion secondary battery.
　以下、充放電サイクル試験の手順について説明する。 Hereinafter, the procedure of the charge / discharge cycle test will be described.
　作製した電池を２５℃で０．３ＣＡ相当の電流で４．２０Ｖまで充電し、その後４．２０Ｖで電流が０．０３Ｃになるまで定電圧充電を行った。３０分休止後に０．３ＣＡ相当の定電流で２．７Ｖまで定電流放電を行った。これを３サイクル行って初期化し、３サイクル目の電池容量を測定し、測定された電池容量を初期電池容量とした。初期電池容量は１．１５Ａｈであった。 The fabricated battery was charged to 4.20 V at a current corresponding to 0.3 CA at 25 ° C., and then charged at a constant voltage until the current became 0.03 C at 4.20 V. After 30 minutes of rest, constant current discharge was performed to 2.7 V with a constant current corresponding to 0.3 CA. This was repeated for 3 cycles to initialize the battery capacity at the third cycle, and the measured battery capacity was defined as the initial battery capacity. The initial battery capacity was 1.15 Ah.
　次に、２５℃で、５００回の充放電サイクルを行った。各サイクルにおいては、１Ｃ相当の電流で４．２０Ｖまで充電し、その後４．２０Ｖで電流が０．０３Ｃになるまで定電圧充電を行った。放電は、１Ｃ相当の電流で２．７Ｖまで放電した。充放電の間には休止を３０分行った。電池容量の測定は上記の通り行った。 Next, 500 charge / discharge cycles were performed at 25 ° C. In each cycle, the battery was charged to 4.20 V with a current corresponding to 1 C, and then constant voltage charging was performed until the current became 0.03 C at 4.20 V. Discharge was discharged to 2.7 V with a current corresponding to 1 C. During charging and discharging, a pause was performed for 30 minutes. The battery capacity was measured as described above.
　以上の得られた結果を用いて、下記式に従って、サイクル容量維持率を算出した。 Using the results obtained above, the cycle capacity retention rate was calculated according to the following formula.
　サイクル容量維持率（％）＝（５００サイクル後の電池容量）／（初期電池容量）
　また、５０℃で、保存試験を行った。０．３ＣＡ相当の電流で４．２０Ｖまで充電し、その後４．２０Ｖで電流が０．０３Ｃになるまで定電圧充電を行った。３０分休止後、５０℃の恒温槽にて３か月間保管した。保管後、恒温槽より取り出して、２５℃で３時間放置後容量を測定した。電池容量の測定は上記の通り行った。３か月間保管後における保存容量維持率は、下記式により算出した。 Cycle capacity retention rate (%) = (battery capacity after 500 cycles) / (initial battery capacity)
Moreover, the storage test was done at 50 degreeC. The battery was charged to 4.20 V with a current corresponding to 0.3 CA, and then constant voltage charging was performed until the current became 0.03 C at 4.20 V. After a 30-minute rest, it was stored for 3 months in a thermostatic bath at 50 ° C. After storage, the sample was taken out from the thermostat and allowed to stand at 25 ° C. for 3 hours, and the capacity was measured. The battery capacity was measured as described above. The storage capacity maintenance rate after storage for 3 months was calculated by the following formula.
　保存容量維持率（％）＝（３か月保管後の電池容量）／（初期電池容量）
　図７は、第一の実施例である図６のシリコンナノ粒子の接合状態を示す模式断面図である。 Storage capacity retention rate (%) = (Battery capacity after 3 months storage) / (Initial battery capacity)
FIG. 7 is a schematic cross-sectional view showing a bonded state of the silicon nanoparticles of FIG. 6 which is the first embodiment.
　本図において、熱膨張酸化黒鉛７０１の表面には、シリコンナノ粒子７０２が、一定の面積を有する平坦部７０３を介して接合した構造となっている。熱膨張酸化黒鉛７０１は、薄膜構造を有しており、形状は、楕円形あるいは長方形に近く、最も長い部分の長さは、１００μｍ程度である。また、膜厚は１００ｎｍ以下であり、酸化条件および熱膨張条件を工夫することにより、１０ｎｍ以下にすることも可能である。特に、膜厚が１０ｎｍ以下の場合には、比表面積が１００ｍ２／ｇ以上になり、より多くのシリコンナノ粒子７０２を表面に接合することが可能となる。当該比表面積の更に望ましい範囲は、１００～１０００ｍ２／ｇである。 In this figure, the surface of a thermally expanded graphite oxide 701 has a structure in which silicon nanoparticles 702 are bonded via a flat portion 703 having a certain area. The thermally expanded graphite oxide 701 has a thin film structure, the shape is close to an ellipse or a rectangle, and the length of the longest portion is about 100 μm. Further, the film thickness is 100 nm or less, and it is possible to make it 10 nm or less by devising the oxidation condition and the thermal expansion condition. In particular, when the film thickness is 10 nm or less, the specific surface area is 100 m 2 / g or more, and more silicon nanoparticles 702 can be bonded to the surface. Further preferred range of the specific surface area is 100 ~ 1000m 2 / g.
　一方、シリコンナノ粒子７０２は、ほぼ球形であり、その平均曲率よりも小さい曲率を有する平坦部７０３を介して熱膨張酸化黒鉛７０１と接合している。シリコンナノ粒子７０２の熱膨張酸化黒鉛７０１との接合面を除いた、平均曲率半径の２倍、すなわち平均直径は１００ｎｍ以下であり、更に望ましくは３０ｎｍ以下である。シリコンナノ粒子７０２は、気相成長法により、熱膨張酸化黒鉛７０１の表面に作製するため、両者の接合面は、必然的に熱膨張酸化黒鉛７０１の表面形状に沿った形態となる。熱膨張酸化黒鉛７０１の表面は、ほぼ平坦であり、場所によっては緩やかな曲率を持っている。 On the other hand, the silicon nanoparticles 702 are substantially spherical and are joined to the thermally expanded graphite oxide 701 through a flat portion 703 having a curvature smaller than the average curvature. Excluding the bonding surface of the silicon nanoparticle 702 with the thermally expanded graphite oxide 701, it is twice the average radius of curvature, that is, the average diameter is 100 nm or less, more preferably 30 nm or less. Since the silicon nanoparticles 702 are produced on the surface of the thermally expanded graphite oxide 701 by a vapor phase growth method, the joint surface between the two is inevitably in a form along the surface shape of the thermally expanded graphite oxide 701. The surface of the thermally expanded graphite oxide 701 is almost flat and has a gentle curvature depending on the location.
　また、シリコンナノ粒子７０２と熱膨張酸化黒鉛７０１との接合部（平坦部７０３）では、シリコン（ケイ素）と炭素との共有結合により接合している。そのため、その接合力は非常に強固であり、また両者間の電気伝導特性も非常に良好である。 In addition, at the joint (flat part 703) between the silicon nanoparticle 702 and the thermally expanded graphite oxide 701, the silicon nanoparticle 702 and the carbon are joined by a covalent bond between silicon (silicon) and carbon. Therefore, the bonding force is very strong, and the electrical conduction characteristics between the two are also very good.
　図８は、第一の実施例の負極活物質である、シリコンと炭素とから成る複合材料の構造を模式的に示したものである。 FIG. 8 schematically shows the structure of a composite material composed of silicon and carbon, which is the negative electrode active material of the first embodiment.
　本図においては、熱膨張酸化黒鉛８０１の表面に複数個のシリコンナノ粒子８０２が、一定の面積を介して炭素質材料の表面と接合している。 In this figure, a plurality of silicon nanoparticles 802 are joined to the surface of the carbonaceous material through a certain area on the surface of the thermally expanded graphite oxide 801.
　熱膨張酸化黒鉛８０１は、薄膜構造を有しており、形状は、楕円形あるいは長方形に近く、最も長い部分の長さは１００μｍ程度である。また、膜厚は１００ｎｍ以下であり、酸化条件および熱膨張条件を工夫することにより、１０ｎｍ以下にすることも可能である。特に膜厚が１０ｎｍ以下の場合は、比表面積が１００ｍ２／ｇ以上になり、より多くのシリコンナノ粒子８０２を表面に接合することが可能となる。 The thermally expanded graphite oxide 801 has a thin film structure, the shape is close to an ellipse or a rectangle, and the length of the longest portion is about 100 μm. Further, the film thickness is 100 nm or less, and it is possible to make it 10 nm or less by devising the oxidation condition and the thermal expansion condition. In particular, when the film thickness is 10 nm or less, the specific surface area is 100 m 2 / g or more, and more silicon nanoparticles 802 can be bonded to the surface.
　一方、シリコンナノ粒子８０２は、ほぼ球形であり、その平均曲率よりも小さい曲率を有する平坦部を介して、熱膨張酸化黒鉛８０１と接合している。シリコンナノ粒子８０２の熱膨張酸化黒鉛８０１との接合面を除いた、平均曲率半径の２倍、すなわち平均直径は１００ｎｍ以下であり、更に望ましくは３０ｎｍ以下である。シリコンナノ粒子８０２は、気相成長法により熱膨張酸化黒鉛８０１の表面に作製するため、両者の接合面は、必然的に熱膨張酸化黒鉛８０１の表面形状に沿った形態となる。熱膨張酸化黒鉛８０１の表面は、ほぼ平坦であり、場所によっては緩やかな曲率を持っている。 On the other hand, the silicon nanoparticles 802 are substantially spherical and are joined to the thermally expanded graphite oxide 801 through a flat portion having a curvature smaller than the average curvature. Excluding the bonding surface of the silicon nanoparticles 802 with the thermally expanded graphite oxide 801, the average radius of curvature is twice, that is, the average diameter is 100 nm or less, and more preferably 30 nm or less. Since the silicon nanoparticles 802 are formed on the surface of the thermally expanded graphite oxide 801 by a vapor phase growth method, the joint surface between the two is inevitably in a form along the surface shape of the thermally expanded graphite oxide 801. The surface of the thermally expanded graphite oxide 801 is almost flat and has a gentle curvature depending on the location.
　また、同一の熱膨張酸化黒鉛８０１の表面に接合したシリコンナノ粒子８０２の個数は、気相成長法において、成長条件を変えることにより、ある程度の制御が可能である。基本的には、成長時間を長くすることにより、個数を増加させることが可能である。 Also, the number of silicon nanoparticles 802 bonded to the surface of the same thermally expanded graphite oxide 801 can be controlled to some extent by changing the growth conditions in the vapor phase growth method. Basically, the number can be increased by increasing the growth time.
　また、シリコンナノ粒子８０２と熱膨張酸化黒鉛８０１との接合部では、シリコンと炭素との共有結合により接合している。そのため、その接合力は非常に強固であり、また両者間の電気伝導特性も非常に良好である。 Also, the joint between the silicon nanoparticle 802 and the thermally expanded graphite oxide 801 is joined by a covalent bond between silicon and carbon. Therefore, the bonding force is very strong, and the electrical conduction characteristics between the two are also very good.
　本実施例においては、熱膨張酸化黒鉛８０１とシリコンナノ粒子８０２との質量比は１：１である。 In this embodiment, the mass ratio of the thermally expanded graphite oxide 801 and the silicon nanoparticles 802 is 1: 1.
　上記の構造を有するシリコンと炭素とから成る複合材料を負極活物質として用いたリチウムイオン二次電池を作製した。その結果、負極活物質の質量当たりの電池容量が１０００ｍＡｈ／ｇで、５０００サイクル後の保存容量維持率が９５％と、高容量で長寿命のリチウムイオン二次電池を実現することができた。 A lithium ion secondary battery using a composite material composed of silicon and carbon having the above structure as a negative electrode active material was produced. As a result, a battery capacity per mass of the negative electrode active material was 1000 mAh / g, and a storage capacity retention rate after 5000 cycles was 95%, and a high capacity and long life lithium ion secondary battery could be realized.
　図９は、第二の実施例の負極活物質を示す模式断面図である。 FIG. 9 is a schematic cross-sectional view showing the negative electrode active material of the second embodiment.
　本実施例においては、接合面以外のシリコンナノ粒子９０２の表面が、炭素を主成分とする被膜９０３に覆われている点が第一の実施例と異なる。 This embodiment is different from the first embodiment in that the surface of the silicon nanoparticle 902 other than the bonding surface is covered with a coating 903 mainly composed of carbon.
　熱膨張酸化黒鉛９０１の表面には、シリコンナノ粒子９０２が一定の面積を有する平坦部９０４を介して接合した構造となっている。さらに、接合面（平坦部９０４）以外のシリコンナノ粒子９０２の表面が、炭素を主成分とする被膜９０３に覆われた構造となっている。熱膨張酸化黒鉛９０１は、薄膜構造を有しており、形状は、楕円形あるいは長方形に近く、最も長い部分の長さは１００μｍ程度である。また、膜厚は１００ｎｍ以下であり、酸化条件および熱膨張条件を工夫することにより、１０ｎｍ以下にすることも可能である。特に膜厚が１０ｎｍ以下の場合は、比表面積が１００ｍ２／ｇ以上になり、より多くのシリコンナノ粒子９０２を表面に接合することが可能となる。 The surface of the thermally expanded graphite oxide 901 has a structure in which silicon nanoparticles 902 are bonded via a flat portion 904 having a certain area. Further, the surface of the silicon nanoparticles 902 other than the bonding surface (flat portion 904) is covered with a coating 903 containing carbon as a main component. The thermally expanded graphite oxide 901 has a thin film structure, the shape is close to an ellipse or a rectangle, and the length of the longest portion is about 100 μm. Further, the film thickness is 100 nm or less, and it is possible to make it 10 nm or less by devising the oxidation condition and the thermal expansion condition. In particular, when the film thickness is 10 nm or less, the specific surface area is 100 m 2 / g or more, and more silicon nanoparticles 902 can be bonded to the surface.
　一方、シリコンナノ粒子９０２は、ほぼ球形であり、その平均曲率よりも小さい曲率を有する平坦部９０４を介して熱膨張酸化黒鉛９０１と接合している。シリコンナノ粒子９０２の熱膨張酸化黒鉛９０１との接合面を除いた、平均曲率半径の２倍、すなわち平均直径は１００ｎｍ以下であり、更に望ましくは３０ｎｍ以下である。シリコンナノ粒子９０２は、気相成長法により、熱膨張酸化黒鉛９０１の表面に作製するため、両者の接合面は、必然的に熱膨張酸化黒鉛９０１の表面形状に沿った形態となる。熱膨張酸化黒鉛９０１の表面は、ほぼ平坦であり、場所によっては緩やかな曲率を持っている。 On the other hand, the silicon nanoparticles 902 are substantially spherical, and are joined to the thermally expanded graphite oxide 901 through a flat portion 904 having a curvature smaller than the average curvature. Excluding the bonding surface of the silicon nanoparticles 902 with the thermally expanded graphite oxide 901, it is twice the average radius of curvature, that is, the average diameter is 100 nm or less, and more preferably 30 nm or less. Since the silicon nanoparticles 902 are produced on the surface of the thermally expanded graphite oxide 901 by a vapor phase growth method, the joint surface between the two is necessarily in a form along the surface shape of the thermally expanded graphite oxide 901. The surface of the thermally expanded graphite oxide 901 is almost flat and has a gentle curvature depending on the location.
　また、シリコンナノ粒子９０２と熱膨張酸化黒鉛９０１との接合部では、シリコンと炭素との共有結合により接合している。そのため、その接合力は非常に強固であり、また両者間の電気伝導特性も非常に良好である。 In addition, the joint between the silicon nanoparticle 902 and the thermally expanded graphite oxide 901 is joined by a covalent bond between silicon and carbon. Therefore, the bonding force is very strong, and the electrical conduction characteristics between the two are also very good.
　上記のように、熱膨張酸化黒鉛９０１の表面に、シリコンナノ粒子９０２が接合した構造を形成した後、プロピレンを原料ガスとした熱気相成長法により、シリコンナノ粒子９０２の熱膨張酸化黒鉛９０１との接合部以外の表面に、ナノグラフェンの多層構造（グラファイトの単一層が積層された構造）を有する、炭素を主成分とする被膜９０３を形成した。炭素を主成分とする被膜９０３は、１０ｎｍである。場合によっては、１～３０ｎｍの膜厚を有する炭素を主成分とする被膜９０３を用いることも可能である。炭素を主成分とする被膜９０３の膜厚は、熱気相成長法において、成長時間を変化させることにより、調整可能である。炭素を主成分とする被膜９０３をシリコンナノ粒子９０２の表面に被覆することにより、シリコンナノ粒子９０２の表面の酸化を防止し、安定した電気伝導性を確保できるため、それを負極活物質として用いたリチウムイオン二次電池の長寿命化が可能である。 As described above, after forming a structure in which the silicon nanoparticles 902 are bonded to the surface of the thermally expanded graphite oxide 901, the thermally expanded graphite oxide 901 of the silicon nanoparticles 902 is formed by thermal vapor deposition using propylene as a raw material gas. A film 903 mainly composed of carbon and having a multilayer structure of nanographene (a structure in which a single layer of graphite is laminated) is formed on the surface other than the joint portion. The film 903 mainly composed of carbon is 10 nm. In some cases, it is possible to use a coating 903 mainly composed of carbon having a thickness of 1 to 30 nm. The film thickness of the coating 903 containing carbon as a main component can be adjusted by changing the growth time in the thermal vapor deposition method. By covering the surface of the silicon nanoparticle 902 with the coating 903 containing carbon as a main component, the surface of the silicon nanoparticle 902 can be prevented from being oxidized and stable electric conductivity can be secured. The life of the lithium ion secondary battery can be extended.
　また、図９では、熱膨張酸化黒鉛９０１の表面に一個のシリコンナノ粒子９０２が接合した構造を示したが、少なくとも複数個のシリコンナノ粒子９０２が、一定の面積の接合面を介して、同一の熱膨張酸化黒鉛９０１の表面と接合している構造を用いることも可能である。 FIG. 9 shows a structure in which one silicon nanoparticle 902 is bonded to the surface of thermally expanded graphite oxide 901. However, at least a plurality of silicon nanoparticles 902 are the same through a bonding surface having a certain area. It is also possible to use a structure bonded to the surface of the thermally expanded graphite oxide 901.
　上記の構造を有するシリコンと炭素とから成る複合材料を負極活物質として用いたリチウムイオン二次電池を作製した。その結果、負極活物質の質量当たりの電池容量が１０００ｍＡｈ／ｇで、８０００サイクル後の保存容量維持率が９５％と、高容量で長寿命のリチウムイオン二次電池を実現することができた。 A lithium ion secondary battery using a composite material composed of silicon and carbon having the above structure as a negative electrode active material was produced. As a result, it was possible to realize a high-capacity and long-life lithium ion secondary battery with a battery capacity per mass of the negative electrode active material of 1000 mAh / g and a storage capacity retention rate of 950 cycles after 95%.
　図１０は、リチウムイオン二次電池の負極の構造を示したものである。 FIG. 10 shows the structure of the negative electrode of the lithium ion secondary battery.
　本図に示す負極は、熱膨張酸化黒鉛１００１の表面にシリコンナノ粒子１００２が接合した複合材料を負極集電体１００３上にプレス成形した構造になっている。この場合に、シリコンナノ粒子１００２の表面に、図９の炭素を主成分とする被膜９０３が形成されていてもよい。 The negative electrode shown in this drawing has a structure in which a composite material in which silicon nanoparticles 1002 are bonded to the surface of thermally expanded graphite oxide 1001 is press-molded on a negative electrode current collector 1003. In this case, a film 903 mainly composed of carbon in FIG. 9 may be formed on the surface of the silicon nanoparticle 1002.
　なお、図示していないが、負極活物質は、バインダと混合して負極集電体１００３に塗布することが望ましい。 Although not shown, it is desirable that the negative electrode active material is mixed with a binder and applied to the negative electrode current collector 1003.
　図１１は、熱膨張酸化黒鉛の表面にシリコンナノ粒子を形成するための熱気相成長装置を示す概略図である。 FIG. 11 is a schematic view showing a thermal vapor phase growth apparatus for forming silicon nanoparticles on the surface of thermally expanded graphite oxide.
　本図において、熱気相成長装置は、原料設置部１１０２を有する反応炉１１０１を備えている。反応炉１１０１は、石英製であり、直径が５ｃｍ、長さが４０ｃｍである。反応炉１１０１には、水素を供給する第一の配管、水素とともに四塩化シリコン（ＳｉＣｌ４）を供給する第二の配管、及び排ガスを外部に放出する第三の配管が接続されている。第二の配管には、シリコンの原料となる液体の四塩化シリコン（ＳｉＣｌ４）を入れた容器１１０３が設けてあり、水素ガス（Ｈ２）でバブリングすることにより、四塩化シリコン（ＳｉＣｌ４）を反応炉１１０１に導入するようになっている。第三の配管には、水酸化ナトリウム水溶液を入れた容器１１０４が設けてあり、有害な酸性ガス等を吸収し除去することができるようになっている。 In this figure, the thermal vapor phase growth apparatus includes a reaction furnace 1101 having a raw material installation unit 1102. The reaction furnace 1101 is made of quartz and has a diameter of 5 cm and a length of 40 cm. The reaction furnace 1101 is connected to a first pipe for supplying hydrogen, a second pipe for supplying silicon tetrachloride (SiCl 4 ) together with hydrogen, and a third pipe for discharging exhaust gas to the outside. The second pipe is provided with a container 1103 in which liquid silicon tetrachloride (SiCl 4 ) serving as a silicon raw material is placed. By bubbling with hydrogen gas (H 2 ), silicon tetrachloride (SiCl 4 ) is provided. Is introduced into the reactor 1101. The third pipe is provided with a container 1104 containing a sodium hydroxide aqueous solution so as to absorb and remove harmful acid gases and the like.
　ここで、水素は、反応炉１１０１内を還元雰囲気とし、熱膨張酸化黒鉛の表面に存在する酸素を還元除去するためのものである。よって、還元雰囲気とすることができる気体であれば、水素以外のものであっても使用可能である。 Here, the hydrogen is used for reducing and removing oxygen present on the surface of the thermally expanded graphite by setting the inside of the reaction furnace 1101 as a reducing atmosphere. Therefore, any gas other than hydrogen can be used as long as it can be a reducing atmosphere.
　第一の配管及び第二の配管には、流量制御部１１０５、１１０６（Mass Flow Controller）及び栓１１０７が設けてあり、水素及び四塩化シリコンの流量をそれぞれ調整することができるようになっている。 The first pipe and the second pipe are provided with flow controllers 1105 and 1106 (Mass Flow Controller) and a plug 1107 so that the flow rates of hydrogen and silicon tetrachloride can be adjusted respectively. .
　四塩化シリコンの２０℃における蒸気圧は３０ｋＰａであり、バブリング導入すると、四塩化シリコンの導入量は３４％となる。流量制御部１１０５、１１０６及び栓１１０７を用いて流量を調整することにより、四塩化シリコンの導入量（水素と四塩化シリコンとの混合比）を制御することができる。 The vapor pressure of silicon tetrachloride at 20 ° C. is 30 kPa. When bubbling is introduced, the amount of silicon tetrachloride introduced is 34%. The amount of silicon tetrachloride introduced (mixing ratio of hydrogen and silicon tetrachloride) can be controlled by adjusting the flow rate using the flow rate control units 1105 and 1106 and the plug 1107.
　流量を調整する代わりに、四塩化シリコンを冷却する方式を採用してもよい。 * Instead of adjusting the flow rate, a method of cooling silicon tetrachloride may be adopted.
　負極活物質を製造するための手順（ナノシリコンを熱膨張酸化黒鉛の表面に成長させる手順）の一例は、下記のとおりである。 An example of a procedure for producing a negative electrode active material (a procedure for growing nanosilicon on the surface of thermally expanded graphite oxide) is as follows.
　原料容器に熱膨張酸化黒鉛を入れ、反応炉１１０１の内部の原料設置部１１０２に設置した。第一の配管には、水素を流量２００ｍＬ／ｍｉｎで流し、第二の配管は閉じた状態とした。この状態で、反応炉１１０１を室温から１０００℃まで、１０℃／ｍｉｎでの速度で昇温した。 The thermally expanded graphite oxide was placed in the raw material container and installed in the raw material installation unit 1102 inside the reaction furnace 1101. Hydrogen was allowed to flow through the first pipe at a flow rate of 200 mL / min, and the second pipe was closed. In this state, the temperature of the reactor 1101 was increased from room temperature to 1000 ° C. at a rate of 10 ° C./min.
　次に、１０００℃に達したところで、の水素流量を１００ｍＬ／ｍｉｎに変更し、第二の配管の水素流量を１００ｍＬ／ｍｉｎに設定した。この条件により、１７％の四塩化シリコンを導入することができる。 Next, when the temperature reached 1000 ° C., the hydrogen flow rate was changed to 100 mL / min, and the hydrogen flow rate of the second pipe was set to 100 mL / min. Under this condition, 17% silicon tetrachloride can be introduced.
　１０００℃で１時間成長した後、第二の配管を閉じ、第一の配管の水素流量を２００ｍＬ／ｍｉｎに変更し、１０００℃で３０分間保持し、その後自然冷却した。このようにして、熱膨張酸化黒鉛の表面に、平均直径が３０ｎｍのシリコンナノ粒子を形成することができた。シリコンナノ粒子の複合材料全体に対する比率は２０質量％である。シリコンナノ粒子の比率は、成長時間を変えることにより調整可能である。なお、シリコンナノ粒子の負極活物質全体に対する割合は、２０～８０質量％であることが望ましい。 After growing at 1000 ° C. for 1 hour, the second pipe was closed, the hydrogen flow rate of the first pipe was changed to 200 mL / min, held at 1000 ° C. for 30 minutes, and then naturally cooled. In this way, silicon nanoparticles having an average diameter of 30 nm could be formed on the surface of the thermally expanded graphite oxide. The ratio of silicon nanoparticles to the total composite material is 20% by mass. The ratio of silicon nanoparticles can be adjusted by changing the growth time. The ratio of silicon nanoparticles to the whole negative electrode active material is desirably 20 to 80% by mass.
　また、シリコンナノ粒子の表面を、炭素を主成分とする被膜で覆う場合には、上記の自然冷却前に、１０℃／ｍｉｎの速度で降温し、８００℃に達したところで、第一の配管を用いて、プロピレンガスを５％含有するアルゴンガスを流量２００ｍＬ／ｍｉｎで導入し、１時間経過した後、２００ｍＬ／ｍｉｎの純アルゴンガスに切換え、３０分間保持し、その後、自然冷却する。これにより、シリコンナノ粒子の表面に、厚さ１０ｎｍの炭素を主成分とする被膜を作製することが可能である。なお、このようにして形成された被膜を構成する炭素は、ナノグラフェン構造を有するものとなる。 When the surface of the silicon nanoparticles is covered with a film containing carbon as a main component, the temperature is lowered at a rate of 10 ° C./min before the natural cooling, and when the temperature reaches 800 ° C., the first pipe Then, argon gas containing 5% propylene gas is introduced at a flow rate of 200 mL / min, and after 1 hour, it is switched to 200 mL / min pure argon gas, held for 30 minutes, and then naturally cooled. As a result, it is possible to produce a coating film mainly composed of carbon having a thickness of 10 nm on the surface of the silicon nanoparticles. In addition, the carbon which comprises the film formed in this way has a nano graphene structure.
　次に、熱膨張酸化黒鉛の作製方法についても説明する。 Next, a method for producing thermally expanded graphite oxide will also be described.
　平均直径が１０～１００μｍのフレーク状グラファイトを濃硫酸、硝酸ナトリウムおよび過マンガン酸カリウムを含む溶液中に数日間浸漬し、撹拌する。これは、いわゆるＨｕｍｍｅｒｓ法である。このＨｕｍｍｅｒｓ法により酸化した後、さらにアルゴン雰囲気で、６００～１１００℃で３０分～３時間、熱処理を行うことにより熱膨張酸化黒鉛を作製した。 * Flake graphite having an average diameter of 10 to 100 μm is immersed in a solution containing concentrated sulfuric acid, sodium nitrate and potassium permanganate for several days and stirred. This is the so-called Hummers method. After oxidation by the Hummers method, heat-expanded graphite oxide was produced by further heat treatment in an argon atmosphere at 600 to 1100 ° C. for 30 minutes to 3 hours.
　熱膨張酸化黒鉛のＣ軸方向の膜厚は、１．０～１００．０ｎｍであり、酸化条件および発泡処理条件を変えることにより、ある程度調整することが可能である。 The film thickness in the C-axis direction of the thermally expanded graphite oxide is 1.0 to 100.0 nm, and can be adjusted to some extent by changing the oxidation conditions and the foaming treatment conditions.
　熱膨張酸化黒鉛の他の作製方法について以下に説明する。 Another method for producing thermally expanded graphite oxide will be described below.
　フレーク状グラファイトと、過酸化ベンゾイルの粉末とを混ぜ、１１０℃で１０分間熱処理することにより酸化する。これを、さらにアルゴン雰囲気で、６００～１１００℃で３０分～３時間、熱処理を行うことにより、熱膨張酸化黒鉛とする。 Flaked graphite and benzoyl peroxide powder are mixed and oxidized by heat treatment at 110 ° C. for 10 minutes. This is further subjected to heat treatment at 600 to 1100 ° C. for 30 minutes to 3 hours in an argon atmosphere to obtain thermally expanded graphite oxide.
　図１２Ａは、本発明の負極活物質を構成するシリコンナノ粒子の形状を示す模式斜視図である。図１２Ｂは、図１２Ａのシリコンナノ粒子の側面図である。これらの図は、シリコンナノ粒子を球形、熱膨張酸化黒鉛との接合面を平面と仮定した場合を示したものである。 FIG. 12A is a schematic perspective view showing the shape of silicon nanoparticles constituting the negative electrode active material of the present invention. FIG. 12B is a side view of the silicon nanoparticles of FIG. 12A. These figures show the case where the silicon nanoparticles are assumed to be spherical and the joint surface with the thermally expanded graphite oxide is assumed to be a flat surface.
　接合面の面積をＡ２、それ以外の球形部分の面積をＡ１とする。さらに、球の半径をｒとし、球の中心から接合面までの距離をｘｒ（ただし０＜ｘ＜１）とすると、全面積Ａ１＋Ａ２に対するＡ２の割合は、（ｘ－１）／（ｘ－３）となる。 The area of the joint surface is A 2 and the area of the other spherical portion is A 1 . Moreover, the radius of the sphere and r, and the distance from the center of the sphere to the joining surfaces and xr (provided that 0 <x <1), the ratio of A 2 to the total area A 1 + A 2 is, (x-1) / (X-3).
　図６の熱膨張酸化黒鉛上に成長したシリコンナノ粒子の透過電子顕微鏡写真において、矢印で示したシリコンナノ粒子の形状から、シリコンナノ粒子の熱膨張酸化黒鉛表面に対する接触角は９０°以上であり、濡れ性が悪いと判断できる。このように濡れ性が悪い場合、ｘは０から１の範囲で変化すると考えられる。ｘ＝０の場合、Ａ２／（Ａ１＋Ａ２）は０．３３となる。ｘが大きくなるにつれてＡ２／（Ａ１＋Ａ２）は小さくなる。しかしながら、Ａ２／（Ａ１＋Ａ２）が小さくなると、シリコンナノ粒子と熱膨張酸化黒鉛との接合力が弱くなり、安定な接合が得られない。概ね、Ａ２／（Ａ１＋Ａ２）が０．１以上であると、十分な接合が得られると考えられる。この場合、ｘは０．７８以下となる。 In the transmission electron micrograph of the silicon nanoparticles grown on the thermally expanded graphite in FIG. 6, the contact angle of the silicon nanoparticles with respect to the thermally expanded graphite surface is 90 ° or more from the shape of the silicon nanoparticles indicated by arrows. It can be judged that the wettability is poor. Thus, when wettability is bad, it is thought that x changes in the range of 0 to 1. When x = 0, A 2 / (A 1 + A 2 ) is 0.33. As x increases, A 2 / (A 1 + A 2 ) decreases. However, when A 2 / (A 1 + A 2 ) decreases, the bonding force between the silicon nanoparticles and the thermally expanded graphite oxide becomes weak, and stable bonding cannot be obtained. In general, it is considered that sufficient bonding is obtained when A 2 / (A 1 + A 2 ) is 0.1 or more. In this case, x is 0.78 or less.
　上記の実施例においては、炭素質材料として熱膨張酸化黒鉛を用いたが、炭素質材料は、微細黒鉛、カーボンナノチューブ又はカーボンナノホーンであっても望ましい結果が得られる。これらは、熱膨張酸化黒鉛と同様に、規則的な炭素原子の配列を有するため、導電性等の電気的な特性が高い。 In the above embodiment, thermally expanded graphite oxide was used as the carbonaceous material, but desirable results can be obtained even if the carbonaceous material is fine graphite, carbon nanotube or carbon nanohorn. Since these have a regular arrangement of carbon atoms like the thermally expanded graphite oxide, they have high electrical characteristics such as conductivity.
　４０１、７０１、８０１、９０１、１００１：熱膨張酸化黒鉛、４０２、５０１、６０１、７０２、８０２、９０２、１００２：シリコンナノ粒子、６０２：接合面、７０３、９０４：平坦部、９０３：炭素を主成分とする被膜、１００３：負極集電体、１１０１：反応炉、１１０２：原料設置部、１１０３、１１０４：容器、１１０５、１１０６：流量制御部、１１０７：栓、１３１０：正極、１３１１：セパレータ、１３１２：負極、１３１３：電池缶、１３１４：正極集電タブ、１３１５：負極集電タブ、１３１６：内蓋、１３１７：内圧開放弁、１３１８：ガスケット、１３１９：正温度係数抵抗素子、１３２０：電池蓋。 401, 701, 801, 901, 1001: Thermally expanded graphite oxide, 402, 501, 601, 702, 802, 902, 1002: Silicon nanoparticles, 602: Bonding surface, 703, 904: Flat part, 903: Mainly carbon Coating as component, 1003: negative electrode current collector, 1101: reaction furnace, 1102: raw material installation unit, 1103, 1104: container, 1105, 1106: flow rate control unit, 1107: plug, 1310: positive electrode, 1311: separator, 1312 : Negative electrode, 1313: battery can, 1314: positive electrode current collecting tab, 1315: negative electrode current collecting tab, 1316: inner lid, 1317: internal pressure release valve, 1318: gasket, 1319: positive temperature coefficient resistance element, 1320: battery lid.

Claims (15)

  1. 　電気伝導性を有する炭素質材料と、シリコンナノ粒子と、を含み、前記シリコンナノ粒子は、前記炭素質材料の表面に化学的に結合した接合面を有することを特徴とするリチウムイオン二次電池用負極活物質。 A lithium ion secondary battery comprising: a carbonaceous material having electrical conductivity; and silicon nanoparticles, wherein the silicon nanoparticles have a bonding surface chemically bonded to a surface of the carbonaceous material. Negative electrode active material.
  2. 　前記炭素質材料と前記シリコンナノ粒子との結合は、炭素とケイ素との共有結合であることを特徴とする請求項１記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the bond between the carbonaceous material and the silicon nanoparticles is a covalent bond between carbon and silicon.
  3. 　前記シリコンナノ粒子は、前記接合面以外の表面に炭素被覆層を有することを特徴とする請求項１又は２に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the silicon nanoparticles have a carbon coating layer on a surface other than the bonding surface.
  4. 　前記シリコンナノ粒子の平均直径は、１００ｎｍ以下であることを特徴とする請求項１～３のいずれか一項に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the silicon nanoparticles have an average diameter of 100 nm or less.
  5. 　前記シリコンナノ粒子は、気相成長法により前記炭素質材料の表面に形成されたものであることを特徴とする請求項１～４のいずれか一項に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the silicon nanoparticles are formed on the surface of the carbonaceous material by a vapor deposition method. material.
  6. 　前記炭素被覆層は、気相成長法により形成されたものであることを特徴とする請求項３～５のいずれか一項に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to any one of claims 3 to 5, wherein the carbon coating layer is formed by a vapor phase growth method.
  7. 　前記炭素被膜層は、ナノグラフェンの多層構造であることを特徴とする請求項３～６のいずれか一項に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to any one of claims 3 to 6, wherein the carbon coating layer has a multilayer structure of nanographene.
  8. 　前記炭素質材料の比表面積は１００ｍ２／ｇ以上であることを特徴とする請求項１～７のいずれか一項に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the specific surface area of the carbonaceous material is 100 m 2 / g or more.
  9. 　前記炭素質材料は、熱膨張酸化黒鉛、微細黒鉛、カーボンナノチューブ又はカーボンナノホーンであることを特徴とする請求項１～８のいずれか一項に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the carbonaceous material is thermally expanded graphite oxide, fine graphite, carbon nanotube, or carbon nanohorn.
  10. 　前記シリコンナノ粒子の負極活物質全体に対する割合は、２０～８０質量％であることを特徴とする請求項１～９のいずれか一項に記載のリチウムイオン二次電池用負極活物質。 The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 9, wherein a ratio of the silicon nanoparticles to the whole negative electrode active material is 20 to 80% by mass.
  11. 　請求項１～１０のいずれか一項に記載のリチウムイオン二次電池用負極活物質と、結着剤と、を含むことを特徴とする負極合剤。 A negative electrode mixture comprising the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 10 and a binder.
  12. 　請求項１～１０いずれか一項に記載のリチウムイオン二次電池用負極活物質と、負極集電体と、を含むことを特徴とする負極。 A negative electrode comprising the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 10 and a negative electrode current collector.
  13. 　請求項１～１０のいずれか一項に記載のリチウムイオン二次電池用負極活物質を用いたことを特徴とするリチウムイオン二次電池。 A lithium ion secondary battery comprising the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 10.
  14. 　電気伝導性を有する炭素質材料と、シリコンナノ粒子と、を含み、前記シリコンナノ粒子は、前記炭素質材料の表面に化学的に結合した接合面を有するリチウムイオン二次電池用負極活物質を製造する方法であって、還元雰囲気で前記炭素質材料を加熱し、前記炭素質材料の表面にケイ素を含む気体を導入し、前記炭素質材料の表面に前記シリコンナノ粒子を形成することを特徴とするリチウムイオン二次電池用負極活物質の製造方法。 A carbonaceous material having electrical conductivity; and silicon nanoparticles, wherein the silicon nanoparticles comprise a negative electrode active material for a lithium ion secondary battery having a bonding surface chemically bonded to the surface of the carbonaceous material. A method of manufacturing, wherein the carbonaceous material is heated in a reducing atmosphere, a gas containing silicon is introduced to the surface of the carbonaceous material, and the silicon nanoparticles are formed on the surface of the carbonaceous material. A method for producing a negative electrode active material for a lithium ion secondary battery.
  15. 　前記還元雰囲気は、水素雰囲気であり、前記気体は、四塩化シリコンであることを特徴とする請求項１４記載の製造方法。 The manufacturing method according to claim 14, wherein the reducing atmosphere is a hydrogen atmosphere, and the gas is silicon tetrachloride.
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