WO2023140192A1 - Negative electrode active material for lithium ion secondary batteries, negative electrode active material layer for lithium ion secondary batteries, negative electrode for lithium ion secondary batteries, and lithium ion secondary battery - Google Patents

Negative electrode active material for lithium ion secondary batteries, negative electrode active material layer for lithium ion secondary batteries, negative electrode for lithium ion secondary batteries, and lithium ion secondary battery Download PDF

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WO2023140192A1
WO2023140192A1 PCT/JP2023/000795 JP2023000795W WO2023140192A1 WO 2023140192 A1 WO2023140192 A1 WO 2023140192A1 JP 2023000795 W JP2023000795 W JP 2023000795W WO 2023140192 A1 WO2023140192 A1 WO 2023140192A1
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negative electrode
active material
electrode active
ion secondary
lithium ion
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PCT/JP2023/000795
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French (fr)
Japanese (ja)
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慎 藤田
敬史 毛利
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Tdk株式会社
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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
    • H01M4/13—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
    • H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
    • 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
    • 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
    • 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
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • 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 for lithium ion secondary batteries, a negative electrode active material layer for lithium ion secondary batteries, a negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery.
  • This application claims priority based on Japanese Patent Application 2022-007758, Japanese Patent Application 2022-007761 and Japanese Patent Application 2022-007766 filed in Japan on January 21, 2022, the contents of which are incorporated herein.
  • Lithium-ion secondary batteries are widely used as power sources for mobile devices such as mobile phones and laptop computers, and hybrid cars.
  • the capacity of lithium-ion secondary batteries mainly depends on the active material of the electrodes.
  • Graphite is generally used as a negative electrode active material, but there is a demand for a negative electrode active material with a higher capacity than graphite.
  • Silicon (Si) is attracting attention as a negative electrode active material having a much larger theoretical capacity than that of graphite (372 mAh/g).
  • a negative electrode active material containing silicon undergoes a large volume expansion during charging.
  • the volume expansion of the negative electrode active material causes deterioration in the cycle characteristics of the battery.
  • the volume of the negative electrode active material expands, for example, the negative electrode active material is damaged, a conductive path between the negative electrode active materials is cut, separation occurs at the interface between the negative electrode active material layer and the current collector, cracks occur in the SEI (Solid Electrolyte Interphase) coating, and decomposition of the electrolyte occurs. These degrade the cycle characteristics of the battery.
  • Patent Document 1 describes that the cycle characteristics are improved by defining the aspect ratio of the silicon particles and the inclination angle of the silicon particles with respect to the current collector.
  • Cycle characteristics are an important parameter of lithium-ion secondary batteries, and methods that can improve cycle characteristics using methods other than the method described in Patent Document 1 are being investigated.
  • the present disclosure has been made in view of the above problems, and aims to provide a lithium ion secondary battery with excellent cycle characteristics.
  • the negative electrode active material for a lithium ion secondary battery according to the first aspect contains silicon particles having an average particle size of 0.1 ⁇ m or more and 10 ⁇ m or less, an average aspect ratio of 0.60 or more and 0.99 or less, and an average circularity of 0.80 or more and 0.99 or less.
  • the negative electrode active material for a lithium ion secondary battery according to the above aspect may have an average circularity of 0.82 or more and 0.985 or less.
  • the negative electrode active material for a lithium ion secondary battery according to the above aspect may have an average aspect ratio of 0.65 or more and 0.80 or less.
  • the silicon particles may have an average circularity of 0.920 or more and 0.985 or less and an average aspect ratio of 0.80 or more and 0.97 or less.
  • the silicon particles may have an average particle diameter of 1 ⁇ m or more and 7 ⁇ m or less.
  • the silicon particles may have an average particle size of 3 ⁇ m or more and 7 ⁇ m or less.
  • the negative electrode active material for a lithium ion secondary battery according to the aspect described above may further include a coating layer that coats at least part of the silicon particles.
  • the coating layer contains one or more materials selected from the group consisting of lithium fluoride, magnesium oxide, magnesium phosphate, and magnesium fluoride.
  • the coating layer may have an average thickness of 5 nm or more and 500 nm or less.
  • the negative electrode active material layer for lithium ion secondary batteries according to the second aspect includes the negative electrode active material for lithium ion secondary batteries according to the aspect described above.
  • a negative electrode for a lithium ion secondary battery according to a third aspect includes the negative electrode active material for a lithium ion secondary battery according to the aspect described above.
  • a lithium ion secondary battery according to a fourth aspect includes the lithium ion secondary battery negative electrode according to the above aspect, a positive electrode, and an electrolyte.
  • a lithium ion secondary battery using the negative electrode active material for a lithium ion secondary battery according to the above aspect has excellent cycle characteristics.
  • FIG. 3 is a cross-sectional view of a negative electrode active material layer according to the first embodiment;
  • FIG. 1 is a scanning electron microscope image of silicon particles after aging treatment.
  • FIG. 5 is a schematic diagram of a negative electrode active material according to a first modified example;
  • 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment;
  • a negative electrode active material according to the first embodiment is used in a lithium ion secondary battery and contains silicon particles.
  • the silicon particles may be a silicon alloy, a silicon compound, or a silicon composite in addition to single silicon. Silicon particles may be crystalline or amorphous.
  • a silicon alloy is represented, for example, by X n Si.
  • X is a cation.
  • X is, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K or the like.
  • n satisfies 0 ⁇ n ⁇ 0.5.
  • a silicon compound is, for example, a silicon oxide denoted by SiO x .
  • x satisfies 0.8 ⁇ x ⁇ 2, for example.
  • the silicon oxide may consist of only SiO2 , may consist of only SiO, or may be a mixture of SiO and SiO2 .
  • silicon oxide may be partially deficient in oxygen.
  • a silicon composite is, for example, at least part of the surface of silicon or silicon compound particles coated with a conductive material.
  • the conductive material is, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or the like.
  • Si—C silicon carbon composite
  • the average particle size of the silicon particles is 0.1 ⁇ m or more and 10 ⁇ m or less, preferably 0.5 ⁇ m or more and 8 ⁇ m or less, and more preferably 1 ⁇ m or more and 7 ⁇ m or less.
  • the median diameter (D50) can be obtained as the average particle diameter using a particle size distribution measuring device (eg, manufactured by Malvern Panalytical).
  • a particle size distribution analyzer for example, the average particle size of 50,000 particles is obtained.
  • the average particle size can be obtained using at least 100 silicon particles confirmed in the cross-sectional image.
  • FIG. 1 is a cross-sectional image of a negative electrode active material layer containing silicon particles.
  • Cross-sectional images can be measured with a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the silicon particles in the electrode can be confirmed from the difference in contrast, and the white portions in FIG. 1 are the silicon particles.
  • the portion confirmed in gray in FIG. 1 is the binder, and the portion confirmed in black is the void. Silicon particles can be extracted from the contrast and numbered.
  • silicon particles negative electrode active material
  • silicon particles can be extracted from an image by setting a contrast threshold and binarizing it. Then, the diameters of at least 100 extracted silicon particles are obtained. The obtained frequency of the diameter of each silicon particle is graphed, and the mode is taken as the average particle diameter. When the shape of the silicon particles is irregular, the diameter of the long axis is used to calculate the average particle size.
  • the average aspect ratio of the silicon particles is 0.60 or more and 0.99 or less, preferably 0.65 or more and 0.98 or less, and more preferably 0.80 or more and 0.97 or less. Moreover, the average aspect ratio of the silicon particles may be 0.60 or more and 0.90 or less, or may be 0.65 or more and 0.80 or less. When the aspect ratio is within this range, stress concentration is less likely to occur at a specific portion during expansion and contraction of the silicon particles, and the silicon particles are less likely to break. Further, when the aspect ratio is within the range, the silicon particles are easily oriented in one direction, and the surface of the negative electrode active material layer becomes flat. As a result, the adhesion between the negative electrode active material layer and the negative electrode current collector is improved.
  • the average aspect ratio of silicon particles is obtained using the aspect ratio of at least 100 silicon particles.
  • the aspect ratio is obtained by dividing the short axis length by the long axis length of the silicon particles to be measured.
  • the average aspect ratio can be obtained using a particle size distribution measuring device if the silicon particles are available in the form of particles, and if it is difficult to separate the silicon particles as shown in Fig. 1, it can be obtained using a cross-sectional image.
  • a particle size distribution measuring apparatus for example, the mode of aspect ratios of 50,000 particles is obtained, and when using a cross-sectional image, for example, the mode of aspect ratios of 100 particles is obtained.
  • the average circularity of the silicon particles is 0.80 or more and 0.99 or less, preferably 0.820 or more and 0.985 or less, more preferably 0.910 or more and 0.988 or less, and still more preferably 0.920 or more and 0.985 or less.
  • the average circularity is within this range, it becomes difficult for stress to concentrate on a specific portion during expansion and contraction of the silicon particles, and a sufficient contact area between the conductive aid and the binder and the silicon particles can be ensured.
  • the average circularity of silicon particles is obtained using the circularity of at least 100 silicon particles.
  • the degree of circularity is obtained by dividing the circumference of a circle having the same area as the silicon particle to be measured by the circumference of the silicon particle to be measured.
  • the average circularity can be obtained using a particle size distribution analyzer if silicon particles are available in the form of particles.
  • a particle size distribution analyzer for example, the mode of circularity of 50,000 particles is obtained.
  • the average circularity can be obtained using a cross-sectional image.
  • cross-sectional images for example, the circularity of each of 100 particles is obtained. Specifically, silicon particles are extracted from the image, and the perimeter and area of each silicon particle are obtained. Also, the circumferential length of a circle having the same area as the calculated area of each silicon particle is calculated to calculate the degree of circularity of each silicon particle. Then, the mode of circularity of the silicon particles is taken as the average circularity.
  • the average particle diameter, average aspect ratio, and average circularity of the silicon particles present in the electrode after charging and discharging of the lithium ion secondary battery and the untreated silicon particles available in the form of particles do not necessarily match.
  • the average particle diameter, average aspect ratio, and average circularity of the silicon particles referred to here are, in principle, defined as numerical values after one or more charge/discharge cycles.
  • the average particle size, average aspect ratio and average circularity of the silicon particles before charge/discharge satisfy the above ranges, the cycle characteristics of the lithium ion secondary battery are improved.
  • the silicon particles may have an oxide film on the surface.
  • the oxide layer includes silicon oxide.
  • the film thickness of the oxide film is, for example, 1 nm or more and 100 nm or less, preferably 3 nm or more and 50 nm or less.
  • the thickness of the oxide film is, for example, the average thickness of the oxide film formed on 100 silicon particles obtained from the cross-sectional image.
  • the silicon particles used for the negative electrode active material according to the first embodiment can be produced, for example, by melting silicon and then solidifying it again. Molten silicon is rounded by surface tension. Silicon particles used for the negative electrode active material according to the first embodiment can be produced by, for example, an atomization method or a thermal plasma method. However, the average particle diameter, average circularity and average aspect ratio of the silicon particles change depending on the conditions during production by these methods. In order to obtain the desired shape of the silicon particles, manufacturing conditions are set and controlled.
  • the conditions for producing silicon particles are, for example, as follows.
  • silicon particles having an average particle size of 1 ⁇ m or more and 8 ⁇ m or less are melted as raw materials.
  • the average particle size of the raw material is one of the parameters that affect the particle size of the silicon particles.
  • the melting temperature, melting time, cooling temperature, cooling rate, etc. are parameters that affect the shape of the silicon particles.
  • the melting temperature of silicon is, for example, 1200° C. or higher and 12000° C. or lower.
  • the melting time of silicon is, for example, 1 s or more and 300 s or less.
  • the cooling temperature for solidifying silicon is, for example, 15° C. or higher and 800° C. or lower.
  • the cooling rate when solidifying silicon is, for example, 5° C./s or more and 10000° C./s or less. When the cooling rate is high, the crystallinity of the particles is lowered, lithium tends to diffuse uniformly during charging and discharging, and the cycle characteristics of the lithium ion secondary battery are improved. A cooling rate of 1000° C./s or more is preferable.
  • the atmosphere in which silicon is melted and cooled is preferably an inert atmosphere such as Ar or nitrogen.
  • an oxide film forming process is performed.
  • the oxide film can be formed, for example, by aging treatment.
  • the aging process is a process of exposing the silicon particles to an environment with a humidity of 80% and a temperature of 25° C. for a predetermined time or longer.
  • the dew point during the aging treatment is, for example, 21°C. It is desirable that the atmosphere in which the aging treatment is performed has an oxygen concentration of 20% or more.
  • Aging treatment is carried out gently in an environment in which low-intensity ultraviolet rays are applied. Aging treatment can be performed using a fluorescent lamp, an LED, or the like having an appropriate wavelength.
  • the entire particles are uniformly oxidized.
  • FIG. 2 is a scanning electron microscope image of silicon particles after aging treatment.
  • the silicon particles are polycrystalline.
  • the thickness of the oxide film varies depending on the crystal planes that appear on the surface of the silicon particles.
  • the silicon particles are polycrystalline, the lithium diffusion paths are multidirectional, and the expansion of the electrode is made uniform.
  • it is preferable that a plurality of silicon particles (second silicon particles) having a size of several nanometers (second silicon particles) are supported on the surface of the silicon particles.
  • the second silicon particles improve the oxidation rate of the silicon particles.
  • the oxide film becomes sufficiently thicker than the natural oxide film by performing the aging treatment.
  • FIG. 3 is a cross-sectional schematic diagram of the negative electrode active material 1 according to the first modified example.
  • a negative electrode active material 1 according to the first modification includes silicon particles 2 and a coating layer 3 .
  • the silicon particles 2 are similar to the silicon particles described above.
  • the coating layer 3 contains one or more materials selected from the group consisting of lithium fluoride (e.g. LiF), magnesium oxide (e.g. Mg 2 O), magnesium phosphate (e.g. Mg 3 (PO 4 ) 2 ) and magnesium fluoride (e.g. Mg 2 F).
  • lithium fluoride e.g. LiF
  • magnesium oxide e.g. Mg 2 O
  • magnesium phosphate e.g. Mg 3 (PO 4 ) 2
  • magnesium fluoride e.g. Mg 2 F
  • the coating layer 3 only needs to cover at least part of the surface of the silicon particles 2 .
  • the coating layer 3 may cover the entire surface of the silicon particles 2 .
  • the coating layer 3 prevents direct contact between the electrolytic solution and the silicon particles 2 in the lithium ion secondary battery.
  • an irreversible reaction (side reaction) occurs in which the electrolytic solution undergoes reductive decomposition. Lithium used for this irreversible reaction cannot contribute to subsequent charge/discharge.
  • the coating layer 3 suppresses irreversible reactions and improves the cycle characteristics of the lithium ion secondary battery.
  • the coating layer 3 can be formed on the surface of the silicon particles 2 after the silicon particles 2 are produced.
  • the coating layer 3 can be produced by simultaneously mixing the silicon particles 2 and a material for the coating layer 3 (one or more materials selected from the group consisting of lithium fluoride, magnesium oxide, magnesium phosphate, and magnesium fluoride) in a reaction vessel (for example, mechanochemical treatment).
  • a material for the coating layer 3 one or more materials selected from the group consisting of lithium fluoride, magnesium oxide, magnesium phosphate, and magnesium fluoride
  • the surface of the silicon particles 2 may be coated with the coating layer 3 using a vapor phase method such as a liquid phase method, an atomic layer deposition (ALD) method, or a sputtering method, in which the silicon particles 2 are immersed in a solution containing a material for the coating layer 3.
  • ALD atomic layer deposition
  • the second silicon particles are preferably present at the interface between the silicon particles 2 and the coating layer 3 .
  • the second silicon particles strengthen the physical bond between the silicon particles 2 and the coating layer 3 and strengthen the electrical continuity.
  • the negative electrode active material according to the first embodiment improves the cycle characteristics of lithium ion secondary batteries. Silicon particles with a defined shape are less likely to break when the silicon particles expand and contract. In addition, when the contact area between the electrolyte and the silicon particles is large, side reactions caused by the contact between the electrolyte and the silicon particles are likely to occur, but by defining the shape of the silicon particles, excessive contact between the electrolyte and the silicon particles can be suppressed. Further, by defining the shape of the silicon particles, the adhesion between the negative electrode active material, the conductive aid, the binder, and the like is improved.
  • the cycle characteristics of the lithium-ion secondary battery are further improved. This is probably because the oxide film suppresses the side reaction between the electrolyte and the silicon particles and increases the mechanical resistance of the silicon particles.
  • the coating layer 3 prevents direct contact between the electrolytic solution and the silicon particles 2. As a result, the irreversible reaction of reductive decomposition of the electrolytic solution is suppressed, and the cycle characteristics of the lithium ion secondary battery are improved.
  • FIG. 4 is a schematic diagram of a lithium ion secondary battery according to the first embodiment.
  • a lithium-ion secondary battery 100 shown in FIG. 4 includes a power generation element 40, an exterior body 50, and an electrolyte (for example, a non-aqueous electrolyte).
  • the exterior body 50 covers the periphery of the power generation element 40 .
  • the power generation element 40 is connected to the outside through a pair of terminals 60 and 62 connected to the power generation element 40 .
  • a non-aqueous electrolyte is contained in the exterior body 50 .
  • FIG. 4 illustrates the case where there is one power generation element 40 in the exterior body 50, a plurality of power generation elements 40 may be stacked.
  • the power generation element 40 includes a separator 10 , a positive electrode 20 and a negative electrode 30 .
  • the power generation element 40 may be a laminate in which these are laminated, or a wound body in which a structure in which these are laminated is wound.
  • the cathode 20 has, for example, a cathode current collector 22 and a cathode active material layer 24 .
  • the cathode active material layer 24 is in contact with at least one surface of the cathode current collector 22 .
  • the positive electrode current collector 22 is, for example, a conductive plate.
  • the positive electrode current collector 22 is, for example, a metal thin plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22 .
  • the average thickness of the positive electrode current collector 22 is, for example, 10 ⁇ m or more and 30 ⁇ m or less.
  • the positive electrode active material layer 24 contains, for example, a positive electrode active material.
  • the positive electrode active material layer 24 may contain a conductive aid and a binder as needed.
  • the positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions and counter anions.
  • the positive electrode active material may be organic.
  • the positive electrode active material may be polyacetylene,
  • the positive electrode active material may be a non-lithium containing material.
  • Lithium-free materials are, for example, FeF 3 , conjugated polymers containing organic conductive materials, Chevrell phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, and the like. Only one of the lithium-free materials may be used, or a plurality of materials may be used in combination.
  • the positive electrode active material is a lithium-free material, for example, it is first discharged. Lithium is inserted into the positive electrode active material by discharging.
  • lithium may be chemically or electrochemically pre-doped into a positive electrode active material that does not contain lithium.
  • the conductive aid enhances the electronic conductivity between the positive electrode active materials.
  • conductive aids include carbon powder, carbon nanotubes, carbon materials, metal fine powders, mixtures of carbon materials and metal fine powders, and conductive oxides.
  • carbon powder include carbon black, acetylene black, and ketjen black.
  • Metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, or the like.
  • the content of the conductive aid in the positive electrode active material layer 24 is not particularly limited.
  • the content of the conductive aid is 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 5% by mass or less with respect to the total mass of the positive electrode active material, the conductive aid, and the binder.
  • the binder in the positive electrode active material layer 24 binds the positive electrode active materials together.
  • a known binder can be used.
  • the binder is preferably insoluble in the electrolytic solution, has oxidation resistance, and has adhesiveness.
  • the binder is, for example, fluororesin.
  • the binder is, for example, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), mixtures thereof.
  • PVDF is particularly preferable as the binder used for the positive electrode active material layer.
  • the binder content in the positive electrode active material layer 24 is not particularly limited.
  • the binder content is 1% by mass or more and 15% by mass or less, preferably 1.5% by mass or more and 5% by mass or less with respect to the total mass of the positive electrode active material, the conductive aid, and the binder.
  • the binder content is low, the adhesive strength of the positive electrode 20 is weakened. If the binder content is high, the binder is electrochemically inactive and does not contribute to the discharge capacity, so the energy density of the lithium ion secondary battery 100 is low.
  • the negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34 .
  • the negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32 .
  • the negative electrode current collector 32 is, for example, a conductive plate.
  • the negative electrode current collector 32 can be the same as the positive electrode current collector 22 .
  • the negative electrode active material layer 34 contains a negative electrode active material and a binder.
  • the negative electrode active material layer may contain a conductive aid, a dispersion stabilizer, and the like, if necessary.
  • the negative electrode active material described above is used as the negative electrode active material.
  • the same conductive aid and binder as those used for the positive electrode 20 can be used.
  • the binder in the negative electrode 30 may be, for example, cellulose, styrene/butadiene rubber, ethylene/propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc., in addition to those listed for the positive electrode 20 .
  • the cellulose may be, for example, carboxymethylcellulose (CMC).
  • Separator 10 is sandwiched between positive electrode 20 and negative electrode 30 .
  • the separator 10 separates the positive electrode 20 and the negative electrode 30 and prevents short circuit between the positive electrode 20 and the negative electrode 30 .
  • the separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30 . Lithium ions can pass through the separator 10 .
  • the separator 10 has, for example, an electrically insulating porous structure.
  • the separator 10 is, for example, a monolayer or laminate of polyolefin films.
  • Separator 10 may be a stretched film of a mixture such as polyethylene or polypropylene.
  • the separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene and polypropylene.
  • Separator 10 may be, for example, a solid electrolyte.
  • Solid electrolytes are polymer solid electrolytes, oxide-based solid electrolytes, and sulfide-based solid electrolytes, for example.
  • Separator 10 may be an inorganic coated separator.
  • the inorganic coated separator is obtained by coating the surface of the above film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica.
  • the inorganic coated separator has excellent heat resistance and suppresses deposition of transition metals eluted from the positive electrode onto the surface of the negative electrode.
  • the electrolytic solution is enclosed in the exterior body 50 and impregnates the power generating element 40 .
  • the electrolytic solution is not limited to a liquid electrolyte, and may be a solid electrolyte.
  • the non-aqueous electrolyte has, for example, a non-aqueous solvent and an electrolytic salt.
  • the electrolytic salt is dissolved in a non-aqueous solvent.
  • the solvent includes, for example, any one of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound.
  • the solvent may contain any mixture of these.
  • Cyclic carbonate compounds are, for example, ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, vinylene carbonate and the like.
  • Chain carbonate compounds are, for example, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like.
  • Cyclic ester compounds include, for example, ⁇ -butyrolactone. Chain ester compounds are, for example, propyl propionate, ethyl propionate, ethyl acetate and the like.
  • An electrolytic salt is, for example, a lithium salt.
  • the electrolyte is, for example, LiPF6 , LiClO4 , LiBF4 , LiCF3SO3 , LiCF3CF2SO3 , LiC ( CF3SO2)3, LiN(CF3SO2 ) 2 , LiN ( CF3CF2SO2 ) 2 , LiN ( CF3SO2 ) ( C4F9SO2 ), LiN ( CF3CF2CO ) 2 , LiBOB, LiN ( FSO2 ) 2 and the like .
  • Lithium salt may be used individually by 1 type, and may use 2 or more types together. From the point of view of the degree of ionization, the electrolyte preferably contains LiPF6 .
  • the divergence of the electrolytic salt in the carbonate solvent at room temperature is preferably 10% or more.
  • the electrolytic solution is preferably, for example, LiPF 6 dissolved in a carbonate solvent.
  • concentration of LiPF 6 is, for example, 1 mol/L.
  • the polyimide resin contains a large amount of aromatics, the polyimide resin may exhibit soft carbon-like charging behavior.
  • the electrolyte is a carbonate electrolyte solvent containing a cyclic carbonate, it is possible to uniformly react lithium with the polyimide.
  • the cyclic carbonate is preferably ethylene carbonate, fluoroethylene carbonate or vinylene carbonate.
  • the exterior body 50 seals the power generation element 40 and the non-aqueous electrolyte therein.
  • the exterior body 50 prevents the leakage of the non-aqueous electrolyte to the outside and the intrusion of moisture into the inside of the lithium ion secondary battery 100 from the outside.
  • the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52, for example, as shown in FIG.
  • the exterior body 50 is a metal laminate film in which a metal foil 52 is coated from both sides with polymer films (resin layers 54).
  • metal foil can be used as the metal foil 52 .
  • a polymer film such as polypropylene can be used for the resin layer 54 .
  • the material forming the resin layer 54 may be different between the inner side and the outer side.
  • a polymer with a high melting point such as polyethylene terephthalate (PET) or polyamide (PA) can be used as the outer material, and polyethylene (PE) or polypropylene (PP) can be used as the inner polymer film material.
  • PET polyethylene terephthalate
  • PA polyamide
  • PE polyethylene
  • PP polypropylene
  • Terminals 62 and 60 are connected to positive electrode 20 and negative electrode 30, respectively.
  • the terminal 62 connected to the positive electrode 20 is a positive terminal
  • the terminal 60 connected to the negative electrode 30 is a negative terminal.
  • Terminals 60 and 62 are responsible for electrical connection with the outside.
  • Terminals 60, 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing.
  • Terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.
  • the lithium ion secondary battery 100 is manufactured by preparing a negative electrode 30, a positive electrode 20, a separator 10, an electrolytic solution, and an outer package 50, and assembling them. An example of a method for manufacturing the lithium ion secondary battery 100 will be described below.
  • the negative electrode 30 is manufactured, for example, by sequentially performing a slurry preparation process, an electrode application process, a drying process, and a rolling process.
  • the slurry preparation process is a process of mixing a negative electrode active material (silicon particles), a binder, a conductive aid, and a solvent to prepare a slurry.
  • a negative electrode active material silicon particles
  • a binder a binder
  • a conductive aid a solvent
  • a solvent a solvent
  • the negative electrode active material the one whose shape is controlled as described above is used.
  • the negative electrode active material may be one in which a coating layer is formed on the surface of silicon particles. Aggregation of the negative electrode active material can be suppressed by adding a dispersion stabilizer to the slurry.
  • the slurry preparation process is a process of mixing a negative electrode active material, a binder, a conductive aid and a solvent to prepare a slurry.
  • Solvents are, for example, water, N-methyl-2-pyrrolidone, and the like.
  • the composition ratio of the negative electrode active material, the conductive material, and the binder is preferably 70 wt % to 100 wt %:0 wt % to 10 wt %:0 wt % to 20 wt %. These mass ratios are adjusted so that the total is 100 wt %.
  • a container made of metal such as SUS is preferable for the container used when preparing the slurry.
  • the electrostatic capacity of the oxide film on the surface of the silicon particles increases.
  • the polar solvent prevents repulsion between the conductive aid and silicon particles. By suppressing these repulsions, it is possible to prevent a decrease in the capacity of the lithium ion secondary battery.
  • the negative electrode active material may be a composite obtained by mixing active material particles and a conductive material while applying a shearing force.
  • the active material particles are mixed by applying a shearing force to such an extent that the active material particles are not degraded, the surfaces of the active material particles are coated with the conductive material.
  • the particle size of the negative electrode active material can be adjusted by the degree of mixing. Further, the negative electrode active material after production may be sieved to make the particle size uniform.
  • the electrode application step is a step of applying slurry to the surface of the negative electrode current collector 32 .
  • the slurry application method is not particularly limited.
  • a slit die coating method and a doctor blade method can be used as a slurry coating method.
  • the slurry is applied, for example, at room temperature.
  • the drying process is the process of removing the solvent from the slurry.
  • the slurry-applied negative electrode current collector 32 is dried in an atmosphere of 80.degree. C. to 350.degree.
  • the rolling process is performed as needed.
  • the rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34 .
  • the rolling process is performed by, for example, a roll press device.
  • the positive electrode 20 can be produced in the same procedure as the negative electrode 30.
  • a commercially available product can be used for the separator 10 and the outer package 50 .
  • the positive electrode 20 and the negative electrode 30 are laminated so that the separator 10 is positioned between them to produce the power generation element 40 .
  • the power generating element 40 is a wound body
  • the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side of the separator.
  • the power generation element 40 is enclosed in the exterior body 50 .
  • a non-aqueous electrolyte is injected into the exterior body 50 .
  • the power generation element 40 is impregnated with the non-aqueous electrolyte by depressurizing, heating, or the like.
  • the lithium ion secondary battery 100 is obtained by applying heat or the like to seal the exterior body 50 .
  • the power generation element 40 may be impregnated with the electrolytic solution. After injecting the liquid into the power generation element, it is preferable to leave it still for 24 hours.
  • the lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics because the negative electrode active material contains silicon particles with a predetermined shape.
  • Example 1 A positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 ⁇ m.
  • a positive electrode slurry was prepared by mixing a positive electrode active material, a conductive aid, a binder, and a solvent.
  • Li x CoO 2 was used as the positive electrode active material.
  • Acetylene black was used as the conductive aid.
  • Polyvinylidene fluoride (PVDF) was used as the binder.
  • N-methyl-2-pyrrolidone was used as the solvent.
  • a positive electrode slurry was prepared by mixing 97 parts by mass of a positive electrode active material, 1 part by mass of a conductive aid, 2 parts by mass of a binder, and 70 parts by mass of a solvent. The amount of the positive electrode active material supported in the dried positive electrode active material layer was 25 mg/cm 2 .
  • a positive electrode active material layer was formed by removing the solvent from the positive electrode slurry in a drying oven. The positive electrode active material layer was pressed with a roll press to produce a positive electrode.
  • a negative electrode slurry was prepared. Silicon particles having an average particle size of 6.2 ⁇ m, an average circularity of 0.954, and an average aspect ratio of 0.91 were used as the negative electrode active material added to the negative electrode slurry. The average particle size, average circularity and average aspect ratio were obtained by measuring 50000 particles using a particle size distribution meter manufactured by Malvern Panalytical. The silicon particles were exposed to an environment of 25° C. and 80% humidity for 7 days as an aging treatment. A silicon oxide film with an average thickness of 15 nm was formed on the surface of the exposed silicon particles. Carbon black was used as the conductive aid. A polyimide resin was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. N-methyl-2-pyrrolidone was mixed with 90 parts by mass of a negative electrode active material, 5 parts by mass of a conductive aid, and 5 parts by mass of a binder to prepare a negative electrode slurry.
  • the negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 ⁇ m and dried.
  • the amount of the negative electrode active material supported in the dried negative electrode active material layer was 2.5 mg/cm 2 .
  • the negative electrode active material layer was pressed with a roll press and then baked at 300° C. or higher for 5 hours in a nitrogen atmosphere.
  • an electrolytic solution was prepared.
  • FEC fluoroethylene carbonate
  • EC ethylene carbonate
  • DEC diethyl carbonate
  • an additive for improving output, an additive for suppressing gas, an additive for improving cycle characteristics, and an additive for improving safety performance were added to the electrolytic solution.
  • LiPF 6 was used as the electrolytic salt. The concentration of LiPF 6 was 1 mol/L.
  • Cycle characteristics of lithium ion secondary batteries were measured. Cycle characteristics were measured using a secondary battery charge/discharge test device (manufactured by Hokuto Denko Co., Ltd.).
  • the battery was charged to a battery voltage of 4.2 V by constant current charging at a charging rate of 1 C (current value at which charging ends in 1 hour when constant current charging is performed at 25° C.), and discharged to a battery voltage of 2.5 V by constant current discharging at a discharge rate of 1.0 C.
  • the discharge capacity after charging/discharging was detected to obtain the battery capacity Q1 before the cycle test.
  • Battery capacity Q1 was 3705 mAh/g.
  • the battery for which the battery capacity Q 1 was obtained as described above was charged at a constant current charge of 1C until the battery voltage reached 4.2 V, and then discharged at a constant current discharge rate of 1C until the battery voltage reached 2.5 V.
  • the charge/discharge was counted as one cycle, and 300 cycles of charge/discharge were performed. After that, the discharge capacity after 300 cycles of charging and discharging was detected, and the battery capacity Q2 after 300 cycles was obtained. From the battery capacities Q 1 and Q 2 obtained above, the capacity retention rate E after 300 cycles was obtained.
  • the capacity retention rate of Example 1 was 98%.
  • the lithium ion secondary battery was disassembled, and the cross section of the negative electrode active material layer was measured with a scanning electron microscope.
  • the average particle size and average aspect ratio of the negative electrode active material determined from the images were not significantly different from those measured using a particle size distribution meter.
  • Examples 2 to 19 differ from Example 1 in that the average particle size, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used in the negative electrode active material were changed.
  • the average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles.
  • the thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
  • Comparative Examples 1 to 6 differ from Example 1 in that the average particle diameter, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used in the negative electrode active material were changed.
  • the average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles.
  • the thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
  • Examples 1-19 had a higher capacity retention rate and better cycle characteristics than Comparative Examples 1-6.
  • Comparative Example 1 the silicon particles have a small average particle size and a large contact area with the electrolytic solution. Therefore, in Comparative Example 1, lithium was consumed in the irreversible reaction (side reaction) between silicon and the electrolytic solution, and the capacity retention rate was low. Comparative Example 2 has a large average particle size of the silicon particles. Large silicon particles are likely to crack during expansion and contraction, and it is considered that the capacity retention rate of Comparative Example 2 was low.
  • Comparative Example 3 has a low average circularity and tends to cause stress concentration during expansion and contraction. It is considered that the silicon particles of Comparative Example 3 were easily cracked during expansion and contraction, and the capacity retention rate of Comparative Example 3 was low. Comparative Example 4 has a high average circularity and a small contact area with the conductive aid and the binder. It is considered that the silicon particles of Comparative Example 4 could not ensure sufficient adhesion to the conductive aid and the binder when the negative electrode active material expanded and contracted, and the capacity retention rate of Comparative Example 4 was low.
  • Comparative Example 5 has a low average aspect ratio and tends to cause stress concentration during expansion and contraction. It is considered that the silicon particles of Comparative Example 5 were easily cracked during expansion and contraction, and the capacity retention rate of Comparative Example 5 was low.
  • the average aspect ratio is close to 1, and the silicon particles are less likely to be oriented in one direction. When the silicon particles are oriented in one direction, the surface of the negative electrode active material layer is flattened, and the adhesion to the negative electrode current collector is improved. In Comparative Example 6, peeling occurred at the interface between the negative electrode active material layer and the negative electrode current collector, and the capacity retention ratio was low. This is probably because Comparative Example 6 has an average aspect ratio close to 1 and the silicon particles are difficult to align in one direction.
  • Comparative Example 6 is considered to have insufficient adhesion between the negative electrode active material layer and the negative electrode current collector.
  • Examples 20-30 differ from Example 1 in that the average particle size, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used for the negative electrode active material were changed.
  • the average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles.
  • the thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
  • Examples 7 to 10 differ from Example 1 in that the average particle diameter, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used for the negative electrode active material were changed.
  • the average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles.
  • the thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
  • Examples 20-30 had a higher capacity retention rate and better cycle characteristics than Comparative Examples 7-10.
  • Comparative Example 7 the silicon particles have a small average particle size and a large contact area with the electrolytic solution. Therefore, in Comparative Example 7, lithium was consumed in the irreversible reaction (side reaction) between silicon and the electrolytic solution, and the capacity retention rate was low. Comparative Example 8 has a large average particle size of the silicon particles. Large silicon particles are likely to crack during expansion and contraction, and it is considered that the capacity retention rate of Comparative Example 8 was low.
  • Comparative Example 9 has a low average aspect ratio and tends to cause stress concentration during expansion and contraction. It is considered that the silicon particles of Comparative Example 9 were easily cracked during expansion and contraction, and the capacity retention rate of Comparative Example 9 was low. In Comparative Example 10, the average circularity of the silicon particles is outside the predetermined range, and the silicon particles can be considered to be amorphous. Therefore, it is considered that stress was concentrated on a specific portion during expansion and contraction of the silicon particles, resulting in a low capacity retention rate.
  • Example 31 silicon particles with a coating layer formed thereon were used as the negative electrode active material. A coating layer was formed on the surface of the silicon particles after the aging treatment.
  • the material constituting the coating layer was LiF.
  • the coating layer was prepared by putting 5.4 g of silicon particles and 0.72 g of LiF powder into a mechanochemical reactor (manufactured by Hosokawa Micron Corporation, product name: Circulating Mechanofusion (registered trademark) System AMS) and causing a mechanochemical reaction at the interface between the silicon particles and LiF powder.
  • the configuration of the lithium ion secondary battery was the same as in Example 1 except for the silicon particles.
  • Examples 32 to 52 differ from Example 31 in that at least one of the material constituting the coating layer, the average particle size, average circularity and average aspect ratio of the silicon particles used in the negative electrode active material, the thickness of the oxide film, and the thickness of the coating layer was changed.
  • the average particle size of the silicon particles was controlled by changing the conditions for producing the silicon particles.
  • the thickness of the oxide film was controlled by changing the aging treatment conditions.
  • the thickness of the coating layer was controlled by changing the conditions for forming the coating layer.
  • Examples 32 and 50 used Mg 2 O as the material constituting the coating layer
  • Examples 33 and 51 used Mg 2 F as the material constituting the coating layer
  • Examples 34 and 52 used Mg 3 (PO 4 ) 2 as the material constituting the coating layer. Then, in Examples 32 to 52, the discharge capacity and the capacity retention rate were obtained in the same manner as in Example 1.
  • Comparative Examples 11 and 12 differ from Example 32 in that at least one of the average particle diameter, average circularity, average aspect ratio, oxide film thickness, and coating layer thickness of the silicon particles used in the negative electrode active material was changed. Then, in Comparative Examples 11 and 12, similarly to Example 1, the discharge capacity and the capacity retention rate were obtained.
  • Comparative Example 13 Comparative Example 13 differs from Example 31 in that no coating layer was formed.
  • the average particle size, average circularity, and average aspect ratio of the silicon particles of Comparative Example 13 are also different from those of Example 31. Then, in Comparative Example 13, the discharge capacity and the capacity retention rate were obtained in the same manner as in Example 1.
  • Examples 31-52 had a higher capacity retention rate and better cycle characteristics than Comparative Examples 11-13.
  • Comparative Example 11 the silicon particles have a small average particle size and a large contact area with the electrolytic solution. Therefore, in Comparative Example 11, lithium was consumed in the irreversible reaction (side reaction) between silicon and the electrolytic solution, and the capacity retention rate was low. Comparative Example 12 has a large average particle size of the silicon particles. Large silicon particles are likely to crack during expansion and contraction, and it is considered that the capacity retention rate of Comparative Example 12 was low.
  • Comparative Example 13 has a low average circularity and a low average aspect ratio, and is considered to have a low capacity retention rate due to the influence of particle cracking due to stress concentration.

Abstract

This negative electrode active material for lithium ion secondary batteries comprises silicon particles having an average diameter of 0.1-10 µm inclusive, an average aspect ratio of 0.60-0.99 inclusive and an average circularity of 0.80-0.99 inclusive.

Description

リチウムイオン二次電池用負極活物質、リチウムイオン二次電池用負極活物質層、リチウムイオン二次電池用負極及びリチウムイオン二次電池Negative electrode active material for lithium ion secondary battery, negative electrode active material layer for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery
　本発明は、リチウムイオン二次電池用負極活物質、リチウムイオン二次電池用負極活物質層、リチウムイオン二次電池用負極及びリチウムイオン二次電池に関する。本願は、２０２２年１月２１日に、日本に出願された特願２０２２－００７７５８、特願２０２２－００７７６１及び特願２０２２－００７７６６に基づき優先権を主張し、その内容をここに援用する。 The present invention relates to a negative electrode active material for lithium ion secondary batteries, a negative electrode active material layer for lithium ion secondary batteries, a negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery. This application claims priority based on Japanese Patent Application 2022-007758, Japanese Patent Application 2022-007761 and Japanese Patent Application 2022-007766 filed in Japan on January 21, 2022, the contents of which are incorporated herein.
　リチウムイオン二次電池は、携帯電話、ノートパソコン等のモバイル機器やハイブリットカー等の動力源として、広く用いられている。 Lithium-ion secondary batteries are widely used as power sources for mobile devices such as mobile phones and laptop computers, and hybrid cars.
　リチウムイオン二次電池の容量は、主に電極の活物質に依存する。黒鉛は、負極活物質として一般に利用されているが、黒鉛より高容量な負極活物質が求められている。シリコン（Ｓｉ）は、黒鉛の理論容量（３７２ｍＡｈ／ｇ）に比べてはるかに大きな理論容量をもつ負極活物質として、注目されている。 The capacity of lithium-ion secondary batteries mainly depends on the active material of the electrodes. Graphite is generally used as a negative electrode active material, but there is a demand for a negative electrode active material with a higher capacity than graphite. Silicon (Si) is attracting attention as a negative electrode active material having a much larger theoretical capacity than that of graphite (372 mAh/g).
　シリコンを含む負極活物質は、充電時に大きな体積膨張を伴う。負極活物質の体積膨張は、電池のサイクル特性の低下の原因となる。負極活物質が体積膨張すると、例えば、負極活物質が破損したり、負極活物質の間の導電パスが切断したり、負極活物質層と集電体の界面で剥離が生じたり、ＳＥＩ（Ｓｏｌｉｄ　Ｅｌｅｃｔｒｏｌｙｔｅ　Ｉｎｔｅｒｐｈａｓｅ）被膜にクラックが生じ電解液の分解等が生じる。これらは、電池のサイクル特性を低下させる。 A negative electrode active material containing silicon undergoes a large volume expansion during charging. The volume expansion of the negative electrode active material causes deterioration in the cycle characteristics of the battery. When the volume of the negative electrode active material expands, for example, the negative electrode active material is damaged, a conductive path between the negative electrode active materials is cut, separation occurs at the interface between the negative electrode active material layer and the current collector, cracks occur in the SEI (Solid Electrolyte Interphase) coating, and decomposition of the electrolyte occurs. These degrade the cycle characteristics of the battery.
　例えば、特許文献１には、シリコン粒子のアスペクト比及びシリコン粒子の集電体に対する傾斜角を規定することで、サイクル特性が向上することが記載されている。 For example, Patent Document 1 describes that the cycle characteristics are improved by defining the aspect ratio of the silicon particles and the inclination angle of the silicon particles with respect to the current collector.
特開２０１９－１４９３３３号公報JP 2019-149333 A
　サイクル特性はリチウムイオン二次電池の重要なパラメータであり、特許文献１に記載の方法以外の方法を用いてサイクル特性を改善できる方法が検討されている。 Cycle characteristics are an important parameter of lithium-ion secondary batteries, and methods that can improve cycle characteristics using methods other than the method described in Patent Document 1 are being investigated.
　本開示は上記問題に鑑みてなされたものであり、サイクル特性に優れるリチウムイオン二次電池を提供することを目的とする。 The present disclosure has been made in view of the above problems, and aims to provide a lithium ion secondary battery with excellent cycle characteristics.
　上記課題を解決するため、以下の手段を提供する。 In order to solve the above issues, we provide the following means.
（１）第１の態様にかかるリチウムイオン二次電池用負極活物質は、平均粒子径が０．１μｍ以上１０μｍ以下、平均アスペクト比が０．６０以上０．９９以下、平均円形度が０．８０以上０．９９以下のシリコン粒子を含む。 (1) The negative electrode active material for a lithium ion secondary battery according to the first aspect contains silicon particles having an average particle size of 0.1 μm or more and 10 μm or less, an average aspect ratio of 0.60 or more and 0.99 or less, and an average circularity of 0.80 or more and 0.99 or less.
（２）上記態様にかかるリチウムイオン二次電池用負極活物質は、平均円形度が０．８２以上０．９８５以下でもよい。 (2) The negative electrode active material for a lithium ion secondary battery according to the above aspect may have an average circularity of 0.82 or more and 0.985 or less.
（３）上記態様にかかるリチウムイオン二次電池用負極活物質は、平均アスペクト比が０．６５以上０．８０以下でもよい。 (3) The negative electrode active material for a lithium ion secondary battery according to the above aspect may have an average aspect ratio of 0.65 or more and 0.80 or less.
（４）上記態様にかかるリチウムイオン二次電池用負極活物質において、前記シリコン粒子は、平均円形度が０．９２０以上０．９８５以下、平均アスペクト比が０．８０以上０．９７以下でもよい。 (4) In the negative electrode active material for a lithium ion secondary battery according to the above aspect, the silicon particles may have an average circularity of 0.920 or more and 0.985 or less and an average aspect ratio of 0.80 or more and 0.97 or less.
（５）上記態様にかかるリチウムイオン二次電池用負極活物質において、前記シリコン粒子は、平均粒子径が１μｍ以上７μｍ以下でもよい。 (5) In the negative electrode active material for a lithium ion secondary battery according to the aspect described above, the silicon particles may have an average particle diameter of 1 μm or more and 7 μm or less.
（６）上記態様にかかるリチウムイオン二次電池用負極活物質において、前記シリコン粒子は、平均粒子径が３μｍ以上７μｍ以下でもよい。 (6) In the negative electrode active material for a lithium ion secondary battery according to the aspect described above, the silicon particles may have an average particle size of 3 μm or more and 7 μm or less.
（７）上記態様にかかるリチウムイオン二次電池用負極活物質は、前記シリコン粒子の少なくとも一部を被覆する被覆層をさらに備えてもよい。前記被覆層は、フッ化リチウム、酸化マグネシウム、リン酸マグネシウム、フッ化マグネシウムからなる群から選択される一種以上の材料を含む。 (7) The negative electrode active material for a lithium ion secondary battery according to the aspect described above may further include a coating layer that coats at least part of the silicon particles. The coating layer contains one or more materials selected from the group consisting of lithium fluoride, magnesium oxide, magnesium phosphate, and magnesium fluoride.
（８）上記態様にかかるリチウムイオン二次電池用負極活物質において、前記被覆層の平均厚さは、５ｎｍ以上５００ｎｍ以下でもよい。 (8) In the negative electrode active material for a lithium ion secondary battery according to the aspect described above, the coating layer may have an average thickness of 5 nm or more and 500 nm or less.
（９）第２の態様にかかるリチウムイオン二次電池用負極活物質層は、上記態様にかかるリチウムイオン二次電池用負極活物質を含む。 (9) The negative electrode active material layer for lithium ion secondary batteries according to the second aspect includes the negative electrode active material for lithium ion secondary batteries according to the aspect described above.
（１０）第３の態様にかかるリチウムイオン二次電池用負極は、上記態様にかかるリチウムイオン二次電池用負極活物質を含む。 (10) A negative electrode for a lithium ion secondary battery according to a third aspect includes the negative electrode active material for a lithium ion secondary battery according to the aspect described above.
（１１）第４の態様にかかるリチウムイオン二次電池は、上記態様にかかるリチウムイオン二次電池用負極と、正極と、電解質と、を含む。 (11) A lithium ion secondary battery according to a fourth aspect includes the lithium ion secondary battery negative electrode according to the above aspect, a positive electrode, and an electrolyte.
　上記態様に係るリチウムイオン二次電池用負極活物質を用いたリチウムイオン二次電池は、サイクル特性に優れる。 A lithium ion secondary battery using the negative electrode active material for a lithium ion secondary battery according to the above aspect has excellent cycle characteristics.
第１実施形態にかかる負極活物質層の断面図である。3 is a cross-sectional view of a negative electrode active material layer according to the first embodiment; FIG. エージング処理後のシリコン粒子の走査型電子顕微鏡像である。1 is a scanning electron microscope image of silicon particles after aging treatment. 第１変形例にかかる負極活物質の模式図である。FIG. 5 is a schematic diagram of a negative electrode active material according to a first modified example; 第１実施形態に係るリチウムイオン二次電池の模式図である。1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment; FIG.
　以下、実施形態について、図を適宜参照しながら詳細に説明する。以下の説明で用いる図面は、特徴をわかりやすくするために便宜上特徴となる部分を拡大して示している場合があり、各構成要素の寸法比率等は実際とは異なっていることがある。以下の説明において例示される材料、寸法等は一例であって、本発明はそれらに限定されるものではなく、その要旨を変更しない範囲で適宜変更して実施することが可能である。 Hereinafter, embodiments will be described in detail with reference to the drawings as appropriate. In the drawings used in the following description, characteristic portions may be enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratios and the like of each component may differ from the actual. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be implemented with appropriate modifications without changing the gist of the invention.
「負極活物質」
　第１実施形態にかかる負極活物質は、リチウムイオン二次電池に用いられ、シリコン粒子を含む。シリコン粒子は、単体のシリコンの他に、シリコン合金、シリコン化合物、シリコン複合体でもよい。シリコン粒子は、結晶質でも非晶質でもよい。 "Negative electrode active material"
A negative electrode active material according to the first embodiment is used in a lithium ion secondary battery and contains silicon particles. The silicon particles may be a silicon alloy, a silicon compound, or a silicon composite in addition to single silicon. Silicon particles may be crystalline or amorphous.
　シリコン合金は、例えば、ＸｎＳｉで表される。Ｘは、カチオンである。Ｘは、例えば、Ｂａ、Ｍｇ、Ａｌ、Ｚｎ、Ｓｎ、Ｃａ、Ｖ、Ｃｒ、Ｍｎ、Ｆｅ、Ｃｏ、Ｎｉ、Ｃｕ、Ｚｎ、Ｇｅ、Ｙ、Ｚｒ、Ｎｂ、Ｍｏ、Ｗ、Ａｕ、Ｔｉ、Ｎａ、Ｋ等である。ｎは、０≦ｎ≦０．５を満たす。 A silicon alloy is represented, for example, by X n Si. X is a cation. X is, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K or the like. n satisfies 0≦n≦0.5.
　シリコン化合物は、例えば、ＳｉＯｘで表記される酸化シリコンである。ｘは、例えば、０．８≦ｘ≦２を満たす。酸化シリコンは、ＳｉＯ２のみからなってもよいし、ＳｉＯのみからなってもよいし、ＳｉＯとＳｉＯ２との混合物でもよい。また酸化シリコンは、酸素の一部が欠損していてもよい。 A silicon compound is, for example, a silicon oxide denoted by SiO x . x satisfies 0.8≦x≦2, for example. The silicon oxide may consist of only SiO2 , may consist of only SiO, or may be a mixture of SiO and SiO2 . In addition, silicon oxide may be partially deficient in oxygen.
　シリコン複合体は、例えば、シリコン又はシリコン化合物の粒子の表面の少なくとも一部に、導電性材料が被覆したものである。導電性材料は、例えば、炭素材料、Ａｌ、Ｔｉ、Ｆｅ、Ｎｉ、Ｃｕ、Ｚｎ、Ａｇ、Ｓｎ等である。例えば、シリコン炭素複合化材料（Ｓｉ－Ｃ）は複合体の一例である。 A silicon composite is, for example, at least part of the surface of silicon or silicon compound particles coated with a conductive material. The conductive material is, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or the like. For example, a silicon carbon composite (Si—C) is one example of a composite.
　シリコン粒子の平均粒子径は０．１μｍ以上１０μｍ以下であり、好ましくは０．５μｍ以上８μｍ以下であり、より好ましくは１μｍ以上７μｍ以下である。 The average particle size of the silicon particles is 0.1 μm or more and 10 μm or less, preferably 0.5 μm or more and 8 μm or less, and more preferably 1 μm or more and 7 μm or less.
　シリコン粒子を粒子の状態で入手可能な場合は、粒度分布測定装置（例えば、Malvern Panalytical社製）を用いてメディアン径（Ｄ５０）を平均粒子径として求めることができる。粒度分布測定装置を用いる場合は、例えば、５００００個の粒子の粒子径の平均を求める。 If the silicon particles are available in the form of particles, the median diameter (D50) can be obtained as the average particle diameter using a particle size distribution measuring device (eg, manufactured by Malvern Panalytical). When using a particle size distribution analyzer, for example, the average particle size of 50,000 particles is obtained.
　図１に示すように、シリコン粒子が電極内にありシリコン粒子の分離が困難な場合は、断面画像で確認される少なくとも１００個のシリコン粒子を用いて平均粒子径を求めることができる。 As shown in FIG. 1, if the silicon particles are inside the electrode and it is difficult to separate the silicon particles, the average particle size can be obtained using at least 100 silicon particles confirmed in the cross-sectional image.
　図１は、シリコン粒子を含む負極活物質層の断面画像である。断面画像は、走査型電子顕微鏡（ＳＥＭ）で測定できる。電極内のシリコン粒子は、コントラストの違いから確認でき、図１の白く確認される部分がシリコン粒子である。図１の灰色で確認される部分がバインダーであり、黒く確認される部分が空隙である。コントラストからシリコン粒子を抽出し、ナンバリングすることができる。 FIG. 1 is a cross-sectional image of a negative electrode active material layer containing silicon particles. Cross-sectional images can be measured with a scanning electron microscope (SEM). The silicon particles in the electrode can be confirmed from the difference in contrast, and the white portions in FIG. 1 are the silicon particles. The portion confirmed in gray in FIG. 1 is the binder, and the portion confirmed in black is the void. Silicon particles can be extracted from the contrast and numbered.
　例えば、コントラストの閾値を設定し２値化することで画像からシリコン粒子（負極活物質）を抽出できる。そして抽出された少なくとも１００個のシリコン粒子の径をそれぞれ求める。求められたそれぞれのシリコン粒子の径の頻度をグラフ化し、最頻値を平均粒子径とする。シリコン粒子の形状が不定形の場合は、長軸の径を平均粒子径の算出に用いる。 For example, silicon particles (negative electrode active material) can be extracted from an image by setting a contrast threshold and binarizing it. Then, the diameters of at least 100 extracted silicon particles are obtained. The obtained frequency of the diameter of each silicon particle is graphed, and the mode is taken as the average particle diameter. When the shape of the silicon particles is irregular, the diameter of the long axis is used to calculate the average particle size.
　シリコン粒子の平均アスペクト比は、０．６０以上０．９９以下であり、好ましくは０．６５以上０．９８以下であり、より好ましくは０．８０以上０．９７以下である。またシリコン粒子の平均アスペクト比は、０．６０以上０．９０以下でもよく、０．６５以上０．８０以下でもよい。アスペクト比が当該範囲にあると、シリコン粒子の膨張収縮時に特定の部分に応力集中が生じにくくなり、シリコン粒子が破損しにくくなる。またアスペクト比が当該範囲内にあると、シリコン粒子が一方向に配向しやすくなり、負極活物質層の表面が平坦になる。その結果、負極活物質層と負極集電体との密着性が向上する。 The average aspect ratio of the silicon particles is 0.60 or more and 0.99 or less, preferably 0.65 or more and 0.98 or less, and more preferably 0.80 or more and 0.97 or less. Moreover, the average aspect ratio of the silicon particles may be 0.60 or more and 0.90 or less, or may be 0.65 or more and 0.80 or less. When the aspect ratio is within this range, stress concentration is less likely to occur at a specific portion during expansion and contraction of the silicon particles, and the silicon particles are less likely to break. Further, when the aspect ratio is within the range, the silicon particles are easily oriented in one direction, and the surface of the negative electrode active material layer becomes flat. As a result, the adhesion between the negative electrode active material layer and the negative electrode current collector is improved.
　シリコン粒子の平均アスペクト比は、少なくとも１００個以上のシリコン粒子のアスペクト比を用いて求められる。アスペクト比は、測定対象のシリコン粒子の短軸長を長軸長で割って求められる。 The average aspect ratio of silicon particles is obtained using the aspect ratio of at least 100 silicon particles. The aspect ratio is obtained by dividing the short axis length by the long axis length of the silicon particles to be measured.
　平均アスペクト比は、平均粒子径と同様に、シリコン粒子を粒子の状態で入手可能な場合は粒度分布測定装置を用いて求めることができ、図１に示すようにシリコン粒子の分離が困難な場合は断面画像を用いて求めることができる。粒度分布測定装置を用いる場合は、例えば、５００００個の粒子のアスペクト比の最頻値を求め、断面画像を用いる場合は、例えば、１００個の粒子のアスペクト比の最頻値を求める。 Similar to the average particle size, the average aspect ratio can be obtained using a particle size distribution measuring device if the silicon particles are available in the form of particles, and if it is difficult to separate the silicon particles as shown in Fig. 1, it can be obtained using a cross-sectional image. When using a particle size distribution measuring apparatus, for example, the mode of aspect ratios of 50,000 particles is obtained, and when using a cross-sectional image, for example, the mode of aspect ratios of 100 particles is obtained.
　シリコン粒子の平均円形度は、０．８０以上０．９９以下であり、好ましくは０．８２０以上０．９８５以下であり、より好ましくは０．９１０以上０．９８８以下であり、さらに好ましくは０．９２０以上０．９８５以下である。平均円形度が当該範囲内にあると、シリコン粒子の膨張収縮時に特定の部分に応力集中しにくくなり、かつ、導電助剤及びバインダーとシリコン粒子との接触面積を十分確保できる。 The average circularity of the silicon particles is 0.80 or more and 0.99 or less, preferably 0.820 or more and 0.985 or less, more preferably 0.910 or more and 0.988 or less, and still more preferably 0.920 or more and 0.985 or less. When the average circularity is within this range, it becomes difficult for stress to concentrate on a specific portion during expansion and contraction of the silicon particles, and a sufficient contact area between the conductive aid and the binder and the silicon particles can be ensured.
　シリコン粒子の平均円形度は、少なくとも１００個以上のシリコン粒子の円形度を用いて求められる。円形度は、測定対象のシリコン粒子と同面積の円の円周長を測定対象のシリコン粒子の周囲長で割って求められる。 The average circularity of silicon particles is obtained using the circularity of at least 100 silicon particles. The degree of circularity is obtained by dividing the circumference of a circle having the same area as the silicon particle to be measured by the circumference of the silicon particle to be measured.
　平均円形度は、平均粒子径と同様に、シリコン粒子を粒子の状態で入手可能な場合は粒度分布測定装置を用いて求めることができる。粒度分布測定装置を用いる場合は、例えば、５００００個の粒子の円形度の最頻値を求める。また図１に示すようにシリコン粒子の分離が困難な場合は、断面画像を用いて平均円形度を求めることができる。断面画像を用いる場合は、例えば、１００個の粒子のそれぞれの円形度を求める。具体的には、画像からシリコン粒子を抽出し、それぞれのシリコン粒子の周囲長と面積を求める。また求められたそれぞれのシリコン粒子の面積と同面積の円の円周長を算出し、それぞれのシリコン粒子の円形度を算出する。そして、シリコン粒子の円形度の最頻値を平均円形度とする。 Similar to the average particle diameter, the average circularity can be obtained using a particle size distribution analyzer if silicon particles are available in the form of particles. When using a particle size distribution analyzer, for example, the mode of circularity of 50,000 particles is obtained. In addition, when it is difficult to separate the silicon particles as shown in FIG. 1, the average circularity can be obtained using a cross-sectional image. When using cross-sectional images, for example, the circularity of each of 100 particles is obtained. Specifically, silicon particles are extracted from the image, and the perimeter and area of each silicon particle are obtained. Also, the circumferential length of a circle having the same area as the calculated area of each silicon particle is calculated to calculate the degree of circularity of each silicon particle. Then, the mode of circularity of the silicon particles is taken as the average circularity.
　ここで、リチウムイオン二次電池の充放電を行った後に電極内に存在するシリコン粒子と、粒子の状態で入手可能な未処理のシリコン粒子とは、平均粒子径、平均アスペクト比及び平均円形度が必ずしも一致するとは限らない。ここでいう、シリコン粒子の平均粒子径、平均アスペクト比及び平均円形度は、原則として１回以上充放電を行った後の数値を規定している。一方で、充放電前のシリコン粒子の平均粒子径、平均アスペクト比及び平均円形度が上記範囲を満たしていても、リチウムイオン二次電池のサイクル特性は向上する。 Here, the average particle diameter, average aspect ratio, and average circularity of the silicon particles present in the electrode after charging and discharging of the lithium ion secondary battery and the untreated silicon particles available in the form of particles do not necessarily match. The average particle diameter, average aspect ratio, and average circularity of the silicon particles referred to here are, in principle, defined as numerical values after one or more charge/discharge cycles. On the other hand, even when the average particle size, average aspect ratio and average circularity of the silicon particles before charge/discharge satisfy the above ranges, the cycle characteristics of the lithium ion secondary battery are improved.
　シリコン粒子が酸化シリコン以外の場合、シリコン粒子は、表面に酸化被膜を有してもよい。酸化被膜は、シリコンの酸化物を含む。酸化被膜の膜厚は、例えば、１ｎｍ以上１００ｎｍ以下であり、好ましくは３ｎｍ以上５０ｎｍ以下である。酸化被膜の膜厚は、例えば、断面画像から求められる１００個のシリコン粒子に形成された酸化被膜の厚みの平均値である。 When the silicon particles are other than silicon oxide, the silicon particles may have an oxide film on the surface. The oxide layer includes silicon oxide. The film thickness of the oxide film is, for example, 1 nm or more and 100 nm or less, preferably 3 nm or more and 50 nm or less. The thickness of the oxide film is, for example, the average thickness of the oxide film formed on 100 silicon particles obtained from the cross-sectional image.
　第１実施形態に係る負極活物質に用いられるシリコン粒子は、例えば、シリコンを溶融した後、再度、固化することで作製できる。溶融したシリコンは、表面張力をうけて丸くなる。第１実施形態に係る負極活物質に用いられるシリコン粒子は、例えば、アトマイズ法、熱プラズマ法で作製できる。ただし、これらの方法で作製する際の条件によって、シリコン粒子の平均粒子径、平均円形度及び平均アスペクト比は変化する。シリコン粒子の形状を所定の形状にするためには、作製条件を設定し、これらを制御する。 The silicon particles used for the negative electrode active material according to the first embodiment can be produced, for example, by melting silicon and then solidifying it again. Molten silicon is rounded by surface tension. Silicon particles used for the negative electrode active material according to the first embodiment can be produced by, for example, an atomization method or a thermal plasma method. However, the average particle diameter, average circularity and average aspect ratio of the silicon particles change depending on the conditions during production by these methods. In order to obtain the desired shape of the silicon particles, manufacturing conditions are set and controlled.
　シリコン粒子の作製条件は、例えば、以下である。熱プラズマ法を用いる場合、原料として平均粒子径が１μｍ以上８μｍ以下のシリコン粒子を溶融する。原料の平均粒径は、シリコン粒子の粒径に影響を及ぼすパラメータの一つである。またシリコン粒子の形状に影響を及ぼすパラメータとして、溶融温度、溶融時間、冷却温度、冷却速度等がある。シリコンの溶融温度は、例えば、１２００℃以上１２０００℃以下とする。シリコンの溶融時間は、例えば、１ｓ以上、３００ｓ以下とする。シリコンを固化する際の冷却温度は、例えば、１５℃以上８００℃以下とする。シリコンを固化する際の冷却速度は、例えば、５℃／ｓ以上１００００℃／ｓ以下とする。冷却速度が速いと、粒子の結晶化度が下がり、充放電時のリチウムの拡散が均一になりやすく、リチウムイオン二次電池のサイクル特性が向上する。冷却速度は１０００℃／ｓ以上が好ましい。またシリコンを溶融し、冷却するときの雰囲気はＡｒまたは窒素などの不活性雰囲気が好ましい。またノズルを用いて冷却空間に溶融シリコンを導入する場合は、ノズルの径、形状、長さ、ノズルへの溶融シリコンの流量を設計する。これらもシリコン粒子の形状に影響を及ぼす。ノズルの径、形状、長さ、ノズルへの溶融シリコンの流量は、事前検討を行い、装置に合わせた条件を決定する。 The conditions for producing silicon particles are, for example, as follows. When the thermal plasma method is used, silicon particles having an average particle size of 1 μm or more and 8 μm or less are melted as raw materials. The average particle size of the raw material is one of the parameters that affect the particle size of the silicon particles. Further, the melting temperature, melting time, cooling temperature, cooling rate, etc. are parameters that affect the shape of the silicon particles. The melting temperature of silicon is, for example, 1200° C. or higher and 12000° C. or lower. The melting time of silicon is, for example, 1 s or more and 300 s or less. The cooling temperature for solidifying silicon is, for example, 15° C. or higher and 800° C. or lower. The cooling rate when solidifying silicon is, for example, 5° C./s or more and 10000° C./s or less. When the cooling rate is high, the crystallinity of the particles is lowered, lithium tends to diffuse uniformly during charging and discharging, and the cycle characteristics of the lithium ion secondary battery are improved. A cooling rate of 1000° C./s or more is preferable. The atmosphere in which silicon is melted and cooled is preferably an inert atmosphere such as Ar or nitrogen. When the molten silicon is introduced into the cooling space using a nozzle, the diameter, shape and length of the nozzle and the flow rate of the molten silicon to the nozzle are designed. These also influence the shape of the silicon particles. The diameter, shape and length of the nozzle and the flow rate of the molten silicon to the nozzle are examined in advance and determined according to the equipment.
　またシリコン粒子の表面に酸化被膜を形成する場合は、酸化被膜の形成処理を行う。酸化被膜は、例えば、エージング処理によって形成できる。エージング処理は、シリコン粒子を湿度８０％、温度２５℃の環境下に、所定時間以上曝露する処理である。エージング処理時の露点は、例えば、２１℃とする。エージング処理を行う雰囲気として、酸素濃度は２０％以上であることが望ましい。エージング処理は、強度の弱い紫外線を照射する環境でゆるやかに行われる。エージング処理は、適切な波長をもった蛍光灯やＬＥＤなどを用いて行うことができる。また粉体を定期的に攪拌もしくは回転させながらエージング処理を行うことで、粒子全体が均一に酸化される。 In addition, when forming an oxide film on the surface of the silicon particles, an oxide film forming process is performed. The oxide film can be formed, for example, by aging treatment. The aging process is a process of exposing the silicon particles to an environment with a humidity of 80% and a temperature of 25° C. for a predetermined time or longer. The dew point during the aging treatment is, for example, 21°C. It is desirable that the atmosphere in which the aging treatment is performed has an oxygen concentration of 20% or more. Aging treatment is carried out gently in an environment in which low-intensity ultraviolet rays are applied. Aging treatment can be performed using a fluorescent lamp, an LED, or the like having an appropriate wavelength. In addition, by performing the aging treatment while periodically stirring or rotating the powder, the entire particles are uniformly oxidized.
　図２は、エージング処理後のシリコン粒子の走査型電子顕微鏡像である。図２に示すシリコン粒子の表面には粒界がある。シリコン粒子は多結晶体であることが好ましい。シリコン粒子の表面の現れている結晶面によって酸化被膜の厚みが異なる。またシリコン粒子が多結晶体であると、リチウム拡散経路が多方向になり、電極の膨張が均一化される。また、シリコン粒子は、表面に数ナノサイズのシリコン粒子（第２のシリコン粒子）を複数担持することが好ましい。第２のシリコン粒子は、シリコン粒子の酸化速度を向上させる。酸化被膜は、エージング処理を行うことで、自然酸化膜と比較して十分厚くなる。 FIG. 2 is a scanning electron microscope image of silicon particles after aging treatment. There are grain boundaries on the surfaces of the silicon particles shown in FIG. Preferably, the silicon particles are polycrystalline. The thickness of the oxide film varies depending on the crystal planes that appear on the surface of the silicon particles. Also, when the silicon particles are polycrystalline, the lithium diffusion paths are multidirectional, and the expansion of the electrode is made uniform. In addition, it is preferable that a plurality of silicon particles (second silicon particles) having a size of several nanometers (second silicon particles) are supported on the surface of the silicon particles. The second silicon particles improve the oxidation rate of the silicon particles. The oxide film becomes sufficiently thicker than the natural oxide film by performing the aging treatment.
　図３は、第１変形例に係る負極活物質１の断面模式図である。第１変形例に係る負極活物質１は、シリコン粒子２と被覆層３とを備える。シリコン粒子２は、上述のシリコン粒子と同様である。 FIG. 3 is a cross-sectional schematic diagram of the negative electrode active material 1 according to the first modified example. A negative electrode active material 1 according to the first modification includes silicon particles 2 and a coating layer 3 . The silicon particles 2 are similar to the silicon particles described above.
　被覆層３は、フッ化リチウム（例えばＬｉＦ）、酸化マグネシウム（例えばＭｇ２Ｏ）、リン酸マグネシウム（例えばＭｇ３（ＰＯ４）２）、フッ化マグネシウム（例えば、Ｍｇ２Ｆ）からなる群から選択される一種以上の材料を含む。 The coating layer 3 contains one or more materials selected from the group consisting of lithium fluoride (e.g. LiF), magnesium oxide (e.g. Mg 2 O), magnesium phosphate (e.g. Mg 3 (PO 4 ) 2 ) and magnesium fluoride (e.g. Mg 2 F).
　被覆層３は、シリコン粒子２の表面の少なくとも一部を被覆していればよい。被覆層３は、シリコン粒子２の表面の全てを被覆していてもよい。 The coating layer 3 only needs to cover at least part of the surface of the silicon particles 2 . The coating layer 3 may cover the entire surface of the silicon particles 2 .
　被覆層３は、リチウムイオン二次電池中で、電解液とシリコン粒子２とが直接接することを阻害する。シリコン粒子２と電解液とが接すると、電解液が還元分解する不可逆反応（副反応）が生じる。この不可逆反応に用いられたリチウムは、その後の充放電に寄与できなくなる。被覆層３は、不可逆反応を抑制し、リチウムイオン二次電池のサイクル特性を向上する。 The coating layer 3 prevents direct contact between the electrolytic solution and the silicon particles 2 in the lithium ion secondary battery. When the silicon particles 2 and the electrolytic solution come into contact with each other, an irreversible reaction (side reaction) occurs in which the electrolytic solution undergoes reductive decomposition. Lithium used for this irreversible reaction cannot contribute to subsequent charge/discharge. The coating layer 3 suppresses irreversible reactions and improves the cycle characteristics of the lithium ion secondary battery.
　被覆層３は、シリコン粒子２を作製した後に、シリコン粒子２の表面に形成できる。 The coating layer 3 can be formed on the surface of the silicon particles 2 after the silicon particles 2 are produced.
　被覆層３は、シリコン粒子２と被覆層３になる材料（フッ化リチウム、酸化マグネシウム、リン酸マグネシウム、フッ化マグネシウムからなる群から選択される一種以上の材料）とを反応容器内で同時混合する（例えば、メカノケミカル処理）ことで作製できる。この他、被覆層３になる材料を含む溶液にシリコン粒子２を浸漬する液相法、原子層堆積（ＡＬＤ）法やスパッタリング法等の気相法を用いて、シリコン粒子２の表面を被覆層３でコーティングしてもよい。なお、シリコン粒子の表面に数ナノサイズのシリコン粒子（第２のシリコン粒子）が担持されている場合、第２のシリコン粒子はシリコン粒子２と被覆層３との界面にあることが望ましい。このような構造であった場合、第２のシリコン粒子は、シリコン粒子２と被覆層３との物理的な結合を強化し、電気的な連続性を強める。 The coating layer 3 can be produced by simultaneously mixing the silicon particles 2 and a material for the coating layer 3 (one or more materials selected from the group consisting of lithium fluoride, magnesium oxide, magnesium phosphate, and magnesium fluoride) in a reaction vessel (for example, mechanochemical treatment). In addition, the surface of the silicon particles 2 may be coated with the coating layer 3 using a vapor phase method such as a liquid phase method, an atomic layer deposition (ALD) method, or a sputtering method, in which the silicon particles 2 are immersed in a solution containing a material for the coating layer 3. In addition, when several nano-sized silicon particles (second silicon particles) are carried on the surfaces of the silicon particles, the second silicon particles are preferably present at the interface between the silicon particles 2 and the coating layer 3 . With such a structure, the second silicon particles strengthen the physical bond between the silicon particles 2 and the coating layer 3 and strengthen the electrical continuity.
　第１実施形態にかかる負極活物質は、リチウムイオン二次電池のサイクル特性を向上する。形状が規定されたシリコン粒子は、シリコン粒子が膨張収縮した際に破損しにくい。また電解液とシリコン粒子との接触面積が広いと、電解液とシリコン粒子とが接触することにより生じる副反応を起こしやすくなるが、シリコン粒子の形状を規定することで、電解液とシリコン粒子との過剰な接触を抑制できる。またシリコン粒子の形状を規定することで、負極活物質と導電助剤及びバインダー等との密着が向上する。 The negative electrode active material according to the first embodiment improves the cycle characteristics of lithium ion secondary batteries. Silicon particles with a defined shape are less likely to break when the silicon particles expand and contract. In addition, when the contact area between the electrolyte and the silicon particles is large, side reactions caused by the contact between the electrolyte and the silicon particles are likely to occur, but by defining the shape of the silicon particles, excessive contact between the electrolyte and the silicon particles can be suppressed. Further, by defining the shape of the silicon particles, the adhesion between the negative electrode active material, the conductive aid, the binder, and the like is improved.
　またシリコン粒子の表面に酸化被膜が形成されていると、リチウムイオン二次電池のサイクル特性がさらに向上する。これは、酸化被膜が電解液とシリコン粒子との副反応を抑制し、シリコン粒子の機械的耐性を高めるためと考えられる。 Also, if an oxide film is formed on the surface of the silicon particles, the cycle characteristics of the lithium-ion secondary battery are further improved. This is probably because the oxide film suppresses the side reaction between the electrolyte and the silicon particles and increases the mechanical resistance of the silicon particles.
　またシリコン粒子２の表面に被覆層３が形成されていると、被覆層３が電解液とシリコン粒子２との直接接触を阻害する。その結果、電解液が還元分解する不可逆反応が抑制され、リチウムイオン二次電池のサイクル特性が向上する。 Also, if the coating layer 3 is formed on the surface of the silicon particles 2, the coating layer 3 prevents direct contact between the electrolytic solution and the silicon particles 2. As a result, the irreversible reaction of reductive decomposition of the electrolytic solution is suppressed, and the cycle characteristics of the lithium ion secondary battery are improved.
「リチウムイオン二次電池」
　図４は、第１実施形態にかかるリチウムイオン二次電池の模式図である。図４に示すリチウムイオン二次電池１００は、発電素子４０と外装体５０と電解質（例えば、非水電解液）とを備える。外装体５０は、発電素子４０の周囲を被覆する。発電素子４０は、発電素子４０に接続された一対の端子６０、６２によって外部と接続される。非水電解液は、外装体５０内に収容されている。図４では、外装体５０内に発電素子４０が一つの場合を例示したが、発電素子４０が複数積層されていてもよい。 "Lithium-ion secondary battery"
FIG. 4 is a schematic diagram of a lithium ion secondary battery according to the first embodiment. A lithium-ion secondary battery 100 shown in FIG. 4 includes a power generation element 40, an exterior body 50, and an electrolyte (for example, a non-aqueous electrolyte). The exterior body 50 covers the periphery of the power generation element 40 . The power generation element 40 is connected to the outside through a pair of terminals 60 and 62 connected to the power generation element 40 . A non-aqueous electrolyte is contained in the exterior body 50 . Although FIG. 4 illustrates the case where there is one power generation element 40 in the exterior body 50, a plurality of power generation elements 40 may be stacked.
（発電素子）
　発電素子４０は、セパレータ１０と正極２０と負極３０とを備える。発電素子４０は、これらが積層された積層体でも、これらを積層した構造物を巻回した巻回体でもよい。 (power generation element)
The power generation element 40 includes a separator 10 , a positive electrode 20 and a negative electrode 30 . The power generation element 40 may be a laminate in which these are laminated, or a wound body in which a structure in which these are laminated is wound.
＜正極＞
　正極２０は、例えば、正極集電体２２と正極活物質層２４とを有する。正極活物質層２４は、正極集電体２２の少なくとも一面に接する。 <Positive electrode>
The cathode 20 has, for example, a cathode current collector 22 and a cathode active material layer 24 . The cathode active material layer 24 is in contact with at least one surface of the cathode current collector 22 .
［正極集電体］
　正極集電体２２は、例えば、導電性の板材である。正極集電体２２は、例えば、アルミニウム、銅、ニッケル、チタン、ステンレス等の金属薄板である。重量が軽いアルミニウムは、正極集電体２２に好適に用いられる。正極集電体２２の平均厚みは、例えば、１０μｍ以上３０μｍ以下である。 [Positive collector]
The positive electrode current collector 22 is, for example, a conductive plate. The positive electrode current collector 22 is, for example, a metal thin plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22 . The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.
［正極活物質層］
　正極活物質層２４は、例えば、正極活物質を含む。正極活物質層２４は、必要に応じて、導電助剤、バインダーを含んでもよい。 [Positive electrode active material layer]
The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive aid and a binder as needed.
　正極活物質は、リチウムイオンの吸蔵及び放出、リチウムイオンの脱離及び挿入（インターカレーション）、又は、リチウムイオンとカウンターアニオンのドープ及び脱ドープを可逆的に進行させることが可能な電極活物質を含む。 The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions and counter anions.
　正極活物質は、例えば、複合金属酸化物である。複合金属酸化物は、例えば、コバルト酸リチウム（ＬｉＣｏＯ２）、ニッケル酸リチウム（ＬｉＮｉＯ２）、マンガン酸リチウム（ＬｉＭｎＯ２）、リチウムマンガンスピネル（ＬｉＭｎ２Ｏ４）、及び、一般式：ＬｉＮｉｘＣｏｙＭｎｚＭａＯ２の化合物（一般式中においてｘ＋ｙ＋ｚ＋ａ＝１、０≦ｘ＜１、０≦ｙ＜１、０≦ｚ＜１、０≦ａ＜１、ＭはＡｌ、Ｍｇ、Ｎｂ、Ｔｉ、Ｃｕ、Ｚｎ、Ｃｒより選ばれる１種類以上の元素）、リチウムバナジウム化合物（ＬｉＶ２Ｏ５）、オリビン型ＬｉＭＰＯ４（ただし、Ｍは、Ｃｏ、Ｎｉ、Ｍｎ、Ｆｅ、Ｍｇ、Ｎｂ、Ｔｉ、Ａｌ、Ｚｒより選ばれる１種類以上の元素又はＶＯを示す）、チタン酸リチウム（Ｌｉ４Ｔｉ５Ｏ１２）、ＬｉＮｉｘＣｏｙＡｌｚＯ２（０．９＜ｘ＋ｙ＋ｚ＜１．１）である。正極活物質は、有機物でもよい。例えば、正極活物質は、ポリアセチレン、ポリアニリン、ポリピロール、ポリチオフェン、ポリアセンでもよい。 The positive electrode active material is, for example, a composite metal oxide.複合金属酸化物は、例えば、コバルト酸リチウム（ＬｉＣｏＯ ２ ）、ニッケル酸リチウム（ＬｉＮｉＯ ２ ）、マンガン酸リチウム（ＬｉＭｎＯ ２ ）、リチウムマンガンスピネル（ＬｉＭｎ ２ Ｏ ４ ）、及び、一般式：ＬｉＮｉ ｘ Ｃｏ ｙ Ｍｎ ｚ Ｍ ａ Ｏ ２の化合物（一般式中においてｘ＋ｙ＋ｚ＋ａ＝１、０≦ｘ＜１、０≦ｙ＜１、０≦ｚ＜１、０≦ａ＜１、ＭはＡｌ、Ｍｇ、Ｎｂ、Ｔｉ、Ｃｕ、Ｚｎ、Ｃｒより選ばれる１種類以上の元素）、リチウムバナジウム化合物（ＬｉＶ ２ Ｏ ５ ）、オリビン型ＬｉＭＰＯ ４ （ただし、Ｍは、Ｃｏ、Ｎｉ、Ｍｎ、Ｆｅ、Ｍｇ、Ｎｂ、Ｔｉ、Ａｌ、Ｚｒより選ばれる１種類以上の元素又はＶＯを示す）、チタン酸リチウム（Ｌｉ ４ Ｔｉ ５ Ｏ １２ ）、ＬｉＮｉ ｘ Ｃｏ ｙ Ａｌ ｚ Ｏ ２ （０．９＜ｘ＋ｙ＋ｚ＜１．１）である。 The positive electrode active material may be organic. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene.
　正極活物質は、リチウム非含有の材料でもよい。リチウム非含有の材料は、例えば、ＦｅＦ３、有機導電性物質を含む共役系ポリマー、シェブレル相化合物、遷移金属カルコゲン化物、バナジウム酸化物、ニオブ酸化物等である。リチウム非含有の材料は、いずれか一つの材料のみを用いてもよいし、複数組み合わせて用いてもよい。正極活物質がリチウム非含有の材料の場合は、例えば、最初に放電を行う。放電により正極活物質にリチウムが挿入される。このほか、正極活物質がリチウム非含有の材料に対して、化学的又は電気化学的にリチウムをプレドープしてもよい。 The positive electrode active material may be a non-lithium containing material. Lithium-free materials are, for example, FeF 3 , conjugated polymers containing organic conductive materials, Chevrell phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, and the like. Only one of the lithium-free materials may be used, or a plurality of materials may be used in combination. When the positive electrode active material is a lithium-free material, for example, it is first discharged. Lithium is inserted into the positive electrode active material by discharging. In addition, lithium may be chemically or electrochemically pre-doped into a positive electrode active material that does not contain lithium.
　導電助剤は、正極活物質の間の電子伝導性を高める。導電助剤は、例えば、カーボン粉末、カーボンナノチューブ、炭素材料、金属微粉、炭素材料及び金属微粉の混合物、導電性酸化物である。カーボン粉末は、例えば、カーボンブラック、アセチレンブラック、ケッチェンブラック等である。金属微粉は、例えば、銅、ニッケル、ステンレス、鉄等の粉である。 The conductive aid enhances the electronic conductivity between the positive electrode active materials. Examples of conductive aids include carbon powder, carbon nanotubes, carbon materials, metal fine powders, mixtures of carbon materials and metal fine powders, and conductive oxides. Examples of carbon powder include carbon black, acetylene black, and ketjen black. Metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, or the like.
　正極活物質層２４における導電助剤の含有率は特に限定されない。例えば、正極活物質、導電助剤、バインダーの総質量に対して導電助剤の含有率は、０．５質量％以上２０質量％以下であり、好ましくは１質量％以上５質量％以下である。 The content of the conductive aid in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive aid is 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 5% by mass or less with respect to the total mass of the positive electrode active material, the conductive aid, and the binder.
　正極活物質層２４におけるバインダーは、正極活物質同士を結合する。バインダーは、公知のものを用いることができる。バインダーは、電解液に溶解せず、耐酸化性を有し、接着性を有するものが好ましい。バインダーは、例えば、フッ素樹脂である。バインダーは、例えば、ポリフッ化ビニリデン（ＰＶＤＦ）、ポリビニルアルコール（ＰＶＡ）、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリアミド（ＰＡ）、ポリイミド（ＰＩ）、ポリアミドイミド（ＰＡＩ）、ポリベンゾイミダゾール（ＰＢＩ）、ポリエーテルスルホン（ＰＥＳ）、ポリアクリル酸及びその共重合体、ポリアクリル酸及びその共重合体の金属イオン架橋体、無水マレイン酸をグラフト化したポリプロピレン（ＰＰ）又はポリエチレン（ＰＥ）、これらの混合物である。正極活物質層に用いるバインダーは、ＰＶＤＦが特に好ましい。 The binder in the positive electrode active material layer 24 binds the positive electrode active materials together. A known binder can be used. The binder is preferably insoluble in the electrolytic solution, has oxidation resistance, and has adhesiveness. The binder is, for example, fluororesin. The binder is, for example, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), mixtures thereof. There is. PVDF is particularly preferable as the binder used for the positive electrode active material layer.
　正極活物質層２４におけるバインダーの含有率は特に限定されない。例えば、正極活物質、導電助剤、バインダーの総質量に対してバインダーの含有率は、１質量％以上１５質量％以下であり、好ましくは１．５質量％以上５質量％以下である。バインダーの含有率が少ないと、正極２０の接着強度が弱まる。バインダーの含有率が高いと、バインダーは電気化学的に不活性で放電容量に寄与しないため、リチウムイオン二次電池１００のエネルギー密度が低くなる。 The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content is 1% by mass or more and 15% by mass or less, preferably 1.5% by mass or more and 5% by mass or less with respect to the total mass of the positive electrode active material, the conductive aid, and the binder. When the binder content is low, the adhesive strength of the positive electrode 20 is weakened. If the binder content is high, the binder is electrochemically inactive and does not contribute to the discharge capacity, so the energy density of the lithium ion secondary battery 100 is low.
＜負極＞
　負極３０は、例えば、負極集電体３２と負極活物質層３４とを有する。負極活物質層３４は、負極集電体３２の少なくとも一面に形成されている。 <Negative Electrode>
The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34 . The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32 .
［負極集電体］
　負極集電体３２は、例えば、導電性の板材である。負極集電体３２は、正極集電体２２と同様のものを用いることができる。 [Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate. The negative electrode current collector 32 can be the same as the positive electrode current collector 22 .
［負極活物質層］
　負極活物質層３４は、負極活物質とバインダーとを含む。負極活物質層は、必要に応じて、導電助剤、分散安定剤等を含んでもよい。負極活物質は、上述の負極活物質を用いる。 [Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer may contain a conductive aid, a dispersion stabilizer, and the like, if necessary. The negative electrode active material described above is used as the negative electrode active material.
　導電助剤及びバインダーは、正極２０と同様のものを用いることができる。負極３０におけるバインダーは、正極２０に挙げたものの他に、例えば、セルロース、スチレン・ブタジエンゴム、エチレン・プロピレンゴム、ポリイミド樹脂、ポリアミドイミド樹脂、アクリル樹脂等でもよい。セルロースは、例えば、カルボキシメチルセルロース（ＣＭＣ）でもよい。 The same conductive aid and binder as those used for the positive electrode 20 can be used. The binder in the negative electrode 30 may be, for example, cellulose, styrene/butadiene rubber, ethylene/propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc., in addition to those listed for the positive electrode 20 . The cellulose may be, for example, carboxymethylcellulose (CMC).
＜セパレータ＞
　セパレータ１０は、正極２０と負極３０とに挟まれる。セパレータ１０は、正極２０と負極３０とを隔離し、正極２０と負極３０との短絡を防ぐ。セパレータ１０は、正極２０及び負極３０に沿って面内に広がる。リチウムイオンは、セパレータ１０を通過できる。 <Separator>
Separator 10 is sandwiched between positive electrode 20 and negative electrode 30 . The separator 10 separates the positive electrode 20 and the negative electrode 30 and prevents short circuit between the positive electrode 20 and the negative electrode 30 . The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30 . Lithium ions can pass through the separator 10 .
　セパレータ１０は、例えば、電気絶縁性の多孔質構造を有する。セパレータ１０は、例えば、ポリオレフィンフィルムの単層体、積層体である。セパレータ１０は、ポリエチレンやポリプロピレン等の混合物の延伸膜でもよい。セパレータ１０は、セルロース、ポリエステル、ポリアクリロニトリル、ポリアミド、ポリエチレン及びポリプロピレンからなる群より選択される少なくとも１種の構成材料からなる繊維不織布でもよい。セパレータ１０は、例えば、固体電解質であってもよい。固体電解質は、例えば、高分子固体電解質、酸化物系固体電解質、硫化物系固体電解質である。セパレータ１０は、無機コートセパレータでもよい。無機コートセパレータは、上記のフィルムの表面に、ＰＶＤＦやＣＭＣなど樹脂とアルミナやシリカなどの無機物の混合物を塗布したものである。無機コートセパレータは、耐熱性に優れ、正極から溶出した遷移金属の負極表面への析出を抑制する。 The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of polyolefin films. Separator 10 may be a stretched film of a mixture such as polyethylene or polypropylene. The separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene and polypropylene. Separator 10 may be, for example, a solid electrolyte. Solid electrolytes are polymer solid electrolytes, oxide-based solid electrolytes, and sulfide-based solid electrolytes, for example. Separator 10 may be an inorganic coated separator. The inorganic coated separator is obtained by coating the surface of the above film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic coated separator has excellent heat resistance and suppresses deposition of transition metals eluted from the positive electrode onto the surface of the negative electrode.
＜電解液＞
　電解液は、外装体５０内に封入され、発電素子４０に含浸している。電解液は、液系の電解質に限られず、固体での電解質でもよい。非水電解液は、例えば、非水溶媒と電解塩とを有する。電解塩は、非水溶媒に溶解している。 <Electrolyte>
The electrolytic solution is enclosed in the exterior body 50 and impregnates the power generating element 40 . The electrolytic solution is not limited to a liquid electrolyte, and may be a solid electrolyte. The non-aqueous electrolyte has, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in a non-aqueous solvent.
　溶媒は、一般にリチウムイオン二次電池に用いられている溶媒であれば特に限定はない。溶媒は、例えば、環状カーボネート化合物、鎖状カーボネート化合物、環状エステル化合物、鎖状エステル化合物のいずれかを含む。溶媒は、これらを任意の割合で混合して含んでもよい。環状カーボネート化合物は、例えば、エチレンカーボネート（ＥＣ）、プロピレンカーボネート（ＰＣ）、フルオロエチレンカーボネート、ビニレンカーボネート等である。鎖状カーボネート化合物は、例えば、ジエチルカーボネート（ＤＥＣ）、エチルメチルカーボネート（ＥＭＣ）等である。環状エステル化合物は、例えば、γ－ブチロラクトン等である。鎖状エステル化合物は、例えば、プロピオン酸プロピル、プロピオン酸エチル、酢酸エチル等である。 There are no particular restrictions on the solvent as long as it is a solvent that is generally used in lithium-ion secondary batteries. The solvent includes, for example, any one of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may contain any mixture of these. Cyclic carbonate compounds are, for example, ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, vinylene carbonate and the like. Chain carbonate compounds are, for example, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. Cyclic ester compounds include, for example, γ-butyrolactone. Chain ester compounds are, for example, propyl propionate, ethyl propionate, ethyl acetate and the like.
　電解塩は、例えば、リチウム塩である。電解質は、例えば、ＬｉＰＦ６、ＬｉＣｌＯ４、ＬｉＢＦ４、ＬｉＣＦ３ＳＯ３、ＬｉＣＦ３ＣＦ２ＳＯ３、ＬｉＣ（ＣＦ３ＳＯ２）３、ＬｉＮ（ＣＦ３ＳＯ２）２、ＬｉＮ（ＣＦ３ＣＦ２ＳＯ２）２、ＬｉＮ（ＣＦ３ＳＯ２）（Ｃ４Ｆ９ＳＯ２）、ＬｉＮ（ＣＦ３ＣＦ２ＣＯ）２、ＬｉＢＯＢ、ＬｉＮ（ＦＳＯ２）２等である。リチウム塩は、１種を単独で使用してもよく、２種以上を併用してもよい。電離度の観点から、電解質はＬｉＰＦ６を含むことが好ましい。カーボネート溶媒中の室温における電解塩の乖離度は１０％以上であることが好ましい。 An electrolytic salt is, for example, a lithium salt. The electrolyte is, for example, LiPF6 , LiClO4 , LiBF4 , LiCF3SO3 , LiCF3CF2SO3 , LiC ( CF3SO2)3, LiN(CF3SO2 ) 2 , LiN ( CF3CF2SO2 ) 2 , LiN ( CF3SO2 ) ( C4F9SO2 ), LiN ( CF3CF2CO ) 2 , LiBOB, LiN ( FSO2 ) 2 and the like . Lithium salt may be used individually by 1 type, and may use 2 or more types together. From the point of view of the degree of ionization, the electrolyte preferably contains LiPF6 . The divergence of the electrolytic salt in the carbonate solvent at room temperature is preferably 10% or more.
　電解液は、例えば、カーボネート溶媒にＬｉＰＦ６を溶解させたものが好ましい。ＬｉＰＦ６の濃度は、例えば、１ｍｏｌ／Ｌである。ポリイミド樹脂が芳香族を多く含む場合、ポリイミド樹脂がソフトカーボンのような充電挙動を示すことがある。電解液が、環状カーボネートを含むカーボネート電解液溶媒の場合、均一にポリイミドにリチウムを反応させることができる。この場合、環状カーボネートは、エチレンカーボネート、フルオロエチレンカーボネート、ビニレンカーボネートが好ましい。 The electrolytic solution is preferably, for example, LiPF 6 dissolved in a carbonate solvent. The concentration of LiPF 6 is, for example, 1 mol/L. When the polyimide resin contains a large amount of aromatics, the polyimide resin may exhibit soft carbon-like charging behavior. When the electrolyte is a carbonate electrolyte solvent containing a cyclic carbonate, it is possible to uniformly react lithium with the polyimide. In this case, the cyclic carbonate is preferably ethylene carbonate, fluoroethylene carbonate or vinylene carbonate.
＜外装体＞
　外装体５０は、その内部に発電素子４０及び非水電解液を密封する。外装体５０は、非水電解液の外部への漏出や、外部からのリチウムイオン二次電池１００内部への水分等の侵入等を抑止する。 <Exterior body>
The exterior body 50 seals the power generation element 40 and the non-aqueous electrolyte therein. The exterior body 50 prevents the leakage of the non-aqueous electrolyte to the outside and the intrusion of moisture into the inside of the lithium ion secondary battery 100 from the outside.
　外装体５０は、例えば図４に示すように、金属箔５２と、金属箔５２の各面に積層された樹脂層５４と、を有する。外装体５０は、金属箔５２を高分子膜（樹脂層５４）で両側からコーティングした金属ラミネートフィルムである。 The exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52, for example, as shown in FIG. The exterior body 50 is a metal laminate film in which a metal foil 52 is coated from both sides with polymer films (resin layers 54).
　金属箔５２としては例えばアルミ箔を用いることができる。樹脂層５４には、ポリプロピレン等の高分子膜を利用できる。樹脂層５４を構成する材料は、内側と外側とで異なっていてもよい。例えば、外側の材料としては融点の高い高分子、例えば、ポリエチレンテレフタレート（ＰＥＴ）、ポリアミド（ＰＡ）等を用い、内側の高分子膜の材料としてはポリエチレン（ＰＥ）、ポリプロピレン（ＰＰ）等を用いることができる。 For example, aluminum foil can be used as the metal foil 52 . A polymer film such as polypropylene can be used for the resin layer 54 . The material forming the resin layer 54 may be different between the inner side and the outer side. For example, a polymer with a high melting point such as polyethylene terephthalate (PET) or polyamide (PA) can be used as the outer material, and polyethylene (PE) or polypropylene (PP) can be used as the inner polymer film material.
＜端子＞
　端子６２、６０は、それぞれ正極２０と負極３０とに接続されている。正極２０に接続された端子６２は正極端子であり、負極３０に接続された端子６０は負極端子である。端子６０、６２は、外部との電気的接続を担う。端子６０、６２は、アルミニウム、ニッケル、銅等の導電材料から形成されている。接続方法は、溶接でもネジ止めでもよい。端子６０、６２は短絡を防ぐために、絶縁テープで保護することが好ましい。 <Terminal>
Terminals 62 and 60 are connected to positive electrode 20 and negative electrode 30, respectively. The terminal 62 connected to the positive electrode 20 is a positive terminal, and the terminal 60 connected to the negative electrode 30 is a negative terminal. Terminals 60 and 62 are responsible for electrical connection with the outside. Terminals 60, 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. Terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.
「リチウムイオン二次電池の製造方法」
　リチウムイオン二次電池１００は、負極３０、正極２０、セパレータ１０、電解液、外装体５０をそれぞれ準備し、これらを組み上げて作製される。以下、リチウムイオン二次電池１００の製造方法の一例を説明する。 "Manufacturing method of lithium ion secondary battery"
The lithium ion secondary battery 100 is manufactured by preparing a negative electrode 30, a positive electrode 20, a separator 10, an electrolytic solution, and an outer package 50, and assembling them. An example of a method for manufacturing the lithium ion secondary battery 100 will be described below.
　負極３０は、例えば、スラリー作製工程、電極塗布工程、乾燥工程、圧延工程を順に行って作製される。 The negative electrode 30 is manufactured, for example, by sequentially performing a slurry preparation process, an electrode application process, a drying process, and a rolling process.
　スラリー作製工程は、負極活物質（シリコン粒子）、バインダー、導電助剤及び溶媒を混合してスラリーを作る工程である。負極活物質は、上述の形状が制御されたものを用いる。負極活物質は、シリコン粒子の表面に被覆層が形成されたものでもよい。スラリーに分散安定剤を添加すると、負極活物質の凝集を抑制できる。 The slurry preparation process is a process of mixing a negative electrode active material (silicon particles), a binder, a conductive aid, and a solvent to prepare a slurry. As the negative electrode active material, the one whose shape is controlled as described above is used. The negative electrode active material may be one in which a coating layer is formed on the surface of silicon particles. Aggregation of the negative electrode active material can be suppressed by adding a dispersion stabilizer to the slurry.
　スラリー作製工程は、負極活物質、バインダー、導電助剤及び溶媒を混合してスラリーを作る工程である。溶媒は、例えば、水、Ｎ－メチル－２－ピロリドン等である。負極活物質、導電材、バインダーの構成比率は、質量比で７０ｗｔ％～１００ｗｔ％：０ｗｔ％～１０ｗｔ％：０ｗｔ％～２０ｗｔ％であることが好ましい。これらの質量比は、全体で１００ｗｔ％となるように調整される。スラリー作製時に用いる容器はＳＵＳ等の金属製のものが好ましい。溶媒にＮ－メチル－２－ピロリドン等の極性溶媒を用いた場合、シリコン粒子の表面の酸化被膜の静電容量が大きくなる。極性溶媒は、導電助剤とシリコン粒子の反発を防ぐ。これらの反発を抑えることで、リチウムイオン二次電池の容量低下を防ぐことができる。 The slurry preparation process is a process of mixing a negative electrode active material, a binder, a conductive aid and a solvent to prepare a slurry. Solvents are, for example, water, N-methyl-2-pyrrolidone, and the like. The composition ratio of the negative electrode active material, the conductive material, and the binder is preferably 70 wt % to 100 wt %:0 wt % to 10 wt %:0 wt % to 20 wt %. These mass ratios are adjusted so that the total is 100 wt %. A container made of metal such as SUS is preferable for the container used when preparing the slurry. When a polar solvent such as N-methyl-2-pyrrolidone is used as the solvent, the electrostatic capacity of the oxide film on the surface of the silicon particles increases. The polar solvent prevents repulsion between the conductive aid and silicon particles. By suppressing these repulsions, it is possible to prevent a decrease in the capacity of the lithium ion secondary battery.
　負極活物質は、活物質粒子と導電性材料とをせん断力を加えながら混合し、複合化したものでもよい。活物質粒子が変質しない程度にせん断力を加えて混合すると、活物質粒子の表面が導電性材料で被覆される。また当該混合の程度により負極活物質の粒径を調整できる。また作製後の負極活物質を篩にかけて、粒径をそろえてもよい。 The negative electrode active material may be a composite obtained by mixing active material particles and a conductive material while applying a shearing force. When the active material particles are mixed by applying a shearing force to such an extent that the active material particles are not degraded, the surfaces of the active material particles are coated with the conductive material. In addition, the particle size of the negative electrode active material can be adjusted by the degree of mixing. Further, the negative electrode active material after production may be sieved to make the particle size uniform.
　電極塗布工程は、負極集電体３２の表面に、スラリーを塗布する工程である。スラリーの塗布方法は、特に制限はない。例えば、スリットダイコート法、ドクターブレード法をスラリーの塗布方法として用いることができる。スラリーは、例えば、室温で塗布する。 The electrode application step is a step of applying slurry to the surface of the negative electrode current collector 32 . The slurry application method is not particularly limited. For example, a slit die coating method and a doctor blade method can be used as a slurry coating method. The slurry is applied, for example, at room temperature.
　乾燥工程は、スラリーから溶媒を除去する工程である。例えば、スラリーが塗布された負極集電体３２を、８０℃～３５０℃の雰囲気下で乾燥させる。 The drying process is the process of removing the solvent from the slurry. For example, the slurry-applied negative electrode current collector 32 is dried in an atmosphere of 80.degree. C. to 350.degree.
　圧延工程は、必要に応じて行われる。圧延工程は、負極活物質層３４に圧力を加え、負極活物質層３４の密度を調整する工程である。圧延工程は、例えば、ロールプレス装置等で行われる。 The rolling process is performed as needed. The rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34 . The rolling process is performed by, for example, a roll press device.
　正極２０は、負極３０と同様の手順で作製できる。セパレータ１０及び外装体５０は、市販のものを用いることができる。 The positive electrode 20 can be produced in the same procedure as the negative electrode 30. A commercially available product can be used for the separator 10 and the outer package 50 .
　次いで、作製した正極２０及び負極３０の間にセパレータ１０が位置するようにこれらを積層して、発電素子４０を作製する。発電素子４０が捲回体の場合は、正極２０、負極３０及びセパレータ１０の一端側を軸として、これらを捲回する。 Next, the positive electrode 20 and the negative electrode 30 are laminated so that the separator 10 is positioned between them to produce the power generation element 40 . When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side of the separator.
　最後に、発電素子４０を外装体５０に封入する。非水電解液は外装体５０内に注入する。非水電解液を注入後に減圧、加熱等を行うことで、発電素子４０内に非水電解液が含浸する。熱等を加えて外装体５０を封止することで、リチウムイオン二次電池１００が得られる。なお、外装体５０に電解液を注入するのではなく、発電素子４０を電解液に含浸してもよい。発電素子への注液後は、２４時間静置することが好ましい。 Finally, the power generation element 40 is enclosed in the exterior body 50 . A non-aqueous electrolyte is injected into the exterior body 50 . After injecting the non-aqueous electrolyte, the power generation element 40 is impregnated with the non-aqueous electrolyte by depressurizing, heating, or the like. The lithium ion secondary battery 100 is obtained by applying heat or the like to seal the exterior body 50 . Instead of injecting the electrolytic solution into the exterior body 50, the power generation element 40 may be impregnated with the electrolytic solution. After injecting the liquid into the power generation element, it is preferable to leave it still for 24 hours.
　第１実施形態にかかるリチウムイオン二次電池１００は、負極活物質が所定の形状のシリコン粒子を含むため、サイクル特性に優れる。 The lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics because the negative electrode active material contains silicon particles with a predetermined shape.
　以上、本発明の実施形態について図面を参照して詳述したが、各実施形態における各構成及びそれらの組み合わせ等は一例であり、本発明の趣旨から逸脱しない範囲内で、構成の付加、省略、置換、及びその他の変更が可能である。 As described above, the embodiments of the present invention have been described in detail with reference to the drawings, but each configuration and combination thereof in each embodiment are examples, and addition, omission, replacement, and other modifications of the configuration are possible without departing from the scope of the present invention.
「実施例１」
　厚さ１５μｍのアルミニウム箔の一面に、正極スラリーを塗布した。正極スラリーは、正極活物質と導電助剤とバインダーと溶媒とを混合して作製した。 "Example 1"
A positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm. A positive electrode slurry was prepared by mixing a positive electrode active material, a conductive aid, a binder, and a solvent.
　正極活物質は、ＬｉｘＣｏＯ２を用いた。導電助剤は、アセチレンブラックを用いた。バインダーは、ポリフッ化ビニリデン（ＰＶＤＦ）を用いた。溶媒は、Ｎ－メチル－２－ピロリドンを用いた。９７質量部の正極活物質と、１質量部の導電助剤と、２質量部のバインダーと、７０質量部の溶媒を混合して、正極スラリーを作製した。乾燥後の正極活物質層における正極活物質の担持量は、２５ｍｇ／ｃｍ２とした。正極スラリーから乾燥炉内で溶媒を除去し、正極活物質層を作成した。正極活物質層をロールプレスで加圧し、正極を作製した。 Li x CoO 2 was used as the positive electrode active material. Acetylene black was used as the conductive aid. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. A positive electrode slurry was prepared by mixing 97 parts by mass of a positive electrode active material, 1 part by mass of a conductive aid, 2 parts by mass of a binder, and 70 parts by mass of a solvent. The amount of the positive electrode active material supported in the dried positive electrode active material layer was 25 mg/cm 2 . A positive electrode active material layer was formed by removing the solvent from the positive electrode slurry in a drying oven. The positive electrode active material layer was pressed with a roll press to produce a positive electrode.
　次いで、負極スラリーを作製した。負極スラリーに添加する負極活物質は、平均粒子径が６．２μｍ、平均円形度が０．９５４、平均アスペクト比が０．９１のシリコン粒子を用いた。平均粒子径、平均円形度及び平均アスペクト比は、Malvern Panalytical社製の粒度分布計を用いて、５００００個の粒子を測定して求めた。シリコン粒子は、エージング処理として、２５℃、湿度８０％の環境下に７日間曝露した。曝露後のシリコン粒子の表面には、平均厚み１５ｎｍの酸化シリコン被膜が形成されていた。導電助剤は、カーボンブラックを用いた。バインダーは、ポリイミド樹脂を用いた。溶媒は、Ｎ－メチル－２－ピロリドンを用いた。９０質量部の負極活物質と、５質量部の導電助剤と、５質量部のバインダーとを、Ｎ－メチル－２－ピロリドンに混合して、負極スラリーを作製した。 Next, a negative electrode slurry was prepared. Silicon particles having an average particle size of 6.2 μm, an average circularity of 0.954, and an average aspect ratio of 0.91 were used as the negative electrode active material added to the negative electrode slurry. The average particle size, average circularity and average aspect ratio were obtained by measuring 50000 particles using a particle size distribution meter manufactured by Malvern Panalytical. The silicon particles were exposed to an environment of 25° C. and 80% humidity for 7 days as an aging treatment. A silicon oxide film with an average thickness of 15 nm was formed on the surface of the exposed silicon particles. Carbon black was used as the conductive aid. A polyimide resin was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. N-methyl-2-pyrrolidone was mixed with 90 parts by mass of a negative electrode active material, 5 parts by mass of a conductive aid, and 5 parts by mass of a binder to prepare a negative electrode slurry.
　そして、厚さ１０μｍの銅箔の一面に、負極スラリーを塗布し、乾燥させた。乾燥後の負極活物質層における負極活物質の担持量は、２．５ｍｇ／ｃｍ２とした。負極活物質層は、ロールプレスで加圧した後、窒素雰囲気下、３００℃以上で５時間、焼成した。 Then, the negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm and dried. The amount of the negative electrode active material supported in the dried negative electrode active material layer was 2.5 mg/cm 2 . The negative electrode active material layer was pressed with a roll press and then baked at 300° C. or higher for 5 hours in a nitrogen atmosphere.
　次いで、電解液を作製した。電解液の溶媒は、フルオロエチレンカーボネート（ＦＥＣ）：エチレンカーボネート（ＥＣ）：ジエチルカーボネート（ＤＥＣ）＝１０体積％：２０体積％：７０体積％とした。また電解液には、出力向上用添加剤、ガス抑制添加剤、サイクル特性改善添加剤、安全性能改善添加剤などを添加した。電解塩は、ＬｉＰＦ６を用いた。ＬｉＰＦ６の濃度は１ｍｏｌ／Ｌとした。 Next, an electrolytic solution was prepared. The solvent of the electrolytic solution was fluoroethylene carbonate (FEC):ethylene carbonate (EC):diethyl carbonate (DEC)=10% by volume:20% by volume:70% by volume. In addition, an additive for improving output, an additive for suppressing gas, an additive for improving cycle characteristics, and an additive for improving safety performance were added to the electrolytic solution. LiPF 6 was used as the electrolytic salt. The concentration of LiPF 6 was 1 mol/L.
（評価用リチウムイオン二次電池の作製）
　作製した負極と正極とを、正極活物質層と負極活物質層とが互いに対向するように、セパレータ（多孔質ポリエチレンシート）を介して積層して積層体を得た。この積層体を、アルミラミネートフィルムの外装体内に挿入して周囲の１箇所を除いてヒートシールすることにより閉口部を形成した。そして、最後に、外装体内に上記電解液を注入した後に、残りの１箇所を真空シール機によって減圧しながらヒートシールで密封して、リチウムイオン二次電池を作製した。作製後のリチウムイオン二次電池は、２４時間静置した。 (Production of lithium ion secondary battery for evaluation)
The produced negative electrode and positive electrode were laminated via a separator (porous polyethylene sheet) so that the positive electrode active material layer and the negative electrode active material layer faced each other to obtain a laminate. This laminate was inserted into an outer package made of an aluminum laminate film, and heat-sealed except for one peripheral portion to form a closed portion. Finally, after the electrolytic solution was injected into the exterior body, the remaining one portion was heat-sealed while the pressure was reduced by a vacuum sealer to fabricate a lithium ion secondary battery. The produced lithium-ion secondary battery was allowed to stand for 24 hours.
（３００サイクル後容量維持率の測定）
　リチウムイオン二次電池のサイクル特性を測定した。サイクル特性は、二次電池充放電試験装置（北斗電工株式会社製）を用いて行った。 (Measurement of capacity retention rate after 300 cycles)
Cycle characteristics of lithium ion secondary batteries were measured. Cycle characteristics were measured using a secondary battery charge/discharge test device (manufactured by Hokuto Denko Co., Ltd.).
　充電レート１Ｃ（２５℃で定電流充電を行ったときに１時間で充電終了となる電流値）の定電流充電で電池電圧が４．２Ｖとなるまで充電を行い、放電レート１．０Ｃの定電流放電で電池電圧が２．５Ｖとなるまで放電を行った。充放電終了後の放電容量を検出し、サイクル試験前の電池容量Ｑ１を求めた。電池容量Ｑ１は、３７０５ｍＡｈ／ｇであった。 The battery was charged to a battery voltage of 4.2 V by constant current charging at a charging rate of 1 C (current value at which charging ends in 1 hour when constant current charging is performed at 25° C.), and discharged to a battery voltage of 2.5 V by constant current discharging at a discharge rate of 1.0 C. The discharge capacity after charging/discharging was detected to obtain the battery capacity Q1 before the cycle test. Battery capacity Q1 was 3705 mAh/g.
　上記で電池容量Ｑ１を求めた電池を、再び二次電池充放電試験装置を用い、充電レート１Ｃの定電流充電で電池電圧が４．２Ｖとなるまで充電を行い、放電レート１Ｃの定電流放電で電池電圧が２．５Ｖとなるまで放電を行った。上記充放電を１サイクルとカウントし、３００サイクルの充放電を行った。その後、３００サイクル充放電終了後の放電容量を検出し、３００サイクル後の電池容量Ｑ２を求めた。上記で求めた電池容量Ｑ１、Ｑ２から、３００サイクル後の容量維持率Ｅを求めた。容量維持率Ｅは、Ｅ＝Ｑ２／Ｑ１×１００で求められる。実施例１の容量維持率は、９８％であった。 Using the secondary battery charge/discharge test device again, the battery for which the battery capacity Q 1 was obtained as described above was charged at a constant current charge of 1C until the battery voltage reached 4.2 V, and then discharged at a constant current discharge rate of 1C until the battery voltage reached 2.5 V. The charge/discharge was counted as one cycle, and 300 cycles of charge/discharge were performed. After that, the discharge capacity after 300 cycles of charging and discharging was detected, and the battery capacity Q2 after 300 cycles was obtained. From the battery capacities Q 1 and Q 2 obtained above, the capacity retention rate E after 300 cycles was obtained. The capacity retention rate E is obtained by E=Q 2 /Q 1 ×100. The capacity retention rate of Example 1 was 98%.
　またサイクル特性評価後のリチウムイオン二次電池を分解し、負極活物質層の断面を走査型電子顕微鏡で測定した。画像から求められた負極活物質の平均粒子径、平均アスペクト比は、粒度分布計を用いて測定されたものと大きく差はなかった。 In addition, after the cycle characteristics evaluation, the lithium ion secondary battery was disassembled, and the cross section of the negative electrode active material layer was measured with a scanning electron microscope. The average particle size and average aspect ratio of the negative electrode active material determined from the images were not significantly different from those measured using a particle size distribution meter.
「実施例２～１９」
　実施例２～１９は、負極活物質に用いられるシリコン粒子の平均粒子径、平均円形度、平均アスペクト比及び酸化被膜の厚さを変更した点が実施例１と異なる。シリコン粒子の平均粒子径、平均円形度及び平均アスペクト比は、シリコン粒子を作製する際の溶融条件、冷却条件を変更することで制御した。シリコン粒子の酸化被膜の厚さは、エージング処理の時間を変更することで制御した。 "Examples 2 to 19"
Examples 2 to 19 differ from Example 1 in that the average particle size, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used in the negative electrode active material were changed. The average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles. The thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
「比較例１～６」
　比較例１～６は、負極活物質に用いられるシリコン粒子の平均粒子径、平均円形度、平均アスペクト比及び酸化被膜の厚さを変更した点が実施例１と異なる。シリコン粒子の平均粒子径、平均円形度及び平均アスペクト比は、シリコン粒子を作製する際の溶融条件、冷却条件を変更することで制御した。シリコン粒子の酸化被膜の厚さは、エージング処理の時間を変更することで制御した。 "Comparative Examples 1 to 6"
Comparative Examples 1 to 6 differ from Example 1 in that the average particle diameter, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used in the negative electrode active material were changed. The average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles. The thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
　実施例１～１９及び比較例１～６の結果を以下の表１にまとめた。 The results of Examples 1-19 and Comparative Examples 1-6 are summarized in Table 1 below.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
　実施例１～１９は、比較例１～６と比較して容量維持率が高く、サイクル特性が優れていた。 Examples 1-19 had a higher capacity retention rate and better cycle characteristics than Comparative Examples 1-6.
　比較例１は、シリコン粒子の平均粒子径が小さく、電解液との接触面積が大きい。そのため、比較例１は、シリコンと電解液との不可逆反応（副反応）にリチウムが消費され、容量維持率が低くなったと考えられる。比較例２は、シリコン粒子の平均粒子径が大きい。大きなシリコン粒子は、膨張収縮時に割れやすく、比較例２の容量維持率が低くなったと考えらえる。 In Comparative Example 1, the silicon particles have a small average particle size and a large contact area with the electrolytic solution. Therefore, in Comparative Example 1, lithium was consumed in the irreversible reaction (side reaction) between silicon and the electrolytic solution, and the capacity retention rate was low. Comparative Example 2 has a large average particle size of the silicon particles. Large silicon particles are likely to crack during expansion and contraction, and it is considered that the capacity retention rate of Comparative Example 2 was low.
　比較例３は、平均円形度が低く、膨張収縮時に応力集中が生じやすい。比較例３のシリコン粒子は膨張収縮時に割れやすく、比較例３の容量維持率が低くなったと考えらえる。比較例４は、平均円形度が高く、導電助剤やバインダーとの接触面積が少ない。比較例４のシリコン粒子は、負極活物質の膨張収縮時に、導電助剤やバインダーとの密着を十分確保できず、比較例４の容量維持率が低くなったと考えられる。 Comparative Example 3 has a low average circularity and tends to cause stress concentration during expansion and contraction. It is considered that the silicon particles of Comparative Example 3 were easily cracked during expansion and contraction, and the capacity retention rate of Comparative Example 3 was low. Comparative Example 4 has a high average circularity and a small contact area with the conductive aid and the binder. It is considered that the silicon particles of Comparative Example 4 could not ensure sufficient adhesion to the conductive aid and the binder when the negative electrode active material expanded and contracted, and the capacity retention rate of Comparative Example 4 was low.
　比較例５は、平均アスペクト比が低く、膨張収縮時に応力集中が生じやすい。比較例５のシリコン粒子は膨張収縮時に割れやすく、比較例５の容量維持率が低くなったと考えらえる。比較例６は、平均アスペクト比が１に近く、シリコン粒子の一方向への配向が生じにくい。シリコン粒子が一方向に配向すると、負極活物質層の表面が平坦化し、負極集電体との密着性が向上する。比較例６は、負極活物質層と負極集電体との界面に剥離が生じ、容量維持率が低くなったと考えられる。これは、比較例６は、平均アスペクト比が１に近く、シリコン粒子が一方向に配向しにくいためと考えられる。シリコン粒子が一方向に配向すると、負極活物質層の表面が平坦化し、負極集電体との密着性が向上する。比較例６は、負極活物質層と負極集電体との密着性が十分ではないと考えられる。 Comparative Example 5 has a low average aspect ratio and tends to cause stress concentration during expansion and contraction. It is considered that the silicon particles of Comparative Example 5 were easily cracked during expansion and contraction, and the capacity retention rate of Comparative Example 5 was low. In Comparative Example 6, the average aspect ratio is close to 1, and the silicon particles are less likely to be oriented in one direction. When the silicon particles are oriented in one direction, the surface of the negative electrode active material layer is flattened, and the adhesion to the negative electrode current collector is improved. In Comparative Example 6, peeling occurred at the interface between the negative electrode active material layer and the negative electrode current collector, and the capacity retention ratio was low. This is probably because Comparative Example 6 has an average aspect ratio close to 1 and the silicon particles are difficult to align in one direction. When the silicon particles are oriented in one direction, the surface of the negative electrode active material layer is flattened, and the adhesion to the negative electrode current collector is improved. Comparative Example 6 is considered to have insufficient adhesion between the negative electrode active material layer and the negative electrode current collector.
「実施例２０～３０」
　実施例２０～３０は、負極活物質に用いられるシリコン粒子の平均粒子径、平均円形度、平均アスペクト比及び酸化被膜の厚さを変更した点が実施例１と異なる。シリコン粒子の平均粒子径、平均円形度及び平均アスペクト比は、シリコン粒子を作製する際の溶融条件、冷却条件を変更することで制御した。シリコン粒子の酸化被膜の厚さは、エージング処理の時間を変更することで制御した。 "Examples 20-30"
Examples 20 to 30 differ from Example 1 in that the average particle size, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used for the negative electrode active material were changed. The average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles. The thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
「比較例７～１０」
　実施例７～１０は、負極活物質に用いられるシリコン粒子の平均粒子径、平均円形度、平均アスペクト比及び酸化被膜の厚さを変更した点が実施例１と異なる。シリコン粒子の平均粒子径、平均円形度及び平均アスペクト比は、シリコン粒子を作製する際の溶融条件、冷却条件を変更することで制御した。シリコン粒子の酸化被膜の厚さは、エージング処理の時間を変更することで制御した。 "Comparative Examples 7 to 10"
Examples 7 to 10 differ from Example 1 in that the average particle diameter, average circularity, average aspect ratio, and oxide film thickness of the silicon particles used for the negative electrode active material were changed. The average particle size, average circularity and average aspect ratio of the silicon particles were controlled by changing the melting conditions and cooling conditions when producing the silicon particles. The thickness of the oxide film on the silicon particles was controlled by changing the aging treatment time.
　実施例２０～３０及び比較例７～１０の結果を以下の表２にまとめた。 The results of Examples 20-30 and Comparative Examples 7-10 are summarized in Table 2 below.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　実施例２０～３０は、比較例７～１０と比較して容量維持率が高く、サイクル特性が優れていた。 Examples 20-30 had a higher capacity retention rate and better cycle characteristics than Comparative Examples 7-10.
　比較例７は、シリコン粒子の平均粒子径が小さく、電解液との接触面積が大きい。そのため、比較例７は、シリコンと電解液との不可逆反応（副反応）にリチウムが消費され、容量維持率が低くなったと考えられる。比較例８は、シリコン粒子の平均粒子径が大きい。大きなシリコン粒子は、膨張収縮時に割れやすく、比較例８の容量維持率が低くなったと考えらえる。 In Comparative Example 7, the silicon particles have a small average particle size and a large contact area with the electrolytic solution. Therefore, in Comparative Example 7, lithium was consumed in the irreversible reaction (side reaction) between silicon and the electrolytic solution, and the capacity retention rate was low. Comparative Example 8 has a large average particle size of the silicon particles. Large silicon particles are likely to crack during expansion and contraction, and it is considered that the capacity retention rate of Comparative Example 8 was low.
　比較例９は、平均アスペクト比が低く、膨張収縮時に応力集中が生じやすい。比較例９のシリコン粒子は膨張収縮時に割れやすく、比較例９の容量維持率が低くなったと考えらえる。比較例１０は、シリコン粒子の平均円形度が所定の範囲外であり、シリコン粒子が不定形であると考えられえる。そのため、シリコン粒子の膨張収縮時に特定の部分に応力集中し容量維持率が低くなったと考えられる。 Comparative Example 9 has a low average aspect ratio and tends to cause stress concentration during expansion and contraction. It is considered that the silicon particles of Comparative Example 9 were easily cracked during expansion and contraction, and the capacity retention rate of Comparative Example 9 was low. In Comparative Example 10, the average circularity of the silicon particles is outside the predetermined range, and the silicon particles can be considered to be amorphous. Therefore, it is considered that stress was concentrated on a specific portion during expansion and contraction of the silicon particles, resulting in a low capacity retention rate.
「実施例３１」
　実施例３１は、被覆層が形成されたシリコン粒子を負極活物質とした。被覆層は、エージング処理後のシリコン粒子の表面に形成した。実施例３１において、被覆層を構成する材料はＬｉＦとした。被覆層はシリコン粒子５．４ｇとＬｉＦ粉末０．７２ｇとをメカノケミカル反応装置（ホソカワミクロン株式会社製、製品名：循環型メカノフュージョン（登録商標）　システムＡＭＳ）に投入し、シリコン粒子とＬｉＦ粉末との界面でメカノケミカル反応を生じさせることで作製した。リチウムイオン二次電池のシリコン粒子を除く構成は、実施例１と同様とした。 "Example 31"
In Example 31, silicon particles with a coating layer formed thereon were used as the negative electrode active material. A coating layer was formed on the surface of the silicon particles after the aging treatment. In Example 31, the material constituting the coating layer was LiF. The coating layer was prepared by putting 5.4 g of silicon particles and 0.72 g of LiF powder into a mechanochemical reactor (manufactured by Hosokawa Micron Corporation, product name: Circulating Mechanofusion (registered trademark) System AMS) and causing a mechanochemical reaction at the interface between the silicon particles and LiF powder. The configuration of the lithium ion secondary battery was the same as in Example 1 except for the silicon particles.
「実施例３２～５２」
　実施例３２～５２は、被覆層を構成する材料、負極活物質に用いられるシリコン粒子の平均粒子径、平均円形度及び平均アスペクト比、酸化被膜の厚さ、被覆層の厚さの少なくとも一つを変更した点が実施例３１と異なる。シリコン粒子の平均粒子径は、シリコン粒子を作製する際の条件を変更することで制御した。酸化被膜の厚さは、エージング処理の条件を変更することで制御した。被覆層の厚さは、被覆層を形成する際の条件を変更することで制御した。実施例３２及び５０は被覆層を構成する材料をＭｇ２Ｏとし、実施例３３及び５１は被覆層を構成する材料をＭｇ２Ｆとし、実施例３４及び５２は被覆層を構成する材料をＭｇ３（ＰＯ４）２とした。そして、実施例３２～５２では、実施例１と同様に、放電容量及び容量維持率を求めた。 "Examples 32-52"
Examples 32 to 52 differ from Example 31 in that at least one of the material constituting the coating layer, the average particle size, average circularity and average aspect ratio of the silicon particles used in the negative electrode active material, the thickness of the oxide film, and the thickness of the coating layer was changed. The average particle size of the silicon particles was controlled by changing the conditions for producing the silicon particles. The thickness of the oxide film was controlled by changing the aging treatment conditions. The thickness of the coating layer was controlled by changing the conditions for forming the coating layer. Examples 32 and 50 used Mg 2 O as the material constituting the coating layer, Examples 33 and 51 used Mg 2 F as the material constituting the coating layer, and Examples 34 and 52 used Mg 3 (PO 4 ) 2 as the material constituting the coating layer. Then, in Examples 32 to 52, the discharge capacity and the capacity retention rate were obtained in the same manner as in Example 1.
「比較例１１、１２」
　比較例１１、１２は、負極活物質に用いられるシリコン粒子の平均粒子径、平均円形度、平均アスペクト比、酸化被膜の厚さ、被覆層の厚さの少なくとも一つを変更した点が実施例３２と異なる。そして、比較例１１，１２では、実施例１と同様に、放電容量及び容量維持率を求めた。 "Comparative Examples 11 and 12"
Comparative Examples 11 and 12 differ from Example 32 in that at least one of the average particle diameter, average circularity, average aspect ratio, oxide film thickness, and coating layer thickness of the silicon particles used in the negative electrode active material was changed. Then, in Comparative Examples 11 and 12, similarly to Example 1, the discharge capacity and the capacity retention rate were obtained.
「比較例１３」
　比較例１３は、被覆層を形成しなかった点が実施例３１と異なる。また比較例１３のシリコン粒子の平均粒子径、平均円形度、平均アスペクト比も実施例３１と異なる。そして、比較例１３では、実施例１と同様に、放電容量及び容量維持率を求めた。 "Comparative Example 13"
Comparative Example 13 differs from Example 31 in that no coating layer was formed. The average particle size, average circularity, and average aspect ratio of the silicon particles of Comparative Example 13 are also different from those of Example 31. Then, in Comparative Example 13, the discharge capacity and the capacity retention rate were obtained in the same manner as in Example 1.
　実施例３１～５２及び比較例１１～１３の結果を以下の表３にまとめた。 The results of Examples 31-52 and Comparative Examples 11-13 are summarized in Table 3 below.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　実施例３１～５２は、比較例１１～１３と比較して容量維持率が高く、サイクル特性が優れていた。 Examples 31-52 had a higher capacity retention rate and better cycle characteristics than Comparative Examples 11-13.
　比較例１１は、シリコン粒子の平均粒子径が小さく、電解液との接触面積が大きい。そのため、比較例１１は、シリコンと電解液との不可逆反応（副反応）にリチウムが消費され、容量維持率が低くなったと考えられる。比較例１２は、シリコン粒子の平均粒子径が大きい。大きなシリコン粒子は、膨張収縮時に割れやすく、比較例１２の容量維持率が低くなったと考えらえる。 In Comparative Example 11, the silicon particles have a small average particle size and a large contact area with the electrolytic solution. Therefore, in Comparative Example 11, lithium was consumed in the irreversible reaction (side reaction) between silicon and the electrolytic solution, and the capacity retention rate was low. Comparative Example 12 has a large average particle size of the silicon particles. Large silicon particles are likely to crack during expansion and contraction, and it is considered that the capacity retention rate of Comparative Example 12 was low.
　比較例１３は、平均円形度および平均アスペクト比が低く、応力集中に伴う粒子割れの影響により、容量維持率が低くなったと考えられる。 Comparative Example 13 has a low average circularity and a low average aspect ratio, and is considered to have a low capacity retention rate due to the influence of particle cracking due to stress concentration.
１０　セパレータ
２０　正極
２２　正極集電体
２４　正極活物質層
３０　負極
３２　負極集電体
３４　負極活物質層
４０　発電素子
５０　外装体
５２　金属箔
５４　樹脂層
６０、６２　端子
１００　リチウムイオン二次電池 10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior 52 Metal foil 54 Resin layers 60, 62 Terminal 100 Lithium ion secondary battery

Claims (11)

  1. 　平均粒子径が０．１μｍ以上１０μｍ以下、
    　平均アスペクト比が０．６０以上０．９９以下、
    　平均円形度が０．８０以上０．９９以下、のシリコン粒子を含む、リチウムイオン二次電池用負極活物質。     an average particle size of 0.1 μm or more and 10 μm or less;
    an average aspect ratio of 0.60 or more and 0.99 or less;
    A negative electrode active material for a lithium ion secondary battery, comprising silicon particles having an average circularity of 0.80 or more and 0.99 or less.
  2. 　前記シリコン粒子は、平均円形度が０．８２以上０．９８５以下である、請求項１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, wherein the silicon particles have an average circularity of 0.82 or more and 0.985 or less.
  3. 　前記シリコン粒子は、平均アスペクト比が０．６５以上０．８０以下である、請求項１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, wherein the silicon particles have an average aspect ratio of 0.65 or more and 0.80 or less.
  4. 　前記シリコン粒子は、平均円形度が０．９２０以上０．９８５以下、平均アスペクト比が０．８０以上０．９７以下である、請求項１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, wherein the silicon particles have an average circularity of 0.920 or more and 0.985 or less and an average aspect ratio of 0.80 or more and 0.97 or less.
  5. 　前記シリコン粒子は、平均粒子径が１μｍ以上７μｍ以下である、請求項１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, wherein the silicon particles have an average particle size of 1 µm or more and 7 µm or less.
  6. 　前記シリコン粒子は、平均粒子径が３μｍ以上７μｍ以下である、請求項１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, wherein the silicon particles have an average particle size of 3 µm or more and 7 µm or less.
  7. 　前記シリコン粒子の少なくとも一部を被覆する被覆層をさらに備え、
    　前記被覆層は、フッ化リチウム、酸化マグネシウム、リン酸マグネシウム、フッ化マグネシウムからなる群から選択される一種以上の材料を含む、請求項１に記載のリチウムイオン二次電池用負極活物質。     Further comprising a coating layer that covers at least a portion of the silicon particles,
    2. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein said coating layer contains one or more materials selected from the group consisting of lithium fluoride, magnesium oxide, magnesium phosphate and magnesium fluoride.
  8. 　前記被覆層の平均厚さは、５ｎｍ以上５００ｎｍ以下である、請求項７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 7, wherein the coating layer has an average thickness of 5 nm or more and 500 nm or less.
  9. 　請求項１に記載のリチウムイオン二次電池用負極活物質を含む、リチウムイオン二次電池用負極活物質層。 A negative electrode active material layer for a lithium ion secondary battery, comprising the negative electrode active material for a lithium ion secondary battery according to claim 1.
  10. 　請求項１に記載のリチウムイオン二次電池用負極活物質を含む、リチウムイオン二次電池用負極。 A negative electrode for lithium ion secondary batteries, comprising the negative electrode active material for lithium ion secondary batteries according to claim 1.
  11. 　請求項１０に記載のリチウムイオン二次電池用負極と、正極と、電解質と、を含む、リチウムイオン二次電池。 A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode according to claim 10, a positive electrode, and an electrolyte.
PCT/JP2023/000795 2022-01-21 2023-01-13 Negative electrode active material for lithium ion secondary batteries, negative electrode active material layer for lithium ion secondary batteries, negative electrode for lithium ion secondary batteries, and lithium ion secondary battery WO2023140192A1 (en)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008016446A (en) * 2006-06-09 2008-01-24 Canon Inc Powder material, electrode structure using powder material, power storage device having the electrode structure, and manufacturing method of powder material
JP2012156055A (en) * 2011-01-27 2012-08-16 Toyota Industries Corp Lithium ion secondary battery
WO2016152718A1 (en) * 2015-03-24 2016-09-29 日本電気株式会社 Lithium ion secondary battery
WO2021117567A1 (en) * 2019-12-10 2021-06-17 三菱マテリアル株式会社 Fine silicon particles and production method thereof

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008016446A (en) * 2006-06-09 2008-01-24 Canon Inc Powder material, electrode structure using powder material, power storage device having the electrode structure, and manufacturing method of powder material
JP2012156055A (en) * 2011-01-27 2012-08-16 Toyota Industries Corp Lithium ion secondary battery
WO2016152718A1 (en) * 2015-03-24 2016-09-29 日本電気株式会社 Lithium ion secondary battery
WO2021117567A1 (en) * 2019-12-10 2021-06-17 三菱マテリアル株式会社 Fine silicon particles and production method thereof

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