WO2019203303A1 - Alumina powder and slurry containing the same, alumina porous film and laminated separator comprising the same, and nonaqueous electrolyte secondary battery and production method therefor - Google Patents

Alumina powder and slurry containing the same, alumina porous film and laminated separator comprising the same, and nonaqueous electrolyte secondary battery and production method therefor Download PDF

Info

Publication number
WO2019203303A1
WO2019203303A1 PCT/JP2019/016636 JP2019016636W WO2019203303A1 WO 2019203303 A1 WO2019203303 A1 WO 2019203303A1 JP 2019016636 W JP2019016636 W JP 2019016636W WO 2019203303 A1 WO2019203303 A1 WO 2019203303A1
Authority
WO
WIPO (PCT)
Prior art keywords
alumina
porous film
separator
alumina powder
secondary battery
Prior art date
Application number
PCT/JP2019/016636
Other languages
French (fr)
Inventor
Hirotaka Ozaki
Hiroyuki Ando
Syusaku HARA
Kosuke Kurakane
Original Assignee
Sumitomo Chemical Company, Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sumitomo Chemical Company, Limited filed Critical Sumitomo Chemical Company, Limited
Priority to KR1020207029860A priority Critical patent/KR20210003745A/en
Publication of WO2019203303A1 publication Critical patent/WO2019203303A1/en

Links

Images

Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/411—Organic material
    • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
    • H01M50/417—Polyolefins
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/44—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
    • H01M10/4235—Safety or regulating additives or arrangements in electrodes, separators or electrolyte
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/431—Inorganic material
    • H01M50/434—Ceramics
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
    • H01M50/451—Separators, membranes or diaphragms characterised by the material having a layered structure comprising layers of only organic material and layers containing inorganic material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • H01M50/491—Porosity
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/80—Compositional purity
    • 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
    • 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
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present disclosure relates to an alumina powder which is suitable for forming an alumina porous film.
  • the present disclosure also relates to a slurry containing this alumina powder.
  • the present disclosure further relates to an alumina porous film containing the alumina powder, and a laminated separator comprising the alumina porous film.
  • the present disclosure also relates to a nonaqueous electrolyte secondary battery including a positive electrode, negative electrode or separator comprising an alumina porous film, and a production method therefor.
  • a nonaqueous electrolyte secondary battery especially a lithium ion secondary battery, has been used in household compact equipment such as a cell phone or a personal computer because of having high energy density.
  • application to automobiles has also been accelerated, in addition to the compact equipment.
  • the nonaqueous electrolyte secondary battery is a battery using an organic solvent-based electrolytic solution, and includes a positive electrode and a negative electrode.
  • a conventional nonaqueous electrolyte secondary battery further includes a separator disposed between these electrode plates.
  • a separator in a lithium ion secondary battery for example, a microporous sheet made of a polyolefin-based resin is used.
  • a separator made of the microporous sheet made of a polyolefin-based resin plays a role of holding the safety of the lithium ion secondary battery by a shutdown function of the separator when short circuit occurs between electrodes inside the battery. Specifically, heat generated at the portion where short circuit occurs causes blocking of holes of the separator, which prevents lithium ions from moving. As a result, the battery function at the hole-blocked portion is lost. However, when the temperature of the battery rapidly exceeds 150°C by heat generated momentarily due to the short circuit, the separator may undergo rapid shrinkage to expand the short circuit portion between the positive electrode and the negative electrode. In this case, the battery may fall into an overheated state where the temperature reaches several hundred degrees or higher, leading to a problem in view of safety.
  • Patent Literature 1 JP 2013-168361 A (Patent Literature 1) and WO 2016/098579 A1 (Patent Literature 2) propose an inorganic oxide powder having special protrusions.
  • Patent Literature 2 disclose that an inorganic oxide porous film formed using such inorganic oxide powder can ensure ion permeability since sufficient porosity can be held by special protrusions of the inorganic oxide powder.
  • a slurry containing an alumina powder is usually coated on a target (e.g., base material), followed by drying of the slurry. Since the alumina powder has high hardness, when using an alumina powder having unevenness, a surface of the base material may be abraded (abrasive). Especially, the inorganic oxide powder of Patent Literatures 1 and 2 are highly abrasive because of its shape of having special protrusions, and there was limitation on the coating step.
  • An object of an embodiment of the present invention is to provide an alumina powder which is suitable for forming an alumina porous film having a surface with high chemical stability, and is less abrasive, and an alumina slurry.
  • An object of another embodiment of the present invention is to provide an alumina porous film having a surface with high chemical stability and its usage, and a method for producing a nonaqueous electrolyte secondary battery comprising the alumina porous film having a surface with high chemical stability.
  • Embodiments of the present invention include the following aspects.
  • Aspect 1 of the present invention provides an alumina powder which has an average three-dimensional particle unevenness of 3.5 or less, and a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) is 0.7 or less.
  • Aspect 2 of the present invention provides the alumina powder according to aspect 1, in which an abundance ratio of the number of particles having a sphere equivalent diameter of less than 0.3 ⁇ m is 40% or less.
  • Aspect 3 of the present invention provides the alumina powder according to aspect 1 or 2, which has a nitrogen adsorption BET specific surface area of 1 m 2 /g to 15 m 2 /g.
  • Aspect 4 of the present invention provides the alumina powder according to any one of aspects 1 to 3, which has an average particle diameter of 2.0 ⁇ m or less.
  • Aspect 5 of the present invention provides an alumina slurry including: the alumina powder according to any one of aspects 1 to 4; a binder; and a solvent.
  • Aspect 6 of the present invention provides an alumina porous film including: the alumina powder according to any one of aspects 1 to 4; and a binder.
  • Aspect 7 of the present invention provides a laminated separator including: the alumina porous film according to aspect 6; and a separator, the alumina porous film being provided on a surface of the separator.
  • Aspect 8 of the present invention provides a nonaqueous electrolyte secondary battery comprising the alumina porous film according to aspect 6, wherein the alumina porous film is provided on at least one of surfaces of a positive electrode, a negative electrode and a separator.
  • Aspect 9 of the present invention provides a method for producing a nonaqueous electrolyte secondary battery, the method including: applying the alumina slurry according to aspect 5 on at least one of surfaces of a positive electrode, a negative electrode and a separator; and drying the alumina slurry to form an alumina porous film.
  • the alumina powder of the embodiment of the present invention it is possible to obtain an alumina slurry which is less abrasive, and it is also possible to form an alumina porous film having a surface with high chemical stability.
  • the alumina powder of the embodiment of the present invention it is possible to provide an alumina porous film having a surface with high chemical stability, an alumina slurry which is suitable for producing the alumina porous film and is less abrasive, a laminated separator including the alumina porous film, and a method for producing a nonaqueous electrolyte secondary battery including the alumina porous film.
  • Fig. 1 is a schematic view for explaining three-dimensional particle unevenness.
  • Fig. 2 is a schematic view for explaining a pore diameter inside the coating film and a pore volume inside the coating film.
  • Embodiments of the present invention will be described in detail below.
  • the expression “to” when used for a range of a value, the range includes the upper and lower numerical values thereof.
  • the embodiments of the present invention include embodiments of an alumina powder, an alumina slurry, an alumina porous film, a laminated separator including the alumina porous film, and a nonaqueous electrolyte secondary battery including the alumina porous film. These embodiments will be sequentially described below.
  • An alumina powder according to an embodiment of the present invention is an alumina powder in which an average three-dimensional particle unevenness is 3.5 or less, and a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) is 0.7 or less (hereinafter sometimes referred to as “alumina powder of an embodiment of the present invention” or simply referred to as “alumina powder”).
  • alumina powder of an embodiment of the present invention or simply referred to as “alumina powder”.
  • the average three-dimensional particle unevenness of alumina particles constituting the alumina powder is 3.5 or less. Namely, the shape of alumina particles has relatively small unevenness.
  • the average three-dimensional particle unevenness is preferably 3.0 or less, and more preferably 2.8 or less.
  • the average three-dimensional particle unevenness is preferably 2.0 or more, more preferably 2.2 or more, and particularly more preferably 2.4 or more.
  • three-dimensional particle unevenness is a shape parameter of individual alumina particle constituting the alumina powder.
  • the three-dimensional particle unevenness is by the following equation (1) based on a particle volume V ( ⁇ m 3 ) of a target particle and a volume of a rectangular parallelepiped La ⁇ Lb ⁇ Lc ( ⁇ m 3 ) circumscribing the target particle:
  • Three-dimensional particle unevenness La ⁇ Lb ⁇ Lc/V --- (1)
  • La denotes a major diameter of particle
  • Lb denotes a middle diameter of particle
  • Lc denotes a minor diameter of particle
  • La, Lb and Lc are orthogonal to one another.
  • FIG. 1 shows a schematic view for explaining the three-dimensional particle unevenness.
  • the “average three-dimensional particle unevenness” can be obtained as an index showing a feature of the particle shape by calculating the three-dimensional particle unevenness from 100 or more particles using the equation (1).
  • the “average three-dimensional unevenness” as used herein is an average of the three-dimensional particle unevenness calculated using the equation (1) for 100 or more arbitrary particles.
  • alumina is a relatively hard substance
  • a target e.g., base material
  • the alumina powder may abrade the surface of a base material.
  • the alumina powder of the embodiment of the present invention can suppress the abrasive because of being formed of alumina particles having relatively small unevenness.
  • the three-dimensional images of the target particles can be obtained in the following ordinary manner: a sample for evaluation obtained by curing a particle fixing resin (epoxy resin, etc.) containing a predetermined amount of an alumina powder dispersed therein is cut by FIB processing at predetermined intervals to acquire the predetermined number of continuous cross-sectional SEM images (i.e., continuous slice images) by repeatedly obtaining the cross-sectional SEM images using a scanning electron microscope (SEM), and then the continuous slice images obtained are synthesized while being subjected to position correction.
  • a specific evaluation procedure of the three-dimensional particle unevenness fabricating method of samples for continuous slice images, and calculation method for V, La, Lb and Lc using the three-dimensional quantitative analysis software
  • ⁇ Ratio of (Pyridine Adsorption BET Specific Surface Area)/ (Nitrogen Adsorption BET Specific Surface Area) is 0.7 or less> It is desirable that the alumina powder used in an alumina porous film for a nonaqueous electrolyte secondary battery has low reactivity with an electrolytic solution.
  • a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) (hereinafter sometimes referred to as “ratio of specific surface area”) of the alumina powder of the embodiment of the present invention is 0.7 or less.
  • a large ratio of a specific surface area means that a basic substance such as pyridine is easily adsorbed on the surface of the alumina powder as compared with a neutral substance such as nitrogen. Namely, it is considered that the alumina powder having a large ratio of a specific surface area has a relatively high acidic nature. It is presumed that the alumina powder having a small ratio of a specific surface area of the embodiment of the present invention has a lesser acidic nature and has low reactivity with the electrolytic solution. The embodiment of the present invention is not limited to this presumption.
  • the ratio of a specific surface area of the alumina powder of the embodiment of the present invention is preferably 0.5 or less.
  • the alumina powder according to the embodiment of the present invention further satisfies the following definitions.
  • ⁇ Abundance Ratio of the Number of Particles having Sphere Equivalent Diameter of less than 0.3 ⁇ m is 40% or less>
  • fine particles having a sphere equivalent diameter of less than 0.3 ⁇ m preferably accounts for 40% or less.
  • particle diameter is one of parameters of alumina particles, means a diameter d of a sphere having the volume equal to the particle volume V ( ⁇ m 3 ) of the alumina powder, and is the value satisfying the following equation (2).
  • V 4 ⁇ /3 ⁇ (d/2) 3 --- (2) It is possible to obtain an abundance ratio of the number of particles having a particle diameter of less than 0.3 ⁇ m by calculating “particle diameter” using the above equation (2) for 100 or more particles.
  • pores are formed by a void between alumina particles.
  • Fine alumina particles can intrude between alumina particles having a relatively large particle diameter. Therefore, when a lot of fine alumina particles are included, the void formed between alumina particles having a relatively large particle diameter might be easily blocked by fine alumina particles, thus degrading the function as a porous film.
  • the alumina powder having low content of fine particles is suited to form an alumina porous film.
  • an alumina powder in which the content of fine particles having a sphere equivalent diameter of less than 0.3 ⁇ m is suppressed to as low as 40% or less, and it is possible to improve the porosity (i.e., ion permeability) when the alumina porous film is formed.
  • the abundance ratio of the number of small particles having a sphere equivalent diameter of less than 0.3 ⁇ m is more preferably 35% or less, and particularly preferably 30% or less. Ideally, the abundance ratio of small particles having a sphere equivalent diameter of less than 0.3 ⁇ m is set at 0%, but it is difficult in practice. The abundance ratio of the number of particles is generally 10% or more, and more realistically 15% or more.
  • a nitrogen adsorption BET specific surface area of the alumina powder of the embodiment of the present invention is preferably in a range of 1 m 2 /g to 15 m 2 /g, more preferably 2 to 10 m 2 /g, and most preferably 3 to 8 m 2 /g. If the nitrogen adsorption BET specific surface area is in the above range, the binding property with a binder is improved when an alumina porous film is formed by the method mentioned later, thus obtaining an alumina porous film having high strength.
  • the nitrogen adsorption BET specific surface area is determined by the one-point method in accordance with the method defined in JIS-Z8830 (2013) using a nitrogen gas as an adsorption gas.
  • Preferable purity of alumina constituting the alumina powder of the embodiment of the present invention varies depending on the purposes.
  • Purity of alumina (hereinafter sometimes referred to as alumina purity) means percentage (% by mass) of aluminum oxide when the total content of all components contained in the alumina powder of the embodiment of the present invention is 100% by mass.
  • Alumina purity (% by mass) of the alumina powder is determined by the following calculation formula, assuming that the total of the mass of alumina and the total mass of oxides other than alumina, for example, SiO 2 , Na 2 O, MgO, CuO, Fe 2 O 3 and ZrO 2 is 100 (% by mass).
  • SiO 2 , Na 2 O, MgO, CuO, Fe 2 O 3 and ZrO 2 are defined as impurities with respect to the oxide ( ⁇ alumina) as a basis.
  • Alumina purity (% by mass) 100 - total mass (% by mass) of impurities
  • Each mass of SiO 2 , Na 2 O, MgO, CuO and Fe 2 O 3 , which are impurities, can be determined by converting each content of Si, Na, Mg, Cu and Fe obtained by measuring an evaluation sample through solid-state emission spectroscopy into the mass of the oxide corresponding to each element (SiO 2 , Na 2 O, MgO, CuO, Fe 2 O 3 , ZrO 2 ), whereas the mass of ZrO 2 , which is remaining impurity, can be determined by converting the content of Zr obtained by measuring an evaluation sample through ICP atomic emission spectroscopy into the mass of the oxide corresponding to each element (SiO 2 , Na 2 O, MgO, CuO, Fe 2 O 3 , ZrO 2 ).
  • the purity is preferably set at 90% by mass or more.
  • each content of impurities (oxides of Si, Na, Mg, Cu, Fe, Zr, etc.) in alumina is preferably set at 10% by mass or less. This makes it possible to suppress degradation of electrical insulation properties of the alumina porous film due to impurities and to reduce the mixing amount of a metallic foreign substance which can cause short circuit.
  • the purity of the alumina powder is preferably 90% by mass or more, more preferably 99% by mass or more, and particularly preferably 99.9% by mass or more.
  • ⁇ Average Particle Diameter is 2.0 ⁇ m or less>
  • An average particle diameter of the alumina powder according to the embodiment of the present invention is preferably 2.0 ⁇ m or less, more preferably 1.5 ⁇ m or less, and most preferably 1.0 ⁇ m or less.
  • “Average particle diameter” defined in the present description means a particle diameter D50 equivalent to 50% cumulative percentage by mass measured by a laser diffraction method (median diameter). If the average particle diameter of the alumina powder is 2.0 ⁇ m or less, it is possible to form an alumina porous film having an appropriate thickness, thus making it possible to maintain high energy density when used in a nonaqueous electrolyte secondary battery.
  • the average particle diameter of the alumina powder is preferably 0.1 ⁇ m or more, more preferably 0.2 ⁇ m or more, and most preferably 0.4 ⁇ m or more. If the average particle diameter of the alumina powder is 0.1 ⁇ m or more, it is possible to suppress the amount of binders to be added to bind particles to an appropriate amount when the alumina porous film is formed. Therefore, the ion permeability of the alumina porous film obtained is not easily inhibited by the binders, which makes it possible to enhance charge and discharge characteristic of the battery.
  • the alumina powder according to the embodiment of the present invention can be produced by using an alumina material produced by a known method as a raw material of the alumina powder and crushing the alumina material under appropriate conditions.
  • the alumina material as a raw material of the alumina powder is sometimes referred to as “raw alumina”.
  • surface protective agent refers to one which suppresses over-crushing during crushing by appropriately protecting the surface of the raw alumina.
  • Use of the surface protective agent makes it easy to set an abundance ratio of the number of fine alumina particles (particles having a sphere equivalent diameter of less than 0.3 ⁇ m) in a preferable range (40% or less) defined in the present application.
  • Use of the surface protective agent during crushing makes it possible to rapidly crush the raw alumina to a target particle size.
  • the surface protective agent it is also possible to impart preferable surface characteristic by protecting the surface of the alumina powder after crushing.
  • an appropriate surface protective agent it is possible to decrease the reactivity of the crushed alumina powder with an electrolytic solution. Therefore, use of the surface protective agent during crushing of the alumina powder makes it possible to obtain an alumina powder suitable for an alumina porous film for a nonaqueous electrolyte secondary battery.
  • Examples of the method for producing a raw alumina include a Bayer’s method, an ammonium alum method, an aluminum underwater discharge method, an ammonium aluminum carbonate hydroxide method (AACH method), an organoaluminum hydrolysis method (aluminum alkoxide method) and the like.
  • AACH method ammonium aluminum carbonate hydroxide method
  • aluminum alkoxide method an organoaluminum hydrolysis method
  • a raw alumina can be produced by calcining aluminum hydroxide obtained from bauxite.
  • aluminum hydroxide as a raw material for producing the raw alumina is sometimes referred to as “raw aluminum hydroxide”.
  • a method for producing a raw alumina using the Bayer’s method will be described in detail below.
  • the particle shape and particle size of the raw aluminum hydroxide there is no particular limitation on the particle shape and particle size of the raw aluminum hydroxide, and aluminum hydroxide having an average particle diameter of about 10 ⁇ m to about 100 ⁇ m is usually used. It is possible to use a calcination furnace such as a rotary kiln or a tunnel kiln for calcination of the raw aluminum hydroxide.
  • the calcination temperature of the raw aluminum hydroxide, the temperature rise rate to the calcination temperature, and the calcination time are appropriately selected so as to obtain alumina having desired physical properties.
  • the calcination temperature of the raw aluminum hydroxide is, for example, 1,000°C or higher and 1,450°C or lower, and preferably 1,000°C or higher and 1,350°C or lower.
  • the temperature rise rate when the temperature is raised to this calcination temperature is usually 30°C/hour or more and 500°C/hour or less.
  • the calcination time of the raw aluminum hydroxide is usually 0.5 hour or more and 24 hours or less, and preferably 1 hour or more and 20 hours or less.
  • the raw aluminum hydroxide may be calcined, for example, in an air atmosphere, or an inert gas atmosphere such as a nitrogen gas or argon gas atmosphere, or may be calcined in an atmosphere with high partial water vapor pressure, as in a gas furnace for calcination by combustion of a propane gas.
  • an atmosphere with high partial water vapor pressure the obtained particles are easily densified by the effect of water vapor, unlike calcination in the air atmosphere. In this way, the raw alumina can be produced using the Bayer’s method.
  • the alumina powder is obtained by crushing a raw alumina produced by a known method.
  • the raw alumina can be crushed by a known method using, for example, a vibration mill, a beads mill, a jet mill or the like, and may be crushed in a dry or wet state.
  • an appropriate surface protective agent is used. This makes it possible to easily control the average three-dimensional particle unevenness and the abundance ratio of the number of particles having a sphere equivalent diameter of 0.3 ⁇ m in a preferable range.
  • the crushing time is appropriately adjusted so as not to perform over-crushing. Too long crushing time might cause excessive crushing of alumina particles into fine particles, thus increasing the abundance ratio of the number of particles having a sphere equivalent diameter of 0.3 ⁇ m or less.
  • the surface protective agent is used during crushing.
  • the surface protective agent suitable to restrict the average three-dimensional particle unevenness include monohydric alcohols such as methanol, ethanol, 1-propanol and 2-propanol; glycols such as propylene glycol, polypropylene glycol, ethylene glycol and polyethylene glycol; amines such as triethanolamine; and higher fatty acids such as palmitic acid, stearic acid and oleic acid.
  • These surface protective agent can be used alone, or two or more surface protective agents can also be used in combination.
  • Glycols, especially propylene glycol, ethylene glycol, polypropylene glycol and polyethylene glycol are preferable.
  • the surface protective agent is capable of having the function of not only protecting the surface of the alumina powder after crushing, but also inactivating the surface of the alumina powder.
  • the function of inactivating the surface is capable of decreasing the reactivity between the alumina powder and the electrolytic solution.
  • Examples of such surface protective agent include monohydric alcohols such as methanol, ethanol, 1-propanol and 2-propanol; and glycols such as ethylene glycol, polyethylene glycol, propylene glycol and polypropylene glycol. From the viewpoint of low volatility, glycols, especially ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol are preferable.
  • the surface protective agent is preferably in the form of liquid having an average molecular weight of about 200 to 600.
  • the amount of the surface protective agent to be added is usually 0.01 to 2.0 parts by mass, preferably 0.05 to 1.0 parts by mass, and more preferably 0.1 to 0.5 part by mass, relative to 100 parts by mass of the raw alumina. Excessive amount of the surface protective agent leads to saturation of the effect of the surface protective agent. If the amount is too small, the effect of the surface protective agent is not sufficiently exerted.
  • the obtained alumina particles of the alumina powder do not have special protrusions as disclosed in Patent Literatures 1 and 2.
  • An alumina slurry according to an embodiment of the present invention includes the alumina powder according to the embodiment of the present invention, a binder and a solvent.
  • a binder has the function of binding between alumina particles when an alumina porous film is formed, and bonding between an alumina porous layer and a base material (e.g., separator, negative electrode and/or positive electrode).
  • a known binder can be used and, usually, the binder is mainly composed of an organic substance.
  • binder examples include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and a tetrafluoroethylene-hexafluoropropylene copolymer (FEP); polyacrylic acid derivatives such as polyacrylic acid, methyl polyacrylate, ethyl polyacrylate and hexyl polyacrylate; polymethacrylic acid derivatives such as polymethacrylic acid, methyl polymethacrylate, ethyl polymethacrylate and hexyl polymethacrylate; polyamide, polyimide, polyamideimide, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, a styrene-butadiene rubber, carboxymethyl cellulose (hereinafter referred to as CMC), polyacrylonitrile and derivatives thereof, polyethylene, polypropylene, PVDF
  • tetrafluoroethylene hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid and hexadiene.
  • These copolymers may be used alone, or two or more copolymers may be used as a mixture.
  • solvent Disposable Medium
  • solvents for example, water, alcohol, acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, xylene, cyclohexanone, or mixed solvents thereof.
  • the mixing amount of the respective components constituting the alumina slurry is not particularly limited as long as the alumina slurry can be appropriately coated on a base material or a support to form an alumina porous film having desired characteristics.
  • the mixing amount of the binder can be set at 0.1 to 20 parts by mass and the mixing amount of the solvent can be set at 10 to 500 parts by mass, relative to 100 parts by mass of the alumina powder according to the embodiment of the present invention
  • additives may be further added to the alumina slurry.
  • the additives include dispersants, thickeners, leveling agents, antioxidants, defoamers, pH adjustors containing an acid or alkali, and additives having the function of suppressing the side reaction such as decomposition of an electrolytic solution. These additives are preferably those which are removed during the step of forming an alumina porous film and do not remain in the alumina porous film.
  • each content of the additives is preferably restricted to the amount which does not significantly inhibit characteristics of the alumina slurry and the alumina porous film to be formed therefrom.
  • the content of the additive is preferably 10 parts by mass or less relative to 100 parts by mass of alumina.
  • the alumina slurry containing the alumina powder is coated on a base material such as a positive electrode, a negative electrode or a separator, or a support so as to form an alumina porous film.
  • alumina has a hardness higher than that of the material used as a base material (metal, resin, etc.). If alumina particles have unevenness, the unevenness might damage the surface of the base material to cause abrasion of the base material when the alumina slurry is coated on the base material. Therefore, in the case of the alumina slurry using alumina particles having large unevenness, there is a need to pay enough attention so as not to cause abrasion during coating on the base material.
  • the average three-dimensional particle unevenness of alumina particles contained in the alumina powder of the embodiment of the present invention is as small as 3.5 or less, so that abrasion of the base material due to alumina particles hardly occurs when the alumina slurry is coated on the surface of the base material.
  • the alumina slurry of the embodiment of the present invention can be prepared by mixing the alumina powder of the embodiment of the present invention, a binder and a solvent in a predetermined mixing amount, and dispersing the alumina powder in the solvent by a known method. It is possible to use, as the method of dispersing the alumina powder, a stirring method using a known planetary mixer, or a dispersing method using ultrasonic vibration or a beads mill.
  • An alumina porous film according to an embodiment of the present invention includes the alumina powder of an embodiment of the present invention and a binder. Such alumina porous film can be produced, for example, using the alumina slurry according to the embodiment of the present invention. Since alumina is a substance having high heat resistance and insulation properties, the alumina porous film including the alumina powder also has high heat resistance and insulation properties. Therefore, the alumina porous film of the embodiment of the present invention is suitable for an insulating porous film for protecting a separator used in a secondary battery from high temperature and the like.
  • the alumina porous film is particular suitable for disposing between a negative electrode and a separator.
  • the alumina porous film may be disposed on a surface facing the negative electrode of both surfaces of the separator.
  • the alumina porous film may also be disposed on the surface (one surface or both surfaces) of the negative electrode.
  • the porosity by volume of the alumina porous film is preferably 20 to 90%, and more preferably 30 to 70%. If the alumina porous film has appropriate porosity, it is possible to improve the ion permeability while ensuring the function as the insulating porous film when using in the secondary battery.
  • the average diameter of pores (average pore diameter) of the alumina porous film is preferably 1 ⁇ m or less, and more preferably 0.7 ⁇ m or less.
  • average pore diameter refers to a median value (D50) of a pore diameter. In the present description, it is also referred to as “average pore diameter D50”. If the alumina porous film has appropriate average pore diameter, it is possible to improve the ion permeability while ensuring the function as the insulating porous film when using in the secondary battery.
  • the volume ratio of pores having a pore diameter of 0.2 ⁇ m or less is preferably 30% by volume or less, and more preferably 25% by volume or less.
  • the volume ratio of pores having a pore diameter of more than 0.5 ⁇ m is preferably 10% by volume or more, and more preferably 15% by volume or more.
  • the volume ratio (% by volume) of pores having a pore diameter of 0.2 ⁇ m or less can be determined by “total pore volume of pores having a pore diameter of 0.2 ⁇ m or less”/“total pore volume of all pores inside the coating film” ⁇ 100.
  • the volume ratio (% by volume) of pores having a pore diameter of more than 0.5 ⁇ m can be determined by “total pore volume of pores having a pore diameter of more than 0.5 ⁇ m”/“total pore volume of all pores inside the coating film” ⁇ 100.
  • Air permeability of the alumina porous film is preferably 10 seconds/100 mL ⁇ m or less, and more preferably 9 seconds/100 mL ⁇ m or less, in terms of the Gurley value per 1 ⁇ m of the thickness. If the alumina porous film has small Gurley value (high air permeability), it is possible to further improve the ion permeability when using in the secondary battery.
  • An alumina porous film can be formed by coating an alumina slurry on a base material or a support, and removing a solvent (dispersion medium) in the alumina slurry.
  • a laminated separator according to an embodiment of the present invention comprises the alumina porous film of the embodiment of the present invention on the surface of the embodiment of the separator.
  • the laminated separator according to the embodiment of the present invention includes the above-mentioned alumina porous film, and the separator comprising the alumina porous film on the surface thereof.
  • “separator” broadly means a film for separating a positive electrode and a negative electrode of a battery, and includes a separator used in a secondary battery (e.g., nonaqueous electrolyte secondary battery).
  • the alumina porous film can be laminated on one surface or both surfaces of the separator.
  • the air permeability of the laminated separator is preferably 30 to 1,000 seconds/100 mL, and more preferably 50 to 800 seconds/100 mL, in terms of the Gurley value. If the laminated separator has the above air permeability, it is possible to obtain sufficient ion permeability when the laminated separator is used as a nonaqueous electrolyte secondary battery member.
  • Small Gurley value (high air permeability) of the laminated separator means high porosity of the laminated separator and coarse laminated structure of the laminated separator. Excessively small Gurley value (excessively high air permeability) of the laminated separator might cause a decrease in strength of the separator, leading to insufficient shape stability at particularly high temperature. Meanwhile, excessively large Gurley value (excessively low air permeability) of the laminated separator would make it impossible to obtain sufficient ion permeability when the laminated separator is used as a nonaqueous electrolyte secondary battery member, leading to degradation of battery characteristics of a nonaqueous electrolyte secondary battery.
  • the separator suitable for a laminated separator is generally formed of a porous film made of a resin.
  • the separator suitable or the present application will be described in detail below.
  • a separator is a membranous porous film disposed between a positive electrode and a negative electrode in a secondary battery, a gas or liquid being permeable from one surface to the other surface.
  • the separator includes a large number of pores therein and the pores are connected with each other.
  • the separator can be produced from a porous and membranous base material containing a polyolefin-based resin as a main component (polyolefin-based porous base material). The separator is melted and changed into a non-porous separator when the battery generates heat to impart the shutdown function.
  • the separator may be formed of a single layer or plural layers.
  • the piercing strength of the separator is preferably 3N or more. If the piercing strength is too small, the separator may be broken by positive and negative electrode active material particles leading to short circuit of positive and negative electrodes, during laminating and winding operations of positive and negative electrodes and a separator of the battery assembling process, a clamping operation of the wound group, or application of pressure to the battery from the outside.
  • the piercing strength of the separator (the porous film) is preferably 10N or less, and more preferably 8N or less.
  • the thickness of the separator may be appropriately determined taking the thickness of nonaqueous electrolyte secondary battery member constituting the nonaqueous electrolyte secondary battery into consideration, and is preferably 4 to 40 ⁇ m, more preferably 5 to 30 ⁇ m, and still more preferably 6 to 15 ⁇ m.
  • the porosity by volume of the separator is preferably 20 to 80%, and more preferably 30 to 75%. If the separator has an appropriate porosity, it is possible to retain a larger amount of an electrolytic solution and to reliably prevent (shutdown) a current at a lower temperature when an excessively large current flows.
  • the average diameter (average pore diameter) of pores of the separator is preferably 0.3 ⁇ m or less, and more preferably 0.05 ⁇ m or more and 0.14 ⁇ m or less.
  • “average pore diameter” refers to a median value (D50) of the pore diameter. In the present description, it is also referred to as “average pore diameter D50”.
  • the separator has an appropriate average pore diameter, it is possible to not only sufficiently maintain the ion permeability of the separator, but also suppress various particles (e.g., active material particles eluted and precipitated from an electrode plate in the charge and discharge process of the secondary battery, and alumina particles which were fallen off from the alumina porous film) from entering into pores of the separator.
  • various particles e.g., active material particles eluted and precipitated from an electrode plate in the charge and discharge process of the secondary battery, and alumina particles which were fallen off from the alumina porous film
  • the proportion of the polyolefin-based component in the separator is usually 50% by volume or more, preferably 90% by volume or more, and more preferably 95% by volume or more, relative to the entire porous film. It is preferable that a high-molecular weight component having a weight average molecular weight of 5 ⁇ 10 5 to 15 ⁇ 10 6 is included in the polyolefin-based component in the separator. It is particularly preferable that the strength of the separator increases when a polyolefin component having a weight average molecular weight of 1,000,000 or more is contained.
  • the polyolefin-based resin suitable for the separator examples include a high-molecular weight homopolymer or copolymer obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene and 1-hexene.
  • the separator can include a single layer (resin single-layer film) or a plurality of layers (resin multi-layer film) of a resin film of a polyolefin-based resin.
  • the separator made of the resin multi-layer film include those in which a single layer of a polyethylene film (PE film) is disposed between two layers of a polypropylene film (PP film) and those in which three layers are laminated (PP/PE/PP laminated film).
  • a high-molecular weight polyethylene composed mainly of ethylene is particularly preferable.
  • the separator may contain components other than polyolefin as long as the function of the layer is not impaired.
  • the air permeability of the separator is usually in a range of 30 to 500 seconds/100 mL, and preferably 50 to 300 seconds/100 mL, in terms of the Gurley value. If the separator has appropriate air permeability, sufficient ion permeability can be obtained.
  • the basis weight of the separator is usually 4 to 20 g/m 2 , preferably 4 to 12 g/m 2 , and more preferably 5 to 10 g/m 2 , in view of the fact that the strength, thickness, handleability and weight, and weight energy density and volume energy density of the battery can be increased when using as a separator of a nonaqueous electrolyte secondary battery.
  • a laminated separator can be produced by forming an alumina porous film on the surface of a separator using an alumina slurry.
  • An example of a method for producing a laminated separator includes a method in which an alumina slurry is directly coated on the surface of a separator and then a solvent (dispersion medium) of the alumina slurry is removed.
  • Another example of a method for producing a laminated separator includes a method in which an alumina slurry is coated on the surface of a support and a solvent (dispersion medium) of the alumina slurry is removed to form an alumina porous film, and then this alumina porous film is compression-bonded to a separator and the support is peeled off from the alumina porous film.
  • Still another example of a method for producing a laminated separator includes a method in which an alumina slurry is coated on the surface of a support and a separator is compression-bonded to the coated surface to transfer the alumina slurry to the separator, and then a solvent (dispersion medium) of the alumina slurry is removed.
  • a further example of a method for producing a laminated separator includes a method in which a separator is immersed in an alumina slurry to perform dip coating, and then a solvent (dispersion medium) of the alumina slurry is removed.
  • an alumina porous film may be sequentially formed on the other surface of a separator after forming an alumina porous film on one surface thereof.
  • the alumina porous film may be simultaneously formed on both surfaces of the separator.
  • the surface of the separator may be optionally subjected to a hydrophilization treatment.
  • a separator can be prepared by the following method.
  • a separator containing a polyolefin-based resin as a main component is preferably produced, for example, by the following method when the porous film contains an ultrahigh-molecular weight polyolefin and a low-molecular weight hydrocarbon having a weight average molecular weight of 10,000 or less.
  • the separator by the method including the step (1) of kneading an ultrahigh-molecular weight polyolefin, a low-molecular weight hydrocarbon having a weight average molecular weight of 10,000 or less, and a pore-forming agent to obtain a polyolefin resin composition, the step (2) of rolling the polyolefin resin composition using a rolling roll to form a sheet (rolling step), the step (3) of removing the pore-forming agent from the sheet obtained in the step (2), and the step (4) of drawing the sheet obtained in the step (3) to obtain a porous film (separator). Before the operation of removing the pore-forming agent in the sheet in the step (3), the operation of drawing the sheet in the step (4) may be performed.
  • the low-molecular weight hydrocarbon examples include a low-molecular weight polyolefin such as polyolefin wax, and a low-molecular weight polymethylene such as Fischer-Tropsch wax.
  • the weight average molecular weight of the low-molecular weight polyolefin and low-molecular weight polymethylene is preferably 200 or more and 3,000 or less. If the weight average molecular weight is 200 or more, there is no fear of evaporation during formation of a porous film. It is preferable that the weight average molecular weight is 3,000 or less since mixing with the ultrahigh-molecular weight polyolefin is performed more uniformly.
  • Examples of the pore-forming agent include an inorganic filler and a plasticizer.
  • Examples of the inorganic filler include an inorganic filler which can be dissolved in an aqueous solvent containing an acid, an aqueous solvent containing an alkali, or an aqueous solvent composed mainly of water.
  • Examples of the inorganic filler which can be dissolved in the aqueous solvent containing an acid, include calcium carbonate, magnesium carbonate, barium carbonate, zinc oxide, calcium oxide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and calcium sulfate, and calcium carbonate is preferable in view of the fact that it is inexpensive and is easy to obtain a fine powder.
  • Examples of the inorganic filler, which can be dissolved in the aqueous solvent containing an alkali include silicic acid and zinc oxide, and silicic acid is preferable since it is inexpensive and is easy to obtain a fine powder.
  • Examples of the inorganic filler, which can be dissolved in the aqueous solvent composed mainly of water include calcium chloride, sodium chloride and magnesium sulfate.
  • plasticizer examples include liquid paraffin, and a low-molecular weight nonvolatile hydrocarbon compound such as mineral oil.
  • a nonaqueous electrolyte secondary battery comprises an electrode group including a positive electrode, a negative electrode and a separator; and a nonaqueous electrolytic solution; and further comprises an alumina porous film of an embodiment of the present invention on at least one surface of the positive electrode, the negative electrode and the separator.
  • Examples of the form of the electrode group suitable for the nonaqueous electrolyte secondary battery of the embodiment of the present invention include an electrode group (laminated type electrode group) formed by laminating a positive electrode, a negative electrode and a separator, or an electrode group (wound type electrode group) formed by laminating a positive electrode, a negative electrode and a separator and then winding the laminate.
  • the alumina porous film to be used has a high porosity and a high proportion of pores having a large pore diameter (e.g., a volume ratio of pores having a pore diameter of more than 0.5 ⁇ m), the lithium ion permeability is excellent and also the rectification effect of lithium ions becomes satisfactory. It is considered that an alumina powder having a small proportion of fine alumina particles having a sphere equivalent diameter of less than 0.3 ⁇ m enables the formation of an alumina porous film having a high porosity and a high proportion of pores having a large pore diameter. Use of such alumina porous film enables an improvement in battery characteristics of the nonaqueous electrolyte secondary battery.
  • a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) is 0.7 or less.
  • a nonaqueous electrolyte secondary battery of the embodiment of the present invention includes the step of forming an alumina porous film on at least one surface of electrode plates (positive electrode, negative electrode) and a separator. Specific one embodiment includes the step of coating the above alumina slurry on at least one surface of a positive electrode, a negative electrode and a separator and drying the alumina slurry to form an alumina porous film.
  • Common positive and negative electrodes have an electrode mixture layer containing an electrode active material (a positive electrode active material or a negative electrode active material) and a binder on the surface thereof. When the alumina slurry is coated on an electrode plate (a positive electrode or a negative electrode), the alumina slurry can be coated on the electrode mixture layer included in those electrodes.
  • An electrode group including a positive electrode, a negative electrode and a separator, and an alumina porous film formed on the surface of any one of the positive electrode, the negative electrode and the separator are housed in a container for a battery (e.g., battery can) and a nonaqueous electrolytic solution is injected into the container, thus obtaining a nonaqueous electrolyte secondary battery.
  • a battery e.g., battery can
  • a nonaqueous electrolyte secondary battery using a wound type electrode group including an negative electrode having an alumina porous film formed on the surface a more specific production method will be described.
  • an electrode mixture layer containing a negative electrode active material is formed on the surface of the negative electrode.
  • An alumina slurry is coated on this electrode mixture layer and the alumina slurry is dried, thus forming an alumina porous film on the surface of the negative electrode.
  • One end of a negative electrode lead is connected to a lead joining portion of the negative electrode, and one end of a positive electrode lead is connected to a lead joining portion of the positive electrode, respectively.
  • the positive electrode and the negative electrode are laminated via a separator and the laminate is wound to constitute a wound type electrode group.
  • This electrode group is housed in a battery can in a state where an insulating ring is disposed at the upper and lower portions of this wound type electrode group, followed by injection of an electrolytic solution in the battery can and further sealing of the opening of the battery can with a battery cap.
  • the thickness of the alumina porous film by adjusting the thickness of a coating film in a wet state after coating, the weight ratio of a resin to microparticles, the solid component concentration of the alumina slurry (sum of the concentration of the resin and the concentration of microparticles) and the like. It is possible to use, as the support, for example, a film made of a resin, a belt made of metal, or a drum.
  • the method for coating the alumina slurry to an electrode plate, a separator or a support may be a method capable of realizing required basis weight and coated area and is not particularly limited.
  • a conventionally known method can be employed as the method for coating the alumina slurry.
  • Such method include a gravure coater method, a small-diameter gravure coater method, a reverse roll coater method, a transfer roll coater method, a kiss coater method, a dip coater method, a knife coater method, an air doctor blade coater method, a blade coater method, a rod coater method, a squeeze coater method, a cast coater method, a bar coater method, a die coater method, a screen printing method and a spray coating method.
  • a gravure coater method a small-diameter gravure coater method, a reverse roll coater method, a transfer roll coater method, a kiss coater method, a dip coater method, a knife coater method, an air doctor blade coater method, a blade coater method, a rod coater method, a squeeze coater method, a cast coater method, a bar coater method, a die coater method, a screen printing method and a spray coating method.
  • the method for removing a solvent (dispersion medium) is generally a method by drying.
  • the drying method include a natural drying method, a fan drying method, a heat drying method and a vacuum drying method, and may be any method as long as the solvent (dispersion medium) can be sufficiently removed.
  • a usual drying device can be used for drying.
  • heating is performed to remove the solvent (dispersion medium) from the coating film of the alumina slurry formed on the separator, heating is performed at a temperature at which the air permeability of the separator is not degraded, specifically 10 to 120°C, and more preferably 20 to 80°C, in order to avoid degradation of the air permeability due to shrinkage of pores of the separator.
  • the thickness of the alumina porous film formed by the above-mentioned method is preferably 0.5 to 15 ⁇ m (per one surface), more preferably 2 to 10 ⁇ m (per one surface), and still more preferably 2 to 5 ⁇ m (per one surface), when a separator is used as a base material and an alumina porous film is laminated on one or both surfaces of the separator to form a laminated separator.
  • the thickness of the alumina porous film is preferably 1 ⁇ m or more (0.5 ⁇ m or more on one surface) since it is possible to sufficiently prevent internal short circuit due to breakage of a battery in a laminated separator comprising an alumina porous film and to maintain the retaining amount of an electrolytic solution in the alumina porous film.
  • the thickness of the alumina porous film is preferably 30 ⁇ m or less (15 ⁇ m or less on one surface) in total of both surfaces since it is possible to suppress an increase in permeation resistance of ions such as lithium ions over the entire laminated separator comprising an alumina porous film to thereby prevent degradation of a positive electrode and degradation of rate characteristic and cycle characteristic when charge and discharge cycle is repeated, and to suppress an increase in distance between a positive electrode and a negative electrode to thereby prevent an increase in size of a nonaqueous electrolyte secondary battery.
  • the physical properties when a porous layer is laminated on both surfaces of a porous film, the physical properties at least indicate physical properties of the alumina porous film laminated to the surface opposite to a positive electrode in the porous film when a secondary battery is assembled.
  • the basis weight per unit area (per one surface) of the alumina porous film may be appropriately determined taking the strength, thickness, weight, and handleability of the laminated separator into account.
  • the basis weight is usually 1 to 20 g/m 2 , preferably 4 to 15 g/m 2 , and more preferably 4 to 12 g/m 2 .
  • the basis weight of the alumina porous film is preferably in the above range since it is possible to increase the weight energy density and volume energy density of a nonaqueous electrolyte secondary battery including the laminated separator comprising the alumina porous film as the member to thereby reduce the weight of the battery.
  • Al hydroxide obtained by the Bayer’s method was calcined in a gas furnace to obtain a raw alumina having a BET specific surface area of 3.7 m 2 /g and an average particle diameter of 54 ⁇ m.
  • a surface protective agent propylene glycol
  • Allumina Powder No. P2 Aluminum hydroxide obtained by the Bayer’s method was calcined in a gas furnace to obtain a raw alumina having a BET specific surface area of 4.2 m 2 /g and an average particle diameter of 52 ⁇ m. To the raw alumina, 0.2% by mass of a surface protective agent (propylene glycol) was added, followed by mixing and further crushing using the following jet mill to obtain an alumina powder.
  • Jet Mill - Apparatus: PJM-280SP, manufactured by Nippon Pneumatic Kogyo - Feed rate of raw alumina powder: 10 kg/h - Gauge pressure of air supply port during crushing: 0.7 MPa
  • the cured sample was fixed to a sample stage for SEM and then a Pt-Pd thin film was vacuum-deposited on the surface of the sample by the vacuum deposition method.
  • the sample with the vacuum-deposited thin film was set to FIB-SEM (HELIOS 600, manufactured by FEI) and processed by focused ion beam (FIB) of gallium accelerated at an accelerating voltage of 30 kV to form a hole of 44.2 ⁇ m or more in depth, and then a cross-section in arbitrary direction was exposed as the first observing surface. Because of three-dimensional measurement, the same results are obtained regardless of the cross-sectional direction.
  • the first observation surface was observed by SEM at an acceleration voltage of 2.1 kV.
  • the range to be observed was set at 51.2 ⁇ m ⁇ 44.2 ⁇ m.
  • the FIB processing was again applied thereto with a thickness of 50 nm in a depth direction of the sample to newly expose an observation surface (second observation surface).
  • the second observation surface was observed by SEM in the same manner as in the first observation surface.
  • the FIB processing and SEM observation were repeated to acquire a series of SEM images with respect to the observation surfaces arranged in parallel at intervals of 50 nm in the sample.
  • the scale of the SEM observation was set at 50 nm/pix for X-axis and Y-axis, and 50 nm/pix for Z-axis.
  • X-axis is an axis parallel to the sample surface in the SEM observation surface
  • Y-axis is an axis in the depth direction orthogonal to the sample surface in the SEM observation surface
  • XY plane is the SEM observation surface
  • Z-axis is an axis in the FIB cutting direction orthogonal to the SEM observation surface in the sample surface.
  • measuring area The area to be subjected to three-dimensional quantitative analysis (this is referred to as “measuring area”) was set at a portion of the above-mentioned observation portion (51.2 ⁇ m ⁇ 44.2 ⁇ m ⁇ 25 ⁇ m). The measuring area had a size of about 48 ⁇ m ⁇ about 20 ⁇ m ⁇ about 20 ⁇ m. Particles passing through the boundary surface of the measuring area were removed from measurement target.
  • a particle volume V, a major diameter La of particle, a middle diameter Lb of particle and a minor diameter Lc of particle were determined for each of measured particles, and then a sphere equivalent diameter d and a three-dimensional particle unevenness were respectively calculated by substituting these values in the following equations (1) and (2).
  • An average of the three-dimensional particle unevenness obtained in calculation was determined and shown in Table 2.
  • An abundance ratio of the number of particles having a sphere equivalent diameter d of less than 0.3 ⁇ m was shown in Table 2.
  • the number of particles in the measuring area is about 1,000 to 2,000, and the average was determined by measuring all particles.
  • the obtained pyridine adsorption BET specific surface area is divided by the nitrogen adsorption BET specific surface area obtained by the above method to determine “ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area)”.
  • a polishing machine (Model 5627-56) and a sample rotating machine (Model 7705-2) manufactured by Marumoto Kogyo Co., Ltd.
  • a ferrite chip (6 mm ⁇ 1.5 mm ⁇ 12 mm) was polished while feeding an alumina dispersed slurry onto a buffing cloth (Model NO-101, manufactured by Marumoto Kogyo Co., Ltd., 250 mm ⁇ ), and then a difference in thickness ( ⁇ m) before and after polishing was regarded as a ferrite polishing rate.
  • the alumina dispersed slurry was fabricated by ultrasonic dispersion of 35 g of an alumina powder in 1,750 mL of a 0.2% by mass aqueous sodium hexametaphosphate solution for 30 minutes. Polishing conditions are as follows. - Alumina dispersed slurry dropping rate: 15 g/min - Ferrite chip polishing area: 6 mm ⁇ 12 mm (72 mm 2 ) - Buff rotation speed: 400 rpm - Load: 400 g - Polishing time: 60 min
  • an alumina powder and 2 mg of an electrolytic solution were sealed into an aluminum laminate bag in a glove box maintained at a dew point of -30°C or lower, and then the mass of the aluminum laminate bag after sealing was measured.
  • the specific gravity and volume of the aluminum laminate bag after sealing were measured by the Archimedes method and then the aluminum laminate bag was subjected to a heat treatment at 85°C for 72 hours. After the heat treatment, the specific gravity and volume of the aluminum laminate bag were measured by the Archimedes method, and a change in volume before and after the heat treatment was calculated as the amount of gas generated.
  • the alumina powder having physical property values in Table 2 within the following range respectively was evaluated as “Good”. In Table 2, the value deviated from the following rage was underlined.
  • “average three-dimensional particle unevenness” and “ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area)” indicate the physical property value indispensable for the alumina powder according to the embodiment of the present invention, and other physical property values indicate preferable physical property values in the alumina powder according to the embodiment of the present invention.
  • alumina porous films F1 to F4 were respectively formed.
  • An alumina porous film was formed on the surface of a base material (separator).
  • a base material As the base material (separator), a porous film made of polyethylene (thickness: 12.4 ⁇ m, basis weight: 7.5 g/m 2 , air permeability (Gurley value): 184 seconds/100 mL) was used.
  • an alumina porous film was formed on the treated surface by the following method.
  • the slurry On the surface of the base material pretreated by corona discharge irradiation, the slurry was gravure-coated using a tabletop test coater manufactured by Yasui Seiki Inc. The slurry thus coated was dried at a drying temperature of 50°C to form an alumina porous film. Using a laminated porous film in which the alumina porous film is formed on the surface of the base materials (laminated separator), physical properties of the alumina porous film were measured.
  • the thickness of a laminated porous film was measured by a high-accuracy digital measuring instrument “VL-50A” manufactured by Mitutoyo Corporation. By subtracting the thickness (12.4 ⁇ m) of a base material porous film from the thickness of the measured laminated porous film, the thickness of the alumina porous film was calculated.
  • a laminated porous film was impregnated with an epoxy resin, followed by curing of the epoxy resin. After a cured sample was fixed to a sample stage, the FIB processing was applied to the sample using FIB-SEM (HELIOS 600, manufactured by FEI) to remove the epoxy resin on the top surface to thereby expose the surface of the alumina porous film. Then, the surface (surface of the alumina porous film) was observed by SEM at an acceleration voltage of 2.1 kV.
  • the FIB processing was again applied thereto with a thickness of 20 nm in the depth direction (in the thickness direction of the alumina porous film) of the sample to newly form a cross-section, and the cross-section was observed by SEM.
  • the FIB processing and SEM observation of the cross-section were repeated at intervals of 20 nm to acquire continuous slice images including the entire thickness of the alumina porous film, and then continuous slice images were obtained by the image analysis software [Avizo ver. 6.0, manufactured by Visualization Sciences Group].
  • the scale of the SEM observation was set at 19.2 nm/pix for X-axis and Y-axis, and 20 nm/pix for Z-axis.
  • X-axis and Y-axis are axes orthogonal to each other on the surface parallel to the surface of the alumina porous film
  • Z-axis is an axis which is orthogonal to X-axis and Y-axis and is in the thickness direction of the alumina porous film.
  • Three-dimensional analysis of the coating film was applied to the acquired continuous slice images using the quantitative analysis software TRI/3D-BON-FCS (version: BON-FCS R10.01.10.29-H-64: manufactured by RATOC SYSTEM ENGINEERING CO., LTD.) to calculate a pore diameter inside the coating film, a pore volume and a porosity.
  • TRI/3D-BON-FCS version: BON-FCS R10.01.10.29-H-64: manufactured by RATOC SYSTEM ENGINEERING CO., LTD.
  • the minor diameter Thickness, the major diameter Width, the distance between branching points Length, the total volume BV of the void portion and the total volume TV of an analysis region were determined to define pores inside the coating film, and then a pore diameter inside coating film, a pore volume and a porosity were calculated by the following methods.
  • Minor diameter Thickness and “major diameter Width” mean a minor diameter and a major diameter in a cross-section of pores, respectively.
  • Distance between branching points Length means a distance between adjacent branching points.
  • Branching point means a point where two (or more) thin lines intersect when each pore is represented by a thin line.
  • a pore diameter inside the coating film was determined from the following equation (3).
  • the equation (3) is for determination of an average diameter of pores (pore diameter) and means that the sum of the minor diameter (Thickness) and the major diameter (Width) of pores shown in FIG. 2 is divided by 2.
  • Pore diameter inside coating film (Thickness + Width)/2 --- (3)
  • a pore volume inside the coating film was determined by the following equations (4) and (5).
  • the equation (4) is for calculation of a cross-sectional area (CS) of pores from a minor diameter (Thickness) and a major diameter (Width).
  • the equation (5) is for calculation of a pore volume from the obtained cross-sectional area and length of pores (distance between branching points (Length)).
  • CS (Thickness/2) ⁇ (Width/2) ⁇ ⁇ --- (4)
  • Pore volume inside coating film CS ⁇ Length --- (5)
  • “Porosity of Coating Film (Alumina Porous Film)” “Porosity” in the embodiment of the present invention is a parameter indicating voids inside an alumina porous film and can be determined by three-dimensional analysis of the alumina porous film in an analysis area. By using three-dimensional analysis, he particle portion and the void portion in the alumina porous film are identified by binarizing to obtain the total volume (BV) of the void portion.
  • the porosity is defined by the following equation (6) which indicates that the total volume (BV) of the void portion is dividing by the total volume (TV) of the analysis area.
  • Porosity (% by volume) BV/TV ⁇ 100 --- (6)
  • Pore distribution inside the coating film was determined from the obtained pore diameter and pore volume inside the coating film, and then a volume ratio of pores having a pore diameter of 0.2 ⁇ m or less (which means “total pore volume of pores having a pore diameter of 0.2 ⁇ m or less”/“total pore volume of pores inside the coating film” ⁇ 100 (% by volume)) and a volume ratio of pores having a diameter of more than 0.5 ⁇ m (which means “total pore volume of pores having a pore diameter of more than 0.5 ⁇ m”/ “total pore volume of pores inside the coating film” ⁇ 100 (% by volume)) were calculated.
  • This pore distribution inside the coating film was determined by setting the area of 17.6 ⁇ m ⁇ 11.3 ⁇ m ⁇ (observation depth) ⁇ m as the measuring range.
  • the observation depth was determined according to the thickness of each sample and was found to be appropriately 3 to 5 ⁇ m (approximately 600 to 1,000 ⁇ m 3 in terms of volume in the observation area).
  • An average pore diameter D50 was determined from pore distribution inside the coating film (alumina porous film).
  • Air Permeability of Alumina Porous Film In accordance with JIS P8117 (2009), air permeability X (Gurley value) (seconds/100 mL) was measured by a Gurley type densometer manufactured by Toyo Seiki Seisaku-Sho, Ltd.
  • Air permeability per 1 ⁇ m of the thickness (Gurley value per 1 ⁇ m of the thickness) (seconds/100 mL ⁇ m) of an alumina porous film was determined by subtracting air permeability (184 seconds/100 mL in terms of Gurley value) of a base material (separator) from the obtained air permeability X (Gurley value) of a laminated porous film, followed by dividing by the thickness T ( ⁇ m) of the alumina porous film.
  • Air permeability per 1 ⁇ m of thickness of alumina porous film (X - 184)/T where X is air permeability (Gurley value) (seconds/100 mL) of a laminated porous film and T is a thickness ( ⁇ m) of an alumina porous film.
  • the alumina porous film having physical property values in Table 3 within the following range respectively was evaluated as “Good”.
  • Table 3 the value deviated from the following rage was underlined.
  • Each physical property value defined herein indicates a preferable range of the physical property value in the alumina porous film according to the embodiment of the present invention, and it should not be understood that there is a need for the alumina porous film according to the embodiment of the present invention to necessarily satisfy these physical property values.
  • Nonaqueous Electrolyte Secondary Battery ⁇ Method for Fabricating Nonaqueous Electrolyte Secondary Battery>
  • a laminated separator including a polyolefin porous film and an alumina porous film formed on the surface thereof
  • nonaqueous electrolyte secondary batteries B1, B3 and B4 were respectively fabricated.
  • Fabrication conditions of positive and negative electrodes included in the nonaqueous electrolyte secondary batteries B1, B3 and B4, and an assembling method of a nonaqueous electrolyte secondary battery are as follows.
  • a positive electrode was fabricated.
  • LiNi 0.5 Mn 0.3 Co 0.2 O 2 /conductive material/PVDF (weight ratio of 92/5/3) as an electrode active material is coated on the surface of an aluminum foil.
  • the positive electrode material was processed such that a positive electrode active material layer has an area of 45 mm ⁇ 30 mm and the portion where the positive electrode active material layer is not formed (portion where an aluminum foil is exposed) around the positive electrode active material layer has a width of 13 mm, thus obtaining a positive electrode.
  • the positive electrode active material layer formed on the positive electrode had a thickness of 58 ⁇ m, a density of 2.50 g/cm 3 , and the positive electrode had a positive electrode capacity of 174 mAh/g.
  • a negative electrode was fabricated.
  • graphite/styrene-1,3-butadiene copolymer/sodium carboxymethyl cellulose (weight ratio of 98/1/1) as a negative electrode active material is coated on the surface of a copper foil.
  • the negative electrode material was processed such that a negative electrode active material layer has an area of 50 mm ⁇ 35 mm and the portion where the negative electrode active material layer is not formed (portion where a copper foil is exposed) around the negative electrode active material layer has a width of 13 mm, thus obtaining a negative electrode.
  • the negative electrode active material layer formed on the negative electrode had a thickness of 49 ⁇ m, a density of 1.40 g/cm 3 , and the negative electrode had a negative electrode capacity of 372 mAh/g.
  • a positive electrode, a laminated separator and a negative electrode were laminated (disposed) in this order to obtain a nonaqueous electrolyte secondary battery member.
  • the laminated separator was disposed such that a layer of an alumina porous film faced the positive electrode.
  • the positive electrode and the negative electrode were positioned such that the whole of a main surface of the positive electrode active material layer of the positive electrode was included in a range of a main surface of the negative electrode active material layer of the negative electrode when observing in the lamination direction.
  • the size of the laminated separator was set at the size lager than that of the positive electrode and the negative electrode.
  • the laminated separator was positioned against the positive electrode and the negative electrode such that the laminated separator entirely covers the positive electrode and the negative electrode when observing in the lamination direction.
  • the nonaqueous electrolyte secondary battery member was put in a bag made by laminating an aluminum layer and a heat seal layer, and 0.25 mL of a nonaqueous electrolyte solution was poured into the bag.
  • the nonaqueous electrolyte solution to be used was an electrolyte solution at 25°C prepared by dissolving a LiPF 6 solution having the concentration of 1.0 mol/L in a mixed solvent of ethylmethyl carbonate, diethyl carbonate and ethylene carbonate at a volume ratio of 50:20:30.
  • the bag was heat-sealed while the pressure inside the bag was reduced, thus fabricating a nonaqueous secondary battery.
  • the nonaqueous secondary battery had a design capacity of 20.5 mAh.
  • CC-CV charge is a charge method of charging at a set constant current and maintaining the voltage while restricting the current to low level after reaching a predetermined voltage.
  • CC discharge is a method of discharging to a predetermined voltage at a set constant current.
  • the discharge current value was set at 0.2 C, 1 C, 5 C, 10 C and 20 C.
  • a charge-discharge test of the discharge current value 0.2 C ⁇ 3 cycles, 1 C ⁇ 3 cycles, 5 C ⁇ 3 cycles, 10 C ⁇ 3 cycles and 20 C ⁇ 3 cycles (15 cycles in total) was performed in this order at 55°C, where one cycle means to perform CC-CV charge and then CC discharge.
  • the nonaqueous electrolyte secondary battery after subjecting to 100 cycles of charge and discharge was subjected to CC-CV charge at a charge current value of 1 C (final current condition of 0.02 C) at a voltage in a range of 2.7 to 4.2 V, and then subjected to CC discharge while changing a discharge current value (discharge rate).
  • the discharge current value was set at 0.2 C, 1 C, 5 C, 10 C and 20 C.
  • a rate characteristic after 100 cycles was calculated by a ratio of (20 C discharge capacity)/ (0.2 C discharge capacity), where (0.2 C discharge capacity) is the discharge capacity at the third cycle of a charge-discharge test performed at a discharge current value 0.2 C and (20 C discharge capacity) is the discharge capacity at the third cycle of a charge-discharge test performed at a discharge current value 20 C.
  • the Secondary battery having initial rate characteristic before the cycle test of 60% or more as well as rate characteristic after 100 cycles of 20% or more is evaluated as “excellent battery characteristics”.
  • the alumina powders Nos. P1 and P2 are aluminum powders produced by the production method of the embodiment of the present invention using a surface protective agent, and all physical property values were excellent. Therefore, the alumina porous films F1 and F2 formed using those alumina powders exhibited satisfactory results in all of the porosity, the average pore diameter, the volume ratio of pores having a pore diameter of 0.2 ⁇ m or less and the volume ratio of pores having a pore diameter of more than 0.5 ⁇ m.
  • the nonaqueous electrolyte secondary battery B1 using the alumina porous film F1 had excellent electrical characteristics.
  • the alumina powders Nos. P1 and P2 exhibited a low ferrite polishing rate and were less abrasive because of small average three-dimensional particle unevenness.
  • the alumina powders P1 and P2 contain a small amount of particles having a sphere equivalent diameter of less than 0.3 ⁇ m, and therefore a volume ratio of pores having a pore diameter of 0.2 ⁇ m or less of the alumina porous films F1 and F2 were decreased and a volume ratio of pores having a pore diameter of more than 0.5 ⁇ m thereof were increased.
  • the alumina porous films F1 and F2 exhibited decreased Gurley value per 1 ⁇ m of the thickness (i.e., air permeability per 1 ⁇ m of the thickness increased, leading to satisfactory Li ion permeability), and thus exhibited excellent battery characteristics (e.g., secondary battery B1).
  • alumina powders P1 and P2 Because of a small ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area), it can be said that the alumina powders P1 and P2 exhibit small pyridine adsorption amount (less acidic nature). Therefore, a small amount of gas was generated when the alumina powders P1 and P2 are introduced into an electrolytic solution, and thus the alumina powders were excellent in electrolytic solution stability. As a result, it is considered that a secondary battery comprising an alumina porous film formed from such alumina powder hardly causes degradation of the electrolytic solution, and the secondary battery was excellent in cycle characteristic (e.g., secondary battery B1 comprising an alumina porous film F1 formed from an alumina powder P1).
  • the alumina powders Nos. P3 and P4 are alumina powders produced without using a surface protective agent.
  • the alumina powders Nos. P3 and P4 exhibited a large abundance ratio of the number of particles having a sphere equivalent diameter of less than 0.3 ⁇ m. Therefore, the alumina porous films F3 and F4 formed using those alumina powders exhibited a large volume ratio of pores having a pore diameter of 0.2 ⁇ m or less and a small volume ratio of pores having a pore diameter of more than 0.5 ⁇ m.
  • the Gurley value per 1 ⁇ m of the thickness of the alumina porous films F3 and F4 decreased (i.e., air permeability per 1 ⁇ m of the thickness decreased, leading to degradation of Li ion permeability), and thus the secondary batteries B3 and B4 exhibited poor battery characteristics.
  • the alumina powders Nos. P3 and P4 were produced without using a surface protective agent and therefore exhibited a large ratio of (pyridine adsorption BET specific surface area/ (nitrogen adsorption BET specific surface area). Namely, it can be said that the alumina powders Nos. P3 and P4 exhibit a large pyridine adsorption amount (high acidic nature). Therefore, a large amount of gas was generated when the alumina powders P3 and P4 are introduced into an electrolytic solution, and thus the alumina powders were inferior in electrolytic solution stability. As a result, it is considered that secondary batteries B3 and B4 comprising alumina porous films F3 and F4 formed from the alumina powders P3 and P4 easily causes degradation of the electrolytic solution, and the secondary batteries are inferior in cycle characteristics.
  • the alumina powder No. P5 exhibited a large ferrite polishing rate and was highly abrasive because of large average three-dimensional particle unevenness.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Organic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Geology (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Ceramic Engineering (AREA)
  • Manufacturing & Machinery (AREA)
  • Cell Separators (AREA)
  • Secondary Cells (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
  • Battery Electrode And Active Subsutance (AREA)

Abstract

The alumina powder of the present disclosure has an average three-dimensional particle unevenness of 3.5 or less, and has a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) of 0.7 or less.

Description

ALUMINA POWDER AND SLURRY CONTAINING THE SAME, ALUMINA POROUS FILM AND LAMINATED SEPARATOR COMPRISING THE SAME, AND NONAQUEOUS ELECTROLYTE SECONDARY BATTERY AND PRODUCTION METHOD THEREFOR
The present disclosure relates to an alumina powder which is suitable for forming an alumina porous film. The present disclosure also relates to a slurry containing this alumina powder. The present disclosure further relates to an alumina porous film containing the alumina powder, and a laminated separator comprising the alumina porous film. The present disclosure also relates to a nonaqueous electrolyte secondary battery including a positive electrode, negative electrode or separator comprising an alumina porous film, and a production method therefor.
A nonaqueous electrolyte secondary battery, especially a lithium ion secondary battery, has been used in household compact equipment such as a cell phone or a personal computer because of having high energy density. In recent years, application to automobiles has also been accelerated, in addition to the compact equipment.
The nonaqueous electrolyte secondary battery is a battery using an organic solvent-based electrolytic solution, and includes a positive electrode and a negative electrode. For the purpose of electrically insulating a positive electrode and a negative electrode, a conventional nonaqueous electrolyte secondary battery further includes a separator disposed between these electrode plates. As the separator in a lithium ion secondary battery, for example, a microporous sheet made of a polyolefin-based resin is used.
A separator made of the microporous sheet made of a polyolefin-based resin plays a role of holding the safety of the lithium ion secondary battery by a shutdown function of the separator when short circuit occurs between electrodes inside the battery. Specifically, heat generated at the portion where short circuit occurs causes blocking of holes of the separator, which prevents lithium ions from moving. As a result, the battery function at the hole-blocked portion is lost. However, when the temperature of the battery rapidly exceeds 150°C by heat generated momentarily due to the short circuit, the separator may undergo rapid shrinkage to expand the short circuit portion between the positive electrode and the negative electrode. In this case, the battery may fall into an overheated state where the temperature reaches several hundred degrees or higher, leading to a problem in view of safety.
JP 2013-168361 A (Patent Literature 1) and WO 2016/098579 A1 (Patent Literature 2) propose an inorganic oxide powder having special protrusions. These patent Literatures disclose that an inorganic oxide porous film formed using such inorganic oxide powder can ensure ion permeability since sufficient porosity can be held by special protrusions of the inorganic oxide powder.
[PLT 1] JP 2013-168361 A
[PLT 2] WO 2016/098579 A1
In order to form an alumina porous film for a separator, a slurry containing an alumina powder is usually coated on a target (e.g., base material), followed by drying of the slurry. Since the alumina powder has high hardness, when using an alumina powder having unevenness, a surface of the base material may be abraded (abrasive). Especially, the inorganic oxide powder of Patent Literatures 1 and 2 are highly abrasive because of its shape of having special protrusions, and there was limitation on the coating step.
From the viewpoint of ensuring the safety of the nonaqueous electrolyte secondary battery, it is important that an alumina porous film to be used in the nonaqueous electrolyte secondary battery exhibits low reactivity with an electrolytic solution. Therefore, there is required an alumina powder suited to produce an alumina porous film having a surface with high chemical stability. However, in Patent Literatures 1 and 2, no consideration is given to the reactivity with an electrolytic solution.
An object of an embodiment of the present invention is to provide an alumina powder which is suitable for forming an alumina porous film having a surface with high chemical stability, and is less abrasive, and an alumina slurry. An object of another embodiment of the present invention is to provide an alumina porous film having a surface with high chemical stability and its usage, and a method for producing a nonaqueous electrolyte secondary battery comprising the alumina porous film having a surface with high chemical stability.
Embodiments of the present invention include the following aspects.
Aspect 1 of the present invention provides an alumina powder which has an average three-dimensional particle unevenness of 3.5 or less, and
a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) is 0.7 or less.
Aspect 2 of the present invention provides the alumina powder according to aspect 1, in which an abundance ratio of the number of particles having a sphere equivalent diameter of less than 0.3 μm is 40% or less.
Aspect 3 of the present invention provides the alumina powder according to aspect 1 or 2, which has a nitrogen adsorption BET specific surface area of 1 m2/g to 15 m2/g.
Aspect 4 of the present invention provides the alumina powder according to any one of aspects 1 to 3, which has an average particle diameter of 2.0 μm or less.
Aspect 5 of the present invention provides an alumina slurry including: the alumina powder according to any one of aspects 1 to 4; a binder; and a solvent.
Aspect 6 of the present invention provides an alumina porous film including: the alumina powder according to any one of aspects 1 to 4; and a binder.
Aspect 7 of the present invention provides a laminated separator including: the alumina porous film according to aspect 6; and a separator, the alumina porous film being provided on a surface of the separator.
Aspect 8 of the present invention provides a nonaqueous electrolyte secondary battery comprising the alumina porous film according to aspect 6, wherein the alumina porous film is provided on at least one of surfaces of a positive electrode, a negative electrode and a separator.
Aspect 9 of the present invention provides a method for producing a nonaqueous electrolyte secondary battery, the method including: applying the alumina slurry according to aspect 5 on at least one of surfaces of a positive electrode, a negative electrode and a separator; and drying the alumina slurry to form an alumina porous film.
According to the alumina powder of the embodiment of the present invention, it is possible to obtain an alumina slurry which is less abrasive, and it is also possible to form an alumina porous film having a surface with high chemical stability. By using the alumina powder of the embodiment of the present invention, it is possible to provide an alumina porous film having a surface with high chemical stability, an alumina slurry which is suitable for producing the alumina porous film and is less abrasive, a laminated separator including the alumina porous film, and a method for producing a nonaqueous electrolyte secondary battery including the alumina porous film.
Fig. 1 is a schematic view for explaining three-dimensional particle unevenness. Fig. 2 is a schematic view for explaining a pore diameter inside the coating film and a pore volume inside the coating film.
Embodiments of the present invention will be described in detail below. In the present description, when the expression “to” is used for a range of a value, the range includes the upper and lower numerical values thereof.
The embodiments of the present invention include embodiments of an alumina powder, an alumina slurry, an alumina porous film, a laminated separator including the alumina porous film, and a nonaqueous electrolyte secondary battery including the alumina porous film. These embodiments will be sequentially described below.
<Alumina Powder>
An alumina powder according to an embodiment of the present invention is an alumina powder in which an average three-dimensional particle unevenness is 3.5 or less, and a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) is 0.7 or less (hereinafter sometimes referred to as “alumina powder of an embodiment of the present invention” or simply referred to as “alumina powder”).
Each definition in the embodiments of the present invention will be described in detail below.
<Average Three-Dimensional Particle Unevenness is 3.5 or less>
In the alumina powder of the embodiment of the present invention, the average three-dimensional particle unevenness of alumina particles constituting the alumina powder is 3.5 or less. Namely, the shape of alumina particles has relatively small unevenness.
In the alumina powder according to the embodiment of the present invention, the average three-dimensional particle unevenness is preferably 3.0 or less, and more preferably 2.8 or less. The average three-dimensional particle unevenness is preferably 2.0 or more, more preferably 2.2 or more, and particularly more preferably 2.4 or more.
Here, “three-dimensional particle unevenness” is a shape parameter of individual alumina particle constituting the alumina powder. The three-dimensional particle unevenness is by the following equation (1) based on a particle volume V (μm3) of a target particle and a volume of a rectangular parallelepiped La × Lb × Lc (μm3) circumscribing the target particle:
Three-dimensional particle unevenness = La × Lb × Lc/V --- (1)
where La denotes a major diameter of particle, Lb denotes a middle diameter of particle, Lc denotes a minor diameter of particle, and La, Lb and Lc are orthogonal to one another. FIG. 1 shows a schematic view for explaining the three-dimensional particle unevenness. The “average three-dimensional particle unevenness” can be obtained as an index showing a feature of the particle shape by calculating the three-dimensional particle unevenness from 100 or more particles using the equation (1). The “average three-dimensional unevenness” as used herein is an average of the three-dimensional particle unevenness calculated using the equation (1) for 100 or more arbitrary particles.
Since alumina is a relatively hard substance, when a slurry containing the alumina powder is coated on a target (e.g., base material) so as to form an alumina porous film or the like, the alumina powder may abrade the surface of a base material. The larger the unevenness of alumina particles, the more property of abrading the base material (this is referred to as “abrasive”) is enhanced.
The alumina powder of the embodiment of the present invention can suppress the abrasive because of being formed of alumina particles having relatively small unevenness.
It is possible to obtain a particle volume V, a major diameter La of particle, a middle diameter Lb of particle and a minor diameter Lc of particle, which are required to determine the three-dimensional particle unevenness, by analyzing three-dimensional images of the target particles. The three-dimensional images of the target particles can be obtained in the following ordinary manner: a sample for evaluation obtained by curing a particle fixing resin (epoxy resin, etc.) containing a predetermined amount of an alumina powder dispersed therein is cut by FIB processing at predetermined intervals to acquire the predetermined number of continuous cross-sectional SEM images (i.e., continuous slice images) by repeatedly obtaining the cross-sectional SEM images using a scanning electron microscope (SEM), and then the continuous slice images obtained are synthesized while being subjected to position correction. A specific evaluation procedure of the three-dimensional particle unevenness (fabricating method of samples for continuous slice images, and calculation method for V, La, Lb and Lc using the three-dimensional quantitative analysis software) will be described in detail in Examples.
<Ratio of (Pyridine Adsorption BET Specific Surface Area)/ (Nitrogen Adsorption BET Specific Surface Area) is 0.7 or less>
It is desirable that the alumina powder used in an alumina porous film for a nonaqueous electrolyte secondary battery has low reactivity with an electrolytic solution. A ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) (hereinafter sometimes referred to as “ratio of specific surface area”) of the alumina powder of the embodiment of the present invention is 0.7 or less. A large ratio of a specific surface area means that a basic substance such as pyridine is easily adsorbed on the surface of the alumina powder as compared with a neutral substance such as nitrogen. Namely, it is considered that the alumina powder having a large ratio of a specific surface area has a relatively high acidic nature. It is presumed that the alumina powder having a small ratio of a specific surface area of the embodiment of the present invention has a lesser acidic nature and has low reactivity with the electrolytic solution. The embodiment of the present invention is not limited to this presumption.
Small ratio of a specific surface area enables reduction in amount of a gas to be generated by a contact between the alumina powder and the electrolytic solution. Therefore, it is possible to suppress degradation of cycle characteristic of a secondary battery due to gas generation in the battery. The ratio of a specific surface area of the alumina powder of the embodiment of the present invention is preferably 0.5 or less.
Using a specific surface area measuring apparatus, the pyridine adsorption BET specific surface area is determined by the multi-point method in accordance with the method defined in JIS-Z8830 (2013) using a pyridine gas as an adsorption gas.
It is preferable that the alumina powder according to the embodiment of the present invention further satisfies the following definitions.
<Abundance Ratio of the Number of Particles having Sphere Equivalent Diameter of less than 0.3 μm is 40% or less>
In the alumina powder of the embodiment of the present invention, fine particles having a sphere equivalent diameter of less than 0.3 μm preferably accounts for 40% or less. Here, “particle diameter” is one of parameters of alumina particles, means a diameter d of a sphere having the volume equal to the particle volume V (μm3) of the alumina powder, and is the value satisfying the following equation (2).
V = 4π/3 × (d/2)3 --- (2)
It is possible to obtain an abundance ratio of the number of particles having a particle diameter of less than 0.3 μm by calculating “particle diameter” using the above equation (2) for 100 or more particles.
When an alumina porous film is formed using the alumina powder, pores are formed by a void between alumina particles. Fine alumina particles can intrude between alumina particles having a relatively large particle diameter. Therefore, when a lot of fine alumina particles are included, the void formed between alumina particles having a relatively large particle diameter might be easily blocked by fine alumina particles, thus degrading the function as a porous film. In other words, the alumina powder having low content of fine particles is suited to form an alumina porous film. Particularly preferred is an alumina powder in which the content of fine particles having a sphere equivalent diameter of less than 0.3 μm is suppressed to as low as 40% or less, and it is possible to improve the porosity (i.e., ion permeability) when the alumina porous film is formed.
The abundance ratio of the number of small particles having a sphere equivalent diameter of less than 0.3 μm is more preferably 35% or less, and particularly preferably 30% or less.
Ideally, the abundance ratio of small particles having a sphere equivalent diameter of less than 0.3 μm is set at 0%, but it is difficult in practice. The abundance ratio of the number of particles is generally 10% or more, and more realistically 15% or more.
It is possible to simultaneously determine the particle volume V and the diameter d of a sphere required to determine the sphere equivalent diameter in the above-mentioned analysis of three-dimensional images of particles. The method for calculating V and d will be described in detail in Examples.
<Nitrogen Adsorption BET Specific Surface Area is 1 m2/g to 15 m2/g>
A nitrogen adsorption BET specific surface area of the alumina powder of the embodiment of the present invention is preferably in a range of 1 m2/g to 15 m2/g, more preferably 2 to 10 m2/g, and most preferably 3 to 8 m2/g. If the nitrogen adsorption BET specific surface area is in the above range, the binding property with a binder is improved when an alumina porous film is formed by the method mentioned later, thus obtaining an alumina porous film having high strength. Using a specific surface area measuring apparatus, the nitrogen adsorption BET specific surface area is determined by the one-point method in accordance with the method defined in JIS-Z8830 (2013) using a nitrogen gas as an adsorption gas.
Preferable purity of alumina constituting the alumina powder of the embodiment of the present invention varies depending on the purposes.
Purity of alumina (hereinafter sometimes referred to as alumina purity) means percentage (% by mass) of aluminum oxide when the total content of all components contained in the alumina powder of the embodiment of the present invention is 100% by mass.
Alumina purity (% by mass) of the alumina powder is determined by the following calculation formula, assuming that the total of the mass of alumina and the total mass of oxides other than alumina, for example, SiO2, Na2O, MgO, CuO, Fe2O3 and ZrO2 is 100 (% by mass). SiO2, Na2O, MgO, CuO, Fe2O3 and ZrO2 are defined as impurities with respect to the oxide (α alumina) as a basis.
Alumina purity (% by mass) = 100 - total mass (% by mass) of impurities
Each mass of SiO2, Na2O, MgO, CuO and Fe2O3, which are impurities, can be determined by converting each content of Si, Na, Mg, Cu and Fe obtained by measuring an evaluation sample through solid-state emission spectroscopy into the mass of the oxide corresponding to each element (SiO2, Na2O, MgO, CuO, Fe2O3, ZrO2), whereas the mass of ZrO2, which is remaining impurity, can be determined by converting the content of Zr obtained by measuring an evaluation sample through ICP atomic emission spectroscopy into the mass of the oxide corresponding to each element (SiO2, Na2O, MgO, CuO, Fe2O3, ZrO2).
In the case of the alumina powder which is used for producing an alumina porous film for a secondary battery, the purity is preferably set at 90% by mass or more. Namely, each content of impurities (oxides of Si, Na, Mg, Cu, Fe, Zr, etc.) in alumina is preferably set at 10% by mass or less. This makes it possible to suppress degradation of electrical insulation properties of the alumina porous film due to impurities and to reduce the mixing amount of a metallic foreign substance which can cause short circuit.
The purity of the alumina powder is preferably 90% by mass or more, more preferably 99% by mass or more, and particularly preferably 99.9% by mass or more.
<Average Particle Diameter is 2.0 μm or less>
An average particle diameter of the alumina powder according to the embodiment of the present invention is preferably 2.0 μm or less, more preferably 1.5 μm or less, and most preferably 1.0 μm or less. “Average particle diameter” defined in the present description means a particle diameter D50 equivalent to 50% cumulative percentage by mass measured by a laser diffraction method (median diameter). If the average particle diameter of the alumina powder is 2.0 μm or less, it is possible to form an alumina porous film having an appropriate thickness, thus making it possible to maintain high energy density when used in a nonaqueous electrolyte secondary battery. The average particle diameter of the alumina powder is preferably 0.1 μm or more, more preferably 0.2 μm or more, and most preferably 0.4 μm or more. If the average particle diameter of the alumina powder is 0.1 μm or more, it is possible to suppress the amount of binders to be added to bind particles to an appropriate amount when the alumina porous film is formed. Therefore, the ion permeability of the alumina porous film obtained is not easily inhibited by the binders, which makes it possible to enhance charge and discharge characteristic of the battery.
(Method for Producing Alumina Powder)
A method for producing an alumina powder of an embodiment of the present invention will be described below.
The alumina powder according to the embodiment of the present invention can be produced by using an alumina material produced by a known method as a raw material of the alumina powder and crushing the alumina material under appropriate conditions. In the present description, the alumina material as a raw material of the alumina powder is sometimes referred to as “raw alumina”.
It is effective to crush a raw alumina produced, for example, by a known method (e.g., Bayer’s method) after adding an appropriate surface protective agent. In the present description, “surface protective agent” refers to one which suppresses over-crushing during crushing by appropriately protecting the surface of the raw alumina. Use of the surface protective agent makes it easy to set an abundance ratio of the number of fine alumina particles (particles having a sphere equivalent diameter of less than 0.3 μm) in a preferable range (40% or less) defined in the present application.
Use of the surface protective agent during crushing makes it possible to rapidly crush the raw alumina to a target particle size.
According to types of the surface protective agent, it is also possible to impart preferable surface characteristic by protecting the surface of the alumina powder after crushing. When the raw alumina is crushed using, for example, an appropriate surface protective agent, it is possible to decrease the reactivity of the crushed alumina powder with an electrolytic solution. Therefore, use of the surface protective agent during crushing of the alumina powder makes it possible to obtain an alumina powder suitable for an alumina porous film for a nonaqueous electrolyte secondary battery.
Examples of the method for producing a raw alumina include a Bayer’s method, an ammonium alum method, an aluminum underwater discharge method, an ammonium aluminum carbonate hydroxide method (AACH method), an organoaluminum hydrolysis method (aluminum alkoxide method) and the like. In the case of the Bayer’s method, a raw alumina can be produced by calcining aluminum hydroxide obtained from bauxite. In the present description, aluminum hydroxide as a raw material for producing the raw alumina is sometimes referred to as “raw aluminum hydroxide”.
A method for producing a raw alumina using the Bayer’s method will be described in detail below.
There is no particular limitation on the particle shape and particle size of the raw aluminum hydroxide, and aluminum hydroxide having an average particle diameter of about 10 μm to about 100 μm is usually used. It is possible to use a calcination furnace such as a rotary kiln or a tunnel kiln for calcination of the raw aluminum hydroxide.
The calcination temperature of the raw aluminum hydroxide, the temperature rise rate to the calcination temperature, and the calcination time are appropriately selected so as to obtain alumina having desired physical properties. The calcination temperature of the raw aluminum hydroxide is, for example, 1,000°C or higher and 1,450°C or lower, and preferably 1,000°C or higher and 1,350°C or lower. The temperature rise rate when the temperature is raised to this calcination temperature is usually 30°C/hour or more and 500°C/hour or less. The calcination time of the raw aluminum hydroxide is usually 0.5 hour or more and 24 hours or less, and preferably 1 hour or more and 20 hours or less.
The raw aluminum hydroxide may be calcined, for example, in an air atmosphere, or an inert gas atmosphere such as a nitrogen gas or argon gas atmosphere, or may be calcined in an atmosphere with high partial water vapor pressure, as in a gas furnace for calcination by combustion of a propane gas. Usually, when calcined in an atmosphere with high partial water vapor pressure, the obtained particles are easily densified by the effect of water vapor, unlike calcination in the air atmosphere.
In this way, the raw alumina can be produced using the Bayer’s method.
A method for producing an alumina powder from a raw alumina will be described in detail below.
The alumina powder is obtained by crushing a raw alumina produced by a known method.
The raw alumina can be crushed by a known method using, for example, a vibration mill, a beads mill, a jet mill or the like, and may be crushed in a dry or wet state. During crushing, an appropriate surface protective agent is used. This makes it possible to easily control the average three-dimensional particle unevenness and the abundance ratio of the number of particles having a sphere equivalent diameter of 0.3 μm in a preferable range.
The crushing time is appropriately adjusted so as not to perform over-crushing. Too long crushing time might cause excessive crushing of alumina particles into fine particles, thus increasing the abundance ratio of the number of particles having a sphere equivalent diameter of 0.3 μm or less.
As mentioned above, the surface protective agent is used during crushing. Examples of the surface protective agent suitable to restrict the average three-dimensional particle unevenness include monohydric alcohols such as methanol, ethanol, 1-propanol and 2-propanol; glycols such as propylene glycol, polypropylene glycol, ethylene glycol and polyethylene glycol; amines such as triethanolamine; and higher fatty acids such as palmitic acid, stearic acid and oleic acid. These surface protective agent can be used alone, or two or more surface protective agents can also be used in combination. Glycols, especially propylene glycol, ethylene glycol, polypropylene glycol and polyethylene glycol are preferable.
The surface protective agent is capable of having the function of not only protecting the surface of the alumina powder after crushing, but also inactivating the surface of the alumina powder. The function of inactivating the surface is capable of decreasing the reactivity between the alumina powder and the electrolytic solution. Examples of such surface protective agent include monohydric alcohols such as methanol, ethanol, 1-propanol and 2-propanol; and glycols such as ethylene glycol, polyethylene glycol, propylene glycol and polypropylene glycol. From the viewpoint of low volatility, glycols, especially ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol are preferable.
There is no particular limitation on the molecular weight of polyethylene glycol and polypropylene glycol to be used as the surface protective agent. In view of ease of addition, the surface protective agent is preferably in the form of liquid having an average molecular weight of about 200 to 600.
The amount of the surface protective agent to be added is usually 0.01 to 2.0 parts by mass, preferably 0.05 to 1.0 parts by mass, and more preferably 0.1 to 0.5 part by mass, relative to 100 parts by mass of the raw alumina. Excessive amount of the surface protective agent leads to saturation of the effect of the surface protective agent. If the amount is too small, the effect of the surface protective agent is not sufficiently exerted.
The obtained alumina particles of the alumina powder do not have special protrusions as disclosed in Patent Literatures 1 and 2.
<Alumina Slurry>
An alumina slurry according to an embodiment of the present invention includes the alumina powder according to the embodiment of the present invention, a binder and a solvent.
(Binder)
A binder has the function of binding between alumina particles when an alumina porous film is formed, and bonding between an alumina porous layer and a base material (e.g., separator, negative electrode and/or positive electrode). A known binder can be used and, usually, the binder is mainly composed of an organic substance.
Specific examples of the binder include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and a tetrafluoroethylene-hexafluoropropylene copolymer (FEP); polyacrylic acid derivatives such as polyacrylic acid, methyl polyacrylate, ethyl polyacrylate and hexyl polyacrylate; polymethacrylic acid derivatives such as polymethacrylic acid, methyl polymethacrylate, ethyl polymethacrylate and hexyl polymethacrylate; polyamide, polyimide, polyamideimide, polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, a styrene-butadiene rubber, carboxymethyl cellulose (hereinafter referred to as CMC), polyacrylonitrile and derivatives thereof, polyethylene, polypropylene, an aramid resin or salts thereof. These compounds may be used alone, or two or more compounds may be used as a mixture, or may be used as a mixture with a copolymer mentioned later.
It is also possible to use, as the binder, a copolymer of two or more materials selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid and hexadiene. These copolymers may be used alone, or two or more copolymers may be used as a mixture.
(Solvent (Dispersion Medium))
It is possible to use, as the solvent suitable for an alumina slurry, known solvents, for example, water, alcohol, acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, xylene, cyclohexanone, or mixed solvents thereof.
The mixing amount of the respective components constituting the alumina slurry is not particularly limited as long as the alumina slurry can be appropriately coated on a base material or a support to form an alumina porous film having desired characteristics. For example, the mixing amount of the binder can be set at 0.1 to 20 parts by mass and the mixing amount of the solvent can be set at 10 to 500 parts by mass, relative to 100 parts by mass of the alumina powder according to the embodiment of the present invention
In order to stabilize dispersion of the alumina powder or to improve coatability of the alumina slurry, various additives may be further added to the alumina slurry. Examples of the additives include dispersants, thickeners, leveling agents, antioxidants, defoamers, pH adjustors containing an acid or alkali, and additives having the function of suppressing the side reaction such as decomposition of an electrolytic solution. These additives are preferably those which are removed during the step of forming an alumina porous film and do not remain in the alumina porous film. When the additives remain in the alumina porous film, it is desired to select additives which are chemically and physically stable such that the additives do not exert a significant influence on a battery reaction when the alumina porous film is used in a nonaqueous electrolyte secondary battery. Each content of the additives is preferably restricted to the amount which does not significantly inhibit characteristics of the alumina slurry and the alumina porous film to be formed therefrom. For example, the content of the additive is preferably 10 parts by mass or less relative to 100 parts by mass of alumina.
The alumina slurry containing the alumina powder is coated on a base material such as a positive electrode, a negative electrode or a separator, or a support so as to form an alumina porous film. Usually, alumina has a hardness higher than that of the material used as a base material (metal, resin, etc.). If alumina particles have unevenness, the unevenness might damage the surface of the base material to cause abrasion of the base material when the alumina slurry is coated on the base material. Therefore, in the case of the alumina slurry using alumina particles having large unevenness, there is a need to pay enough attention so as not to cause abrasion during coating on the base material.
However, the average three-dimensional particle unevenness of alumina particles contained in the alumina powder of the embodiment of the present invention is as small as 3.5 or less, so that abrasion of the base material due to alumina particles hardly occurs when the alumina slurry is coated on the surface of the base material.
(Method for Producing Alumina Slurry)
The alumina slurry of the embodiment of the present invention can be prepared by mixing the alumina powder of the embodiment of the present invention, a binder and a solvent in a predetermined mixing amount, and dispersing the alumina powder in the solvent by a known method. It is possible to use, as the method of dispersing the alumina powder, a stirring method using a known planetary mixer, or a dispersing method using ultrasonic vibration or a beads mill.
<Alumina Porous Film>
An alumina porous film according to an embodiment of the present invention includes the alumina powder of an embodiment of the present invention and a binder. Such alumina porous film can be produced, for example, using the alumina slurry according to the embodiment of the present invention.
Since alumina is a substance having high heat resistance and insulation properties, the alumina porous film including the alumina powder also has high heat resistance and insulation properties. Therefore, the alumina porous film of the embodiment of the present invention is suitable for an insulating porous film for protecting a separator used in a secondary battery from high temperature and the like. The alumina porous film is particular suitable for disposing between a negative electrode and a separator. For example, the alumina porous film may be disposed on a surface facing the negative electrode of both surfaces of the separator. The alumina porous film may also be disposed on the surface (one surface or both surfaces) of the negative electrode.
The porosity by volume of the alumina porous film is preferably 20 to 90%, and more preferably 30 to 70%. If the alumina porous film has appropriate porosity, it is possible to improve the ion permeability while ensuring the function as the insulating porous film when using in the secondary battery.
The average diameter of pores (average pore diameter) of the alumina porous film is preferably 1 μm or less, and more preferably 0.7 μm or less. Here, “average pore diameter” refers to a median value (D50) of a pore diameter. In the present description, it is also referred to as “average pore diameter D50”. If the alumina porous film has appropriate average pore diameter, it is possible to improve the ion permeability while ensuring the function as the insulating porous film when using in the secondary battery.
The lesser fine pores of the alumina porous film, the more the ion permeability can be improved when using in the secondary battery. The volume ratio of pores having a pore diameter of 0.2 μm or less is preferably 30% by volume or less, and more preferably 25% by volume or less. The volume ratio of pores having a pore diameter of more than 0.5 μm is preferably 10% by volume or more, and more preferably 15% by volume or more.
The volume ratio (% by volume) of pores having a pore diameter of 0.2 μm or less can be determined by “total pore volume of pores having a pore diameter of 0.2 μm or less”/“total pore volume of all pores inside the coating film” × 100. The volume ratio (% by volume) of pores having a pore diameter of more than 0.5 μm can be determined by “total pore volume of pores having a pore diameter of more than 0.5 μm”/“total pore volume of all pores inside the coating film” × 100.
Air permeability of the alumina porous film is preferably 10 seconds/100 mL×μm or less, and more preferably 9 seconds/100 mL×μm or less, in terms of the Gurley value per 1 μm of the thickness. If the alumina porous film has small Gurley value (high air permeability), it is possible to further improve the ion permeability when using in the secondary battery.
(Method for Producing Alumina Porous Film)
An alumina porous film can be formed by coating an alumina slurry on a base material or a support, and removing a solvent (dispersion medium) in the alumina slurry.
<Laminated Separator>
A laminated separator according to an embodiment of the present invention comprises the alumina porous film of the embodiment of the present invention on the surface of the embodiment of the separator. Namely, the laminated separator according to the embodiment of the present invention includes the above-mentioned alumina porous film, and the separator comprising the alumina porous film on the surface thereof. In the present description, “separator” broadly means a film for separating a positive electrode and a negative electrode of a battery, and includes a separator used in a secondary battery (e.g., nonaqueous electrolyte secondary battery). The alumina porous film can be laminated on one surface or both surfaces of the separator.
The air permeability of the laminated separator is preferably 30 to 1,000 seconds/100 mL, and more preferably 50 to 800 seconds/100 mL, in terms of the Gurley value. If the laminated separator has the above air permeability, it is possible to obtain sufficient ion permeability when the laminated separator is used as a nonaqueous electrolyte secondary battery member.
Small Gurley value (high air permeability) of the laminated separator means high porosity of the laminated separator and coarse laminated structure of the laminated separator. Excessively small Gurley value (excessively high air permeability) of the laminated separator might cause a decrease in strength of the separator, leading to insufficient shape stability at particularly high temperature. Meanwhile, excessively large Gurley value (excessively low air permeability) of the laminated separator would make it impossible to obtain sufficient ion permeability when the laminated separator is used as a nonaqueous electrolyte secondary battery member, leading to degradation of battery characteristics of a nonaqueous electrolyte secondary battery.
The separator suitable for a laminated separator is generally formed of a porous film made of a resin. The separator suitable or the present application will be described in detail below.
(Separator)
A separator is a membranous porous film disposed between a positive electrode and a negative electrode in a secondary battery, a gas or liquid being permeable from one surface to the other surface. The separator includes a large number of pores therein and the pores are connected with each other.
The separator can be produced from a porous and membranous base material containing a polyolefin-based resin as a main component (polyolefin-based porous base material).
The separator is melted and changed into a non-porous separator when the battery generates heat to impart the shutdown function. The separator may be formed of a single layer or plural layers.
The piercing strength of the separator is preferably 3N or more. If the piercing strength is too small, the separator may be broken by positive and negative electrode active material particles leading to short circuit of positive and negative electrodes, during laminating and winding operations of positive and negative electrodes and a separator of the battery assembling process, a clamping operation of the wound group, or application of pressure to the battery from the outside. The piercing strength of the separator (the porous film) is preferably 10N or less, and more preferably 8N or less.
The thickness of the separator may be appropriately determined taking the thickness of nonaqueous electrolyte secondary battery member constituting the nonaqueous electrolyte secondary battery into consideration, and is preferably 4 to 40 μm, more preferably 5 to 30 μm, and still more preferably 6 to 15 μm.
The porosity by volume of the separator is preferably 20 to 80%, and more preferably 30 to 75%. If the separator has an appropriate porosity, it is possible to retain a larger amount of an electrolytic solution and to reliably prevent (shutdown) a current at a lower temperature when an excessively large current flows.
The average diameter (average pore diameter) of pores of the separator is preferably 0.3 μm or less, and more preferably 0.05 μm or more and 0.14 μm or less. Here, “average pore diameter” refers to a median value (D50) of the pore diameter. In the present description, it is also referred to as “average pore diameter D50”. If the separator has an appropriate average pore diameter, it is possible to not only sufficiently maintain the ion permeability of the separator, but also suppress various particles (e.g., active material particles eluted and precipitated from an electrode plate in the charge and discharge process of the secondary battery, and alumina particles which were fallen off from the alumina porous film) from entering into pores of the separator.
The proportion of the polyolefin-based component in the separator is usually 50% by volume or more, preferably 90% by volume or more, and more preferably 95% by volume or more, relative to the entire porous film. It is preferable that a high-molecular weight component having a weight average molecular weight of 5 × 105 to 15 × 106 is included in the polyolefin-based component in the separator. It is particularly preferable that the strength of the separator increases when a polyolefin component having a weight average molecular weight of 1,000,000 or more is contained.
Examples of the polyolefin-based resin suitable for the separator include a high-molecular weight homopolymer or copolymer obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene and 1-hexene. The separator can include a single layer (resin single-layer film) or a plurality of layers (resin multi-layer film) of a resin film of a polyolefin-based resin. Examples of the separator made of the resin multi-layer film include those in which a single layer of a polyethylene film (PE film) is disposed between two layers of a polypropylene film (PP film) and those in which three layers are laminated (PP/PE/PP laminated film). A high-molecular weight polyethylene composed mainly of ethylene is particularly preferable. The separator may contain components other than polyolefin as long as the function of the layer is not impaired.
The air permeability of the separator is usually in a range of 30 to 500 seconds/100 mL, and preferably 50 to 300 seconds/100 mL, in terms of the Gurley value. If the separator has appropriate air permeability, sufficient ion permeability can be obtained.
The basis weight of the separator is usually 4 to 20 g/m2, preferably 4 to 12 g/m2, and more preferably 5 to 10 g/m2, in view of the fact that the strength, thickness, handleability and weight, and weight energy density and volume energy density of the battery can be increased when using as a separator of a nonaqueous electrolyte secondary battery.
(Method for Producing Laminated Separator)
A laminated separator can be produced by forming an alumina porous film on the surface of a separator using an alumina slurry.
An example of a method for producing a laminated separator includes a method in which an alumina slurry is directly coated on the surface of a separator and then a solvent (dispersion medium) of the alumina slurry is removed.
Another example of a method for producing a laminated separator includes a method in which an alumina slurry is coated on the surface of a support and a solvent (dispersion medium) of the alumina slurry is removed to form an alumina porous film, and then this alumina porous film is compression-bonded to a separator and the support is peeled off from the alumina porous film.
Still another example of a method for producing a laminated separator includes a method in which an alumina slurry is coated on the surface of a support and a separator is compression-bonded to the coated surface to transfer the alumina slurry to the separator, and then a solvent (dispersion medium) of the alumina slurry is removed.
A further example of a method for producing a laminated separator includes a method in which a separator is immersed in an alumina slurry to perform dip coating, and then a solvent (dispersion medium) of the alumina slurry is removed.
In the case of producing a laminated separator comprising an alumina porous film on both surfaces of a separator, an alumina porous film may be sequentially formed on the other surface of a separator after forming an alumina porous film on one surface thereof. Alternatively, the alumina porous film may be simultaneously formed on both surfaces of the separator.
Before forming an alumina porous film on the surface of the separator, the surface of the separator may be optionally subjected to a hydrophilization treatment.
Preferable conditions (basis weight, thickness, coating method, method for removing a solvent) of the alumina porous film in the laminated separator will be mentioned later.
A separator can be prepared by the following method.
A separator containing a polyolefin-based resin as a main component is preferably produced, for example, by the following method when the porous film contains an ultrahigh-molecular weight polyolefin and a low-molecular weight hydrocarbon having a weight average molecular weight of 10,000 or less.
Namely, it is possible to obtain the separator by the method including the step (1) of kneading an ultrahigh-molecular weight polyolefin, a low-molecular weight hydrocarbon having a weight average molecular weight of 10,000 or less, and a pore-forming agent to obtain a polyolefin resin composition, the step (2) of rolling the polyolefin resin composition using a rolling roll to form a sheet (rolling step), the step (3) of removing the pore-forming agent from the sheet obtained in the step (2), and the step (4) of drawing the sheet obtained in the step (3) to obtain a porous film (separator). Before the operation of removing the pore-forming agent in the sheet in the step (3), the operation of drawing the sheet in the step (4) may be performed.
Examples of the low-molecular weight hydrocarbon include a low-molecular weight polyolefin such as polyolefin wax, and a low-molecular weight polymethylene such as Fischer-Tropsch wax. The weight average molecular weight of the low-molecular weight polyolefin and low-molecular weight polymethylene is preferably 200 or more and 3,000 or less. If the weight average molecular weight is 200 or more, there is no fear of evaporation during formation of a porous film. It is preferable that the weight average molecular weight is 3,000 or less since mixing with the ultrahigh-molecular weight polyolefin is performed more uniformly.
Examples of the pore-forming agent include an inorganic filler and a plasticizer. Examples of the inorganic filler include an inorganic filler which can be dissolved in an aqueous solvent containing an acid, an aqueous solvent containing an alkali, or an aqueous solvent composed mainly of water.
Examples of the inorganic filler, which can be dissolved in the aqueous solvent containing an acid, include calcium carbonate, magnesium carbonate, barium carbonate, zinc oxide, calcium oxide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and calcium sulfate, and calcium carbonate is preferable in view of the fact that it is inexpensive and is easy to obtain a fine powder. Examples of the inorganic filler, which can be dissolved in the aqueous solvent containing an alkali, include silicic acid and zinc oxide, and silicic acid is preferable since it is inexpensive and is easy to obtain a fine powder. Examples of the inorganic filler, which can be dissolved in the aqueous solvent composed mainly of water, include calcium chloride, sodium chloride and magnesium sulfate.
Examples of the plasticizer include liquid paraffin, and a low-molecular weight nonvolatile hydrocarbon compound such as mineral oil.
<Nonaqueous Electrolyte Secondary Battery>
A nonaqueous electrolyte secondary battery according to an embodiment of the present invention comprises an electrode group including a positive electrode, a negative electrode and a separator; and a nonaqueous electrolytic solution; and further comprises an alumina porous film of an embodiment of the present invention on at least one surface of the positive electrode, the negative electrode and the separator.
Examples of the form of the electrode group suitable for the nonaqueous electrolyte secondary battery of the embodiment of the present invention include an electrode group (laminated type electrode group) formed by laminating a positive electrode, a negative electrode and a separator, or an electrode group (wound type electrode group) formed by laminating a positive electrode, a negative electrode and a separator and then winding the laminate.
If the alumina porous film to be used has a high porosity and a high proportion of pores having a large pore diameter (e.g., a volume ratio of pores having a pore diameter of more than 0.5 μm), the lithium ion permeability is excellent and also the rectification effect of lithium ions becomes satisfactory.
It is considered that an alumina powder having a small proportion of fine alumina particles having a sphere equivalent diameter of less than 0.3 μm enables the formation of an alumina porous film having a high porosity and a high proportion of pores having a large pore diameter. Use of such alumina porous film enables an improvement in battery characteristics of the nonaqueous electrolyte secondary battery.
In the alumina powder of the embodiment of the present invention, a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) is 0.7 or less.
When a porous film obtained from an alumina powder having the above small ratio is used in a nonaqueous electrolyte secondary battery, the reaction between the nonaqueous electrolyte secondary battery and the electrolytic solution does not easily occur (in this way, difficulty in reaction with the electrolytic solution is referred to as “electrolytic solution stability of alumina porous film” in the present description). When using an alumina porous film having excellent electrolytic solution stability, degradation of the electrolytic solution is suppressed, thus enabling an improvement in cycle characteristic of the nonaqueous electrolyte secondary battery. Therefore, it is possible to produce a nonaqueous electrolyte secondary battery in which excellent electrical characteristics are maintained over a long period.
(Method for Producing Nonaqueous Electrolyte Secondary Battery)
A nonaqueous electrolyte secondary battery of the embodiment of the present invention includes the step of forming an alumina porous film on at least one surface of electrode plates (positive electrode, negative electrode) and a separator. Specific one embodiment includes the step of coating the above alumina slurry on at least one surface of a positive electrode, a negative electrode and a separator and drying the alumina slurry to form an alumina porous film. Common positive and negative electrodes have an electrode mixture layer containing an electrode active material (a positive electrode active material or a negative electrode active material) and a binder on the surface thereof. When the alumina slurry is coated on an electrode plate (a positive electrode or a negative electrode), the alumina slurry can be coated on the electrode mixture layer included in those electrodes.
An electrode group including a positive electrode, a negative electrode and a separator, and an alumina porous film formed on the surface of any one of the positive electrode, the negative electrode and the separator are housed in a container for a battery (e.g., battery can) and a nonaqueous electrolytic solution is injected into the container, thus obtaining a nonaqueous electrolyte secondary battery.
Taking, as an example, a nonaqueous electrolyte secondary battery using a wound type electrode group including an negative electrode having an alumina porous film formed on the surface, a more specific production method will be described. On the surface of the negative electrode, an electrode mixture layer containing a negative electrode active material is formed. An alumina slurry is coated on this electrode mixture layer and the alumina slurry is dried, thus forming an alumina porous film on the surface of the negative electrode. One end of a negative electrode lead is connected to a lead joining portion of the negative electrode, and one end of a positive electrode lead is connected to a lead joining portion of the positive electrode, respectively. The positive electrode and the negative electrode are laminated via a separator and the laminate is wound to constitute a wound type electrode group. This electrode group is housed in a battery can in a state where an insulating ring is disposed at the upper and lower portions of this wound type electrode group, followed by injection of an electrolytic solution in the battery can and further sealing of the opening of the battery can with a battery cap.
It is possible to control the thickness of the alumina porous film by adjusting the thickness of a coating film in a wet state after coating, the weight ratio of a resin to microparticles, the solid component concentration of the alumina slurry (sum of the concentration of the resin and the concentration of microparticles) and the like. It is possible to use, as the support, for example, a film made of a resin, a belt made of metal, or a drum.
The method for coating the alumina slurry to an electrode plate, a separator or a support may be a method capable of realizing required basis weight and coated area and is not particularly limited. A conventionally known method can be employed as the method for coating the alumina slurry. Specific examples of such method include a gravure coater method, a small-diameter gravure coater method, a reverse roll coater method, a transfer roll coater method, a kiss coater method, a dip coater method, a knife coater method, an air doctor blade coater method, a blade coater method, a rod coater method, a squeeze coater method, a cast coater method, a bar coater method, a die coater method, a screen printing method and a spray coating method.
The method for removing a solvent (dispersion medium) is generally a method by drying. Examples of the drying method include a natural drying method, a fan drying method, a heat drying method and a vacuum drying method, and may be any method as long as the solvent (dispersion medium) can be sufficiently removed. A usual drying device can be used for drying.
When heating is performed to remove the solvent (dispersion medium) from the coating film of the alumina slurry formed on the separator, heating is performed at a temperature at which the air permeability of the separator is not degraded, specifically 10 to 120°C, and more preferably 20 to 80°C, in order to avoid degradation of the air permeability due to shrinkage of pores of the separator.
The thickness of the alumina porous film formed by the above-mentioned method is preferably 0.5 to 15 μm (per one surface), more preferably 2 to 10 μm (per one surface), and still more preferably 2 to 5 μm (per one surface), when a separator is used as a base material and an alumina porous film is laminated on one or both surfaces of the separator to form a laminated separator.
The thickness of the alumina porous film is preferably 1 μm or more (0.5 μm or more on one surface) since it is possible to sufficiently prevent internal short circuit due to breakage of a battery in a laminated separator comprising an alumina porous film and to maintain the retaining amount of an electrolytic solution in the alumina porous film. Meanwhile, the thickness of the alumina porous film is preferably 30 μm or less (15 μm or less on one surface) in total of both surfaces since it is possible to suppress an increase in permeation resistance of ions such as lithium ions over the entire laminated separator comprising an alumina porous film to thereby prevent degradation of a positive electrode and degradation of rate characteristic and cycle characteristic when charge and discharge cycle is repeated, and to suppress an increase in distance between a positive electrode and a negative electrode to thereby prevent an increase in size of a nonaqueous electrolyte secondary battery.
In the following description with respect to physical properties of the alumina porous film, when a porous layer is laminated on both surfaces of a porous film, the physical properties at least indicate physical properties of the alumina porous film laminated to the surface opposite to a positive electrode in the porous film when a secondary battery is assembled.
The basis weight per unit area (per one surface) of the alumina porous film may be appropriately determined taking the strength, thickness, weight, and handleability of the laminated separator into account. The basis weight is usually 1 to 20 g/m2, preferably 4 to 15 g/m2, and more preferably 4 to 12 g/m2. The basis weight of the alumina porous film is preferably in the above range since it is possible to increase the weight energy density and volume energy density of a nonaqueous electrolyte secondary battery including the laminated separator comprising the alumina porous film as the member to thereby reduce the weight of the battery.
The embodiments of the present invention will be described in detail by way of Examples, but the embodiments of the present invention are not limited only to the following Examples.
In Examples, a measurement was made of physical properties of an alumina powder, physical properties of an alumina porous film produced using the alumina powder, and electrical characteristics of a nonaqueous electrolyte secondary battery comprising the alumina porous film. A method for fabricating samples used for the measurement, and methods for measuring physical properties and electrical characteristics are as follows.
<Method or Fabricating Alumina Powder>
Fabrication conditions of each alumina powder are shown in Table 1 and described in detail below.
(Alumina Powder No. P1)
Aluminum hydroxide obtained by the Bayer’s method was calcined in a gas furnace to obtain a raw alumina having a BET specific surface area of 3.7 m2/g and an average particle diameter of 54 μm. To the raw alumina, 0.2% by mass of a surface protective agent (propylene glycol) was added, followed by mixing and further crushing using the following continuous beads mill with classification mechanism to obtain an α phase alumina powder (No. P1).
(Continuous Beads Mill with Classification Mechanism)
- Classification point (maximum particle diameter): set at 3 μm
- Crushing media: φ5 mm zirconia beads
- Ratio (crushing media/ raw alumina mass): 7
- Feed of raw alumina powder: measuring loss in mass (loss-in-weight) type
(Alumina Powder No. P2)
Aluminum hydroxide obtained by the Bayer’s method was calcined in a gas furnace to obtain a raw alumina having a BET specific surface area of 4.2 m2/g and an average particle diameter of 52 μm. To the raw alumina, 0.2% by mass of a surface protective agent (propylene glycol) was added, followed by mixing and further crushing using the following jet mill to obtain an alumina powder.
(Jet Mill)
- Apparatus: PJM-280SP, manufactured by Nippon Pneumatic Kogyo
- Feed rate of raw alumina powder: 10 kg/h
- Gauge pressure of air supply port during crushing: 0.7 MPa
(Alumina Powder No. P3)
Aluminum hydroxide obtained by the Bayer’s method was calcined in a gas furnace to obtain a raw alumina having a BET specific surface area of 3.7 m2/g and an average particle diameter of 54 μm. The raw alumina was crushed using the following ball mill to obtain an alumina powder. During crushing, a surface protective agent was not added.
(Ball Mill)
- Crushing media: φ30mm alumina ball
- Ratio (crushing media/ raw alumina mass): 6
- Crushing time: particle size control (average particle diameter D50 = 0.5 μm)
(Alumina Powder No. P4)
Under the same conditions as in the alumina powder No. P1, except that the surface protective agent was not added, an alumina powder was fabricated.
(Alumina Powder No. P5)
Aluminum hydroxide obtained by the aluminum alkoxide method was calcined in a gas furnace to obtain a raw alumina having a BET specific surface area of 4.0 m2/g and an average particle diameter of 7.8 μm. The raw alumina was crushed using the following jet mill to obtain an alumina powder. During crushing, a surface protective agent was not added.
(Jet Mill)
- Apparatus: PJM-380SP, manufactured by Nippon Pneumatic Kogyo
- Feed rate of raw alumina powder: 15 kg/h
- Gauge pressure of air supply port during crushing: 0.7 MPa
Figure JPOXMLDOC01-appb-T000001
<Physical Properties of Alumina Powder>
Physical properties of the alumina powders thus fabricated were measured in the following manner. The measurement results are shown in Table 2.
(1. Average Three-Dimensional Particle Unevenness, Abundance Ratio of the number of Particles having Sphere Equivalent Diameter of less than 0.3 μm)
In 100 parts by weight of an epoxy resin, 2 parts by weight of a dispersant (Phosphoric acid polyester BYK-111, manufactured by BYK) and 2 parts by weight of an alumina powder were dispersed. After vacuum deaeration, 12 parts by weight of a curing agent was charged. The alumina dispersed epoxy resin obtained was poured into a mold made of a silicon resin and then cured.
The cured sample was fixed to a sample stage for SEM and then a Pt-Pd thin film was vacuum-deposited on the surface of the sample by the vacuum deposition method. The sample with the vacuum-deposited thin film was set to FIB-SEM (HELIOS 600, manufactured by FEI) and processed by focused ion beam (FIB) of gallium accelerated at an accelerating voltage of 30 kV to form a hole of 44.2 μm or more in depth, and then a cross-section in arbitrary direction was exposed as the first observing surface. Because of three-dimensional measurement, the same results are obtained regardless of the cross-sectional direction. The first observation surface was observed by SEM at an acceleration voltage of 2.1 kV. The range to be observed (observation range) was set at 51.2 μm × 44.2 μm. After observing the first observation surface, the FIB processing was again applied thereto with a thickness of 50 nm in a depth direction of the sample to newly expose an observation surface (second observation surface). The second observation surface was observed by SEM in the same manner as in the first observation surface. The FIB processing and SEM observation were repeated to acquire a series of SEM images with respect to the observation surfaces arranged in parallel at intervals of 50 nm in the sample. The scale of the SEM observation was set at 50 nm/pix for X-axis and Y-axis, and 50 nm/pix for Z-axis. Here, X-axis is an axis parallel to the sample surface in the SEM observation surface, Y-axis is an axis in the depth direction orthogonal to the sample surface in the SEM observation surface, XY plane is the SEM observation surface, and Z-axis is an axis in the FIB cutting direction orthogonal to the SEM observation surface in the sample surface. This series of SEM images were processed by the image analysis software (Avizo ver. 6.0, manufactured by Visualization Sciences Group) to acquire continuous slice images of the sample with respect to the observation portion corresponding to a rectangular parallelepiped in an observation range (51.2 μm × 44.2 μm) × 25 μm.
Using continuous slice images of this sample, three-dimensional quantitative analysis (quantitative analysis of three-dimensional structure) of alumina particles included in the sample was performed to calculate a three-dimensional particle unevenness and a sphere equivalent diameter. For the three-dimensional quantitative analysis, a 3D-PRT particle analysis option of the quantitative analysis software TRI/3D-BON-FCS (version: BON-FCS R10.01.10.29-H-64: manufactured by RATOC SYSTEM ENGINEERING CO., LTD.) was used.
In the three-dimensional quantitative analysis, data of continuous slice images were firstly read by the quantitative analysis software TRI/3D-PRT and a median filter (3D, 3 × 3) was applied to remove noises. Subsequently, three-dimensionally isolated particles were respectively identified by labeling, and then particles to be measured were specified. The area to be subjected to three-dimensional quantitative analysis (this is referred to as “measuring area”) was set at a portion of the above-mentioned observation portion (51.2 μm × 44.2 μm × 25 μm). The measuring area had a size of about 48 μm × about 20 μm × about 20 μm. Particles passing through the boundary surface of the measuring area were removed from measurement target.
A particle volume V, a major diameter La of particle, a middle diameter Lb of particle and a minor diameter Lc of particle were determined for each of measured particles, and then a sphere equivalent diameter d and a three-dimensional particle unevenness were respectively calculated by substituting these values in the following equations (1) and (2). An average of the three-dimensional particle unevenness obtained in calculation was determined and shown in Table 2. An abundance ratio of the number of particles having a sphere equivalent diameter d of less than 0.3 μm was shown in Table 2.
Three-dimensional particle unevenness = La × Lb × Lc/V --- (1)
V = 4π/3 × (d/2)3 --- (2)
When the sphere equivalent diameter d and the average of the three-dimensional particle unevenness are determined, there is a need to set the number of particles to be measured at 100 or more. This makes it possible to approximately converge the average of the measured values. In this Example, the number of particles in the measuring area is about 1,000 to 2,000, and the average was determined by measuring all particles.
(2. Nitrogen Adsorption BET Specific Surface Area)
Using “Flow Sorb III 2310” manufactured by Shimadzu Corporation as a specific surface area measuring apparatus, a BET specific surface area was determined by the nitrogen absorption method (one point method) in accordance with the method defined in JIS-Z8830 (2013). The measurement conditions are as follows.
- Carrier gas: nitrogen/helium mixed gas
- Filling amount of sample: 0.1 g
- Pretreatment conditions of sample: treatment at 200°C for 20 min
- Nitrogen adsorption temperature: liquid nitrogen temperature (-196°C or lower)
- Nitrogen desorption temperature: room temperature (about 20°C)
(3. Ratio of (Pyridine Adsorption BET Specific Surface Area)/ (Nitrogen Adsorption BET Specific Surface Area))
Using “BELSORP-18” manufactured by MicrotracBEL Corp., a pyridine adsorption BET specific surface area was measured by the multi-point method. The measurement conditions are as follows. Calculation was performed by analyzing under the conditions: adsorption cross-sectional area of pyridine: 0.285 nm2 and analysis range of pyridine adsorption BET specific surface area: P/P0 = 0.1 to 0.3.
- Filling amount of sample: 1 g
- Pretreatment conditions of sample: treated under vacuum at 150°C for 5 hours
- Pyridine thermostatic chamber temperature: 80°C
- Pyridine adsorption temperature: 50°C
- Saturated vapor pressure: 9.607 kPa
- Initial introduction pressure: 0.267 kPa
- Maximum adsorption pressure: P/P0 = 0.9
- Adsorption equilibrium time: 300 seconds
The obtained pyridine adsorption BET specific surface area is divided by the nitrogen adsorption BET specific surface area obtained by the above method to determine “ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area)”.
(4. Average Particle Diameter D50)
Using a laser particle size distribution analyzer [“Microtrac MT3300EXII”, manufactured by MicrotracBEL Corp.], a particle diameter D50 equivalent to 50% cumulative percentage by mass measured by a laser diffraction method was regarded as an average particle diameter. In the measurement, ultrasonic dispersion was performed for 5 minutes in a 0.2% by mass aqueous sodium hexametaphosphate solution, and a refractive index was set at 1.76.
(5. Ferrite Polishing Rate)
In order to examine a property in which a target such as a base material is polished with an alumina powder (abrasive), a ferrite polishing rate was measured. The ferrite polishing rate was measured using only alumina powders Nos. P1, P2 and P5. In the present description, an alumina powder having the ferrite polishing rate of 20 μm or less is evaluated as “less abrasive”.
Using a polishing machine (Model 5627-56) and a sample rotating machine (Model 7705-2) manufactured by Marumoto Kogyo Co., Ltd., a ferrite chip (6 mm × 1.5 mm × 12 mm) was polished while feeding an alumina dispersed slurry onto a buffing cloth (Model NO-101, manufactured by Marumoto Kogyo Co., Ltd., 250 mmφ), and then a difference in thickness (μm) before and after polishing was regarded as a ferrite polishing rate. Here, the alumina dispersed slurry was fabricated by ultrasonic dispersion of 35 g of an alumina powder in 1,750 mL of a 0.2% by mass aqueous sodium hexametaphosphate solution for 30 minutes. Polishing conditions are as follows.
- Alumina dispersed slurry dropping rate: 15 g/min
- Ferrite chip polishing area: 6 mm × 12 mm (72 mm2)
- Buff rotation speed: 400 rpm
- Load: 400 g
- Polishing time: 60 min
(6. Gas Generation Amount)
In order to examine the electrolytic solution stability, the amount of gas generated when an alumina powder is introduced into an electrolytic solution (gas generation amount) was measured. In the present description, an alumina powder having the gas generation amount of 60 mL or less is evaluated as “excellent electrolytic solution stability”.
After vacuum-drying an alumina powder at 120°C for 8 hours, 1 g of an alumina powder and 2 mg of an electrolytic solution were sealed into an aluminum laminate bag in a glove box maintained at a dew point of -30°C or lower, and then the mass of the aluminum laminate bag after sealing was measured. As the electrolytic solution, a LiPF6 solution (1 mol/L, ethylene carbonate:ethylmethyl carbonate:diethyl carbonate = 30% by volume:50% by volume:20% by volume) manufactured by KISHIDA CHEMICAL Co., Ltd. was used. Before a heat treatment, the specific gravity and volume of the aluminum laminate bag after sealing were measured by the Archimedes method and then the aluminum laminate bag was subjected to a heat treatment at 85°C for 72 hours. After the heat treatment, the specific gravity and volume of the aluminum laminate bag were measured by the Archimedes method, and a change in volume before and after the heat treatment was calculated as the amount of gas generated.
Figure JPOXMLDOC01-appb-T000002
The alumina powder having physical property values in Table 2 within the following range respectively was evaluated as “Good”. In Table 2, the value deviated from the following rage was underlined. Of the respective physical property values defined herein, “average three-dimensional particle unevenness” and “ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area)” indicate the physical property value indispensable for the alumina powder according to the embodiment of the present invention, and other physical property values indicate preferable physical property values in the alumina powder according to the embodiment of the present invention.
- Average three-dimensional particle unevenness: 3.5 or less
- Abundance ratio of the number of particles having a sphere equivalent diameter of less than 0.3 μm: 40% or less
- Nitrogen adsorption BET specific surface area: 1 to 15 m2/g
- Ratio (pyridine adsorption BET specific surface area/nitrogen adsorption BET) of Specific Surface Area: 0.7 or less
- Average particle diameter D50: 2.0 μm or less
- Ferrite polishing rate: 20 μm or less
- Gas generation amount: 60 mL or less
<Method for Forming Alumina Porous Film>
Next, using alumina powders Nos. P1 to P4, alumina porous films F1 to F4 were respectively formed. An alumina porous film was formed on the surface of a base material (separator).
As the base material (separator), a porous film made of polyethylene (thickness: 12.4 μm, basis weight: 7.5 g/m2, air permeability (Gurley value): 184 seconds/100 mL) was used. After pretreating one surface of the base material by corona discharge irradiation, an alumina porous film was formed on the treated surface by the following method.
CMC; product No. 1110 manufactured by DAICEL FINECHEM LTD. (3 parts by weight), isopropyl alcohol (51.6 parts by weight), pure water (292 parts by weight) and an alumina powder (100 parts by weight) were sequentially mixed with stirring and then ultrasonic-dispersed for 10 minutes. The obtained mixture was further dispersed using CLEARMIX (“CLM-0.8S”, manufactured by M Technique Co., Ltd.) for 21 minutes. The obtained dispersion was filtered through a wire mesh having an opening size of 10 μm to prepare an alumina slurry.
On the surface of the base material pretreated by corona discharge irradiation, the slurry was gravure-coated using a tabletop test coater manufactured by Yasui Seiki Inc. The slurry thus coated was dried at a drying temperature of 50°C to form an alumina porous film. Using a laminated porous film in which the alumina porous film is formed on the surface of the base materials (laminated separator), physical properties of the alumina porous film were measured.
<Physical Properties of Alumina Porous Film>
Physical properties of the alumina porous film thus fabricated were measured in the following manner. The measurement results are shown in Table 3.
(1. Thickness of Alumina Porous Film)
The thickness of a laminated porous film was measured by a high-accuracy digital measuring instrument “VL-50A” manufactured by Mitutoyo Corporation. By subtracting the thickness (12.4 μm) of a base material porous film from the thickness of the measured laminated porous film, the thickness of the alumina porous film was calculated.
(2. Basis Weight of Alumina Porous Film)
After cutting a laminated porous film into a square in size of 8 cm × 8 cm, the weight W (g) was measured to calculate a basis weight (g/m2) = W/(0.08 m × 0.08 m) of the laminated porous film. By subtracting the basis weight (7.5 g/m2) of the base material porous film therefrom, the basis weight of the alumina porous film was calculated.
(3. Pore Diameter inside Coating Film (Alumina Porous Film), Volume Ratio of Pore, Porosity, etc.)
A laminated porous film was impregnated with an epoxy resin, followed by curing of the epoxy resin. After a cured sample was fixed to a sample stage, the FIB processing was applied to the sample using FIB-SEM (HELIOS 600, manufactured by FEI) to remove the epoxy resin on the top surface to thereby expose the surface of the alumina porous film. Then, the surface (surface of the alumina porous film) was observed by SEM at an acceleration voltage of 2.1 kV. After observation, the FIB processing was again applied thereto with a thickness of 20 nm in the depth direction (in the thickness direction of the alumina porous film) of the sample to newly form a cross-section, and the cross-section was observed by SEM. In this way, the FIB processing and SEM observation of the cross-section were repeated at intervals of 20 nm to acquire continuous slice images including the entire thickness of the alumina porous film, and then continuous slice images were obtained by the image analysis software [Avizo ver. 6.0, manufactured by Visualization Sciences Group]. The scale of the SEM observation was set at 19.2 nm/pix for X-axis and Y-axis, and 20 nm/pix for Z-axis. Here, X-axis and Y-axis are axes orthogonal to each other on the surface parallel to the surface of the alumina porous film, and Z-axis is an axis which is orthogonal to X-axis and Y-axis and is in the thickness direction of the alumina porous film.
Three-dimensional analysis of the coating film was applied to the acquired continuous slice images using the quantitative analysis software TRI/3D-BON-FCS (version: BON-FCS R10.01.10.29-H-64: manufactured by RATOC SYSTEM ENGINEERING CO., LTD.) to calculate a pore diameter inside the coating film, a pore volume and a porosity.
In the three-dimensional quantitative analysis, data of continuous slice images were firstly read by TRI/3D-BON-FCS and a median filter (3D, 3 × 3) was applied, and then the particle portion and the void portion were identified by binarization using Auto-LW.
With respect to the void portion identified by the above processing, noises were removed under the conditions of 2D Ers Sml = 1 and 3D Ers Sml = 5. Then, the value of the Thickness parameter was subjected to a calculation processing under the conditions of MIL = 0.5, NdNd = 1.5 and NdTm = 2.0. The minor diameter Thickness, the major diameter Width, the distance between branching points Length, the total volume BV of the void portion and the total volume TV of an analysis region were determined to define pores inside the coating film, and then a pore diameter inside coating film, a pore volume and a porosity were calculated by the following methods.
Here, the minor diameter Thickness, the major diameter Width and the distance between branching points Length will be described with reference to FIG. 2. “Minor diameter Thickness” and “major diameter Width” mean a minor diameter and a major diameter in a cross-section of pores, respectively. “Distance between branching points Length” means a distance between adjacent branching points. “Branching point” means a point where two (or more) thin lines intersect when each pore is represented by a thin line.
(Pore Diameter inside Coating Film (Alumina Porous Film))
A pore diameter inside the coating film was determined from the following equation (3). The equation (3) is for determination of an average diameter of pores (pore diameter) and means that the sum of the minor diameter (Thickness) and the major diameter (Width) of pores shown in FIG. 2 is divided by 2.
Pore diameter inside coating film = (Thickness + Width)/2 --- (3)
(Pore Volume inside Coating Film (Alumina Porous Film))
A pore volume inside the coating film was determined by the following equations (4) and (5). The equation (4) is for calculation of a cross-sectional area (CS) of pores from a minor diameter (Thickness) and a major diameter (Width). The equation (5) is for calculation of a pore volume from the obtained cross-sectional area and length of pores (distance between branching points (Length)).
CS = (Thickness/2) × (Width/2) × π --- (4)
Pore volume inside coating film = CS × Length --- (5)
(Porosity of Coating Film (Alumina Porous Film))
“Porosity” in the embodiment of the present invention is a parameter indicating voids inside an alumina porous film and can be determined by three-dimensional analysis of the alumina porous film in an analysis area. By using three-dimensional analysis, he particle portion and the void portion in the alumina porous film are identified by binarizing to obtain the total volume (BV) of the void portion. The porosity is defined by the following equation (6) which indicates that the total volume (BV) of the void portion is dividing by the total volume (TV) of the analysis area.
Porosity (% by volume) = BV/TV × 100 --- (6)
(Pore Distribution of Coating Film (Alumina Porous Film))
Pore distribution inside the coating film was determined from the obtained pore diameter and pore volume inside the coating film, and then a volume ratio of pores having a pore diameter of 0.2 μm or less (which means “total pore volume of pores having a pore diameter of 0.2 μm or less”/“total pore volume of pores inside the coating film” × 100 (% by volume)) and a volume ratio of pores having a diameter of more than 0.5 μm (which means “total pore volume of pores having a pore diameter of more than 0.5 μm”/ “total pore volume of pores inside the coating film” × 100 (% by volume)) were calculated. This pore distribution inside the coating film was determined by setting the area of 17.6 μm × 11.3 μm × (observation depth) μm as the measuring range. The observation depth was determined according to the thickness of each sample and was found to be appropriately 3 to 5 μm (approximately 600 to 1,000 μm3 in terms of volume in the observation area).
(Average Pore Diameter D50)
An average pore diameter D50 was determined from pore distribution inside the coating film (alumina porous film).
(4. Air Permeability of Alumina Porous Film)
In accordance with JIS P8117 (2009), air permeability X (Gurley value) (seconds/100 mL) was measured by a Gurley type densometer manufactured by Toyo Seiki Seisaku-Sho, Ltd. Air permeability per 1 μm of the thickness (Gurley value per 1 μm of the thickness) (seconds/100 mL×μm) of an alumina porous film was determined by subtracting air permeability (184 seconds/100 mL in terms of Gurley value) of a base material (separator) from the obtained air permeability X (Gurley value) of a laminated porous film, followed by dividing by the thickness T (μm) of the alumina porous film.
It is possible to determine the air permeability per 1 μm of the thickness of alumina porous film by the following equation:
Air permeability per 1 μm of thickness of alumina porous film = (X - 184)/T
where X is air permeability (Gurley value) (seconds/100 mL) of a laminated porous film and T is a thickness (μm) of an alumina porous film.
Figure JPOXMLDOC01-appb-T000003
The alumina porous film having physical property values in Table 3 within the following range respectively was evaluated as “Good”. In Table 3, the value deviated from the following rage was underlined. Each physical property value defined herein indicates a preferable range of the physical property value in the alumina porous film according to the embodiment of the present invention, and it should not be understood that there is a need for the alumina porous film according to the embodiment of the present invention to necessarily satisfy these physical property values.
- Porosity: 20 to 90% by volume
- Average pore diameter D50: 1 μm or less
- Volume ratio of pores having a pore diameter of 0.2 μm or less: 30% by volume or less
- Volume ratio of pores having a pore diameter of more than 0.5 μm: 10% by volume or more
- Air permeability per 1 μm of thickness (Gurley value per 1 μm of the thickness): 10 seconds/100 mL×μm or less
<Method for Fabricating Nonaqueous Electrolyte Secondary Battery>
Next, using a laminated separator (including a polyolefin porous film and an alumina porous film formed on the surface thereof) prepared for the measurement of physical properties of alumina porous films F1, F3 and F4, nonaqueous electrolyte secondary batteries B1, B3 and B4 were respectively fabricated. Fabrication conditions of positive and negative electrodes included in the nonaqueous electrolyte secondary batteries B1, B3 and B4, and an assembling method of a nonaqueous electrolyte secondary battery are as follows.
(Fabrication of Positive Electrode)
Using a commercially available positive electrode material (aluminum foil having a positive electrode active material layer formed thereon), a positive electrode was fabricated. Regarding the commercially available positive electrode material, LiNi0.5Mn0.3Co0.2O2/conductive material/PVDF (weight ratio of 92/5/3) as an electrode active material is coated on the surface of an aluminum foil. The positive electrode material was processed such that a positive electrode active material layer has an area of 45 mm × 30 mm and the portion where the positive electrode active material layer is not formed (portion where an aluminum foil is exposed) around the positive electrode active material layer has a width of 13 mm, thus obtaining a positive electrode. The positive electrode active material layer formed on the positive electrode had a thickness of 58 μm, a density of 2.50 g/cm3, and the positive electrode had a positive electrode capacity of 174 mAh/g.
(Fabrication of Negative Electrode)
Using a commercially available negative electrode material (copper foil having a negative electrode active material layer formed thereon), a negative electrode was fabricated. Regarding the commercially available negative electrode material, graphite/styrene-1,3-butadiene copolymer/sodium carboxymethyl cellulose (weight ratio of 98/1/1) as a negative electrode active material is coated on the surface of a copper foil. The negative electrode material was processed such that a negative electrode active material layer has an area of 50 mm × 35 mm and the portion where the negative electrode active material layer is not formed (portion where a copper foil is exposed) around the negative electrode active material layer has a width of 13 mm, thus obtaining a negative electrode. The negative electrode active material layer formed on the negative electrode had a thickness of 49 μm, a density of 1.40 g/cm3, and the negative electrode had a negative electrode capacity of 372 mAh/g.
(Assembling of Nonaqueous Electrolyte Secondary Battery)
In a laminate pouch, a positive electrode, a laminated separator and a negative electrode were laminated (disposed) in this order to obtain a nonaqueous electrolyte secondary battery member. In this case, the laminated separator was disposed such that a layer of an alumina porous film faced the positive electrode. The positive electrode and the negative electrode were positioned such that the whole of a main surface of the positive electrode active material layer of the positive electrode was included in a range of a main surface of the negative electrode active material layer of the negative electrode when observing in the lamination direction. The size of the laminated separator was set at the size lager than that of the positive electrode and the negative electrode. The laminated separator was positioned against the positive electrode and the negative electrode such that the laminated separator entirely covers the positive electrode and the negative electrode when observing in the lamination direction.
Subsequently, the nonaqueous electrolyte secondary battery member was put in a bag made by laminating an aluminum layer and a heat seal layer, and 0.25 mL of a nonaqueous electrolyte solution was poured into the bag. The nonaqueous electrolyte solution to be used was an electrolyte solution at 25°C prepared by dissolving a LiPF6 solution having the concentration of 1.0 mol/L in a mixed solvent of ethylmethyl carbonate, diethyl carbonate and ethylene carbonate at a volume ratio of 50:20:30. The bag was heat-sealed while the pressure inside the bag was reduced, thus fabricating a nonaqueous secondary battery. The nonaqueous secondary battery had a design capacity of 20.5 mAh.
<Electrical Characteristics of Nonaqueous Electrolyte Secondary Battery>
Electrical characteristics of the nonaqueous electrolyte secondary battery thus fabricated were measured in the following manner. The measurement results are shown in Table 4.
(1. Initial Rate Characteristic before Cycle Test)
Each of nonaqueous electrolyte secondary batteries B1, B3 and B4 was subjected to 4 cycles of initial charge and discharge at a temperature of 25°C, where one cycle means to perform a CC-CV charge at a charge current value of 0.2 C (final current condition of 0.02 C) at a voltage in a range of 2.7 V to 4.1 V and then a CC discharge at a discharge current value of 0.2 C (note that a current value 1 C refers to a current value at which a rated capacity derived from a one hour rate discharged capacity is discharged in one hour, same shall apply hereinafter). Herein, CC-CV charge is a charge method of charging at a set constant current and maintaining the voltage while restricting the current to low level after reaching a predetermined voltage. CC discharge is a method of discharging to a predetermined voltage at a set constant current.
The nonaqueous electrolyte secondary battery after subjecting to initial charge and discharge, was subjected to CC-CV charge (final current condition of 0.02 C) at a charge current value of 1 C and then subjected to CC discharge while changing a discharge current value (discharge rate). The discharge current value was set at 0.2 C, 1 C, 5 C, 10 C and 20 C. Using the same nonaqueous electrolyte secondary battery, a charge-discharge test of the discharge current value 0.2 C × 3 cycles, 1 C × 3 cycles, 5 C × 3 cycles, 10 C × 3 cycles and 20 C × 3 cycles (15 cycles in total) was performed in this order at 55°C, where one cycle means to perform CC-CV charge and then CC discharge.
A ratio of the discharge capacity at the third cycle of a charge-discharge test performed at a discharge current value 0.2 C (0.2 C discharge capacity) to the discharge capacity at the third cycle of a charge-discharge test performed at a discharge current value 20 C (20 C discharge capacity), (20 C discharge capacity/0.2 C discharge capacity), was calculated as initial rate characteristic before the cycle test.
(2. Rate Characteristic after 100 Cycles)
After measuring initial rate characteristic before the cycle test, the nonaqueous electrolyte secondary battery was subjected to 100 cycles of charge and discharge at a temperature of 55°C, where one cycle means to perform a CC-CV charge at a charge current value of 1 C (final current condition of 0.02 C) at a voltage in a range of 2.7 to 4.2 V and then a CC discharge at a discharge current value of 10 C.
The nonaqueous electrolyte secondary battery after subjecting to 100 cycles of charge and discharge was subjected to CC-CV charge at a charge current value of 1 C (final current condition of 0.02 C) at a voltage in a range of 2.7 to 4.2 V, and then subjected to CC discharge while changing a discharge current value (discharge rate). The discharge current value was set at 0.2 C, 1 C, 5 C, 10 C and 20 C. Using the same nonaqueous electrolyte secondary battery, a charge-discharge test of the discharge current value 0.2 C × 3 cycles, 1 C × 3 cycles, 5 C × 3 cycles, 10 C × 3 cycles and 20 C × 3 cycles (15 cycles in total) was performed in this order at 55°C assuming that one cycle of CC-CV charge and CC discharge was performed.
A rate characteristic after 100 cycles was calculated by a ratio of (20 C discharge capacity)/ (0.2 C discharge capacity), where (0.2 C discharge capacity) is the discharge capacity at the third cycle of a charge-discharge test performed at a discharge current value 0.2 C and (20 C discharge capacity) is the discharge capacity at the third cycle of a charge-discharge test performed at a discharge current value 20 C.
In the present description, The Secondary battery having initial rate characteristic before the cycle test of 60% or more as well as rate characteristic after 100 cycles of 20% or more is evaluated as “excellent battery characteristics”.
Figure JPOXMLDOC01-appb-T000004
The Secondary battery having physical property values in Table 4 which are within the following range respectively was evaluated as “Good”. In Table 4, the value deviated from the following rage was underlined. Each measured value defined herein indicates a preferable range of the measured value in the nonaqueous electrolyte secondary battery according to the embodiment of the present invention, and it should not be understood that there is a need for the nonaqueous electrolyte secondary battery according to the embodiment of the present invention to necessarily satisfy these physical property values.
- Initial rate characteristic before cycle test: 60% or more
- Rate characteristic after 100 cycles: 20% or more
The test results of Tables 1 to 4 will be considered below.
The alumina powders Nos. P1 and P2 are aluminum powders produced by the production method of the embodiment of the present invention using a surface protective agent, and all physical property values were excellent. Therefore, the alumina porous films F1 and F2 formed using those alumina powders exhibited satisfactory results in all of the porosity, the average pore diameter, the volume ratio of pores having a pore diameter of 0.2 μm or less and the volume ratio of pores having a pore diameter of more than 0.5 μm. The nonaqueous electrolyte secondary battery B1 using the alumina porous film F1 had excellent electrical characteristics.
The alumina powders Nos. P1 and P2 exhibited a low ferrite polishing rate and were less abrasive because of small average three-dimensional particle unevenness.
The alumina powders P1 and P2 contain a small amount of particles having a sphere equivalent diameter of less than 0.3 μm, and therefore a volume ratio of pores having a pore diameter of 0.2 μm or less of the alumina porous films F1 and F2 were decreased and a volume ratio of pores having a pore diameter of more than 0.5 μm thereof were increased. As a result, the alumina porous films F1 and F2 exhibited decreased Gurley value per 1 μm of the thickness (i.e., air permeability per 1 μm of the thickness increased, leading to satisfactory Li ion permeability), and thus exhibited excellent battery characteristics (e.g., secondary battery B1).
Because of a small ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area), it can be said that the alumina powders P1 and P2 exhibit small pyridine adsorption amount (less acidic nature). Therefore, a small amount of gas was generated when the alumina powders P1 and P2 are introduced into an electrolytic solution, and thus the alumina powders were excellent in electrolytic solution stability. As a result, it is considered that a secondary battery comprising an alumina porous film formed from such alumina powder hardly causes degradation of the electrolytic solution, and the secondary battery was excellent in cycle characteristic (e.g., secondary battery B1 comprising an alumina porous film F1 formed from an alumina powder P1).
The alumina powders Nos. P3 and P4 are alumina powders produced without using a surface protective agent. The alumina powders Nos. P3 and P4 exhibited a large abundance ratio of the number of particles having a sphere equivalent diameter of less than 0.3 μm. Therefore, the alumina porous films F3 and F4 formed using those alumina powders exhibited a large volume ratio of pores having a pore diameter of 0.2 μm or less and a small volume ratio of pores having a pore diameter of more than 0.5 μm. As a result, the Gurley value per 1 μm of the thickness of the alumina porous films F3 and F4 decreased (i.e., air permeability per 1 μm of the thickness decreased, leading to degradation of Li ion permeability), and thus the secondary batteries B3 and B4 exhibited poor battery characteristics.
The alumina powders Nos. P3 and P4 were produced without using a surface protective agent and therefore exhibited a large ratio of (pyridine adsorption BET specific surface area/ (nitrogen adsorption BET specific surface area). Namely, it can be said that the alumina powders Nos. P3 and P4 exhibit a large pyridine adsorption amount (high acidic nature). Therefore, a large amount of gas was generated when the alumina powders P3 and P4 are introduced into an electrolytic solution, and thus the alumina powders were inferior in electrolytic solution stability. As a result, it is considered that secondary batteries B3 and B4 comprising alumina porous films F3 and F4 formed from the alumina powders P3 and P4 easily causes degradation of the electrolytic solution, and the secondary batteries are inferior in cycle characteristics.
The alumina powder No. P5 exhibited a large ferrite polishing rate and was highly abrasive because of large average three-dimensional particle unevenness.

Claims (9)

  1. An alumina powder which has an average three-dimensional particle unevenness of 3.5 or less, and
    a ratio of (pyridine adsorption BET specific surface area)/ (nitrogen adsorption BET specific surface area) is 0.7 or less.
  2. The alumina powder according to claim 1, wherein an abundance ratio of the number of particles having a sphere equivalent diameter of less than 0.3 μm is 40% or less.
  3. The alumina powder according to claim 1 or 2, which has a nitrogen adsorption BET specific surface area of 1 m2/g to 15 m2/g.
  4. The alumina powder according to any one of claims 1 to 3, which has an average particle diameter of 2.0 μm or less.
  5. An alumina slurry comprising: the alumina powder according to any one of claims 1 to 4; a binder; and a solvent.
  6. An alumina porous film comprising: the alumina powder according to any one of claims 1 to 4; and a binder.
  7. A laminated separator comprising: the alumina porous film according to claim 6; and a separator, the alumina porous film being provided on a surface of the separator.
  8. A nonaqueous electrolyte secondary battery comprising the alumina porous film according to claim 6, wherein the alumina porous film is provided on at least one of surfaces of a positive electrode, a negative electrode and a separator.
  9. A method for producing a nonaqueous electrolyte secondary battery, the method comprising:
    applying the alumina slurry according to claim 5 on at least one of surfaces of a positive electrode, a negative electrode and a separator; and
    drying the alumina slurry to form an alumina porous film.

PCT/JP2019/016636 2018-04-19 2019-04-18 Alumina powder and slurry containing the same, alumina porous film and laminated separator comprising the same, and nonaqueous electrolyte secondary battery and production method therefor WO2019203303A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
KR1020207029860A KR20210003745A (en) 2018-04-19 2019-04-18 Alumina powder and slurry containing the same, alumina porous film and laminated separator containing the same, and non-aqueous electrolyte secondary battery and production method thereof

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2018-080838 2018-04-19
JP2018080838A JP6971193B2 (en) 2018-04-19 2018-04-19 Alumina powder and a slurry containing it, an alumina porous membrane and a laminated separator provided with the alumina powder, a non-aqueous electrolytic solution secondary battery and a method for producing the same.

Publications (1)

Publication Number Publication Date
WO2019203303A1 true WO2019203303A1 (en) 2019-10-24

Family

ID=68240262

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2019/016636 WO2019203303A1 (en) 2018-04-19 2019-04-18 Alumina powder and slurry containing the same, alumina porous film and laminated separator comprising the same, and nonaqueous electrolyte secondary battery and production method therefor

Country Status (3)

Country Link
JP (1) JP6971193B2 (en)
KR (1) KR20210003745A (en)
WO (1) WO2019203303A1 (en)

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR102258452B1 (en) 2017-05-23 2021-06-02 나이키 이노베이트 씨.브이. Midsole with graded response

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH05330819A (en) * 1992-05-29 1993-12-14 Sumitomo Chem Co Ltd Dry pulverizing method for alumina
US20010036438A1 (en) * 2000-04-27 2001-11-01 Koji Yamamoto Method for producing an inorganic oxide powder
JP2014040580A (en) * 2012-07-27 2014-03-06 Sumitomo Chemical Co Ltd Production method of laminate porous film
EP2978045A1 (en) * 2013-03-21 2016-01-27 Zeon Corporation Slurry for lithium ion secondary battery porous film, production method therefor, separator for lithium ion secondary battery, and lithium ion secondary battery

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP5362132B2 (en) 2012-01-20 2013-12-11 住友化学株式会社 Inorganic oxide powder, inorganic oxide-containing slurry, lithium ion secondary battery using the slurry, and manufacturing method thereof
CN108807823B (en) 2014-12-17 2021-05-04 住友化学株式会社 Inorganic oxide powder, slurry containing same, nonaqueous electrolyte secondary battery, and method for producing same

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH05330819A (en) * 1992-05-29 1993-12-14 Sumitomo Chem Co Ltd Dry pulverizing method for alumina
US20010036438A1 (en) * 2000-04-27 2001-11-01 Koji Yamamoto Method for producing an inorganic oxide powder
JP2014040580A (en) * 2012-07-27 2014-03-06 Sumitomo Chemical Co Ltd Production method of laminate porous film
EP2978045A1 (en) * 2013-03-21 2016-01-27 Zeon Corporation Slurry for lithium ion secondary battery porous film, production method therefor, separator for lithium ion secondary battery, and lithium ion secondary battery

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
HASEGAWA, MASAHIRO ET AL.: "Effect of Liquid Additives and Behavior of Alumina Powder in Ultrafine Grinding of Alumina", JOURNAL OF THE SOCIETY OF POWDER TECHNOLOGY, vol. 39, 2002, Japan, pages 736 - 742, XP055602049 *

Also Published As

Publication number Publication date
KR20210003745A (en) 2021-01-12
JP2019189478A (en) 2019-10-31
JP6971193B2 (en) 2021-11-24

Similar Documents

Publication Publication Date Title
KR101907363B1 (en) Alumina, slurry containing the same, and porous alumina film, laminated separator, nonaqueous electrolyte secondary battery using the same, and method for manufacturing nonaqueous electrolyte secondary battery
EP3401281B1 (en) Alumina powder, alumina slurry, alumina-containing coating layer, multilayer separation membrane and secondary battery
TWI577635B (en) Inorganic oxide powder and a slurry containing an inorganic oxide, and a lithium ion secondary battery using the same and a method for producing the same
EP3128597B1 (en) Inorganic oxide powder, slurry containing same, nonaqueous electrolyte secondary battery, and method for manufacturing nonaqueous electrolyte secondary battery
JP6193333B2 (en) Separator and manufacturing method thereof
EP3817096A1 (en) Separator having fine pattern, wound body, and non-aqueous electrolyte battery
US20180351147A1 (en) Alumina and slurry containing the same, and alumina porous film using the same, laminated separator, nonaqueous electrolyte secondary battery and method for manufacturing nonaqueous electrolyte secondary battery
WO2019203303A1 (en) Alumina powder and slurry containing the same, alumina porous film and laminated separator comprising the same, and nonaqueous electrolyte secondary battery and production method therefor
JPWO2018088004A1 (en) Alumina, slurry containing the same, alumina porous film using the same, laminated separator, non-aqueous electrolyte secondary battery, and method of manufacturing non-aqueous electrolyte secondary battery
CN113226988A (en) Alumina, alumina slurry, alumina film, laminated separator, non-aqueous electrolyte secondary battery, and method for producing same
KR20210108430A (en) α-alumina, slurry, porous membrane, laminated separator, and non-aqueous electrolyte secondary battery and manufacturing method thereof
JP2019061972A (en) Separator for nonaqueous electrolyte secondary battery
JP2020001249A (en) Porous film, separator for secondary battery and secondary battery

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 19787986

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 19787986

Country of ref document: EP

Kind code of ref document: A1