Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
  • H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/113—Silicon oxides; Hydrates thereof
  • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
  • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M10/00—Secondary cells; Manufacture thereof
  • H01M10/05—Accumulators with non-aqueous electrolyte
  • H01M10/052—Li-accumulators
  • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/04—Processes of manufacture in general
  • H01M4/049—Manufacturing of an active layer by chemical means
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/30—Particle morphology extending in three dimensions
  • C01P2004/32—Spheres
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/51—Particles with a specific particle size distribution
• • 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
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
  • C01P2004/62—Submicrometer sized, i.e. from 0.1-1 micrometer
• • 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
  • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M2004/021—Physical characteristics, e.g. porosity, surface area
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M2300/00—Electrolytes
  • H01M2300/0017—Non-aqueous electrolytes
  • H01M2300/0065—Solid electrolytes
  • H01M2300/0068—Solid electrolytes inorganic
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• This invention relates to a nonaqueous electrolyte secondary battery comprising hydrophobic spherical silica particles.
• the nonaqueous electrolyte secondary battery is constructed of a positive electrode, a negative electrode, and a separator interposed therebetween.
• a secondary battery has the problem that if the water content inside the battery increases, the electrolyte is degraded thereby to generate acid.
• the acid thus formed promotes side reactions, for example, degradation of solid electrolyte interface at the negative electrode, and dissolution of positive electrode active material. There eventually arise problems like a lowering of battery capacity and an increase of internal resistance.
• Patent Document 1 discloses the technique of admixing hydrophobized inorganic powder into the separator.
• the separator material itself is hydrophobic.
• this technique of imparting hydrophobicity to the separator material is not necessary insofar as no hydrophilic inorganic substance is further added to the separator material.
• Patent Document 2 discloses the technique of coating an electrode material to an electrode current collector, and forming a layer of hydrophobic material on the surface of the electrode, for thereby preventing water from penetrating into the electrode material.
• the hydrophobic material is coated on the surface of the electrode material to form a layer which is effective for preventing water penetration. Accordingly, while water can be absorbed in or enter the electrode material during the preparation or coating of the electrode material, it is substantially impossible to evaporate off this water because of the overlying hydrophobic material layer.
• Patent Document 3 proposes the technique of removing water contained in electrode material.
• An electrode material is coated onto an electrode current collector and then sintered.
• various additives to construct the electrode material for example, binder can undergo decomposition reaction during sintering. Accordingly, it is not preferable to apply this technique to the battery manufacture.
• Patent Document 4 discloses addition of hydrophobic inactive particles to the electrode material itself.
• the hydrophobizing method is by treating ordinary hydrophilic inorganic oxide with a hydrophobic substance (e.g., hydrophobic silane compound) while the particle size and shape of the resulting hydrophobic inert particles are not specified.
• a simple mixture of the hydrophobic inactive particles (in Patent Document 4) with the electrode material is difficult to efficiently inhibit water entry or water-aided side reactions because the inorganic oxide particles are poorly dispersible. Even when entry of water from the exterior is prevented, it is difficult to effectively remove water and moisture from within the electrode material.
• Patent Document 1 JP-A 2001-093498
• Patent Document 2 JP-A 2002-015728
• Patent Document 3 KR 1995-0002099
• Patent Document 4 JP 5160544 (WO 2008/023890)
• An object of the invention is to provide a nonaqueous electrolyte secondary battery wherein hydrophobicity is exerted over all components of the battery for inhibiting absorption and entry of water in the battery.
• the invention pertains to a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte.
• the inventors have found that when hydrophobic spherical silica particles which have been treated by a specific method so as to be hydrophobic and have a specific average particle size, particle size distribution, and average circularity are incorporated into the electrode material of which the positive and/or negative electrode is made, the particles are effective for inhibiting absorption and entry of water in the battery, particularly during the battery manufacture. By virtue of this inhibitory effect, the battery is improved in high-temperature storage characteristics.
• the invention provides a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a nonaqueous electrolyte.
• the positive and/or negative electrode is made of an electrode material containing hydrophobic spherical silica particles.
• the hydrophobic spherical silica particles are introduced R 1 SiO 3/2 units wherein R 1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 20 carbon atoms on surfaces of hydrophilic spherical silica particles consisting essentially of SiO 2 units, and further introduced R 2 3 SiO 1/2 units wherein R 2 is independently a substituted or unsubstituted monovalent hydrocarbon group of 1 to 6 carbon atoms on the surfaces having R 1 SiO 3/2 units introduced therein.
• the spherical silica particles have an average particle size of 5 nm to 1.00 ⁇ m, a particle size distribution D 90 /D 10 of up to 3, and an average circularity of 0.8 to 1.
• the hydrophobic spherical silica particles are obtained through the steps of (A1) subjecting a tetrafunctional silane compound, a partial hydrolytic condensate thereof or a combination thereof to hydrolysis and condensation to form hydrophilic spherical silica particles consisting essentially of SiO 2 units, (A2) introducing R 1 SiO 3/2 units wherein R 1 is as defined above on surfaces of the hydrophilic spherical silica particles, and (A3) further introducing R 2 3 SiO 1/2 units wherein R 2 is as defined above on the surfaces having R 1 SiO 3/2 units introduced thereon.
• the hydrophobic spherical silica particles are obtained through the steps of (A1) forming hydrophilic spherical silica particles, (A2) first hydrophobic surface treatment with trifunctional silane compound, and (A3) second hydrophobic surface treatment with monofunctional silane compound.
• the step (A1) of forming hydrophilic spherical silica particles includes subjecting a tetrafunctional silane compound having the general formula (I):
• R 3 is each independently a monovalent hydrocarbon group of 1 to 6 carbon atoms, a partial hydrolyzate thereof or a mixture thereof to hydrolysis and condensation in a mixture of a hydrophilic organic solvent and water in the presence of a basic substance, thereby forming a dispersion of hydrophilic spherical silica particles consisting essentially of SiO 2 units in the solvent mixture.
• the first hydrophobic surface treatment (A2) includes adding a trifunctional silane compound having the general formula (II):
• R 1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 20 carbon atoms
• R 4 is each independently a monovalent hydrocarbon group of 1 to 6 carbon atoms, a partial hydrolyzate thereof or a mixture thereof to the dispersion from step (A1), for conducting surface treatment of the hydrophilic spherical silica particles, thereby obtaining a dispersion of spherical silica particles having R 1 SiO 3/2 units (wherein R 1 is as defined above) introduced on their surface in the solvent mixture.
• the second hydrophobic surface treatment (A3) includes adding a silazane compound having the general formula (III):
• R 2 is each independently a substituted or unsubstituted monovalent hydrocarbon group of 1 to 6 carbon atoms, a monofunctional silane compound having the general formula (IV):
• R 2 is as defined above and X is an OH group or hydrolyzable group, or a mixture thereof to the dispersion from step (A2), for conducting surface treatment of the spherical silica particles having R 1 SiO 3/2 units introduced thereon, thereby introducing R 2 3 SiO 1/2 units (wherein R 2 is as defined above) on their surface.
• the positive electrode is made of a positive electrode material containing 0.1 to 5% by weight as solids of the hydrophobic spherical silica particles based on the weight of the positive electrode material.
• the negative electrode is made of a negative electrode material containing 0.1 to 5% by weight as solids of the hydrophobic spherical silica particles based on the weight of the negative electrode material.
• the invention is effective for inhibiting absorption and entry of water in a nonaqueous electrolyte secondary battery.
• One embodiment of the invention is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a nonaqueous electrolyte.
• the positive and/or negative electrode is made of an electrode material containing hydrophobic spherical silica particles.
• the hydrophobic spherical silica particles are obtained by introducing R 1 SiO 3/2 units wherein R 1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 20 carbon atoms on surfaces of hydrophilic spherical silica particles consisting essentially of SiO 2 units, and further introducing R 2 2 SiO 1/2 units wherein R 2 is independently a substituted or unsubstituted monovalent hydrocarbon group of 1 to 6 carbon atoms on the surfaces having R 1 SiO 3/2 units introduced therein.
• the spherical silica particles should have an average particle size of 5 nm to 1.00 ⁇ m, a particle size distribution D 90 /D 10 of up to 3, and an average circularity of 0.8 to 1.
• R 1 SiO 3/2 units wherein R 1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 20 carbon atoms on surfaces of hydrophilic spherical silica particles consisting essentially of SiO 2 units, and further introduced R 2 3 SiO 1/2 units wherein R 2 is independently a substituted or unsubstituted monovalent hydrocarbon group of 1 to 6 carbon atoms on the surfaces having R 1 SiO 3/2 units introduced thereon, and (2) they have an average particle size of 5 nm to 1.00 ym, a particle size distribution D 90 /D 10 of up to 3, and an average circularity of 0.8 to 1.
• the hydrophobic spherical silica particles are obtained by introducing R 1 SiO 3/2 units on surfaces of hydrophilic spherical silica particles consisting essentially of SiO 2 units, and further introducing R 2 3 SiO 112 units on the surfaces.
• the hydrophobic spherical silica particles are obtained, for example, by effecting hydrolysis and condensation of a tetrafunctional silane compound to form a sol-gel method silica precursor having a submicron particle size (i.e., hydrophilic spherical silica particles), and subjecting it to a specific hydrophobic surface treatment. After the hydrophobic treatment, there are obtained hydrophobic spherical silica particles which have a submicron particle size, i.e., maintain the primary particle size of the silica precursor, are free of agglomerates, and capable of absorbing water in electrodes.
• a submicron particle size i.e., hydrophilic spherical silica particles
• hydrophilic spherical silica particles “consisting essentially of SiO 2 units” means that particles are basically composed of SiO 2 units and have silanol groups at least on their surface as is well known in the art. This means that sometimes, some of hydrolyzable groups (hydrocarbyloxy groups) on the starting material, tetrafunctional silane compound of the general formula (I) or partial hydrolytic condensate thereof (sometimes collectively referred to as tetrafunctional silane compound) may be left, in minor amounts, on the surface or in the interior of particles, without conversion to silanol groups.
• the hydrophobic spherical silica particles After hydrophobic surface treatment, the hydrophobic spherical silica particles are hydrophobized on their surface, but silanol groups remain in their interior.
• the hydrophobic spherical silica particles which are incorporated in an electrode, can effectively absorb internal moisture during electrode preparation and thereafter, contributing to electrode stability. It is noted that residual silanol groups can be confirmed by solid NMR spectroscopy.
• the hydrophobic spherical silica particles should have an average particle size of 5 nm to 1.00 ⁇ m, preferably 10 to 300 nm, more preferably 30 to 200 nm, and even more preferably 30 to 100 nm. If the particle size is less than 5 nm, the effect of inhibiting entry of water into the battery may become insufficient. If the particle size exceeds 1.00 ⁇ m, the effect of removing water from the electrode material may become insufficient.
• the term “average particle size” of spherical silica particles is a volume basis median diameter in the particle size distribution measurement by the laser diffraction scattering method.
• the hydrophobic spherical silica particles should have a D 90 /D 10 value of up to 3, preferably up to 2.9, provided that D 90 /D 10 is an index of particle size distribution.
• the lower limit of D 90 /D 10 is not critical for the reason that the lower the D 90 /D 10 value, the better are the results. Most often, the lower limit of D 90 /D 10 is about 2. It is noted that D 10 designates a particle diameter corresponding to cumulative 10% by volume counting from the smaller side, and D 90 designates a particle diameter corresponding to cumulative 90% by volume counting from the smaller side, both in the particle size distribution as measured by the laser diffraction scattering method.
• a D 90 /D 10 value of up to 3 indicates a sharp particle size distribution. A sharp particle size distribution ensures that hydrophobic spherical silica particles are uniformly dispersed in the electrode material, which is effective for inhibiting entry of water or moisture.
• the term “spherical” includes true spheres and somewhat deformed spheres, and refers to those spheres having an average circularity of 0.8 to 1, preferably 0.92 to 1.
• the circularity is defined as the circumference of a circle equal to the area of a particle divided by the peripheral length of the particle, which may be determined by analysis of particle images taken under an electron microscope or the like. It is also preferred from the standpoint of imparting good fluidity that the spherical silica particles be primary particles.
• the hydrophobic spherical silica particles are obtained through the steps of (A1) subjecting a tetrafunctional silane compound, a partial hydrolytic condensate thereof or a combination thereof to hydrolysis and condensation to form hydrophilic spherical silica particles consisting essentially of S10 2 units, (A2) introducing R 1 SiO 3/2 units on surfaces of the hydrophilic spherical silica particles, and (A3) further introducing R 2 3 SiO 1/2 units on the surfaces having R 1 SiO 3/2 units introduced thereon.
• the preferred method for preparing hydrophobic spherical silica particles involves:
• the step (A1) of forming hydrophilic spherical silica particles includes subjecting a tetrafunctional silane compound having the general formula (I):
• R 3 is each independently a monovalent hydrocarbon group of 1 to 6 carbon atoms, a partial hydrolyzate thereof or a mixture thereof to hydrolysis and condensation in a mixture of a hydrophilic organic solvent and water in the presence of a basic substance, thereby forming a dispersion of hydrophilic spherical silica particles consisting essentially of SiO 2 units in the solvent mixture.
• R 3 is each independently a monovalent hydrocarbon group of 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, and more preferably 1 to 2 carbon atoms.
• exemplary of the monovalent hydrocarbon group are alkyl groups such as methyl, ethyl, propyl, and butyl, and aryl groups such as phenyl. Of these, methyl, ethyl, propyl, and butyl are preferred, with methyl and ethyl being most preferred.
• Preferred examples of the tetrafunctional silane compound having formula (I) include tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane, and tetraphenoxysilane. Of these, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane are preferred, with tetramethoxysilane and tetraethoxysilane being most preferred. Those tetraalkoxysilanes whose alkoxy moiety has a smaller carbon count are preferred for the purpose of obtaining spherical silica particles having a smaller particle size.
• Exemplary of the partial hydrolyzate of tetrafunctional silane compound are alkyl silicates such as methyl silicate and ethyl silicate.
• the hydrophilic organic solvent is not particularly limited as long as the tetrafunctional silane compound having formula (I), partial hydrolytic condensate thereof and water are soluble or miscible.
• Suitable solvents include alcohols, cellosolves such as methyl cellosolve, ethyl cellosolve, butyl cellosolve, and cellosolve acetate, ketones such as acetone and methyl ethyl ketone, and ethers such as dioxane and tetrahydrofuran, which may be used alone or in admixture of two or more.
• the alcohols and cellosolves are preferred, with the alcohols being more preferred.
• the alcohols used herein include alcohols having the general formula (V):
• R 5 is a monovalent hydrocarbon group of 1 to 6 carbon atoms.
• R 5 is a monovalent hydrocarbon group of 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, and more preferably 1 to 2 carbon atoms.
• Typical of the monovalent hydrocarbon group are alkyl groups such as methyl, ethyl, propyl, isopropyl, and butyl. Of these, methyl, ethyl, propyl, and isopropyl are preferred, with methyl and ethyl being most preferred.
• Examples of the alcohol having formula (V) include methanol, ethanol, propanol, isopropanol and butanol, with methanol and ethanol being preferred. For the reason that the particle size of spherical silica particles increases as the carbon count of alcohol increases, methanol is most preferred for the purpose of obtaining spherical silica particles with a smaller particle size.
• water is preferably used in such amounts that 0.5 to 5 moles, more preferably 0.6 to 2 moles, and even more preferably 0.7 to 1 mole of water is present per mole of total hydrocarbyloxy groups on the tetrafunctional silane compound.
• the hydrophilic organic solvent and water are preferably mixed such that the weight ratio of organic solvent to water may range from 0.5/1 to 10/1, more preferably from 3 to 9, and even more preferably from 5 to 8. As the proportion of hydrophilic organic solvent increases, the particle size of spherical silica particles becomes smaller.
• Suitable basic substances include ammonia, dimethylamine, and diethylamine, which may be used alone or in admixture. Inter alia, ammonia and diethylamine are preferred, with ammonia being most preferred.
• a predetermined amount of the basic substance is dissolved in water to form an aqueous basic solution, which may be mixed with the hydrophilic organic solvent.
• An appropriate amount of the basic substance is 0.01 to 2 moles, more preferably 0.02 to 0.5 mole, and even more preferably 0.04 to 0.12 mole per mole of total hydrocarbyloxy groups on the tetrafunctional silane compound. As the amount of the basic substance used is reduced within the range, the particle size of spherical silica particles becomes smaller.
• the hydrolysis and condensation of the tetrafunctional silane compound may be carried out by the well-known technique, specifically by adding the tetrafunctional silane compound to a solvent mixture of the hydrophilic organic solvent and water containing the basic substance.
• a temperature of 5 to 60° C. and a time of 0.5 to 10 hours are preferred. As the temperature becomes higher within the range, the particle size of spherical silica particles becomes smaller.
• This dispersion (A1) typically has a concentration of 3 to 15% by weight, preferably 5 to 10% by weight.
• Step (A2) of first hydrophobic surface treatment with trifunctional silane compound includes adding a trifunctional silane compound having the general formula (II):
• R 1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 20 carbon atoms
• R 4 is each independently a monovalent hydrocarbon group of 1 to 6 carbon atoms, a partial hydrolyzate thereof or a mixture thereof to the dispersion from step (A1), for conducting surface treatment of the hydrophilic spherical silica particles, thereby obtaining a dispersion of spherical silica particles having R 1 SiO 3/2 units (wherein R 1 is as defined above) introduced on their surface in the solvent mixture.
• This step (A2) is essential to prevent spherical silica particles from agglomerating during the subsequent concentration step (A′). If agglomeration is not restrained, the resulting hydrophobic spherical silica particles may fail to maintain the primary particle size and lose the water removal ability.
• R 1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 20 carbon atoms, preferably 1 to 3 carbon atoms, and more preferably 1 to 2 carbon atoms.
• exemplary of the monovalent hydrocarbon group R 1 are alkyl groups such as methyl, ethyl, n-propyl, isopropyl, butyl, and hexyl. Of these, methyl, ethyl, n-propyl, and isopropyl are preferred, with methyl and ethyl being most preferred.
• substituted forms of monovalent hydrocarbon groups in which some or all hydrogen atoms are substituted by halogen atoms such as fluorine, chlorine or bromine, preferably fluorine.
• R 4 is each independently a monovalent hydrocarbon group of 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms, and more preferably 1 to 2 carbon atoms.
• exemplary of the monovalent hydrocarbon group R 4 are alkyl groups such as methyl, ethyl, propyl, and butyl. Of these, methyl, ethyl, and propyl are preferred, with methyl and ethyl being most preferred.
• Exemplary of the trifunctional silane compound having formula (II) are trialkoxysilanes including methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, isopropyltrimethoxysilane, isopropyltriethoxysilane, butyltrimethoxysilane, butyltriethoxysilane, hexyltrimethoxysilane, trifluoropropyltrimethoxysilane, and heptadecafluorodecyltrimethoxysilane, and partial hydrolyzates thereof, which may be used alone or in admixture.
• methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, and partial hydrolyzates thereof are preferred, with methyltrimethoxysilane, methyltriethoxysilane, and partial hydrolyzates thereof being most preferred.
• An appropriate amount of the trifunctional silane compound having formula (II) added is 0.001 to 1 mole, more preferably 0.01 to 0.1 mole, and even more preferably 0.01 to 0.05 mole per mole of silicon atoms in the hydrophilic spherical silica particles. If the addition amount is less than 0.001 mole, the resulting hydrophobic particles may be less dispersible. An addition amount in excess of 1 mole may allow spherical silica particles to agglomerate together in step (A2).
• step (A2) a trifunctional silane compound having formula (II), a partial hydrolyzate thereof or a mixture thereof (sometimes collectively referred to as trifunctional silane compound) is added to the dispersion from step (A1) for thereby conducting surface treatment of the hydrophilic spherical silica particles.
• the hydrophilic spherical silica particles having R 1 SiO 3/2 units introduced on their surface are dispersed in the solvent mixture.
• This dispersion (A2) preferably has a concentration of 3% by weight to less than 15% by weight, more preferably 5 to 10% by weight. Outside the range, a lower concentration may lead to a drop of productivity whereas a higher concentration may allow spherical silica particles to agglomerate together in step (A2).
• step (A2) may be followed by a concentration step (A′) of removing a portion of the hydrophilic organic solvent and water from the dispersion from step (A2). That is, the dispersion (A2) may be concentrated into a concentrated dispersion (A′).
• the means of removing a portion of the hydrophilic organic solvent and water may be, for example, distillation or vacuum distillation.
• the temperature may be selected as appropriate depending on the hydrophilic organic solvent and its proportion, and a temperature of 60 to 110° C. is typically used.
• a hydrophobic solvent may be added prior to or during the concentration step. Suitable hydrophobic solvents include hydrocarbon and ketone solvents, which may be used alone or in admixture. Examples include toluene, xylene, methyl ethyl ketone, and methyl isobutyl ketone, with methyl isobutyl ketone being preferred.
• the concentrated dispersion (A′) should preferably contain 15 to 40% by weight, more preferably 20 to 35% by weight, and even more preferably 25 to 30% by weight of spherical silica particles.
• a concentration of at least 15% by weight ensures smooth progress of the subsequent step (A3) whereas a concentration in excess of 40% by weight may cause spherical silica particles to agglomerate together during step (A3).
• the concentration step (A′) is effective to prevent a failure in the subsequent step (A3), i.e., the phenomenon that a silazane compound having formula (III), a monofunctional silane compound having formula (IV) or a mixture thereof serving as a surface treating agent can react with the hydrophilic organic solvent and water to make the surface treatment insufficient, allowing the resulting hydrophobic spherical silica particles to agglomerate together in subsequent drying so that the hydrophobic spherical silica particles fail to maintain the primary particle size, thus preventing effective drying and recovery.
• a failure in the subsequent step (A3) i.e., the phenomenon that a silazane compound having formula (III), a monofunctional silane compound having formula (IV) or a mixture thereof serving as a surface treating agent can react with the hydrophilic organic solvent and water to make the surface treatment insufficient, allowing the resulting hydrophobic spherical silica particles to agglomerate together in subsequent drying so that the hydrophobic
• the step (A3) of second hydrophobic surface treatment with monofunctional silane compound includes adding a silazane compound having the general formula (III):
• R 2 is each independently a substituted or unsubstituted monovalent hydrocarbon group of 1 to 6 carbon atoms, a monofunctional silane compound having the general formula (IV):
• R 2 is as defined above and X is an OH group or hydrolyzable group, or a mixture thereof to the optionally concentrated dispersion from step (A2) or (A′), for conducting surface treatment of the spherical silica particles having R 1 SiO 3/2 units introduced thereon, thereby introducing R 2 3 SiO 1/2 units (wherein R 2 is as defined above) on their surface.
• the silazane compound and monofunctional silane compound may be used alone or in admixture of two or more.
• R 2 is each independently a substituted or unsubstituted monovalent hydrocarbon group of 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, and more preferably 1 to 2 carbon atoms.
• exemplary of the monovalent hydrocarbon group R 2 are alkyl groups such as methyl, ethyl, propyl, isopropyl, and butyl. Of these, methyl, ethyl, and propyl are preferred, with methyl and ethyl being most preferred.
• substituted forms of monovalent hydrocarbon groups in which some or all hydrogen atoms are substituted by halogen atoms such as fluorine, chlorine or bromine, preferably fluorine.
• X is a hydroxyl or hydrolyzable group.
• hydrolyzable group are chlorine, alkoxy, amino, and acyloxy groups. Inter alia, alkoxy and amino groups are preferred, with the alkoxy groups being more preferred.
• silazane compound having formula (III) examples include hexamethyldisilazane and hexaethyldisilazane, with hexamethyldisilazane being preferred.
• Examples of the monofunctional silane compound having formula (IV) include monosilanol compounds such as trimethylsilanol and triethylsilanol, monochlorosilanes such as trimethylchlorosilane and triethylchlorosilane, monoalkoxysilanes such as trimethylmethoxysilane and trimethylethoxysilane, monoaminosilanes such as trimethylsilyldimethylamine and trimethylsilyldiethylamine, and monoacyloxysilanes such as trimethylacetoxysilane.
• monosilanol compounds such as trimethylsilanol and triethylsilanol
• monochlorosilanes such as trimethylchlorosilane and triethylchlorosilane
• monoalkoxysilanes such as trimethylmethoxysilane and trimethylethoxysilane
• monoaminosilanes such as trimethylsily
• the silazane compound and/or monofunctional silane compound is added in an amount of 0.1 to 0.5 mole, more preferably 0.2 to 0.4 mole, and even more preferably 0.25 to 0.35 mole per mole of silicon atoms of the hydrophilic spherical silica particles. If the addition amount is less than 0.1 mole, the resulting particles may be less dispersible. More than 0.5 mole of the compound may be uneconomical.
• step (A3) a silazane compound having formula (III), a monofunctional silane compound having formula (IV), or a mixture thereof is added to the optionally concentrated dispersion from step (A2) or (A′), whereby the spherical silica particles having R 1 SiO 3/2 units introduced thereon are surface treated with the relevant compound. There are obtained hydrophobic spherical silica particles further having R 2 3 SiO 1/2 units introduced on their surface.
• R 1 SiO 3/2 units are preferably introduced in such an amount to provide 0.001 to 1 mole, more preferably 0.01 to 0.1 mole, and even more preferably 0.01 to 0.05 mole of silicon atoms in R 1 SiO 3/2 units; and R 2 3 SiO 1/2 units are preferably introduced in such an amount to provide 0.1 to 0.5 mole, more preferably 0.2 to 0.4 mole, and even more preferably 0.25 to 0.35 mole of silicon atoms in R 2 3 SiO 1/2 units, both per mole of silicon atoms of the hydrophilic spherical silica particles.
• the hydrophobic spherical silica particles are obtained as dispersed in the solvent mixture.
• the dispersion (A3) preferably has a concentration of 15 to 40% by weight.
• the hydrophobic spherical silica particles are recovered in powder form.
• the hydrophobic spherical silica particles are incorporated in an electrode material of which a positive and/or negative electrode of a nonaqueous electrolyte secondary battery is made.
• the particles may be incorporated in either the electrode material for positive electrode or the electrode material for negative electrode, or both.
• Preferably the particles are incorporated in both the positive and negative electrode materials.
• hydrophobic spherical silica particles incorporated in the electrode material If the proportion of hydrophobic spherical silica particles incorporated in the electrode material is too high, the hydrophobic spherical silica particles function as electric resistance, thus adversely affecting the rate characteristics of the battery, rather than exerting the effect of absorbing water in the battery and the effect of inhibiting entry of water into the battery.
• the content (in % by weight of solids) of hydrophobic spherical silica particles is 0.1 to 5% by weight, more preferably 0.5 to 5% by weight of the positive electrode material, and 0.1 to 5% by weight, more preferably 0.5 to 5% by weight of the negative electrode material.
• the nonaqueous electrolyte secondary battery is defined as comprising a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a nonaqueous electrolyte.
• the nonaqueous electrolyte secondary battery is a lithium ion secondary battery.
• the positive electrode material comprises a positive electrode active material, conductive agent, binder, viscosity modifier, and the like.
• the positive electrode active materials may be used alone or in admixture of two or more. Suitable positive electrode active materials include lithium, lithium-containing complex oxides, and composite metals such as NbSe 2 . Also included are sulfides and oxides of metals exclusive of lithium such as TiS 2 , MoS 2 , NbS 2 , ZrS 2 , VS 2 , V 2 O 5 , MoO 3 , and Mg(V 3 O 8 ) 2 . For increasing the energy density, lithium complex oxides based on Li p MetO 2 are preferred wherein Met is preferably at least one element of cobalt, nickel, iron and manganese and p has a value in the range: 0.05 ⁇ p ⁇ 1.10.
• Illustrative examples of the lithium complex oxides include LiCoO 2 , LiNiO 2 , LiFeO 2 , and Li q Ni r Co 1-r O 2 (wherein q and r have values varying with the charged/discharged state of the battery and usually in the range: 0 ⁇ q ⁇ 1 and 0.7 ⁇ r ⁇ 1) having a layer structure, LiMn 2 O 4 having a spinel structure, and rhombic LiMnO 2 .
• a substitutional spinel type manganese compound LiMet s Mn 1-s O 4 wherein Met is titanium, chromium, iron, cobalt, nickel, copper, zinc or the like and s has a value in the range: 0 ⁇ s ⁇ 1.
• the lithium complex oxide may be prepared, for example, by grinding and mixing a carbonate, nitrate, oxide or hydroxide of lithium and a carbonate, nitrate, oxide or hydroxide of a transition metal in accordance with the desired composition, and firing at a temperature in the range of 600 to 1,000° C. in an oxygen atmosphere.
• Organic materials may also be used as the positive electrode active material. Examples include polyacetylene, polypyrrole, poly-p-phenylene, polyaniline, polythiophene, polyacene, and polysulfide.
• the negative electrode material comprises a negative electrode active material, conductive agent, binder, viscosity modifier, and the like.
• the negative electrode active materials may be used alone or in admixture of two or more.
• Suitable negative electrode active materials are carbon materials including non-graphitizable carbon, graphitizable carbon, graphite, pyrolytic carbons, coke, vitreous carbons, fired organic polymers, carbon fibers, and active carbon.
• materials capable of occluding and releasing lithium ions and containing at least one of metal and semi-metal elements silicon; composites in which silicon nanoparticles are dispersed in a silicon compound; silicon oxides of the formula: SiO x wherein 0.5 ⁇ x ⁇ 1.6; silicon-free metal oxides represented by the formula: MO a wherein M is at least one metal selected from among Ge, Sn, Pb, Bi, Sb, Zn, In and Mg and “a” is a positive number of 0.1 to 4; and lithium complex oxides (which may contain silicon) represented by the formula: LiM b O c wherein M is at least one metal selected from among Ge, Sn, Pb, Bi, Sb, Zn, In, Mg and Si, b is a positive number of 0.1 to 4, and c is a positive number of 0.1 to 8.
• Specific examples include GeO, GeO 2 , SnO, SnO 2 , Sn 2 O 3 , Bi 2 O 3 , Bi 2 O 5 , Sb 2 O 3 , Sb 2 O 4 , Sb 2 O 5 , ZnO, In 2 O, InO, In 2 O 3 , MgO, Li 2 SiO 3 , Li 4 SiO 4 , Li 2 Si 3 O 7 , Li 2 Si 2 O 5 , Li 8 SiO 6 , Li 6 Si 2 O 7 , Li 4 Ge 9 O 7 , Li 4 Ge 9 O 2 , Li 5 Ge 8 O 19 , Li 4 Ge 5 O 12 , Li 5 Ge 2 O 7 , Li 4 GeO 4 , Li 2 Ge 7 O 15 , Li 2 GeO 3 , Li 2 Ge 4 O 9 , Li 2 SnO 3 , Li 8 SnO 6 , Li 2 PbO 3 , Li 7 SbO 5 , LiSbO 3 , Li 3 SbO 4 , Li 3 BiO 5 , Li 6 Bi
• silicon (Si), composites having silicon nanoparticles dispersed in a silicon compound, and silicon oxides SiO x wherein 0.5 ⁇ x ⁇ 1.6 are preferred.
• the negative electrode material is in the form of particles, preferably having an average particle size of 0.01 to 30 ⁇ m, more preferably 0.1 to 10 ⁇ m, and even more preferably 0.5 to 6 ⁇ m.
• the average particle size is a cumulative 50% by volume diameter D 50 , i.e., a particle diameter corresponding to cumulative 50% by volume in the particle size distribution measurement by the laser diffraction scattering method.
• the positive electrode material comprises a positive electrode active material, conductive agent, binder, viscosity modifier, and the like.
• the positive electrode material comprises 90 to 98% of the active material, 0.5 to 5.0% of the conductive agent, 0.5 to 5.0% of the binder, and 0% or 0.1 to 3.0% of the viscosity modifier, expressed in % by weight of solids.
• the negative electrode material comprises a negative electrode active material, conductive agent, binder, viscosity modifier, and the like.
• the negative electrode material comprises 75 to 98%, more preferably 80 to 98% of the active material, 1 to 20%, more preferably 2 to 10% of the conductive agent, 1 to 20%, more preferably 3 to 10% of the binder, and 0% or 0.1 to 3.0% of the viscosity modifier, expressed in % by weight of solids.
• Suitable conductive agents include metal powders and fibers such as Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, and Si, and various types of graphite such as natural graphite, synthetic graphite, various coke powders, meso-phase carbon, vapor grown carbon fibers, pitch-based carbon fibers, PAN-based carbon fibers, and fired resins. They may be used alone or in admixture.
• Suitable binders include polyimide resins, polyamide resins, and polyamide-imide resins, specifically polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR). They may be used alone or in admixture.
• PVDF polyvinylidene fluoride
• SBR styrene-butadiene rubber
• Suitable viscosity modifiers include carboxymethyl cellulose, sodium polyacrylate, other acrylic polymers and fatty acid esters. They may be used alone or in admixture.
• the positive or negative electrode material may be shaped into a positive or negative electrode form by the following exemplary procedure.
• the positive or negative electrode material comprising the active material, conductive agent, binder and other additives is combined with a solvent suitable for dissolving or dispersing the binder, such as N-methylpyrrolidone or water, and kneaded into a paste mix, which is applied in sheet form to a current collector.
• the current collector may be copper foil, nickel foil or any other materials which are typically used as the electrode current collector while its thickness and surface treatment are not critical.
• the method of shaping the mix into a sheet is not particularly limited and any well-known methods may be used.
• the positive or negative electrode shaped form has a thickness of about 3 to 500 ⁇ m, though the thickness is not critical.
• the separator interposed between the positive and negative electrodes is not particularly limited as long as it is stable to the electrolytic solution and holds the solution effectively.
• the separator is most often a porous sheet or non-woven fabric of polyolefins such as polyethylene, polypropylene and copolymers thereof and aramide resins. Such sheets may be used as a single layer or a laminate of multiple layers. Ceramics such as metal oxides may be deposited on the surface of sheets. Porous glass and ceramics are employed as well.
• Exemplary of the nonaqueous electrolyte are light metal salts.
• the light metal salts include salts of alkali metals such as lithium, sodium and potassium, salts of alkaline earth metals such as magnesium and calcium, and aluminum salts. A choice may be made among these salts and mixtures thereof depending on a particular purpose.
• lithium salts examples include LiBF 4 , LiClO 4 , LiPF 6 , LiAsF 6 , CF 3 SO 3 Li, (CF 3 SO 2 ) 2 NLi, C 4 F 9 SO 3 Li, CF 3 CO 2 Li, (CF 3 CO 2 ) 2 NLi, C 6 F 5 SO 3 Li, C 8 F 17 SO 3 Li, (C 2 F 5 SO 2 ) 2 NLi, (C 4 F 9 SO 2 )(CF 3 SO 2 )NLi, (FSO 2 C 6 F 4 )(CF 3 SO 2 )NLi, ((CF 3 ) 2 CHOSO 2 ) 2 NLi, (CF 3 SO 2 ) 3 CLi, (3,5-(CF 3 ) 2 C 6 F 3 ) 4 BLi, LiCF 3 , LiAlCl 4 , and C 4 BO 8 Li, which may be used alone or in admixture.
• the nonaqueous solvent is not particularly limited as long as it can serve for the nonaqueous electrolytic solution.
• Suitable solvents include aprotic high-dielectric-constant solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, and ⁇ -butyrolactone; and aprotic low-viscosity solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, dipropyl carbonate, diethyl ether, tetrahydrofuran, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,3-dioxolan, sulfolane, methylsulfolane, acetonitrile, propionitrile, anisole, acetic acid esters, e.g., methyl acetate and propionic acid esters.
• aprotic high-dielectric-constant solvents such as ethylene carbonate, propy
• ionic liquids containing imidazolium, ammonium and pyridinium cations.
• the counter anions are not particularly limited and include BF 4 ⁇ , PF 6 ⁇ and (CF 3 SO 2 ) 2 N ⁇ .
• the ionic liquid may be used in admixture with the foregoing solvent for nonaqueous electrolytic solution.
• a glass based inorganic solid electrolyte, polyether gel, silicone gel, silicone polyether gel, acrylic gel, silicone acrylic gel, acrylonitrile gel, poly(vinylidene fluoride) or the like may be included as polymer component. These ingredients may be polymerized prior to or after casting. They may be used alone or in admixture.
• various additives may be added to the nonaqueous electrolytic solution.
• examples include an additive for improving cycle life such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate and 4-vinylethylene carbonate, an additive for preventing over-charging such as biphenyl, alkylbiphenyl, cyclohexylbenzene, t-butylbenzene, diphenyl ether, and benzofuran, and an additive for acid removal and water removal purposes such as various carbonate compounds (e.g., carbon dioxide gas), carboxylic acid anhydrides, nitrogen- and sulfur-containing compounds.
• carbonate compounds e.g., carbon dioxide gas
• carboxylic acid anhydrides e.g., nitrogen- and sulfur-containing compounds.
• the nonaqueous electrolyte secondary battery further comprises a casing for receiving the foregoing components in a tight seal manner.
• the battery may take any desired shape without particular limits.
• the battery is of the coin type wherein electrodes and a separator, all punched into coin shape, are stacked, or of the rectangular or cylinder type wherein electrode sheets and a separator are spirally wound.
• a 3-liter glass reactor equipped with a stirrer, dropping funnel and thermometer was charged with 989.5 g (to give a weight ratio of methanol to water of 5.4) of methanol, 135.5 g (3.6 moles per mole of tetramethoxysilane) of water, and 66.5 g (0.38 mole of ammonia per mole of tetramethoxysilane) of 28 wt % aqueous ammonia, which were mixed.
• 436.5 g (2.87 moles) of tetramethoxysilane was added dropwise over 6 hours. After the completion of dropwise addition, stirring was continued for a further 0.5 hour to conduct hydrolysis, yielding a suspension of hydrophilic spherical silica particles.
• the glass reactor was equipped with an ester adaptor and condenser tube.
• the dispersion from step (A2) was heated at 60-70° C., whereby methanol and water were distilled off in a total amount of 1,021 g, yielding a concentrated dispersion of hydrophobic spherical silica particles in the solvent mixture.
• the concentration of hydrophobic spherical silica particles in the concentrated dispersion was 28% by weight.
• step (A′) To the concentrated dispersion from step (A′) at room temperature, 138.4 g (0.86 mole, to give a molar ratio to silicon atoms of hydrophilic spherical silica particles of 0.3) of hexamethyldisilazane was added. The dispersion was heated at 50-60° C. for 9 hours for reaction, that is, trimethylsilylating reaction on silica particles in the dispersion. From the dispersion, the solvents were distilled off at 130° C. under a vacuum of 6,650 Pa, yielding 186 g of hydrophobic spherical silica particles (1).
• the hydrophilic spherical silica particles from step (A1) were analyzed by Measurement Method 1 as described below.
• the hydrophobic spherical silica particles obtained through steps (A1) to (A3) were analyzed by Measurement Methods 2 to 4 as described below. The results are shown in Table 1.
• the silica particle suspension was added to methanol so as to give a concentration of 0.5% by weight of silica particles.
• the suspension was ultrasonified for 10 minutes for dispersing the particles.
• the particle size distribution of thus dispersed particles was measured by a Nanotrac particle size distribution analyzer by the dynamic light scattering/laser Doppler method (trade name UPA-EX150 by Nikkiso Co., Ltd.).
• the median diameter on volume basis is reported as the average particle size.
• the median diameter is a particle diameter at cumulative 50% by volume when the particle size distribution is expressed as cumulative distribution.
• the silica particles were added to methanol so as to give a concentration of 0.5% by weight of silica particles. This was ultrasonified for 10 minutes for dispersing the particles.
• the particle size distribution of thus dispersed particles was measured by a Nanotrac particle size distribution analyzer by the dynamic light scattering/laser Doppler method (trade name UPA-EX150 by Nikkiso Co., Ltd.). The median diameter on volume basis is reported as the average particle size.
• the particle size distribution D 90 /D 10 was determined from measurements of D 10 and D 90 wherein D 10 and D 90 correspond to a particle diameter at cumulative 10% and 90% by volume counting from the smaller side in the particle size distribution measurement, respectively.
• Sample particles were observed under an electron microscope S-4700 (Hitachi, Ltd., magnifying power ⁇ 10 5 ) for inspecting their shape. Not only true spheres, but also somewhat deformed spheres are included in the “spherical” particles.
• the particle shape was evaluated in terms of circularity when a particle is projected as a two-dimensional image. Particles are regarded as “spherical” when they have a circularity in the range of 0.8 to 1.
• the circularity is defined as the circumference of a circle equal to the area of a particle divided by the peripheral length of the particle.
• a 3-liter glass reactor equipped with a stirrer, dropping funnel and thermometer was charged with 623.7 g of methanol, 41.4 g of water, and 49.8 g of 28 wt % aqueous ammonia, which were mixed.
• 1,163.7 g of tetramethoxysilane and 418.1 g of 5.4 wt % aqueous ammonia were concurrently added dropwise over 6 hours and 4 hours, respectively.
• stirring was continued for a further 0.5 hour to conduct hydrolysis, yielding a suspension of silica particles.
• the glass reactor was equipped with an ester adaptor and condenser tube.
• 1,440 g of methyl isobutyl ketone was added to the dispersion of surface treated silica particles from step (A2).
• the dispersion was heated at 80 to 110° C. for 7 hours, whereby methanol and water were distilled off.
• step (A′) To the concentrated dispersion from step (A′) at room temperature, 357.6 g of hexamethyldisilazane was added. The dispersion was heated at 120° C. for 3 hours for reaction, that is, trimethylsilylating reaction on silica particles. From the dispersion, the solvents were distilled off under vacuum, yielding 472 g of hydrophobic spherical silica particles (3). The silica particles (3) were analyzed as in Synthesis Example 1, with the results shown in Table 1.
• VMC silica (trade name SOCl by Admatechs Co., Ltd., VMC: vaporized metal combustion). With stirring, 1 g of deionized water was added. With the reactor closed, stirring was continued at 60° C. for 10 hours. The reactor was cooled to room temperature, after which with stirring, 2 g of hexamethyldisilazane was added. With the reactor closed, stirring was continued for a further 24 hours. The reactor was heated at 120° C., whereupon the residual reactant and ammonia formed were removed while feeding nitrogen gas. There was obtained 100 g of hydrophobic spherical silica particles (5). The silica particles (5) were analyzed as in Synthesis Example 1, with the results shown in Table 1.
• VMC silica (trade name SOCl by Admatechs Co., Ltd.). With stirring, 1 g of deionized water was added. With the reactor closed, stirring was continued at 60° C. for 10 hours. The reactor was cooled to room temperature, after which with stirring, 1 g of methyltrimethoxysilane was added. With the reactor closed, stirring was continued for a further 24 hours. With stirring, 2 g of hexamethyldisilazane was added. With the reactor closed, stirring was continued for a further 24 hours. The reactor was heated at 120° C., whereupon the residual reactants and ammonia formed were removed while feeding nitrogen gas. There was obtained 101 g of hydrophobic irregular silica particles (6). The silica particles (6) were analyzed as in Synthesis Example 1, with the results shown in Table 1.
• a paste mix was prepared by adding 95 wt % of LiCoO 2 as positive electrode active material, 2.5 wt % of conductive carbon black (Super-P) as conductive agent, and 2.5 wt % of polyvinylidene fluoride (PVdF) as binder to N-methyl-2-pyrrolidone (NMP) solvent.
• the hydrophobic spherical silica particles (1) was added to the paste in an amount of 0.1 wt % (calculated as solids) based on the total weight of positive electrode material, completing the paste mix.
• the paste mix was coated onto an aluminum foil, dried, and compressed to form a positive electrode (shaped form).
• Another paste mix was prepared by adding 95 wt % of synthetic graphite as negative electrode active material, 2.5 wt % of conductive carbon black (Super-P) as conductive agent, and 2.5 wt % of PVdF as binder to NMP solvent.
• the hydrophobic spherical silica particles (1) was added to the paste in an amount of 0.1 wt % (calculated as solids) based on the total weight of negative electrode material, completing the paste mix.
• the paste mix was coated onto an copper foil, dried, and compressed to form a negative electrode (shaped form).
• a separator (Celgard 2400 by Hoechst Celanese Corp.) was interposed between the positive and negative electrodes.
• EC/EMC electrolyte containing 1 M (mol/L) of LiP 6 lithium salt was injected into the separator. In this way, a lithium secondary battery was manufactured.
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (1) were not added to the positive electrode.
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (1) were not added to the negative electrode.
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (2) were used instead of (1).
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (3) were used instead of (1) in an amount of 2 wt % (calculated as solids) based on the total weight of each of negative and positive electrode materials.
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (3) were used instead of (1) in an amount of 5 wt % (calculated as solids) based on the total weight of each of negative and positive electrode materials.
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (4) were used instead of (1) in an amount of 3 wt % (calculated as solids) based on the total weight of each of negative and positive electrode materials.
• a lithium secondary battery was manufactured as in Example 1 except that the hydrophobic spherical silica particles were added to neither of the positive and negative electrodes.
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (5) were used instead of (1) in an amount of 2 wt % based on the total weight of each of negative and positive electrode materials.
• a lithium secondary battery was manufactured as in Example 1 except that hydrophobic spherical silica particles (6) were used instead of (1) in an amount of 2 wt % based on the total weight of each of negative and positive electrode materials.
• the electrodes prepared in Examples and Comparative Examples were measured for water content using a moisture counter AQ-7 (Hiranuma Sangyo Corp.) according to the Karl Fischer method.
• the lithium secondary batteries manufactured in Examples and Comparative Examples were fully charged and stored at 60° C. for 2 weeks.
• the batteries after storage were measured for capacity.
• a ratio (percent) of the capacity after storage to the initial capacity is reported in Table 2.
• the lithium secondary batteries manufactured in Examples and Comparative Examples were charged to 4.2 V. While the batteries were discharged at a current flow of 0.5 C or 5 C, a discharge capacity was measured. A ratio (percent) of the discharge capacity at 0.5 C to the discharge capacity at 5 C is reported in Table 2.
• the hydrophobic spherical silica particles within the scope of the invention are relatively hygroscopic and fully hydrophobic so that when the hydrophobic spherical silica particles within the scope of the invention are added to positive and negative electrodes, the resulting positive and negative electrodes have significantly reduced water contents, as seen from Table 2.
• the capacity of the battery after high-temperature storage is as high as at least 80% of the initial capacity.
• the discharge capacity ratio indicative of a ratio of low-current discharge capacity to high-current discharge capacity is also as high as at least 79%.

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