JP2014137871A - Battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery separator reduced in internal resistance.SOLUTION: In a battery separator comprising a nonwoven fabric base material and a porous film layer containing an alumina-based material, the nonwoven fabric base material is a nonwoven fabric base material in which an alumina-based material is incorporated.

Description

  The present invention relates to a battery separator.

  Conventionally, as a separator of a lithium battery, a polyolefin film having fine pores that have penetrated has been used. These separators suppress heat generation by increasing the internal resistance of the battery when the battery malfunctions and generate heat, and the through-holes are melted and closed. It has been responsible for suppressing battery explosion due to runaway.

  However, in applications that require charging and discharging with a large current, such as batteries for hybrid vehicles and uninterruptible power supplies, research on electrode composition has made it possible to suppress thermal runaway explosions, and conversely, rapid battery internals Due to the increase in temperature, in order to avoid electrode contact due to thermal contraction of the separator, there is an increasing demand for a separator having high heat resistance and low internal resistance.

  In response to this demand, Patent Document 1 proposes a separator made of a porous support material having pores and a porous ceramic material that closes the pores, polymer fibers for the flexible support material, zirconium for the porous ceramic material, Silica and alumina are used. By using a low-density support and a porous ceramic in combination, the porosity in the separator and the heat resistance can be improved.

  However, since such a separator is produced by coating a dispersion of a porous ceramic material on a flexible support, if the polymer fiber density of the flexible support is low, the strength of the support is reduced. There is a problem, and the dispersion of the coated porous ceramic material is detached from the support, so that a porous ceramic layer that is uniform in the surface direction cannot be obtained, resulting in a defective separator and an internal short circuit occurs. There was a problem that occurred. Moreover, when the polymer fiber density is too high, a problem that the thickness is increased occurs. In order to impart an appropriate polymer fiber density and suppress an increase in thickness, Patent Document 2 manufactures a support using a wet papermaking method that is excellent in polymer fiber dispersibility. It is disclosed that a support having a balance between strength and thickness can be obtained while using ultrafine fibers.

  However, in this method, when the hot-melt binder fiber for maintaining the strength of the support is deformed due to the influence of heat treatment or the like, ideally, the strength is increased only at the entangled portion of the fiber. In some cases, the binder fiber that develops develops into a film by fusing the fibers to form a film, and the vertical through-holes of the support, which would have been conventionally formed, are buried. In particular, when fine fibers with high electrolyte retention are used, a dense matrix is formed, and there is a high possibility that the fibers melt by heat, and this is actually manifested as an increase in internal resistance during battery operation. It was causing the problem.

Japanese Patent No. 4594098 JP 2009-230975 A

  An object of the present invention is to provide a battery separator with reduced internal resistance.

  As a result of intensive studies, it has been found that the above-described problems can be solved by the present invention described below.

  A battery separator comprising a porous membrane layer containing an alumina-based material and a nonwoven fabric base material, wherein the nonwoven fabric base material is a nonwoven fabric base material in which an alumina-based material is internally added. Battery separator.

  By using the battery separator of the present invention, a battery separator having a low internal resistance can be obtained.

  The alumina-based material of the present invention is α, β, γ-alumina typified by alumina oxide, boehmite, pseudoboehmite typified by hydroxylated alumina, or the like. In particular, boehmite is a preferable material because various forms such as a plate shape, a granular shape, and a fiber shape can be obtained by a hydrothermal synthesis method. The alumina-based material is used as fine particles, and the particle diameter is preferably 10 nm to 10 μm, more preferably 12 nm to 5 μm. The particle size is determined by sufficiently diluting the inorganic oxide with water and measuring this with a laser scattering type particle size measuring device (trade name: 3300EX2 manufactured by Microtrac Co., Ltd.), and the obtained center particle size (D50, volume average). ) As the particle size.

  The nonwoven fabric substrate of the present invention is preferably produced by a wet papermaking method. The wet papermaking method is a method in which fibers are dispersed in water at a low concentration, a fiber matrix is made on a net, and matrix strength is given by drying or heat treatment to obtain a sheet-like support. There are various types of fibers used for batteries, such as cellulose, polyethylene, polypropylene, polyester, and polyamide, and cellulose, polypropylene, polyester, and the like are particularly preferable materials. Such a material is manufactured in advance into a long fiber shape by a hot melting method or a precipitation method, and then cut into a predetermined length to be used as a short fiber. The fiber length suitable for the wet papermaking method is preferably 1 to 30 mm, more preferably 3 to 15 mm. Moreover, although it is preferable to contain the short fiber of a fineness 1.0 decitex (dt) or less, when producing a thinner battery separator, it is more preferable to contain the short fiber of 0.6 dt or less.

  In a general wet papermaking method, fibers are dispersed in water at a low concentration of about 0.01 to 0.2%, and then made from water using a net such as a long net, a short net, or a round net, A web can be formed. In this process, when the alumina-based material, which is fine particles, coexists with the fiber in water and then agglomerates on the fiber surface, the web containing the fiber and the alumina-based material is finished. The finished web is hot-pressed through a drying process to become a nonwoven fabric substrate. When the alumina material is aggregated in water, various aggregating agents such as nonionic, anionic, cationic and amphoteric are used while adjusting the pH. A particularly preferable method for agglomerating the alumina-based material is a method in which an anionic flocculant is added, and then a cationic flocculant is further added for aggregation.

  Anionic flocculants include water-soluble polymers such as polycarboxylic acids and carboxylic acid-modified water-soluble acrylic resins, polyacrylamide, acid-modified polyacrylamide, acid-modified polyethylene glycol, acid-modified polyvinyl alcohol, and carboxylic acid-modified cellulose. It is used as an alkali metal or alkaline earth metal salt such as ammonium salt, sodium or potassium. Examples of the cationic flocculant include water-soluble polymers such as acrylic resins containing quaternary ammonium salts, polyacrylamide, polyethylene glycol, polyvinyl alcohol, chitosan, modified chitosan, polydiallyldimethylammonium salts.

  Agglomerates of the alumina-based material formed by the polymer flocculant are captured by the short fibers and become a form (internal addition) attached to the fiber surface in the web. The size of the obtained aggregate is preferably about several hundred nm to 10 μm. If it is too fine, it cannot be captured by the short fibers, leading to a decrease in yield. In order to prevent such a decrease in yield, fibrillated cellulose having a finer fiber diameter may be used in combination. Further, if the amount of internal addition is too large, the strength physical properties of the support deteriorate, so the content ratio of the alumina-based material to the short fibers is preferably 1 to 50% by mass, more preferably 2 to 20% by mass. It is.

After the obtained web is dried, the thickness is made uniform by hot pressing to form a nonwoven fabric substrate. In order to make the battery separator of the present invention thin, the nonwoven fabric base material preferably has a thickness of 10 to 25 μm, and the porosity is preferably 30 to 80%. More preferably, the thickness is 12 to 18 μm and the porosity is 40 to 70%. Preferably the weight per unit area is 5~18g / ^{m 2,} more preferably from 8~15g / ^{m 2.} The heating temperature during the hot press treatment is preferably 180 to 250 ° C, more preferably 200 to 245 ° C. The nip pressure between the rolls that imparts smoothness is preferably 190 to 1800 N / cm, and more preferably 400 to 1500 N / cm.

  The battery separator of the present invention is a battery separator composed of a porous membrane layer containing an alumina-based material and a nonwoven fabric substrate, and is a porous separator containing an alumina-based material on the nonwoven fabric substrate. A membrane layer is provided. The alumina-based material may penetrate into the nonwoven fabric substrate. For alumina-based materials, first, a dispersion using a surfactant or the like is prepared, and a coating solution is prepared by combining a binder such as a polymer latex and a thickening agent, and then applied onto the nonwoven fabric substrate. Is done.

  A preferred surfactant is a nonionic surfactant such as a fatty acid ester. A particularly preferred material is an aliphatic glyceryl compound such as glyceryl laurate. Preferred binders include acrylic resins, styrene-acrylic resins, styrene butadiene rubber (SBR) resins, modified polyolefin resins, and latexes thereof. A preferred thickener is carboxymethylcellulose.

  As a dispersion method, mixing with a stirrer is sufficient when the concentration is low. However, when the concentration exceeds several percent, the overall viscosity and the like increase, and therefore, it is preferable to use a homogenizer or a bead mill. A preferable stirring and mixing time is about several minutes to 40 hours, more preferably about 1 hour to 20 hours.

The porous membrane layer can be obtained by applying or casting a non-woven fabric base material with a coating liquid containing an alumina-based material and drying it. As an application or casting method, an air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll coater, die coater, reverse roll coater, transfer roll coater, spray coater, etc. The method used can be used. The coating amount of the porous membrane layer is preferably 0.5 to 50 g / ^{m 2} by dry weight, more preferably from 1 to 30 g / ^{m 2.} It is also possible to adjust the thickness of the obtained battery separator by further applying a heat calendering treatment after drying. The preferable thickness of the battery separator having a porous support is 10 to 30 μm, more preferably 12 to 25 μm.

  The obtained separator is cut and sandwiched between electrode materials for a lithium battery, an electrolyte is injected, the battery is sealed, and a lithium battery is obtained. The material constituting the positive electrode is mainly an active material and a conductive agent such as carbon black, a binder such as polyvinylidene fluoride or styrene butadiene rubber, and the active material includes lithium cobaltate, lithium nickelate, lithium manganate. In addition, lithium manganese composite oxides such as lithium nickel manganese cobaltate (NMC) and lithium aluminum manganate (AMO), and lithium iron phosphate are used. These are mixed and apply | coated on the aluminum foil which is a collector, and become a positive electrode.

The material constituting the negative electrode is mainly an active material, a conductive agent, and a binder. As the active material, graphite, amorphous carbon material, silicon, lithium, lithium alloy and the like are used. These are mixed and applied onto a copper foil as a current collector to form a negative electrode. In a lithium battery, a separator is sandwiched between a positive electrode and a negative electrode, and an electrolytic solution is impregnated therein to provide ionic conductivity and conduct. In the lithium battery, a non-aqueous electrolytic solution is used, but in general, this is composed of a solvent and a supporting electrolyte. As the solvent, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and vinylene carbonate having an additive function, vinyl Carbonate system such as ethylene carbonate. As the supporting electrolyte, in addition to lithium hexafluorophosphate and lithium tetrafluoroborate, an organic lithium salt such as LiN (SO ₂ CF ₃ ) ₂ is also used. Ionic liquids can also be used.

  As the exterior body, a metal cylindrical can such as aluminum or stainless steel, a rectangular can, a sheet-type exterior body using a laminate film obtained by laminating aluminum foil with polypropylene, polyethylene, polyethylene terephthalate, polybutylene terephthalate, or the like can be used. Further, it can be used by stacking and stacking, or it can be used by rotating in a cylindrical shape.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited to these at all.

Example 1
[Preparation of nonwoven fabric substrate]
In the following configuration, dispersed short fibers and an alumina-based material were mixed.

Stretched polyethylene terephthalate (PET) short fibers (fineness 0.1 dt, fiber length 3 mm) 60.0 parts by mass Unstretched PET short fibers (fineness 0.2 dt, fiber length 4 mm) 40.0 parts by mass Alumina-based material (manufactured by Sumitomo Chemical) , Trade name: AA03) 5.0 parts by weight Papermaking adhesive (manufactured by Danitrix, trade name: Acrypers (registered trademark) P-NS) 0.1 parts by weight

Next, 0.1 parts by mass of a dissolved flocculant (trade name: Himax SC-103, manufactured by Hymo Co., Ltd.) is added to agglomerate the alumina-based material, and it is rolled up with a short mesh to obtain a basis weight of 11 g / m. No. ² web was prepared, dried, and then subjected to hot pressing at 220 ° C. to obtain a nonwoven fabric substrate (1) having a thickness of 15 μm.

[Preparation of alumina fine particle dispersion]
The materials having the following constitution were mixed and dispersed by a bead mill to obtain a dispersion liquid (1).

Alumina-based material (manufactured by Sumitomo Chemical Co., Ltd., trade name: AA03) 40.0 parts by mass Pure water 160.0 parts by mass Glyceryl laurate (manufactured by Taiyo Chemical Co., Ltd., trade name: Tirabazole (registered trademark) W-01) 0.2 parts by mass Carboxymethylcellulose (Nippon Paper Chemicals, trade name: MAC200HC) 0.1 parts by mass

  Next, the binder (latex) and thickener and the dispersion liquid (1) were combined with the following formulation to prepare a coating liquid (1).

Dispersion (1) 10.0 parts by mass 0.4 mass% carboxymethylcellulose aqueous solution (manufactured by Nippon Paper Chemicals, trade name: MAC500HC) 4.0 parts by mass 5 mass% binder dispersion (manufactured by Nippon A & L, SBR latex, Product name: AL-2001) 2.0 parts by mass

[Preparation of separator]
The obtained coating liquid (1) was apply | coated on the nonwoven fabric base material, and it was made to dry at 120 degreeC, and obtained the separator (1) of thickness 24 micrometers.

(Example 2)
Except that the alumina-based material used in Example 1 was changed to boehmite particles (manufactured by Daimei Chemical Co., Ltd.) and the nonwoven fabric base material (2) and the coating liquid (2) were produced, the same method as in Example 1, A separator (2) having a thickness of 25 μm was obtained.

(Comparative Example 1)
[Preparation of nonwoven fabric substrate]
Stretched PET staple fiber (fineness 0.1 dt, fiber length 3 mm) 60.0 parts by mass Unstretched PET staple fiber (fineness 0.2 dt, fiber length 4 mm) 40.0 parts by mass Papermaking adhesive (manufactured by Dianitics, trade name: Acrypers (registered trademark) P-NS) 0.1 parts by mass

With the above configuration, a web having a basis weight of 11 g / m ² was prepared, dried, and then subjected to thermal calendering at 220 ° C. to obtain a comparative nonwoven fabric substrate (1) having a thickness of 15 μm.

  On this comparative nonwoven fabric substrate (1), the coating liquid (1) used in Example 1 was applied and dried at 120 ° C. to obtain a comparative separator (1) having a thickness of 25 μm.

(Comparative Example 2)
The coating liquid (2) obtained in Example 2 was applied to the comparative nonwoven fabric substrate (1) produced in Comparative Example 1 and dried at 120 ° C. to obtain a comparative separator (2) having a thickness of 25 μm.

[Evaluation of battery characteristics]
On the aluminum foil, lithium manganate, acetylene black, and polyvinylidene fluoride were applied at a mass ratio of 100/5/3 at a mass ratio of about 200 g / m ² , the solvent was dried, and further pressed to produce a positive electrode. On the other hand, spherical artificial graphite, acetylene black, and polyvinylidene fluoride were coated on a copper foil at a mass ratio of 85/15/5 in an amount of 100 g / m ² , dried and pressed to prepare a negative electrode.

  The separator obtained in the example was sandwiched between the obtained electrodes with the coated surface on the negative electrode side, and the electrolyte solution for lithium batteries manufactured by Kishida Chemical (solvent: EC / DEC = 3/7 (volume ratio)), support Electrolyte: Lithium hexafluorophosphate 1 mol / l) was added dropwise and sealed in an aluminum foil laminate film under reduced pressure to prepare a lithium battery. Next, the produced lithium battery was charged to 4.2 V at 0.2 C, and then discharged at 0.2 C. At this time, the ratio of the discharge capacity initially performed under the condition of 0.2 C to the charge capacity was measured. Further, the internal resistance was measured with the voltage drop value taken as the voltage 30 minutes after the start of discharge under the condition of 0.2 C (discharge time of 300 minutes). The results are given in Table 1.

  As apparent from Table 1, in the battery separator composed of a porous membrane layer containing alumina fine particles and a nonwoven fabric base material, the nonwoven fabric base material is a nonwoven fabric base material into which alumina fine particles are internally added. When the battery separator of the present invention was used, the internal resistance during actual operation of the battery was reduced.

  The battery separator of the present invention can be used as a separator for a lithium battery or a separator for a capacitor.

Claims (1)

  A battery separator comprising a porous membrane layer containing alumina fine particles and a nonwoven fabric base material, wherein the nonwoven fabric base material is a nonwoven fabric base material in which alumina fine particles are internally added. Battery separator.