JP2016062723A - Battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery separator which is obtained by manufacturing a porous ceramic layer including a microfibrillated cellulose with stability.SOLUTION: A battery separator comprises: a support body including polyester fiber; and a porous ceramic layer including microfibrillated cellulose. The surface of the fiber of the support body is coated with acid-modified polyolefin.SELECTED DRAWING: None

Description

  The present invention relates to a battery separator and a method for producing a battery separator.

  In recent years, as batteries have increased in size and output, such as automobile batteries for automobiles and power storage power sources, demands for imparting heat resistance to battery separators that are constituent elements of lithium secondary batteries have increased. As a battery separator used in a lithium secondary battery, a polyolefin porous film is the mainstream. However, in vehicles for vehicles with particularly high charge / discharge current values, the battery can operate stably even when the internal temperature of the battery exceeds the heat-resistant temperature of the polyolefin porous film used as a battery separator. Is desired. For example, the heat resistant temperature of a polypropylene porous film is about 130 ° C. Therefore, the polyolefin porous membrane battery separator is in a state of insufficient heat resistance. Such a situation can be seen not only in lithium secondary batteries but also in magnesium-air batteries and lithium-air batteries. Furthermore, there is a similar tendency even when these battery separators are used as primary batteries.

  In a lithium secondary battery, as a method of imparting heat resistance to a battery separator, a method of imparting heat resistance by filling a polyolefin porous membrane with a ceramic filler (Patent Document 1), a porous ceramic membrane is applied to a polypropylene porous membrane. A method of laminating (Patent Document 2) and the like has been studied. However, as long as a polyolefin-based material is used for the separator, heat resistance exceeding 130 ° C. cannot be imparted. Further, a polytetrafluoroethylene (PTFE) porous film (Patent Document 3), a polyamide porous film, a polysulfone porous film, a polyimide porous film (Patent Document 4) and the like having high heat resistance have been proposed. A PTFE porous membrane can be produced by stretching a PTFE film, but the film strength is weak and is not suitable for a thin porous membrane. The polysulfone porous membrane has a complicated manufacturing process because polysulfone is dissolved in an organic solvent, the solvent is replaced with water, and the like is made porous by spinodal decomposition. In the case of polyamide porous membranes and polyimide porous membranes, the synthesis process of aromatic polyamide and aromatic polyimide and the formation of porous membranes proceed simultaneously, so it is difficult to control and there are many restrictions on the use for batteries that should be generalized .

  Apart from these, a separator using an aromatic polyester material (Patent Document 5) has also been proposed. This is intended to satisfy both heat resistance and porosity by combining a porous ceramic membrane layer with a support using polyethylene terephthalate (PET) fibers as an aromatic polyester. At this time, it has been proposed that microporous cellulose is contained in the porous ceramic layer to facilitate the adjustment of the pores (Patent Document 6). However, when the porous ceramic layer is formed by impregnating the support with a coating liquid containing microfibrillated cellulose and ceramic fine particles, the cellulose fibers stabilize the ceramic fine particles in the coating liquid. The support made of fibers could not lift it, and as a result, the ceramic fine particles and the microfibrillated cellulose were concentrated in the coating solution, resulting in a problem that stable production was impossible.

JP 2005-259680 A JP 2005-38854 A JP 2004-127545 A JP-A-53-74572 JP 2006-507635 A JP 2010-67653 A

  An object of the present invention is to provide a battery separator obtained by stably producing a porous ceramic layer containing microfibrillated cellulose.

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

  A battery separator having a support containing polyester fiber and a porous ceramic layer containing microfibrillated cellulose, wherein the fiber surface of the support is coated with an acid-modified polyolefin. .

  According to the present invention, a battery separator obtained by stably producing a porous ceramic layer containing microfibrillated cellulose can be obtained.

  The battery separator of the present invention (hereinafter sometimes abbreviated as “separator”) is a battery separator having a support containing polyester fibers and a porous ceramic layer containing microfibrillated cellulose. Further, the battery separator is characterized in that the fiber surface of the support is coated with an acid-modified polyolefin.

  Examples of the polyester fiber include polyester fibers such as polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene isophthalate. These polyester fibers may be used alone or in combination of two or more. Among these, polyethylene terephthalate (PET), which is excellent in heat resistance, is preferable when used for a battery separator of a lithium secondary battery.

  The support of the present invention contains polyester fiber and is produced by a dry method or a wet method. The dry method is a method in which short fibers having a fiber length of 30 mm to 120 mm are defibrated in air to form a web and bonded at a temperature equal to or higher than the heat fusion temperature of the binder type polyester fiber to give strength. The wet method is a process in which short fibers with a fiber length of 0.5 mm to 20 mm are defibrated in water and then spun out of the water with a circular net, short net, or long net. This is a method in which fibers are thermally fused to give strength. In a dry method or a wet method, fibers may be entangled with a needle or water flow with respect to the web. The uniformity of the support is improved in the wet method than in the dry method. Due to the difference in the length of short fibers, the dry method is superior to the wet method for the strength of the support.

  Examples of the binder-type polyester fiber include polyester fibers having low crystallinity that are not heat-stretched at the stage of producing the fibers, and polyester fibers having a molecular weight reduced to lower the melting point. The binder-type polyester fiber can be fused at about 80 to 130 ° C.

The support can be made thin or uniform by a process such as a thermal calendar or a calendar. The support preferably has a basis weight of 6 to 25 g / m ² , a thickness of 10 to 50 μm, and a porosity of 30 to 80%. More preferably, the basis weight is 8 to 15 g / m ² , the thickness is 12 to 25 μm, and the porosity is 40% to 70%. In addition to polyester fibers, it is possible to use rayon, cellulose acetate, lyocell, pulp, acrylic, aramid, polyolefin and the like in the support.

  The fiber surface of the support is coated with acid-modified polyolefin. The acid-modified polyolefin is a carboxylic acid-modified polypropylene, polyethylene, poly α-olefin, or the like. It can be obtained by adding maleic anhydride as a carboxylic acid to these polyolefins. The amount of maleic anhydride added is preferably 0.1 to 5% by mass. The method of coating the fiber surface of the support with acid-modified polyolefin is to apply the acid-modified polyolefin pre-emulsified to the support and heat-treat at a temperature equal to or higher than the film formation temperature. Is coated with an acid-modified polyolefin. The coating amount is preferably 0.1 to 10% by mass of the support, more preferably 0.5 to 2% by mass, although it depends on the basis weight of the support and the fiber diameter. When the fiber surface of the support is coated with acid-modified polyolefin, that is, when an acid-modified polyolefin layer is formed on the fiber surface of the support, the carboxylic acid appears on the surface and the microfibrils contained in the porous ceramic layer It becomes the bonding point of cellulose oxide and ceramic fine particles.

  In the present invention, the porous ceramic layer contains microfibrillated cellulose. Microfibrillated cellulose is a high-purity α-cellulosic body, such as cotton pulp, linter pulp, and dissolved pulp. It is a cellulose fiber having fine cellulose (nanocellulose) obtained by pulverization and defibration with a mortar grinder or the like as a constituent element. This cellulose has a hydrophilic hydroxyl group on the surface and has a fine fibrous portion of about 10 to 200 nm, and has a high correlation with ceramic fine particles in water and the porous ceramic layer. Can be a skeleton. If the content of the microfibrillated cellulose is too large, nanocellulose forms a film and the electrolyte solution cannot be retained. Therefore, the content is preferably 5% by mass or less, more preferably 3% by mass or less of the porous ceramic layer. Further, the lower limit amount of the microfibrillated cellulose is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more because if the amount is too small, the effect becomes difficult.

  In the present invention, the porous ceramic layer contains microfibrillated cellulose and ceramic fine particles. Examples of the ceramic fine particles include metal oxides such as alumina, silica, magnesia, ceria, titania and zirconia, and fine particles of hardly soluble salts such as barium sulfate, aluminum phosphate, magnesium hydroxide and boehmite. The particle size of the ceramic fine particles is preferably 0.05 to 2.0 μm, more preferably 0.1 to 1.0 μm. The particle size is obtained by sufficiently diluting the inorganic particles with water, and measuring the particle size with a laser scattering type particle size measuring device (trade name: 3300EX2 manufactured by Microtrack Co., Ltd.). The obtained center particle size (D50, volume average) It is.

  In the porous ceramic layer, a binder can be further used to stabilize the layer. As such a binder, acrylic resin, styrene-acrylic resin, styrene butadiene rubber (SBR) resin, acid-modified polyolefin resin, and the like can be used. In particular, acid-modified polyolefin resin is excellent. When the binder is made into a latex, it can be applied in an aqueous system. In addition, when a water-soluble cellulose such as sodium carboxymethylcellulose or methylcellulose is used in combination with an aqueous system to form a porous ceramic layer, the coatability is improved. The content of the binder is preferably 0.5 to 20% by mass of the porous ceramic layer, and more preferably 1 to 8% by mass.

Examples of the coating method include casting (cast coating) method, dipping (dip) method, doctor blade method, knife coating method, bar coating method, gravure coating method, screen coating method, spray coating method, roll coating method and the like. It is done. The amount of the porous ceramic layer is preferably 0.5 to 100 g / ^{m 2,} more preferably from 1 to 50 g / ^{m 2.} Moreover, as a coating speed, 0.2-50 m / min is preferable. After coating, a porous ceramic layer is formed through a drying process. The drying temperature is preferably 70 ° C. to 160 ° C., but when latex is used, it is preferably heated to a temperature higher than the film forming temperature.

  As a battery separator, thickness becomes like this. Preferably it is 10-60 micrometers, More preferably, it is 12-35 micrometers. The necessary pore distribution is preferably 0.1 to 10 μm, more preferably 0.2 to 4 μm. The internal porosity is preferably 30 to 90%, more preferably 40 to 80%. After the porous ceramic is formed on the support by coating, it is also possible to adjust the thickness of the battery separator by performing calendar treatment or thermal calendar treatment.

  Battery separators are used in lithium secondary batteries, lithium air batteries, magnesium air batteries, and the like. For example, in a lithium secondary battery, a battery separator is cut and sandwiched between electrode materials for a lithium secondary battery, an electrolyte is injected, the battery is sealed, and a lithium secondary battery is obtained. .

  The material constituting the positive electrode of the lithium secondary battery is mainly an active material and a conductive agent such as carbon black, a binder such as polyvinylidene fluoride and SBR, and the active material includes lithium cobaltate, lithium nickelate, A lithium manganese complex oxide such as lithium manganate, lithium nickel manganese cobaltate (NMC) or lithium aluminum manganate (AMO), or lithium iron phosphate is used. These are mixed and coated on an aluminum foil as a current collector to form 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, lithium titanate, or the like is used. These are mixed and applied onto a copper foil as a current collector to form a negative electrode. In a lithium secondary 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 secondary battery, a non-aqueous electrolyte solution is used. Generally, this is composed of a solvent and a supporting electrolyte. As the solvent, for example, 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. Dimethoxyethane (DME) can also be used. 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. Further, a gel electrolyte in which a lithium salt is dissolved in a gel polymer such as polyethylene glycol or a derivative thereof, polymethacrylic acid derivative, polysiloxane or a derivative thereof, or polyvinylidene fluoride 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.

  As a lithium air battery, it is inserted between metal lithium and a positive electrode catalyst layer, an electrolyte is injected, and a water repellent film or the like is used together to form a lithium air battery. In the positive electrode catalyst layer, a catalyst such as platinum, palladium, iron, or carbon nanotube is supported on a porous carbon body, and oxygen is reduced. As the electrolytic solution, alkaline water such as potassium hydroxide and sodium hydroxide is used.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited to these at all. In Examples, parts and percentages are based on mass unless otherwise specified.

Example 1
[Preparation of Support 1]
A support 1 having a basis weight of 7.5 g / m ² was prepared by a wet method with the following fiber configuration. The drying temperature at this time was 130 ° C. Further, the dried support 1 was subjected to a heat calendar treatment at 200 ° C., and the thickness was adjusted to 14 μm.

PET fiber (0.06 dtex, 3 mm) 40 parts by mass PET fiber (0.1 dtex, 3 mm) 35 parts by mass Binder-type PET fiber (0.2 dtex, 3 mm) 25 parts by mass

[Coating with acid-modified polyolefin]
The support 1 is coated with an acid-modified polyolefin latex (manufactured by Maruyoshi Chemical Co., Ltd., acid-modified polypropylene emulsion, trade name: MNP8000) at a solid content of 1.5 g / m ² and heat-treated at 130 ° C. The fiber surface was coated with acid-modified polyolefin.

[Preparation of separator]
A coating liquid 1 having the following constitution was produced.

Magnesium hydroxide fine particles (Kyowa Chemical Industry, trade name: 200-06H) 300 parts by mass 40% sodium polyacrylate (Kao, trade name: Poaz 520) 4.0 parts by mass 0.6% sodium carboxymethylcellulose (Japan) Made of paper, trade name: MAC500LC) 200 parts by mass 1% microfibrillated cellulose (manufactured by Daicel Finechem, trade name: Celish (registered trademark) KY100G) 500 parts by weight 40% polypropylene emulsion (manufactured by Maruyoshi Chemical, trade name: MNP-) 8000)
30 parts by weight water 1300 parts by weight

  When the support 1 was impregnated with the coating liquid 1 and dried at 80 ° C., and continuous coating was performed to a thickness of 15 μm, stable coating was possible.

(Comparative Example 1)
While the fiber surface of the support 1 obtained in Example 1 was not coated with acid-modified polyolefin, the coating liquid 1 was impregnated in the same manner as in Example 1 to produce a separator for a comparative battery having a thickness of 15 μm. Since the coating amount gradually decreased, the coating was interrupted. Aggregates were formed in the coating solution. That is, stable coating could not be performed with a predetermined coating amount.

  The battery separator of the present invention can be used as a battery separator such as a lithium secondary battery or a lithium air battery.

Claims (1)

  A battery separator having a support containing polyester fiber and a porous ceramic layer containing microfibrillated cellulose, wherein the fiber surface of the support is coated with an acid-modified polyolefin. .