JP6061735B2 - Battery separator and battery separator manufacturing method - Google Patents

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, there is a demand for a battery that can operate stably even in a region where the internal temperature exceeds the heat-resistant temperature of 130 ° C. of polypropylene used for the polyolefin porous membrane, in particular, for a vehicle battery having a large charge / discharge current value. However, conventional polyolefin porous membranes are in a state of insufficient heat resistance. Such a situation is seen not only in lithium secondary batteries but also in magnesium-air batteries and lithium-air batteries, and the same tendency is observed when these are used as primary batteries.

  In a lithium secondary battery, in order to impart heat resistance, a method for imparting heat resistance by filling a polyolefin porous membrane with an inorganic filler (Patent Document 1), a method for laminating an inorganic porous membrane on a polypropylene porous membrane (patent) Document 2) is being studied. However, as long as a polyolefin-based material is used for the separator, heat resistance exceeding 130 ° C. cannot be imparted. In addition, a polytetrafluoroethylene (PTFE) porous film (Patent Document 3), a polyamide 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 aromatic polyester. May be hydrolyzed by acid or alkali generated in the electrolyte, and there was a problem in reliability when the lithium secondary battery was used under severe conditions. In particular, when producing a support using PET short fibers by a wet papermaking method that is easy and highly uniform, in addition to stretched PET fibers with high crystallinity, unstretched PET fibers are used as binder fibers. Although used in combination (Patent Document 6), the hydrolysis resistance of the unstretched PET fiber is lower than that of the stretched PET fiber, which leaves a problem.

JP 2005-259680 A JP 2005-38854 A JP 2004-127545 A JP-A-53-74572 JP 2006-507635 A JP 2009-230975 A

  An object of the present invention is to provide a battery separator having improved hydrolysis resistance.

  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.

(1) In a battery separator containing a stretched polyester fiber and an unstretched polyester fiber and a fine material, the fiber surface of the fiber matrix is covered with a polyolefin film layer. Characteristic battery separator.
(2) The battery separator according to the above (1), wherein the fine substance has a fiber shape or a particle shape.
(3) a step of forming a fiber matrix comprising stretched polyester fibers and unstretched polyester fibers, a step of coating the fiber surface of the fiber matrix with a polyolefin film layer by applying polyolefin fine particles, a fiber matrix The manufacturing method of the separator for batteries which includes the process of providing a fine substance in a battery.
(4) A step of forming a fiber matrix comprising a stretched polyester fiber, an unstretched polyester fiber and a fine substance, and a step of coating the fiber surface of the fiber matrix with a polyolefin film layer by applying polyolefin fine particles The manufacturing method of the separator for batteries containing this.
(5) The method for producing a battery separator as described in (3) or (4) above, wherein the fine substance has a fiber shape or a particle shape.

  According to the present invention, a battery separator having heat resistance and improved hydrolysis resistance can be obtained.

  The battery separator of the present invention (hereinafter sometimes abbreviated as “separator”) is a battery separator containing a fiber matrix containing stretched polyester fibers and unstretched polyester fibers, and a fine material. The battery separator is characterized in that the fiber matrix is coated with a polyolefin film layer.

  Examples of the polyester fiber include polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene isophthalate. These 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 stretched polyester fiber is a fiber in which polyester is oriented by stretching the fiber by applying heat and tensile strength when spinning the fiber. It is a fiber having high strength and heat resistance with a melting point or softening point exceeding 200 ° C. On the other hand, unstretched polyester fiber is a fiber that is also used as a binder, and is formed without including a stretching step. In some cases, the fiber contains a polymer in which a functional group such as a sulfonic acid group that inhibits orientation is introduced at the terminal. The heat-sealing temperature of the unstretched polyester fibers is 80 ° C. to 130 ° C., and heat-sealing can be performed between the unstretched polyester fibers or between the stretched polyester fibers and the unstretched polyester fibers.

  In the present invention, a fiber matrix containing stretched polyester fibers and unstretched polyester fibers is used. This fiber matrix is manufactured by a dry method or a wet method. In the dry method, short fibers having a fiber length of 30 mm to 120 mm are defibrated in the air to form a web, entangled with a needle or a water stream, and then bonded at a temperature equal to or higher than the heat fusion temperature of the unstretched polyester fiber, to the fiber matrix. It is a method of giving strength. The wet method is a method in which short fibers with a fiber length of 1.5 mm to 20 mm are defibrated in water and then spun out of the water with a circular net, short net, or long net. Is a method of heat-sealing to give strength. The uniformity of the fiber matrix is improved in the wet method than in the dry method. Due to the difference in the length of short fibers, the strength of the fiber matrix is superior to the wet method by the dry method.

The fiber matrix can be made thin or uniform by a process such as a thermal calendar or calendar. The fiber matrix 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 the fiber matrix, in addition to polyester fibers, rayon, cellulose acetate, lyocell, pulp, acrylic, aramid, polyolefin and the like can be used in combination.

  The stretched polyester fiber is preferably 60 to 98% by mass and more preferably 70 to 90% by mass with respect to the total fiber. Moreover, it is preferable that it is 2-40 mass% with respect to all the fibers, and, as for an unstretched polyester fiber, it is more preferable that it is 10-30 mass%.

  The fiber matrix is coated with a polyolefin membrane layer. Examples of the polyolefin that can be used for imparting hydrolysis resistance include polyethylene and polypropylene. Further, a polyolefin having a modification rate of 5% or less and a film forming temperature of 80 ° C. or more is preferable. The modification rate and the film formation temperature are important, and a polyolefin having a higher modification rate and a lower film formation temperature lowers the hydrolysis resistance. The amount of polyolefin added is preferably 1 to 10% by mass of the fiber matrix, more preferably 2 to 8% by mass. If the amount of adhesion is low, the fiber matrix may not be coated and hydrolysis resistance may be reduced. If the amount of adhesion is too high, the porosity required as a battery separator may be hindered. As an attaching method, coating by a dipping method using polyolefin fine particles (latex) is preferable. After coating, the film is dried and heated at a film formation temperature or higher to form a polyolefin film layer.

  The fiber matrix performs pore adjustment to become a battery separator. Pore adjustment is adjusting the pore distribution of the battery separator. By optimizing the pore distribution, effects such as retention of electrolyte, prevention of separation and separation of electrode active material and conductive agent such as acetylene black from the electrode, and suppression of dendrite growth in the case of secondary batteries are obtained. It is done. In order to adjust the pore distribution, fine substances are contained in the battery separator, and there are generally three methods.

  The first is a method in which a fine porous material is used and a new porous layer containing the fine material is formed on the fiber matrix. Examples of the fine fiber-shaped material include ultrafine fibers, and fibrillated aramid fibers, fibrillated cellulose fibers, bacterial cellulose fibers, and the like can be used. A fibrillated fiber is a fiber that is refined by dividing a crystalline fiber in water in the orientation direction of the polymer. However, when these fibers are taken out from the water alone, they must be formed into a film and lose their porosity. A dispersion containing these ultrafine fibers can be applied or impregnated into a fiber matrix to form a porous layer.

In addition, a method in which a polymer is dissolved in an organic solvent and blown out during electrolysis to produce a porous layer containing continuous ultrafine fibers having a fiber diameter of 50 nm to 200 nm (electrospinning method), high There is a method in which molecules are dissolved in an organic solvent and blown into a poor solvent to produce a porous layer containing ultrafine fibers having a fiber diameter of 300 nm or less. The amount of the porous layer containing ultrafine fibers is preferably 1 to 10 g / m ² , more preferably ² to 8 g / m ² .

  As a second method, a fine porous material having a particle shape is used, and a new porous layer containing the fine material is formed in the fiber matrix. Examples of the fine particles having a particle shape include inorganic particles. Examples of the inorganic particles include metal oxides such as alumina, silica, titania and zirconia, and fine particles of hardly soluble salts such as barium sulfate and aluminum phosphate. The porous layer containing α-alumina is excellent in heat resistance and hydrolysis resistance and is a preferable material. The particle diameter of the inorganic particles is preferably 0.1 μm to 2.0 μm, more preferably 0.3 to 1.5 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 order to impart flexibility to the porous layer containing inorganic particles, it is preferable to hybridize with an organic substance having flexibility. As the organic substance, a polymer is particularly preferable. As such a polymer, acrylic resin, styrene-acrylic resin, styrene butadiene rubber (SBR) resin, modified polyolefin resin, and the like can be used. In particular, from the viewpoint of hydrolysis resistance, the modified polyolefin resin is excellent. If the polymer is made into a latex, it can be applied in an aqueous system. In addition, in order to form a porous layer by coating in an aqueous system, when water-soluble cellulose such as sodium carboxymethyl cellulose or methyl cellulose is used in combination, the coating property is improved.

Examples of the coating method include casting, dipping, doctor blade, knife coating, bar coating, gravure coating, screen coating, spraying, and roll coating. The amount of the porous layer containing inorganic particles is preferably 1 to 100 g / ^{m 2,} more preferably from 2 to 50 g / ^{m 2.} Moreover, as a coating speed, 0.2-50 m / min is preferable. A porous ceramic membrane layer is formed through a drying process after coating. The drying temperature is preferably from 70 ° C. to 160 ° C., but when a polyolefin-based latex is used, it is necessary to heat it to a film forming temperature or higher.

  As a third method, ultrafine fibers and inorganic particles are introduced at the stage of forming the fiber matrix. In particular, when a fiber matrix is produced by a wet method, a dispersion containing stretched polyester fibers, unstretched polyester fibers, and fines can be prepared in water and sprinkled at a time. This method has an advantage that the manufacturing method can be simplified, but there is a problem that the amount of the fine material that can be introduced into the fiber matrix varies, and there is a problem that the fine material settles during the production. It is difficult to obtain a pore distribution as compared with the first and second methods.

  The three methods may be used alone or in combination. As the battery separator, the thickness is preferably 10 to 60 μm, more preferably 12 to 35 μm, and the necessary pore distribution is preferably 0.1 μm to 10 μm, more preferably 0.2 μm to 4 μm, The internal porosity is preferably 30% to 90%, more preferably 40% to 80%.

  Battery separators are used in lithium secondary batteries, lithium air batteries, magnesium air batteries, and the like. The separator for the lithium secondary battery is cut and sandwiched between electrode materials for the lithium secondary battery, and an electrolyte is injected to seal the battery, thereby forming a lithium secondary battery. 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 and SBR, and the active material includes lithium cobaltate, lithium nickelate, lithium manganate, nickel A lithium manganese composite oxide such as lithium manganese cobaltate (NMC) or lithium aluminum manganate (AMO), lithium iron phosphate, or the like 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, 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.

  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.

Example 1
[Fabrication of fiber matrix 1]
A fiber matrix having a basis weight of 12 g / m ² was prepared by a wet method with the following short fiber configuration. The drying temperature at this time was 130 ° C.

Stretched PET fiber (0.06 dtex, 3 mm) 40 parts by weight Stretched PET fiber (0.1 dtex, 3 mm) 35 parts by weight Unstretched PET fiber (0.2 dtex, 3 mm) 25 parts by weight

Next, a thermal calendar process was performed at 200 ° C. to prepare a fiber matrix 1 having a thickness of 17 μm. Next, polyolefin latex (available from Unitika, Arrow Base (registered trademark) SE-1205J2) is applied to the fiber matrix 1 at a solid content of 1.0 g / m ² , heat-treated at 105 ° C., and the fiber matrix 1 is formed. Covered with a polyolefin membrane layer.

  A coating liquid 1 having the following constitution was produced.

α-alumina fine particles (manufactured by Sumitomo Chemical Co., Ltd., trade name: AA07) 100 parts by mass sodium pyrophosphate 0.1 part by mass sodium polyacrylate (trade name: Poaz 520) 0.2 parts by mass sodium carboxymethylcellulose (Nippon Paper Industries) Chemical product, trade name: MAC500LC) 0.9 parts by mass polyolefin latex (product of Unitika, trade name: Arrow Base (registered trademark) SD-1205J2) 15 parts by weight water 180 parts by weight

  The coating liquid 1 was impregnated into a fiber matrix 1 covered with a polyolefin film layer, dried at 80 ° C., and then heat treated at 105 ° C. to produce a battery separator 1 having a thickness of 25 μm.

  Next, the battery separator 1 was immersed in a 0.2N potassium hydroxide aqueous solution heated to 60 ° C. for 72 hours, washed with water, and dried to obtain a battery separator 2.

(Comparative Example 1)
The fiber matrix 1 obtained in Example 1 was impregnated with the coating liquid 1 to produce a separator 1 for a comparative battery having a thickness of 25 μm. The obtained comparative battery separator 1 was immersed in a 0.2N potassium hydroxide aqueous solution heated to 60 ° C. for 72 hours, washed with water, and dried to obtain a comparative battery separator 2.

(Example 2)
A fiber matrix containing fine particles was obtained with the following constitution. The drying temperature at this time was 130 ° C.

Stretched PET fiber (0.06 dtex, 3 mm) 40 parts by weight Stretched PET fiber (0.1 dtex, 3 mm) 35 parts by weight Unstretched PET fiber (0.2 dtex, 3 mm) 25 parts by weight Boehmite fine particles (manufactured by Daimei Chemical, trade name: C20) 50 parts by mass sodium carboxymethylcellulose (Daiichi Kogyo Seiyaku Co., Ltd., trade name: Cellogen (registered trademark) BSH-12) 0.2 parts by mass cationized cellulose (manufactured by Kao, Poise C-150L) 0.1 parts by mass

Next, a thermal calendar process was performed at 200 ° C. to prepare a fiber matrix 2 having a thickness of 27 μm. Next, a polyolefin latex (available from Unitika, Arrow Base (registered trademark) SE-1205J2) is applied to the fiber matrix 2 at a solid content of 2.5 g / m ² , heat-treated at 105 ° C., and a polyolefin film layer. A battery separator 3 having a thickness of 27 μm was prepared. Next, the battery separator 3 was immersed in a 0.2N potassium hydroxide aqueous solution heated to 60 ° C. for 72 hours, washed with water, and dried to obtain a battery separator 4.

Example 3
Using the following configuration, a homogenizer (manufactured by PRIMI, trade name: TK HOMODERS model 2.5) was stirred at 3000 rpm for 2 hours to prepare a fine cellulose dispersion.

Concentration 10 mass% Fine cellulose fiber (Daicel Finechem, Selish (registered trademark) KY-100G) 100 parts by mass Sodium carboxymethyl cellulose (Nippon Paper Chemicals, trade name: MAC800LC) 2.0 parts by mass Water 400 parts by mass

This dispersion was cast on the fiber matrix 1 produced in Example 1 to give a fiber matrix containing fine cellulose fibers corresponding to a basis weight of 1.2 g / m ² . Next, a thermal calendar process was performed at 200 ° C. to produce a fiber matrix 3 having a thickness of 18 μm. This fiber matrix 3 is coated with a polyolefin latex (available from Unitika, Arrow Base (registered trademark) SE-1205J2) at a solid content of 2.5 g / m ² , heat-treated at 105 ° C., and a battery separator having a thickness of 18 μm. 5 was produced. Next, the battery separator 5 was immersed in a 0.2N potassium hydroxide aqueous solution heated to 60 ° C. for 72 hours, washed with water, and dried to obtain a battery separator 6.

[Strength properties of separator]
Each separator was cut into a width of 5 cm and a length of 20 cm, and the tensile strength was measured by a method specified in JIS P 8113 using a tabletop material testing machine (trade name: STA-1150, manufactured by ORIENTEC) (Sample Tsumi The interval is 10 cm, and the speed is 100 mm / min). The maximum load was measured, this was taken as the tensile strength, and the results are given in Table 1.

[Measurement of average pore size of separator]
The average pore diameter of the separator was measured using a porometer (trade name: Capillary Flow Porometer CEP-1500-A, manufactured by PMI), and the results are given in Table 1.

[Measurement of air permeability of separator]
The Gurley air permeability of each separator was measured with a Gurley type air permeability tester (manufactured by Toyo Seiki Seisakusho, Densometer), and the results are given in Table 1.

[Evaluation of battery characteristics]
On the aluminum foil, 200 g / m ² of lithium manganate, acetylene black and polyvinylidene fluoride were applied at a mass ratio of 100/5/3, and 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.

  Each separator was sandwiched between the obtained electrodes, and an electrolyte for a lithium secondary battery manufactured by Ube Industries (solvent: EC / DEC / DME = 1/1/1 (volume ratio), supporting electrolyte: hexafluorophosphoric acid Lithium, 1 mol / l) was added dropwise and sealed in an aluminum foil laminate film under reduced pressure to produce a lithium secondary battery. Next, the produced lithium secondary battery was charged and discharged at 4.2 V, and the ratio (%) of the charge capacity to the discharge capacity was determined at the first time (initial) and the 500th time. The results are given in Table 1.

  From Table 1, in the battery separators 1, 3 and 5 in which the fiber matrix is coated with the polyolefin film layer, the strength properties, average pore diameter, Gurley permeability even after alkaline treatment (battery separators 2, 4 and 6). There was no significant difference in evaluation of the temperament and charge / discharge capacity ratio. However, in the comparative battery separator 1 in which the fiber matrix is not coated with the polyolefin film layer, not only the reduction rate of the charge / discharge capacity ratio after 500 times is large, but also after alkali treatment (comparative battery separator 2), The characteristic in evaluation has also collapsed. The comparative battery separators 1 and 2 whose characteristics are destroyed by alkali treatment are not only difficult to use in lithium secondary batteries used under harsh conditions, but also in batteries using an alkaline aqueous solution such as a lithium air battery as an electrolyte. Not suitable for use.

  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 (5)

A battery separator comprising a fiber matrix comprising stretched polyester fibers and unstretched polyester fibers, and a fine material, wherein the fiber surface of the fiber matrix is coated with a polyolefin film layer Separator for use.   The battery separator according to claim 1, wherein the fine substance has a fiber shape or a particle shape.   A step of forming a fiber matrix comprising stretched polyester fibers and unstretched polyester fibers, a step of coating the fiber surface of the fiber matrix with a polyolefin film layer by applying polyolefin fine particles, and a fine substance in the fiber matrix The manufacturing method of the separator for batteries including the process of providing.   A battery comprising a step of forming a fiber matrix comprising stretched polyester fibers, unstretched polyester fibers and fines, and a step of coating the fiber surface of the fiber matrix with a polyolefin film layer by applying polyolefin fine particles Manufacturing method for the separator.   The method for producing a battery separator according to claim 3 or 4, wherein the fine substance has a fiber shape or a particle shape.