JP2014175232A - Cell separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cell separator in which internal resistance is reduced.SOLUTION: A carboxyl group is applied to the surface of a cell separator comprising fine cellulose fiber having an I type crystal shape. Examples of a method for applying a carboxyl group includes: a method in which a hydroxy group on the surface of fine cellulose fiber is heated under coexistence of maleic acid anhydride, is subjected to dehydration condensation, and is washed with alkali; a method in which alkali such as sodium hydroxide or potassium hydroxide is catalyzed under the coexistence of monochloroacetic acid or monochloroacetic acid sodium, and the hydroxy group of the surface of cellulose is carboxymethylated; a method in which, using 2, 2, 6, 6-tetramethylpiperidine-1-oxy radical as oxydation catalyst, C6 of glucose, which is the principal unit of cellulose, is specifically oxidized under alkali condition, and a carboxyl group is introduced.

Description

  The present invention relates to a battery separator.

  The present invention relates to a battery separator (hereinafter sometimes abbreviated as “separator”).

  Conventionally, as a separator for a lithium secondary battery, a polyolefin film having fine pores that have penetrated has been used. This battery separator suppresses heat generation by increasing the internal resistance of the battery by increasing the internal resistance of the battery when the battery generates heat due to abnormalities, and the internal resistance of the lithium cobalt oxide as an electrode agent It has been responsible for suppressing battery explosion due to thermal 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 order to meet this demand, Patent Document 1 proposes a separator made of a flexible support having a hole and a porous ceramic material that closes the hole. Polymer fibers are used for the flexible support, and zirconium, silica, and alumina are used for the porous ceramic. It is described that the porosity in the separator and the heat resistance can be improved by using a low-density support and a porous ceramic in combination.

  However, such a separator is produced by coating a dispersion of a porous ceramic material on a flexible support. When the polymer fiber density of the flexible support is low, there is a problem that the strength of the support decreases. In addition, the coated dispersion of the porous ceramic material is detached from the support, and a porous ceramic layer that is uniform in the surface direction cannot be obtained, resulting in a defective separator and an internal short circuit. There was a thing. On the other hand, if the polymer fiber density is too high, there arises a problem that the thickness increases. In Patent Document 2, in order to impart an appropriate polymer fiber density and suppress an increase in thickness, a support is produced 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.

[1] A battery separator comprising fine cellulose fibers having an I-type crystal form having a carboxyl group on the surface.
[2] A battery separator comprising at least a porous membrane layer containing inorganic fine particles and a porous support, wherein the porous membrane layer has an I-type crystal form having a carboxymethylated surface. A separator for a battery, comprising a fine cellulose fiber.

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

  The fine cellulose fiber having the I-type crystal form in the present invention refers to non-wood pulp such as cotton pulp and cotton linter pulp, wood pulp such as dissolving pulp and kraft pulp, etc. Fibrillated fibers that are divided and refined in the direction of pulp fibers, and cellulose fibers such as bacterial cellulose, the fiber diameter is 1 μm or less, the fiber length is 5 mm or less, 1 μm or more, preferably 2 mm or less, preferably 10 μm or more. Refers to things.

  In this invention, the carboxyl group is provided to the surface of the fine cellulose fiber which has I type crystal form. As a method for imparting a carboxyl group, a method of heating the hydroxyl group on the surface of fine cellulose fibers in the presence of maleic anhydride, dehydrating and condensing, washing with alkali, sodium hydroxide in the presence of monochloroacetic acid or sodium monochloroacetate, And a method of catalyzing an alkali such as potassium hydroxide to carboxymethylate the hydroxyl group on the cellulose surface, and using 2,2,6,6-tetramethylpiperidine-1-oxy radical as an oxidation catalyst, For example, a method of specifically oxidizing C6 of glucose, which is the main unit, and introducing a carboxyl group can be used. When both are salts with sodium or the like, the redispersibility in water is dramatically improved.

  The battery separator of the present invention contains fine cellulose fibers having an I-type crystal form with a carboxyl group added to the surface (hereinafter sometimes abbreviated as “carboxyl group-added I-type fine cellulose fibers”). . As such a battery separator, for example, a battery separator made of a nonwoven fabric containing carboxyl group-provided I-type fine cellulose fibers, a porous support such as a porous film, a woven fabric, a nonwoven fabric, and a knitted fabric; Separator for battery, porous film, woven fabric, non-woven fabric, knitted fabric and the like having a porous membrane layer containing a fine type cellulose fiber, a carboxyl group-provided type I fine cellulose fiber and inorganic fine particles Examples include battery separators having a porous membrane layer containing as a constituent element. Among these, the battery separator comprising the porous support and the porous membrane layer containing the carboxyl group-added type I fine cellulose fiber and the inorganic fine particles is excellent in strength and heat resistance. preferable.

  Examples of the porous support include a porous film, a woven fabric, a nonwoven fabric, and a knitted fabric. Examples of the material for the porous support include polyester, polyolefin, polyamide, aramid, and cellulose. As the porous support, it is a nonwoven fabric using fibers such as polyester, polyolefin, polyamide, aramid, cellulose, etc., and it is particularly preferable to use a nonwoven fabric using polyester or polyolefin, which is easy to obtain heat resistance and fine fibers. preferable. The nonwoven fabric can be produced by various methods such as a wet papermaking method, a dry method, and an electrostatic spinning method. Since various fibers can be selected, the uniformity is high, and the internal resistance can be lowered, a nonwoven fabric produced by a wet papermaking method is preferable.

  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 solvent-spun cellulose, polyethylene, polypropylene, polyester, and polyamide. Solvent-spun 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. 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 porous support 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 inorganic fine particles contained in the porous membrane layer include fine particles of oxides such as silica, alumina, and titanium oxide; hardly soluble salts such as barium sulfate and aluminum phosphate. In particular, α, β, γ-alumina typified by alumina, boehmite, pseudoboehmite typified by hydroxylated alumina, and the like are useful materials. Boehmite is a particularly 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 particle size of such inorganic fine particles is preferably 10 nm to 10 μm, more preferably 12 nm to 5 μm. The particle size is obtained by sufficiently diluting the inorganic fine particles with water, and measuring this with a laser scattering type particle size measuring device (trade name: 3300EX2 manufactured by Microtrac Co., Ltd.). Was the particle diameter.

  The battery separator of the present invention can be produced by applying a dispersion of inorganic fine particles to a porous support to form a porous membrane layer. This porous membrane layer contains carboxyl group-provided type I fine cellulose fibers. This carboxyl group-provided type I fine cellulose fiber spreads between inorganic fine particles, improves its dispersibility, and imparts interparticle bonding properties during film formation and the strength of the porous membrane layer. In general, a high molecular weight type soluble carboxymethylcellulose sodium has a degree of polymerization of about 1000 to 2000 and a molecular chain length of about submicron, but the fiber length of the carboxyl group-added type I fine cellulose fiber in the present invention is 1 μm. As described above, there is also an effect of suppressing the formation of pinholes, and the effect is enormous. The content of the carboxyl group-provided type I fine cellulose fiber in the present invention is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 2%, based on the content of the inorganic fine particles. % By mass.

  The inorganic fine particles are first dispersed in water. Since the carboxyl group-provided type I fine cellulose fiber behaves as an anion body in water as a sodium salt or the like, the inorganic fine particles are also preferably anionized and dispersed. As the material imparting anionic property, sodium carboxymethyl cellulose (CMC), sodium polycarboxylate and a modified product thereof are preferable. These assist the dispersibility of the ceramic particles as a surfactant. The addition amount is preferably 0.01% by mass to 5% by mass and more preferably 0.1% by mass to 2% by mass with respect to the content of the inorganic fine particles.

  Furthermore, a binder such as an acrylic resin, a styrene acrylic resin, a styrene butadiene rubber (SBR) resin, or a modified polyolefin resin can be added in order to give the dispersion of the inorganic fine particles more membrane strength of the porous membrane layer. These binders are preferably used in a latex state. The binder improves the binding property of the inorganic fine particles in the drying step. In this dispersion, the latex is preferably an anion. If the amount of the binder added is too large, it forms a film and increases the internal resistance of the battery separator, so the amount added is 0.2% to 10% by weight of the inorganic fine particles. Is more preferable, and 0.8 to 5% by mass is more preferable.

  As a method for dispersing the coating solution, mixing with a stirrer is sufficient when the concentration is low. However, if the concentration exceeds several percent, the overall viscosity and the like are increased, so a homogenizer, a bead mill, or the like can be used. preferable. 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 coating liquid containing inorganic fine particles on a porous support 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 battery 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 battery 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. 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.

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

[Production of carboxyl group-added type I fine cellulose fiber]
100 parts by mass of isopropanol is added to 100 parts by mass of fibrillated cellulose (made by Daicel Chemical Industries, trade name: Selish (registered trademark) KY-100G, concentration: 10% by mass) obtained by micronizing cotton linter pulp by an explosion method. The mixture was cooled to 10 ° C., and 40 parts by mass of 2N aqueous sodium hydroxide solution was slowly added dropwise with stirring. Next, after heating to 70 ° C. and dropping 40 parts by mass of chloroacetic acid and stirring for 1 hour, it was filtered and washed with a 0.05N aqueous sodium hydroxide solution and water to give carboxyl group-provided type I fine cellulose fibers. Produced.

Example 1
[Preparation of porous support]
Stretched polyethylene terephthalate (PET) short fibers (fineness 0.1 dt, fiber length 5 mm) 75.0 parts by mass Unstretched PET short fibers (fineness 0.2 dt, fiber length 5 mm) 25.0 parts by weight sodium carboxymethylcellulose (Daiichi Kogyo) Product name: Serogen (registered trademark) BSH-12, manufactured by Pharmaceutical Co., Ltd. In a composition of 0.1 part by mass, dispersed in water, and PET fibers are rolled up with a short mesh, and a web having a basis weight of 8.5 g / m ² is obtained. After producing and drying, a hot pressure treatment was performed at 220 ° C. to obtain a porous support (1) made of a nonwoven fabric having a thickness of 15 μm.

[Preparation of coating liquid]
Boehmite (manufactured by Daimei Chemical Co., Ltd., trade name: C20) 250.0 parts by mass pure water 400.0 parts by mass sodium polyacrylate (product of Kao, trade name: Poaz 520, concentration 40% by mass) 2.0 parts by mass of homogenizer (PRIMI, TKHOMODERSER model 2.5) was used, and it stirred at 1200 rpm for 1 hour.

  Next, carboxyl group-provided type I fine cellulose fibers (concentration 0.4 mass%) 200 mass parts, SBR latex (manufactured by Nippon A & L, trade name: AL-2001, concentration 48.3 mass%) 25 mass parts, carboxymethyl cellulose 200 parts by mass of sodium (manufactured by Nippon Paper Chemicals, trade name: MAC500LC, concentration 0.6% by mass) was added, and the mixture was further stirred for 1 hour at a rotational speed of 2500 rpm to prepare a coating liquid (1).

[Preparation of separator]
The obtained coating liquid (1) was impregnated into the porous support (1) and dried at 100 ° C. to obtain a separator (1) having a thickness of 35 μm.

(Example 2)
[Preparation of porous support]
Stretched polyethylene terephthalate (PET) short fiber (fineness 0.1 dt, fiber length 5 mm) 75.0 parts by mass Unstretched PET short fiber (fineness 0.2 dt, fiber length 5 mm) 24.0 parts by weight Carboxyl produced in Example 1 Group-provided type I fine cellulose fiber 1.0 part by weight sodium carboxymethylcellulose (Daiichi Kogyo Seiyaku, trade name: Serogen (registered trademark) BSH-12) 0.1 mass part, dispersed in water, PET fiber Was rolled up with a short net to produce a web having a basis weight of 8.5 g / m ² , dried and then subjected to hot pressing at 220 ° C. to obtain a porous support (2) made of a nonwoven fabric having a thickness of 15 μm. .

[Preparation of coating liquid]
Boehmite (manufactured by Daimei Chemical Co., Ltd., trade name: C20) 250.0 parts by mass pure water 400.0 parts by mass sodium polyacrylate (product of Kao, trade name: Poaz 520, concentration 40% by mass) 2.0 parts by mass of homogenizer (PRIMI, TKHOMODERSER model 2.5) was used, and it stirred at 1200 rpm for 1 hour.

  Next, 25 parts by mass of SBR latex (manufactured by Nippon A & L, trade name: AL-2001, concentration 48.3 mass%), sodium carboxymethyl cellulose (trade name: Serogen (registered trademark) BSH-12, manufactured by Daiichi Kogyo Seiyaku), 400 parts by mass of (concentration: 0.6% by mass) was added, and the mixture was further stirred for 1 hour at a rotational speed of 2500 rpm to prepare a coating liquid (2).

[Preparation of separator]
The obtained coating liquid (2) was impregnated into the porous support (2) and dried at 100 ° C. to obtain a separator (2) having a thickness of 30 μm.

(Comparative Example 1)
Instead of the carboxyl group-provided type I fine cellulose fibers used in Example 1, unmodified fine cellulose fibers (manufactured by Daicel Chemical Industries, trade name: Selish (registered trademark) KY-100S, concentration 0.4 mass%) Using the same treatment, a comparative separator (1) having a thickness of 28 μm was obtained.

(Comparative Example 2)
The same treatment was performed using sodium carboxymethylcellulose (manufactured by Nippon Paper Chemical Co., Ltd., trade name: MAC500LC, concentration 0.6 mass%) instead of the carboxyl group-provided type I fine cellulose fibers used in the examples, and the thickness was 38 μm. Comparative separator (2) was obtained.

(Comparative Example 3)
Instead of the carboxyl group-provided type I fine cellulose fibers used in Example 2, unmodified fine cellulose fibers (manufactured by Daicel Chemical Industries, trade name: Selish (registered trademark) KY-100S, concentration 0.4 mass%) were used. Using the same treatment, a comparative separator (3) having a thickness of 32 μm was obtained.

[Measurement of air permeability and average pore diameter]
The air permeability of the obtained separator was measured with a Gurley densometer manufactured by Toyo Seiki. Also, the average pore size was measured by Porous Materials Inc. Measured with a Capillary Flow Porometer CEP-1500A. The results are given in Table 1.

[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 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 facing the negative electrode side, and the electrolyte for lithium batteries (Ube Industries, solvent: EC / DEC / DME = 1/1/1 (volume Ratio), supporting electrolyte: lithium hexafluorophosphate 1 mol / l) was dropped, and sealed in an aluminum foil laminate film under reduced pressure to produce 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.

  In a battery separator having a porous membrane layer containing inorganic fine particles and a porous support as at least constituent elements, in Comparative Example 1, the porous membrane layer contains fine cellulose fibers. The dispersibility of the fibers is uneven and the average pore diameter is not small. Also in Comparative Example 2 in which the porous membrane layer contains molecular sodium carboxymethylcellulose, the average pore diameter is widened. On the other hand, in the separator of Example 1 in which the porous membrane layer contains carboxyl group-provided type I fine cellulose, the average pore diameter was reduced and the internal resistance was successfully reduced. .

  Moreover, in the comparative example 3, although the porous support body contains the fine cellulose fiber, the average pore diameter cannot be made small. On the other hand, in the separator of Example 2 in which the porous support contains carboxyl group-provided type I fine cellulose, the average pore diameter was reduced and the internal resistance was successfully reduced. .

  Comparing Examples 1 and 2, when the porous membrane layer containing inorganic fine particles contains carboxyl group-provided type I fine cellulose fibers, the average pore diameter can be reduced and the ratio of discharge capacity / charge capacity is also high. It was confirmed that the internal resistance was also lowered.

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

Claims (2)

  A battery separator comprising fine cellulose fibers having an I-type crystal form having a carboxyl group on the surface.   A battery separator comprising a porous membrane layer containing inorganic fine particles and a porous support at least as constituents, wherein the porous membrane layer has a fine cellulose having an I-type crystal form whose surface is carboxymethylated A battery separator characterized by comprising a fiber.