JP2014160580A - Cell separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thinned separator whose resistivity is low.SOLUTION: The cell separator includes a porous film layer containing inorganic particles on a porous support, wherein the inorganic particles are inorganic salt particles, and the porous film layer contains a water-soluble cellulose derivative. It is preferable that a solubility product of the inorganic salt particles is equal to or less than 10.

Description

  The present invention relates to a battery separator.

  Conventionally, as a battery separator (hereinafter sometimes abbreviated as “separator”) used in a lithium battery, a polyolefin film having fine pores 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 internal 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 Documents 1 to 3 propose a separator made of a flexible support having a hole and a porous ceramic material that closes the hole. By using a porous film or non-woven fabric as a flexible support with holes, and using a ceramic material with excellent heat resistance as the porous membrane layer, even if the battery runs out of heat, the electrode contacts due to shrinkage. It is attracting attention because it can be suppressed. In addition to oxides such as aluminum oxide, magnesium oxide, silicon oxide, titanium oxide, zinc oxide and zirconium oxide, titanates such as barium titanate and lead titanate have been proposed as porous ceramic materials. Basically, it is an oxide and is limited in material.

  On the other hand, in Patent Documents 4 to 6, proposals have been made to suppress thermal shrinkage by using inorganic fine particles as fillers in conventional olefin film separators. As fillers of inorganic fine particles used, carbonates such as calcium carbonate and magnesium carbonate, aluminum silicates such as talc, clay and mica, or magnesium silicates and clay minerals which are composites thereof, magnesium sulfate, barium sulfate Such as sulfates, hydroxides and oxides. Patent Documents 4 to 6 are characterized in that these inorganic fine particles are used in a dispersed state in a polyolefin film that is inactive in a lithium battery. A state in which the inorganic fine particles described in Patent Documents 4 to 6 are combined with the perforated flexible support described in Patent Documents 1 to 3, and the inorganic fine particles are present on the surface of the support. It was difficult to say that the mode used in the above has been fully studied.

Japanese Patent No. 4594098 Special table 2008-503049 gazette JP 2011-113770 A Japanese Patent No. 4563008 Japanese Patent No. 4929593 JP 2012-43629 A

  An object of the present invention is to provide a battery separator having a low resistance and a reduced thickness.

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

[1] In a battery separator having a porous membrane layer containing inorganic particles on a porous support, the inorganic particles are inorganic salt particles, and the porous membrane layer contains a water-soluble cellulose derivative. Battery separator, characterized by
[2] The battery separator according to [1], wherein the solubility product of the inorganic salt particles is 10 ⁻⁹ or less.

  In the present invention, the battery internal resistance of the battery using the battery separator of the present invention is reduced, and a battery separator with a reduced thickness can be obtained.

  The battery separator of the present invention is a battery separator having a porous membrane layer containing inorganic particles on a porous support, the inorganic particles are inorganic salt particles, and the porous membrane layer is water-soluble cellulose. It is characterized by containing a derivative.

  The water-soluble cellulose derivative in the present invention is a cellulose derivative synthesized by modifying a part of hydroxyl group of β-glucose molecule bonded linearly by a glycosidic bond to enable water-solubilization. Represents a compound modified with a carboxymethoxy group, a methoxy group, a hydroxyethoxy group, or a hydroxyproxy group. A derivative substituted with a carboxymethoxy group is called carboxymethylcellulose (CMC) and can be water-solubilized with sodium salt or the like. Methylcellulose containing only methoxy groups dissolves only in low-temperature water and has a thermal gel property that gels an aqueous solution when the temperature rises. Moreover, it is excellent in foaming property and foaming property, and can behave like a nonionic polymer surfactant. In general, by combining a methoxy group with a hydroxyethoxy group or a hydroxypropoxy group, water solubility and thermal gel properties can be controlled. In addition, cellulose derivatives such as cellulose acetate and ethyl cellulose are known, but cellulose acetate and ethyl cellulose do not dissolve in water.

The water-soluble cellulose derivative is used in combination with inorganic salt particles to form a porous membrane layer. The inorganic salt may be finely divided and dispersed in advance to form a porous film layer. However, the inorganic salt may be mixed in an ionic state to precipitate a salt, and at this point, the fine particles may be formed into a porous film. At this time, the size of the precipitated inorganic salt particles can be controlled by pH, temperature, water-soluble cellulose derivative, surfactant and the like. Since the steps of dispersion and precipitation in water are included and the porous membrane layer does not collapse in the production process, the solubility product of the inorganic salt is preferably small, and the solubility product in water at 25 ° C. is preferably 10 ^{−. 9} or less, more preferably 10 ⁻¹⁰ or less.

  Further, since the battery is exposed to an acidic or alkaline substance, the inorganic salt needs to have a certain degree of acid resistance and alkali resistance. Anions capable of imparting such properties are sulfate ion, phosphate ion, pyrophosphate ion and the like. More specifically, preferable salts include phosphates such as calcium phosphate, magnesium phosphate, aluminum phosphate, nickel phosphate, and zinc phosphate, and pyrophosphates such as calcium pyrophosphate, magnesium pyrophosphate, aluminum pyrophosphate, and zinc pyrophosphate. And sulfates such as acid salts and barium sulfate. In particular, barium sulfate is an excellent material that has excellent insulating properties and is stable except for thermal acid.

  In the present invention, various polymer binders can be used in combination with the porous membrane layer in order to improve the adhesion between the inorganic salt particles and the adhesion between the inorganic salt particles and the porous support. In particular, when polyester fibers or polypropylene fibers that are difficult to bond are used for the porous support, it is preferable to use a latex polymer binder as the polymer binder. As the polymer binder, polyolefin, styrene-butadiene, acrylic, or the like can be used. The content of the polymer binder is preferably 0.5 to 20% by mass of the porous membrane layer, and more preferably 1 to 8% by mass.

  In this invention, in order to improve the intensity | strength as a battery separator, you may have a porous support body with the porous membrane layer. 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 preferable to use a porous support using polyester from the viewpoint of chemical and thermal stability in the battery, and a nonwoven fabric is used because of excellent handling and strength. Is preferred. The nonwoven fabric can be manufactured by a wet method, a dry method, an electrostatic spinning method, or the like. As a porous support body, it is preferable that it is 10-25 micrometers in thickness, and it is preferable that a porosity is 30-80%. More preferably, the thickness is 12 to 18 μm and the porosity is 40 to 70%.

The porous membrane layer can be obtained by coating or casting on a porous support, gelling, and drying. 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 performing another heat calendering process 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 formed. 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 styrene butadiene rubber, and as the active material, lithium cobaltate, lithium nickelate, lithium manganate, A lithium manganese composite oxide such as lithium nickel manganese cobaltate (NMC) or lithium aluminum manganate (AMO), lithium iron phosphate, or the like is used. These are mixed and applied onto 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 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 a lithium battery, a non-aqueous electrolyte solution is used. Generally, 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, an organic lithium salt such as LiN (SO ₂ CF ₃ ) ₂ is used in addition to lithium hexafluorophosphate and lithium tetrafluoroborate. 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
The following materials were stirred for 3 hours with a homogenizer (manufactured by PRIMIX, TK HODISPER Model 2.5, rotation speed 2500 rpm) to prepare dispersion (1).

Precipitated barium sulfate (product name: BARIACE (registered trademark) B-54, manufactured by Sakai Chemical Industry)
100 parts by mass sodium metaphosphate 0.1 parts by mass special carboxylic acid type polymer activator (trade name: Demol (registered trademark) P manufactured by Kao) 0.5 parts by mass distilled water

  100 parts by mass of the obtained dispersion (1), 100 parts by mass of 0.9% by mass aqueous carboxymethylcellulose solution (manufactured by Nippon Paper Chemicals Co., Ltd., MAC500LC) and styrene-butadiene rubber (SBR) latex (manufactured by Nippon A & L, products) 6.2 parts by mass of name: AL-2001, concentration 48.3% by mass) was added to prepare a coating liquid (1).

Next, with a constitution of 60 parts by mass of stretched regular polyethylene terephthalate (PET) fiber (0.1 dtex, length 3 mm) and 40 parts by mass of unstretched PET fiber (0.2 dtex, length 4 mm), the basis weight is obtained by a wet papermaking method. A web of 10 g / m ² was produced. The drying temperature at this time was 130 ° C. Next, a thermal calendar process was performed at 220 ° C. to prepare a porous support (1) having a thickness of 15 μm. The obtained coating liquid (1) was impregnated into a porous support and dried at 100 ° C. to obtain a separator (1) having a thickness of 25 μm.

(Example 2)
100 parts by mass of a sodium carboxymethylcellulose aqueous solution having a concentration of 1.2% by mass (manufactured by Nippon Paper Chemicals Co., Ltd., MAC500LC), 24 parts by mass of barium chloride dihydrate (formula weight 244.26 g / mol), 0.03 mass of sodium metaphosphate Part, 0.4 parts by mass of a special carboxylic acid type polymer activator (trade name: Demol (registered trademark) P, manufactured by Kao) was added and dissolved. Next, while slowly stirring with a homogenizer (Primics, TK HODISPER Model 2.5), 14 parts by mass of sodium sulfate (formula weight 142.04 g / mol) was gradually added, and the rotational speed was 2000 rpm. To a barium sulfate dispersion (2).

  2 parts by mass of styrene-butadiene rubber (SBR) latex (manufactured by Nippon A & L, trade name: AL-2001, concentration 48.3% by mass) is added to 100 parts by mass of the prepared dispersion (2), and the coating liquid is added. (2) was produced. Next, the porous support (1) produced in Example 1 was impregnated with the coating liquid (2), dried at 80 ° C., washed with water to remove excess sodium chloride, and a separator (2) having a thickness of 21 μm. Got.

(Example 3)
The separator (3) was obtained by changing the precipitated barium sulfate in Example 1 to calcium sulfate.

(Comparative Example 1)
A comparative separator (1) was produced by changing the sodium carboxymethylcellulose of Example 1 to polyvinyl alcohol (Kuraray, trade name: Poval PVA124).

(Comparative Example 2)
The carboxymethylcellulose sodium of Example 2 was changed to polyvinyl alcohol (manufactured by Kuraray, trade name: POVAL PVA217) to obtain a comparative separator (2).

(Comparative Example 3)
The porous support (1) was used as a comparative separator (3).

[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. In addition, the average pore diameter 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, 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 prepare a positive electrode. On the other hand, spherical artificial graphite, acetylene black, and polyvinylidene fluoride were coated at a mass ratio of 85/15/5 on a copper foil at a rate 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, and the electrolyte solution for lithium batteries (solvent: EC / DEC = 3/7 (volume ratio)) manufactured by Kishida Chemical, 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.

  From the above results, from Examples 1 to 3, it is a battery separator having a porous membrane layer containing inorganic particles on a porous support, the inorganic particles are inorganic salt particles, and the porous membrane layer The battery separator containing water-soluble cellulose derivative functioned effectively as a battery separator used for lithium batteries. However, from Comparative Examples 1 and 2, when polyvinyl alcohol was used instead of the water-soluble cellulose derivative, the characteristics were greatly reduced.

From the comparison between Example 1 and Example 3, when the inorganic salt particles are barium sulfate (solubility product 1.1 × 10 ⁻¹⁰ ), calcium sulfate (solubility product 9.1 × 10 ⁻⁶ ) In the battery separator manufacturing process, a porous membrane layer having excellent porosity was obtained, and a good battery separator could be obtained.

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

Claims (2)

  In a battery separator having a porous membrane layer containing inorganic particles on a porous support, the inorganic particles are inorganic salt particles, and the porous membrane layer contains a water-soluble cellulose derivative. Battery separator. The battery separator according to claim 1, wherein the solubility product of the inorganic salt particles is 10 ⁻⁹ or less.