JP2015046330A - Battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excellent battery separator suppressed in characteristic deterioration in prolonged heating.SOLUTION: In a battery separator, a support and a porous ceramic layer containing ceramic fine particles are combined. The ceramic fine particles are amphoteric ceramic fine particles, and have a surface coated with a basic oxide. The basic oxide is preferably a magnesium oxide.

Description

  The present invention relates to a battery separator.

  Conventionally, as a separator of a lithium secondary battery, a polyolefin porous 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 discloses that the battery is thermally runaway by combining a porous ceramic layer containing porous ceramic fine particles with a porous support such as a nonwoven fabric. There has been proposed a method capable of suppressing electrode contact without causing thermal contraction of the separator even in the case of occurrence of heat. This method is an excellent method because the porous ceramic layer can permeate the surface and the inside of the separator and can impart high electrolyte retention and heat resistance.

  On the other hand, Patent Documents 2 and 3 propose a method of providing heat resistance by providing a porous ceramic layer containing ceramic fine particles on one side of a porous film. In particular, Patent Document 2 describes that titanium oxides such as alumina, zirconia, silica, titania, barium titanate, lead titanate, strontium titanate, and composite oxides thereof are effective for the porous ceramic layer.

  In particular, silica having a high insulating property and alumina that forms a unique aggregate structure are very promising materials (see, for example, Patent Document 4). There was a problem, such as the deterioration of was observed.

Japanese Patent No. 4594098 Special table 2008-503049 gazette Japanese Patent No. 4499851 International Publication No. 2008/114727 Pamphlet

  An object of the present invention is to provide an excellent battery separator in which deterioration of characteristics over time is suppressed.

  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] A battery separator in which a porous ceramic layer containing ceramic fine particles and a support are combined, wherein the ceramic fine particles are amphoteric ceramic fine particles, and the surface of the ceramic fine particles is coated with a basic oxide. Battery separator.
[2] The battery separator according to [1], wherein the basic oxide is magnesium oxide.

  In the present invention, it is possible to obtain an excellent battery separator in which deterioration of characteristics over time is suppressed.

  The battery separator of the present invention is a battery separator in which a porous ceramic layer containing ceramic fine particles and a support are combined. The ceramic fine particles are amphoteric ceramic fine particles, and the surface of the ceramic fine particles is coated with a basic oxide. It is characterized by.

The ceramic fine particles in the present invention are amphoteric ceramic fine particles. Examples of amphoteric ceramic fine particles include zinc oxide, titanium oxide, alumina, and beryllium oxide. Amphoteric ceramic fine particles are oxides having both properties of adsorbing protons (releasing hydroxyl groups) and releasing, depending on the dissociation state of the surface hydroxyl groups. In the battery, it is used after sufficiently removing the water adsorbed on the surface of the oxide, but a trace amount of water remains, and the surface hydroxyl groups of the ceramic fine particles cannot be completely removed. When exposed to the electrolyte in this state, surface hydroxyl groups release protons and adsorb lithium ions. Protons generated by the adsorption of lithium ions react with anions such as PF ₆ ⁻ which are electrolyte counter ions, and in this case, HF (hydrogen fluoride) and PF ₅ are generated. Further, hydrogen fluoride attacks the electrode agent and degrades battery characteristics. In particular, in the environment where the battery is heated, this reaction is promoted, and the generated hydrogen fluoride is adsorbed by the amphoteric ceramic fine particles. Water is generated, the reaction is chained, and the separator itself is deteriorated.

  Examples of the basic oxide in the present invention include potassium oxide, lithium oxide, cesium oxide, barium oxide, strontium oxide, calcium oxide, and magnesium oxide. As basic oxides that can be handled in an aqueous system, strontium oxide, calcium oxide, and magnesium oxide having low solubility in hydroxides are preferable materials, and in particular, magnesium oxide having low solubility is the most preferable material. Magnesium oxide does not liberate protons even if it has a surface hydroxyl group due to a small amount of water, so lithium ions cannot be substituted and only adsorbed on the surface, suppressing the generation of hydrogen fluoride that causes changes in the chemical structure of the battery. it can.

  In the present invention, the surface of the amphoteric ceramic fine particles needs to be coated with a basic oxide. The surface coating method will be described in the case where the basic oxide is magnesium oxide. Surface coating method in which magnesium salt soluble in water such as magnesium acetate coexists with amphoteric ceramic fine particles, neutralized to precipitate magnesium hydroxide, heated, taken out of water and sintered to make magnesium oxide, There is a surface coating method in which, after coexisting in water, it is taken out from water as it is, sintered as it is, and magnesium ions are converted into magnesium oxide. In the latter, since the surface treatment is performed only in a state where magnesium ions are adsorbed, the volume or structure of the amphoteric ceramic fine particles may be caused by heat treatment, etc. The former method is preferable because it can cover more and is less likely to cause volume change or structural change.

  Since the surface of the amphoteric ceramic fine particles is coated with a basic oxide so that the surface hydroxyl groups of the amphoteric ceramic fine particles need to be blocked, the entire surface of the amphoteric ceramic fine particles is preferably uniformly coated. The coating amount is preferably 0.5% by mass to 100% by mass of the amphoteric ceramic fine particles, more preferably 2.0% by mass to 60% by mass, and 5.0% by mass to 40% by mass. More preferably.

  The particle size of the amphoteric ceramic fine particles coated with the basic oxide is important because it is deeply related to the internal voids of the porous ceramic layer, preferably 0.3 μm or more and 5.0 μm or less, more preferably 0.5 μm or more. 3.0 μm or less. The particle size is determined by sufficiently diluting the ceramic fine particles with water, and measuring this with a laser scattering type particle size measuring device (trade name: 3300EX2 manufactured by Microtrac Co., Ltd.). The resulting center particle size (D50, volume average) It is.

  In the present invention, the porous ceramic layer can contain a water-soluble cellulose derivative. The ceramic fine particles are dispersed in water to form a coating liquid that forms a porous ceramic layer. In this case, if a water-soluble cellulose derivative is contained, the water-soluble cellulose derivative becomes a dispersibility aid in water, and the coating liquid is thickened to improve the setting property of the coating liquid on the support. be able to. As the dispersing agent at the time of dispersion, various surfactants such as nonionic, anionic, and cationic, and salts for imparting electric charge to the ceramic fine particles can be used in combination.

  The water-soluble cellulose derivative in the present invention is a cellulose derivative synthesized by modifying a part of hydroxyl groups of β-glucose molecules bonded linearly by glycosidic bonds so as to be water-soluble. , 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 ammonium salt. 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, solubility and thermal gel properties can be controlled by combining a hydroxyethoxy group or a hydroxypropoxy group with a methoxy group. In addition, water-soluble cationized cellulose can also be used. However, cellulose derivatives such as cellulose acetate and ethyl cellulose cannot be used because they are water-insoluble cellulose derivatives that do not dissolve in water. The water-soluble cellulose derivative can be used in combination with ceramic fine particles to form a porous ceramic layer and reduce internal resistance. If the content of the water-soluble cellulose derivative is too large, a film is formed around the voids in the drying step and independent voids are formed. Therefore, the content is preferably 5% by mass or less of the porous ceramic layer, more preferably 3 It is below mass%.

  Various polymer binders can be used in combination in order to improve the adhesion between the ceramic particles and the adhesion between the support and the ceramic particles. In particular, when polyester or polypropylene, which is difficult to bond, is used for the 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 ceramic layer, and more preferably 1 to 8% by mass.

  In the present invention, a porous ceramic layer is combined with a support in order to improve the strength as a battery separator. Examples of the support include a porous film, a woven fabric, a nonwoven fabric, and a knitted fabric. Examples of the material for the support include polyester, polyolefin, polyamide, aramid, and cellulose. The support is a non-woven fabric using fibers such as polyester, polyolefin, polyamide, aramid, cellulose, etc., and is particularly excellent in heat resistance and easy to obtain fine fibers, using non-woven fabric using polyester or polyolefin fibers. Is preferred. The nonwoven fabric can be produced by various methods such as a wet method, a dry method, and an electrostatic spinning method. As a 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 ceramic layer can be obtained by applying or casting a coating liquid containing ceramic fine particles on a support, gelling in some cases, and then 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 ceramic 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 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 secondary battery, an electrolyte is injected, the battery is sealed, and a lithium secondary 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 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 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 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 type 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.

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

(Example 1)
40 parts by mass of alumina (manufactured by Daimei Chemical Co., Ltd., granular boehmite, trade name: C06) was dispersed in 200 parts by mass of a 2% by mass magnesium acetate aqueous solution heated to 80 ° C. Next, sodium hydroxide was slowly added to adjust the pH to 8, and heating was continued for 12 hours. This was diluted in 1000 parts by mass of water, filtered and washed repeatedly to remove sodium acetate. Furthermore, it was dried at 120 ° C. to obtain magnesium hydroxide-coated alumina. Further, this material was heated at 350 ° C. for 12 hours to obtain magnesium oxide-coated alumina.

  Next, the following material was stirred for 3 hours with a homogenizer (manufactured by Primex, trade name: TK HODISPER Model 2.5, rotation speed 2500 rpm) to prepare dispersion (1).

Magnesium hydroxide-coated alumina 40 parts by weight Sodium laurate 0.01 parts by weight Special carboxylic acid type polymer activator (trade name: Poaz 520 manufactured by Kao) 0.5 parts by weight distilled water 100 parts by weight

  To 100 parts by mass of the obtained dispersion (1), a sodium carboxymethylcellulose aqueous solution having a concentration of 0.6% by mass (made by Nippon Paper Industries Co., Ltd., trade name: MAC500LC) and an acrylic resin (manufactured by JSR, trade name: TRD-202A) 4.0 parts by mass of 40.2% by mass) was added to prepare a coating liquid (1).

Next, 75 g of stretched regular polyethylene terephthalate (PET) fiber (0.1 dtex, length 3 mm) and 25 parts by weight of unstretched PET fiber (0.2 dtex, length 4 mm), and a basis weight of 9 g by a wet method. A web of / m ² was produced. The drying temperature at this time was 130 ° C. Next, a thermal calendar process was performed on the web at 220 ° C. to prepare a support (1) which was a nonwoven fabric having a thickness of 15 μm. The obtained coating liquid (1) was impregnated into the support (1) and dried at 100 ° C. to produce a separator (1) having a thickness of 25 μm.

(Example 2)
A separator (2) having a thickness of 25 μm was prepared in the same manner except that the magnesium hydroxide-coated alumina in Example 1 was changed to magnesium oxide-coated alumina.

(Comparative Example 1)
In Example 1, instead of magnesium oxide-coated alumina, alumina (granular boehmite manufactured by Daimei Chemical Co., Ltd., trade name: C06) was used as it was to obtain a comparative separator (1).

(Comparative Example 2)
The following materials were stirred for 3 hours with a homogenizer (manufactured by PRIMIX, trade name: TK HODISPER Model 2.5, rotation speed 2500 rpm) to prepare a comparative dispersion.

Alumina (manufactured by Daimei Chemical Co., Ltd., trade name: C06) 36 parts by mass magnesium hydroxide (manufactured by Kyowa Chemical Industry, trade name: Kisuma (registered trademark) 5P) Name: Poise 520) 0.5 parts by weight distilled water 120 parts by weight

  To 100 parts by mass of the obtained comparative dispersion, 100 parts by mass of 0.6 mass% sodium carboxymethylcellulose aqueous solution (manufactured by Nippon Paper Industries, trade name: MAC500LC) and acrylic resin (manufactured by JSR, trade name: TRD-202A, concentration 40). .2 mass%) was added to 4.0 parts by mass to prepare a comparative coating solution. Next, the support (1) was impregnated with the comparative coating liquid and dried at 100 ° C. to prepare a comparative separator (2) having a thickness of 25 μm.

(Comparative Example 3)
In Example 1, instead of magnesium hydroxide-coated alumina, magnesium hydroxide (manufactured by Kyowa Chemical Industry, trade name: Kisuma (registered trademark) 5P) was used as it was to obtain a comparative separator (3).

[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, 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 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.

  A separator was sandwiched between the obtained electrodes and heated at 95 ° C. under reduced pressure of 0.01 MPa or less for 12 hours, and then an electrolyte for a lithium secondary battery manufactured by Ube Industries (trade name: Purelite, 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 by reducing the pressure, and the lithium secondary battery was Produced. Next, the produced lithium secondary 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. Next, the battery was left to stand at 90 ° C. for 24 hours, and then charged and discharged under the same conditions of 0.2 C, and the ratio between the discharge capacity and the charge capacity was measured.

  From comparison between Examples 1 and 2 and Comparative Example 1, it is understood that when amphoteric ceramic fine particles coated with a basic oxide are used, a battery that can withstand heating with time is obtained without degrading the performance of the initial battery characteristics. . Comparative Example 2 is a system in which alumina and magnesium hydroxide are used in combination, but with this, the surface of alumina which is amphoteric ceramic fine particles remains, and the performance with heating deteriorates. Furthermore, in Comparative Example 3, only magnesium hydroxide was used as the ceramic fine particles, but the internal resistance increased, resulting in an undesirable result. From the above, it was the battery produced according to the example that good initial characteristics were obtained and that the capacity decrease with time of heating could be suppressed. Moreover, when Example 1 and Example 2 are compared, when a basic oxide is magnesium oxide, it turns out that the capacity | capacitance fall by heating aging can be suppressed more.

  The battery separator of the present invention can be used as a capacitor separator in addition to a lithium secondary battery separator.

Claims (2)

  A battery separator comprising a composite of a porous ceramic layer containing ceramic fine particles and a support, wherein the ceramic fine particles are amphoteric ceramic fine particles, and the surface of the ceramic fine particles is coated with a basic oxide. Separator for use.   The battery separator according to claim 1, wherein the basic oxide is magnesium oxide.