JP2015170441A - Separator for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for battery excellent in durability, in which reduction of the charge discharge capacity of a battery is less, even if the battery is held under a high temperature environment.SOLUTION: In a separator for battery having a porous ceramic layer containing heteromorphic silica having an elongated shape, e.g., rodlike, bend-like or branch-like, and a basic oxide, e.g., a chloride of calcium or magnesium, a nitrate salt, a sulfamate, a formate, or an acetate, and a porous support, surface of the heteromorphic silica is coated with magnesium hydroxide. The separator can be used not only as the separator for lithium secondary battery, but also as the separator for a capacitor.

Description

  The present invention relates to a battery separator.

  Conventionally, as a battery separator (hereinafter sometimes abbreviated as “separator”) used in 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 In order to prevent the separator from being thermally contracted due to an increase in temperature and coming into contact with the electrode, 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 a battery is provided with heat resistance by combining a porous support having a hole such as a nonwoven fabric and a porous ceramic layer containing porous ceramic fine particles. There has been proposed a method capable of suppressing electrode contact without causing thermal contraction of the separator even when thermal runaway occurs. 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. 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 as the ceramic fine particles. In Patent Document 3, silica, magnesium oxide, alumina, calcium oxide, titania, zinc oxide, tin oxide and the like are listed as ceramic fine particles.

  In particular, silica having high insulation is an inexpensive and very promising material, but has left the problem of deteriorating battery characteristics over time with respect to the electrolyte used in lithium secondary batteries.

Japanese Patent No. 4594098 Special table 2008-503049 gazette Japanese Patent No. 4499851

  The subject of this invention is providing the battery separator excellent in durability.

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

  A battery separator having a porous ceramic layer containing a deformed silica and a basic oxide and a porous support, wherein the surface of the deformed silica is coated with magnesium hydroxide. Separator.

  In the present invention, a battery separator having excellent durability can be obtained.

  The deformed silica in the present invention is a silica having an elongated shape such as a rod shape, a bent shape, a branched shape, etc. obtained by hydrothermal synthesis by adding a salt that forms a basic oxide to an active silicic acid that is a raw material silica. . The active silicic acid is a silica sol obtained by subjecting water glass to a cation exchange treatment and adjusting the pH to an acidic atmosphere. As such salts, calcium, magnesium chloride, nitrate, sulfamate, formate, acetate and the like are preferable materials. The addition amount is 0.15% by mass to 1.5% by mass in terms of CaO and MgO (basic oxide), the pH is returned from neutral to alkaline, and from 85 ° C. to 200 ° C., 0.5% Heat for 20 hours to obtain deformed silica having a predetermined structure. During hydrothermal synthesis, if the silica concentration is too high, the silica concentration is preferably 10 to 40% by mass due to problems such as the gelation of the silica sol or the sudden bond bonding that makes the reaction uncontrollable. More preferably, it is 15-30 mass%. Such deformed silica is, for example, ST-UP, ST-UP-S, ST-UP-M, ST-OUP, ST, which are specially shaped products in the trade name Snowtex (registered trademark) manufactured by Nissan Chemical. -Available in grades such as UP-SO and ST-UP-MO.

  In the present invention, a porous ceramic layer is formed by containing irregular shaped silica and a basic oxide as ceramic fine particles. Basic oxides are fine particles such as particles and plates, and form an aggregated structure on the fiber surface in the porous support to adjust the pore distribution of the porous support. When the silica-based material reacts with hydrogen fluoride generated by decomposition of the electrolytic solution, the bond between silicon and oxygen is cut off to generate gaseous silicon fluoride, which causes a serious problem in battery life. In order to remove hydrogen fluoride, a basic oxide is included. As the basic oxide, calcium oxide, magnesium oxide, magnesium hydroxide and the like are preferable. Magnesium oxide and magnesium hydroxide are particularly preferable materials because the fluoride can be stably fixed on the surface.

  The particle size of the basic oxide is preferably 0.1 μm or more and 1.0 μm or less. Fine particles of less than 0.1 μm form dense ceramic aggregates with deformed silica in the porous support, and the porous ceramic layer does not spread over the entire porous support. Therefore, the maximum pore diameter is increased and the battery characteristics are deteriorated. Further, if the particle size exceeds 1.0 μm, the coating amount of the porous ceramic layer necessary for stabilizing the battery characteristics increases and the thickness of the separator is increased. It is 0.2 μm or more and 0.8 μm or less. The particle diameter of the ceramic fine particles is an average value of the particle diameters of ten ceramic fine particles measured with a transmission electron microscope (TEM) or a scanning electron microscope (SEM).

  Furthermore, the present invention is characterized in that the deformed silica is coated with magnesium hydroxide and stabilized with respect to the electrolytic solution. In order to make magnesium hydroxide exist on the surface of the deformed silica, an aqueous magnesium salt such as magnesium acetate or magnesium citrate is added to the dispersion of the deformed silica while keeping the pH of the solution in an alkaline state. Magnesium hydroxide is precipitated. The addition amount is preferably 0.1 to 20% by mass and more preferably 0.5 to 10% by mass with respect to the deformed silica in terms of magnesium hydroxide. If the amount is small, the effect may not be obtained. If the amount is too large, the film formability of deformed silica may be impaired. After coating with magnesium hydroxide, the dispersion of the deformed silica is heated at 40 ° C. to 90 ° C. to stabilize the dispersion state.

  In the present invention, the magnesium hydroxide-coated irregular shaped silica and the basic oxide are preferably dispersed at the same time. Dispersed at the same time means that when the coating liquid is produced, both fine particles are mixed and mechanical shear is added so that fine particles are stably dispersed in water. Means that. In general, in the case of a coating liquid containing two or more kinds of dispersed particles, first, aggregation occurs between the same particles during drying. When this agglomeration phenomenon occurs, a high-density porous ceramic layer (high-density layer) composed of fine particles and a low-density porous ceramic layer (low-density layer) composed of large particles are separately formed. In particular, when a high-density layer made of deformed silica is formed inside the separator, the internal resistance of the separator increases. However, when two types of particles are mixed at the same time and shear is applied, the fine particles are dispersed so as to cover the surface of the large particles, so that the formed porous ceramic layer has a locally high density. The structure which does not have a layer easily is taken, and a high porosity can be formed as the whole separator.

  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 dispersion aid in water, and the coating liquid is thickened to improve the setting property of the coating liquid on the porous support. Can be improved. In order to improve the dispersibility of ceramic fine particles and the affinity between the porous support and the ceramic fine particles, various surfactants such as nonionic, anionic and cationic, salts for imparting electric charges to the ceramic fine particles, etc. Can also be used together. Examples of the surfactant include sodium laurate and dodecylbenzene sulfonic acid.

  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 to form independent voids, so that it is preferably 10% by mass or less, more preferably 5% of the porous ceramic layer. It is below mass%.

  Various polymer binders can be used in combination in order to improve the adhesion between the ceramic fine particles and the adhesion between the porous support and the ceramic fine particles. In particular, when polyester or polypropylene that is difficult to bond is 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 ceramic layer, and more preferably 1 to 6% by mass.

  In the present invention, a porous ceramic layer is formed on a porous support. 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. The porous support is a non-woven fabric using fibers such as polyester, polyolefin, polyamide, aramid, cellulose, etc., which is particularly excellent in heat resistance, stable against electrolyte solution, and easy to obtain fine fibers. It is preferable to use a nonwoven fabric using polyester or polyolefin fibers. The nonwoven fabric can be produced by various methods such as a wet method, a dry method, and an electrostatic spinning method. The porous support preferably has a thickness of 10 to 25 μm, and the porosity is preferably 40 to 80%. More preferably, the thickness is 12 to 18 μm, and the porosity is 50 to 75%.

  When a nonwoven fabric (wet nonwoven fabric) made by a wet method is used as a porous support, the surface of the porous support is hydrophilized by the surfactant remaining on the fiber surface during paper making. Then, an aggregated structure in which the above-mentioned deformed silica is stabilized appears. The size (diameter) of the aggregated structure is about submicron to several microns, and grows while adhering to the fiber surface with anisotropic silica.

The porous ceramic layer can be obtained by applying or casting a coating liquid containing ceramic fine particles on a porous 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 20 g / m ² by dry mass, and more preferably 1 to 10 g / m ² . The preferable thickness of the battery separator is 10 to 30 μm, and 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 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 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 (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ), 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.

[Preparation of porous support]
With a configuration of 65 parts by mass of stretched regular polyethylene terephthalate (PET) fiber (0.1 dtex, length 3 mm) and 35 parts by mass of unstretched PET fiber (0.2 dtex, length 4 mm), the basis weight is 8.1 g by a wet method. A web of / m ² was produced. The drying temperature at this time was 130 ° C. Next, the web was subjected to thermal calendering treatment at 220 ° C. to prepare a porous support which was a wet nonwoven fabric having a thickness of 16 μm.

Example 1
50 parts by mass of a magnesium acetate aqueous solution having a concentration of 1% by mass was added to a mixed solution of the following materials and heated at 60 ° C. for 2 hours.

Water 50 parts by mass Sodium polycarboxylate 0.03 parts by mass Deformed silica (manufactured by Nissan Chemical Industries, trade name: Snowtex (registered trademark) ST-UP, concentration 20% by mass)
100 parts by mass

  Next, 20 parts by mass of magnesium hydroxide (manufactured by Kyowa Chemical Industry, trade name: 200-06H, average particle size 0.6 μm) is added, and a dispersing machine (product name: TK HOMO, manufactured by Tokushu Kika Kogyo Co., Ltd.) is added. MIXER) and stirred at a rotational speed of 7000 rpm for 60 minutes to prepare a dispersion.

The following materials were added to this dispersion to prepare a coating solution, impregnated into a porous support, and dried at 100 ° C. to prepare a separator having a thickness of 20 μm.

0.6% by weight sodium carboxymethylcellulose (Nippon Paper Industries, trade name: Sunrose (registered trademark) MAC500LC) 100 parts by weight 40% by weight acrylic latex (JSR, trade name: TRD202A) 3 parts by weight

(Comparative Example 1)
The mixture of the following materials was stirred for 60 minutes at a rotational speed of 7000 rpm using a disperser (product name: TK HOMO MIXER, manufactured by Tokushu Kika Kogyo Co., Ltd.) to prepare a dispersion.
Water 50 parts by mass Sodium polycarboxylate 0.03 parts by mass Deformed silica (manufactured by Nissan Chemical Industries, trade name: Snowtex (registered trademark) ST-UP, concentration 20% by mass)
100 parts by mass of magnesium hydroxide (Kyowa Chemical Industry, trade name: 200-06H, average particle size 0.6 μm)
20 parts by mass

The following materials were added to this dispersion to prepare a coating solution, impregnated into a porous support, and dried at 100 ° C. to prepare a separator having a thickness of 20 μm.

0.6% by weight sodium carboxymethylcellulose (Nippon Paper Industries, trade name: Sunrose (registered trademark) MAC500LC) 100 parts by weight 40% by weight acrylic latex (JSR, trade name: TRD202A) 3 parts by weight

[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. The internal resistance was measured with a voltage drop value of 30 minutes after the start of discharge under the condition of 0.2 C (discharge time of 300 minutes). Next, the battery was left to stand at 90 ° C. for 3 days, and then charged and discharged under the condition of 0.2 C, and the ratio between the discharge capacity and the charge capacity (charge / discharge capacity ratio) was measured. The results are given in Table 1.

  In the examples, since the surface of the deformed silica was coated with magnesium hydroxide as compared with the comparative example, the durability was improved.

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

Claims (1)

  A battery separator having a porous ceramic layer containing a deformed silica and a basic oxide and a porous support, wherein the surface of the deformed silica is coated with magnesium hydroxide. Separator.