JP2016012548A - Battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery separator which is thin in thickness and enables the achievement of a low internal resistance of a battery, and in which the distribution of micropores is controlled.SOLUTION: A battery separator comprises: a porous base including organic fibers; and a ceramic porous layer. The ceramic porous layer includes glass ceramic bodies. The glass ceramic bodies are each a glass ceramic body of needle-like boehmite, and 5 to 30 in aspect ratio and 1.0-10.0 μm in a long axis thereof.

Description

  The present invention relates to a battery separator.

  Conventionally, as a lithium secondary battery separator, a polyolefin film having fine pores that have penetrated has been used. This separator, when the battery is abnormal and generates heat, melts and closes the fine pores that pass therethrough, and increases the internal resistance of the battery, thereby suppressing the heat generation and the lithium cobalt oxide as the 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 In order to avoid electrode contact due to thermal contraction of the separator that occurs due to an increase in temperature, there is an increasing demand for a separator having high heat resistance and low internal resistance.

  Patent Document 1 describes a method of suppressing a short circuit caused by dendrite generated in a secondary battery by using a porous layer made of a metal oxide in combination with a separator. Further, Patent Document 2 describes a method of suppressing internal short-circuit by forming a metal oxide porous protective film on the electrode surface. As a result of these studies, it became clear that a ceramic porous layer containing a ceramic body such as alumina or silica can be used in a lithium secondary battery. The study of using it to improve the heat resistance of the mosquito has started.

  The separator using the ceramic porous layer is roughly classified as in Patent Document 3, using a low-density porous substrate typified by a nonwoven fabric, and a separator in which this void is filled with the ceramic porous layer, As in Patent Document 4, two types of separators are known in which a ceramic porous layer is coated on one or both surfaces of a conventional polyolefin microporous membrane to impart heat resistance. In any case, the ceramic porous layer needs to have a sufficient porosity because it is necessary to reduce the internal resistance of the separator. In addition, the pore distribution of the ceramic porous layer is an important design factor in order to maintain the electrode isolation performance.

  In order to meet such a demand, Patent Documents 5 and 6 propose plate-type boehmite, boehmite having a secondary particle structure in which primary particles are continuous, and the like as ceramic bodies. However, the plate-like ceramic body is easy to take a laminated structure, and when the plate-like surface is arranged in parallel with the electrode surface, the particles are an insulating layer, so that a dead zone where current does not easily flow is formed on the electrode surface. Therefore, it was easy to cause an increase in internal resistance and a decrease in battery capacity. In the case of a granular ceramic body, the phenomenon seen in such a plate-shaped ceramic body is avoided, but in order to obtain a sufficient pore distribution in the ceramic porous layer, the ceramic porous layer is made thicker. Therefore, in order to adjust the pore distribution of the separator, it was necessary to use a microporous membrane in combination.

  On the other hand, in Patent Document 7, a polyolefin microporous membrane and a needle-like filler are used in combination, but the needle-like filler is used for suppressing thermal shrinkage of the polyolefin microporous membrane, and has an internal resistance as a ceramic porous layer. It has not been considered at all to reduce the thickness or adjust the pore distribution.

Japanese Patent No. 3423338 Japanese Patent No. 3371301 Japanese Patent No. 4981220 Japanese Patent No. 4460028 JP 2008-243825 A International Publication No. 2008/114727 Pamphlet JP 2012-3938 A

  An object of the present invention is to provide a battery separator whose pore distribution is controlled, thin, and having low internal resistance of the battery.

  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.

  In a battery separator comprising a porous substrate made of organic fibers and a ceramic porous layer, the ceramic body contained in the ceramic porous layer has an aspect ratio of 5 to 30 and a major axis of 1.0 μm. A battery separator characterized by being acicular boehmite of 10.0 μm.

  In the present invention, it is possible to obtain a battery separator with a controlled pore distribution and a small thickness and low internal resistance.

  The battery separator of the present invention is a battery separator containing a porous substrate made of organic fibers and a ceramic porous layer, and the ceramic body contained in the ceramic porous layer has an aspect ratio of 5 to 30, and A battery separator characterized by being acicular boehmite having a major axis of 1.0 μm to 10.0 μm.

  Acicular boehmite can be obtained by hydrothermal synthesis at a temperature of 190 ° C. to 250 ° C. in the presence of water while stirring aluminum hydroxide as a raw material in the presence of a magnesium compound. If the aspect ratio (major axis length / minor axis length ratio) is less than 5, it is difficult to adjust the pore distribution of the ceramic porous layer. If the aspect ratio exceeds 30, the boehmite aggregates. This is not preferable because redispersion becomes difficult and preparation of a coating liquid for producing a ceramic porous layer becomes difficult. Therefore, in the present invention, the aspect ratio of acicular boehmite is 5 to 30, more preferably 10 to 30. The major axis is preferably 1.0 to 10.0 μm, more preferably 1.5 to 5.0 μm. The minor axis is preferably 0.05 μm to 0.25 μm. The long axis and the short axis in the present invention are observed and measured with a scanning electron microscope, and are average values of five acicular boehmites.

  As a magnesium compound used for hydrothermal synthesis, magnesium salt of organic acid and magnesium sulfate are preferable. As the organic acid, acetic acid, propionic acid, lactic acid or the like that does not cause thermal decomposition during hydrothermal synthesis is used. The mixing ratio is preferably a magnesium / aluminum molar ratio of 0.5 / 100 to 1/1, and more preferably 1/100 to 1/10.

  The obtained acicular boehmite is redispersed in water or a solvent and impregnated or coated on a porous substrate made of organic fibers. In the production process, an aqueous dispersion is easy to handle and is a preferred method. Acicular boehmite is dispersed together with a dispersing agent, deagglomerated and then impregnated or coated on a porous substrate. The impregnated acicular boehmite spreads randomly while filling the voids between the organic fibers while maintaining a part of the bonding between the organic fibers, thereby realizing adjustment of the pore distribution and high internal voids. This is an effect that can be realized by a synergistic effect of the porous substrate made of organic fibers and the ceramic porous layer containing acicular boehmite. In the present invention, since only the acicular boehmite tends to increase the viscosity of the dispersion, other ceramic bodies may be combined to form a ceramic porous layer. Examples of other ceramic bodies include granular or plate-shaped beamart, alumina, and granular silica. After preparing a dispersion of acicular boehmite, a polymer for binding a ceramic body is added to obtain a coating solution. A latex-like polymer may be used. The ceramic bodies can be bound with a silane coupling agent or the like instead of the polymer.

The amount applied to the porous substrate is preferably 1 g / m ² to 10 g / m ² , more preferably 1.5 g / m ² to 4 g / m ² . If the coating amount is small, pin holes may be formed and the discharge capacity may not be obtained. If the amount is too large, the thickness of the separator increases and the internal resistance may increase.

  As the dispersant, phosphates such as sodium hydrogen phosphate and sodium hexametaphosphate, sodium polycarboxylate, and sodium salt of carboxymethyl cellulose are used. As the polymer, SBR (styrene butadiene rubber) or acrylic polymer can be used. As described above, a latex-like material can also be used. When a silane coupling agent is used, it is preferable that the ceramic porous layer is formed in advance and then impregnated with the silane coupling agent to impart the inter-ceramic bonding ability. As the silane coupling agent to be used, it is preferable to use a silane coupling agent such as orthoethoxysilane, orthomethoxysilane, methyltriethoxysilane, methyltrimethoxysilane, diethyldimethoxysilane, and the like. It can also be used in combination.

In the present invention, the porous substrate containing organic fibers includes organic fibers such as high-strength polypropylene, polyethylene terephthalate and other aromatic polyesters, aromatic polyamides typified by aramid, polysulfone, and cellulose. Examples of the woven fabric, nonwoven fabric, and woven fabric include nonwoven fabrics when importance is attached to the uniformity of the substrate. In order to further impart heat resistance of the porous substrate, glass fibers, ceramic fibers, or the like may be included. The nonwoven fabric is produced by a dry method or a wet method, but when the uniformity of the substrate is important, the wet method is preferable. The wet method is a method in which fibers are dispersed in water and then rolled up with a papermaking net such as a long net or a circular net. In the wet method, it is easy to combine ultrafine fibers, high uniformity, and a thin nonwoven fabric can be produced. The fiber used is preferably a short fiber having a length of 1.5 mm to 15 mm, more preferably 3 mm to 10 mm, and an ultrafine fiber having a diameter of preferably 10 μm or less, more preferably 7 μm or less. The basis weight of the nonwoven fabric is preferably 5 g / m ² to 15 g / m ² , and more preferably 7 g / m ² to 10 g / m ² . The obtained non-woven fabric is subjected to a heat calender treatment, and can be used by adjusting the thickness preferably from 8 μm to 20 μm, more preferably from 10 μm to 16 μm. The separator is obtained by impregnating or coating the obtained porous substrate with a coating liquid containing acicular boehmite.

  As an impregnation or coating 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. are used. You could use the method you had. 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 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, 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 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.

<Synthesis of acicular boehmite>
While stirring 0.92 parts by mass of magnesium acetate tetrahydrate, 2.0 parts by mass of aluminum hydroxide, and 4.0 parts by mass of water in an autoclave, the mixture was heated to 205 ° C. at a heating rate of 80 ° C./hour. This state was maintained for 5 hours. After the reaction, the mixture was naturally cooled, filtered, washed with water and dried at 105 ° C. to obtain acicular boehmite (1) having a major axis of 3.0 μm and a minor axis of 0.15 μm (aspect ratio 20). Furthermore, when the element ratio of oxygen / magnesium / aluminum of the acicular boehmite (1) was measured with an energy dispersive X-ray spectrometer (EDS) attached to a scanning electron microscope JSM-6610LV manufactured by JEOL, 2.32 / It was 0.03 / 1.00.

<Preparation of porous substrate>
Fibers and adhesives were mixed at the following composition ratios to prepare fiber dispersions.
Stretched polyethylene terephthalate (PET) short fiber 40.0 parts by mass (fineness 0.3 dt, fiber length 3 mm)
Stretched polyethylene terephthalate short fiber 30.0 parts by mass (fineness 0.1 dt, fiber length 3 mm)
Unstretched polyethylene terephthalate short fiber 30.0 parts by mass (fineness 0.2 dt, fiber length 3 mm)
Sodium laurate 0.05 parts by mass Polypropylene glycol (triol type, molecular weight 3000) 0.05 parts by mass (Daiichi Kogyo Seiyaku, trade name: Cellogen (registered trademark) BSH-12) 0.20 parts by mass

Using this fiber dispersion, it was rolled up with a circular net and short net combination paper machine and dried at a Yankee temperature of 160 ° C. to prepare a web having a basis weight of 9.0 g / m ² . Next, strength was imparted with a hot-press roll at 190 ° C. to obtain a porous substrate (1) which is a wet polyester nonwoven fabric having a thickness of 15 μm.

(Example 1)
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 dispersion (1).

Acicular boehmite (1) 10 parts by weight sodium hexametaphosphate 0.1 part by weight Special carboxylic acid type polymer activator (product name: Poaz 520) 0.2 part by weight ion-exchanged water 150 parts by weight

To the obtained dispersion liquid (1), 50 parts by mass of a 0.6 mass% sodium carboxymethylcellulose aqueous solution (manufactured by Nippon Paper Chemicals, trade name: MAC500LC) and acrylic latex (manufactured by JSR, trade name: TRD202A, concentration 40. 2 parts by mass) was added to prepare a coating liquid (1). Next, the porous substrate (1) is impregnated into the coating liquid (1) and dried, and a separator (1) having a dry weight (coating amount) of the ceramic porous layer of 3.0 g / m ² and a thickness of 21 μm is obtained. Obtained.

(Example 2)
The acicular boehmite (1) was heated at 450 ° C. for 6 hours to obtain acicular boehmite (2) having an oxygen / magnesium / aluminum element ratio of 2.00 / 0.03 / 1.00. A separator (2) having a thickness of 21 μm was obtained in the same manner as in Example 1 except that the acicular boehmite (1) was changed to the acicular boehmite (2).

(Example 3)
While stirring 0.92 parts by weight of magnesium acetate tetrahydrate and 2.0 parts by weight of aluminum hydroxide having a diameter of 10 μm and 4.0 parts by weight of water in an autoclave, the heating rate was 190 ° C. at 80 ° C./hour. And kept in this state for 5 hours. After the reaction, the mixture was naturally cooled, filtered, washed with water and dried at 105 ° C. to obtain acicular boehmite (3) having a major axis of 10 μm and a minor axis of 0.4 μm (aspect ratio 25). Using the obtained acicular boehmite (3), a separator (3) having a dry mass of the ceramic porous layer of 2.4 g / m ² and a thickness of 21 μm was obtained in the same manner as in Example 1.

Example 4
While stirring 0.92 parts by weight of magnesium acetate tetrahydrate, 2.0 parts by weight of aluminum hydroxide having a diameter of 1 μm, and 4.0 parts by weight of water in an autoclave, the temperature is increased to 190 ° C. at a heating rate of 80 ° C./hour. And kept in this state for 4 hours. After the reaction, the mixture was naturally cooled, filtered, washed with water and dried at 105 ° C. to obtain acicular boehmite (4) having a major axis of 1.0 μm and a minor axis of 0.2 μm (aspect ratio 5). Using the obtained acicular boehmite (4), a separator (4) having a dry weight of the ceramic porous layer of 5.0 g / m ² and a thickness of 22 μm was obtained in the same manner as in Example 1.

(Example 5)
The following material was stirred for 3 hours in the same manner as the preparation of the dispersion liquid (1) to prepare a dispersion liquid (2).

Acicular boehmite (1) 5 parts by weight granular boehmite (manufactured by Daimei Chemicals, trade name: C20) 40 parts by weight sodium hexametaphosphate 0.1 part by weight Special carboxylic acid type polymer activator (Poise 520) 0.2 parts by weight ion 100 parts by weight of replacement water

To the obtained dispersion (2), a 0.6 mass% sodium carboxymethylcellulose aqueous solution (manufactured by Nippon Paper Chemicals, trade name: MAC500LC) and 100 parts by mass of acrylic latex (manufactured by JSR, trade name: TRD202A, concentration 40. 2 parts by mass) was added in an amount of 4.5 parts by mass to prepare a coating liquid (2). Next, the porous substrate (1) was impregnated with the coating liquid (2) and dried to obtain a separator (5) having a dry mass of the ceramic porous layer of 8.5 g / m ² and a thickness of 26 μm.

(Comparative Example 1)
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.

Granular boehmite (manufactured by Daimei Chemical Co., Ltd., trade name: C20) 60 parts by mass sodium hexametaphosphate 0.1 part by mass Special carboxylic acid type polymer activator (product of Kao, trade name: Poaz 520) 0.1 parts by mass ion-exchanged water 100 parts by mass

To 100 parts by mass of the obtained comparative dispersion, 100 parts by mass of a 0.6 mass% sodium carboxymethylcellulose aqueous solution (manufactured by Nippon Paper Industries Co., Ltd., MAC500LC) and acrylic latex (manufactured by JSR, trade name: TRD202A, concentration: 40.2 mass%) Was added to prepare a comparative coating solution. Next, the comparative coating liquid was impregnated with the porous substrate (1) to prepare a comparative separator (1) having a dry mass of the ceramic porous layer of 9.0 g / m ² and a thickness of 26 μm.

[Internal porosity]
The specific porosity of PET was 1.38, the specific gravity of boehmite was 3.04, and the internal porosity was obtained from the separator mass and thickness. The results are given in Table 1.

[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.

[Measurement of membrane resistance]
A separator impregnated with an electrolyte was sandwiched between cylindrical copper having a diameter of 3 cm and a diameter of 1.5 cm. The electrolyte is 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: 1 mol of lithium hexafluorophosphate / L) was used. The resistance value between the electrodes was measured at a bias voltage of 20 kHz and 10 mV using Electrochemical Measurement Unit SI-1280B manufactured by Solatron using both copper as electrodes. 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, 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 applied 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 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 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 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.

  The separators of Examples 1 and 2 and Comparative Example 1 have the same calculated internal porosity, but in the separators of Examples 1 and 2 using acicular boehmite, the internal porosity is small even if the amount of the ceramic porous layer is small. The resistance was small and the average pore diameter was controlled to be small. From the comparison of Examples 1 to 4, when the length of the acicular boehmite is shortened and the aspect ratio is decreased, the average pore diameter tends to be increased, and the thickness of the separator is slightly increased. Met. In Example 5, although acicular boehmite and granular boehmite were used together, the tendency for thickness to increase further was seen. From the comparison between Example 5 and Comparative Example 1, in the separator that does not use acicular boehmite, the average pore diameter increases and the ratio of the discharge capacity to the charge capacity decreases even with the same internal porosity. It was. Moreover, in Comparative Example 1, the thickness increased to 26 μm. From the above results, the separator of the present invention was an excellent separator with controlled pore distribution, thin and low internal resistance.

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

Claims (1)

In a battery separator comprising a porous substrate made of organic fibers and a ceramic porous layer,
A battery separator, wherein the ceramic body contained in the ceramic porous layer is acicular boehmite having an aspect ratio of 5 to 30 and a major axis of 1.0 μm to 10.0 μm.