JPH05258740A - Separator for battery - Google Patents

Abstract

PURPOSE: To provide a separator for battery having characteristics for losing ion transparency and interrupting a current when the separator is heated at a predetermined temperature or more. CONSTITUTION: A separator for battery is constituted by a first polymer having a predetermined fusion temperature and is comprised of a porous polymer layer having a fine hole therein and a second polymer existing in a fine hole of the porous polymer layer and having the predetermined fusion temperature lower than the fusion temperature of the first polymer. A relative quantity of the second polymer with respect to the first polymer exists in a state in which the second polymer does not close a fine bole in front of heating at a certain temperature of more; however, when the polymer is heated at a certain temperature or more the second polymer is fused, the inside of the fine hole is closed, that ion transparency is offset, and a current is interrupted.

Description

Detailed Description of the Invention

[0001]

[Object of the Invention]

FIELD OF THE INVENTION The present invention relates to a separator used in primary batteries and secondary batteries such as lithium batteries. More specifically, the present invention relates to a battery separator having a property of losing its ion permeability when the temperature inside the battery exceeds a predetermined temperature and rises abnormally.

[0002]

2. Description of the Related Art Electrolyte separators used in batteries, lithium batteries, electrolytic condensers, etc. used a lot of electrolytic paper such as kraft paper and Manila hemp in the past, but recently Non-woven fabrics, porous polyolefin membranes, and the like, which have higher strength and a lower short circuit (short circuit between electrodes) ratio, are being used.

Here, particularly in a lithium battery or the like, since it is designed to pass a high-density current, the temperature inside the battery rises rapidly when a short circuit occurs between electrodes. This is because a short circuit causes an overcurrent to flow, and the overcurrent accelerates a chemical reaction at the cathode. A rapid temperature rise in this battery is extremely dangerous, and it is 14 in a high-speed battery without a protection mechanism.
A temperature of more than 0 ° C. is reached quickly, causing a risk of fire due to discharge of vapor of the organic solvent used in the electrolytic solution and a risk of battery explosion. Various measures are required to avoid such dangers, and the market demand for such measures is extremely high from the principle of safety first.

As a means for solving such a problem, a material that fuses when heated above its melting point is used as a separator while maintaining the strength as a separator, so that the separator material itself melts during abnormal heating. A method has been considered in which the fine pores are blocked by the attachment and the ion permeability is lost, whereby the flow path of the ions, that is, the current, is closed to prevent the overcurrent from flowing. Polyolefins are conceivable as the material used for such a purpose, but since the mechanical strength of polyolefin decreases as the melting point decreases, the material that can actually be used as the separator material is the melting temperature of polypropylene or high-density polyethylene. Is above 130 ° C. When these materials are used, the ion flow path is not completely closed even after the temperature inside the battery actually reaches 130 ° C. or higher, and the current continues to flow. With this, there is a high risk of fire and explosion due to abnormal heating, and it is not very practical.

Further, recently, a porous film of a polymer having a high melting point is used as a supporting layer, and a porous film of a polymer having a low melting point is laminated on the supporting layer to obtain a melting point of the low melting point polymer. A separator having a multi-layer structure in which the low melting point polymer melts to form an impermeable layer at the above temperature is considered. As an example of the separator having such a structure, a battery separator constituted by a polyolefin-based multi-layered porous membrane in JP-A-62-1857 can be used.
Japanese Patent No. 59947 discloses a battery separator in which a crosslinked polyethylene film and an uncrosslinked polyethylene film are laminated. U.S. Pat. No. 4,741,9
No. 79 discloses a battery separator in which a non-woven fabric is coated with wax. In addition, Japanese Patent Laid-Open No. 2-7
Japanese Patent No. 5152 discloses a separator for a lithium battery, which is composed of a porous membrane in which a layer of a fusible non-woven fabric is heat-sealed to form a multilayer on a support made of a porous or microporous polymer. . For reference, an outline of a cross-sectional structure of these multilayer porous membranes, that is, a porous membrane having a multilayer structure in which a fusible porous polymer layer is laminated on a porous polymer layer as a supporting layer is shown in FIG. .

However, in the battery separator composed of these conventional porous polymer membranes,
There were the following problems.

That is, in the case of a small battery such as a lithium battery, it is required from the market demand that the film thickness of the separator be thin and uniform in order to increase the energy density per weight and volume. In the above-mentioned multi-layer structure, there are problems that the film thickness becomes thick and it is difficult to make the film thickness uniform due to the multi-layer structure. In addition, in these multilayer structures,
When the films were laminated, there were many problems that the multi-layered film was curved to one side or peeled off due to the difference in heat shrinkage. Further, when the temperature rises, problems often occur in the state of the film, such as wrinkles on the film and peeling between the films due to the difference in the coefficient of thermal expansion of the film. Further, when the porous films having different melting temperatures are bonded together, the skin layer is generated between the films due to the heating at the time of adhesion, and the permeability of the multilayer film is lost, or the skin layers block the micropores. In some cases, the situation may occur. Furthermore, in order to increase the strength as a separator while maintaining a predetermined film thickness, it is necessary to use a high molecular weight polyolefin as a support layer and increase the film thickness of the support layer.
As a result, it is necessary to make the film thickness of the low melting temperature layer extremely thin, but it was technically extremely difficult to uniformly bond the very thin porous film.

[0008]

[Constitution of the invention]

Means for Solving the Problems The present inventors have found that when the temperature inside a battery abnormally rises above a predetermined temperature, its ion permeability disappears and the current flow path is cut off. In the development of, as a result of intensive research to solve the above-mentioned conventional problems, as a result, the first polymer having a predetermined melting temperature,
A porous polymer layer having micropores therein, and a second polymer existing inside the micropores of the porous polymer layer and having a predetermined melting temperature lower than the melting temperature of the first polymer. And the relative amount of the second polymer with respect to the first polymer is such that before heating above a certain temperature, the second polymer exists in a state in which the second polymer does not block the micropores. A separator is formed by a porous polymer membrane having a single-layer structure, which is characterized in that the second polymer is melted when heated to a temperature higher than or equal to the temperature to close the inside of the fine pores and lose its permeability. It was found that the above-mentioned desired properties can be obtained and the above-mentioned various problems can be solved by the formation, and the present invention has been completed.

That is, the battery separator of the present invention is composed of the first polymer having a predetermined melting temperature,
From a porous polymer layer having micropores therein and a second polymer existing inside the micropores of the porous polymer layer and having a predetermined melting temperature lower than the melting temperature of the first polymer The second polymer is characterized in that it has a single-layer structure, and the second polymer exists in a state in which it does not block the fine pores before heating to a certain temperature or higher. When heated to a temperature higher than the temperature, the second polymer melts and closes the inside of the fine pores, the permeability of the separator disappears, and the current flow path is blocked.

FIG. 1 shows a schematic cross section of a battery separator formed of a single-layer porous membrane of the present invention. As is clear from the figure, in the battery separator according to the present invention, in the wall portion of the fine pores present inside the porous polymer layer of the first polymer having a predetermined melting temperature which constitutes the separator, A layer of a second fusible polymer having a predetermined melting temperature that is lower than the melting temperature of the first polymer is deposited in an amount that does not block the micropores. In a normal state, ions can move between the anode / cathode through the micropores (Fig. 1-a), but when the temperature rises abnormally to the melting point of the second fusible polymer or more, The polymer of No. 2 melts and becomes a fluid state to block the micropores, and the movement of ions through the micropores is blocked (FIG. 1-b).

In the battery separator of the present invention, any one can be used as the first polymer constituting the porous polymer layer as long as it has a high melting temperature that can withstand use as a battery separator. It can be used, but those having high crystallinity and high tensile strength when formed into a film are preferable. Further, as long as the object of the present invention is not violated, it is possible to improve the strength and rigidity by crosslinking. Since it is desirable that the porous polymer layer in the battery separator has heat resistance and high strength, it is considered preferable to increase crystallinity, increase molecular weight, or introduce physical crosslinking. . However, if the molecular weight or the degree of crosslinking is too high, it may not be possible to form a film, which is not desirable. In addition, as a crosslinking method,
In addition to known cross-linking with an electron beam or radiation, cross-linking with a silane coupling agent or a peroxide can be used.

An example of the first polymer having such properties is a molecular weight of about 3 having a melting point of 150 to 180 ° C.
10,000 to 800,000 polypropylene, for example, Hipol under the trade name of Mitsui Nisseki Polymer Co., Ltd., Noblen under the trade name of Mitsui Toatsu Chemical Co., Ltd., Chisso Polypro under the trade name of Chisso Co., Idemitsu Oil Chemical name: Idemitsu Polypro, trade name: Mitsubishi Yuka Co., Ltd., trade name Mitsubishi Polypro, Tonen Kagaku: trade name Tonen Polypro, etc., but the separator of the present invention is used. It goes without saying that other polymers having the above properties can be widely used depending on the type of battery and the like.

In the present invention, the second constituent of the meltable phase
The polymer is selected according to the temperature at which the ion permeability of the separator should be eliminated, and the separator is used as long as it has a predetermined melting temperature lower than the melting temperature of the first polymer selected from this viewpoint. A wide variety can be used depending on the structure of the battery and the like. For example, when used as a separator for a lithium secondary battery using an organic solvent, it is desirable to use a polymer having a melting temperature of 80 to 120 ° C. as the second polymer. Further, copolymers having various compositions can be used, provided that the melting temperature condition is satisfied. Examples of such a second polymer include polyethylene and its copolymers such as low density polyethylene (LDPE), for example, Mitsubishi Polyethylene-LD under the trade name of Mitsubishi Kasei.
Misoran manufactured by Mitsui Petrochemical Co., Ltd., Sumitomo Chemical Co., Ltd.
Product name Sumikasen manufactured by Mitsui Petrochemical Industry Co., Ltd., product name Ultzex manufactured by Mitsui Petrochemical Co., Ltd., product name Frosen manufactured by Sumitomo Seika Chemicals Ltd., product name manufactured by Nippon Petrochemical Co., Ltd. Density polyethylene (LLDPE), for example,
Sumitomo Chemical Co., Ltd. trade name Sumikasen-L, Idemitsu Petrochemical Co., Ltd. trade name Idemitsu Polyethylene-L, Nippon Petrochemical Co., Ltd. trade name Linirex, ethylene-vinyl acetate copolymer, ethylene -Butadiene copolymer, ethylene-pentadiene copolymer, ethylene-methacrylic acid copolymer, ethylene-acrylic acid ester copolymer, ethylene-methacrylic acid ester copolymer, ethylene-propylene copolymer, ethylene-propylene- Examples include diene terpolymer (EPDM), terpolymer with ethylene-maleic anhydride and other monomers.

In the battery using the separator of the present invention, various characteristics such as strength and heat resistance of the separator itself and the temperature required to lose its ion permeability and the temperature at which the battery is usually used are determined. Then, the separator of the present invention can be formed by appropriately combining and selecting the above-mentioned first and second polymers. For example,
When the single-layer porous polymer film of the present invention is used as a separator for a lithium secondary battery, polypropylene having a melting temperature of 130 ° C. or higher is used as the first polymer and about 95 to 120 ° C. as the second polymer. When low density polyethylene having a melting temperature of 0.91 to 0.94 g / cc is used, the strength of the separator itself is also preferable, and a very useful separator which rapidly loses its permeability at a predetermined dangerous temperature is obtained. .

The amount ratio of the first polymer and the second polymer forming the single-layer porous polymer film forming the separator of the present invention is such that the porous film formed by the first polymer is in a normal state. It is necessary that the micropores are not blocked by the second polymer, and that the micropores are blocked by the melting of the second polymer. Such desirable ratios include the thickness of the porous membrane, the size and shape of the fine pores in the membrane, the method for producing the porous membrane, the second
The optimum ratio can be determined by experiments and the like, though it varies depending on the method of introducing the polymer of (1) into micropores and the like. For example, when a porous film made of the first polymer having a porosity of 50% and a thickness of 25 μm is used, the ratio of the first polymer to the second polymer is
50/50 to 95/5 (weight ratio) is desirable.

Further, in order to enhance the binding property between the first polymer and the second polymer, a polymer obtained by copolymerizing an unsaturated carboxylic acid with a polyolefin, such as Bondine or Mitsubishi Oil manufactured by Sumitomo Chemical Co., Ltd. Chemical name: Yukaron, a polyolefin graft material, Mitsui Petrochemical Co., Ltd. product name: Admer, Sumitomo Chemical Co., Ltd. product name: Sumifarm, Asahi Kasei Kogyo Co., Ltd. product name: A component such as Nex may be added as a binder. Furthermore, various additives such as an antioxidant, an ultraviolet absorber, a lubricant, an anti-blocking agent, etc. are added to the first and second polymers according to the present invention, if necessary, within a range that does not impair the object of the present invention. be able to.

As the method for producing the battery separator of the present invention using the above-mentioned first and second polymers, various methods known in the art can be used. Hereinafter, examples of the method for producing the battery separator of the present invention are shown, but these are merely representative examples, and in addition to this, a wide variety of methods are used for producing the battery separator of the present invention. be able to.

The first and second polymers constituting the single-layer porous membrane forming the battery separator of the present invention and the third polymer incompatible with them are melt-kneaded to form a film. The IPN (interpenetrating polymer) has a continuous layer in which polymer molecules are intertwined with each other.
A polymer film having a network structure is obtained. Next, a single-layer porous polymer membrane according to the structure of the present invention can be produced by extracting only the third polymer using a solvent that dissolves only the third polymer. According to this method, the battery separator formed of the single-layer porous polymer film of the present invention can be obtained very easily. For reference, polymer alloy (published by Kyoritsu Publishing, edited by The Society of Polymer Science, Takashi Inoue, Shoji Ichihara), page 1.3, Figure 1.3, IPN
A conceptual diagram of the structure is shown.

The third polymer material used in such a method is incompatible with any of the two kinds of polymers constituting the porous membrane forming the separator of the present invention, and forms an IPN structure with these polymers. However, any polymer can be used as long as it is a polymer that can be extracted and removed only with a specific solvent. Here, the specific solvent means a solvent capable of dissolving only the third polymer without dissolving the first and second polymers at a predetermined temperature, for example, 50 ° C. One skilled in the art can empirically and empirically determine the third polymer that can be used for a particular combination of first and second polymers and the solvent that can be used for that combination. . Here, specific examples of the third polymer that can be used include styrene / hydrogenated butadiene / styrene block copolymer (SEBS), such as Kraton G (trade name) manufactured by Shell Chemical Co., Ltd., and styrene. -Hydrogenated isoprene / styrene and styrene / hydrogenated isoprene block copolymer (SEPS), for example, Septon (trade name) manufactured by Kuraray Co., Ltd., and the like. S
The structural formulas of EBS and SEPS are shown in FIG. These SE
BS and SEPS form an IPN structure when melt-kneaded with a polyolefin such as polypropylene or polyethylene, and after forming a film using such a kneaded product, S with a specific solvent.
A porous membrane having through holes can be prepared by extracting only EBS or SEPS. Specific solvents that can be used for SEPS or SEPS as the specific solvent in the method include toluene, xylene, cyclohexane, methylcyclohexane, methyl ethyl ketone, carbon tetrachloride, chloroform and the like. Further, in addition to these solvents, solvents having solubility parameter values of 7.5 to 9.3 are considered to be solvents that roughly meet this purpose.

Two typical examples of the method for producing the battery separator formed of the single-layer porous membrane of the present invention are given above, but in addition to this, extrusion by the inflation method, the T-die method, or the like. A known plastic film forming technique such as a casting method or a casting method can be adopted. Further, stretching can be carried out in order to increase the strength of the finally obtained porous film. For example, a film produced by a predetermined film-forming method is stretched at a temperature not lower than the glass transition point of the first polymer and lower than the melting point, and then, if necessary, annealed at a temperature higher than the stretching temperature. Thus, the residual stress inside the film is relaxed, and then the solvent is used to extract and remove only the third polymer, whereby a porous film having high strength can be formed. At this time, the draw ratio is 1.1 to 20 times, and particularly 1.1 to 20 times.
5 times is desirable.

The porosity of the porous polymer membrane constituting the battery separator of the present invention is preferably 80% or less, 4
0 to 70% is particularly preferable. When the porosity is 30% or less,
When used as a separator, the electrolyte retainability decreases and the possibility of dry-up increases, and when the porosity exceeds 80%, the strength of the film decreases sharply and the separator cannot withstand actual use. Disappear. Further, when the porosity exceeds 80%, the amount of the second polymer must be increased in order to block the fine pores of the porous layer by melting the second polymer, and the second polymer is The compositional balance with the second polymer is completely lost, and most of the second polymer is composed of the second polymer. In this case, the strength of the porous film cannot be obtained, and breakage due to dendrites generated in the negative electrode or short circuit between the electrodes due to pressing tends to occur.

The porosity is expressed by the following equation.

Porosity = (pore volume / porous membrane volume) × 100 [where, pore volume = (n-butyl alcohol porous membrane weight-absolute dry porous membrane weight) /0.81] The porosity of the porous membrane is controlled by the amount of the third polymer added in the formulation. This is because the third polymer is extracted and removed after forming the film to form pores thereafter. Here, the third polymer is preferably added in an amount of 0.5 to 9 times the total amount of the first polymer and the second polymer to obtain a porous film. When the amount of the third polymer is 0.5 times or less of the total amount of the first polymer and the second polymer, the porosity becomes low and the desired porosity cannot be obtained. If the amount of the polymer exceeds 9 times the total amount of the first polymer and the second polymer, the strength of the porous film cannot be maintained, which is not practical.

Further, by annealing the film composed of the first polymer, the second polymer and the third polymer at a temperature equal to or lower than the melting temperature of the second polymer, the porosity is higher than that of the non-annealed film. Can be raised.
This is because by annealing the film, the stress of each polymer during film formation can be relieved, and the shrinkage in the thickness direction after extracting and removing the third polymer from the film can be reduced.

Further, the desirable average pore diameter of the porous polymer membrane constituting the separator of the present invention is 0.05 μm.
It is preferably from 10 to 10 μm, more preferably from 0.1 to 5 μm. If the average pore diameter is smaller than 0.05 μm, the equivalent series resistance becomes large and the battery performance cannot be fully exhibited, and if it is larger than 10 μm, the occurrence rate of inter-electrode short circuit increases dramatically due to the generation of dendrites. Resulting in. The pore size of the porous membrane is determined by how finely the third polymer is dispersed in the mixture of the first polymer and the second polymer during melt-kneading. In other words, the pore size is determined by how to reduce the structural period of the separated phase of each polymer. This is because the third polymer is extracted and removed after forming the film to form pores thereafter. Therefore, the pore diameter of the porous membrane can be adjusted by changing the temperature and shear stress during melt-kneading, and the melt viscosity of the first polymer, the second polymer and the third polymer. For example, when the first polymer, the second polymer, and the third polymer are melt-kneaded with a kneader, the kneading temperature is lowered to perform melt-kneading in a state where the melt viscosity of each polymer is high, whereby the porosity of the present invention is improved. The pore size of the membrane can be reduced. On the contrary, when the temperature at the time of melt-kneading is increased, the pore diameter of the obtained porous film becomes large. Further, when the shear stress during melt-kneading is increased, the pore diameter of the obtained porous membrane becomes smaller, and conversely when it is weakened, the pore diameter of the obtained porous membrane becomes larger. Further, when a polymer having a high molecular weight is used as the third polymer to increase the melt viscosity, a porous membrane having a small pore size can be obtained. On the contrary, when a polymer having a small molecular weight is used as the third polymer to reduce the melt viscosity, a porous film having a large pore size can be obtained. By combining these events, the pore size of the obtained porous membrane can be easily changed.

The thickness of the porous polymer film constituting the battery separator of the present invention is preferably 15 to 200 μm, particularly preferably 20 μm to 120 μm. When the film thickness is less than 15 μm, the strength of the separator is remarkably reduced, and a problem of short circuit between electrodes due to pressing or breaking of the dendrite is likely to occur.
If it is thicker than μm, the volume occupied by the separator inside the battery becomes large, and it becomes impossible to meet the market demands such as miniaturization and high energy density.

Various batteries can be manufactured using the battery separator of the present invention. In particular, it is extremely useful to use the separator of the present invention in a battery in which it is required to prevent the risk of fire or explosion by blocking the flow path of the current when the temperature inside the battery abnormally rises above a certain temperature. Is.

In this sense, at present, the usefulness of the separator of the present invention is considered to be most effectively exerted in a lithium battery using an organic electrolytic solution. Combinations of an anode material, a cathode material, an electrolytic solution and the like in such a lithium battery are well known in the art, and the separator of the present invention can be used for any combination of these constituent elements. As a preferable organic electrolytic solution that can be used in such a lithium battery, a solution obtained by dissolving lithium perchlorate in a mixed solution of propylene carbonate and 1,2-dimethoxyethane can be mentioned. Further, as a preferable combination of the other solvent and the electrolyte, butyrolactone, propylene carbonate, dimethoxyethane, dioxolane, tetrahydrofuran, 1,2-dimethoxypropane as a solvent, or any other solvent usually used for forming a battery having a lithium anode is used. As a lithium salt soluble in the solvent, lithium trifluorometasulfonate (CF ₃ SO ₃ Li), LiAsF ₆ , LiB
Examples are F ₄ , LiCl _{4 and the} like. Further, useful cathode materials include metallic lithium, lithium-aluminum alloys, lithium-silicon alloys, lithium-boron alloys and 1
Examples include metals of Group A and 2A. Examples of the metal that can be used as the current collector and the support include nickel, stainless steel, aluminum and titanium. A wide range of various anode active materials can be used, and examples thereof include MnO ₂ , TiS ₂ , FeS ₂ , FeS, MoS ₂ and Cu.
Examples thereof include O, V ₆ O ₁₃ , Bi ₂ O _3, and various forms of polyfluorocarbon.

Of course, as described above, the separator of the present invention can be used in lithium batteries other than the above-mentioned ones and batteries other than lithium batteries.

[0030]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Details of various tests used in the following examples are set forth below.

The film thickness was measured with a dial gauge, the average pore size was determined from photographs of the surface and cross section obtained by a scanning electron microscope, and the porosity was
The weight of the porous polymer membrane sample cut into a size of 20 mm × 20 mm when impregnated with n-butyl alcohol and the weight of the sample when it was in an absolutely dry state were respectively calculated, and the values obtained by the following formulas were used. is there.

Porosity = (pore volume / porous membrane volume) × 100 [where, pore volume = (n-butyl alcohol porous membrane weight-absolute dry porous membrane weight) /0.81] The state of blockage inside the fine pores of the porous membrane due to the melting of the second polymer was examined by ion permeability. That is, the obtained porous membrane was heated at a predetermined temperature for a predetermined time and then placed in the apparatus shown in FIG. 4 to examine the permeation of ions. The apparatus shown in FIG. 4 is arranged so that a porous membrane can be placed between two containers. One container contains pure water, and the other container contains a 1% solution of sodium chloride and a surfactant. 50 cc of each solution was added to reduce the surface tension below the critical surface tension of the porous membrane. Due to the difference in concentration between the two solutions, the ions migrate toward the pure water through the porous membrane. The flow of ions moving into this pure water was measured with a conductivity meter over time, and compared with the amount of ion permeation before heating, depending on whether or not the permeation of ions was blocked after heating, the second polymer It was judged whether or not the inside of the fine pores of the porous film was melted and blocked. Also,
The cross section of the porous membrane was observed with a scanning electron microscope before and after the melting test to determine whether the polymer was molten.

[0034]

Example 1 Polypropylene having a melting temperature of 169 ° C. (manufactured by Mitsui Nisseki Polymer Co., Ltd., trade name Hypol B200,
Melt flow rate 0.5g / 10 minutes, density 0.91g
/ Cm ³ ) 50 parts by weight, low-density polyethylene having a melting temperature of 109 ° C. (manufactured by Nippon Petrochemical Co., Ltd., trade name Nisseki Lexron L501, melt flow rate 7.0 g / 10 minutes,
Density 0.917 g / cm ³ ) 50 parts by weight, styrene-hydrogenated isoprene-styrene block polymer (Kuraray Co., Ltd., trade name Septon 2007, melt flow rate 4 g / 10 minutes, density 0.93 g / cm ³ ) 100 parts by weight. A part was sufficiently melt-kneaded at 200 to 220 ° C. using a kneader, and a film was produced by extrusion molding with a T die. Furthermore, this film is 180 ° C, 200 kg / c
It was pressed for 1 minute under the condition of m ² to produce a film having a film thickness of 150 μm. The film was immersed in cyclohexane to completely extract and remove the styrene-hydrogenated isoprene-styrene block polymer, and the surface of the film was washed with new cyclohexane to form a single-layer porous film composed of polypropylene and polyethylene. A film was obtained. The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2.
It was shown to. From the results in Table 2, it was found that the membrane heated to 115 ° C lost the ion permeability. Electron micrographs of the cross section of the porous film before and after the heating test at 115 ° C. are shown in FIGS. 5-a and 5-b, respectively. From these photographs, it is observed that the micropores of the porous film are closed after heating.

[0035]

Example 2 In the same manner as in Example 1, 60 parts of polypropylene (B200) and 40 parts of polyethylene (L501) were used.
Part, styrene-hydrogenated isoprene-styrene block polymer (Septon 2007) 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. It was The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2. From the results of Table 2, it was found that the porous membrane lost the ion permeability after the heating test at each temperature. In addition, electron micrographs of the cross section of the porous film before and after the heating test at 115 ° C. are shown in FIGS. 6-a and 6-b, respectively. From these photographs, it is observed that the micropores of the porous film are closed after heating.

[0036]

Example 3 In the same manner as in Example 1, 70 parts of polypropylene (B200) and 30 parts of polyethylene (L501) were used.
Part, styrene-hydrogenated isoprene-styrene block polymer (Septon 2007) 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. It was The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2. From the results in Table 2, 1
It was found that after heating to 15 ° C. and 130 ° C., the amount of ion permeation was significantly reduced, that is, the ion permeability was lost.

[0037]

Example 4 In the same manner as in Example 1, 80 parts of polypropylene (B200) and 20 parts of polyethylene (L501) were used.
Part, styrene-hydrogenated isoprene-styrene block polymer (Septon 2007) 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. It was The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2. From the results of Table 2, it was found that the amount of ion permeation slightly decreased after heating. However, the remarkable decrease as in Examples 1 to 3 was not observed.

[0038]

Example 5 In the same manner as in Example 1, 70 parts of polypropylene (B200), 30 parts of polyethylene (manufactured by Tosoh Corporation, Petrosen 356, melt flow rate 100, density 0.914 g / cm ³ ), styrene-hydrogenation. Isoprene-styrene block polymer (Septon 200
7) A film composed of 100 parts was prepared, and the styrene-hydrogenated isoprene-styrene block polymer was extracted and removed with cyclohexane to obtain the porous membrane of the present invention. The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Further, IR analysis also revealed that the porous film did not contain a styrene component and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1. The results of the ion permeability test are shown in Table 2.
It was shown to. From the results in Table 2, it was found that the amount of permeation of ions was significantly suppressed after heating at 105 ° C and 115 ° C.

[0039]

Example 6 Polypropylene having a melting temperature of 130 ° C. or higher (manufactured by Mitsui Nisseki Polymer Co., Ltd., trade name Hypol F60)
0) 80 parts by weight, low-density polyethylene having a melting temperature of 104 ° C. (trade name: FLOWCEN G801 manufactured by Sumitomo Seika Chemicals Ltd.) 2
0 parts by weight, styrene-hydrogenated isoprene-styrene block polymer (Kuraray Co., Ltd., trade name Septon 200)
6) 100 parts by weight was sufficiently melt-kneaded at 220 ° C. using a kneader, and a film was prepared by extrusion molding with a T die. Next, the film was immersed in cyclohexane for 1 hour to completely extract the styrene-hydrogenated isoprene-styrene block polymer, and the surface of the film was washed with new cyclohexane to form a single layer porous film composed of polypropylene and polyethylene. A quality film was obtained. The film showed a weight loss due to the content of styrene-hydrogenated isoprene-styrene block polymer due to the weight change before and after extraction. Also, by IR analysis,
It was found that this porous film did not contain a styrene component, and a single-layer porous film composed of polypropylene and polyethylene was obtained. The properties of the obtained film are shown in Table 1.

This porous film was immersed in propylene carbonate, which is a poor solvent for polyolefin, and heated at 100 ° C. for 3 minutes. The cross section of the film was observed with a scanning electron microscope. Unlike before, it was found that the micropores were blocked by heat. Electron micrographs of the cross sections of the porous film obtained in this example before and after the heating test are shown in FIGS. 7 and 8, respectively.

The results of the ion permeability test of the obtained membrane are shown in Table 2. From the results of Table 2, it was found that the amount of ions permeated was significantly suppressed in the heating test at 115 ° C. Table 1: Properties of each porous membrane ─────────────────────────────── Example 1 2 3 4 5 6 6 ───────────────────────────── PE amount (%) 50 40 30 20 30 20 Porosity (%) 33.2 38.9 40.8 45.2 35.6 50 Pore diameter (μm) 1 0.8 0.8 0.8 0.9 1.2 Thickness (μm) 120 120 120 120 120 100 ──────────────────────────── ──── Table 2: Temporal change in conductivity before and after heating ─────────────────────────────────── ─ Conductivity (μS / cm) ─────────────────────────────── Time (hr) 0 1 2 3 ─── ───────────────────────────────── Example 1 Before heating 1.24 303 619 894 115 ° C After heating 1.24 6.55 5.14 5.47 ────────── ────────────────────────── Example 2 Before heating 1.42 177 368 543 105 ° C After heating 0.77 3.82 7.8 11.05 ─────── ──────────────────────── Before heating 1.42 145 338 514 115 ° C After heating 0.77 2.53 2.93 3.76 ──────────── ─────────────────── Before heating 1.42 276 593 903 130 ℃ After heating 0.77 5.72 4.85 4.87 ───────────────── ─────────────────── Example 3 Before heating 1.42 211 407 590 105 ° C After heating 0.77 59.8 124 195 ────────────── ───────────────── Before heating 1.42 159 351 504 115 ° C After heating 0.77 11.3 29.6 44.9 ─────────────────── ──────────── Before heating 0.77 127 276 415 415 130 ° C After heating 0.77 9.74 7.58 11.35 ─────────── ───────────────────────── Example 4 Before heating 1.51 290 630 888 120 ° C After heating 1.3 222 496 689 ──────── ─────────────────────── Before heating 1.51 306 760 1112 140 ° C After heating 1.9 165 382 610 ───────────── ─────────────────────── Example 5 Before heating 1.25 229 430 705 105 After heating at 1.62 12 21.7 31.7 ────────── ───────────────────── Before heating 1.25 189 347 547 115 ° C After heating 1.25 14.2 20.2 28.5 ─────────────── ──────────────── Before heating 1.62 205 394 580 130 ℃ After heating 1.39 54.8 125 202 ──────────────────── ──────────────── Example 6 Before heating 1.30 386 801 1179 115 ° C After heating 1.30 5.44 8.09 12.4 ───── ────────────────────────────── * Heating time = 1 minute for Examples 1, 2, 3, 5, and 6 Example 4 is 10 minutes.

As described above, the battery separator according to the present invention has good ion permeability at room temperature, but when heated above a predetermined temperature, the micropores therein are closed. Therefore, it has a property of losing ion permeability and interrupting a flow path of a current, and is extremely useful as a separator having an unusual heating safety for a lithium secondary battery, for example. Further, even when compared with a film having a multilayer structure conventionally used for this purpose, it solves various problems included in these conventional ones, and an increase in electric resistance value at a predetermined temperature is extremely high. Because it is sharp,
It is extremely useful. INDUSTRIAL APPLICABILITY The battery separator according to the present invention is extremely useful in various batteries for which it is desired to cut off current at a certain temperature or higher, and its industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of a cross-sectional structure of a single-layer porous polymer film constituting a battery separator according to the present invention. FIG. 1-a shows a normal state before heating to a predetermined temperature, and FIG. 1-b shows a state where the fine holes are closed after heating to a predetermined temperature.

FIG. 2 is a diagram schematically showing a cross-sectional structure of a conventional separator made of a multilayered porous film having a fusible layer.

FIG. 3 Styrene / hydrogenated butadiene / styrene block copolymer (SEBS) and styrene / hydrogenated isoprene /
It is a figure which shows the structure of a styrene block copolymer (SEPS).

FIG. 4 is a schematic diagram of an apparatus for measuring ion permeation and determining whether the porous membrane is occluded.

5 is an electron micrograph at a magnification of 5000 times showing the fiber shape of the single-layer porous membrane of the present invention obtained in Example 1. FIG. FIG. 5-a is an electron micrograph showing the shape of the fiber before heating the porous membrane to 115 ° C., and FIG. 5-b is showing the shape of the fiber after heating the porous membrane to 115 ° C.

FIG. 6 is an electron micrograph at a magnification of 5000 times showing the fiber shape of the single-layer porous membrane of the present invention obtained in Example 2. FIG. 6-a is an electron micrograph showing the shape of the fiber before heating the porous membrane to 115 ° C., and FIG. 6-b is the shape of the fiber after heating the porous membrane to 115 ° C.

FIG. 7: Magnification 2 showing the shape of the fibers of the single-layer porous membrane of the present invention obtained in Example 6 before heating to 100 ° C.
It is a 000 times electron micrograph.

FIG. 8: Magnification 2 showing the shape of the fibers of the single-layer porous membrane of the present invention obtained in Example 6 after heating to 100 ° C.
It is a 000 times electron micrograph.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yuichi Mori 1642-212 B-4, Kamariya-cho, Kanazawa-ku, Yokohama, Kanagawa

Claims (3)

[Claims] 1. A porous polymer layer composed of a first polymer having a predetermined melting temperature and having fine pores therein, and a first polymer existing inside the fine pores of the porous polymer layer. Is composed of a second polymer having a predetermined melting temperature that is lower than the melting temperature of the polymer, and the relative amount of the second polymer to the first polymer is
Before being heated above a certain temperature, the second polymer exists in a state where it does not block the fine pores, but when heated above a certain temperature, the second polymer melts and the inside of the fine pores is melted. A separator for a battery formed by a porous film having a single-layer structure, which is characterized in that the amount is such that it closes the surface and loses its permeability. 2. A first polymer having a predetermined melting temperature, a second polymer having a predetermined melting temperature lower than the melting temperature of the first polymer, and incompatible with the first and second polymers. An IPN (interpenetrating polymer networ) having a continuous layer in which polymer molecules are intertwined with each other by melt-kneading a third polymer to form a film.
The method for producing a battery separator according to claim 1, wherein k) a polymer film having a structure is produced, and then only the third polymer is extracted with a solvent that dissolves only the third polymer. . 3. A battery comprising the battery separator according to claim 1.