JP4588136B2 - Battery separator and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a safety and heat resistance provided with a shutdown function based on a porous film having a porous layer of a polyamide-based, for example, a polymetaphenylene isophthalamide-based resin and a porous layer made of a polymer that is heated and melted at an elevated temperature. And a battery separator having excellent mechanical properties.
[0002]
[Prior art]
Conventionally, lithium ion batteries or lithium secondary batteries that use non-aqueous electrolytes are highly expected due to their excellent characteristics such as high energy density and high voltage, light weight and small size. Although it is an electronic component, the battery temperature rises simultaneously due to a short circuit inside and outside the battery, causing problems such as deformation, eruption of contents, and ignition, and various safety measures must be taken. Therefore, it is described in Japanese Patent Publication Nos. 59-14494 and 59-36939 that a porous film made of an aromatic polyamide having excellent heat resistance and chemical resistance is used as a separator. It has been suggested that it can be used. JP-A-5-335005, JP-A-7-78608, and JP-A-7-37571 propose using a nonwoven fabric or paper-like sheet made of aramid fibers for a battery separator. The thinner the separator, the smaller the internal electrical resistance and the better the electrical properties. On the other hand, the film, nonwoven fabric, and paper sheet have a thickness of 50 μm or less and do not have sufficient strength, and have excellent homogeneity. Is very difficult to manufacture industrially.
[0003]
As described above, the separator is an important electronic member interposed between the positive and negative electrodes in the battery, but in recent years, the separator has a shutdown function in order to solve the contradictory demands of electrical characteristics and safety expected from the separator. The idea to make is made. The shutdown means that when the battery temperature rises due to troubles such as overcharge and external short circuit, the porous film as a separator is melted and the pores are closed to interrupt the current. For this reason, a porous film made of polyethylene or polypropylene, which is capable of producing a thin porous film, is used as a support, and a heat-meltable plugging material having a lower melting point is attached thereto, so that the plugging material can be formed at high temperatures. JP-A-60-52 and JP-A-60-136161 have proposed to provide a shutdown function by heating and melting so as to close the pores of the porous film. However, since a thermoplastic film of polyethylene or polypropylene is used as a base material, the heat resistance is low, and there is a problem that the use is very limited in terms of safety.
[0004]
[Problems to be solved by the invention]
It is an object of the present invention to provide a battery separator having a high heat resistance and a shutdown function excellent in safety, mechanical characteristics, and electrical characteristics.
[0005]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have provided a shutdown function by forming a porous layer made of an amide polymer on a porous film made of ultrahigh molecular weight polyolefin, and has heat resistance and safety. The present inventors have found that a porous film having excellent mechanical characteristics and electrical characteristics can be formed. That is, the configuration of the present invention is as follows.
1. A composite porous membrane having at least two layers of a layer (a) having a void made of an aromatic polyamide and a layer (b) composed of a porous membrane made of a heat-meltable thermoplastic polymer, the thermoplastic polymer The heat deformation temperature, which is the starting temperature for closing due to heat shrinkage, is 60 ° C. to 150 ° C., the heat resistance strength of the aromatic polyamide is 300 ° C. or more, and the heat is generated at the temperature at which the battery is abnormally heated. A battery separator characterized in that a plastic polymer melts to close a void, and the polyamide layer retains its shape without melting.
2. 2. The battery separator according to 1 above, wherein the layer (a) has a thickness of 5 to 50 μm, and the layer (b) has a thickness of 5 to 100 μm.
3. 3. The battery separator according to 1 or 2 above, wherein the porosity of the layer (a) is 30% to 70% and the porosity of the layer (b) is 30% to 70%.
4). 4. The battery separator according to any one of the above 1 to 3, wherein the aromatic polyamide is a polymetaphenylene isophthalamide resin, and the thermoplastic polymer is an ultrahigh molecular weight polyolefin having a molecular weight of 400,000 or more. .
5). The composite porous membrane of porosity separator for any of the batteries 1 to 4 above, wherein it is a 30 to 70% - coater.
6). A method for producing a battery separator, comprising forming a porous layer of polymetaphenylene isophthalamide resin on both sides or one side of a porous membrane made of ultrahigh molecular weight polyolefin.
[0006]
The thermoplastic polymer used in the present invention is not particularly limited, and examples thereof include a polyester resin typified by polyethylene terephthalate and an aliphatic polyamide resin typified by 2,6-nylon. These thermoplastic resins must be melted or deformed at an elevated temperature, that is, a temperature formed by abnormal heat generation of the battery, to close the hole. For this purpose, the heat deformation temperature (the temperature at which the hole is closed due to heat shrinkage) is 60. ° C to 150 ° C.
[0007]
Among these, as the thermoplastic polymer, polyolefin is preferable, and an ultrahigh molecular weight polyolefin having a weight average molecular weight of 400,000 or more and preferably 800,000 or more is preferable. Specific examples of the ultrahigh molecular weight polyolefin include polymers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene. Among them, particularly high density ultrahigh molecular weight polyethylene is preferable.
[0008]
As a method for producing a porous film made of polyolefin, a method in which a polyolefin solution is die-extruded and a gel composition obtained by cooling is made porous by stretching, for example, Japanese Patent Publication No. 2-232242 and Japanese Patent Publication No. 5-56251. And Japanese Patent Publication No. 3-64344.
[0009]
The porous membrane may have a thickness of 5 μm to 50 μm, preferably 7 μm to 30 μm, more preferably 10 μm to 20 μm, and a porosity of 30% to 70%, preferably 40% to 65%, more preferably 50 % To 60%. Porosity is apparent density, true density to porosity (vol%) = [(true density−apparent density) / true density] × 100
Can be obtained.
[0010]
Next, the porous layer made of an aromatic polyamide formed on the ultrahigh molecular weight polyolefin porous film will be described.
[0011]
The porous membrane layer can have a thickness of 5 μm to 100 μm, preferably 7 μm to 50 μm, more preferably 10 μm to 30 μm, and a porosity of 30% to 80%, preferably 40% to 70%, more preferably It can be 50% to 65%.
[0012]
Aromatic polyamides need to maintain the self-supporting property of the porous membrane made of the thermoplastic resin even at the heat deformation temperature of the thermoplastic resin, but in order to satisfy such a requirement, the heat resistant strength (specifying that the mechanical strength is maintained) It is sufficient that the temperature of (i.e., the temperature at which the decrease in elastic modulus is confirmed when viscoelasticity is measured while raising the temperature) is 300 ° C. or higher.
[0013]
Such an aromatic amide polymer is preferably a metaphenylene isophthalamide polymer, which is a polycondensation of a meta-oriented aromatic diamine and a meta-oriented aromatic dicarboxylic acid halide, and further has a copolymerization ratio of 40 with respect to the aforementioned monomers. % Is a polymer obtained by polycondensation with para-oriented aromatic diamine, para-oriented aromatic dichloride, aliphatic diamine, and aliphatic dicarboxylic acid, and the amide bond is in the meta position of the aromatic ring or in an oriented position equivalent thereto. It consists essentially of combined repeating units. Specifically, meta-oriented aromatic diamines include 1,3-phenylenediamine, 1,6-naphthalenediamine, 1,7-naphthalenediamine, 2,7-naphthalenediamine, 3,4'-biphenyldiamine, and the like. Examples of the oriented aromatic dicarboxylic acid include isophthalic acid, 1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid, 3,4-biphenyldicarboxylic acid, and the like. Further, the copolymerizable monomer specifically includes para-phenylene diamine, 4,4′-diaminobiphenyl, 2-methyl-paraphenylene diamine, 2-chloro-paraphenylene diamine, and 2,6-naphthalenediamine as para-oriented aromatic diamines. , Etc., para-oriented aromatic dicarboxylic acid dichloride is terephthalic acid chloride, biphenyl-4,4′-dicarboxylic acid chloride, 2,6-naphthalenedicarboxylic acid chloride, etc. For aliphatic diamines, hexanediamine, decanediamine, dodecanediamine , Ethylenediamine, hexamethylenediamine, and the like, and aliphatic dicarboxylic acids include ethylenedicarboxylic acid, hexamethylenedicarboxylic acid, and the like, but are not limited thereto.
[0014]
The formation method of the layer having voids made of the above aromatic polyamide is not particularly limited, but for convenience, a solution of the aromatic polyamide resin is placed on both sides or one side of the thermoplastic polymer, especially a porous membrane made of ultrahigh molecular weight polyolefin. The method of apply | coating and then solidifying and forming a porous layer is mentioned.
[0015]
Examples of the polyamide solvent include, but are not limited to, polar solvents such as N-methyl-2-pyrrolidone, N, N-dimethylacetamide, and N, N-dimethylformamide.
[0016]
The concentration of the polyamide solution is 3 to 30 wt%, more preferably 10 to 15 wt%. Moreover, in order to improve the solubility of this polyamide, a monovalent or divalent cation metal salt can be used. The metal salt may be present in an amount of 0 to 50 wt% with respect to the polymer, and specific examples include calcium chloride, lithium chloride, lithium nitrate, and magnesium chloride.
[0017]
For the formation of the porous layer, the polyamide solution is cast on a porous film made of polyolefin and introduced into a coagulation bath. Examples of the film forming method include die extrusion, coating, and dip method. The coagulation bath uses a mixed solution of an amide solvent and a non-solvent which is inert and compatible with the amide solvent. Specific examples of the amide solvent include N-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide, and preferably N-methyl-2-pyrrolidone is used. Water is used as a non-solvent that is inert and compatible with the amide solvent. Moreover, 1-10 wt% of metal salts can be used with respect to the coagulation bath for the purpose of adjusting the pore diameter. Specific examples include calcium chloride, lithium chloride, lithium nitrate, and magnesium chloride. The concentration of the coagulation bath is 20% or more and 80% or less, more preferably 60 to 70% with respect to the whole of the amide solvent. The coagulation bath temperature is 0 ° C. or higher and 80 ° C. or lower, more preferably 30 ° C. or higher and 60 ° C. or lower. When the amide solvent concentration is less than 20% and the temperature is less than 0 ° C., the surface of the prepared porous membrane has insufficient pores, resulting in a porous membrane with low air permeability. When the concentration exceeds 80% and the temperature exceeds 80 ° C., the polymer is granulated and does not form a porous film.
[0018]
The polyamide film thus solidified is introduced into water and washed. The temperature at this time is not particularly limited.
[0019]
After washing with water, the polyamide membrane is immersed in a mixed solution of an amide solvent and a non-solvent which is inert and compatible with the amide solvent. This promotes crystallization of the porous film. The mixed solution is a mixture of an amide solvent and a non-solvent that is inert and compatible with the amide solvent. Specifically, N-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide and the like can be mentioned, and N-methyl-2-pyrrolidone is preferably used. Further, water is used as a non-solvent which is inert to the amide solvent and is a phase solution. The concentration of the mixed solution is 30% or more and 80% or less, more preferably 50 to 70% of the amide solvent. The temperature is 50 ° C. or higher and 98 ° C. or lower, more preferably 60 ° C. or higher and 90 ° C. or lower. If the amide solvent concentration exceeds 80%, the polyamide forming the porous film dissolves and the structure of the porous film is destroyed. If it is less than 30%, plasticization is insufficient and crystallization does not proceed. If the temperature is less than 50 ° C., crystallization does not proceed, and if it exceeds 98 ° C., the polyamide forming the porous film is dissolved and the structure of the porous film is destroyed.
[0020]
Thereafter, the polyamide membrane is again introduced into water and washed. Further, this high crystallization treatment may be carried out continuously from the coagulation bath.
The porous membrane obtained after this is dried.
The composite porous membrane thus formed has a total thickness of 10 μm to 150 μm, preferably 14 μm to 80 μm, more preferably 20 μm to 50 μm. The overall porosity is 30% to 70%, preferably 40% to 65%, more preferably 50% to 60%.
[0021]
【The invention's effect】
The present invention provides a composite porous membrane for a battery separator having a shutdown function and excellent in heat resistance, safety, mechanical properties, and electrical properties.
[0022]
【Example】
Hereinafter, preferred embodiments of the present invention will be described with reference to examples, but the present invention is not limited to the examples.
[0023]
[Example 1]
10 wt% of polymetaphenylene isophthalamide (Conex (registered trademark) manufactured by Teijin Limited) was dissolved in 90 wt% of N-methyl-2-pyrrolidone. At this time, 47 wt% calcium chloride was dissolved in the polymer as a dissolution aid. This solution was cast on an ultra high molecular weight polyethylene porous film (Toljin-DMSoltech Co., Ltd. Solpour (registered trademark)) at a thickness of 100 μm, and then N-methyl-2-pyrrolidone 60 wt% and water 40 wt%. Was immersed in a coagulation bath at 30 ° C. for 10 minutes, washed with water and dried at 110 ° C. The composite porous membrane has a porosity of 60%, has continuous pores, has an air permeation time of 250 sec / 100 ml at room temperature ((JISL 1096-1990 6.27 breathability), and is heated to 170 ° C. for 5 min. Then, the hole was closed, and it was confirmed that the air permeation time at room temperature was ∞ sec / 100 ml ((JISL 1096-1990 6.27 breathability) and the shutdown property was obtained. At this time, no deformation or shrinkage of the film itself was observed. It was.
[0024]
[Comparative example]
Ultra-high molecular weight polyethylene porous film (Toljin-DS Soltech Co., Ltd. Solpour (registered trademark)) has a porosity of 50%, has continuous pores, and the air permeation time at room temperature is 250 sec / 100 ml. ((JISL 1096-1990 6.27 breathability), begins to deform and contract at 70 ° C. or higher, closes when heated at 170 ° C. for 5 minutes, and air permeation time at room temperature is ∞ sec / 100 ml ((JIS L 1096-1990 6.27 breathability) and a shutdown property was obtained, but the film itself was greatly deformed and contracted.

Claims (6)

A composite porous membrane having at least two layers of a layer (a) having a void made of an aromatic polyamide and a layer (b) made of a porous membrane made of a heat-meltable thermoplastic polymer,
The heat deformation temperature, which is the temperature at which pore closing starts due to heat shrinkage of the thermoplastic polymer, is 60 ° C. to 150 ° C., and the heat resistance strength of the aromatic polyamide is 300 ° C. or more,
A battery separator characterized in that the thermoplastic polymer melts and closes voids at a temperature formed by abnormal heat generation of the battery , and the polyamide layer retains its shape without melting.   The battery separator according to claim 1, wherein the thickness of the layer (a) is 5 µm to 50 µm, and the thickness of the layer (b) is 5 µm to 100 µm.   The battery separator according to claim 1 or 2, wherein the porosity of the layer (a) is 30% to 70%, and the porosity of the layer (b) is 30% to 70%.   4. The battery separator according to claim 1, wherein the aromatic polyamide is a polymetaphenylene isophthalamide resin, and the thermoplastic polymer is an ultrahigh molecular weight polyolefin having a molecular weight of 400,000 or more. Tar. The composite porous membrane separator for any battery porosity of the preceding claims, wherein it is a 30 to 70 percent of the - coater.   A method for producing a battery separator, comprising forming a porous layer of polymetaphenylene isophthalamide resin on both sides or one side of a porous membrane made of ultrahigh molecular weight polyolefin.